Single neuron activity in the hypothalamic paraventricular nucleus (PVN) was recorded during horizontal and 45° head-down tilt suspension in unanesthetized rats. When the rats were raised in the head-down position, 13 of 34 neurons (38%) in the PVN changed activity as follows: gradual decrease ( 11 13, type I), or gradual increase ( 2 13, type II). Responses of 6 type I and 1 type II neuron were smaller during horizontal suspension. Intracerebroventricular or intraperitoneal injection of hypertonic saline caused an increase in activity of 4 of 5 type I neurons tested. This hyperosmotic stimulation had no effect during suppressed activity induced by head-down tilt. These results suggest that the activity of PVN neurons is altered during body suspension, probably by information from baroreceptors in the thoracic activity, proprioreceptors and/or the vestibular organ, and these neurons might be involved in regulation of the autonomic and neuroendocrine systems.
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