Immunocytochemical distribution of the cannabinoid CB1 receptor in the primate neocortex: a regional and laminar analysis

Delta-9-tetrahydrocannabinol (Delta9-THC) has profound effects on higher cognitive functions, and exposure to Delta9-THC has been associated with the appearance or exacerbation of the clinical features of schizophrenia. These actions appear to be mediated via the CB1 receptor, the principal cannabinoid receptor expressed in the brain. However, the distribution of the CB1 receptor in neocortical regions of the primate brain that mediate cognitive functions is not known. We therefore investigated the immunocytochemical localization of the CB1 receptor in the brains of macaque monkeys and humans using antibodies that specifically recognize the N- or C-terminus of the CB1 receptor. In monkeys, intense CB1 immunoreactivity was observed primarily in axons and boutons. Across neocortical regions of the monkey brain, CB1-immunoreactive (IR) axons exhibited considerable heterogeneity in density and laminar distribution. Neocortical association regions, such as the prefrontal and cingulate cortices, demonstrated a higher density, and exhibited a unique laminar pattern of CB1-IR axons, compared with primary sensory and motor cortices. Similar regional and laminar distributions of CB1-IR axons were also present in the human neocortex. CB1-IR axons had more prominent varicosities in human tissue, but this difference appeared to represent a postmortem effect as similar morphological features increased in unperfused monkey tissue as a function of postmortem interval. In electron microscopy studies of perfused monkey prefrontal cortex, CB1 immunoreactivity was predominantly found in axon terminals that exclusively formed symmetric synapses. The high density, distinctive laminar distribution, and localization to inhibitory terminals of CB1 receptors in primate higher-order association regions suggests that the CB1 receptor may play a critical role in the circuitry that subserves cognitive functions such as those that are disturbed in schizophrenia.