The SK channel blocker apamin inhibits slow afterhyperpolarization currents in rat gonadotropin‐releasing hormone neurones

Abstract

Gonadotropin-releasing hormone (GnRH) neurones play an essential role in the hypothalamopituitary-gonadal axis. As for other neurones, the discharge pattern of action potentials is important for GnRH neurones to properly function. In the case of a luteinizing hormone (LH) surge, for example, GnRH neurones are likely to continuously fire for more than an hour. For this type of firing, GnRH neurones must have a certain intrinsic property. To address this issue, we investigated the voltage-gated Ca2+ currents and Ca 2+ -activated voltage-independent K + currents underlying afterhyperpolarization, because they affect cell excitability. Dispersed GnRH neurones from adult GnRH-EGFP (enhanced green fluorescent protein) transgenic rats were cultured overnight and then used for an electrophysiological experiment involving the perforated patch-clamp configuration. The GnRH neurones showed five subtypes of voltage-gated Ca 2+ currents, i.e. the T-, L-, N-, P/Q-and R-types. The GnRH neurones also showed a slow afterhyperpolarization current (I sAHP ), but not a medium one. It is reported that the SK channel blocker apamin (10 pM-100 nM) blocks a medium afterhyperpolarization current but not an I sAHP . In contrast to previous reports, the I sAHP observed in rat GnRH neurones was potently blocked by apamin. In addition, the GnRH neurones expressed transcripts for SK1-3 channels. The results indicate that rat GnRH neurones express all five subtypes of voltage-gated Ca 2+ channels and exhibit an apamin-sensitive I sAHP , which may allow continuous firing in response to a relatively strong depolarizing input.

Two Slow Calcium-Activated Afterhyperpolarization Currents Control Burst Firing Dynamics in Gonadotropin-Releasing Hormone Neurons

“…The one exception to this is the GnRH neuron, for which the apaminsensitive current persists for several seconds in both mice and rats (Kato et al, 2006;Spergel, 2007;Liu and Herbison, 2008). As GnRH neurons express mRNA for all three SK channels (Bosch et al, 2002;Kato et al, 2006) it is very likely that SK channels underlie the apamin-sensitive currents (sI AHP-SK ) in these cells. However, it is unclear why the sI AHP-SK decay is so prolonged in these cells and best modeled by a two phase dynamic comprised of a rapid (ϳ0.3 s) and a long decay (ϳ19 s).…”

“…In nearly all neurons examined, the SK channel is implicated in mediating the majority of the mAHP with little direct role in the sAHP Pedarzani and Stocker, 2008). The one exception to this is the GnRH neuron, for which the apaminsensitive current persists for several seconds in both mice and rats (Kato et al, 2006;Spergel, 2007;Liu and Herbison, 2008). As GnRH neurons express mRNA for all three SK channels (Bosch et al, 2002;Kato et al, 2006) it is very likely that SK channels underlie the apamin-sensitive currents (sI AHP-SK ) in these cells.…”

“…Ca 2ϩ coming in from outside then stimulates the release of additional Ca 2ϩ from the ER, via IP 3 -gated Ca 2ϩ channels, resulting in an amplified Ca 2ϩ transient. Prior studies in GnRH neurons have shown that an apaminsensitive SK channel controls spike frequency adaptation in these cells (Kato et al, 2006;Liu and Herbison, 2008). This channel (sI AHP-SK ) was modeled to help terminate electrical bursting, while another slow Ca 2ϩ -sensitive K ϩ current (sI AHP-UCL ), was necessary to set the IBI in GnRH neurons.…”

Kisspeptin inhibits a slow afterhyperpolarization current via protein kinase C and reduces spike frequency adaptation in GnRH neurons

“…It has been reported that GnRH neurons express both an apamin-sensitive I sAHP and an apamin-insensitive I sAHP (22,29). Therefore, to examine whether kisspeptin inhibits the apamin-insensitive I sAHP , we first examined the effects of kisspeptin on the apamin-insensitive currents after apamin application.…”

“…GnRH neurons express a mAHP current and both an apamin-sensitive and an apamin-insensitive I sAHP (5,8,22,29,30,51). In most central neurons, the mAHP controls action potential discharge frequency, whereas the sAHP is largely responsible for producing spike frequency adaptation, i.e., decreased firing in the face of a sustained depolarizing stimulus (47).…”