Corticotropin-releasing hormone receptor type 1-deficiency enhances hippocampal serotonergic neurotransmission: an in vivo microdialysis study in mutant mice.

Corticotropin-releasing hormone plays an important role in the coordination of various responses to stress. Previous research has implicated both corticotropin-releasing hormone and the serotonergic system as causative factors in the development and course of stress-related psychiatric disorders such as major depression. To delineate the role of the corticotropin-releasing hormone receptor type 1 (CRH-R1) in the interactions between corticotropin-releasing hormone and serotonergic neurotransmission, in vivo microdialysis was performed in CRH-R1-deficient mice under basal (home cage) and stress (forced swimming) conditions. Hippocampal dialysates were used to measure extracellular levels of serotonin and its metabolite 5-hydroxyindoleacetic acid, and free corticosterone levels to monitor the status of the hypothalamic-pituitary-adrenocortical axis. Moreover, behavioural activity was assessed by visual observation and a scoring paradigm.Both wild-type and heterozygous mutant mice showed a clear diurnal rhythm in free corticosterone. Free corticosterone concentrations were, however, lower in heterozygous mutant mice than in wild-type animals and undetectable in homozygous CRH-R1-deficient mice. Homozygous CRH-R1-deficient mice showed enhanced hippocampal levels of 5-hydroxyindoleacetic acid but not of serotonin during the light and the dark phase of the diurnal cycle, which may point to an enhanced synthesis of serotonin in the raphe-hippocampal system. Moreover, the mutation resulted in higher behavioural activity in the home cage during the light but not during the dark period. Forced swimming caused a rise in hippocampal serotonin followed by a further increase after the end of the stress paradigm in all genotypes. Homozygous and heterozygous mutant mice showed, however, a significantly amplified serotonin response to the forced swimming as compared to wild-type control animals.We conclude that CRH-R1-deficiency results in reduced hypothalamic-pituitary-adrenocortical axis activity, in enhanced synthesis of serotonin during basal conditions, and in an augmented response in extracellular levels of serotonin to stress. These data provide further evidence for the intricate relationship between corticotropin-releasing hormone and serotonin and the important role of the CRH-R1 herein.