Presynaptic Na/Ca action potentials in unmyelinated axons of olfactory cortex.
(1) Pial surface slices of guinea-pig olfactory cortex were cut to have a thickness of 150 micron. Action potentials were recorded from the sectioned ends of the unmyelinated afferent axons originating from the lateral olfactory tract (LOT). These potentials were prolonged by the K-channel blocker 3,4-diaminopyridine (0.1 mmol/l) and further lengthened by tetraethylammonium (10 mmol/l). The action potential was also greatly prolonged by partly replacing the K+ in the bathing solution by Cs+. (2) These prolonged action potentials were shortened by Cd2+; Gd3+ (gadolinium); Ni2+; Mn2+; Co2+, in order of potency. The residual early component of the action potential was tetrodotoxin (TTX) sensitive. In contrast, the LOT action potential was little affected by Ca-channel blockade. (3) Organic Ca-channel blockers either had no effect (0.05 mmol/l nifedipine), or depressed the early and later phases of the prolonged action potential equally (0.05-0.5 mmol/l verapamil or 0.05-0.2 mmol/l diltiazem). (4) A propagated action potential was also obtained in solution containing TTX and low Na+. This potential was supported by Ca2+, Sr2+ or Ba2+ and completely suppressed by Cd2+. (5) The later parts of the action potential, after K-channel blockade, had a pharmacological sensitivity towards Ca-channel blockers matching that of synaptic transmission. This suggests the falling phase of the action potential is caused by charge carrier (mainly Ca2+) passing through Ca-channels that have similar properties to, or are the same as those which open prior to transmitter release.
Scholfield
C N
CN
Department of Physiology, Queen's University, Belfast, Northern Ireland.
D015761:4-Aminopyridine / Q000031:analogs & derivatives*; D000200:Action Potentials / Q000187:drug effects; D000077770:Amifampridine; D000631:Aminopyridines / Q000494:pharmacology; D000679:Amygdala / Q000502:physiology; D000818:Animals; D001369:Axons / Q000502:physiology*; D002118:Calcium / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002413:Cations, Divalent; D002540:Cerebral Cortex / Q000502:physiology*; D005260:Female; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008297:Male; D011188:Potassium / Q000494:pharmacology; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
D000631:Aminopyridines; D002121:Calcium Channel Blockers; D002413:Cations, Divalent; D013757:Tetraethylammonium Compounds; D013779:Tetrodotoxin; D019789:Tetraethylammonium; D015761:4-Aminopyridine; D000077770:Amifampridine; D011188:Potassium; D002118:Calcium