HYPOTHALAMIC GABAERGIC INFLUENCES ON TREADMILL EXERCISE RESPONSES IN RATS

Microinjection of GABAergic antagonists in the posterior hypothalamus (PH) produces exercise-like adjustments in cardiovascular function. To test the hypothesis that a hypothalamic GABAergic mechanism within th e PH modulates the cardiovascular adjustments to dynamic exercise in c onscious animals, Sprague-Dawley rats (n = 10) were instrumented with bilateral guide cannula directed at the PH, an arterial cannula, and D oppler flow probes on the iliac and mesenteric arteries. Saline (100 n l) or the GABA(A) receptor agonist muscimol (125 ng.100 nl(-1)) was bi laterally injected into the PH during treadmill exercise (20 m min(-1) ). Microinjection of saline had no effect on mean arterial pressure (M AP), heart rate (HR), mesenteric vascular resistance (MR), or iliac va scular resistance (IR) during exercise. Microinjection of muscimol dur ing exercise produced no significant changes in MAP (mean change +/- S E; +0 +/- 1 mmHg), HR (+17 +/- 12 b min(-1)), or MR (+7 +/- 13%). Howe ver, microinjection of muscimol produced a significant increase in IR during exercise (16 +/- 6%). In addition, muscimol significantly decre ased treadmill run time (saline = 19.6 +/- 0.4 min; muscimol = 17.8 +/ - 0.6 min) and produced behavioral effects (including mild sedation) t hat were most evident after exercise. The results of these experiments suggest that while the posterior hypothalamic GABAergic system may mo dulate iliac blood flow during exercise in rats, this system does not modulate HR and MR responses to dynamic exercise.