Permissive role of corticotropin-releasing factor in the acute stress-induced prolactin release in female rats

Distribution of Corticotrophin-Releasing Factor Type 1 Receptor-Like Immunoreactivity in the Rat Pituitary

Corticotrophin-releasing factor (CRF) regulates the hypothalamic-pituitary-adrenal axis response to stress through its type 1 receptor (CRF₁ ) in the corticotrophs of the anterior pituitary. Although CRF₁ mRNA expression has been confirmed in the rat pituitary, the distribution pattern of CRF₁ protein in the pituitary has not been reported. Therefore, we generated an antiserum against the amino acid fragment corresponding to the 177-188 sequence of the first extracellular loop of the rat CRF₁ . Using the antiserum, CRF₁ -like immunoreactivity (CRF₁ -LI) was detected in the anterior lobe cells of the rat pituitary where some of them expressed intense signals. CRF₁ -LI also appeared in the intermediate lobe cells and on the fibre-like elements of the posterior lobe of the pituitary. Dual immunofluorescence labelling showed that corticotrophs exhibited the highest percentage of CRF₁ (male: 27.1 ± 3.0%, female: 18.0 ± 3.0%), followed by lactotrophs (male: 6.7 ± 3.0%, female: 12.1 ± 1.3%), gonadotrophs (male: 2.6 ± 1.0%, female: 7.5 ± 0.5%), thyrotrophs (male: 2.9 ± 0.1%, female: 5.3 ± 1.2%) and somatotrophs (male: 1.1 ± 0.3%, female: 1.2 ± 0.5%). The percentage of CRF₁ -LI-positive cells that were corticotrophs was significantly higher in male rats than in female rats, whereas CRF₁ -LI-positive lactotrophs and gonadotrophs were significantly higher in female rats than in male rats. Almost all of the melanotrophs were positive for CRF₁ in the intermediate lobe (98.9 ± 0.2%). CRF₁ -LI and the percentage of CRF₁ -LI in corticotrophs were decreased in the anterior pituitary, and the distribution patterns were altered from a diffuse to punctate one by adrenalectomy; the changes were restored by treatment with dexamethasone (100 μg/kg bw). These results suggest that CRF₁ is involved in the modulation of the functions of the pituitary; moreover, protein expression and the distribution patterns of CRF₁ are regulated by glucocorticoids in the rat anterior pituitary.

Pituitary CRH-binding protein and stress in female mice

The CRH-binding protein (CRH-BP) binds CRH with very high affinity and inhibits CRH-mediated ACTH release from anterior pituitary cells in vitro, suggesting that the CRH-BP functions as a negative regulator of CRH activity. Our previous studies have demonstrated sexually dimorphic expression of CRH-BP in the murine pituitary. Basal CRH-BP expression is higher in the female pituitary, where CRH-BP mRNA is detected in multiple anterior pituitary cell types. In this study, we examined stress-induced changes in CRH-BP mRNA and protein expression in mouse pituitary and assessed the in vivo role of CRH-BP in modulating the stress response. Pituitary CRH-BP mRNA was greater than 200-fold more abundant in females than males, and restraint stress increased pituitary CRH-BP mRNA by 11.8-fold in females and 3.2-fold in males as assessed by qRT-PCR. In females, restraint stress increased CRH-BP mRNA levels not only in POMC-expressing cells, but also in PRL-expressing cells. The increase in female pituitary CRH-BP mRNA following stress resulted in significant increases in CRH-BP protein 4-6h after a 30-minute restraint stress as detected by [(125)I]-CRH:CRH-BP cross-linking analyses. Based on this temporal profile, the physiological role of CRH-BP was assessed using a stressor of longer duration. In lipopolysaccharide (LPS) stress studies, female CRH-BP-deficient mice showed elevated levels of stress-induced corticosterone release as compared to wild-type littermates. These studies demonstrate a role for the pituitary CRH-BP in attenuating the HPA response to stress in female mice.

The CRH-R₁ receptor mediates luteinizing hormone, prolactin, corticosterone and progesterone secretion induced by restraint stress in estrogen-primed rats

Acute stress has been shown to modify hypothalamus-pituitary-gonadal (HPG) axis activity. Corticotropin-releasing hormone (CRH), the principal regulator of the hypothalamus-pituitary-adrenal (HPA) axis, has been implicated as a mediator of stress-induced effects on the reproductive axis. The role of the specific CRH receptor subtypes in this response is not completely understood. In the current study, we investigated the role of the CRH-R(1) receptor on luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin (PRL), progesterone (P) and corticosterone (CT) secretion in stress-induced responses under the influence of estrogen (E(2)). Estrogen-primed ovariectomized rats (estradiol cypionate, 10 μg sc) received an i.v. administration of antalarmin (0.1 or 1mg/kg), a selective CRH-R(1) antagonist, or vehicle before restraint stress for 40 min. Seven blood samples were collected from two experimental groups (one from 10:00 h to 14:00 h and the other from 10:00 h to 18:00 h). An increase of plasma LH induced by restraint acute-stress was followed by alteration of the secretion pattern in the estrogen-induced afternoon surge. In a similar manner, we observed a suppression of the afternoon surge in plasma FSH, a delay of E(2)-induced PRL secretion, and an increase in plasma P and CT. Antalarmin attenuated stress-induce LH increase, decreased CT and P secretion and blocked the stress effects on PRL secretion. These findings suggest that CRH-R(1) mediates, at least in part, the restraint stress effects on the HPA, PRL, and reproductive axes.

Novel expression of type 1 corticotropin-releasing hormone receptor in multiple endocrine cell types in the murine anterior pituitary

The CRH family of ligands signals via two distinct receptors, CRH-R1 and CRH-R2. Previous studies localized CRH-R1 and CRH-R2 to a subset of anterior pituitary corticotropes and gonadotropes, respectively. However, numerous studies have indicated that stress and CRH activity can alter the secretion of multiple anterior pituitary hormones, suggesting a broader expression of the CRH receptors in pituitary. To examine this hypothesis, the in vivo expression of CRH-R1 and CRH-R2 mRNA was further characterized in adult mouse pituitary. Quantitative RT-PCR analysis demonstrated that CRH-R1 mRNA is greater than 100-fold more abundant than CRH-R2 mRNA in male and female mouse pituitaries. Dual in situ hybridization analysis identified cell-specific CRH-R1 expression in the anterior pituitary. At least half of the CRH-R1-positive cells expressed proopiomelanocortin-mRNA (50% in females; 70% in males). In females, a significant percentage of the cells expressing CRH-R1 also expressed transcript for prolactin (40%), LHbeta (10%), or TSH (3%), all novel sites of CRH-R1 expression. Similarly in males, a percentage of CRH-R1-positive cells expressed prolactin (12%), LHbeta (13%), and TSH (5%). RT-PCR studies with immortalized murine anterior pituitary cell lines showed CRH-R1 and/or CRH-R2 expression in corticotropes (AtT-20 cells), gonadotropes (alphaT3-1 and LbetaT2 cells), and thyrotropes (alphaTSH cells). Whereas CRH-R1 expression in corticotropes is well established, the presence of CRH-R1 mRNA in a subset of lactotropes, gonadotropes, and thyrotropes establishes these cell types as novel sites of murine CRH-R1 expression and highlights the pituitary as an important site of interaction between the hypothalamus-pituitary-adrenal and multiple endocrine axes.

Stress-induced changes in sleep in rodents: models and mechanisms

Psychological stressors have a prominent effect on sleep in general, and rapid eye movement (REM) sleep in particular. Disruptions in sleep are a prominent feature, and potentially even the hallmark, of posttraumatic stress disorder (PTSD) (Ross, R.J., Ball, W.A., Sullivan, K., Caroff, S., 1989. Sleep disturbance as the hallmark of posttraumatic stress disorder. American Journal of Psychiatry 146, 697-707). Animal models are critical in understanding both the causes and potential treatments of psychiatric disorders. The current review describes a number of studies that have focused on the impact of stress on sleep in rodent models. The studies are also in Table 1, summarizing the effects of stress in 4-h blocks in both the light and dark phases. Although mild stress procedures have sometimes produced increases in REM sleep, more intense stressors appear to model the human condition by leading to disruptions in sleep, particularly REM sleep. We also discuss work conducted by our group and others looking at conditioning as a factor in the temporal extension of stress-related sleep disruptions. Finally, we attempt to describe the probable neural mechanisms of the sleep disruptions. A complete understanding of the neural correlates of stress-induced sleep alterations may lead to novel treatments for a variety of debilitating sleep disorders.

Restraint increases prolactin and REM sleep in C57BL/6J mice but not in BALB/cJ mice

Sleep is generally considered to be a recovery from prior wakefulness. The architecture of sleep not only depends on the duration of wakefulness but also on its quality in terms of specific experiences. In the present experiment, we studied the effects of restraint stress on sleep architecture and sleep electroencephalography (EEG) in different strains of mice (C57BL/6J and BALB/cJ). One objective was to determine if the rapid eye movement (REM) sleep-promoting effects of restraint stress previously reported for rats would also occur in mice. In addition, we examined whether the effects of restraint stress on sleep are different from effects of social defeat stress, which was found to have a non-REM (NREM) sleep-promoting effect. We further measured corticosterone and prolactin levels as possible mediators of restraint stress-induced changes in sleep. Adult male C57BL/6J and BALB/cJ mice were subjected to 1 h of restraint stress in the middle of the light phase. To control for possible effects of sleep loss per se, the animals were also kept awake for 1 h by gentle handling. Restraint stress resulted in a mild increase in NREM sleep compared with baseline, but, overall, this effect was not significantly different from sleep deprivation by gentle handling. In contrast, restraint stress caused a significant increase in REM sleep compared with handling in the C57BL/6J mice but not in BALB/cJ mice. Corticosterone levels were significantly and similarly elevated after restraint in both strains, but prolactin was increased only in the C57BL/6J mice. In conclusion, this study shows that the restraint stress-induced increase in REM sleep in mice is strongly strain dependent. The concomitant increases in prolactin and REM sleep in the C57BL/6J mice, but not in BALB/cJ mice, suggest prolactin may be involved in the mechanism underlying restraint stress-induced REM sleep. Furthermore, this study confirms that different stressors differentially affect NREM and REM sleep. Whereas restraint stress promotes REM sleep in C57BL/6J mice, we previously found that in the same strain, social defeat stress promotes NREM sleep. As such, studying the consequences of specific stressful stimuli may be an important tool to unravel both the mechanism and function of different sleep stages.

An ether stressor increases REM sleep in rats: possible role of prolactin

Sleep alterations after a 1-min exposure to ether vapor were studied in rats to determine if this stressor increases rapid eye-movement (REM) sleep as does an immobilization stressor. Ether exposure before light onset or dark onset was followed by significant increases in REM sleep starting approximately 3-4 h later and lasting for several hours. Non-REM (NREM) sleep and electroencephalographic slow-wave activity during NREM sleep were not altered. Exposure to ether vapor elicited prolactin (Prl) secretion. REM sleep was not promoted after ether exposure in hypophysectomized rats. If the hypophysectomy was partial and the rats secreted Prl after ether exposure, then increases in REM sleep were observed. Intracerebroventricular administration of an antiserum to Prl decreased spontaneous REM sleep and inhibited ether exposure-induced REM sleep. The results indicate that a brief exposure to ether vapor is followed by increases in REM sleep if the Prl response associated with stress is unimpaired. This suggests that Prl, which is a previously documented REM sleep-promoting hormone, may contribute to the stimulation of REM sleep after ether exposure.

The influence of postnatal handling on adult neuroendocrine and behavioural stress reactivity

Environmental stimuli during early stages of life can influence the development of an organism and may result in permanent changes in adult behaviour and physiology. In the present study we investigated the influence of early postnatal handling on adult neuroendocrine and behavioural stress reactivity in Wistar rats. Pups were subjected to handling from postnatal day 1-21. The young were taken from the nest every day for 15 min and each of the pups was handled separately. Control nests were left undisturbed. When the animals had reached an adult age of 3-4 months they were individually housed and subjected to a series of tests to measure their stress reactivity. In the first experiment we established adult behavioural coping with stressors and anxiety in the following series of tests: open field test, shock prod defensive burying test, elevated plus maze and conditioned fear test. Collectively, the data clearly indicate that handled animals are characterized by a lower stress-induced anxiety. Yet, handled and control animals do not differ in their general way of coping with stressors. Although the lower anxiety in handled animals is often reflected in a higher activity, they are not more active per se. In a second experiment, animals were provided with a permanent jugular vein canula for repeated blood sampling to determine stress hormones: noradrenaline, adrenaline, prolactin and corticosterone. Animals were subjected to a novelty test and a conditioned fear test. The neuroendocrine response profile is consistent with the conclusion that handled animals are less anxious than controls but are not different in their general strategy of coping with stressors. The handled animals showed an attenuated adrenaline, prolactin and corticosterone response. Yet, in neither of the two tests there was a difference in noradrenaline response, a typical marker for an active coping strategy. Interestingly, the differences in neuroendocrine reactivity already appeared in response to a mild novelty challenge when there were no clear behavioural differences yet. The neuroendocrine measures are in line with the behavioural data but more sensitively reflect the differences between handled and control animals.

Hyper-reactive hypothalamo-pituitary-adrenocortical axis in rats bred for high anxiety-related behaviour

Psychiatric patients suffering from anxiety disorders or endogenous depression exhibit increased activity in their hypothalamo-pituitary-adrenocortical (HPA) axis. Recently, two Wistar rat lines, bred for high (HAB) and low (LAB) anxiety-related behaviour on the elevated plus-maze, were described as a unique psychopathological animal model (1). The present study focused on the HPA axis reactivity of HAB and LAB animals to an emotional stressor. Thus, adult male HAB and LAB animals, fitted with jugular vein catheters 5 days prior to the experiment, were exposed to an open arm of the elevated plus-maze for 5 min. Whereas basal levels of ACTH and corticosterone were similar in both lines, HAB rats showed higher plasma concentrations at 5 and 15 min following stressor exposure (both hormones and both time points: P<0.01 vs LAB). Furthermore, increased basal (P<0.05 vs LAB) and stimulated (P<0.01 vs LAB) prolactin concentrations in HAB rats were found. In contrast to ACTH, corticosterone and prolactin, plasma oxytocin and vasopressin levels did not differ between HAB and LAB animals; oxytocin, but not vasopressin, responding to open arm exposure with a significant increase in both lines (P<0.05). In conclusion, particularly due to the association between inborn anxiety and HPA axis hyper-reactivity, the HAB rat represents a promising animal model for further investigation of the relationship between emotional disturbance and neuroendocrine activity.

Serum prolactin concentrations in the captive female African elephant (Loxodonta africana): potential effects of season and steroid hormone interactions

Research was conducted to determine whether seasonal changes in prolactin secretion occur in nonpregnant female African elephants and to examine potential functional interrelationships between secretion of prolactin, cortisol, and progesterone. Weekly blood samples were taken for 18 months from four female African elephants and the sera were analyzed by RIA for progesterone, cortisol, and prolactin concentrations. There was no significant effect of season on serum concentrations of prolactin. Estrous cycles averaged 14 weeks in length and were composed of a 9-week luteal phase and a 5-week follicular phase (based on progesterone concentrations consistently >200 and <200 pg/ml, respectively). Estrous cycle synchronicity was evident between pairs of elephants. Serum concentrations of prolactin (3.91 +/- 0.69 ng/ml; range: 0.84-15.8 ng/ml) were significantly lower during the luteal, compared with the follicular, phase (P < 0.0001; t test) and were positively correlated with serum concentrations of cortisol (r = 0.14; P < 0.05). Mean (+/-SE) serum concentration of cortisol was 5.7 +/- 1.3 ng/ml (range: 1.4-19.3 ng/ml), and concentrations of this adrenal steroid were negatively correlated with progesterone concentrations (r = -0.15; P < 0.01). Increased serum concentrations of prolactin detected during the follicular phase suggest that this hormone may be regulated by ovarian estrogens and may play a role in modulating ovarian function in the elephant.

Anterior pituitary response to stress: time-related changes and adaptation

A wide array of physical and psychological stressors alter the secretion of anterior pituitary hormones. However, both the qualitative and the quantitative features of the stressors as well as its duration markedly influence the final endocrine response. In addition, among all anterior pituitary hormones, only ACTH and prolactin levels appear to reflect the intensity of the stress experienced by the animals. Although physical stressors show a somewhat specific neuroendocrine profile, the response of the pituitary-adrenal (PA) and sympathomedulloadrenal axes are common to almost all stressors. After an initial stimulatory effect of stress, an inhibition of all anterior pituitary hormones, except ACTH, can be found provided the stressor is intense enough. The mechanisms responsible for this biphasic response to stress are likely to be located at sites above the pituitary. When the animals are repeatedly exposed to the same stressor, some behavioural and physiological consequences of stress exposure are reduced, suggesting that the animals become adapted to the stimulus. This process has been also termed habituation. Among all the pituitary hormones, only ACTH and prolactin levels are reduced as a consequence of repeated exposure to the same (homotypic) stressor, although some negative results have been reported. However, it has been recently reported that subtle changes in the characteristics of the stressors or in their regularity can greatly influence adaptation, and these factors might explain failure to find adaptation of ACTH and prolactin in some works. Habituation of ACTH and prolactin, when observed, appears to be specific for the chronically applied stressor so that the potentiality of the PA axis and prolactin to respond to a novel (heterotypic) stressor can be preserved. In the case of the PA axis, an intact or potentiated response to a novel stressor is observed in spite of presumably negative feedback exerted by daily stress-induced glucocorticoid release and the high resting levels of glucocorticoids. This phenomenon has been termed as facilitation and can be unmasked alternating stress. Although with the exception of the PA axis, developmental aspects of anterior pituitary response to stress have been poorly studied, available data suggest that dramatic changes occur in some hormones during weaning, with some, but less profound, change thereafter. Responsiveness to stressors appears to mature with age, but developmental patterns differ among the various anterior pituitary hormones.

Effects of postnatal handling of rats on emotional, HPA-axis, and prolactin reactivity to novelty and conflict

The present studies evaluated whether or not postnatal handling (PH) (administered during the first 21 days of life) could enduringly improve coping behavior with novel and/or conflict situations. To this purpose, different groups of naive male rats (control and PH-treated) were submitted in separate experiments to 1 of the 3 following situations: an emotional reactivity test (in 4-month-old animals), an open-field session followed by endocrine measurements (in 7-month-old animals) and a punished drinking test (in 11-month-old animals). PH effects were significant in the 3 situations: handled animals were less resistant to capture or to the handling manouvers involved in the emotional reactivity test: the hormonal responses (corticosterone, prolactin, and ACTH changes) during and after an open-field stress were less intense, and PH effects lasted up to 11 months in the punished drinking test, as measured by a higher number of punished responses and less time spent freezing by handled animals during the punished period. The results are discussed in relation to previous evidence showing a long-lasting reduction of fearfulness in rats due to postnatal handling.