Modulation of embryonic chick motoneuron glutamate sensitivity by interneurons and agonists

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Abstract
Embryonic chick motoneurons grown in culture together with other spinal cord cells are more sensitive to L-glutamate than are sorted motoneurons grown in isolation. After 6 d in vitro, the difference in peak sensitivity reached 6-fold. Comparable increases in aspartate and kainate currents were observed, indicating that both G1 and G2 amino acid receptors were affected. Elimination of proliferating non-neuronal cells from mixed spinal cord cell cultures by addition of cytosine arabinoside (ara C) did not prevent the increase in motoneuron chemosensitivity, so the induction is probably due to the presence of interneurons. In contrast to their effect on glutamate response, interneurons did not affect the sensitivity of motoneurons to the inhibitory neurotransmitters GABA and glycine. Glutamate receptors expressed by sorted and unsorted motoneurons are identical in terms of their ED50, reversal potential, mean channel open time, and conductance, implying that the increased sensitivity of motoneurons in mixed cultures is due to an increase in the number of open channels. In addition to an increase in the number of channels, the distribution of glutamate sensitivity over the surface of individual motoneurons was altered in interneuron-containing cultures. The sensitivity of isolated motoneurons was greatest at the soma and decreased with distance along major processes, but the sites of highest sensitivity on motoneurons in mixed cultures occurred along their processes. Sharp peaks identified by focal iontophoresis of glutamate were separated by areas of lower sensitivity. The inductive effect of interneurons cannot be due to glutamate, the most likely excitatory interneuron-motoneuron transmitter in 6 d chick cultures.(ABSTRACT TRUNCATED AT 250 WORDS)