Effect of opiate on receptor-mediated adenylate cyclase-cAMP signal system in NG-LNCXiNOS cell with the stable expression of inducible nitric oxide synthase gene

AIM: To study the effect of opiate on receptor-mediated adenylate cyclase (AC)-cAMP signal system in NG-LNCXiNOS cell with the stable expression of inducible nitric oxide synthase gene. METHODS: The intracellular cAMP content was measured with ³H-cAMP protein binding assay. The effect of opioid agonists on binding properties of opioid receptor was assessed using ³H-Etor binding assay with intact cells. RESULTS: The forskolin-stimulated cAMP accumulation was inhibited by opioid agonists in a concentration-dependent manner. The order of inhibitory potencies was DPDPE(D-Pen², D-Pen⁵-enkephalin) > DADLE(D-Ala², D-Leu⁵-enkephalin) > etorphine > morphine with IC₅₀ values of 2.0 × 10^-8, 2.1 × 10^-8, 3.7 × 10^-8 and 1.7 × 10 mol · L^-1, respectively. Such acute inhibitory effect of opioid agonist on adenylate cyclase (AC) activity could be remarkably blocked by the addition of naloxone and completely abolished by pretreatment of the cells with pertussis toxin(PTX), suggesting that acute inhibition of opioid agonists on AC-cAMP system may be regulated via a receptor-mediated and PTX-sensitive G protein pathway. After NG-LNCXiNOS cells were pretreated with DPDPE and DADLE for 48 hours, opioid agonists elevated intracellular cAMP concentration in the presence and absence of naloxone. Chronic exposure of the cells to DPDPE and DADLE resulted in reduction of Bmax values and increase of K_D values. Long-term treatment of neuronal cells with opioid agonists and naloxone-precipitated cell "withdrawal" can lead to receptor desensitization in a time-dependent and a dose-dependent manner, and the down-regulation of opiate receptor. CONCLUSION: A cellular model in opioid tolerance and dependence was successfully established. It may be useful for studying the cross-talk regulation of AC-cAMP and NO-cGMP signal pathway on the development of opiate tolerance and addiction.