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Although the excitatory amino acid (EAA) receptor agonist N-methyl-d-aspartate (NMDA) can exert profound stimulatory effects on the neuroendocrine reproductive axis of Syrian hamsters, the exact relationship between NMDA receptors and LHRH neurones is unclear. In the present study, in situ hybridization histochemistry was performed on sections of hamster brain using an 35S-labelled riboprobe to the EAA receptor gene, NMDAR1. A high content of NMDA receptor mRNA was detected not only in brain areas classically associated with specific NMDA binding (for example, hippocampus and cerebral cortex) but also in the hypothalamus, in particular the ventromedial–arcuate area; diffuse hybridization of the riboprobe also occurred in the medial–septal area and diagonal band of Broca, regions of the hamster brain in which the LHRH neuronal perikarya are primarily located. In a separate experiment, RNA was extracted from immortalized LHRH neurones (GT1–1 and GT1–7 cells) and used for northern analysis with a 32P-labelled NMDAR1 riboprobe. Clear-cut hybridization occurred with RNA bands of approximately 4.2 and 4.4 kb from the two LHRH neuronal subtypes. These findings suggest that at least some of the stimulatory action of EAAs on LHRH secretion is likely to be exerted directly at the level of the LHRH neurones rather than being mediated through interneurones. Furthermore, the demonstration of abundant NMDA receptor gene expression within hypothalamic areas that lie outside the blood–brain barrier adds plausibility to the concern that EAAs of dietary origin, such as monosodium glutamate, have the capacity to perturb the normal secretory activity of neuroendocrine circuits of the hypothalamus.
