Synaptic Innervation of the GnRH Neuron Distal Dendron in Female Mice

GnRH neuron cell bodies are scattered throughout the basal forebrain but funnel their projections to the median eminence to release GnRH into the pituitary portal system to control fertility. Prior studies have shown that GnRH neurons located in the anterior hypothalamus send projections to the median eminence that have characteristics of both dendrites and axons. These unusual structures have been termed "dendrons." To address whether the dendron is unique to anterior hypothalamic GnRH neurons or is also a characteristic of more rostral GnRH neurons, we used viral vector‒mediated GnRH neuron‒specific tract-tracing coupled with CLARITY optical clearing. Individual rostral preoptic area GnRH neurons in female mice were identified to elaborate processes up to 4 mm in length that exhibited spines and projected all the way to the median eminence before branching into multiple short axons. The synaptic innervation patterns of distal GnRH neuron dendrons and their short axons in the vicinity of the median eminence were examined using electron microscopy. This revealed the presence of a high density of synaptic inputs to distal dendrons at the border of the median eminence. In contrast, no synapses were detected on any GnRH neuron axons. These studies demonstrate that GnRH neurons in the rostral preoptic area project dendrons to the edge of the median eminence, whereupon they branch into multiple short axons responsible for GnRH secretion. The dense synaptic innervation of these distal dendrons likely represents an efficient mechanism for controlling GnRH secretion required for fertility.

Pulse and Surge Profiles of Luteinizing Hormone Secretion in the Mouse

Using a new tail-tip bleeding procedure and a sensitive ELISA, we describe here the patterns of LH secretion throughout the mouse estrous cycle; in ovariectomized mice; in ovariectomized, estradiol-treated mice that model estrogen-negative and -positive feedback; and in transgenic GNR23 mice that exhibit allele-dependent reductions in GnRH neuron number. Pulsatile LH secretion was evident at all stages of the estrous cycle, with LH pulse frequency being approximately one pulse per hour in metestrous, diestrous, and proestrous mice but much less frequent at estrus (less than one pulse per 4 h). Ovariectomy resulted in substantial increases in basal and pulsatile LH secretion with pulses occurring approximately every 21 minutes. Chronic treatment with negative-feedback, estradiol-filled capsules returned LH pulse frequency to intact follicular phase levels, although pulse amplitude remained elevated. On the afternoon of proestrus, the LH surge was found to begin in a highly variable manner over a 4-hour range, lasting for more than 3 hours. In contrast, ovariectomized, estradiol-treated, positive-feedback mice exhibited a relatively uniform surge onset at approximately 0.5 hour prior to lights out. Gonadectomized wild-type and heterozygous GNR23 (∼200 GnRH neurons) male mice exhibited an LH pulse every 60 minutes. Homozygous GNR23 mice (∼80 GnRH neurons) had very low basal LH concentrations but continued to exhibit small amplitude LH pulses every 90 minutes. These studies provide the first characterization in mice of pulse and surge modes of LH secretion across the estrous cycle and demonstrate that very few GnRH neurons are required for pulsatile LH secretion.