The development of excitatory capability in Aplysia californica bag cells observed in cohorts.

The bag cells of Aplysia release egg laying hormone in sexually mature animals. Bag cells cannot sustain the long-lasting excitatory afterdischarge (AD) required for hormone release prior to sexual maturity (T.A. Nick, L.K. Kaczmarek, T.J. Carew, Ionic currents underlying developmental regulation of repetitive firing in Aplysia bag cell neurons, J. Neurosci. 1996;16:7583-7598; L.A. Fieber, Characterization of Na(+) and Ca(2+) currents in bag cells of sexually immature Aplysia californica, J. Exp. Biol. 1998;201:745-754). To investigate the development of bag cell excitability, whole-cell voltage-clamp experiments were executed in dissociated bag cells from four cohorts (batches) of hatchery-reared A. californica maintained at 13-15 degrees C. K(+) current densities, representing the sum of at least four different outward K(+) currents (Nick et al., 1996), declined significantly as a function of age, beginning at least 2-3 months before sexual maturity. The K(+) current decreases coincided with the first appearance of Na(+) and Ca(2+) currents in bag cells, which occurred at ages 6-7 months. Whole cell K(+) currents were not decreased significantly by a cAMP analog earlier than 1 month prior to the onset of reproductive activity. The frequency of observing Na(+) currents in whole cell recordings was low for developmental times earlier than sexual maturity. In one winter batch, both control and PMA-treated Na(+) currents increased significantly with age, and PMA-treated current densities were significantly greater than controls, but the other two batches studied had significant differences in Na(+) current frequency only at sexual maturity. Ca(2+) currents were reliably measured in more cells than were Na(+) currents. The Ca(2+) current frequency increased significantly with maturity in one winter batch. Ca(2+) currents were significantly increased by phorbol ester treatment beginning 6-8 weeks before reproductive activity in the two winter batches. These observations support the hypothesis that bag cell excitability is not fully developed until shortly before sexual maturity.