The development of responsiveness to substance P and glutamate in the spinal motoneurons of rat fetuses.

The development of responsiveness of motoneurons to substance P (SP) and glutamate was investigated in the isolated spinal cord of the rat fetus at embryonic days 13.5-21.5. The motoneurons at embryonic day 14.5 first responded to SP with a slow depolarization. The responsiveness to SP increased by embryonic days 19.5-21.5. In a low Ca2+ solution, responsiveness was reduced after embryonic day 17.5. Glutamate caused a slow depolarization of motoneurons from embryonic day 13.5. The responsiveness increased until embryonic day 17.5 and decreased thereafter.