D₂ dopamine receptor activation facilitates endocannabinoid- mediated long-term synaptic depression of GABAergic synaptic transmission in midbrain dopamine neurons via cAMP-protein kinase A signaling

Endocannabinoid (eCB) signaling mediates short-term and long-term synaptic depression (LTD) in many brain areas. In the ventral tegmental area (VTA) and striatum, D₂ dopamine receptors cooperate with group I metabotropic glutamate receptors (mGluRs) to induce eCB-mediated LTD of glutamatergic excitatory and GABAergic inhibitory (I-LTD) synaptic transmission. Because D₂ receptors and group I mGluR agonists are capable of inducing the release of eCBs, the predominant hypothesis is that the cooperation between these receptors to induce eCB-mediated synaptic depression results from the combined activation of type I cannabinoid (CB₁) receptors by the eCBs. By determining the downstream effectors for D₂ receptor and group I mGluR activation in VTA dopamine neurons, we show that group I mGluR activation contributes to I-LTD induction by enhancing eCB release and CB ₁ receptor activation. However, D₂ receptor activation does not enhance CB₁ receptor activation, but facilitates I-LTD induction via direct inhibition of cAMP-dependent protein kinase A (PKA) signaling. We further demonstrate that cAMP/PKA signaling pathway is the downstream effector for CB₁ receptors and is required for eCB-mediated I-LTD induction. Our results suggest that D₂ receptors and CB₁ receptors target the same downstream effector cAMP/PKA signaling pathway to induce I-LTD and D₂ receptor activation facilitates eCB-mediated I-LTD in dopamine neurons not by enhancing CB ₁ receptor activation, but by enhancing its downstream effects.