Stimulus control of the maternal pheromone in the lactating rat

Chemical Identification of "Maternal Signature Odors" in Rat

Newborn altricial mammals need just after birth to locate their mother's nipples for suckling. In this precocious behavior, including for the human baby, maternal odor via the olfactory process plays a major role. Maternal odor emitted by lactating females or by amniotic fluid (AF) attracts pups, but the chemical identity of this attractant has not yet been elucidated. Here, using behavioral tests and gas chromatography coupled with mass spectrometry (GC-MS) techniques, we show that AF extracts from rat pregnant female, nipples, ventral skin, milk, and nest extracts of mother contained 3-6 active substances. AF extracts contained 3 active compounds: ethylbenzene, benzaldehyde, and benzyl alcohol, and their mixture in similar proportions to those found in AF extracts, in a ratio, respectively, of 1:1:12 (700 ng), attracts pups as putative maternal attractant substances (MAS). These 3 AF substances have already been identified in milk, nipples, ventral wash, and nest extracts of mother, but not in feces. Moreover, anethole flavor incorporated in pregnant rat and mother's diet is also detected in AF, nipples, milk, and nest extracts and the pups are attracted to anethole odor, but not in the case of the no-anethole pups. MAS, combined with diet flavors present in the AF bath, represent olfactory signals as "maternal signature odors" (MSO) that are learned by fetus and pups. These findings open the way to improved understanding of the neurobiology of early olfactory learning and of the importance of evolutionarily conserved survival behavior in many mammal species.

Pituitary--adrenal responsiveness of rat mothers to noxious stimuli and stimuli produced by pups

Studies on the response of female rats to various stresses during the course of lactation showed a marked reduction in pituitary-adrenal activity. Maximum suppression coincided with the period of maximum lactation, about 14 days after parturition. Both pituitary corticotropin and the release of corticosterone were significantly reduced. Females showing maternal behaviour but not lactating did not exhibit this buffering of the stress response. Another aspect of pituitary-adrenal activity has been studied in lactating female rats. Females, although they have a buffered stress response, responded differently, in terms of pituitary-adrenal activity, to stimuli emitted by the pups: pups which are exposed to noxious stimuli elicit a much greater pituitary-adrenal response in lactating females than do pups which are merely handled. Separation from the pups does not elicit a pituitary-adrenal response. This difference in response is modified if the infants are malnourished and is not observed in virgin animals that are not lactating.

MPEP, an antagonist of metabotropic glutamate receptors, exhibits anticonvulsant action in immature rats without a serious impairment of motor performance

An antagonist of type I metabotropic glutamate receptors MPEP was found to exhibit anticonvulsant action in adult rodents. Present experiments were focused on action of this drug against pentetrazol-induced motor seizures in immature rats 12-, 18- and 25-days old. Dose of pentetrazol (100 mg/kg s.c.) was chosen to elicit minimal clonic seizures and (after a longer latency) generalized tonic-clonic seizures. Pretreatment with MPEP (doses from 10 to 80 mg/kg i.p.) resulted in a dose-dependent suppression of the tonic phase of generalized tonic-clonic seizures in all age groups studied. Efficacy of MPEP was higher and the effect lasted longer in 12- than in 25-day-old rats. In addition, minimal clonic seizures were suppressed in 18-day-old rats. Motor abilities of immature animals were not compromised by MPEP in doses of 20 and/or 40 mg/kg i.p., only righting reflex was a little slowed down in 12- and 18-day-old rats. In contrast to antagonists of ionotropic glutamate receptors anticonvulsant doses of MPEP do not induce unwanted side effects in motor performance of developing rats.

Manipulations during the second, but not the first, week of life increase susceptibility to cocaine self-administration in female rats

We compared the effects of manipulations during week 1 vs week 2 of life on the propensity to self-administer cocaine. Pups received daily subcutaneous saline injections, were handled briefly, or remained undisturbed during their respective treatment periods. Animals handled during the second week of life exhibited increased locomotor response to novelty when tested on postnatal day (PND) 48, compared to all other groups. Rats were implanted with jugular catheters on PND 70 and then given the opportunity to self-administer (0.125 mg/kg/infusion) cocaine for 5 consecutive days (1 h sessions). The dose was then raised to 0.25 mg/kg/infusion for 5 days and to 0.5 mg/kg/infusion for the final 5 days of testing. Only animals manipulated during the second week of life acquired drug-taking behavior. These effects were both stimulus- and gender-specific. Females handled during the second week of life acquired cocaine self-administration (SA) at the lowest dose, and females injected during the second week of life acquired at the intermediate dose. Males injected during the second week of life showed a similar, but more variable, drug-taking pattern. There were no group differences in serum corticosterone response to novelty, although relative to undisturbed animals and those manipulated in the first week of life, female animals manipulated during the second week of life had lower basal expression of hippocampal glucocorticoid receptor mRNA in adulthood. We conclude that the second week of life in the rodent is a sensitive period during which manipulations result in a more vulnerable phenotype for the acquisition of cocaine SA.

Neuroethology of olfactory preference development

Young mammals come to approach the odor of their mother, a response that facilitates their survival during early life. Young rats induce a cascade of events in their mother to induce the emission of her odor. The pups increase circulating prolactin levels, which increases food intake and the emission of large quantities of cecotrophe containing the maternal odor. This odor is synthesized by the action of cecal microorganisms and changes with maternal diet. The diet-dependence of the odor requires the pups to acquire their attraction to the odor postnatally. The acquisition of this preference occurs when an odor is paired with the tactile stimulation that pups receive during maternal care. The action of the tactile stimulation appears to be mediated by noradrenaline. The development of this type of olfactory attraction is accompanied by changes in the regions of the olfactory bulb that are responsive to the attractive odor. Metabolic, anatomical, and neurophysiological changes in response to the attractive odor emerge in such regions of the bulb after early olfactory preference training.

Familiar contextual odors promote discrimination learning in preweanling but not in older rats

For rats 16- or 28-days-old postnatal, we tested the source of the facilitation in instrumental learning provided by odors from home nest materials. Three experiments confirmed that acquisition of a spatial discrimination (to escape footshock) was facilitated not only by the presence of typical nest odors in the training context, but also by the presence of a non-rat odor (banana) to which the animals had been familiarized for 4 hr/day for 7 days. There was a borderline tendency for a similar facilitation after familiarization to the latter odor for only 10 min. These effects occurred for rats 16 days postnatal but not for those 28 days old. The experiments also confirmed that the prior exposure to non-rat odors did not in itself, in the absence of that odor during learning, affect discriminated escape learning, and that enhanced affinity for a contextual odor is not a sufficient condition for enhancement of learning in its presence. There was some indication that the 16-day-old rat was more likely to select a discriminative odor to guide their choice of spatial locations if a familiar contextual odor was present.

Development of olfactory bulb organization in precocial and altricial rodents

The structural organization of the olfactory bulbs of spiny mice, Norway rats and Mongolian gerbils was followed over the course of their development. The pups of all 3 species normally begin to approach the odor of their dams at a time when their olfactory bulbs are at a similar stage of development. The data suggest that there may be a common aspect of olfactory bulb development that underlies the onset of olfactory guided approach behavior in rodents.

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.

Maternal pheromone in Rattus norvegicus

A replication was undertaken of the finding that rat pups between 14 and 28 days of age will approach an attractant (maternal pheromone) contained in the excrement of lactating female rats. Preweaning pups from three strains of Rattus norvegicus (hooded PVG/C, Wistar, and Sprague-Dawley) were tested under different apparatus conditions, with different stimuli, and with different pretest experience. Despite attempts to maximize the attractant olfactory properties of stimulus material no clear evidence of maternal pheromone could be demonstrated. The use of olfactory cues, and an ability of rat pups to discriminate maternal odor, were shown. But such behavior falls short of that which would merit the use of the term pheromone.

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.

The maternal pheromone and bile acids in the lactating rat

Bile was drawn from virgin rats and from postpartum rats that were with young for 5, 12, 21, and 30 days, respectively. The bile thus drawn was analyzed enzymatically after chromatographic separation to test an hypothesis relating cholic acid and one of its metabolites, deoxycholic acid, to the appearance of the maternal pheromone. Our finding that cholic acid, but not deoxycholic acid, reached a peak that was tied specifically to the period of pheromonal emission led us to advance a revised hypothesis. We now think that cholic acid alone, or more likely a cholic metabolite other than deoxycholic acid, underlies the appearance of the pheromone.

Effects of the home environment on withholding behaviors and conditioning in infant and neonatal rats

Rats 16 days old received passive-avoidance training in the presence or absence of home litter cues. Rats trained in the context of home litter cues learned the passive avoidance reliably faster than rats trained in isolation. In the presence of home litter cues, 16-day-old rats also exhibited more adultlike spontaneous alternation. Pavlovian conditioning of rats trained at 2 days of age was studied in the presence and absence of conspecifics. These experiments suggest that deficiencies in inhibitory behaviors and conditioning associated with immaturity can be alleviated when the testing environment is made more similar to the home environment.

Bile, prolactin, and the maternal pheromone

When bile from females that had been lactating for 21 days was injected into the cecum of male rats it induced release of a maternal pheromone. Males injected with bile drawn from females in which prolactin had been inhibited, or from females that had been lactating for only 5 days, did not emit the pheromone. These data suggest a sex difference in the way prolactin alters the composition of bile so that the female can emit the maternal pheromone while the male normally cannot.

Maternal pituitary-adrenal responsiveness as a function of differential treatment of rat pups

The effects of litter separation and pup treatment on the maternal pituitary-adrenal system were investigated in 3 experiments. Lactating females did not show a pituitary-adrenal response to separation from their pups. However, the lactating females showed an increase in plasma corticosterone when their pups had been briefly removed and then returned. If, in addition, the pups were subjected to a noxious stimulus (electric shock) during the 2-min separation, mothers showed a further increase in corticosterone. When pups were returned after 3 hr of separation, mothers again showed a differential pattern of corticoid responsiveness. The magnitude of the mother's pituitary adrenocortical response depended upon the intensity of treatment given to the pups. These data lend physiological support to behavioral studies which have shown that maternal behavior is differentially influenced by pup-produced stimuli.

The responses of one-day old Acomys cabirinus pups to naturally occurring chemical stimuli

The role of olfactory stimuli in the mediation of the behavior of 26--36 hour old spiny mice (Acomys cabirinus) was assessed through a series of six experiments. The major results of these experiments are: 1. One day old spiny mice showed a preference for soiled bedding from their own home-cage as compared to clean bedding in a simultaneous two-choice preference test. 2. No significant preference was displayed for home-cage bedding vs. bedding from the cage of a conspecific litter and parents. 3. Soiled bedding from the home-cage was preferred over soiled bedding from the cage of a nulliparous adult female conspecific. 4. A reduced mean latency to leave an area of clean bedding was found in comparison to mean latencies to leave soiled bedding from the subjects' home-cage or from the cage of a conspecific litter. 5. No differential activity levels were displayed on home-cage vs. clean bedding. It was concluded that by the second day of life, Acomys pups have developed attachment to chemical stimuli produced by specific classes of conspecifics--and that the early appearance of such attachments is a function of the high degree of sensory and motor precocity of the Acomys pups at birth. The adaptive value of such rapidly developed attachments in precocial as contrasted to altricial species was discussed.