Effect of organophosphate administration on the expression of pro-opiomelanocortin-derived peptides in motoneurones.

Adult male mice were treated with di-isopropylfluorophosphate (DFP), a neurotoxic organophosphate which crosses the blood brain barrier, or ecothiophate (phospholine iodide) which has been reported to be non-neurotoxic. The presence of immunoreactivity for the pro-opiomelanocortin (POMC)-derived peptides beta-endorphin and alpha-melanotropin was examined in intramuscular nerves in diaphragm, extensor digitorum longus (EDL) and soleus muscles, and in motoneurones in the cervical and lumbar regions of the spinal cord, up to 5 days after the treatment. The incidence of both immunoreactive intramuscular nerves and immunoreactive ventral horn cells was increased after administration of either DFP or ecothiophate but the extent of the increase and the time course depended on the muscle type or spinal cord region being examined. The effect of the treatment on the expression of POMC mRNA was investigated using in situ hybridization. The proportion of ventral horn cells which expressed the mRNA was significantly increased by 3h after ecothiophate administration. It reached a peak at 24 h but had returned to normal by 48 h. The results indicate that organophosphates can increase the expression of POMC-derived peptides in motoneurones whether the compounds cross the blood-brain barrier freely or not. This effect may be a consequence of their action to inhibit acetylcholinesterase at the neuromuscular junction and may be part of a regenerative response.