Persistent sodium current drives conditional pacemaking in CA1 pyramidal neurons under muscarinic stimulation.
Hippocampal CA1 pyramidal neurons are normally quiescent but can fire spontaneously when stimulated by muscarinic agonists. In brain slice recordings from mouse CA1 pyramidal neurons, we examined the ionic basis of this activity using interleaved current-clamp and voltage-clamp experiments. Both in control and after muscarinic stimulation, the steady-state current-voltage curve was dominated by inward TTX-sensitive persistent sodium current (I(NaP)) that activated near -75 mV and increased steeply with depolarization. In control, total membrane current was net outward (hyperpolarizing) near -70 mV so that cells had a stable resting potential. Muscarinic stimulation activated a small nonselective cation current so that total membrane current near -70 mV shifted to become barely net inward (depolarizing). The small depolarization triggers regenerative activation of I(NaP), which then depolarizes the cell from -70 mV to spike threshold. We quantified the relative contributions of I(NaP), hyperpolarization-activated cation current (I(h)), and calcium current to pacemaking by using the cell's own firing as a voltage command along with specific blockers. TTX-sensitive sodium current was substantial throughout the entire interspike interval, increasing as the membrane potential approached threshold, while both Ih and calcium current were minimal. Thus, spontaneous activity is driven primarily by activation of I(NaP) in a positive feedback loop starting near -70 mV and providing increasing inward current to threshold. These results show that the pacemaking "engine" from I(NaP) is an inherent property of CA1 pyramidal neurons that can be engaged or disengaged by small shifts in net membrane current near -70 mV, as by muscarinic stimulation.
Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115.
Jason
J
Yamada-Hanff
Bruce P
BP
Bean
D000109:Acetylcholine / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D000831:Animals, Newborn; D001683:Biological Clocks / Q000187:drug effects*; D056547:CA1 Region, Hippocampal / Q000166:cytology*; D018678:Cholinergic Agents / Q000494:pharmacology*; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005260:Female; D018756:GABA Antagonists / Q000494:pharmacology; D066298:In Vitro Techniques; D008297:Male; D051379:Mice; D009116:Muscarine / Q000494:pharmacology*; D009532:Nickel / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010721:Phosphinic Acids / Q000494:pharmacology; D010852:Picrotoxin / Q000494:pharmacology; D011412:Propanolamines / Q000494:pharmacology; D017966:Pyramidal Cells / Q000187:drug effects*; D011743:Pyrimidines / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D026941:Sodium Channel Blockers / Q000494:pharmacology; D015222:Sodium Channels / Q000187:drug effects / Q000502:physiology*; D014633:Valine / Q000031:analogs & derivatives / Q000494:pharmacology
D000109:Acetylcholine; D018678:Cholinergic Agents; D018691:Excitatory Amino Acid Antagonists; D018756:GABA Antagonists; D009116:Muscarine; D009532:Nickel; D010721:Phosphinic Acids; D010852:Picrotoxin; D011412:Propanolamines; D011743:Pyrimidines; D011810:Quinoxalines; D026941:Sodium Channel Blockers; D015222:Sodium Channels; D014633:Valine; C062865:2,3-dioxo-6-nitro-7-sulfamoylbenzo(f)quinoxaline; C082246:ICI D2788; C085075:CGP 55845A; C022838:nickel chloride; C056639:2-amino-5-phosphopentanoic acid