Reduced vasopressin receptors activation mediates the anti-depressant effects of fluoxetine and venlafaxine in bulbectomy model of depression

RATIONALE

In response to stress, corticotropin releasing hormone (CRH) and vasopressin (AVP) are released from the hypothalamus, activate their receptors (CRHR1, CRHR2 or AVPr1b), and synergistically act to induce adrenocorticotropic hormone (ACTH) release from the anterior pituitary. Overstimulation of this system has been frequently associated with major depression states.

OBJECTIVE

The objective of the study is to assess the role of AVP and CRH receptors in fluoxetine and venlafaxine effects on the expression of depression-related behavior.

METHODS

In an animal model of depression (olfactory bulbectomy in mice, OB), we evaluated the effects of fluoxetine or venlafaxine (both 10 mg/kg/day) chronic administration on depression-related behavior in the tail suspension test. Plasma levels of AVP, CRH, and ACTH were determined as well as participation of their receptors in the expression of depression related-behavior and gene expression of AVP and CRH receptors (AVPr1b, CRHR1, and CRHR2) in the pituitary gland.

RESULTS

The expression of depressive-like behavior in OB animals was reversed by treatment with both antidepressants. Surprisingly, OB-saline mice exhibited increased AVP and ACTH plasma levels, with no alterations in CRH levels when compared to sham mice. Chronic fluoxetine or venlafaxine reversed these effects. In addition, a significant increase only in AVPr1b gene expression was found in OB-saline.

CONCLUSION

The antidepressant therapy used seems to be more likely related to a reduced activation of AVP rather than CRH receptors, since a positive correlation between AVP levels and depressive-like behavior was observed in OB animals. Furthermore, a full restoration of depressive behavior was observed in OB-fluoxetine- or venlafaxine-treated mice only when AVP was centrally administered but not CRH.

Differential effects of fluoxetine and venlafaxine on memory recognition: possible mechanisms of action

Serotonin-specific reuptake inhibitors (SSRI) and serotonin-norepinephrine reuptake inhibitors (SNRI) are antidepressant drugs commonly used to treat a wide spectrum of mood disorders (Wong and Licinio, 2001). Although they have been clinically used for more than 50 years, the molecular and cellular basis for the action of SSRIs and SNRIs is not clear. Considering that the changes in gene expression involved in the action of antidepressant drugs on memory have not been identified, in this study we investigated the impact of chronic treatment with a SSRI (fluoxetine) and a SNRI (venlafaxine) on the mRNA expression of genes related to memory cascade in the mouse hippocampus, namely, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), nitric oxide synthase 1 (NOS1), neurotrophic tyrosine kinase receptor type 2 (TrKB), mitogen-activated protein kinases (MAPK/ERK) and serotonin transporter (SERT). Animals treated with fluoxetine 10 mg/Kg/day for 28 days showed a significant decrease in the percentage of time spent in the novel object recognition test (p≤0.005) and induced MAPK1/ERK2 down-regulation (p=0.005). Our results suggest that the effect on cognition could probably be explained by fluoxetine interference in the MAPK/ERK memory pathway. In contrast, chronic treatment with venlafaxine did not reduce MAPK1/ERK2 expression, suggesting that MAPK1/ERK2 down-regulation is not a common effect of all antidepressant drugs. Further studies are needed to examine the effect of chronic fluoxetine treatment on the ERK-CREB system, and to determine whether there is a causal relationship between the disruption of the ERK-CREB system and the effect of this antidepressant on memory performance.