Four Excitatory Postsynaptic Ionotropic Receptors Coactivated at the Motoneuron-Renshaw Cell Synapse

Renshaw cells (RCs) are spinal interneurons excited by collaterals of the axons of motoneurons (MNs). They respond to a single motoneuronal volley by a surprisingly long (tens of milliseconds) train of action potentials. We have analyzed this synaptic response in spinal cord slices of neonatal mice in light of recent observations suggesting that the MN axons release both acetylcholine and glutamate. We found that the RC synaptic current involves four components of similar amplitudes mediated by two nicotinic receptors (nAChRs, tentatively identified as alpha(7) homomers and alpha(4)beta(2) heteromers) and two glutamate receptors (AMPARs and NMDARs). The decay time constants of the four components cover a wide range: from 3.6 +/- 2.2 ms (alpha(7) nAChRs) to 54.6 +/- 19.5 ms ( NMDARs, at -45 mV). The RC discharge can be separated into an initial doublet of high-frequency action potentials followed by later spikes with a variable latency and longer interspike intervals. The initial doublet involves the four ionotropic receptors as well as endogenous voltage-dependent conductances. The late discharge depends on NMDARs, but these receptors must be primed by the initial depolarization. The activation of the NMDARs is prolonged by the fact that their slow deactivation is further slowed by depolarization. The formation of the initial doublet is favored by hyperpolarization, whereas the late discharge is favored by depolarization. This suggests that in physiological conditions the pattern of discharge of the RC in response to a MN input may alternate between a phasic and a tonic response.