Cannabinoids Shape Synaptic Activity and Adult Neurogenesis in the Zebrafish Pallium

The endocannabinoid system regulates neuronal activity and plasticity, but its role in non-mammalian vertebrates remains poorly understood. In zebrafish (Danio rerio), the pallium processes cognitive functions such as memory, learning, and emotional behavior. This region expresses cannabinoid receptors and undergoes continuous neuronal remodeling through adult neurogenesis. Here, we investigate whether cannabinoid receptor type 1 (CB1R) modulates synaptic activity and adult neurogenesis in zebrafish pallial circuits. Using immunofluorescence and single-cell mRNA analysis, we mapped CB1R expression in the pallium and found it to be distributed in a scattered pattern within the dorsomedial (Dm) and dorsolateral (Dl) regions, predominantly in glutamatergic neurons. Electrophysiological recordings showed that acute application of rimonabant, a CB1R antagonist, reduced the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) without altering intrinsic or other synaptic properties, suggesting a tonic role for CB1R in modulating synaptic transmission. Additionally, prolonged rimonabant treatment (13 days) significantly reduced ERK phosphorylation, a marker of neuronal activity, further supporting the involvement of CB1R in maintaining basal synaptic activity in the pallium. To assess whether cannabinoid signaling shapes adult neurogenesis, we analyzed the proliferation of neural stem cells (NSCs) and maturation of adult-born neurons. Acute phytocannabinoid exposure resulted in a reduction in NSC proliferation, specifically in the anterior Dm. To assess the neurogenic outcome, the cannabinoid treatment was administered during neuronal maturation (12-24 days after BrdU labeling). We observed an increase in the number of 25-day-old neurons (BrdU⁺, HuC/D⁺) in both Dm and Dl regions. This effect was reverted by the CB1R antagonist rimonabant. These results indicate that cannabinoid signaling modulates synaptic activity and neuronal integration, highlighting a conserved control of neurogenesis by the endocannabinoid system across vertebrates.