Monocular visual deprivation at the critical period modulates photic evoked responses

BrainRe,arch Bulletin,Vol. 36. No. 6, pp. 545-548, 1995 Copyright © 1995ElsevierScienceLtd Printedin the USA.All rights reserved 0361-9230/95 $9.50 + .00

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Monocular Visual Deprivation at the Critical Period Modulates Photic Evoked Responses HONG QU YAN,* MALCOLM L. MAZOW* AND NACHUM DAFNY1-1 Departments of *Ophthalmologyand tNeurobiology & Anatomy, The University of Texas Medical School at Houston, 6431 Fannin Street, Houston, TX 77225 [Received 28 January 1994; Accepted 22 September 1994] ABSTRACT: Photic evoked responses were recorded horn the strlate corWx of Long-Evans hooded normal (control) rats and from monocular visual deprivaUon (MD) ,~-;=. The averaged vi -

ronal processes that constitute each component are generally measured as changes in amplitude of the voltage peak of the wave [22]. Monocular sensory (eye) deprivation in early infancy and childhood results in defective vision. The lateral geniculate cellular mass and the visual cortex participating in vision are affected in this condition. Monocular occlusion results in dramatic shift of the ocular dominance of the cortical neurons away from the closed eye [2,3,9,24,35]. The visual acuity is dramatically diminished and contrast sensitivity is depressed, that is, the deprived eye becomes amblyopic [7]. Extensive studies of visual evoked responses have been used [20]. However, very few of these studies were conducted with the monocular visual deprivation preparation. The aim of this study was to utilize the rat VERs as a tool to detect the deficiencies produced by 1 month of monocular visual deprivation (MD) during the critical period in young rats in order to set up a simple animal model of neuroplasticity.

s.~ evoked responses(AV~) were obtat.~ from both hemiei~ md la~o~aeem~Nd~mn t m t t m the oonnlatem erie the i p ~ ad~ cortex with relation to 'ilhe n ~ de,pdved eye. The AVF..R recorded folk)v~ng binocular photic stimulaition after 1 month of m o n o ~ depr~ demonstrated that the two visual corbtxes responded ~ . In the contralatwal hemiq:hereof lhe visual cortex (related to the MD eye), all three c~npmlmlts (P2, N2 and P3) of the AVER of the MD rats had significant incmaus in their peak amplitude as compared to the control recordings. In the i p d a b x a l cortex, ~ e amplitude of component P2 and N2 was signiflcanlJy reduced as a result of 1 month of MD. Compadng the AVER amplibJdes of the two homotopic sites of the visual cortex obtained from the control group reveaht no differences between the two hemispheres but markedly significant diffemm:~ in P2, H2 and 1=3components for the MD group. B a u d on the literature, the possibilitythat the monocular vismd deprivation at the critical period in early developmentalstlagemodulatN the AVER as a result from the neurocytologtcai alteration from altering of GABA and ACh within the stTtate cortex was discusse(L In conclusion, the AVER is a reliable and practical mMhod for studying the effects of monocular deprivation and neuroplasticity in the rat visual cortex.

MATERIALS AND METHODS Six male and six female 10-day-old Long-Evans hooded rats (Harlan Spragne-Dawley Company, Indianapolis, IN) from three different litters were used. Animals were kept on a 12:12 light/ dark cycle with lights on at 0700 prior to and during the experiment. All 12 rats were marked individually by toe clipping and were randomly divided into two groups, the normal control group (6 rats) and monocular deprivation (MD) group (6 rats). All animals were weighed every other day. After weaning, on postnatal day 22 (P-22), all pups were given free access to laboratory chow and water [36,37]. Monocular vision deprivation was performed under ether anesthesia by means of the left eyelid suturing prior to eye opening on postnatal day 14 (P-14; time of natural eye opening). The eyelid remained sutured for 30 days (or more) until P-45. This procedure effectively spans the whole length of the critical period in rats [30], and produces an amblyopic animal model [7,24]. Control animals were exposed to ether together with the MD rats. After day P-45, both the matching control and the MD rats were anesthetized with intraperitoneal (lP) injection of urethane (1.2 g/kg; Sigma, St. Louis, MO), and their heads secured in a stereotactic instrument (David Kopf, Tujunga, CA). The skull

KEY WORDS: Hooded rat, Monocular visual deprivation, Visual evoked response.

INTRODUCTION A primary function of the nervous system is to receive incoming information that has particular significance for the animal, to convert this sensory input into an electrical signal, and to transmit this electrical signal to higher centers. Therefore, electrogenesis recording in the primary sensory structures of the central nervous system (CNS) during sensory stimulation is used to study the physiological mechanism underlying this process. Visual evoked responses (VERs) are useful indicators of the functional integrity of the nervous system [10]. VERs are recorded as a series of electrical events (waves), which are reflections of undedying neuronal processes. Each wave may represent either a single component or multiple components, and alterations in the neu-

LTo whom requests for reprints should be addressed. Supported by Hermann Eye Fund, Houston, Texas and Research to Prevent Blindness, Inc., New York City. 545

546

YAN, MAZOW AND DAFNY

Body Weight Gain 220

200 180

E (.9

160 140

components of the AVER (P2, N2 and P3) were measured peakto-peak [6,37] and used for further analysis. Mean _+ SE was calculated for each of the amplitude (P2, N2 and P3). Statistical significance of the difference within or between the control and the MD groups was determined with a twotailed Student's t-test both for paired and unpaired observations and one-way analysis of variance (ANOVA) for independent comparisons between and within groups [38]. Changes associated with the treatment (i.e., the MD rats compared to the control) were considered significant when p-values were less than 0.05 (p < 0.05). RESULTS

V

.¢: 120 ._~ 100 80 0 en

60 40

+_.1~ Contr°l] MD

20 0 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46

Postnatal

Days

FIG. 1. The mean value of the body weight gain of the control and the monoculardeprived (MD) rats from postnatal day 14 (P-14) to postnatal day 46 tP-46). The bars indicate the standard deviation. skin and the muscle were removed, and 2-mm-diameter holes were drilled above the appropriate regions of the skull. Two silver ball electrodes (0.5-mm-diameter) were placed on both sides of the visual cortex (at Bregma -7.3 ram, Lateral 4.5 mm) according to the atlas of Paxinos and Watson [25] and connected to a Grass P511 preamplifier (Grass Instruments, Quincy, MA) via its emitter follower and to a storage oscilloscope as well as a minicomputer (NIC 1070, signal averaging, Nicolet, Madison, WI). Both eyes were kept open by placing of plastic tings around the external portion of the eye bulbs under the nictating membrane. Photic stimulation was provided by a Grass photo stimulator (intensity setting I = 8, which is about 18/.t joules/ cm 2) through its stroboscope situated l0 cm from the dilated pupils (two drops of atropine sulfate 0.5%) and triggered by a pulse from a Devices Digitimer [36,37]. The visual evoked response experiments were performed in electrophysiological testing cages in a darkened room. The rats were given at least 20 min to recover from the surgical procedure prior to the beginning of the recording session. During the experiment, the body temperature of the animal was monitored by Tele-Thermometer (Yellow Springs Instrument Co., Yellow Springs, OH) and maintained at 37 _+ 2*(2 by a heating pad. To test the ability of the visual cortex to respond to the photic stimulation, stimuli were presented every 2.5 s, and repeated 32 times to obtain one average from each hemisphere. Averaged visual evoked responses (AVER) were plotted by an x - y plotter. A total of four averaged responses were obtained from the respective hemispheres. Each AVER exhibited initial positive peak (Pl) followed by negative (Nl), positive (P2), negative (N2) and positive (P3) components, respectively. As components Pl and N1 in some recordings were difficult to define, only the consistent

On postnatal day 14 (P-14), the rats weighed 20.6 _ 4.2 g (Mean _+ SD) and were divided into two groups, control (n = 6) and MD (n = 6) group. On postnatal day 46 (P-46), the MD rats weighed 165.7 _+ 23.3 g and the control rats were 181.7 _~ 17.2 g. There were no significant differences in body weight between control and MD rats after I month of monocular deprivation qo > 0.05). Figure 1 summarizes the weight gain of these two groups and shows that they were similar in their nutritional status. The visual evoked response obtained from the striate cortex following photic stimuli exhibited an initial positive peak (PI), followed by a first negative peak (N 1), and a large positive-negative-positive wave composed of P2, N2, P3 components (Fig. 2). Since in some recording components PI and NI were difficult to define, only P2, N2 and P3 were analyzed. The effects of 1 month of monocular deprivation on the AVER amplitudes compared to normal controls objects are sum-

MD Contralateral Hemisphere

Control Left Hemisphere

P

P3

P2 P3

N.

N1

~j/

0pV

J 100 ms

Control Right Hemisphere

MD Ipsilateral Hemisphere

FIG. 2. The typical waveform of averaged visual evoked responses (AVER) from the representative control animal (left upper and lower) and from the representative monocular deprived (MD) rat (right upper and lower).

PHOTIC-EVOKED RESPONSES AND NEUROPLASTICITY

marized in Table 1. In Table 1 the observations were summarized in three ways. In A, all of the amplitude values from both hemispheres for each AVER component (P2, N2 and P3) were combined and compared between the control and MD group. B lists the observations of the AVER components obtained from the left hemispheres only, which are the ipsilateral sides to the MD eye in the experimental animals. C lists the observations obtained from the right hemispheres. In the MD group, this recording was obtained from the contralateral side to the MD eye. In Table 1A the recording obtained from the MD rats shows an increase in their amplitude resulting from the visual deprivation. In Table IB the opposite phenomenon was observed that is, reduction in the AVER of the experimental animals compared to their matching control group. When the AVER recording from the contralateral visual cortex related to the MD eye was compared to their matching control group, a remarkable increase for all the three AVER components P2, N2 and P3 were obtained (p < 0.001, Table IC). Comparing the response amplitudes between the two sides of the visual cortex (ipsilateral side to contralateral side for the MD rats and left side to right side for the control), there were markedly significant differences in P2, N2 and P3 for the MD group (p < 0.001), but there were no differences for the control group (see Table 1).

547

TABLE 1 AVERAGED VISUAL EVOKED RESPONSES (AVER) RECORDED FROM THE INTACT RATS (CONTROL) AND TIlE MONOCULAR DEPRIVED (MD) RATS

A-Both hemisphere Control MD B-Left hemisphere Control MD C-Right hemisphere Control MD

P2

N2

P3

288 ~ 13 299 ~ 20

339 _ 13 411 +_ 35*

187 ± 13 315 _ 38~t

300 _~ 18 193 _ 15:~

349 ± 23 252 +__241

180 __. 19 168 ~ 17

277 ± 18 365 _ 29*

329 ± 14 570 ± 48:~

194 ± 19 394 ± 39:~

A: Measurementobtained from both visual cortexes (VCx). B: Measurement obtained from the left/ipsilateral VCx related to the MD eye for the MD rats and the left VCx for the control rats. C: Measurement obtained from the right/contralateralVCx with relation to the MD eye for the MD rats and the right VCx for the control rats. P2, N2 and P3 indicate the second and third positive and negative components of the AVER, respectively. Mean __. SE are the values of the amplitude of the components in microvolts. Significanceof difference between the control and MD groups is given by asterisks: *p < 0.05; "['p< 0.01; ~p < 0.001.

DISCUSSION It is generally accepted that field potentials (i.e., AVER) are mainly due to summated postsynaptic potentials, although the synchronous presynaptic activity may also contribute to the early AVER components. It has been shown in numerous studies that evoked responses are highly intracellularly correlated with postsynaptic potentials, as well as with extraceUularly recorded unit activity. Evoked responses, which are initiated by a synchronized volley of impulses through an afferent channel, are a useful tool for the investigation of sensory pathways, because they combine the conveniences of time locking in the test procedure with the advantage of recording the activity of a sizable neuronal population in parallel. Therefore, recording of this neuronal activity is a useful means for studying the effects of MD on the visual cortex, as changes in activity reflect the population response of a group of neurons to the MD. The AVER obtained from normal (control) animals in this study is similar to those reported by others and by us [5,6,11,27,37]. The AVER recorded following binocular photic stimulation after 1 month monocular deprivation demonstrated that the two visual cortexes responded differently. In the ipsilateral cortex (related to the MD eye), the amplitude of component P2 and N2 was significantly reduced as a result of 1 month of MD. In the contralateral hemisphere of the visual cortex, all three components (P2, N2 and P3) of the AVER of the MD rats had significant increases in their peak value as compared to the control recordings. In normal animals, the left and the right eyes drive nearly equal numbers of cortical neurons. When vision in one eye is blurred or occluded during the critical period in postnatal development, neuroanatomical changes in the nerve terminal occurs, for example, a reduction in the complexity of the axonal terminal arbors and shortening in axonal length of the geniculocortical afferents, while afferents serving the nondeprived eye expanded, which results in axonal rearrangement [1,12,13,14,31], and the cortical domains devoted to the deprived eye undergo a substantial shrinkage while those of the nondeprived eye expand [ 15,32]. These result in the reduction of the territories assigned to the lateral geniculate body afferents in the primary visual cortex

driven by the deprived eye and the complementary expansion of those occupied by the inputs from the nondeprived eye [24,32]. In rats, monocular lid-closure from the time of eye opening to 45 days (or more) of age produces a reduction in the number of spines on apical shafts of pyramidal cells in visual cortex contralateral to the deprived eye [12,13,14,31]. Monocular lid-closure causes also an increase in the density of axospinal and axodendritic synapses in the binocular segment of the visual cortex connected with the function eye and a shrinkage of neurons in the lateral genicular body connected with the deprived eye [8,13,14]. The reduction of the nerve terminals and the simplification of the complexity of the axonal arbors in the visual cortex may result in decrease of the AVER recorded from the ipsilateral cortex, while increasing in nerve terminals and axonal arbors in the contralateral cortex resulted in increase of the AVER obtained from the MD animals compared to the control. An additional explanation in interpreting the present observations is that the anatomical changes resulted from MD alters the neurotransmitter composition in the visual cortex. Monocular deprivation seems to reduce the concentration of GABA associated with the deprived eye in rats and monkey [16,26] as well as glutamate subtype receptors [28]. Parvalbumin is a calcium binding protein that in the neocortex colocalizes with a subpopulation of GABAergic neurons. Monocular deprivation results in a dramatic reduction of parvalbumin-like immunoreactivity in the visual cortex contralateral to the deprived eye in rats [4]. Recent studies have revealed rapid modification in transmitter and receptor function in the striate cortex (area 17) after brief MD. For instance, changes in immunohistochemicallydetectable levels of GABA, GAD, GABA^ receptors, and neuropeptides in the cortical domains of the deprived eye [17,18,19,29]. It is possible to assume that changes in the neurocytological composition in the MD rats resulted in modulation of GABA and ACh [21] which cause the differences obtained in the AVER recorded from the ipsilateral vs. the contralateral areas. A commonly held view is that the second positive deflection (P2) reflects a GABA-mediated superficial hyperpolarization [11]. Singer [33] reported that

548

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intracellular inhibitory postsynaptic potentials (IPSPs) to electrical stimulation were only slightly impaired after extensive monocular deprivation. However, Tsumoto and Suda [34] reported a strong reduction in IPSP amplitude after monocular deprivation, even after 1 - 2 months of normal binocular vision. In conclusion, the A V E R is a reliable and practical method for studying the effects of monocular deprivation in the rat visual cortex. ACKNOWLEDGEMENTS We thank Ms, Diana Parker for secretarial assistance. REFERENCES I. Antonini, A.; Stryker, M. P. Rapid remodeling of axonal arbors in the visual cortex. Science 260:1819-1821 ; 1993. 2. Baker, F. H.: Grigg, P.; yon Noorden, G. K. Effects of visual deprivation and strabismus of the response of neurons in the visual cortex of the monkey, including studies on the striate and prestriate cortex in normal animals. Brain Res. 66:185-208; 1974. 3. Berardi, N.; Carmignoto, G.; Cremisi, F.; Domenici, L.; Maffei, L.; Parisi, V.; Pizzorusso, T. NGF prevents the change in ocular dominance distribution induced by monocular deprivation in the rat visual cortex. J, Physiol. (Lond.) 434: 14P; 1991. 4. Cellerino, A.; Siciliano, R.; Domenici, L.; Maffei, L. Parvalbumin immunoreactivily: A reliable marker for the effects of monocular deprivation in rat visual cortex. Neurosci. 51(4):749-753; 1992. 5. Creel, D. J.; Dustman, R. E.; Beck, E. C. Visually evoked responses in rat, guinea pig, cat, monkey and man. Exp. Neuroh 40:351-366; 1973+ 6. Dafny, N.; McClung, R.; Strada, S. J. Neurophysiological properties of the pineal body. I. Field potentials. Life Sci. 16:611-620; 1975. 7. Domenici, L.; Berardi, N.; Carmignoto, G.; Vantini, G.; Maffei, L. Nerve growth factor prevents the amblyopic effects of monocular deprivation. Proc. Natl. Acad. Sci. USA 88:8811-8815; 1991. 8. Domenici, L.; Cellerino, A.; Maffei, L. Monocular deprivation effects in the rat visual cortex and lateral geniculate nucleus are prevented by nerve growth factor (NGF). I1. Lateral geniculate nucleus. Proc. R. Soc. Lond. B. 251:25-31; 1993. 9. Dr~iger, U. C. Observations on monocular deprivation in mice. J. Nem'ophysiol. 41:28-42; 1978. 10. Dyer, R. S. The use of sensory evoked potentials in toxicology. Fundam. Appl. Toxicol. 5:24-40; 1985. 11. Dyer, R. S.; Clark, C. C.; Boyes, W. K. Surface distribution of flashevoked and pattern reversal-evoked potentials in hooded rats. Brain Res. Bull. 18:227-234; 1987. 12. Fifkova, E. The effect of monocular deprivation on the synaptic contacts in the visual cortex. J. Neurobiol. 1:285-294; 1970. 13. Fifkova, E. The effect of unilateral deprivation on visual centres in rats. J. Comp. Neuroh 140:431-438; 1970. 14. Fifkova, E. Effects of monocular deprivation on synaptic density of the visual cortex in hooded rats. Anat. Rec. 193:537; 1979. 15. Friedlander+ M. J.; Martin, K. A. C.; Wassenhove-McCarthy, D. Effects of monocular visual deprivation on genicuiocortical innervation. J. Neurosci. 11:3268-3288; 1991. 16. Hendry, S. H.; Jones, E. G. Reduction in the number of immunostained GABAergic neurons in deprived eye dominance columns of monkey area 17. Nature 320:750-753; 1986. 17. Hendry, S. H.; Jones, E. G. Activity-dependentregulation of GABA expression in visual cortex of adult monkeys. Neuron 1:701-712; 1988. 18. Hendry, S. H.: Jones, E. G.; Burstein, N. Activity-dependent regulation of tachykinin-like immunoreactivity in neurons of monkey visual cortex. J. Neurosci. 8:1225-1238; 1988.

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