Flattening the Corticosterone Rhythm Attenuates 5-HT1A Autoreceptor Function in the Rat: Relevance for Depression

Both glucocorticoids and selective serotonin reuptake inhibitors (SSRIs) alter aspects of 5-HT function including somatodendritic 5-HT 1A autoreceptor sensitivity. Many depressed patients prescribed SSRIs have pre-existing flattened diurnal gluococorticoid rhythm. In these patients, interactions between flattened glucocorticoid rhythm and chronic SSRIs, which impact on the SSRI's ability to elevate forebrain 5-HT, may alter clinical efficacy. To address this issue rats underwent implantation of slow-release corticosterone (75 mg pellet s.c.) (to flatten the glucocorticoid rhythm) or sham surgery, and injection of fluoxetine (10 mg/kg/day i.p., 12 days) or vehicle. Using microdialysis in the frontal cortex we found that (21 h after the last injection) extracellular 5-HT was elevated in fluoxetine-or corticosterone-treated animals, but not in those treated with corticosterone plus fluoxetine. In fluoxetine-treated animals, blockade of terminal reuptake by local perfusion of fluoxetine increased 5-HT to the same level as it did in controls, suggesting normal terminal 5-HT release after chronic fluoxetine. However, 5-HT levels following local reuptake blockade in both the corticosterone and corticosterone plus fluoxetine groups were lower than controls, suggesting a corticosterone-induced decrease in terminal release. Finally in fluoxetine, corticosterone, and corticosterone plus fluoxetine groups, there was marked 5-HT 1A receptor desensitization, evidenced by attenuation of the decrease in 5-HT release following systemic fluoxetine injection. The data indicate that, despite desensitization of 5-HT 1A autoreceptors, concurrent flattened glucocorticoid rhythm compromises the ability of SSRIs to elevate forebrain 5-HT. These findings suggest a potential mechanism for the reduced antidepressant efficacy of SSRIs in those patients with pre-existing glucocorticoid abnormalities.

Section: Introductionmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

Altered Glucocorticoid Rhythm Attenuates the Ability of a Chronic SSRI to Elevate Forebrain 5-HT: Implications for the Treatment of Depression

Gartside

Leitch

Young

2003

“…In particular, the 5-HT 1A receptor (5-HT 1A R) is very susceptible to modulation by stress and HPA-axis activation and is well known to play an important role in the pathophysiology of mood disorders (Lopez et al, 1998;Cryan and Leonard, 2000;Leitch et al, 2003). Accumulating evidence points also to an important role for the neurotrophin brainderived neurotrophic factor (BDNF) in both the pathophysiology of mood disorders and in the therapeutic effects of antidepressants (Duman, 2002;Castren, 2004;Hashimoto et al, 2004).…”

Section: Introductionmentioning

confidence: 99%

Metabotropic Glutamate Receptor Subtype 7 Ablation Causes Dysregulation of the HPA Axis and Increases Hippocampal BDNF Protein Levels: Implications for Stress-Related Psychiatric Disorders

Mitsukawa

Mombereau

Lötscher

et al. 2005

124

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 À/À ) 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 À/À 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 À/À animals, (ii) similar increases were seen with 5-HT 1A receptor transcripts, which are thought to be directly controlled by the transcription factor GR and finally (iii) mGluR7 À/À 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 À/À 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 À/À 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.

“…We have previously shown that implantation of slowrelease corticosterone pellets in adrenally intact adult rats produces a flattened glucocorticoid rhythm, in which levels are clamped midway between the normal morning nadir and evening peak, and there is no change in overall glucocorticoid exposure (Leitch et al, 2003). Here we used this model to determine the effects of flattened glucocorticoid rhythm on hippocampal expression of mRNAs coding for three neuronal proteins using in situ hybridization histochemistry.…”

Section: Introductionmentioning

confidence: 99%

Flattening the Glucocorticoid Rhythm causes Changes in Hippocampal Expression of Messenger RNAs Coding Structural and Functional Proteins: Implications for Aging and Depression

Gartside

Leitch

McQuade

et al. 2002

Self Cite

Subtle changes in glucocorticoid levels, including a flattening of the diurnal rhythm with raised nadir, are prevalent, being characteristic of both aging and major depression. Both these conditions are also associated with deficits in hippocampally mediated cognitive functions. We hypothesized that this profile of glucocorticoid levels causes structural and functional changes in the hippocampus, which in turn may engender cognitive deficits. We implanted slow-release corticosterone pellets into adrenally intact adult male rats to produce a flattened glucocorticoid rhythm with levels clamped midway between the normal nadir and zenith. Using density profile analysis we measured hippocampal expression of messenger RNAs encoding structural and functional proteins. In rats with a flattened glucocorticoid rhythm, the expression of the mRNA coding for microtubule associated protein-2b (MAP2b) was reduced in CA3 relative to sham-operated controls, but unchanged in dentate gyrus and CA1. In contrast, the expression of the mRNA coding the alpha subunit of calciumcalmodulin dependent kinase (CAMKIIa) was reduced in dentate gyrus in animals with a flattened glucocorticoid rhythm, but unchanged in CA3. The expression of the mRNA coding the synaptic vesicle protein synaptophysin was unchanged in both CA3 and dentate gyrus. The data indicate that a flattening of the normal diurnal glucocorticoid rhythm decreases the hippocampal expression of mRNAs coding key structural and functional proteins, and does so in a regionally selective manner. The data may have relevance for cognitive deficits characteristic of aging and depression.