The identification of thalamocortical axon terminals in barrels of mouse Sml cortex using immunohistochemistry of anterogradely transported lectin (Phaseolus vulgaris-leucoagglutinin)

The anterograde transport and immunohistochemical demonstration of the lectin, Phaseolus vulgaris-leucoagglutinin (PHA-L) has been used to label thalamocortical axon terminals in barrels of mouse SmI cortex. The reaction product is visible with both the light and electron microscopes so that the distribution of axons and the types of synapses they form can be determined.     

The microtubule plus-end-tracking protein TACC3 promotes persistent axon outgrowth and mediates responses to axon guidance signals during development

Background

Formation of precise neuronal connections requires proper axon guidance. Microtubules (MTs) of the growth cone provide a critical driving force during navigation of the growing ends of axons. Pioneer MTs and their plus-end tracking proteins (+TIPs) are thought to play integrative roles during this navigation. TACC3 is a?+?TIP that we have previously implicated in regulating MT dynamics within axons. However, the role of TACC3 in axon guidance has not been previously explored.

Results

Here, we show that TACC3 is required to promote persistent axon outgrowth and prevent spontaneous axon retractions in embryonic Xenopus laevis neurons. We also show that overexpressing TACC3 can counteract the depolymerizing effect of low doses of nocodazole, and that TACC3 interacts with MT polymerase XMAP215 to promote axon outgrowth. Moreover, we demonstrate that manipulation of TACC3 levels interferes with the growth cone response to the axon guidance cue Slit2 ex vivo, and that ablation of TACC3 causes pathfinding defects in axons of developing spinal neurons in vivo.

Conclusion

Together, our results suggest that by mediating MT dynamics, the?+?TIP TACC3 is involved in axon outgrowth and pathfinding decisions of neurons during embryonic development.

     

Synapses formed by identified retinogeniculate axons during the segregation of eye input.

The synaptic organization of identified retinogeniculate axons was studied during the prenatal development of eye-specific layers in the LGN of the cat. During this period, retinogeniculate axons undergo stereotyped morphological changes. Retinogeniculate axons originating from one eye and passing through LGN territory destined to be solely innervated by the other eye (inappropriate territory) initially give rise to many side branches. As the eye-specific layers emerge, these axons elaborate extensive terminal arbors within territory appropriate to their eye of origin and concurrently retract their side branches from inappropriate territory (Sretavan and Shatz, 1986). These transient side branches may therefore represent a morphological substrate for the observed functional convergence of inputs from the two eyes onto common LGN neurons during prenatal development (Shatz and Kirkwood, 1984). This possibility was investigated by examining whether identified axons and their side branches form synapses in inappropriate territory. Three retinogeniculate axons from two fetuses aged embryonic day 53 (E53) and E57 were filled with HRP in an in vitro preparation, prior to being processed for electron microscopy (EM). The HRP-filled axons, originating from the contralateral eye, were first reconstructed at the light microscope level. The portion of axon passing through the center of ipsilaterally innervated layer A1 was then serially sectioned and reconstructed by EM. Two sets of 450 serial EM sections revealed that all three contralateral axons established synaptic contacts in ipsilateral territory. Many of these synapses were made by side branches and a few were even formed by the main axon trunks. Both side branches and trunks formed mainly en passant asymmetrical contacts that were associated with spherical synaptic vesicles and that were apposed to immature dendritic elements and dendritic shafts. For comparison, a portion of the same E53 axon within the future contralateral layer A was also serially sectioned and reconstructed for EM. Within this contralateral zone, the E53 axon formed synaptic contacts similar to those established in the ipsilateral region, except that in the appropriate zone they contained significantly more synaptic vesicles. These results demonstrate that axons from the contralateral eye can establish synapses in territory simultaneously innervated by the ipsilateral eye, both via side branches and by means of contacts along the main axon trunk. Thus, the development of eye-specific layers is accompanied by the formation and subsequent elimination of synapses that almost certainly represent a morphological substrate for the known transient functional convergence of inputs from the two eyes.     

Gender specific processing of eye contact within the human medial temporal lobe.

Eye contact is a powerful social stimulus for human and non-human primates. However, it is unclear whether brain mechanisms that interpret eye contact are sensitive to gender. Here we show that human brain responses to eye contact are indeed gender specific. Recording event-related potentials directly from the medial temporal lobes, we found that eye contact elicited specific responses in men only when they saw female faces. Conversely, women responded specifically to eye contact only when they saw pictures of men. Thus, the human medial temporal lobes subserve specifically the processing of eye contact with persons of the opposite gender.     

Modeling LGN responses during free-viewing: a possible role of microscopic eye movements in the refinement of cortical orientation selectivity.

Neural activity appears to be essential for the normal development of the orientation-selective responses of cortical cells. It has been proposed that the correlated activity of LGN cells is a crucial component for shaping the receptive fields of cortical simple cells into adjacent, oriented subregions alternately receiving ON- and OFF-center excitatory geniculate inputs. After eye opening, the spatiotemporal structure of neural activity in the early stages of the visual pathway depends not only on the characteristics of the environment, but also on the way the environment is scanned. In this study, we use computational modeling to investigate how eye movements might affect the refinement of orientation tuning in the presence of a Hebbian scheme of synaptic plasticity. Visual input consisting of natural scenes scanned by varying types of eye movements was used to activate a spatiotemporal model of LGN cells. In the presence of different types of movement, significantly different patterns of activity were found in the LGN. Specific patterns of correlation required for the development of segregated cortical receptive field subregions were observed in the case of micromovements, but were not seen in the case of saccades or static presentation of natural visual input. These results suggest an important role for the eye movements occurring during fixation in the refinement of orientation selectivity.     

Neuronal addition and retinal expansion during growth of the crucian carp eye

Using standard paraffin technique the addition of new cells in crucian carp retinas was examined. Between eye diameters 4.4 and 10.0 mm the number of ganglion cells increases from 103,000 to 205,000, INL cells from 1.5 to 3 million, cones from 250,000 to 900,000, and rods from 2 to 9 million. Concomitantly retinal area increases fivefold and the cell densities decrease by 37% for the cones, 57% for th e INL cells, and 58% for the ganglion cells, while the rod density remains stable. In relation to the rods the cell ratios at different retinal loci undergo marked changes during growth. The contributions to retinal growth by addition of new neurons and by expansion of the retina have been determined for the different retinal layers. The layer of rods grows exclusively by addition of new rod mosaic. In the cone layer 81% of growth is due to addition of new cone mosaic. In the inner nuclear layer (INL) 56% of growth is due to addition of new cells and in the ganglion cell layer 52% is due to cell addition. In each case retinal expansion accounts for the remainder of increase in retinal area. On morphological grounds six cone types can be found in the crucian carp retina. Their ratios are constant during retinal growth and at different retinal loci.     

Prenatal development of retinal ganglion cell axons: segregation into eye-specific layers within the cat's lateral geniculate nucleus

The morphological changes in individual retinal ganglion cell axons associated with the formation of the eye-specific layers in the dorsal lateral geniculate nucleus (LGN) were studied during the prenatal development of the cat's visual system. Previous work has shown that the pattern of segregated eye inputs found in the adult arises from an immature state in which inputs from the two eyes are intermixed within the nucleus (Shatz, 1983). Here, this developmental process is examined at its fundamental unit of connectivity--the individual retinal ganglion cell axon. To do so, an in vitro method was used to label fetal cat optic tract axons with HRP at various times during development between embryonic day 38 (E38) and postnatal day 2 (P2) (gestation = 65 d). The results presented here are based on reconstructions of 172 axons. During the initial period of intermixing (E38-43), axons are relatively simple in morphology. Many axons studied at the earliest ages (E38) end in growth cones and have very few branches along the main axon trunk as they traverse the nucleus. By E43, the number of side branches given off along the main axon trunk has increased and most axons also have a simple terminal arbor. Over the next 2 weeks (E43-55), the majority of axons are studded with side branches and the terminal arbor is well defined. Then, between E55 and birth, axons lose their side branches and the eye-specific layers appear. By birth, nearly all axons have a smooth trunk and an elaborate terminal arbor restricted to the LGN layer appropriate to the eye of axon origin. When the number of side branches per axon was quantified, the time course of appearance and subsequent loss of side branches was found to parallel the time course of the initial intermixing of inputs and subsequent reduction in territory shared by the two eyes as determined from previous intraocular injection experiments. Our results also showed that the side branches along each axon were located primarily within LGN territory destined to be occupied by the other eye. Thus, the side branches are likely to represent a morphological substrate for the intermixing of inputs from the two eyes. These observations suggest that the segregation of eye input to the LGN involves two fundamental and simultaneous events. One event is the remodeling of the branching pattern along the length of the main axon trunk so that the side branches present early on are eliminated and the main axon trunk becomes smooth.(ABSTRACT TRUNCATED AT 400 WORDS)     

Rescaling of the retinal map of visual space during growth of the kitten's eye

We have measured the angle between the visual axis and the axis projected from the center of the optic disk in 35 cats ranging in age from two weeks to adulthood. Our results show that this angle,a, declines from around 27° in very young kittens to about 16° in adult cats, with most of the change occurring during the first 6 weeks after birth. We interpret this change as reflecting a progressive contraction of the area of object space projected onto the retina. For this to occur, the posterior nodal distance of the eye's optical system must increase by a larger factor than the transvers extent of the retina. This process undoubtedly contributes to maturation of the kitten's visual function, causing a reduction of the size of neuronal receptive fields and an enhancement of spatial resolution.     

Eye‐specific segregation and differential fasciculation of developing retinal ganglion cell axons in the mouse visual pathway

Prior to forming and refining synaptic connections, axons of projection neurons navigate long distances to their targets. While much is known about guidance cues for axon navigation through intermediate choice points, whether and how axons are organized within tracts is less clear. Here we analyze the organization of retinal ganglion cell (RGC) axons in the developing mouse retinogeniculate pathway. RGC axons are organized by both eye‐specificity and topography in the optic nerve and tract: ipsilateral RGC axons are segregated from contralateral axons and are offset laterally in the tract relative to contralateral axon topographic position. To identify potential cell‐autonomous factors contributing to the segregation of ipsilateral and contralateral RGC axons in the visual pathway, we assessed their fasciculation behavior in a retinal explant assay. Ipsilateral RGC neurites self‐fasciculate more than contralateral neurites in vitro and maintain this difference in the presence of extrinsic chiasm cues. To further probe the role of axon self‐association in circuit formation in vivo, we examined RGC axon organization and fasciculation in an EphB1^?/? mutant, in which a subset of ipsilateral RGC axons aberrantly crosses the midline but targets the ipsilateral zone in the dorsal lateral geniculate nucleus on the opposite side. Aberrantly crossing axons retain their association with ipsilateral axons in the contralateral tract, indicating that cohort‐specific axon affinity is maintained independently of guidance signals present at the midline. Our results provide a comprehensive assessment of RGC axon organization in the retinogeniculate pathway and suggest that axon self‐association contributes to pre‐target axon organization.     

Social defeat promotes specific cytokine variations within the prefrontal cortex upon subsequent aggressive or endotoxin challenges

Stressful experiences typically have short-lived neuroendocrine and neurochemical effects, but the processes leading to these biological alterations may be sensitized so that later challenges promote exaggerated responses. As stressors and immunogenic insults have both been associated with inflammatory immune variations within the brain, we assessed whether a social defeat stressor would result in augmented corticosterone release and mRNA expression of pro-inflammatory cytokines within the prefrontal cortex (PFC) upon later social defeat (sensitization) or endotoxin (lipopolysaccharide: LPS) challenges (cross-sensitization). In the absence of a prior stressor experience, the social defeat challenge did not affect prefrontal interleukin (IL)-1β or tumor necrosis factor (TNF)-α mRNA expression, but increased that of IL-6, whereas LPS increased the expression of each cytokine. Among mice that had initially been repeatedly defeated, IL-1β and TNF-α expression was enhanced after the social defeat challenge, whereas this was not evident in response to the LPS challenge. In contrast, the initial social defeat stressor had protracted effects in that increase of IL-6 expression was limited upon subsequent challenge with either social defeat or LPS. Previous social stressor experiences also limited the corticosterone rise ordinarily elicited by either social defeat or LPS treatment. It seems that a powerful stressor, such as social defeat, may have persistent effects on later corticosterone and cytokine responses to different types of stressful insults (social versus systemic challenges), but the nature of the effects varies with the specific process assessed.     

The composition and organization of axonally transported proteins in the retinal ganglion cells of the guinea pig

We labeled the proteins of guinea pig retinal ganglion cells with [³⁵S]methionine and analyzed the axonally transported polypeptides by means of sodium dodecyl sulfate gel electrophoresis. Five groups of transported polypeptides could be distinguished by their characteristic times of initial appearance in segments of the axons of the retinal ganglion cells. The times of initial appearance of the groups corresponded to maximum transport velocities ranging from greater than 200 mm/day to 0.5 mm/day. We directly compared these transported polypeptides to polypeptides undergoing axonal transport in the retinal ganglion cells of the rabbit. Electrophoretically similar polypeptides were transported at the same relative velocities in the two animals. Our results lead to the following conclusions.

(1) The basic composition and organization of axonally transported proteins is probably a general constant feature of mammalian retinal ganglion cells, implying that the correct organization is important for the proper functioning of these neurons. Therefore, the results obtained by the analysis of individual model systems should have general significance.

(2) Four discontinuities in the transport process (in addition to the 5 discontinuities represented by the major transport groups) were revealed by a consideration of subtle differences between the rabbut and guinea pig, as well as differences in the rate of disappearance of label from individual polypeptides within each transport group.

(3) The guinea pig should provide a useful model system for studying axonal transport, especially for immunological studies, since antibodies against axonally transported proteins of the guinea pig can be conveniently prepared in the rabbit.

(4) While the structure (as reflected by electrophoretic mobility) of most major axonally transported polypeptides appears to be conserved over the evolutionary period (about 30 million years) separating two orders of mammals, the electrophoretic mobility of one neurofilament-associated polypeptide, H, was abnormally variant between the two species.

Evidence for estrogen receptor in cell nuclei and axon terminals within the lateral habenula of the rat: Regulation during pregnancy

The habenular complex is involved in several estrogen-dependent reproductive behaviors in female rats, namely, sexual behavior, maternal behavior, and postpartum sexual behavior. Although it is known that estrogen acts in other brain regions to mediate these behaviors, it is not known whether estrogen may also act directly on the habenular complex. To address this possibility, we examined this region for the presence of estrogen receptor (ER). This analysis was carried out in separate experiments by using in situ hybridization, immunocytochemistry at the light and electron microscopic levels, and steroid autoradiography. Neurons within the lateral habenula (LHb), but not the medial habenula, express ER mRNA, contain ER immunoreactivity (ER-ir) in their nuclei, and concentrate radiolabelled estradiol, providing strong evidence for the presence of functional ER in the lateral habenula. There were also ER-ir containing punctate fibers within the LHb, which, at the electron microscopic level, in part, proved to be axons and presynaptic axonal terminals. Both the level of ER-ir in cell nuclei and the density of ER-ir fibers within the LHb were regulated during the course of pregnancy and the postpartum period, suggesting that the sensitivity of the LHb to estrogen may be altered during this time. Taken together, these results demonstrate that the LHb is likely a more estrogen-sensitive region than was previously considered, and they suggest alternative mechanisms of action for ER. ER within the LHb may play a critical role in the involvement of the LHb in estrogen-dependent female reproductive behaviors. J. Comp. Neurol. 392:330–342, 1998. © 1998 Wiley-Liss, Inc.     

The diversity and pattern of glia during axon pathway formation in the Drosophila embryo

Enhancer trap lines have been used to generate a collection of molecular lineage markers specific for different subsets of glia in the Drosophila embryo. Using these markers, we have been able to describe the diversity and pattern of glia along the major axon pathways in the embryonic central and peripheral nervous system. Just as these and other studies show the great diversity of embryonic glia, so too the enhancer trap lines described here point to a remarkable degree of molecular heterogeneity, and probably a concomitant functional specificity, of the embryonic glia.     

The organization of retinal maps within the dorsal and ventral lateral geniculate nuclei of the rabbit

The cytoarchitectonic subdivisions in the rabbit's dorsal and ventral lateral geniculate nuclei have been related to the several retinal maps that can be defined in terms of the distribution of retinal axons within these nuclei. Destruction of different retinal sectors was combined with intravitreal injections of ³H-proline, so that the distribution of fiber degeneration and autoradiographic label in the geniculate nuclei could be used to define the retinal maps in each nucleus, and to compare the two nuclei with each other. The two nuclei show surprisingly similar patterns of organization. Each is made up of a laminated alpha sector that curves around a relatively cell-sparse beta sector. Two morphologically distinct layers of each alpha sector receive contralateral retinal afferents and between these there is a small region in receipt of ipsilateral afferents. In each nucleus, the lines of projection that represent single points in visual space pass perpendicular to the layers of the alpha sector and continue an almost straight course into the beta sector. Quantitative comparisons of the retinal maps show that the relative volumes devoted to the representation of segments of the visual field are approximately the same in the two nuclei.     

The antidepressant sertraline improves the phenotype, promotes neurogenesis and increases BDNF levels in the R6/2 Huntington's disease mouse model

Huntington's disease (HD) is an inherited progressive neurodegenerative disorder characterized by progressive movement, psychiatric and cognitive disturbances. Previous studies have indicated that HD pathogenesis may be mediated in part by loss of brain derived neurotrophic factor (BDNF). Antidepressants selectively blocking serotonin reuptake can increase BDNF levels, and also may increase neurogenesis. Here we report that an SSRI antidepressant, sertraline, prolongs survival, improves motor performance, and ameliorates brain atrophy in the R6/2 HD mouse model. Six-week-old R6/2 mice and nontransgenic control mice were administered either sertraline or vehicle daily. Motor function was assessed in an accelerating rotarod test and evaluated at 10 weeks. R6/2 mice exhibited reduced time on the rod. Sertraline treatment improved the motor performance in R6/2 mice, but did not affect nontransgenic mice. R6/2 mice showed significant striatal atrophy which was reduced by sertraline treatment. These beneficial effects of sertraline are associated with enhanced neurogenesis and increased BDNF levels in brain treated with sertraline. The effective serum and brain levels of sertraline are comparable to the levels achieved in human antidepressant treatment. Our findings provide evidence that sertraline is neuroprotective in this HD model. Successful treatment with sertraline in depressed HD patients has been reported; moreover, sertraline is safe and well-tolerated for long-term administration, including in HD patients. Our findings suggest that a clinical trial of SSRI treatment in order to retard disease progression in human HD may be warranted.     

Scaffolding the retina: The interstitial extracellular matrix during rat retinal development

PurposeTo examine the expression of interstitial extracellular matrix components and their role during retinal development.Material and methodsFibronectin (FN), collagen IV (Coll IV) and laminin 5 (Lam 5) expression in rat retinas from developmental stages E17 to adult were studied. In addition, PN5 full-thickness retinas were cultured for 7 days with dispase, which selectively cleaves FN and Coll IV, at either 0.5 U/ml or 5.0 U/ml for 3 or 24 h. Eyecups and retinal cultures were examined morphologically using hematoxylin and eosin staining and immunohistochemistry.ResultsColl IV, Lam 5 and FN were all transiently expressed in the interstitial matrix of the retinal layers during development. The retinal layers in dispase treated explants was severely disturbed in a dose and time dependent manner.ConclusionsFN, Lam 5 and Coll IV, are present in the interstitial extracellular matrix during rat retinal development. Enzymatic cleavage of FN and Coll IV early in the lamination process disrupts the retinal layers implicating their pivotal role in this process.     

The deficit syndrome and eye tracking disorder may reflect a distinct subtype within the syndrome of schizophrenia

Although many researchers suspect that schizophrenia is a heterogeneous syndrome, identification of valid subtypes has been surprisingly difficult. Both the deficit syndrome and eye tracking disorder have been proposed as meaningful subtypes within schizophrenia. Despite a significant amount of research in these two areas, most of it has been conducted in parallel. Recent evidence suggests that the two phenomena are associated directly with each other. A review of the literature reveals that they share a common pattern of validators, ranging widely from risk factors to neurocognitive measures, that distinguishes them from other subjects with schizophrenia. This common pattern of validators suggests that the two phenomena share an underlying pathophysiology. Furthermore, a modest amount of evidence suggests that the deficit syndrome and eye tracking disorder each reflect a distinct subtype of schizophrenia, not simply a more severe form. If valid, then the evidence would suggest that the deficit syndrome and eye tracking disorder reflect the same distinct subtype within the syndrome of schizophrenia. Although this theory is in the early stages of development, it may prove useful for dissecting the heterogeneity in schizophrenia.