Endocannabinoids produced upon action potential firing evoke a Cl⁻ current via type-2 cannabinoid receptors in the medial prefrontal cortex

The functional presence of type-2 cannabinoid receptors (CB₂Rs) in layer II/III pyramidal neurons of the rat medial prefrontal cortex (mPFC) was recently demonstrated. In the present study, we show that the application of the endocannabinoids (eCBs) 2-arachidonoylglycerol (2-AG) and methanandamide [a stable analog of the eCB anandamide (AEA)] can activate CB₂Rs of mPFC layer II/III pyramidal neurons, which subsequently induces a Cl⁻ current. In addition, we show that action potential (AP) firing evoked by 20-Hz current injections results in an eCB-mediated opening of Cl⁻ channels via CB₂R activation. This AP-evoked synthesis of eCBs is dependent on the Ca²⁺ influx through N-type voltage-gated calcium channels. Our results indicate that 2-AG is the main eCB involved in this process. Finally, we demonstrate that under physiologically relevant intracellular Cl⁻ conditions, 20-Hz AP firing leads to a CB₂R-dependent reduction in neuronal excitability. Altogether, our data indicate that eCBs released upon action potential firing can modulate, through CB₂R activation, neuronal activity in the mPFC. We discuss how this may be a mechanism to prevent excessive neuronal firing.