Neurotoxic action of 2,5-hexanedione on the autonomic nervous system: ultrastructural and functional alterations in the rat sympathetic superior cervical ganglion

Proapoptotic effects of 2,5‑hexanedione on pheochromocytoma cells via oxidative injury

N‑hexanes are prominent environmental pollutants that are able to cause neurotoxicity in vivo and in vitro. Central and peripheral neuropathies induced by n‑hexane exposure are a major health concern. 2,5‑Hexanedione (2,5‑HD) is the most significant neurotoxic metabolite of n‑hexane; however, little is known regarding the underlying mechanism of its neurotoxicity. Thus, the aim of the present study was to investigate the damaging effects of 2,5‑HD on pheochromocytoma PC12 cells, and to explore the underlying mechanism. Cell viability was tested using a Cell Counting Kit‑8 method, and the leakage of lactate dehydrogenase (LDH) from cells was measured using an LDH assay kit. Glutathione peroxidase (GSHPx) and superoxide dismutase (SOD) activities, and the level of malondialdehyde (MDA) were determined using corresponding assay kits. Apoptotic cells were detected using an annexin V‑fluorescein isothiocyanate/propidium iodide (PI) apoptosis kit, and were subsequently observed by fluorescence microscopy. The relative expression levels of cleaved‑caspase‑3, Bcl‑associated‑X protein (Bax) and Bcl‑2 were identified by western blotting. The results revealed that 2,5‑HD was able to decrease the viability of PC12 cells and promoted the leakage of LDH in a concentrationdependent manner. Further analysis demonstrated that 2,5‑HD decreased the activity of the antioxidative enzymes, SOD and GSHPx, and led to an increase in the levels of MDA in the supernatant of cultured PC12 cells. The annexin V/PI staining results revealed that the numbers of apoptotic cells were increased following treatment with 2,5‑HD. In addition, 2,5‑HD (5 and 10 mmol/l) led to significant increases in the expression levels of caspase‑3 and Bax, with the concomitant downregulation of Bcl‑2. The antioxidant N‑acetylcysteine was identified to antagonize 2,5‑HD‑stimulated cleaved‑caspase‑3 and Bax upregulation, and Bcl‑2 downregulation. Collectively, the results of the present study suggested that 2,5‑HD exerts proapoptotic effects on PC12 cells via oxidative injury. These findings may be applied in the development of novel therapeutic strategies to treat neurological disorders associated with nhexane exposure.

Atropine reverses antinociception induced by 2,5-hexanedione in rats

2,5-Hexanedione is a n-hexane metabolite with neurotoxic properties. We have previously demonstrated that acute administration of 2,5-hexanedione causes analgesia in the tail-flick test in rats. In the present investigation, we examined the possible involvement of a cholinergic component in the 2,5-hexanedione-induced antinociception, since literature data indicate that this hexacarbon compound may act as a competitive inhibitor of acetylcholinesterase and that cholinesterase inhibitors are analgesic to rodents. Rats were treated with saline or with 5 or 25 mg/kg atropine (intraperitoneally) 10 min. before the injection of vehicle or 800 mg/kg 2,5-hexanedione (intraperitoneally). 2,5-Hexanedione caused a significant increase in tail-flick latencies at 10, 30, 60 and 90 min. after hexacarbon injection. Atropine (5 or 25 mg/kg) partially reversed the analgesia caused by 2,5-hexanedione at 60 and 90 min. When effects of 2,5-hexanedione on brain acetylcholinesterase was assessed in vitro, the results demonstrated that a competitive component is involved in enzyme inhibition. Taken together, these data support the involvement of a cholinergic (muscarinic) component in 2,5-hexanedione-induced analgesia.

Total number and mean cell volume of neocortical neurons in rats exposed to 2,5-hexanedione with and without acetone

The toxicological effects of 2,5-hexanedione (2,5-HD) alone and combined with acetone on the number and size of neurons in the cerebral cortex of rats were evaluated with stereological techniques. Thirty rats were equally divided into three groups: One control, one receiving 0.5% 2,5-HD, and one receiving 0.5% 2,5-HD and 0.5% acetone in the drinking water for seven weeks. Unbiased estimates of the total number of neocortical neurons, as well as the mean neuronal nuclei and cell body volumes were obtained from systematically sampled 3.5-microns sections. The total number of neurons in the 2,5-HD group was significantly smaller than the control group (p less than 0.05, one-tailed t-test). Both test groups showed significant changes in the mean cell body volume: Compared with the control group, animals exposed to 2,5-HD had 11% smaller cell body volumes while animals exposed to 2,5-HD and acetone had 13% larger cell body volumes. These data represent the first unbiased estimation of mean cell volume in toxicology. We propose the nucleator method as an efficient and accurate tool for estimating quantitative changes in toxicological research.

2,5-Hexanedione-induced accumulations of neurofilament-immunoreactive material throughout the rat autonomic nervous system

In rats intoxicated with 2,5-hexanedione, nerve fibres supplying virtually all visceral organs showed large numbers of densely immunoreactive accumulations of neurofilament-like material, of fusiform, elongated, smoothly tapering morphology. In the gut, round to oval, morphologically different lesions were also present, and abnormal neurofilament-immunoreactive accumulations were revealed in oesophageal terminal end-plates. An extensive damage to autonomic nerve fibres, which are largely non-myelinated, was thus revealed in 2,5-hexanedione intoxication. The observed diversity in lesion morphology may suggest heterogeneity in cytoskeletal and/or associated proteins among autonomic neurons.

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 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.