IONIC CURRENTS UNDERLYING DEVELOPMENTAL REGULATION OF REPETITIVE FIRING IN APLYSIA BAG CELL NEURONS

We have investigated the developmental regulation of the ability to fi re repetitively in the bag cell neurons of Aplysia californica, a neur onal system in which the behavioral effects of repetitive firing are w ell characterized. Adult bag cell neurons exhibit an afterdischarge, c onsisting of prolonged depolarization and repetitive firing, which cau ses the release of several peptides from these neurons that induce egg -laying behaviors. Afterdischarge can be triggered in vitro by a varie ty of stimuli, including electrical stimulation and exposure to the po tassium channel blocker tetraethyl ammonium chloride (TEA). In contras t to adults, juvenile neurons did not exhibit afterdischarge in respon se to pleural-abdominal connective shock or TEA. Juvenile neurons did exhibit, however, prolonged depolarizations in the presence of TEA, pe rhaps reflecting the anlage of the mechanism responsible for afterdisc harge in the adult. To investigate developmental mechanisms underlying the regulation of repetitive firing, we compared ionic currents in ad ult and juvenile bag cell neurons. We found that during the period in which these neurons acquire the capacity to fire repetitively, a numbe r of currents are regulated: (1) three K+ currents decrease (Ca2+-depe ndent K+ and two components of voltage-dependent delayed-rectifier Kcurrent); (2) A-type K+ current increases; and (3) two Ca2+ currents i ncrease (basal and pKC-activated). This pattern is consistent with the increase in the ability to fire repetitively that we observe during m aturation: our results indicate that developmental control of repetiti ve firing in this system is accompanied by selective regulation of spe cific ionic currents which, after maturation, play important roles in generating the afterdischarge and triggering egg-laying behaviors.