Glutamate receptors in spinal motoneurons after sciatic nerve transection

Astrocytic expression of HIV-1 viral protein R in the hippocampus causes chromatolysis, synaptic loss and memory impairment

BACKGROUND

HIV-infected individuals are at an increased risk of developing neurological abnormalities. HIV induces neurotoxicity by host cellular factors and individual viral proteins. Some of these proteins including viral protein R (Vpr) promote immune activation and neuronal damage. Vpr is known to contribute to cell death of cultured rat hippocampal neurons and suppresses axonal growth. Behavioral studies are limited and suggest hyperactivity in the presence of Vpr. Thus Vpr may play a role in hippocampal loss of function. The purpose of this study is to determine the ability of HIV-1 Vpr production by astrocytes in the hippocampus to cause neurological deficits and memory impairments.

METHODS

We tested the performance of rats in novel object and novel location tasks after hippocampal infusion with astrocytes expressing HIV-1 Vpr. Synaptic injury and morphological changes were measured by synaptophysin immunoreactivity and Nissl staining.

RESULTS

Vpr-infused rats showed impaired novel location and novel object recognition compared with control rats expressing green fluorescent protein (GFP). This impairment was correlated with a significant decrease in synaptophysin immunoreactivity in the hippocampal CA3 region, suggesting synaptic injury in HIV-1 Vpr-treated animals. In addition, Nissl staining showed morphological changes indicative of neuronal chromatolysis in the Vpr group. The Vpr-induced neuronal damage and synaptic loss suggest that neuronal dysfunction caused the spatial and recognition memory deficits found in the Vpr-infused animals.

CONCLUSIONS

In this study, we demonstrate that HIV-1 Vpr produced by astrocytes in the hippocampus impairs hippocampal-dependent learning. The data suggest Vpr is a neurotoxin with the potential to cause learning impairment in HIV-1 infected individuals even under conditions of limited viral replication.

WITHDRAWN: H-reflex up-conditioning after sciatic nerve transection and regeneration may increase VGLUT-1 terminals and GluR2/3 immunoreactivity in spinal motoneurons

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Neural plasticity after peripheral nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory and autonomic functions conveyed by the lesioned nerves to the denervated segments of the body, due to the interruption of axons continuity, degeneration of nerve fibers distal to the lesion and eventual death of axotomized neurons. Injuries to the peripheral nervous system may thus result in considerable disability. After axotomy, neuronal phenotype switches from a transmitter to a regenerative state, inducing the down- and up-regulation of numerous cellular components as well as the synthesis de novo of some molecules normally not expressed in adult neurons. These changes in gene expression activate and regulate the pathways responsible for neuronal survival and axonal regeneration. Functional deficits caused by nerve injuries can be compensated by three neural mechanisms: the reinnervation of denervated targets by regeneration of injured axons, the reinnervation by collateral branching of undamaged axons, and the remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of specificity in target reinnervation; plasticity in human has, however, limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain, hyperreflexia and dystonia. Recent research has uncovered that peripheral nerve injuries induce a concurrent cascade of events, at the systemic, cellular and molecular levels, initiated by the nerve injury and progressing throughout plastic changes at the spinal cord, brainstem relay nuclei, thalamus and brain cortex. Mechanisms for these changes are ubiquitous in central substrates and include neurochemical changes, functional alterations of excitatory and inhibitory connections, atrophy and degeneration of normal substrates, sprouting of new connections, and reorganization of somatosensory and motor maps. An important direction for ongoing research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, but are also able to modulate central nervous system reorganization, amplifying those positive adaptive changes that help to improve functional recovery but also diminishing undesirable consequences.

Nerve injury, axonal degeneration and neural regeneration: basic insights

Axotomy or crush of a peripheral nerve leads to degeneration of the distal nerve stump referred to as Wallerian degeneration (WD). During WD a microenvironment is created that allows successful regrowth of nerve fibres from the proximal nerve segment. Schwann cells respond to loss of axons by extrusion of their myelin sheaths, downregulation of myelin genes, dedifferentiation and proliferation. They finally aline in tubes (Büngner bands) and express surface molecules that guide regenerating fibres. Hematogenous macrophages are rapidly recruited to the distal stump and remove the vast majority of myelin debris. Molecular changes in the distal stump include upregulation of neurotrophins, neural cell adhesion molecules, cytokines and other soluble factors and their corresponding receptors. Axonal injury not only induces muscle weakness and loss of sensation but also leads to adaptive responses and neuropathic pain. Regrowth of nerve fibres occurs with high specificity with formerly motor fibres preferentially reinnervating muscle. This involves recognition molecules of the L2/HNK-1 family. Nerve regeneration occurs at a rate of 3-4 mm/day after crush and 2-3 mm/day after sectioning a nerve. Nerve regeneration can be fostered pharmacologically. Upon reestablishment of axonal contact Schwann cells remyelinate nerve sprouts and downregulate surface molecules characteristic for precursor/premyelinating or nonmyelinating Schwann cells. At present it is unclear whether axonal regeneration after nerve injury is impeded in neuropathies.

Early and transient increase in spontaneous synaptic inputs to the rat facial motoneurons after axotomy in isolated brainstem slices of rats

Section of motor nerve fibers (axotomy) elicits a variety of morphofunctional responses in the motoneurons in the motor nuclei. Later than the fifth post-operational day after section of the facial nerve, synapse elimination occurs in the facial motoneuron pool, leading to gradual abolishment of synaptic input-driven activities of the axotomized motoneurons. However, it remains unknown how the amount of synaptic input changes during this period between the axotomy and the synaptic elimination. Here we examined a hypothesis that axotomy of the motoneurons itself modifies the synaptic inputs to the motoneurons. One day after axotomy, the postsynaptic currents, mostly mediated by non-N-methyl-D-aspartic acid (non-NMDA) receptors, recorded from the axotomized facial motoneurons in the acute slice preparations of the rats were of higher frequency and larger amplitude than those in the intact motoneurons. This difference was not observed after the third post-operational day and appeared earlier than the changes in the electrophysiological properties and increase in the number of dead neurons in the axotomized motor nucleus. The larger postsynaptic current frequency of the axotomized motoneurons was observed both in the absence and in the presence of tetrodotoxin citrate, suggesting that increased excitability and facilitated release underlie the postsynaptic current frequency increase. These results suggest that synaptic re-organization occurs in the synapses of motoneurons at an early stage following axotomy.

Spinal Axonal Injury Induces Brief Downregulation of Ionotropic Glutamate Receptors and No Stripping of Synapses in Cord-Projection Central Neurons

Spinal cord injury often damages the axons of cord-projecting central neurons. To determine whether their excitatory inputs are altered following axonal injury, we used rat rubrospinal neurons as a model and examined their excitatory input following upper cervical axotomy. Anterograde tracing showed that the primary afferents from the cerebellum terminated in a pattern similar to that of control animals. Ultrastructurally, neurons in the injured nucleus were contacted by excitatory synapses of normal appearance, with no sign of glial stripping. Since cerebellar fibers are glutamatergic, we examined the expression of ionotropic receptor subunits GluR1-4 and NR1 for AMPA and NMDA receptors, respectively, in control and injured neurons using immunolabeling methods. In control neurons, GluR2 appeared to be low as compared to GluR1, GluR3, and GluR4, while NR1 labeling was intense. Following unilateral tractotomy, the levels of expression of each subunit in axotomized neurons appeared to be normal, with the exception that they were lower than those of control neurons of the nonlesioned side at 2-6 days postinjury. These findings suggest that axotomized neurons are only temporarily protected from excitotoxicity. This is in sharp contrast to the responses of central neurons that innervate peripheral targets, in which both synaptic stripping and reduction of their ionotropic glutamate receptor subunits persist following axotomy. The absence of an injury-induced trimming of afferents and stripping of synapses and the lack of a persistent downregulation of postsynaptic receptors might enable injured cord-projection neurons to continue to control their supraspinal targets during most of their postinjury survival. Although this may support neurons by providing trophic influences, it nevertheless may subject them to excitotoxicity and ultimately lead to their degenerative fate.

Spinal axonal injury transiently elevates the level of metabotropic glutamate receptor 5, but not 1, in cord-projection central neurons

In investigating the effect of spinal injury on cord-projection central neurons, we found that rat rubrospinal neurons retained glutamatergic afferents and, in general, ionotropic glutamate receptor expression following spinal axotomy. Since glutamate also acts on second-messenger-coupled metabotropic receptors, the expression of group I metabotropic glutamate receptors, mGluR1 and mGluR5, was examined following similar treatment. mGluR1 expression began to decline in the perikarya 2 days postlesion and a day later in the neuropil. The decline slowed down by the fifth day and recovered in both the perikarya and neuropil 1 week postlesion. However, expression in both the perikarya and neuropil declined again and persisted up to 2 years postlesion. Similarly, the mGluR5 displayed an early transient decrease and returned to normal levels by 7 days post-lesion. However, rather than progressing to a secondary decline, the expression of mGluR5 increased to levels dramatically higher than those of control nuclei at 2-4 weeks postlesion, subsiding again by 8 weeks, and remaining low up to 2 years postinjury. Although mGluR5 has been shown to save cultured neurons from excitotoxic cell death, its elevated expression in the present model corresponds in time to an increased input/output relationship and excitability of the injured neurons as well as a period of maximal somatic shrinkage and cell loss. In addition to the cell bodies and dendrites, axon-like profiles also contain mGluR1. Their decrease following rubrospinal axotomy suggests that axonal injury may also compromise the presynaptic regulation of afferent activities onto injured cord-projection central neurons.

Changes of high-affinity choline transporter CHT1 mRNA expression during degeneration and regeneration of hypoglossal nerves in mice

The high-affinity choline transporter CHT1 works for choline uptake in the presynaptic terminals of cholinergic neurons. We examined its expression in the hypoglossal nucleus after unilateral hypoglossal nerve transection in mice by fluorescent in situ hybridization. One week after axotomy, CHT1 mRNA expression was lost in all hypoglossal motoneurons in the lesioned side. Two weeks after axotomy, CHT1 mRNA started to be re-expressed in a few motoneurons that recovered connections to tongue muscles as revealed by retrograde labeling with Fast Blue. After 4 weeks, most of axotomized hypoglossal motoneurons were reconnected and re-expressed CHT1 mRNA as strongly as control neurons, and the regenerating cholinergic axons established mature neuromuscular junctions. These results suggest that the establishment of motor innervation is critical for CHT1 mRNA expression in hypoglossal neurons after axotomy.

The facial nerve axotomy model

Experimental models such as the facial nerve axotomy paradigm in rodents allow the systematic and detailed study of the response of neurones and their microenvironment to various types of challenges. Well-studied experimental examples include peripheral nerve trauma, the retrograde axonal transport of neurotoxins and locally enhanced inflammation following the induction of experimental autoimmune encephalomyelitis in combination with axotomy. These studies have led to novel insights into the regeneration programme of the motoneurone, the role of microglia and astrocytes in synaptic plasticity and the biology of glial cells. Importantly, many of the findings obtained have proven to be valid in other functional systems and even across species barriers. In particular, microglial expression of major histocompatibility complex molecules has been found to occur in response to various types of neuronal damage and is now regarded as a characteristic component of "glial inflammation". It is found in the context of numerous neurodegenerative disorders including Parkinson's and Alzheimer's disease. The detachment of afferent axonal endings from the surface membrane of regenerating motoneurones and their subsequent displacement by microglia ("synaptic stripping") and long-lasting insulation by astrocytes have also been confirmed in humans. The medical implications of these findings are significant. Also, the facial nerve system of rats and mice has become the best studied and most widely used test system for the evaluation of neurotrophic factors.

Ventral root avulsion leads to downregulation of GluR2 subunit in spinal motoneurons in adult rats

It has been observed that motor neuron death is induced in adult rats by ventral root avulsion which involves pulling out the spinal cord root. Since motor neurons are reported to be selectively vulnerable to alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor-mediated injury in vitro, we investigated changes in the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-receptor subunits in rat spinal motor neurons after ventral root avulsion. The L4-L5 ventral roots of adult Sprague-Dawley rats were avulsed by an extravertebral extraction procedure. After an appropriate survival time, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-receptor subunits were detected immunohistochemically in the L4-L5 segments. Ventral root avulsion resulted in a 60% loss of motor neurons by 14 days after surgery. GluR2 labeling in motor neurons was markedly decreased after avulsion, but before the onset of motor neuron death, while the GluR1 and GluR4 labeling of motor neurons remained unchanged. Intrathecal administration of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-receptor antagonists rescued a significant number of injured motor neurons from cell death. In contrast, N-methyl-D-aspartate-receptor antagonists did not prevent motor neuron death. Since the presence of GluR2 subunit renders heteromeric alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors Ca(2+)-impermeable, the downregulation of GluR2 may result in increased formation of GluR2-lacking, Ca(2+)-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors in motor neurons and could contribute to motor neuron death after ventral root avulsion.

AMPA/kainate receptors in mouse spinal cord cell-specific display of receptor subunits by oligodendrocytes and astrocytes and at the nodes of Ranvier

Spinal cord white matter is susceptible to AMPA/kainate (KA)-type glutamate receptor-mediated excitotoxicity. To understand this vulnerability, it is important to characterize the distribution of AMPA/KA receptor subunits in this tissue. Using immunohistochemistry and laser confocal microscopy, we studied the expression sites of AMPA/KA receptor subunits in mouse spinal cord. The white matter showed consistent immunoreactivity for AMPA receptor subunit GluR2/3 and KA receptor subunits GluR6/7 and KA2. In contrast, antibodies against GluR1, GluR2, GluR4 (AMPA), and GluR5 (KA) subunits showed only weak and occasional labeling of white matter. However, gray matter neurons did express GluR1 and GluR2, as well as GluR2/3. The white matter astrocytes were GluR2/3 and GluR6/7 immunopositive, while the gray matter astrocytes displayed primarily GluR6/7. Both exclusively and abundantly, KA2 labeled oligodendrocytes and myelin, identified by CNPase expression. Interestingly, myelin basic protein, another myelin marker, showed less correlation with KA2 expression, placing KA2 at specific CNPase-containing subdomains. Focal points of dense KA2 labeling showed colocalization with limited, but distinct, axonal regions. These regions were identified as nodes of Ranvier by coexpressing the nodal marker, ankyrin G. Overall, axonal tracts showed little, if any, AMPA/KA receptor expression. The proximity of oligodendrocytic KA2 to the axonal node and the paucity of axonal AMPA/kainate receptor expression suggest that excitotoxic axonal damage may be secondary and, possibly, mediated by oligodendrocytes. Our data demonstrate differential expression of glutamate AMPA and KA receptor subunits in mouse spinal cord white matter and point to astrocytes and oligodendrocytes as potential targets for pharmacological intervention in white matter glutamate excitotoxicity.

Neuroprotective action of melatonin on neonatal rat motoneurons after sciatic nerve transection

The neuronal isoform of nitric oxide synthase (nNOS), a NADPH-dependent diaphorase, is considered to play a role in motoneuron death induced by sciatic nerve transection in neonatal rats. Neuronal loss in these circumstances has been correlated with nitric oxide (NO) production and NADPH-diaphorase positivity in motoneurons after axotomy. In the present study we looked for a possible protective effect of melatonin, an antioxidant agent and inhibitor of nNOS, on spinal motoneurons after axonal injury. Neonatal Wistar rats (P2) were submitted to sciatic nerve transection and allowed to survive to P7. Melatonin at doses of 1, 5, 10, 50 and 100 mg/kg was given subcutaneously before and at intervals after the surgery. Controls operated in the same way received dilution vehicle or no treatment. The animals were killed by perfusion of fixative and the spinal cord was examined in serial paraffin sections. The motoneurons of the sciatic pool were counted in the axotomized and contralateral sides. Immunohistochemistry for nNOS and glial fibrillary acidic protein was used to evaluate nNOS expression in the axotomized cells and the astrocytic response. We found that melatonin at doses of 1-50 mg/kg decreased neuronal death. Astrocytic hypertrophy in melatonin treated animals was less intense. There were no differences in nNOS expression between treated and control rats, and surviving motoneurons of the sciatic pool did not express the enzyme, suggesting that nNOS may not be involved in neuronal death or survival in these experimental conditions. Possible mechanisms of melatonin neuroprotection, which was equally effective at doses of 1-50 mg/kg, are discussed. Doses of 50 and 100 mg/kg caused failure to thrive, seizures or death. The fact that neuroprotective doses were far smaller than toxic ones should encourage testing of melatonin in neurologic diseases.

Drebrin expression is increased in spinal motoneurons of rats after axotomy

Drebrin has been known to act on actin filaments at dendritic spines to cause morphological change, and might be related to the plasticity of synaptic transmission. In the present study, changes of drebrin were examined immunohistochemically in the spinal motoneurons of rats following unilateral sciatic nerve transection. Drebrin-immunoreactivity (-ir) in the motoneurons was significantly increased on the lesioned side after 3 days. Confocal laser-scanning microscopic images of the motoneurons revealed conspicuous increase in drebrin in the submembranous region of the cells. After 10 weeks, drebrin-ir on the lesioned side decreased to a level not significantly different from that on the unlesioned side. The results suggested that drebrin played important roles in synaptic restoration in axotomized motoneurons.

Changes of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors in layer V of epileptogenic, chronically isolated rat neocortex

In vivo chronic partial isolation of neocortical islands results in epileptogenesis that involves pyramidal neurons of layer V. To test whether an alteration in glutamate receptors might contribute to the epileptiform activity, we analysed the time-course of light microscopic changes in expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors using subunit-specific antibodies. The isolation caused a rapid down-regulation of immunoreactivity for GluR1 and GluR2/3 subunits in deep layer V pyramidal neurons within the neocortical island which was evident 24h post-lesion, and within three days was reduced to about 40-60% of the control level. Many pyramidal cells in deep layer V completely lacked GluR2. Between one and four weeks of survival, down-regulation of GluR2/3 and GluR2 involved the majority of pyramidal layer V neurons, except for cells in the upper part of layer V, and those within narrow areas of all sub-laminae of layer V ("micro-islands"). Initial down-regulation was also observed one to three days post-lesion for subunits 1 and 2 of the N-methyl-D-aspartate receptor, but in contrast to GluR2/3 immunoreactivity, NMDAR2A/B immunoreactivity was enhanced three weeks post-lesion. The present data provide evidence for plastic changes in glutamate receptors in neurons of partially isolated neocortical island. A sub-population of layer V neurons remains relatively unaffected, and would presumably be capable of generating fast glutamatergic synaptic potentials necessary for the development of synchronous epileptiform activity.

Factors regulating AMPA-type glutamate receptor subunit changes induced by sciatic nerve injury in rats

Excitatory glutamatergic neurotransmission at Ia afferent-motoneuron synapses is enhanced shortly after physically severing or blocking impulse propagation of the afferent and/or motoneuron axons. We considered the possibility that these synaptic changes occur because of alterations in the number or properties of motoneuron alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors. Therefore, we quantitatively analyzed glutamate receptor (GluR)1, GluR2/3, and GluR4 AMPA subunit immunoreactivity (ir) in motoneurons 3, 7, or 14 days after axotomy or continuous tetrodotoxin (TTX) block of the sciatic nerve. GluR1-ir remained low in experimental and control motoneurons with either treatment and at any date. However, there was a large reduction of GluR2/3-ir (peak at 7 days >60% reduced) and a smaller, but statistically significant, reduction of GluR4-ir (around 10% reduction at days 3, 7, and 14) in axotomized motoneurons. TTX sciatic blockade did not affect AMPA subunit immunostainings. Axonal injury or interruption of the trophic interaction between muscle and spinal cord, but not activity disruption, appears therefore more likely responsible for altering AMPA subunit immunoreactivity in motoneurons. These findings also suggest that synaptic plasticity induced by axotomy or TTX block, although similar in the first week, could be related to different mechanisms. The effects of axotomy or TTX block on motoneuron expression of the metabotropic glutamate receptor mGluR1a were also studied. mGluR1a-ir was also strongly decreased after axotomy but not after TTX treatment. The time course of the known stripping of synapses from the cell somas of axotomized motoneurons was studied by using synaptophysin antibodies and compared with AMPA and mGluR1a receptor changes. Coverage by synaptophysin-ir boutons was only clearly decreased 14 days post axotomy and not at shorter intervals or after TTX block.

Axotomy of the sciatic nerve differentially affects expression of metabotropic glutamate receptor mRNA in adult rat motoneurons

Previous studies indicated that axotomy exposes motoneurons to glutamatergic excitotoxic stress and protection from glutamatergic overactivation might be crucial for survival. Depending on the experimental model and the subtype involved, activation of metabotropic glutamate receptors (mGluRs) may either enhance excitotoxicity or exert protective effects. To investigate a possible involvement of mGluRs in neuronal rescue mechanisms after axotomy we have monitored the distribution of mGluR mRNA with in situ hybridization in adult rat motoneurons 1, 2, 3, and 4 weeks after sciatic nerve transection. Motoneurons in sham-operated control animals expressed mGluR 1, 4, and 7 mRNA. The mGluR1 mRNA signal was reduced to 49.6+/-6.9% as compared to the contralateral side 2 weeks after axotomy and 31.2+/-8.3% after 4 weeks. The mGluR4 signal declined to 22.1+/-5.1% after 1 week and 10.2+/-1.6% after 2 weeks, remaining stable thereafter. During the entire observation period the mRNA for mGluR7 was not significantly altered. Axotomy did not change the overall number of motoneurons on the ipsi- or contralateral side. The differential regulation of mGluR subtypes may be part of an adaptive cell program that helps to rescue adult motoneurons from excitotoxic cell death during the stress induced by peripheral denervation.

Changes in expression of NMDA receptor subunits in the rat lumbar spinal cord following neonatal nerve injury

The vulnerability of motoneurones to glutamate has been implicated in neurological disorders such as amyotrophic lateral sclerosis but it is not known whether specific receptor subtypes mediate this effect. In order to investigate this further, the expression of N-methyl-D-aspartate (NMDA) receptor subunits was studied during the first three post-natal weeks when motoneurones are differentially vulnerable to injury following neonatal nerve crush compared to the adult. Unilateral nerve crush was carried out at day 2 after birth (P2) which causes a decrease of 66% in motoneurone number by 14 days (P14). To study receptor expression in identified motoneurones, serial section analysis was carried out on retrogradely labelled common peroneal (CP) motoneurones by combined immunocytochemistry and in situ hybridization (ISH). mRNA levels were also quantified in homogenates from lumbar spinal cords in which the side ipsilateral to the crush was separated from the contralateral side. The NR1 subunit of the NMDA receptor was widely distributed in the spinal cord being expressed most strongly in motoneurone somata particularly during the neonatal period (P3-P7). The NR2 subunits were also expressed at higher levels in the somata and dendrites of neonatal motoneurones compared to older animals. NR2B mRNA was expressed at low to moderate levels throughout the studied period whereas NR2A mRNA levels were low until P21. Following unilateral nerve crush, an initial decrease in NR1 mRNA occurred at one day after nerve crush (P3) in labelled CP motoneurones ipsilateral to the crush which was followed by a significant increase in NR1 subunit expression at 5 days post-injury. This increase was bilateral although reaching greater significance ipsilateral to the crush compared with sham-operated animals. A significant increase in NR1 and NR2B mRNA post injury was also detected in spinal cord homogenates. In addition, the changes in levels of NR1 and NR2B mRNA were reflected by comparable bilateral changes at P7 in receptor protein determined by quantitative immunocytochemical analysis of NR1 and NR2 subunit expression in identified CP motoneurones indicating a co-ordinated regulation of receptor subunits in response to injury.

Organization of ionotropic glutamate receptors at dendrodendritic synapses in the rat olfactory bulb

Dendrodendritic synapses between mitral (or tufted) and granule cells of the olfactory bulb play a major role in the processes of odor discrimination and olfactory learning. Release of glutamate at these synapses activates postsynaptic receptors on the dendritic spines of granule cells, as well as presynaptic NMDA receptors in the mitral cell membrane. However, immunocytochemical studies have failed to demonstrate the presence of ionotropic glutamate receptors in granule cell dendrites. By using a postembedding immunogold procedure, we describe here the precise organization of neurotransmitter receptors at dendrodendritic synapses. We show that there is a selective localization of glutamate and GABA receptors at asymmetric and symmetric synaptic junctions, respectively. In addition, we demonstrate that NMDA and AMPA receptors are clustered at postsynaptic specializations on granule cell spines and that they are extensively colocalized. Conversely, glutamate receptors do not appear to be concentrated in clusters on mitral cell dendrites, suggesting that the presynaptic effects of glutamate are mediated by a small complement of extrasynaptic receptors. By analyzing the subsynaptic distribution of the NR1 and GluR2/3 subunits, we show that they are distributed along the entire extent of the postsynaptic specialization, indicating that both NMDA and AMPA receptors are available for dendrodendritic signaling between mitral and granule cells. These results indicate that the principles recently found to underlie the organization of glutamate receptors at axospinous synapses also apply to dendrodendritic synapses.

Effect of sympathectomy on the expression of NMDA receptors in the spinal cord

The expression of NMDA receptors in the intermediolateral (IML) region of the upper thoracic spinal cord, was studied in 3 week old rats. The effect of section of the cervical sympathetic nerve on neuronal cell number and receptor expression was examined up to two weeks after the operation. Age-matched sham-operated and unoperated animals were used as controls. It was shown using quantitative autoradiography with the NMDA receptor antagonist [(3)H]MK-801 (dizocilpine maleate), that there was a marked downregulation of receptors in all groups of animals, beginning at approximately 4 weeks of age. However after sympathectomy, which resulted in the death of 44% of neurones in the IML by 7 days, there was a significant increase in receptor density per neurone compared to sham-operated controls. In the control animals there was a significant increase in the Kd value of the binding between 21 and 24 days after birth indicating an increased expression of a low affinity receptor, but no such increase was seen after axotomy. The results are consistent with two populations of NMDA receptors being transiently expressed in the IML in developing animals, and the higher affinity receptor being down-regulated between 4 and 5 weeks of age. The presence of the high affinity receptor subtype may predispose neurones to die after axotomy.

Long-term changes of ionotropic glutamate and GABA receptors after unilateral permanent focal cerebral ischemia in the mouse brain

Long-term hyperexcitability was found after unilateral, permanent middle cerebral artery occlusion in exofocal neocortical areas of the adult mouse [Mittmann et al. (1998) Neuroscience 85, 15-27]. The aim of the present study was to test the hypothesis in an identical paradigm of ischemia. whether alterations in the densities of both excitatory and inhibitory amino acid receptors may underlie these pathophysiological changes. Alterations in densities of [3H]dizocilpine, [3H]D,L-amino-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, [3H]kainate and [3H]muscimol binding sites were demonstrated with quantitative in vitro receptor autoradiography. All binding sites were severely reduced in the core of the ischemic lesion. A completely different reaction was found in the exofocal, histologically inconspicuous parts of the somatosensory cortex and the more remote neocortical areas of both hemispheres. The [3H]muscimol binding sites were significantly reduced four weeks after ischemia in the motor cortex, hindlimb representation area and exofocal parts of the primary and secondary somatosensory cortices of both hemispheres. The focus of the reduction in [3H]muscimol binding sites was found in lower layer V and upper layer VI. Contrastingly, the densities of [3H]dizocilpine binding sites were found to be increased in these areas, whereas those of [3H]D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and [3H]kainate binding sites did not show significant changes. The [3H]dizocilpine binding site density increased predominantly in layers III and IV. All binding sites were also reduced in the retrogradely reacting, gliotic part of the ipsilateral ventroposterior thalamic nucleus, whereas the [3H]D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid binding sites were increased in the surround of the ipsilateral nucleus and no changes in binding sites were seen in the whole contralateral nucleus. We conclude that permanent local ischemia leads to a long-term and widespread impairment of the normal balance between binding sites of excitatory and inhibitory neurotransmitter receptors in neocortical areas far away from the focus of the post-ischemic tissue damage. The imbalance comprises an up-regulation of the [3H]dizocilpine binding sites in the ion channels of N-methyl-D-aspartate receptors and a down-regulation of [3H]muscimol binding sites of the GABA(A) receptors in the ipsi- and contralateral neocortex. These changes at the receptor level explain the previously observed hyperexcitability with the appearance of epileptiform field potentials and the long duration of excitatory postsynaptic potentials four weeks after ischemia.

Central neuron-glial and glial-glial interactions following axon injury

Axon injury rapidly activates microglial and astroglial cells close to the axotomized neurons. Following motor axon injury, astrocytes upregulate within hour(s) the gap junction protein connexin-43, and within one day glial fibrillary acidic protein (GFAP). Concomitantly, microglial cells proliferate and migrate towards the axotomized neuron perikarya. Analogous responses occur in central termination territories of peripherally injured sensory ganglion cells. The activated microglia express a number of inflammatory and immune mediators. When neuron degeneration occurs, microglia act as phagocytes. This is uncommon after peripheral nerve injury in the adult mammal, however, and the functional implications of the glial cell responses in this situation are unclear. When central axons are injured, the glial cell responses around the affected neuron perikarya appears to be minimal or absent, unless neuron degeneration occurs. Microglia proliferate, and astrocytes upregulate GFAP along central axons undergoing anterograde, Wallerian, degeneration. Although microglia develop into phagocytes, they eliminate the disintegrating myelin very slowly, presumably because they fail to release molecules which facilitate phagocytosis. During later stages of Wallerian degeneration, oligodendrocytes express clusterin, a glycoprotein implicated in several conditions of cell degeneration. A hypothetical scheme for glial cell activation following axon injury is discussed, implying the injured neurons initially interact with adjacent astrocytes. Subsequently, neighbouring resting microglia are activated. These glial reactions are amplified by paracrine and autocrine mechanisms, in which cytokines appear to be important mediators. The specific functional properties of the activated glial cells will determine their influence on neuronal survival, axon regeneration, and synaptic plasticity. The control of the induction and progression of these responses are therefore likely to be critical for the outcome of, for example, neurotrauma, brain ischemia and chronic neurodegenerative diseases.

Expansion of the dendritic tree of motoneurons innervating neck muscles of the adult cat after permanent axotomy

The morphologic characteristics of neck motoneurons with intact axons were compared with those of neck motoneurons that had been permanently axotomized for 11 to 17 weeks. Motoneurons were identified antidromically, intracellularly stained with horseradish peroxidase (HRP) and examined after reconstructions of their entire dendritic tree. Axotomized motoneurons differed qualitatively and quantitatively from motoneurons with intact axons. The distal branches of axotomized motoneurons exhibited two novel features: some gave rise to tangled appendages that exhibited growth cone-like specializations resembling lamellipodia and filopodia; others followed a meandering path and had unusually large diameters. These branches showed a discontinuous pattern of staining that was similar to the appearance of myelinated axons stained intra-axonally with HRP. A quantitative analysis of the dendritic trees of 13 completely reconstructed dendritic trees (five axotomized motoneurons and eight motoneurons with intact axons) showed that total dendritic surface area, total dendritic length, and total number of branches increased 38, 34, and 215%, respectively, after axotomy. These measurements were confirmed by comparing the sizes of a larger number of motoneurons (16 axotomized and 21 intact), calculated on the basis of correlations between dendritic tree size and proximal dendritic diameter. We conclude, therefore, that neck motoneurons, in contrast to other types of motoneurons, expand their dendritic trees after axotomy. It is suggested that this expansion is a consequence of two mechanisms: one involves dendritic growth, possibly leading to new synaptic connections; the other causes a conversion of some dendrites into axons.

A differential and time-dependent decrease in AMPA-type glutamate receptor subunits in spinal motoneurons after sciatic nerve injury

After sciatic transection a strong decrease in immunoreactivity occurred, starting at 2 days. After 6, 10, 14, and 20 days survival only 5% of the sciatic motoneurons were strongly labeled for GluR2/3 against 80% in the control situation. From Day 20, GluR2/3 labeling started to increase again, reaching near normal levels at Day 80 after sciatic transection. In contrast, after sciatic crush, the decrease in GluR2/3 labeling in motoneurons was less pronounced and returned to normal in 30 days. In all animals, the GluR1 and GluR4 labeling of motoneurons remained unchanged after sciatic transection or crush. It is concluded that sciatic nerve injury leads to a strong, time-dependent decrease in the expression of GluR2 and 3 subunits in the corresponding motoneurons. As a consequence, AMPA receptors with a different subunit composition may be assembled, leading to a change in the functional properties of these receptors. Moreover, if they lack the GluR2 subunit, they may become calcium permeable.

Heterogeneity of AMPA receptors in the dorsal column nuclei of the rat

We have combined immunocytochemistry with retrograde tracing to demonstrate that projecting neurons in the gracile and cuneate nuclei express predominantly the GluR3 subunit of the AMPA receptor while interneurons in these nuclei express predominantly the GluR1 subunit. Interneurons expressing the GluR2 subunit are also present. It is speculated that the two classes of interneurons may release different inhibitory transmitters.