Effects of amyloid peptides on cell viability and expression of neuropeptides in cultured rat dorsal root ganglion neurons: a role for free radicals and protein kinase C

Chronic pain caused by nerve injury and inflammation is more common in the elderly. However, mechanisms underlying this phenomenon are unclear. Higher sensitivity of sensory neurons to free radicals has been suggested as one possibility. The production of free radicals can be induced by various agen ts, including the highly toxic protein beta -amyloid (A beta), which is fou nd in higher amounts in the brains of Alzheimer's Disease patients. In dors al root ganglion (DRG) cultures exposed to A beta, we examined cellular tox icity and peptide expression, in particular calcitonin gene-related peptide (CGRP), a peptide which is abundantly expressed by nociceptive afferents a nd is known to be involved in pain processes. Exposure of cultured rat DRG neurons to A beta (25-35) or A beta (1-40) (10 or 20 mum for 24-96 h) incre ased trypan blue-stained cells in a concentration- and time-dependent manne r, thus, indicating cellular toxicity. These treatments also increased the number of CGRP immunoreactive (IR) neurons while decreasing the number of n europeptide Y- and galanin-IR neurons. The free radical scavenger, superoxi de dismutase, attenuated both the toxicity and neuropeptide changes induced by A beta, thus, suggesting that oxidative stress probably contributes to these effects. Exposure of cultured DRG neurons to A beta also increased th e number of protein kinase C alpha (PKC alpha)-IR neurons. The PKC inhibito rs, chelerythrine chloride and Go6976, significantly augmented A beta -indu ced cellular toxicity while attenuating the increases in CGRP-and PKC alpha -IR cells, supporting the notion of a protective role for PKC in A beta in sults. These in vitro data suggest that A beta peptides may, in addition to causing neurotoxicity, regulate neuropeptide expression in primary afferen ts. This finding could be relevant to the higher incidence of neuropathic p ain that occurs with ageing.