Modulation of Cardiovascular Depressant Action of Clonidine by Pentobarbital Anesthesia in Rats

The influence of pentobarbital anesthesia (5mg/100g, i.p.) on the cardiovascular depressor effect of clonidine administered intracerebroventricularly (i.c.v.) and intravenously (i.v.) was examined in rats. In conscious rats the hypotensive potency of i.v. clonidine [effective dose of clonidine which induced a decrease in blood pressure of 15mmHg; ED15=7 (μg/kg)] is greater than that of i.c.v. clonidine (ED15=24.3), while in anesthetized rats that of i.c.v. clonidine (ED15-0.7μg/kg) was greater than that of i.v. clonidine (ED15= 3.5μg/kg). Pentobarbital anesthesia potentiated the hypotensive potency of i.c.v. clonidine in conscious rats by 50 times, while it potentiated that of i.v. clonidine only by 2 times. The bradycardic potency of i.v. and i.c.v, clonidine was also potentiated by pentobarbital anesthesia. These pieces of evidence suggest that in conscious rat, the hypotensive action of i.c.v. clonidine is opposed by a centrally mediated hypertensive effect of the drug and that pentobarbital anesthesia suppresses the clonidine sensitive pressor center and so potentiates the hypotensive effect of i.c.v. clonidine.