Organisation of the amygdalo-thalamic pathways in rats

This study examines the organisation of the pathways from the amygdala to t he thalamus. Amygdaloid nuclei (medial, central, basolateral and olfactory groups) of Sprague-Dawley rats were injected with biotinylated dextran usin g stereotaxic coordinates and their brains were then aldehyde-fixed and pro cessed using standard methods. We have three major findings. First, the amy gdala has a distinct set of projections to particular nuclei of the thalamu s. The thalamic nuclei with the heaviest amygdaloid terminations include th e zona incerta, the mediodorsal and the midline nuclei. Second, nuclei of d ifferent amygdaloid groups project to the thalamus in slightly different pa tterns. For example, some groups of nuclei project to the thalamic reticula r nucleus (e.g. medial, olfactory) whilst others do not (e.g. central, baso lateral). Thus, there is a certain amount of heterogeneity within the amygd aloid projections to the thalamus. Third, when we compare our results on th e amygdalothalamic pathways to the many previous descriptions of the thalam o-amygdaloid pathways, we note that they are largely out of register. In ot her words, some of the thalamic nuclei that project to a given group of amy gdaloid nuclei do not necessarily receive a projection back from that same amygdaloid nucleus. Hence, there is no substrate for a strong feed-back rel ationship between the thalamus and the amygdala, as there has been shown fo r other centres of the brain (e.g. between the thalamus and neocortex).