Effects of locus coeruleus stimulation on neuronal activities of dorsal lateral geniculate nucleus and perigeniculate reticular nucleus of the rat.

In rats anesthetized with urethane, a stimulating electrode was introduced to the locus coeruleus by observing the antidromic field response to single shock stimulation of the dorsal pathway of noradrenergic axons. Effects of locus coeruleus stimulation were studied on activities of relay neurons and intrinsic interneurons of the dorsal lateral geniculate nucleus and on those of neurons in the perigeniculate reticular nucleus. The intrinsic interneurons and the perigeniculate reticular neurons are believed to exert inhibition upon the relay neurons. The relay neurons were activated by repetitive stimulation of locus coeruleus; spontaneous discharges were increased in rate and the threshold of response to single shock stimulation of the optic nerve was lowered. The activation was rarely seen in rats pretreated with alpha-methyl-p-tyrosine. Iontophoretic application of phentolamine, an alpha-blocker, effectively antagonized the activation, whereas an iontophoretic beta-blocker and cholinergic blockers were virtually ineffective. The activation of the relay neurons was suggested to be due to a direct action of noradrenaline, released by locus coeruleus stimulation. Locus coeruleus stimulation inhibited the interneurons and activated the perigeniculate reticular neurons; spontaneous or light-evoked discharges were suppressed in the interneurons and tonic discharges were elicited in the perigeniculate reticular neurons. These effects of locus coeruleus stimulation were mimicked by noradrenaline applied iontophoretically. Activation of the perigeniculate reticular neurons was antagonized by an iontophoretic alpha-blocker but not by a beta-blocker. Two special features emerge from the present results: (1) the locus coeruleus exerts different effects upon the two neuronal constituents of the dorsal lateral geniculate nucleus, excitation of the relay neurons and inhibition of the intrinsic interneurons; (2) a suggestion previously advocated that locus-coeruleus-induced excitation of the lateral geniculate relay neurons would be due to inhibition of inhibitory neurons (disinhibition) does not hold true, at least with respect to the perigeniculate reticular neurons; the latter neurons have been proved to exert a powerful inhibition upon the geniculate relay neurons and they are excited by stimulation of the locus coeruleus.