Structure of a cannabinoid receptor and functional expression of the cloned cDNA

Neuronal localization of cannabinoid receptors in the basal ganglia of the rat

Cannabinoid receptors have recently been characterized and localized using a high-affinity radiolabeled cannabinoid analog in section binding assays. In rat brain, the highest receptor densities are in the globus pallidus and substantia nigra pars reticulata. Receptors are also dense in the caudate-putamen. In order to determine the neuronal localization of these receptors, selective lesions of key striatal afferent and efferent systems were made. Striatal neurons and efferent projections were selectively destroyed by unilateral infusion of ibotenic acid into the caudate-putamen. The nigrostriatal pathway was selectively destroyed in another set of animals by infusion of 6-hydroxydopamine into the medial forebrain bundle. After 2- or 4-week survivals, slide-mounted brain sections were incubated with ligands selective for cannabinoid ([3H]CP 55,940), dopamine D1 3H]SCH-23390) and D2 ([3H]raclopride) receptors, and dopamine uptake sites ([3H]GBR-12935). Slides were exposed to 3H-sensitive film. The resulting autoradiography showed ibotenate-induced losses of cannabinoid, D1 and D2 receptors in the caudate-putamen and topographic losses of cannabinoid and D1 receptors in the globus pallidus, entopeduncular nucleus, and substantia nigra pars reticulata at both survivals. Four weeks after medial forebrain bundle lesions (which resulted in amphetamine-induced rotations), there was loss of dopamine uptake sites in the striatum and substantia nigra pars compacta but no change in cannabinoid receptor binding. The data show that cannabinoid receptors in the basal ganglia are neuronally located on striatal projection neurons, including their axons and terminals. Cannabinoid receptors may be co-localized with D1 receptors on striatonigral neurons. Cannabinoid receptors are not localized on dopaminergic nigrostriatal cell bodies or terminals.

Cerebellar metabolic activation by delta-9-tetrahydro-cannabinol in human brain: a study with positron emission tomography and 18F-2-fluoro-2-deoxyglucose

We investigated the effects of acute i.v. administration of 2 mg of delta 9-tetrahydrocannabinol (THC) on regional brain glucose metabolism using 18F-2-fluoro-2-deoxyglucose and positron emission tomography (PET) in eight normal subjects. Subjects were tested twice: during baseline conditions and 30-40 min after THC administration. Changes in global cerebral glucose metabolism in response to THC were variable: three subjects showed an increase, three showed a decrease, and two showed no change. In contrast, all subjects showed an increase in normalized metabolism in the cerebellum following THC administration. Cerebellar changes were the only significant regional metabolic changes due to THC administration. The increase in metabolic activity in the cerebellum was correlated with the subjective sense of THC intoxication and with plasma THC concentration. Cerebellar localization of metabolic effects due to THC administration corresponds well with the high density of cannabinoid receptors known to be in this area.

Cannabinoids and the hippocampal glucocorticoid receptor: recent findings and possible significance

It has long been recognized that cannabinoids, including delta 9-tetrahydrocannabinol (THC), the major psychoactive substance of marijuana, bear structural similarities to steroid hormones. The hippocampal region of the brain is particularly rich in glucocorticoid receptors (GCRs), and the region also displays dense autoradiographic binding by synthetic cannabinoids. The present report summarizes studies conducted on cannabinoid interaction with hippocampal GCRs, both in vivo and in vitro. Young rats treated for 8 months with THC displayed anatomic and cellular changes in the hippocampus similar to those seen in older, untreated rats, or in rats treated with high levels of glucocorticoids. Binding of [3H]dexamethasone in cytosol prepared from adrenalectomized rat hippocampus was reduced in the presence of 100-fold molar excess of unlabeled THC. However, further increases of THC concentration, to 20,000-fold excess, could displace no more than 50% of radiolabeled dexamethasone. Scatchard analysis of the binding produced a parallel competition plot for THC, versus the plot for dexamethasone, which may reflect a noncompetitive or allosteric interaction with hippocampal GCR. Cannabidiol, a nonpsychoactive cannabinoid, displayed less competition than THC in all parameters. Treatment of adrenalectomized rats for 14 days with 10 mg/kg THC produced down-regulation of hippocampal GCR binding in a manner also reported following high glucocorticoid administration. Although an initial oral administration of THC to intact rats stimulated release of plasma corticosterone, daily repetition of treatment for 7 and 14 days failed to elicit further corticosterone secretion. Taken together, the results indicate that THC may possess some agonist-like properties of glucocorticoids at the hippocampal GCR site.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction of the fertilizing capacity of sea urchin sperm by cannabinoids derived from marihuana. I. Inhibition of the acrosome reaction induced by egg jelly

delta 9-Tetrahydrocannabinol (THC) and two other major cannabinoids derived from marihuana--cannabidiol (CBD) and cannabinol (CBN)--inhibit fertilization in the sea urchin Strongylocentrotus purpuratus by reducing the fertilizing capacity of sperm (Schuel et al., 1987). Sperm fertility depends on their motility and on their ability to undergo the acrosome reaction upon encountering the egg's jelly coat. Pretreatment of S. purpuratus sperm with THC prevents triggering of the acrosome reaction by solubilized egg jelly in a dose (0.1-100 microM) and time (0-5 min)-dependent manner. Induction of the acrosome reaction is inhibited in 88.9 +/- 2.3% of sperm pretreated with 100 microM THC for 5 min, while motility of THC-treated sperm is not reduced compared to solvent (vehicle) and seawater-treated controls. The acrosome reaction is inhibited 50% by pretreatment with 6.6 microM THC for 5 min and with 100 microM THC after 20.8 sec. CBN and CBD at comparable concentrations inhibit the acrosome reaction by egg jelly in a manner similar to THC. THC does not inhibit the acrosome reaction artificially induced by ionomycin, which promotes Ca2+ influx, and nigericin, which promotes K+ efflux. THC partially inhibits (20-30%) the acrosome reaction induced by A23187, which promotes Ca2+ influx, and NH4OH, which raises the internal pH of the sperm. Addition of monensin, which promotes Na+ influx to egg jelly or to A23187, does not overcome the THC inhibition. Inhibition of the egg jelly-induced acrosome reaction by THC produces a corresponding reduction in the fertilizing capacity of the sperm. The adverse effects of THC on the acrosome reaction and sperm fertility are reversible.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of cannabinoid receptors by photoaffinity labelling

A novel [125I]-labelled photoaffinity ligand designed to detect cannabinoid binding sites has been used in mouse brain preparations and in cultured S49 mouse lymphoma cells. The ligand, 2-iodo-5'-azido-delta 8-THC, shows a high affinity for sites in both brain (Kd = 5.60 pM) and whole cell (Kd = 9.38 pM) systems. Photolabelling studies with brain samples revealed the existence of four ligand-protein adducts, of estimated molecular weights 85.5, 62.1, 30.0 and 25.5 kDa, that were diminished by prior exposure to 8 microM THC. A similar study with S49 cells gave adducts with apparent molecular weights of 62.1, 34.4, 16.9 and 13.5 kDa. The ligand produces a typical cannabinoid cataleptic response in mice suggesting that possibly one or more of the binding sites may be involved in some of the receptor mediated actions of THC.

Heroin addiction: neurobiology, pharmacology, and policy

The neurobiology of drug addiction is being clarified. Research is revealing the anatomic pathways of primary drug reinforcement (reward) in the brain and the molecular architecture of the receptors on which addictive drugs act. All addictive drugs mimic (or occasionally block) the actions of some neurotransmitter; in the case of heroin or methadone an endogenous opioid, probably beta-endorphin. The groundwork is being laid for understanding Dole and Nyswander's "metabolic disease" concept as a concrete neurochemical abnormality of the endogenous opioid system. Thus, a stronger basis is developing for regarding methadone maintenance as a means of replacing a neurohormonal deficiency. Three practical conclusions that can be drawn from this model are: (1) methadone dosage must be adequate (never less than 50-80 mg); (2) it should be more widely accepted that some patients may require lifelong methadone maintenance; and (3) longer-acting, better stabilizing methadone congeners, such as LAAM and its metabolites, should be brought into general use.

Receptors for neurotransmitters and related substances

Advances in techniques for cloning neurotransmitter receptors have revealed new targets for selective drug design. Cell systems for more efficient expression of cloned receptor genes have also been developed. Knowledge of the nature of ligand-binding sites is now becoming available and this should aid in the design of better drugs with fewer side effects.

Current developments in G-protein-coupled receptors

The rate at which receptors have been cloned has recently increased dramatically--existing families have been extended and new families created. The rapid cloning by homology of 'orphan receptors' has also stimulated the development of a new reverse pharmacology.

Effects of pre- and perinatal exposure to hashish extracts on the ontogeny of brain dopaminergic neurons

The changes induced by maternal exposure to cannabinoids in the maturation of nigrostriatal, tuberoinfundibular and mesolimbic dopaminergic activities of rat offspring 15-40 days old were studied. In the striatum, tyrosine hydroxylase activity was constantly decreased during cannabinoid exposure in males. This decrease was correlative to increased number of D1 and D2 dopaminergic receptors. Both effects were also observed after the drug withdrawal caused by weaning on day 24. In females, the most consistent effect appeared on day 20, when decreased dopamine content and number of D1 receptors were observed. Both effects disappeared after drug withdrawal, but the reduction in the number of D1 receptors was again observed 40 days after birth. In the limbic area, cannabinoid exposure caused a decrease in the number of D1 receptors in 15-day-old females, along with decreases in the content of dopamine and its metabolite, L-3,4-dihydroxyphenylacetic acid. Changes in receptors disappeared on subsequent days, but increases in L-3,4-dihydroxyphenylacetic acid content and in its ratio with dopamine (L-3,4-dihydroxyphenylacetic acid/dopamine) were observed on day 20 followed by a decrease in the neurotransmitter content on day 30. In males, tyrosine hydroxylase activity increased on day 30, followed by an increase in L-3,4-dihydroxyphenylacetic acid content and L-3,4-dihydroxyphenylacetic acid/dopamine ratio on day 40. In the hypothalamus, the cannabinoid effects were always manifested after the cessation of drug exposure. Thus, a rise in L-3,4-dihydroxyphenylacetic acid/dopamine ratio was observed in 30-day-old females, and it was followed by a decrease on day 40, accompanied by a decrease in the anterior pituitary content of dopamine. Rise in prolactin release was not significant. In males, tyrosine hydroxylase activity was increased 30 days after birth, while L-3,4-dihydroxyphenylacetic acid content decreased. On day 40, L-3,4-dihydroxyphenylacetic acid content increased, paired to a rise in L-3,4-dihydroxyphenylacetic acid/dopamine ratio and anterior pituitary content of dopamine and to a decrease in the prolactin release. Perinatal exposure to cannabinoids altered the normal development of nigrostriatal, mesolimbic and tuberoinfundibular dopaminergic neurons, as reflected by changes in several indices of their activity. These changes were different regarding the sex and brain areas. Cannabinoid effects were more marked and constant in the striatum of males, while alterations in limbic neurons were mostly transient and those in hypothalamic neurons occurred after drug withdrawal. A long-term impact of these early changes on the neurological processes of adulthood is plausible.

Structure and function of G protein coupled receptors

Application of a molecular genetic techniques has allowed the isolation and identification of more than 50 members of the G protein-coupled receptor family. Their specificities range from sensory receptors such as the opsins and odorant receptors through those for the amines, peptides and other small molecules to those for glycoprotein hormones. These studies make it clear that traditional pharmacological methods, often underestimate receptor diversity. G protein-coupled receptors share a common structure consisting of 7 transmembrane alpha helical segments. Receptor structure-function relationships are discussed in the light of results obtained by site-directed mutagenesis and the construction of chimeric receptors. Studies which have allowed the identification of ligand-binding domains, and of sequences defining G protein specificity as well as those involved in receptor desensitization and downregulation are also discussed.