CHARACTERIZATION OF NA-CALIFORNICA( AND CA2+ CURRENTS IN BAG CELLS OFSEXUALLY IMMATURE APLYSIA)

The neurosecretory bag cells of sexually mature Aplysia californica re lease egg-laying hormones as part of the reproductive process after a train of action potentials termed afterdischarge. Whole-cell voltage-c lamp experiments were performed in cultured cells from sexually immatu re A. californica to characterize the inward voltage-gated currents fo r Na+ and Ca2+. The goal of these experiments was to investigate the r egulation of excitability during sexual maturation. Na+ currents in ba g cells of immature A. californica were similar in several ways to tho se of mature animals, The Na+ currents activated at voltages less nega tive than -30 mV and peaked at 10-20 mV in artificial sea water, The t ime course and pharmacology of bag cell Na+ currents were similar to t hose of bag cells from mature A. californica, although the Na+ current density was lower in immature A. californica. Na+ currents were inhib ited by tetrodotoxin (50 nmoll(-1)). The Na+ current was relatively in sensitive to depolarized holding potentials (V-h), maintaining approxi mately 50% of peak current amplitude present at V-h = -70 mV throughou t the activation range at V-h = -30 mV, In experiments using a 1 s dep olarized Vh prior to a test pulse, the half-inactivation voltage (V-h) was - 27 mV, Recovery of immature Na+ current from steady-state inact ivation at V-h = -30 mV had a time constant (tau) of 9.5 ms, significa ntly slower than in mature animals, Ca2+ currents of immature A. calif ornica activated at approximately -30 mV and peaked at approximately 2 0 mV with 11 mmoll(-1) Ba2+ as the charge carrier, The principle diffe rences from mature Ca2+ currents were the low density of the immature Ca2+ currents and their 'rundown' in whole-cell recordings, The pharma cology and V-1/2 of bag cell Ca2+ currents were similar to those of L- type Ca2+ currents in mature cells, The Ca2+ currents were inhibited 6 1 +/- 10% by nifedipine (10 mu moll(-1)) and were unaffected by omega- conotoxin GVIA (10 mu moll(-1)). The Ca2+ currents were relatively ins ensitive to depolarized V-h, activating maximally at V-h = -90, -70 an d -50 mV, and maintaining 50% of this peak current amplitude throughou t the activation range at V-h = -30 mV, The V-1/2 was -23 mV in experi ments in which cells were subjected to a 1 s depolarized V-h prior to a test pulse. Na+ current amplitudes were maintained or increased duri ng 1 min of 4 Hz test pulses in bag cells at V-h = -70 mV and V-h = -3 0 mV. In contrast, Ca2+ current run-down occurred during 1 min of 4 Hz test pulses in seven of 10 cells at V-h = -70 mV and in 12 of 12 cell s at V-h = -30 mV. The observed scarcity of Na+ and Ca2+ currents in i mmature bag cells as well as the specific characteristics of immature bag cell Ca2+ currents make repetitive action potential firing and hor mone release less likely than in mature bag cells.