Long-term recordings of networks of immortalized GnRH neurons reveal episodic patterns of electrical activity

The CNS controls reproduction through pulsatile secretion of gonadotropin-releasing hormone (GnRH). Episodic increases in the firing rate of unidentified hypothalamic neurons have been associated with downstream markers of GnRH secretion. Whether this episodic electrical activity is intrinsic to GnRH neurons, intrinsic to other "pulse generator" neurons that drive GnRH neurons, or a combination of these is unknown. To determine if GnRH neurons display episodic firing patterns in isolation from other cell types, immortalized GnRH neurons (GT1-7 cells) were cultured on multiple microelectrode arrays. Long-term, multi-site recordings of GT1-7 cells revealed repeated episodes of increased firing rate with an interval of 24.8 +/- 1.3 (SE) min that were completely eliminated by tetrodotoxin, a sodium channel blocker. This pattern was comprised of active units that fired independently as well as coincidentally, suggesting the overall pattern of electrical activity in GT1-7 cells emerges as a network property. The A-type potassium-channel antagonist 4-aminopyridine (1 mM) increased both firing rate and GnRH secretion, demonstrating the presence of A-type currents in these cells and supporting the hypothesis that electrical activity is associated with GnRH release. Physiologically relevant episodic firing patterns are thus an intrinsic property of immortalized GnRH neurons and appear to be associated with secretion. The finding that overall activity is derived from the sum of multiple independent active units within a network may have important implications for the genesis of the GnRH secretory pattern that is delivered to the target organ. Specifically, these data suggest not every GnRH neuron participates in each secretory pulse and provide a possible mechanism for the variations in GnRH-pulse amplitude observed in vivo.