Metabotropic glutamate receptor subtype 7 ablation causes dysregulation of the HPA axis and increases hippocampal BDNF protein levels:: Implications for stress-related psychiatric disorders

Regulation of neurotransmission via group-III metabotropic glutamate receptors (mGluR4, -6, -7, and -8) has recently been implicated in the pathophysiology of affective disorders, such as major depression and anxiety. For instance, mice with a targeted deletion of the gene for mGluR7 (mGluR7(-/-)) showed antidepressant and anxiolytic-like effects in a variety of stress-related paradigms, including the forced swim stress and the stress-induced hyperthermia tests. Deletion of mGluR7 reduces also amygdala- and hippocampus-dependent conditioned fear and aversion responses. Since the hypothalamic-pituitary-adrenal (HPA) axis regulates the stress response we investigate whether parameters of the HPA axis at the levels of selected mRNA transcripts and endocrine hormones are altered in mGluR7-deficient mice. Over all, mGluR7(-/-) mice showed only moderately lower serum levels of corticosterone and ACTH compared with mGluR7(+/+) mice. More strikingly however, we found strong evidence for upregulated glucocorticoid receptor (GR)-dependent feedback suppression of the HPA axis in mice with mGluR7 deficiency: (i) mRNA transcripts of GR were significantly upregulated in the hippocampus of mGluR7(-/-) animals, (ii) similar increases were seen with 5-HT1A receptor transcripts, which are thought to be directly controlled by the transcription factor GR and finally (iii) mGluR7(-/-) mice showed elevated sensitivity to dexamethasone-induced suppression of serum corticosterone when compared with mGluR7(+/+) animals. These results indicate that mGluR7 deficiency causes dysregulation of HPA axis parameters, which may account, at least in part, for the phenotype of mGluR7(-/-) mice in animal models for anxiety and depression. In addition, we present evidence that protein levels of brain-derived neurotrophic factor are also elevated in the hippocampus of mGluR7(-/-) mice, which we discuss in the context of the antidepressant-like phenotype found in those animals. We conclude that genetic ablation of mGluR7 in mice interferes at multiple sites in the neuronal circuitry and molecular pathways implicated in affective disorders.