1. ¹Department of Pharmacological Sciences, Center of Neuropharmacology, University of Milano and Center of Excellence on Neurodegenerative Diseases, University of Milano, Milano, Italy
2. ²Genetics Unit, IRCCS Centro S Giovanni di Dio-FBF, Brescia, Italy
3. ³Department of Pharmacology, Tohoku University, Sendai, Japan
4. ⁴Department of Biomedical Sciences and Biotechnology, University of Brescia, Brescia, Italy

Correspondence: Dr M Popoli, Center of Neuropharmacology, Department of Pharmacological Sciences, University of Milan, Via Balzaretti 9, Milano 20133, Italy. Tel: +39 02 5031 8361; Fax: +39 02 5031 8278; E-mail: maurizio.popoli@unimi.it

Received 10 December 2003; Revised 17 March 2004; Accepted 15 April 2004; Published online 12 May 2004.

Abstract

Regulation of gene expression is purported as a major component in the long-term action of antidepressants. The transcription factor cAMP-response element-binding protein (CREB) is activated by chronic antidepressant treatments, although a number of studies reported different effects on CREB, depending on drug types used and brain areas investigated. Furthermore, little is known as to what signaling cascades are responsible for CREB activation, although cAMP-protein kinase A (PKA) cascade was suggested to be a central player. We investigated how different drugs (fluoxetine (FLX), desipramine (DMI), reboxetine (RBX)) affect CREB expression and phosphorylation of Ser¹³³ in the hippocampus and prefrontal/frontal cortex (PFCX). Acute treatments did not induce changes in these mechanisms. Chronic FLX increased nuclear phospho-CREB (pCREB) far more markedly than pronoradrenergic drugs, particularly in PFCX. We investigated the function of the main signaling cascades that were shown to phosphorylate and regulate CREB. PKA did not seem to account for the selective increase of pCREB induced by FLX. All drug treatments markedly increased the enzymatic activity of nuclear Ca²⁺/calmodulin (CaM) kinase IV (CaMKIV), a major neuronal CREB kinase, in PFCX. Activation of this kinase was due to increased phosphorylation of the activatory residue Thr¹⁹⁶, with no major changes in the expression levels of - and -CaM kinase kinase, enzymes that phosphorylate CaMKIV. Again in PFCX, FLX selectively increased the expression level of MAP kinases Erk1/2, without affecting their phosphorylation. Our results show that FLX exerts a more marked effect on CREB phosphorylation and suggest that CaMKIV and MAP kinase cascades are involved in this effect.