Marijuana is one of the most commonly used drugs of abuse in the United States. The active ingredient of marijuana, delta 9THC, acts on specific receptors, termed cannabinoid receptors, which are abundantly expressed in many regions of the brain. Although relatively few studies have been performed to determine their function in these regions a principle role for these receptors appears to be in the presynaptic regulation of neurotransmitter release. Our own preliminary data examining the effects of cannabinoids on neurotransmitter release from rat brain slices suggests that cannabinoid receptor activation produces a marked inhibition of the release of ACh in the hippocampus and also to lesser extent in the nucleus accumbens. The strong inhibitory effects of cannabinoid receptor activation on hippocampal ACh release may account for the well known short-term memory impairment resulting from marijuana use. In addition to the extensively documented behavioral effects of cannabinoid agonists, recent behavioral and electrophysiological observations in rodents have indicated that a newly developed cannabinoid antagonist, SR 141716A, produces effects on its own in the brain. This suggests the presence in the brain of an endogenous tonic cannabinoid tone. In our studies we have found that SR 141716A and the related compound, AM281, on their own produce a potent and large enhancement of ACh release in hippocampal slices. Taken together these observations thus suggest that this new class of compounds may have therapeutic uses distinct from those of cannabinoid agonists. For example, ameliorating the memory deficits seen in Alzheimer's disease. In this proposal we will extend our preliminary findings on the effects of cannabinoid receptor ligands on neurotransmitter release. Our specific goals are two-fold. First, we will use ACh release from hippocampal slices as a CNS assay system to evaluate the efficacy of new cannabinoid compounds developed by A. Makryannis. Second, we will obtain an explanation for the approximately two-fold enhancement of ACh release from the hippocampus produced by low concentrations of SR 141716A and AM281. This suggests either that cannabinoid receptors are constitutively active and produce a tonic inhibition of ACh release or else that these antagonists are antagonizing the effects of an endogenous agonist that is co-released by electrical stimulation of the slices. For the latter case an examination of the effects of new anandamide uptake inhibitors and amidase inhibitors in the slice preparation would help to confirm this compound as the endogenous ligand for cannabinoid receptors. These experiments will thus provide data using a biologically relevant in vitro CNS preparation on the potency and efficacy of newly developed cannabinoid receptor ligands and this in turn may provide information that is of value in understanding how marijuana produces its psychotropic effects in the brain. Furthermore, these experiments will give information on the mode of action of cannabinoid antagonists and could suggest novel therapeutic uses for this new class of compounds.
