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Development of the light response in neonatal mammalian rods
G M Ratto1, D W Robinson, B Yan
1Physiological Laboratory, Cambridge, UK.
Insights
Neonatal mammals have low light sensitivity due to underdeveloped photoreceptors. This is caused by insufficient 11-cis retinal, a key component of rhodopsin, which can be restored to adult levels with treatment.
Area of Science:
- Visual neuroscience
- Developmental biology
- Photoreceptor physiology
Background:
- Neonatal mammals exhibit significantly lower light sensitivity compared to adults.
- This reduced sensitivity in infants and rats has been observed, but the underlying cause within photoreceptors remains unclear.
Purpose of the Study:
- To investigate the sensitivity of single neonatal rat rod photoreceptors.
- To determine if photoreceptor function contributes to the low visual sensitivity in neonates.
- To identify the specific molecular factors limiting neonatal photoreceptor sensitivity.
Main Methods:
- Recording the light response of single neonatal rat rod cells.
- Comparing the sensitivity of neonatal rods to adult rods.
- Assessing the effect of 9-cis retinal treatment on neonatal rod sensitivity.
Main Results:
- Single neonatal rat rods demonstrated considerably lower light sensitivity than adult rods.
- The response to a single photoisomerization was normal, indicating functional opsin.
- Treatment with 9-cis retinal restored neonatal rod sensitivity to adult levels.
Conclusions:
- The reduced sensitivity of neonatal rat photoreceptors is primarily due to a low concentration of the chromophore 11-cis retinal.
- Opsin is present in adequate amounts, but its function is limited by the availability of 11-cis retinal.
- This finding highlights the critical role of chromophore availability in establishing adult-level visual sensitivity during development.
Abstract:
The sensitivity to light is low in many neonatal mammals when compared with that in the adult. In human infants at one month of age, for example, the dark-adapted sensitivity for detection of large stimuli is 50 times lower than in the adult, and in rats the overall sensitivity of the neonatal retina is also low compared with the adult. This low sensitivity in the neonate has been attributed to a number of factors, but the possibility that the photoreceptors themselves might be an important limitation on the overall visual sensitivity has not so far been clearly established. Here we record the light response of single neonatal rat rods and find that the sensitivity is considerably lower than in the adult. The response to a single photoisomerization is normal in the neonate, and the sensitivity deficit can therefore be attributed to a low level of functional rhodopsin. Opsin, the protein component of rhodopsin, must be present in normal amounts, as the sensitivity can be restored to adult levels by treating the retina with 9-cis retinal, an active homologue of the native chromophore 11-cis retinal. The low sensitivity of photoreceptors in the neonate can therefore be attributed mainly to a low concentration of 11-cis retinal in the developing retina.