VULNERABILITY OF CA1 NEURONS TO GLUTAMATE IS DEVELOPMENTALLY-REGULATED

Although it is well documented that glutamate receptor subtypes are di fferentially expressed during central nervous system development postn atally, how glutamate affects neurons during postnatal development is unclear. We therefore examined the development of the intrinsic neuron al response to glutamate receptor activation by studying single, hippo campal CA1 neurons that had been acutely dissociated from newborn (P-1 -3), 1 week old (P-6-8), and 3 week old (P-21-25) rats. Using laser sc anning confocal microscopy and the calcium dye Fluo-3, we made time-la pse studies of the effects of glutamate stimulation on free intracellu lar calcium ([Ca2+](i)) and simultaneous changes in neuronal morpholog y. In P-21-25 neurons, glutamate increased [Ca2+](i) fluorescence, and caused marked somal swelling, blebbing, and retraction of dendrites i nto the soma. These major morphological changes were followed by sudde n loss of intracellular fluorescence, indicative of a loss of membrane integrity and cell death. In P-6-8 neurons, glutamate increased [Ca2](i) to the same extent, but this increase was not followed by either major morphological changes or loss of membrane integrity. In P-1-3 ne urons, glutamate increased [Ca2+](i) minimally, and no morphologic cha nges were observed. P-1-3 neurons dissociated without enzymatic digest ion demonstrated glutamate responses identical to responses seen in ne urons dissociated with enzymatic digestion. In the presence of MK-801 (15 mu M), glutamate still increased [Ca2+](i) and caused cell death i n P-21-25 neurons, but the latency to these effects more than tripled. This late, MK-801-resistant [Ca2+](i) increase was not eliminated by DNQX or Ni2+/Cd2+, suggesting that this increase is mediated by metabo tropic receptors. These findings demonstrate that (I) hippocampal neur ons from newborns are intrinsically less vulnerable to glutamate toxic ity than neurons from 3 week old animals, and (2) multiple glutamate r eceptor subtypes affect the magnitude of the [Ca2+](i) increase in res ponse to glutamate in the neuronal microenvironment.