ON THE MECHANISM OF PRE-SYNAPTIC AUTORECEPTOR-MEDIATED INHIBITION OF TRANSMITTER SYNTHESIS IN DOPAMINERGIC NERVE-TERMINALS

The effect of apomorphine (APO) upon dopamine (DA) synthesis and release from rat striatal slices was studied. The synthesis of DA was measured by incubating the slices in Krebs-Ringer phosphate (KRP) medium of variable ionic compositon containing L-tyrosine[14C-U] as DA precursor. A superfusion system was used to study both spontaneous and K+-induced release of labeled DA from striatal slices. The addition of APO directly to the normal KRP medium markedly blocked the formation of [14C]DA from [14C]Tyr with an IC50 [median inhibitory concentration] of 1.8 .times. 10-7 M. Haloperidol (4 .times. 10-7 M), a known DA antagonist, produced a shift to the right of the concentration-response curve for APO inhibition on DA synthesis, whereas the DA antagonist (+)butaclamol (4 .times. 10-7 M) completely reversed the inhibition caused by APO (2 .times. 10-7 M). DA uptake blockers, such as benztropine (2 .times. 10-6 M) or cocaine (1 .times. 10-5 M), did not affect the ability of APO to inhibit DA synthesis. The .alpha.2-adrenergic agonist clonidine produced only a mild inhibition and the .beta.-adrenergic agonist isoproterenol produced no inhibition of [14C]DA formation. APO was able to inhibit DA formation in the absence of Ca in the incubation medium or in the presence of high external Ca concentrations (4, 8 and 24 mM) which depress the rate of DA synthesis. Incubation conditions that cause an increase of free intraneuronal Ca concentrations, such as Na+-free medium, the presence of ouabain (1 .times. 10-4 M), or K+ depolarization, dramatically abolished or impaired the ability of APO to inhibit DA synthesis in striatal slices. It was not possible to demonstrate any change in spontaneous and K+ (27 mM)-induced release of DA in the presence of APO concentrations that produced a marked inhibition of DA synthesis. Tissue slices can evidently be used as a valuable experimental tool to study the inhibitory effect of APO on DA synthesis; this effect occurs through an interaction of APO with presynaptic DA autoreceptors located in striatal dopaminergic nerve terminals. The results do not correlate with the autoreceptor-mediated inhibition of DA synthesis occurring through regulation of Ca influx into the DA nerve terminals. A sensitivity to high intraneuronal Ca concentration may exist during the events that mediate APO-DA autoreceptor interaction and DA synthesis inhibition. DA-synthesis-modulating autoreceptors apparently do not participate in the modulation of DA release.