The Journal of Neuroscience, July 15, 1999, 19(14):5750-5757
-Opioid Tolerance and Dependence in Cultures of Dopaminergic
Midbrain Neurons
Friedrich C.
Dalman1 and
Karen L.
O'Malley2
Departments of 1 Anesthesiology and
2 Anatomy & Neurobiology, Washington University School of
Medicine, St. Louis, Missouri 63110
Repeated cocaine exposure upregulates 
opioids and their
receptors in the mesocorticolimbic system; the ensuing -mediated dysphoria appears to contribute to addiction and withdrawal. As a
potential rehabilitation strategy to reverse cocaine-induced sensitization, the present study used tritiated dopamine release assays
to examine the induction of -opioid tolerance in cultured mesencephalic neurons. Administration of the agonist U69,593 inhibited tetrodotoxin-sensitive, spontaneous (EC50 = 1.5 nM), and potassium-stimulated (EC50 = 10 nM) release. These effects were blocked by pertussis
toxin and by the antagonist nor-binaltorphimine. The 2 d
agonist exposure (1 µM) caused a shift in the U69,593 dose-response curve that was greater in the potassium-stimulated paradigm (140-fold) than in the spontaneous release assay (sixfold). These results were attributable to the attenuation of -receptor signaling mechanisms and to dependence. In the stimulated release assay, attenuation of signaling caused by 4 hr of U69,593 exposure recovered with a half-life of 1.1 hr, whereas attenuation after 144 hr
of exposure recovered slowly
(t1/2 = 20 hr). In the spontaneous release assay, attenuation of -opioid signaling occurred slowly (t1/2 = 22 hr), and
resensitization after a 144 hr exposure was rapid
(t1/2 < 1 hr). -Opioid
dependence was observed after 144 hr of U69,593 exposure. Thus multiple
mechanisms of adaptation to -opioid exposure occur in
mesocorticolimbic neurons. These data support the idea that the
administration of opioids might facilitate drug rehabilitation.
Key words:
-opioid receptor; tolerance; dependence; mesencephalon; dopaminergic neurons; primary neuronal culture; neurotransmitter release; U69,593
Copyright © 1999 Society for Neuroscience 0270-6474/99/19145750-08$05.00/0
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