Chronic treatment with the synthetic glucocorticoid (GC) prednisolone has been reported in association with many detrimental health effects. In addition to well-documented adverse metabolic effects, there is evidence for memory impairments in these patients. GCs are known to have effects on memory, either enhancing or impairing dependant upon timing of exposure. Cell experiments have shown that synthetic GCs such as methyl-prednisolone (MPL) cause an alteration in timing of glucocorticoid receptor (GR) activation. In contrast to the rapid pulsatile GR activation associated with natural GC hormones corticosterone and cortisol, synthetic GCs including MPL induce a prolonged GR activation profile. Therefore we have now investigated the action of MPL in vivo. We report that a single subcutaneous injection of MPL into adrenalectomised rats induced prolonged GR activity in discrete brain regions vital to learning and memory, including the hippocampus, prefrontal and perirhinal cortices. Three-day treatment in adrenal-intact rats (1 mg/ml MPL in drinking water) supressed endogenous corticosterone secretion, induced significant GR activation during the circadian peak and loss of circadian GR nadir consistent with prolonged MPL-induced GR activation. Interestingly, radiotelemetry data showed a significant dysregulation of circadian activity rhythms in the MPL treated rats. To test if MPL treatment resulted in impaired hippocampal or perirhinal-dependent memory, rats were tested in object location and object recognition tasks. MPL treated rats were able to discriminate between novel and familiar in the object recognition and object location tests following a 1 h delay. However at a 6 h delay, MPL treated rats were not able to discriminate. Therefore, we conclude that chronic MPL treatment impairs both perirhinal-dependent recognition memory and hippocampal-dependent spatial memory following a 6 h delay, but not a 1 h delay. Our experimental model may provide a robust method for determining the mechanisms underlying memory deficits in patients treated with prednisolone and other synthetic GCs.
02 Nov 2015 - 04 Nov 2015