Kisspeptin Neuron-Specific and Self-Sustained Calcium Oscillation in the Hypothalamic Arcuate Nucleus of Neonatal Mice: Regulatory Factors of its Synchronization

Activation of ionotropic and group I metabotropic glutamate receptors stimulates kisspeptin neuron activity in mice

Different populations of hypothalamic kisspeptin (KISS1) neurons located in the rostral periventricular area of the third ventricle (RP3V) and arcuate nucleus (ARC) are thought to generate the sex-specific patterns of gonadotropin secretion. These neuronal populations integrate gonadal sex steroid feedback with internal and external cues relayed via the actions of neurotransmitters and neuropeptides. The excitatory amino acid neurotransmitter glutamate, the main excitatory neurotransmitter in the brain, plays a role in regulating gonadotropin secretion, at least partially through engaging KISS1 signaling. The expression and function of individual glutamate receptor subtypes in KISS1 neurons, however, are not well characterized. Here, we used GCaMP-based calcium imaging and patch-clamp electrophysiology to assess the impact of activating individual ionotropic (iGluR) and group I metabotropic (mGluR) glutamate receptors on KISS1 neuron activity in the mouse RP3V and ARC. Our results indicate that activation of all iGluR subtypes and of group I mGluRs, likely mGluR1, consistently drives activity in the majority of KISS1 neurons within the RP3V and ARC of males and females. Our results also revealed, somewhat unexpectedly, sex- and region-specific differences. Indeed, activating (S)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type iGluRs evoked larger responses in female ARCKISS1 neurons than in their male counterparts whereas activating group I mGluRs induced larger responses in RP3VKISS1 neurons than in ARCKISS1 neurons in females. Together, our findings suggest that glutamatergic neurotransmission in KISS1 neurons, and its impact on the activity of these cells, might be sex- and region-dependent in mice.

Mechanism of Arcuate Kisspeptin Neuron Synchronization in Acute Brain Slices From Female Mice

The mechanism by which arcuate kisspeptin (ARNKISS) neurons co-expressing glutamate, neurokinin B, and dynorphin intermittently synchronize their activity to drive pulsatile hormone secretion remains unclear in females. In order to study spontaneous synchronization within the ARNKISS neuron network, acute brain slices were prepared from adult female Kiss1-GCaMP6 mice. Analysis of both spontaneous synchronizations and those driven by high frequency stimulation of individual ARNKISS neurons revealed that the network exhibits semi-random emergent excitation dependent upon glutamate signaling through AMPA receptors. No role for NMDA receptors was identified. In contrast to male mice, ongoing tachykinin receptor tone within the slice operated to promote spontaneous synchronizations in females. As previously observed in males, we found that ongoing dynorphin transmission in the slice did not contribute to synchronization events. These observations indicate that a very similar AMPA receptor-dependent mechanism underlies ARNKISS neuron synchronizations in the female mouse supporting the "glutamate two-transition" model for kisspeptin neuron synchronization. However, a potentially important sex difference appears to exist with a more prominent facilitatory role for tachykinin transmission in the female.

BDNF and KISS-1 Levels in Maternal Serum, Umbilical Cord, and Placenta: The Potential Role of Maternal Levels as Effect Biomarker

Brain-derived neurotrophic factor (BDNF) and kisspeptin-1 (KISS-1) regulate placental development and fetal growth. The predictive value of maternal serum BDNF and KISS-1 concentrations for placental and umbilical cord levels has not yet been explored. The influence of prenatal lead (Pb) and cadmium (Cd) exposure and maternal iron status on BDNF and KISS-1 levels is also unclarified and of concern. In a pilot cross-sectional study with 65 mother-newborn pairs, we analyzed maternal and cord serum levels of pro-BDNF, mature BDNF, and KISS-1, BDNF, and KISS-1 gene expression in placenta, Pb and Cd in maternal and umbilical cord blood (erythrocytes), and placenta. We conducted a series of in vitro experiments using human primary trophoblast cells (hTCs) and BeWo cells to verify main findings of the epidemiological analysis. Strong and consistent correlations were observed between maternal serum levels of pro-BDNF, mature BDNF, and KISS-1 and corresponding levels in umbilical serum and placental tissue. Maternal red blood cell Pb levels were inversely correlated with serum and placental KISS-1 levels. Lower expression and release of KISS-1 was also observed in Pb-exposed BeWo cells. In vitro Pb exposure also reduced cellular BDNF levels. Cd-treated BeWo cells showed increased pro-BDNF levels. Low maternal iron status was positively associated with low BDNF levels. Iron-deficient hTCs and BeWo cells showed a consistent decrease in the release of mature BDNF. The correlations between maternal BDNF and KISS-1 levels, placental gene expression, and umbilical cord serum levels, respectively, indicate the strong potential of maternal serum as predictive matrix for BDNF and KISS-1 levels in placentas and fetal sera. Pb exposure and iron status modulate BDNF and KISS-1 levels, but a clear direction of modulations was not evident. The associations need to be confirmed in a larger sample and validated in terms of placental and neurodevelopmental function.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12403-023-00565-w.

Mechanism of kisspeptin neuron synchronization for pulsatile hormone secretion in male mice

The mechanism by which arcuate nucleus kisspeptin (ARN^KISS) neurons co-expressing glutamate, neurokinin B, and dynorphin intermittently synchronize their activity to generate pulsatile hormone secretion remains unknown. An acute brain slice preparation maintaining synchronized ARN^KISS neuron burst firing was used alongside in vivo GCaMP GRIN lens microendoscope and fiber photometry imaging coupled with intra-ARN microinfusion. Studies in intact and gonadectomized male mice revealed that ARN^KISS neuron synchronizations result from near-random emergent network activity within the population and that this was critically dependent on local glutamate-AMPA signaling. Whereas neurokinin B operated to potentiate glutamate-generated synchronizations, dynorphin-kappa opioid tone within the network served as a gate for synchronization initiation. These observations force a departure from the existing "KNDy hypothesis" for ARN^KISS neuron synchronization. A "glutamate two-transition" mechanism is proposed to underlie synchronizations in this key hypothalamic central pattern generator driving mammalian fertility.

Crosstalk between kisspeptin and gonadotropin-inhibitory hormone in the silence of puberty: preclinical evidence from a calcium signaling study

Kisspeptin and gonadotropin-inhibitory hormone (GnIH) are among suggested neuroendocrine modulators of reproductive function. Intracellular calcium signaling is a critical component in the regulation of a variety of physiological and pathological processes including neurotransmitter release, and, therefore, can be used as signaling indicator for investigating the involvement of kisspeptin, GnIH, and gonadotropin-releasing hormone (GnRH) release. Hence, this study investigated the effects of kisspeptin and GnIH on calcium signaling using immortalized hypothalamic cells (rHypoE-8) as a model. Kisspeptin neurons were loaded with the ratiometric calcium dye (Fura-2 AM, 1 μmol) and intracellular free calcium ([Ca²⁺]_i) responses were quantified using digital fluorescence imaging system. Kisspeptin-10 (100, 300, and 1000 nM) caused a significant increase in [Ca²⁺]_i in rHypoE-8 cells (n = 58, n = 64, and n = 49, respectively, p < 0.001). The kisspeptin receptor antagonist, P234, inhibited the calcium responses to kisspeptin (p < 0.001, n = 32). GnIH (100 and 1000 nM), alone, did not cause any significant change in the mean basal [Ca²⁺]_i levels in kisspeptin cells, but GnIH attenuated the kisspeptin-evoked [Ca²⁺]_i transients (n = 47, p < 0.001). This novel findings of [Ca²⁺]_i signaling in in vitro setting implicate that kisspeptin and GnIH may exert their effects on hypothalamus-pituitary-gonadal (HPG) axis by modulating kisspeptin neurons. These results also implicate that kisspeptin neurons may have an autocrine regulation.

Heterogeneity in GnRH and kisspeptin neurons and their significance in vertebrate reproductive biology

Vertebrate reproduction is essentially controlled by the hypothalamus-pituitary-gonadal (HPG) axis, which is a central dogma of reproductive biology. Two major hypothalamic neuroendocrine cell groups containing gonadotropin-releasing hormone (GnRH) and kisspeptin are crucial for control of the HPG axis in vertebrates. GnRH and kisspeptin neurons exhibit high levels of heterogeneity including their cellular morphology, biochemistry, neurophysiology and functions. However, the molecular foundation underlying heterogeneities in GnRH and kisspeptin neurons remains unknown. More importantly, the biological and physiological significance of their heterogeneity in reproductive biology is poorly understood. In this review, we first describe the recent advances in the neuroendocrine functions of kisspeptin-GnRH pathways. We then view the recent emerging progress in the heterogeneity of GnRH and kisspeptin neurons using morphological and single-cell transcriptomic analyses. Finally, we discuss our views on the significance of functional heterogeneity of reproductive endocrine cells and their potential relevance to reproductive health.

GABAergic Inhibition of Presynaptic Ca2+ Transients in Respiratory PreBötzinger Neurons in Organotypic Slice Cultures

GABAergic somatodendritic inhibition in the preBötzinger complex (preBötC), a medullary site for the generation of inspiratory rhythm, is involved in respiratory rhythmogenesis and patterning. Nevertheless, whether GABA acts distally on presynaptic terminals, evoking presynaptic inhibition is unknown. Here, we begin to address this problem by measuring presynaptic Ca²⁺ transients in preBötC neurons, under rhythmic and non-rhythmic conditions, with two variants of genetically encoded Ca²⁺ indicators (GECIs). Organotypic slice cultures from newborn mice, containing the preBötC, were drop-transduced with jGCaMP7s, or injected with jGCaMP7f-labeling commissural preBötC neurons. Then, Ca²⁺ imaging combined with whole-cell patch-clamp or field stimulation was obtained from inspiratory preBötC neurons. We found that rhythmically active neurons expressed synchronized Ca²⁺ transients in soma, proximal and distal dendritic regions, and punctate synapse-like structures. Expansion microscopy revealed morphologic characteristics of bona fide synaptic boutons of the en passant and terminal type. Under non-rhythmic conditions, we found that bath application of the GABA_A receptor agonist muscimol, and local microiontophoresis of GABA, reduced action potential (AP)-evoked and field stimulus-evoked Ca²⁺ transients in presynaptic terminals in inspiratory neurons and commissural neurons projecting to the contralateral preBötC. In addition, under rhythmic conditions, network rhythmic activity was suppressed by muscimol, while the GABA_A receptor antagonist bicuculline completely re-activated spontaneous activity. These observations demonstrate that the preBötC includes neurons that show GABAergic inhibition of presynaptic Ca²⁺ transients, and presynaptic inhibition may play a role in the network activity that underlies breathing.

The KiNG of reproduction: Kisspeptin/ nNOS interactions shaping hypothalamic GnRH release

Gonadotropin-releasing hormone (GnRH) is the master regulator of the hypothalamic-pituitary-gonadal (HPG) axis, and therefore of fertility and reproduction. The release pattern of GnRH by the hypothalamus includes both pulses and surges. However, despite a considerable body of evidence in support of a determinant role for kisspeptin, the mechanisms regulating a GnRH pulse and surge remain a topic of debate. In this review we challenge the view of kisspeptin as an absolute "monarch", and instead present the idea of a Kisspeptin-nNOS-GnRH or "KiNG" network that is responsible for generating the "GnRH pulse" and "GnRH surge". In particular, the neuromodulator nitric oxide (NO) has opposite effects to kisspeptin on GnRH secretion in many respects, acting as the Yin to kisspeptin's Yang and creating a dynamic system in which kisspeptin provides the "ON" signal, promoting GnRH release, while NO mediates the "OFF" signal, acting as a tonic brake on GnRH secretion. This interplay between an activator and an inhibitor, which is in turn fine-tuned by the gonadal steroid environment, thus leads to the generation of GnRH pulses and surges and is crucial for the proper development and function of the reproductive axis.

Multiplexed CRISPR-Cas9 system in a single adeno-associated virus to simultaneously knock out redundant clock genes

The mammalian molecular clock is based on a transcription-translation feedback loop (TTFL) comprising the Period1, 2 (Per1, 2), Cryptochrome1, 2 (Cry1, 2), and Brain and Muscle ARNT-Like 1 (Bmal1) genes. The robustness of the TTFL is attributed to genetic redundancy among some essential clock genes, deterring genetic studies on molecular clocks using genome editing targeting single genes. To manipulate multiple clock genes in a streamlined and efficient manner, we developed a CRISPR-Cas9-based single adeno-associated viral (AAV) system targeting the circadian clock (CSAC) for essential clock genes including Pers, Crys, or Bmal1. First, we tested several single guide RNAs (sgRNAs) targeting individual clock genes in silico and validated their efficiency in Neuro2a cells. To target multiple genes, multiplex sgRNA plasmids were constructed using Golden Gate assembly and packaged into AAVs. CSAC efficiency was evident through protein downregulation in vitro and ablated molecular oscillation ex vivo. We also measured the efficiency of CSAC in vivo by assessing circadian rhythms after injecting CSAC into the suprachiasmatic nuclei of Cas9-expressing knock-in mice. Circadian locomotor activity and body temperature rhythms were severely disrupted in these mice, indicating that our CSAC is a simple yet powerful tool for investigating the molecular clock in vivo.

Ultradian Rhythms in the Hypothalamic Arcuate Nucleus Kisspeptin Neurons and Developmental Processes

Numerous physiological processes in nature have multiple oscillations within 24 h, that is, ultradian rhythms. Compared to the circadian rhythm, which has a period of approximately one day, these short oscillations range from seconds to hours, and the mechanisms underlying ultradian rhythms remain largely unknown. This review aims to explore and emphasize the implications of ultradian rhythms and their underlying regulations. Reproduction and developmental processes show ultradian rhythms, and these physiological systems can be regulated by short biological rhythms. Specifically, we recently uncovered synchronized calcium oscillations in the organotypic culture of hypothalamic arcuate nucleus (ARN) kisspeptin neurons that regulate reproduction. Synchronized calcium oscillations were dependent on voltage-gated ion channel-mediated action potentials and were repressed by chemogenetic inhibition, suggesting that the network within the ARN and between the kisspeptin population mediates the oscillation. This minireview describes that ultradian rhythms are a general theme that underlies biological features, with special reference to calcium oscillations in the hypothalamic ARN from a developmental perspective. We expect that more attention to these oscillations might provide insight into physiological or developmental mechanisms, since many oscillatory features in nature still remain to be explored.