Biol. Bull. 207: 157. (October 2004)
© 2004 Marine Biological Laboratory
Membrane Properties of Two Subtypes of Skate Bipolar Cells
Haohua Qian1,2,
Richard L. Chappell1,3,
Stephen Redenti1,3 and
Harris Ripps1,2
1 Marine Biological Laboratory, Woods Hole, Massachusetts
2 University of Illinois at Chicago, College of Medicine, Chicago, Illinois
3 Hunter College and The Graduate Center, CUNY, New York, New York
Bipolar cells in the vertebrate retina are second order neurons that transmit visual information from rod and cone photoreceptors to the amacrine and ganglion cells of more proximal retinal layers. In most species, they consist typically of multiple subtypes that differ in their morphology, synaptic connections, and response properties. The individual subtypes of bipolar cell are thought to carry different aspects of the visual signal through the retina, and they often exhibit unique membrane properties and neurotransmitter receptors.
The skate retina presents an unusual situation. Unlike other vertebrate species, it contains only one class of photoreceptor, namely rods; and, thus far, only two morphologically and pharmacologically distinct subtypes of bipolar cell have been identified. The large-field bipolar cells, with extensive dendritic arbors, are glycine-insensitive, whereas the small-field bipolar cells, which have only one or two dendritic branches, are sensitive to glycine. In the present study, we explored further the membrane properties of these two subtypes of skate bipolar cell. Solitary bipolar cells were obtained by enzymatic and mechanical dissociation of the skate retina, and kept in modified L-15 medium for 1 to 3 days at 14 °C. Membrane currents, measured with whole-cell patch-clamp recording techniques, revealed that the two subtypes of bipolar cell exhibited different voltage-activated current responses. Both hyperpolarizing and depolarizing command voltages activated large membrane conductances on the small-field, glycine-sensitive bipolar cells. On the other hand, large-field (glycine-insensitive) bipolar cells were much less voltage sensitive. In addition, the inward currents activated by hyperpolarizing voltages displayed faster kinetics in small-field bipolar cells than those activated in large-field bipolar cells; and a clear difference was seen in the magnitude of the TEA-sensitive potassium currents present on the two cell types. In conclusion, two morphologically distinct subtypes of bipolar cell are seen in the all-rod skate retina, and they exhibit different voltage-activated current profiles and express different populations of neurotransmitter receptors.
Support: NIH Grant EY-12028 (HQ); Fight for Sight, PSC/CUNY Grant 66257-0035, and NCRR/NIH RCMI Award RR-03037 (RLC); and NIH Grant EY-06516 and RPB (HR).
