GABAergic and cortical and subcortical glutamatergic axon terminals contain CB1 cannabinoid receptors in the ventromedial nucleus of the hypothalamus

Background

Type-1 cannabinoid receptors (CB₁R) are enriched in the hypothalamus, particularly in the ventromedial hypothalamic nucleus (VMH) that participates in homeostatic and behavioral functions including food intake. Although CB₁R activation modulates excitatory and inhibitory synaptic transmission in the brain, CB₁R contribution to the molecular architecture of the excitatory and inhibitory synaptic terminals in the VMH is not known. Therefore, the aim of this study was to investigate the precise subcellular distribution of CB₁R in the VMH to better understand the modulation exerted by the endocannabinoid system on the complex brain circuitries converging into this nucleus.

Methodology/Principal Findings

Light and electron microscopy techniques were used to analyze CB₁R distribution in the VMH of CB₁R-WT, CB₁R-KO and conditional mutant mice bearing a selective deletion of CB₁R in cortical glutamatergic (Glu-CB₁R-KO) or GABAergic neurons (GABA-CB₁R-KO). At light microscopy, CB₁R immunolabeling was observed in the VMH of CB₁R-WT and Glu-CB₁R-KO animals, being remarkably reduced in GABA-CB₁R-KO mice. In the electron microscope, CB₁R appeared in membranes of both glutamatergic and GABAergic terminals/preterminals. There was no significant difference in the percentage of CB₁R immunopositive profiles and CB₁R density in terminals making asymmetric or symmetric synapses in CB₁R-WT mice. Furthermore, the proportion of CB₁R immunopositive terminals/preterminals in CB₁R-WT and Glu-CB₁R-KO mice was reduced in GABA-CB₁R-KO mutants. CB₁R density was similar in all animal conditions. Finally, the percentage of CB₁R labeled boutons making asymmetric synapses slightly decreased in Glu-CB₁R-KO mutants relative to CB₁R-WT mice, indicating that CB₁R was distributed in cortical and subcortical excitatory synaptic terminals.

Conclusions/Significance

Our anatomical results support the idea that the VMH is a relevant hub candidate in the endocannabinoid-mediated modulation of the excitatory and inhibitory neurotransmission of cortical and subcortical pathways regulating essential hypothalamic functions for the individual''s survival such as the feeding behavior.