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.