Cholinergic depolarization recruits a persistent Ca²⁺ current in Aplysia 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 Aplysia californica, 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 Ca²⁺ current. Hence, we hypothesized that this current is activated by the acetylcholine-induced depolarization and sought a selective Ca²⁺ current blocker. Rapid Ca²⁺ current evoked by 200-ms depolarizing steps in voltage-clamped cultured bag cell neurons demonstrated a concentration-dependent sensitivity to Ni²⁺, Co²⁺, Zn²⁺, and verapamil but not Cd²⁺ or ω-conotoxin GIVa. Leak subtraction of Ca²⁺ current evoked by 10-s depolarizing steps using the IC₁₀₀ (concentration required to eliminate maximal current) of Ni²⁺, Co²⁺, Zn²⁺, or verapamil revealed persistent Ca²⁺ current, demonstrating persistent current block. Only Co²⁺ and Zn²⁺ did not suppress the acetylcholine-induced current, although Zn²⁺ appeared to impact additional channels. When Co²⁺ was applied during an acetylcholine-induced depolarization, the amplitude was reduced; furthermore, protein kinase C activation, previously established to enhance the persistent Ca²⁺ current, extended the depolarization. Therefore, the persistent Ca²⁺ 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.NEW & NOTEWORTHY Ionotropic acetylcholine receptors mediate brief synaptic communication, including in bag cell neurons of the sea snail Aplysia. However, this study demonstrates that cholinergic depolarization can open a voltage-gated persistent Ca²⁺ 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 Ca²⁺ influx and perpetuating the activity necessary for behavior.