Unmyelinated nerve fibres in feline acrylamide neuropathy

Neuropathology of skin denervation in acrylamide-induced neuropathy

Previous studies have established the neurotoxicity and pathology of acrylamide to large-diameter nerves. It remains unclear (1) whether small-diameter sensory nerves are vulnerable to acrylamide and (2) if so, how the pathology evolves during intoxication. We investigated the influence of acrylamide on small-diameter sensory nerves by studying the pathology of sensory nerve terminals in the skin. The neurotoxic effects of acrylamide (400 ppm in drinking water) on mice were assessed by immunostaining the skin with protein gene product 9.5, a ubiquitin C-terminal hydrolase, particularly useful for demonstrating cutaneous nerve terminals. Within 5 days of acrylamide administration (the initial stage), epidermal nerves showed two major changes: (1) terminal swelling and (2) increased branching. There was a progressive reduction in epidermal nerve density (END) thereafter. Fifteen days after acrylamide intoxication (the late stage), reduction in END became evident (25.22 +/- 2.19 fibers/mm vs 41.74 +/- 2.60 fibers/mm in control mice, P < 0.003). At this stage, there was significant dermal nerve degeneration with ultrastructural demonstrations of vacuolar changes. These findings establish the pathological consequences of acrylamide neurotoxicity in cutaneous sensory nerves with far-reaching implications: (1) providing an animal system to study "dying-back" pathology of nociceptive nerves and (2) forming the ultrastructural foundation for interpreting the pathology of cutaneous nerve degeneration in skin biopsies.

Neurofilaments are nonessential to the pathogenesis of toxicant-induced axonal degeneration

Axonal neurofilament (NF) accumulations occur before development of symptoms and before other pathological changes among idiopathic neurodegenerative diseases and toxic neuropathies, suggesting a cause-effect relationship. The dependence of symptoms and axonal degeneration on neurofilament accumulation has been tested here in a transgenic mouse model (Eyer and Peterson, 1994) lacking axonal NFs and using two prototypic toxicant models. Chronic acrylamide (ACR) or 2,5-hexanedione exposure resulted in progressive and cumulative increases in sensorimotor deficits. Neurobehavioral tests demonstrated similar expression of neurotoxicity in transgenic (T) mice and their nontransgenic (NT) littermates (containing normal numbers of axonal NFs). Axonal lesions were frequently observed after exposure to either toxicant. Quantitation of ACR-induced lesions demonstrated the distal location of pathology and equal susceptibility of T and NT axons. We conclude that axonal NFs have no effect on neurotoxicity and the pattern of pathology in these mammalian toxic neuropathies. These results also suggest that the role of neurofilament accumulation in the pathogenesis of neurodegenerative diseases requires careful evaluation.

Cutaneous nerve degeneration induced by acrylamide in mice

Acrylamide is a neurotoxin producing distal axonopathy. Previous studies mainly focused on large-diameter motor and sensory nerves, and the influences of acrylamide neurotoxicity on small-diameter sensory nerves in the skin remained elusive. We investigated skin innervation in mice intoxicated by acrylamide. Small-diameter sensory nerves in the skin degenerated after acrylamide intoxication. Epidermal nerve swelling was the earliest sign of acrylamide intoxication, with 29.5+/-2.4% of swollen epidermal nerves in the initial stage (P<0.001). There was a trend of progressive loss of epidermal nerves with a significantly reduced epidermal nerve density in the late stage (P<0.003). In the mean time, degenerating dermal nerves exhibited a beaded appearance. These results suggest the scenario of small-diameter cutaneous nerve degeneration in acrylamide neurotoxicity: beginning with epidermal nerve terminal swelling in the initial stage and resultant epidermal nerve depletion in the late phase.

Acrylamide-induced neuropathic changes in rat enteric nerves: similarities with effects of streptozotocin-diabetes

The effect of acrylamide intoxication (a widely used model for autonomic neuropathy) on the fluorescence intensity and density of catecholamine- and peptide-containing nerve fibres and tissue content of noradrenaline and the peptides vasoactive intestinal polypeptide, calcitonin gene-related peptide, substance P and neuropeptide Y in the enteric nerves of rat ileum was examined. Histochemical and immunohistochemical techniques were used to localize catecholamine- and peptide-containing nerve fibres. The tissue content of noradrenaline was measured using high-performance liquid chromatography, and an enzyme-linked immunosorbent assay technique was used to determine the tissue content of the peptides investigated. Acrylamide intoxication caused a significant decrease in the density of catecholamine-containing nerve fibres and tissue content of noradrenaline in the myenteric plexus of rat ileum. A decrease in tissue content and immunoreactivity of calcitonin gene-related peptide and an increase in vasoactive intestinal polypeptide was seen in the myenteric plexus of ileum from acrylamide-intoxicated rats. In the submucous plexus, the acrylamide treatment caused a decrease in calcitonin gene-related peptide immunoreactivity and an increase in vasoactive intestinal polypeptide and neuropeptide Y immunoreactivity. There was no change in either tissue content or immunoreactivity of substance P in both myenteric and submucous plexuses of the treated rat ileum. These changes have a striking similarity with those found in the enteric nerves of streptozotocin-diabetic rat ileum, suggesting the possible presence of an underlying common mechanism(s) in the development of neuropathic changes in the autonomic nerves of acrylamide-intoxicated and streptozotocin-diabetic rats.

Effects of acrylamide on cotransmission in perivascular sympathetic and sensory nerves

The effects of chronic administration of acrylamide on sympathetic and sensory nerves were examined in the mesenteric artery of rabbits. The noradrenaline (NA) content of the artery was significantly decreased and the total contractile response to electrical field stimulation (4-64 Hz) markedly reduced in the acrylamide group. This was not due to an impairment of the contractility of the smooth muscle or to alterations in the postjunctional receptors. At 16 Hz, only the purinergic component of sympathetic cotransmission was significantly reduced by acrylamide. At 64 Hz, both the purinergic and the adrenergic components were significantly decreased. Field stimulation of the artery pretreated with guanethidine and precontracted with NA produced a frequency-dependent relaxation which was prevented by capsaicin and thus mediated by perivascular sensory nerves. In contrast to its effects on sympathetic cotransmission, acrylamide resulted in a trend, although not significant, towards increased responses at each frequency studied (2-16 Hz). 2-Methylthio-ATP (2Me-S-ATP) caused significantly greater relaxation following acrylamide treatment while vasodilator responses to calcitonin gene-related peptide and substance P were unchanged. It is concluded that, in addition to its known action in producing neuropathy in myelinated somatic motor and sensory nerves, acrylamide causes damage to unmyelinated perivascular sympathetic fibres. Purinergic mechanisms may be particularly susceptible to acrylamide since both the purinergic component of sympathetic vasoconstriction and the relaxation in response to 2Me-S-ATP were affected by acrylamide treatment.

Direct measurement of fast axonal organelle transport in the sciatic nerve of rats treated with acrylamide

The effects of acrylamide on fast axonal transport have been measured primarily using the indirect methods of isotope or enzyme accumulation. We report the first direct evaluation of the effects of subchronic acrylamide dosing (150, 300, or 500 mg/kg total dose, i.e., 50 mg/kg, 2x/wk, for 1.5, 3, 5 wk, respectively) on the fast axonal transport motility machinery itself using video-enhanced differential interference contrast optics with digital image processing and computer analysis. Four principle observations were made: (1) Rapid anterograde transport was not affected at any dosage level within 1 wk after cessation of dosing. (2) A high cumulative dosage (500 mg/kg total) of acrylamide or bisacrylamide produced approximately 7-18% decrease in the rate of retrograde transport in both myelinated and unmyelinated axons. (3) Lower dosages of acrylamide (150 or 300 mg/kg total) produced an increase in retrograde transport rates in myelinated axons only. (4) During the "recovery" phase for the 500 mg/kg acrylamide animals (i.e., 3 or 5 wk after the last dosage of acrylamide) the rate of anterograde transport in the myelinated axons was decreased at 3 wk but not at 5 wk, and the rate of retrograde transport in the myelinated axons returned to control levels while the retrograde transport in the unmyelinated axons continued at abnormally slow speeds. The application of this new technique to evaluate the neurotoxic effects of acrylamide provides evidence of dynamic changes in the axonal transport motility machinery itself and differential effects on myelinated versus unmyelinated fibers.

Abnormalities of sympathetic sudomotor function in experimental acrylamide neuropathy

The possible involvement of sympathetic sudomotor function by acrylamide intoxication was investigated in the mouse, and compared with nerve conduction studies and global motor tests. Acrylamide (40 mg/kg, 3 days per week, 8 weeks) was given per os to a group of mice (A1). Their motor ability to stand on the rotarod was impaired from day 11, reaching a minimum between 46 and 60 days. The number of pilocarpine reactive sweat glands (SG), evaluated by the silicone mold technique, was similar to controls at 40 days and slightly decreased at 54 days. Another group of mice (A2), given acrylamide at a higher dose (50 mg/kg, 5 days per week, 5 weeks), showed abnormalities on the rotarod by 11 days, a progressive decrease of muscle action potential (CMAP) amplitude, and significantly decreased number of reactive SG from 15 days, with respect to controls. Comparatively, sudomotor dysfunction was milder and appeared later in time than alphamotor involvement, being noticeable only after severe poisoning. The decrease in SG response is attributable to damage by acrylamide intoxication of postganglionic sudomotor nerve fibers, which are unmyelinated sympathetic efferents.

Changes in sympathetic and endothelium-mediated responses in the rabbit central ear artery after acrylamide treatment

The effect of acrylamide intoxication on the innervation and local control of the rabbit central ear artery was investigated. There was no difference in the noradrenaline, neuropeptide Y and calcitonin gene-related peptide tissue content between control and experimental animals. There was, however, a slight reduction in catecholamine histofluorescence. Although the contractile efficiency of the rabbit central ear artery as measured by responses to potassium chloride was unchanged, nerve-mediated contractile responses were significantly attenuated in acrylamide-treated animals. Contractile responses induced by exogenous alpha,beta-methylene ATP were markedly increased after acrylamide treatment, in contrast to contractions induced by exogenous noradrenaline which were attenuated at maximal concentrations. Modulatory effects of nerve-mediated contractile responses by neuropeptide Y were unaffected by acrylamide intoxication. It therefore appears that acrylamide intoxication damages sympathetic cotransmission, perhaps with preferential action on the purinergic component. Endothelium-dependent relaxant responses to acetylcholine and substance P were attenuated in acrylamide-treated animals, whereas relaxant responses mediated by calcitonin gene-related peptide (endothelium independent) were unaffected. The question of whether the damage to the endothelial cell action is a primary effect, or a secondary consequence of sympathetic nerve damage, is discussed.

Murine susceptibility to organophosphorus-induced delayed neuropathy (OPIDN)

This study reports that CD-1 strain mice are neuropathologically and biochemically responsive to acute doses of tri-ortho-cresyl phosphate (TOCP). Young (25-30 g) male and female animals were exposed (po) to a single dose of TOCP (580-3480 mg/kg) and sampled for neurotoxic esterase (NTE) activity at 24 and 44 hr postexposure and for neuropathic damage 14 days later. Biochemically, high intragroup variability existed at the lower doses, and at higher levels of TOCP exposure (i.e., greater than or equal to 1160 mg/kg), mean brain NTE inhibition never exceeded 68%. Hen and mouse brain NTE activity, assayed in vitro for sensitivity to inhibition by tolyl saligenin phosphate (TSP), the active neurotoxic metabolite of TOCP, showed similar IC50 values. Histologically, highly variable spinal cord damage was recorded throughout treatment groups and mean damage scores followed a dose-response pattern with no apparent correlation to threshold (i.e., greater than or equal to 65%) inhibition of brain NTE activity. Topographically, axonal degeneration in the mouse spinal cord predominated in the lateral and ventral columns of the upper cervical cord. Unlike the rat, which displays degeneration in the upper cervical cord's dorsal columns (i.e., gracilis fasciculus) in response to TOCP intoxication, treated mice showed minimal damage to this tract. To examine this discrepancy further, ultrastructural morphometric analysis of axon diameters in the cervical cord was performed in control mice and rats. These results indicated that in both species, the largest diameter (greater than or equal to 4 microns) axons are housed in the ventral columns of the cervical spinal cord, suggesting that axon length and diameter may not be the only criteria underlying fiber tract vulnerability in OPIDN.

Acrylamide-induced remodelling of perikarya in rat superior cervical ganglia

To extend investigations on how acrylamide affects neuronal perikarya, we studied post-ganglionic cell bodies in the superior cervical ganglia of control and intoxicated rats (50 mg/kg/day for 9-10 days) with light and electron microscopy and with stereology. By light microscopy, perikarya of intoxicated rats showed peripheral Nissl bodies and nuclear eccentricity. Ultrastructurally, many experimental neurons showed augmented Nissl bodies, often extending from nucleus to plasma membrane; cisternae of granula endoplasmic bodies often appeared in orderly stacked configurations. Intoxicated neurons had increased numbers of normal-appearing mitochondria; some mitochondrial profiles appeared in clumps and others were hypertrophied. Compared to control neurons where Golgi complex most often aligned in a circumnuclear position, experimentals showed reductions in amount and loss of orientation. In some perikarya the pattern of eccentricity of nucleus, peripheral mantle of Nissl, and increased mitochondria gave a distinctive chromatolysis-like appearance. Ultrastructural stereologic analysis showed significantly increased volume fractions of Nissl (P less than 0.001) and mitochondria (P less than 0.002), a trend towards decreased Golgi, and no change in lysosomes, confirming the ultrastructural findings. These data indicate that cell body remodelling is a widespread phenomenon, specific for different neuronal types, and that it probably acts importantly in the pathogenesis of disease.

Circulatory control in canine acrylamide neuropathy

The baroreflex regulation of arterial blood pressure and heart rate was examined in dogs with acrylamide-induced peripheral neuropathy. In anaesthetized animals the carotid sinuses were isolated from the systemic circulation and perfused. The carotid sinus and vagus nerves were kept intact. The maximum baroreflex gain (the ratio of the change in systemic pressure to a change in sinus pressure) was significantly greater in the treated animals. A step increase in carotid sinus pressure produced long periods of asystole and sustained bradycardia in the animals with neuropathy. The alterations in the baroreceptor reflex responses were consistent with dysfunction of baroreceptor fibres in the vagus.

Autonomic neuropathy in the trembler mouse

There is little detailed histological information concerning the autonomic nervous system in hereditary demyelinating neuropathies in man. An opportunity was therefore taken to study the autonomic nervous system of the trembler mouse which suffers from a dominantly inherited peripheral neuropathy. Schwann cell myelination in trembler vagus and splanchnic nerves was abnormal. Morphometric analysis of myelinated and unmyelinated fibres in these nerves showed a marked reduction in myelinated fibre density distribution, whilst unmyelinated fibre densities were within the control range. The trembler vagus contained increased numbers of large diameter unmyelinated fibres probably as a result of trembler Schwann cell failure to form myelin around axons of the appropriate diameter for myelination. The trembler splanchnic nerve, however, contained increased numbers of small diameter unmyelinated fibres, possibly postganglionic fibres which fail to achieve their expected diameters.

The pathology of allyl chloride neurotoxicity in mice

Allyl chloride is known to produce a neuropathy in man after occupational exposure to its vapour. The present study describes the neuropathy which develops in mice given allyl chloride by mouth. Mice were dosed three times weekly with 300 or 500 mg/kg allyl chloride for periods from 2-17 weeks. Functional disability was observed in some animals. Apart from evidence of focal kidney damage in 70% of dosed mice, pathological changes were restricted to the nervous system. Nerve fibre degeneration was found in many peripheral nerves and in roots, tending to be more marked distally and to affect more motor than sensory nerves. Degenerated fibres were also found in dorsal, ventral and lateral columns of the spinal cord. Males were more severely affected than females. Increased numbers of filaments were on early axonal change, occurring multifocally and apparently preceding axonal degeneration. No neuronal death was observed, but occasional anterior horn and dorsal root ganglion cells showed some morphological changes. Vacuolated lesions mainly due to swelling of astrocytes and their processes were found in the ventral horn in cervical and lumbar regions of spinal cord. Animals appeared to become tolerant to allyl chloride after continuous dosing. This neuropathy appears to be a central-peripheral distal type of axonopathy.