Postnatal development of corticotectal neurons in the kitten striate cortex: a quantitative study with the horseradish peroxidase technique

Postnatal development of striate cortical neurons projecting to the superior colliculus (SC) was studied in cats, ranging in age from newborn to adult, by injection of horseradish peroxidase (HRP) into the SC. At birth HRP-labelled cells were widely distributed throughout the cortex between the splenial and suprasylvian sulci, although a very rough topographic correspondence seemed to exist between the striate cortex and SC. The labelled cells were confined to layer V of the cortex, as in the adult. They were very densely packed and their somas were already pyramidal in shape although very slender. During the third to eighth days, apical dendrites of a substantial number of cells, mostly located in the upper bank of the splenial sulcus, were filled with HRP up to layer I and their somas were larger than those of cells located near the crown of the lateral and postlateral gyri. At the eighth day and thereafter, the distribution of the labelled cells across the visual cortex was not so widespread as that seen in the newborn kittens. The dendritic arborization pattern of labelled cells became nearly adultlike at the four week, and its full maturation was seen at the eighth week. A quantitative analysis of the cross-sectional areas of the cells and their packing density in layer V of the cortex revealed that (1) the size of cells increased very rapidly during the second week and became almost adultlike at the fifth week; (2) the density of cells reduced dramatically during the second week and thereafter at a low rate until the eighth week; and (3) the ratio of the labelled to unlabelled cells in layer V decreased remarkably also during the second week. These results suggest that an elimination of axon collaterals of corticotectal cells or their death may take place mostly during the second week of age, when eye-opening occurs in kittens. By comparison with previous data on functional development of the SC, it is also suggested that the maturation of visual response properties of SC neurons may depend on postnatal development of corticotectal cells.     

Postnatal development of the electrical activity of rat nigrostriatal dopaminergic neurons

Extra- and intracellular recordings were obtained in vivo from dopaminergic nigrostriatal neurons in rat pups ranging in age from postnatal day (PD) 1 to PD28, and in adult rats. Neurons from PD1-3 rats were active at very low rates in a random pattern, rarely showed bursting activity, and often exhibited long periods of up to several minutes of silence. Spontaneous spikes were of relatively low amplitude and long duration. The mean firing rate increased and became more regular over time, and short bursts consisting of only 2 spikes were observed. By the second postnatal week, the initial segment component of the spontaneous spike resembled that seen in adults, but the somadendritic component was still relatively small, and there was often a very marked temporal delay between the two. Near the end of the second postnatal week, neurons exhibited a transient phase of pacemaker-like activity. Mean firing rates continued to increase with time, as did the incidence and complexity of bursting activity. The spontaneous firing rate, pattern and spike morphology approached adult values by the fourth postnatal week. Antidromic responses from neostriatum were obtained as early as PD1, and consisted of a significantly greater proportion of full initial segment-soma dendritic spikes compared to nigrostriatal neurons from adult rats. There was usually a long delay between the initial segment and somadendritic components of the spike. Mean antidromic latency and mean antidromic threshold did not vary significantly from PD1-3 to adults. Axonal conduction velocity reached maximal adult values by PD16-21. Neostriatal-evoked orthodromic responses consisted principally of a poststimulus inhibition whose duration decreased from PD1 through adulthood. Pure excitatory responses were very rarely observed at any age. Intracellular recordings from PD2, PD3 and PD5 rats revealed striatal-evoked inhibitory postsynaptic potentials in non-dopaminergic nigral neurons with a mean onset latency (9.8 +/- 3.8 ms) which did not differ from that previously reported for adult rats.     

Transient Colocalization of Parvalbumin and Calbindin D28k in the Postnatal Cerebral Cortex: Evidence for a Phenotypic Shift in Developing Nonpyramidal Neurons

In the adult rat cerebral cortex the calcium-binding proteins parvalbumin and calbindin D28k are present in essentially non-overlapping populations of GABAergic interneurons. These proteins follow different developmental patterns in the cortex: calbindin D28k-immunoreactive nonpyramidal neurons are abundant until the second postnatal week and decrease markedly thereafter; it is at this time that parvalbumin immunoreactivity develops in cortical nonpyramidal neurons. To determine whether parvalbumin-immunoreactive neurons derive from calbindin D28k-positive cells we used double-immunofluorescence studies for both calcium-binding proteins, together with combined immunocytochemistry for calbindin D28k and in situ hybridization for parvalbumin mRNA during postnatal development. Double-labelled cells were found in all cortical layers between P9 and P21, coinciding with the onset of parvalbumin expression. The percentage of colocalization of the two calcium-binding proteins depended on the age and layer examined. Colocalization reached a peak (80–100%) during the second postnatal week in layers II–IV and VI and decreased thereafter to adult levels by the end of the third postnatal week. Double-labelled neurons were rare in layer V at all ages studied. The present results indicate a phenotypic shift during the development of some cortical interneurons that halts the expression of calbindin D28k while parvalbumin expression starts. These findings agree with lineage analyses reporting that different types of nonpyramidal neuron arise from a common progenitor.     

Visuocortical epileptiform discharges in rabbits: Differential effects on neuronal development in the lateral geniculate nucleus and superior colliculus

We have studied the effects of interictal epileptiform discharges originating from the striate cortex on the development of the receptive field characteristics of neurons in the lateral geniculate nucleus (LGNd) and superior colliculus (SC) in neonatal rabbits. The paroxysmal discharges were generated by twice-daily injections of penicillin into an implanted cannula. Control injections of penicillin + penicillinase were given to the other striate cortex of the same animal. Similar experimental procedures were used to study the effect of such projected discharges on the LGNd neurons in adult rabbit.The results of the first experiment show that cortical epileptiform discharges, initiated in neonatal rabbits 7–9 days of age and continuing to 20–25 days of age, retard the normal development of LGNd cells. There was an abnormal increase of indefinite cells, cells failing to respond to any light stimulation, and a concurrent decrease of cells with concentric or uniform receptive fields. The second experiment shows that the effect of such interictal discharges is age-dependent; they did not cause any changes on the receptive fields of LGNd cells in adult rabbit. That these epileptiform discharges, occuring early in life, had long-lasting effects is demonstrated in the third experiment. An abnormal increase of uniform cells and a concurrent decrease of concentric cells was still present in adult rabbits which had interictal discharges in the striate cortex limited to the period of 7–9 dats to 21–25 days of age. The fourth experiment shows that the interictal discharges in neonatal rabbits do not affect the normal receptive field development of neurons in the SC.The present results demonstrate that asymptomatic interictal epileptiform discharges, produced without focal structural damages in immature brain, can affect the development of neuronal connectivity. These results may have some clinical implications in relation to our understanding about the learning and developmental disabilities exhibited in children who had episodic seizure discharges.     

Rapid rearrangement of intrinsic tangential connections in the striate cortex of normal and dark-reared kittens: lack of exuberance beyond the second postnatal week.

Clustered intrinsic connections in the kitten striate cortex originate from an unclustered, diffusely organized pattern prevailing during the first postnatal week. We have studied the progress of this reorganization and its dependence on visual input by determining the topographies of the intrinsic tangential connections at various postnatal ages by means of axonal tracing methods. Neurons were labeled either by diffusion of the carbocyanin dye DiI in animals ranging in age between 1 day and 30 days, or by retrograde transport of fluorescent microspheres in animals ranging in age between 7 days and 11 months. Quantitative evaluation of retrogradely labeled neurons revealed that during the first postnatal week, intrinsic tangential connections are organized in an unclustered fashion. During the second postnatal week a rapid rearrangement of connections occurs and is complete around postnatal day 11. The main events taking place during the course of this rearrangement are a decrease in the density of tangential connections and an arrangement of them in a clustered fashion. Once the clusters have been formed, the periodicity of the clustered pattern of connections and the size and distinctness of the clusters do not change. This means that the system of clustered tangential connections is adult-like at the end of the second postnatal week. Dark rearing affects neither the rapid rearrangement of horizontal connections into an adult-like system of clusters, nor the integrity of this clustered topography until the end of the first postnatal month. The overall distribution and the lateral extent of the tangential connections remain about the same during the postnatal period and are not affected by dark rearing until the end of the first postnatal month. We conclude that the clustered system of tangential connections in the cat's striate cortex is determined innately.     

Bihemispheric Axonal Bifurcation of the Afferents to the Visual Cortical Areas during Postnatal Development in the Rat

Numerous cortical neurons in the juvenile and adult rat project to visual areas of both hemispheres whereas the vast majority of subcortical structures projecting to the visual cortex send strictly ipsilateral projections (Dreher et al., 1990). In the present study, the authors have sought to determine whether this pattern of axonal bifurcation in the connectivity of the visual areas undergoes a change during postnatal development. Two retrograde fluorescent dyes were used, fast blue (FB) and diamidino yellow (DY). Large multiple injections of one of the dyes were placed in all visual areas of one hemisphere and a small injection of the other dye was placed in area 17 of the opposite hemisphere. Labelled neurons were observed in subcortical and cortical structures on the side of the small injection. The experiments were performed on ten neonatal albino rat pups aged between 3 and 12 postnatal days (p.n.d.) at the time of injection and the results were compared with those obtained in the juvenile and adult animals, as reported in the preceding paper. In the thalamus of newborn animals, neurons belonging to nuclei located away from the midline send strictly ipsilateral cortical projections. However, in the midline nuclei of the intralaminar thalamic complex, a small region of overlap was observed between neurons projecting ipsilaterally and neurons projecting contralaterally in animals aged less than 9 postnatal days. In addition, in these neonatal animals a small number of bilaterally projecting neurons was detected in this region of overlap. In all other subcortical structures examined (ventral tegmental area, diagonal band of Broca, claustrum), the laterality of the projection was the same in the newborn and the adult animals. In particular, in the claustrum of neonatal animals, as in adult animals, there was a large contingent of contralaterally projecting neurons and only a very small number of bilaterally projecting neurons. The results in the cortex contrast with those observed in subcortical structures. Whereas ipsilaterally projecting neurons were distributed in a broadly similar way in newborn and adult animals, the laminar and areal distribution of contralaterally projecting neurons in newborn animals clearly differed from those observed in the adult animals. Furthermore, double labelled neurons were more numerous in animals aged less than 12 days than in adults. The proportions of such bilaterally projecting neurons were computed with respect to the numbers of neurons sending ipsilateral projections to area 17. These proportions are constant at all ages in the claustrum and cortical area 8. In areas 18a, 29 and 35 on the other hand, the proportions of bilaterally projecting neurons increase after 5 days and reach a peak in the period extending from 9 to 11 days of age when more than half of the neurons projecting ipsilaterally also send an axonal branch to the contralateral cortex. In cortical areas 29 and 35, this peak is followed by a sudden drop to the adult level at 12 postnatal days, whereas the return to the adult level is gradual in area 18a. These results demonstrate that, in subcortical structures and in cortical area 8, the laterality of the afferent connections to the visual cortex does not change during postnatal development. By contrast, cortical areas 18a, 29 and 35 go through a stage when numerous cells send bifurcating connections to both hemispheres. The timing of the decrease in proportions of bilaterally projecting neurons in these areas suggests that numerous neurons retract their callosal axonal branch when the adult pattern of callosal connectivity is established at 9 - 11 days of age.     

Postnatal development of cholinergic neurons in the mesopontine tegmentum revealed by histochemistry

Cholinergic neurons in the laterodorsal tegmental nucleus (LDT) and pedunculopontine tegmental nucleus (PPT) play a role in the regulation of several kinds of behavior. Some of them, such as locomotion, motor inhibition or sleep, show dramatic changes at a certain period of postnatal development. To understand the neural substrate for the development of these physiological functions, we studied the development of cholinergic neurons in the LDT and PPT of postnatal and adult rats using histochemical staining of NADPH-diaphorase (NADPH-d) and immunohistochemical staining of choline acetyltransferase (ChAT) and the vesicular acetylcholine transporter (VAChT). At postnatal day 1 (P1), ChAT- and VAChT-stained cells localized more dorsally than those of NADPH-d-stained cells, and at P7 their distributions became similar to those of NADPH-d-stained cells. The number of NADPH-d-stained cells increased rapidly after birth, reaching the adult level by P7. In contrast, the number of ChAT- and VAChT-stained cells and the intensity of their staining decreased from P1 to P3 and then increased through P21. The volume of the LDT increased during the second postnatal week. These findings indicate that cholinergic neurons in the LDT develop their cholinergic properties during the second postnatal week and mature functionally thereafter. We discuss these results in light of the several physiological functions regulated by the cholinergic neurons in the mesopontine tegmentum.     

Postnatal development of glutamatergic, GABAergic, and cholinergic neurotransmitter phenotypes in the visual cortex, lateral geniculate nucleus, pulvinar, and superior colliculus in cats

We have analyzed the postnatal development of glutamatergic/aspartergic, GABAergic, and cholinergic neurotransmitter systems in the visual cortical Areas 17 and 18, lateral geniculate nucleus (LGN), pulvinar, and the visual and non-visual parts of superior colliculus (SC) in kittens. High-affinity uptake of D-aspartate (HA D-Asp), glutamate decarboxylase (GAD), and choline acetyltransferase (ChAT) activities were measured as a means of probing the development of the respective transmitter systems. HA D-Asp exceeded the adult level several-fold in all areas during the postnatal period which corresponded with the period of maximal dendritic/axonal branching patterns and synapse densities in the respective regions. GAD exhibited a gradual increase towards adult levels during the first month. The adult level was reached during postnatal week (PNW) 5-6 in Areas 17 and 18, during PNW3 within LGN, pulvinar, and the visual part of SC. In the nonvisual part of SC, the adult GAD level was reached as early as PNW2. ChAT exhibited biphasic developmental profiles in Areas 17 and 18. An initial peak of near adultlike activity in PNW2 was followed by a decline and subsequently by a slow increase towards adult levels during PNW5-17. ChAT developed very slowly in LGN and pulvinar, and in the latter structure only approximately 70% of the adult activity had been attained by PNW17. In both subdivisions of SC, ChAT had reached adult levels during PNW3-5. Dark-rearing from birth until PNW6 moderately attenuated GAD development in all areas and increased ChAT activity in Areas 17 and 18 but did not affect development of HA D-Asp in any part of the kitten visual system. Our neurochemical findings in the developing cat visual system are consistent with available evidence regarding localization of neurotransmitter systems, as well as postnatal changes in terms of cytoarchitectonics, synaptogenesis, functional development, and susceptibility to neonatal dark-rearing in visual pathways.     

Specificity in the Development of Vertical Connections in Cat Striate Cortex

The main efferent axons of pyramidal cells in layer 2/3 in the adult cat striate cortex make collateral connections specifically within layer 2/3 and layer 5 and avoid the intervening layer 4. Intracellular dye injections in vitro were used to determine how, during early postnatal development, this precise pattern of laminar connections was achieved. These investigations revealed that the pattern of collateral outgrowth was specific from the very earliest time that axons began sprouting collaterals. During the first postnatal week, sprouts were seen exclusively within layers 2/3 and 5; no evidence for a transient connection to layer 4 was observed. Furthermore, collaterals emerged simultaneously within layers 2/3 and 5, despite the large difference in the postmigratory ages of the two layers. By the end of the second postnatal week, the adult number of collaterals was achieved. Further elaboration of the local arbors occurred by repeated branching of already existing collaterals, rather than by addition of new collaterals to the main axon. These results demonstrate that the formation of local connections between cortical layers is highly specific, in contrast to the development of clustered horizontal connections by these same cells within layers 2/3 and 5, which involves extensive remodelling of local connections.     

Postnatal maturation of nonpyramidal neurons in the visual cortex of the cat

A Golgi study of nonpyramidal neurons in the visual cortex of kittens aged from 1 to 80 days revealed that different neuronal types undergo a differential sequence of maturation. The earliest nonpyramidal cells to differentiate are large multipolar cells of layers 3-5, which appear around birth and whose axons gradually establish long lateral, intracortical connections. They are followed by spiny stellate cells of layer 4, which appear in the first postnatal, week, and by neuroglioform cells in layers 4 and 5, a cell type which at 10 days displays a highly differentiated axonal plexus. In general, most classes of local axon cells can be identified by the end of the second week, though still possessing a very immature morphology, the axonal-tuft cells of layer 2 maturing later, in the third week. With some exceptions, most neurons exhibit an adultlike axonal arborization by the end of the first month; however, immature chandelier terminals are observed until the 40th day, and in kittens aged from 30 to 80 days, the vertical terminal segments of chandelier cells are larger than in the adult. Some neuronal types seem to present an exuberant growth of axonal fibers in the late postnatal period and a subsequent reduction up to the adult stage.     

Postnatal changes in the distribution of acetylcholinesterase in kitten striate cortex

We have traced the postnatal development of axons and cells in kitten striate cortex that contain acetylcholinesterase (AChE) by using a modification of Koelle's histochemical method. The maturation of AChE-positive axons was not found to be fully complete until at least 3 months of age, and was characterized by several distinct developmental trends. AChE-positive fibers in layers IVc-VI proliferate rapidly after birth until, by 4 weeks postnatal, they appear to exceed the adult density. They remain at this level as late as 8 weeks and then decrease to the adult density by 13 weeks. In contrast, the AChE-positive fibers in layer I do not show a substantial increase in density until 6 weeks of age and the adult level is not achieved before 3 months postnatal. Finally, the density of AChE-positive fibers in layers II and III appears to increase gradually from birth until the mature pattern is reached at about 6 weeks. AChE could also be localized histochemically to cell bodies whose position and appearance depended on postnatal age. Stained cells first appeared in the white matter subjacent to layer VI shortly after birth. By 2 weeks of age, most cells in layer VI were also AChE positive. The staining of these cells gradually disappears over the next 2 months until, at 3 months of age, there are no AChE-positive cells in cat striate cortex. However, a subpopulation of stained neurons appears in layer V by 1 year of age that persists throughout adulthood. The possible contributions of acetylcholine and AChE to the postnatal development of kitten striate cortex are discussed.     

Two converging brainstem pathways mediating circling behavior

Ipsiversive circling results from stimulation of the rostromedial tegmentum (RMT) or medial pons (PONS), and contraversive circling results from stimulation of the superior colliculus (SC). To determine whether these sites are functionally connected, the collision method of Shizgal, Bielajew, Corbett, Skelton and Yeomans (1980) was used in rats. Pairs of stimulation pulses were presented to two sites, and the degree of collision between stimulation-evoked action potentials was assessed by measuring the frequency required for circling at short and long intrapair conditioning-testing (C-T) intervals. Collision was evidenced when the required frequencies were higher at short C-T intervals than at long C-T intervals. Collision of 46-62% was observed between RMT and PONS, and collision of 15-29% was observed between SC and PONS. Sites from which collision was obtained were located along the trajectories of the medial tegmental tract and the crossed tectospinal pathway. Refractory periods in all sites were similar, ranging from 0.3 to 1.7 ms. Conduction velocities of axons connecting RMT and PONS and SC and PONS were comparable, ranging from 0.8 to 13.3 m/s and 1.7 to 13.8 m/s, respectively, with lower conduction velocities associated with more ventral pontine sites. Thus, RMT and PONS, and SC and PONS are connected by myelinated axons that mediate circling.     

Cell death organizes the postnatal development of the trigeminal innervation of the cerebral vasculature

In the adult trigeminal ganglion single cell bodies that innervate the middle cerebral artery (MCA) are different from but situated near to one or more cell bodies that innervate the forehead (O'Connor and Van der Kooy, submitted). Multiple fluorescent retrograde axonal tracing in postnatal day 3-90 rats was employed to describe the development of this adult pattern of trigeminal projections. We found that close to 90% of the cells that innervate the MCA at postnatal day 5 (PND 5) are eliminated by PND 90. Less than 20% of the ganglion cells innervating the forehead die over the same postnatal period. Subpopulations of cells in the ganglion were observed to have a maximal rate of death during different postnatal periods. First, 15-20% of the cells throughout the ophthalmic division die between PND 5 and PND 10. Second, a small population of cells that had early projections to the contralateral MCA die out completely by PND 22. Third, cells with a projection only to the MCA die primarily between PND 10 and PND 54. Fourth, during the first postnatal week there are many cells that project to both the MCA and the forehead; however, 90% of this population dies by PND 90. This elimination is observed latest in the postnatal period, with these cells exhibiting their greatest rate of cell death between PND 22 and PND 90. Thus, cell death is the primary postnatal mechanism that produces this organization in the ophthalmic division of the trigeminal ganglion and retraction of axonal collaterals is a minor mechanism. We suggest that the latest period of death in cells with divergent artery and forehead projections as well as the ultimate persistence of some artery projecting cells beyond PND 90, may be due to the larger peripheral fields of innervation of these trigeminal ganglion cells.     

Cytochrome oxidase activity in rat retinal ganglion cells during postnatal development

In this study, the metabolic activity of rat retinal ganglion cells during postnatal development has been examined in vivo using cytochrome oxidase histochemistry. The intensity of staining was measured by optical densitometry. The activity of cytochrome oxidase in retinal ganglion cells progressively increased from postnatal day 0 (P0) and reached a peak during the second week of postnatal development (P10-P14) and declined thereafter. Our data show that the increased levels of cytochrome oxidase seen in developing retinal ganglion cells occur at the same time, when neuronal maturity and synaptogenesis reach their peaks.     

Postnatal development of phrenic motoneurons in the cat

The postnatal growth of phrenic motoneurons in the cat was studied using retrograde transport of horseradish peroxidase (HRP). The mean somal surface area of these developing motoneurons increased 2.5 times from day 3 to adult while the mean somal volume increased four-fold. This change in mean somal surface area during postnatal development was found to be correlated with the change in mean axonal conduction velocity measured from phrenic motoneurons.     

Properties of single motor units of the extensor digitorum brevis in elderly humans

The axonal conduction velocity and voluntary discharge properties of single motor units of the extensor digitorum brevis were studied in elderly subjects. The results were compared with the findings of a previous study of younger subjects. The range of the axonal conduction velocities differed only slightly between the age groups, the elderly subjects having velocities of 25-52 m/sec and the younger subjects 30-54 m/sec. In both age groups, motor units firing tonically within the range of 10-30 Hz had axonal conduction velocities below 45 m/sec, and motor units firing mainly phasically within the range of 20-60 Hz had axonal conduction velocities above 40 m/sec. Motor units with an intermediate type of discharge pattern were more frequently seen in the elderly. In the elderly subjects, these motor units had axonal conduction velocities distributed within the whole range, while in the younger subjects, they had axonal conduction velocities in the middle of the range. An autopsy study showed an increased proportion of muscle fibers with intermediate characteristics of ATPase staining in the elderly subjects. It is suggested that the decreased differentiation of the muscle fiber histochemistry might be secondary to the decreased differentiation of the motoneuron firing properties.     

The ipsilateral retinocollicular projection in the rabbit: an autoradiographic study of postnatal development and effects of unilateral enucleation

The postnatal development of the ipsilateral retinocollicular projection in the rabbit and the effects of unilateral enucleation (performed on the day of birth, day 0) on that development were studied by using the anterograde axonal transport of tritiated proline injected intraocularly. Material from 1-, 6-, and 10-day-old (i.e., at days 1, 6, and 10) and adult animals was examined. On day 1, autoradiographically labelled optic fibers from the ipsilateral eye formed distinct patches and bands within the superior colliculus (SC), which were restricted primarily to the lateral one-half and anterior one-third to one-half of the nucleus. At subsequent ages no major changes in the location of this projection were found for normal animals or animals enucleated on day 0 (0-DE animals). From dorsal-view reconstructions, the pattern of the ipsilateral projection appeared wedge-shaped with a broad base aligned with the lateral SC border for all normal and 0-DE animals at the various postnatal ages examined. In normal animals the surface area of this projection increased with age and maintained a constant proportion of the increasing surface area of the total SC. In 0-DE animals the surface area of the projection initially increased more rapidly than in normal animals. Thus, by day 6 the area was already within the normal adult range but did not exceed this range at later postnatal ages. The only obvious difference in the appearance of the ipsilateral retinocollicular projection between normal and 0-DE animals at corresponding ages was an enhanced radial distribution of the projection across laminae in the 0-DE animals. Taken together these findings suggest that, in the rabbit, once topographically appropriate connections are established between the SC and the ipsilateral retinal projection, they are maintained regardless of substantial postnatal growth of the SC and removal of the contralateral retinal projection to the SC.     

Development of antioxidant enzymes in rat brain and in reaggregation culture of fetal brain cells.

The development of antioxidant enzymes in rat brain and reaggregation cultures of fetal brain cells was studied from embryonic day 15 to postnatal day 45. Both in vivo and in culture, the copper-zinc superoxide dismutase activity first increased and then decreased with age, whereas the manganese superoxide dismutase activity increased throughout the period. Catalase showed a maximum activity at day 5 after birth, thereafter decreasing to adult level around day 30, both in vivo and in culture. The glutathione peroxidase activity increased from the first week after birth and reached adult level at day 45. In culture, the activity of this enzyme was slightly lower. The good correlation between the development of the antioxidant enzymes in vivo and in culture suggests that reaggregation cultures might be a valuable system for studying defense mechanisms against free radicals in the brain.     

Changes in rapidly transported proteins in developing hamster retinofugal axons

Proteins synthesized in retinal ganglion cells and conveyed to the terminals of optic tract axons in the rapid phase of axonal transport were analyzed at different developmental stages in the hamster. Animals between 2 d of age and adulthood were labeled intraocularly with 35S-methionine, and after a 4 hr survival time, the superior colliculus was dissected out, subjected to subcellular fractionation, and radiolabeled proteins in the particulate fraction analyzed by 2-dimensional gel electrophoresis and fluorography. The previously identified growth-associated phosphoprotein, GAP-43 (GAP-48, B-50, F1, pp46), was synthesized and transported at high levels in the neonate, but these levels declined precipitously after the second postnatal week. Immunohistochemical studies using a monospecific antibody showed that GAP-43 was localized along the entire length of retinal axons in the optic tract and target areas in P2 animals but was virtually absent in the adult visual pathway. By metabolic labeling, 2 proteins with molecular weights of about 230 kDa also showed a sharp decrease during development. In contrast, acidic proteins of 27 and 64 kDa, which were barely detectable in the neonate, increased steadily to become the most heavily labeled proteins of rapid axonal transport by the second postnatal week. Another group of proteins, of about 94-110 kDa, also rose to peak levels after birth but then declined. Temporal correlations between the molecular changes described here and the known anatomical events in optic tract development suggest that the synthesis and transport of particular membrane proteins may be directly related to the sequence of morphological changes.     

Developmental study of factors controlling microtubule in vitro cold-stability in rat cerebrum

The ability of crude cerebrum supernatant to stabilize, at a low temperature, cold-labile microtubules purified from beef brain was studied during development in the rat. The cold-stabilizing activity of the supernatant was low during the first postnatal week, rose significantly during the second postnatal week, and thereafter continued to increase to the adult level. The partial purification of stable-tubule-only polypeptide (STOP) from the supernatant showed that high amounts of this protein exist at all ages, especially in young animals. The age-related increase in the microtubule cold-stabilizing activity of the cerebrum supernatant resulted from a developmental decrease in factors inhibiting STOP activity. The present study shows that (1) a close temporal correlation exists between the in vitro and in situ acquisition of cold-stable microtubules during brain development, (2) STOP activity may account for this acquisition, and (3) STOP activity is controlled by inhibiting factors that decrease with age, in turn allowing increased stability of the microtubular apparatus, a necessary condition for the development of neuronal processes.