WHOLE-CELL RECORDINGS OF LUMBAR MOTONEURONS DURING LOCOMOTOR-LIKE ACTIVITY
IN THE IN VITRO NEONATAL RAT SPINAL CORD.
S. Hochman and B. J. Schmidt.
Department of Physiology, Faculty of Medicine, University of Manitoba,
CANADA R3E 0W3.
APStracts 4:279N, 1997.
ABSTRACT
Whole-cell current and voltage clamp recordings were obtained from lumbar
motoneurons in the isolated neonatal rat spinal cord, in order to characterize
the behavior of motoneurons during neurochemically-induced locomotor-like
activity. Bath application of 5-HT (10-100 (M) in combination with NMDA (1-12
(M) initially produced tonic membrane depolarization (mean = 26 mV), increased
input resistance, decreased rheobase, and increased spike inactivation in
response to depolarizing current pulse injections. After the initial tonic
depolarization, rhythmic fluctuations of the motoneuron membrane potential
(locomotor drive potentials; LDPs) developed that were phasically modulated in
association with ventral root discharge. The peak and trough voltage levels of
the LDP fluctuated above and below the membrane potential recorded immediately
before the onset of rhythmic activity. Similarly, firing frequency was
modulated above and below pre-locomotion firing rates (in those motoneurons
which displayed neurochemically-induced tonic firing immediately before the
onset of rhythmic activity). These observations are consistent with an
alternation between phasic excitatory and inhibitory synaptic drives. The
amplitude of LDPs and rhythmic excitatory drive current increased with
membrane depolarization from -80 mV to - 40 mV, and then decreased with
further depolarization, thus displaying non-linear voltage-dependence. Faster
frequency, small amplitude voltage fluctuations were observed superimposed on
the depolarized phase of LDPs. In some motoneurons the trajectory of these
superimposed fluctuations was consistent with a synaptic origin, while in
other cells the regular sinusoidal appearance of the fluctuations and the
occurrence of superimposed plateau potentials was more compatible with the
activation of an intrinsic membrane property. One motoneuron displayed
exclusively excitatory phasic drive, and another motoneuron was characterized
by inhibitory phasic drive alone, during rhythmic activity. These findings are
compatible with the concept of a central pattern generator which is capable of
delivering both excitatory and inhibitory drive to motoneurons during
locomotion. The data also suggests that the rhythmic excitatory and inhibitory
outputs of the hypothetical half-center model can be dissociated and operate
in isolation.
Received 22 August 1996; accepted in final form 29 September 1997.
APS Manuscript Number J678-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 29 October 1997