Evidence for olfactory function in utero

Olfactory Development and Dysfunction: Involvement of Microglia

Olfactory deficits are increasingly recognized in a variety of neurological, neurodevelopmental, psychiatric, and viral diseases. While the pathology underlying olfactory loss is likely to differ across diseases, one shared feature may be an immune response mediated by microglia. Microglia orchestrate the brain's response to environmental insults and maintain neurodevelopmental homeostasis. Here, we explore the potential involvement of microglia in olfactory development and loss in disease. The effects of microglia-mediated immune response during development may be of special relevance to the olfactory system, which is unique in both its vulnerability to environmental insults as well as its extended period of neurogenesis and neuronal migration.

Network Dynamics in the Developing Piriform Cortex of Unanesthetized Rats

The time course of changes in functional cortical activity during early development has been extensively studied in the rodent visual system. A key period in this process is the time of eye opening, which marks the onset of patterned visual input and active vision. However, vision differs from other systems in that it receives limited patterned sensory input before eye opening, and it remains unclear how findings from vision relate to other systems. Here, we focus on the development of cortical network activity in the olfactory system-which is crucial for survival at birth-by recording field potential and spiking activity from piriform cortex of unanesthetized rat pups from birth (P0) to P21. Our results demonstrate that odors evoke stable 10-15 Hz oscillations in piriform cortex from birth to P15, after which cortical responses undergo rapid changes. This transition is coincident with the emergence of gamma oscillations and fast sniffing behavior and preceded by an increase in spontaneous activity. Neonatal network oscillations and their developmental dynamics exhibit striking similarities with those previously observed in the visual, auditory, and somatosensory systems, providing insight into the network-level mechanisms underlying the development of sensory cortex in general and olfactory processing in particular.

Diet-based assortative mating through sexual imprinting

Speciation is facilitated by "magic traits," where divergent natural selection on such traits also results in assortative mating. In animal populations, diet has the potential to act as a magic trait if populations diverge in consumed food that incidentally affects mating and therefore sexual isolation. While diet-based assortative mating has been observed in the laboratory and in natural populations, the mechanisms causing positive diet-based assortment remain largely unknown. Here, we experimentally created divergent diets in a sexually imprinting species of mouse, Peromyscus gossypinus (the cotton mouse), to test the hypothesis that sexual imprinting on diet could be a mechanism that generates rapid and significant sexual isolation. We provided breeding pairs with novel garlic- or orange-flavored water and assessed whether their offspring, exposed to these flavors in utero and in the nest before weaning, later preferred mates that consumed the same flavored water as their parents. While males showed no preference, females preferred males of their parental diet, which is predicted to yield moderate sexual isolation. Thus, our experiment demonstrates the potential for sexual imprinting on dietary cues learned in utero and/or postnatally to facilitate reproductive isolation and potentially speciation.

OPEN RESEARCH BADGES

This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.n1qq6v3.

Sensitive Periods, Vasotocin-Family Peptides, and the Evolution and Development of Social Behavior

Nonapeptides, by modulating the activity of neural circuits in specific social contexts, provide an important mechanism underlying the evolution of diverse behavioral phenotypes across vertebrate taxa. Vasotocin-family nonapeptides, in particular, have been found to be involved in behavioral plasticity and diversity in social behavior, including seasonal variation, sexual dimorphism, and species differences. Although nonapeptides have been the focus of a great deal of research over the last several decades, the vast majority of this work has focused on adults. However, behavioral diversity may also be explained by the ways in which these peptides shape neural circuits and influence social processes during development. In this review, I synthesize comparative work on vasotocin-family peptides during development and classic work on early forms of social learning in developmental psychobiology. I also summarize recent work demonstrating that early life manipulations of the nonapeptide system alter attachment, affiliation, and vocal learning in zebra finches. I thus hypothesize that vasotocin-family peptides are involved in the evolution of social behaviors through their influence on learning during sensitive periods in social development.

Trpm5 expression in the olfactory epithelium

The Ca²⁺-activated monovalent cation channel Trpm5 is a key element in chemotransduction of taste receptor cells of the tongue, but the extent to which Trpm5 channels are expressed in olfactory sensory neurons (OSNs) of the main olfactory epithelium (MOE) of adult mice as part of a specific pheromonal detection system is debated. Here, we used a novel Trpm5-IRES-Cre knockin strain to drive Cre recombinase expression, employed previously validated Trpm5 antibodies, performed in situ hybridization experiments to localize Trpm5 RNA, and searched extensively for Trpm5 splice variants in genetically-labeled, Trpm5-expressing MOE cells. In contrast to previous reports, we find no evidence for the existence in adult mouse OSNs of the classical Trpm5 channel known from taste cells. We show that Trpm5-expressing adult OSNs express a novel Trpm5 splice variant, Trpm5-9, that is unlikely to form a functional cation channel by itself. We also demonstrate that Trpm5 is transiently expressed in a subpopulation of mature OSNs in the embryonic olfactory epithelium, indicating that Trpm5 channels could play a specific role in utero during a narrow developmental time window. Ca²⁺ imaging with GCaMP3 under the control of the Trpm5-IRES-Cre allele using a newly developed MOE wholemount preparation of the adult olfactory epithelium reveals that Trpm5-GCaMP3 OSNs comprise a heterogeneous group of sensory neurons many of which can detect general odorants. Together, these studies are essential for understanding the role of transient receptor potential channels in mammalian olfaction.

Learned recognition of maternal signature odors mediates the first suckling episode in mice

BACKGROUND

Soon after birth, all mammals must initiate milk suckling to survive. In rodents, this innate behavior is critically dependent on uncharacterized maternally derived chemosensory ligands. Recently, the first pheromone sufficient to initiate suckling was isolated from the rabbit. Identification of the olfactory cues that trigger first suckling in the mouse would provide the means to determine the neural mechanisms that generate innate behavior.

RESULTS

Here we use behavioral analysis, metabolomics, and calcium imaging of primary sensory neurons and find no evidence of ligands with intrinsic bioactivity, such as pheromones, acting to promote first suckling in the mouse. Instead, we find that the initiation of suckling is dependent on variable blends of maternal "signature odors" that are learned and recognized prior to first suckling.

CONCLUSIONS

As observed with pheromone-mediated behavior, the response to signature odors releases innate behavior. However, this mechanism tolerates variability in both the signaling ligands and sensory neurons, which may maximize the probability that this first essential behavior is successfully initiated. These results suggest that mammalian species have evolved multiple strategies to ensure the onset of this critical behavior.

Developmental consequences and biological significance of mother-infant bonding

Mother-infant bonding is universal to all mammalian species. Here, we review how mutual communication between the mother and infant leads to mother-infant bonding in non-primate species. In rodents, mother-infant bond formation is reinforced by various pup stimuli, such as tactile stimuli and ultrasonic vocalizations. Evidence suggests that the oxytocin neural system plays a pivotal role in each aspect of the mother-infant bonding, although the mechanisms underlying bond formation in the brain of infants has not yet been clarified. Impairment of mother-infant bonding strongly influences offspring sociality. We describe the negative effects of mother-infant bonding deprivation on the neurobehavioral development in rodent offspring, even if weaning occurs in the later lactating period. We also discuss similar effects observed in pigs and dogs, which are usually weaned earlier than under natural conditions. The comparative understanding of the developmental consequences of mother-infant bonding and the underlying mechanisms provide insight into the biological significance of this bonding in mammals, and may help us to understand psychiatric disorders related to child abuse or childhood neglect.

Chromosomal location-dependent nonstochastic onset of odor receptor expression

As odorant receptors (ORs) are thought to be critical determinants of olfactory sensory neuron (OSN) axon targeting and organization, we examined the spatiotemporal onset of mice ORs expression from the differentiation of OSNs in the olfactory placode to an aging olfactory epithelium. ORs were first detected in the placode at embryonic day 9 (E9), at the onset of OSN differentiation but before axon extension. By E13, 22 of 23 ORs were expressed. Onset of individual OR expression was diverse; levels and patterns of expression were unique for each OR. Regional distribution of ORs within zones of the olfactory epithelium appeared stable across development; adult-like patterns were observed by E13. Finally, analysis of OR expression and chromosomal location suggests that ORs are not stochastically expressed; they show evidence of coordinated expression. Collectively, these studies demonstrate that ORs are not equally represented in the "olfactome" across an animal's lifespan.

The rodent accessory olfactory system

The accessory olfactory system contributes to the perception of chemical stimuli in the environment. This review summarizes the structure of the accessory olfactory system, the stimuli that activate it, and the responses elicited in the receptor cells and in the brain. The accessory olfactory system consists of a sensory organ, the vomeronasal organ, and its central projection areas: the accessory olfactory bulb, which is connected to the amygdala and hypothalamus, and also to the cortex. In the vomeronasal organ, several receptors-in contrast to the main olfactory receptors-are sensitive to volatile or nonvolatile molecules. In a similar manner to the main olfactory epithelium, the vomeronasal organ is sensitive to common odorants and pheromones. Each accessory olfactory bulb receives input from the ipsilateral vomeronasal organ, but its activity is modulated by centrifugal projections arising from other brain areas. The processing of vomeronasal stimuli in the amygdala involves contributions from the main olfactory system, and results in long-lasting responses that may be related to the activation of the hypothalamic-hypophyseal axis over a prolonged timeframe. Different brain areas receive inputs from both the main and the accessory olfactory systems, possibly merging the stimulation of the two sensory organs to originate a more complex and integrated chemosensory perception.

Early functional development of interneurons in the zebrafish olfactory bulb

In the adult olfactory bulb (OB) of vertebrates, local GABAergic interneurons (INs) mediate recurrent and lateral inhibition between the principal neurons of the OB, the mitral cells (MCs), and play pivotal roles in the processing of odor-evoked activity patterns. The properties and functions of INs in the developing OB are, however, not well understood. We studied the functional development of INs in the OB of living zebrafish larvae 3-6 days postfertilization using anatomical techniques and in-vivo two-photon Ca2+ imaging. We identified MCs and INs by cell-type-specific expression of transgenic fluorescent markers and found that the IN:MC ratio was lower than in the adult fish. Moreover, the fraction of INs responding with Ca2+ signals to a set of natural odors was substantially lower than in adults. Odors of different chemical classes evoked overlapping patterns of Ca2+ signals that were concentrated in the center of the IN layer. The GABA(A) receptor agonists GABA and muscimol strongly suppressed odor responses, whereas a GABA(A) receptor antagonist enhanced responses and altered the spatial distribution of odor-evoked activity. These results indicate that IN odor responses at early developmental stages are sparse and exhibit no obvious chemotopic organization. Nevertheless, GABAergic signaling is already inhibitory at early stages of OB development and strongly influences odor-evoked activity patterns. Hence, INs already participate in the processing of odor information at very early stages of OB development even though the majority of INs emerge only at later stages.

Approach to the fetal movements: a pilot study of six cases

The aim of this study was to verify developmental aspects of fetal movements through 2D ultrasonic recordings of normal fetuses, product of low risk pregnancies. A qualitative analysis of the several types of movements was performed in a sample of six fetuses, each four weeks, from 12th to 40th gestational week. In the close period to 20th gestational week was registered the largest prevalence of movements. It was observed that certain movements disappear, and others stay or appear through period. Complex movements (breathing, deglutition and suction) were more frequent after the 20th week than before. In spite of the fetuses have presented a general motor pattern, individual variations were observed. These findings are in consonance with literature, toward the existence of certain motor patterns, with emergence and disappearance of the several types of movements according to the gestational age, and individual variations that suggest specificity.

Interactions between prenatal ethanol exposure and postnatal learning about ethanol in rat pups

Exposure to ethanol during the last days of gestation (1 or 2/kg) has been shown to induce greater ethanol intake as well as enhanced ethanol palatability in infant rats compared to pups without previous experience with the drug. This higher acceptance of ethanol seems to result from the prenatal association between the chemosensory aspects of ethanol and its reinforcing properties; the latter mediated at least in part by the opioid system. A series of experiments analyzed how this prenatal experience with ethanol affected postnatal conditioning of the flavor of ethanol. In the first experiment, pups exposed prenatally to ethanol (2g/kg during gestational days 17-20) were tested for acquisition and extinction of a conditioned aversion to the taste of ethanol (conditioned stimulus), employing lithium chloride as the aversive unconditioned stimulus. In the next two experiments, pups with the same prenatal exposure to ethanol were tested for the extinction of an aversion, employing three (Experiment 2) or one (Experiment 3) conditioning trials, using a high dose of ethanol (3g/kg) as the aversive agent. The first experiment showed that prenatal ethanol exposure delayed the postnatal acquisition of a conditioned aversion to the taste of ethanol. In the next two experiments, the same effect was observed during the extinction trials. These results provide additional support to the hypothesis of an appetitive associative memory established prenatally as a consequence of fetal ethanol exposure.

Housing pregnant mice (Mus musculus) in small groups facilitates the development of odor-based homing in offspring

Infant mice (Mus musculus) born to dams housed in isolation throughout pregnancy (IsoPreg) begin differentially approaching homenest bedding over clean bedding on Postnatal Day 6. Offspring of dams housed with 2 other potentially pregnant conspecifics (SocPreg) display such homing behavior on Day 4. Earlier onset of homing reflects facilitated olfactory responsiveness in SocPreg pups, rather than qualitative or quantitative differences in IsoPreg versus SocPreg nest odors, body growth, or motoric capabilities. Exposing pregnant IsoPreg dams to SocPreg bedding also accelerated homing onset in the offspring, though not to the same extent as the full social context. Thus, it appears that the facilitation of homing is mediated through the pregnant dam by a combination of chemical cues and other social stimuli.

Increased palatability of ethanol after prenatal ethanol exposure is mediated by the opioid system

Previous studies have shown that prenatal exposure to a moderate dose of ethanol (2 g/kg) during the last days of gestation of the rat (17-20) not only increases postnatal intake of the drug but also enhances the palatability of ethanol's taste when measured with a taste reactivity test. Prenatal administration of the opioid antagonist naloxone, together with ethanol, reduces ethanol intake. The aim of the present study was to analyze whether this decreased intake of ethanol after the administration of naloxone is accompanied by a reduction in ethanol's palatability. Results show that preweanling rats exposed prenatally to ethanol alone displayed more ethanol intake and more ingestive responses in reaction to its taste than non-exposed pups. Simultaneous prenatal administration of naloxone with ethanol prevented both the increased intake of ethanol and the higher amount of appetitive responses to its taste. These results indicate that the opioid system plays an important role in the effect of enhanced palatability of ethanol's taste after its prenatal exposure. Results also support the hypothesis of a conditioned response established in utero as a consequence of the association between ethanol's chemosensory and reinforcing aspects, the latter mediated by the opioid system.

Increased preference for ethanol in the infant rat after prenatal ethanol exposure, expressed on intake and taste reactivity tests

BACKGROUND

Previous studies have shown that prenatal exposure during gestational days 17 to 20 to low or moderate doses of ethanol (1 or 2 g/kg) increases alcohol intake in infant rats. Taking into account that higher consumption does not necessarily suggest a preference for alcohol, in the present study, the hedonic nature of the prenatal experience was analyzed further with the use of a taste reactivity test.

METHOD

General activity, wall climbing, passive drips, paw licking, and mouthing in response to intraoral infusions of alcohol, water, and a sucrose-quinine solution (which resembles alcohol taste in rats) were tested in 161 preweanling 14-day-old rat pups that were prenatally exposed to 0, 1, or 2 g/kg of alcohol during gestational days 17 to 20. Consumption of those substances was measured during the taste reactivity test and on postnatal day 15.

RESULTS

Pups that were prenatally exposed to both doses of ethanol displayed lower levels of general activity and wall climbing than controls in response to ethanol. Infant rats that were treated prenatally with both doses of ethanol showed higher intake of the drug and also more mouthing and paw licking in response to ethanol taste. Only pups that were exposed to the higher ethanol dose in utero generalized those responses to the sucrose-quinine compound.

CONCLUSIONS

These results seem to indicate that for the infant rat, the palatability of ethanol is enhanced after exposure to the drug during the last days of gestation.

Fetal learning with ethanol: correlations between maternal hypothermia during pregnancy and neonatal responsiveness to chemosensory cues of the drug

BACKGROUND

Fetuses learn about ethanol odor when the drug is present in the amniotic fluid. Prenatal learning comprising ethanol's chemosensory cues also suggests an acquired association between ethanol's chemosensory and postabsorptive properties. Ethanol-related thermal disruptions have been implicated as a significant component of the drug's unconditioned properties. In the present study, ethanol-induced thermal changes were analyzed in pregnant rats subjected to a moderate ethanol dose. This thermal response was later tested for its correlation with the responsiveness of the progeny to ethanol and nonethanol chemosensory stimuli.

METHODS

During gestational day (GD) 14, pregnant rats were subjected to a minor surgical procedure to place a subcutaneous telemetric thermal sensor in the nape of the neck. During GDs 17 to 20, females received a daily intragastric administration of ethanol (2 g/kg) or water, using solutions kept at room temperature. Maternal body temperatures were recorded before and after (4 consecutive hours) the administration of water or ethanol. Newborns representative of both prenatal treatments were tested in terms of behavioral activity elicited by the smell of ethanol or of a novel odorant (cineole). A third group of pups were tested in response to unscented air stimulation.

RESULTS

Ethanol administration during late gestation induced reliable maternal hypothermia, a thermal disruption greater than that observed in water-treated females. It was systematically observed that maternal ethanol-induced hypothermia negatively correlated with neonatal motor reactivity elicited by ethanol olfactory stimulation. No other significant correlations were observed in terms of responsiveness to cineole or to unscented air in animals prenatally exposed to ethanol or water.

CONCLUSIONS

In conjunction with prior research, the present results indicate that fetal ethanol exposure may yield learning of an association between ethanol's sensory and unconditioned properties. Ethanol-induced hypothermia during late gestation seems to represent a significant component of ethanol's unconditioned consequences. Specifically, ethanol-related thermal disruptions in the womb are highly predictive of neonatal responsiveness to ethanol's chemosensory cues that are known to be processed by the near-term fetus.

Development of the vomeronasal receptor epithelium and the accessory olfactory bulb in sheep

The morphological development of the vomeronasal organ (VNO) and accessory olfactory bulb (AOB) of the sheep from anlage to birth were studied by classical and histochemical methods using embryos and fetuses obtained from an abattoir with ages estimated from crown-to-rump length. Both VNO and AOB developed in a biologically logical sequence and completed their morphological development around day 98, at entry into the last third of the gestation period. A lectin with specificity for oligomeric N-acetylglucosamine labeled the sensory epithelium of the VNO, the vomeronasal nerves, and the nervous and glomerular layers of the AOB before birth. These results suggest that the vomeronasal system, which is well developed and functional in adult sheep, may be able to function at or even before birth in these animals (whereas in rodents, for example, this is precluded by the AOB not completing its development until after birth).

Morphogenesis and growth of the soft tissue and cartilage of the vomeronasal organ in pigs

The morphology of the soft tissue and supporting cartilage of the vomeronasal organ of the fetal pig was studied from early stages to term. Specimens obtained from an abattoir were aged by crown-to-rump distance. Series of transverse sections show that some time before birth all structures--cartilage, connective tissue, blood vessels, nerves, glands and epithelia--are well developed and very similar in appearance to those of the adult. Furthermore, in transmission electron microscopy photomicrographs obtained at this stage the vomeronasal glands exhibit secretory activity.

Prenatal exposure to ethanol increases ethanol consumption: a conditioned response?

Administration of a moderate dose (2 g/kg) of ethanol to the pregnant rat during the last days of gestation (17-20) has been observed to increase the offspring's ethanol consumption on postnatal day 14. This effect was not observed with a 1-g/kg dose (Experiment 1a). When pups were tested during adolescence (day 28) an augmented ethanol intake was observed in female rats exposed to the low dose of ethanol and in male rats exposed to the moderate one (Experiment 1b). The effect of increased ethanol intake in preweanling rats was reduced after naloxone was administered, together with ethanol, to the pregnant dam (Experiment 2). Postnatal reexposure to ethanol, together with a naloxone injection, also decreased ethanol intake in pups exposed prenatally to the drug (Experiment 3). All these results seem to support the hypothesis of a conditioned preference learned in utero as a consequence of the association between the orosensory characteristics of ethanol and its reinforcing properties, apparently mediated by the opioid system.

Anatomic mapping of neuronal odor responses in the developing rat olfactory bulb

Behavioral evidence indicates that altricial mammals possess olfactory function at early developmental ages, before the olfactory bulb has matured anatomically. We monitored the early anatomic and functional development of the olfactory bulb in rat pups stimulated with odors using in situ localization of c-fos mRNA to identify responsive postsynaptic neurons. Odor-specific spatial patterns of neuronal activation in the glomerular layer were evident from birth, were sharply defined rather than diffuse, and remained relatively unchanged in terms of their bulbar distribution during the first 3 postnatal weeks. In neonates, focal postsynaptic responses in the glomerular layer occurred in the form of clusters of activated tufted neurons. Broad zones of activated mitral cells were located beneath these cell clusters, with scattered neurons in the underlying granule cell layer also expressing c-fos. The cellular composition of these functional neuronal groups shifted from predominantly output neurons at the earliest ages, to increasing incorporation of interneurons as they developed postnatally. The characteristic distribution of activated neurons in the mature glomerular layer, in which the boundaries of individual glomeruli are precisely defined by cells expressing c-fos, emerged near the end of the first week. Broad zones of cRNA hybridization in the mitral cell layer became increasingly restricted as the size of the activated granule cell population increased postnatally, correlating with the functional maturation of inhibitory circuitry. These results provide evidence that the types and distributions of neurons collectively activated by sensory input to glomeruli change as the rat olfactory bulb matures and that distinct, functional odor maps in the glomerular layer are established from birth.

Clustered distribution of cAMP-dependent protein kinase regulatory isoform RI alpha during the development of the rat brain

cAMP is a ubiquitous second messenger, which acts mainly through specific protein kinases that consist of two regulatory and two catalytic subunits. An unsolved problem in cAMP physiology is how it can regulate so many cellular functions through this simple enzymatic cascade. A tentative explanation is related to the different biochemical properties of the four regulatory subunit isoforms (RI alpha and RI beta, RII alpha and RII beta) and to their differential cell and tissue distribution. For example, detergent insoluble aggregates of RI alpha are present in some cholinergic neurons of the adult rat brain. Rat brains, from the embryonic stage to old age, were examined for the presence of highly concentrated clusters of RI alpha. They are present only in some neurons of restricted brain areas, for a limited time span. During development, labeled neurons appear in different brain areas after neuron migration, at a stage of advanced functional maturation. They have their greatest expression after birth but before sexual maturation, and then they slowly decline, persisting only in a few brain areas throughout life. The first appearance, time course, and eventual disappearance is different in the different brain areas: RI alpha clusters appear in brainstem, hypothalamus, and accessory olfactory bulb at a late embryonic stage; in the main olfactory bulb, hippocampus, and medial thalamic nuclei shortly after birth; and in the cortex as late as in the third and fourth postnatal week. During the rat's lifespan, the distribution of these peculiar RI alpha clusters undergo changes that may contribute to shape neuronal responses differentially to agents modifying cAMP levels.

Immunohistochemical studies on the differential maturation of three types of olfactory organs in the rats

Differential maturation of three types of olfactory organs, the olfactory epithelium (OE), the vomeronasal organ (VNO) and the septal olfactory organ of Masera (MO), was examined immunohistochemically in embryonic and newborn rats by the use of antiprotein gene product 9.5 (PGP 9.5) serum. These olfactory organs were derived in common from the olfactory placode as neuroepithelia. In the OE, PGP 9.5-immunopositive olfactory cells first appeared at 13 days of gestation. The OE maturated completely, and showed the same cytological features as in the adult at 20 days of gestation. The MO first appeared as a dense mass of PGP 9.5-immunopositive sensory cells on the most ventrocaudal part of the nasal septum at 15 days of gestation and was evidently isolated from the OE by the decrease of immunopositive cells in the intercalated epithelium between the OE and the MO at 20 days of gestation. However, even at 7 days after birth, the MO did not complete its development and contained sensory cells aggregating in the mass. The VNO was separated from the nasal cavity at 13 days of gestation as a tubular structure of a neuroepithelium including PGP 9.5-immunopositive sensory cells. These cells gradually increased in number in the sensory epithelium of the VNO and extended their dendritic processes to the free surface at 7 days after birth. These findings clarified the differential maturation of these olfactory organs. That is, the OE completes its development before birth, while the MO and VNO after birth.

Expression of glycoproteins in the vomeronasal organ reveals a novel spatiotemporal pattern of sensory neurone maturation

The main olfactory and the accessory olfactory systems are both anatomically and functionally distinct chemosensory systems. The primary sensory neurones of the accessory olfactory system are sequestered in the vomeronasal organ (VNO), where they express pheromone receptors, which are unrelated to the odorant receptors expressed in the principal nasal cavity. We have identified a 240 kDa glycoprotein (VNO(240)) that is selectively expressed by sensory neurones in the VNO but not in the main olfactory neuroepithelium of mouse. VNO(240) is first expressed at embryonic day 20.5 by a small subpopulation of sensory neurones residing within the central region of the crescent-shaped VNO. Although VNO(240) was detected in neuronal perikarya at this age, it was not observed in the axons in the accessory olfactory bulb until postnatal day 3.5. This delayed appearance in the accessory olfactory bulb suggests that VNO(240) is involved in the functional maturation of VNO neurones rather than in axon growth and targeting to the bulb. During the first 2 postnatal weeks, the population of neurones expressing VNO(240) spread peripherally, and by adulthood all primary sensory neurones in the VNO appeared to be expressing this molecule. Similar patterns of expression were also observed for NOC-1, a previously characterized glycoform of the neural cell adhesion molecule NCAM. To date, differential expression of VNO-specific molecules has only been reported along the rostrocaudal axis or at different apical-basal levels in the neuroepithelium. This is the first demonstration of a centroperipheral wave of expression of molecules in the VNO. These results indicate that mechanisms controlling the molecular differentiation of VNO neurones must involve spatial cues organised, not only about orthogonal axes, but also about a centroperipheral axis. Moreover, expression about this centroperipheral axis also involves a temporal component because the subpopulation of neurones expressing VNO(240) and NOC-1 increases during postnatal maturation.

A genetic approach to trace neural circuits

Mammalian nervous system function involves billions of neurons which are interconnected in a multitude of neural circuits. Here we describe a genetic approach to chart neural circuits. By using an olfactory-specific promoter, we selectively expressed barley lectin in sensory neurons in the olfactory epithelium and vomeronasal organ of transgenic mice. The lectin was transported through the axons of those neurons to the olfactory bulb, transferred to the bulb neurons with which they synapse, and transported through the axons of bulb neurons to the olfactory cortex. The lectin also was retrogradely transported from the bulb to neuromodulatory brain areas. No evidence could be obtained for adverse effects of the lectin on odorant receptor gene expression, sensory axon targeting in the bulb, or the generation or transmission of signals by olfactory sensory neurons. Transneuronal transfer was detected prenatally in the odor-sensing pathway, but only postnatally in the pheromone-sensing pathway, suggesting that odors, but not pheromones, may be sensed in utero. Our studies demonstrate that a plant lectin can serve as a transneuronal tracer when its expression is genetically targeted to a subset of neurons. This technology can potentially be applied to a variety of vertebrate and invertebrate neural systems and may be particularly valuable for mapping connections formed by small subsets of neurons and for studying the development of connectivity as it occurs in utero.

Neonatal responsiveness to alcohol odor and infant alcohol intake as a function of alcohol experience during late gestation

It has been previously suggested that maternal alcohol intoxication during the last days of pregnancy promotes fetal experiences that include chemosensory processing of the drug. In this study pregnant Wistar-derived rats were administered saline or one of two alcohol doses (1 or 2 g/kg) during gestational days 17-20. Immediately after birth, pups were tested in regard to motor-eliciting properties of the odor of amniotic fluid or alcohol, or of these stimuli presented as a configuration. Saline controls showed significantly shorter duration of overall motor activity and head movements when stimulated with the biological cue (amniotic fluid) than when exposed to a novel stimulus (ethanol alone or configured with the amniotic fluid). The opposite pattern was found in pups with prenatal experience with the higher alcohol dose. In a second experiment, the impact of similar alcohol treatments on infant consumption of different tastants, including alcohol and a configuration of sucrose and quinine, was tested. This configuration appears to mimic psychophysical properties of ethanol. Consumption of water, sucrose, or quinine was unaffected by the prenatal status of the subjects. Antenatal alcohol experience with the lower alcohol dose (1 g/kg) increased both alcohol and sucrose-quinine consumption. The 2 g/kg alcohol animals also ingested more sucrose-quinine relative to saline controls. As a whole, the results confirm the hypothesis that an intrauterine alcohol sensory memory selectively affects neonatal recognition of the alcohol's olfactory attributes and infant intake of either alcohol or solutions that share certain sensory equivalence with this psychopharmacological agent.

Prenatal development of the mammalian vomeronasal organ

The vomeronasal organ (VNO) originates from the medial wall of the olfactory pit shortly after the middle of the embryonic period in mammals. The Anlage stage consists of a cellular bud that grows dorsally, caudally, and towards the midline leaving a groove. The following stage, Early Morphogenesis, includes the closure of the vomeronasal groove to form a parasagittal blind-ended tube in the nasal septum, which opens into the nasal and/or oral cavities. The lumen adopts a crescent shape while the epithelial lining differentiates into an increasingly wider epithelium on the concave side and a gradually thinner epithelium on the convex side. The former goes on to occupy a medial position and develops neuroblasts among supporting and undifferentiated cells, with supporting cell nuclei tending to align in the upper rows. The lateral "non-sensory" epithelium furrows, giving a kidney-shaped appearance to the VNO cross section. The next stage, Late Morphogenesis is extended up to a difference in thickness between both epithelia becomes similar to the adult, generally by birth. An increasing number of ciliary generation complexes, larger and more abundant microvilli, and an evident glycocalyx are observed in the neuroepithelium at the luminal surface, while enzymatic activities become more intense. The non-sensory epithelium appears quite mature save for its luminal surface, which is still devoid of cilia. Blood capillaries penetrate the most basal region of the neuroepithelium and vomeronasal glands are very few and immature. At birth, some neurons appear well developed to support certain functionality; however, persistence of architectural, histochemical, and ultrastructural signs of immaturity, suggests that full performance of the VNO does not occur in newborn mammals, but in prepubertal ages.

CaBPs and other immunohistochemical markers of the human vomeronasal system: a comparison with other mammals

After more than two centuries of almost sporadic inquiry as to the existence and function of the human vomeronasal system (VNS), the last decade has seen a resurgent interest in it. The principal question vexing many laboratories is whether adult humans retain the VNS that clearly develops during fetal growth. Additional questions are whether the structurally defined fetal VNS has any function role, and if this structure and function extend into postnatal life. One research tool that has been successfully used to identify key components of the mammalian VNS has been immunohistochemistry (IHC). This technique has clearly defined the vomeronasal receptor neurons in the vomeronasal organ, the vomeronasal nerve that projects into the central nervous system, and the target of this nerve, the accessory olfactory bulb. This review will discuss immunohistochemical studies that have identified these features in the mammalian VNS, and relate them to structural and IHC studies of the fetal and adult human VNS. Suggestions as to future studies to clarify the status of the human VNO also are offered.

Prenatal differentiation of mouse vomeronasal neurones

The vomeronasal organ (VNO) subserves basic chemosensory functions in rodents, mainly related to sexual behaviour. In order to understand early stages of the VNO structural maturation, we have undertaken an immunocytochemical analysis of the VNO of fetal mice. Our results demonstrate that Olfactory Marker Protein (OMP), a marker of differentiated chemosensory cells, is already expressed in vomeronasal neurones and their fibres projecting to the accessory olfactory bulb during the last week of gestation. However, in contrast to the adult, where its expression is restricted to the medial sensory neuronal component of the VNO, during fetal development OMP is also present in cells located in the lateral non-sensory epithelial component. Some other markers of nasal chemosensory neurones, such as GAP-43/B-50, Protein Gene Product 9.5 (PGP 9.5) and carnosine are also transiently expressed in this ectopic site. These results indicate that (i) significant morphological and biochemical maturation of the VNO is achieved before birth; (ii) transient cell populations, sharing the biochemical profile of the vomeronasal chemosensory receptors, occur in ectopic areas during fetal development.

Prenatal and postnatal ethanol exposure influences preweanling rats' behavioral and autonomic responding to ethanol odor

The specific question was how prenatal and/or postnatal experience with ethanol influences cardiac and behavioral responses to the odor of ethanol on postnatal day (PD) 16. In each of two experiments, pregnant rats were given ethanol or water on gestational days 17-20. Offspring were exposed on PD12 to one of three conditions: intragastric administration of 6% ethanol, indirect exposure to ethanol from littermates, or no treatment. Results of Experiment 1 indicated that, regardless of prenatal ethanol exposure, 16-day-olds exposed on PD12 either directly or indirectly to ethanol expressed a greater increase in HR in response to ethanol odor than pups not postnatally exposed to ethanol. In Experiment 2, in which a lower ethanol dose was used postnatally, an interaction between pre- and postnatal ethanol exposure was observed; that is, pups exposed pre- and postnatally to ethanol showed the greatest increases in HR and the smallest increases in motor activity in response to ethanol odor. In both experiments motor activity was dissociated from increases in HR. The results are discussed in terms of what is learned, prenatally and postnatally, in association with the chemosensory properties of ethanol.

Local cerebral glucose utilization in fetal guinea pigs at 0.75 gestation

OBJECTIVE

Using the 2-deoxyglucose method, measurements of local cerebral glucose utilization in large fetal animals are very difficult and expensive. To circumvent these problems we recently modified the 2-deoxyglucose method for use in the fetal guinea pig in utero (Berger et al., J Neurochem 1994; 63: 271-279). The present study was designed to measure the rates of local cerebral glucose utilization in fetal guinea pigs at 0.75 of gestation.

STUDY DESIGN

After intravenous injection of 14C 2-deoxyglucose into the dams, local cerebral glucose utilization of the fetuses was measured from the time integral of the tracer in the maternal plasma and the autoradiographically determined concentration of the tracer in various parts of the fetal brain.

RESULTS

Fetal cerebral glucose utilization was low as compared to adult animals and varied in different brain structures from 19 +/- 4 to 29 +/- 7 mumol/100 g/min.

CONCLUSION

This study demonstrates the feasibility to measure local cerebral glucose utilization in undisturbed fetal guinea pigs in utero. We conclude that the low rate of cerebral glucose utilization and its small overall variability may reflect the neurological immaturity of the fetal brain.

Multiple factors shape development of olfactory glomeruli: insights from an insect model system

The antennal system of the moth Manduca sexta is a useful model for studies of the development of olfactory glomeruli, the complex synaptic structures that typically underlie the initial processing of olfactory input in vertebrates and invertebrates. In this review, we summarize cellular events in the construction of glomeruli in Manduca and highlight experiments that reveal factors that influence glomerulus development. By methodically manipulating each of various cell types, both neuronal and glial, that contribute to glomerular architecture, we have found that: olfactory receptor axons lay a template for developing glomeruli, stabilization of the template by glial cells is necessary to permit subsequent steps in development of the glomeruli, and the hormone that regulates adult development causes production of adequate numbers of glial cells. Neither electrical activity nor the presence of a serotonin-containing neuron that persists throughout development is required for a glomerular pattern to develop; these factors might, however, influence the synaptic organization of individual glomeruli.

Differential expression of odorant-binding protein genes in rat nasal glands: implications for odorant-binding proteinII as a possible pheromone transporter

We examined the distribution and ontogeny of two odorant-binding proteins in the rat at various stages of development from newborn to adult using northern blot and in situ hybridization methods. Our results demonstrated spatial segregation between odorant-binding protein and odorant-binding proteinII in nasal glandular tissues. Odorant-binding protein messenger RNA was expressed in the glandular system opening into the nasal vestibule, whereas odorant-binding proteinII messenger RNA was seen in the posterior glands of the nasal septum and in the vomeronasal glands. In addition, odorant-binding protein and odorant-binding proteinII messenger RNA levels increased during early postnatal stages with time courses that paralleled the anatomical development of the main olfactory system and the vomeronasal system, respectively. Our results suggest that odorant-binding proteinII functions as a pheromone transporter in the vomeronasal system.

Salicylate-induced changes in auditory thresholds of adolescent and adult rats

Shifts in auditory intensity thresholds after salicylate administration were examined in postweanling and adult pigmented rats at frequencies ranging from 1 to 35 kHz. A total of 132 subjects from both age levels were tested under two-way active avoidance or one-way active avoidance paradigms. Estimated thresholds were inferred from behavioral responses to presentations of descending and ascending series of intensities for each test frequency value. Reliable threshold estimates were found under both avoidance conditioning methods, and compared to controls, subjects at both age levels showed threshold shifts at selective higher frequency values after salicylate injection, and the extent of shifts was related to salicylate dose level.

Fos protein immunoreactivity in the developing olfactory bulbs of normal and naris-occluded rats

Immediate early genes such as c-fos may be a route through which extracellular events affect genomic expression. Expression of immediate early genes is important in the transcriptional regulation necessary for the normal development of the nervous system. Developmental patterns of Fos protein (the product of c-fos immediate early gene expression) were studied in the main olfactory bulb of the rat using immunocytochemistry. Embryonic Day 21 (E21, the last prenatal day), as well as Postnatal Day 0 (P0), P1, P5, P10, P15, P20 and P30 subjects were examined. Although staining was absent in the E21 bulb, there was a rapid onset of Fos synthesis within hours after birth. Distribution of Fos-immunoreactive (Fos-ir) nuclei corresponded to the sequence of bulb maturation: numerous mitral/tufted and granule cells were labeled on P0, followed by the appearance of Fos-ir in the nuclei of periglomerular cells and an increase in the number of stained granule cells with development. Surgical closure of an external naris on P1 resulted in a 70% reduction in the number of Fos-ir granule cell nuclei as early as 2 h after the manipulation. During the next 30 days, levels of Fos staining further diminished in experimental bulbs when compared to their contralateral controls. Nevertheless, electrical stimulation of the contralateral bulb in P20 pups resulted in a robust increase of Fos labeling in most main and accessory olfactory bulb mitral cells and in many granule and periglomerular neurons, suggesting that the experimental bulbs remain competent to express Fos protein.

Pheromonally accelerated puberty is enhanced by previous experience of the same stimulus

The influence of rearing conditions on pheromone-induced puberty acceleration was tested on Swiss mice. Litters were reared in one of three conditions: with either both parents, or with two females, or finally with two females in the presence of urinary pheromonal cues from adult males, which are known to induce puberty acceleration. Nine days after weaning the females were exposed to either prepubertal or adult male urine. In the groups reared with either both parents or with two females and the pheromonal cues from stranger males, females treated with adult male urine presented heavier uteri and more cornified vaginal smears than females reared in the same conditions but subsequently treated with prepubertal males urine. In the group reared simply with two females, the differences in both uterus weight and vaginal smears did not reach statistical significance. The data support the notion that early experience of pheromonal cues may influence the response to pheromones in a later period, even if the preweaning exposure to males had no direct influence on early signs of puberty onset.

Olfactory experience modulated apoptosis in the developing olfactory bulb

Early sensory stimulation plays a key role in shaping the structure and function of the developing olfactory system. Here, we provide the first direct evidence for apoptotic cell death in the olfactory bulbs of rat pups during normal development and we also demonstrate that olfactory deprivation by unilateral naris occlusion causes a dramatic increase in apoptotic cell death in the glomerular and granule cell layers of the deprived bulb. The accessory olfactory bulbs displayed a remarkably high basal level of apoptosis but the occluded accessory bulb did not differ in that regard from the control accessory bulb. These results suggest that apoptosis may be an important mechanism by which the olfactory system can adjust its cell numbers in response in sensory stimuli experienced in early life, thereby underlying one form of plasticity in the developing olfactory system.

Differentiation of the olfactory epithelium during development

Maturation of the olfactory epithelium was studied in the rat. As a developmental index of the olfactory epithelium, quantitation was based on the thickness and the number of cell layers of the epithelium. Mean epithelial thicknesses were 84 microns on embryonic day (E) 19, 94 microns on postnatal day (P) 0, 98 microns on P10 and 57 microns in adult. Average numbers of cell layers were 7 (E19), 8 (P0), 10 (P10) and 7 (adult). Thus the olfactory epithelium progressively increased in thickness between P0-P10, and later decreased in thickness as well as in number of cell layers toward adult. To clarify the number of mature receptor cells sending their axons to the olfactory bulb, the percentages of receptor cell labeling (number of labeled cells/total number of receptor cells) were estimated with an aid of retrograde fluorescent tracer injected into the olfactory bulb. As a result, their percentages were 34% (P1-P3), 50% (P8-P10) and 78% (adult). The ablation experiment of the bulb on P1 pups and adult rats revealed that the percentages of receptor cell degeneration were 31% in neonates (P1) and 76% in adults, confirming the results obtained from the retrograde tracing. Thus the present study shows the substantial increase of mature receptor cells during development and may provide a morphological basis in understanding of the functional maturity of olfaction.