Serotonin release in the rat brain cortex is inhibited by neuropeptide Y but not affected by ACTH1-24, angiotensin II, bradykinin and delta-sleep-inducing peptide

Neuropeptide Y interaction with dopaminergic and serotonergic pathways: interlinked neurocircuits modulating hedonic eating behaviours

Independent from homeostatic needs, the consumption of foods originating from hyperpalatable diets is defined as hedonic eating. Hedonic eating can be observed in many forms of eating phenotypes, such as compulsive eating and stress-eating, heightening the risk of obesity development. For instance, stress can trigger the consumption of palatable foods as a type of coping strategy, which can become compulsive, particularly when developed as a habit. Although eating for pleasure is observed in multiple maladaptive eating behaviours, the current understanding of the neurobiology underlying hedonic eating remains deficient. Intriguingly, the combined orexigenic, anxiolytic and reward-seeking properties of Neuropeptide Y (NPY) ignited great interest and has positioned NPY as one of the core neuromodulators operating hedonic eating behaviours. While extensive literature exists exploring the homeostatic orexigenic and anxiolytic properties of NPY, the rewarding effects of NPY continue to be investigated. As deduced from a series of behavioural and molecular-based studies, NPY appears to motivate the consumption and enhancement of food-rewards. As a possible mechanism, NPY may modulate reward-associated monoaminergic pathways, such as the dopaminergic and serotoninergic neural networks, to modulate hedonic eating behaviours. Furthermore, potential direct and indirect NPYergic neurocircuitries connecting classical homeostatic and hedonic neuropathways may also exist involving the anti-reward centre the lateral habenula. Therefore, this review investigates the participation of NPY in orchestrating hedonic eating behaviours through the modulation of monoaminergic pathways.

Serotonin and beyond-a tribute to Manfred Göthert (1939-2019)

Manfred Göthert, who had served Naunyn-Schmiedeberg's Arch Pharmacol as Managing Editor from 1998 to 2005, deceased in June 2019. His scientific oeuvre encompasses more than 20 types of presynaptic receptors, mostly on serotoninergic and noradrenergic neurones. He was the first to identify presynaptic receptors for somatostatin and ACTH and described many presynaptic receptors, known from animal preparations, also in human tissue. In particular, he elucidated the pharmacology of presynaptic 5-HT receptors. A second field of interest included ligand-gated and voltage-dependent channels. The negative allosteric effect of anesthetics at peripheral nACh receptors is relevant for the peripheral clinical effects of these drugs and modified the Meyer-Overton hypothesis. The negative allosteric effect of ethanol at NMDA receptors in human brain tissue occurred at concentrations found in the range of clinical ethanol intoxication. Moreover, the inhibitory effect of gabapentinoids on P/Q Ca2+ channels and the subsequent decrease in AMPA-induced noradrenaline release may contribute to their clinical effect. Another ligand-gated ion channel, the 5-HT3 receptor, attracted the interest of Manfred Göthert from the whole animal via isolated preparations down to the cellular level. He contributed to that molecular study in which 5-HT3 receptor subtypes were disclosed. Finally, he found altered pharmacological properties of 5-HT receptor variants like the Arg219Leu 5-HT1A receptor (which was also shown to be associated with major depression) and the Phe124Cys 5-HT1B receptor (which may be related to sumatriptan-induced vasospasm). Manfred Göthert was a brilliant scientist and his papers have a major impact on today's pharmacology.

Y1 receptors regulate aggressive behavior by modulating serotonin pathways

Neuropeptide Y (NPY) is pivotal in the coordinated regulation of food intake, growth, and reproduction, ensuring that procreation and growth occur only when food is abundant and allowing for energy conservation when food is scant. Although emotional and behavioral responses from the higher brain are known to be involved in all of these functions, understanding of the coordinated regulation of emotion/behavior and physiological functions is lacking. Here, we show that the NPY system plays a central role in this process because ablation of the Y1 receptor gene leads to a strong increase in territorial aggressive behavior. After exposure to the resident-intruder test, expression of c-fos mRNA in Y1-knockout mice is significantly increased in the medial amygdala, consistent with the activation of centers known to be important in regulating aggressive behavior. Expression of the serotonin [5-hydroxytryptamine (5-HT)] synthesis enzyme tryptophan hydroxylase is significantly reduced in Y1-deficient mice. Importantly, treatment with a 5-HT-1A agonist, (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide, abolished the aggressive behavior in Y1-knockout mice. These results suggest that NPY acting through Y1 receptors regulates the 5-HT system, thereby coordinately linking physiological survival mechanisms such as food intake with enabling territorial aggressive behavior.

Alterations in neuropeptide Y and Y1 receptor mRNA expression in brains from an animal model of depression: region specific adaptation after fluoxetine treatment

To investigate the possible link between neuropeptide Y (NPY) and depression, we analyzed NPY and its receptors in different limbic-related regions in the Flinder sensitive line (FSL), a genetic animal model of depression. In situ hybridization histochemistry was used to measure mRNA expression levels of NPY and NPY receptors, Y1 and Y2, in the FSL as compared to the control Flinder resistant Line rats (FRL). In the FSL rats, NPY mRNA expression levels were significantly decreased in the nucleus accumbens and CA regions, but increased in the arcuate nucleus and anterior cingulate cortex. Y1 receptor mRNA expression was decreased in different cortical regions (retrosplenial, anterior cingulate, and occipital) and in the hippocampal dentate gyrus. Y2 mRNA expression levels did not differ between FSL and FRL animals. The effect of the antidepressant drug fluoxetine (a serotonin reuptake inhibitor) in the two rat strains was also studied. There was an increase of the NPY mRNA hybridization signal in the arcuate nucleus of both strains following the antidepressant treatment (10 micromol/kg; daily for 14 days). However, in other brain regions, fluoxetine administration caused a differential effect on the induction of NPY-related genes in the two rat strains: in the CA region and dentate gyrus NPY mRNA expression was increased in the FSL, but decreased in the FRL. In contrast, Y1 mRNA levels tended to be decreased by fluoxetine in the nucleus accumbens of the FSL rats, but increased in the FRL. These findings suggest an involvement of the Y1, but not the Y2, receptor subtype in depressive disorder. Overall, the results appear to sustain the importance of the FSL rats as an animal model of depression in view of the impairment of NPY genes and the ability of fluoxetine treatment to normalize NPY-related gene expression selectively in this strain.

Cracking the riddle of cancer anorexia

During tumor growth, anorexia and reduced food intake are among the major causes leading to malnutrition and eventually cachexia, which negatively affect patients' outcome. Consistent evidence from our laboratories in rats and humans indicates a key role for ventromedial hypothalamic (VMH) serotonergic system in the development of cancer anorexia. Thus, we postulated that during cancer, increased plasma tryptophan levels (the precursor of serotonin) lead to increased cerebrospinal fluid tryptophan concentrations and increased VMH serotonin synthesis, which then mediates the occurrence of anorexia. However, recent data strongly suggest that factors other than tryptophan supplied to the central nervous system might be involved in the pathogenesis of reduced food intake during tumor growth. Particularly, a significant role appears to be played by interleukin-1 (IL-1). We recently showed that IL-1 infusion in normal rats causes changes in food intake and its determinants, meal number and meal size, similar to those characterizing cancer anorexia, thus supporting the involvement of this cytokine in the development of anorexia. Interestingly, IL-1 and the VMH serotonergic system appear to be closely linked: peripherally infused IL-1 increases brain tryptophan and serotonin concentrations, while intracerebrally infused IL-1 increases neuronal firing rate and serotonin release. We therefore hypothesize that during tumor growth, increased production/secretion of IL-1 occurs, which facilitates the tryptophan supply to the brain. IL-1 can then also act on the VHM itself, where IL-1 receptors exist, to increase its neuronal activity and serotonin release. In other words, we believe that centrally acting IL-1 increases hypothalamic neuronal firing rate and serotonin release, while peripherally acting IL-1 is critical in supplying the hypothalamus with the precursor, tryptophan, in order to maintain the high rate of serotonin synthesis. Also, additional factors recently proposed as mediators of anorexia (including neuropeptide Y and nitric oxide) appear to be part of the hypothesized pathogenic mechanism.

Neuropeptide Y perfused in the preoptic area of rats shifts extracellular efflux of dopamine, norepinephrine, and serotonin during hypothermia and feeding

This study examined the localized action of neuropeptide Y (NPY) on monoamine transmitter activity in the hypothalamus of the unrestrained rat as this peptide induced hypothermia, spontaneous feeding or both responses simultaneously. A guide tube was implanted in the anterior hypothalamic pre-optic area (AH/POA) of Sprague-Dawley rats. Then either control CSF vehicle or NPY in a dose of either 100 ng/microliter or 250 ng/microliter was perfused by push-pull cannulae in this structure in the fully sated, normothermic rat. Successive perfusions were carried out at a rate of 20 microliters/min for 6.0 min with an interval of 6.0 min elapsing between each. Samples of perfusate were assayed by HPLC for their levels of dopamine (DA), norepinephrine (NE), serotonin (5-HT) and their respective metabolites. Whereas control CSF was without effect on body temperature (Tb) or feeding, repeated perfusions of NPY over 3.0 hr caused dose-dependent eating from 4 to 39 g of food, hypothermia of 0.9 to 2.3 degrees C or both responses concurrently. As the rats consumed 11-39 g of food, the efflux of NE, MHPG, DOPAC and 5-HT was enhanced significantly, whereas during the fall in Tb the efflux of NE, DOPAC and 5-HIAA from the AH/POA increased. When the Tb of the rat declined simultaneously with eating behavior, the levels in perfusate of DOPAC and HVA increased significantly while MHPG declined. During perfusion of the AH/POA with NPY the turnover of NE declined while DA and 5-HT turnover increased during hypothermia alone or when accompanied by feeding. These results demonstrate that the sustained elevation in NPY within the AH/POA causes a selective alteration in the activity of the neurotransmitters implicated in thermoregulation, satiety and hunger. These findings suggest that both DA and NE comprise intermediary factors facilitating the action of NPY on neurons involved in thermoregulatory and ingestive processes. The local activity of NPY on hypothalamic neurons apparently shifts the functional balance of serotonergic and catecholaminergic neurons now thought to play a primary role in the control of energy metabolism and caloric intake.

The role of alpha-1 adrenergic receptors in the stimulating effect of neuropeptide Y (NPY) on rat behavioural activity

The role of noradrenergic alpha-1 receptors in increasing the behavioural activity after neuropeptide Y (NPY) injection into the frontal cortex was examined in rats. NPY (1 micrograms/1 microliter) injected into the frontal cortex of rats with chronically implanted cannulae, increased their locomotor and exploratory activity in the open field test. Similar effects were observed after injection of the alpha-1-adrenergic agonist phenylephrine (PH) (3-19 micrograms/1 microliter). Behavioural stimulation after NPY or PH was totally abolished by pretreatment with benextramine (1.73-173 micrograms/1 microliter), a compound blocking NPY and alpha-adrenergic receptors. The NPY-induced activation of behaviour was also abolished by i.p. injection of prazosine (3 mg/kg), a specific alpha-1-adrenergic receptor antagonist. It is concluded that: 1) NPY injected into the rat frontal cortex induces an increase in the locomotor and exploratory activity of the animals; and 2) indirect activation of alpha-1-adrenergic receptor seems to play a crucial role in the observed behavioural effects.

Norepinephrine, dopamine, and 5-HT release from perfused hypothalamus of the rat during feeding induced by neuropeptide Y

In the unrestrained rat, the hyperphagic-like ingestion of food evoked by the sustained elevation of neuropeptide-Y (NPY) in the hypothalamus was correlated with the release and turnover of monoaminergic transmitters in this structure. A single guide tube was implanted stereotaxically in the perifornical region of the hypothalamus for localized push-pull perfusion of an artificial CSF vehicle or NPY1-36 in a concentration of 10, 50, or 100 ng/1.0 microliters. After the rat was fully satiated, a site reactive to NPY was perfused repeatedly at a rate of 20 microliters/min for 6.0 min with an interval of 6.0-12 min elapsing between each perfusion. Samples of perfusate were analyzed by HPLC with coulometric detection for DA, HVA, DOPAC, NE, MHPG, 5-HT, and 5-HIAA. Although control perfusions were without effect on feeding or monoamine activity, NPY evoked mean cumulative intakes of food of 14 +/- 2.4, 25.6 +/- 3.0 and 26.5 +/- 3.2 g in response to 10, 50, or 100 ng/microliter concentrations of NPY, respectively, over the 4.0-5.0 hr test interval. HPLC analyses showed that during feeding the release of both NE and DA was enhanced significantly. The turnover of both catecholamines likewise increased significantly as reflected by the elevated levels of MHPG, DOPAC and HVA. However, neither the basal efflux of 5-HT nor its turnover, as reflected by the output of 5-HIAA, was affected during feeding induced by NPY perfused in the hypothalamus. These results suggest that a sustained elevation of NPY in the hypothalamus causes a perturbation in the basal activity of NE and DA which are both implicated in the neuronal mechanism regulating normal eating behavior. Thus, these catecholamine neurotransmitters are envisaged to comprise an intermediary step in the functional role played by NPY in the hypothalamus in integrating the control of energy metabolism and caloric intake.