Postnatal development of biotinylated dextran amine-labeled corpus callosum axons projecting from the visual and auditory cortices to the visual cortex of the rat.

The distribution and morphology of developing corpus callosum (CC) axons in rat visual cortex was studied by unilateral application of the in vivo anterograde tracer biotinylated dextran amine (BDA) to the visual or auditory cortex of newborns through adults. Changes in the distribution and morphology of CC axons during development were observed. Following BDA placement only in visual cortex, nearly all CC projections were to visual cortex (homotopic CC projections). At postnatal day (PND) 5-8, labeled CC axons were found throughout the contralateral visual cortex, including area 17; these CC axons could be followed from the white matter to layer I. By PND 13, few CC axons were found in medial area 17, indicating the existence of transitory CC axons in area 17 at younger ages. Morphological changes were investigated at the area 17/18a border and showed that CC axon collaterals were not formed until PND 8, and terminal arbors were not visible until PND 13; by PND 17, the adult CC-axon terminal pattern was present. At all ages, only a few heterotopic CC projections from visual to auditory cortex were found in the gray matter, although many labeled CC axons extended laterally into the white matter underlying the auditory cortex. Following BDA placement only in auditory cortex, CC projections to both auditory (homotopic CC projections) and visual (heterotopic CC projections) cortex were observed. At all ages, the homotopic CC projections were present throughout the auditory cortex, but were not distributed homogeneously; densely labeled CC axons showed a distinct columnar organization. The heterotopic CC projections were present in all visual cortical areas, including medial area 17, in significant numbers until PND 24, but were mostly eliminated by PND 28, at which time a labeling pattern similar to the adult was found. Thus, most of the heterotopic CC projections were transitory. The present study confirms the existence of transitory CC axons projecting through all layers of the visual cortex, as revealed by DiI, and extends the DiI results by showing that these transitory CC axons arise from both homotopic and heterotopic origins. Furthermore, different sources of transitory CC axons have different timetables for elimination.

Effect of prenatal alcohol exposure on midsagittal commissure size in rats.

BACKGROUND: Fetal alcohol exposure in humans can cause a variety of brain and behavioral abnormalities. The brain abnormalities include defects in the corpus callosum that range from total absence (agenesis) to reduction in size or thickness. Determination of the critical alcohol level or time period of exposure to produce these effects is difficult because of the lack of control of possible mitigating factors. METHODS: The present study was undertaken to examine possible relationships between midsagittal corpus callosum dimensions and prenatal alcohol level as measured by blood alcohol concentration, as well as prenatal period of exposure as measured by first- or second- or first- plus second-trimester equivalents in a rat model. In addition to the corpus callosum, the hippocampal and anterior commissures were also examined. Pregnant mothers were given a single daily dose of alcohol by intragastric gavage; four different doses were tested in different mothers. Peak blood alcohol concentration was determined at one of four hourly intervals thereafter. Control pregnant mothers were pairfed to individual alcohol treated mothers and handled accordingly, but no alcohol was given. Other controls consisted of normal, untreated pregnant mothers. RESULTS: The results show all measures of corpus callosum and anterior commissure were not affected by any dose of alcohol during any time period. However, higher BAC levels during prolonged periods of alcohol exposure were associated with reduced size of the hippocampal commissure. CONCLUSIONS: The results suggest that additional experimental factors not included in the present study are responsible for the effects observed in humans.

The role of pioneer neurons in the development of mouse visual cortex and corpus callosum.

The primordial plexiform neuropil is very critical to neocortical development. The pioneer neurons, mainly Cajal-Retzius cells in the marginal zone, and subplate neurons in the subplate, differentiate from the primordial plexiform neuropil. In this study, the development of corpus callosum, visual cortex, and subcortical pathways has been observed in C57BL/6 mice with various methods, such as DiI labeling in vitro and in vivo, Dil and DiA in vitro double labeling, immunocytochemistry, and in vivo BrdU and Fast Blue labeling. As early as E14, the primordial plexiform neuropil can be found in the telencephalic wall, and it contains many pioneer neurons. On E15 the primordial plexiform neuropil differentiates into the marginal zone and the subplate. Cajal-Retzius cells exist in the marginal zone, and subplate neurons are in the subplate. Either Cajal-Retzius cells or subplate neurons have long projections toward the ganglionic eminence, suggesting that they migrate tangentially from the ganglionic eminence. Cajal-Retzius cells are involved in radial migration, and subplate neurons participate to guide pathfinding of subcortical pathways. This study reveals how the pioneer neurons, through radial and tangential migration, play an important role in neocortical formation and in the pathfinding of the corpus callosum and subcortical pathways. Furthermore, DiI labeling in vivo has demonstrated the presence of pioneer neurons all along the corpus callosum pathway, especially in the midline. This suggests that pioneer neurons may also play a role in guiding the pathfinding of the corpus callosum.

Neuropeptide Y- and somatostatin-immunoreactive axons in the corpus callosum during postnatal development of the rat.

Corpus callosum (CC) projections in adult mammals were generally thought to be excitatory and to use excitatory amino acids as their transmitters. Little information has been available about the electrical properties and neurochemical status of developing CC connections. The present study investigated the chemical status of rat CC axons during postnatal development by using antibodies to neuropeptide Y (NPY) and to somatostatin (SOM). Both NPY-immunoreactive (ir) and SOM-ir axons were found in the CC of the rat from newborn through adult; however, the number of SOM-ir CC axons is less than that of NPY-ir CC axons at corresponding ages. The density of both NPY-ir and SOM-ir CC axons initially increased, then peaked, and finally decreased to the mature level. In the adult, only a few NPY-ir and SOM-ir CC axons were found in the CC. These results indicate that many NPY-ir and SOM-ir CC axons are transitory during early postnatal development. The results also suggest that the functions of CC connections in adult mammals may be different from that of developing ones. The present results as well as the previous results demonstrate that both developing and mature CC connections are chemically heterogeneous.

Disruption of the Saccharomyces cerevisiae homologue to the murine fatty acid transport protein impairs uptake and growth on long-chain fatty acids.

The yeast Saccharomyces cerevisiae is able to utilize exogenous fatty acids for a variety of cellular processes including beta-oxidation, phospholipid biosynthesis, and protein modification. The molecular mechanisms that govern the uptake of these compounds in S. cerevisiae have not been described. We report the characterization of FAT1, a gene that encodes a putative membrane-bound long-chain fatty acid transport protein (Fat1p). Fat1p contains 623 amino acid residues that are 33% identical and 54% with similar chemical properties as compared with the fatty acid transport protein FATP described in 3T3-L1 adipocytes (Schaffer and Lodish (1994) Cell 79, 427-436), suggesting a similar function. Disruption of FAT1 results in 1) an impaired growth in YPD medium containing 25 microM cerulenin and 500 microM fatty acid (myristate (C14:0), palmitate (C16:0), or oleate (C18:1)); 2) a marked decrease in the uptake of the fluorescent long-chain fatty acid analogue boron dipyrromethene difluoride dodecanoic acid (BODIPY-3823); 3) a reduced rate of exogenous oleate incorporation into phospholipids; and 4) a 2-3-fold decrease in the rates of oleate uptake. These data support the hypothesis that Fat1p is involved in long-chain fatty acid uptake and may represent a long-chain fatty acid transport protein.

A modification of biotinylated dextran amine histochemistry for labeling the developing mammalian brain.

Biotinylated dextran amine (BDA) has proven to be an excellent anterograde tracer in adult mammalian brains, having some advantages over other anterograde tracers such as Phaseolus vulgaris-leucoagglutinin (PHA-L) and biocytin. However, results are inferior when BDA is used in neonatal mammals. To improve the sensitivity and quality of BDA labeling in neonatal mammalian brains, the tetramethylbenzidine-sodium tungstate (TMB-ST) method for horseradish peroxidase (HRP) histochemistry was modified and used in BDA histochemistry. After BDA application to the visual cortex of neonatal rat and cat, contralateral and ipsilateral cortical and subcortical regions were examined for BDA-labeled exons and terminals. The modified BDA histochemistry produced corpus callosum (CC) axons in neonatal rat and cat that were heavily and continuously labeled. The distribution, trajectories, branching and termination of individual CC axons, and even possible axon-axon contracts, were clearly identified in exquisite detail, even at low magnification. The quality of BDA labeling in the ipsilateral lateral geniculate nucleus and superior colliculus was similar to that of the CC axonal labeling. These results indicate that the modified BDA histochemistry provides a very sensitive and reliable approach to revealing the detailed distribution and morphology of projecting axons and terminals in the developing mammalian nervous system.

The corpus callosum provides a massive transitory input to the visual cortex of cat and rat during early postnatal development.

Studies of corpus callosum development in cat revealed that the callosum must be intact during postnatal month 1 if normal visual development is to occur [11-20,25]. The use of DiI, a lipophilic carbocyanine dye that is an in vitro membrane tracer, permits a detailed search for morphological evidence to account for these functional results because many cells can be simultaneously labeled in their entirety. To search for morphological evidence, the corpus callosum was labeled in vitro with DiI in tissue from cats aged 2-277 days old [21]. To determine whether there was consistent callosal development in mammals, similar studies were carried out in tissue from rats aged 0 days old through adult [22]. Hemispheres were coronally sectioned 1-24 months later. Sections were reconstructed in photomontages to show the overall distribution of corpus callosum projections, as well as provide details about the locations of individual corpus callosum axons and their presumed terminals. The distribution of corpus callosum projections, examined in visual cortex of cat and rat, changed significantly during development. During early postnatal development, callosal axons extended throughout visual cortex to layer I. Numerous varicosities on callosal axons were located en passant and at axon terminals in layer I. In the following weeks, the density of callosal projections was reduced in all cortical areas, although many axons still extended to layer I. By postnatal month 2 the callosal axons were predominantly near the borders between adjacent cortical areas. Thus, for several postnatal weeks, many elaborately formed transitory corpus callosum axons are distributed throughout visual cortex. The transitory callosal axons appear to have terminals in layers I-VI. If some of these terminals were to for synapses, the corpus callosum could provide an extensive input to layers I-VI throughout visual cortex while the majority of cortical microcircuitry is being established.

Confirmation of the existence of transitory corpus callosum axons in area 17 of neonatal cat: an anterograde tracing study using biotinylated dextran amine.

Corpus callosum (CC) axons in visual cortex were labeled anterogradely by in vivo biotinylated dextran amine (BDA) in neonatal cat at postnatal day (PND) 6, 10 and 15. Labeled CC axons were distributed throughout the visual cortex including medial area 17. The number of CC axons in medial area 17 increased from PND 6 to PND 10, and then decreased from PND 10 to PND 15. At PND 15, few CC axons could be followed into the grey matter in medial area 17. Thus, BDA labels transitory CC axons that extend through all cortical layers in medial area 17, confirming the results revealed by in vitro DiI labeling.

Neuropeptide Y immunoreactive axons in the corpus callosum of the cat during postnatal development.

Many immunocytochemical studies have identified different types of neurotransmitters localized in the corpus callosum (CC) axons in the adult mammal. Few studies have looked at the development of different neurochemically identified CC systems. Previous studies on the development of cat CC axons have indicated that a large number of transitory CC axons project to the cortex during early postnatal development. The present study focuses on the development of one neurochemically identified group of CC axons in the cat, labeled with an antibody against neuropeptide Y (NPY), to determine if this group participates in transitory CC axonal growth. Cats at specified ages from birth to adulthood were studied with a routine method of immunocytochemistry for antiserum to NPY. NPY-immunoreactive (ir) CC axons were detected at all stages examined, from newborn to adult; the peak density occurred during postnatal weeks (PNW) 3-4. During PNW 1-2, the density of NPY-ir CC axons increased gradually; some NPY-ir axons at this age had growth cones located within the CC bundle between the cerebral hemispheres. The density of the NPY-ir CC axons decreased gradually during PNW 5-7, and from PNW 8 to maturity only a few NPY-ir CC axons were observed. These results indicate that at least two types of NPY-ir CC axons (i.e., transitory and permanent) exist during development, and that most of these axons are eliminated or only express NPY-ir for a short period during development. The results also indicate that neurochemical subsets of CC axons participate in the extensive transitory growth observed by means of the membrane tracer DiI but they may follow unique developmental timetables.

Urea-derived cyanate forms epsilon-amino-carbamoyl-lysine (homocitrulline) in leukocyte proteins in patients with end-stage renal disease on peritoneal dialysis.

Carbamoylated proteins have been located by using a site-specific polyclonal antihomocitrulline antibody and a fluorescent secondary antibody in leukocytes from patients with end-stage renal disease who were undergoing maintenance continuous ambulatory peritoneal dialysis. A covalent reaction with urea-derived cyanate and the epsilon-amino group of lysine forms homocitrulline residues in carbamoylated proteins. Isocyanic acid, the reactive form of cyanate, is spontaneously formed from urea in aqueous solution at physiologic pH and temperature. In washed, fixed monolayers of cells, an intracellular fluorescent antigen-antibody complex was located throughout the cytoplasm of polymorphonuclear neutrophils (PMNs) and monocytes from 11 patients with blood urea nitrogen (BUN) levels ranging from 32 to 102 mg/dl who were undergoing dialysis for 2 to 135 months. A punctate fluorescence present in the cell surface proteins of living cells demonstrated that lysine residues in the external domain of proteins were carbamoylated, forming homocitrulline. In contrast, we found a perinuclear fluorescence in PMNs in normal subjects with no history of renal insufficiency and BUN levels of 6 to 19 mg/dl. This suggests that homocitrulline is located in carbamoylated proteins within the perinuclear membrane, a structural organelle continuous with the endoplasmic reticulum. It appears that continuous exposure to urea-derived cyanate in low levels results in increasing carbamoylation of stable proteins over the PMN's lifetime. When normal PMNs were exposed to 120 mmol/L cyanate ion in vitro for 10 to 30 minutes, the ability of PMNs to release microbicidal superoxide was strongly inhibited. Thus protein carbamoylation may provide a regulatory mechanism. The altered function of PMNs in renal disease may be due in part to the posttranslational modification of proteins by urea-derived cyanate.

Transitory corpus callosum axons projecting throughout developing rat visual cortex revealed by Dil.

Anatomical tracing was used to determine the extent and distribution of CC axons in mammalian visual cortex. Postnatal development of rat CC was studied by in vitro callosal labeling with the lipophilic carbocyanine dye Dil in 59 rats. Solid Dil crystals were placed in the mid-sagittal region of the CC in aldehyde-fixed brain slabs. Coronal sections through visual cortex were photographed and reconstructed to show the overall distribution of Dil-labeled callosal projections as well as the locations of individual callosal axons and their presumed synaptic boutons. During postnatal weeks 1 and 2, CC axons were found to project to layer I throughout the entire mediolateral extent of areas 17, 18a, and 18b. Numerous varicosities on callosal axons are located en passant and at axon terminals in layer I. During postnatal week 3 the tangential density of callosal projections was significantly reduced, so that fewer callosal axons extended to layer I throughout areas 17, 18a, and 18b than in younger postnatal rats. However, at this age some CC axons could still be found extending to layer I throughout the mediolateral extent of areas 17, 18a, and 18b. By postnatal week 4 the tangential distribution of callosal projections was greatly restricted; most callosal axons projecting to layer I were located at the borders of the visual cortical areas. Nevertheless, there were still callosal axons projecting into cortex and terminating in supragranular and infragranular layers in areas 17, 18a, and 18b; this was most pronounced in area 18a. Thus, in the rat there are many elaborately formed transitory CC axons projecting throughout visual cortex for several weeks postnatal. These projections extend to layer I and have varicosities in all cortical layers. With increasing age, fewer axons extended to layer I; subsequently most axons not at cytoarchitectonic borders fail to extend to layer I. If some of the varicosities on the transitory rat callosal axons were to form synapses, there would be extensive opportunities for the CC to provide input to all layers of visual cortical areas while cortical microcircuitry is being established. The same type of study in the cat has shown similar results during early postnatal development. Cat CC axons project to all parts of primary and association visual cortical areas; even in regions found to be acallosal in the adult, the neonatal callosal axons extend through all layers of cortex to reach layer I (Elberger, 1993).(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of transitory corpus callosum axons projecting to developing cat visual cortex revealed by DiI.

Functional studies of the development of the corpus callosum in the cat have shown that an intact callosum during postnatal month 1 is necessary for normal visual development. In vivo tracing techniques have not provided enough information on corpus callosum connectivity to fully evaluate the evidence for a morphological mechanism for the functional effects of neonatal callosum section. However, lipophilic in vitro membrane tracers permit a more detailed search for such evidence because the entire limit of many cells can be labeled simultaneously. To investigate the morphological basis for the observed functional results in cats, the corpus callosum was labeled in vitro with the carbocyanine dye, DiI. Crystals of DiI were placed in the midsagittal callosum in tissue from 2 to 277-day-old cats. Tissue was coronally sectioned 3-22 months later. Sections were photographed and reconstructed to show the overall distribution of corpus callosum projections, as well as the locations of individual corpus callosum axons and their presumed terminals. The distribution of corpus callosum projections, examined in cortical areas 17-19, 7, and posterior medial lateral suprasylvian cortex, changes significantly during development. During postnatal week 1, callosal axons extend throughout these cortical areas to layer I. Numerous varicosities on callosal axons are located en passant and at axon terminals in layer I. During postnatal week 2, the density of callosal projections is reduced in all cortical areas, although many axons still extend to layer I. By postnatal month 2, the callosal axons extending to layer I are predominantly near the border with adjacent cortical areas; in the nonborder regions of these areas, many axons extend to layer VI while a much smaller number of axons extend to layers II-V. By postnatal month 3, the callosal projections to supragranular layers are almost exclusively restricted to cytoarchitectonic border regions; in the remaining regions, including medial area 17, there are occasional axons extending to the supragranular layers and only a moderate number of axons extending to infragranular layers. Thus, a substantial number of elaborately formed transitory corpus callosum axons, distributed throughout visual cortex, exist for several weeks during postnatal development; in area 17, these axons are found in central through peripheral visual field representations. The transitory callosal axons appear to have axon terminals in layer I as well as en passant terminals while extending through layers II-VI. If some of these terminals were to form synapses, there would be extensive opportunities for the corpus callosum to provide input to layers I-VI throughout visual cortex during the period of development in which cortical microcircuitry is being established.

HRP reacted with the chromogen o-tolidine produces whole-cell reaction product at light and electron microscope levels: negative effects of sucrose and Golgi staining on benzidine reactions.

Studies of pathway microcircuitry often require electron microscope analysis. To facilitate these analyses, methods for labeling cells in their entirety are extremely useful. Furthermore, such a method would be most useful if the label would be completely confined by the cell membrane so that second labels for synapse identification could be used. No existing method reliably and repeatably produces this kind of a result. In seeking to develop such a method, a seldom-used chromogen for horseradish peroxidase (HRP) was found which produced superlative results for light and electron microscope analysis. o-Tolidine (3,3'-dimethylbenzidine) reacted with HRP produces a very electron-dense reaction product distributed uniformly throughout the cytoplasm and nucleoplasm; membranes are unobscured so that mitochondria, lysosomes, Golgi apparati and endoplasmic reticula are well defined. The reaction product extends into cellular processes of all sizes, including processes with cell bodies not within the plane of section, and is easily visualized at even the lowest electron microscope magnification. The HRP reaction product is completely confined by the cell membrane, thus terminals presynaptic to labeled cells remain distinct. However, the o-tolidine/HRP reaction product is negatively affected by exposure to oxidizers. In tissue exposed to sucrose before or after being reacted with o-tolidine, the HRP reaction product is less electron dense and is found only in lysosomes outside the nucleus or occasionally in proximal cellular processes. The o-tolidine/HRP reaction product is similarly affected when exposed to potassium dichromate for Golgi staining. For some other benzidine compounds, the chromogen/HRP reaction product is also negatively affected by exposure to these chemicals. Therefore, o-tolidine is a superior chromogen for HRP, labeling cells with details similar to that found for cells intracellularly injected with HRP.

Double-labeling of tissue containing the carbocyanine dye DiI for immunocytochemistry.

The fluorescent carbocyanine dye DiI can be used for retrograde and anterograde labeling of neuronal pathways. To investigate the possible neurochemical identity of DiI-labeled neuronal cell bodies and terminals, we used a procedure for double-labeling of the same tissue with antisera to specific neuroactive substances. This procedure involves visualizing the immunohistochemical label with an FITC-conjugated secondary antiserum. Both labels can be viewed in the same tissue by fluorescence microscopy, and individual cell bodies and processes double-labeled with DiI and antiserum can be identified by switching between filter sets appropriate for rhodamine (to see the DiI labeling) and for fluorescein (to see the immunhistochemical labeling). The method has been used with primary antisera to excitatory and inhibitory amino acid neurotransmitters, as well as to neuropeptides, and is likely to be useful with antibodies against a wide variety of substances. Several other immunocytochemical methods were found to be incompatible with DiI labeling.

Spatial frequency thresholds of single striate cortical cells in neonatal corpus callosum sectioned cats.

Following section of the corpus callosum at 1-6 postnatal weeks in cats, behavioral visual acuity was measured binocularly and monocularly from 6-29 postnatal weeks; physiological determination of spatial frequency thresholds of single striate cortical cells was performed when the cats were at least 8 months old. Results were compared between cats with callosum section at each postnatal week, as well as with normal cats. Cats with callosotomy at 1-3 postnatal weeks had deficits in behavioral visual acuity, and the deficits were greatest in the youngest operated cats. Cats with callosotomy at 1-2 postnatal weeks failed to resolve as high spatial frequencies as did normal cats, and the resolution of the 1 week operated cats was lower than the resolution of the 2 week operated cats. Cats with callosotomy at 3-6 postnatal weeks had spatial frequency thresholds that were equivalent to those of normal cats. To determine what kinds of striate cells had reduced spatial resolution following neonatal corpus callosum section, cells were categorized according to class (Simple, Complex), receptive field location (Central, Peripheral), and monocular behavioral acuity eye performance (Better Eye, Worse Eye). Cats with corpus callosum section during postnatal week 1 had the lowest spatial resolution for all cell categories compared to all groups tested. However, cats with callosum section during postnatal week 2 had normal spatial frequency thresholds for Simple, Central and Better Eye categories. The cats with callosum section in postnatal weeks 3-6 had normal spatial frequency thresholds for all cell categories. For corpus callosum sectioned cats with and without visual deficits, and for normal cats, visual acuity measured behaviorally is significantly related to visual acuity measured physiologically. The results show that neonatal corpus callosum section in cats can affect behavioral visual acuity, as well as the spatial frequency thresholds of many categories of striate cortical cells. However, callosum section at different ages affects different populations of cortical cells. Furthermore, the results suggest that neonatal corpus callosum section may directly affect a single fundamental property of cells in primary visual cortex with a resulting disruption of many visual functions.

Selective labeling of visual corpus callosum connections with aspartate in cat and rat.

Tritiated neurotransmitter candidates were unilaterally injected in visual cortical regions with abundant corpus callosum connections. D-aspartate (Asp) or gamma-aminobutyric acid (GABA) was injected along the area 17/18 border in cat, and the area 17/18a and 17/18b borders in rat. Retrograde Asp label was found contralaterally in supragranular and infragranular laminae in areas 17, 18, 19, PMLS, and PLLS in cat, and in areas 29, 18b, 17, and 18a in rat. No contralateral GABA label was found in cat or rat. Thus, the cat and rat corpus callosum may use Asp or a related substance as a neurotransmitter.

Binocularity and single cell acuity are related in striate cortex of corpus callosum sectioned and normal cats.

Cats with corpus callosum section at 4-37 postnatal days underwent electrophysiological recording in striate cortex after they reached adulthood. Single cells were examined to determine both their ocular dominance and spatial frequency threshold (acuity). Data were analyzed for each cat according to the extent of binocular interaction (binocularity) and the mean striate acuity. Both visual functions were found to be significantly related to the age at which the corpus callosum section occurred, with the greatest deficits in visual function resulting from callosum section at the younger ages. There was a significant relationship between striate binocularity and acuity in the callosum sectioned, as well as in normal, cats. This suggests that visual resolution is at least partially determined by the ability to integrate information from both eyes.

Developmental interactions between the corpus callosum and the visual system in cats.

The neonatal corpus callosum is involved in the development of pathways or mechanisms that coordinate the inputs from the two eyes. Several related visual functions are permanently altered by the absence of the callosum during early development. As determined behaviorally and by visual evoked potentials, the normal amount of 90 degrees of the visual field in which both eyes respond to stimulation is almost completely eliminated. Also, there is a reduced number of binocular cells in striate cortical regions representing most of the visual field. In addition, behaviorally measured visual acuity is reduced. Changes in acuity and striate binocularity only result when a callosal section occurs during a critical period of the first postnatal month, and the earlier the surgery, the greater the changes. The lack of myelination during the first postnatal month indicates that the conduction properties of the callosum are poorly developed during its critical period. The pattern of callosal connectivity is probably significant for its role in development, but not all callosal fibers are necessary for normal visual development. Developmental plasticity of callosal connections has been demonstrated for striate cortex, but now it has also been demonstrated for the claustrum. Thus, the callosal role in regions representing both central and peripheral visual field, in neocortical and non-neocortical brain areas should be reassessed.

Plasticity of claustroneocortical connections via the corpus callosum in the cat.

The cat's distribution of claustral cells that project to the contralateral visual cortex via the corpus callosum was examined. Horseradish peroxidase (HRP) was applied to severed callosal axons to label a heterogeneous population of callosal connections. Cats reared with optically induced strabismus, and Siamese cats, had HRP-filled cells extending more ventrally in the claustrum than in controls. In these groups the compaction of labeled cells was higher than in controls and the amount of increased labeled area was not dependent on the resulting eye alignment. This indicates possible plasticity of visual claustrocallosal connectivity in cats.

Norepinephrine is a presynaptic input to visual corpus callosum cells.

The neurochemical identity of presynaptic inputs to cell bodies of corpus callosum fibers (callosal cells) located in visual cortex of the cat was investigated using a double labeling technique. Callosal cells were labeled by horseradish peroxidase applied to the severed ends of callosal fibers, and [3H]norepinephrine (NE) was injected into the crown of the lateral gyrus corresponding to the visual cortical area 17/18 border to label synapses. The tissue surrounding the injection sites was processed for electron microscope autoradiography. The results show that NE can be localized to cortical synapses contacting callosal cells in visual cortex of the cat.