Exposure to prenatal stress blocks full masculinization of several sexually dimorphic nuclei in the brain and spinal cord of male rats. We now compare the adult volume of the medial amygdala (MA) and two of its component cell groups, posterodorsal (MePD) and posteroventral (MePV), in prenatally stressed male rats and nonstressed males and females. Previous reports of sex differences (male > female) in the overall size of the MA and the MePD component were confirmed, and we identified a previously unreported sex difference (male > female) in MePV. Prenatal stress had no effect on the size of the total MA, or of the MePD or MePV in males. Maternal stress attenuates the surge in plasma testosterone (T) which normally occurs on days 18 and 19 of gestation in male rats. This brief suppression of T during prenatal development leads to incomplete masculinization of some sexually dimorphic features of the CNS (i.e. the SDN-MPOA of the hypothalamus, and SNB and DLN of the spinal cord) but not others (i.e. the MA, MePD, and MePV). The selective effects of prenatal stress on neural differentiation may be due to differences in the onset and duration of the periods when each of these structures is most sensitive to T and/or its metabolites.

From previous research, the ultrasonic vocalizations of male mice (Mus domesticus) to female mouse urine were hypothesized to be learned as a result of classical conditioning during adult heterosexual encounters. According to this interpretation, a previously neutral conditioned stimulus in female urine comes to elicit vocalizations as a result of its association with some other unknown unconditioned stimulus associated with adult females. However, the research from which this hypothesis was derived utilized urine collected from females housed in metabolic cages. Three experiments further examined the classical conditioning hypothesis using two types of female urine: (i) metabolic-cage-collected urine and (ii) freshly voided urine. Experiment 1 demonstrated that, in contrast to vocalizations to metabolic-cage-collected urine, adult heterosexual experience was not necessary for males to vocalize to freshly voided female urine. In addition, unlike metabolic-cage-collected urine (Experiment 3), freshly voided urine remained a potent stimulus for eliciting vocalizations during repeated testing (Experiments 2 and 3). Finally, freshly voided urine appeared to cause a previously neutral stimulus (cotton swab) to acquire ultrasound eliciting properties (Experiment 2). We suggest from these findings that two chemosignals that elicit vocalizations from males may exist in female mouse urine: (i) a potent, but volatile or easily degraded, unconditioned stimulus to which males vocalize without sexual experience and (ii) a nonvolatile, chemically stable conditioned stimulus.

A computer-assisted image analysis technique was used to measure the adult volume of the sexually dimorphic nucleus of the medial preoptic area (SDN-MPOA) in prenatally stressed male rats and in groups of non-stressed males and females. The SDN-MPOA of male offspring from dams stressed daily (i.e. three 45-min exposures to physical restraint and bright light) during the last week of pregnancy was significantly smaller than in males not exposed to stress, but was larger than in females. Maternal stress has been shown to attenuate the surge in fetal plasma testosterone (T) which normally occurs on days 18 and 19 of gestation in male rats. The present results suggest that suppression of T during prenatal development leads to an incomplete masculinization of the SDN-MPOA in male rats. There was no difference in SDN-MPOA volume between males that exhibited the ejaculatory pattern when tested with estrous females and males that failed to ejaculate in either the control or prenatal stress group. SDN-MPOA volume does not appear to be predictive of masculine ejaculatory performance.

Five experiments examined the hormonal regulation of the precopulatory reproductive behavior of male housemice of two genotypes (DBA/2J inbreds and C57BL/6J X AKR/J hybrids). The two precopulatory behaviors examined were preferences for female urinary odors and ultrasonic courtship vocalizations to anesthetized females. The preferences were then used to make inferences about odor attractiveness. Gonadally intact hybrid males were highly attracted to the airborne urinary odors of female mice but were either indifferent to, or exhibited less attraction to, male urinary odors. Castration decreased male attraction to female odor such that castrated males were equally attracted to male and female odors. Normal levels of attraction could be maintained in castrated hybrid males by Silastic implants of either testosterone or estradiol. While Silastic implants of dihydrotestosterone (DHT) were also effective in maintaining attraction in hybrids, this hormone was ineffective in inbreds. The effectiveness of estradiol, DHT, and testosterone in maintaining attraction following castration was paralleled in castrated hybrids by the effects of these hormones in maintaining courtship vocalizations to females. In contrast to the genotype-specific effects of DHT upon behavior, DHT was effective in both genotypes in maintaining seminal vesicle weight. Estradiol, on the other hand, which was quite effective in maintaining both precopulatory behaviors in hybrids, had little effect upon seminal vesicle weight. Thus these experiments dissociate the behavioral effects of steroids from their effects upon peripheral morphology. We suggest that testosterone can activate precopulatory behaviors following either aromatization or 5-alpha reduction but that genetic variability somehow gives rise to strain differences in DHT responsiveness.

Two experiments were conducted to examine 70-kHz ultrasonic courtship vocalizations by adult male mice (Mus musculus) to novel odors following exposure to these odors in infancy and/or adulthood. The research was performed in hopes of better understanding the experiences giving rise to vocalizations to naturally occurring chemosignals. Experiment 1 demonstrated that adult males normally do not vocalize to the urine of female rats but would come to do so if adult female mice odorized with female rat urine were repeatedly encountered postpubertally. On the other hand, encountering their own mother odorized with female rat urine from birth until weaning did not promote vocalizations to the urine of female rats. Experiment 2 was designed to examine vocalizations to the urine of female mice whose urinary odor was altered by the ingestion of fenugreek, a spice. Although the magnitude of the effect was smaller in this experiment, greater amounts of vocalization again were seen by males that as adults encountered females that had ingested fenugreek. Males with such experience also showed a small but significant elevation in vocalizations to the fenugreek odor itself. Again, experience with the novel odor during infancy was not associated with elevated vocalizations during adulthood either to fenugreek-altered urine or to the fenugreek odor itself. Thus vocalizations to two different novel odors occurred only after an adult male had encountered an adult female odorized with the novel odor. On the other hand, none of the novel odor experiences eliminated vocalizations to the naturally occurring chemosignal in female mouse urine.(ABSTRACT TRUNCATED AT 250 WORDS)

Previous research was consistent with the hypothesis that urinary chemosignals from female mice (Mus musculus) serve as a conditioned stimulus (CS) for the elicitation of male ultrasonic courtship vocalizations while some other unknown aspect of the female serves as an unconditioned stimulus (US). According to this hypothesis adult heterosexual experience is necessary for males to pair the urinary CS with the US. Three experiments further examined this hypothesis. Experiment 1 demonstrated that the hypothesized US was not female behavior. Experience with anesthesized males and females was just as effective as experience with awake conspecifics. Experiments 2 and 3, however, question the primacy of the Classical Conditioning hypothesis. In both experiments sexually naive males were allowed contact with either normal females or female surrogates. The female surrogates were neonatally castrated males (Experiment 2) or hypophysectomized females (Experiment 3), both of which appeared to possess the hypothesized US but not the urinary CS on the basis of previous research. While exposure to normal females caused the highest level of vocalization to urine from normal females, exposure to the two classes of female surrogates also resulted in vocalizations to the urine of normal females. Thus under some circumstances, males do not require experience with a normal female to emit ultrasounds to urine from normal females. Factors in addition to Classical Conditioning must be operating to account fully for the role of adult heterosexual experience in causing female urine to come to elicit male courtship vocalization.