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 w ith the previous studies using CB1 receptor autoradiography and messen ger RNA in situ hybridization. However, because of its greater resolut ion, immunohistochemistry allowed identification of particular neurona l cells and fibers that possess cannabinoid receptors. CB1-like immuno reactivity was found in axons, cell bodies and dendrites, where it app eared as puncta in somata and processes. Both intensely and moderately or lightly stained neurons were observed. The intensely stained neuro ns were dispersed and only occur in cortical structures including hipp ocampal formation and olfactory bulb. Moderately or lightly stained ne urons were found in caudate-putamen and amygdala. In the hippocampal f ormation only intensely stained neurons were observed. The cell bodies of pyramidal neurons in CA1 and CA3 fields appeared to be unstained b ut surrounded by a dense plexus of immunoreactive fibers. The granule cells in the dentate area were also immunonegative. Many intensely sta ined neurons were located at the base of the granule cell layer. CB1-l ike immunoreactive neurons and fibers were also found in the somatosen sory, cingulate, perirhinal, entorhinal and piriform cortices, in clau strum, amygdaloid nuclei, nucleus accumbens and septum. Beaded immunor eactive fibers were detected in periaqueductal gray, nucleus tractus s olitarius, 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 molecu lar layer in the cerebellum close to the Purkinje cell layer. The CB1 receptor is widely distributed in the forebrain and has a more restric ted distribution in the hindbrain and the spinal cord. It appears to b e expressed on cell bodies, dendrites and axons. According to the loca tion and morphology, many, but not all, CB1-like immunoreactive neuron s appear to be GABAergic. Therefore, cannabinoids and cannabinoid rece ptors may play a role in modulating GABAergic neurons. (C) 1997 IBRO. Published by Elsevier Science Ltd.