Role of the Y5 neuropeptide Y receptor in limbic seizures

KO's and organisation of peptidergic feeding behavior mechanisms

Feeding behavior results from complex interactions arising between numerous neuromediators, including classical neurotransmitters and neuropeptides present in hypothalamic networks. One way to unravel these complex mechanisms is to examine animal models with a deletion of genes coding for the different neuropeptides involved in the regulation of feeding. The aim of this review is to focus on feeding and body weight regulation in mice lacking neuropeptide Y (NPY), melanocortins (POMC), corticotropin-releasing hormone, melanin-concentrating hormone, or bombesin-like peptides respectively. The phenotypes, which relate to the deletion of gene coding for the peptides, rarely include changes in body weight and food intake, indicating therefore the existence of redundant mechanisms to compensate for the loss of the peptide. The phenotype is much more marked when the gene deletion is targeted towards the functioning of the peptidergic machinery, e.g. the receptors and especially the POMC and NPY receptors, as well as one subtype of bombesin receptor (BRS-3). These knockout models are also interesting when examining the role of environmental and social factors in the determination of feeding behavior. They have granted us better knowledge of all these integrated and complex mechanisms. Moreover, they are also valuable tools for pharmacological studies when specific antagonists are lacking. From the information obtained by the study of knockouts, it is possible to determine certain targets for selective drugs that could be efficient for the pharmacological treatment of obesity. However, at the present state of our knowledge, it seems necessary to target several peptides in order to get good results with weight loss. It will also be imperative to associate these multitherapies with changes in eating and behavioral habits, in order to obtain complete effectiveness and long-lasting results.

Neuropeptide Y Y5 receptor protein in the cortical/limbic system and brainstem of the rat: expression on gamma-aminobutyric acid and corticotropin-releasing hormone neurons

Neuropeptide Y displays diverse modes of action in the CNS including the modulation of cortical/limbic function. Some of these physiological actions have been at least partially attributed to actions of neuropeptide Y on the Y5 receptor subtype. We utilized an antibody raised against the Y5 receptor to characterize the distribution of this receptor subtype in the rat cortical/limbic system and brainstem. Y5-like immunoreactivity was located primarily in neuronal cell bodies and proximal dendritic processes throughout the brain. In the cortex, Y5 immunoreactivity was limited to a subpopulation of small gamma-aminobutyric-acid interneurons (approximately 15 microm diameter) scattered throughout all cortical levels. Double label immunofluorescence was also used to demonstrate that all of the Y5 immunoreactive neurons in the cortex displayed intense corticotropin releasing hormone immunoreactivity. The most intense Y5 immunoreactive staining in the hippocampus was located in the pyramidal cell layer of the small CA2 subregion and the fasciola cinerea, with lower levels of staining in the hilar region of the dentate gyrus and CA3 subregion of the pyramidal cell layer. Nearly all of the Y5 immunoreactive neurons in the hilar region of the hippocampus displayed gamma-aminobutyric-acid immunoreactivity. In the brainstem, Y5 immunoreactivity was most intense in the Edinger-Westphal nucleus, locus coeruleus and the mesencephalic trigeminal nucleus. The present study provides neuroanatomical evidence for the possible sites of action of the neuropeptide Y/Y5 receptor system in the control of cortical/limbic function. The presence of Y5 immunoreactivity on cell bodies and proximal dendritic processes in specific regions of the hippocampus suggests that this receptor functions to modulate postsynaptic activity. These data also suggest that the neuropeptide Y/Y5 system may play a role in the modulation of a specific population of GABAergic neurons in the cortex, namely those that contain corticotropin-releasing hormone. The location of the Y5 receptor immunoreactivity fits with the known physiological actions of neuropeptide Y and this receptor.

Neurobiological responses to ethanol in mutant mice lacking neuropeptide Y or the Y5 receptor

We have previously shown that voluntary ethanol consumption and resistance are inversely related to neuropeptide Y (NPY) levels in NPY-knockout (NPY -/-) and NPY-overexpressing mice. Here we report that NPY -/- mice on a mixed C57BL/6Jx129/SvEv background showed increased sensitivity to locomotor activation caused by intraperitoneal (ip) injection of 1.5 g/kg of ethanol, and were resistant to sedation caused by a 3.5-g/kg dose of ethanol. In contrast, NPY -/- mice on an inbred 129/SvEv background consumed the same amount of ethanol as wild-type (WT) controls at 3%, 6%, and 10% ethanol, but consumed significantly more of a 20% solution. They exhibited normal locomotor activation following a 1.5-g/kg injection of ethanol, and displayed normal sedation in response to 2.5 and 3.0 g/kg of ethanol, suggesting a genetic background effect. Y5 receptor knockout (Y5 -/-) mice on an inbred 129/SvEv background showed normal ethanol-induced locomotor activity and normal voluntary ethanol consumption, but displayed increased sleep time caused by 2.5 and 3.0 g/kg injection of ethanol. These data extend previous results by showing that NPY -/- mice of a mixed C57BL/6Jx129/SvEv background have increased sensitivity to the locomotor activation effect caused by a low dose of ethanol, and that expression of ethanol-related phenotypes are dependent on the genetic background of NPY -/- mice.

Plastic changes in neuropeptide Y receptor subtypes in experimental models of limbic seizures

PURPOSE

Neuropetide Y (NPY)-mediated neurotransmission in the hippocampus is altered by limbic seizures. The functional consequences of this change are still unresolved and clearly depend on the type of NPY receptors involved. NPY Y2 and Y1 receptors are increased on mossy fiber terminals and decreased on granule cell dendrites after seizures, respectively. We investigated (a) whether seizures modify the NPY Y5 receptors in the hippocampus, and (b) the effect of an agonist at Y2/Y5 receptors and antagonists at Y1 receptors on acute and chronic seizure susceptibility.

METHODS

Limbic seizures were induced in rats by electrical stimulation of the dorsal hippocampus, leading to stage 5 kindled seizures, or by intrahippocampal or systemic injections of kainic acid. Pentylentetrazol was administered to epileptic rats to assess their enhanced susceptibility to seizures. NPY Y5 receptor protein was measured in hippocampal homogenates using a specific polyclonal antibody and quantitative Western blotting.

RESULTS

Y5 receptors (57-kD band) were transiently decreased (23 to 35%) in all hippocampal subregions 2 and 7 days, but not 2.5 hours, after seizures induced by systemic kainic acid. A minor band of 51 kD was reduced significantly in CA3 and dentate gyrus, although it was increased in CA1, 30 days after seizures, suggesting long-term posttranslational changes in this protein. NPY Y5 receptors were increased by 200% in total homogenate from the stimulated hippocampus 2 days but not 30 days after fully kindled seizures. Intracerebral injections of NPY 13-36 (Y2/Y5 receptor agonist) or BIBP 3225 and BIBO 3304 (selective Y1 receptor antagonists) decreased seizure susceptibility in rats.

CONCLUSIONS

These results indicate that NPY Y5 receptors change after limbic seizures and suggest that NPY receptors may provide novel target(s) for the treatment of epilepsy.

Decreased levels of neuropeptide Y(5) receptor binding sites in two experimental models of epilepsy

It has been suggested that the anticonvulsant effects of neuropeptide Y (NPY) could be mediated by the activation of Y(2) and/or Y(5) receptors. NPY Y(1) receptor levels are known to decrease and Y(2) to increase in rat models of epilepsy. By using an autoradiographic approach, we investigated whether epilepsy models (kainic acid and kindling) are also associated with changes in Y(5) receptors. Compared with naive controls, [125I][Leu(31), Pro(34)]PYY/BIBP3226-insensitive (Y(5)) binding sites in the hippocampus (strata oriens and radiatum of CA3 and CA1) and in the neocortex (superficial layers) were unchanged in sham-stimulated rats, but reduced by approximately 50% in kindled rats (seven days after the last stimulus evokes seizure), and further reduced (to approximately -90%) 1h after a kindled seizure. Additionally, Y(5) receptor binding sites in the hippocampus and in the neocortex were unchanged 6h after kainic acid injection, but were highly reduced at 12 and 24h. No changes in Y(5) binding levels were found in the dentate gyrus and the pyramidal cell layer of the hippocampus. The present data suggest that changes in Y(5) receptor levels occur in epilepsy models. These changes may play a role in seizure expression and/or in the maintenance of kindling hyperexcitability.

Expression of a novel neuropeptide Y receptor subtype involved in food intake: an in situ hybridization study of Y5 mRNA distribution in rat brain

Our group has reported on the cloning of a novel rat neuropeptide Y (NPY) receptor involved in NPY-induced food intake, the Y5 receptor. The distribution in rat brain of the mRNA encoding this receptor has been determined by in situ hybridization histochemistry, using radiolabeled oligonucleotide probes. Control experiments were carried out in cell lines transfected with either rat Y1 or rat Y5 cDNAs. With the exception of the cerebellum, only the antisense probes yielded hybridization signal in rat brain tissue sections. A number of brain regions contained hybridization signals indicative of Y5 mRNA localization. Chief among these were various hypothalamic nuclei, including the medial preoptic nucleus, the supraoptic nucleus, the paraventricular nucleus, and the lateral hypothalamus. Other regions with substantial hybridization signals included the midline thalamus, parts of the amygdala and hippocampus, and some midbrain and brain-stem nuclei. In general a low density of Y5 mRNA was observed in most cortical structures, with the exception of the cingulate and retrosplenial cortices, each of which contained a moderate abundance of Y5 hybridization signal. The distribution of this receptor mRNA is consistent with a role for the Y5 receptor in food intake and also suggests involvement in other processes mediated by NPY.

Electroconvulsive stimuli enhance both neuropeptide Y receptor Y1 and Y2 messenger RNA expression and levels of binding in the rat hippocampus

Repeated electroconvulsive stimulations and other seizure modalities produce an increase in neuropeptide Y synthesis and local release in the rat hippocampus, and perhaps as a consequence, a change in the concentration of neuropeptide Y binding sites in the same region. The aim of the present study was to determine possible changes in the expression of neuropeptide Y receptor subtypes affected by repeated stimulations in the hippocampus. Rats were exposed to 14 daily stimulations, and the brains were removed 24h after the last stimulation. For in vitro receptor autoradiography and in situ hybridisation histochemistry, the brains were frozen, sectioned, and levels of neuropeptide Y binding sites and messenger RNA expressions were determined quantitatively on sections from the same animals. In order to determine the contribution of different neuropeptide Y receptor subtypes, serial sections were incubated with either 125I-labelled peptide YY alone or the same radio-labelled peptide mixed with an excess of a number of displacing compounds with affinity for either neuropeptide Y receptor subtype Y1, Y2, or both. Binding studies revealed that the majority of peptide YY binding sites was represented by Y2, and that electroconvulsive stimulations reduced the binding capacity or the concentration of this receptor. A prominent reduction of Y1-preferring binding sites was determined in the dentate gyrus, and to a lesser extent in the CA1 and CA3 regions. Similarly, the treatment produced a significant reduction of Y2-preferring binding sites in the CA1 and CA3 region, but not in the granular cell layer of the dentate gyrus. Using semi-quantitative in situ hybridization, Y1 receptor messenger RNA level in the granular cell layer of the dentate increased by the stimulations. In the same region, Y2 receptor messenger RNA was expressed in low to undetectable amounts, but after the repeated stimulations, this transcript was found in moderate to high levels. These data suggest that the neuropeptide Yergic system in the dentate gyrus and the pyramidal cell layer are affected by the treatment, and that this includes both Y1 and Y2 receptor subtypes. Because levels of messenger RNA and binding are distinctly regulated, the turnover of both Y1 and Y2 molecules is strongly increased under electroconvulsive stimulations, suggesting that the intrahippocampal neuropeptide Yergic neurotransmission is also increased under the stimulations.

Neurodegenerative and morphogenic changes in a mouse model of temporal lobe epilepsy do not depend on the expression of the calcium-binding proteins parvalbumin, calbindin, or calretinin

The functional role of the calcium-binding proteins parvalbumin, calretinin, and calbindin D-28k for epileptogenesis and long-term seizure-related alterations of the hippocampal formation was assessed in single- and double-knockout mice, using a kainate model of mesial temporal lobe epilepsy. The effects of a unilateral intrahippocampal injection of kainic acid were assessed at one day, 30 days, and four months post-injection, using various markers of GABAergic interneurons (GABA-transporter type 1, GABA(A)-receptor alpha1 subunit, calretinin, calbindin D-28k, somatostatin, and neuropeptide Y). Parvalbumin-deficient, parvalbumin/calbindin-deficient, and parvalbumin/calretinin-deficient mice exhibited no difference in cytoarchitecture of the hippocampal formation and in the number, distribution, or morphology of interneurons compared to wild-type mice. Likewise, mutant mice were not more vulnerable to acute kainate-induced excitotoxicity or to long-term effects of recurrent focal seizures, and exhibited the same pattern of neurochemical alterations (e.g., bilateral induction of neuropeptide Y in granule cells) and morphogenic changes (enlargement and dispersion of dentate gyrus granule cells) as wild-type animals. Quantification of interneurons revealed no significant difference in neuronal vulnerability among the genotypes.These results indicate that the calcium-binding proteins investigated here are not essential for determining the neurochemical phenotype of interneurons. Furthermore, they are not protective against kainate-induced excitotoxicity in this model, and do not appear to modulate the overall level of excitability of the hippocampus. Finally, seizure-induced changes in gene expression in granule cells, which normally express high levels of calcium-binding proteins, apparently were not affected by the gene deletions analysed.

Behavioral characterization of neuropeptide Y knockout mice

An extensive behavioral characterization was conducted with mice lacking the gene for neuropeptide Y (NPY) including response to 24 and 48 h fast and challenge with small molecule antagonists of NPY receptors implicated in mediating the feeding effects of NPY (i.e., Y1 and Y5). In addition, wildtype (WT) and NPY knockout (KO) mice were tested in locomotor monitors, elevated plus maze, inhibitory avoidance, acoustic startle, prepulse inhibition, and hot plate assays. One of the major findings was that the NPY KO mice have a reduced food intake relative to WT controls in response to fasting. Also, based on data from the behavioral models, the NPY KO mice may have an anxiogenic-like phenotype, and appear to be hypoalgesic in the hot plate paradigm. The data from these studies provide further evidence of involvement of NPY in energy balance, anxiety, and possibly nociception.