An ultrastructural study of the synaptic contacts of alpha 1-motoneuron axon collaterals. II. Contacts in lamina VII

Escape from homeostasis: spinal microcircuits and progression of amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), loss of motoneuron function leads to weakness and, ultimately, respiratory failure and death. Regardless of the initial pathogenic factors, motoneuron loss follows a specific pattern: the largest α-motoneurons die before smaller α-motoneurons, and γ-motoneurons are spared. In this article, we examine how homeostatic responses to this orderly progression could lead to local microcircuit dysfunction that in turn propagates motoneuron dysfunction and death. We first review motoneuron diversity and the principle of α-γ coactivation and then discuss two specific spinal motoneuron microcircuits: those involving proprioceptive afferents and those involving Renshaw cells. Next, we propose that the overall homeostatic response of the nervous system is aimed at maintaining force output. Thus motoneuron degeneration would lead to an increase in inputs to motoneurons, and, because of the pattern of neuronal degeneration, would result in an imbalance in local microcircuit activity that would overwhelm initial homeostatic responses. We suggest that this activity would ultimately lead to excitotoxicity of motoneurons, which would hasten the progression of disease. Finally, we propose that should this be the case, new therapies targeted toward microcircuit dysfunction could slow the course of ALS.

Segregation of glutamatergic and cholinergic transmission at the mixed motoneuron Renshaw cell synapse

In neonatal mice motoneurons excite Renshaw cells by releasing both acetylcholine (ACh) and glutamate. These two neurotransmitters activate two types of nicotinic receptors (nAChRs) (the homomeric α₇ receptors and the heteromeric α*ß* receptors) as well as the two types of glutamate receptors (GluRs) (AMPARs and NMDARs). Using paired recordings, we confirm that a single motoneuron can release both transmitters on a single post-synaptic Renshaw cell. We then show that co-transmission is preserved in adult animals. Kinetic analysis of miniature EPSCs revealed quantal release of mixed events associating AMPARs and NMDARs, as well as α₇ and α*ß* nAChRs, but no evidence was found for mEPSCs associating nAChRs with GluRs. Bayesian Quantal Analysis (BQA) of evoked EPSCs showed that the number of functional contacts on a single Renshaw cell is more than halved when the nicotinic receptors are blocked, confirming that the two neurotransmitters systems are segregated. Our observations can be explained if ACh and glutamate are released from common vesicles onto spatially segregated post-synaptic receptors clusters, but a pre-synaptic segregation of cholinergic and glutamatergic release sites is also possible.

NADPH-diaphorase reactivity and Fos-immunoreactivity within the ventral horn of the lumbar spinal cord of cats submitted to acute muscle inflammation induced by injection of carrageenan

The NADPH-diaphorase activity and Fos-immunoreactivity within the ventral horn of the lumbar spinal cord were studied in cats with acute unilateral myositis following injection of carrageenan into the m.m. gastrocnemius-soleus. In carrageenan-injected cats maximum in the mean number of intensely stained NADPH-diaphorase reactive (NADPH-dr) neurons was found in lamina VII (+100%) and VIII (+33%) of the contralateral ventral horn of the L6/L7 segments as compared with control animals. The maximumal level of Fos-immunoreactivity was registered in the same laminae with ipsilateral predominance (39.3±4.6 and 7.6±0.9 cells), in comparison with the contralateral side (13.6±0.8 and 5.5±0.6 cells, respectively; P<0.05). We also visualized low-intensely stained and double labelled (Fos immunoreactive+low-intensely stained NADPH-dr) multipolar and fusiform Renshaw-like cells (RLCs) within the ventral horn on both sides of the L6/L7 segments in carrageenan-injected cats. We visualized the double labelled (Fos-ir+NADPH-dr) multipolar and fusiform Renshaw-like cells (RLCs) within the ventral horn on both sides of the L6/L7 segments in carrageenan-injected cats. A significant difference in the mean number of RLCs was recorded between the ipsi- and contralateral sides in the lamina VII (13.6±2.5 vs. 4.9±0.7 cells, respectively). We suppose that activation of inhibitory RLCs in ipsilateral lamina VII could be directed on attenuation of activation of motoneurons during muscle pain development. Our study showed that a significant contralateral increase in the number of NADPH-dr cells is accompanied by an ipsilateral increase in c-Fos expression in lamina VII. These data may suggest that NADPH-dr neurons of the contralateral ventral horn through commissural connections also involved in the maintenance of the neuronal activity associated with acute muscle inflammation. It is also hypothesized, that during acute myositis, plastic changes in the ventral horn activate the processes of disinhibition due to an increase in the number of NADPH-d-reactive neurons in the spinal gray matter.

Postsynaptic muscarinic m2 receptors at cholinergic and glutamatergic synapses of mouse brainstem motoneurons

In many brain areas, few cholinergic synapses are identified. Acetylcholine is released into the extracellular space and acts through diffuse transmission. Motoneurons, however, are contacted by numerous cholinergic terminals, indicating synaptic cholinergic transmission on them. The muscarinic m2 receptor is the major acetylcholine receptor subtype of motoneurons; therefore, we analyzed the localization of the m2 receptor in correlation with synapses by electron microscopic immunohistochemistry in the mouse trigeminal, facial, and hypoglossal motor nuclei. In all nuclei, m2 receptors were localized at the membrane of motoneuronal perikarya and dendrites. The m2 receptors were concentrated at cholinergic synapses located on the perikarya and most proximal dendrites. However, m2 receptors at cholinergic synapses represented only a minority (<10%) of surface m2 receptors. The m2 receptors were also enriched at glutamatergic synapses in both motoneuronal perikarya and dendrites. A relatively large proportion (20-30%) of plasma membrane-associated m2 receptors were located at glutamatergic synapses. In conclusion, the effect of acetylcholine on motoneuron populations might be mediated through a synaptic as well as diffuse type of transmission.

Axonal projections of Renshaw cells in the thoracic spinal cord

Renshaw cells are widely distributed in all segments of the spinal cord, but detailed morphological studies of these cells and their axonal branching patterns have only been made for lumbosacral segments. For these, a characteristic distribution of terminals was reported, including extensive collateralization within 1-2 mm of the soma, but then more restricted collaterals given off at intervals from the funicular axon. Previous authors have suggested that the projections close to the soma serve inhibition of motoneurons (known to be greatest for the motor nuclei providing the Renshaw cell excitation) but that the distant projections serve mainly the inhibition of other neurons. However, in thoracic segments, inhibition of motoneurons is known to occur over two to three segments (20-40 mm) from the presumed somatic locations of the Renshaw cells. Here, we report the first detailed morphological study of Renshaw cell axons outside the lumbosacral segments, which investigated whether this different distribution of motoneuron inhibition is reflected in a different pattern of Renshaw cell terminations. Four Renshaw cells in T7 or T8 segments were intracellularly labeled with neurobiotin in anesthetized cats and their axons traced for distances ≥6 mm from the somata. The only morphological difference detected within this distance in comparison with Renshaw cells in the lumbosacral cord was a minimal taper in the funicular axons, where in the lumbosacral cord this is pronounced. Patterns of termination were virtually identical to those in the lumbosacral segments, so we conclude that these patterns are unrelated to the pattern of motoneuronal inhibition.

Early presymptomatic cholinergic dysfunction in a murine model of amyotrophic lateral sclerosis

Sporadic and familiar amyotrophic lateral sclerosis (ALS) cases presented lower cholinergic activity than in healthy individuals in their still preserved spinal motoneurons (MNs) suggesting that cholinergic reduction might occur before MN death. To unravel how and when cholinergic function is compromised, we have analyzed the spatiotemporal expression of choline acetyltransferase (ChAT) from early presymptomatic stages of the SOD1(G93A) ALS mouse model by confocal immunohistochemistry. The analysis showed an early reduction in ChAT content in soma and presynaptic boutons apposed onto MNs (to 76%) as well as in cholinergic interneurons in the lumbar spinal cord of the 30-day-old SOD1(G93A) mice. Cholinergic synaptic stripping occurred simultaneously to the presence of abundant surrounding major histocompatibility complex II (MHC-II)-positive microglia and the accumulation of nuclear Tdp-43 and the appearance of mild oxidative stress within MNs. Besides, there was a loss of neuronal MHC-I expression, which is necessary for balanced synaptic stripping after axotomy. These events occurred before the selective raise of markers of denervation such as ATF3. By the same time, alterations in postsynaptic cholinergic-related structures were also revealed with a loss of the presence of sigma-1 receptor, a Ca2+ buffering chaperone in the postsynaptic cisternae. By 2 months of age, ChAT seemed to accumulate in the soma of MNs, and thus efferences toward Renshaw interneurons were drastically diminished. In conclusion, cholinergic dysfunction in the local circuitry of the spinal cord may be one of the earliest events in ALS etiopathogenesis.

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.

Recurrent excitation of motoneurons in the isolated spinal cord of newborn rats detected by whole-cell recording

Motoneurons in the isolated spinal cord of newborn rats (1- to 6-day-old) were visualized using infrared videomicroscopy. Whole-cell recordings were performed from the neurons under observation. Stimulation of a sciatic nerve which adjoined ventral roots elicited postsynaptic currents (PSCs) in 28 out of 88 motoneurons. When membrane potentials were changed, some PSCs reversed at around -70 mV, which was compatible with the chloride equilibrium potential calculated. Thus, they were considered to be recurrent inhibitory, namely Renshaw inhibition. On the other hand, we detected PSCs which reversed at +3.3 mV on average. They were interpreted as excitatory based on the level of the reversal potential which was similar to that of orthodromic excitatory PSCs. To determine the origin of the antidromic excitatory inputs, the effect of d-tubocurarine (10 microM) on the PSCs was examined. In three out of five motoneurons, the excitatory currents were eliminated. Therefore, it was concluded that the excitatory inputs, if not all, are mediated via the axon collaterals. Furthermore, it was found that the locations of motoneurons receiving the recurrent inhibitory and the excitatory PSCs were different in the spinal cord.

Morphological analysis of external urethral and external anal sphincter motoneurones of cat

A previous electrophysiological study suggested that there is a common correlation among axonal conduction velocity, input resistance, and size of motoneurones regardless of the muscle type innervated by a motoneurone and that the dendritic arborization is less developed in sphincter than in hindlimb motoneurones. To verify the previous suggestions, cell bodies, axons, and dendrites of external urethral sphincter (EUS) and external anal sphincter (EAS) motoneurones were studied after intracellular labelling with horseradish peroxidase in cats anaesthetised with pentobarbitone. All the cell bodies were located in Onuf's nucleus. The soma diameter (19.7-50.0 microns) was positively correlated with the axonal conduction velocity, and the plots of these two variables were extrapolations from similar plots obtained for hindlimb alpha-motoneurones, supporting the suggestion of the preceding report. Half of motoneurones had recurrent axon collaterals, which terminated within Onuf's nucleus and the ventral border of lamina VIII. The diameter of the cell body with collaterals was significantly larger than that without collaterals. Dendrites extended in five directions: dorsal, medial, lateral, rostral, and caudal. Dorsally directed dendrites of both EUS and EAS motoneurones coursed in lamina VII toward the intermediolateral nucleus, and dendrites of EAS motoneurones further extended toward the intermediomedial nucleus. Long dendrites directed rostrally or caudally within Onuf's nucleus, more prominently in EUS motoneurones. EAS motoneurones had longer end branches and relatively larger number of end branches and summed length and surface area of a dendrite compared with EUS. Among relations, the dendritic surface area and the dendritic volume were tightly correlated with the diameter of the first-order dendrite. The latter relation was almost identical between EUS, EAS, and hindlimb alpha- and gamma-motoneurones. The large values for the dendritic-to-soma surface area ratio (31 in EUS, 50 in EAS) indicated the well-developed dendritic arborization comparable to hindlimb motoneurones. The ratio of sum sigma (daughter diameter)3/2 to the 3/2 power of the parent diameter was 0.99 in EUS and 1.08 in EAS motoneurones, indicating that the 3/2 power constraint at branching points is well satisfied, and Rall's equivalent-cylinder model is applicable to sphincter motoneurones.

Synaptic terminal coverage of primate triceps surae motoneurons

This study examined the synaptic terminal coverage of primate triceps surae (TS) motoneurons at the electron microscopic level. In three male pigtail macaques, motoneurons were labeled by retrograde transport of cholera toxin-horseradish peroxidase that was injected into TS muscles bilaterally and visualized with tetramethylbenzidine stabilized with diaminobenzidine. Somatic, proximal dendritic, and distal dendritic synaptic terminals were classified by standard criteria and measured. Overall and type-specific synaptic terminal coverages and frequencies were determined. Labeled cells were located in caudal L5 to rostral S1 ventral horn and ranged from 40 to 74 microns in diameter (average, 54 microns). The range and unimodal distribution of diameters, the label used, and the presence of C terminals on almost all cells indicated that the 15 cell bodies and associated proximal dendrites analyzed here probably belonged to alpha-motoneurons. Synaptic terminals covered 39% of the cell body membrane, 60% of the proximal dendritic membrane, and 40% of the distal dendritic membrane. At each of these three sites, F terminals (flattened or pleomorphic vesicles, usually symmetric active zones, average contact length 1.6 microns) were most common, averaging 52%, 56%, and 58% of total coverage and 56%, 57%, and 58% of total number of cell bodies, proximal dendrites, and distal dendrites respectively. S terminals (round vesicles, usually asymmetric active zones, average contact length 1.3 microns) averaged 24%, 29%, and 33% of coverage and 33%, 35%, and 36% of number at these three sites, respectively. Thus, S terminals were slightly more prominent relative to F terminals on distal dendrites than on cell bodies. C terminals (spherical vesicles, subsynaptic cisterns associated with rough endoplasmic reticulum, average contact length 3.5 microns) constituted 24% and 11% of total terminal coverage on cell bodies and proximal dendrites, respectively, and averaged 11% and 6% of terminal number at these two locations. M terminals (spherical vesicles, postsynaptic Taxi bodies, some with presynaptic terminals, average contact length 2.7 microns) were absent on cell bodies and averaged 3% and 7% of total coverage and 2% and 5% of terminals on proximal and distal dendrites, respectively. Except for M terminals, which tended to be smaller distally, terminal contact length was not correlated with location. Total and type-specific coverages and frequencies were not correlated with cell body diameter. Primate TS motoneurons are similar to cat TS motoneurons in synaptic terminal morphology, frequency, and distribution. However, primate terminals appear to be smaller, so that the fraction of membrane covered by them is lower.

Distribution of cholinergic neurons in the chick spinal cord during embryonic development. Comparison of ChAT immunocytochemistry with AChE histochemistry

The location of cholinergic neurons was studied during the development of the chick embryo spinal cord. A comparison between choline acetyltransferase (ChAT) immunocytochemistry and acetylcholinesterase (AChE) histochemistry was performed. ChAT-positive neurons could be detected only from embryonic day 9 (E9) onwards by the FITC technique and from E12 onwards by the PAP technique. These neurons were located mainly in the medial and lateral motor columns in the ventral horn of the gray matter and some of them were observed in the intermediate region of the spinal cord. AChE-containing cell bodies were much more numerous than the ChAT immunoreactive ones and were distributed in the ventral horn of the gray matter, the intermediate gray region and mostly off the apical part of the dorsal horn. ChAT should provide a reliable and specific marker for cholinergic neurons.

Evidence for Renshaw cell-motoneuron decoupling during tonic vestibular stimulation in man

The influence of static head-body tilts in the sagittal plane on the activity of Renshaw cells coupled to the soleus extensor alpha-motoneurons was studied in eight human subjects. Head-body rotation was carried out using a tilting seat and its effect was evaluated at 80 degrees (normal sitting position) and at 40 degrees of backward inclination (nose-up). Renshaw cell activity was assessed through a specially designed method of paired H-reflexes first described by Bussel and Pierrot-Deseilligny. alpha-Motoneuron excitability was also independently studied by mapping a reference H-reflex amplitude as a function of static head-body displacements. In almost all subjects Renshaw cell activity was increased at 40 degrees backward inclination with respect to control values at 80 degrees. These changes were attributed to the tonic labyrinthine reflexes capable of decoupling Renshaw cell activity from their motoneurons when the body was tilted backward from the upright position. We discuss the hypothetical functional modalities of the recurrent inhibitory circuit during postural adjustments elicited by labyrinthine input.

Regeneration by supernumerary axons with synaptic terminals in spinal motoneurons of cats

Axons in the central nervous system (CNS) of mammals do not normally regrow if they are cut, which severely limits restoration of function after injury. We have studied the reactions of adult cat spinal alpha-motoneurons after chronic transection of their axons in the periphery by labelling single cells with horseradish peroxidase. Twelve weeks after the operation, about a third of the axotomized cells had developed a 'supernumerary' axon originating from the cell-body region. These supernumerary axons had variable trajectories and termination fields in the ipsilateral spinal cord but generally anomalous projections. Ultrastructural examination shows that they give rise to boutons that form morphologically normal synaptic contacts with neuronal profiles, although they contain dense-cored vesicles not normally seen in central terminals of alpha-motor axons. We conclude that axotomized neurons in the mammalian CNS may be able to form new synaptic contacts by means of supernumerary axons in the absence of local damage.

Variability of axonal arborizations hides simple rules of construction: a topological study from HRP intracellular injections

Intracellular staining with horseradish peroxidase (HRP) allows the analysis of the extent and diversity of axonal field, as the Golgi techniques did for dendritic fields. In this study, we have used such HRP injections to investigate possible rules of construction that underlie the variability in the observed morphological patterns of axons. The Mauthner cell of the teleost provides a suitable material for such a work since it receives inhibitory inputs from two distinct classes of cells that can be identified physiologically prior to their intracellular staining: those that contribute to a recurrent collateral network and those that are part of the commissural vestibulovestibular pathway. The distribution of their terminal boutons over the M-cell surface was quantified and their axonal arborizations were analyzed topologically with the help of a centripetal method of terminal ordering. The results indicated that both types of interneurons have qualitatively a similar distribution of boutons over this target, encompassing the soma, initial portions of the main dendrites, and the cap-dendrites, and that furthermore, the terminals have a tendency to form clusters, the proportion of which is relatively constant regardless of the total number of boutons established by a given afferent cell. In respect to their projection on the M-cell, the two populations differ mainly in the number of established contacts, which averaged 44.1 +/- 28.2 (n = 10) and 14.4 +/- 11.3 (n = 43) for collateral and commissural interneurons, respectively. These differences reflect the variations in axonal arborizations. A topological analysis has revealed that branching occurs mainly as bifurcations, whereas in contrast three or four segments issuing from a branch point are only occasionally observed at the level of the last segment; for each cell, the distribution of boutons within orders is not random and rather follows an unimodal distribution, centered on the mean order (theta); the terminals of each subset, or class, of neurons have their own pattern of distribution within orders; and finally the mean order (theta) is linked to the number of terminal segments (nT) by the relation theta = 1.28 log2 nT. This parameter is of importance: it characterizes the axonal arborization and it allows one to predict the number of terminal segments of individual neurons. Similar analysis of camera lucida reconstruction of axonal arborizations of various cell types studied by other investigators suggests that this relation can be generalized.(ABSTRACT TRUNCATED AT 400 WORDS)

Electron microscopic observations on recurrent axon collateral boutons of a triceps surae gamma-motoneuron in the cat

Boutons from the recurrent axon collaterals of an adult cat gastrocnemius gamma-motoneuron were studied after intracellular labelling with horseradish peroxidase (HRP). Light and electron microscopic observations revealed that the studied gamma-motoneuron possessed 6 dendrites with 39 dendritic end branches totally and that its axon gave off two axon collaterals with together 8 synaptic boutons of either en passant or terminal type. Both collateral trees were confined to the ventral part of lamina VII. Four of the synaptic boutons were studied electron microscopically. They were all found to be S-type boutons containing spherical vesicles. In two cases the bouton could be further subclassified as a type T bouton. All of the studied boutons made synaptic contact with thin dendrites of unknown origin.

Dimensions and branching patterns of triceps surae alpha-motor axons and their recurrent axon collaterals in the spinal cord during the postnatal development of the cat

Triceps surae alpha motoneurons in the cat were stained intracellularly with horseradish peroxidase (HRP) at different postnatal ages from birth to the adult stage. The motor axons and axon collaterals were studied with regard to length, diameter and branching pattern. The postnatal increase of internodal length, measured as the distance between two subsequent axon collateral origins, was about 100% which paralleled the total length increase of the main axon in the grey matter. The axon collaterals were unmyelinated at birth and branched exclusively dichotomously until after 3 weeks of age when a substantial fraction of the branching points gave off 3-5 daughter branches. This was interpreted as signs of a fusion between neighboring branching points during the period of myelination of the axon collaterals. The length analysis of the collaterals indicated that the postnatal elimination of collateral branches described previously is preferentially located in the distal parts of the collateral tree.

Light microscopic observations on cat Renshaw cells after intracellular staining with horseradish peroxidase. I. The axonal systems

Five intracellularly HRP-stained Renshaw cells were subjected to light microscopic analysis of the trajectories, branching patterns, and projections of the axonal systems. The cell bodies were located ventrally in lamina VII. In three neurons the axon originated from the cell body and in the remaining two cells from a dendrite. After a 600-870-microns distance the axons entered the ventral funiculus, where all of them continued rostrally. Two axons also gave off a caudal branch in the funiculus. The diameters of the main axons varied between 2.1 and 10.0 microns. The main axons gave off one to four first-order collaterals before entering the ventral funiculus and up to three collaterals could be seen to originate from the same node of Ranvier. In the ventral funiculus up to five first-order collaterals could be traced from the same main axon. The axon collateral trees were often very extensive and daughter branches up to the 22nd order were observed. The distance between two successive branching points varied between 4 and 410 microns. A large number of boutonlike swellings were found along (59%) or at the ends of the collateral branches. At the most, 1,278 swellings originated from a single axon collateral tree. Most of the swellings were located in lamina IX, but they also appeared ventrally and dorsolaterally in lamina VII.

Light microscopic observations on cat Renshaw cells after intracellular staining with horseradish peroxidase. II. The cell bodies and dendrites

The cell bodies and dendritic trees of five lumbosacral Renshaw cells of adult cats were studied in the light microscope (LM) after intracellular injection with horseradish peroxidase (HRP). The cell bodies were all located in the ventral part of lamina VII. The dendrites extended up to 0.7 mm from the cell body into the neighbouring parts of laminae VIII and IX as well as into more dorsal parts of lamina VII. The dendritic branching was sparse and about half the dendrites were unbranched. The mean diameter of the cell body was positively correlated to both the combined and mean diameters of the first-order dendrites. Between four and eight dendrites originated from the cell bodies. The number of dendritic end-branches, the combined dendritic length, the mean dendritic length from the cell body to the termination of the end branches, the distance from the cell body to the termination of the most remote end-branch, the dendritic surface area, and the dendritic volume all correlated positively with the diameter of the parent first-order dendrite. The dendritic tapering was somewhat more pronounced in the Renshaw cells than previously observed in alpha- and gamma-motoneurons. The present data are discussed in relation to previous morphological observations on Renshaw cells and alpha- and gamma-motoneurons.

Influence of Renshaw cells on the response gain of hindlimb extensor muscles to sinusoidal labyrinth stimulation

The contraction of limb extensor muscles during side-down roll tilt of the animal depends upon an increased discharge of excitatory vestibulospinal (VS) neurons (alpha-response) and a reduced discharge of inhibitory reticulospinal (RS) neurons of the medulla (beta-response), both acting on ipsilateral limb extensor motoneurons. In the decerebrate cat, a modulation of the multiunit EMG activity was clearly present in forelimb extensors, but was extremely weak or absent in hindlimb extensors. Experiments performed in decerebrate cats with the deefferented GS muscle fixed at a constant length have shown that Renshaw (R)-cells, monosynaptically coupled with gastrocnemius-soleus (GS) motoneurons, were either unresponsive or displayed only very weak, small amplitude alpha-responses to sinusoidal stimulation of labyrinth receptors elicited during slow head rotation after bilateral neck deafferentation. This effect was attributed to excitatory VS volleys acting on GS motoneurons and, through their recurrent collaterals, on the related R-cells. In these instances the recurrent inhibition of the GS motoneurons contributed to the very low gain of the EMG response of the corresponding muscles to labyrinth stimulation. Intravenous injection of an anticholinesterase (eserine sulphate, 0.05-0.1 mg/kg) at a dose that in previous experiments increased the firing rate of medullary RS neurons, while decreasing the decerebrate rigidity, slightly increased the discharge rate of R-cells linked with the GS motoneurons in the animal at rest; these findings suggest that the RS system inhibits the extensor motoneurons by exciting the related R-cells. All the R-cells, which prior to the injection were either unresponsive or showed an alpha-response to head rotation (at 0.026-0.15 Hz, +/- 10 degrees), after eserine sulphate showed a beta-response for the same parameters of labyrinth stimulation. In particular, a reduced discharge of the R-cells linked with the GS motoneurons occurred during side-down head rotation as shown for the majority of the RS neurons. It appears therefore that the same R-cells, which in the normal decerebrate cat responded to the excitatory VS volleys acting through the GS motoneurons, were now decoupled from their input motoneurons during head rotation, thus behaving as if they underwent the most efficient direct excitatory control of the RS system. The reduced discharge of the R-cells linked with the GS motoneurons during side-down head rotation would lead to disinhibition of these motoneurons, thus enhancing the response gain of the corresponding muscle to labyrinth stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Postnatal changes in the termination pattern of recurrent axon collaterals of triceps surae alpha-motoneurons in the cat

alpha-Motoneurons innervating the triceps surae and short plantar muscles were stained intracellularly with horseradish peroxidase (HRP) in 0-44-day-old kittens and adult cats. The terminal arborizations of the recurrent axon collaterals in the spinal cord were studied in the light microscope (LM). The short plantar motoneurons lacked axon collaterals in all age groups. With a few exceptions in the youngest kittens (0-1 days of age), the projection field of the axon collaterals of triceps surae motoneurons did not change during development. The exceptional motoneurons had axon collaterals projecting ventromedial to the adult termination areas in Rexed's laminae VII and IX. Within all parts of the projection field, there was a substantial postnatal reduction in the number of axon collateral swellings, interpreted as synaptic terminals, and a total elimination of short and thin axonal processes without swellings. The findings are discussed in relation to earlier demonstrated loss of synaptic terminals on the motoneurons and elimination of polyneuronal innervation of muscle fibers postnatally.

Organization and morphological characteristics of cholinergic neurons: an immunocytochemical study with a monoclonal antibody to choline acetyltransferase

Choline acetyltransferase (ChAT), the acetylcholine (ACh) synthesizing enzyme, has been localized immunocytochemically with a monoclonal antibody in light and electron microscopic preparations of rat central nervous system (CNS). The antibody was an IgG1 subclass immunoglobulin that removed ChAT activity from solution. The specificity of the antibody and immunocytochemical methods has been confirmed by the demonstration of ChAT-positive neurons in a number of well-characterized cholinergic systems. For example, ChAT-positive reaction product was present in the cell bodies of spinal and cranial nerve motoneurons, as well as in their axons and terminations as motor end-plates in skeletal muscle. In addition, the somata of preganglionic sympathetic and parasympathetic neurons were ChAT-positive. The specificity of staining was further supported by a lack of reaction product in several groups of neurons thought to use neuroactive substances other than acetylcholine. No specific staining was observed in control specimens. The findings indicated that ChAT had an extensive intraneuronal distribution in many cholinergic neurons, being present in cell bodies, dendrites, axons and axon terminals. ChAT-positive somata were found in the medial septum and diagonal band, the medial habenula, and the basal nucleus of, the forebrain, 3 regions that are sources of cholinergic afferents to the hippocampus, interpeduncular nucleus and cerebral cortex, respectively. In addition, positively stained cell bodies were present within the cerebral cortex. ChAT-positive punctate structures were observed in the ventral horn of the spinal cord, where electron microscopic studies demonstrated that some of these structures were synaptic terminals. Other regions containing numerous ChAT-positive puncta included the hippocampus, the interpeduncular nucleus and the cerebral cortex. The light microscopic appearance of these putative cholinergic terminals varied among different brain regions. Large punctate structures related to well-defined post-synaptic elements were characteristic of some regions, such as the ventral horn of the spinal cord, while smaller punctate structures and varicose fibers with a diffuse pattern of organization distinguished other regions, such as the cerebral cortex.

An ultrastructural study of serially sectioned Renshaw cells. III. Quantitative distribution of synaptic boutons

The quantitative distribution of synaptic boutons on 17 presumed Renshaw cells has been studied ultrastructurally. All 17 neurons were postsynaptic to axon collateral boutons of intracellularly HRP-stained triceps surae alpha-motoneurons and were located in lamina VII, ventromedially to the main motor nuclei. In each of the presumed Renshaw cells, the values for mean length and mean area of apposition, percentage synaptic covering, and packing density of S-type, F-type, and S + F-type boutons were estimated on the cell body and in two dendritic compartments. The F/S percentage synaptic covering ratio was also calculated. The previously demonstrated differences within the present group of neurons, with respect to the site of axonal origin, were not accompanied by any corresponding differences in the quantitative distribution of synaptic boutons. However, it is suggested that the presumed Renshaw cells may possibly fall into two categories with respect to the F/S percentage synaptic covering ratio. The results are discussed in relation to previous studies on the neuronal architecture and synaptic types on the same presumed Renshaw cells, as well as in relation to earlier observations on the quantitative distribution of boutons on central neurons, particularly spinal alpha-motoneurons.

Trophism between C-type axon terminals and thoracic motoneurones in the cat

1. Quantitative ultrastructural examinations of axon terminals synapsing with normal alpha-motoneurones in segment T9 of cat spinal cord provided estimates of their numbers, sizes and synaptic structure. One synapse, the C type, derived from short-axon propriospinal segmental interneurones, was studied in detail.2. The numbers, sizes and post-synaptic structure of normal C-type synapses at T9 were compared with similar estimates from material provided by cats subjected to partial central deafferentation by double spinal hemisection at T5 and T10 between 7 days and 2 years previously.3. The proportion of C-type synapses present progessively increased from 1% in normal cats to 8.8% 200 days following hemisection, and had still attained a level of 3.1% by 2 years; these increases imply that the absolute number of C-type synapses underwent increase.4. Mean sizes of C-type synapses increased from 4.0 mum (normal) to 5.8 mum (200 days) and retained their enlarged sizes up to 2 years (5.9 mum). Furthermore, while 84% of C-type synapses were under 6 mum in length in normal motoneurones, 48% were over 6 mum long 200 days post-operatively.5. The unique post-synaptic structure of C-type synapses also proliferated following partial central deafferentation of the motoneurones. The elongated cistern, increased numbers and individual lengths of lamellae of the associated underlying rough endoplasmic reticulum indicated a trophic interaction between the presynaptic C terminal and its post-synaptic motoneurone.6. Counts of ribosomes ;bound' to lamellae of the subsynaptic rough endoplasmic reticulum, and of the lamellae-associated polyribosomes interposed between individual lamellae for normal and 200 day post-operative C-type synapses indicated an over-all post-operative increase in capacity for local subsynaptic protein synthesis topographically directed towards this type of axon terminal.7. The observed greater increase in frequency of ribosomes ;bound' to the rough endoplasmic reticulum, together with an over-all proliferation of this structure, specificially indicated an increased capacity for synthesis of protein for utilization in sites remote from those of synthesis (e.g. a trans-synaptic passage of protein).8. A hypothesis is advanced on the basis of the above results relating both pre- and post-synaptic changes in structure to an increased functional activation of the segmental short-axon propriospinal interneurones forming the C-type synapses, as a compensatory response to partial central deafferentation of spinal motoneurones.

Evidence for a postnatal elimination of terminal arborizations and synaptic boutons of recurrent motor axon collaterals in the cat

Triceps surae alpha motoneurons in cats of different postnatal ages were stained intracellularly with horseradish peroxidase (HRP). The recurrent axon collateral trees of the neurons were studied light microscopically. A large reduction of the number of axon collateral end branches and swellings, interpreted as synaptic boutons, was found to occur during the first two weeks of postnatal life.

An ultrastructural study of serially sectioned Renshaw cells. II. Synaptic types

Boutons and synaptic contacts on 17 presumed Renshaw cells were studied ultrastructurally. All 17 neurons were postsynaptic to axon collateral boutons of intracellularly HRP-stained triceps and surae alpha-motoneurons and located in lamina VII, ventromedially to the main motor nuclei. The boutons and synaptic contacts could be classified into two main categories on the basis of synaptic vesicles: S-type boutons with spherical synaptic vesicles and F-type boutons with flattened vesicles, the alpha-motoraxon collateral boutons falling into the S-category. In addition, some S-type boutons containing neurofilaments and some being apposed by small presynaptic boutons were observed. The results are discussed to earlier observations on the synaptology of central neurons, particularly spinal alpha-motoneurons.

An ultrastructural study of serially sectioned Renshaw cells. I. Architecture of the cell body, axon hillock, initial axon segment and proximal dendrites

Seventeen neurons which were postsynaptic to axon collateral boutons of intracellularly HRP-stained triceps surae alpha-motoneurons were studied ultrastructurally. All 17 neurons were situated in lamina VII, ventro-medially to the main motor nuclei. This and other facts support the assumption that the observed neurons are morphological correlates to the physiologically defined Renshaw cells. The contours of the cell bodies, as observed in the midnucleolus plane, were elongated. The axons originated either from the cell bodies or from dendrites. The number of dendrites of each neuron varied between 3 and 7. The appearance of the presumed Renshaw cells was also compared with that of a larger sample of neurons from the ventral part of lamina VII which was studied light microscopically in semithin sections. It was suggested that the Renshaw cells belong to the larger and more elongated neurons in the area.