MUSCIMOL ACTS IN DORSOMEDIAL BUT NOT PARAVENTRICULAR HYPOTHALAMIC NUCLEUS TO SUPPRESS CARDIOVASCULAR EFFECTS OF STRESS

Both the dorsomedial hypothalamic nucleus (DMH) and the paraventricula r hypothalamic nucleus (PVN) have been implicated in the neural contro l of the cardiovascular response to stress. We used the GABA(A) agonis t muscimol to inhibit neuronal activation and attempted to identify hy pothalamic nuclei required for the cardiovascular response to air stre ss. Chronically instrumented rats received bilateral injections of eit her 80 pmol of muscimol or 100 nl of saline vehicle into the DMH, the PVN, or an intermediate area (including the rostral edge of the DMH an d the region between the two nuclei) and were placed immediately in a restraining tube and subjected to 20 min of air stress, In all rats, a ir stress after vehicle injections caused marked increases in heart ra te (137 +/- 6 beats/min) and blood pressure (26 +/- 2 mmHg). Microinje ction of muscimol into the DMH suppressed the heart rate and blood pre ssure response by 85 and 68%, respectively. Identical microinjection o f muscimol into the intermediate area between the DMH and the PVN atte nuated the increases in heart rate by only 46% and in blood pressure b y 52%. In contrast, similar injections into the vicinity of the PVN fa iled to alter the cardiovascular response to air stress. These finding s demonstrate that muscimol-induced inhibition of neuronal activity in the region of the DMH blocks air stress-induced increases in heart ra te and arterial pressure, whereas similar treatment in the area of the PVN has no effect.