Brain Research, 419 (1987) 397-402 397 Elsevier BRE 22467 Voltage clamp analysis of lamprey neurons role of N-methyI-D-aspartate receptors in fictive locomotion L.E. Moore, R.H. Hill and S. Grillner Nobel Institute for Neurophysiology, Karolinska lnstitutet, Stockholm (Sweden) and Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston, TX 77550 (U. S.A.) (Accepted 26 May 1987) Key words: N-Methyl-D-aspartate receptor; Fictive locomotion; Voltage clamp; Impedance; Admittance; Voltage-dependent conductance; Excitatory synaptic current; Lamprey Spinal neurons in the lamprey have been subjected to a voltage clamp analysis of the excitatory currents generated during fictive lo- comotion with particular reference to the phasic activation of voltage dependent N-methyl-D-aspartate (NMDA) receptors. Voltage- clamped neurons observed during NMDA-induced fictive swimming show excitatory and inhibitory synaptic currents in phase with the ipsilateral and contralateral ventral root discharges, respectively. The excitatory synaptic currents showed a marked voltage depen- dence suggesting that potential sensitive conductances such as the NMDA ionophore are involved in the synaptic events underlying rhythmic locomotor activity. The effect of NMDA receptor activation during application of tetrodotoxin has also been analyzed dur- ing NMDA-induced pacemaker-like oscillations. Such NMDA-induced oscillations are essentially abolished during the voltage clamp. In the presence of NMDA current voltage plots reveal a negative slope conductance in the potential range of the inherent oscillations. The addition of tetraethyl ammonium (TEA) to the NMDA solution enhanced a net steady state inward current by more than 10-fold due to a partial block of the outward currents. A kinetic analysis was done with a frequency domain technique using a white noise stim- ulus to linearly perturb the membrane potential over a wide range of frequencies. The analysis revealed that the induced negative con- ductance leads to a response which is nearly 180 ° out of phase with the stimulus at low frequencies. This is an unstable condition which leads to the depolarizing phase of the induced oscillations. L-Glutamate and related amino acids activate exci- tatory amino acid (EAA) receptors, which can be further subdivided into 3 subtypes based on their re- sponse to N-methyl-o-aspartate (NMDA), kainate and quisqualate 7,16,19,27. The NMDA-activated channel exhibits a marked voltage dependence which only oc- curs in the presence of physiological levels of Mg 2+ (refs. 11,18,19,22). However, in Mg2+-free solutions NMDA receptor mediated excitatory postsynaptic potentials (EPSP's) have been demonstrated and NMDA induces an increase in the membrane con- ductance like that of other EAA receptors 1,9,18,19. The physiological significance of the NMDA receptor ac- tivation through the release of the Mg 2+ block with a moderate depolarization is at present uncertain. Bath-applied NMDA will elicit the motor pattern un- derlying locomotion in the lamprey spinal cord 4't3,25'26 and, at the single cell level, NMDA receptor activa- tion can elicit pacemaker-like membrane potential oscillations12'24 (see Fig. 1A). These oscillations are due to an interaction between the intrinsic membrane channels of the oscillating neurons and the activation of NMDA channels, which in other systems have been shown to exhibit a negative slope conduc- tance 14'24 and to have a conductance for Na + and Ca 2+ (ref. 17). Excitatory spinal interneurons, reti- culospinal neurons, sensory neurons and axons 3'5'6'9 release a transmitter, perhaps glutamate 14, which ac- tivates kainate/quisqualate and/or NMDA receptors and they have similar characteristics in lamprey and mammalian neuronal systems 9. The object of this study was to directly measure NMDA induced cur- rents in lamprey neurons with the voltage clamp method and to further explore the physiological role of NMDA receptor activation known to occur in the neuronal network underlying locomotion 2,8.9,21. Correspondence: S. Grillner, Nobel Institute for Neurophysiology, Karolinska Institutet, Box 60400, S-104 01 Stockholm, Sweden.