BLOCKING CA2-DEPENDENT SYNAPTIC RELEASE DELAYS MOTONEURON DIFFERENTIATION IN THE RAT SPINAL-CORD()

Development of motoneuron electrical properties and excitability was s tudied in spinal cord explants of rat embryos cultured for 1-3 weeks. The morphological organization of the spinal cord and synaptic inputs onto motoneurons were maintained in organ culture. The rate of differe ntiation of motoneuron resting potential and increase in membrane exci tability was similar in vitro and in vivo, suggesting that these prope rties were regulated by cellular signals or extracellular differentiat ion-promoting factors that were preserved in culture. However, maturat ion of input resistance, action potential threshold and action potenti al maximum rate of rise was slower than in vivo. Culturing spinal cord explants with their dorsal root ganglia attached did not facilitate m otoneuron differentiation. The role of newly formed synaptic pathways in regulating the changes in motoneuron electrical properties was stud ied in the presence of blockers of synaptic transmission. Motoneuron d ifferentiation was delayed in spinal cords cultured in the presence of TTX, indicating that electrical activity influenced the time course o f their development. However, blocking synaptic transmission with anta gonists of glutamate, glycine, and GABA(A) receptors did not affect th e rate of motoneuron differentiation, suggesting that maturation of mo toneuron phenotype was independent of activation of these transmitter- gated channels. Incubating spinal cords in medium containing high-K+, which increased the frequency of spontaneous potentials, reversed the inhibitory effect of TTX. Similar to TTX action, motoneuron developmen t was retarded when synaptic release was chronically blocked with eith er tetanus toxin or omega-conotoxin, a Ca2+ channel blocker. These fin dings suggested that electrical activity in spinal cord explants modul ated motoneuron differentiation via Ca2+-dependent synaptic release of neurotransmitters or neurotrophic factors.