Neonatal ACTH administration elicits long-term changes in forebrain monoamine innervation. Subsequent disruptions in hypothalamic-pituitary-adrenal and gonadal function

Fetal alcohol-induced hyperactivity is reversed by treatment with the PPARα agonist fenofibrate in a rat model

INTRODUCTION

Exposure to alcohol in utero is linked to the development of a wide range of psychobehavioral changes, notably hyperactivity and attention deficit, with complex underlying pathological and functional mechanisms. Although the currently available treatments for hyperactivity have been studied in children exposed to alcohol in utero, the efficacy of these compounds is subject to debate and has prompted efforts to identify new pharmacological targets.

METHOD

In a rat model of early alcohol exposure (i.e., in utero and during lactation), we studied the effect of the lipid-lowering peroxisome proliferator-activated receptor (PPAR) alpha activator fenofibrate on psychobehavioral impairments.

RESULTS

In the young rat, early exposure to alcohol perturbs locomotor behavior and induces prepubertal hyperactivity and postpubertal hypoactivity. The hyperactivity, usually observed at the end of the fifth week of life, was prevented by the administration of fenofibrate, which also had a beneficial effect on the accompanying attention deficit by reinforcing sustained attention.

CONCLUSION

Our results with fenofibrate suggest that the pharmacological modulation of nuclear receptors such as PPAR-alpha may constitute a new therapeutic approach to managing the psychobehavioral disorders associated with early alcohol exposure.

Early exposure to haloperidol or olanzapine induces long-term alterations of dendritic form

Exposure of the developing brain to a wide variety of drugs of abuse (e.g., stimulants, opioids, ethanol, etc.) can induce life-long changes in behavior and neural circuitry. However, the long-term effects of exposure to therapeutic, psychotropic drugs have only recently begun to be appreciated. Antipsychotic drugs are little studied in this regard. Here, we quantitatively analyzed dendritic architecture in adult mice treated with paradigmatic typical- (haloperidol) or atypical (olanzapine) antipsychotic drugs at developmental stages corresponding to fetal or fetal plus early childhood stages in humans. In layer 3 pyramidal cells of the medial and orbital prefrontal cortices and the parietal cortex and in spiny neurons of the core of the nucleus accumbens, both drugs induced significant changes (predominantly reductions) in the amount and complexity of dendritic arbor and the density of dendritic spines. The drug-induced plasticity of dendritic architecture suggests changes in patterns of neuronal connectivity in multiple brain regions that are likely to be functionally significant.

Psychometric evaluation of the Mainland Chinese version of the Edinburgh Postnatal Depression Scale

BACKGROUND

The Edinburgh Postnatal Depression Scale (EPDS) has been validated in many countries, but not in Mainland China.

OBJECTIVES

This study investigated the reliability and validity of the Mainland Chinese version of the EPDS.

DESIGN

A three-stage design was used for this study. Stage I consisted of a multi-stepped process of forward and backward translation, using a panel of six experts to test content validity. Stage II established the psychometric properties of the EPDS by examining the convergent, discriminant and construct validity, internal consistency and stability of the scale. Stage III established its sensitivity, specificity and the optimal cutoff score of the EPDS according the DSM-IV-TR criteria using the Structured Clinical Interview.

SETTING

Three regional public hospitals in Chengdu.

PARTICIPANTS

A convenience sample was composed of 312 and 451 pregnant women at 28-36 weeks' gestation.

METHOD

Translation and back-translation of the original English instrument and content validation by an expert panel. Receiver operating characteristic (ROC) analysis was carried out to evaluate the global functioning of the scale. The Beck Depression Inventory (BDI), Dyadic Adjustment Scale (DAS) and standard SF-12 Health Survey (SF-12) were used to investigate the convergent, discriminant and construct validity. The Cronbach's alpha reliability coefficient, split-half reliability and test-retest reliability were used to examine the internal consistency and stability of the scale.

RESULTS

The translation process was rigorously conducted to ensure that equivalence was established. Content validity was confirmed by a satisfactory level of agreement with a content validity index (CVI) of 0.93. The area under curve (AUC) was 85.6% and the logistic estimate for the threshold score was 9.5 (sensitivity, 80.0%; specificity, 83.03%) for clinical depression. Convergent and construct validity was supported and discriminant validity suggested that the EPDS successfully discriminated among the non-depressed, mildly and clinically depressed groups. The split-half reliability of the EPDS was 0.76, Cronbach's alpha was 0.79 and test-retest reliability was 0.85.

CONCLUSIONS

Based on the results of this psychometric testing, the Mainland Chinese version of the EPDS is considered ready for use in the routine screening of pregnant women. It is hoped that with this type of cross-cultural information, the universality of the constructs of the instrument can be reliably demonstrated.

Periadolescent stress exposure exerts long-term effects on adult stress responding and expression of prefrontal dopamine receptors in male and female rats

Recent research has demonstrated that experiential/environmental factors in early life can program the adult stress response in rats, and this is manifest as altered hypothalamic-pituitary-adrenocortical activity and behavior in response to a stressor. Very little work has been devoted to investigating whether the environment during adolescence plays a similar role in modulating ongoing developmental processes and how this might affect adult stress responding. Periadolescent predator odor (PPO) exposure was used here as a naturalistic model of repeated psychological stress. Behavioral and endocrine responses to PPO changed across the exposure period, and behavioral alterations persisted into adulthood. While adolescent rats showed pronounced avoidance responses upon initial PPO exposure, hyperactivity increased across the exposure period, especially in females. Corticosterone (cort) responses to stressor exposure also changed in females, with higher physiological baseline levels observed at the end of the exposure period. In adulthood, relative to rats who had received a control manipulation during adolescence, PPO-exposed rats were more fearful in a novel open field and displayed altered responses to a predator odor stress test in adulthood. Moreover, lower levels of the D2 dopamine (DA) receptor were measured in prefrontal (infralimbic and dorsopeduncular) cortices of PPO-exposed rats. These findings suggest that the adolescent period may represent a sensitive period during which developmental programming of the stress response occurs.

A comparative study of pregnant women attending a tertiary obstetric unit and a district general hospital with a previous history of postnatal depression

The aim of this study was to compare pregnant women with a previous history of postnatal depression, attending a Tertiary Obstetric Unit in Belfast and a District General Hospital in Newry. Women with a previous history of postnatal depression recorded at booking for their most current pregnancy were studied between January 2001 and May 2002. A total of 443 women (6.6%) in Belfast had a history of postnatal depression, compared with 113 women (6%) in Newry. The most common age range was 31 - 35 years in both centres. Some 69% of women in Belfast compared with 81% of women in Newry were given treatment. Fluoxetine was the commonly prescribed drug for each centre. During this subsequent pregnancy, 19 women (4.3%) in Belfast and five women (4.4%) in Newry remained on antidepressants. There was one woman in each centre who took an overdose during this pregnancy. More women in Newry were given treatment. During the subsequent pregnancy, the percentages of women from each unit requiring antidepressant therapy were similar.

Anxiety disorders in children with epispadias-exstrophy

OBJECTIVES

To evaluate the hypothesis that anxiety disorders are common comorbid conditions in children with the epispadias-exstrophy complex.

METHODS

Twenty consecutive outpatient exstrophy subjects ranging in age from 5 to 22 years were assessed using a formalized semistructured psychiatric evaluation and were categorized according to the Diagnostic and Statistical Manual of Mental Disorders, version IV, criteria. Disorders were identified if subjects endured moderate to severe impairment in their home, academic, and social environments.

RESULTS

All 20 subjects met the criteria for at least one anxiety disorder; 19 met criteria for more than one anxiety disorder. The adolescent subjects described a gradual waning of some specific symptoms some time after surgical correction of the physical conditions (eg, incontinence) but intensifying sexual anxiety with age.

CONCLUSIONS

The epispadias-exstrophy complex appears to be associated with clinically significant vulnerabilities for anxiety disorders in children.

Prolonged deleterious effects of neonatal handling on reproductive parameters of pubertal male rats

The purpose of the present study was to investigate the long-lasting effects of neonatal handling on reproductive parameters of male rats. Neonatal handling (pups separated from their mothers, kept isolated at environmental temperature for 20 min and submitted to 1 min of tactile stimulation) was applied from post partum Days 1 to 14 (a stress-hyporesponsive period, SHRP) and the animals were killed at puberty (61 days of age). The number of mature spermatids and the daily sperm production were estimated in homogenates from the right testes and cauda epididymidis. Histometric parameters (diameter of seminiferous tubule, germinal epithelium thickness and number of Sertoli cells) were evaluated in paraplast sections of the left testes. The association of the slightly aversive stimuli applied during the SHRP proved to have lasting deleterious effects on male reproduction, causing lower testicular weight and reduced values of seminiferous tubule diameter and germinal epithelium thickness at puberty, which resulted in a 25% reduction in the daily sperm production and in the number of mature spermatids. Similarly, the number of Sertoli cells per tubular cross section was 20% smaller and the weight and number of spermatozoa were reduced more than 40% in the cauda epididymidis of animals handled.

Rat estrous cycle influences the sexual diergism of HPA axis stimulation by nicotine

We previously reported that female rats had significantly greater hypothalamic-pituitary-adrenal (HPA) axis responses to cholinergic stimulation by nicotine (NIC) than did male rats. Females in defined estrous cycle stages, however, were not studied because of sample size limitations. We further explored this finding by determining HPA axis responses to two doses of NIC in female rats (N = 101) during different estrous cycle stages, and in males (N = 69). NIC doses were: 0.3 mg/kg, which provided the greatest female-male difference in the earlier study, and 0.5 mg/kg, which stimulated the HPA axis similarly in the two sexes. Plasma AVP, ACTH, and corticosterone were measured. Proestrous and estrous females had higher ACTH responses to NIC (0.3 mg/kg) compared to metestrous and diestrous females, and compared to males. ACTH responses to NIC (0.5 mg/kg) were similar, regardless of estrous cycle stage or sex. Males had higher AVP responses to both NIC doses compared to females in all estrous cycle stages. Corticosterone responses followed the ACTH responses, except that females in all estrous stages started from a higher corticosterone baseline compared to males. These results are similar to our earlier findings across the estrous cycle with non-specific cholinergic stimulation by physostigmine and suggest that the nicotinic system contributes to the differential HPA axis responses to cholinergic challenge across the estrous cycle.

Accelerating sensory recovery after sciatic nerve crush: non-selective versus melanocortin MC4 receptor-selective peptides

Melanocortin receptor ligands accelerate functional recovery after peripheral nerve crush. It is not known which mechanism is involved or via which melanocortin receptor this effect occurs, albeit indirect evidence favours the melanocortin MC4 receptor. To test whether the melanocortin MC4 receptor is involved in the effects of melanocortins on functional recovery, we used melanocortin compounds that distinguish the melanocortin MC4 receptor from the melanocortin MC1, MC3 and MC5 receptors on basis of selectivity and agonist/antagonist profile. Activation and binding studies indicated that the previously described peptides JK1 (Ac-Nle-Gly-Lys-D-Phe-Arg-Trp-Gly-NH2) and [D-Tyr4]melanotan-II ([D-Tyr4]MTII. Ac-Nle-c[Asp-His-D-Tyr-Arg-Trp-Lys]NH2) are selective for the rat melanocortin MC4 receptor as compared to the rat melanocortin MC3 and MC5 receptors, but are also potent on the melanocortin MC1 receptor. Both peptides did not accelerate sensory recovery in rats with a sciatic nerve crush, whereas the non-selective melanocortin agonist melanotan-II (MTII, Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]NH2) was effective. The melanocortin MC3/MC4 receptor antagonist SHU9119 (Ac-Nle-c[Asp-His-D-Nal(2)-Arg-Trp-Lys]NH2) also enhanced sensory recovery. This effect was probably not due to interaction with the melanocortin MC4 receptor, since JK46 (Ac-Gly-Lys-His-D-Nal(2)-Arg-Trp-Gly-NH2), a selective melanocortin MC4 receptor antagonist, was ineffective. Taken together, these data suggest that melanocortins do not accelerate sensory recovery via interaction with the melanocortin MC4 receptor. From the known melanocortin receptors, only the involvement of the melanocortin MC5 receptor in acceleration of recovery could not be excluded.

Systemic adrenocorticotropic hormone administration down-regulates the expression of corticotropin-releasing hormone (CRH) and CRH-binding protein in infant rat hippocampus

Systemic adrenocorticotropic hormone (ACTH) administration is a first-line therapy for the treatment of infantile spasms, an age-specific seizure disorder of infancy. It is proposed that exogenous ACTH acts via negative feedback to suppress the synthesis of corticotropin-releasing hormone (CRH), a possible endogenous convulsant in infant brain tissue. The aim of this study was to determine whether systemic ACTH treatment in infant rats down-regulates the hippocampal CRH system, including CRH, CRH-binding protein (CRH-BP), and CRH receptors (CRH-R1 and CRH-R2). Daily i.p. injection of ACTH for 7 consecutive days (postnatal days 3-9) elevated serum corticosterone levels 20-fold measured on postnatal day 10, indicating systemic absorption and circulation of the ACTH. Semiquantitative reverse transcriptase-PCR demonstrated that both CRH and CRH-BP mRNA obtained from the hippocampi of ACTH-injected infant rats was significantly depressed relative to saline-injected animals. Comparable reductions in both CRH and CRH-BP synthesis were further demonstrated with radioimmunoassay. In contrast, neither CRH-R1 nor CRH-R2 mRNA was altered by ACTH treatment, relative to saline-injected rats. This latter finding was confirmed electrophysiologically by measuring the enhancement of hippocampal population spikes by exogenous CRH, also showing no differences between ACTH- and saline-injected rats. The results of this study support the proposal that systemic ACTH treatment down-regulates CRH expression in infant brain, perhaps contributing to the therapeutic efficacy observed during treatment of infantile spasms.

Assessment and treatment of depression during pregnancy: an update

Depression occurs commonly during pregnancy, and women with recurrent depression are at particularly high risk for depressive illness in this setting. Though the use of psychotropic medications during pregnancy raises concerns, there are data to support the use of certain antidepressants, including fluoxetine and the tricyclic antidepressants. Data on the newer SSRI antidepressants is gradually accumulating and is encouraging. None of the SSRIS or TCAs have been associated with an increased risk of congenital malformation. However, information on the long-term neurobehavioral effects of these medications still remains limited. As depression during pregnancy carries risk for both the mother and child, it is crucial to diagnose depression in this setting and to provide appropriate treatment strategies. Further data on nonpharmacologic and pharmacologic strategies is needed to aid in the treatment of this challenging clinical population. The clinician must weigh the relative risks of various treatment options and take into account individual patient wishes. Such a process will lead to thoughtful treatment choices, which with close clinical follow-up can minimize the risk for maternal morbidity.

Effects of prenatal hydrocortisone acetate exposure on fertility and sexual behavior in male rats

The aim of the present study was to investigate the effects of hydrocortisone during the prenatal period and its later repercussions on the fertility and sexual behavior of male rats. Pregnant rats were treated (s.c.) with hydrocortisone acetate, at 1.5 mg/day on the 17th, 18th, and 19th days of gestation. Decreased body weight and no alteration in anogenital distance were observed in male offspring. Adulthood, presented reductions of body weight, plasma testosterone levels, and seminal-vesicle wet weight without secretion as well as no alteration in the wet weights of the testes, epididymis, and seminal vesicle with secretion in the treated group. Males exposed to hydrocortisone during the prenatal period were able to mate with normal females, which became pregnant but exhibited an increased number of post-implantation losses. In spite of this, these treated males exhibited decreased male sexual behavior and the appearance of female sexual behavior after these male rats were castrated and pretreated with exogenous estrogen. These results indicate that exposure to hydrocortisone in the later stages of pregnancy may have a long-term effect on the fertility and sexual behavior of male rats, suggesting an incomplete masculinization and defeminization of the central nervous system.

Estrous cycle influences on sexual diergism of HPA axis responses to cholinergic stimulation in rats

Central cholinergic systems differentially modulate hypothalamic-pituitary-adrenal (HPA) axis activity in female and male animals (sexual diergism). We previously reported that male rats had significantly greater HPA axis responses to stimulation by physostigmine (PHYSO), an acetylcholinesterase (AChE) inhibitor, compared to females. Females in defined estrous cycle stages, however, were not studied because of sample size limitations. We, therefore, determined HPA axis responses to stimulation by PHYSO in females during all estrous cycle stages (n = 78), and in male rats (n = 75). Plasma arginine vasopressin (AVP), adrenocorticotropic hormone (ACTH), and corticosterone (CORT) were measured. Estrous cycle stage was determined by light microscopy of vaginal smears. Proestrous and estrous females had higher ACTH and CORT responses compared to metestrous and diestrous females. Males had higher ACTH and AVP responses compared to females in all cycle stages. CORT responses followed the ACTH responses, except that females started from a higher baseline in all estrous stages, compared to males. These results suggest that cholinergic regulation of the HPA axis differs among females across stages of the estrous cycle, as well as between males and females. These effects are likely due to differences in circulating sex steroids and their actions within the brain.

Changes of leptin and metabolic hormones in preterm infants: a longitudinal study in early postnatal life

OBJECTIVES

Very little is known concerning the physiological role of leptin and growth in the early postnatal period and the association of leptin with other metabolic hormones in preterm infants. This study aims to investigate these relationships, and to explore the longitudinal and dynamic profile of leptin and metabolic hormones including insulin, ACTH, cortisol and FT4 in this category of patient. We also postulate that a rapid increase in body weight and body mass index in the first few weeks of life may be associated with a corresponding increase in serum leptin if the 'adipoinsular axis' is active at this stage.

DESIGN

A longitudinal study in a cohort of preterm infants < 34 weeks gestation for the first five weeks of postnatal life.

PATIENTS AND METHODS

Sixty-one preterm newborns < 34 weeks gestation were prospectively enrolled. Blood samples were collected in the morning when the newborns were 24 h of age (day 1), and on days 5, 14 and 35 of life. Serum leptin, insulin, cortisol, FT4, glucose and plasma ACTH concentrations were analysed using standard biochemical methods. Spearman's correlation coefficient was used to assess the inter-relationship of different metabolic hormones on the first day of life, and the relationship between metabolic hormones on day 1 and anthropometric or clinical parameters. The mixed-effects models were further used for analysing the multiple longitudinal measurements, and also for comparing the hormone concentrations between day 1 (baseline) and their corresponding levels on days 5, 14 and 35.

RESULTS

Serum leptin on day 1 (baseline) was significantly associated with serum insulin (r = 0.30, P < 0.05) and insulin:glucose ratio (r = 0.29, P < 0.05) in infants < 34 weeks gestation. Female preterm infants had significantly higher serum leptin (P < 0.05) and insulin (P < 0.05) levels than male infants. In addition, the duration between the last dose of antenatal dexamethasone and delivery significantly influenced the serum concentrations of leptin (r = - 0.27, P < 0.05), cortisol (r = 0.52, P < 0.001), plasma ACTH (r = 0.28, P < 0.05) and insulin:glucose ratio (r = - 0.27, P < 0.05) on the first day of life. Despite significant increase in body weight (P < 0.00001) and body mass index (P < 0.00001) by day 35 when compared to birth weight, no corresponding significant changes were observed for serum leptin, insulin and FT4. However, there was an increasing though statistically nonsignificant trend in serum leptin after day 14 of life.

CONCLUSION

This study characterized the longitudinal profile of leptin and metabolic hormones in preterm infants < 34 weeks gestation in the first 5 weeks of life. Serum leptin was significantly associated with serum insulin and insulin:glucose ratio supporting the hypothesis that an 'adipoinsular axis' exists and is likely to be functional before 34 weeks of gestation. Although a significant increase in body weight was demonstrated by day 35, no significant corresponding changes occurred with regard to serum leptin. We postulate that the limited quantity of adipose tissue at this stage of development might have contributed to this observation. Moreover, our results also showed that the duration between the last dose of antenatal corticosteroid and delivery could influence the postnatal concentrations of adipoinsular and pituitary-adrenal hormones. Thus, it is possible that antenatal dexamethasone might affect fetal growth and development via these neuroendocrine pathways in early intrauterine life.

David and Goliath - the slingshot that started the neuropeptide revolution

This review in honor of David de Wied summarizes the work done in my laboratory that first indicated that adrenocorticotropic hormone (ACTH) has a direct effect on the neuromuscular system. Cold stress or ACTH and its related peptides alpha-melanocyte-stimulating hormone (alpha-MSH ) and beta-lipotropin improve the electromechanical characteristics of adrenalectomized and hypophysectomized rats. ACTH-(1-39) accelerates the return of motor and sensory function and improves the morphological characteristics of the motor endplate after peripheral nerve crush. The non-corticotropic fragments ACTH-(4-10), alpha-MSH, the ACTH-(4-9) analogue Organon 2766 (Org 2766) or the ACTH-(4-10) analogue Biomeasure 22015 (BIM 22015) improve electrophysiological and morphological parameters of the regenerating neuromuscular system. ACTH-(4-10) immunoreactivity, present in ventral horn motor neurons in low levels, is decreased ipsilaterally following ipsilateral nerve crush but increases both ipsilaterally and contralaterally if injured animals are treated with ACTH-(4-10) indicating a neuroprotective action. Similarly, Org 2766 appears to have a protective action in the brain following nigrostriatal lesions. In developmental studies, perinatal exposure to ACTH peptides improves the structure of the neuromuscular junction, accelerates the maturation of electromechanical properties and enhances nerve-muscle integration and nerve regeneration. Perinatal exposure to these peptides decreases adult male sexual behavior, a change correlated with increased serotinergic input within the medial preoptic area. Similar changes occur in female rats and appear to be long-lasting. In tissue culture studies, both Org 2766 and BIM 22015 promote neurite outgrowth in the absence of nerve growth factor, indicating a neurotrophic role for these peptides.

Prenatal morphine exposure differentially alters TH-immunoreactivity in the stress-sensitive brain circuitry of adult male and female rats

Previously, we demonstrated that exposure to morphine during gestation increases hypothalamic norepinephrine (NE) content and turnover rate in adult male rats and decreases these measures in adult females. To investigate the basis of these alterations, the present study examined the effects of prenatal exposure to morphine on tyrosine hydroxylase immunoreactivity (TH-IR) in the brains of adult male and female progeny. In male rats, prenatal morphine exposure significantly increased the density of TH-IR in cells and fibers in the caudal paraventricular nucleus of the hypothalamus (PVN) and locus coeruleus (LC), but had no effects in the lateral hypothalamus (LH). In female rats that were ovariectomized (OVX), prenatal morphine exposure significantly decreased the density of TH-IR in cells and fibers in the LC. Interestingly, an injection of estrogen in OVX control females reduced the mean optical density of TH-IR in the LC, but it was ineffective in drug-exposed females in the same brain region. Estrogen injections also reduced the mean optical density of TH-IR in the LH but not in the PVN of females, regardless of prenatal drug exposure. Thus, the present study suggests that prenatal morphine exposure induces long-term, sex-specific alterations in TH-IR in the PVN and LC of adult progeny.

The growth hormone axis and cognition: empirical results and integrated theory derived from giant transgenic mice

Sleep is required for the consolidation of memory for complex tasks, and elements of the growth-hormone (GH) axis may regulate sleep. The GH axis also up-regulates protein synthesis, which is required for memory consolidation. Transgenic rat GH mice (TRGHM) express plasma GH at levels 100-300 times normal and sleep 3.4 h longer (30%) than their normal siblings. Consequently, we hypothesized that they might show superior ability to learn a complex task (8-choice radial maze); 47% of the TRGHM learned the task before any normal mice. All 17 TRGHM learned the task, but 33% of the 18 normal mice learned little. TRGHM learned the task significantly faster than normal mice (p < 0.05) and made half as many errors in doing so, even when the normal nonlearners were excluded from the analysis. Whereas normal mice expressed a linear learning curve, TRGHM showed exponentially declining error rates. The contribution of the GH axis to cognition is conspicuously sparse in literature syntheses of knowledge concerning neuroendocrine mechanisms of learning and memory. This paper synthesizes the crucial role of major components of the GH axis in brain functioning into a holistic framework, integrating learning, sleep, free radicals, aging, and neurodegenerative diseases. TRGHM show both enhanced learning in youth and accelerated aging. Thus, they may provide a powerful new probe for use in gaining an understanding of aspects of central nervous system functioning, which is highly relevant to human health.

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.