On the mechanism of presynaptic autoreceptor-mediated inhibition of transmitter synthesis in dopaminergic nerve terminals

Abstract

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 composition 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]tyrosine with an ic 50 of 1.8 × 10 −7 M. Haloperidol (4 × 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 × 10 −7 M) completely reversed the inhibition caused by APO (2 × 10 −7 M). DA uptake blockers, such as benztropine (2 × −6 M) or cocaine (1 × 10 −5 M), did not affect the ability of APO to inhibit DA synthesis. Furthermore, the α 2-adrenergic agonist clonidine produced only a mild inhibition and the β-adrenergic agonist isoproterenol produced no inhibition of [ 14C]DA formation. APO was able to inhibit DA formation both in the absence of calcium in the incubation medium or in the presence of high external calcium concentrations (4, 8 and 24 mM) which depress the rate of DA synthesis. Incubation conditions that cause an increase of free intraneuronal calcium concentrations, such as Na +-free medium, the presence of ouabain (1 × 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. The results reported in this work indicate that tissue slices can be used as a valuable experimental tool to study the inhibitory effect of APO on DA synthesis, and that this effect occurs through an interaction of APO with presynaptic DA autoreceptors located in striatal dopaminergic nerve terminals. The results obtained are not in keeping with the hypothesis that this autoreceptormediated inhibition of DA synthesis occurs through regulation of calcium influx into the DA nerve terminals. However, the possibility is raised that a sensitivity to high intraneuronal calcium concentration exists during the events that mediate APO-DA autoreceptor interaction and DA synthesis inhibition. It is further suggested that DA-synthesis-modulating autoreceptors do not participate in the modulation of DA release.