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The Journal of Neuroscience, September 1, 2004, 24(35):7718-7726; doi:10.1523/JNEUROSCI.1240-04.2004

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Cellular/Molecular
Muscarinic Receptor Modulation of Slow Afterhyperpolarization and Phasic Firing in Rat Supraoptic Nucleus Neurons

Masoud Ghamari-Langroudi and Charles W. Bourque

Centre for Research in Neuroscience, Montreal General Hospital and McGill University, Montreal, Quebec H3G 1A4, Canada

A slow posttrain afterhyperpolarization (sAHP) was studied in rat magnocellular neurosecretory cells (MNCs) in vitro. The sAHP was isolated from other afterpotentials by blocking the depolarizing afterpotential (DAP) with Cs⁺ and the medium afterhyperpolarization (mAHP) with apamin. The sAHP amplitude increased logarithmically with activity (3 mV per e-fold increase in number of impulses) and, when firing stopped, decayed exponentially with a time constant of 2 sec. The sAHP was associated with increased membrane conductance, and its amplitude varied linearly with voltage, reversing at the K⁺ equilibrium potential. The sAHP was blocked by Cd²⁺ but not by charybdotoxin or iberiotoxin, blockers of intermediate- and big-conductance-type Ca²⁺-dependent K⁺ (K_Ca) channels. The sAHP was reversibly inhibited by muscarine, an effect antagonized by atropine, indicating involvement of muscarinic cholinergic receptors. Muscarine did not affect Ca²⁺-dependent features of action potentials, DAPs, or the mAHP in MNCs, indicating selective modulation of K_Ca channels causing the sAHP. Muscarinic inhibition of the sAHP enhanced plateau potentials and increased the mean firing rate and duration of afterdischarges that followed spike trains evoked from voltages near threshold. Similarly, the frequency and duration of the spontaneous phasic bursts that characterize physiologically activated vasopressin-releasing MNCs were enhanced by muscarine. MNCs thus express apamin- and voltage-insensitive K_Ca channels that mediate an sAHP. The activity dependence and kinetics of the sAHP cause it to mask DAPs in a manner that attenuates the amplitude of plateau potentials. Muscarinic inhibition of the sAHP provides an effective mechanism for promoting phasic firing in MNCs.

Key words: vasopressin; supraoptic; afterdischarge; neurohypophysis; burst; muscarinic

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Received April 2, 2004; revised July 19, 2004; accepted July 19, 2004.

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