One hypothesis regarding the etiology of schizophrenia proposes that disruption of the dopaminergic innervation of the prefrontal cortex leads to an increase in dopamine (DA) transmission in subcortical regions. In the present study, we examined the effect of 6-hydroxydopamine lesions of the medial prefrontal cortex (mPFC) dopamine innervation on the spontaneous electrophysiological activity of ventral tegmental DA neurons recorded in vivo. DA cell activity was assessed along three dimensions: (1) the relative proportion of DA neurons exhibiting spontaneous activity, (2) their basal firing rate, and (3) the mean percentage of spikes fired in bursts. In lesioned rats, DA neurons in the ventral tegmental area (VTA) exhibited a significantly slower mean firing rate, as well as a significant reduction in the percentage of spikes fired in bursts relative to controls. In contrast, depletion of DA in the mPFC did not have a significant effect on the relative proportion of VTA DA neurons exhibiting spontaneous activity. We suggest that by reducing the basal electrophysiological activity of VTA DA neurons, mPFC DA depletion may lead to an increase in the level of responsivity of the system to excitatory stimuli. Thus, the magnitude of increase in action potential-dependent DA release that occurs in response to a challenge may be augmented in lesioned rats.
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