Muscimol injection into the ventral posterolateral nucleus of the thalamus impairs tactile reward-seeking behavior but preserves affective vocalization in male rats.

Ultrasonic vocalizations (USVs) are a useful tool for evaluating emotion and motivation in rodents. Rhythmic stroking in rats induces positive affective 50-kHz USVs and reward-seeking behavior. This response involves the dorsal column tract as the ascending pathway; however, supraspinal mechanisms remain unknown. We hypothesized that the ventral posterolateral nucleus of the thalamus (VPL), which receives somatosensory inputs from the spinal cord via the dorsal column-medial lemniscus pathway, might be involved in positive 50-kHz USV production and motivated tactile reward-induced behavior. To test the hypothesis, we used young adult male rats before and after inactivation of the VPL with bilateral infusion of muscimol, a potent γ-aminobutyric acid (GABA)-A type receptor agonist. We measured 50-kHz USVs, approach latency, and spontaneous behaviors pre- and post-muscimol injection, focusing on two emotional (during rhythmic stroking and receiving reward) and motivational (after rhythmic stroking and reward-seeking) conditions. Increased GABAergic inhibition in the VPL completely impaired approach behavior without causing obvious sensory and motor dysfunctions, but only slightly affected 50-kHz USVs. The present data indicate that the GABAergic system in the VPL is substantially involved in execution of reward-seeking behavior but not in the 50-kHz USVs induced by tactile rewards.