Differential projection of the posterior paralaminar thalamic nuclei to the amygdaloid complex in the rat.

The thalamic paralaminar nuclei that border the medial and ventral edges of the medial geniculate body, viz. the suprageniculate nucleus (SG), the posterior intralaminar nucleus (PIN), the medial division of the medial geniculate nucleus (MGm), and the peripeduncular nucleus (PP), are regarded as important extralemniscal relay nuclei for sensory stimuli and as an important link for the direct transmission of sensory stimuli to the amygdala. Each of these thalamic nuclei receives a unique pattern of afferent input but an unresolved question is, how each of these thalamic nuclei project to the amygdala and whether there are zones of convergence and/or non-overlapping regions within amygdaloid target nuclei. Small injections of PHA-L or Miniruby, which were made into single thalamic nuclei at different rostrocaudal levels, revealed a non-uniform distribution of anterogradely labeled axons within the amygdaloid complex. Injections into the SG, MGm, and rostral PIN predominantly labeled axons in the laterodorsal and lateroventral portions of the lateral nucleus of the amygdala (LA). Axons from the MGm were located rather in the dorsal part of the LA, whereas SG-derived axons were concentrated in the ventrolateral part of the LA. Injections into the PP labeled axons predominantly in the medial part of the LA, whereas after injections into the caudal PIN axons were seen in the entire LA. In addition, the PIN projects heavily to the anterior basomedial nucleus and medial division of the central nucleus, whereas this projection is virtually absent from the other thalamic nuclei. The lateral part of the central nucleus and the basal nucleus of the amygdala are spared by axons from the thalamic paralaminar nuclei. The present results suggest that, despite a considerable degree of convergence of the thalamoamygdaloid projection in the lateral nucleus, each thalamic nucleus plays a unique role in the transmission of sensory stimuli to the amygdala and in the modulation of intraamygdaloid circuits.