Effects of social context on endocrine function and Zif268 expression in response to an acute stressor in adolescent and adult rats

There is a paucity of studies comparing social buffering in adolescents and adults, despite their marked differences in social behavior. We investigated whether greater effects of social buffering on plasma corticosterone concentrations and expression of Zif268 in neural regions after an acute stressor would be found in adolescent than adult rats. Samples were obtained before and after 1 h of isolation stress and after either 1 or 3 h of recovery back in the colony with either a familiar or unfamiliar cage partner. Adolescent and adult rats did not differ in plasma concentrations of corticosterone at any time point. Corticosterone concentrations were higher after 1 h isolation than at baseline (p < 0.001), and rats with a familiar partner during the recovery phase had lower corticosterone concentrations than did rats with an unfamiliar partner (p = 0.02). Zif268 immunoreactive cell counts were higher in the arcuate nucleus in both age groups after isolation (p = 0.007) and in the paraventricular nucleus of adolescents than adults during the recovery phase irrespective of partner familiarity. There was a significant decrease in immunoreactive cell counts after 1 h isolation compared to baseline in the basolateral amygdala, central nucleus of the amygdala, and in the pyramidal layer of the hippocampus (all p < 0.05). An effect of partner familiarity on Zif268 immunoreactive cell counts was found in the granule layer of the dentate gyrus irrespective of age (higher in those with a familiar partner, p = 0.03) and in the medial prefrontal cortex in adolescents (higher with an unfamiliar partner, p = 0.02). Overall, the acute stress and partner familiarity produced a similar pattern of results in adolescents and adults, with both age groups sensitive to the social context.     

The effect of periaqueductal gray lesions on responses to age-specific threats in infant rats

During early ontogeny infant rats show specific responses to a variety of age-dependent threatening situations. When isolated from nest and dam, they emit ultrasonic vocalizations and show decreased reactivity to noxious stimulation, or analgesia. When exposed to an unfamiliar adult male, they become immobile and analgesic. The midbrain periaqueductal gray (PAG) is an important area within the circuitry that controls responses to threatening stimuli in the adult. Little is known about the functions of the PAG in early life. It was hypothesized that the PAG mediates the responses to the age-specific threats social isolation and male exposure in the infant rat. Rat pups were lesioned electrolytically either in the lateral or the ventrolateral PAG on postnatal day 7, tested in social isolation on day 10, and exposed to a male on day 14. On day 10 during isolation, ultrasonic vocalizations and isolation-induced analgesia were decreased in both lesion groups. On day 14, male-induced immobility and analgesia were decreased in ventrally lesioned animals. In conclusion, the PAG seems to play a developmentally continuous role in age-specific responses to threat such as ultrasonic vocalization, analgesia, and immobility.     

Regional vascular responses to an acute stressor in spontaneously hypertensive and Wistar-Kyoto rats

The regional vascular responses to an acute intermittent footshock stressor were examined in Wistar-Kyoto and spontaneously hypertensive rats using the pulsed Doppler flow probe method. While both strains exhibited vascular resistance changes, which were typical of the 'defense response', the changes in regional vascular resistance were exaggerated in the spontaneously hypertensive rats compared to the Wistar-Kyoto. Despite these alterations in vascular resistance, and greater increases in heart rate in the spontaneously hypertensive rats, there were no differences in the pressor responses footshock between the Wistar-Kyoto and spontaneously hypertensive rats. These findings indicate that very specific regional hemodynamic changes accompany the characteristic sympathetic nervous system hyperresponsiveness of spontaneously hypertensive rats to environmental stress.     

c-fos proto-oncogene changes in relation to REM sleep duration.

Auditory stimulation has been shown to increase REM sleep periods in cats and humans. This effect has been attributed to an elevation of the level of excitability in a variety of brain stem neuronal groups. Fos-like immunostaining (FLI) has been useful in constructing maps of post-synaptic neuronal activity with single cell resolution, and has been suggested to be tightly correlated with ongoing neuronal activity. This study used FLI to quantify neurons from structures expressing c-fos in brain stem areas in animals with normal REMs and compared them with those showing extended REM periods. The results basically indicated that brain stem areas which in other studies have been described as having REM-ON cells, showed an increase in FLI, while no FLI changes occurred in areas described as having REM-OFF cells. These results are discussed in terms of the possibility that REM maintenance is related to a widespread increase in brain stem excitability.     

The medial prefrontal cortex differentially regulates stress-induced c-fos expression in the forebrain depending on type of stressor

The medial prefrontal cortex (mPFC) plays an important inhibitory role in the hypothalamic-pituitary-adrenal (HPA) axis response. The involvement of the mPFC appears to depend on the type of stressor, preferentially affecting 'psychogenic' stimuli. In this study, we mapped expression of c-fos mRNA to assess the neural circuitry underlying stressor-specific actions of the mPFC on HPA reactivity. Thus, groups of mPFC-lesioned and sham-operated rats were restrained for 20 min or exposed to ether fumes for 2 min. In both cases, the animals were killed at 40 min from the onset of stress. Interestingly, bilateral lesions of the mPFC significantly enhanced c-fos mRNA expression in the hypothalamic paraventricular nucleus of restrained animals, an effect that was paralleled by potentiation of circulating ACTH concentrations in these animals. On the other hand, lesions of the mPFC did not affect neither PVN c-fos mRNA expression nor plasma ACTH concentrations in animals exposed to ether. Lesions of the mPFC also enhanced c-fos activation in the medial amygdala following restraint, but not following ether exposure. Additional regions whose activity was affected by mPFC lesions or stressor differences included the ventrolateral division of the bed nucleus of the stria terminalis, CA3 hippocampus, piriform cortex, and dorsal endopiriform nucleus. Expression of c-fos mRNA was nearly absent in the central amygdala of all stressed animals, regardless of lesion. Furthermore, prefrontal cortex lesions did not change stress-induction levels of c-fos in the CA1 hippocampus, dentate gyrus, anteromedial division of the bed nucleus of the stria terminalis, lateral septum, and claustrum. Taken together, this study indicates that the medial prefrontal cortex differentially regulates cellular activation of specific stress-related brain regions, thus exerting stressor-dependent inhibition of the HPA axis.     

Long-term changes in mineralocorticoid and glucocorticoid receptor occupancy following exposure to an acute stressor

Stressors produce rapid activation of the hypothalamic-pituitary-adrenal axis, which typically resolves within 60-90 min following termination of the stressor. In addition, some stressors such as inescapable tailshock (IS) also produce elevated basal levels of corticosterone (CORT), and reduced serum levels of corticosteroid binding globulin (CBG). The elevated basal levels of CORT produced by IS are only observed at the trough of the circadian rhythm of CORT secretion, and are sustained for 2-3 days following stressor termination. The goal of the following experiments was to determine the extent to which the elevated basal levels of CORT observed following IS exposure produced greater corticosteroid receptor occupancy in the brain and pituitary. To do so, rats (n=8-10 per group) received either sham or bilateral adrenalectomy (with CORT replacement in their drinking water; 25 microg/ml) and were given 3 days to recover. Rats were then exposed to 100 ISs (1.6 mA, 5 s each) administered on a 60 s variable intertrial interval, or remained in their home cages. As seen previously, IS produced an increase in basal CORT (5 microg/dl) and a decrease in CBG (30% decrease). Rats were sacrificed 24 h following IS for trunk blood samples and brain dissections. IS exposure had very little effect on corticosteroid receptor protein expression as determined by mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) binding levels in ADX rats. In addition, no changes in whole cell GR levels (as detected by Western blot) were observed in sham rats exposed to IS. On the other hand, IS exposure led to greater occupancy of MR (ranging from 25%-50%) in hippocampus, hypothalamus, pituitary, and posterior cortex. IS also produced greater occupancy of GR (approximately 20%) in hypothalamus and posterior cortex. These long-term changes in corticosteroid receptor activation, evident 24 h after IS exposure, may be responsible for some of the long-term neural, behavioral and immune changes observed following this acute stress procedure.     

Pubertal-related changes in hypothalamic-pituitary-adrenal axis reactivity and cytokine secretion in response to an immunological stressor

Pubertal development is marked by profound changes in stress reactivity. For example, following a brief stressor, such as foot shock, ether inhalation or restraint, prepubertal rats display a prolonged adrenocorticotrophic hormone (ACTH) and corticosterone response that takes twice as long to return to baseline compared to adults. Pubertal‐related differences in the recovery of the hormonal stress response following a more protracted systemic stressor, such as an immunological challenge, have not yet been investigated. Moreover, it is unclear whether an immunological stressor leads to a differential cytokine response in animals before and after pubertal maturation. To examine these issues, we used a single injection of lipopolysaccharide (LPS; 0.1 mg/kg) to induce a hormonal stress and innate immune response and measured plasma ACTH, corticosterone, and the pro‐inflammatory cytokines interleukin (IL)‐1β and IL‐6 in prepubertal and adult male rats 0, 2, 4, 6, 8, or 24 h after LPS exposure. In a follow‐up experiment, we assessed neural activation, as indexed by FOS immunohistochemistry, in the paraventricular nucleus of the hypothalamus (PVN) in prepubertal and adult males 0, 4, 8, or 24 h after a 0.1 mg/kg injection of LPS. By contrast to the prolonged response observed in prepubertal animals following a variety of acute stressors, we found that corticosterone and IL‐6 responses induced by LPS recover toward baseline faster in prepubertal compared to adult rats. Along with these different peripheral responses, we also found that LPS‐induced neural activation in the PVN of prepubertal animals showed a faster return to baseline compared to adults. Together, these data indicate that prepubertal and adult animals react in distinct ways, both peripherally and centrally, to an immunological stressor.     

Developmental changes in cerebrospinal fluid pressure and resistance to absorption in rats

Cerebrospinal fluid (c.s.f.) pressure and resistance to absorption have been studied in exteriorized fetuses (18 to 21 days gestation) from pregnant rats anaesthetised with pentobarbitone and in similarly anaesthetised postnatal rats at 1, 5, 10 and 30 days after birth and in adults. The resting c.s.f. pressure was low in fetuses (17-21 mm H2O) and it rose to 26-27 mm H2O at 1 and 5 days after birth. By 10 days after birth the pressure was 34 mm H2O after which time there was no further change. The plateau pressure response to different infusion rates was linear, but only up to pressures around 7-fold higher than the resting c.s.f. pressure. Hence, resistance to absorption was calculated for plateau pressures up to 140 mm H2O in fetuses, to 200 mm H2O in 1- and 5-day rats and to 240 mm H2O for 10-day, 30-day and adult rats. Resistance to absorption was not significantly different for the two measurement sites (lateral ventricle and subarachnoid space) at any age studied, showing that fluid can move freely through the ventricular system in young rats. The resistance to absorption was low in the fetuses, 10.8 and 16.3 mm H2O min/microliters at 18-19 and 20-21 days gestation, respectively. There was a sharp increase in resistance to 39.2 mm H2O min/microliters at 1 day after birth and thereafter there was a decrease to 6.8 mm H2O min/microliters at 30 days and this was similar to adult values. This decrease in outflow resistance from the first day after birth may be related to the increase in c.s.f. pressure and secretion rate that occur after birth in the rat.     

Developmental changes in hypothalamic toll-like-receptor 4 mRNA expression and the effects of lipopolysaccharide on such changes in female rats

Hypothalamic pro-inflammatory cytokine expression exhibits a weaker response to lipopolysaccharides (LPS) during the early neonatal period than during the later developmental period. Although toll-like receptor 4 (TLR4), which recognizes bacterial molecules, activates pro-inflammatory cytokine responses, the developmental changes in hypothalamic TLR4 expression have not been evaluated. In this study, the hypothalamic TLR4 mRNA levels of saline-injected and LPS-injected rats were measured during the neonatal, pre-pubertal, and post-pubertal periods. The rats’ hypothalamic TLR4 mRNA levels gradually increased from the neonatal to pubertal period and were altered by the injection of LPS at all examined ages (postnatal day (PND) 5, 15, 25, and 42). LPS injection resulted in decreased hypothalamic TLR4 mRNA expression at PND5, whereas it increased hypothalamic TLR4 mRNA expression at PND15, 25, and 42. After the injection of LPS, the hypothalamic mRNA levels of the pro-inflammatory cytokines interleukin (IL)-1β, tumor necrosis factor α, and IL-6 were attenuated during the early developmental period and increased acutely on PND42. The expression profiles of these pro-inflammatory cytokines exhibited similar, but not entirely consistent, changes to those displayed by TLR4 during the developmental period. Hypothalamic TLR4 mRNA expression gradually increased throughout the developmental period, whereas the mRNA expression levels of the pro-inflammatory cytokines increased acutely at PND42. Thus, it is assumed that hypothalamic TLR4 hypoactivity contributes to the low sensitivity of pro-inflammatory cytokines to LPS during the early developmental period.     

HPA axis response to a psychological stressor in generalised social phobia

Social phobia may be associated with a dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis. In this study we determined HPA axis responsivity to a psychological stressor in patients with social phobia and compared them to healthy controls. Fifteen patients with DSM IV social phobia with a mean score of 77.7 on the Liebowitz Social Anxiety Scale and 15 age and sex matched controls underwent the stressor consisting of mental arithmetic and a short term memory test performed in front of an audience. Plasma levels of cortisol and corticotropin were measured at various intervals throughout the test. Although baseline measures of cortisol did not differ between patients (319.8+/-34.6 nmol/l) and controls (279.5+/-42.7 nmol/l)(t=0.7, df=28, P<0.5) nor did baseline corticotropin values (8.6+/-2.1 pg/ml vs 13.7+/-2.0 pg/ml respectively) (t=-1.8, df=28, P<0.08) this stressor resulted in a significantly greater delta max cortisol response (the difference between baseline values and the maximum increase during the stressor) in patients (167.1+/-23.7 nmol/l) than in controls (106.7+/-16 nmol/l) (t=2.1, df=28, P<0.04). There was no significant difference in delta max corticotropin between groups (patients 8.8+/-2.1 pg/ml vs controls 9.1+/-1.9 pg/ml) (t=-0.08, df=28, P<0.9). This preliminary study indicates that patients with social phobia appear to have a hyper-responsive adrenocortical response to psychological stress.     

Developmental changes of synaptojanin expression in the human cerebrum and cerebellum.

Synaptojanin is a highly abundant polyphosphoinositide phosphatase in nerve terminals, and has been thought to play roles in clathrin-mediated synaptic vesicle endocytosis and signaling. In order to determine the broader role of synaptojanin in the central nervous system, we examined synaptojanin expression in the cerebrum and cerebellum from the fetal to the adult period by means of immunohistochemical and Western blot analyses. Immunohistochemistry consistently revealed the localization of synaptojanin in Cajal--Retzius cells, cortical plate neurons, subplate neurons, intermediate neurons, germinal matrix cells and the ventricular neuroepithelium of the fetal cerebrum. In the fetal cerebellum, synaptojanin immunoreactivity was localized in the external granular cell layer, Purkinje cell layer neuropil, cytoplasm of Purkinje cells and internal granular cells. The immunoreactivity in these structures was decreased around birth. After birth, the synaptojanin immunoreactivity of cortical neurons in the cerebrum, Purkinje cell layer neuropil, and internal granular cells and Purkinje cells in the cerebellum increased and reached a plateau after 11 years of age. These results were consistent with the intensity observed on Western blot analysis. These developmental changes of synaptojanin suggest a broader role in not only synaptic vesicle recycling, but also the regulation of neuronal migration and synaptogenesis in the fetal cerebrum and cerebellum.     

Developmental changes in progenitor cell responsiveness to cytokines.

Multipotent progenitor cells have been identified within periventricular generative zones of the developing and adult brain. To determine whether the environmental responsiveness of these cells changes during development, progenitor cells were cultured from embryonic, postnatal, and adult rat brain in the presence of either basic fibroblast growth factor (bFGF) or epidermal growth factor (EGF). Embryonic cells cultured as intact progenitor neurospheres proliferated more robustly in response to bFGF than to EGF, whereas proliferation of postnatal and adult progenitor cells was enhanced more by EGF than bFGF. Progenitor cells generated in the presence of either bFGF or EGF had the capacity to generate neurons, astrocytes, and oligodendrocytes at all developmental stages. Most embryonic and neonatal bFGF-generated cells differentiated predominantly into neurons, whereas late stage embryonic and neonatal EGF-generated progenitors largely remained in an undifferentiated state. However, later postnatal and adult progenitor species, irrespective of whether they were generated in the presence of bFGF or EGF, gave rise preferentially to astrocytes. Treatment with bone morphogenetic protein (BMP)2 or BMP7 enhanced astroglial differentiation and suppressed oligodendroglial differentiation of both EGF- and bFGF-generated progenitor species, suggesting that the effects of the BMPs are not dependent on EGF receptor activation. Thus, while central nervous system (CNS) progenitor cells retain multipotent capacity and responsiveness to the BMPs throughout development, they exhibit significant changes in other cellular response properties, perhaps reflecting differences in the requirements for specific generative versus regenerative events.     

Activation of serotonin-immunoreactive cells in the dorsal raphe nucleus in rats exposed to an uncontrollable stressor

The dorsal raphe nucleus (DRN) and its serotonergic terminal regions have been suggested to be part of the neural substrate by which exposure to uncontrollable stressors produces poor escape responding and enhanced conditioned fear expression. Such stressor exposure is thought to selectively activate DRN serotonergic neurons in such a way as to render them transiently sensitized to further input. As a result of this sensitized state, behavioral testing procedures are thought to cause excess serotonergic activity in brain regions that control these behaviors. The present studies were conducted to investigate activity in the DRN following exposure to escapable and yoked, inescapable tailshock. Neural activity was characterized using immunohistochemistry to detect the immediate early gene product Fos in serotonin-immunoreactive cells in the DRN. Inescapable tailshock led to greater serotonergic neural activity than did escapable tailshock, supporting the hypothesis that uncontrollable stressors preferentially activate serotonergic neurons in the DRN.     

Developmental changes in hypothalamic oxytocin and oxytocin receptor mRNA expression and their sensitivity to fasting in male and female rats

Oxytocin (OT) affects the central nervous system and is involved in a variety of social and non-social behaviors. Recently, the role played by OT in energy metabolism and its organizational effects on estrogen receptor alpha (ER-α) during the neonatal period have gained attention. In this study, the developmental changes in the hypothalamic mRNA levels of OT, the OT receptor (OTR), and ER-α were evaluated in male and female rats. In addition, the fasting-induced changes in the hypothalamic mRNA levels of OT and the OTR were evaluated. Hypothalamic explants were taken from postnatal day (PND) 10, 20, and 30 rats, and the mRNA level of each molecule was measured. Hypothalamic OT mRNA expression increased throughout the developmental period in both sexes. The rats’ hypothalamic OTR mRNA levels were highest on PND 10 and decreased throughout the developmental period. In the male rats, the hypothalamic mRNA levels of ER-α were higher on PND 30 than on PND 10. On the other hand, no significant differences in hypothalamic ER-α mRNA expression were detected among the examined time points in the female rats, although hypothalamic ER-α mRNA expression tended to be higher on PND 30 than on PND 10. Significant positive correlations were detected between hypothalamic OT and ER-α mRNA expression in both the male and female rats. Hypothalamic OT mRNA expression was not affected by fasting at any of the examined time points in either sex. These results indicate that hypothalamic OT expression is not sensitive to fasting during the developmental period. In addition, as a positive correlation was detected between hypothalamic OT and ER-α mRNA expression, these two molecules might interact with each other to induce appropriate neuronal development.     

Developmental changes in hypothalamic melatonin levels of male rats

Hypothalamic melatonin levels of Long-Evans male rats were studied at three ages (25, 55-60 and 90 days), at four times of the day in the autumn (6:00, 12:00, 18:00 and 24:00), and at two times (12:00 and 24:00) in the spring using radiommunoassay. Melatonin levels increased markedly at noon at 55-60 days of age, compared with the levels at the same time of the day at 25 and 90 days. This increase persisted in autumn and spring. The 24-hr pattern in hypothalamic melatonin was the inverse of that in the pineal, with the levels at noon higher than those at midnight. This pattern was detectable at 25 days of age although the difference in melatonin between 12:00 and 24:00 hr was not great. The day/night difference was prominent by 55-60 days of age and disappeared by adulthood (90 days). This 24-hr pattern was similar in spring and autumn in the three ages studied. Although in the 55-60-day-old group the melatonin ratio (noon/midnight) was the same in autumn and spring, the absolute levels of melatonin in spring were significantly lower. The findings are consistent with the general concept of a modulatory role of melatonin in control of hypothalamo-hypophyseal GnRH and gonadotropin function, and the timing of the developmental maturation of this neuroendocrine axis. Demonstration of the mechanism of melatonin's action at the hypothalamic level will be facilitated by further definition of quantitative developmental changes.