In the age ranges and samples in which we found that ERO was significant for the onset of alcohol use or alcohol dependence, it was the lower values which characterize the risk factor, which is consistent with the results in adolescents and young adults in the studies by Rangaswamy et al. , Kamarajan et al. , and Gilmore et al. . In those investigations high risk groups had lower ERO values than the low risk groups. That no effects of regular alcohol use on ERO values were found is consistent with similar results obtained by Perlman et al. .It is important to note that the objectives of the twin studies considered here and of this study are quite different. The twin studies investigate the presence of a ‘‘disease’’ condition, although exactly which condition varies considerably among studies. The objective of this study, as a survival analysis, is to analyze the factors contributing to an event, the onset of a condition. Once the condition has come to pass, it is not of further interest in survival analysis. The genetic effects which produce the condition are only significant at the onset of the condition, and their effects persist only if the subsequent onset of the condition in other subjects is attributable to them. In the twin studies post-onset presence of the condition is part of the outcome analyzed. That is, in the longitudinal studies using multistage models, the affected subjects are retained throughout the study subsequent to their becoming affected, while in the survival analysis method used in this study, the affected subjects are removed from consideration in the study once they have become affected, and no longer influence the results. Therefore, although the use of a longitudinal multi-stage model in van Beek et al. and Baker et al. enables genetic influences to have age-specific characteristics,vertical growing garden these effects are modeled as persisting through time as a result of an effect at a single age range. If early onset alcohol use is associated with the more genetically determined form of alcoholism then it would be expected that genetic factors leading to early drinking and dependence would be manifest. Our results are consistent with this hypothesis.
The pattern of genetic results obtained here, albeit from a single gene, is weighted towards the strongest effects manifesting themselves in the youngest age range. However, most twin studies find low genetic influences at younger ages and increases in genetic influences with age , although not all twin studies have this conclusion . These results can be understood after examination of the populations from which the twin samples are drawn and the outcomes which are modeled. The samples in the twin studies are drawn from the general population, not from the densely affected families which form the bulk of the sample used here. Thus genetic effects will be more difficult to find in the twin studies, particularly for the rarer, more genetically affected conditions. In a number of studies outcome definitions are broad, and are not subject to as strong genetic effect as more restricted outcomes such as alcohol dependence or externalizing disorders.The discovery of the cannabinoid receptors and endocannabinoid ligands has generated a great deal of interest in identifying opportunities for the development of novel cannabinergic therapeutic drugs. Such an effort was first undertaken three decades ago by a number of pharmaceutical industries, but was rewarded with only modest success. However, the newly acquired knowledge on the physiological roles of the endocannabinoid system has significantly enhanced these prospects. At the June 27, 2004 workshop ‘‘Future Directions in Cannabinoid Therapeutics II: From the Bench to the Clinic’’, sponsored by the University of California Center for Medicinal Cannabis Research, we on the Scientific Planning Committee were asked to identify the areas of research with the most immediate promise for the development of novel therapeutic agents.
The Committee identified four broad areas involving modulation of the endocannabinoid system as particularly promising in this regard: agonists for central CB1 cannabinoid receptors and peripheral CB2 receptors, antagonists of CB1 receptors, inhibitors of endocannabinoid deactivation, and endocannabinoid-like compounds that act through mechanisms distinct from CB1 and CB2 receptors activation. Below, we summarize the data presented at the Workshop and the consensus of its participants on the most exciting opportunities for drug discovery.Two endogenous agonists of cannabinoid receptors have been well characterized and are now widely used in research: anandamide , and 2-arachidonoylglycerol . Both molecules derive chemically from the polyunsaturated fatty acid, arachidonic acid, which is used in nature as the starting material for other important signaling compounds, such as the eicosanoids. Additional endocannabinoid-related compounds present in the body include virodhamine, which may act as an endogenous antagonist of CB1 receptors, and arachidonoylserine, which may engage an as-yet-uncharacterized cannabinoid-like receptor expressed in the vasculature. As is well-known, the Cannabis plant contains more than 60 cannabinoids, which include -D9 -tetrahydrocannabinol , cannabigerol, cannabidiol, cannabinol, cannabichromene and cannabicyclol. Attention has been mostly focused on D9 -THC, because of its multiple biological properties. Nevertheless, less studied compounds such as cannabidiol may also be important, although we do not yet know at which receptors they may act to achieve their effects. In addition to these plant-derived cannabinoids, an extensive set of synthetic cannabinergic agonists has been developed over the last 30 years. Products of these efforts include CP-55940 , created by opening one of the rings of the tricyclic D9 -THC structure and introducing other small changes in its structure; HU-210 , a very potent cannabinoid agonist resembling some D9 -THC metabolites; and WIN55212-2 , which belongs to an altogether different class of chemicals, the aminoalkylindoles.
Additionally, the metabolically stable synthetic analog of anandamideR-methanandamide is routinely used as a pharmacological probe to circumvent the short half life of the natural substance. Two important new additions to this armamentarium under discussion at the workshop include a peripherally acting cannabinoid agonist in preclinical development by Novartis for the treatment of neuropathic and inflammatory pain , andBAY-387271 , a centrally acting cannabinoid agonist in Phase II clinical studies for the treatment of stroke. The interest of the pharmaceutical industry in the application of cannabinoid agonists to the treatment of pain conditions is not recent. Indeed, most of the compounds now in experimental use derive from such an interest. Historically however cannabinoid agonist development has not proved clinically fruitful, largely because of the profound psychotropic side effects of centrally active cannabinoid agonists, hence the attention given to peripherally acting cannabinoids, which exhibit significant analgesic efficacy and low central activity in animal models. Neuroprotection is a relatively new area for cannabinoid agonists, but one that appears to be already well advanced. Preclinical studies have made a convincing case for the efficacy of cannabinoid agents not only in experimental brain ischemia, but also in models of Parkinson’s disease and other forms of degenerative brain disorders. The results of a Phase II clinical trial with BAY-387271 are awaited with great excitement. Also highlighted during the conference were various derivatives of cannabidiol. Particularly interesting in this regard was the compound -7-hydroxy-4#- dimethylheptyl-cannabidiol a hydroxylated, dimethylheptylated cannabidiol,vertical growing greenhouse structurally related to HU-210. Like D9 -THC, 7-OH-DMH-CBD is a potent inhibitor of electrically evoked contractions in the mouse vas deferens. However, 7-OH-DMH-CBD does not significantly bind to either CB1 or CB2 receptors and its inhibitory effects on muscle contractility are not blocked by CB1 or CB2 receptor antagonists, suggesting that the compound may target an as-yet-uncharacterized cannabinoid-like receptor. This hypothesis is reinforced by pharmacological experiments, which suggest that 7-OH-DMH-CBD displays anti-inflammatory and intestinal-relaxing properties, but does not exert overt psychoactive effects in mice. However, the nature of this hypothetical receptor and its relationship to other cannabinoid-like sites in the vasculature and in the brain hippocampus remains to be determined.Another way to reduce central side effects is to target peripheral CB2 receptors, which are expressed in the brain only during inflammatory states and even then are limited to microglia. Selective CB2 receptor agonists include the compounds AM1241 and HU-308 . Compounds of this type offer a great deal of promise in the treatment of pain and inflammation. Studies conducted on multiple animal models of pain have shown that the CB2-selective agonist AM1241 has robust analgesic effects and is very potent in neuropathic pain models.
These effects are maintained in CB1-deficient, but not in CB2-deficient mice.Cannabinoid analogs are currently in development or being tested by Astra, Bayer, Endo, GSK, GW Pharmaceuticals, Indevus, Kadmus Pharmaceuticals, MAK Scientific, and Pharmos. Some of the obvious issues in drug development are efficacy, side effects, and regulatory scheduling. With regard to efficacy, the primary clinical targets are pain, stroke, nausea, appetite, and cough. Potential side effects in addition to the well-known central psychotropic actions include cardiovascular and immune perturbations, as well as fertility problems. Finally, a major question from the perspective of drug industry is one of regulatory scheduling: that is, whether any drug acting either directly or indirectly through the endocannabinoid system will be subject to the restrictive scheduling of D9 -THC, or whether scheduling will be determined by side effects, as is the case with most drugs. A large number of pharmaceutical companies have started active CB1 antagonist programs, mostly as a result of the clinical success of SR141716A , the first CB1 antagonist to be developed. This molecule has successfully completed Phase III studies and is anticipated to become available within a year for the treatment of obesity and tobacco addiction. Rimonabant is an inverse CB1 agonist with a Ki of 11 nM at the CB1 receptors and 1640 nM at CB2. Additional agents currently in development include SLV-326 and LY320135 . However, all of these compounds are inverse agonists. A series of neutral antagonists has been generated, but remains not as well characterized in the literature. Examples of this class are the compounds O-2654 and AM5171 . As noted above, therapeutic areas for cannabinoid antagonists include obesity, drug addiction and perhaps CNS disorders.The mechanism by which cannabinoid antagonists exert their anti-obesity effects is still not fully understood. Data on rimonabant presented at the workshop identified two possibilities. First, there is a loss of appetite. Mutant mice that are deficient in CB1 receptors eat less than wild-type controls. Second, there is an increase in metabolic rate and a loss of fat mass. These effects may be linked, on the one hand, to the ability of rimonabant to affect corticotropin-releasing hormone , as suggested by the fact that CB1 receptors colocalize with CRH receptors in the hypothalamus. This may be significant for explaining the drug’s effects on appetite drive, as it is known that CRH is anorexigenic. On the other hand, mice that lack CB1 receptors display a hyperactivity of the hypothalamicpituitary-adrenal axis, with increases in both ACTH and corticosterone. This phenotype may be important in regard to overall metabolic rate. Another possible mediator of the long-lasting effect on body weight reduction unrelated to altered food intake is the adipocyte, because CB1 receptor activation causes lipogenesis, which is blocked by rimonabant.CB1 cannabinoid receptors are present on the cell surface of neurons within the brain reward circuitry. Furthermore, endocannabinoids may be released from dopamine neurons in the ventral tegmental area , and from medium spiny neurons in the nucleus accumbens of the brain reward circuit. Additionally, endocannabinoids and D9 -THC activate CB1 receptors and by doing so regulate reward strength and drug craving. Though we do not know how this occurs, it is likely that these mechanisms extend to all drugs of abuse, because collectively these drugs show the propensity to increase VTA dopamine neuron activity, which might be coupled to augmented endocannabinoid production from the dopamine neurons themselves. Finally, cannabinoid receptor antagonists block the effects of endocannabinoids in these reward circuits. Preclinical work shows that priming injections of cannabinoid agonists reinstate heroin-seeking behavior after a prolonged period of abstinence in rats trained to self-administer heroin. The cannabinoid antagonist rimonabant fully prevents heroin-induced reinstatement of heroin-seeking behavior. Additionally, rimonabant significantly attenuates cannabinoid-induced reinstatement of heroinseeking behavior. All these findings clearly support the hypothesis of a functional interaction between opioid and cannabinoid systems in the neurobiological mechanisms of relapse and might suggest a potential clinical use of cannabinoid antagonists for preventing relapse to heroin abuse.