Pathophysiology of the spinal cord studied in vitro

We describe the use of isolated hemisected mouse spinal cords for pathophysiological investigations and analyze the responses evoked and recorded with suction electrodes in spinal roots. Dorsal root (DR) recordings from preparations in control solution show a directly evoked fiber volley (FV); an early postsynaptic spike generated by neurons in spinal gray matter and picked up by volume conduction (DRR1); and a 'slow' dorsal root potential (DRP). The 'conventional' dorsal root reflex (here termed DRR2) was absent or very small in control medium but became very prominent in elevated bath [Ca2+]. DRP and DRR2 but not DRR1 are depressed by GABAA antagonists. Recordings from VR contain the electrotonically conducted VRepsp and superimposed monosynaptic reflex discharge (VRR1). Rarely in control medium but regularly in elevated bath [Ca2+] a GABA-dependent late reflex (VRR2) appears (see also Duchen, 1986). The effects of varying bath concentrations of K+, Ca2+ and Mg2+ on evoked responses are briefly summarized. Irregularly timed spontaneous discharges appear in DR and VR recordings when [Ca2+] is elevated above 1.8 or 2.4 mM, and when [Mg2+] is lowered to 0.4 mM. In hypoxic solution synaptically transmitted responses fail in 10 to 20 min, but persist longer when [Ca2+] is elevated. Unexpectedly, spreading depression (SD)-like responses were recorded in some preparations during hypoxia. Following hypoxia, after synaptically transmitted responses recovered, spontaneous activity developed in DR and VR recordings.