Glutamate-Mediated Slow Synaptic Currents in Neonatal Rat Deep Dorsal Horn Neurons in vitro. Miller, Barbara A. and Clifford J. Woolf. Department of Anatomy and Developmental Biology, University College London, Gower Street, London, WC1E 6BT, England.
APStracts 3:0078N, 1996.
SUMMARY AND CONCLUSIONS
1. The role of glutamate in slow excitatory synaptic transmission between small-diameter primary afferents and deep dorsal horn neurons was examined in neonatal rat spinal cord in vitro using the whole-cell voltage-clamp technique. 2. Single-shock electrical stimulation of large-diameter A[beta]- fibers evoked a short latency (< 10 ms) fast (< 500 ms) excitatory postsynaptic current (EPSC). Stimulation of small-diameter A[delta]- and C- fibers resulted, in addition, in a slowly rising and decaying EPSC (lasting up to 14 s) following the fast EPSC. The slow EPSC was never observed with stimulation of A[beta]-fibers. 3. Two patterns of EPSC were observed, "Type-1" and "Type-2", which differed in their time course (lasting up to 1 s and 14 s, respectively). The Type-1 response was biphasic, with a fast monosynaptic component followed by an invariant, presumably monosynaptic, late slow component. The Type-2 response was multiphasic, with a fast monosynaptic component followed by a slow component composed of fast polysynaptic currents superimposed on a slow current.4. The fast monosynaptic component had a linear conductance, whereas, the late slower component of the A[beta]-fiber-evoked response had a negative slope conductance at holding potentials more negative than -23 mV. Both currents reversed at a membrane potential of -1.2 +/- 2.8 mV. 5. Using selective non-NMDA and NMDA-receptor antagonists (CNQX or NBQX and D-AP5, respectively) we showed that both the early fast (A[beta]-fiber evoked) and the late slow (A[delta]- and C-fiber evoked) components were mediated by non-NMDA and NMDA receptors. CNQX suppressed both the early fast and late slow components of the compound EPSC. Whereas, D-AP5 suppressed the polysynaptic currents of the early fast component and the late slow component without significantly affecting the early fast monosynaptic component. 6. Slow EPSCs summated on low-frequency (1 or 10 Hz), repetitive stimulation and produced long-duration "tail" currents on cessation of the stimulus. The amount of temporal summation was proportional to the duration of the slow EPSC and the frequency of stimulation. 7. Our results suggest that slow ionotropic glutamate receptor-mediated EPSCs produced by the stimulation of small- diameter primary afferents play an important role in activity-dependent synaptic plasticity in the dorsal horn. 

Received 16 January 1996; accepted in final form 11 April 1996.
APS Manuscript Number J19-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 1 May 96