Ionic and pharmacological properties of reciprocal inhibition in Xenopus embryo motoneurones.
1. Properties of rhythmic, compound mid-cycle inhibitory post-synaptic potentials (i.p.s.p.s), which constitute one of the three main synaptic drives to motoneurones during fictive swimming in Xenopus embryos, have been examined using ionic and pharmacological manipulation. 2. Mid-cycle i.p.s.p.s are Cl- dependent. They are reversed by intracellular Cl- injection and attenuated by lowered extracellular Cl- concentration. 3. In response to bath application of 100 microM-glycine or 100 microM-gamma-aminobutyric acid (GABA), motoneurones show a decrease in cell input resistance of 24 +/- 2.9 M omega (mean +/- S.E. of mean) or 16 +/- 3.7% and 26 +/- 6.0 M omega or 14 +/- 2.0% respectively. This is associated with a weak hyperpolarization or depolarization of 0 +/- 1.5 mV and -3 +/- 1.4 mV respectively. Both responses can be made strongly depolarizing by intracellular Cl- injection. 4. The response to glycine is blocked by 1 microM-strychnine but is largely unaffected by bicuculline below 50 microM. The response to GABA is largely blocked by 10 microM-bicuculline but is unaffected by 1 microM-strychnine. Both strychnine and bicuculline are therefore specific antagonists in the amphibian embryo preparation. Glycine and GABA are both partially antagonized by 10 microM-picrotoxin. 5. Mid-cycle i.p.s.p.s recorded in motoneurones during fictive swimming are reduced in amplitude by 0.5-1 microM-strychnine but are largely unaffected by 40 microM-bicuculline. In embryos immobilized by ventral root transection, 100 microM-tubocurarine, a likely GABA antagonist in the embryo, has no effect on mid-cycle inhibition. Glycine is suggested to be the probable transmitter released by commissural interneurones and mediating mid-cycle inhibition during fictive swimming, acting to increase conductance of Cl-.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002712:Chlorides / Q000494:pharmacology; D003292:Convulsants / Q000494:pharmacology; D005998:Glycine / Q000494:pharmacology; D009046:Motor Neurons / Q000502:physiology*; D009433:Neural Inhibition; D013550:Swimming; D013569:Synapses / Q000502:physiology*; D014403:Tubocurarine / Q000494:pharmacology; D014981:Xenopus / Q000196:embryology*; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D002712:Chlorides; D003292:Convulsants; D005680:gamma-Aminobutyric Acid; D005998:Glycine; D014403:Tubocurarine