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

Abstract

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.

Main Points

• Intracellular CB₁ receptors are localized in astroglial mitochondria. About 12% of the mitochondria in the hippocampal astrocytes express the receptor.
• This novel finding extends the subcellular compartmentalization of the CB₁ receptor in the brain.

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