Neuroscience Letters, 41 (1983) 277-282 Elsevier Scientific Publishers Ireland Ltd.

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MONOCULAR STIMULATION INCREASES DEOXYGLUCOSE UPTAKE IN IPSILATERAL PRIMARY OPTIC CENTERS OF THE RAT

AM! ISSEROFF and YIGAL MADAR

Department o f Isotope Research, The Weizmann Institute o f Science, 76 100 Rehovot (Israel)

(Received February 21st, 1983; Revised version received August 23rd, 1983; Accepted August 24th, 1983)

Key words: deoxyglucose - dorsal lateral geniculate nucleus - laterality - rats - superior colliculus - vision

We present data from autoradiography of 2-deoxyglucose (2-DG), which indicate that despite the limited uncrossed optic innervation of the rat, monocular stimulation produces considerable activation of ipsilateral primary visual centers. Compared to binocularly occluded rats, intact, monocularly occlud- ed ones showed substantial and statistically significant increases in 2-DG uptake throughout the dorsal lateral genicula~.e nucleus and superficial layers of the superior colliculus ipsilateral to the stimulated eye, as well as larger increases in contralateral areas, in contrast, stimulation of mono-enucleated rats did not produce observable increases in 2-DG uptake in ipsilateral optic centers, confirming previous results. This effect may be due to functional depression in the denervated areas.

The rat visual system is thought to be highly lateralized [15], since over 90°/0 of optic tract fibers cross at the chiasm [10]. Few cells respond to ipsilateral input in the dorsal lateral geniculate nucleus (DLGN) and superficial gray of the superior colliculus (SC) [13, 18], and stimulation of the remaining eye in mono-enucleated albino [7, 20] or pigmented [21] rats produces virtually no ipsilateral increase in up- take of 2-deoxyglucose (2-DG), a measure of functional activity [7, 19]. However,, there is ample reason to expect at least some functional response to ipsilateral stimulation in primary optic centers. Despite its paucity, the uncrossed innervation may be of considerable functional importance, since it provides for binocularity throughout the central third of the visual field [1-3] and can mediate visual pattern discrimination [2]. Functional activity in the ipsilateral DLGN and SC should reflect the contributions of the ipsilateral optic tract, of bilateral input from various sub- cortical sources [11], and input relayed over callosal and corticofugal pathways [4, 14, 17, 20].

We report that ipsilateral optic centers display significant and substantial in- creases in 2-DG uptake during stimulation of intact monocularly occluded pigmented rats. By contrast, stimulation following mono-enucleation produces no

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observable change in 2-DG uptake in ipsilaterai visual structures. This inexcitability may reflect diaschisis depression [12] in the denervated DLGN and SC.

Eighteen Manchester hooded rats were distributed equally among binocular oc- clusion, monocular occlusion and monocular enucleationgroups. Enucleation was performed under ether anesthesia 30-60 days before the 2-DG assay. Prior to the assay, each rat was secured in a restrainer; occlusion groups were lightly anesthetiz- ed with ether, opaque occluders were placed under the eyelids, and the lids pulled shut and sealed with black tape. Rats of all 3 groups were alio~ed 0.5 h to recover from anesthesia and habituate to the restrainer. They were the~a injected i.p. with 5 #Ci/100 g [~4C]2-DG, placed facing the stimulation apparatus at a distance of ap- proximately 40 cm and stimulation was directed at one visual hemifield; thus, in- terhemispheric comparisons in the binocularly occluded group controlled for light leakage. Stimuli were generated on two stacked television monitors having 38 cm wide x 30 cm high screens. The upper monitor was angled downward, and both were mounted on a pivot, allowing for stimulation of the entire visual field. A bright (~ 1000 iux) 30 x 5 cm bar was shown on each screen against a dark (0-20 lux) background at orientations of 0, 45 and 90 ° from vertical, and moved perpendicular to its long axis at I or 1.5 sweeps per sec. Every 20 sweeps, the monitors were pivoted to cover a different portion of the eye. All combinations of sweep rates and orientations were presented to all areas of the eye within any 5 min period.

To assess the residual effects of enucleation upon baseline uptake of 2-DG, 3 rats were tested in total darkness 30 days after mono-enucleation. These rats were restrained, both eyes were occluded and a black shroud was placed over their heads. Following ! h of adaptation in a darkened booth, they were injected with 2-DG and placed in a small lightproof (no fogging of X-ray film within 10 min) enclosure with baffled air inlets.

Forty-five min after injection of 2-DG, rats were anesthetized with sodium pen- tobarbital and perfused intracardially with 1% formalin in phosphate buffer [5]; brains were removed and frozen, and 30 ~,m cryostat sections were used to prepare autoradiograms on X-ray film. Optical densities (O.D.s) were measured on images of the autoradiograms that were digitized and enlarged 11 x by a microcomputer- based image analysis system [8]. Each measurement covered 150 #m x 190 am in the original autoradiograms, and was repeated in 2-4 sections, in the superficial gray of the superior coiliculus (SC), 8 measurements were made in each hemisphere, and in the DLGN 4 different areas were measured as shown in Fig. 1, to assess the distribution of activity within these structures.

O.D.s were converted to uptake using a calibration curve based on quadruplicate exposures of 8 standards of known activity. Uptake in SC was divided by the average of measurements in the stratum iaconosum moleculare of posterior hip- pocampus and reticular formation, and uptake in DLGN was divided by that of ven- trobasal thalamic nuclei and anterior hippocampus. This procedure controlled for possible variations in overall uptake due to non-specific activation, and reduced variability due to differences in section thickness or exposure time.

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T A B L E I

A C T I V A T I O N OF V I S U A L A R E A S BY M O N O C U L A R S T I M U L A T I O N AS I N D I C A T E D BY

R E L A T I V E U P T A K E O F 2 - D E O X Y G L U C O S E

SC, superficial gray of superior colliculus; DLGN, dorsal lateral geniculate nucleus; Contra, hemisphere contralateral to stimulated eye (intact eye for mono-enucleates kept in :darkness); Ipsi, ipsilaterai to stimulated (intact) eye. Loci of measurements are indicated in Fig. 1. Binocularly occluded rats were ex- posed to visual stimulation aimed at one side of the head to cont~rol for possible light leakage. Values shown are

uptake in visual areas mean + S.E.M. of x 100

uptake in reference areas

*Significantly (P<0.01 2-tailed) different from binocular occlusion group

Binocular Monocular Monocular Monocular occlusion occlusion enucleation enucleation

(darkness)

Contra DLGN 94.7 + 4.4 142.7 + 6.9* 127.3 + 2.3* 92.1 + 4.2

SC 97.6 + 4.3 143.1 ± 2.3* 133.1 + 4.9* 95.2 + 3.7

ipsi DLGN 93.3 + 4.1 120.8 + 6.3* 93.9 + 4.2 92.5 ± 3.1

SC 95.1 + 3.2 120.3 + 3.1" 92.9 + 3.2 93.1 : 2.9

As expected, in the contralateral SC and DLGN, both mono-enucleated and mono-occluded groups exposed to visual stimulation showed significantly (P< 0.01 or better by 2-tailed t-test) greater 2-DG uptake than binocularly occluded controls (Table I). Uptake in the contralateral DLGN and SC of mono-enucleated rats was marginally (0.10> P>0.05) below that of the mono-occluded group. However, in the ipsilateral DLGN and SC of mono-occluded intact rats, uptake averaged 120.8 +_ 6.3 and 120.3 + 3.1 respectively, significantly (P<0.01) above the mc:~ns of 93.3 +_ 4.1 and 95.1 + 3.2 observed in binocularly occluded rats. These increases were not confined to the known projection areas of the ipsilateral optic tract [6, 10, 18], as should be evident from Fig. 1. Thus, ipsilateral uptake was significantly greater in the mono-occluded group than in the bino-occluded controls at each point measured with the exception of posteromedial SC, which showed only a marginal (0.10> P>0.05) increase. In the ipsilateral DLGN and SC of the mono-enucleated group overall uptake averaged 93.9 +_ 4.2 and 92.9 _+ 3.2 respectively, not dif- ferent from the means of 93.3 + 4.1 and 95.1 + 3.2 observed in binocularly occlud- ed rats, and significantly (P<0.01) below the values obtained in the monocularly occluded group; identical results were obtained in comparisons of individual loci

within DLGN and SC. As shown in Table I, 2-DG uptake in optic structures of mono-enucleated rats

maintained in total darkness was similar in both hemispheres, and means were not significantly different from those observed in the ipsilateral DLGN and SC of

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i ̧

["ig. i. Comb ~ter-generated pseudo-brightness representations oi 2-DG autoradiograms of coronal s'~'c- tions from the brains of binocularl~ occluded (left columrtJ and monocularly occluded (right colut,mJ mrs. Relative optical density has been mapped into 5 brightness levels: white, 0-75% of reference values; light gray, 76-90°:0; meditm] gray 91-105%; dark gray, 106-120°,'o; black, over 120°,10. Top to bottom: successively more posterior sections through the lexel of the lateral geniculate nuclei (abovej and superior colliculus (belox~). The right hemisphere was ipsilateral to the stimulated eye. Arrows shoxv approximate loci of measurements in dorsal lateral geniculate nucleus and superior colliculus.

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monocularly stimulated mono-enucleates and in the binocularly occluded intact group. Apparently, enucleation virtually eliminates the response to ipsilateral stimulation, but has little effect on baseline uptake. Denervation also seems to depress areas ipsilateral to the removed eye, and contralateral to the stimulated one, since uptake in the stimulated hemisphere of mono-enucleates was marginally below ti~at of intact rats.

In the ipsilateral DLGN and SC of intact monocularly occluded rats, stimulation produced increases in uptake of 2-DG which were 57.5070 and 55.2°70, respectively, of those obtained contralaterally. These increases are far greater than would be ex- pected from electrophysiological or anatomic data [3, 6, 10, 13, 18], though 2-DG uptake is not always equivalent to electrophysiologicai excitation [16]. The propor- tion of ipsilaterai activity due to the uncrossed optic tract as opposed to indirect in- puts is not known; it might be est':mated by studying uptake of 2-DG during monocular stimulation of intact albino rats, in which the decussation of optic fibers is more complete [9].

The devoted assistance of Ms. S. Fieldust is gratefully acknowledged. This research was funded by Grant 3/83 from the Israel Psychobiology Institute, Charles Smith Family Foundation.

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