Background light adaptation of the retinal neuronal adaptive system. II. Dynamic effects

The response of the neuronal adaptive system to background illumination and readaptation to dark in the immature retina

PURPOSE

Developmental characteristics of the neuronal adaptive system of the retina, focusing on background light (BGL) adaptation and readaptation functions, were studied by measuring the oscillatory response (SOP) of the electroretinogram (ERG).

METHODS

Digitally filtered and conventional ERGs were simultaneously recorded. Rats aged 15 and 17 days were studied during exposure to BGLs of two mesopic intensities and during readaptation to dark.

RESULTS

Results were compared to adult rats. In 'low mesopic' BGL SOP instantly dropped significantly to about half of its dark-adapted (DA) value contrary to mature rats, in which the SOP significantly increased. In 'high mesopic' BGL SOP decreased to about 20% and 30% of DA values in immature and adult rats, respectively. The process of recovery of SOP in darkness lacked the transient enhancement immediately as BGL was turned off, characteristic of adult rats. There were no major age differences in adaptive behaviour of a-wave. In young rats, recovery of b-wave was relatively slower.

CONCLUSIONS

Properties of BGL adaptation and readaptation functions of the neuronal adaptive system in baby retina differed compared to the adult one by being less forceful and more restrained. Handling of mesopic illumination and recovery in the dark was immature. Development of these functions of the neuronal adaptive system progresses postnatally and lags behind that of the photoreceptor response and seems to be delayed also compared to that of the bipolar response.

Low-hemorrhage-risk surgical approach to expose the optic nerve in rabbits without bony removal and rectus resection

OBJECTIVE

A low-hemorrhage-risk surgical approach to expose the optic nerve (ON) in rabbits through the orbital process of the frontal bone without removal of the bony orbit and resection of the rectus muscle was explored and assessed in this study. This approach will be used to investigate a new visual prosthesis that requires intraorbital ON stimulation with penetrating electrodes. Animals Chinese Albino rabbits (n = 10).

METHODS

Rabbits were classified into a surgery and a control group (five in each). In the surgery group, the ON exposure was explored by the newly proposed surgical approach. Surgical time, blood loss, visually evoked potentials (VEP) at four different scheduled time points, and H&E-stained histology of the ON at one month after surgery were recorded and analyzed to assess the ease and safety of the approach.

RESULTS

The average surgical time for the ON exposure was 16.40 +/- 1.14 min with average blood loss of 0.52 +/- 0.08 mL. Within the one-month follow-up, the ON exhibited a naturally reversible conduction change in terms of VEP amplitude. Histological examination of the ON was unremarkable. A postoperative mild ptosis of the surgical eye resolved within one month after surgery.

CONCLUSION

The ease and safety of this new surgical approach allowed it to be easily used by non-expert operators and widely applied in rabbit experiments for various research purposes requiring exposure of the ON.

Frequency spectrum and amplitude analysis of dark- and light-adapted oscillatory potentials in albino mouse, rat and rabbit

We studied frequency spectrum, implicit time and amplitude of oscillatory potentials (OPs) in albino mice, rats, and rabbits. Oscillatory potentials were extracted digitally from dark- and light-adapted electroretinograms (ERGs) recorded with a protocol commonly used in our laboratory. The frequency spectra of OPs were analyzed by using Fast Fourier Transform (FFT). Oscillatory potential amplitudes were calculated via numerically integrating the power spectrum. Oscillatory potential frequency spectra vary among species and are light-intensity dependent. In dark-adapted ERG, mouse and rat OPs have one major component with a frequency peak at approximately 100 Hz. Rabbits show multiple frequency peaks with a low frequency peak around 75 Hz. In all the three species, the implicit time of light-adapted OP is longer than that of the dark-adapted OPs. At a given intensity, mice have the highest OP responses. Our data suggest that the commonly used bandpass of 75 Hz (or even 100 Hz) to 300 Hz for OP extraction is insufficient in these animals. In order to acquire the complete OP responses from the ERG signals, it is necessary to determine the OP frequency spectrum. In this study, the lower end cutoff frequency was set at 40 Hz in mice, 65 Hz in rats and rabbits.

Dopaminergic modulation and rod contribution in the generation of oscillatory potentials in the tiger salamander retina

The roles of rod and cone input and of dopamine in the generation of oscillatory potentials were studied in tiger salamander retina. Under scotopic conditions, oscillations were elicited with a green, but not a red stimulus. With mesopic background illumination, both stimuli caused oscillations. Addition of quinpirole to a mesopic retina eliminated oscillations while SKF-38393 had no effect. Similarly, addition of sulpiride to a light-adapted retina elicited oscillatory activity, but SCH 22390 had no effect. These results suggest that oscillatory potentials are elicited through activation of the rod pathway and are modulated by dopamine through D2-receptors.