Through expression of a glucocorticoid receptor (GR) antisense RNA in brain, we have produced transgenic mice with an hyperactive hypothalamic-pituitary-adrenocortical (HPA) system similar to that seen in depressed patients. This model supports the hypothesis that disturbed corticosteroid receptor regulation could be the primary factor responsible for both the CRH/AVP hyperdrive that leads to increased activity of the HPA system, and the premature escape from the cortisol suppressant action of dexamethasone seen in affective disorders. Although normalisation of the hyperactive HPA system occurs during successful antidepressant therapy of depressive illness, these improvements do not correlate with changes in monoaminergic neurotransmitter systems, suggesting that unknown mechanisms of action may be operative. Work from my laboratory was the first to show that different types of antidepressants increased glucocorticoid receptor (GR) mRNA. We found increased GR mRNA levels irrespective of the preferential inhibitory action of antidepressant on the monoamine neurotransmitter re-uptake and showed increased GR gene transcription in antidepressant-treated mouse fibroblast cells that do not possess monoamine re-uptake mechanisms. We measured changes in glucocorticoid response in cells transfected with a glucocorticoid-sensitive reporter plasmid (MMTV-CAT) and observed increased glucocorticoid-stimulated CAT activity when the cells were treated with antidepressant. A different chimaeric gene construct consisting of a fragment of the GR gene promoter region fused to the CAT gene allowed more direct measurement of antidepressant action and increased CAT activity was also seen when cells transfected with this construct were treated with antidepressant. Finally, GR mRNA concentration and glucocorticoid binding activity were increased in brain tissues of animals chronically treated with antidepressant. The time course of antidepressant actions on corticosteroid receptors coincides with their long-term actions on HPA system activity and follows closely that of clinical improvement of depression. This suggests that antidepressant-induced changes in brain corticosteroid receptors may underlie the observed simultaneous decrease in circulating ACTH and corticosterone levels and the decreased adrenal size. Some of these effects may be mediated through CRH since, in antidepressant-treated transgenic mice hypothalamic CRH mRNA levels were decreased. From this work we have formulated the hypothesis that a primary action of antidepressants could be the stimulation of corticosteroid receptor gene expression that renders the HPA system more susceptible to feedback inhibition by cortisol. The resultant decrease in HPA system activity could induce secondary changes in glucocorticoid-sensitive gene expression and lead to redressment of neurotransmitter imbalance. This work opens up a completely new insight into antidepressant drug action and suggests a line of approach to the development of new drugs by focusing on this action.