Developmental disruption of serotonin transporterfunction impairs cerebral responses to whiskerstimulation in miceTakanori Esaki*, Michelle Cook*, Kazuaki Shimoji†, Dennis L. Murphy‡, Louis Sokoloff*§, and Andrew Holmes¶Laboratories of *Cerebral Metabolism and‡Clinical Science, National Institute of Mental Health,¶Section on Behavioral Science and Genetics, Laboratory forIntegrative Neuroscience, National Institute on Alcohol Abuse and Alcoholism, and†Positron Emission Tomography Department, Clinical Center, NationalInstitutes of Health, Bethesda, MD 20892Contributed by Louis Sokoloff, March 8, 2005There is growing evidence that serotonin (5-hydroxtryptamine,5-HT) has major influences on brain development in mammals.Genetic and pharmacological disruption of 5-HT signaling duringearly postnatal development in rodents causes neuroanatomicalcortical abnormalities, including malformations in the somatosen-sory cortex. Possible functional consequences of this developmen-tal perturbation by 5-HT are not yet understood. We have exam-ined the effects of deletion of the 5-HT transporter (5-HTT) gene onsomatosensory responses to sensory stimulation in mice. Localcerebral glucose utilization (lCMRglc) was measured by the quan-titative 2-deoxy[14C]glucose method during unilateral whiskerstimulation in awake adult mice. lCMRglcwas increased by stimu-lation but to a markedly lesser extent in 5-HTTⴚ/ⴚmice than in5-HTTⴙ/ⴙcontrols in each of four major stations in the whisker-to-barrel cortex pathway (the spinal and principal sensory trigem-inal nuclei, the ventral posteromedial thalamic nucleus, and thebarrel region of the somatosensory cortex). Lowering brain 5-HTlevels by administration of the selective tryptophan hydroxylaseinhibitor p-chlorophenylalanine on postnatal days 0 and 1 restoredthe metabolic responses to functional activation in the whisker-to-barrel cortex pathway in adult 5-HTTⴚ/ⴚmice. These resultsindicate that functional deficits in this pathway in 5-HTTⴚ/ⴚmicemay be due to excessive postnatal 5-HT activity. With or withoutpostnatal p-chlorophenylalanine treatment, 5-HTTⴚ/ⴚmice exhib-ited lower resting (unstimulated) lCMRglcthan did 5-HTTⴙ/ⴙcon-trols in the whisker-to-barrel cortex pathway and throughout thebrain. These findings have implications for understanding thepotential long-term consequences of genetic and pharmacologicaldisruption of 5-HT neurotransmission on cerebral functions duringcritical periods of postnatal development.cerebral glucose utilization 兩 cerebral metabolism 兩 2-deoxy[14C]glucoseIn addition to functioning as a neurotransmitter in the adultcentral nervous system, increasing evidence points to a majorrole for serotonin (5-hydroxtrypt amine, 5-HT) in mammalianbrain development. The midbrain 5-HT-producing raphe neu-rons exhibit differentiation and forebrain innervation relativelyearly in utero. Subsequently, 5-HT acts to modulate cell div ision,neuronal migration, cell differentiation, and synaptogenesis(1–3). Perturbation of brain 5-HT levels during the neonatalperiod has been shown to disrupt normal cortical developmentin rodents. Postnatal depletion of 5-HT retards rodent corticaldevelopment (4–9). Conversely, genetically and pharmacologi-cally induced elevations of brain 5-HT levels cause abnormaldevelopment of cortical c ytoarchitecture (10–12).The rodent whisker-to-somatosensory cortex pathway pro-vides an excellent model system for studying the effects ofdisr uptions of 5-HT system neurotransmission on cortical de-velopment and function. During late embryonic to early post-nat al development, the rodent mystacial whiskers form one-to-one connections by means of the brainstem trigeminal complexand the ventral posteromedial thalamus with dedicated neuronalclusters (barrels) in layer IV of the primary somatosensoryc ortex (13). 5-HT activity in thalamoc ortical neurons during theneonat al period is essential to the formation of normal whiskerbarrels (5, 6, 14–19). The 5-HT transporter (5-HTT) and the5-HT1Breceptor subtype (5-HT1BR) exhibit high transient ex-pression in somatosensory and other cortical regions duringearly postnatal life (16, 20–25). Genetic inactivation of the5-HTT or another major regulatory component of the 5-HTsystem, monoamine oxidase A (MAOA), prevents the develop-ment of topographically organ ized whisker-barrel fields in themouse somatosensory cortex (10, 11, 26, 27). These c ytoarchi-tectural defects appear to be caused by excessive levels of 5-HTin the brain during a critical period of development; they can berescued by depletion of brain 5-HT during postnatal day (P) 0and P1 (10, 11, 27).A lthough the neuroanatomical deficits caused by perturbationof 5-HT functions have been consistently observed, little isk nown about the consequences of these perturbations onfunctional activities during sensitive periods of cortical devel-opment. In the present study, the functional integrity of themouse whisker-to-somatosensory cortex pathway during sensorystimulation was assessed in mutant mice lacking the 5-HTT(5-HTT⫺/⫺). Local cerebral glucose utilization (lCMRglc) wasmeasured during unilateral vibrissal stimulation in awake adultmice by the 2-deoxy[14C]gluc ose method (28). Functional acti-vation of lCMRglchas previously been shown to be a highlysensitive assay for quantif ying neural responses to sensorystimuli in awake unanesthetized mice (29, 30). In addition, to testthe hypothesis that any impair ments in sensory function foundin the whisker-to-somatosensory cortex pathway in 5-HTT⫺/⫺mice might be due to excess 5-HT levels in the postnatal brain,mut ant and normal mice were treated with the selective tryp-tophan hydroxylase inhibitor p-chlorophenylalanine (PCPA)during P0 and P1 and then examined for their lCMRglcresponsesto whisker stimulation during adulthood.Materials and MethodsChemicals. PCPA was purchased from Research Biochemicals(Natick, MA), and 2-deoxy-D-[1-14C]glucose (deoxy[14C]glucose)[specific activity ⫽ 53 mCi兾mmol (1 Ci ⫽ 37 GBq)] was obtainedfrom DuPont兾NEN.Animals. 5-HTT⫺/⫺mice were generated as described in ref. 31.For the present study, the 5-HTT null mutation was backcrossedonto a C57BL兾6J background. 5-HTT⫺/⫺and 5-HTT⫹/⫹miceAbbreviations: 5-HT, 5-hydroxtryptamine (serotonin); 5-HTT, serotonin transporter;5-HT1BR, 5-HT1Breceptor subtype; lCMRglc, local cerebral glucose utilization; MAOA, mono-amine oxidase A; PCPA,