Dopamine release from pharmacologically distinct storage pools in rat striatum following stimulation at frequency of neuronal bursting.

The increase of dopamine overflow in the rat caudate and nucleus accumbens following repeated stimulation of the median forebrain bundle (MFB) at short intervals and the role of alpha-methyl-p-tyrosine (AMPT) and reserpine sensitive storage pools in evoked dopamine overflow were investigated by in vivo voltammetry. In contrast to stimulation at 30-s intervals and longer, stimulation of the MFB at 5-s intervals led to increased dopamine release following each consecutive stimulation. This effect was much larger in the caudate than in the nucleus accumbens. We consider that the increase in dopamine release is due to translocation of dopamine from a reserpine-sensitive storage pool and this accounts for the increase of the rate of refilling of the readily releasable pool. The shorter the intervals between stimulation and the higher the frequency (with a plateau at 30 Hz), then the greater the fraction of dopamine which originates from the reserpine-sensitive storage pool. Exocytotic release of dopamine from the AMPT-sensitive storage pool does not seem to depend on the intervals between stimulation. We propose that the vesicles in presynaptic dopaminergic terminals near the outer membrane are more sensitive to AMPT. Distantly located vesicles have a relatively higher sensitivity to reserpine. Experiments using repeated stimulation at as low a frequency as 10-20 Hz, revealed that this phenomenon may take place under physiological conditions following bursting of dopamine neurones.