GABA is the major inhibitory neurotransmitter of the central nervous system in mammals. We are studying GABA function in C. elegans and have identified genes required for GABA neurotransmission. The neurotransmitter GABA inhibits muscle contraction in C. elegans. Muscimol, a GABA agonist, inhibits muscle contractions in wild-type animals. By contrast,
unc-25 mutants, which lack GABA immunoreactivity, display a hypercontracted (shrinker) phenotype. Killing the GABAergic VD and DD motorneurons also leads to the hypercontracted phenotype. These results suggest that GABA released from these motorneurons inhibits body muscle contractions. The
unc-49 gene product mediates the inhibitory GABA input.
unc-49 mutants are similarly hypercontracted and are completely resistant to muscimol, which implies that
unc-49 encodes either a GABA receptor or a postsynaptic component required for receptor function. GABA may provide a stimulatory function in defecation.
unc-25 animals lack contractions of the expulsion muscles during defecation and are constipated. Furthermore, killing two GABAergic neurons, AVL and DVB, phenocopies this expulsion defect. In the simplest model, GABA released from AVL and DVB directly stimulates muscle contraction; alternatively, AVL and DVB may act through an intervening neuron. Because
unc-49 mutants have normal expulsions, GABA cannot act through the
unc-49 product in expulsion muscle contraction, and a second receptor mechanism must exist. A GABA agonist induces muscle contraction. More direct evidence that GABA can stimulate muscle contraction comes from experiments with muscimol.
unc-25 unc-49 double mutants lack expulsion muscle contractions and are resistant to inhibitory effects on muscle contraction by muscimol. Expulsions in such animals can be restored by muscimol or high concentrations of GABA. This rescue is not blocked by killing AVL and DVB, indicating that muscimol is acting postsynaptic to AVL and DVB, perhaps directly on the muscles. AVL and DVB utilize exogenous GABA. Providing exogenous GABA at low concentrations to
unc-25 worrns also rescues the expulsion defect. However, GABA rescue at these concentrations can be blocked by killing both AVL and DVB or by addition of nipecotic acid, a GABA reuptake blocker. We conclude that GABA at low concentrations is not acting directly on the muscle, but rather is taken up, presumably via a specific GABA transport protein, and then released by AVL and DVB. Thus, the only functional defect in
unc-25 appears to be a lack of GABA. A number of mutants exist that display the shrinker phenotype or expulsion defects (S.M. and H.R.H., in preparation; Thomas, Genetics 124:855-872,1990). The pharmacological techniques described here will allow us to to distinguish mutations causing presynaptic expulsion defects (affecting AVL and DVB or a preceding step) from those causing postsynaptic expulsion defects (presumably in the muscle).