PERSISTENT AMPA RECEPTOR STIMULATION ALTERS [CA2-NEURONS(](I) HOMEOSTASIS IN CULTURES OF EMBRYONIC DOPAMINERGIC)

The effect of the ionotropic glutamate receptor agonist, AMPA, on intr acellular Ca2+ concentrations ([Ca2+](i)) was studied in dopaminergic neurons present in primary cultures of ventral tegmental mesencephalon of 14 day rat embryos. Exposure of cells to 10 mu M AMPA for 1 min in creased [Ca2+](i) by 2-3 fold in dopaminergic and other neurons and th is response was obliterated within 5 min by superfusion with AMPA-free incubation buffer. In dopaminergic neurons, 1 min or 5 min exposure t o 50 mu M AMPA increased [Ca2+](i) 3 to 5 times over control values. T his rise in [Ca2+](i) persisted even after a 20 min superfusion with A MPA-free media, whereas, [Ca2+](i) in non-dopaminergic neurons was rev ersed to control Values during this time. Preincubation (2 min) of cul tured cells with NBQX or the L-type channel blocker, nifedipine, but n ot with MK-801 blunted the rise of [Ca2+](i) in dopaminergic and other neurons. Pretreatment with 2 mu M NBQX Shifted the dose response curv e for AMPA to the right without changing the basal [Ca2+](i). The pres ence of 10 mu M dantrolene, a blocker of Ca2+ release from intracellul ar stores, did not alter the initial rise of [Ca2+](i) elicited by 50 mu M AMPA, but prevented the destabilization of Ca2+ homeostasis by fa cilitating the recovery to normal of basal [Ca2+](i). Exposure to 50 m u M AMPA (5 min) caused an irreversible increase of[Ca2+](i) in dopami nergic neurons and cell death was manifested by propidium iodide uptak e 6-7 h after AMPA exposure. The present results show that in dopamine rgic neurons the increase of [Ca2+](i) elicited by activation of AMPA receptors is mediated by sustained Ca2+ flux through voltage-gated cha nnels and through a Ca2+-dependent Ca2+ release from intracellular sto res. Protracted stimulation of AMPA and kainate receptors caused death of dopaminergic neurons before that of other neurons present in the s ame culture. The increased vulnerability of dopaminergic neurons to AM PA may represent a mechanism of pathological relevance during ontogene sis.