Administrations of the glucocorticoid receptor antagonist (anti-glucocorticoid, RU38486) and the mineralocorticoid antagonist (anti-mineralocorticoid, RU28318) followed by frequent, sequential blood sampling were employed to investigate the possible role the brain mineralocorticoid receptor (MR, type I) and glucocorticoid receptor (GR, type II) have in the regulation of basal and stress-induced adrenocortical secretion in the rat. The anti-mineralocorticoid and anti-glucocorticoid were administered subcutaneously (s.c.) at doses of 2.5 mg and 1.0 mg/100 g body weight, respectively. Both antagonists were also given intracerebro-ventricularly (i.c.v.) at a dose of 100 ng/rat. Under basal non-stressed conditions (at the diurnal trough in the morning), injections of either saline, anti-glucocorticoid (s.c. or i.c.v.) or anti-mineralocorticoid (s.c.) did not have effect on the plasma corticosterone level. The anti-mineralocorticoid given intracerebroventricularly, however, caused an elevation of plasma corticosterone up to 60 min after the injection. Exposure of the rats to a novel environment resulted in a large increase in the plasma corticosterone level, which was slightly reduced in the rats treated with the anti-glucocorticoid. In vehicle-treated rats, the level returned to basal values at 90 min, while in the anti-glucocorticoid- and anti-mineralocorticoid-treated groups, it remained elevated for prolonged periods. The present study thus shows that (1) the anti-glucocorticoid RU38486 via the brain GR has no effect on the basal plasma corticosterone level in the morning but interferes with a glucocorticoid negative feedback following stress and (2) the anti-mineralocorticoid RU28318 via the brain MR elevates the basal plasma corticosterone level and enhances adrenocortical secretion following stress. Accordingly, both antagonists caused prolonged adrenocortical secretion following stress. Such an effect caused by the anti-mineralocorticoid is probably due to an enhanced stress responsiveness resulting from a blockade of the limbic MR and that caused by the anti-glucocorticoid resulting from a blockade of GR involved in the termination of the stress response.

In conclusion, PBMC of CFS patients display an increased sensitivity to glucocorticoids, which cannot be explained by number or affinity of the GR but should rather be attributed to molecular processes beyond the actual binding of the ligand to the GR.

The antinociceptive effect of morphine (5 mg/kg body weight i.p.) in rats subjected to various experimental manipulations of the pituitary-adrenocortical system was studied. The absence of adrenal steroids increased the sensitivity to morphine. The following findings suggest that glucocorticosteroids have a long-lasting influence on opioid-induced antinociception, even when the steroids have been removed by adrenalectomy. First, when rats were adrenalectomized in the morning under basal conditions of pituitary-adrenocortical activity (plasma corticosterone level < 1 µg%), the subsequent hypersensitivity to morphine was more pronounced than when rats were adrenalectomized in the evening (plasma corticosterone level 18.4 µg%). This difference in hypersensitivity to morphine-induced antinociception following adrenalectomy either in the morning or in the evening persisted for at least 2 weeks. Second, exposure to a novel environment (stress of a new cage) or administration of corticosterone (10 mg/kg body weight s.c.) prior to morning adrenalectomy decreased the sensitivity to morphine measured 1 week later. Third, RU 38486, a glucocorticoid antagonist, injected in the lateral cerebral ventricle prior to the evening adrenalectomy increased subsequent morphine antinociception. In attempts to understand the long-term effect on morphine antinociception, the opioid receptor sites were quantified by an in vivo procedure. Quantitative autoradiography of binding sites labeled after intravenous administration of a tracer dose of [³H]-diprenorphine showed a decrease in retention of the labeled opioid in cortical and midbrain regions of rats adrenalectomized in the evening when compared with rats operated in the morning. In conclusion, the data suggest that a neuroen-docrine state invoked by the actual plasma corticosterone level at the time of adrenalectomy has a long-lasting influence on the degree of post-adrenalectomy opioid-induced antinociception. These long-term alterations of opioid responsiveness are reflected in altered binding to the opioid receptor system in the brain.

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