2-3-dioxo-6-nitro-7-sulfamoylbenzo(f)quinoxaline and nickel-chloride

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.

Topics: Acetylcholine; Action Potentials; Animals; Animals, Newborn; Biological Clocks; CA1 Region, Hippocampal; Cholinergic Agents; Excitatory Amino Acid Antagonists; Female; GABA Antagonists; In Vitro Techniques; Male; Mice; Muscarine; Nickel; Patch-Clamp Techniques; Phosphinic Acids; Picrotoxin; Propanolamines; Pyramidal Cells; Pyrimidines; Quinoxalines; Sodium Channel Blockers; Sodium Channels; Valine

R-type calcium channels in postsynaptic spines signal through functional calcium microdomains to regulate a calcium/calmodulin-sensitive potassium channel that in turn regulates postsynaptic hippocampal long-term potentiation (LTP). Here, we ask whether R-type calcium channels in presynaptic terminals also signal through calcium microdomains to control presynaptic LTP. We focus on presynaptic LTP at parallel fiber to Purkinje cell synapses in the cerebellum (PF-LTP), which is mediated by calcium/calmodulin-stimulated adenylyl cyclases. Although most presynaptic calcium influx is through N-type and P/Q-type calcium channels, blocking these channels does not disrupt PF-LTP, but blocking R-type calcium channels does. Moreover, global calcium signaling cannot account for the calcium dependence of PF-LTP because R-type channels contribute modestly to overall calcium entry. These findings indicate that, within presynaptic terminals, R-type calcium channels produce calcium microdomains that evoke presynaptic LTP at moderate frequencies that do not greatly increase global calcium levels.

Topics: Adenosine A1 Receptor Antagonists; Analysis of Variance; Animals; Animals, Newborn; Calcium; Calcium Channel Blockers; Calcium Channels, R-Type; Calcium Signaling; Cerebellum; Dose-Response Relationship, Drug; Electric Stimulation; Excitatory Amino Acid Antagonists; GABA Antagonists; In Vitro Techniques; Long-Term Potentiation; Membrane Microdomains; Neural Pathways; Nickel; omega-Agatoxin IVA; omega-Conotoxin GVIA; Patch-Clamp Techniques; Phosphinic Acids; Piperidines; Presynaptic Terminals; Propanolamines; Purkinje Cells; Pyrazoles; Quinoxalines; Rats; Rats, Sprague-Dawley; Sodium Channel Blockers; Spider Venoms; Tetrodotoxin; Xanthines