Critical period for the monocular deprivation effect in rats: assessment with sweep visually evoked potentials

Extended plasticity of visual cortex in dark-reared animals may result from prolonged expression of cpg15-like genes

cpg15 is an activity-regulated gene that encodes a membrane-bound ligand that coordinately regulates growth of apposing dendritic and axonal arbors and the maturation of their synapses. These properties make it an attractive candidate for participating in plasticity of the mammalian visual system. Here we compare cpg15 expression during normal development of the rat visual system with that seen in response to dark rearing, monocular blockade of retinal action potentials, or monocular deprivation. Our results show that the onset of cpg15 expression in the visual cortex is coincident with eye opening, and it increases until the peak of the critical period at postnatal day 28 (P28). This early expression is independent of both retinal activity and visual experience. After P28, a component of cpg15 expression in the visual cortex, lateral geniculate nucleus (LGN), and superior colliculus (SC) develops a progressively stronger dependence on retinally driven action potentials. Dark rearing does not affect cpg15 mRNA expression in the LGN and SC at any age, but it does significantly affect its expression in the visual cortex from the peak of the critical period and into adulthood. In dark-reared rats, the peak level of cpg15 expression in the visual cortex at P28 is lower than in controls. Rather than showing the normal decline with maturation, these levels are maintained in dark-reared animals. We suggest that the prolonged plasticity in the visual cortex that is seen in dark-reared animals may result from failure to downregulate genes such as cpg15 that could promote structural remodeling and synaptic maturation.

Long-term potentiation of thalamocortical transmission in the adult visual cortex in vivo

It has been suggested that NMDA receptor-dependent synaptic strengthening, like that observed after long-term potentiation (LTP), is a mechanism by which experience modifies responses in the neocortex. We report here that patterned (theta burst) stimulation of the dorsal lateral geniculate nucleus reliably induces LTP of field potentials (FPs) evoked in primary visual cortex (Oc1) of adult rats in vivo. The response enhancement is saturable, long-lasting, and dependent on NMDA receptor activation. To determine the laminar locus of these changes, current source density (CSD) analysis was performed on FP profiles obtained before and after LTP induction. LTP was accompanied by an enhancement of synaptic current sinks located in thalamorecipient (layer IV and deep layer III) and supragranular (layers II/III) cell layers. We also examined immunocytochemical labeling for the immediate early gene zif-268 1 hr after induction of LTP. In concert with the laminar changes observed in CSD analyses, we observed a significant increase in the number of zif-268-immunopositive neurons in layers II-IV that occurred over a wide extent of Oc1. Last, we investigated the functional consequences of LTP induction by monitoring changes in visually evoked potentials. After LTP, we observed that the cortical response to a full-field flash was significantly enhanced and that responses to grating stimuli were increased across a range of spatial frequencies. These findings are consistent with growing evidence that primary sensory cortex remains plastic into adulthood, and they show that the mechanisms of LTP can contribute to this plasticity.

Visual deprivation alters development of synaptic function in inner retina after eye opening

Visual deprivation impedes refinement of neuronal function in higher visual centers of mammals. It is often assumed that visual deprivation has minimal effect, if any, on neuronal function in retina. Here we report that dark rearing reduces the light-evoked responsiveness of inner retinal neurons in young mice. We also find that 1 to 2 weeks after eye opening, there is a surge (>4-fold) in the frequency of spontaneous excitatory and inhibitory synaptic events in ganglion cells. Dark rearing reversibly suppresses this surge, but recovery takes >6 days. Frequency changes are not accompanied by amplitude changes, indicating that synaptic reorganization is likely to be presynaptic. These findings indicate there is a degree of activity-dependent plasticity in the mammalian retina that has not been previously described.

Developmental inhibitory gate controls the relay of activity to the superficial layers of the visual cortex

A developmental reduction in the radial transmission of synaptic activity has been proposed to underlie the end of the critical period for experience-dependent modification in layers II/III of the visual cortex. Using paired-pulse stimulation, we investigated in visual cortical slices how the propagation of synaptic activity to the superficial layers changes during development and how this process is affected by sensory experience. The results can be summarized as follows. (1) Layers II/III responses to repetitive stimulation of the white matter become increasingly depressed between the third and sixth week of postnatal development, a time course that parallels the end of the critical period. (2) Paired-pulse depression is reduced after dark rearing and also by blocking inhibitory synaptic transmission. (3) Paired-pulse depression and its regulation by age and sensory experience is more pronounced when stimulation is applied to the white matter than when applied to layer IV. Together, these results are consistent with the idea that the maturation of intracortical inhibition reduces the capability of the cortex to relay incoming high-frequency patterns of activity to the supragranular layers.

Reduced visual acuity impairs place but not cued learning in the Morris water task

The Morris water task is a standard method for testing spatial learning in rodents. In a place version of the task, animals utilize multiple visual cues to learn the location of a hidden platform. The ability of animals to locate a cued platform is often used to qualitatively test for possible non-cognitive contributions to deficient place learning, including reduced visual function. We investigated the role of visual acuity in water maze performance quantitatively by depriving rats of pattern vision during a critical period for visual plasticity, which reduced their acuity by approximately 27% and then tested them in typical place and cued platform configurations of the Morris water task. Animals with reduced visual acuity had a significant deficit in place learning, but eventually reached the same escape latency as non-deprived animals. Deprived and non-deprived animals, however, did not differ in their ability to locate a cued platform following place learning. These data indicate that reduced visual acuity in rats can influence measurement of their place learning and that a typical cued platform version of the task cannot detect a modest, but significant, visual deficit.

Experience-dependent plasticity of visual acuity in rats

Rats have become a popular model for investigating the mechanisms underlying ocular dominance plasticity; however, no quantitative assessment of the effects of visual deprivation on behavioural acuity has been reported in this species. We measured the spatial acuity of monocularly and binocularly deprived rats with a visual discrimination task. The average spatial acuity of normal rats and rats deprived of vision after postnatal day 40 was approximately 1 cycle/degree. Monocular deprivation up to postnatal day 40 resulted in a 30% decrease in acuity and there was no recovery after 8 months. Identical binocular deprivation produced a comparable but significantly smaller reduction in acuity. The deleterious effects of monocular and binocular deprivation on visual acuity indicate that the development of cortical receptive field properties related to spatial tuning are affected by both monocular and binocular deprivation. The similarities in the effects of visual deprivation on visual acuity between rats and other mammals confirm that rats are a good model system for studying the cellular and molecular mechanisms underlying experience-dependent visual plasticity.

Development of NR1, NR2A and NR2B mRNA in NR1 immunoreactive cells of rat visual cortex

In cells marked for N-methyl-D-aspartate receptors (NMDARs), we studied the relationship between the sensitive period for monocular deprivation and the expression of rat NMDAR subunits, NR2A and NR2B. In the rat the sensitive period ends sometime after postnatal day 50 (P50). Previous studies of the development of these subunit mRNAs focused on animals prior to the end of the sensitive period and did not examine the visual cortex specifically. We used a monoclonal antibody to the NR1 subunit of the receptor to identify cells containing NMDARs. We then used in situ hybridization to label the same sections for NR2A or NR2B mRNA. In an additional experiment we labeled sections for NR1 mRNA to see if the developmental profile was similar at both the mRNA and protein level. We used five animals at each of four ages: P22, P30, P45 and P90. Staining for NR2B mRNA, but not for NR2A mRNA, decreased dramatically from P22 to P45. Staining for NR1 mRNA declined dramatically between P22 and P45 even though most cells remained strongly immunopositive for the NR1 protein during this time. This discrepancy suggests that significant NR1 regulation occurs after gene transcription. Because most of the decrease in NR1 mRNA and NR2B mRNA occurs by P30, transcriptional regulation of these subunits does not easily explain the loss of sensitivity to monocular deprivation, which occurs around P50. The changes are, in fact, more closely synchronized with the beginning of experience-dependent plasticity than with its end.

Developmental changes in the expression of NMDA receptor subunits (NR1, NR2A, NR2B) in the cat visual cortex and the effects of dark rearing

The present study used Western blots to determine changes in the level of expression of the three major NMDA receptor subunits, NR1, NR2A, and NR2B, in relation to the 'critical period' in cat visual cortex. NR2A rose dramatically (10-fold) from very low levels at 1 week to a peak at 5 weeks and gradually declined into adulthood (twofold). NR2B showed a similar time course to NR2A, but the rise from 1 to 5 weeks was of lesser magnitude (twofold). NR1 was expressed at comparable levels at 1, 5, 10 weeks and declined markedly (fivefold) in older animals. No significant effects of dark rearing on the levels of NR2B and NR1 were found. However, NR2A expression was significantly elevated in normal compared to dark reared visual cortex (twofold) at 5 weeks and significantly elevated in dark reared compared to normal visual cortex at 20 weeks (twofold). The close agreement between NR2A expression and both the time course of the critical period and the effects of dark rearing on that time course further indicates a role of this subunit in visual cortical critical period plasticity.

Neonatal lead exposure impairs development of rodent barrel field cortex

Childhood exposure to low-level lead can permanently reduce intelligence, but the neurobiologic mechanism for this effect is unknown. We examined the impact of lead exposure on the development of cortical columns, using the rodent barrel field as a model. In all areas of mammalian neocortex, cortical columns constitute a fundamental structural unit subserving information processing. Barrel field cortex contains columnar processing units with distinct clusters of layer IV neurons that receive sensory input from individual whiskers. In this study, rat pups were exposed to 0, 0.2, 1, 1.5, or 2 g/liter lead acetate in their dam's drinking water from birth through postnatal day 10. This treatment, which coincides with the development of segregated columns in the barrel field, produced blood lead concentrations from 1 to 31 microg/dl. On postnatal day 10, the area of the barrel field and of individual barrels was measured. A dose-related reduction in barrel field area was observed (Pearson correlation = -0.740; P < 0.001); mean barrel field area in the highest exposure group was decreased 12% versus controls. Individual barrels in the physiologically more active caudoventral group were affected preferentially. Total cortical area measured in the same sections was not altered significantly by lead exposure. These data support the hypothesis that lead exposure may impair the development of columnar processing units in immature neocortex. We demonstrate that low levels of blood lead, in the range seen in many impoverished inner-city children, cause structural alterations in a neocortical somatosensory map.

Postnatal development of NR1, NR2A and NR2B immunoreactivity in the visual cortex of the rat

N-Methyl-D-aspartate receptors (NMDARs) are critically involved in some types of synaptic plasticity. The NMDAR subunits NR1, NR2A and NR2B are developmentally regulated, and it has been proposed that developmental changes in their expression may underlie developmental changes in cortical plasticity. Age-dependent change in cortical plasticity is most commonly measured by the monocular deprivation effect, which occurs during a critical period between P22 and P50 in the rat. Although the development of NMDAR subunits has been studied from birth through the fourth postnatal week, there is only meager information from older ages when visual plasticity ends. We hypothesized that there will be significant age-dependent change in expression of NR1, NR2A or NR2B between P22, when the cortex is plastic, and P90, when it is not. We applied specific antibodies recognizing NR1, NR2A and NR2B to the primary visual cortex at P14, P22, P30, P45 and P90. We found age-dependent changes in NR1-IR that were negatively correlated with changes in NR2A-IR; these subunits are not regulated in unison. In contrast, NR2A-IR and NR2B-IR were positively correlated. NR2A-IR and NR2B-IR both passed through a developmental minimum around P45, then recovered to approximately their P22 level. NR1-IR passed through a maximum at P45. There were no significant differences between P22 and P90. These results do not support the simple hypothesis that the loss of plasticity corresponds to a simple transition from juvenile levels of NMDAR subunit proteins to new adult levels. On the other hand, the results do confirm the hypothesis that there are significant changes in processing of NMDAR proteins during the time that plasticity is lost. How these changes of IR relate to synaptic transmission and plasticity needs to be clarified.

Bidirectional, experience-dependent regulation of N-methyl-D-aspartate receptor subunit composition in the rat visual cortex during postnatal development

In the visual cortex, as elsewhere, N-methyl-D-aspartate receptors (NMDARs) play a critical role in triggering long-term, experience-dependent synaptic plasticity. Modifications of NMDAR subunit composition alter receptor function, and could have a large impact on the properties of synaptic plasticity. We have used immunoblot analysis to investigate the effects of age and visual experience on the expression of different NMDAR subunits in synaptoneurosomes prepared from rat visual cortices. NMDARs at birth are comprised of NR2B and NR1 subunits, and, over the first 5 postnatal weeks, there is a progressive inclusion of the NR2A subunit. Dark rearing from birth attenuates the developmental increase in NR2A. Levels of NR2A increase rapidly (in <2 hr) when dark-reared animals are exposed to light, and decrease gradually over the course of 3 to 4 days when animals are deprived of light. These data reveal that NMDAR subunit composition in the visual cortex is remarkably dynamic and bidirectionally regulated by sensory experience. We propose that NMDAR subunit regulation is a mechanism for experience-dependent modulation of synaptic plasticity in the visual cortex, and serves to maintain synaptic strength within an optimal dynamic range.