Exp. Bye Res. (1977) 25, 9-17
Differences in Optokinetic Nystagmus between Albino and Pigmented Rabbits R. W. HAHNENBERGER Institute
of Medical
(Received 19 January
Pharmacoloyy,
Uppsala
University,
Uppsala,
/Sweden
1976 and ilz revised form 22 October 191‘6, London)
Optokinetic nystagmus was tested in albino and pigmented rabbits. The response was significantly weaker in the albino animals. The albino rabbits always had an unidirectional optokinetic nystagmus under monocular viewing conditions : they reacted only, if the moving screen moved from the temporal side through the visual field of the viewing eye. Pigmented rabbits had a weak but consistent bidirectional optokinetio nystagmus. This difference may be related to the abnormal decussation of the albinotic visual pathways and/or degeneration
in the albinotic retina. Key
Words: albino rabbit;
pigmented rabbit;
optokinetic
nystagmus;
visual pathways.
1. Introduction During recent studies in our laboratory on interrupted axoplasmic flow in the optic nerve of rabbits it became evident that it was necessary to test visual function in the animals at regular intervals over extended periods of time. Optokinetic nystagmus (OKN) seemed to be a suitable test. In preliminary experiments albinos displayed a poor OKN, whereas pigmented rabbits responded more regularly and to higher stimulus frequencies. Similar conclusions have been reached previously (Fukuda and Tokita, 1957). It was therefore decided to investigate if these differences between albino and pigmented rabbits are really characteristic and this paper reports the findings.
2. Materials and Methods Animals Twelve albino (New Zealand White) and 20 pigmented rabbits (of different breeds), weighing bet\\reen 1.6 and 3.5 kg were used. The sexes were equally represented and the animals were fed with a diet intended for pregnant rabbits. Free access to water was allowed. The animal was first put in a box (36 x 20 x 18 cm in size) and foam rubber pieces were then packed firmly around, so that the rabbit was restrained “elastically”. This kind of handling in combination with the special diet reduced the numbers of vertebral fractures, which were otherwise quite frequent. The head of the animal was immobilized by a headholder. Optokinetic dwm The restrained animal was placed on a round table 50 cm in diameter, which could be lifted into an optokinetic drum 63.5 cm in diameter, 66 cm in. height and closed at the top. The inner black wall of the drum was patterned with 18 equidistant vertical white stripes 1.6 cm wide. The upper front teeth of the animal were 10 cm from the inner wall. As the rotating stripes entered the visual field in the nasal direction, they were thus seen at a distance decreasing from 48 to 15 cm. The drum could be rotated electrically clockwise or counterclockwise at continuously variable angular velocities between l.O”/sec and
10
R.
IA'.
HAHNENBERGER
lZO)“/sec, which were recorded by an infrared photocell, mounted close to a reflector on the outside of the drum. This reflector was designed so that both the velocity and direction of the drum movement were recorded. The interior of the drum was illuminated by an array of 32 identical light bulbs, with nominal ratings of 0.84 W when operated at 28 V. These bulbs could be switched OII in three combinations differing by a factor of 4, i.e. two, eight and 32 bulbs and one of five fixed operating voltages could be selected. The brightest illumination of 22 lx (measured with luxmeter, Metrux K, Metrawatt GmbH, Niirnberg, West Germany) was thus obtained with 32 light bulbs operating at 24 V, while the dimmest condition was two bulbs at 4.11 V, the luminance differing in these two extreme cases by a factor of 48. The relative brightness in the drum was measured by a barrier layer photocell (Clairex Photoconductive cell type C1707L) with maximal peak of sensitivity at 610.5 nm. The voltage of the variable and highly stable d.e. power supply was measured with a digital volt meter (Keathley 168 Autoranging DMM). The different illumination levels between 40 and 48 arbitrary units were obtained by stepwise alteration of the combinations of operating voltages and the numbers of bulbs lit. Since the spectral sensitivity of the photocell was not the same as that of the eye, the lower illumination levels, with reduced bulb voltages, were physiologically lower than indicated by the cell. No attempt was made to compensate for this. To check the possible influence of differences in visual acuity, the following additional experiments were made: four pigmented and three albino rabbits were placed with their heads in the center of a large drum (150 cm in diameter) containing 18 5-cm wide white stripes. The interior illumination was adjusted to 20 Ix with a 60 W bulb and thus corresponded to approximately 4s arbitrary units of the small drum. The animals tested viewed the moving screen binocularly. The drum was rotated with angular velocities of 1*5”, 6” and 18”/sec, either clockwise or counterclockwise.
A needle electrode was inserted at each canthus of one eye to record the cornea-retinal potential changes. These electrodes were not implanted permanently and electrode positioning was therefore slightly different at each new insertion. In this way it was only possible to compare the amplitude quantit,atively within any one trial. Attempts to calibrate the OKN by a defined vestibular nystagmus stimulus failed. The response was very variable and habituation soon occurred. The cornea-retinal potential changes were recorded on the DC-channel of a type 428 Mingograph after amplification by a DC-amplifier (Sanborn 350-1500A with the subunit 350-2). Cut off was set at 15 Hz. We never experienced serious baseline drift. The leads were arranged so that an upward deflection of the recording indicated a counterclockwise (seen from above) movement of the eyes. Thus, an upward directed slow phase in the OKN recording was induced by A counterclockwise movement of the drum. This was designated as a regular OKN.
Procedure of testing In the first part of the experiment the animals were tested by the following schedule: The illumination was kept constant at 24 V and 32 bulbs (4s) and the drum rotated at three different velocities: 1.5”, 6” and 18’/sec. This test was done both with neither eye occluded and with the left eye occluded by an nluminium plate fitted to the anaesthetized cornea. In the second part of the experiment the illumination was set at the lowest level (40) and the animal allowed to adapt for 40 min. With neither eye occluded the drum was then moved clockwise at 1.5, 3, 4.5, 6, 7.5, 9, 12, 15 and 18”/sec for at least one minute at each
OPTOKINETIC
NYXTAG&IUS
IX
RABBIT
11
velocity. Speed changes were made without stopping the drum and took about 30-60 sec. dfter testing with this program at 40 it was then repeated with six higher illuminations. The animal was allowed to adapt for 2 min at each illumination level before testing. While testing, all room and instrument lights were turned off, except for a 13.8 R red lamp. In the sections Results and Discussion the “unidirectional” responses refer only to the monocularly viewing animal. 3. Results
Typical records from an albino and a pigmented rabbit are shown in Figs 1 and 2. Results of the two groups of 12 albinos and 14 pigmented rabbits are summarized in Table I. They were all tested under similar conditions: the illumination was kept, constant at ils and the drum rotated around the “binocular” (Fig. 1 and Table I) or the “monocular” animal (Fig. 2 and Table I) in a clockwise or counterclockwise direction at several angular velocities. There was no significant difference due to the direction of rotation, when the moving drum was viewed binocularly. There was: however, a large difference between the two groups of rabbits. Already at the lowest,
FIG. 1. Recording of an albino and a pigmented rabbit tested with constant illumination (48) at 1,5”/sec, R”/sec, 4.50/set, 6’/sec, 12’/sec and lS”/sec velocities of the optokinetic drum. Both eyes were open. c = clockwise, cc = counterclockwise direction of the drum (seen from above). Each trace embraces 30 sec. Lower traces: speed recorder.
angular velocity of 1*5’/sec, where all rabbits tested responded, the albinos followed the moving drum with an average of 7 beats/min while pigmented rabbits had 10 beats/min. With increasing drum speed the groups diverged even more. At Go/see the albino’s frequency had only increased to 10 beats/min, while in the pigmented rabbits it rose to 27 beats/min. At the highest tested angular velocity only two out of 12 albino rabbits were able to produce a nystagmus while all pigmented rabbits were
12
R.
W.
HAHNENBERGER
still able to do so with a further increase of the mean beat frequency up to 48 beats/ min. When one eye was o&kled the direction in which the drum moved was of paramount importance. Rotation of the drum from the temporal to the nasal side of the viewing eye, provoked a regular sustained nystagmus, which resembled the nystagmus induced TABLE
I
Frequency of nystagmus beats per minute
Drum
velocity
Rabbit,
A
Binocular Direction counter clockwise (cc)
7.08 t h1.00
n-3.
viewing of drum Clockwise
P
A
6°/s~c
4 10.92 30.60 10.08 53.44 +
+
t
N.S.
+
t
N.S.
--f
1'
a
658*+
4 48.71 ~6.05
X.8.
+
& IO.00 50.90 9.83 53.35
Differences in Optokinetic Nystagmus between Albino and Pigmented Rabbits R. W. HAHNENBERGER Institute
of Medical
(Received 19 January
Pharmacoloyy,
Uppsala
University,
Uppsala,
/Sweden
1976 and ilz revised form 22 October 191‘6, London)
Optokinetic nystagmus was tested in albino and pigmented rabbits. The response was significantly weaker in the albino animals. The albino rabbits always had an unidirectional optokinetic nystagmus under monocular viewing conditions : they reacted only, if the moving screen moved from the temporal side through the visual field of the viewing eye. Pigmented rabbits had a weak but consistent bidirectional optokinetio nystagmus. This difference may be related to the abnormal decussation of the albinotic visual pathways and/or degeneration
in the albinotic retina. Key
Words: albino rabbit;
pigmented rabbit;
optokinetic
nystagmus;
visual pathways.
1. Introduction During recent studies in our laboratory on interrupted axoplasmic flow in the optic nerve of rabbits it became evident that it was necessary to test visual function in the animals at regular intervals over extended periods of time. Optokinetic nystagmus (OKN) seemed to be a suitable test. In preliminary experiments albinos displayed a poor OKN, whereas pigmented rabbits responded more regularly and to higher stimulus frequencies. Similar conclusions have been reached previously (Fukuda and Tokita, 1957). It was therefore decided to investigate if these differences between albino and pigmented rabbits are really characteristic and this paper reports the findings.
2. Materials and Methods Animals Twelve albino (New Zealand White) and 20 pigmented rabbits (of different breeds), weighing bet\\reen 1.6 and 3.5 kg were used. The sexes were equally represented and the animals were fed with a diet intended for pregnant rabbits. Free access to water was allowed. The animal was first put in a box (36 x 20 x 18 cm in size) and foam rubber pieces were then packed firmly around, so that the rabbit was restrained “elastically”. This kind of handling in combination with the special diet reduced the numbers of vertebral fractures, which were otherwise quite frequent. The head of the animal was immobilized by a headholder. Optokinetic dwm The restrained animal was placed on a round table 50 cm in diameter, which could be lifted into an optokinetic drum 63.5 cm in diameter, 66 cm in. height and closed at the top. The inner black wall of the drum was patterned with 18 equidistant vertical white stripes 1.6 cm wide. The upper front teeth of the animal were 10 cm from the inner wall. As the rotating stripes entered the visual field in the nasal direction, they were thus seen at a distance decreasing from 48 to 15 cm. The drum could be rotated electrically clockwise or counterclockwise at continuously variable angular velocities between l.O”/sec and
10
R.
IA'.
HAHNENBERGER
lZO)“/sec, which were recorded by an infrared photocell, mounted close to a reflector on the outside of the drum. This reflector was designed so that both the velocity and direction of the drum movement were recorded. The interior of the drum was illuminated by an array of 32 identical light bulbs, with nominal ratings of 0.84 W when operated at 28 V. These bulbs could be switched OII in three combinations differing by a factor of 4, i.e. two, eight and 32 bulbs and one of five fixed operating voltages could be selected. The brightest illumination of 22 lx (measured with luxmeter, Metrux K, Metrawatt GmbH, Niirnberg, West Germany) was thus obtained with 32 light bulbs operating at 24 V, while the dimmest condition was two bulbs at 4.11 V, the luminance differing in these two extreme cases by a factor of 48. The relative brightness in the drum was measured by a barrier layer photocell (Clairex Photoconductive cell type C1707L) with maximal peak of sensitivity at 610.5 nm. The voltage of the variable and highly stable d.e. power supply was measured with a digital volt meter (Keathley 168 Autoranging DMM). The different illumination levels between 40 and 48 arbitrary units were obtained by stepwise alteration of the combinations of operating voltages and the numbers of bulbs lit. Since the spectral sensitivity of the photocell was not the same as that of the eye, the lower illumination levels, with reduced bulb voltages, were physiologically lower than indicated by the cell. No attempt was made to compensate for this. To check the possible influence of differences in visual acuity, the following additional experiments were made: four pigmented and three albino rabbits were placed with their heads in the center of a large drum (150 cm in diameter) containing 18 5-cm wide white stripes. The interior illumination was adjusted to 20 Ix with a 60 W bulb and thus corresponded to approximately 4s arbitrary units of the small drum. The animals tested viewed the moving screen binocularly. The drum was rotated with angular velocities of 1*5”, 6” and 18”/sec, either clockwise or counterclockwise.
A needle electrode was inserted at each canthus of one eye to record the cornea-retinal potential changes. These electrodes were not implanted permanently and electrode positioning was therefore slightly different at each new insertion. In this way it was only possible to compare the amplitude quantit,atively within any one trial. Attempts to calibrate the OKN by a defined vestibular nystagmus stimulus failed. The response was very variable and habituation soon occurred. The cornea-retinal potential changes were recorded on the DC-channel of a type 428 Mingograph after amplification by a DC-amplifier (Sanborn 350-1500A with the subunit 350-2). Cut off was set at 15 Hz. We never experienced serious baseline drift. The leads were arranged so that an upward deflection of the recording indicated a counterclockwise (seen from above) movement of the eyes. Thus, an upward directed slow phase in the OKN recording was induced by A counterclockwise movement of the drum. This was designated as a regular OKN.
Procedure of testing In the first part of the experiment the animals were tested by the following schedule: The illumination was kept constant at 24 V and 32 bulbs (4s) and the drum rotated at three different velocities: 1.5”, 6” and 18’/sec. This test was done both with neither eye occluded and with the left eye occluded by an nluminium plate fitted to the anaesthetized cornea. In the second part of the experiment the illumination was set at the lowest level (40) and the animal allowed to adapt for 40 min. With neither eye occluded the drum was then moved clockwise at 1.5, 3, 4.5, 6, 7.5, 9, 12, 15 and 18”/sec for at least one minute at each
OPTOKINETIC
NYXTAG&IUS
IX
RABBIT
11
velocity. Speed changes were made without stopping the drum and took about 30-60 sec. dfter testing with this program at 40 it was then repeated with six higher illuminations. The animal was allowed to adapt for 2 min at each illumination level before testing. While testing, all room and instrument lights were turned off, except for a 13.8 R red lamp. In the sections Results and Discussion the “unidirectional” responses refer only to the monocularly viewing animal. 3. Results
Typical records from an albino and a pigmented rabbit are shown in Figs 1 and 2. Results of the two groups of 12 albinos and 14 pigmented rabbits are summarized in Table I. They were all tested under similar conditions: the illumination was kept, constant at ils and the drum rotated around the “binocular” (Fig. 1 and Table I) or the “monocular” animal (Fig. 2 and Table I) in a clockwise or counterclockwise direction at several angular velocities. There was no significant difference due to the direction of rotation, when the moving drum was viewed binocularly. There was: however, a large difference between the two groups of rabbits. Already at the lowest,
FIG. 1. Recording of an albino and a pigmented rabbit tested with constant illumination (48) at 1,5”/sec, R”/sec, 4.50/set, 6’/sec, 12’/sec and lS”/sec velocities of the optokinetic drum. Both eyes were open. c = clockwise, cc = counterclockwise direction of the drum (seen from above). Each trace embraces 30 sec. Lower traces: speed recorder.
angular velocity of 1*5’/sec, where all rabbits tested responded, the albinos followed the moving drum with an average of 7 beats/min while pigmented rabbits had 10 beats/min. With increasing drum speed the groups diverged even more. At Go/see the albino’s frequency had only increased to 10 beats/min, while in the pigmented rabbits it rose to 27 beats/min. At the highest tested angular velocity only two out of 12 albino rabbits were able to produce a nystagmus while all pigmented rabbits were
12
R.
W.
HAHNENBERGER
still able to do so with a further increase of the mean beat frequency up to 48 beats/ min. When one eye was o&kled the direction in which the drum moved was of paramount importance. Rotation of the drum from the temporal to the nasal side of the viewing eye, provoked a regular sustained nystagmus, which resembled the nystagmus induced TABLE
I
Frequency of nystagmus beats per minute
Drum
velocity
Rabbit,
A
Binocular Direction counter clockwise (cc)
7.08 t h1.00
n-3.
viewing of drum Clockwise
P
A
6°/s~c
4 10.92 30.60 10.08 53.44 +
+
t
N.S.
+
t
N.S.
--f
1'
a
658*+
4 48.71 ~6.05
X.8.
+
& IO.00 50.90 9.83 53.35
