Robert C. Drugan

Central and peripheral benzodiazepine receptors: Involvement in an organism's response to physical and psychological stress

The present review discusses the current knowledge of the molecular pharmacology and neuroanatomical and subcellular localization of both the central benzodiazepine/GABA-chloride ionophore receptor complex and the peripheral benzodiazepine receptor. It then reviews all of the literature to date on how these two receptor sites are modulated by environmental stress. The possible role of these sites in learning and memory is also discussed. Finally, a theoretical model is presented which examines the differential, and perhaps complementary, alterations of these two sites in an organisms response to stress.

Impact of water temperature and stressor controllability on swim stress-induced changes in body temperature, serum corticosterone, and immobility in rats

The present study compared the effects of three different water temperatures (20, 25, and 30 degrees C) and stressor controllability on several physiological and behavioral endpoints in an intermittent swim stress paradigm. The escape latency of rats in the 20 and 25 degrees C water was less than that observed for the 30 degrees C group. Both escape and yoked groups at 20 and 25 degrees C exhibited moderate to severe hypothermia following the swim stress session that returned to prestress levels 30-40 min post-stress. At 30 degrees C core body temperature (Tb) only decreased by 1 degree C for either swim group. Following swim, serum corticosterone (CORT) levels were significantly elevated in both escape and yoked groups in comparison to confined and home cage controls. The confined control group showed a significant elevation that was approximately halfway between the home cage control and the swim stress groups. At 30 degrees C, there was still a significant elevation of serum CORT in both swim groups in comparison to confined and home cage controls. Therefore, 30 degrees C appears to be the optimal water temperature to evaluate stress controllability effects in the current paradigm. In a final experiment, swim stressor controllability effects were examined in a 5 min forced swim test (FST) 24 h following the initial stress exposure. Rats exposed to yoked-inescapable swim stress at 30 degrees C exhibited more immobility than their escapable swim stress and confined counterparts, while the escape and confined controls did not differ. These results demonstrate that the behavioral deficits observed in the FST are attributable to the stress of inescapable swim and not swim stress per se.

The protective effects of stress control may be mediated by increased brain levels of benzodiazepine receptor agonists

Control over stress protects against many of the deleterious effects of stress exposure, but the endogenous mediators responsible for these prophylactic effects have remained elusive. Using behavioral pharmacology, in vitro radioligand binding and neurochemical analyses, we demonstrate that exposure to escapable stress results in brain and behavior changes reminiscent of benzodiazepine administration. The stress control group shows significant protection against picrotoxinin-induced seizures, reductions in [35S]t-butylbicyclophosphorothionate (TBPS) binding and a 3-fold increase of benzodiazepine-like substances in brain in comparison to both yoked-inescapable shock and non-shock controls. These observations suggest that coping behavior leads to the release of endogenous benzodiazepine-like compounds in brain which protect the organism from stress pathology.

Sex differences in open-field behavior in response to the β-carboline FG 7142 in rats

Sex differences in animal models of anxiety and depression that employ external stimuli have been previously reported. This study examined the effect of gender on pharmacologically induced anxiety in rats coupled with novel handling, injection, and activity in an open-field test. The anxiogenic compound FG 7142 significantly decreased male open-field exploratory at 5, 10, and 20 mg/kg, while rearing behavior was decreased only at the 20 mg/kg dose. Female rats were more resistant to the effects of FG 7142 on open-field exploratory behavior, decreasing open-field activity, and rearing behavior only after the administration of a dose of 40 mg/kg. In addition, a significant sex difference was observed in the open-field activity of rats injected with the vehicle control. Male rats were less active but showed a similar level of rearing behavior when compared to female rats. The greater sensitivity of male rats to the activity-suppressant effects of FG 7142 could explain the sex differences observed in several other animal models of anxiety and depression.

Differential effects of anxiogenic central and peripheral benzodiazepine receptor ligands in tests of learning and memory

Previous research has demonstrated that low doses of anxiogenic central benzodiazepine receptor (CBR) ligands, the beta-carbolines, improve performance in various learning and memory tests in animals if administered prior to training. The present experiments compared the effect of a beta-carboline (FG 7142) with that of a pharmacologically distinct anxiogenic compound, a peripheral benzodiazepine receptor (PBR) ligand, 4′-chlorodiazepam (Ro5-4864), in two tests of learning and memory in rats. As expected, FG 7142 significantly improved performance in a passive avoidance test. Ro5-4864 was without effect. In a shuttlebox escape test, Ro5-4864 significantly impaired performance while FG 7142 had no effect. The effect of Ro5-4864 was antagonized by the specific peripheral benzodiazepine receptor antagonist, PK 11195. These results indicate that the differential impact of CBR and PBR anxiogenic ligands on performance in aversively-motivated learning tests may be a reflection of their distinct pharmacologies.

Stress resilience and vulnerability: The association with rearing conditions, endocrine function, immunology, and anxious behavior

BACKGROUND The current study explored the underlying behavioral, endocrine, and immune markers of vulnerability to stress-induced depression, and the impact of rearing environments on adult functioning. METHOD Adult Sprague-Dawley rats (n=195) were reared in either Maternal Separation (MS), Early Weaning and Isolation (EWI), or Non-Handled (NH) conditions. Anxiety behavior was assessed using the emergence test at mean postnatal day (PND) 60. Stress-induced depressive behavior was measured at mean PND 86 using an intermittent cold water swim stress and swim escape test (SET) paradigm. Immediately following the SET, and in a sample of naïve controls (N=31), trunk blood was collected to assay for serum corticosterone (CORT) and spleens were removed for determination of Concanavalin A (Con-A) stimulated T-cell proliferation. RESULTS Stress vulnerable rats (top tertile of SET swim time) were characterised by increased anxiety-like behavior, greater post-stress CORT concentrations, and a significantly higher Con-A induced T-cell proliferative response compared to stress resilient rats (bottom tertile of SET swim time). The EWI rearing condition was a contributing factor in predicting total swim escape time, however MS was not. MS offspring did have double the basal level of CORT than NH offspring, suggestive of a hyperfunctioning HPA axis. CONCLUSION The swim stress animal model enabled observation of stress vulnerability and resilience; results point towards the existence of distinct behavioral, endocrine, and immunological profiles of the vulnerable and resilient animal, which may have important implications for mental health and stress research.

Intermittent cold water swim stress increases immobility and interferes with escape performance in rat.

The behavioral consequences of intermittent, 5 s cold-water swims (15 degrees C) or confinement were assessed 24 h after stress in a 5 min forced swim test or an instrumental swim escape test (SET). The SET was conducted with temporal and instrumental parameters similar to the shock-motivated shuttle escape test. The tests detected significantly increased immobility in the forced swim test and increased latency to escape in the SET. These results extend previous findings with intermittent swim stress and provide evidence that intermittent swim stress produces behavioral deficits similar to other stress models. This new model may be a useful tool for exploring the physiological mechanisms underlying the stress response.

Environmentally induced changes in peripheral benzodiazepine receptors are stressor and tissue specific

The stress-induced changes in peripheral benzodiazepine receptors (PBR) can be observed in a number of different tissues, depending upon the nature and chronicity of the aversive experience. In addition, virtually all stress procedures that cause rapid changes in PBR simultaneously increase the physical activity or metabolic rate of the subjects. The present study analyzed the contributions of rapid alterations in activity or metabolic rate with and without aversive stimulation and their subsequent impact on PBR. Mechanically induced increases in activity by forced running stress results in a significant reduction in [3H]Ro 5-4864 binding to PBR in olfactory bulb, opposite to the PBR changes in this tissue following forced cold-water swim stress. Pharmacological induction of increased locomotor activity as well as metabolic rate by d-amphetamine causes a significant increase in cardiac PBR binding, again, opposite to the response typically observed following inescapable shock stress. Finally, administration of the anxiogenic beta-carboline, FG-7142, causes increases in both hippocampus and adrenal gland PBR binding reminiscent of acute noise stress exposure. These experiments demonstrate that increased locomotor activity or metabolic rate alone is not a necessary and sufficient condition for previous stress-induced changes in PBR. Conversely, increased metabolic rate coupled with an aversive stimulus appears to be an important factor for inducing stress-like changes in PBR. This data, coupled with previous reports, suggests that rapid alterations in these sites are stressor and tissue dependent. Finally, we propose that the PBR may be involved in many aspects of the stress response including: a) a blowarning system in adrenal gland, b) participation in stress-induced hypertension via renal PBR, and c) a modulator of stress-induced immunosuppression and subsequent recovery of function or recuperation by actions on immune cells.

Escapable stress modulates retention of spatial learning in rats: Preliminary evidence for involvement of neurosteroids

The ability to escape stress has been shown to protect an organism from many of the deleterious effects of stress exposure. It has been suggested that this amelioration could be mediated by the release of an endogenous benzodiazepine-like substance in the brain demonstrated 2 h poststress. Since benzodiazepines possess amnestic as well as antianxiety actions, the possibility of memory alterations in coping subjects was evaluated. Animals were randomly assigned to three groups: escapable shock, yoked inescapable shock, and no shock. Immediately poststress, all subjects were trained in a circular water maze. Acquisition and retention data were obtained in a between-subjects design at three different retention intervals (2 h, 4 h, and 24 h postshock). Results revealed no significant group differences in acquisition of the spatial learning task between groups. However, subjects in the escapable stress group had enhanced retention at 2 h postshock but were significantly impaired 24 h postshock relative to yoked-inescapable-shock and nonshock controls. Blockade of the synthesis of neuroactive GABAA positive steroids by 4-MA, a 5-alpha reductase enzyme inhibitor, blocked this effect. Thus, neuroactive steroid metabolites may play a critical role in the escapable-stress-induced retention deficit seen at 24 h poststress. These observations suggest that altered memory in the escapable-shock subjects may impart stress resiliency by reducing proactive interference of prior stress on subsequent learning and physiology.

Stress controllability influences the ataxic properties of both ethanol and midazolam in the rat.

Rats were administered either 80 escapable shocks or yoked inescapable shocks, were then injected with saline or several ataxic doses of either ethanol or midazolam, and then had their motoric impairment assessed by Rotarod performance. No motoric impairment was observed following saline injection. However, inescapable shock impaired Rotarod performance in response to both ethanol and midazolam at 2 hr, but not immediately poststress. Conversely, escapable shock reduced the ataxic potency of ethanol, although it had no influence on midazolam-induced ataxia. These results indicate functional alterations in behavioral reactivity to low doses of several classes of central nervous system depressants by psychological dynamics of stress exposure. Our findings demonstrate the impact of stress controllability on behavioral reactivity to two classes of drugs of abuse.