Differential α2-mediated inhibition of dopamine and noradrenaline release in the parietal and occipital cortex following noradrenaline transporter blockade

Parietal and occipital cortices, while densely innervated by noradrenalin 2 (NA) projections, possess a comparatively sparse dopamine 2 (DA) innervation, even sparser than the prefrontal cortex. We previously reported that reboxetine and desipramine, two selective norepinephrine transporter (NET) blockers, at doses that maximally increase DA in the prefrontal cortex, do not increase DA in the parietal and occipital cortices. In the present study, we performed a full dose-response study of the effect of systemic reboxetine and desipramine on DA and NA in dialysates from the parietal and occipital cortices. Seven doses of reboxetine (0.1, 0.25, 0.5, 1.0, 2.5, 5.0 and 10 mg/kg) and four doses of desipramine (0.25, 1.0, 2.5 and 5.0 mg/kg) were tested. Reboxetine and desipramine differentially affected dialysate DA as compared with NA. Reboxetine increased DA maximally by about 100% after doses of 0.25-0.5 mg/kg and showed a bell-shaped dose-response function in both areas; desipramine did not affect DA in the parietal cortex and increased it in the occipital cortex only at 2.5 mg/kg. NA was maximally increased by 275% by 0.5-2.5 mg/kg reboxetine and by about 300% by 5.0 mg/kg desipramine with a more linear dose-response curve. The mechanism of peculiar dose-response function of dialysate DA after reboxetine and desipramine was further investigated by testing the effect of drugs on dialysate DA and NA under alpha(2) receptor blockade. Under local perfusion of the occipital cortex with idazoxan, an otherwise ineffective dose of reboxetine and desipramine (5 mg/kg) became effective in raising extracellular DA. In contrast, the effect of reboxetine on NA was potentiated, while that of desipramine was not affected. These results suggest that, in the parietal and occipital cortices, extracellular NA, raised by NET blockade, exerts a preferential inhibitory influence on DA release by acting on local alpha(2) receptors, thus accounting for the bell-shaped feature of the dose-response function of drugs on dialysate DA in these areas.