Central Olfactory System Research Articles

Maps of Odorant Molecular Features in the Mammalian Olfactory Bulb

The olfactory bulb (OB) is the first relay station of the central olfactory system in the mammalian brain and contains a few thousand glomeruli on its surface. Because individual glomeruli represent a single odorant receptor, the glomerular sheet of the OB forms odorant receptor maps. This review summarizes the emerging view of the spatial organization of the odorant receptor maps. Recent studies suggest that individual odorant receptors are molecular-feature detecting units, and so are individual glomeruli in the OB. How are the molecular-feature detecting units spatially arranged in the glomerular sheet? To characterize the molecular-feature specificity of an individual glomerulus, it is necessary to determine the molecular receptive range (MRR) of the glomerulus and to compare the molecular structure of odorants within the MRR. Studies of the MRR mapping show that 1) individual glomeruli typically respond to a range of odorants that share a specific combination of molecular features, 2) each glomerulus appears to be unique in its MRR property, and 3) glomeruli with similar MRR properties gather together in proximity and form molecular-feature clusters. The molecular-feature clusters are located at stereotypical positions in the OB and might be part of the neural representation of basic odor quality. Detailed studies suggest that the glomerular sheet represents the characteristic molecular features in a systematic, gradual, and multidimensional fashion. The molecular-feature maps provide a basis for understanding how the olfactory cortex reads the odor maps of the OB.

Temporal dynamics and latency patterns of receptor neuron input to the olfactory bulb.

Odorants are first represented in the brain by distributed patterns of activity in the olfactory bulb (OB). Although neurons downstream of sensory inputs respond to odorants with temporally structured activity, sensory inputs to glomeruli are typically described as static maps. Here, we imaged the temporal dynamics of receptor neuron input to the OB with a calcium-sensitive dye in the olfactory receptor nerve terminals in anesthetized mice. We found that diverse, glomerulus- and odorant-dependent temporal dynamics are present even at this initial input stage. Instantaneous spatial patterns of receptor input to glomeruli changed both within and between respiration cycles. Glomerular odorant responses differed in amplitude, latency, rise time, and degree of modulation by sniffing in an odorant-specific manner. Pattern dynamics within the first respiration cycle recurred in a similar manner during consecutive cycles. When sniff rate was increased artificially, pattern dynamics were preserved in the first sniff but were attenuated during subsequent sniffs. Temporal response properties were consistent across individuals on a coarse regional scale and on a fine scale of individual glomeruli. Latency and magnitude of glomerular inputs were only weakly correlated and might therefore convey independent odorant information. These data demonstrate that glomerular maps of primary sensory input to the OB are temporally dynamic. These dynamics may contribute to the representation of odorant information and affect information processing in the central olfactory system of rodents.

Functional Magnetic Resonance Imaging Study of Human Olfaction and Normal Aging

The function of human olfaction declines with advancing age. An important question centers on whether functional alterations to olfactory brain structures accompany age-related behavioral changes. In the present study, we tested the hypothesis that aged adults have intact though reduced activity in the central olfactory system using functional magnetic resonance imaging (fMRI). University of Pennsylvania Smell Identification Test (UPSIT) was used to test the smell function of 11 young (23.9 +/- 1.6 years) and 8 aged (66.4 +/- 4.4 years) healthy participants. Then, the participants received fMRI at 3.0 T with lavender and spearmint as stimulants. After fMRI, the participants provided ratings for the odorants' intensity and pleasantness. The average UPSIT score of the aged adults was 34.1 +/- 1.5, which was significantly lower than that of the young adults (37.3 +/- 1.1) (p =.0004). Both age groups showed significant activation in major olfactory brain structures, including the primary olfactory cortex, entorhinal cortex, hippocampus and parahippocampal cortex, thalamus, hypothalamus, orbitofrontal cortex, and insular cortex and its extension into the inferior lateral frontal region. The aged adults showed less brain activity in olfactory structures (p =.022), consistent with lower ratings of odor intensity and UPSIT scores. Activation intensity in bilateral primary olfactory cortex areas and right insular cortex was also comparatively weaker (p <.019). Results demonstrate that significant activation in aged adults can be observed in all the olfactory brain structures that are activated in young adults, but with lower activation volume and intensity. This finding provides a necessary baseline for further investigations in olfaction and aging.

Developmental changes in the structure and function of the central olfactory system in gregarious and solitary desert locusts

Desert locusts are guided by olfactory cues in different behavioural contexts. In order to understand the basis for the variable olfactory guided behaviour displayed by different developmental stages and by solitary and gregarious locusts, we investigated their central olfactory system with neuroanatomical and neurophysiological methods. The primary olfactory centre of the brain, the antennal lobe (AL), increases in size during development due to an increased number and size of glomeruli. These glomeruli are innervated by a constant number of projection neurons that display increased dendritic arborizations during the development of the locust. The anatomical parameters do not differ between gregarious and solitary locusts. In parallel with the observed neuroanatomical changes, neurophysiological changes in response spectra and response specificity of AL neurons were found. During development, the percentage of neurons responding specifically to aggregation pheromone components decreases, whereas an increase in both pheromone-generalists and plant-pheromone generalist neurons is observed. The percentage of neurons responding to green leaf volatiles, however, remains constant. A decrease in the number of nymph blend-specific neurons was also observed. Our data show that anatomical and physiological properties of the AL and its neurons to a large extent reflect the changes in olfactory guided behaviour during development and between phases. The majority of our results are also in accordance with findings that the number of olfactory receptor neurons increases during development, resulting in increasing convergence on AL neurons.

The minimum number of neurons in the central olfactory pathway in relation to its function: a retrograde fiber tracing study.

The present study was aimed at determining the functionally essential size of the neuronal population in the central olfactory nervous system. Using conditioned rats who had learnt to avoid repellent (cycloheximide) solution by olfaction, varying degrees of injuries were made to the lateral olfactory tract, a major central olfactory pathway connecting the olfactory bulb to the olfactory cortex. After examining their olfactory ability to discriminate cycloheximide solution from water, intact bulbar projection neurons (mitral cells) with fiber connections to the olfactory cortex were quantified using a retrograde fiber tracing technique. The numbers of retrogradely labeled mitral cells from the rats with normal olfaction ranged between 20 and 92% of the control value, while those numbers from the anosmic rats ranged between 0 and 22%. We conclude that the functionally essential neuronal population is approximately one-fifth of the total in the central olfactory pathway, a presumed threshold value in terms of the ability to avoid cycloheximide solution by olfactory discrimination.

Juvenile-Hormone-Mediated Plasticity of Aggregation Behaviour and Olfactory Processing in Adult Desert Locusts

In desert locusts Schistocerca gregaria, aggregation behaviour is elicited by aggregation pheromones. In this study, we show that the behavioural response to the major and most potent adult aggregation pheromone component, phenylacetonitrile, is age- and juvenile-hormone-dependent. Furthermore, we show that juvenile hormone influences the responsiveness of olfactory interneurons in the antennal lobe to aggregation pheromone, whereas the responsiveness of antennal receptor neurons is not changed. Old locusts and locusts injected with juvenile hormone, in contrast to young locusts and locusts deprived of juvenile hormone through allatectomy, i.e. after surgical removal of the gland producing this hormone, do not display any aggregation behaviour, as indicated by long-term behavioural observations. The lack of positive olfactory-guided behaviour coincides with an impairment of the central olfactory system, which displays a lower number of neurons responding to aggregation pheromone. Indirect and direct actions of juvenile hormone at different levels of the central nervous system may thus contribute to the regulation and modulation of behavioural responsiveness in the locust.

Structural plasticity in the olfactory system of adult spiny lobsters: postembryonic development permits life-long growth, turnover, and regeneration

Caribbean spiny lobsters (Panulirus argus) rely on their sense of olfaction for many behaviours. Growth of their olfactory systems, and maintenance of olfactory function, is ensured by structural change that occurs continuously throughout life. In this paper, we review recent studies on postembryonic development in the olfactory system of P. argus and several other decapod species. Major structural change occurs in both the peripheral and central olfactory systems; it includes addition and loss of olfactory receptor neurons (ORNs), aesthetasc and other sensilla, and interneurons associated with the olfactory lobes of the brain. From these studies it is clear that continuous growth and turnover of olfactory tissue is a normal process in decapod crustaceans. In addition, we describe for the first time mechanisms that enable the peripheral olfactory system of spiny lobsters to regenerate after injury. We monitored the regeneration of olfactory tissue usingin vivo incorporation of the cell proliferation marker 5- bromo-2′-deoxyuridine (BrdU). Our results show that regeneration after partial antennular amputation, which reduces the length of the antennule and thereby the number of ORNs, occurs as a result of upregulation of the normal mode of ORN addition and down-regulation of loss. In contrast, localized injury to aesthetasc sensilla, which causes the associated ORNs to degenerate but does not reduce antennular length, is followed by local regeneration of olfactory tissue.

In goldfish the discriminative ability for odours persists after reduction of the olfactory epithelium, and rapidly returns after olfactory nerve axotomy and crossing bulbs.

Goldfish are ideal vertebrates for the study of regeneration within the peripheral and the central olfactory system. The present behavioural investigations studied the effects of bilateral lesions on the animals' ability to qualitatively discriminate two amino acids (10(-6) M) and their performance in two more difficult tasks: (i) rewarded amino acid applied in a lower concentration, and (ii) rewarded stimulus contaminated. A 50 and 85% reduction of the olfactory epithelium resulted in no recordable behavioural deficit. After axotomy of olfactory nerves and lateral olfactory tractotomy, fishes were anosmic for seven to ten days. Following replacement of sensory cells in the epithelium, and after regeneration of olfactory tract fibres a full functional recovery i.e. a highly specific regeneration, was recorded. After three surgical modifications of the olfactory bulbs' position, (i) crossing olfactory tracts and bulbs, (ii) crossing tracts and turning bulbs, and (iii) turning bulbs upside down, a full functional recovery was recorded for amino-acid discrimination in a similar concentration. A permanent, and similar slight deficit was, however, found during application of different concentrations, and of contaminated stimuli when medial lateral halves of the bulb were in 'incorrect' position (i) and (ii), or olfactory bulbs were positioned in the vicinity of the contralateral epithelium (i) and (ii).

Beta-amyloid deposition and neurofibrillary tangle formation in the olfactory bulb in ageing and Alzheimer's disease.

Impaired olfaction, hyposmia or anosmia are part of the clinical phenotype in neurodegenerative disorders including Alzheimer's disease (AD). It has been proposed that the most severely affected areas are interconnected with the central olfactory system in contrast to the relative sparing of other sensory areas which lack olfactory connections. The pathology of the first synaptic relay in the olfactory pathway, the olfactory bulb (OB), has been studied in AD, but the results have been inconsistent. In order to define more fully the pathology of the OB, we analysed 15 AD and 15 control cases, using amyloid and tau immunohistochemistry on serial sections. This study demonstrates for the first time that all layers of the OB are severely affected in AD and in normal ageing. The principal effector cells of the OB, the mitral cells, developed neurofibrillary tangles (NFTs) both in AD and in controls. All the cases, with the exception of two of the controls, contained NFTs. Amyloid immunoreactivity was detected in diffuse, primitive, classical and compact deposits in AD, while five control cases contained mainly diffuse deposits. We did not find a correlation between amyloid deposition and NFT formation. Among the control cases, two contained neither amyloid nor NFTs, eight had NFTs but no amyloid and only five had both NFTs and amyloid. All the AD cases had NFT and amyloid deposition. Our data suggest that the earlier pathology in the OB is NFT formation and more than ten NFTs/section is compatible with 93.3% diagnostic accuracy for AD.

EXERCISE INCREASES BLOOD FLOW TO LOCOMOTOR, VESTIBULAR, CARDIORESPIRATORY AND VISUAL REGIONS OF THE BRAIN

876 The purpose of this study was to test the hypothesis that regional brain blood flow distribution is altered by locomotor exercise. Brain blood flow was measured (radiolabeled microsphere technique) in miniature swine (n=6) during standing and treadmill exercise at 70 and 100% VO2MAX. Average brain blood flow increased from 57 ± 6 ml/min/100 g at rest to 70 ± 8 ml/min/100 g at 70%VO2MAX and 72 ± 5 ml/min/100 g at 100%VO2MAX. Perfusion of cortical areas of the brain was not altered by exercise, but flow to subcortical locomotor regions (midbrain, pons, rostral and caudal medulla, spinal cord) and the cerebellar hemispheres was elevated. Blood flow to regions associated with the maintenance of equilibrium (ventral vermis and floccular lobe of cerebellum and vestibular apparatus), cardiorespiratory regulation (pons and medulla), vision (dorsal occipital cortex and superior colliculi), and memory (hippocampus) was increased during submaximal and maximal exercise. In contrast, perfusion of structures of the central auditory pathway (cochlear nuclei, inferior colliculi and temporal cortex) and olfactory system (olfactory bulbs and rhinencephalon) was not elevated during exercise. These data indicate that subcortical locomotor regions and the cerebellum are actively involved in the control of treadmill locomotion, and that the neural activity of regions involved in the maintenance of balance, cardiorespiratory control and vision are heightened during submaximal and maximal exercise. Supported by NIH AM-37098, HL-36088 and NASA NAGW-4842, NAG5-3754.

Cellular and molecular neuropathology of the olfactory epithelium and central olfactory pathways in Alzheimer's disease and schizophrenia.

Specific deficits in odor detection threshold, identification, and memory have been recognized in a variety of disorders including the neurodegenerative disorder, Alzheimer's disease (AD), and the psychiatric illness, schizophrenia, which is likely due to abnormalities in neurodevelopment. Neuropathological abnormalities in peripheral and central olfactory systems have been described in both disorder. In the olfactory, epithelium, dystrophic neurites that are immunoreactive for tau, neurofilaments and other polypeptides, as well as deposits of beta-amyloid have been observed, and these findings have been thought to contribute to the olfactory dysfunction of these disorders. However, similar findings also occur in the olfactory epithelium of many normal individuals and those with various other neurodegenerative diseases. In contrast, neuropathological studies have reported selective vulnerability of central olfactory pathways for the accumulation of neurofibrillary pathology in AD, and for cytoarchitectural, neuronal morphometric, and cytoskeletal protein abnormalities suggestive of abnormal neurodevelopment in schizophrenia. Thus, it is likely that the olfactory impairments associated with these diseases are due to damage within central olfactory pathways, and that they are further amplified by the less specific impairments associated with age-related sensory neuroepithelial abnormalities. Finally, both the olfactory epithelium and central olfactory pathways represent model systems in which to study the neurobiology of these disorders, which ultimately may yield clues with diagnostic and therapeutic utility.

Electron microscopic study of synaptogenesis and myelination of the olfactory centers in developing rats.

Development of the central olfactory system was studied in the rat with an electron microscope at three main structures: the olfactory bulb, the lateral olfactory tract, and the primary olfactory cortex (the piriform cortex). As a parameter of development, the synaptic density was examined quantitatively in the bulbar glomerulus and layer Ia (termination of bulbofugal fibers) of the piriform cortex. which are the key stations of the olfactory pathway. The synaptic densities in the glomerulus and those in layer Ia were 5.7% and 4.6% on embryonic day 19, 15.8% and 12.5% on postnatal day (P) 0, and 57.3% and 37.2% on P10, as compared with the adult (100%). As another parameter of development, the density of myelinated axons in the lateral olfactory tract was examined quantitatively. The densities of myelinated axons in the tract were 0% on P5, 15.1% on P10, and 73.5% on P21 of the adult density. Maturation in the tract was still progressing, even at P21, in terms of bundle formation and the thickness of myelin sheaths. The results show that synaptogenesis in the bulbar glomerulus is followed by synaptogenesis in layer Ia of the piriform cortex, and that myelination in the lateral olfactory tract occurs over a prolonged period, even in the stages after P21.

Rat hepatic glutathione S-transferase-mediated embryotoxic bioactivation of ethylene dibromide.

The embryotoxic effects of ethylene dibromide (EDB) bioactivation, mediated by purified rat liver glutathione S-transferases (GST), were investigated using rat embryos in culture. Significant EDB metabolism was observed with rat liver GST purified by affinity chromatography (specific activity of 188 +/- 11.3 nmol/min/mg protein). The reaction was enzymatic in nature and the conjugation rate was proportional to the concentration of EDB (up to 0.75 mM) and the enzyme present in the reaction medium. EDB activation by 100 units (1 unit = 1 nmol of glutathione consumed per min) of purified rat liver GST caused a significant reduction in general development as measured by crown-rump length, yolk sac diameter, somite number, and the composite score for different morphological parameters (Brown and Fabro methodology). Structures most significantly affected were the central nervous and olfactory systems as well as the yolk sac circulation and allantois. The results of this study clearly indicate that under in vitro conditions, bioactivation of EDB by GST can lead to embryotoxicity.

A novel model to assess developmental toxicity of dihaloalkanes in humans: bioactivation of 1,2-dibromoethane by the isozymes of human fetal liver glutathione S-transferase.

Glutathione S-transferase (GST) isozymes from human fetal liver (16-18 weeks gestation) were purified by affinity chromatography followed by ion-exchange high performance liquid chromatography (HPLC). The purified isozymes were used to investigate toxicity of 1,2-dibromoethane(EDB) in an in vitro model of rat embryos in culture as passive targets. At least five isozymes of GST were found in the human fetal liver. Two anionic forms [pI values 5.5 (P-2) and 4.5 (P-3)] and one basic form [pI value 8.7 (P-6)] were clearly separated. The presence of two near-neutral forms was also identified. All the isozymes of the human fetal liver GSTs tested metabolized EDB (specific activities were 2.1, 7.0, and 2.0 mumol of GSH consumed/min/mg protein for P-2, P-3, and P-6 isozymes, respectively). Covalent binding of EDB to DNA and protein was 144% and 212% higher, respectively, with the P-3 anionic isozyme when compared to the P-6 basic isozyme of GST. No covalent binding to either protein or DNA was observed with the P-2 isozyme. EDB bioactivation by the GST isozyme P-3 (15 units; 1 unit = 1 nmol of GSH consumed/min) resulted in toxicity to cultured rat embryos. Significant reductions of crown rump length, yolk sac diameter, and the composite score of morphological parameters (Brown and Fabro method) were observed. The central nervous system, optic and olfactory systems, and the hind limb were most significantly affected. The results of this investigation suggest that EDB may be classified as a suspected developmental toxicant in humans.

Differential expression of mRNAs for the NGF family of neurotrophic factors in the adult rat central olfactory system

The cellular localization of mRNAs for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT3), in the rat central olfactory system was evaluated with in situ hybridization of 35S-labeled cRNA probes. In the main olfactory bulb, low levels of NGF and BDNF mRNA expression were detected. NGF mRNA was restricted to the glomerular region while BDNF mRNA was predominantly localized to the granule cell layer. No cellular hybridization to NT3 cRNA was seen. The accessory olfactory bulb did not express detectable levels of mRNA for any of the three related neurotrophic factors. Areas which receive olfactory bulb afferents expressed comparatively high levels of both NGF and BDNF mRNA. Cell labeling with cRNAs for NGF and BDNF occurred throughout the cellular layers of the anterior olfactory nucleus and in layers 2 and 3 of rostral piriform cortex. BDNF mRNA expression in these areas appeared more robust than that of NGF mRNA, while NT3 mRNA was not detectable. In contrast, tenia tecta exhibited dense labeling with the cRNAs for all three neurotrophic factors. The localization of NGF mRNA to primary target neurons of the olfactory nerve in the periglomerular region of the main olfactory bulb suggests that bulb cells may influence the ingrowth and continual turnover of olfactory sensory afferents. However, as there is a strong correlation between the distribution of neurotrophic factor mRNAs within rostral olfactory structures and the distribution of centrifugal cholinergic afferents, it is more likely that bulb-derived NGF, and possibly BDNF, act on the cholinergic neurons of the basal forebrain.

Neurovirulence of Two Clonally Related Herpes Simplex Virus Type 1 Strains in a Rabbit Seizure Model

Herpes simplex virus type 1 (HSV-1) strains vary widely with regard to neurovirulence, but their tropism for specific central nervous system structures and their ability to induce seizures are poorly defined. We have used the clonally related +GC and -GC strains of HSV-1 to define the pathophysiological basis of neurovirulence in a rabbit model. Following intranasal inoculation, +GC infection was nearly uniformly fatal while -GC infection was asymptomatic. The +GC infected animals developed electroencephalographic (EEG) abnormalities which preceded severe motor seizures. Tropism of the +GC strain for specific CNS nerve centers and the expression of viral antigens within them correlated with its virulence. Although both viruses invaded and replicated within the brain, +GC replicated to slightly higher titers and expressed more abundant viral antigen than -GC. The relatively less efficient replication of -GC appeared to correlate with its temperature-sensitive phenotype in vitro. Both +GC and -GC antigens were found in cerebral cortical layers IV-VI, and in several central nervous system trigeminal and olfactory system structures. However, +GC spread more completely throughout the brain to involve the amygdala, nucleus accumbens, several brainstem nuclei and the locus ceruleus. The +GC antigens were also found in cerebral cortical layer I of animals that developed seizures. These results indicate that the ability of HSV-1 to induce electrophysiologic brain abnormalities is associated with its ability to replicate within specific brain nerve centers.

Simulation of chaotic EEG patterns with a dynamic model of the olfactory system.

The main parts of the central olfactory system are the bulb (OB), anterior nucleus (AON), and prepyriform cortex (PC). Each part consists of a mass of excitatory or inhibitory neurons that is modelled in its noninteractive state by a 2nd order ordinary differential equation (ODE) having a static nonlinearity. The model is called a KOe or a KOi set respectively; it is evaluated in the "open loop" state under deep anesthesia. Interactions in waking states are represented by coupled KO sets, respectively KIe (mutual excitation) and KIi (mutual inhibition). The coupled KIe and KIi sets form a KII set, which suffices to represent the dynamics of the OB, AON, and PC separately. The coupling of these three structures by both excitatory and inhibitory feedback loops forms a KIII set. The solutions to this high-dimensional system of ODEs suffice to simulate the chaotic patterns of the EEG, including the normal low-level background activity, the high-level relatively coherent "bursts" of oscillation that accompany reception of input to the bulb, and a degenerate state of an epileptic seizure determined by a toroidal chaotic attractor. An example is given of the Ruelle-Takens-Newhouse route to chaos in the olfactory system. Due to the simplicity and generality of the elements of the model and their interconnections, the model can serve as the starting point for other neural systems that generate deterministic chaotic activity.

Olfactory mechanisms in the control of maternal aggression, appetite, and fearfulness: effects of lesions to olfactory receptors, mediodorsal thalamic nucleus, and insular prefrontal cortex.

During lactation the female rat is hyperphagic, aggressive toward adult conspecifics, and less fearful than usual. In the first experiment the importance of olfactory receptors was investigated by surgically removing the olfactory epithelium of the nasal cavity. Mother rats subjected to this treatment consumed significantly less food and weighed less than sham-operated females. Moreover, experimental subjects displayed a dramatic decrease in maternal aggression. Fear behavior (sound-elicited freezing), on the other hand, was not affected by the lesions. The mediodorsal thalamic nucleus and the prefrontal insular cortex form part of the central olfactory system. The second experiment assessed the involvement of this olfactory-related thalamocortical system and the behavioral profile of mother rats. It was found that whereas the thalamic and cortical lesions left food intake and fear behavior unaffected, they significantly decreased the frequency with which the mother would attack an intruder male placed into her home cage. The sense of smell appears, according to the present experiments, to play a crucial role in maternal aggression.

Olfactory dysfunction in alzheimer's disease

Recent evidence suggesting involvement of the central olfactory system in Alzheimer's disease (AD) was assessed by contrasting olfactory and visual discrimination in 15 probable AD subjects and 6 healthy aged controls. Control subjects performed better than AD subjects on both tasks. Mild AD subjects performed better than moderate AD subjects on visual discrimination, although the two groups did not differ on olfactory discrimination. Positron emission tomographic (PET) studies with (F‐18)2‐fluoro‐2‐deoxy‐D‐glucose performed on 11 of the AD subjects showed no relation between olfactory performance and cerebral metabolic asymmetries, but relations to visual discrimination were found.

The development of axonal connections in the central olfactory system of rats.

The development of the cytoarchitecture and axonal connections of the central olfactory system were studied in fetal and neonatal rats from E16. In contrast to neocortical development, the olfactory cortex lacks a distinct cortical plate. In the piriform cortex and the olfactory tubercle the cellular laminae emerge simultaneously, while in the anterior olfactory nucleus, there are morphogenetic gradients from superficial to deep as well as from caudal to rostral which parallel the known cytogenetic gradients. Parallel morphogenetic and cytogenetic gradients are also present in the lateral to medial axis of the olfactory tubercle. The projection from the olfactory bulb and the associational projections from the piriform cortex begin to develop well before birth. At E17 fibers from the bulb are limited to the lateral olfactory tract (LOT) and the molecular layer just deep to it, and then spread out caudally, laterally, and medially away from the LOT. This sequence of innervation parallels and predicts the density of innervation in the adult: those areas which are innervated first (such as the piriform cortex deep to the LOT) ultimately receive the heaviest innervation; conversely, those areas which are innervated very late (such as the medial olfactory tubercle) receive the lightest projection. The intracortical projections from the anterior and posterior piriform cortex extend into layer I ipsilaterally by E20 and obtain their adult distribution by the middle of the first postnatal week. On the other hand, fibers from the anterior olfactory nucleus and the entorhinal area do not reach their full adult extent until the second postnatal week. Similarly, the crossed projection of the anterior piriform cortex to the contralateral posterior piriform cortex does not grow into layer I until this later time. The timing of fiber ingrowth showed no relation to the trajectory or eventual areal or laminar termination of fibers. As with the olfactory bulb projection, the timing may influence the density of termination. Centrifugal fibers to the bulb are demonstrable around the time of birth both by the retrograde transport of horseradish peroxidase (HRP) and by the anterograde transport of 3H-leucine. The arrival of additional fibers during the remainder of the first postnatal week parallels the known cytogenetic and morphogenetic gradients in the areas in which they arise. The projections of the olfactory cortex to the lateral hypothalamic area and the mediodorsal thalamic nucleus are evident before birth. This correlates with the early generation of the cells which give rise to these projections.