The dimethylheptyl derivative of (-)-delta 8-tetrahydrocannabinol reduces the turnover rate of gamma-aminobutyric acid in the septum and nucleus accumbens

Immunohistochemical distribution of cannabinoid CB1 receptors in the rat central nervous system

Immunohistochemical distribution of cannabinoid receptors in the adult rat brain was studied using specific purified antibodies against the amino-terminus of the CB1 receptor. Our results generally agree well with the previous studies using CB1 receptor autoradiography and messenger RNA in situ hybridization. However, because of its greater resolution, immunohistochemistry allowed identification of particular neuronal cells and fibers that possess cannabinoid receptors. CB1-like immunoreactivity was found in axons, cell bodies and dendrites, where it appeared as puncta in somata and processes. Both intensely and moderately or lightly stained neurons were observed. The intensely stained neurons were dispersed and only occur in cortical structures including hippocampal formation and olfactory bulb. Moderately or lightly stained neurons were found in caudate-putamen and amygdala. In the hippocampal formation only intensely stained neurons were observed. The cell bodies of pyramidal neurons in CA1 and CA3 fields appeared to be unstained but surrounded by a dense plexus of immunoreactive fibers. The granule cells in the dentate area were also immunonegative. Many intensely stained neurons were located at the base of the granule cell layer. CB1-like immunoreactive neurons and fibers were also found in the somatosensory, cingulate, perirhinal, entorhinal and piriform cortices, in claustrum, amygdaloid nuclei, nucleus accumbens and septum. Beaded immunoreactive fibers were detected in periaqueductal gray, nucleus tractus solitarius, spinal trigeminal tract and nucleus, dorsal horn and lamina X of the spinal cord. A triangular cap-like mass of immunoreactivity was found to surround the basal part of the Purkinje cell body in the cerebellum. Only small, lightly stained cells were found in the molecular layer in the cerebellum close to the Purkinje cell layer. The CB1 receptor is widely distributed in the forebrain and has a more restricted distribution in the hindbrain and the spinal cord. It appears to be expressed on cell bodies, dendrites and axons. According to the location and morphology, many, but not all, CB1-like immunoreactive neurons appear to be GABAergic. Therefore, cannabinoids and cannabinoid receptors may play a role in modulating GABAergic neurons.

The activation of cannabinoid receptors in striatonigral GABAergic neurons inhibited GABA uptake

Cannabinoid receptors (CNRs) in basal ganglia are located on striatal efferent neurons which are gamma-aminobutiric acid (GABA)-containing neurons. Recently, we have demonstrated that CN-induced motor inhibition is reversed by GABA-B, but not GABA-A, receptor antagonists, presumably indicating that the activation of CNRs in striatal outflow nuclei, mainly in the substantia nigra, should be followed by an increase of GABA concentrations into the synaptic cleft of GABA-B receptor synapses. The present study was designed to examine whether this was originated by increasing GABA synthesis and/or release or by decreasing GABA uptake. We analyzed: (i) GABA synthesis, by measuring the activity of glutamic acid decarboxylase (GAD) and GABA contents in brain regions that contain striatonigral GABAergic neurons, after in vivo administration of CNs and/or the CNR antagonist SR141716; (ii) [3H]GABA release in vitro in the presence or the absence of a synthetic CN agonist, HU-210, by using perifusion of small fragments of substantia nigra; and (iii) [3H]GABA uptake in vitro in the presence or the absence of WIN-55,212-2, by using synaptosomes obtained from either globus pallidus or substantia nigra. Results were as follows. Delta9-tetrahydrocannabinol (delta9-THC) and HU-210, did not alter neither GAD activity nor GABA contents in both the striatum and the ventral midbrain at any of the two times tested, thus suggesting that CNs apparently failed to change GABA synthesis in striatonigral GABAergic neurons. A similar lack of effect of HU-210 on in vitro [3H]GABA release, both basal and K+-evoked, was seen when this CN was added to perifused substantia nigra fragments, also suggesting no changes at the level of GABA release. However, when synaptosome preparations obtained from the substantia nigra were incubated in the presence of WIN-55,212-2, a decrease in [3H]GABA uptake could be measured. This lowering effect was specific of striatonigral GABAergic neurons since it was not observed in synaptosome preparations obtained from the globus pallidus. In summary, the activation of CNRs located on striatonigral GABAergic neurons, which primarily access to GABA-B receptor synapses, was accompanied by a reduction in neurotransmitter uptake, thus prolonging the presence of GABA into the synaptic cleft. This mechanism might underly the CN-induced motor inhibition through the potentiation of the inhibitory effect of GABA on neuronal activity, in particular of nigrostriatal dopaminergic neurons.

Role of the subthalamic nucleus in cannabinoid actions in the substantia nigra of the rat

The effect of cannabinoids on the excitatory input to the substantia nigra reticulata (SNr) from the subthalamic nucleus was explored. For this purpose a knife cut was performed rostral to the subthalamic nucleus to isolate the subthalamic nucleus and the SNr from the striatum, a major source of cannabinoid receptors to the SNr. The data showed that the cannabinoid agonist WIN55,212-2 blocked the increase in the firing rate of SNr neurons induced by stimulation of the subthalamic nucleus with bicuculline. Furthermore, the cannabinoid antagonist SR141716A antagonized the effect of the cannabinoid agonist. This study showed that cannabinoids regulate not only the striatonigral pathway, as previously reported, but also the subthalamonigral pathway. The opposite influences of these two inputs to the SNr, inhibitory and excitatory respectively, suggest that endogenous cannabinoids play a major role in the physiological regulation of the SNr.

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.

An EEG investigation on nabilone, a synthetic cannabinoid, in rabbits

In this paper, the electroencephalographic (EEG) and behavioural effects of nabilone, a derivative compound from delta 9-tetrahydrocannabinol (delta 9-THC), are studied in comparison with those of delta 8-tetrahydrocannabinol (delta 8-THC) in rabbits. The results show that nabilone requires lower doses than delta 8-THC to elicit cortical spike-and-wave complexes, but higher doses to disrupt hippocampal theta rhythm. These data are discussed in connection with the clinical actions of nabilone in man.