Differential projection patterns of superior and inferior collicular neurons onto posterior paralaminar nuclei of the thalamus surrounding the medial geniculate body in the rat.

The thalamic nuclei at the medial border of the medial geniculate body (i.e. the suprageniculate nucleus, the medial division of the medial geniculate nucleus, the posterior intralaminar nucleus and the peripeduncular nucleus) which relay sensory information to the amygdala are thought to receive convergent input from multiple sites. In order to delineate the organization of these multimodal thalamic nuclei, the locations of superior and inferior collicular neurons projecting to these nuclei were studied by means of retrograde transport methods. Small injections of the tracer Miniruby were made into single paralaminar thalamic nuclei. Injections of Miniruby into the suprageniculate nucleus labelled predominantly neurons in the stratum opticum of the superior colliculus, whereas injections into the medial division of the medial geniculate body, the posterior intralaminar nucleus and the peripeduncular nucleus labelled predominantly neurons in the deep layers of the superior colliculus. These injections also labelled neurons in the inferior colliculus. The majority of retrogradely labelled neurons were found in the external nucleus of the inferior colliculus and here predominantly in layer 2. Injections focused onto the medial division of the medial geniculate body additionally labelled magnocellular neurons in layer 3 of the external nucleus and a few neurons in the central nucleus. More ventrally located injections, focused onto the posterior intralaminar and peripeduncular nucleus, almost exclusively labelled neurons in layer 1 of the external nucleus and the dorsal part of the dorsal nucleus. After injections into the suprageniculate nucleus, only neurons in layer 2 were found. Neurons in the central nucleus of the inferior colliculus were only found after injections that involved the medial division of the medial geniculate body. The present results suggest that, despite a considerable degree of convergence in this thalamic region, each of these thalamic nuclei receives a unique pattern of projections from the superior and inferior colliculi. It appears that the thalamic nuclei may be concerned mainly, but not exclusively, with a single sensory modality, and give rise to parallel multimodal and unimodal pathways to the amygdala.