Proc. Nati. Acad. Sci. USAVol. 88, pp. 8811-8815, October 1991Neurobiology

Nerve growth factor prevents the amblyopic effects ofmonocular deprivation

(visual plasticity/neurotrophic factors/rat)

LUCIANO DOMENICI*t, NICOLETTA BERARDI*, GIORGIO CARMIGNOTOt, GUIDO VANTINI*,AND LAMBERTO MAFFEI*§*Istituto di Neurofisiologia, Consiglio Nazionale delle Ricerche, 56100 Pisa, Italy; tFidia Neurobiological Research Laboratories, 35031 Abano Terme, Italy;and §Scuola Normale Superiore, 56100 Pisa, Italy

Communicated by James M. Sprague, June 3, 1991

ABSTRACT Monocular deprivation early in life causesdramatic changes in the functional organization ofmammalianvisual cortex and severe reduction in visual acuity and contrastsensitivity of the deprived eye. We tested whether or not thesechanges could be from competition between the afferents fromthe two eyes for a target-derived neurotrophic factor. Ratsmonocularly deprived during early postnatal developmentwere treated with repetitive intraventricular injections ortopical administration of nerve growth factor. The effects ofmonocular deprivation were then assessed electrophysiologi-cally. In untreated animals visual acuity and contrast sensitiv-ity of the deprived eye were strongly reduced, whereas in nervegrowth factor-treated animals these parameters were normal.

Manipulations of the visual environment during early post-natal life (critical period) of mammals lead to dramaticchanges in organization of the visual cortex. For example,monocular deprivation by temporary closure of one eyerenders neurons in the striate cortex largely unresponsive tovisual stimulation of the deprived eye, shifting ocular-dominance distribution in favor of the nondeprived eye (1-3).The deprived eye becomes amblyopic-i.e., visual acuity isdramatically impaired, and contrast sensitivity is depressed(4-6).The effects of monocular deprivation have usually been

ascribed to competition between the two monocular inputs tobinocular cortical neurons. Since this binocular-competitionhypothesis was proposed by Wiesel and Hubel (1), severalfactors have been shown to prevent the plastic changesinduced in visual cortex by monocular deprivation during thecritical period (7-11). However, no one has sought to deter-mine for what the afferents from the two eyes might becompeting during early development.We propose the hypothesis that this competition between

afferents is for a neurotrophic factor, the production ofwhichis activity dependent. The activity in afferents from thedeprived eye could be insufficient or inappropriate for thenecessary production (or uptake) of neurotrophic factor,leading to decreased synaptic efficacy of these afferents. Totest this hypothesis we assessed whether an exogenoussupply of nerve growth factor (NGF) (13) could prevent theeffects of monocular deprivation on the ocular-dominancedistribution, visual acuity, and contrast sensitivity of thedeprived eye in rats during the critical period (12). Our datashow that NGF, given exogenously, preserves visual acuityand contrast sensitivity of the deprived eye. That NGF alsoprevents the change in ocular-dominance distribution hasbeen reported (14).

MATERIALS AND METHODSA total of 59 Long-Evans hooded rats were used in theseexperiments: 40 rats for visual evoked potentials (VEP) andcell recordings, 4 rats for the estimation of NGF diffusion,and 15 rats for determination of choline acetyltransferase(ChoAcT) activity.VEP and Cell Recordings. Animal treatment. We used 11

normal and 29 monocularly deprived rats. Rats were monoc-ularly deprived for 1 mo by suturing one eyelid from postnatalday 14 (P14; time of natural eye opening) to P45. Thisprocedure effectively spans the whole length of the criticalperiod in rat (12). Ten rats were only monocularly deprived.Thirteen rats were monocularly deprived and repetitivelyinjected (2 Al) into the lateral ventricle with B-NGF (1-1.6,tg/1xl)/buffered saline (nine rats), with cytochrome c (1,4g/,41)/buffered saline (two rats), or with vehicle alone (tworats). Rats were injected every other day for 1 mo through amicrosyringe connected with a cannula (26 gauge) insertedthrough a skull hole 1 mm lateral and in correspondence withbregma, entering the lateral ventricle (15). When dye (Pon-tamine sky blue) was injected by this procedure, it wasinvariably found in the ventricles. In the other six rats,monocular deprivation was combined with local applicationto the visual cortex of one hemisphere of a small (=-1 mm2)piece of fibrine (Zimospuma, 80% fibrine/20% NaCl, Bal-dacci Laboratories, Pisa, Italy) placed 1 mm anterior withrespect to the visual cortex. Fibrine was soaked with 2 pLI ofeither NGF at 1.6 ug/IL/buffered saline (four rats) or vehiclealone (two rats). Treatments were repeated every 2 days for2 weeks by injecting, through a microcannula, 1 pA of theappropriate solution into the fibrine patch. After 1 week theold patch was replaced with another one. In these ratsmonocular deprivation was continued for only 2 weeks.Eyelid suture, injections, and topical applications were doneunder chloral hydrate anesthesia (0.4 g/kg i.p.).Recordings. Of the 11 normal rats, recordings were made

of four rats after P45, of two rats at P19, of two rats at P21,and of three rats at P30. All monocularly deprived rats weretested at the end of the deprivation period (1 mo for intra-ventricular treatment and 2 weeks for local treatment). Re-cordings were made in urethane-anesthetized rats (1200mg/kg i.p.). VEP evoked by different visual stimuli to eithereye were recorded by inserting a micropipette, filled with 3M NaCl, into the binocular portion of either visual cortex(binocular area 17 or area OC1B). Eyes of the animals werefixed by means of metal rings. Correct location of themicropipette was checked by recording single-unit activity atthe beginning of the experimental session and ascertainingthat (i) the cell receptive fields were in the central portion of

Abbreviations: NGF, nerve growth factor; VEP, visual evokedpotentials; ChoAcT, choline acetyltransferase; Pn, postnatal day n.

tTo whom reprint requests should be addressed.

8811

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8812 Neurobiology: Domenici et al.

the visual field (within 200 from the vertical meridian) and inthe upper quadrant and (it) binocular units could be recorded.In three rats recordings were also made in the monocularportion of the visual cortex (monocular area 17 or area

OCiM), as judged by location of the cell receptive fields andocular dominance. The visual stimuli were vertical gratings ofdifferent spatial frequencies generated on a display (HPHewlett-Packard model 1300 A, mean luminance 12 cycles /in2) positioned 20 cm from the rat eyes and centered on thepreviously determined receptive fields. The gratings werealternated in phase with a fixed temporal frequency, from the2-4 Hz range. The signals were filtered and amplified in aconventional manner, computer-averaged, and analyzed.For each condition (visual cortex, viewing eye, spatial fre-quency, contrast) at least 400 responses were averaged. Foreach record the amplitude, phase, and relative power of thefirst 12 harmonics were measured. For the temporal frequen-cies used, signals consisted mainly of the second harmonic(relative power >70%). For this reason, amplitude of thesecond harmonic in each record (1/2 peak-to-trough ampli-tude) was taken as the VEP amplitude for that condition.Noise level for a given condition (temporal frequency ofalternation, viewing eye, visual cortex) was considered to bethe amplitude of the second harmonic in records where thestimulus was covered with a translucent screen. To assess thespatial resolution value (visual acuity), gratings of maximumavailable contrast were used (70%6); spatial frequency wasprogressively increased until signal was indistinguishablefrom noise. Visual acuity was measured as the highest spatial

frequency that still evoked a response above noise level. Totest whether animals needed any optical correction, record-ings were made with and without lenses of different dioptricpower placed before the rat eyes. The lens that impartedhighest visual acuity was considered optimal. The contrastthreshold at a given spatial frequency was evaluated byextrapolating to zero voltage (noise level) the linear regres-sion through a contrast response curve (VEP amplitude vs.logarithm of stimulus contrast) (16, 17). Contrast sensitivityis the reciprocal of contrast threshold.

Determination of ChoAcT Activity. Fifteen rats were used,all monocularly deprived at P14 and treated intraventricularlywith either NGF (n = 10) or cytochrome c (n = 5) under theabove-mentioned protocols. These rats were sacrificed at P45by cervical dislocation. Brains were rapidly removed, and theoccipital cortices were dissected on ice and stored at -800Cuntil assayed. ChoAcT activity was determined by describedprocedures (18, 19) with minor modifications. Enzyme ac-tivity was expressed on the basis of protein content (20).

Diffusion of NGF. To estimate diffusion ofNGF, a piece offibrine loaded with 1 lI (0.08 ,ACi; 1 Ci = 37 GBq) of125I-labeled NGF (specific activity 72.2 ,Ci/jtg) was appliedto the cortical surface (anterior-posterior bregma, left 2 mm)in four rats. The animals were sacrificed 24 hr later, and thewhole brains were dissected and frozen. Serial transversesections of the neocortex 1 mm thick were cut from theinjection site in a rostrocaudal direction. The radioactivity inthe sections was counted by a y counter (model Cobra,Hewlett-Packard).

Table 1. Visual acuities in monocularly deprived and monocularly deprived NGF-treated rats

Visual acuity, cycles/degree

Nondeprived eye Deprived eye

Animal Treatment Ipsi Contra Ipsi ContraDepl MD 1.0 1.1 0.45 0.35Dep2 MD 1.0 0.35Dep3t MD 1.1 1.0 0.25 0.5Dep4 MD 1.0 1.0 0.45 0.5Dep5t MD 1.1 0.3 0.5Dep6 MD 1.1 1.05 0.3Dep7 MD 0.9 1.0 0.3 0.35Dep8 MD 0.9 0.35Dep9 MD 1.0 1.0 0.3 0.35DeplOt MD 1.0 0.45Mean ± SD 1.0 ± 0.07 1.0 ± 0.04 0.34 ± 0.08** 0.4 ± 0.07**

Contl MD + saline 1.1 1.1 0.2 0.35Cont2 MD + saline 1.0 0.25 0.4Cont3 MD + cyt c 1.1 1.2 0.35Cont4 MD + cyt c 1.1 1.1 0.35 0.5Mean + SD 1.1 ± 0.08 0.26 ± 0.07** 0.4 ± 0.07*

NGF1 MD + NGF 1.1 1.0NGFx2 MD + NGF 0.9 0.85 1.1NGF2 MD + NGF 1.0 0.9 0.75 1.0NGF3 MD + NGF 0.8 1.1 0.8 1.1NGFx1 MD + NGF 0.9 1.1 0.9 1.1NGF4 MD + NGF 1.2 1.1 0.9 0.7NGFy2 MD + NGF 0.95 0.95NGF5 MD + NGF 1.1 1.0 0.9 0.8NGFx3 MD + NGF 0.8 1.0Mean ± SD 1 ± 0.14 1.04 ± 0.08 0.85 ± 0.06 0.97 ± 0.14

Visual acuity was assessed for both eyes in 10 monocularly deprived (Dep) rats, 4 monocularlydeprived rats with intraventricular injections of saline or cytochrome (cyt) c (Cont) and 9 monocularlydeprived rats with intraventricular injections of NGF (NGF). VEP were recorded in the binocularportion ofboth visual cortices. Ipsi, ipsilateral cortex recording; contra, contralateral cortex recording.Significance of difference between mean visual acuity for the deprived eye in the ipsilateral andcontralateral cortex with respect to mean visual acuity for the nondeprived eye is given by asterisks:*P < 0.002, **P < 0.001. MD, monocular deprivation.tRats for which recordings in the monocular portion of area 17 were also made.

Proc. Natl. Acad. Sci. USA 88 (1991)

Proc. NaMl. Acad. Sci. USA 88 (1991) 8813

Statistical Analysis. Statistical significance of the differ-ences between groups has been evaluated with a two-tailedStudent's t test both for paired (Figs. 1 and 2) and unpaired(tables and text) observations. Differences were consideredsignificant when P < 0.05.

RESULTSIn adult pigmented rats, the curve relating VEP amplitude tostimulus spatial frequency (VEP spatial-frequency curve) isapproximately low-pass-shaped for spatial frequencies >0.1cycles/degree, with the estimated visual acuity being -1.2cycles/degree (21), in accordance with behavioral visualacuity (22). The visual acuity found in normal rats agreed withthe reported value (1 ± 0.15 cycles/degree, n = 4). As inother mammals (23-26), the adult value for visual acuity isgradually reached during postnatal visual development. Infour pups aged 19 (n = 2) or 21 days (n = 2), mean visualacuity for the contralateral eye (binocular cortex) was 0.48 +

0.06 cycles/degree; in three 30-day-old rats mean visualacuity was 0.9 + 0.05 cycles/degree.The effects of 1 mo of monocular deprivation on visual

acuity of untreated, NGF-treated, and sham-treated rats arereported in Table 1. Clearly visual acuity in the deprived eyeis strongly reduced with respect to the nondeprived eye in alluntreated rats, both in the ipsilateral (0.34 ± 0.08 cycles/degree) and contralateral (0.4 ± 0.08 cycles/degree) visualcortex. Visual acuity for the nondeprived eye was normal (1cycles/degree). We note that the effects of monocular dep-rivation on visual acuity are only apparent when recordingfrom the binocular portion of the visual cortex. When re-cording from both the binocular and the monocular visualcortex, mean visual acuity for the deprived eye was 0.48 +0.05 cycles/degree (n = 3) in the binocular and 0.9 + 0.05cycles/degree (n = 3) in the monocular visual cortex, thedifference being highly significant (P < 0.002).

In all NGF-treated rats the effects of monocular depriva-tion on visual acuity of the deprived eye were largely absent.Mean visual acuity was 0.85 ± 0.06 cycles/degree in theipsilateral and 0.97 ± 0.14 cycles/degree in the contralateralcortex; neither value differed significantly from the corre-sponding one for the nondeprived eye.The reduction in visual acuity found in sham-treated mon-

ocularly deprived rats was equivalent to that found in un-treated rats (Table 1).The whole VEP spatial-frequency curves obtained in mon-

ocularly deprived and monocularly deprived NGF-treatedrats are reported in Fig. 1 A and B; the mean relative VEPamplitudes for deprived and nondeprived eye are plotted asa function of the stimulus spatial frequency only for thoseanimals that were recorded from both hemispheres for botheyes. The mean visual acuity for each eye is also reported inFig. 1 (symbols on abscissa). Data for the ipsilateral andcontralateral cortex are plotted separately. In untreated rats(Fig. 1A) not only is the visual acuity for the deprived eyesignificantly lower in both cortices (P <0.001), but the wholecurve is affected. Indeed, the signal amplitude for the de-prived eye is significantly reduced (P < 0.01) at all spatialfrequencies tested in both cortices.

In rats with intraventricular NGF injections (Fig. 1B) bothmean visual acuity and mean relative VEP amplitude for thedeprived eye did not differ significantly from the correspond-ing values in the nondeprived eye.Neither treatment with saline nor treatment with cy-

tochrome c was effective in preserving VEP amplitude andvisual acuity in the deprived eye (Fig. 2).We also tested whether exogenous NGF wouldprevent the

loss in contrast sensitivity resulting from monocular depri-vation. Contrast threshold for various spatial frequencies wasmeasured in two monocularly deprived rats and in two

CONTRALATERAL

A MONOCULARLY DEPRIVED

LLU

D

-J

LUIllJ

Q

0.5

0.2 -

0.1

IPSILATERAL

RATS 0 NONDEPRIVEDEYE*DEPRIVED

---EYE

Y,~~~~~~~~"'0.1 0.2 0.5 1 0.1 0.2 0.5 1

B MONOCULARLY DEPRIVED NGF-TREATED RATS

I'

--I).1 0.2 0.5 1

1 * *0.1 0.2 0.5 1

SPATIAL FREQUENCY (c/deg)

FIG. 1. (A and B) Effects of monocular deprivation in untreatedrats (A) and rats treated intraventricularly with NGF (B). Leftcolumn, VEP recorded in the cortex contralateral to stimulated eye;right column, VEP recorded in the cortex ipsilateral to stimulatedeye. Mean relative VEP amplitude is reported as a function ofstimulus spatial frequency. Contrast of the visual stimuli was 30-40%6in all cases, except for stimulation of the deprived eye in monocularlydeprived untreated rats, in which case contrast was 40-50%6. Onlydata from animals for which recordings existed for both eyes in bothcortices were plotted (n = 5 in A and in B). For each rat VEPamplitude for deprived and nondeprived eye at each spatial fre-quency was normalized to VEP amplitude for nondeprived eye at 0.2cycles/degree (c/deg). Normalized data were then averaged. Meanabsolute VEP amplitudes found for nondeprived eye at 0.2 cycles/degree were as follows: A, 21 ± 4 1AV (contralateral cortex); B, 21 ±6,uV (contralateral cortex). Vertical bars represent SDs. Noise levelwas, on average, 2,uV. Symbols on abscissa correspond to the meanresolution values; the resolution value was assessed with gratings ofmaximum available contrast (70%6) by increasing grating spatialfrequency until signal was indistinguishable from noise. The highestspatial frequency still evoking a reliable signal was called theresolution value. Horizontal bars are SDs. Difference between visualacuity of deprived and nondeprived eye is significantin A (P <0.001)and not significant in B (P > 0.05).

monocularly deprived NGF-treated rats. In the nondeprivedeye (Fig. 3, o), the contrast thresholds were similar to thoseestimated electrophysiologically and behaviorally for hoodedrats (21, 22), whereas in the deprived eye of untreated ratsthis parameter was increased at all spatial frequencies tested(Fig. 3, 0). Contrast thresholds for the deprived eye ofNGF-treated rats were essentially normal for spatial frequen-cies lower than 0.8 cycles/degree (Fig. 3,*).Thus intraventricular injections of NGF prevent the loss in

visual acuity and in contrast sensitivity otherwise induced bymonocular deprivation.

Effects of repetitive intraventricular NGF injections on thefunctional activity of visual cortical neurons were furtherevaluated with single-unit recordings. Spontaneous activityof single cells was recorded for 1-2 min in the binocularcortex of normal and of monocularly deprived NGF-treatedrats. The mean spontaneous activity was 10 + 5 spikes per

Neurobiology: Domenici et al.

8814 Neurobiology: Domenici et al.

CONTRALATERAL CORTEX

Lu 1.5-nD 1-

< 0.5-- -

LU 0.2-

I-LU 0.1a:

o NONDEPRIVED EYE

* DEPRIVED EYE

I0.1 0.2 0.5 1 2

SPATIAL FREQUENCY (c/deg)

FIG. 2. Effects of monocular deprivation in rats intraventricu-larly treated with cytochrome c (n = 2) or saline (n = 2). Meanrelative VEP amplitude, normalized as for Fig. 1, reported as afunction of the stimulus spatial frequency. The absolute mean VEPamplitude at 0.2 cycles/degree for the nondeprived eye is 18 NV (SD= 5). Data were recorded in the cortex contralateral to the stimulatedeye. See Fig. 1 legend. Difference between resolution values for thedeprived and nondeprived eye is significant (P < 0.002).

sec in normal rats (n = 24), 9 ± 6 spikes per sec inNGF-treated rats (2 days from the last injection, n = 22), and8 ± 6 spikes per sec in NGF-treated rats recorded 2-4 hr afterthe last injection (n = 20). The differences are not significant.In addition, the orientation selectivity of single cells inresponse to light bars projected on a screen centered on thecell receptive field was evaluated. Orientation-selective cellswere 65% (n = 30) in normal rats and 66% (n = 30) inmonocularly deprived NGF-treated rats.

In four monocularly deprived rats, NGF was appliedlocally to the visual cortex contralateral to the deprived eye,and the VEP was recorded. The diffusion ofNGF, estimatedin four rats with 125I-labeled NGF applied to the corticalsurface in correspondence with bregma, was =4 mm frombregma, 24 hr later (Table 2). No radioactivity was detectedin the untreated cortex. The visual acuity of the deprived eyewas in the normal range (0.94 ± 0.11 cycles/degree) inNGF-treated cortex but was significantly reduced in un-treated cortex (0.45 ± 0.1 cycles/degree). The visual acuityfor nondeprived eye was normal in both cortices (0.9 ± 0.05cycles/degree). Local application of saline alone failed to

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F-z0(9

3020 2

1inv _

5 -

2-

0

F- - I 70.1 0.2 0.5 1 2

SPATIAL FREQUENCY(c /deg )

FIG. 3. Mean contrast sensitivity from two monocularly depriveduntreated rats and two monocularly deprived rats treated intraven-tricularly with NGF. o, Nondeprived eye, n = 4; *, deprived eye ofNGF-treated rats (n = 2); *, deprived eye of untreated rats (n = 2).SD for the contrast sensitivity in the nondeprived eye was between3 (for spatial frequency of 0.1 cycles/degree) and 1 (for spatialfrequency of 0.8 cycles/degree).

Table 2. Cortical diffusion of NGFMean radioactivity,

Distance from cpm/mg of wetapplication site, mm tissue

0-1 23.3 ± 5.611-2 20.7 ± 4.142-3 17.48 ± 4.63-4 13.34 ± 4.874-5 2.06 ± 0.65-6 0.76 ± 0.44

In four rats 125I-labeled NGF (1 ,ul, specific activity 72.2 ,uCi4&g)was applied on the cortex (bregma, L2 mm). Twenty-four hours laterserial transverse sections of the neocortex, 1 mm thick, were cutfrom the application site in a rostrocaudal direction, and radioactivityof each section was counted by a fy counter. Mean radioactivity andSD detected for each section (cpm/mg of wet tissue) are indicated.

prevent the reduction in visual acuity for the deprived eye(0.4 cycles/degree). These data demonstrate that the ambly-opic effects of monocular deprivation can also be preventedby local application of NGF to the primary visual cortex.A possible interaction of exogenous NGF with the cholin-

ergic input to the visual cortex was examined by measuringChoAcT activity in the visual cortices of monocularly de-prived rats treated with either NGF (n = 10) or cytochromec (n = 5). ChoAcT activities in the visual cortices of NGF-and cytochrome c-treated rats did not differ significantly(Table 3).

DISCUSSIONMonocular deprivation in rats during the critical period (12)substantially decreases visual acuity of the deprived eye.This decrease is found in only the binocular portion of thevisual cortex, as expected for a process linked to binocularcompetition (27, 28).We have found that supply ofthe neurotrophic factorNGF

to monocularly deprived rats prevents the loss of visualacuity and contrast sensitivity in the deprived eye. This resultsuggests that NGF preserves functional input from the de-prived eye to visual cortex. This interpretation is supportedby the finding (14) that in monocularly deprived NGF-treated

Table 3. ChoAcT activity in NGF- and cytochrome c-treatedmonocularly deprived rats

Treatment

MD1 cyt cMD2 cyt cMD3 cyt cMD4 cyt cMD5 cyt cMean + SD

MD6 NGFMD7 NGFMD8 NGFMD9 NGFMD10 NGFMD11 NGFMD12 NGFMD13 NGFMD14 NGFMD15 NGFMean ± SD

ChoAct,activity34723115351433313767

3439 + 2403179293030503273302937263258374635383468

3319 ± 288ChoAcT activity (nmol/hr per mg ofprotein) in the occipital cortex

of 45-day-old rats after 1 mo of monocular deprivation (MD) asso-ciated with intraventricular injections of either cytochrome c (cyt c)or NGF. Difference between the two means is not significant. c/deg,cycles/degree.

Proc. Natl. Acad. Sci. USA 88 (1991)

Proc. Natl. Acad. Sci. USA 88 (1991) 8815

rats ocular-dominance distribution is normal: the percentageof cells receiving binocular input is similar to that of normalrats. In monocularly deprived rats, either sham- or cy-tochrome c-treated, the percentage of binocular units ishalved, and most cortical neurons are dominated by thenondeprived eye.The data from control animals indicate that the effects of

NGF are not aspecific, resulting, for example, from animalhandling or anesthesia. A specific role for NGF iq develop-ment of the mammalian visual cortex is in accor'dance withthe presence of both NGF and NGF receptors in the neomor-tex of newborn, as well as in adult rats (29-31) add primates(32). Interestingly, the NGF content in the rat neocortex (29)and primate occipital cortex (32) is higher during the first partof the critical period, later decreasing to adult level.The mechanism underlying these actions of NGF in the

visual system is unknown, although several possibilities canbe proposed. With few possible exceptions, NqIF-docu-mented neurotrophic action in the central nervous system isexerted on the cholinergic neurons of the basal forebrain (forreview, see ref. 33). One explanation for our results could,therefore, be (i) an action of NGF on cholinergic input fromthe basal forebrain to the visual cortex. In the present studies,however, cholinergic neurons do not seem to be the majorlocus of NGF action because ChoAcT activity in visualcortex was unchanged by NGF treatment.Another possibility is that (ii) NGF increases the electrical

activity of cortical neurons, as may occur with PC-12 cells(34, 35). Increased electrical activity of visual cortical cellswould be expected to antagonize the effects of monoculardeprivation, as described by Shaw and Cynader (9) forglutamate infusion. Such an explanation seems unlikely be-cause single-cell recordings during NGF treatment failed todetect either an increase in spontaneous discharge or analteration in cell responses to visual stimuli. These findingsalso suggest that NGF does not impair the transmission ofeither excitatory (11) or inhibitory (8) visual information.

Still another possibility is that (iii) NGF interferes with thenormal development of visual cortex. Were this so, thefunctional properties of visual cortex in adult NGF-treatedrats should resemble the properties found for young pups atthe beginning ofNGF treatment. Because visual acuity foundat P20 was much lower than that in adult rats and visual acuityof both deprived and nondeprived eyes of NGF-treated ratswas normal, this last hypothesis also appears unlikely.The most probable explanation for our findings is that (iv)

NGF preserves functional 'input from the deprived eye tovisual cortex through a specific, direct action pn visualneurons.Whether this action is in accord with the neurotrophic

hypothesis and where this action is exerted remain to beascertained.

We thank Dr. S. Skaper for reading the manuscript and for helpfuldiscussion; we also thank M. Antoni, G. Cappagli, Y. Alpigiani, andA. Tacchi for technical assistance. Dr. G.- Tinivella made thecomputer program for VEP acquisition and analysis. Drs. T. Piz-

zorusso and V. Parisi participated in some of the experiments. N.B.is Associate Professor at the Department of General and Environ-mental Physiology, University of Napoli, Napoli, Italy.

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