Afferent connections to the amygdaloid complex of the rat and cat. I. Projections from the thalamus.

By the use of the retrograde transport of horseradish peroxidase (HRP), the projections from the thalamus to the amygdala were investigated in the rat and cat, with main emphasis on the former species. HRP was injected stereotactically by microiontophoresis in the various amygdaloid nuclei. Several control procedures including the use of different approaches for the introduction of the micropipette were undertaken to eliminate the possibility of misinterpretation due to uptake of the protein by adjacent structures or fibers en passant. The paraventricular and paratenial nuclei of the thalamus were found to project throughout the entire amygdaloid complex. The medial geniculate complex and the basal nucleus of the ventromedial complex (the thalamic taste relay) mainly project to the centromedial part of the amygdala. The basolateral nucleus is the main recipient of a hitherto undescribed bilateral thalamo-amygdaloid pathway originating in the interanteromedial nucleus. The parafasciculare nucleus projects to the central nucleus of the amygdala; in the cat mainly to the lateral subdivision of the central nucleus. The findings are discussed in relation to previous anatomical and electrophysiological studies centered on the amygdaloid complex. Special reference is made to the possible role of the thalamo-amygdaloid connections in the conveyance of sensory information to the amygdala.