Autor: Eva-Maria Löken

"Immunohistochemical and behavioral studies of postsynaptic serotonin1a receptor effects on adult neurogenesis" With around 322 Million affected people worldwide and an increasing prevalence, depression is one of the most prevalent mental illnesses. The exact pathophysiological mechanisms of this disease have not been fully elucidated. In addition pharmacological therapy of depression comes along with a high non-responder rate and numerous adverse drug reactions. Further understanding of the etiology of depression is required to develop novel antidepressants with better efficacy and fewer adverse drug reactions. Studies of humans and animals suggest a dysregulation of the serotonergic system as well as alterations of adult neurogenesis in the development of depression. The 5-HT1A receptor, a subtype of the serotonin receptor family, was focussed in research and seems to play a significant role in the etiopathology of depression and the regulation of adult neurogenesis. The 5-HT1A receptor is presynaptically located as an autoreceptor on serotonergic neurons in the raphe and postsynaptically as a heteroreceptor in the projection regions of serotonergic neurons such as the hippocampus. The well-established transgenic mouse model with an overexpression of postsynaptic 5-HT1A receptor (OE mouse) offers a good possibility to specifically investigate the effects of this receptor on adult neurogenesis, depression-like behavior, and hippocampus-dependent learning. Previous studies with OE mice indicate an antidepressant and proneurogenic effect of the postsynaptic 5-HT1A receptor. However, in these studies untreated or one-time treated mice were tested and, thus, compensatory mechanisms cannot be excluded. The present study aimed at analyzing the effects of chronic 5-HT1A receptor activation on adult neurogenesis, depression-like behavior and hippocampusdependent learning in OE mice compared to wildtype (WT) mice. Furthermore, it is known that the serotonergic system is involved in the regulation of exerciseinduced adult neurogenesis. However, the proneurogenic effect in exercise-induced adult neurogenesis has not been related to any serotonin receptor-subtype yet. In this study, the involvement of the postsynaptic 5-HT1A receptor in exercise-induced adult neurogenesis was analyzed in the OE model. Both male and female mice were tested due to gender-specific differences in the prevalence and pathophysiology of the 5 HT1A receptor in depression as well as gender-specific differences in OE mice found in previous studies. After chronic 8-OH-DPAT or vehicle administration and in vivo labeling with BrdU, immunohistochemical studies for quantification of cell proliferation and survival in the dentate gyrus of male and female OE and WT mice were carried out. For analyzing exercise-induced adult neurogenesis a subgroup of both OE and WT mice had access or no access to a running wheel, respectively. Depression-like behavior of chronic 8-OH-DPAT or vehicle-treated OE and WT mice and untreated control animals was studied using the forced swim test and sucrose preference test. Differences in hippocampus-dependent learning of OE and WT animals were tested in the novel object recognition test and the novel object location test. Voluntary wheel running was able to increase cell proliferation and survival in WT and OE mice. Considering the reduced distance traveled by OE mice, postsynaptically located 5-HT1A receptors are assumed to mediate a proliferative effect in exercise-induced adult neurogenesis. Chronic 5-HT1A receptor activation did not result in increased cell proliferation or survival in either the transgenic mouse model or in WT animals. Female OE mice even showed a lower survival rate after chronic 5-HT1A receptor activation compared to WT animals and, correspondingly, depression-like behavior. The studies on hippocampus-dependent learning revealed no differences between OE and WT animals or the different treatment groups according to the results of cell survival. It is assumed that, in addition to 5-HT1A receptor desensitization after chronic receptor activation, mainly stress-causing factors such as injection and isolation were responsible for the present results. In conclusion, the results of our study, together with the results of a recent study on stress behavior of the OE mouse, indicate an increased stress sensitivity of the OE mouse. An interaction of sex hormones with postsynaptic 5-HT1A receptors as well as a decreased basal brain 5-HT concentration in female OE animals may result in a reduced cell survival rate and depression-like behavior of female OE animals following chronic 5-HT1A receptor activation. An analysis of stress response, the measurement of stress hormone concentrations such as corticosterone in blood and the determination of basal brain 5-HT concentrations in female transgenic mice are required to strengthen this assumption.