Neuropeptide Y (NPY) and peptide YY (PYY) receptors in rat brain

Microbiota-gut-brain axis as a regulator of reward processes

Our gut harbours trillions of microorganisms essential for the maintenance of homeostasis and host physiology in health and disease. In the last decade, there has been a growing interest in understanding the bidirectional pathway of communication between our microbiota and the central nervous system. With regard to reward processes there is accumulating evidence from both animal and human studies that this axis may be a key factor in gating reward valence. Focusing on the mesocorticolimbic pathway, we will discuss how the intestinal microbiota is involved in regulating brain reward functions, both in natural (i.e. eating, social or sexual behaviours) and non-natural reinforcers (drug addiction behaviours including those relevant to alcohol, psychostimulants, opioids and cannabinoids). We will integrate preclinical and clinical evidence suggesting that the microbiota-gut-brain axis could be implicated in the development of disorders associated with alterations in the reward system and how it may be targeted as a promising therapeutic strategy. Cover Image for this issue: https://doi.org/10.1111/jnc.15065.

Characterization of neuropeptide Y2 receptor protein expression in the mouse brain. I. Distribution in cell bodies and nerve terminals

Neuropeptide Y (NPY), a 36-amino-acid peptide, mediates biological effects by activating Y1, Y2, Y5, and y6 receptors. NPY neurons innervate many brain regions, including the hypothalamus, where NPY is involved in regulation of a broad range of homeostatic functions. We examined, by immunohistochemistry with tyramide signal amplification, the expression of the NPY Y2 receptor (Y2R) in the mouse brain with a newly developed rabbit polyclonal antibody. Y2R immunoreactivity was specific with its absence in Y2R knockout (KO) mice and in adjacent sections following preadsorption with the immunogenic peptide (10(-5) M). Y2R-positive processes were located in many brain regions, including the olfactory bulb, some cortical areas, septum, basal forebrain, nucleus accumbens, amygdala, hippocampus, hypothalamus, substantia nigra compacta, locus coeruleus, and solitary tract nucleus. However, colchicine treatment was needed to detect Y2R-like immunoreactivity in cell bodies in many, but not all, areas. The densest distributions of cell bodies were located in the septum basal forebrain, including the bed nucleus, and amygdala, with lower density in the anterior olfactory nucleus, nucleus accumbens, caudal striatum, CA1, CA2, and CA3 hippocampal fields, preoptic nuclei lateral hypothalamus, and A13 DA cells. The widespread distribution of Y2R-positive cell bodies and fibers suggests that NPY signaling through the Y2R is common in the mouse brain. Localization of the Y2R suggests that it is mostly presynaptic, a view supported by its frequent absence in cell bodies in the normal mouse and its dramatic increase in cell bodies of colchicine-treated mice.

Comparative distribution of neuropeptide Y Y1 and Y5 receptors in the rat brain by using immunohistochemistry

Neuropeptide Y (NPY) Y1 and Y5 receptor subtypes mediate many of NPY's diverse actions in the central nervous system. The present studies use polyclonal antibodies directed against the Y1 and Y5 receptors to map and compare the relative distribution of these NPY receptor subtypes within the rat brain. Antibody specificity was assessed by using Western analysis, preadsorption of the antibody with peptide, and preimmune serum controls. Immunostaining for the Y1 and Y5 receptor subtypes was present throughout the rostral-caudal aspect of the brain with many regions expressing both subtypes: cerebral cortex, hippocampus, hypothalamus, thalamus, amygdala, and brainstem. Further studies using double-label immunocytochemistry indicate that Y1R immunoreactivity (-ir) and Y5R-ir are colocalized in the cerebral cortex and caudate putamen. Y1 receptor ir was evident in the central amygdala, whereas both Y1- and Y5-immunoreactive cells and fibers were present in the basolateral amygdala. Corresponding with the physiology of NPY in the hypothalamus, both Y1R- and Y5R-ir was present within the paraventricular (PVN), supraoptic, arcuate nuclei, and lateral hypothalamus. In the PVN, Y5R-ir and Y1R-ir were detected in cells and fibers of the parvo- and magnocellular divisions. Intense immunostaining for these receptors was observed within the locus coeruleus, A1-5 and C1-3 nuclei, subnuclei of the trigeminal nerve and nucleus tractus solitarius. These data provide a detailed and comparative mapping of Y1 and Y5 receptor subtypes within cell bodies and nerve fibers in the brain which, together with physiological and electrophysiological studies, provide a better understanding of NPY neural circuitries.

Expression of the neuropeptide Y Y1 receptor in the CNS of rat and of wild-type and Y1 receptor knock-out mice. Focus on immunohistochemical localization

The distribution of neuropeptide Y (NPY) Y1 receptor-like immunoreactivity (Y1R-LI) has been studied in detail in the CNS of rat using a rabbit polyclonal antibody against the C-terminal 13 amino acids of the rat receptor protein. The indirect immunofluorescence technique with tyramide signal amplification has been employed. For specificity and comparative reasons Y1 knock-out mice and wild-type controls were analyzed. The distribution of Y1R mRNA was also studied using in situ hybridization. A limited comparison between Y1R-LI and NPY-LI was carried out.A widespread and abundant distribution of Y1R-LI, predominantly in processes but also in cell bodies, was observed. In fact, Y1R-LI was found in most regions of the CNS with a similar distribution pattern between rat and wild-type mouse. This staining was specific in the sense that it was absent in adjacent sections following preadsorption of the antibody with 10(-5) M of the antigenic peptide, and that it could not be observed in sections of the Y1 KO mouse. In contrast, the staining obtained with an N-terminally directed Y1R antiserum did not disappear, strongly suggesting unspecificity. In brief, very high levels of Y1R-LI were seen in the islands of Calleja, the anterior olfactory nucleus, the molecular layer of the dentate gyrus, parts of the habenula, the interpeduncular nucleus, the mammillary body, the spinal nucleus of the trigeminal, caudal part, the paratrigeminal nucleus, and superficial layers of the dorsal horn. High levels were found in most cortical areas, many thalamic nuclei, some subnuclei of the amygdaloid complex, the hypothalamus and the nucleus of the stria terminalis, the nucleus of the solitary tract, the parabrachial nucleus, and the inferior olive. Moderate levels of Y1R-LI were detected in the cornu Ammonis and the subicular complex, many septal, some thalamic and many brainstem regions. Y1R staining of processes, often fiber and/or dot-like, and occasional cell bodies was also seen in tracts, such as the lateral lemniscus, the rubrospinal tract and the spinal tract of the trigeminal. There was in general a good overlap between Y1R-LI and NPY-LI, but some exceptions were found. Thus, some areas had NPY innervation but apparently lacked Y1Rs, whereas in other regions Y1R-LI, but no or only few NPY-positive nerve endings could be detected. Our results demonstrate that NPY signalling through the Y1R is common in the rat (and mouse) CNS. Mostly the Y1R is postsynaptic but there are also presynaptic Y1Rs. Mostly there is a good match between NPY-releasing nerve endings and Y1Rs, but 'volume transmission' may be 'needed' in some regions. Finally, the importance of using proper control experiments for immunohistochemical studies on seven-transmembrane receptors is stressed.

Regional distribution of Y-receptor subtype mRNAs in rat brain

Molecular cloning techniques have recently led to the identification of a growing number of neuropeptide Y-receptor subtypes, suggesting possible subtype-specific involvement in different physiological processes. Here we report the first study which determines and compares the mRNA expression of all four cloned functional Y-receptor subtypes (Y1, Y2, Y4 and Y5) in consecutive sections of the rat brain on a cellular level, using a uniform in situ hybridization technique. Our results demonstrate that Y-receptor subtype mRNA expression is widely distributed throughout the rat brain. Interestingly, coexpression of all four Y-receptors, at different levels, is particularly evident within the limbic system, including the hypothalamus, hippocampus, amygdala, piriform and cingulate cortices and tegmental areas, all of which are heavily involved in behaviour, emotion and homeostatic regulation. Particularly interesting is the demonstration that Y5-receptor mRNA expression always coincides with the presence of Y1-receptor mRNA (although not vice versa), possibly due to the overlapping organization and transcriptional control of their genes. However, it is also clear that several brain nuclei display preferential expression of one or a selective combination of Y-receptor subtype mRNAs. Furthermore, it is evident that there is regionalization of expression within certain loci which express all four receptor subtype mRNAs, particularly within the paraventricular and arcuate hypothalamic nuclei. Our results suggest that some of neuropeptide Y's (NPY) effects may be mediated through one particular subtype, whereas other physiological processes might require the coordinated action of different subtypes within the same or discrete areas.

The effect of hypothalamic peptide YY on hippocampal acetylcholine release in vivo: implications for limbic function in binge-eating behavior

Central injection of peptide YY (PYY) in sated rats produces the most powerful stimulating effect of food intake known to date. The neural mechanisms by which PYY regulates appetite are not clear but may be important because abnormal levels of PYY have been implicated in the neurobiology of bulimia nervosa. Interactions between brain acetylcholine (ACh) and PYY had not been studied. Therefore, the present experiments were designed to explore the in vivo release of ACh from the hippocampus (HPC) of rats in response to hypothalamic infusion of PYY. Hippocampal ACh release was found to increase 400% in response to 10 microg PYY. In a separate experiment, blockade of the same area of the HPC with bilateral intracerebral injections of 3.5 microg scopolamine did not affect intake stimulated by intrahypothalamic injection of 4 microg PYY. Furthermore, a third experiment showed, for the first time, that PYY (2.5-10.0 microg) can elicit robust feeding when infused directly into the HPC. The significance of these findings to the activation of limbic functions such as memory, reinforcement, and obsessional processes that accompany human binge-eating syndromes is discussed.

Neuropeptide Y and limbic seizures

Since its discovery in 1982, neuropeptide Y (NPY), a 36 amino-acid member of the pancreatic polypeptide family, has received considerable attention in the field of neuroscience. Originally isolated from porcine brain /86/, NPY is one of the most abundant and widely distributed peptides in the central nervous system. In the brain, NPY is present in the hypothalamus, limbic structures, cerebral cortex, brainstem and striatum /2,71/. Because of the widespread distribution of NPY, it has been implicated in the modulation of a variety of behaviors, including, but not limited to, circadian rhythms /1/, memory retention /33/, feeding /19,56/, sympathetic control of cardiovascular function /89/ and anxiety /42,43/. These functions have been reviewed elsewhere and will not be discussed in great detail here. The present review is intended to provide an overview of recent work implicating a role for NPY in limbic seizures.

The ventral lateral geniculate nucleus and the intergeniculate leaflet: interrelated structures in the visual and circadian systems

The ventral lateral geniculate nucleus (vLGN) and the intergeniculate leaflet (IGL) are retinorecipient subcortical nuclei. This paper attempts a comprehensive summary of research on these thalamic areas, drawing on anatomical, electrophysiological, and behavioral studies. From the current perspective, the vLGN and IGL appear closely linked, in that they share many neurochemicals, projections, and physiological properties. Neurochemicals commonly reported in the vLGN and IGL are neuropeptide Y, GABA, enkephalin, and nitric oxide synthase (localized in cells) and serotonin, acetylcholine, histamine, dopamine and noradrenalin (localized in fibers). Afferent and efferent connections are also similar, with both areas commonly receiving input from the retina, locus coreuleus, and raphe, having reciprocal connections with superior colliculus, pretectum and hypothalamus, and also showing connections to zona incerta, accessory optic system, pons, the contralateral vLGN/IGL, and other thalamic nuclei. Physiological studies indicate species differences, with spectral-sensitive responses common in some species, and varying populations of motion-sensitive units or units linked to optokinetic stimulation. A high percentage of IGL neurons show light intensity-coding responses. Behavioral studies suggest that the vLGN and IGL play a major role in mediating non-photic phase shifts of circadian rhythms, largely via neuropeptide Y, but may also play a role in photic phase shifts and in photoperiodic responses. The vLGN and IGL may participate in two major functional systems, those controlling visuomotor responses and those controlling circadian rhythms. Future research should be directed toward further integration of these diverse findings.

Characterization of neuropeptide Y receptor subtypes in the normal human brain, including the hypothalamus

The aim of the present study was to investigate the existence and distribution of neuropeptide Y receptor subtypes in various regions of the normal human brain using the peptide YY derivative receptor probes, [125I][Leu31,Pro34]polypeptide YY/Y1 and [125I]polypeptide YY(3-36)/Y2, in addition to the non-selective ligand [125I]polypeptide YY. Membrane binding assays performed with post mortem frontal cortex homogenates revealed that [125I]polypeptide YY and [125I]polypeptide YY(3-36) bound in a time- and protein concentration-dependent manner. Very low amounts of specific [125I][Leu31,Pro34]polypeptide YY binding could be detected even in the presence of high amounts of protein, contrasting with results obtained with [125I]polypeptide YY and [125I]polypeptide YY(3-36), a preferential Y2 receptor probe. Analysis of saturation isotherms revealed that [125I]polypeptide YY(3-36) bound to a single class of high-affinity sites (0.5-2 nM). Significantly higher binding capacities were evident for [125I]polypeptide YY(3-36) as compared to [125I][Leu31,Pro34]polypeptide YY, suggesting that the human frontal cortex, in contrast to the rat, is mostly enriched with Y2 receptors. Ligand selectivity profile confirmed the hypothesis that polypeptide YY(3-36), neuropeptide Y and polypeptide YY but not the [Leu31,Pro34] derivatives are potent competitors of [125I]polypeptide YY and [125I]polypeptide YY(3-36) binding sites. Autoradiographic studies demonstrated further that cortical areas, as well as most other regions of the human brain, are particularly enriched with Y2/[125I]polypeptide YY(3-36) sites, while only low to very low amounts of Y1 binding were detected except in the dentate gyrus of the hippocampal formation. In the human hypothalamus, a preponderance of Y2 binding sites was also noted. Taken together, these results clearly establish that the distribution of the Y1 and Y2 receptor subtypes in human is different from the rodent brain, the Y2 subtype being most abundant in the human brain.

Autoradiographic localization of receptors for neuropeptide Y (NPY) in the brain of broiler and leghorn chickens (Gallus domesticus)

Broiler and leghorn chickens show an extreme difference in ingestive and reproductive behavior. As neuropeptide Y (NPY) influences both behaviors the goal of this study was to elucidate the distribution, expression and affinity of NPY binding sites in broiler and leghorn chicken brain. By means of in vitro autoradiography, sections of chicken brains were incubated with 3H-NPY as tracer and NPY as displacer. Scatchard analysis revealed a curvilinear plot suggesting two subtypes of the NPY binding site in the chicken brain, a high affinity one (KD = 2-4 nM) and one with a lower affinity (KD = 18-24 nM). Binding sites for NPY are localized with high density in the different subdivisions of the neostriatum and the hyperstriatum, the cerebellum, the nucleus septalis lateralis and medialis, the nucleus ruber and the nucleus tractus solitarii. A lower density of NPY binding sites was found in the different subdivisions of the striatum, the nucleus mesencephalicus lateralis pars dorsalis, the paleostriatum, the archistriatum intermedium pars ventralis, the nucleus geniculatus lateralis, the nucleus taeniae, the locus ceruleus, the nucleus rotondus, the nucleus habenularis medialis, the nucleus dorsomedialis anterior (rostralis) thalami, the pituitary and the area of the hypothalamus with its nuclei such as the nucleus paraventricularis magnocellularis and the nucleus preopticus medialis. Comparison of the localization of NPY binding sites in the brains of broilers and leghorns showed no differences but the density of both receptor types is two to three times higher in broilers than in leghorns.

Neuropeptide tyrosine is expressed in ensheathing cells around the olfactory nerves in the rat olfactory bulb

The olfactory bulbs of young and adult normal rats and of colchicine-treated rats and of some other species were analysed for the presence of neuropeptide Y and neuropeptide Y messenger RNA, using immunohistochemistry at the light- and electron-microscopic levels and with in situ hybridization. In the rat and mouse, but not in monkey and guinea-pig, neuropeptide Y-like immunoreactivity and neuropeptide Y messenger RNA were observed in ensheathing cells in the olfactory nerve layer of the olfactory bulb and within nerve bundles in the olfactory mucosa. Double staining experiments revealed that neuropeptide Y-like immunoreactivity was often present in a restricted compartment, mainly the Golgi apparatus, of S-100 protein-positive ensheathing cells. After colchicine treatment a different distribution of neuropeptide Y-like immunoreactivity and neuropeptide Y messenger RNA was observed. Thus, in the outer olfactory nerve layer both neuropeptide Y-like immunoreactivity and neuropeptide Y messenger RNA disappeared, whereas in the inner part messenger RNA levels remained high and neuropeptide Y-like immunoreactivity was observed in many granule-like structures distributed diffusely in the cytoplasm. The present findings suggest that neuropeptide Y may be involved in the control of regeneration, growth and/or guiding of the axons of the olfactory sensory neurons, the only mammalian neurons known to have a continuous renewal and growth during adult life.

Benextramine-neuropeptide Y (NPY) binding site interactions: characterization of 3H-NPY binding site heterogeneity in rat brain

Pre-incubation of rat brain membranes with 200 microM benextramine followed by extensive dilution and washing to remove unbound ligand reduced Bmax for N-[propionyl-3H]-NPY (3H-NPY) specific binding by 61% relative to control membranes treated identically but in the absence of benextramine. When rat brain membranes were co-incubated with 3H-NPY and 57 microM benextramine, there was a significant shift to the right; the apparent Kd for 3H-NPY binding increased two-fold relative to control membranes. These data are consistent with the hypothesis that benextramine is a competitive and irreversible ligand for a population (60-65%) of rat brain NPY binding sites. 'Paired tube' assays were then used to determine the selectivity of these benextramine-sensitive and insensitive 3H-NPY binding site populations. PYY, NPY and NPY13-36 each displaced 100% of 3H-NPY from rat brain membrane binding sites both in the absence and presence of 1 mM benextramine. In contrast, [Leu31,Pro34]NPY displayed the same binding site selectivity as benextramine in displacing 65% of 3H-NPY from specific binding sites on untreated rat brain membranes, and it failed to displace 3H-NPY from membranes treated with 1 mM benextramine. Thus the selectivity of the benextramine-insensitive 3H-NPY binding site population--PYY > = NPY > NPY13-36 >> [Leu31,Pro34]NPY--is characteristic of a Y2-like NPY binding site population, while the benextramine-sensitive 3H-NPY binding sites appear to be a Y1-like binding site population.

[Leu31-Pro34] neuropeptide Y identifies a subtype of 125I-labeled peptide YY binding sites in the rat brain

Subtypes of the neuropeptide Y (NPY) receptor in the rat brain were identified by the use of the selective Y-1 analog, [Leu34-Pro34] NPY. In rat brain homogenate binding studies, [Leu31-Pro34] NPY was found to produce a partial inhibition of 100 pM 125I-labeled peptide YY (PYY) binding with a plateau at 50-1000 nM [Leu31-Pro34] NPY resulting in a 70% inhibition of binding. The C-terminal fragment NPY 13-36, a putative Y-2 agonist, exhibited very little selectivity in rat brain homogenates. Scatchard analysis of 125I-labeled PYY binding to rat brain homogenate yielded biphasic plots with Kd values of 40 and 610 pM. Inclusion of 100 nM [Leu31-Pro34] NPY was found to eliminate the low affinity component of 125I-labeled PYY binding leaving a single, high affinity binding site with a Kd of 68 pM. In autoradiographic studies, displacement curves indicated that [Leu31-Pro34] NPY completely inhibited binding in the cerebral cortex with little effect on the binding in the hypothalamus. On the other hand NPY 13-36 inhibited binding in the hypothalamus at low concentrations but required higher concentrations to inhibit binding in the cerebral cortex. Other brain regions such as the hippocampus, appeared to contain both subtypes. Subsequent to these studies, a quantitative autoradiographic map was conducted using 50-100 pM 125I-labeled PYY in the presence and absence of [Leu31-Pro34] NPY which produced a selective displacement of binding in certain distinct brain regions. These areas included the cerebral cortex, certain thalamic nuclei and brainstem while ligand binding was retained in other brain regions including the zona lateralis of the substantia nigra, lateral septum, nucleus of the solitary tract and the hippocampus. Numerous brain regions appeared to contain both receptor subtypes. Therefore, the Y-1 and Y-2 receptor subtypes exhibited a somewhat distinct distribution in the brain. In addition, 125I-labeled PYY appears to label the Y-2 receptor with relatively higher affinity when compared to the Y-1 receptor.

Specific binding sites for 125I-endothelin-1 in the porcine and human spinal cord

Specific binding sites for 125I-endothelin-1 (125I-ET-1) in the spinal cord were investigated using quantitative receptor autoradiographic and chemical cross-linking methods. The binding sites were highly concentrated in porcine and human spinal cord areas corresponding anatomically to the dorsal horn (Rexed's laminae I-III), an area around the central canal (lamina X) and the principal part of the intermediolateral nucleus (IMLp). The localization of the binding sites differed from those of 125I-omega-conotoxin GVIA (125I-CgTx) and 125I-Bolton-Hunter substance P (125I-BH-SP), with the exception that the IMLp shared 125I-ET-1 with 125I-CgTx and 125I-BH-SP binding sites. Specific 125I-ET-1 binding sites in the areas examined were characteristically single and of high affinity. There were no differences between the potencies of unlabeled ET family peptides, ET-1, ET-2, ET-3 and sarafotoxin S6b at inhibiting 125I-ET-1 binding to the areas. Chemical cross-linking studies showed that 125I-ET-1 and 125I-ET-3 mainly bound to a protein with molecular mass of 43 kDa in the porcine and human thoracic spinal cord membranes. The present finding shows the neuronal significance of this newly discovered peptide in the spinal cord.

Receptor-selective analogs demonstrate NPY/PYY receptor heterogeneity in rat brain

Neuropeptide Y (NPY) receptors are heterogeneous, consisting of at least two subclasses, Y1 and Y2. We sought evidence for differential expression of NPY receptor subtypes in the rat brain. Tissue was incubated with 125I-peptide YY (PYY) which labels NPY and PYY binding sites. The Y1-selective agonist, p[Pro34]NPY, and the Y2-selective agonist, pNPY 13-36, were used as displacing ligands. Autoradiographic analyses of regional receptor binding demonstrated heterogeneity across brain regions. We conclude that Y1- and Y2-receptors may be independently expressed in the brain. While the predominate NPY/PYY receptor subtype in the brain is Y2, there are also Y1-receptors in some brain regions such as the superficial layers of the parietal cortex.