Contrasting hemodynamic responses elicited by stimulation of the paraventricular (PVH) and median preoptic (mnPO) nuclei of the hypothalamus in the rat

The paraventricular (PVH) and median preoptic (mnPO) nuclei of the hypothalamus play important roles in mediating hemodynamic responses to stress. Others have proposed that inhibitory projections exist from the mnPO to the PVH. Therefore, we hypothesized that stimulation of the mnPO would evoke opposite cardiovascular responses compared to activation of the PVH. Male Sprague‐Dawley rats anesthetized with urethane were instrumented for recording arterial pressure, heart rate, and hindquarters (Hq) and mesenteric (Mes) blood flows. D‐L‐homocysteic acid (DLH) was microinjected (100 pmol in 100 nl) into the mnPO and the PVH while recording hemodynamic responses. Microinjection of DLH in the mnPO elicited a decrease in arterial pressure due, in part, to Hq and Mes vasodilation. In contrast, DLH in the PVH evoked an increase in arterial pressure due, in part, to an increase in Mes vascular resistance and despite Hq vasodilation. The hemodynamic pattern elicited from the PVH more closely resembled responses observed in conscious rats to acute stress. Since opposite responses were evoked from these two nuclei, our data support the existence of an inhibitory pathway from the mnPO to the PVH. Moreover, our findings suggest that activation of the PVH simulates the hemodynamic response pattern to behavioral stress. Supported by USPHS DA17371 and HL091440.