Chapter 11 Physiology of the Adult Gonadotropin-Releasing Hormone Neuronal Network © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 11. 1 Distribution of GnRH neurons in the mouse brain FIGURE 11.1 Distribution of GnRH neurons in the mouse brain. Schematic diagrams showing the distribution of GnRH neurons (black dots) in the sagittal (top) and coronal (bottom) planes. The GnRH neuron continuum extends from the olfactory bulb through to the median eminence (ME), with the great majority of GnRH neurons residing within three arbitrarily defined anatomical brain regions: the medial septal (MS) population, the rostral preoptic area (rPOA) population, and the caudal-most anterior hypothalamic area (AHA) neurons. Note the projection zone for GnRH neurons in the median eminence. Coronal views at the three locations indicated on the sagittal plane are shown below where the concept of the “inverted Y” distribution is more clearly observed. Abbreviations: ac, anterior commissure; oc, optic chiasm; vdbb, vertical limb of the diagonal band of Broca; OVLT, organum vasculosum of the lamina terminals; 3V, third ventricle. Source: Diagram courtesy of Dr Michel Herde. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 11. 2 GnRH neuron morphology in the mouse and sheep FIGURE 11.2 GnRH neuron morphology in the mouse and sheep. The typical morphology of GnRH neurons in the mouse is shown by the cells in (A) and (B), with (B) exhibiting a clear “spiny” morphology following GnRH immunocytochemistry. The two cells in (C) exhibit the more complex and variable morphology of GnRH cell bodies in the sheep. (D), a montage showing the soma and dendritic structure of an adult mouse GnRH neuron filled with biocytin in situ. Note the length of the primary dendrite (over 500 μm) and the high density of spines throughout the length of the dendrite (two highlighted insets). © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 11.3 Typical firing patterns of adult rodent GnRH neurons in vitro (top) and in vivo (bottom). The three top traces show the typical bursting, continuous, and silent types of firing exhibited by adult GnRH neurons in the acute brain slice preparation using cell-attached recording mode. The bottom two recordings show cell-attached firing patterns from GnRH neurons in vivo in the anesthetized adult mouse. Each deflection up or down represents a single action potential. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 11. 4 Electrical properties of adult rodent GnRH neurons FIGURE 11.4 Electrical properties of adult rodent GnRH neurons. (A) Schematic (not to scale) diagram of the action potential and after potentials in GnRH neurons and the underlying currents. The action potential is generated in the normal manner by activation of voltage-gated sodium channels (Na) with repolarization occurring through inward rectifier (IRK) and large-conductance calciumactivated potassium (BK) channels. The A-type potassium current (A) is indicated to suppress the initiation of the action potential. Following the action potential, several different voltage- and calciumdependent channels are activated to create the after depolarization potential (ADP) and after hyperpolarization potential (AHP). These include the activation of a slow voltage-gated sodium channel (sNa) to generate the ADP and a slow small-conductance calcium-activated potassium channel (SK) and even slower calcium-activated potassium channel (UCL) that generates the AHP. (B) Experimental trace showing simultaneous recording of bursts of action potentials (top, each number shows the number of action potentials in the burst) and intracellular calcium transients (bottom) occurring in adult GnRH neurons. (C) Schematic diagram indicating known channels involved in generating burst firing in GnRH neurons. The sequence starts with a spike (1) that (2) initiates calcium entry through voltage- gated calcium channels (VGCC) and (3) activates the slow sodium channel (sNa) that, in turn, drives the ADP promoting burst © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 11. 5 Biosynthesis of GnRH and GAP FIGURE 11.5 Biosynthesis of GnRH and GAP. Top panel depicts the structure of the GnRH gene with the 5′ regulatory sequence of human, rat, and mouse overlaid and specific regions of known function highlighted (see text). Middle panel depicts GnRH RNA and various splicing and processing intermediates. Bottom panel shows translation and posttranslational modifications that result in the final GnRH decapeptide. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 11. 6 Modes of GABA action upon GnRH neurons FIGURE 11.6 Modes of GABA action upon GnRH neurons. Presynaptic GABA terminals from the RP3V and other brain sites release GABA into the synaptic space where it activates GABAA receptors to initiate phasic transmission and GABAB receptors to suppress excitability. GABA spilling outside of the synapse activates extrasynaptic GABAA receptors, comprised of delta GABAA receptor subunits that generate a tonic membrane hyperpolarization. GABA diffusing back to the nerve terminal can activate presynaptic GABAB receptors that act to inhibit calcium entry into the terminal and suppress presynaptic vesicular release. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 11.7 Sites of kisspeptin neurons in the hypothalamus and their projections in the mouse brain. Left: The locations of kisspeptin cells bodies (dots) in the hypothalamus of the rodent, sheep, and monkey are shown. Source: These schematic diagrams are based on data reported in Ref. 435–437. Right: the projections of AVPV (solid lines) and ARN (dotted lines) kisspeptin neurons are shown on a schematized horizontal flat-map of the mouse brain. Abbreviations: Aq, cerebral aqueduct; ARN, arcuate nucleus; AVPV, anteroventral periventricular nucleus; ac, anterior commissure; BNST, bed nucleus of the stria terminalis; DMN, dorsomedial nucleus; LS, lateral septum; LHA, lateral hypothalamic area; LPO, lateral preoptic area; LV, lateral ventricle; MEPO, median preoptic nucleus; MPA, medial preoptic area; MPN, medial preoptic nucleus; mtt, mammilothalamic tract; oc, optic chiasm; OVLT, organum vasculosum of the lamina terminalis; PAG, periaqueductal gray; PVN, paraventricular nucleus; PVPO, periventricular nucleus; vmn, ventromedial nucleus. Source: Published with permission from Yeo and Herbison438. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 11. 8 GnRH neuron synchronization FIGURE 11.8 GnRH neuron synchronization. Schematic diagram showing two potential models for GnRH neuron synchronization. In the Intrinsic Model, the GnRH neurons alone are able to form reciprocal communication with one another thus allowing a buildup of network excitability. The episodic nature of this activity might be driven by one subset of GnRH neurons (shaded) that determine the pace of the interconnectedness. In the Extrinsic Model, the synchronization of the GnRH neurons is generated and controlled by a remote set of neurons with pulse-generating properties. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 11. 9 Effects of estradiol on GnRH secretion in vivo FIGURE 11.9 Effects of estradiol on GnRH secretion in vivo. Portal plasma GnRH concentrations in two OVX ewes showing their response to parenteral 17β-estradiol administration. Note that GnRH secretion is clearly pulsatile with sharp episodes of GnRH secretion. Following estradiol there is a relatively rapid phase of negative feedback with reduced amplitude and frequency of GnRH pulses, which is then followed approximately 12 h later by an increase in pulsatile and baseline GnRH resulting in the GnRH surge. Source: Adapted with permission from Caraty et al. (1989).632 © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 11. 10 Model of estradiol feedback actions FIGURE 11.10 Model of estradiol feedback actions. Schematic diagram showing proposed modes of estradiol action within the GnRH neuron network for positive and negative feedback. α and β indicate location of ERα and ERβ, respectively. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 11. 11 Model of progesterone feedback actions FIGURE 11.11 Model of progesterone feedback actions. Schematic diagram showing proposed modes of progesterone action within the GnRH neuron network for positive and negative feedback. Abbreviations: PR, progesterone receptor; PgRMC1, progesterone receptor membrane component 1. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 11.12 Model of integrated circadian, estradiol, and progesterone influences within the GnRH neuronal network. Left: Schematized profiles of circulating progesterone (prog.), estradiol, and the daily circadian signal are given above the profile of GnRH secretion in ovariectomized (OVX) and intact rats. In OVX rats, the secretion of estradiol and progesterone is minimal, and this reveals “free-running” high amplitude, high frequency GnRH pulsatility that is not strongly entrained by the circadian input in adults. In intact female rats, significant progesterone secretion is only observed coincident with the onset of the GnRH surge (light gray vertical line). Whereas the initial period of progesterone secretion exerts a stimulatory feedback effect (marked as +ve), further boosting estrogen positive feedback, the subsequent progesterone secretion exerts negative feedback actions (marked as −ve). Estradiol feedback is proposed to exist as two separate feedback mechanisms that run simultaneously; the positive feedback pathway is entrained by the rising estradiol levels of the follicular phase and “breaks through” the relatively constant negative feedback pathway (marked as −ve) once every cycle to generate the GnRH surge. The precise timing of the surge is entrained by circadian inputs (black boxes) to the positive feedback pathway. Right: Schematized model showing the nature of the different pathways within the GnRH neuronal network. Circadian inputs regulate GnRH neurons through direct and indirect pathways. Both estradiol (E2) and progesterone (P4) positive feedback act primarily through an ERα/PR-expressing transsynaptic mode. In contrast, the negative feedback actions of both estradiol and progesterone appear to be multimodal utilizing direct, transsynaptic and glial cell modalities to suppress the activity of GnRH neurons. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition