Parafascicular thalamus recruits motor and autonomic responses.

The parafascicular nucleus (Pf), part of the intralaminar thalamic nuclei, has been implicated in diverse functions such as attention, nociception, and behavioral flexibility, yet its precise contributions to behavior remain poorly defined. In this study, we used optogenetics to study the role of Pf projection neurons using high-resolution and continuous measures to quantify both skeletomotor and autonomic behavioral outputs, as well as motivational valence. Using optogenetic stimulation of Vglut2+ Pf neurons in male and female mice, combined with high-resolution tracking of movement kinematics, pupil diameter, and heart rate, we found that Pf stimulation elicits a stereotyped sequence of responses spanning multiple effector systems. Optogenetic activation produces rapid ipsiversive orienting, bilateral orofacial movements, followed by slower autonomic responses, including pupil constriction and bradycardia. These effects show distinct latencies. Finally, we found that Pf stimulation could be aversive, as mice avoided areas where stimulation was delivered. Together, our results indicate that Pf outputs can ultimately influence both skeletomotor outputs, like turning, and autonomic outputs, like pupil constriction. These findings broaden our understanding of Pf function and identify the Pf as a hub that links orienting behavior, parasympathetic regulation, and motivational valence.