Recordings were made 3 to 4 weeks after optic nerve section and 1
to 10 days after intravitreal injection. Rats were anesthetized with
urethane (1200 mg/kg), kept warm by a feedback-controlled electric
blanket and placed in a conventional head holder. In cats, anesthesia
was induced by an intramuscular injection of ketamine (30 mg/kg;
Ketalar, Parke–Davis, Morris Plains, NJ). An endotracheal tube and a
venous cannula were then inserted. The animal was paralyzed with an
intravenous infusion of a 0.2% solution of pancuronium bromide
(Pavulon; Organon Teknika, Durham, NC) at a rate of 0.1 to 0.2 ml/kg
per hour and artificially ventilated. Anesthesia was maintained
throughout the recording session by intravenous infusion of sodium
thiopental (3.5 mg/kg per hour). Silver wire electrodes were introduced
into the vitreous humor through a minimal incision on the lateral side
of the eyeball. Recordings were made from left and right eyes
separately, with the other eye fully covered. Recordings were made
between the two vitreous electrodes or between the electrode in the
vitreous and an electrode placed on the tongue. The band-pass of the
amplifier was 0.1 Hz to 500 Hz.
Rats were dark-adapted for 40 minutes before recording commenced, and
darkness was maintained throughout except for the light stimulus. Three
modes of light stimulation were used. In one, dark-adapted responses
were evoked with an 8-msec flash of a monitor screen subtending
approximately 30° at the eye and of intensity up to 28
cd/m2. Interstimulus intervals were 2 minutes or
longer, and during this time the screen was dark. In the second mode,
light-adapted responses were obtained using the same screen stimulator.
The stimulus was a square-wave variation of intensity of mean
brightness 18 cd/m2 (maximum range, 0–36
cd/m2. The frequency of stimulation was 0.5 Hz,
and the screen brightness remained high for 1 second, then low for 1
second. In the third series of experiments dark-adapted responses were
obtained using a flash unit that produced flashes of 1.5-msec duration
and 15, 000 cd/m2 intensity, equivalent to
105,000 scotopic troland/sec. Neutral density filters were used to
attenuate the flash for intensity–response relationships. The
background was kept dark, and the interstimulus interval was at least 2
minutes. The a-wave was measured from the baseline to the first
negative peak, and the b-wave was measured from negative to positive
peaks. In a few experiments we recorded ERG responses to bright flashes
with an amplifier bandwidth extended up to 3 kHz. In these instances
oscillatory potentials could be observed in the normal ERG, but they
were almost absent in the optic nerve sectioned eye.