Neonatal monosodium glutamate lesions alter neurosensitivity to ethanol in adult mice

Proopiomelanocortin peptides are not essential for development of ethanol-induced behavioral sensitization

BACKGROUND

Behavioral sensitization is a result of neuroadaptation to repeated drug administration and is hypothesized to reflect an increased susceptibility to drug abuse. Proopiomelanocortin (POMC) derived peptides including beta-endorphin and alpha-melanocyte stimulating hormone have been implicated in development of behavioral sensitization and the reinforcing effects of alcohol and other drugs of abuse. This study used a genetically engineered mouse strain that is deficient for neural POMC to directly determine if any POMC peptides are necessary for the development of ethanol-induced locomotor sensitization.

METHODS

Adult female mice deficient for POMC in neurons only (Pomc(-/-)Tg/Tg, KO) and wildtype (Pomc(+/+)Tg/Tg, WT) littermates were injected once daily with either saline or ethanol (i.p.) for 12 to 13 days. On ethanol test day (day 13 or 14) all mice from both treatment groups received an i.p. injection of ethanol immediately before a 15-minute analysis of locomotor activity. Blood ethanol concentration (BEC) was measured on ethanol test day immediately following the test session. Baseline locomotor activity was measured for 15 minutes after a saline injection 2 days later in both groups.

RESULTS

There was no significant difference in BEC between genotypes (WT = 2.11 +/- 0.06; KO = 2.03 +/- 0.08 mg/ml). Both WT and nPOMC-deficient mice treated repeatedly with ethanol demonstrated a significant increase in locomotor activity on test day when compared to repeated saline-treated counterparts. In addition, mice of both genotypes in the repeated saline groups showed a significant locomotor stimulant response to acute ethanol injection.

CONCLUSIONS

Central POMC peptides are not required for either the acute locomotor stimulatory effect of ethanol or the development of ethanol-induced locomotor sensitization. While these peptides may modulate other ethanol-associated behaviors, they are not essential for development of behavioral sensitization.

Ethanol injected into the hypothalamic arcuate nucleus induces behavioral stimulation in rats: an effect prevented by catalase inhibition and naltrexone

It is suggested that some of the behavioral effects of ethanol, including its psychomotor properties, are mediated by beta-endorphin and opioid receptors. Ethanol-induced increases in the release of hypothalamic beta-endorphin depend on the catalasemic conversion of ethanol to acetaldehyde. Here, we evaluated the locomotor activity in rats microinjected with ethanol directly into the hypothalamic arcuate nucleus (ArcN), the main site of beta-endorphin synthesis in the brain and a region with high levels of catalase expression. Intra-ArcN ethanol-induced changes in motor activity were also investigated in rats pretreated with the opioid receptor antagonist, naltrexone (0-2 mg/kg) or the catalase inhibitor 3-amino-1,2,4-triazole (AT; 0-1 g/kg). We found that ethanol microinjections of 64 or 128, but not 256 microg, produced locomotor stimulation. Intra-ArcN ethanol (128 microg)-induced activation was prevented by naltrexone and AT, whereas these compounds did not affect spontaneous activity. The present results support earlier evidence indicating that the ArcN and the beta-endorphinic neurons of this nucleus are necessary for ethanol to induce stimulation. In addition, our data suggest that brain structures that, as the ArcN, are rich in catalase may support the formation of ethanol-derived pharmacologically relevant concentrations of acetaldehyde and, thus be of particular importance for the behavioral effects of ethanol.

Infusions of acetaldehyde into the arcuate nucleus of the hypothalamus induce motor activity in rats

AIMS

The hypothalamic arcuate nucleus (ARH) is one of the brain regions with the highest levels of catalase expression. Acetaldehyde, metabolized from ethanol in the CNS through the actions of catalase, has a role in the behavioral effects observed after ethanol administration. In previous studies acetaldehyde injected in the lateral ventricles or in the substantia nigra reticulata (SNR) mimicked the behavioral stimulant effects of centrally administered ethanol.

MAIN METHODS

In the present study we assessed the effects of acetaldehyde administered either into the ARH into a dorsal control or into the third ventricle on locomotion and rearing observed in 30 min sessions in an open field.

KEY FINDINGS

Acetaldehyde injected into the ARH induced horizontal locomotion and rearing for 20 min. In contrast, administration of acetaldehyde into a control site dorsal to the ARH did not have any effect on locomotion. Although acetaldehyde administration into the third ventricle also induced locomotion, the time course for the effect in this area was different from the time course following ARH injections. Acetaldehyde in the ARH produced a long lasting induction of locomotion, while with intraventricular injections the effects disappeared after 5 min.

SIGNIFICANCE

The present results are consistent with previous studies demonstrating that acetaldehyde is an active metabolite of ethanol, which can have locomotor stimulant properties when administered in the ventricular system of the brain or into specific brain nuclei. Some brain nuclei rich in catalase (i.e.; SNR and ARH) could be mediating some of the locomotor stimulant effects of ethanol through its conversion to acetaldehyde.

Calorie restriction alters physical performance but not cognition in two models of altered neuroendocrine signaling

A major neuroendocrinological effect of calorie restriction (CR) is induction of neuropeptide Y (NPY) in the arcuate nucleus (ARC). Aside from its appetite-stimulating effects, NPY is thought to be involved in the modulation of behavioral processes including anxiety and learning and memory. In the present study physical fitness, anxiety, and learning/memory-related tasks were assessed in mice lacking NPY or a functional ARC after dietary manipulation by CR. Physical fitness was improved by CR when measured by inclined screen and rotarod, and this diet effect was not affected by NPY or ARC status. As has been observed previously, the NPY knockout mice displayed heightened anxiety in an open field. This phenotype was not fully recapitulated in the ARC-lesioned model. CR affected neither total locomotor activity in the open field nor thigmotaxic behavior in these models. Neither NPY nor CR had a significant effect on Morris water maze performance; however, ARC-damaged mice were unable to learn the task, and this deficit was not corrected by CR. We conclude that despite established effects of CR on ARC signaling, our results suggest a mechanistic separation between the two where behavior is concerned.

Catalase inhibition in the Arcuate nucleus blocks ethanol effects on the locomotor activity of rats

Previous studies have demonstrated that there is a bidirectional modulation of ethanol-induced locomotion produced by drugs that regulate brain catalase activity. In the present study we have assessed the effect in rats of intraperitoneal, intraventricular or intracraneal administration of the catalase inhibitor sodium azide in the locomotor changes observed after ethanol (1 g/kg) administration. Our results show that sodium azide prevents the effects of ethanol in rats locomotion not only when sodium azide was systemically administered but also when it was intraventricularly injected, then confirming that the interaction between catalase and ethanol takes place in Central Nervous System (CNS). Even more interestingly, the same results were observed when sodium azide administration was restricted to the hypothalamic Arcuate nucleus (ARC), a brain region which has one of the highest levels of expression of catalase. Therefore, the results of the present study not only confirm a role for brain catalase in the mediation of ethanol-induced locomotor changes in rodents but also point to the ARC as a major neuroanatomical location for this interaction. These results are in agreement with our reports showing that ethanol-induced locomotor changes are clearly dependent of the ARC integrity and, especially of the POMc-synthesising neurons of this nucleus. According to these data we propose a model in which ethanol oxidation via catalase could produce acetaldehyde into the ARC and to promote a release of beta-endorphins that would activate opioid receptors to produce locomotion and other ethanol-induced neurobehavioural changes.

Opposite effects of acute versus chronic naltrexone administration on ethanol-induced locomotion

Several studies have pointed out that the mu opioid receptor (MOR) can play a key role in some of the behavioural effects of ethanol. In the present study, the implication of the MOR in ethanol-induced locomotion in mice was assessed. First, the effects of the administration of different naltrexone doses (0.001-1.000 mg/kg) on the locomotor changes produced by ethanol (2.5 g/kg) were evaluated. In a second set of experiments, the ability of repeated naltrexone (6 mg/kg) administrations to modify the effects of ethanol was also assessed on mice locomotion. The results of the present study revealed that an acute naltrexone administration reduced dose-dependently ethanol-induced locomotion. Conversely, after repeated naltrexone injections, a transient boost of ethanol induced locomotor activity was observed. Thus, the results of the present study revealed that the effects of these naltrexone pretreatments on ethanol-induced locomotion are similar to the previously described changes on MOR activity. Moreover, the same (acute and chronic) naltrexone pretreatments produced similar changes on the locomotion of mice after a challenge with morphine (a MOR agonist), but not after tert-butanol (an alcohol which does not release beta-endorphins) administration. Therefore, our results are discussed in terms of the proved ability of ethanol to promote the release of beta-endorphins and, consequently, to activate the MOR.

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.

Consequences of monosodium glutamate or goldthioglucose arcuate nucleus lesions on ethanol-induced locomotion

It has been suggested that the endogenous opioid system, especially beta-endorphins, may play an important role in the behavioral effects of ethanol. The main site of beta-endorphin synthesis in the brain is the hypothalamic arcuate nucleus (ARC). In the present study, we used the neurotoxins monosodium glutamate (MSG) or goldthioglucose (GTG) to produce a selective ARC lesion and to assess its effects on the locomotion observed after ethanol administration. The results show that MSG or GTG pre-treatment produces a blockade of the increased locomotion produced by the injection of low and moderate doses of ethanol (0.5 and 1.5 g/kg, respectively). These effects were observed in the absence of any change in blood ethanol levels. On the other hand, MSG (but not GTG) pre-treatment enhanced the locomotor depression produced by higher doses of this alcohol (2.5 g/kg). Finally, caffeine (10 mg/kg)-induced locomotion was unaffected by the aforementioned neurotoxic agents. Thus, taken together, the present results suggest that MSG and GTG administration produce a blockade of the stimulating effects of ethanol on locomotion in mice and thus provides further support for a role of the ARC in the behavioral effects observed after ethanol administration.

L-glutamate abolishes differential responses to alcohol deprivation in mice

Hybrid mice produced by crossing CBA and C57BL lines were deprived of alcohol for three days after prolonged prior access. Subsequently, when access was first given to a flavored 30% alcohol solution, about half of the mice showed a greatly elevated rate of drinking during the first 1.5 h, characteristic of the alcohol-deprivation effect (ADE), but other mice showed no increase. Repeating the test three weeks later showed that having or lacking an ADE is a stable group characteristic. Behavioral differences were found on cross-maze and slip funnel tests between mice that had an ADE and those that lacked it. Topical application of L-glutamate to the frontal cortex prevented the subsequent elevation of alcohol drinking during the first 1.5 h after deprivation but did not alter drinking during the remaining 22.5 h. L-glutamate treatment also affected those cross-maze behaviors found to be related to the ADE. The results suggest that frontal cortex neurons sensitive to L-glutamate are necessary for the ADE and that comparisons between hybrid mice having and lacking an ADE might be used for determining the neuronal mechanisms responsible for the effect.

Ethanol alters monoamines in specific mouse brain regions

Ethanol (3.5 g/kg 60 min post-IP injection) produced the following changes in regional brain monoamine levels and in the respective metabolite/neurotransmitter ratios: for the noradrenergic system, MHPG was decreased in the amygdala and increased in the hypothalamus, while the MHPG/NE ratio was increased in the prefrontal cortex and the hypothalamus. For the dopaminergic system, DA was decreased in the olfactory tubercle, DOPAC was increased in the prefrontal cortex and septum, and DOPAC/DA was increased in the prefrontal cortex, septum, striatum, and hypothalamus. HVA was increased in the prefrontal cortex and septum, while HVA/DA was increased in the same regions plus the olfactory bulb. 3MT was decreased in the olfactory tubercle and striatum. The serotonergic system was not altered. The results demonstrate that ETOH produces selective regional changes in the concentration and utilization of monoamines in mouse brain with a predominant influence on dopaminergic systems and a lesser effect on noradrenergic activity.

Gender differences in the effects of ethanol on C57BL/6 mice

The influence of gender on the stimulatory and depressant effects of ethanol was examined in C57BL/6 (C57) mice. In Experiment 1, locomotor activity was assessed in young (2-month-old) male and female mice injected intraperitoneally (IP) with stimulatory (1.5 g/kg) or depressant (2.5 g/kg) doses of ethanol. Both the stimulatory and the depressive effects of ethanol were greater in young male than female C57 mice, and the gender difference was unrelated to blood ethanol concentration (BEC). In Experiment 2, older (9-month-old) male and female mice were given ethanol (2.5 g/kg) either by IP injection or gavage to determine if the gender differences in BEC and ethanol effects observed in the first experiment depended upon the route of ethanol administration. In this experiment, ethanol reduced locomotor activity more in males than females whether given by gavage or IP injection, and the males had higher BECs than females at the time of testing. Thus, the differences in the behavioral effects of ethanol appeared to be related to BEC. The greater depressive effect of ethanol on older male mice in this experiment is consistent with an earlier report of prolonged ethanol-hypnosis in older male C57 mice. Therefore, differences in BEC could account for the gender differences in the behavioral effects of ethanol on older but not young mice. The gender difference in BEC of mice obtained in the present and earlier reports is opposite to that reported for humans.(ABSTRACT TRUNCATED AT 250 WORDS)