Long-Term Effect of Moderate Hypoxia and Chronic Administration of Fluoxetine during the Neonatal Period on Cognitive and Stress-Hormonal Functions in Adult Male Rats.

We studied the effect of moderate neonatal normobaric hypoxia on the indicators of spatial learning, memory, and reactivity of the hypothalamic-pituitary-adrenocortical system in adult male Wistar rats. The pharmacological effect of chronic injections of the serotonin reuptake inhibitor fluoxetine during the neonatal period on the studied behavioral and the physiological indices was evaluated. Hypoxia impaired spatial training, increased the short-term memory performance, but did not change long-term memory and stress indicator in response to its testing. The use of fluoxetine normalized learning, but did not change memory indicators and the stress-induced level of corticosterone in blood plasma in the hypoxic rats and control animals. New results indicate a protective effect of fluoxetine in the neonatal period under conditions of moderate normobaric hypoxia.

Sexual Dimorphism in the Effect of Neonatal Inflammatory Pain on Stress Reactivity of Hormonal Response and Cognitive Functions in Adult Rats.

The effect of moderate neonatal stress induced by inflammatory pain in rat pups of both sexes on the hormonal response and cognitive processes in adult animals was studied in the Morris water maze. No significant differences in spatial learning and memory were found in experimental rats exposed to neonatal inflammatory pain vs. control animals. However, experimental rats exhibited sex differences in long-term spatial memory whose efficiency was higher in males vs. females. After long-term memory testing, stress responsiveness of the hypothalamic-pituitary-adrenocortical axis, as assessed by the plasma corticosterone level in the formalin test, was higher in experimental males vs. females. Only experimental females exhibited differences between short-term and long-term memory, with the efficiency being higher in the former. Thus, sexual dimorphism was found in the effect of neonatal nociceptive stress on long-term spatial memory in adult rats: experimental males vs. females demonstrated more effective long-term memory combined with a higher stress reactivity of the hormonal response.

Effects of Stress Exposure during Adolescent Period on Inflammatory Pain Response, Psychoemotional Behavior, and Action of Antidepressants in Prenatally Stressed Adult Male Rats.

We studied the effects of stress exposure during the adolescent period of development (SAPD) on the parameters of inflammatory painful response and the level of depression-like behavior in prenatally stressed adult male rats. In addition, we analyzed the effects of selective serotonin (5-HT) reuptake inhibitor fluoxetine and 5-HT1A receptor agonist buspirone injected chronically to pregnant mothers for correction of behavioral disturbances caused by prenatal stress in their adult male progeny. In the formalin test, SAPD decreased integrated at the supraspinal level pain-like response that was increased by prenatal stress; under these conditions, buspirone and fluoxetine were ineffective in contrast to their antinociceptive action on spinally integrated pain-like response increased by SAPD. In the forced swimming test, SAPD had no effect on the level of depression-like behavior in prenatally stressed males; no differences in plasma corticosterone level were found in these animals.

Prenatal Stimulation of 5-HT1A Receptors Improves Adaptive Behavior in Prenatally Stressed Rats.

Various types of adaptive behavior during the prepubertal period were analyzed in the offspring of rats receiving chronic injections of serotonin (5-HT) reuptake inhibitor fluoxetine, 5-HT1A receptor agonist buspirone, or their combination starting from gestation day 9 and subjected to immobilization stress from the 15th day of pregnancy until delivery. Prenatal stress increased pain sensitivity, prolonged inflammatory pain response, and increased the levels of anxiety and depression. Chronic administration of drugs acting through 5-HT1A receptors to pregnant rats improved the studied behavioral parameters in their offspring. Differences in the pain sensitivity were found between the effect of drug combination and each of them separately, and in the level of depression between combined administration and fluoxetine alone.

Effect of Fluoxetine in Prenatal Period on Nociceptive System Reactivity and Psychoemotional Behavior in Young Female Rats.

We studied the effect of chronic injections of serotonin reuptake inhibitor fluoxetine to rats during pregnancy on physiological and behavioral characteristics of female offspring during the prepubertal period. Fluoxetine reduced body weight in newborn females, but this parameter was restored to normal values by the age of 25 days. Fluoxetine also increased animal anxiety, but did not change the level of depressive behavior and cognitive capacities. It was shown that chronic injections of physiological solution to pregnant female enhanced nociceptive responses in the offspring during the prepubertal period, while fluoxetine neutralized the consequences of invasive intervention, which was expressed in a lower level of pain reaction in the offspring. This indicates the antinociceptive effect of fluoxetine.

Long-Term Effects of Neonatal Pain and Stress on Reactivity of the Nociceptive System.

The influence of inflammatory pain and/or weaning stress at different terms of neonatal development on functional activity of the nociceptive system during adulthood was studied in rats. Repeated stress in 1-2-day-old rat pups (a premature baby model) enhanced pain sensitivity to peripheral inflammation in both males and females. Repeated inflammatory pain experienced by male pups aged 1-2 or 7-8 days (models of preterm and full-term baby), even in presence of mother, enhanced pain behavior under conditions of repeated inflammatory pain in adulthood. Pain sensitivity in adult animals before (hot plate test) and after formation of the inflammatory focus (formalin test) depended on the age when the animals were subjected to the injury, type of exposure, and on animal sex. The priority data obtained by us will help to understand the mechanisms of long-term effects of early injuries and are important for pediatricians and neonatologists.

The Role of 5-HT1A Receptors in Long-Term Adaptation of Newborn Rats to Hypoxia.

We studied the effects of neonatal hypoxia on adaptive behavior of rats during prepubertal and pubertal periods in the control and after repeated injections of 5-HT1A receptor agonist buspirone. Hypoxia enhanced the inflammatory nociceptive response and exacerbated the depressive-like behavior. Repeated injections of buspirone starting from the neonatal period produced a long-term normalizing effect on the inflammatory nociceptive response and psychoemotional behavior disturbed by hypoxia. The protective effect of buspirone can result from strengthening of the adaptive potencies of the serotoninergic system via activation of 5-HT1A receptors that up-regulate secretion of trophic factor S100ß under conditions of serotonin deficiency typical of rats exposed to neonatal hypoxia. Buspirone promotes recovery of the afferent and efferent connections of the raphe nuclei with the prefrontal cortex and spinal cord involved in integration of the anti-nociceptive and psychoemotional systems.

[LONG-TERM EFFECTS OF STRESSFUL IMPACTS DURING NEONATAL PERIOD OF DEVELOPMENT ON NOCICEPTIVE SYSTEM AND PSYCHOEMOTIONAL BEHAVIOR].

Long-term effects of peripheral pain, brief stress of maternal isolation and their combination in one-day- and repeatedly two-day-old rat pups or seven-day- and repeatedly eight-day-old pups were investigated on indices of basal pain (in the absence of inflammation), a prolonged response to again evoked inflammation in the formalin test, the level of anxiety and depression and of spatial learning ability in the animals as adults (90-day-olds). Changes in the indices under investigation were revealed in adult rats depending on the type of early life impact and age during which it occurred: inflammatory pain induced hypoalgesia in the test of hot plate and deterioration in spatial learning ability in the Morris test; the stress of maternal isolation on the first and second days, but not seventh and eighth days of life caused a strengthening of formalin-induced pain response. Regardless of the age when rat pups were exposed to impact, again induced inflammation evoked strengthening of pain response when the animals reached adulthood. In the forced swim test the time of immobility was increased in rats exposed early to any from impacts investigated. The effect of combination pain with stress did not exceed the effect of each impact separately. The new data contribute to the urgent problem of the long-term effects of the damaging impacts of pain and stress during the neonatal period on the functional activity of the tonic nociceptive system and behavior.

[INVOLVEMENT OF 5-HT1A RECEPTORS IN LONG-TERM INFLUENCE OF INFLAMMATORY PAIN AND STRESS IN EARLY AGE ON REACTIVITY OF NOCICEPTIVE SYSTEM].

Effects of inflammatory pain, short stress of maternal isolation and combination of these impacts in 1-day-old and repeatedly 2-day-old rat pups (neonatal period of development) on the indices of generalized pain and the inflammatory pain response were studied on the rats during the adulthood. To study the involvement of 5-HT1A receptors in the long-term impact of neonatal effects on pain sensitivity we used a chronic injection of 5-HT1A receptor agonist buspirone during the prepuberal period of rats which as newborn experienced similar impacts (control, injection of saline). It was found that in adult rats in which inflammatory pain and stress of maternal isolation during the first two days of life caused changes in pain sensitivity, buspirone normalized the indices of basic pain in the hot plate test and the pain response in the formalin test; the combination of these impacts did not cause any changes in the pain sensitivity, and the effect of buspirone did not appear. Thus, effects of buspirone found in this study suggest that 5-HT1A receptors are involved in the long-term influence of the studied adverse neonatal impacts on the reactivity of the nociceptive system.

[THE DAMAGING EFFECTS AT AN EARLY AGE ALTER PAIN SENSITIVITY IN ADULT FEMALE RATS, CORRECTION WITH BUSPIRONE].

Most studies on the damaging effects of pain and stress impacts in the neonatal period of development on pain sensitivity were performed on individuals of the male sex. In the present study, we investigated the influence of inflammatory pain and/or stress of isolation from the mother in newborn female rats to pain sensitivity when they reached adulthood; an attempt was undertaken to correct identified deviations using 5-HT1A-receptor agonist buspirone. Adult females exposed to early pain displayed increased hypoalgesia in the hot plate test and rats subjected to stress of isolation from the mother showed increased hyperalgesia in the formalin test. The pain and subsequent isolation from the mother did not change pain sensitivity in the adult females. The chronic injection of buspirone from 25 th to 39 th day of life to females subjected to inflammatory pain and isolation from the mother in the neonatal period caused the normalization of pain sensitivity when they reached adulthood. It is found that the prepubertal period is a critical period for the correction of deviations caused by damaging impacts in neonatal rata in the functional activity of the nociceptive system.

[DIFFERENCES IN ADAPTIVE BEHAVIORS IN ADOLESCENT MALE AND FEMALE RATS EXPOSED AS NEWBORNS TO INFLAMMATORY PAIN OR STRESS].

In adolescent rats (25-35-day-old) exposed as newborns (the first and repeatedly second days) to adverse impacts (inflammatory pain, stress of short-term maternal separation or their combination) sex dimorphism was revealed in pain behavior under conditions of similar peripheral inflammation. According to the priority data obtained, strengthening of pain-related response in the formalin test was found in males, whereas pain sensitivity in females was not changed, that is pain experienced by them as newborns did not affect the system reactivity to the same chemical irritant in the adolescent period. However, the rats of both sexes, who experienced short-term stress of maternal deprivation (60 min-during the first and the second days of life), displayed increased pain sensitivity in the formalin test. Combined effect of inflammatory pain and maternal deprivation in newborns did not alter pain sensitivity in both adolescent males and adolescent females. The male and female rats exposed as newborns to maternal deprivation displayed a decrease of the anxiety level in the elevated plus maze; the rats, exposed to each of the above-mentioned early impacts showed a decline of adaptive behavior in the forced swimming test; the males exposed to pain and combined impacts demonstrated impairment of spatial learning in Morris labyrinth. Thus, we pioneered in demonstrating sex differences in the effects of inflammatory pain in newborn pups on pain sensitivity in the formalin test in adolescent rats. Separation of the influence of early stress or pain was revealed in adolescent females in the formalin test: maternal deprivation induced hyperalgesia, whereas pain failed to change functional activity of the tonic nociceptive system.

[EFFECT OF PRENATAL STRESS ON SEROTONERGIC NEURONS IN DORSAL RAPHE NUCLEUS AND ON PAIN BEHAVIOR DURING NEONATAL PERIOD].

The effects of prenatal stress on immunocytochemical reaction on serotonin (5-HT) in the dorsal raphe nucleus (DRN) of the brainstem were investigated in 7-day-old male rat pups exposed to impact of pain in the formalin test (a control is an injection of saline). A strengthening effect of prenatal stress on pain behavior was revealed in animals in the formalin test. Prenatal stress decreased the number of 5-HT-immunoreactive neurons as compared with that in prenatally non-stressed animals in both pups with inflammatory pain and the controls. There were no differences in the number of 5-HT-immunoreactive cells between pups with inflammatory pain and saline in both prenatally non-stressed animals and prenatally stressed ones. The data suggest that stress of maternal separation during the experiment hides the difference in immunocytochemical result in the rat pups with inflammatory pain and in the control. Thus, it was demonstrated for the first time that in 7-day-old rat pups the 5-HT-ergic neurons in DRN of the brainstem are a target of the prenatal stress.

[Long-term changes in adaptive behavior of rats after neonatal inflammatory pain].

In this study we addressed the tonic nociceptive system functional activity in the formalin test, anxiety- and depression-like behaviors and spatial learning in adolescent male rats exposed in the neonatal development to repeated inflammatory pain peripheral stimulation. The following groups of 25-day-old rats were used after being exposed on days 7 and 8 to: 1) formalin-induced inflammatory pain with maternal separation for 60 min (FS), 2) the same inflammatory pain stimulation without maternal separation (FWS), 3) physiological saline injection with maternal separation for 1 h (SS), 4) physiological saline injection without maternal separation (SWS) and 5) no stimulation (intact rats). The data obtained indicate that pain caused in 7-8-day-old rat pups by formalin injection into the plantar pad of the hind paw manifests by adolescence (day 25 as a strengthened inflammatory response under the analogous painful stimulation in the formalin test, adaptive behavior disorder in the forced swimming test and spatial learning disability. Our findings that a short-term repeated maternal deprivation of the 7-8-day-old rat pups without inflammatory pain increases the depression-like behavior are also of particular interest. Thus, a repeated inflammatory pain during the neonatal development brings about significant changes in the adaptive behaviors studied as well as in spatial learning in adolescent rats.

[Repeated inflammatory pain syndrome in newborn male rats changes adaptive behavior during adolescent period of development].

The study is devoted to the investigation of the effects of the repeated inflammatory pain syndrome of newborn male rat pups at the age of one and two days of life on characteristics of adaptive behaviors in the formalin test, the elevated-plus maze, the forced swim test and also in the swimming pool spatial test in these animals at the age of 25 days. The new data are enhancement of functional activity of the tonic nociceptive system, an increase in depression like behavior, impairment of spatial learning. A priority fact has been obtained that indicates that a short (60 min) repeated maternal deprivation of newborn rat pups (a stressful impact) without inflammatory noxious impact results in changes of the adaptive behaviors in 25-day-old animals. The evaluation of pain patterns organized at different levels of the central nervous system in the formalin test let us to find differences between effects of early painful and stressful impacts on the behavioral indices under study.

Tonic pain response during inflammation and stress-induced corticosterone variations in prenatally stressed infant rats: effects of maternal buspirone injected during pregnancy.

We studied the effects of injections of 5-HT1A-agonist buspirone to pregnant rats before stress exposure on corticosterone level in the dynamics of stress response to inflammatory-induced pain in 7-day-old offspring. During the period of the hypothalamic-pituitary-adrenal system hyporeactivity, the pain response in the formalin test was associated with stress-related corticosterone variations. Maternal buspirone normalized the pain reaction in prenatally stressed rats during all periods of the formalin test and modified the dynamics of the corticosterone response. In 1 day after the formalin test, the basal level of this hormone in blood plasma remained increased. Maternal buspirone increased the resistance of the nociceptive and stress-systems to inflammatory-induced pain response in prenatally stressed rats.

[Long-term effects of stress in critical periods of development on reactivity of adult female rats].

Effects of stress during different periods of ontogeny, namely, the prenatal, prepubertal, or their combination (prenatal+prepubertal), on the indices of psychoemotional and tonic pain-related behaviors, as well as corticosterone reactivity after pain behavior were investigated in adult 90-day-old female Wistar rats. Our data show for the first time, the similarity of effects of prenatal (immobilization stress of a rat dam during the last week of pregnancy) and prepubertal (forced swimming, pain-related response in the formalin test) stresses on the indices under study, an increase in the time of immobility and in licking duration, but the difference between effects of combined stress on these indices. Pain-related response increased corticosterone in prepubertally stressed rats while in prenatally stressed rats, decreased it. In rats experienced combined stress, formalin-induced pain increased corticosterone as compared with that in prenatally, but not in prepubertally stressed rats. A positive correlation between pain-related reaction and stressed hormonal response was revealed in prepubertally stressed animals. So, long-term effects of stress during critical periods of ontogeny determine stress reactivity of behavioral and hormonal responses in adult female rats.

[Effects of prenatal influences of buspirone and stress on indices of inflammatory pain response and depressive behaviour in adult rats].

New data on complex realization of anxiolytic, antidepressive and antinociceptive effects of prenatal injections of a 5-HT1A agonist buspirone were obtained in prenatally stressed adult rats. Buspirone was injected to female rats from the 9th to 21st days of pregnancy; during the last week of pregnancy buspirone was injected 10 min before immobilization stress. In the adult offspring of both sexes, behavioral indices of tonic pain response in the formalin test and the indices of depression in the forced swim test were investigated. The choice of the target was defined in accordance with available literature data on the role of 5-HT1A receptors in the mechanisms of prenatal stress, of formation of the ascending link of the nociceptive system, of development of depression, and in the mechanisms of the treatment ofnociceptive information. Prenatal stress increased the duration of licking and the time of immobility, the indices of tonic pain and depression in the rats of both sexes. Buspirone evoked the decrease of the indices investigated in prenatally stressed rats in both tests in comparison with the relevant indices in prenatally stressed rats that were not subjected to buspirone. Thereby, it has been demonstrated that buspirone normalized the indices of the tonic pain response modified by prenatal stress; a considerable decrease of the index of depression suggests that there are differences in mechanisms of antinociceptive and antidepressive effects of buspirone. The data on complex realization of anxiolytic, antidepressive and antinociceptive effects ofbuspirone stimulate the attention of clinicians and prompt further investigations in this direction.

Early and delayed effects of hypoxia during the infantile period on behavioral and hormonal reactions of rats.

We studied the early and delayed effects of hypoxia during the infantile period on the behavioral reactions and corticosterone concentration in male rats. The elevation of corticosterone concentration, decrease in the immobility time (forced swimming test), and increase in the nociceptive response (formalin test) were observed in 7-day-old rats immediately after hypoxia. Adult animals exposed to hypoxia at the age of 7 days exhibited elevated basal corticosterone level and lengthened immobility time. Hypoxia had the same effect on plasma corticosterone concentration in 7-day-old and adult rats. Changes in corticosterone concentration after forced swimming were shown to differ in hypoxic animals and non-hypoxic specimens. Studying the dynamics of age-related variations in the test parameters will contribute to the understanding of pathogenetic mechanisms and development of new methods for pharmacological correction of postnatal changes in CNS after hypoxia during early ontogeny.

Interrelationship between measures of pain reactions in inflammation and levels of depression in prenatally stressed rat pups.

The interrelationship between measures of pain reactions (number of flexion + shaking patterns) in the formalin test and the level of depression (duration of immobility) in the forced swimming (Porsolt) test was studied in prenatally stressed rat pups aged 7-8 days. Two series of experiments were performed, with different sequences of tests separated by intervals of one day. In the first series of experiments, the Porsolt test was performed first; in the second series, the formalin test was performed before forced swimming. The sequence of tests was found to have different effects on measures of pain and depression and their correlation in prenatally stressed and unstressed rat pups. The effects of the sequence of the depression test (before or after the formalin test) on measures of depression were different in prenatally unstressed and stressed rat pups. In the former there were no differences between the two test sequences, while in prenatally stressed rat pups the first sequence showed a significant increase in the duration of immobility. The order of testing had no effect on the pain response--there were no differences between the numbers of flexion + shaking patterns in either prenatally stressed rat pups or unstressed animals; measures of the pain response were significantly greater in the sequence in which the formalin test was followed by the Porsolt test in prenatally stressed individuals as compared with unstressed animals. A positive correlation between study parameters was seen in the first series in prenatally unstressed rat pups, while there was a negative correlation in prenatally stressed animals. In the second series, there were no significant relationships between measures. Thus, the sequelae of postnatal stress, as imposed by each test the day before the final test, were apparent only in prenatally stressed animals in terms of the level of depression.