Functional interaction between 5-HT6 receptors and hypothalamic-pituitary-adrenal axis:: Cognitive implications

The serotonin 5-HT6 receptor has become a promising target for the treatment of neuropsychological diseases, such as affective disorders. Increasing evidence implicates stress and its effector system, the hypothalamic-pituitary-adrenal (HPA) axis, in the neurobiology of depression. In addition, there are important memory disturbances in stress-related psychiatric disorders that have been associated to an impairment of the HPA axis reactivity. The aim of the present work is to study the functional interactions between 5-HT6 receptors and HPA axis. In a situation of increased HPA axis responsiveness (maternal separation, MS) no differences were found in the expression of 5-HT6 gene in the hippocampus or frontal cortex, although serotonin levels were higher in the frontal cortex of MS rats. 5-HT6 receptor mRNA expression increased significantly in the hippocampus in a situation of decreased glucocorticoid levels, such as adrenalectomy. Cognitive deficits associated to HPA dysfunction, such those found in the MS model, were fully reversed by administration of SB271046, a selective 5-HT6 receptor antagonist. A chronic treatment with SB271046 did not modify CRF mRNA levels in the hypothalamus, but there was a higher glucocorticoid receptor density in the hippocampus compared to control. In contrast, in the frontal cortex, treatment with SB271046 induced a significant decrease in glucocorticoid receptor density. These data suggest that expression of 5-HT6 receptors might be differentially regulated depending on levels of circulating adrenal corticoids. These results are discussed in terms of therapeutical approaches to the treatment of behavioral (depressive-like) and cognitive disturbances associated to an altered response to stress. (C) 2007 Elsevier Ltd. All rights reserved.