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

Abstract

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 ␣ 7 homomers and ␣ 4 ␤ 2 heteromers) and two glutamate receptors (AMPARs and NMDARs). The decay time constants of the four components cover a wide range: from 3.6 Ϯ 2.2 ms (␣ 7 nAChRs) to 54.6 Ϯ 19.5 ms (NMDARs, at Ϫ45 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.

Coapplication of noisy patterned electrical stimuli and NMDA plus serotonin facilitates fictive locomotion in the rat spinal cord

“…This result suggests that the antidromic pathways activated by FListim could not readily access the locomotor CPG that needs to be stimulated by stronger electrical pulses (Mentis et al 2005) than the ones used in the present investigation. This notion accords with the demonstration that inhibitory Renshaw cells activate different postsynaptic receptors on motoneurons, depending on the stimulation frequency (McCrea et al 1980;Lamotte d'Incamps and Ascher 2008). As one aim of the current report was to identify patterns of stimulation with minimal deleterious effects and with sustained efficiency on the locomotor CPG, it seemed desirable to carry further experiments with weak intensity FListim applied to a single DR.…”

Mechanisms Shaping the Slow Nicotinic Synaptic Current at the Motoneuron-Renshaw Cell Synapse

“…This concentration of D-APV, which was sufficient to block the NMDAdependent EPSCs recorded at negative potentials (Lamotte d'Incamps and Ascher, 2008), turned out to be insufficient to block the responses recorded at positive potentials, particularly those induced by repetitive stimulations (see Results). Complete blockade could be obtained by the addition of a glycine site blocker, 5,7-dichloro-kynurenic acid (DCKA) (20 M) (Lowe, 2003).…”

“…The RCs were found in the region of lamina VII where the axons of the MNs converge to make two to three axon bundles. They were identified by their characteristic response to a single ventral rootlet (VR) stimulation (a train of action potentials) in the cell-attached mode (Lamotte d'Incamps and Ascher, 2008), then voltage-clamped in the whole-cell configuration. Patch pipettes had an initial open-tip resistance of 3-6 M⍀.…”

“…In all experiments the glutamatergic components of the MN-RC response (Lamotte d'Incamps and Ascher, 2008) were suppressed by adding 2,3-dioxo-6-nitro-7-sulfonyl-benzo[f]quinoxaline (NBQX) (2 M) to block the AMPA receptors and D(Ϫ)-2-amino-5-phosphonovaleric acid (D-APV) (50 M) to block NMDA receptors. This concentration of D-APV, which was sufficient to block the NMDAdependent EPSCs recorded at negative potentials (Lamotte d'Incamps and Ascher, 2008), turned out to be insufficient to block the responses recorded at positive potentials, particularly those induced by repetitive stimulations (see Results).…”

“…One of the few central cholinergic synapses in which the presynaptic axons can be selectively stimulated is the MN-RC synapse (Eccles et al, 1954;Dourado and Sargent, 2002), the organization of which is typical of central synapses (Alvarez et al, 1999). The EPSCs induced by MNs at this synapse have both glutamatergic and nicotinic components (Mentis et al, 2005;Nishimaru et al, 2005) and the nicotinic component involves both homomeric ␣7 and heteromeric nAChRs (Lamotte d'Incamps and Ascher, 2008). In the present study we analyzed in voltage-clamp conditions the EPSCs mediated by heteromeric nAChRs to evaluate the evidence for spillover after a repetitive stimulation of the MNs as well as after inactivation of AChE.…”