SPONTANEOUS RHYTHMIC BURSTS INDUCED BY PHARMACOLOGICAL BLOCK OF INHIBITION IN LUMBAR MOTONEURONS OF THE NEONATAL RAT SPINAL CORD. Bracci, Enrico, Laura Ballerini and Andrea Nistri. Biophysics Sector, International School for Advanced Studies (S.I.S.S.A.), Via Beirut 4, 34013 Trieste, Italy.
APStracts 2:0299N, 1995.
SUMMARY AND CONCLUSIONS
1. The effects of blocking GABA and glycine mediated synaptic transmission by bicuculline and strychnine on the neonatal rat isolated spinal cord were investigated by intracellular recording from motoneurons using current-clamp and voltage-clamp techniques and by extracellular recording from homologous ventral roots of the L5 segment. 2. Bicuculline per se evoked irregular bursts of motoneuron membrane potential often comprising individual events fused together. Strychnine alone did not elicit spontaneous bursting in the large majority of preparations. Simultaneous application of bicuculline and strychnine consistently induced regular rhythmic bursts (frequency about 2/min, duration about 7s) comprising a rapid depolarization followed by large amplitude oscillations. 3. Burst frequency, duration and intraburst oscillation timecourse were independent of motoneuron membrane potential. Burst and oscillation amplitude decreased with membrane depolarization and, under voltage clamp conditions, inverted polarity near 0 mV. 4. The regular bursts produced by bicuculline and strychnine were blocked by 6-cyano-7- nitroquinoxaline-2,3-dione (CNQX), tetrodotoxin (TTX) or Cd 2+ . 5. NMDA antagonists (R-5-aminophosphonovalerate, APV or 3-((RS)-2-carboxypiperazine-4- yl)-propyl-1-phosphonate, CPP) reversibly blocked or slowed down bursting induced by bicuculline and strychnine. Addition of cyclothiazide to the bicuculline and strychnine solution increased bursting frequency while preserving the regular burst structure; under these conditions bursts became insensitive to CPP. 6. In the presence of bicuculline and strychnine, 5- hydroxytryptamine (5-HT) increased burst frequency and decreased burst duration in a dose dependent fashion. 7. In the presence of bicuculline and strychnine L5 ventral roots developed synchronous rhythmic activity with timecourse similar to that recorded from individual motoneurons. The rhythmic activity was accelerated by 5-HT on both roots, in accordance with observations on single motoneurons. 8. Rhythmic bursts thus appear to result from large, synchronous synaptic events generated by a network modulated by 5- HT and highly sensitive to variations in efficacy of glutamatergic synaptic transmission. These results show that in the rat spinal cord highly patterned motor output can occur despite block of inhibition. 

Received 7 June 1995; accepted in final form 7 September 1995.
APS Manuscript Number J366-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95