Evaluation of food intake and Fos expression in serotonergic neurons of raphe nuclei after intracerebroventricular injection of adrenaline in free-feeding rats

Abstract

Previous studies indicate that the modification of adrenergic neurotransmission in median raphe nucleus (MRN) enhances or removes an inhibitory influence on food intake, possibly serotonergic, due to a presence of serotonin-producing neurons in that nucleus. Therefore, the aim of this study is evaluated whether the activity of neurons in the MRN and dorsal raphe nucleus (DRN) are affected by intracerebroventricular injection of adrenaline (AD) in free-feeding rats. Male Wistar rats with guide cannulae chronically implanted in the lateral ventricle were injected with AD followed by evaluation of ingestive behavioral parameters. Behavior was monitored and the amount of food ingested was assessed. The highest dose (20 nmol) of AD was the most effective dose in increasing food intake. Subsequently, AD 20 nmol was injected to study neuronal activity indicated by the presence of Fos protein and its co-localization with serotonergic neurons in the MRN and DRN of naive rats with or without access to food during the recording of behavior. The administration of AD 20 nmol increased Fos expression and double labeling with serotonergic neurons in the DRN in rats with access to food, but not in animals without access. No statistically significant changes in Fos expression were observed in the MRN in any of the experimental conditions tested. These results suggest that DRN serotonergic and non-serotonergic neurons are activated by post-prandial signals. In contrast, the absence of Fos expression in the MRN suggests that this nucleus does not participate in the circuit involved in the control of post-prandial satiety.

Keywords: Adrenaline; Dorsal raphe nucleus; Food intake; Fos protein; Median raphe nucleus; Serotonin.