Physiological basis of anisometropic amblyopia.

In the visual cortex of kittens that have received their only visual experience while wearing a high-power lens before one eye, most neurons are dominated by input from the normal eye. Moreover, contrast sensitivity and resolving power are lower for stimulation through the originally defocused eye, mimicking psychophysical results from human anisometropic amblyopes.

Monocular deprivation: morphological effects on different classes of neurons in the lateral geniculate nucleus.

Retrograde axonal transport of horseadish peroxidase from areas 17 and 18 of the cat's visual cortex labels, principally, the small (X) and large (Y) cells, respectively, of the lateral geniculate nucleus. Quantitative analysis of the sizes of these morphologically identified neurons after monocular deprivation shows that the arrest of cell growth in the deprived laminae involves mainly Y cells.

Size adaptation: a new aftereffect.

If, after prolonged observation of a striped pattern, one views a grating of the same orientation with somewhat narrower bars, then the bars seem even thinner than in fact they are. Broader bars seem broader still. This finding implies a system of size-detecting channels in humnan vision. The phenomenon may underlie many of the classical figural aftereffects.

Stereoscopic depth aftereffect produced without monocular cues.

Random-dot stereograms when used as adaptation stimuli can influence the perceived depth of similar test stimuli. Adaptation for 1 minute is sufficient to evoke this three-dimensional aftereffect for several seconds. This aftereffect must occur after stereopsis because prior to stereopsis no relevant monocular cues exist in these adaptation and test stimuli.

PLC-beta1, activated via mGluRs, mediates activity-dependent differentiation in cerebral cortex.

During development of the cerebral cortex, the invasion of thalamic axons and subsequent differentiation of cortical neurons are tightly coordinated. Here we provide evidence that glutamate neurotransmission triggers a critical signaling mechanism involving the activation of phospholipase C-beta1 (PLC-beta1) by metabotropic glutamate receptors (mGluRs). Homozygous null mutation of either PLC-beta1 or mGluR5 dramatically disrupts the cytoarchitectural differentiation of 'barrels' in the mouse somatosensory cortex, despite segregation in the pattern of thalamic innervation. Furthermore, group 1 mGluR-stimulated phosphoinositide hydrolysis is dramatically reduced in PLC-beta1-/- mice during barrel development. Our data indicate that PLC-beta1 activation via mGluR5 is critical for the coordinated development of the neocortex, and that presynaptic and postsynaptic components of cortical differentiation can be genetically dissociated.

How do thalamic axons find their way to the cortex?

A cascade of simple mechanisms influences thalamic innervation of the neocortex. The cortex exerts a remote growth-promoting influence on thalamic axons when they start to grow out, becomes growth-permissive when the axons begin to invade, and later expresses a 'stop signal', causing termination in layer 4. However, any part of the thalamus will innervate any region of developing cortex in culture, and the precise topographic distribution of thalamic fibres in vivo is unlikely to depend exclusively on regional chemoaffinity. The 'handshake hypothesis' proposes that axons from the thalamus and from early-born cortical preplate cells meet and intermingle in the basal telencephalon, whereafter thalamic axons grow over the scaffold of preplate axons, and become 'captured' for a waiting period in the subplate layer below the corresponding part of the cortex. The bizarre pattern of development of thalamic innervation in the mutant reeler mouse provides strong evidence that thalamic axons are guided by preplate axons.

Morphology and growth patterns of developing thalamocortical axons.

It is increasingly evident that the actions of guidance factors depend critically on the cellular and molecular context in which they operate. For this reason we examined the growth cone morphology and behavior of thalamic fibers in the relatively natural environment of a slice preparation containing the entire pathway from thalamus to cortex. Axons were labeled with DiI crystals and imaged with a laser-scanning confocal microscope for up to 8 hr. Their behavior was analyzed in terms of morphology, extension rates, shape of trajectory, frequency of branching, and percentage of time spent in advance, pause, and retraction. Thalamic fibers had distinct and stereotyped growth patterns that related closely to their position; within the striatum growth cones were small and elongated, rarely extending filopodia or side branches. Axons grew quickly, in straight trajectories, with minimal pauses or retractions. When they reached the ventral intermediate zone, axons slowed down, often coming to a complete stop for up to several hours, and their growth cones became larger and more complex. During pauses there were continuous extensions and retractions of filopodia and/or side branches. When advance resumed, it was often to a different direction. These results demonstrate consistent regional variations in growth patterns that identify an unexpected decision region for thalamic axons. They provide the basis for examining the roles of guidance cues in an accessible yet intact preparation of the thalamocortical pathway and allow for an evaluation of previously suggested pathfinding mechanisms.

Formation of cortical fields on a reduced cortical sheet.

Theories of both cortical field development and cortical evolution propose that thalamocortical projections play a critical role in the differentiation of cortical fields (; ). In the present study, we examined how changing the size of the immature neocortex before the establishment of thalamocortical connections affects the subsequent development and organization of the adult neocortex. This alteration in cortex is consistent with one of the most profound changes made to the mammalian neocortex throughout evolution: cortical size. Removing the caudal one-third to three-fourths of the cortical neuroepithelial sheet unilaterally at an early stage of development in marsupials resulted in normal spatial relationships between visual, somatosensory, and auditory cortical fields on the remaining cortical sheet. Injections of neuroanatomical tracers into the reduced cortex revealed in an altered distribution of thalamocortical axons; this alteration allowed the maintenance of their original anteroposterior distribution. These results demonstrate the capacity of the cortical neuroepithelium to accommodate different cortical fields at early stages of development, although the anteroposterior and mediolateral relationships between cortical fields appear to be invariant. The shifting of afferents and efferents with cortical reduction or expansion at very early stages of development may have occurred naturally in different lineages over time and may be sufficient to explain much of the phenotypic variation in cortical field number and organization in different mammals.

Regional specialization in the golden hamster's retina.

Ganglion cell swere counted and measured in whole mounts of the hamster's retina, stained with methylene blue. Their density varies between about 1,000/mm2 at the edge of the retina to about 5-6,000/mm2 in a broad area centralis centred about 1.9 mm(39 deg) directly temporal to the optic disk. Maps of cell density show a long horizontal extension of the dense area in the nasal direction. The sizes of ganglion cell somata fall into two main groups--small cells (5-11 mum diameter) and large cells (greater than 11 mum), the latter including a small proportion of giant cells (greater than 17 mum). All three classes of cells are maximal in density in the area centralis, although the small cells are relatively more numerous there. The total number of cells is about 114,000 with about 63,000 small cells and about 51,000 large. The optic nerve contains about 69,000 unmyelinated axons and about 50,000 myelinated axons, suggesting that the latter are the fibres of the larger ganglion cells. It is likely that the projections of the centres of the areae centrales of the two eyes are normally divergent in space; they are therefore not on "corresponding retinal points."

Functional organization in the visual cortex of the golden hamster.

The visual cortex of the golden hamster was studied by means of multi-unit and single unit recording, which revealed three separate retinotopic maps of the visual field in the posterior cortex. V1, corresponding to cyto-architectonic area 17, has the contralateral temporal field represented medially, the central visual field (extending about 10 deg ipsilateral) represented laterally and the lower field anteriorly. The borders of the map, especially for the upper field, seem to be more restricted than the whole visual field available to the contralateral hemiretina: V1 probably does not represent the extreme periphery of the field. A large fraction of V1 has binocular input, for up to about 50 deg lateral to the vertical midline. There is a retinotopic reversal near the representation of the vertical midline where V1 meets V2 (corresponding to the more lateral "area 18a"). There is another retinotopic reversal at the extremity of the contralateral field representation, where V1 meets Vm (the medial visual area, corresponding to "area 18"). V2 and Vm each contain a reduced mirror image version of the map in V1. Almost all isolated single units in V1 have receptive fields that can be classified as radially symmetrical (60%) or asymmetrical (35%). Symmetrical fields have ON (13%), OFF (4%), ON-OFF (30%) or "SILENT" (12%) central areas when plotted with flashing spots. There are minor but not striking differences between these groups in their field sizes, velocity preferences and so on. They almost invariably prefer moving to stationary stimuli but are not selective for orientation or direction of movement. Asymmetrical fields are of four types, three of which (type 1, 11%; type 2, 17%; and type 3, 2%) are orientation selective and resemble simple, complex and hypercomplex cells in the cat cortex. Some of these have direction as well as orientation preference. Axial movement detectors (5%) have a selectivity for one axis of motion, and thus prefer one orientation of edge, but respond equally well to movement of a spot. Vertical and horizontal orientation preferences, especially the latter, are much the most common. There is some evidence of clustering of cells according to receptive field type and, possibly, preferred orientation. Asymmetrical cells are, relatively somewhat rarer in the deeper cortical layers. Within the binocular segment, fully 89% of cells are binocularly driven and the receptive fields are similar in the two eyes. Receptive fields tend to increase in size away from the area centralis representation and, in a complementary fashion, the magnification factor decreases from up to 0.1 mm/deg at the area centralis representation to about 0.02 mm/deg for the peripheral field.

Functional organization in the superior colliculus of the golden hamster.

The superior colliculus of the golden hamster was investigated by means of multi-unit and single unit recording. The retinotopic map, which probably embraces a projection from the entire retina of the contralateral eye, is organized as in other vertebrates, with the central field represented in the anterior colliculus, the upper field medially. Magnification factor is fairly uniform and is about 0.02 mm/deg. There is a small binocular segment (where almost half of all neurones have input from the ipsilateral eye) in the anterior colliculus, representing the area of field around the area centralis and the anterior pole of the field. In the more superficial layers, units have small (about 10 deg diameter) receptive fields, which can be classified as symmetrical, responding to slow movement (80%), very fast movement detectors (6%), directional movement detectors (13%) and axial movement detectors (1%). In the deeper layers, below the stratum opticum, receptive field size increases dramatically and many cells habituate rapidly, making them sensitive only to new events. Receptive fields can be classified as movement detectors (89%), directional movement detectors (10%) and axial movement detectors (2%). All directional receptive fields, at least in the upper visual field, have an upward component in their directional preferences. About 42% of deeper layer cells have somatic sensory input, responding to light touch on the fur or whiskers of the contralateral half of the body. Some 5% of cells respond to complex sounds on the contralateral side of the animal. Many of these somatic and auditory cells also have visual receptive fields and, throughout the colliculus, there is general correspondence between the maps of visual space, auditory space and the body surface. This correlation may be important in the regulation of orienting behaviour towards novel peripheral stimuli.

Projections to the visual cortex in the golden hamster.

Retrograde transport of horseradish peroxidase (HRP) was used to determine the origins of afferent connexions to the visual cortex (areas 17, 18a and 18b) in the hamster. The distribution of neurons projecting to the visual cortex from other cortical areas, from the thalamus and from the brainstem was studied using a computer technique for three-dimensional reconstruction. There is a topographically organized projection from the dorsal lateral geniculate nucleus to area 17, but probably to no other of the areas studied. The lateral posterior nucleus of the thalamus (LP) projects to area 18a and weakly to area 17. The lateral nucleus (L) projects to area 18b and also, probably, weakly to area 17. The cortical projections from LP and L are also organized topographically but relatively grossly compared with the geniculo-cortical pathway. There are reciprocal association projections between area 17 and areas 18a and 18b. Areas 18a projects weakly to 18b. The main commissural connexions of the posterior neocortex are between the area 17/18a boundary zones in the two hemispheres, with little between the bodies of area 17. Labelled neurons were found bilaterally in the locus coeruleus, more ipsilaterally than contralaterally, after multiple injections into the visual cortex: single, small injections sometimes resulted in the labelling of a single cell body in the locus coeruleus.

Postnatal development of the ipsilateral retinocollicular projection and the effects of unilateral enucleation in the golden hamster.

Anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was used to study the normal development of the ipsilateral retinocollicular projection in golden hamsters, and to examine the effect of enucleation of the other eye at birth. In neonatal animals there were retinal fibers and sparsely distributed granular labeling in the superficial layers of the ipsilateral superior colliculus over its entire areal extent. Differences in the uncrossed projections of normal and enucleated animals first became clear at day 5. In normal animals, retinal fibers withdrew from the superficial layers of the superior colliculus, and the projection became concentrated in the stratum opticum, where denser clumps of label in the rostral part of the superior colliculus were first seen at day 5. In enucleated animals, the retinal projection persisted in the most superficial layers, and the density of labeling was higher than in normals. The very sensitive WGA-HRP technique showed retinal fibers extending to the caudal pole of the superior colliculus at all ages: even in normal animals more than 2 weeks of age some fibers reached as far as the inferior colliculus. When the shrunken size of the superior colliculus in the enucleated animals was taken into account, the total areal distribution of the ipsilateral projection was similar in normal animals and enucleates. The major difference between the two groups was in the higher density of ipsilateral labeling, especially in the caudal part of the superior colliculus, and in its more superficial laminar distribution in the enucleated animals.

Pyramidal neurons in layer 5 of the rat visual cortex. I. Correlation among cell morphology, intrinsic electrophysiological properties, and axon targets.

Previous work has established two structure/function correlations for pyramidal neurons of layer 5 of the primary visual cortex of the rat. First, cells projecting to the superior colliculus have thick apical dendrites with a florid terminal arborization in layer 1, whereas those projecting to the visual cortex of the opposite hemisphere have thinner apical dendrites that terminate below layer 1, without a terminal tuft (e.g., Hallman et al.: J Comp Neurol 272:149, '90). Second, intracellular recording combined with dye injection has revealed two classes of cells: the first has a thick, tufted apical dendrite and fires a distinctive initial burst of two or more impulses, of virtually fixed, short interspike interval, in response to current injection; and the other, with a slender apical dendrite lacking a terminal tuft, tends to have a longer membrane time constant and higher input resistance, and does not fire characteristic bursts (e.g., Larkman and Mason: J Neurosci 10:1407, '90). The present study combined intracellular recording in isolated slices of rat visual cortex and injection of carboxyfluorescein, to reveal soma-dendritic morphology, with prior injection of rhodamine-conjugated microspheres into the superior colliculus or contralateral visual cortex to label neurons according to the target of their axons. This permitted a complete correlation of morphology, intrinsic electrophysiological properties, and identity of the projection target for individual pyramidal cells. Neurons retrogradely labeled from the opposite visual cortex were found in all layers except layer 1 while those labeled from the superior colliculus lay exclusively in layer 5. Within layer 5 interhemispheric cells were more concentrated in the lower half of the layer but extensively overlapped the distribution of corticotectal cells. Every cell studied that projected to the superior colliculus was of the bursting type and had a thick apical dendrite with a terminal tuft. Every cell in this study projecting to the opposite visual cortex was a "nonburster" and had a slender apical dendrite with fewer oblique branches that ended without a terminal tuft, usually in the upper part of layer 2/3. Interhemispheric cells also had rounder, less conical somata and generally had fewer basal dendrites than corticotectal neurons. Many cells with the physiological and morphological characteristics of interhemispheric cells were not back-labeled from the opposite visual cortex, implying that pyramidal cells of this type can have other projection targets (e.g., other cortical sites in the ipsilateral hemisphere).(ABSTRACT TRUNCATED AT 400 WORDS)

Pyramidal neurons in layer 5 of the rat visual cortex. II. Development of electrophysiological properties.

Two major classes of pyramidal neurons can be distinguished in layer 5 of the adult rat visual cortex. Cells of the "thick/tufted" type have stout apical dendrites with terminal tufts, and most of them project to the superior colliculus (Larkman and Mason: J Neurosci 10:407, '90; Kasper et al.: J Comp Neurol, this issue, 339:459-474). "Slender/untufted" cells have thinner apical trunks with no obvious terminal tufts, and a substantial proportion of them project to the contralateral visual cortex. These two types also differ in their intrinsic electrophysiological features. In this study we describe the postnatal maturation of the electrophysiological and synaptic properties of layer 5 pyramidal neurons and relate these findings to the morphological development and divergence of the two cell types. Living slices were prepared from the visual cortex of rats aged between postnatal day 3 (P3) and young adults and maintained in vitro. Stable intracellular impalements were obtained from a total of 63 pyramidal cells of layer 5 at various ages, which were injected with biocytin so that morphological and electrophysiological data could be obtained from the same cell. Before P15, injection of a single cell sometimes stained a cluster of neurons of similar morphology, probably as a result of dye coupling. The incidence of such clustering and the number of neurons within each cluster decreased with age. There was no obvious difference in electrophysiological properties between cells in clusters and age-matched, noncoupled neurons. From P5, the apical dendrites of neurons could easily be classified as "thick/tufted" or "slender/untufted." On average, the resting potential became more negative, and membrane time constant and input resistance decreased with age. Electrophysiological differences between the "thick/tufted" and "slender/untufted" cell types did not become apparent until the third postnatal week, after which the "thick/tufted" cells on average had lower input resistances and slightly faster time constants than "slender/untufted" cells. The current-voltage relations of the neurons became progressively more nonlinear during maturation, with both rapid inward rectification and time-dependent rectification or "sag" becoming more prominent. There were also changes in the amplitude and waveform of action potentials, which generally approached adult values by 3 weeks of age. Action potential threshold became more negative, both in absolute terms and relative to the resting membrane potential. Action potentials became larger in peak amplitude and of shorter duration, with both rise and fall times decreasing progressively during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Pyramidal neurons in layer 5 of the rat visual cortex. III. Differential maturation of axon targeting, dendritic morphology, and electrophysiological properties.

This paper describes the early morphological and physiological development of pyramidal neurons in layer 5 of the rat visual cortex in relation to the targets chosen by their axons. Cells were prelabeled by retrograde transport from the superior colliculus or the contralateral visual cortex and intracellularly injected either in fixed slices or after recording in living slices. In the adult, corticotectal cells have thick apical dendrites with an extensive terminal arborization extending into layer 1, and fire characteristic bursts of action potentials when injected with a depolarizing current; interhemispheric cells have slender apical dendrites that terminate without a terminal tuft, usually in layer 2/3, and they display a more regular firing pattern (Kasper et al.: J Comp Neurol, this issue, 339:459-474). At embryonic day E18 (when axons of the two classes of cells are already taking different routes towards their targets) and E21, pyramidal-like cells throughout the cortical plate all have similar soma-dendritic morphology, with spindle-shaped cell bodies and few, short basal dendrites but apical dendrites that all end in distinct tufts in the marginal zone. At postnatal day P3, after the axons of both cell classes have reached their targets, all pyramidal neurons in layer 5 still have distinct terminal arborizations in layer 1, though they vary in complexity and extent. The somata are now more mature (round to ovoid in shape), and the basal dendritic tree has extended. As early as P5, all cells studied could be clearly classified as tufted or untufted (considerably earlier than previously reported; Koester and O'Leary: J Neurosci 12:1382, '92), and these features correlated precisely with the projection target, as in the adult. Measurement showed that although interhemispheric cells lose their terminal tufts, in general the trunks of their apical dendrites do not withdraw but continue to grow, at roughly the same rate as those of corticotectal cells. The two classes of layer 5 pyramidal neurons differentiate from each other in three distinct phases: pathway selection by axons precedes the loss of the apical tuft by interhemispheric cells, and these morphological characteristics are established 10 days before the onset of burst-firing in corticotectal cells. These three steps may be guided by different molecular signals.

Development of thalamocortical projections in the South American gray short-tailed opossum (Monodelphis domestica).

We determined the time-course and general pattern of thalamocortical development of Monodelphis domestica by tracing projections with carbocyanine dye in fixed postnatal brains between postnatal day 2 (P2) and P30. By P2, the first neurons have migrated to form the preplate of the lateral cortex and have sent out axons into the intermediate zone. By P3, fibers from the preplate of more dorsal cortex have entered the intermediate zone, and, by P5, they reach the primitive internal capsule. Crystal placements in the dorsal thalamus at P2-P3 reveal thalamic axons extending down through the diencephalon and growing out through the internal capsule among groups of back-labelled cells that already project into the thalamus. Thalamic axons arrive at the cortex after the arrival of cells of the true cortical plate has split the preplate into marginal zone and subplate. Axons from the ventral part of the dorsal thalamus reach the lateral cortex by P5: Dorsal thalamic fibers arrive at the extreme dorsal cortex by P9. The deeper layers of the cortex appear to mature relatively earlier in Monodelphis than in eutherian mammals, and the subplate becomes less distinct. Thalamic fibers and their side branches proceed into the cortex without an obvious period of waiting in the subplate, but they do not penetrate the dense cortical plate itself. Monodelphis could provide an excellent model species, because the development of its thalamocortical connections is entirely an extrauterine process: The period P0-P15 corresponds to that of E12-P0 in the rat.

Analysis of connectivity: neural systems in the cerebral cortex.

The mammalian cerebral cortex is composed of many distinct areas, which are very richly interconnected. The very large number of connections between cortical areas require analysis to be undertaken before reliable conclusions about the organization of neural systems in the cortex can be drawn. We review the methodology and results of two means of analysing central nervous connectivity, hierarchical analysis and optimization analysis. We conclude that these methods are reliable methods for analysing neural connectivity data, and that their results concur. The analyses indicate that all major cortical sensory systems are organized hierarchically, some central sensory systems are divided structurally into several "streams" of processing, the cortical motor system is embedded in the cortical somatosensory system, the frontal and limbic structures are connectionally associated, and that these frontal and limbic areas are invariably associated with the least peripheral sensory processing regions, and are therefore connectionally central. Finally, we discuss the differences on this common plan between the organizations of the cat and primate that these analyses reveal.

Phospholipase C-beta1 expression correlates with neuronal differentiation and synaptic plasticity in rat somatosensory cortex.

Receptor-mediated signal transduction is thought to play an important role in neuronal differentiation and the modification of synaptic connections during brain development. The intracellular signalling molecule phospholipase C-beta1 (PLC-beta1), which is activated via specific neurotransmitter receptors, has recently been implicated in activity-dependent plasticity in the cat visual cortex. PLC-beta1 has been shown to be concentrated in an intermediate compartment-like organelle, the botrysome, which is present in 5-week-old, but not adult, cat cortical neurons. We have characterized the spatial and temporal regulation of PLC-beta1 expression in the developing rat cerebral cortex. PLC-beta1-positive botrysome-like organelles are observed during early postnatal cortical development, but not at postnatal day 14 or later stages. In the postnatal somatosensory cortex, there is also striking spatial variation in diffuse neuropilar immunoreactivity of layer IV and above, in a pattern corresponding to the thalamocortical recipient zones known as barrels. This expression pattern is specific to the developing barrel field and is most distinct at postnatal days 4-7, when cellular components of barrels are capable of activity-dependent modification. During later stages of cortical maturation, stained botrysomes disappear, expression of PLC-beta1 is down-regulated and only diffuse immunoreactivity remains in dendritic processes. Our results are consistent with a role for PLC-beta1 in activity-dependent, receptor-mediated neuronal plasticity during development of the somatosensory cortex.

Delayed onset of Huntington's disease in mice in an enriched environment correlates with delayed loss of cannabinoid CB1 receptors.

Huntington's disease (HD) is a late onset progressive genetic disorder characterised by motor dysfunction, personality changes, dementia and premature death. The disease is caused by an unstable expanded trinucleotide (CAG) repeat encoding a polyglutamine stretch in the IT15 gene for huntingtin, a protein of unknown function. Transgenic mice expressing exon one of the human HD gene with an expanded polyglutamine region develop many features of human HD. Exposure of these mice to an "enriched" environment delays the onset of motor disorders and slows disease progression [Nature 404 (2000) 721]. We have compared the levels of receptor binding of a range of basal ganglia neurotransmitter receptors believed to be important in HD, in normal mice and R6/1 transgenic HD mice housed in either enriched or standard laboratory environments. HD mice housed in a normal environment show a loss of cannabinoid CB1 and dopamine D1 and D2 receptors in the striatum and the corresponding output nuclei of the basal ganglia. HD mice exposed to an enriched environment show equivalent loss of D1 and D2 receptors as their "non-enriched" counterparts; in contrast, the "enriched" mice show significantly less depletion of CB1 receptors. In the brains of humans diagnosed with HD cannabinoid CB1 receptors are selectively lost from the basal ganglia output nuclei prior to the development of other identifiable neuropathology [Neuroscience 97 (2000) 505]. Our results therefore show that an enhanced environment slows the rate of loss of one of the first identifiable neurochemical deficits of HD. This suggests that delaying the loss of CB1 receptors, either by environmental stimulation or pharmacologically, may be beneficial in delaying disease progression in HD patients.