Response properties of single units in areas of rat auditory thalamus that project to the amygdala - I. Acoustic discharge patterns and frequency receptive fields

Projections from the auditory thalamus to the amygdala have been implicated in the processing of the emotional signficance of auditory stimuli. In order to further our understanding of the contribution of thalamoamygdala projections to auditory emotional processing, acoustic response properties of single neurons were examined in the auditory thalamus of chloral hydrate-anesthetized rats. The emphasis was on the medial division of the medial geniculate body (MGm), the suprageniculate nucleus (SG), and the posterior intralaminar nucleus (PIN), thalamic areas that receive inputs from the inferior colliculus and project to the lateral nucleus of the amygdala (AL). For comparison, recordings were also made from the specific thalamocortical relay nucleus, the ventral division of the medial geniculate body (MGv). Responses latencies were not statistically different in MGv, MGm, PIN, and SG, but were longer in the posterior thalamic region (PO). Overall, frequency tuning functions were narrower in MGv than in the other areas but many cells in MGm were as narrowly tuned as cells in MGv. There was some organization of MGv, with low frequencies represented dorsally and high frequencies ventrally. A similar but considerably weaker organization was observed in MGm. While the full range of frequencies tested (1-30 kHz) was represented in MGv, cells in MGm, PIN, and SG tended to respond best to higher frequencies (16-30 kHz). Thresholds were higher in PIN than in MGv (other areas did not differ from MGv). Nevertheless, across the various areas, the breadth of tuning was inversely related to threshold, such that more narrowly tuned cells tended to have lower thresholds. Many of the response properties observed in MGm, PIN, and SG correspond with properties found in AL neurons and thus add support to the notion that auditory responses in AL reflect thalamoamygdala transmission.