Voluntary exercise alters GABA(A) receptor subunit and glutamic acid decarboxylase-67 gene expression in the rat forebrain.

Voluntary exercise improves stress coping and lowers anxiety. Because of the role of GABA in these processes, we investigated changes in the central GABAergic system in rats with free access to a running wheel for 4 weeks. The control animals had no access to a running wheel. Using insitu hybridisation histochemistry, we studied changes in gene expression of various GABA(A) receptor subunits as well as the GABA-synthesising enzyme glutamic acid decarboxylase-67 (GAD67) in the forebrain. There were region-specific decreases in alpha2, beta3 and gamma2 subunit mRNA expression and region-specific increases in beta1 subunit expression. The alpha5 and delta subunits, in the forebrain specifically associated with extrasynaptic GABA(A) receptors in the hippocampus, showed differential increases in expression levels. Expression of GAD67 mRNA was increased in many forebrain regions including all hippocampal cell layers, peri-paraventricular nucleus, bed nucleus stria terminalis, nucleus accumbens core and motor cortex, suggesting that long-term voluntary exercise enhances forebrain GABA synthesis capacity but in a region-specific manner. Thus, regular performance of exercise results in extensive changes in the forebrain GABAergic system that may be implicated in the changes in stress sensitivity and emotionality observed in exercising subjects.

The ultradian and circadian rhythms of free corticosterone in the brain are not affected by gender: an in vivo microdialysis study in Wistar rats.

Recently, we described that free corticosterone levels in the brain of male Wistar rats, as assessed by in vivo microdialysis, show an ultradian rhythm with a pulse frequency of 1.2 pulses/h. To establish whether gender influences brain free corticosterone rhythms, we studied free corticosterone levels in the female Wistar rat under baseline and stressful conditions using microdialysis in the hippocampus. Analysis of the data with the PULSAR algorithm revealed that hippocampal free corticosterone levels show a clear ultradian pattern in female rats with a pulse frequency of 1.16+/-0.05 pulses/h between 09.00 h and 21.00 h. Further analysis showed that the pulse amplitude is significantly higher during the late afternoon/early night (15.00-21.00 h) than during the morning/early afternoon (09.00-15.00 h) phase (0.13+/-0.03 versus 0.07+/-0.01 microg/dl, respectively, P < 0.05). Pulse characteristics were extremely reproducible as demonstrated by the almost identical pulse parameters derived from two consecutive 24-h periods [pulse frequency: 1.13+/-0.09 and 1.19+/-0.08 pulses/h; pulse amplitude: 0.11+/-0.05 and 0.10+/-0.02 microg/dl for day 1 and day 2 (09.00-21.00 h) respectively, P > 0.05]. Both exposure to a novel environment and forced swim stress increased hippocampal free corticosterone levels. However, the stress-induced rise reached higher levels and was more prolonged after forced swimming (area under the curve: 46.84+/-9.25 and 12.08+/-1.69 arbitrary units for forced swimming and novelty stress respectively, P = 0.01). Importantly, the ultradian rhythm was rapidly restored after termination of the stress response. This is the first demonstration that the female rat brain is exposed to free corticosterone levels that follow a circadian as well as an ultradian pattern and show almost identical pulse characteristics as recently reported in male animals. These observations are of significance for further investigations into the dynamics of glucocorticoid action in the brain of both genders.

Long-term voluntary exercise and the mouse hypothalamic-pituitary-adrenocortical axis: impact of concurrent treatment with the antidepressant drug tianeptine.

We investigated whether voluntary exercise and concurrent antidepressant treatment (tianeptine; 20 mg/kg/day; 4 weeks) exert synergistic effects on the mouse hypothalamic-pituitary-adrenocortical (HPA) axis. Animals had access to a running wheel, were treated with the antidepressant, or received both conditions combined. Control mice received no running wheel and no drug treatment. Exercise resulted in asymmetric changes in the adrenal glands. Whereas sedentary mice had larger left adrenals than right ones, this situation was abolished in exercising animals, mainly due to enlargement of the right adrenal cortex. However, antidepressant treatment alone was ineffective whereas the combination of antidepressant treatment and exercise resulted in an enlargement of both adrenal cortices. In these respective conditions, the levels of tyrosine hydroxylase (TH) mRNA expression in the left and right adrenal medullas varied greatly in parallel to the changes observed in the adrenal cortex sizes. TH mRNA expression in the locus coeruleus of exercising mice was significantly increased irrespective of concomitant tianeptine treatment. Corticotrophin-releasing factor mRNA levels in the hypothalamic paraventricular nucleus were decreased after voluntary exercise but were unaffected by tianeptine. Exercise, particularly in combination with tianeptine treatment, resulted in decreased early morning baseline plasma levels of corticosterone. If animals were exposed to novelty (i.e. a mild psychological stressor), a decreased response in plasma corticosterone levels was observed in the exercising mice. By contrast, after restraint, a mixed physical and psychological stressor, exercising mice showed an enhanced response in plasma corticosterone compared to the controls; a response which was even further boosted in exercising mice concomitantly treated with tianeptine. Under either condition, plasma adrenocorticotrophic hormone levels were not different between groups. Thus, voluntary exercise impacts substantially on HPA axis regulation. Concurrent tianeptine treatment results in synergistic actions, mainly at the adrenal level, affecting both its structure and function.

Psychological stress increases hippocampal mineralocorticoid receptor levels: involvement of corticotropin-releasing hormone.

We investigated whether acute stressors regulate functional properties of the hippocampal mineralocorticoid receptor (MR), which acts inhibitory on hypothalamic-pituitary-adrenocortical activity. Exposure of rats to forced swimming or novelty evoked a significant rise in density of MR immunoreactivity in all hippocampal subfields after 24 hr, whereas exposure to a cold environment was ineffective. Time course analysis revealed that the effect of forced swimming on MR peaked at 24 hr and returned to control levels between 24 and 48 hr. In pyramidal neurons of CA2 and CA3, marked rises were already observed after 8 hr. Radioligand binding assays showed that corticotropin-releasing hormone (CRH) injected intracerebroventricularly into adrenalectomized rats also produced a rise in hippocampal MR levels; an effect for which the presence of corticosterone, but not dexamethasone, at the time of injection was a prerequisite. Moreover, pretreatment with the CRH receptor antagonist (d-Phe(12),Nle(21,38),alpha-Me-Leu(37))-CRH(12-41) blocked the effect of forced swimming on hippocampal MR levels. To investigate whether the rise in MR levels had any functional consequences for HPA regulation, 24 hr after forced swimming, a challenge test with the MR antagonist RU 28318 was conducted. The forced swimming exposed rats showed an enhanced MR-mediated inhibition of HPA activity. This study identifies CRH as an important regulator of MR, a pathway with marked consequence for HPA axis regulation. We conclude that the interaction between CRH and MR presents a novel mechanism involved in the adaptation of the brain to psychologically stressful events.