Differential actions of dizocilpine (MK-801) on the mesolimbic and mesocortical dopamine systems: role of neuronal activity.

The significance of impulse activity in the dopamine neurons of the ventral tegmental area for the dopamine release evoked by systemic administration of the psychotomimetic drug dizocilpine (MK-801) was investigated. Dual probe microdialysis was utilized in freely moving rats implanted with one probe in the ventral tegmental area and a second ipsilateral probe in either the nucleus accumbens or the medial prefrontal cortex. Dialysates were analyzed with high-performance liquid chromatography with electrochemical detection for dopamine. The ventral tegmental area was perfused with the sodium channel blocker tetrodotoxin (1 microM) or vehicle (perfusion solution). A total of 2 h after the onset of tetrodotoxin perfusion of the ventral tegmental area, MK-801 (0.1 mg/kg) was injected subcutaneously. Tetrodotoxin perfusion of the ventral tegmental area significantly reduced dialysate levels of dopamine both in the nucleus accumbens and the medial prefrontal cortex to approximately 30% of baseline. When given alone, MK-801 caused a significant, i.e. 50%, increase in extracellular dopamine levels in the nucleus accumbens, and an even larger increase in the medial prefrontal cortex, i.e. 150%. Tetrodotoxin perfusion of the ventral tegmental area completely blocked the systemic MK-801 induced increase in extracellular concentrations of dopamine in the nucleus accumbens. However, the MK-801-evoked increase in dopamine levels in the medial prefrontal cortex was not significantly affected. Thus, the present results allow the conclusion that basal dopamine output in mesolimbic and mesocortical dopamine nerve terminal regions is predominantly dependent on nerve impulses generated in the ventral tegmental area. Moreover, also the MK-801 evoked dopamine release in the mesolimbic projection is almost entirely dependent on the impulse activity of the dopamine neurons, in agreement with our previous results. However, the MK-801 evoked dopamine release in the mesocortical projection is, in contrast, largely independent of the nerve impulse activity in the dopamine cells. The dysfunctions of mesolimbic and mesocortical dopamine neurons induced by systemic administration of non-competitive NMDA receptor antagonists may have direct bearing on the neurobiology of psychotic states, in particular as regards the generation of emotional and cognitive impairments.