Local facilitation of plateau potentials in dendrites of turtle motoneurones by synaptic activation of metabotropic receptors

1. The spatial distribution of synaptic facilitation of plateau potentials in dendrites of motoneurones was investigated in transverse sections of the spinal cord of the turtle using differential polarization by applied elect ric fields. 2. The excitability of motoneurones in response to depolarizing current pul ses was increased following brief activation of either the dorsolateral fun iculus (DLF) or the medial funiculus (MF) even when synaptic potentials wer e eliminated by antagonists of ionotropic receptors. 3. The medial and lateral compartments of motoneurones were differentially polarized by the electric field generated by passing current between two el ectrodes on either side of the preparation. In one direction of the field l ateral dendrites were depolarized while the cell body and medial dendrites were hyperpolarized (S- configuration). With current in the opposite direct ion the cell body and medial dendrites were depolarized while lateral dendr ites were hyperpolarized (S+ configuration). 4. Following brief activation of the DLF the excitability and the generatio n of plateau potentials were facilitated during differential depolarization of the lateral dendrites but not during differential depolarization of the cell body and medial dendrites. Following brief activation of the MF the e xcitability and generation of plateau potentials were facilitated during di fferential depolarization of the cell body and medial dendrites but not dur ing differential depolarization of the lateral dendrites. 5. It is concluded that the synaptic facilitation of the dihydropyridine-se nsitive response to depolarization is compartmentalized in turtle motoneuro nes.