Esmail Meisami

Effects of olfactory deprivation on postnatal growth of the rat olfactory bulb utilizing a new method for production of neonatal unilateral anosmia.

Considerable research in the last decade has shown that the postnatal development of the brain in altricial mammals such as the rat is significantly affected if the sensory input to the brain is altered during the early postnatal periods when the brain growth and development is rapid and criticall,Z,5,7, a,10. Even though the detailed mechanisms of these effects are still far from clear, nevertheless it is noteworthy that this generalization is based almost exclusively on studies on the visual system 1. It is therefore reasonable to investigate the existence of similar effects in other sensory systems, especially if the rat is used as the experimental animal. It is known that in this animal vision is not the dominant sense, and in fact the eyes open during the end of the second postnatal week when considerable brain growth and development has already taken place. In this paper, we wish to suggest that the olfactory system, in particular the olfactory bulb (OB) of the rat, is a very interesting system for early sensory deprivation studies. The rat, being a macrosmatic animal, has very well developed olfactory bulbs, and for nutritional needs seems to rely heavily on olfactory cues, especially during the neonatal period when, paradoxically, the olfactory bulb is still, in many important respects, a very immature structure (unpublished observations). Therefore, as a first step, we attempted to investigate the effects of olfactory deprivation on the growth of olfactory bulbs. Since each OB receives its direct afferent sensory input solely from the ipsilateral olfactory mucosa, it is best to investigate the effects of olfactory deprivation unilaterally, for under such conditions the contralateral OB can serve as the control structure, making comparisons much more meaningful. We had recently shown that t he growth and postnatal appearance of the i

A quantitative study of the effects of early unilateral olfactory deprivation on the number and distribution of mitral and tufted cells and of glomeruli in the rat olfactory bulb

Anatomic effects of early unilateral olfactory deprivation on the developing olfactory bulb were investigated in the albino rat. Unilateral anosmia was experimentally induced by neonatal cauterization of the left or right nare; regenerating the tissue permanently blocked the nostril. The anosmic olfactory bulb (ipsilateral to the blocked nare) and its contralateral counterpart, serving as the normal control, were compared for the following quantitative anatomic parameters: total number and distribution of mitral and tufted cells and olfactory glomeruli; average diameters of mitral cells and glomeruli; relative dimensions of the bulb and its layers. The effects on mitral cells and glomeruli were studied at the ages of 25 and 60 days and at 2 years; other studies were carried out in the 25-day-old animals only. In the normal mature bulb, the number of mitral cells, tufted cells and glomeruli was found to be about 70,000, 160,000 and 3000, respectively. It was found that the absence of early olfactory stimulation was invariably correlated with a significant and permanent loss of mitral cells, amounting to more than a quarter of the total number. This loss apparently occurred rapidly during the first 3 weeks, as it was already evident by day 25 and did not increase appreciably with prolongation of deprivation. Tufted cells were apparently even more susceptible, because their number decreased by about 45%. As evident from this distribution profiles, the loss of mitral and tufted cells occurred uniformly throughout the bulb. It is shown that these differences were due neither to inherent interbulbar differences, nor to a hyperplasia in the normal bulb. Early anosmia had no significant effects on the number or average diameter of the glomeruli. Morphometric studies revealed that the dimensions and thickness of layers (internal and external plexiform and granular) of the anosmic bulb were significantly reduced. It is suggested that early olfactory stimulation is necessary for survival of the developing mitral and tufted cells; thus the first 2-3 postnatal weeks, covering the final developmental stages of these cells, would constitute a vulnerable period in the development of the rat olfactory system.

Lasting effects of early olfactory deprivation on the growth, DNA, RNA and protein content, and Na-K-ATPase and AChE activity of the rat olfactory bulb

It is shown that when one of the olfactory bulbs (OB) of the rat is deprived of olfactory stimulation from birth, as compared to the normal OB on the opposite side, significant and permanent reductions in its growth and in several biochemical parameters take place as follows: weight gain, 25%; total DNA, 20--30%; total RNA, 30%; total protein, 30%; total Na-K-ATPase activity, 50%; total AChE activity, 20%. The concentrations of DNA, RNA and protein and the specific activity of AChE were not significantly affected but the specific activity of Na-K-ATPase was significantly reduced. In general, these interbulbar differences were seen in every experimental animal and were not due to hypertrophy or hyperplasia of the normal OB. These results suggest that olfactory stimulation during the early postnatal period has a significant influence on cell proliferation and cell growth as well as on the proliferation of neuronal membranes and synapses in the developing OB. It appears that the effects of olfactory deprivation are exerted during the first few weeks after birth when the bulb shows its most rapid period of growth and development, since (with the exception of DNA) the magnitude of the interbulbar difference did not increase appreciably by prolongation of olfactory deprivation beyond the early period. Thus in the rat the existence of a critical period encompassing the second and third postnatal weeks is suggested during which the developing OB appears to be especially vulnerable to the absence of olfactory stimulation.

Early olfactory deprivation and the mitral cells of the olfactory bulb: a Golgi study

Golgi impregnation was used to study the morphology, morphometry and number of mitral cells in control and stimulus‐deprived olfactory bulbs (OB) of 25‐day‐old albino rats. The rats were made unilaterally anosmic by closure of one nostril in the early neonatal period. The previously described reductions in the size of OB and thickness of OB layers were confirmed. Quantitative comparison of the control and anosmic OB revealed that in the odor deprived OB the number of Golgi impregnated mitral cells was 40% less and the impregnation of dendritic neuropil in the external plexiform layer was greatly reduced. Of the remaining mitral cells of the odor‐deprived OB, a small portion exhibited an ‘atrophic’ appearance with markedly reduced impregnation of the intracellular material, while the majority showed relatively ‘normal’ morphology. Morphometric measurements and statistical comparison of the means revealed that in the remaining ‘normal’ mitral cells, there were significant reductions in the following parameters: (1) the size of the cell soma as measured by the vertical and transverse diameters (17 and 12%); (2) length of the apical dendrite (27%); (3) diameter of the glomerular tuft (22%); (4) total length of the basal dendrites (18%); (5) diameter of the dendritic field of the basal dendrites (22%). The pattern of impregnation did not differ between the control OB of the experimental rats and the normal OB of normal rats. The results indicate that an absence of olfactory stimulation during the postnatal period exerts deleterious transneuronal influences on the developing mitral neurons, leading to atrophy and loss for some and diminished somatic and dendritic growth for others.

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.

Influence of early anosmia on the developing olfactory bulb.

Publisher Summary This chapter discusses the effects of olfactory deprivation during the postnatal period on the morphological and neurochemical features of the developing rat olfactory bulb. Each olfactory bulb receives its entire afferent input solely from its ipsilateral olfactory mucosa. This anatomical situation provides an ideal system, where the effects of unilateral olfactory deprivation on the ipsilateral olfactory bulb may be properly compared with its control counterpart—that is, the contralateral bulb. The chapter argues that the developing olfactory bulb of the rat is a suitable model for studies on the central neural correlates of early sensory deprivation and shows that early olfactory deprivation in this animal, whether produced by unilateral neonatal closure of one nostril or bilateral chemical destruction of olfactory receptor neurons, leads to marked reduction in the growth of the olfactory bulb, accompanied by a state of chemical underdevelopment and anatomic atrophy. Specifically, early olfactory deprivation causes significant reductions in weight, total protein, DNA, RNA, and Na,KATPase and acetylcholinesterase activities, as well as a marked reduction in the total number of mitral and tufted cells. It is suggested that during postnatal development, the presence of olfactory receptor activity is necessary for the proper development of the olfactory bulbs.

Retarding effect of early anosmia on growth of the body, brain, olfactory bulbs, and cerebellum and its implications for the development of the olfactory system in the rat

Abstract The effect of neonatal anosmia caused by destruction of the olfactory neuroepithelium with zinc sulfate or by bilateral bulbotomy (transection of bulbar stalks) on the growth (weight gain) of body, brain, and brain regions (cerebellum and olfactory bulbs) was studied in the albino rat. Compared with their appropriate controls (i.e., pups with zinc sulfate injected into the stomach or with monolateral bulbotomy), the anosmic rats exhibited significantly higher mortality as well as reduced body and brain growth. Body and brain growth retardation was observed thoughout the first 2 postnatal months, and was evident at both the whole brain and specific brain regions (cerebellum and olfactory bulbs). In general, and at least to the end of the second postnatal month, these effects appeared to be irreversible. We suggest that the observed retardations in body and brain growth resemble those experienced by animals undernourished during the early postnatal period, indicating that the anosmic pups by virtue of the loss of needed olfactory cues are undermourished. This implies a cricial functional role for the structurally immature olfactory system of the neonate rat, which is discussed in relation to the development of this system and particularly to the concept of minimum structural requirements for function.

Deficient lipid-protein interaction in mitochondrial cytochrome oxidase from newborn and old rat brain

In contrast to the mature brain, Arrhenius plots of mitochondrial cytochrome c oxidase from newborn and old rat brain show no breaks (transition temperature); the apparent activation energy of the enzyme, in the physiological range of temperature, is also significantly higher. The results indicate deficient lipid-protein interaction as well as differential fluidity status in the inner mitochondrial membranes of the newborn and aged brain. These changes may contribute to the known deficiency in oxygen consumption of the newborn and old brain.

The asymmetries in morphology, desoxyribonucleic acid, protein and ribonucleic acid of the two hemispheres induced by monolateral removal of the olfactory bulb and retrobulbar structures in neonatal rats

Abstract Monolateral removal of the olfactory bulb and the olfactory penduncular structures in the neonatal rat resulted in a profound morphological and biochemical asymmetry between the two hemispheres. The experimental hemisphere, ipsilateral to the lesion, protruded into the space normally occupied by the olfactory bulb and showed enlarged ventricles. The brain loci were displaced rostrally in this hemisphere. The desoxyribonucleic acid, ribonucleic acid and protein contents of the experimental hemisphere at 25 and 60 days of age were all significantly lower than that found in the control hemisphere contralateral to the lesion. To resolve between the two possible causes of this asymmetry (i.e. atrophy of the experimental or hypertrophy of the control hemisphere) hemispheres of the asymmetric brain were compared with homonymous hemispheres of unoperated normal rats. This comparison revealed that the asymmetry is basically due to an excess of desoxyribonucleic acid synthesis in the control hemisphere, which continues even after postnatal day 25, on the one hand, and a dearth of protein in the experimental hemisphere on the other. This finding implies two separate mechanisms for the processes that underlie the asymmetries observed for these two substances. Our results demonstrate two important characteristics of the developing brain. Firstly they indicate that removal of the olfactory bulb and the olfactory peduncle can produce considerable changes in the hemispheres, and secondly they unravel the strong latent potential of the brain for cell proliferation beyond the usual period of cell division in the brain. As in these experiments regulation of cell division is affected, this system might serve as a model for the study of aberrant cell division found in tumor formation and the process of carcinogenesis.