Tara J. Nyhuis

Hypothalamic Pituitary Adrenal Axis Responses to Low-Intensity Stressors are Reduced After Voluntary Wheel Running in Rats

Regular physical exercise is beneficial for both physical and mental health. By contrast, stress is associated with deleterious effects on health and there is growing evidence that regular physical exercise counteracts some of the effects of stress. However, most previous studies have suggested that prior exercise does not alter the acute hypothalamic pituitary adrenal (HPA) axis responses to stress. The present series of studies provides evidence that in rats, 6 weeks (but not 1 or 3 weeks) of voluntary wheel running reduces the HPA axis responses to lower‐intensity stressors such as an i.p. saline injection, exposure to a novel environment or exposure to moderate intensity noise, but not to more intense stressors such as predator odour exposure or restraint. Daily exercise does not appear to be necessary for the reduction in HPA axis responses, with intermittent access (24 h out of each 72‐h period) to a running wheel for 6 weeks, resulting in similar decrements in adrenocorticotrophic hormone and corticosterone release in response to 85 dBA noise exposure. Data from in situ hybridisation for c‐fos mRNA are consistent with the hypothesis that voluntary exercise results in a decrease in HPA axis responsiveness to a low‐intensity stressor at a central level, with no changes in primary sensory processing. Together, these data suggest that 6 weeks of daily or intermittent exercise constrains the HPA axis response to mild, but not more intense stressors, and that this regulation may be mediated at a central level beyond the primary sensory input.

Chronic voluntary wheel running facilitates corticosterone response habituation to repeated audiogenic stress exposure in male rats

Voluntary exercise is associated with the prevention and treatment of numerous physical and psychological illnesses, yet the mechanisms by which it confers this protection remain unclear. In contrast, stress, particularly under conditions of prolonged or repeated exposure when glucocorticoid levels are consistently elevated, can have a devastating impact on health. It has been suggested that the benefits of physical exercise may lie in an ability to reduce some of the more deleterious health effects of stress and stress hormones. The present series of experiments provides evidence that voluntary exercise facilitates habituation of corticosterone but not adrenocorticotropin hormone responses to repeated stress presentations. After 6 weeks of running wheel access or sedentary housing conditions, rats were exposed to 11 consecutive daily 30 min presentations of 98 dB noise stress. Similar corticosterone responses in exercised rats and sedentary controls were observed following the first, acute stress presentation. While both groups demonstrated habituation of corticosterone secretory responses with repeated noise stress exposures, the rate of habituation was significantly facilitated in exercised animals. These results suggest that voluntary exercise may reduce the negative impact of prolonged or repeated stress on health by enhancing habituation of the corticosterone response ultimately reducing the amount of glucocorticoids the body and brain are exposed to.

Acute and chronic effects of ferret odor exposure in Sprague-Dawley rats

This manuscript describes several behavioral and functional studies evaluating the capacity of ferret odors to elicit a number of acute and long-term responses in male Sprague-Dawley rats. Acute presentation elicits multiple responses, suggesting that ferret odor, likely from skin gland secretions, provides an anxiogenic-like stimulus in this strain of rats. Compared to cat odor, however, ferret odor did not produce rapid fear conditioning, a result perhaps attributable to methodological factors. Inactivation of the olfactory system and medial nucleus of the amygdala, combined with induction of the immediate-early gene c-fos, suggest the necessity of the accessory olfactory system in mediating the effects of ferret odor. Repeated exposures to ferret odor produce variable habituation of neuroendocrine and behavioral responses, perhaps indicative of the lack of control over the exact individual origin or concentration of ferret odor. Ferret odor induces rapid and long-term body weight regulation, thymic involution, adrenal hyperplasia and facilitation of the neuroendocrine response to additional challenges. It is argued that the use of such odors is exquisitely suited to investigate the brain regions coordinating anxiety-like responses and the long-term changes elicited by such stimuli.

Conditioned Fear Inhibits c-fos mRNA Expression in the Central Extended Amygdala

We have shown previously that unconditioned stressors inhibit neurons of the lateral/capsular division of the central nucleus of the amygdala (CEAl/c) and oval division of the bed nucleus of the stria terminalis (BSTov), which form part of the central extended amygdala. The current study investigated whether conditioned fear inhibits c-fos mRNA expression in these regions. Male rats were trained either to associate a visual stimulus (light) with footshock or were exposed to the light alone. After training, animals were replaced in the apparatus, and 2 h later injected remotely, via a catheter, with amphetamine (2 mg/kg i.p.), to induce c-fos mRNA and allow inhibition of expression to be measured. The rats were then presented with 15 visual stimuli over a 30 minute period. As expected, fear conditioned animals that were not injected with amphetamine, had extremely low levels of c-fos mRNA in the central extended amygdala. In contrast, animals that were trained with the light alone (no fear conditioning) and were injected with amphetamine had high levels of c-fos mRNA in the CEAl/c and BSTov. Animals that underwent fear conditioning, and were re-exposed to the conditioned stimulus after amphetamine injection had significantly reduced levels of c-fos mRNA in both the BSTov and CEAl/c, compared to the non-conditioned animals. These data suggest that conditioned fear can inhibit neurons of the central extended amygdala. Because these neurons are GABAergic, and project to the medial CEA (an amygdaloid output region), this may be a novel mechanism whereby conditioned fear potentiates amygdaloid output.

Accessory and main olfactory systems influences on predator odor-induced behavioral and endocrine stress responses in rats

Exposures to predator odors are very effective methods to evoke a variety of stress responses in rodents. We have previously found that ferret odor exposure leads to changes in endocrine hormones (corticosterone and ACTH) and behavior. To distinguish the contributions of the main and accessory olfactory systems in these responses, studies were designed to interfere with these two systems either independently, or simultaneously. Male Sprague-Dawley rats were treated with 10% zinc sulfate (ZnSO(4)), which renders rodents anosmic (unable to smell) while leaving the accessory olfactory areas intact, or saline, in Experiment 1. In Experiment 2, the vomeronasal organs of rats were surgically removed (VNX) to block accessory olfactory processing, while leaving the main olfactory system intact. And in the third experiment both the main and accessory olfactory areas were disrupted by combining the two procedures in the same rats. Neither ZnSO(4) treatment nor VNX alone reliably reduced the increased corticosterone response to ferret odor compared to strawberry odor, but in combination, they did. This suggests that processing through the main or the accessory olfactory system can elicit the endocrine stress response to ferret odor. VNX alone also did not affect the behavioral responses to the ferret odor. ZnSO(4) treatment, alone and in combination with VNX, led to changes in behavior in response to both ferret and strawberry odor, making the behavioral results less clearly interpretable. Overall these studies suggest that both the main and accessory olfactory systems mediate the neuroendocrine response to predator odor.

Physical activity, but not environmental complexity, facilitates HPA axis response habituation to repeated audiogenic stress despite neurotrophin mRNA regulation in both conditions.

Stress exacerbates several physical and psychological disorders. Voluntary exercise can reduce susceptibility to many of these stress-associated disorders. In rodents, voluntary exercise can reduce hypothalamic-pituitary-adrenocortical (HPA) axis activity in response to various stressors as well as upregulate several brain neurotrophins. An important issue regarding voluntary exercise is whether its effect on the reduction of HPA axis activation in response to stress is due to the physical activity itself or simply the enhanced environmental complexity provided by the running wheels. The present study compared the effects of physical activity and environmental complexity (that did not increase physical activity) on HPA axis habituation to repeated stress and modulation of brain neurotrophin mRNA expression. For six weeks, male rats were given free access to running wheels (exercise group), given 4 objects that were repeatedly exchanged (increased environmental complexity group), or housed in standard cages. On week 7, animals were exposed to 11 consecutive daily 30-min sessions of 98-dBA noise. Plasma corticosterone and adrenocorticotropic hormone were measured from blood collected directly after noise exposures. Tissue, including brains, thymi, and adrenal glands was collected on Day 11. Although rats in both the exercise and enhanced environmental complexity groups expressed higher levels of BDNF and NGF mRNA in several brain regions, only exercise animals showed quicker glucocorticoid habituation to repeated audiogenic stress. These results suggest that voluntary exercise, independent from other environmental manipulations, accounts for the reduction in susceptibility to stress.

Disruption of neuroendocrine stress responses to acute ferret odor by medial, but not central amygdala lesions in rats

Investigations of the neural pathways associated with responses to predators have implicated the medial amygdala (MeA) as an important region involved in defensive behaviors. To our knowledge, however, the involvement of the MeA in neuroendocrine responses to predator odor exposure has not been investigated. Therefore, the present study examined the effects of MeA disruption in rats exposed to ferret or control odor on hypothalamo-pituitary-adrenocortical (HPA) axis activation. Bilateral lesions of the MeA were made in Sprague-Dawley rats with the neurotoxin ibotenic acid (10 microg/microl; 0.3 microl / side). As a control for regional specificity, additional groups of rats were given lesions in the central amygdala (CeA). One week after recovery, the rats were exposed to ferret or strawberry control towels in small cages to examine HPA axis responses as determined by plasma corticosterone and adrenocorticotropin hormone (ACTH) levels. Rats with complete bilateral MeA but not CeA lesions displayed significantly less corticosterone and ACTH release compared to sham-operated control rats only in the ferret odor conditions. These results suggest that the MeA is an important structure involved in the HPA axis responses to predator odors, in support of previous studies investigating behavioral responses under similar conditions.

Effects of voluntary wheel running on heart rate, body temperature, and locomotor activity in response to acute and repeated stressor exposures in rats.

Stress often negatively impacts physical and mental health but it has been suggested that voluntary physical activity may benefit health by reducing some of the effects of stress. The present experiments tested whether voluntary exercise can reduce heart rate, core body temperature and locomotor activity responses to acute (novelty or loud noise) or repeated stress (loud noise). After 6 weeks of running-wheel access, rats exposed to a novel environment had reduced heart rate, core body temperature, and locomotor activity responses compared to rats housed under sedentary conditions. In contrast, none of these measures were different between exercised and sedentary rats following acute 30-min noise exposures, at either 85 or 98 dB. Following 10 weeks of running-wheel access, both groups displayed significant habituation of all these responses to 10 consecutive daily 30-min presentations of 98 dB noise stress. However, the extent of habituation of all three responses was significantly enhanced in exercised compared to sedentary animals on the last exposure to noise. These results suggest that in physically active animals, under some conditions, acute responses to stress exposure may be reduced, and response habituation to repeated stress may be enhanced, which ultimately may reduce the negative and cumulative impact of stress.

Evidence for a lack of phasic inhibitory properties of habituated stressors on HPA axis responses in rats

This experiment tested the hypothesis that habituation to repeated stressor exposures is produced by phasic inhibitory influence on the neural circuitry that normally drives the paraventricular nucleus of the hypothalamus and subsequently the adrenocortical hormone response to psychological stress. Such a process would be expected to lower the acute response to a novel stressor when experienced concurrently with a habituated stressor. Rats were exposed to restraint or no stress conditions for 14 consecutive days. On the 15th day, the rats were exposed to the control condition (no stress), acute restraint, loud noise, or restraint and loud noise concurrently. Blood was taken and assayed for ACTH and corticosterone and brains were collected to examine c-fos messenger RNA expression in several brain areas. As predicted, the rats that received the same (homotypic) stressor repeatedly and again on the test day displayed low levels of ACTH and corticosterone, similar to the control conditions (i.e., showed habituation). All rats that received a single novel stressor on the test day, regardless of prior stress history, exhibited high levels of ACTH and corticosterone. The rats that received two novel stressors also displayed high levels of ACTH and corticosterone, but little evidence of additivity was observed. Importantly, when a novel stressor was concurrently given with a habituated stressor on the test day, no reduction of HPA axis response was observed when compared to previously habituated rats given only the novel stressor on the test day. In general, c-fos mRNA induction in several stress responsive brain areas followed the same patterns as the ACTH and corticosterone data. These data suggest that habituation of the adrenocortical hormone response to psychological stressors is not mediated by phasic inhibition of the effector system.

Lack of contextual modulation of habituated neuroendocrine responses to repeated audiogenic stress.

Exposure to stress reliably activates the hypothalamo-pituitary-adrenocortical (HPA) axis response in rodents, which is significantly reduced (habituated) following repeated exposures. In the current study, it was first established that HPA axis response habituation to repeated loud noise lasted for at least 4 weeks in rats. In the next experiment, a contextual extinction procedure following repeated loud noise exposures failed to restore the habituated HPA axis response. Although an additional study indicated some recovery of responses when the context was modified on a test day following habituation, this effect could be mostly attributed to the familiarity with the contextual cues. A final study confirmed that rats could distinguish between the contexts used and further indicated that context preexposures reduce acute HPA axis responses to loud noise. These studies therefore provide no support for the hypothesis that contextual cues regulate HPA axis response habituation.