Ascending fiber projections from the midbrain central gray to the ventromedial hypothalamus in the rat

In 18 urethane-anesthetized ovariectomized female rats, 54 neurons in and around the hypothalamic ventromedial nucleus were transynaptically activated by electrical stimulation of the mesencephalic central gray matter. Ventromedial nucleus afferents from either the dorsal longitudinal fascicle (N = 10) or the medial forebrain bundle (N = 8) were disrupted by a small knife cut. In animals which lacked inputs from the dorsal longitudinal fascicle by a cut posterior to the ventromedial nucleus, transynaptic activation followed central gray stimulation in 20 cells; 26 cells were driven from the central gray in animals deprived of afferents through the medial forebrain bundle by a cut placed laterally to the ventromedial nucleus. Two patterns of synaptic activation followed electrical stimulation at 0.5 Hz: one type of response occurred during a latency range of 4 to 20 ms with fluctuation of each response less than 5 ms, the other type was characterized by diffuse activation for 50 ms following central gray stimulation. It was noted that the former time-locked response occurred in 19 among 20 cells in the posterior-cut animals, and the latter diffuse response was seen in 20 of 26 cells in the lateral-cut animals. Electrical stimulation of the ventromedial nucleus in another group of seven animals with the posterior cut caused antidromic activation of 19 central gray cells at latencies of 4.8 to 21.0 ms. Thus, central gray axons in the medial forebrain bundle were shown to have direct access to the ventromedial nucleus and multisynaptic pathways intervened for those in the dorsal longitudinal fascicle.