Response properties of single units in areas of rat auditory thalamus that project to the amygdala. II. Cells receiving convergent auditory and somatosensory inputs and cells antidromically activated by amygdala stimulation.

The purpose of this study was to further our understanding of the contribution of auditory thalamoamygdala projections to conditioned emotional memories formed when auditory and noxious somatosensory stimuli are associated. Single unit activity was recorded in the acoustic thalamus of chloral hydrate-anesthetized rats in response to auditory (white noise, clicks, tones) and somatosensory (foot-shock) stimulation. The thalamic areas focused on were 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 both the inferior colliculus and the spinal cord and that 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), which receives projections from the inferior colliculus but not from the spinal cord. Auditory but not somatosensory responses were recorded from MGv, while both auditory and somatosensory responses were frequently found in MGm, PIN, and SG. In these areas, convergent auditory and somatosensory responses were more frequently found rostrally than caudally. Within a thalamic subregion, the acoustic response properties of the convergence cells were not different from the response properties of unimodal auditory cells. Some cells that responded to somatosensory but not auditory stimuli showed a potentiated response when tested with simultaneous presentation of auditory and somatosensory stimuli. In some studies, thalamic cells that project to the amygdala were antidromically activated by stimulation of the AL. Consistent with anatomical tracing results, antidromically activated cells were found in MGm, PIN, and SG, but not in MGv. Antidromically activated cells were more likely to respond to auditory stimuli than to somatosensory stimuli, but unimodal somatosensory and convergence cells were also found. These findings, which provide the first characterization of acoustic response properties of multimodal cells in the auditory thalamus and of cells in the auditory thalamus that project to amygdala, suggest insights into the emotional functions of the thalamoamygdala pathway.