P.1.018 - The impact of selective CREB deletion in serotonergic neurons on the effects of fluoxetine administration and antidepressive behaviour

Abstract

The majority of current antidepressant therapies is based on the enhancement of monoaminergic transmission observed directly after drug administration, yet alleviation of depressive symptoms occurs weeks later. Therefore, the mechanism of molecular changes arising during chronic antidepressant treatment is intensively researched. Many studies have pointed to an important role of cAMP response element binding (CREB) protein in the mechanism of antidepressant drug action, however the data are inconclusive. Generally, an increase of CREB level or activity after chronic antidepressant treatment was postulated, but in other studies opposite effects were observed [1]. Moreover, CREB-deficient animals show antidepressant-like phenotype. However, the feedback from knock-out studies could have been disturbed by cAMP response element modulator (CREM) upregulation occurring upon CREB loss [2], which was not taken into consideration. In our studies we investigated the role of CREB in the mechanism of antidepressant treatment using novel, inducible transgenic mouse model lacking CREB selectively in serotonergic neurons [3], maintained in CREM-deficient background (Creb1TPH2CreERT2Crem-/- mice). The animals have been phenotypically characterized showing no impairments in their basal behavior. However, single Creb1TPH2CreERT2 mutants of both sexes presented drug-resistant phenotype in tail suspension test after fluoxetine treatment, whereas male but not female double mutants (Creb1TPH2CreERT2Crem-/-) reacted to the drug similar to control animals [4]. The aim of current studies was to assess the molecular changes induced by chronic fluoxetine (10 mg/kg i.p., 21 days, 1x daily) treatment in hippocampus and prefrontal cortex (PFC). Animals were sacrificed 24h after last injection and the brain structures were collected. Assessment of mRNA expression for Creb1 and Bdnf genes was measured by RT-PCR using TaqMan probes. The levels of phosphorylated CREB, total CREB and BDNF proteins were studied using Western Blot. The results were analyzed using two-way ANOVA, followed by Fisher’s LSD test. We observed no effect in mRNA expression of Creb1 and Bdnf genes in control animals after fluoxetine treatment, nor in any of studied mutants. Moreover, no changes in phosphorylation of CREB or total CREB protein were visible in any of studied group. On the other hand, BDNF protein was strongly increased in response to drug in hippocampus of control males and females (by 212% and 120%, respectively), and in PFC of control females only (by 47%). Both treated and non-treated Creb1TPH2CreERT2 single mutant mice did not show any changes in BDNF in both studied brain structures; the result was significantly different from the effect observed in control animals after fluoxetine administration. Creb1TPH2CreERT2Crem-/- double mutants presented only non-significant tendency of increased BDNF protein level in hippocampus (both males and females) and PFC (females only). Despite the absence of any effect in CREB mRNA or protein levels observed in both mutant lines, the BDNF upregulation in fluoxetine-treated control mice suggests increased activity of CREB in hippocampus after antidepressant treatment as observed by other investigators [5]. Lack of molecular or behavioral changes after fluoxetine treatment in Creb1TPH2CreERT2 single mutant animals suggests that compensatory role of CREM upregulation in the inducible line may have different effects from that observed previously in constitutive CREB knockouts.