Development and role of GABA(A) receptor-mediated synaptic potentials during swimming in postembryonic Xenopus laevis tadpoles

We have investigated the contribution of GABA, receptor activation to swimm ing in Xenopus tadpoles during the first day of postembryonic development. Around the time of hatching stage (37/8), bicuculline (10-50 mu M) causes a decrease in swim episode duration and cycle period, suggesting that GABA, receptor activation influences embryonic swimming. Twenty-four hours later, at stage 42, GABA, receptor activation plays a more pronounced role in mod ulating larval swimming activity. Bicuculline causes short, intense swim ep isodes with increased burst durations and decreased cycle periods and rostr ocaudal delays. Conversely, the allosteric agonist, 5 beta-pregnan-3 alpha- ol-20-one (1-10 mu M) or the uptake inhibitor, nipecotic acid (200 mu M) ca use slow swimming with reduced burst durations and increased cycle periods. These effects appear to be mainly the result of GABA release from the spin al terminals of midhindbrain reticulospinal neurons but may also involve sp inal GABAergic neurons. Intracellular recordings were made using KCl electr odes to reverse the sign and enhance the amplitude of chloride-dependent in hibitory postsynaptic potentials (IPSPs). Recordings from larval motoneuron s in the presence of strychnine (1-5 mu M), to block glycinergic IPSPs, pro vided no evidence for any GABAergic component to midcycle inhibition. GABA potentials were observed during episodes, but they were not phase-locked to the swimming rhythm. Bicuculline (10-50 mu M) abolished these sporadic pot entials and caused an apparent decrease in the level of tonic depolarizatio n during swimming activity and an increase in spike height. Finally, in mos t larval preparations, GABA potentials were observed at the termination of swimming. In combination with the other evidence, our data suggest that mid hindbrain reticulospinal neurons become involved in an intrinsic pathway th at can prematurely terminate swim episodes. Thus during the first day of la rval development, endogenous activation of GABA, receptors plays an increas ingly important role in modulating locomotion; and GABAergic neurons become involved in an intrinsic descending pathway for terminating swim episodes.