Retinal transplants let rats see the light.

Vision researchers have reported the first behavioral evidence that retinal cells transplanted into experimentally blinded animals can restore their ability to sense and respond to light. Another group reports that transplanted retinal cells can make the appropriate connections within a animal's brain.

Although the findings shold out hope for treating currently incurable vision disorders involving the retina -- such as retinitis pigmentosa and mascular degeneration -- the researchers say it could take years to perfect retinal transplants for use in humans. And they cuation that it is still unclear whether such transplants could provide even limited vision to blind people with retinal defects.

Manuel del Cerro and his colleagues at the University of Rochester (N.Y.) grafted fetal rat retinal cells into one retina of each of nine rats previously blinded by continuous exposure to bright light. During behavioral tests, the engrafted rats showed a greater ability to detect light than did a control group of nine blinded rats receiving no transplants, del Cerro reported in New Orleans last week at the annual meeting of the Society for Neuro-science.

His group checked for light-sensing ability by measuring how high the rats jumped when startled by a loud noise preceded by a flash of light. The researchers found that the engrafted rats were significantly less startled by the noise than were the control rats. This suggests that the engrafted rats saw the light and leaned to brace themselves for the upcoming noise, the Rochester group asserts.

To confirm that hypothesis, del Cerro and his colleagues next transplanted retinal cells into one retina of each of the control rats. The new engrafted rats became less easily startled, again suggesting that the transplants had restored the ability to detect light.

"This indicates that the grafted [cells] were sensitive to light and made effective [cell-to-cell] contacts within the [eye of the] host," says del Cerro. "It also indicates that the visual information . . . was transmitted to the proper critical regions in [each rat's] brain."

Two years ago, researchers at Washington University in St. Louis reported transplanting gelating patches containing retinal cells into blinded rats (SN: 11/4/89, p.297). They found that the transplanted cells took up increased quantities of glucose after exposure to light, suggesting that the cells were functioning. But the researchers did not test whether these cells gave the rats the ability to perceive light.

In separate work described at last week's meeting, Raymond D. Lund of the University of Pittsburgh and P.J. Coffey of the University of Sheffield, England, discovered that patches of retinal cells grafted atop the midbrains of rats can grow and connect with brain areas that process and respond to visual stimuli. Lund reports that shining a light on the exposed transplant caused the pupil to constrict as if the eye had sensed the light. Moreover, he says, the rats with retinal transplants in their brains halted their normal activities when he illuminated their cages, although they could not tell the direction of the light.

In a third presentation at the meeting, Albert Aguayo and colleagues at McGill University in Montreal reported the results of using transplanted nerve grafts to guide the regrowth of damaged retinal cells' long, tail-like axons. They had previously found in hamsters that such grafts helped severed retinal axons reconnect to the appropriate brain regions (SN: 10/14/89, p.244). In their new work, they discovered that the regenerated axons formed a normal number of connections with brain cells.