Introduction
The interaction between glutamatergic and dopaminergic systems of the central nervous system is heavily implicated in the etiology of several disease states, including Parkinson’s disease and schizophrenia. Application of ionotropic glutamate agonists to the striatum increases extracellular dopamine levels as measured by microdialysis via a TTX-insensitive mechanism¹. The goal of this study was to selectively examine the impact of glutamate receptors on impulse-dependent dopamine release in the striatum. Electrically evoked dopamine release in slices is completely TTX- and Ca²⁺-sensitive², and so provides a valid and useful model of impulse-dependent dopamine release. Therefore, we used fast scan cyclic voltammetry (FSCV) to monitor electrically evoked dopamine release in rat striatal slices. FSCV allows subsecond temporal resolution and has the potential of differentiating between changes in the ratio of release and uptake.

Methods
Coronal slices (400 m m thick) were perfused with artificial cerebrospinal fluid (aCSF: 126 mM NaCl, 25 mM NaHCO₃, 11 mM dextrose, 2.5 mM KCl, 2.4 mM CaCl₂, 1.2 mM MgCl₂ and 1.2 mM NaH₂PO₄; pH=7.4; saturated with 95%O₂/5%CO₂ at 34^oC). Dopamine detection was carried out by FSCV which involves the application of a triangular potential waveform to the microelectrode (7 m m in diameter, 100 m m in length) and measurement of the current due to the oxidation of dopamine and subsequent reduction of dopamine-o-quinone at the electrode surface. The voltage scan rate was 300 V/s and scans were repeated every 100 ms. A bipolar stimulating electrode was used to deliver stimulation (12 pulses, 60 Hz, 70 m A rms, 5 min interval between stimuli). Both the stimulating electrode and working electrode were placed about 100 m m beneath the surface of the slice. Three stimuli were performed, then the perfusion buffer was switched to one containing a drug of interest, and three additional stimuli were performed.

Results and Discussion
Evoked dopamine release was readily detected by voltammetry, as confirmed by examination of background subtracted cyclic voltammograms (not shown). The evoked responses were prevented by addition of TTX (10 m M) and by removal of Ca²⁺ from the perfusion buffer. Figure 1 shows that A) nomifensine (1 m M) increased the amplitude and the duration of the response, B) kynurenate (a broad-spectrum ionotropic glutamate receptor antagonist, 100 m M) increased the amplitude of the response by ~60%, C) kainate (100 m M) decreased the amplitude of the response by ~30% and D) NMDA ( 100 m M, without Mg²⁺) decreased the amplitude of the response by ~50%. Relative to nomifensine, the glutamatergic drugs had little effect on the duration of the response.

Figure 1 Effects of nomifensine, kynurenate, kainate and NMDA on voltammetrically recorded evoked dopamine release in striatal slices. Predrug stimulus responses are shown with solid symbols and postdrug responses are shown with open symbols.

Consistent with an earlier report³ these results demonstrate that striatal ionotropic glutamate receptors inhibit evoked dopamine release by a local mechanism. Furthermore, the actions of the glutamate antagonists, reported here and in a previous study¹, clearly demonstrate that impulse-dependent dopamine release is under the inhibitory control of endogenous glutamate. These results do not, however, demonstrate that a direct presynaptic mechanism is involved and further mechanistic studies are required to resolve this question. The concept that schizophrenia involves a decrease in the glutamatergic input to the basal ganglia has been difficult to reconcile with the hyperdopaminergic features of the disorder. The finding of this study may be significant in understanding the etiology of schizophrenia. (Supported by NIH: MH29670, MH 45156, NS 19608 and NS 31442.)

References
1. Keefe KA, Zigmond MJ and Abercrombie ED (1992) Extracellular dopamine in striatum: influence of nerve impulse activity in medial forebrain bundle and local glutamatergic input. Neuroscience 47: 325-332.

2. Kennedy RT, Jones SR and Wightman RM (1992) Dynamic observation of dopamine autoreceptor effects in rat striatal slices. J. Neurochem 59: 449-455.

3. Iravani MM and Kruk ZL (1996) Real-time effects of NMDA on dopamine release in slices of rat caudate putamen: a study using fast cyclic voltammetry. J. Neurochem 66: 1076-1085.