Composition of the excitatory drive during swimming in two amphibian embryos: Rana and Bufo.

It has recently been shown that spinal neurons in Xenopus embryos receive cholinergic and electrotonic excitation during swimming, in addition to the well documented excitatory amino acid (EAA)-mediated excitation. We have now examined the composition of the excitatory drive during swimming in embryos of two further amphibian species, Rana and Bufo, which have somewhat different motor patterns. Localised applications of antagonists show that presumed motoneurons in Rana and Bufo embryos receive both cholinergic and FAA input during swimming. There is also a further chemical component which is blocked by Cd2+ and a small Cd(2+)-insensitive component, which is usually non-rhythmic. Rhythmic Cd(2+)-insensitive components, presumed to be phasic electrotonic potentials, were only seen in a small proportion of Bufo neurons and in no Rana neurons. While EAA and cholinergic inputs therefore appear to be consistent features of excitatory drive for swimming in amphibian embryo motoneurons, electrotonic input apparently occurs less commonly. Antagonist specificity was tested using applied agonists in Rana. Results of these tests also suggested that the further, unidentified Cd(2+)-sensitive component seen during swimming could represent an incomplete block of AMPA receptor-mediated excitation.