Organization of projections to temporal cortex originating in the thalamic posterior intralaminar nucleus of the rat.

Thalamic nuclei surrounding the medial geniculate body, among which the posterior intralaminar nucleus (PIN) is one of the largest, have great importance in fear-potentiated emotional behavior. Due to limited knowledge of the distribution of the cortical projections of the PIN, the connections between the temporal neocortex and the PIN were investigated by means of axonal transport of Phaseolus vulgaris leucoagglutinin or Mini-ruby. After iontophoretic injections of either tracer, anterogradely labeled terminals showed a broad, but not a diffuse, distribution in temporal and adjacent cortices (perirhinal, secondary auditory, visceral, secondary somatosensory, agranular insular cortices). A common projection to all areas was found in the upper layer I except for perirhinal cortex, where this projection was confined to the basal layer I. In selected cortical fields (ectorhinal, perirhinal, visceral cortices), an additional projection to layers III/IV was found. The corticofugal projection to the PIN originated from pyramidal neurons in layer V and - in some regions - in layer VI. The present results demonstrate a distinct and selective projection of the PIN to several areas of the temporal neocortex, which may activate inter- and intra-areal cortical circuits during processing of auditory stimuli.