ReviewIdentified GnRH neuron electrophysiology: A decade of study
Introduction
The gonadotropin-releasing hormone (GnRH) neuronal system is the final common pathway for central regulation of fertility. Through its regulation of the synthesis and secretion of the pituitary gonadotropins luteinizing hormone (LH, often used as a surrogate marker for GnRH release) and follicle-stimulating hormone (FSH), GnRH exerts control over the hypothalamo–pituitary gonadal axis. Multiple inputs from the internal environment (e.g., steroid hormones, energy balance, stress) and external environment (e.g., photoperiod, pheromones, endocrine disruptors) can alter fertility through central actions that ultimately alter the release of GnRH. Even before the sequencing of the GnRH decapeptide (Baba et al., 1971), both the innate function of this system and its response to various stimuli were intensely studied. During much of this time, the ability to tease out the neurobiological mechanisms underlying observations of altered GnRH or LH release was limited by the technical approaches available and the scattered distribution of GnRH neurons. It was possible, for example, to determine if a factor acted centrally by intracerebroventricular application or treatment of hypothalamic explants or cultures and to speculate about upstream afferents through the localization of steroid receptors or expression of the immediate early gene cFos (Hoffman et al., 1990, Lehman and Karsch, 1993, Shivers et al., 1983), knife cut studies (Blake and Sawyer, 1974) or localized administration of steroids (McManus et al., 2005). But understanding the intrinsic, synaptic and network properties of these neurons was very limited in mammalian systems. In contrast, electrophysiological work in the teleost dwarf gourami provided considerable insight into pacemaker activities of the terminal nerve GnRH neurons, which can be easily identified by the eye (Oka, 1995, Oka and Matsushima, 1993).
Despite the limitations largely precluding direct study of mammalian GnRH neurons, a great deal of knowledge about the reproductive neuroendocrine system was acquired and has served as the basis for designing and interpreting experiments that are now taking advantage of new model systems. The pioneering work of Ernst Knobil's group describing circhoral oscillations of LH in the circulation (Dierschke et al., 1970), the importance of episodic GnRH stimulation for pituitary function (Belchetz et al., 1978, Wildt et al., 1981) and central electrical correlates with LH release (Wilson et al., 1984) still provides the fundamental description of this system as a pulse generator and drives many current research questions. Several groups independently developed methods to measure directly GnRH release (Caraty and Locatelli, 1988, Clarke and Cummins, 1982, Levine et al., 1985, Levine and Ramirez, 1980). Both immortalized GnRH neuron cell lines (Mellon et al., 1990, Radovick et al., 1991) and primary embryonic cultures derived from the migratory pathway of GnRH neurons first in the primate (Terasawa et al., 1993) and later in the mouse (Fueshko and Wray, 1994) also provided insight into the function of this system. None of these approaches, however, allowed examination of native adult GnRH neurons in the context of at least a portion of their normal network using the powerful electrophysiological methods that were being applied to other central neuronal cell types that could be identified by anatomical location.
Martin Kelly's group was the first to study individual GnRH neurons using electrophysiological recordings in acutely prepared brain slices, demonstrating that GnRH neurons were acutely hyperpolarized by estradiol in the guinea pig model (Kelly et al., 1984). This and subsequent work in this model (Lagrange et al., 1995, Wagner et al., 1998) identified the first intrinsic properties of mammalian GnRH neurons. The effort needed to accumulate GnRH neurons, however, was heroic; in the first study, which utilized Procion yellow labeling during blind sharp electrode recordings followed by post hoc identification, only 5 of 102 cells studied were GnRH positive on post hoc examination.
About a decade ago, promoter-driven reporter genes were applied to the identification of GnRH neurons. The cell-specifically and strongly expressed GnRH promoter is well suited to this approach and was used to drive expression of modified jellyfish Aequorea victoria green fluorescent protein (GFP), beta galactosidease or the calcium indicator Pericam (Han et al., 2004, Jasoni et al., 2007, Kanda et al., 2010, Kato et al., 2003, Skynner et al., 1999, Spergel et al., 1999, Suter et al., 2000a, Wayne et al., 2005) in GnRH neurons from mouse, rat and medaka. Primarily using the GFP models, considerable progress has been made over the past decade in the understanding of the neurobiology of GnRH neurons. These studies will form the basis of this review. The reader is also pointed to a recent excellent review on the intracellular calcium dynamics in GnRH neurons (Jasoni et al., In press) and previous reviews on the physiology of the GnRH neuronal system (Herbison, 2006, Moenter et al., 2003).
Section snippets
An overview of electrophysiological approaches used in the GnRH system
The purpose of this review is not to provide a complete primer on electrophysiological approaches, which are available elsewhere (Hille, 2001, Sakman and Neher, 1995). What follows is a brief overview of the different approaches that have been used on GnRH neurons, their plusses and minuses and the types of data that can be acquired with each.
Intrinsic properties of GnRH neurons
Observations of the episodic activity of multiunit electrical activity and hormone release at the whole animal level (Moenter et al., 1992, Wilson et al., 1984) coupled with the electrophysiology magnocellular neuroendocrine neurons (Andrew and Dudek, 1983, Dutton and Dyball, 1979, Dutton et al., 1978, Nordmann and Stuenkel, 1986, Wakerley et al., 1978, Wakerley et al., 1975) showing that action potential firing, in particular burst firing, was needed for hormone release, generated several
Steroid modulation of GnRH neuron firing pattern and underlying conductances
A critical aspect of the GnRH neuronal system is its feedback regulation by steroid hormones. GnRH neurons express the beta isoform of the estradiol receptor (Herbison and Pape, 2001, Hrabovszky et al., 2000, Hrabovszky et al., 2001), but most other steroid receptors have only rarely, if at all, been detected in GnRH neurons. This has led to an overall view in the field that steroid regulation of GnRH neurons engages steroid-sensitive afferent neurons (Wintermantel et al., 2006). Direct
Neurotransmitters, neuromodulators and GnRH neuron function
When discussing either acute or chronic effects of steroids on GnRH neurons, the likelihood of intermediate neurons being involved necessitates an examination of these factors. The primary conveyors of fast synaptic transmission (mediated by ionotropic vs. metabotropic receptors) are GABA and glutamate. GnRH neurons receive spontaneous glutamatergic transmission via both AMPA/KA and NMDA receptors (Chen and Moenter, 2009, Christian et al., 2009, Spergel et al., 1999, Suter, 2004), but the
Coordination of GnRH neurons
The episodic nature of GnRH release mandates some level of coordination among these cells. Studies in GT1 cells suggested that not every cell participates in every burst of activity (Nunemaker et al., 2001). Similarly, oscillations in intracellular calcium levels indicate coordination among a part of the population (Abe et al., 2008). Unfortunately, relatively little experimental evidence can positively support a mechanism for how the GnRH neuronal network produces pulses, however some findings
Summary
The past decade has seen a major increase in our knowledge of the intrinsic and synaptic properties of GnRH neurons. The expanding repertoire of intrinsic and synaptic properties can continue to be incorporated into sophisticated models that generate new hypotheses to be tested about the network. Questions we are now poised to address include the following: What are the roles of non-GnRH neuron and non-neuronal elements in coordinating the network? How do the various conductances that
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2022, Current Opinion in Endocrine and Metabolic ResearchCitation Excerpt :The GnRH neuron is an integral part of the pulse generator as it secretes GnRH to the pituitary; however, this does not necessarily mean the generator is solely located within the GnRH neuronal population. As mentioned above, the pulsatile nature of GnRH release can be externally modulated or even wholly driven by upstream inputs, with the obvious possibility being stimulation by kisspeptin [52]. Given that kisspeptin acts directly on GnRH neurons [53,54] and is a strong candidate for influencing GnRH release, the model previously developed by Chen et al. [17] was expanded to account for the impact of kisspeptin on the behaviour of the GnRH neuron [55].
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2020, Frontiers in NeuroendocrinologyCitation Excerpt :Implicit in all of these models was the assumption that the ultimate pattern of GnRH secretion results from neural integration of various afferent pathways mediating the positive and negative feedback signals at the GnRH neuron cell body. As such, very many studies have compared the morphological and electrical properties of the GnRH neuron soma during conditions simulating estrogen negative and positive feedback (Moenter 2010, Herbison 2015). This mini-review first describes the proposed model and then provides a biological basis for its components and operation.
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2016, Current Opinion in NeurobiologyCitation Excerpt :Gravid females respond to the courtship hum of the males, and their sensitivity to the frequency range of the hum increases during the summer when they lay eggs. Across all vertebrates, GnRH1 neurons exhibit coordinated, pulsatile activity, which is postulated to overcome a signaling threshold in the pituitary [35•]. Gonadotropin-producing cells of the pituitary rapidly desensitize to tonic levels of GnRH1 [36••].
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