Changes in action potential duration alter reliance of excitatory synaptic transmission on multiple types of Ca2+ channels in rat hippocampus.
It has been established that multiple types of Ca2+ channels participate in triggering neurotransmitter release at central synapses, but there is uncertainty about the nature of their combined actions. We investigated synaptic transmission at CA3-CA1 synapses of rat hippocampal slices and asked whether the dependence on omega-CTx-GVIA-sensitive N-type channels and omega-Aga-IVA-sensitive P/Q-type Ca2+ channels can be altered by physiological mechanisms. The reliance on multiple types of Ca2+ channels was not absolute but depended strongly on the amount of Ca2+ influx through individual channels, which was manipulated by prolonging the presynaptic action potential with the K+ channel blocker 4-aminopyridine (4-AP) and by varying the extracellular Ca2+ concentration ([Ca2+]o). We quantified the influence of spike broadening on Ca2+ influx through various Ca2+ channels by imposing mock action potentials on voltage-clamped cerebellar granule neurons. In field recordings of the EPSP in hippocampal slices, action potential prolongation increased the EPSP slope by 2-fold and decreased its reliance on either N-type or P/Q-type Ca2+ channels. The inhibition of synaptic transmission by N-type channel blockade was virtually eliminated in the presence of 4-AP, but it could be restored by lowering [Ca2+]o. These results rule out a scenario in which a significant fraction of presynaptic terminals rely solely on N-type channels to trigger transmission. The change in sensitivity to the neurotoxins with 4-AP could be explained in terms of a nonlinear relationship between Ca2+ entry and synaptic strength, which rises steeply at low [Ca2+]o, but approaches saturation at high [Ca2+]o. This relationship was evaluated experimentally by varying [CA2+]o in the absence and presence of 4-AP. One consequence of this relationship is that down-modulation of presynaptic Ca2+ channels by various modulators would increase the relative impact of spike broadening greatly.
Wheeler
D B
DB
Neurosciences Program, Stanford University School of Medicine, California 94305, USA.
Randall
A
A
Tsien
R W
RW
D015761:4-Aminopyridine / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology*; D005260:Female; D006624:Hippocampus / Q000737:chemistry* / Q000502:physiology; D008297:Male; D018377:Neurotransmitter Agents / Q000378:metabolism; D010455:Peptides / Q000494:pharmacology; D026902:Potassium Channel Blockers; D051381:Rats; D017207:Rats, Sprague-Dawley; D013111:Spider Venoms / Q000494:pharmacology; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*; D013997:Time Factors; D020867:omega-Agatoxin IVA; D020866:omega-Conotoxin GVIA
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D018377:Neurotransmitter Agents; D010455:Peptides; D026902:Potassium Channel Blockers; D013111:Spider Venoms; D020867:omega-Agatoxin IVA; D020866:omega-Conotoxin GVIA; D015761:4-Aminopyridine; D002118:Calcium