INHIBITORY EFFECT OF NITRIC-OXIDE ON NEURONAL-ACTIVITY IN THE PERIAQUEDUCTAL GREY-MATTER OF THE RAT

Experiments were carried out in urethane-anaesthetized rats to examine the effect of nitric oxide (NO) on neuronal activity within the dorso lateral sector of the midbrain periaqueductal grey matter (FAG), an ar ea which is rich in NO-synthesizing neurones. NADPH-dependent diaphora se histochemistry revealed small NO synthase-containing perikarya, 15. 4+/-3.1 mu m (mean +/- SEM) in diameter, in a longitudinal column in t he dorsolateral sector of the FAG. The labelled cell bodies were surro unded by a dense meshwork of stained fibres and processes in which unl abelled neurones were embedded. In order to establish whether NO was g enerated when NO donors were ejected iontophoretically from micropipet tes, a chemiluminescence method was used to estimate the output of NO in vitro after iontophoresis of two chemically different classes of NO donor: the sydnonimine 3-morpholino-sydnonimin-hydrochloride (SIN 1) and the nitrosothiol S-nitroso glutathione (SNOG). Iontophoresis of bo th NO donors into 200 mu l aliquots of 165 mM NaCl using ejection curr ents between 6000 and 18000 nA . min produced a current-related increa se in the concentration of NO. Iontophoresis of SIN 1 in vivo produced a reproducible, current-related inhibition of firing in 40 of 59 neur ones in the dorsolateral FAG. In 8 of 10 neurones the effect of SIN 1 was significantly reduced after iontophoresis of methylene blue (10-30 nA for 2.7-5 min). The inhibition took up to 7 min to develop and las ted for up to 13 min. Inhibitory responses to GABA were not affected b y methylene blue. Iontophoresis of SNOG also inhibited ongoing activit y of 18 of 24 neurones tested in the PAG. The experiments demonstrate firstly that NO donors can be used in vivo to deliver NO in the vicini ty of neurones by iontophoresis from micropipettes. Secondly, NO appea rs to inhibit neuronal activity within the PAG.