Behavioral findings suggest that the effects of neostriatal glutamate and ascorbate are opposed to those of neostriatal dopamine.Recent evidence also indicates that glutamate and ascorbate are linked via a carrier-mediated heteroexchange process, suggesting that ascorbate may act through the glutamate system to influence behavior. In order to assess glutamate-ascorbate interactions and their influence on the behavioral output of the basal ganglia, glutamate and homocysteic acid (a glutamate reuptake blocker) as well as NMDA antagonists were infused into the neostriatum of freely moving rats while extracellular neostriatal ascorbate was monitored via electrochemically modified carbon-fiber electrodes. Neostriatal 3,4-dihydroxyphenylacetic acid (DO-PAC), a major dopamine metabolite, also was recorded in order to assess the dependency of any drug effect on the nigrostriatal dopamine system. lntraneostriatal infusions of L-glutamate (1 pg/pl), but not L-homocysteic acid (30 rglpl), elevated extracellular neostriatal ascorbate levels. Neither of these drugs had any effect on neostriatal DOPAC or overt behavioral activity. lntraneostriatal infusion of the noncompetitive NMDA antagonist dirocilpine (MK-801; 3 pg/pl) or the competitive NMDA antagonist 3-(2-carboxypiperazin-4-yl)-propyl-1 -phosphonene (CPPene; 5 rg/Al) decreased neostriatal ascorbate but had no effect on neostriatal DO-PAC. Both dizocilpine and CPPene activated behavior in intact and sham-lesioned animals as well as in animals with near-total depletions of neostriatal dopamine following a 6-hydroxydopamine lesion. When administered systemically, however, dizocilpine (1 .O mg/kg) significantly increased neostriatal DOPAC. This effect appears to be regulated via midbrain NMDA receptors, in that this effect was completely abolished by electrolytic lesions of the substantia nigra pars reticulata. In addition, systemic dizocilpine substantially lowered the level of extracellular ascorbate (in both intact/sham and substantia nigra-lesioned animals), but subsequent intraneostriatal infusions of glutamate increased ascorbate levels to the'same extent as that observed in undrugged animals.Taken together, these results suggest that neostriatal NMDA receptors regulate basal, but not glutamate-stimulat-

A local infusion of d-amphetamine (d-AMPH) into the dorsal raphe nucleus (DRN) inhibited neuronal activity in this site and produced a mirror-image excitation in the ventrolateral, but not dorsomedial, neostriatum. This effect, which was mimicked by &methoxy-N,N-dimethyltryptamine, a serotonin autoreceptor agonist, was not altered by pretreatment with cu-methyl-p-tyrosine. Similar regional differences in neostriatal activity were obtained following an electrolytic lesion of the DRN or an intraperitoneal injection of d-AMPH. In fact, whereas 1.0 mg/kg of d-AMPH accelerated ventrolateral activity and inhibited dorsomedial neurons, 7.5 mg/kg produced the opposite effect. At both doses, however, DRN activity was inversely related to firing rate in the ventrolateral, but not dorsomedial, neostriatum. These results indicate that only certain regions of the neostriatum are responsive to changes in DRN activity and that these regions respond differently to systemic injections of d-AMPH than other neostriatal sites.