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The use and abuse of cannabis can be associated with significant pathophysiology, however, it remains unclear whether (1) acute administration of Δ-9-tetrahydrocannabinol (THC) during early adulthood alters the cannabinoid type 1 (CB1 ) receptor localization and expression in cells of the brain, and (2) THC produces structural brain changes. Here we use electron microscopy and a highly sensitive pre-embedding immunogold method to examine CB1 receptors in the hippocampus cornu ammonis subfield 1 (CA1) 30 min after male mice were exposed to a single THC injection (5 mg/kg). The findings show that acute exposure to THC can significantly decrease the percentage of CB1 receptor immunopositive terminals making symmetric synapses, mitochondria, and astrocytes. The percentage of CB1 receptor-labeled terminals forming asymmetric synapses was unaffected. Lastly, CB1 receptor expression was significantly lower at terminals of symmetric and asymmetric synapses as well as in mitochondria. Structurally, CA1 dendrites were significantly larger, and contained more spines and mitochondria following acute THC administration. The area of the dendritic spines, synaptic terminals, mitochondria, and astrocytes decreased significantly following acute THC exposure. Altogether, these results indicate that even a single THC exposure can have a significant impact on CB1 receptor expression, and can alter CA1 ultrastructure, within 30 min of drug exposure. These changes may contribute to the behavioral alterations experienced by young individuals shortly after cannabis intoxication.

The cannabinoid CB1 receptor localizes to the glutamatergic parallel fiber (PF) terminals of the cerebellar granule cells and participates in synaptic plasticity, motor control and learning that are impaired in CB1 receptor knockout (CB 1 -KO) mice. However, whether ultrastructural changes at the PF-Purkinje cell (PC) synapses occur in CB 1 -KO remains unknown. We studied this in the vermis of the spinocerebellar lobule V and the vestibulocerebellar lobule X of CB 1 -KO and wild-type (CB 1 -WT) mice by electron microscopy. Lobule V, but not lobule X, of CB 1 -KO had significantly less and longer synapses than in CB 1 -WT. PF terminals were significantly larger in both lobules of CB 1 -KO with no changes in PC dendritic spines. The PF terminals in lobule V of CB 1 -KO contained less synaptic vesicles and lower vesicle density; by contrast, vesicle density in lobule X of CB 1 -KO remained unchangeable relative to CB 1 -WT. There were as many vesicles in lobule V of CB 1 -KO as in CB 1 -WT, but their distribution decreased drastically at 300 nm of the active zone. In lobule X of CB 1 -KO, less vesicles were found within 150 nm from the presynaptic membrane; however, no vesicles were at 450-600 nm of the active zone. A significant higher amount of synaptic vesicles close to the active zone in lobule V and X of CB 1 -KO was observed. In conclusion, the absence of CB1 receptors strikingly and distinctively impacts on the ultrastructural architecture of the PF-PC synapses located in cerebellar lobules that differ in vulnerability to damage and motor functions.

Type 1 cannabinoid (CB1 ) receptors are widely distributed in the brain. Their physiological roles depend on their distribution pattern, which differs remarkably among cell types. Hence, subcellular compartments with little but functionally relevant CB1 receptors can be overlooked, fostering an incomplete mapping. To overcome this, knockin mice with cell-type-specific rescue of CB1 receptors have emerged as excellent tools for investigating CB1 receptors' cell-type-specific localization and sufficient functional role with no bias. However, to know whether these rescue mice maintain endogenous CB1 receptor expression level, detailed anatomical studies are necessary. The subcellular distribution of hippocampal CB1 receptors of rescue mice that express the gene exclusively in dorsal telencephalic glutamatergic neurons (Glu-CB1 -RS) or GABAergic neurons (GABA-CB1 -RS) was studied by immunoelectron microscopy. Results were compared with conditional CB1 receptor knockout lines. As expected, CB1 immunoparticles appeared at presynaptic plasmalemma, making asymmetric and symmetric synapses. In the hippocampal CA1 stratum radiatum, the values of the CB1 receptor-immunopositive excitatory and inhibitory synapses were Glu-CB1 -RS, 21.89% (glutamatergic terminals); 2.38% (GABAergic terminals); GABA-CB1 -RS, 1.92% (glutamatergic terminals); 77.92% (GABAergic terminals). The proportion of CB1 receptor-immunopositive excitatory and inhibitory synapses in the inner one-third of the dentate molecular layer was Glu-CB1 -RS, 53.19% (glutamatergic terminals); 2.30% (GABAergic terminals); GABA-CB1 -RS, 3.19% (glutamatergic terminals); 85.07% (GABAergic terminals). Taken together, Glu-CB1 -RS and GABA-CB1 -RS mice show the usual CB1 receptor distribution and expression in hippocampal cell types with specific rescue of the receptor, thus being ideal for in-depth anatomical and functional investigations of the endocannabinoid system. J. Comp. Neurol. 525:302-318, 2017. © 2016 Wiley Periodicals, Inc.