Modulation of a post-synaptic cell's response to neurotransmitter is an important mechanism for regulating the synaptic strength, a process critical for diverse forms of learning and memory. Our laboratory has been attempting to understand the molecular events that regulate the sensitivity of a specific post-synaptic cell type, the vulval muscles, to the neurotransmitter serotonin, released endogenously by the HSN motorneurons. In earlier work, we found that mutations in two genes encoding putative voltage-gated calcium channel subunits,
unc-2 and
unc-36, caused hypersensitivity and failure to desensitize to serotonin. This suggested that a voltage-sensitive calcium channel containing the UNC-2 and UNC-36 proteins activates a calcium-dependent process leading to serotonin desensitization. Based on expression studies and mosaic analysis, we proposed that the UNC-2 calcium channel might act by regulating the neurotransmitter release from modulatory neurons, perhaps the VCs. To identify more molecules that participate in the regulation of serotonin response, we surveyed known Egl-c Unc mutants for serotonin hypersensitivity. Mutations in at least 6 genes (
unc-8,
unc-10,
unc-20,
unc-35,
unc-75, and
unc-77) conferred both constitutive egg-laying and hypersensitivity to serotonin. We found that one of these genes,
unc-20, is required for the proper guidance of the VC axons and positioning of the VC cell bodies. Mutations in several other genes have also been isolated (by other groups) in screens for aldicarb-resistant mutants, suggesting that the function of their gene products is to facilitate release of ACh and possibly other neurotransmitters. In fact we determined that both
snt-1 mutants (defective in synaptotagmin) and
cha-1 mutants (defective in choline acetyltransferase) were serotonin hypersensitive, and that
cha-1 mutants were also strongly Egl-c, indicating that ACh is an inhibitory modulator of egg-laying behavior and serotonin response. Analysis of nicotinic ACh receptor mutants suggested that this modulation is mediated through a ligand-gated ion channel containing the UNC-38 protein as a subunit. We are currently investigating the site of action of this receptor, as well as the site of ACh release. In the future, we hope to use physiological approaches, such as calcium imaging, to study the processes involved in serotonin desensitization. Progress in developing these techniques will (hopefully!) be discussed.