Elsevier

Neuropharmacology

Volume 43, Issue 7, December 2002, Pages 1148-1157
Neuropharmacology

Regulation of cAMP phosphodiesterase mRNAs expression in rat brain by acute and chronic fluoxetine treatment. An in situ hybridization study

https://doi.org/10.1016/S0028-3908(02)00220-4Get rights and content

Abstract

Changes in brain cyclic AMP (cAMP) have been suggested to underlie the clinical action of antidepressant treatments. Also, a regionally-selective regulation of cAMP-specific phosphodiesterases (PDEs) has been demonstrated for some antidepressants. To further investigate the effects of antidepressant treatments on PDEs, we examined the expression of different cAMP-specific PDEs in the brain of rats treated (1 and 14 days) with fluoxetine 3 mg/kg day. The mRNAs coding for PDE4A, PDE4B, PDE4D, and the five known PDE4D splice variants were analyzed by in situ hybridization on 45 brain structures of acute and chronic fluoxetine-treated rats. We also examined the binding sites for the putative antidepressant drug [3H]rolipram, a PDE4-selective inhibitor. In some brain areas single fluoxetine administration increased the density of the mRNA of all PDE4 isozymes, except PDE4D and PDE4D5. Chronic fluoxetine treatment increased PDE4A mRNA levels and decreased those for PDE4B, PDE4D and PDE4D1 mRNAs in some brain regions. The study was complemented with the analysis of the expression of the transcripts of BDNF. Chronic fluoxetine treatment down-regulated the expression of BDNF. These results show that the expression of PDE4 isozymes is modulated by a clinically relevant fluoxetine dose. The significance of these changes in PDE4 expression to the antidepressant effect of fluoxetine is discussed.

Introduction

Selective serotonin reuptake inhibitors (SSRIs) are extensively used in the treatment of major depression. They exert their therapeutic effect enhancing brain serotonergic activity by preventing serotonin (5-HT) reuptake and thereby increasing its concentration in the synaptic cleft. But all antidepressant treatments require an administration of at least two weeks before inducing a clinically significant improvement. The mechanisms underlying in this delay in the therapeutic effect of SSRIs remain unknown.

Cyclic AMP (cAMP) plays an important role in signal transduction processes (Houslay, 1998). A role of the cAMP pathway has been suggested in several CNS diseases such as depression (Duman et al., 1997) and Alzheimer’s disease (Cowburn et al., 1996, Bonkale et al., 1999). The hydrolysis of cAMP is regulated by a family of cyclic nucleotide phosphodiesterases (PDEs). Twelve members of this family have been identified until now on the basis of their substrate specificities, kinetic properties, allosteric regulators, inhibitor sensitivities and amino acid sequences (Conti and Jin, 1999, Soderling and Beavo, 2000, Houslay, 2001). Families 4, 7 and 8 specifically hydrolyze cAMP. The PDE4 family has four members (PDE4A through PDE4D) (Houslay, 1998), the PDE7 family, two (PDE7A, PDE7B) (Conti and Jin, 1999) and the PDE8 two (PDE8A, PDE8B) (Fisher et al., 1998, Hayashi et al., 1998) encoded by different genes.

Selective inhibitors of cAMP-specific PDEs have been proposed for the treatment of CNS disorders, such as depression (O’Donnell and Frith, 1999), multiple sclerosis (Genain et al., 1995), ischaemia-reperfusion injury (Kato et al., 1995), Alzheimer’s (McGeer and McGeer, 1995) and Parkinson’s diseases (Hulley et al., 1995). Rolipram, a PDE4 selective inhibitor, has antidepressant properties (Wachtel, 1983), yet its side effects prevented a widespread clinical use (Fleischhacker et al., 1992).

The long-term treatment with antidepressant drugs results in an increase of the levels of cAMP and of the expression of CREB (cyclic adenosine monophosphate response element-binding protein) in specific brain regions (Nibuya et al., 1996, Duman et al., 1997). These treatments also increased the expression of the brain-derived neurotrophic factor (BDNF), which has been suggested to play a role in the therapeutic action of antidepressant drugs (Duman et al., 1997, Siuciak et al., 1997).

The main objective of the present study was to examine the putative involvement of PDE4s in the mechanism of action of antidepressant drugs. For this, we analyzed the effects of single and repeated fluoxetine treatment on the regulation of the expression of different cAMP-specific PDE4s in a large number of brain structures. The study was complemented by the analysis of the expression of BDNF and [3H]rolipram binding on the same brain structures.

Section snippets

Animals and treatments

Adult male Wistar rats (200–300 g) were purchased from Iffa Credo (Lyon, France). Animals were acclimated to standard laboratory conditions (14-h light/10-h dark cycle) with free access to rat chow and water. Each rat was used only once for experimentation, and all the procedures conformed to the European Communities Council directive of November 24, 1986 (86/609/EEC). Animals were conscious and freely moving at all times throughout the experimental procedure. Rats were randomly assigned to

Results

We have examined the effects of the acute and chronic treatment with the selective serotonin reuptake inhibitor fluoxetine on the expression of the four members of the PDE4 family (PDE4A, PDE4B, PDE4C and PDE4D, including its five mRNA splice variants), and BDNF mRNAs, by in situ hybridization histochemistry. We have also analyzed the effects of fluoxetine treatments on the density of [3H]rolipram binding sites by autoradiography. Figure 1 shows representative autoradiograms of rat coronal

Discussion

The aim of the present work was to study the alterations on the transcription levels of the mRNAs coding for PDE4A, PDE4B, PDE4D and its five splice forms, in the rat brain upon acute and chronic treatment with a 5-HT transporter inhibitor, fluoxetine. When studying the correlation between the changes in the levels of both 3H-rolipram binding sites and mRNA coding for each of the PDE4 isozymes analyzed, several considerations should be taken into account: (i) 3H-rolipram labels only the high

Acknowledgements

This work was supported, by grants from Fundació La Marató de TV3 (#1017/97), CICYT (SAF1999-0123 and 2FD97-0395). S.P.-T. is a recipient of a fellowship from CIRIT (Generalitat de Catalunya) and X.M. from CIRIT (Centre de Referència de la Generalitat de Catalunya).

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      Citation Excerpt :

      Further studies showed that fluoxetine administration reduced PDE4D3 expression in the cingulate cortex, while PDE4D4 expression was increased in the frontal and frontoparietal cortex. Moreover, expression of PDE4D1 and PDE4D5 was decreased in several areas of the hippocampus (Miró et al., 2002). These studies indicate that the earlier results, showing unaltered levels of PDE4D expression after fluoxetine treatment, most likely represent the net result of opposing changes in different PDE4D splice variants.

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    1

    These two authors have contributed equally to the work.

    2

    Permanent address: Research Center, Almirall Prodesfarma S.A., Cardener 68-74, E-08024 Barcelona, Spain.

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