We have investigated the effects of noradrenaline (NA) on the spontaneous firing activity of red nucleus (RN) neurons recorded extracellularly in anesthetized rats by using an in vivo electrophysiological technique. Microiontophoretic applications of NA (5-100 nA for 30 s) modified the background firing rate in 99 out of 124 neurons and three different patterns of response were observed in distinct cells. In 61% of the responding neurons NA decreased the mean firing rate, whereas 22% of the neurons responded to NA application with an increase of their spiking activity; in a smaller group of cells (17%) NA exerted a biphasic inhibitory/excitatory effect on the spontaneous firing rate. The effects of NA were reversible and dose-dependent. From histological examination, the neurons responding to NA with a purely inhibitory effect were scattered throughout the RN. On the other hand, the neurons responding to NA with an excitation were found to be more numerous in the dorso-medial part of the RN, whereas the neurons in which NA induced biphasic effects appeared to be segregated in the outer lateral portion of the RN. The α2-adrenoceptor antagonist yohimbine completely blocked the inhibitory effect of NA but was unable to antagonize the excitatory response. In addition, the inhibitory effect of NA was mimicked by clonidine, a selective agonist of α2-adrenoceptors; clonidine had no effect on those cells which responded to NA with an increase of the mean firing rate. The excitatory effect of NA was mimicked by the β-receptor agonist isoprenaline and was antagonized by timolol, a selective antagonist of α2-adrenoceptors. Isoprenaline was ineffective in those cells in which NA exerted inhibitory responses. Taken together, our results indicate that the inhibitory effects of NA on the firing activity of rat RN neurons were mediated by α2-adrenoceptors, whereas β-adrenoceptors were responsible for the excitatory effects.
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