Release of catecholamines in the locus coeruleus of freely moving and anaesthetized normotensive and spontaneously hypertensive rats: effects of cardiovascular changes and tail pinch

Noradrenaline turnover has been found to be increased in the locus coeruleu s of young spontaneously hypertensive rats (SHR). There is also evidence th at the noradrenergic projection from the locus coeruleus to the posterior h ypothalamus contributes to the development of genetic hypertension. To inve stigate whether the release of noradrenaline and dopamine in the locus coer uleus is modified in genetic hypertension, this brain region of adult SHR a nd normotensive Wistar-Kyoto (WKY) rats was superfused with artificial cere brospinal fluid through a push-pull cannula. Dopamine and noradrenaline rel eased in the superfusate were determined radioenzymatically. There was no d ifference in the basal release of noradrenaline and dopamine in the locus c oeruleus of conscious, anaesthetized or diazepam-treated adult WKY rats and SHR. In conscious animals, a rise in blood pressure elicited by intravenou s infusion of phenylephrine enhanced the release of noradrenaline and dopam ine in both strains to the same extent. Intravenous infusion of sodium nitr oprusside elicited a fall in blood pressure and also increased to the same degree the release of noradrenaline and dopamine in the locus coeruleus of normotensive and hypertensive conscious rats. In anaesthetized rats, barore ceptor activation by phenylephrine decreased the release of noradrenaline a nd dopamine, while sodium nitroprusside lowered blood pressure and enhanced the release rates of the two catecholamines. Treatment of conscious rats w ith diazepam (10 mg/kg, i.p., 120 min prior to starting collection of the s uperfusate) abolished the phenylephrine-evoked release of catecholamines ob served in conscious animals. The sensory stimulus tail pinch led to a sligh t increase in blood pressure. In conscious animals, this aversive stimulus led to enhanced release of noradrenaline and dopamine that lasted longer in SHR than in WKY rats. The release of catecholamines evoked by tail pinch w as abolished in rats treated with diazepam, as well as in anaesthetized ani mals. Our findings show that in adult rats, genetic hypertension does not m odify the release of noradrenaline and dopamine in the locus coeruleus. Sin ce in anaesthetized rats increases in blood pressure diminish, while decrea ses in blood pressure enhance, the release of noradrenaline and dopamine, i t seems that both amines possess a counteracting, hypertensive function in the rat locus coeruleus. When baroreceptor activation by phenylephrine is c arried out on conscious animals, stress predominates and the release of cat echolamines is enhanced. This study demonstrates the importance of the nora drenergic system of the locus coeruleus in central cardiovascular control a nd in emotional, stress and pain-regulating processes.