Abstract
Vagus nerve stimulation (VNS) enhances extinction of conditioned fear in rats. Previous findings support the hypothesis that VNS effects on extinction are due to enhanced consolidation of extinction memories through promotion of plasticity in extinction-related brain pathways however, alternative explanations are plausible. According to one hypothesis, VNS may produce a hedonic effect and enhance extinction through counter-conditioning. According to another hypothesis, VNS reduces anxiety during exposure and this weakens the association of conditioned stimuli with aversive conditioned responses. The present set of experiments (1) used conditioned place preference (CPP) to identify potential rewarding effects associated with VNS and (2) examined the peripheral effects of VNS on anxiety and extinction enhancement. Male Sprague-Dawley rats were surgically implanted with cuff electrodes around the vagus nerve and subjected to a CPP task in which VNS and sham stimulation were each paired with one of two distinct contexts over the course of 5 d. Following this procedure, rats did not show a place preference, suggesting that VNS is not rewarding or aversive. The role of the peripheral parasympathetic system in the anxiolytic effect of VNS on the elevated plus maze was examined by blocking peripheral muscarinic receptors with intraperitoneal administration of methyl scopolamine prior to VNS. Methyl scopolamine blocked the VNS-induced reduction in anxiety but did not interfere with VNS enhancement of extinction of conditioned fear, indicating that the anxiety-reducing effect of VNS is not necessary for the extinction enhancement.