Neurochemical regulation of feeding in the rat: facilitation by alpha-noradrenergic, but not dopaminergic, receptor stimulants.

Rats ate voraciously after intraventricular injections of the alpha-noradrenergic receptor stimulant clonidine. Intraventricular administration of l-norepinephrine also facilitated feeding, but similar injections of dopamine and apomorphine (a dopamine receptor stimulant) were ineffective and even tended to suppress feeding. Clonidine was 100 times more potent than norepinephrine and increased the intake of both the ordinary diet of powdered food and a highly palatable wet food. The anorexic action of amphetamine was reversed by centrally administered clonidine. These observations suggest "respondent" rather than "operant" regulation of feeding by noradrenergic systems. That is, in relation to noradrenergic mechanisms, feeding appears to be a respondent which is sensitized or disinhibited by activation of alpha-noradrenergic receptors, rather than an operant which is reinforced by the release of norepinephrine.