Organization of projections to temporal cortex originating in the thalamicposterior intralaminar nucleus of the rat

Thalamic nuclei surrounding the medial geniculate body, among which the pos terior intralaminar nucleus (PIN) is one of the largest, have great importa nce in fear-potentiated emotional behavior. Due to limited knowledge of the distribution of the cortical projections of the PIN, the connections betwe en the temporal neocortex and the PIN were investigated by means of axonal transport of Phaseolus vulgaris leucoagglutinin or Miniruby. After iontopho retic 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, agranula r insular cortices). A common projection to all areas was found in the uppe r 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. Th e corticofugal projection to the PIN originated from pyramidal neurons in l ayer 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 tem poral neocortex, which may activate inter- and intra-areal cortical circuit s during processing of auditory stimuli.