The role of presynaptic receptors in the release and synthesis of ³H-dopamine by slices of rat striatum

Summary

Striatal slices were continuously superfused with l-3,5-³H-tyrosine (50 μCi/ml) and ³H-H₂O [index of ³H-dopamine (³H-DA) synthesis] and ³H-DA estimated in 0.5 ml (2.5 min) superfusate fractions. Depolarization with 50 mM K⁺ for 7.5 min induced a marked increase in ³H-DA release and a biphasic effect on synthesis (slight increase in the first fraction followed by a significant decrease in the third and fourth fractions). The decrease in the rate of ³H-H₂O formation induced by K⁺ was not related to modifications of the specific activity of tyrosine in tissues. The possibility that the inhibition of synthesis was due to alterations in DA concentration in the synaptic cleft was examined. Benztropine in a concentration which produced inhibition of DA uptake (10⁻⁶ M) increased the K⁺ induced overflow of ³H-DA but failed to alter the inhibition of synthesis. On the other hand, when the powerful neuroleptic fluphenazine was added to the superfusion medium in a concentration which only weakly blocked ³H-DA uptake (10⁻⁶ M) it potentiated ³H-DA release and prevented the inhibition of synthesis both in the absence or presence of benztropine. A similar effect was seen following the in vivo treatment of rats with fluphenazine (2 mg/kg; 1 1/2 h before sacrifice). The addition of exogenous DA (0.6×10⁻⁶ M) or NA (10⁻⁶ M) to the superfusion medium increased ³H-DA outflow and reduced DA synthesis while isoproterenol (10⁻⁶ M) was without effect. The DA inhibitory effect on synthesis was still observed in the presence of benztropine (10⁻⁶ M) while the NA effect was prevented. This concentration of benztropine blocked both DA and NA uptake. The administration of fluphenazine (10⁻⁶ M) significantly prevented the decrease in ³H-DA synthesis induced by exogenous DA and partially prevented the effect of NA. In addition, the effect of exogenous DA on the inhibition of synthesis was still seen in the presence of 2-amino-4-hydroxy-6,7-dimethyl-5,6–7,8-tetrahydropteridine hydrochloride (DMPH₄) (to protect against end-product inhibition). The present results provide direct support for the concept that activation of presynaptic DA receptors located on DA terminals in the striatum of the rat results in an inhibition of synthesis and release of the transmitter.