Cholinergic depolarization recruits a persistent Ca current in bag cell neurons.

Many behaviors and types of information storage are mediated by lengthy changes in neuronal activity. In bag cell neurons of the hermaphroditic sea snail , a transient cholinergic synaptic input triggers an ∼30-min afterdischarge. This causes these neuroendocrine cells to release egg laying hormone and elicit reproductive behavior. When acetylcholine is pressure-ejected onto a current-clamped bag cell neuron, the evoked depolarization is far longer than the current evoked by acetylcholine under voltage clamp, suggesting recruitment of another conductance. Our earlier studies found bag cell neurons to display a voltage-dependent persistent Ca current. Hence, we hypothesized that this current is activated by the acetylcholine-induced depolarization and sought a selective Ca current blocker. Rapid Ca current evoked by 200-ms depolarizing steps in voltage-clamped cultured bag cell neurons demonstrated a concentration-dependent sensitivity to Ni, Co, Zn, and verapamil but not Cd or ω-conotoxin GIVa. Leak subtraction of Ca current evoked by 10-s depolarizing steps using the IC (concentration required to eliminate maximal current) of Ni, Co, Zn, or verapamil revealed persistent Ca current, demonstrating persistent current block. Only Co and Zn did not suppress the acetylcholine-induced current, although Zn appeared to impact additional channels. When Co was applied during an acetylcholine-induced depolarization, the amplitude was reduced; furthermore, protein kinase C activation, previously established to enhance the persistent Ca current, extended the depolarization. Therefore, the persistent Ca current sustains the acetylcholine-induced depolarization and may translate brief cholinergic input into afterdischarge initiation. This could be a general mechanism of triggering long-term change in activity with a short-lived input. Ionotropic acetylcholine receptors mediate brief synaptic communication, including in bag cell neurons of the sea snail . However, this study demonstrates that cholinergic depolarization can open a voltage-gated persistent Ca current, which extends the bag cell neuron response to acetylcholine. Bursting in these neuroendocrine cells results in hormone release and egg laying. Thus, this emphasizes the role of ionotropic signaling in reaching a depolarized level to engage Ca influx and perpetuating the activity necessary for behavior.