BARORECEPTOR INHIBITION OF THE LOCUS-COERULEUS NORADRENERGIC NEURONS

The locus coeruleus is involved in the regulation of blood pressure. T he present study was undertaken to address the question of how the blo od pressure, in turn, changes the activity of the locus coeruleus neur ons via the action of baroreceptors. In chloralose- and urethane-anest hetized rats, the central cut end of the aortic depressor nerve, which does not contain chemoreceptor afferents in this species, was stimula ted electrically to excite baroreceptor afferents after bilateral vago tomy and sectioning of the carotid sinus nerve. Single train-pulse sti mulation of the aortic depressor nerve provoked the inhibition of ongo ing activity in 48% of locus coeruleus neurons tested, but 30% of them responded by excitation with subsequent inhibition. However, when the train-purse stimulation was repeated with a frequency of 5 Hz, which is close to that of the heartbeat in the rat, all neurons were markedl y inhibited. Another series of experiments was conducted in vagotomize d and carotid sinus nerve-sectioned rats with intact aortic depressor nerves. When blood pressure was elevated by an intravenous injection o f a presser agent, methoxamine or angiotensin II, or by rapid blood lo ading, ongoing activities of all locus coeruleus neurons tested were s uppressed. In contrast, intravenously injected nitroprusside, a depres sor agent, increased the activity of locus coeruleus neurons. In rats with all nerves preserved, rapid blood loading markedly inhibited the activity of these neurons. Such inhibition was partially but significa ntly attenuated by bilateral sectioning of the aortic depressor nerves . It is concluded that activation of aortic baroreceptors provokes inh ibitory or excitatory-inhibitory response in locus coeruleus neurons; phasic activation of the baroreceptor afferents with repetitive train- pulse stimulations at a frequency close to that of the heartbeat produ ces only inhibition, suggesting that, under natural conditions, the lo cus coeruleus neurons are tonically inhibited by ongoing input from ar terial baroreceptors.