Relationship of cannabinoid CB1 receptor and cholecystokinin immunoreactivity in monkey dorsolateral prefrontal cortex.

Exposure to cannabis impairs cognitive functions reliant on the circuitry of the dorsolateral prefrontal cortex (DLPFC) and increases the risk of schizophrenia. The actions of cannabis are mediated via the brain cannabinoid 1 receptor (CB1R), which in rodents is heavily localized to the axon terminals of cortical GABA basket neurons that contain cholecystokinin (CCK). Differences in the laminar distribution of CB1R-immunoreactive (IR) axons have been reported between rodent and monkey neocortex, suggesting that the cell type(s) containing CB1Rs, and the synaptic targets of CB1R-IR axon terminals, may differ across species; however, neither the relationship of CB1Rs to CCK-containing interneurons, nor the postsynaptic targets of CB1R and CCK axon terminals, have been examined in primate DLPFC. Consequently, we compared the distribution patterns of CB1R- and CCK-IR structures, determined the proportions of CB1R and CCK neurons that were dual-labeled, and identified the synaptic types and postsynaptic targets of CB1R- and CCK-IR axon terminals in macaque monkey DLPFC. By light microscopy, CB1R- and CCK-IR axons exhibited a similar laminar distribution, with their greatest densities in layer 4. Dual-label fluorescence experiments demonstrated that 91% of CB1R-IR neurons were immunopositive for CCK, whereas only 51% of CCK-IR neurons were immunopositive for CB1R. By electron microscopy, all synapses formed by CB1R-IR axon terminals were symmetric, whereas CCK-IR axon terminals formed both symmetric (88%) and asymmetric (12%) synapses. The primary postsynaptic target of both CB1R- and CCK-IR axon terminals forming symmetric synapses was dendritic shafts (81-88%), with the remainder targeting cell bodies or dendritic spines. Thus, despite species differences in laminar distribution, CB1Rs are principally localized to CCK basket neuron axons in both rodent neocortex and monkey DLPFC. These axons target the perisomatic region of pyramidal neurons, providing a potential anatomical substrate for the impaired function of the DLPFC associated with cannabis use and schizophrenia.

Synaptic targets of calretinin-containing axon terminals in macaque monkey prefrontal cortex.

The coordinated activity of specific populations of pyramidal cells and GABA-containing, local circuit neurons in the primate prefrontal cortex (PFC) appears to be critical for working memory. Different subclasses of GABA-containing neurons can be distinguished by their content of the calcium-binding proteins parvalbumin (PV) and calretinin (CR). The postsynaptic targets of PV-containing cells have been well characterized in the primate PFC, but the postsynaptic targets of CR-containing neurons in this cortical region remain unknown. In the present study, we used immuno-electron microscopy to examine the synaptic type and postsynaptic targets of CR-immunoreactive (IR) axon terminals in the superficial and deep layers of macaque monkey PFC. Labeled axon terminals formed both symmetric and asymmetric synapses. Within the superficial layers, 93% of the synapses formed by CR-IR were symmetric, whereas in the deep layers the labeled axon terminals forming synapses were more evenly divided between symmetric (57%) and asymmetric (43%). The primary postsynaptic target of these two populations of CR-IR axon terminals also differed; unlabeled dendritic shafts were the predominant target of the symmetric synapses, whereas dendritic spines were the most common target of the asymmetric synapses. In addition, the mean cross-sectional area of the terminals forming asymmetric synapses was significantly larger than that of the terminals forming symmetric synapses. The presence of CR-IR asymmetric synapses suggested that they might arise from neurons that do not utilize GABA; indeed, dual-label fluorescent immunocytochemistry revealed that a subpopulation (23%) of CR-containing neurons in monkey PFC were not GABA-IR. These findings indicate that the synaptology of CR-containing neurons is more heterogeneous than that of PV-containing cells and suggests that the contributions of CR-containing neurons to cognitive processes mediated by the PFC may be more diverse.