Different temperature dependence of carrier-mediated (cytoplasmic) and stimulus-evoked (exocytotic) release of transmitter: a simple method to separate the two types of release.

The temperature dependence of transmitter release associated with axonal conduction, evoked by ligand-gated mechanism and by reversed operation of plasma membrane transporter was studied in superfused slice preparation. When the temperature was reduced from 37-17 degrees C the release of [3H]noradrenaline ([3H]NA) and [3H]dopamine ([3H]DA) in response to field stimulation was significantly enhanced in slice preparations of the hippocampus and main olfactory bulbs. The release of [3H]dopamine evoked by a ligand-gated mechanism (nicotine receptor stimulation) was potentiated at low temperature (12 degrees C). In contrast, when transmitter release was evoked by ouabain, a drug inhibiting Na+/K+-activated ATPase and thereby increasing [Na+]i the release of [3H]GABA was enhanced. This release was very sensitive to cooling (Q10=3.5 between 37 degrees C and 27 degrees C), indicating that the release was induced by a reversed operation of the transporter. The excessive release of [3H]NA from the hippocampal slice in response to oxygen and glucose deprivation (simulation of ischemia) was also inhibited in a temperature-dependent manner. Because at low temperatures (17-12 degrees C) only one type of release mechanism (exocytosis) is operational and carrier-mediated uptake and release is inhibited, this temperature condition provides a method to study the mode of action of different drugs (e.g. amphetamine) and the effect(s) of certain conditions (e.g. ischemia) on the mechanisms of transmitter release, specifically whether they exert their transmitter releasing effect through an exocytotic process or through the reversed operation of plasma membrane transporter. This finding also suggests that it would be important to re-examine mechanistic conclusions based on results from electrophysiological, neurochemical and pharmacological studies that have been carried out at room temperature (approximately 20 degrees C). In particular because transmitter release associated with the action potential, diffusion, receptor kinetics, active transport in both directions (uptake and release) and the probability of transmitter release are all temperature dependent, it would seem important to carry out experiments involving these processes at physiological temperature (37 degrees C).