EFFECTS OF NALOXONE AND D-PHE-CYS-TYR-D-TRP-ARG-THR-PEN-THR-NH2 AND THE PROTEIN-KINASE INHIBITORS H7 AND H8 ON ACUTE MORPHINE-DEPENDENCE AND ANTINOCICEPTIVE TOLERANCE IN MICE

Previous studies measuring opioid inhibition of cyclic adenosine monop hosphate in SH-SY5Y cells supported the hypothesis that continuous ago nist stimulation causes a gradual conversion of the mu opioid receptor to a sensitized or constitutively active state termed mu. Conversion to mu was prevented by the kinase inhibitor H7, but not its close an alog H8. Naloxone was proposed to act as a negative antagonist (invers e agonist) blocking mu activity, whereas D-Phe-Cys-Tyr-D-Trp-Arg-Thr- Pen-Thr-NH2 (CTAP) appeared to act as a neutral antagonist having no e ffect on mu activity. Initial in vivo results indicated that mu* acti vity may play a role in narcotic tolerance and dependence (Wang et al. , Life Sci. 54: PL339-PL350 1994). Our study explores the pharmacology of H7 and H8, naloxone and CTAP in mice after induction of acute tole rance and dependence induced by a single s.c. dose of morphine (100 mg /kg). Physical dependence was defined by withdrawal jumping induced by i.p. naloxone injections 4 hr after the morphine dose, the time of ma ximal physical dependence. Neither H7 nor H8 (50 nmol or less) induced jumping, affected morphine antinociception or produced significant be havioral effects, when injected by the intracerebroventricular (i.c.v. ) or intrathecal (i.th.) routes. When given 30 min before the naloxone challenge, H7, but not H8, significantly reduced naloxone jumping by i.c.v. injection. Administration of naloxone into the central nervous system, rather than by i.p. administration, required coinjection by bo th i.c.v. and i.th. routes to elicit full withdrawal jumping (30 nmol at each site). in contrast, the putative neutral antagonist CTAP cause d little withdrawal jumping when coinjected i.c.v. and i.th., as expec ted if modulation of mu activity played a role in dependence. However , CTAP was capable of partially reversing naloxone (i.p.) induced jump ing when given either i.c.v. or i.th., indicating that CTAP competes w ith naloxone at mu. Moreover, these results demonstrate that both spi nal and supraspinal sites are required for full opioid withdrawal jump ing in mice. Antinociceptive tolerance was also evaluated by determini ng the response to morphine in the 55 degrees C warm-water tail-flick test. Morphine pretreatment (100 mg/kg, s.c., -5 hr) produced antinoci ceptive tolerance as shown by a 2.7-fold increase in the calculated mo rphine A(50) value. Tolerance was reversed by H7, but not H8, treatmen t (50 nmol, i.c.v., -30 min). These results are consistent with the hy pothesis that a sensitized or constitutively active mu state plays a role in narcotic tolerance and dependence.