Role of nitric oxide and serotonin in modulation of the cardiovascular defence response evoked by stimulation in the periaqueductal grey matter in rats.

In rats anaesthetised with alphaxalone/alphadolone, electrical stimulation (10 s trains of 1 ms pulses at 80 Hz, 40-80 microA) in the dorsolateral and lateral periaqueductal grey matter (PAG), the midbrain defence area, evoked a pressor response with tachycardia and vasodilatation in the hindlimb. Microinjection of 200 nl 0.66 mM 5HT, but not 200 nl 165 mM NaCl, at the site of stimulation attenuated the components of the PAG-evoked response by 75-98%. The effect of 5HT was significantly reduced by prior intracerebroventricular injection of 100 microg N-nitro-L-arginine methyl ester (an inhibitor of nitric oxide synthase) but not 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 involves nitric oxide.