Influence of control of shock on subsequent shock-elicited aggression

Prefrontal endocannabinoids, stress controllability and resilience: A hypothesis

Stressor exposure is a predisposing risk factor for many psychiatric conditions such as PTSD and depression. However, stressors do not influence all individuals equally and in response to an identical stressor some individuals may be vulnerable while others are resilient. While various biological and behavioral factors contribute to vulnerability versus resilience, an individual's degree of control over the stressor is among the most potent. Even with only one experience with control over stress, behavioral control has been shown to have acute and long-lasting stress-mitigating effects. This suggests that control both blunts the response to acute stress and prepares the subject to be resilient to future stressors. In this review, we first summarize the evidence which suggests the ventromedial prefrontal cortex (vmPFC) is a critical component of stressor controllability circuits and a locus of neuroplasticity supporting the acute and long-lasting consequences of control. We next review the central endocannabinoid (eCB) system as a possible mediator of short and long-term synaptic transmission in the vmPFC, and offer a hypothesis whereby eCBs regulate vmPFC circuits engaged when a subject has control over stress and may contribute to the encoding of acute stress coping into long lasting stressor resilience.

Periaqueductal Grey differential modulation of Nucleus Accumbens and Basolateral Amygdala plasticity under controllable and uncontrollable stress

Resilience has been conceptualized in part as a dynamic process that includes the ability to adapt to stressful conditions. As such it encompasses the extent to which neural plasticity may be promoted. The current study examined metaplasticity by referring to the “plasticity of synaptic plasticity” in a neural circuit composed of the basolateral amygdala (BLA) and the nucleus accumbens (NAcc), using behavioural stress controllability with or without preceding stimulation of the dorsal periaqueductal gray (i.e. dPAG priming). A tendency for increased plasticity in the controllable versus the uncontrollable group was found in both the BLA and NAcc. dPAG priming suppressed NAcc LTP in all groups, but it suppressed BLA LTP only in the uncontrollable group, demonstrating dissociation between either controllable and uncontrollable groups or the NAcc and BLA. Thus, metaplasticity in the dPAG-BLA-NAcc circuit regulated differentially by controllable or uncontrollable stress may underlie stress coping, and thus contribute to stress-related psychopathologies.

Effects of Shock Controllability on Subsequent Aggressive and Defensive Behaviors in the Cockroach (Periplaneta Americana)

Male cockroaches ( Periplaneta americana) were exposed to either escapable, inescapable, or no shock in an escape task for three consecutive days. 24 hr. later they were placed individually in an aquarium with a naive cockroach and the frequency of aggressive behavior and defensive behavior was recorded by a blind observer. The inescapable shock group of cockroaches displayed less aggressive behavior and a greater tendency to retreat from social encounter than the escapable shock or no shock groups.

The role of prior stressor controllability and the dorsal raphé nucleus in sucrose preference and social exploration

Research investigating how control over stressors affects behavior often utilizes freezing and shuttle escape learning as the behavioral endpoints. These endpoints have been argued to reflect anxious or depressed states, but these descriptions are problematic. The present study sought to determine the impact of stressor controllability and the dorsal raphé nucleus (DRN) on sucrose preference and juvenile social exploration, putative measures of anhedonia and anxiety that are commonly used in studies of stress per se. In Experiment 1 rats were exposed to escapable stress (ES) or yoked-inescapable stress (IS) tailshocks. In Experiment 2 ES or IS was given 7 days before all rats received IS. In Experiment 3 the DRN was inactivated during IS by microinjection of 8-OH-DPAT. Sucrose preference and social exploration were tested for several days after stress. A fourth experiment confirmed that juvenile social exploration is sensitive to traditional beta-carboline and benzodiazepine manipulations. Both ES and IS reduced sucrose preference, but only IS reduced social exploration. Prior treatment with ES prevented the effect of IS on social exploration but did not prevent the effect of IS on sucrose preference and inactivation of the DRN prevented the effect of IS on social exploration but did not change sucrose preference. The present results indicate that social exploration but not sucrose preference is sensitive to prior stressor controllability, and that DRN activation mediates the effect of IS on social exploration. We argue that DRN-5-HT activation mediates a state of generalized anxiety produced by uncontrollable stress and that juvenile social exploration is a useful behavioral endpoint in stressor controllability studies.

Long-term neuroendocrine and behavioural effects of a single exposure to stress in adult animals

There is now considerable evidence for long-lasting sequels of stress. A single exposure to high intensity predominantly emotional stressors such as immobilisation in wooden-boards (IMO) induces long-term (days to weeks) desensitization of the hypothalamic-pituitary-adrenal (HPA) response to the same (homotypic) stressor, whereas the response to novel (heterotypic) stressors was enhanced. In addition, long-lasting changes in behaviour have been described after a single exposure to brief or more prolonged sessions of shocks, predator, predator odour, underwater stress or a combination of three stressors on 1 day. The most consistent changes are reduced entries into the open arms of the elevated plus-maze and enhanced acoustic startle response, both reflecting enhanced anxiety. However, it is unclear whether there is any relationship between the intensity of the stressors, as evaluated by the main physiological indexes of stress (e.g. HPA axis), the putative traumatic experience they represent and their long-term behavioural consequences. This is particularly critical when trying to model post-traumatic stress disorders (PTSD), which demands a great effort to validate such putative models.

Irt>t schedule controlled behavior in ‘learned-helpless’ rats: Effects from a cannabinoid agonist

Human depression is partly a congenital disorder. Aspects of the behavior accompanying depression can be magnified by genetic manipulation of bred animal species. Learned Helplessness (LH) is a trait-mark behavior that successfully breeds in rodents. Here, 'congenital' LH (cLH) rats were trained to recognize and respond to 12s long interval cues (irt>12s schedule). Rats compliant to an irt>t schedule will space responses evenly and respond rhythmically. Irt>t schedule derived data are plotted in histograms showing irt (interresponse time) frequencies. A pause response peak emerges, for outbred rats, at irt values approximating the minimum interval for reinforcement. cLH rats [n=9] complied poorly to schedule contingencies when diluent (vehicle) was injected before testing. Moderate and high dose injections of a CB 1 receptor selective agonist drug (AM 411), however, increased operant schedule compliance and normalized the cLH rats' irt>t histogram distributions. Performance indicators for cLH rats are presented alongside coordinate measures from a comparison group [n=5] of normally bred Sprague-Dawley (SD) rats. In both cLH and SD rats, treatment session histograms revealed shifts of the pause response peak not accompanied by a change in motor responsiveness. The irt>12s histogram shifts were absent when AM 411 dosages were arranged to follow pre-medication injections of a CB 1 receptor selective antagonist drug (AM 251). In short, AM 411 increased timing acuity in rats prone to behavioral despair but had opposite timing effects in normally bred SD rats.

Exposure to the stressor environment prevents the temporal dissipation of behavioral depression/learned helplessness

BACKGROUND

Exposure to uncontrollable stressors such as inescapable shock (IS) produces a set of behavioral changes such as poor escape learning that have been called behavioral depression and learned helplessness. This paradigm has been proposed to be a model of depression and of anxiety-related disorders such as posttraumatic stress disorder (PTSD). However, the behavioral changes persist for only a few days after the stressor, rendering the phenomenon questionable as a model. However, the original traumatic experience is re-experienced in PTSD and rumination occurs in depression. In a series of experiments we therefore sought to determine whether behavioral depression/learned helplessness could be made to endure by periodically "reminding" the subject of the original IS experience.

METHODS

Rats exposed to IS were tested for escape learning at various times thereafter. In different experiments the subjects were exposed to the environment in which IS had occurred at differing points in the interval between IS and escape testing.

RESULTS

Exposure to the environment in which IS had occurred prolonged the duration of behavioral depression/learned helplessness, and repeated exposures prolonged it indefinitely. This effect required exposure to the cues that had been present during IS (i.e., reminding) and was not duplicated by exposure to other stressors or stress environments.

CONCLUSIONS

Behavioral depression/learned helplessness can be maintained over time by processes that may be similar to those occurring in depression and PTSD, thereby strengthening the possibility that this paradigm is indeed a reasonable model of these disorders.

The effects of operant control over food and light on the behaviour of domestic hens

In intensive farming systems, the animals have little control over important elements in their environments. For instance, food of a pre-set type is delivered at set times, and the lighting schedule is controlled by the farmer. It has been suggested that low levels of environmental control over important events may reduce welfare by increasing passivity and stress. This study aimed to evaluate the effect of providing control over food and lighting additional to a restricted regime on the behaviour of small groups of laying hens (Gallus gallus domesticus). Twelve pens, each containing five birds, were paired to give six controlling and six non-controlling pens. These pairs of pens were yoked, such that birds in the controlling pens were able to make an operant response to gain access to extra food and light, whilst the yoked pens also received these outcomes but were unable to control their occurrence. The birds were kept continuously in the experimental conditions for 9 weeks. Records were made of general behaviour and activity, aggression and plumage damage scores, every 2 weeks. Data on key-pecking and egg production were continuously recorded throughout the experiment. The controlling birds used the operant keys to open the feeder for an average of 92min and to turn on the light for 46min per pen per day. The high number of key-pecks indicates that the birds were motivated to make use of the keys to control access to additional food and light. The non-controlling treatment pens showed significantly higher levels of preening and resting. Contrary to previous studies the use of operant feeders in this experiment did not induce a high level of feather pecking or aggressive interactions, as there was no significant difference between treatments. During the experiment the non-controlling hens laid significantly more eggs than the controlling hens. The results suggest that lack of control over these particular environmental events induced mild stress in the non-controlling pens of birds, and that further investigations into the effect of lack of control on welfare would be warranted.

Group housing of mice increases immobility and antidepressant sensitivity in the forced swim and tail suspension tests

The forced swim test and tail suspension test are often used in laboratory practice to identify compounds that possess antidepressant-like activity. This experiment was conducted to determine whether housing conditions per se influence the response of mice in these antidepressant screening procedures. Male NIH Swiss mice were housed individually or in groups (five per cage) for 8 weeks prior to testing. After 8 weeks, the animals were exposed to the forced swim and tail-suspension tests. Group housed mice displayed high levels of immobility in the forced swim and tail suspension tests. Desipramine injection 60 min prior testing, in doses 7.5 and 15 mg/kg, produced significant reductions in the immobility time in forced swimming and tail suspension tests. Individually housed mice, when exposed to these tests, displayed lower levels of immobility with a magnitude comparable to the effect of desipramine in group housed mice. Desipramine given to individually housed mice did not reduce the duration of immobility either in the forced swim test or in the tail suspension test. These results indicate that both tests are sensitive to housing conditions. This observation suggests that long lasting group housing may be critical to the behavioral response in these preclinical screening procedures in mice.

Behavioral analysis of stress controllability effects in a new swim stress paradigm

Previous animal stress studies have illustrated the marked impact of coping on subsequent behavior and physiology by using shock as the stressor. The current study evaluates the generality of shock stress controllability effects in a new swim stress paradigm on several dependent measures: behavioral despair, analgesia, shuttlebox escape, and alcohol reactivity. In this new paradigm, rats in the escape group are able to learn the behavioral response as evidenced by significant reduction in the acquisition of a lever press response. Both escape and yoked subjects showed "behavioral despair" in comparison to both restrained and home cage controls when tested 24 h later. In the standard shuttlebox escape task 24-h post-stress, no group differences emerged, although a trend for poorer performance in the yoked subjects was evident. No group differences were observed in pain sensitivity after the first or second forced swim exposure. Finally, stress controllability effects were observed in behavioral reactivity to alcohol 2-h post-stress as measured by rotarod performance. This effect is opposite to the previous observations with the tailshock stress controllability paradigm. These results suggest that (1) there are certain similarities, but some fundamental differences between the behavioral endpoints measured following intermittent swim stress in comparison to the well-established effects of the intermittent tailshock stress model and (2) the qualitative nature of a stressor may markedly influence the behavioral and physiological consequences of stress and coping.

Involvement of delta-opioid receptors in the effects induced by endogenous enkephalins on learned helplessness model

Pharmacological, neurochemical and behavioural findings support a possible role of endogenous opioids in clinical depression. There is evidence from animal studies that delta-opioid receptors are involved in several behavioural responses to opioids, including motivational activities. In the present study, the mixed enkephalin catabolism inhibitor, RB 101 (N(R,S)-2-benzyl-3[(S)-(2-amino-4-methylthiobutyldithio]-1-oxoprop yl)-L-phenylalanine benzyl ester) (1.25, 2.5 and 5 mg/kg), induced a dose-dependent antidepressant-like effect in a learned helplessness model. Thus, RB 101 reversed escape deficits in rats previously subjected to inescapable shocks, suggesting the involvement of endogenous enkephalins in depression. Similar effects were observed after administration of the selective delta-opioid receptor agonist, BUBU (Tyr-D.Ser-(O-tert-butyl)-Gly-Phe-Leu-Thr(O-Tet-butyl-OH) (1 and 2 mg/kg). Moreover, RB 101 effects were antagonized by administration of naltrindole (NTI) (0.1 mg/kg), which points to a preferential involvement of delta-opioid receptors in this enkephalin-controlled behaviour. As RB 101 has been reported to be almost devoid of opiate-related side-effects, it could represent a promising alternative in the treatment of depressive patients who are unresponsive to, or intolerant of, classical antidepressants.

The elevated plus-maze is not sensitive to the effect of stressor controllability in rats

The present experiments examined the sensitivity of the elevated plus-maze to the effects of stressor controllability. Previous work had established that inescapable but not an equal amount of escapable electric tail shock reduced social interaction. The present experiments demonstrate that prior exposure to shock alters elevated plus-maze behavior, but that this effect is not sensitive to the escapability of the shock. These experiments include a replication of the usual pharmacologic effects of benzodiazepine ligands (2 mg/kg diazepam; 0.4 mg/kg methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate) to demonstrate the sensitivity of the elevated plus-maze procedures used. The results provide additional support for the idea that the social interaction and elevated plus-maze measures of "anxiety" are sensitive to different processes.

Implication of endogenous opioid system in the learned helplessness model of depression

The involvement of opioid system on the learned helplessness model of depression was investigated. Animals preexposed to inescapable shocks were treated with either Met-enkephalin, Leu-enkephalin, morphine, imipramine, naloxone, RB 38A (a mixed inhibitor of enkephalin degrading enzymes), or RB 38B (a selective inhibitor of neutral endopeptidase EC 3.4.24.11). Stimulation of opioid system by either opioid agonists or enkephalin catabolism inhibitors reversed the escape deficit induced by shock pretreatment. In contrast, administration of naloxone potentiated the effect of inescapable shocks. Imipramine reduced the number of escape failures in this test, and this effect was antagonized by naloxone. These results point to the involvement of the endogenous opioid system in this model of depression.

Inescapable shock induces the opposite changes of the plus-maze test behavior in rats with divergent coping strategy

Behavior in the elevated plus-maze and plasma corticosterone levels were investigated in Koltushi high-avoidance (KHA) and low-avoidance (KLA) rat strains subjected to inescapable shock (IS) or daily handling for 10 days. These strains have been genetically selected on the basis of divergent acquisition of a conditioned avoidance response in a two-way shuttle-box. Naive KHA rats were more anxious than KLA ones. Following exposure to IS, the time spent in open arms was increased in KHA rats but decreased in KLA rats. After handling, the time spent in open arms increased only in KHA rats and became similar in both strains. There were no differences in plasma corticosterone levels between naive animals. Exposure to either IS or handling increased the plasma corticosterone levels in KLA rats. In KHA rats, handling reduced the plasma corticosterone levels, and exposure to IS had no effect. The present results suggest that the stress-induced changes in anxiety levels depend on the coping strategy of the subject.

Decreased forebrain [35S]TBPS binding and increased [3H]muscimol binding in rats that do not develop stress-induced behavioral depression

Recent evidence suggests that anxiety and its biological concomitants may be involved in the pathophysiology of depression. In the present study, the in vitro radioligand binding of [3H]flunitrazepam, [3H]muscimol and [35S]t-butylbicyclophosphorothionate (TBPS) sites on the benzodiazepine/GABA chloride ionophore receptor complex (BGRC) was examined using the learned helplessness paradigm. Only rats which did not develop the syndrome showed a significant increase in [3H]muscimol binding in cerebral cortex and a decrease in [35S]TBPS binding in cerebral cortex and hippocampus in comparison to naive controls. For both ligands, this represented a change in Bmax rather than a change in affinity. Adrenalectomy had no impact on these alterations indicating that critical endogenous factors are not manufactured by the adrenal glands. These findings suggest that the BGRC in the forebrain may be a site mediating the 'coping' ability of rats that do not develop the learned helplessness syndrome. The possible involvement of neurosteroids in this effect is discussed.

Hippocampal neuropeptide Y mRNA is reduced in a strain of learned helpless resistant rats

The learned helpless rat is considered to be one of the better animal models of depression. A genetically inbred strain with a high vulnerability to develop helplessness (LH), as well as a highly resistant strain (NLH) have both been developed. Since the brain peptide neuropeptide Y (NPY) is involved in the regulation of a number of behaviors known to be altered in clinical depression as well as in learned helplessness, we measured the relative level of NPY mRNA in the hippocampus and cortex of control Sprague Dawley (SD), LH and NLH rats. We find that NLH rats have approximately a 30-35% decrease in basal hippocampal NPY mRNA compared with SD and LH rats. By contrast, cortical NPY mRNA and hippocampal pre-proenkephalin and somatostatin mRNA levels were not significantly different in the 3 strains. The data suggest that the regulation of NPY gene expression may be involved in the reduced vulnerability of NLH rats to develop learned helplessness.

Antidepressants reverse the inhibition of shock-induced aggression elicited by a prior inescapable shock

Animals were exposed to long-duration inescapable shock (IS) and six days later submitted in pairs to a foot-shock session in order to induce shock-elicited aggression (SIF). Shocked rats subsequently displayed a lower aggressive response as compared to unshocked animals. This reduction was prevented by repeated treatment with different antidepressant drugs administered either prior or following IS exposure. In addition, rats chronically administered with antidepressant drugs before the IS showed less inactivity during the application of the uncontrollable aversive event. These data indicate that persistent administration with these pharmacological compounds prevent the induction and impede the further expression of the reduced aggressive response induced by a previous IS.

Control of food delivery in food-deprived rats mediates analgesia

An experiment was carried out to investigate the effects of control of food delivery, in food-deprived rats, on analgesia. Tail flick latency was used as an index of pain sensitivity and naloxone reversibility of analgesia was used as the criterion for opioid involvement. Food-deprived rats were submitted to one of two schedules of food delivery. The 'contingent' group could control the delivery of food by lever-pressing. The 'non-contingent' group received the same number of food pellets but delivery of food was independent of lever-pressing behaviour. Animals in the 'control' group were placed in the test chamber but did not receive any pellets. Subjects were tested on 6 consecutive days, each test session being of 10 min duration. Half of the animals in each group received an intraperitoneal injection of saline (0.5 ml) prior to each test session, the other half received an intraperitoneal injection of naloxone (5 mg/kg in 0.5 ml saline) prior to each session. Both contingent and non-contingent food delivery resulted in a significant post-test analgesia. The analgesia was noticeably greater when food delivery was non-contingent and this analgesia was reversible by naloxone. The findings suggest that non-contingent food delivery, in food-deprived rats, elicits an opioid analgesia, whilst contingent food delivery elicits a non-opioid analgesia.

Adinazolam both prevents and reverses the long-term reduction of daily activity produced by inescapable shock

The behavioral consequences of exposure to stressors such as inescapable shock are usually quite transitory if testing is conducted in an environment different from that in which the stressor was administered. Daily running activity is an exception in that it remains depressed for several weeks following experience with inescapable shock. In the present experiments we found the administration of the triazolbenzodiazepine adinazolam able to both reduce this long-term activity reduction produced by inescapable shock when acutely administered before the inescapable shock, and to reverse the effect when chronically administered after the inescapable shock. Classic 1,4-benzodiazepines such as diazepam have been able to prevent such effects when acutely administered before inescapable shock, but cannot reverse these effects when provided after the inescapable shock. Conversely, classic antidepressants such as desipramine have been unable to prevent these behavioral effects when given before inescapable shock in acute form, but can reverse the effects with chronic administration following the inescapable shock. Our observations that adinazolam can both prevent and reverse the effects of inescapable shock are consistent with reports that this agent has both anxiolytic and antidepressant effects in clinical use.

A pretest procedure reliably predicts performance in two animal models of inescapable stress

Rats exposed to inescapable tailshock fail to learn a shuttle-escape task 24 hours later, an effect referred to as "learned helplessness." However, within most rat strains only 10-50% of the animals tested develop this syndrome. In the present study a significant correlation was found between rats that displayed learned helplessness on the first test and those that displayed learned helplessness on a second test performed either 2 weeks (r = .80, p less than 0.001) or 4 weeks (r = .74, p less than 0.001) later. An analysis of the mean session latency of the shuttlebox task in these two tests suggested a bimodal distribution of animals that failed and learned. A significant correlation was found between individual rats that learned this task on the first test and those which learned this task 2 or 4 weeks later. Similarly, in the "behavioral despair" test, a significant correlation was observed for floating time for individual rats on the first test and on the second test either 2 (r = .72, p less than 0.001) or 4 weeks (r = .63, p less than 0.001) later. However, for the forced-swim test, a unimodal and rather graded response was observed across individual subjects. Thus, performance on the first round predicted performance on the second round in both models. When rats experienced the learned helplessness paradigm on round 1 and the behavioral despair paradigm in round 2, there was no correlation between rats that displayed helplessness following inescapable tailshock and the rats that demonstrated "behavioral despair" on a later test.(ABSTRACT TRUNCATED AT 250 WORDS)

Stress-induced behavioral depression in the rat is associated with a decrease in GABA receptor-mediated chloride ion flux and brain benzodiazepine receptor occupancy

Rats exposed to inescapable tailshock exhibit deficits in learning a simple shuttlebox escape task 24 h later. This syndrome has been termed 'behavioral depression' or 'learned helplessness', and is a model of stress-induced depression. In the present study a significant (25%) decrease in GABA receptor-mediated chloride ion flux as measured by muscimol-stimulated 36Cl- uptake in synaptoneurosomes was found in the cerebral cortices of rats that failed the shuttlebox task as compared to naive control rats. Rats which were exposed to tailshock and subsequently learned the escape task did not show a significant difference in muscimol-stimulated 36Cl- uptake as compared to naive control rats. Similarly, rats that failed to learn the shuttlebox escape task had significantly lower in vivo [3H]Ro15-1788 specific binding in cerebral cortex (43%), hippocampus (35%) and striatum (33%) as compared to naive control rats. In cerebellum and hypothalamus, there were significant reductions in specific [3H]Ro15-1788 binding in both animals that failed and animals that learned the shuttlebox escape task as compared to naive controls. To control the stress of the footshock associated with the shuttlebox escape task, we investigated the effect of gridshock in which total footshock received was equivalent to that received by rats who failed the shuttlebox task. There were no differences in muscimol-stimulated 36Cl- uptake or in vivo [3H]Ro15-1788 specific binding between naive controls and rats administered footshock independent of a learning task. These data suggest that the development of stress-induced behavioral depression may be associated with a decrease in GABA receptor-mediated chloride channel function.

Inescapable versus escapable shock modulates long-term potentiation in the rat hippocampus

A group of rats was trained to escape low-intensity shock in a shuttle-box test, while another group of yoked controls could not escape but was exposed to the same amount and regime of shock. After 1 week of training, long-term potentiation (LTP) was measured in vitro in hippocampal slices. Exposure to uncontrollable shock massively impaired LTP relative to exposure to the same amount and regime of controllable shock. These results provide evidence that controllability modulates plasticity at the cellular-neuronal level.

Effect of nifedipine on the shuttlebox escape deficit induced by inescapable shock in the rat

The behavioural effect of subchronic treatment with calcium channel antagonists (nifedipine, verapamil) and with imipramine was assessed in rats subjected to inescapable shock (IS). The effect of subchronic treatment with nifedipine and imipramine on specific [3H]nitrendipine ([3H]NDP) binding was investigated in frontal cortex of naive rats and in rats given IS then tested for shuttlebox escape. The rats showed a severe impairment in escape behaviour after IS. Imipramine and nifedipine significantly reduced FR1 and FR2 escape deficits. Verapamil had no effect. A small but significant increase in the number of [3H]NDP binding sites (Bmax) was seen in rats exposed to the shuttlebox escape test independent of a previous exposure to IS. Imipramine had no influence on Bmax in any of the groups. Nifedipine did not affect [3H]NDP binding in naive rats but decreased Bmax in rats subjected to IS and the shuttlebox escape test. The comparable ability of nifedipine and imipramine to reverse the shuttlebox escape deficit induced by IS argues for a possible antidepressant activity of nifedipine. The biochemical data indicate that cortical [3H]NDP binding sites are not correlated to performance in the shuttlebox escape test.

Long-term effects of inescapable stress on daily running activity and antagonism by desipramine

The behavioral consequences of exposure to stressors such as inescapable shock are usually transitory if testing is conducted in an environment different from that in which the stressor was administered. The behaviors tested have generally been motivated by discrete stimuli in the environment (e.g., activity in reaction to shock) or have been part of homeostatic regulatory mechanisms (e.g., eating). Here we investigated the effects of inescapable shock on a behavior that is not so tightly tied to motivating and reinforcing conditions, daily activity in a familiar home cage/running wheel environment. Rats lived in the wheel environment for 44-85 days before treatment. Inescapable shock produced only a transient reduction of water intake and body weight, but daily running was depressed for 14-42 days (the maximum period studied) depending on the conditions. This long-term effect on activity occurred despite the fact that shock was administered in an environment very different from the animal's home running wheel environment. The activity reduction was reversed by desipramine in a dose dependent fashion. Indeed, the activity of inescapably shocked animals treated with the optimum dose of desipramine exceeded that of control animals undergoing neither stress nor drug treatment. The maximum effect of desipramine required 7 days of treatment. Desipramine did not affect the activity of control subjects.

Neurochemical and electrophysiological events underlying stress-induced depression in an animal model

This paper has described an animal (rodent) model of depression in which depression-like characteristics are produced by exposure of animals to stressful events that they cannot control. This model, called "stress-induced depression," appears to be mediated by stress-induced changes in brain norepinephrine (NE), and evidence now indicates that a large depletion of NE in the Locus Coeruleus region of the brain stem (LC) is critical for producing the behavioral disturbance seen in this model. To explain the functional significance of NE depletion in the LC, it has been suggested that this change results in decreased stimulation, (i.e., functional blockade) of alpha-2 receptors that normally inhibit firing of LC neurons. Consequently, LC neurons should be disinhibited in stress-induced depression. The final part of this paper describes findings from electrophysiological measurement of LC activity. Prior to measuring changes occurring in stress-induced depression, studies examined the basic role played by alpha-2 receptors in LC activity. These receptors were found to regulate the responsivity of LC neurons to excitatory stimuli, so that pharmacological blockade of these receptors increased the firing of LC neurons to excitatory input. When LC activity was then measured in stressed animals showing behavioral depression, responsivity of LC neurons to excitatory input was elevated in comparison to animals not exposed to stress. Also, administration of an alpha-2 blocking drug could not increase responsivity of LC neurons in stressed animals, which further indicated that alpha-2 receptors in the LC region were functionally blocked in these animals. Finally, the amount that LC neurons increased their firing to an excitatory stimulus (i.e., an index of alpha-2 receptor blockade) was highly correlated with the amount of behavioral depression seen in an activity test conducted just prior to electrophysiological measurement. These electrophysiological findings indicate that LC neurons are disinhibited in stress-induced depression, and that this disinhibition is particularly characterized by increased responsivity of LC neurons to excitatory input, which is indicative of alpha-2 receptor blockade. These findings further support the view that the LC is involved in stress-induced depression, and are consistent with a proposed mechanism that attributes behavioral disturbance in the model to disinhibition of LC neurons arising from subnormal activation of inhibitory alpha-2 receptors.

Stressors in the learned helplessness paradigm: effects on body weight and conditioned taste aversion in rats

Changes in body weight and taste aversion in the learned helplessness paradigm were examined. In Experiment 1, adult male Sprague-Dawley rats drank saccharin or a control solution, followed by either 100 inescapable shocks or simple restraint. Rats were weighted daily and were tested for saccharin aversion two days after the stress session. Shocked rats gained less weight in the days after stress than restrained controls. Saccharin aversion was apparent only among rats that had consumed saccharin before the stress session. Experiment 2 examined whether control over shock affected body weight or taste aversion. Home-cage controls were included to assess the effects of restraint alone. In addition, the combined effects of shock and a toxin on aversion were studied. Rats drank saccharin solution, followed by escapable or inescapable shock, restraint, or no treatment. Then half of each group was injected with saline; the other half was injected with lithium chloride. As in Experiment 1, shock reduced body weight relative to restraint or no treatment, and shock produced a taste aversion among saline-treated rats. However, shock attenuated the aversion produced by lithium chloride, as did simple restraint. There were no differences in body weight or taste aversion between escapably and inescapably shocked rats. These results suggest a role for stress in the anorexia and weight loss associated with clinical depression and may have implications for theories of learning and learned helplessness.

Neurochemical and behavioral consequences of mild, uncontrollable shock: effects of PCPA

The present experiments examined the role of the serotonergic system in the behavioral deficit produced by uncontrollable shock. In Experiment 1: Establishment of model, the behavioral potential of the Sprague-Dawley rat was defined. When exposed to mild uncontrollable stress such as a 0.8 mA electric footshock, a significant percentage of rats developed a shock escape deficit which was evident when subsequently placed in a shock escape paradigm. Serotonin depletion was produced by chronic treatment with p-chlorophenylalanine. Biogenic amine levels and 5-HT levels were monitored in various brain areas using HPLC. Following chronic treatment with PCPA, the shock escape capability of the Sprague-Dawley rat was assessed. The severe depletion of 5-HT in various brain regions was highly correlated with a dramatic improvement in the shock escape scores. Thus, the detrimental effects of exposure to a mild course of inescapable shock can be prevented by chronic treatment with PCPA. These experiments implicate the serotonergic system as a possible mediator of the "learned helplessness" phenomenon.

Validation criteria for animal models of human mental disorders: learned helplessness as a paradigm case

Three sets of criteria are proposed for assessing animal models of human mental disorders: predictive validity (performance in the test predicts performance in the condition being modelled), face validity (phenomenological similarity) and construct validity (theoretical rationale). The problems inherent in each of these validation procedures are discussed, and their application to the learned helplessness model of depression is examined. It is concluded that whilst the model has good predictive validity, important questions about face validity remain unanswered, and construct validity has not yet been established. The distinctions between animal models and some related experimental procedures are also discussed.

Coping and seizure susceptibility: control over shock protects against bicuculline-induced seizures

Rats were either given 80 escapable shocks, yoked inescapable shocks, restraint or given no treatment. Two hours later all subjects received i.p. injection of bicuculline (4, 6 or 8 mg/kg) and were immediately tested for latency to initial myoclonic jerk and clonus. The latency to clonic convulsion was dramatically affected by prior shock treatment, and the direction of this change depended upon the escapability/inescapability of the shock. Subjects that were given escapable shock showed a delay of onset to seizure, while subjects inescapably shocked demonstrated a decreased latency to clonus in comparison to restrained and naive controls. It was also demonstrated that if the subjects were tested immediately following a stress experience, both the 80 escapable and inescapable shock condition protected against bicuculline-induced seizures in comparison to the control condition. Finally Experiment 2 confirmed a previous finding that less stress, i.e., 20 inescapable shocks, protects against seizures when the animals are challenged with bicuculline either immediately or 2 h later. Our suggestion is that control over stress may facilitate GABAergic transmission, and this may be the mechanism whereby coping protects against the behavioral and physiological disruption produced by exposure to a stressor.

Librium prevents the analgesia and shuttlebox escape deficit typically observed following inescapable shock

Administration of a benzodiazepine, chlordiazepoxide (CDP), prior to exposure to inescapable shock prevented both the long-term analgesia and the shuttle-escape deficit typically observed following inescapable shock. If given only prior to testing, CDP had little effect. The protective effects of CDP were determined not to be a result of state dependency or a general facilitatory effect of the drug on escape performance. It is suggested that the induction of anxiety or fear by inescapable shock is critical in mobilizing endogenous changes such as transmitter depletion which are thought to be responsible for the deficits observed.

The validity of animal models of depression

Eighteen animal models of depression are reviewed in relation to three sets of validating criteria. Of the 18 models, five could only be assessed for predictive validity, seven could be assessed for predictive and face validity, and six could potentially have predictive, face and construct validity. Some traditional models (reserpine reversal, amphetamine potentiation) are rejected as invalid; the models with the highest overall validity are the intracranial self-stimulation, chronic stress and learned helplessness models in rats, and the primate separation model.

Evidence for a serotonergic mechanism of the learned helplessness phenomenon

The present experiments examined the role of the serotonergic system in the learned helplessness phenomenon. In Experiment 1, a 200 mg/kg dose of 1-tryptophan injected 30 min prior to testing disrupted acquisition of Fixed Ratio 2 shuttle escape behavior. In Experiment 2, a 100 mg/kg dose of 5-HTP produced interference with the acquisition of the escape response. Furthermore, this interference was prevented by treatment with the serotonergic antagonist methysergide. In Experiment 3, animals were pretreated with a subeffective dose of 1-tryptophan in combination with subeffective exposure to inescapable shock. These animals showed a deficit in the acquisition of FR-2 shuttle escape. In Experiment 4, combined exposure to a subeffective dose of 5-HTP and inescapable shock (40 trials) resulted in an acquisition deficit. This deficit was reversed by methysergide. Experiment 5 showed that the detrimental effects of exposure to prolonged (80 trials) of inescapable shock can be prevented by treatment with methysergide. These studies implicate the serotonergic system as a possible mediator of the learned helplessness phenomenon.

Species-specific defense reactions and avoidance learning. An evaluative review

Bolles (1970) proposed a theory of avoidance learning, the species-specific defense reaction (SSDR) hypothesis, which emphasized innate constraints on the response repertoire of rats in aversive situations and minimized the role of reinforcement in avoidance learning. The present paper describes Bolles' (1970, 1971, 1972, 1975, 1978) development of SSDR theory and reviews the empirical tests of its assumptions and predictions. It is concluded that the SSDRs described by Bolles, along with some others, are highly probable in aversive situations but that the response repertoire is not limited to them. Further, there is strong evidence for reinforcement effects in the establishment and maintenance of at least some avoidance responses.

Tumor rejection in rats after inescapable or escapable shock

Rats experienced inescapable, escapable, or no electric shock 1 day after being implanted with a Walker 256 tumor preparation. Only 27 percent of the rats receiving inescapable shock rejected the tumor, whereas 63 percent of the rats receiving escapable shock and 54 percent of the rats receiving no shock rejected the tumor. These results imply that lack of control over stressors reduces tumor rejection and decreases survival.

Controllability, coping behavior, and stress-induced analgesia in the rat

Exposure to painful or stressful stimuli produces an analgesic reaction which can persist for 1-2 h post-stress. In the typical stress-induced analgesia study the subject is not permitted to alter or exert control over the aversive event to which it is exposed. That is, its behavior affects neither the duration or intensity of the event. The experiments reported here attempted to determine whether this inability of the subject to control the aversive event is an important determinant of stress-induced analgesia, or whether simple exposure to painful events is a sufficient condition for its production. In the first experiment rats were given either escapable electric shocks (the subject's behavior could terminate the shock), equal amounts of inescapable shock, or no shock. Tail-flick to radiant heat was assessed 30 min later. The group given inescapable shock was strongly analgesic, while the group given an equal amount of escapable shock was only mildly analgesic. Thus the controllability of the shock or the availability of a coping response determined the antinociceptive reaction which followed. The second experiment revealed that this differential effect of controllability on tail-flick responding is masked, shortly after the end of the shock session, by a transient analgesic effect of shock which is not sensitive to the controllability dimension. The implications of these results for stress-induced analgesia and the activation of opioid systems are discussed.