Description

Balanced excitation and inhibition activities are critical for proper neural circuit function. The C. elegans locomotor circuit is regulated via the coordinated activities of cholinergic excitation and GABAergic inhibition that promote muscle contraction and relaxation, respectively. A gain-of-function mutation in a neuronal acetylcholine receptor subunit gene, acr-2 [acr-2(gf)], causes both increased cholinergic excitation and decreased GABAergic inhibition to body muscles, resulting in overall hyperactivity of the locomotor circuit. acr-2(gf) mutant animals show Uncoordinated(Unc) movement and spontaneous shrinking, or, convulsion (Jospin et al. 2009).

Acetylcholine activates nicotinic receptors, which are ion channels that mediate fast synaptic transmission, and muscarinic receptors, which are G-protein coupled receptors that modulate neuronal activity via intracellular signaling (Albuquerque et al. 2009; Jones et al. 2012). As implied by their names, these receptors have different pharmacology that distinguish their activities. Similar to nicotine, the muscarinic agonist arecoline is used as a recreational drug, particularly in Asia, where it is often chewed in a preparation from the betel quid plant known as paan (WHO 2004). In C. elegans, arecoline has been used to study muscarinic functions in pharyngeal pumping and the motor circuit, where muscarinic signaling serves a modulatory role (Lackner et al. 1999; Robatzek et al. 2001; Steger and Avery 2004). Wild-type animals exposed to arecoline have slightly elevated cholinergic release, exhibiting, for example, faster locomotion and increased abundance and activity of synaptic proteins in cholinergic neurons (Chan et al. 2012; Chan and Sieburth 2012; Lackner et al. 1999).

Previous studies from our lab found that loss of function mutations in sphk-1, a lipid kinase, can suppress acr-2(gf) phenotypes (McCulloch et al. 2017). sphk-1 is an effector for muscarinic signaling (Chan et al. 2012; Chan and Sieburth 2012). Based on the interaction between acr-2(gf) and sphk-1, we tested the effect of arecoline on acr-2(gf) mutant animals. As arecoline is a cholinergic agonist and may likely induce parallel elevation of cholinergic signaling, we expected to observe an enhancement of acr-2(gf) phenotypes. However, to our surprise, arecoline treatment strongly suppressed the convulsion rate of acr-2(gf) animals. We observed a dose-dependent suppression of convulsion after 3hrs of arecoline treatment (Figure 1A). The Unc phenotype of these animals was also strongly suppressed, with animals moving rapidly across the plate following drug treatment. This effect of arecoline is reversible, and the convulsion phenotype almost fully recovered after 1hr off drug (Figure 1B).

Arecoline is one of many muscarinic agonists that have been identified and used in pharmacological studies of cholinergic signaling, and different muscarinic agonists can have preferential effects on different receptor subtypes. Arecoline is a relatively non-specific muscarinic agonist, able to act through all of the vertebrate subtypes (Rang et al. 2012). Additionally, arecoline has been shown to activate nicotinic receptors, although with much less potency (Papke et al. 2015). We next wanted to determine if the observed arecoline effect was a general property of muscarinic agonists. We tested two other drugs, pilocarpine and oxotremorine, which have specificity for the excitatory M1 muscarinic receptor subtype. For example, pilocarpine has been used for decades to induce frontal temporal lobe seizure in murine models, and this action is via the M1 receptor in the brain (Hamilton et al. 1997). Oxotremorine is also a highly specific muscarinic agonist (Rang et al. 2012). We observed that these muscarinic agonists showed varied effects on acr-2(gf) behaviors. Pilocarpine treatment enhanced acr-2(gf) in a dose-dependent manner, which would be consistent with this drug stimulating cholinergic activity. In contrast, Oxotremorine had an effect more similar to arecoline, although suppression was only observed at the lowest concentration tested, 1mM (Figure 1C).

Together, these data show that, in the context of acr-2(gf) induced circuit hyperactivity, cholinergic agonists can have varied effects, and imply additional targets of these drugs in C. elegans. Future studies will involve identifying the pathways and receptors that mediate these different functions.