Dendritic calcium channels and their activation by synaptic signals in auditory coincidence detector neurons

The avian nucleus laminaris (NL) encodes the azimuthal location of low-frequency sound sources by detecting the coincidence of binaural signals. Accurate coincidence detection requires precise developmental regulation of the lengths of the fine, bitufted dendrites that characterize neurons in NL. Such regulation has been suggested to be driven by local, synaptically mediated, dendritic signals such as Ca²⁺. We examined Ca²⁺ signaling through patch clamp and ion imaging experiments in slices containing nucleus laminaris from embryonic chicks. Voltage-clamp recordings of neurons located in the NL showed the presence of large Ca²⁺ currents of two types, a low voltage-activated, fast inactivating Ni²⁺ sensitive channel resembling mammalian T-type channels, and a high voltage-activated, slowly inactivating Cd²⁺ sensitive channel. Two-photon Ca²⁺ imaging showed that both channel types were concentrated on dendrites, even at their distal tips. Single action potentials triggered synaptically or by somatic current injection immediately elevated Ca²⁺ throughout the entire cell. Ca²⁺ signals triggered by subthreshold synaptic activity were highly localized. Thus when electrical activity is suprathreshold, Ca ²⁺ channels ensure that Ca²⁺ rises in all dendrites, even those that are synaptically inactive.