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Two well-characterized cannabinoid receptors (CBrs), CB1 and CB2, mediate the effects of cannabinoids and marijuana use, with functional evidence for other CBrs. CB1 receptors are expressed primarily in brain and peripheral tissues. For over a decade several laboratories were unable to detect CB2 receptors in brain and were known to be intensely expressed in peripheral and immune tissues and have traditionally been referred to as peripheral CB2 CBrs. We have reported the discovery and functional presence of CB2 cannabinoid receptors in mammalian brain that may be involved in depression and drug abuse and this was supported by reports of identification of neuronal CB2 receptors that are involved in emesis. We used RT-PCR, immunoblotting, hippocampal cultures, immunohistochemistry, transmission electron microscopy, and stereotaxic techniques with behavioral assays to determine the functional expression of CB2 CBrs in rat brain and mice brain exposed to chronic mild stress (CMS) or those treated with abused drugs. RT-PCR analyses supported the expression of brain CB2 receptor transcripts at levels much lower than those of CB1 receptors. In situ hybridization revealed CB2 mRNA in cerebellar neurons of wild-type but not of CB2 knockout mice. Abundant CB2 receptor immunoreactivity (iCB2) in neuronal and glial processes was detected in brain and CB2 expression was detected in neuron-specific enolase (NSE) positive hippocampal cell cultures. The effect of direct CB2 antisense oligonucleotide injection into the brain and treatment with JWH015 in motor function and plus-maze tests also demonstrated the functional presence of CB2 cannabinoid receptors in the central nervous system (CNS). Thus, contrary to the prevailing view that CB2 CBrs are restricted to peripheral tissues and predominantly in immune cells, we demonstrated that CB2 CBrs and their gene transcripts are widely distributed in the brain. This multifocal expression of CB2 immunoreactivity in brain suggests that CB2 receptors may play broader roles in the brain than previously anticipated and may be exploited as new targets in the treatment of depression and substance abuse.

Cocaine and methylphenidate block uptake by neuronal plasma membrane transporters for dopamine, serotonin, and norepinephrine. Cocaine also blocks voltagegated sodium channels, a property not shared by methylphenidate. Several lines of evidence have suggested that cocaine blockade of the dopamine transporter (DAT), perhaps with additional contributions from serotonin transporter (5-HTT) recognition, was key to its rewarding actions. We now report that knockout mice without DAT and mice without 5-HTT establish cocaine-conditioned place preferences. Each strain displays cocaine-conditioned place preference in this major mouse model for assessing drug reward, while methylphenidate-conditioned place preference is also maintained in DAT knockout mice. These results have substantial implications for understanding cocaine actions and for strategies to produce anticocaine medications.Cocaine use is a principal drug abuse problem in the United States and other countries, contributing to substantial morbidity and mortality among the millions of individuals who use it each year (1). No current medication provides effective treatment for cocaine dependence (2). These facts give particular importance to defining the sites for cocaine reward in the brain so that they can be more accurately targeted by potential therapeutic agents.Several lines of evidence have provided support for a role of the dopamine transporter (DAT) as a primary site for cocaine reward. Structure-activity studies document good correlations between psychostimulant properties in tests of reward and their abilities to block DAT; poorer correlations are noted with their potencies in blocking other transporters (3, 4). Dopaminergic lesions blunt cocaine influences in model systems that test reward (5-7). Psychostimulants enhance dopamine release from dopaminergic circuits (8). Transgenic mice that overexpress DAT display enhanced cocaine-conditioned place preference (G.R.U., et al., unpublished observations). Finally, ''indifference'' to cocaine has been inferred from the reduced cocaine-stimulated locomotion recently described in mice that lack DAT (9, 10).There are also limitations to postulated direct relationships between DAT blockade and psychostimulant-induced reward. Among these are the failure of several compounds that potently inhibit dopamine uptake, including mazindol, to display substantial abuse liability in humans or animal model studies (11-13). Because mazindol potently inhibits dopamine and norepinephrine transport, but only weakly inhibits serotonin transport, this difference from cocaine could conceivably contribute to a distinct profile on tests of reward (14-16). These and other more indirect lines of evidence support the idea that cocaine's inhibition of serotonin uptake could also provide an alternative and plausible molecular site for contributions to cocaine reward (17)(18)(19).To test the dopamine-or serotonin-transporter dependence of cocaine reward, we have constructed DAT knockout mice and assessed cocaine-conditioned plac...