Y1 receptors in the nucleus accumbens: ultrastructural localization and association with neuropeptide Y

Hedonic eating, obesity, and addiction result from increased neuropeptide Y in the nucleus accumbens during human brain evolution

The nucleus accumbens (NAc) is central to motivation and action, exhibiting one of the highest densities of neuropeptide Y (NPY) in the brain. Within the NAc, NPY plays a role in reward and is involved in emotional behavior and in increasing alcohol and drug addiction and fat intake. Here, we examined NPY innervation and neurons of the NAc in humans and other anthropoid primates in order to determine whether there are differences among these various species that would correspond to behavioral or life history variables. We quantified NPY-immunoreactive axons and neurons in the NAc of 13 primate species, including humans, great apes, and monkeys. Our data show that the human brain is unique among primates in having denser NPY innervation within the NAc, as measured by axon length density to neuron density, even after accounting for brain size. Combined with our previous finding of increased dopaminergic innervation in the same region, our results suggest that the neurochemical profile of the human NAc appears to have rendered our species uniquely susceptible to neurophysiological conditions such as addiction. The increase in NPY specific to the NAc may represent an adaptation that favors fat intake and contributes to an increased vulnerability to eating disorders, obesity, as well as alcohol and drug dependence. Along with our findings for dopamine, these deeply rooted structural attributes of the human brain are likely to have emerged early in the human clade, laying the groundwork for later brain expansion and the development of cognitive and behavioral specializations.

Neuropeptides Modulate Feeding via the Dopamine Reward Pathway

Dopamine (DA) is a catecholamine neurotransmitter widely distributed in the central nervous system. It participates in various physiological functions, such as feeding, anxiety, fear, sleeping and arousal. The regulation of feeding is exceptionally complex, involving energy homeostasis and reward motivation. The reward system comprises the ventral tegmental area (VTA), nucleus accumbens (NAc), hypothalamus, and limbic system. This paper illustrates the detailed mechanisms of eight typical orexigenic and anorexic neuropeptides that regulate food intake through the reward system. According to recent literature, neuropeptides released from the hypothalamus and other brain regions regulate reward feeding predominantly through dopaminergic neurons projecting from the VTA to the NAc. In addition, their effect on the dopaminergic system is mediated by the prefrontal cortex, paraventricular thalamus, laterodorsal tegmental area, amygdala, and complex neural circuits. Research on neuropeptides involved in reward feeding can help identify more targets to treat diseases with metabolic disorders, such as obesity.

Neuropeptide Y modulates excitatory synaptic transmission and promotes social behavior in the mouse nucleus accumbens

Social interactions define the human experience, but these integral behaviors are disrupted in many psychiatric disorders. Social behaviors have evolved over millennia, and neuromodulatory systems that promote social behavior in invertebrates are also present in mammalian brains. One such conserved neuromodulator, neuropeptide Y (NPY), acts through several receptors including the Y1r, Y2r, and Y5r. These receptors are present in brain regions that control social behavior, including the nucleus accumbens (NAc). However, whether NPY modulates NAc neurotransmission is unknown. Using whole-cell patch-clamp electrophysiology of NAc neurons, we find that multiple NPY receptors regulate excitatory synaptic transmission in a cell-type specific manner. At excitatory synapses onto D1+ MSNs, Y1r activity enhances transmission while Y2r suppresses transmission. At excitatory synapses onto D1- MSNs, Y5r activity enhances transmission while Y2r suppresses transmission. Directly infusing NPY or the Y1r agonist [Leu³¹, Pro³⁴]-NPY into the NAc significantly increases social interaction with an unfamiliar conspecific. Inhibition of an enzyme that breaks down NPY, dipeptidyl peptidase IV (DPP-IV), shifts the effect of NPY on D1+ MSNs to a Y1r dominated phenotype. Together, these results increase our understanding of how NPY regulates neurotransmission in the NAc and identify a novel mechanism underlying the control of social behavior. Further, they reveal a potential strategy to shift NPY signaling for therapeutic gain.

The Role of Neuropeptide Y in the Nucleus Accumbens

Neuropeptide Y (NPY), an abundant peptide in the central nervous system, is expressed in neurons of various regions throughout the brain. The physiological and behavioral effects of NPY are mainly mediated through Y1, Y2, and Y5 receptor subtypes, which are expressed in regions regulating food intake, fear and anxiety, learning and memory, depression, and posttraumatic stress. In particular, the nucleus accumbens (NAc) has one of the highest NPY concentrations in the brain. In this review, we summarize the role of NPY in the NAc. NPY is expressed principally in medium-sized aspiny neurons, and numerous NPY immunoreactive fibers are observed in the NAc. Alterations in NPY expression under certain conditions through intra-NAc injections of NPY or receptor agonists/antagonists revealed NPY to be involved in the characteristic functions of the NAc, such as alcohol intake and drug addiction. In addition, control of mesolimbic dopaminergic release via NPY receptors may take part in these functions. NPY in the NAc also participates in fat intake and emotional behavior. Accumbal NPY neurons and fibers may exert physiological and pathophysiological actions partly through neuroendocrine mechanisms and the autonomic nervous system.

NPY and Gene Therapy for Epilepsy: How, When,... and Y

Neuropeptide Y (NPY) is a neuropeptide abundantly expressed in the mammalian central and peripheral nervous system. NPY is a pleiotropic molecule, which influences cell proliferation, cardiovascular and metabolic function, pain and neuronal excitability. In the central nervous system, NPY acts as a neuromodulator, affecting pathways that range from cellular (excitability, neurogenesis) to circuit level (food intake, stress response, pain perception). NPY has a broad repertoire of receptor subtypes, each activating specific signaling pathways in different tissues and cellular sub-regions. In the context of epilepsy, NPY is thought to act as an endogenous anticonvulsant that performs its action through Y2 and Y5 receptors. In fact, its overexpression in the brain with the aid of viral vectors can suppress seizures in animal models of epilepsy. Therefore, NPY-based gene therapy may represent a novel approach for the treatment of epilepsy patients, particularly for pharmaco-resistant and genetic forms of the disease. Nonetheless, considering all the aforementioned aspects of NPY signaling, the study of possible NPY applications as a therapeutic molecule is not devoid of critical aspects. The present review will summarize data related to NPY biology, focusing on its anti-epileptic effects, with a critical appraisal of key elements that could be exploited to improve the already existing NPY-based gene therapy approaches for epilepsy.

Actions of Neuropeptide Y on Synaptic Transmission in the Lateral Habenula

Neuropeptide Y is a peptide neuromodulator with protective roles including anxiolytic and antidepressant-like effects in animal models of depression and post-traumatic stress disorder. The lateral habenula (LHb) is a brain region that encodes aversive information and is closely related with mood disorders. Although LHb neurons express NPY receptors, the physiological roles of NPY in this region remain uninvestigated. In this study, we examined the actions of NPY on synaptic transmission in the LHb using whole cell patch clamp recording. We observed that NPY inhibited excitatory neurotransmission in a subset of LHb neurons whereas potentiating in a small population of neurons. Inhibitory transmission remained unchanged by NPY application in a subset of neurons but was reduced in the majority of LHb neurons recorded. The overall outcome of NPY application was a decrease in the spontaneous firing rate of the LHb, leading to hypoactivation of the LHb. Our observations indicate that although NPY has divergent effects on excitatory and inhibitory transmission, NPY receptor activation decreases LHb activity, suggesting that the LHb may partly mediate the protective roles of NPY in the central nervous system.

Neuropeptide Y activity in the nucleus accumbens modulates feeding behavior and neuronal activity

BACKGROUND

Neuropeptide Y (NPY) is a hypothalamic neuropeptide that plays a prominent role in feeding and energy homeostasis. Expression of the NPY Y1 receptor (Y1R) is highly concentrated in the nucleus accumbens (Acb), a region important in the regulation of palatable feeding. In this study, we performed a number of experiments to investigate the actions of NPY in the Acb.

METHODS

First, we determined caloric intake and food choice after bilateral administration of NPY in the Acb in rats on a free-choice diet of saturated fat, 30% sucrose solution, and standard chow and whether this was mediated by the Y1R. Second, we measured the effect of intra-Acb NPY on neuronal activity using in vivo electrophysiology. Third, we examined co-localization of Y1R with enkephalin and dynorphin neurons and the effect of NPY on preproenkephalin messenger RNA levels in the striatum using fluorescent and radioactive in situ hybridization. Finally, using retrograde tracing, we examined whether NPY neurons in the arcuate nucleus projected to the Acb.

RESULTS

In rats on the free-choice, high-fat, high-sugar diet, intra-Acb NPY increased intake of fat, but not sugar or chow, and this was mediated by the Y1R. Intra-Acb NPY reduced neuronal firing, as well as preproenkephalin messenger RNA expression in the striatum. Moreover, Acb enkephalin neurons expressed Y1R and arcuate nucleus NPY neurons projected to the Acb.

CONCLUSIONS

NPY reduces neuronal firing in the Acb resulting in increased palatable food intake. Together, our neuroanatomical, pharmacologic, and neuronal activity data support a role and mechanism for intra-Acb NPY-induced fat intake.

Presynaptic inhibition of glutamate release by neuropeptides: use-dependent synaptic modification

Neuropeptides are signaling molecules that interact with G-protein coupled receptors located both pre- and postsynaptically. Presynaptically, these receptors are localized in axons and terminals away from presynaptic specializations. Neuropeptides are stored in dense core vesicles that are distinct from the clear synaptic vesicles containing classic neurotransmitters such as glutamate and GABA. Because they require a stronger Ca(2+) signal than synaptic vesicles, dense core vesicles do not release neuropeptides with single action potentials but rather require high-frequency trains. Thus, neuropeptides only modulate strongly stimulated synapses, providing negative or positive feedback. Many neuropeptides have been found to inhibit glutamate release from presynaptic terminals, and the major mechanism is likely direct interaction of betagamma G-protein subunits with presynaptic proteins such as SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor). The use of mouse genetic models and specific receptor antagonists are beginning to unravel the function of inhibitory neuropeptides. The opioid receptors kappa and mu, which are activated by endogenous opioid peptides such as dynorphin, enkephalin, and possibly the endomorphins, are important in modulating pain transmission. Dynorphin, nociceptin/orphanin FQ, and somatostatin and its related peptide cortistatin appear to play a role in modulation of learning and memory. Neuropeptide Y has important functions in ingestive behavior and also in entraining circadian rhythms. The existence of neuropeptides greatly expands the computational ability of the brain by providing additional levels of modulation.

Neuropeptide tyrosine and pain

Research during the past two decades supports a complex role for neuropeptide tyrosine (NPY) and two of its associated receptors, the Y1 receptor and the Y2 receptor, in the modulation of pain, in addition to regeneration and survival mechanisms at the spinal level. Thus, NPY has been shown to both cause and reduce pain, in addition to having biphasic effects. Recent research has focused on the distribution of the spinal NPY-mediated system. Here, we propose various possible scenarios for the role of NPY in pain processing, based on its actions at different sites (axon versus cell body), through different receptors (Y1 receptor versus Y2 receptor) and/or types of neuron (ganglion neurons and intraganglionic cross-excitation versus interneurons versus projection neurons).

Role of brain neuroactive steroids in the functional interplay between the GABA(A) and the NPY-Y1 receptor mediated signals in the amygdala

Various lines of evidence suggest a functional interaction between GABA(A) and Neuropeptide Y (NPY)-Y(1) receptor (Y(1)R) mediated transmissions in various brain regions, which can be important in the regulation of sedation, feeding, anxious behaviour and neuronal excitability. By using a transgenic mouse model carrying the murine Y(1)R gene promoter fused to the lacZ reporter gene (Y(1)R/LacZ mice), we showed that prolonged pharmacologically or physiologically induced changes in the cerebrocortical concentrations of the neuroactive steroids 3alpha-hydroxy-5alpha-pregnan- 20-one (3alpha,5alpha TH PROG) and tetrahydrodeoxycorticosterone (3alpha,5alpha TH DOC) increases Y(1)R/LacZ transgene expression in the central and medial amygdala, an effect similar to that induced by long-term treatment with positive modulators of the GABA(A) receptor complex (diazepam or abecarnil). We also demonstrated that fluctuations in the cerebrocortical concentrations of 3alpha,5alpha-TH PROG and 3alpha,5alpha TH DOC during voluntary ethanol consumption and ethanol withdrawal induces a marked increase in Y(1)R gene expression that becomes apparent 48 h after withdrawal. These data provide evidence that neuroactive steroids may play an important role in the functional interaction between the GABA(A) receptor and NPY-Y(1)R mediated pathways in the amygdala, which might represent an important regulatory mechanism for modulation of several functions, including ethanol withdrawal.

Astrocytes in the arcuate nucleus and median eminence that take up a fluorescent dye from the circulation express leptin receptors and neuropeptide Y Y1 receptors

Following systemic injection, several different dyes and markers are found to accumulate rapidly in cells in the arcuate nucleus and median eminence, and the capillaries in this region appear specialised for exchange of molecules. The present study used hydroxystilbamidine (FluoroGold equivalent) to identify cells that take up molecules from the circulation in these regions; 2-6 h following injection, uptake was seen in the external and intermediate zones of the median eminence and the adjacent ventral part of the arcuate nucleus, but not in other regions of the hypothalamus. The labelled cells were small; double-labelling experiments revealed that they expressed glial fibrillary acid protein (GFAP), but not NeuN, Agouti-related protein (AgRP) or beta-endorphin. They had the morphology of astrocytes and were readily distinguished from tanycytes by staining for vimentin. Many of these labelled astrocytes also expressed leptin receptors and neuropeptide Y Y1 receptors. The surrounding neurons that expressed these receptors did not take up this dye. This demonstrates that astrocytes take up molecules from the circulation in the median eminence and adjacent arcuate nucleus, and may have a significant signalling role in regulation of food intake.

Peripheral and central administration of a selective neuropeptide Y Y1 receptor antagonist suppresses ethanol intake by C57BL/6J mice

BACKGROUND

Neuropeptide Y (NPY) is a 36-amino acid neuromodulator that is expressed throughout the central nervous system. Recent genetic and pharmacological evidence suggests that the NPY Y1 receptor modulates ethanol intake. To further characterize the role of the Y1 receptor, we examined voluntary ethanol consumption by mice after administration of [(-)-2-[1-(3-chloro-5-isopropyloxycarbonylaminophenyl)ethylamino]-6-[2-(5-ethyl-4-methyl-1,3-tiazol-2-yl)ethyl]-4-morpholinopyridine] (compound A), a novel and selective Y1 receptor antagonist (Y1RA) that acts centrally on brain receptors when administered peripherally.

METHODS

C57BL/6J mice were habituated to drinking a 10% (v/v) ethanol solution by using a two-bottle-choice procedure and were then given an intraperitoneal (ip) injection (5 ml/kg) of the Y1RA (0, 25, 50, or 75 mg/kg). In a second study, mice were given intracerebroventricular infusion of the Y1RA (0, 30, or 100 microg). Finally, we determined whether the Y1RA alters open-field locomotor activity, ethanol-induced sedation (3.8 g/kg, ip), or blood ethanol levels.

RESULTS

Relative to control treatment, ip injection (50 and 75 mg/kg) and intracerebroventricular infusion (100 microg) of the Y1RA significantly reduced ethanol consumption and food intake without altering water drinking. However, the Y1RA did not alter open-field locomotor activity, ethanol-induced sedation, or blood ethanol levels.

CONCLUSIONS

These data indicate that acute blockade of the NPY Y1 receptor with a systemically bioavailable NPY Y1RA reduces voluntary ethanol consumption by C57BL/6J mice. These results are consistent with observations that hypothalamic infusion of NPY increases ethanol drinking by rats.

Effect of a graded single constriction of the rat sciatic nerve on pain behavior and expression of immunoreactive NPY and NPY Y1 receptor in DRG neurons and spinal cord

In the present study, the rat sciatic nerve was constricted to varying degrees using only one ligature with a very thin polyethylene sheath placed between nerve and ligature thread. Complete nerve transection was studied for comparison. With a 40-80% constriction of the nerve we observed allodynia to a similar extent as in the so-called Bennett model based on four loose ligatures. We also monitored changes in the expression of neuropeptide Y (NPY) and the NPY Y1 receptor (Y1R) in the lumbar 4-5 dorsal root ganglia (DRG) and dorsal horn and found upregulation of NPY and downregulation of the Y1R in DRG neurons after injury. These results indicate that similar peptide and receptor changes occur in this model as after axotomy and in other nerve injury models, although the immunohistochemical and behavioral changes seem to be dependent on the degree of constriction of the nerve. Thus, it seems relevant to monitor the degree of constriction when evaluating pain and other post-injury events. The possibility that some of the changes in NPY-ergic neurotransmission are related to the generation of allodynia is discussed; as well as the possibility to use this mononeuropathic model based on a single ligature nerve constriction (SLNC) as a complementary approach to other widely used pain models.

Estrogen-induced mu-opioid receptor internalization in the medial preoptic nucleus is mediated via neuropeptide Y-Y1 receptor activation in the arcuate nucleus of female rats

The endogenous peptides beta-endorphin (beta-END) and neuropeptide Y (NPY) have been implicated in regulating sexual receptivity. Both beta-END and NPY systems are activated by estrogen and inhibit female sexual receptivity. The initial estrogen-induced sexual nonreceptivity is correlated with the activation and internalization of mu-opioid receptors (MORs), in the medial preoptic nucleus (MPN). Progesterone reverses the estrogen-induced activation/internalization of MOR and induces the sexual receptive behavior lordosis. To determine whether NPY and endogenous opioids interact, we tested the hypothesis that estrogen-induced MOR activation is mediated through NPY-Y1 receptor (Y1R) activation. Retrograde tract tracing demonstrated Y1Ron beta-END neurons that projected to the MPN. Sex steroid modulation of MOR in the MPN acts through NPY and the Y1R. Estradiol administration or intracerebroventricular injection of NPY activated/internalized Y1R in the arcuate nucleus and MOR in the MPN of ovariectomized (OVX) rats. Moreover, the selective Y1R agonist [Leu31, Pro34]-Neuropeptide Y (LPNY) internalized MOR in the MPN of OVX rats. The Y1R antagonist (Cys31, Nva34)-Neuropeptide Y (27-36)2 prevented estrogen-induced Y1R and MOR activation/internalization. NPY reversed the progesterone blockade of estradiol-induced Y1R and MOR internalization in the arcuate nucleus and MPN, respectively. Behaviorally, LPNY inhibited estrogen plus progesterone-induced lordosis, and the MOR-selective antagonist D-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr amide reversed LPNY-induced inhibition of lordosis. These results suggest that a sequential sex steroid activation of NPY and MOR circuits regulates sexual receptivity.

Neuropeptide Y inhibits capsaicin-sensitive nociceptors via a Y1-receptor-mediated mechanism

Neuropeptide Y (NPY) is expressed in certain primary afferent fibers, is up-regulated in response to tissue injury and is capable of inhibiting nociceptive behavior at the spinal level. However, the spinal mechanism(s) for NPY-evoked antinociception is unknown. In this study, we evaluated the hypothesis that agonists at the NPY Y1 receptor subtype (Y1-R) inhibit exocytosis from the capsaicin-sensitive class of nociceptors. Using in vitro superfusion of rat dorsal spinal cord slices, pre-treatment with the Y1-R agonist [Leu(31)Pro(34)]NPY significantly inhibited capsaicin-evoked release of immunoreactive calcitonin gene-related peptide with an EC(50) value of 10.6 nM. This inhibitory effect was concentration dependent, significantly attenuated by pre-treatment with the Y1 receptor antagonist BIBP3226 and reproduced by synthetic NPY. Examination of adult rat dorsal root ganglia using double immunofluorescent labeling revealed frequent co-localization of Y1 receptor immunoreactivity in vanilloid receptor type 1-immunoreactive neurons, indicating that Y1 agonists may directly modulate the capsaicin-sensitive class of nociceptors. Collectively, these results indicate that NPY is capable of inhibiting capsaicin-sensitive neurons via a Y1 receptor mechanism, suggesting the mechanisms for spinal NPY-induced antinociception is due, at least in part, to inhibition of central terminals of capsaicin-sensitive nociceptors.

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.

Changes in NPY-mediated modulation of hippocampal [3H]D-aspartate outflow in the kindling model of epilepsy

The anticonvulsant effect of NPY may depend on Y(2) and/or Y(5) receptor-mediated inhibition of glutamate release in critical areas, such as the hippocampus. However, Y(2) and Y(5) receptor levels have been reported to increase and decrease, respectively, in the epileptic hippocampus, implicating that the profile of NPY effects may change accordingly. The aim of this study was to evaluate the differential effects of NPY on glutamate release in the normal and in the epileptic hippocampus. Thus, we pharmacologically characterized the effects of NPY on the release of [(3)H]D-aspartate, a valid marker of endogenous glutamate, from synaptosomes prepared from the whole hippocampus and from the three hippocampal subregions (dentate gyrus and CA1 and CA3 subfields) of control and kindled rats, killed 1 week after the last stimulus-evoked seizure. In the whole hippocampus, NPY does not significantly affect stimulus-evoked [(3)H]D-aspartate overflow. In synaptosomes prepared from control rats, NPY significantly inhibited 15 mM K(+)-evoked [(3)H]D-aspartate overflow only in the CA1 subfield (approx. -30%). Both Y(2) and Y(5) receptor antagonists (respectively, 1 microM BIIE0246 and 1 microM CGP71683A) prevented this effect, suggesting the involvement of both receptor types. In contrast, in synaptosomes prepared from kindled rats NPY significantly inhibited 15 mM K(+)-evoked [(3)H]D-aspartate overflow in the CA1 subfield and in the dentate gyrus (approx. -30%). Only the Y(2) (not the Y(5)) antagonist prevented these effects. These data indicate a critical role for the Y(2) receptor in the inhibitory control of glutamate release in the kindled hippocampus and, thus, suggest that the anticonvulsant effect of NPY in the epileptic brain is most likely Y(2), but not Y(5), receptor-mediated.

Y1 receptor of neuropeptide Y as a glial marker in proliferative vitreoretinopathy and diseased human retina

The Y1 receptor of neuropeptide Y (NPY) has been demonstrated in glial cells of astrocytic lineage in vitro. We have studied the immunohistochemical expression of Y1 receptors in the glia of the diseased human retina, in tissue samples obtained after surgery for proliferative vitreoretinopathy. In this condition, glia and other cell types migrate and form epi- or subretinal membranes. Both diseased retinas (n = 8) and PVR membranes (n = 43) contained numerous Y1-immunoreactive cells. In the diseased retina, the Y1 antiserum labeled cells with the morphological radial pattern characteristic of Müller cells, whereas in the membranes, label appeared in a large population of elongate cells, measuring up to 250 microm. In both retina and membranes, double labeling demonstrated that the vast majority of Y1-immunoreactive cells were also labeled by a glial fibrillary acidic protein (GFAP) antibody, indicating their glial origin. Retinal regions devoid of GFAP immunoreactivity also lacked the Y1 label. None of these markers was detected in Müller cells of normal retina. Y1 immunoreactivity did not co-localize with smooth muscle actin immunoreactivity, a marker of myofibroblasts. Expression of Y1 receptors would characterize reactive and proliferating glial cells of the diseased retina and could perhaps be involved in the proliferation of injured glial cells causing regrowth of PVR membranes and the consequent secondary retinal detachments.

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.

Distribution of neuropeptide Y Y1 receptors in rodent peripheral tissues

Using a sensitive immunohistochemical technique, the localization of neuropeptide Y (NPY) Y1-receptor (Y1R)-like immunoreactivity (LI) was studied in various peripheral tissues of rat. Wild-type (WT) and Y1R-knockout (KO) mice were also analyzed. Y1R-LI was found in small arteries and arterioles in many tissues, with particularly high levels in the thyroid and parathyroid glands. In the thyroid gland, Y1R-LI was seen in blood vessel walls lacking alpha-smooth muscle actin, i.e., perhaps in endothelial cells of capillaries. Larger arteries lacked detectable Y1R-LI. A distinct Y1R-immunoreactive (IR) reticulum was seen in the WT mouse spleen, but not in Y1R-KO mouse or rat. In the gastrointestinal tract, Y1R-positive neurons were observed in the myenteric plexus, and a few enteroendocrine cells were Y1R-IR. Some cells in islets of Langerhans in the pancreas were Y1R-positive, and double immunostaining showed coexistence with somatostatin in D-cells. In the urogenital tract, Y1R-LI was observed in the collecting tubule cells of the renal papillae and in some epithelial cells of the seminal vesicle. Some chromaffin cells of adrenal medulla were positive for Y1R. The problem of the specificity of the Y1R-LI is evaluated using adsorption tests as well as comparisons among rat, WT mouse, and mouse with deleted Y1R. Our findings support many earlier studies based on other methodologies, showing that Y1Rs on smooth muscle cells of blood vessels mediate NPY-induced vasoconstriction in various organs. In addition, Y1Rs in other cells in parenchymal tissues of several organs suggest nonvascular effects of NPY via the Y1R.

Ultrastructural localization of neuropeptide Y Y1 receptors in the rat medial nucleus tractus solitarius: relationships with neuropeptide Y or catecholamine neurons

Neuropeptide Y (NPY) Y1 receptor (Y1-R) agonists influence cardiovascular regulation. These actions may involve NPY- and catecholamine-containing neurons in the medial nucleus of the solitary tract (mNTS), at the level of the area postrema. The cellular sites through which Y1-R agonists may interact with NPY and catecholamines in the mNTS, however, are not known. To determine potential sites of action for Y1-R agonists, and their relationship to NPY or catecholamines in the mNTS, we used electron microscopic immunocytochemistry for the detection of sequence-specific antipeptide antisera against Y1-R alone or in combination with antisera against NPY or the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Analyses were conducted in the rat mNTS, at the level of the area postrema. Y1-R was found mainly in small unmyelinated axons and axon terminals but also in some somata and dendrites as well as a small number of glia. Within axon terminals, labeling for Y1-R was often present on dense core vesicles and small synaptic vesicles as well as extrasynaptic areas of the plasmalemma. Some Y1-R-labeled terminals also contained NPY or TH, suggesting that agonists of Y1-R may influence the release of NPY or catecholamines in the mNTS. In addition, Y1-R was found in dendrites that received asymmetric excitatory-type synapses from unlabeled axon terminals. Some of these dendrites contained NPY or TH, which indicates that Y1-R may be targeted for functional activation within NPY- or catecholamine-expressing neurons in the mNTS. These results demonstrate that Y1-R is a presynaptic receptor in NPY- or catecholamine-containing axon terminals within the mNTS as well as a postsynaptic receptor on NPY- or catecholamine-containing neurons that are contacted by axon terminals that likely contain excitatory amino acid transmitters. Agonists of Y1-R in the mNTS may thus affect cardiovascular regulation by modulating NPY, catecholamine, and excitatory amino acid transmission.

NPY Y1 receptors are present in axonal processes of DRG neurons

Using a sensitive immunohistochemical method, the localization of the neuropeptide Y (NPY) Y1 receptor (Y1R) was studied in contralateral and ipsilateral dorsal root ganglion (DRG) neurons of rats subjected to different unilateral manipulations with focus on their axonal processes and projection areas. Y1R-like immunoreactivity (LI) was observed in the contralateral sciatic nerve and dorsal roots of lesioned rats, and double staining revealed colocalization with calcitonin gene-related peptide (CGRP). Y1R-LI was also seen in fibers close to and even within the epidermis. A fairly small number of nerve endings double-labeled for Y1R and CGRP were present in the dorsal horn. After unilateral crush of the sciatic nerve Y1R- and CGRP-LI accumulated in the same axons proximal to the lesion. After dorsal rhizotomy CGRP-LI was strongly reduced in the ipsilateral dorsal horn. No certain change was observed for Y1R- or NPY-LI, but Y1R/CGRP double-labeled nerve endings disappeared after the lesion. These results strongly suggest centrifugal transport of Y1Rs in DRG neurons, mainly to the peripheral sensory branches. To what extent these Y1Rs are functional has not been analyzed here, but a recent study on Y1R null mice provides evidence for involvement of prejunctional Y1Rs in peripheral sensory functions

Role of dopaminergic system in core part of nucleus accumbens in hyperlocomotion and rearing induced by MK-801 in rats: a behavioral and in vivo microdialysis study

We investigated modification of the MK-801 effect on motor activity and extracellular amines concentration by 6-hydroxydopamine (6-OHDA)-induced lesion of core nucleus accumbens (cACC) of rats. In vivo microdialysis-HPLC showed that the concentrations (fmol/microl) of dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and serotonin were 0.738 +/- 0.135, 155.34 +/- 41.01 and 0.334 +/- 0.024, respectively, in the cACC of intact rats. The DOPAC/DA ratio was 264.24 +/- 94.01. Unilateral lesion of the cACC with 6-OHDA (8 microg/microl) substantially reduced DA (-93%) and DOPAC (-97%) in desipramine (30 mg/kg, i.p.)-pretreated rats (6-OHDA+DMI rats) as compared to the 65% reduction rate of both amines in saline-pretreated rats (6-OHDA+saline rats). Moreover, DOPAC was reduced by 72% in 6-OHDA+DMI rats. MK-801 increased DOPAC (426-467%) and DOPAC/DA ratio (180-230%) in intact rats. On the other hand, MK-801 increased DA by 154% and 505% in 6-OHDA+saline and 6-OHDA+DMI rats, respectively. 6-OHDA reduced the effect of MK-801 on DOPAC and DOPAC/DA ratio. In the behavioral studies, MK-801 (0.01-0.3 mg/kg, i.p.) increased locomotor activity and rearing of intact rats. Bilateral 6-OHDA+DMI lesion of the cACC caused greater reduction in the effect of MK-801 (0.1 mg/kg) than that of the shell nucleus accumbens. These results suggest that increased extracellular DOPAC concentration (but not DA) and DOPAC/DA ratio in the cACC plays an important role in MK-801-hyperactivity.

Microdialysates of amines and metabolites from core nucleus accumbens of freely moving rats are altered by dizocilpine

In vivo microdialysis combined with high performance liquid chromatography (HPLC) with electrochemical detection, was used to study the effect of MK-801 (0.1 mg/kg i.p.) on extracellular concentrations of dopamine (DA) 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT), norepinephrine (NE) and DOPAC/DA ratio in intact, 6-hydroxydopamine (6-OHDA)-lesioned, DSP4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzyl-amine hydrochloride)-lesioned and reserpine-treated rats. The results revealed high basal DA (0.735+/-0.05 fmol/microl), DOPAC (195.93+/-20.18 fmol/microl) and NE (0.585+/-0.01 fmol/microl), low 5-HT (0.334+/-0.032 fmol/microl) and high DOPAC/DA ratio (265.11+/-20.73) in intact cACC. 6-OHDA alone (8 microg/2 microl) depleted DA (-66%), DOPAC (-65%), and NE (-62%). On the other hand, in desipramine (DMI)-pretreated rats, 6-OHDA induced a large depletion of DA (-94%), DOPAC (-97%) and reduced DOPAC/DA ratio (-73%), but increased NE to 142% of intact and 369% of 6-OHDA-lesioned rats. DSP4 (50 mg/kg) decreased NE (-97%), DOPAC (-75%) and DOPAC/DA ratio (-69%). Reserpine (5 mg/kg s.c.) significantly decreased DOPAC (-84%), DOPAC/DA ratio (-81%), 5-HT (-69%) and NE (-86%), but nonsignificantly increased DA. In the intact rats, MK-801 did not change DA, but increased DOPAC and DOPAC/DA ratio. In 6-OHDA-lesioned rats, MK-801 increased DA, whereas in 6-OHDA+DMI rats MK-801 additionally increased DOPAC and DOPAC/DA ratio. DSP4 and reserpine reduced the ability of MK-801 to increase DOPAC and DOPAC/DA ratio. MK-801 did not change NE concentration in dialysates collected from intact rats, but increased that from 6-OHDA+DMI-lesioned rats. In DSP4-lesioned and reserpine-treated rats, MK-801 increased NE but to a level lower than that observed in the intact rats. These results suggest that systemic administration of a low dose of MK-801, which induces profound locomotor stimulation without stereotypy, increases DOPAC and DOPAC/DA ratio in the cACC of intact rats, whereas it additionally increases the depleted DA and NE concentrations especially in 6-OHDA-lesioned rats pretreated with DMI.

Immunohistochemical localization of the neuropeptide Y Y1 receptor in rat central nervous system

The diverse effects of neuropeptide Y (NPY) are mediated through interaction with G-protein coupled receptors. Pharmacological analysis suggests the Y1 receptor mediates several of NPY's central and peripheral actions. We sought to determine the distribution of Y1 protein throughout the rat central nervous system by means of indirect immunofluorescence using the tyramide signal amplification method and a novel, amino terminally-directed Y1 antisera. This antisera was verified as specific for Y1 by solution-phase competition ELISA, Western blot and in situ blocking experiments. High concentrations of Y1 immunoreactivity were found in the claustrum, piriform cortex (superficial layer), arcuate hypothalamic nucleus, interpeduncular nucleus, paratrigeminal nucleus, and lamina II of the spinal trigeminal nucleus and entire spinal cord. Moderate levels of Y1 immunoreactivity were found the in the main olfactory bulb, dorsomedial part of suprachiasmatic nucleus, paraventricular hypothalamic nucleus, ventral nucleus of lateral lemniscus, pontine nuclei, mesencephalic trigeminal nucleus, external cuneate nucleus, area postrema, and nucleus tractus solitarius. Low levels of Y1 immunostaining were distributed widely throughout layers II-III of the cerebral cortex (i.e., orbital, cingulate, frontal, parietal, insular, and temporal regions), nucleus accumbens core, amygdalohippocampal and amygdalopiriform areas, dentate gyrus, CA1 and CA2 fields of hippocampus, principal and oral divisions of the spinal trigeminal nucleus, islands of Calleja and presubiculum. These findings are discussed with reference to previously reported receptor autoradiography, immunohistochemistry and mRNA analyses to further support the role of Y1 in NPY-mediated biology.

Association of neuropeptide Y Y1 receptors with glutamate-positive and NPY-positive neurons in rat hippocampal cultures

The hippocampus is particularly enriched with neuropeptide tyrosine (NPY) and NPY receptors including the Y1, Y2 and Y5 subtypes. We have previously reported on the enrichment of cultured rat hippocampal neurons in specific [125I][Leu31, Pro34]PYY/BIBP3226-sensitive (Y1) binding sites and Y1 receptor mRNAs [St-Pierre et al. (1998) Br. J. Pharmacol., 123, p183]. We have now identified which cell types express the Y1 receptor. The majority of Y1 receptors, visualized using either the radiolabeled probe [125I][Leu31,Pro34]PYY or two antibodies directed against distinct domains of the Y1 receptor, was expressed in neurons as revealed by neuron-specific enolase (NSE) immunostaining. One antibody was directed against the second extracelllular loop of the Y1 receptor (amino acids 185-203) whereas the second was directed against the intracellular C-terminal loop (amino acids 355-382). The labelling was evident over both perikarya and processes. Neurons labelled by the various Y1 receptor probes were mostly glutamate-positive as revealed by double immunostaining. Most interestingly, a number of NPY-positive cultured hippocampal neurons were also enriched with the Y1 receptor, suggesting that this subtype may act as an autoreceptor to regulate NPY release in the hippocampus. These results thus provide an anatomical basis for the modulation of glutamate and NPY release by the Y1 receptor in the hippocampus.

Multiple receptors for neuropeptide Y in the hippocampus: putative roles in seizures and cognition

Neuropeptide Y (NPY) is widely distributed throughout the central nervous system (CNS) and is one of the most conserved peptides in evolution, suggesting an important role in the regulation of basic physiological functions, including learning and memory. In addition, experimental studies have suggested that NPY, together with its receptors, may have a direct implication in several pathological disorders, including epilepsy/seizure. NPY-like immunoreactivity and NPY receptors have been shown to be present throughout the brain, but is concentrated in the hippocampus. The hippocampal formation has been repeatedly implicated in the modulation of cognition, as well as the pathogenesis of seizure. This review will concentrate on the hippocampal distribution of NPY, its receptors and the putative role played by this peptide in seizure, together with the regulation of cognitive function associated with learning and memory.

Morphological evidence for direct interaction between arcuate nucleus neuropeptide Y (NPY) neurons and gonadotropin-releasing hormone neurons and the possible involvement of NPY Y1 receptors

Neuropeptide Y (NPY) neurons in the arcuate nucleus of the hypothalamus (ARH) have been shown to play an important role in modulating LH secretion. One mechanism by which the ARH NPY system may regulate LH secretion is by modulating GnRH neuronal function. Thus, the present study examined whether the ARH NPY system provided direct input to GnRH cell bodies in the preoptic area (POA), as well as to their nerve terminals in the median eminence (ME). The possible involvement of the NPY Y1 receptor subtype in mediating the effects of NPY was also investigated. Lactating rats were used in these studies because they have increased hypothalamic NPY content, especially in the ARH/ME areas, making it easier to detect NPY fibers and terminals. The anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L), was iontophoresed into the ARH of lactating rats; and triple-label immunofluorescence was performed, with the aid of confocal microscopy, to visualize NPY, PHA-L, and GnRH. GnRH cell bodies were found scattered throughout the organum vasculosum laminae terminalis (OVLT)/POA region, and NPY/ PHA-L double-labeled fibers were found in very close proximity to numerous GnRH perikarya. In the ME, double-labeled NPY/PHA-L fibers were found in the inner and external zones, and they were found in close proximity to GnRH neuronal fibers. Using a NPY Y1 specific antibody, double-label immunofluorescence was performed to examine whether the Y1 receptor subtype was expressed in GnRH neurons. No convincing Y1-positive staining was found in GnRH cell bodies in the OVLT/POA region. However, abundant Y1-positive fiber and cell staining were observed throughout the region, and Y1-positive fibers were found in close apposition to GnRH cell bodies. In contrast, numerous GnRH nerve fibers and terminals in both the OVLT and ME were colocalized with Y1-positive staining. The results of this study suggest that ARH NPY neurons come in close contact with GnRH neurons and may provide direct input to both GnRH cell bodies in the POA region and to their nerve terminals in the ME. The Y1 receptor subtype may be directly involved in NPY modulation of GnRH secretion from its nerve terminals.