The role of potassium ion loss in the anoxic impairment of respiration of rat cerebral-cortex slices.

Synaptosomes are now known to possess a high degree. of metabolic independence during incubation in physiological medium, containing glucose as a substrate (see review by de Belleroche & Bradford, 1973). Under these conditions, depolarizing agents induce the calcium-dependent release of physiologically active compounds, including acetylcholine (de Belleroche &Bradford, 19726), aspartate, glutamate, y-aminobutyrate (de Belleroche & Bradford, 1972a) and noradrenaline (Blaustein et al., 1972) from cerebral cortex, glycine, aspartate and y-aminobutyrate from spinal cord (Osborne et al., 1973), and releasingfractors from hypothalamus (Edwardson et al., 1972). In the present study, this experimental system was used to study the metabolism and release of dopamine (3,4-dihydroxyphenethylamine) from synaptosomes prepared from corpus striatum where it is likely to act as an inhibitory transmitter as judged by histochemical, electrophysio-logical and biochemical evidence (see review by Hornykiewicz, 1973). Both electrical pulses and amphetamine induce dopamine release in vivo from corpus striatum from striatal synaptosomes pre-loaded with radioactively labelled dopamine (Ferris et al., 1972). To characterize nerve-terminal compartments of dopamine (Javoy & Glowinski, 1971) involved in specific release processes, the release of both endogenous dopamine and dopamine synthesized in situ from radioactively labelled precursors [L-tyrosine and DL-dopa (3,4-dihydroxyphenylalanine)] was measured. Synaptosomes were isolated from sheep corpus striatum by conventional methods (Gray & Whittaker, 1962) and incubatedin the formof a'synaptosomebed'aspreviously described (de Belleroche & Bradford, 1972a). Oxygen consumption by the striatal synap-tosomes was linear over at least 90min of incubation at a rate of 61.0k4.0 (6)pmol of oxygen consumed/h per lOOmg of protein, which was comparable with the rates for synaptosomes isolated from other brain regions. Endogenous dopamine was measured fluorimetically after separation with alumina by the methods of Anton & Sayre (1962) and Laverty & Taylor (1968). Radioactively labelled dopamine was isolated together with non-radioactive carrier by using a cation-exchange resin (Zeocarb 225) before liquid-scintillation counting. Electrical pulses, elevated potassium (5 6 m ~) and D-and L-amphetamine (0.1 1 9 m ~) induced the release of endogenous dopamine from synaptosome beds substantially above the control amounts (Table 1). By using radioactively labelled DL-dopa, radioactive dopamine was readily formed. Labelled dopamine was released during incubation and the amount released was increased by amphetamine but not by depolarizing agents during incubation periods of up to 30min. The D-isomer was most potent in this respect. However, with incubation times of greater than 30min potassium stimulation in the presence of D-amphetamine caused an increase in the release of radioactive dopamine. This …