Physiology & Behavior 104 (2011) 900–905

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Sex and age differences in the impact of the forced swimming test on the levels ofsteroid hormones

Lucía Martínez-Mota a,⁎, Rosa-Elena Ulloa b, Jaime Herrera-Pérez a,d,Roberto Chavira c, Alonso Fernández-Guasti d

a Instituto Nacional de Psiquiatría “Ramón de la Fuente Muñiz,” Calz. Mexico Xochimilco 101, San Lorenzo Huipulco, Tlalpan, Mexico City, 14370, Mexicob Psicofarmacología del Desarrollo, Hospital Psiquiátrico Infantil “Juan N. Navarro,” San Buenaventura 86, Belisario Domínguez, Tlalpan, Mexico City, 14080, Mexicoc Instituto Nacional de Ciencias Médicas y Nutrición “Salvador Zubirán,” Vasco de Quiroga 15, Sección XVI, Tlalpan, Mexico City, 14000, Mexicod Departamento de Farmacobiología, CINVESTAV, Calz. De los Tenorios 235, Granjas Coapa, Tlalpan, Mexico City, 14330, Mexico

⁎ Corresponding author at: Laboratorio de Farmacologíde Psiquiatría Ramón de la Fuente Muñiz, Calzada MéxicHuipulco, Tlalpan, Mexico City 14370, Mexico. Tel.: +556559980.

E-mail addresses: lucia@imp.edu.mx (L. Martínez-M(R.-E. Ulloa), jherrera@imp.edu.mx (J. Herrera-Pérez), r(R. Chavira), jfernand@cinvestav.mx (A. Fernández-Gua

0031-9384/$ – see front matter © 2011 Elsevier Inc. Aldoi:10.1016/j.physbeh.2011.05.027

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 January 2011Received in revised form 21 May 2011Accepted 24 May 2011

Keywords:DepressionSteroid hormone levelsForced swimming testNeurodevelopmentPrepubertal ratsEstrogens

Compared with the adult disorder, depression in children exhibits differences in its neurobiology, particularlyin the HPA axis regulation. The bases of such differences can be evaluated in animal models of depression. Theobjective of the present study was to determine age and sex differences of Wistar rats in the forced swimmingtest (FST). The influence of sex and age on corticosterone, estrogens and testosterone serum levels was alsodetermined. Prepubertal rats showed immobility, swimming and climbing behaviors during the pre-test andtest sessions. In addition, in the prepubertal animals, no sex differences were found during the pre-test andtest sessions. Age comparisons indicated no differences in the female groups, however adult males exhibitedmore immobility and less swimming than young males, in both FST sessions. The young and female ratsshowed less immobility behavior and increased levels of estrogens after the FST. The present results indicatethat the FST is an animal model suitable to evaluate depressive-like behaviors in prepubertal subjects and toexplore behavioral changes related to neurodevelopment.

a Conductual, Instituto Nacionalo-Xochimilco 101, San Lorenzo2 55 41605054; fax: +52 55

ota), eulloa@hotmail.comchavira2002@prodigy.net.mxsti).

l rights reserved.

© 2011 Elsevier Inc. All rights reserved.

1. Introduction

Major Depressive Disorder (MDD) is a psychiatric disordercharacterized by sadness, anhedonia, disturbances in sleep, appetiteand locomotor activity as well as guilt, death and suicide ideas [1]. Thedisorder is frequently recurrent and chronic, and has been associatedwith suicide risk and psychosocial dysfunction [2]. Prevalence rates ofMDD in children range from 0.4% to 2.5% [3].

The clinical characteristics of MDD in children and adolescents aresimilar to those described for adults, including the symptom ofirritability as equivalent of adult sadness. However, age differenceshave been reported in the neurobiology of the disorder; in particular,children and adolescents do not respond to tricyclic antidepressants[4]. Such differences could be partly explained by the different rate ofdevelopment of the serotonergic (5HT) and noradrenergic (NA)systems, where the former reaches an earlier maturity. In adults, sexand gender differences in various aspects of depression, including rate

of depression relapse [5] and response to selective serotonin reuptakeinhibitors (SSRIs) [6] have been reported. Most of these sexdifferences have been associated with hormonal factors, primarilychanges in ovarian steroid secretions [6].

The animal models predictive of drug-antidepressant actions havebeen extensively used for the study of the neural substratesunderlying depressive behavior [7], through examining the behaviorof adult animals after their exposure to acute or chronic stress (forcedswimming test or chronic mild stress, respectively) or by inducingdepressive-like behaviors by pharmacological or environmentalchanges during the perinatal period [8,9]. However, few animalmodels have explored depressive-like behaviors in prepubertalanimals [10,11] considering sex differences.

The FST is an extensively usedmodel in which a behavioral changeis induced by an acute stress [12,13]. The model has shown predictivevalidity [7,12], pharmacological selectivity [14], construct- [15] andface-validity [12,13]. In this test, animals are forced to swim. After asingle pretest session, most subjects showed an increased immobilitywhen retested for swimming 24 h later, i.e., they showed despairreflected as immobility defined as floating without struggling andmaking only those movements necessary to keep the head above thewater. Immobility-time decreases or increases are interpreted asantidepressive- or depressive-like actions, respectively [12]. Inaddition to immobility the animals show active behaviors that reflectthe animal interest to actively try to avoid this aversive situation. Such

901L. Martínez-Mota et al. / Physiology & Behavior 104 (2011) 900–905

active behaviors have been divided in two categories: swimming andclimbing (vide infra). The modified version of this model [16] permitsto infer the participation of different neurotransmitters in the effect ofantidepressant drugs. Thus, the decrease in immobility accompaniedby increase in swimming denotes the activation of the serotoninergicsystem while the increase in climbing indicates the activation of thecatecholaminergic systems [17].

The FST has successfully revealed differences in depressive-likebehaviors and response to pharmacological treatments according tostrains, species [18], sex and endocrine conditions. Sex differenceshave been described by several authors indicating that adult male ratsexhibit higher immobility and lower active behaviors than females[19,20], although several authors have reported contradictory results[21,22]. Such results variation could be due by plotting or ignoring thebehavioral changes associated with the estrous cycle, which clearlyshows lower immobility and higher active behaviors during proes-trus–estrus [23,24]. Age differences in FST have been poorly studied.The immobility behavior emerges at 21 PN and begins to stabilize at26 PN [25]; however, it is not knownwhether prepubertal rats exhibitsimilar patterns of active behaviors and immobility than adults and ifsex differences occur.

In adult and young (rats over 21 days) subjects of both sexes, acuteand chronic stress increased the levels of CRH, ACTH and glucocor-ticoids [26] while only in adult females it increased the levels ofgonadal hormones [27]. However, the impact of acute stress producedby the FST on the levels of gonadal hormones in prepubertal subjectsis unknown. Considering that prepubertal subjects are also vulnerableto stress and that depression in this age group can occur as aconsequence of acute stressful events [28,29], the present researchproposes to determine differences between prepubertal (PN35) andadult (PN90) male and female rats in their performance on the forcedswimming test (FST) and to characterize the putative changes incorticosterone, estradiol and testosterone serum levels induced by thetest. It was hypothesized that age and sex differences would occur inthe development of despair in the FST and that corticosterone andgonadal hormones´ levels would increased after the FST.

2. Materials and methods

2.1. Animals

Male and female Wistar rats were reared from our colony inthe Instituto Nacional de Psiquiatría Ramón de la Fuente Muñiz, andhoused with cohorts of the same age and sex. Animals from two agegroups were studied simultaneously: adult (90 days old) andprepubertal animals (35–37 days) were housed four-five per cage(60×40×20 cm) in a room under inverted light/dark cycle conditions(12/12 h; lights on at 2200 hours) with ad libitum access to water andfood. Adult female rats were selected in vaginal estrus in order tocontrol the hormonal cyclic influence on the FST ([23]) and thevariations in steroid hormone levels. Prepuberty was established bychecking the vaginal opening in females and the preputial separationin males. The rats were kept in a separate room four hours before thetests. The “local ethical committee for animal use” approved theprotocol for these experiments (Number 3370.1). Animal manage-ment was done according to the Mexican official norm for the use andcare of laboratory animals: “NOM-062-ZOO-1999”.

2.2. Forced swimming test

The modified forced swimming test was used in this study [17].Swimming sessions were conducted by placing the rat in a glasscylinder (46 cm tall x 20 cm in diameter) containing water (30 cmdepth) at 23–25 °C. The FST consisted of two swimming sessions 24 hapart. In the first session (pre-test) rats were allowed to swim during15 min. Twenty four hours later, animals were subjected to a 5 min

swimming session (test). At the end of each session, rats wereremoved from the jar, dried with paper towel and placed in a cageduring 15 min before returning them to the home cages. Pre-test andtest were videotaped and scored by two observers who were blind tothe animal's sex.

A time-sampling techniquewasused to score, every5 s, thepresenceof the following behaviors: a) immobility, floating without strugglingand making only those movements necessary to keep the head abovethewater; b) swimming, activemotions, i.e., animalsmoving anddivingaround the jar or c) climbing, rats making active movements with theirforepaws in and out thewater, usually directed against thewall. Resultswere expressed as mean number of counts±s.e.m. of the behaviorseach 5 min.

2.3. Experimental design

In adult animals, the pre-test session induces a gradual increase ofimmobility and a decrease of the active behaviors [30].The develop-ment of these behaviors in prepubertal rats was analyzed. To thatpurpose prepubertal male and female (n=10 or n=11, respectively)and adult (n=8 or n=8, respectively) rats were randomized andassigned to independent groups and subjected to the pre-test sessionin the FST. Twenty-four hours later these rats were subjected to thetest session to establish the possible behavioral changes subsequentto the former stress session.

2.4. Vaginal smears

To control the possible hormonal influence on the FST perfor-mance of adult females [23], a vaginal smear was daily taken beforethe pre-test and test sessions. Estrous cycle phases were establishedbased on vaginal cytology [31]. Results of estrous phase wereconsidered as a pool in the analysis of the behavioral data.

2.5. Steroid levels in FST- and non-FST-exposed rats

Serum levels of corticosterone (C), estradiol (E2) and testosterone(T) were measured in control subjects (non-FST) and in ratssubmitted to the FST. Animals (n=8 per group) were sacrificed bydecapitation 30 min after the test [32] or after two days of handling(for the non-FST groups). Trunk blood was collected in cold tubes andcentrifuged (4000 rpm for 25 min at 4 °C) to obtain serum samplesthat were stored at −4 °C. Total corticosterone (C), estradiol (E2) andtestosterone (T) concentrations were measured by radioimmunoas-say using commercial kits (TKRC1, TKE21 or TKTT1 Diagnostic ProductCorporation, for C, E2 and T, respectively). The procedure usedantibody-coated tubes in which 125I-labelled C E2 or T, competed withfree C, E2 and T, respectively, in the sample for antibody sites. Totalsteroid quantity was determined using a calibration curve. Thesensitivity limits for C, E2 and T were 5 ng/ml, 0.13 pg/ml and0.0045 ng/ml, respectively. The inter-assay and intra-assay variabil-ities were: 6.95 and 6.15% for C, 8.41 and 7.56% for E2, and 8.9 and7.23% for T.

2.6. Statistics

The pre-test session behaviors (immobility, swimming andclimbing) were compared by a three-way analysis of variance(ANOVA) including the factors age, sex and interval within the pre-test (the behaviors were recorded each five minutes along a fifteenminute period), followed by a two-way repeated measures ANOVA toreveal interactions between the factors for females and males,respectively. Tukey was used as the post hoc test. In the test sessionthe behaviors were analyzed with a two-way ANOVA (with thefactors: age and sex) followed by Tukey's test. Determinations ofsteroid hormones were analyzed by a three-way ANOVA (considering

902 L. Martínez-Mota et al. / Physiology & Behavior 104 (2011) 900–905

the exposure to the FST, age and sex as factors) followed by two-wayANOVAs (considering sex and age) within FST or control groups andTukey's as a post-hoc test. The statistics was carried out using theSigma Stat software, version 3.1. A value of pb0.05 was considered asstatistically significant.

3. Results

3.1. Vaginal smear

The observation of the vaginal smears indicated that 60% of theadult female rats executed the pre-test in the diestrus–metestrusphase, while 40% of rats were tested in proestrus–estrus. Twenty-fourhours later 90% of the females exhibited a vaginal estrus before thetest session. Tomatch, in the group of rats non-exposed to the FST 80%of the females were also sacrificed in estrus.

3.2. Age and sex differences in the pretest session of the FST

Fig. 1 compares the development of immobility (panel a),swimming (panel b) and climbing (panel c) between prepubertaland adult, male and female rats, in the first FST session (pretest)analyzed in intervals of 5 min. In the first interval, even the young rats

Fig. 1. Comparison of pre-test execution between young and adult, male and female rats.Abscise axis shows the mean and s.e.m. of immobility (a), swimming (b) and climbing(c) scores in 5-min intervals during the 15-min of pre-test. Tukey's test: * pb0.05 versusthe age-matched group in each 5-min periods; & pb0.05 versus their respective controlgroup in the 5-min interval.

showed either immobility, swimming or climbing behaviors at thebeginning of the pre-test; an increase of immobility and a decrease inactive behaviors were observed at the subsequent periods (10 and15 min) attaining a maximal change at the last interval. Interestingly,young animals showed similar levels of immobility and activebehaviors in the first 5 minutes interval than adult females; whileadult males exhibited a higher immobility accompanied by lowerlevels of active behaviors as compared with young males. In theanalysis of the pre-test session, the three-way ANOVA showed asignificant main age effect on immobility (F1,102=27.74, pb0.001),swimming (F1,102=8.35, p=0.005) and climbing F1,102=16.39,pb0.001), a main sex effect on climbing (F1,102=4.43, p=0.03) anda main interval effect on immobility (F2,102=50.25, pb0.001),swimming (F2,102=3.70, p=0.028) and climbing (F1,102=144.96,pb0.001).

The two-way RMANOVA showed that in males the interaction ofage and interval was statistical significant for immobility (F2,12=7.53,p=0.008) and climbing (F2,12=6.21, p=0.014). The two-way RMANOVA in adult rats showed that interaction sex and interval wasstatistically significant for immobility (F2,12=5.44, p=0.021).

3.3. Sex and age comparisons in the test session

Fig. 2 shows the behavior displayed by male and female rats ofboth age groups after a second exposition to the FST. The two-wayANOVA showed that immobility was modified by the interaction ofsex and age, but not by the independent factors (sex F1,31=1.01p=0.32; age F1,31=0.09, p=0.76, interaction F1,31=4.38,p=0.045), however the active behaviors were influenced by allfactors (swimming: sex F1, 31=6.34, p=0.01, age F1,31=3.64,p=0.06, interaction F1,31=15.95, p b0.001; climbing: sexF1,31=3.69, p=0.06, age F1,31=10.37, p=0.003, interactionF1,31=4.82, p=0.036). These data show that adult males displaythe highest levels of immobility and climbing accompanied by thelower levels of swimming as compared with youngmales and females

Fig. 2. Differences in the post-test execution by age and sex in the FST rat. Immobility(a); swimming (b); climbing (c) behaviors. Asterisks over bars represent comparisonversus the other sub-groups (Tukey's test: * pb0.05, ** pb0.01, *** pb0.001).

903L. Martínez-Mota et al. / Physiology & Behavior 104 (2011) 900–905

of both ages. These last three groups showed similar levels of the threebehaviors.

3.4. Steroid levels in FST- and non-FST-exposed rats

Table 1 shows changes in corticosterone, estradiol and testoster-one serum concentrations in control and experimental rats. The three-way ANOVA showed that the corticosterone levels were influenced bysex (F1,60=22.05, pb0.001) and FST (F1,60=6.22, p=0.01), but notby age (F1,60=0.80, p=0.37) or by the interaction between thesefactors (F1,60=1.44, p=0.23). Under basal conditions, i.e., in animalsnot exposed to the FST, the levels of corticosterone were much higherin prepubertal females as compared with those found in the otherthree groups, which showed similar values. The exposition to FSTproduced an increase in the corticosterone levels in adult rats. Inyoung there was not an increase in the levels of corticosterone;indeed, in young females there was a trend to a decrease in the levelsof this steroid. In correspondence, the two-way ANOVA showeddifferences in the interaction of age and FST (for age F 1,60=0.56,p=0.45; for FST F1,60=4.32, p=0.040; for interaction F1,60=11.40,p=0.001).

Estradiol levels were different by sex (F1,60=15.20, pb0.001), age(F1,60=14.20, pb0.001) and FST (F1,60=46.58, pb0.001), but not bytheir interaction (F1,60=0.67, p=0.41). Female rats showed higherbasal levels of estradiol than males. In young animals, regardless oftheir sex, the FST induced a drastic increase in the levels of estradiol.In adults, this stress also produced an increase in the levels of thishormone that were roughly a third of those seen in young. The two-way ANOVA showed differences by age (F1,60=11.45, pb0.001); byFST (F1,60=37.55, pb0.001); and their interaction (F1,60=14.47,pb0.001).

Finally, testosterone levels changed by sex (F1,60=16.12,pb0.001) and age (F1,60=10.77, pb0.001), but not by FST exposition(F1,60=1.83, p=0.18) or their interaction (F1,60=1.26, p=0.26). Asexpected, only adultmales showed baseline high levels of this gonadalsteroid. The FST did not alter the levels of testosterone. The two-wayANOVA illustrated differences by sex (F1,60=16.09, pb0.001); by age(F1,60=10.75, p=0.002); and their interaction (F1,60=11.47,p=0.001).

4. Discussion

In the present study we found that prepubertal male and femalerats exhibited an increase in immobility and a decrease in activebehaviors along the pre-test session. In the test session (done 24 hlater) adult males showed an increased immobility and climbingaccompanied by a reduced swimming, the behavioral pattern ofyoung subjects was similar to that of adult females. Under basalconditions young females, adult males and adult females showed thehighest levels of corticosterone, testosterone and estradiol, respec-tively. After the FST, corticosterone levels increased in adult animals

Table 1Differences in corticosterone, estradiol and testosterone in exposed and non exposed to the

Adult

Male Female

Control FST Control

Corticosterone ng/mL Mean (±SE) 345.94 607.56 461.95(43.71) (44.32#) (55.92)

Estradiol pg/mL Mean (±SE) 3.92 8.32 12.05(0.97) (2.62) (2.09*)

Testosterone ng/mL Mean (±SE) 1.14 1.88 0.05(0.20*) (0.72*) (0.05)

Rats exposed and non-exposed to FST (n=8 per group) were sacrificed to obtain trunk bloodFST groups; # pb0.05 versus the sex- and age-matched control group (non-FST).

but not in young, while estradiol drastically augmented in all rats.These findings support the hypothesis regarding age differences onFST related behaviors.

4.1. Age and sex differences in FST

Acute stress is used in animal models to induce behavioral,physiologic and neural changes relative to depression in humans [33].Themodified version of the FST [17] is a model that includes a pre-testsession required to induce despair, reflected as a gradual increase ofimmobility and a decrease of active behaviors usually accompanied bya raise in corticosterone serum levels [30,34,35]. The current studyconfirms the behavioral changes reported in adult rats [30] andevidences a similar behavioral profile for prepubertal animals. Presentresults also agree with previous reports on the ontogeny of thesebehaviors, where immobility emerges at day 21 PN and begin tostabilize at day 26 PN [25].

The enhanced swimming behavior observed in young and adultfemale rats, suggests that, in comparison with adult males, theypresent a different coping response to acute stress [19]. In addition toa higher immobility, adult males showed an enhanced climbing. Toremind, changes in climbing have been associated with the catechol-aminergic transmission, while swimming has been related with theserotonergic system [16,17]. After these data it seems that in adultmales – in contrast with young animals and adult females – theparticipation of the noradrenergic/dopaminergic systems prevailsover the serotonergic in response to the stress produced by the FST.This difference could be underlied by testosterone which is known tostimulate the dopaminergic transmission [36]. In support, recentevidence shows that males respond better to catecholaminergic andfemales to serotonergic-antidepressants [6,37,38].

4.2. Corticosterone and FST

Interestingly, this report shows that in animals non-submitted tostress the levels of corticosterone in young females were higher thanthose shown by young males or adult subjects. This difference hasbeen previously reported by Patchev et al. [26] who showed that thecorticosterone release in 40 days animals non submitted to stress is asexually dimorphic response that depends upon the organizationalaction of gonadal hormones. The effect of gonadal hormones on HPAaxis should be considered in prepubertal and adult animals.Testosterone is converted intracellularly to estradiol and dihydrotes-tosterone (DHT) and these metabolites regulate HPA axis. DHT is alsometabolized to compounds such as 5α-androstan-3β,17β-diol (3β-diol), which are biologically significant. The enzymes responsible forthis conversion can be detected in prepubertal animals, and sometabolites such as (3β-diol). It is possible that estrogen levels foundin adults after stress facilitate the activation of HPA axis by their actionon ERα, meanwhile estrogens and 3β-diol acting on ERβ reduce theHPA axis activation in young animals [39,40].

FST animals.

Prepubertal

Male Female

FST Control FST Control FST

687.99 342.69 390.73 701.48 536.79(40.53#) (46.76) (48.61) (81.16) (38.77)15.81 5.37 26.16 8.37 40.70(3.17#) (1.76) (3.08#) (1.83) (6.63#)0.31 0.25 0.24 0.22 0.22(0.12) (0.08) (0.03) (0.02) (0.02)

. Results of Tukey's test: * pb0.05 versus the other subgroups in the control (non-FST) or

904 L. Martínez-Mota et al. / Physiology & Behavior 104 (2011) 900–905

The brain reacts to acute and chronic stress by increasing theactivity of the HPA axis [41]. This mechanism has been consistentlyreported to be altered in a 40-60% of depressed patients [42]. In thepresent study corticosterone levels in adult rats rose after FST, data inagreementwith other reports [21,35]. The age differences in the levelsof corticosterone might be explained by the time course ofcorticosterone response to the FST. Thus, the lower corticosteronelevels observed in young animals 30 min after the test could beunderlied by the kinetic changes in this hormone after the stress. Toexplore this idea, comparative studies on the kinetics of the hormonalstress response should be conducted. Alternatively, the sensitivity ofHPA axis to a stressor like the FST may be effectively decreased inprepubertal rats. The adrenal response to stress in rats begins at 14 PNdays [43], and present data suggest that at 35–37 PN days it is stillblunted as compared to adults. In support, it has been reported thatthe increase of ACTH in response to the administration of theexcitatory aminoacid N-methyl-D-aspartate were less pronounced in43 days old prepubertal rats than in adults, possibly due to age-relatedchanges in the sensitivity of CRH neurons [44]. In addition, thehypercortisolemia reported for depressed adults is absent in de-pressed children and adolescents [4]; indeed, evidence of a decreasedcortisol secretion, rather than an increase, in depressed children isemerging [11]. Our data showing a mild decrease in corticosteroneafter the FST in young females agrees with this idea.

4.3. Gonadal hormones and FST

The estradiol levels in the group of adult females corresponded tothose found during the estrous phase [45]. The levels of this steroid inprepubertal rats are in line with those reported in developing animals[46]. This hormone peaked after FST in young animals and adultfemales. In this last group the increase in estradiol could be associatedwith the high levels of corticosterone or to the activation of the HPAaxis [32], evidencing the bidirectional relationship between HPA axisand estradiol. In prepubertal rats the concentration of estradiolincreased drastically after the FST, without an apparent relationshipwith increases in corticosterone levels. The mechanism underlyingthe increase in estradiol in young may involve the stimulation ofGnRH evoked by stress at central levels. In agreement, GnRH isstimulated by excitatory amino acids (such as N-methyl-aspartate)and opioid peptides released in response to stress [44,47] and FSTevokes the release of opioid peptides and activates NMDA receptors[48,49].

The estradiol levels rose after FST in adult females and prepubertalanimals. In these groups the source of estradiol could be the gonadssince ovaries and testes have a high content of aromatase [50,51], theenzyme that converts androgens to estrogens. This enzyme could beactivated by different factors such as acute stress. The estradiolconcentrations in adult females and prepubertal rats could be relatedwith the reduction of immobility and the increased swimmingobserved during the test, revealing the activation of the serotonergicsystem [52,53]. Several data support this hypothesis: 1) theadministration of 17ß-estradiol to castrated female or male ratsincrease swimming and reduce immobility in the FST; 2) estradiolfacilitates the effect of SSRIs, such as fluoxetine, in both male andfemale rats in this model, and 3) the 5-HT1A receptor antagonist,WAY 100635, blocks the antidepressant effect of estradiol in FST[38,52–54]. Further studies examining the effect of estrogen antag-onists on swimming behavior should be conducted in order todetermine the role of estrogens on FST in young and adult rats.

The baseline levels of testosterone in adult males are on aphysiological range [55]. Recent reports show that in adult malesstress increases testosterone possibly by stimulating the gonadotro-phin secretion [56]. Present data argue against this observation sincethe levels of this hormone were similar between control and animalsexposed to the FST. The nature of this difference is unknown although

the type and intensity of stressors should be considered: while astrong stress eliminates the pulse of LH [57], a moderate stress doesnot alter the levels of LH [44].

4.4. Limitations

Although our results show that FST induces a differential effect ongonadal hormones and corticosterone in adult and young rats,changes in other androgens and progestins should be explored inanimals subjected to the FST. Fluctuations on progestins andandrogens occur concomitant with variations in estrogens along theestrous cycle or other reproductive states in rodents and importantlycontribute to modulate depressive-like behaviors [27].

5. Conclusions

In conclusion, these results suggest that the FST is a suitable modelto evaluate depressive like behaviors in prepubertal rats. The changesin corticosterone and gonadal hormones could mediate the sex andage differences in the behavioral responses in the FST.

Acknowledgments

Authors wish to thank Miss Sandra López Martínez for technicalassistance and Miss Gabriela López for her assistance in thepreparation of manuscript. This work was partially financed by the“Consejo Nacional de Ciencia y Tecnología (CONACyT) (grants 62020and 50888).

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