Energy utilization in the induced release of gamma-aminobutyric acid from synaptosomes

Requirement of ATP for exocytotic release of catecholamines from digitonin-permeabilized adrenal chromaffin cells

Cultured chromaffin cells were preincubated with digitonin to deplete endogenous ATP from the cell cytoplasm. Catecholamine release from these digitonin-pretreated cells was then studied in the presence and absence of exogenous ATP to elucidate a possible involvement of the cytoplasmic ATP in the exocytotic process. The preincubation of the cells with digitonin in the ATP-free permeabilizing medium resulted in a marked decline of the releasing response to a calcium challenge. Furthermore, the declined activity of catecholamine release caused by digitonin pretreatment was restored by the presence of ATP, but not by other adenine nucleotides, and this recovery was observed in a manner dependent on the concentration of ATP. These findings, therefore, seem to indicate that a decrease in the releasing activity of the digitonin-pretreated cells may be due to the removal of endogenous ATP from the cytoplasmic space of the cells, thus suggesting that the cytoplasmic ATP may be involved in the exocytotic mechanism of catecholamine secretion.

Metabolic properties of nerve endings isolated from rat brain

Energy metabolism was studied in nerve endings isolated from 3-week-old rat brain. Concentrations of glycogen, glucose, ATP, phosphocreatine and lactate were lower in synaptosomes than in the intact brain. The consumption of these endogenous substrates, the ability to generate high-energy phosphate, and the production of ammonia were determined in aerobic and anaerobic conditions. Unlike nerve tissue in general, synaptosomes preferentially utilized endogenous ATP and phosphocreatine stores which, on incubation in the absence of exogenous substrates, were emptied long before glycogen stores were exhausted. The optimal medium for respiratory studies was found to have electrolyte concentrations equal to the extracellular fluid. The synaptosomes had an endogenous respiration rate of 6.3 nmol O2 mg prot. min, measured with an oxygen electrode, and it probably reflects consumption of their glycogen stores. Glucose usually had no effect on the respiration rate of synaptosomes, but sometimes increased it slightly. However, after incubation in the presence of arsenate synaptosomes showed an increase in respiration when glucose was added. ADP, when added with glucose, also stimulated respiration. Pyruvate and succinate always increased respiration, succinate usually having the stronger effect. The present results show that isolated nerve endings are metabolically intact, which justifies their use in research on neurotransmission. In addition, opposite to the present consensus, synaptic transmission does not seem primarily to depend on the availability of glucose but rather on local stores of high-energy phosphate compounds.

GABA binding processes in rat brain and liver

Although these studies have provided some further insight into GABA binding processes associated with transport and receptor-activation, such attempts remain limited due to the lack of specificity of the ligands used to displace these components. Thus, BMI appeared to interact with both transport and synaptic receptor sites for GABA. Perhaps certain GABA-agonists (e.g., muscimol, isoguvacine) will be shown to interact more specifically with synaptic GABA-receptors? So-called "specific" binding of GABA and its analogues, or antagonists might not be the issue to be kept in focus in this type of experiment, especially since it cannot be separated into presynaptic, postsynaptic and non-synaptic components. The specificity of an interaction of ligand with membrane site may be determined by other requirements of the particular system concerned. In any case, it seems essential to examine the binding sites, themselves, in order to define more clearly which of these are involved in receptor-activation and in uptake. An approach involving the isolation and purification of GABA-receptors should provide evidence for specific binding of GABA to its postsynaptic receptors.