Collateral projections of single neurons in the posterior thalamic region to both the temporal cortex and the amygdala: a fluorescent retrograde double-labeling study in the rat.

It has been reported that the acoustic thalamus of the rat sends projection fibers to both the temporal cortical areas and the lateral amygdaloid nucleus to mediate conditioned emotional responses to an acoustic stimulus. In the present study, fluorescent retrograde double labeling with Fast Blue and Diamidino Yellow has been used in the rat to examine whether single neurons in the posterior thalamic region send axon collaterals to both the temporal cortical areas and lateral amygdaloid nucleus. One of the tracers was injected into the lateral amygdaloid nucleus and the other into the temporal cortical areas close to the rhinal sulcus. Neurons double-labeled with both tracers were found mainly in the posterior intralaminar nucleus and suprageniculate nucleus, and to a lesser extent in the subparafascicular nucleus and medial division of the medial geniculate nucleus. No double-labeled neurons were seen in either the dorsal or ventral division of the medial geniculate nucleus. When one of the tracers was injected into the lateral amygdaloid nucleus and the other into either the dorsal portion of the temporal cortex, the dorsal portion of the entorhinal cortex, or the posterior agranular insular cortex, no double-labeled neurons were found in the posterior thalamic region. The present results indicate that a substantial number of single neurons in the acoustic thalamus project to both the limbic cortical areas and lateral amygdaloid nucleus by way of axon collaterals. These neurons may be implicated in affective and autonomic components of responses to multi-sensory stimuli, including acoustic ones.