Ultrastructural evidence for the existence of a direct hypothalamic--vagal descending pathway.

The ultrastructural changes in the presynaptic nerve terminals in the dorsal vagal nucleus were studied in rats with bilateral electrolytic lesions placed in the dorso-medio-caudal hypothalamic area in the sagittal plane of the dorsomedial nuclei, just behind these nuclei in the P3,5--P3,7 interval. On days 3, 4 and 5 following placement of the hypothalamic lesions, degeneration of some of the presynaptic profiles in the dorsal vagal nucleus was found in the experimental animals. The data obtained provide evidence for the existence of descending hypothalamic axons terminating in the medulla oblongata on the neurons of the dorsal vagal nucleus. The possible involvement of the descending hypothalamic--vagal nerve pathway in the control of endocrine pancreas is discussed.