Localisation of cannabinoid receptors in the rat brain using antibodies to the intracellular C-terminal tail of CB1

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Abstract
The CB1‐type cannabinoid receptor mediates physiologic effects of Δ9‐tetrahydrocannabinol, the psychoactive ingredient of the drug marijuana. In this report, the authors analyse the expression of CB1 in the rat brain by using antibodies to the C‐terminal 13 amino acids of the receptor. Western blot analysis of rat brain membranes revealed a prominent immunoreactive band with a molecular mass (≈53 kDa) consistent with that predicted for CB1 from the rat cDNA sequence. In addition, however, less intense immunoreactive bands corresponding to glycosylated (≈62 kDa) and putative N‐terminally shorter (≈45 kDa) isoforms of CB1 were detected. The distribution of CB1‐immunoreactivity in rat brain was similar to the distribution of binding sites for radiolabelled cannabinoids, with high levels of expression in the olfactory system, the hippocampal formation, the basal ganglia, the cerebellum, and the neocortex. This provides important evidence that CB1 is likely to be largely responsible for mediating effects of cannabinoids in the brain. CB1 immunoreactivity was associated with nerve fibre systems and axon terminals but was not detected in neuronal somata. This is consistent with the presynaptic inhibitory effects of cannabinoids on neurotransmitter release in the brain. Detailed immunocytochemical analysis of anatomically or functionally related regions of the brain revealed the location of CB1 receptors within identified neural circuits. Determination of the cellular and subcellular location of CB1 within known neuronal circuits of the brain provides an anatomic framework for interpretation of the neurophysiologic and behavioural effects of cannabinoids. J. Comp. Neurol. 422:159–171, 2000.