Interaction Between the Serotoninergic and Dopaminergic Systems in d-Fenfluramine-Induced Activation of c-fos and jun B Genes in Rat Striatal Neurons *A. M. Gardier, ‡§R. Moratalla, ‡B. Cue ´llar, ‡M. Sacerdote, *B. Guibert, †H. Lebrec, and §A. M. Graybiel *Laboratoire de Neuropharmacologie UPRES EAD MENRT, IFR-ISIT Institut de Signalisation et Innovation The ´rapeutique, †Laboratoire de Toxicologie, INSERM U 461, IFR-ISIT Faculte ´ de Pharmacie, Universite ´ Paris-Sud, Cha ˆtenay-Malabry, and Laboratoire de Pharmacologie, Faculte ´ de Pharmacie, Universite ´ de Picardie Jules Verne, Amiens, France; ‡Instituto Cajal de Neurobiologı ´a, Consejo Superior de Investigaciones Cientı ´ficas, Madrid, Spain; and §Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A. Abstract: To test for the relative contributions of the dopa- minergic and serotoninergic systems in the striatum to the effects of d-fenfluramine, an indirect serotonin receptor ag- onist, we assessed the expression of Fos/Jun proteins in- duced by d-fenfluramine given alone or in the presence of dopaminergic or serotoninergic agents. To determine the neuronal targets of d-fenfluramine in the striatum, we iden- tified the phenotypes of striatal neurons in which d-fenflur- amine induced Fos expression. Our results demonstrated that d-fenfluramine evokes nuclear expression of Fos/Jun B proteins in the striatum, and that the Fos expression was dose-dependent and accompanied by transient induction of c-fos mRNA. Fos expression was blocked by p-chloro- amphetamine, a serotoninergic neurotoxin. Pretreatment with SCH 23390, a D 1 -dopamine receptor antagonist, led to a marked decrease in Fos/Jun B expression in the caudo- putamen, but not in the cortex, whereas pretreatment with methiothepin, a nonselective serotonin 5-HT 1 receptor an- tagonist, blocked Fos expression completely in the cortex and only partially in the caudoputamen. The expression of Fos/Jun B in the striatum occurred mainly in dynorphin- containing neurons and in a subpopulation of striatal inter- neurons that exhibited NADPH-diaphorase activity. Most of the enkephalin-containing neurons of the striatum did not show Fos/Jun B staining. These results suggest that the mechanism by which d-fenfluramine induces c-fos and jun B expression in the rat caudoputamen depends at least in part on activation of the dopaminergic system by serotonin. Key Words: Dexfenfluramine —Striatum— c-Fos—Jun B— Dynorphin—p-Chloroamphetamine. J. Neurochem. 74, 1363–1373 (2000). The innovative method for mapping neuronal activity by monitoring the nuclear expression of immediate-early genes (IEGs) has proven to be particularly successful in identifying selective neuroanatomical loci targeted by drugs (Hughes and Dragunow, 1995). We have used this method to approach the question of what neurotransmit- ter receptors are responsible for the activation of IEGs in the striatum following an acute exposure to d-fenflur- amine. d-Fenfluramine acts both by releasing serotonin [5-hydroxytryptamine (5-HT)] from nerve terminals and by blocking 5-HT reuptake (Samanin and Garattini, 1990; Appel et al., 1991). Evidence suggests that most of the behavioral effects of this drug, including stimulation of the hypothalamo–pituitary–adrenal axis, decrease in food intake by enhancing satiety, and increase in ther- mogenesis, are mediated through the activation of brain serotoninergic systems. d-Fenfluramine has been shown in in vivo microdialysis studies to increase extracellular 5-HT levels in the caudoputamen and nucleus accum- bens, as well as in the frontal cortex and hypothalamus (Laferrere and Wurtman, 1989; Balcioglu and Wurtman, 1998). All of these effects of d-fenfluramine have also been observed with d,l-fenfluramine, the racemic mix- ture of the d and l isomers of fenfluramine (Auerbach et al., 1989; Sabol et al., 1992; Kirby et al., 1995). Immunohistochemical studies have shown that Fos- like proteins are expressed rapidly and transiently in specific regions of the rat brain following a single ad- ministration of d-fenfluramine to rodents (Li and Row- land, 1993; Li et al., 1994). Consistent with the sero- toninergic-related behavioral effects of d-fenfluramine, the brain regions showing the most intense Fos immu- nostaining include regions receiving a dense serotonin- Received July 21, 1999; revised manuscript received November 9, 1999; accepted November 9, 1999. Address correspondence and reprint requests to Dr. R. Moratalla at Instituto Cajal de Neurobiologı ´a, Consejo Superior de Investigaciones Cientı ´ficas, Dr. Arce no. 37, Madrid 28002, Spain. E-mail: moratalla@cajal.csic.es Abbreviations used: 5-HIAA, 5-hydroxyindole-3-acetic acid; 5-HT, 5-hydroxytryptamine or serotonin; IEG, immediate-early gene; PBS, phosphate-buffered saline; PCA, p-chloroamphetamine. 1363 Journal of Neurochemistry Lippincott Williams & Wilkins, Inc., Philadelphia © 2000 International Society for Neurochemistry