A sensory neuron subpopulation with unique sequential survival dependence on nerve growth factor and basic fibroblast growth factor during development

We characterized a subpopulation of dorsal root ganglion (DRG) sensory neur ons that were previously identified as preferential targets of enkephalins. This group, termed P-neurons after their "pear" shape, sequentially requir ed nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) for survival in vitro during different developmental stages. Embryonic P-neuron s required NGF, but not bFGF. NGF continued to promote their survival, alth ough less potently, up to postnatal day 2 (P2). Conversely, at P5, they nee ded bFGF but not NGF, with either factor having similar effects at P2. This trophic switch was unique to that DRG neuronal group. In addition, neither neurotrophin-3 (NT-3) nor brain-derived neurotrophic factor influenced the ir survival during embryonic and postnatal stages, respectively. The expres sion of NGF (Trk-A) and bFGF (flg) receptors paralleled the switch in troph ic requirement. No single P-neuron appeared to coexpress both Trk-A and flg . In contrast, all of them coexpressed flg and substance P, providing a spe cific marker of these cells. Immunosuppression of bFGF in newborn animals g reatly reduced their number, suggesting that the factor was required in viv o. bFGF was present in the DRG and spinal cord, as well as in skeletal musc le, the peripheral projection site of P-neurons, as revealed by tracer DiIC (18)3. The lack of requirement of NT-3 for survival and immunoreactivity fo r the neurofilament of 200 kDa distinguished them from muscle proprioceptor s, suggesting that they are likely to be unmyelinated muscle fibers. Collec tively, their properties indicate that P-neurons constitute a distinct subp opulation of sensory neurons for which the function may be modulated by enk ephalins.