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Sensory experience profoundly shapes neural circuitry of juvenile brain. Although the visual cortex of adult rodents retains a capacity for plasticity in response to monocular visual deprivation, the nature of this plasticity and the neural circuit changes that accompany it remain enigmatic. Here we investigate differences between adult and juvenile ocular dominance plasticity using Fourier optical imaging of intrinsic signals in mouse visual cortex. This comparison reveals that adult plasticity takes longer than in the juvenile mouse, is of smaller magnitude, has a greater contribution from the increase in response to the open eye, and has less effect on the hemisphere ipsilateral to the deprived eye. Binocular deprivation also causes different changes in the adult. Adult plasticity is similar to juvenile plasticity in its dependence on signaling through NMDA receptors. We propose that adult ocular dominance plasticity arises from compensatory mechanisms that counterbalance the loss of afferent activity caused by visual deprivation.
The remarkable behavioral adaptability to altered sensory environments is one of the most prominent features of the brain. Normal sensory experience during an animal’s early life is required for establishment and maintenance of proper function of its sensory systems, and sensory deprivation in early life can exert a persistent and devastating effects (Jacobson et al., 1981). Responses of neurons in the visual cortex to the two eyes provide a dramatic example of experience-dependent plasticity of a sensory cortical circuit, one that has been studied over more than four decades (Wiesel, 1982). In ocular dominance (OD) plasticity, brief deprivation of patterned vision in one eye leads to a reduction in the responses of visual cortical neurons to the deprived eye and a matching increase in responses to the open eye. Sensitivity to this manipulation is high in juvenile animals during a critical period in early life, after which the cortical neurons become less sensitive to monocular deprivation (MD).
There is also plasticity in adult sensory cortex, including the visual cortex (Buonomano and Merzenich, 1998; Fox and Wong, 2005). An understanding of cortical plasticity in the adult brain is important because it has the potential to provide a rational basis for therapy. The same manipulations that induce plasticity in the critical period are reported to have similar, but somewhat slower effects in the adult (Sawtell et al., 2003; Pham et al., 2004). However, near the close of the critical period, there are dramatic differences in the anatomical rearrangements and laminar-specific physiological changes produced by MD that begins at different ages (LeVay et al., 1980; Daw et al., 1992). These studies would suggest that the neural circuits and plasticity mechanisms in primary visual cortex are not affected identically by MD carried out in adulthood and during the critical period.
Here we compare the plasticity induced in mouse visual cortex by monocular and binocular deprivation during the critical period with plasticity during adulthood. We sought to determine whether the plasticity at different ages was similar in character and differed only in rate or extent, or whether there were also qualitative differences. We hoped that such a side-by-side comparison would reveal differences if they existed. We used optical imaging of the intrinsic signal of cortical activity for our measurements of visual responses to stimulation through the two eyes because it provides quantitative unbiased estimates of cortical responses that are not affected by the modest sample of single neurons from which conclusions about ocular dominance have conventionally been drawn.
Our results reveal clear qualitative differences between critical period and adult plasticity in mouse visual cortex. It will be important for future work to elucidate the specific cellular mechanisms and neural circuits that retain the capacity for plasticity in the adult.
All experiments were conducted in accordance with protocols approved by the University of California, San Francisco Institutional Animal Care and Use Committee.
For monocular and binocular deprivation, one or both eyelids of C57BL/6 mice (Simonsen Laboratories, Gilroy, CA) were sutured shut under anesthesia induced by 2-3% isofluorane in oxygen otherwise according to the procedures described previously (Gordon and Stryker, 1996). Ages of animals at the beginning of MD were as follows: P16-18 for pre-critical period animals, P24-29 for critical period animals and 12-13 weeks old for adult animals. Animals were checked daily for the first few days after surgery and occasionally thereafter to make sure that the eyelids were completely sealed closed by the scar tissue. Mice whose eyelid fusion had a hole or those with any indications of corneal damage or cataract were removed from the study before imaging. Animals were maintained on a 12hr light/dark cycle with ad libitum access to food and water.
Surgical preparation, visual stimuli, optical imaging of intrinsic signals, and data analysis were carried out as described previously (Gordon and Stryker, 1996; Kalatsky and Stryker, 2003, Cang et al., 2005). Briefly, mice were anesthetized with an intraperitoneal injection of Nembutal (50 mg/kg) supplemented by chlorprothixene (10 mg/kg i.m.) and placed in a stereotaxic frame. Atropine (0.3 mg/kg, s.c.) and dexamethazone (2 mg/kg, s.c.) were administered additionally to suppress secretions and brain edema, respectively. The animal’s temperature was maintained at 37.5C by a rectal thermoprobe feeding back to a heating pad. The heart rate was continuously monitored through electrocardiograph leads attached to the animal. This enabled us to keep the depth of anesthesia stable. A tracheotomy and a craniotomy were performed. The exposed left occipital cortex was covered with agarose and a coverslip to form an imaging window. The eyes were protected with a thin layer of silicon oil. For visual stimuli, a horizontal bar (2 degrees in height and 20 degrees in width) drifting up or down with a period of 8 s was presented for 320 s (i.e. 40 cycles) on a high refresh rate monitor positioned 25 cm in front of the animal. The axis of drifting was shifted 5 degrees rightward from the midline to elicit responses in the center of binocular zone. Optical images of visual cortex were acquired continuously under a 610-nm illumination over the stimulation period with a Dalsa 1M30 CCD camera (Dalsa, Waterloo, Canada) equipped with a Nikon 135 x 50 mm tandem lens. The Fourier component of light reflectance changes matched to the stimulus frequency was extracted pixel-by-pixel from the image stream to generate amplitude and phase maps of cortical intrinsic signals. In each imaging session, a set of four images was taken by visualizing each eye’s response (i.e. the contralateral or ipsilateral eye) to each direction (i.e. upward or downward). Then, a map with absolute retinotopy and average magnitude for each eye (called “average map”) was computed from a pair of the upward and downward maps by stimulus reversal and averaging. The magnitudes of response in these maps are fractional changes in reflectance. For calculation of ocular dominance, the magnitude maps were smoothed by a uniform 5x5 filter and thresholded at 30% of peak response amplitude to define a response region. The extent of central binocular zone was defined by maps of the ipsilateral eye’s responses. The ocular dominance index (ODI) was calculated by averaging (C-I)/(C+I), where C and I represent the response magnitude of each pixel to the contralateral and ipsilateral eyes, respectively, over all responsive pixels in the region. The ODI ranges from +1 to −1, where a positive value indicates a contralateral bias, and a negative value an ipsilateral bias. Maximum magnitude values are also presented as a measure of response strength separately for each eye. The ODIs and maximum magnitude values obtained independently from multiple sessions were averaged to determine representative values for each animal. For clarification, we have expressed all magnitudes after multiplication by 104 so that they can be written as small, positive numbers.
Imaging data from 102 mice are reported in Results. Measurements in 4 additional mice were not completed when initial responses to contralateral eye stimulation were less than 0.9 x 10-4, about a third of normal, a sign of poor responsiveness. The data from these 4 mice were excluded from analysis.
The competitive NMDA receptor antagonist (RS)-CPP (((R,S)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid)) (Tocris, Ellisville, Missouri) was dissolved in saline at a concentration of 1-1.5 mg/ml and the drug solution was injected intraperitoneally at a dose of 10-15 mg/kg every 24 hours (Villarreal et al., 2002). To animals with MD, first CPP injection was made 4-6 hours before MD surgery.
Visual cortices were dissected using a blunted 16-gauge needle from 2mm-thick occipital cortex slices cut on an acrylic matrix. Each tissue was separately homogenized by a 1.5 ml tube and a plastic pestle in 100 μl of buffer containing 60 mM Tris-Hcl, pH 6.8, 2% SDS, 5mM EDTA and 1x Halt Proteinase Inhibitor Cocktail (Pierce, Rockford, IL). Homogenates were then incubated at 100 C° for 2 min. Total protein concentrations were quantitated by BCA Protein Assay Kit (Piarce). Protein samples (5 μg protein/lane) were resolved by 7.5 % SDS-PAGE gels and transferred onto PVDF membrane. Blots were incubated with rabbit polyclonal anti-Zif268 (otherwise called as Egr-1) antibody (Santa Cruz Biotechnologies, Santa Cruz, CA) or mouse monoclonal anti-beta III tubulin antibody (Clone TUJ1, Chemicon, Temecula, CA) followed by incubation with HRP-conjugated anti-rabbit or anti-mouse secondary antibody (Jackson Immunoreseach laboratories, West Grove, PA), respectively. Signal was detected on film with ECLplus reagents (Amershan Biosciences, Buckingamshire, UK). Film images were then captured with a CCD camera and optical density of bands was measured with NIH Image J software. The relative amount of Zif268 protein was determined by in-blot comparison with diluted standards and was normalized to that of beta III tubulin from the corresponding lane.
Visual acuity of unrestrained, freely-moving mice was measured with the virtual optomotor system (Prusky et al., 2004). Briefly, a mouse was placed onto an elevated platform in the center of a virtual-reality chamber which comprises four 17-inch LCD monitors arranged into a square. Vertical sinusoidal gratings of 100% contrast were drawn on the screens at various spatial frequencies (0.042-0.514 c/d) and were rotated at a speed of 12 deg/s by a Macintosh software (OptoMotry, CerebralMechanics, Lethbride, Alberta, Canada). The mouse’s behavior was observed for 12 minutes through a video camera positioned immediately above the platform. When the mouse faced the screen and moved its head smoothly in parallel with the rotation of the gratings, it was judged that the mouse tracked the gratings and could see them. To assess the acute behavioral effects of CPP injection, measurements were carried out before and after the CPP administration in the same animals. At the end of each session, the highest spatial frequency that elicited noticeable tracking behavior was recorded as the threshold. Thresholds for the left and right eyes were determined separately with clockwise and counterclockwise rotation of the gratings, respectively. An average of those two values was taken as the representative threshold for each animal.
All data represent mean ± SEM. Statistical significance between two groups in imaging, biochemical and behavioral experiments was calculated by Mann-Whitney test, two-tailed unpaired t-test and Wilcoxon matched pairs test, respectively.
We used Fourier analysis-based intrinsic signal optical imaging to measure visual responses and plasticity in the binocular zone of mouse visual cortex (Kalatsky and Stryker, 2003; Cang et al., 2005). A 2 deg x 20 deg light bar was swept at 10 deg/sec upward or downward through the binocular visual field at 8-sec intervals, and images of reflected light at 610 nm were captured in a CCD camera at 7.5 Hz and 512 x 512 pixel x 16-bit resolution (Fig. 1Aa). Responses of each pixel at the stimulus frequency were extracted by Fourier analysis. Data about magnitude and retinotopic position of the response at each pixel were obtained as amplitude and phase values of the Fourier components, respectively (Fig.1Ab). For presentation, these two parameters are jointly depicted in a “polar map”, where the response magnitude is expressed as brightness and the retinotopy as hue (for example, see Fig. 1D). The ocular dominance index (ODI) was calculated by averaging the ratios of response magnitudes to the two eyes across the response area (see Methods). This technique provides a quantitative, unbiased measurement of response magnitude. The relative responses to the two eyes measured by optical imaging are closely correlated with similar measures made by single-unit recording (Gordon and Stryker, 1996; Cang et al., 2005).
We first compared the effects of brief 4-5 day MD on the responsiveness of the visual cortex to the two eyes when the deprivation was carried out at the peak of the critical period with the effects of the same period of deprivation in younger and older animals. In non-deprived control mice of all ages, the magnitude of the contralateral-eye response was about 1.5-1.7 times larger than that of the ipsilateral eye. This gave rise to an average OD index of about 0.22 (Fig. 1B). In the pre-critical period (MD starting at P16-18), mice subjected to 4-5 d MD showed only slight plasticity, with an OD barely lower than the non-deprived control mice (0.22 ± 0.026 vs. 0.16 ± 0.021, p=0.20) (Fig.1Ba). This minor reduction in OD index was mediated by a small decrease in the deprived eye’s response (2.80 ± 0.20 vs. 2.31 ± 0.03, p=0.10) (Figs 1Ca and 1Da). In contrast, at the peak of the critical period (MD starting at P24-29) mice showed great plasticity after brief MD, with a significantly lower OD index than the non-deprived control group (0.22 ± 0.016 vs. 0.02 ± 0.025, p<0.01) (Fig.1Bb). The degree of this OD shift was comparable to the OD shift observed in single-unit recordings (Gordon and Stryker, 1996). This plasticity was principally the result of a dramatic decrease of the deprived-eye response (2.96 ± 0.27 vs. 2.07 ± 0.09, p<0.05) (Figs 1Cb and 1Db). At 12 weeks of age, when mice are considered to be adult, brief MD produced a much weaker OD shift than during the critical period (0.26 ± 0.022 vs.0.18 ± 0.039, p=0.10) (Fig.1Bc). This small shift in OD index resulted from a small decrease of the deprived eye response (2.68 ± 0.34 vs. 2.02 ± 0.26, p=0.15) (Figs 1Cc and 1Dc). Taken together, these results suggest that brief MD causes a substantial decrease in deprived-eye responsiveness only at the peak of the critical period, with much smaller reductions before and after.
We next examined the effects of varying durations of MD on cortical responses to the two eyes in adult mice and in juvenile mice during the critical period. In juvenile mice, a very short MD for 1-2 d elicited a significant, intermediate shift in OD (0.12 ± 0.022) (Fig. 2A). A brief MD for 4-5 d gave a near-saturating OD shift, with a mean ODI of 0.02 ± 0.025 (Figs 1Bb and and2A).2A). As with single-unit recording (Gordon and Stryker, 1996), longer MD caused only modest additional change in OD toward the slightly negative value observed at 13-22d after MD (-0.04 ± 0.026) (Fig. 2A). The OD shift in the critical period was the result of an initial decrease in the deprived-eye response followed by a smaller increase in the response of the non-deprived eye (Fig. 2B, deprived eye: no MD 2.96 ± 0.27 vs 4-22d MD 2.15 ± 0.09, p<0.05; non-deprived eye: 0-5dMD 1.85 ± 0.11 vs 7-22dMD 2.22 ± 0.13, p<0.05). Overall, these results are consistent with earlier studies in cats (Mioche and Singer, 1989) and mice (Frenkel and Bear, 2004), and demonstrate that the OD shift during the critical period is rapid, large, and involves an initial sharp decrease of the deprived-eye response (Fig. 2E).
By contrast, in adult mice, 4-5 day of MD was not effective in inducing a significant OD shift (Figs. 1Bc and and2C).2C). However, 7-day MD did result in a significant OD shift (0.07 ± 0.028, p<0.01 vs. control) (Fig. 2C). Longer MD, for 13-22 days, gave rise to an OD index similar to that caused by 7-8 d MD (0.06 ± 0.028, p<0.05 vs. control). This indicates that 7-8 day MD has a saturating effect on the OD of adult visual cortical responses and that the saturating MD in adulthood does not produce as great a change as was seen in the critical period. Thus, OD plasticity in adulthood (aODP) is slower in time course and milder in degree than the OD shift in the critical period.
An analysis of changes in response magnitude for each eye showed that aODP was initiated by a small decrease in the deprived eye’s responsiveness at 4-5 days after MD (Figs.1Cc and and2D).2D). To our surprise, however, the response magnitude of the deprived eye at 7 days after MD appeared to recover from this weakening, returning to a level comparable to that of non-deprived control mice (2.74 ± 0.29). The magnitude of the non-deprived eye response also increased markedly after deprivations longer than 7 days (2.32 ± 0.16, p < 0.01 0-5d MD versus 7-22d MD) (Figs.2D and and2E).2E). The response magnitudes for both eyes at 13-22 d after MD were qualitatively indistinguishable from those after 7-8 d MD (2.56 ± 0.36 for contra and 2.23 ± 0.25 for ipsi), confirming that a week of MD is sufficient to induce a full expression of aODP. The delayed increases in responses to both eyes are much more substantial in aODP than in the critical period form, and the reduction in response to the deprived eye is much less so. Collectively, these results suggest that aODP is not merely a slower form of the OD plasticity in the critical period but rather is mediated by a different series of response changes, one in which strengthening plays a key role.
OD plasticity in the critical period has been shown to be competitive, with complementary changes in the responses to the two eyes and little effect of binocular deprivation (BD) (Gordon and Stryker, 1996). To investigate further how visual activity from the two eyes interacts in aODP, we next examined the effects of the ipsilataral–eye MD and BD in adult mice and in juvenile mice during the critical period.
In juvenile mice, a brief ipsilateral-eye MD for 4-5 days produced substantial plasticity, of a similar magnitude but opposite in direction to the plasticity produced by deprivation of the other eye (0.38 ± 0.012 experimental vs 0.22 ± 0.016 control, p < 0.05) (Fig. 3A). This finding suggests that capacity for OD plasticity is symmetrical between the two hemispheres during the critical period. An examination of response magnitude revealed that plasticity after ipsilateral-eye MD was mediated primarily by a decrease in the deprived eye’s responsiveness (from 1.85 ± 0.11 to 1.26 ± 0.17, p < 0.05) (Figs. 3B and 3C). This observation matches the decrease in the contralateral eye’s responsiveness after contralateral-eye MD and thus indicates that a rapid loss of responsiveness to the deprived eye is a common mechanism for MD-induced OD shifts in the critical period.
Confirming previous single-unit findings (Gordon and Stryker, 1996), a brief period of BD in juvenile mice for 4-5 d produced no change in OD (0.20 ± 0.015, p=0.38) (Fig. 3A). Response magnitudes for both eyes after BD did not differ significantly from those of non-deprived control mice (2.72 ± 0.12 for contralateral eye, p=0.90 vs control, 1.91 ± 0.16; p=0.71) (Figs. 3B and 3C). Taken together, results of ipsilateral-eye MD and BD in juvenile mice reveal a similar capacity for plasticity for input from the ipsilateral eye as that for the contralateral eye input during the critical period. This symmetry is reflected in the lack of an OD shift when both eyes are deprived.
In adult mice, the effects of deprivation on the contralateral and ipsilateral pathways are no longer symmetrical. Ipsilateral-eye MD for 4-15 days produced no significant change in OD and only the slightest change in the average values (control 0.21 ± 0.020; 4-5 d ipsi MD, 0.24 ± 0.038, p=0.39 vs control; 7-15d ipsi MD 0.26 ± 0.027, p= 0.19 vs control) (Fig.3D). The degree of the OD shift after 7-15 d ipsilateral-eye MD was much smaller (36%) than that after the contralateral MD. The response magnitude of the deprived ipsilateral eye also did not change significantly (Figs. 3E and 3F). Response magnitude of the non-deprived contralateral eye after 7-15d ipsilateral MD also changed little, increasing slightly but not significantly (control 3.05 ± 0.21; 7-15d ipsi MD 3.34 ± 0.25, p=0.56 vs control).
The striking asymmetry in the adult between ipsilateral- and contralateral-eye deprivation raises the possibility that adult plasticity depends on the relative strength of the input being deprived. The effects of BD in the adult give further support to this property of adult plasticity. Adult mice subjected to 7-day BD exhibited a significant shift in OD (from 0.21 ± 0.020 to 0.12 ± 0.021, p < 0.05) (Fig. 3D). The surprising OD shift induced by BD had the same direction of change as the OD shift after the contralateral-eye MD but was milder (69%) in degree. Response magnitudes for both eyes after 7-day BD appeared larger than that of non-deprived controls, significantly so for the ipsilateral eye (from 2.19 ± 0.12 to 3.23 ± 0.24, p < 0.05) although not for the contaralateral eye (3.74 ± 0.30, p=0.19) (Figs 3E and 3F). The result of both ipsilateral-eye MD and BD suggest that plasticity in the adult visual cortex is simply not much affected by the presence or absence of visual stimulation through the ipsilateral eye. When the ipsilateral eye is deprived alone, there is no significant plasticity, and when it is deprived together with the contralateral eye, the effects are similar to those of depriving the contralateral eye alone. Taken together, these results suggest that aODP obeys its own plasticity rules, different from those that operate during the critical period.
Finally, to gain insights into possible difference in the molecular mechanisms that underlie critical period and adult plasticity, we investigated the role of NMDA-receptor activation in the OD shifts of juvenile and adult mice. We selectively blocked NMDA receptor activity by repeated intraperitoneal injection of 10-15 mg/kg of the competitive antagonist CPP ((R,S)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid) during a period of MD. This drug has been shown to cross the blood-brain barrier to the brain after systemic administration and to inhibit the NMDA receptor activity effectively as long as 24 h after injection in rodents (Villarreal et al., 2002; Frenkel et al., 2006). Indeed, in control juvenile and adult mice, we confirmed that intraperitoneally administered CPP significantly suppressed NMDA receptor-dependent expression of the immediate early gene product Zif268 within visual cortex at 24 h after injection (Mataga et al., 2001) (Fig. 4A). This biochemical finding allowed us to schedule daily CPP injections for continuous inhibition of NMDA receptor activity in visual cortex. Behavioral observation during the first 30-60 min after injection confirmed that this dose of CPP did not cause any obvious behavioral abnormality such as hyperlocomotion, stereotypy, or lethargy, and casual observation did not distinguish between experimental and control animals. Quantitative assessment of visually-evoked optokinetic behavior further supported these observations: visual acuity thresholds did not change 30-90 min after CPP injection in juvenile mice (CPP Pre 0.340 ± 0.014 cyc/deg vs. CPP Post 0.5 0.349 ± 0.012 cyc/deg; p=0.68) and only minimal suppression was recognizable at 30-90 min (CPP Pre 0.378 ± 0.004 cyc/deg vs. CPP Post 0.5 0.352 ± 0.004 cyc/deg; p<0.05) but not at 3-6 hours after injection in adult mice (CPP Post 3 0.374 ± 0.004 cyc/deg; p=0.70 vs. CPP Pre, Supplementary Figure 1).
We first examined the effect of NMDA receptor inhibition on the OD shift after brief MD during the critical period. CPP (10 mg/kg) was first injected 4-6 hour prior to the start of MD and was repeatedly administered every 24 hr until the day before imaging (Fig. 4B). The OD shift after 4-5d MD in juvenile mice was completely blocked by the CPP treatment (Fig. 4C). The OD index of CPP-treated deprived mice remained within the range of that of the non-deprived control and was significantly higher than that of control deprived mice (0.19 ± 0.028, p=0.70 vs non-deprived control; p < 0.05 vs deprived control). CPP treatment by itself, with no MD, had no effect on OD (Fig. 4C) or on response magnitude (Figs. 4D and 4E). Measurements of response magnitudes following MD showed that CPP treatment prevented the loss of responsiveness to the deprived eye (Figs. 4D and 4E). These findings suggest that the blockade of plasticity by CPP treatment was mediated by its selective action on NMDA receptors rather than by a more general inhibition of neural activity. They are in agreement with previous single-unit findings in cats that OD plasticity in the critical period is NMDA-receptor dependent (Kleinschmidt et al, 1987).
In adult mice, as in juvenile mice, the effectiveness of CPP on blocking NMDA receptors in visual cortex was confirmed by marked reduction of Zif268 expression at 24 h after injection (Fig.4A). Plasticity was almost completely blocked by CPP treatment throughout the 7-day period of MD necessary to reveal plasticity in control adult animals (ODI = 0.21 ± 0.022, p=0.90 vs control, p<0.05 vs 7d MD) (Figs 5A and 5B). To determine whether the response strengthening phase of aODP is separable from its earlier phase characterized by the reduction in deprived-eye response, we blocked NMDA receptor activation in another group of adult animals only during the last 3 days of the 7-day period of MD (Fig. 5A). Consistent with this hypothesis, these mice showed at most a very small OD shift, comparable to untreated animals following 4 days of MD (Fig. 5B). As expected, CPP treatment during a 4-day MD in adult animals, had no significant effect (Supplementary Fig. 2). Notably, the CPP treatment did not impair visual responses (Fig.5C). These results support the hypothesis that aODP, like critical period plasticity, requires NMDA receptor activation.
In this study, we have made a detailed comparison of the effects of visual deprivation on binocular cortical responses in adult and juvenile mice using the Fourier optical imaging technique. The results are summarized in Fig.6. In visual cortex of juvenile mice, 4-day MD is sufficient to elicit an NMDA receptor-dependent, near-saturating OD shift (Fig.6A). This rapid shift is mediated primarily by a large reduction of the deprived eye’s response (Fig.6A). Ipsilateral MD produced an OD shift that is comparable in magnitude but opposite in sign to that caused by contralateral MD. BD has no effect on responses to the two eyes (Fig.6B). In contrast, 7-day MD of the contralateral eye is required to produce a significant and saturating OD shift in adult visual cortex. This shift was initiated by a small and transient decrease in the deprived eye’s response at 4 days after MD, and was followed by an increase of the non-deprived eye’s response at 7 days after MD and a recovery of the deprived eye’s response from its initial decrease (Fig.6C). This plasticity process in adulthood is also dependent on NMDA receptor activation. MD of the weaker ipsilateral eye is very much less effective in eliciting an OD shift than that of the contralateral eye. Binocular deprivation (BD) caused a modest OD shift similar to the effect of MD or the contralateral eye alone (Fig.6D). These findings reveal that aODP differs qualitatively from that in the critical period in several respects.
Experience-dependent OD plasticity in adult rodent visual cortex has been studied with evoked potentials (Sawtell et al., 2003; Pham et al., 2004; Sale et al., 2007), single-unit recording (Antonini et al., 1999; Fischer et al., 2007), measurements of visual acuity (Prusky et al., 2006), immediate early gene expression (Tagawa et al., 2005) and intrinsic signal optical imaging (Hofer et al., 2006b) . The present study adds two novel findings to the previous literature and confirms others. (1) We have shown that aODP consists of two phases that are even more distinct than those in juvenile animals: an initial reduction of contralateral deprived-eye response, followed by a second phase in which responses to both eyes increase considerably. The mechanism of adult OD plasticity was first described as being mediated almost exclusively by potentiation of the non-deprived response (Sawtell et al., 2003; Hofer et al., 2006b), although this view has been expanded to include both depression of the deprived eye response and potentiation of the open-eye response (Frenkel et al., 2006). The potent recovery of the deprived eye response from its transient depression that we find here is novel and shows some similarity to the potentiation of the response to the deprived vibrissa after long-term whisker deprivation in rat barrel cortex (Glazewski and Fox, 1996). (2) We find here that BD in adult mice, unlike in juveniles, induces a substantial OD shift. This novel observation is a qualitative difference between plasticity during critical period and adulthood. (3) We also find in adult but not juvenile animals a clear asymmetry between the two hemispheres in the effects of deprivation in the adult, which may be related to the effects of BD. This asymmetry is a remarkable feature of aODP. For both BD and ipsilateral-eye deprivation, it appears that reducing the activity of the less-effective ipsilateral eye by lid suture has little effect on plasticity mechanisms. Asymmetry of adult OD plasticity has been observed previously in VEP studies (Sawtell.et al., 2003; Pham et al., 2004) but not in an optical imaging study that used episodic rather than Fourier imaging (Hofer et al., 2006b). We have no explanation for the greater ipsilateral plasticity that was observed in that case. (4) We have shown that aODP can be observed under barbiturate anesthesia. Earlier findings suggested that adult OD plasticity may have been overlooked because barbiturate anesthetics whose actions augment cortical GABAergic inhibition might mask it (for reviews, see Hofer et al., 2006a; Karmarkar and Dan, 2006). Indeed, acute pentobarbital administration masked the effect of adult MD in VEP recordings (Pham et al., 2004). Our use of chlorprothixene in combination with barbiturate anesthesia apparently averts this issue.
Taken together with the earlier studies, our new results on adult OD plasticity suggest that the visual cortex does not abruptly lose its capacity for plasticity at the end of the critical period but rather that the characteristics of plasticity change over the course of maturation. One would expect that the precise neural connections that are altered as a result of MD and BD are also different at different ages. Future research will be needed to reveal which changes in connections within the visual cortex are responsible for the different changes in neuronal responses observed in the critical period and adulthood.
We summarize basic features of juvenile and adult plasticity in Table 1. Critical period plasticity is primarily mediated by NMDA-receptor dependent, rapid and substantial weakening of the deprived-eye response within 4 days. It is triggered by imbalanced visual inputs provided by MD of either eye, and its capacity is symmetrical in the two hemispheres regardless of the different strengths of the ipsilateral- and contralateral-eye inputs. These features are consistent with a role for juvenile plasticity in the developmental refinement of cortical circuitry for binocular vision. On the other hand, aODP is a slower process, more modest in degree, that begins with a transient weakening of the deprived-eye response and is followed by its subsequent recovery and the strengthening of the open-eye response over 7 days. While it shares a dependence on NMDA receptor activation with critical period plasticity, it differs in that the magnitude of aODP depends strongly on the strength of the input being deprived, which results in asymmetry of plasticity between the two hemispheres. It is also triggered by changes in absolute activity level brought about by BD. Therefore, aODP appears primarily to be a compensatory process for maintaining activity levels in the adult visual cortex.
The synaptic mechanisms that underlie the distinctive properties of aODP are not yet known. There are, however, age-dependent changes in the mechanisms of NMDA-receptor dependent long-term potentiation (LTP) in rodent cortex (Yoshimura et al., 2003). Homeostatic synaptic scaling may also contribute to adult cortical plasticity (Turrigiano and Nelson, 2004). A recent study demonstrates that activity-dependent synaptic scaling in mouse visual cortex persists into adulthood (Goel and Lee, 2007). The dependence of aODP of the activity of the dominant, contralateral-eye input, as indicated by the asymmetry of MD and the plasticity following BD, suggests further that one should look to compensatory mechanisms like synaptic scaling, rather than to synapse-specific plasticity mechanisms, for its explanation.
Comparing experience-dependent plasticity in rodent visual cortex with that in somatosensory cortex reveals remarkable similarities between these processes (Fox and Wang, 2005). Characteristics such as a critical period for depression and a persistence of potentiation with a slower time course in adults may be common core features across different cortical areas. Functional plasticity is known to be extensive in adult life in auditory and somatosensory cortex from rodents to primates (Buonomano and Merzenich, 1998) and in the forebrain and even midbrain of birds (Knudsen, 2002; Tumer and Brainard, 2007). Functional OD plasticity after the closure of the critical period for ocular dominance columns as defined by the thalamocortical input to layer 4 has long been known, even in the animals held up as examples of a rigid critical period, such as cats and monkeys (Daw et al., 1992; LeVay et al., 1980). In few cases do we know the anatomical changes that mediate the widespread functional plasticity in adults, or even the extent to which the functional plasticity represents rewiring as opposed to changes in the strengths of existing connections. Recent longitudinal observations of the plasticity of anatomical connections in adult cortex span a wide range, from apparent rigidity to extensive plasticity (Trachtenberg et al., 2002; Grutzendler et al., 2002; Lee et al., 2006). Full elucidation of adult plasticity mechanisms at the cellular and molecular levels is also lacking. The further study of adult plasticity will be important for aiding recovery from cortical injury as well as, potentially, for enhancing learning capacity in adulthood.
We are grateful to Professors Juan Korenbrot, David Sretavan and Matthew LaVail for sharing their equipment and expertise and to Maha Abdulla for helping behavioral experiments. This work was supported by grants from the NIH to M.P.S. M. S. is a recipient of Uehara Memorial Foundation and the Japan Society for the Promotion of Science Fellowships