Merce Giralt

Evidence that the Pituitary-Adrenal Axis Does Not Cross-Adapt to Stressors: Comparison to Other Physiological Variables

The effects of previous chronic immobilization stress on the physiological responses of male rats to a novel chronic stressor (shock) were studied. Previous chronic exposure to immobilization reduced adrenocorticotropin (ACTH) and lactate responses to acute immobilization stress without altering the response to a novel acute stressor (tail shock). When subjected to chronic tail shock, body weight inhibition caused by chronic shock was greater in the rats not previously exposed to chronic immobilization, which suggests that there is cross-adaptation between different stressors. However, adrenocorticotropin adaptation to chronic shock was impaired by previous chronic immobilization. These data indicate that the existence of cross-adaptation to stressors might depend on the variable measured, the central nervous system pathways controlling the pituitary-adrenal axis being, apparently, resistant to cross-adaptation. This lack of cross-adaptation at certain levels can assure the maintenance of an adequate response to unknown environmental stimuli.

Individual housing does not influence the adaptation of the pituitary-adrenal axis and other physiological variables to chronic stress in adult male rats

Although the influence of housing conditions on the physiological response to stress has been extensively studied for several years, no attempts have been made to investigate the effect of this variable on the capacity for adaptation to chronic stress. To this end, adult male rats were housed either individually or in groups of four per cage and subjected to 2 hr of daily immobilization stress for 14 days. Housing did not influence any of the physiological variables measured either in unstressed or in stressed rats except the corticosterone response to stress which was higher in individually housed rats. Of the behavioral measures, individual housing significantly decreased defecation rate in the novel environment. Other behavioral measures were not influenced by housing. Chronic stress significantly reduced ambulation but no significant interaction between housing and chronic stress was observed. Taken together, these data indicate that a short period of individual housing did not affect the physiological and behavioral consequences of repeated exposure to chronic stress.

Sildenafil (Viagra) ameliorates clinical symptoms and neuropathology in a mouse model of multiple sclerosis

Cyclic GMP (cGMP)-mediated pathways regulate inflammatory responses in immune and CNS cells. Recently, cGMP phosphodiesterase inhibitors such as sildenafil, commonly used to treat sexual dysfunction in humans including multiple sclerosis (MS) patients, have been reported to be neuroprotective in animal models of stroke, Alzheimer’s disease, and focal brain lesion. In this work, we have examined if sildenafil ameliorates myelin oligodendrocyte glycoprotein peptide (MOG35–55)-induced experimental autoimmune encephalomyelitis (EAE), a mouse model of MS. We show for the first time that treatment with sildenafil after disease onset markedly reduces the clinical signs of EAE by preventing axonal loss and promoting remyelination. Furthermore, sildenafil decreases CD3+ leukocyte infiltration and microglial/macrophage activation in the spinal cord, while increasing forkhead box transcription factor 3-expressing T regulatory cells (Foxp3 Tregs). However, sildenafil treatment did not significantly affect MOG35–55-stimulated proliferation or release of Th1/Th2 cytokines in splenocytes but decreased ICAM-1 in spinal cord infiltrated cells. The presence of reactive astrocytes forming scar-like structures around infiltrates was enhanced by sildenafil suggesting a possible mechanism for restriction of leukocyte spread into healthy parenchyma. These results highlight novel actions of sildenafil that may contribute to its beneficial effects in EAE and suggest that treatment with this widely used and well-tolerated drug may be a useful therapeutic intervention to ameliorate MS neuropathology.

Inhibition of corticosteroid-binding globulin caused by a severe stressor is apparently mediated by the adrenal but not by glucocorticoid receptors

The effect of stress on serum corticosteroid-binding globulin (CBG) was studied in adult male Sprague-Dawley rats. CBG was measured either by a homologous radioimmunoassay (RIA) or by a binding assay (BA) using 3H-corticosterone. Exposure of adult male rats to a severe stressor such as immobilization (IMO) for 1 h did not alter serum CBG levels, but a significant decrease was found after 6 and especially 24 h IMO. This decrease was not observed after 24 h exposure to a milder treatment such as food and water deprivation. The effect of different periods of exposure to two stressors, IMO or restraint, was also studied. The following results were obtained: serum CBG levels were reduced by IMO, but not by restraint; IMO-induced reduction of CBG levels was always observed 24 h after starting exposure to IMO, independently of the actual period of exposure to the stressor; and IMO-induced inhibition of CBG was proportional to the hours of exposure to the stressor. Although IMO-induced inhibition of CBG was prevented by adrenalectomy, a role for glucocorticoid acting through their classical type II receptors is unclear as far as treatment of rats with the glucocorticoid receptor antagonist RU486 (100 mg/kg) did not prevent the inhibition caused by IMO. The present data clearly indicate that acute exposure to a stressor is able to decrease CBG levels provided that duration of exposure to the stressor and its intensity are high and that the effect is tested at least 6 h after the onset of stress. The effect appears to be mediated by some adrenal factor(s) other than glucocorticoids.

Effects of chronic immobilization stress on GH and TSH secretion in the rat: Response to hypothalamic regulatory factors

The effect of chronic immobilization (2 h/day) for 13 days on basal and stress levels of GH and TSH, and their response to various hypothalamic regulatory factors was studied in male Sprague-Dawley rats. Chronic immobilization (IMO) resulted in reduced serum TSH levels in stress situations but not in resting conditions. GH secretion was inhibited both in resting and stress situations. Chronic IMO impaired both GH and TSH responses to GRH and TRH, respectively, but also to another peptide (VIP) stimulatory for the two hormones. Whereas somatostatin administration inhibited GH secretion in control but not in chronic IMO rats, its inhibitory effect on TSH was slight and similar in the two experimental groups. The present results suggest that chronic exposure to a severe stressor such as IMO alters GH and TSH secretion, at least in part by changes in the response of the pituitary to the hypothalamic regulatory factors. The actual influence of chronic IMO on the release of these peptides into the median eminence remains to be studied.

Previous chronic ACTH administration does not protect against the effects of acute or chronic stress in male rats

The effect of previous chronic ACTH administration on the physiological response to acute and chronic immobilization stress was studied in adult male rats. Chronic ACTH administration slightly reduced food intake and drastically inhibited body weight gain. Serum corticosterone levels were similar in saline- and ACTH-treated rats twenty hours after the last administration. However, the corticosterone response to 1 hr immobilization was greatly reduced by previous ACTH administration. When the exposure to the stressor was prolonged up to 18 hours, the corticosterone response was similar in saline, and ACTH-treated rats. While body weight loss caused by starvation and acute stress was lower in ACTH-treated rats, stomach ulceration was greater in the latter animals. Although ACTH-treated rats showed higher body weight gain than saline-treated animals during chronic immobilization, this was probably due to catch-up growth as food intake inhibition caused by the stressor was similar in the two groups. Pituitary-adrenal adaptation to the repeated stressor was the same in saline- and ACTH-treated rats. Therefore, the effects of previous ACTH treated on the physiological response to either acute or chronic stress appear to be mixed, depending on the variable studied.

Previous chronic chlorimipramine treatment did not modify some physiological responses to acute and chronic stress in rats

The effects of previous chronic administration of the tricyclic antidepressant drug chlorimipramine (CMI) on some physiological responses of adult male rats to stress has been studied. CMI significantly reduced food intake and body weight gain, but did not alter either adrenal weight or basal serum corticosterone levels. Corticosterone response to 1 h of immobilization stress was the same in saline and CMI-treated rats. When the rats previously treated with CMI were subjected to chronic immobilization stress, it was found that the drug did not alter the anorexic effects of the stressor, but reduced the rate of adaptation of adrenocorticotropin response to stress. These data indicate that previous chronic CMI administration does not prevent changes in the secretory activity of the pituitary-adrenal system or the reduction of food intake and body weight caused by strong stressors.

Metallothioneins I/II are involved in the neuroprotective effect of sildenafil in focal brain injury.

We recently reported that administration of the non-selective cyclic GMP-phosphodiesterase (cGMP-PDE) inhibitor zaprinast to cortically cryoinjured rats results three days post-lesion in reduced neuronal cell death that was associated to decreased macrophage/microglial activation and oxidative stress and increased astrogliosis and angiogenesis. Similar effects have been observed in cryoinjured animals overexpressing metallothioneins I/II (MT-I/II), metal-binding cysteine-rich proteins that are up-regulated in response to injury. In this work we have examined the effect of administration of the selective PDE5 inhibitor sildenafil (10mg/kg, sc) 2h before and 24 and 48h after induction of cortical cryolesion in wild-type and MT-I/II-deficient mice. Our results show that in wild-type animals sildenafil induces similar changes in glial reactivity, angiogenesis and antioxidant and antiapoptotic effects in the cryolesioned cortex as those observed in rats with zaprinast, indicating that inhibition of PDE5 is responsible for the neuroprotective actions. However, these effects were not observed in mice deficient in MT-I/II. We further show that sildenafil significantly increases MT-I/II protein levels in homogenates of lesioned cortex and MT-I/II immunostaining in glial cells around the lesion. Taken together these results indicate that cGMP-mediated pathways regulate expression of MT-I/II and support the involvement of these proteins in the neuroprotective effects of sildenafil in focal brain lesion.

Chronic stress induced changes in LH secretion: The contribution of anorexia associated to stress

The effects of chronic intermittent immobilization (IMO) on serum LH levels of adult male rats were studied. Chronic IMO (2 h daily for 13 days) did not alter basal LH levels, but abolished the LH response to acute stressors (IMO and tailshock). The inhibition of LH caused by acute exposure to IMO for 4 or 18 h was similar in control and chronic IMO rats. Also the LH response to exogenous LHRH administration was normal in chronically stressed rats. When a group of rats eating the same amount of food as that eaten by immobilized rats was introduced (pair-fed), an inhibition of LH response to acute stressors quite similar to that found in chronic IMO rats was observed. These data indicate that chronic stress-induced inhibition of LH release caused by short-term exposure to acute stressors was located above the pituitary and was mainly due to anorexia accompanying daily exposure to the stressor.

Long-lasting effects of chronic chlorimipramine treatment of rats on exploratory activity on a hole-board, and on immobility in the force swimming test

Abstract The study concerned the effects of acute and chronic clomipramine administration to male rats on exploratory activity in a novel environment (hole-board) and on immobility in the forced swimming test. Acute clomipramine administration did not alter either exploratory activity on a hole-board as measured 3 or 20 h after drug administration, or immobility in the forced swimming test as measured 20 h after drug administration. Approximately 20 h after the last injection of clomipramine, the rats chronically treated with the drug showed reduced exploratory activity on the hole-board. In contrast, chronic clomipramine treatment significantly increased the activity in the forced swimming test. The effects of the drug on exploratory and forced swimming activities persisted for 14 days after the cessation of clomipramine administration. These data indicate that chronic clomipramine administration exerted profound and long-lasting effects on central nervous system function. The long-lasting action of the drug on behaviour in the forced swimming test might explain the long-term beneficial effect of antidepressant drugs in counteractings behavioral depression.