Direct synaptic connections of axons from superior colliculus with identified thalamo-amygdaloid projection neurons in the rat: Possible substrates of a subcortical visual pathway to the amygdala

The aim of the present study was to identify synaptic contacts from axons o riginating in the superior colliculus with thalamic neurons projecting to t he lateral nucleus of the amygdala. Axons from the superior colliculus were traced with the anterograde tracers Phaseolus vulgaris leucoagglutinin or the biotinylated and fluorescent dextran amine "Miniruby." Thalamo-amygdalo id projection neurons were identified with the retrograde tracer Fluoro-Gol d. Injections of Fluoro-Gold into the lateral nucleus of the amygdala label ed neurons in nuclei of the posterior thalamus which surround the medial ge niculate body, viz. the suprageniculate nucleus, the medial division of the medial geniculate body, the posterior intralaminar nucleus, and the peripe duncular nucleus. Anterogradely labeled axons from the superior colliculus terminated in the same regions of the thalamus. Tecto-thalamic axons origin ating from superficial collicular layers were found predominantly in the su prageniculate nucleus, whereas axons from deep collicular layers were detec ted in equal density in all thalamic nuclei surrounding the medial genicula te body. Double-labeling experiments revealed an overlap of projection area s in the above-mentioned thalamic nuclei. Electron microscopy of areas of o verlap confirmed synaptic contacts of anterogradely labeled presynaptic pro files originating in the superficial layers of the superior colliculus with retrogradely labeled postsynaptic profiles of thalamo-amygdaloid projectio n neurons. These connections may represent a subcortical pathway for visual information transfer to the amygdala. (C) 1999 Wiley-Liss, Inc.