AMPA ELICITS LONG-LASTING, PARTLY HYPOTHERMIA-SENSITIVE CALCIUM RESPONSES IN ACUTELY DISSOCIATED OR CULTURED EMBRYONIC BRAIN-STEM CELLS

This study aimed at testing if, and under which conditions, long-lasti ng cytosolic calcium responses can be induced in dissociated embryonic brain cells exposed to alpha-amino-hydroxy-5-methyl-4-isoxazole-propi onic acid (AMPA) receptor agonists. Rat brainstem cells (gestation day s 13-14; mean crown-rump lengths 8-11 mm) were mechanically dissociate d and loaded with the fluorescent calcium marker Fluo-3 after in vitro delays ranging from 20 min to 6 days. The cells were exposed to vario us concentrations of AMPA, domoic acid or kainic acid. The evoked fluo rescence changes, indicating variations of cytosolic calcium, were rec orded and analysed either with a video-microscope or a laser cytometer . Even at the earliest stages, non-desensitizing (or partly desensitiz ing) calcium responses to AMPA were found. In addition, sequential exp osure to AMPA followed either by domoic acid, or by AMPA in the presen ce of aniracetam, revealed the existence of cells bearing predominantl y desensitizing receptors. The non-desensitizing as well as desensitiz ing response components were blocked by 6,7-dinitroquinoxaline-2,3-dio ne (DNQX). When the experiments were conducted at 24 degrees C, the cy tosolic calcium levels generally returned close to pre-stimulus baseli ne levels after washout. In contrast, when the working temperature was slightly raised (to 27 degrees C), complex secondary calcium rises we re observed not only during prolonged stimulation, but also after shor t agonist application. The calcium modulation might be correlated with some form of cellular ''learning'' in the embryonic brain. Under part icular conditions, where the regulation processes are either switched off by cell programmes or simply overloaded, the cascade of events com prising secondary calcium rises may lead to cell death.