How do animals distinguish between sensory stimuli? We are studying a neural circuit in C. elegans in which the ASH sensory neuron, detect noxious chemical and mechanical stimuli and signal to the worm to move backward. Three stimuli are detected by the ASH neurons: touch to the nose, high osmolarity and volatile repellants. Genetic and molecular evidence suggests that different signals are produced at the ASH-interneuron synapses in response to the different stimuli. The
glr-1 gene encodes a glutamate receptor required specifically for response to nose touch. GLR-1 is expressed in the interneurons which receive synaptic input from ASH, suggesting that neurotransmitter release from ASH in response to nose touch is different than release in response to osmotic stimuli and volatile repellants. To better understand ASH signaling, we screened for suppressors of
glr-1 (
n2461).
glr-1 might be suppressed by mutations which cause ASH to use the other neurotransmission pathway(s) in response to nose touch; thus suppressors may give us insight into differential release of neurotransmitters. We isolated two suppressors from 6500 genomes. One suppressor, (
nu349) , maps to chromosome V and also suppresses a deletion allele,
glr-1(
ky176).
nu349 has three phenotypes: it is egg-laying defective, it does not respond to gentle touch to the body and it suppresses the nose touch defect of
glr-1. The second suppressor,
nu348, has no additional phenotypes. I killed ASH with a laser in the suppressed animals and tested their response in the nose touch assay. Both suppressors became nose touch defective after ablation of ASH indicating that they suppress
glr-1(
n2461) by restoring function of the ASH-interneuron synapse. To look at the restored synaptic function more closely we tested whether suppressed animals could habituate to the nose touch stimulus. Wild type animals back up 9 out of 10 times in response to nose touch. However, after 40 successive touches to the nose, wild type animals only back up in response to 4 out of the next 10 touches.
nu349 mutants continue to respond 9 out of 10 times after the same treatment, indicating that they fail to habituate to nose touch. There are several ways habituation can be altered, but the process is still poorly understood. In order to facilitate the cloning of the gene I have isolated three new alleles of
nu349 in a UV noncomplementation screen of 7340 haploid genomes.
nu349 maps near a gap in the region which has recently been sequenced by the genome sequencing project. I am refining the map position of
nu349 by looking for polymorphisms in the UV alleles with cosmid probes and probes derived from the YAC sequence. I am also trying to rescue the mutant with genes identified by the YAC sequence. I am also continuing the mapping of
nu348.