Differential projection patterns of superior and inferior collicular neurons onto posterior paralaminar nuclei of the thalamus surrounding the medialgeniculate 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 co nvergent 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 ret rograde transport methods. Small injections of the tracer Miniruby were made into single paralaminar t halamic nuclei. Injections of Miniruby into the suprageniculate nucleus lab elled predominantly neurons in the stratum opticum of the superior collicul us, whereas injections into the medial division of the medial geniculate bo dy, the posterior intralaminar nucleus and the peripeduncular nucleus label led predominantly neurons in the deep layers of the superior colliculus. Th ese injections also labelled neurons in the inferior colliculus. The majori ty of retrogradely labelled neurons were found in the external nucleus of t he inferior colliculus and here predominantly in layer 2. Injections focuse d onto the medial division of the medial geniculate body additionally label led magnocellular neurons in layer 3 of the external nucleus and a few neur ons 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 infer ior colliculus were only found after injections that involved the medial di vision of the medial geniculate body. The present results suggest that, despite a considerable degree of converge nce in this thalamic region, each of these thalamic nuclei receives a uniqu e pattern of projections from the superior and inferior colliculi, It appea rs that the thalamic nuclei may be concerned mainly, but not exclusively, w ith a single sensory modality, and give rise to parallel multimodal and uni modal pathways to the amygdala.