Spontaneous dorsal root potentials arise from interneuronal activity in the isolated frog spinal cord

Abstract

Spontaneous dorsal root potentials (sDRPs) were recorded from the dorsal roots of the isolated frog spinal cord using sucrose gap techniques. sDRPs were always negative (depolarizing) in sign and ranged in size from about 100 μV to 6.0 mV. The largest sDRPs were 25–40% of the amplitude of DRPs evoked by stimulation of adjacent dorsal roots. Hypoxia or accumulation of extracellular K + ions did not appear responsible for the generation of this spontaneous activity since exposing the cord to unoxygenated Ringer's solution decreased sDRPs and K +-sensitive microelectrodes indicated that only small changes in extracellular K + (approximately 0.15 mM) were produced coincidently with the largest sDRPs. Chemically-mediated synaptic transmission was found to be necessary for the production of sDRPs because the addition of Mn 2+ or Mg 2+ ions or tetrodotoxin to the Ringer's solution or reduction of its Na + concentration blocked sDRPs, whereas application of 4-aminopyridine enhanced them. It did not seem that a direct action of GABA on afferent fiber terminals was responsible for the generation of spontaneous potentials since an increase in sDRPs was seen after: application of the GABA antagonists, bicuculline and picrotoxin; exposure to the glutamic acid decarboxylase inhibitor, semicarbazide (which significantly reduced the concentration of GABA in the cord); and lowering of the external Cl − concentration. Similarly taurine is probably not significant since the taurine antagonist, TAG, increased the amount of spontaneous activity. On the other hand, (−)-baclofen, which is thought to reduce excitatory amino acid release, d,l-α-aminoadipic acid, α,ɛ-diaminopimelic acid, and 2-amino-4-phosphonobutyric acid, which are believed to be selective postsynaptic excitatory amino acid antagonists, and [ d-Pro 2- d-Phe 7- d-Trp 9]-substance P, a postsynaptic blocker of the action of substance P, markedly and reversibly reduced sDRPs. Experiments were performed on isolated cords without supraspinal or afferent input; therefore sDRPs must be generated by intraspinal structures. It would seem that interneurons are responsible because addition of mephenesin or pentobarbital — compounds which inhibit polysynaptic reflex transmission involving interneurons — reduced the production of sDRPs. sDRPs may result from the action of excitatory transmitters such as l-glutamate, l-aspartate, or substance P released by interneuronal firing in the spinal cord. Moreover, because sDRPs were increased by application of yohimbine, corynanthine and propanolol and reduced by haloperidol, such interneurons may be under descending control of adrenergic and dopaminergic fibers.