Pharming for Genes in Neurotransmission: Combining Chemical and Genetic Approaches in Caenorhabditis elegans

Abstract

Synaptic transmission is central to nervous system function. Chemical and genetic screens are valuable approaches to probe synaptic mechanisms in living animals. The nematode Caenorhabditis elegans is a prime system to apply these methods to discover genes and dissect the cellular pathways underlying neurotransmission. Here, we review key approaches to understand neurotransmission and the action of psychiatric drugs in C. elegans. We start with early studies on cholinergic excitatory signaling at the neuromuscular junction, and move into mechanisms mediated by biogenic amines. Finally, we discuss emerging work toward understanding the mechanisms driving synaptic plasticity with a focus on regulation of protein translation.

Keywords: Pharmacology; acetylcholine; dopamine; ion channels; neuromodulation; neuromuscular junction; neuronal circuit; protein synthesis; psychotic drugs; serotonin; synaptic vesicle release.

Figure 1.

Pharmacology screens in C. elegans. The germ cells of preadult worms are mutagenized with chemicals or radiation on agar plates. After culturing for two generations to allow recessive phenotypes to arise, worms are treated with a chosen neuroactive drug on agar plates or in liquid. Drug-resistant animals are then isolated and cultured, and their mutations are mapped to genes conferring drug-resistance.

Figure 2.

Genes operating in post- and presynaptic cells of the neuromuscular junction identified in levamisole and aldicarb resistance screens. Levamisole resistance mutations primarily affect genes operating in the postsynaptic muscle, including subunits of the levamisole sensitive pentameric ACh receptor, their assembly factors in the endoplasmic reticulum (ER), vesicular transporters, and receptor clustering proteins. A major class of genes identified in aldicarb resistance (ric) screens are synaptic vesicle cycle factors located in the presynaptic terminal.

Figure 3.

Distinct pathways of fluoxetine and imipramine induced egg-laying. The imipramine pathway (blue) requires the SER-4 receptor in the head neurons and the HSN neuron to stimulate egg-laying in the vulva muscle, but its intermediate effectors and interneuron linking signals from the head neurons have not been determined. The fluoxetine pathway (red) also requires the HSN neuron and signals parallel to the 5-HT receptor SER-1, converging on EGL-30, through unknown receptor(s). In addition, both drugs inhibit the C. elegans SERT homologue MOD-5.

Figure 4.

Long-term associative memory (LTAM) assay scheme. In these experiments, worms are “trained” by alternating exposure to a chemoattractant in the presence of food and starvation over a number of intervals. The trained worms are cultured on plates with food lacking the chemoattractant for a long period and subsequently assayed for improved taxis toward the chemoattractant.