FLUOROMETRIC-DETERMINATION OF ELECTRICALLY-EVOKED INCREASE IN INTRACELLULAR CALCIUM IN CULTURED CEREBELLAR GRANULE CELLS

A technique is described to measure the electrically evoked increase i n intracellular calcium in cerebellar granule cells cultured on glass coverslips and preloaded with FURA-2. To minimize light scattering, th e coverslip containing the granules was placed in the fluorimeter cuve tte at a 30-degrees angle to the exciting light beam. The cuvette was provided with 2 platinum electrodes so as to stimulate the neurons wit h a tangential field. The [Ca2+li transients were maximized by omittin g Mg2+. The fluorescence peaks were directly related to the pulse (1 m s, 100 mA) frequency and to the train length. The responses were compl etely tetrodotoxin- and [Ca2+]o-dependent and could be replicated 5-6 times at 5-min intervals. At the stimulation rate of 20 Hz for 5 s, a condition ensuring submaximal peaks, the [Ca2+]i rose from the basal l evels of 41 +/- 2.7 nmol/l to 89.6 +/- 5.8 nmol/l. The participation o f various membrane channels in the electrically induced [Ca2+]i increa se was demonstrated. 4-Aminopyridine (1 mM) increased the height of th e peaks to 240%. Both nifedipine (10 muM) and omega-conotoxin (1 muM) reduced the transients by about 25%. The residual response (in the abs ence of Mg2+) depended mostly on the release of endogenous glutamate a s it proved sensitive to NMDA, AMPA and t-ACPD receptor antagonists. S ince a technique to measure the electrically evoked release of D[H-3]a spartate is presently available, the parallel determination of release and of [Ca2+]i in twin populations of cultured granule cells is possi ble.