[Transsynaptic degeneration in the rabbit olfactory bulb following peripheric deafferentation. Ultrastructural study]

Morphological study of the effects of intranasal zinc sulfate irrigation on the mouse olfactory epithelium and olfactory bulb

The effects of intranasal zinc sulfate (ZnSO4) irrigation on the morphology of the olfactory epithelium and olfactory bulb were studied in mice with short survival times (as early as 1 day) and with long survival times (up to 593 days) after the irrigation procedure. As in several previous studies, the olfactory epithelium was completely destroyed within a few days after the ZnSO4 treatment. Within 2-4 days, the septum and turbinates were covered by a new, cuboidal epithelium, the cells of which differed significantly from any cells normally seen in the olfactory epithelium. Slowly, over several months, small areas of the olfactory epithelium regenerated in many of the animals. The ultrastructural changes occurring in the olfactory bulb from 1 to 25 days (the reactive stage) were characterized by degenerating olfactory axons and axon terminals, hypertrophy of astroglial cell processes, and proliferation of or extravasation by phagocytic cells. By 25 days after intranasal ZnSO4 irrigation, the number of reactive glial processes and phagocytic cells returned to normal. In some mice with survival times of 150 days or longer, there was reinnervation of small areas of the olfactory bulb by regenerated olfactory axons. These new olfactory axons innervated only superficial glomeruli or the outer portions of deeper glomeruli, but they formed synaptic contacts with mitral/tufted cells and periglomerular cells that did not differ from control animals. These findings were supported by tract-tracing experiments with 3H-amino acids and by behavioral analysis. In summary, the ultrastructural changes observed in the olfactory bulb in this study were not significantly different from those observed after surgical lesions of the olfactory epithelium or nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of the effects of climbing fiber deafferentation in adult and weanling rats

The climbing fiber input to the cerebellar cortex was destroyed using both electrolytic and chemical (3-acetylpyridine) lesions. The long-term effects of climbing fiber deafferentation on the ansiform lobule of weanling and adult rats were examined at both the light and electron microscopic levels. Image analysis of Golgi-impregnated Purkinje cells indicated a significantly lower number of smooth branches and spiny branchlets following climbing fiber deafferentation of both adult and weanling rats. The results suggest that the lower number of smooth branches and spiny branchlets following climbing fiber deafferentation of the weanling rat is the result of a loss of postnatal growth rather than transneuronal degeneration. Ultrastructural evidence is provided in confirmation of these quantitative findings. Formation of ectopic dendritic spines was found following climbing fiber deafferentation of the weanling rat, but not the adult. It is shown that ectopic spines and the denervated dendritic thorns of these animals were synaptically innervated by the parallel fiber system and basket axons. The formation of ectopic spines on climbing fiber deafferentated Purkinje cells may represent a form of dendritic plasticity. Ultrastructurally, the dendritic arborizations of weanling deafferentated Purkinje cells showed no signs of transneuronal degeneration. However, the primary response to climbing fiber deafferentation in the adult rat was marked transneuronal degeneration of the Purkinje cell dendrites. It is suggested that the inability of the adult Purkinje cell to form ectopic spines and to replace the excitatory postsynaptic potential of the climbing fiber varicosity is directly related to the Purkinje cell's subsequent transneuronal degeneration.

Aging in the rat olfactory system: relative stability of piriform cortex contrasts with changes in olfactory bulb and olfactory epithelium

Previous studies have quantified growth and atrophy of the olfactory bulb and olfactory epithelium of the Sprague-Dawley rat from maturity to senescence. Major events occurring in these structures include changes in the volume of mitral cells and changes in the number of septal olfactory receptors. These effects are large, consist of a growth phase followed by atrophy, and are temporally related in that events in the olfactory epithelium precede those in the mitral cells. A hypothesis of aging based on transneuronal effects would predict that these changes would be similarly transmitted to the next synaptic station in the olfactory pathway. Therefore, cells and synapses of the piriform cortex were studied in rats 3, 12, 18, 24, 27, 30, and 33 months of age. Alternate Vibratome sections through brains perfused with mixed aldehydes were processed for light and electron microscopy. No significant age effects were found for the volumes of cortical laminae Ia and Ib. Both numerical and surface density of synaptic apposition zones in layer Ia, formed primarily by mitral cell axons, were stable with age. A modest (18%) but significant decline in the proportion of layer Ia occupied by dendrites and spines was mirrored by an increase in the proportion of glial processes; no change in the proportion of axons and terminals was observed. Neither nuclear volume, nor soma volume, nor numerical density of layer II neurons changed with age. Thus, contacts made in the piriform cortex by mitral cell axons remain relatively stable in senescence, despite the marked volumetric changes in the mitral cell somata, changes which were confirmed again in this study. Age-related dendritic regression in layer II neurons may be attributable to functional deafferentation subsequent to reduced receptor input to mitral cells.

Long-term effect of mossy fiber degeneration in the rat

Mossy fiber-deafferentated rats (20) were permitted to survive from 34 to to 120 days and subsequently examined following Golgi-Cox preparation or after processing for electron microscopy. The primary response to mossy fiber deafferentation was transneuronal degeneration of the granule cell system. Morphological evidence is provided that suggests that the mossy fiber varicosity plays an important role in the fragmentation and removal of the granule cell digitiform dendrite. Computer-assisted image analysis of Golgi-impregnated Purkinje cells indicated significant losses in both smooth branch and spiny branchlet numbers following loss of the mossy fiber input. Ultrastructural examination revealed that a secondary transneuronal degeneration occurred within the dendritic arborization of both Purkinje cells and molecular layer interneurons. Although an overall reduction in the number of dendritic spines occurred along the terminal branchlets following mossy fiber deafferentation, several of the existing spines underwent marked changes in length, with some elongating to more than twice their size. By increasing the length of their spines, denervated Purkinje cells may acquire new synaptic contacts.

Transsynaptic regulation of olfactory bulb catecholamines in mice and rats

Norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylalanine (DOPA), and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured simultaneously by high performance liquid chromatography with electrochemical detection in extracts of olfactory bulbs at various intervals after chemical or surgical deafferentation. Chemical deafferentation of mice by intranasal irrigation with Triton X-100 or of rats by olfactory axotomy resulted in a rapid progressive decline of DA and DOPAC and an associated rise in NE in the olfactory bulb. However, after several weeks, these values returned to prelesion levels concomitant with reinnervation of the bulb by the afferent neurons. In contrast, deafferentation by procedures known to prevent reinnervation of the bulb by the afferent chemoreceptor neurons (i.e., a ZnSo4 solution in mice or a surgical procedure in rats) completely blocked the return to pre-lesion values of DA, DOPAC, and NE. The specificity of these effects was demonstrated by the inability of intranasal administration of the neurotoxin 6-hydroxydopamine to alter DA levels, resulting instead in a significant decline in olfactory bulb NE content. These data demonstrate that the DA content of the olfactory bulb can be influenced by either chemical or surgical modulation of the afferent pathway in two different species. This offers additional support for our hypothesis of transsynaptic regulation of intrinsic DA neurons of the bulb by the afferent olfactory chemoreceptor neurons.

Aging in the rat olfactory system: correlation of changes in the olfactory epithelium and olfactory bulb

Two regions closely linked synaptically (olfactory epithelium and olfactory bulb) have been compared in an age-graded series of rats. Previous findings of growth and atrophy of constituent elements in Sprague-Dawley Wisconsin (SD) rats have now been confirmed in Charles River (Crl) rats. In the olfactory bulbs of Charles River (Crl) rats, the volume of layers, the number of olfactory axodendritic synapses in the glomeruli, the total volume of glomerular dendrites, and the size of mitral cell bodies all approximately double between 3 and 24--27 months, and then all decrease by 36 months. Unlike SD rats, however, no loss in the number of mitral cells occurs in Crl rats, and the increase in volume of the olfactory bulbs from 3 to 24 months is approximately double that of SD rats. In the olfactory epithelium the total number of septal olfactory receptors more than doubles between 3 and 18--24 months and then declines markedly, as does the volume of olfactory axons in olfactory bulb glomeruli. Comparison of the regression lines for change in number of septal receptors with that of the size of mitral cell bodies discloses that the decline in number of receptors begins several months earlier than the decline in mitral cell size. This suggests that the atrophic changes in the olfactory bulb may in part be secondary to changes in the receptors of the olfactory epithelium. Numbers of synapses in the glomeruli appear to decline less markedly with age than the number of receptors, and a significant increase in number of synapses per receptor occurs in the oldest group studied (33 months), suggesting a compensatory increase in the relative number of synapses per receptor in the surviving receptors.

Regeneration of vomeronasal nerves into the main olfactory bulb in the mouse

After surgical section of the vomeronasal nerves the neurosensory cells in the vomeronasal epithelium die. Electron microscopy has been used to demonstrate that their axons, and synaptic terminals in the accessory olfactory bulb degenerate and are removed by phagocytic astroglia. The vacated postsynaptic sites in the accessory bulb persist, and are not re-innervated, either by vomeronasal or olfactory axons, as long as 150 days post-operatively. However, new neurosensory cells which are produced in the vomeronasal epithelium after vomeronasal nerve section do form axons. Light and electron microscope autoradiography of axonally transported material has been used to show that some of these axons grow back into the cranial cavity and form glomeruli in the main olfactory bulb, in regions where it is damaged or de-afferented. The regenerated vomeronasal glomeruli contain synapses between vomeronasal nerve terminals and dendrites of main bulb neurons.

Ligand binding studies in the mouse olfactory bulb: identification and characterization of a L-[3H]carnosine binding site

Binding sites for the dipeptide L-carnosine (beta-alanyl-L-histidine) have been detected in membranes prepared from mouse olfactory bulbs. The binding of L-[3H]-carnosine was saturable, reversible and stereospecific and had a Kd of about 770 nM. The stereospecific binding of L-carnosine represented about 30% of the total binding at pH 6.8, and decreased markedly with increasing pH. Binding was stimulated by calcium, unaffected by zinc, magnesium or manganese and inhibited by sodium and potassium. Carnosine binding was sensitive to trypsin and phospholipases A and C, but not to neuraminidase. Nystatin and filipin, which interact with membrane lipids, also interferred with binding. Some peptide analogues of carnosine were potent inhibitors of binding, but a variety of drugs serving as potent inhibitors in other binding systems had no effect on carnosine binding. Carnosine binding to mouse olfactory bulb membranes was 15-fold higher than that seen in membranes prepared from cerebral hemispheres, 5-fold higher than that seen in membranes prepared from cerebral hemispheres, 5-fold higher than in cerebellum membranes and 3-fold higher than in membranes from spinal medulla and the olfactory tubercle-lateral olfactory tract area. Binding sites for 6 other radiolabeled receptor ligands were also detected in bulb membranes. Peripheral deafferentation of the olfactory bulbs by intranasal irrigation with ZnSO4 led to a loss greater than 90% of the L-[3H]carnosine binding in 4--5 days with much smaller losses in binding of the other 6 ligands over a 180-day observation period. This initial loss of carnosine binding after denervation was due to a loss of binding site stereo-specificity followed by a loss of binding sites. The characteristics of the carnosine binding site in olfactory bulb fulfil 6 of the 7 criteria considered relevant for a functional receptor.

Electron microscope autoradiographic evidence for specific transneuronal transport in the mouse accessory olfactory bulb

The distribution of radioactive material was examined autoradiographically 8 h after application of [3H] proline to the vomeronasal organ in mice. Labelled material was transported along the axons of the vomeronasal nerves to their terminals in the glomerular layer of the accessory olfactory bulb (AOB). A lesser but consistent amount of radioactivity was found in the external plexiform layer (EPL) of the AOB. Electron microscopic autoradiography was used to determine which of the components of the EPL contained this labelled material. The method of proportional grain counts showed that the highest concentration of silver grains lay over the mitral cell dendrites, which are the elements immediately postsynaptic to the vomeronasal nerve axons. However, a fairly high proportion of grains also lay over the peripheral processes of granule cells. By application of a method of 'crossfire analysis' (which is explained in detail) it was possible to show that the observed grain distribution is best explained by the assumption that the radioactive material is confined to mitral cells, and the labelling over granule cell processes is due to crossfire from these sources. Im one animal at 5 days after [3H]proline administration label was found to have extended from mitral cells to granule cells, suggesting that the transsynaptically transported radioactive material, which was confined to the mitral cells at 8 h, may have become further redistributed at longer survivals. In a control experiment, [3H]proline was applied directly to the surface of the AOB. This gave rise to a completely different distribution of radioactivity in the EPL: radioactive material was present in all tissue components.

Denervation in the primary olfactory pathway of mice. IV. Biochemical and morphological evidence for neuronal replacement following nerve section

Unilateral olfactory nerve section was performed in the mouse. Three biochemical markers of the olfactory chemoreceptor neurons: carnosine, carnosine synthetase activity and the olfactory marker protein, were measured in the olfactory bulb and epithelium. Parallel observations were made by light microscopy as well as at the ultrastructural level. The specific biochemical markers decrease rapidly in both bulb and epithelium and reach a minimum by the end of the first week after surgery. They then slowly return to 80% of control values by one month. Carnosinase activity in epithelium was essentially unaffected. These biochemical observations coincide temporally with the onset of degenerative changes seen morphologically, in both the bulb and epithelium. The degenerative changes persist for up to two weeks in the bulb and for about one week in the epithelium. At this time basal cell division and differentiation begins in the epithelium with subsequent regrowth of olfactory axons into the glomerular layer of the olfactory bulb with ther reappearance of olfactory axon terminals. The temporal coincidence of these biochemical and morphological observations suggests they are manifestations of the same process, and is consistent with the idea that the olfactory chemoreceptor neurons are perhaps unique in being able to be replaced from undifferentiated stem cells.

Rapid dendritic atrophy following deafferentation: an EM morphometric analysis

The nucleus laminaris (NL), a third-order brain stem auditory nucleus in birds, receives afferents to its dorsal dendrites from the ipsilateral nucleus magnocellularis (NM), while the ventral dendrites of NL neurons are innervated by axons from the contralateral NM via the crossed dorsal cochlear tract (CTrX). The CTrx was transected in young chickens and, 96 h later, NL was examined for cytological changes. A morphometric analysis of electron micrographs from lesioned and shamd axon terminals were localized almost entirely to the ventral neuropil region of NL. (2) The volume density of dendrite in the ventral region of NL of lesioned animals was reduced by 85% compared to both the dorsal dendritic region of the same animals and the ventral dendritic region of sham-operated control animals. (3) The frequency with which primary dendrites were encountered in the ventral neuropil of lesioned animals was 81% lower than sham-operated controls. (4) Frequently, there was an apparent reduction in the amount of rough endoplasmic reticulum, Golgi apparatus and cytoplasmic granularity in the deafferented part of the NL neurons. (5) The data indicate that the cytological integrity of dendritic processes cna be specifically, profoundly and very rapidly compromised by removing a significant proportion of their afferents, suggesting that a tonic influence may be exerted by the presynaptic terminals.