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Vol. 281, Issue 1, 54-61, 1997
Program in Neuroscience, Department of Veterinary and Comparative
Anatomy, Pharmacology and Physiology, Washington State University,
Pullman, Washington
Male Sprague-Dawley rats that were naive or that had been treated with
five daily saline or cocaine injections (15 mg/kg i.p.) were
subsequently challenged with an injection of cocaine, and extracellular
dopamine content in the medial prefrontal cortex (mPFC) was measured
using in vivo microdialysis. Cocaine challenge increased
extracellular dopamine levels from base line in all three groups of
rats, but the augmentation was significantly reduced in the
cocaine-pretreated group, compared with the saline-pretreated group. In
contrast, mPFC dopamine levels were not different among groups after
challenge with systemic d-amphetamine. To test whether repeated cocaine treatment led to altered releasability of dopamine from mPFC terminals, challenge with KCl (10, 30 or 100 mM) or d-amphetamine (3, 30 or 300 µM) was made
via infusion through the dialysis probe into the mPFC.
No differences in dopamine levels were found between treatment groups
for either drug at any dose. To determine whether the effects of
cocaine were mediated by local actions within mPFC dopamine terminals,
a cocaine challenge was administered through the microdialysis probe
(1, 10 or 100 µM). In contrast to the systemic cocaine challenge,
local infusion of cocaine elicited a significant increase in daily
cocaine-pretreated rats, compared with saline-pretreated controls, at
the lowest dose tested, with no differences at the higher two doses. In
summary, daily cocaine-pretreated rats demonstrated a suppressed mPFC
dopamine response to subsequent systemic, but not local, cocaine
challenge. The results suggest that this apparent tolerance is not due
to altered releasability of dopamine from mPFC terminals and may rely
on altered afferent regulation of mesocortical dopamine neurons.
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