Ionic and pharmacological properties of reciprocal inhibition in Xenopus embryo motoneurones.

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Abstract
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 motoneurons during fictive swiming 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 .mu.M-glycine or 100 .mu.M-.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 .mu.M-strychnine but is largely unaffected by bicuculline below 50 .mu.M. The response to GABA is largely blocked by 10 .mu.M-bicuculline but is unaffected by 1 .mu.M-strychnine. Both strychnine and bicuculline are therefore specific antagonists in the amphibian embryo preparation. Glycine and GABA are both partially antagonized by 10 .mu.M-picrotoxin. 5. Mid-cycle i.p.s.p.s recorded in motoneurones during fictive swimming are reduced in amplitude by 0.5-1 .mu.M-strychnine but are largely unaffected by 40 .mu.M-bicuculline. In embryos immobilized by ventral root transection, 100 .mu.M-tubocurarine, a likely GABA antagonist in the embryo, hasno effect on mid-cycle inhibition. Glycine is suggested to be the probable transmitter release by commissural interneurones and mediating mid-cycle inhibition during fictive swimming, acting to increase conductance of Cl-.