Development and further characterization of a small subclass of rat olfactory receptor neurons that shows immunoreactivity for the HSP70 heat shock protein

We previously described a rat olfactory receptor neuron (ORN) subpopulation [the 2A4(+) ORNs] that shows uniquely strong reactivity with antibodies to the 70-kD heat shock protein (HSP70) family of molecular chaperones (Carr et al. [1994] J. Comp. Neurol. 348:150-160). The 2A4(+)ORNs are dispersed t hrough zones II-IV of the olfactory epithelium (OE), and their axons projec t to only two or three glomeruli that are located consistently in each olfa ctory bulb (OB). To date, the 2A4(+)ORN subpopulation is the only cell popu lation to show such distinct HSP70 immunoreactivity as well as the most dis crete ORN subpopulation to be so labeled. The present report shows that 2A4 (+)ORN neurons first appear between postnatal days 7 (P7) and P10. Initiall y, low cell numbers rise to a density of 0.1 2A4(+)ORNs/mm OE length by P14 , plateau at 0.9 2A4(+)ORNs/mm by P49, then fall to adult values of 0.4 cel ls/mm. Autoradiographic birthdating indicates that almost all of these earl y appearing 2A4(+)ORNs are generated postnatally, in contrast to the prenat al generation of all ORN subpopulations characterized to date by their expr ession of olfactory receptor protein mRNAs. A developmentally related incre ase in the mean depth of 2A4(+)ORNs within the OE also occurs. In the OB, i nitial 2A4(+)axonal projections are to only two or three glomeruli, as in a dults. Slight but significant rostral shifts in (+)glomerular location occu r with development. The 2A4( +)ORN immunoreactivity was found to be due to expression of HSP70, the dominant stress-inducible member of the HSP70 fami ly, rather than constitutively expressed HSC70. In addition, despite their presence in rat OE, no 2A4(+)ORNs were found in mice, gerbils, guinea pigs, or hamsters. (C) 1999 Wiley-Liss, Inc.