Changes in terminal sprout formation in rat sternocostalis muscle during chronic intoxication with 2,5 hexanedione

Qualitative and quantitative morphological studies of the sternocostalis muscle innervation were made on rats chronically intoxicated with 2,5 hexanedione (2,5 HD) using the zinc iodide-osmium (ZIO) technique. Two distinct phases were seen in the events at the motor endplate. First, the number of motor endplates forming spontaneous terminal sprouts was found to increase linearly with time and, from the third week onward, the sprouts appeared to become progressively elongated. This latter change was associated with the appearance of swollen axons within intramuscular nerve bundles. Second, from the sixth week onward, wallerian degeneration of nerve fibers was seen and terminal sprouts began to make new arborizations on muscle fibers. By the eighth week, this occurred in as many as 66% of the rats, and collateral sprouting was also observed at this time. The occurrence of increased spontaneous terminal sprouting due to altered neuromuscular function is discussed in the light of axonal changes resulting from neurofilament accumulation following 2,5 HD intoxication.

The evolution of intracellular responses to acrylamide in rat spinal ganglion neurons

Acrylamide (30 mg or 50 mg/kg/day, 5 days each week) was injected intraperitoneally into rats for up to 4 weeks. Lumbar spinal ganglia, spinal cord and lumbrical muscle spindles were examined by light and electron microscopy at various times during this period. The first abnormalities in spinal ganglion neurons were seen at 7 days when an apparent increase in numbers of mitochondria, some being hypertrophic, were found in a few large light cells. This was 10 days before any significant Wallerian degeneration was found in muscle spindle sensory fibres. Mitochondrial changes became more marked with time and were later associated with RER disruption, loss of neurofilaments and peripheral displacement of the nucleus thus mimicking chromatolysis of the axon reaction. All these changes began, however, before axon degeneration. Evidence of increased satellite cell activity was maximal at 21 days. These changes are discussed in the light of the possibility that calcium entry into the cell may be seriously increased early in the intoxication as a direct result of the presence of acrylamide and that many of these cellular features are secondary responses to such an event. Distal degeneration of axons seems likely to be secondary to the perikaryal changes.

Effects of acrylamide and other sulfhydryl compounds in vivo and in vitro on staining of motor nerve terminals by the zinc iodide-osmium technique

The zinc iodide-osmium technique blackens motor nerve terminals by selectively staining synaptic vesicles. Intraperitoneal injections of acrylamide (30 mg/kg/day, 5 times each week) cause inhibition of staining by this technique so that approximately one third of the end-plates in rat sternocostalis muscle are unstained after 24 hours, and by 17 days more than 70% are unstained. This is not associated with nerve fiber degeneration. A similar inhibition of staining can also be shown after prior incubation of the sternocostalis muscle in 4 mM acrylamide in oxygenated Ringer's solution. Intraperitoneal injection of the thiol group blocker N-ethylmaleimide also causes marked inhibition of staining of motor end-plates by this method. Dithiothreitol, which prevents the oxidation of thiol groups, will partly prevent the inhibition of staining by both acrylamide and N-ethylmaleimide, when given in vivo.

Effects of acrylamide and some other sulfhydryl reagents on spontaneous and pathologically induced terminal sprouting from motor end-plates

The innervation of the normal rat sternocostalis muscle exhibits a constant low level of short spontaneous terminal sprouts visible in zinc iodide-osmium (ZIO) and in methylene blue-stained preparations. Acrylamide inhibits these spontaneous sprouts in a dose-dependent manner. This inhibition is mimicked by N-ethylmaleimide (a sulfhydryl group blocking agent) and can be nullified when acrylamide is given after the sulfhydryl group protecting agent, dithiothreitol (DTT). This could not be reversed by giving DTT 3 hours after acrylamide. Furthermore, when given alone DTT increases the level of spontaneous terminal sprouting seen in ZIO and in methylene blue-stained preparations. These findings suggest that the binding of acrylamide to sulfhydryl groups is involved in the inhibitory process. Acrylamide also reduces the number and length of the reactionary terminal sprouts, seen in ZIO and in methylene blue-stained preparations, that follow partial denervation or local injection of botulinum toxin. These inhibitory effects are long-lasting; recovery still has not fully occurred 4 weeks after a single dose of acrylamide (50 mg/kg). The roles of glutathione and other sulfhydryl components of axons are discussed in relation to the mechanism of acrylamide neurotoxicity.

The effects of chronic isoniazid intoxication on motor end plate sprouting in rat sternocostalis muscle and on responses to partial denervation and local botulinum toxin

Chronic dosing of rats with isoniazid (INH) leads to an increase in the incidence of short "spontaneous" sprouts on motor end plates in the rat sternocostalis muscle. After partial denervation there is a slight increase in terminal sprouting after 1 week of dosing: this changes to a significant decrease from 2 to 6 weeks of dosing. The same is noted after local botulinum toxin injection, and in both conditions sprout length is significantly reduced. In vitro studies show that glutathione, cysteine and cystathionine all increase the incidence of short, "spontaneous" sprouts from end plates, while homocysteine and cystine have no effect. These findings are interpreted in the light of the hypothesis that in INH intoxication there may be a reduction of available axonal glutathione and cysteine due to inhibition of the pyridoxal phosphate-dependent enzymes cystathionine synthetase and cystathioninase .

Effects of acrylamide and botulinum toxin on horseradish peroxidase labelling of trigeminal motor neurons in the rat

The extent of neuronal labelling with horseradish peroxidase (HRP) was examined in rat trigeminal motor neurons at various stages of acrylamide intoxication, following HRP uptake by motor nerve endings in the anterior digastric muscle. After 7 days of acrylamide (5 daily injections of 30 mg acrylamide/kg body weight), the pattern of HRP labelling was altered from normal, and this changed pattern persisted without further alteration when survival time and dosage were increased to 14 and 21 days (equivalent to 10 X 30 mg/kg and 15 X 30 mg/kg respectively). Six hours after HRP injection, the number of labelled cells in the ipsilateral trigeminal motor nucleus was reduced in treated animals compared to controls. By 18 hours, cells containing label were present in similar numbers to controls; but by 24 hours, the number in treated animals had fallen again, unlike controls in which labelling remained constant between 6 and 24 hours. At longer intervals, this reduction in labelling continued, but more slowly, so that by 96 hours after HRP injection, numbers of labelled cells were again similar in poisoned and control animals. One and three days after a single intramuscular injection of 0.05 microgram botulinum toxin type A, HRP labelling in the trigeminal motor neurons was unaffected, although at three days after toxin, mild chromatolytic changes could be seen in a few of the neurons.

Cytochemical staining characteristics of peripheral nodes of Ranvier in hexacarbon intoxication

Changes in the distribution of stainable gap substance and subaxolemmal density at peripheral nodes of Ranvier in hexacarbon-intoxicated rats have been studied by light and electron microscopy. Cupric ion binding to the nodal gap substance was seen in normal nodes as a discrete annulus by the formation with ferrocyanide of Hatchett's Brown reaction product. Staining of osmicated fibres with ferrocyanide ions alone gave a deposit of black reaction product at the subaxolemmal region at nodes of Ranvier. Paranodal distension by neurofilamentous masses and separation of the terminal myelin loops in the early phase of paranodal dilatation produced no change in the distribution of the two kinds of stainable material. Paranodal myelin retraction with increases both in nodal gap width and nodal axon diameter resulted in displacement and attenuation of both stained regions. Axonal protrusion at the nodal region tended only to displace the stained gap substance, but sometimes it resulted in its attenuation. Occasionally loss of subaxolemmal staining was found. The possible functional relevance of these abnormal findings is discussed in relation to changes in conduction in affected nerves.

Distortions of the nodes of Ranvier from axonal distension by filamentous masses in hexacarbon intoxication

A study has been made of the structural changes of nodal and paranodal regions of the nodes of Ranvier of peripheral nerves of rats in which marked accumulations of neurofilaments have occurred within axons under the influence of 2,5-hexanediol over 10 weeks. The neurofilamentous masses caused distension of the axon at two points of apparent weakness as they attempted to slide through the axonal constriction at the nodes. Principally, a spiral axonal protrusion pushed into the zone of unattached myelin loops in the proximal paranodal spinous bracelet of Nageotte. This led to a conical widening of the paranodal constriction and considerable attenuation of the overlying myelin. No degeneration of the myelin occurred however. Alternatively, or additionally, a protrusion occurred of the axon at the nodal region which increased the nodal gap width and occasionally compressed and displaced the adjacent distal paranodal constriction which could have led to some obstruction of axoplasmic flow. Swelling of distal paranodal regions occurred later and was usually associated with proximal swelling. It was also accompanied by evidence suggesting transnodal passage of filamentous material. Sometimes, however, striking nodal constriction occurred in association with symmetrical paranodal swelling. These observations suggest that the spiral glial-axonal relationships at nodes of Ranvier are capable of marked deformation that might allow the intra-axonal neurofilamentous masses to move distally. These findings are discussed in relation to the structural features of the paranodal constrictions.

Ultrastructural features of the Purkinje cell damage caused by acrylamide in the rat: a new phenomenon in cellular neuropathology

Dosing rats with acrylamide leads to the formation in Purkinje cells of juxtanuclear clusters of tubular and vesicular smooth endoplastic reticulum (SER). A microtubule organizing centre forms in relation to these clusters and together they appear to move to the cell surface, where protrusions of plasmalemma form, often with overlying synaptic attachments, containing densely packed tubular and vesicular SER membranes. Usually the microtubule organizing centre immediately underlies this. Subsequently, appearances suggest that astroglial intrusions occur internal to the protrusions described above to which the tubulo-vesicular material appears to be transferred. During these events the organization of the cytoplasm of the Purkinje cell is grossly disturbed with apparent loss and disarray of rough endoplasmic reticulum (RER) and of polyribosomes. This temporal sequence of events can be followed after a single dose of acrylamide. In chronically intoxicated animals vacuolation and swelling of dendrites takes place and the Purkinje cell may die after all stages of the cellular transformations have been present. These unique events appear to be confined to Purkinje cells and are considered probably to be the result of a primary disturbance to SER synthesis caused by acrylamide. It is argued that the changes taking place in acrylamide intoxication in neurons that lead to degeneration in long axons are probably of the same general kind.

Bilateral innervation of the anterior digastric muscle by trigeminal motor neurons

The locations and numbers of trigeminal motor neurons supplying the anterior digastric muscle in the rat were observed following retrograde transport of horseradish peroxidase. Unilateral injection of horseradish peroxidase into the muscle resulted in bilateral labelling of cells in the ventromedial region of the caudal trigeminal nucleus. The numbers of cells on the contralateral side were consistently lower than those in the ipsilateral nucleus (30 cells compared to 171). Bilateral injection of horseradish peroxidase resulted in higher numbers of cells on both sides than was seen ipsilaterally after unilateral injection: these numbers corresponded roughly to the sum of ipsilateral and contralateral labelled cells after a single injection. The contralateral reaction product survived removal of the opposite muscle and nerve immediately before injection. Ablation of one trigeminal motor nucleus caused degeneration in the opposite nucleus, and some degenerating fibres crossed the mid-line at the level of the motor nucleus, in the bundle of fibres decussating in the dorsal pons.

Blood-brain barrier integrity and host responses in experimental metastatic brain tumours

The effect of brain implants of Walker 256 carcinosarcoma tumour cells on the integrity of the blood-brain barrier was examined in rats using labelled albumin, horseradish peroxidase and trypan blue. Barrier integrity was intact within 1 hour of implantation but was gradually reduced within the tumour after 3.5 days. This was related to alterations in the fine structure of the tumour capillaries. Dissociated tight junctions were apparent within the tumour centre, but no fenestrated endothelium or gap junctions were observed by electron microscopy.

The effects of 2,5-hexanedione on axonal regeneration after nerve crush in the rat

The pattern of recovery of myelinated axons in the posterior tibial nerve after crushing was studied in rats chronically intoxicated with 2,5-hexanedione. It was given for 2 weeks before crushing (200 mg/kg i.p. 5 times a week) or additionally for two further weeks after the nerve crush. Two animals were examined from each group at approximately 1,2,3,4 and 8 weeks later. Return of function in poisoned animals was slower than in the controls. The numbers of regenerating myelinated fibres was severely reduced in poisoned animals up to 4 weeks later, but by 8 weeks the numbers equalled those in the control nerves. Marked impairment of initiation of neurite outgrowth was found, but once begun, axonal growth was comparable to controls and myelination occurred normally. Above the crush for 10 mm, filament-filled axonal swellings were found in poisoned animals accompanied by varying amounts of retrograde axonal degeneration. These findings are discussed in relation to the role of normal neurofilaments in axonal growth and the effects of probably cross-linking of these by 2,5-hexanedione on regenerating neurites.

The effects of acrylamide on beta-glucuronidase and acid phosphatase activities in rat sciatic nerve above and below a ligature

The enzymes beta-glucuronidase and acid phosphatase have been assayed by histochemical and by fluorimetric methods above and below a tightly tied ligature on the rat sciatic nerve over the subsequent 10 days. These findings have been compared with similarly treated animals also given four daily doses of acrylamide (50 mg/kg). The following have been found: 1 during this time, acrylamide at this dose causes slight increases in beta-glucuronidase in untied sciatic nerves, detectable both histochemically and fluorimetrically; 2 below the ligature both enzyme activities were greatly increased and this was slightly reduced in the acrylamide-dosed animals; 3 there was a mild rise in beta-glucuronidase activity in the 1.0 cm above the ligature in undosed animals demonstrable both histochemically and fluorimetrically; 4 that in the acrylamide-dosed animals there was a marked rise in beta-glucuronidase activity both 1.0 cm and 2.0 cm above the ligature which was intensified at 7 days and at 10 days after dosing. This was demonstrated both histochemically and fluorimetrically. These results are discussed in the light of the known sheath cell and axonal responses to acrylamide intoxication.

The pathokinetics of acrylamide intoxication: a reassessment of the problem

Acrylamide was given intraperitoneally to rats (30 mg/kg/day, five times/week) for 3 weeks, and the nervous and muscle tissues were examined by conventional methods over 5 weeks. Three striking cellular changes were observed. 1 Scattered degeneration of many Purkinje cells from 5 days onwards. 2 Widespread swelling and argyrophilia of nerve terminals from 10 days in both PNS and CNS. Motor and sensory endings were equally affected in all muscles examined. Synaptic and preterminal swelling also occurred in spinal cord, brain stem, and in certain cerebellar terminals. Degeneration occasionally followed this change, particularly in sensory nerve fibres, but not necessarily. 3 Chromatolysis in spinal ganglion cells and occasionally in anterior horn cells from 7 days onwards before the onset of axonal degeneration. This unique sequence of events is discussed in the light of the metabolic and other changes described by earlier authors.

The early evolution of neurofilamentous accumulations due to 2,5-hexanediol in the optic pathways of the rat

Rats were given 2,5-hexanediol in their water for more than 5 weeks. The changes in the optic pathways were studied both qualitatively and quantitatively. Increase in 10-nm filaments within axons was noticeable from 10 days onwards in the superior colliculus and in the brachium of the superior colliculus. From then onwards there was a steady increase in the number of affected axons, and their gross enlargement occurred. The proportion of affected fibres in the brachia was in fact few, but all fibres were of retinotectal origin. Less than half of those in the superior colliculus that were swollen were of retinal origin. Measurement of axon diameter versus myelin sheath thickness showed gross relative thinning of the latter. Axon degeneration did not occur but there was increasing ultrastructural evidence of impairment of transport of organelles both centrifugally and centripetally as the filamentous masses accumulated.

Selective loss of Purkinje cells from the rat cerebellum caused by acrylamide and the responses of beta-glucuronidase and beta-galactosidase

Acrylamide (30 mg/kg) given daily to rats five times each week for 3 weeks leads to progressive loss of Purkinje cells. The necrotic cells begin to be visible from the third day and their numbers reach a peak at the time when the dosing ceases at 18 days. They are less frequent thereafter, but are still visible almost 3 weeks later in small numbers. The density of Purkinje cells per millimeter falls to about 70% of normal at the 7th day, and a similar degree of reduction of the neuronal marker enzyme, beta-galactosidase, is found over the same time scale. By contrast, while there is a brisk macrophage/microglial response in the molecular layer to the loss of the Purkinje cell dendrites, the increase in beta-glucuronidase activity is relatively minor and is not significantly different from normal until after the 21st day. These responses are discussed in the context of the use of lysosomal enzyme activities in the assay of certain neurotoxic lesions.

Recovery from 2,5-hexanediol intoxication of the retinotectal tract of the rat. An ultrastructural study

Rats were given 2,5-hexanediol, a metabolite of n-hexane, in the drinking water until they developed a marked degree of paresis over about 7 weeks and were then allowed to recover naturally. The time course and the manner of removal of the neurofilamentous masses accumulated within axons caused by the intoxication were followed by electron microscopy over the subsequent 8 weeks. The neurofilamentous masses slowly disappeared completely from the axons of this tract, without there being any degeneration, over 6-7 weeks. They disappeared first from the fibres in the brachium of the superior colliculus, perhaps by transport towards the terminals, and later from the axons within the superior colliculus itself. Particularly in preterminal fibres in the superior colliculus the filamentous accumulations became permeated by a network of smooth endoplasmic reticulum which may have played a part in the removal of the filaments. Accumulations of mitochondria and dense bodies in preterminal regions, presumed to be caused by obstruction to retrograde transport, disappeared pari passu with loss of the filaments. The significance of these events in relation to neurofilament metabolism is discussed.

The pattern of recovery of axons in the nervous system of rats following 2,5-hexanediol intoxication: a question of rheology?

Rats have been dosed with 2,5-hexanediol for 48 days and then allowed to recover. The changes in the accumulations of neurofilamentous masses in various pathways in CNS and PNS have been followed by light microscopy over the subsequent 9 weeks. It was found that many CNS pathways allow the argyrophilic masses to pass to their terminals from whence they subsequently disappear, usually over 5-6 weeks. Little or no axonal degeneration is seen where this happens. The same occurs in many peripheral nerves, particularly cranial nerves. However, in many tracts in the spinal cord and in many axons in the longer peripheral nerves, filamentous masses remain and becomes associated with axon degeneration, and, in tracts, gliosis. The importance of paranodal constrictions at nodes of Ranvier which tend to be greater in larger diameter axons is emphasized as a likely mechanism for the axon degeneration which largely took place during the recovery period.

Comparison between the early changes in isoniazid intoxication and the chromatolytic response to nerve ligation in spinal ganglion cells of the rat

The early effects of acute isoniazid intoxication and the early responses of the neuron to axotomy have been compared in lumbar spinal ganglion cells in the rat. The cells were studied by light and electron microscopy during the 4 days after a single oral dose (2g/kg) of isoniazid and for four days after application of a tight ligature to the sciatic nerve at the level of the sciatic notch. After axotomy a greater proportion of cells showed changes visible by light and electron microscopy than after INH. Small dark neurons were affected more rapidly and more severely by both insults than the large light cells as evidenced principally by changes in their ribosomal organization. Large light cells showed little response to INH. The first response to axotomy was visible about 12 h after injury. The response to INH began 24 h after the single oral dose. The early cell responses to INH therefore apparently precede axon degeneration which is first seen 72 h later and are thus probably a direct toxic effect.

Ultrastructural changes in axons caused by acrylamide above a nerve ligature

The ultrastructural features of the ascending degeneration produced by acrylamide in peripheral nerves above the point of nerve ligature have been described. Before the onset of signs of axonal degeneration abundant accumulation of smooth endoplasmic reticulum and vesicles occur particularly proximal to nodes of Ranvier, but also in internodal regions. Some mitochondrial accumulations occur distal to nodes of Ranvier. The changes closely resemble those found in axons up to 4 mm above a nerve crush in normal animals and appear to be essentially non-specific in nature. Their significance in relation to the subsequent axonal degeneration can only be guessed at.

Enhancement of sensitivity to acrylamide after nerve ligature

Acrylamide given to rats after damage to the posterior tibial nerve by a tight ligature causes retrograde degeneration of axons. The number of fibres affected and the extent of degeneration towards the cell body are dose dependent. The effect is the same whether the acrylamide is given immediately after ligature or one week later. The degeneration is delayed several days after giving acrylamide. Before axonal disintegration occurs, focal accumulations of SER, vesicle and multivesicular bodies are seen beneath the axon membrane. Associated with these changes larger vacuoles occur, either in the axon or external to it. It is suggested that these early changes are a response by the axon and perikaryon to the local metabolic damage caused by the toxic chemical. Somewhat similar changes also occur in early predegenerative INH intoxication, and before the axon degeneration in O-P neurotoxicity.