GABAergic and cortical and subcortical glutamatergic axon terminals contain CB1 cannabinoid receptors in the ventromedial nucleus of the hypothalamus.
BACKGROUND
Type-1 cannabinoid receptors (CB(1)R) are enriched in the hypothalamus, particularly in the ventromedial hypothalamic nucleus (VMH) that participates in homeostatic and behavioral functions including food intake. Although CB(1)R activation modulates excitatory and inhibitory synaptic transmission in the brain, CB(1)R 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(1)R in the VMH to better understand the modulation exerted by the endocannabinoid system on the complex brain circuitries converging into this nucleus.


RESULTS
Light and electron microscopy techniques were used to analyze CB(1)R distribution in the VMH of CB(1)R-WT, CB(1)R-KO and conditional mutant mice bearing a selective deletion of CB(1)R in cortical glutamatergic (Glu-CB(1)R-KO) or GABAergic neurons (GABA-CB(1)R-KO). At light microscopy, CB(1)R immunolabeling was observed in the VMH of CB(1)R-WT and Glu-CB(1)R-KO animals, being remarkably reduced in GABA-CB(1)R-KO mice. In the electron microscope, CB(1)R appeared in membranes of both glutamatergic and GABAergic terminals/preterminals. There was no significant difference in the percentage of CB(1)R immunopositive profiles and CB(1)R density in terminals making asymmetric or symmetric synapses in CB(1)R-WT mice. Furthermore, the proportion of CB(1)R immunopositive terminals/preterminals in CB(1)R-WT and Glu-CB(1)R-KO mice was reduced in GABA-CB(1)R-KO mutants. CB(1)R density was similar in all animal conditions. Finally, the percentage of CB(1)R labeled boutons making asymmetric synapses slightly decreased in Glu-CB(1)R-KO mutants relative to CB(1)R-WT mice, indicating that CB(1)R was distributed in cortical and subcortical excitatory synaptic terminals.


CONCLUSIONS
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.
Department of Neurosciences, Faculty of Medicine and Dentistry, Basque Country University, Leioa, Spain.
Leire
L
Reguero
Nagore
N
Puente
Izaskun
I
Elezgarai
Juan
J
Mendizabal-Zubiaga
Miren Josune
MJ
Canduela
Ianire
I
Buceta
Almudena
A
Ramos
Juan
J
Suárez
Fernando
F
Rodríguez de Fonseca
Giovanni
G
Marsicano
Pedro
P
Grandes
D000818:Animals; D002540:Cerebral Cortex / Q000166:cytology / Q000378:metabolism*; D059330:GABAergic Neurons / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism*; D007150:Immunohistochemistry; D051379:Mice; D018345:Mice, Knockout; D017729:Presynaptic Terminals / Q000378:metabolism* / Q000648:ultrastructure; D043884:Receptor, Cannabinoid, CB1 / Q000378:metabolism*; D014697:Ventromedial Hypothalamic Nucleus / Q000166:cytology / Q000378:metabolism* / Q000648:ultrastructure; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D043884:Receptor, Cannabinoid, CB1; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid