Up-regulation of Neuropeptide Y Receptors in the Hypothalamus of Monosodium Glutamate-lesioned Sprague-Dawley Rats

Mechanisms for AgRP neuron-mediated regulation of appetitive behaviors in rodents

The obesity epidemic is a major health and economic burden facing both developed and developing countries worldwide. Interrogation of the central and peripheral mechanisms regulating ingestive behaviors have primarily focused on food intake, and in the process uncovered a detailed neuroanatomical framework controlling this behavior. However, these studies have largely ignored the behaviors that bring animals, including humans, in contact with food. It is therefore useful to dichotomize ingestive behaviors as appetitive (motivation to find and store food) and consummatory (consumption of food once found), and utilize an animal model that naturally displays these behaviors. Recent advances in genetics have facilitated the identification of several neuronal populations critical for regulating ingestive behaviors in mice, and novel functions of these neurons and neuropeptides in regulating appetitive behaviors in Siberian hamsters, a natural model of food foraging and food hoarding, have been identified. To this end, hypothalamic agouti-related protein/neuropeptide Y expressing neurons (AgRP neurons) have emerged as a critical regulator of ingestive behaviors. Recent studies by Dr. Timothy Bartness and others have identified several discrete mechanisms through which peripheral endocrine signals regulate AgRP neurons to control food foraging, food hoarding, and food intake. We review here recent advances in our understanding of the neuroendocrine control of ingestive behaviors in Siberian hamsters and other laboratory rodents, and identify novel mechanisms through which AgRP neurons mediate appetitive and consummatory behaviors.

Antidepressant-like activity of the neuropeptide Y Y5 receptor antagonist Lu AA33810: behavioral, molecular, and immunohistochemical evidence

RATIONALE

It has recently been found that chronic treatment with the highly selective, brain-penetrating Y5 receptor antagonist, Lu AA33810 [N-[[trans-4-[(4,5-dihydro [1] benzothiepino[5,4-d] thiazol-2-yl) amino] cyclohexyl]methyl]-methanesulfonamide], produces antidepressant-like effects in the rat chronic mild stress model.

OBJECTIVE

In the present study, we investigated the possible antidepressant-like activity of Lu AA33810 in rats subjected to glial ablation in the prefrontal cortex (PFC) by the gliotoxin L-AAA, which is an astroglial degeneration model of depression.

RESULTS

We observed that Lu AA33810 administered intraperitoneally at a single dose of 10 mg/kg both reversed depressive-like behavioral changes in the forced swim test (FST) and prevented degeneration of astrocytes in the mPFC. The mechanism of antidepressant and glioprotective effects of Lu AA33810 has not been studied, so far. We demonstrated the contribution of the noradrenergic rather than the serotonergic pathway to the antidepressant-like action of Lu AA33810 in the FST. Moreover, we found that antidepressant-like effect of Lu AA33810 was connected with the influence on brain-derived neurotrophic factor (BDNF) protein expression. We also demonstrated the antidepressant-like effect of Lu AA33810 in the FST in rats which did not receive the gliotoxin. We found that intracerebroventricular injection of the selective MAPK/ERK inhibitor U0126 (5 μg/2 μl) and the selective PI3K inhibitor LY294002 (10 nmol/2 μl) significantly inhibited the anti-immobility effect of Lu AA33810 in the FST in rats, suggesting that MAPK/ERK and PI3K signaling pathways could be involved in the antidepressant-like effect of Lu AA33810.

CONCLUSION

Our results indicate that Lu AA33810 exerts an antidepressant-like effect and suggest the Y5 receptors as a promising target for antidepressant therapy.

Neural and hormonal control of food hoarding

Many animals hoard food, including humans, but despite its pervasiveness, little is known about the physiological mechanisms underlying this appetitive behavior. We summarize studies of food hoarding in humans and rodents with an emphasis on mechanistic laboratory studies of species where this behavior importantly impacts their energy balance (hamsters), but include laboratory rat studies although their wild counterparts do not hoard food. The photoperiod and cold can affect food hoarding, but food availability is the most significant environmental factor affecting food hoarding. Food-deprived/restricted hamsters and humans exhibit large increases in food hoarding compared with their fed counterparts, both doing so without overeating. Some of the peripheral and central peptides involved in food intake also affect food hoarding, although many have not been tested. Ad libitum-fed hamsters given systemic injections of ghrelin, the peripheral orexigenic hormone that increases with fasting, mimics food deprivation-induced increases in food hoarding. Neuropeptide Y or agouti-related protein, brain peptides stimulated by ghrelin, given centrally to ad libitum-fed hamsters, duplicates the early and prolonged postfood deprivation increases in food hoarding, whereas central melanocortin receptor agonism tends to inhibit food deprivation and ghrelin stimulation of hoarding. Central or peripheral leptin injection or peripheral cholecystokinin-33, known satiety peptides, inhibit food hoarding. Food hoarding markedly increases with pregnancy and lactation. Because fasted and/or obese humans hoard more food in general, and more high-density/high-fat foods specifically, than nonfasted and/or nonobese humans, understanding the mechanisms underlying food hoarding could provide another target for behavioral/pharmacological approaches to curb obesity.

Direct and indirect effects of neuropeptide Y and neurotrophin 3 on myelination in the neonatal brains

Neuropeptide Y (NPY) is expressed in the developing central nervous system, however, its role in the brain development remains unclear. In this study, C57/B6 mice were intraperitoneally administered 1 nmol/capita/day of NPY, 10 nmol/capita/day of an NPY-receptor 1-specific antagonist (Y1R-A), or NPY and Y1R-A simultaneously (NPY+Y1R-A) from postnatal day (P) 7 to P14. Recombinant NPY reached the P14 cerebrum in 1 hour. These treatments didn't significantly affect body weight gain or P14 brain weight. The ratio of myelinated axons to total axons in the parietal cerebrum was significantly higher in the NPY group than in the control group. The expression of myelin basic protein (MBP)-mRNA in the cerebrum was significantly higher in the NPY group than in the control group and was significantly lower in the NPY+Y1R-A group than in the NPY group, while it was significantly higher in the NPY+Y1R-A group than in the control group. In cultured oligodendroglioma-derived B12 cells, NPY didn't influence the MBP-mRNA expression, while neurotrophin 3 (NT3) increased MBP mRNA via receptor-type tyrosine kinase type C (Trk C). NPY administration significantly increased NT3-mRNA expression in the P14 cerebrum as deduced by quantitative real-time PCR. The change in phosphorylated Trk C (P-Trk C) was proportional to that of the NT3-mRNA expression, and the proportion of P-Trk C was higher in the NPY group than in the control group. These results suggest that NPY, partially via Y1R, induces NT3 which, via Trk C phosphorylation, accelerates myelination by oligodendrocytes in the mouse brain during the neonatal period.

Arcuate nucleus destruction does not block food deprivation-induced increases in food foraging and hoarding

The mechanisms underlying the control of food intake are considerably better understood than those underlying the appetitive ingestive behaviors of foraging and hoarding of food, despite the prevalence of the latter across species including humans. Neuropeptide Y (NPY) and Agouti-related protein (AgRP), two orexigenic neuropeptides known to stimulate food intake in a variety of species, applied centrally to Siberian hamsters increases foraging and especially hoarding with lesser increases in food intake. Both are expressed in the arcuate nucleus (Arc) and their synthesis increases with food deprivation, a naturally-occurring stimulus that markedly increases foraging and hoarding in Siberian hamsters. Therefore, we tested whether destruction of Arc neurons blocks these ingestive behaviors. This was accomplished either by microinjecting NPY conjugated to saporin (NPY-SAP) bilaterally into the Arc to kill NPY receptor-bearing neurons or via neonatal monosodium glutamate (MSG) treatment. For both methods, Arc cresyl violet staining (cell density) and NPY and Y1 receptor-immunoreactivity (ir) were significantly decreased. Although baseline foraging and food hoarding were not affected, food deprivation-induced increased food hoarding was surprisingly exaggerated approximately 100% with both types of Arc destruction. We found a substantial amount of remaining NPY-ir fibers, likely emanating from the brainstem, and a significant up-regulation of Y1 receptors in Arc NPY projections areas (hypothalamic paraventricular nucleus and perifornical area) after Arc denervation and their activation may have accounted for the exaggerated increases. The converging evidence from both Arc destruction methods suggests an intact Arc is not necessary for food deprivation-induced increases in food foraging and hoarding.

Responsiveness of obese Zucker rats to [D-Trp34]-NPY supports the targeting of Y5 receptor for obesity treatment

The increased synthesis and release of neuropeptide Y (NPY) in the hypothalamus participate in the development of overeating and obesity in the Zucker fa/fa rat. The orexigenic effects of NPY are mediated through the Y1 and Y5 receptors. The substitution of [D-Trp34] in the NPY amino-acid sequence increases selectivity without lowering potency at the Y5 receptor. In the present study, to address the role of the NPY Y5 receptor in obesity, we investigated the acute effect of [D-Trp 34]-NPY in lean and obese Zucker rats. Obese rats were markedly hyperphagic (27.1 +/- 0.6 vs. 18.7 +/- 0.4 (lean) g/day; p < 0.01). Injection of [D-Trp34]-NPY in the lateral brain ventricle at a dose of 16 microg stimulated food intake to the same extent in both lean (p < 0.01) and obese (p < 0.01) rats 1 h after injection. This effect was still observed after 6 h (p < 0.01). These results indicate, therefore, that the obese rats are responsive to [D-Trp34]-NPY. They support the role of the neuropeptide Y5 receptor in the regulation of food intake and suggest that NPY Y5 antagonism might be useful for treating obesity.

Sensory and autonomic nerve changes in the monosodium glutamate-treated rat: a model of type II diabetes

Rats that had been injected with monosodium glutamate (MSG) neonatally were studied for up to 70 weeks and compared with age-matched control rats to study changes in glucose tolerance and in sympathetic and sensory nerves. At 61 and 65 weeks of age, there were significant differences in glucose tolerance between the MSG and control groups, and the MSG group had raised fasting blood glucose. These changes were not associated with changes in the number of beta-cells in the islets of Langerhans. In addition, the diabetic MSG-treated rats had central obesity and cataracts. Hypoalgesia to thermal stimuli was present in MSG-treated rats as early as 6 weeks and persisted at 70 weeks. However, no differences were observed in the distribution of substance P, the neurokinin-1 receptor or calcitonin gene-related peptide in the dorsal horn of L3-L5 at this age (70 weeks). Diabetic MSG-treated animals at 65 and 70 weeks of age had significantly reduced noradrenaline concentrations in the heart, tail artery and ileum, while concentrations in the adrenal gland and corpus cavernosum were significantly increased. There was also a significant increase in adrenal adrenaline, dopamine and serotonin, largely attributable to changes in weight of the adrenal gland in the MSG-treated animals. The results indicate that MSG-treated animals develop a form of type II diabetes by about 60 weeks of age, and that there are significant changes in amine levels in various tissues associated with these developments.

Central but not peripheral glucoprivation is impaired in monosodium glutamate-treated rats

In the present study, newborn male Wistar rats were injected, subcutaneously, five times, every other day, with monosodium glutamate (MSG, 4 g/kg bw) or saline (as control, C), during the neonatal period. MSG animals developed destruction of the arcuate nuclei (ARC) with absence of NPY-immunoreactive cell bodies, which impaired both the food intake (baseline) and the 2-deoxy-D-glucose (2DG) glucoprivic feeding response. Increases in the immunoreactivity of corticotropin-releasing hormone-cell bodies in the paraventricular nuclei might have developed to compensate for the atrophy of the pituitary in MSG-treated rats. After systemic 2DG injection, neither the C nor the MSG rats increased their food intake, but they showed similar hyperglycemic responses, whereas plasma free fatty acids (FFA) increased only in the C group. In other groups, 2DG, norepinephrine (NE), neostigmine (NEO) and saline were intracerebroventricularly (i.c.v.) administered. In this condition, impairment of the hyperglycemic and food intake responses, associated to a lower increase in plasma FFA levels, were observed. As opposed to this, the MSG treatment gives support to NE effects, enhancing food intake, as well as plasma glucose and FFA levels. After NEO, plasma glucose increased only in the MSG group, while plasma FFA levels were elevated in the C rats. Taken together, the results obtained after MSG treatment point to a separate neural control of the hyperglycemic response and of the lipid mobilization when stimulated by central 2DG, NE or NEO administration. It seems likely that the excitatory neural pathway that controls lipid metabolism and is present in C rats was destroyed by the MSG treatment.

Development of neurological reflexes and motor coordination in rats neonatally treated with monosodium glutamate

Monosodium glutamate (MSG) treatment of neonatal rats causes neuronal degeneration in various brain areas and leads to several neurochemical, endocrinological and behavioral alterations. However, relatively little is known about the development of neurological reflexes and motor coordination of these animals. Therefore, the aim of the present study was to examine the neurobehavioral development of newborn rats treated with MSG. Rats received MSG at postnatal days 1, 3, 5, 7, and 9. Appearance of neural reflexes and reflex performance as well as motor coordination were examined for 5 weeks after birth. The efficacy of MSG treatment was confirmed by histological examination of the arcuate nucleus. We found that MSG treatment delayed the appearance of forelimb placing, forelimb grasp and righting reflexes, besides the retarded somatic development. The treated pups performed surface righting in significantly longer times. Also, worse performance was observed in the foot-fault and rota-rod tests. However, MSG-treated rats reached control levels by the end of the fifth postnatal week. These results show that MSG treatment does not cause permanent alterations in the neurobehavioral development, only delays the appearance of some reflexes and leads to temporary changes in reflex performance and motor coordination signs.