α-1 Adrenoceptor Activation in the Dorsal Raphe Nucleus Decreases Food Intake in Fasted Rats

Abstract

The dorsal raphe (DR) nucleus is involved in a myriad of physiological functions, such as the control of sleep-wake cycle, motivation, pain, energy balance, and food intake. We have previously demonstrated that in ad libitum fed rats the intra-DR administration of phenylephrine, an α-1 receptor agonist, does not affect food intake, whereas clonidine, an α-2 receptor agonist, potently stimulates food intake. These results indicated that in fed rats an increased adrenergic tonus blocked food intake, since the activation of α-2 auto-receptors, which decreases pre-synaptic release of adrenaline/noradrenaline, affected food intake. Thus, in this study we assessed whether the response to adrenergic stimuli would differ after overnight fasting, a situation of low adrenergic activity in the DR. Intra-DR administration of adrenaline and noradrenaline blocked food intake evoked by overnight fasting. Similarly, phenylephrine administration decreased hunger-induced food intake. These changes in food intake were accompanied by changes in other behaviors, such as increased immobility time and feeding duration. On the other hand, intra-DR administration of clonidine did not affect food-intake or associated behaviors. These results further support the hypothesis that in fed animals, increased adrenergic tonus in DR neurons inhibiting feeding, while in fasted rats the adrenergic tonus decreases and favors food intake. These data indicate a possible mechanism through which adrenergic input to the DRN contributes to neurobiology of feeding.

Keywords: adrenergic receptor; dorsal raphe (DR); food intake; hunger; phenylephrine.

FIGURE 1

Confirmation of injection sites into DR of fasted rats. (A) Photomicrograph of a stained section, showing injection site into the DR. (B) Injection sites at the bregma level:7, 80 mm; other injection sites were located at-7.32 to-8.04 mm to bregma. Aq, aqueduct; DR, dorsal raphe nucleus; DMPAG, dorsomedial periaqueductal gray; LPAG, lateral periaqueductal gray; VLPAG, ventrolateral periaqueductal gray; mlf, medial longitudinal fasciculus. Scale bar = 100 μm. *, location of each injection site.

FIGURE 2

Food and water intake after injection of adrenaline (AD) or noradrenaline (NA) into DR of fasted rats. (A) Changes in the amount of food intake after injection with vehicle (VEH) or AD at 6, 20, and 60 nmol doses into DR of fasted rats. (B) Changes in the amount of water intake after treatment with VEH or AD at 6, 20, and 60 nmol doses into DR of fasted rats. (C) Correlation between water and food intake after administration of AD (6, 20, 60 nmol) or VEH into DR. (D) Changes in the amount of food intake after injection with VEH or NA at 6, 20, and 60 nmol doses into DR of fasted rats. (E) Changes in the amount of water intake after injection with VEH or NA at 6, 20, and 60 nmol doses into DR of fasted rats. (F) Correlation between water and food intake after administration of NA (6, 20, 60 nmol) or VEH into DR. In all experiments, separate rats were used for each dose; each rat received only a single injection of drug or vehicle. Data represent the mean ± SEM *p < 0.05 vs. vehicle group. One-way ANOVA followed by Tukey’s post hoc test (n = 6–8 per group). Pearson’s correlation test *p < 0.05.

FIGURE 3

Food and water intake after injection of the α-1 adrenoceptor agonist phenylephrine (PHE) or the α-2 adrenoceptor agonist clonidine (CLO) into DR of fasted rats. (A) Changes in the amount of food intake after injection with vehicle (VEH) or PHE at 6 and 20 nmol doses into DR of fasted rats. (B) Changes in the amount of water intake after treatment with VEH or PHE at 6 and 20 nmol doses into DR of fasted rats. (C,D) Effect of the PHE 20 nmol injection into the decussation of the superior cerebellar peduncle (xscp) on food intake in fasted rats. MR = median raphe nucleus; Aq = cerebral aqueduct; DR = dorsal raphe nucleus; NS = non-significant. (E) Changes in the amount of food intake after injection with VEH or CLO at 6 and 20 nmol doses into DR of fasted rats. (F) Changes in the amount of water intake after injection with VEH or CLO at 6 and 20 nmol doses into DR of fasted rats. In all experiments, separate rats were used for each dose; each rat received only a single injection of drug or vehicle. Data represent the mean ± SEM *p < 0.05 vs. vehicle group. One-way ANOVA followed by Tukey’s post hoc test (n = 6–8 per group).

FIGURE 4

Proposed role of noradrenergic circuits in the DR in food intake regulation in rats. In the fed state, there is a tonic activation of α-1 adrenoceptors into this nucleus, which facilitates the release of a signal that inhibits food intake (possibly via 5-HT release into proensephalic areas) into prosencephalic areas. On the other hand, in fasted rats the intensity of this endogenous noradrenergic activity seems to decline. We also speculate that peripheral satiety signals, such as CCK, could indirectly modify neuronal activity in the DR through noradrenergic neurons located in the NTS that send neuronal input to DR neurons. DR, dorsal raphe nucleus; NTS, nucleus of the solitary tract; AD, adrenaline; NA, noradrenaline; CLO, clonidine; PHE, phenylephrine.