Phenylephrine into the median raphe nucleus evokes an anxiolytic-like effect in free-feeding rats but does not alter food intake in free feeding rats

This study investigated the role of MnR α1-adrenergic receptors in the control of anxiety-like and feeding behaviors and attempted to reveal a possible functional association between both behaviors. The α1-adrenergic agonist phenylephrine (PHE) (at doses of 0.2, 2, 6, 20 nmol) or saline was injected into the MnR or into the pontine nucleus (Pn) of free-feeding rats. The animals were exposed to the elevated plus maze to analyse spatial–temporal and ethological variables. Subsequently, the ingestive and non-ingestive behaviors were recorded during 30 min and feeding and drinking behaviors were measured. Both in the elevated plus-maze and in the feeding chamber, all PHE doses injected into the MnR decreased the risk assessment frequency, an ethological parameter of anxiolytic-like effect. The spatial–temporal variables remained unchanged after PHE treatment. Feeding behavior was not affected by PHE into the MnR. The anxiety-like or ingestive behaviors were not affected by PHE treatment in the Pn, an area adjacent to the MnR. These data indicate that α1-adrenergic receptors within MnR participate in the control of anxiety-like behaviors. The absence of effects on feeding behavior after MnR α1-adrenergic activation could be due to an elevated α1-adrenergic tonus and its possible strong facilitatory influence on 5-HT neurons within MnR. Furthermore, the present results suggest that anxiety-like and feeding behaviors controled by MnR adrenergic circuits operate by independent neural pathways.

Research highlights▶ Anxiolytic-like effect is mediated by activation of α1-adrenergic receptors in the MnR. ▶ Feeding behavior did not change after phenylephrine injections into the MnR. ▶ Nutritional status influences the effect on feeding after MnR adrenergic receptors activation. ▶ The anxiolytic-like effect seem to be circumscribed to the MnR.