Deconstructing 5-HT6 receptor effects on striatal circuit function.

Area of Science:

• Neuroscience
• Behavioral Neuroscience
• Molecular Psychiatry

Background:

• Medium spiny neurons (MSNs) in the dorsal striatum are critical for instrumental learning and habit formation.
• 5-hydroxytryptamine 6 receptors (5-HT6) are expressed in both direct and indirect MSN pathways, suggesting a role in modulating their opposing functions.
• Previous studies showed 5-HT6 receptor overexpression in MSNs broadly impacts learning and habit reversal.

Purpose of the Study:

• To investigate the distinct roles of 5-HT6 receptors in direct and indirect MSN pathways within the dorsal striatum.
• To elucidate how 5-HT6 receptor signaling influences instrumental learning and behavioral flexibility.

Main Methods:

• Utilized novel viral vectors for selective overexpression of 5-HT6 receptors in direct or indirect pathway MSNs in specific dorsal striatum sub-regions (DMS and DLS).
• Assessed effects on initial acquisition of a discrete instrumental learning task.
• Evaluated behavioral flexibility in rats overtrained on a repetitive pressing task using omission contingency training and probe testing.

Main Results:

• Overexpression of 5-HT6 receptors in direct pathway MSNs of the posterior DMS enhanced initial learning acquisition.
• Overexpression in indirect pathway MSNs of the posterior DMS impaired initial learning acquisition.
• Selective 5-HT6 receptor increase in DLS indirect pathway MSNs facilitated behavioral flexibility in overtrained rats.

Conclusions:

• 5-HT6 receptor signaling in MSNs differentially modulates instrumental learning and habit-related behaviors based on pathway and striatal sub-region.
• Findings support the hypothesis that 5-HT6 receptors mediate balanced serotonin activation of opposing MSN pathways, promoting adaptive behavioral control.