CENTRAL MODULATION OF STRETCH RECEPTOR NEURONS DURING FICTIVE LOCOMOTION IN LAMPREY. VINAY, Laurent, Jean-Yves BARTHE and Sten GRILLNER. The Nobel Institute for Neurophysiology, Department of Neuroscience, Karolinska Institutet, Box 60400, S-171 77 Stockholm, Sweden.
APStracts 3:0055N, 1996.
SUMMARY AND CONCLUSIONS
1. In lamprey, stretch receptor neurons (SRNs) also referred to as edge cells are located along the lateral margin of the spinal cord. They sense the lateral movements occurring in each swim cycle during locomotion. The isolated lamprey spinal cord in vitro was used to investigate the activity of SRNs during fictive locomotion induced by bath applied N -methyl- [delta] - aspartate (NMDA). Intracellular recordings with potassium acetate filled electrodes showed that 63% of SRNs had a clear locomotor-related modulation of their membrane potential. 2. Of the modulated SRNs, two thirds had periods of alternating excitation and inhibition occurring during the ipsilateral and the contralateral ventral root bursts respectively. The phasic hyperpolarization could be reversed into a depolarizing phase following the injection of chloride ions into the cells, which revealed a chloride-dependent synaptic drive. The remaining modulated SRNs were phasically inhibited during ipsilateral motor activity. 3. Experiments with barriers partitioning the recording chamber with the spinal cord into three pools, allowed an inactivation of the locomotor networks within one pool by washing out NMDA from the pool in which the SRN was recorded. This resulted in a marked reduction, but not an abolishment, of the amplitude of the membrane potential oscillations. Both the excitatory and the inhibitory phases were reduced, resulting from removal of input from inhibitory and excitatory interneurons projecting from the adjacent pools. If the glycine receptor antagonist strychnine (1 [mu]M) was applied in one pool, the phasic hyperpolarizing phase disappeared without affecting the excitatory phase. 4. Bath application of the [gamma] -aminobutyric acid (GABA) A receptor antagonist, bicuculline (50-100 [mu]M) blocked the spontaneous large unitary inhibitory post-synaptic potentials (IPSPs) which occurred without a clear phasic pattern. Bicuculline had no significant effect on the peak to peak amplitude of the locomotor- related membrane potential oscillations. The inhibition in SRNs therefore has a dual origin: glycinergic interneurons provide phasic inhibition, while the GABA system can exert a tonic inhibition via GABA A receptors. 5. These data show that, in addition to the stretch evoked excitation, which SRNs receive during each locomotor cycle, most of them also receive excitation from the central pattern generator network during the ipsilateral contraction, which may ensure a maintained high level of sensitivity to stretch during the shortening phase of the locomotor cycle. This arrangement is analogous to the efferent control of muscle spindles exerted by [gamma] -motoneurons in mammals, which as a rule are co-activated with [alpha] -motoneurons to the same muscle ( [alpha] - [gamma] linkage). 

Received 18 July 1995; accepted in final form 8 March 1996.
APS Manuscript Number J460-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 27 March 96