Decreased serotonin (5-HT) transmission is thought to underlie several mental diseases, including depression and feeding disorders. However, whether deficits in genes encoding G protein-coupled receptors may down-regulate the activity of 5-HT neurons is unknown currently. Based on recent evidence that stress-induced anorexia may involve 5-HT(4)receptors (5-HT(4)R), we measured various aspects of 5-HT function in 5-HT(4)R knock-out (KO) mice. When compared to dorsal raphe nucleus (DRN) 5-HT neurons from wild-type mice, those from 5-HT(4)R KO mice exhibited reduced spontaneous electrical activity. This reduced activity was associated with diminished tissue levels of 5-HT and its main metabolite, 5-hydroxyindole acetic acid (5-HIAA). Cumulative, systemic doses of the 5-HT uptake blocker citalopram, that reduced 5-HT cell firing by 30% in wild-type animals, completely inhibited 5-HT neuron firing in the KO mice. This effect was reversed by administration of the 5-HT(1A) receptor (5-HT(1A)R) antagonist, WAY100635, in mice of both genotypes. Other changes in DRN of the KO mice included increases in the levels of 5-HT plasma membrane transporter sites and mRNA, as well as a decrease in the density of 5-HT(1A)R sites without any change in 5-HT(1A) mRNA content. With the exception of increased 5-HT turnover index in the hypothalamus and nucleus accumbens and a decreased density of 5-HT(1A)R sites in the dorsal hippocampus (CA1) and septum, no major changes were detected in 5-HT territories of projection, suggesting region-specific adaptive changes. The mechanisms whereby 5-HT(4)R mediate a tonic positive influence on the firing activity of DRN 5-HT neurons and 5-HT content remain to be determined.