GABAergic and cortical and subcortical glutamatergic axon terminals contain CB 1 cannabinoid receptors in the ventromedial nucleus of the hypothalamus

Abstract

Background: Type-1 cannabinoid receptors (CB 1R) are enriched in the hypothalamus, particularly in the ventromedial hypothalamic nucleus (VMH) that participates in homeostatic and behavioral functions including food intake. Although CB 1R activation modulates excitatory and inhibitory synaptic transmission in the brain, CB 1R 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 CB 1R 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 CB 1R distribution in the VMH of CB 1R-WT, CB 1R-KO and conditional mutant mice bearing a selective deletion of CB 1R in cortical glutamatergic (Glu-CB 1R-KO) or GABAergic neurons (GABA-CB 1R-KO). At light microscopy, CB 1R immunolabeling was observed in the VMH of CB 1R-WT and Glu-CB 1R-KO animals, being remarkably reduced in GABA-CB 1R-KO mice. In the electron microscope, CB 1R appeared in membranes of both glutamatergic and GABAergic terminals/preterminals. There was no significant difference in the percentage of CB 1R immunopositive profiles and CB 1R density in terminals making asymmetric or symmetric synapses in CB 1R-WT mice. Furthermore, the proportion of CB 1R immunopositive terminals/preterminals in CB 1R-WT and Glu-CB 1R-KO mice was reduced in GABA-CB 1R-KO mutants. CB 1R density was similar in all animal conditions. Finally, the percentage of CB 1R labeled boutons making asymmetric synapses slightly decreased in Glu-CB 1R-KO mutants relative to CB 1R-WT mice, indicating that CB 1R 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. © 2011 Reguero et al.