Behavioral deficits in monosodium glutamate rats: specific changes in the structure of feeding behavior

Altered behavior of adult obese rats by monosodium l-glutamate neonatal treatment is related to hypercorticosteronemia and activation of hypothalamic ERK1 and ERK2

OBJECTIVES

Obesity is a metabolic and hormonal disorder with serious social and psychological impacts. There is a close relationship among obesity, neuroendocrine homeostasis and behavioral patterns. However, few data are available in the literature regarding this subject. This study assessed behavior and memory of adult obese rats by monosodium l-glutamate (MSG) neonatal treatment or highly palatable dietary treatment.

METHODS

MSG obesity was induced by subcutaneous injections of MSG (4 mg/g) during the first 5 days of life (Ob-MSG); control group (C-MSG), received saline solution equimolar. Both groups were fed with commercial chow. To induce dietary obesity, 21-day-old rats were assigned to two experimental diets: highly palatable diet (Ob-Diet) and control diet (C-Diet) composed of commercial chow. Ninety-day-old animals were submitted to behavioral assessment by the open-field test and short- and long-term memory by the object recognition test. Biometric variables were obtained, the Lee index was calculated and mass of retroperitoneal and perigonadal fat pads was measured. Furthermore, an altered behavioral profile was investigated by quantification of plasmatic corticosterone, expression, and activity of hypothalamic extracellular signal-regulated kinase protein (ERK) 1 and 2.

RESULTS

Increased Lee index and fat pads were observed in Ob-MSG and Ob-Diet groups. Ob-MSG presented a higher level of anxiety and impaired long-term memory compared to C-MSG, while there was no difference between Ob-Diet and C-Diet. The Ob-MSG group presented a higher level of plasmatic corticosterone and increased phosphorylation of hypothalamic ERK1 and 2.

DISCUSSION

Both treatments induced obesity but only Ob-MSG showed altered behavioral parameters, which is related to increased concentration of corticosterone and hypothalamic ERK1 and 2 activation.

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

Monosodium-glutamate (MSG) is neurotoxic for brain regions devoid of blood-brain barrier when it is injected at high doses during the neonatal period. Neuropeptide Y (NPY) neurons in the arcuate nucleus are particularly sensitive to MSG treatment. But, despite of the large decrease of this potent orexigenic peptide, feeding behavior is only slightly affected. We hypothesized that the hypothalamic NPY receptor system might be modified in these rats. The present study characterizes hypothalamic NPY and NPY receptors in normal and MSG-treated rats. MSG-treated rats were lighter (p < 0.01) and ate 17% less than the control rats (p < 0.01). NPY levels in the mediobasal and mediodorsal hypothalamus were reduced in MSG-treated rats compared to normal rats (-26% and -43%, p < 0.05 and p < 0.01, respectively). Combined hypothalamic Y1 and Y5 NPY receptor density was increased in MSG-treated rats compared to normal rats (+25%, p < 0.04), but affinity remained unaltered. Blockade with a selective Y1 antagonist showed that the Y1 receptor subtype represented more than 90% of the combined Y1 and Y5 receptor populations. The up-regulation of the NPY receptors is an element necessary to maintain food intake at a sufficient level to allow survival and growth of the lesioned rats.

Steroid-induced sexual differentiation of the developing brain: multiple pathways, one goal

Hormone exposure, including testosterone and its metabolite estradiol, induces a myriad of effects during a critical period of brain development that are necessary for brain sexual differentiation. Nuclear volume, neuronal morphology, and astrocyte complexity are examples of the wide range of effects by which testosterone and estradiol can induce permanent changes in the function of neurons for the purpose of reproduction in adulthood. This review will examine the multitude of mechanisms by which steroid hormones induce these permanent changes in brain structure and function. Elucidating how steroids alter brain development sheds light on how individual variation in neuronal phenotype is established during a critical period.

Effects of the neonatal treatment with monosodium glutamate on myenteric neurons and the intestine wall in the ileum of rats

BACKGROUND

The neonatal administration of a 4 mg/g dose of monosodium glutamate (MSG) to rodents leads to neuronal death in the hypothalamus arcuate nucleus, which leads in turn to obesity in the adult animal. However, few studies have investigated the effects on the enteric nervous system. This study evaluated the effects of the neonatal administration of MSG on the frequency and morphometry of the myenteric as well as the ileum wall morphometry of adult Wistar male rats.

METHODS

Whole-mount preparations of ileum samples were stained by the Giemsa or NADH-diaphorase histochemical methods. For histological processing, hematoxylin and eosin staining was used.

RESULTS

The treatment with MSG led to obesity, as shown by the higher values for Lee's index and the weights of periepididimal and retroperitoneal adipose tissues. The Giemsa staining revealed a significantly larger neuronal density in the MSG group, which is explained by smaller physical growth and a reduction in the weight of the small intestine. The mean neuronal profile did not change between groups. The NADH-diaphorase-positive neuronal subpopulation kept its neuronal density but its average cellular profile was reduced in the MSG group. A morphometric analysis of the intestinal wall, muscular layer, villi, and intestinal crypts showed that their characteristics did not change.

CONCLUSIONS

The treatment with MSG did not cause alteration of the total myenteric population of the ileum, but it influenced the NADH-diaphorase-positive subpopulation. From the maintenance of the morphometric parameters of the ileum intestinal wall, we inferred that intestinal function was preserved in obese animals.

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.

Behavioral deficits in adult rats treated neonatally with glutamate

The present study evaluated long-term behavioral consequences of neonatal monosodium-l-glutamate (MSG) treatment in rats. The pups received MSG (3 mg/g sc) daily from postnatal day (PD) 5-12. Data from an automatic activity monitor showed that locomotion of MSG-treated females and males aged 56 and 84 days was significantly reduced. Beginning PD 120, three behavioral tests were performed. As compared to the controls, in the elevated plus maze test, modified to evaluate the adaptive form of spatial memory, MSG-treated animals of both sex had significantly prolonged start and transfer latencies. In the social recognition test, assessing olfactory working memory, MSG-treated males displayed a reduced interest in the juvenile conspecific as the stimulus partner during both the initial exposure and re-exposure performed 30 min later. In the open field test, a significant decrease in the habituation rate was found in MSG-treated animals. Sex-dependent differences in behavioral performance were suggested in the open field and elevated plus maze tests. Behavioral changes are discussed in light of the deficits in perception and processing of visual and olfactory stimuli.

Basomedial hypothalamic injections of neuropeptide Y conjugated to saporin selectively disrupt hypothalamic controls of food intake

Neuropeptide Y (NPY) conjugated to saporin (NPY-SAP), a ribosomal inactivating toxin, is a newly developed compound designed to selectively target and lesion NPY receptor-expressing cells. We injected NPY-SAP into the basomedial hypothalamus (BMH), just dorsal to the arcuate nucleus (ARC), to investigate its neurotoxicity and to determine whether ARC NPY neurons are required for glucoprivic feeding. We found that NPY-SAP profoundly reduced NPY Y1 receptor and alpha MSH immunoreactivity, as well as NPY, Agouti gene-related protein (AGRP), and cocaine and amphetamine-related transcript mRNA expression in the BMH. NPY-SAP lesions were localized to the injection site with no evidence of retrograde transport by hindbrain NPY neurons with BMH terminals. These lesions impaired responses to intracerebroventricular (icv) leptin (5 microg/5 microl x d) and ghrelin (2 microg/5 microl), which are thought to alter feeding primarily by actions on ARC NPY/AGRP and proopiomelanocortin/cocaine and amphetamine-related transcript neurons. However, the hypothesis that NPY/AGRP neurons are required downstream mediators of glucoprivic feeding was not supported. Although NPY/AGRP neurons were destroyed by NPY-SAP, the lesion did not impair either the feeding or the hyperglycemic response to 2-deoxy-D-glucose-induced blockade of glycolysis use. Similarly, responses to glucagon-like peptide-1 (GLP-1, 5 microg/3 microl icv), NPY (5 microg/3 microl icv), cholecystokinin octapeptide (4 microg/kg ip), and beta-mercaptoacetate (68 mg/kg ip) were not altered by the NPY-SAP lesion. Thus, NPY-SAP destroyed NPY receptor-expressing neurons in the ARC and selectively disrupted controls of feeding dependent on those neurons but did not disrupt peptidergic or metabolic controls dependent upon circuitry outside the BMH.

PVN activation is suppressed by repeated hypoglycemia but not antecedent corticosterone in the rat

The mechanism(s) underlying hypoglycemia-associated autonomic failure (HAAF) are unknown. To test the hypothesis that the activation of brain regions involved in the counterregulatory response to hypoglycemia is blunted with HAAF, rats were studied in a 2-day protocol. Neuroendocrine responses and brain activation (c-Fos immunoreactivity) were measured during day 2 insulin-induced hypoglycemia (0.5 U insulin x 100 g body x wt(-1) x h(-1) iv for 2 h) after day 1 hypoglycemia (Hypo-Hypo) or vehicle. Hypo-Hypo animals demonstrated HAAF with blunted epinephrine, glucagon, and corticosterone (Cort) responses and decreased activation of the medial hypothalamus [the paraventricular (PVN), dorsomedial (DMH), and arcuate (Arc) nuclei]. To evaluate whether increases in day 1 Cort were responsible for the decreased hypothalamic activation, Cort was infused intracerebroventricularly (72 microg) on day 1 and the response to day 2 hypoglycemia was measured. Intracerebroventricular Cort infusion failed to alter the neuroendocrine response to day 2 hypoglycemia, despite elevating both central nervous system and peripheral Cort levels. However, day 1 Cort blunted responses in two of the same hypothalamic regions as Hypo-Hypo (the DMH and Arc) but not in the PVN. These results suggest that decreased activation of the PVN may be important in the development of HAAF and that antecedent exposure to elevated levels of Cort is not always sufficient to produce HAAF.

Neonatal monosodium glutamate treatment alters rat intestinal muscle reactivity to some agonists

The following study is an investigation of the changes in the contractile reactivity of visceral muscles in response to agonists and alterations in metabolic parameters after neonatal rat treatment with monosodium-L-glutamate. This treatment markedly sensitizes ileum and colon preparations to adenosine-5'-triphosphate (ATP) stimulation and also increases the colon activity to acetylcholine (p<0.05). Response to bradykinin remained unchanged, while ileum activity to angiotensin II was characterized by a reduction in the maximal tension (E(max)) and an increase in the EC(50) (p<0.05) value. The responses of nonintestinal muscle preparations from monosodium-glutamate-treated rats to both ATP and bradykinin did not show a significant difference when compared to the controls. This treatment diminished food intake, feces excretion and increased plasma insulin, nonesterified fatty acids and triglyceride concentrations (p<0.001). These results suggest that the changes in intestinal muscle activity, in response to agonists, can be due to metabolic alterations as well as the monosodium glutamate action on enteric neurons and/or smooth muscle receptors.

Neonatal monosodium glutamate alters circadian organization of feeding, food anticipatory activity and photic masking in the rat

In rodents, parenteral administration of monosodium glutamate (MSG) induces marked degeneration of the retina and arcuate nucleus (AN) and disrupts daily rhythms of food intake. We quantified the effects of neonatal MSG (2 mg/g SC, postnatal days 1, 3, 5, 7, 9) on the expression of feeding and activity rhythms in adult rats under schedules of light-dark (LD), constant dark (DD), restricted daily feeding and total food deprivation. AN lesions were confirmed by neuropeptide Y (NPY) immunocytochemistry and Nissl stain. Compared to age-matched control rats, the amplitude (quantified as LD ratios) of daily food intake and food-bin activity rhythms was significantly attenuated in MSG rats in LD 12:12 and on the first day of DD. Control rats, but not MSG rats, showed lower amplitude rhythms in DD compared to LD. The phase angle of feeding and activity rhythms did not differ between groups in either condition. In a short LD cycle (2:2), control rats, but not MSG rats, showed significant inhibition (masking) of activity during the 2 h light periods. When food access was restricted to a 4 h daily meal, MSG rats showed enhanced expression and persistence of food-entrained anticipatory activity rhythms by comparison with control rats. These results indicate that attenuation of daily feeding rhythms in MSG rats is due in part to loss of direct inhibitory effects of light on behavior, and that the AN likely modulates, but does not mediate entrainment of feeding-related rhythms to daily cycles of LD or food access.

Evidence that hypothalamic neurotensin signals leptin effects on feeding behavior in normal and fat-preferring rats

Leptin inhibits food intake when it is injected in the periphery or in the central nervous system. It is likely that its action is not only mediated by the inhibition of orexigenic peptides such as neuropeptide Y. Therefore, we characterized the pharmacological and physiological relationships of leptin with neurotensin (NT), a central feeding inhibitor. Firstly, we investigated the central interactions of leptin and NT. Intracerebro-ventricular (ICV) injection were done in normal Long-Evans rats. NT had a short lasting (30 minutes; p<0.01) inhibitory effect on spontaneous food intake measured at the beginning of the dark phase whereas the effect of ICV leptin was observed after 24 hours (p<0.001). Co-injection with leptin potentiated NT effect at 30 minutes (p<0. 001) and prolonged it for 30 additional minutes (p<0.01). In addition, NT potentiated the effect of leptin at 30 and 60 minutes (p<0.02 and p<0.001 respectively) but not at 24 hours. Secondly, we observed that NT concentrations were augmented in selective brain areas in fat-preferring rats (+ 34% for hypothalamic NT; p<0.03). This increase was observed in the parvocellular part of the paraventricular nucleus (PVNp) only and was associated with an increase in circulating leptin levels (+ 75%; p<0.003). Interestingly, plasma leptin and NT in the PVNp were strongly correlated (r=0.57; p<0.003), suggesting changes of NT processing or release in this nucleus. These results strongly suggest that the short-term anorexigenic effects of leptin in normal rats are at least partly mediated by changes in NT processing or release. They also suggest that these processes take place in the hypothalamus, most probably in the PVNp and that they might be sensitive to fat ingestion. Therefore, the neurotensin increase observed in fat-preferring rats would limit the overconsumption of energy, a physiological mechanism translated by leptin.