Posterior hypothalamus cholinergic stimulation-induced activation of anterior hypothalamic area neurons is enhanced in spontaneously hypertensive rats

We have previously demonstrated that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins in rats and that activities of these AHA neurons are enhanced in spontaneously hypertensive rats (SHR). In addition, we have demonstrated that cholinergic mechanisms in the posterior hypothalamic nucleus (PHN) are involved in the activation of AHA angiotensin-II-sensitive neurons. It has been suggested that cholinergic function in the posterior hypothalamus is enhanced in SHR and that this hyperactivity plays a role in hypertension in SHR. In the present study, we examined whether the PHN cholinergic stimulation-induced activation of AHA angiotensin-II-sensitive neurons is altered in SHR. Male 15- to 16-week-old SHR and age-matched Wistar Kyoto rats (WKY) were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Microinjection of the cholinoceptor agonist carbachol, the cholinesterase inhibitor physostigmine and the excitatory amino acid glutamate into the PHN caused increases in firing rate of AHA angiotensin-II-sensitive neurons in anesthetized WKY and SHR. The increase in firing rate of AHA neurons induced by these drugs was enhanced in SHR as compared to WKY. The enhancement of the physostigmine-induced activation of AHA neurons in SHR was similar to that of the carbachol-induced activation of AHA neurons in SHR. The enhancement of the glutamate-induced activation of AHA neurons in SHR was similar to that of the carbachol-induced activation of AHA neurons in SHR. Microinjection of scopolamine, a cholinoceptor antagonist, into the PHN caused a small but significant decrease of firing rate of AHA angiotensin-II-sensitive neurons in SHR but not in WKY. These findings indicate that the PHN cholinergic stimulation-induced activation of AHA angiotensin-II-sensitive neurons is enhanced in SHR and that PHN cholinergic mechanisms are involved in tonic activation of angiotensin-II-sensitive neurons in the AHA of SHR. It appears that the enhancement of the PHN cholinergic stimulation-induced activation of AHA neurons in SHR results mainly from the enhanced neural reactivity to angiotensins in AHA neurons of SHR.