Changes in food intake and anxiety-like behaviors after clonidine injected into the median raphe nucleus.

Serotonergic neurons in the median raphe nucleus (MnR) are stimulated by alpha(1)-adrenergic agonists and inhibited by alpha(2) agonists. This study investigated the participation of MnR alpha(2)-adrenergic receptors in the control of anxiety-like behavior and feeding as an attempt to establish a functional association between these behaviors. The alpha(2)-adrenergic agonist clonidine (CLON) was injected into the MnR (0, 0.2, 2, 6, 20nmol), into the pontine nucleus (Pn) or into the mesencephalic reticular formation (mRt) (0.2, 20nmol) of free-feeding rats. The animals were exposed to the elevated plus-maze to evaluate spatial-temporal and ethological variables. Subsequently, the ingestive and non-ingestive behaviors were recorded during 30min and the quantity of food and water consumed were measured. The lowest dose of CLON injected into the MnR decreased the total risk assessment (TRA) frequency, an ethological parameter of anxiolytic-like effect, but did not change feeding behavior. The highest dose of CLON injected into the MnR increased the TRA frequency, an anxiogenic-like effect. Similar result was observed after CLON injected into the Pn and mRt at the highest dose. In addition, clonidine at the highest dose caused hyperphagy accompanied by a reduction in the latency to start eating and an increase in feeding frequency when injected into the MnR but not in the Pn or mRt. These data indicate that MnR alpha(2)-adrenergic receptors participate in the control of anxiety-like and feeding behaviors, probably decreasing the facilitatory influence on MnR serotonergic neurons. The present results suggest that these behaviors involve independent neural pathways.