Relationships between mesolimbic dopamine function and eating behavior.

These experiments are the first to correlate a specific stimulus of eating--the orosensory stimulation by sucrose--to mesolimbic dopaminergic activity. The pharmacological results and the increase in hypothalamic DOPAC/DA are convergent evidence for this relationship. The meaning of the relationship is not clear. The possibility that the decreased intake after dopaminergic receptor antagonist treatment and the increase in hypothalamic DOPAC/DA is related to the ingestive movements necessary for sham feeding seems unlikely for the following reasons: first, at least one dose of the antagonist had a preferential inhibition on sucrose intake compared to water intake; second, the D1 and D2 selective receptor antagonists decreased the initial rate of sucrose intake, but did not affect the rate of decay of sucrose intake over a 30-min test; third, in the second neurochemical experiment (Table 1), the volume ingested during 9 min of sham feeding 10% sucrose was not significantly different from the volume ingested from sham feeding 40% sucrose, but there was a significantly larger increase in the hypothalamic DOPAC/DA after 40% sucrose. Of course, a finer measurement of movements, such as lick rate, may reveal a significant difference that would correlate with the metabolic change. Fourth, striatal DOPAC/DA did not change during sham feeding of sucrose. Fifth, Bailey et al. demonstrated that a response-frequency summation analysis of the inhibitory effect of pimozide on sucrose intake did not reveal a motoric deficit. Thus, we interpret the increase of hypothalamic DOPAC/DA during the sham feeding of sucrose as evidence that activation of mesolimbic dopaminergic terminals in the hypothalamus is necessary for the normal processing of the central sensory and/or positive-reinforcing information produced by oral sucrose stimulation. Experiments are in progress to test this hypothesis and to attempt to distinguish between the sensory and positive-reinforcing effects of sucrose during sham feeding. In addition to generating this specific hypothesis concerning sucrose and hypothalamic DA activity, the results of these experiments suggest that the sham feeding preparation will be useful for the analysis of the important problem of the natural reinforcing properties of sweet taste. And finally, since sham feeding of sucrose is a form of oral self-stimulation, it provides a new experimental tool for comparing the role of central DA mechanisms in the positive-reinforcing effects of food, psychostimulant self-administration, and intracranial electrical self-stimulation.