The effect of neonatal handling on adult feeding behavior is not an anxiety‐like behavior

Cyclooxygenase-2 inhibitors alleviated depressive and anxious-like behaviors in mice exposed to lipopolysaccharide: Involvement of oxidative stress and neuroinflammation

Depression and anxiety disorders have their pathophysiologies linked to inflammation and oxidative stress. In this context, celecoxib (CLX) and etoricoxib (ETR) inhibit cyclooxygenase 2 (COX-2), an enzyme expressed by cells involved in the inflammatory process and found in the brain. Studies have been using CLX as a possible drug in the treatment of depression, although its mechanisms at the central nervous system level are not fully elucidated. In this study, the effects of CLX and ETR on behavioral, oxidative, and inflammatory changes induced by systemic exposure to Escherichia coli lipopolysaccharide (LPS) were evaluated in adult male swiss mice. For ten days, the animals received intraperitoneal injections of LPS at 0.5 mg/kg. From the sixth to the tenth day, one hour after LPS exposure, they were treated orally with CLX (15 mg/kg), ETR (10 mg/kg), or fluoxetine (FLU) (20 mg/kg). Twenty-four hours after the last oral administration, the animals underwent evaluation of locomotor activity (open field test), predictive tests for depressive-like behavior (forced swim and tail suspension tests), and anxiolytic-like effect (elevated plus maze and hole board tests). Subsequently, the hippocampus, prefrontal cortex and striatum were dissected for the measurement of oxidative and nitrosative parameters (malondialdehyde, nitrite, and glutathione) and quantification of pro-inflammatory cytokines (IL-1β and IL-6). LPS induced depressive and anxious-like behavior, and treatment with CLX or ETR was able to reverse most of the behavioral changes. It was evidenced that nitrosative stress and the degree of lipid peroxidation induced by LPS were reduced in different brain areas after treatment with the drugs, as well as the endogenous defense system against free radicals was strengthened. CLX and ETR also significantly reduced LPS-induced cytokine levels. These data are expected to expand information on the role of inflammation in depression and anxiety and provide insights into possible mechanisms of COX-2 inhibitors in psychiatric disorders with a neurobiological basis in inflammation and oxidative stress.

Repeated gentle handling or maternal deprivation during the neonatal stage increases adult male rats' baseline orofacial pain responsiveness

OBJECTIVE

Early life experiences have been found to have a long-lasting effect on brain development in adult life. The purpose of this study was to determine whether neonatal manipulation could alter orofacial pain responsiveness in adult rats METHODS: In the first 21 days of life, male rats were exposed to gentle handling or maternal deprivation (MD) procedures to establish models of handled and MD rats, respectively. The rats were assigned to three of the following experimental groups at the age of two months: intra-dental capsaicin (100 µg), intra-lip formalin (50 µL), and repeated nitroglycerin (NTG) (5 mg/rat/ip) infusion. In addition, there were three drug vehicle groups and three groups that received capsaicin, formalin, or NTG without prior handling or MD procedures. The behaviors were recorded following the pain induction.

RESULTS

Spontaneous pain behaviors in the first phase of formalin test was significantly increased in MD (p < 0.01) and handled rats in comparison with the vehicle group (p < 0.05). The second-phase data showed that formalin-induced spontaneous pain behaviors was increased in rats- treated with MD as compared to either vehicle or handled+formalin groups (p < 0.001). Capsaicin-induced dental pulp nociception was increased in the MD group in comparison with the capsaicin (p < 0.001) and capsaicin+handled (p < 0.001) groups. Moreover, NTG-induced migraine-like behaviors symptoms were increased in the MD group as compared to control and handled groups (p < 0.05).

CONCLUSIONS

In this study neonatal gentle handling or MD treatment increased orofacial pain in adulthood, showing early life experiences permanent effects on the development of trigeminal circuits in the brain.

Energy drinks and alcohol in a binge drinking protocol in Wistar rats: Male and female behavioral and reproductive effects

The consumption of energy drinks is common among adolescents and young adults. The possible effects (mainly behavioral and reproductive) of ingestion in this population remain unknown. For this reason, this study aimed to evaluate the behavioral and reproductive effects of energy drinks and their main constituents (caffeine and taurine), as well as their combinations with alcohol, via a binge drinking protocol in male and female Wistar rats during puberty. In this study, 100 male and 100 female rats were treated with a binge drinking protocol 3 days a week over 4 weeks from postnatal day (PND) 28 to PND 60, which included 10 mL/kg by oral gavage of distilled water, energy drink, caffeine (3.2 mg/kg), taurine (40 mg/kg), and their combinations with alcohol (2 g/kg). The animals were evaluated by behavioral tests from PND 56 to PND 60 (open field, plus maze and object recognition) and reproductive parameters (estrous cycle regularity, weight of sexual organs, oocyte quality, spermatid and sperm count, sperm morphology and testosterone level). Locomotor activity was increased in females in the groups combined with alcohol (except alcohol + caffeine) and in the caffeine group. Long-term memory was increased in males in the caffeine and taurine groups even when combined with alcohol. The combination of energy drinks and alcohol did not have significant effects on the reproductive parameters of either sex of rats during puberty. We concluded that energy drinks (and their main constituents) and alcohol combinations did not cause alterations in reproductive profiles, and locomotor activity and long-term memory were increased in females and males, respectively.

Persistence of neural function in animals submitted to seizure-suppressing scale-free nonperiodic electrical stimulation applied to the amygdala

Based on the rationale that neural hypersynchronization underlies epileptic phenomena, nonperiodic stimulation (NPS) was designed and successfully tested as an electrical stimulus with robust anticonvulsant action. Considering the scale-free temporal structure of NPS mimics natural-like activity, here we hypothesized its application to the amygdala would induce minor to none impairment of neural function in treated animals. Wistar rats underwent gold-standard behavioral tests such as open field (OF), elevated plus-maze (EPM), novel object recognition, and social interaction test in order to evaluate the functions of base-level anxiety, motor function, episodic memory, and sociability. We also performed daily (8 days, 6 h per day) electrophysiological recordings (local field potential/LFP and electromyography) to assess global forebrain dynamics and the sleep-wake cycle architecture and integrity. All animals displayed an increased proportion of time exploring new objects, spent more time in the closed arms of the EPM and in the periphery of the OF arena, with similar numbers of crossing between quadrants and no significant changes of social behaviors. In the sleep-wake cycle electrophysiology experiments, we found no differences regarding duration and proportion of sleep stages and the number of transitions between stages. Finally, the power spectrum of LFP recordings and neurodynamics were also unaltered. We concluded that NPS did not impair neural functions evaluated and thus, it may be safe for clinical studies. Additionally, results corroborate the notion that NPS may exert an on-demand only desynchronization effect by efficiently competing with epileptiform activity for the physiological and healthy recruitment of neural circuitry. Considering the very dynamical nature of circuit activation and functional activity underlying neural function in general (including cognition, processing of emotion, memory acquisition, and sensorimotor integration) and its corruption leading to disorder, such mechanism of action may have important implications in the investigation of neuropsychological phenomena and also in the development of rehabilitation neurotechnology.

Effects of subchronic inhalation exposure to an organophosphorus insecticide compound containing dichlorvos on wistar rats' otoacoustic emissions

INTRODUCTION

Considering that previous studies suggest that pesticides may cause hearing disorders in humans, as well as the lack of studies proving the specific mechanisms of injury and the difficulty of separating concomitant etiological factors of the hearing damage, such as noise and vibration, it is important to develop studies using animal models to elucidate the effects of exposure to those substances isolated from other hearing damage etiologies.

OBJECTIVE

To evaluate if the exposure to a dichlorvos based organophosphorus insecticide may induce ototoxicity.

METHODS

36 male Wistar rats were assigned to 3 groups (12 rats/group): control (exposed to water), positive control (treated with cisplatin to induce hearing damage) and experimental (exposed to dichlorvos based organophosphorus insecticide). The amplitude of distortion product otoacoustic emissions in the frequencies of 4, 6, 8, 10 and 12kHz was evaluated before and after exposure, as well as systemic toxicity signs, body mass gain and plasma cholinesterase. Open field and plus maze tests were performed in 24 rats: experimental (n=8), control (n=8) and positive control group (n=8 introduced new rats to induce anxiolytic activity) to evaluate the locomotor activity and anxiety, respectively.

RESULTS

There was no significant change in body mass gain and plasma cholinesterase in the dichlorvos based organophosphorus insecticide group, however, the animals showed transient piloerection, depression and dyspnea during exposure. The behavior was not affected in any group. The frequencies of 8 and 10kHz were significantly affected bilaterally in the insecticide group, which also showed a significant difference of the control in 10kHz on the right and 8 and 10kHz on the left ear.

CONCLUSION

Subchronic inhalation exposure to dichlorvos based organophosphorus insecticide induced ototoxicity in the cochlear function of rats without relevant systemic toxicity.

Dry needling increases antioxidant activity and grip force in a rat model of muscle pain

BACKGROUND

Muscle pain syndromes (MPS) are one of the main causes of functional, structural and metabolic problems, being associated with tissue oxidative damage. Although dry needling is widely used in the treatment of MPS, there is little scientific evidence of its efficacy and underlying mechanisms of action.

OBJECTIVES

To investigate the effects of different dry needling techniques on thermal and mechanical hyperalgesia, locomotor and functional activity, and oxidative stress markers in a rat model of muscle pain.

METHODS

A total of 48 male Wistar rats underwent injection of the gastrocnemius muscle with control neutral saline (pH 7) and remained untreated (Saline group), or acidic saline (pH 4) and remained untreated (ASA group) or received pregabalin (PG group), deep needling (DN group), superficial needling (SN group) or twitch needling (TN group) with n = 8 rats per group. Mechanical (von Frey test) and thermal hyperalgesia (acetone test), muscle edema (assessed with a caliper), strength and muscle function (grip force evaluation), surface thermography and locomotor and exploratory activities (open field test) were evaluated. The animals were then euthanized, and the gastrocnemius muscle was excised for assessment of oxidative analyses of lipid peroxidation with thiobarbituric acid reactive species (TBA-RS) and total glutathione (GSH) levels.

RESULTS

All treatments significantly improved muscle strength and function when compared to the AS group (p < 0.05). Pregabalin reduced locomotor and exploratory activities, while the TN intervention increased the antioxidant response (p < 0.05).

CONCLUSION

Dry needling improved strength, functionality and locomotor activity in a rat model of muscle pain. Twitch needling induced an antioxidant effect.

Neurodevelopmental testing of mice in the neonatal period does not affect their locomotor activity, depressive- and anxiety-like behaviour in adolescence

Neonatal life is a sensitive period of brain plasticity. There are concerns that pre-weaning handling may therefore alter behavioural phenotype in adolescence or adulthood. Since neurodevelopment tests require daily manipulation with pups, later behavioural outcomes might be biased by repeated handling during suckling period. The aim of our study was to assess whether daily neurodevelopmental testing would cause alterations in behavioural phenotype. Sixty-four CD1 mice were randomized to tested and not tested group. In the tested group, maturation of physical features and neurodevelopment were monitored daily from postnatal day 1-21 between 9 and 11 AM. After weaning, battery of behavioural tests to monitor anxiety-like, depressive, or repetitive behaviour was performed in all mice. We revealed no significant between-group differences in performance of these tests. Our data did not confirm the assumption that early neurodevelopment testing during suckling period affects behavioural phenotype in adolescence.

Early-life stress affects behavioral and neurochemical parameters differently in male and female juvenile Wistar rats

Neonatal handling is an early life stressor that leads to behavioral and neurochemical changes in adult rats in a sex-specific manner and possibly affects earlier stages of development. Here, we investigated the effects of neonatal handling (days 1-10 after birth) on juvenile rats focusing on biochemical parameters and olfactory memory after weaning. Male neonatal handled rats performed more crossings on the hole-board task, increased Na⁺ /K⁺ -ATPase activity in the olfactory bulb, and decreased acetylcholinesterase activity in the hippocampus versus non-handled males. Female neonatal handled animals increased the number of rearing and nose-pokes on the hole-board task, decreased glutathione peroxidase activity, and total thiol content in the hippocampus versus non-handled females. This study reinforces that early life stress affects behavioral and neurochemical parameters in a sex-specific manner even before the puberty onset.

Maternal deprivation alters nociceptive response in a gender-dependent manner in rats

The present study aimed at investigating both the early and long-term effects of maternal deprivation as well as gender on neuromotor reflexes, anxiety behavior and thermal nociceptive responses. A total of 64 Wistar rats pups (32 males, 32 females) were utilized and were deprived of their mother for 3 h/daily, from postnatal day 1 (P1) until P10. Successively, animals were divided into 2 groups: control group (C) - pups no subjected to intervention; and the maternal-deprived group (MD): pups subjected to maternal deprivation. The neuromotor reflexes were evaluated through the righting reflex and negative geotaxis tests; the exploratory behavior by open field test (OFT); the anxiety-like behavior by elevated plus-maze test (EPM); the thermal nociceptive responses byhot plate (HP) and tail-flick (TFL) tests. All the animals subjected to maternal deprivation showed a delayed reflex response at P8 in the negative geotaxis test. In contrast, the OFT at P20 identified an effect of gender on the outer crossings and grooming as well as an interaction between gender and maternal deprivation on latency. Additionally, effect of maternal deprivation in the open and closed arms as well as gender effect in the protected head-dipping (PHD) and non-protected head-dipping (NPHD) were observed at P20 (EPM). In contrast, there were a gender effect on latency and an interaction between gender and maternal deprivation on rearing at P42. Moreover, in nociceptive tests was observed an analgesic effect induced by maternal deprivation; however, in the TFL test, only deprived females showed this effect. Surprisingly, only control animals presented an ontogeny nociceptive effect in the HP testat P21 and P43, which may be related to an increase in the inhibitory nociceptive pathways throughout life. In this way, we suggest maternal deprivation to be able to anticipate the maturation of the inhibitory nociceptive pathway. In conclusion, maternal deprivation induced a delayed reflex response at P8 and altered the anxiety and nociceptive behaviors according to the time after exposure to this stressor, in a gender-specific manner.

Transcranial direct current stimulation (tDCS) reverts behavioral alterations and brainstem BDNF level increase induced by neuropathic pain model: Long-lasting effect

INTRODUCTION

Neuropathic pain (NP) is a chronic pain modality that usually results of damage in the somatosensory system. NP often shows insufficient response to classic analgesics and remains a challenge to medical treatment. The transcranial direct current stimulation (tDCS) is a non-invasive technique, which induces neuroplastic changes in central nervous system of animals and humans. The brain derived neurotrophic factor plays an important role in synaptic plasticity process. Behavior changes such as decreased locomotor and exploratory activities and anxiety disorders are common comorbidities associated with NP.

OBJECTIVE

Evaluate the effect of tDCS treatment on locomotor and exploratory activities, and anxiety-like behavior, and peripheral and central BDNF levels in rats submitted to neuropathic pain model.

METHODS

Rats were randomly divided: Ss, SsS, SsT, NP, NpS, and NpT. The neuropathic pain model was induced by partial sciatic nerve compression at 14 days after surgery; the tDCS treatment was initiated. The animals of treated groups were subjected to a 20 minute session of tDCS, for eight days. The Open Field and Elevated Pluz Maze tests were applied 24 h (phase I) and 7 days (phase II) after the end of tDCS treatment. The serum, spinal cord, brainstem and cerebral cortex BDNF levels were determined 48 h (phase I) and 8 days (phase II) after tDCS treatment by ELISA.

RESULTS

The chronic constriction injury (CCI) induces decrease in locomotor and exploratory activities, increases in the behavior-like anxiety, and increases in the brainstem BDNF levels, the last, in phase II (one-way ANOVA/SNK, P<0.05 for all). The tDCS treatment already reverted all these effects induced by CCI (one-way ANOVA/SNK, P<0.05 for all). Furthermore, the tDCS treatment decreased serum and cerebral cortex BDNF levels and it increased these levels in the spinal cord in phase II (one-way ANOVA/SNK, P<0.05).

CONCLUSION

tDCS reverts behavioral alterations associated to neuropathic pain, indicating possible analgesic and anxiolytic tDCS effects. tDCS treatment induces changes in the BDNF levels in different regions of the central nervous system (CNS), and this effect can be attributed to different cellular signaling activations.

Stress during the gestational period modifies pups' emotionality parameters and favors preference for morphine in adolescent rats

Experimental animal studies have shown that early life periods are highly vulnerable to environmental factors, which may exert prolonged impact on HPA axis function and on subsequent neurochemical and behavioral responses in adulthood. Here we evaluated the influence of environmental stressful situations in two different early life stages on stress-related behaviors, and morphine-conditioned place preference (CPP), which is indicative of addiction. While in the gestational stress (Gest-S) dams were exposed to daily sessions of chronic mild stress (CMS) for 2 weeks, in the postnatal stress (post-NS) the offspring were exposed daily to neonatal isolation from postnatal day (PND) 2 to PND 9 for 60 min. Animals exposed to post-NS showed lesser anxiety in different behavioral paradigms (elevated plus maze-EPM and defensive burying test-DBT) as well as increased exploratory behavior (open-field task-OFT), and no preference for morphine in CPP. In contrast, animals exposed to Gest-S showed increased corticosterone plasma levels together with anxiety symptoms and greater preference for morphine following three days of drug withdrawal. Our findings indicate that the gestational period is critical for stress, whose effects may be manifest throughout life. On the other hand, post-NS can trigger neuroadaptations able to overcome emotional consequences of early life. We hypothesized that Gest-S is able to modify responses to opioids along adulthood, which may facilitate development of addiction to these drugs.

How postnatal insults may program development: studies in animal models

During the postnatal period, the nervous system is modified and shaped by experience, in order to adjust it to the particular environment in which the animal will live. This plasticity, one of the most remarkable characteristics of the nervous system, promotes adaptive changes, but it also makes brain more vulnerable to insults. This chapter will focus on the effects of interventions during the postnatal development in animal models of neonatal handling (usually up to 15 min of handling) and maternal separation (usually at least for 3 h). Sex-specific changes and effects of prepubertal stress such as social isolation later on in life were also considered. These interventions during development induce long-lasting traces in the pups' nervous system, which will be reflected in changes in neuroendocrine functions, including the hypothalamus-pituitary-adrenal and hypothalamus-pituitary-gonadal axes; anxiety and cognitive performance; and feeding, sexual, and social behavior. These enduring changes may be adaptive or maladaptive, depending on the environment in which the animal will live. The challenge researchers facing now is to determine how to reverse the deleterious effects that may result from early-life stress exposure.

Hypercaloric diet modulates effects of chronic stress: a behavioral and biometric study on rats

Obesity is a chronic disease that has been associated with chronic stress and hypercaloric diet (HD) consumption. Increased ingestion of food containing sugar and fat ingredients (comfort food) is proposed to "compensate" chronic stress effects. However, this eating habit may increase body fat depositions leading to obesity. This study evaluated behavioral/physiological parameters seeking to establish whether there is an association between the effects of HD intake and stress, and to test the hypothesis that the development of anxious behavior and obesity during chronic stress periods depends on the type of diet. Sixty-day-old male Wistar rats (n = 100) were divided into four groups: standard chow, hypercaloric diet, chronic stress/standard chow and chronic stress/hypercaloric diet. Chronic stress was induced by restraint stress exposure for 1 h/day, for 80 d. At the end of this period, rat behavior was evaluated using open-field and plus-maze tests. The results showed that HD alone increased weight gain and adipose deposition in subcutaneous and mesenteric areas. However, stress reduced weight gain and adipose tissue in these areas. HD also increased naso-anal length and concurrent stress prevented this. Behavioral data indicated that stress increased anxiety-like behaviors and comfort food reduced these anxiogenic effects; locomotor activity increased in rats fed with HD. Furthermore, HD decreased corticosterone levels and stress increased adrenal weight. The data indicate that when rats are given HD and experience chronic stress this association reduces the pro-obesogenic effects of HD, and decreases adrenocortical activity.

Neonatal hypoxic-ischemic encephalopathy reduces c-Fos activation in the rat hippocampus: evidence of a long-lasting effect

The effect of neonatal hypoxic-ischemic encephalopathy (HIE) on maturation of nociceptive pathways has been sparsely explored. To investigate whether neonatal HIE alters neuronal activity, nociceptive behavior, and serum neuroplasticity mediators (brain-derived neurotrophic factor [BDNF] and tumor necrosis factor-α [TNF]) in the short, medium, and long term. Neonate male Wistar rats were randomized to receive a brain insult that could be either ischemic (left carotid artery ligation [LCAL]), hypoxic (8% oxygen chamber), hypoxic-ischemic (LCAL and hypoxic chamber), sham-ischemic, or sham-hypoxic. Neuronal activity (c-Fos activation at region CA1 and dentate gyrus of the hippocampus), nociceptive behavior (von Frey, tail-flick, and hot-plate tests), neuroplasticity mediators (BDNF, TNF), and a cellular injury marker (lactase dehydrogenase [LDH]) were assessed in blood serum 14, 30, and 60 days after birth. Neonatal HIE persistently reduced c-Fos activation in the ipsilateral hippocampal region CA1; however, contralateral c-Fos reduction appeared only 7 weeks after the event. Neonatal HIE acutely reduced the paw withdrawal threshold (von Frey test), but this returned to normal by the 30th postnatal day. Hypoxia reduced serum LDH levels. Serum neuroplasticity mediators increased with age, and neonatal HIE did not affect their ontogeny. Neonatal HIE-induced reduction in neuronal activity occurs acutely in the ipsilateral hippocampal region CA1 and persists for at least 60 days, but the contralateral effect of the insult is delayed. Alterations in the nociceptive response are acute and self-limited. Serum neuroplasticity mediators increase with age, and remain unaffected by HIE.

Neonatal handling: an overview of the positive and negative effects

As one of the first rodent models designed to investigate the effects of early-life experiences, the neonatal handling paradigm has helped us better understand how subtle changes in the infant environment can powerfully drive neurodevelopment of the immature brain in typical or atypical trajectories. Here, we review data from more than 50 years demonstrating the compelling effects of neonatal handling on behavior, physiology, and neural function across the lifespan. Moreover, we present data that challenge the classical view of neonatal handling as an animal model that results only in positive/beneficial outcomes. Indeed, the overall goal of this review is to offer the suggestion that the effects of early-life experiences-including neonatal handling-are nuanced rather than unidirectional. Both beneficial and negative outcomes may occur, depending on the parameters of testing, sex of the subject, and neurobehavioral system analyzed.

Lifetime behavioural changes after exposure to anaesthetics in infant rats

The aim of this study was to assess the effect of acute use of general anaesthetic with or without a surgical procedure, at post-natal day 14 (P14), on behavioural responses in the short-, medium- and long-term, evaluated in open field (OF) and elevated plus-maze (EPM) tests. Fourteen-day-old male Wistar rats were divided into two experimental designs (ED): inhalation and intravenous anaesthetic, and these groups were subdivided into: 1st ED - control (C), isoflurane (ISO), isoflurane/surgery (ISO-SUR); 2nd ED - control (C), fentanyl/S(+)-ketamine (FK) and fentanyl+ketamine-s/surgery (FK-SUR). In the OF the following were found: (a) in the 1st ED: an increase in the locomotor activity in the ISO group at P14, and ISO and ISO-SUR groups at P30; the ISO-SUR group showed a reduced latency to leave the first quadrant at P30 and P60; (b) in the 2nd ED: FK and FK-SUR groups presented increased locomotor activity at P30, and the FK group showed a reduction in the number of faecal boluses. In the EPM the following were found: FK and FK-SUR groups presented an increase in the number of non-protected head-dipping (NPHD) movements and in the number of entries and time spent in open arms at P30; the FK group showed an increased number of protected head-dipping movements, NPHD and entries and time spent in the open arms at P60. The behavioural changes observed may be related to locomotor activity (1st ED) and anxiety level (2nd ED) and they may result from changes in neurotransmitters/hormones (DA, 5HT, CRH) and glutamate/NMDA receptors, respectively.

Na+, K+ ATPase activity is reduced in amygdala of rats with chronic stress-induced anxiety-like behavior

In this study, we examined the effects of two chronic stress regimens upon anxiety-like behavior, Na(+), K(+)-ATPase activity and immunocontent, and oxidative stress parameters (antioxidant enzymes and reactive oxygen species production) in the amygdala. Male rats were subjected to chronic unpredictable and to chronic restraint stress for 40 days. Subsequently, anxiety-like behavior was examined. Both stressed groups presented increased anxiety-like behavior. Reduced amygdalal Na(+), K(+)-ATPase activity in the synaptic plasma membranes was also observed, without alterations in the amygdala immunocontent. In addition, when analyzing oxidative stress parameters, only superoxide dismutase activity was decreased in the amygdala of animals subjected to unpredictable stress. We conclude that both models of chronic stress lead to anxiety-like behavior and decreased amygdalal Na(+), K(+)-ATPase activity, which appears not to be related to oxidative imbalance. The relationship between this decreased activity and anxiety-like behavior remains to be studied.

Long-term effects of neonatal handling on mu-opioid receptor levels in the brain of the offspring

Neonatal handling is an experimental paradigm of an early experience which permanently alters hypothalamic-pituitary-adrenal axis function resulting in increased ability to cope with stress, and decreased emotionality. In the present work we investigated the effect of neonatal handling on adult rat brain mu-opioid receptor levels, since the opioid system is known to play an important role in emotional processing, anxiety and stress responses. Neonatal handling resulted in increased levels of mu-opioid receptors in the basolateral and central amygdaloid nuclei, in the CA3 and CA4 hippocampal areas, in the ventral tegmental area, the nucleus accumbens and the prefrontal cortex. Handled animals of both sexes had lower anxiety as measured in the elevated plus maze. The increased mu receptor levels could participate in the molecular mechanisms underlying the well-documented decreased stress and anxiety responses of handled animals.

Both infantile stimulation and exposure to sweet food lead to an increased sweet food ingestion in adult life

We have reported that neonatal handling leads to increased sweet food preference in adult life. Our aim was to verify if these differences in feeding behavior appear before puberty, and whether other types of intervention in periadolescence (such as exposure to toys) could interfere with sweet food consumption later in life. Nests of Wistar rats were (1) non-handled or (2) handled (10 min/day) on days 1-10 after birth. Males from these groups were subdivided in two subgroups: one was habituated to sweet food (Froot Loops-Kellogs) in a new environment for 4 days and tested for sweet food preference at age 27 days, before submitting to a new habituation and test for sweet food ingestion again in adult life. The other subgroup was habituated and tested only in adulthood. In another set of experiments, neonatally non-handled rats were exposed or not to a new environment with toys in periadolescence, and tested for sweet food ingestion as adults. Neonatal handling increases sweet food consumption only if the habituation and tests are performed after puberty. Interestingly, infant exposure to sweet food had a similar effect as neonatal handling, since controls that were exposed to sweet food at age 22 to 27 days increased their ingestion as adults. Exposure to toys in periadolescence had the same effect. We suggest that an intervention during the first postnatal days or exposure to an enriched environment later in the pre-pubertal period leads to behavioral alterations that persist through adulthood, such as increased sweet food ingestion.

Neonatal tactile stimulation reverses the effect of neonatal isolation on open-field and anxiety-like behavior, and pain sensitivity in male and female adult Sprague-Dawley rats

It is well known that early life events induce long-lasting psychophysiological and psychobiological influences in later life. In rodent studies, environmental enrichment after weaning prevents the adulthood behavioral and emotional disturbances in response to early adversities. We compared the behavioral effect of neonatal isolation (NI) with the effect of NI accompanied by tactile stimulation (NTS) to determine whether NTS could reverse or prevent the effects of NI on the adulthood behavioral and emotional responses to environmental stimuli. In addition, we also examined the sex difference of the NTS effect. Measurements of body weights, an open-field locomotor test, an elevated plus maze test, a hot-plate test, and a contextual fear-conditioning test were performed on postnatal day 60. As compared with rats subjected to NI, rats subjected to NTS showed significantly higher activity and exploration in the open-field locomotor test, lower anxiety-like behavior in the elevated plus maze test, and significantly prolonged latencies in the hot-plate test, and this effect was equal among males and females. In the contextual fear-conditioning test, whereas NTS significantly reduced the enhanced freezing time due to NI in females, no significant difference in the freezing time between NI and NTS was found in males. These findings indicate that adequate tactile stimulation in early life plays an important role in the prevention of disturbances in the behavioral and emotional responses to environmental stimuli in adulthood induced by early adverse experiences.

Long lasting sex-specific effects upon behavior and S100b levels after maternal separation and exposure to a model of post-traumatic stress disorder in rats

This study was undertaken to verify if repeated long-term separation from dams would affect the development of parameters related to post-traumatic stress disorder (PTSD) after animals are subjected to inescapable shock when adults. Wistar rats were subjected to repeated maternal separation during post-natal days 1-10. When adults, rats from both sexes were submitted to a PTSD model consisting of exposure to inescapable footshock, followed by situational reminders. We observed long-lasting effects of both interventions. Exposure to shock increased fear conditioning. Anxiety-like behavior was increased and exploratory activity decreased by both treatments, and these effects were more robust in males. Additionally, basal corticosterone in plasma was decreased, paralleling effects observed in PTSD patients. Levels of S100B protein in serum and cerebrospinal fluid (CSF) were measured. Levels in serum correlated with the effects observed in anxiety-like behavior, increasing in males exposed to shock, and presenting no effect in females. S100B in CSF was increased in females submitted to maternal separation during the neonatal period. These results suggest that, in rats, an early stress experience such as maternal separation may aggravate some effects of exposure to a stressor during adult age, and that this effect is sex-specific. Additionally, data suggest that the increased S100B levels, observed in serum, have an extracerebral origin, possibly mediated by an increase in the noradrenergic tonus. Increased S100B in brain could be related to its neurotrophic actions.

Sex differences in the basolateral amygdala: the extracellular levels of serotonin and dopamine, and their responses to restraint stress in rats

The sex difference in the emotional response to stress suggests a sex-specific stress response in the amygdala. To examine the sex difference in extracellular levels of serotonin (5HT) and dopamine (DA) in the basolateral amygdala (BLA) and their responses to restraint stress, in vivo microdialysis studies were performed in male and female rats. In experiment I, dialysates were collected from the BLA at 15-min intervals under the freely moving condition. Mean extracellular levels of 5HT or DA in the BLA were higher in male rats than in female rats. In experiment II, rats were subjected to restraint stress for 60 min to examine the stress response of 5HT or DA levels. Although restraint stress significantly increased extracellular 5HT levels in both sexes of rats, female rats showed a greater response than male rats. Moreover, restraint stress significantly increased extracellular DA levels in female rats, but not in male rats. In experiment III, rats were subjected to restraint stress for 30 min to examine behavioral responses to restraint stress. Although no sex difference was observed in the number of audible vocalizations, male rats defecated a larger number of fecal pellets than female rats. In experiment IV, rats were tested for freezing behavior to examine contextual fear responses. Conditioned male rats showed a longer freezing time than conditioned female rats. We found sex differences in the extracellular levels of 5HT and DA in the BLA and their responses to restraint stress, which may be involved in the sex-specific emotional response to stress in rats.