Activation of lateral geniculate neurons by norepinephrine: Mediation by an α-adrenergic receptor

Abstract

Adrenergic receptors in the vicinity of neurons in the lateral geniculate nucleus (LGN) of the rat were pharmacologically characterized using extracellular single-cell recording and microiontophoretic techniques. Application of norepinephrine (NE) at low iontophoretic currents (1–15 nA) produced a delayed activation of most LGN neurons. This activation was mimicked by various sympathomimetic amines. The relative potency series of agonists was typical of postsynaptic α-adrenergic receptors:epinephrine>NE>phenylephrine⩾ α-methylnorepinephrine>dopamine>isoproterenol. The α-antagonists phentolamine, piperoxane and WB-4101, when applied at low iontophoretic currents (<10nA), produced a selective, dose-dependent and reversible blockade of the response to NE. The β-antagonist sotalol had weak and variable effects at these currents. At low currents, the presynaptic α-agonist clonidine was also able to block the response to NE but, at higher currents, produced a partial activation of some units, suggesting that it is a weak agonist. The ability of sympathomimetic amines to activate LGN neurons correlates well with their reported affinities for brainα 1-adrenoceptors labeled with [ 3H]WB-4101. It is concluded that NE activates neurons in the LGN via a postsynaptic orα 1-adrenergic receptor.