Contribution of Ca²⁺‐Activated K⁺ Channels to Hyperpolarizing After‐Potentials and Discharge Pattern in Rat Supraoptic Neurones

Abstract

The contribution of Ca(2+)-activated K(+) channels to hyperpolarizing after-potentials (HAP) of action potentials, to spike-frequency adaptation and thus to the shaping of discharge pattern, was examined in rat supraoptic magnocellular neurosecretory cells. In addition, the expression of BK channels and SK3 subunits of SK channels was studied using double immunofluorescence detection. The presence of BK channels and SK3 subunits was detected in many supraoptic neurones containing either vasopressin or oxytocin. Current-clamp recordings of current-induced spike trains revealed that HAPs comprise a fast and a slow HAP (fHAP and sHAP). Correlation analyses revealed that the increase of the fHAP in amplitude and spike broadening were correlated to a moderate gradual increase of the interspike interval and thus to weak spike-frequency adaptation. By contrast, marked prolongation of the interspike interval and strong spike-frequency adaptation depended on the appearance and on the amplitude of the sHAP. The sHAP and spike-frequency adaptation were blocked by cadmium, as well as by the SK channel antagonist apamin. The fHAP was attenuated by the BK channel antagonist iberiotoxin (IbTX), by the BK/IK channel antagonist charybdotoxin (ChTX) and by apamin. ChTX attenuated fHAPs throughout the entire spike train. By contrast, the IbTX-induced attenuation of the fHAP was restricted to the initial part of the spike train, while the apamin-induced attenuation slowly increased with the progression of the spike train. These results suggest that strong spike-frequency adaptation in supraoptic neurones essentially depends on the generation of the sHAP by activation of SK channels. Comparison of effects of IbTX, ChTX and apamin suggests a complementary contribution of SK-, BK- and IK-channels to fHAPs.

Hypertonicity increases NO production to modulate the firing rate of magnocellular neurons of the supraoptic nucleus of rats

“…Several hypotheses have been proposed to explain the generation and properties of the AHP. In MNCs, Bourque et al (1985) showed that calciumactivated potassium channels are involved in the generation of the AHP phase, which precedes the beginning of the ADP and the subsequent firing of an action potential (Greffrath et al, 2004). GhamariLangroudi and Bourque (2000) have shown the presence of hyperpolarization-activated and cyclic nucleotide-gated cation channels (HCNs) in MNCs which could determine the properties of the ADP.…”

Voltage-activated ion channels and Ca2+-induced Ca2+ release shape Ca2+ signaling in Merkel cells

“…Because mechanoreceptors are proposed to be tuned to a whisker's resonant frequency [50], it is possible that this differential expression of BK Ca channels in Merkel cells contribute to tuning at the cellular level, as they do in chick auditory hair cells [48,51,52]. In addition to tuning sensory receptor cells, BK Ca channels regulate neurotransmitter release in sensory cells [29,53] and excitability in a variety of systems [54][55][56][57]. Moreover, BK Ca provides critical feedback regulation of contractility of mechanically sensitive smooth muscle [58][59][60][61].…”

Small conductance Ca2+-activated K+ channels regulate firing properties and excitability in parasympathetic cardiac motoneurons in the nucleus ambiguus

“…Therefore, we assumed that SK channels significantly contribute to transient outward currents and I AHP . To study SK outward currents, we used TTX (0.5 M) and TEA (1 mM) to block Na ϩ and reduce voltage-dependent K ϩ and BK channels (sensitive to low concentrations of TEA) (20). In the following experiments, TTX and TEA were routinely added in the bath Fig.…”