Ion conductances related to development of repetitive firing in mouse retinal ganglion neuronsin situ
✍ Scribed by Rothe, Thomas ;J�ttner, Ren� ;B�hring, Robert ;Grantyn, Rosemarie
- Book ID
- 101260104
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 445 KB
- Volume
- 38
- Category
- Article
- ISSN
- 0022-3034
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✦ Synopsis
In the retina, the ability to encode graded depolarizations into spike trains of variable frequency appears to be a specific property of retinal ganglion neurons (RGNs). To deduce the developmental changes in ion conductances underlying the transition from single to repetitive firing, patch-clamp recordings were performed in the isolated mouse retina between embryonic day 15 (E15) and postnatal day 5 (P5). Immature neurons of the E15 retina were selected according to their capacity to generate voltage-activated Na ؉ currents (I Na(V) ). Identification of P5 RGNs was based on retrograde labeling, visualization of the axon, or the amplitude of I Na(V) . At E15, half of the cells were excitable but none of them generated more than one spike. At P5, all cells were excitable and a majority discharged in tonic fashion. Ion conductances subserving maintenance of repetitive discharge were identified at P5 by exposure to low extracellular Ca 2؉ , Cd 2؉ , and charybdotoxin, all of which suppressed repetitive discharge. -Conotoxin GVIA and nifedipine had no effect. We compared pas-sive membrane properties and a variety of voltage-activated ion channels at E15 and P5. It was found that the density of high voltage-activated (HVA) Ca 2؉ currents increased in parallel with the development of repetitive firing, while the density of Ni 2؉ -sensitive low voltageactivated (LVA) Ca 2؉ currents decreased. Changes in density and activation kinetics of tetrodotoxin-sensitive Na ؉ currents paralleled changes in firing thresholds and size of action potentials, but seemed to be unrelated to maintenance of repetitive firing. Densities of A-type K ؉ currents and delayed rectifier currents did not change. The results suggest that HVA Ca 2؉ channels, and among them a toxin-resistant subtype, are specifically engaged in activation of Ca 2؉ -sensitive K ؉ conductance and thereby account for frequency coding in postnatal RGNs.