Prolonged sensitivity to monocular deprivation in dark-reared cats: effects of age and visual exposure

Involvement of muscarinic acetylcholine receptors in regulation of kitten visual cortex plasticity

Receptor autoradiographic methods specific for M3 and M1 muscarinic acetylcholine receptors were used to investigate the development and input-dependent laminar redistribution of these receptor populations during the critical period for kitten visual cortex plasticity. Analysis of the binding curves of [3H]4-diphenylacetyl-N-methyl-piperidine (4-DAMP) and [3H]pirenzepine (PZ) indicated that these two ligands bound heterogeneously to muscarinic acetylcholine receptors with different affinities. While [3H]4-DAMP showed a high affinity for M3 receptors and much lower affinities for M1 and M2 receptors, [3H]PZ displayed higher affinity for M1 receptors. By carefully choosing concentrations of labelled and unlabeled ligands, the patterns of laminar distribution for both receptor subtypes within visual cortex were obtained. Both receptors were most concentrated in cortical layer IV immediately after birth and during the most sensitive period of visual cortex plasticity. The binding density for both receptor subtypes thinned out progressively in this layer to concentrate in more superficial layers as plasticity waned with age. Moreover, interruption of visual or spontaneous input to visual cortex induced either by lesion or by tetrodotoxin infusion into lateral geniculate nucleus prevented the developmental redistribution of these receptors from layer IV to superficial layers, that is, the pattern of laminar distribution remained that of the age at which the lesion or tetrodotoxin infusion into the lateral geniculate nucleus was performed. The results indicate that the developmental expression of M3 and M1 muscarinic acetylcholine receptors in kitten visual cortex depends on cortical inputs.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of cDNA clones expressed selectively during the critical period for visual cortex development by subtractive hybridization

We have used the method of subtractive hybridization to isolate cDNA clones of mRNAs expressed in abundance in the visual cortex of 30-day-old kittens but absent or in lower abundance in the adult cat visual cortex. Of 12,000 colonies screened, 200 clones which hybridized to the subtracted probe were isolated and characterized. Northern blots confirmed the specificity of the vast majority of the isolated clones. 120 of the 200 clones were sequenced and the EMBL and GenBank (release 76) database were searched for known identities using FASTA and BLAST programs. Twenty-seven of these sequenced clones were identifiable. The identities showed that these sequences code for proteins involved in a variety of cellular processes. These include cell-cell interaction (TAPA-1, contactin, tachykinin receptor, phospholipase A2), cellular remodeling (C1q beta isoform, heat shock protein), neurofilament assembly (alpha tubulin and alpha internexin), neurotransmitter release (VAMP-2, amphiphysin, carboxypeptidase E, scg 10 and proton channel), energy metabolism (mitochondrial hinge protein, ADP/ATP transporter, cytochrome oxidase subunits), RNA processing (helix destabilizing protein, ribonucleoprotein) and protein synthesis (eIF-4A initiation factor, ribosomal protein S27). The results show that gene expression in the kitten visual cortex differs rather little from that of the adult visual cortex since over 98% of the sequences appear common. The relatively rare kitten-specific sequences are likely to form the basis for the critical period plasticity in this system.

Immunohistochemical study of the pattern of rapid expression of C-Fos protein in the visual cortex of dark-reared kittens following initial exposure to light

Brief alterations to the nature of the visual input during critical periods in the early life of cats and monkeys can result in rapid anatomical and physiological changes in the central visual pathways. The immediate early genes (IEGs) represent a possible way in which these changes could be mediated since the protein products of a number of these genes have been shown to be induced rapidly in neurons in response to a variety of transsynaptic stimuli. Immunohistochemical methods were employed to examine the tempo and pattern of expression of Fos, the protein product of the c-fos gene, induced in the visual cortex of kittens dark-reared from birth to 30 days of age by brief periods of binocular visual exposure. In visual cortical area 17, the number of Fos immunoreactive cells increased rapidly from virtually zero in control kittens that received no visual exposure, to reach high levels in animals that received between 1 and 2 hours of visual experience. Immunoreactive cells were absent in the dorsal lateral geniculate nucleus, but were numerous in the ventral lateral geniculate nucleus, and in area 17, were most numerous in the extragranular layers (2, 3 and 6) but sparse in lower layer 4 and layer 5, and virtually absent in layer 1. Substantial constitutive Fos immunoreactivity was observed in area 17 of normal 30-day-old kittens but very few immunopositive cells were evident in adult animals. However, Fos immunoreactivity was observed in the visual cortex of a dark-reared (for 30 days) adult animal following a brief period of visual exposure, a finding that suggests that Fos might serve other roles in the visual cortex in addition to those it might play uniquely during development. It is suggested that Fos, in combination with the protein products of a select number of other IEGs, may mediate a variety of rapid changes in the visual cortex including those that underlie visual system plasticity during early postnatal life.

Edridge-Green Lecture. Competition and cooperation in visual development

Studies of the effect of visual deprivation on cells in the lateral geniculate nucleus (LGN) show that there are two distinct sensitive periods in the monkey during which different reactions between the visual pathways related to the two eyes predominate and requirements for recovery from deprivation differ. The first extends from birth to about 8 weeks of age. The main interaction between the pathways from the two eyes is competitive, segregation of cortical ocular dominance columns occurs during this early period and monocular deprivation results initially in hypertrophy of undeprived LGN cells, with later parallel shrinkage of both deprived and undeprived parvocellular cells. Simply reopening the closed eye produces no recovery but reverse suture is effective in reversing some of the changes. The second sensitive period starts from about 8 weeks of age, although the peak of the later sensitivity appears to be at 7-9 months of age and some effect is still present at 12-18 months. During this later phase a cooperative interaction between the pathways related to the two eyes is necessary for normal development and in the absence of this selective shrinkage of both deprived and undeprived parvocellular LGN cells occurs. Simply reopening an eye during this late sensitive period allows recovery of these cells to normal size.

The Critical Period for Experience-dependent Plasticity in a System of Binocular Visual Connections in Xenopus laevis: Its Extension by Dark-rearing

Following surgical rotation of an eye, the Xenopus 'intertectal' system is capable of a vision-dependent alteration of its connectivity, that restores spatial registration of binocular maps on the optic tectum. In the preceding paper (Keating and Grant, Eur. J. Neurosci., 4, 27 - 36, 1992), we reported that this capacity undergoes a progressive, age-dependent restriction during a critical period around the time of metamorphosis, so that rotations produced in animals aged >/=3 months postmetamorphosis normally evoke no such alteration of the system. Here we examine whether this restriction is rigidly age-dependent or whether vision can influence its profile. We report that in animals dark-reared from embryonic stage 35 through the critical period to 3 months, 1 year or even 2 years after metamorphosis, rotations instituted at those ages now result in intertectal reorganization if a period of normal vision is allowed after the operation. Similarly, intertectal alteration was also seen in animals eye-rotated at larval stage 58, then dark-reared just for the duration of the critical period, and subsequently returned, at 3 months of age, to a normal visual environment. We conclude, therefore, that the normal developmental restriction in the plasticity of the Xenopus intertectal system is not strictly age-dependent, but that vision contributes to the process by activating the underlying plasticity mechanisms.

Morphology and distribution of neurons and glial cells expressing beta-adrenergic receptors in developing kitten visual cortex

The morphology and distribution of cells expressing beta-adrenergic receptors has been studied in developing kitten visual cortex using a monoclonal antibody which recognizes both beta-1 and beta-2 adrenergic receptors. We found specific populations of neurons and glial cells which express beta-adrenergic receptor immunoreactivity in the kitten visual cortex. In adult animals, the receptors are most concentrated in the superficial and deep cortical layers (layers I, II, III and VI). About 50% of the stained neural cells in adult cat visual cortex are glial cells. Most of the immunoreactive neurons in layers III and V are pyramidal cells while those in layers II and IV are more likely to be nonpyramidal cells. In neonatal kittens, staining is weaker than that in adult cats and it appears to be concentrated in neurons of the deep cortical layers and in the subcortical plate and white matter. Only a few immunoreactive glial cells were found at this age. Receptor numbers increase after birth and by 24 days of age, the laminar distribution of beta-adrenergic receptors approaches that of adult animals. Immunoreactive glial cells in the white matter show a progressive increase in number throughout postnatal development.

Neuron loss and addition in developing zebra finch song nuclei are independent of auditory experience during song learning

In zebra finches early auditory experience is critical for normal song development. Young males first listen to and memorize a suitable song model and then use auditory feedback from their own vocalizations to mimic that model. During these two phases of vocal learning, song-related brain regions exhibit large, hormone-induced changes in volume and neuron number. Overlap between these neural changes and auditory-based vocal learning suggests that processing and acquiring auditory input may influence cellular processes that determine neuron number in the song system. We addressed this hypothesis by measuring neuron density, nuclear volume, and neuron number within the song system of normal male zebra finches and males deafened prior to song learning (10 days of age). Measures were obtained at 25, 50, 65, and 120 days of age, and included four song nuclei: the hyperstriatum ventralis pars caudalis or higher vocal center (HVc), Area X, the robust nucleus of the archistriatum (RA), and the lateral magnocellular nucleus of the anterior neostriatum (IMAN). In both HVc and Area X, nuclear volume and neuron number increased markedly with age in both normal and deafened birds. The volume of RA also increased with age and was not affected by early deafening. In IMAN, deafening also did not affect the overall age-related loss of neurons, although at 25 days neuron number was slightly less in deafened than in normal birds. We conclude that while the addition and loss of neurons in the developing song system may provide plasticity essential for song learning, these changes do not reflect learning.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of dark rearing on the time course of the critical period in cat visual cortex

The effects of dark rearing on the time course of the postnatal critical period for monocular deprivation (MD) in visual cortex were determined in cats who experienced 2 days of MD at various postnatal ages. In normal development, plasticity (susceptibility to MD) was very low at 3 weeks, rose sharply at 6 weeks, and gradually declined over the next 10 weeks. This developmental profile was dramatically altered by dark rearing which slowed the entire time course of the critical period. In dark reared cats, plasticity rose steadily over the first 12 weeks, and was maintained at 16 weeks. There was a crossover of the two profiles of the critical period such that at young ages (6 weeks) normal cats were more plastic than dark reared cats while at later ages (after 9 weeks) dark reared cats were more plastic. A second experiment indicated that dark rearing slowed down the progression of the critical period even after it had been initiated by a period of normal vision. MD produced substantial effects after the normal critical period in cats who were reared normally for the first 6-8 weeks of life and then placed in darkness until 5 months of age. The results are discussed in terms of a simple model of the accelerating effect of visual input and the decelerating effect of total darkness on the time course of the critical period.

Age-dependent changes in acetylcholinesterase activity in the primary somatosensory cortex of the cat

As cortical reorganization in cat somatosensory cortex has been shown to be age-dependent and acetylcholinesterase and acetylcholine have been implicated in the shaping of sensory responses during the developmental process, we decided to investigate the biochemical changes that occur in acetylcholinesterase during postnatal development of the primary somatosensory cortex in normal cat. Somatosensory cortices were removed from cats at various ages between 4 and 144 postnatal days. Three fractions (total, membrane-bound and soluble) were analyzed for activity (esterase assay and sedimentation analysis) and amount of acetylcholinesterase (electrophoresis). Results indicated that both esterase activity levels and amounts were characterized by 4 distinct phases which included a large step increase in all fractions between postnatal days 10 and 12: a gradual rise between days 12 and 28: a 'dip' during the 42- to 82-day interval, and a subsequent recovery. Results may be attributed to concomitant developmental events. Furthermore, we suggest that the observed changes may relate to age-dependent differences in somatosensory cortex reorganization that occur after spinal cord transection.

Physiological and pathophysiological roles of excitatory amino acids during central nervous system development

Recent studies suggest that excitatory amino acids (EAAs) have a wide variety of physiological and pathophysiological roles during central nervous system (CNS) development. In addition to participating in neuronal signal transduction, EAAs also exert trophic influences affecting neuronal survival, growth and differentiation during restricted developmental periods. EAAs also participate in the development and maintenance of neuronal circuitry and regulate several forms of activity-dependent synaptic plasticity such as LTP and segregation of converging retinal inputs to tectum and visual cortex. Pre- and post-synaptic markers of EAA pathways in brain undergo marked ontogenic changes. These markers are commonly overexpressed during development; periods of overproduction often coincide with times when synaptic plasticity is great and when appropriate neuronal connections are consolidated. The electrophysiological and biochemical properties of EAA receptors also undergo marked ontogenic changes. In addition to these physiological roles of EAAs, overactivation of EAA receptors may initiate a cascade of cellular events which produce neuronal injury and death. There is a unique developmental profile of susceptibility of the brain to excitotoxic injury mediated by activation of each of the EAA receptor subtypes. Overactivation of EAA receptors is implicated in the pathophysiology of brain injury in several clinical disorders to which the developing brain is susceptible, including hypoxia-ischemia, epilepsy, physical trauma and some rare genetic abnormalities of amino acid metabolism. Potential therapeutic approaches may be rationally devised based on recent information about the developmental regulation of EAA receptors and their involvement in the pathogenesis of these disorders.

The development of N-methyl-D-aspartate receptors in cat visual cortex

During a critical period of early postnatal development, the visual cortex of kittens is susceptible to experience-dependent modifications of neuronal response properties. Recently, the activation of N-methyl-D-aspartate (NMDA) receptors has been identified as an indispensable prerequisite for the induction of such modifications. We therefore investigated developmental changes in the density and distribution of NMDA receptors and questioned whether these showed a relation to the time course of the critical period. We determined the proportion of [3H]glutamate binding sites that were displaced by the NMDA receptor antagonist 2-amino-5-phosphonovalerate (APV) on 10-microns-thick cryostat sections of the primary visual cortex. The overall density of APV-sensitive [3H]glutamate binding sites increased dramatically between the second and the fourth week and stayed at this level throughout the critical period. Towards the end of the critical period, these binding sites decreased and finally reached adult values that were slightly above those of 2-week-old kittens. APV-sensitive binding sites were present in all cortical layers of the age groups investigated. While the general pattern of developmental changes was similar in all layers, slight differences existed in the time course. These observations are compatible with the notion that NMDA receptor activation is required for the expression of use-dependent change of response properties in the kitten visual cortex. Furthermore, they suggest as a possible reason for the decline of malleability towards the end of the critical period the reduction of NMDA receptors.

The sensitive period for the morphological effects of monocular deprivation in two nuclei of the tectofugal pathway of zebra finches

Previous experiments with 2-deoxyglucose (2-DG) suggested the existence of a critical period for the effects of monocular deprivation in the nucleus rotundus of zebra finches. The present study concerns the time course of this sensitive period for the morphological effects of monocular deprivation in two areas of the tectofugal visual pathway of zebra finches, the nucleus rotundus of the thalamus and the telencephalic ectostriatum. Cell size and volume changes were measured in birds subjected to 40 days of unilateral eye closure starting at ages spaced regularly throughout the first 70 days of life. The results show that monocular deprivation markedly affects cell size in both areas if the treatment starts at one or 10 days posthatch. The differences between deprived and non-deprived neurons decline monotonically with increasing visual experience prior to deprivation. However, deprivation onset at day 40 again causes as severe effects as early monocular closure. Deprivation as from day 50 or later no longer leads to abnormalities. The measurements of the volume of the nucleus rotundus parallel the cell size measurements, with the exception that the second increase in sensitivity occurs with deprivation onset at day 50 instead of day 40. These data indicate that the time course of the sensitive period for the effects of monocular deprivation may be double-peaked: the sensitivity for external stimuli declines from hatch until day 30, but has another peak at 40-50 days of life. The definite end of the sensitive period, as determined with this method, can therefore be assumed to be at around day 50-60.

Experiments to study recovery of lateral geniculate nucleus cell size following monocular lid closure and reverse suture in infant monkeys

Following monocular eyelid closure at birth in macaque monkeys, reverse suture at 3 weeks of age cancels the difference in size between deprived and undeprived parvocellular lateral geniculate nucleus (LGN) cells by causing hypertrophy of the initially deprived cells. This means that two weeks after reversal cells in both initially deprived and initially undeprived parvocellular laminae are approximately 15% larger than normal. However, long term survival shows that there is a second phase of change during which all parvocellular cells shrink, but particularly the initially undeprived cells which become considerably smaller than the initially deprived cells. Reopening the secondarily closed eye after a short period of reversal resulted in normal cell sizes in one animal, but two animals developed marked squints and had very small parvocellular cells in all laminae, and one animal developed bilateral amblyopia. Simply reopening the eye of two animals after two months of late closure started at 7 months of age reversed the shrinkage of all parvocellular cells which is caused by late closure.

Role of visual experience in promoting segregation of eye dominance patches in the visual cortex of the cat

Transneuronal autoradiography was used to study the role of visual experience in the development of ocular dominance patches in the cat. In order to assess quantitatively the effects of visual deprivation, image analysis was used to measure the profiles of grain density in layer IV. Fourier power spectra of these profiles were computed to give a measure of the amplitudes and frequencies of the fluctuations in grain density that were present. Deprivation of normal patterned vision by binocular lid suture or by rearing in total darkness from shortly after birth abolished the dominant periodicity (of about 1.1 mm) in the distribution of left and right eye afferents in layer IV of area 17. A dominant periodicity of about 2.2 mm was, however, present in area 18 of both normal and dark-reared animals. Visual deprivation was not able to reverse segregation. One animal reared normally for 6 weeks was placed in the dark for a further 28 weeks and showed normal periodicities in the distribution of geniculate inputs to area 17. Another animal given 128 hours of experience and kept in the dark for the rest of the time until it was 12 weeks old also showed normal segregation. To determine the minimum amount of visual experience necessary for segregation to occur, four animals were given 8-, 24-, 48-, and 128-hour periods of visual experience and were studied at 12 weeks of age. Eight hours of experience had no detectable effect on segregation; periodicities of intermediate amplitude were present in animals that received 24 and 48 hours of experience, while 128 hours of experience resulted in periodicities of normal amplitude. Recovery from visual deprivation was studied by rearing kittens from birth in the dark for varying periods and then returning them to the normally lit colony room for periods of 6 to 22 weeks. Recovery from 6 weeks of dark rearing was found to be complete; much less recovery occurred following periods of 8 to 25 weeks of initial deprivation, and no recovery at all occurred after 30 weeks of deprivation. It is concluded that the spontaneous activity present in the geniculocortical afferents of dark-reared and lid-sutured cats is not adequate to drive normal periodic segregation in area 17, though it can do so in area 18. Between 48 and 128 hours of visual experience, given before 8 weeks of age, appears to be necessary and sufficient for normal periodic segregation of geniculate afferents in area 17 of the cat.

Quantitative analysis of the choline acetyltransferase-immunoreactive axonal network in the cat primary visual cortex: II. Pre- and postnatal development

The pre- and postnatal development of cholinergic projections was investigated in the cat striate cortex by applying immunohistochemical methods based on a monoclonal antibody against choline acetyltransferase (ChAT). The earliest age investigated was gestational day 54. At this stage a sparse network of ChAT(+) fibers was distributed throughout the striate cortex. Subsequent postnatal maturation of ChAT(+) fibers was characterized by an increase in fiber density that started in layer VI and gradually progressed toward more superficial layers. By 4 weeks of age the density of ChAT(+) fibers and varicosities had reached adult levels in layers V and VI but was still subnormal in layers I-IV. The mature pattern of cholinergic innervation was established by 13 weeks of age. There was no evidence for developmental gradients in the anteroposterior and mediolateral directions within area 17. These results indicate that the cholinergic projection to striate cortex develops continuously in an inside-out sequence as is characteristic for most cortical maturation processes. There was no indication that striate cortex receives an especially dense cholinergic input during the critical period.

Reversal of the physiological effects of monocular deprivation in adult dark-reared cats

If kittens are dark-reared for 4 months and subsequently monocularly sutured, cells in area 17 become dominated by the experienced eye. We now find that the effects of monocular deprivation in adult dark-reared cats can be reversed by suturing the experienced eye and allowing the cat to use the deprived eye, an effect that has previously been shown only in young kittens. The presence of continuous or nearly continuous visual experience during infancy is required for the critical period to exhaust itself--brief periods of visual experience will not suffice.

Dark rearing prolongs physiological but not anatomical plasticity of the cat visual cortex

Recent studies (Cynader and Mitchell, '80; Mower et al., '81) have shown that total dark rearing prolongs susceptibility to the physiological effects of monocular deprivation (MD) in visual cortex beyond the normal age limits. The present study addressed whether this delayed physiological plasticity is accompanied by delayed anatomical plasticity in the geniculocortical pathway. Ocular dominance (OD) columns as defined by transsynaptic autoradiography following injection of 3H proline into one eye were studied both qualitatively and quantitatively in 17 cats. Compared to normal rearing (N-3), both binocular eyelid suture (N-2) and total dark rearing (N-3) resulted in incomplete segregation of OD columns in area 17. This apparent immaturity after binocular deprivation, however, did not reflect a delayed capacity for development and plasticity. Visual experience after dark rearing produced no marked changes. In cats who experienced MD after dark rearing, injection of either the nondeprived (N-2) or deprived eye (N-3) resulted in a nearly uniform distribution of label throughout layer IV of area 17. The same result occurred with binocular vision after dark rearing (N-1). MD from birth, however, produced expansion of columns from the nondeprived eye (N-1) and contraction of columns from the deprived eye (N-1). MD imposed after 4 months of normal vision resulted in normal OD columns (N-1). Electrophysiological studies revealed a high proportion of binocular cells within layer IV in cats who experienced monocular or binocular vision after dark rearing. Outside of layer IV there were clear environmental effects on OD of single cells in these cats. Measurements of cell sizes in the clateral geniculate nucleus showed shrinkage of cells innervated by the deprived eye when MD was initiated at birth (N-3). MD after dark rearing (N-4) produced no differences in cell sizes. It is concluded that visual input is necessary for the formation of normal OD columns, the critical period for formation and environmental modification of OD columns is limited to early life, and the physiological effects of visual experience after dark rearing reflect changes occurring beyond the geniculocortical pathway.

Survival of early monocular deprivation effects in cortical cells of kittens following prolonged dark rearing

To study whether early visual experience survives the absence of consequent visual stimulation during development, experimental kittens were reared in the dark for 5-13.5 months following monocular deprivation (MD) periods of 2-11 weeks which were initiated at the time of natural eye opening (MD-dark). For comparison, experimental kittens, normally reared after equivalent MD periods (MD-bino.), were also studied. Cats raised with permanent MD, dark-reared cats and normal cats, served as controls. The proportion of responsive cells was considerably reduced by the dark-rearing. It was especially reduced for the MD-dark kittens following monocular deprivation limited to the first postnatal month (58.3% responsive cells) in comparison to the equivalent group of MD-bino. kittens (80.5%). This is also in keeping with the diminution in cortical responsiveness obtained in the kittens which were dark-reared from birth (55.5%). The responsiveness level found in the normal control cats was 87.3%. It was found that the duration of the MD period prior to the dark-rearing period was directly related to the ocular dominance (OD) distribution of cortical cells. The susceptibility period to MD in both MD-dark and MD-bino. groups ends at approximately 3 months of age; the lower limit for the susceptibility period is at approximately 1-2 weeks after natural eye opening. The main period of sensitivity within this period of time is the first 4 postnatal weeks following natural eye opening. It is concluded that once the effect of monocular deprivation has been established, it will survive for the rest of the cat's life, even under conditions of complete absence of consequent visual stimulation. Furthermore, a certain degree of consolidation of the MD effect takes place in the light (i.e. in MD-bino. cats) despite their return to normal binocular vision. A somewhat opposite occurrence takes place in the dark (in MD-dark cats) with a tendency for masking of the MD effect previously induced in the light to be found.