Deconstructing 5-HT6 receptor effects on striatal circuit function.

Medium spiny neurons (MSNs) constitute 95% of neurons in the dorsal striatum subdivided into direct (striatonigral) and indirect (striatopallidal) pathways. Whereas D1 and D2 receptors and several neuropeptides, including dynorphin and enkephalin, are differentially expressed in these neurons, 5-hydroxytryptamine 6 receptors (5-HT6) are expressed in both pathways. Previous results demonstrate that concurrent 5-HT6 receptor overexpression in MSNs of both pathways in the dorsomedial striatum (DMS) interferes with instrumental learning and that 5-HT6 overexpression in the dorsolateral striatum (DLS) relieves rats from inflexible habitual behaviors. We hypothesized that 5-HT6 receptor-mediated co-activation of both pathways interferes with the differential activation/inhibition of direct/indirect pathways by dopamine. To test this idea, we cloned novel viral vectors to selectively overexpress 5-HT6 receptors in direct or indirect pathway MSNs to deconstruct their role in modulating instrumental learning and habitual responding. We found that increasing 5-HT6 receptor expression in either direct or indirect pathway MSNs of the posterior DMS selectively enhanced or impaired initial acquisition of a discrete instrumental learning task respectively, though all rats were ultimately able to learn the task. In a separate set of experiments, 5-HT6 receptor overexpression in indirect pathway MSNs of the DLS facilitated behavioral flexibility in rats overtrained on a repetitive pressing task using a variable interval schedule of reinforcement, during an omission contingency training session and subsequent probe testing. Together these findings further the notion that 5-HT6 signaling causes balanced activation of opposing MSN pathways by serotonin in sub-regions of the dorsal striatum allowing for more reflective modalities of behavior.