Ontogeny of biogenic amine systems and modification of indole levels upon adult sexual behavior in the rat

Male, female and androgenized female rats treated on Days 9, 10 and 11 after birth with either 5-HTP or saline were tested for female sexual behavior in adulthood. It was hypothesized that such pharmacological manipulations of the developing serotonergic system in which Day 12 sex differences have been reported might have an influence on the expression of lordosis behavior. No effect of 5-HTP was found in either normal or androgenized females, even though fluormetric analysis indicated a marked increase in endogenous levels of 5-HT at Day 12. However, males treated with 5-HTP has significantly higher 5-HT levels than 5-HT treated control or androgenized females. In addition, fluorometric analysis of norepinephrine, dopamine and serotonin from hypothalamus, mesencephalon and cortex was performed on male and female rats on Days 8, 10, 12 and 14 postnatally to examine the development of various transmitter systems during the early postnatal period.     

In vivo evidence for a greater brain tryptophan hydroxylase capacity in female than in male rats

Summary Previous studies have revealed that brain levels of tryptophan, 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) are moderately higher in female than in male rats. Since tryptophan hydroxylase is only about half saturated with substrate, the greater availability of precursor in female rats might contribute to their higher 5-hydroxyindole levels. The present investigation was aimed at clarifying whether there is a sex difference in central tryptophan hydroxylase capacity. Hence, both sexes received a high dose of l-tryptophan (400 mg/kg), which resulted in a tenfold increase in brain tryptophan concentrations and presumably a virtual saturation of tryptophan hydroxylase. Following such treatment, 5-hydroxytryptophan (5-HTP) levels, measured after l-amino acid decarboxylase inhibition, were compared in males and females. Both in saline-and l-tryptophan-treated rats, 5-HTP levels were generally higher in females. In another group of animals, receiving 400 mg/kg of l-tryptophan as sole treatment, 5-HT and 5-HIAA concentrations were measured. As in the case of 5-HTP, the higher 5-HT and 5-HIAA levels observed in females persisted after l-tryptophan treatment. The present data suggest that brain tryptophan hydroxylase activity is greater in females; this sex difference probably contributes to the higher 5-HT and 5-HIAA levels in females.Send offprint requests to M. Carlsson at the above address     

Selective androgen insensitivity of hepatic drug-metabolizing enzymes in senescent mice

The normal sexual dimorphism in murine hepatic hexobarbital metabolism (i.e. females greater than males) was found to be absent in senescent animals. Hexobarbital metabolism, expressed as microsomal activity of hexobarbital hydroxylase and hexobarbital-induced sleep time, in senescent male mice was similar to that in females, but significantly greater than that found in young adult males. No age-related changes in hexobarbital metabolism were observed in intact females. In addition, experiments involving gonadectomies and testosterone administration indicated that both male and female senescent mice were insensitive to the normally repressive effects of androgens on hexobarbital hydroxylase. In contrast, the sexual dimorphism in the activity of p-nitrophenol UDP-glucuronosyltransferase was maintained in the senescent mice as well as the usual responsiveness to testosterone regulation. Furthermore, the growth-promoting effects of androgen on the kidneys and seminal vesicles were similarly expressed in young and old mice. Thus, our results suggest the development of an age-dependent and selective insensitivity of hexobarbital hydroxylase to androgenic regulation in the aging mouse.     

Effects of methylphenidate on the hyperemotional behavior in olfactory bulbectomized mice by using the hole-board test

The most consistent behavioral changes caused by olfactory bulbectomy are hyperemotional responses such as hyperactivity in a novel environment. However, the changes in the emotional behavior of mice after undergoing olfactory bulbectomy have not yet been described in detail. The effects of methylphenidate on the hyperemotional behavior of olfactory bulbectomized (OBX) mice were examined by using the hole-board test. Mice (4-week-old) were subjected to olfactory bulbectomy, and the behavioral test was performed 2 weeks after surgery. OBX mice showed a significant increase in the number of head-dips as compared to the sham-operated mice. This increase was significantly decreased after treatment with methylphenidate (10 microg/kg, s.c.). The norepinephrine (NE) turnover ratio in the frontal cortex in OBX mice was significantly less than that in the sham-operated mice. However, the decreased NE ratio in OBX mice normalized after treatment with methylphenidate. Our results suggest that the increased head-dipping behavior in OBX mice might reflect an impulsive-like behavior. In addition, we proposed that the improvement in the noradrenergic abnormalities in the frontal cortex due to methylphenidate treatment may play a key role in the improvement of impulsive-like behaviors observed in OBX mice.     

Influence of olfactory bulbectomy and subsequent imipramine treatment on 5-hydroxytryptaminergic presynapses in the rat frontal cortex: behavioural correlates

1. Alterations of 5-hydroxytryptaminergic mechanisms are thought to play a special role in the pathogenesis of depression and antidepressant treatments are assumed to restore these changes.
2. We have used one of the most reliable models of depression, the olfactory bulbectomized rat to study the long term consequences of this manipulation and of subchronic imipramine treatment on two parameters of 5-hydroxytryptaminergic presynapses, 5-hydroxytryptamine (5-HT) transporter density and tryptophan hydroxylase apoenzyme concentration, in the frontal cortex as well as on active avoidance learning several weeks after bulbectomy.
3. The B_max value of [³H]-paroxetine binding and the concentration of the 5-HT synthesizing enzyme were both significantly elevated in the frontal cortex of bulbectomized rats compared to sham-operated controls.
4. Imipramine treatment, either by daily injections or by subcutaneous implantation of slow release imipramine-containing polymers reduced the elevated tryptophan hydroxylase apoenzyme levels in the frontal cortex of bulbectomized, but not of sham-operated control rats and restored the deficient learning performance of bulbectomized rats.
5. Both effects were more pronounced after continuous drug administration by imipramine-releasing polymers compared to daily i.p. injections.
6. These findings indicate that bulbectomy leads to a compensatory 5-hydroxytryptaminergic hyperinnervation of the frontal cortex. Chronic antidepressant treatment seems to attenuate the increased output of the 5-hydroxytryptaminergic projections in the frontal cortex through the destabilization of the rate limiting enzyme of 5-HT synthesis of the 5-hydroxytryptaminergic nerve endings in this brain region.

     

Relation of spontaneous wet dog shakes and copulatory behavior in male rats

Pharmacological manipulation of 5-HT2 activity has yielded equivocal effects on male rat sexual behavior. Both facilitation and inhibition of copulation have been reported following treatment with 5-HT2 antagonists. "Wet dog shake" (WDS), a component of the Serotonin Behavioral Syndrome, is largely mediated by 5-HT2 receptors. The present series of experiments were aimed at determining whether WDS might yield a spontaneous behavioral measure of 5-HT2 activity that converges on the pharmacological data. In Experiment 1, spontaneous WDS was recorded in 58 male rats paired with receptive females. Normal copulators (Studs) exhibited significantly less WDS than did noncopulators (Duds). In Experiment 2, the males were additionally paired with males and nonreceptive females. In these situations, WDS did not discriminate Duds from Studs, but Studs scores differed from each other across the three different partner conditions. Lastly, in Experiment 3, treatment with the selective 5-HT2 agonist DOI potently inhibited copulatory responses and increased WDS. Overall, the data from the three experiments suggest that 5-HT2 activity mediates an inhibition of male rat sexual behavior.     

Effects of amitriptyline and isocarboxazid on 5-hydroxytryptophan induced head twitches in mice

Effects of amitriptyline and isocarboxazid on brain 5-HT and 5-HIAA were examined in relation to their action on 5-HTP induced head twitches. Amitriptyline reduced 5-HTP induced head twitches but isocarboxazid increased them. Both amitriptyline and isocarboxazid caused a significant increase of brain 5-HT concentration in 5-HTP treated mice. Amitriptyline also caused a significant increase of 5-HIAA concentration, while isocarboxazid reduced 5-HIAA concentration in the brains of 5-HTP treated mice. Probenecid, which significantly increased 5-HIAA concentration without affecting brain 5-HT concentration in 5-HTP treated mice, reduced 5-HTP induced head twitches. These results suggest that 5-HTP induced head twitches might be induced by an increase of 5-HT concentration, and reduced by an increase of 5-HIAA or a decrease of 5-HT concentration in the brains of mice.     

Sex behavior of male and female Wistar rats affected by the serotonin agonist 8-OH-DPAT.

Four experiments were carried out to test the stimulatory effects of 8-OH-DPAT on various aspects of "masculine" sexual behavior of male and female rats and on the sexual attractivity of male rats. In Experiment 1 8-OH-DPAT (0.2 mg/kg) stimulated ejaculation frequency in middle-aged (approx. 15 months old) males, both sexually inactive and active subjects. There was a coinciding decrease in total number of mounts, intromissions, intromissions to first ejaculation and latency to first ejaculation. In Experiment 2 the effects of two doses (0.2 and 0.4 mg/kg) 8-OH-DPAT on the first ejaculation cycle were investigated. Especially, the higher dose made a high percentage (45-55%) of males to ejaculate "prematurely," i.e., at the first or second intromission. Latency to ejaculation decreased. With the higher dose, 25-35% of the males ejaculated extravaginally. In Experiment 3 8-OH-DPAT did not make males more attractive for an estrous female than saline-treated males, as judged by the time spent in their vicinity. However, estrous females received much more ejaculations from the tethered 8-OH-DPAT males, with the lowest latencies to first ejaculation, than from the saline-treated males. In Experiment 4 8-OH-DPAT stimulated mounting behavior in female rats only when they were long-term treated with testosterone. In that condition also shortest latencies to first mount were found with 8-OH-DPAT treatment.     

Imipramine administered during an infantile period modifies sex difference in L-5-hydroxytryptophan-induced head shakes in rats

Five mg/kg imipramine or desipramine was injected to infantile rats. L-5-Hydroxytryptophan-induced head shakes were assessed when rats were mature. The saline- and desipramine-treated adult male rats exhibited more sustained response to L-5-HTP than females. The time course of the head shake frequency in the imipramine-treated male and female rats showed a pattern between control males and females, resulting in no significant sex difference. The results suggest that infantile exposure to imipramine induces an alteration of the serotonergic neurons of the brain.     

The effects of chronic lithium chloride administration on some behavioural and immunological changes in the bilaterally olfactory bulbectomized rat

The effect of chronic lithium chloride (3 mmol/kg for 15 days) on the 'open field' activity and some aspects of immune function was studied in bilaterally olfactory bulbectomized and sham-operated rats. Chronic lithium chloride administration did not reverse the hyperactivity of the bulbectomized rats in the 'open field', neither did it affect the growth rate significantly. Lithium chloride treatment reversed the deficit in the neutrophil phagocytic response and slightly reversed the deficit in lymphocyte proliferation induced by mitogens that resulted from bulbectomy. The reduction in neutrophil catalase activity, and the increase in superoxide dismutase activity, that occurred following bulbectomy was reversed by chronic lithium treatment. However, the reduction in the proportion of lymphocytes, and the increase in the proportion of neutrophils that occurred in the bulbectomized rats was not reversed significantly by chronic lithium treatment. Chronic lithium treatment reversed the deficit in noradrenaline in the amygdaloid cortex and hypothalamus of the bulbectomized rats and the reduction in the dopamine concentration in the amygdaloid cortex. No changes in the concentrations of 5-HT and 5-HIAA could be detected in any of the brain regions studied. Thus there is a disparity between the lack of effect of lithium chloride on 'open field' behaviour and its beneficial effects in correcting some of the immune and neurotransmitter deficits which were observed in the bulbectomized rats.     

Effects of phenylalanine and 5-hydroxytryptophan on seizure severity in mice

The degree of audiogenic seizure was measured in DBA/2J (phenylalanine hydroxylase deficient) mice as a function of dietary phenylalanine (Phe) and injected 5-hydroxytryptophan (5-HTP), the precursor of serotonin (5-HT). Phe was shown to exacerbate seizures significantly, and seizure severity was found to be directly related to dietary concentration when animals were not treated with exogenous 5-HTP. 5-HTP was observed to significantly ameliorate seizures. The seizure-intensifying effect of Phe was reversible by 5-HTP injection and protection against seizures was directly related to 5-HTP concentration for animals on a high Phe diet. The results of this study indicate that Phe and 5-HTP are mutually antagonistic in modulating audiogenic seizure susceptibility.     

Postnatal function following prenatal reserpine exposure in rats: neurobehavioral toxicity

Behavioral and neurochemical analyses were conducted on preweanling CD rats prenatally exposed to either 0, 0.375 or 0.750 mg/kg/day reserpine SC on gestation days 12-15. Offspring body weights were taken on test days, and pups were tested for negative geotaxis responding on postnatal day 8, developmental activity on days 12, 16 and 20, and auditory startle habituation on day 19 or 20. In addition, brains were removed from culled pups on day 1, 1 male and 1 female/litter on day 8, and animals tested for activity on day 21. Neurochemical assays were performed on whole brains from 1- and 8-day-old pups, and on caudate nucleus, frontal cortex and hippocampus of day 21 rats. Treatment resulted in dose-related decreases in maternal weight gain over gestation and mean pup weight at birth. Changes in the normal developmental activity pattern were both sex and dose dependent in treated rats. In auditory startle habituation experiments, rats exhibited a dose-related decrease in response amplitude and rate of habituation. In the day 21 females, caudate nucleus dopamine (DA) and serotonin (5-HT) concentrations and DA-receptor binding were decreased in a dose-dependent manner. Males showed less dramatic, but similar trends in caudate changes. However, hippocampal 5-HT and 5-HT receptor binding were significantly reduced only in females. Thus, sex-related behavioral alterations were accompanied by sex-related neurochemical changes, and females generally were more affected than males by prenatal reserpine treatment. The significant decrease in activity and auditory startle amplitude in the females is consistent with the suggested down regulation of the DA system in regional brain areas.     

Impulsive choice as a predictor of acquisition of IV cocaine self- administration and reinstatement of cocaine-seeking behavior in male and female rats

Drug abuse and impulsive choice are related in humans. In female rats, impulsive choice predicted the rate of acquisition of IV cocaine self-administration. The objectives of the present experiments were to: (a) compare impulsive choice in males and females, (b) extend previous research on impulsive choice and acquisition of cocaine self-administration to males, and (c) compare males and females during maintenance, extinction, and reinstatement of cocaine-seeking behavior. Male and female rats were trained on an adjusting delay task in which a response on one of two levers yielded one food pellet immediately, and a response on the other resulted in three pellets after an adjusting delay that decreased after responses on the immediate lever and increased after responses on the delay lever. A mean adjusted delay (MAD) was used as the quantitative measure of impulsivity. In Experiment 1, MADs were analyzed for sex differences. In Experiment 2, acquisition of cocaine self-administration was examined in rats selected for high (HiI; MADs < or =9 seconds) or low (LoI; MADs > or =13 seconds) impulsivity. In Experiment 3, HiI and LoI groups were compared on maintenance and extinction of cocaine self-administration and cocaine-primed reinstatement of drug-seeking behavior. There were no sex differences in impulsive choice; however, HiI male and female rats acquired cocaine self-administration faster than their LoI counterparts. LoI females responded more on a cocaine-associated lever during maintenance and extinction than HiI females, but HiI females showed greater reinstatement of cocaine-seeking behavior than all other groups at the highest dose tested (15 mg/kg). Thus, individual differences in impulsive choice were associated with differences in cocaine-seeking behavior. Impulsive choice and sex may be additive vulnerability factors in certain phases of drug abuse.     

Effects of lithium chloride injections on rank-related fighting, maternal aggression and locust-killing responses in naive and experienced 'TO' strain mice

Three experiments investigated lithium chloride's (LiCl) effects on three forms of aggression in male ‘TO’ strain mice. Models of aggression investigated, included attack by preisolated males on male conspecifics (rank-related or intermale fighting), attack by lactating females on male intruders (matermal aggression) and the locust-killing response (a form of predatory aggression?) In the first study, injections of naive male mice with 0.2 and 0.4 mEq of LiCl resulted in marked declines in rank-related fighting. The effects of this treatment on locust killing could not be assessed, as this activity was already at a low incidence in controls. In a second experiment, LiCl injection had little influence on the locust-killing response in selected, experienced male killer mice. As in Experiment 1, rank-related fighting was suppressed by this treatment. The third experiment revealed that LiCl injections did not influence either maternal aggression or locust killing in naive females and predatory aggression in experienced-killer females. These results provide further support for the contention that these three behaviors have very different physiological bases. The data suggests that one should be cautious when extrapolating between different models of aggression even within the same species.     

Strain independent elevation of hepatic mono-oxygenase enzymes in female mice

1. Hepatic microsomal drug metabolizing activity was compared in male and female AL/N, Balb/c, Balb/cJ, CD-1, C57BL/6J, C57BL/10J, DBA/2, and DBA/2J mice. 2. Cytochrome P-450-dependent hexobarbital hydroxylase and aminopyrine N-demethylase activities were sexually dimorphic with apparent maximal velocities consistently higher in females. Hexobarbital-induced sleeping times were greater in males, corresponding to their lower hepatic mono-oxygenase enzyme activities measured in vitro. 3. No murine sex differences were observed for the hydroxylation of aniline, while UDP-glucuronyltransferase activity was strain dependent with either no sexual dimorphism or higher activities in males. 4. Testectomy resulted in an elevation of hepatic hexobarbital metabolism to female levels in all strains examined. Thus, decreased hepatic cytochrome P-450-dependent xenobiotic metabolism in adult male mice results from the suppressive effects of gonadal androgens. 5. Sexually dimorphic patterns of hepatic hexobarbital and aminopyrine metabolism in adult mice are opposite in orientation and lower in magnitude than the well established relationship in rats in which these same substrates are metabolized at a rate 3 to 5 fold higher in intact males as compared to females or gonadectomized males.     

Alteration of behavioral sex differentiation by exposure to estrogenic compounds during a critical neonatal period: effects of zearalenone, methoxychlor, and estradiol in hamsters

The present study was designed to determine if neonatal exposure to the estrogenic mycotoxin zearalenone or the weakly estrogenic pesticide methoxychlor could masculinize and/or defeminize the behavior of female hamsters. Neonatal hamsters were given a single sc injection of either zearalenone (1 mg/pup), methoxychlor (1 mg/pup), 17 beta-estradiol (E2) (40 micrograms/pup), or the vehicle 2 days after birth. After puberty, behavioral estrous cyclicity was measured. The females were then ovariectomized, treated with the male hormone testosterone, and tested for their ability to mount a receptive female (a behavior not normally displayed by female hamsters). Females treated neonatally with estradiol or zearalenone were masculinized but not defeminized, an effect consistent with perinatal exposure to low doses of sex hormones. Females in these two treatment groups displayed normal 4-day behavioral estrous cycles, but following ovariectomy and testosterone treatment they mounted a sexually receptive female at a frequency comparable to the males. Methoxychlor-treated females did not differ from controls. The mounting behavior of similarly treated males was unaffected by any of the chemicals. However, males receiving estradiol treatment had smaller testes, seminal vesicles, and cauda epididymides and 57% had epididymal cysts. These results demonstrate that a single exposure to a weakly estrogenic chemical like zearalenone during a critical developmental period can cause the brain to differentiate in a manner inconsistent with the female's genetic sex. This enables the female to respond to the activational influence of testosterone as an adult and readily mount a sexually receptive female. The failure of methoxychlor to alter reproductive development in the current study may be due to an inability of the neonatal hamster to convert methoxychlor to estrogenic metabolites.     

Chemosensory cues are essential for mating-induced dopamine release in MPOA of male Syrian hamsters.

The medial preoptic area (MPOA) is crucial for male sex behavior. Dopamine (DA) is released in MPOA during copulation, and contributes to the reinforcing effects of mating. The aim of the present study was to identify sensory stimuli responsible for mating-induced DA release. Specifically, we determined if chemosensory cues are essential for mating-induced MPOA DA release using in vivo microdialysis in male Syrian hamsters. Hamsters were used because chemosensory cues from the olfactory mucosa and vomeronasal organ are essential for sexual behavior in this species. Sexually experienced adult male hamsters were implanted with a microdialysis guide cannula over MPOA. At the same time, males received sham olfactory bulbectomy (Sham Bx, n = 11), bilateral bulbectomy (Bibx, n = 6), or unilateral bulbectomy (Ubx) ipsilateral (Ipsi Ubx, n = 9) or contralateral (Contra Ubx, n = 8) to the microdialysis probe. This model takes advantage of the predominantly ipsilateral projections of the olfactory bulbs. Microdialysis samples were collected from the MPOA during baseline, exposure to a receptive female, and after removal of female. Extracellular DA was measured using high-performance liquid chromatography with electrochemical detection. During mating, DA increased in MPOA of Sham Bx males (to 146.7 +/- 17.5% of baseline). Bibx males did not mate, and MPOA DA did not increase (96.1 +/- 15.8% of baseline). Although both groups of Ubx males mated to ejaculation, MPOA DA increased significantly only in Contra Ubx males (to 161.8 +/- 35.3% of baseline), and not in males with Ipsi Ubx (107.6 +/- 11.5% of baseline). The results demonstrate that chemosensory cues are essential for MPOA DA release during mating in male Syrian hamsters.     

The effects of novelty-seeking phenotypes and sex differences on acquisition of cocaine self-administration in selectively bred High-Responder and Low-Responder rats

Individual differences in exploratory behavior can predictably influence psychostimulant self-administration behavior. Male rats that exhibit a high degree of locomotor activity in a novel environment (High Responders, HR) will self-administer cocaine more readily than males exhibiting low levels of novelty-induced locomotion (Low Responders, LR). The present experiment investigates the combined influences of the sex of an individual and individual phenotypes in novelty-induced locomotion to predispose animals to acquire cocaine self-administration behavior, in male and female rats selectively bred for the HR-LR phenotypes. We first established that HR females, like their male counterparts, exhibit a dramatically greater locomotor response to novelty and less anxiety-like behavior than do LR females. While locomotor behavior was subtly influenced by estrous stage, with both HR and LR females showing increased activity during metestrus and diestrus compared to proestrus and estrus, the effect did not obscure HR-LR differences. When male and female HR-LR animals were trained to self-administer cocaine (2 h/day, 5 days/wk x 3 wk, 0.2 mg cocaine/kg/infusion), HR males and females acquired cocaine self-administration significantly faster than their LR counterparts. Furthermore, HR females self-administered significantly more cocaine than all other groups. In conclusion, female rats, like males, exhibit HR-LR phenotypes that predict rapidity of acquiring cocaine self-administration. Moreover, HR females self-administer more cocaine than HR males and both LR groups.     

Effects of olfactory bulbectomy and estrogen on tyrosine hydroxylase and glutamic acid decarboxylase in the nigrostriatal and mesolimbic dopamine systems of adult female rats.

Following olfactory bulbectomy (BULBX), ovariectomized female rats show enhanced behavioral sensitivity to estradiol benzoate (EB) as measured by an index of sexual receptivity, lordosis responding. We have proposed previously that alterations in EB sensitivity which also are produced by septal destruction reflect disruptions of gamma-aminobutyric acid (GABA) inhibitory feedback on dopamine (DA) cell bodies in the midbrain, which may be inhibitory to the expression of lordosis. Since the olfactory system as well as the septum receives mesolimbic DA projections, in the present study we examined the effects of EB given to bulbectomized and sham-operated rats on tyrosine hydroxylase (TH) activity in the olfactory tubercle (OT), nucleus accumbens septi and corpus striatum. Glutamic acid decarboxylase (GAD) activity was measured in the ventral tegmental region (VTR) and the substantia nigra (SN). Rats received 2 μg EB/day for 3 days and were tested on Day 4 in a 20 mount behavioral test. Ten bulbectomized rats demonstrating enhanced behavioral sensitivity to EB and all sham-operated rats (N = 10) were selected for further study. Five rats in each group received 0.05 ml oil/day X 3 (BULBX-oil, SHAM-oil), and five received 2 μg EB/day X 3 (BULBX-EB, SHAM-EB). Rats were sacrificed on Day 4. Subsequent assays revealed a bulbectomy-dependent decrease in TH activity in the striatum, and a lesion plus EB-dependent decrease in TH activity in the OT. GAD activity was slightly but significantly suppressed in the VTR in the SHAM-EB group relative to that in the SHAM-oil group. Rats of the BULBX-EB group failed to exhibit decreased GAD activity. Thus, bulbectomy may result in enhanced behavioral sensitivity to EB due to disruptions in GABA-DA interactions, which are similar to those observed following septal destruction and which result in diminished behaviorally inhibitory DA tone.     

Intracranial cycloheximide: effect on male mouse sexual behavior and plasma testosterone.

Cycloheximide (Cyclo), an inhibitor of protein synthesis, infused bilaterally into the preoptic area (POA) of intact B6D2F male mice significantly inhibited male sexual behavior when the males were presented with receptive females 12 hr after treatment. The few males that ejaculated appeared to copulate normally. This finding suggests that Cyclo acts primarily by inhibiting sexual arousal rather than sexual performance. The inhibition of sexual behavior was not observed when the males were tested 84 hr after treatment. After exposure to an estrous female, plasma testosterone levels were measured in males with POA infusions of Cyclo or saline vehicle. No significant difference was found, but both groups had significantly higher levels of plasma testosterone than males not exposed to estrous females. It is suggested that the interference with sexual behavior by Cyclo was not due to interference with the neuroendocrine mechanisms controlling blood andorgen levels, but due to Cyclo acting directly on the neural circuits controlling sexual responsiveness.