Longitudinal distribution of components of excitatory synaptic input to motoneurones during swimming in young Xenopus tadpoles: experiments with antagonists

Recent studies have revealed that the excitatory synaptic input to spinal motoneurones during fictive swimming in Xenopus tadpoles has three main components: glutamatergic (Glu) from premotor excitatory interneurones, nicotinic cholinergic (nACh) from more rostral motoneurones, and electrotonic coupling from neighbouring motoneurones. During swimming, these components sum to produce two kinds of excitation: phasic excitation (EPSPs) underlying spikes, and tonic depolarization. We have investigated the longitudinal distribution of these excitatory synaptic inputs to presumed motoneurones at different positions along the spinal cord using intracellular recording techniques. Different antagonists (10 μ m dihydro-β-erythroidine (DHβE) for nicotinic ACh receptors (nAChRs), 2 mM kynurenate (Kyn) for glutamate receptors (GluRs), and 100 μ m Cd 2+ for all chemical synapses) were microperfused very locally to unmask the relative contributions of these components to the total excitatory drive, and their distribution along the spinal cord during swimming. If the potentials remaining when all chemical components were blocked by Cd 2+ were subtracted from potentials recorded after blocking nAChRs and GluRs with DHβE plus Kyn, a small unidentified component was observed. This component was blocked by the specific AMPA antagonist 6-nitro-7-sulphamoylbenzo( f) quinoxaline-2,3-dione (NBQX, 5 μ m ), so is glutamate mediated. We used the potential measurements to calculate the relative synaptic conductances of the different synaptic inputs, and conclude that: (a) there is a rostral-caudal gradient in input during EPSPs and tonic depolarization; (b) the glutamatergic component accounts for most of the excitation, and decreases caudally; (c) cholinergic and electrotonic components are relatively constant in different positions along the spinal cord; and (d) these two components provide an increasing proportion of the input in more caudal neurones. We propose that the glutamate components of excitation are fundamental to rhythm generation in the brainstem and rostral cord, while the electrotonic and cholinergic components ensure that the central pattern generator activates motoneurones effectively in all parts of the spinal cord.