Nonlinear relationship between impulse flow, dopamine release and dopamine elimination in the rat brain in vivo

Central dopaminergic neurons exhibit two kinds of discharge activity: single spikes and bursts of two to six action potentials. Since these neurons can switch from one discharge pattern to the other whereas the mean discharge rate remains little affected, bursts may be more potent in triggering the release of their neurotransmitter, dopamine. Electrical stimulations mimicking the bursting pattern were actually twice as potent as regularly spaced stimulations to enhance the dopamine extracellular concentration. This suggested that dopamine release might be facilitated by increasing the impulse flow frequency. The high extracellular overflow evoked by a burst might also be due to accumulation of the released dopamine whereas, at lower frequencies, dopamine might be readily eliminated between every action potential. In the present study the dopamine overflow evoked by electrical stimulation of the dopaminergic pathway was measured in vivo by carbon fibre electrodes combined with continuous amperometry. We observed a small facilitation of the release per pulse during stimulations mimicking a burst but only in mesolimbic areas. The high extra-cellular dopamine level evoked by a burst was mainly due to accumulation of the released dopamine.