Effects and mechanism of action of isatin, a MAO inhibitor, on in vivo striatal dopamine release

• In situ infusion of isatin increases the in vivo dopamine release from rat striatum in a concentration-dependent manner.
• Isatin-induced dopamine release is almost completely blocked by the absence of Ca++ in perfusion medium.
• The release of dopamine induced by isatin depends on voltage-sensitive sodium channels activation.
• Treatment with reserpine decreased but not completely abolished the effect of isatin on striatal dopamine release.
• The blockade of the DAT with nomifensine did not alter the effect of isatin on striatal dopamine levels.

Isatin is an endogenous indole that inhibits monoamine oxidase (MAO), being more selective for MAO-B than MAO-A isoform. By inhibiting MAO, isatin increases dopamine levels in the brain and, in animal models of Parkinson’s disease (PD) isatin is able to prevent dopamine depletion. Contradictorily, some studies indicate that isatin did not increase striatal dopamine levels, although it was able to improve the motor signs in PD model. Given these conflicting data, our aim was to study the effects and neurochemical mechanisms of action of isatin on in vivo dopamine release from rat dorsal striatum using brain microdialysis technique in conscious and freely moving animals. Our results showed that intrastriatal administration of 1, 5 or 10 mM isatin, for 1 h, significantly increased dopamine levels to 355 ± 104%, 700 ± 72%, and 1241 ± 146%, when compared with basal values, respectively. The highest concentration of isatin (10 mM) was used to investigate whether the dopamine overflow is due to an exocytotic release or due to a possible action on dopamine transporter (DAT). The removal of Ca++ from medium, administration of TTX (10 μM), or pretreatment with reserpine (10 mg/kg) significantly decreased by 90%, 83%, and 78%, respectively, the effect of isatin on dopamine levels. The blockade of DAT with nomifensine (50 μM) did not alter the effect of isatin; and isatin significantly increased the depolarization-evoked release of dopamine. These results suggest that isatin-induced dopamine release depends on vesicular dopamine content, and takes place due to a previous entry of Ca++ and terminal depolarization.