Participation of excitatory amino acid receptors in the slow excitatory synaptic transmission in the rat spinal dorsal horn in vitro.

The participation of excitatory amino acid (EAA) receptors in the responses of deep dorsal horn neurons to repetitive stimulation of dorsal roots was investigated using a spinal slice preparation and current-clamp and voltage-clamp techniques. Using EAA receptor and substance P (SP) receptor antagonists and current-clamp, slow excitatory synaptic response evoked by 10-20 Hz stimulation consisted of two depolarizing components: an initial component lasting 1-5 s and a late-one of 1-3 min duration. The initial and late components of the slow excitatory postsynaptic currents (EPSCs) can also be distinguished on the basis of their voltage-dependence and sensitivity to Mg2+ ions, D-2-amino-5-phosphonovalerate (D-APV) and 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (CNQX). In the presence of Mg2+, the initial component of the slow EPSC increased with membrane hyperpolarization, whereas the late component decreased. In a zero-Mg2+ medium, the initial component was potentiated, but the late component was reduced, or unchanged. CNQX reduced the initial component. In a zero-Mg2+ solution, or at membrane potentials positive to -55 mV in 1 mM Mg2+, D-APV reduced or even abolished the initial component, whereas the late component was not modified by D-APV. We propose that slow excitatory synaptic response evoked in deep dorsal horn neurons by repetitive stimulation of primary afferents has two components, an initial transient component that requires activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors, and a late longer-lasting peptidergic component that has been already described (Brain Res., 290 (1984) 336-341.