Optogenetic activation of GnRH neurons reveals minimal requirements for pulsatile luteinizing hormone secretion

The mechanisms responsible for generating the pulsatile release of gonadotropins from the pituitary gland are unknown. We develop here a methodology in mice for controlling the activity of the gonadotropin-releasing hormone (GnRH) neurons in vivo to establish the minimal parameters of activation required to evoke a pulse of luteinizing hormone (LH) secretion. Injections of Cre-dependent channelrhodopsin (ChR2)-bearing adeno-associated virus into the median eminence of adult GnRH-Cre mice resulted in the selective expression of ChR2 in hypophysiotropic GnRH neurons. Acute brain slice experiments demonstrated that ChR2-expressing GnRH neurons could be driven to fire with high spike fidelity with blue-light stimulation frequencies up to 40 Hz for periods of seconds and up to 10 Hz for minutes. Anesthetized, ovariectomized mice had optical fibers implanted in the vicinity of GnRH neurons within the rostral preoptic area. Optogenetic activation of GnRH neurons for 30-s to 5-min time periods over a range of different frequencies revealed that 10 Hz stimulation for 2 min was the minimum required to generate a pulse-like increment of LH. The same result was found for optical activation of GnRH projections in the median eminence. Increases in LH secretion were compared with endogenous LH pulse parameters measured from ovariectomized mice. Driving GnRH neurons to exhibit simultaneous burst firing was ineffective at altering LH secretion. These observations provide an insight into how GnRH neurons generate pulsatile LH secretion in vivo.