The Pressor Response Induced by Repeated Injections of Neostigmine into the Central Nervous System is Desensitized in Adrenalectomized, But Not in Intact Rats

To investigate whether or not pressor responses to repeated stimulation of central cholinoceptive neurons are desensitized, mean arterial blood pressure (MAP) and heart rate (HR) were measured following repeated injections of neostigmine, an acetylcholinesterase inhibitor, into the third cerebral ventricle in conscious, unrestrained intact or adrenalectomized (ADX) rats. Neostigmine (5× 10-9 or 5×10-8mol) in 1μl saline increased MAP dose-dependently and increased HR in intact rats. The peak values in MAP and HR after three repeated injections at 4 hour intervals did not wane. Neostigmine (5×10-8mol) also increased MAP in ADX rats, and the peak values after the first injection were higher in the ADX rats than in the intact rats. The pressor responses to the second and third injection, however, were less than to the first injection in the ADX rats. HR responses to the repeated injections in the ADX rats were identical to those in the intact rats. These findings suggest that the adrenal gland plays a role in antagonizing the development of desensitization in the neostigmine-induced pressor response.