Inactivation of the median raphe nucleus increases intake of sucrose solutions: a microstructural analysis

Characterizing ingestive behavior through licking microstructure: Underlying neurobiology and its use in the study of obesity in animal models

Ingestive behavior is controlled by multiple distinct peripheral and central physiological mechanisms that ultimately determine whether a particular food should be accepted or avoided. As rodents consume a fluid they display stereotyped rhythmic tongue movements, and by analyzing the temporal distribution of pauses of licking, it is possible through analyses of licking microstructure to uncover dissociable evaluative and motivational variables that contribute to ingestive behavior. The mean number of licks occurring within each burst of licking (burst and cluster size) reflects the palatability of the consumed solution, whereas the frequency of initiating novel bouts of licking behavior (burst and cluster number) is dependent upon the degree of gastrointestinal inhibition that accrues through continued fluid ingestion. This review describes the analysis of these measures within a context of the behavioral variables that come to influence the acceptance or avoidance of a fluid, and the neurobiological mechanisms that underlie alterations in the temporal distribution of pauses of licks. The application of these studies to models of obesity in animals is also described.

Chemogenetic inhibition of cells in the paramedian midbrain tegmentum increases locomotor activity in rats

Pronounced hyperactivity can be produced by lesions or pharmacological inhibition of cells in the median raphe nucleus (MR) located in the paramedian midbrain tegmentum. In the current study we examined whether a similar effect can be seen after chemogenetic inhibition of cells in this region using the DREADD (Designer Receptors Exclusively Activated by Designer Drugs) approach. We found that the DREADD ligand clozapine-N-oxide (CNO) increased locomotor activity in animals expressing the inhibitory DREADD hM4Di, but not those injected with a control virus in the MR. The effect was of rapid onset and short duration and persisted for at least four months after virus injections. Histological examination of the brains indicated that labeled fibers followed the known projection patterns of the MR to a variety of forebrain and midbrain structures. These findings confirm the role of the MR region in the control of locomotion and suggest that the DREADD technique may be a useful approach to the study of the functional architecture of this complex area. Methodological and interpretive aspects of DREADD studies are discussed.

Ingestive and locomotor behaviours induced by pharmacological manipulation of <Alpha>-adrenoceptors into the median raphe nucleus

The present study evaluated the involvement of α-adrenoceptors of the median raphe nucleus (MRN) in satiated rats, in food and water intake and motor behaviour. Control groups were treated with saline (SAL) or adrenaline (ADR), injected into the MRN seven minutes after injection of the vehicle used to solubilize the antagonists, propylene glycol (PLG) or SAL. Experimental groups were treated with an α-adrenoceptor antagonist, prazosin (α1, 20 or 40 nmol) or yohimbine (α2, 20 or 40 nmol) or phentolamine (non-selective α, 20 or 40 nmol), followed (later) by injection of ADR or SAL. Behaviour was recorded for 30 min. The injection of ADR and the blockade of α1 receptors resulted in hyperphagia whereas blocking α2 or α1 and α2 simultaneously did not change feeding behaviour. Pre-treatment with prazosin, followed by injection of ADR was not able to cause an increase in the amount of food ingested, while the higher dose of the α1 antagonist reduced the latency to start feeding. Pre-treatment with prazosin also caused hyperactivity. However, pre-treatment with phentolamine or yohimbine was able to block ADR-induced feeding. The present study supports the hypothesis that there is a tonic activation of α1-adrenoceptors in the MRN in satiated rats, which activates an inhibitory influence in areas that control food intake. Injection of ADR seems to activate α2 receptors, resulting in a decrease in the availability of endogenous catecholamines, which reduces the release of the signal that inhibits food intake, leading to hyperphagia.

Dopamine is differentially involved in the locomotor hyperactivity produced by manipulations of opioid, GABA and glutamate receptors in the median raphe nucleus

The median raphe nucleus (MR) has been shown to exert a powerful influence on behavioral arousal and marked locomotor hyperactivity can be produced by intra-MR injections of a variety of drugs including GABAA and GABAB agonists, excitatory amino acid antagonists, and μ- and δ-opioid agonists. Other studies have indicated that the MR exerts an inhibitory influence on ascending dopamine systems, suggesting that MR induced alterations in activity may be mediated through changes in dopaminergic transmission. In the present study, we explored this possibility by examining whether systemic administration of the preferential D2 dopamine antagonist haloperidol is able to antagonize the hyperactivity produced by intra-MR injections of various drugs. We found that haloperidol completely blocked the locomotor response to intra-MR injections of the μ-opioid receptor agonist DAMGO and the δ-opioid receptor agonist DPDPE. In marked contrast, at doses which abolished the locomotor response to systemic amphetamine, haloperidol had no effect on the hyperactivity induced by intra-MR injections of GABAA agonist muscimol, the GABAB agonist baclofen, or the kainate/quisqualate antagonist pBB-PZDA, even though it suppressed baseline activity in these same animals. These results indicate that there must be at least two mechanisms capable of influencing behavioral arousal within the MR region, one of which is dependent on D2 dopamine receptors and the other is not.

Effects of muscimol in the nucleus accumbens shell on salt appetite and sucrose intake: a microstructural study with a comment on the sensitization of salt intake

Previous work has demonstrated that injections of the γ-aminobutyric acidA (GABAA) agonist muscimol into the nucleus accumbens shell (AcbSh) induce pronounced increases in the intake of solid foods and sucrose solutions, but do not potentiate water intake. In order to clarify the range of situations in which inactivation of the AcbSh potentiates ingestive behavior, we examined the effects of muscimol injections on the intake of a 3% NaCl solution in sodium-depleted animals. Although sodium-depleted subjects avidly consumed this solution, muscimol injections had no effect either on the volume consumed or on a variety of microstructural licking parameters. In contrast, in these same animals, muscimol injections significantly increased licking of a 10% sucrose solution. These results suggest that inactivation of the AcbSh may selectively increase the intake of foods, but not that of other homeostatically relevant ingestates. Examination of microstructural parameters suggested that the effect of muscimol on sucrose intake was not mediated by alterations in the "palatability" of the sucrose solution. We also observed that sodium-depleted subjects displayed significantly larger salt intakes after their second experience with sodium depletion than their first, and microstructural analysis in this case indicated that this sensitization effect was produced in a manner consistent with the animals showing increased "hedonic responsiveness" to the salt solution.