Lisa E. Kalynchuk

Effect of repeated corticosterone injections and restraint stress on anxiety and depression-like behavior in male rats

Repeated stress is an important risk factor for the development of depression. However, the mechanism by which stress influences depression is largely unknown, in part due to the fact that few animal models of repeated stress produce robust changes in depression-like behavior. The purpose of the present study was to characterize the effect of repeated corticosterone (CORT) injections and repeated restraint stress on anxiety and depression-like behavior in male rats. Rats received CORT injections (40 mg/kg, s.c.), vehicle injections, restraint stress (6 h/day), or handling once per day for 21 consecutive days prior to the behavioral testing. The rats were then tested for changes in fearful/anxious behavior in the open-field and social interaction tests and for depression-like behavior in the forced swim test. The repeated CORT injections had no significant effect on activity levels or anxiety in the open-field or social interaction tests. However, they did increase depression-like behaviors in the forced swim test. Repeated restraint stress had no significant effect on anxiety or depression-like behavior on any of the behavioral tests. These results suggest that repeated CORT injections warrant further investigation as an animal model to study the role of stress in depression.

Postnatal Inflammation Increases Seizure Susceptibility in Adult Rats

There are critical postnatal periods during which even subtle interventions can have long-lasting effects on adult physiology. We asked whether an immune challenge during early postnatal development can alter neuronal excitability and seizure susceptibility in adults. Postnatal day 14 (P14) male Sprague Dawley rats were injected with the bacterial endotoxin lipopolysaccharide (LPS), and control animals received sterile saline. Three weeks later, extracellular recordings from hippocampal slices revealed enhanced field EPSP slopes after Schaffer collateral stimulation and increased epileptiform burst-firing activity in CA1 after 4-aminopyridine application. Six to 8 weeks after postnatal LPS injection, seizure susceptibility was assessed in response to lithium–pilocarpine, kainic acid, and pentylenetetrazol. Rats treated with LPS showed significantly greater adult seizure susceptibility to all convulsants, as well as increased cytokine release and enhanced neuronal degeneration within the hippocampus after limbic seizures. These persistent increases in seizure susceptibility occurred only when LPS was given during a critical postnatal period (P7 and P14) and not before (P1) or after (P20). This early effect of LPS on adult seizures was blocked by concurrent intracerebroventricular administration of a tumor necrosis factor α (TNFα) antibody and mimicked by intracerebroventricular injection of rat recombinant TNFα. Postnatal LPS injection did not result in permanent changes in microglial (Iba1) activity or hippocampal cytokine [IL-1β (interleukin-1β) and TNFα] levels, but caused a slight increase in astrocyte (GFAP) numbers. These novel results indicate that a single LPS injection during a critical postnatal period causes a long-lasting increase in seizure susceptibility that is strongly dependent on TNFα.

Effect of different doses of corticosterone on depression-like behavior and HPA axis responses to a novel stressor.

Stress is recognized to precipitate depressive illness, yet the specific relationship between stress, glucocorticoids and depression is not well understood. We have recently shown that repeated corticosterone (CORT) injections reliably increase depression-like behavior on the forced-swim test in rats, suggesting that glucocorticoids can precipitate depressive symptomatology. The purpose of this experiment was to determine the extent to which the effects of CORT on depression-like behavior depend on (1) the dose-injected and (2) the duration of treatment. Rats received either acute or repeated injections of vehicle, 10, 20 or 40 mg/kg of CORT, and then were subjected to the forced-swim test. Serum CORT levels were assessed after the 21-day injection period, and 30 and 60 min after the onset of forced-swim testing. Repeated, but not acute, CORT injections decreased body weight and increased immobility behavior in the forced-swim test in a dose-dependent manner. In addition, all doses of repeated CORT injections suppressed CORT release after the novel stress of forced-swim testing. Our results demonstrate that glucocorticoids increase depression-like behavior in rats in a dose-dependent manner and disrupt normal HPA axis function. These results support the hypothesis that high levels of cortisol contribute to the etiology of depressive symptomatology in humans.

Corticosterone increases depression-like behavior, with some effects on predator odor-induced defensive behavior, in male and female rats.

This experiment examined the effect of repeated corticosterone injections on anxiety and depression-like behavior in male and female rats. Rats received either corticosterone or vehicle injections for 21 consecutive days prior to behavioral testing in the forced swim, open-field, and predator odor tests. The corticosterone injections significantly increased depression-like behavior in the forced swim test in both male and female rats but had no significant effect on anxiety in the open-field test. In the predator odor test, the corticosterone injections significantly increased a subset of defensive behaviors in the male rats. These results suggest that repeated exposure to corticosterone increases depression-like behavior, with some effects on anxiety, and that male rats may be more affected than female rats by this manipulation.

Behavioral and neurobiological consequences of prolonged glucocorticoid exposure in rats: Relevance to depression

Stress is a critical environmental trigger for the development of clinical depression, yet little is known about the specific neurobiological mechanisms by which stress influences the development of depressive symptomatology. Animal models provide an efficient way to study the etiology of human disorders such as depression, and a number of preclinical models have been developed to assess the link between stress, glucocorticoids, and depressive behavior. These mode ls typically make use of repeated exposure to physical or psychological stressors in rodents or other small laboratory animals. This review focuses primarily on a recently developed preclinical model of depression that uses exogenous administration of the stress hormone corticosterone (CORT) in rodents instead of exposure to physical or psychological stressors. Repeated CORT administration in rats or mice produces reliable behavioral and neurobiological alterations that parallel many of the core symptoms and neurobiological changes associated with human depression. This provides an opportunity to study behavior and neurobiology in the same animal, so that the neurobiological factors that underlie specific symptoms can be identified. Taken together, these findings suggest that exogenous CORT administration is a useful method for studying the relationship between stress, glucocorticoids, and depression. Further study with this model may provide important new data regarding the neurobiological bases of depression.

Long-term amygdala kindling in rats as a model for the study of interictal emotionality in temporal lobe epilepsy

Temporal lobe epileptics often experience profound interictal (i.e. between seizure) emotional disturbances, such as fear, anxiety, and depression. Although the presence of this interictal emotionality has been well documented, little progress has been made in identifying its precise nature and cause because it is not amenable to experimental analysis in clinical populations. Accordingly, there is much to gain by studying the fundamental nature and neural basis of interictal emotionality using animal models. Kindling is a widely studied animal model of temporal lobe epilepsy in which daily electrical stimulation of certain brain regions results in the gradual progression and intensification of limbic motor seizures. Several investigators have found that partial and short-term kindling produce robust changes in emotional behavior in both cats and rats. Recently, our laboratory has developed a new model to study interictal emotionality using long-term kindling in rats. These long-term kindled rats display profound changes in fearful and defensive behavior which last for at least two months after the final stimulation. We are now beginning to use this model to study the neural mechanisms underlying the development and expression of interictal emotionality.

Changes in emotional behavior produced by long-term amygdala kindling in rats

The effects of long-term amygdala kindling on emotional behavior were investigated. In Experiment 1, rats received 99 basolateral amygdala, central amygdala, or sham stimulations. The rats in both kindled groups displayed more resistance to capture from an open field and more open-arm activity on an elevated plus maze than did the sham control rats. In Experiment 2, rats received either 20, 60, or 100 amygdala stimulations or sham stimulations. Compared to the sham controls, the kindled rats explored less during the first 30s in a novel open field, avoided the central area of the open field, resisted being captured from the open field, and engaged in more open-arm activity on the elevated plus maze. The magnitude of these effects was greatest in the 100-stim rats and least in the 20-stim rats. Together, these results suggest that long-term amygdala kindling in rats is a useful model for studying the emotionality associated with temporal lobe epilepsy.

Repeated exposure to corticosterone increases depression-like behavior in two different versions of the forced swim test without altering nonspecific locomotor activity or muscle strength

We have recently shown that repeated high dose injections of corticosterone (CORT) reliably increase depression-like behavior on a modified one-day version of the forced swim test. The main purpose of this experiment was to compare the effect of these CORT injections on our one-day version of the forced swim test and the more traditional two-day version of the test. A second purpose was to determine whether altered behavior in the forced swim test could be due to nonspecific changes in locomotor activity or muscle strength. Separate groups of rats received a high dose CORT injection (40 mg/kg) or a vehicle injection once per day for 21 consecutive days. Then, half the rats from each group were exposed to the traditional two-day forced swim test and the other half were exposed to our one-day forced swim test. After the forced swim testing, all the rats were tested in an open field and in a wire suspension grip strength test. The CORT injections significantly increased the time spent immobile and decreased the time spent swimming in both versions of the forced swim test. However, they had no significant effect on activity in the open field or grip strength in the wire suspension test. These results show that repeated CORT injections increase depression-like behavior regardless of the specific parameters of forced swim testing, and that these effects are independent of changes in locomotor activity or muscle strength.

Object-recognition and spatial learning and memory in rats prenatally exposed to ethanol.

Prenatal ethanol exposure can produce cognitive and behavioral impairments. In the present study, rats from prenatal ethanol (E), pair-fed (PF), and ad libitum-fed control (C) treatment conditions were tested on the object-recognition delayed-nonmatching-to-sample (DNMS) task with nonrecurring items and on the spatial-navigation Morris water maze task. In Experiment 1, there were no significant differences among groups in object-recognition learning and memory, distractibility, or response perseveration on the DNMS task. In Experiment 2, the same rats were tested in the water maze; E rats took significantly longer to learn the task than did the PF or C rats. These data suggest that the mechanisms underlying spatial cognitive abilities are more vulnerable to the teratogenic effects of prenatal ethanol exposure than those underlying object-recognition abilities.

Repeated exposure to corticosterone, but not restraint, decreases the number of reelin-positive cells in the adult rat hippocampus.

Stress is an important risk factor for the emergence of depression, but little is known about the neurobiological mechanisms by which stress might promote depressive symptomatology. Much of the research on this topic has focused on stress-induced changes in hippocampal plasticity, specifically the idea that decreased hippocampal plasticity could be a precipitating factor for depression. Interestingly, recent evidence has described a regulatory role for the extracellular matrix protein reelin in important aspects of neural plasticity within the hippocampus and dentate gyrus. Given this association between reelin and hippocampal plasticity, we investigated whether repeated exposure to corticosterone or physical restraint might decrease reelin expression in specific hippocampal regions. Rats were subjected to either 21 days of corticosterone injections or physical restraint and then sacrificed so that the number of reelin-positive cells throughout the hippocampus and dentate gyrus could be quantified using immunohistochemistry. Our results revealed a significant decrease in the number of reelin-positive cells in the CA1 stratum lacunosum and the subgranular zone of the dentate gyrus in rats that received corticosterone, but not in rats that received restraint. Interestingly, these results parallel our previous observation that corticosterone increases depression-like behavior but physical restraint does not. These novel findings suggest that altered reelin signaling could play a role in the expression of depressive symptomatology after exposure to high levels of glucocorticoids.