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

Changes in Dendritic Spine Morphology and Density of Granule Cells in the Olfactory Bulb of Anguilla anguilla (L., 1758): A Possible Way to Understand Orientation and Migratory Behavior

Simple Summary

The olfactory bulb can process odour cues through granular cells (GCs) and dendritic spines, changing their synaptic plasticity properties and their morphology. The GCs’ dendritic spines density and morphology were analysed in Anguilla anguilla, considering the olfaction as a driver involved in fish orientation and migration. For the head and neck morphology, spines were classified as mushroom, long thin, stubby, and filopodia. Spines’ density decreased from juvenile migrants to no-migrant stages and increased in the adult migrants. Spines’ density was comparable between glass and silver eels as an adaptation to migration, while at non-migrating phases, spines’ density decreased. For its phylogenetic Elopomorph attribution and its complex life cycle, A. anguilla could be recommended as a model species to study the development of dendritic spines in GCs of the olfactory bulb. Considering the role of olfaction in the orientation and migration of A. anguilla, the modification of environmental stimuli (ocean alterations and climate change) could represent contributing factors that threaten this critically endangered species.

Abstract

Olfaction could represent a pivotal process involved in fish orientation and migration. The olfactory bulb can manage olfactive signals at the granular cell (GC) and dendritic spine levels for their synaptic plasticity properties and changing their morphology and structural stability after environmental odour cues. The GCs’ dendritic spine density and morphology were analysed across the life stages of the catadromous Anguilla anguilla. According to the head and neck morphology, spines were classified as mushroom (M), long thin (LT), stubby (S), and filopodia (F). Total spines’ density decreased from juvenile migrants to no-migrant stages, to increase again in the adult migrant stage. Mean spines’ density was comparable between glass and silver eels as an adaptation to migration. At non-migrating phases, spines’ density decreased for M and LT, while M, LT, and S density increased in silver eels. A great dendritic spine development was found in the two migratory phases, regressing in trophic phases, but that could be recreated in adults, tracing the migratory memory of the routes travelled in juvenile phases. For its phylogenetic Elopomorph attribution and its complex life cycle, A. anguilla could be recommended as a model species to study the development of dendritic spines in GCs of the olfactory bulb as an index of synaptic plasticity involved in the modulation of olfactory stimuli. If olfaction is involved in the orientation and migration of A. anguilla and if eels possess a memory, these processes could be influenced by the modification of environmental stimuli (ocean alterations and rapid climate change) contributing to threatening this critically endangered species.

Olfaction and Multiple Chemical Sensitivity

In this paper, a description of olfactory anatomy is presented, followed by a brief review of modern procedures for testing olfactory function. Information from the sole study which has quantitatively examined olfactory function in patients with apparent multiple chemical sensitivity (MCS) is presented. In essence, this study suggests that MCS is associated with increased nasal airflow resistance, respiration rate, heart rate, and scores on the Beck Depression Inventory, but not with significant changes in odor detection threshold sensitivity to phenyl ethyl alcohol and methyl ethyl ketone, the two target stimuli evaluated. Whether MCS patients evidence hypersensitivity to other chemicals is unknown.

Effect of early experience on neuronal and behavioral responses to con- and heterospecific odors in closely related Mus taxa: epigenetic contribution in formation of precopulatory isolation

BACKGROUND

The most effective learning occurs during sensitive periods. Olfactory plasticity to main social olfactory cues is limited to a critical period to a large degree. The objective was to evaluate the influence of early olfactory experience on the behavioral and neuronal responses of males to con- and heterospecific odors of receptive females in two species, M. musculus (subspecies musculus, wagneri) and M. spicilegus, and thus to determine the potential role of epigenetic contribution in the formation of precopulatory isolation.

RESULTS

Males were reciprocally cross-fostered shortly after the birth and were tested for response to con- and heterospecific urine odors of estrus females using two-choice tests at 70-85 days of age. Neuronal activity of non- and cross-fostered males was evaluated at 90-110 days of age in the MOB and AOB to con- and heterospecific female odor using fMRI (MEMRI). Non-cross-fostered males of three taxa demonstrated a strong preference for odor of conspecific females compared to odor of heterospecific ones. Spicilegus-nursed musculus preferred odor of heterospecific females. Wagneri-nursed spicilegus and spicilegus-nursed wagneri did not demonstrate significant choice of con - or heterospecific female odor. The level of MRI signal obtained from the evaluation of manganese accumulation in AOB neurons was significantly higher when the odor of conspecific estrus females was exposed, compared to urine exposure of heterospecific females. The response pattern changed to the opposite in males raised by heterospecific females. Response patterns of neuronal activity in the MOB to con- and heterospecific female odors were different in cross-fostered and control males.

CONCLUSION

The maternal environment, including odor, had a greater effect on the level of MRI signal in the AOB than the genetic relationships of the recipient and the donor of the odor stimulus. Behavioral and neuronal responses to con- and heterospecific odors changed in closely related Mus taxa as a result of early experience. We demonstrated the importance of early learning in mate choice in adulthood in mice and the possibility of epigenetic contribution in the formation of precopulatory reproductive isolation.

The influences of age on olfaction: a review

Decreased olfactory function is very common in the older population, being present in over half of those between the ages of 65 and 80 years and in over three quarters of those over the age of 80 years. Such dysfunction significantly influences physical well-being and quality of life, nutrition, the enjoyment of food, as well as everyday safety. Indeed a disproportionate number of the elderly die in accident gas poisonings each year. As described in this review, multiple factors contribute to such age-related loss, including altered nasal engorgement, increased propensity for nasal disease, cumulative damage to the olfactory epithelium from viral and other environmental insults, decrements in mucosal metabolizing enzymes, ossification of cribriform plate foramina, loss of selectivity of receptor cells to odorants, changes in neurotransmitter and neuromodulator systems, and neuronal expression of aberrant proteins associated with neurodegenerative disease. It is now well established that decreased smell loss can be an early sign of such neurodegenerative diseases as Alzheimer's disease and sporadic Parkinson's disease. In this review we provide an overview of the anatomy and physiology of the aging olfactory system, how this system is clinically evaluated, and the multiple pathophysiological factors that are associated with its dysfunction.

Brain plasticity and behavior

Brain plasticity refers to the brain's ability to change structure and function. Experience is a major stimulant of brain plasticity in animal species as diverse as insects and humans. It is now clear that experience produces multiple, dissociable changes in the brain including increases in dendritic length, increases (or decreases) in spine density, synapse formation, increased glial activity, and altered metabolic activity. These anatomical changes are correlated with behavioral differences between subjects with and without the changes. Experience-dependent changes in neurons are affected by various factors including aging, gonadal hormones, trophic factors, stress, and brain pathology. We discuss the important role that changes in dendritic arborization play in brain plasticity and behavior, and we consider these changes in the context of changing intrinsic circuitry of the cortex in processes such as learning.

Marked restoration of density and total number of mature (knob-bearing) olfactory receptor neurons in rats recovering from early hypothyroid-induced growth retardation

Our recent studies have shown that restoration of thyroid function in developing hypothyroid rats results in upregulation of olfactory neurogenesis and compensatory proliferation of olfactory receptor neurons (ORN) in the olfactory epithelium (OE) (Paternostro and Meisami, Dev. Brain Res., 76 (1993) 151-161; ibid., 83 (1994) 151-162). It was not clear, however, whether the newly forming ORNs undergo complete maturational stages. To determine the effects of restoration of thyroid function on maturation of ORNs, the density and total number of mature ORNs were estimated in the OE of euthyroid and hypothyroid rats at postnatal days 1, 12, 25 and 90 and the results were compared with those in rats allowed to recover from early thyroid deficiency at weaning (day 25). As a marker for mature ORNs, and on the basis of one olfactory dendritic knob per ORN, the density and total number of the olfactory knobs were determined in the entire extent of the OE covering the nasal septum. Hypothyroidism was induced by adding propylthiouracil (PTU) to the drinking water (1 g/l) from birth until days 12, 25 or 90 of age. Recovery from hypothyroidism was induced by withdrawal of PTU at day 25, leading to restoration of thyroid function and somatic growth recovery. The density of olfactory knobs was determined in 1 microm semi-thin sections stained with toluidine blue. In the normal rats, the number of olfactory knobs (= mature ORNs) increased 8.5- and 3-fold during postnatal days 1-25 and 25-90 respectively, reaching a mean value of 4 X 10(6)/septal OE, compared to 2.8- and 1.4-fold, respectively, for the hypothyroid rats. This led to deficits of 51% and 76% in the number of mature ORNs in the 25- and 90-day-old hypothyroid rats. In rats allowed to recover, the number of mature ORNs increased 4.5-fold during postnatal days 25-90 (3 X > hypothyroid rats and 1.5 X > controls). The results indicate marked upregulation of the maturational process of the ORNs and their compensatory accretion within the OE of the recovery group. The recovery process was not complete however, as indicated by a remaining deficit of about 25% in the total number of mature ORN, compared to normal 90-day controls. Thus thyroid hormones are essential for accretion of new mature ORNs in both the suckling and postweaning rats. Also, the ORNs show a remarkable ability to recover from severe early hypothyroid-induced growth retardation and attain normal mature state.

Synaptophysin and synaptoporin expression in the developing rat olfactory system

The expressions of two closely related synaptic vesicle antigens synaptophysin and synaptoporin were examined in the olfactory system of the adult rat and during pre- and postnatal development. In the adult, immunocytochemistry showed that the continuously regenerating olfactory receptor neurons (primary neurons) produce both synaptophysin and synaptoporin which were localized in the cell bodies of the receptor neurons in the olfactory epithelium, their dendrites, axonal processes in the olfactory nerve and their terminals in the olfactory bulb glomeruli. Furthermore, ultrastructural analysis revealed synaptophysin- and synaptoporin-immunoreactivities associated with synaptic vesicles in most olfactory receptor axonal terminals impinging on dendrites of the mitral and tufted neurons (secondary neurons in the olfactory bulb circuitry) in the olfactory glomeruli. In like manner, tufted neurons, granule and periglomerular neurons (interneurons in the olfactory bulb circuitry) express both synaptophysin and synaptoporin. In contrast, mitral neurons expressed only the synaptophysin antigen which was likewise associated with mitral axonal terminals in their target the olfactory cortex. The patterns of synaptophysin and synaptoporin expressions in mitral neurons (synaptophysin only) and tufted neurons (synaptophysin and synaptoporin) were similar in prenatal, postnatal and adult rats as revealed by immunocytochemistry and in situ hybridization. However, the biosynthesis of synaptophysin and synaptoporin by granule and periglomerular neurons, olfactory bulb interneurons, occurred mainly postnatally.

Olfactory imprinting

The term imprinting is used to refer to biologically relevant learning during a sensitive period defined by a particular developmental stage or physiological state. Although olfactory imprinting may occur at any age, and some of the best-studied paradigms involve adult animals, recent reports of long-term memory for odorants experienced during prenatal life present a particular challenge to our understanding of olfactory learning. Firstly, it is possible that these paradigms represent a form of exposure learning based on mechanisms different to the more familiar associative paradigms. Secondly, given the substantial addition of neural elements occurring during the perinatal period, these paradigms raise the question as to how the olfactory system, and eventually the brain, is able to acquire and retain information under conditions of major neural growth and change.

Olfactory capacities in aging and Alzheimer's disease. Psychophysical and anatomic considerations

Age-related alterations in the ability to smell are well documented. For example, more than three-fourths of individuals over the age of 80 have major difficulty detecting and identifying odors. Furthermore, olfactory dysfunction is among the first signs of Alzheimer's disease (AD). Indeed, the olfactory pathways of patients with AD evidence disproportionate numbers of neuritic plaques and neurofibrillary tangles relative to other sensory pathways, suggesting that the olfactory system may be the site of first involvement of the AD process. In this article, the literature related to age- and AD-related alterations in olfactory perception has been briefly reviewed, and several current hypotheses regarding the physiologic basis for these changes discussed.

Morphology of a sensory neuron in Drosophila is abnormal in memory mutants and changes during aging

Several mutations in Drosophila impair learning and the cAMP cascade. We report here that the fine morphology of an identified mechanosensory neuron is abnormal in two of these mutants, dunce (dnc) and rutabaga (rut). The neuron innervating the antero-notopleural bristle was filled with horseradish peroxidase and studied at the light- and electron-microscopy level. In the mutants dnc and rut, this neuron has an abnormally large number of side branches and varicosities in a defined segment of the axon. In wild-type flies, age tends to decrease the number of side branches and variacosities in the same axonal segment that is affected by the mutations. Ultrastructural studies are compatible with the interpretation that the varicosities are potential synaptic sites. The results suggest that the cAMP cascade plays a role in shaping neuronal connectivity.

Lack of thyroid hormones but not their excess affects the maturation of olfactory receptor neurons: a quantitative morphologic study in the postnatal rat

To study quantitatively actions of thyroid hormones on maturation of olfactory receptor neurons (ORN), surface density and total number of receptor knobs (1 knob/ORN) were measured in 1 mu sections from septal olfactory epithelium of newborn, 12- and 25 day normal, hypo- and hyperthyroid rats. Hypothyroidism was induced by adding to drinking water n-propylthiouracil (0.1% w/v) from birth. Hyperthyroidism was induced by daily injection of pups with T4 (1-thyroxine, 0.3 microgram/g b.w., s.c.). Experimental pups showed all the signs of hypo- and hyperthyroidism. Between days 1-25, normal pups showed marked increase in surface area of septal olfactory epithelium (6x), total number (12x) and surface density (#/mm2, 2x) of mature ORNs. Thyroid deficient rats showed, by day 12, marked reductions in epithelial surface area and total number of mature ORNs; these and the surface density deficits became very pronounced by 25 day (30% area, 27% density, 47% # mature ORNs). Hyperthyroid rats, however, did not show an increase in any of these parameters over controls. Although total number of ORNs (mature and immature), as measured by number of nuclei, was also reduced in hypothyroid pups, surface density was not altered, indicating that maturation of ORNs, but not their local accretion is altered in thyroid deficiency. The results indicate that thyroid hormones are essential for normal proliferative expansion of olfactory epithelium and for maturation of ORNs postnatally. These actions of thyroid hormones are not increased or accelerated by excess T4 suggesting saturation of the hormone receptor system at the normal plasma level.

A morphometric comparison of the olfactory epithelium of newborn and weanling rabbits

As part of a study of the development of olfactory function in the rabbit, a morphometric analysis of the olfactory epithelium in newborn and 30-day-old animals was carried out. Surface area, thickness and cell densities of the olfactory epithelium were compared in hematoxylin-eosin stained serial sections through the nasal cavities of 4 newborn and 3 weanling rabbits. While the basic structure of the olfactory cavity changed little with age, a large quantitative development in the epithelium was observed. The pattern of growth appeared uniform and resulted in a 3-fold increase in total surface area from about 1 cm2 per side in the newborn to about 3 cm2 in the weanling, and an increase in thickness from approximately 65 microns to about 90 microns. The increase in thickness was due mainly to a disproportionate, 5-fold increase in the number of olfactory neurons. This resulted in a total of about 32 million cells per side by day 30, and represented an increase in the ratio of neurons to basal cells of 7:1 to 10:1, and neurons to supporting cells of 2:1 to 4:1. While such an increase in the number of primary neurons presumably improves the animal's perceptual abilities, it nevertheless raises the question as to how perceptual constancy can be maintained during a period of such rapid neural change.

A proposed relationship between increases in the number of olfactory receptor neurons, convergence ratio and sensitivity in the developing rat

In the rat neonate, as in other altricial young, olfaction, in contrast to vision and hearing, is functional at birth, being critically important in feeding, growth and other aspects of infant-mother interaction. Yet, olfactory sensitivity, i.e., the ability to detect odors at low levels, is poor in the newborn, improving dramatically in the first few postnatal weeks. To find a neural explanation for this phenomenon at the level of peripheral olfactory system, we present quantitative light microscopic data which reveal that during the suckling period of postnatal development the surface area of the olfactory receptor sheet and the total number of olfactory receptor neurons increase by about 8- and 12-fold respectively, being about 15 mm2 and 1.0 million on each nasal half of the newborn. Since the number of mitral cells, the principal relay neurons of the olfactory bulb, is already established at birth, at about 40,000 per olfactory bulb, it may be estimated that the convergence ratio of the olfactory neurons to mitral cells increases by more than 10-fold in the suckling period. We propose that the increased number of primary sensory afferent units and the higher convergence upon the central relay cells enhances the physiological capacities of the olfactory afferent pathway, increasing the opportunity for spatial summation and facilitation. The latter changes may lead to reduced olfactory thresholds and improved sensitivity with development. The relative contribution of these peripheral changes in enhancement of olfactory sensitivity during growth is discussed in the light of our knowledge on the developing olfactory system.

Spine density on olfactory granule cell dendrites is reduced in rats reared in a restricted olfactory environment

The Golgi technique was used to study the development of spines on the external dendrites of granule cells in the olfactory bulbs of young rats that had been continuously exposed to cyclohexanone vapor, deodorized air, or rat odors. Exposure to deodorized air for 3 weeks from postnatal day 1 reduced spine density on the medial and lateral sides of the bulb, whilst exposure to cyclohexanone reduced spine density on the lateral side only. These reductions were evident in other animals exposed for 7 weeks. With all treatments, spine density reached a maximum at postnatal day 21 and decreased markedly during the next month.