LONG-TERM DEPRESSION OF C-FIBER-EVOKED SPINAL FIELD POTENTIALS BY STIMULATION OF PRIMARY AFFERENT A-DELTA-FIBERS IN THE ADULT-RAT

Long-term potentiation (LTP) of spinal C-fibre-evoked field potentials can be induced by brief electrical stimulation of afferent C-fibres, by natural noxious stimulation of skin or by acute nerve injury. Here, we report that in urethane anaesthetized, adult rats prolonged high f requency burst stimulation of the sciatic nerve at A delta-fibre stren gth produced long-term depression (LTD) of C-fibre-evoked field potent ials, and also depressed the increased amplitudes of C-fibre-evoked fi eld potentials recorded after LTP had been established (depotentiation ). Electrical stimulation of A beta-fibres failed to induce LTD or dep otentiation. In spinalized rats, prolonged A delta-fibre conditioning stimulation induced LTP rather than LTD of C-fibre-evoked field potent ials. Thus, tonic descending inhibition may determine the direction of plastic changes in C-fibre-mediated synaptic transmission. Spinal app lication of the N-methyl-D-aspartic acid receptor antagonist D-APV blo cked induction of LTD in intact rats and LTP in spinalized rats. The p resently described LTD and the depotentiation of established LTP of C- fibre-evoked field potentials in spinal dorsal horn may underlie some forms of prolonged analgesia induced by peripheral nerve stimulation p rocedures.