Pii: S0306-4522(96)00445-9

–Noradrenaline and adrenergic agonists were tested on pacemaker-like and silent neurons of the rat rostral ventrolateral medulla using intracellular recordings in coronal brainstem slices as well as in punches containing only the rostral ventrolateral medullary region. Noradrenaline (1–100 μM) depolarized or increased the frequency of discharge of all cells tested in a dose-dependent manner. The noradrenaline-induced depolarization was associated with an apparent increase in cell input resistance at low concentrations and a decrease or no significant change at higher concentrations. Moreover, it was voltage dependent and its amplitude decreased with membrane potential hyperpolarization. Noradrenaline caused a dose-related increase in the frequency and amplitude of spontaneous inhibitory postsynaptic potentials. The á1-adrenoceptor antagonist prazosin (0.5 μM) abolished the noradrenaline depolarizing response as well as the noradrenaline-evoked increase in synaptic activity and unmasked an underlying noradrenaline dose-dependent hyperpolarizing response associated with a decrease in cell input resistance and sensitive to the á2-adrenoceptor antagonist yohimbine (0.5 μM). The á1-adrenoceptor agonist phenylephrine (10 μM) mimicked the noradrenaline depolarizing response associated with an increase in membrane resistance as well as the noradrenaline-induced increase in synaptic activity. The á2-adrenoceptor agonists UK-14,304 (1–3 μM) and clonidine (10–30 μM) produced only a small hyperpolarizing response, whereas the â-adrenoceptor agonist isoproterenol (10–30 μM) had no effect. Baseline spontaneous postsynaptic potentials were abolished by strychnine (1 μM), bicuculline (30 μM) or both. However, only the strychnine-sensitive postsynaptic potentials had their frequency increased by noradrenaline or phenylephrine and they usually occurred with a regular pattern. Tetrodotoxin (1 μM) eliminated 80–95% of baseline spontaneous postsynaptic potentials and prevented the increase in synaptic activity evoked by noradrenaline and phenylephrine. Similar results were obtained in rostral ventrolateral medulla neurons impaled in both coronal slices and punches of the rostral ventrolateral medulla. It is concluded that noradrenaline could play an important inhibitory role in the rostral ventrolateral medulla via at least two mechanisms: an á2-adrenoceptor-mediated hyperpolarization and an enhancement of inhibitory synaptic transmission through activation of á1-adrenoceptors located on the somatic membrane of glycinergic interneurons. Some of these interneurons exhibit a regular discharge similar to the pacemaker-like neurons and might, at least in part, constitute a central inhibitory link in the baroreceptor–vasomotor reflex pathway. Copyright ? 1997 IBRO. Published by Elsevier Science Ltd.