Differential responses of the hypothalamo-pituitary-adrenocortical axis to acute restraint stress in Hatano high- and low-avoidance rats

Differential Hypothalamic-pituitary-adrenal Response to Stress among Rat Strains: Methodological Considerations and Relevance for Neuropsychiatric Research

The hormones of the hypothalamic-pituitary-adrenal (HPA) axis, particularly glucocorticoids (GCs), play a critical role in the behavioral and physiological consequences of exposure to stress. For this reason, numerous studies have described differences in HPA function between different rodent strains/lines obtained by genetic selection of certain characteristics not directly related to the HPA axis. These studies have demonstrated a complex and poorly understood relationship between HPA function and certain relevant behavioral characteristics. The present review first remarks important methodological considerations regarding the evaluation and interpretation of resting and stress levels of HPA hormones. Then, it presents works in which differences in HPA function between Lewis and Fischer rats were explored as a model for how to approach other strain comparisons. After that, differences in the HPA axis between classical strain pairs (e.g. High and Low anxiety rats, Roman high- and low-avoidance, Wistar Kyoto versus Spontaneously Hypertensive or other strains, Flinder Sensitive and Flinder Resistant lines) are described. Finally, after discussing the relationship between HPA differences and relevant behavioral traits (anxiety-like and depression-like behavior and coping style), an example for main methodological and interpretative concerns and how to test strain differences is offered.

Influence of exercise and emotional stresses on secretion of prolactin and growth hormone in Thoroughbred horses

The secretion of prolactin and growth hormone in response to exercise and emotional stresses was investigated in Thoroughbreds. Two experiments were performed: one with loading of only exercise stress and one with simultaneous loading of exercise and emotional stresses. Exercise stress was loaded in 4 steps using a treadmill for horses: pre-exercise period (5 min), walking period (6.5 min), galloping period (3 min), and cooling down period (10 min). Emotional stress was loaded by showing a loud video of an audience at a racetrack during the walking period. The results clearly demonstrated that exercise stress rapidly increased the secretion of prolactin and growth hormone and that secretion of them persisted for a specific period after the exercise. In addition, emotional stress promoted prolactin secretion.

Stress-Induced Enhanced Long-Term Potentiation and Reduced Threshold for N-Methyl-D-Aspartate Receptor- and β-Adrenergic Receptor-Mediated Synaptic Plasticity in Rodent Ventral Subiculum

Stress is a biologically relevant signal and can modulate hippocampal synaptic plasticity. The subiculum is the major output station of the hippocampus and serves as a critical hub in the stress response network. However, stress-associated synaptic plasticity in the ventral subiculum has not been adequately addressed. Therefore, we investigated the impact of a single exposure to an inherently stressful two-way active avoidance conditioning on the induction of long-term potentiation (LTP) at CA1-subiculum synapses in ventral hippocampal slices from young adult rats 1 day after stressor exposure. We found that acute stress enhanced LTP and lowered the induction threshold for a late-onset LTP at excitatory CA1 to subicular burst-spiking neuron synapses. This late-onset LTP was dependent on the activation of β-adrenergic and glutamatergic N-methyl-D-aspartate receptors and independent of D1/D5 dopamine receptor activation. Thereby, we present a cellular mechanism that might contribute to behavioral stress adaptation after acute stressor exposure.

Maternal traits during lactation period reduce the anxiety-related behavior in male offspring: Results from a fostering study in Hatano rats

The Hatano strains of the Sprague Dawley rats have been selectively bred to create high- (HAA) and low- (LAA) active avoidance variants. We previously reported that HAA rats display more anxiety-related behavior than LAA rats, but whether this strain difference is affected by postnatal environmental factors remains unclear. In this study, we performed in- and cross-fostering between the HAA and LAA strains and investigated the effect of postnatal maternal traits on the emotional responses in each strain of the male offspring. We evaluated the effect of the fostering treatment on the emotional responses of the male offspring using the elevated plus maze test. The male LAA offspring reared by HAA dam showed higher anxiety-related behavior than those reared by LAA dam. Next, we quantified and typed various maternal behavior under the in- and cross-fostering conditions during the lactation period using a snapshot sampling method. This method allowed us to evaluate potential maternal traits that may influence the emotional responses of the offspring observed in our first experiment. We found that HAA dams showed long-term resting without offspring and offspring arrangement compared with LAA dams. These findings suggest that postnatal environmental factors may alter anxiety-related behavior in the male LAA offspring and that less direct contact with their offspring during the lactation period may induce anxiety-related behavior in male offspring.

Phoenixin influences the excitability of nucleus of the solitary tract neurones, effects which are modified by environmental and glucocorticoid stress

Phoenixin (PNX) is a neuropeptide shown to play roles in the control of reproduction. The nucleus of the solitary tract (NTS), a critical autonomic integrating centre in the hindbrain, is one of many areas with dense expression of PNX. Using coronal NTS slices obtained from male Sprague-Dawley rats, the present study characterised the effects of PNX on both spike frequency and membrane potential of NTS neurones. Extracellular recordings demonstrated that bath-applied 10 nmol L^-1 PNX increased the firing frequency in 32% of NTS neurones, effects which were confirmed with patch-clamp recordings showing that 50% of NTS neurones tested depolarised in response to application of the peptide. Surprisingly, the responsiveness to PNX in NTS neurones then declined suddenly to 9% (P < 0.001). This effect was subsequently attributed to stress associated with construction in our animal care facility because PNX responsiveness was again observed in slices from rats delivered and maintained in a construction-free facility. We then examined whether this loss of PNX responsiveness could be replicated in rats placed on a chronic stress regimen involving ongoing corticosterone (CORT) treatment in the construction-free facility. Slices from animals treated in this way showed a similar lack of neuronal responsiveness to PNX (9.1 ± 3.9%) within 2 weeks of CORT treatment. These effects were specific to PNX responsiveness because CORT treatment had no effect on the responsiveness of NTS neurones to angiotensin II. These results are the first to implicate PNX with respect to directly controlling the excitability of NTS neurones and also provide intriguing data showing the plasticity of these effects associated with environmental and glucocorticoid stress levels of the animal.

Hatano rats are a suitable metabolic syndrome model for studying feeding behavior, blood pressure levels, and percent body fat

Currently, metabolic syndrome is a worldwide concern. Thus, it is imperative to understand the mechanism of metabolic syndrome by establishing various metabolic syndrome models. In this study, we used Hatano high-avoidance animals (HAA) and low-avoidance animals (LAA), both derived from Sprague–Dawley rats by selective breeding to determine high- or low-avoidance rates in shuttle-box active avoidance tests. HAA and LAA rats have some strain differences related to eating and appetite. Therefore, we determined whether Hatano rats could be used as a metabolic syndrome model. We compared food intake, body weights, blood pressure levels, plasma component levels, and fat contents between HAA and LAA rats. The HAA rats showed more active eating, higher blood pressure, higher percentage fat, and higher triglyceride levels than the LAA rats—these features correspond to some of the risk factors associated with metabolic syndrome. Our study suggests that HAA rats can be considered as a metabolic syndrome model by focusing on their feeding behavior, blood pressure levels, and percent body fat.

Effects of pregnancy experience on ovarian senescence and longevity in Hatano rats bred for high- and low-avoidance learning

We investigated the effects of pregnancy experience on ovarian senescence and longevity using two inbred strains of Hatano rats. These strains have been selectively bred for high- and low-avoidance animals (HAA and LAA, respectively), but the HAA line has a slower onset of ovarian senescence and a shorter lifespan compared with the LAA line. The onset of abnormal estrous cycles and survival curves were compared between nulliparous and parous rats in each line. In the HAA line, pregnancy experience did not change the onset of ovarian senescence but increased longevity. This suggests that a pituitary tumor, which is a causal factor for accelerated mortality in this line, developed slowly in parous rats. In the LAA line, pregnancy experience delayed the onset of ovarian senescence and reduced the incidence of mammary tumors but did not increase longevity because of an increased frequency of constipation with megacolon. These data suggest that the effects of pregnancy experience on ovarian senescence and longevity depend on the reproductive characteristics of the rat strains.

Hatano rats selectively bred for high- and low-avoidance learning: an overview

This review compiles the results of a series of studies on Hatano high- and low-avoidance animals (HAA and LAA, respectively) established at the Hatano Research Institute, Food and Drug Safety Center, Japan. The HAA and LAA lines were selected and bred from Sprague Dawley rats for high and low avoidance learning, respectively, in a shuttlebox task since 1985. Although Hatano rats were selected only based on their behavioral traits in the active avoidance task, strain differences between the HAA and LAA lines were also observed in their stress responses and reproductive functions. However, the most noticeable finding of Hatano rats is a matched result in both active and passive avoidance tasks. The HAA and LAA lines are useful for next-generation toxicological studies, because the hereditary characters of behaviors or endocrine functions are well controlled.

Male Hatano low-avoidance rats show more active sexual behavior with lower plasma testosterone than high-avoidance rats

Two inbred strains of Sprague-Dawley rats, known as the Hatano high- and low-avoidance animals (HAA and LAA respectively), have been selectively bred for high versus low rates of avoidance responses in a shuttle-box avoidance task. To investigate differences in the sexual behavior of Hatano rats, male HAA, LAA and SD rats were tested from 12 to 15 weeks of age. LAA rats exhibited more rapid and frequent sexual behavior than HAA or SD rats, and such differences increased with repeated sexual experience. Plasma testosterone levels tended to be lower in LAA rats than in HAA or SD rats, suggesting that active sexual behavior in LAA rats is not related to these levels. Strain differences in mating behavior between HAA and LAA rats may be caused by emotional responses to novelty.

Genetic variation in low-dose effects of neonatal DES exposure in female rats

To confirm genetic variation in low-dose effects of diethylstilbestrol (DES), two inbred strains of rats, which have been selectively bred for high- and low-avoidance learning (HAA and LAA, respectively), were used in this study. LAA rats characteristically show later sexual maturation, earlier reproductive senescence, and lower body weight as compared to HAA rats. Female neonates of each strain were daily administered DES by oral gavage at doses of 0 (vehicle only), 0.05 and 0.5μg/kg for the first 5days after birth. As a result, early onset of abnormal estrous cycles was observed during the same period in HAA and LAA rats treated with 0.5μg/kg. However, accelerated puberty and excessive body weight gains were observed only in LAA rats treated with 0.05 and 0.5μg/kg. These results suggest that the effects of neonatal DES exposure vary with the genetic background of the female rats used.

Stress-reactive rats (high-avoidance female rats) have a shorter lifespan than stress-nonreactive rats (low-avoidance female rats)

Although Hatano high-avoidance and low-avoidance rats (HAA and LAA, respectively) have been selectively bred for good versus poor avoidance learning, HAA rats are known to be more reactive to stress than LAA rats. In this study, HAA and LAA female rats were compared during reproductive aging by observing estrous cycles from 8 to 11 months of age. Furthermore, these rats were allowed to live out their natural lifespans, that is, until 24 months of age, in order to compare their survival and to clarify the relationship between reproductive aging and tumor development. At eight months of age, 2 of 35 HAA rats and 20 of 35 LAA rats had abnormal estrous cycles. The median lifespan of the HAA rats (673 days) was shorter than that of the LAA rats (733 days). The incidence of pituitary neoplasia was higher in the HAA rats than in the LAA rats. These results suggest that HAA female rats (i.e., stress-reactive rats) have a shorter lifespan than LAA female rats (i.e., stress-nonreactive rats) and develop pituitary neoplasia, which was one of the causal factors in their accelerated mortality. However, the onset of an age-matched abnormal cycle did not correspond with their lifespan.

Effects of an extended photoperiod on gonadal function and condition of hair coats in Thoroughbred colts and fillies

The effects of an extended photoperiod (EP) in Thoroughbreds colts and fillies from winter at one year old to spring at two years old on the gonadal functions, coat condition, and endocrine changes were investigated. Sixty-two Thoroughbreds (31 colts and 31 fillies) reared in the Hidaka Training and Research Center (Hidaka), Japan Racing Association were used. Thirty of them (15 colts and 15 fillies) were reared under EP conditions from December 20 to April 10, and the remaining 32 horses were reared under natural light alone as a control group. For EP, a 100-watt white bulb was set near the ceilings of stalls, and lighting conditions of 14.5-hr light and 9.5-hr dark periods were established. Blood was collected from the jugular vein once a month from October at one year old to February at two years old in both colts and fillies, and then twice a month in colts and weekly in fillies after March, and the coat condition was evaluated in January and April in 56 horses. To investigate endocrine changes, the plasma concentrations of prolactin, luteinizing hormone (LH), follicle-stimulating hormone (FSH), immunoreactive (ir-) inhibin, testosterone, estradiol-17β and progesterone were measured. No significant difference was noted in the coat condition between the two groups in January, but they changed from winter to summer coats (molting of winter coats) in April in the EP group compared with the control group. Regarding endocrine changes, the plasma concentrations of prolactin, FSH, ir-inhibin and testosterone were significantly higher in the EP colts than in the control group from January to April. The plasma concentrations of LH tended to rise in the EP colts from January to April compared with the control group. In the EP fillies, the plasma concentrations of prolactin, LH, ir-inhibin, estradiol-17β and progesterone were significantly higher during January and April, but a significantly high level of FSH was noted in the control than EP group in January. The ovulation day was advanced in the EP fillies compared with the control group. The present study clearly demonstrated that EP treatment during rearing advanced the molting of winter coats in both colts and fillies. These results suggested to be due to the action of prolactin being increased by EP treatment. In addition, EP treatment stimulated the hypothalamus-pituitary-gonadal axis even in yearlings, and advanced ovulation in fillies. Since EP treatment-induced changes in the yearlings were within the physiological range, and the method is safe and simple, EP treatment may be an effective technique in horse husbandry.

Male Hatano high-avoidance rats show high avoidance and high anxiety-like behaviors as compared with male low-avoidance rats

Our prime objective was to establish an optimal model animal for studying avoidance learning and memory in rodents. The two-way rat inbred strains of Hatano high- (HAA) and low-avoidance (LAA) animals were originally selected and bred in accordance with their high or low performance respectively in the shuttle-box active avoidance task. Previous studies demonstrated that they have clear strain differences in endocrine stress response, which is related to acquisition of aversive learning and emotional reactivity. To evaluate the effect of selection by the shuttle-box task on avoidance performance and emotional reactivity, male Hatano rats underwent passive avoidance, open field and elevated plus maze tests. The present results show that the avoidance performance in the passive task was significantly greater in HAA rats than in LAA rats. Furthermore, HAA rats showed high anxiety-like behaviors compared with LAA rats in open field and elevated plus maze tests. Taken together, this study demonstrated that 1) selection and breeding of Hatano HAA and LAA strain rats by shuttle-box task had been properly carried out with the criterion of high and low avoidance performance respectively and that 2) HAA rats were predisposed to high anxiety compared with LAA rats. These results indicated that Hatano HAA and LAA rats can be useful models for studying avoidance learning and memory.

Lactation deficit in OFA hr/hr rats may be caused by differential sensitivity to stress compared with Wistar and Sprague Dawley rats

OFA hr/hr (OFA) rats present a major lactation deficit that impairs offspring survival. To explore whether abnormal stress responsiveness causes this deficit, we compared their hormonal (prolactin, progesterone, and corticosterone) responses to stress (room change and 2-min ether exposure) with those of Wistar and Sprague Dawley (SD) rats. We tested responses during the estrous cycle, pregnancy, lactation, after ovariectomy, and ovarian steroid hormone priming, and responses to suckling. We evaluated hypothalamic expression of receptors for prolactin (PRLRlong) and the isoforms of receptors for progesterone (PRA and B) and estrogen (ERα and β) in late pregnancy. We tested whether administration of an anxiolytic (diazepam) improved lactation. Ether exposure increased circulating levels of the three hormones in the three strains of rats, cycling and ovariectomized, but was less effective in pregnancy and lactation. Elevated estrogen level (estrus and estradiol-treated ovariectomized rats) potentiated the prolactin response more in SD and OFA rats than in Wistar rats. Elevated progesterone level (late pregnancy, lactation, progesterone-treated ovariectomized rats) inhibited the prolactin response less in OFA than in SD or Wistar rats. Ether exposure inhibited the prolactin and oxytocin responses to suckling only in OFA rats. Diazepam treatment increased pup survival rate and the prolactin response to suckling. Hypothalamic total PR mRNA content, assayed by RT-PCR, was higher in pregnant OFA rats compared with SD and Wistar rats, but the PRB/PRA protein ratio determined by Western blot was lowest in Wistar rats, intermediate in OFA rats, and highest in SD rats. The heightened sensitivity to stress of lactating OFA rats may contribute to their lactational deficit and be caused by a combination of hypoprolactinemia and reduced inhibitory capacity of progesterone.

Endocrinological differences between Hatano high- and low-avoidance rats during early two-way avoidance acquisition

Hatano high (HAA)- and low (LAA)-avoidance rats were selected from Sprague-Dawley rats genetically on the basis of their active avoidance behavior in a shuttle-box test. The purpose of this study was to investigate stress-related alterations of hormones corticotropin-releasing hormone (CRH), arginine-vasopressin (AVP), prolactin, and adrenocorticotropin (ACTH) in the brain and blood during early avoidance acquisition using two lines of Hatano rats. In paraventricular nucleus (PVN) of the hypothalamus, the CRH levels in HAA rats were significantly increased after shuttle-box tasks compared with before the tasks, whereas the CRH levels in LAA rats significantly decreased after shuttle-box tasks compared with before the tasks. In the HAA rats, the CRH and AVP levels in the median eminence decreased after shuttle-box tasks, whereas there were no significant differences in the levels between before and after shuttle-box tasks in LAA rats. The plasma concentrations of ACTH were significantly higher in HAA rats than in LAA rats after shuttle-box tasks. These results show that the response of CRH-ACTH was higher in HAA rats than in LAA rats. This phenotype may be an important reason for the high avoidance rates of shuttle-box tasks in HAA rats. These endocrine differences in early avoidance acquisition may be involved in regulation of their avoidance responses in the shuttle-box task.

The secretory pattern and source of immunoreactive prolactin in pregnant African (Loxodonta africana) and Asian (Elephas maximus) elephants

The objective of the present study was to define the secretion of prolactin (PRL) in pregnant African and Asian elephants. Levels of immunoreactive (ir-) PRL in serum and placental homogenates were measured by a heterologous radioimmunoassay (RIA) based on an ovine and human RIA system, and the localization of ir-PRL in the placenta was detected by immunohistochemistry using anti-human PRL. Circulating ir-PRL clearly showed a biphasic pattern during pregnancy in African and Asian elephants. Serum levels of ir-PRL started to increase from the 4 - 6th month of gestation and reached the first peak level around the 11-14th month. A second peak of circulating ir-PRL levels was observed around the 18-20th month of gestation followed by an abrupt decline after parturition. In contrast, in a case of abortion of an African elephant, the second peak of ir-PRL was not observed, and the levels remained low for about four months until parturition. The weight of the fetus delivered at the 17th month of gestation was 23.5 kg, which was quite small compared with normal fetuses in previous reports. Ir-PRL was detected in placental homogenates, and immunolocalization was observed in trophoblasts in both the African and Asian elephants, indicating that the placenta is the source of ir-PRL during pregnancy in elephants. The present results clearly demonstrated that circulating ir-PRL shows a biphasic pattern during normal pregnancy and that the placenta appears to be an important source of circulating ir-PRL during pregnancy in both African and Asian elephants.

Prolactin induces phosphorylation of the STAT5 in adrenal glands of Hatano rats during stress

AIMS

To investigate the signaling of prolactin (PRL) in the adrenal gland during stress in Hatano high- (HAA) and low-avoidance (LAA) rats.

MAIN METHODS

Adrenal glands of both strains were collected at 0, 15 and 30 min after stress. The protein levels of phosphorylated STAT5 and the mRNA levels of melanocortin receptor 2 (MC2R) and PRL receptor (PRLR) were analyzed. Furthermore, the effects of bromocriptine-induced hypoprolactinemia on adrenocortical responses to stress were investigated.

KEY FINDINGS

Adrenocorticotropic hormone (ACTH) concentrations in HAA were greater than LAA, while the difference in PRL concentrations were found only at 120 min after stress induction. No strain differences were observed in corticosterone or progesterone in response to stress. The stress-induced increase in MC2R mRNA expression was higher in HAA, but there was a lowered PRLR mRNA expression. STAT5 become highly phosphorylated in response to stress in both strains, but bromocriptine led to a reduction the STAT5 phosphorylation. Exposure to bromocriptine was associated with a reduction in plasma PRL in response to stress in both strains, while the ACTH levels were not altered. However, the decrease in corticosterone and progesterone in response to stress was observed only in bromocriptine-treated LAA rats.

SIGNIFICANCE

These data show that PRL plays an important role in the regulation of corticosterone and progesterone release in LAA but not in HAA during stress. These results suggest that PRL increase in response to stress, and it acts on the adrenal cortex and thereby plays an important physiologic role in protecting against acute stress.

Differences in adrenocortical secretory and gene expression responses to stimulation in vitro by ACTH or prolactin between high- and low-avoidance Hatano rats

Rats of the Hatano high-avoidance (HAA) and low-avoidance (LAA) strains have been genetically selected on the basis of their two-way active avoidance behavior, and have different endocrine responses to stress. The present study focused on the adrenal steroid hormone responses of the Hatano strains and identifies differences in regulation of the adrenal cortex in vitro of HAA and LAA rats. Although incubation with prolactin (PRL) and/or adrenocorticotrophic hormone (ACTH) resulted in a dose-dependent increase of corticosterone and progesterone release by adrenal cells from both HAA and LAA male rats, the responses were markedly increased for adrenal cells from LAA rats as compared with HAA rats. This finding suggested that adrenal glands of HAA rats are less sensitive to PRL and/or ACTH than adrenals from LAA rats. Several possible intra-adrenal regulators were investigated. The basal level of expression of steroidogenic acute regulatory protein (StAR) and the long form of the PRL receptor (PRLR-L) mRNAs was higher in adrenals of LAA rats. ACTH treatment of adrenal cells from HAA rats resulted in statistically significant increases in melanocortin receptor 2 (MC2R) mRNA expression, while neither ACTH nor PRL altered MC2R mRNA expression in adrenal cells of LAA rats. Conversely, the increase in PRLR-L mRNA expression induced by PRL was observed only in adrenal cells from LAA rats. Treatment of adrenal cells with PRL and/or ACTH increased the expression of StAR and CYP11A1 mRNAs for both Hatano strains. However, the induction of StAR mRNA expression was higher in LAA rats, but the CYP11A1 response was lower. These findings indicate that adrenal cells of the LAA strain have higher sensitivity to secretagogues than those of the HAA strain. These results suggest that PRL may also be important in stimulating secretion of adrenal steroid hormones.

Exposure to stressors during juvenility disrupts development-related alterations in the PSA-NCAM to NCAM expression ratio: potential relevance for mood and anxiety disorders

Childhood trauma is associated with higher rates of both mood and anxiety disorders in adulthood. The exposure of rats to stressors during juvenility has comparable effects, and was suggested as a model of induced predisposition for these disorders. The neural cell adhesion molecule (NCAM) and its polysialylated form PSA-NCAM are critically involved in neural development, activity-dependent synaptic plasticity, and learning processes. We examined the effects of exposure to stressors during juvenility on coping with stressors in adulthood and on NCAM and PSA-NCAM expression within the rat limbic system both soon after the exposure and in adulthood. Exposure to stressors during juvenility reduced novel-setting exploration and impaired two-way shuttle avoidance learning in adulthood. Among naive rats, a development-related decrease of about 50% was evident in the PSA-NCAM to NCAM expression ratio in the basolateral amygdala, in the CA1 and dentate gyrus regions of the hippocampus, and in the entorhinal cortex. In juvenile-stressed rats, we found no such decrease, but rather an increase in the polysialylation of NCAM ( approximately 50%), evident soon after the exposure to juvenile stress and also in adulthood. Our results suggest that exposure to stressors during juvenility alters the maturation of the limbic system, and potentially underlies the predisposition to exhibit stress-related symptoms in adulthood.

Physiological roles of prolactin in the adrenocortical response to acute restraint stress

The present study characterized the different hormonal responses to stress in the endocrine milieu with different circulating levels of prolactin (PRL) and examined the direct effects of PRL on adrenal steroidogenic responses to adrenocorticotropic hormone (ACTH) using experimentally induced hyperprolactinemia and hypoprolactinemia male rat models. Hyperprolactinemia was induced by transplantation of two adult female rat anterior pituitary glands under the kidney capsule for 2 weeks, and hypoprolactinemia was induced by daily subcutaneous injection of 2-Bromo-alpha-Ergocryptine (CB-154) for 2 weeks. Under stress conditions, the peak levels of ACTH were significantly higher in hypoprolactinemia than normal rats. Meanwhile, the peak levels of corticosterone and progesterone were significantly higher in hyperprolactinemia than in normal and hypoprolactinemia stressed rats. Results of in vitro experiments showed that adrenocortical cells in hyperprolactinemia exhibited higher basal levels of corticosterone and progesterone rats than normal and hypoprolactinemia rats. The stimulatory effect of ACTH on corticosterone and progesterone release was higher in hyperprolactinemia than hypoprolactinemia rats. In addition, PRL increased the stimulatory effect of ACTH-induced corticosterone secretion in all rat models. These results suggest that hypoprolactinemia and hyperprolactinemia rats exhibit marked differences in the response of their hypothalamic-pituitary-adrenal (HPA) axis during acute restrain stress. Additionally, these studies emphasize that the adrenal cortex might be more sensitive to ACTH stimulation in endocrine milieu with high levels of PRL resulting in high corticosterone and progesterone release.

Inhibition of stress-induced adrenocorticotropin and prolactin secretion mediating hypophysiotropic factors by antagonist of AMPA type glutamate receptor

Glutamate is the dominant excitatory neurotransmitter in a large number of physiological processes including neuroendocrine regulation. Some pharmacological studies have shown that different subtypes of glutamate receptor, such as the N-methyl-D-aspartic acid (NMDA) and alpha-amino-3-hydroxy-5-methy-4-isoxazolepropionic acid (AMPA) receptors, are involved in stress-induced adrenocorticotropin (ACTH) and prolactin secretion. However, the roles of the respective glutamate receptors and the mechanism of ACTH and prolactin secretion during stress via these receptors have not been investigated in detail. In the present study, we evaluated the role of AMPA-type glutamate receptor in ACTH and prolactin regulation under restraint stress in adult male rats. Male rats pretreated with a selective AMPA receptor antagonist, 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX; 50 microg), through a lateral ventricle cannula were stressed by immobilization. Administration of NBQX inhibited ACTH and prolactin secretion in response to restraint stress. However, NBQX had no significant effects on the activity of tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine biosynthesis, as measured by the accumulation of 3, 4-dihydroxyphenylalanine (DOPA). In addition, administration of NBQX suppressed stress-induced prolactin secretion in the male rats pretreated with alpha-MT, an inhibitor of dopamine synthesis, and infused with dopamine solution (2.5 microg/200 microl/10 min). These results indicated that the effects of NBQX on prolactin secretion might be mediated by non-dopamine mechanisms. The contents of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) in the median eminence (ME) of the male rats decreased during restraint stress; however, the fluctuations in CRH and AVP were eliminated by NBQX administration. These results suggest that stress-induced ACTH and prolactin release mediated by neurotransmission via AMPA receptors might be partly attributable to hypophysiotropic regulatory factors in the hypothalamus.

Direct Effects of Prolactin on Adrenal Steroid Release in Male Hatano High-Avoidance (HAA) Rats May be Mediated Through Janus Kinase 2 (Jak2) Activity

Prolactin (PRL) has been proposed to directly stimulate corticosterone release. However, the role of PRL on adrenocortical function in male HAA rats has not been fully clarified. The aim of this study was to investigate the effect of PRL on the secretion of corticosterone and progesterone using an in vitro cell culture system in male rats. Administration of PRL (10(-7) and 10(-6) M) resulted in dose-dependent increases in corticosterone and progesterone release. Cotreatment with PRL produced an increase in the stimulatory effects of ACTH-induced corticosterone and progesterone secretion. However, the PRL-induced corticosterone and progesterone releases were significantly reduced by treatment with AG490, a specific Janus kinase 2 (Jak2) inhibitor. In addition, administration of AG490 blunted the significant inhibition of ACTH-induced corticosterone and progesterone secretion by PRL. These results demonstrated that PRL could act directly on the adrenal gland to drive corticosterone and progesterone secretion in male rats. Additionally, the results emphasize that PRL stimulation of adrenal steroid release may be mediated through Jak2 activity.

Gastric ulceration and expression of prolactin receptor in the brain in Hatano high- and low-avoidance rats

Recently, prolactin was shown to inhibit the development of stress-induced ulcers. However, the mechanism for suppression of gastric ulcers by prolactin has not been clarified. Hatano high-avoidance (HAA) and low-avoidance (LAA) strains of rats were originally selected and bred from Sprague-Dawley rats based on shuttle-box tasks. The present study focused on the relationships among gastric ulceration and endocrine response with special reference to prolactin secretion and restraint stress in water of HAA and LAA rats. The restraint stress induced an elevation of plasma concentrations of ACTH, corticosterone, and prolactin. Peak levels of plasma ACTH during stressful condition were significantly higher in HAA rats than in LAA rats, while peak levels of prolactin were significantly lower in HAA rats than in LAA rats. The gastric erosion index was significantly higher in HAA rats than in LAA rats 7 h after restraint stress in water. The numbers of prolactin- receptor-positive cells determined by immunohistochemistry in the paraventricular nucleus was significantly increased in LAA rats than in HAA rats 7 h after restraint stress in water. These results indicate that HAA rats were more sensitive than LAA rats to restraint stress in water. The strain differences in gastric ulceration under stress may be involved in peripheral prolactin secretion and central prolactin receptor expression. The expression of prolactin receptor in the paraventricular nucleus may be important in suppressing gastric ulceration.

Immunological characteristics of Hatano high-and low-avoidance rats

Hatano high- and low-avoidance (HAA and LAA) rats have been genetically selected on the basis of their two-way active avoidance behavior, and have been shown to differ in other behavioral and hormonal parameters. Since close interconnections among the nervous, endocrine and immune systems have been well documented, these two strains might possess differences in aspects of immunological action. In Experiment 1, plasma levels of IgG, IgM, complement 3 (C3), classical pathway hemolytic complement (CH50) and beta(2)-microglobulin were compared between males of the two strains at 5 and 24 weeks of age. Plasma levels of IgG and CH50 were lower in LAA than HAA rats at 5 weeks of age, whereas those differences disappeared at 24 weeks of age. There were no differences between the two strains in plasma levels of IgM, C3 and beta(2)-microglobulin. In Experiment 2, antibody production to sheep red blood cells (SRBC) and mitogen-induced lymphocyte proliferation were compared between 12-week-old males of the two strains. Antibody responses in the PFC assay, plasma anti-SRBC-IgM levels and spleen weights were higher in LAA than HAA rats. LPS-induced lymphocyte proliferation was greater in LAA than HAA rats. It was concluded that HAA rats show earlier development of immunological development, but that antibody production and mitotic response of B lymphocytes may be more pronounced in adult LAA than HAA rats. The strain differences observed in the immunological response may indicate the usefulness of using Hatano rats in studies of behavioral-immunological relationships.