Intracellular recordings were made from antidromically identified motoneurons in transverse (500 μm) lumbar spinal cord slices of neonatal (12-20 day) rats. Electrical stimulation of ventral rootlets evoked, with or without an antidromic spike or initial segment potential, a depolarizing response (latency, 1-4.2 ms), a hyperpolarizing response (latency, 1.5-3.5 ms), or a combination of two preceding responses in 38, 6, and 8% of motoneurons investigated. The hyperpolarizing response was reversibly eliminated by low Ca2+ (0.25 mM), d-tubocurarine (d-Tc; 10 μM) or strychnine (1 μM), suggesting that this response represents an inhibitory postsynaptic potential (IPSP) mediated by glycine or a related substance released from inhibitory interneurons subsequent to their activation by axon collaterals in a manner analogous to the Renshaw cell circuitry described for the cat motoneurons. The depolarizing responses were excitatory postsynaptic potentials (EPSPs), because they could be graded by varying the stimulus intensity and were reversibly abolished in low Ca2+ solution. Membrane hyperpolarization increased the amplitude of EPSPs, and the mean extrapolated reversal potential was -4 mV. EPSPs were augmented, rather than diminished, by dihydro-β-erythroidine (1 μM) or d-Tc, arguing against a role of recurrent motor axon collaterals in initiating the responses. The conduction velocity of the fibers initiating the EPSPs ranged from 0.35 to 0.96 m/s, indicating that these fibers were unmyelinated. Furthermore, the EPSP exhibited a constant delay when the stimulus frequency was varied from 1 to 5 Hz, and the synaptic delay estimated by extrapolation was <1 ms, suggesting that it was a monosynaptic event. After complete separation of the ventral and dorsal horns by a knife cut, stimulation of ventral rootlets could still evoke an EPSP in motoneurons. Superfusion of the slices with the nonselective glutamate receptor antagonist kynurenic acid (0.2-1 mM) or the selective quisqualate/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) (0.5-1 μM) reversibly diminished the EPSPs. EPSPs evoked by stimulation of dorsal and ventral rootlets exhibited different latency and waveform in the same motoneurons. The results provide evidence that activation of ventral root afferents evoked an EPSP mediated by glutamate or a related substance in a population of motoneurons. Furthermore, the afferent pathway mediating the EPSP appears to be monosynaptic and confined to the ventral horn.
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