In the present study the effects of microiontophoretically applied acetylcholine and NMDA were investigated on neurones of the avian nucleus isthmi. Whereas acetylcholine affected equally cell firing in the two parts of the nucleus, the magnocellular (Imc) and the parvocellular (Ipc) division, the effect of NMDA was predominantly confined to Ipc. The NMDA induced firing was specifically blocked by the antagonist CPP, while having no effect on acetylcholine. Our results support earlier findings that isthmo-tectal feedback loops can be modulated by distinct mechanisms in separate divisions of nucleus isthmi.
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