Superior colliculus projections to midline and intralaminar thalamic nuclei of the rat

The superior colliculus (SC) projections to the midline and intralaminar thalamic nuclei were examined in the rat. The retrograde tracer cholera toxin beta (CTb) was injected into one of the midline thalamic nuclei-paraventricular, intermediodorsal, rhomboid, reuniens, submedius, mediodorsal, paratenial, anteroventral, caudal ventromedial, or parvicellular part of the ventral posteriomedial nucleus-or into one of the intralaminar thalamic nuclei-medial parafascicular, lateral parafascicular, central medial, paracentral, oval paracentral, or central lateral nucleus. After 10-14 days, the brains from these animals were processed histochemically, and the retrogradely labeled neurons in the SC were mapped. The lateral sector of the intermediate gray and white layers of the SC send axonal projections to the medial and lateral parafascicular, central lateral, paracentral, central medial, rhomboid, reuniens, and submedius nuclei. The medial sector of the intermediate and deep SC layers project to the parafascicular and central lateral thalamic nuclei. The paraventricular thalamic nucleus is innervated almost exclusively by the medial sectors of the deep SC layers. The superficial gray and optic layers of the SC do not project to any of these thalamic areas. The discussion focuses on the role these SC-thalamic inputs may have on forebrain circuits controlling orienting and defense (i.e., fight-or-flight) reactions.