Altered lipid metabolism in 2,5-hexanedione-induced testicular atrophy and peripheral neuropathy in the rat

Neurotoxic effects of 2,5-hexanedione on growing peripheral nerve axons of rat fetuses

Peripheral nerves of rat fetuses were used to investigate the potential neurotoxicity of 2,5-hexanedione (2,5-HD) on developing axons. Pregnant female Sprague-Dawley rats were injected subcutaneously with 680 mg/kg of 2,5-HD once a day from day 12 of gestation (GD12) through GD16. On GD20 live fetuses were removed from the uterus, and the sciatic nerves were examined morphologically. Upon electron-microscopy affected nerves showed axons which were aggregated and fused together; the number of large axons was increased, but there were no axons aggregated with neurofilaments.

Critical review of the toxicity of methyl n-butyl ketone: risk from occupational exposure

Methyl n-butyl ketone (MBK) was considered rather harmless until an outbreak of peripheral neuropathy occurred in 1973 among workers exposed to MBK. MBK easily penetrates the skin; pulmonary retention is approximately 80-85% in man. Distribution is widespread with highest levels in blood and liver; MBK also reaches the fetal tissues. MBK metabolism probably depends on the route of exposure, and is very similar to that of n-hexane. The critical organ is the nervous system. These effects find expression as peripheral neuropathy, with potential for serious effects of the central nervous system. From the viewpoint of neurotoxicity, 2,5-hexanedione is the most important metabolite. The neurotoxicity is potentiated by several compounds, while MBK itself potentiates the toxicity of other chemicals. From animal experiments, a no-adverse-effect level (NAEL) could not be established. Peripheral neuropathy may develop in workers exposed to only a few ppm of MBK. The difference in the Occupational Exposure Limits for MBK and n-hexane, as established by several organizations, is questioned in view of the neurotoxic effects of these substances.

Effects of methyl ethyl ketone pretreatment on hepatic mixed-function oxidase activity and on in vivo metabolism of n-hexane

1. Male Fischer-344 rats were given methyl ethyl ketone (MEK; 1.87 ml/kg), a potentiator of the neurotoxicity of n-hexane, by gavage for 4 days prior to a single inhalation exposure to n-hexane (1000 ppm). 2. Samples of blood, liver, testis and sciatic nerve were obtained and analysed for n-hexane, MEK and their metabolites by g.l.c.-mass spectrometry. 3. Pretreatment with MEK increased the concentrations of 2,5-hexanedione (2,5-HD; the proximal neurotoxin) in blood, sciatic nerve and testis relative to concentrations in the tissues in sham-treated controls. 4. Concentrations of 2,5-dimethylfuran, a metabolite of 2,5-HD, were increased in all four tissues tested. 5. After 1-7 days treatment with MEK, the activity of 7-ethoxycoumarin O-deethylase was increased (up to 500%), but benzphetamine N-demethylase activity was virtually unaffected. 6. Hence, the potentiating effects of MEK on the neurotoxicity of n-hexane appear to arise, at least in part, from the activating effects of MEK on selected microsomal enzymes responsible for n-hexane activation.

Rat testis during 2,5-hexanedione intoxication and recovery. II. Dynamics of pyrrole reactivity, tubulin content, and microtubule assembly

Charles River CD rats (200 g) were intoxicated with 1% 2,5-hexanedione (2,5-HD) in the drinking water for 5 weeks followed by a 17-week recovery period. Pyrrole reactivity of testis proteins increased early during intoxication and then returned toward normal during recovery. Testis tubulin content first increased as germ cells were lost and then fell over time while atrophy was maintained. Purified testis tubulin demonstrated a decreased nucleation time for microtubule assembly at 2 weeks, maintained this alteration throughout intoxication, and then returned to normal assembly kinetics during recovery. The assembly abnormality was accompanied by the presence of a unique crosslinked tubulin species. These findings support the hypothesis that alterations in Sertoli cell microtubules result in germ cell loss following 2,5-HD exposure.

Rat testis during 2,5-hexanedione intoxication and recovery. I. Dose response and the reversibility of germ cell loss

The histopathology of the testicular injury induced by 2,5-hexanedione (2,5-HD) exposure was examined in the rat. Charles River CD rats (200 g) were intoxicated by consuming 1% 2,5-HD in the drinking water or by intraperitoneal injection of the toxicant. Both neurotoxic and subneurotoxic exposures were studied, the total dose ranging from 40 to 211 mmol/kg. The following results were obtained: (1) there was a time delay between administration of the toxicant and development of the testicular injury, (2) Sertoli cell vacuolation in stages associated with the meiotic metaphase was the first histological sign of cellular injury at all doses, (3) subneurotoxic doses produced selective defects in germ cells in stages I-VIII of the spermatogenic cycle, (4) both subneurotoxic and neurotoxic doses produced germ cell necrosis and generalized sloughing of germ cells, and (5) intensive intoxication followed by a 17-week recovery period resulted in an absence of all postspermatogonial germ cells from the seminiferous epithelium of three of five treated rats. These data demonstrate that 2,5-hexanedione-induced testicular atrophy occurs at exposure levels below those producing clinical neurotoxicity and that, within the time frame of this study, the testicular injury is at least partially irreversible.

Structural basis of gamma-diketone neurotoxicity: non-neurotoxicity of 3,3-dimethyl-2,5-hexanedione, a gamma-diketone incapable of pyrrole formation

The chronic exposure to gamma-diketones results in the formation of giant neurofilament (NF)-containing axonal enlargements, followed by axonal degeneration in peripheral axons. Based on the specific ability of gamma-diketones to react with primary amino groups to form pyrroles, and the observation of such reaction with NF protein in vitro and with other proteins in vivo, it has been proposed that pyrrole formation at primary amino groups of NF protein is responsible for the neurotoxicity of gamma-diketones. We have tested this hypothesis through an investigation of the neurotoxicity in rats of 3,3-dimethyl-2,5-hexanedione (3,3-DMHD), a gamma-diketone which is incapable of forming pyrroles. 3,3-DMHD was found to produce only a slight alteration of axonal caliber and no clinical neurotoxicity after up to 12 weeks of administration, at a dose over 20 times that for which its isomer 3,4-dimethyl-2,5-hexanedione (3,4-DMHD) produced massive focal NF-containing axonal enlargements and complete paralysis in 4 weeks. These results support the view that the pyrrole-forming capability of gamma-diketones is the initial molecular event in the pathogenesis of gamma-diketone neurotoxicity.

Molecular mechanisms of diketone neurotoxicity

The important industrial and commercial solvents n-hexane and methyl n-butyl ketone undergo metabolic conversion in experimental animals and man to the neurotoxic gamma-diketone 2,5-hexanedione. Several molecular mechanisms of action have been proposed to explain the pathogenesis of gamma-diketone neuropathy. Such a mechanism must account for the target organ specificity, neurofilament accumulation, structure/activity relationships, in vivo covalent binding, and apparent direct axonal toxicity encountered in this syndrome. It has been proposed that the gamma-diketones exert their effects by reaction with sulfhydryl moieties of energy-producing axonal glycolytic enzymes, with resultant disruption of axoplasmic transport. Others have suggested that reaction instead occurs with lysine moieties of axonal cytoskeletal proteins to form alkyl pyrrole adducts, leading to damaging physicochemical changes in these proteins. Additional hypotheses involve inhibition of axonal sterologenesis, alterations in nerve membrane properties, and reduced neurofilament proteolysis within the nerve terminal. Although a comprehensive mechanism of action for the gamma-diketones remains to be demonstrated, much progress has been made toward this goal. Ultimate success awaits elucidation of the interactions of the neurotoxic diketones with axonal components at the molecular level. Previous reviews have addressed the historical, pharmacokinetic, and neuropathological aspects of this neuropathy. The present critique will examine proposed molecular mechanisms for the gamma-diketones with regard to theoretical considerations and experimental evidence.

Effects of hexafluoroacetone on Leydig cell steroidogenesis and spermatogenesis in the rat

Steroidogenesis was investigated in Leydig cell-enriched fractions isolated from the testes of rats dosed dermally with 130 mg/kg/day hexafluoroacetone (HFA) for 14 days and from pair-fed control rats. Compared to controls, Leydig cells from HFA-treated rats exhibited decreased incorporation of [14C]acetate (78%) and [3H]mevalonate (41%) into sterols and steroids. Testosterone was decreased 50% in the testes of HFA-treated rats. Incubation of Leydig cells from untreated rats with 1.0 mM HFA did not affect steroidogenesis. HFA treatment led to the development of histopathological lesions in the testes within 24 hr after a single dose; with daily dosing the lesions became progressively more severe. Despite the development of severe lesions, HFA treatment did not affect the blood levels of luteinizing hormone or testosterone; however, follicle-stimulating hormone was slightly elevated (48%) after 14 days of treatment. The data indicate that steroidogenesis is inhibited in Leydig cells of HFA-treated rats; the inhibition is not due to a direct or immediate effect of HFA, nor does it appear to be hormonally mediated.

Neurotoxic esterase in rooster testis

Neurotoxic esterase (NTE) is the putative target protein in the nervous system for the initiation of organophosphorus-induced delayed neuropathy. Here it is reported that NTE activity is present in rooster testis. Complete titration of rooster testis phenyl valerate esterases with paraoxon shows that about 15% of the enzymic activity is resistant to paraoxon. NTE activity after complete mipafox titration accounts for 30% of paraoxon-resistant phenyl valerate esterases and corresponds to 7.93 +/- 0.39 nmol/min/mg of protein (mean +/- SD, n = 7). Testis NTE is inhibited in vitro similarly to brain NTE by several organophosphorus compounds. Subcellular fractionation studies of the testis indicate that most NTE activity is particle bound. Testis NTE is also inhibited in vivo by several organophosphorus esters but to a lesser extent than brain NTE. Birds doses with organophosphorus compounds, causing delayed neuropathy, became grossly ataxic, but no testicular pathology was noted by light microscopy in roosters killed 15 days after administration. Serum testosterone levels also measured 15 days after dosing were not different from those of a control group. Recovery of NTE activity was faster in testis than in brain (4 days vs 6 days to recover to 50% of initial activity) in animals that received a high dose of an organophosphorus ester which cause delayed neuropathy.

Ultrastructural alterations in hexafluoroacetone-induced testicular atrophy in the rat

Testicular atrophy was induced in rats by dermal application of hexafluoroacetone (HFA) at 39 or 130 mg/kg/day for 14 days, but not at a dosage of 13 mg/kg/day. Affected germ cells were mostly spermatids and to a much lesser extent spermatocytes; spermatogonia were unaffected. Late spermatids were retained in Sertoli cells and showed degenerative changes. Sertoli cells exhibited cytoplasmic vacuolation, distended endoplasmic reticulum, and a marked increase in lipid droplets. Leydig cells exhibited a slight increase in lipid droplets, fewer mitochondria, and diminution and segregation of the agranular endoplasmic reticulum from mitochondria. A correlation between ultrastructural and biochemical changes in HFA-induced testicular atrophy is presented.

Electrophysiologic deficits in peripheral nerve as a discriminator of early hexacarbon neurotoxicity

To determine the extent of neurotoxicity of parenterally administered hexacarbons, male Sprague-Dawley rats were given either n-hexane or 2,5-hexanedione for 35 consecutive days. Electrophysiologic measurements showed a lengthening of the sciatic and sural nerve action potentials (slower conduction velocities) and increased refractory periods. These effects correlated with a shift in the nerve membrane sensitivity to potassium-induced depolarization. A similar effect can be induced by ouabain, an Na+, K+-ATPase inhibitor. These effects were seen with both n-hexane and 2,5-hexane-dione. Although the treated animals gained weight more slowly than controls, they showed no loss of motor function when tested behaviorally, and there were no signs of histopathology in the peripheral nerves. These results show that hexacarbons produce a neurotoxicity that can be demonstrated by changes in nerve excitability, prior to overt behavioral neurotoxicity. Furthermore, these electrophysiologic changes may be related to a hexacarbon-induced disruption of nerve-membrane ATPase activity.

The effect of 3,4-dimethyl substitution on the neurotoxicity of 2,5-hexanedione. I. Accelerated clinical neuropathy is accompanied by more proximal axonal swellings

The neurotoxicity of the gamma-diketone, 3,4-dimethyl-2,5-hexanedione, was studied in rats and compared to the known neurotoxicity of the parent compound, 2,5-hexanedione. The test compound was found to be 20 to 30 times more potent on a molar basis than hexanedione. In addition, unlike the distal axonal changes associated with hexanedione, the neurofilamentous swellings following exposure to the dimethyl analog occurred more proximally in the axon, with a preponderance in the anterior horn and lateral tracts of the spinal cord, and in the anterior roots. Since alkyl substitution causes branched-chain compounds to cyclize more rapidly than unbranched analogs, the greater neurotoxicity of the dimethyl compound implicates pyrrole formation in the pathogenesis of n-hexane neuropathy. Furthermore, the location of the axonal swellings induced with 3,4-dimethyl 2,5-hexanedione suggests that there is a common mechanism of injury for the entire class of neurofilament neuropathies, providing a continuum between the intraspinal swellings of beta, beta'-iminodipropionitrile (IDPN) and the distal axonopathies of 2,5-hexanedione, carbon disulfide, and acrylamide. In addition, lower doses of 3,4-dimethyl-2,5-hexanedione for longer periods of time led to a shift in the location of the axonal swellings to include more distal sites. These observations support the hypothesis that covalent crosslinking of the stable neurofilament is the primary event in the molecular pathogenesis of these toxic neuropathies, and that the rate of crosslinking of neurofilaments determines the proximodistal location of the axonal swelling.

Effects of hexafluoroacetone on testicular morphology and lipid metabolism in the rat

Rats dosed dermally with 39 or 130 mg/kg/day hexafluoroacetone sesquihydrate (HFA) for 14 days developed moderate or severe testicular atrophy, respectively; rats dosed with 13 mg/kg/day HFA for 14 days did not. Histologic evaluation of the testes revealed that spermatids, followed by spermatocytes, were the germ cells most affected by HFA; spermatogonia and Sertoli cells appeared to be less vulnerable. Lipogenesis from [3H]acetate and[14C]glucose was investigated in vitro in testes from HFA-treated and pair-fed control rats. Triacylglycerol and phospholipid synthesis was increased whereas sterol synthesis was decreased in testes from HFA-treated rats. Vitamin A and zinc were measured in the testes of control and HFA-treated rats; no differences in the levels of these nutrients were observed between the two groups. The data support the hypothesis that altered lipid metabolism, in particular sterol metabolism, is associated with the development of HFA-induced testicular atrophy.

The morphogenesis of testicular degeneration induced in rats by orally administered 2,5-hexanedione

The neurotoxic hexacarbon 2,5-hexanedione (2,5-HD) produces testicular atrophy in experimental animals. To examine the morphogenesis of the testicular lesion, 1.0% 2,5-HD was provided in the drinking water of adult F-344 rats for up to 6 weeks. After 3 weeks of administration, there were occasional large vacuoles in the basal region of the germinal epithelium. At 4 weeks, these vacuoles were much larger and more numerous; electron microscopy demonstrated that they were derived from the smooth endoplasmic reticulum. The vacuoles were preferentially associated with stages 12, 13, 14, and 1 of the spermatogenic cycle. Additionally, at 4 weeks there was a significant decrease in the number of tubules in stages 7 and 13, and a concomitant increase in the percentage of tubules in stages 3, 5, and 6. By Week 5, most Golgi-phase and cap-phase spermatids were visibly affected, showing margination of nuclear chromatin, and were becoming dissociated from Sertoli cells. Frequent multinucleated giant cells were seen and electron microscopy of these cells suggested that they were derived from fused spermatocytes or spermatids. After 6 weeks, fewer giant cells were present, most tubules contained cellular debris, and many showed empty lumina encircled by a thin ring of cytoplasm near the basement membrane. Interstitial tissue appeared unaffected. These studies indicate that the Sertoli cell is probably an initial target for 2,5-HD action in the testis.