Identification of glutamate receptor subtype mRNAs in gonadotropin-releasing hormone neurons in rat brain

The aims of the present study were to determine: 1. If glutamate neurites can provide input to gonadotropin-releasing hormone (GnRH) neurons; 2. Which glutamate receptor subtype mRNAs are expressed in GnRH neurons; and 3. If GnRH neurons synthesize kainate 2 receptor (KA(2)) protein. Immunohistochemical double stainings for GnRH and glutamate or for GnRH and KA(2)-receptor protein were applied to rat brain sections containing the medial septum-diagonal band and preoptic area or the median eminence; in addition, dual in situ hybridization studies were carried out with digoxygenin-labeled cRNA probes encoding GnRH in combination with S-35-labeled cRNA probes encoding the glutamate receptor subtypes GluR(1-4), KA(2), NMDAR(1), or NMDA R(2A-D). The results show that GnRH neurons are surrounded by glutaminergic neurites, which form puncta-like close appositions with the GnRH perikarya, and that an extensive overlap exists in the distribution of GnRH-positive axon terminals and glutaminergic neurites in the median eminence. Similarly, KA(2)-receptor immunoreactivity is present in the perikarya of many GnRH neurons and in their axon terminals in the median eminence. Dual in situ hybridization experiments show that about 32% of all digoxygenin-labeled GnRH neurons also contain KA(2)-receptor mRNA, 17% contain NMDA R(2A) mRNA, 8% contain NMDR R(1), whereas <5% of the GnRH neurons express measurable amounts of GluR(1-4) or NMDAR(2B-D) mRNA. The results suggest that glutaminergic neurons innervate the GnRH neuronal system directly through activation of KA(2) receptors on GnRH neurons, whereas the effects of AMPA and NMDA on GnRH release are likely to be exerted indirectly through interneurons.