Photoreceptor Voltage Dependent Calcium Channels but not Bipolar Cell Activity is Required for Normal Ribbon Synapse Formation

Abstract

Abstract: : Purpose: Transmission between photoreceptors and bipolar cells is mediated by release of neurotransmitter at the ribbon synapse. We have identified two mouse lines, nob (Pardue et al., IOVS. 39:2443) and CNS-ß₂ null that lack a bipolar cell response as determined by absence of the ERG b-wave. The thickness of the OPL in the nob mouse is normal, whereas the OPL in CNS-ß₂ null mice is considerably thinner. To examine the mechanism of this defect, synapse morphology and the expression pattern of synaptic proteins was examined during development. Methods: OPL synapse morphology was examined in both lines of mice and compared to control littermates using electron microscopy and immunohistochemistry of synapse specific proteins: synaptophysin, bassoon and B16. Results: The ribbon synapses in the OPL of nob mice appear normal. In contrast, the CNS-ß₂ null mice have an almost complete loss of the ribbon synapses. Analyses of CNS-ß₂ null and control mice at 7 days of age indicate that ribbons synapses form in similar numbers. Immunohistochemical analyses of bassoon expression at 7 days of age shows a horseshoe shaped staining pattern in controls. In the CNS-ß₂ null mice, there is a punctate staining pattern and there is a greatly reduced number of horseshoe shaped structures. Conclusion: These data indicate that the invagination of the bipoar cells into the terminal of the photoreceptor may not occur in the CNS-ß₂ null mice. The fact that these mouse models lack ERG b-waves but only the CNS-ß₂ null shows abnormalities in OPL synaptic architecture indicates bipolar cell function is not required for ribbon synapse formation. In contrast, the presence of the photoreceptor voltage dependent calcium channel is required normal formation of the ribbon synapse.

Keywords: 564 retinal development • 445 ion channels • 594 synapse