Hyperphagia caused by muscimol injection in the nucleus raphe dorsalis of rats: its control by 5-hydroxytryptamine in the nucleus accumbens

GABAergic interneurons' feedback inhibition of dorsal raphe-projecting pyramidal neurons of the medial prefrontal cortex suppresses feeding of adolescent female mice undergoing activity-based anorexia

Anorexia Nervosa (AN) is characterized by voluntary food restriction, excessive exercise and extreme body weight loss. AN is particularly prevalent among adolescent females experiencing stress-induced anxiety. We used the animal model, activity-based anorexia (ABA), which captures these characteristics of AN, to reveal the neurobiology underlying individual differences in AN vulnerability. Dorsal raphe (DR) regulates feeding and is recruited when coping inescapable stress. Through chemogenetic activation, we investigated the role of mPFC pyramidal neurons projecting to DR (mPFC→DR) in adolescent female mice's decision to eat or exercise following ABA induction. Although the DREADD ligand C21 could activate 44% of the mPFC→DR neurons, this did not generate significant group mean difference in the amount of food intake, compared to control ABA mice without chemogenetic activation. However, analysis of individuals' responses to C21 revealed a significant, positive correlation between food intake and mPFC→DR neurons that co-express cFos, a marker for neuronal activity. cFos expression by GABAergic interneurons (GABA-IN) in mPFC was significantly greater than that for the control ABA mice, indicating recruitment of GABA-IN by mPFC→DR neurons. Electron microscopic immunohistochemistry revealed that GABAergic innervation is 60% greater for the PFC→DR neurons than adjacent Layer 5 pyramidal neurons without projections to DR. Moreover, individual differences in this innervation correlated negatively with food intake specifically on the day of C21 administration. We propose that C21 activates two antagonistic pathways: (1) PFC→DR pyramidal neurons that promote food intake; and (2) GABA-IN in the mPFC that dampen food intake through feedback inhibition of mPFC→DR neurons.

Under- to over-eating: how do serotonin receptors contribute?

In numerous pathological states, the brain can restrict food intake to a lethal level despite mounting requirements for energy as seen in adolescents with anorexia nervosa. How the brain reduces food intake to the point of death while eating is a cornerstone of survival that remains just as ‘cryptic’ as the association between anorexia and overeating. This review provides a recent snapshot of the neural underpinnings of the rewarding effects of anorexia that may compete with the adaptive decision-making process to eat, and with survival instinct. Among a plethora of factors, impaired activity of the serotonin receptors in the reward system underlies the ability of animals to self-impose food restriction, and the transition from under- to over-eating. However, the triumvirate association between serotonin, overeating and addiction appears unlikely. Considering the implication of the serotonin receptors in the hypothalamus, anorexia and bulimia nervosa could result from an impairment of a ‘synchronic activity’ between the autonomic and voluntary nervous systems.

GABA and behavior: the role of receptor subtypes

The discovery of different GABA receptor subtypes has stimulated research relating this neurotransmitter to a variety of behavioral functions and clinical disorders. The development of new and specific GABAergic compounds has made it possible to try to identify the specific functions of these receptors. The purpose of the present review is to evaluate the data regarding the functions of the GABA receptor subtypes in different behaviors such as motor function, reproduction, learning and memory, and aggressive-defensive behaviors. A description of GABAergic functions (stress, peripheral effects, thermoregulation) that might directly or indirectly affect behavior is also included. The possible involvement of GABA in different neurological and psychiatric disorders is also discussed. Although much research has been done trying to identify the possible role of GABA in different behaviors, the role of receptor subtypes has only recently attracted attention, and only preliminary data are available at present. It is therefore evident that still much work has to be done before a clear picture of the behavioral significance of these receptor subtypes can be obtained. Nevertheless, existing data are sufficient to justify the prediction that GABAergic agents, in the near future, will be much used in the field of behavioral pharmacology. It is hoped that the present review will contribute to this. Some specific suggestions concerning the most efficient way to pursue future research are also made.

Administration of 8-hydroxy-2-(Di-n-propylamino)tetralin in raphe nuclei dorsalis and medianus reduces serotonin synthesis in the rat brain: differences in potency and regional sensitivity

Following administration of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.04-5.0 micrograms/0.5 microliter) in the raphe nucleus dorsalis (DR) or medianus (MR), the synthesis of serotonin (5-HT), as assessed by the accumulation of 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition, was measured in various regions of the rat CNS. At all doses, 8-OH-DPAT in the DR significantly reduced 5-HTP accumulation in the striatum, nucleus accumbens, cortex, and prefrontal cortex, whereas even the highest dose had no effect in the hippocampus, hypothalamus, and spinal cord. One microgram of 8-OH-DPAT in the MR significantly reduced 5-HTP accumulation in the nucleus accumbens and prefrontal cortex, and 5 micrograms had an effect in all the areas except the striatum and spinal cord. One and 5 micrograms of 8-OH-DPAT, administered in either the DR or MR, did not significantly modify the accumulation of dihydroxyphenylalanine in the striatum and nucleus accumbens. The results confirm that DR and MR have different sensitivities to 5-HT1A receptor agonists, and that activation of 5-HT1A receptors in these nuclei produces different effects on 5-HT synthesis in different brain regions.

Different effects of fenfluramine isomers and metabolites on extracellular 5-HIAA in nucleus accumbens and hippocampus of freely moving rats

The effects of optical isomers of fenfluramine and their metabolites, d- and l-norfenfluramine on the serotonergic system were studied in the nucleus accumbens and hippocampus of freely moving rats by in vivo voltammetry. Both isomers and the metabolites induced a slow, sustained decrease in 5HIAA but only d-fenfluramine and its metabolite, d-norfenfluramine, increased the 5HIAA levels in nucleus accumbens shortly after injection, the increase being greater after the metabolite. No effect could be detected in the hippocampus after the higher dose of d-fenfluramine.

Monoamine transmitter activity in lateral hypothalamus during its perfusion with insulin or 2-DG in sated and fasted rat

A unique profile of neurochemical events is proposed to occur in the diencephalon which is contingent upon the nutrient status of the animal. In this first of a series of investigations, we selected the lateral hypothalamus (LH) in order to determine its specific resting profile of monoaminergic neurotransmitters and their principal metabolites. The neuronal pattern of activity was studied during sated and fasted conditions as well as during a local glucoprivic challenge to the LH. After permanent guide cannulae for push-pull perfusion were implanted in female Sprague-Dawley rats, the LH was perfused repeatedly with an artificial CSF, at a rate of 20 microliters/min, in order to collect a series of 5.0 min samples. Aliquots of each perfusate were assayed directly using a high performance liquid chromatography system with electrochemical detection (HPLC-EC) for pg/microliter concentrations of norepinephrine (NE), dopamine (DA) and serotonin (5-HT). In comparison to the basal levels of amines during the sated condition, when the rat was food-deprived for 20-22 hr, the release of NE, DA, and 5-HT was significantly lower than that observed under the sated condition. Further, the turnover of NE in the LH was concurrently attenuated as reflected by the lower levels of MHPG in the perfusate, thus demonstrating the modification in catecholamine activity produced in the LH by the condition of hunger. When either 10 micrograms/microliters 2-deoxy-D-glucose (2-DG) or 4.0 mU/microliter insulin was incorporated into the CSF perfused in the LH, the efflux of DA was significantly enhanced independent of the state of satiation. In addition, the proportion of both NE and DA to 5-HT was likewise increased by either of these centrally acting substances, while the turnover of 5-HT was enhanced and NE and DA turnovers were reduced. Perfusion of 2-DG in the LH of the fasted rat caused a significant reduction in catecholamine turnover in terms of MHPG/NE, VMA/NE, DOPAC/DA and HVA/DA ratios. Moreover, 2-DG increased NE/5-HT while lowering the NE/DA ratio, and enhanced simultaneously the 5-HTOL/5-HT ratio. In the sated rat, 2-DG attenuated the release of 5-HT from the animal's LH, whereas insulin caused a shift in the proportions of NE/5-HT and DA/5-HT. Further, the peptide served to reduced the efflux of 5-HT, enhanced the turnover of 5-HT while diminishing DA turnover, and shifted the metabolism of NE from MHPG to VMA.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative studies of the ingestive behaviors produced by microinjections of muscimol into the midbrain raphe nuclei of the ventral tegmental area of the rat

Microinjection of the GABA-A agonist muscimol into the median (MR) or dorsal (DR) raphe nuclei or the ventral tegmental area (VTA) of non-deprived rats induced intense feeding and drinking in a dose-dependent and site-specific manner. Lower doses of muscimol were required to increase food intake, spillage and water intake with injections into the MR than with injections into the other two sites. These data demonstrate that the MR is a more sensitive site for the elicitation of ingestive behavior than either the DR or the VTA.

Eating caused by neuropeptide-Y injection in the paraventricular hypothalamus: response to (+)-fenfluramine and (+)-amphetamine in rats

(+)-Fenfluramine and (+)-amphetamine have been compared for their ability to reduce food intake in food-deprived rats or eating caused by injecting neuropeptide-Y in the paraventricular hypothalamus of free feeding rats. (+)-Fenfluramine at doses ranging from 0.625 to 5 mg kg-1 reduced eating caused by neuropeptide-Y more effectively than it did the food intake of food-deprived rats, whereas (+)-amphetamine (dose range 0.625-2.5 mg kg-1) reduced both types of eating to a similar extent. The results confirm that (+)-fenfluramine, although less potent than (+)-amphetamine in reducing eating by food-deprived rats, markedly reduces overeating caused by various endogenous substances or stress in free feeding rats. The physiological significance of the neuropeptide-Y-induced eating and its control by (+)-fenfluramine remains to be elucidated.