Signal transduction mechanisms of K+-Cl- cotransport regulation and relationship to disease.

The K+-Cl- cotransport (COT) regulatory pathways recently uncovered in our laboratory and their implication in disease state are reviewed. Three mechanisms of K+-Cl- COT regulation can be identified in vascular cells: (1) the Li+-sensitive pathway, (2) the platelet-derived growth factor (PDGF)-sensitive pathway and (3) the nitric oxide (NO)-dependent pathway. Ion fluxes, Western blotting, semi-quantitative RT-PCR, immunofluorescence and confocal microscopy were used. Li+, used in the treatment of manic depression, stimulates volume-sensitive K+-Cl- COT of low K+ sheep red blood cells at cellular concentrations <1 mM and inhibits at >3 mM, causes cell swelling, and appears to regulate K+-Cl- COT through a protein kinase C-dependent pathway. PDGF, a potent serum mitogen for vascular smooth muscle cells (VSMCs), regulates membrane transport and is involved in atherosclerosis. PDGF stimulates VSM K+-Cl- COT in a time- and concentration-dependent manner, both acutely and chronically, through the PDGF receptor. The acute effect occurs at the post-translational level whereas the chronic effect may involve regulation through gene expression. Regulation by PDGF involves the signalling molecules phosphoinositides 3-kinase and protein phosphatase-1. Finally, the NO/cGMP/protein kinase G pathway, involved in vasodilation and hence cardiovascular disease, regulates K+-Cl- COT in VSMCs at the mRNA expression and transport levels. A complex and diverse array of mechanisms and effectors regulate K+-Cl- COT and thus cell volume homeostasis, setting the stage for abnormalities at the genetic and/or regulatory level thus effecting or being affected by various pathological conditions.

Focal aggregation of voltage-gated, Kv2.1 subunit-containing, potassium channels at synaptic sites in rat spinal motoneurones.

Delayed rectifier K+ currents are involved in the control of alpha-motoneurone excitability, but the precise spatial distribution and organization of the membrane ion channels that contribute to these currents have not been defined. Voltage-activated Kv2.1 channels have properties commensurate with a contribution to delayed rectifier currents and are expressed in neurones throughout the mammalian central nervous system. A specific antibody against Kv2.1 channel subunits was used to determine the surface distribution and clustering of Kv2.1 subunit-containing channels in the cell membrane of alpha-motoneurones and other spinal cord neurones. In alpha-motoneurones, Kv2.1 immunoreactivity (-IR) was abundant in the surface membrane of the soma and large proximal dendrites, and was present also in smaller diameter distal dendrites. Plasma membrane-associated Kv2.1-IR in alpha-motoneurones was distributed in a mosaic of small irregularly shaped, and large disc-like, clusters. However, only small to medium clusters of Kv2.1-IR were observed in spinal interneurones and projection neurones, and some interneurones, including Renshaw cells, lacked demonstrable Kv2.1-IR. In alpha-motoneurones, dual immunostaining procedures revealed that the prominent disc-like domains of Kv2.1-IR are invariably apposed to presynaptic cholinergic C-terminals. Further, Kv2.1-IR colocalizes with immunoreactivity against postsynaptic muscarinic (m2) receptors at these locations. Ultrastructural examination confirmed the postsynaptic localization of Kv2.1-IR at C-terminal synapses, and revealed clusters of Kv2.1-IR at a majority of S-type, presumed excitatory, synapses. Kv2.1-IR in alpha-motoneurones is not directly associated with presumed inhibitory (F-type) synapses, nor is it present in presynaptic structures apposed to the motoneurone. Occasionally, small patches of extrasynaptic Kv2.1-IR labelling were observed in surface membrane apposed by glial processes. Voltage-gated potassium channels responsible for the delayed rectifier current, including Kv2.1, are usually assigned roles in the repolarization of the action potential. However, the strategic localization of Kv2.1 subunit-containing channels at specific postsynaptic sites suggests that this family of voltage-activated K+ channels may have additional roles and/or regulatory components.

Comparison of the morphological and electrotonic properties of Renshaw cells, Ia inhibitory interneurons, and motoneurons in the cat.

The morphological and electrotonic properties of 4 motoneurons, 8 Ia inhibitory interneurons, and 4 Renshaw cells were compared. The morphological analysis, based on 3-D reconstructions of the cells, revealed that dendrites of motoneurons are longer and more extensively branched. Renshaw cells have dendrites that are shorter and simpler in structure. Dendrites of Ia inhibitory interneurons could be as long as those of motoneurons but the branching structure resembled that of Renshaw cells. Compartmental models were used to determine the electrotonic properties of the paths from each dendritic terminal to the soma. The attenuations of steady-state voltage changes in motoneurons were 3 and 7 times larger than in Ia inhibitory interneurons and Renshaw cells, respectively. The same relative order was observed for current attenuation and electrotonic length. The dendritic input resistances in Renshaw cells were 2 and 4 times larger than in Ia inhibitory interneurons and motoneurons, respectively. The difference in these electrotonic properties increased during higher synaptic activity as modeled by a decrease of Rm. The peak amplitudes of voltage transients at sites of brief, synaptic-like changes in conductance were highly dependent on cell class and were largest in Renshaw cells and smallest in motoneurons. In combination with class-specific differences in the attenuation of transient voltage signals, this led to large differences in the peak amplitudes of somatic voltage transients. Differences in the rise times and half-widths of the voltage transients were observed as well. Thus, based on passive properties, each cell class has a unique set of input/output properties.

The Cav2.1/alpha1A (P/Q-type) voltage-dependent calcium channel mediates inhibitory neurotransmission onto mouse cerebellar Purkinje cells.

The effects of voltage-dependent calcium channel (VDCC) antagonists on spontaneous inhibitory postsynaptic currents (sIPSCs) in mouse Purkinje cells were examined using in vitro cerebellar slices. The inorganic ion Cd2+ reduced sIPSC amplitude and frequency. No additional block was seen with the Na+ channel antagonist tetrodotoxin (TTX) suggesting that all action potential-evoked inhibitory GABA release was mediated by high-voltage-activated VDCCs. No evidence was found for involvement of Cav1/alpha1C and alpha1D (L-type), Cav2.2/alpha1B (N-type) or Cav2.3/alpha1E (R-type) high-voltage-activated VDCCs or low-voltage-activated Cav3/alpha1G, alpha1H and alpha1I (T-type) VDCCs in mediating presynaptic GABA release. Blockade of sIPSCs by 200 nM omega-agatoxin IVA implicated the Cav2.1/alpha1A (P/Q-type) subtype of high-voltage-activated VDCCs in mediating inhibitory transmission. Inhibition by omega-agatoxin IVA was similar to that seen with Cd2+ and TTX. Selective antibodies directed against the Cav2.1 subunit revealed staining in regions closely opposed to Purkinje cell somata. Cav2.1 staining was colocalized with staining for antibodies against glutamic acid decarboxylase and corresponded well with the pericellular network formed by GABAergic basket cell interneurons. Antibody labelling of Cav2.3 revealed a region-specific expression. In the cerebellar cortex anterior lobe, Cav2.3 staining was predominantly somatodendritic; whilst in the posterior lobe, perisomatic staining was seen primarily. However, electrophysiological data was not consistent with a role for the Cav2.3 subunit in mediating presynaptic GABA release. No consistent staining was seen for other Cav (alpha1) subunits. Electrophysiological and immunostaining data support a predominant role for Cav2.1 subunits in mediating action potential-evoked inhibitory GABA release onto mouse Purkinje cells.

K-Cl cotransport: immunohistochemical and ion flux studies in human embryonic kidney (HEK293) cells transfected with full-length and C-terminal-domain-truncated KCC1 cDNAs.

Coupled K and Cl movements are mediated by several isoforms of the K-Cl cotransporter (COT) encoded by the KCC genes. The ubiquitous KCC1 isoform, important for cell volume and ion homeostasis, has 12 transmembrane domains (Tmds), and cytoplasmic N- and C-terminal domains (Ntd and Ctd). This study investigates the cellular localization of KCC1 by confocal microscopy, activation of K-Cl COT by various non-osmotic and osmotic interventions with net unidirectional K and Rb fluxes at 37( degrees )C, and the effect of Ctd deletion on K-Cl COT regulation. Human embryonic kidney (HEK293) cells were transfected with full-length (fl) rabbit (rb)KCC1 and - CtdKCC1 cDNAs obtained after truncation at nucleotide 2011. Normal cells exposed to polyclonal anti-Ctd antibodies against Ctd epitopes within a 77 amino acid sequence (a.a.943-1020) revealed granular membrane and cytoplasmic immunostaining, presumably endogenous KCC1. Additional diffuse membrane and cytoplasmic immunofluorescence in flKCC1-transfected cells was absent in -CtdKCC1-transfected cells. Monoclonal antibodies against a c-myc epitope at the protein Ntd showed both membrane and cytosolic fluorescence. Basal and N-ethylmaleimide (NEM)-stimulated Rb influxes through K-Cl COT, calculated as Cl-dependent Rb fluxes, were 2-3-fold higher in flKCC1-transfected than in normal cells. NEM stimulation of K-Cl COT was highest in flKCC1-transfected cells, significantly lower in stably and abrogated in transiently -CtdKCC1-transfected cells. Furosemide, calyculin and genistein inhibited basal and NEM-stimulated K-Cl COT in normal and transfected cells. Staurosporine and hydroxylamine were ineffective stimulators. No effect of pH(0) changes (6.3-8.4) was observed in basal or NEM-stimulated K-Cl COT, in both normal and transfected cells. However, inhibition by NEM occurred at pH(0) 8.4. Furthermore, in a Cl-independent manner, NEM lowered cell K content by >30% and hypotonicity (210-70mOsM) stimulated furosemide-sensitive Rb influx and K loss. Thus, in cultured normal and KCC1-transfected cells, K-Cl COT shows significant differences from erythrocytes, and NEM and cell swelling open furosemide-sensitive and Cl-independent K/Rb channels. Failure of K-Cl COT in cells transfected with Ctd-truncated KCC1 to respond to NEM suggests a role of the Ctd for signal transduction.

K-Cl co-transport: immunocytochemical and functional evidence for more than one KCC isoform in high K and low K sheep erythrocytes.

K-Cl co-transport (COT) is significantly higher in low K (LK), L-antigen (L) positive, than in high K (HK), M-antigen (M) positive, sheep red blood cells (SRBCs) and is inhibited by sheep allo-anti-L1 antibody. To answer the question of whether this difference in K-Cl co-transport activity resides at the level of the transporter or its regulation, a combined immunocytochemical and functional approach was taken. At least four isoforms of K-Cl COT encoded by different KCC genes are known, with 12 transmembrane domains and cytoplasmic C- and N-terminal domains (Ctd and Ntd, respectively). Polyclonal anti-rat (rt)KCC1 antibodies against a fusion peptide with 77 amino acids from the Ctd of rtKCC1 and anti-human (h)KCC3 against an 18-aa peptide from the Ntd of hKCC3, were prepared in rabbits (rb). Two distinctly separate protein bands of 180 and 145 kDa molecular mass were detected in hemoglobin-free ghosts from RBCs of two LK (one homozygous LL and one heterozygous LM) and one HK (homozygous MM) sheep by Western blots with rb anti-rtKCC1 and rb anti-hKCC3. Confocal microscopy showed specific immunostaining of KCC1 with rb anti-rtKCC1, and of KCC3 with rb anti-hKCC3, in white ghosts from both LK and HK SRBCs. To test the functional heterogeneity of K-Cl COT, the effect of the anti-L1 antibody was assessed on K-Cl COT activated by the kinase inhibitor staurosporine. Incubation of LK SRBCs with anti-L1 serum inhibited by 30% staurosporine-stimulated K-Cl COT suggesting that approximately two-thirds of the transport activity is independent of the L1 antigen. That staurosporine altered the L1 antigen/antibody reaction is unlikely since the action of another antibody, anti-Lp, stimulating the Na/K pump flux, was not modified. The present results, in conjunction with earlier work, lead to the hypothesis that the partial anti-L1 inhibition of K-Cl COT may be related to the molecular KCC dimorphism, seen in these cells with anti-KCC1 and anti-KCC3 antibodies.

Nociceptors for the 21st century.

This review summarizes recent developments in the context of the neurochemical classification of nociceptors and explores the relationships between functionally and neurochemically defined subgroups. Although the complete picture is not yet available, several lines of intriguing evidence suggest that despite the complexity and diversity of nociceptor properties, a relatively "simple" neurochemical classification fits well with several recently identified molecular characteristics.

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.

A functional role for the two-pore domain potassium channel TASK-1 in cerebellar granule neurons.

Cerebellar granule neurons (CGNs) are one of the most populous cells in the mammalian brain. They express an outwardly rectifying potassium current, termed a "standing-outward" K(+) current, or IK(SO), which does not inactivate. It is active at the resting potential of CGNs, and blocking IK(SO) leads to cell depolarization. IK(SO) is blocked by Ba(2+) ions and is regulated by activation of muscarinic M(3) receptors, but it is insensitive to the classical broad-spectrum potassium channel blocking drugs 4-aminopyridine and tetraethylammonium ions. The molecular nature of this important current has yet to be established, but in this study, we provide strong evidence to suggest that IK(SO) is the functional correlate of the recently identified two-pore domain potassium channel TASK-1. We show that IK(SO) has no threshold for activation by voltage and that it is blocked by small extracellular acidifications. Both of these are properties that are diagnostic of TASK-1 channels. In addition, we show that TASK-1 currents expressed in Xenopus oocytes are inhibited after activation of endogenous M(3) muscarinic receptors. Finally, we demonstrate that mRNA for TASK-1 is found in CGNs and that TASK-1 protein is expressed in CGN membranes. This description of a functional two-pore domain potassium channel in the mammalian central nervous system indicates its physiological importance in controlling cell excitability and how agents that modify its activity, such as agonists at G protein-coupled receptors and hydrogen ions, can profoundly alter both the neuron's resting potential and its excitability.

Light microscopic observations on the relationships between 5-hydroxytryptamine-immunoreactive axons and dorsal spinocerebellar tract cells in Clarke's column in the cat.

Serotonin (5-HT) exerts a variety of effects on the excitability of motoneurons, interneurons, and ascending tract cells. Spinocerebellar-tract cells in the dorsal horn receive synaptic connections from serotoninergic axons, but little is known about the relationships between serotoninergic axons and dorsal spinocerebellar tract (DSCT) cells in Clarke's column. We studied these relationships by using a combination of immunohistochemical localization of 5-HT-immunoreactive boutons and intracellular staining with horseradish peroxidase (HRP) or neurobiotin of identified DSCT cells in vivo. In the adult cat, Clarke's column displayed a lower density of 5-HT-immunoreactive axons and boutons than adjacent regions of the spinal gray matter. Eleven intracellularly stained DSCT cells were analyzed with light microscopy, and six of these cells were entirely reconstructed in three dimensions. A total of 3,739 close appositions (340+/-101 per postsynaptic neuron: mean +/- SD) were observed on the labeled DSCT cells. The majority (97%) of the appositions were formed on dendrites, including proximal and distal branches, with an average density of about 1.4 appositions per 1,000 microm2 of dendritic membrane. These results indicate that the bulbo-spinal serotoninergic system(s) provide direct innervation of Clarke's column-DSCT cells in the upper lumbar spinal cord and that the inputs are spread widely over all regions of the target neurons' soma-dendritic membrane.

Distribution of 5-hydroxytryptamine-immunoreactive boutons on immunohistochemically-identified Renshaw cells in cat and rat lumbar spinal cord.

A combination of anti-gephyrin- and anti-calbindin D28k-immunoreactivity was used to identify 129 and 171 Renshaw cells and their dendrites in cat and rat lumbar spinal cord, respectively. Using anti-5-hydroxytryptamine-immunoreactivity to label serotoninergic fibers and boutons, 1048 serotoninergic boutons were observed in close contact with the immunolabeled Renshaw cells, with an average of 4.4 and 2.8 close contacts on cat and rat Renshaw cells, respectively. Two-thirds of the observed appositions were formed on the somatic membrane.

Distribution of cholinergic contacts on Renshaw cells in the rat spinal cord: a light microscopic study.

1. Cholinergic terminals in the rat spinal cord were revealed by immunohistochemical detection of the vesicular acetycholine transporter (VAChT). In order to determine the relationships of these terminals to Renshaw cells, we used dual immunolabelling with antibodies against gephyrin or calbindin D28k to provide immunohistochemical identification of Renshaw cells in lamina VII of the ventral horn. 2. A total of 50 Renshaw cells were analysed quantitatively using a computer-aided reconstruction system to provide accurate localization of contact sites and determination of somatic and dendritic surface area. Dendrites could be traced for up to 413 microm from the soma in calbindin D28k-identified Renshaw cells and up to 184 microm in gephyrin-identified cells. 3. A total of 3330 cholinergic terminals were observed on 50 Renshaw cells, with a range of 21-138 terminal appositions per cell (mean 66.6 +/- 25.56 contacts per cell). The vast majority (83.5 %) of the terminals were apposed to dendrites rather than the soma. The overall density of cholinergic contacts increased from a little above 1 per 100 microm2 on the soma and initial 25 microm of proximal dendrites to 4-5 per 100 microm2 on the surface of dendritic segments located 50-250 microm from the soma. Single presynaptic fibres frequently formed multiple contacts with the soma and/or dendrites of individual Renshaw cells. 4. VAChT-immunoreactive terminals apposed to Renshaw cells varied in size from 0.6 to 6.9 microm in diameter (mean 2.26 +/- 0.94; n = 986) and were on average smaller than the cholinergic C-terminals apposed to motoneurones, but larger than VAChT-immunoreactive terminals contacting other ventral horn interneurones. 5. The high density and relatively large size of many cholinergic terminals on Renshaw cells presumably correlates with the strong synaptic connection between motoneurones and Renshaw cells. The fact that the majority of contacts are distributed over the dendrites makes the motoneurone axon collateral input susceptible to inhibition by the prominent glycinergic inhibitory synapses located on the soma and proximal dendrites. The relative positions and structural features of the excitatory cholinergic and inhibitory glycinergic synapses may explain why Renshaw cells, although capable of firing at very high frequency following motor axon stimulation, appear to fire at relatively low rates during locomotor activity.

Calbindin D28k expression in immunohistochemically identified Renshaw cells.

Double immunofluorescence was utilized to determine whether Renshaw cells contain calbindin D28k immunoreactivity. Renshaw cells were identified by their characteristic expression patterns of gephyrin immunoreactivity in sections of rat and cat lumbar spinal cord. In the rat, all neurons classified as Renshaw cells (n = 487) also contained calbindin D28k-immunoreactivity, and all calbindin D28k-immunoreactive cells located in the ventral-most region of lamina VII expressed the characteristic gephyrin labeling and morphology of Renshaw cells. In the cat, fewer than half of the Renshaw cells (47%; n = 128) were double-labeled. In both species, occasional calbindin D28k-immunoreactive Renshaw cells were identified within motor nuclei in lamina IX. The distinctive immunolabeling of Renshaw cells allowed us to estimate that there are about 250 Renshaw cells in each ventral horn of the fourth lumbar segment of rat spinal cord, and about 750 cells per ventral horn in the L6 segment of the cat. We conclude that the functional properties of Renshaw cells, including their ability to fire action potentials at high rates, likely require specific homeostatic mechanisms including strong intracellular calcium buffering, the precise mechanisms of which may vary between species.

Diversity of structure and function at mammalian central synapses.

Our appreciation of the relationship between synaptic structure and function, and in particular our understanding of quantal synaptic transmission, is derived from classical studies on the neuromuscular junction. However, physiological studies of quantal transmission at mammalian CNS synapses have produced a variety of results, and thus no consensus of opinion has emerged. This variability could be due, in part, to experimental and analytical limitations or to differences in the structural and functional features of central synapses, or both. Some of the experimental limitations have recently been overcome by the use of novel preparations that permit direct measurement of quantal synaptic events in the CNS. Although these studies reveal similarities between the synaptic mechanisms of the neuromuscular junction and CNS synapses, important differences and specializations are also evident. The purpose of this review is to highlight the structural and functional diversity of synapses in the mammalian CNS, and to discuss the potential relevance of structural features to synaptic function.

Distribution of 5-hydroxytryptamine-immunoreactive boutons on alpha-motoneurons in the lumbar spinal cord of adult cats.

Recent studies have shown that at least some of the functional effects of serotonin (5-HT) on motoneuron excitability are direct and are mediated via postsynaptic 5-HT receptors on motoneurons. To determine the spatial distribution of direct inputs from the serotonin system on the proximal and distal dendrites of individual motoneurons, we examined identified motoneurons in vivo with a combination of immunohistochemical localization of 5-HT-immunoreactive boutons and intracellular staining with horseradish peroxidase. Seventeen intracellularly stained motoneurons from 12 adult cats were analyzed with light microscopy. Quantitative analysis of 5-HT boutons apposed to dendrites of five representative motoneurons that were entirely reconstructed in three dimensions (each from the lumbosacral spinal cord of a different animal) revealed a total of 7,848 contacts (1,570+/-487 contacts/postsynaptic neuron; mean +/- SD) over the dendrites of these cells. Analysis of contacts on the soma of two of these cells, and on the somas of an additional 12 intracellularly stained motoneurons, revealed a wide range of somatic contacts (11-211 contacts/cell) on motoneuron cell bodies, with an average of 52 contacts/cell. These results indicate that the vast majority of 5-HT-immunoreactive boutons are apposed to dendritic branches rather than to the somatic surface of motoneurons. The spatial distribution of contacts essentially matched the distribution of surface membrane area of the postsynaptic neuron, resulting in a relatively uniform density of contacts (<1/100 microm2) on proximal and distal dendrites. Consequently, the frequency of contacts was higher on the proximal dendritic compartments where available membrane area is greater. There was no preferential distribution of contacts to particular dendrites. Light/electron microscopic correlations were performed on 21 boutons that contacted dendrites (n = 7) of three motoneurons from different animals. At the electron microscope level, most appositions (18/21; 85.7%) selected by our light microscopic criteria were confirmed as direct contacts when the 5-HT boutons were examined through serial sections. Synaptic junctions, generally small and symmetric, were positively identified in only a subset of these cases (n = 6; 28.6%), in part due to the obscuring effects of the peroxidase histochemical precipitate present in both pre- and postsynaptic profiles. A few 5-HT boutons (3/21; 14.3%) selected as contacts by our light microscopic criteria were in fact separated from the adjacent labeled dendrites; in two of these three cases, the separation was due to intrusion of very thin glial lamellae (<0.3 microm in cross section). These results indicate that the bulbospinal serotonergic system(s) provide a significant, direct synaptic input to spinal motoneurons that innervate hindlimb muscles. The nature of the modulatory actions exerted by such widespread synaptic inputs will affect all regions of the somatodendritic membrane and will ultimately depend on the nature of the 5-HT receptors present over different parts of the postsynaptic neuron's dendritic tree.

Effect of sciatic nerve transection or TTX application on enzyme activity in rat spinal cord.

To clarify the differential effects on spinal circuitry caused by physical vs functional disconnection from the periphery, we compared changes produced by 3-, 7- or 14-day unilateral sciatic axotomy or tetrodotoxin (TTX) nerve blockade on the abundance or activity of NADPH diaphorase (NDP), cytochrome oxidase (CO) and acid phosphatase (AP) in the spinal cord. Following axotomy, AP and NDP were decreased in the dorsal horn and increased in large cells in the dorsolateral motor nuclei while CO was decreased in ventral horn neuropil. TTX induced a decrease of CO in the ventral horn and NDP in the dorsal horn. This suggests that physical vs functional disconnection causes modulation of distinct intracellular pathways in sensory afferents, dorsal horn neurons and motoneurons.

Downregulation of metabotropic glutamate receptor 1a in motoneurons after axotomy.

Axotomized motoneurons display drastic modifications in synaptic structure and function related to their disconnection from the periphery and establishment of a regenerative metabolic functional mode. The molecular basis of these modifications is not fully understood. Here we describe changes in metabotropic glutamate receptor 1a (mGluR1a)-immunoreactivity 3, 7 or 14 days after unilateral aciatic transection. mGluR1a-immunoreactivity was distributed throughout the somatic cytoplasm and somatodendritic membrane of uninjured motoneurons and was significantly reduced in axotomized motoneurons. This reduction was observed at 3 days and grew progressively over 2 weeks. These findings suggest that downregulation of mGluR1a could contribute to reduced excitatory neurotransmission in axotomized motoneurons.

Cell-type specific organization of glycine receptor clusters in the mammalian spinal cord.

Glycinergic synapses play a major role in shaping the activity of spinal cord neurons. The spatial organization of postsynaptic receptors is likely to determine many functional parameters at these synapses and is probably related to the integrative capabilities of different neurons. In the present study, we have investigated the organization of gephyrin expression along the dendritic membranes of alpha- and gamma-motoneurons, Ia inhibitory interneurons, and Renshaw cells. Gephyrin is a protein responsible for the postsynaptic clustering of glycine receptors, and the features of gephyrin and glycine receptor alpha(1)-subunit immunofluorescent clusters displayed similar characteristics on ventral horn spinal neurons. However, the density of clusters and their topographical organization and architecture varied widely in different neurons and in different dendritic regions. For motoneurons and Ia inhibitory interneurons, cluster size and complexity increased with distance from the soma, perhaps as a mechanism to enhance the influence of distal synapses. Renshaw cells were special in that they displayed an abundant complement of large and morphologically complex clusters concentrated in their somas and proximal dendrites. Serial electron microscopy confirmed that the various immunoreactivity patterns observed with immunofluorescence accurately parallel the variable organization of pre- and postsynaptic active zones of glycinergic synapses. Finally, synaptic boutons from single-labeled axons of glycinergic neurons (Ia inhibitory interneurons) were also associated with postsynaptic receptor clusters of variable shapes and configurations. Our results indicate that mechanisms regulating receptor clustering do so primarily in the context of the postsynaptic neuron identity and localization in the dendritic arbor.

Distribution of immunoreactivity for the beta 2 and beta 3 subunits of the GABAA receptor in the mammalian spinal cord.

The localization of GABAA receptors in cat and rat spinal cord was analyzed using two monoclonal antibodies specific for an epitope shared by the beta 2 and beta 3 subunits of the receptor. beta 2/beta 3-subunit immunoreactivity was the most intense in inner lamina II, lamina III, and lamina X, and it was the least intense in lamina IX. In laminae I-III, generally, the staining had a rather diffuse appearance, but the surfaces of small cell bodies in these laminae were outlined clearly by discrete labeling, as were many cell bodies and dendrites in deeper laminae. Rhizotomy experiments and ultrastructural observations indicated that beta 2/beta 3-subunit immunoreactivity in the dorsal horn was largely localized in intrinsic neuropil elements rather than in the terminals of primary afferent fibers, even though labeling overlapped with the terminal fields of different types of primary afferents and was also detected on the membranes of dorsal root ganglion neurons. With few exceptions (most notably, a highly immunoreactive group of dorsolaterally located cells in the cat lumbar ventral horn), motoneurons expressed low levels of beta 2/beta 3-subunit immunoreactivity. Labeling of neuronal membranes was fairly continuous, but focal accumulations of beta 2/beta 3-subunit immunoreactivity were also detected using immunofluorescence. Focal "hot spots" correlated ultrastructurally with the presence of synaptic junctions. Dual-color immunofluorescence revealed that focal accumulations of beta 2/beta 3-subunit immunoreactivity were frequently apposed by glutamic acid decarboxylase (GAD)-immunoreactive terminals. However, the density of continuous-membrane beta 2/beta 3 immunolabeling and GAD terminal density were not correlated in many individual neurons. The results suggest the existence of "classical" (synaptic) and "nonclassical" (paracrine) actions mediated via spinal cord GABAA receptors. The study also revealed the relative paucity of beta 2/beta 3-subunit immunoreactivity postsynaptic to certain GABAergic terminals, particularly those presynaptic to motoneurons or primary afferent terminals.

Electrotonic architecture of cat gamma motoneurons.

1. Experimental measures of input resistance, RN, and responses to brief hyperpolarizing current pulses were obtained in identified gamma-motoneurons in pentobarbital-anesthetized cats using conventional sharp micropipettes. The same cells were subsequently injected with horseradish peroxidase and completely reconstructed. In two cells, the electrophysiological and morphological data were of sufficient quality to permit estimation of specific membrane resistance, Rm, using biologically plausible ranges of specific cytoplasmic resistance, Ri, and membrane capacitance, Cm. 2. A combination of steady-state and dynamic computer models were employed to reconcile cell morphology with RN and the trajectories of the voltage decay following brief current pulses delivered to the soma. Simulated transient responses matched the tails of the observed transient when generated with the same current injections used experimentally. With Cm < or = 1.0 microF cm-2, the most satisfactory fits were obtained when the values of Rm assigned to the soma, Rms, were much smaller than the spatially uniform value assigned to the dendrites, Rmd and Ri = 60-70 omega cm. With Cm = 1.0 microF cm-2, Rms ranged from 260 to 427 omega cm2, whereas Rmd was approximately 33 K omega cm2. With Cm = 0.8 microF cm-2, Rms ranged from 235 to 357 omega cm2 and Rmd was between 62 and 68 K omega cm2. When Rm was constrained to be spatially uniform (i.e., Rm = Rms), implausibly high values of Cm (2.5-5.0 microF cm-2; Ri = 70 omega cm) were required to match the observed tail time constant, tau o,peel, but the simulated transients did not otherwise match those obtained experimentally. 3. With best fit values of Rms and Rmd, both gamma-motoneurons were electronically relatively compact (80% of total membrane area within 0.85 length constants from the soma). However, the calculated average steady-state inward attenuation factor (AFin) for voltages generated at any point within the dendrites increased rapidly with distance from the soma, reaching levels of < or = 90 and < or = 45 for the proximal 80% of membrane area for the respective motoneurons in the presence of a somatic shunt (Rms << Rmd). If we assume that the somatic shunt is an artifact of sharp micropipette penetration (i.e., that Rms = Rmd for uninjured cells), then AFin decreased to < or = 20 and < or = 15, respectively, for the proximal 80% of cell membrane.(ABSTRACT TRUNCATED AT 400 WORDS)