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.     

Endogenous opioids and their role in odor preference acquisition and consolidation following odor-shock conditioning in infant rats.

We assessed the neurochemical basis of olfactory learning induced by presentations of odor and moderate shock in infant rats. Paradoxically, shock conditioning produces an odor preference in 8-day-olds, but an odor aversion in 12-day-olds. Studies have demonstrated the importance of opioids in early olfactory learning; their specific role remains undefined. In this study, postnatal Days 8 and 12 pups were systemically injected with naltrexone, a nonspecific opioid antagonist, or saline and received either paired or backward presentations of odor-moderate shock or odor-only presentations. Blocking the opioid system during conditioning disrupted acquisition of the Day 8 odor preference, but not the Day 12 odor aversion. Additional Day 8 pups were given naltrexone posttraining. Naltrexone not only blocked consolidation of an odor preference but also yielded an odor aversion. These results suggest that the opioid system has a critical role in both olfactory learning and consolidation of odor preferences during the sensitive period.     

The effects of olfactory experience on nest odor preferences and pup retrieval in rats

Previous studies have shown that maternal rats are attracted to their own bedding and that of other dams but not to that of virgins. The present studies were designed to test two hypotheses: First, that the addition of a novel botanical odor to the bedding of maternal animals would result in subsequent preferences for such an odor. Second, such a preference should enable dams to discriminate between pups odorized with familiar and unfamiliar odors as measured by differential retrieval of the pups. Both hypotheses were confirmed. In addition, we found that preferences established during a first pregnancy and lactation affected the retrieval of pups during a second lactational period, but could be overturned by new olfactory experience during that second lactational period. © 1993 John Wiley & Sons, Inc.     

Olfactory, thermal, and tactual influences on infantile ultrasonic vocalization in rats.

The amount of time infant rats 3-13 days old spent emitting ultrasounds was greater when pups were placed in an empty dish than when placed in a dish containing clean bedding or soiled bedding from the nest. Pups from 5-13 days of age vocalized more when on the clean bedding than when on the soiled bedding. When placed on cloth covered dishes, pups vocalized most to the empty dish and least to the dish containing soiled bedding. The odor of clean bedding elicited less ultrasonic vocalization than the odor of the empty dish only when less than 7 days old. Pups vocalized more to room temperature than to heated conditions. The results indicate that olfactory, tactual, and thermal nest cues influence infantile ultrasonic vocalization in the rat.     

The role of diet in mother-infant reciprocity in the spiny mouse

One-day-old spiny mouse pups responded preferentially to bedding soiled by lactating conspecifies fed the same diet as their mothers. Following this test, pups were fostered onto different-diet females. When retested at 84–96 hr of age, no preferences were shown for bedding soiled by a female fed the biological mother's diet vs bedding of a female fed the different diet. When tested again at 120–132 hr of age, however, the pups preferred the bedding associated with their foster mother's diet. In a 2nd experiment, recently parturient females retrieved 1-day-old pups born of same-diet females faster than pups born to novel-diet females. These results indicate that pup preferences for chemical cues produced by lactating conspecifics can be altered by sufficient exposure to a 2nd female maintained on a different diet and that neonatal chemical cues, like maternal chemical stimuli, are diet-dependent.     

Learned aversion to soiled bedding in male Mongolian gerbils

This study was designed to determine whether the conditioned aversion paradigm could be extended to aversion to soiled bedding in the male Mongolian gerbil. In Experiment 1, sixty subjects were exposed to a glass petri dish filled with soiled bedding placed in the center of a one-meter-square open field for two 5-min sessions 48 hours apart. Half of the subjects received an injection of lithium chloride immediately after session 1, while the rest received saline. The LiCl subjects learned to avoid the soiled bedding in session 2, as indicated by significantly longer latency to first approach and lower frequency of approach to the stimulus. Experiment 2 examined conditioned aversion to an empty petri dish. No aversion was formed, thus strengthening the argument that the gerbils were learning an aversion specific to soiled bedding. The research results demonstrate olfactory aversion of this type is possible in gerbils, and suggest that it may have an important influence on their social behavior.     

Maternal and paternal pheromones in gerbils

Mongolian gerbil pups are attracted to maternal and parental nest odors. They approach these odors in preference to the odors of either virgin females or clean bedding. The pups do not distinguish between maternal or parental odors and odors of unfamiliar lactating females. Thus lactating female gerbils produce a maternal pheromone similar to that of rats and other rodents. Attraction to the pheromone develops by two weeks of age and persists for at least three weeks. The primary source of the pheromone is the mother's enlarged ventral scent gland. Removing this gland eliminates maternal attractiveness. Development of attraction to maternal scent gland odors depends on prior exposure to them. Pups raised by mothers lacking scent glands do not approach lactating females that have scent glands. The father's presence during development alters the responses of gerbil pups to odors of other males. Pups raised by both parents avoid odors of other pairs with young, whereas pups raised by their mothers only do not. Pups are not attracted to their father's odors.     

Olfactory development in gerbil pups

In a 2-choice situation, Mongolian gerbil pups show a preference for home-cage bedding odor that appears during the 2nd postnatal week and is consistently high between Days 8 and 14. Pups show responses to certain nonanimal odors as early as Day 4, indicating that the motor capacity to orient to odors precedes the emergence of nest odor preference. Increases in pups' nest odor preference appear to parallel increases in the frequency of maternal nest-building, an activity which can eject pups from the nest. This coincidence suggests that olfaction helps displaced pups to return to their nests.     

Gabaergic control of olfactory learning in young rats.

Olfactory learning in young rats correlates with neural plasticity in the olfactory bulb, and involves noradrenergic modulation of reciprocal dendrodendritic synapses between mitral cells and GABAergic granule cells. The purpose of this study was to examine, in vivo, the consequences of manipulating bulbar GABA transmission during training. In the first experiment, postnatal day 11 rat pups were trained in an olfactory associative learning task with citral odor and foot shock as the conditioned and unconditioned stimuli, respectively. The pups received continuous infusion of saline or the GABA(A) receptor agonist muscimol into the olfactory bulbs throughout a 30-min training session. The pups were then tested on postnatal day 12 for a preference for or an aversion to citral odor. Saline-infused control pups developed an aversion to citral odor. The GABA(A) receptor agonist muscimol impaired this aversive learning in a dose-dependent manner. In the second experiment, pups were exposed to the odor for 30 min while receiving continuous intrabulbar infusion of a low or high dose of the GABA(A) receptor antagonist bicuculline, without any other reinforcer. Depending on whether a low (0.2 nmol/bulb) or high (1.0 nmol/bulb) dose of bicuculline was infused, the pups showed a preference or an aversion for citral odor after infusion of low and high doses, respectively. These results indicate that disinhibition of mitral cells in the olfactory bulb is critical for olfactory learning in young rats, and suggest that the degree of disinhibition is an important determinant in acquiring either preference or aversion for the conditioned odor.     

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.     

Female discrimination of male odors correlated with male genotype at the T locus: A response to T-locus or H-2-locus variability?

Female house mice (Mus musculus), derived from several populations of wild-caught mice, were tested for their ability to discriminate between males whose genotype at the T locus was +/+ and those whose genotype was +/t, using odor cues alone. Females spent more time near the odors or +/+ males than near the odors of +/t males. This preference was independent of the T-locus genotype of the female and the particular type of t allele carried by either the male or the female. A female's preference, however, did appear to be related to the genotype of her parents. Females with one +/t parent were more likely to prefer +/t males than were females whose parents were both +/+. In a second experiment 18 females were tested with odors from soiled bedding of recombinant males whose genotype varied at the T locus but who were similar at the H-2 locus. As a control, these 18 females were also tested with bedding of wild-derived +/+ and +/t^{w semilethal} males. Females tested with recombinant males preferred odors of males not carrying lethal t alleles over those of males carrying two lethal t alleles, indicating that T-locus variability, not H-2-locus variability, is responsible for odor differences between +/+ and +/t males. Female responses to odors of recombinanat males did not differ from those to odors of +/+ and +/t^{w semilethal} males. Responses of mice to odor differences associated with T-locus variability may have evolved independently of responses to odor variability associated with the H-2 locus.This work was supported by NIH Grant HD 15997-01.     

Preferences of pre- and post-weanling long-evans rats for nest odors

Three experiments were conducted on the preferences of infant rats for nest odors. Pre-weanling rats preferred their own nest odor to nests of strange litters and showed low preferences for the nest odors of virgin females. Nest odors of unmated males were investigated less than the pup's own nest odor but more than the odors of virgin females. Male and female rats did not differ in odor preferences. Rats reared with both their dam and sire did not differ in their odor preferences from rats reared with their dam alone when tested at 16–20 days of age, but when tested at 33–38 days of age they showed a greater preference for male odors. Post-weanling rats did not show a preference for the nest odors of lactating females and began to show sex differences in their preferences.     

Temporal parameters of responsiveness to maternal pheromone in Acomys cahirinus

In a series of two-choice preference tests, 15-and 20-day-old A. cahirinus pups were found to prefer chemical cues produced by a lactating female over such cues emanating from a nonlactating female. Twenty-five-day-old pups, however, showed reliable preferences for bedding material soiled by a nonlactating female when paired against either lactating female-soiled bedding or clean bedding material. Although 25-day-old pups are no longer responsive to chemical stimuli produced by lactating females (maternal pheromone), 2-day-old pups are attracted to such cues emanating from lactating females as late as 35 and 38 days postpartum. Therefore, there does not appear to be close temporal synchrony between the waning of responsiveness to maternal pheromone and the production thereof.     

Influence of the odorization of the rearing environment on the development of odor-guided behavior in rat pups

The study describes a phenomenon in which attraction of rat pups to artificial odors disappeared after the first week of life. In experiment 1, pups were continually exposed to either: (1) the normal odors present in the litter or (2) the same odors enriched with one of four artificial odors present in the dam's food. Pups were tested daily with the odors of normal or adulterated soiled shavings from their nests. The results show that attraction to the normal test shavings lasted throughout the testing period (PN 1-7). However, pups raised on odorized shavings exhibited an attraction to the artificial odor until day 6 only, not on day 7. In experiment 2, pups were tested with dam's artificially adulterated food. The results show that the artificial odor, and not the food odor, was responsible for the lack of attraction on day 7. Experiment 3 was carried out to determine whether the date on which attraction to artificial odors changed might be specific to postnatal day 7 or whether the duration of odor exposure in test conditions was the important factor. The results suggest that the age of pup is the more important variable.     

Centrally and peripherally induced anosmia: influences on maternal behavior in lactating female rats.

Bilateral removal of the olfactory bulbs was found to produce deficits in several components of the maternal behavior of lactating, primiparous female rats. More dead pups were found in the cages of bilaterally bulbectomized females shortly after parturition than in the cages of unilaterally bulbectomized, sham operated, or unoperated females. Futhermore, bilaterally bulbectomized females showed deficits in pup retrieval relative to females in the other surgical conditions, and pups reared by bilaterally bulbectomized females weighed less at weaning than pups reared by females in the other surgical conditions. In contrast to bilateral bulbecomy, zinc sulphate treatments had no influence on pup mortality at parturition. However, zinc sulphate treated females did exhibit deficits in pup retrieval relative to saline treated females.     

Development of sex differences in the response of spiny mouse pups to adult male odors

Using a three-choice preference test, olfactory-mediated investigatory activity in response to adult male urine odor was examined in a precocially active rodent, the spiny mouse (Acomys cahirinus) aged between 3-26 days. Temporally related sex differences were seen in the time spent in the presence of the odors of father's or unfamiliar adult male's urine, or distilled (control) water. Neither male nor female pups discriminated between odors from the father and strange adult males. After the first olfactory test, when the pups were aged between four and six days, male pups strongly preferred to stay in the vicinity of urine odors of adult males, whereas female pups avoided odors of adult males and remained in the enclosure with the control odor source. To our knowledge this is the first time that such a behavioral sex difference related to olfaction has been shown to occur in young rodent pups. We suggest that the sexually dimorphic response of the pups is associated with the development of later sex differences in behavior.     

Consolidation and expression of a shock-induced odor preference in rat pups is facilitated by opioids

To support nipple attachment and huddling, rat pups must learn to approach and prefer maternal odor. Similar to other altricial species, rat pups have a sensitive period for learning this odor preference, which ends around postnatal day (PN) 10 and coincides with the emergence of walking. One characteristic of this sensitive period is that an odor paired with moderate shock elicits an odor preference. After PN10, this behavioral training produces an odor aversion, although pain threshold remains unchanged. Recently, we demonstrated that the endogenous opioid system might be a key element in the acquisition of the shock-induced odor preference during the sensitive period since antagonism of this system disrupts odor preference learning. In older pups, acquisition of a shock-induced odor aversion was unaffected by opioid system manipulation. The purpose of these experiments was to further elucidate the role of opioids in infant olfactory learning through assessment of memory consolidation and expression during and after the sensitive period. In Experiment 1, we demonstrate that naltrexone (NTX), a nonspecific opioid antagonist, given immediately following odor-shock conditioning during the sensitive period, blocks odor preference formation and yields an odor aversion. However, the same treatment does not disrupt consolidation of an odor aversion in older pups. In Experiment 2, we demonstrate that during the sensitive period, NTX disrupts expression of the shock-induced odor preference, but not the learned odor aversion in older pups. Results using this model of attachment suggest that opioids have an important role in the acquisition, consolidation, and expression of early olfactory preferences. Furthermore, since prenatal drug exposure is known to alter the endogenous opioid system, these results highlight the capacity of prenatal opiate exposure to disrupt early infant learning and attachment.     

Dietary influences in the preferences of pre-weanling Long-Evans rats for the anal excreta of adult males

Male rats which ingest feces of lactating females produce anal excreta which is attractive to pups. This excreta is as attractive as that of lactating females and more attractive than the excreta of males eating a diet of lab chow only. Anal excreta of males which consume feces of lactating females is more attractive to pups than excreta of males that consume feces of nonlactating females. These results suggest that group or colony odors may develop from rats eating feces of other colony members and that infant rats may be more attracted to adults that have an odor which is similar to the maternal odor than adults which have different odors.     

Effects of cycloheximide on the retention of olfactory learning and maternal experience effects in postpartum rats

We have previously shown that cycloheximide, a protein synthesis inhibitor, blocks retention of experimentally based material responding in postpartum rats. In the following study, we investigated further the effect of this drug on maternal experience effects and, in particular, on the retention of olfactory learning. Dams were injected SC with cycloheximide or saline following a one-hour maternal experience with pups scented with one of two artificial odorants. When tested eight days later, saline but not cycloheximide animals demonstrated a preference for the odor which they had experience on pups. Animals were then tested for maternal responsiveness with pups that were scented with an odor that was either the same or different from the one used in the maternal experience. All animals receiving cycloheximide, regardless of the pup odor at testing, exhibited long latencies to become maternal, replicating our earlier finding that the retention of learning in maternal behavior is susceptible to disruption by protein synthesis inhibitors. A similar disruption of the experience effect was found when the pup odor at testing was different from the exposure odor. This suggests that olfactory learning may normally play a role in the mediation of postpartum experience effects.