Molecular effects of subchronic andchronic treatment with escitalopram inthe rat central nervous system

A clear understanding of the mechanisms behind depressionand its treatment is a critical issue for amelioratethe effectiveness of existing antidepressants. Acutely,antidepressant drugs increase synaptic concentrations ofmonoamines, but clinical efficacy requires several weeksof continuous treatment, proposing a key role for timedependentneural adaptations, perhaps induced by acutesynaptic actions, in their therapeutic efficacy.Escitalopram is the S(+) enantiomer of citalopram, oneof the most widely prescribed serotonin selective reuptakeinhibitor (SSRIs) antidepressants. In the chronic mildstress model of depression sucrose intake was alreadynormalized after one week of treatment.We evaluated the effects of a subchronic or a chronictreatment with escitalopram on expression levels of someof the main targets of antidepressant drugs such as theneurotrophin Brain Derived Neurotrophic Factor (BDNF),the transcription factors cAMP Response Element Binding(CREB) [1] Protein and Calcium Responsive Factor(CaRF).Sprague-Dawley rats were treated for 7 days (subchronic)or 21 days (chronic) with either escitalopram(10 mg/kg die i.p) or saline (1 mL/kg die); BDNF, CREBand CaRF mRNAs were evaluated using RNAse ProtectionAssay in hippocampus and prefrontal cortex.No difference was observed on BDNF, CREB andCaRF expression in the hippocampus of rats treatedsubchronically with escitalopram with respect to the grouptreated with saline. In contrast a significant inductionof BDNF mRNA was observed in prefrontal cortexof escitalopram-treated animals with respect to salinetreated ones. CaRF expression patterns were similar.Escitalopram for 7 days caused a significant induction ofCaRF mRNA with respect to the group treated with saline(p<0.05; Dunnett t-test), on the other hand CREB mRNAremained unaffected. Following a chronic treatment withEscitalopram, BDNF, CREB and CaRF mRNA levels weresignificantly decreased with respect to the group treatedwith saline in hippocampus (p<0.05; Dunnett t-test),while a 21 day treatment with escitalopram failed toproduce changes in gene expression in prefrontal cortex.These results showed that escitalopram is able todifferentially affect BDNF, CREB and CaRF expressionwith respect to treatment duration and that the observedeffects are area-specific.Consequently, to further investigate the possiblemolecular mechanisms underlying the observed effectson gene expression we evaluated by western blottingsome of the main signalling pathways regulating CREB aswell as BDNF expression, such as p38 MAPK (Mitogen-Activated Protein Kinase), CaMKII (Calcium CalmodulineKinase), ERK 1/2 (Extracellular Signal-Regulated Kinase)and CREB itself [1].Our study demonstrates that:1. A subchronic treatment with escitalopram inducesBDNF and CaRF expression in prefrontal cortexprobably through activation of p38 MAPK signallingpathway.2. A 21 day escitalopram treatment reduces hippocampalBDNF, CaRF, CREB gene expression and also CREBphosphorylated nuclear levels.Spatially distinct signalling pathways may be involvedin mediating the differential effect on gene expressionobserved following either a subchronic or a chronictreatment with escitalopram.