Prolonged cannabinoid exposure alters GABAA receptor mediated synaptic function in cultured hippocampal neurons

Developing cannabinoid-based medication along with marijuana's recreational use makes it important to investigate molecular adaptations the endocannabinoid system undergoes following prolonged use and withdrawal. Repeated cannabinoid administration results in development of tolerance and produces withdrawal symptoms that may include seizures. Here we employed electrophysiological and immunochemical techniques to investigate the effects of prolonged CB1 receptor agonist exposure on cultured hippocampal neurons. Approximately 60% of CB1 receptors colocalize to GABAergic terminals in hippocampal cultures. Prolonged treatment with the cannabinamimetic WIN 55,212-2 (+ WIN, 1 μM, 24 h) caused profound CB1 receptor downregulation accompanied by neuronal hyperexcitability. Furthermore, prolonged + WIN treatment resulted in increased GABA release as indicated by increased mIPSC frequency, a diminished GABAergic inhibition as indicated by reduction in mIPSC amplitude and a reduction in GABAA channel number. Additionally, surface staining for the GABAA β2/3 receptor subunits was decreased, while no changes in staining for the presynaptic vesicular GABA transporter were observed, indicating that GABAergic terminals remained intact. These findings demonstrate that agonist-induced downregulation of the CB1 receptor in hippocampal cultures results in neuronal hyperexcitability that may be attributed, in part, to alterations in both presynaptic GABA release mechanisms and postsynaptic GABAA receptor function demonstrating a novel role for cannabinoid-dependent presynaptic control of neuronal transmission.

Research Highlights
► CB1 receptor predominantly colocalize at GABAergic terminals in cultured neurons.
► Prolonged CB1 agonist exposure downregulates CB1 receptor.
► Profound neuronal hyperexcitability following CB1 agonist withdrawal.
► Decreased GABAA receptor number may underlie CB1 agonist induced hyperexcitability.