Membrane potential measured during potassium-evoked release of noradrenaline from rat brain neurons: effects of normorphine.

A single slice of rat pons that contained the locus ceruleus (LC) or two slices of cerebellum were loaded with [3H]noradrenaline; superfusion with high (35 or 60 mM) potassium solutions evoked a release of 3H. In the presence of normorphine, the release of 3H evoked by 35 mM potassium and 60 mM potassium was reduced. In some of those experiments in which the release of 3H from the LC slice was measured, an intracellular microelectrode was used to measure membrane potential. This showed that solutions of increased potassium concentration depolarized the neurons to a potential at which inward calcium currents flowed (calcium action potentials occurred). Normorphine hyperpolarized the neurons; during this hyperpolarization the depolarization caused by 35 mM potassium did not reach the threshold for significant calcium entry. The results suggest that the inhibition by normorphine of transmitter release evoked by solutions of raised potassium concentration could result in part from the membrane hyperpolarization caused by the normorphine.