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Olfaction in utero: Can the rodent model be generalized? a

B. Schaal & P. Orgeur



CNRS, Paris, France and INRA , Nouzilly, France Published online: 29 May 2007.

To cite this article: B. Schaal & P. Orgeur (1992) Olfaction in utero: Can the rodent model be generalized?, The Quarterly Journal of Experimental Psychology Section B: Comparative and Physiological Psychology, 44:3-4, 245-278 To link to this article:

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Olfaction in Utero: Can the Rodent Model be Generalized? B. Schaal and P. Orgeur Downloaded by [Florida International University] at 04:30 19 December 2014

CNRS, Paris, France and INRA, Nouzilly, France In this article we discuss five requirements that theoretically must be fulfilled for transnatal chemosensory learning to occur in three placental species, the rat, the sheep, and the human, viz. (1) minimum or partial maturity of nasal chemoreceptor systems, (2) efficient odorivector compounds in the fetal environment, (3) the ability to memorize chemosensory information across birth, (4) perinatal continuity in chemical signals, (5) neonatal ability to detect air-borne odorants previously experienced in the aquatic environment. A substantial body of data is reviewed for the rat, in which fetal chemosensation is now firmly established. The less studied ovine perinate also shows preliminary evidence of nasal chemoreception and of postnatal retention of prenatally experienced odorants. Concerning the human species, we discuss extensive anatomical data supporting nasochemoreception in utero, but as yet no direct or indirect functional demonstration is provided. Furthermore, the strongest evidence of odorivector compounds in amniotic fluid is from human data. The results presented allow generalization of chemosensory functioning in utero in the species considered.

An increasing body of data demonstrates that the learning abilities of the newborn organism do not commence with birth. It is delivered to the postnatal world with earlier sensory experience, which can result in the biasing of neonatal behaviour. The paradigmatic example of such prenatal preparedness is the transnatal continuity in chemoreception mechanisms in the rat. This early continuity optimizes vital responses of the newborn animal to specific environmental tasks, e.g. locating and seizing a lactating nipple. The complex processes underlying this chemosensory thread can therefore be considered as an ontogenetic adaptation (Oppenheim, 1981; Requests for reprints should be sent to Benoist Schaal, Laboratoire de Psychobiologie de I’Enfant, EPHE-CNRS URA 315, 41 rue Gay-Lussac, 75005 Paris, France. This research was supported by grant no. 8960388 of the Ministtre de la Recherche et de la Technologie, by URA 315 (Psychobiologie de I’Enfant) and URA 1291 (Physiologie de la Reproduction) of the Centre National de la Recherche Scientifique. The authors are grateful to P. Hepper and to two anonymous referees for their comments on an earlier draft and to A.Y. Jacquet and C. Kervella for their generous assistance.

0 1992 The Experimental Psychology Society

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Smotherman & Robinson, 1986), as they serve to improve the ability of the organism to survive and develop normally during a specific transition in development. Is the prenatal acquisition of cues having a postnatal signal value a mechanism of ontogenetic adaptation generalizable to species other than the murine rodents? It can reasonably be expected that shared constraints during the natal transition should determine isomorphic optimizing trends in sensory, motor, and physiological responses in the perinates of other eutherian mammals. To survive, they all must swiftly develop an efficient suckling pattern in order to gain alimentary fuel and passive immunization. If some of the features of perinatal environments are submitted to a shared source of variability, then the postnatal ecology will be endowed with predictable properties. As Hofer (1988, pp. 7-8) puts it, the function of maternal regulations over the fetal chemical ecology might be to preadapt the fetus to otherwise non-predictable features of the postnatal environment. Mammalian females may therefore have been evolutionarily driven to create specialized sensory cues concurrently with newborns equipped to detect and integrate them. Many structural and functional similarities found in the neonate-mother interactions of mammals (e.g. Moltz & Rosenblum, 1983) may lead one to extrapolate findings revealed in one species to another, yet undocumented, one. This generalization perspective does not appear to be illicit, because structures and processes at work in the ontogenetic process under scrutiny are most probably homologous (for chemoreceptors, cf. Graziadei, 1977; Halasz, 1990; for placental physiology, cf. Steven, 1975). Marked interspecific differences might, however, seriously limit the validity of inferences from one placental species to another. Some lines of specific differences relevant to the chemical senses in the perinatal period are: (1) the gestation span, which may affect the duration of prenatal exposure to chemical stimulations; ( 2 ) the sensory dominance at birth, which may modulate the behavioural usefulness of chemical stimuli in the perinatal environments; (3) the combination of chemoreceptors present in the nasal passages, which conditions what is being detected; (4) differing structural and functional properties of the placenta may affect the openness of amniotic environment to external influences; (5) maternal direct or indirect contribution to the olfactory overlap between pre- and postnatal environments can differ across species; (6) the vocality of the species may provide cues that can implement, or even supplant, an olfactory tether by an acoustic one; (7) genetic and experiential determinants of early odour preferences can be balanced differently. Despite these potential interspecies differences, all mammalian fetuses possess structurally sophisticated nasochemoreceptive systems; nevertheless, chemosensory function has been documented in very few, with the

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exception of the rat. At least five requirements are to be fulfilled for prenatal odour acquisition to occur in any placental species, and for this to influence postnatal behaviour: (1) chemoreceptive systems must reach sufficient maturation to support minimal functional activity; (2) efficient chemical stimuli must be present in the fetal environment; (3) the fetal brain must be able to store chemosensory information across the birth episode; (4) similar chemical signals must be present in the fetal and neonatal environments; ( 5 ) the neonate must show detection abilities for air-borne chemical signals previously detected in the fetal aqueous medium. The present essay is intended to weigh how far the perinatal chemosensory adaptation described in the rat can be extended to other mammalian species. If much has been accomplished for murine rodents (viz. rat and mouse), it is appropriate to assess the present knowledge we have for other species. The discussion will centre first around the rat as a heuristic reference, and then around two species for which comparative data are increasingly available: the sheep and the human. The five requirements emphasized above will be taken as an analytic device to structure existing material related to perinatal chemoreception in these three species. The focus of the discussion will mainly be on nasal chemoreceptors.


I. THE RAT MODEL 1. Functional Status of Fetal Nasal Chemoreceptors At least five anatomically distinct chemoreceptive subsystems can be found in the nasal fossae of rodents: the main olfactory (including the main glomerular complex), vomeronasal, trigeminal, septal, and terminal subsystems (cf. Meredith, 1983). The maturational schedule of these subsystems is heterogeneous, but it is largely completed in ufero,although it also continues long after birth (Meisami, 1989). Several excellent reviews have surveyed the prenatal growth of chemoreceptors in murine rodents (Brunjes & Frazier, 1986; Farbman, 1986; Pedersen, Greer, & Shepherd, 1986; Shepherd, Pedersen, & Greer, 1987), and the reader is referred to them for more details. Taken together, these data show that nasochemoreceptors are structurally advanced and have the capacity to detect chem'Following Smotherman and Robinson (1988a), several facts suggest that nasal chemoreception prevails over oral chemoreception in fetal chemical responsiveness. For example, stimuli having low olfactory power (sucrose or quinine hydrochloride) do not trigger reliable motor activity changes in fetal rats, whereas a compound with low gustatory power (citral) elicits intense fetal response. In the same vein, gustatory stimuli directly introduced into the amniotic fluid did not induce clear changes in the swallowing activity of the sheep fetus (Mistretta & Bradley, 1977).

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ical stimuli in a fluid medium towards the end of gestation. From day E20 (20th day of gestation), externalized rat fetuses also react (but more weakly) to odorants delivered in a gaseous medium (Smotherman & Robinson, 1990a). Tests performed on prematurely delivered pups further attest to the “amphibious” nature of chemoreceptive competence in rat fetuses. When prematurely delivered (Day E20) and isolated during the first hour in either amniotic fluid (AF), mint, or control atmosphere (Smotherman, Robinson, La VallCe, & Hennessy, 1987), higher motor activity and survival rates were noted in pups exposed to AF odour as compared to the mint odour. This outcome is interpreted as a consequence of disrupted psychobiological balance of the pups by an unfamiliar odour. It is thus clear that the fetuses of murine rodents possess nasal chemoreceptors ready for functioning from the end of the gestation period. Which olfactory stimuli are they exposed to in ufero?

2. Amniotic Ecology Both the mother and the fetus contribute to the amniotic pool, which directly bathes the fetal chemoreceptors (Liley, 1972; Renfree, Hensleigh, & McLaren, 1975). The molecular weight cut-off for olfactory efficacy in air being around 400 d and placental barrier being easily crossed by molecules until 1,OOO d (Seeds, 1980), it can be expected that virtually all stable aerial odorivector compounds are transferable to the fetus. The majority of studies of placental transfer have, however, been conducted with the aim of modelling homologous human mechanisms and not for a better understanding of the fetal chemical environment per se. Hence, to the best of our knowledge, no studies have focused on the placental transfer of odorigenic substances. Indirect evidence of maternal diet-mediated influence is provided by an experiment by Hepper (1988), wherein pups of dams fed with raw garlic during gestation displayed a preference for garlic flavour at 12 days of age. The pathway through which aromatic compounds gain access to the chemoreceptors and the chemoreceptor subsystem concerned remains uncertain. The most evident access route is via the fetal external milieu, i.e. by AF-borne chemicals flowing through the nasal-retronasal passages. When injected into the AF surrounding mice fetuses (at Day E18), fluorescent microparticles were absorbed in the whole nasal cavity; by contrast, no evidence of fluorescence was found in the vomeronasal organ (VNO). A closer examination revealed that the VNO lumen is not patent in fetal mice, but it is in fetal rats (Coppola & O’Connell, 1989), and so different chemosensory subsystems might thus be activated prenatally in such closely related species. It is known that fetal rats swallow (Lev & Orlic, 1972) and inhale AF (Becker, King, Marsh, & Wyrick, 1965), and thus may renew

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the A F surrounding the chemoreceptors. But two hindrances might slow down the access of putative odorants to chemodetectors: (1) The volume of A F decreases and its viscosity increases with advancing gestation (Marsh, King, & Becker, 1963), two changes that could reduce the efficacy of A F as odorant carrier. (2) The olfactory mucosa is progressively embedded in the aqueous mucous layer (visible around Day El8 in rats; Farbman, 1986); this mucous sheet should not disturb the stimulation process, however, as both media presumably share similar diffusion rates. The other path to the nasochemoreceptors might be the hematogenic way. Maruniak, Silver, and Moulton (1983) have demonstrated that the intravenous injection of an odorant is followed by an electro-olfactographic (EOG) response after several seconds in the adult rat. But as yet the hypothesis of hematogenic chemosensation has not been tested empirically in the fetus.

3. Fetal Memorization of Chemosensory Information Since the initial studies published in 1982 by Blass’s and Smotherman’s groups, several studies have confirmed that prenatal exposure to odorous compounds can result in long-term memories. Most experiments used an a posteriori strategy to demonstrate fetal acquisition by testing postnatal retention of prenatal chemical treatment. A first group of studies applied aversive conditioning techniques (for references, see Table 1). Briefly, this procedure demonstrated that rat pups exposed to an odour cue injected in utero contingently with a noxious stimulus [lithium chloride (LiCl) injected into the peritoneum] avoid any substrate bearing that odour in choice tests. Control pups exposed to saline-LiCI association did not show avoidance of the target flavour. The acquired aversion was specific to the aroma experienced in utero, i.e. it did not generalize to other odorants. This aversive effect was shown in different situations where the pups could sense the odour source either by both oral and nasal chemoreceptors (nipple search tests), or by exclusive nasal chemoreceptive inputs (open-field, running maze tests). A second group of studies used a less intrusive strategy by which nearterm fetuses were merely exposed to an artificial odorant after its injection into the amniotic sac (Table 1). At delivery, these pups were given to a foster dam, so that they could not experience the target odour potentially transferred to the mother’s milk. When exposed to citral prenatally by intra-amniotic injection, one-hour-old rat pups attached with short latency to unwashed nipples in a citral-saturated atmosphere, but they did not seize washed or olfactory intact nipples in the absence of citral vapours (Pedersen & Blass, 1981, 1982). The expression of this early preference was, however, conditional on a short postnatal exposure to citral contingently with tactile or pharmacologic activation. A similar experiment in


SCHAAL AND ORGEUR TABLE 1 Summary of Experiments Showing Postnatal Consequences of Prenatal Chemosensory Experience in the Rat, Indicating Odorants Used, Fetal Exposure Ages, Postnatal Ages of Choice Test and References

Prenatal Treatment

Postnatal Consequences

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I . Conditioned flavour aversion AF odorized with apple at E20 (IAinj. + IPLiCl inj.)

P10: behavioural inhibition when apple odour suffused in test chamber (Smotherman, 1982a).

AFodorized with apple at E20 (IA inj. IP LiCl inj.)

P16: aversion to nipples painted with apple odour (Stickrod et al., 1982a).

AF odorized with apple at E20 (IAinj. + IPLiCl inj.)

P16: aversion to apple odour in an openfield test (Stickrod et al., 1982b).


2. Mere exposure to an artijicial odorant

AFodorized with citral at E20 (IA inj.)

Ih: nipple seizing when citral in atmosphere (Pedersen & Blass, 1982).

AFodorized with ethanol at E l 9 (IA inj.)

P8: preference for ethanol odour in an open-field test (Chotro & Molina, 1990; exp. 1).

AFodorized with ethanol at El9 (IA inj.)

P9: ingestive preference for ethanolflavoured solution (Chotro & Molina, 1990; exp. 2).

AFodorized with lemon or orange at E l 9 (IA inj.)

Preference for lemon (PS) or orange (P12) odour in an open-field test (Chotro & Molina. 1990;Hepper, 1990).

AF odorized with garlic during gestation (maternal ingestion)

P12: preference for garlic odour in doublechoice test (Hepper, 1988).

AF odorized with apple at E20 (IA inj.)

P60: ingestive preference for apple’ flavoured solution (Smotherman, 1982b).

Abbreviations: AF: amniotic fluid; IA: intra-amniotic; inj.: injection; IP: intraperitoneal; E20: embryonic day 20; P10: postnatal day 10.

mice resulted in reduced avoidance to nipples coated with the prenatally experienced odour of vanilla (Kodama, 1987). Previous attempts of olfactory familiarization in fetal rats using vanilla did not bring about significant results, however (Blass & Pedersen, 1979), suggesting that all odorants are not equally efficient. Finally, similar results were obtained when fetal rats were exposed to aromas ingested throughout gestation by the dam (Hepper, 1988): the pups showed a locational preference for the familiar odour (garlic) when presented along with a novel odour (onion). These experiments demonstrate that the rodent fetus is able to detect and retain transnatally the chemical features of its manipulated amniotic

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environment. The range of different odorants memorized also suggests that the fetal brain is able to integrate varied olfactory qualities. This fetal learning is keenly selective, as it allows neonatal discrimination of sometimes closely related aromas (e.g. garlic vs. onion). The time of prenatal exposure necessary for inducing an appetitive effect in neonates ranges from periods of 2-3 days to periods as brief as 10 min (e.g. Chotro & Molina, 1990). Aversion conditioning is obtained after a single C S U S pairing episode. Both fetal appetitive and aversive odour learning can persist for relatively long periods, sometimes until weaning and even into adulthood (Table 1). Odour cue acquisition sessions and retention tests performed between two fetal periods add further information to the features of the chemical stimuli to which fetal rats have access. Rat fetuses that received aversive conditioning at El7 could differentiate at E l 9 the conditioned stimulus from various test stimuli dissimilar in intensity or quality (Smotherman & Robinson, 1987). A further experiment indicated that repeated intraoral injections of lemon odour induced a progressive waning of behavioural responsiveness in E19-day old fetal rats, indicative of habituation-like processes (Smotherman & Robinson, 1988a). Additionally, when compared for their behavioural response to a mint solution infused in the mouth, is, presented with mint for the first timenaive fetuses (at day E19)-that exhibited a sharp and transient movement suppression, whereas preexposed fetuses-that is, having had mint injected in their A F two days earlier (day E17)-did not show this pattern of response (Smotherman & Robinson, 1988a). It is thus evident that rat fetuses can discriminate between chemical stimulations along the familiarity-novelty dimension. These findings support the view that the chemoreceptor systems of the rat fetuses have the competence to extract the main informational content of chemical stimulations, i.e. their quality, intensity, and familiarity. The evidence of short- and long-term behavioural effects of previous experience with odorants in utero, as revealed by habituation, familiarization, or conditioning procedures, attest to an active fetal intake of this information and to its utilization in the experimental or natural contexts. This is true whether the testing takes place between two successive prenatal periods or between a prenatal and a postnatal period.

4. Transnatal Continuity of Chemical Signals It is clear from the results presented above that mainly volatile compounds can ensure a thread of chemical continuity between the pre- and postnatal niches.’ 2This affirmation must, however, be nuanced in rodents, as the vomeronasal chemoreceptors might detect compounds of very low volatility (Halpern, 1987).

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In the rat, the parturient dam contributes directly by her intense selfdirected licking to the spreading of AF on her nipples and on the ventral fur (Roth & Rosenblatt, 1967). The crucial role of this AF deposit was shown by Teicher and Blass’s (1977) odour removal and restitution experiment. In that experiment, the AF painted on prewashed nipples was obtained on the last day of gestation from pregnant dams unfamiliar to the pups tested. This suggests that any AF has attractant properties for newborn rats, fresh samples being more effective than older ones. The fact that the AF held its efficiency even after 40-day cold storage suggests that the target compound, or mixture of compounds, is chemically stable. The alteration of the natural AF odour by introducing a dominant artificial note in utero indicates that the chemical continuity can depend on very simple compounds. Finally, the mother may contribute to the transnatal continuity by producing similar odour qualities in AF and colostrum, both substances being subjected to the common underlying influence of her diet. Existing data in rats demonstrate maternal dietary influences on both milk and AF odorous properties. The milk of a lactating dam carries aromatic constituents reflecting her diet, and at weaning pups show a preference for a diet bearing the flavour of the food that the mother ate during lactation (Galef & Henderson, 1972; Galef & Sherry, 1973). Similarly, as cited above, the pregnant female’s diet also determines flavour preferences in preweanling rat pups (Hepper, 1988). No chemical characterization of the volatile compounds naturally present in fetal and neonatal niches has been attempted so far.

5. Neonatal Sensing of Natural Prenatal Chemical Signals The results discussed in the sections above clearly demonstrate that within a few hours of birth rat pups are exquisitely sensitive to artificial olfactants to which they have been exposed in very different conditions of stimulation, several days or hours earlier. Neonatal rats and mice also detect the odour of unmanipulated AF (cf. Kodama, 1990; Teicher & Blass, 1977). In an additional study, 8-hr-old rat pups were subjected to a double-choice test between (1) AF odour obtained from own mother and (2) AF taken from an unfamiliar parturient dam, both series of females having had the same food regimen during pregnancy (Hepper, 1987). A significantly greater proportion of pups oriented to their own AF. To ensure that this response could not be acquired through exposure with AF present in the postnatal niche, a second sample of pups was given to a foster dam between birth and testing; the same outcome was attained, however, indicating that the preference for own AF is acquired through prenatal experience.



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It can be concluded that volatile chemicals contained in AF have two confounded functions in murine newborns: (1) a general effect on the psychobiological balance of the pup, inducing behavioural activation and orienting responses, and (2) a recognition cue that is unique, individually discriminable, to pups having shared a similar olfactory profile in the womb. The chemical substrates mediating both effects might be different.

II. THE OVINE MODEL 1. Functional Status of Fetal Chemoreceptors The sheep fetus has been extensively scrutinized for the development of its ability to sense tastes (for reviews, cf. Mistretta, 1990; Mistretta & Bradley, 1986). In contrast, knowledge on the development of the ovine nasal chemoreceptors remains extremely scanty. No reference could be located on the main olfactory subsystem (cf. Kratzing, 1969). The vomeronasal (VNO) receptors appeared to have an adult-like morphology in the newborn lamb (Kratzing, 1971), and hence probably in the near-term fetus. Tactile responsiveness in the nasal fields innervated by the sensitive afferents of the trigeminal nerve suggests that the common chemical sense may be functional very early in gestation (adult-like cortical evoked responses could be triggered by intranasal tactile stimulations in 110-day old fetuses; Molliver, 1975). Despite the scarceness of the anatomical bases, functional activity of the nasal chemoreceptors could be shown recently by our group (Schaal et al., 1991a, 1991b). The intranasal infusions of flavoured solutions [citral (CI) and 2-methyl-2-thiazoline (MT) solutions in isotonic saline (IS), which also served as the control stimulus] to ovine fetuses induced differential heart rate variations. The most dramatic effects were triggered by MT infusions, which induced reliable fetal heart rate (FHR) decelerations in both fetuses. Infusions of CI induced a mean weak accelerative effect in one fetus, but not in the other. Finally, no significant FHR variations were obtained in either fetus in response to IS injections. Control infusions being inefficient in eliciting significant FHR changes, the potential confounding somesthesic effects associated with the fluid injections could be excluded. In addition, the fact that injections were made into the nasal passages rules out FHR changes potentially elicited by the laryngeal chemoreflex. The weak effect of CI on the present experiment conditions appeared surprising compared to the clear results obtained in fetal rats (cf. Section 1.3) and newborn lambs (cf. Section 11.2). The differential effectiveness of the stimuli might be related to differences in intensity, hydrophilicity, or hedonic dimensions value of both odorants (cf. discussion in Schaal et al.,



1991a). But in any case these results show that near-term ovine fetuses are able to sense smells presented in a liquid medium.

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2. Amniotic Ecology The ovine conceptus is composed of two main fluid pools enclosed in the allantoic and the amniotic sacs, the latter containing the fetus itself. The volume of the amniotic compartment peaks at 0.7-1.5 1 and declines nearterm to 0.2-0.5 1 (Wintour et al., 1978). Early A F is isotonic with maternal and fetal plasmas but tends towards hypotony from the beginning of the third trimester (Mellor & Slater, 1971; Wintour, 1986), a trend indicating long-term chemical changes during gestation and increasing contribution of the fetus to its chemical ecology by the production of hypotonic urine (Seeds, 1973). The volume of urine produced daily in the last third of gestation increases from about 240 to 720 mVday (Gresham, Rankin, & Makowski, 1972; Wintour, 1986). A corresponding amount of A F is swallowed by the ovine fetus (200-800 myday; Bradley & Mistretta, 1973). The amniotic environment thus appears to be in a state of rapid turnover. Whether or not the nasal chemoreceptors are able to monitor these spontaneous changes is not documented in sheep. Neither do data on the oral chemoreceptors produce clear results. Although Mistretta and Bradley (1977) showed an inconsistent effect of taste on swallowing activity (when sweet or bitter tastants were injected in the AF), Harding, Bocking, & Sigger (1984) found that infusions of distilled water, hypotonic saline, or isotonic sucrose into the upper trachea affected swallowing, as compared to infusions of AF, tracheal fluid, or isotonic saline. Exogenous (i.e. maternal) influences on the amniotic ecology are mainly exerted through the placenta, but also to a lesser extent through nonplacental routes. The cotyledonary epitheliochorial placenta of the sheep has been described as less permeable to certain solutes (viz. Na+) than is the hemochorial placenta of rodents or primates (Faber, 1973; Flexner, Gellhorn & Baltimore, 1942; Robinson, Atkinson, Jones, & Sibley, 1988). Nevertheless, the pregnant ewe is currently used to model maternal-fetal exchanges in humans. Several passive and active mechanisms of placental transfer have been extensively investigated in the ovine model (Szeto, 1982). Research effort was mainly concentrated on the transfer of water, dissolved gases, elementary nutrients (hexoses, amino acids, fatty acids), fetal wastes, or substances of clinical interest, however, and chemical variations of the A F after normal feeding have not yet been investigated. But several points from the available results can be extrapolated to the maternal-fetal transfer of volatile substances: (1) Their natural access to the maternal blood stream is mainly through ingestion; it may also be possible through the inhalation of odorigenic materials, as attested by experiments where volatile compounds (e.g. analgesics, nitrous oxide) administered to

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pregnant ewes were recovered from fetal plasma and A F (Blechner et al., 1969). (2) The transfer of odorigenic compounds from maternal to fetal circulation is mainly limited by the physico-chemical properties of the molecule. In an attempt to classify compounds transferred according to size and lipophilicity, Seeds (1980) has proposed four classes: (i) small, highly soluble compounds diffusing quickly to the fetus (2-5 min) and to amniotic fluid; (ii) larger-sized (200-600 d), exclusively hydrophilic compounds whose concentration peaks 30-60 min after maternal infusion and culminates in A F after 4-24 hr; (iii) larger-sized lipophilic compounds, which cross rapidly (2-3 min) to the fetus, but slowly to the AF; (iv) compounds that cross to the fetus but are metabolized in the fetal liver. Odorivector substances generally containing a hydrophobic region (Ohloff, 1986) should correspond to the compounds quickly transferred to the fetus. (3) As in the rat, several access routes of potential odorants to fetal receptors can be considered: (i) the hematogenic route, which is undocumented in sheep; (ii) the stimulation of chemoreceptors by the fetal external milieu (AF) can be effective either by compounds passing across the fetal body itself (urine) or across the fetal membranes. The more documented access of compounds from maternal plasma to A F is through the fetal renal system, the molecule being excreted in urine and later taken in by the oral or nasal routes. Another more direct way of elimination of mother-transferred compounds is through the fetal pulmonary fluids (Riggs et al., 1987). An additional source of chemical exchanges between the mother and A F takes place at the richly vascularized fetal membranes and bypasses the fetal metabolism (Mellor, 1980).

3. Memorization of Fetal Chemosensory Information and Its Sensing by the Neonate There is preliminary evidence that the newborn lamb retains some information from its prenatal chemical ecology. In a preliminary experiment (Schaal & Orgeur, 1990), 30 fetuses (Ile de France breed) were exposed to three conditions of prenatal odorization: (1) Group A lambs were exposed to CI (citral) fragrance through maternal diet only during the two last weeks of pregnancy; after birth they were maintained in a box so they could not have access to the udder; (2) Group B lambs experienced CI odour via both maternal diet and postnatal exposure to CI-enriched atmosphere; finally (3) Group C lambs were left in an olfactory intact perinatal environment. Each group consisted of 10 lambs. Prenatal exposure was achieved by feeding ewes twice a day with grains soaked with highly concentrated CI (12.5%, emulsioned in distilled water). For the postnatal exposure, a water solution of CI (1,7%) was painted on the walls of a box that prevented the lamb from having direct contact with the mother, while she had access to the lamb. In order to avoid contamination, control and

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CI-treated ewes were raised in two separate pens by different persons. At the time they were able to stand, lambs were tested in a 5-min double choice test (Schaal, Orgeur, & Poindron, 1990) between (1) non-odorized A F obtained from a parturient ewe and (2) a weak CI-solution (1,7%) in distilled water. The rationale for choosing these stimuli was to present the lambs with two odour sources theoretically equal in familiarity for both treated groups (A and B). The specific hypothesis was that both odour sources should have attractive properties for the lambs of Groups A and B (B lambs being more attracted by citral than A lambs), but not for the control lambs. CI should indeed be avoided by naive lambs as a consequence of its novelty. Orientation time of the lamb’s muzzle to both stimuli was taken as the dependent variable. The orientation responses of the three groups of lambs in the behavioural test are given in Figure 1. It appears that lambs exposed to CI prenatally (Group A), or lambs exposed to CI pre- and postnatally (Group B), displayed no differential responsiveness towards both odour sources. In contrast, control lambs show a significantly longer orientation time towards A F when presented along with CI (Wilcoxon’s test, onetailed, p

Olfaction in utero: can the rodent model be generalized?

In this article we discuss five requirements that theoretically must be fulfilled for transnatal chemosensory learning to occur in three placental spe...
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