Natural nociceptive stimuli (pinching or heating) produce 5-20 Hz tonic firing in C- and A-delta DRG neurons (1;2). Transcutaneous stimulation at low to medium pain intensities causes hypoalgesic effects, that could be related to the induction of long-term depression (LTD) at the dorsal root ganglion (DRG) synapses onto spinal dorsal horn (DH) neurons (3), but this aspect has not been well studied. In pairs of co-cultured rats DRG and DH neurons we have studied the effect of different durations of DRG neurons 5Hz low-frequency firing (LFF) on LTD induction properties in glutamatergic primary afferent synapses. Using dual whole-cell patch clamp recording we analyzed the monosynaptic AMPA receptor-mediated eEPSC initiated in the DH neurons by AP of small DRG neurons (10-25 µm diameters). LFF was induced by 5Hz depolarizing impulses (30 ms, 20 – 50 pA) under current clamp mode. LTD was induced by LFF of single DRG neuron with different durations. LFF during 60 sec did not alter the eEPSC amplitudes, but significantly increased the coefficient of variation (CV; 56.8±11.5%, n=5; P<0.002) and the paired-pulse ratio (PPR; 37.8±11.4%, n=5; P< 0.05), and decreased the release probability (p; 21.9±5.6%, n=5; P< 0.05) calculated using binomial analysis. Data are expressed as percentage of the mean post-LFF value relative to the respective mean pre-LFF controls (± S.E.M.); statistical significance was evaluated using Student’s t-test. LFF for 120 sec led to LTD (eEPSCs decreased to 14±3.3%, n=13; P<0.005); no changes in CV, PPR and p were found. LFF for 160, 200, 240 and 360 sec induced robust LTD: eEPSCs decreased to 37.3±4.3%, n=9; 48.1±3.5%, n=7; 58.3±2.5%, n=8, and 57.3±2.8%, n=8, respectively; P<0.001). LTD induced by these LFF durations was not accompanied by significant changing in PPR and p, but showed an increase in CV: 24.4±8.6%, n=6; 35.1±11.2%, n=6; 37.7±12.8%, n=7, and 38.1±9.3, n=8, respectively. LTD magnitude was correlated with the enhancement of the CV at different LFF duration (r2=0.96). Miniature EPSCs did not change in amplitude and variability during LTD induced by LFF with different durations. LTD was unaffected by D-AP5 20 µM and by GABAA- and glycine receptor blockers bicuculline 10 µM and strychnine 1 µM respectively. Data suggest a possibility of induction 5Hz LFF-induced LTD in the glutamatergic primary afferent synapses in the DH. The magnitude of LTD depends on the duration of DRG neuron LFF. Different mechanisms could be involved in LTD expression according to duration of LFF. Thus, 60 sec LFF induces presynaptic changes, but not change in eEPSC LTD, whereas LTD elicited by 120 and 160 sec LFF affected the postsynaptic site. LTD initiated by longer LFF (200 – 360 sec) probably was caused by a silencing of functional synapses (4;5) without changes in glutamate release probability.