Assessing the roles of glutamatergic and cholinergic synaptic drive in thecontrol of fictive swimming frequency in young Xenopus tadpoles

This paper investigates the proposal that the frequency of the swimming cen tral pattern generator in young Xenopus tadpoles is partly determined by th e population of glutamatergic premotor interneurons active on each cycle. D uring fictive swimming spinal neurons also receive cholinergic and electrot onic excitation from motoneurons. As frequency changes during swimming we m ake two predictions: first, since most motoneurons fire very reliably at al l frequencies, the electrotonic and nicotinic drive from motoneurons should remain constant, and second, when swimming frequency decreases, the glutam atergic drive should decrease as the number of active premotor excitatory i nterneurons decreases. We have tested these predictions by measuring the ex citatory synaptic drive to motoneurons as frequency changes during fictive swimming. The components of synaptic drive were revealed by the local micro perfusion of strychnine together with different excitatory antagonists. Aft er blocking the nicotinic acetylcholine receptor, the mainly glutmatergic e xcitatory synaptic drive still changed with frequency. However, when glutam ate receptors or all chemical transmission was blocked, el;citation did not change with frequency. Our predictions are confirmed, suggesting that prem otor excitatory interneurons are a major factor in frequency control in the tadpole central pattern generator and that motoneurons provide a stable ba ckground excitation.