Adrenocortical response to stress in fasted and unfasted artificially reared 12-day-old rat pups

Recent studies have compared artificially reared (AR) rats with dam-reared rats on behavioral outcomes but, despite the fact that they are deprived of their mothers during the stress hyporesponsive period (SHRP), little is known about the effects of AR on the development of the stress response. In this study, the corticosterone (CORT) response to a stressor (saline injection ip) on postnatal Day 12 was assessed in rat pups that had been either dam-reared (DR) or artificially reared since Day 5. In the preceding 24 hr, half the pups in the DR group were maternally deprived (DEP). To control for the food deprivation consequent to maternal deprivation in these groups, half the pups in the AR groups also underwent 24-hr food deprivation (DEP). In the nondeprived condition AR pups did not differ from DR pups on untreated CORT levels or on levels at 1-hr poststress (i.e., all rats demonstrated low levels of CORT characteristic of the SHRP). In contrast, both maternally deprived DR pups and food-deprived AR pups exhibited increased untreated CORT levels as well as a significant increase at 30-min poststress, but CORT elevations were lower in the AR groups than in the DR groups. Thus, long-term maternal deprivation through artificial rearing in rats does not affect the reduced CORT levels and reduced CORT responsiveness associated with the SHRP; however, if animals are food deprived, then all show increased basal CORT levels and a greater CORT response to stress, although this response is lower in AR groups than in DR groups. These results suggest that rat pups artificially reared with adequate nutrition will still exhibit the SHRP.     

Modulation of attentional inhibition by norepinephrine and cortisol after psychological stress.

Two of the most salient physiological responses to stress are increased norepinephrine (NE) and cortisol (CORT) activities. However, it is unclear how these neurochemical events affect cognition, especially attention. We examined the effects of mild psychological stress on selective attention, as assessed by the negative priming (NP) paradigm. Salivary measures of the stress hormone CORT and alpha-amylase (a correlate of NE) were assayed to probe the relationship between the stress response and attentional inhibition. Healthy subjects (N = 20) engaged in the attention task, which was then followed by 15 min of a stressful video game before a return to the attentional task. Baseline saliva samples were obtained before the experiment began, 1 min after the video-game stressor, and 20 min post-stress. Subjects showed a significant reduction in NP and a decrease in reaction time (RT) after the video game. Moreover, alpha-amylase levels increased significantly after the stressor, indicating the role of NE in the acute stress response. While CORT levels remained unchanged after stress, CORT correlated significantly with both NP scores and RT after the stressor. These results imply that mild psychological stress can significantly alter attentional processes. Given the increase in alpha-amylase and the correlation between attention and CORT after stress, it seems likely that attentional processes are under tight control by brain systems which mediate the fight-or-flight response.     

Increased ACTH and corticosterone secretion induced by different methods of paradoxical sleep deprivation

The methods used to induce paradoxical sleep (PS) deprivation are believed to be stressful. In the present study, two methods were compared in regard to their ability to activate the hypothalamic-pituitary-adrenal (HPA) axis. Animals were placed on multiple large (MLP) or small (MSP) platforms or on single large (SLP) or small (SSP) platforms and blood sampled at the end of a 4-day period of PS deprivation (experiment 1) or on Days 1 (short-term) and 4 (long-term) of PS deprivation (experiment 2). ACTH and corticosterone (CORT) levels were determined by RIA. The results of experiment 1 showed that all experimental animals presented increased ACTH response, compared to controls. CORT levels, however, were only elevated in MSP animals, suggesting increased adrenal sensitivity. Experiment 2 showed that ACTH levels of MSP animals were higher than MLP and SSP animals, and that animals placed on the multiple platform tanks showed the highest ACTH levels on Day 4 of manipulation. CORT levels were elevated in the animals kept over small platforms, and these levels where higher on Day 1 than basal and further elevated on Day 4 of PS deprivation. These results indicate that the multiple platform technique induces a distinct activation of the HPA axis, and that PS deprivation may act as an additional stressor.     

Consistency of cholecystokinin satiety effect across deprivation levels and motivational states

A gastrointestinal hormone, cholecystokinin (CCK), has recently been implicated in the regulation of meal size. The consistency of the CCK satiety effect was examined across deprivation levels and motivational states. In a series of experiments rats were food deprived for varying amounts of time and injected with various doses of the CCK octapeptide before consuming a test meal of a liquid diet. In Experiment 1, 20 rats were deprived for 5 or 19 hr and injected with 0, 15, and 40 Ivy dog units/kg (U/kg) of CCK and in Experiment 2, 18 rats were 48 hr deprived and were injected with 0, 40, or 80 U/kg of CCK. In Experiment 3, 12 rats were deprived for 92 hr and received 80 U/kg of CCK. In all experiments CCK produced a dose-related suppression in food intake. CCK did not appear to become less effective as deprivation increased: 15 U/kg suppressed intake by approximately 30% at 5 and 19 hr deprivation; 40 U/kg suppressed intake by approximately 50% at all three deprivation levels; 40 U/kg suppressed intake by approximately 72% at 48 hr deprivation and 66% at 92 hr deprivation. In Experiment 4, the effects of CCK on food consumed in absence of hunger (0 hr deprivation) were observed by administering hypertonic saline to food-sated rats before presentation of a liquid diet. Under these conditions 40 U/kg of CCK suppressed intake by 76%. An additional experiment indicated that the increased inhibitory effects observed in the latter experiment were not due to the added variable of thirst. Thus under a wide variety of deprivation conditions and under varying motivational states CCK is remarkably consistent in its inhibitory effects on food intake, which are best described by a constant percent of control intake.     

Persistent stress-induced elevations of urinary corticosterone in rats

Exposure of rats to inescapable stressors (IS) results in persistent elevations in plasma corticosterone (CORT), which are selective to the trough of the circadian rhythm. Although affective disorders (depression, anxiety) in humans are also characterized by persistent hypothalamic-pituitary-adrenal axis (HPAA) activation, the predominant measure of HPAA activation in clinical studies is 24-h urinary cortisol. To facilitate interspecies comparisons regarding the persistent effects of stress on HPAA activity, we compared the effects of IS on plasma and urinary CORT in rats. Male Sprague-Dawley rats were exposed to three 2-h sessions of IS (40, 2.0 mA tailshocks) or remained in their home cages. The 24-h urine samples were collected daily from 2 days prior to stress to 5 days after stressor cessation, then weekly for 3 weeks. In addition, plasma samples were obtained at 08:00 (trough) and 20:00 hours (peak) for the first 3 days after stressor cessation and weekly for 3 weeks thereafter. Consistent with our earlier work, plasma CORT elevations were apparent in the trough, but not the peak samples for 3 days after stressor cessation. The 24-h urinary CORT levels were elevated during stressor exposure, and remained elevated for 3 days after stressor cessation. Persistent stress-induced urinary CORT elevations in rats are reminiscent of the clinical HPAA abnormalities described for major depression and affective disorders.     

Time course of the effect of maternal deprivation on the hypothalamic-pituitary-adrenal axis in the infant rat.

Prolonged (i.e., 24-hr) maternal deprivation leads to a marked disinhibition of the infant rat's adrenocortical response to stress and/or ACTH. In the following study we examined the time course over which these effects develop. Pups were maternally deprived for varying lengths of time (i.e., 0, 2, 4, 8, & 24 hr); at the end of this period, corticosterone (CORT) secretion in response to stress (novelty or novelty plus saline injection) and ACTH injection was measured. Basal levels of CORT increased progressively over time in 7- and 11- (but not 3-) day-old pups. CORT release in response to stress followed a similar pattern. In contrast, ACTH injection resulted in marked increases in CORT levels regardless of the length of maternal deprivation in 3-day-old animals; at older ages, however, 24 hr of deprivation led to a much larger increase. These findings support the hypothesis that the hypothalamic-pituitary-adrenal axis of the neonatal rat is subject to maternal regulation.     

白松片对慢性应激抑郁模型大鼠行为学及血浆CORT、ACTH的影响

目的：观察白松片对慢性应激抑郁大鼠模型行为学和血浆CORT、ACTH含量的影响。方法：SD雄性大鼠28只随机分为空白对照组、模型对照组、氟西汀对照组及白松片试验组，选用慢性轻度不可预见性应激加孤养造模，观察各组大鼠敞箱实验和液体消耗等行为学指标变化，采用放射免疫方法检测大鼠血浆皮质醇(CORT)和促肾上腺皮质激素(ACTH)含量。结果：慢性应激抑郁大鼠体重增加缓慢，敞箱实验中的水平运动、垂直运动得分、清洁动作次数显著减少，中央格停留时间显著延长；糖水消耗明显下降，纯水消耗显著增多，而且其血浆皮质醇和促肾上腺皮质激素含量增加。氟西汀和白松片均显著改善慢性应激抑郁大鼠模型的行为学和神经内分泌变化。结论：慢性轻度不可预见性应激可使大鼠行为及神经内分泌发生异常改变，引起抑郁状态，白松片对此具有一定拮抗作用。     

Maternal regulation of the adrenocortical response in preweanling rats.

In the following studies, we investigated the effects of 24-h maternal deprivation on the infant's hypothalamic-pituitary-adrenal system. Experiment 1 examined the effect of deprivation on the infant's corticosterone (CORT) response to adrenocorticotropin hormone (ACTH) injection. At all ages studied, deprivation resulted in a potentiation of the response. At some ages, deprived nontreated pups had higher CORT levels than nondeprived pups. Experiment 2 examined the ontogeny of the deprivation-induced stress response, and the capacity of the mother to inhibit it. From 8 days of age onwards, deprived animals showed a CORT response to saline injection that was either absent or far smaller in nondeprived pups. Saline-induced CORT secretion was diminished, or prevented, by returning the infant to its dam. Maternal reunion had no effect on ACTH-induced CORT elevations. Finally, Experiment 3 investigated the effects of deprivation over a more extended period of time. In maternally deprived pups, ACTH-induced CORT elevations persisted for at least 2 h following reunion, but by 6 h had returned to baseline. These data suggest that maternal factors are involved in the regulation of the responsiveness of the pup's hypothalamic-pituitary-adrenal system.     

The effect of apomorphine on genital reflexes in male rats deprived of paradoxical sleep

Drugs that stimulate dopamine (DA) systems can stimulate sexual arousal in male rats and humans, and previous work has shown that cocaine enhances genital reflexes [penile erection (PE) and ejaculation (EJ)] in rats deprived of paradoxical sleep (PS). The present study sought to expand the latter finding by assessing the effects of DA receptor agonist apomorphine in sleep-deprived rats. Apomorphine in doses ranging from 10 to 240 microg/kg was administered intraperitoneally to rats that had been deprived of sleep for 4 days and to normal controls, and the incidence of PEs and EJs was measured for 60 min. Sleep deprivation alone induced PE and this effect was potentiated by apomorphine, with maximal effects occurring with the 120 microg/kg dose; results for this dose group differed from those of PSD groups treated with 0, 10, 20, 40, 80, and 240 microg/kg of apomorphine. Sleep deprivation alone also induced spontaneous EJ, but this response was not potentiated by apomorphine in the dose range tested. We suggest that the potentiating effects of apomorphine on PE are likely due to PSD-induced DA receptor supersensitivity.     

Food deprivation dissociates pancreatic glucagon's effects on satiety and hepatic glucose production at dark onset

We compared the effects of different durations of pretest food deprivation on pancreatic glucagon's (PG) satiating and glycogenolytic actions in order to test the hypothesis that stimulation of hepatic glucose production causes PG's satiety effect. Rats were maintained on a 12:12 LD cycle (lights off: 1015) and deprived of food 45 min or 8, 12, 18, or 24 hr before intraperitoneal injection of 400 micrograms/kg PG. Testing began at 1015, the beginning of the dark phase. Food intake was not inhibited after 45 min of pretest food deprivation (30 min change, 2.5 +/- 4.0%, p greater than 0.05), but was inhibited after 8 or more hr food deprivation. The largest inhibitory effect, 16.2 +/- 3.8%, p less than 0.01, occurred after 8 hr food deprivation. In separate experiments, rats were food deprived 45 min or 8 hr, similarly injected, and killed 10 min after refeeding for blood and liver samples. Hepatic glycogen content at meal onset was higher in rats deprived 45 min than in rats deprived 8 hr (3.2 +/- 0.3 vs. 1.7 +/- 0.3% liver weight, p less than 0.01), and PG injection produced a higher level of hepatic vein blood glucose in the less deprived rats (196 +/- 5 vs. 168 +/- 12 mg/dl, p less than 0.05). Thus, in rats tested at the beginning of the dark phase of the LD cycle after 45 min or 8 hr food deprivation, there is an inverse relation between PG's potencies to inhibit food intake and to stimulate hepatic glucose production.(ABSTRACT TRUNCATED AT 250 WORDS)     

Psychosocial maternal stress during pregnancy: effects on reproduction for F0 and F1 generation laboratory rats

The impact of daily social maternal stress on reproduction parameters was studied in F0 and F1 generation female Long-Evans rats. Chronic maternal stress was induced in pregnant females (F0 females) by 2 h social confrontation with a dominant female throughout pregnancy. Social stress of F0 females was associated with lower maternal body mass investment in litters. While maternal stress did not cause a decline in the F0 female mass, it resulted in reduced litter sizes and lower litter masses. The individual body mass of offspring from stressed (= prenatally stressed offspring, PS) and control F0 generation mothers (= prenatal control offspring, PC) did not differ at birth. However, PS offspring grew faster during lactation and were therefore heavier than PC offspring at weaning. Reproduction parameters of F1 generation females were determined until an age of 180 days. Investigation revealed that PS females did not differ from PC females in any reproduction parameter assessed, except for higher PS offspring body mass at birth. It was also observed that a higher percentage of PS females gave birth outside the core breeding period during the light (= inactive) period. This study shows that exposure to a relatively mild social stressor during pregnancy alters female reproduction in rats. However, there was no indication of higher infant mortality which is often associated with severe laboratory stress. We argue that the reduced litter sizes in stressed F0 mothers represent an adaptive strategy from an evolutionary point of view.     

Estrogen receptor beta in the paraventricular nucleus of hypothalamus regulates the neuroendocrine response to stress and is regulated by corticosterone

The function of the second nuclear estrogen receptor, estrogen receptor beta (ERbeta), in the brain is largely unknown. The present study tested whether 1) ERbeta in the paraventricular nucleus (PVN) of the hypothalamus has a direct role in the hypothalamic-pituitary-adrenal (HPA) axis-mediated stress function, and 2) whether corticosterone (CORT) can regulate ERbeta gene expression in the PVN in the intact, cycling female rat. To test the first hypothesis a pure estrogen receptor antagonist, ICI182, 780, was microinjected into the PVN bilaterally and stress-induced CORT response to an acute stressor (15 min restraint) was measured at 0, 15, 30, 60 and 90 min time points. Estrogen antagonist-injected rats showed inhibited CORT levels at the peak (15 min) of the stress response compared with vehicle-injected animals. To test the second hypothesis, ERbeta mRNA levels were measured in the PVN using in situ hybridization histochemistry following sham surgery, adrenalectomy, and adrenalectomy with low or high CORT replacement. Adrenalectomy reduced ERbeta mRNA expression in the PVN, whereas CORT replacement fully reversed this effect in a dose-dependent fashion. Both antagonist inhibition of CORT response and CORT-mediated regulation of ERbeta mRNA were found to be estrus cycle-dependent in the intact, cycling female. These data suggest that ERbeta in the PVN may critically modulate the HPA axis response to stress and is, in turn, regulated by circulating CORT.     

Aging increases basal but not stress-induced levels of corticosterone in the brain of the awake rat

The main purpose of this study was to evaluate the effect of aging on plasma and free corticosterone (CORT) levels in the brain in basal conditions and in response to an acute stressor. Microdialysis experiments were performed in the hippocampus (HC) and the prefrontal cortex (PFC) of young adult (6 months) and aged (24 months) male Wistar rats. Basal free levels of CORT in the HC and the PFC were higher in aged animals. Restraint stress increased plasma CORT and free CORT levels in the HC and the PFC both in young and aged animals. However, while the increase of plasma CORT was higher in aged rats compared with young rats, the increases of free CORT in the HC and the PFC were not different between these two groups of rats. These results suggest that the changes produced by aging in the brain may be related to the enhanced basal levels of free CORT and not to the CORT increases in response to stress.     

The influence of forepaw palmar sensorimotor deprivation on learning and memory in young rats

This study examined the effects of early palmar forepaw sensorimotor deprivation on learning and memory in rats. Sensorimotor deprivation was performed on 18-day-old male rats. Controls were sham operated. Studies were performed on rats aged 18, 25, 35, 45 and 60 days. Morris water maze testing was used to assess learning and memory. Long-term potentiation (LTP) was assessed by electrophysiological means in slices obtained from the hippocampal Schaffer collateral pathway. Nissl staining was performed to assess pyramidal cell number in hippocampal CA1 and CA3 regions. Hippocampal N-methyl-d-aspartate receptor 1 (NMDAR1) mRNA and protein levels were assessed. Learning and short-term memory were significantly depressed in 25 and 35 day old sensorimotor deprived rats (P<0.01). LTP was also significantly depressed in sensorimotor deprived rats at these ages, while hippocampal CA1 pyramidal cell counts were significantly decreased (P<0.05). CA3 cell numbers were significantly lower in 25-day-old sensorimotor deprived rats (P<0.05). Both NMDAR1 mRNA and protein levels were significantly lower in sensorimotor deprived rats aged 25 and 35 days (P<0.05). These findings indicate that palmar surface forepaw sensorimotor deprivation impairs subsequent learning and memory in young rats. Decreased hippocampal pyramidal cell numbers and altered NMDAR1 expression may underlie this impairment.     

Decreased NaCl sensitivity in zinc-deprived rats.

NaCl thresholds and ability to discriminate between NaCl and sucrose were assessed in rats using an operant discrimination conditioning procedure before and during moderate and severe zinc deprivation and during zinc supplementation. NaCl thresholds were approximately 1 mM before dietary zinc manipulation. They increased in all zinc-deprived rats tested 10 and 17 days after initiation of deprivation but did not change in pair-fed controls maintained on supplemental zinc. Threshold changes were greater for those rats severely zinc deprived than for those only moderately deprived and were greater as the period of deprivation lengthened. Plasma zinc concentrations decreased significantly in deprived rats from values obtained at baseline, values in severely deprived rats being significantly lower than in those only moderately deprived. Although zinc-deprived rats discriminated NaCl from sucrose, they made more discrimination errors than controls. Following 24 days of zinc supplementation, previously deprived rats exhibited no significant improvement in gustatory performance, although their body weight increased and plasma zinc concentrations increased; but these later changes were not significant. These results demonstrate that zinc deprivation induces decreased gustatory sensitivity and confirm a role for zinc in taste.     

Increases in paradoxical sleep as a result of amygdaloid stimulation.

Holtzman rats were partially and selectively deprived of paradoxical sleep (PS) for a 16 1/2 hr period and allowed to rest for a 7 1/2 hr period each day. Test animals were given mild (60 Hz, 50 muA) unilateral amygdaloid stimulation for one min each day of a 5 day test period. There was a significant increase in PS during the rest periods of test animals on stimulation days when compared to nonstimulated controls. The effect did not last beyond a 5 day period. Only subconvulsive behaviour was observed in two test animals. The similarity to the PS increases in this study to those seen in conventional learning and other stiuations are discussed.     

Effects of mild food deprivation on the estrous cycle of rats

It has long been known that severe food deprivation disrupts the estrous cycle. One of the main problems with behavioral tasks that use food for reinforcement is the requirement that the animal be food deprived. This manipulation could be problematic in studies using female animals, since it may interfere with the estrous cycle of the animals. The purpose of the present study was to investigate: (1) the effect of mild food deprivation on four different strains of rats, (2) factors in the food deprivation procedure that could affect the estrous cycle, and (3) the possible effect of enriched diets during food deprivation on the estrous cycle. A comparison of the estrous cycle in four different rat strains revealed differences in the reliability of the estrous cycle even before the onset of food deprivation. Fischer, Long-Evans, and Sprague-Dawley rats all showed reliable cycle patterns. This was not the case for Brown Norway rats. During food deprivation, the cycle of the Fischer rats was disrupted, whereas the Long-Evans and Sprague-Dawley animals continued to cycle. Both the rate of weight loss and the percent of ad libitum body weight were related to cessation of the estrous cycle. However, enriching an animal's diet with sugar or oil additives delayed the disruption of the estrous cycle. Additionally, animals resumed cycling when returned to ad libitum weight levels. The present findings suggest that when animals need to be food deprived, preference should be given to using Long-Evans or Sprague-Dawley rats. If Fischer rats must be used, they should not be deprived below 90-95% of their ad libitum body weight. Strategies for future food deprivation studies are discussed, as well as a comparison of the effects of mild and severe food deprivation.     

Noradrenergic neurons expressing Fos during waking and paradoxical sleep deprivation in the rat

Noradrenaline is known to induce waking (W) and to inhibit paradoxical sleep (PS or REM). Both roles have been exclusively attributed to the noradrenergic neurons of the locus coeruleus (LC, A6), shown to be active during W and inactive during PS. However, the A1, A2, A5 and A7 noradrenergic neurons could also be responsible. Therefore, to determine the contribution of each of the noradrenergic groups in W and in PS inhibition, rats were maintained in continuous W for 3 h in a novel environment or specifically deprived of PS for 3 days, with some of them allowed to recover from this deprivation. A double immunohistochemical labeling with Fos and tyrosine hydroxylase was then performed. Thirty percent of the LC noradrenergic cells were found to be Fos-positive after exposure to the novel environment and less than 2% after PS deprivation. In contrast, a significant number of double-labeled neurons (up to 40% of the noradrenergic neurons) were observed in the A1/C1, A2 and A5 groups, after both novel environment and PS deprivation. After PS recovery and in control condition, less than 1% of the noradrenergic neurons were Fos-immunoreactive, regardless of the noradrenergic group. These results indicate that the brainstem noradrenergic cell groups are activated during W and silent during PS. They further suggest that the inhibitory effect of noradrenaline on PS may be due to the A1/C1, A2 and to a lesser degree to A5 neurons but not from those of the LC as previously hypothesized.     

Effects of various stressors on milk release in the rat.

This study investigated the effect of four stimuli on milk release (MR), namely, sound, nociception, novelty, and restraint. The role of the ensuing adrenocortical response in the suppression of MR was also evaluated. Plasma corticosterone (CORT) levels were measured at 0 (basal), 15, 30, and 60 min during the suckling sessions to determine whether elevated CORT normally associated with stress could be inhibitory to MR. Compared to nonstressed lactators, dams exposed to sound demonstrated no suppression in MR, but a significant increase in plasma CORT. Pain did not alter milk yield and elevated CORT only at the end of the first hour of exposure. During novelty, MR was suppressed and again CORT was only elevated at the end of the sampling period. Restraint decreased milk yield and increased CORT. During novelty, MR appeared to be regulated by an adrenal factor, which remains to be identified. The peripheral opiates seem to be partially involved during restraint. In conclusion, not all types of aversive stimuli interfere with MR. Of those which do, different mechanisms seem to be implicated depending upon the nature of the stressor. Furthermore, reduced MR during stress is not a direct consequence of increased CORT.     

Stress responses of adolescent male and female rats exposed repeatedly to cat odor stimuli,and long‐term enhancement of adult defensive behaviors

In order to characterize the short‐ and long‐term effects of repeated stressor exposure during adolescence, and to compare the effects of using two sources of cat odor as stressor stimuli, male and female adolescent rats (postnatal day (PND) ～ 38–46) were exposed on five occasions to either a control stimulus, a cloth stimulus containing cat hair/dander, or a section of cat collar previously worn by a cat. Relative to control stimulus exposure, activity was suppressed and defensive behavior enhanced during exposure to either cat odor stimulus (most pervasively in rats exposed to the collar). Only cloth‐exposed rats showed elevated levels of corticosterone (CORT), and only after repeated stressor exposure, but interestingly, rats exposed to the collar stimulus during adolescence continued to show increased behavioral indices of anxiety in adulthood. In this group, the time an individual spent in physical contact with a cagemate during the final adolescent exposure was negatively related to stress‐induced CORT output in adulthood, which suggests that greater use of social support during adolescent stress may facilitate adult behavioral coping, without necessitating increased CORT release. These findings demonstrate that adolescent male and female rats respond defensively to cat odor stimuli across repeated exposures and that exposure to such stressors during adolescence can augment adult anxiety‐like behavior in similar stressful conditions. These findings also suggest a potential role for social behavior during adolescent stressor exposure in mediating long‐term outcomes. © 2012 Wiley Periodicals, Inc. Dev Psychobiol 55: 551–567, 2013