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

Pathological classification of equine recurrent laryngeal neuropathy

Recurrent Laryngeal Neuropathy (RLN) is a highly prevalent and predominantly left-sided, degenerative disorder of the recurrent laryngeal nerves (RLn) of tall horses, that causes inspiratory stridor at exercise because of intrinsic laryngeal muscle paresis. The associated laryngeal dysfunction and exercise intolerance in athletic horses commonly leads to surgical intervention, retirement or euthanasia with associated financial and welfare implications. Despite speculation, there is a lack of consensus and conflicting evidence supporting the primary classification of RLN, as either a distal ("dying back") axonopathy or as a primary myelinopathy and as either a (bilateral) mononeuropathy or a polyneuropathy; this uncertainty hinders etiological and pathophysiological research. In this review, we discuss the neuropathological changes and electrophysiological deficits reported in the RLn of affected horses, and the evidence for correct classification of the disorder. In so doing, we summarize and reveal the limitations of much historical research on RLN and propose future directions that might best help identify the etiology and pathophysiology of this enigmatic disorder.

The mechanisms of neurotoxicity and the selective vulnerability of nervous system sites

The spatial heterogeneity of the structure, function, and cellular composition of the nervous system confers extraordinary complexity and a multiplicity of mechanisms of chemical neurotoxicity. Because of its relatively high metabolic demands and functional dependence on postmitotic neurons, the nervous system is vulnerable to a variety of xenobiotics that affect essential homeostatic mechanisms that support function. Despite protection from the neuroglia and blood-brain barrier, the central nervous system is prone to attack from lipophilic toxicants and those that hijack endogenous transport, receptor, metabolic, and other biochemical systems. The inherent predilection of chemicals for highly conserved biochemical systems confers selective vulnerability of the nervous system to neurotoxicants. This chapter discusses selective vulnerability of the nervous system in the context of neuron-specific decrements (axonopathy, myelinopathy, disruption of neurotransmission), and the degree to which neuronal damage is facilitated or ameliorated by surrounding nonneural cells in both the central and peripheral nervous systems.

Increased thermal stability of collagen after 2,5-hexanedione intoxication in rats

The present study examined the ability of 2,5-hexanedione (2,5-HD) to cross-link the fibrillary protein, collagen, in vivo. The thermal stability of collagen measured as the isometric contraction-relaxation force was studied in tail tendons from rats, receiving 2,5-HD intraperitoneally for 2 or 6 weeks (1 g/kg/week, divided into 3 doses). The maximum contraction force was significantly increased after 6 weeks of intoxication as compared to controls, indicating an increase in covalent cross-linking between collagen molecules. Likewise, in vitro incubation of rat tail tendons in 2,5-HD for 24 hr induced an increase in thermal isometric contraction force.

Morphologic changes induced in vitro by 2,5 hexanedione

The effects of 2,5 hexanedione (2,5 HD), a metabolite of n-hexane, were investigated in different in vitro systems. A human mammary carcinoma cell line, a human melanoma cell line, and fetal mouse neuronal cells in primary culture were considered. Light and electron microscopic observations demonstrated clearly that changes in cell proliferation can be detected. Furthermore, morphologic differentiative phenomena characterized by a noticeable increase in cell protrusions and dendriticlike processes can occur. Differences in the features of these processes were also detected between the different cell lines. These data can indicate non-neuronal cells as possible further targets of the toxicant. The possibility could be hypothesized that toxic neuropathies are generalized disorders, being neuronal system exceptionally vulnerable to 2,5 HD. Moreover, results obtained suggest that the sensitivity of in vitro systems could represent a useful tool in studying the mechanisms of action of the neurotoxicant 2,5 HD.

Cytofluorometric quantification of somatopetal axonal transport: effects of a conditioning lesion and 2,5-hexanedione

Cytofluorometric quantification of axonally transported fluorescein isothiocyanate (FITC)-labelled wheat germ agglutinin (WGA) from the injection site in the snout area was performed in the facial nucleus at various times during regeneration of one of the facial nerves. Measurements were made on single neurons on both operated and non-operated sides in three different groups of mice 8, 12 and 16 days after a nerve crush, Group 1: (control group) animals with a nerve crush, Group 2: animals with a conditioning lesion (nerve injury) made 3 days before the nerve crush, and Group 3: animals exposed daily to 2,5-hexanedione from 2 weeks before nerve crush until killing. A conditioning lesion caused a more rapid return of transport in regenerating nerves but there was no evidence for an increase in the total amount of transported FITC-WGA. For mice exposed to 2,5-hexanedione a transient increase of tracer transport in regenerating nerves could be demonstrated on day 12 after nerve crush. On day 16, however, a reduction of transport was seen in both operated and non-operated nerves. This study shows that it is possible experimentally to manipulate the influx of macromolecules to the nerve cell body from the periphery during nerve regeneration, and the present method offers the opportunity to study quantitatively the effects of various treatments on reinnervation of a muscle.

Anterograde components of axonal transport in motor and sensory nerves in experimental 2,5-hexanedione neuropathy

Anterograde slow and fast axonal transport was examined in rats intoxicated with 2,5-hexanedione (1 g/kg/week) for 8 weeks. Distribution of radioactivity was measured in 3-mm segments of the sciatic nerve after labelling of proteins with [35S]methionine or [3H]leucine and glycoproteins with [3H]fucose. The axonal transport of the anterograde slow components was examined after 25 (SCa) and 10 days (SCb), in motor and sensory nerves. SCa showed an increased transport velocity in motor (1.25 +/- 0.08 mm/day versus 1.01 +/- 0.05 mm/day) and in sensory nerves (1.21 +/- 0.13 mm/day versus 1.06 +/- 0.07 mm/day). The relative amount of labelled protein in the SCa wave in both fiber systems was also increased. SCb showed unchanged transport velocity in motor as well as in sensory nerves, whereas the amount of label was decreased in the motor system. Anterograde fast transport in motor nerves was examined after intervals of 3 and 5 h, whereas intervals of 2 and 4 h were used for sensory nerves. Velocities and amounts of labelled proteins of the anterograde fast component remained normal. We suggest that the increase in protein transport in SCa reflects axonal regeneration.

Effects of dithiobiuret intoxication on motor end plates in sternocostalis and hindlimb muscles of female rats

Daily dosing with 1-3 mg/kg dithiobiuret for 4-5 days causes progressive, generalised muscle weakness which is fatal in about 50% of cases on day 4 or 5. Survivors recover mobility by day 7 and appear normal, although still weak. Striking changes in the motor nerves and motor end plates can be observed before and during the development of this weakness, using the zinc iodide-osmium staining technique. The terminal internodes of intramuscular axons become densely stained: later this may extend back into the main intramuscular (i.m.) nerves, and is often followed by axonal degeneration. Many motor end plates lose their branching form and become globular, and profuse terminal sprouting develops before any nerve degeneration appears. Following axonal degeneration, collateral sprouting becomes prominent, and, within four weeks of beginning the dose regime, restores the normal appearance of the innervation. This pattern of response was observed clearly in the whole mounts of the sternocostalis muscle: similar but less marked changes occurred in the lumbrical muscles, while in the soleus and tibialis anterior muscles, loss of end plates seemed to be a more common response. Despite marked differences in the severity of the functional disability, the i.m. changes were similar in juvenile and adult rats. These changes are related to previous electrophysiological findings on the possible mechanism of action of dithiobiuret.

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 .