The role of glutamatergic receptors in the hypoxia-induced increases of [Ca2+]i in the superior colliculus in fetal rats: in vivo optical recordings

Abstract

Using the optical recordings, we studied the role of non-NMDA and NMDA receptors in the hypoxia-induced increases of [Ca2+]i in the superior colliculus (SC) in a fetal rat, which was still connected with the dam by the umbilical cord. The changes of [Ca2+]i in the SC were recorded as fractional changes in the intensity of fluorescent dye (Fura-2) by an optical recording system. Hypoxia was caused by occlusion of the umbilical cord with a small clip. Focal stimulation of the SC was made by a single and train pulse at currents of 0.5-3 mA via a bipolar electrode inserted into the SC. The umbilical cord occlusion caused marked increases in [Ca2+]i. After the pretreatment of CNQX and AP-5, the occlusion-induced increase in [Ca2+]i was remarkably reduced. Picrotoxin enhanced the occlusion-induced increase in [Ca2+]i. Focal SC stimulation induced increases in [Ca2+]i of the SC regions. This response almost completely disappeared during umbilical cord occlusion. At 3 hrs after reperfusion of umbilical blood flow, the SC stimulation-induced increases in [Ca2+]i remained markedly attenuated. These small [Ca2+]i responses were antagonized by CNQX but not by AP-5. These results suggest that hypoxia could alter the function of NMDA receptors in fetal SC neurons, which may be associated with an innate hypoxia tolerance in the fetal brain. [Jpn J Physiol 54 Suppl:S140 (2004)]