THE PRESSOR-RESPONSE INDUCED BY REPEATED INJECTIONS OF NEOSTIGMINE INTO THE CENTRAL-NERVOUS-SYSTEM IS DESENSITIZED IN ADRENALECTOMIZED, BUTNOT IN INTACT RATS

To investigate whether or not pressor responses to repeated stimulatio n of central cholinoceptive neurons are desensitized, mean arterial bl ood pressure (MAP) and heart rate (HR) were measured following repeate d injections of neostigmine, an acetylcholinesterase inhibitor, into t he third cerebral ventricle in conscious, unrestrained intact or adren alectomized (ADX) rats. Neostigmine (5 x 10(-9) or 5 x 10(-8) mol) in 1 mu l saline increased MAP dose-dependently and increased HR in intac t rats. The peak values in MAP and HR after three repeated injections at 4 hour intervals did not wane. Neostigmine (5 x 10(-8) mol) also in creased MAP in ADX rats, and the peak values after the first injection were higher in the ADX rats than in the intact rats. The presser resp onses to the second and third injection, however, were less than to th e first injection in the ADX rats. HR responses to the repeated inject ions 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.