Preclinical studies suggest that substance P (SP) neurokinin 1 (NK1) receptor antagonists are efficient in the treatment of anxiety and depression. This therapeutic activity could be mediated via stimulation of serotonin (5-HT) neurons located in the dorsal raphe nucleus (DRN), which receive important SP-NK1 receptor immunoreactive innervations. The present study examined the effects of intraraphe injection of SP on extracellular 5-HT levels in the frontal cortex, ventral hippocampus, and DRN by using intracerebral microdialysis in conscious mice. Intraraphe SP injection dose dependently decreased cortical 5-HT release, whereas no effects were detected in the ventral hippocampus. Cortical effects were blocked by the selective NK1 receptor antagonist N- [[2-methoxy-5-[5-(trifluoromethyl)tetrazol-1-yl]phenyl]methyl]-2-phenylpiperidin-3-amine (GR205171) and completely dampened in mice lacking NK1 receptors. Furthermore, genetic (in knockout 5-HT 1A ÏȘ/ÏȘ mice) or pharmacological inactivation of 5-HT 1A autoreceptors blocked cortical responses to SP. Contrasting with its cortical effects, intraraphe SP injection increased 5-HT outflow in the DRN in wild-type mice; this effect was potentiated by a local perfusion of the selective 5-Finally, SP-induced changes in frontal cortex and DRN dialysate 5-HT levels were blocked by the DRN perfusion of the âŁ-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate ionotropic receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). These data support the hypothesis that SP-induced overactivation of 5-HT 1A autoreceptors within the DRN limits cortical 5-HT release. A better knowledge of the complex relationship between tachykininergic, serotonergic, and glutamatergic systems within the DRN might help better understand the pathophysiology and subsequent treatment of depression.Substance P (SP), a small peptide that belongs to the tachykinins family with neurokinins A and B, is widely distributed in the brain, specifically in limbic regions and brainstem nuclei such as the dorsal raphe nucleus (DRN) (Froger et al., 2001;Commons et al., 2002;Lacoste et al., 2006). In several species, including rodents and humans, SP distribution overlaps with that of its high-affinity NK1 receptor (Ribeiro-da-Silva and Hokfelt, 2000). Recent clinical and preclinical studies have pointed out the potential therapeutic action of SP (neurokinin 1) receptor antagonists in major depressive disorders (Kramer et al., 1998;Chahl, 2006). Data obtained from NK1 receptor knockout mice have suggested that the antidepressant-like action of NK1 receptor inactivation may result, at least in part, from an increase in central 5-HT neurotransmission through functional desensitization B.P.G. was the recipient of a fellowship from La Fondation pour la Recherche Medicale (FRM) during the performance of this work.Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org. doi:10.1124/mol.107.040113.ABBREVIATIONS: SP, substance P; DRN, dorsal raphe nucleus; NK,...