Introduction: Adolescence is an important phase in brain maturation, specifically it is a time during which weak synapses are pruned and neural pathways are strengthened. Adolescence is also a time of experimentation with drugs, including cannabis, which may have detrimental effects on the developing nervous system. The cannabinoid type 1 receptor (CB₁) is an important modulator of neurotransmitter release and plays a central role in neural development. Neurotrophic factors such as brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin receptor kinase B (TrkB), are also critical during development for axon guidance and synapse specification. Objective: The objective of this study was to examine the effects of the phytocannabinoids, Δ⁹-tetrahydrocannabinol (THC) and cannabidiol (CBD), on the expression of BDNF, its receptor TrkB, and other synaptic markers in the adolescent mouse hippocampus. Materials and Methods: Mice of both sexes were injected daily from P28 to P49 with 3 mg/kg THC, CBD, or a combination of THC/CBD. Brains were harvested on P50, and the dorsal and ventral hippocampi were analyzed for levels of BDNF, TrkB, and several synaptic markers using quantitative polymerase chain reaction, western blotting, and image analyses. Results: THC treatment statistically significantly reduced transcript levels of BDNF in adolescent female (BDNF I) and male (BDNF I, II, IV, VI, and IX) hippocampi. These changes were prevented when CBD was co-administered with THC. CBD by itself statistically significantly increased expression of some transcripts (BDNF II, VI, and IX for females, BDNF VI for males). No statistically significant changes were observed in protein expression for BDNF, TrkB, phospho-TrkB, phospho-CREB (cAMP response element-binding protein), and the synaptic markers, vesicular GABA transporter, vesicular glutamate transporter, synaptobrevin, and postsynaptic density protein 95. However, CB₁ receptors were statistically significantly reduced in the ventral hippocampus with THC treatment. Conclusions: This study found changes in BDNF mRNA expression within the hippocampus of adolescent mice exposed to THC and CBD. THC represses transcript expression for some BDNF variants, and this effect is rescued when CBD is co-administered. These effects were seen in both males and females, but sex differences were observed in specific BDNF isoforms. While a statistically significant reduction in CB₁ receptor protein in the ventral dentate gyrus was seen, no other changes in protein levels were observed.

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