Criteria for assigning a neurotransmitter function in C. elegans:
For detailed information on specific neurotransmitters in C. elegans, see: Neurotransmitter Table
By classical definition, for a substance to be accepted as a neurotransmitter of a particular neuron, it must meet certain criteria1;
Presence of the chemical within the cell. The chemical is either synthesized by the neuron or is taken up from other cells that release it.2
Stimulus-dependent release. It is released in appropriate quantities by the neuron upon stimulation.
Action on postsynaptic cell. Exogenous application of the substance in appropriate amounts mimics the action of the endogenously-released substance on the postsynaptic cell or organ.
Mechanism for removal. [Note, not always included as a criterion] A specific mechanism exists to remove the substance from the synaptic cleft, i.e., by degradation or reuptake.
1See: Criteria That Define a Neurotransmitter, in Neuroscience by Purves et al., 2nd Ed., 2001, Sinauer Associates.
2Although most cells that release a neurotransmitter synthesize the neurotransmitter themselves, this criterion has recently been expanded beyond synthesis alone, to include rare cells that do not synthesize, but instead take up the neurotransmitter from the vicinity and then re-release it.
For technical reasons, the traditional criteria cannot be met for putative neurotransmitters in C. elegans, although the advent of new microfluidic devices coupled with optical stimulation may soon overcome the difficulties currently encountered in the worm for isolation and analysis of substances following stimulated release (i.e. traditional physiological techniques to identify a chemical as a neurotransmitter). Nevertheless, a large body of 'circumstantial' evidence indicates that various neurotransmitters are used by specific neurons in the worm. Hence, a modified set of criteria may have to suffice to identify the neurotransmitter used by a particular neuron in the worm although traditional criteria cannot fully be met.
Reasonable criteria for identifying a neurotransmitter in C. elegans:
I. Localization in the neuron of:
- The putative neurotransmitter by:
Localization by such techniques should be corroborated by identification of the
genuine substance in the whole animal, e.g., by HPLC.
- Immunocytochemistry or
- Other histological technique (e.g., formaldehyde-induced fluorescence)
- Biosynthetic, transport (vesicular and/or reuptake) or catabolic enzymes3 typically
associated with the neurotransmitter by:
Similar to (A) above, localization should be corroborated by identification of
the enzymatic activities in whole worms, and loss of such activity in
appropriate mutants. In the case of transporters, substrate specificity may be tested by expression
in heterologous systems such as the Xenopus oocyte.
- Immunocytochemistry or other histological techniques
- Expression of genes encoding such proteins as seen by:
- Reporter genes
- In situ RNA hybridization (not commonly done in C. elegans)
3Reuptake transporters and catabolic enzymes may be also found in postsynaptic partners,
other nearby cells, or in the nearby extracellullar space (for catabolic enzymes such as AChE), mediating
removal or otherwise stopping the action of the neurotransmitter in the synaptic cleft.
II. Behavioral effects of:
- the neurotransmitter, agonists, or antagonists
- inhibitors of biosynthetic, transport or catabolic enzymes
- mutants with altered neurotransmitter expression, altered function of transport or catabolic enzymes, or receptors
Behavioral effects should be consistent with known connectivity of neurons in which the putative neurotransmitter is thought to function.
III. Localization of neurotransmitter receptors in known or putative postsynaptic cells
To be reliable as evidence for specific neurotransmitter function, the specificity of the receptor should be determined physiologically, and not simply inferred from gene sequence.
Ancillary supportive evidence
Evidence from other nematodes with clearly homologous neurons, including categories I, II and III above, as well as traditional electrophysiological and pharmacological methods (e.g., using the large parasitic nematode Ascaris)
This section should be cited as: Loer, C.§ 2010. Criteria for neurotransmitters in Caenorhabditis elegans, in WormAtlas. doi:10.3908/wormatlas.5.201
§To whom correspondence should be addressed. Curtis Loer: firstname.lastname@example.org