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|Title:||AN OSMOSENSITIVE M-LIKE K+ CURRENT THAT MAY REGULATE FIRING PATTERNS IN RAT SUPRAOPTIC NEURONS|
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|Authors/Affiliations:||1 Wenbo Zhang*; 1 W.R.A. Kosala Rajapaksha; 1 Thomas Fisher; |
1 University of Saskatchewan, Saskatoon, SK, Canada
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|Content:||Objectives: The magnocellular neurosecretory cells of the hypothalamus (MNCs) change their electrical behaviour in response to small changes in external osmolality through the regulation of osmosensitive ion channels. The rate and pattern of action potentials generated in the MNC somata determine the release of vasopressin, which is the primary hormone that regulates osmolality by controlling water excretion from the kidney. The mechanisms underlying the osmolality-induced changes in MNC firing are incompletely understood. Our studies will explore the properties and function of a recently identified K+ current that may be involved in regulating firing patterns in MNCs.|
Materials and Methods: MNCs were acutely isolated from rat supraoptic nuclei and whole cell patch-clamp recordings were made at room temperature. Single unit extracellular recordings were made in rat hypothalamic explants. The osmolalities of the external recording solutions were adjusted by adding mannitol.
Results: A voltage-gated, 4-AP- and TEA-insensitive slowly activating outward current displayed a significant increase in about 65% of MNCs when the osmolality of the external solution was increased from 295 mOsm (12.3 ▒ 1.3pA/pF) to 325 mOsm (21.4 ▒ 1.8 pA/pF). The M-current selective blockers linopirdine (150 ÁM) and XE 991 (5 ÁM) suppressed this current. The IC50 of XE 991 blockade is about 5 μM, which is consistent with a selective affect on M-currents. Two M-current openers, retigabine (10 ÁM) and flupirtine (10 ÁM), significantly increased the current and shifted the voltage dependence of its activation to more negative potentials. E-4031, a potent and specific blocker of ERG K+ channels, did not significantly block this current. Results from RT-PCR and immunocytochemical experiments suggest that MNCs express KCNQ2, KCNQ3, KCNQ4, and KCNQ5, but not KCNQ1. Studies using single unit extracellular recording in rat hypothalamic explants showed that 10 ÁM XE 991 increased MNC firing rate and the duration of spontaneous bursts and that 20 ÁM retigabine decreased firing rate or caused a cessation of firing in rat MNCs.
Conclusion: These data suggest that this slowly activating osmosensitive current is an M-like K+ current and that it may play an important role in regulating firing patterns in MNCs. This is the first demonstration of a voltage dependent current in the MNC somata that is modulated by external osmolality. M-currents are known to play key roles in regulating neuronal firing frequency and excitability in many central neurons. Slow activation of the osmosensitive M-like K+ current during firing might suppress activity by hyperpolarizing the cell and this could contribute to the transition between continuous and burst firing. This research was supported by a Regional Partnership Program research grant from CIHR and the Saskatchewan Health Research Foundation to T.E.F. and a CGS Doctoral Research Award from the CIHR to W.Z.
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