Rat supraoptic magnocellular neurones show distinct large conductance, Ca²⁺‐activated K⁺ channel subtypes in cell bodies versus nerve endings

Abstract

Rat magnocellular neurones with cell bodies in the supraoptic and paraventricular nuclei of the hypothalamus send their axonal nerve endings into the neurohypophysis, where they release oxytocin or vasopressin into the bloodstream during highly specific firing patterns. Peptide release from the neurohypophysis is closely controlled by an interplay between the duration and frequency of the action potential burst, and the silence that separates the bursts generated by these neurones (Cazalis et al. 1985). Thus, understanding of peptide release from the hypothalamicneurohypophysial system requires characterization of the conductances that shape action potentials in both the cell bodies and their axonal endings. The physical separation of cell bodies and nerve endings in this system allows a unique opportunity to elucidate the differences in neuronal excitability, and underlying conductances in different cellular domains. Large conductance Ca¥-activated K¤ (BK) channels are ubiquitous in neurones. They typically exhibit high permeability and selectivity for K¤, and activation upon 1. Large conductance, Ca¥-activated K¤ (BK) channels were identified in freshly dissociated rat supraoptic neurones using patch clamp techniques. 2. The single channel conductance of cell body BK channels, recorded from inside-out patches in symmetric 145 mÒ K¤, was 246·1 pS, compared with 213 pS in nerve ending BK channels (P < 0·01). 3. At low open probability (Pï), the reciprocal of the slope in the ln(NPï)-voltage relationship (N, number of available channels in the patch) for cell body and nerve ending channels were similar: 11 vs. 14 mVper e-fold change in NPï, respectively. 4. At 40 mV, the [Ca¥]é producing half-maximal activation was 273 nÒ, as opposed to > 1·53 ìÒ for the neurohypophysial channel, indicating the higher Ca¥ sensitivity of the cell body isochannel. 5. Cell body BK channels showed fast kinetics (open time constant, 8·5 ms; fast closed time constant, 1·6 and slow closed time constant, 12·7 ms), identifying them as 'type I' isochannels, as opposed to the slow gating (type II) of neurohypophysial BK channels. 6. Cell body BK activity was reduced by 10 nÒ charybdotoxin (NPï, 37% of control), or 10 nÒ iberiotoxin (NPï, 5% of control), whereas neurohypophysial BK channels are insensitive to charybdotoxin at concentrations as high as 360 nÒ. 7. Whilst blockade of nerve ending BK channels markedly slowed the repolarization of evoked single spikes, blockade of cell body channels was without effect on repolarization of evoked single spikes. 8. Ethanol reversibly increased neurohypophysial BK channel activity (EC50, 22 mÒ; maximal effect, 100 mÒ). In contrast, ethanol (up to 100 mÒ) failed to increase cell body BK channel activity. 9. In conclusion, we have characterized BK channels in supraoptic neuronal cell bodies, and demonstrated that they display different electrophysiological and pharmacological properties from their counterparts in the nerve endings. Keywords:

Alcohol Tolerance in Large-Conductance, Calcium-Activated Potassium Channels of CNS Terminals Is Intrinsic and Includes Two Components: Decreased Ethanol Potentiation and Decreased Channel Density

“…Earlier findings indicated that the acute actions of ethanol differ between cellular compartments of magnocellular neurons, with BK channels located in the terminals potentiated by ethanol, whereas BK channels in the cell bodies were insensitive to ethanol (Dopico et al, 1999b). Single-channel recordings showed that the acute actions of ethanol targeted the gating of the channels: the contribution of long openings to the total time spent in the open Figure 6.…”

Somato-dendritic mechanisms underlying the electrophysiological properties of hypothalamic magnocellular neuroendocrine cells: A multicompartmental model study

“…1, the model contains several voltage-gated currents: the transient voltage-gated sodium current (I Na ) (Tanaka et al 1999), the delayed rectifier potassium current (I K(DR) ) , the A-type transient potassium current (I A ) , and L-and Ntype Ca 2+ currents (I Ca,L , I Ca,N ) (Foehring and Armstrong 1996;Joux et al 2001). It also contains the following Ca 2+ -dependent currents: a K + current mediated by large conductance K + (BK) channels (Dopico et al 1999), a K + current mediated by small conductance K + (SK) channels (Kirkpatrick and Bourque 1996), a non-selective cation (CAN) current (I CAN ) (Ghamari-Langroudi and Bourque 2002), and a slowly activated K + current in OT neurons Armstrong 1995,1997).…”

“…Activation of the SK channels is voltage independent and highly sensitive to intracellular Ca 2+ (Hille 2001;Sah and Davies 2000); BK channel activation is both voltage-and Ca 2+ -dependent (Vergara et al 1998;Dopico et al 1999). While activation of BK channels contributes to the falling phase of individual action potentials and the generation of the fast after hyperpolarizing potential (AHP) or hyperpolarizing after potential in many types of neurons (Lancaster and Adams 1986;Lancaster and Nicoll 1987;MacDermott and Weight 1982;Womack and Khodakhah 2002), including MNCs (Dopico et al 1999), SK channel activation is responsible for the generation of the AHP following a train of action potentials (Armstrong et al 1994;Bourque and Brown 1987;Greffrath et al 1998;Kirkpatrick and Bourque 1996;Lancaster and Adams 1986;Lancaster and Nicoll 1987;Sah and Bekkers 1996). This apamin-sensitive AHP is intermediate between the fast AHP (~50 ms) and "slow" AHP (lasting >5 s) (GhamariLangroudi and Bourque 2004).…”

“…Parameters of these channels in the model were based on data obtained in rat supraoptic MNCs (Dopico et al 1999) (Fig. 2(c)).…”

Essential role for smooth muscle BK channels in alcohol-induced cerebrovascular constriction

“…However, EtOH inhibition of BK channels is not universal, because both channel activation (18,19) and refractoriness (20) were observed. EtOH action on BK channels varies amongst slo subunit isoforms (21), cell domains (20), and bilayer lipid species (19). Thus, EtOH action on channels heterologously expressed may differ dramatically from drug action on the native channel studied in intact cerebrovascular myocytes.…”