The Evidence for Classical Neurotransmitters in C. elegans Neurons
by Curtis M. Loer and James B. Rand

This table covers the classical small molecule neurotransmitters for which there is substantial published evidence that they are present, and are likely to function in specific C. elegans neurons. This evidence falls under 'criterion I' as outlined in Criteria for assigning a neurotransmitter function in C. elegans. Other molecules that may serve as neurotransmitters in C. elegans, but for which there is little or no published evidence, are generally not included here. This table also does not typically include substances released by non-neuronal cells that may act in a hormonal or transmitter-like function. [See Hobert (2013) for a presentation of  'the case for and against other neurotransmitter systems' and other transmitter-related aspects of the 'neuronal genome.'] See also Pereira et al. (2015), Gendrel et al. (2016) and Serrano-Saiz et al., 2017 for extensive treatments of neurotransmitters in C. elegans, and the Hobert Lab Neurotransmitter Map Resource.

In the literature cited, there are occasional disagreements about which specific cells show expression in reporter fusion transgenics. We have included in this table published cell "identifications" even for cases in which there was a likely mis-identification. (We have attempted to identify such instances in the footnotes.) In some cases, the situation is less clear, and could reflect real expression differences arising from reporter constructs with different regions of sequence, integration sites, etc. Each Summary List of neurons represents our best judgment based on the evidence available. In general, when conflicts arise, we defer to studies that are 1) more recent, 2) comprehensive, and/or 3) use constructs more likely to represent the true expression pattern (e.g., fosmid or single-copy insertions into native loci).

Acetylcholine (ACh)
Biogenic Amines
   Dopamine
   Tyramine (TA) / Octopamine (OA)
   Serotonin (5HT)
Gamma-aminobutyric acid (GABA)
Glutamate (Glu)

Neurons currently lacking classical neurotransmitter assignments

 Footnotes
Abbreviations
Acknowledgments and General References
How to cite this document

All Neurons with Neurotransmitter IDs - Excel spreadsheet [now updated from the previous WormAtlas 2016 version]
[Note: The neuron list comes originally from Individual Neurons on WormAtlas; the tables are mostly derived with permission from the tables in Pereira et al. (2015) and Serrano-Saiz et al., 2017  supplemental materials.]

ACETYLCHOLINE (ACh)
Summary List of ACh neurons
DESCRIPTION GENE NAME DETECTION METHOD LOCALIZATION REFERENCES
ACh NA Radioenzymatic assay Whole animal Hosono et al., 1987; Hosono & Kamiya, 1991; Nguyen et al., 1995
Synthesis
acetylcholine synthesis
Choline Acetyltransferase (ChAT) cha-11
 Enzymatic assay Whole animal Rand & Russell, 1984
ChAT cha-11
 Antibody Same as unc-17 (see below)2 Duerr et al., 2008
Transport
Vesicular Acetylcholine Transporter (VAChT) unc-171
 Antibody AIA, AIY Altun-Gultekin et al., 2001
VAChT unc-171
 Antibody ALN, AS1-11, DA1-9, DB1-7, HSN (faint, h), PLN, SDQ, URA, URB, VA1-12, VB1-11, VC1-6(h), many others Duerr et al., 2008
VAChT unc-171
 Reporter transgenics IL2, URA, URB Zhang et al., 2014
VAChT unc-171
 Reporter transgenics PCB(m), PCC(m), PVX(m), PVY(m), SPC(m), SPV(m) Garcia et al., 2001; LeBouef et al., 2014
VAChT unc-171
 Reporter transgenics SMB Kim et al., 2015
VAChT unc-171
 Reporter transgenics (fosmid3), Antibody ADF, AIA, [AIM]4, AIN, AIY, ALN, AS1-11(1-11), ASJ, AVA, AVB, AVD, AVE, AVG5, AWB, CA1-9(m), CEM(m), DA1-9, DB1-7, DVA6, DVE(m)7, DVF(m)7, HOB(m), HSN(h), I15, I35, IL2, M15, M25, M4, M5, MC5, PCB(m), PCC(m), PDA, PDB, PDC(m), PGA(m), PLN, PVC, PVN5, PVP, PVV(m), PVX(m), PVY(m), PVZ(m), R1A(m), R2A(m), R3A(m), R4A(m), R6A(m), RIB8, RIF, RIH, RIR, RIV, RMD, RMF, RMH, SAA5, SAB, SDQ, SIA, SIB, SMB, SMD, SPC(m), SPV(m), URA, URB, URX, VA1-12, VB1-11, VC1-6(h)9 Pereira et al., 2015
VAChT unc-171 Reporter transgenics (fosmid3) DX1/2(m)7 Serrano-Saiz et al., 2017
Choline Transporter
(ChT)		 cho-1
 Reporter transgenics Most cholinergic (i.e., unc-17-expressing) neurons Okuda et al., 2000; Matthies et al., 2006; Mullen et al., 2007
ChT cho-1 Reporter transgenics IL2, URA Zhang et al., 2014
ChT cho-1 Reporter transgenics (fosmid3) Expressed in all unc-17-expressing neurons EXCEPT AVG, CA7-9(m), DX1/2(m)7, HSN(h), I1, I3, M1, M2, MC, PVN, SAA, and VC4-5(h); also expressed in R8A(m), R8B(m) Pereira et al., 2015; Serrano-Saiz et al., 2017
ChT cho-1 Uptake assay10 NA - heterologous expression (Xenopus oocyte) Okuda et al., 2000
Postsynaptic Choline/Acetylcholine Transporter snf-6
 Reporter transgenics Neuromuscular junctions of body wall muscle, vulval and enteric muscles; a few unidentified neurons Kim et al., 2004
Catabolism
acetylcholine catabolism
Acetylcholinesterase (AChE) Total NA Histochemistry11 Strong, reliable staining in nerve ring, ventral ganglion, pharyngeal-intestinal valve and anal depressor region; more variable staining in VNC, DNC and PAG Culotti et al., 1981
AChE class A ace-1
 Enzymatic assay Whole animal Johnson et al., 1981
AChE class B ace-2
 Enzymatic assay Whole animal Johnson et al., 1981; Culotti et al., 1981
AChE class C ace-3
 Enzymatic assay Whole animal Kolson et al., 1985a; Kolson et al., 1985b; Johnson et al., 1988
AChE ace-1
 Reporter transgenics CEP, OLL, pm5, body wall muscles, vulval muscles, anal sphincter muscle Culetto, 1999
AChE ace-1
 Reporter transgenics (fosmid3) CEP, OLL Pereira et al., 2015
AChE ace-2
 Reporter transgenics IL cells, AWB(?), AWC, additional head neurons, pm5, PVC, PVQ, PDA, hyp 8-11 Combes et al., 2003
AChE ace-2
 Reporter transgenics (fosmid3) AS1-11, AVA, AVB, AVD, AVE, DA1-9, DB1-7, DVA, M4, PDA, RIH, VA1-1, VB1-11, others Pereira et al., 2015
AChE ace-3/ace-412
 Reporter transgenics pm3, pm4, pm5, pm7, CAN, some body muscles Combes et al., 2003
AChE ace-3/ace-412 Reporter transgenics AIA, DVA, IL2, PDA, RIH, RIV, RMD, SIA, SMD, URA, URB, URX, others Pereira et al., 2015
Related Mutant Phenotypes / Other Supportive Evidence
ACh cha-1
 ACh levels reduced or absent Hosono et al., 1987
ACh unc-17
 ACh levels elevated Hosono et al., 1987
ChAT cha-1
 ChAT enzymatic activity is reduced or absent; hypomorphic cha-1 mutants are Unc and Ric (resistant to inhibitors of cholinesterase); null mutants arrest shortly after hatching (lethal). Rand & Russell 1984; Hosono et al., 1987; Rand, 1989
VAChT unc-17
 Hypomorphic unc-17 mutants are Unc and Ric; null mutants arrest shortly after hatching (lethal). Brenner, 1974; Rand & Russell 1984; Alfonso et al., 1993
VAChT unc-17
 Mutant analysis indicates cholinergic function in pharyngeal neuron MC. Raizen et al., 1995
VAChT unc-17
 Mutant analysis indicates cholinergic function in neuron IL2. Lee et al., 2012
AChE ace-1 Class A - AChE activity is absent. ace-1 mutants have no behavioral phenotype. Johnson et al., 1981
AChE ace-2 Class B - AChE activity is absent. Histochemical staining4 is reduced in ace-2 mutants, and completely eliminated in ace-2; ace-1 double mutants. ace-2 mutants are hypersensitive to Aldicarb, but have no behavioral phenotype, but ace-2; ace-1 double mutants are Unc. Culotti et al., 1981
AChE ace-3 Class C- AChE activity is absent. ace-3 mutants have no behavioral phenotype; ace-3; ace-1 and ace-2 ; ace-3 double mutants have ~wild type behavior; ace-2 ; ace-3; ace-1 triple mutants arrest as L1s (lethal). Johnson et al., 1988

SUMMARY - Cholinergic Neurons (adult): Hermaphrodite (n = 160), Male (n = 193)

By Class (with numbers and notes):
ADF(2), AIA(2), AIM(2)4, AIN(2), AIY(2), ALN(2), AS1-11, ASJ(2), AVA(2), AVB(2), AVD(2), AVE(2), AVG5, AWB(2), CA1-9(m)4, CEM(4, m), DA1-9, DB1-7,  DVA6, DVE(m), DX1(m)7, DX2(m)7, HOB(m), HSN(2, h), I1(2)5, I35, IL2(6), M15, M2(2)5, M4, M5, MC(2)5, PCB(2, m), PCC(2, m), PDA, PDB, PDC(m)8, PGA(m)8, PLN(2), PVC(2), PVN(2)5, PVP(2), PVV(m), PVX(m), PVY(m), PVZ(m), R1A(2, m), R2A(2, m), R3A(2, m), R4A(2, m), R6A(2, m), RIB(2)8, RIF(2), RIH, RIR, RIV(2), RMD(6), RMF(2), RMH(2), SAA(4)5, SAB(3), SDQ(2), SIA(4), SIB(4), SMB(4), SMD(4), SPC(2, m), SPV(2, m), URA(4), URB(2), URX(2), VA1-12, VB1-11, VC1-6(h) 

By Body Region:
Head: ADF(2), AIA(2), AIM(2)4, AIN(2), AIY(2), AS1, ASJ(2), AVA(2), AVB(2), AVD(2), AVE(2), AVG5, AWB(2), CEM(4, m), DA1, DB1-2, IL2(6), RIB(2)8, RIF(2), RIH, RIR, RIV(2), RMD(6), RMF(2), RMH(2), SAA(4)5, SAB(3), SIA(4), SIB(4), SMB(4), SMD(4), URA(4), URB(2), URX(2), VA1, VB1-2   
Pharynx: I1(2)5, I35, M15, M2(2)5, M4, M5, MC(2)5
Ventral nerve cord & body: AS2-10, CA1-9(m)4, DA2-7, DB3-7, HSN(2, h), SDQ(2), VA2-11, VB3-11, VC1-6 (h)   
Tail: ALN(2), AS11, DA8-9, DVA6, DVE(m), DX1(m)7, DX2(m)7, HOB(m), PCB(2, m), PCC(2, m), PDA, PDB, PDC(m)8, PGA(m)8, PLN(2), PVC(2), PVN(2)5, PVP(2), PVV(m), PVX(m), PVY(m), PVZ(m), R1A(2, m), R2A(2, m), R3A(2, m), R4A(2, m), R6A(2, m), SPC(2, m), SPV(2, m), VA12

In all Summary lists, By Body Region - Somas found in the retrovesicular ganglion (RVG) are listed as head neurons; those in the preanal ganglion (PAG) are listed as tail neurons.

 
BIOGENIC AMINES
DOPAMINE (3-hydroxytyramine, dihydroxyphenylethylamine)
Summary List of Dopaminergic Neurons
DESCRIPTION GENE NAME DETECTION METHOD LOCALIZATION REFERENCES
Dopamine NA HPLC + ED Whole animal Sanyal et al., 2004
Dopamine NA Formaldehyde induced fluorescence (FIF) CEP, ADE, PDE, R5A(m), R7A(m), R9A(m) Sulston et al., 1975
Synthesis
dopamine synthesis
Tyrosine Hydroxylase (TH) cat-2
 Reporter transgenics CEP, ADE, PDE, R5A(m), R7A(m), R9A(m),
SPSo(m)13		 Lints & Emmons, 1999; Flames & Hobert, 2009 ; LeBouef et al., 2014
Aromatic L-Amino Acid Decarboxylase (AADC)17 bas-1
 Reporter transgenics CEP, ADE, PDE, R5A(m), R7A(m), R9A(m), SPSo(m)13; see also 5HT neurons Hare & Loer, 2004; Flames & Hobert, 2009 ; LeBouef et al., 2014
BH4 Cofactor Synthesis and Regeneration
BH4 synthesis
Tetrahydrobiopterin (BH4) is an essential cofactor of aromatic amino acid hydroxylases, including TH and TPH, and other enzymes (reviewed by Werner et al., 2011).15 BH4 is synthesized in 3-4 steps (left); the final synthetic enzyme (or enzymes) in C. elegans is unknown. When used as a cofactor, BH4 is oxidized to P4C; BH4 can be regenerated in two reduction steps (right).
GTP Cyclohydrolase I (GTPCH1) cat-4
 Reporter transgenics CEP, ADE, PDE, R5A(m), R7A(m), R9A(m); see also 5HT neurons Sze et al., 2002; Flames & Hobert, 2009
GTPCH1 cat-4 Enzymatic assay Whole animal Loer et al., 2015
Pyruvoyl Tetrahydropterin Synthase (PTPS) ptps-116
 Reporter transgenics Various unidentified neurons and non-neuronal cells; see also 5HT neurons Zhang et al., 2014; Loer et al., 2015
PTPS ptps-1 Enzymatic assay Whole animal Loer et al., 2015
Pterin Carbinolamine Dehydratase (PCBD) pcbd-116
 Reporter transgenics Various unidentified neurons and non-neuronal cells; see also 5HT neurons Zhang et al., 2014; Loer et al., 2015
Quinoid Dihydropterin Reductase (QDPR)17 qdpr-116
 Reporter transgenics CEP, other unidentified cells; see also 5HT neurons Zhang et al., 2014; Loer et al., 2015
Transport
Vesicular Monoamine Transporter (VMAT) cat-1
 Antibody19 CEP, ADE, PDE; see also 5HT, OA, and TA neurons Duerr et al., 1999
VMAT cat-1
 Reporter transgenics CEP, ADE, PDE, R5A(m), R9A(m); see also 5HT, OA, and TA neurons Flames & Hobert, 2009
VMAT cat-1
 Reporter transgenics (fosmid3) CEP, ADE, PDE, R5A(m), R7A(m), R9A(m); see also 5HT, OA, and TA neurons Serrano-Saiz et al., 2017
VMAT cat-1
 Uptake assay21 NA - heterologous expression (CV-1 cells) Duerr et al., 1999
(Plasma Membrane) Dopamine Transporter (DAT) dat-1
 Reporter transgenics CEP, ADE, PDE, R5A(m), R7A(m), R9A(m) Nass et al., 2001; Nass et al., 2002; Flames & Hobert, 2009
DAT dat-1 Antibody CEP, ADE, PDE(rare) McDonald et al, 2007
DAT dat-1
 Uptake assay22 NA - heterologous expression (HeLa cells) Jayanthi et al., 1998
DAT dat-1
 Uptake assay, patch
clamping recording23		 Heterologous expression (tsA-201 cells) and cultured embryonic C. elegans dopaminergic neurons (Pdat-1::GFP cells) Carvelli et al., 2004
Catabolism
dopamine succinylation & acetylation

[Note: Monoamine catabolism pathways are only partially characterized in C. elegans. N-acetylation and N-succinylation are likely significant destinations for monoamines including dopamine (Artyukhin et al., 2013). Oxidation via AMX-2/MAO is likely, similar to the well-characterized pathways in other animals (best known from vertebrates, especially mammals). There are no clear orthologs of catechol-o-methyl transferase (COMT); however, worm comt genes encode proteins with a methyltransferase associated domain also found in mammalian COMT24. Several different monoamine inactivating modifications are found among invertebrate phyla, including N-acetylation, γ-glutamylation, β-alanylation, sulfation, etc. (see review by Sloley, 2004), although it is not always clear whether these are associated with neurons, or that the modifications are catabolic in nature.]
Arylalkylamine N-Acetyltransferase
(AA-NAT)		 anat-1? Enzymatic assay Whole animal Migliori et al., 2012
Monoamine Oxidase (MAO) amx-125
Reporter transgenics ~30 head and tail neurons including ASJ, IL2, other amphid neurons; PHA, PHB, 3 other tail neurons; not expressed in dopaminergic cells. Expressed in nearly all cells in the embryo. Filkin et al., 2007; Kaushal 2008
MAO amx-225
Reporter transgenics 4 pairs amphid neurons: ASJ, and most likely ASH, ASE, AWB, PHA, PHB Hostettler et al., 2017
MAO amx-225
Reporter transgenics Intestine, neurons Filkin et al., 2007

Aldehyde Dehydrogenase (ALDH)

 alh-126 Reporter transgenics Nervous system, including head neurons, neurons along body, PVT, intestine, head mesodermal cell, rectal gland cells, etc. McKay et al., 200327

ALDH

 alh-626 Reporter transgenics Body wall muscle, hypodermis, unidentified cells in head, unidentified cells in tail (adult) McKay et al., 200327

Succinic Semialdehyde Dehydrogenase (SSADH)

 alh-726 Reporter transgenics Intestine, rectal gland cells, nervous system, head neurons (adult) McKay et al., 200327

ALDH

 alh-1026 Reporter transgenics Intestine, nervous system, tail neurons (adult) McKay et al., 200327
Related Mutant Phenotypes / Other Supportive Evidence
Dopamine cat-1, cat-2, cat-4 Dopamine levels (by HPLC + ED) are reduced to about 40% of wildtype in each of these three mutants. Sanyal et al., 2004
TH cat-2 Mutant lacks dopamine by FIF; see also 5HT-related phenotypes. Sulston et al., 1975
AADC bas-1 Mutant lacks dopamine by FIF; Dopaminergic cells do not become serotonin-immunoreactive with 5HTP treatment; see also 5HT-related phenotypes. Sawin et al., 2000; Loer & Kenyon, 1993
AADC bas-1 Age-related decline in bas-1 mRNA levels and reporter expression correlate with reduced FIF in CEPs. Yin et al., 2014
GTPCH1 cat-4 Mutant lacks dopamine by FIF; see also 5HT-related phenotypes. Sulston et al., 1975; Desai et al., 1988; Loer et al., 2015
GTPCH1 cat-4 Mutant lacks GPCH1 enzymatic activity. Loer et al., 2015
PTPS ptps-1 Mutant lacks dopamine by FIF; see also 5HT-related phenotypes. Loer et al., 2015
PTPS ptps-1 Mutant lacks PTPS enzymatic activity. Loer et al., 2015
QDPR qdpr-1 Mutant has reduced dopamine by FIF, especially combined with cat-4 reduction-of-function mutation; see also 5HT-related phenotypes. Loer et al., 2015
PCBD pcbd-1 Mutant has reduced dopamine by FIF, especially combined with cat-4 reduction-of-function mutation; see also 5HT-related phenotypes. Loer et al., 2015
VMAT cat-1 Dopamine by FIF in mutant is reduced in processes and increased in somas; mutant is phenocopied by reserpine (VMAT blocker). Sulston et al., 1975
VMAT cat-1 Expression of human VMAT1 or VMAT2 protein in mutant partially rescues behavioral phenotypes, and restores 5HT & dopamine-induced fluorescence (GAIF). Duerr et al., 1999
VMAT cat-1 Null mutants are deficient in dopamine-mediated behaviors. Duerr et al., 1999
DAT dat-1 Null mutants lack dopamine uptake in cultured embryonic dopaminergic cells. Carvelli et al., 2004
DAT dat-1 Mutants are protected from 6-OHDopamine-induced dopaminergic neuron degeneration, consistent with role of DAT-1 in dopamine uptake. Nass et al., 2001; Nass et al., 2005
DAT dat-1 Mutants display 'swimming induced paralysis' (SWIP) phenotype consistent with excess extrasynaptic dopamine; SWIP is reduced by pretreatment with reserpine, and abolished in cat-2; dat-1 double mutant. McDonald et al., 2007

SUMMARY - Dopaminergic Neurons (adult): Hermaphrodite (n = 8), Male (n = 14+2)13

By Class (with numbers and notes):
ADE(2), CEP(4), PDE(2), R5A(2, m), R7A(2, m), R9A(2, m), SPSo(2, m)13

By Body Region:
Head: ADE(2), CEP(4)    
Ventral nerve cord & body: PDE(2)    
Tail (male only): R5A(2), R7A(2), R9A(2), SPSo(2)13

 

TYRAMINE (TA) and OCTOPAMINE (OA)
Summary List of TA & OA Neurons
DESCRIPTION GENE NAME DETECTION METHOD LOCALIZATION REFERENCES
TA NA TLC Whole animal Alkema et al., 2005
OA NA Radioenzymatic assay 28 Whole animal Horvitz et al., 1982
OA NA HPLC + ED Whole animal Alkema et al., 2005
Synthesis
octopamine synthesis
Tyrosine Decarboxylase (TDC) tdc-1
 Enzymatic assay Whole animal Alkema et al., 2005
TDC tdc-1
 Reporter transgenics, Antibody RIC, RIM, uv129, gonadal sheath cells Alkema et al., 2005
TDC tdc-1
 Reporter transgenics RIC, RIM, HOA(m), R8A(m)30, R8B(m)30 Serrano-Saiz et al., 2017
Tyramine Beta-Hydroxylase (TBH) tbh-1
 Reporter transgenics, Antibody RIC, gonadal sheath cells Alkema et al., 2005; Suo et al., 2006
Transport - TA and OA are both likely substrates for transport by VMAT
Vesicular Monoamine Transporter (VMAT)18 cat-1 Reporter transgenics RIC; see also dopaminergic, 5HT neurons Duerr et al., 1999
VMAT cat-1
 Reporter transgenics (fosmid3) RIC, RIM, HOA(m); see also dopaminergic, 5HT neurons Serrano-Saiz et al., 2017
VMAT cat-1
 Antibody19 RIC; see also dopaminergic, 5HT neurons Duerr et al., 1999
VMAT cat-1
 Uptake assay21 TA and OA are competitive inhibitors of dopamine and 5HT uptake by CAT-1 in heterologous expression (CV-1 cells). Duerr et al., 1999
Catabolism
tyramine succinylation

[Note: Monoamine catabolism pathways are only partially characterized in C. elegans. Succinylation appears to be a significant pathway for TA and OA (Artyukhin et al., 2013). TA and OA may also be catabolized via MAO, likely AMX-2. See Dopamine catabolism for further notes on possible monoamine metabolism, and lists of genes that may be involved.]
Related Mutant Phenotypes / Other Supportive Evidence
TA tdc-1 Mutant lacks TA (by TLC). Alkema et al., 2005
TA, OA tdc-1 Mutant lacks both tyramine- and octopamine succinyl ascarosides. Artyukhin et al., 2013
TA, OA tbh-1 Mutants lacks octopamine succinyl ascarosides; excess succinylated derivative of tyramine is produced. Artyukhin et al., 2013
OA tdc-1, tbh-1 Mutants lack OA (by HPLC+ ED). Alkema et al., 2005
TDC tdc-1 TDC enzymatic activity is absent or strongly reduced. Alkema et al., 2005

SUMMARY - Tyraminergic Neurons (adult): Hermaphrodite (n = 2), Male (n = 3-730)

By Class (with numbers and notes):
HOA(m), possibly R8A(2, m), R8B(2, m)30, RIM(2), uv1(4, h)29

By Body Region:
Head: RIM(2)
Tail (male only): HOA, possibly R8A(2), R8B(2)30
Ventral nerve cord &body: uv1(4, h)29 (non-neuronal)
SUMMARY - Octopaminergic Neurons (adult): Hermaphrodite (n = 2), Male (n = 2)

Head: RIC(2)
Body: gonadal sheath (non-neuronal)
 

SEROTONIN (5HT, 5-hydroxytryptamine)
Summary List of 5HT Neurons
DESCRIPTION GENE NAME DETECTION METHOD LOCALIZATION REFERENCES
5HT NA HPLC + ED Whole animal Sanyal et al., 2004
5HT NA HPLC + ED Whole animal Wang et al., 2017
5HT NA Formaldehyde induced fluorescence (FIF)31 NSM Horvitz et al., 1982
5HT NA Glyoxylic acid induced fluorescence (GAIF)32 NSM, ADF, AIM, RIH, HSN(h), VC4-5(h) Duerr et al., 1999
5HT NA Antibody NSM, ADF, AIM, RIH, HSN(h), VC4-5(h)33, CA1-4(m)34, CP1-6(m), RPAG(m)36, R1B(m), R3B(m), R9B(m) Desai et al., 1988; Loer & Kenyon, 1993; Duerr et al., 1999; Jia & Emmons, 2006
5HT NA Antibody ASG 23 Pocock & Hobert, 2010
5HT NA Antibody Extracellular 5HT-immunoreactivity observed following optogenetic stimulation of NSM, ADF [traditional criterion - stimulation-dependent release]. Tatum et al., 2015
5HT NA Antibody NSM, ADF, AIM4,35, RIH, URX(weak)35, I5(weak)35, CEM(m,weak)35, HSN(h), CP1-6(m), PGA(m)36, PVW(m, weak)37, R1B(m, weak), R3B(m), R9B(m) Serrano-Saiz et al., 2017
Synthesis
serotonin synthesis
Tryptophan Hydroxylase (TPH) tph-1
 Reporter transgenics NSM, ADF, [AIM, RIH]39, HSN(h), CP1-6(m), R1B(m), R3B(m), R9B(m) Sze et al., 2000
TPH tph-1 Reporter transgenics ASG38 Pocock & Hobert, 2010
TPH tph-1 Reporter transgenics (fosmid3) NSM, ADF, HSN(h), CP1-6(m), R1B(m), R3B(m), R9B(m) Serrano-Saiz et al., 2017
TPH tph-1 Reporter transgenics VC4-5(h)33 Mondal et al., 2018
Aromatic L-Amino Acid Decarboxylase (AADC)14 bas-1
 Reporter transgenics NSM, ADF, [AIM, RIH]39, HSN(h), CP1-6(m), R1B(m), R3B(m), R9B(m); see also dopaminergic neurons
 Hare & Loer, 2004; Flames & Hobert, 2009
BH4 Cofactor Synthesis and Regeneration
BH4 synthesis
Tetrahydrobiopterin (BH4) is an essential cofactor of aromatic amino acid hydroxylases, including TH and TPH, and other enzymes (reviewed by Werner et al., 2011).15 BH4 is synthesized in 3-4 steps (left); the final synthetic enzyme (or enzymes) in C. elegans is unknown. When used as a cofactor, BH4 is oxidized to P4C; BH4 can be regenerated in two reduction steps (right).
GTP Cyclohydrolase I (GTPCH1) cat-4
 Reporter transgenics NSM, ADF, HSN(h), CP1-6(m), R1B(m), R3B(m), R9B(m); see also Dopaminergic neurons Sze et al., 2002 ; Flames & Hobert, 2009
Pyruvoyl Tetrahydropterin Synthase (PTPS) ptps-1
 Reporter transgenics NSM, ADF, HSN(h), VC4-5(h), other unidentified cells; see also Dopaminergic neurons Zhang et al., 2014; Loer et al., 2015
Pterin Carbinolamine Dehydratase (PCBD) pcbd-116
 Reporter transgenics Various unidentified neurons and non-neuronal cells; see also Dopaminergic neurons Zhang et al., 2014; Loer et al., 2015
Quinoid Dihydropterin Reductase (QDPR) qdpr-116
 Reporter transgenics NSM, ADF, other unidentified cells; see also Dopaminergic neurons Zhang et al., 2014; Loer et al., 2015
Transport
Vesicular Monoamine Transporter (VMAT) cat-1
 Antibody19 NSM, ADF, AIM, male VNC & tail cells; see also Dopaminergic neurons Duerr et al., 1999
VMAT cat-1
 Reporter transgenics NSM, ADF, AIM40, RIH, HSN(h), VC4-5(h), CP1-6(m), RPAG(m)36, R1B(m), R3B(m), R9B(m); see also DA, OA and TA neurons Duerr et al., 1999; Nurrish et al., 1999; Duerr et al., 2001; Flames & Hobert, 2009
VMAT cat-1
 Reporter transgenics NSM, ADF, RIH, HSN(h), CP1-6(m), PGA(m, weak), PVW(male only, weak)37, R1B(m), R3B(m), R9B(m); see also DA, OA and TA neurons Serrano-Saiz et al., 2017
VMAT cat-1
 Uptake assay21 NA - heterologous expression (CV-1 cells) Duerr et al., 1999
Serotonin Reuptake Transporter (SERT) mod-5
 Reporter transgenics NSM, ADF, AIM, RIH, other neuronal and non-neuronal cells Jafari et al., 2011; Barrière et al., 2014
SERT mod-5
 Antibody NSM, AIM Jafari et al., 2011
SERT mod-5
 Uptake assay41 NA - heterologous expression (HEK293 cells) Ranganathan et al., 2001

Catabolism
serotonin oxidation
[Note: Monoamine catabolism pathways are only partially characterized in C. elegans. Acetylation and succinylation appear to be a significant destinations for 5HT (Artyukhin et al., 2013). 5HT is also metabolized by MAO, and further to 5HIAA. See Dopamine catabolism for further notes on possible monoamine metabolism, and lists of genes that may be involved.]
Serotonin N-Acetyltransferase (SNAT) anat-1? Enzymatic assay Whole animal Muimo & Isaacs, 1993
Arylalkylamine N-Acetyltransferase
(AA-NAT)		 anat-1?42 Enzymatic assay Whole animal Migliori et al., 2012
Monoamine Oxidase (MAO) amx-125 Reporter transgenics ~30 head and tail neurons including ASJ, IL2, other amphid neurons; PHB, 3 other tail neurons Filkin et al., 2007; Kaushal 2008
MAO amx-225 Reporter transgenics Intestine, neurons Filkin et al., 2007
MAO amx-225 Reporter transgenics Pharynx, Intestine, vulval cells, rectal epithelial cells Schmid et al., 2015
MAO amx-225 Reporter transgenics Pharyngeal muscle, intestine, NSM, other head neurons, vulval HSN(h), anus Wang et al., 2017

Aldehyde Dehydrogenase (ALDH)

 alh-126
 Reporter transgenics Nervous system, including head neurons, neurons along body, PVT, intestine, head mesodermal cell, rectal gland cells, etc. McKay et al., 200327
Related Mutant Phenotypes / Other Supportive Evidence
TPH tph-1 Mutant lacks serotonin immunoreactivity (5HT-IR). Sze et al., 2000
GTPCH1 cat-4 Mutant lacks 5HT-IR (or is greatly reduced); see also Dopamine-related phenotypes. Desai et al., 1988; Loer & Kenyon, 1993; Loer et al., 2015
PTPS ptps-1 Mutant lacks 5HT-IR; see also Dopamine-related phenotypes. Loer et al., 2015
QDPR qdpr-1 Mutant has reduced 5HT-IR, especially combined with cat-4 reduction-of-function mutation; see also Dopamine-related phenotypes. Loer et al., 2015
PCBD pcbd-1 Mutant has reduced 5HT-IR, especially combined with cat-4 reduction-of-function mutation; see also Dopamine-related phenotypes. Loer et al., 2015
AADC bas-1 Mutant lacks 5HT-IR (or is greatly reduced); 5HT-IR rescued by exogenous 5HT but not 5HTP (5-hydroxytryptophan); see also Dopamine-related phenotypes. Loer & Kenyon, 1993; Weinshenker et al., 1995; Sawin et al., 2000
AADC bas-1 Age-related decline in bas-1 mRNA levels and reporter expression correlate with reduced NSM 5HT-IR. Yin et al., 2014
VMAT cat-1 Mutant lacks serotonin FIF in NSM processes, but shows increased FIF in somas; reduced 5HT-IR. Horvitz et al., 1982; Loer & Kenyon, 1993
VMAT cat-1 Expression of human VMAT1 or VMAT2 protein in mutant partially rescues behavioral phenotypes, and restores 5HT & dopamine-induced fluorescence (GAIF). Duerr et al., 1999
SERT mod-5 Mutant phenotype consistent with increased presynaptic serotonin, phenocopied by serotonin-specific reuptake inhibitors (SSRIs) such as fluoxetine, partially phenocopied by less-specific tricyclics such as imipramine; mutant is hypersensitive to exogenous serotonin. Ranganathan et al., 2001
SERT mod-5 Mutants lack 5HT-IR in AIM and RIH; fluoxetine or imipramine treatment reduces or eliminates 5HT-IR in AIM and RIH. In mutants, expression of mod-5 cDNA in AIM restores 5HT-IR; expression in other neurons causes ectopic 5HT-IR. Kullyev et al., 2010; Jafari et al., 2011
SERT mod-5 In tph-1; mod-5 double mutants, addition of 5HT fails to rescue loss of 5HT-IR. [Addition of 5HT in tph-1 single mutants rescues 5HT-IR in many neurons.] Jafari et al., 2011
MAO amx-2 Mutants have elevated 5HT levels; treatment with 5HIAA rescues altered vulval induction phenotype in mutant. Schmid et al., 2015
MAO amx-2 Mutants have elevated 5HT levels, and greatly reduced 5HIAA levels. Mutants in grk-2, which elevate AMX-2 levels, have greatly reduced 5HT levels, and greatly elevated 5HIAA levels. Wang et al., 2017
SUMMARY - Serotonergic Neurons (adult): Hermaphrodite (n = 11 or 13)38, Male (n = 20 or 22)38

By Class (with numbers and notes):
ADF(2), AIM(2)4, 35, ASG(2)38, CP1-6(m), HSN(2, h), NSM(2), PGA(m), R1B(2, m), R3B(2, m), R9B(2, m), RIH, VC4-5(h)

By Body Region:
Head: ADF(2), AIM(2)4, 35, RIH, [ASG(2)38]   
Pharynx: NSM(2)   
Ventral nerve cord & body: HSN(2, h), VC4-5(h), CP1-6(m)   
Tail (male only): PGA(m), R1B(2, m), R3B(2, m), R9B(2, m)

Other possibly serotonergic neurons (not included in above totals): CA1-4(m)34, CEM(2, m)35, I535, PHB(2)43, URX35
 

GAMMA-AMINOBUTYRIC ACID (GABA, gamma-aminobutyrate)
Summary List of GABA Neurons
DESCRIPTION GENE NAME DETECTION METHOD LOCALIZATION REFERENCES
GABA NA Antibody AVL, DD1-6, DVB, RIS, RME, VD1-13 McIntire et al., 1993b
GABA NA Antibody ALA44, AVA45, AVB45, AVJ45, AVL, DD1-6, DVB, EF1-4(m), R2A(m), R6A(m), R9B(m), RIB, RIS, RME, SMDD/V45, VD1-13, GLR, hmc, muscle Gendrel et al., 2016
GABA NA Antibody CP9(m), EF1-4(m), R2A(m, weak), R6A(m), R9B(m, weak) Gendrel et al., 2016; Serrano-Saiz et al., 2017
Synthesis
GABA synthesis
There are other potential pathways for GABA synthesis. In the mammalian CNS, GABA is also synthesized from the polyamine putrescine via the enzymes diamine oxidase (DAO) and an aldehyde dehydrogenase (Seiler & Al-Therib, 1974; Kim et al., 2015); such synthesis occurs in cells that do not express GAD. [Note: there are no apparent DAO homologs in C. elegans.] There may also be alternate pathways from putrescine to GABA via n-acetylation (Seiler & Al-Therib, 1974). See also Glutamate synthesis.
Glutamatic Acid Decarboxylase (GAD) unc-25
 Reporter transgenics AVL, DD1-6, DVB, RIS, RME, VD1-13 Jin et al., 1999
GAD unc-25 Reporter transgenics (CRISPR-endogenous locus3) AVL, DD1-6, DVB, EF1-4(m), RIB, RIS, RME, VD1-13 Gendrel et al., 2016
GAD unc-25 Reporter transgenics (CRISPR-endogenous locus3) CP9(m) Serrano-Saiz et al., 2017
Transport
Vesicular GABA Transporter (VGAT) unc-47 Reporter transgenics AVL, DD1-6, DVB, RIS, RME, VD1-13 McIntire et al., 1997
VGAT unc-47 Reporter transgenics AVL, DD1-6, DVB , RIS, RME, SDQ(weak), SIAD, VD1-13 Barrière & Ruvinsky, 2014
VGAT unc-47 Reporter transgenics (fosmid3) AVL, DD1-6, DVB , EF1-4(m), R6A(m), RIB, RIS, RME, SMDD/V, VD1-13 Gendrel et al., 2016
VGAT unc-47 Reporter transgenics (fosmid3) CP9(m), EF1-4(m), R2A(m), R6A(m), R9B(m); Sex-shared cells with adult male only expression: ADF, AS10 or DA7, AS11, PDB, PHC, PVN; Loses expression in adult male: PQR; Cells that express unc-47 reporter but lack GABA-IR (possibly Glycinergic?): CA1-4(m)34, CEM(m), CP1-6(m), CP8(m), DVE(m), DVF(m), HOA(m), PDC(m), PGA(m), PVX(m), PVY(m), PVZ(m), R1A(m), R3A(m), R3B(m), R5A(m), R5B(m), R8A(m), R9B(m), PCB, PCC, SPC(m) Serrano-Saiz et al., 2017
'VGAT Chaperone'46 unc-46
 Reporter transgenics AVL, DD1-6, DVB , RIS, RME, VD1-13, plus a few unidentified neurons Schuske et al., 2007
'VGAT Chaperone'46 unc-46 Reporter transgenics AVL, DD1-6, DVB, RIS, RME, SIAD, VD1-13 Barrière & Ruvinsky, 2014
'VGAT Chaperone'46 unc-46 Reporter transgenics (fosmid3) AVL, DD1-6, DVB, EF1-4(m), R9B(m)47, RIB, RIS, RME,VD1-13 Gendrel et al., 2016
GABA Transporter (GAT) snf-11
 Reporter transgenics AVL, [DD1-6,]48 DVB , [PVQ,]48 RIS, RME, [VD1-13]48 Jiang et al., 2005
GAT snf-11
 Reporter transgenics, antibody AVL, DVB, RID, RIS, RME, 2 neurons in pharynx, 2 neurons in RVG, muscles (body wall, anal, uterine)48 Mullen et al., 2006
GAT snf-11
 Reporter transgenics (fosmid3) ALA44, AVF, AVL, RIB, RME, VD12(m)49, GLR, hmc, muscle Gendrel et al., 2016
GAT snf-11
 Reporter transgenics (fosmid3) CP9(m)49 Serrano-Saiz et al., 2017
GAT snf-11 Uptake assay50 NA - heterologous expression (HRPE cells and Xenopus oocytes) Jiang et al., 2005
GAT snf-11 Uptake assay50 NA - heterologous expression (Xenopus oocyte) Mullen et al., 2006
Catabolism
tyramine succinylation

[Note: GABA catabolism has not been characterized in C. elegans. Although both GABA-T and SSADH (see below) are important in GABA catabolism in the mammalian brain (Tillakaratne et al., 1995), there is currently no evidence they serve the same functions in C. elegans (i.e., no revealing mutant phenotypes), but see below regarding recent gta-1 (GABA-T orthologous gene) expression data. Also, GABA-T transfers an amino group from GABA to α-Ketoglutarate to form Glutamate and SSA, so this reaction is also a potential source of glutamate. See also note26 regarding putative aldehyde dehydrogenases other than SSADH.]
GABA Transaminase
(GABA-T) 		gta-1
Reporter transgenics Body wall muscle, head neurons, unidentified cells McKay et al., 200327; Meissner et al., 2011
GABA Transaminase
(GABA-T) 		gta-1
Reporter transgenics (fosmid3) Ubiquitous51 Gendrel et al., 2016
Succinic Semialdehyde Dehydrogenase (SSADH) alh-726
 Reporter transgenics Intestine, rectal gland cells, hypodermis, nervous system, head neurons McKay et al., 200327
Related Mutant Phenotypes / Other Supportive Evidence
GAD unc-25 Mutant lacks GABA immunoreactivity; mutant has 'shrinker' phenotype. McIntire et al., 1993b
VGAT unc-47 Mutant has 'shrinker' phenotype characteristic of GABA loss of function, but elevated cellular GABA immunoreactivity. McIntire et al., 1997
VGAT Chaperone unc-46 Mutant has 'shrinker' phenotype. McIntire et al., 1993b; Schuske et al., 2007
GAT snf-11 Mutants show GABA-dependent aldicarb resistance. Mullen et al., 2006
GAT snf-11 GABA-dependent behaviors are not rescued by exogenous GABA in unc-25; snf-11 double mutants. Mullen et al., 2006
GAT snf-11 Mutants fail to accumulate GABA in cultured embryonic cells. Mullen et al., 2006
SUMMARY - GABAergic Neurons (adult): Hermaphrodite (n = 32), Male (n = 43)

By Class (with numbers and notes):
AVL, CP9(m), DD1-6, DVB, EF1-4(m), R2A(m), R6A(m), R9B(m), RIB(2), RIS, RME(4), SMDD/V(4), VD1-13

By Body Region:
Head: AVL, DD1, RIB(2), RIS, RME(4), SMDD/V(4), VD1-2    
Ventral nerve cord & body: CP9(m), DD2-5, VD3-11   
Tail: DD6, DVB, EF1-4(m), R2A(m), R6A(m), R9B(m) VD12-13

Other possibly GABAergic neurons (not included in above totals): AVA, AVB, AVJ; possible 'GABA clearance' neurons: ALA, AVF
 

GLUTAMATE (Glu, Glutamic Acid)
Summary List of Glu Neurons
Synthesis
Glu structure		 The amino acid Glu is involved in intermediary metabolism, and is found in proteins, and so is present in all cells and tissues; therefore, enzymes for Glu metabolism are unlikely to be specific markers of a glutamatergic neuron. Glu can be generated by amination of α-ketoglutarate (from the TCA cycle) by glutamate dehydrogenase, or deamination of glutamine by glutaminase. In mammals, neurons don't synthesize Glu (or GABA) de novo from glucose, but are supplied glutamine by astrocytes, which also perform most uptake of synaptically released Glu (see reviews Bak et al., 2006; McKenna, 2007). As in mammals, glutaminases in C. elegans (e.g., glna-1, glna-2, glna-3) are not expressed specifically in glutamatergic neurons (Serrano-Saiz et al., 2013). The vesicular glutamate transporter (VGluT / EAT-4) appears to be the only specific marker of glutamatergic neurons.
DESCRIPTION GENE NAME DETECTION METHOD LOCALIZATION REFERENCES
Transport
Vesicular Glutamate Transporter (VGluT) eat-452
 Reporter transgenics M3, I5, ADA, ALM, ASH, ASK, AUA, AVM, FLP, IL1, LUA, OLL, OLQ, PLM, PVD, PVR Lee et al., 1999 ; Mano et al., 2007
VGluT eat-452
 Reporter transgenics Many head neurons, including ADL, AFD, AIB, AIM, AIZ, ASH, ASE, ASG, ASK, AUA, AVA, AVE, AWB, AWC, RIA, etc. Ohnishi et al., 2011
VGluT eat-452
 Reporter transgenics (fosmid3) M3, MI, I2, I5, ADA, ADL, AFD, AIB, AIM, AIZ, ALM, AQR, ASE, ASG, ASH, ASK, AUA, AVM, AWC, BAG, DVC, FLP, IL1, LUA, OLL, OLQ, PHA, PHB, PHC, PLM, PQR, PVD, PVQ, PVR, RIA, RIG, RIM, URY, 20 more male-specific cells (see below) Serrano-Saiz et al., 2013
VGluT eat-452
 Reporter transgenics (fosmid3) Male-specific cells only: CP0(m, weak), CP5(m, weak), CP6(m, weak), CP7(m), HOA(m), PCA(m), PVV(m), R2B(m, weak), R5A(m), R6B(m, weak), R9A(m, weak) Serrano-Saiz et al., 2017
VGluT vglu-253 Reporter transgenics (fosmid3) AIA, skin cells Serrano-Saiz et al., 2020
Plasma Membrane Glutamate Transporter (PmGluT) glt-154
 Reporter transgenics Muscle, hypodermis Mano et al., 2007
PmGluT glt-3
 Reporter transgenics Excretory canal cell, pharynx Mano et al., 2007
PmGluT glt-455
 Reporter transgenics AUA, RIA, IL2 Mano et al., 2007
PmGluT glt-5
 Reporter transgenics Pharynx Mano et al., 2007
PmGluT glt-6
 Reporter transgenics Excretory canal cell, pharynx marginal cells Reported27 in Mano et al., 2007
PmGluT glt-7
 Reporter transgenics Excretory canal cell Mano et al., 2007
Related Mutant Phenotypes/ Other Supportive Evidence
VGluT eat-4 Mutant has pharyngeal phenotypes similar to glutamate receptor avr-15 mutant, but pharynx responds normally to exogenous Glu, indicating presynaptic function. Dent et al., 1997
VGluT eat-4 Mutant phenotypes (hyperactive foraging, reduced pharyngeal pumping rate) are rescued by expression of human VGluT via eat-4 promoter. Lee et al., 2008
PmGluT glt-1 - glt-7 Individual glt mutants have increased Glu-dependent behaviors. Mano et al., 2007
PmGluT glt-3, glt-4, glt-6
glt-3, glt-4, glt-7		 Triple glt mutants have strongly increased Glu-dependent behaviors. Mano et al., 2007
SUMMARY - Glutamatergic Neurons (adult): Hermaphrodite (n = 79), Male (n = ~98)

By Class (with numbers and notes):
ADA(2), ADL(2), AFD(2), AIB(2), AIM(2)5, AIZ(2),  ALM(2), ASE(2), ASG(2)38, ASH(2), ASK(2), AQR, AUA(2), AVM, AWC(2), BAG(2), CP0, CP5-7, DVC, FLP(2),  HOA(m), I2(2), I543, IL1(6), LUA(2), M3(2), MI, OLL(2), OLQ(4),  PCA(2, m) PHA(2), PHB(2)43, PHC(2), PLM(2), PVD(2), PVQ(2), PQR(2), PVR, PVV(m), R2B(2, m), R5A(2, m), R6B(2, m), R9A(2, m), RIA(2), RIG(2), RIM(2), URY(4)

By Body Region:
Head: ADA(2), ADL(2), AFD(2), AIB(2), AIM(2)5, AIZ(2), ASE(2), ASG(2)38, ASH(2), ASK(2), AQR, AUA(2), AWC(2), BAG(2), FLP(2), IL1(6), OLL(2), OLQ(4), RIA(2), RIG(2), RIM(2), URY(4)
Pharynx: M3(2), MI, I2(2), I543    
Ventral nerve cord & body: ALM(2), AVM, CP0, CP5, CP6, CP7   
Tail: DVC, HOA(m), LUA(2), PCA(2, m) PHA(2), PHB(2)43, PHC(2), PLM(2), PVD(2), PVQ(2), PQR(2), PVR, PVV(m), R2B(2, m), R5A(2, m), R6B(2, m), R9A(2, m)

 

Neurons currently lacking classical neurotransmitter assignments (list taken from Serrano-Saiz et al., 2017)

Sex-shared neurons: ASI, AVF, AVH, AVJ, AVK, AWA, BDU, CAN20, PVM, PVT, PVW [only in hermaphrodites], RID, RIP, RMG, I4, and I6
Male-specific neurons: MCM, DVE, DVF, DX3/4, R4B, R5B, R7B, and SPD
 
Notes
    1 The genes unc-17 and cha-1 are coexpressed in an operon, controlled by a single promoter upstream of unc-17 (Alfonso et al., 1994a).
    2 Because of the genomic structure of the unc-17-cha-1 locus, it is likely that the genes are co-expressed in the same neurons. However, expression of cha-1 was confirmed only for the unc-17 -expressing neurons identified in the Duerr et al., 2008 study.
    3 Most reporter transgenics to date have been made with a few kbp of upstream genomic sequence fused to the reporter (transcriptional, or translational near the N-terminus), and therefore may lack important regulatory elements (distant upstream, within coding, and downstream). Larger fosmid-based reporters typically have 35-40 kbp of genomic sequence, both upstream and downstream, with the reporter fused to the full length coding sequence at the C-terminus, or separated from the gene of interest by an SL2-splice site creating an artificial operon (e.g., see Dolphin & Hope, 2006; Sarov et al., 2006; Tursun et al., 2009). Therefore, transgenics with these fosmid (or BAC) constructs may be more likely to reproduce the expression pattern of the endogenous locus. Issues with both small fusion reporter constructs and fosmid reporters that are extrachromosomal or inserted into ectopic genomic locations may also progressively be resolved with CRISPR-generated knock-in reporters of various types inserted as single copies into the endogenous locus.
    4 AIM neurons are dual-transmitter glutamatergic/serotonergic in hermaphrodites (Serrano-Saiz et al., 2013); in males, AIM neurons are glutamatergic/serotonergic until L3, then switch to a cholinergic/serotonergic dual-transmitter phenotype (Pereira et al., 2015).
    5 These cells express unc-17 but not cho-1 (Pereira et al., 2015).
    6 DVA was originally (mis)identified as DVC by Duerr et al., 2008.
    7 Pereira et al., 2015 originally identified 2 male-specific tail neurons as DVE & DVF; Serrano-Saiz et al., 2017 revised this identification to be DX1/2.
    8 Expression of cho-1 is strong in RIB, but unc-17 expression is very weak (Pereira et al., 2015).
    9 VC4-5 express unc-17 but not cho-1 (Pereira et al., 2015); VC4-5 also release serotonin (Duerr et al., 1999).
    10 Uptake was Na+-dependent, and blocked by 1 μm HC3 (criterion for high-affinity uptake) (Okuda et al., 2000).
    11 Staining with acetylthiocholine (Method: Karnovsky & Roots, 1964).
    12 Although the genes ace-4 and ace-3 are coexpressed in a bi-cistronic operon (controlled by a single promoter upstream of ace-4), the level of ace-4 mRNA is exceedingly low, and there is no detectable ACE-4 protein or enzymatic activity present in worms (Combes et al., 2000). Also, there is no known ace-4-orthologous gene in nematodes outside of genus Caenorhabditis; only an ace-3 gene is present. .
    13 Spicule socket cells SPSo ('non-neuronal' support cells) comprise 2 syncitial cells with 4 nuclei each that likely release dopamime to promote sperm release, acting much like neurons (LeBouef et al., 2014). Interestingly, SPSo cells do not appear to express DAT-1, and SPSo-mediated behavior is not rescued by exogenous dopamine (LeBouef et al., 2014).
    14 AADC is also known as 5HTP Decarboxylase or Dopa Decarboxylase (DDC); in animals, the enzyme generally has broad substrate specificity, catalyzing both serotonin and dopamine synthesis (reviewed by Zhu & Jorio, 1995).
    15 BH4 is also required for the function of other aromatic amino acid hydroxylases such as phenylalanine hydroxylase (pah-1), and the lipid metabolic enzyme alkylglycerol monooxygenase (agmo-1); therefore, expression of BH4 synthesis and regeneration genes is not neuron-specific. They are highly expressed in the hypodermis, and also in the intestine (Loer et al., 2015). BH4 is also required for nitric oxide synthase (NOS) function; however, NOS has not been identified in C. elegans (Gusarov et al. 2013).
    16 Reporters for these BH4 synthesis and regeneration genes to date have shown limited expression in identified dopaminergic and serotonergic neurons (Zhang et al., 2014; Loer et al., 2015), despite mutant phenotypes indicating function in those cells promoting serotonin and dopamine synthesis (Loer et al., 2015).
    17 QDPR is also known as Dihydropteridine reductase (DHPR).
    18 Vesicular monoamine transporter (VMAT) is used in common by all monoaminergic neurons to transport the neurotransmitter into synaptic vesicles for release (Eiden & Weihe, 2011). In C. elegans, this includes neurons using dopamine, tyramine, octopamine, serotonin and perhaps one or more as yet unidentified monoamines20.
    19 VMAT colocalizes with synaptic vesicles (Duerr et al., 1999).
    20 Expression of a  cat-1/VMAT fosmid (but no other monoaminergic markers) may indicate CAN uses an unidentified monoamine (Serrano-Saiz et al., 2017).
    21 VMAT's relative affinity for monoamine substrates: dopamine ~ tyramine > serotonin > norepinephrine ~ octopamine > histamine (Duerr et al., 1999).
    22 DAT expressed in human cells mediates Na+ and Cl--dependent uptake of dopamine better than norepinephrine or other transmitters. Transport by DAT was blocked by tricyclics (especially imipramine) and other monoamine transport inhibitors (Jayanthi et al., 1998).
    23 DAT mediates Na+ and Cl--dependent uptake of dopamine in both heterologous (tsA-201) and cultured native C. elegans cells, and shows electrogenic activity (Carvelli et al., 2004).
    24 Among the named C. elegans comt genes, four encode proteins orthologous to mammalian COMT-domain containing protein 1 (COMT-D1), and are most similar to bacterial and plant known and putative O-methyltransferases, including Caffeic Acid O-Methyltransferase (also abbreviated COMT) and Caffeoyl CoA O-Methyltransferase (CCoAOMT, Ferrer et al., 2005). It is plausible that one or more of the worm COMT proteins could act on a catechol-containing substrate based on similarity to CCoAMTs - i.e., the substrate caffeoyl CoA has a catechol structure that is methylated on the 3-hydroxyl by CCoAMT. The worm COMT proteins, however, have very limited or no significant sequence homology to mammalian COMTs except among S-adenosylmethionine binding residues (SAM aka AdoMet, the methyl donor) found in all AdoMet-dependent methyltransferases (e.g., see Martin & McMillan, 2002). Currently, there is no evidence suggesting comt gene function in neurotransmitter metabolism; no informative mutant phenotypes (by RNAi) and no expression patterns have been reported.
    25 Among the amx gene encoded proteins, the AMX-2 predicted protein is most similar to a mammalian monoamine oxidase (MAO-A), and has been shown to metabolize serotonin (Schmid et al., 2015). AMX-1 may be a histone demethylase (orthologous to lysine-specific histone demethylases). The protein encoded by amx-3 (not listed) is most similar to polyamine and spermine oxidases; no expression pattern has been reported to date.
    26 There are 13 identified alh genes that encode proteins orthologous or highly similar to mammalian aldehyde dehydrogenases (ALDHs). Those shown here have relevant reporter expression patterns. To date, there are no expression patterns reported for alh-2, alh-3, alh-4, alh-5, alh-11, and alh-12; alh-8 is expressed in muscle mitochondria (Meissner et al., 2011); alh-9 is expressed only in hypodermis, during embryogenesis (Mounsey et al., 2002); alh-13 is expressed in the adult intestine (McKay et al., 2003). See also GABA catabolism regarding alh-7 / SSADH. Aldehyde dehydrogenases convert a wide array of both endogenous and exogenous aldehydes to carboxylic acids in amino acids, biogenic amines, carbohydrates, lipids, etc. In humans, the same ALDH (ALDH1A1) that converts DOPAL to DOPAC in dopminergic neurons elsewhere converts retinal to retinoic acid, and oxidizes the ethanol metabolite acetaldehyde (Marchitti et al., 2008). ALDH1A1 is also capable of mediating GAD-independent GABA synthesis in mammalian brain via an alternative pathway from putrescine (Seiler & Al-Therib, 1974; Kim et al., 2015).
    27 Found in Hope Expression Pattern DB (Reference: Hope et al., 1996).
    28 Method of  P. D. Evans, 1978.
    29 The uterine uv1 cells appear to be neuroendocrine. They express neurosecretory proteins and neuropeptides, contain neuroscretory vesicles, and likely release tyramine to inhibit egg laying (Alkema et al., 2005 and references therein).
    30 Neither R8A or R8B express a cat-1/VMAT fosmid, making it unclear whether they actually use tyramine as a neurotransmitter (Serrano-Saiz et al., 2017).
    31 FIF for serotonin in C. elegans is weak and extremely labile (Horvitz et al., 1982), which likely explains why it has been seen only in the NSM neurons.
    32 GAIF method of de la Torre, 1980; comparable to FIF, but apparently more robust in C. elegans.
    33 5HT-IR in VC4-5 can be weak and unreliable (Duerr et al., 1999), and/or varies with antiserum and staining protocol. Although at least one smaller tph-1 reporter construct does show expression, Serrano-Saiz et al., 2017 did not observe 5HT-IR or cat-1/VMAT fosmid reporter expression in VC4-5.
    34 5HT-IR in CA neurons (CA1-4) is rare (Loer & Kenyon, 1993), but is not reported by others (e.g., Serrano-Saiz et al., 2017). CA neurons also do not express other serotonergic marker genes. Strong 5HT-IR in likely CA homologs is observed in other nematodes, including other Caenorhabditis species (Loer & Rivard, 2007).
    35 Serrano-Saiz et al., 2017 suggest that AIM, CEM, I5 and URX are '5HT clearance' neurons - they saw no expression of tph-1 or cat-1/VMAT in the cells. Unlike the other cells, which are weakly 5HT immunoreactive, AIM stains strongly and reliably with serotonin antibodies.
    36 The male-specific, unpaired 'right preanal ganglion' (RPAG) neuron was first identified as either PDC or PGA (Loer & Kenyon, 1993); it is now known to be PGA (Serrano-Saiz et al., 2017).
    37 5HT-IR and cat-1/VMAT fosmid expression in PVW is sexually dimorphic, found only in the adult male (Serrano-Saiz et al., 2017).
    38 ASG is glutamatergic, but 5HT-IR and tph-1 reporter expression appear after exposure to hypoxic conditions (Pocock & Hobert, 2010).
    39 Expression of tph-1 and bas-1 in AIMs and RIH is rare with most reporters; these cells may be '5HT-absorbing' via MOD-5/SERT (and serotonin-releasing) rather than 5HT-synthesizing neurons (Kullyev et al., 2010; Jafari et al., 2011). No expression was observed in the RIH and AIM (and VC4-5) neurons with tph-1 fosmid reporter (Serrano-Saiz et al., 2017).
    40 Reported CAT-1/VMAT antibody staining in AIM (Duerr et al., 1999) may actually be in the tyraminergic RIM, based on a cat-1 fosmid reporter that otherwise fully replicates previous antibody staining (Serrano-Saiz et al., 2017).
    41 MOD-5 expressed in mammalian cells mediates Na+-dependent uptake of 5HT but not dopamine, histamine, norepinephrine, GABA, Glu, or Gly. Uptake is blocked by SSRIs, tricyclics, and non-specific monoamine transport inhibitors (Ranganathan et al., 2001).
    42 AA-NAT activity may be catabolic for serotonin and/or synthetic for melatonin. Although there are no clear orthologs of hydroxyindole-o-methyltransferase (HIOMT) in C. elegans, synthesis of melatonin, and a reporter expression pattern for a very weak homolog (Y74C9A.3/homt-1) has been reported (Tanaka et al., 2007; Migliori et al., 2012).
    43 I5 and PHB are glutamatergic (Serrano-Saiz et al., 2013), and may also be serotonergic (Sawin et al., 2000).
    44 Cells / tissues likely mediate GABA clearance and/or recycling; they do not express unc-47 (VGAT) (Gendrel et al., 2016).
    45 Anti-GABA staining in AVA, AVB and AVJ is weak; the cells express no other GABA-related marker genes (Gendrel et al., 2016).
    46 UNC-46 is a BAD-LAMP-related protein required for trafficking UNC-47/VGAT to synaptic vesicles at synapses, specifically in GABAergic neurons in worms. Rescuing UNC-46-GFP fusion proteins co-localize with synaptic varicosities (Schuske et al., 2007).
    47 Expression is weak and inconsistent.
    48 There are discrepancies between the snf-11 expression patterns reported by Jiang et al. (2005) and those reported by Mullen et al. (2006); the Jiang et al observations are likely due to an error in promoter construction (see Mullen et al., 2006).
    49 Male-specific expression of  snf-11 in VD12 reported in Gendrel et al., 2016 is instead expression in the male-specific CP9 (Serrano-Saiz et al., 2017).
    50 SNF-11 expressed in Xenopus oocytes (Jiang et al. 2005; Mullen et al., 2006) or in mammalian HRPE cells (Jiang et al. 2005) mediates Na+- and Cl--dependent high affinity uptake of GABA; uptake is blocked by GAT inhibitors nipecotic acid and SKF89976A (Mullen et al., 2006).
    51 Ubiquitous expression of gta-1 (single GABA-T orthologous gene) is consistent with a role beyond GABA metabolism (Gendrel et al., 2016).
    52 Lee et al., 1999 reported eat-4 expression in NSM, AVJ, and IL2, and Ohnishi et al., 2011 reported eat-4 expression in the AVA, AVE, SIB, RMD and ASJ neurons; however, subsequent analysis with the same transgenic reporter constructs did not replicate those cell IDs (Serrano-Saiz et al., 2013).
    53 Reporter expression localizes to vesicular structures in the soma, indicating VGLU-2 may not be involved in synaptic vesicle function; however, vglu-2 mutants have AIN-associated olfactory behavior defects indicating function. In C. elegans alone (not other sequenced Caenorhabditis species), there is a degenerate duplicate gene, vglu-3, for which reporters show no expression (Serrano-Saiz et al., 2019).
    54 There is no glt-2 (the sequence originally named glt-2 was found to be a splice variant of glt-1). Although all glt genes are listed here, it seems unlikely that all are involved in neuronal glutamate use.
    55 glt-4 may be a primarily presynaptic neuronal GluT.
Abbreviations
  • Anatomical: (DNC) dorsal nerve cord; (h) hermaphrodite-specific cell; (m) male-specific cell; (PAG) preanal ganglion; (RVG) retrovesicular ganglion; (VNC) ventral nerve cord
  • Methodological: (5HT-IR) Serotonin immunoreactivity; (GABA-IR) GABA immunoreactivity; (HPLC) high performance liquid chromatography; (ED) electrochemical detection; (TLC) thin layer chromatography; (FIF) formaldehyde induced fluorescence; (GAIF) glyoxylic acid induced fluorescence
  • Proteins/Enzymes: (AADC) aromatic L-amino acid decarboxylase; (AA-NAT) arylalkylamine N-acetyltransferase; (AChE) acetylcholinesterase; (AGMO) alkylglycerol monooxygenase; (ChAT) choline acetyltransferase; (ChT) choline transporter; (DAT) dopamine reuptake transporter; (GAT) GABA transporter; (GTPCH1) GTP cyclohydrolase I; (MAO) monoamine oxidase; (PAH) phenylalanine hydroxylase; (PmGluT) plasma membrane glutamate transporter; (PCBD) pterin carbinolamine dehydratase; (PTPS) pyruvoyl tetrahydropterin synthase; (QDPR) quinoid dihydropterin reductase [aka DHPR]; (SR) sepiapterin reductase; (SNAT) serotonin N-acetyltransferase; (SERT) serotonin reuptake transporter; (TPH) tryptophan hydroxylase [aka TrpH]; (TBH) tyramine beta-hydroxylase; (TDC) tyrosine decarboxylase; (TH) tyrosine hydroxylase; (VAChT) vesicular acetylcholine transporter; (VGAT) vesicular GABA transporter; (VGluT) vesicular glutamate transporter; (VMAT) vesicular monoamine transporter
Acknowledgements and General References

We thank Oliver Hobert for extensive comments and suggestions; members of the Hobert lab (some now former) Marie Gendrel, Laura Pereira and Esther Serrano-Saiz; also Nate Schroeder for other comments and corrections. Thanks to Alec Knapp and Daniel Sykora (Loer lab) for help in proof-reading information and links in the table (2016 version).

Beside primary sources listed in the table above, other sites and reviews of C. elegans literature were helpful in assembling this information, including the following:

See here for more information on the Criteria for Assigning a Neurotransmitter Function in C. elegans.
To see all monoamine synthesis and degradation pathways refer to Monoamine Pathways Chart.

How to Cite this Document

This section should be cited as: Loer, CM§ & Rand, JB (2022). The Evidence for Classical Neurotransmitters in Caenorhabditis elegans, in WormAtlas. doi:10.3908/wormatlas.5.200
§To whom correspondence should be addressed. Curtis Loer: cloer@sandiego.edu

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