Expression of ALZ-50 immunoreactivity in the developing principal sensory nucleus of the trigeminal nerve: effect of transecting the infraorbital nerve

In vivo magnetic resonance imaging at 11.7 Tesla visualized the effects of neonatal transection of infraorbital nerve upon primary and secondary trigeminal pathways in rats

Using 11.7T ultra high-field T2-weighted MRI, the present study aimed to investigate pathological changes of primary and secondary trigeminal pathways following neonatal transection of infraorbital nerve in rats. The trigeminal pathways consist of spinal trigeminal tract, trigeminal sensory nuclear complex, medial lemniscus, ventromedial portion of external medullary lamina and ventral posterior nucleus of thalamus. By selecting optimum parameters of MRI such as repetition time, echo time, and slice orientation, this study visualized the trigeminal pathways in rats without any contrast agents. Pathological changes due to the nerve transection were found at 8 weeks of age as a marked reduction of the areas of the trigeminal pathways connecting from the injured nerve. In addition, T2-weighted MR images of the trigeminal nerve trunk and the spinal trigeminal tract suggest a communication of CSF through the trigeminal nerve between the inside and outside of the brain stem. These results support the utility of ultra high-field MRI system for noninvasive assessment of effects of trigeminal nerve injury upon the trigeminal pathways.

Development and critical period plasticity of the barrel cortex

In primary sensory neocortical areas of mammals, the distribution of sensory receptors is mapped with topographic precision and amplification in proportion to the peripheral receptor density. The visual, somatosensory and auditory cortical maps are established during a critical period in development. Throughout this window in time, the developing cortical maps are vulnerable to deleterious effects of sense organ damage or sensory deprivation. The rodent barrel cortex offers an invaluable model system with which to investigate the mechanisms underlying the formation of topographic maps and their plasticity during development. Five rows of mystacial vibrissa (whisker) follicles on the snout and an array of sinus hairs are represented by layer IV neural modules ('barrels') and thalamocortical axon terminals in the primary somatosensory cortex. Perinatal damage to the whiskers or the sensory nerve innervating them irreversibly alters the structural organization of the barrels. Earlier studies emphasized the role of the sensory periphery in dictating whisker-specific brain maps and patterns. Recent advances in molecular genetics and analyses of genetically altered mice allow new insights into neural pattern formation in the neocortex and the mechanisms underlying critical period plasticity. Here, we review the development and patterning of the barrel cortex and the critical period plasticity.

Astrocytes promote peripheral nerve injury-induced reactive synaptogenesis in the neonatal CNS

Neonatal damage to the trigeminal nerve leads to "reactive synaptogenesis" in the brain stem sensory trigeminal nuclei. In vitro models of brain injury-induced synaptogenesis have implicated an important role for astrocytes. In this study we tested the role of astrocyte function in reactive synaptogenesis in the trigeminal principal nucleus (PrV) of neonatal rats following unilateral transection of the infraorbital (IO) branch of the trigeminal nerve. We used electrophysiological multiple input index analysis (MII) to estimate the number of central trigeminal afferent fibers that converge onto single barrelette neurons. In the developing PrV, about 30% of afferent connections are eliminated within 2 postnatal weeks. After neonatal IO nerve damage, multiple trigeminal inputs (2.7 times that of the normal inputs) converge on single barrelette cells within 3-5 days; they remain stable up to the second postnatal week. Astrocyte proliferation and upregulation of astrocyte-specific proteins (GFAP and ALDH1L1) accompany reactive synaptogenesis in the IO nerve projection zone of the PrV. Pharmacological blockade of astrocyte function, purinergic receptors, and thrombospondins significantly reduced or eliminated reactive synaptogenesis without changing the MII in the intact PrV. GFAP immunohistochemistry further supported these electrophysiological results. We conclude that immature astrocytes, purinergic receptors, and thrombospondins play an important role in reactive synaptogenesis in the peripherally deafferented neonatal PrV.

Prenatal exposure to ethanol affects postnatal neurogenesis in thalamus

The number of neurons in the ventrobasal thalamus (VB) in the adolescent rat is unaffected by prenatal exposure to ethanol. This is in sharp contrast to other parts of the trigeminal-somatosensory system, which exhibit 30-35% fewer neurons after prenatal ethanol exposure. The present study tested the hypothesis that prenatal ethanol exposure affects dynamic changes in the numbers of VB neurons; such changes reflect the sum of cell proliferation and death. Neuronal number in the VB was determined during the first postnatal month in the offspring of pregnant Long-Evans rats fed an ethanol-containing diet or pair-fed an isocaloric non-alcoholic liquid diet. Offspring were examined between postnatal day (P) 1 and P30. The size of the VB and neuronal number were determined stereologically. Prenatal exposure to ethanol did not significantly alter neuronal number on any individual day, nor was the prenatal generation of VB neurons affected. Interestingly, prenatal ethanol exposure did affect the pattern of the change in neuronal number over time; total neuronal number was stable in the ethanol-treated pups after P12, but it continued to rise in the controls until P21. In addition, the rate of cell proliferation during the postnatal period was greater in ethanol-treated animals. Thus, the rate of neuronal acquisition is altered by ethanol, and by deduction, there appears to be less ethanol-induced neuronal loss in the VB. A contributor to these changes is a latent effect of ethanol on postnatal neurogenesis in the VB and the apparent survival of new neurons.

Conversion of functional synapses into silent synapses in the trigeminal brainstem after neonatal peripheral nerve transection

One of the major consequences of neonatal infraorbital nerve damage is irreversible morphological reorganization in the principal sensory nucleus (PrV) of the trigeminal nerve in the brainstem. We used the voltage-clamp technique to study synaptic transmission in the normal and the denervated PrV of neonatal rats in an in vitro brainstem preparation. Most of the synapses in the PrV are already functional at birth. Three days after peripheral deafferentation, functional synapses become silent, lacking AMPA receptor-mediated currents. Without sensory inputs from the whiskers, silent synapses persist through the second postnatal week, indicating that the maintenance of AMPA receptor function depends on sensory inputs. High-frequency (50 Hz) electrical stimulation of the afferent pathway, which mimics sensory input, restores synaptic function, whereas low-frequency (1 Hz) stimulation has no effect. Application of glycine, which promotes AMPA receptor exocytosis, also restores synaptic function. Therefore, normal synaptic function in the developing PrV requires incoming activity via sensory afferents and/or enhanced AMPA receptor exocytosis. Sensory deprivation most likely results in AMPA receptor endocytosis and/or lateral diffusion to the extrasynaptic membrane.

Expression of bcl-2, bax, and caspase-3 in the brain of the developing rat

Naturally occurring neuronal death (NOND) is generally considered to be apoptotic. Apoptosis is an active form of cell death in which the regulation of specific proteins produces anti- or pro-apoptotic signals. Two of the protein families involved in this regulation are the bcl proteins and caspases. A quantitative immunoblotting technique was used to examine the temporal expression of bcl-2, bax, and two isoforms of caspase 3 (an active 20 kDa isoform and the inactive 32 kDa precursor) throughout the developing neuraxis. Long-Evans rat fetuses were collected on gestational day (G) 16 and G19, and pups were harvested on postnatal day (P) 0, P3, P6, P12, P21, and P30. Brains were divided into five segments: cortex, thalamus, midbrain, medulla/pons, and cerebellum. In general, the expression of bax increased and the ratio of bcl-2 expression to bax expression decreased concurrent with published data on the onset of NOND in a given area. The timing of these events was paralleled by an increase in the expression of active caspase 3. Unlike the bcl proteins, caspase 3 expression returned toward fetal levels as the brain matured. The timing of the changes in bcl protein and caspase expression show that both protein families are involved in promoting neuronal death. Reductions in caspase expression (and not bcl-2 and bax expression) are key to ending the period of NOND.

Apoptotic cascade of neurons in the subcortical sensory relay nuclei following the neonatal infraorbital nerve transection

A terminal transferase-mediated dUTP nick end labeling (TUNEL) method was utilized for detection of neuronal death in the subcortical relay nuclei of the trigeminosensory system following the infraorbital nerve transection in newborn rats. At 18-24 h after injury, numerous TUNEL-positive profiles were found within the ventroposteromedial thalamic nucleus (VPM) contralateral to the injury, whereas the VPM on the ipsilateral side and of the age-matched normal control contained only a few profiles per section. Electron microscopy revealed that the TUNEL-positive profiles were apoptotic neurons. The ventral part of the ipsilateral brainstem sensory trigeminal nuclear complex (the nucleus principalis, and the subnuclei oralis and interpolaris) exhibited statistically significant 65-70% increase in number of apoptotic neurons compared to the contralateral side. Taken together with our previous study [T. Sugimoto, C. Xiao, H. Ichikawa, Neonatal primary neuronal death induced by capsaicin and axotomy involves an apoptotic mechanism, Brain Res. 807 (1998) 147-154], the present results demonstrated a cascade of apoptosis in the primary, secondary and tertiary order sensory neurons along the neuroaxis.

Expression of p53 and ALZ-50 immunoreactivity in rat cortex: effect of prenatal exposure to ethanol

Neuronal death is an active process that results in the upregulation of antigens recognized by ALZ-50 and p53. Since prenatal exposure to ethanol can induce the postnatal death of cortical neurons, we examined the effects of ethanol on the in vivo expression of both the ALZ-50-positive antigen and p53. Pregnant rats were fed one of three diets, a liquid diet containing ethanol (Et), an isocaloric and isonutritive diet (Ct), or chow and water (Ch). Segments of frontoparietal cortex from fetuses and pups were examined for ethanol-induced changes (a) in the expression of ALZ-50 and p53 immunoreactivity using a quantitative immunoblotting assay and (b) in the distribution of ALZ-50- and p53-positive cells using immunohistochemistry. In control rats, ALZ-50 identified a 56-kDa peptide that was transiently expressed postnatally and peak expression occurred on postnatal day (P) 6 to P12. In Et-treated rats, peak expression was attained earlier (on P3) and was about three times of that achieved in the controls. The anti-p53 antibody identified three proteins (28, 56, and 58 kDa). Peak expression in control rats occurred during the second postnatal week and only the 58-kDa protein was expressed in appreciable amounts in adult cortex. Each p53-positive protein was affected by ethanol exposure. The 28- and 56-kDa p53-positive proteins were affected by ethanol much in the same way as was the ALZ-50-positive antigen. That is, the timing and amount of peak expression were earlier and lower, respectively, in the Et-treated rats. The postnatal expression of the 58-kDa protein was halved following prenatal exposure to ethanol. Both ALZ-50 and anti-p53 immunoprecipitated proteins are p53- and ALZ-50-positive, respectively. Thus, ethanol alters the expression of the ALZ-50- and p53-positive proteins and presumably the timing of neuronal death in the developing cortex. The parallel effects of prenatal ethanol exposure on the 56-kDa ALZ-50-positive antigen and the 28- and 56-kDa p53-positive proteins and the coprecipitation of the proteins are consistent with the notion that ALZ-50 recognizes a form of p53.

Neonatal transection of the infraorbital nerve increases the expression of proteins related to neuronal death in the principal sensory nucleus of the trigeminal nerve

Neonatal lesion of the primary afferents in the infraorbital nerve causes the death of one-third of the neurons in the second-order target, the principal sensory nucleus of the trigeminal nerve (PSN). We examined the expression of two candidate 'death' proteins, p53 and the antigen recognized by the antibody ALZ-50, in the normal and deafferented PSN. In addition, the effect of neonatal transection of the infraorbital nerve (a major component of the trigeminal nerve) on protein expression was examined. The expression of c-fos in the developing PSN was also studied as an index of metabolic activity. Protein expression was measured using quantitative analyses of immunoblots and immunohistochemical preparations. The expression of p53- and ALZ-50-immunoreactivity in the normal PSN peaked during the first postnatal week. Transection of the infraorbital nerve directly affected the expression of p53 and the ALZ-50-positive antigen. The immunoblots showed that whereas p53 amounts were unaffected by the lesion, ALZ-50 expression was significantly upregulated in the ipsilateral PSN 2 h and 2 days postlesion. The density of p53- and ALZ-50-immunoreactive neurons was significantly higher in the ventral ipsilateral PSN (i.e., the target of the transected infraorbital nerve) than in the contralateral PSN. c-fos expression selectively and transiently rose in the ventral ipsilateral PSN within 2 h of the lesion. Thus, both p53 and the ALZ-50-positive antigen are involved in neuronal death. In light of data suggesting that ALZ-50 recognizes a phosphorylated form of p53, we conclude that neuronal death in the developing nervous system involves the post-translational modification of an existing protein, p53. The increase in ALZ-50 expression apparently occurs during a catabolic phase of neuronal death, as indicated by the increase in c-fos expression.

c-neu oncoprotein in developing rostral cerebral cortex: relationship to epidermal growth factor receptor

The c-neu oncoprotein, p185c-neu, is a transmembrane tyrosine kinase that shares structural similarities with the receptor for epidermal growth factor (EGFr). We used immunoblots, immunoprecipitation, and immunohistochemistry 1) to test the hypothesis that p185c-neu and EGFr are coordinately expressed in central nervous system tissue and 2) to assess the spatiotemporal expression of both the c-neu oncoprotein and EGFr in the rostral cerebral cortex. In nondenaturing gels, anti-c-neu antibody identified high molecular weight proteins (about 300-400 kDa) that were reduced by EDTA to a molecular weight of 180-200 kDa. Sodium dodecylsulfate polyacrylamide gel electrophoresis broke down this protein into an array of smaller peptides, which were expressed prenatally, transiently during the first three postnatal weeks, or in the adult. Perinatally, c-neu immunoreactivity was evident in subplate neurons, ascending processes of neurons in the cortical plate, and ventricular zone cells. During the second postnatal week, cells throughout cortex expressed somatodendritic immunostaining, but, in the adult, c-neu immunoreactivity was expressed only by pyramidal neurons in layer V and by glia in the white matter and ependyma. EGFr-positive proteins behaved in the nondenaturing gels as did c-neu-positive oncoproteins, suggesting that both proteins naturally formed dimers. This contention was supported by the EGFr-or c-neu immunolabeling of tissue that was previously immunoprecipitated with anti-c-neu or anti-EGFr, respectively. The pattern of EGFr immunolabeling in the developing and mature cortex was virtually identical to that described for c-neu immunoreactivity. Cortical neurons express the c-neu oncoprotein and EGFr, probably as heterodimers. The specific immunolabeling of layer V neurons in the adult cortex with anti-c-neu and anti-EGFr suggests that the p185c-neu ligand and EGF regulate the activity of corticofugal systems. The expression of different c-neu- and EGFr-positive peptides is developmentally defined and may be related to specific ontogenetic events.

Development of trigeminal nucleus principalis in the rat: effects of target removal at birth

Little is known about how neurons develop in the trigeminal nucleus principalis (PrV) despite their acknowledged role in establishing whisker-related patterns in the thalamus and cortex. Golgi-impregnated PrV cells were studied in newborn, 4-day-old and adult rats. Adult neurons typically had short dendrites that were confined to a hemisphere around the soma. In contrast, at birth PrV neurons had radial trees and more primary dendrites than did adults, but adult-like numbers of dendritic spines. By day 4, most neurons had eccentric dendritic trees and the numbers of primary dendrites per neuron were adult-like, yet spines were more prevalent than in adults and newborns. Thus, it appears that there is a pruning of the dendritic tree during the first postnatal week. To assess the role of retrograde signals from the thalamus on PrV development, the right thalamus was destroyed at birth. By postnatal day 6, the number of neurons in the left PrV was 59% of that in the right PrV, PrV transverse area was reduced by 21%, cell density was reduced by 48%, and somatic diameter was increased by 36%, relative to the intact right PrV. By contrast, in the left V subnucleus interpolaris, which has only a weak thalamic projection, these measures were unaffected. Thus, neonatal thalamic lesions selectively depopulated the PrV. The morphology of PrV neurons was affected by the thalamic lesions: e.g. the total dendritic length, the number of dendritic branch points and the total number of spines were increased. The number of primary dendrites and the tree's eccentricity, area, and volume of influence were unaffected by the lesion. The structure of neurons in subnucleus interpolaris was unaffected by the lesion. Thus, normal afferent patterning is insufficient for normal development of PrV cells. Interactions among dendrites and retrograde signals from a target are also important.

Appearance of neuronal S-100 beta during development of the rat brain

In addition to being an astroglial protein, S-100 beta is localised in distinct populations of neurons in the adult rat hindbrain. We report, here, the expression of S-100 beta in both neurons and glia of the rat brain during development. Prenatally, S-100 beta immunoreactivity was confined to glial cells close to the germinal zone. After birth, S-100 beta positive glial cells were seen mainly in the brainstem and cerebellum, while only a few were detected in cerebral cortex and hippocampus. The number of S-100 beta containing glial cells increased steadily during the first 2 postnatal weeks after which the adult pattern was attained. No S-100 beta containing neurons were present prenatally. The first S-100 beta containing neurons were seen in the mesencephalic trigeminal nucleus at postnatal day 1 (P1), and in the motor trigeminal nucleus at P3. Neuronal S-100 beta immunoreactivity in other nuclei was mostly attained from the 10th to the 21st postnatal day. The neuronal S-100 beta immunoreactivity was first detected in the cell nuclei during development, then increased in the cytoplasm with ages. A nuclear staining in many immunoreactive neurons persisted until the adult. It usually took 1 to 2 weeks for neuronal S-100 beta to attain the adult staining pattern, i.e., heavy staining of the cytoplasm and processes, after its appearance. The forebrain never contained S-100 beta positive neurons. The S-100 beta is first expressed in glial cells, suggesting it is primarily of the glial origin. Coupled with neurotrophic effects of the protein, the time course of neuronal S-100 beta expression during the critical period of neuronal development implies that it may be involved in neuronal differentiation and maturation.

Developmental expression of a 56 kDa protein isolated from rat neocortex

The expression of ALZ-50 immunoreactivity in the immature rat cortex was assessed using a variety of immunochemical techniques. The antigen recognized by ALZ-50 was isolated using immunoaffinity column chromatography. Unlike the ALZ-50-positive antigen found in the brains of people with Alzheimer's Disease (mol. wt. 68 kDa), the principal antigen expressed in the immature rat had a molecular weight of 56 kDa. The temporal expression of ALZ-50-positive antigens was determined with immunoblotting techniques. The expression of the 56 kDa protein appeared during the last prenatal week, increased to a peak during the second postnatal week and waned in the third postnatal week. Immunohistochemically, neonatal cortex contained many ALZ-50-positive neurons in the cortical subplate and the intermediate zone and a smattering of neurons in the cortical plate. The temporal expression of the ALZ-50-positive 56 kDa antigen was similar to that for fetal tau (tau-1); however, the principal protein identified by the anti-tau-1 antibody was considerably larger (being about 68-70 kDa). In contrast to ALZ-50, tau-1-immunoreactive elements were distributed in the subplate, the deep cortical plate, the intermediate zone and the subventricular zone. tau-1-positive axons passing through the superficial intermediate zone were conspicuous. Thus, the biochemical and anatomical evidence show that ALZ-50 and tau-1 recognize distinct proteins.

Numbers of neurons in the developing principal sensory nucleus of the trigeminal nerve: enhanced survival of early-generated neurons over late-generated neurons

The overproduction and subsequent death of neurons is a common phenomenon in the developing vertebrate central nervous system (CNS). We tested the hypothesis that the survival of a neuronal subpopulation is related to its time of origin. Neuronal survival was examined in a well-defined CNS structure, the principal sensory nucleus of the trigeminal nerve (PSN) of the rat. The changes in the total number of PSN neurons and in the numbers of early- and late-generated neurons (i.e., neurons heavily labeled by a single injection of [3H] thymidine on G12 or G14, respectively), between gestational day (G) 16 and postnatal day (P) 10 were determined. The total number of neurons in the PSN rose prenatally to a maximum of 40,600 on G18.5. The increase in neuronal number correlates to the period of migration. More than half of the neurons that successfully migrated to the PSN were lost by P10. The patterns for the changes in the numbers of early- and late-generated neurons were similar; however, there were significant differences between the two subpopulations. The maximum number of early-generated neurons (4,250) was attained on G18.2 and subsequently 58.9% of these neurons were lost. In contrast, the maximum number of late-generated neurons (5,050) was attained on G20.0 and 66.6% of these neurons were lost by P10. Therefore, it appears that the survivability of early generated neurons is greater than for late-generated neurons. This enhanced survivability presumably results from a competitive advantage that early-generated neurons have for forming synapses or gaining access to trophic factor(s) that are in limited supply.

Alz 50 as a reagent to assess animal models of Alzheimer's disease

As will be apparent from a reading of the papers included in this issue, one reagent that has been used extensively in assessing the effects of the beta/A4 peptide on rodent neurons is the monoclonal antibody Alz 50. A variety of model systems have been employed, in which beta/A4 is delivered to neurons both in vitro, and in several different ways, in vivo. While Alz 50 can be a useful reagent in these studies, there are limitations to the utility of this antibody, and it is important to recognize that appropriate controls must be performed to establish the specificity of observed immunoreactivity.