Intraocular tetrodotoxin in goldfish hinders optic nerve regeneration

Lidocaine Nerve Block Diminishes the Effects of Therapeutic Electrical Stimulation to Enhance Nerve Regeneration in Rats

BACKGROUND

Although electrical stimulation (ES) can improve nerve regeneration, the impact of nerve block, such as lidocaine (Lido), on the therapeutic benefits of ES remains unclear. We used a rat tibial nerve transection-and-repair model to explore how either preoperative (PreOp) or postoperative (PostOp) nerve block affects ES-related improvement in regeneration.

METHODS

Lewis rats were used in 1 of 2 studies. The first evaluated the effects of extraneural Lido on both healthy and injured nerves. In the second study, rats were randomized to 5 experimental groups: No ES (negative control), PreOp Lido, ES + PreOp Lido, PostOp + ES, and ES (positive control). All groups underwent tibial nerve transection and repair. In both studies, nerves were harvested for histological analysis of regeneration distal to the injury site.

RESULTS

Application of extraneural Lido did not damage healthy or injured nerve based on qualitative histological observations. In the context of nerve transection and repair, the ES group exhibited improved axon regeneration at 21 days measured by the total number of myelinated fibers compared with No ES. Fiber density and percentage of neural tissue in the ES group were greater than those in both No ES and PreOp Lido + ES groups. ES + PostOp Lido was not different from No ES or ES group.

CONCLUSIONS

Extraneural application of Lido did not damage nerves. Electrical stimulation augmented nerve regeneration, but Lido diminished the ES-related improvement in nerve regeneration. Clinical studies on the effects of ES to nerve regeneration may need to consider nerve block as a variable affecting ES outcome.

Activity-dependent development of spontaneous bioelectric activity in organotypic cultures of rat occipital cortex

The development of spontaneous bioelectric activity (SBA) in organotypic tissue cultures (OTCs) from rat occipital cortex was studied by means of extracellular recording techniques in OTCs grown normally for 6-51 days in vitro (DIV), and in OTCs in which SBA had been silenced from DIV 4 on for 2 to 3 weeks by elevating the Mg(2+) levels in the growth medium. The proportions of spontaneously active neurones increased from about 25% at 6-14 DIV to more than 80% beyond the third week in vitro. Mature neurones discharged at shorter intervals and more vigorously than immature neurones; the developmental increase in firing rate was not significant, however. In OTCs 6-14 DIV the majority of spontaneously active neurones fired sluggishly in a regular manner. The remaining neurones fired action potentials in the form of discrete bursts resembling interictal activity in vivo. The proportions of these neurones increased from about 40% at 6-14 DIV to more than 80% beyond the third week in vitro. During development in vitro the mean burst duration increased from 3.5 s to about 8 s whereas the mean burst rate (between 0.7-1 bursts/min) remained constant. Activity-deprived neurons had low firing rates and fired action potentials in the form of discrete bursts with a mean burst rate of 0.4/min. The proportions of spontaneously active neurons, the variability of neuronal firing and the viability of the explants either were not altered by the activity blockade or had recovered to control values after 5-6 days in normal growth medium. We conclude that in OTCs of rat neocortex the absence of SBA during development in vitro delays the maturation of excitatory mechanisms responsible for the developmental increase in firing intensity. The development of burst firing modes is less affected by activity blockade.

Functional differentiation of ganglion cells from multipotent progenitor cells in sliced retina of adult goldfish

Multipotent progenitor cells at the retinal margin of adult goldfish give rise to all cell types in the rest of the retina. We took advantage of this spatial arrangement of progenitor and mature cells in slices of peripheral retina, to investigate the appearance and maturation of voltage-activated Na(+) current. We divided the peripheral retina into three broad regions (marginal, intermediate, and mature) on the basis of their morphological development. Whole-cell patch-clamp recordings were performed in ruptured-patch mode, so that cells from which currents were recorded could be identified by Lucifer Yellow fills. No voltage-activated Na(+) current was detected in the slender, peripherally located marginal cells. Voltage-activated Na(+) currents were detected in rounded cells found alongside or near marginal cells, facing the vitreal side of the retina. Some of these "intermediate cells" had a long axon-like process which ran along the vitreal surface. Intermediate cells adjacent to the marginal region tended to have smaller Na(+) currents than intermediate cells closer to the mature region. On average, the maximum Na(+) current amplitude recorded from intermediate cells was roughly 6-fold smaller than that of mature ganglion cells. In addition, the activation threshold of the Na(+) current in intermediate cells was nearly 14 mV more positive than that of mature ganglion cells. The results indicate that voltage-activated Na(+) current, as a possible marker of retinal ganglion cells, begins to develop well before these cells migrate to their adult position within the retina.

Modification of gonadotropin releasing hormone (GnRH) mRNA expression in the retinal-recipient Thalamus

Although the environmental cues that trigger reproductive behaviors are known for many species, the mechanisms through which these signals influence the neurochemistry of the brain to produce behavior have been elusive. In this study, we describe a retinally modulated system of gonadotropin releasing hormone (GnRH) producing neurons in the thalamus of the plainfin midshipman fish, Porichthys notatus. Previously, we cloned and sequenced the cDNA for prepro-GnRH in midshipman. Here, using in situ hybridization, we localized prepro-GnRH mRNA to the ventrolateral nucleus of the thalamus, three divisions of the preoptic area, the ganglion of the terminal nerve, and the olfactory bulb. Since the thalamus, terminal nerve ganglion, and preoptic area have been associated with visual functions, we investigated the retinal connections in midshipman. In particular, biocytin tract tracing delineated a reciprocal connection between the ventrolateral nucleus of the thalamus and the retina. Retinofugal projections are exclusively contralateral. Experimental manipulation of this retinalthalamic loop through complete optic nerve transection shows that GnRH mRNA expression in the contralateral ventrolateral nucleus may be influenced by the retina. We hypothesize that a reciprocal retinothalamic GnRH circuit is important in modulating the expression of seasonal reproductive behaviors.

Transient presence of GABA in astrocytes of the developing optic nerve

Immunostaining and high-pressure liquid chromatography (HPLC) were used to study the developmental time course of astrocytic gamma-aminobutyric acid (GABA) expression in rat optic nerve. GABA immunostaining was carried out on cultured astrocytes, and on whole optic nerve. Confocal scanning laser microscopy was used to obtain optical sections in excised whole tissue in order to localize the cellular origins of GABA within the relatively intact optic nerve. GABA immunoreactivity was localized in astrocytes identified by GFAP staining; GABA staining was most intense in early neonatal optic nerve and attenuated over 3 weeks of postnatal development. The staining was pronounced in the astrocyte cell bodies and processes but not in the nucleus. There was a paucity of GABA immunoreactivity by postnatal day 20, both in culture and in whole optic nerve. A biochemical assay for optic nerve GABA using HPLC indicated a relatively high concentration of GABA in the neonate, which rapidly attenuated over the first 3 postnatal weeks. Immunoreactivity for the GABA synthesis enzyme glutamic acid decarboxylase (GAD) was pronounced in neonates but also attenuated with development. These results indicate that GABA and the GABA synthesis enzyme GAD are localized in astrocytes of optic nerve, and that their expression is transient during postnatal development.

Ependymin, a brain extracellular glycoprotein, and CNS plasticity

Ependymin, a glycoprotein of the brain ECF, has been implicated in the neurochemistry of memory and neuronal regeneration. Three behavioral experiments (swimming with a float, avoidance conditioning, and classical conditioning) in the goldfish and one in the mouse (T-maze learning) indicate that ependymin has a role in the synaptic changes that take place in the consolidation step of memory formation and the activity-dependent phase of sharpening of goldfish retinotectal connections during neuronal regeneration. The ECF concentration of the protein was found to decrease after the goldfish learned to associate a light stimulus (CS) with the subsequent arrival of a shock (US): paired CS-US gave changes whereas an unpaired presentation of CS-US gave no changes relative to the unstimulated controls. Ependymin is present in ECF as a mixture of three disulfide-linked dimers of two acidic (alpha and beta) polypeptide chains (37 kDa and 31 kDa). Upon removal of its N-linked glycan fragment by N-glycosidase F, the beta chain yields gamma-ependymin (26 kDa). Determinations of the amino acid sequence of gamma-ependymin indicate that it is a unique protein with no long sequence homologies to any known polypeptide. There are, however, small segments (5-7 amino acids long) with homologies to fibronectin, laminin, and tubulin. Ependymin has the capacity to polymerize into FIP (after activation by phosphorylation) in response to events that deplete ECF calcium. FIP is insoluble in 2% SDS in 6 M urea, 10 mM Ca2+Ac2, 100% acetic acid, chloroform/methanol (2/1), saturated KCNS, and even 100% trifluoroacetic acid. FIP was found to be present in goldfish brain and to be formed as a labeled product in vivo. Ependymin's FIP-forming property was used to propose a molecular hypothesis for generating synaptic changes in response to local extracellular depletions of calcium at sites of "associating inputs." The model assumes that, following NMDA receptor stimulation, the translocated PKC that is generated activates extracellular ependymin by converting it to its phosphorylated form using presynaptically released ATP. The hypothesis was tested in studies of LTP of rat hippocampal slices at CA1. After LTP, new sites that stained with antisera to ependymin, visible at 100x, were obtained in its potentiated radiatum in the CA1 region but not in the unpotentiated CA3. Electron microscopic studies showed that the horseradish peroxidase reaction product obtained was localized at synaptic clefts and postsynaptic regions. The results suggest that FIP may be formed at extracellular and postsynaptic loci where multiple associating inputs interact at CA1.

Adrenergic regulation of visuocortical plasticity: a role of the locus coeruleus system

Noradrenaline-beta-adrenoceptor-mediated neural plasticity in cat visual cortex exemplifies clearly established roles of the locus coeruleus system in brain function. The prime role of the noradrenaline-beta-adrenoceptor system in the regulation of ocular dominance plasticity is discussed in this chapter and includes a newly invented paradigm of ocular dominance changes under anesthesia and paralysis without benefit of visual attention. Based on our recent findings, we have sought to integrate positive contributions of muscarinic cholinergic receptors to the beta-adrenoceptor-mediated regulatory processes. The issue of "activity dependency" is important and we recognize the necessity of designing new studies in which relationships between activity dependency within the visual pathway and global neurochemical/cellular factors can be tested directly. Further, we critically reviewed the involvement of gamma-aminobutyric acidA type receptors and N-methyl-D-aspartate receptors in the regulation of ocular dominance plasticity.

Role of activity in the selection of new electrical synapses between adult Helisoma neurons

Our aim was to determine whether neural activity in the form of sodium-dependent action potentials play a role in the formation, maintenance and specificity of electrical synapses between regenerating neurons. We axotomized buccal neurons of the mollusc, Helisoma trivolvis, and placed ganglia into organ culture in the absence or presence of tetrodotoxin (TTX), a specific sodium channel blocker. Electrical coupling was measured using intracellular microelectrodes positioned within the soma of identified neurons. Neurite outgrowth was assessed by epifluorescence microscopy after filling neurons by iontophoresis with Lucifer yellow. Previous studies found that two days after axotomy transient electrical synapses form between heterologous neurons (e.g. buccal neurons 4 and 5). Five days after axotomy these transient connections disappeared and a new electrical synapse was stabilized between the paired buccal neurons 5. To determine whether blocking neural activity with TTX affected the specificity and formation of new electrical synapses, we examined electrical coupling between the heterologous neurons 4 and 5 two days after axotomy, and the paired buccal neurons 5 five days after axotomy. Our electrophysiological recordings indicated that different neurons in the buccal ganglion varied in their sensitivity to TTX (i.e. sensitivity of buccal neurons 19 greater than 5 greater than 4), but spontaneous activity was abolished in all 3 neurons by 2 x 10(-5) M TTX. Furthermore, the inhibitory effects of TTX occurred within seconds of superfusion and persisted for at least 6 days. Inhibition of activity by TTX could be reversed after superfusion with normal saline. Neurite outgrowth from axotomized neurons was not appreciably altered in the presence of TTX. Furthermore, no differences in the incidence of electrical coupling or the coupling resistance were detected between neurons 4 and 5 two days after axotomy and organ culture in the presence of TTX. However, electrical coupling between the symmetrically paired neurons 5 was elevated in the presence of TTX after 5 days. We conclude from these results that neural activity in the form of sodium-dependent action potentials does not play an important role in the formation or breaking of transient electrical synapses during neuronal regeneration in the mollusc Helisoma trivolvis.

Disturbance of refinement of retinotectal projection in chick embryos by tetrodotoxin and grayanotoxin

The role of neuronal activity in the refinement of the retinotectal projection in chick embryos was studied using tetrodotoxin (TTX) which blocks sodium channels and grayanotoxin I (GTX) which opens the channels. Optic nerve fibers were traced by the fluorescent dye DiI. The toxins were injected into the eyeball during the refinement phase of retinotectal projection (either the 13th, 14th or 15th day of incubation). A tiny crystal of DiI was placed in the temporal part of the retina. In the control embryos, fibers caudally overshooting and arborizations outside the terminal zone were found before maturation of retinotectal projection. Overshooting fibers regressed linearly, and aberrant arborizations reduced quadratically, and then the precise retinotopic map is formed by stage 44 (18th day of incubation). Both TTX and GTX interfered with the regression of overshooting fibers and arborizations outside the terminal zone. The initial action of the toxins are reverse, but their final effect may be comparable, and they may interfere with synchronous firing of the neighboring fibers. Our results emphasize a role of neuronal activity in the refinement of retinotectal projection.

Activity-driven sharpening of the regenerating retinotectal projection: effects of blocking or synchronizing activity on the morphology of individual regenerating arbors

Both blocking activity with intraocular tetrodotoxin (TTX) and synchronizing activity with a xenon strobe light (1 Hz) prevent retinotopic sharpening of regenerating optic projection in goldfish (Meyer, 1983; Schmidt, 1985; Cook and Rankin, 1986). In this study, we tested, in both normal and regenerating projections, the effects of these two treatments on individual optic arbors. Arbors were stained via anterograde transport of HRP, drawn in camera lucida from tectal whole mounts, and analyzed for spatial extent in the plane of the retinotopic map, order of branching, number of branch endings, depth of termination, and the caliber of the parent axon. In normal tectum, fine, medium, and coarse caliber axons gave rise to small, medium, and large arbors, which averaged 127 microns, 211 microns and 275 microns in horizontal extent, and terminated at characteristic depths. All three classes averaged roughly 21 branch endings. Optic arbors that regenerated with normal patterns of activity returned to a roughly normal appearance by 6-11 weeks postcrush: the same three calibers of axons gave rise to the same three sizes of arbors at the same depths, but they were much less stratified and well on average about 16% larger in horizontal extent. At this time point, arbors regenerated under TTX or strobe were on the average 71 and 119% larger, respectively, than the control-regenerated arbors (larger in all classes), although they had approximately the same number of branch endings and were equally poorly stratified. Synapses formed under strobe were also normal in appearance. Thus the only significant effect of both strobe and TTX treatment was to enlarge the spatial extent of arbor branches. Arbors that were not regenerating were very slightly (but significantly) enlarged by TTX block of activity or strobe illumination. As previous staining showed that regenerating axons initially make widespread branches and later retract many of those branches (Schmidt, Turcotte, Buzzard, and Tieman, 1988; Stuermer, 1988), the present findings support the idea that blocking activity or synchronizing activity prevents retinotopic sharpening by interfering with the elimination of some of the errant branches.

Pathfinding and target selection of goldfish retinal axons regenerating under TTX-induced impulse blockade

To define the extent to which impulse blockade interferes with the morphological changes of regenerating retinal axons during their growth through the tectum, axons were deprived of activity by repeated intraocular injections of TTX. At intervals between 24 and 189 days after optic nerve section (ONS), a defined group of TTX-silenced axons and of axons with normal activity (controls) were labeled by applications of HRP to the ventro- or dorsotemporal retina. The trajectories of these labeled axons were traced in DAB processed tectal wholemounts. As in controls, TTX-blocked axons went through a phase of exploratory growth at early regeneration stages (24 to 80 days after ONS). Coursing in abnormal routes, the axons initially distributed their growing endings widely over the tectum. Axons with and without activity extended side branches with growth cones and filopodia over all regions of the tectum. These ramifications were of similar dimensions for the TTX-blocked and control axons. Despite abnormal routes and branching over inappropriate territories, axons showed a preference for the rostral tectum. At late regeneration stages (120-189 days after ONS), axons had lost their side branches and their growth cones. Their preterminal segments exhibited striking bends, suggesting that they had undergone course corrections to achieve access to the retinotopic target. Axonal processes had disappeared from the caudal tectum, and the preferential accumulation of axons over the rostral tectum had increased. The majority of the TTX-blocked and control axons ended in terminal arbors at retinotopic regions. The labeled arbors of the TTX-group were no larger than those of the control group. The arbors of each group lay close together in a continuous cluster in the TTX-group as well as in two-thirds of the control group. In the other one-third of the control group, however, terminal arbors were aggregated into separate patches. The clusters of the TTX-blocked axons covered between 2.2 and 3.9% (mean 2.95%) of the tectal surface and the clusters and/or patches of active axons between 1.9 and 3.4% (mean 2.7%). Thus the terminal arbor clusters of the TTX-silenced axons were not significantly larger than those of the active axons. These data show that retinal ganglion cell impulse activity is required for neither the extension of side branches in the early exploratory phase of regeneration nor for the withdrawal of these branches nor for the establishment of target-directed routes and the deployment of normal-size terminal arbors at retinotopic loci.(ABSTRACT TRUNCATED AT 400 WORDS)

Postnatal development of the visual corpus callosum: the influence of activity of the retinofugal projections

Visual callosal projections were studied in normal adult rabbits, and in adult rabbits in which normal development was manipulated by monocular enucleation on the first or seventh postnatal day, or by abolition of retinal physiological activity by repeated application of tetrodotoxin (TTX) beginning on postnatal day 7. Animals given control vehicle injections, and animals enucleated on postnatal day 7 did not differ from normal in the tangential extent of their callosal zone which is limited to the lateral one-third of area 17. In contrast, animals enucleated on the day of birth and animals given TTX vitreous injections beginning on postnatal day 6-7 are similar in that the tangential extent of their callosal cell zone extends approximately through the lateral two-thirds of area 17. The results suggest that different mechanisms underly the effects of removal of the eye, and abolition of retinal activity, and that the critical period for the effective manipulation of these two mechanisms is different.

Activity sharpens the regenerating retinotectal projection in goldfish: sensitive period for strobe illumination and lack of effect on synaptogenesis and on ganglion cell receptive field properties

The regenerating optic nerve of goldfish first reestablishes a rough retinotopic map on the contralateral tectum and then sharpens it. Disruption of visual activity, either by blocking activity with intraocular tetrodotoxin (TTX; Schmidt and Edwards, 1983) or by synchronizing activity with strobe illumination (Schmidt and Eisele, 1985), disrupts the sharpening process: the map is correctly oriented but the multiunit receptive fields at each point average 25-40 degrees in diameter. In order to test whether strobe and TTX interfere with the same mechanism, we have tested whether their sensitive periods are the same, and whether strobe, like TTX treatment, does not affect either ganglion cell receptive field properties or synaptogenesis. In parallel studies, we exposed fish to 2 weeks of either strobe illumination or intraocular TTX beginning at various times after crush and determined via electrophysiological recordings that the periods of sensitivity were nearly identical. There was no effect of either treatment during the first 2 weeks (before the fibers arrive at the tectum), maximal disruption of sharpening between 14 and 50 days (the period of rapid synaptogenesis), decreasing disruption between 50 and 125 days, and no effect beyond that point or in the normal projection. In addition, long strobe exposures of up to 142 days produced no greater disruptions than shorter 2-3-week exposures, indicating no cumulative effect. The reestablishment of synaptic transmission in tectum, assayed by recording field potentials elicited by optic nerve shock, was not affected by stroboscopic illumination. Finally, individual ganglion cells, recorded intraretinally following long-term strobe exposure, had receptive fields that were normal both in size and in their characteristic responses to light-on, to light-off, or to both on and off. These findings support the hypothesis that strobe-like TTX prevents retinotopic refinement by preventing the correction of errors initially made by the ingrowing optic axons (Schmidt et al., 1988).

Metabolic changes in the superior colliculus after retinal receptor loss--neurotrophic interactions in the inactive visual system

In the mature rat, direct denervation by means of eye enucleation resulted in a temporary metabolic depression followed by "recovery" in primary visual centers as determined by the 2-deoxyglucose technique (4). After unilateral destruction of the retinal receptor layer by means of intense light, the superior colliculus (SC) demonstrated this same depression-recovery process. Because receptor destruction is believed to silence ongoing ganglion cell activity, and because the SC changes occurred whether or not ganglion cells sustained damage, it appeared that direct denervation of colliculus neurons was not necessary to initiate the depression-recovery sequence and that lack of activity or "disuse" was the critical factor. The silencing effect of the receptor destruction was confirmed when tetrodotoxin (TTX) injections into the damaged eye 2 months after damaging light exposure only slightly affected metabolic activity in the recovered colliculus. Binocular TTX injections in unilaterally light-damaged rats after 2 months of recovery resulted in greater depression in the normal colliculus than in the "recovered" colliculus, again suggesting that increases in glucose metabolism over time reflected physiological adjustments in the SC to loss of afferent activity. The strong depression in the SC fed by the normal eye after TTX injection confirmed that tonic retinal afferent activity was important to the metabolic integrity of the SC and that cessation of such activity could lead to at least to depression in the system. In a final group of 2-month recovery animals the light-damaged eye was enucleated. Presumably, if withdrawal of afferent activity is solely responsible for initiating the depression-recovery sequence, the destruction of already silenced retinal ganglion cells would have no effect on the recovered SC. This was not found to be the case. In fact, enucleation reinstated the metabolic depression in the recovered SC and demonstrated that denervation per se resulted in depression of glucose metabolism in postsynaptic neurons. Even in the absence of impulse activity, visual system neurons maintained trophic interactions.

Expression of membrane currents in rat neocortical neurons in serum-free culture. I. Inward currents

The gigaseal whole-cell voltage clamp technique has been used to investigate the timing of expression and properties of voltage-dependent inward currents in cultured neocortical pyramidal-shaped neurons. The dissociated primary cultures of synchronized (same cell cycle), growth arrested (G1-phase) and birth-dated neurons from fetal rat (E18) were maintained in a serum-free medium. The earliest inward current is expressed within 24 h. This current is carried by Na+ and the channels are located in distal neurites at discrete sites. The Na+ channels near the cell body are expressed after 5 days in culture, at which time the neuritic Na+ current persists. The magnitude of the current near the soma increases with age of the neuron. The Na+ current is blocked by both tetrodotoxin (TTX) and nitrendipine. The sensitivity to nitrendipine changes with age of the culture. The results suggest that Na channels expressed early during neuronal development have some structural component common also to Ca2+ channels.

Staining of regenerated optic arbors in goldfish tectum: progressive changes in immature arbors and a comparison of mature regenerated arbors with normal arbors

Individual optic arbors, normal and regenerated, were stained via anterograde transport of HRP and viewed in tectal whole mounts. Camera lucida drawings were made of 119 normal optic arbors and of 242 regenerated arbors from fish 2 weeks to 14 months postcrush. These arbors were analyzed for axonal trajectory, spatial extent in the horizontal plane, degree of branching, number of branch endings, average depth, and degree of stratification. Normal optic arbors ranged in size from roughly 100 to 400 microns across in a continuous distribution, had an average of 20 branch endings with average of fifth-order branching, and were highly stratified into one of three planes within the major optic lamina (SO-SFGS). Small arbors arising from fine-caliber axons terminated in the most superficial plane of SO-SFGS; large arbors from coarse axons terminated in the superficial and middle planes; and medium arbors from medium-caliber axons terminated in the middle and deep planes of SO-SFGS, as well as deeper in the central gray and deep white layers. Arbors from central tectum tended to be much more tightly stratified than those in the periphery. No other differences between central and peripheral arbors were noted. Mature regenerated arbors (five months or more postcrush) were normal in their number of branch endings, order of branching, and depth of termination. Their branches covered a wider area of tectum, partially because of their early branching and abnormal trajectories of branches. Axonal trajectories were often abnormal with U-turns and tortuos paths. Fine-, medium-, and coarse-caliber axons were again present and gave rise to small, medium, and large arbors at roughly the same depths as in the normals. There was frequently a lack of stratification in the medium and large arbors, which spanned much greater depths than normal. Overall, however, regenerates reestablished nearly normal morphology except for axonal trajectory and stratification. Early in regeneration, the arbors went through a series of changes. At 2 weeks postcrush, regenerated axons had grown branches over a wider-than-normal extent of tectum, though they were sparsely branched and often tipped with growth cones. At 3 weeks, the branches were more numerous and covered a still wider extent (average of five times normal), many covering more than half the tectal length or width. At 4-5 weeks smaller arbors predominated, although a few enlarged arbors were present for up to 8 weeks. Additional small changes occurred beyond 8 weeks as the arbors became progressively more normal in appearance.(ABSTRACT TRUNCATED AT 400 WORDS)

Topographic refinement of the goldfish retinotectal projection: sensitivity to stroboscopic light at different periods during optic nerve regeneration

When the severed optic nerve of a goldfish regenerates, the restored retinotectal projection is at first only grossly topographic. Refinement occurs later, by a mechanism that is thought to depend on correlation in the electrical activity of neighbouring retinal ganglion cells because it can be blocked by exposure to tetrodotoxin or diffuse stroboscopic (strobe) light. To study the sensitivity of retinotectal map refinement to strobe light at different periods during regeneration, four equivalent groups of goldfish with severed right optic nerves and ablated right lenses were interchanged, at 21 day intervals, between strobe (S) and diurnal (D) light to generate four different exposure sequences. After 84 days, a localized iontophoretic injection of WGA-HRP was made into each left tectum to label retinal ganglion cells with terminal arbors at the injection site, and the degree of clustering of the labelled cells was estimated statistically to assess map refinement. Retinae exposed to the sequences SDDS, SSDD or DSSD were broadly similar to each other and to those seen previously after exposure for similar total periods to diurnal light, constant light or strobe light with the lens in place. However, those kept in diurnal light for the first 42 days and in strobe light thereafter (DDSS) revealed significantly less refinement, equivalent to that seen previously after just 42-44 days in diurnal light. Thus diffuse strobe light itself neither sharpens nor unsharpens the regenerated map: its immediate effect seems only to be the indefinite postponement of whatever refinement would otherwise have occurred.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinal ganglion cell death is not prevented by application of tetrodotoxin during optic nerve regeneration in the frog Hyla moorei

Following extracranial optic nerve crush in the adult frog Hyla moorei, regeneration takes place to restore topographically organised visual projections, despite partial depletion of the retinal ganglion cell population. In the present study, the right optic nerve was crushed and tetrodotoxin (TTX) repeatedly injected into the right eye to abolish electrical activity mediated by sodium channels in ganglion cell axons. At 70-78 days post-crush, the number and distribution of live cells in the ganglion cell layer were assessed from cresyl violet-stained wholemounts. After regeneration, cell numbers in TTX-treated animals fell by a mean of 32.6% in comparison with their unoperated partner retinae. This value was very similar to the 32.4% mean fall found after regeneration for animals receiving no injections of TTX. Furthermore, distributions of surviving cells were comparable for the two groups. We conclude that sodium-mediated electrical activity within retinal ganglion cells does not control the extent or pattern of their death during optic nerve regeneration.

Neural activity in the regenerating optic nerve of the goldfish

1. Retinal ganglion cells of one eye were axotomized in goldfish either by sectioning the contralateral optic tract or by ablating the contralateral lobe of the optic tectum. Between 2 and 40 days later, multiunit activity in response to diffuse light flashes was recorded from the axotomized and normal optic nerves, and from the optic tectum. 2. Two days after tract section, the amplitude of the integrated multiunit response of the axotomized nerve was normal. By 16 days it had fallen to 15% of control values, at which time visual responses carried by the regenerating tract were first recorded in tectum. Activity in the axotomized nerve then recovered gradually. 3. After ablation of one tectal lobe, multiunit responses in the axotomized nerve had not recovered by 40 days. 4. Integrated spontaneous activity in the axotomized nerve was depressed with a similar time course to the depression of light-evoked activity, both after tract section and tectal ablation. 5. Retinal ganglion cell nuclear size, a morphological indicator of the cell body reaction, varied inversely with evoked activity, whether axotomy was by tract section or by tectal ablation. 6. Electrically evoked compound action potentials of normal amplitude could be recorded from an axotomized nerve despite depressed responses to light flashes. 7. It is concluded that optic nerve axotomy in goldfish reduces the number of optic fibres carrying impulses and/or the frequency of their discharge. The effect is closely linked to morphological changes occurring in the retinal ganglion cell bodies. Recovery of impulse activity and morphology depends upon the regenerating optic fibres innervating an appropriate target.

Preferential loss of collaterals from goldfish retinal axons in the optic tract is delayed by tetrodotoxin

Following optic nerve section (ONS) in goldfish, the right eye was repeatedly injected with tetrodotoxin (TTX) and the left eye with Ringer solution. At various survival periods after ONS, horseradish peroxidase (HRP) was applied to small groups of axons in dorsal or ventral retina in both eyes. Counts of labeled regenerating retinal axons show that 43 +/- 4.6% of regenerating axons course through the inappropriate brachium of the optic tract between 20 and 65 days after ONS. The amount of misrouted axons declines to 30 +/- 6.1% between 70 and 80 days after ONS. Under TTX blockade the reduction of misrouted axons is delayed but reaches 27 +/- 3.6% at 150 days after ONS.

Activity-dependent sharpening of the regenerating retinotectal projection in goldfish: relationship to the expression of growth-associated proteins

During regeneration of the optic nerve in goldfish, manipulations that disrupt the transmission of patterned visual information, if applied within the so-called 'sensitive period', lead to the formation of a diffuse retinotopic map (Schmidt, Cell. Mol. Neurobiol., 5 (1985) 65). The present study examined: (a) whether the sensitive period (14-50 days postcrush) coincides with the period in which specific 'growth-associated proteins' are present in the regenerating optic nerve terminals; and (b) whether manipulations that alter physiological activity during the sensitive period influence the expression of these proteins. Following bilateral optic nerve crush, goldfish regenerated their optic nerves either under normal illumination conditions (control), in total darkness, or with physiological activity suppressed in the nerve by intraocular injections of tetrodotoxin (TTX). At various times postcrush, proteins conveyed from the retina to the developing nerve endings were visualized by labeling the eye with [35S]methionine and then analyzing, by 2-dimensional gel electrophoresis and fluorography, radiolabeled proteins present in the optic tectum 15 h later. Rapidly-transported proteins that underwent large, specific increases during regeneration included the previously described 48 kDa growth-associated protein (GAP-48); labeling of GAP-48 was maximal during axonal outgrowth and then declined, but still remained well above background levels throughout the 'sensitive period'. Another group of rapidly-transported proteins, mol. wt. = 110-140 kDa (HMW), followed a similar time course, while levels of a 28 kDa protein peaked at 2 weeks and then declined rapidly. Thus, activity-dependent 'sharpening' processes occur during a period in which the levels of GAP-48 and HMW remain elevated in the nerve terminals.(ABSTRACT TRUNCATED AT 250 WORDS)

Accelerated neural network formation in rat cerebral cortex cultures chronically disinhibited with picrotoxin

Our aim was to determine if chronic blockade of GABAergic inhibitory synaptic activity, monitored electrophysiologically at the neuronal level, would affect synapse formation and ultrastructure in dissociated fetal rat cerebral cortex cultures. This was achieved by adding picrotoxin to the serum-free growth medium in a dose that induced continuous epileptiform discharges throughout the culture period. Light and electron microscopic analysis suggested an accelerated synaptic network formation in the experimental cultures during the first 2 weeks in vitro. The elimination of excess synapses (mainly on spines), which normally takes place during the fourth week in vitro, occurred 1 week earlier in the presence of picrotoxin. Finally, the experimental cultures showed smaller spine synapses throughout the entire culture period. Because these effects were opposite those induced by chronic tetrodotoxin-blockade of spontaneous bioelectric activity in a previous study, the underlying causal factor could be the respective intensification and suppression of neuronal activity in the two experiments. An appropriate balance between excitatory and inhibitory synaptic drive seems therefore to be important for normal maturation of neocortical circuitry.

Fast axonally transported proteins in regenerating goldfish optic nerve: effect of abolishing electrophysiological activity with TTX

Blocking neural activity with intraocular tetrodotoxin (TTX) hinders regeneration of goldfish optic axons, and prevents the refinement of the retinotopic map that is formed in the optic tectum. The latter effect is not observed with TTX treatment confined to the first two weeks of regeneration, but is produced when the TTX treatment is delayed until after this time. In the present study, 2-dimensional gel electrophoresis was used to analyse the effects of two different schedules of TTX treatment (0-9 days or 14-32 days) on incorporation of [3H]proline into individual proteins conveyed by fast axonal transport in the optic nerve. The labelling of many of these proteins was somewhat reduced by either schedule of TTX treatment, but a number of proteins showed a larger reduction as a result of the delayed treatment. These included some glycoproteins, as well as a protein of about 45 kDa and pI 4.5, which shows greatly increased synthesis during regeneration, and which is probably identical to the 'growth-associated protein' GAP-43. By contrast, cytoskeletal proteins (alpha- and beta-tubulin and actin) were unaffected by the delayed TTX treatment. It is possible that the differential effects of the early and delayed TTX treatments on various transported proteins may account for differences in the effect of these treatments on the retinotectal projection.

Induction of the action potential in innervated slow muscle fibers of the frog. Effects of tetrodotoxin, vincristine and colchicine

Experiments were made to induce action potentials in innervated slow muscle fibers of Rana temporaria. Drugs were applied to the sciatic nerve or its spinal roots by one of the following methods: implantation of a silastic cuff containing tetrodotoxin (TTX); subepineural injection of TTX combined with subcutaneous injections of vincristine; and epineural application of colchicine or vincristine. Twelve to 50 days later slow fibers responded to direct or indirect stimulation with an action potential. A substantial number of these fibers were innervated by 2 or 3 slow motor axons. It is concluded that these effects of TTX and colchicine (vincristine) are due to inhibition of the axoplasmic transport of an unidentified substance. The results support the hypothesis that the inexcitability of normal slow fibers is due to a 'trophic' effect of their motor axons.

Formation of retinotopic connections: selective stabilization by an activity-dependent mechanism

During regeneration of the optic nerve in goldfish, the ingrowing retinal fibers successfully seek out their correct places in the overall retinotopic projection on the tectum. Chemospecific cell-surface interactions appear to be sufficient to organize only a crude retinotopic map on the tectum during regeneration. Precise retinotopic ordering appears to be achieved via an activity-dependent stabilization of appropriate synapses and is based upon the correlated activity of neighboring ganglion cells of the same receptive-field type in the retina. Four treatments have been found to block the sharpening process: (a) blocking the activity of the ganglion cells with intraocular tetrodotoxin (TTX), (b) rearing in total darkness, (c) correlating the activation of all ganglion cells via stroboscopic illumination and (d) blocking retinotectal synaptic transmission with alpha-bungarotoxin (alphaBTX). These experiments support a role for correlated visually driven activity in sharpening the diffuse projection and suggest that this correlated activity interacts within the postsynaptic cells, probably through the summation of excitatory postsynaptic potentials (EPSPs). Other experiments support the concept that effective synapses are stabilized: a local postsynaptic block of transmission causes a local disruption in the retinotectal map. The changes that occur during this disruption suggest that each arbor can move to maximize its synaptic efficacy. In development, initial retinotectal projections are often diffuse and may undergo a similar activity-dependent sharpening. Indirect retinotectal maps, as well as auditory maps, appear to be brought into register with the direct retinotopic projections by promoting the convergence of contacts with correlated activity. A similar mechanism may drive both the formation of ocular dominance patches in fish tectum and kitten visual cortex and the segregation of different receptive-field types in the lateral geniculate nucleus. Activity-dependent synaptic stabilization may therefore be a general mechanism whereby the diffuse projections of early development are brought to the precise, mature level of organization.

Eye-specific segregation of optic afferents in mammals, fish, and frogs: the role of activity

Eye-specific patches or stripes normally develop in the visual cortex and superior colliculus of many (but not all) mammals and are also formed, after surgically produced binocular innervation, in the optic tectum of fish and frogs. The segregation of ocular dominance patches or columns has been studied using a variety of anatomical pathway-tracing techniques, by electrophysiological recording of postsynaptic units or field potentials, and by the 2-deoxyglucose method following visual stimulation of only one eye. In the tectum of both fish and frogs and in the cortex and colliculus of mammals, eye-specific patches develop from initially diffuse, overlapping projections. Of the various mechanisms that might cause such segregation, the evidence favors an activity-dependent process that stabilizes synapses from the same eye because of their correlated activity. First, several environmental manipulations affect the segregation of afferents in visual cortex: strabismus and alternate monocular exposure apparently enhance segregation, whereas dark rearing slows the segregation process, and monocular deprivation causes the experienced eye to form larger patches at the expense of those of the deprived eye. Second, blocking activity in both eyes is effective in preventing the segregation both in the tectum of fish and frog and in the visual cortex of cat. With the eyes blocked, alternate stimulation of the optic nerves permits the segregation of ocular dominance, at least onto single cells in the cat visual cortex. These findings are discussed in terms of an activity-dependent stabilization of those synapses having correlated activity (those from neighboring ganglion cells within one eye) but not of those lacking correlated activity (those from left and right eyes). We suggest that the eye-specific patches represent a compromise between total segregation of the projections from the two eyes and the formation of a single continuous retinotopic map across the surface of the cortex or tectum.

Synaptogenesis in rat cerebral cortex cultures is affected during chronic blockade of spontaneous bioelectric activity by tetrodotoxin

Reaggregated occipital cortex cells of 19-day-old fetal rats were grown in a serum-free, chemically defined medium, and chronically exposed to impulse-blocking levels of tetrodotoxin (TTX) in order to study the role of bioelectric activity in synaptogenesis. As judged by phase-contrast microscopy, no differences were noticed in the development of neuronal networks in the TTX-treated vs control cultures. In addition, when TTX was withdrawn from experimental cultures at any stage of development, bioelectric activity qualitatively comparable to that of the control cultures appeared within 1 min. However, quantitative stereological EM analysis revealed a significant retardation in synapse formation and ultrastructural maturation of synaptic junctions during the first 3 weeks. Around 23 days in vitro, the central zone of the reaggregates in control cultures started to degenerate, but not earlier then day 27 in TTX-treated cultures. During this time, the control, but not the experimental cultures showed (in intact tissue regions mainly situated at the outside of the aggregates) a large and selective loss of spine synapses. It is concluded that functional blockade not only retards the early growth and maturation of synaptic networks but also prevents the later occurring selective loss of spine synapses.

Selective retinal reinnervation of a surgically created tectal island in goldfish. I. Light microscopic analysis

Through anatomical and physiological studies of the regenerating retinotectal projection of goldfish, we sought to determine whether the establishment of a topographic projection is attained through a refinement of an initially less precise pattern of innervation. A 1-mm-wide mediolateral strip of caudal tectum was removed so that a small island of tectal tissue was spared at the caudal pole, and the contralateral nerve was either crushed (TIX) or left intact (TI). The presence of regenerated axons in the ablated zone and the reinnervation of the caudal island were assessed with anterograde and retrograde labeling methods in the following postoperative intervals: early, 20-50 days; middle, 50-110 days; and late, more than 170 days. The anterograde radioautographic method revealed that the appropriate layers of the tectal island became reinnervated by optic axons during the early period. During the middle and late periods, one to several large, discrete bundles bridging the lesion zone along the surface of exposed subtectal structures were readily identified both by radioautography and by anterograde or retrograde labeling following application of horseradish peroxidase to the transected optic nerve or tectal island, respectively. In contrast, the anterograde horseradish peroxidase method did not reveal axon bundles extending caudal to the half-tectum in the absence of a tectal island. Among TIX cases, retrograde horseradish peroxidase labeling of the contralateral nasal retina was more widespread in the middle period than in the late period, a result we interpret as reflecting an improvement in topographical precision with time. The area of retinal labeling among TIX cases in the late period was similar to that following caudal tectal injection in cases with simple nerve crush, although it was still elevated above normal control values. Physiological maps indicated a focal representation of the nasal retina in the tectal island in both periods and did not reveal a transient extreme convergence of retinal input. These findings are discussed in relation to Sperry's chemoaffinity theory.

Stroboscopic illumination and dark rearing block the sharpening of the regenerated retinotectal map in goldfish

Blocking activity with intraocular tetrodotoxin prevents the sharpening of the retinotectal map formed during regeneration of the optic nerve. If (under normal conditions) the initially diffuse map sharpens because of correlated activity in neighboring but not distant ganglion cells, then sharpening should also be prevented merely by disrupting the spatiotemporal correlation in the pattern of activity. To test this idea, fish were exposed during regeneration to stroboscopic illumination in a featureless environment, or were maintained in complete darkness. The regenerating cells remained visually responsive after axotomy, and the xenon strobe effectively drove each ganglion cell at a constant latency. The maps formed in the strobe-reared fish were normally oriented, but the multiunit receptive fields were greatly enlarged, averaging 32 degrees. In control regenerates, multiunit receptive fields averaged only 11-12 degrees, nearly the same as for single units. Dark rearing, which allows only spontaneous activity, also resulted in enlarged multiunit receptive fields, averaging more than 28 degrees. Both effects parallel those reported previously with tetrodotoxin block. The mature projection did not become diffuse as a result of the strobe rearing, and the sensitive period corresponded to the early stage of synaptogenesis (20-34 days). Periods of normal visual exposure after 35 days produced very little sharpening of the diffuse maps produced during either strobe or dark rearing. The results are attributed to an activity-dependent stabilization of developing synapses. The correlated firing of neighboring ganglion cells could allow postsynaptic summation of their responses, and the retention of those more effective, retinotopically placed synapses might then occur via a Hebbian mechanism.

Hexacarbon neuropathy: cell body changes are early, dynamic, and specific

To study the evolution of cell body alterations during toxic neuropathy we exposed rats to the prototype neurotoxin 2,5-hexanedione and examined perikarya of lumbar dorsal root ganglia with electron microscopy and stereology at three stages of neuropathy. Compared to unintoxicated controls, neurons from rats with incipient (four weeks) and intermediate (six to seven weeks) neuropathy showed dispersion of Nissl substance and significant decreases (p less than 0.001) in the volume fractions of Nissl bodies, but not of mitochondria or Golgi apparatus. However, at advanced (twelve to fourteen weeks) stages the volume fraction of Nissl bodies had increased and no longer differed from that of controls; distinct chromatolysis-like changes also became prominent. To evaluate the specificity of this remodeling we compared current morphometric results to data from rats exposed to acrylamide monomer and found significant differences (p less than 0.001) in the volume fractions of Nissl bodies and mitochondria. We conclude: (1) in axonopathy, cell body remodeling occurs early and advances as a dynamic, evolving process, and (2) distinct differences in the patterns of cell body changes can distinguish the neuropathies studied, implying distinct cell body functions.

Intraocular injection of tetrodotoxin in goldfish decreases fast axonal transport of [3H]glucosamine-labeled materials in optic axons

When physiological activity in goldfish visual system was abolished by repeated intraocular injection of tetrodotoxin (TTX), the fast axonal transport of radioactive amino acid-labeled protein in the optic axons was unaltered. However, the TTX treatment reduced the amount of [3H]glucosamine-labeled glycolipids that were axonally transported to the optic tectum, and may have decreased their rate of turnover in the tectum. A similar though smaller effect was observed for glucosamine-containing glycoproteins. These alterations in axonal transport may be the basis for at least some of the deleterious effects of TTX on axonal regeneration in this system.

The re-establishment of synaptic transmission by regenerating optic axons in goldfish: time course and effects of blocking activity by intraocular injection of tetrodotoxin

Intraocular injections of tetrodotoxin were used to block activity for 27 days in normal fish and for the first 27 or 31 days of regeneration in fish with one optic nerve crushed. Synaptic activity was then assessed by a current source-density analysis of field potentials evoked by optic nerve shock at different times following the TTX treatment. In normal fish, the lack of activity for 4 weeks had no significant effect on the maintenance of synaptic strength. Likewise, in fish with nerve crush, lack of activity did not prevent the regenerating optic fibers from forming synapses that were nearly as effective as those formed in controls injected with the citrate buffer vehicle. The earliest synapses were formed at the rostromedial corner of the tectum (where the tract enters) at 20 days after nerve crush, when fibers had not yet reached the caudal areas. By 28 days synaptic potentials could be recorded everywhere on the surface of the tectum in both controls and TTX injected fish. However, the latency of the responses with TTX were longer, suggesting a smaller caliber of fiber, which is consistent with an earlier finding of decreased axonal transport in TTX fish. Maturation of the regenerating fibers proceeded slowly in both TTX and control fish. After more than 5 months, the projections were nearly normal but still not completely normal.

Activity sharpens the map during the regeneration of the retinotectal projection in goldfish

In the regenerating retinotectal projection of goldfish, we have used intraocular injections of tetrodotoxin (TTX) to determine whether activity plays a role in organizing or refining the retinotopic map. Repeated injections produced a continuous 27-day block without producing extraocular effects or causing deleterious effects in the retinal ganglion cells. The retinotectal maps regenerated in the TTX fish were normally organized but the multiunit receptive fields were grossly enlarged. In control regenerates, 1-3 units (arbors of retinal ganglion cell axons) were simultaneously recorded at each penetration and their combined receptive field averaged 11-12 degrees, nearly the same as for single units. In TTX fish each penetration yielded at least 5-10 units whose receptive fields were clustered over a wider area averaging 27 degrees across. Individual ganglion cell receptive fields were assessed both by tectal and by intraretinal recording and were not enlarged. Many fish were recorded up to 4 months after the release from TTX block, but no further refinement of the maps occurred. If the nerve was recrushed and regenerated a second time without TTX, a normal map was formed, ruling out any permanent changes in the retinal ganglion cells or in the tectum. Blocks during various portions of the regeneration process showed that lack of activity during the process of axonal elongation (first 2 weeks) does not cause enlargement of the multiunit receptive fields, but lack of activity during the period of synapse formation and maturation (14-34 days) does. The results are discussed in terms of an activity-dependent stabilization of synapses. Neighboring retinal ganglion cells are known to fire in a statistically correlated fashion and this could help in their elimination of incorrect branches following an early period of diffuse connections.