FUNCTIONAL SIGNIFICANCE OF MU OPIOID RECEPTOR TURNOVER

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This is a request for a Scientist Development Award (K21). The research in this proposal is designed to determine the relationship of mu-opioid receptor turnover to biochemical and pharmacological indices of opioid receptor function. Investigating the relationship of opioid receptor occupancy with biochemical and pharmacological indices of opioid receptor function is pertinent for understanding the mechanisms that determine the potency and efficacy of opioids in whole animals. Preliminary data suggest that when the density of mu-opioid receptors is decreased in rat brain through receptor alkylation with beta-funaltrexamine (beta-FNA), the reinforcing effects of heroin return prior to the return prior to the return of receptor binding. These data suggest that mu-opioid receptor turnover should be measured using both biochemical and pharmacological assays of receptor function, rather than receptor binding alone as indices of receptor turnover. The objective of this research is to determine the relevance of mu-opioid receptor turnover as measured with receptor binding or biochemical assays of receptor function (inhibition of adenylyl cyclase and stimulation of GTPase) to the in vivo effects of mu-opioid agonists. Specific aim 1 tests the hypothesis that the pharmacological activity of opioids in whole animals is restored when a relatively small proportion of mu-opioid receptors become functionally coupled to G-proteins. The time course of the effects of beta-FNA i.c.v. on mu-receptor-mediated stimulation of GTPase and inhibition of adenylyl cyclase will be compared to the effects on morphine-induced analgesia and hyperlocomotion. The second specific aim tests the hypothesis that mu=opioid receptor turnover is more rapid in pharmacologically-relevant brain regions, and that receptor binding experiments do not determine this due to the fact that these receptor populations represent a relatively small proportion of those in the whole brain. The experiments that will test this hypothesis are similar to those in specific aim 1, except that quantitative receptor autoradiography will be used to assess the density of mu-opioid receptors in discrete brain regions following receptor alkylation. The third specific aim tests the hypothesis that receptor alkylation will result in an upregulation of receptor biosynthesis in pharmacologically-relevant regions of the brain. Following receptor alkylation, mu-opioid receptor mRNA will be quantitated in various brain regions at different time points and compared to the return of the pharmacological effects of morphine. Specific aim 4 tests the hypothesis that inhibition of mu-opioid receptor biosynthesis will be most effective at altering the pharmacological effects of opiates when administered into relevant brain regions. These experiments will involve administering anti-sense oligodeoxynucleotides for mu-opioid receptor mRNA into brain regions that contain the highest amount of mu-opioid receptor mRNA or in which the mRNA is increased to the greatest extent following receptor alkylation.