Effects of picrotoxin on potassium accumulation and dorsal root potentials in the frog spinal cord

Abstract

The effects of picrotoxin on the changes of extracellular potassium concentration (Δ[K+]e), field potentials and dorsal root potentials evoked by afferent stimulation, were studied in the isolated spinal cord of the frog. Δ[K+]e was measured with potassium selective micro-electrodes. In a normal Ringer's solution the Δ[K+]e evoked by a single pulse applied to a dorsal root did not exceed 0.05 mM. In solutions containing picrotoxin (10−7−10−5m) the Δ[K+]e increased to 0.06–0.1 mm. At higher concentrations (10−4−10−3m) of picrotoxin the Δ[K+]e reached 3–6 mm and spontaneous elevations of [K+]e were observed synchronously with the dorsal root potentials. The latter were depressed by 20–40% and considerably prolonged. The time constant of their ascending phase increased from 9 to 10 ms to 30–40 ms. The second component of the negative field potential, recorded from the intermediate region, increased and its time course corresponded to that of the evoked dorsal root potentials. Impulse activity of motoneurones and interneurones evoked by afferent stimulation was greatly enhanced. Picrotoxin (10−4−5.10−4m) was found to have no effect on the ‘asynaptic’ component of evoked dorsal root potentials, which is resistant to 20 mm MgSO4 and to the absence of Ca2+. It is therefore unlikely that the depressant effect of picrotoxin on the evoked dorsal root potentials is produced by its direct action on the potassium conductance of primary afferents.The findings are consistent with a dual mechanism of dorsal root potentials. The fast component of evoked dorsal root potentials which is depressed by picrotoxin is apparently produced by activation of axo-axonic synapses at the primary afferents, while the slow component is due to transient accumulation of extracellular K+. The potassium component of the evoked dorsal root potentials becomes dominant in solutions with high concentrations of picrotoxin (10−4−10−3m) when impulse transmission is greatly enhanced.