Regulatory aspects of nigrostriatal dopaminergic neurons

In the urethane-anesthetized rat, electrical stimulation (10 Hz, 30 s, 250 microA) of the medial forebrain bundle (MFB), at 20-min intervals over an 8-h period, combined with intracerebral microdialysis in the striatum caused: an undiminished increase in the release of dopamine (DA) with each stimulation episode; a decreased efflux of 3,4-dihydroxyphenylacetic acid (DOPAC) and 4-hydroxy-3-methoxyphenylacetic acid (HVA) after the first stimulation only; a delayed increased efflux of DOPAC with no change in HVA; and a poststimulation depression of firing of dopaminergic neurons in the substantia nigra (before, 3.1 +/- 0.7 Hz; after, 1.9 +/- 1.0 Hz; P < 0.05). After the last stimulation episode, the release of DA declined to prestimulation values, while the increased efflux of DOPAC persisted for three more hours. After the infusion of tetrodotoxin (4.0 x 10(-7) M, 1.5 microliters, 1.0 microliters/min) into the MFB, the basal release of DA was reduced (P < 0.05), while the efflux of DOPAC and HVA was increased (P < 0.05). A model is proposed suggesting that: (1) during increased release of DA in the striatum, the metabolism of DA is decreased; (2) inhibition of nigrostriatal dopaminergic neurons is the usual cause of increased synthesis and metabolism of DA in the striatum; and (3) increased release of DA, and increased synthesis and metabolism of DA in the striatum are not causally linked and are noncoupled processes.