Persistent Ca Current Contributes to a Prolonged Depolarization in Aplysia Bag Cell Neurons

Tam AK, Geiger JE, Hung AY, Groten CJ, Magoski NS. Persistent Ca current contributes to a prolonged depolarization in Aplysia bag cell neurons. J Neurophysiol 102: 3753–3765, 2009. First published October 14, 2009; doi:10.1152/jn.00669.2009. 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 Ca 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 Ca ; thus we tested for persistent Ca 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 Ca current, was enhanced when Ba was the permeant ion. The rapid and persistent Ca current, but not the cation current, were Ni sensitive. Consistent with the persistent current contributing to the response, Ni reduced the amplitude of a prolonged depolarization evoked under current clamp. Finally, protein kinase C activation enhanced the rapid and persistent Ca current as well as increased the prolonged depolarization when elicited by an action potential-independent stimulus. Thus the prolonged depolarization arises from Ca influx triggering a cation current, followed by voltage-dependent activation of a persistent Ca current and is subject to modulation. Such synergy between currents may represent a common means of achieving activity-dependent changes to excitability.