Autoradiographic and histological studies of postnatal neurogenesis. IV. Cell proliferation and migration in the anterior forebrain, with special reference to persisting neurogenesis in the olfactory bulb

Cultured epithelioid astrocytes migrate after transplantation into the adult rat brain

A highly purified population of dividing epithelioid astrocytes has been prepared from postnatal rat corpus callosum. These cells were labelled in culture by incorporation of either [3H]thymidine or fluorescent microspheres and transplanted in a fibrin clot into the hippocampi of adult syngeneic rats. Transplanted cells divided in vivo and progressively migrated into the host brain from the site of implantation up to distances of about 1 mm. After a 1-week survival, transplant cells stained strongly for glial fibrillary acidic protein and had the thick sinuous processes characteristic of stellate astrocytes. Artefactual transfer of radiolabel to host cells was ruled out by control experiments in which either the proportion of transplant cells that were radiolabelled was varied or radiolabelled transplant cells were killed prior to implantation. Astrocyte migration over the first days after implantation was determined to occur at a rate of approximately 100 microns per day. Transplant cells moved into both grey and white matter areas of the host brain and over the migratory period were commonly observed to be associated with blood vessels. Some transplant cells were directly juxtaposed against neuronal perikarya and dendrites. Many labelled astrocytes were located in areas that were apparently completely free of damage caused by implantation. These results define a class of mature astrocytic cells that have the ability to migrate through the adult brain. The existence of pathways for cell movement in the adult CNS has implications for the mechanisms of tissue remodelling after injury and transplantation, for regenerative repair of the CNS, and for the dynamics of cell-cell contacts in the normal adult mammalian brain.

Effects of prenatal protein deprivation on postnatal development of granule cells in the fascia dentata

The effect of prenatal protein deprivation on the postnatal development of granule cells in the fascia dentata in the rat was studied at 15, 30, 90, and 220 days of age. The granule cells showed a significant reduction in cell size, decreased number of synaptic spines throughout their dendritic extent, and reduced complexity of dendritic branching in the outer two-thirds of the molecular layer. All of these deficits were present at 15 days and persisted throughout the study (220 days). The least deficits in synaptic spine density occurred at 90 days and in dendritic branching at 30 days. Partial restitution of earlier, more severe deficits was associated primarily with maturational events occurring in the protein deprived rats, whereas later increases in deficits were related primarily to a failure of the protein deprived rats to keep pace with neuronal development occurring in the controls. The present results are similar to those noted in our previous study in this journal of the effect of a low protein diet (8% casein) on these neurons that extended from pregnancy until the time of sacrifice at 30, 90, and 220 days of age (Cintra et al., '90; 532:271-277). Taken together, these two studies suggest that the postnatal adaptation of the granule cells to prenatal protein deprivation is primarily due to events that occur during pregnancy and that the site of predilection for the deficit is their dendrites in the outer two-thirds of the molecular layer of the fascia dentata.

Cell proliferation in the subependymal layer of the postnatal marmoset, Callithrix jacchus

Cells of the subependymal layer (SEL) have been shown to be capable of continued postnatal cell division throughout life in rodents. To determine if the primate brain behaves similarly, proliferative activity in the SEL of the marmoset has been investigated by tritiated thymidine autoradiography and bromodeoxyuridine immunocytochemistry. Both methods revealed the presence of DNA-synthesizing cells at all postnatal ages studied. The labelling index (LI), low at birth, reached a peak of almost 4% at one month but decreased gradually thereafter. In animals older than two years the LI was extremely low and labelled cells were rarely seen anywhere in the brain. The cell density of the SEL, in contrast to the low LI, was highest in neonates and decreased linearly with increasing age. Bromodeoxyuridine immunoreactivity revealed the distribution of proliferating cells in the SEL and neighbouring regions. Such cells were most abundant around the anterior lateral ventricle where the SEL was most evident. Proliferating cells were numerous in neonates, though not adjacent to the ependyma where counts for the LI were made, and were mainly located dorsally and ventrally at the junctions of the corpus callosum and caudate nucleus.

Substance P- and opioid-immunoreactive structures in olfactory centers of the cat: adult pattern and postnatal development

Substance P (SP)-ir and opioid-ir structures were studied in the cat main olfactory bulb (MOB), accessory olfactory bulb (AOB), and olfactory peduncle. In the MOB, the opioid-ir and the majority of the SP-ir neurons belong to the granule cell type. SP-ir granule cells reside in the deeper granule cell layer, whereas opioid-ir granule cells reside in the superficial granule cell layer, internal plexiform, and mitral cell layer. Many granule cells are observed in the external plexiform and glomerular layer. Other granule cells were found in the bulbar/peduncular white matter, the taenia tecta, and the genu of the corpus callosum. A new substance P-ir cell type was identified in the glomerular layer. This cell type was also identified by using the technique of intracellular injection of Lucifer Yellow. The cell type corresponds neither to the external tufted type nor to the short axon cell types described so far. The AOB resembles the MOB with respect to large numbers of SP-ir and opioid-ir granule cells. In addition, a few opioid-ir neurons, probably superficial mitral cells, were found in the glomerular layer. The AOB is surrounded by islands of immunoreactive granule cells, which connect to the granule cell layer by extremely long processes. Opioid-ir and SP-ir beaded axons pass through the olfactory peduncle terminating on granule cells, and ascend as far as the glomerular layer. All subdivisions of the anterior olfactory nucleus (AON) contain immunoreactive terminal fields. Afferent fibers and terminal plexuses derive from a population of immunoreactive neurons located predominantly in the region of the septo-olfactory junction. They have large somata. Their axons form recurrent collaterals, some of which run rostrally in the peduncular white matter. Others ascend caudally towards the septal region. The fibers seem to remain ipsilaterally, since the olfactory limb of the anterior commissure and the commissure proper are devoid of SP-ir and opioid-ir fibers. During development SP and opioid immunoreactivity were found only in differentiated granule cells. The peptides were not detectable in migrating or immature granule cells, as identified in Golgi-impregnated material. The granule cell population largely develops during postnatal life. The number of opioid-ir granule cells increases slowly and continuously, reaching the adult level not before the sixth postnatal month. Strikingly, SP-ir granule cell number increases fast and reaches a transient peak during the second month. Thereafter it declines (40% decrease) to the adult density, which is similar to that of opioid-ir granule cells.

Premitotic DNA synthesis in the brain of the adult frog (Rana esculenta L.): an autoradiographic 3H-thymidine study

Replicative synthesis of DNA in the brain of the adult frog was studied by light microscope autoradiography. Animals collected during the active period (May-June) and in hibernation (January) were used. In active frogs, 3H-thymidine labelling occurred mainly in the ependymal cells which line the ventricles. The mean labelling index (LI%) was higher in the ependyma of the lateral and fourth ventricles than in the ependyma of the lateral diencephalon and tectal parts of the mesencephalon. In the recessus infundibularis and preopticus the number of labelled cells (LCs) was several times greater than in the lateral parts of the third ventricle. LCs were seen subependymally only occasionally. The incidence of LCs in the parenchyma of the brain was much lower in most regions than in the ventricular ependyma; LCs were mainly small and, from their nuclear morphology, they were glial cells. The LI% reached the highest value in the septum hippocampi and in the nucleus entopeduncularis. In these locations, LCs were larger and closer in size to the nerve cells of these regions. From comparison with data obtained earlier in the brain of mammals, it is evident that the distribution of proliferating cells in the olfactory and limbic system is phylogenetically conservative. The occurrence of pyknotic cells in the same areas which contain LCs, suggests that cell division reflects in part the process of cell renewal observed in mammals. However, proliferating cells could also be linked to the continuous growth observed in non-mammalian vertebrates. In hibernating frogs, LCs and pyknoses were not seen or were found occasionally, which further indicates the functional significance of both processes.

Retention of J1/tenascin and the polysialylated form of the neural cell adhesion molecule (N-CAM) in the adult olfactory bulb

To gain insight into the cellular and molecular mechanisms underlying neurogenesis in adult mouse olfactory bulb, several adhesion molecules expressed by glial cells and neurons were investigated. In the germinal zone of the olfactory bulb, the subependymal layer of the rostral region of the lateral ventricles, two adhesion molecules are detectable that are characteristic of early morphogenetic events: J1/tenascin and the polysialylated form, the so-called embryonic form, of N-CAM. The polysialylated form of N-CAM is expressed by most cells in the subependymal layer, and by some astrocytes and neurons in the granular layer adjacent to the subependymal layer. This suggests that bipotential precursor cells retain expression of the embryonic form during their migration from the subependymal layer and during the first stages of differentiation into neurons and glia. Expression of the polysialylated form of N-CAM is also retained in monolayer cultures of six-day-old olfactory bulbs, 55 days after seeding in vitro. J1/tenascin was detectable in the subependymal layer using two monoclonal antibodies. The immunostaining pattern was different between the two antibodies and more restricted to the subependymal layer than when staining with polyclonal J1 antibodies was performed, indicating that J1/tenascin exists in distinct isoforms. Finally, our observations suggest that, in the adult olfactory bulb, L1 is not only a neuron-neuron adhesion molecule, but it may also be involved in neuron-glia interactions, since it is found at contact sites between these two cell types. L1, therefore, may be a neuron-glia adhesion molecule in some parts of the CNS, while it is not in others.

Modification of olfactory bulb synaptic inhibition by early unilateral olfactory deprivation

Early unilateral olfactory deprivation produces large structural and neurochemical changes in the olfactory bulb, the first central relay for olfactory information. The functioning of deprived bulbs was examined in the present report by using paired-pulse stimulation of the lateral olfactory tract. Paired-pulse stimulation reflects interactions between mitral/tufted cells and granule cells, as well as the modulatory effects of centrifugal and intra-bulbar association fibers. Paired-pulse stimulation produced inhibition of mitral/tufted cells in control animals at PN20-PN22. This inhibition was significantly enhanced in littermates deprived of olfactory input from PN1 to PN20-PN22. Suppression of mitral/tufted cell single-unit spontaneous activity following single-pulse stimulation of the lateral olfactory tract (LOT) was similarly enhanced in deprived bulbs. These results suggest that early olfactory deprivation significantly modifies subsequent olfactory system function.

Morphology of neuropeptide Y-immunoreactive neurons in the cat olfactory bulb and olfactory peduncle: postnatal development and species comparison

The distribution and morphology of Neuropeptide Y-immunoreactive (NPY-ir) neurons in the olfactory bulb and the olfactory peduncle was studied in the adult cat and rat, and the common marmoset Callithrix jacchus. Significant species differences were not observed. In all three species, the population of NPY-ir neurons is localized in the white matter extending from the main olfactory bulb to the border of the striatum. The neurons are characterized by a conspicuously looping axonal ramification pattern with some major collaterals running toward the olfactory bulb and others running toward the internal olfactory tract. The former, ipsilateral projection terminates in the granule cell layer of the main and accessory olfactory bulb and in layer II/III of the anterior olfactory nucleus. Reconstruction of the latter projection has revealed that the fibers are continuous with the olfactory limb of the anterior commissure and the anterior commissure proper suggesting a commissural contralateral projection. The analysis of the postnatal development of the cat NPY neuron system supports this assumption in a very clear-cut way. In young animals growing fibers are observed to cross the brachium of the commissure. The NPY neuron system develops postnatally. The maximum cell number is reached during the third postnatal week. The appearance of more and more NPY-ir neurons slightly precedes the formation of the terminal fields and of the fiber projection in the internal olfactory tract. The density of this early fiber projection by far exceeds the fiber density observed in the adult. Later in development the fiber density in the olfactory limb and the anterior commissure becomes considerably reduced. In contrast, the plexus density in the anterior olfactory nucleus and the granule cell layer of the main and accessory olfactory bulb undergoes only a slight reduction, and the NPY-ir cell number remains roughly constant. These observations suggest that the ipsilateral NPY-ir projection remains largely unchanged, in contrast to the contralateral projection, which exists to a large extent only for the first four postnatal months. The observation that the NPY neuron system gives rise to a contralateral projection does not support a classification of NPY neurons as short axon cells.

Postnatal neurogenesis in the nucleus sphericus of the lizard, Podarcis hispanica

Autoradiography was used to demonstrate the genesis and migration of cells in the nucleus sphericus of perinatal, juvenile and adult lizards, Podarcis hispanica (Sauria, Lacertidae). Following intraperitoneal injections of [3H]thymidine (5 microCi/g b. w.) and survival times of 1, 7, 18 and 28 days, labelled cells were found in the ependyma, marginal layer and mural layer of the nucleus sphericus in specimens of all ages. After short survival times, most labelled cells were located in the ependymal layer. Longer survival times resulted in labelling of cells mainly in the mural layer. At intermediate survival times, a substantial number of labelled cells was also found in the marginal layer. The time course of labelling suggests that neuronal proliferation takes place in the ependyma surrounding the ventricle. Subsequently, the newly formed neurons migrate centripetally and are recruited into the mural layer.

Early postnatal cellular proliferation and survival in the olfactory bulb and rostral migratory stream of normal and unilaterally odor-deprived rats

Unilateral naris occlusion in rats on postnatal Day 1 results in dramatic decreases in the size of specific olfactory bulb cell populations when pups are examined 30 days later (Frazier and Brunjes: J. Comp. Neurol. 269: 355-370, '88). The observed reductions must result from alterations in cell proliferation and/or survival, alternatives examined in the present study. During early postnatal development, most cells destined for the bulb are produced in regions caudal to the structure and migrate to the bulb in the massive rostral migratory stream. The dynamics of the stream were examined in both normal rats and pups with a single naris closed on Day 1. 3H-thymidine injections were made on postnatal Days 2, 5, 10, 20, and 30. Groups of pups were killed 2 hours later to assess patterns of proliferation and 24 hours later to gauge initial stages of migration. A gradient of labeled cells was observed in the stream, with higher levels occurring at more caudal locations. The supply of cells to the bulb peaked on Day 5 and was still substantial as late as Day 30. The deprivation procedure did not affect patterns of cell labeling at any stage tested, indicating the procedure does not affect early cellular proliferation. A third group of pups was examined 30 days after thymidine injection to assess both time of cell origin and survival rates. Dark granule cells and glia in the granule cell layer were produced at a consistent rate until Day 20 with cells added during the period evenly spread throughout the layer. Light granule and periglomerular cell production decreased dramatically after P5. Thirty days after injections on P2, fewer labeled dark granule cells and their associated glia were found in deprived bulbs, indicating that enhanced cell death plays a major role in the deprivation-induced decrease in cell number.

Induction of tyrosine hydroxylase expression in rat forebrain neurons

Olfactory nerve input is required for the normal expression of tyrosine hydroxylase (TH) by dopaminergic neurons in the glomerular region of the rodent main olfactory bulb. To determine whether the olfactory nerve exerts a similar influence on neurons in other brain regions, we performed unilateral bulbectomies in rat pups on postnatal day 5-7 and examined the brains 2-6 months later, after the regenerated olfactory nerve had penetrated the forebrain. Tissue was stained for TH, dopamine beta-hydroxylase (DBH) and olfactory marker protein immunoreactivity. We observed novel TH-immunoreactivity in neurons located in those areas of the adult forebrain which received olfactory nerve fibers, particularly the rostral extension of the subependymal layer. Many of these neurons resembled the periglomerular cells of the olfactory bulb. No cell staining for DBH was observed in these areas, suggesting the possible dopaminergic phenotype of these neurons. Our data indicate that afferent regulation of neurotransmitter expression by the olfactory nerve is not limited to the cells of the olfactory bulb.

Postnatal neurogenesis in the olfactory bulbs of a lizard. A tritiated thymidine autoradiographic study

Autoradiographically labelled cells were observed in the olfactory bulbs of perinatal, young and adult specimens of the lizard Podarcis hispanica following intraperitoneal injection of tritiated thymidine (5 muCi/g b.wt). After survival times of 7, 18 and 28 days labelled cells were found in the granular layer of both main and accessory bulbs. A few labelled cells were observed in the ependyma, mitral and glomerular layer. In the main olfactory bulb, one week of survival time resulted in labelling of cells in the innermost part of the granular layer. Longer survival times (up to 4 weeks), resulted in labelling of cells mainly in the outermost part of the granular layer. This spatio-temporal gradient was not observed in the accessory bulb. Nevertheless, longer survival times resulted in greater number of labelled cells located in the dorsal and ventral parts of the granular layer of the accessory bulb.

Noradrenergic modulation of olfactory bulb excitability in the postnatal rat

Noradrenergic centrifugal inputs to the rat olfactory bulb mainly terminate on granule cells, which are inhibitory interneurons. In the mature bulb, norepinephrine suppresses granule cell activity, thus increasing the excitability of the primary output neurons of the bulb. However, since the majority of granule cells develop postnatally, the effectiveness of noradrenergic inputs to the bulb during development is unclear. The present report describes the postnatal development of noradrenergic modulation of olfactory bulb function by examining the effects of noradrenergic beta-receptor agonists and antagonists on paired-pulse inhibition at the granule cell/mitral cell reciprocal synapse. The results demonstrate that noradrenergic modulation of olfactory bulb excitability emerges during the first postnatal week in the rat. These results suggest that noradrenergic centrifugal control of olfactory bulb activity appears early during postnatal development, and thus is capable of playing an important role in pup responses to olfactory cues early in life.

Neuropeptide- and neurotransmitter-related immunoreactivities in the developing rat olfactory bulb

The development of neuropeptide and neurotransmitter-related immunoreactivities in the rat olfactory bulb were investigated immunohistochemically by using antisera raised against substance P (SP), cholecystokinin-8 (CCK), neurotensin (NT), leucine-enkephalin or methionine-enkephalin-Arg6-Gly7-Leu8 (ENK), somatostatin (SOM), neuropeptide Y (NPY) and tyrosine hydroxylase (TH). Results obtained for the adult olfactory bulb confirmed previous observations, except for SP-like immunoreactive (SP-IR) granule cells in the main olfactory bulb (MOB) and NT-IR neurons around the modified glomerular complex (MGC) (Teicher et al., Brain Res. 194:530-535, 1980). SP-, CCK- and NT-IR neurons were observed in the MOB of the rat fetus. SP-IR neurons also appeared in the accessory olfactory bulb (AOB). Among them, NT-IR neurons in the MOB and SP-IR neurons in the AOB were observed on embryonic day 16. SP- and CCK-IR neurons in the MOB appeared on embryonic day 18. Most of these neurons were presumed to be projecting neurons. SOM-, NPY-, ENK- and TH-IR neurons appeared in the newborn rats. The number and intensity of immunostaining of these neurons continued to increase with age, producing the adult pattern, except for NT-IR neurons in the MGC and SP-IR neurons in the mitral cell layer of the AOB, which were more numerous and intensely stained in young animals.

Number of olfactory marker protein-containing receptor cells is influenced by developmental stage of the olfactory bulb

In vitro studies on E15 rat embryos have shown that the number of receptor neurons containing olfactory marker protein (OMP) is markedly increased when olfactory mucosa (OM) is cultured in direct contact with the presumptive olfactory bulb (POB). This facilitatory influence is tissue-specific; that is, it is absent when the POB is substituted with other nervous or non-nervous tissues. In the present quantitative immunohistochemical study, we show that the enhancing influence of the POB is also a stage-specific phenomenon. The number of OMP-containing receptor cells is greatest when E15 OM is cultured with POB of the same age. If the POB was taken from a less mature (E13) or from a more mature (E17) embryo, the number of OMP-containing cells was greatly reduced.

Deviations in brain development due to caloric undernutrition and scope of their prevention by rehabilitation: alterations in the power spectra of the EEG of areas of the neocortex and limbic system

Wistar rats underwent caloric undernutrition so as to produce undernourished pups (F1, both male and female), which were put on a reduced diet throughout life, causing deficits in body weight of 40-70%, and in brain weight of 14-20% by adulthood. These chronic undernourished rats were mated and their progeny (F2), which had suffered gestational and postnatal undernutrition, were undernourished, like their parents, throughout life until the moment of use in this study. At this degree of chronic undernutrition, rats do not appear to be sick or morbid, but only small-for-age, they are active and can reproduce. The computer-generated power spectra of the EEG of two neocortical areas (visual and motor) and of 3 regions of the limbic system (cingulate, hippocampus and olfactory bulb) of the chronically undernourished pups were obtained at different ages from birth to adulthood, and compared with the normal ontogenetic patterns of corresponding areas obtained from age-matched controls. In another group of rats, the undernutrition was imposed on normal pups from the day of weaning (21st day), i.e., after the major part of the brain growth spurt is over. Finally, the effect of restituting normal rehabilitatory nutrition from the day of weaning on the chronic group was also studied. The results show that all brain regions studied were affected by the chronic caloric undernutrition. The deviations from the normal consist of: (1) an initial lag or delay in the development of the EEG amplitude and activity by several days, (2) a later development of an abnormally high amplitude or power in the EEG power spectra by about the weaning age, and (3) an imbalance in the occurrence of the characteristics of the EEG of the respective areas of the brain (the low and high frequency spindles of slow sleep, the slow and fast theta activities of paradoxical sleep of the hippocampus, the delta rhythm, etc.). While the chronic undernutrition was continuing there was also a certain degree of spontaneous reduction in the magnitude of abnormality after about 80 days of age, instead of worsening with age, perhaps due to an intrinsic adaptational readjustment to the stress of the nutritional deprivation. The results reveal a variation in the susceptibility of development to these abnormalities. Typical examples of the range of variation of the effects are presented in the results showing that there can be nearly normal EEG patterns in some undernourished subjects (about 14-22%), and also that there can be abnormal EEG power spectra in a proportion of normals (up to about 20%).(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical demonstration of radial glia in the developing rat olfactory bulb with antibodies to glial fibrillary acidic protein

Olfactory bulbs of 8-, 12-, 16- and 30-day-old rats were studied by means of immunocytochemistry using antibodies to glial fibrillary acidic protein (GFAP) and with the Toluidine blue-staining of semithin sections. Until day 12 the GFAP-reaction revealed a radial glia system, the fibres of which extended from the axial ventricular cleft to the surface. From day 16 onwards radial fibres were gradually replaced by typical astrocytes. The lack of proliferative activity within the bulb during the early postnatal period suggests that its cells are generated at and migrating from an external site. An intensely proliferating area was detected in the frontal lobe subventricular layer from where a bundle of migratory cells extends into the bulb. Radial glia may thus be of importance in guiding the migration of cells from this axial bundle to more peripheral regions of the olfactory bulb.

One-trial olfactory learning enhances olfactory bulb responses to an appetitive conditioned odor in 7-day-old rats

The expression of a conditioned odor preference and focal uptake of [14C]2-deoxyglucose (2-DG) within the olfactory bulb was assessed in neonatal rat pups that had undergone a single olfactory classical conditioning trial. At 6 days of age, rat pups were simultaneously exposed for 10 min to an odor (peppermint) and to a reinforcing tactile stimulation similar to that received from the dam. Three control groups received only the odor, only the stimulation, or neither of these stimuli. The next day, pups were either assessed for differential olfactory bulb activity using the 2-DG technique or tested for their olfactory preference behavior. Only pups that received simultaneous odor and tactile stimulation exhibited an attraction to the conditioned odor in the two-odor choice test. Furthermore, such pups had greater focal 2-DG uptake in the olfactory bulb glomeruli that were responsive to the odor than pups in all other groups. Thus, the olfactory bulb responds differentially to an odor which has acquired attractive value.

Abrupt decrease in synaptic inhibition in the postnatal rat olfactory bulb

Olfactory bulb responses to paired-pulse stimulation of the lateral olfactory tract were examined in urethane-anesthetized rats, aged 5 days to adult. Brief inter-pulse intervals resulted in a depression of test responses at all ages. The magnitude of this depression decreased dramatically between postnatal days 19 and 20 to approach adult levels. Longer inter-pulse intervals resulted in a facilitation of test response amplitude in adult animals. This facilitation was evident at adult levels by postnatal day 10. These results suggest that both inhibitory and facilitatory synaptic mechanisms appear early in the course of rat olfactory bulb development. Furthermore, presumed granule cell-mediated inhibition is present at unusually high levels in the developing bulb, decreasing sharply between days 19 and 20.

Golgi studies on the development of granule cells of the rat olfactory bulb with reference to migration in the subependymal layer

The morphology and the development of the cells in the subependymal layer and of granule cells of the olfactory bulb were examined by Nissl and Golgi staining in postnatal rats. The subependymal layer around the anterior lateral ventricle extends into the center of the olfactory bulb. The mitotic indexes in the subependymal layer are high at the level of the anterior horn of the lateral ventricle and very low inside the olfactory bulb during the first 3 weeks after birth. Golgi-stained subependymal cells are classified into two main groups. One group consists of smoothly contoured bipolar cells with leading processes tipped by large growth cones and with trailing processes. They make up a majority of Golgi-stained subependymal cells during the first 3 weeks of age, and smaller numbers of them continue to exist at 37 and 60 days. They migrate with their growth cones oriented toward the olfactory bulb from the level of the anterior lateral ventricle into the granular layer of the olfactory bulb, where they differentiate into the definitive granule cells: their somata enlarge; the leading processes elongate, branch, sprout many gemmules, and become the peripheral processes; and the trailing processes become the basal dendrites. The other group contains relatively large cells with many cytoplasmic processes that are considered to belong to the glial cell line.

Early X-irradiation of rats--II. Effect on granule cells and their dendrodendritic synapses in the olfactory bulb

Low, repeated doses of X-rays from a Co60 source were used to impair the development of the granule cells and their dendritic terminals in the olfactory bulb, and the resulting effect was studied under light and electron microscopes at 9 days of age. Irradiation of rats from embryonic day 18 (in utero) to postnatal day 5 resulted, among others, in maldevelopment of the (internal) granule cell and external plexiform layers. This was accompanied by a decrease in the number and the density of the granule cells, and the remaining granule cells contained less ribosomes, regardless of their position within the layer. This implies that both supposed subtypes of granule cells were effected. In the external plexiform layer, a reduced number of mature dendrodendritic synapses and signs of harmed granule gemmules were observed. The results suggest that intrauterinal plus postnatal irradiation with low, repeated doses of X-rays may be an effective tool impairing the development of prenatally forming neurons.

Early X-irradiation of rats. 1. Methodological description and morphological observations on the olfactory bulb

X-ray irradiation of rats in utero and during early postnatal days was performed using low, fragmented doses from a 60Co radiation source. Different radiation schedules were applied in order to impair the development of certain neurons. The effect of radiation on the main olfactory bulb was characterized in several ways. Early X-ray irradiation resulted in retardation of dams and young pups and an underdevelopment of the bulb and its main histological layers, especially the granule cell and external plexiform layers, but a disturbed glomerular development was also apparent. The resulting damage was proportional to the magnitude of the irradiation. The present technique seems to be useful in eliciting damage in separate populations of pre- or postnatally forming neurons.

Relative contribution of brain and peripheral connections to postnatal growth and cell accretion in the rat olfactory bulb

Total weight and DNA (cell number) determination in whole olfactory bulbs (OB) revealed that unilateral transection of the olfactory peduncle in newborn rats causes 2 X more (40%) deficit in cell number in the operated OB than section of the olfactory nerve. The deficit due to combined operations ('isolation' of OB) were additive. Similar effects were observed on weight gain. It is inferred that both olfactory nerve and sources in the brain exert marked influences on postnatal cell accretion and growth in the OB and that the greater share of the brain is due to its direct contribution of new microneurons via the 'rostral migratory stream'.

Neuronal changes in the forebrain of mice following penetration by regenerating olfactory axons

Following total, unilateral bulbectomy in neonatal mice, the olfactory sensory axons regrow from a reconstituted population of sensory neurons, cross the lamina cribrosa, and invade the spared forebrain that has leaned forward toward the anteroventral wall of the cranial cavity. The sensory axons invade several regions of the spared forebrain, at times penetrating deeply into the brain parenchyma. These axons terminate in characteristic globose structures resembling the glomeruli of the olfactory bulb. However, they can be distinguished from the latter by the absence of periglomerular cells. These ectopic glomerular structures are formed by the commingling of the olfactory axon terminals and the dendrites of brain neurons that lie in their proximity. Previously we have established that synaptic contacts occur between the sensory axon terminals and the dendrites of the brain neurons. Our present study describes large neurons, resembling mitral cells, that expand their dendrites into the intracerebral glomeruli. These neurons are recognized by virtue of their relatively large diameter, their selective stainability with silver methods, and the unorthodox arrangement of their dendrites in comparison with the neurons of the region. Their appearance is contingent upon the presence of ectopic glomeruli. The possibility is discussed that the large argyrophilic neurons may be derived from developing neuronal elements of the brain.

Changes in granule cells of the ferret olfactory bulb associated with imprinting on prey odours

The maturation of the granule cells of the ferret olfactory bulb around the time of odour imprinting has been examined. Rapid Golgi impregnation studies revealed a temporal overshoot in the development of the spines on the external and internal dendrites of the granule cells. In contrast, the number of somatic spines decreased continuously. Electron microscopical examinations of the synaptic contacts in the external plexiform layer revealed that the time course of synapse and reciprocal synapse formation was similar to that of the formation of the spines on the external dendrites. The results were taken as evidence that both the Golgi and the electron microscopical investigations described the same developmental process of postnatal synaptic rearrangement.

Early appearance of inhibition in the neonatal rat olfactory bulb

The functional development of inhibition in the rat olfactory bulb was examined in the present study. Inhibition of presumed mitral cell spontaneous activity following stimulation of the lateral olfactory tract was present by postnatal day 5, the youngest age tested. The duration of this inhibition was greatest in young animals, decreasing after postnatal day 15. Possible mechanisms of this enhanced inhibition in neonates were discussed.

Absence of correlations between glutamine-synthetase activity and dysmyelination-associated modifications of astroglia in the brain of murine mutants

Glutamine Synthetase (GS) activity was investigated in cerebellum (ce), cerebral cortex (cc), olfactory bulb (ob), and medulla oblongata (mo) of murine dysmyelinating mutants for correlations with modifications of astroglia associated with genetic dysmyelination. One of these mutants, jimpy, develops a strong gliosis throughout the CNS. The other three mutants: shiverer, mld, and quaking, exhibit various astrocytic responses to dysmyelination, but reduced gliosis if any. Comparison between CNS areas in control animals showed a higher GS activity in the olfactory bulb than in the cerebral cortex, medulla, and cerebellum. The developmental patterns of GS activity were similar in mutants and in controls in all four areas investigated. Data on Jimpy suggest that GS activity is not associated with reactive astrocytes.

Olfactory bulb responses after odor aversion learning by young rats

The increased olfactory bulb response by young rats to familiar odors was not observed in response to odors which have attained their familiarity in aversive situations. Odor experience associated with toxicosis induced a behavioral aversion to the odor which was not accompanied by the enhanced uptake of [14C]2-deoxyglucose (2-DG) that accompanies attractive familiar odors. A single odor exposure on day 17 was sufficient to induce a small increase in 2-DG uptake in specific glomerular areas. We hypothesize that a different neural substrate underlies familiarity associated with an aversive odor than that associated with an attractive odor.

Olfactory deprivation enhances normal spine loss in the olfactory bulb of developing ferrets

Ferrets show a sensitive phase in their postnatal development during which they can become imprinted to food odors. At the same time the number of granule cell spines in the olfactory bulb reaches a maximum, declining significantly thereafter. In ferrets, exposed continuously to saturated levels of geraniol odor in the cage environment, the normal decline in spine number (occurring between day 60 and 90) is significantly enhanced. No such effects were observed during earlier ontogenetic phases. This late postnatal phase is further associated with a marked and significant decrease in total brain weight. The significance of these events to olfactory imprinting and plasticity in the developing brain is discussed.

Evoked olfactory responses in neonatal rats

Although there is behavioral evidence that rat pups respond to odors, the physiological basis of the response is unclear. In the present study, eight different odors were presented to 44 anesthetized rat pups between the ages of 0 and 20 days, and recordings of electrical activity were made from the olfactory bulb. The major finding was that, contrary to earlier neurophysiological findings, even on the first day of life--when spontaneous electrical activity is minimal--evoked potentials, in the form of synchronized waveforms two to four times the amplitude of the background level of spontaneous activity, can be recorded to a variety of odor stimuli.

Ultrastructural characterization of synaptic terminals formed on newly generated neurons in a song control nucleus of the adult canary forebrain

The fine structure of synaptic terminals contacting neurons generated in the forebrain of adult male canaries was investigated by autoradiography and electron microscopy. The procedure for labeling the new neurons included pretreating adult canaries with 3H-thymidine and sacrificing them 23-45 days later. Neurons were identified as newly generated by the presence of 3H-thymidine in the cell nucleus. The new neurons in the nucleus hyperstriatum ventralis, pars caudalis (HVc) were identified by autoradiography and light microscopy and examined with electron microscopy. Several types of synaptic terminals contacted the cell body and proximal dendrites of the newly formed neurons. Synaptic junctions were formed by terminals that contained spherical, agranular vesicles, large dense-core vesicles and spherical, agranular vesicles, and pleomorphic or flattened synaptic vesicles. Terminals that contained spherical vesicles were most often associated with asymmetric synaptic densities, and terminals that contained pleomorphic or flattened vesicles formed symmetric junctions. New neurons were also contacted by small terminals that contained few vesicles and had little pre- or postsynaptic density associated with the junction; these terminals may be a special type or may be in the process of developing their synaptic contact with the new neuron. In addition, rare terminals that appeared to be degenerating or to contain debris from other degenerating neural elements contacted new neurons. In summary, these data indicate that the new neurons, which are known to be inserted into existing neural networks, receive synaptic input from at least three different sources.

Acetylcholinesterase activity in the developing olfactory bulb: a biochemical study on normal maturation and the influence of peripheral and central connections

In the newborn rat olfactory bulb (OB), the specific activity of acetylcholinesterase (AChE) is about 20% of the adult value. During postnatal development, the specific activity remains unchanged until day 10; a growth spurt of 5X is observed between days 10 and 25, when adult values (90 nmoles ACh/min/mg protein; 6 nmol/min/mg wet wt.) are reached. However, total activity shows continuous increase slowly at first and rapidly between days 10 to 30, reaching a plateau by day 60. Between birth to day 60 total activity increases 65X. To determine the influence of peripheral and central connections on the development of AChE activity in the OB, rats were subjected to unilateral olfactory denervation and/or transection of olfactory peduncle, carried out either neonatally or at day 30; the bulbs were assayed a month later (days 30 and 60 respectively). It was found that in both neonatal and 30-day-old rats, denervation caused a 15% decrease in total activity, while transection led to more than 60% reduction. In the older rats, the reduction due to transection represented the degenerative loss of activity, but in the neonatally transected bulbs the growth of some cholinergic elements continued although very slowly. Between birth to day 30, total AChE activity increased only 12X in completely isolated OB, 25X in transected OB and 40X in denervated OB, compared to 45-50X in control OB. In the transected and isolated bulbs specific activity of AChE was also reduced significantly (25-50% depending on age and operation). These results suggest that while centrifugal fibers are the main source of cholinergic activity in the mature as well as the developing OB, the olfactory nerve and some intrabulbar sources such as cholinergic cells or cholinoceptive membranes also contribute to AChE activity in the OB. These intrinsic sources of AChE activity can persist and even show some growth in the developing olfactory bulb in the absence of the centrifugal fibers and/or the olfactory afferents.

The effects of protein deprivation on neuronal migration in rats

Although there is extensive literature documenting the effects of undernutrition on brain development, most studies have been concerned with cell differentiation with little attention given to neuronal migration. In a rat model we have investigated the effect of chronic protein deprivation on cell migration from the anterior lateral ventricle to the olfactory bulb. By using standard autoradiographic techniques and comparing the position of heavily labeled cells within the migratory stream, we estimated the migration rate to be 100 microns/h in 25%-casein-diet rats and between 33 and 70 microns/h in 8%-casein-diet rats. We therefore conclude that migration is slowed in chronically protein-deprived rats and this slowed migration may be related to subsequent abnormalities of cell differentiation seen in protein-deprived rats.

Ontogeny of electrical activity of main olfactory bulb in freely-moving normal and malnourished rats

Electroencephalographic activity (EEG) was recorded from the main olfactory bulb (MOB) in freely-moving, normally-nourished, (NP, normal-protein diet) and malnourished (LP, low-protein diet) rats from 4 days of age through adulthood. MOB EEG was analyzed for dominant frequency components using power spectral techniques. For NP rats, a single dominant frequency component (induced wave) was present in the MOB EEG at 4-6 days of age. From 10 days of age through adulthood, the MOB EEG contained two dominant frequency components (induced and intrinsic waves). Both the induced wave and intrinsic waves increased in center-frequency to reach maturity at approximately 30 days of age. Rats reared on low-protein diets (8% casein, prenatal and postnatal) displayed relatively permanent retardation in the development of induced wave center-frequencies and a delay in the development of the intrinsic wave center-frequencies. These results closely parallel the morphological development of the MOB in normally-nourished and malnourished rats.

Proliferation of primary sensory neurons in adult rat dorsal root ganglion and the kinetics of retrograde cell loss after sciatic nerve section

This study was aimed at measuring the kinetics of retrograde death among primary sensory neurons axotomized by transection of the ipsilateral sciatic nerve in adult rats. Using electrophysiological and retrograde transport methods, we first determined that most sciatic afferents enter the spinal cord along the L4 and L5 dorsal roots (DRs), and that about 54% of the cells in the L4 and L5 dorsal root ganglia (DRGs) project an axon into the sciatic nerve. Knowing this value, we could then calculate the rate of loss of axotomized neurons from the overall rate of neuron loss in the DRGs at different times after the lesion. Following unilateral sciatic neurectomy, we found a steady falloff in the ratio of DRG neurons on the operated versus the intact control sides in cresyl-violet-stained serial paraffin sections. We were surprised to note, however, that on the control side there was a steady increase in the cell count with age. Counts done on a series of unoperated rats of various ages confirmed this natural increase. Overall, new neurons accrete at an average rate of 18.1 cells per day to the combined L4 and L5 DRGs, nearly doubling their numbers during the adult life of the animal. The new cells add mostly to the small-diameter neuronal compartment. Evidence from neonatally operated rats indicates that the decline in the ratio of neurons in operated versus control DRGs following sciatic nerve section in the adult results more from a halt in the accretion of new neurons to the sciatic compartment than from frank cell death. From our data, we calculate that the loss of axotomized neurons occurs at a rate of only about 8% per 100 postoperative days.

Evidence for olfactory function in utero

Pregnant rats received 2-[14C]deoxy-D-glucose (2DG) intravenously on the last day of gestation, and their fetuses were delivered 1 hour later by cesarean section. Fetal brains showed high 2DG uptake spread throughout the accessory olfactory bulb and little or no differential uptake in the main olfactory bulb. These findings demonstrate that functional activity occurs in the accessory olfactory bulb in utero and suggest that the accessory olfactory system may be the pathway by which fetal rats detect the odor quality of their intrauterine milieu.

Development of the serotonergic system in the rat embryo: an immunocytochemical study

The development of central serotonergic neurons has been examined immunocytochemically utilizing an antiserum to serotonin (5-HT). Cells of the B4-B9 complex are first detected early on embryonic day 13 (E13; 7 mm crown rump length, CRL) and increase rapidly in number through E15 when they appear as bilateral columns situated from just caudal to the mesencephalic flexure to the pontine flexure. Aggregation of cells into subgroups is apparent soon after 5-HT neurons leave the ventricular zone, allowing the identification of certain subdivisions of the B4-B9 complex long before they assume their adult locations. The initial detection of 5-HT immunoreactive cells in the medulla occurs 1-2 days after the appearance of cells in the B4-B9 complex, although it has been reported that the time of origin of medullary raphe neurons (B1-B3) occurs before that of raphe neurons in the midbrain and pons (B4-B9). The first medullary 5-HT neurons, comprising the B3 subdivision occur ventro-laterally on E14 (10-11 mm CRL) at least 1-2 days before midline 5-HT neurons are visualized in the B1 and B2 groups. Thus, in contrast to cells in the B4-B9 complex, medullary 5-HT neurons complete much of their migration before they can be detected immunocytochemically, indicating that the time of onset of transmitter synthesis and storage may differ during differentiation of cells sharing a common neurotransmitter phenotype. The formation of ascending 5-HT fiber projections occurs rapidly from cells of the B4-B9 complex. Within 24 hours after the initial detection of 5-HT fiber immunoreactivity in such cells at E13, their axons are seen entering the caudal diencephalon (E14). These fibers have traversed the diencephalon and floor of the telencephalon by E15-E16 and reach the frontal neocortical pole by E17. The main ascending bundle of 5-HT axons courses through the diencephalon in the vicinity of the medial forebrain bundle, although some fibers also diverge and travel along certain pre-existing non-5HT pathways. However, examples are also found of acute directional changes in 5-HT fiber growth which do not appear to be associated with pre-formed non-5HT pathways. The pattern of ascending fiber outgrowth suggests a priority routing system which provides certain regions with 5-HT axons in a preferential sequence irrespective of the distance of these areas from 5-HT cell groups or from major bundles of ascending 5-HT fibers.

Neuronal production, migration, and differentiation in a vocal control nucleus of the adult female canary brain

The vocal control nucleus designated HVc (hyperstriatum ventrale, pars caudalis) of adult female canaries expands in response to systemic testosterone administration, which also induces the females to sing in a male-like manner. We became interested in the possibility of neurogenesis as a potential basis for this phenomenon. Intact adult female canaries were injected with [3H]thymidine over a 2-day period. Some birds were given testosterone implants at various times before thymidine. The birds were sacrificed 5 wk after hormone implantation, and their brains were processed for autoradiography. In parallel control experiments, some birds were given implants of cholesterol instead of testosterone. All birds showed considerable numbers of labeled neurons, glia, endothelia, and ventricular zone cells in and around HVc. Ultrastructural analysis confirmed the identity of these labeled neurons. Cholesterol- and testosterone-treated birds had similar neuronal labeling indices, which ranged from 1.8% to 4.0% in HVc. Thus, neurogenesis occurred in these adults independently of exogenous hormone treatment. Conversely, both glial and endothelial proliferation rates were markedly stimulated by exogenous testosterone treatment. We determined the origin of the thymidine-incorporating neurons by sacrificing two thymidine-treated females soon after their thymidine injections, precluding any significant migration of newly labeled cells. Analysis of these brains revealed no cells of neuronal morphology present in HVc but a very heavily labeled ventricular zone overlying HVc. We conclude that neuronal precursors exist in the HVc ventricular zone that incorporate tritiated thymidine during the S phase preceding their mitosis; after division these cells migrate into, and to some extent beyond, HVc. This ventricular zone neurogenesis seems to be a normally occurring phenomenon in intact adult female canaries.

Postnatal proliferation and maturation of olfactory bulb neurons in the rat

Mitral cells are formed prenatally whereas most granule cells originate postnatally. Material was taken from 2-day-old, 14-day-old, 28-day-old, and adult rat olfactory bulbs and processed for rapid Golgi or Cresyl Violet staining. We show that the number of granule cell bodies/mitral cell body increases from 7.0 to 46.3 during the first two weeks of life; most mitral cells appear morphologically functional during the first postnatal week; few granule cells appear to be functional until the second postnatal week; and the number of short axon interneurons increases dramatically during the second postnatal week. We conclude the newborn rats have an intact afferent pathway from olfactory receptors to primary cortex that lacks the extensive interneuronal circuitry characteristic of adults.

Changes in the total number of dentate granule cells in juvenile and adult rats: a correlated volumetric and 3H-thymidine autoradiographic study

The total number of granule cells in the dentate gyrus was estimated in 17 male rats, four each aged 30, 120, and 200 days, and five aged 365 days. There is a substantial 35-43% linear increase between 1 month and 1 year. Two parameters of the granular layer are involved in the numerical change. First, total granular layer volume grows linearly with age. Second, average volume of a single granule cell nucleus in the ventral dentate gyrus decreases with age. Older rats tend to have a larger granular layer filled with more and smaller cells. In another group of 21 male rats, 3H-thymidine injections were given on four consecutive days during juvenile (30-33, n = 6) and adult life (60-63, n = 5; 120-123, n = 6; 180-183, n = 4). All animals survived to 200 days of age. The proportion of labeled mature granule cells and labeled presumptive granule cell precursors were determined in anatomically-matched slices. With older ages at injection, there is a decline in labeled mature granule cells and a concurrent increase in labeled precursors. These data are compatible with the constant level of granule cell increase determined volumetrically. Most of the late granule cells originate nearly simultaneously along the base of the main bulk of the granular layer; very few are found in the dorsal tip (septal extreme) and ventral tip (temporal extreme). This study is the first demonstration of a net numerical gain in a neuronal population during adulthood in the mammalian brain. Since the granule adulthood in the mammalian brain. Since the granule cells play a pivotal role in hippocampal function, these data suggest that their influence grows with age.

Light microscope differentiation of two populations of rat olfactory bulb granule cells

Examination of glycolmethacrylate embedded olfactory bulbs of normal rats revealed that the granule cells of the accessory olfactory bulb were dissimilar from the majority of main olfactory bulb granule cells. Cells from these structures can be discriminated on the basis of spherical form. nuclear diameter, nucleoplasm staining, basal dendritic arborizations, and susceptibility to loss after neonatal X-irradiation. Based on their staining qualities, we have called these cells light and dark granule cells. In the normal rat the dark granules make up about 85% of granule cells in the main olfactory bulb and is the cell type preferentially killed by neonatal X-irradiation. Timing of postnatal X-irradiation and consequent differential loss of the dark type of main olfactory bulb granule cell suggests that the light type is largely prenatally formed.