Pre- and Postsynaptic Expression Mechanisms of Long-Term Depression at Rat Glutamatergic Primary Afferent Synapses on Dorsal Horn Spinal Cord Neurons in Co-Culture

Abstract

In co-culture of the dorsal root ganglion (DRG) and the dorsal horn (DH) neurons, we studied the long-term depression (LTD) caused by low-frequency (5 s−1) stimulation (LFS) of the DRG neurons. Dual whole-cell patch clamp recording in the pairs of DRG and DH neurons was used. The monosynaptic AMPA-receptor-mediated evoked excitatory postsynaptic currents (eEPSC), initiated in the DH neurons by LFS of the DRG neurons, and were analyzed. LFS with durations from 60 to 360 s affected the LTD magnitude, and different expression mechanisms were involved in LTD. The amplitude and variability of miniature EPSC were not changed during LTD. LFS during 60 s did not alter the eEPSC amplitudes, but it significantly increased the coefficient of variation (CV; by 56.8% ± 11.5%, n = 5; P < 0.002) and the paired-pulse ratio (PPR; by 37.8% ± 11.4%, n = 5; P < 0.05), and it decreased the release probability (p; by 21.9% ± 5.6%, n = 5; P < 0.05) calculated using binomial analysis. LFS during 120 s led to LTD (eEPSC decreased by 14% ± 3.3%, n = 13; P < 0.005); no changes in CV, PPR and p were found. LFS for 160, 200, 240 and 360 s induced robust LTD: eEPSC decreased by (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 such LFS durations was not accompanied by a significant change 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 in the CV value at different LFS durations (r = 0.96). Our data suggest that the different mechanisms could be involved in LTD expression according to LFS duration. Thus, LFS (60 s) induced presynaptic changes, without changes in eEPSC, whereas in LTD induced by LFS (120 and 160 s), the postsynaptic site was affected. LTD initiated by longer LFS (200 − 360 s) is probably caused by a silencing of functional synapses without changes in glutamate release probability.