Beneficial action of glucocorticoid treatment on neuromuscular transmission during early motor nerve degeneration

The effect of high-dose methylprednisolone on laser-induced retinal injury in primates: an electron microscopic study

BACKGROUND

Previously we reported an ameliorative effect of high-dose methylprednisolone in laser injury to monkey retinas. The ultrastructural modification by methyl-prednisolone has not been examined.

METHODS

Cynomolgus monkeys were given severe (grade III) retinal laser burns and treated with an intravenous megadose of methylprednisolone. Pathologic features of the retinal lesions with or without methylprednisolone treatment were evaluated by light and electron microscopy.

RESULTS

Ultrastructurally, the treated lesions showed rapid recanalization of choriocapillaris; proliferation of retinal pigment epithelium to replace the necrotic and damaged cells, resulting in rapid re-establishment of blood retinal barrier; mild macrophagic activity; and rapid reformation of the outer limiting membrane by Mueller cells.

CONCLUSION

A high dose of methylprednisolone affected the responses of the choriocapillaris, retinal pigment epithelium, photoreceptor cells and Mueller cells to laser injury, showing an overall beneficial effect. These modifications might be ascribed to methylprednisolone's anti-inflammatory action, protection of the microcirculation and anti-lipid peroxidation effect.

Neuroprotective actions of glucocorticoid and nonglucocorticoid steroids in acute neuronal injury

1. The glucocorticoid steroid methylprednisolone (MP) has been shown to enhance chronic recovery after human spinal cord injury when administered in a 24-hr high-dose regimen beginning within 8 hr. The doses of MP that affect this improved recovery have been demonstrated to inhibit posttraumatic spinal cord lipid peroxidation (LP), which has been postulated to be a key event in the secondary injury-induced degenerative cascade. 2. The molecular mechanism of action of the steroid appears to involve intercalation into the cell membrane and blockade of the propagation of peroxidative reactions. At a physiological level, the inhibition of injury-induced LP has been found to result in an attenuation of progressive posttraumatic ischemia and energy failure together with an augmented reversal of intracellular calcium accumulation. However, MP also acts directly to retard secondary neuronal degeneration as observed in studies showing the steroid's ability to slow the anterograde degeneration of experimentally injured cat soleus motor nerves. 3. The duplication of this effect by the nonsteroidal lipid antioxidant alpha-tocopherol supports the notion that is indeed a manifestation of the inhibition of posttraumatic LP. Moreover, the efficacy of MP in limiting lipid peroxidation and secondary spinal cord or motor nerve degeneration has also been duplicated by a nonglucocorticoid 21-aminosteroid tirilazad mesylate (U-74006F), which suggests the independence of the antioxidant and glucocorticoid effects of MP.

Preservation of motor nerve function during early degeneration by the 21-aminosteroid anti-oxidant U74006F

The effects of 5 days of pretreatment with the 21-aminosteroid anti-oxidant U74006F have been examined on the rate of functional degeneration of cat soleus motor nerve terminals after axon section. Female cats were dosed for 5 days with either 7.7, 13.0 or 30.0 mg/kg (average doses) of U74006F p.o. twice daily followed by unilateral sciatic nerve section at the hip level on day 5. On day 7, the bilateral in vivo soleus nerve muscle prep. was set up to assess the neuromuscular functional status of the 48 h degenerating soleus nerve terminals in comparison to the contralateral non-sectioned preparation. In untreated cats, the ratio of the nerve-evoked (0.4 Hz) contractile tension of the 48 h nerve-sectioned to that of the contralateral non-sectioned was only 52 +/- 8%. U74006F pretreatment produced a dose-related improvement with the 13.0 mg/kg dose having the best effect; the ratio was 86 +/- 5% (P less than 0.01 vs untreated). The maintenance of tetanic tension during a 10 s period of 100 Hz nerve stimulation was also improved by the 13.0 mg/kg dose from only 54.0 +/- 5.2% in untreated animals to 72.2 +/- 5.7 (P less than 0.02). These results show a preservation of motor nerve function during early degeneration by the anti-oxidant U74006F thus providing further evidence for a free radical-mediated process in anterograde degeneration.

Nerve conduction studies in chickens given phenyl saligenin phosphate and corticosterone

Clinical signs of delayed neuropathy were induced in adult white leghorn chickens given the organophosphorus ester phenyl saligenin phosphate (PSP, 2.5 mg/kg im) 22-24 d before assessment of nerve conduction parameters. Damage to the myelinated sensory portion of the sciatic nerve was indicated by abnormal compound action potentials in treated chickens. In particular, the amplitude of the A beta response was markedly reduced. In addition, the A beta fibers did not respond normally to increasing stimulation intensity. These parameters were more like controls in chickens that had been given PSP and 30 ppm corticosterone for 11 d, beginning 1 d before PSP administration. These studies indicated that nerve conduction parameters could distinguish peripheral nerve damage in chickens given PSP and improvement could be noted in chickens treated with corticosterone.

Types of adrenocorticoids and their effect on organophosphorus-induced delayed neuropathy in chickens

The present study examined the effects of a glucocorticoid and a mineralocorticoid on organophosphorus-induced delayed neuropathy (OPIDN) as previous investigations have indicated that an endogenous steroid with both properties could alter this syndrome in chickens. The glucocorticoid triamcinolone and the mineralocorticoid deoxycorticosterone were provided in the diet beginning 1 day before and continuing 10 days after triortho-tolyl phosphate (TOTP, 360 mg/kg po), phenyl saligenin phosphate (PSP, 2.5 mg/kg im), and diisopropyl phosphorofluoridate (DFP, 1 mg/kg sc). In a manner similar to that seen with corticosterone, a low concentration (0.1 ppm) of triamcinolone reduced and a high concentration (10 ppm) exacerbated clinical signs. Concentrations of deoxycorticosterone under 80 ppm also partially delayed or ameliorated ataxia induced by TOTP, PSP, and DFP, but a combination of 0.1 ppm triamcinolone and 80 ppm deoxycorticosterone was not more effective than triamcinolone alone. Peripheral nerve damage was noted in all chickens given organophosphorus compounds, whether or not they had been given corticoids. Both steroids induced hydroxylase activity, but effects on most other enzyme systems examined were unremarkable. High concentrations of triamcinolone (10 ppm) could, however, also reduce liver cytochrome P450 levels and liver cholinesterase activity. Exacerbation of OPIDN was most notable in chickens under highest stress, as indicated by elevated heterophil-to-lymphocyte ratios. The clinical, pathological, biochemical, and hematological indices of exposure to adrenocorticoids and agents inducing OPIDN in chickens were, therefore, similar for both a synthetic glucocorticoid and the endogenous steroid corticosterone.

Intensive anti-oxidant pretreatment retards motor nerve degeneration

Intensive pretreatment of cats with a combination of the antioxidants D-alpha-tocopherol (200 IU) and selenium (50 micrograms) once daily for 5 days (p.o.) was found to significantly preserve the functional capacity of degenerating soleus motor nerve terminals (measured at 48 h after axon section at the hip) in the in vivo soleus nerve-muscle preparation. The preservation of function was apparent in terms of: a greater soleus contractile response to nerve stimulation at low frequencies, a more rapid recovery from D-tubocurarine-induced neuromuscular block, and a better maintenance of tetanic contractile tension during high-frequency nerve stimulation. The ability of antioxidants to retard the anterograde axonal degeneration (i.e. 'Wallerian') process suggests that lipid peroxidation may be a fundamental mechanism of neuronal degeneration.

Altered regulation of lesion-induced synaptogenesis by adrenalectomy and corticosterone in young adult rats

Quantitative electron microscopy was used to examine the effect of circulating glucocorticoids on the removal of degenerating synapses and the replacement of lost synaptic contacts in young adult rats that follow partial denervation of the hippocampal dentate gyrus. Subjects were adrenalectomized prior to subcutaneous implantation of pellets containing a specified concentration of corticosterone and subsequent unilateral ablation of the entorhinal cortex. Animals maintained at high circulating concentrations of glucocorticoids were significantly retarded in the early phase of degenerating synapse removal and in the rate of synaptic replacement. Subjects maintained at extremely low concentrations of glucocorticoids were also significantly retarded in the early stages of synapse removal but showed an early replacement of lost synaptic contacts followed by a dramatic decrease in the rate of replacement. By 60 days after the lesion both groups of animals showed synapse replacement equivalent to young adult controls while significant amounts of degenerating synapses still remained in the denervated neuropil. The results demonstrate that circulating glucocorticoids can exert a marked influence on lesion-induced synaptic replacement in the hippocampal dentate gyrus.

Effect of supplemental corticosterone and social stress on organophosphorus-induced delayed neuropathy in chickens

Adult White Leghorn roosters maintained for 3 months in an environment of low social stress (LSS) were given triorthotolyl phosphate (TOTP), 180 mg/kg po, 24 h after initial exposure to either an environment of high social stress (HSS) or exogenously administered corticosterone (200 ppm in diet). The environment of HSS or the administration of corticosterone were continued for 10 days after TOTP administration. It was noted that the heterophil/lymphocyte ratio, a hematological indicator of stress, was similar in both groups and both had similar scores for clinical signs of delayed neuropathy. Both heterophil/lymphocyte ratios and clinical scores were significantly lower in chickens kept in the LSS environment and not fed corticosterone than in chickens kept in the HSS environment or fed corticosterone.

Methylprednisolone treatment of an organophosphorus-induced delayed neuropathy

Diisopropylfluorophosphate (DFP) injected into the femoral artery of cats causes a localized organophosphorus-induced delayed neuropathy (OPIDN). Gait disturbances develop in the treated leg 14 days after DFP exposure and reaches a maximum at 21 to 28 days after DFP. In vivo high-frequency conditioning of soleus motor nerve endings evokes stimulus-bound repetitive neural discharges (SBR) and an obligatory potentiation of the muscle contractile response (PTP). In this OPIDN model, SBR and PTP are maximally suppressed at 21 to 28 days after DFP. A high-dose regimen of methylprednisolone started 30 to 40 min after DFP exposure and lasting for 20 days prevented the development of OPIDN. In the methylprednisolone-DFP treated cats, SBR and PTP functions were not suppressed and not different from those in untreated normal cats.

Target neurons for [3H]corticosterone in the rat spinal cord

Rats were injected with [3H]corticosterone and their spinal cords processed for autoradiography. Nuclear concentration of the tritiated steroid was observed in motor neurons in cervical, thoracic, lumbar, and sacral regions. Nuclear concentrations of [3H]corticosterone was also observed in certain neurons of laminae VII and III in all levels of the spinal cord. Injection of unlabeled corticosterone 5 min before the [3H]corticosterone injection abolished the nuclear concentration of the adrenal steroid. These results provide the morphological basis for a direct genomic action of corticosterone in spinal motor neurons and in neurons of laminae III and VII.

Methylprednisolone preservation of motor nerve function during early degeneration

Intensive pretreatment of cats with methylprednisolone acetate (8 mg/kg, i.m. once daily for 7 days) was found to significantly preserve the functional capabilities of degenerating soleus motor nerve terminals, as measured at 48 h after motor axon section in the in vivo soleus nerve-muscle preparation. This effect is similar to that reported previously for triamcinolone, another glucocorticoid.