Plasticity in a subpopulation of gonadotropin-releasing hormone neurons at the time of the pre-ovulatory surge.
Gonadotropin-releasing hormone (GnRH) neurons are the key output cells of the neuronal network controlling fertility.
In females, fluctuations in the activity of GnRH neurons result in cyclical changes in gonadal steroid hormones (e.g. estrogen) that, in turn, feed back to the brain and modulate GnRH neuron activities.
The mechanism through which estrogen feedback to the brain regulates GnRH neuronal activity to drive ovulation remains unclear.
However, a subset of GnRH neurons (~40%) is known to be activated at the time of the pre-ovulatory GnRH/LH (leutinising hormone) surge, as indicated by the expression of the immediate-early gene, cFos.
We aimed to determine whether estrogen feedback at this time induces synaptic plasticity, particularly within this activated subpopulation of GnRH neurons.
To do this, transgenic mice expressing green fluorescent protein (GFP) exclusively in GnRH neurons were used to quantify dendritic spine density (a correlative measure of excitatory synaptic input).
Five GnRH-GFP mice were put through a surge protocol which involves simulation of the negative estrogen feedback using estradiol capsules, and then on day 6 a subcutaneous bolus injection of estrogen to mimic the positive feedback stage prior to ovulation.
Five GnRH-GFP mice were treated with vehicle as controls, whereby sesame oil was used instead of estrogen.
Double-labelling immunofluorescence was performed for GFP (indicating GnRH neurons) and Fos (indicating activation), and confocal microscopy was used to visualise the cells.
A range of activated (Fos-positive) and non-activated (Fos-negative) GnRH neurons from each mice were imaged to be analysed.
We counted the number of somatic and dendritic spiny processes, and, using the results, calculated the total spine density (number of spines/μm) for activated and non-activated cells from the estrogen-treated animals, and for randomly selected cells from the vehicle controls (all non-activated, Fos negative).
We found a dramatically consistent increase (60%, P<0.0001) in total spine density within the subpopulation of GnRH neurons that were activated compared to not activated at the time of the surge.
This implies that during this time, activated neurons were receiving more excitatory synaptic input than others.
Additionally, total spine density was not different between non-activated GnRH neurons from surged animals and GnRH neurons from vehicle-treated animals.
This strongly suggests that there is morphological plasticity specifically within a subpopulation of GnRH neurons that are activated at the time of the GnRH/LH surge.
This novel finding establishes the existence of a functionally distinct group of GnRH neurons, which could be an important step in working out the way ovulation and fertility is regulated.
Supported by an Otago School of Medical Sciences Summer Research Scholarship.