Research report
Mice with reduced brain-derived neurotrophic factor expression show decreased choline acetyltransferase activity, but regular brain monoamine levels and unaltered emotional behavior

https://doi.org/10.1016/j.molbrainres.2003.11.002Get rights and content

Abstract

The “neurotrophin hypothesis” of depression predicts that depressive disorders in humans coincide with a decreased activity and/or expression of brain-derived neurotrophic factor (BDNF) in the brain. Therefore, we investigated whether mice with a reduced BDNF expression due to heterozygous gene disruption demonstrate depression-like neurochemical changes or behavioral symptoms. BNDF protein levels of adult BDNF+/− mice were reduced to about 60% in several brain areas investigated, including the hippocampus, frontal cortex, striatum, and hypothalamus. The content of monoamines (serotonin, norepinephrine, and dopamine) as well as of serotonin and dopamine degradation products was unchanged in these brain regions. By contrast, choline acetyltransferase activity was significantly reduced by 19% in the hippocampus of BDNF+/− mice, indicating that the cholinergic system of the basal forebrain is critically dependent on sufficient endogenous BDNF levels in adulthood. Moreover, BDNF+/− mice exhibited normal corticosterone and adrenocorticotropic hormone (ACTH) serum levels under baseline conditions and following immobilization stress. In a panel of behavioral tests investigating locomotor activity, exploration, anxiety, fear-associated learning, and behavioral despair, BDNF+/− mice were indistinguishable from wild-type littermates. Thus, a chronic reduction of BDNF protein content in adult mice is not sufficient to induce neurochemical or behavioral alterations that are reminiscent of depressive symptoms in humans.

Introduction

Recent basic and clinical studies have provided evidence for a “neurotrophin hypothesis” of depression and antidepressive treatment [1], [9], [31], [56], [61]. This concept postulates that plasticity-related changes, like hippocampal atrophy in depressed patients, are related to a decreased expression or function of brain-derived neurotrophic factor (BDNF) and/or its high-affinity receptor TrkB [1], [9], [39], [56], [61]. This theory is supported by the fact that BDNF is found in high concentrations in the hippocampus and cerebral cortex—brain areas known to play a role in depression [15], [26], [30]. Furthermore, BDNF expression in these areas is decreased by stress exposure, which is currently the only experimental manipulation to induce depression-like states in rodents [51], [52], [55]. Moreover, chronic, but not acute, treatment with antidepressants as well as electroconvulsive therapy induce increased levels of BDNF mRNA and protein, mainly in the hippocampus [8], [46], [64]. Local cerebral administration of BDNF itself is reported to exert antidepressant-like effects in animal models of depression [49], [50].

The hypothesis has been put forward that mice with compromised BDNF–TrkB signaling pathways could provide a genetic murine model of depression, which would be reflected by neurochemical/neuroendocrinological alterations in specific brain regions as well as by characteristic changes of emotional behaviors [29], [47]. Therefore, we investigated whether mice with a heterozygous BDNF knockout exhibit differences in monoamine levels in various forebrain areas because the monoaminergic hypothesis of depression predicts that depression is related to an impairment of neurotransmission by serotonin (5-HT), norepinephrine (NE), and, most likely also, dopamine (DA) [3], [4], [48]. A disinhibition of the hypothalamic–pituitary–adrenal (HPA) system is regarded as a hallmark neuroendocrinological correlate for major depressive episodes in human patients [2], [38]. Furthermore, persistent changes of the HPA system represent a risk factor for the occurrence or recurrence of a depressive episode [2], [38]. Therefore, we analyzed the HPA system of BDNF heterozygous mice under baseline conditions and following stress exposure. Furthermore, the mice were subjected to a test battery for emotional behaviors, analyzing locomotion and exploration as well as anxiety- and depression-related behaviors. On the basis of earlier studies [20], [28], [29] and due to the fact that women bear a higher risk than men to develop major depressive episodes, we decided mainly to work with female mice in the present study [25].

Section snippets

Animals

BDNF+/− male mice on a mixed C57BL/6×SV129 background were supplied by Jackson Laboratories (Bar Harbor, ME, USA) and used for breeding. They were crossed to C57BL/6 female mice obtained from Charles River (Sulzfeld, Germany). All experiments were performed with offspring of this breeding. Experimental animals were weaned at 28 days of age, transferred to standard cages, and housed under standard conditions on a reversed 12-h light–dark cycle (light: 1800–0600 h). We decided to maintain the

Biochemical analyses

BDNF+/− mice demonstrated a reduction of BDNF protein expression to about 60% in all brain areas analyzed by ELISA (i.e., hippocampus, frontal cortex, striatum, and hypothalamus of the right hemisphere) (Fig. 1A). In contrast, BDNF+/− mice showed normal NGF levels in these brain regions (Fig. 1B). Analyses of monoaminergic neurotransmitter systems (5-HT, NE, and DA) revealed regular tissue levels of all three biogenic amines and also of their degradation products 5-HIAA and HVA (Table 1). These

Discussion

The present study examined potential neurochemical, neuroendocrinological, and behavioral changes in adult female mice with a heterozygous mutation of the BDNF gene. We showed that these mice have a ca. 60% reduction of BDNF protein levels in several forebrain regions (Fig. 1). Our aim was to test one of the predictions of the “neurotrophin hypothesis of depression” (i.e., that the reduction of BDNF levels could induce depression-like features in these mice). However, BDNF+/− animals exhibited

Supplementary Files

Acknowledgements

We gratefully acknowledge the excellent technical assistance of Astrid Arnswald and Gala Sadovska. This work was supported by grants from the Deutsche Forschungsgemeinschaft (GA 427/4-2 to P.G. and HO 2201/2-1 to H.H.).

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    Supplementary data associated with this article can be found in the online version at doi:10.1016/j.molbrainres.2003.11.002.

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    Both authors made equal contributions to this paper.

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