PROJECTIONS OF THE AMYGDALA TO THE THALAMUS IN THE CYNOMOLGUS MONKEY

The projections of the amygdala to the thalamus in cynomolgus monkeys (M. fascicularis) were studied with both anterograde and retrograde axonal tracing techniques. Horseradish peroxidase (HRP) was injected into medial and midline thalamic sites in 5 animals, and [3H] amino acids were injected into selected amygdaloid regions in a total of 13 hemispheres in 10 animals. The 2 types of tracer experiments demonstrated the origins, course and terminal pattern of amygdaloid projections to 2 thalamic nuclei-medialis dorsalis (MD) and reuniens. Almost all of the amygdaloid nuclei contribute projections to MD, though the greatest proportion arise from the basal group and terminate in discrete, interlocking patches within the medial, magnocellular portion of MD. In addition to this major projection, the central and medial amygdaloid nuclei send a lighter projection to the lateral portion of nucleus reuniens. The amygdalothalamic projections took a variety of routes out of the amygdala before the large majority joined the inferior thalamic peduncle and entered the rostral head of the thalamus where they turned caudally toward their targets. A small number of amygdalothalamic fibers may also run in the stria terminalis.