Glutamate regulation of GnRH neuron excitability.

The gonadotropin-releasing hormone (GnRH) neuronal network is the master controller of the reproductive axis. It is widely accepted that the amino acid transmitters GABA and glutamate play important roles in controlling GnRH neuron excitability. However, remarkably few studies have examined the functional role of direct glutamate regulation of GnRH neurons. Dual-labeling investigations have shown that GnRH neurons express receptor subunits required for AMPA, NMDA and kainate signaling in a heterogeneous manner. Electrophysiological and calcium imaging studies have confirmed this heterogeneity and shown that while the majority of adult GnRH neurons express AMPA/kainate receptors, only small sub-populations have functional NMDA or metabotropic glutamate receptors. Accumulating evidence suggests that one important role of direct glutamate signaling at GnRH neurons is for their activation at the time of puberty. Whereas in vivo studies have indicated the importance of NMDA signaling within the whole of the GnRH neuronal network, including afferent neurons and glia, investigations at the level of the GnRH neuron suggest that peripubertal changes in AMPA receptor expression may be dominant in the mouse. The sources of glutamatergic inputs to the GnRH neurons are only just beginning to be examined and include the anteroventral periventricular nucleus as well as the possibility that GnRH neurons may use glutamate as a neurotransmitter in recurrent collateral innervation. It is expected that a full understanding of the glutamatergic regulation of GnRH neurons will provide significant insight into the mechanisms underlying their control of reproductive function.