Effect of chronic administration of morphine, U-50, 488H and [D-Pen2, D-Pen5]enkephalin on the concentration of cGMP in brain regions and spinal cord of the mouse.

The effects of chronic administration and subsequent withdrawal of mu-, kappa- and delta-opioid receptor agonists on the levels of cyclic GMP in several brain regions and spinal cord of mice were determined in an attempt to further study the role of NO cascade in opioid actions. The agonists at mu-, kappa- and delta-opioid receptor included morphine, U-50,488H and DPDPE, respectively. Tolerance to morphine was associated with highly significant increases in cGMP levels in corpus striatum (41%), cortex (36%), midbrain (73%) and cerebellum (51%) relative to controls. Abstinence caused increases in cGMP levels in corpus striatum (61%) and pons and medulla (45%). Tolerance to U-50,488H resulted in increases in cGMP levels in midbrain (52%) whereas abstinence from U-50,488H increased the cGMP levels in pons and medulla (76%). Tolerance to DPDPE was associated with increases in cGMP levels in hypothalamus (12%) and pons and medulla (33%) but decreases in cerebellum (66%) and spinal cord (58%). Abstinence from DPDPE produced increases in cGMP levels in pons and medulla (14%) but decreases in cerebellum (67%) and spinal cord (50%). Overall treatment with morphine and U-50,488H produced increases in cGMP levels in brain regions whereas DPDPE produced decreases in brain regions and spinal cord. Previous studies have shown that chronic administration of mu- and kappa-opioid receptor agonists induce NO synthase (NOS) in certain brain regions and that the inhibitors of NO synthase attenuate tolerance to mu- and kappa- but not to delta-opioid receptors agonists. Since activation of NO increases the production of cGMP, the present results demonstrating alterations of cGMP levels by mu-, kappa- and delta-opioid receptor agonists are consistent with the behavioral results with NOS inhibitors on tolerance to mu-, kappa- and delta-opioid receptor agonists.