Effects Of Inescapable Shock Research Articles

Microglia serve as a neuroimmune substrate for stress-induced potentiation of CNS pro-inflammatory cytokine responses

Prior exposure to a stressor can potentiate CNS pro-inflammatory immune responses to a peripheral immune challenge. However, the neuroimmune substrate(s) mediating this effect has not been determined. The present investigation examined whether microglia serve as this neuroimmune substrate given that microglia are the primary immune effector cell in the CNS. The effect of inescapable shock (IS) on glial activation (MHC II, CD11b, Iba-1, and GFAP) and regulatory markers (CD200) in vivo, and microglia pro-inflammatory responses (interleukin-1β; IL-1β) to lipopolysaccharide (LPS) ex vivo, were assessed in rat hippocampus. IS upregulated the microglia activation marker MHC II 24 h post-IS, while the astroglia marker GFAP was unaffected. IS also downregulated the neuronal glycoprotein CD200, which functions to hold microglia in a quiescent state. Moreover, IS potentiated the pro-inflammatory response to LPS ex vivo 24 h post-IS in isolated hippocampal microglia. Finally, the behavioral controllability of shock was manipulated and the effect of escapable (controllable) shock was comparable to the effect of IS on hippocampal microglia responses to LPS ex vivo. The present results suggest that stress can activate microglia, thereby sensitizing the pro-inflammatory reactivity of microglia to immunogenic stimuli.

Learned helplessness: Validity and reliability of depressive-like states in mice

The learned helplessness paradigm is a depression model in which animals are exposed to unpredictable and uncontrollable stress, e.g. electroshocks, and subsequently develop coping deficits for aversive but escapable situations (J.B. Overmier, M.E. Seligman, Effects of inescapable shock upon subsequent escape and avoidance responding, J. Comp. Physiol. Psychol. 63 (1967) 28–33 [15]). It represents a model with good similarity to the symptoms of depression, construct, and predictive validity in rats. Despite an increased need to investigate emotional, in particular depression-like behaviors in transgenic mice, so far only a few studies have been published using the learned helplessness paradigm. One reason may be the fact that—in contrast to rats (B. Vollmayr, F.A. Henn, Learned helplessness in the rat: improvements in validity and reliability, Brain Res. Brain Res. Protoc. 8 (2001) 1–7)—there is no generally accepted learned helplessness protocol available for mice. This prompted us to develop a reliable helplessness procedure in C57BL/6N mice, to exclude possible artifacts, and to establish a protocol, which yields a consistent fraction of helpless mice following the shock exposure. Furthermore, we validated this protocol pharmacologically using the tricyclic antidepressant imipramine. Here, we present a mouse model with good face and predictive validity that can be used for transgenic, behavioral, and pharmacological studies.

Electrolytic lesions and pharmacological inhibition of the dorsal raphe nucleus prevent stressor potentiation of morphine conditioned place preference in rats.

Exposure to a single session of uncontrollable inescapable shock (IS), but not to identical controllable escapable shock, produces a potentiation of morphine's rewarding properties that is unusual in that the stressor can be given a number of days before the drug administration in an environment quite different from the drug context. Many other behavioral outcomes of stressors that depend on the uncontrollability of the stressor are mediated by alterations in serotonergic (5-HT) neurons within the dorsal raphe nucleus (DRN). The present experiments examined the role of the DRN and 5-HT in mediating the effect of IS on the rewarding properties of morphine as assessed by conditioned place preference (CPP). In experiment 1, subjects received small electrolytic lesions of the DRN and were tested for morphine (3.0 mg/kg, SC) CPP after IS or control treatment. In experiment 2, subjects received an intra-DRN microinjection of the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 1.0 microg/0.5 microl) either before IS or before morphine (3.0 mg/kg, SC) injections during CPP testing. IS potentiated morphine CPP in controls, but both DRN lesion and intra-DRN 8-OH-DPAT, either before IS or before morphine administration, completely blocked this effect. These data implicate alterations in DRN 5-HT neurons in the potentiation of morphine reward produced by uncontrollable stress.

The Role of Corticotropin-Releasing Hormone in the Dorsal Raphe Nucleus in Mediating the Behavioral Consequences of Uncontrollable Stress

Inescapable shock (IS) produces subsequent interference with escape behavior and increased fear conditioning that has been linked to increased activity and release of serotonin (5-HT) from neurons within the caudal dorsal raphe nucleus (DRN) both at the time of IS and later behavioral testing. Extrahypothalamic corticotropin-releasing hormone (CRH) has been implicated in many stress-related phenomena and has recently been shown to increase DRN 5-HT activity in the same caudal DRN area at which IS increases 5-HT activity. The current set of studies therefore examined the role of CRH in mediating the behavioral sequelae of IS. Intra-DRN microinjection of the nonselective CRH receptor antagonist d-Phe CRH (12-41) blocked the IS-induced behavioral changes when administered before IS but not when administered before later behavioral testing. Furthermore, intra-DRN administration of CRH in the absence of IS dose-dependently mimicked the effects of IS and interfered with escape behavior and increased fear conditioning 24 hr later. This effect was specific to injection of CRH into the caudal DRN and was not produced by microinjection into the rostral DRN. Intracerebroventricular CRH produced escape deficits and potentiated fear conditioning 24 hr later at only much higher doses, further confirming the site specificity of the effects. The potential role of the caudal DRN in states of anxiety is discussed.

Modulation of the locomotor properties of morphine and amphetamine by uncontrollable stress

We have recently demonstrated that exposure to a single session of inescapable shock (IS), but not to identical amounts and distributions of escapable shock (ES), increases the rewarding properties of morphine, as measured by conditioned place preference (CPP). Interestingly, we also found that exposure to IS has no effect, or even interferes with amphetamine CPP. The present study explored whether the potentiating effect of IS on morphine reward, but not amphetamine reward, would generalize to the locomotor properties of these drugs. The locomotor response to morphine and amphetamine was measured 120 h following exposure to either IS or home cage control (HCC) treatment. On test day, the activity of all subjects was measured for 1 h before and 3 h after drug administration. The results demonstrated that exposure to IS potentiated the locomotor response to morphine, while having no effect on the response to amphetamine. An additional study investigated whether the effects of IS on the locomotor properties of morphine were sensitive to stressor controllability, by comparing the influence of IS, ES, or control treatment. Again, IS potentiated the locomotor properties of morphine, while exposure to ES and control treatment had no effect.

The neurosteroid pregnanolone prevents the anxiogenic-like effect of inescapable shock in the rat.

The ability of progesterone (P4) and its neurosteroid metabolite, 3alpha-OH-5beta-pregnan-20-one (pregnanolone) in protecting against the anxiogenic-like effect of inescapable shock (IS) in male rats was examined, as these steroids exert anxiolytic, anticonvulsant, and ataxic effects similar to the benzodiazepines (BZ), drugs shown to prevent IS-induced anxiogenesis. Adult male rats were injected with pregnanolone (8 mg/kg, SC), P4 (4 mg/rat) or its appropriate vehicle before exposure to IS. Twenty-four hours later, animals were injected with the steroid or its vehicle and then tested in the elevated plus-maze. In a control experiment, animals were injected with chlordiazepoxide (CDP, 15 mg/kg, IP) or vehicle before IS, and tested in the plus-maze 24 h later. Whereas CDP or pregnanolone before IS resulted in the loss of the anxiogenic-like response seen 24 h after IS, P4 before IS did not protect against the anxiogenic-like effect of IS. The acute anxiolytic-like effect of pregnanolone and P4 was lost in animals that were injected with vehicle before the IS, but was observed in animals that were injected with the steroids before IS. Moreover, P4 injection in non-shocked animals was associated with an anxiogenic-like response observed 24 h after steroid administration. The protection against the effect of IS afforded by a GABAergic neurosteroid is comparable to that observed with BZs, and thus provides further evidence of an allosteric relationship between the neurosteroid and BZ binding site on the GABA(A) receptor complex.

Effects of inescapable shock and conditioned fear on the release of excitatory and inhibitory amino acids in the locus coeruleus

We investigated the importance of endogenous amino acids in the locus coeruleus in inescapable electric shock and conditioned fear. In naive rats and in rats exposed to noise (N), light (L) and electric shock (S) or to N + L only, the locus coeruleus was superfused with artificial cerebrospinal fluid through a push-pull cannula and the release of GABA, taurine, glutamate, aspartate, serine and glutamine was determined in the superfusate by HPLC after derivatization with o-phthaldialdehyde. Locomotor activity, arterial blood pressure and heart rate were telemetrically monitored. The placement of naive rats or conditioned rats from their home cage to a chamber provided with a grid-floor for shock virtually did not change the release rates of the amino acids in the locus coeruleus. Motility was enhanced in naive and conditioned rats to a similar extent. Blood pressure and heart rate were enhanced in conditioned rats only. Exposure to N + L + S for 5 min greatly enhanced the release rates of all determined amino acids in the locus coeruleus. In conditioned rats the increase in release of most amino acids lasted longer than in naive rats. Electric shock also enhanced motility, blood pressure and heart rate. In conditioned rats, motility and cardiovascular changes were more pronounced and/or lasted longer than in naive rats. Exposure of conditioned rats to the conditioned stimuli N + L for 5 min led to an increased release of taurine and aspartate. The enhanced release of taurine lasted 30 min. Exposure to N + L did not affect the release rates of amino acids in naive rats. N + L did not influence motility but arterial blood pressure and heart rate were elevated in conditioned rats. The findings show that inescapable electric shock enhances the release of several amino acids in the locus coeruleus, while conditioned fear selectively increases the outflow of taurine and aspartate. Moreover, conditioned fear prolongs the response of excitatory and inhibitory amino acids to electric shock. The results suggest that an excitatory amino acid (aspartate) and an inhibitory amino acid (taurine) of the locus coeruleus are implicated in conditioned fear.

Opioid-dependent effects of inescapable shock on escape behavior and conditioned fear responding are mediated by the dorsal raphe nucleus

Manipulations of the dorsal raphe nucleus (DRN) modulate the behavioral effects of exposure to inescapable shock (IS). Opiate agonists and antagonists also influence the impact of IS, but the role of the DRN in mediating these effects is unknown. The opiate antagonist naltrexone micro-injected into the region of the DRN immediately prior to IS prevented both the escape deficit and the enhancement of fear conditioning that occur 24 h later. Intra-DRN naltrexone administered at the time of later behavioral testing reduced, but did not eliminate, these effects of prior IS. Conversely, the opiate agonist morphine, in combination with a subthreshold number of 20 IS trials, induced an escape deficit and enhanced conditioned fear 24 h later. Microinjections of naltrexone into the dorsolateral periaqueductal gray area did not alter the effects of IS and electrolytic lesions of the DRN prevented the effect of the morphine-20 IS trial combination. The role of opioids in mediating the behavioral effects of IS is discussed.

Inhibition of adenosine deaminase by erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) mimics the effect of inescapable shock on escape learning in rats.

Three experiments examined the role of adenosine neuroregulation in the production of shuttle-escape deficits caused by prior exposure to inescapable electric shock in rats (learned helplessness). Intracerebroventricular administration of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), a selective adenosine deaminase inhibitor, mimicked the effect of earlier inescapable shock at a dose of 2.5 microM in previously restrained rats. Performance deficits produced by EHNA or by earlier exposure to inescapable shock were reversed by intraperitoneal injection of 10 mg/kg caffeine, an adenosine receptor antagonist. Finally, preexposure to an ineffective number of shocks interacted in synergy with an ineffective pretest dose (1.0 microM) of EHNA to maximize shuttle-escape latencies. These data implicate endogenous adenosine neuroregulation as a proximate mechanism in learned helplessness and conservation-withdrawal.

Analysis of the importance of controllable versus uncontrollable stress on subsequent behavioral and physiological functioning

Original observations of the effects of stress exposure on behavioral, physiological and pathological indices were documented in the mid 1960s [J.B. Overmier, Interference with avoidance behavior: failure to avoid traumatic shock, J. Exp. Psychol. 78 (1968) 340–343 [12]; J.B. Overmier, M.E.P. Seligman, Effects of inescapable shock upon subsequent escape and avoidance learning, J. Comp. Physiol. Psychol. 63 (1967) 28–33 [13]; M.E.P. Seligman, S.F. Maier, Failure to escape traumatic shock, J. Exp. Psychol. 74 (1967) 1–9 [15]; J.M. Weiss, Effects of coping responses on stress, J. Comp. Physiol. Psychol. 65 (1968) 251–260 [18]]. Studies employing the triadic design (e.g. escapable stress, yoked-inescapable stress and no stress) indicated that the deficits following stress exposure were not caused by stress per se, rather the uncontrollability of the stress was the critical determinant. In this paradigm, the first group (escape) receives exposure to an environmental event that it can “control” by performing a behavioral response. Stress control or coping behavior includes the ability to alter the onset, duration, intensity or pattern of an aversive experience [S.F. Maier, M.E.P. Seligman, Learned helplessness: theory and evidence, J. Exp. Psychol.: Gen. 105 (1976) 3–46 [10]]. The second group is “yoked” to its escape partner and receives the identical physical stressor as its escape counterpart, but there is no behavioral response that the yoked subject can make to alter the outcome. The third group (naive) receives no stress exposure and is either restrained in the experimental apparatus or remains in the home cage until subsequent testing. Researchers using this triadic design should be aware of the concerns of certain investigators [R.M. Church, Systematic effect of random error in the yoked control design, Psychol. Bull. 62 (1964) 122–131 [3]; E.A. Wasserman, Response bias in the yoked control procedure, Behav. Brain Sci. 11 (1988) 477–478 [17]] who have raised important issues about the validity of the yoked control design because of the possibility of systematic biases. For example, individual differences in stress reactivity may result in random error in the yoked control group. This point will be addressed further in Section 5. This procedure allows the investigator to analyze the contributions of the importance of psychological dynamics of stress on a variety of dependent measures including: behavioral, pharmacological, neurochemical and immunological indices.

Inescapable shock-induced potentiation of morphine analgesia in rats: involvement of opioid, GABAergic, and serotonergic mechanisms in the dorsal raphe nucleus.

Exposure of rats to inescapable shock (IS) potentiated the analgesic response to a low dose (1 mg/kg) of morphine 24 hr later. This effect was blocked by naltrexone (10 micrograms), diazepam (5 micrograms), or 8-hydroxy-2-(di-n-propylamine)-tetralin (8-OH-DPAT; 1 microgram) microinjected into the dorsal raphe nucleus (DRN) 15 min before IS. When microinjected into the DRN at the time of tail-flick testing, 8-OH-DPAT also effectively prevented this effect. Further, intra-DRN administration of a beta-carboline mimicked the effects of IS, because rats treated with methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (1 microgram) and simply restrained displayed potentiated morphine analgesia 24 hr later. These data suggest that this phenomenon shares mechanisms in common with other effects of IS at the level of the DRN.

Stress and adenosine: II. Adenosine analogs mimic the effect of inescapable shock on shuttle-escape performance in rats.

In 3 experiments, the authors examined the role of adenosine regulation in escape deficits produced by earlier exposure to inescapable shock in rats (learned helplessness). Adenosine analogs injected before escape testing mimicked the effect of earlier inescapable shock, with the magnitude of the deficit varying with dose and drug specificity for A2 adenosine receptors. Agonist-induced and stress-induced escape deficits were eliminated by pretest treatment with the centrally acting adenosine receptor antagonist theophylline but not the peripheral antagonist 8-[p-sulfophenyl]-theophylline. Finally, preexposure to an ineffective number of inescapable shocks interacted in synergy with an ineffective pretest injection of adenosine agonist to maximize deficits in escape performance. These data implicate energy regulation and a central compensatory action by adenosine in the aspects of helplessness related to conservation-withdrawal.

Sex differences in the behavioral consequences of inescapable footshocks depend on time since shock

In two experiments, the effects of inescapable shock on subsequent shuttle-box escape performance were studied in male and female rats. Effects of treatment with short-duration shocks (2 s) were studied after 1- and 24-hour intervals (Experiment 1), and effects of long-duration shocks (6 s) were studied after 24- and 72-hour intervals (Experiment 2). Experience with inescapable shock resulted in a serious disruption of escape performance in both males and females. A large increment in escape latencies was found both during fixed ratio 1 and fixed ratio 2 escape training; however, effects of inescapable shock were more pronounced in males than in females. In Experiment 1, sex differences were most obvious after the short 1-hour interval whereas, in Experiment 2, sex differences were only present after 24 hours and not after 72 hours. Shuttle activity during 2-min adaptation prior to shock-escape training was reduced in both males and females treated with IS, and this effect was somewhat stronger in males than in females. The data of these experiments show that male rats are more sensitive to the consequences of exposure to inescapable aversive stimulation than female rats. It is proposed that the time-dependency of the sex differences in behavioral consequences of treatment with inescapable shock may be related to sex differences in transient neurochemical or hormonal changes induced by inescapable shock.

No spatial learning impairment following exposure to inescapable shock

Uncontrollable stressors produce changes both in hippocampal physiology and in cognitive performance. It has been suggested that some of the cognitive effects of uncontrollable shock may be mediated by disturbances in hippocampal processes. We evaluated the prediction that inescapable shock should produce a learning deficit in a task known to be particularly sensitive to disruption in hippocampal function-the Morris water-maze spatial-learning task. Exposure to a single session of inescapable shock of the type typically employed in learned helplessness experiments produced no apparent effect on performance regardless of whether shock was given 30 min or 6 h prior to water-maze training or whether animals were tested at retention intervals of 0, 1, 4, or 24 h following training. Similarly, no effect of inescapable shock was observed when shock treatment and water-maze training were distributed over three daily sessions. These results are inconsistent with the hypothesis that the effects of inescapable shock on cognitive performance result from disturbances in hippocampal function.

Sex-dependent effects of aversive stimulation on holeboard and elevated plus-maze behavior

The behavioral effects of exposure to inescapable shocks (IS) were studied in both the holeboard and the elevated plus-maze, 24 and 72 h after IS in male and female Wistar rats. The following effects were observed at the 24-h interval. In both sexes, head-dipping in the holeboard was reduced by IS, whereas general activity (ambulation and rearing) was reduced in males and not in females. Furthermore, the results of a correlation analysis indicate that previous exposure to IS disrupts the dissociation observed in control groups between exploratory activity directed at the holes (head-dipping) and general activity in the holeboard (ambulation and rearing). Effects of IS on plus-maze behavior could be observed in a clear suppression of rearing in males and not in females. IS did not affect time spent on the open arms. At the 72-h interval, IS affected head-dipping in the holeboard only in males and not in females. The present findings show that the effect of IS on specified behavioral elements is sex-dependent, with stronger and longer-lasting effects in males than in females.

Effects of signaling inescapable shock on subsequent escape learning: Implications for theories of coping and "learned helplessness."

The present experiments reveal that shuttle-escape performance deficits are eliminated when exteroceptive cues are paired with inescapable shock. Experiment 1 indicated that, as in instrumental control, a signal following inescapable shock eliminated later escape performance deficits. Subsequent experiments revealed that both forward and backward pairings between signals and inescapable shock attenuated performance deficits. However, the data also suggest that the impact of these temporal relations may be modulated by qualitative aspects of the cues because the effects of these relations depended upon whether an increase or decrease in illumination (Experiment 2) or a compound auditory cue (Experiment 4) was used. Preliminary evidence suggests that the ability of illumination cues to block escape learning deficits may be related to their to reduce contextual fear (Experiment 3). The implications of these data for conceptions of instrumental control and the role of fear in the etiology of effects of inescapable shock exposure are discussed.

The effects of inescapable shock on the retention of a previously learned response in an appetitive situation with delay of reinforcement

During Phase 1 of this experiment, all rats received a delay of (food) reward in a straight alley. During Phase 2, rats received escapable, inescapable, or no footshock in an operant chamber. The results, which supported a motivational interpretation of learned helplessness, showed that only inescapable shock reduced speed in a response previously attained (running in the straight alley; Phase 3).

Influence of p-chloroamphetamine and methysergide on the escape deficits provoked by inescapable shock.

The effects of serotonergic manipulations on the escape interference engendered by exposure to inescapable shock were assessed. Consistent with the view that the behavioral interference was related to reductions of serotonin (5-HT), treatment with the 5-HT receptor blocker methysergide mimicked the effects of inescapable shock in that it increased escape latencies. Conversely, acute treatment with a moderate dose of the 5-HT releasing agent p-chloroamphetamine (PCA) effectively antagonized the escape interference ordinarily provoked by inescapable shock. The effects of PCA were behaviorally distinguishable from the previously observed effects of catecholamine stimulants. Whereas catecholamine stimulants applied prior to either inescapable shock or escape testing eliminated the interference, PCA antagonized the behavioral disruption only if it was administered before testing. It is suggested that catecholamine alterations may be fundamental in the provocation of the interference, whereas 5-HT variations may contribute to the expression of the behavioral disturbance.

Effects of inescapable shock on maze performance as a function of age in mice.

The hypothesis was investigated that aged animals receiving an experimental treatment of inescapable shock would suffer greater deficits in swim T-maze performance than either their younger shocked counterparts or their age-matched nonshocked controls. Three age groups of male C57BL/6J mice were studied: (a) young-mature (8-9 mo); (b) middle-aged (15-16 mo); and (c) aged (26-27 mo). Equal numbers (n = 16) from each age group were assigned to either a treatment group, which received 3 days of inescapable shock (6 sec, 200 microA) during a 60-min session on a variable time schedule (VI-60 sec), or a control group, which was placed inside the apparatus for an equal amount of time. Beginning 24 hr after the last shock session, swim maze training consisted of 10 daily trials over 5 consecutive sessions. The hypothesis was partially confirmed with respect to the latency to swim to the choice-point of the T-maze. Only the aged treatment group demonstrated significantly impaired performance. With respect to the measure of learning (percent correct responses) in the alternation task, inescapable shock significantly impaired performance; however, the effect was a weak one and there was no evidence of differential impairment across age.

Ventromedial septal lesions in rats reduce the effects of inescapable shock on escape performance and analgesia.

Lesions of the ventromedial septum reduced or eliminated several effects of exposure to inescapable shock in rats, whereas lesions of the dorsolateral septum did not. Experiment 1 demonstrated that ventromedial septal lesions reduced the loss in body weight produced by inescapable shock and eliminated the subsequent (24 hr later) interference with escape performance (learned helplessness). Experiment 2 demonstrated that ventromedial septal lesions reduced the analgesia that occurs immediately following inescapable shock and the analgesia reinstated by exposure to escapable shocks 24 hr later. These findings, in conjunction with the findings that ventromedial septal lesions also reduce the secretion of corticosterone (Kelsey, 1975) and stomach erosions (Kelsey, Note 1) produced by inescapable shocks, indicate that ventromedial septal lesions reduce several responses to inescapable shock and suggest that possibility that all of these effects may reflect a unitary deficit. It is hypothesized that ventromedial septal lesions reduce these effects of exposure to inescapable shock either by reducing the ability of the rats to learn that their responses and shocks were uncorrelated or by reducing the emotional impact of this lack of correlation.