κ-Opioid tolerance and dependence in cultures of dopaminergic midbrain neurons

Repeated cocaine exposure upregulates κ opioids and their receptors in the mesocorticolimbic system; the ensuing κ-mediated dysphoria appears to contribute to addiction and withdrawal. As a potential rehabilitation strategy to reverse cocaine-induced κ sensitization, the present study used tritiated dopamine release assays to examine the induction of κ-opioid tolerance in cultured mesencephalic neurons. Administration of the κ 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 κ 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 κ-receptor signaling mechanisms and to dependence. In the stimulated release assay, attenuation of κ 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 (t 1/2 = 20 hr). In the spontaneous release assay, attenuation of κ-opioid signaling occurred slowly (t 1/2 = 22 hr), and resensitization after a 144 hr exposure was rapid (t 1/2 < 1 hr). κ- Opioid dependence was observed after 144 hr of U69,593 exposure. Thus multiple mechanisms of adaptation to κ-opioid exposure occur in mesocorticolimbic neurons. These data support the idea that the administration of κ opioids might facilitate drug rehabilitation.