Persistent Ca2+Current Contributes to a Prolonged Depolarization inAplysiaBag Cell Neurons

Abstract

Neurons may initiate behavior or store information by translating prior activity into a lengthy change in excitability. For example, brief input to the bag cell neurons of Aplysia results in an approximate 30-min afterdischarge that induces reproduction. Similarly, momentary stimulation of cultured bag cells neurons evokes a prolonged depolarization lasting many minutes. Contributing to this is a voltage-independent cation current activated by Ca2+entering during the stimulus. However, the cation current is relatively short-lived, and we hypothesized that a second, voltage-dependent persistent current sustains the prolonged depolarization. In bag cell neurons, the inward voltage-dependent current is carried by Ca2+; thus we tested for persistent Ca2+current in primary culture under voltage clamp. The observed current activated between −40 and −50 mV exhibited a very slow decay, presented a similar magnitude regardless of stimulus duration (10–60 s), and, like the rapid Ca2+current, was enhanced when Ba2+was the permeant ion. The rapid and persistent Ca2+current, but not the cation current, were Ni2+sensitive. Consistent with the persistent current contributing to the response, Ni2+reduced the amplitude of a prolonged depolarization evoked under current clamp. Finally, protein kinase C activation enhanced the rapid and persistent Ca2+current as well as increased the prolonged depolarization when elicited by an action potential-independent stimulus. Thus the prolonged depolarization arises from Ca2+influx triggering a cation current, followed by voltage-dependent activation of a persistent Ca2+current and is subject to modulation. Such synergy between currents may represent a common means of achieving activity-dependent changes to excitability.