GnRH (Gonadotrophin Releasing Hormone)

This week we'll be delving into the details for two reproductive hormones: GnRH and kisspeptin! For the purposes of this post, we'll be discussing GnRH.

GnRH neurons are scattered throughout the hypothalamus, with the highest concentration in the preoptic area of the hypothalamus (the preoptic area is near the optic chiasm). Interestingly enough, GnRH neurons actually originated from the olfactory epithelium in the nose before migrating into the hypothalamus. It is thought that neural cell adhesion molecules and cytokines may help influence axonal outgrowth and brain development.

One of the most important cytokines in GnRH neuron development and migration is probably SDF-1. SDF-1 and its receptor, CXCR4, are highly expressed during development in the cerebellum, hippocampus, neocortex and nasal compartment. CXCR4 and/or SDF-1 gene deletions are embryolethal.

GnRH neurons are very unique in their morphology. They have two long, axonal-like projections that go to the median eminence (the network of blood vessels connecting the hypothalamus and anterior pituitary- see here), but a closer inspection of biotin-stained neurons suggests that these axonal-like projections might actually be more dendritic in nature as they have structures known as "dendritic spines." As such, these projections are sometimes called dendrites or "dendrons."

GnRH neuronal "dendrons" are mostly in close apposition to "dendrons" from other GnRH neurons, forming "dendritic bundles." "Dendrons" also share synapses (i.e. they synapse onto the same axon from another neuron). These features help GnRH neurons to communicate with each other, which might help coordinate the pulsatile release of GnRH. "Dendrons" also extend outside the blood-brain barrier into the OVLT, which might help these neurons to obtain input from external stimuli.

As mentioned before, "dendrons" have dendritic spines. Dendritic spines are areas where neurons receive input, and are usually found in areas with GABAergic or glutamatergic transmission. The number of spines can fluctuate- there is a dramatic increase before puberty, and there are also increases in spine density around the time of the GnRH/LH surge in activated neurons.

Welp, that was a short and sweet post. Less typing! Yay!