Localization of the cannabinoid type‐1 receptor in subcellular astrocyte compartments of mutant mouse hippocampus

Astroglial type‐1 cannabinoid (CB₁) receptors are involved in synaptic transmission, plasticity and behavior by interfering with the so‐called tripartite synapse formed by pre‐ and post‐synaptic neuronal elements and surrounding astrocyte processes. However, little is known concerning the subcellular distribution of astroglial CB₁ receptors. In particular, brain CB₁ receptors are mostly localized at cells' plasmalemma, but recent evidence indicates their functional presence in mitochondrial membranes. Whether CB₁ receptors are present in astroglial mitochondria has remained unknown. To investigate this issue, we included conditional knock‐out mice lacking astroglial CB₁ receptor expression specifically in glial fibrillary acidic protein (GFAP)‐containing astrocytes (GFAP‐CB₁‐KO mice) and also generated genetic rescue mice to re‐express CB₁ receptors exclusively in astrocytes (GFAP‐CB₁‐RS). To better identify astroglial structures by immunoelectron microscopy, global CB₁ knock‐out (CB₁‐KO) mice and wild‐type (CB₁‐WT) littermates were intra‐hippocampally injected with an adeno‐associated virus expressing humanized renilla green fluorescent protein (hrGFP) under the control of human GFAP promoter to generate GFAPhrGFP‐CB₁‐KO and ‐WT mice, respectively. Furthermore, double immunogold (for CB₁) and immunoperoxidase (for GFAP or hrGFP) revealed that CB₁ receptors are present in astroglial mitochondria from different hippocampal regions of CB₁‐WT, GFAP‐CB₁‐RS and GFAPhrGFP‐CB₁‐WT mice. Only non‐specific gold particles were detected in mouse hippocampi lacking CB₁ receptors. Altogether, we demonstrated the existence of a precise molecular architecture of the CB₁ receptor in astrocytes that will have to be taken into account in evaluating the functional activity of cannabinergic signaling at the tripartite synapse.