Descending hypothalamic pathways with cardiovascular function in the cat: A silver impregnation study

To identify descending pathways with cardiovascular function originating in the hypothalamus, unilateral electrolytic lesions were made in adult cats at hypothalamic sites from which arterial hypertension and cardioacceleration were elicited by electrical stimulation. After a survival period of 5 to 11 days, histological sections of the hypothalamus, midbrain, pons, medulla, and spinal cord were stained using modifications of the Nauta techniques for degenerating axons and terminals. The main caudal projection from lesions in the dorsal hypothalamic region could be traced ipsilaterally into the periaqueductal gray and was found to continue in the region of the dorsal longitudinal fasciculus throughout the midbrain, pons, and medulla. Fibers terminated primarily in the Edinger-Westphal nucleus, in the inferior colliculus, in a region dorsolateral to the superior olivary nucleus, and in the parahypoglossal region. The degeneration patterns resulting from lesions in the posterior hypothalamic region and in the ventrolateral hypothalamic-medial forebrain bundle regions were essentially similar. Lesions in these regions demonstrated the existence of dorsomedial and ventrolateral descending pathways (i) through the medial hypothalamus and periaqueductal gray and (ii) through the medial forebrain bundle and ventrolateral mesencephalic reticular formation. The projection through the periaqueductal gray was similar to that traced in animals with lesions in the dorsal hypothalamic region, but dense terminal degeneration was seen bilaterally in the caudal aspect of the accessory cuneate nucleus and the medial aspect of the nucleus tractus solitarius. The medial forebrain bundle partially decussated just rostral to the red nucleus and descended bilaterally into the region of the ventral tegmental area of Tsai, where some fibers terminated. The pathway continued through and around the lateral reticular nucleus in the medulla and into the spinal cord. Dense terminal degeneration was noted in the intermediomedial nucleus primarily in segments from C8 to T3, but extended to the lower thoracic segments as low as T12. No terminal degeneration was observed in the upper cervical, lumbar, or sacral cord, or in the intermediolateral nucleus of the thoracolumbar cord. These results suggest the existence of two distinct descending pathways that may be involved in the mediation of cardiovascular responses elicited by stimulation of the hypothalamus.