Cannabinoids and morphine produce antinociceptive effects in combination, which are far greater in magnitude than the effects of either drug alone. Cannabinoid-induced release of endogenous opioids results in antinociceptive effects of the cannabinoids and enhancement of morphine, A low dose combination of delta9-THC (THC) and morphine produces antinociception without the development of tolerance, while antinoceptive doses of THC and morphine alone produce tolerance. Tolerance to THC or morphine is accompanied by change sin opioid receptor protein levels that may underlie tolerance. However, no change in receptor proteins is observed with the low dose combination of THC and morphine. Thus, prevention of behavioral tolerance results in prevention of biochemical correlates of behavioral tolerance. Similarly, we have implicated c-AMP dependent protein kinase (PKA) in the actions of both cannabinoids and opioids. A PKA inhibitor has been shown to reverse tolerance to both THC and to morphine. Reversal of tolerance to THC with the PKA inhibitor restores THC-induced release of dynorphin. Therefore, reversal of behavioral tolerance results in reversal of the correlated biochemical events associated with tolerance. Thus, our proposed work is designed to answer several pertinent questions raised by our studies. We have yet to answer the important question of how the activation of the BC1 receptor modulates opioid release. We will determine if the THC/morphine interaction involves an interaction with common G-proteins using agonist-stimulated [35S]GTPgammaS binding and determine the role of G proteins in tolerance to the drugs. In addition, we will evaluate down-stream effectors in nociceptive pathways distal to and modulated by opioid release, such as Substance P and CGRP. We hypothesize that opioid-induced decreases in the release of Substance P and CGRP are a contribute to THC/morphine enhancement. Thus, our Specific Aims are designed to capitalize upon the cannabinoid/opioid synergy in order understand the response (plasticity) of neuronal substrates to both THC and opioid tolerance. The ability to better define those processes involved in tolerance may enable us to produce long term antinociceptive effects with the THC/morphine combination, while preventing the neuronal biochemical changes that accompany tolerance.
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