The generation of complex behaviors often requires the coordinated activity of diverse sets of neural circuits in the brain. Activation of neuronal circuits drives behavior. Inappropriate signaling can contribute to cognitive disorders such as epilepsy, Parkinson’s, and addiction (Nordberg et al., 1992; Quik and McIntosh, 2006; Steinlein et al., 2012). The molecular mechanisms by which the activity of neural circuits is coordinated remain unclear. What are the molecules that regulate the timing of neural circuit activation and how is signaling between various neural circuits achieved? While much work has attempted to address these points, answers to these questions have been difficult to ascertain, in part owing to the diversity of molecules involved and the complex connectivity patterns of neural circuits in the mammalian brain.

My thesis work addresses these questions in the context of the nervous system of an invertebrate model organism, the nematode Caenorhabditis elegans. The locomotory circuit contains two subsets of motor neurons, excitatory and inhibitory, and the body wall muscle. Dyadic synapses from excitatory neurons coordinate the simultaneous activation of inhibitory neurons and body wall muscle. Here I identify a distinct class of ionotropic acetylcholine receptors (ACR-12R) that are expressed in GABA neurons and contain the subunit ACR-12. ACR-12R localize to synapses of GABA neurons and facilitate consistent body bend amplitude across consecutive body bends. ACR-12Rs regulate GABA neuron activity under conditions of elevated ACh release. This is in contrast to the diffuse and modulatory role of ACR-12 containing receptors expressed in cholinergic motor neurons (ACR-2R) (Barbagallo et al., 2010; Jospin et al., 2009). Additionally, I show transgenic animals expressing ACR-12 with a mutation in the second transmembrane domain [ACR-12(V/S)] results in spontaneous contractions. Unexpectedly, I found expression of ACR-12 (V/S) results in the preferential toxicity of GABA neurons. Interestingly loss of presynaptic GABA neurons did not have any obvious effects on inhibitory NMJ receptor localization. Together, my thesis work demonstrates the diverse roles of nicotinic acetylcholine receptors (nAChRs) in the regulation of neuronal activity that underlies nematode movement. The findings presented here are broadly applicable to the mechanisms of cholinergic signaling in vertebrate models.