GnRH neurons exhibit increased excitability during the afternoon of proestrus in the rodent leading to increased synthesis and neurosecretion of GnRH. We have focused on the cellular mechanisms underlying this increase in excitability both in GnRH neurons and in presynaptic neurons including those expressing kisspeptin. Kisspeptin neurons are vital for reproduction, and the rostral periventricular (RP3V) group synapse on GnRH neurons and are positively regulated by E2. Using sensitive single cell RT-PCR and whole-cell recording techniques, we have demonstrated that 17β-estradiol (E2) alters mRNA expression of numerous signaling molecules including T-type calcium, hyperpolarization activated (HCN) and canonical transient receptor potential (TRPC) channels in a dose-dependent manner, all of which collaborate to increase the excitability of GnRH neurons. Kisspeptin binding to its cognate G protein-coupled receptor (GPR54) on GnRH neurons activates TRPC channels via phosphatidylinositol-4,5,-biphosphate (PIP2) depletion and cSRC tyrosine kinase activation leading to prolonged excitation of GnRH neurons. Kisspeptin also inhibits a slow afterhyperpolarization current, which reduces spike frequency adaptation in GnRH neurons and prolongs burst firing. E2 likewise increases the expression of T-type calcium and HCN channels in RP3V kisspeptin neurons. Therefore, proestrous levels of E2 alter the excitability and burst firing of kisspeptin neurons, which provides the excitatory drive onto GnRH neurons necessary for the preovulatory surge of GnRH.
Supported by NIH grants NS043330, NS038809, DK068098.