Persistent behavioural changes in rats following inescapable shock stress: a potential model of posttraumatic stress disorder.

Behavioural changes in rats two weeks after inescapable shock stress were studied using a shuttle-box task (active avoidance/escape). Rats exposed to inescapable shock stress two weeks beforehand showed more frequent avoidance responses and greater within-group variation in both avoidance response and general activity during the test than rats in a control group. General activity during an unstressful period (before starting the test) did not differ significantly between groups. This relatively prolonged increase in responsiveness to external stimuli in the stressful context may be a useful experimental model of posttraumatic stress disorder, especially in relation to hypervigilance.     

To move or not: previous experience in a runway avoidance task determines the appearance of hippocampal Type 2 sensory processing theta

Rats in a runway avoidance task responded to a test shock probe with a period of immobility lasting from 2 to 6s. The shock avoidance-trained group displayed hippocampal theta during the immobility response. The inescapable shock group, in contrast, displayed large amplitude irregular activity (LIA). Following reversal training to escapable shock, all shock avoidance-trained rats responded with LIA and inescapable shock trained rats, reversed to shock avoidance, displayed theta.     

Enhanced retrieval of unpleasant memory in helpless rats

Several studies in humans have indicated an association between enhanced retrieval of unpleasant events and depressive mood. No analogy has so far been demonstrated in animals, however. The purpose of the present study was to determine the effect of learned helplessness on the retrieval of unpleasant memory in rats. Animals initially exposed to a single unpleasant event in a passive avoidance task were subjected, respectively, to inescapable, escapable, or no shock stress exposure. A retention test conducted 48 hr following stress exposure showed an enhanced performance for the passive avoidance task in rats subjected to inescapable shock stress. This improved performance was not observed in escapable or no shock stress groups. This finding in the learned helplessness condition in rats, is qualitatively similar to that found in humans in depressive mood.     

Inhibitory avoidance learning altered ensemble activity of amygdaloid neurons in rats

In this study, we examined single-unit activity in the amygdala before and after a rat had acquired an inhibitory avoidance task. Long-Evans rats with microwires chronically implanted into the central nucleus (CeA) or basolateral complex (BLC) of the amygdala were acclimatized to the apparatus of a step-through inhibitory avoidance task for three sessions. On the fourth session, rats in the experimental group received an inescapable footshock (3 mA, 1 s) as they stepped from the lit side into the dark side of the task apparatus, whereas rats in the control group received the same amount of shock on a different apparatus. All rats were tested for retention in the task apparatus 1 day after shock training. The experimental rats showed better retention than the controls as they stayed longer in the lit side. Ensemble unit activities were recorded in the amygdala nuclei from the indwelling wire bundles during the acclimation and test sessions. The data collected from well-isolated amygdala units showed that neuronal discharge habituated from the first to the third acclimation session. In the test session, the experimental group, but not the control group, showed elevated firing rates in the CeA or BLC neurons located on either side of the brain. These findings provide the first piece of evidence showing that learning of an inhibitory avoidance task leads to an increase in amygdala neuronal discharges during a retention test.     

Changes in mu opiate receptors following inescapable shock

Rats exposed to inescapable shock exhibit profound hypoalgesia. Pharmacological evidence has suggested that changes in endogenous opiate activity may be responsible for the hypoalgesic response. We measured the binding of [3H]DAGO, a selective mu-opiate receptor agonist, in brains of rats exposed to no shock, inescapable shock, or escapable shock. Binding of [3H]DAGO in the midbrains of rats in the inescapable shock group was decreased relative to the other two groups. The decrease in binding appeared to result from a decrease in number of mu-receptors and not a change in affinity. These results support the hypothesis that inescapable shock produces long-term changes in endogenous opiate systems.     

Avoidance perseveration during extinction training in Wistar-Kyoto rats: an interaction of innate vulnerability and stressor intensity

Given that avoidance is a core feature of anxiety disorders, Wistar-Kyoto (WKY) rats may be a good model of anxiety vulnerability for their hypersensitivity to stress and trait behavioral inhibition. Here, we examined the influence of strain and shock intensity on avoidance acquisition and extinction. Accordingly, we trained WKY and Sprague-Dawley (SD) rats in lever-press avoidance using either 1.0-mA or 2.0-mA foot-shock. After extinction, neuronal activation was visualized by c-Fos for overall activity and parvalbumin immunoreactivity for gamma-aminobutyric acid (GABA) neuron in brain areas linked to anxiety (medial prefrontal cortex and amygdala). Consistent with earlier work, WKY rats acquired lever-press avoidance faster and to a greater extent than SD rats. However, the intensity of foot shock did not differentially affect acquisition. Although there were no differences during extinction in SD rats, avoidance responses of WKY rats trained with the higher foot shock perseverated during extinction compared to those WKY rats trained with lower foot shock intensity or SD rats. WKY rats trained with 2.0-mA shock exhibited less GABAergic activation in the basolateral amygdala after extinction. These findings suggest that inhibitory modulation in amygdala is important to ensure successful extinction learning. Deficits in avoidance extinction secondary to lower GABAergic activation in baslolateral amygdala may contribute to anxiety vulnerability in this animal model of inhibited temperament.     

Exposure to inescapable but not escapable shock increases extracellular levels of 5-HT in the dorsal raphe nucleus of the rat

The effects of escapable and yoked inescapable electric tailshocks on extracellular levels of serotonin (5-HT) in the dorsal raphe nucleus were measured by in vivo microdialysis. In comparison to either control rats or to their own preshock baseline, rats exposed to inescapable shock showed an increase in extracellular 5-HT within 25 min of shock initiation, and 5-HT levels continued to rise during the remainder of the shock session. Rats that were exposed to comparable shock treatment, but that were given the opportunity to escape, did not show an increase in 5-HT. Rats that were restrained but not shocked also did not show an increase in 5-HT. These results add further support to suggestions that serotonergic changes occur in the dorsal raphe nucleus during inescapable shock and that such changes may contribute to the behavioral effects of inescapable shock.     

Relationships of plasma catecholamines to open-field behavior after inescapable shock

Male rats with indwelling jugular catheters were exposed to inescapable shock or no shock, and ambulation and defecation were measured 24 h later in an open field. Plasma catecholamine levels were determined from blood samples taken before and during pretreatment as well as before and after testing for aftereffects on open-field behavior. Shocked animals showed higher plasma catecholamine levels during the shock session and lower locomotor activity in the open field. Open-field activity was negatively correlated in shocked animals with both plasma catecholamines before and during shock and also with plasma epinephrine before open-field testing. Defecation was only positively correlated with plasma norepinephrine before open-field testing. Thus, the reduced open-field activity after inescapable shock may indicate heightened fear or anxiety which may also be present when shocked animals are tested for their performance in more complex tasks.     

Blockade of alpha1 adrenoreceptors in the dorsal raphe nucleus prevents enhanced conditioned fear and impaired escape performance following uncontrollable stressor exposure in rats

Previous research has shown that the effect of exposure to uncontrollable stressors on conditioned fear responding and escape behavior in rats is dependent on serotonergic neural activity in the dorsal raphe nucleus (DRN). The role that norepinephrine released in the DRN plays in producing the behavioral consequences of exposure to inescapable tail shock in rats was investigated in the present study. The selective alpha1 adrenoreceptor antagonist benoxathian was injected into the DRN before exposure to inescapable tail shock or before behavioral testing conducted 24 h later. Benoxathian prevented the impairment of escape responding produced by inescapable shock, but did not reverse this effect when given before testing. The enhancement of conditioned fear produced by prior inescapable shock was attenuated by benoxathian administered before inescapable shock or before behavioral testing. These results support the view that noradrenergic input to the DRN is necessary to produce the behavioral effects of inescapable tail shock.     

Stress controllability and plasma prolactin levels in the rat

The effects of stress controllability on plasma prolactin levels were investigated in the rat. Animals were exposed to 60 trials of either escapable shock, inescapable shock or no shock. Rats in the escapable shock group exhibited higher levels of prolactin when compared to animals that could not control shock as well as no-shock controls. The results point to the involvement of coping factors in prolactin level changes induced by stress stimuli.     

Predator odor exposure facilitates acquisition of a leverpress avoidance response in rats

We have previously reported that prior exposure to inescapable tailshock stress increased avoidance responding 24 hours later. We argued previously that this might model the avoidance behavior characteristic of post-traumatic stress disorder (PTSD). The current experiment was conducted to determine whether a more ethologically relevant stressor would produce similar effects on avoidance responding. Therefore, rats were restrained for 2 hours and exposed to trimethylthiazoline (TMT), a component of fox feces, restrained only, or served as home cage controls. Twenty-four hours later, subjects received a 4-hour escape–avoidance session. Animals exposed to TMT made more escape responses overall, and made more avoidance responses than the other two groups by the 4th hour of the session. Differences between the TMT-exposed animals and restraint alone could not be explained by differences in corticosterone (CORT) levels. Results are discussed in terms of the possible neural changes induced by TMT exposure and the relationship to the behavioral aspects of PTSD or acute stress.     

Alterations in brain norepinephrine metabolism and behavior induced by environmental stimuli previously paired with inescapable shock

After exposure to a single session of inescapable footshock, rats show deficits in escape performance 24 h later when required to lever press on a fixed ratio (FR-3) schedule. Footshock stress produces an immediate increase in brain levels of 3-methoxy-4-hydroxyphenylglycol sulfate (MHPG-SO₄), a major metabolite of norepinephrine in rat brain. Twenty-four hours after a single or repeated session(s) of footshock stress, when levels of MHPG-SO₄ returned to baseline, increases in brain levels of MHPG-SO₄, crouching and defecation behavior were elicited in rats by neutral environmental stimuli that had been previously paired with inescapable footshock stress. These results suggest that sensitization or conditioning of noradrenergic neuronal systems may be induced by environmental stimuli previously paired with stress, and may help to explain, at least in part, the deficits in escape performance observed 24 h after exposure to inescapable shock.     

Escape deficits induced by inescapable shock and metabolic stress are reversed by adenosine receptor antagonists

We examined the relationship between metabolic stress, brain adenosine regulation, and the learned helplessness effect in four experiments in rats. Glucoprivation and metabolic inhibition were induced by treating previously restrained (nonshocked) rats with 2-deoxy-D-glucose (2DG) shortly before escape testing. Experiment 1 demonstrated that 2-deoxy-D-glucose impairs escape performance in a dose-dependent manner. Experiment 2 showed that 2-deoxy-D-glucose and shock induced escape deficits are completely reversed by peripheral administration of the adenosine receptor antagonist caffeine. This result indicates that both inescapable shock and 2-deoxy-D-glucose result in compensatory adenosine regulation which, in turn, mediates the behavioral impairment. Experiment 3 determined that 8-[p-sulfophenyl]-theophylline, a peripheral adenosine receptor antagonist, fails to reverse the escape deficit resulting from metabolic stress, whereas centrally acting theophylline does. Experiment 4 showed that the behavioral impairments from both 2-deoxy-D-glucose and inescapable shock are reversed by intracranial ventricular (icv) caffeine treatment. The results of Experiments 3 and 4 indicate that the enhanced adenosine regulation and the ensuing performance deficit resulting from 2-deoxy-D-glucose treatment occurred in the central nervous system. These data are discussed in terms of the metabolic demands of neuronal over-activation during escape testing in inescapably shocked rats and the loss of normal behavioral function due to compensatory adenosine regulation in the brain.     

Stress-induced suppression of experimental allergic encephalomyelitis in the rat.

Numerous experiments have demonstrated that physical stress can alter immunological parameters. However, little attention has been paid to the interrelationship between stress and autoimmune processes. The present study was designed to determine the influence of electric shock and sound stress on the development of experimental allergic encephalomyelitis (EAE). Ten-week-old male DA rats highly susceptible to EAE were used. Rats were subjected to the stress procedure during 19 days either before or after immunization with intradermal injection of 0.1 ml of an emulsion containing guinea pig spinal cord (20 mg/rat) in an equal volume of complete Freund's adjuvant (CFA). In addition, rats received subcutaneous injection of Bordetella pertussis in the dorsum of the same foot. Electric stress procedure consisted of 80 inescapable, unpredictable tail shocks (5 s, 1 mA) delivered at the same time each day. Sound stress procedure consisted of exposure of rats to a 90 dB fire alarm bell which rings 60 times for 5 s during one hour, at the same time of the day. Rats were observed daily for clinical signs of EAE and survived animals were sacrificed on day 20 after immunization. The brain and spinal cord sections were examined histologically for mononuclear cell infiltrates characteristics for EAE. The results clearly indicate that inescapable tail shocks suppressed the appearance and development of EAE when rats were subjected to stress procedure during 19 days after immunization, but not when rats were stressed during 19 day before the induction of EAE. On the other hand, in rats exposed to sound stress there was only delay in the onset of the disease.     

Voluntary wheel running produces resistance to inescapable stress-induced potentiation of morphine conditioned place preference

In rodents, exposure to acute inescapable, but not escapable, stress potentiates morphine conditioned place preference (CPP), an effect that is dependent upon hyperactivation of serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN). Six weeks of voluntary wheel running constrains activation of DRN 5-HT neurons during exposure to inescapable stress. Six weeks of voluntary wheel running before inescapable stress blocked stress-induced potentiation of morphine CPP.     

Stress-induced resistance to anaphylactic shock.

There have been many reports of the immunomodulatory effects of stress, but the influence of stress on anaphylaxis has been given little attention till now. In this study we investigated the influence of tail-shock stress on the course of anaphylactic shock (AS) in the rat. For this purpose, rats were sensitized to ovalbumin and subjected to stress procedure before the induction of AS. In the first series of experiments we used chronic (4 day) stress consisted of 80 inescapable tail shocks delivered at the same time each day. Anaphylactic shock was induced 24 hours later by intraperitoneal injection of 3 mg of ovalbumin. Results showed that stressed rats exhibited lower intensity of three investigated parameters of AS: clinical signs, hematocrit values, and drop of rectal temperature. In order to investigate whether acute stress procedure could also influence course of AS, rats were given various shock doses of ovalbumin immediately after the end of acute (1 day) tail-shock stress. Anti-anaphylactic effect of acute stress was demonstrated to be dose-dependent: the greatest protective effect was in animals that received the highest shocking dose of ovalbumin. Finally, we examined the duration of protective effect of acute inescapable tail shocks on AS, and these results showed that observed anti-AS phenomenon disappears 72 hours after the end of acute stress session.     

Scopolamine blocks the effects of swim stress on memory retrieval in rats

Summary This study examined whether application of swim stress improved retrieval of a passive avoidance memory and if pretreatment with the anticho-linergic agent, scopolamine, blocked this effect on memory retrieval. Animals initially given a passive avoidance training session were subjected to either a two or four swim stress sessions (15 min each) with or without prior treatment of scopolamine (0.05 or 0.1 mg/kg). The retrieval performance in passive avoidance test and motor activity was assessed 24 hr after the last swim stress session. In an independent control experiment, the passive avoidance training and test were conducted respectively, 24 and 72 hr after the last of four swim stress sessions with or without prior injection of scopolamine (0.1 mg/kg). The results showed an enhanced performance for the passive avoidance task in rats subjected to four swim stress sessions in both experiments and scopolamine given 30 min prior to each stress session diminished this performance of animals in the passive avoidance test. Two swim stress sessions with or without scopolamine treatment caused no significant effects on the retrieval performance. Also, no significant difference was observed among the groups in motor activity following any of the stress treatments in the open field test. These results, thus suggested for the first time, a relationship among swim stress, cholinergic activity and avoidance memory processes.     

Increased septal 5-HIAA efflux in rats that do not develop learned helplessness after inescapable stress

Learned helplessness is a behavioral deficit that can be induced by exposure to inescapable stress. Previous studies have implicated the lateral septum in mediating this phenomenon, and in this brain region, serotonin plays an important role in the development, maintenance, prevention, and reversal of learned helplessness behavior. Using the technique of in vivo microdialysis, we measured the efflux of serotonin (5-HT), dopamine (DA), and their respective metabolites, 5-hydroxyindoleacetic acid (5-HIAA) and 3, 4-dihydroxyphenylacetic acid (DOPAC), from the lateral septum of rats that either developed or did not develop learned helplessness. During the microdialysis session all rats were subjected to restraint stress. Control groups included na?ve, home cage rats as well as tested control rats that were subjected to the identical handling, restraint, and shuttlebox testing as the rats that received inescapable shock. Overall, levels of 5-HIAA were significantly higher in non-helpless rats. There were no significant effects of restraint or differences in levels of 5-HT, DA, or DOPAC. We propose that this increase in 5-HIAA is indicative of an overall increase in serotonin metabolism in the lateral septum of rats that do not become helpless after inescapable stress. This increased serotonin metabolism in the lateral septum may protect the animal from adverse behavioral consequences of inescapable stress. J. Neurosci. Res. 61:101-106, 2000. Published 2000 Wiley-Liss, Inc.     

Activity Wheel Running Reduces Escape Latency and Alters Brain Monoamine Levels After Footshock

We examined the effects of chronic activity wheel running on brain monoamines and latency to escape foot shock after prior exposure to uncontrollable, inescapable foot shock. Individually housed young (∼50 day) female Sprague-Dawley rats were randomly assigned to standard cages (sedentary) or cages with activity wheels. After 9–12 weeks, animals were matched in pairs on body mass. Activity wheel animals were also matched on running distance. An animal from each matched pair was randomly assigned to controllable or uncontrollable inescapable foot shock followed the next day by a foot shock escape test in a shuttle box. Brain concentrations of norepinephrine (NE), dopamine (DA), dihydroxyphenylacetic acid (DOPAC), 5-hydroxytryptamine (5-HT), and 5-hydroxyindole acetic acid (5-HIAA) were assayed in the locus coeruleus (LC), dorsal raphe (DR), central amygdala (AC), hippocampus (CA1), arcuate nucleus, paraventricular nucleus (PVN), and midbrain central gray. After prior exposure to uncontrollable foot shock, escape latency was reduced by 34% for wheel runners compared with sedentary controls. The shortened escape latency for wheel runners was associated with 61% higher NE concentrations in LC and 44% higher NE concentrations in DR compared with sedentary controls. Sedentary controls, compared with wheel runners, had 31% higher 5-HIAA concentrations in CA1 and 30% higher 5-HIAA concentrations in AC after uncontrollable foot shock and had 28% higher 5-HT and 33% higher 5-HIAA concentrations in AC averaged across both foot shock conditions. There were no group differences in monoamines in the central gray or in plasma prolactin or ACTH concentrations, despite 52% higher DA concentrations in the arcuate nucleus after uncontrollable foot shock and 50% higher DOPAC/DA and 17% higher 5-HIAA/5-HT concentrations in the PVN averaged across both foot shock conditions for sedentary compared with activity wheel animals. The present results extend understanding of the escape-deficit by indicating an attenuating role for circadian physical activity. The altered monoamine levels suggest brain regions for more direct probes of neural activity after wheel running and foot shock.