Learned helplessness: an experimental model of the DST in rats

Post-Stress Fructose and Glucose Ingestion Exhibit Dissociable Behavioral and Physiological Effects

An acute traumatic event can lead to lifelong changes in stress susceptibility and result in psychiatric disease such as Post-Traumatic Stress Disorder (PTSD). We have previously shown that access to a concentrated glucose solution for 24 hours beginning immediately after trauma decreased stress-related pathology in the learned helplessness model of PTSD and comorbid major depression. The current study sought to investigate the peripheral physiological effects of post-stress glucose consumption. We exposed 128 male Sprague-Dawley rats to inescapable and unpredictable 1-milliamp electric tail shocks or simple restraint in the learned helplessness procedure. Rats in each stress condition had access to a 40% glucose solution, 40% fructose solution, or water. Blood and liver tissue were extracted and processed for assay. We assessed corticosterone, corticosteroid-binding globulin (CBG), glucose, and liver glycogen concentrations at various time points following stress. We found that rats given access to glucose following exposure to traumatic shock showed a transient rise in blood glucose and an increase in liver glycogen repletion compared to those that received water or fructose following exposure to electric shock. We also found that animals given glucose following shock exhibited reduced free corticosterone and increased CBG compared to their water-drinking counterparts. However, this difference was not apparent when glucose was compared to fructose. These data suggest that post-stress glucose prophylaxis is likely not working via modulation of the HPA axis, but rather may provide its benefit by mitigating the metabolic challenges of trauma exposure.

Temporal parameters of post-stress prophylactic glucose treatment in rats

Acute trauma can lead to life-long changes in susceptibility to psychiatric disease, such as post-traumatic stress disorder (PTSD). Rats given free access to a concentrated glucose solution for 24 h beginning immediately after trauma failed to show stress-related pathology in the learned helplessness model of PTSD and comorbid major depression. We assessed effective dosing and temporal constraints of the glucose intervention in three experiments. We exposed 120 male Sprague-Dawley rats to 100, 1 mA, 3-15 s, inescapable and unpredictable electric tail shocks (over a 110-min period) or simple restraint in the learned helplessness procedure. Rats in each stress condition had access to a 40% glucose solution or water. We measured fluid consumption under 18-h free access conditions, or limited access (1, 3, 6, 18 h) beginning immediately after trauma, or 3-h access with delayed availability of the glucose solution (0, 1, 3, 6 h). We hypothesized that longer and earlier access following acute stress would improve shuttle-escape performance. Rats exposed to traumatic shock and given 18-h access to glucose failed to show exaggerated fearfulness and showed normal reactivity to foot shock during testing as compared to their water-treated counterparts. At least 3 h of immediate post-stress access to glucose were necessary to see these improvements in test performance. Moreover, delaying access to glucose for more than 3 h post-trauma yielded no beneficial effects. These data clearly identify limits on the post-stress glucose intervention. In conclusion, glucose should be administered almost immediately and at the highest dose after trauma.

Rigid patterns of effortful choice behavior after acute stress in rats

Physical effort is a common cost of acquiring rewards, and decreased effort is a feature of many neuropsychiatric disorders. Stress affects performance on several tests of cognition and decision making in both humans and nonhumans. Only a few recent reports show impairing effects of stress in operant tasks involving effort and cognitive flexibility. Brain regions affected by stress, such as the medial prefrontal cortex and amygdala, are also implicated in mediating effortful choices. Here, we assessed effort-based decision making after an acute stress procedure known to induce persistent impairment in shuttle escape and elevated plasma corticosterone. In these animals, we also probed levels of polysialyted neural cell adhesion molecule (PSA-NCAM), a marker of structural plasticity, in medial frontal cortex and amygdala. We found that animals that consistently worked for high magnitude rewards continued to do so, even after acute shock stress. We also found that PSA-NCAM was increased in both regions after effortful choice experience but not after shock stress alone. These findings are discussed with reference to the existing broad literature on cognitive effects of stress and in the context of how acute stress may bias effortful decisions to a rigid pattern of responding.

Caution When Diagnosing Your Mouse With Schizophrenia: The Use and Misuse of Model Animals for Understanding Psychiatric Disorders

Animal models are widely used in biomedical research, but their applicability to psychiatric disorders is less clear. There are several reasons for this, including 1) emergent features of psychiatric illness that are not captured by the sum of individual symptoms, 2) a lack of equivalency between model animal behavior and human psychiatric symptoms, and 3) the possibility that model organisms do not have (and may not be capable of having) the same illnesses as humans. Here, we discuss the effective use, and inherent limitations, of model animals for psychiatric research. As disrupted-in-schizophrenia 1 (DISC1) is a genetic risk factor across a spectrum of psychiatric disorders, we focus on the results of studies using mice with various mutations of DISC1. The data from a broad range of studies show remarkable consistency with the effects of DISC1 mutation on developmental/anatomical endophenotypes. However, when one expands the phenotype to include behavioral correlates of human psychiatric diseases, much of this consistency ends. Despite these challenges, model animals remain valuable for understanding the basic brain processes that underlie psychiatric diseases. We argue that model animals have great potential to help us understand the core neurobiological dysfunction underlying psychiatric disorders and that marrying genetics and brain circuits with behavior is a good way forward.

The effect of glucose on hippocampal-dependent contextual fear conditioning

BACKGROUND

The metabolic challenge of trauma disrupts hippocampal functioning, which is necessary for processing the complex co-occurring elements comprising the traumatic context. Poor contextual memory of trauma may subsequently contribute to intrusive memories and overgeneralization of fear. Glucose consumption following trauma may be a means to protect hippocampal functioning and contextual fear learning. This study experimentally examined the effect of glucose on hippocampal-dependent contextual learning versus cued fear learning in humans.

METHODS

Forty-two male participants underwent cued conditioning with an unconditional stimulus (US) (shock) paired with a discrete conditional stimulus (geometric shape) and context conditioning (requiring hippocampal processing) with a US unpredictably paired with a background context (picture of room). Participants were then blindly randomized to consume either a 25 g glucose or sweet-tasting placebo drink and returned for a test phase 24 hours later. Measures included acoustic startle response, US expectancy, blood glucose levels, and arousal ratings.

RESULTS

The glucose group showed superior retention of hippocampal-dependent contextual learning at test relative to the placebo group, as demonstrated by acoustic startle response and US expectancy ratings. Glucose and placebo groups did not differ on any measure of cued fear learning at test.

CONCLUSIONS

This study provides experimental evidence that in mildly stressed humans postconditioning glucose consumption improves retention of hippocampal-dependent contextual learning but not cued learning. Ultimately, glucose consumption following trauma may be a means of improving learning about the traumatic context, thereby preventing subsequent development of symptoms of posttraumatic stress.

Reflections on the interaction of psychogenic stress systems in humans: the stress coherence/compensation model

Although stress simultaneously affects and causes changes in central nervous system systems together with the sympathetic nervous system and the hypothalamus-pituitary-adrenal axis, this interaction and its behavioral consequences are rarely assessed. The current paper first describes the different systems involved in the perception and processing of stressful stimuli on an anatomical and functional level, and the available measures to assess changes in these systems. It then explores, based on theoretical and empirical grounds, the interaction of the systems. This is followed by a review of previous stress models, and how these attempted to integrate the interaction of these systems. Then, it complements previous models by suggesting a complementary regulation of the stress systems, and discusses potential behavioral consequences. Finally, based on the three-system approach to assess stress it is argued that psychological measures, together with physiological and endocrine measures are indispensable. However, the lack of consensus on how to best assess the central and sympathetic nervous system components of stress make it more difficult to include measures of all systems routinely in future stress studies. Thus, the paper closes by giving some recommendations on how to include a minimum of feasible stress measures for all systems involved in stress processing and regulation.

What can we know from pituitary-adrenal hormones about the nature and consequences of exposure to emotional stressors?

Exposure to stress induces profound physiological and behavioral changes in the organisms and some of these changes may be important regarding stress-induced pathologies and animal models of psychiatric diseases. Consequences of stress are dependent on the duration of exposure to stressors (acute, chronic), but also of certain characteristics such as intensity, controllability, and predictability. If some biological variables were able to reflect these characteristics, they could be used to predict negative consequences of stress. Among the myriad of physiological changes caused by stress, only a restricted number of variables appears to reflect the intensity of the situation, mainly plasma levels of ACTH and adrenaline. Peripheral hypothalamic-pituitary-adrenal (HPA) hormones (ACTH and corticosterone) are also able to reflect fear conditioning. In contrast, the activation of the HPA axis is not consistently related to anxiety as evaluated by classical tests such as the elevated plus-maze. Similarly, there is no consistent evidence about the sensitivity of the HPA axis to psychological variables such as controllability and predictability, despite the fact that: (a) lack of control over aversive stimuli can induce behavioral alterations not seen in animals which exert control, and (b) animals showed clear preference for predictable versus unpredictable stressful situations. New studies are needed to re-evaluate the relationship between the HPA axis and psychological stress characteristics using ACTH instead of corticosterone and taking advantages of our current knowledge about the regulation of this important stress system.

A comparison of the welfare of sows in different housing conditions

Twelve sows in good quality unstrawed stalls, three groups of five sows in strawed pens with individual feeding stalls and sows in a 38-sow group in a strawed yard with an electronic sow feeder were compared during the first four parities. They originated from the same source, were about 9 months of age and in the 7th week of their first pregnancy at the start of the experiment and were kept in adjacent rooms in a building, cared for by the same staff and given the same diets at a rate of 2·2 kg/day per animal. No new animals were added to the groups or stalls during the study and animals returned to the same condition after periods in farrowing and service accommodation. Using a wide range of welfare indicators, it was clear that stall-housed sows had more problems than group-housed sows and that tliese problems were worse in the fourth than in the first pregnancy. By the fourth pregnancy, stall-housed sows spent proportionately 0·14 of time showing activities which were clearly stereotypies and much time on activities which were sometimes stereotyped, i.e. ‘drinking’ and rooting or chewing at pen fittings making a total of proportionately 0·50 of time. Comparable figures for group-housed sows were much lower (0·037 and 0·081 in total). Stall-housed sows were also more aggressive than group-housed by the fourth pregnancy and their body weights were lower. There were no differences using physiological or immunological tests or measures of reproductive output. When the two group-housing systems were compared, sows in the electronic feeder system showed more fighting, especially soon after initial mixing, but fewer total agonistic interactions than sows in groups of five during the first pregnancy. Oral stereotypies were slightly higher in small groups, perhaps because of smaller pen space, than in larger groups but much lower than in stalls. By the fourth pregnancy there were few differences between sows in small and large groups and all seemed to have adapted well to the conditions. Evaluation of welfare in different housing systems requires use of a wide range of measures and of long-term studies.

Persistent depressive state after chronic stress in rats is accompanied by HPA axis dysregulation and reduced prefrontal dopaminergic neurotransmission

Exposure to stress is thought to play an important role in the etiology of depression. Dysregulation of the hypothalamo-pituitary-adrenal (HPA) axis characterized by glucocorticoid negative feedback resistance is frequently observed in human depressives. Additionally, dysfunctions of the dopaminergic and serotonergic systems in the prefrontal cortex (PFC) are thought to be involved in the development of a depressive state. In rats, chronic stress induces a behaviorally depressive state, concomitant with dysregulation of the HPA axis and reductions in dopaminergic and serotonergic transmissions in the PFC. Considering that dysregulation of the HPA axis is associated with relapse and persistency of depression, it is possible that the chronic stress-induced depressive state persists during long-term rest after its exposure. In the present study, we examined this possibility in rats and found that the behaviorally depressive state in the rotarod test, negative feedback resistance in the dexamethasone suppression test, and a decrease in the extracellular concentration of dopamine but not serotonin in the PFC persisted for 3 months following a 4-week stress session. These results suggest that dysregulation of the HPA system and reduced dopaminergic transmission in the PFC underlies persistent behavioral depression following chronic stress.

Dexamethasone and stressor-magnitude regulation of stress-induced transcription of HPA axis secretagogues in the rat

Regulation of the production of hypothalamic-pituitary-adrenal (HPA) axis secretagogues, corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP), may be differentially sensitive to the negative feedback effects of glucocorticoids. We chose to study this phenomenon by examining the ability of dexamethasone to influence CRH and AVP heteronuclear RNA (hnRNA) levels in an escapable/inescapable (ES/IS) foot-shock stress paradigm. On Day 1, adult male rats were subjected to either ES or IS foot-shock; on Day 2, saline or dexamethasone (100 microg/kg) was administered 2 h prior to the stressor. We found that ES/IS foot-shock stimulated similar robust increases in plasma adrenocorticotrophic hormone (ACTH) and corticosterone concentrations, and medial parvocellular division of the paraventricular nucleus (mpPVN) AVP and CRH hnRNA and c-fos mRNA levels in saline-treated ES/IS rats. Dexamethasone pretreatment suppressed ACTH and corticosterone levels similarly in IS and ES animals. Dexamethasone pretreatment also suppressed mpPVN CRH and AVP hnRNA levels at 30 min. However, by 120 min, the mpPVN AVP hnRNA levels in dexamethasone-treated rats were similar to those measured in the saline group. We also found that rats that received the most shocks on Day 1 had greater HPA axis activation on Day 2. We conclude that the magnitude of the foot-shock stressor, determined by learned and immediate cues, is important in determining the magnitude of the HPA response.

Antidepressive-like effects of electroacupuncture in rats

Here, we investigate the effects of electroacupunture on the depressive-like symptoms in learned helplessness and forced swim tests in rats. Electroacupuncture stimulation (EA) was provided at ST-36 (Zusanli) and SP-6 (Sanyinjiao) acupoints. A positive control group was treated with imipramine. To verify the effects of EA over serotonergic system, other additional groups received daily, for three days, p-chlorophenylalanine and after two days, were submitted to behavioral tests. EA, like imipramine, enhanced the successful active avoidance in the learned helplessness and diminished the time spent in immobility position in the forced swim test, without affecting the number of squares crossed in the open field test. The administration of p-chlorophenylalanine abolished the antidepressive-like effect of EA. EA generates a clear antidepressant effect in two different animal models of depression, and this effect is related, at least in part, to the serotonergic system.

Single and repeated stress-induced modulation of phospholipase C catalytic activity and expression: role in LH behavior

PI-PLC, a critical enzyme of the phosphoinositide (PI) signaling pathway, mediates many physiological functions in the brain, including cellular plasticity. Stress-induced learned helplessness (LH) in animals serves as a model of behavioral depression. Recently, we observed that repeated stress prolongs the duration of LH behavior in rats, enabling us to compare neurobiologic abnormalities in acute and chronic depression. Here we examine whether LH behavior is associated with alterations in phospholipase C (PLC), and whether repetition of inescapable shock has similar or dissimilar effects on PLC to those of the single-stress paradigm. Rats were exposed to inescapable shock either once on day 1, or twice, on days 1 and 7. Rats were tested for escape latency on days 2 and 4 after day 1 inescapable shock or on days 2, 8, and 14 after day 1 and 7 inescapable shock. PI-PLC activity and mRNA and protein expression of three different PLC isozymes were determined in the frontal cortex and hippocampus. Higher escape latencies were observed in LH rats tested on day 2 after single inescapable shock and on day 14 after repeated inescapable shock. Single inescapable shock reduced PI-PLC activity in the frontal cortex and hippocampus of LH rats. On the other hand, repeated inescapable shock not only reduced PI-PLC activity in these brain areas of LH rats but also selectively decreased the expression of PLC beta1 and PLC gamma1 isozymes. Our results suggest different responsiveness at the level of PI-PLC after single vs repeated stress, and that reductions in PLC may be critical in the pathophysiology of depression and other stress-related disorders.

Differential regulation of serotonin (5HT)2A receptor mRNA and protein levels after single and repeated stress in rat brain: role in learned helplessness behavior

Stress-induced learned helplessness in animals serves as a model of behavioral depression and other stress-related disorders. Our recent report that repeated stress prolongs the duration of learned helplessness behavior in rats may be important since acute and recurrent disorders may have different responsive mechanisms. To examine the role of serotonergic (5HT) mechanisms in such behavior, we studied the expression of 5HT2A receptors in different brain areas of rats, and further investigated whether the alterations in expression of 5HT2A receptors are similar after single versus repeated stress. Rats exposed to inescapable shock once on day 1, or twice, on day 1 and day 7, were tested for escape latency on days 2 and 4, or day 14, respectively. Higher escape latencies were observed on day 2 after single, and on day 14 after repeated shock. Whereas the single-stress paradigm produced a significant decrease of 5HT2A receptor mRNA and protein expression in hippocampus of non-learned helpless and learned helpless rats as compared with tested controls, repeated stress resulted in increase in frontal cortex but decrease in hippocampus and hypothalamus of learned helpless rats only, as compared with tested control rats. These results demonstrate differential regulation of 5HT2A receptors in LH rats after single and repeated stress, which may be critical in the pathophysiology of depression/other stress-related disorders.

Effects of rolipram, a phosphodiesterase 4 inhibitor, in combination with imipramine on depressive behavior, CRE-binding activity and BDNF level in learned helplessness rats

The brain cAMP regulating system and its downstream elements play a pivotal role in the therapeutic effects of antidepressants. We previously reported the increase in activities of phosphodiesterase 4, a major phosphodiesterase isozyme hydrolyzing cAMP, in the frontal cortex and hippocampus of learned helplessness rats, an animal model for depression. The present study was undertaken to examine the combination of effects of rolipram, a phosphodiesterase 4 inhibitor, with imipramine, a typical tricyclic antidepressant, on depressive behavior in learned helplessness rats. Concurrently, cAMP-response element (CRE)-binding activity and brain-derived neurotrophic factor (BDNF) levels related to the therapeutic effects of antidepressants were determined. Repeated administration of imipramine (1.25-10 mg/kg, i.p.) or rolipram (1.25 mg/kg, i.p.) reduced the number of escape failures in learned helplessness rats. Imipramine could not completely ameliorate the escape behavior to a level similar to that of non-stressed rats even at 10 mg/kg. However, repeated coadministration of rolipram with imipramine (1.25 and 2.5 mg/kg, respectively) almost completely eliminated the escape failures in learned helplessness rats. The reduction of CRE-binding activities and BDNF levels in the frontal cortex or hippocampus in learned helplessness rats were ameliorated by treatment with imipramine or rolipram alone. CRE-binding activities and/or BDNF levels of the frontal cortex and hippocampus were significantly increased by treatment with a combination of rolipram and imipramine compared to those in imipramine-treated rats. These results indicated that coadministration of phosphodiesterase type 4 inhibitors with antidepressants may be more effective for depression therapy and suggest that elevation of the cAMP signal transduction pathway is involved in the antidepressive effects.

Saiko-ka-ryukotsu-borei-to, a herbal medicine, ameliorates chronic stress-induced depressive state in rotarod performance

Exposure to chronic stress is thought to play an important role in the etiology of depression. This disorder has been shown to involve disruption of the hypothalamo-pituitary-adrenal (HPA) system and dysfunction of the prefrontal cortex (PFC). We have demonstrated that chronic stress in rats induces similar HPA disruption or a depressive state caused by a reduction of dopaminergic and serotonergic transmission in the PFC. We have also shown that saiko-ka-ryukotsu-borei-to, a herbal medicine, prevents such chronic stress-induced HPA disruption. However, the behavioral and neurochemical bases of this drug remain unclear. Here we examined the effects of saiko-ka-ryukotsu-borei-to on the depressive behavioral state and the reduction of transmission resulting from chronic stress. The chronic stress was induced by water immersion and restraint (2 h/day) for 4 weeks followed by recovery for 10 days. The treatment with saiko-ka-ryukotsu-borei-to (100, 300, or 1000 mg/kg p.o.) ameliorated the stress-induced depressive state in a dose-dependent manner, evaluated by a rotarod test. A microdialysis study indicated that the drug treatment significantly prevented the chronic stress-induced decreases in extracellular concentrations of dopamine and serotonin in the PFC. These results suggest that saiko-ka-ryukotsu-borei-to ameliorates the chronic stress-induced depressive state based on the prevention of PFC dysfunction. These findings provide important information for treatment of depression.

Negative feedback functions in chronically stressed rats: role of the posterior paraventricular thalamus

A gradual decrement in hypothalamic-pituitary-adrenal (HPA) activity is observed following repeated exposure to the same stressor, such as repeated restraint. This decrement, termed habituation, may be partly due to alterations in corticosterone-mediated negative feedback inhibition of the HPA axis. We have previously found that the posterior division of the paraventricular thalamus (pPVTh) regulates habituated HPA activity without altering HPA responses to acute stress. Therefore, in the present study, we examined the role of the pPVTh in delayed feedback inhibition of plasma corticosterone responses to repeated restraint. Dexamethasone was administered subcutaneously 2 h prior to 30 min restraint to induce delayed negative feedback inhibition of the HPA axis. In the first experiment, we determined that a 0.05-mg/kg dose of dexamethasone produced submaximal suppression of corticosterone responses to acute restraint and used this dose in the remainder of the experiments. In Experiment 2, we examined dexamethasone-induced feedback inhibition to corticosterone responses to a single or eighth restraint exposure since negative feedback functions in chronically stressed rats are not well studied. We found that corticosterone levels following dexamethasone treatment were similar in repeatedly restrained compared to acutely restrained rats. In Experiment 3, we lesioned the pPVTh and examined dexamethasone-induced feedback inhibition of corticosterone responses to a single or eighth exposure to restraint. pPVTh lesions attenuated dexamethasone-induced inhibition of corticosterone at 30 min in chronically stressed rats but had no effect in acutely stressed rats. These data suggest that negative feedback functions are maintained in rats exposed to repeated restraint and implicate the pPVTh as a site that contributes to these negative feedback functions specifically under chronic stress conditions.

Differential effects of chronic escapable versus inescapable stress on male syrian hamster (Mesocricetus auratus) reproductive behavior

Stress decreases sexual activity, but it is uncertain which aspects of stress are detrimental to reproduction. This study used an escapable/inescapable stress paradigm to attempt to dissociate physical from psychological components of stress, and assess each component's impact on reproductive behavior in the male Syrian hamster (Mesocricetus auratus). Two experiments were completed using this protocol where two animals receive the same physical stressor (an electric footshock) but differ in the psychological aspect of control. One group (executive) could terminate the shock for themselves as well as a second group (yoked) by pressing a bar. Experiment 1 demonstrated a significant increase in plasma glucocorticoids at the end of a single 90-min stress session with no difference in glucocorticoid levels between the executive and yoked groups at any time point. Experiment 2 quantified male reproductive behavior prior to and immediately following 12 days of escapable or inescapable stress in executive, yoked, and no-stress control hamsters (n = 12/group). Repeated-measures analysis of variance revealed a number of significant changes in reproductive behavior before and after stress in the three treatment groups. The most striking difference was a decrease in hit rate observed only in the animals that could not control their stress (yoked group). Hit rate in the executive males that received the exact same physical stressor but could terminate the shock by pressing a bar was nearly identical to control animals that never received any foot shock. Therefore, we conclude that coping or control can ameliorate the negative effects of stress on male reproductive behavior.

Saiko-ka-ryukotsu-borei-to, an herbal medicine, prevents chronic stress-induced disruption of glucocorticoid negative feedback in rats

Exposure to stress is known to precipitate or exacerbate many neuropsychiatric disorders such as depression. Abnormality of the neuroendocrine system, as shown by increased adrenal weight and attenuated glucocorticoid negative feedback, is frequently seen in depression. The aim of the present study is to clarify the usefulness of saiko-ka-ryukotsu-borei-to, an herbal medicine, in the treatment of abnormality of the neuroendocrine system using an experimental stress-depression model. Rats were subjected to water immersion and restraint for 2 h daily for 4 weeks (chronic stress), followed by recovery for 10 days. Saiko-ka-ryukotsu-borei-to was administered during the stress and recovery periods (100, 300, or 1000 mg/kg daily, p.o.) or only during the recovery period (1000 mg/kg). After the recovery period, the adrenal weight was measured, and glucocorticoid feedback ability was evaluated by a dexamethasone suppression test using 30 microg/kg dexamethasone. The administration of saiko-ka-ryukotsu-borei-to during the stress and recovery periods prevented the stress-induced increase in adrenal weight or the attenuated negative feedback in a dose-dependent manner. The administration of saiko-ka-ryukotsu-borei-to during the recovery period alone also ameliorated the abnormality of the neuroendocrine system. These results indicate that saiko-ka-ryukotsu-borei-to is effective against chronic stress-induced abnormality of the neuroendocrine system. Because some symptoms and symptomatic relapses in depressives are attributed to dysfunction of the hypothalamo-pituitary-adrenal axis, the present findings provide information important for prevention and treatment of depression.

Dopamine-receptor stimulation in the prefrontal cortex ameliorates stress-induced rotarod impairment

Exposure to chronic stress is thought to play an important role in the etiology of depression. In this disorder, dopaminergic dysfunction in the prefrontal cortex (PFC) is thought to be involved. Indeed, chronic stress reduces dopaminergic transmission in the rat PFC or induces a behaviorally depressive state. However, a relationship between the reduced dopaminergic activity and the behavior of the chronically stressed rats has not been proven. Here, we examined the effects of local application of a dopamine Type I (D(1)) receptor-specific agonist, SKF 81297, in the PFC on the chronic-stress-induced depressive state using a rotarod test. The chronic stress produced by water immersion and restraint for 4 weeks followed by recovery for 10 days impaired the rotarod performance without changing the traction performance or locomotor activity. Although intra-PFC infusion of 1 or 10 ng of SKF 81297 did not affect this impairment, 100 ng of SKF 81297 significantly ameliorated it. These results suggest that the chronic-stress-induced depressive state is caused by a D(1) receptor-mediated hypodopaminergic mechanism in the PFC. These findings will further understanding of the mechanisms underlying the pathophysiology of depression.

Stressor controllability and learned helplessness research in the United States: sensitization and fatigue processes

Recent work in the learned helplessness paradigm suggests that neuronal sensitization and fatigue processes are critical to producing the behavioral impairment that follows prolonged exposure to an unsignaled inescapable stressor such as a series of electric tail shocks. Here we discuss how an interaction between serotonin (5-HT) and corticosterone (CORT) sensitizes GABA neurons early in the pretreatment session with inescapable shock. We propose that this process eventually depletes GABA, thus removing an important form of inhibition on excitatory glutamate transmission in the amygdala, hippocampus, and frontal cortex. When rats are re-exposed to shock during shuttle-escape testing 24 hrs later, the loss of inhibition (as well as other excitatory effects) results in unregulated excitation of glutamate neurons. This state of neuronal over-excitation rapidly compromises metabolic homeostasis. Metabolic fatigue results in compensatory inhibition by the nucleoside adenosine, which regulates neuronal excitation with respect to energy availability. The exceptionally potent form of inhibition associated with adenosine receptor activation yields important neuroprotective benefits under conditions of metabolic failure, but also precludes the processing of information in fatigued neurons. The substrates of adaptive behavior are removed; performance deficits ensue.

Chronic stress differentially regulates glucocorticoid negative feedback response in rats

Exposure to chronic stress is thought to play an important role in the etiology of depression. In this disorder, a disrupted negative feedback response to exogenous glucocorticoids on cortisol secretion has been indicated. However, the regulation of glucocorticoid negative feedback by chronic stress is not fully understood. In the present study, we investigated the effects of chronic stress administered by water immersion and restraint (2 h/day) for four weeks on the glucocorticoid feedback in rats. In the acutely (one-time) stressed rats, the basal plasma corticosterone (CORT) level was markedly elevated, remained at high levels for 5 h after the termination of stress, and then decreased. In the chronically stressed rats, the CORT level was initially elevated similarly, but rapidly decreased at 2 h. In the dexamethasone (DEX) suppression test, the peak CORT level in response to stress was not suppressed by DEX in the acutely stressed rats, but was significantly suppressed in the chronically stressed rats. In contrast, the suppressive effects of DEX on the basal CORT secretion in naive rats were attenuated in the chronically stressed rats. In the chronically stressed hippocampus, which plays an important role in the regulation of the glucocorticoid feedback response, the binding of [3H]DEX was decreased and the increased response of activator protein-1 induced by acute stress was abolished. These results suggest that chronic stress induces a hypersuppressive state for induced CORT secretion in response to acute stress, which is caused by partial habituation, coping, and adaptation to the stressor, whereas it induces a hyposuppressive state for the basal CORT secretion, which is caused by glucocorticoid receptor downregulation. These mechanisms may be involved in the stress-induced neural abnormalities observed in depression.

Dexamethasone suppression of corticosteroid secretion: evaluation of the site of action by receptor measures and functional studies

A dose of dexamethasone was determined in rats (50 micrograms/kg s.c.) that suppressed the corticosterone response to restraint stress by 80%. Corticosteroid receptor occupancy estimates found that the 50 micrograms/kg s.c. dose of dexamethasone had no significant effect on available glucocorticoid receptor (GR) or mineralocorticoid receptor (MR) binding in brain regions (hypothalamus, hippocampus and cortex); on the other hand dexamethasone produced a selective and significant decrease in available GR in peripheral tissues (pituitary and spleen). Functional studies showed that the 50 micrograms/kg s.c. dose of dexamethasone completely blocked the effects of corticotropin-releasing hormone (CRH; 0.3-3.0 micrograms/kg i.p.) on corticosterone secretion, but did not inhibit the corticosterone response to an adrenocorticotropin hormone (ACTH; 2.5 I.U./kg i.p.) challenge. These studies indicate that this dose of dexamethasone exerts its inhibitory effects on the HPA axis primarily by acting at GR in the pituitary. The plasma dexamethasone levels produced by this dose of dexamethasone are similar to those present in humans the afternoon after an oral dexamethasone suppression test (DST), a time at which many depressed patients escape from dexamethasone suppression. These results support and extend other studies which suggest that the DST provides a direct test of the effects of increased GR activation in the pituitary on ACTH and cortisol secretion.

The effect of stressor controllability on stress-induced neuropeptide mRNA expression within the paraventricular nucleus of the hypothalamus

Many stressors elicit changes in corticotrophin (CRH), enkephalin (ENK), and neurotensin (NT) mRNA levels within the medial parvocellular region of the paraventricular nucleus of the hypothalamus (mpPVN), and the pattern of changes in mRNA levels appears to depend on the physical characteristics of the stressor. We questioned whether psychologically distinct stressors would cause different patterns of neuropeptide mRNA expression within the PVN. Psychologically distinct stressors were created by employing a paradigm of escapable (controllable) vs. non-escapable (yoked) tail shock. An adult male rats could terminate the stress stimulus by performing wheel-turning behaviour; his behaviour also terminated the stress for his yoked partner, who had no control over the termination of the shock. Four h post-stress, brains were collected and processed for in-situ hybridization histochemistry. Tail-shock stress stimulated a significant increase in CRH, ENK, and NT mRNA levels within the mpPVN. The number of CRH identified neurones coexpressing AVP mRNA was also significantly elevated in both stress groups. Moreover, the pattern and magnitude of the stress-induced increases in mRNA was similar in both stress groups. Additionally, no stress-induced changes in CRH mRNA levels were observed in the central nucleus of the amygdala. In sum, two psychologically distinct stressors, escapable vs. yoked tail shock stress, stimulated similar increases in CRH, NT, ENK, and AVP mRNA levels within the mpPVN. These results suggest that physical attributes of a stress, rather than psychological, may be the more important factors in determining the PVN mRNA response.

Principles of psychoneuroendocrinology

The goal of this article is to describe some of the central nervous system circuits involved in the regulation of the hypothalamopituitary-adrenocortical (HPA) axis, with an emphasis on animal models believed to mimic the human experience of emotional stress. First, the basic constitutive elements of the HPA axis that control glucocorticoid secretion are reviewed. A description of the neural systems assumed to regulate the activity of the HPA axis, both anatomically and functionally, follows. It is argued that hypothalamic, septal and bed nucleus of the stria terminalis neurons are involved in the regulation of the HPA axis by situations eliciting emotional responses.

Poststress glucose mitigates behavioral impairment in rats in the "learned helplessness" model of psychopathology

Three experiments examined the effects of poststress glucose treatment in the learned helplessness model of psychopathology in rats. In experiment 1, rats were given access to water or 40% aqueous glucose immediately following exposure to inescapable tailshocks or simple restraint in a 2 x 2 factorial design. Inescapably shocked rats failed to drink the glucose solution during the poststress interval and failed to show any improvement 24 hours after stress induction in shuttle-escape performance. Consequently, all rats received preexposure to a sweetened glucose cocktail in an attempt to increase poststress ingestion following inescapable shock treatment in experiment 2. Under these conditions, poststress intake of the glucose cocktail eliminated behavioral impairment in inescapably shocked rats relative to water-treated shocked rats and water- and glucose-treated restrained controls. Experiment 3 demonstrated that glucose prophylaxis occurs in the absence of sucrose when rats are preexposed to a 40% glucose solution prior to stress induction.

Richard L. Solomon and learned helplessness

This article reviews the course of development of research on a currently popular explanatory approach to dysfunctional behavior, the learned helplessness analysis. The early history is prominent in this review as it reflects the inspirations of Richard L. Solomon, a scholar who fostered the resurgence of psychologists' interests in Pavlovian conditioning in the 1950s and 1960s. Current research is characterized as having four separate themes: elaboration of "symptoms," elucidating the role of fear, explicit modeling, and extensions involving attributional constructs.

Corticosterone is involved in foot shock-induced inactivity in rats

Inescapable shock (IS) exposure induces behavioral inactivity, related to behavioral alterations in subsequent tests (i.e., escape failure, and inactivity during shuttle box task). Metyrapone (150 mg/kg, IP), a corticosterone (CS) synthesis inhibitor, administered 3 h prior to IS reduced inactivity during this aversive experience. Forty-eight hours later, when these rats were submitted to a shuttle box task, a reduction in both escape failure and inactivity was observed. These effects were reversed by CS (20 mg/kg, SC) and dose dependent of the synthetic glucocorticoid dexamethasone, both administered 1 h before IS. When metyrapone was administered 3 h before the shuttle box task to IS-exposed animals, escape failures and inactivity were markedly reduced. This effect was subsequently reversed by CS. The dynamics of changes in serum CS concentrations after both IS and shuttle box task paralleled behavioral changes. Animals injected with metyrapone before IS, which displayed active behavior, showed serum CS levels stable at their basal levels after shock, and their secretion pattern was quite attenuated after the shuttle box task, whereas vehicle-, CS alone-, and metyrapone + CS-injected animals showed higher serum CS concentrations post-IS, which slowly decreased to their corresponding basal levels. CS secretion after the shuttle box task was similar for the three groups: it had the same magnitude as after IS, though the decrease was faster. In all groups, animals displayed passive behavior. These results indicate that glucocorticoids are involved in the onset and expression of passive behaviors induced by uncontrollable stressors. Therefore, it is possible to suggest a functional relationship between CS released by exposure to inescapable stressor and the behavioral strategies adopted by rats under this stressful condition.

Increased 5-HT2 receptor-mediated behavior 11 days after shock in learned helplessness rats

In the learned helplessness procedure, rats can be differentiated into two distinct groups. Learned helplessness (LH) rats do not learn to escape a controllable shock while non-learned helplessness (NLH) rats learn this response. This deficit in performance in LH rats lasted for 11 days. In LH rats, pretreatment with acute desipramine (15 mg/kg i.p.) or chronic diazepam (0.95 mg/kg/day p.o. for 7 days) did not produce recovery from this deficit of performance, but pretreatment with chronic desipramine (17.7 mg/kg/day p.o. for 7 days) or chronic mianserin (6.1 mg/kg/day p.o. for 7 days) led to recovery. Before presentation of uncontrollable shock, there was no difference between LH and NLH rats, but 11 days after the shock, head shakes induced by (+/-)-1-(2,5-demethoxy-4-iodophenyl)-2-aminopropane (DOI) in LH rats was significantly more frequent than those in NLH and naive rats without change of [3H]ketanserin binding. The basal corticosterone level was higher in LH rats than in NLH rats. These findings suggest that the learned helplessness model is a reliable animal model of depression accompanied by 5-HT2 receptor hypersensitivity.

Effects of ACTH and ACTH 4-10 on aversive memory retrieval in rats

The aim of the study was to examine whether ACTH and ACTH-fragment 4-10, given before the test would produce a selectively enhanced retrieval of aversive memories, in the same way as preexposure to inescapable footshocks, in rats. For this purpose animals conditioned in a T-maze with appetitive (10% sucrose) and aversive (2.0 mA footshock) events were administered (s.c.) a single dose of 10, 20 or 40 ug/rat of ACTH or 5, 10 or 20 ug/rat of ACTH-fragment 4-10, 20-min before testing. The retention test conducted in the same training apparatus 72-hrs after conditioning showed a dose-dependent increase in latencies to enter the previously shocked goalarm with the absence of such a difference in responding to the nonshocked goalarm, in ACTH and ACTH 4-10 treated groups. This differential response was not observed in saline treated rats. This effect of peptides on memory retrieval was similar to that seen following inescapable footshock in rats. The results suggest the possible involvement of ACTH in the differential enhancement of memory of helplessness condition.

Vasopressin, immobilization, and the dexamethasone suppression test in rats

To investigate the role of the hormone vasopressin (VP) in mediating the response of the body to stress, corticosterone levels of VP-containing (LE) rats and VP-deficient (DI) rats were compared following administration of the dexamethasone suppression test (DST) under stressed and nonstressed conditions. The stressor utilized was immobilization, an acute physical stressor. Dexamethasone (DEX), a synthetic glucocorticoid, was injected subcutaneously at a dose of 0.025 mg/kg. This dose of DEX was found to significantly suppress plasma corticosterone in the nonstressed animals (both DI and LE) via feedback inhibition of the hypothalamic-pituitary-adrenocortical (HPA) axis. In the stressed situation, however, LE animals exhibited "escape" from DEX suppression, whereas DI animals did not. Escape indicates a resistance of the HPA axis to the suppressive action of DEX. Thus, in the absence of corticotropin-releasing factor, which is inhibited by DEX, VP alone appears to be sufficient to elicit significant corticosterone release. These results support the hypothesis that VP plays an important role in the regulation of glucocorticoid release in acute stress via the HPA axis.

Corticosterone influences forced swim-induced immobility

The effect of corticosterone (CS) synthesis inhibition with metyrapone-a blocker of the 11 beta-hydroxylase (150 mg/kg IP)-on immobility time during the forced swim test was recorded. Immobility time was measured during a 15-min forced swim (test). Twenty-four hours later rats were subjected to an additional 5 min forced swim (retest). In one experiment, metyrapone or vehicle was administered 3 h before the initial test, while CS (0, 5, 10, or 20 mg/kg SC) was administered 1 h prior to the initial test. Metyrapone significantly reduced immobility time during both test and retest. This effect was reverted in a dose-dependent fashion by CS. In a second experiment, animals exposed to the initial test 24 h before were injected with metyrapone or vehicle 3 h before the retest, while CS (0, 10, or 20 mg/kg SC) was administered 1 h prior the retest. Metyrapone, administered before the retest, reduced immobility time and CS partially reverted metyrapone effect. In another group of animals, serum CS concentrations were evaluated before and after test and retest. In vehicle groups, the high immobility time during test and retest was associated with high CS serum concentrations poststress. In animals receiving metyrapone prior to the initial test, the reduced immobility time was related to low levels of CS after the test and an attenuated secretion following the retest. Moreover, CS (20 mg/kg) and metyrapone+CS groups had high CS levels before the test, which remained high 2 h after the test, although after the retest, both groups showed a pattern of CS secretion similar to that observed in vehicle animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the antiglucocorticoid RU 38486 on the induction of learned helpless behavior in Sprague-Dawley rats

Learned helplessness (LH) is induced by exposure to an inescapable or uncontrollable stressor which results in an inability to escape or avoid the same stressor when subsequently presented in a different context. In order to understand which central mechanisms may influence the expression of the learned helpless phenotype, we have pursued an experimental approach that seeks to elucidate the behavioral effects of glucocorticoid (GC) hormones in this animal model of depression. We have previously shown that the induction of LH behavior is enhanced by adrenalectomy, an effect that is reversed by corticosterone. In this study, our aim was to attempt to locate CNS sites responsible for the observed effects of glucocorticoids on learned helpless behavior by introducing the type II GC receptor antagonist, RU 38486 to discrete brain regions. We did not observe a significant effect in LH with acute systemic, acute dentate gyrus or intracerebroventricular injection of RU 38486 in contrast to previous studies using the Porsolt swim test, another animal model of depression. However, we were able to observe a significant change upon chronic administration to the dentate gyrus. These findings suggest that glucocorticoids exert a long-term influence on stress-induced behavior, presumably by affecting glucocorticoid responsive genes in the dentate gyrus.

Modulation of [3H]paroxetine binding to the 5-hydroxytryptamine uptake site in an animal model of depression

The effects of learned helplessness on the 5-hydroxytryptamine (5-HT) uptake site were studied in rats using [3H]paroxetine binding. This ligand was chosen because it was demonstrated to label directly the 5-HT uptake site whereas the [3H]imipramine binding site has been demonstrated to be heterogeneous in nature. Moreover, [3H]imipramine appears to bind to a presynaptic recognition site different from the uptake site. Exposure to uncontrollable shock training and testing resulted in an overall increase in [3H]paroxetine binding in all the groups studied [nonhelpless (NLH), learned helpless (LH), spontaneously helpless (SPLH)] as compared to naive controls (NC). However, the increase in [3H]paroxetine binding was significantly higher in the LH and SPLH groups. The maximum number of [3H]paroxetine binding sites in the rat hippocampus was increased significantly in learned helpless rats (LH and SPLH) at day 4 and day 30 after the shock escape test as compared to NC and NLH rats. By contrast, in the rat hypothalamus the maximum number of [3H]paroxetine binding sites was reduced significantly in the LH rats as compared to naive controls and NLH rats during the same time course. There was no change in [3H]paroxetine binding sites in any other brain regions examined in LH, NLH, and NC rats. The results suggest that a hippocampal hypothalamic connection might play a role in the serotonergic mediation of learned helpless behavior.

Tricyclic antidepressants, thyroid function, and their relationship with the behavioral responses in rats

We first studied the effects of tricyclic antidepressants (TCAs) on thyroid function in rats in the learned helplessness paradigm. TCAs (clomipramine 32 mg/kg, desipramine 16, 24 mg/kg, or imipramine 8, 16, 32 mg/kg per day) were injected IP for 5 consecutive days. Blood samples were collected 1 hr after the last administration of the antidepressant for radioimmunoassay determination of triiodothyronine (T3) and thyrotropin. Whereas inducing helplessness did not result in any change in T3 and thyroid-stimulating hormone (TSH) levels, TCA therapy dose dependently decreased the T3 levels without changing TSH levels in helpless animals and in naive control rats. To further the investigation, the effects of TCAs on thyroid function were examined using two models of experimentation, one involving diabetes induction, the other using food deprivation; both are known to induce a resistance to TCAs that is reversible under T3 treatment. In both models, a decreased T3 level existed prior to the TCA administration. Although they had no effect on behavior, TCAs further decreased the T3 levels in diabetic and food-restricted rats. This study confirms that TCAs decrease thyroid function and suggests that the antidepressant effect of TCAs is not related to their T3 decreasing effects.

Differential sensitivity to dexamethasone suppression in an animal model of the DST

The present study reports the feedback suppression of basal and stimulated corticosterone secretion in rats by low doses of dexamethasone (DEX). DEX suppression of basal secretion 6 hr after administration was observed with doses as low as 0.005 mg/kg. The lowest dose capable of suppressing basal corticosterone levels for 24 hr with a return to normal values by 36 hr was established to be 0.025 mg/kg. The ability of DEX to decrease corticosterone responses to physostigmine, morphine, immobilization, and ether stress was determined. Although the magnitude of the rise in corticosterone did not differ significantly among these evocative stimuli, the degree to which DEX attenuated these responses varied. The response to morphine was completely prevented by 0.025 mg/kg and the rises following ether or immobilization were decreased significantly. In contrast, the response to physostigmine was not affected by DEX. With a higher dose of DEX (0.25 mg/kg), responses to morphine, ether, and immobilization were completely eliminated, but the response to physostigmine was only attenuated partway. The time course of the suppression in basal levels, the attenuation of several stimuli for corticosterone secretion, and the "escape" of physostigmine-induced corticosterone secretion resemble the clinical Dexamethasone Suppression Test of endogenous depression and suggest that this test might be useful in the study of animal models of depression.

Suppression of feeding and body weight by inescapable shock: modulation by quinine adulteration, stress reinstatement, and controllability

Three experiments examined food intake and body weight in rats after exposure to one session of intermittent, inescapable electric shock. Quinine adulteration and shock both suppressed feeding (Experiment 1); recovery of feeding after shock was impeded when quinine adulteration was combined with a mild daily stress reinstatement (Experiment 2). Body weight also was suppressed by shock (Experiments 1 and 2); control over shock provided some protection against this deficit (Experiment 3). These results suggest roles for "finickiness" and vulnerability to mild stressors in the maintenance of eating disorders associated with stress and depression. The findings also may have implications for interpretation of deficits in appetitively motivated behaviors after stress.

Gonadotropin-releasing hormone (GnRH) as antidepressant: a psychopharmacological animal study

Because the secretion of gonadotrophic hormones is disturbed in some depressive states, it has been hypothesized that gonadotropin-releasing hormone (GnRH) has antidepressant properties in humans, but no clear information has emerged from clinical trials. The lack of experimental psychopharmacological data prompted us to investigate the effects of GnRH on the 'learned helplessness' behavioral model of depression in rats. GnRH was injected i.p. at doses of 0.06, 0.25, 0.50, 1 and 2 mg/kg per day. GnRH significantly reduced the number of escape failures at doses of 1 mg/kg per day or higher during the first shuttle-box session and at doses of 0.25 mg/kg per day onwards during the third shuttle-box session. These antidepressant-like effects of GnRH were similar to those observed with the tricyclic antidepressants imipramine (32 mg/kg per day) or clomipramine (32 mg/kg per day) in the same model. Moreover, while the induction of learned helplessness behaviour resulted in a fall in the plasma levels of FSH and LH, normal values of these hormones could be restored by a behaviorally effective GnRH regimen. From these data it can be suggested that GnRH exhibits an interesting antidepressant-like activity in rats.

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.