Background light adaptation of the retinal neuronal adaptive system. II. Dynamic effects Mildred el Azazi 1 , Ling Wang 2, *, Anders Eklund 3 & Lillemor Wachtmeister 2 1 S:t Erik’s Eye Hospital, Karolinska Institute, Stockholm, Sweden; 2 Department of Clinical Sciences, Oph- thalmology, Umea ˚ University, Umea ˚, Sweden; *Present address: Department of Ophthalmology, Rui Jin Hospital, Shanghai Second Medical University, Shanghai 200025, China; 3 Department of Biomedical Engi- neering and Informatics, Umea ˚ University Hospital, Umea ˚, Sweden Accepted 28 October 2004 Key words: background light adaptation, electroretinography, oscillatory potentials, rat Abstract The dynamic effects of continuous exposure to light on the neuronal adaptive system of the retina, as indicated by the oscillatory response (OPs) of the electroretinogram (ERG) were studied in the albino rat. Digitally filtered OPs and the a- and b-waves of the corneal ERG were simultaneously recorded in dark adaptation, during continuous light adaptation to four levels of background light (BGL) changing in steps of two log units from 1.43 · 10 )6 cd/m 2 , referred to as ‘low and high scotopic, low and high mesopic’ levels. Exposed to ‘high scotopic’ BGL the total oscillatory response (SOP) significantly enhanced within the first minute, whereas the amplitudes of the a- and b-waves were unaffected. In ‘low mesopic’ BGL the SOP increased within the first minute, whereas the a- and b-waves significantly decreased. ‘High mesopic’ BGL instantaneously and profoundly reduced both the SOP and the slow potentials. The individual OPs changed in amplitudes mainly within the first minute of BGL. In general, the earlier OPs (O1 and O2) reacted more to the two ‘scotopic’ BGL levels, whereas the later OPs (O3 and O4) were more affected by the relatively brighter two ‘mesopic’ conditions. In conclusion, the rapid increase of the OPs within the first minute of ‘high scotopic’ and ‘low mesopic’ BGL exposure may rep- resent a rudimentary light adaptational effect in the rod-dominated rat retina. These findings also sug- gest that the neuronal adaptive mechanism of the retina seems to be a robust system, probably attaining preservation of visual abilities in the rat on exposure to light. Introduction The oscillatory potentials (OPs) are high fre- quency components of the electroretinogram (ERG) superimposed on the slower b-wave. To be elicited the OPs require a series of sharply ris- ing, suprathreshold light stimuli presented to the dark or light adapted retina [1–3]. Systematical studies of the OPs in the human retina have shown that the OPs reflect both photopic and scotopic activity, as well as an interaction between the two [3–7]. Further, the OPs are known to be highly sensitive to fast changes of illumination, and this specific property has also been used to optimize the OPs [2, 3, 5, 8]. The processes underlying fast changes of retinal sensi- tivity occurring at a postreceptoral level are still not completely elucidated. The OPs offer a tool to contribute to the understanding of the neuro- nal adaptation taking place in the inner retina. Although sparse of cones, the complement of cones in the rat retina is not much smaller than in the human retina, and the retina of the rat possesses a well-documented OP activity [9–14]. In a previous study in the rat the OP activity was found to have increased after 6 min exposure to steady background light (BGL) corresponding to 1.43 · 10 )6 , 1.43 · 10 )4 and Documenta Ophthalmologica (2004) 109:201–213 Ó Springer 2005