Ca2+-Dependent and Ca2+-Independent [3H]GABA Release from Rat Brain Synaptosomes: the Effect of Temperature

The main inhibitory neurotransmitter GABA in the mammalian brain is distributed in the nerve terminals between two pools, vesicular (synaptic vesicles) and cytosolic. GABA is released from these pools by different mechanisms; there are calcium-activated exocytotic release and calcium-independent sodium-dependent release from the cytosolic pool (resulting from the membrane GABA transporter reversal). We investigated the influence of temperature on ³H]GABA release from rat brain synaptosomes, which was induced by stimulation of both these processes. In addition, we used -latrotoxin as a stimulant of ³H]GABA release. Synaptosomes from the rat brain were used in the experiments. 4-Aminopyridine (4-AP) and high KCl] were applied to stimulate calcium-activated and calcium-independent ³H]GABA release, respectively. 4-AP-evoked ³H]GABA release was of the same intensity at 37 and 25°C (10.1 ± 1.2 and 10.1 ± 0.8% of total ³H]GABA incorporated into the synaptosomes, respectively). The effect of 4-AP on the ⁴⁵Ca²⁺ influx into synaptosomes was also temperature-independent: 0.775 ± 0.075 and 0.725 ± 0.100 nmol/min/mg of protein at 37 and 25°C, respectively. A drop in the effect of 4-AP was observed only at 15°C. When synaptosomes were depolarized with 50 mM KCl, a temperature decrease from 37°C to 25°C resulted in a twofold drop in the ³H]GABA release, from 20.5 ± 1.4 to 10.3 ± 0.7%; at 15°C ³H]GABA release dropped to less than one-third of the norm (6.0 ± 0.5%). -Latrotoxin-stimulated ³H]GABA release was diminished from 32.5 ± 2.5 at 37°C to 17.2 ± 1.3 at 25°C and 5.9 ± 0.4% at 15°C and was not affected by the presence or absence of calcium in the medium. It seems likely that the observed effect of temperature can be interpreted as based on the temperature dependence of the -latrotoxin insertion into the membrane. It is suggested that the pattern of the temperature sensitivity of GABA release from the synaptosomes can be used as a criterion for identification of the mode of neurotransmitter release.