Sexual and olfactory preference in noncopulating male rats

In some species including rats, mice, gerbils, and rams, apparently normal males fail to copulate when repeatedly tested with receptive females. These animals are called "noncopulators (NC)," and the cause of this behavioral deficit is unknown. It has been shown that NC rats do not have hormonal alterations or deficits in the mechanisms that control penile function. The present study was designed to examine (Experiment 1) whether NC male rats prefer receptive females to sexually active males. In addition, the olfactory preference for bedding soiled from estrous or for anestrous bedding was investigated. These tests were performed in NC and copulating (C) male rats when the subjects were intact, gonadectomized (GDX), or GDX and treated with high doses of testosterone propionate (TP). Our results demonstrate that NC rats do not display sexual behavior even after high TP treatment. While C male rats have a clear preference for receptive females as opposed to a sexually active male, NC rats do not. In all hormonal conditions, the preference shown by NC rats for estrous bedding was significantly reduced in comparison to that seen in C rats. TP treatment in NC rats did not modify either partner or odor preference. In Experiment 2, we evaluated if NC rats are feminized and if it could be easier to induce feminine-like behavior by hormone treatment with estradiol benzoate (EB) or with EB plus progesterone (P) (EB+P). Odor preference for estrous or male bedding under these hormonal conditions was also compared. No differences between NC and C rats were found in feminine sexual behavior. In the olfactory test, we found that NC rats prefer odors from receptive females as opposed to male odors, but this preference is reduced compared to that of C rats. Males treated with EB or EB+P show no preference for female odors. These results demonstrate that treatment with EB or EB+P does not increase feminine sexual behavior in NC rats.     

Olfactory preference and Fos expression in the accessory olfactory system of male rats with bilateral lesions of the medial preoptic area/anterior hypothalamus

In the present study we evaluated if a medial preoptic area/anterior hypothalamus lesion affects the olfactory preference toward soiled bedding from receptive females in comparison to bedding from anestrous females or clean bedding. In the second part of the study we evaluated the accessory olfactory system response to estrous bedding with Fos immunoreactivity to determine if the preoptic lesions modify the processing of sexually relevant olfactory cues. Before medial preoptic area/anterior hypothalamus lesions, male rats spent more time investigating estrous bedding as opposed to anestrous or clean bedding. After the lesion, subjects showed no preference between estrous and anestrous bedding; that is, males spent the same amount of time investigating both types of bedding. These two odors were investigated more than clean bedding. Increments in Fos immunoreactivity neurons were seen in structures of the accessory olfactory system after exposure to soiled estrous bedding [granular layer of the accessory olfactory bulb, anterior-dorsal medial amygdala, posterior-dorsal medial amygdala, bed nucleus of the stria terminalis]. These results suggest that bilateral destruction of the medial preoptic area/anterior hypothalamus modify male olfactory preference in such a way that subjects spend the same time smelling and investigating bedding from estrous and anestrous females. This change in olfactory preference is not associated with alterations in the processing of sexually relevant olfactory cues by the accessory olfactory system.     

Olfactory discrimination and incentive value of non copulating and sexually sluggish male rats

Some apparently healthy male rats fail to copulate despite being tested on repeated occasions with receptive females and are called non copulating (NC) rats. NC rats sniff and lick the female genitals, and show normal erectile and ejaculatory functions and hormonal levels. Sexually sluggish (S) male rats take a long time to ejaculate or sometimes they don't achieve ejaculation when tested repeatedly with receptive females. The aim of the present study was to determine if NC and S males can discriminate sexually relevant olfactory cues such as urine from estrous or anestrous female and urine from sexually experienced males. We also tested odors like amyl acetate and mint using an olfactory discrimination test. In a second experiment we evaluated if a sexually receptive female has a preference for a copulating (C) male, for a NC male, or for a S male in a sexual incentive motivation test. This would let us determine if a NC and an S male are equally attractive than a C male to a sexually receptive female. The olfactory test revealed that C, NC and S males have the same ability to discriminate sexually relevant odors. As well, all males clearly discriminate non sexual odors like amyl acetate and mint suggesting that NC and S male rats do not have alterations in their olfactory system. With respect to the sexual incentive motivation test, females spend the same time in the incentive zone of the NC and C males. As well, females spent the same time in the incentive zone of S and C males. These results demonstrate that NC, S and C males are equally attractive to receptive females.     

Disparate effects of small medial amygdala lesions on noncontact erection,copulation, and partner preference

Male rats with radiofrequency lesions in the anterior medial amygdala (MeAa) or the posterior medial amygdala (MeAp), respectively, were tested for copulation and for noncontact erection (NCE; evoked by inaccessible estrous females) in a chamber in which the male was located between estrous and anestrous females. Barriers allowed only olfactory and auditory interaction between animals. With conscious females as stimuli, MeAp lesions virtually eliminated NCEs, and MeAa lesions moderately impaired them, without affecting the normal preference for estrous over anestrous females. When tested with anesthetized females to remove auditory stimulation, few males with lesions had NCEs. Only the males with MeAp lesions had a significant reduction in preference for estrous over anestrous anesthetized females. Neither MeAa nor MeAp lesions had an effect on copulatory behavior. MeAp lesions may have caused a reduced sensitivity to--or impaired processing of--estrous odors, thereby preventing NCE without disrupting copulatory behavior.     

The influence of estrogen, testosterone and progesterone on partner preference, receptivity and proceptivity

The influence of Estradiol benzoate (EB), Testosterone propionate (TP) and Progesterone (P) on the female's partner preference for sexually active males was investigated and compared to levels of receptive and proceptive behaviors observed in a tethered male test situation. Doses of EB (1 micrograms), TP (500 micrograms) and P were selected on the basis of previous investigations indicating that female rats treated with these dosages will show comparable levels of lordosis behavior. The results indicate that TP stimulates partner preference for sexually active males over estrous females in ovariectomized female rats. Females treated with EB tended to prefer the company of sexually active males more than Oil-treated females and less and TP-treated females. However, preference behavior of EB-treated females was not significantly different from that of Oil- or TP-treated females. Additional treatment with P (100 micrograms) did not influence partner preference of Oil-, EB- or TP-treated females. In the tethered male tests, P stimulated proceptivity of EB- or TP-treated females and receptivity of EB-treated females. Significant differences in proceptive and receptive behaviors between EB- or TP-treated were not found. Although facilitation of receptive and in particular, proceptive behaviors were found to be generally accompanied by an increased partner preference for males, it is concluded that gonadal hormones are differentially affecting aspects of female rat sexuality: Relative to the activation of receptive behavior, TP was found to be more effective (than EB) to increase preference for a male; P (given to EB- or TP-treated females) was found to stimulate receptive and proceptive behaviors considerably, while being ineffective to stimulate preference for a male.     

Characterization of 50-kHz ultrasonic vocalizations in male and female rats

Male and female rats emit ultrasonic vocalizations in reproductive encounters. While estrous bedding has been used to elicit vocalizations of males, the number of responses is variable. We report a reliable method to assess vocalizations using exposure to a stimulus animal. The stimulus rat is placed behind a wire barrier for 5 min, then removed. Vocalizations are then recorded for 5 min. Experiment 1 validated this method and it was used for subsequent experiments. In Experiment 2, male rats were castrated and tested for the restoration of vocalizations. In one group, males were allowed to copulate freely; in the other, females had vaginal masks to prevent ejaculation, but not mounting. Vocalizations were restored only in males allowed to ejaculate. In Experiment 3, we measured vocalizations in sexually nai;ve and sexually experienced males following exposure to either castrated (CAS) males, testosterone (T)-treated males, ovariectomized (OVX) females, or OVX females receiving estrogen plus progesterone (E+P). Males vocalized most after exposure to E+P females, whether they were sexually experienced or naive. However, the rate of vocalizations was significantly higher after exposure to E+P females when the males were sexually experienced. In Experiment 4, we measured vocalizations in females following exposure to CAS males, T-treated males, OVX females, or E+P females. Females vocalized most after exposure to T-treated males. Our results show that (1) sexual experience facilitates vocalizations in male rats, (2) vocalizations are highest after exposure to hormonally receptive conspecifics, and (3) ultrasonic signaling is a sensitive index for assessing the hormonal disposition of conspecifics.     

Mating preference and sexual reinforcement in female rats

Intact female rats chose between sexually active males and inactive males or females in a runway-choice apparatus. In either a discrete choice or continuous choice format, sexually receptive female rats demonstrated a preference for sexually active males over the other two incentive animals. No preference was demonstrated when females were sexually non-receptive. The results demonstrate that, for sexually receptive female rats, copulation can have reinforcing effects distinct from the reinforcing effects of more general social and exploratory factors.     

Hormonal control of receptivity,proceptivity and sexual motivation

The results of three experiments using ovariectomized female rats are reported. In each experiment, females were individually tested in large arena with a partner (or partners) tethered so as to give the unrestrained female control over the occurrence of social interaction. In the first experiment we show that ovariectomized females will repeatedly approach a sexually active male and allow themselves to be mounted. Other forms of female initiated contact are common. These include “grooming” and “nosing” the male, crawling over or under the male, the “push-past” the male, genital sniffing, and following the male. For convenience we refer to these as feminine social contacts. Treatment with ovarian hormones induces sexual receptivity, increases the frequency with which females approach males, and increases the display of sexual soliciting behaviors such as hopping and darting. In this setting the frequency of feminine social contacts is decreased. In a second experiment we show that hopping/darting is frequently displayed by intensely receptive females when they are tested with sexually active males, but these beahviors are almost never displayed in the presence of either a sexually passive male or a female. The frequency of feminine social contacts varies predictably with the setting: contact frequency is low when receptive females are tested with active males; contact frequency is high when females are tested with a passive male or a female. In the third experiment we used a preference test paradigm to measure sexual motivation. This involved testing a female with a sexually active male and a sexually inactive castrate at the same time. Treatment with ovarian hormones increases a female's preference for the sexually active male over the castrate, and by this criterion hormonal stimulation can be said to increase sexual motivation. The results of this experiment, using a very simple test for sexual motivation, form the basis for a forthcoming series of papers dealing with the effects of brain damage upon reflexive and volitional components of feminine sexual behavior.     

Participation of the olfactory system in the control of approach behavior of the female rat to the male

The amounts of time spent by females in the sector of an open field close to the cage housing a normal male or a castrated male were measured in order to quantitate the tendency of the female to reach physical proximity to a sexually active male (androtropism). Intact proestrous or ovariectomized females primed with 100 micrograms of estradiol benzoate/kg b. wt. (EB) or EB plus 2 mg progesterone/kg b. wt. (P) spent significantly more time close to the sexually active (intact) male than in the proximity of the orchidectomized male. In order to determine whether olfactory clues were sufficient for female rats to distinguish between intact and castrated males, the males were removed from the stimulus cages, leaving the soiled bedding in place. Ovariectomized rats primed with EB or EB plus P clearly preferred proximity to the cage where the intact male had been living. No preference was evident after transection of olfactory nerves in proestrous rats or in ovariectomized rats primed with EB plus P. Resection of the vomeronasal organ also suppressed preference. These results indicate that olfactory input is necessary and sufficient for androtropism to occur, and suggest that the accessory olfactory system is involved in the analysis of olfactory signals used by female rats to identify the endocrine status of prospective sexual partners. In a different group of animals, it was demonstrated that destruction of the posteromedial cortical amygdaloid nucleus also suppressed preference for the intact male. It is proposed that this structure serves as a relay station for the analysis and integration of olfactory input significant for the motivational control of sexual behavior in the female rat.(ABSTRACT TRUNCATED AT 250 WORDS)     

Homosexual and heterosexual partner preference in ovariectomized female rats: effects of testosterone, estradiol and mating experience

Ovariectomized female rats, longterm treated with estradiol benzoate (EB, 20 micrograms, 3 x/week) or testosterone propionate (TP, 0.4 mg, 3 X/week), were tested for homo- or heterosexual partner preference behavior with either tethered animals (sexually active male vs. estrous female) or animals behind a wire mesh. A preference score was calculated by subtracting the time spent in the vicinity of the stimulus female from the time spent in the vicinity of the stimulus male. Thus, a positive score indicates preference for the male (heterosexual preference), a negative score preference for the female (homosexual preference). Two weeks of EB treatment caused a clearcut preference for the male incentive. This heterosexual preference was significantly different from the indistinct preference of the TP-treated females. Sexual interaction with a freely moving active male (with or without a vaginal mask which prevented intromissions) did not alter the preference for the male of the EB-treated females. It did affect, however, the preference behavior of the TP-treated females significantly: it changed in the direction of the sexually active estrous incentive female, i.e., a homosexual preference. This change in preference could not be attributed to the experience of penile intromissions, it occurred despite the presence of a vaginal mask. Apparently, being mounted by a sexually active male had a negative reinforcing value in the TP-treated female rats and provoked a homosexual partner preference.     

Attraction to male pheromones and sexual behaviour show different regulatory mechanisms in female mice

In rodents, female sexual behaviour is under hormonal control. The attraction females show for male-derived nonvolatile chemicals (pheromones) can be regarded as the first step of this behaviour, but it is unknown whether this attraction is also modulated by sexual steroids. To test this possibility, ovariectomized adult female mice with no experience of chemical signals from adult males were randomly assigned to four groups that received oil (control), progesterone, estradiol (E) or estradiol+progesterone (E+P) injections, respectively. Females were then tested for their attraction to male-soiled bedding and, subsequently, for their proceptive behaviour when confronted to adult males. Females showed attraction to male-soiled bedding irrespective of the hormonal treatment, whereas only those females treated with E or E+P showed proceptive behaviour. Therefore, in contrast to proceptive and copulatory behaviour, the female attraction to male pheromones is independent of sexual steroids, thus indicating that those parts of the vomeronasal system involved in this attraction do not respond to steroids. In summary, sexual behaviour in female mice can be seen as a two-step process. First, females are attracted by male pheromones, a process which is independent of their hormonal status. After encountering the males, females show proceptive behaviour only in estrous, when fertilization is more likely. The attraction exerted by male sexual pheromones promotes female autostimulation that might ensure anticipatory endocrine changes leading to ovulation by the time of sexual intercourse.     

Individual recognition of female hamsters by males: role of chemical cues and of the olfactory and vomeronasal systems

Sexually satiated male hamsters preferred to investigate and to mount an anesthetized, estrous, novel female over a similarly presented female with which the male had become satiated (the Coolidge effect); likewise, such males preferred a novel female recently mated with another male over the familiar female but showed no preference between fresh and mated novel females. Thus the Coolidge effect is at least partly dependent on discrimination of a new female by chemical cues. Another experiment indicated that transfer of a male's own scent during mating is not involved in discrimination between familiar and novel females. Flank gland secretion of females were sufficient for individual discrimination by males, whereas head region scents and vaginal secretions were not sufficient. The presence of female's flank glands was not, however, necessary for such discrimination. Lesions of or removal of the vomeronasal organ did not disrupt the preferences of sexually satiated males for a novel female, but elimination of main olfactory system function by ZnSO4 treatment of the olfactory mucosa did abolish such preferences. Thus olfactory cues are sufficient for individual discrimination of novel females by sexually satiated male hamsters, and such recognition leads to increased sexual arousal. These processes are mediated by the main olfactory system but not the vomeronasal accessory-olfactory system.     

Olfactory bulb removal: effects on sexual behavior and partner-preference in male rats

Control and bilaterally bulbectomized male rats were tested in an arena where the male could choose to spend time with (and mate with) a sexually receptive female, a nonreceptive female, or be in a neutral compartment. Control males mated with, and showed a strong preference for, sexually receptive females. Bulbectomy virtually eliminated mating. In addition, bulbectomized males showed no preference for a receptive female over a nonreceptive female, and spent their time equally between the receptive female, the nonreceptive female, and the neutral compartment. Effects of bulbectomy on preference and copulation could be consequences of a severely impaired ability to smell--the perception of odors may be essential for sexual arousal, or the absence of preference and copulation after bulbectomy might reflect a deficit in the male's ability to make odor-dependent classification of conspecifics as appropriate sexual partners. Or the behavioral effects of bulbectomy might reflect a disruption of tonic input to the forebrain that has little or nothing to do with the sensory impairment that follows bulb removal. But whatever the reason, in partner-preference tests bulbectomized males show a striking indifference to the sexual status of females, and it seems likely that the failure to mate is causally linked to this effect of surgery.     

On the organization of partner preference behavior in female Wistar rats

The possible prenatal organizing effects of testosterone (T) on adult sexual partner preference, i.e., sexual orientation in female rats, were studied through prenatal exposure (days 11-22) of female fetuses to the antiandrogens flutamide (Sch 13521; 4'-nitro-3'-trifluoromethylisobutyranilide; 5 or 10 mg/day; Experiment 1) or anandron [RU 23908; 5,5-dimethyl-3-(4-nitro-3-(trifluoromethyl)phenyl)- 2,4-imidazolidinedione; 35 mg/kg/day; Experiment 2]. The neonatal organizing effects of T were further studied by giving T, dihydrotestosterone (DHT) or oil within 9 h after birth to female pups (Experiment 3). In adulthood sexual orientation was ascertained, after ovariectomy followed by hormone treatment, in an automated open field (AOF), with stimulus animals behind wire mesh, and in a 3-compartment box (3-CB), with stimulus animals tethered. When given the choice between an estrous female and a sexually active male in the AOF, flutamide females, as well as controls, preferred the male partner. After long-term T treatment and 3 weekly pair-tests with an estrous female, flutamide females as well as controls switched their preference to the estrous female partner. In anadron females similar results were obtained. Thus the prenatal antiandrogens had no significant effect on sexual orientation in female rats. This suggests that adult sexual orientation in female rats is not organized prenatally through endogenous T. The change in preference after sexual experience corroborates earlier findings from our laboratory. When given the choice between an estrous female and a sexually active male in the 3-CB (sexual interaction with incentives possible), neonatally DHTP-treated females preferred the male; neonatally TP- or oil-treated females showed no preference.(ABSTRACT TRUNCATED AT 250 WORDS)     

The reproductive stage and experience of sexually receptive mothers alter their preference for pups or males

Female rats show postpartum estrus, a unique stage in their reproductive cycle in which they are able to display maternal and sexual responses at the same time. To assess the relative value of pups or males for sexually receptive mothers with different hormonal profiles and reproductive experiences, we employed a 3-point star maze with 3 choice compartments containing: pups, a sexually active male, or no stimulus (neutral). Cycling maternal and nonmaternal females in late proestrus, independently of their previous reproductive experience, strongly preferred the male to the pups, although most postpartum estrous dams did not exhibit preference for the male. The majority of the postpartum primiparous females did not prefer the litter's chamber either, but a previous reproductive experience strongly determined their preference for the pups. These results suggest that the hormonal changes of the proestrus, in contrast to those of the postpartum estrus, promote a strong preference for the male that is not diminished by the maternal condition. Conversely, the endocrine changes of the postpartum facilitate the effect of previous reproductive experience in strengthening the incentive value of the pups.     

Neuronal Fos activation in olfactory bulb and forebrain of male rats having erections in the presence of inaccessible estrous females.

Volatile odors from estrous female rats are necessary and sufficient to induce non-contact penile erections in male rats. It is not known whether these pheromones are detected by the accessory as opposed to the main olfactory system or whether they are processed by forebrain regions that receive olfactory inputs. Using nuclear Fos immunoreactivity as a marker of neuronal activation, we asked how the detection and processing of distal cues from inaccessible estrous females, which elicited non-contact penile erections, compared with the processing of sensory cues from soiled estrous bedding which did not elicit non-contact penile erections. In Experiment 1, groups of sexually experienced males were given one of five treatments. A control group was placed on clean bedding. A second group displayed non-contact penile erections when exposed to the smell, sight and sound of an estrous female restrained behind a permeable barrier. A third group was exposed to the same stimuli as the second (an estrous female) but failed to exhibit non-contact penile erections during the first hour of testing. A fourth group was placed on soiled estrous bedding, and a fifth group was allowed two ejaculations with an estrous female. All males were perfused with 4% paraformaldehyde 2 h after the onset of these respective treatments, and their brains were later processed for Fos immunoreactivity. Non-contact penile erections were observed in males that were exposed to distal cues from an estrous female but not in males exposed to soiled estrous bedding. Males that displayed non-contact penile erections or that were exposed to estrous bedding showed significantly more neuronal Fos immunoreactivity than clean-bedding controls in the nucleus accumbens core and shell, anterior and posterior medial amygdala, bed nucleus of the stria terminalis and the medial preoptic nucleus. Even greater neuronal Fos responses occurred in these regions in mated males. In Experiment 2 these same treatments were given to another cohort of sexually experienced males. Increased neuronal Fos immunoreactivity was observed in the granule and mitral cell layers of the accessory olfactory bulb of males that were either mated or exposed to estrous bedding, but not in males that displayed non-contact penile erections in response to distal cues from an estrous female. The volatile odors which presumably caused non-contact penile erections failed to stimulate significant neuronal Fos immunoreactivity in five main olfactory bulb sites examined. Even so, it seems likely that these pheromones are detected via the main olfactory system and are subsequently processed by the same projection circuit that responds to other pheromones present in estrous bedding that are incapable of eliciting non-contact penile erections.     

Peripheral anosmia attenuates female-enhanced aggression in male rats

It is well established that male rats with prior access to sexually active females show enhanced offensive aggression toward unfamiliar male intruders. The present study assessed the importance of the sense of smell for this facilitatory effect. It was found in 2 independent experiments that anosmia, induced peripherally by surgically removing the olfactory epithelium and cutting the olfactory nerves, reduced baseline levels of offensive aggression and significantly attenuated the female-enhanced aggression effect. It was also found that sexual performance of anosmic rats was context-dependent, in that it was more impaired in the homecage environment than in standard observation cages. In contrast to sham-operated males, the experimental animals showed no preference for estrous over anestrous females in a mate choice test. Anosmic males did not appear more fearful than controls, as assessed in a hyponeophagia test, but they showed less exploratory behavior (rearing and head-dipping) in the hole-board test, and less rearing activity in automated activity boxes.     

Acute increase in plasma corticosterone level in female mice evoked by pheromones

Exposure of a singly reared estrous female to grouped females or their bedding during 20 min resulted in an increase in the plasma corticosterone level during the first 10 min. The reaction of the adrenal glands was similar to the excitation evoked by stress connected with moving females from their own cage to a new, clean cage. The highest level of the adrenal hormone was present in estrous females after exposure to male bedding during 10 min. This was followed by a dramatic decrease during the next 90 min. It is suggested that male pheromones, through activation of the pituitary-adrenal system and acute release of corticosterone, participate in the stimulation of sexual behavior in receptive female mice.     

Inactivation of the medial preoptic area/anterior hypothalamus by lidocaine reduces male sexual behavior and sexual incentive motivation in male rats

Permanent bilateral lesions of the medial preoptic area anterior hypothalamus (MPOA/AH) produce a drastic inhibition of male sexual behavior in all species studied to date. The present experiment was designed to evaluate if temporal inactivation of the MPOA/AH by infusions of lidocaine also inhibits sexual behavior in male rats. This would allow us to rule out the possibility that the behavioral effects observed after damage of the MPOA/AH could be associated with plastic changes induced by the lesion in other brain regions. We also evaluated sexual incentive motivation in males after the infusion of lidocaine in a test in which copulation is not possible but where males maintain approach behavior to the estrous females despite repeated testing. The percentage of animals displaying mounts, intromissions and ejaculation was significantly reduced while mount and intromission latency were prolonged after infusion of lidocaine. No changes were observed in sexual behavior after infusion of lidocaine in animals with cannulae outside the MPOA/AH suggesting that the inhibitory effects are specific to this brain region. Sexual incentive motivation was also affected by administration of lidocaine. Males consistently showed a clear preference for the sexually receptive female except when infused with lidocaine. After the infusion of the compound a significant reduction in the time spent in the incentive zone of the stimulus female was observed. These results support the hypothesis that neurons of the MPOA/AH are involved in the control of male sexual motivation.     

Effects of neonatal castration and testosterone treatment on sexual partner preference in the ferret

Groups of male and female ferrets were tested in a T maze to determine whether they preferred to approach and interact with a sexually active male or an estrous female. Control male and female ferrets gonadectomized (GX) on postnatal Day 35 and tested in adulthood while receiving no hormone or testosterone (T) displayed no significant preference. When given estradiol benzoate (EB), however, control males preferred stimulus females whereas control females preferred stimulus males. When tested in adulthood with EB treatment, males GX on postnatal Day 5 showed a significant reduction in their approach to stimulus females, although they did not switch their preference to stimulus males and thereby resemble control females. Female ferrets GX on postnatal Day 5 and given a high dosage of T over postnatal Days 5–20 showed a significant reduction in their approach to stimulus males, although they did not switch their preference to stimulus females, and thereby resemble control males. The results suggest that extended perinatal exposure of male ferrets to T is required for the development of a sociosexual preference for females.