NEURONAL AND NON-NEURONAL CELL RESPONSES TO PERIPHERAL NERVE INJURY IN BALB/C, ATHYMIC, C57BL/6J AND C57BL/WLD{SUPERSCRIPT S} MICE

Abstract

A review of the literature reveals that the mechanisms of neuron death and the responses of non-neuronal cells to peripheral nerve injury are not fully known. This study combined immunohistochemistry and nicotinamide adenine dinucleotide hydrogen phosphate-diaphorase (NADPH-d) histochemistry with electron microscopy to investigate the neuronal and non-neuronal cell responses to peripheral neurectomy in different strains of neonatal and adult mice. The study addressed several issues concerning neuron death and responses of microglia, astrocytes and macrophages after peripheral nerve injury. Athymic, C57BL/6J and C57BL/W/d' mice were used, each of which has special characteristics best suited for the designs of the various experiments. BALB/c mice were used as control. The results showed that apoptotic cells were present in the white and gray matters of the L4 cord segment of newborn BALB/c mice. Their numbers gradually declined to a constant level at 4 days after birth. After sciatic (Sc) neurectomy in 5-day-old BALB/c mice, apoptotic glioblasts in the process of transforming into oligodendrocytes, apoptotic neurons which were probably interneurons, and some unidentifiable apoptotic cells were observed in the dorsal horn and dorsal column of the L4 cord segment as early as 4 hours after Sc neurectomy. These peaked at 16 hours and were hardly observed at 2 days after operation. Necrotic motoneurons were observed in the ventrolateral region of the L4 cord segment and about 60% of the motoneurons were eventually lost at 15 days after Sc neurectomy in 5-day old BALB/c mice. Although an increase in NADPH-d expression in the ventrolateral region of the L4 cord segment was noted after Sc neurectomy in 1-day-old BALB/c mice, there was no such increase after Sc neurectomy in 5-day-old BALB/c mice. Nitric oxide was not responsible for the death of the axotomized motoneurons. The glutamate receptor subtype 2/3 in the Sc motoneurons was downregulated after Sc neurectomy in both neonates and adults. The significance of this finding awaits pharmacological elucidation. Although microglia did not play any role in nerve regeneration, they might exert some neuroprotective functions. When present in smaller number and not being able to adequately express the complement receptor type 3 (CR3) antigenicity as in the neonate, they might fail to redeem the axotomized Sc motoneurons from death. This is supported by the observation of a faster motoneuron death in neonatal athymic mice which showed a smaller number of microglia and a tardy expression of their CR3 antigenicity when compared to their littermate BALB/c mice. Astrocyte response to axotomy occurred faster in neonates than in adults despite the fact that astrocytes were less developed in neonates. Of the many possibilities, the different functional status of the astrocytes between neonate and adults should be considered. The results of the study had also shed light on the signals eliciting microglia and macrophage responses. The different time frames for microglia reaction and macrophage recruitment after common peroneal (CP) or Sc nerve injury in C57BL/6J and C57BL/Wld' mice is a clear indication that different types of signals were involved. The signal for macrophage recruitment could be the presence of degenerated materials. The signal for microglia response could be simply a cut or crush of the peripheral nerve which alerts the microglia for some neuroprotective function, failing which they would become phagocytotic, a phenomena that has been demonstrated in the neonates in this study. The results of the present study may have possible therapeutic applications in the treatment of neuronal traumas or diseases. In addition to providing neurotrophic factors for damaged neurons, additional measures may be taken to strengthen the neuroprotective functions of the surrounding microglia and astrocytes.