A novel class of compounds that bind to sphingosine-1 phosphate (S1P, formerly called EDG) lysolipid receptors is strikingly similar to cannabinoids in their behavioral activity (Martin """"""""tetrad"""""""" behaviors characteristic of cannabinoids) and localization in the central nervous system (CNS) but their role in CNS functions has not been investigated. The regional distribution of S1P-with cannabinoid (CBI)-mediated Gprotein activity in the CNS reveals co-localization in areas involved in nociceptive transmission. CB1 and S1P receptors have significant sequence homology and both act via G-proteins to produce intracellular effects. In addition, both FTY720, a sphingosine analogue and sphingosine are competitive CB1 antagonists, and CB1 and S1P agonists produce less than additive levels of G-protein activation in regions where colocalize. Cannabinoids such as delta-9-tetrahydrocannabinol (THC) have also been shown to release endogenous opioids spinally, synergize with opioids in the production of antinociception, and prevent the expression of tolerance and physical dependence to morphine. CB1 and opioid receptors co-localize and may heterodimerize in areas involved with nociception in the CNS. Thus, it is apparent that not only do CB receptors co-localize with S1P receptors, but also with opioid receptors. S1P/ CB1 system similarities led us to evaluate S1P/opioid interactions. S1P-induced antinociception is non-CB1; non-CB2 mediated, but is completely blocked by opioid antagonists. S1P and opioids produce less-than-additive G-protein activation in brain regions. In addition, S1P, like THC, enhances the antinociceptive effects of morphine. We hypothesize that S1P, like THC, will release endogenous opioids, enhance morphine and will also block tolerance to morphine. These hypotheses will be addressed pharmacogenetically by the use of endogenous opioid and opioid receptor knockouts, as well as S1P receptor knockout mice; pharmacologically, via the use of specific antagonists for the opioid receptors and S1P receptors, and biochemically, in rats via a direct quantification of the release of endogenous opioids into cerebrospinal fluid following administration of S1P. These findings will be critical in understanding the role of the endogenous lysolipid receptor systems in the CNS and could lead to identification of novel therapeutic targets in the treatment of pain with reduction of opioid-induced side effects, possibly to include the prevention of tolerance and physical dependence. ? ? ?
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