EXOCYTOSIS AND SELECTIVE NEURITE CALCIUM RESPONSES IN RAT CEREBELLAR GRANULE CELLS DURING FIELD STIMULATION

The free calcium concentration, [Ca2+](c), in fura-2-loaded rat cerebe llar granule cells was investigated by digital imaging during trains o f uniform field stimuli in order to compare the ability of calcium cha nnels in somata and neurites to respond to brief, physiologically rele vant depolarizations. Very few somata responded to 20 Hz trains of 1 m s pulses, while virtually all neurites showed an extensive increase wh ich was rapidly reversed when stimulation was terminated. In contrast, both somata and neurites responded when cells were depolarized with 5 0 mM KCl. The field stimuli evoked a tetrodotoxin-sensitive increase i n Na+ concentration in both somata and neurites. When 4-aminopyridine, which inhibits delayed K+ currents in these cells, was present during the field stimulus both somata and neurites increased their [Ca2+](c) , suggesting that prolongation of the duration of depolarization is re quired for somatic Ca2+ channel activation. The neurite response did n ot depend on the orientation of the neurite relative to the applied fi eld, The neurite response was insensitive to nifedipine (1 mu M) and o mega-agatoxin-IVA (30 nM) but was uniformly inhibited by omega-conotox in-GVIA (30% inhibition at 1 mu M) and omega-conotoxin-MVIIC (44% inhi bition at 5 mu M). The two inhibitors were not additive. The neurite [ Ca2+](c) response was insensitive to the combination of ionotropic glu tamate receptor antagonists. Field stimulation caused the exocytosis o f the fluorescent probe FM1-43 previously loaded during KCI depolariza tion, suggesting that presynaptic Ca2+ channels contribute to the fiel d-evoked neurite response.