Teresa E. Foley

Freewheel Running Prevents Learned Helplessness/Behavioral Depression: Role of Dorsal Raphe Serotonergic Neurons

Serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN) are implicated in mediating learned helplessness (LH) behaviors, such as poor escape responding and expression of exaggerated conditioned fear, induced by acute exposure to uncontrollable stress. DRN 5-HT neurons are hyperactive during uncontrollable stress, resulting in desensitization of 5-HT type 1A (5-HT1A) inhibitory autoreceptors in the DRN. 5-HT1A autoreceptor downregulation is thought to induce transient sensitization of DRN 5-HT neurons, resulting in excessive 5-HT activity in brain areas that control the expression of learned helplessness behaviors. Habitual physical activity has antidepressant/anxiolytic properties and results in dramatic alterations in physiological stress responses, but the neurochemical mediators of these effects are unknown. The current study determined the effects of 6 weeks of voluntary freewheel running on LH behaviors, uncontrollable stress-induced activity of DRN 5-HT neurons, and basal expression of DRN 5-HT1A autoreceptor mRNA. Freewheel running prevented the shuttle box escape deficit and the exaggerated conditioned fear that is induced by uncontrollable tail shock in sedentary rats. Furthermore, double c-Fos/5-HT immunohistochemistry revealed that physical activity attenuated tail shock-induced activity of 5-HT neurons in the rostral–mid DRN. Six weeks of freewheel running also resulted in a basal increase in 5-HT1A inhibitory autoreceptor mRNA in the rostral–mid DRN. Results suggest that freewheel running prevents behavioral depression/LH and attenuates DRN 5-HT neural activity during uncontrollable stress. An increase in 5-HT1A inhibitory autoreceptor expression may contribute to the attenuation of DRN 5-HT activity and the prevention of LH in physically active rats.

Long-term voluntary wheel running is rewarding and produces plasticity in the mesolimbic reward pathway.

The mesolimbic reward pathway is implicated in stress-related psychiatric disorders and is a potential target of plasticity underlying the stress resistance produced by repeated voluntary exercise. It is unknown, however, whether rats find long-term access to running wheels rewarding, or if repeated voluntary exercise reward produces plastic changes in mesolimbic reward neurocircuitry. In the current studies, young adult, male Fischer 344 rats allowed voluntary access to running wheels for 6 weeks, but not 2 weeks, found wheel running rewarding, as measured by conditioned place preference (CPP). Consistent with prior reports and the behavioral data, 6 weeks of wheel running increased ΔFosB/FosB immunoreactivity in the nucleus accumbens (Acb). In addition, semi quantitative in situ hybridization revealed that 6 weeks of wheel running, compared to sedentary housing, increased tyrosine hydroxylase (TH) mRNA levels in the ventral tegmental area (VTA), increased delta opioid receptor (DOR) mRNA levels in the Acb shell, and reduced levels of dopamine receptor (DR)-D2 mRNA in the Acb core. Results indicate that repeated voluntary exercise is rewarding and alters gene transcription in mesolimbic reward neurocircuitry. The duration-dependent effects of wheel running on CPP suggest that as the weeks of wheel running progress, the rewarding effects of a night of voluntary wheel running might linger longer into the inactive cycle thus providing stronger support for CPP. The observed plasticity could contribute to the mechanisms by which exercise reduces the incidence and severity of substance abuse disorders, changes the rewarding properties of drugs of abuse, and facilitates successful coping with stress.

The consequences of uncontrollable stress are sensitive to duration of prior wheel running

The behavioral consequences of uncontrollable stress, or learned helplessness (LH) behaviors, are thought to involve hyperactivity of serotonergic (5-HT) neurons in the dorsal raphe nucleus (DRN). Other brain regions implicated in LH and capable of affecting 5-HT systems, such as the bed nucleus of the stria terminalis (BNST), amygdala, and habenula, could contribute to DRN 5-HT hyperactivity during uncontrollable stress. Six weeks of wheel running prevents LH and attenuates uncontrollable stress-induced c-Fos expression in DRN 5-HT neurons, although the duration of wheel running necessary for these effects is unknown. In the current study, 6 but not 3, weeks of wheel running blocked the shuttle box escape deficit and exaggerated fear produced by uncontrollable tail shock in sedentary rats. Corresponding to the duration-dependent effects of wheel running on LH behaviors, 6 weeks of wheel running was required to attenuate uncontrollable stress-induced 5-HT neural activity, indexed by c-Fos protein expression, in the DRN and c-Fos expression in the lateral ventral region of the BNST. Wheel running, regardless of duration, did not affect c-Fos expression anywhere in the amygdala or habenula. These data indicate that the behavioral effects of uncontrollable stress are sensitive to the duration of prior physical activity and are consistent with the hypothesis that attenuation of DRN 5-HT activity contributes to the prevention of LH by wheel running. The potential role of the BNST in the prevention of LH by wheel running is discussed.

Wheel running alters serotonin (5-HT) transporter, 5-HT1A, 5-HT1B, and alpha1b-adrenergic receptor mRNA in the rat raphe nuclei

BACKGROUND Altered serotonergic (5-HT) neurotransmission is implicated in the antidepressant and anxiolytic properties of physical activity. In the current study, we investigated whether physical activity alters factors involved in the regulation of central 5-HT neural activity. METHODS In situ hybridization was used to quantify levels of 5-HT transporter (5-HTT), 5-HT(1A), 5-HT(1B), and alpha(1b)-adrenergic receptor (alpha(1b) ADR) messenger ribonucleic acids (mRNAs) in the dorsal (DRN) and median raphe (MR) nuclei of male Fischer rats after either sedentary housing or 3 days, 3 weeks, or 6 weeks of wheel running. RESULTS Wheel running produced a rapid and lasting reduction of 5-HT(1B) mRNA in the ventral DRN. Three weeks of wheel running decreased 5-HTT mRNA in the DRN and MR and increased alpha(1b) ADR mRNA in the DRN. After 6 weeks of wheel running, 5-HTT mRNA remained reduced, but alpha(1b) ADR mRNA returned to sedentary levels. Serotonin(1A) mRNA was increased in the MR and certain DRN subregions after 6 weeks only. CONCLUSIONS Data suggest that the central 5-HT system is sensitive to wheel running in a time-dependent manner. The observed changes in mRNA regulation in a subset of raphe nuclei might contribute to the stress resistance produced by wheel running and the antidepressant and anxiolytic effects of physical activity.

A behavioral analysis of the impact of voluntary physical activity on hippocampus-dependent contextual conditioning.

Voluntary physical activity induces molecular changes in the hippocampus consistent with improved hippocampal function, but few studies have explored the effects of wheel running on specific hippocampal‐dependent learning and memory processes. The current studies investigated the impact of voluntary wheel running on learning and memory for context and extinction using contextual fear conditioning which is known to be dependent on the hippocampus. When conditioning occurred prior to the start of 6 weeks of wheel running, wheel running had no effect on memory for context or extinction (assessed with freezing). In contrast, when wheel running occurred for 6 weeks prior to conditioning, physical activity improved contextual memory during a retention test 24 h later, but did not affect extinction learning or memory. Wheel running had no effect on freezing immediately after foot shock presentation during conditioning, suggesting that physical activity does not affect the acquisition of the context—shock association or alter the expression of freezing, per se. Instead, it is argued that physical activity improves the consolidation of contextual memories in the hippocampus. Consistent with improved hippocampus‐dependent context learning and memory, 6 weeks of wheel running also improved context discrimination and reduced the context pre‐exposure time required to form a strong contextual memory. The effect of wheel running on brain‐derived neurotrophic factor (BDNF) messenger ribonucleic acid (mRNA) in hippocampal and amygdala subregions was also investigated. Wheel running increased BDNF mRNA in the dentate gyrus, CA1, and the basolateral amygdala. Results are consistent with improved hippocampal function following physical activity.

Learned helplessness is independent of levels of brain-derived neurotrophic factor in the hippocampus

Reduced levels of brain-derived neurotrophic factor (BDNF) in the hippocampus have been implicated in human affective disorders and behavioral stress responses. The current studies examined the role of BDNF in the behavioral consequences of inescapable stress, or learned helplessness. Inescapable stress decreased BDNF mRNA and protein in the hippocampus of sedentary rats. Rats allowed voluntary access to running wheels for either 3 or 6 weeks prior to exposure to stress were protected against stress-induced reductions of hippocampal BDNF protein. The observed prevention of stress-induced deceases in BDNF, however, occurred in a time course inconsistent with the prevention of learned helplessness by wheel running, which is evident following 6 weeks, but not 3 weeks, of wheel running. BDNF suppression in physically active rats was produced by administering a single injection of the selective serotonin reuptake inhibitor fluoxetine (10 mg/kg) just prior to stress. Despite reduced levels of hippocampal BDNF mRNA following stress, physically active rats given the combination of fluoxetine and stress remained resistant against learned helplessness. Sedentary rats given both fluoxetine and stress still demonstrated typical learned helplessness behaviors. Fluoxetine by itself reduced BDNF mRNA in sedentary rats only, but did not affect freezing or escape learning 24 h later. Finally, bilateral injections of BDNF (1 mug) into the dentate gyrus prior to stress prevented stress-induced reductions of hippocampal BDNF but did not prevent learned helplessness in sedentary rats. These data indicate that learned helplessness behaviors are independent of the presence or absence of hippocampal BDNF because blocking inescapable stress-induced BDNF suppression does not always prevent learned helplessness, and learned helplessness does not always occur in the presence of reduced BDNF. Results also suggest that the prevention of stress-induced hippocampal BDNF suppression is not necessary for the protective effect of wheel running against learned helplessness.

Role of central β-adrenergic receptors in regulating proinflammatory cytokine responses to a peripheral bacterial challenge

Elevation of proinflammatory cytokines in the brain have potent effects on altering physiological, behavioral, and cognitive processes. The mechanism(s) by which brain cytokines are induced during a peripheral immune challenge remains unclear since microorganisms/cytokines do not cross the blood-brain barrier (BBB). Recent studies indicate that central beta-adrenergic receptors (beta-ADRs) may mediate brain interleukin-1beta (IL-1) production. This has direct implications for the production of brain cytokines during a peripheral immune response since peripheral pathogens and cytokines rapidly stimulate brainstem catecholamine neurons via peripheral nerves and circumventricular pathways. Studies here examine the role of central beta-ADRs in regulating brain cytokine production following peripheral Escherichia coli (E. coli) challenge. Rats were centrally administered propranolol (beta-ADR antagonist) or vehicle followed by peripheral E. coli or saline and sacrificed 6h later for measurement of cytokines. Pre-treatment with propranolol completely blocked the induction of brain IL-1 following E. coli. Surprisingly, central propranolol also attenuated E. coli-induced peripheral cytokines. To examine whether the attenuated peripheral cytokine response following central propranolol administration was due leakage of propranolol into the general circulation and blockade of peripheral beta-blockade, nadolol (beta-ADR antagonist that does not cross the BBB) was administered peripherally prior to E. coli. Nadolol administration did not block central cytokine production following E. coli, but instead enhanced both peripheral and central proinflammatory cytokine production. Furthermore, central administration of isoproterenol (beta-ADR agonist) results in a time-dependent increase in brain IL-1 production. These data demonstrate central beta-ADRs may play a critical role to induce brain IL-1, while peripheral beta-ADRs inhibit cytokine response to bacterial challenge.

Elevated central monoamine receptor mRNA in rats bred for high endurance capacity: Implications for central fatigue

Although alteration to peripheral systems at the skeletal muscle level can contribute to ones ability to sustain endurance capacity, neural circuits regulating fatigue may also play a critical role. Previous studies demonstrated that increasing brain serotonin (5-HT) release is sufficient to hasten the onset of exercise-induced fatigue, while manipulations that increase brain dopamine (DA) release can delay the onset of fatigue. These results suggest that individual differences in endurance capacity could be due to factors capable of influencing the activity of 5-HT and DA systems. We evaluated possible differences in central fatigue pathways between two contrasting rat groups selectively bred for high (HCR) or low (LCR) capacity running. Using quantitative in situ hybridization, we measured messenger RNA (mRNA) levels of tryptophan hydroxylase (TPH), 5-HT transporter (5-HTT), 5-HT1A and 5-HT1B autoreceptors, dopamine receptor-D2 (DR-D2) autoreceptors and postsynaptic receptors, and dopamine receptor-D1 (DR-D1) postsynaptic receptors, in discrete brain regions of HCR and LCR. HCR expressed higher levels of 5-HT1B autoreceptor mRNA in the raphe nuclei relative to LCR, but similar levels of TPH, 5-HTT, and 5-HT1A mRNA in these areas. Surprisingly, HCR expressed higher levels of DR-D2 autoreceptor mRNA in the midbrain, while simultaneously expressing greater DR-D2 postsynaptic mRNA in the striatum compared to LCR. There were no differences in DR-D1 mRNA levels in the striatum or cortex between groups. These data suggest that central serotonergic and dopaminergic systems may be involved in the mechanisms by which HCR have delayed onset of exercise-induced fatigue compared to LCR.

Brain Activation Patterns at Exhaustion in Rats That Differ in Inherent Exercise Capacity

In order to further understand the genetic basis for variation in inherent (untrained) exercise capacity, we examined the brains of 32 male rats selectively bred for high or low running capacity (HCR and LCR, respectively). The aim was to characterize the activation patterns of brain regions potentially involved in differences in inherent running capacity between HCR and LCR. Using quantitative in situ hybridization techniques, we measured messenger ribonuclease (mRNA) levels of c-Fos, a marker of neuronal activation, in the brains of HCR and LCR rats after a single bout of acute treadmill running (7.5–15 minutes, 15° slope, 10 m/min) or after treadmill running to exhaustion (15–51 minutes, 15° slope, initial velocity 10 m/min). During verification of trait differences, HCR rats ran six times farther and three times longer prior to exhaustion than LCR rats. Running to exhaustion significantly increased c-Fos mRNA activation of several brain areas in HCR, but LCR failed to show significant elevations of c-Fos mRNA at exhaustion in the majority of areas examined compared to acutely run controls. Results from these studies suggest that there are differences in central c-Fos mRNA expression, and potential brain activation patterns, between HCR and LCR rats during treadmill running to exhaustion and these differences could be involved in the variation in inherent running capacity between lines.

40 Activation of the acute stress response promotes host defense and T cell dependent immunological memory against bacterial challenge

role in the development of IFN-a (cytokine)-induced behavioral change. To further explore the potential role of CRF in IFN-a-induced behavioral alterations, we have examined IFN-a-induced neuroendocrine, immune, and behavioral responses in rhesus monkeys. To first determine whether IFN-a activates relevant signaling pathways in rhesus monkeys, we evaluated the expression of phosphoSTAT1 (a major signaling pathway activated by IFN-a) in monkey peripheral blood mononuclear cells (PBMCs) treated with rHu-IFN-a (1000 IU/ml) in vitro using both flow cytometry and Western blot. IFN-a increased phospho-STAT-1 in monkey PBMCs at 15, 30, and 60 min, a pattern similar to that seen in humans. We then investigated the acute and chronic effects of IFN-a on neuroendocrine, immune, and behavioral responses in these animals. Consistent with the activation of upstream neuroendocrine secretagogues including CRF, acute administration of IFN-a (10 and 20 MIU/m2) to rhesus monkeys markedly increased ACTH and cortisol as well as IL-6 compared to saline in 8 young adult (6 male, 2 female) animals. We additionally treated 4 monkeys (2 male and 2 female), with IFN-a (20 MIU/m2) or saline for 4 weeks. Interestingly, the neuroendocrine and immune responses to chronic IFN-a exposure in monkeys appeared to depend on the social status of the animal. Whereas acute elevations in cortisol, ACTH, and IL-6 concentrations returned to saline levels by week 2 in dominant animals, the concentration of these hormones remained high throughout IFN-a treatment in subordinant animals. Behavioral changes included the induction of huddling (2 out 4 animals) and marked aggressive behavior in one of the dominant monkeys. Neither aggression nor huddling was present during saline treatment. Of note huddling has been previously described in monkeys treated with icv CRF. Taken together, these data provide evidence that rhesus monkeys exhibit similar immune, neuroendocrine, and behavioral responses to IFN-a as humans, and support the idea that activation of CRF pathways may be involved. Future studies using CRF antagonists will further elucidate the role of CRF in IFN-a induced neuroendocrine, immune, and behavioral changes, thus providing insight into novel approaches to the treatment of cytokine-induced behavioral syndromes.