A STUDY OF THE METABOLISM AND RELEASE OF DOPAMINE AND AMINO ACIDS FROM NERVE ENDINGS ISOLATED FROM SHEEP CORPUS STRIATUM

Abstract— Synaptosomes prepared from sheep corpus striatum showed a linear rate of respiration over a 90 min period of incubation in Krebs-bicarbonate medium containing glucose (10 mm ) and the rate of respiration was stimulated by electrical pulses. Dopamine was released from synaptosome beds to the medium by either electrical pulses or 56mm -K⁺ (10min), increasing 108% and 76% respectively above control levels of release. The presence of d- or 1-amphetamine (0.12mm ) in the incubation medium (40 min) increased the accumulation of dopamine in the medium by 310 and 275% respectively and 56mm -K⁺ also caused a significant increase in the release of glutamate, GABA and aspartate. Radioactively labelled dopamine was synthesized by the synaptosomes from l -¹⁴C]tyrosine, l -DOPA or dl -DOPA, and electrical pulses caused a 35% increase in the rate of dopamine production from U-¹⁴C] tyrosine. No increased release of ¹⁴C]dopamine in response to depolarizing stimuli was found to occur when synaptosome beds were transferred from medium containing radioactive precursors to fresh medium for further incubation (20 min). In the presence of 1- and d-amphetamine, accumulation of ¹⁴C-labelled doparnine in the incubation media was increased 129% and 380% respectively, the latter was partially depressed by absence of calcium from the medium. Three radioactively labelled metabolites formed by synaptosomes during incubation in dl -2-¹⁴C]DOPA were detected; the major ones were dihydroxyphenylacetic acid and homovanillic acid and the third was unidentified. When the synaptosome beds were transferred to medium containing no radioactive precursors, it was found that labelled dihydroxyphenylacetic acid was 7 times more abundant than labelled dopamine in the incubation medium (20 min) and one-third as abundant in the synaptosomes. The dihydroxyphenylacetic acid n Ci/dopamine n Ci ratio was greatly affected by K⁺ stimulation, decreasing 52% and 34% in the incubation medium and synaptosomes respectively. A pathway of dihydroxyphenylacetic acid degradation was shown to occur through decarboxylation. These results are discussed in terms of the compartmentation of dopamine and its metabolism. It is proposed that one pool of dopamine is released by depolarizing agents and during the period of incubation it is replaced by synthesis from the endogenous tyrosine (19.5 nmol/100 mg protein) and not by the labelled dopamine in the synaptosome. The synaptosomal pool of dopamine which is radioactively labelled after pulse labelling with dl -2-¹⁴C]DOPA appears to be prone to oxidation to DOPAC and homovanillic acid which are preferentially released from the synaptosomes.