Quantitative and qualitative changes in AMPA receptor expression during spinal cord development

Ontogeny of NMDA receptor-mediated morphine tolerance in the postnatal rat

N-methyl-D-aspartate (NMDA) receptor antagonists are effective in inhibiting the development of morphine tolerance in adult rats. But NMDA receptors undergo dramatic change during the first few weeks of the postnatal life in the rat, and it is unknown whether NMDA receptor antagonists can inhibit the acquisition of opiate tolerance in the developing organism. Here, we investigated the effects of two NMDA receptor antagonists MK-801 and dextromethorphan on the development of morphine tolerance in 7-, 14-, and 21-day-old rats. NMDA receptor antagonists are not effective in attenuating morphine tolerance in the neonatal rat whereas they were partially effected in the 14-day-old and fully effective in rats as old or older than 21 days of age. These data suggest that there exists a transition age, around the second postnatal week in the rat, for the NMDA receptor to play a role in the development of morphine tolerance.

The AMPA receptor subunit GluR1 regulates dendritic architecture of motor neurons

The morphology of the mature motor neuron dendritic arbor is determined by activity-dependent processes occurring during a critical period in early postnatal life. The abundance of the AMPA receptor subunit GluR1 in motor neurons is very high during this period and subsequently falls to a negligible level. To test the role of GluR1 in dendrite morphogenesis, we reintroduced GluR1 into rat motor neurons at the end of the critical period and quantitatively studied the effects on dendrite architecture. Two versions of GluR1 were studied that differed by the amino acid in the "Q/R" editing site. The amino acid occupying this site determines single-channel conductance, ionic permeability, and other essential electrophysiologic properties of the resulting receptor channels. We found large-scale remodeling of dendritic architectures in a manner depending on the amino acid occupying the Q/R editing site. Alterations in the distribution of dendritic arbor were not prevented by blocking NMDA receptors. These observations suggest that the expression of GluR1 in motor neurons modulates a component of the molecular substrate of activity-dependent dendrite morphogenesis. The control of these events relies on subunit-specific properties of AMPA receptors.

Development of posture and locomotion: an interplay of endogenously generated activities and neurotrophic actions by descending pathways

The adult pattern of locomotion is observed at the end of the second postnatal week in the rat. The in vitro spinal cord isolated from immature rats has served as a valuable preparation to study the mechanisms underlying the development of locomotion. Although the rat is unable to walk at birth, because of an immature posture, its spinal cord networks can generate at least two kinds of motor patterns in vitro. One activity is called 'fictive locomotion' because it shares several common features with locomotion observed in vivo. This fictive locomotor pattern is rarely observed spontaneously and its release requires either pharmacological or electrical stimulation of the spinal cord. A second endogenously generated activity observed in this preparation occurs spontaneously and exhibits phase relationships between motor outputs that are quite different from the fictive locomotor pattern. Here we review some of the developmental functions this spontaneous activity may subserve. It is likely a major trigger for the maturation of lumbar networks in the fetus, at a stage when inputs from both the periphery and supraspinal structures are weak. Pathways descending from the brainstem arrive in the lumbar enlargement during the last week in utero and the first two postnatal weeks. These pathways, through the neurotransmitters they contain, especially monoamines, are essential for the expression of some neuronal properties and may regulate several ongoing developmental processes.

Differing mechanisms for glutamate receptor aggregation on dendritic spines and shafts in cultured hippocampal neurons

We have explored the ability of axons from spinal and hippocampal neurons to aggregate NMDA- and AMPA-type glutamate receptors on each other as a way of exploring the molecular differences between their presynaptic elements. Spinal axons, which normally cluster only AMPA-type glutamate receptors on other spinal neurons, cluster both AMPA- and NMDA-type glutamate receptors on the dendritic shafts of hippocampal interneurons but are ineffective at clustering either subtype of glutamate receptor on the dendritic spines of hippocampal pyramidal neurons. Conversely, hippocampal axons appear to be multipotent, capable of clustering both AMPA- and NMDA-type glutamate receptors on hippocampal interneurons and pyramidal cells. The secretion of the neuronal activity-regulated pentraxin (Narp) by hippocampal axons is restricted to contacts with interneurons. Exogenous application of Narp to cultured hippocampal neurons results in clusters of both NMDA- and AMPA-type glutamate receptors on hippocampal interneurons but not hippocampal pyramidal neurons. Because Narp displays no ability to directly aggregate NMDA receptors, we propose that Narp aggregates NMDA receptors in hippocampal interneurons indirectly through cytoplasmic coupling to synaptic AMPA receptors. Furthermore, our data suggest the existence of a novel molecule(s), capable of forming excitatory synapses on dendritic spines.

Quantitative measurement of glutamate receptor subunit protein expression in the postnatal rat spinal cord

Glutamate is the major excitatory neurotransmitter in the CNS and its effects on neurons are dependent on the type and composition of glutamate receptors with which it interacts. In this study, the protein expression levels of several ionotropic glutamate receptor subunits (N-methyl-D-aspartate (NMDA) subunits NR1, NR2A, NR2B, and alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptor subunits GluR1, GluR2, GluR4) were quantified in particulate preparations from rat spinal cord at various ages after birth. We found that all six subunits showed high expression in the early postnatal period, followed by a subsequent decline as the rats matured to adults. The levels of two subunits (NR2A and GluR4) were found to initially increase during the first postnatal week prior to the decline to adult levels. The high levels of expression observed of these subunits in the early postnatal period may have implications for mechanisms of neural injury and cell death in the immature nervous system that involve cation influx through ionotropic glutamate receptors.

Postural modifications and neuronal excitability changes induced by a short-term serotonin depletion during neonatal development in the rat

Serotonin (5-HT) plays an important role both in the development and in the recovery of locomotion after spinalization in vertebrates. We investigated the contribution of the serotonergic system to the maturation of the lumbar motoneurons and networks in the neonatal rat. A 5-HT synthesis inhibitor, p-chlorophenylalanine (PCPA), was administered daily from the first postnatal day (P0) onward. This protocol depleted serotonin in the spinal cord within 3-4 d, as demonstrated by immunohistochemistry. PCPA-treated rats exhibited postural changes characterized by lesser flexion at the knee and ankle levels and lesser extension of the hip. Posture was asymmetric, suggesting possible deficits in the interlimb coordination. Intracellular recordings were made at P3-5 from motoneurons innervating different hindlimb muscles, using the in vitro brainstem-spinal cord-nerve-attached preparation. In PCPA-treated rats, the conduction velocity of motoneurons was increased, and their excitability was decreased (because of higher rehobase and input conductance) compared with sham animals. In accordance with postural observations, changes were more pronounced in hip extensor/knee flexor than in ankle extensor motoneurons. The maturation of repetitive firing properties was stopped by PCPA treatment, although PCPA, applied in vitro, had no effect on membrane properties. The spontaneous endogenously generated activity, which is a characteristic of immature networks, was increased in PCPA-treated rats, suggesting that developing lumbar networks are sensitive to 5-HT levels. Serotonin may play a critical role during development in regulating the balance between the excitability of motoneurons and that of interneurons. Interneuronal excitability is crucial for the activity-dependent development of spinal cord networks.

AMPA-type glutamate receptor subunits 2/3 in the human trigeminal sensory ganglion and subnucleus caudalis from prenatal ages to adulthood

Using immunohistochemistry, the occurrence and distribution of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) glutamate receptor subunits GluR2/3 is shown in the human trigeminal ganglion and subnucleus caudalis from 20 weeks of gestation to adulthood. In the trigeminal ganglion a subpopulation of GluR2/3-like immunoreactive (LI) primary sensory neurons occurred at all examined ages, amounting to about 20% of all ganglion cells in the earliest pre-term newborn and in the adult, to about 30% at 24 and 32 weeks of gestation, and peaking to about 40% in the neonate. At all ages examined, GluR2/3-LI neurons were heterogeneous in size, although in the adult most of the labeled perikarya were large-sized, with a mean cell diameter above 35 microm. In the trigeminal subnucleus caudalis, positive elements could be first detected at 30 weeks of gestation and persisted at all other examined ages. At pre- and perinatal ages, the immunoreactivity was restricted to neuronal perikarya in the superficial layers and in the marginal zone of the nucleus. In the adult tissue, the subnucleus caudalis harbored a loose meshwork of varicose thread- and dot-like elements in the superficial layers and numerous immunoreactive neurons, distributed in lamina I, substantia gelatinosa, and in the superficial zone of the magnocellular region.

Opiate withdrawal during development: are NMDA receptors indispensable?

Despite decades of research, the mechanisms that underlie opiate tolerance, dependence and withdrawal remain elusive. Evidence accumulated over the past ten years suggests that the NMDA receptor plays a central role in mediating the neuroplasticity induced by chronic opiate administration in adult animals. Yet, during ontogeny, the NMDA receptor complex undergoes qualitative developmental changes, which renders some of the basic assumptions for a role of the NMDA receptor in opiate withdrawal invalid in infants. Recent data indicate that NMDA receptor antagonists are not effective in blocking morphine tolerance, dependence and withdrawal in the neonatal rat. Roles for other glutamate receptor types (e.g. metabotropic glutamate receptors) have also been proposed recently. In this article, the latest evidence that characterizes the dynamic roles of glutamate receptors in these phenomena during ontogeny will be discussed.

Subcellular localization of calcium-permeable AMPA receptors in spinal motoneurons

Activation of Ca(2+)-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors has been linked to potent effects on survival and dendritic outgrowth of spinal motoneurons. Ca(2+) permeability of AMPA receptors is controlled by the GluR2 subunit. Whole-cell electrophysiological studies have suggested that GluR2-containing and GluR2-lacking AMPA receptors may coexist in individual motoneurons. However, there has not been a direct demonstration of heterogeneity in AMPA receptor subunit composition in single motoneurons, nor of distinct subcellular distributions of GluR2-containing and GluR2-lacking receptors. In the present study, we have used confocal microscopy, immunocytochemistry and Ca(2+) imaging to characterize the subcellular localization of AMPA receptors in cultured rat spinal motoneurons. Immunoreactivity for GluR2 and GluR4 was concentrated in clusters, the vast majority of which were found in dendrites at synapses. Double-labelling for GluR2 and GluR4 revealed variability in relative expression of GluR2 and GluR4 between clusters within individual motoneurons; most AMPA receptor clusters were immunoreactive for both GluR2 and GluR4, but a significant minority of clusters were immunoreactive for GluR2 only or for GluR4 only. The majority of GluR2-immunonegative AMPA receptor clusters was present in dendrites, but the relative proportion of GluR2-immunonegative and GluR2-immunopositive clusters was similar in dendrites and soma. Imaging of [Ca(2+)](i) rises triggered by AMPA receptor activation confirmed Ca(2+) influx in motoneuron dendrites. These findings strongly support a model in which GluR2-containing and GluR2-lacking AMPA receptors coexist in motoneurons, clustered at synapses, and mixed in a relative proportion that varies considerably between cell membrane microdomains.

Inhibition of morphine withdrawal by the NMDA receptor antagonist MK-801 in rat is age-dependent

This study investigated the effects of the NMDA receptor antagonist MK-801 on the development of morphine dependence in 7-, 14-, and 21-day-old rat pups. For 6.5 days, starting at 1, 8, or 15 days of age, rats were pretreated with MK-801 (0.03 or 0.1 mg/kg, bid) or saline; 15 min later, morphine sulfate (10 mg/kg) or saline was injected to induce opiate dependence. On the afternoon of the seventh day, pups were injected with MK-801 (0.1 mg/kg) or saline and 15 min later with naltrexone (1 mg/kg) to precipitate withdrawal. Pups were then placed in a warm chamber with the litter and their behavior scan-sampled every 15 sec for a total of 15 min. MK-801 failed to inhibit morphine withdrawal in the 7-day-old rat, but did attenuate the development of morphine dependence in both the 14- and 21-day-old rats. These results suggest that the NMDA receptor is not functionally active in opiate withdrawal until around the second to third week of postnatal life in the rat and that there exists a transition period for the NMDA receptor to play a role in the development of opiate dependence and withdrawal.

The neurobiology of pain: developmental aspects

Invasive procedures that would be painful in children and adults are frequently performed on infants admitted to the neonatal intensive care unit. This article discusses sensory responses to these procedures in the immature nervous system and highlights the fact that, in addition to causing distress and delayed recovery, pain in infancy is also a developmental issue. First, the immaturity of sensory processing within the newborn spinal cord leads to lower thresholds for excitation and sensitization, therefore potentially maximizing the central effects of these tissue-damaging inputs. Second, the plasticity of both peripheral and central sensory connections in the neonatal period means that early damage in infancy can lead to prolonged structural and functional alterations in pain pathways that can last into adult life.

Perinatal development of lumbar motoneurons and their inputs in the rat

The rat is quite immature at birth and a rapid maturation of motor behavior takes place during the first 2 postnatal weeks. Lumbar motoneurons undergo a rapid development during this period. The last week before birth represents the initial stages of motoneuron differentiation, including regulation of the number of cells and the arrival of segmental and first supraspinal afferents. At birth, motoneurons are electrically coupled and receive both appropriate and inappropriate connections from the periphery; the control from supraspinal structures is weak and exerted mainly through polysynaptic connections. During the 1st postnatal week, inappropriate sensori-motor contacts and electrical coupling disappear, the supraspinal control increases gradually and myelin formation is responsible for an increased conduction velocity in both descending and motor axons. Both N-methyl-D-aspartate (NMDA) and non-NMDA receptors are transiently overexpressed in the neonatal spinal cord. The contribution of non-NMDA receptors to excitatory amino acid transmission increases with age. Activation of gamma-aminobutyric acid(A) and glycine receptors leads to membrane depolarization in embryonic motoneurons but to hyperpolarization in older motoneurons. The firing properties of motoneurons change with development: they are capable of more repetitive firing at the end of the 1st postnatal week than before birth. However, maturation does not proceed simultaneously in the motor pools innervating antagonistic muscles; for instance, the development of repetitive firing of ankle extensor motoneurons lags behind that of flexor motoneurons. The spontaneous embryonic and neonatal network-driven activity, detected at the levels of motoneurons and primary afferent terminals, may play a role in neuronal maturation and in the formation and refinement of sensorimotor connections.

AMPA receptor subunit expression in trigeminal neurons during postnatal development

Trigeminal motoneurons (Mo5) and mesencephalic trigeminal neurons (Me5) are important constituents of the neural circuitry responsible for jaw movements. Non-N-methyl-D-aspartate (NMDA) glutamate receptors are a critical component of the brainstem circuitry responsible for reflex and centrally activated jaw movements; however, little is known about the expression of these receptors in neonatal oral-motor circuitry. Receptor immunohistochemistry using affinity-purified polyclonal antibodies directed against GluR1, GluR2/3/4c, and GluR4, respectively, and a monoclonal antibody directed against the GluR2 subunit, were used in rats at postnatal day (P)1, P3, P5, P10, P19-21, P32-35, and P60 to describe the expression of the alpha-amino-d-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor in Mo5 and Me5 neurons. In Mo5, immunoreactivity was noted for all antibodies throughout the time frame sampled. Neurons in caudal portions of Me5 displayed immunoreactivity to each antibody except at P60 when GluR2 immunoreactivity was absent. Neurons located in rostral Me5 displayed GluR2/3/4c and GluR4 immunoreactivity throughout the time frame, GluR1 immunoreactivity emerged at P3 and a transient expression of GluR2 expression was observed between P10 and P32-35. The lack of labeling of some neurons in both regions, coupled with differences in temporal expression, suggests that there are differences in the AMPA receptor phenotype within and between Mo5 and Me5 during postnatal development. Differences in AMPA subunit composition suggest a complex role for AMPA-mediated glutamatergic neurotransmission in brainstem circuits controlling jaw movements.

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.

Increased immunoreactive labeling of the spinal N-methyl-D-aspartate R1 receptors after dorsal root ganglionectomy in the rats

The N-methyl-D-aspartate (NMDA) receptor plays an important role in the development of the autotomy after dorsal root ganglionectomy (DRGn). In this study, we further investigated the expression of the NMDAR1 in the spinal cord of the rats after right DRGn by immunohistochemical analyses. Computerized densitometric analysis of the NMDAR1 immunoreactivity was done and the integrated optical density (IOD) of the superficial laminae of the dorsal horn of the spinal cord was measured. The immunoreactive labeling of the NMDAR1 was increased in the cervical spinal cord ipsilateral to the DRGn from day 5 to 14 after DRGn. The ratio of the right/left IOD of the rats receiving DRGn was significantly higher than the rats in the sham-operated group and the control group (P<0.05). The expression of the NMDAR1 increased gradually to reach the peak at day 7 after DRGn (mean right/left IOD ratio=1.52), then decreased thereafter. The increased expression of the NMDAR1 at day 7 was suppressed by MK-801 (NMDA receptor antagonist) administered immediately after DRGn, but not by normal saline or 1,2,3,4-tetrahydro-6-nitro-2, 3-dioxo-benzo[f] quinoxaline-7-sulfonamide (NBQX, non-NMDA receptor antagonist). The results indicated that the expression of the NMDAR1 in the superficial laminae of the dorsal horn of the spinal cord was increased after DRGn and the time course was compatible with the onset and development of the autotomy induced by DRGn.

Experience-dependent development of spinal motor neurons

Locomotor activity in many species undergoes pronounced alterations in early postnatal life, and environmental cues may be responsible for modifying this process. To determine how these events are reflected in the nervous system, we studied rats reared under two different conditions-the presence or absence of gravity-in which the performance of motor operations differed. We found a significant effect of rearing environment on the size and complexity of dendritic architecture of spinal motor neurons, particularly those that are likely to participate in postural control. These results provide evidence that neurons subserving motor function undergo activity-dependent maturation in early postnatal life in a manner analogous to sensory systems.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Molecular factors underlying selective vulnerability of motor neurons to neurodegeneration in amyotrophic lateral sclerosis

Current research evidence suggests that genetic factors, oxidative stress and glutamatergic toxicity, with damage to critical target proteins and organelles, may be important contributory factors to motor neuron injury in amyotrophic lateral sclerosis (ALS). Various molecular and neurochemical features of human motor neurons may render this cell group differentially vulnerable to such insults. Motor neurons are large cells with long axonal processes which lead to requirements for a high level of mitochondrial activity and a high neurofilament content compared to other neuronal groups. The lack of calcium buffering proteins parvalbumin and calbindin D28k and the low expression of the GluR2 AMPA receptor subunit may render human motor neurons particularly vulnerable to calcium toxicity following glutamate receptor activation. Motor neurons also have a high perisomatic expression of the glutamate transporter protein EAAT2 and a very high expression of the cytosolic free radical scavenging enzyme Cu/Zn superoxide dismutase (SOD1) which may render this cell group vulnerable in the face of genetic or post-translational alterations interfering with the function of these proteins. More detailed characterisation of the molecular features of human motor neurons in the future may allow the strategic development of better neuroprotective therapies for the benefit of patients afflicted by ALS.

Silent NMDA receptor-mediated synapses are developmentally regulated in the dorsal horn of the rat spinal cord

In vitro whole cell patch-clamp recording techniques were utilized to study silent pure-N-methyl-D-aspartate (NMDA) receptor-mediated synaptic responses in lamina II (substantia gelatinosa, SG) and lamina III of the spinal dorsal horn. To clarify whether these synapses are present in the adult and contribute to neuropathic pain, transverse lumbar spinal cord slices were prepared from neonatal, naive adult and adult sciatic nerve transected rats. In neonatal rats, pure-NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) were elicited in SG neurons either by focal intraspinal stimulation (n = 15 of 20 neurons) or focal stimulation of the dorsal root (n = 2 of 7 neurons). In contrast, in slices from naive adult rats, no silent pure-NMDA EPSCs were recorded in SG neurons following focal intraspinal stimulation (n = 27), and only one pure-NMDA EPSC was observed in lamina III (n = 23). Furthermore, in rats with chronic sciatic nerve transection, pure-NMDA EPSCs were elicited by focal intraspinal stimulation in only 2 of 45 SG neurons. Although a large increase in Abeta fiber evoked mixed alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and NMDA receptor-mediated synapses was detected after sciatic nerve injury, Abeta fiber-mediated pure-NMDA EPSCs were not evoked in SG neurons by dorsal root stimulation. Pure-NMDA receptor-mediated EPSCs are therefore a transient, developmentally regulated phenomenon, and, although they may have a role in synaptic refinement in the immature dorsal horn, they are unlikely to be involved in receptive field plasticity in the adult.

AMPA glutamate receptors and respiratory control in the developing rat: anatomic and pharmacological aspects

The developmental role of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) glutamate receptors in respiratory regulation remains undefined. To study this issue, minute ventilation (V(E)) was measured in 5-, 10-, and 15-day-old intact freely behaving rat pups using whole body plethysmography during room air (RA), hypercapnic (5% CO(2)), and hypoxic (10% O(2)) conditions, both before and after administration of the non-N-methyl-D-aspartate (NMDA) receptor antagonist 1,2,3, 4-tetrahydro-6-nitro-2,3-dioxobenzo[f]quinoxaline-7-sulfonamide disodium (NBQX; 10 mg/kg ip). In all age groups, V(E) during RA was unaffected by NBQX, despite reductions in breathing frequency (f) induced by increases in both inspiratory and expiratory duration. During hypoxia and hypercapnia, V(E) increases were similar in both NBQX and control conditions in all age groups. However, tidal volume was greater and f lower after NBQX. To determine if AMPA receptor-positive neurons are recruited during hypoxia, immunostaining for AMPA receptor (GluR2/3) and c-fos colabeling was performed in caudal brain stem sections after exposing rat pups at postnatal ages 2, 5, 10, and 20 days, and adult rats to room air or 10% O(2) for 3 h. GluR2/3 expression increased with postnatal age in the nucleus of the solitary tract (NTS) and hypoglossal nucleus, whereas a biphasic pattern emerged for the nucleus ambiguus (NA). c-fos expression was enhanced by hypoxia at all postnatal ages in the NTS and NA and also demonstrated a clear maturational pattern. However, colocalization of GluR2/3 and c-fos was not affected by hypoxia. We conclude that AMPA glutamate receptor expression in the caudal brain stem is developmentally regulated. Furthermore, the role of non-NMDA receptors in respiratory control of conscious neonatal rats appears to be limited to modest, albeit significant, regulation of breathing pattern.

Ionotropic glutamate receptor expression in human spinal cord during first trimester development

Quantitative receptor autoradiography and immunoblotting were used to study the expression and distribution of AMPA, kainate and NMDA receptors in first trimester human spinal cord obtained from elective abortions ranging from 4 to 11.5 weeks of gestational age. Spinal cord tissue sections were processed for receptor autoradiography with the ligands [3H]AMPA, [3H]kainate and [3H]MK-801 and the optical density was measured separately in a dorsal region (alar plate) and ventral region (basal plate) of the autoradiographs. Binding sites for all three ligands were demonstrated already at 4-5.5 weeks of gestation and increased continuously during the first trimester both in the dorsal and ventral regions. [3H]AMPA binding to both high- and low-affinity sites increased from undetectable levels to about 35 and 400 fmol/mg tissue, respectively, during this period. A temporal difference in the distribution of [3H]AMPA binding sites was observed. The early homogeneous pattern of [3H]AMPA binding in both alar and basal plates had changed to a heterogeneous pattern at 11 weeks of gestation with the highest density of [3H]AMPA binding sites in the superficial layers of the immature dorsal horn. [3H]kainate and [3H]MK-801 binding sites were densely and homogeneously distributed already at 4 weeks, and steadily increased six- and two-fold, respectively, to about 100 fmol/mg tissue at 11.5 weeks of gestation. Immunoreactive bands corresponding to the NMDA receptor subunits NR1, NR2A, NR2B, NR2C and NR2D were demonstrated by immunoblotting at the earliest between 4.5 and 7 weeks and increasing concentrations were seen up to 11 weeks of gestation. These results suggest that AMPA, kainate and NMDA receptors are expressed in the human spinal cord early in embryogenesis.

Building blocks of pain: the regulation of key molecules in spinal sensory neurones during development and following peripheral axotomy

The pathways, synapses and molecules involved in pain processing in the newborn are not only required to trigger repair and recuperation but are also involved in the process of forming a mature nervous system. Sensory neurons in the dorsal root ganglion and dorsal horn express a phenomenal array of molecules which contribute to their structural and functional characteristics and many of these are developmentally regulated both pre- and postnatally. In order to understand nociceptive signalling and pain in the neonate we need a clear picture of that regulation. This review concentrates on the changing expression of selected key molecules, receptors and channels in the embryo, neonate and adult, which both characterise the sensory neuron and contribute to its response to painful stimuli in normal and pathological conditions.

The distribution of neurons expressing calcium-permeable AMPA receptors in the superficial laminae of the spinal cord dorsal horn

The superficial dorsal horn is a major site of termination of nociceptive primary afferents. Fast excitatory synaptic transmission in this region is mediated mainly by release of glutamate onto postsynaptic AMPA and NMDA receptors. NMDA receptors are known to be Ca2+-permeable and to provide synaptically localized Ca2+ signals that mediate short-term and long-term changes in synaptic strength. Less well known is a subpopulation of AMPA receptors that is Ca2+-permeable and has been shown to be synaptically localized on dorsal horn neurons in culture (Gu et al., 1996) and expressed by dorsal horn neurons in situ (Nagy et al., 1994; Engelman et al., 1997). We used kainate-induced cobalt uptake as a functional marker of neurons expressing Ca2+-permeable AMPA receptors and combined this with markers of nociceptive primary afferents in the postnatal rat dorsal horn. We have shown that cobalt-positive neurons are located in lamina I and outer lamina II, a region strongly innervated by nociceptors. These cobalt-positive neurons colocalize with afferents labeled by LD2, and with the most dorsal region of capsaicin-sensitive and IB4- and LA4-positive afferents. In contrast, inner lamina II has a sparser distribution of cobalt-positive neurons. Some lamina I neurons expressing the NK1 receptor, the receptor for substance P, are also cobalt positive. These neurons are likely to be projection neurons in the nociceptive pathway. On the basis of all of these observations, we propose that Ca2+-permeable AMPA receptors are localized to mediate transmission of nociceptive information.

Excitatory amino acids and synaptic transmission in embryonic rat brainstem motoneurons in organotypic culture

We used brainstem motoneurons recorded in organotypic slice co-cultures maintained for more than 18 days in vitro, together with multibarrel ionophoretic applications of glutamate receptor agonists and bath applications of specific blocking agents, to study the responses of rat brainstem motoneurons to glutamate receptor activation, and the contribution of these receptors to synaptic transmission. Differentiated brainstem motoneurons in vitro are depolarized by glutamate, N-methyl-d-aspartate (NMDA) and dl-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) iontophoresis, and express NMDA, AMPA and also specific kainate receptors, as evidenced by (+/-)2-amino-5-phosphonovaleric acid (APV)- and (-)1-(4-aminophenyl)-3-methyl-carbamoyl-4-methyl-7, 8-methylenedioxy-3,4-dihydro-5H-2,3-benzo-diazepine [GYKI 53784 (LY303070)]-resistant depolarizations. Electrical stimulations applied to the dorsal part of the explant trigger excitatory synaptic potentials with latencies distributed in three regularly spaced groups. Excitatory postsynaptic potentials (EPSPs) in the earliest group have a similar latency and time course and correspond to monosynaptic activation. EPSPs in later groups have more scattered latencies and time courses and correspond to polysynaptic activation. Monosynaptic EPSPs are insensitive to the specific NMDA blocker APV, and are completely and reversibly suppressed by the non-competitive AMPA receptor antagonist GYKI 53784 (LY303070). Detailed analysis of the spontaneous excitatory synaptic activity shows that APV decreases the frequency of spontaneous EPSPs without modifying their shape or amplitude. We conclude that excitatory synapses on brainstem motoneurons in vitro are mainly activated through AMPA receptors (AMPA-Rs). NMDA receptors (NMDA-Rs) are present in the membrane, but are located either at extrasynaptic sites or silent synapses, and are not directly involved in synaptic transmission on motoneurons. On the contrary, NMDA receptors contribute to synaptic transmission within the premotor interneuronal network.

The role of nitric oxide and NMDA receptors in the development of motor neuron dendrites

Nitric oxide (NO) has been implicated in the establishment of precise synaptic connectivity throughout the neuroaxis in several species. To determine the contribution of NO to NMDA receptor-dependent dendritic growth in motor neurons, we administered the NMDA antagonist MK-801 to wild-type mice and neuronal nitric oxide synthase (nNOS) knock-out mice between postnatal days 7 and 14. Compared to saline-treated wild-type animals the number of dendritic bifurcations was significantly reduced in nNOS knock-out animals and MK-801-treated wild-type animals. There was no significant difference in dendritic bifurcation between MK-801-treated wild-type, MK-801-treated nNOS knock-out, and saline-treated nNOS knock-out animals, suggesting that nNOS knock-out and NMDA receptor block had similar effects. The path of the longest dendrite and the number of primary dendrites was the same in all treatment groups, indicating an effect specific to bifurcation. Sholl analysis revealed that differences in bifurcation numbers occurred between 160 and 320 micrometers from the cell body, the distance at which second, third, and fourth order dendrites are most prevalent. Dendrite order analyses confirmed a significant reduction in numbers, but not lengths, of third and fourth order dendrites in nNOS knock-out and drug-treatment groups. Finally, immunohistochemical examination of the developing spinal cord indicated that NMDA receptors and nNOS are colocalized within interneurons surrounding the motor neuron pool. These results support the view that at least part of NMDA receptor-dependent arborization of motor neuron dendrites is mediated by the local production of NO within the developing spinal cord.

The origin of spontaneous activity in developing networks of the vertebrate nervous system

Spontaneous neuronal activity has been detected in many parts of the developing vertebrate nervous system. Recent studies suggest that this activity depends on properties that are probably shared by all developing networks. Of particular importance is the high excitability of recurrently connected, developing networks and the presence of activity-induced transient depression of network excitability. In the spinal cord, it has been proposed that the interaction of these properties gives rise to spontaneous, periodic activity.

Excitotoxic death of a subset of embryonic rat motor neurons in vitro

We have used cultures of purified embryonic rat spinal cord motor neurons to study the neurotoxic effects of prolonged ionotropic glutamate receptor activation. NMDA and non-NMDA glutamate receptor agonists kill a maximum of 40% of the motor neurons in a concentration- and time-dependent manner, which can be blocked by receptor subtype-specific antagonists. Subunit-specific antibodies stain all of the motor neurons with approximately the same intensity and for the same repertoire of subunits, suggesting that the survival of the nonvulnerable population is unlikely to be due to the lack of glutamate receptor expression. Extracellular Ca2+ is required for excitotoxicity, and the route of entry initiated by activation of non-NMDA, but not NMDA, receptors is L-type Ca2+ channels. Ca2+ imaging of motor neurons after application of specific glutamate receptor agonists reveals a sustained rise in intracellular Ca2+ that is present to a similar degree in most motor neurons, and can be blocked by appropriate receptor/channel antagonists. Although the lethal effects of glutamate receptor agonists are seen in only a subset of cultured motor neurons, the basis of this selectivity is unlikely to be simply the glutamate receptor phenotype or the level/pattern of rise in agonist-evoked intracellular Ca2+.

Distribution and developmental changes in metabotropic glutamate receptor messenger RNA expression in the rat lumbar spinal cord

Using in situ hybridisation, the regional distribution of primary transcripts and splice variants of all metabotropic glutamate receptor subtypes (mGluR) currently known to be expressed in the spinal cord have been studied in the lumbar enlargement of the rat spinal cord. In adult animals, the messenger RNA of the mGluR subtypes 1, 5, 3, 4 and 7 were differentially expressed. The transcripts of mGluR1 and 5 were most abundant with mGluR5 messenger RNA being concentrated in the superficial dorsal horn. In contrast, the mGluR2 transcript was not detectable with the sensitivity of the method. Secondly, age related changes (postnatal days 1, 7, 12, 21) in the postnatal expression of mGluR1-5 and 7 transcripts have been investigated. mGluR1 and 7 messenger RNA showed a general decrease in spinal expression from postnatal day 1 to day 21. Quantitative densitometry showed high mGluR3 and 5 messenger RNA levels especially in the superficial dorsal horn at birth, however these levels decreased with age. In addition to changes in density, the regional distribution of mGluR3 messenger RNA was altered with postnatal development. Up to postnatal day 12, mGluR3 messenger RNA expression was almost exclusively restricted to the spinal grey matter, but with postnatal day 21 a strong additional expression in the white matter occurred. Distribution of mGluR4 messenger RNA showed little change in the dorsal horn, however motoneuronal expression emerged during development. These changes may suggest different roles for mGluRs in the maturation of spinal transmission of the rat nervous system.

Glutamate receptor phenotypes in the auditory brainstem and mid-brain of the developing rat

Glutamate receptors mediate most excitatory synaptic transmission in the adult vertebrate brain, but their activation in developing neurons also influences developmental processes. However, little is known about the developmental regulation of the subunits composing these receptors. Here we have studied age-dependent changes in the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) and N-methyl-D-aspartate (NMDA) receptor subunits in the cochlear nucleus complex (CN), the superior olivary complex (SOC), the nuclei of the lateral lemniscus, and the inferior colliculus of the developing rat. In the lateral superior olive, the medial nucleus of the trapezoid body, and the ventral nucleus of the lateral lemniscus, the distribution of AMPA receptor subunits changed drastically with age. While GluR1 and GluR2 subunits were highly expressed in the first 2 postnatal weeks, GluR4 staining was detectable only thereafter. GluR1 and GluR2 immunoreactivities rapidly decreased during the third postnatal week, with the GluR1 subunits disappearing from most neurons. In contrast, the adult pattern of the distribution of AMPA receptor subunits emerged gradually in most of the other auditory nuclei. Thus, progressive as well as regressive events characterized AMPA receptor development in some nuclei, while a monotonically maturation was seen in other regions. In contrast, the staining patterns of NMDA receptor subunits remained stable or only decreased during the same period. Although our data are not consistent with a generalized pattern of AMPA receptor development, the abundance of GluR1 subunits is a distinctive feature of early AMPA receptors. As similar AMPA receptors are present during plasticity periods throughout the brain, neurons undergoing synaptic and structural remodelling might have a particular need for these receptors.

Presence of a non-NMDA glutamate receptor subtype in the sympathetic nervous system of neonatal swine

For the first time, the GluR-1 subtype of AMPA receptor was identified in the sympathetic nervous system of neonatal swine, an animal model of human development and heart disease. The rationale was to seek evidence of a role ascribed to glutamate in cardiorespiratory regulation in the laboratory rat. The receptor was demonstrated with the avidin-biotin immunoperoxidase technique by using an affinity-purified polyclonal antibody judged to be specific to Glu-R1 in several species. Glu-R1 immunoreactivity was regionally distributed in the thoracic spinal gray, and present intracellularly in neurons and within the surrounding neuropil. Sympathetic preganglionic neurons in the intermediolateral cell column of upper and lower thoracic spinal segments were intensely labeled and surrounded by labeled neuropil. High concentrations of Glu-R1 distinguished laminae II: substantia gelatinosa and the outer region of lamina III. Laminae I and V of the dorsal horn but not IV contained immunolabeled neurons. Arrays of moderately immunoreactive perikarya extended from an intermediate zone of laminae VII to the central gray. Glia and perivascular processes were not labeled, confirming previous observations [Tachibana, M., Wenthold, R.J., Morioka, H., Petralia, R.S., 1994. Light and electron microscopic immunocytochemical localization of AMPA-selective glutamate receptors in the rat spinal cord. J. Comp. Neurol. 344, 431-454]. Neuronal staining patterns corroborated evidence in rats indicating a postsynaptic localization of Glu-R1 associated with plasma membranes and cytoplasmic organelles [Martin, L.J., Blackstone, C.D., Levey, A.I., Huganir, R.L., Price, D.L., 1993. AMPA glutamate receptor subunits are differentially distributed in rat brain. Neuroscience 53, 327-358.; Rubio, M.E., Wenthold, R.J., 1997. Glutamate receptors are selectively targeted to postsynaptic sites in neurons. Neuron 18, 939-950]. Our data predict a role for L-glutamate in postnatal development of cardiorespiratory reflexes in swine.

Network bursting by organotypic spinal slice cultures in the presence of bicuculline and/or strychnine is developmentally regulated

Organotypic cocultures of dorsal root ganglia and spinal cord from embryonic rats provides direct access to spinal interneurons in a culture system in which the cytoarchitectural organization of the spinal cord slice is maintained. This preparation was used to investigate the possible induction of rhythmic behaviour at different times of development in vitro. Spontaneous rhythmic bursts induced by coapplication of strychnine (1 microM) and bicuculline (20 microM) were observed with patch-clamp recordings from ventral interneurons. Ventral horn interneurons consistently developed a very regular pattern of activity which was superimposed on a background of sustained synaptic activity. The pattern of the spontaneous bursting following application of strychnine and bicuculline showed a developmentally regulated difference in frequency between two distinct stages of in vitro development.

Neurodegeneration in excitotoxicity, global cerebral ischemia, and target deprivation: A perspective on the contributions of apoptosis and necrosis

In the human brain and spinal cord, neurons degenerate after acute insults (e.g., stroke, cardiac arrest, trauma) and during progressive, adult-onset diseases [e.g., amyotrophic lateral sclerosis, Alzheimer's disease]. Glutamate receptor-mediated excitotoxicity has been implicated in all of these neurological conditions. Nevertheless, effective approaches to prevent or limit neuronal damage in these disorders remain elusive, primarily because of an incomplete understanding of the mechanisms of neuronal death in in vivo settings. Therefore, animal models of neurodegeneration are crucial for improving our understanding of the mechanisms of neuronal death. In this review, we evaluate experimental data on the general characteristics of cell death and, in particular, neuronal death in the central nervous system (CNS) following injury. We focus on the ongoing controversy of the contributions of apoptosis and necrosis in neurodegeneration and summarize new data from this laboratory on the classification of neuronal death using a variety of animal models of neurodegeneration in the immature or adult brain following excitotoxic injury, global cerebral ischemia, and axotomy/target deprivation. In these different models of brain injury, we determined whether the process of neuronal death has uniformly similar morphological characteristics or whether the features of neurodegeneration induced by different insults are distinct. We classified neurodegeneration in each of these models with respect to whether it resembles apoptosis, necrosis, or an intermediate form of cell death falling along an apoptosis-necrosis continuum. We found that N-methyl-D-aspartate (NMDA) receptor- and non-NMDA receptor-mediated excitotoxic injury results in neurodegeneration along an apoptosis-necrosis continuum, in which neuronal death (appearing as apoptotic, necrotic, or intermediate between the two extremes) is influenced by the degree of brain maturity and the subtype of glutamate receptor that is stimulated. Global cerebral ischemia produces neuronal death that has commonalities with excitotoxicity and target deprivation. Degeneration of selectively vulnerable populations of neurons after ischemia is morphologically nonapoptotic and is indistinguishable from NMDA receptor-mediated excitotoxic death of mature neurons. However, prominent apoptotic cell death occurs following global ischemia in neuronal groups that are interconnected with selectively vulnerable populations of neurons and also in nonneuronal cells. This apoptotic neuronal death is similar to some forms of retrograde neuronal apoptosis that occur following target deprivation. We conclude that cell death in the CNS following injury can coexist as apoptosis, necrosis, and hybrid forms along an apoptosis-necrosis continuum. These different forms of cell death have varying contributions to the neuropathology resulting from excitotoxicity, cerebral ischemia, and target deprivation/axotomy. Degeneration of different populations of cells (neurons and nonneuronal cells) may be mediated by distinct or common causal mechanisms that can temporally overlap and perhaps differ mechanistically in the rate of progression of cell death.

Postnatal changes in responses of rat dorsal horn cells to afferent stimulation: a fibre-induced sensitization

1. In vivo extracellular recordings were made of 171 dorsal horn cells in both superficial and deep laminae in urethane-anaesthetized newborn rats aged 3, 6, 10 and 21 days, and their response to single and repeated stimuli to primary afferent fibres investigated. 2. No long-latency spike responses were evoked in response to C fibre stimulation in pups at postnatal day 3 (P3) or P6, while by P10, 35 % of cells had a C fibre response. Latencies of response to A fibre skin stimulation were very long and varied widely in the youngest animals, particularly in superficial cells, but mean latencies decreased with postnatal age, from 33.1 +/- 2.78 ms at P3 to 7.3 +/- 0.3 ms at P21. The mean number of spikes evoked by a single A fibre skin stimulus was remarkably consistent between cells and not significantly different in superficial and deep laminae at each age. The mean value of 5.1 +/- 0.6 at P3 increased to 7.0 +/- 1.4 at P10. 3. Repeated stimulation of cutaneous A fibres at 0.5 Hz at twice the threshold level did not significantly alter the magnitude of the evoked response but led to shifts in latency, or 'latency jitter', which decreased with age. Deeper cells displayed more latency jitter than superficial cells. 4. Repeated stimulation of cutaneous A fibres at 0.5 Hz at twice the threshold level produced considerable sensitization in a population of dorsal horn cells in the neonate. This sensitization was unlike the classic C fibre-evoked 'wind-up' observed in adult dorsal horn. The direct A fibre-evoked activity did not increase, but the background activity increased during repetitive stimulation leading to a prolonged after-discharge beyond the stimulation period. At P6, 33 % of cells were sensitized, displaying a mean after-discharge of 70.6 +/- 18 s. At P10, only 6 % were sensitized, with a mean after-discharge of 63 s, and by P21, sensitization was no longer observed. 5. The present study demonstrates that the postsynaptic activity evoked in neonatal dorsal horn cells by cutaneous afferents differs considerably from that in adults. The results may account for the known behavioural reflex sensitization to low-intensity cutaneous stimulation observed in neonatal rats and man.

Maturation of the biphasic behavioral and heart rate response in the formalin test

The biological processes that mediate and modulate the perception of pain in the infant animal are not well studied and thus nociception during early development is poorly understood. In the adult animal, injection of formalin into the hind paw produces distinct phases of behavioral and autonomic responses: an early nociceptive response followed by a period of quiescence and a later second phase that matches or exceeds the initial response. The delayed reaction of the second phase has been suggested to be a model of inflammation-induced changes in neuronal sensitivity. Studies in the infant rat have demonstrated that the first phase is present in the fetus and neonate but the onset of the second phase is later maturing. We report here that the first phase occurs in 7- to 35-day-old pups in the formalin test when measured behaviorally and in 14- to 35-day-old pups when assessed by increased heart rate. However, the behavioral response in second phase is greatly attenuated or absent in 7- or 14-day-old pups, a finding consistent with that of others, appearing first at 21 days of age. The biphasic tachycardic response was not noted until even later, at 35 days of age. These data confirm that the neural mechanisms that mediate the secondary behavioral phase in the formalin test are late maturing, that the biphasic cardiovascular response does not occur until substantially later, after weaning, and that the behavioral and cardiovascular responses are dissociated developmentally.

Functional AMPA/kainate receptors in human embryonic and foetal central nervous system

Here, functional AMPA/kainate receptors in human embryonic (5.5-7.5 gestational weeks) and foetal (8-10 gestational weeks) central nervous system tissue, shown by the cobalt labeling method, are reported. Specific agonist-induced cobalt incorporation was detected in brainstem and spinal cord cells, even in the youngest embryo studied. T-AMPA or kainate, but also vegetal toxins such as L-BOAA or acromelate, induced accumulation of cobalt. In contrast, no labeling was observed after exposure to KCl or NMDA. Cobalt labeled cells were particularly prominent in motor regions of brainstem and spinal cord. Co-application of the diuretic agent cyclothiazide, a desensitization blocker at AMPA receptors, dramatically increased the number of stained cells, which was particularly obvious in sensory regions, suggesting different receptor properties in motor versus sensory regions. This is the first study providing evidence for functional AMPA/kainate receptors, permeable to divalent cations, in brainstem and spinal cord at an early stage of human central nervous system development. Since many developmental processes are influenced by the modulation of cytosolic calcium, exposure at critical stages of embryogenesis to food or drug substances modifying the activity of AMPA/kainate receptors may alter brain development.

GluR1 and GluR2/3 subunits of the AMPA-type glutamate receptor are associated with particular types of neurone in laminae I-III of the spinal dorsal horn of the rat

GluR1 and GluR2 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor are expressed at high levels by neurones in laminae I-III of rat spinal dorsal horn, an area which contains numerous, densely packed small neurones. In order to determine whether these subunits are expressed by inhibitory or excitatory neurones, we combined pre-embedding immunocytochemistry with antibodies that recognize either GluR1, or an epitope common to GluR2 and 3, with postembedding detection of gamma-aminobutyric acid (GABA) and glycine. Most (78%) of the neurones with GluR1-immunoreactivity were GABA-immunoreactive, and some of these were also glycine-immunoreactive, whereas nearly all (97%) of the GluR2/3-immunoreactive neurones were not GABA- or glycine-immunoreactive. We carried out double-immunofluorescence and confocal microscopy to provide further information on the neurochemistry of cells that express these subunits. As expected, all neurotensin- and virtually all somatostatin-immunoreactive cells (which are thought to be excitatory interneurones) were GluR2/3- but not GluR1-immunoreactive, whereas parvalbumin-containing cells (most of which are GABAergic) possessed GluR1-, but usually not GluR2/3-immunoreactivity. Neurones that contained nitric oxide synthase (most of which are GABAergic) were more variable, with 57% GluR1-immunoreactive and 41% GluR2/3-immunoreactive. Cholinergic neurones in lamina III (which are also GABAergic) invariably showed each type of GluR-immunoreactivity. These results suggest that neuronal populations in laminae I-III have characteristic patterns of GluR expression: GluR1 is particularly associated with inhibitory neurones, and GluR2 with excitatory neurones. This makes it likely that some of the AMPA receptors present on the inhibitory interneurones lack the GluR2 subunit, and may therefore have significant Ca2+-permeability.

Neuronal apoptosis and necrosis following spinal cord ischemia in the rat

We examined the characteristics of neuronal death induced by ischemia in the spinal cord. Spinal cord ischemia was induced in Long-Evans rats by occlusion of the descending aorta with a 2F Fogarty catheter for 20 min (model 1) or more limited aortic occlusion (15 min) coupled with blood volume reduction (model 2); rats were sacrificed 6 h-7 days later. The animals developed variable paraparesis in model 1 and reliable paraplegia in model 2. The extent of histopathological spinal cord damage, being maximal in the lumbar cord, correlated well with the severity of paraparesis. Two distinct types of spinal cord neuronal death were observed, consistent with necrosis and apoptosis. Neuronal necrosis was seen in gray matter laminae 3-7, characterized by the rapid (6 h) onset of eosinophilia on hematoxylin/eosin-stained sections, and gradual (1-7 days) development of eosinophilic ghosting. Although TUNEL positivity was present, disintegration of membranes and cytoplasmic organelles was seen under electron microscopy. Neuronal apoptosis was seen after 1-2 days in dorsal horn laminae 1-3, characterized by both TUNEL positivity and electron microscopic appearance of nuclear chromatin aggregation and the formation of apoptotic bodies. DNA extracted from the ischemic lumbar cord showed internucleosomal fragmentation (laddering) on gel electrophoresis. These data suggest that distinct spinal cord neuronal populations may undergo necrosis and apoptosis following transient ischemic insults.

In vivo and in vitro studies of glycine- and glutamate-evoked acetylcholinesterase release from spinal motor neurones: implications for amyotrophic lateral sclerosis/motor neurone disease pathogenesis

To investigate the spinal cellular structures and molecular mechanisms involved in acetylcholinesterase (AChE) release evoked by both glycine (GLY) and glutamate (GLU)--responses that might play a role in chronic neurotoxicity--we analysed AChE histochemistry and histology upon systemic administration of aspartate (ASP), and conducted in vitro experiments in synaptosomes and slices prepared from mouse spinal ventral horns. Upon superfusion and incubation exposure of these preparations to GLY- and GLU-receptor agonists, we assayed both tissue content and release of AChE, butyrylcholinesterase and lactic dehydrogenase. Histochemical reduction of motor neurone (MN) AChE, calcium dependency, decreases in intracellular AChE and the ratio amongst molecular forms released, suggest that both synaptosomal GLY-evoked AChE release (GLY-EAR) and GLU-receptor-elicited AChE release (GEAR) have release sites located at MN presynaptic terminals. These responses exhibited remarkable postnatal regulation. GEAR seems to be mediated through alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate receptors after the fourth postnatal week and through both NMDA and non-NMDA receptors at earlier stages. Sustained rises of extracellular AChE might link acute excitotoxic injury with several long-lasting pathways leading to chronic neurotoxicity, since AChE molecular properties include: (1) the ability to block cholinergic mechanisms that protect MN against overactivity; (2) activation of ATP-dependent potassium channels; (3) promotion of neurite and axon outgrowth; and possibly (4) stimulation of brain macrophage migration and activation.

Ontogeny of non-NMDA glutamate receptors in rat barrel field cortex: II. Alpha-AMPA and kainate receptors

The ontogeny of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate (KA) glutamate receptors in rat barrel field cortex was characterized by using receptor autoradiography and immunocytochemistry. A somatotopic pattern of AMPA receptors with fewer [3H]AMPA sites in barrel centers than in surrounding cortex did not emerge until postnatal day 10 (P10). After reaching a peak density at P14, the density of [3H]AMPA receptors declined in both barrel centers and surrounding cortex. Compared with AMPA receptors, the density of [3H]KA sites at all ages was low, a somatotopic expression of [3H]KA sites was missing, and the developmental curve for [3H]KA sites was more shallow than that for [3H]AMPA binding sites. A differential ontogeny of AMPA and KA receptors in barrel field cortex was also demonstrated in immunocytochemical studies with antibodies to the AMPA receptor subunits GluR1 and GluR2,3 and the KA receptor subunits GluR6,7. GluR1 and GluR2,3 staining was more dense in barrel septa than in barrel centers; this pattern persisted into adulthood. GluR1 and GluR2,3 receptors were localized to cell bodies and dendrites as well as the neuropil, but different populations of cortical neurons expressed these receptors. At P10, KA receptor subunits GluR6,7 exhibited a contrasting pattern to that of AMPA receptor subunits, with slightly more neuropil staining in barrel centers than in surrounding cortex. After that point, the somatotopic pattern of GluR6,7 subunit expression was lost. The contrasting developmental patterns of expression of the AMPA and KA receptors in the barrel field suggest that they may play different roles in the whisker-to-barrel pathway.

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.

Synchronized overproduction of AMPA, kainate, and NMDA glutamate receptors during human spinal cord development

Quantitative receptor autoradiography was used to map the distribution in the developing human spinal cord of the three types of ionotropic glutamate receptors. N-methyl-D-Aspartate (NMDA) receptors were labeled with [3H]glutamate, kainic acid (KA) receptors were labeled with [3H]KA, and alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionate (AMPA) receptors were labeled with [3H]AMPA. In the adult, labeling of all three receptor subtypes is largely restricted to the substantia gelatinosa (SG) in the dorsal horn, with very low level labeling elsewhere in the spinal gray matter. In marked distinction, in late fetal life, high level ligand binding is seen throughout the spinal gray matter. In early postnatal life, binding sites diminish in all regions, but least so in the SG, until the adult pattern emerges. Thus a coordinated transient high level of ionotropic glutamate receptor expression occurs within the developing spinal cord. Saturation analysis of ligand binding shows that the affinity of [3H]KA and [3H]AMPA binding is not developmentally regulated. In contrast, the affinity of [3H]glutamate binding to the NMDA receptor in the fetal ventral horn is three-fold greater than in the adult ventral horn. Thus, in addition to quantitative changes in glutamate receptor expression, qualitative changes occur in the expression of NMDA receptors during development. The distinct glutamate receptor phenotype of fetal and early postnatal spinal cord cells suggests that alterations in the excitable properties of these cells plays an important role in activity-dependent development and in susceptibility to excitotoxic injury.

Postnatal ontogeny of glutamate receptors in the rat nucleus tractus solitarii and ventrolateral medulla

The nucleus tractus solitarii and the ventrolateral medulla are two brainstem regions involved in regulation of autonomic functions. Glutamate (Glu) receptors localized within these two regions play a key role in neural control of swallowing and breathing and in blood pressure regulation. In the present study, postnatal changes in global [3H]Glu binding and in [3H]Glu binding to N-methyl-D-aspartate (NMDA) receptors were analyzed in the nucleus tractus solitarii and the ventrolateral medulla using in vitro receptor autoradiography. Similar results were obtained in both regions. When expressed as density values (fmol/mg tissue), both global and NMDA-sensitive Glu binding increased by approximately 50-70% between birth and postnatal day 9 (P9) and then decreased until P30. When expressed as binding per nucleus (i.e., after correction for tissue growth), global Glu binding still increased between birth and P9 and decreased between P9 and P30 whereas NMDA-sensitive binding increased until P9 and remained stable thereafter. Saturation studies showed a postnatal increase in Glu receptor number per nucleus, which occurred mainly between birth and P9, and a decrease in Glu receptor affinity between P9 and adulthood. These results indicate that dramatic changes in glutamatergic neurotransmission occur in the nucleus tractus solitarii and the ventrolateral medulla during the first month of postnatal life. They suggest that both neonates and young animals may not be fully mature as regard to central regulation of autonomic functions.

Introduction of the glutamate receptor subunit 1 into motor neurons in vitro and in vivo using a recombinant herpes simplex virus

We developed and characterized a recombinant herpes simplex virus vector and used it to introduce the complementary DNA encoding glutamate receptor subunit 1 flip into postmitotic motor neurons. Infection of purified motor neurons in vitro with this vector resulted in selective, high-level expression of glutamate receptor subunit 1 immunoreactivity in nearly 100% of the neurons. Patch-clamp experiments demonstrated that the protein product of the glutamate receptor subunit 1 flip transgene assembles into functional alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor channels. Herpes simplex virus-glutamate receptor subunit 1 flip was introduced into spinal cord cells by direct injection into the ventral horn and selectively into motor neurons by sciatic nerve injection. High levels of expression were sustained for at least one week and were accompanied by changes in the ionic permeability of AMPA receptors in transgene-expressing neurons. Throughout the first week of infection, there was little evidence for toxicity. Herpes simplex virus provides a versatile tool for manipulating the glutamate receptor phenotype of postmitotic neurons and will permit study of the role of individual glutamate receptor subunits in neuronal physiology and pathophysiology.

Receptor localization in the mammalian dorsal horn and primary afferent neurons

The dorsal horn of the spinal cord is a primary receiving area for somatosensory input and contains high concentrations of a large variety of receptors. These receptors tend to congregate in lamina II, which is a major receiving center for fine, presumably nociceptive, somatosensory input. There are rapid reorganizations of many of these receptors in response to various stimuli or pathological situations. These receptor localizations in the normal and their changes after various pertubations modify present concepts about the wiring diagram of the nervous system. Accordingly, the present work reviews the receptor localizations and relates them to classic organizational patterns in the mammalian dorsal horn.

Intrinsic NMDA-induced oscillations in motoneurons of an adult vertebrate spinal cord are masked by inhibition

Low-frequency membrane potential oscillations were induced in motoneurons (MNs) of isolated hemisected frog spinal cords during N-methyl-D-aspartate (NMDA) application. Oscillations required the presence of physiological Mg2+ and preincubation with strychnine, whereas incubation with bicuculline or phaclofen was not effective. Oscillations were evident in intracellular recordings from single MNs and simultaneous extracellular recordings from lumbar ventral roots. In Mg(2+)-free solution, MNs exhibited irregular transient membrane potential depolarizations that were blocked by D,L-2-amino-5-phosphonopentanoic acid (APV) but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Generation and maintenance of membrane potential oscillations required specific NMDA receptor activation. Oscillations were antagonized by APV but not by CNQX. Strychnine preincubation was required for NMDA to induce oscillations, but was not critical in maintaining them, because oscillations persisted after removal of strychnine. Therefore oscillations are suggested to be an inherent property of the spinal neuronal circuitry. Tetrodotoxin (TTX) blocked spike activity and had a bimodal effect on membrane potential oscillations. Oscillations initially were blocked by TTX, but reappeared spontaneously after 10-40 min. This suggests that maintenance of oscillations, once evoked, does not involve MN firing. Na+ entry through TTX-insensitive Na+ channels and/or NMDA receptor channels, trans-membrane Ca2+ flux, Ca2+ release from intracellular stores, and Ca2+ activated K+ channels were critical in controlling the amplitude and frequency of membrane potential oscillations. It is hypothesized that these unmasked intrinsic oscillations in adult frog spinal cord MNs may represent a premetamorphic spinal oscillator involved in tadpole swimming that becomes suppressed during metamorphosis as strychnine-sensitive inhibition becomes more pronounced.

C-fos can be induced in the neonatal rat spinal cord by both noxious and innocuous peripheral stimulation

The development of spinal cord nociceptive pathways has been investigated in neonatal rat pups using the expression of Fos immunoreactivity in laminae I and II cells produced by high and low intensity skin stimulation. Noxious pinch of the hindpaw evoked a clear response in the newborn rat pup, which was not significantly different from that seen at postnatal day (P) 21. Low intensity touch stimulation also produced a significant fos response in laminae I and II cells at P3 which was 60% that of the pinch response. This was reduced to 27% of the pinch response by P10 and was gone by P21. Electrical stimulation through percutaneous electrodes showed that A beta fibre stimulation also produced a fos response at P3 that was not significantly different from that produced by C fibre stimulation. By P21 and P30 the response to C fibre stimulation was much greater and the response to A fibre stimulation was not significantly above background. The results suggest that in the neonatal spinal cord, low threshold A fibres are able to activate pathways in lamina I and II of the dorsal horn that in the adult are predominantly nociceptive.

Differential regional distribution of AMPA receptor subunit messenger RNAs in the human spinal cord as visualized by in situ hybridization

The electrophysiological characteristics of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors vary with their subunit composition. The establishment of the subunit distribution is an essential step in the understanding of the function of these receptors. In the spinal cord, AMPA receptors are involved in normal and, possibly, pathological processes. Using in situ hybridization histochemistry with radiolabelled oligonucleotides as probes, we have studied the distribution of AMPA receptor subunit messenger RNAs (spliced flip and flop variants of glutamate receptor subunits A-D) in the human post mortem spinal cord. Transcripts for flip variants were preferentially expressed in the superficial dorsal horn, with a dorsoventral decreasing gradient of the signals. Transcripts for flop variants were also abundantly present in all layers of the gray matter, with the highest signal being observed for glutamate receptor subunit Bflop. Accordingly, flop forms were predominant in areas other than the superficial dorsal horn. This differential distribution of transcripts in the dorsal horn suggests that the subunit composition of AMPA receptors varies with the afferent inputs; AMPA receptors on neurons in the superficial dorsal horn, where terminals of thin primary afferents conducting noxious information are located, contain more flip forms, whereas neurons in the deep dorsal horn, where thick primary afferents mediating innocuous stimuli terminate, have AMPA receptors which are mainly composed of flop forms of glutamate receptor subunits A and B. The relatively high abundance of glutamate receptor subunit B transcripts in the superficial laminae of the dorsal horn indicates that AMPA receptors in these laminae have lower Ca2+ permeability. In addition, the relative abundance of glutamate receptor subunits Bflip and Dflop may show that AMPA receptors in the superficial dorsal horn have slow desensitization, while those of motor neurons have rapid desensitization.