GnRH Neurons Elaborate a Long-Range Projection with Shared Axonal and Dendritic Functions

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 bluelight 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.GnRH | luteinizing hormone | pulse | optogenetics

Section: Discussionsupporting

confidence: 77%

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

Campos

Herbison

2014

“…The ARN kisspeptin, or KNDy cells, synthesize numerous neuropeptidergic and classic transmitters (20,23,35) and innervate only the distal processes of GnRH neurons in mice, either at the level of the dendron or their nerve terminals in the median eminence (36,37). We found that 2-Hz stimulation of ARN KNDy neurons was completely ineffective in modulating LH secretion.…”

Section: Discussionmentioning

confidence: 74%

Selective optogenetic activation of arcuate kisspeptin neurons generates pulsatile luteinizing hormone secretion

Han

McLennan

Czieselsky

et al. 2015

160

111

Normal reproductive functioning in mammals depends upon gonadotropin-releasing hormone (GnRH) neurons generating a pulsatile pattern of gonadotropin secretion. The neural mechanism underlying the episodic release of GnRH is not known, although recent studies have suggested that the kisspeptin neurons located in the arcuate nucleus (ARN) may be involved. In the present experiments we expressed channelrhodopsin (ChR2) in the ARN kisspeptin population to test directly whether synchronous activation of these neurons would generate pulsatile luteinizing hormone (LH) secretion in vivo. Characterization studies showed that this strategy targeted ChR2 to 70% of all ARN kisspeptin neurons and that, in vitro, these neurons were activated by 473-nm blue light with high fidelity up to 30 Hz. In vivo, the optogenetic activation of ARN kisspeptin neurons at 10 and 20 Hz evoked high amplitude, pulse-like increments in LH secretion in anesthetized male mice. Stimulation at 10 Hz for 2 min was sufficient to generate repetitive LH pulses. In diestrous female mice, only 20-Hz activation generated significant increments in LH secretion. In ovariectomized mice, 5-, 10-, and 20-Hz activation of ARN kisspeptin neurons were all found to evoke LH pulses. Part of the sex difference, but not the gonadal steroid dependence, resulted from differential pituitary sensitivity to GnRH. Experiments in kisspeptin receptor-null mice, showed that kisspeptin was the critical neuropeptide underlying the ability of ARN kisspeptin neurons to generate LH pulses. Together these data demonstrate that synchronized activation of the ARN kisspeptin neuronal population generates pulses of LH.GnRH | kisspeptin | optogenetics | arcuate nucleus | gonadal steroids

“…These distal GnRH neuron projections, termed dendrons, are unusual in that they display both dendrite-and axon-like properties and are regulated directly by multiple neural inputs (35,36). We show here that ARN KISS neurons likely release kisspeptin for ∼1 min to generate an LH pulse.…”

Section: Discussionmentioning

confidence: 79%

Definition of the hypothalamic GnRH pulse generator in mice

Clarkson

Han

Piet

et al. 2017

312

287

The pulsatile release of luteinizing hormone (LH) is critical for mammalian fertility. However, despite several decades of investigation, the identity of the neuronal network generating pulsatile reproductive hormone secretion remains unproven. We use here a variety of optogenetic approaches in freely behaving mice to evaluate the role of the arcuate nucleus kisspeptin (ARN KISS ) neurons in LH pulse generation. Using GCaMP6 fiber photometry, we find that the ARN KISS neuron population exhibits brief (∼1 min) synchronized episodes of calcium activity occurring as frequently as every 9 min in gonadectomized mice. These ARN KISS population events were found to be near-perfectly correlated with pulsatile LH secretion. The selective optogenetic activation of ARN KISS neurons for 1 min generated pulses of LH in freely behaving mice, whereas inhibition with archaerhodopsin for 30 min suppressed LH pulsatility. Experiments aimed at resetting the activity of the ARN KISS neuron population with halorhodopsin were found to reset ongoing LH pulsatility. These observations indicate the ARN KISS neurons as the long-elusive hypothalamic pulse generator driving fertility.fertility | GnRH | kisspeptin | pulse | arcuate nucleus