INHIBITION BY MORPHINE OF THE CARDIOVASCULAR AND BEHAVIORAL-RESPONSESEVOKED BY CENTRALLY ADMINISTERED SUBSTANCE-P IN CONSCIOUS RATS

The effect of endogenous opioid receptor stimulation on the central ca rdiovascular and behavioral actions of substance P (SP) was examined i n conscious rats. SP (55 pmol) injected intracerebroventricularly (i.c .v.) elicited increases in mean arterial pressure, heart rate, and ste reotyped behavioral activation such as exploring and grooming, which w ere considered to be parts of the cardiovascular defense reaction. Int ravenous (i.v.) pretreatment with morphine (2.5 and 5.0 mg/kg) attenua ted the cardiovascular and behavioral responses produced by SP i.c.v. dose-dependently. The i.v. pretreatment with naloxone (10 mg/kg) had n o effect on the central SP-induced response. Presser responses elicite d by i.c.v. injection of corticotropin-releasing factor or angiotensin II were also attenuated by pretreatment with i.v. morphine (5.0 mg/kg ). Our results showed that endogenous opioid receptor stimulation anta gonizes the central cardiovascular and behavioral actions of SP. Morph ine may not influence the primary site of action of SP but does influe nce the central neural pathway which conveys the SP-induced sympatheti c activation signal.