Alteration of neuronal synaptic complement during regeneration and axonal sprouting of rat spinal cord

Changes in the distribution of synaptic potentials from bulbospinal neurones following axotomy in cat thoracic spinal cord

1. Plasticity in functional connections of expiratory bulbospinal neurones was investigated by measurement of terminal potentials (TPs) and focal synaptic potentials (FSPs), recorded with spike-triggered averaging in the thoracic spinal cord of anaesthetized, paralysed cats. These measurements were made in normal cats and in those which had previously been subjected to spinal cord lesions that transected the axons of the bulbospinal neurones in the segment below that under investigation, either about 2 weeks or about 16 weeks previously. 2. In both groups of operated animals bulbospinal neurones with firing properties and conduction velocities similar to normal were present. The extracellular recordings that were averaged to reveal TPs and FSPs were made on two standard grids, each consisting of eight sites spaced 0.25 mm apart on two electrode tracks. One grid was positioned at a rostral and one at a caudal location within one segment (T7-T9). 3. Tn the normal animals TPs and FSPs were larger and/or more common at rostral sites than at caudal sites, by a factor of about 1.7. In both 2 week and 16 week animals, TPs and FSPs were observed, both showing normal tine courses and latencies. At rostral sites in 2 week and 16 week animals the amplitudes and/or the frequency of occurrence of TPs and FSPs were similar to normal, as was the case fir caudal sites in the 2 week animals. However, at caudal sites in the 16 week animals the FSPs were mole common and/or significantly larger than normal, with the increase particularly marked on the lateral track, being equivalent to a factor of about 2. A corresponding increase in the amplitude and/or frequency of occurrence of TPs at caudal lateral sites was also seen, but was not significant. 4. The results are interpreted as evidence for short-range sprouting of the bulbospinal axons and the formation by them of new connections in the caudal parts of the segments concerned.

Structural changes of anterior horn neurons and their synaptic input caudal to a low thoracic spinal cord hemisection in the adult rat: a light and electron microscopic study

Structural changes in lumbosacral ventral horn neurons and their synaptic input were studied at 3, 10, 21, 42, and 90 days following low thoracic cord hemisection in adult rats by light microscopic examination of synaptophysin immunoreactivity (SYN-IR) and by electron microscopy. There was an ipsilateral transient decrease in SYN-IR at the somal and proximal dendritic surfaces of anterior horn neurons which extended caudally from the site of injury over a postoperative (p.o.) period of 42 days. Concomitantly, at 21 days p.o., perineuronal SYN-IR started to recover in upper lumbar segments. By 90 days p.o., a normal staining pattern of SYN was noted in upper and mid lumbar segments, but the perineuronal SYN-IR was still slightly below normal levels in low lumbar and sacral segments. Electron microscopy revealed ultrastructural changes coincident with the alterations in SYN-IR. At 3 days p.o., phagocytosis of degenerating axon terminals by activated microglial cells was observed at the somal and proximal dendritic surfaces of ventral horn neurons. These changes were most prominent up to two segments caudal to the lesion. At 10 days p.o., advanced stages of bouton phagocytosis were still detectable in all lumbosacral motor nuclei. Additionally, abnormal axon terminals, with a few dispersed synaptic vesicles and accumulations of large mitochondria, appeared at the scalloped somal surfaces of anterior horn neurons. At 21 days p.o., several large lumbosacral motoneurons had developed chromatolysis-like ultrastructural alterations and motoneuronal cell bodies had become partially covered by astrocytic lamellae. At 42 days p.o., there was a transient appearance of polyribosomes in some M-type boutons. In addition, at 42 and 90 days p.o., a few degenerating motoneurons were detected in all lumbosacral segments, but most displayed normal neuronal cell bodies contacted by numerous intact synapses as well as by astrocytic processes. In contrast to these striking alterations of synaptic input at somal and proximal dendritic surfaces of motoneurons, relatively few degenerating boutons were detected in the neuropil of motor nuclei at all the p.o. times studied. We suggest that the preferential disturbance of the predominantly inhibitory axosomatic synapses on ventral horn neurons may be involved in the mechanisms which influence the well-established increase in motoneuronal excitability after spinal cord injury.

Sprouting of peripherally regenerating primary sensory neurones in the adult central nervous system

We have studied the ability of primary afferent neurones to proliferate within the grey matter of the dorsal horn following the degeneration of other, nearby, afferent fibres. The peripheral branches of primary afferents have the capacity to regenerate successfully over long distances, and we have examined the possibility that when they are so doing, the neurones' status changes to facilitate greatly the sprouting of afferent fibres within the dorsal horn. "Spared root" preparations (rhizotomies of L3, L4, L6, S1, and the caudal half of L5, sparing the rostral half of the L5 dorsal root) were made in adult rats. In some animals (acute preparations) the distribution of the central terminals of the spared root was assessed by labelling the sciatic nerve with WGA-HRP at the time of the rhizotomies. In other animals (chronic preparations), symmetrical bilateral spared roots were made and the sciatic nerve on one side was concomitantly crushed to trigger regrowth of the peripheral branches of these axons. Eight to 10 weeks later the sciatic nerves on both sides were labelled with HRP-WGA. In the acute preparations the reaction product was found in a limited rostrocaudal and mediolateral region of the dorsal horn. In lamina II (the lamina of densest labelling) the labelled terminals occupied an average of 1.17 +/- 0.21 mm2. In chronic preparations, the area of labelled terminals on the side of the uncrushed sciatic nerve was 1.34 +/- 0.28 mm2 (not significantly different from acute animals). However, the labelled area on the side of the crushed sciatic nerve was significantly greater, averaging 2.17 +/- 0.14 mm2.(ABSTRACT TRUNCATED AT 250 WORDS)

A physiological study of the monosynaptic reflex responses of cat spinal alpha-motoneurons after partial lumbosacral deafferentation

In adult cats the whole S1 and rostral half of the L7 dorsal roots were cut on the left side of the spinal cord to produce a partial monosynaptic deafferentation of the ipsilateral alpha-motoneurons. Three, 6 or 12 weeks later, monosynaptic reflexes (MSRs) were recorded from the L6, L7 and S1 ventral roots or from various peripheral nerves during stimulation of the L6 and remaining parts of the L7 dorsal roots. Also, monosynaptic excitatory postsynaptic potentials (EPSPs) were recorded intracellularly in different types of medial gastrocnemius alpha-motoneurons of the L7 segment during stimulation of various hind limb muscle nerves. The right side with an identical acute deafferentation served as control. On the chronically lesioned side the MSRs were increased in size, also during post-tetanic potentiation. The monosynaptic EPSPs had increased amplitudes in all motoneuron types, but the relation in EPSP size between different motoneuron types as well as between different synergistic inputs remained largely unchanged. EPSP rise times were not changed, and aberrant monosynaptic connections from non-synergist muscles were not observed. It is concluded that the extent of reactive reflex changes may be related to both the number of vacant synaptic sites and the degree of functional synergism between the eliminated and remaining monosynaptic pathways. Possible underlying mechanisms are discussed.

Plasticity of catecholaminergic terminals in rat paraventricular hypothalamic nucleus after 6-hydroxydopamine lesion: an emphasis on bouton sizes and synaptic frequency

Catecholaminergic (CA) nerve terminals in the paraventricular hypothalamic nucleus (PVN) of adult rats were studied at 4, 21, 56 and 180 days after a single injection of 6-hydroxydopamine (6-OHDA) neurotoxin into the right lateral ventricle of the brain. We previously described and quantified the extent of CA terminal sprouting in the PVN after 6-OHDA lesions. For this communication we studied parameters, specifically the bouton sizes and the synaptic frequencies of CA terminals during the renewal process, and evaluated how changes of these parameters are related to axonal sprouting. The CA boutons were identifiable in the electron microscope by exhibiting small granular vesicles (SGVs) after central administration of 5-hydroxydopamine (5-OHDA) marker. The marked CA boutons were measured and further categorized according to whether or not they were associated with distinct synaptic specializations at various post-lesion stages. The average sizes of CA boutons were strikingly similar in their diameters (1.0 micron) for both control and experimental tissues. However, CA boutons larger than 2.1 micron were rare and seen more often in the experimental tissues with 6-OHDA lesion and were sustained up to 180 days after lesions. Catecholaminergic profiles with ultrastructural features of growth cones were also seen in the PVN following the 6-OHDA lesions, indicating that there is growth activity in the PVN after 6-OHDA lesion. There were 33% of CA boutons in the PVN from the control tissues that appeared to have synaptic contacts.(ABSTRACT TRUNCATED AT 250 WORDS)

Thoracic dorsal funicular lesions affect the bouton patterns on, and diameters of, layer VB pyramidal cell somata in rat hindlimb cortex

The effect of spinal dorsal funicular lesions (T 12) upon the frequency of boutons on, and diameters of the somata of pyramidal cells in layer VB of hindlimb cortex was studied. Adult rats sustained bilateral damage to either the dorsal column (DC, n = 10) alone or DC combined with the corticospinal tract (CS) (DC + CS, n = 34) and were utilized 1, 2, 3, 7, 14, 30, 45, 60, 90, or 120 days postoperatively (DPO). Neurons randomly sampled from 44 lesioned and 13 unoperated cases were analyzed for the number of silver-impregnated boutons (Rasmussen method) on the circumference of the soma as well as diameters of the soma, nucleus, and nucleolus. Analyses of variance comparing across lesioned and normal groups were significant for bouton counts on the soma (P less than 0.01), and diameters (long axis) of somata (P less than 0.01) and their nuclei (P less than 0.05). Both lesioned groups exhibited significant decreases from normal for these latter three parameters. With respect to survival time for the DC + CS-lesioned animals we noted the following: (1) Bouton counts on the soma significantly decreased below normal between 1 and 60 DPO; this decrease was most dramatic during the first three days postlesion. (2) Somal diameter (long axis) significantly decreased below normal between 2 and 120 DPO (except at 14 and 90 DPO). (3) Nuclear diameter (long axis) significantly decreased below normal only at 90 DPO. (4) Bouton counts on somata of neurons in layers VB and IV [Ganchrow and Bernstein, 1981] of hindlimb cortex correlated negatively and significantly across 120 postlesion days. The rapid shrinkage and reduced afferentation of layer VB somata during the first week following DC + CS lesions suggest initial, retrograde reactions to CS axotomy. Since bouton counts on layer VB somata were significantly less (P less than 0.05) in DC- than DC + CS-lesioned rats, it is hypothesized that CS axotomy regulated a set-point for increased afferentation which was maintained on the shrunken somata between 7 and 120 DPO.

Effects of nerve growth factor and its antibodies on sprouting of sensory axons following spinal cord hemisection

Axons were counted in rat dorsal roots three segments cranial and three segments caudal to a spinal hemisection done at birth. The animals were placed in three groups: (1) hemisected (HEMI), (2) hemisected and given nerve growth factor (NGF) daily (HEMI + NGF) and (3) hemisected and given antibodies to NGF daily (HEMI + anti-NGF). They were sacrificed at 1 month. In the HEMI and HEMI + NGF groups, there were 20% and 13% more unmyelinated axons, respectively, in roots of the operated side. We interpret these differences to be the result of sprouting of unmyelinated axons as a result of denervation, and exogenous NGF does not seem to affect the response. By contrast, the unmyelinated axon responses in HEMI + anti-NGF animals are different. These animals have approximately 7% more unmyelinated axons on the denervated side and 50% more axons on the normal side than in the other animals we studied. The results on the anti-NGF animals are interpreted as indicating sprouting in response to a chemical denervation caused by NGF removal and the response is more dramatic on the unoperated side.

Quantitative ultrastructural stereology of synapses in nucleus dorsalis after a peripheral nerve injury at birth

Utilizing recent techniques in quantitative stereology, this investigation studied the synaptology of nucleus dorsalis (Clarke's column) in 12-week-old rats whose sciatic nerves were crushed in the 1st postnatal day. Four morphometric variables were analyzed at the levels of L1 and L3 spinal cord segments: total surface area of synaptic contact zones per unit volume (SV), total length of synaptic contact zones per unit area (LA), average length of synaptic membrane (L), and numerical density of synapses per unit volume (NV). The original raw data were corrected for Holmes's effect. The results indicated that peripheral nerve crush at birth induced a transganglionic change in central sensory terminals with a loss of numerous synapses. A significant loss (P less than 0.001) of about 32% in the SV and LA and a significant loss (P less than 0.001) of about 36% in the NV were observed on the experimental side. There was no preferential loss of synapses in either segment. The mean synaptic membrane length showed no significant difference between the control and experimental sides. The control values of the four morphometric variables calculated for L3 were lower than those calculated for L1. The loss of synapses after a peripheral nerve lesion was probably due to the loss of sensory neurons and their central processes, but there were other possibilities.

Recovery from partial deafferentation increases 2-deoxyglucose uptake in distant spinal segments

2-Deoxy[14C]glucose autoradiography was used to study the responsiveness of the partially deafferented rat spinal cord to electrical stimulation of low-threshold afferent fibers. Unilateral extradural dorsal rhizotomies were carried out at L3 to S2, sparing L5. Postoperative sensory deficits were appropriate to the extent of the lesion. Acute, 7 day, and 14 to 20 day postrhizotomy animals and unoperated controls were anesthetized prior to isotope injection and electrical stimulation of Ia fibers in the L5 root. Quantitative densitometry was carried out on enlarged autoradiographs, subdividing the spinal gray matter into laminar divisions drawn from the corresponding stained sections. Optical densities from stimulated and unstimulated sides were compared using paired t tests for each experimental group at each lumbosacral segment (L1 to S2) and at T13 for the day 14 to 20 animals. This procedure provided an objective basis for statistical comparisons between homologous areas even where the differences in density were small. Unoperated animals showed activation in 11 of 23 dorsal horn zones extending to L2 and never involving the base of the dorsal horn (lamina V). Acute and day 7 groups did not appreciably differ from the control group except for activation of lamina V within the L4 segment. In the most delayed group, 18 dorsal horn regions were activated, extending to L1 with an additional zone in T13 . Lamina V contained significant labeling in three segments. In no group was there increased labeling of the ventral horn. The results are interpreted as showing that stimulus-related, neural activity increases after a 2-week delay in regions of spinal cord distant from the normal zone of significant metabolic change. This increase in neural activity during recovery is discussed in relation to time-dependent electrophysiologic, structural, and metabolic responses to deafferentation. The longitudinal spread of dorsal horn activation by preserved afferent fibers in the spared root may facilitate more effective central transmission of sensory information.

Long-term survival of peripheral axons that have reinnervated the spinal cord

Adult cats received grafts after the fashion of Barnes and Worall by anastomosing the central stump of a ventral L7 root to the central stump of a dorsal L6 root. After regeneration periods from 4 weeks to 316 weeks (79 months) the regenerated axons were identified by the horseradish peroxidase technique. In some animals, regenerated boutons were identified using electron microscopy. With shorter regeneration times, most of the labeled axons were seen in the white matter of the dorsal funiculus. In the longest surviving cat, labeled axons were seen in the entire medial half of the dorsal horn grey matter. Boutons derived from regenerated axons appeared typical of CNS boutons, showing none of the morphologic characteristics of the motor end plate.

Restoration of function in external intercostal motoneurones of the cat following partial central deafferentation

The activity of external intercostal motoneurones in the cat was studied under anaesthesia and paralysis before and after partial central deafferentation caused by single or double ipsilateral hemisections of the thoracic spinal cord. The normal efferent inspiratory discharges recorded from external intercostal nerve filaments caudal to the upper lesion were greatly reduced acutely, but activity of approximately normal intensity and phase at eupneoic levels of CO2 was restored within a few days and remained at similar levels for up to two years. The patterns of the restored activity were abnormal, with more discharges of alpha-motoneutrones during expiration than normal and a stronger modulation of the discharges by the respiratory pump than normal. A common abnormal component of the restored activity was a tonic discharge in hypocapnic apnoea, often modulated by the respiratory pump. This activity was never seen in normal animals or in those with acute lesions under similar conditions of anaesthesia. Synchronization of the discharges of alpha-motoneurones caudal to the upper lesion was studied by constructing cross-correlation histograms between paired groups of motoneurones, each group being represented by the discharges in one filament. Synchronization was stronger than normal, usually extending over a time course of +/- 20 to +/- 50 ms (broad-peak synchronization). This synchronization was particularly strong for the discharges in hypocapnic apnoea. We conclude that the restored activity was derived in large part from abnormal tonic (non-respiratory-phased) inputs, partly proprioceptive in origin, probably involving spinal cord interneurones with abnormally synchronized discharges. This conclusion is supported by intracellular measurements including respiratory drive potentials, synaptic noise and average common excitation potentials.

Time course of segmental reflex changes after chronic spinal cord hemisection in the rat

In a companion paper (Hultborn & Malmsten 1983 a) it was described that, in the cat, direct excitatory reflexes became larger on the side of a chronic spinal hemisection than on the other side. In this paper the time course for change of the size of excitatory reflexes after spinal cord hemisection in the rat is described. Ipsilateral reflexes obtained by stimulation of cut dorsal roots L 4 and L 5 were recorded in the corresponding cut ventral roots. It was found that reflexes became larger on the lesioned (left) side than on the opposite side. For both mono- and polysynaptic reflexes there was a marked peak in left/right ratio on reflex size at 2 to 6 days post lesion. The ratio then returned to normal but increased again from about 21 days until at least 135 days. In control rats reflexes were larger on the right side, in agreement with findings in the cat.

Changes in segmental reflexes following chronic spinal cord hemisection in the cat. I. Increased monosynaptic and polysynaptic ventral root discharges

The effect of a chronic spinal cord hemisection on segmental reflex transmission was studied in cats. Recordings of ventral root responses were made after a terminal transection below the initial lesion to eliminate descending influence through the intact spinal half. Procedures to ensure comparability between sides, that are lacking in earlier work on this experimental model, were introduced in the present work. It was demonstrated that there was an increase in reflex size on the lesioned (left) side, relative to the other side, of both mono- and polysynaptic reflexes. The reflex changes were found at all survival times studied (from 2 to 515 days). In control animals mono- and polysynaptic reflexes were found to be larger on the right side. It is discussed that side symmetry of reflex size is not to be presupposed even in a normal population. Detailed clinical examinations of lesioned animals were not performed, but a tendency for enhancement of the ipsilateral knee-jerk was found. There was no syndrome of spasticity and general motor recovery was very good.

Progression of change following median nerve section in the cortical representation of the hand in areas 3b and 1 in adult owl and squirrel monkeys

In an earlier study (Neuroscience 8, 33-55, 1983), we found that the cortex representing the skin of the median nerve within parietal somatosensory fields 3b and 1 was completely occupied by 'new' inputs from the ulnar and radial nerves, 2-9 months after the median nerve was cut and tied in adult squirrel and owl monkeys. In this report, we describe the results of studies directed toward determining the time course and likely mechanisms underlying this remarkable plasticity. Highly detailed maps of the hand surface representation were derived in monkeys before, immediately after, and at subsequent short and intermediate time stages after median nerve section. In one monkey, maps were derived before nerve section, immediately after nerve section, and 11, 22 and 144 days later. Thus, direct comparisons in cortical map structure could be made over time in this individual monkey. In other experiments, single maps were derived at given post-section intervals. These studies revealed that: (1) large cortical sectors were 'silenced' by median nerve transection. (2) Significant inputs restricted to the dorsum of the radial hand and the dorsum of digits 1, 2 and 3 were immediately 'unmasked' by median nerve transection. (3) These immediately 'unmasked' regions were topographically crude, and represented only fragments of this dorsal skin. They were transformed, over time, into very large, highly topographic and complete representations of dorsal skin surfaces. (4) Representations of bordering glabrous skin surfaces progressively expanded to occupy larger and larger portions of the former median nerve cortical representational zone. (5) These 'expanded' representations of ulnar nerve-innervated skin surfaces sometimes moved, in entirety, into the former median nerve representational zone. (6) Almost all of the former median nerve zone was driven by new inputs in a map derived 22 days after nerve section. At shorter times (3, 6 and 11 days), 'reoccupation' was still incomplete. (7) Very significant changes in map dimensions within and outside of the former median skin cortical field were seen after the 'reoccupation' of the deprived cortex by 'new' inputs was initially completed. (8) Progressive changes were recorded within the original ulnar and radial nerve cortical representational zones, as skin surfaces originally overtly represented wholly within these regions expanded into the former median nerve zone. (9) Throughout the studied period, the cortical representational loci of many skin sites appeared to change continually and often markedly. (10) The locations of map discontinuities also shifted significantly over time. (11) Concomitant with changes in representational magnification over time, inverse changes in receptive field sizes were recorded.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of spinal dorsal horn neurons after partial chronic deafferentation by dorsal root transection

Unilateral transections of 1-3 lumbar dorsal roots were performed in 13 adult cats to investigate the effect of partial deafferentation on dorsal horn neurons. Eleven to 45 days after deafferentation various parameters of spontaneous and evoked activity of 169 neurons were measured and compared to the data of 168 neurons from previous experiments recorded under identical experimental conditions except that these animals had not been deafferented. Eighty-six of the units encountered were located in the segment of transected dorsal root(s) and 82 in the caudally adjacent segment. No significant differences could be observed in the functional properties of these two samples of units. Most parameters measured indicate that either no change at all in responsiveness or signs of decreased excitability occurred in the partially deafferented neurons compared to units recorded in control animals. Discharges evoked by noxious skin heating indicate a linear relationship between discharge frequency and skin temperature. This kind of encoding curve could also be measured during a reversible cold block of the spinal cord at segment L1. The mean encoding curves before and during spinal blockade were not different in deafferented compared to corresponding curves measured in control animals. The only finding that could be interpreted as an indication for increased excitability of partially deafferented neurons was that the mean frequency of spontaneous discharges of a subsample of heat-sensitive neurons was higher in deafferented compared to control animals. Possible mechanisms are discussed.

Bouton renewal patterns in rat hindlimb cortex after thoracic dorsal funicular lesions

The transneuronal effect of bilateral, dorsal funicular lesions (T 12) on the frequency of boutons on cells in layer IV of hindlimb cortex was studied. Adult rats were utilized 1, 2, 3, 7, 14, 30, 45, 60, 90, or 120 days postoperative (DPO), and tissue was processed for the light microscopic visualization of silver-impregnated boutons (Rasmussen method). Bouton counts were taken on soma, or along 5- and 10-micrometers segments or proximal dendrite branching from soma. The soma diameter also was measured on those neurons chosen for bouton counts on the circumference of the soma. Statistically significant, increased afferentation on soma and proximal dendrite occurred during the first postlesion week relative to longer survival times; bouton counts on the proximal dendrite showed a trend (not statistically significant) toward increases above normal. These data mirror similar, consistent increases in bouton counts reported in thalamic ventroposterolateral nucleus of these same cases. At 14 DPO, bouton counts on the soma decreased below normal (P less than 0.005) and, except at 60 DPO, remained so through 120 DPO (P less than 0.025). Bouton counts on the proximal dendrite also decreased below normal at 14 DPO (P less than 0.005), thereafter exhibiting either periodic (along 5-micrometers) or extended (along 10-micrometers) periods in significant decreases from normal. Correlations in lesioned cases between the number of boutons on the soma and either bouton counts on proximal dendrite or soma diameter were positive and statistically significant (P less than 0.005 in all correlations). Possible anterograde (via the dorsal column-medial lemniscal system) and/or retrograde (via the corticospinal tract) transneuronal mediation of these effects in hindlimb cortex is discussed.

Separation of radiolabelled protein from brain and spinal cord of spinal hemisected rats on SDS polyacrylamide slab gels

After hemisection of the spinal cord in rats, alterations in amino acid incorporation into protein have been observed in brain and spinal cord. Proteins mediating these changes have been studied in the present experiment. Male, Long-Evans hooded rats were given either a laminectomy and dura incision (sham) at spinal segment T2, a left spinal cord hemisection, or no operative procedures. One hour prior to utilization at 1, 3 and 14 days postoperation, amimals were injected subcutaneously with 200 microCi of [3H]-L-lysine and 200 microCi of [3H]-L-amino acid mixture. Samples were prepared for electrophoresis and proteins separated on linear gradient SDS polyacrylamide slab gels (7.5-20% acrylamide). The Gel slabs were cut into slices and processed for scintillation counting. In right somatomotor cortex, there was evidence for a general stimulation of amino acid incorporation at one day postoperation in animals which received surgery. At other postoperation intervals, significant increases in proteins in the regions of 90-150,000, 50-65,000 and 35-45,000 molecular weight (MW) were observed. In spinal cord, significant increases in proteins in the regions of 55-65,000, 30-45,000 and 15,000 MW were observed over postoperation time in all animals receiving surgery compared to normals. A large, significant increase in the radioactivity of proteins in the region of 18-20,000 MW were observed in the spinal cord lesion site of spinal hemisected animals alone at three days postoperation. These results indicate that increases in amino acid incorporation into the brain and spinal cord of laminectomized and spinal hemisected animals observed previously are theresult of both specific and nonspecific changes in protein incorporation of precursors.

Effect of puromycin treatment on the regeneration of hemisected and transected rat spinal cord

The effect of puromycin on spinal cord regeneration was studied following implantation into the site of spinal cord hemi- or transection of Gel-foam saturated with puromycin (1 mM) in a saline carrier, implantation of Gel-foam sponge saturated with saline (carrier control), or lesion alone (lesion control). The spinal cords of 107 rats were studied with light and electron microscopy 7, 14, 30, 60 and 90 days postoperative (DPO). Spinal cord hemisected animals developed a dense cicatrix at the site of lesion replete with connective tissue, blood vessles, and myelinated and unmyelinated nerve fibres which could be traced to peripheral sources. Rostrally at the C.N.S.--cicatrix interface, there were reactive neuroglial cells, occasional nerve fibres and finger-like projections of spinal cord (due to cavitation lesions) which contained neuroglia, axons and dendrites. Implantation of saline in Gel-foam resulted in the same morphology as in hemisected animals except for increased lesion size due to mechanical factors and decreased cicatrix density during the first 30 DPO. Puromycin treatment resulted in a cicatrix with initial decreased cell density but which contained a new class of nerve fibres at 30 DPO. These nerve fibres were oriented in a rostro-caudal direction, were unmyelinated, 0.1-0.2 micron in diameter and had expanded smooth endoplasmic reticulum. Some of these nerve fibres were degenerating at 30 DPO and all were absent by 60 DPO. The puromycin-treated spinal cord within 200 micron rostral to the basal lamina contained nerve terminal conglomerates, which resembled boutons, in fascicles from 30-90 DPO (duration of experiment). Hemisection of the spinal cord by crushing 1-1 1/2 segments rostral to the site of puromycin implantation at 30 DPO resulted in degeneration of these nerve fibres in the cicatrix as well as the degeneration of nerve terminal conglomerates just rostral to the basal lamina. The regenerative capacity of the spinal cord is discussed in relationship to these findings.

Amino acid incorporation in medulla, pons, midbrain and cortex following spinal cord hemisection in the cebus monkey (Cebus apella)

The effects of spinal cord hemisection on protein labeling in the medulla, pons, midbrain, and cortex in the Cebus monkey were studied by measuring the uptake of [3H]lysine into the trichloroacetic acid (TCA) precipitable and non-precipitable fractions of brain. Animals were sacrificed 3, 6 or 13 days after hemisection. Two normal animals and a 3-day sham operate were utilized as controls. Differences included individual animals, operated vs. unoperated animals, brain areas, and variation of brain areas over all postoperative days. The overall trend in brain radioactivity over postoperative days indicates stress and non-stress related components of brain response to spinal injury. Left-right side differences (L greater than R) were noted in the uptake of [3H]lysine into specific areas of brain over postoperative time, including leg areas of motor cortex, occipital cortex, and superior colliculus. A biphasic action of leg sensory cortical areas over time was noted, where L greater than R for the first 6 postoperative days and R greater than L at day 13. The factors possibly mediating radiolabeled amino acid uptake into protein are discussed in relation to demonstrated anatomical and neurochemical regenerative processes.

Synaptic frequency alteration on rat ventral horn neurons in the first segment proximal to spinal cord hemisection: an ultrastructural statistical study of regenerative capacity

To study the regenerative capacity of the spinal cord in adult rat, presynaptic boutons were classified as S (spherical vesicles), F (flattened vesicles) and C complexes, and analysed statistically on alpha-motoneuron somata and lamina VII interneurons on the operated side in the first segment rostral to a spinal cord hemisection. Following chloral hydrate anesthesia left spinal cord hemisections were made on twenty adult rats (225 gms) at vertebral level T-2. Animals were prepared for electron microscopy at 7, 14, 30, 45, 60 and 90 DPO and compared with normals. All counts were made on coded material and subjected to statistical analysis. The normal frequency of presynaptic bouton types on alpha-motoneuron somata at 30 DPO. At 45 DPO, massive degeneration with concomitant synaptic remodeling resulted in a return to near normal frequencies of S and F presynaptic boutons. At 60 and 90 DPO a gain in S presynaptic boutons and a concomitant loss in F presynaptic boutons resulted in frequencies different from normal and decreased absolute numbers of presynaptic boutons. The interneuron somata also exhibited alterations over the postoperative period. There was a reversal of frequency of presynaptic boutons at 45 DPO. However unlike on alpha-motoneuron somata the frequency of S and F presynaptic boutons returned to normal at 60 and 90 DPO. The alpha-motoneuron somata appeared to be cyclically and nonselectively reinnervated by ventral horn interneurons over 90 DPO.

An electron microscopic study of lesion-induced synaptogenesis in the dentate gyrus of the adult rat. II. Reappearance of morphologically normal synaptic contacts

Intact synapses in the denervated area of the rat dentate gyrus are reduced to 14% of those normally present 2-4 days following a unilateral entorhinal lesion. By 160-240 days after lesion, the former entorhinal terminal zone is repopulated with new synapses. In all, there is more than a 5-fold increase in the density of intact synapses in the denervated zone between 2 and 240 days post-lesion, and the denervated zone of the molecular layer is restored to 80% of control values. The synapses are Gray type I and are formed on simple and complex spines which closely resemble those normally present. A few boutons have an abnormally large number of synaptic junctions. Reinnervation seems to progress at differential rates. Synapses are rapidly regained up to 30 days after operation, but thereafter the reacquisition of synaptic connections is much slower. Reinnervation is more rapid in the portion of the denervated zone nearest the granule cells, where the maximal densities are attained within 30 days. The time course of reinnervation differed from that of degeneration. A portion of the new synapses in the reinnervated molecular layer appear to arise by the assembly of new synaptic junctions. Over time, the number of post-synaptic contact sites along a given length of dendritic surface recovers, suggesting the formation of new synaptic sites. Our data indicate that granule cells retain a capacity even into adulthood to manufacture, position and assemble postsynaptic components of a synapse and, in concert with reactive afferents, form normal-appearing synapses.

Early changes in protein synthesis following spinal cord hemisection in the Cebus monkey (Cebus apella)

The short term effects of spinal cord hemisection on protein synthesis in the Cebus monkey were studied by measuring the uptake of [3H]lysine into the trichloroacetic acid (TCA) precipitable and non-precipitable fractions of spinal cord. Animals were sacrificed 3, 6, or 13 days after hemisection. Two normal animals and a 3 day sham operate were utilized as controls. All data were pooled and subjected to analysis of variance. Significant group differences were resolved by a standad t-test over pooled operative group samples and a paired t-test for left-right differences within groups. Compared to normal animals, significant increases in the mean uptake of precursor into TCA precipitable and non-precipitable fractions of spinal cord samples were noted in all postoperative groups. The observed increase in the uptake of [3H]lysine into TCA soluble and insoluble fractions at 3 days after hemisection were attributed to stress, as were increases in the activity of the TCA insoluble fractions in the sham operate. By 6 days, the increases attributed to stress in the average protein activity seemed to subside, resulting in an overall decrease in the TCA precipitalbe protein activity with an increase in TCA soluble fraction activity. Between 6 and 13 days the activity of the TCA soluble and insoluble fractions returned to the 3 day levels with a sharp increase in the activity of both fractions in the area of the hemisection. This increase was attributed to increases in neuronal, neuroglial, and infiltrating cell metabolism possibly associated with regenerative changes.

Regeneration of retinal axons into the goldfish optic tectum

The growth of regenerating retinal axons into the central portion of the optic tectum of adult goldfish was examined with the light and electron microscopes. Optic tracts were cut and, two days to five months later, the animals were perfused and the tecta prepared for microscopy. Regenerating axons first reached central regions of the tectum seven to ten days postoperatively. Regenerating axons appear in very large numbers and travel in fascicles in the stratum opticum (SO) and in the adjacent neuropil, the stratum fibrosum et griseum superficiale (SFGS). In the SO, the fascicles are bordered by glial cells and degenerating debris. Within the SFGS, however, the fascicles do not seem to be similarly associated with glial cells and degenerating debris. The youngest regenerating axons are very slender processes, containing microtubules but few or no neurofilaments or dense granular material. By 10 to 14 days postoperatively, neurofilaments can be seen and, in addition, large numbers of vesicles with dense cores appear. The vesicles with dense cores increase in numbers until about 28 days postoperatively and then become quite rare. That vesicles with dense cores were seen in regenerating axons in both SO and SFGS during the period of growth into the tectum but were not seen in axon terminals at any time, suggests that they may be concerned with axon elongation. During the period one month to five months postoperatively, the regenerating axons gradually increase in diameter but do not reach preoperative sizes, suggesting that the regenerative changes may still be occurring. Remyelination is delayed and proceeds slowly. Many axons remain unmyelinated for as long as five months postoperatively.

Increases in collateral axonal growth rostral to a thoracic hemisection in neonatal and weanling rat

The spinal cords of newborn and weanling rats were hemisected at the mid-thoracic level. Control studies revealed that Fink-Heimer positive debris was absent in the gray matter at three months postoperative. The remaining animals were given a second lesion, a high cervical spinal hemisection, at five to seven months after the original thoracic hemisection. The pattern of degeneration rostal to the thoracic lesion was compared with similar regions of the spinal cord from animals receiving only a cervical hemisection at the adult stage. In neither experimental group of doubly hemisected rats was there any degeneration observed below the thoracic lesion site, even though no glial or connective tissue scar had formed in animals originally operated at birth. Thus no regeneration had occurred. At least one segment above the initial hemisection: 1. the majority of degenerating axons were localized toward the lateral edge of the spinal cord, especially in the doubly lesioned neonatal group; 2. the erae of ipsilateral white matter was reduced more in the neonatal than the weanling operates; 3. there was an upward shift in axonal diameter of ipsilateral fibers in both the region of the rubrospinal tract and the ventrolateral portion of the lateral funiculus of the doubly hemisected rats when compared with the cervically lesioned controls; 4. a significantly greater amount of degeneration was present in lamina VII of Rexed in both the neonatal and weanling experimental operates (p less than 0.05 weanling; p less than 0.001 neonate); 5. no mean difference in area was seen between the ipsilateral and contralateral gray matter in any group for the segments of the spinal cord in which the judgements and measurements were taken. These data suggest that there has been sprouting of axons from descending nerve tracts rostral to the thoracic lesion in both the neonatal and weanling experimental groups. The question remains whether the sprouting of descending nerve tracts is from collateral of axons which normally project rostral to the thoracic hemisection and are not cut by the thoracic lesion (collateral sprouting) or from collaterals of lesioned axons (regenerative sprouting). Present evidence favors collateral sprouting, expecially in the neonatal operate where much retrograde cell death appears to have taken place.

Ventral horn synaptology in the rat

The synaptology of the normal ventral horn of the rat was studied. Presynaptic boutons were classified as S (Spherical vesicles), F (flattened vesicles), and G (predominance of 700-1200 A granular vesicles). In addition, Cf, Cs, M, and T synaptic complexes were defined and quantitated. Synaptology was studied on alpha-motoneuron somata, alpha-motoneuron primary dendrites, peripheral dendrites and interneuron somata. In addition, organelles were quantified for the pre- and postsynaptic members of the synaptic complex. All counts were made on coded material and these data were analyzed statistically. Motoneuron somata had significantly more (P less than 0.01) F (58%) than S (33%) boutons. This was also the case for the motoneuron primary dendrite (P less than 0.01; F, 61%; S, 37%). The small dendrites had more (P less than 0.05) S (56%) than F (44%) boutons. More Cf bulbs (P less than 0.01) were found on motoneuron somata (9%) than on motoneuron primary dendrites (2%) or interneruon somata (3%). The C complex presynaptic bouton contained spherical (Cs) or flattened (Cf) synaptic vesicles which were attributed to the fixation employed. Cf bulbs were not observed on small dendrites. G bulbs were observed (less than 1%) only on small dendrites M bulbs were not observed on any postsynaptic structure. The boutons of the motoneuron primary dendrites (15% of total afferents) and peripheral dendrites (14% of total afferents) were frequently branched whereas there was significantly (P less than 0.01) less branching of boutons on motoneuron and interneuron somata. Small postsynaptic subsurface cisterns were associated with boutons of both the S and F type on all structures. In addition, these cisterns were observed in motoneuron somata (4%) and interneuron somata (2%) without an accompanying bouton. C postsynaptic organelles were observed in motoneuron somata (3%) and primary dendrites (1%) with an overlying neuroglial cell process and no presynaptic bouton. The synaptology of the rat ventral horn is comparable to that in the cat and monkey. However, M (R) and P bulbs were not observed in the rat. This could be due to the sampling method which indicated that synapses with less than 1% occurrence fall at the level of statistical resolution in quantitative electron microscopy. The presence of postsynaptic specialization usually associated with presynaptic boutons with no presynaptic component may be a reflection of the dynamics of normal bouton renewal in the rat ventral horn.

A fine structural study of degenerative-regenerative pathology in the surgically deafferentated lateral vestibular nucleus of the rat

An experiment was designed to examine the course of degeneration, phagocytosis, and regeneration in the central nervous system following surgical deafferentation. The anterior cerebellar vermis was ablated in young male rats. The animals were sacrificed by perfusion at postoperative times ranging from 24 hrs to 6 months. The lateral vestibular nuclei, to which the anterior cerebellar vermis projects, were processed for electron microscopy. Degenerating synaptic terminals, of the dark variety, were seen from 24 hrs to five days postoperatively. Phagocytosis of degenerating terminals occurred during this time. Degenerating axons persisted through 6 months survival, and phagocytosis of these degenerating axons were observed. Astrocyte scar formation began at 1 month postoperatively. The relative number of axosomatic synaptic terminals containing flattened vesicles ("F" terminals; presumed inhibitory in function) increased in operated animals. The highest F scores were found from 24 hrs to two weeks postoperatively, and then the F scores declined through six months. The significance of these sprouting activities is discussed in relation to the abortive sprouting phenomenon described by Ramon y Cajal.