Interactions between neuropeptide Y and alpha 2-adrenoceptors in selective rat brain regions

Enhanced inhibitory control by neuropeptide Y Y5 receptor blockade in rats

RATIONALE

The neuropeptide Y (NPY) system acts in synergy with the classic neurotransmitters to regulate a large variety of functions including autonomic, affective, and cognitive processes. Research on the effects of NPY in the central nervous system has focused on food intake control and affective processes, but growing evidence of NPY involvement in attention-deficit/hyperactivity disorder (ADHD) and other psychiatric conditions motivated the present study.

OBJECTIVES

We tested the effects of the novel and highly selective NPY Y5 receptor antagonist Lu AE00654 on impulsivity and the underlying cortico-striatal circuitry in rats to further explore the possible involvement of the NPY system in pathologies characterized by inattention and impulsive behavior.

RESULTS

A low dose of Lu AE00654 (0.03 mg/kg) selectively facilitated response inhibition as measured by the stop-signal task, whereas no effects were found at higher doses (0.3 and 3 mg/kg). Systemic administration of Lu AE00654 also enhanced the inhibitory influence of the dorsal frontal cortex on neurons in the caudate-putamen, this fronto-striatal circuitry being implicated in the executive control of behavior. Finally, by locally injecting a Y5 agonist, we observed reciprocal activation between dorsal frontal cortex and caudate-putamen neurons. Importantly, the effects of the Y5 agonist were attenuated by pretreatment with Lu AE00654, confirming the presence of Y5 binding sites modulating functional interactions within frontal-subcortical circuits.

CONCLUSIONS

These results suggest that the NPY system modulates inhibitory neurotransmission in brain areas important for impulse control, and may be relevant for the treatment of pathologies such as ADHD and drug abuse.

Neuropeptide Y, alpha-melanocyte-stimulating hormone, and monoamines in food intake regulation

Obesity is increasing in severity and prevalence in the United States and represents a major public health issue. No effective pharmacologic treatment leading to sustained weight loss currently exists. The growing interest in the regulation of food intake stems from the current drug treatments for obesity, almost all of which interfere with the monoamine system. Our knowledge of potential interactions between the orexigenic and anorexigenic pathways is limited and fragmented, making the development of targeted drug therapy for obesity difficult. The present review of the interaction of neuropeptides and monoamines emphasizes the complexity of the central mechanisms that regulate feeding behavior. Two main systems are implicated in food intake regulation: neuropeptide Y (NPY) and pro-opiomelanocortin. alpha-Melanocyte-stimulating hormone is a tridecapeptide cleaved from pro-opiomelanocortin that acts to inhibit food intake. The predominant NPY orexigenic receptors are NPY-Y1 and NPY-Y5, and the two anorexigenic melanocortin receptors involved in hypothalamic food intake control are MC3-R and MC4-R. Both neuropeptides interact with monoamines in the hypothalamus to control physiologic states such as hunger, satiation, and satiety. Serotonin suppresses food intake and body weight, acting mainly through the serotonin 1B receptor. Dopamine regulates hunger and satiety by acting in specific hypothalamic areas, through the D1 and D2 receptors. Noradrenaline activation of alpha1- and beta2-adrenoceptors decreases food intake, and stimulation of the alpha2-adrenoceptor increases food intake. A better understanding of the detailed mechanisms underlying the pathogenesis of hyperphagia and hypophagia is needed to develop new therapeutic approaches to obesity.

Neuropeptide Y and [Leu31,Pro34]neuropeptide Y potentiate potassium-induced noradrenaline release in the paraventricular nucleus of the aged rat

This microdialysis study investigated the effects of NPY and the Y1 selective agonist [Leu31, Pro34]NPY on basal and potassium-stimulated noradrenaline release in the PVN of 18-month-old anaesthetised male Sprague-Dawley rats. Microdialysate noradrenaline, DOPAC and HVA concentrations were measured by HPLC after i.c.v. administration of 2 nmol NPY, [Leu31, Pro34]NPY or vehicle. [Leu31, Pro34]NPY produced a significant 40% reduction in basal noradrenaline concentration (P < 0.05). Aged rats had blunted noradrenaline response to potassium stimulation, however stimulated noradrenaline release was similar in 18-month-old NPY-treated animals and 3-month-old saline treated age controls (2.8 and 3.2 times resting, respectively). [Leu31, Pro34]NPY induced a significantly greater release of noradrenaline in response to KC1 (5.0 times resting, P < 0.05). Thus, in 18-month-old animals with reduced endogenous hypothalamic NPY content, administration of NPY or [Leu31, Pro34]NPY increased potassium-induced noradrenaline release to levels seen in 3-month-old rats. This effect may be mediated by an NPY Y1 receptor.

Alterations in cyclic AMP generation and G protein subunits following transient ischemia in gerbil hippocampus

We examined alterations in the cyclic AMP generating system and G protein subunits in gerbil hippocampus following 10 min of transient ischemia. In hippocampal slices, basal and isoproterenol- and forskolin-stimulated cyclic AMP accumulations were markedly increased at 6 and 24 h after ischemia. Interestingly, both the inhibition of forskolin-stimulated cyclic AMP and the potentiation of beta-adrenoceptor-stimulated cyclic AMP by a gamma-aminobutyric acidB receptor agonist were attenuated at these time points. Ischemia did not affect the immunolabeling of any of the G protein alpha subunits; only that of beta subunits was significantly decreased, by 28.2%, 4 days after ischemia. In contrast, pertussis toxin-catalyzed [32P]ADP ribosylation declined progressively during the late recirculation period, reaching a significant reduction (25.4%) at 6 h after ischemia. These results suggest that ischemia affects the heterotrimeric conformation (alpha beta gamma) of Gi/Go during the recirculation period, thereby leading to increased cyclic AMP production. Because cyclic AMP-dependent protein kinase A modulates the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-kainate receptor channels, postischemic sensitization of the cyclic AMP generating system may contribute to neuronal degeneration in the hippocampus.

Neuropeptide Y potentiation of potassium-induced noradrenaline release in the hypothalamic paraventricular nucleus of the rat in vivo

NPY is co-localised with catecholamines in the brain and periphery. Noradrenaline and NPY are present in high concentrations in the PVN of the hypothalamus, an area implicated in autonomic regulation. This microdialysis study examined whether NPY can modulate rat PVN noradrenaline release in vivo, as has been shown in vitro. Basal and K(+)-stimulated noradrenaline release was measured after i.c.v. administration of 2 nmol NPY or vehicle. No effect of NPY was observed on basal release, however a significant doubling of K(+)-induced release was observed, both 60 and 150 min following i.c.v. NPY. This raises the possibility that NPY may potentiate rather than inhibit brain noradrenaline release in vivo.

CNS effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides, 1986-1993

The centrally mediated effects of peptides as published in the journal Peptides from 1986 to 1993 are tabulated in two ways. In one table, the peptides are listed alphabetically. In another table, the effects are arranged alphabetically. Most of the effects observed after administration of peptides are grouped, wherever possible, into categories such as cardiovascular and gastrointestinal. The species used in most cases has been rats; where other animals were used, the species is noted. The route of administration of peptides and source of information also are included in the tables, with a complete listing provided at the end. Many peptides have been shown to exert a large number of centrally mediated effects.

Regional distribution of putative NPY Y1 receptors and neurons expressing Y1 mRNA in forebrain areas of the rat central nervous system

Using monoiodinated radioligands of peptide YY (PYY), and the recently introduced neuropeptide Y (NPY) analogue [Leu31, Pro34]NPY, receptor binding sites of the Y1 and Y2 type were localized in the rat brain by quantitative in vitro autoradiography. The binding specificity and affinity of both radiolabelled ligands were analysed by ligand binding studies employing rat brain membrane homogenates from cerebral cortex and hippocampus. Using in situ hybridization histochemistry, the regional distribution and cellular localization of mRNA encoding the Y1 receptor were investigated in rat brain sections and compared to the distribution of Y1-specific binding sites. PYY binds to both Y1 and Y2 receptors, while long C-terminal fragments such as NPY13-36 and NPY-16-36 bind preferentially to Y2 receptors. [Leu31,Pro34]NPY is a specific agonist for Y1 receptors. Highest densities of [125I]PYY binding sites were found in the cerebral cortex, the thalamus, the lateral septum, the hippocampus and the mesencephalic dopaminergic areas. In order to block putative Y2 receptors, a series of [125I]PYY binding experiments was performed in the presence of NPY13-36 (1 microM), a Y2 preferring C-terminal fragment. High densities of binding sites remained present in the cerebral cortex, the thalamus and the medial mammillary nucleus when NPY13-36 was present in the incubation medium. Furthermore, these areas were highly enriched with [125I][Leu31,Pro34]NPY binding sites. In contrast, the hippocampal complex had its binding capacity reduced by approximately 50%, while the lateral septum and mesencephalic dopaminergic areas had their binding capacities reduced even further. Linear regression analysis showed a high degree of correspondence between [125I][Leu31,Pro34]NPY binding and that obtained with [125I]PYY in the presence of 1 microM NPY13-36, suggesting that the two independent approaches to visualizing Y1 binding sites are comparable. In situ hybridization histochemistry revealed high levels of Y1 mRNA in the granular cell layer of the hippocampal dentate gyrus, several thalamic nuclei and the hypothalamic arcuate nucleus. Moderate levels of Y1 mRNA were seen in the frontoparietal cortex, several thalamic nuclei, the hippocampal pyramidal layers, the subiculum, the olfactory tubercle, the claustrum and a number of hypothalamic nuclei. The mesencephalon, the amygdala and most basal ganglia showed very low levels of hybridization. The present study further clarifies the anatomical distribution of multiple NPY binding sites within the central nervous system of the rat, and extends earlier suggestions that Y1 and Y2 receptor types are present within the central nervous system.

Neuroendocrine regulation of thyrotropin-releasing hormone (TRH) in the tuberoinfundibular system

[...] It is now required to list each part needed for mucous excretion. They are two ducts in the brain substance, then a thin portion of membrane shaped as the infundibulum, then the gland that receives the tip of this infundibulum and the ducts that drive the mucus (pituita) from this gland to the palate and nares. [...] and I said that one (duct) [...] from the middle of the common cavity (third ventricle) descends [...] into the brain substance, and the end of this duct is [...] the sinus of the gland where the brain mucus is collected [...].