Epidemiological studies show that tobacco smoking reduces the prevalence of Parkinson's disease. One explanation for these findings is that tobacco-derived nicotine protects substantia nigra dopaminergic neurons (DNs) from degeneration. To examine this possibility we established, in collaborated with the lab of Guy Caldwell (University of Alabama), a C. elegans model for nicotine-induced protection of DNs. Using this model we have shown that chronic nicotine exposure protects DNs against 6-OHDA toxicity via a signaling pathway involving activation of mitochondrial stress. We also identified four nAChR subunits needed for nicotine-induced protection of DNs (Nourse et al., iScience 2021). ACR-20, one of these "protective" nAChR subunits, was previously shown to form a homomeric receptor (Bauer et al., Mol. Pharmacol. 2014). To examine whether nicotine-induced protection depends on the ACR-20 homomeric receptor we examined the effects of betaine, an agonist of this receptor. We find that chronic betaine exposure like chronic nicotine exposure protects DNs in a nAChR-dependent manner. Protective effects of betaine are, however, distinct from the protective effects of nicotine; pre-exposure to betaine, unlike similar nicotine exposure, is not protective; while, exposure to betaine during and after 6-OHDA treatment, like similar nicotine exposure, is protective. The protective effects of betaine and the differences in their dynamics, relative to the protective effects of nicotine, are consistent with two distinct nAChRs functioning to protect DNs. In addition, differences between effects of nicotine and betaine suggest that the neuroprotective effects of betaine, unlike these of nicotine, do not involve long lasting gene expression changes. This explanation is consistent with the finding that betaine exposure, unlike nicotine exposure, does not enhance expression of mitochondrial stress markers. Of note, efficacy of nicotine as an agonist for ACR-23, closely related to ACR-20, is very low (Rufener et al., PLOS Path. 2013). This raises a question concerning the role of ACR-20 in nicotine-induced protection. We are now examining the possibility that exposure to nicotine, shown to upregulate
acr-20 expression (Polli et al., Neurotox. 2015), sensitizes DNs to the protective effects of ACR-20 activation during and after 6-OHDA treatment.