Impaired function of bipolar cells in the Royal College of Surgeons rat

The retina of the Royal College of Surgeons (RCS) strain of rat, which is b eing used as an animal model for human retinal degenerations, has been empl oyed in the study of the function of second order neurons. By about the 33r d postnatal day the dendritic branching of isolated bipolar cells is more s parse than in bipolar cells of the normal rat retina, but their GABA channe ls are as in the normal rat retina. The normally occurring light-induced di stal potassium increase has been used as the indicator of the functional co mpetence of second order neurons in the isolated RCS rat retina. These are dependent upon the integrity of ionotropic and metabotropic synapses. At ab out the 22nd postnatal day MgCl2 enlarges the light-induced distal potassiu m increase in the young RCS rat retina as in the normal rat retina. It seem s that MgCl2 does not block the metabotropic synapses of on-bipolar cells. At about postnatal day 33, at which time the photoreceptors of the RCS rat retina had become severely damaged, the size of this light-induced distal p otassium increase was not changed, but it was abolished by MgCl2. This indi cates that bipolar cells are still active but that the synaptic function of on-bipolar cells has become vulnerable to IMgCl2. The conclusion is that a t a time when photoreceptor degeneration is already severe bipolar cells ar e still active, but that on-bipolars, mainly rod bipolar cells, have some f unctional deficit.