Distribution and morphology of myelinated perikarya and dendrites in the olfactory bulb of primates

The olfactory bulb as the entry site for prion-like propagation in neurodegenerative diseases

Olfactory deficits are present in numerous neurodegenerative disorders and are accompanied by pathology in related brain regions. In several of these disorders, olfactory disturbances appear early and are considered as prodromal symptoms of the disease. In addition, pathological protein aggregates affect olfactory regions prior to other regions, suggesting that the olfactory system might be particularly vulnerable to neurodegenerative diseases. Exposed to the external environment, the olfactory epithelium and olfactory bulb allow pathogen and toxin penetration into the brain, a process that has been proposed to play a role in neurodegenerative diseases. Determining whether the olfactory bulb could be a starting point of pathology and of pathology spread is crucial to understanding how neurodegenerative diseases evolve. We argue that pathological changes following environmental insults contribute to the initiation of protein aggregation in the olfactory bulb, which then triggers the spread of the pathology within the brain by a templating mechanism in a prion-like manner. We review the evidence for the early involvement of olfactory structures in neurodegenerative diseases and the relationship between neuropathology and olfactory function. We discuss the vulnerability and putative underlying mechanisms by which pathology could be initiated in the olfactory bulb, from the entry of pathogens (promoted by increased permeability of the olfactory epithelium with aging or inflammation) to the sensitivity of the olfactory system to oxidative stress and inflammation. Finally, we review changes in protein expression and neural excitability triggered by pathogenic proteins that can promote pathogenesis in the olfactory bulb and beyond.

Propagation of alpha-synuclein pathology from the olfactory bulb: possible role in the pathogenesis of dementia with Lewy bodies

Olfactory limbic structures, like the amygdala, the entorhinal, and the piriform cortices, are closely involved in cognitive processes. Thus, besides olfactory dysfunctions, it is conceivable that the compromise of these structures can lead to cognitive impairment. The olfactory bulb is affected by alpha-synuclein pathology in almost all cases of both Parkinson's disease and dementia with Lewy bodies. The clinical distinction between these disorders relies on the timing in the appearance of dementia in relationship to motor symptoms. Typically, it occurs late in the course of Parkinson's disease, and within the first year in dementia with Lewy bodies. The close anatomical proximity of the olfactory bulb with limbic regions, together with the early occurrence of cognitive impairment that is observed in dementia with Lewy bodies, raise the question whether the propagation of alpha-synuclein pathology in this condition might originate in the olfactory bulb, spreading from there to other limbic structures, and thereby reaching the associative neocortex. This review will describe the anatomical basis of the olfactory system and discuss the evidence of potential spreading pathways from the olfactory bulb that could support the presence of early dementia in the setting of Lewy body disorders.

Olfaction in Parkinson's disease and related disorders

Olfactory dysfunction is an early 'pre-clinical' sign of Parkinson's disease (PD). The present review is a comprehensive and up-to-date assessment of such dysfunction in PD and related disorders. The olfactory bulb is implicated in the dysfunction, since only those syndromes with olfactory bulb pathology exhibit significant smell loss. The role of dopamine in the production of olfactory system pathology is enigmatic, as overexpression of dopaminergic cells within the bulb's glomerular layer is a common feature of PD and most animal models of PD. Damage to cholinergic, serotonergic, and noradrenergic systems is likely involved, since such damage is most marked in those diseases with the most smell loss. When compromised, these systems, which regulate microglial activity, can influence the induction of localized brain inflammation, oxidative damage, and cytosolic disruption of cellular processes. In monogenetic forms of PD, olfactory dysfunction is rarely observed in asymptomatic gene carriers, but is present in many of those that exhibit the motor phenotype. This suggests that such gene-related influences on olfaction, when present, take time to develop and depend upon additional factors, such as those from aging, other genes, formation of α-synuclein- and tau-related pathology, or lowered thresholds to oxidative stress from toxic insults. The limited data available suggest that the physiological determinants of the early changes in PD-related olfactory function are likely multifactorial and may include the same determinants as those responsible for a number of other non-motor symptoms of PD, such as dysautonomia and sleep disturbances.

Oligodendrocyte/myelin-immunoreactivity in the developing olfactory system

Immunocytochemistry was used to characterize oligodendrocyte maturation in the developing mammalian olfactory system. Postnatal day 10-16, 20, 30 and adult rats were examined, as well as postnatal day 20, 30, 40 and adult Monodelphis domestica (the grey, short-tailed opossum). In rats, oligodendrocyte/myelin-immunoreactivity first appears in the accessory olfactory bulb by day 11, with labeling rapidly increasing throughout the entire bulb over the next five days. An adult pattern of immunoreactivity, characterized by dense labeling in the granule cell layer, sparse immunoreactivity in the external plexiform layer, and staining along the periphery of glomeruli, is attained by day 30. Staining is apparent in both the lateral olfactory tract and anterior commissure by day 11, and becomes heavy by day 20. While patterns of oligodendrocyte/myelin-immunoreactivity in the adult Monodelphis and rat bulb are similar, staining first appears much later in the opossum (around day 30), and maturation occurs more slowly. For example, rostral-caudal gradients in the development of staining in the anterior commissure were noted which were not seen in the rat. These differences emerge because Monodelphis' slower growth allows more resolution into developmental sequences. Finally, in rats, unilateral naris closure on the day after birth, which significantly alters normal patterns of bulb development, has no effect on the pattern and level of immunoreactivity even after long (30 day) survival periods. In both normal and naris occluded rats, oligodendrocyte/myelin-immunoreactivity is found in caudal aspects of the rat bulb on day 11 and subsequently progresses throughout the entire bulb over the next five days. Patterns in the Monodelphis bulb mirror those observed in the rat, however, staining appears later and progresses more slowly, suggesting Monodelphis is a useful animal for examining early myelin formation.

Localization of tyrosine hydroxylase and olfactory marker protein immunoreactivities in the human and macaque olfactory bulb

These studies utilized specific antisera to examine the distribution and characteristics of tyrosine hydroxylase and olfactory marker protein in the olfactory bulb of the human and macaque monkey. The macaque displayed immunoreactive profiles to both antisera comparable to those described previously for other mammals. Olfactory marker protein antiserum labeled the olfactory nerve layer and glomeruli. Within the glomeruli, labeled processes were interdigitated with unlabeled processes believed to be the postsynaptic dendrites of olfactory bulb neurons. Tyrosine hydroxylase antisera labeled somata surrounding the glomeruli as well as putative dendritic processes with the glomerular neuropil. It appeared that only a subset of juxtaglomerular neurons were immunoreactive. A similar pattern was observed in the human for both antibodies. Fascicles of olfactory marker protein immunoreactive olfactory nerves often coursed long distances into the olfactory bulb prior to arborizing within a glomerulus. The data suggest that olfactory receptor cell axons destined for specific glomeruli fasciculate into bundles prior to reaching the target glomeruli. The immunoreactivity in the human to tyrosine hydroxylase was qualitatively similar to the macaque and other mammals although the number of labeled somata and intraglomerular processes appeared lower. As in the macaque, it appeared that only a subset of juxtaglomerular neurons were labeled.

Myelinated non-axonal neuronal elements in the feline olfactory bulb lack sites with a nodal structural differentiation

Myelinated dendrites in the external plexiform layer (EPL) of the feline olfactory bulb and myelinated axons in the lateral olfactory tract (LOT), were examined by transmission electron microscopy. The results show that the non-axonal myelin sheaths are extremely thin and short and that the number of myelin lamellae does not increase with increasing dendritic diameter. In myelinated LOT axons the sheaths tend to be thicker and the myelin lamellar number increases with axon diameter. Domains with node-like structural characteristics are not encountered along myelinated dendrites, neither between successive myelin sheaths nor where single sheaths terminate. The partly myelinated neuronal perikarya, which occur in the EPL, also lack node-like domains. In contrast, typical nodes are easily found in myelinated LOT axons. In the periglomerular region dendrites and neuronal perikarya are surrounded by non-compacted glial sheets. It is concluded that myelination and node formation are relatively independent events and that morphogenetic glial-neuronal interactions may give different results in different parts of the same neuron.

Myelinated perikarya and dendrites in lateral geniculate nucleus of adult cat following chronic cortical deafferentation

Chronic (one year) cortical deafferentation resulted in the appearance of myelinated neuronal perikarya and dendritic processes in the lateral geniculate nucleus (LGN) of the cat. Four per cent of all nerve cell bodies, both large and small, were observed to be covered partially or totally by compact myelin sheath. The myelination of the dendrites was confined to the most proximal portion of the processes. A comparison of the fine structural features of the myelinated cell bodies and dendrites with those from intact LGN indicate that both surviving projection neurons and local interneurons may become myelinated. It is suggested that the unorthodox myelination of nerve cell somata and dendrites is caused primarily by the massive deafferentation of the nucleus. This indicates a relationship between synaptic input and the process of myelination of the postsynaptic nerve cells.