Sustained pressore responsiveness to prolonged hypothalamic stimulation in awake rats

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Abstract
Whether or not pressor responsiveness changes in unanesthetized rats during recurrent sympathetic excitation was determined by recording blood pressure and heart rate continuously while the posterior hypothalamus was stimulated repeatedly with constant currents. Because preliminary tests showed that telestimulation with a radio-controlled stimulator produced erratic responses, awake rats were routinely stimulated in a conventional manner by connecting them through wires to a square-wave stimulator. Although tachycardia was the most common chronotropic effect, bradycardia also occurred, and both responses were occasionally seen in the same rat at different times. Inhibition of chronotropic responses by combined pharmacologic blockade with propranolol and atropine did not affect corresponding pressor responses in normotensive rats. Renal and spontaneously hypertensive rats always had larger pressor responses than normotensive ones and, despite individual variations, responsiveness generally remained unaltered during 3-6 h of repeated hypothalamic stimulation. In awake normotensive or hypertensive rats cardiovascular responses to posterior hypothalamic stimulation may continue unabated even when stimulation is repeated for hours.