CROSSED PROJECTION NEURONS ARE GENERATED PRIOR TO UNCROSSED PROJECTION NEURONS IN THE LATERAL SUPERIOR OLIVE OF THE RAT

The present study examined in the lateral superior olive (LSO) of the rat whether LSO neurons projecting to the ipsilateral inferior collicu lus (IC) might be generated later than those projecting to the contral ateral IC. Rat fetuses were exposed in utero to 5-bromodeoxyuridine (B rdU), a thymidine analogue, to label neurons proliferating at differen t embryonic stages from day Ell through to E20. Upon reaching adulthoo d, the rats were given unilateral injections of fluoro-gold (FG), a re trograde fluorescent tracer, into the IC. Subsequently, the tissue sec tions of the brains obtained from the rats were immunostained for BrdU to simultaneously detect neurons that were BrdU-positive and/or FG-po sitive. BrdU-positive LSO neurons were found in the rats which had bee n exposed to BrdU during E12-E16. In E12 and E13 BrdU-exposure cases, the vast majority of double-labeled (BrdU-positive and FG-positive) ne urons were seen on the contralateral side to the FG injection. In E14, E15 and E16 BrdU-exposure cases, in contrast, all double-labeled neur ons were found on the ipsilateral side to the FG injection. The distri bution of these double-labeled neurons within the nucleus was diffuse in all the BrdU-exposure cases. Thus, the results indicate that LSO ne urons are generated during E12-E16, that the crossed projection neuron s are generated 1-4 days earlier than the uncrossed projection neurons , and that no topographical relationships exist between the early- and the late-generated populations of the LSO neurons.