Neurotoxic lesions induced by monosodium glutamate result in increased adenopituitary proopiomelanocortin gene expression and decreased corticosterone clearance in rats

Analysis of angiotensin II- and ACTH-driven mineralocorticoid functions and omental adiposity in a non-genetic, hyperadipose female rat phenotype

The hypothalamic damage induced by neonatal treatment with monosodium L -glutamate (MSG) induces several metabolic abnormalities, resulting in a rat hyperleptinemic-hyperadipose phenotype. This study was conducted to explore the impact of the neonatal MSG treatment, in the adult (120 days old) female rat on: (a) the in vivo and in vitro mineralocorticoid responses to ACTH and angiotensin II (AII); (b) the effect of leptin on ACTH- and AII-stimulated mineralocorticoid secretions by isolated corticoadrenal cells; and (c) abdominal adiposity characteristics. Our data indicate that, compared with age-matched controls, MSG rats displayed: (1) enhanced and reduced mineralocorticoid responses to ACTH and AII treatments, respectively, effects observed in both in vivo and in vitro conditions; (2) adrenal refractoriness to the inhibitory effect of exogenous leptin on ACTH-stimulated aldosterone output by isolated adrenocortical cells; and (3) distorted omental adiposity morphology and function. This study supports that the adult hyperleptinemic MSG female rat is characterized by enhanced ACTH-driven mineralocorticoid function, impaired adrenal leptin sensitivity, and disrupted abdominal adiposity function. MSG rats could counteract undesirable effects of glucocorticoid excess, by developing a reduced AII-driven mineralocorticoid function. Thus, chronic hyperleptinemia could play a protective role against ACTH-mediated allostatic loads in the adrenal leptin resistant, MSG female rat phenotype.

Arginine vasopressin is an important regulator in antinociceptive modulation of hypothalamic paraventricular nucleus in the rat

Our previous study has proven that hypothalamic paraventricular nucleus (PVN) stimulation increases pain threshold and PVN cauterization decreases pain threshold. The studied neuropeptides in PVN were investigated to involve to pain modulation in the rat. The results showed that (1) intraventricular injection (icv) of anti-arginine vasopressin (AVP) serum completely reversed pain threshold increase induced by l-glutamate sodium (Glu) injection into the PVN, and local administration (icv) of anti-leucine-enkephalin (L-Ek) serum or anti-beta-endorphin (beta-Ep) serum partly attenuated pain threshold increase induced by Glu injection into the PVN, but pre-treatment of anti-oxytocin (OXT), dynorphinA(1-13) (DynA(1-13)), cholecystokinin-like peptide (CCK), neurotensin (NT), corticotrophin-releasing hormone (CRH), adrenocorticotrophin (ACTH), somatostatin (SST), prolactin-releasing hormone (PRH), angiotensinII (AngII), vasoactive intestinal polypeptide (VIP), melanotropin-releasing hormone (MRH), thyrotropin-releasing hormone (TRH), substance P (SP) or growth hormone-releasing hormone (GHRH) serum (icv) did not influence the analgesic effect of PVN administration with Glu; (2) PVN stimulation with Glu elevated the concentrations of AVP, OXT, CCK, NT, CRH, SST, PRH and DynA(1-13) in PVN perfusion liquid, and could not change the concentrations of L-Ek, beta-Ep, AngII, ACTH, VIP, MRH, TRH, SP and GHRH in PVN perfusion liquid; (3) Pain stimulation increased the concentrations of AVP, L-Ek, beta-Ep, DynA(1-13), CRH and ACTH in PVN perfusion liquid, and did not alter the concentrations of OXT, CCK, NT, SST, PRH, AngII, VIP, MRH, TRH, SP and GHRH in PVN perfusion liquid. The data suggested that AVP played a more important role than the other studied peptides (OXT, L-Ek, beta-Ep, DynA(1-13), CCK, NT, CRH, ACTH, SST, PRH, AngII, VIP, MRH, TRH, SP and GHRH) in PVN antinociceptive progress.

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.

Impact of transient correction of increased adrenocortical activity in hypothalamo-damaged, hyperadipose female rats

OBJECTIVE

To explore the effects of transient correction of enhanced corticoadrenal activity in monosodium L-glutamate (MSG)-damaged female rats on peripheral insulin sensitivity and in vitro retroperitoneal (RP) adipocyte function.

DESIGNS

A dose of 4 mg/g body weight (BW) of MSG or vehicle (CTR) was i.p. injected, once every 2 days, between days 2 and 10 of age, in female rats. Intact and 21 day-operated (sham or adrenal enucleation (AE)) rats from both (CTR and MSG) groups were used for experimentation on day 120 of age. Circulating levels of several hormones, in basal and after i.v. high-glucose load conditions, and RP adiposity morphology and function were then evaluated.

RESULTS

MSG rats developed increased adrenocortical function, hyperadiposity, hyperleptinemia, hyperinsulinemia and decreased peripheral insulin sensitivity. These characteristics were fully reversed after transient correction of corticoadrenal hyperactivity induced by AE. In addition, in vitro experimentation with isolated RP adipocytes indicated that cells from intact MSG animals displayed decreased sensitivity to insulin and dexamethasone stimulation of leptin secretion. Interestingly, adipocyte dysfunction in MSG rats was fully abrogated after AE-induced transient correction of insulinemia, leptinemia and adrenocortical activity. Importantly, the reversion of these metabolic abnormalities, induced by AE for 21 days, in MSG animals did occur, despite no significant changes in BW values.

CONCLUSION

Our results support that the changes in adipocyte characteristics and peripheral insulin resistance, developed in this pseudo-obese female rat model, are mainly due to increased glucocorticoid production. Importantly, appropriate correction of the enhanced adrenocortical activity fully reversed these abnormal functions.

Neuroendocrine changes in adult female rats prenatally exposed to phenytoin

Neuroendocrine changes in fetal hydantoin syndrome have not been described yet. This study was aimed to verify the hypothesis that prenatal exposure to phenytoin influences the stress response of adult female offspring in an animal model. To study possible development of depression like state, hedonic behavior and long-term changes in neuropeptide gene expression in the hypothalamus were investigated. Treatment consisted of per os administration of 150 mg/kg of phenytoin or water daily, from day 7-18 of gestation. Adult female offspring (6 animals per group) were acutely stressed by 1 min handling. Blood was collected in conscious rats via tail artery cannulas before, 1, 15 and 30 min after the handling. Exposure to phenytoin in uterus resulted in increased catecholamine and corticosterone concentrations in response to a mild stressor of 1 min handling in adult offspring. The gestational treatment used in this study did not induce a depression like state nor long-term changes in neuropeptide gene expression in the adult offspring. In conclusion, prenatal exposure to phenytoin treatment enhanced the stress response of adult female offspring. Possible new component of fetal hydantoin syndrome is the increase in catecholamine release in response to a mild stressor in adulthood.

Orexin a stimulates hypothalamic-pituitary-adrenal (HPA) axis function, but not food intake, in the absence of full hypothalamic NPY-ergic activity

Neonatal monosodium L-glutamate (MSG) treatment destroys hypothalamic arcuate nucleus neuronal bodies, thus inducing several metabolic abnormalities. As a result, rats develop a phenotype characterized by hyperleptinemia and by impaired NPY but normal preproorexin hypothalamic mRNAs expression. Thus, our study was designed to explore whether hypothalamic effects of orexin A on food intake and glucocorticoid production develop in the absence of full hypothalamic NPY-ergic activity. For this purpose we evaluated, in control and MSG-treated rats, the consequences of intracerebroventricular (icv) orexin A administration on food intake and changes in circulating levels of ACTH and glucocorticoid. Our results indicate that orexin A icv treatment stimulated hypothalamic-pituitary-adrenal (HPA) axis activity in both MSG-damaged and normal animals, with this response even more pronounced in neurotoxin-damaged rats. Conversely, food intake was only enhanced by icv orexin A injection in normal rats. Our study further supports that acute hypothalamic effects of orexin A on food intake and glucocorticoid production are due to independent neuronal systems. While intact arcuate nucleus activity is needed for the orexinergic effect induced by icv orexin A administration, conversely, orexin A-stimulated HPA axis function takes place even in the absence of full NPY-ergic activity.

Effect of Chronic Emotional Stress on Habituation Processes in Open Field in Adult Rats

In our previous study, repeated stress in the neonatal period was found to slow habituation in the open field in adult rats. The objective of the present study was to investigate how chronic stress can affect habituation processes in the open field when occurring in adulthood. Animals were exposed to 1 week of immobilization on metal boards followed by 1 week of hypokinesis. After the stress procedure, rats were tested in the open field (once daily in 6-min sessions for four consecutive days). Immediately after the last open-field test, animals were decapitated. The rapidity of between- and within-session habituation was lower than in control rats. However, this lowering failed to be statistically significant compared with control rats. On the other hand, time latency to step down from an elevated platform was significantly increased in stress-exposed animals. Four days after the last stressful event, corticotropin-releasing hormone mRNA levels in the paraventricular nucleus were significantly increased, indicating a long-term activation of the hypothalamic-pituitary-adrenocortical axis. The results suggest that, in contrast to neonatal stress exposure, chronic emotional stress in adult rats does not represent a risk factor for the alteration of habituation processes.

Adrenal enucleation in MSG-damaged hyperleptinemic male rats transiently restores adrenal sensitivity to leptin

It is known that the neonatal treatment of rats with monosodium L-glutamate (MSG) induces several metabolic abnormalities, resulting in enhanced adiposity and hyperleptinemia. Our study was designed to explore the consequences of MSG-induced chronic hyperleptinemia on adrenal sensitivity to the inhibitory effect of exogenous leptin. Neonatal male rats treated with MSG or vehicle (controls, CTR) were followed during 150 days in order to study changes observed over development in body weight, food consumption as well as in vivo hypothalamo-pituitary-adrenal (HPA) axis and adipocyte functions. During adulthood, adrenal response to adrenocorticotropin (ACTH) was evaluated both in vitro and in vivo in order to determine the adrenal sensitivity to the inhibitory effect of leptin. For this purpose, sham-operated as well as CTR and MSG rats with bilateral adrenal enucleation (AE) were used. Our results indicate that: (1) between 30 and 150 days of age, MSG animals developed hypophagia, accompanied by arrest in body weight gain, and concomitant enhanced basal levels of all HPA axis components and of leptin; (2) adrenals from of 150-day- old MSG rats displayed an in vitro adrenocortical hyperresponse to ACTH stimulation as well as an adrenal refractoriness to the physiological inhibitory effect of leptin on ACTH-stimulated glucocorticoid output, and (3) bilateral AE in adult MSG-treated rats transiently reversed the MSG-induced hyperleptinemia, restoring normal leptin levels as well as a normal adrenal sensitivity to the inhibitory effect of leptin. Our data indicate that adrenal exposure to the chronically high plasma leptin levels observed in MSG rats is involved in the loss of the inhibitory regulatory effect of leptin at the adrenal level, being therefore, at least in part, responsible for the increased total and free glucocorticoid production measured in MSG adult rats. Furthermore, this study strongly suggests that the adrenal overfunction, frequently associated with different phenotypes of obesity, could be due to an adrenal resistance to the leptin-negative regulation.

Voluntary wheel running modulates glutamate receptor subunit gene expression and stress hormone release in Lewis rats

Lewis rats that are known to be addiction-prone, develop compulsive running if they have access to running wheels. The present experiments were aimed 1) to evaluate the activation of stress systems following chronic and acute voluntary wheel running in Lewis rats by measurement of hormone release and gene expression of neuropeptides related to hypothalamic-pituitary-adrenocortical (HPA) axis activity and 2) to test the hypothesis that wheel running as a combined model of addictive behavior and stress exposure is associated with modulation of ionotropic glutamate receptor subunits in the ventral tegmental area. Voluntary running for three weeks but not for one night resulted in a rise in plasma corticosterone and adrenocorticotropic hormone (ACTH) levels (p<0.05) compared to those in control rats. Principal component analysis revealed the relation between POMC gene expression in the intermediate pituitary and running rate. Acute exposure of animals to voluntary wheel running induced a significant decrease in alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor GluR1 subunit mRNA levels (p<0.01), while repeated voluntary physical activity increased levels of GluR1 mRNA in the ventral tegmentum (p<0.05). Neither acute nor chronic wheel running influenced N-methyl-D-aspartate (NMDA) receptor subunit NR1 mRNA levels in the ventral tegmental area. Thus, the present study revealed changes in AMPA receptor subunit gene expression in a reward-related brain structure as well as an activation of HPA axis in response to compulsive wheel running in Lewis rats. It may be suggested that hormones of HPA axis and glutamate receptors belong to the factors that substantiate higher vulnerability to addictive behavior.

Neonatal administration of monosodium glutamate prevents the development of ethanol- but not psychostimulant-induced sensitization: a putative role of the arcuate nucleus

Lesions of the arcuate nucleus by monosodium glutamate, goldthioglucose and oestradiol valerate treatments are known to prevent the acute stimulating effect of ethanol in mice. On the basis of these results, the current study analysed whether a lesion of the arcuate nucleus by monosodium glutamate was able to block ethanol-induced locomotor sensitization. To produce the arcuate nucleus lesions, pups were injected with saline or monosodium glutamate (4 mg/g body weight) subcutaneously on 5 alternate days, starting on postnatal day one. Sensitization treatments began 10 weeks after the initial lesions. Sensitization training consisted of six trials on alternate days, in which groups of mice were treated with ethanol (2 g/kg) or saline, and then tested in an open-field for the induction of locomotor activity. The present study demonstrated that animals with monosodium glutamate-induced lesions did not develop locomotor sensitization to ethanol. Different groups of mice were used to assay blood ethanol levels and to evaluate the effect of arcuate nucleus lesions on psychostimulant-induced locomotor sensitization. Sensitization to cocaine or amphetamine was spared in monosodium glutamate-pre-treated animals, although the lesion of arcuate nucleus reduced the sensitivity of mice to cocaine. Our findings therefore suggest that the arcuate nucleus may be critical for the neuroadaptations that underlie the behavioural sensitization to ethanol, in contrast to those mediating psychostimulant-induced sensitization.

Repeated citalopram treatment but not stress exposure attenuates hypothalamic-pituitary-adrenocortical axis response to acute citalopram injection

Many experimental, clinical and epidemiological studies have shown a direct connection between exposure to stress or adverse life events and disease, but little is known about the effect of stress on the action of drugs. The aim of this study was to test the hypothesis that previous exposure to stress changes the action of the antidepressant drug citalopram (10 mg/kg, i.p.) on hypothalamic-pituitary-adrenocortical (HPA) axis function, gene expression of selected neuropeptides and serotonin reuptake. Three different stress models were used, which included immobilization, restraint and unpredictable stress stimuli. Samples of plasma for hormone measurement were taken from conscious cannulated animals. Changes in corticotropin-releasing hormone (CRH) and proopiomelanocortin (POMC) gene expression in the paraventricular nucleus of the hypothalamus and the anterior pituitary, respectively, and the ability of citalopram to inhibit serotonin reuptake were investigated. The exposure to three different stress models did not influence citalopram action on individual parameters of HPA axis and on serotonin reuptake. On the other hand, repeated administration of the drug led to significant attenuation of ACTH and CRH mRNA responses. The present results allow to suggest that the stressors used did not influence serotonergic neurotransmission to the extent that would modify HPA axis response to citalopram challenge. Activation of HPA axis by acute citalopram treatment was found to be accompanied by increased CRH gene expression in the hypothalamus. Repeated administration of the drug led to the development of tolerance to activation of central and peripheral components of HPA axis, but not to serotonin reuptake inhibition.

Enhanced glucocorticoid feedback inhibition of hypothalamo-pituitary-adrenal responses to stress in adult rats neonatally treated with dexamethasone

We studied the long-term effect of neonatal treatment with the synthetic glucocorticoid dexamethasone (DEX) on stress responsivity later in life. It was found that the plasma adrenocorticotropin hormone (ACTH) and corticosterone (CORT) responses induced by novelty or conditioned fear stress were markedly attenuated in adult rats that had been neonatally treated with DEX as compared with saline (SAL)-treated controls. Since there were no differences in the heart rate, body temperature, plasma noradrenaline, plasma adrenaline and behavioral responses to these stressors, this points to a deficit within the hypothalamic-pituitary-adrenal (HPA) axis of DEX rats. We found no differences between DEX and SAL rats in basal plasma CORT concentrations measured throughout the circadian cycle, nor in the fraction unbound of CORT circulating under resting conditions, indicating normal tonic regulation of the HPA axis in DEX rats. Since we also found no differences in the hormonal responses induced by intravenous injection of graded doses of ACTH or corticotropin-releasing hormone (CRH), we investigated the sensitivity of the HPA response to stress for inhibition by glucocorticoids. Pretreatment with a low dose of CORT that did not affect the HPA response of SAL rats markedly inhibited the ACTH and CORT responses induced by novelty stress in DEX rats. This strongly suggests that an enhanced corticosteroid feedback underlies the blunted HPA response to stress in DEX rats. Finally, using quantitative immunocytochemistry, we found an increase in arginine-vasopressin (AVP) but not CRH stores in the external zone of the median eminence, suggesting an altered AVP/CRH ratio in the secretory output of the hypophysiotropic paraventricular nucleus. Taken together, our results show that exposure to DEX during early life leads to hyporesponsivity of the HPA axis to stress most likely due to hypersensitivity of the axis for negative feedback by corticosteroids at the suprapituitary level.

Corticotropin-releasing hormone mRNA levels in response to chronic mild stress rise in male but not in female rats while tyrosine hydroxylase mRNA levels decrease in both sexes

Corticotropin-releasing hormone (CRH) and catecholamines are suggested to play a significant role in the pathophysiology of depression. In the present study we investigated gene expression of CRH in the hypothalamic paraventricular nucleus (PVN) and tyrosine hydroxylase (TH) in the locus coeruleus (LC) in an experimental model of depression. A chronic mild stress model was applied in rats of both genders for a three-week period. Anhedonic behaviour, a typical sign of depression-like state, was measured by a sucrose preference test. The chronic mild stress induced a decrease in sucrose preference in both genders. The body weight gain was reduced in males only. The total activity in the open field test was unchanged, however, male rats exposed to chronic mild stress showed enhanced locomotor activity during the first minute of the session, suggesting increased anxiety. Basal plasma corticosterone levels, thymus and adrenal weights measured on the third day after cessation of the stress regimen, were not affected by the stress procedure. Evaluation of CRH mRNA levels in the PVN by in situ hybridisation revealed a significant rise in response to chronic mild stress in males. In females, the basal CRH mRNA levels were higher compared to those in males, but the stress-induced rise was absent. Chronic mild stress resulted in a decrease in TH mRNA levels in the LC. These data demonstrate that chronic mild stress model of depression induces a specific stress response with a reduction of TH gene expression in the LC and clear gender differences in gain of body weight, anxiety-like behaviour, and CRH mRNA levels in the PVN.

Daily profiles of arginine vasopressin mRNA in the suprachiasmatic, supraoptic and paraventricular nuclei of the rat hypothalamus under various photoperiods

Daily rhythm of arginine vasopressin (AVP) mRNA levels in the suprachiasmatic nucleus (SCN) of rats maintained under a short, LD 8:16 photoperiod differed from that of rats maintained under a long, LD 16:8 photoperiod: under the short photoperiod the morning AVP rise occurred significantly later than under the long one. Daily profiles of AVP mRNA in the supraoptic and paraventricular nuclei were not rhythmic and AVP mRNA levels under LD 8:16 did not differ from those under LD 16:8. The data indicate that photoperiod affects selectively the clock driven AVP gene expression in the SCN.

Neonatal stress alters habituation of exploratory behavior in adult male but not female rats

The effect of monosodium-L-glutamate (MSG) administration in the neonatal period on habituation of exploratory behavior related to gender differences was investigated. Rats of both sexes were intraperitoneally treated with MSG (4 mg/g) or hypertonic saline (10% NaCl) on postnatal days 2, 4, 6, 8, and 10. On postnatal day 65, the animals were tested in an open-field test during 4 consecutive days, once daily in 6-min sessions. The rapidity of habituation of exploratory behavior during repeated exposure to the open field (interrupted habituation) and over individual sessions (uninterrupted habituation) was evaluated by using the method of linear regression. Compared to intact controls, there were no significant differences found in interrupted habituation, neither in males nor in females. Uninterrupted habituation in neonatally treated males was slowed down in the first 2 days of testing. No differences in adult behavior between treated groups (MSG and hypertonic saline) were observed, i.e., there were no late effects specific for neonatal MSG administration. In females, uninterrupted habituation was not affected. Males proved to be more sensitive to neonatal stress associated with injections of MSG or hypertonic saline than females, and showed feminine-like habituation in the new environment.

Dissociation of changes in hypothalamic corticotropin-releasing hormone and pituitary proopiomelanocortin mRNA levels after prolonged stress exposure

Repeated immobilization stress for 7 days caused marked increase in corticotropin-releasing hormone (CRH) mRNA in the paraventricular nucleus (PVN, 90% above the control) and proopiomelanocortin (POMC) mRNA in the anterior pituitary (73% above the control) in rats. Prolongation of the stress exposure to 42 days led to a decline of CRH mRNA response (42% above the control) in comparison with that after 7 days, while POMC mRNA persisted at the same level. Observed adaptation of CRH but not POMC responses suggests that other factor(s) next to CRH participate in sustained rise in POMC expression during long-term stress.