title: Voltage-dependent Na(+) currents in mammalian retinal cone bipolar cells.
abstract: Voltage-dependent Na(+) channels are usually expressed in neurons that use spikes as a means of signal coding. Retinal bipolar cells are commonly thought to be nonspiking neurons, a category of neurons in the CNS that uses graded potential for signal transmission. Here we report for the first time voltage-dependent Na(+) currents in acutely isolated mammalian retinal bipolar cells with whole cell patch-clamp recordings. Na(+) currents were observed in approximately 45% of recorded cone bipolar cells but not in rod bipolar cells. Both ON and OFF cone bipolar cells were found to express Na(+) channels. The Na(+) currents were activated at membrane potentials around -50 to -40 mV and reached their peak around -20 to 0 mV. The half-maximal activation and steady-state inactivation potentials were -24.7 and -68.0 mV, respectively. The time course of recovery from inactivation could be fitted by two time constants of 6.2 and 81 ms. The amplitude of the Na(+) currents ranged from a few to >300 pA with the current density in some cells close or comparable to that of retinal third neurons. In current-clamp recordings, Na(+)-dependent action potentials were evoked in Na(+)-current-bearing bipolar cells by current injections. These findings raise the possibility that voltage-dependent Na(+) currents may play a role in bipolar cell function.
authors.affiliation: Department of Anatomy and Cell Biology, Wayne State University School of Medicine, Detroit, Michigan 48201, USA. zhpan@med.wayne.edu
authors.forename: Z H
authors.initials: ZH
authors.lastname: Pan
authors.forename: H J
authors.initials: HJ
authors.lastname: Hu
mesh_terms: D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D001640:Bicuculline / Q000494:pharmacology; D002118:Calcium / Q000493:pharmacokinetics; D003035:Cobalt / Q000494:pharmacology; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D007608:Kainic Acid / Q000494:pharmacology; D008322:Mammals; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D051381:Rats; D020318:Rats, Long-Evans; D018079:Receptors, GABA / Q000502:physiology*; D012160:Retina / Q000166:cytology* / Q000502:physiology; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology*; D012964:Sodium / Q000378:metabolism*; D015222:Sodium Channels / Q000502:physiology*; D013779:Tetrodotoxin / Q000494:pharmacology; D014795:Visual Pathways; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
chemical_list: D018690:Excitatory Amino Acid Agonists; D018756:GABA Antagonists; C087200:GABA-C receptor; D018079:Receptors, GABA; D015222:Sodium Channels; D003035:Cobalt; D013779:Tetrodotoxin; D005680:gamma-Aminobutyric Acid; D012964:Sodium; D015761:4-Aminopyridine; D007608:Kainic Acid; D002118:Calcium; D001640:Bicuculline