Tonic noradrenergic input to neurons in the dorsal raphe nucleus mediates food intake in male mice

The dorsal raphe nucleus (DRN) is essential for the control of food intake. Efferent projections from the DRN extend to several forebrain regions that are involved in the control of food intake. However, the neurotransmitters released in the DRN related to the control of food intake are not known. We have previously demonstrated that a tonic alpha 1 action on DRN neurons contributes to satiety in the fed rats. In this study we investigated the participation of norepinephrine (NE) signaling in the DRN in the satiety response. Intra-DRN administration of NE causes an increase in the 2-hour food intake of sated mice, an effect that was blocked by previous administration of yohimbine, an alpha 2 antagonist. Similarly, Intra-DRN administration of clonidine, an alpha 2 agonist, increases food intake in sated mice. This result indicates that in the satiated mice exogenous NE acts on alpha 2 receptors to increase food intake. Furthermore, administration of phenylephrine, an alpha 1 agonist, decreases food intake in fasted mice and prazosin, an alpha 1 antagonist, increases food intake in the sated mice. Taken together these results indicate that, in a satiated condition, a tonic alpha 1 adrenergic action on the DRN neurons inhibits food intake and that exogenous NE administered to the DRN acts on alpha 2 adrenergic receptors to increase food intake. These data reinforce the intricate neuronal functioning of the DRN and its effects on feeding.