Pharmacological study of the chicken's monocular optokinetic nystagmus: effects of GABAergic agonist and antagonists

Modulation of the zebrafish optokinetic reflex by pharmacologic agents targeting GABAA receptors

Optokinetic reflex (OKR) responses provide a convenient means to evaluate visual, integrative and oculomotor function in larval zebrafish. We measured multiple aspects of the OKR response in zebrafish exposed systemically to compounds altering signaling at GABA_A receptors in order to derive quantitative concentration-response relationships. The GABA_A antagonist picrotoxin caused concentration-dependent decreases in reflex gain, saccade velocity, saccade amplitude, interocular concordance and interocular gain. Conversely, the GABA_A agonist gaboxadol provoked increases in reflex gain, saccade velocity, saccade amplitude and ocular range at low concentrations, and decreases in some of these parameters at higher concentrations. These data show that GABA_A signaling influences multiple aspects of the OKR (including gain, generation of saccades, and coordination between the two eyes) and provide proof of concept that quantitative OKR analysis can be used as a tool for chemical biology and neuropharmacology applications.

Die Fledermaus: regarding optokinetic contrast sensitivity and light-adaptation, chicks are mice with wings

BACKGROUND

Through adaptation, animals can function visually under an extremely broad range of light intensities. Light adaptation starts in the retina, through shifts in photoreceptor sensitivity and kinetics plus modulation of visual processing in retinal circuits. Although considerable research has been conducted on retinal adaptation in nocturnal species with rod-dominated retinas, such as the mouse, little is known about how cone-dominated avian retinas adapt to changes in mean light intensity.

METHODOLOGY/PRINCIPAL FINDINGS

We used the optokinetic response to characterize contrast sensitivity (CS) in the chick retina as a function of spatial frequency and temporal frequency at different mean light intensities. We found that: 1) daytime, cone-driven CS was tuned to spatial frequency; 2) nighttime, presumably rod-driven CS was tuned to temporal frequency and spatial frequency; 3) daytime, presumably cone-driven CS at threshold intensity was invariant with temporal and spatial frequency; and 4) daytime photopic CS was invariant with clock time.

CONCLUSION/SIGNIFICANCE

Light- and dark-adaptational changes in CS were investigated comprehensively for the first time in the cone-dominated retina of an avian, diurnal species. The chick retina, like the mouse retina, adapts by using a "day/night" or "cone/rod" switch in tuning preference during changes in lighting conditions. The chick optokinetic response is an attractive model for noninvasive, behavioral studies of adaptation in retinal circuitry in health and disease.

Responses to Moving Visual Stimuli in Pretectal Neurons of the Small-Spotted Dogfish (Scyliorhinus canicula)

Single-unit recordings were performed from a retinorecipient pretectal area (corpus geniculatum laterale) in Scyliorhinus canicula. The function and homology of this nucleus has not been clarified so far. During visual stimulation with a random dot pattern, 45 (35%) neurons were found to be direction selective, 10 (8%) were axis selective (best neuronal responses to rotations in both directions around one particular stimulus axis), and 75 (58%) were movement sensitive. Direction-selective responses were found to the following stimulus directions (in retinal coordinates): temporonasal and nasotemporal horizontal movements, up- and downward vertical movements, and oblique movements. All directions of motion were represented equally by our sample of pretectal neurons. Additionally we tested the responses of 58 of the 130 neurons to random dot patterns rotating around the semicircular canal or body axes to investigate whether direction-selective visual information is mapped into vestibular coordinates in pretectal neurons of this chondrichthyan species. Again all rotational directions were represented equally, which argues against a direct transformation from a retinal to a vestibular reference frame. If a complete transformation had occurred, responses to rotational axes corresponding to the axes of the semicircular canals should have been overrepresented. In conclusion, the recorded direction-selective neurons in the Cgl are plausible detectors for retinal slip created by body rotations in all directions.

Optokinetic deficits in albino ferrets (Mustela putorius furo): a behavioral and electrophysiological study

We compared the horizontal optokinetic reaction (OKR) and response properties of retinal slip neurons in the nucleus of the optic tract and dorsal terminal nucleus (NOT-DTN) of albino and wild-type ferrets (Mustela putorius furo). In contrast to pigmented ferrets, we were unable to observe OKR in albino ferrets during binocular and monocular viewing using random dot full field stimulation and electro-oculography (EOG). Observations during early postnatal life indicate that regular OKR is present in pigmented pups 3 d after eye opening but is absent at any stage during development in albino ferrets. Unilateral muscimol injections to inactivate all neurons in the NOT-DTN containing GABA(A) and GABA(C) receptors caused spontaneous horizontal nystagmus with slow phases away from the injected hemisphere in albino as well as in pigmented animals. Retinal slip neurons in the NOT-DTN of albino ferrets identified by antidromic activation from the inferior olive and orthodromic activation from the optic chiasm were well responding to intermittent bright light stimuli, but many showed a profound reduction of responsiveness to moving stimuli. The movement-sensitive neurons exhibited no clear direction selectivity for ipsiversive stimulus movement, a characteristic property of these neurons in pigmented ferrets and other mammals. Thus, the defect rendering albino ferrets optokinetically nonresponsive is located in the visual pathway subserving the OKR, namely in or before the NOT-DTN, and not in oculomotor centers.

Pharmacological study of the chicken's monocular optokinetic nystagmus: involvement of the ON retinal channel evidenced by the glutamatergic separation of ON and OFF pathways

The chicken's monocular optokinetic nystagmus (OKN), recorded by the magnetic search coil technique, displays a directional asymmetry, the temporal-nasal (T-N) stimulation being more efficient than the nasal-temporal (N-T) one to evoke the reflex. The intravitreal administration of APB, a glutamate agonist which selectively blocks the ON retinal channel strongly reduced the eye monocular OKN; it also induced spontaneous eye movements in the T-N direction. The intravitreal injection of PDA another glutamate analog, which reduces the OFF channel, while increasing the activity of the ON channel, induced a large increase in OKN velocity gain, especially for a N-T stimulation at the lowest drum speeds. These results indicate the main involvement of the ON retinal channel in the OKN genesis and the inhibitory effect of the OFF channel upon this oculomotor reflex.