Origin of thalamically projecting somatosensory relay neurons in the immature rat.

The locations of spinothalamic (STT) and trigeminothalamic (TTT) neurons in 14-18-day rats using retrograde transport of fluorescent latex microspheres were examined. The aim of this study was to determine whether the connections between the somatosensory relay neurons and the thalamus were established in these immature rats. The majority of the labeled STT and TTT neurons was found in the brainstem and upper cervical cord (C1-C4). These cells were distributed to a number of distinct groups. Among them, the nucleus of trigeminal spinal tract interpolaris (SP51) had the highest number of TTT cells, and the internal basilar nucleus (IB) comprised the largest population of STT cells. Except for the cells located in the dorsal portion of the ventral horn (VHd), most of the labeled STT and TTT cells were contralateral to the thalamic injection sites. This pattern of distribution of the projecting neurons in immature rats is very similar to the pattern observed in adult rats. The development of the STT and TTT projections in 14-18-day rats is therefore largely complete. This result will allow us to extrapolate our understanding of the membrane properties of projection cells obtained in the immature rats to the adult rats.