Effects of some divalent cations on synaptic transmission in frog spinal neurones.

Abstract

Synaptic transmission between dorsal root afferents and motoneurons was studied in the isolated and hemisected spinal cord of frogs [R. temporaria], using intracellular and extracellular recording techniques, and ionic substitutions of divalent cations in the bathing fluid. Delayed components of excitatory post-synaptic potentials (epsp) evoked in motoneurons by dorsal root supramaximal stimuli, and the Ca2+-dependent slow after-hyperpolarization which follows antidromic spikes, were reversibly blocked by superfusing the cords with Ca2+-free media containing Co2+ (4 mM) or Mg2+ (6-10 mM). Short latency e.p.s.p. persisted in these media for > 8 h. The minimum synaptic delay of the Co2+ and Mg2+, resistant epsp, measured from the peak negativity of the extracellularly recorded presynaptic spike to the onset of the epsp, was 0.3 ms at 10 .+-. 1.degree. C. The Co2+, Mg2+-resistant epsp was graded, and could be elicited by stimulation of segmental or adjacent roots. Those evoked by each of 2 adjacent roots showed linear summation when the roots were stimulated simultaneously. The Co2+, Mg2+-resistant epsp decreased in amplitude at stimulating frequencies between 10 and 100 Hz, and with paired stimuli at intervals shorter than 20-40 ms. These reductions in amplitude were paralleled by decreases in amplitude of the presynaptic population spike. Solutions free of divalent ions, containing EGTA ethylene glycol bis(.beta.-aminoethyl ether) tetracetic acid (2 mM) abolished the Co2+, Mg2+-resistant epsp. They remained blocked for a variable time after returning to Ca2+-free Ringer containing Mg2+ (8 mM). Their continued abolition at this stage is probably not due to changes in electrical properties of motoneuronal membranes. Eventually, the Mg2+-resistant epsp started recovering in the Ca2+-free Ringer containing Mg2+. The time of onset of this recovery depended on the duration of exposure to EGTA. Sr2+ (2-11 mM), although less effective than Ca2+, restored the composite epsp evoked by dorsal root supramaximal stimuli, and the Ca2+-dependent slow after-hyperpolarization of the motoneuron. The composite epsp could not be restored with Ba2+ (2-10 mM).