Inhibitory mechanism by polysialic acid for lamina-specific branch formation of thalamocortical axons.

During development, thalamocortical axons form arbors primarily in layer 4 of the neocortex. This lamina-specific branch formation was studied in cultures of rat thalamic explants grown next to chemically fixed cortical slices. After a week in vitro, thalamic axons formed branches specifically in the target layer of fixed cortical slices, regardless of the orientation of the ingrowth. This in vitro system permits a direct assessment of contributions of membrane-associated molecules to thalamic axon branch formation. To this end, the present study uses three enzymatic perturbations: chondroitinase, phosphatidylinositol phospholipase C, or the polysialic acid (PSA)-specific endoneuraminidase (endo N). With endo N pretreatment of cortex, the number of branch points was increased significantly, whereas branch tip length was decreased. In addition, the localization of branch points to the target layer was weakened considerably. These features of branch formation were not altered by the other two enzymatic treatments, except that branch tips were shortened by chondroitinase treatment to the same extent as in endo N treatment. These results suggest that membrane-bound components are involved in lamina-specific branch formation of thalamocortical axons, and in particular that PSA moieties contribute to laminar specificity by inhibiting branch emergence in inappropriate layers.

Development and organization of ocular dominance bands in primary visual cortex of the sable ferret.

Thalamocortical afferents in the visual cortex of the adult sable ferret are segregated into eye-specific ocular dominance bands. The development of ocular dominance bands was studied by transneuronal labeling of the visual cortices of ferret kits between the ages of postnatal day 28 (P28) and P81 after intravitreous injections of either tritiated proline or wheat germ agglutinin-horseradish peroxidase. Laminar specificity was evident in the youngest animals studied and was similar to that in the adult by P50. In P28 and P30 ferret kits, no modulation reminiscent of ocular dominance bands was detectable in the pattern of labeling along layer IV. By P37 a slight fluctuation in the density of labeling in layer IV was evident in serial reconstructions. By P50, the amplitude of modulation had increased considerably but the pattern of ocular dominance bands did not yet appear mature. The pattern and degree of modulation of the ocular dominance bands resembled that in adult animals by P63. Flat mounts of cortex and serial reconstructions of layer IV revealed an unusual arrangement of inputs serving the two eyes in the region rostral to the periodic ocular dominance bands. In this region, inputs serving the contralateral eye were commonly fused along a mediolateral axis, rostral to which were large and sometimes fused patches of ipsilateral input.

Relationship between the ocular dominance and orientation maps in visual cortex of monocularly deprived cats.

The significance of functional maps for cortical plasticity was investigated by imaging of intrinsic optical signals together with single-unit recording in kittens. After even a brief period of monocular deprivation during the height of the critical period, only isolated patches of visual cortex continued to respond strongly to the closed eye. These deprived-eye patches were located on the pinwheel center singularities of the orientation map and consisted of neurons that were poorly selective for stimulus orientation. Neurons in regions surrounding the deprived-eye patches responded only weakly to the deprived eye but were well tuned for the same stimulus orientation that optimally excited them when presented to the open, nondeprived eye. The coincidence of deprived-eye patches with pinwheel center singularities, and the selective loss of orientation tuning within the deprived-eye patches, indicate that the orientation and ocular dominance maps are functionally linked and provide compelling evidence that pinwheel center singularities are important for cortical plasticity.

Ocular dominance peaks at pinwheel center singularities of the orientation map in cat visual cortex.

In the primary visual cortex of monkey and cat, ocular dominance and orientation are represented continuously and simultaneously, so that most neighboring neurons respond optimally to visual stimulation of the same eye and orientation. Maps of stimulus orientation are punctuated by singularities referred to as "pinwheel centers," around which all orientations are represented. Given that the orientation map is mostly continuous, orientation singularities are a mathematical necessity unless the map consists of perfectly parallel rows, and there is no evidence that the singularities play a role in normal function or development. We report here that in cats there is a strong tendency for peaks of ocular dominance to lie on the pinwheel center singularities of the orientation map. This relationship predicts but is not predicted by the tendencies, previously reported, for pinwheels to lie near the center lines of ocular dominance bands and for iso-orientation bands to cross ocular dominance boundaries at right angles. The coincidence of ocular dominance peaks with orientation singularities is likely to reflect a strong underlying functional link between the two visual cortical maps.

The role of activity in the development of long-range horizontal connections in area 17 of the ferret.

Horizontal connections in area 17 of adult cats and ferrets link cells with similar preferred orientations by a patchy network of projections extending several millimeters across the cortex. The maturation of orientation selectivity in ferret area 17 has been demonstrated previously by quantitative single-unit recording and optical imaging to begin at approximately postnatal days (P) 32-P36. We therefore made restricted injections of cholera toxin B-subunit (CTB) or CTB-gold into ferret area 17 at a series of developmental ages and statistically quantified the degree of clustering in plots of retrogradely labeled cells in tangential sections through layer III for comparison to the published values for orientation tuning at each age. At P21, horizontal connections within area 17 lacked patchiness entirely, although clear patches of labeled cells were present in extrastriate areas. By P27, significant clustering of horizontal connections within area 17 was present. A second phase of cluster refinement was observed to occur at approximately P34-P36, coinciding with the emergence of mature orientation tuning and maps. Continuous silencing of cortical action potentials by chronic tetrodotoxin infusion from P21 resulted in a spatially random distribution of retrogradely labeled cells at P34. In contrast, bilateral enucleation from P21 did not prevent the initial development of clustered horizontal connections. We conclude, based on our findings and those of others, that the anatomical specificity of long-range horizontal connections results from an activity-dependent process that initially can use spontaneous activity in the cortical and thalamic networks to establish crude periodic connections and later uses visual cues to refine these connections.

Inhibition of nitric oxide synthase does not prevent ocular dominance plasticity in kitten visual cortex.

1. The neural messenger molecule nitric oxide (NO) has been shown to be involved in several forms of plasticity including hippocampal long-term potentiation. We examined the effects of chronic intracortical infusion of inhibitors of NO synthase (NOS) activity on the plasticity of visual cortical responses following monocular lid suture during the critical period. 2. Single unit recordings (618 cells) made in both the NOS inhibitor-treated (30 mM NG-methyl-L-arginine (L-NMMA), or 22 or 2 mM nitro-L-arginine (L-NOArg)) and saline-treated control hemispheres of barbiturate-anaesthetized, critical-period kittens (n = 8) revealed a profound shift in favour of the non-deprived eye. Shifts were of similar magnitude in hemispheres in which NOS was inhibited and in saline control hemispheres. 3. Subsequent analysis of NOS activity in the same cortical tissue in which recordings had been made showed a pronounced decrease in NOS activity in inhibitor-treated hemispheres. In the region in which all the single unit recordings were made (< 3 mm from the infusion cannula), 22 mM L-NOArg resulted in a reduction of NOS activity to 5.55 +/- 5.33% of control hemisphere NOS activity levels. L-NOArg (2 mM) and L-NMMA (30 mM) also produced clear, but smaller, inhibition of NOS activity. 4. These findings demonstrate that NOS activity is not essential for ocular dominance plasticity in visual cortex.