Elevated free calcium levels in the subretinal space elevate the absolute dark-adapted threshold in hypopigmented mice.

Abundant evidence spanning 25 years demonstrates that hypopigmentation is associated with sensory abnormalities manifested most clearly as elevated absolute dark-adapted thresholds in hypopigmented mice. Here we show that when ocular melanin is increased in the himalayan mouse via alpha-melanocyte stimulating hormone (alpha-MSH) injections, dark-adapted thresholds drop in proportion to the change in ocular melanin. We further measured free calcium concentration with calcium-sensitive microelectrodes in both albino and black mouse retinal eyecups in living subjects. The recordings were done in anesthetized animals as the defect is not present in isolated retinas or in the superfused eye preparation. A double-barreled electrode--pCa and Vref--was used to simultaneously record the calcium concentration and the electroretinogram (ERG) at each of many depths as the electrode was driven through the retina. The position of the electrode was confirmed with ERG and 1,1'-dioctadecyl-3, 3,3',3'-tetramethylindocarbocyanine perchlorate electrode tract reconstruction. Dark-adapted albinos (n = 6) had 1.4 +/- 0.015 mM calcium in the subretinal space compared with 0.80 +/- 0.025 mM in black mice (n = 6). The results of these experiments are consistent with the hypothesis that ocular hypopigmentation causes elevated calcium levels in the subretinal space that in turn mimic light adaptation in hypopigmented mice.

Disruption of laminin beta2 chain production causes alterations in morphology and function in the CNS.

From the elegant studies of Ramon y Cajal (1909) to the current advances in molecular cloning (e.g., Farber and Danciger, 1997), the retina has served as an ideal model for the entire CNS. We have taken advantage of the well described anatomy, physiology, and molecular biology of the retina to begin to examine the role of the laminins, one component of the extracellular matrix, on the processes of neuronal differentiation and synapse formation in the CNS. We have examined the effect of the deletion of one laminin chain, the beta2 chain, on retinal development. The gross development of retinas from laminin beta2 chain-deficient animals appears normal, and photoreceptors are formed. However, these retinas exhibit several pathologies: laminin beta2 chain-deficient mice display abnormal outer segment elongation, abnormal electroretinograms, and abnormal rod photoreceptor synapses. Morphologically, the outer segments are reduced by 50% in length; the outer plexiform layer of mutant animals is disrupted specifically, because only 7% of observed rod invaginating synapses appear normal, whereas the inner plexiform layer is undisturbed; finally, the rate of apoptosis in the mutant photoreceptor layer is twice that of control mice. Physiologically, the electroretinogram is altered; the amplitude of the b-wave and the slope of the b-wave intensity-response function are both decreased, consistent with synaptic disruption in the outer retina. Together, these results emphasize the prominence of the extracellular matrix and, in particular, the laminins in the development and maintenance of synaptic function and morphogenesis in the CNS.