Graded spikes differentially signal neurotransmitter input in cerebrospinal fluid contacting neurons of the mouse spinal cord

The action potential and its all-or-none nature is fundamental to neural communication. Canonically the action potential is initiated once voltage-gated Na ⁺ channels are activated, and their rapid kinetics of activation and inactivation give rise to the action potential’s all-or-none nature. Here we show that cerebrospinal fluid contacting neurons (CSFcNs) surrounding the central canal of the mouse spinal cord employ a different strategy. Rather than using voltage-gated Na ⁺ channels to generate binary spikes, CSFcNs use two different types of voltage-gated Ca ²⁺ channel, enabling spikes of different amplitude. T-type Ca ²⁺ channels generate small amplitude spikes, whereas large amplitude spikes require high voltage-activated Cd ²⁺ sensitive Ca ²⁺ channels. We show that these different amplitude spikes signal input from different transmitter systems; purinergic inputs evoke smaller T-type dependent spikes while cholinergic inputs evoke large T-type independent spikes. Different synaptic inputs to CSFcNs can therefore be signalled by the spike amplitude.