Axotomy-induced apoptosis in adult rat primary sensory neurons

Neuronal death following unilateral axotomy of a sensory nerve has long been inferred from neuronal counts of dorsal root ganglion neurons, using the contralateral ganglia as a control. The counting methods used usually involved the counting of neuronal nucleoli and made assumptions about them which could conceivably be flawed. Very few studies have used direct observations of dying or degenerating neurons to address questions concerning the duration of the period of neuronal death or the mechanisms involved in this process. Here we describe a morphological, morphometric and histochemical study into the nature and duration of sensory neuron death following transection and ligation of the sciatic nerve at mid-thigh level in the adult rat. We show that at least some of this neuronal loss occurs by apoptosis as defined by morphological criteria and in situ end-labelling of damaged DNA. Absolute numbers of apoptotic neurons were counted from serial paraffin sections of ganglia and estimates of neuronal numbers obtained by disector analysis at 1, 2, 3 and 6 months after axotomy. Using this approach we show that axotomy-induced apoptosis begins at around 1 week and continues up to at least 6 months after axotomy.