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Serotonergic modulation of male-like pseudocopulatory behavior in theparthenogenetic whiptail lizard, Cnemidophorus uniparensBrian George Diasa , David Crews b, ?aInstitute for Neuroscience, University of Texas at Austin, Austin, TX 78712, USAbSection of Integrative Biology, University of Texas at Austin, PAT 30, 1 University Station Stop C 0930, Austin, TX 78712, USAReceived 6 March 2006; revised 10 May 2006; accepted 11 May 2006Available online 21 June 2006AbstractHormone–neurotransmitter interactions form an important link through which hormones influence a variety of behavioral processes. Typically,sexualbehaviorisdimorphicwithmalesmountingreceptivefemales.Intheall-femalelizardspeciesCnemidophorusuniparens,individualsdisplaybothmale-like pseudocopulation and female-like receptivity. These respective behavioral states are correlated with high circulating concentrations ofprogesteronefollowingovulationandofestrogenprecedingit.Insexualspecies,serotoninisinvolvedinmale-typicalmounting,and,asreportedhere,inmale-likepseudosexualbehaviorinthisunisexualspecies.Inthefirststudy,C.uniparenswereovariectomizedandtreatedsystemicallywithexogenousandrogen,ahormonalregimenthatresultsinindividualsdisplayingonlymale-likepseudosexualbehavior.Anincreaseinserotoninlevelsinthepreopticarea coupled with the suppression of male-like pseudocopulation was observed in androgen-treated lizards injected with 5-hydroxytryptophan (theprecursor of serotonin) and clorgyline (a monoamine oxidase inhibitor) compared to vehicle-treated controls. Our second experiment involvedovariectomizing lizards and either injecting them with estradiol or implanting them with either an empty (Blank) or a progesterone- or testosterone-containing Silastic capsule. Treatment with para-chlorophenylalanine (an inhibitor of tryptophan hydroxylase) facilitated male-like pseudosexualbehaviordependingonthecirculatinghormonalmilieuanddecreasedserotoninlevelsinthepreopticarea.Ourdatasuggestthatserotoninisinhibitorytomale-likepseudosexualbehaviorinC.uniparensbutmoreimportantlythatthehormonalenvironmentmodulatestheserotoninsystematthelevelofthepreoptic area, with the serotonergic system then establishing behavioral thresholds that allow for this behavior to be “gated”.© 2006 Elsevier Inc. All rights reserved.Keywords: Serotonin; Reptile; Sexual behavior; Thresholds; Neurotransmitters; Hormones; Preoptic areaIntroductionSexual behavior is a composite of male- and female-typicalbehavioral repertoires including male-typical mounting and fe-male-typical receptivity. The mechanisms underlying both behav-iors involve a complex interplay of internal and external factorswithin a spatial and temporal framework. Circulating sex steroidhormones play a pivotal role in the expression of sexual behavior,and neurotransmitters are thought to be an important link betweenhormones and behavior in several species (Hull et al., 1999;Summers, 2001). However, details of how steroid hormones andneurotransmitters interact remain poorly understood.Amongtheneurotransmitters,serotonin(5-HT)isinhibitorytosexualbehaviorina varietyofvertebrates (BitranandHull,1987;Gorzalka et al., 1990). Decreasing 5-HT levels via the systemicadministration of a tryptophan hydroxylase inhibitor (pCPA)increases mounting, intromission, and ejaculatory behavior inintact male rats (Tagliamonte et al., 1969; Ahlenius et al., 1971;Del Fiacco et al., 1974). Conversely, increasing 5-HT levels bychronic administration of fluoxetine to adult male rats suppressesmale-typical sexual behavior (Frank et al., 2000), as does theinjection of 5-HT directly into the preoptic area (POA) of malerats (Verma et al., 1989). Administration of pCPA to ovariecto-mized female rats increases the incidence of mounting behaviorwhen treated females are tested with receptive females, an effectthat appears to be independent of the circulating hormonal milieu(EmeryandSachs,1976;Rodriguez-Sierraetal.,1976;Soderstenet al., 1976; van de Poll et al., 1977).The unisexual whiptail lizard Cnemidophorus uniparens alter-natelydisplaysmale-like pseudocopulationandfemale-likerecep-tivity during the course of the ovarian cycle (reviewed in Crews,Hormones and Behavior 50 (2006) 401–409www.elsevier.com/locate/yhbeh? Corresponding author. Fax: +1 512 471 6078.E-mail address: crews@mail.utexas.edu (D. Crews).0018-506X/$ - see front matter © 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.yhbeh.2006.05.001