Monosodium L-glutamate lesions reduce susceptibility to hypoglycemic feeding and convulsions

Lipidized analogues of the anorexigenic CART (cocaine- and amphetamine-regulated transcript) neuropeptide show anorexigenic and neuroprotective potential in mouse model of monosodium-glutamate induced obesity

AIMS

This study investigates the neuroprotective effects of lipidized analogues of 2-SS-CART(61-102) derived from anorexigenic neuropeptide cocaine- and amphetamine-regulated transcript peptide (CARTp) in light of the link between obesity, its comorbidities, and the development of Alzheimer's disease.

METHODS

We introduce novel lipidized analogues derived from 2-SS-CART(61-102), a specific analogue of natural CART(61-102), with two disulfide bridges. Using hypothermic PC12 cells, we tested the effect of the most potent analogues on Tau phosphorylation. We further described the anorexigenic and neuroprotective potential of subcutaneously (SC) injected lipidized CARTp analogue in a mouse model with prediabetes and obesity induced by neonatal monosodium glutamate (MSG) administration.

RESULTS

Compared to the non-lipidized 2-SS-CART(61-102), all lipidized analogues exhibited a potent binding affinity to PC12 cells and enhanced in vitro stability in rat plasma. Two most potent lipidized analogues attenuated hypothermia-induced Tau hyperphosphorylation at multiple epitopes. Subsequently, chronic SC treatment with palm-2-SS-CART(61-102) significantly decreased body weight and food intake, improved metabolic parameters, decreased level of pTau and increased neurogenesis in hippocampi of obese MSG mice.

CONCLUSION

Our unique CARTp analogue palm-2-SS-CART(61-102) shows promise as a potent anti-obesity and neuroprotective agent.

Remote control of glucose-sensing neurons to analyze glucose metabolism

The central nervous system relies on a continual supply of glucose, and must be able to detect glucose levels and regulate peripheral organ functions to ensure that its energy requirements are met. Specialized glucose-sensing neurons, first described half a century ago, use glucose as a signal and modulate their firing rates as glucose levels change. Glucose-excited neurons are activated by increasing glucose concentrations, while glucose-inhibited neurons increase their firing rate as glucose concentrations fall and decrease their firing rate as glucose concentrations rise. Glucose-sensing neurons are present in multiple brain regions and are highly expressed in hypothalamic regions, where they are involved in functions related to glucose homeostasis. However, the roles of glucose-sensing neurons in healthy and disease states remain poorly understood. Technologies that can rapidly and reversibly activate or inhibit defined neural populations provide invaluable tools to investigate how specific neural populations regulate metabolism and other physiological roles. Optogenetics has high temporal and spatial resolutions, requires implants for neural stimulation, and is suitable for modulating local neural populations. Chemogenetics, which requires injection of a synthetic ligand, can target both local and widespread populations. Radio- and magnetogenetics offer rapid neural activation in localized or widespread neural populations without the need for implants or injections. These tools will allow us to better understand glucose-sensing neurons and their metabolism-regulating circuits.

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.

Astrocytic and microglia cells reactivity induced by neonatal administration of glutamate in cerebral cortex of the adult rats

Recent studies confirm that astrocytes and neurons are associated with the synaptic transmission, particularly with the regulation of glutamate (Glu) levels. Therefore, they have the capacity to modulate the Glu released from neurons into the extracellular space. It has also been demonstrated an intense astrocytic and microglia response to physical or chemical lesions of the central nervous system. However, the persistence of the response of the glial cells in adult brain had not been previously reported, after the excitotoxic damage caused by neonatal dosage of monosodium glutamate (MSG) to newborn rats. In this study, 4 mg/g body weight of MSG were administered to newborn rats at 1, 3, 5, and 7 days after birth, at the age of 60 days the astrocytes and the microglia cells were analyzed with immunohistochemical methods in the fronto-parietal cortex. Double labeling to glial fibrillary acidic protein (GFAP) and BrdU, or isolectin-B(4) and BrdU identified astrocytes or microglia cells that proliferated; immunoblotting and immunoreactivity to vimentin served for assess immaturity of astrocytic intermediate filaments. The results show that the neonatal administration of MSG-induced reactivity of astrocytes and microglia cells in the fronto-parietal cortex, which was characterized by hyperplasia; an increased number of astrocytes and microglia cells that proliferated, hypertrophy; increased complexity of the cytoplasm extension of both glial cells and expression of RNAm to vimentin, with the presence of vimentin-positive astrocytes. This glial response to neuroexcitotoxic stimulus of Glu on the immature brain, which persisted to adulthood, suggests that the neurotransmitter Glu could trigger neuro-degenerative illnesses.

Effect of L-glutamate on cholinergic neurotransmission in various brain regions and during the development of rats, when administered perinatally

Glutamate, as a monosodium salt (MSG) has neurotoxic effects on some brain regions when systemically given to young rats. Few studies have been conducted to establish the mechanisms involved in studying neurotoxicity resulting in neuronal death by glutamate (Glu) and its effects as related to different brain neuropathologies under in-vivo conditions and where the cholinergic system shows vulnerability. Thus, this paper aims to evaluate the binding kinetics of quinuclynidyl benzylate (QNB) to muscarinic receptors for acetylcholine and the activity of choline acetyltransferase (CAT) in rats treated with MSG (4 mg/g on days 1, 3, 5, and 7 after birth) during the rat development stages (days 14, 21, 30, and 60) in different brain regions. The results show that perinatal treatment with MSG significantly decreases the CAT activity and increases the affinity of [3H]-QNB and the number of receptors of the brain cortex during the ages studied. The striatum showed increased CAT activity and BMAX on days 30 and 60 after birth. Affinity and the number of receptors increased in the hippocampus only between days 21 through 60 after birth. NaCl given at MSG equimolar doses only modified the CAT activity but had no effect on the [3H]-QNB binding kinetics in any of the regions studied. The results show that MSG alters cholinergic neurotransmission in the central nervous system (CNS) and induces the development of compensating events suggesting an involvement in neuronal plasticity during the development of rat CNS.

Central effects of monosodium glutamate on feeding behavior in adult Long-Evans rats

Monosodium glutamate (MSG) is known as a neurotoxic molecule when injected neonatally in rats, where it produces a marked decrease in food intake and an increase in adipose tissue mass. But, in adult rats subcutaneous injections of MSG produce a small, dose-dependent increase in food intake. It is not known if this action is centrally or systemically mediated. Therefore, the feeding pattern of adult rats injected intracerebroventricularly with MSG was measured. Seven days after installation of a cannula in the right lateral ventricle, rats were injected either with artificial cerebrospinal fluid or twice with 3 mg/brain MSG within a 3-day interval. The feeding pattern was recorded via a complete computerized system during 24 h. Feeding behavior was significantly modified by MSG treatments. These effects were observed immediately after drug injections, that is, upon the first meal, as well as during the 24 h that followed. For the first meal, modifications in meal size (+285%; p = 0.0001), meal duration (x10; p = 0.0005), postmeal interval (x4; p = 0.0005), and the satiety ratio (-50%; p = 0.01) were observed. During the 24-h postinjection period, modifications in meal number (-3; p = 0.0007), total amount of food eaten (+21%,; p = 0.007), time spent eating (+40%; p = 0.007), meal duration (+53%; p = 0.005), and meal size (+44%; p = 0.01) were noted. When the two MSG injections were compared, differences were also noted. For the first meal, postmeal interval (-50%; p < 0.005) and satiety ratio (-50%; p < 0.005) were decreased after the second injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of monosodium glutamate and gold thioglucose on dietary regulation of sympathetic nervous system activity in rodents

Neonatal administration of monosodium glutamate (MSG) disrupts hypothalamic regulation of a number of neuroendocrine systems. Studies described in this report using techniques of norepinephrine (NE) turnover examined sympathetic nervous system (SNS) activity in heart and interscapular brown adipose tissue (IBAT) of animals given MSG as neonates. Although in every experiment overall rates of NE turnover were lower in MSG-treated mice and rats, the differences were due exclusively to diminished tissue NE content, especially in IBAT. Fractional rates of NE turnover did not differ between groups. In contrast to animals with lesions in the ventromedial hypothalamus produced by gold thioglucose (AuTG) or electric current, MSG-treated mice and rats varied SNS activity in heart and IBAT in accord with changes in nutrient intake. Thus, SNS activity, both at baseline and in response to dietary manipulation, is probably not affected by neonatal MSG administration.

A pharmacological analysis of food intake regulation in rats treated neonatally with monosodium L-glutamate (MSG)

Studies were conducted to examine deficits in food intake regulation in MSG-treated rats that result from known or suspected damage to neurotransmitter systems involved in feeding. Male rats were injected with either MSG (4 mg/g) or sodium chloride on postnatal days 2 and 4 (MSG-Lo) or postnatal days 2, 4, 6 and 8 (MSG-Hi). As adults, MSG-treated and control rats (n = 12/group) were examined for deficits in pharmacologically elicited feeding and other measures of food intake regulation. A second group of MSG-treated (n = 9/group) and control rats (n = 12) were used to measure basal blood pressure and nociceptive reactivity in adulthood. Organ weights, body weight and neuropeptide Y (NPY) content in brain regions were determined at the end of the study. MSG-Hi rats consumed significantly less food than controls during the dark part of the light cycle. Both MSG-Hi and MSG-Lo groups ate significantly less food than controls after a 48-hour fast. MSG-Hi and MSG-Lo rats consumed significantly less food than controls in response to 1.0 mg/kg morphine. MSG-Hi rats consumed significantly less food than controls during the dark phase and significantly more food than controls during the light phase in response to naloxone (1.0 mg/kg). MSG-Lo ate significantly more than controls in response to 0.1 mg/kg guanfacine. MSG-Hi and MSG-Lo showed a significant attenuation in diazepam-stimulated feeding when compared to controls. Blood pressure was significantly lower in both MSG-Hi and MSG-Lo rats compared to controls. Tail flick latencies were not altered by MSG-treatment.(ABSTRACT TRUNCATED AT 250 WORDS)