Peripheral anosmia affects puberty-influencing chemosignals in mice: donors and recipients

A series of five experiments tested the effects of peripheral anosmia on both donors and recipients of urinary chemosignals that accelerate and delay puberty in female mice. Control tests determined that the manipulations, in and of themselves, did not bring about any changes in release or reception of the chemosignals. Rendering mice peripherally anosmic using a solution of zinc sulfate did result in effects on both donors and recipients. For donors, release of the substances in male urine, urine from females in estrus, or urine from pregnant or lactating females, all of which accelerate female puberty, were not influenced by peripheral anosmia. Peripheral anosmia did, however, bring about the cessation of release of the substance in the urine of grouped adult females that delays sexual maturation in conspecific females. Peripherally anosmic young female recipients of the chemosignals were accelerated in their sexual development when the treatment applied involved urine from pregnant or lactating females. However, the acceleratory effects normally produced by male urine or urine from females in estrus did not occur in peripherally anosmic young females. The substance in the urine of grouped adult females did not produce delays in puberty when the recipient females were rendered peripherally anosmic prior to urine treatment.     

Sexual behavior in peripherally anosmic male hamsters

Sexually experienced and inexperienced male hamsters mated normally after they had been rendered temporarily anosmic by intranasal zinc sulfate treatment (Experiment 1), in contrast to the complete absence of sexual behavior following removal of the olfactory bulbs. The efficacy of our zinc sulfate technique was demonstrated by use of an olfactory discrimination training procedure. Hamsters lost the ability to smell both amyl acetate (Experiment 2) and the female hamster post-estrus vaginal discharge (Experiment 3) following zinc sulfate induced anosmia but at the same time retained normal sexual behavior.     

Effects of central and peripheral anosmia on reproduction of female mice

Anosmia, induced either by removal of the olfactory bulbs or by a ZnSO₄ flush of the nasal epithelium, resulted in lengthened and irregular estrous cycles in mice as determined by vaginal lavage. When mated with males, anosmic females conceived but at a rate lower than sham controls. Both central and peripheral anosmia disrupted nest building and maternal care but the severity of the effect was greater among centrally anosmic females. These results indicate that the sense of smell is essential for normal ovarian rhythmicity in the mouse and that bulbectomy produced more severe disruption of nest building and maternal care than anosmia alone.     

Olfaction and sexual receptivity in the female golden hamster

Apparently olfaction is not necessary for the appearance of female sexual receptivity in the golden hamster (Mesocricetus auratus). Cycling females with olfactory bulb lesions or zinc sulfate induced anosmia showed normal levels of lordosis behavior in response to a male. Following copulation these females showed depressions in receptivity and increases in fighting behavior equivalent to those of control females. However, these postcopulatory depressions were not as great in females receiving the anesthetic barbituate, sodium methohexital. Also noted were disruptions in cyclicity in some bulbectomized females.     

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.     

Male copulatory behavior and female maternal behavior in neonatally bulbectomized rats

Olfactory bulbs were destroyed in six-day-old male and female rats, and anosmia was confirmed in adulthood. Bulbectomized males initiated apparently normal sexual activity at the same age as did control animals. As virgins, bulbectomized females did not injure foster young; when allowed to mate and deliver young, they displayed apparently normal maternal behavior. It was concluded that neonatal olfactory bulbectomy does not preclude the development of essentially normal copulatory behavior in the male rat and maternal behavior in the female rat.     

Social-olfactory experience and initiation of copulation in the virgin male rat

Two experiments examined the role of olfactory sensation for initiation of normal copulatory behavior of mature male rats. Experiment 1 compared copulatory performance of peripherally anosmic social isolates with that of intact isolate controls. The results revealed significantly lower mounting, intromission, and ejaculation frequencies of the anosmic group and suggested that nonolfactory cues attain significance via an associative-learning process. Experiment 2 compared three social isolate groups of mature males with varying sexual experience prior to anosmia. Four 1-hr tests were conducted at 5-day intervals. The results indicated comparable copulatory performance of the experienced groups and absence of copulation by the majority of the nonexperienced group. With a 12-day continuous exposure to intact females, all noncopulators eventually impregnated. The results are discussed with reference to the requirement of social experience for initiation of normal sexual performance in the male rat.     

Simultaneous and successive olfactory bulb removal: influences on the mating behavior of male mice

Simultaneous olfactory bulb removal abolished mating behavior in sexually experienced male mice. In contrast to simultaneous bulbectomy, removal of both olfactory bulbs in two operations separated by an interval of 30 days had no effect on the mating behavior of male mice. In successively bulbectomized males, the occurrence or nonocurrence of the opportunity for mating behavior in the interval between the removal of each bulb had no influence on the degree to which mating behavior was spared. It was concluded that the abolition of mating behavior produced by simultaneously removing both olfactory bulbs in male mice is not a consequence of a sensory deficit, but is due to the destruction of tissues directly involved in the mediation of sexual behavior.     

Anosmia and play fighting behavior in prepubescent male and female rats

Prepubescent male and female rats were paired after intranasal treatment with isotonic zinc sulfate or saline solutions to determine the influence of peripheral anosmia upon play fighting behavior. In Experiment 1, male isolate residents pinned intruders significantly more often than intruders pinned residents but anosmia treatment had no influence on pinning behavior. In Experiment 2, males and females differed significantly in all major categories of play fighting but anosmia treatment had no influence on play fighting of either males or females. In Experiment 3, male groups matched for play fighting behavior prior to intranasal zinc or saline treatment were paired with like-treated males. All males in all pairings engaged in play behavior but anosmic males played less than controls. Evidently, intranasal zinc treatment depresses play behavior only when both members of a pair are anosmic. Play behavior is more markedly affected by resident-intruder status and by gender than by peripheral anosmia.     

Effect of bilateral transection of the lateral olfactory tract on the male-induced implantation failure (the Bruce effect) in mice

Bilateral transection of the lateral olfactory tract (LOT) at the rostral level induced anosmia in female mice; by contrast, sectioning of the LOT at more caudal levels failed to induce anosmia in females. Transection of the LOT at all the levels inhibited the alien male-induced implantation failure in newly inseminated mice (the Bruce effect). Sham-operated as well as intact females exhibited a high rate of implantation failure following alien male exposure. The results suggest that the inhibition of the Bruce effect in LOT-transected females is not due to anosmia induced by the operation procedure, but due to interruption of the primary olfactory bulb projections to the posterior parts of the olfactory cortex. Our results rule out the involvement of the nervus terminalis in the Bruce effect. The present report lends support to the involvement of the accessory olfactory system in the transmission of the pheromonal stimulus involved in the male-induced implantation failure.     

Olfactory bulbectomy: emotional behavior and defense responses in the rat

Removal of the olfactory bulbs of male hooded rats caused a reduction in defensive reactions to an unconditioned aversive stimulus (mechanical robot). In the presence of the robot, bulbectomized rats showed significantly less freezing and significantly less suppression of feeding behavior than sham-operated controls. At the same time, the bulbectomized rats were significantly more irritable to handling and showed a significant increase in muricide. These effects could not be attributed to loss of olfactory perception alone. Rats made peripherally anosmic through surgical removal of olfactory afferents were virtually indistinguishable from controls. These results provide further evidence for the view that in addition to their sensory functions, the olfactory bulbs of the rat have important tonic influences on emotional behavior. These tonic influences can occur independently of sensory processing and seem to be critical for the normal expression of emotional behavior or defense responses in this animal.     

Olfactory cues and accessory olfactory bulb lesion: effects on sexual behavior in the cyclic female rat

The influence of olfactory cues and accessory olfactory bulb (AOB) lesion on female sexual behavior was studied in virgin female Wistar rats. In Experiment 1, it appeared that distance or contact exposure to male urine soiled bedding for 8 hours before testing increased sexual receptivity, i.e., the number of receptive females at 18:00-19:00 on proestrus. In Experiment 2, we observed that sexual receptivity at 18:00-19:00 on proestrus was not affected by AOB lesion as compared to sham-operated females. In Experiment 3 the effects of both AOB lesion and olfactory cues were analyzed. Sexual receptivity at 18:00-19:00 on proestrus did not significantly differ in sham-operated and accessory olfactory bulbectomized females both exposed to the odor of male urine. Regarding lordosis quotient in the three experiments, no significant difference was observed. Mechanisms whereby olfactory cues and/or AOB lesion modified female sexual behavior on proestrus in virgin female rats were discussed in the light of previous and present observations.     

Inhibitory effects of centrally and peripherally induced anosmia on mounting behavior in the female rat

Adult ovariectomized female rats received a testosterone-propionate (TP)-filled silastic implant and were tested for mounting behavior seven days later. Exposure to estrous female urine but not to urine from ovariectomized females increased the number of TP-treated females that displayed mounting behavior. Olfactory bulb removal on its own and peripheral anosmia induced by intranasal ZnSO4 application impaired mounting behavior. These results are discussed with respect to the effects of anosmia on copulatory behavior in the male.     

Effects of volatile and nonvolatile chemical signals on male sex behaviors mediated by the main and accessory olfactory systems

Estrous hamster vaginal discharge (HVD) contains both volatile and nonvolatile chemical signals which collectively elicit both male attraction to females and male mating behavior. These two aspects of normal sexual behavior are differentially affected by lesions involving afferents of the main and the accessory olfactory systems. The results of lesions that involve the main olfactory system suggest that it provides information primarily concerning those volatile components of HVD that normally signal the presence of a female. Lesions restricted to the accessory olfactory system do not impair a male's interest in or relative preference for HVD but do significantly interfere with subsequent stages of sexual behavior by reducing the amount of mating obtained upon exposure to HVD. Stimulation of the accessory olfactory system by nonvolatile components of HVD has thus been implicated in the production of mating behavior. In these experiments, male hamsters were attracted to female odor and engaged in significant amounts of mating behavior with surrogate females when only the volatile components of HVD were available to them. These behaviors were further enhanced when both volatile and nonvolatile components of HVD were provided. Lesion studies of the afferents involved reinforce the hypothesis that the main olfactory system is preferentially involved with processing those volatile chemical signals in HVD that denote female attractiveness whereas the accessory olfactory system is preferentially involved with processing those chemical signals, both volatile and nonvolatile, that evoke subsequent steps in male sexual behavior.     

Bulbectomy and sensitivity to estrogen: Anatomical and functional specificity

In order to clarify the influence of the olfactory system on female sexual behavior, ovariectomized rats were given sham operations (SHAM), total bilateral olfactory bulbectomy (TBULBX), partial bulbectomy (PBULBX), anterior olfactory nucleus lesions (AON) or accessory olfactory bulb lesions (AOB), and tested for lordosis behavior. Only TBULBX resulted in increased sensitivity to estradiol benzoate (EB) in that lordosis quotients (LQ) were increased and rejection behavior decreased following administration of 2, 4 or 8 μg EB/kg/day for 3 days. Only TBULBX group rats were anosmic on 2 postoperative tests. TBULBX group rats showed very mild hyperresponsiveness on an emotionality test. Effects of TBULBX on LQ are not due to general sensory hyperresponsiveness or EB-induced hyperresponsiveness since no differences in the quality of lordosis occurred, and no differences occurred in latency to paw-lift on hot plate tests with or without EB. Heightened EB sensitivity in the TBULBX group is not due to adrenal steroids since following adrenalectomy and 8 μg EB/kg treatment, TBULBX group LQ scores were still elevated relative to those of SHAM controls. The LQ scores of PBULBX group rats were intermediate to those of SHAM and TBULBX group rats. Bulbectomy-induced alterations in sensitivity to EB as measured by the LQ do not appear to be due to alterations in “arousal” mechanisms in general. While deficits in olfactory perception might exacerbate the effect, it is unlikely that anosmia per se is sufficient to induce major alterations in the degree of sexual receptivity following EB. The magnitude of behavioral effects of bulbectomy on EB sensitivity may be related, to some extent, to the amount of bulb tissue removed. It is possible that bulbectomy may enhance behavioral sensitivity to EB by disrupting biochemical responses to EB in limbic system structures which normally exert an inhibitory influence over sexual receptivity.     

Effects of ovarian hormones on sexual receptivity, proceptivity, and motivation in olfactory bulbectomized female rats.

The purpose of the present experiment was to assess the effect of olfactory bulbectomy and ovarian hormones on female sexual motivation. Ovariectomized female rats underwent either bilateral bulbectomy or sham surgery. Females received one of four subthreshold hormone treatments: 0% estradiol (E2) plus 500 micrograms progesterone (P), 100% E2 alone, 10% E2 plus 500 micrograms P, or 100% E2 plus 500 micrograms P. Sexual motivation (as indicated by a female's preference for a sexually active male over a castrated male) and proceptivity (dart and ear wiggling sequences) were measured in a three compartment partner preference apparatus. Sexual receptivity (lordosis) was measured separately in a glass arena with a sexually active male. Results showed that olfactory bulb removal facilitates sexual receptivity and proceptivity in females exposed to 10% or 100% E2 in combination with 500 micrograms P. In contrast, sexual motivation was only demonstrated by olfactory bulbectomized females which received 100% E2 in combination with 500 micrograms P. These findings support the hypothesis that olfactory bulbectomy induces a behavioral hypersensitivity to estrogen, and suggest that sexual motivation is an estrogen-mediated response which requires a higher level of estrogen stimulation than sexual receptivity and proceptivity.     

Olfactory bulb removal: influences on the aggressive behaviors of male mice

Bilateral removal of the olfactory bulbs of castrated male mice completely prevented the arousal of aggressiveness by exogenous administration of androgen. Unilateralally bulbectomized mice showed fighting comparable to that shown by sham-operated control mice. It was concluded that earlier demonstrations of the abolition of intermale aggressive behavior in mice following olfactory bulb removal could not be attributed to impairment in pituitary-gonadal function. Although in this experiment bulbectomy completely prevented the androgenic arousal of intermale aggression, bulbectomy did not affect the display of aggressive behavior in a competition for food situation.     

Female and male sexual responses in female cats with ventromedial hypothalamic lesions

The effects of lesions in the ventromedial hypothalamus (VMH) on male and female sexual behavior in female cats were examined. Following preoperative testing for receptivity, proceptivity, and male mating behavior, 27 female cats received either lesions in the anterior or posterior portion of the VMH or sham lesions. Neither of the VMH lesion placements reduced proceptivity and receptivity scores. However, the female mating pattern was significantly altered in that although females in both lesion groups initially allowed a stud male to mount, they usually did not allow the male to intromit. As in rodents, the VMH in the cat appears to be an important neural area for the display of normal female sexual behavior. Low levels of male sexual behavior were seen in the three groups preoperatively, and no changes were observed in the levels of male behavior following lesion placement. Thus, because lesions that disrupted female behavior did not affect male sexual behavior, it appears that the neural areas controlling homotypical and heterotypical sexual behaviors are not necessarily neurally linked.     

Fluoxetine does not prevent interspecific mating between two hamster species

In a recent study we showed that female Syrian hamsters (Mesocricetus auratus) from a laboratory stock readily mated with male Turkish hamsters (M. brandti). We hypothesized that captivity and/or unconscious selection of the most receptive females by researchers or animal caretakers results in heightened female sexual receptivity and reduces the tendency to reject heterospecific males. To test this hypothesis, we decided to decrease female receptivity by injection of fluoxetine, which increases the levels of serotonin in a number of brain loci, including areas involved in mediating sexual behavior, and determine whether such a decrease in receptivity would result in better discrimination of heterospecific males and rejection of such males as mates. We treated estrous female Syrian hamsters with 20 mg/kg fluoxetine or vehicle and paired them with both a conspecific and a heterospecific male (Turkish hamster) in two sequential tests. All females showed similar behavior — they exhibited lordosis toward both conspecific and heterospecific males. We did not observe any sign of aggression. Latency to display lordosis and the duration of lordosis were not affected by fluoxetine. Fluoxetine thus did not have an effect on the behavior of females toward conspecific or heterospecific males. Independent of treatment (fluoxetine or vehicle injection), females did display lordosis significantly faster and for a longer duration in the presence of a conspecific male, even though these differences were small.     

Disruption of urinary odor preference and lordosis behavior in female mice given lesions of the medial amygdala

Previous research showed that axonal inputs to both anterior and posterior subdivisions of the medial amygdala from the main and accessory olfactory bulbs of female mice, respectively, process volatile and non-volatile pheromonal signals from male conspecifics. In the present study we found that bilateral electrolytic lesions that included posterior portions, but not the anterior subdivision alone of the medial amygdala (Me) blocked the preference of estrous female mice to investigate volatile urinary odors from testes-intact vs. castrated males. Similar results were obtained in separate tests in which nasal contact with urinary stimuli was permitted. In addition, total time investigating volatile urinary stimuli was reduced in subjects with posterior Me lesions. Subjects were able to discriminate volatile urinary odors from testes-intact vs. castrated male mice, suggesting that this disruption of odor preference did not result from the inability of females given amygdaloid lesions to discriminate these male urinary odors. Bilateral lesions of the Me that were either restricted to the anterior or posterior subdivisions, or included areas of both regions, caused significant reductions in the display of lordosis behavior in estrous female mice. Our results suggest that the Me is a critical segment of the olfactory circuit that controls both mate recognition and mating behavior in the female mouse.