DIFFERENTIAL DESENSITIZATION OF MU-OPIOID AND DELTA-OPIOID RECEPTORS IN SELECTED NEURAL PATHWAYS FOLLOWING CHRONIC MORPHINE TREATMENT

1 Morphine produces a plethora of pharmacological effects and its chro nic administration induces several side-effects. The cellular mechanis ms by which opiates induce these side-effects are not fully understood . Several studies suggest that regulation of adenylyl cyclase activity by opioids and other transmitters plays an important role in the cont rol of neural function. 2 The aim of this study was to evaluate desens itization of mu- and delta-opioid receptors, defined as a reduced abil ity of opioid agonists to inhibit adenylyl cyclase activity, in four d ifferent brain structures known to be involved in opiate drug actions: caudate putamen, nucleus accumbens, thalamus and periaqueductal gray (PAG). Opiate regulation of adenylyl cyclase in these regions has been studied in control and morphine-dependent rats. 3 The chronic morphin e treatment used in the present study (subcutaneous administration of 15.4 mg morphine/rat/day for 6 days via osmotic pump) induced signific ant physical dependence as indicated by naloxone-precipitated withdraw al symptoms. 4 Basal adenylyl cyclase in the four brain regions was no t modified by this chronic morphine treatment. In the FAG and the thal amus, a desensitization of mu- and delta-opioid receptors was observed , characterized by a reduced ability of Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAMGO; mu), Tyr-D-Pen-Gly-Phe-D-Pen (DPDPE; delta) and [D-Ala(2)]-del torphin-II (DT-II; delta) to inhibit adenylyl cyclase, activity follow ing chronic morphine treatment. 5 The opioid receptor desensitization in FAG and thalamus appeared to be heterologous since the metabotropic glutamate receptor agonists, L-AP4 and glutamate, and the 5-hydroxytr yptamine (5-HT)(1A) receptor agonist, R(+)-8-hydroxy-2-(di-n-propylami no)tetralin hydrobromide (8-OH-DPAT), also showed reduced inhibition o f adenylyl cyclase activity following chronic morphine treatment. 6 In the nucleus accumbens and the caudate putamen, desensitization of del ta-opioid receptor-mediated inhibition without modification of mu-opio id receptor-mediated inhibition was observed. An indirect mechanism pr obably involving dopaminergic systems is proposed to explain the desen sitization of delta-mediated responses and the lack of mu-opioid recep tor desensitization after chronic morphine treatment in caudate putame n and nucleus accumbens. 7 These results suggest that adaptive respons es occurring during chronic morphine administration are not identical in all opiate-sensitive neural populations.