Influence of psychogenic and neurogenic stressors on neuroendocrine and central monoamine activity in fast and slow kindling rats

Activation of PPG neurons following acute stressors differentially involves hindbrain serotonin in male rats

Within the hindbrain, serotonin (5-HT) functions as a modulator of the central glucagon-like peptide-1 (GLP-1) system. This interaction between 5-HT and GLP-1 is achieved via 5-HT2C and 5-HT3 receptors and is relevant for GLP-1-mediated feeding behavior. The central GLP-1 system is activated by various stressors, activates the hypothalamic pituitary adrenocortical (HPA) axis, and contributes to stress-related behaviors. Whether 5-HT modulates GLP-1's role in the stress response in unknown. We hypothesized that the serotonergic modulation of GLP-1-producing neurons (i.e., PPG neurons) is stimuli-specific and that stressed-induced PPG activity is one of the modalities in which 5-HT plays a role. In this study, we investigated the roles of 5-HT2C and 5-HT3 receptors in mediating the activation of PPG neurons in the nucleus tractus solitarius (NTS) following exposure to three different acute stressors: lithium chloride (LiCl), noncontingent cocaine (Coc), and novel restraint stress (RES). Results showed that increased c-Fos expression in PPG neurons following LiCl and RES-but not Coc-is dependent on hindbrain 5-HT2C and 5-HT3 receptor signaling. Additionally, stressors that depend on 5-HT signaling to activate PPG neurons (i.e., LiCl and RES) increased c-Fos expression in 5-HT-expressing neurons within the caudal raphe (CR), specifically in the raphe magnus (RMg). Finally, we showed that RMg neurons innervate NTS PPG neurons and that some of these PPG neurons lie in close proximity to 5-HT axons, suggesting RMg 5-HT-expressing neurons are the source of 5-HT input responsible for engaging NTS PPG neurons. Together, these findings identify a direct RMg to NTS pathway responsible for the modulatory effect of 5-HT on the central GLP-1 system-specifically via activation of 5-HT2C and 5-HT3 receptors-in the facilitation of acute stress responses.

An animal model of genetic predisposition to develop acquired epileptogenesis: The FAST and SLOW rats

Epidemiological data and gene association studies suggest a genetic predisposition to developing epilepsy after an acquired brain insult, such as traumatic brain injury. An improved understanding of genetic determinants of vulnerability is imperative for early disease diagnosis and prognosis prediction, with flow-on benefits for the development of targeted antiepileptogenic treatments as well as optimal clinical trial design. In the laboratory, one approach to investigate why some individuals are more vulnerable to acquired epilepsy than others is to examine unique rodent models exhibiting either vulnerability or resistance to epileptogenesis. This review focuses on the most well-characterized of these models, the FAST (seizure-prone) and SLOW (seizure-resistant) rat strains, which were derived by selective breeding for differential amygdala electrical kindling rates. We describe how these strains differ in their seizure profiles, neuroanatomy, and neurobehavioral phenotypes, both at baseline and after a brain insult, with this knowledge proving fruitful to identify common pathological abnormalities associated with seizure susceptibility and psychiatric comorbidities. It is important to note that accruing data on strain differences in multiple biological processes provides insight into why some individuals may be more vulnerable to epileptogenesis, although future studies are evidently needed to identify the precise molecular and genetic risk factors. Together, the FAST and SLOW rat strains, and other similar experimental models, are invaluable neurobiological tools to investigate the effect of genetic background on acquired epilepsy risk, as well as the poorly understood relationship between epilepsy development and associated comorbidities.

Orexin 2 receptor regulation of the hypothalamic-pituitary-adrenal (HPA) response to acute and repeated stress

Orexins are hypothalamic neuropeptides that have a documented role in mediating the acute stress response. However, their role in habituation to repeated stress, and the role of orexin receptors (OX₁R and OX₂R) in the stress response, has yet to be defined. Orexin neuronal activation and levels in the cerebrospinal fluid (CSF) were found to be stimulated with acute restraint, but were significantly reduced by day five of repeated restraint. As certain disease states such as panic disorder are associated with increased central orexin levels and failure to habituate to repeated stress, the effect of activating orexin signaling via Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) on the hypothalamic-pituitary-adrenal (HPA) response was evaluated after repeated restraint. While vehicle-treated rats displayed habituation of Adrenocorticotropic Hormone (ACTH) from day 1 to day 5 of restraint, stimulating orexins did not further increase ACTH beyond vehicle levels for either acute or repeated restraint. We delineated the roles of orexin receptors in acute and repeated stress using a selective OX₂R antagonist (MK-1064). Pretreatment with MK-1064 reduced day 1 ACTH levels, but did not allow further habituation on day 5 compared with vehicle-treated rats, indicating that endogenous OX₂R activity plays a role in acute stress, but not in habituation to repeated stress. However, in restrained rats with further stimulated orexins by DREADDs, MK-1064 decreased ACTH levels on day 5. Collectively, these results indicate that the OX₂R plays a role in acute stress, and can prevent habituation to repeated stress under conditions of high orexin release.

Effects of a ketogenic diet on ADHD-like behavior in dogs with idiopathic epilepsy

OBJECTIVES

Epilepsy in humans and rodent models of epilepsy can be associated with behavioral comorbidities including an increased prevalence of attention-deficit/hyperactivity disorder (ADHD). Attention-deficit/hyperactivity disorder symptoms and seizure frequency have been successfully reduced in humans and rodents using a ketogenic diet (KD). The aims of this study were (i) to describe the behavioral profile of dogs with idiopathic epilepsy (IE) while on a standardized nonketogenic placebo diet, to determine whether ADHD-like behaviors are present, and (ii) to examine the effect of a ketogenic medium chain triglyceride diet (MCTD) on the behavioral profile of dogs with idiopathic epilepsy (IE) compared with the standardized placebo control diet, including ADHD-like behaviors.

METHODS

A 6-month prospective, randomized, double-blinded, placebo-controlled, crossover dietary trial comparing the effects of the MCTD with a standardized placebo diet on canine behavior was carried out. Dogs diagnosed with IE, with a seizure frequency of at least 3 seizures in the past 3months (n=21), were fed the MCTD or placebo diet for 3months and were then switched to the alternative diet for 3months. Owners completed a validated behavioral questionnaire to measure 11 defined behavioral factors at the end of each diet period to report their dogs' behavior, with three specific behaviors hypothesized to be related to ADHD: excitability, chasing, and trainability.

RESULTS

The highest scoring behavioral factors in the placebo and MCTD periods were excitability (mean±SE: 1.910±0.127) and chasing (mean±SE: 1.824±0.210). A markedly lower trainability score (mean±SE: 0.437±0.125) than that of previously studied canine populations was observed. The MCTD resulted in a significant improvement in the ADHD-related behavioral factor chasing and a reduction in stranger-directed fear (p<0.05) compared with the placebo diet. The latter effect may be attributed to previously described anxiolytic effects of a KD.

CONCLUSIONS

These data support the supposition that dogs with IE may exhibit behaviors that resemble ADHD symptoms seen in humans and rodent models of epilepsy and that a MCTD may be able to improve some of these behaviors, along with potentially anxiolytic effects.

Two seconds is all it takes: European starlings (Sturnus vulgaris) increase levels of circulating glucocorticoids after witnessing a brief raptor attack

Researchers typically study "acute" activation of the hypothalamic-pituitary-adrenal (HPA) axis by measuring levels of circulating glucocorticoids in animals that have been exposed to a predator or a cue from a predator (e.g., odor), or have experienced a standardized capture-and-restraint protocol, all of which are many minutes in duration. However, exposure to predators in the "wild", either as the subject of an attack or as a witness to an attack, is generally much shorter as most depredation attempts upon free-living animals last <5s. Yet, whether a stimulus lasting only seconds can activate the HPA axis is unknown. To determine if a stimulus of a few seconds triggers a glucocorticoid response, we measured levels of corticosterone (CORT; the primary avian glucocorticoid) in wild-caught European starlings (Sturnus vulgaris) after they witnessed a brief (<2-8s) raptor attack upon a conspecific, a human "attack" (i.e., a researcher handling a conspecific), and an undisturbed control. Witnesses of a raptor attack responded with CORT levels comparable to that induced by a standardized capture-and-restraint protocol. Glucocorticoid levels of individuals following the control treatment were similar to baseline levels, and those that witnessed a human "attack" had intermediate levels. Our results demonstrate that witnessing a predator attack of very brief duration triggers a profound adrenocortical stress response. Given the considerable evidence of a role for glucocorticoids in learning and memory, such a response may affect how individuals learn to recognize and appropriately react to predators.

Changes in protein and gene expression of angiotensin II receptors (AT1 and AT2) in aorta of diabetic and hypertensive rats

Diabetes and hypertension have been associated with cardiovascular diseases and stroke. Some reports have related the coexistence of hypertension and diabetes with increase in the risk of developing vascular complications. Recently some studies have shown results suggesting that in the early stages of diabetes and hypertension exist a reduced functional response to vasopressor agents like angiotensin II (Ang II), which plays an important role in blood pressure regulation mechanism through the activation of its AT1 and AT2 receptors. For that reason, the aim of this work was to study the gene and protein expression of AT1 and AT2 receptors in aorta of diabetic SHR and WKY rats. Diabetes was induced by the administration of streptozotocin (60 mg/kg i.p.). After 4 weeks of the onset of diabetes, the protein expression was obtained by western blot and the mRNA expression by RT-PCR. Our results showed that the hypertensive rats have a higher mRNA and protein expression of AT1 receptors than normotensive rats while the AT2 expression remained unchanged. On the other hand, the combination of diabetes and hypertension increased the mRNA and protein expression of AT1 and AT2 receptors significantly. In conclusion, our results suggest that diabetes with hypertension modifies the mRNA and protein expression of AT1 and AT2 receptors. However, the overexpression of AT2 could be associated with the reduction in the response to Ang II in the early stage of diabetes.

Gonadal steroid hormones and the hypothalamo-pituitary-adrenal axis

The hypothalamo-pituitary-adrenal (HPA) axis represents a complex neuroendocrine feedback loop controlling the secretion of adrenal glucocorticoid hormones. Central to its function is the paraventricular nucleus of the hypothalamus (PVN) where neurons expressing corticotropin releasing factor reside. These HPA motor neurons are a primary site of integration leading to graded endocrine responses to physical and psychological stressors. An important regulatory factor that must be considered, prior to generating an appropriate response is the animal's reproductive status. Thus, PVN neurons express androgen and estrogen receptors and receive input from sites that also express these receptors. Consequently, changes in reproduction and gonadal steroid levels modulate the stress response and this underlies sex differences in HPA axis function. This review examines the make up of the HPA axis and hypothalamo-pituitary-gonadal (HPG) axis and the interactions between the two that should be considered when exploring normal and pathological responses to environmental stressors.

Chronically stressed female rats show increased anxiety but no behavioral alterations in object recognition or placement memory: a preliminary examination

Stress, depending on intensity and duration, elicits adaptive or maladaptive physiological effects. Increasing evidence shows those patterns of advantageous versus deleterious physiologic stress effects also exist for some brain functions, including learning and memory. For example, short stress enhances, while chronic stress impairs, performance on numerous cognitive tasks in male rats. In contrast, performance of female rats is enhanced, or not altered, following both short-term and long-term stress exposure on the same behavioral tasks. The current study was designed to better characterize the behavioral effects of sustained chronic restraint stress in female rats. Female Sprague Dawley rats were assigned to a stress (restraint, 6 h/day, 35 days) or control (no stress) condition, weighed weekly, and then tested on open field (OF), object recognition (OR) and object placement (OP) tasks. Stressed females gained less weight during stress than controls. On the OF, there were no group differences in locomotor activity, but stressed females made fewer inner visits than controls, indicating increased anxiety. Both groups successfully performed the OP and OR tasks across all inter-trial delays, indicating intact non-spatial and spatial memory in both control and stress females. The current results provide preliminary evidence that the commonly used chronic restraint stress model may not be an efficient stressor to female rats.

Activation patterns of cells in selected brain stem nuclei of more and less stress responsive rats in two animal models of PTSD - predator exposure and submersion stress

This study had two purposes. First: compare predator and water submersion stress cFos activation patterns in dorsal raphe (DR), locus coeruleus (LC) and periaqueductal gray (PAG). Second: identify markers of vulnerability to stressors within these areas. Rats were either predator or submersion stressed and tested 1.75 h later for anxiety-like behavior. Immediately thereafter, rats were sacrificed and cFos expression examined. In DR, serotonergic cells expressing or not expressing cFos were also counted. Predator and submersion stress increased anxiety-like behavior (in the elevated plus maze- EPM) equally over controls. Moreover, stressed rats spent equally less time in the center of the hole board than handled controls, another indication of increased anxiety-like behavior. To examine vulnerability, rats which were less anxious (LA) and more (highly) anxious (MA) in the EPM were selected from among handled control and stressed animals. LA rats in the stressed groups were considered stress non-responsive and MA stressed rats were considered stress responsive. LA and MA rats did not differ in cFos expression in any brain area, though stressors did increase cFos cell counts in all areas over controls. Intriguingly, the number of serotonergic DR neurons not activated by stress predicted degree of anxiety response to submersion stress only. LA submersion stressed rats had more serotonergic cells than all other groups, and MA submersion stressed rats had fewer serotonergic cells than all other groups, which did not differ. Moreover, these cell counts correlated with EPM anxiety. We conclude that a surplus of such cells protects against anxiogenic effects of submersion, while a paucity of such cells enhances vulnerability to submersion stress. Other data suggest serotonergic cells may exert their effects via inhibition of dorsolateral PAG cells during submersion stress. Findings are discussed with respect to serotonergic transmission in vulnerability to predator stress and relevance of findings for post traumatic stress disorder (PTSD). This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Inescapable but not escapable stress leads to increased struggling behavior and basolateral amygdala c-fos gene expression in response to subsequent novel stress challenge

Control over an aversive experience can greatly impact the organism's response to subsequent stressors. We compared the effects of escapable (ES) and yoked inescapable (IS) electric tail shocks on the hypothalamic-pituitary-adrenal (HPA) axis hormonal (corticosterone and adrenocorticotropic hormone (ACTH)), neural (c-fos mRNA) and behavioral (struggling) response to subsequent restraint. We found that although the HPA axis response during restraint of both previously stressed groups were higher than stress-naïve rats and not different from each other, lack of control over the tailshock experience led to an increase in restraint-induced struggling behavior of the IS rats compared to both stress-naïve and ES rats. Additionally, c-fos expression in the basolateral amygdala was increased selectively in the IS group, and relative c-fos mRNA expression in the basolateral amygdala positively correlated with struggling behavior. Restraint-induced c-fos expression in the medial prefrontal cortex, a brain area critical for mediating some of the differential neurochemical and behavioral effects of ES and IS, was surprisingly similar in both ES and IS groups, lower than that of stress-naïve rats, and did not correlate with struggling behavior. Our findings indicate that basolateral amygdala activity may be connected with the differential effects of ES and IS on subsequent behavioral responses to restraint, without contributing to the concurrent HPA axis hormone response.

The effects of stress on play and home cage behaviors in adolescent male rats

One prominent feature of adolescence is the high frequency of social behaviors, such as play. Engaging in these behaviors appears necessary for proper socio-emotional development as social isolation during adolescence typically leads to behavioral dysfunctions in adulthood. The present experiments examined the effects of stress on social and nonsocial behaviors in group housed adolescent male rats. We found that acute restraint stress led to a complete inhibition of play (e.g., nape contacts and pins) and reduced social investigations in pre- (28 days), mid- (35 days), and late-adolescent (42 and 49 days) animals. A follow-up study, however, found that restraint-induced suppression of play and social investigations was transient such that experimental animals engage in these behaviors at levels similar to those of controls 1 hr after termination of the stressor. We also found that exposure to repeated restraint stress throughout adolescence led to a decrease in social investigations, while leaving play largely unaffected. Interestingly, in all of the experiments, nonsocial behaviors (e.g., eating, drinking, grooming) were unaffected by restraint, suggesting these effects of stress are specific to social behaviors. Together, these data indicate that both acute and repeated stress significantly affect social behaviors during adolescence.

The medial amygdala modulates body weight but not neuroendocrine responses to chronic stress

Stress pathologies such as depression and eating disorders (i.e. anorexia nervosa) are associated with amygdalar dysfunction, which are linked with hypothalamic-pituitary-adrenal axis (HPA) axis hyperactivity. The medial amygdaloid nucleus (MeA), a key output nucleus of the amygdaloid complex, promotes HPA axis activation to acute psychogenic stress and is in a prime position to mediate the deleterious effects of chronic stress on physiology and behaviour. The present study tests the hypothesis that the MeA is necessary for the development of maladaptive physiological changes caused by prolonged stress exposure. Male rats received bilateral ibotenate or sham lesions targeting the MeA and one half underwent 2 weeks of chronic variable stress (CVS) or served as home cage controls. Sixteen hours post CVS, all animals were exposed to an acute restraint challenge. CVS induced thymic involution, adrenal hypertrophy, and attenuated body weight gain and up-regulation of hypothalamic corticotrophin-releasing hormone mRNA expression. Consistent with previous literature, lesions of the MeA dampened stress-induced increases in corticosterone after 30 min of exposure to acute restraint stress. However, this effect was independent of CVS exposure, suggesting that the MeA may not be critical for modulating neuroendocrine responses after chronic HPA axis drive. Interestingly, lesion of the MeA modestly exaggerated the stress-induced attenuation of weight gain. Overall, the data obtained suggest that the MeA modulates the neuroendocrine responses to acute but not chronic stress. In addition, the data suggest that the MeA may be an important neural component for the control of body weight in the face of chronic stress.

Disruption of neuroendocrine stress responses to acute ferret odor by medial, but not central amygdala lesions in rats

Investigations of the neural pathways associated with responses to predators have implicated the medial amygdala (MeA) as an important region involved in defensive behaviors. To our knowledge, however, the involvement of the MeA in neuroendocrine responses to predator odor exposure has not been investigated. Therefore, the present study examined the effects of MeA disruption in rats exposed to ferret or control odor on hypothalamo-pituitary-adrenocortical (HPA) axis activation. Bilateral lesions of the MeA were made in Sprague-Dawley rats with the neurotoxin ibotenic acid (10 microg/microl; 0.3 microl / side). As a control for regional specificity, additional groups of rats were given lesions in the central amygdala (CeA). One week after recovery, the rats were exposed to ferret or strawberry control towels in small cages to examine HPA axis responses as determined by plasma corticosterone and adrenocorticotropin hormone (ACTH) levels. Rats with complete bilateral MeA but not CeA lesions displayed significantly less corticosterone and ACTH release compared to sham-operated control rats only in the ferret odor conditions. These results suggest that the MeA is an important structure involved in the HPA axis responses to predator odors, in support of previous studies investigating behavioral responses under similar conditions.

Acute and chronic effects of ferret odor exposure in Sprague-Dawley rats

This manuscript describes several behavioral and functional studies evaluating the capacity of ferret odors to elicit a number of acute and long-term responses in male Sprague-Dawley rats. Acute presentation elicits multiple responses, suggesting that ferret odor, likely from skin gland secretions, provides an anxiogenic-like stimulus in this strain of rats. Compared to cat odor, however, ferret odor did not produce rapid fear conditioning, a result perhaps attributable to methodological factors. Inactivation of the olfactory system and medial nucleus of the amygdala, combined with induction of the immediate-early gene c-fos, suggest the necessity of the accessory olfactory system in mediating the effects of ferret odor. Repeated exposures to ferret odor produce variable habituation of neuroendocrine and behavioral responses, perhaps indicative of the lack of control over the exact individual origin or concentration of ferret odor. Ferret odor induces rapid and long-term body weight regulation, thymic involution, adrenal hyperplasia and facilitation of the neuroendocrine response to additional challenges. It is argued that the use of such odors is exquisitely suited to investigate the brain regions coordinating anxiety-like responses and the long-term changes elicited by such stimuli.

Struggling behavior during restraint is regulated by stress experience

Restraint elicits a number of physiological stress responses that can be increased or decreased in magnitude based on prior stress history. For instance, repeated exposure to restraint leads to habituation of hypothalamic-pituitary-adrenal (HPA) activation to restraint. In contrast, acute restraint after a different repeated stressor leads to facilitation of HPA activity to the novel stress. Acute restraint also elicits a variety of behaviors, including struggling, but the effect of prior stress in regulating behavioral responses to restraint is not clear. The goal of the present studies was to assess struggling during restraint with or without a prior history of repeated stress. Using automated behavioral analysis software (EthoVision), we quantified struggling during restraint. We found that acutely restrained rats exhibited vigorous struggling behavior that declined during a single restraint period. Repeated restraint lead to habituated struggling behavior, whereas acute restraint after repeated swim elicited facilitated struggling behavior. These effects on struggling were found alongside expected differences in HPA activity. Removing stress-induced increases in corticosterone via adrenalectomy did not significantly affect struggling responses to restraint. Overall, restraint-induced struggling appears to be regulated in a manner similar to HPA responses to restraint, but is not dictated by adrenal hormones.

The role of the read through variant of acetylcholinesterase in anxiogenic effects of predator stress in mice

This study examined the role of the read through variant of acetylcholinesterase (AChE-R) in lasting changes in murine affective behavior produced by a brief predator stress. AChE-R is elevated by stress in limbic cholinergic circuits implicated in anxiogenic effects of predator stress. The expression of AChE-R was blocked with a systemically administered central acting antisense oligonucleotide for AChE-R (EN101). EN101 was injected at multiple points prior to and after a predator stress in male C57 mice. Seven days after the last injection, behavior was tested. Predator stress caused a significant increase in startle amplitude, which EN101 blocked. This effect was specific to EN101, as the negative control inactive form of EN101, INVEN101 was without effect on stress effects on startle. Neither EN101 nor INVEN101 altered the anxiogenic effects of predator stress on behavior in the elevated plus maze, and both drugs partially reduced stress suppression of time active in the hole board. In the light dark box test, INVEN101 exhibited a weak block of stress effects on behavior for reasons which are unclear. Taken together, findings support the view that multiple neural systems are responsible for the different changes in behavior produced by predator stress. Present findings also suggest a role for AChE-R in specific anxiogenic (hyperarousal) effects following predator stress. Since AChE-R manipulations took place starting 23 h prior to predator stress and continued 48 h after predator stress, further research is necessary to determine the role of AChE-R in initiation and/or consolidation of hyperarousal effects of predator stress.

Rats selectively-bred for behavior related to affective disorders: proclivity for intake of alcohol and drugs of abuse, and measures of brain monoamines

Several lines of rats potentially useful for studying affective disorders have been developed in our laboratory though selective breeding for behavioral characteristics. The propensity of these lines to consume alcohol and other drugs of abuse (amphetamine and cocaine) was examined. Also, measurement of the concentration of brain monoamines - norepinephrine, dopamine, and serotonin - as well as estimation of their metabolism by measurement of the major extracellular metabolites of these monoamines was carried out to examine possible relationships of brain chemistry to the behavioral characteristics shown by these lines, as well as to their propensity for drug usage. The lines of rats are: Swim Low-active (SwLo) and Swim High-active (SwHi), which show either very low (SwLo) or very high (SwHi) amounts of motor activity in a swim test; Swim-test Susceptible (Susceptible or SUS) and Swim-test Resistant (Resistant or RES), which are highly susceptible (SUS) or highly resistant (RES) to having their swim-test activity depressed by being exposed to a stressful condition prior to the swim test; and Hyperactive (HYPER), which show spontaneous nocturnal hyperactivity compared to non-selectively bred (i.e., normal) rats as well as both extreme hyperactivity and behavioral depression after being exposed to a stressful condition. Regarding alcohol and drug usage, SUS rats readily consume alcohol while all other lines including non-selected, normal rats do not, and SwLo rats show a strong tendency to consume amphetamine and cocaine. Marked differences in brain monoamines were found between the various lines and normal rats, with salient differences seen in norepinephrine, particularly in the hippocampus, and in dopamine in forebrain regions (striatum and nucleus accumbens).

Involvement of noradrenergic and corticoid receptors in the consolidation of the lasting anxiogenic effects of predator stress

The roles of beta-NER (beta-noradrenergic receptor), GR (glucocorticoid) and mineral corticoid receptors (MR) in the consolidation of anxiogenic effects of predator stress were studied. One minute after predator stress, different groups of rats were injected (ip) with vehicle, propranolol (beta-NER blocker, 5 and 10 mg/kg), mifepristone (RU486, GR blocker, 20 mg/kg), spironolactone (MR blocker, 50 mg/kg), propranolol (5 mg/kg) plus RU486 (20 mg/kg) or the anxiolytic, chloradiazepoxide (CPZ, 10 mg/kg). One week later, rodent anxiety was assessed in elevated plus maze, hole board, light/dark box, social interaction and acoustic startle. Considering all tests except startle, propranolol dose dependently blocked consolidation of lasting anxiogenic effects of predator stress in all tests. GR receptor block alone was ineffective. However, GR block in combination with an ineffective dose of propranolol did blocked consolidation of predator stress effects in all tests, suggesting a synergism between beta-NER and GR. Surprisingly, MR block prevented consolidation of anxiogenic effects in all tests except the light/dark box. CPZ post stress was ineffective against the anxiogenic impact of predator stress. Study of startle was complicated by the fact that anxiogenic effects of stress on startle amplitude manifested as both an increase and a decrease in startle amplitude. Suppression of startle occurred in stressed plus vehicle injected groups handled three times prior to predator stress. In contrast, stressed plus vehicle rats handled five times prior to predator stress showed increases in startle, as did all predator stressed only groups. Mechanisms of consolidation of the different startle responses appear to differ. CPZ post stress blocked startle suppression but not enhancement of startle. Propranolol post stress had no effect on either suppression or enhancement of startle. GR block alone post stress prevented suppression of startle, but not enhancement. In contrast blocking GR and beta-NER together prevented startle enhancement. MR block also prevented startle enhancement. Effects of MR block on startle suppression were not tested. Delay of habituation to startle was found in all stressed rats. Consolidation of delay of habituation was blocked or attenuated by post stress MR block, GR plus beta-NER block and CPZ but not by post stress GR or beta-NER block alone. Taken together, present findings suggest consolidation of lasting anxiogenic effects of predator stress may share some of the same neurochemical mechanisms implicated in some forms of fear memory consolidation. Implications of these findings for the study of stress-induced changes in affect including posttraumatic stress disorder (PTSD) are discussed.

Differential noradrenergic influence on seizure expression in genetically Fast and Slow kindling rat strains during massed trial stimulation of the amygdala

The involvement of alpha(2) noradrenergic receptors during amygdala 'massed' stimulation (MS) was examined in rats that were selectively bred to be seizure-prone (Fast) or seizure-resistant (Slow) to amygdala kindling. The selective alpha(2) noradrenergic agonist guanfacine, or the antagonist idazoxan, was intraperitoneally injected during the MS procedure to study subsequent changes in afterdischarge (AD) threshold, AD duration and behavioral seizure expression. These measurements were again assessed weekly for 2 weeks after the MS treatment. Daily kindling began immediately thereafter. Following 6 stage-5 once daily convulsive seizures, guanfacine or idazoxan were re-administered. With idazoxan, the Slow rats expressed greater numbers of convulsive seizures and longer AD durations compared to guanfacine or saline controls during MS treatment. This pro-convulsive property of idazoxan was absent in Fast rats. By contrast, Fast rats showed enhanced convulsive expression in the presence of guanfacine. In the fully kindled rat, idazoxan and guanfacine differentially impacted seizure duration and severity in the Slow rats, but again not in the Fast rats. These data suggest that some aspect(s) of the alpha(2) noradrenergic system in the Fast and Slow rats are dissimilar and the mechanisms by which these receptors govern seizure genesis and propagation may be genetically controlled and distinct.

The brain 5HTergic response to an acute sound stress in rats with generalized (absence and audiogenic) epilepsy

The brain serotoninergic (5HTergic) system of epileptic subjects can influence their vulnerability to stress. We studied the putative dependency of 5HT neurotransmission parameters on emotional stress, and the presence, types and severity of seizures using rats with genetic generalized (absence and/or audiogenic) epilepsy, of WAG/Rij and Wistar strains. The animals were stressed by exposure to a short aversive noise or left without sound stimulation. Tissue concentrations of 5HT, tryptophan (TRT) and 5-hydroxyindolacetic acid (5HIAA) were assessed by HPLC. The stressor activated the 5HTergic system within thalamus (5HIAA elevated), frontal cortex (5HT, TRT elevated), hypothalamus (increased TRT) in all rats. However, the normal (non-epileptic) rats displayed the highest response in the frontal cortex and the lowest one in the thalamus, as compared to the epileptic rats. Absence-epileptic rats exhibited higher thalamic 5HIAA increase than their controls. Significant correlations existed between propensity of absence epilepsy and 5HTergic parameters measured in the cortex and hypothalamus of absence-epileptic rats. No major difference was found between groups with and without audiogenic epilepsy. The results imply that the stress response depends on the presence of epileptic pathology and the seizure type and severity. The brain 5HT may be involved in the control of the paroxysms and behaviour in absence-epileptic subjects.

Non-associative defensive responses of rats to ferret odor

Predators and their odors offer an ethologically valid model to study learning processes. The present series of experiments assessed the ability of ferret odor to serve as an unconditioned stimulus and examined behavioral and endocrine changes in male Sprague-Dawley rats with single or repeated exposures in a defensive withdrawal paradigm or in their home cages. Rats exposed to ferret odor avoided the ferret odor stimulus more, exhibited greater risk assessment and displayed higher adrenocorticotropin hormone (ACTH) and corticosterone release compared with control odor exposed rats and these measures did not significantly habituate over repeated exposures. Ferret odor exposure did not show associative conditioning effects during extinction trials. However, rats that were pre-exposed to ferret odor only once, as compared to control and repeatedly exposed rats, displayed a sensitized ACTH and corticosterone response to an additional ferret odor exposure in small cages. These experiments suggest that ferret odor is a highly potent unconditioned stimulus that has long lasting effects on behavior and endocrine responses, and further suggests the independence of habituation and sensitization processes.

Play fighting between kindling-prone (fast) and kindling-resistant (slow) rats

Differences in the play behavior of 2 strains of rats suggest that different components of play fighting can be modified independently. The development of play fighting in cross-strain pairs of familiar and unfamiliar rats was examined to determine whether interacting with a non-congruent pair-mate would alter the pattern of play typical for each strain. In both strains, changes in play fighting were observed throughout development, but partner identity appeared to influence play fighting in different ways depending on age. These data suggest that some components of play may be more impervious to changes in social environment than other components.

Relationship of the predatory attack experience to neural plasticity, pCREB expression and neuroendocrine response

Aggression takes at least two, an attacker and a target. This paper will address the lasting consequences of being a target of aggression. We review the lasting impact of predatory attack on brain and behavior in rodents. A single brief unprotected exposure of a rat to a cat lastingly alters affective responses of rats in a variety of contexts. Alterations of these behaviors resembles both generalized anxiety comorbid with post traumatic stress disorder (PTSD), and the hyper arousal expressed in enhanced startle in PTSD. Examination of neural transmission and neural plasticity in limbic circuits implicates changes in transmission in two connecting pathways in many but not all of the behavioral changes. Quantification of the predator encounter reveals that both the behavior of the predator and the reaction of the rat to attack are highly predictive of the effects of predatory attack on molecular biological (pCREB expression) and electrophysiological measures of limbic neuroplastic change. Moreover, a case will be made that the pattern of change of corticosteroid level over three hours after the predator encounter, in interaction with the predatory experience, plays an important part in initiation of lasting changes in brain and behavior.

Stress-induced changes in spatial memory are sexually differentiated and vary across the lifespan

Stress exposure, depending on intensity and duration, elicits adaptive or maladaptive physiological changes. The same general pattern of advantageous versus deleterious stress effects appears to exist for some cognitive functions, particularly spatial learning and memory performance. This article reviews sex differences in response to stress on a variety of spatial tasks. In general, females are more resistant than males to stress-induced impairments on spatial tasks, including the radial arm maze and object placement. In young adulthood, chronic stress (restraint, 6 h per day for 21 days) impairs male performance on both tasks but leads to behavioural enhancements in females. Furthermore, these sex-dependent stress effects are influenced by both organisational and activational oestrogenic effects. Additionally, sex-specific stress responses vary depending on developmental age at the time of stress exposure. Male behavioural stress responses appear fixed across the lifespan (i.e. stress-induced cognitive impairments) whereas female stress responses appear more variable (i.e. stress-induced enhancements observed in young adulthood are different in response to prenatal stress and diminished following stress exposure at old age). These findings underscore the point that many effects obtained in males cannot be generalised to females and highlight the need to investigate the stress response at different ages and in both sexes.

Role of noradrenergic projections to the bed nucleus of the stria terminalis in the regulation of the hypothalamic-pituitary-adrenal axis

The bed nucleus of the stria terminalis (BNST) plays an important role in the regulation of the hypothalamic-pituitary-adrenal (HPA) axis during stress and it is a major extrahypothalamic relay to the paraventricular nucleus of the hypothalamus (PVN) from the amygdala and the hippocampus. In this review, we discuss the anatomical, neurochemical and behavioral evidence that substantiate a role for noradrenergic terminals of the anterior BNST in the regulation of the HPA axis. We propose the hypothesis that BNST noradrenaline (NA) participates in the regulation of the hippocampal inhibitory influence on the HPA axis activation. The observation that NA exerts a tonic inhibitory effect upon glutamatergic transmission in the anterior BNST supports this hypothesis. We also discuss the known mechanisms involved in the regulation of BNST NA extracellular levels and the possible interactions between NA and corticotropin-releasing hormone (CRH), and of CRH with glutamate (GLU) in the regulation of the HPA axis activity exerted by the BNST. The evidence discussed in the present review situates the BNST as a key extrahypothalamic center that relays and integrates limbic and autonomic information related to stress responses suggesting that dysregulation in the functioning of the BNST may underlie the pathophysiology of stress-related psychiatric disorders.

Anxiety levels and wild running susceptibility in rats: assessment with elevated plus maze test and predator odor exposure

It is reported in the literature that nearly 20% of rats are susceptible to displays of wild running (WR) behavior when submitted to high intensity acoustic stimulation. Some characteristics of WR suggest that it can be viewed as a panic-like reaction. This work aimed to test whether WR-sensitive rats show higher levels of anxiety in elevated-plus-maze (EPM) and predator-odor exposure paradigms in comparison with WR-resistant ones. Male adult Wistar rats were submitted to two trials of acoustic stimulation (104 dB, 60 s) in order to assess WR susceptibility. Seven WR-sensitive and 15 WR-resistant rats were evaluated by the EPM test. Other 13 WR-sensitive and 18 WR-resistant animals were submitted to the predator-odor exposure test which consisted of a 10 min-session of free exploration in a specific apparatus containing two odoriferous stimuli: cotton swab imbedded with snake cloacal gland secretion or with iguana feces (control). WR-sensitive rats presented a significantly higher closed-to open-arm-entry ratio in the EPM test. All rats responded with anxiety-like behaviors to the predator odor exposure, although the WR-sensitive ones showed a marked behavioral inhibition regardless of the odor condition. We conclude that WR-sensitive rats present elevated levels of anxiety manifested by means of passive behavioral strategies.

Ferret odor as a processive stress model in rats: neurochemical, behavioral, and endocrine evidence

Predator odors have been shown to elicit stress responses in rats. The present studies assessed the use of domestic ferret odor as a processive stress model. Plasma corticosterone and adrenocorticotropin hormone levels were higher after 30 min of exposure to ferret odor (fur/skin) but not control odors, ferret feces, urine, or anal gland secretions. Behavioral differences were also found between ferret and the control odors as tested in a defensive withdrawal paradigm. In addition, c-fos messenger RNA expression in several brain areas previously associated with processive stress was significantly higher in ferret odor-exposed rat brains than in control odor-exposed brains. These results suggest that ferret odor produces a reliable unconditioned stress response and may be useful as a processive stress model.

Avoidance behavior and brain monoamines in fish

The crucian carp performs a typical avoidance behavior when exposed to olfactory cues from injured skin of conspecifics. They swim rapidly to the bottom and hide in available material. This work examines the effects of skin extract exposure and availability of hiding material on this behavior, and concomitant changes in brain monoaminergic activity in crucian carp. Individual fish were exposed to skin extract in aquaria with or without hiding material. Exposure to skin extract resulted in the expected avoidance behavior consisting of rapid movement towards the bottom of the aquarium. This lasted for 1-2 min. Activity then decreased below the level observed before exposure, suggesting a "freezing" type of avoidance behavior. This behavior was independent of availability of hiding material. Brain dopaminergic activity increased in telencephalon and decreased in the brain stem following skin extract exposure, again independent of availability of hiding material. However, fish kept in aquaria without hiding material showed an elevation of serotonergic activity in the brain stem and the optic tectum compared to fish with available hiding material. Absence of hiding material increased serotonergic activity also without exposure to skin extract. In aquaria with hiding material, the fish stirred up a cloud of fine sediments and showed a more pronounced decrease in locomotor activity in agreement with this being a more efficient freezing or immobile avoidance behavior. These results show that basic components of avoidance behavior and related brain changes are present in the fish brain, in accordance with the common phylogenetic roots of avoidance behavior in all vertebrates.

Pavlovian autoshaping procedures increase plasma corticosterone and levels of norepinephrine and serotonin in prefrontal cortex in rats

Pavlovian autoshaping procedures provide for pairings of a small object conditioned stimulus (CS) with a rewarding substance unconditioned stimulus (US), resulting in the acquisition of complex sequences of CS-directed skeletal-motor responses or autoshaping conditioned responses (CRs). Autoshaping procedures induce higher post-session levels of corticosterone than in controls receiving CS and US randomly, and the enhanced post-session corticosterone levels have been attributed to the appetitive or arousal-inducing effects of autoshaping procedures. Enhanced corticosterone release can be induced by aversive stimulation or stressful situations, where it is often accompanied by higher levels of norepinephrine (NE) and serotonin (5-HT) in prefrontal cortex (PFC) but not in striatum (ST). Effects of autoshaping procedures on post-session corticosterone levels, NE contents in PFC, and 5-HT contents in PFC and ST were investigated in male Long-Evans rats. Post-session blood samples revealed higher corticosterone levels in the CS-US Paired group (n = 46) than in the CS-US Random control group (n = 21), and brain samples revealed higher levels of PFC NE and 5-HT in CS-US Paired group. Striatal 5-HT levels were unaltered by the autoshaping procedures. Autoshaping procedures provide for appetitive stimulation and induce an arousal-like state, as well as simultaneous stress-like changes in plasma corticosterone and monoamine levels in PFC. Autoshaping, therefore, may be useful for the study of endocrine and central processes associated with appetitive conditions.

Working and Reference Memory in Seizure-Prone and Seizure-Resistant Rats: Impact of Amygdala Kindling

In rat selectively bred for different amygdala kindling rates (Fast vs. Slow), comorbid differences in learning were detected. Here, performance was tested in a delayed alternation task before, during, and after kindling. Although similar reference memory was evident, Fast rats showed working memory deficits with increasing delays between information and choice trials. Further, seizures shortly before learning disrupted both reference and working memory in Fast, but not Slow, rats. Weeks after kindling, progressive delays further disrupted Fast rats, but only longer delays disrupted Slow rats. Clearly relevant to individual differences in human epilepsy, a temporal lobe, seizure-prone genetic background in rats provides poorer original learning and easier disruption of new learning by recent and past seizures than a seizure-resistant background.

Chronic cold stress regulates ascending noradrenergic pathways

Chronic exposure to cold in rats alters the activity of locus coeruleus (LC) neurons. In this study we aimed to examine the cellular effect of cold stress on catecholamine neurons, and determine whether this is specific to the LC compared with other catecholamine cell groups. Male Sprague-Dawley rats were subjected to 21 days of isolation under ambient conditions, chronic cold exposure at 5 degrees C, or after chronic cold followed by return to ambient temperature for 7 or 14 days. In the LC, chronic cold exposure significantly reduced tyrosine hydroxylase (TH) mRNA expression by approximately 45% compared with control rats, and remained significantly reduced (approximately 36%) after return to ambient conditions for 7 days; however, expression returned to normal after 14 days' recovery. There were no significant changes in TH mRNA in the substantia nigra or ventral tegmental area. Chronic cold increased expression of alpha2A adrenoceptor mRNA in the LC (approximately 27%). There were decreases in alpha2A expression in the nucleus tractus solitarius; however, this was seen only in rats returned to ambient conditions for 7 days. Additionally, alpha2A mRNA in the caudal ventrolateral medulla (A1 region) increased following cold exposure (approximately 84%) compared with controls. Binding of [125I]iodoclonidine to alpha2-like protein increased in the olfactory bulbs but decreased in the medial amygdala following cold exposure. Collectively, these data indicate robust effects of cold on central catecholamine neurons, not necessarily specific to the LC.

Differential stress reactivity in intact and ovariectomized prepubertal and adult female rats

The pubertal development of the hypothalamic-pituitary-adrenal (HPA) axis has received relatively little experimental attention. As puberty is marked by an increase in the susceptibility to various psychiatric disorders that may be related to HPA dysfunction, it is imperative to elucidate the pubertal development of this neuroendocrine axis. To date, the limited research in this area has been conducted primarily on males. Presently, we investigated stress responsiveness, as measured by both stress hormones (e.g., corticotropin (ACTH) and corticosterone) and gonadal steroids, in intact and ovariectomized prepubertal and adult female rats before and after a 30-min session of restraint stress. We report here that intact prepubertal females exhibit an extended corticosterone stress response (30-45 min longer) compared to intact adults. Moreover, ovariectomized prepubertal females continue to exhibit a prolonged stress-induced corticosterone and progesterone response compared to ovariectomized adults, indicating these protracted responses prior to puberty are independent of ovarian hormones. ACTH levels were not significantly different between intact and ovariectomized prepubertal and adult animals at all the post-stress time points measured, suggesting that the prolonged corticosterone response in prepubertal females is due to an enhanced sensitivity to ACTH at the level of the adrenal cortex. Taken together, these data indicate that stress reactivity changes dramatically during puberty in females. Furthermore, these data demonstrate additional development of the HPA axis during pubertal maturation, resulting in a more quickly terminated stress response in adulthood.

Anxiety in rats selectively bred for Fast and Slow kindling rates: situation-specific outcomes

Rats selectively bred for amygdala excitability, realized by fast or slow kindling epileptogenesis, were previously reported to exhibit differential levels of anxiety. Although the Slow kindling rats generally appeared more anxious in several behavioral tests, under certain test conditions the Fast kindling rats displayed greater anxiety or stressor reactivity. The present investigation confirmed that in a test of anxiety comprising suppression of consumption of a palatable snack in an unfamiliar environment, the Slow kindling rats exhibited greater anxiety and that this effect was attenuated by diazepam. Likewise, the acoustic startle response was greater in the Slow kindling rats. However, the fear-potentiated startle response was more pronounced in Fast kindling rats, particularly among females, irrespective of whether the test parameters elicited moderate or high startle amplitudes. The elevated startle in the Slow rats, and the fear potentiated startle in the Fast rats, were attenuated by diazepam. These data indicate the need to differentiate subtypes of anxiety in animal models, and raise the issue that anxiety elicited by specific environmental triggers may interact with genetically determined trait characteristics.

Differential involvement of amygdaloid CRH system(s) in the salience and valence of the stimuli

Anxiety is a heterogeneous term encompassing not only state or trait characteristics but also a wide range of pathologies such as generalized anxiety disorders, phobias, panic and obsessive-compulsive disorders, acute stress disorder, and posttraumatic stress disorder. Given that diverse forms of anxiety exist, numerous animal models have been developed, which are considered to be useful in identifying mechanisms underlying anxiety states. Examples of such animal models include paradigms that assess the behavioral response to neurogenic (or painful stimuli) or psychogenic stressors or to cues that had previously been associated with painful stimuli. The present report presents data regarding the impact of stressors on corticotropin-releasing hormone (CRH), and relates these to changes in anxiety-like states. Specifically, we demonstrate that (1) psychogenic stressors influence the in vivo release of CRH at the central nucleus of the amygdala (CeA); (2) although CRH changes within the CeA are exquisitely sensitive to stressors, they are also elicited by positive stimuli; and (3) while treatment with diazepam attenuates behavioral signs of anxiety, the CRH release associated with a stressor is unaffected by the treatment. The position is offered that although release of CRH within the CeA is increased under stressful conditions, it is not a necessary condition for the consequent behavioral expression of anxiety-like reactions, at least not in minimally threatening situations. We suggest that the CRH responses at the CeA may be involved in a preparatory capacity and, as such, may accompany a range of emotionally significant stimuli, be they appetitive or aversive.

Chronic stress effects on memory: sex differences in performance and monoaminergic activity

Increasing evidence suggests that the time course of advantageous versus deleterious effects of stress on physiologic function is also apparent in some brain functions, including learning and memory. This article reviews the effects of chronic stress on behavioral performance and, more importantly, shows that sex of the subject, as well as duration and intensity of stress, is an important determinant of the functional/behavioral, neurochemical, and anatomical consequences of the stress. Following chronic stress (7-28 days of restraint, 6 h/day), male and female rats were tested on a visual memory task (object recognition) and two spatial memory tasks (object placement and radial arm maze). At 21 days, stress impaired males on all tasks while females were either enhanced (spatial memory tasks) or not impaired (nonspatial memory tasks). Additionally, the influence of the hypothalamic-pituitary-adrenocortical axis in mediating the sex-specific responses to stress is considered. Behavioral and neurochemical assessments following chronic stress in ovariectomized females, with and without estradiol, suggest that estrogen exerts both organizational and activational influences on the observed sex differences in response to stress. Furthermore, stress differentially affected central transmitter levels in the frontal cortex, hippocampus, and amygdala depending on sex. The possible role of these sex-specific changes in neurotransmitter levels in mediating behavioral differences in response to stress is discussed. While these results are thus far limited to a few studies and require both further investigation and verification, chronic stress appears to be associated with distinct, sex-differentiated behavioral/cognitive and neurochemical responses. We conclude that sex differences must be taken into account when investigating or describing stress and associated sequalae.

Acute and chronic restraint stress alter the incidence of social conflict in male rats

Stress and elevated stress hormone levels are known to alter cognition, learning, memory, and emotional responses. Three weeks of chronic stress or glucocorticoid exposure is reported to alter neuronal morphology in the hippocampus, the amygdala, and the prefrontal cortex, and to decrease neurogenesis in the dentate gyrus. Here we examine the effects of acute and chronic restraint stress exposure on the incidence of emotional responses throughout a 3-week period among adult rat conspecifics. Our data indicate that acute restraint stress (i.e., a single 6-h exposure) results in a significant reduction in aggressive conflicts among stressed males compared to experimental controls. In contrast, on Days 14 and 21, repeatedly restrained rats exhibited significantly more aggressive behaviors than controls. Blood samples taken 18 h after the last restraint session indicate that plasma concentrations of the stress hormone corticosterone (CORT) in stressed rats were equivalent to those of unstressed rats; however, the number of individually initiated aggressive acts observed positively correlated with plasma CORT measures taken at the end of the study. In contrast to studies of psychosocial stress or intruder paradigms, here we observe spontaneous emotional responses to an uncontrollable stressor in the homecage. This study provides a novel examination of the effects of chronic restraint stress on emotional responses in the home environment among cagemates. These results indicate that acute and chronic restraint stress alter the incidence of aggression, and emphasize the relevance of this model of chronic stress to studies of stress-responsive disorders characterized by aggressive behavior.

The effects of chronic mild stress on male Sprague-Dawley and Long Evans rats: I. Biochemical and physiological analyses

The chronic unpredictable mild stress (CMS) is a paradigm developed in animals to model the relatively minor and unanticipated irritants that lead to a state of anhedonia in some individuals. However, the effectiveness of CMS is sometimes difficult to establish, for which unique strain sensitivities has been attributed as one contributing factor. These considerations led us to design the present study, which was an investigation of the corticosterone response to CMS in two outbred rat strains--Sprague-Dawley and Long Evans. Animals were exposed to one of two conditions--control or CMS--for 3 weeks during which body weight and fecal count were regularly monitored. At the end of this period, blood was sampled at a variety of time intervals following induction of a brief restraint stressor. First, a significant effect of CMS on corticosterone levels was evident at time 0 (prior to the application of the acute restraint stressor) in both strains. Second, the typical quadratic pattern of stressor-elicited fluctuations in this measure was similar in both Sprague-Dawley and Long Evans rats, with consistently elevated levels for the first hour following exposure to the acute stressor; near baseline values were observed at 2 h. However, only in the Long Evans strain were CMS related values much less than that observed in the control group after restraint stress. Third, both strains showed a reduced weight gain in the CMS groups relative to control groups. Fourth, spleen and adrenal weights were similar across all groups. Fifth, fecal counts remained stable across weeks of treatment in all groups with the exception of the Long Evans rats exposed to CMS; in this group, average counts were systematically reduced over the treatment period. We conclude that a history of chronic stress significantly blunts corticosterone levels in Long Evans but not Sprague-Dawley rats following exposure to an acute stressor. Physiological indices however are less influenced by this experience, at least when the exposure is limited to 3 weeks.

Conceptual, spatial, and cue learning in the Morris water maze in fast or slow kindling rats: attention deficit comorbidity

Rat lines selectively bred for differences in amygdala excitability, manifested by "fast" or "slow" kindling epileptogenesis, display several comorbid features related to anxiety and learning. To assess the nature of the learning deficits in fast kindling rats, performance was evaluated in several variants of a Morris water-maze test. Regardless of whether the location of the platform was fixed or varied over days (matching-to-place task), the fast rats displayed inferior performance, suggesting both working and reference memory impairments. Furthermore, when the position of the platform was altered after the response was acquired, fast rats were more persistent in emitting the previously acquired response. The poor performance of fast rats was also evident in both cued and uncued tasks, indicating that their disturbed learning was not simply a reflection of a spatial deficit. Moreover, fast rats could be easily distracted by irrelevant cues, suggesting that these animals suffered from an attentional disturbance. Interestingly, when rats received several training trials with the platform elevated, permitting them to develop the concept of facile escape, the performance of fast rats improved greatly. The performance disturbance in fast rats may reflect difficulties in forming a conceptual framework under conditions involving some degree of ambiguity, as well as greater distractibility by irrelevant cues. These various attributes of the fast rats may serve as a potentially useful animal model of disorders characterized by an attention deficit.

Sensitization to the neuroendocrine, central monoamine and behavioural effects of murine tumor necrosis factor-alpha: peripheral and central mechanisms

Systemic administration of murine tumour necrosis factor-alpha (mTNF-alpha; 0.1-2.0 microg, i.p.) dose-dependently increased plasma corticosterone and augmented monoamine utilization within the paraventricular nucleus of the hypothalamus (PVN), locus coeruleus, medial prefrontal cortex (PFC), central and medial amygdala. A time-dependent sensitization was induced in mice, wherein reexposure to mTNF-alpha 28 days (but not 1 day) following the initial cytokine treatment provoked marked signs of illness (diminished activity, ptosis, piloerection) and increased plasma corticosterone levels. Serotonin (5-HT) activity was augmented upon mTNF-alpha reexposure at the 1- or 28-day intervals in the PFC and medial amygdala, respectively. Intracerebroventricular (i.c.v.; 1-500 ng) mTNF-alpha did not promote illness, but modestly increased plasma corticosterone levels. Neither the illness nor the corticosterone changes were subject to a sensitization upon i.c.v. cytokine reexposure. Acute i.c.v. mTNF-alpha increased norepinephrine (NE), 5-HT and dopamine (DA) activity within the PVN and median eminence/arcuate nucleus complex (ME/ARC), and NE utilization within the central amygdala. Subsequent i.c.v. mTNF-alpha further enhanced the hypothalamic monoamine variations. Finally, systemic (i.p.) mTNF-alpha pretreatment did not proactively influence sickness or corticosterone responses upon later i.c.v. cytokine challenge, but augmented locus coeruleus NE activity and 5-HT and DA utilization within the ME/ARC. It is suggested that the sensitization with respect to sickness and corticosterone activity in response to mTNF-alpha reflect the involvement of peripheral mechanisms. Moreover, it appears that mTNF-alpha promotes central neurochemical plasticity through independent central and peripheral mechanisms.

Enhancement of delayed release of dopamine in the amygdala induced by conditioned fear stress in methamphetamine-sensitized rats

Behavior during conditioned fear stress, a form of psychological stress, and the release of dopamine in the amygdala were measured over time using methamphetamine-sensitized rats, which are considered to be a model of hypersensitivity and vulnerability to emotional stress associated with stimulant-induced psychosis and schizophrenia. Dopamine release in the amygdala showed a delayed increase following completion of freezing behavior induced by conditioned fear stress regardless of the presence or absence of methamphetamine-sensitization. Since methamphetamine treatment did not lower the basal level of dopamine in the amygdala, under the conditions of this study, methamphetamine was presumed not to show neurotoxicity. On the other hand, basal dopamine levels after 15 h of repeated electric foot shock were about 40% lower than those in the control group (p<0.0002). In addition, dopamine release following conditioned fear stress in animals repeatedly treated with methamphetamine increased significantly from 40 to 100 min after conditioned fear stress while the duration of freezing behavior or latency of the appearance of grooming were not different from those in the control group. The above results suggested that delayed dopamine release in the amygdala is a phenomenon strongly associated with the emotional context of conditioned fear stress, and hypersensitivity and vulnerability to stress are at least partially involved with the overreaction to stress.

Medullary neurones regulate hypothalamic corticotropin-releasing factor cell responses to an emotional stressor

Hypothalamic-pituitary-adrenal axis activation is a hallmark of the stress response. In the case of physical stressors, there is considerable evidence that medullary catecholamine neurones are critical to the activation of the paraventricular nucleus corticotropin-releasing factor cells that constitute the apex of the hypothalamic-pituitary-adrenal axis. In contrast, it has been thought that hypothalamic-pituitary-adrenal axis responses to emotional stressors do not involve brainstem neurones. To investigate this issue we have mapped patterns of restraint-induced neuronal c-fos expression in intact animals and in animals prepared with either paraventricular nucleus-directed injections of a retrograde tracer, lesions of paraventricular nucleus catecholamine terminals, or lesions of the medulla corresponding to the A1 or A2 noradrenergic cell groups. Restraint-induced patterns of neuronal activation within the medulla of intact animals were very similar to those previously reported in response to physical stressors, including the fact that most stressor-responsive, paraventricular nucleus-projecting cells were certainly catecholaminergic and probably noradrenergic. Despite this, the destruction of paraventricular nucleus catecholamine terminals with 6-hydroxydopamine did not alter corticotropin-releasing factor cell responses to restraint. However, animals with ibotenic acid lesions encompassing either the A1 or A2 noradrenergic cell groups displayed significantly suppressed corticotropin-releasing factor cell responses to restraint. Notably, these medullary lesions also suppressed neuronal responses in the medial amygdala, an area that is now considered critical to hypothalamic-pituitary-adrenal axis responses to emotional stressors and that is also known to display a significant increase in noradrenaline turnover during restraint. We conclude that medullary neurones influence corticotropin-releasing factor cell responses to emotional stressors via a multisynaptic pathway that may involve a noradrenergic input to the medial amygdala. These results overturn the idea that hypothalamic-pituitary-adrenal axis response to emotional stressors can occur independently of the brainstem.

Differential impact of predator or immobilization stressors on central corticotropin-releasing hormone and bombesin-like peptides in Fast and Slow seizing rat

Lines of rats selectively bred for amygdala excitability, as reflected by kindling rates in response to electrical stimulation, also exhibit differences in tests of anxiety. Inasmuch as corticotropin-releasing hormone (CRH) and bombesin (BN) have been associated with anxiety, regional levels and release of these peptides, as well as plasma adrenocorticotropic hormone (ACTH) and corticosterone, were assessed in 'Slow' and 'Fast' seizing rats following predator exposure (ferret) or immobilization. Ferret exposure elicited a greater increase of plasma ACTH and corticosterone concentrations in the Slow than in the Fast rats. In contrast, immobilization provoked a greater rise of plasma ACTH levels in the Fast rats, paralleling the vigorous struggling observed in this line. In Slow rats, stressor exposure elicited increased levels of ir-BN at the anterior hypothalamus, and increased ir-CRH at the median eminence/arcuate nucleus (Me/Arc), paraventricular hypothalamic nucleus (PVN) and pituitary (Pit), whereas decreased levels of ir-BN were found at the nucleus tractus solitarius (NTS). Fast rats likewise showed decreased ir-BN at the NTS, but unlike the Slow rats, ir-CRH was reduced in the Me/Arc, PVN and Pit in response to both stressors. In vivo microdialysis experiments revealed that in response to ferret exposure, the Slow rats showed a greater CRH release at the central nucleus of the amygdala (CeA) as compared to Fast rats. However, immobilization elicited a more pronounced release of CRH in Fast than in Slow rats. Taken together, the results demonstrate that these two lines of rats show differential endocrinological and neurochemical response patterns to these stressors.