DEVELOPMENTAL REGULATION OF 2 DISTINCT NEURONAL PHENOTYPES IN RAT DORSAL-ROOT GANGLIA

In a previous study we described two distinct neuronal phenotypes in r at dorsal root ganglia based on immunocytochemical assays for the neur onal intermediate filament proteins, peripherin and low-molecular-weig ht neurofilaments [Goldstein M. E. et al. (1991) J. Neurosci. Res. 30, 92-104]. In this paper we have extended this classification by using in situ hybridization to localize and evaluate the levels of various c ytoskeletal and neuropeptide messenger RNAs within the peripherin-immu noreactive and peripherin-immunoreactive-negative neurons found in emb ryonic day 15 and 20, postnatal day 2 and adult dorsal root ganglia. W e found in postnatal and adult dorsal root ganglia in vivo that the la rge, peripherin-immunoreactive-negative neurons, which are intensely s tained by low-molecular-weight neurofilament antibodies, also contain high levels of low, medium and high-molecular-weight neurofilament mes senger RNAs, whereas the smaller peripherin-immunoreactive neurons do not. On the other hand, both cell types contained comparable levels of peripherin and alpha-tubulin messenger RNA. The presence of peripheri n messenger RNA but no peripherin immunoreactivity in the large cells suggested either a translational or post-translational regulation of t his polypeptide; or rapid clearance of this protein from the perikaryo n into the axon. In adult dorsal root ganglia, more than 50% of the pe ripherin-immunoreactive neurons also contained high levels of substanc e P and/or calcitonin gene-related peptide messenger RNAs, while less than 20% of the large peripherin-immunoreactive-negative neurons did. The attainment of these phenotypic characteristics during development in vivo was studied by northern blot and in situ hybridization histoch emistry. In early embryonic stages (embryonic days 15-16), virtually a ll neurons were peripherin-immunoreactive and were positive for periph erin, alpha-tubulin and low-molecular-weight neurofilament messenger R NAs, suggesting a homogeneous population. By embryonic day 20, the two adult phenotypes became clearly evident, and were fully established b y postnatal day 2. In cultures of embryonic day 15 dorsal root ganglio n neurons grown in the presence of nerve growth factor, peripherin and low-molecular-weight neurofilament messenger RNAs were expressed in a ll neurons, even after nine days in vitro, similar to embryonic dorsal root ganglia in vivo. Nerve growth factor supplemented by skeletal an d heart muscle extracts did up-regulate neurofilament gene expression, but not to the extent characteristic of the peripherin-immunoreactive -negative adult phenotype. These results suggest that development of t he mature phenotype of dorsal root ganglion neurons occurs by postnata l day 2 in vitro and is dependent upon target contact and/or target-de rived factors.