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Regulation of spontaneous activity and oscillatory spike firing in rat midbrain dopamine neurons recorded in vitro

Intracellular recordings were obtained from identified dopamine (DA) neurons in rat midbrain slices maintained in vitro. DA neuron membranes exhibited pronounced instantaneous and time‐dependent anomalous rectification that showed evidence of maximal activation at average membrane potentials of −63 and −78 mV, respectively. Action potentials were followed by prominent afterhyperpolarizations (AHP) that consisted of two components. The fast component showed evidence of inactivation at −63 mV independent of the initial membrane potential, whereas the longer‐duration, later component increased in amplitude at hyperpolarized potentials. Unlike DA neurons recorded in vivo, there was no evidence of spike frequency adaptation or summation of AHPs with prolonged depolarization‐induced spike trains. Spontaneous spike discharge occurred via an endogenous pacemaker potential that was dependent on both TTX‐sensitive and cobalt‐sensitive processes. Hyperpolarizing prepulses could activate rebound pacemaker discharge, but this rebound activity was progressively blocked with larger‐amplitude hyperpolarizing prepulses.