Introduction: There are few vocal learning animals that are suitable for laboratory study, and so songbirds have unique utility for evaluating drug effects on behavior learned during a critical period of development. We previously found that purified botanically-derived cannabidiol (CBD, ≥98%) mitigates effects of partial ablation of zebra finch HVC, a pre-vocal motor cortical region. Here we expand prior work to determine ability of the euphorigenic cannabis constituent, Δ 9 -tetrahydrocannabinol (THC) to modulate CBD efficacy. Evidence suggests relative abundance of phytocannabinoids within cannabis extracts is an important determinant of activity, with CBD:THC of particular significance. As CBD-enriched extracts have become increasingly available both by prescription and over the counter, differential efficacy associated with distinct phytocannabinoid combinations and relative CBD:THC amounts is of increasing concern. Methods and Results: To evaluate THC modulation of CBD efficacy in mitigating the effects of partial ablation of zebra finch HVC, we have tested 3 mg/kg of purified botanically derived CBD (≥98%) containing 0.02, 0.08, 1, 3 and 5% THC. Results demonstrate differential efficacy on phonology and syntax, consistent with complex, hormetic dose-responses. On phonology, CBD with the lowest THC content (3% CBD + 0.02% THC) improved recovery while that with the highest THC content (3% CBD+5% THC) slowed it. In terms of syntax, all THC concentrations improved recovery time with the higher 3 mg/kg+3% THC being distinctly effective in returning behavior to pre-injury levels, and the highest 3 mg/kg CBD+5% THC for reducing the acute magnitude of syntax disruption. Differential phonology and syntax effects likely involve distinct neural circuits that control vocal learning and production. Understanding these systems-level effects will inform mechanisms underlying both phytocannabinoid action, and learning-dependent vocal recovery. Conclusions: Overall, we have found that efficacy of purified botanically derived CBD (≥98%) to influence vocal recovery varies with THC content in complex ways. This adds to evidence of differential efficacy with phytocannabinoid combinations and ratios thereof and underscores the importance of careful control over cannabis preparations used therapeutically.

The non-euphorigenic phytocannabinoid cannabidiol (CBD) has been used successfully to treat childhood-onset epilepsies. These conditions are associated with developmental delays that often include vocal learning. Zebra finch song, like language, is a complex behavior learned during a sensitive period of development. Song quality is maintained through continuous sensorimotor refinement involving circuits that control learning and production. Within the vocal motor circuit, HVC is a cortical-like region that when partially lesioned temporarily disrupts song structure. We previously found CBD (10 mg/kg/day) improves post-lesion vocal recovery. The present studies were done to begin to understand mechanisms possibly responsible for CBD vocal protection. We found CBD markedly reduced expression of inflammatory mediators and oxidative stress markers. These effects were associated with regionally-reduced expression of the microglial marker TMEM119. As microglia are key regulators of synaptic reorganization, we measured synapse densities, finding significant lesion-induced circuit-wide decreases that were largely reversed by CBD. Synaptic protection was accompanied by NRF2 activation and BDNF/ARC/ARG3.1/MSK1 expression implicating mechanisms important to song circuit node mitigation of oxidative stress and promotion of synaptic homeostasis. Our findings demonstrate that CBD promotes an array of neuroprotective processes consistent with modulation of multiple cell signaling systems, and suggest these mechanisms are important to post-lesion recovery of a complex learned behavior.

The non-euphorigenic phytocannabinoid cannabidiol (CBD) has been used successfully to treat childhood-onset epilepsies. These conditions are associated with developmental delays that often include vocal learning. Zebra finch song, like language, is a complex behavior learned during a sensitive period of development. Song quality is maintained through continuous sensorimotor refinement involving circuits that control learning and production. Within the vocal motor circuit, HVC is a cortical-like region that when partially lesioned temporarily disrupts song structure. We previously found CBD (10 mg/kg/day) improves post-lesion vocal recovery. The present studies were done to understand mechanisms underlying CBD vocal protection. We found CBD-improved vocal recovery is accompanied by reduced expression of inflammatory mediators and oxidative stress markers. These effects were associated with regionally-reduced expression of the microglia marker TMEM119. As microglia are key regulators of synaptic reorganization, we measured synapse densities, finding significant lesion-induced circuit-wide decreases that were largely reversed by CBD. Synaptic protection was accompanied by NRF2 activation and BDNF/ARC/ARG3.1/MSK1 expression implicating mechanisms important to song circuit node mitigation of oxidative stress and promotion of synaptic homeostasis. Our findings indicate CBD improves post-lesion recovery of a complex learned behavior through an array of neuroprotective processes consistent with modulation of multiple cell signaling systems.