Elementary properties of axonal calcium currents in type B photoreceptors in Hermissenda crassicornis.
Axons of the type B photoreceptors form synapses with hair cells and interneurons that are involved in classical conditioning in Hermissenda. We examined the differences in the Ca2+ channels expressed in the soma and axons of the B photoreceptors by direct functional recordings of single-channel currents. Although the soma of the B cells express two Ca2+ current subtypes, a transient BayK 8644-insensitive (approximately 17 pS) current and a sustained BayK 8644-sensitive (approximately 10 pS) current, the axons expressed only the latter. The axonal Ca2+ current activated at potentials positive to -20 mV. Moreover, the Ca2+ channels are distributed heterogeneously along the length of the axon, with the higher channel density (approximately 10-15 channel microm(-2)) occurring at the distal one-third of the isolated axons, with respect to the soma. The regions of Ca2+ channel clusters may represent the presynaptic site of the photoreceptor-interneuron synapses. Furthermore, the high-density clusters of Ca2+ channels may augment postsynaptic responses. The results of the present study represent the first direct recordings of Ca2+ currents at presumed synaptic sites. Expression of different Ca2+ channel subtypes at distinct compartments of the type B photoreceptors may generate diverse Ca2+ domains that may be required for neuronal plasticity in Hermissenda.
Tamse
Catherine T
CT
Center for Neuroscience, Department of Otolaryngology, University of California, Davis, Davis, California 95616, USA.
Yamoah
Ebenezer N
EN
D000200:Action Potentials; D000818:Animals; D001369:Axons / Q000502:physiology*; D015220:Calcium Channels / Q000502:physiology*; D004553:Electric Conductivity; D007700:Kinetics; D008974:Mollusca / Q000502:physiology*; D009473:Neuronal Plasticity; D018408:Patch-Clamp Techniques; D017956:Photoreceptor Cells, Invertebrate / Q000502:physiology*
D015220:Calcium Channels