Contribution of Blockade of the Noradrenaline Carrier to the Increase of Extracellular Dopamine in the Rat Prefrontal Cortex by Amphetamine and Cocaine

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Abstract
This study was performed to investigate the relative role of noradrenaline (NA) and dopamine (DA) carrier blockade in the effects of psychostimulants on DA transmission in the rat prefrontal cortex (PFCX). To this end, changes of extracellular DA and NA in the PFCX and of extracellular DA in the nucleus accumbens (NAc) were measured following the administration of amphetamine and cocaine, which are known to bind to both DA and NA carriers, or GBR 12909, a selective DA carrier blocker. After non-intravenous injection, amphetamine (0.25 and 0.5 mg/kg, s.c.) and cocaine (5 and 10 mg/kg, i.p.) increased extracellular DA in the PFCX to a larger extent than in the NAc, while the reverse applied to GBR 12909 (2.5 and 5 mg/kg, i.p.). These differences were obtained in spite of the fact that the three drugs elicited at each dose level a similar peak increase of extracellular DA in the NAc. Amphetamine and cocaine also increased extracellular NA in the PFCX and this effect was quantitatively similar to that on extracellular DA in the same area. Intravenous doses of cocaine and GBR 12909, corresponding to those which maintain self-administration in the rat, while equieffective in raising extracellular DA in the NAG, had different effects on extracellular DA in the PFCX. In fact, in contrast to cocaine, GBR 12909 increased extracellular DA in the PFCX to a lesser extent than in the NAc or did not modify it at all. The peak increase of extracellular DA in the PFCX was highly correlated to that of NA in the same area but was poorly correlated to the increase of extracellular DA in the NAc. These results suggest that amphetamine and cocaine increase extracellular DA in the PFCX largely through the blockade of the NA carrier. Direct evidence for this hypothesis was provided by the observation that, when the NA carrier was blocked by reverse dialysis of the PFCX with desipramine (1 μM), cocaine and GBR 12909 lost their differences in the ability to increase extracellular DA in the PFCX.

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