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 (in general) include substances released by non-neuronal cells that may act in a hormonal or transmitter-like fashion. [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) for an extensive treatment of neurotransmitters in C. elegans.

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

Footnotes
Abbreviations
Acknowledgements and General References
How to cite this document

All Neurons with Neurotransmitter IDs - downloadable Excel spreadsheet


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, ASn, DAn, DBn, HSN (faint, h), PLN, SDQ, URA, URB, VAn, VBn, VCn(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 ADF4, AIA, [AIM]5, AIN, AIY, ALN, ASn(1-11), ASJ, AVA, AVB, AVD, AVE, AVG6, AWB, CAn(1-9)(m)4, CEM(m), DAn(1-9), DBn(1-7), DVA7, DVE(m), DVF(m), HOB(m), HSN(h)4, I16, I36, IL2, M16, M26, M4, M5, MC6, PCB(m), PCC(m), PDA, PDB, PDC(m)8, PGA(m)8, PLN, PVC, PVN6, PVP, PVV(m)9, PVX(m), PVY(m), PVZ(m), R1A(m), R2A(m), R3A(m), R4A(m), R6A(m)9, RIB10, RIF, RIH4, RIR, RIV, RMD, RMF, RMH, SAA6, SAB, SIA, SIB, SMB, SMD, SDQ, SPC(m), SPV(m), URA, URB, URX, VAn(1-12), VBn(1-11), VCn(1-6)(h)11 Pereira et al., 2015
Choline Transporter (HAChT / 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), DVE(m), DVF(m), HSN(h), I1, I3, M1, M2, MC, PVN, SAA, and VC4-5(h) Pereira et al., 2015
ChT cho-1 Uptake assay12 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
ACh Catabolism
acetylcholine catabolism
Acetylcholinesterase (AChE) Total NA Histochemistry13 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., 1985; 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) ASn(1-11), AVA, AVB, AVD, AVE, DAn(1-9), DBn(1-7), DVA, M4, PDA, RIH, VAn(1-12), VBn(1-11), others Pereira et al., 2015
AChE ace-3/ace-414
Reporter transgenics pm3, pm4, pm5, pm7, CAN, some body muscles Combes et al., 2003
AChE ace-3/ace-414 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 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)

Head: ADF(2)4, AIA(2), AIM(2)5, AIN(2), AIY(2), AS1, ASJ(2), AVA(2), AVB(2), AVD(2), AVE(2), AVG6, AWB(2), CEM(4, m), DA1, DB1-2, IL2(6), RIB(2)10, RIF(2), RIH4, RIR, RIV(2), RMD(6), RMF(2), RMH(2), SAA(4)6, SAB(3), SIA(4), SIB(4), SMB(4), SMD(4), URA(4), URB(2), URX(2), VA1, VB1-2   

Pharynx: I1(2)6, I36, M16, M2(2)6, M4, M5, MC(2)6

Ventral nerve cord & body: AS2-10, CAn(1-9)(m)4, DA2-7, DB3-7, HSN(2, h)4, SDQ(2), VA2-11, VB3-11, VCn(1-6) (h)15    

Tail: ALN(2), AS11, DA8-9, DVA7, DVE(m), DVF(m), HOB(m), PCB(2, m), PCC(2, m), PDA, PDB, PDC(m)8, PGA(m)8, PLN(2), PVC(2), PVN(2)6, PVP(2), PVV(m)9, PVX(m), PVY(m), PVZ(m), R1A(2, m), R2A(2, m), R3A(2, m), R4A(2, m), R6A(2, m)9, SPC(2, m), SPV(2, m), VA12

Note: Somas found in the retrovesicular ganglion (RVG) are listed in the head; those in the preanal ganglion (PAG) are listed in the tail.

 
BIOGENIC AMINES
DOPAMINE (DA, 3-hydroxytyramine, dihydroxyphenylethylamine)
Summary List of DA Neurons
DESCRIPTION GENE NAME DETECTION METHOD LOCALIZATION REFERENCES
DA NA HPLC + ED Whole animal Sanyal et al., 2004
DA NA Formaldehyde induced fluorescence (FIF) CEP, ADE, PDE, R5A(m), R7A(m), R9A(m) Sulston et al., 1975
DA Synthesis
dopamine synthesis
Tyrosine Hydroxylase (TH) cat-2
Reporter transgenics CEP, ADE, PDE, R5A(m), R7A(m), R9A(m),
SPSo(m)16
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)16; 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). 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
Pyruvoyl Tetrahydropterin Synthase (PTPS) ptps-118
Reporter transgenics various unidentified neurons and non-neuronal cells; see also 5HT neurons Zhang et al., 2014; Loer et al., 2015
Pterin Carbinolamine Dehydratase (PCBD) pcbd-118
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)19 qdpr-118
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
Antibody20 CEP, ADE, PDE; see also 5HT, OA, and TA neurons Duerr et al., 1999
VMAT cat-1
Reporter transgenics CEP, ADE, PDE, R5A(m), R7A(m), R9A(m); see also 5HT, OA, and TA neurons Flames & Hobert, 2009
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 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 DA neurons (Pdat-1::GFP cells) Carvelli et al., 2004
DA Catabolism
dopamine succinylation & acetylation

[Note: Monoamine catabolism pathways are largely uncharacterized in C. elegans. N-acetylation and N-succinylation are likely significant destinations for monoamines including dopamine (Artyukhin et al., 2013). Oxidation is also possible based on well-characterized pathways in other animals (best known from vertebrates, especially mammals), but has not been demonstrated. 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 COMT.24 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 DA cells. Expressed in nearly all cells in the embryo. Filkin et al., 2007; Kaushal 2008
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
DA cat-1, cat-2, cat-4 DA 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 DA by FIF; see also 5HT-related phenotypes. Sulston et al., 1975
AADC bas-1 Mutant lacks dopamine by FIF; DA 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 DA by FIF; see also 5HT-related phenotypes. Sulston et al., 1975; Desai et al., 1988; Loer et al., 2015
PTPS ptps-1 Mutant lacks DA by FIF; see also 5HT-related phenotypes. Loer et al., 2015
QDPR qdpr-1 Mutant has reduced DA 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 DA by FIF, especially combined with cat-4 reduction-of-function mutation; see also 5HT-related phenotypes. Loer et al., 2015
VMAT cat-1 DA 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 & DA 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 DA uptake in cultured embryonic DA cells. Carvelli et al., 2004
SUMMARY - Dopaminergic Neurons (adult): Hermaphrodite (n = 8), Male (n = 14+2)16

Head: ADE(2), CEP(4)     Ventral nerve cord & body: PDE(2)     Tail (male only): R5A(2), R7A(2), R9A(2), SPSo(2)16

 

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 27 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, RIM29, UV1, gonadal sheath cells Alkema et al., 2005
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) cat-1 Reporter transgenics RIC; see also DA, 5HT neurons Duerr et al., 1999
VMAT cat-1
Antibody20 RIC; see also DA, 5HT neurons Duerr et al., 1999
VMAT cat-1
Uptake assay21 TA and OA are competitive inhibitors of DA and 5HT uptake by CAT-1 in heterologous expression (CV-1 cells). Duerr et al., 1999
TA and OA Catabolism
tyramine succinylation

[Note: Monoamine catabolism pathways are largely uncharacterized 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. See DA 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 lacks 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 = 2?)
Head: RIM(2)29

SUMMARY - Octopaminergic Neurons (adult): Hermaphrodite (n = 2), Male (n = 2?)
Head: RIC(2)

 

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 Formaldehyde induced fluorescence (FIF)30 NSM Horvitz et al., 1982
5HT NA Glyoxylic acid induced fluorescence (GAIF)31 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)32, CA1-4(m)33, CP1-6(m), RPAG(m)34, 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 Synthesis
serotonin synthesis
Tryptophan Hydroxylase (TPH) tph-1
Reporter transgenics NSM, ADF, [AIM, RIH]36, HSN(h), CP1-6(m), R1B(m), R3B(m), R9B(m) Sze et al., 2000
TPH tph-1 Reporter transgenics ASG35 Pocock & Hobert, 2010
Aromatic L-Amino Acid Decarboxylase (AADC)17 bas-1
Reporter transgenics NSM, ADF, [AIM, RIH]36, HSN(h), CP1-6(m), R1B(m), R3B(m), R9B(m); see also DA 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). 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 DA 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 DA neurons Zhang et al., 2014; Loer et al., 2015
Pterin Carbinolamine Dehydratase (PCBD) pcbd-118
Reporter transgenics various unidentified neurons and non-neuronal cells; see also DA neurons Zhang et al., 2014; Loer et al., 2015
Quinoid Dihydropterin Reductase (QDPR) qdpr-118
Reporter transgenics NSM, ADF, other unidentified cells; see also DA neurons Zhang et al., 2014; Loer et al., 2015
Transport
Vesicular Monoamine Transporter (VMAT) cat-1
Antibody20 NSM, ADF, AIM, male VNC & tail cells; see also DA neurons Duerr et al., 1999
VMAT cat-1
Reporter transgenics NSM, ADF, AIM, RIH, HSN(h), VC4-5(h), CP1-6(m), RPAG(m)34, R1B(m), R3B(m), R9B(m); see also DA neurons Duerr et al., 1999; Nurrish et al., 1999; Duerr et al., 2001; Flames & Hobert, 2009
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 assay37 NA - heterologous expression (HEK293 cells) Ranganathan et al., 2001

5HT Catabolism
serotonin oxidation

[Note: Monoamine catabolism pathways are largely uncharacterized in C. elegans. Acetylation and succinylation appear to be a significant destinations for 5HT (Artyukhin et al., 2013). 5HT may be catabolized via MAO. See DA 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?38 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

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 DA-related phenotypes. Desai et al., 1988; Loer & Kenyon, 1993; Loer et al., 2015
PTPS ptps-1 Mutant lacks 5HT-IR; see also DA-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 DA-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 DA-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 DA-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 & DA 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
SUMMARY - Serotonergic Neurons (adult): Hermaphrodite (n = 11 or 13)35, Male (n = 20 or 22)35

Head: ADF(2)4, AIM(2)5, RIH4, [ ASG(2)35]    Pharynx: NSM(2)   Ventral nerve cord & body: HSN(2, h)4, VC4-5(h)4, CP1-6(m)    Tail (male only): RPAG(m)34, R1B(2, m), R3B(2, m), R9B(2, m)

Other possibly serotonergic neurons (not included in above totals): CA1-4(m)4, 32, I539, PHB(2)

 

GAMMA-AMINOBUTYRIC ACID (GABA, gamma-aminobutyrate)
Summary List of GABA Neurons
DESCRIPTION GENE NAME DETECTION METHOD LOCALIZATION REFERENCES
GABA NA Antibody DDn, VDn, RME, AVL, RIS, DVB McIntire et al., 1993b
Synthesis
GABA synthesis
There are other potential pathways for GABA synthesis. In mammalian CNS, GABA is also synthesized from the polyamine putrescine via the enzymes diamine oxidase (DAO) and an aldehyde dehydrogenase (Seiler & Al-Therib, 1974Kim 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, DDn, DVB , RIS, RME, VDn Jin et al., 1999
Transport
Vesicular GABA Transporter (VGAT) unc-47 Reporter transgenics AVL, DDn, DVB , RIS, RME, VDn McIntire et al., 1997
VGAT unc-47 Reporter transgenics AVL, DDn, DVB , RIS, RME, SIAD, SDQ(weak), VDn Barrière & Ruvinsky, 2014
'VGAT Chaperone'40 unc-46
Reporter transgenics AVL, DDn, DVB , RIS, RME, VDn, plus a few unidentified neurons Schuske et al., 2007
'VGAT Chaperone'40 unc-46 Reporter transgenics AVL, DDn, DVB , RIS, RME, SIAD, VDn Barrière & Ruvinsky, 2014
GABA Transporter (GAT) snf-11
Reporter transgenics AVL, [DDn,]41 DVB , [PVQ,]41 RIS, RME, [VDn]41 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)41 Mullen et al., 2006
GAT snf-11 Uptake assay42 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). 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
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 uptake GABA in cultured embryonic cells. Mullen et al., 2006
SUMMARY - GABAergic Neurons (adult): Hermaphrodite (n = 26), Male (n = 26 +/-?)

Head: AVL, DD1, RIS, RME(4), VD1-2     Ventral nerve cord & body: DD2-5, VD3-11    Tail: DD6, DVB, VD12-13

 

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-443
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-443
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-443
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, RIM29, URY, 20 more male-specific cells Serrano-Saiz et al., 2013
Plasma Membrane Glutamate Transporter (PmGluT) glt-144
Reporter transgenics Muscle, hypodermis Mano et al., 2007
PmGluT glt-3
Reporter transgenics Excretory canal cell, pharynx Mano et al., 2007
PmGluT glt-445
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
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SUMMARY - Glutamatergic Neurons (adult): Hermaphrodite (n = 79), Male (n = ~98)

Head: ADA(2), ADL(2), AFD(2), AIB(2), AIM(2)5, AIZ(2), ASE(2), ASG(2)35, 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)29, URY(4)
Pharynx: M3(2), MI, I2(2), I539    Ventral nerve cord & body: ALM(2), AVM   Tail: DVC, LUA(2), PHA(2), PHB(2)39, PHC(2), PLM(2), PVD(2), PVQ(2), PQR(2), PVR, PVV(m)9, R6A(2, m)9

 
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.
    4 ADF, HSN, RIH and VC4-5 are dual-transmitter - both cholinergic and serotonergic; CA1-4 may also be both cholinergic and serotonergic. (See also note 5 regarding AIM.)
    5 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).
    6 These cells express unc-17 but not cho-1 (Pereira et al., 2015).
    7 DVA was originally (mis)identified as DVC by Duerr et al., 2008.
    8 Either PDC or PGA also contains serotonin (Loer & Kenyon, 1993); see also note 34.
    9 PVV and R6A dual-transmitter, both cholinergic and glutamatergic (Pereira et al., 2015).
    10 Expression of cho-1 is strong in RIB, but unc-17 expression is very weak (Pereira et al., 2015).
    11 VC4-5 express unc-17 but not cho-1 (Pereira et al., 2015); VC4-5 also release serotonin (Duerr et al., 1999).
    12 Uptake was Na+-dependent, and blocked by 1 μm HC3 (criterion for high-affinity uptake) (Okuda et al., 2000).
    13 Staining with acetylthiocholine (Method: Karnovsky & Roots, 1964)
    14 The genes ace-4 and ace-3 are coexpressed in an operon, controlled by a single promoter upstream of ace-4 (Combes et al., 2000).
    15 VC4-5 neurons likely take up serotonin released by HSNs (Duerr et al., 2001).
    16 Spicule socket cells SPSo ('non-neuronal' support cells) comprise 2 syncytial cells with 4 nuclei each that likely release DA 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 DA (LeBouef et al., 2014).
    17 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).
    18 Reporters 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).
    19 QDPR is also known as Dihydropteridine reductase (DHPR).
    20 VMAT colocalizes with synaptic vesicles (Duerr et al., 1999).
    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 DA 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 DA 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). 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 DA 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 RIM cells are dual-transmitter, releasing both tyramine and glutamate (Serrano-Saiz et al., 2013).
    30 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.
    31 GAIF method of de la Torre, 1980; comparable to FIF, but apparently more robust in C. elegans.
    32 5HT-IR in VC4, 5 can be weak and unreliable (Duerr et al., 1999), and/or varies with antiserum and staining protocol. See also notes 11, 15.
    33 5HT-IR in CA neurons (CA1-4) is rare (Loer & Kenyon, 1993).
    34 The male-specific, unpaired 'right preanal ganglion' (RPAG) neuron is not identified, but is likely PDC or PGA (Loer & Kenyon, 1993).
    35 ASG is glutamatergic, but 5HT-IR and tph-1 reporter expression appear after exposure to hypoxic conditions (Pocock & Hobert, 2010).
    36 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).
    37 MOD-5 expressed in mammalian cells mediates Na+-dependent uptake of 5HT but not DA, histamine, norepinephrine, GABA, Glu, or Gly. Uptake is blocked by SSRIs, tricyclics, and non-specific monoamine transport inhibitors (Ranganathan et al., 2001).
    38 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 has been reported, and a reporter expression pattern for a very weak homolog Y74C9A.3/homt-1 (Tanaka et al., 2007; Migliori et al., 2012).
    39 I5 and PHB are glutamatergic (Serrano-Saiz et al., 2013), and may also be serotonergic (Sawin et al., 2000).
    40 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).
    41 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).
    42 SNF-11 expressed in Xenopus oocytes mediates Na+- and Cl --dependent high affinity uptake of GABA; uptake is blocked by GAT inhibitors nipecotic acid
    and SKF89976A (Mullen et al., 2006).
    43 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).
    44 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.
    45 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; (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; (ChAT) choline acetyltransferase; (ChT) choline transporter; (DAT) dopamine reuptake transporter; (GAT) GABA transporter; (GTPCH1) GTP cyclohydrolase I; (MAO) monoamine oxidase; (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; also Nate Schroeder for other comments and corrections. Thanks to Alec Knapp and Daniel Sykora for help in proof-reading information and links in the table.

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:

For information on the criteria for assigning a chemical neurotransmitter function in C. elegans see Neurotransmitter criteria.
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 (2016). The Evidence for Classical Neurotransmitters in Caenorhabditis elegans, in WormAtlas.
§To whom correspondence should be addressed. Curtis Loer: cloer@sandiego.edu

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