A role for potassium currents in the generation of the swimming motor pattern of Xenopus embryos.
1. To assess the role that K+ currents play in the production of the swimming motor pattern in the Xenopus embryo, we have used low doses of the K+ channel blockers, 3,4-diaminopyridine (3,4-DAP; 25-100 microM) and tetraethylammonium (TEA; 500 microM), to reduce K+ currents and investigated the effects on motor output. 2. To confirm that 3,4-DAP and TEA block K+ currents and characterize their actions, we made whole-cell voltage-clamp recordings from acutely isolated spinal neurons. Both 25-100 microM 3,4-DAP and 100-500 microM TEA blocked the sustained K+ current in a dose-dependent manner. 3. Because TEA can block acetylcholine nicotinic receptors on autonomic ganglia, and nicotinic acetylcholine receptors have recently been shown to be present on Xenopus spinal neurons, we have tested both 3,4-DAP and TEA for antagonist action against the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP). Although 500 microM TEA blocked the DMPP-induced depolarization, 25 microM 3,4-DAP did not. 4. In the intact embryo, application of 25-100 microM 3,4-DAP or 500 microM TEA disrupted both the left and right alternation of ventral root discharge and the motor pattern recorded intracellularly from spinal neurons during swimming. Both blockers allowed the firing of an extra action potential at midcycle, which led to a number of different patterns. These patterns were categorized as follows: type A, cycles with midcycle action potentials; type B, the simultaneous firing of neurons on both sides of the cord; and type C, in which one side was active, whereas the other side was inhibited. In both 3,4-DAP and TEA these abnormalities tended to occur at the beginning of swimming episodes. Both blockers also caused a significant increase in the cycle period. Because both 3,4-DAP and TEA produced very similar affects to the motor pattern, we conclude that the perturbations are probably a result of reducing K+ current amplitude. 5. To investigate whether 3,4-DAP and TEA were producing disruptions in the motor pattern by increasing synaptic drive through the broadening of action potentials, we made measurements of spike width, tonic depolarization, and midcycle inhibitory postsynaptic potential (IPSP) amplitude during swimming. Both 3,4-DAP and TEA caused significant but modest spike broadening (20.8 and 29.8%, respectively); however, their effects on tonic depolarization were inconsistent although both blockers increased midcycle IPSP amplitude. 6. To test whether a reduction in K+ currents could plausibly produce the specific motor pattern perturbations that were seen, we have made computer simulations of simplified spinal networks.(ABSTRACT TRUNCATED AT 400 WORDS)
Wall
M J
MJ
School of Biological Sciences, Bristol University, United Kingdom.
Dale
N
N
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D003198:Computer Simulation; D004625:Embryo, Nonmammalian; D005071:Evoked Potentials / Q000502:physiology; D008675:Metamorphosis, Biological / Q000502:physiology*; D008959:Models, Neurological; D009043:Motor Activity / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation*; D009415:Nerve Net / Q000502:physiology; D009474:Neurons / Q000502:physiology; D015221:Potassium Channels / Q000502:physiology*; D011978:Receptors, Nicotinic / Q000502:physiology; D013116:Spinal Cord / Q000502:physiology*; D013126:Spinal Nerve Roots / Q000502:physiology; D013550:Swimming; D009435:Synaptic Transmission / Q000502:physiology*; D014981:Xenopus / Q000502:physiology*
D015221:Potassium Channels; D011978:Receptors, Nicotinic