Neonatal hypothalamic c-fos expression in an excitotoxicity-induced model of precocious puberty

We have used immunocytochemical detection of c-fos expression (FOS-like immunoreactivity, FLI) to establish the site of action of monosodium glutamate (MSG) in neonatal rats in a model of lesion-induced precocious puberty. The primary target appears to be the hypothalamic arcuate nucleus (ARC) but other circumventricular organs (CVO) are also affected (e.g. subfornical organ). Single injections of MSG (1-4 mg/g single dose, postnatal day 2 (P2)) which result in precocious puberty induce an area of edema, surrounded by a ring of FLI in the basal hypothalamus. In contrast, a maximal sub-lethal dose of NMDA (N-methyl-D-aspartate; 3 mg/kg) which produces a different pattern of ARC FLI, without edema, has no effect on puberty. Multiple doses of MSG (4 mg/g P2, P4, P6, P8), consistent with severe ARC damage and resultant sterility, markedly attenuates the FLI response by P8, with no visible edematous reaction following the final injection. In efforts to block the effect of MSG, pretreatment with the NMDA receptor-specific antagonist MK-801 (dizocilpine maleate) prevented the appearance of edema, as well as the onset of precocious puberty. However, MK-801 did not completely eliminate the FLI, but transformed the pattern of staining so that the original edematous area now contained many FOS-positive cells. This remaining MSG-induced FLI could not be eliminated by higher doses of MK-801 or by the non-NMDA antagonist DNQX (6,7-dinitroquinoxaline-2,3-dione). The combination of MK-801 and DNQX was also ineffective. MK-801 or DNQX had no effect on FLI when injected alone (i.e., without MSG) or in combination. The receptor which mediates MSG-induced c-fos expression, in the presence of MK-801 or DNQX, needs to be identified. We conclude, in conjunction with our previous work, that MSG induces precocious sexual maturation via an MK-801-sensitive mechanism associated with an edematous response of the basal hypothalamus. Whether the appearance of edema is indicative of an excitotoxic action of MSG, resulting in the removal of neurons inhibitory to sexual maturation, remains to be established.