IDENTIFICATION OF GLUTAMATE-RECEPTOR SUBTYPE MESSENGER-RNAS IN GONADOTROPIN-RELEASING-HORMONE NEURONS IN RAT-BRAIN

The aims of the present study were to determine: 1. If glutamate neuri tes can provide input to gonadotropin-releasing hormone (GnRH) neurons ; 2. Which glutamate receptor subtype mRNAs are expressed in GnRH neur ons; and 3. If GnRH neurons synthesize kainate 2 receptor (KA(2)) prot ein. Immunohistochemical double stainings for GnRH and glutamate or fo r GnRH and KA(2)-receptor protein were applied to rat brain sections c ontaining the medial septum-diagonal band and preoptic area or the med ian eminence; in addition, dual in situ hybridization studies were car ried out with digoxygenin-labeled cRNA probes encoding GnRH in combina tion with S-35-labeled cRNA probes encoding the glutamate receptor sub types GluR(1-4), KA(2), NMDAR(1), or NMDA R(2A-D). The results show th at GnRH neurons are surrounded by glutaminergic neurites, which form p uncta-like close appositions with the GnRH perikarya, and that an exte nsive overlap exists in the distribution of GnRH-positive axon termina ls and glutaminergic neurites in the median eminence. Similarly, KA(2) -receptor immunoreactivity is present in the perikarya of many GnRH ne urons and in their axon terminals in the median eminence. Dual in situ hybridization experiments show that about 32% of all digoxygenin-labe led 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 expres s measurable amounts of GluR(1-4) or NMDAR(2B-D) mRNA. The results sug gest that glutaminergic neurons innervate the GnRH neuronal system dir ectly through activation of KA(2) receptors on GnRH neurons, whereas t he effects of AMPA and NMDA on GnRH release are likely to be exerted i ndirectly through interneurons.