The role played by melatonin and melatonin receptors in the mammalian retina is not well defined. The lack of data is due to the fact that the vast majority of mouse strains are genetically deficient in synthesizing melatonin in the pineal gland and retina 
, so very few studies have compared retinal physiology in melatonin-proficient and melatonin-deficient mice. This situation is further complicated by the fact that C57/BL6 and other strains may produce a small amount of melatonin; therefore, we cannot completely exclude that this small amount may somehow influence the mouse physiology 
. In the present study, we investigated the effect of aging on retinal function and morphology in a melatonin proficient mouse strain (C3H-f+/+
). Our data indicate that the following: the amplitude of the a and b waves of dark-adapted ERG decrease with age; the daily rhythm in the ERGs is lost by the age of 12 months; the STR at ZT18 is significantly affected by age; the amplitude of the photopic ERG and its daily rhythm are also affected by age; the changes observed in the ERGs are not paralleled by gross alterations in retinal general morphology; and administration of exogenous melatonin affects the ERGs in 12-month-old mice.
Age-related declines in retinal function have been reported for mice 
and for humans 
. Our data are in agreement with previous reports and expand these results by showing that the daily rhythm in the scotopic and photopic ERGs and the STR are both affected by aging (, , ). Interestingly, no further decrease in the amplitude of the a and b waves was observed after 12 months of age for both scotopic and photopic conditions. The observation that the STR is only affected by aging at ZT18 is not surprising, since mice are nocturnal and therefore the visual system is optimized to perform at night when the mice are active.
Previous studies have shown that photoreceptor loss is only observed in mice older than 12 months 
with no significant changes observed before this age 
. Our immunocitochemistry analysis () indicates that many of the retinal biomarkers are not affected by aging in C3H-f+/+
mice. In our retina samples of 12 months age, the only effect seen in the retina (i.e., the retraction of the photoreceptor and sprouting of the bipolar cells dendrites) is congruent with the aging signature already reported in literature 
. This result suggests the future search and quantification of markers, which might help explain our ERG finding, will require a detailed analysis on other components of the signaling pathways, such as the amount of rhodopsin or of other phototransduction proteins, the distribution of mGluR6, or the number of rod and on-cone bipolar cells. Our data also indicate that the decrease in retinal functioning (ERG) and sensitivity (STR) is important for the early detection of photoreceptor dysfunctions, preceding any obvious morphological change in retinal organization.
Several investigations have shown that blood melatonin levels or biosynthesis decline with age 
. It has been also reported that a similar scenario may be present in the retina since the level of Arylalkylamine N-acetyltransferase, a key enzyme for melatonin synthesis, transcription or protein expression, decreases with age 
. Our previous work showed that in melatonin-proficient mice, the daily rhythms observed in ERG parameters are controlled by melatonin 
; therefore, we hypothesized that the observed decline in the amplitudes of the ERG and the loss of the daily rhythm may be due to the decline of melatonin synthesis in older mice. To test this, we administered exogenous melatonin to mice of different ages. Surprisingly, we observed a dose- and age- dependent increase in the responsiveness of the ERG to administration of exogenous melatonin (), which was not rescued by exogenous melatonin. Such a result may suggest that the reduction in the melatonin receptors is responsible for the lack of responsiveness to the administration of exogenous melatonin. Interestingly, a previous study has reported a loss of responsiveness to administration of exogenous melatonin in the suprachiasmatic nucleus (SCN) of aged mice 
; in addition, melatonin receptor expression decreases during aging in human SCN and in patients affected by Alzheimer's 
and Parkinson's diseases 
. Such results may suggest that the lack of responsiveness to the administration of exogenous melatonin is probably due a reduction in the levels of melatonin receptors present in the photoreceptors. Further studies will be required to address this important issue.
Melatonin and its analogues are currently used by millions of people around the world to prevent aging, to improve sleep performance, to ameliorate jet-lag symptoms and to treat depression 
. Our new study indicates that responsiveness to exogenous melatonin is affected by aging; therefore, in some instances, melatonin treatment may not be effective due to lack or reduced sensitivity of its receptors.