Properties and plasticity of synaptic inputs to rat dorsal column neuronesrecorded in vitro

1. The mechanisms regulating the flow of sensory signals and their modifica tion by synaptic interactions in the dorsal column nuclei are incompletely understood. Therefore, we examined the interactions between EPSPs evoked by stimulation of dorsal column and corticofugal fibres in the dorsal column nuclei cells using an in vitro slice technique. 2. Dorsal column EPSPs had briefer durations at depolarised membrane potent ials than corticofugal EPSPs. Superfusion of the NMDA receptor antagonist 2 D(-)-2-amino-5-phosphonovaleric acid (AP5) did not modify dorsal column EPS Ps but reduced corticofugal EPSPs. Application of the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) abolished both dors al column and corticofugal EPSPs in cells held at the resting potential. Th erefore, dorsal column EPSPs were mediated by non-NMDA receptors but cortic ofugal EPSPs revealed both non-NMDA- and NMDA-dependent components. 3. Paired-pulse stimulation of dorsal column fibres elicited a depression o f the second EPSP at pulse intervals of <50 ins; however, paired-pulse stim ulation of corticofugal fibres evoked facilitation of the second EPSP at pu lse intervals of <30 ms. When stimulation of the corticofugal fibres preced ed stimulation of the dorsal column fibres, facilitation of the dorsal colu mn EPSP was observed at pulse intervals of <100 ms. This facilitation was b locked at hyperpolarised membrane potentials or in the presence of AP5, sug gesting activation of NMDA receptors. There was a depression of corticofuga l EPSPs by previous dorsal column stimulation. 4. Dorsal column EPSPs were gradually depressed during stimulation with bar rages at frequencies of >10 Hz, while corticofugal EPSPs were facilitated a nd summated at frequencies >30 Hz. Hyperpolarisation and application of AP5 prevented the facilitation of corticofugal EPSPs. High frequency stimulati on of the corticofugal input elicited a short-lasting AP5-sensitive facilit ation of both corticofugal and dorsal column EPSPS. Depolarising current fa cilitated dorsal column EPSPs but not corticofugal EPSPs. 5. These results indicate that synaptic interactions include different form s of activity-dependent synaptic plasticity, with the participation of NMDA receptors and probably Ca2+ inflow through voltage-gated channels. These c omplex synaptic interactions may represent the cellular substrate of the in tegrative function of the dorsal column nuclei observed in vivo.