V. A. Mikhailenko

Modulation of Signal-Transducing Function of Neuronal Membrane Na+,K by Endogenous Ouabain and Low-Power Infrared Radiation Leads to Pain Relief

Effects of infrared (IR) radiation generated by a low-power Co2-laser on sensory neurons of chick embryos were investigated by organotypic culture method. Low-power IR radiation firstly results in marked neurite suppressing action, probably induced by activation of Na+,K+-ATPase signal-transducing function. A further increase in energy of radiation leads to stimulation of neurite growth. We suggest that this effect is triggered by activation of Na+,K+-ATPase pumping function. Involvement of Na+,K+-ATPase in the control of the transduction process was proved by results obtained after application of ouabain at very low concentrations. Physiological significance of low-power IR radiation and effects of ouabain at nanomolar level was investigated in behavioral experiments (formalin test). It is shown that inflammatory pain induced by injection of formalin is relieved both due to ouabain action and after IR irradiation.

Buspirone before prenatal stress protects against adverse effects of stress on emotional and inflammatory pain-related behaviors in infant rats: age and sex differences.

Prenatal stress strengthens tonic pain and provokes depression. The serotoninergic system is involved in these processes. We recently showed that maternal buspirone, a 5-HT1A receptor agonist, protects against the adverse effects of in utero stress on depression and pain in adult rat offspring. Using a similar maternal treatment with buspirone, we focus here on the infant stage, which is important for the correction of prenatal abnormalities. Maternal buspirone before restraint stress during the last week of pregnancy decreased the time of immobility in the forced swim test in the infant offspring. Prenatal stress increased formalin-induced pain in the second part of the time-course of the response to formalin in males of middle infancy but in the first part of the response in males of late infancy. The effect was reversed by maternal buspirone. Pain dominated in males of both middle and late infancy but the time-course of formalin pain in infant females revealed a slower development of the processes. The results show that the time-course of formalin-induced pain in infant rats reacts to prenatal stress in an age-dependent and sexually dimorphic manner. Our finding of opposite influences of prenatal stress and buspirone before prenatal stress on formalin-induced pain during the interphase indicates that functional maturity of the descending serotonergic inhibitory system occurs in late infancy males (11-day-olds), and 5-HT1A receptors participate in this process. The data provide evidence that maternal treatment with buspirone prior to stress during pregnancy alleviates depression-like and tonic pain-related behaviors in the infant offspring.

Maternal buspirone protects against the adverse effects of in utero stress on emotional and pain-related behaviors in offspring

Previous investigations from our laboratory demonstrated that prenatal stress exacerbates inflammatory pain-related behavior in adult rats and that fetal serotonin (5-HT) is involved in this phenomenon. In the present study we test the hypothesis that injections of buspirone, a 5-HT1A agonist, to rat dams before restraint stress during the last week of pregnancy (between pregnant days 15 and 20) can improve the characteristics of emotional and inflammatory pain-related behaviors in the adult offspring. Buspirone was injected to dams between the 9 and 20 days of pregnancy, during restraint stress, five min before it. The depression-like behavior in the forced swim test, formalininduced pain and body weight were investigated in the adult offspring. Prenatal stress exacerbated the licking behavior, the index of formalin-induced pain, and increased the time of immobility, the index of depression-like behavior. Buspirone normalized the licking behavior and profoundly reduced the time of immobility, which indicates differences in the mechanisms of antinociceptive and antidepressant effects of buspirone. The present new findings demonstrate that adverse influences of prenatal stress on emotional and inflammatory pain-related behaviors can be prevented by using prenatal buspirone, which shows long-term anxiolytic, antidepressant and antinociceptive effects. The new fact of body weight decrease in buspirone+stress males is worth noting in the context of the important problem of body weight gain as a common side effect of treatment with antidepressant drugs.

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.

Inflammatory Pain and Corticosterone Response in Infant Rats: Effect of 5-HT1A Agonist Buspirone Prior to Gestational Stress

Our researches have shown that gestational stress causes exacerbation of inflammatory pain in the offspring; the maternal 5-HT1A agonist buspirone before the stress prevents the adverse effect. The serotonergic system and hypothalamo-pituitary-adrenal (HPA) axis are closely interrelated. However, interrelations between inflammatory pain and the HPA axis during the hyporeactive period of the latter have not been studied. The present research demonstrates that formalin-induced pain causes a gradual and prolonged increase in plasma corticosterone level in 7-day-old male rats; twenty-four hours after injection of formalin, the basal corticosterone level still exceeds the initial basal corticosterone value. Chronic treatments of rat dams with buspirone before restraint stress during gestation normalize in the offspring pain-like behavior and induce during the acute phase in the formalin test the stronger corticosterone increase as compared to the stress hormonal elevation in animals with other prenatal treatments. Negative correlation between plasma corticosterone level and the number of flexes+shakes is revealed in buspirone+stress rats. The new data enhance the idea about relativity of the HPA axis hyporeactive period and suggest that maternal buspirone prior to stress during gestation may enhance an adaptive mechanism of the inflammatory nociceptive system in the infant male offspring through activation of the HPA axis peripheral link.

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.

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 at birth (neonatal days 1 and, repeatedly, 2) to adverse impacts (inflammatory pain, short-term maternal separation stress, or both), sexual dimorphism was found in pain behavior under identical peripheral inflammation conditions. Our priority data indicate an enhancement of pain response in the formalin test in males, whereas in females pain sensitivity did not change, i.e. pain experienced in females at birth did not affect the system reactivity to the same chemical irritant in the adolescent period. However, rats of both sexes exposed to short-term maternal deprivation stress (60 min on neonatal days 1 and 2) exhibited the enhanced pain sensitivity in the formalin test. The combined impact of inflammatory pain and maternal deprivation in neonatal pups did not change pain sensitivity at adolescence both in males and females. Male and female rats exposed to early maternal deprivation exhibited decreased anxiety levels in the elevated plus-maze; rats exposed to each of the above-mentioned early impacts showed a decline in adaptive behavior in the forced swim test; males exposed to pain and combined impacts demonstrated the spatial learning impairment in the Morris labyrinth. Thus, we have pioneered in demonstrating sex-dependent differences in the effect of inflammatory pain in neonatal rat pups on the inflammatory pain sensitivity in adolescent rats. The division of early stress and pain impacts was revealed in adolescent females in the formalin test: while maternal deprivation induced hyperalgesia, pain did not change the tonic nociceptive system functional activity.

Persistent pain responses in inflammation and corticosterone levels in juvenile rats born to adrenalectomized dams

Studies in juvenile (Wistar) rats born to adrenalectomized dams (surgery performed 3–4 weeks before mating) addressed the intensity of behavioral pain responses (numbers of flexion + shaking patterns and durations of licking patterns) induced by foci of inflammation in the formalin test and plasma corticosterone levels on the background of the pain response. Maternal adrenalectomy had no effect on basal corticosterone levels or measures of the intensity of the pain response. In conditions of persistent pain (25 min after injection of formalin), corticosterone levels significantly (p < 0.05) increased in the offspring of both intact and operated dams. Females born to adrenalectomized dams, as compared with females from shamoperated dams, showed higher (p = 0.008) hormone levels in response to persistent pain. There were no differences in measures of the pain responses of the female offspring of adrenalectomized and sham-operated dams. There were no gender differences in measures of the pain response and hormone levels. Thus, pain evoked by acute foci of inflammation in the formalin test activated the hypothalamo-hypophysealadrenal system in 25-day-old rats, though the corticosterone released did not decrease pain intensity, which is consistent with data obtained from the offspring of adrenalectomized dams.

Prenatal Imbalance between Serotonergic and Hypothalamo-Pituitery-Adrenocortical Systems and Body Weight during Different Periods of Rat Ontogeny

287 When studying the effect of prenatal stress on longterm behavioral pain response to inflammation in rats of different ages and sexes, we discovered prenatal programming of nociception and demonstrated an important role of the serotonergic system in this phenomenon [1–4]. The use of different types of stress helps to analyze the development of adaptations to changing prenatal conditions [5]. The weight and size of the fetus and newborn are adequate indices of the body development during prenatal ontogeny [6, 7]. Taking into consideration literature data on the involvement of the serotonergic and hypothalamo-pituitery-adrenocortical (HPA) systems in prenatal stress and the control of feeding behavior and metabolism [8, 9], we studied the body weight of experimental animals.

Long-Term Effects of Chronic Buspirone during Adolescence Reduce the Adverse Influences of Neonatal Inflammatory Pain and Stress on Adaptive Behavior in Adult Male Rats

Neonatal pain and stress induce long-term changes in pain sensitivity and behavior. Previously we found alterations in pain sensitivity in adolescent rats exposed to early-life adverse events. We tested whether these alterations have long-lasting effects and if those effects can be improved by the 5-hydroxytryptamine 1A (5-HT1A) receptor agonist buspirone injected chronically during the adolescent period. This study investigates: (1) effects of inflammatory pain (the injection of formalin into the pad of a hind paw) or stress (short maternal deprivation-isolation, MI), or their combination in 1–2-day-old rats on the adult basal pain, formalin-induced pain, anxiety and depression; (2) effects of adolescent buspirone in adult rats that experienced similar early-life insults. Changes in nociceptive thresholds were evaluated using the hot plate (HP) and formalin tests; levels of anxiety and depression were assessed with the elevated plus maze and forced swim tests respectively. Both neonatal painful and stressful treatments induced long-term alterations in the forced swim test. Other changes in adult behavioral responses were dependent on the type of neonatal treatment. There was a notable lack of long-term effects of the combination of early inflammatory pain and stress of MI on the pain responses, anxiety levels or on the effects of adolescent buspirone. This study provides the first evidence that chronic injection of buspirone in adolescent rats alters antinociceptive and anxiolytic effects limited to adult rats that showed behavioral alterations induced by early-life adverse treatments. These data highlight the role of 5-HT1A receptors in long-term effects of neonatal inflammatory pain and stress of short MI on adaptive behavior and possibility of correction of the pain and psychoemotional behavior that were altered by adverse pain/stress intervention using buspirone during critical adolescent period.