Naloxone blocks 'anxiolytic' effects of neuropeptide Y

Glycyl-l-glutamine attenuates NPY-induced hyperphagia via the melanocortin system

This study aimed to determine whether glycyl-l-glutamine (Gly-Gln; β-endorphin (30-31)), a non-opioid peptide derived from β-endorphin processing, modulates neuropeptide Y (NPY)-induced feeding and hypothalamic mRNA expression of peptide hormones in male broiler chicks. Intracerebroventricular injection of NPY (235 pmol) generated a hyperphagic response in ad libitum chicks within 30 min. Co-administration of Gly-Gln (100 nmol) attenuated this response, inducing a 30 % decrease. This was not attributable to Gly-Gln hydrolysis because co-administration of glycine (Gly) and glutamine (Gln) had no effect on NPY-induced hyperphagia. Gly-Gln injected alone also showed no effect. The hypothalamic pro-opiomelanocortin mRNA expression in the co-injection group was significantly higher than that in the NPY alone group. These data indicate that endogenous Gly-Gln may contribute to regulate feeding behavior via the central melanocortin system in chicks and acts as a counter regulator of the neural activity in energy metabolism.

The effect of opioid receptor blockade on the neural processing of thermal stimuli

The endogenous opioid system represents one of the principal systems in the modulation of pain. This has been demonstrated in studies of placebo analgesia and stress-induced analgesia, where anti-nociceptive activity triggered by pain itself or by cognitive states is blocked by opioid antagonists. The aim of this study was to characterize the effect of opioid receptor blockade on the physiological processing of painful thermal stimulation in the absence of cognitive manipulation. We therefore measured BOLD (blood oxygen level dependent) signal responses and intensity ratings to non-painful and painful thermal stimuli in a double-blind, cross-over design using the opioid receptor antagonist naloxone. On the behavioral level, we observed an increase in intensity ratings under naloxone due mainly to a difference in the non-painful stimuli. On the neural level, painful thermal stimulation was associated with a negative BOLD signal within the pregenual anterior cingulate cortex, and this deactivation was abolished by naloxone.

Genetic variation in human NPY expression affects stress response and emotion

Understanding inter-individual differences in stress response requires the explanation of genetic influences at multiple phenotypic levels, including complex behaviours and the metabolic responses of brain regions to emotional stimuli. Neuropeptide Y (NPY) is anxiolytic and its release is induced by stress. NPY is abundantly expressed in regions of the limbic system that are implicated in arousal and in the assignment of emotional valences to stimuli and memories. Here we show that haplotype-driven NPY expression predicts brain responses to emotional and stress challenges and also inversely correlates with trait anxiety. NPY haplotypes predicted levels of NPY messenger RNA in post-mortem brain and lymphoblasts, and levels of plasma NPY. Lower haplotype-driven NPY expression predicted higher emotion-induced activation of the amygdala, as well as diminished resiliency as assessed by pain/stress-induced activations of endogenous opioid neurotransmission in various brain regions. A single nucleotide polymorphism (SNP rs16147) located in the promoter region alters NPY expression in vitro and seems to account for more than half of the variation in expression in vivo. These convergent findings are consistent with the function of NPY as an anxiolytic peptide and help to explain inter-individual variation in resiliency to stress, a risk factor for many diseases.

Neuropeptide Y administration into the amygdala alters high fat food intake

The orexigenic effects of neuropeptide Y (NPY) are mediated through the hypothalamus, while the anxiolytic effects of NPY appear to be mediated through the amygdala. We hypothesized that intra-amygdalar administration of NPY might alter food preference without changing total food intake. Neuropeptide Y was administered into the central nucleus of the amygdala in both satiated and overnight-fasted rats, and intake and preference for a high fat diet (56%)/low carbohydrate (20%) diet or a low fat (10%)/high carbohydrate (66%) diet were measured. Intra-amygdalar NPY administration in satiated rats did not change total caloric intake, but it did produce a dose-dependent decrease in intake of and preference for high fat diet relative to low fat diet over 24 h. In overnight-fasted rats, intra-amygdalar NPY also decreased the intake and preference for a high fat diet relative to low fat diet over 24 h, without altering total caloric intake. Intra-amygdalar NPY administration did not produce conditioned taste aversions to a novel saccharin solution. These results suggest that amygdalar NPY may have a role in macronutrient selection, without altering total caloric intake.

A lack of μ-opioid receptors modulates the expressions of neuropeptide Y and substance P mRNA

The present study was undertaken to investigate changes in the expressions of neuropeptide Y (NPY) and substance P (SP) in mice lacking mu-opioid receptors. In an in situ hybridization study, in which we compared wild type and mu-opioid receptor knockout mice, NPY mRNA levels were found to be lower in the caudate-putamen and nucleus accumbens of mu-opioid receptor knockout mice. In addition, SP mRNA levels were lower in the ventromedial hypothalamic nucleus of mu-opioid receptor knockout mice. Our findings suggest that a lack of mu-opioid receptors modulates basal NPY mRNA levels in striatal regions and SP mRNA levels in the ventromedial hypothalamic nucleus of the mouse, and that these changes are due to compensatory modulation in the brain.

Aged neuropeptide Y transgenic rats are resistant to acute stress but maintain spatial and non-spatial learning

The behavioral phenotype of five-month-old rats overexpressing neuropeptide Y (NPY) has previously been described [Proc Natl Acad Sci USA 97 (2000) 12852]. In this transgenic rat model, there is central overexpression of prepro-NPY mRNA and NPY peptide in the hippocampus and hypothalamus and decreased Y1 binding sites within the hippocampus. These molecular and neurochemical events led to altered anxiety profile and learning abilities in NPY-overexpressing rats. In the present study, anxiety and learning/memory related behaviors were examined in one-year-old NPY-transgenic rats in order to assess any behavioral changes that may have occurred during the aging process. As observed in 5-month-old overexpressing rats, aged NPY-transgenic animals are resistant to acute physical restraint stress measured by the elevated-plus maze and demonstrate anxiolytic-like activity in the open field. However, in contrast to data in young rats, there was no significant difference between aged wildtype and NPY-transgenic animals in relation to spatial and non-spatial memory as indicated by the (allo- and ego-centric) Morris water maze and object recognition test. It would thus appear that the anxiolytic-like profile observed in young NPY-overexpressing rats is maintained in older animals providing further evidence for a role for NPY in anxious behaviors. However, the cognitive deficits observed in young rats do not appear to occur in older animals suggesting the existence of compensatory mechanisms leading to a reversal of the learning deficits noted in younger animals. These results also provide additional evidence for the mechanistic dissociation between anxiety and cognition-related behaviors modulated by NPY.

FMRI Measurement of CNS Responses to Naloxone Infusion and Subsequent Mild Noxious Thermal Stimuli in Healthy Volunteers

The aims of this study were to assess the effects of a μ-opioid antagonist, naloxone, on endogenous opioid systems and to evaluate the effect of naloxone on the CNS response to mild noxious heat. Doubled-blinded experiments were performed in a cross-over design in 10 healthy male volunteers. Functional magnetic resonance imaging (fMRI) data were collected before and during the infusion and also during thermal stimuli. Increased signal was observed in a number of cortical and subcortical brain regions for naloxone versus saline infusion. Cortical activation was induced in regions including cingulate, prefrontal cortex, and insula. Subcortical regions showing increased signal change included hippocampus and entorhinal cortex. A 46°C stimulus delivered to the back of the hand induced an overall increase in activation in a number of regions in the naloxone group that were not seen in the saline group (e.g., insula, orbitofrontal cortex, thalamus, and hippocampus). These results show that naloxone, even in the absence of psychophysical effects, produces activation in several brain regions that are known to have high levels of μ-opioid receptors and may be involved in endogenous analgesia. Our study is an example of how fMRI can measure subtle changes in brain activation induced by pharmacological agents without cognitive effects.

Synthesis and binding affinity of neuropeptide Y at opiate receptors

Neuropeptide Y and several metabolic fragments were synthesized and evaluated for binding affinity at non-selective opiate receptors. Neuropeptide Y and several C-terminal fragments were shown to bind to non-selective opiate receptors with an affinity similar to that of Leu-enkephalin.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Regulation of neuropeptide Y in the rat amygdala following unilateral olfactory bulbectomy

While the mechanisms are not fully understood, olfactory bulbectomy (OBX) is a well-known rat model of depression and depression-related disorders such as anxiety and aggression. Alterations in neuropeptide Y (NPY) levels in the brain have been linked to depression and have been shown to be involved in the response to stress. This study explored the possible regulation of NPY immunoreactivity in specific regions of the amygdala 14 days after OBX in adult male Sprague-Dawley rats (n=6). Unilateral OBX and immunohistochemistry permitted comparisons of NPY in the ipsilateral amygdala with NPY in the contralateral (sham) amygdala. OBX resulted in significant increases (P<0.05) in NPY immunoreactivity in the anterior medial amygdala (threefold) and the posterior medial amygdala (2.5-fold). These regions receive projections from the accessory olfactory bulb (AOB). In contrast, the anterior and posterolateral cortical nuclei of the amygdala receive projections from the main olfactory bulb (MOB). NPY was not increased in these nuclei. These data show that not only does OBX increase NPY immunoreactivity in the amygdala, but also suggest that the AOB plays a prominent role in this regulation.

Anti-nociceptive effect of neuropeptide Y in the nucleus accumbens of rats: an involvement of opioid receptors in the effect

The present study investigated the effect of neuropeptide Y on nociception in the nucleus accumbens of rats. Intra-nucleus accumbens administration of neuropeptide Y induced dose-dependent increases in the hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats. There were no significant changes in the HWL to both stimulation during 60 min after the administration of NPY to outside of the nucleus accumbens. The anti-nociceptive effect of NPY was blocked by subsequent intra-nucleus accumbens injection of the Y1 receptor antagonist neuropeptide Y 28-36, indicating that Y1 receptor is involved in the neuropeptide Y-induced anti-nociception in the nucleus accumbens. Furthermore, the anti-nociceptive effect of neuropeptide Y was attenuated by intra-nucleus accumbens administration of the opioid antagonist naloxone, suggesting an involvement of the endogenous opioid system in the neuropeptide Y-induced anti-nociception in the nucleus accumbens of rats. Moreover, the neuropeptide Y-induced anti-nociception was attenuated by following intra-nucleus accumbens injection of the selective opioid antagonists nor-binaltorphimine and beta-funaltrexamine, but not by naltrindole, illustrating that mu- and kappa-opioid receptors, not the delta-opioid receptor, were involved in the neuropeptide Y-induced anti-nociception in the nucleus accumbens of rats.