Background: The present study was designed to confirm or refute in human fibroblasts the hypothesized cross-talk elicited via neurotransmitter transduction cascades at the level of protein kinase mediated phosphorylation of the nuclear transcription factor CREB. Methods: Human fibroblasts from normal control subjects were subcultured and incubated at confluency after five growth passages with isoproterenol (stimulation of PKA mediated phosphorylation) and/or phorbol 12-myristate 13-acetate (PMA) (stimulation of PKC mediated phosphorylation) followed by the determination of nuclear CREB-P by immunoblotting, enhanced chemiluminescence and quantitation of the autoradiograms by laser densitometry. Results: Using the nuclear transcription factor CREB as a target, both the activation of the cyclic AMP-PKA pathway by isoproterenol and the activation of the PKC pathway by PMA caused phosphorylation of nuclear CREB. This phosphorylation is additive in nature and appears to occur at the same molecular site, serine133 of CREB. Conclusions: The present results in human fibroblasts demonstrate that the hypothesized cross-talk at the level of protein kinase mediated phosphorylation of transcription factors is no longer hypothetical. Since it is the phosphorylation of nuclear CREB that determines its dimerization and transcriptional activation of programs of CRE containing genes, the results suggest that this convergence of the neurotransmitter signals may be the critical mechanism in gene expression following the administration of antidepressant drugs that affect noradrenergic, serotonergic or both transduction cascades. The results may also provide a rationale for the apparent superior clinical efficacy of dual uptake inhibitors. © 2001 Elsevier Science B.V. All rights reserved.