Macrowell cultures identify a subpopulation of neonatal rat dorsal root ganglionic neurons displaying nerve growth factor independent survival

The role of glial cells in influencing neurite extension by dorsal root ganglion cells

When pretreated with pertussis toxin (PTX), the neurites of adult rat dorsal root ganglion (DRG) cells in mixed cell cultures retract over a period of 2 h following the initial stimulus of removal from the cell culture incubator for brief periods of observation. The purpose of this investigation was to determine whether this PTX-dependent response was specific to any one of the three subpopulations of DRG neurons. However, no neurite retraction response was observed in neuron-enriched populations of cells, or in cultures enriched in isolectin B4 (IB4)-positive neurons or in IB4-negative neurons. But, the addition of non-neuronal cells, and/or medium conditioned by non-neuronal cells, was sufficient to restore the PTX-dependent neurite retraction response, but only in large diameter IB4-negative neurons. In conclusion, we have identified a regulatory response, mediated by Gi/o-proteins, which prevents retraction of neurites in large diameter IB4-negative cells of adult rat DRG. The non-neuronal cells of adult rat DRG constitutively release factor/s that can stimulate neurite retraction of a subset of isolated DRG neurons, but this property of non-neuronal cells is only observed when the Gi/o-proteins of large diameter IB4-negative cells are inhibited.

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 neurons that were previously identified as preferential targets of enkephalins. This group, termed P-neurons after their "pear" shape, sequentially required nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) for survival in vitro during different developmental stages. Embryonic P-neurons required NGF, but not bFGF. NGF continued to promote their survival, although less potently, up to postnatal day 2 (P2). Conversely, at P5, they needed 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 their survival during embryonic and postnatal stages, respectively. The expression of NGF (Trk-A) and bFGF (flg) receptors paralleled the switch in trophic 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 specific marker of these cells. Immunosuppression of bFGF in newborn animals greatly reduced their number, suggesting that the factor was required in vivo. bFGF was present in the DRG and spinal cord, as well as in skeletal muscle, 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 for the neurofilament of 200 kDa distinguished them from muscle proprioceptors, suggesting that they are likely to be unmyelinated muscle fibers. Collectively, their properties indicate that P-neurons constitute a distinct subpopulation of sensory neurons for which the function may be modulated by enkephalins.