R E Otoya | PubMed Health

PubMed1997

Acute and long-lasting effects of neonatal hypoxia on (+)-3-[125I]MK-801 binding to NMDA brain receptors.

The NMDA receptor subtype is the major excitatory mediator for glutamate neurotoxicity. To assess its participation in the noxious effects of postnatal hypoxia, we have characterized the binding of the ionophoric marker of NMDA receptor, dizocilpine (MK-801). Binding of (+)-3-[125I]MK-801 to NMDA brain receptors under nonequilibrium conditions was quantified by in vitro autoradiography in rats exposed to hypoxia induced by 93% N2/6.5% O2 exposure for 70 min on Postnatal Day 4. Acute and long-lasting effects were investigated at 4 h after injury and on Postnatal Day 40. At the acute stage, a transient decrease in binding was found in several specific brain areas, hypothalamus, amygdaloid nuclei, entorhinal cortex, perirhinal cortex, and hippocampus, and no differences were found in temporal cortex, thalamus, and geniculate nucleus, when compared to sham-treated animals. At this early age, there was no increase of binding when slices from both groups were incubated in the presence of glutamate and glycine (Glu/Gly), positive allosteric modulators of MK-801 binding. In the 40-day-old brains, the binding to the NMDA receptors of hypoxiatreated animals was not different with respect to controls in most of the areas studied, but the Glu/Gly stimulation of binding in hypoxic rats showed a reduced, or absent, response to the allosteric modulators. In contrast, control rats showed a remarkable increase of the specific binding induced by the presence of the modulators in the incubation buffer. Binding of (+)-3-[125I]MK-801 was also performed at a higher concentration to clarify whether the altered response to Glu/Gly may be due to differences in the number of channels; however, the density of NMDA receptors at this concentration was similar in both control and hypoxia-treated rats. We conclude that the effect of exposure of newborn rats to hypoxia can generate acute and long-lasting effects on the NMDA receptor. The deleterious action of this kind of noxa on the CNS could be exerted by interference with normal glutamatergic transmission and hence over normal growth and development.

Acute Disease↗

PubMed1996

Decrease of (+)-3-[125I]MK-801 binding to NMDA brain receptors revealed at puberty in rats treated neonatally with monosodium glutamate.

Obesity, altered pattern of gonadal hormone secretion, advanced vaginal opening, irregular cycling, altered sexual behavior and infertility are the effects of the neonatal administration of monosodium glutamate (MSG) to rodents. These are the consequences of lesions located mainly in the hypothalamic region. It is believed that the receptors to N-methyl-D-aspartic acid (NMDA) actively participate in the onset and development of such lesions, on the other hand, they may be altered by neuronal dysfunction as well, seriously compromising the glutamatergic pathways that are involved in the neuroendocrine regulation. To clarify the scope of the lesion induced by MSG and its probable effects on the NMDA receptors, we measured them with a very sensitive ligand for autoradiography, (+)-3-[125I]MK-801. Coronal cuts at the level of the arcuate-median eminence of brains from 4-, 8- and 40-day-old rats treated neonatally with MSG (4 mg/g) or saline (controls) were examined. In the normal hypothalamus, NMDA receptor labelling was higher in the young animals than in the 40-day-old animals, and this was observed in both control and treated rats. NMDA receptor labelling of rats at puberty was very low, and no apparent differences were observed between groups. In contrast, in areas where an increase in NMDA binding sites normally occurs with development, a significant impairment of the normal augmentation of MK-801 binding was revealed. In the hippocampal layers, stratum radiatum and stratum oriens and in the cerebral cortex of 40-day-old rats treated with MSG a lower amount of binding was observed, of about 50% fewer sites compared to the untreated controls at the level of CA3 and in the outer layer of the parietal cortex. These results suggest that at an early stage of the MSG lesion the NMDA receptors located in the hypothalamus and other brain areas are apparently expressed normally, but at puberty the effects of the lesion are revealed in the hippocampus and cerebral cortex by a decrease in the density of binding. Thus, the abnormal neuroendocrine and behavioral responses displayed by the MSG-treated rats may be contributed partially by the alteration of the NMDA receptors in these areas.

Animals↗