Calcium and Small-Conductance Calcium-Activated Potassium Channels in Gonadotropin-Releasing Hormone Neurons before, during, and after Puberty

Abstract

The pubertal increase in GnRH secretion resulting in sexual maturation and reproductive competence is a complex process involving kisspeptin stimulation of GnRH neurons and requiring Ca(2+) and possibly other intracellular messengers. To determine whether the expression of Ca(2+) channels, or small-conductance Ca(2+)-activated K(+) (SK) channels, whose activity reflects cytoplasmic free Ca(2+) concentration, changes at puberty in GnRH neurons, Ca(2+) and SK currents in GnRH neurons were recorded in brain slices of juvenile [postnatal day (P) 10-21], pubertal (P28-P42), and adult (> or =P56) male GnRH-green fluorescent protein transgenic mice using perforated-patch and whole-cell techniques. Ca(2+) currents were inhibited by the Ca(2+) channel blocker Cd(2+) and showed marked heterogeneity but were on average similar in juvenile, pubertal, and adult GnRH neurons. SK currents, which were inhibited by the SK channel blocker apamin and enhanced by the SK and intermediate-conductance Ca(2+)-activated K(+) channel activator 1-ethyl-2-benzimidazolinone, were also on average similar in juvenile, pubertal, and adult GnRH neurons. These findings suggest that whereas Ca(2+) and SK channels may participate in the pubertal increase in GnRH secretion and there may be changes in Ca(2+) or SK channel subtypes, overall Ca(2+) and SK channel expression in GnRH neurons remains relatively constant across pubertal development. Hence, the expected increase in GnRH neuron cytoplasmic free Ca(2+) concentration required for increased GnRH secretion at puberty appears to be due to mechanisms other than altered Ca(2+) or SK channel expression, e.g. increased membrane depolarization and subsequent activation of preexisting Ca(2+) channels after increased excitatory synaptic input.

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

“…This I sAHP current peaks in several hundred milliseconds, decays with a time constant from 1 to 10 s, and is not sensitive to apamin inhibition in most neurons (4,46,47,54). However, GnRH neurons have been found to express both an apamin-insensitive I sAHP and an apaminsensitive I sAHP (22,29,30,51) (present findings). It is most likely that the SK channel mediates the apamin-sensitive I sAHP (and I mAHP ) in GnRH neurons (1).…”

“…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).…”

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

“…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.…”

Synaptic integration in hypothalamic gonadotropin releasing hormone (GnRH) neurons

“…For these reasons, we did not include the T-type calcium, muscarinic potassium, or pacemaker Ca 2+ -carrying inward leak currents from the models of LeBeau et al, (2000). Somata contained Fast Na + , Delayed Rectifier K + (Kusano et al, 1995) and Inward Rectifier K + (Kusano et al, 1995;Wagner et al, 1998), and L-type Ca 2+ conductances (Kusano et al, 1995;Spergel 2007). Fast Na + and Delayed Rectifier K + conductances were incorporated in dendrites and axons.…”