Localization and phenotypic characterization of brainstem neurons activated by rimonabant and WIN55,212-2

The mechanisms by which the CB1 receptor antagonist rimonabant exerts its appetite-suppressing and energy-dissipating effects are still incompletely resolved. To shed further light on the central pathways influenced by CB1 receptor modulation we examined the expression of the immediate early gene c-fos in male Sprague-Dawley rats at 60, 120 and 240 min after intraperitoneal administration of the CB1R antagonist rimonabant (10 mg/kg) and the CB1R agonist WIN55,212-2 (3 mg/kg). Perfusion-fixed brains were processed for immunohistochemistry and the localization of c-Fos immunoreactive neuronal profiles was assessed qualitatively throughout the brain. Nine areas, including specific hypothalamic and brainstem nuclei known to be involved in appetite regulation, were selected for quantitative analyses. Whereas WIN55,212-2 induced c-Fos immunoreactivity in a time-specific manner in the striatum, the central nucleus of amygdala, the hypothalamic paraventricular nucleus and the arcuate nucleus, no significant increases in c-Fos positive nuclei were found in any forebrain areas following rimonabant administration. In contrast, rimonabant and WIN55,212-2 were both found to significantly increase c-Fos immunoreactivity in the brainstem lateral parabrachial nucleus, the nucleus of the solitary tract and the area postrema. To characterize the phenotype of activated neurons in the nucleus of the solitary tract, a triple immunohistochemical staining technique was used to simultaneously label c-Fos protein and tyrosine hydroxylase (TH), GLP-1 or CART. Interestingly, rimonabant was found to significantly increase c-Fos protein expression in TH-positive neurons. Collectively, these results suggest that brainstem areas including ascending catetholaminergic A2/C2 neurons could play a role in rimonabant-induced inhibition of food intake.