ROLE OF NITRIC-OXIDE AND SEROTONIN IN MODULATION OF THE CARDIOVASCULAR DEFENSE RESPONSE EVOKED BY STIMULATION IN THE PERIAQUEDUCTAL GREY-MATTER IN RATS

In rats anaesthetised with alphaxalone/alphadolone, electrical stimula tion (10 s trains of 1 ms pulses at 80 Hz, 40-80 mu A) in the dorsolat eral and lateral periaqueductal grey matter (PAG), the midbrain defenc e area, evoked a presser response with tachycardia and vasodilatation in the hindlimb. Microinjection of 200 nl 0.66 mM 5HT, but not 200 nl 165 mM NaCl, al the site of stimulation attenuated the components of t he PAG-evoked response by 75-98%. The effect of 5HT was significantly reduced by prior intracerebroventricular injection of 100 mu g N-nitro -L-arginine methyl ester (an inhibitor of nitric oxide synthase) but n ot N-nitro-D-arginine methyl ester. Resting cardiovascular parameters did not change significantly following any of these manipulations. The results suggest that serotonin exerts an inhibitory modulation on the excitability of the midbrain defence area by a mechanism which involv es nitric oxide. (C) 1997 Elsevier Science Ireland Ltd.