Neurochemical modulation of ingestive behavior in the ventral pallidum

The nucleus accumbens and its related circuitry have been shown to play an important role in promoting the intake of hedonically desirable food. A previous report has demonstrated that the blockade of GABA(A) receptors in the ventral pallidum (VP), a target of GABAergic projection from the nucleus accumbens, greatly increases food, but not water, intake in satiated rats [Stratford et al. (1999) Brain Research, 825, 199-203]. The present study examined which neurotransmission in the VP is specifically involved in the intake of normally preferred taste stimuli. Microinjections of the GABA(A) antagonist bicuculline selectively increased the intake of saccharin solution but not that of water and quinine solution in water-deprived rats. In contrast, the facilitation of GABA(A) receptors by microinjections of muscimol in the VP generally suppressed the intake of saccharin, water and quinine. The same injections induced strong aversive taste reactivity responses to oral stimulation with not only quinine but also water and saccharin. The local administration of D-Ala2,N-Me-Phe4,Glyol5-enkephalin, a selective mu-opioid receptor agonist, into the VP had time-dependent effects, decreasing saccharine intake early and increasing intake late. Microinjections of SCH-23390, a dopamine D-1 receptor antagonist, in the VP suppressed the intake of saccharin but not water or quinine. Microinjections of sulpiride, the dopamine D-2 receptor antagonist, and 6-cyano-7-nitroquinoxaline-2,3-dione, the AMPA/kainate glutamate receptor antagonist, had no effect on fluid intake. These results reveal that GABA, opioid and D-1 receptors in the VP are involved in the consumption of hedonically positive taste stimuli.