Synaptic adjustment after deafferentation of the superior colliculus of the rat

Ocular dominance plasticity: Molecular mechanisms revisited

Ocular dominance plasticity (ODP) is a type of cortical plasticity operating in visual cortex of mammals that are endowed with binocular vision based on the competition-driven disparity. Earlier, a molecular mechanism was proposed that catecholamines play an important role in the maintenance of ODP in kittens. Having survived the initial test, the hypothesis was further advanced to identify noradrenaline (NA) as a key factor that regulates ODP in the immature cortex. Later, the ODP-promoting effect of NA is extended to the adult with age-related limitations. Following the enhanced NA availability, the chain events downstream lead to the β-adrenoreceptor-induced cAMP accumulation, which in turn activates the protein kinase A. Eventually, the protein kinase translocates to the cell nucleus to activate cAMP responsive element binding protein (CREB). CREB is a cellular transcription factor that controls the transcription of various genes, underpinning neuronal plasticity and long-term memory. In the advent of molecular genetics in that various types of new tools have become available with relative ease, ODP research has lightly adopted in the rodent model the original concepts and methodologies. Here, after briefly tracing the strategic maturation of our quest, the review moves to the later development of the field, with the emphasis placed around the following issues: (a) Are we testing ODP per se? (b) What does monocular deprivation deprive of the immature cortex? (c) The critical importance of binocular competition, (d) What is the adult plasticity? (e) Excitation-Inhibition balance in local circuits, and (f) Species differences in the animal models.

Structural brain plasticity induced by early blindness

It is well established that early blindness results in behavioural adaptations. While the functional effects of visual deprivation have been well researched, anatomical studies are scarce. The aim of this study was to investigate whole brain structural plasticity in a mouse model of congenital blindness. Volumetric analyses were conducted on high-resolution MRI images and histological sections from the same brains. These morphometric measurements were compared between anophthalmic and sighted ZRDBA mice obtained by breeding ZRDCT and DBA mice. Results from MRI analyses using the Multiple Automatically Generated Templates (MAGeT) method showed smaller volume for the primary visual cortex and superior colliculi in anophthalmic compared with sighted mice. Deformation-based morphometry revealed smaller volumes within the dorsal lateral geniculate nuclei and the lateral secondary visual cortex and larger volumes within olfactory areas, piriform cortex, orbital areas and the amygdala, in anophthalmic compared with sighted mice. Histological analyses revealed a larger volume for the amygdala and smaller volume for the superior colliculi, primary visual cortex and medial secondary visual cortex, in anophthalmic compared with sighted mice. The absence of superficial visual layers of the superior colliculus and the thinner cortical layer IV of the primary and secondary visual cortices may explain the smaller volume of these areas, although this was observed in a limited sample. The present study shows large-scale brain plasticity in a mouse model of congenital blindness. In addition, the congruence of MRI and histological findings support the use of MRI to investigate structural brain plasticity in the mouse.

Osteopontin expression in acute immune response mediates hippocampal synaptogenesis and adaptive outcome following cortical brain injury

Traumatic brain injury (TBI) produces axotomy, deafferentation and reactive synaptogenesis. Inflammation influences synaptic repair, and the novel brain cytokine osteopontin (OPN) has potential to support axon regeneration through exposure of its integrin receptor binding sites. This study explored whether OPN secretion and proteolysis by matrix metalloproteinases (MMPs) mediate the initial degenerative phase of synaptogenesis, targeting reactive neuroglia to affect successful repair. Adult rats received unilateral entorhinal cortex lesion (UEC) modeling adaptive synaptic plasticity. Over the first week postinjury, hippocampal OPN protein and mRNA were assayed and histology was performed. At 1-2d, OPN protein increased up to 51 fold, and was localized within activated, mobilized glia. OPN transcript also increased over 50 fold, predominantly within reactive microglia. OPN fragments known to be derived from MMP proteolysis were elevated at 1d, consistent with prior reports of UEC glial activation and enzyme production. Postinjury minocycline immunosuppression attenuated MMP-9 gelatinase activity, which was correlated with the reduction of neutrophil gelatinase-associated lipocalin (LCN2) expression, and reduced OPN fragment generation. The antibiotic also attenuated removal of synapsin-1 positive axons from the deafferented zone. OPN KO mice subjected to UEC had similar reduction of hippocampal MMP-9 activity, as well as lower synapsin-1 breakdown over the deafferented zone. MAP1B and N-cadherin, surrogates of cytoarchitecture and synaptic adhesion, were not affected. OPN KO mice with UEC exhibited time dependent cognitive deficits during the synaptogenic phase of recovery. This study demonstrates that OPN can mediate immune response during TBI synaptic repair, positively influencing synapse reorganization and functional recovery.

In vivo visuotopic brain mapping with manganese-enhanced MRI and resting-state functional connectivity MRI

The rodents are an increasingly important model for understanding the mechanisms of development, plasticity, functional specialization and disease in the visual system. However, limited tools have been available for assessing the structural and functional connectivity of the visual brain network globally, in vivo and longitudinally. There are also ongoing debates on whether functional brain connectivity directly reflects structural brain connectivity. In this study, we explored the feasibility of manganese-enhanced MRI (MEMRI) via 3 different routes of Mn(2+) administration for visuotopic brain mapping and understanding of physiological transport in normal and visually deprived adult rats. In addition, resting-state functional connectivity MRI (RSfcMRI) was performed to evaluate the intrinsic functional network and structural-functional relationships in the corresponding anatomical visual brain connections traced by MEMRI. Upon intravitreal, subcortical, and intracortical Mn(2+) injection, different topographic and layer-specific Mn enhancement patterns could be revealed in the visual cortex and subcortical visual nuclei along retinal, callosal, cortico-subcortical, transsynaptic and intracortical horizontal connections. Loss of visual input upon monocular enucleation to adult rats appeared to reduce interhemispheric polysynaptic Mn(2+) transfer but not intra- or inter-hemispheric monosynaptic Mn(2+) transport after Mn(2+) injection into visual cortex. In normal adults, both structural and functional connectivity by MEMRI and RSfcMRI was stronger interhemispherically between bilateral primary/secondary visual cortex (V1/V2) transition zones (TZ) than between V1/V2 TZ and other cortical nuclei. Intrahemispherically, structural and functional connectivity was stronger between visual cortex and subcortical visual nuclei than between visual cortex and other subcortical nuclei. The current results demonstrated the sensitivity of MEMRI and RSfcMRI for assessing the neuroarchitecture, neurophysiology and structural-functional relationships of the visual brains in vivo. These may possess great potentials for effective monitoring and understanding of the basic anatomical and functional connections in the visual system during development, plasticity, disease, pharmacological interventions and genetic modifications in future studies.

In vivo manganese-enhanced MRI for visuotopic brain mapping

This study explored the feasibility of localized manganese-enhanced MRI (MEMRI) via 3 different routes of Mn(2+) administrations for visuotopic brain mapping of retinal, callosal, cortico-subcortical, transsynaptic and horizontal connections in normal adult rats. Upon fractionated intravitreal Mn(2+) injection, Mn enhancements were observed in the contralateral superior colliculus (SC) and lateral geniculate nucleus (LGN) by 45-60% at 1-3 days after initial Mn(2+) injection and in the contralateral primary visual cortex (V1) by about 10% at 2-3 days after initial Mn(2+) injection. Direct, single-dose Mn(2+) injection to the LGN resulted in Mn enhancement by 13-21% in V1 and 8-11% in SC of the ipsilateral hemisphere at 8 to 24 hours after Mn(2+) administration. Intracortical, single-dose Mn(2+) injection to the visual cortex resulted in Mn enhancement by 53-65% in ipsilateral LGN, 15-26% in ipsilateral SC, 32-34% in the splenium of corpus callosum and 17-25% in contralateral V1/V2 transition zone at 8 to 24 hours after Mn(2+) administration. Notably, some patchy patterns were apparent near the V1/V2 border of the contralateral hemisphere. Laminar-specific horizontal cortical connections were also observed in the ipsilateral hemisphere. The current results demonstrated the sensitivity of MEMRI for assessing the neuroarchitecture of the visual brains in vivo without depth-limitation, and may possess great potentials for studying the basic neural components and connections in the visual system longitudinally during development, plasticity, pharmacological interventions and genetic modifications.

Transient enhancement of inhibition following visual cortical lesions in the mouse superior colliculus

Numerous studies have investigated the effects of lesions of the primary visual cortex (V1) on visual responses in neurons of the superficial layer of the superior colliculus (sSC), which receives visual information from both the retina and V1. However, little is known about the changes in the local circuit dynamics of the sSC after receiving V1 lesions. Here, we show that surround inhibition of sSC neurons is transiently enhanced following V1 lesions in mice and that this enhancement may be attributed to alterations in the balance between excitatory and inhibitory inputs to sSC neurons. Extracellular recordings in vivo revealed that sSC neuronal responses to large visual stimuli were transiently reduced at about 1 week after visual cortical lesions compared with normal mice and that this reduction was partially recovered at about 1 month after the lesions. By using whole-cell patch-clamp recordings from sSC neurons in slice preparations obtained from mice that had received visual cortical lesions at 1 week prior to the recordings, we found cell type-dependent changes in the balance between excitation and inhibition. In non-GABAergic cells, inhibition predominated over excitation, whereas the excitation-inhibition balance did not change in GABAergic neurons. These results suggest that enhanced inhibition may be partially responsible for the reduced responses to large visual stimuli in some sSC neurons. Thus, we propose that the enhanced surround inhibition shortly after visual cortical lesions may prevent hyperexcitability in the sSC local circuit, contributing to reconstructing the finely tuned receptive field organization of sSC neurons after the visual cortical lesions.

Changes in the modulation of retinocollicular transmission through group III mGluRs long after an increase in intraocular pressure in a rat model of glaucoma

Metabotropic glutamate receptors (mGluRs) have been shown to be involved in the modulation of retinocollicular neurotransmission. In glaucoma, retinal ganglion cells (RGCs) degenerate, which may have an implication on this transmission as the superior colliculus is their major central target in the much-used rodent models of the disease. We have investigated this using an in vitro slice preparation of the superior colliculus by eliciting field excitatory postsynaptic potentials (fEPSPs) through optic tract stimulation in a rat ocular hypertension model of glaucoma. Application of the group III mGluR agonist L-AP4 reduced the peak amplitude of the fEPSP in superior colliculus slices through presynaptic mechanisms as previously shown in our lab. At 3 and 16 weeks after surgery, there were no significant differences in the effect of L-AP4 on fEPSP peak amplitude in the superior colliculus slices receiving input from the glaucomatous eyes [elevated intraocular pressure (IOP)] compared to those with input from the unoperated eyes (normal IOP). However, at 32 weeks, the fEPSP peak amplitude was reduced to a significantly greater degree during L-AP4 application in the elevated IOP slices compared to normal IOP slices. At all time points, there were no significant changes in the baseline amplitudes of fEPSPs or the stimulus intensities required to evoke fEPSPs. These results suggest that the modulation of synaptic transmission through group III mGluRs on RGC terminals to the superior colliculus is changed at later stages due to RGC degeneration through IOP elevation. These changes may be compensatory changes possibly through plasticity in the RGC terminals of surviving cells, which may be due to increases in the numbers of group III mGluRs. This result may have implications on further treatment studies carried out using these models of glaucoma as changes in the central visual system may need to be considered along with the retinal changes that occur.

A synaptic strategy for consolidation of convergent visuotopic maps

The mechanisms by which experience guides refinement of converging afferent pathways are poorly understood. We describe a vision-driven refinement of corticocollicular inputs that determines the consolidation of retinal and visual cortical (VC) synapses on individual neurons in the superficial superior colliculus (sSC). Highly refined corticocollicular terminals form 1-2 days after eye-opening (EO), accompanied by VC-dependent filopodia sprouting on proximal dendrites, and PSD-95 and VC-dependent quadrupling of functional synapses. Delayed EO eliminates synapses, corticocollicular terminals, and spines on VC-recipient dendrites. Awake recordings after EO show that VC and retina cooperate to activate sSC neurons, and VC light responses precede sSC responses within intervals promoting potentiation. Eyelid closure is associated with more protracted cortical visual responses, causing the majority of VC spikes to follow those of the colliculus. These data implicate spike-timing plasticity as a mechanism for cortical input survival, and support a cooperative strategy for retinal and cortical coinnervation of the sSC.

Rearrangement of retinogeniculate projection patterns after eye-specific segregation in mice

It has been of interest whether and when the rearrangement of neuronal circuits can be induced after projection patterns are formed during development. Earlier studies using cats reported that the rearrangement of retinogeniculate projections could be induced even after eye-specific segregation has occurred, but detailed and quantitative characterization of this rearrangement has been lacking. Here we delineate the structural changes of retinogeniculate projections in the C57BL/6 mouse in response to monocular enucleation (ME) after eye-specific segregation. When ME was performed after eye-specific segregation, rearrangement of retinogeniculate axons in the dorsal lateral geniculate nucleus (dLGN) was observed within 5 days. Although this rearrangement was observed both along the dorsomedial-ventrolateral and outer-inner axes in the dLGN, it occurred more rapidly along the outer-inner axis. We also examined the critical period for this rearrangement and found that the rearrangement became almost absent by the beginning of the critical period for ocular dominance plasticity in the primary visual cortex. Taken together, our findings serve as a framework for the assessment of phenotypes of genetically altered mouse strains as well as provide insights into the mechanisms underlying the rearrangement of retinogeniculate projections.

Changes in NMDA receptor contribution to synaptic transmission in the brain in a rat model of glaucoma

In the age-related, blinding disease glaucoma, retinal ganglion cells (RGCs) degenerate, possibly affecting glutamatergic retinofugal transmission to the brain. The superior colliculus (SC) is a major central target of retinofugal axons in the rodent, a much used disease model. We investigated the contribution of NMDA-type glutamate receptors to retinocollicular transmission in a rat glaucoma model, using a SC brain slice preparation to determine the sensitivity of synaptic responses to the NMDAR antagonist D-AP5. At 32 weeks after induction of experimental glaucoma, but not earlier, there was an increase in NMDAR contribution to SC synaptic responses in slices receiving input from glaucomatous eyes. This suggests that there are changes in NMDAR function after RGC degeneration in experimental glaucoma, which may represent functional SC compensation through plasticity via NMDARs. This has implications for studies carried out using rodent glaucoma models, especially those evaluating potential treatment strategies, as it suggests that functional changes in the central visual system need to be considered in addition to those in the eye. Furthermore, the data underline the need for early therapeutic intervention in order to pre-empt subsequent central functional changes.

Immunoelectron microscopy reveals the presence of neurofilament proteins in retinal terminals undergoing dark degeneration

After nerve crushing or section, the distal stump undergoes morphological changes described as Wallerian degeneration (WD). Immediately after nerve injury, early ultrastructural alterations occur in the terminal boutons, a process known as terminal degeneration (TD), which occurs before degeneration of the axon and leads to electrophysiological impairment. In this study we investigated the presence of neurofilament (NF) proteins in TD and compared the results with degeneration in the optic nerve. Young adult Wistar rats were submitted to bilateral enucleation and perfused after 24 h, 48 h and 1 week. Optic nerves (ON) and superior colliculus (SC) segments were processed for electron microscopy (EM) and immunoelectron microscopy (IEM) for NF subunits. Analysis of ultrathin sections of SC, at 24 h, revealed terminals undergoing TD. At 48 h and 1 week after enucleation, there was a clear increase in the number of degenerating terminals. The cytoarchitecture of the optic nerve did not change considerably at 24 h, but it was progressively altered at 48 h and 1 week after enucleation, when we observed intense astrogliosis, and most fibers exhibited dark degeneration (DD). The IEM for the NF subunits of normal ON showed gold particles located along the filaments, but we did not observe labeling for neurofilament proteins in normal retinal terminals. However, 48 h after lesion, we observed immunogold particles for the NF proteins in fibers undergoing DD and on terminals undergoing TD. Therefore, we can conclude that NF proteins participate in the process of TD, and this event occurs before complete axonal degeneration, suggesting different mechanisms for TD and DD.

Signs of plasticity and reconnection in spinal cord damage

Experiments on the thalamus, dorsal column nuclei and spinal cord in the adult cat and rat show that partial destruction of afferents to these regions leads to the immediate unmasking of certain types of activity which are normally inhibited. Chronic studies show that some cells begin to respond to afferent nerve impulses after a period in which the cell seems to have lost all inputs. Sprouting of terminals from remaining areas is one possible explanation for these late changes but one must also consider the possibility that the new connections result from axons which were already present but which normally were ineffective.

Embryonic stem cells and retinal repair

In this review we examine the potential of embryonic stem cells (ESCs) for use in the treatment of retinal diseases involving photoreceptors and retinal pigment epithelium (RPE). We outline the ontogenesis of target retinal cell types (RPE, rods and cones) and discuss how an understanding of developmental processes can inform our manipulation of ESCs in vitro. Due to their potential for cellular therapy, special emphasis is placed upon the derivation and culture of human embryonic stem cells (HESCs) and their differentiation towards a retinal phenotype. In terms of achieving this goal, we suggest that much of the success to date reflects permissive in vitro environments provided by established protocols for HESC derivation, propagation and neural differentiation. In addition, we summarise key factors that may be important for enhancing efficiency of retinal cell-type derivation from HESCs. The retina is an amenable component of the central nervous system (CNS) and as such, diseases of this structure provide a realistic target for the application of HESC-derived cellular therapy to the CNS. In order to further this goal, the second component of our review focuses on the cellular and molecular cues within retinal environments that may influence the survival and behaviour of transplanted cells. Our analysis considers both the potential barriers to transplant integration in the retina itself together with the remodelling in host visual centres that is known to accompany retinal dystrophy.

Modulation of GABAergic inhibition in the rat superior colliculus by a presynaptic group II metabotropic glutamate receptor

Previous work has indicated that metabotropic glutamate receptors (mGluRs) modulate visual responses of superior colliculus (SC) neurones in vivo in a variety of ways, in a manner that can be dependent upon visual stimulus properties. How this occurs remains unclear. In this study we aimed to determine how activation of mGluR2 and mGluR3 receptors (Group II) might modulate visual responses, by using field potential and whole-cell patch clamp recording techniques in rat SC slice. Stimulation within the superficial layers of the SC, in the presence of ionotropic glutamate receptor antagonists, evoked IPSCs that were blocked by bicuculline indicating that they are mediated via GABAA receptors. It is likely that these IPSCs were of heterogeneous origin as they showed substantial variation in paired-pulse behaviour. Nevertheless, activation of Group II mGluRs with the group-selective agonist LY354740 (300 nM, bath application) resulted in a reduction of these IPSCs (to 56% of control amplitude), and this was associated with a decrease in paired-pulse depression. At the same concentration, LY354740 did not reduce the EPSC or field-EPSP evoked by stimulation of the retinal input to the SC. The effects of LY354740 on IPSCs were not mimicked by the mGluR3-selective agonist N-acetyl-aspartyl-glutamate (NAAG, 200-500 microM). Stimulation of IPSCs with trains of impulses (10 at 20 Hz) in order to mimic natural activation patterns resulted in sequences of IPSCs that were reduced in amplitude towards the end of the stimulus train. Application of the Group II antagonist LY341495 (100 nM) under these conditions resulted in an increase in later IPSCs in a third of neurones tested. These findings indicate that mGluR2 (but not mGluR3) can selectively modulate GABAergic inhibition in SC, probably via a presynaptic mechanism. Furthermore, these receptors may be activated by synaptically released transmitter during patterns of activation similar to those seen during visual processing. Thus mGluR2 receptors could have a function in activity-dependent modulation of inhibitory processing during visual responses.

Recruitment of local excitatory circuits in the superior colliculus following deafferentation and the regeneration of retinocollicular inputs

The local synaptic connectivity in the superficial gray layer of the superior colliculus (SC) was assessed following retinal ganglion cell axonal regeneration through a peripheral nerve graft into the SC of Lister Hooded rats, using in vitro brain slice techniques. Repair was effected between the ipsilateral eye and SC, following bilateral lesion of optic nerves and ablation of ipsilateral occipital cortex. Deafferentation surgery alone resulted in a complete loss of synaptic potentials of extrinsic origin, once both retinal and cortical inputs were removed. Stimulation of graft insertion sites elicited synaptic responses comprising monosynaptic and network-mediated depolarising events. This activity, together with similar spontaneous bursts of depolarising events and action potential firing, was generated by the activation of non-N-methyl-D-aspartate glutamate receptors. This behaviour may reflect the development of a local recurrent synaptic connectivity following the repair surgery, as both evoked and spontaneous responses developed into large long-lasting bursts of excitatory activity when inhibition mediated by GABA receptors was blocked. These results suggest that the ultrastructural changes in the superficial layers of the SC resulting from deafferentation are reflected functionally at the synaptic level in the target structure even after repair. Such changes are likely to compromise the ability of the target structure to function normally during information processing. Therefore, although axons regenerating along peripheral nerve grafts can make functional synaptic connections, their efficacy in activating the target structure will probably be compromised by local changes in synaptic connectivity.

Transient ischemia of the retina results in massive degeneration of the retinotectal projection: long-term neuroprotection with brimonidine

In adult rats, we have induced retinal ischemia and investigated anterogradely labeled surviving retinal ganglion cell (RGC) afferents to the contralateral superior colliculus (SC). The animals received topically in their left eyes two 5-microl drops of saline or saline-containing 0.5% brimonidine (BMD), 1 h before 90 min of retinal ischemia induced by ligature of the left ophthalmic vessels. Two months after ischemia, the anterogradely transported neuronal tracer cholera toxin B subunit (CTB) was injected in the ischemic eyes and animals were processed 4 days later. As controls and for comparison, the retinotectal innervation of unlesioned age-matched control rats was also examined with CTB. In control and experimental animals, serial coronal sections of the mesencephalon and brainstem were immunoreacted for CTB and the area and thickness of the two most superficial layers of the SC containing densely CTB-labeled profiles were estimated with an image analysis system. Ninety minutes of ischemia resulted 2 months later in reduced density of CTB-labeled profiles in the contralateral SC of the vehicle-treated rats, representing less than one half the area occupied by CTB-labeled profiles in control rats. This resulted in shrinkage of these layers and in the presence of areas virtually devoid of CTB immunoreactivity, suggesting orthograde degeneration of retinal terminals and/or decrease of anterograde axonal transport. Topical pretreatment with BMD resulted 2 months later in CTB immunoreactivity that occupied the superficial layers of the contralateral SC in an area of approximately 86% of that observed in the unlesioned control group of animals, indicating that BMD protects against ischemia-induced degeneration of the retinotectal projection, and preserves anterograde axonal transport.

Loss of calretinin immunoreactive fibers in subcortical visual recipient structures of the RCS dystrophic rat

The retinae of dystrophic Royal College of Surgeons (RCS) rats exhibit progressive photoreceptor degeneration accompanied by pathology of ganglion cells. To date, little work has examined the consequences of retinal degeneration for central visual structures in dystrophic rats. Here, we use immunohistochemistry for calretinin (CR) to label retinal afferents in the superior colliculus (SC), lateral geniculate nucleus, and olivary pretectal nucleus of RCS rats aged between 2 and 26 months of age. Early indications of fiber loss in the medial dystrophic SC were apparent between 9 and 13 months. Quantitative methods reveal a significant reduction in the level of CR immunoreactivity in visual layers of the medial dystrophic SC at 13 months (P < 0.02). In dystrophic animals aged 19-26 months the loss of CR fibers in SC was dramatic, with well-defined patches of fiber degeneration predominating in medial aspects of the structure. This fiber degeneration in SC was accompanied by increased detection of cells immunoreactive for CR. In several animals, regions of fiber loss were also found to contain strongly parvalbumin-immunoreactive cells. Loss of CR fibers was also observed in the lateral geniculate nucleus and olivary pretectal nucleus. Patterns of fiber loss in the dystrophic SC compliment reports of ganglion cell degeneration in these animals and the response of collicular neurons to degeneration is discussed in terms of plasticity of the dystrophic visual system and properties of calcium binding proteins.

Synergistic effects of brain-derived neurotrophic factor and chondroitinase ABC on retinal fiber sprouting after denervation of the superior colliculus in adult rats

Damage to the adult CNS often causes devastating and permanent deficits because of the limited capacity of the brain for anatomical reorganization. The finding that collateral sprouting of uninjured fiber tracts mediates recovery of function prompts the search for experimental strategies that stimulate axonal plasticity after CNS trauma. Here we characterize treatments that promote the sprouting of undamaged retinal afferents into the denervated superior colliculus (SC) after a partial retinal lesion in the adult rat. Delivery of brain-derived neurotrophic factor (BDNF) was performed to enhance the intrinsic potential of retinal ganglion cells to reelongate their axons. Reduction of the neurite growth-inhibitory properties of the adult SC was accomplished via treatment with chondroitinase ABC (C-ABC), which degrades chondroitin sulfate proteoglycans. Retinal axons were labeled via intraocular injections of fluorescently tagged cholera toxin B subunit, and fiber sprouting within the denervated SC was measured by quantitative laser-scanning confocal microscopy 1 week after the retinal lesion. We found that both the administration of BDNF and the injection of C-ABC induce significant sprouting of retinal afferents into the collicular scotoma. Remarkably, the combined treatment with BDNF and C-ABC showed synergistic effects on axon growth. Colocalization analysis with anti-synapsin antibodies demonstrated synapse formation by the sprouting axons. These results suggest that the combined treatment with BDNF and C-ABC can be relevant in therapies for the repair of the damaged adult CNS.

Reinnervation of the pretectum in adult rats by regenerated retinal ganglion cell axons: anatomical and functional studies

We have investigated the specificity of reinnervation and terminal arborization of injured retinal ganglion cell (RGC) axons in the brainstem with the object of studying in a simple situation the degree to which regenerating axons are able to replicate the characteristic patterns of terminal arborization and restore normal function. We have focussed here on the pathway that is responsible for the pupillary light reflex, which is mediated through the olivary pretectal nucleus (OPN). In adult rats, the left optic nerve was transected and a segment of peripheral nerve (PN) graft was used to bridge between the retina and different regions of the ipsilateral brainstem, including the superior colliculus. After 4-13 months, regenerated RGC axons were examined in coronal sections stained for cholera toxin B subunit. RGC axons were found extending into the ipsilateral brainstem for distances of up to 6 mm. Within the pretectum, axons innervated the OPN and the nucleus of the optic tract preferentially, and formed distinctive terminal arbors within each. Within the SC axons extended laterally into the visual layers and formed a different type of arborization. On testing the pupillary light reflex, it was found in best cases to show response amplitudes which were comparable to those recorded from control intact animals. However, unlike normals, the response amplitude tended to diminish with repeated stimulation and also appeared to deteriorate with age, although responses could still be detected in some cases as long as 15 months after grafting. These results indicate that regenerating axons can selectively reinnervate denervated nuclei, where they form typical terminal arborizations, and provide the substrates for restoring functional circuitry.

Ultrastructural transynaptic effects of unilateral cochlear ablation in the gerbil medial superior olive

This study investigated the long-term effects of unilateral hearing loss on the structure of synapses within the gerbil medial superior olivary (MSO) nuclei. Five animals had complete (surgical) left cochlear ablation at postnatal day 18. Previous studies have shown this to produce, within 3 days, significant transneuronal atrophy in the left dendritic field of both MSOs. Electron micrographs from sagittal ultrathin sections through the MSOs of the cochlear-ablated animals were compared to those from unoperated normals. Qualitatively, the ultrastructural features were similar. Most of the axodendritic terminals were R-type (round-type vesicles, putative excitatory) whereas, in the central part of the nucleus, predominated by neuron soma profiles, terminals of P- and F-type (pleomorphic- and flattened-type vesicles, putative inhibitory) were present in equal numbers with R-type terminals. F-type terminals were infrequent and occurred most around lateral parts of the MSO somata. These three types of terminals seen around the somata and proximal dendrites all had extended profiles with multiple, discontinuous appositions. Quantitative analysis revealed that R-type axodendritic terminals became smaller and less densely populated with vesicles where they synapsed onto the remaining dendrites arrayed towards the ablated side of both MSOs, and axosomatic P-type afferent terminals were smaller in the contralateral nuclei. A significant reduction in the number of terminals and synapses occurred in the central, somatic, region of the ipsilateral MSO. However, the terminal vesicle concentration in the remaining terminals increased. The results indicate that cochlear ablation can induce transynaptic reduction in the size of afferent axon terminals within the MSO, and alter their vesicle concentration. These changes are likely to affect the probability of transmitter release and thus influence their signaling power within the nucleus.

Reorganization of descending motor tracts in the rat spinal cord

Following lesion of the central nervous system (CNS), reinnervation of denervated areas may occur via two distinct processes: regeneration of the lesioned fibres or/and sprouting from adjacent intact fibres into the deafferented zone. Both regeneration and axonal sprouting are very limited in the fully mature CNS of higher vertebrates, but can be enhanced by neutralizing the neurite outgrowth inhibitory protein Nogo-A. This study takes advantage of the distinct spinal projection pattern of two descending tracts, the corticospinal tract (CST) and the rubrospinal tract (RST), to investigate if re-innervation of denervated targets can occur by sprouting of anatomically separate, undamaged tracts in the adult rat spinal cord. The CST was transected bilaterally at its entry into the pyramidal decussation. Anatomical studies of the RST in IN-1 antibody-treated rats showed a reorganization of the RST projection pattern after neutralization of the myelin associated neurite growth inhibitor Nogo-A. The terminal arborizations of the rubrospinal fibres, which are normally restricted to the intermediate layers of the spinal cord, invaded the ventral horn but not the dorsal horn of the cervical spinal cord. Moreover, new close appositions were observed, in the ventral horn, onto motoneurons normally receiving CST projections. Red nucleus microstimulation experiments confirmed the reorganization of the RST system. These observations indicate that mature descending motor tracts are capable of significant intraspinal reorganization following lesion and suggests the expression of cues guiding and/or stabilizing newly formed sprouts in the adult, denervated spinal cord.

Quantifying presynaptic terminals at the light microscope level in intact and deafferented central nervous structures

Quantification of presynaptic terminals often requires laborious techniques that involve tissue preparation for ultrastructural analysis. Modern preembedding immunohistochemical techniques provide a high morphological resolution at the light microscope level, thus allowing us to identify immunostained presynaptic boutons using specific antibodies. When absolute density of boutons (D(a)) is analysed for comparison between control and deafferented nervous tissue, quantification may be distorted due to tissue shrinkage that follows deafferentiation. The magnitude of this effect must be, therefore, estimated to correct quantitative data. Using the superior colliculus (SC) as a model, an easily applicable protocol to quantify the density of small size labelled particles in control and deafferented nervous tissue is described. This protocol was used to analyse the effect of neonatal and adult enucleation on the adult pattern of cholinergic input to the rat SC. Statistical treatment of data demonstrated that neonatal enucleation caused a drastic increase in bouton density in the visual collicular layers, stratum zonale (SZ) and stratum griseum superficiale (SGS). The same lesion carried out in adult animals caused an increase in the bouton density exclusively in the SZ.

Plastic reaction of the rat visual corticocollicular connection after contralateral retinal deafferentiation at the neonatal or adult stage: axonal growth versus reactive synaptogenesis

The effects of neonatal or adult enucleation on the final adult pattern of the rat visual corticocollicular (C-Co) connection were studied using the anterograde tracer biotinylated dextranamine 10,000 (BDA) iontophoretically injected in the primary visual cortex. In control animals, column-shaped terminal fields limited to a small portion of the collicular surface were observed. Synaptic boutons were present in all superficial strata of the superior colliculus (SC), with the highest density in the ventral part of the stratum griseum superficiale (SGS). Neonatal enucleation caused a considerable expansion of the contralateral visual C-Co terminal fields, which occupied almost the entire collicular surface, suggesting that axonal sprouting had occurred. In addition, terminal boutons tended to localize more dorsally in these cases compared with controls. Following enucleation in adult animals, no changes were observed with respect to the extension of the terminal fields, although a plastic reaction leading to an increase in the bouton density in the stratum zonale (SZ) and upper SGS was found, reflecting a process of reactive synaptogenesis at these levels. These results show that both neonatal and adult visual C-Co fibers react in response to retinal ablation, although this reaction shows distinct characteristics. Molecular factors, such as growth-associated cytoskeletal proteins operating in the cortical origin, and extracellular matrix components and myelin-associated axonal growth inhibitors acting on the collicular target very likely account for these differences.

Expression of ephrin-A2 in the superior colliculus and EphA5 in the retina following optic nerve section in adult rat

The vertebrate retina projects topographically to visual brain centres. In the developing visual system, gradients of ephrins and Eph receptors play a role in defining topography. At maturity, ephrins but not Ephs are downregulated. Here we show that optic nerve section in adult rat differentially regulates the expression of ephrin-A2 in the superior colliculus (SC) and of EphA5 in the retina. Expression was quantified immunohistochemically; ephrin-A2 levels were also estimated by semiquantitative reverse transcriptase polymerase chain reaction. In the normal SC, ephrin-A2 was expressed at low levels. At 1 month, levels of protein and of mRNA were upregulated across the contralateral SC giving rise to an increasing rostro-caudal gradient. At 6 months, levels had fallen but a gradient remained. In the retina of normal animals, EphA5 was expressed as an increasing naso-temporal gradient. By 1 month, expression was decreased in far temporal retina, resulting in a uniform expression across the naso-temporal axis. We suggest that denervation-induced plastic changes within the SC modify expression of these molecules.

Potential role of endogenous brain-derived neurotrophic factor in long-term neuronal reorganization of the superior colliculus after bilateral visual deprivation

The role of the brain-derived neurotrophic factor (BDNF), the BDNF receptor (TrkB), and the glutamic acid decarboxylase (GAD67) after neonatal, bilateral nerve deafferentiation during postnatal development was investigated in the rat superior colliculus (SC). BDNF and GAD67 mRNA expression were significantly increased in optic (Op) and intermediate gray (InG) layers at 5, 8, 15, and 21 days after birth, but not in adult animals. However, TrkB mRNA expression was not modified at any time tested. At 15 days, where changes in BDNF and GAD67 mRNAs were more evident, an upregulation of the NMDAR(1A) mRNA glutamate receptor in the Op and InG, a modification in the pattern of synaptic zinc in the superficial layers of SC, and an increased synaptophysin immunoreactivity in the Op was found. This indicates the existence of a synergic mechanism between BDNF and NMDA to determine refinement of connections after the loss of visual input in SC.

Reorganization of the corticotectal projections introduced by neonatal monocular enucleation in the Monodelphis opossum and the influence of serotoninergic depletion

The influence of neonatal serotoninergic lesion (performed with s.c. injection of 5,7-dihydroxytryptamine) on the plasticity of the developing corticotectal projection was studied in the gray short-tailed opossum (Monodelphis domestica). As a first step, the placement and density of neurons projecting from the visual cortical areas to the superior colliculus was established in the adult opossum. Injections of retrogradely transported fluorescent dyes into the superior colliculus of intact three-month-old animals labeled neurons of cortical layer V. In this species, there are three visual areas: the striate area and two secondary areas, the laterally placed peristriate area and the medial visual area. The population of the labeled neurons was denser in peristriate and medial visual areas than in the striate area. Secondly, the influence of neonatal monocular enucleation on the extent of this projection was investigated, alone or in combination with a serotoninergic lesion. Injection of dyes into the superior colliculi of three-month-old animals that were unilaterally enucleated on the second postnatal day also labeled neurons of cortical layer V. However, the density of the cortical neurons projecting to the superior colliculus contralateral to the remaining eye was much lower. This reduction was most profound in the striate visual area. No significant modifications of this projection were found on the side ipsilateral to the remaining eye. In another group of opossums, unilateral enucleation on the second postnatal day was combined with serotoninergic lesion. Brains of some of the treated pups were immunostained for serotonin on the fifth postnatal day. At this age, 70-80% of serotoninergic axons in the brain were missing. However, in about three weeks these axons had regrown, and their density in the neocortex was approximately the same as in the control animals. We conclude that severe reduction of the serotoninergic innervation during the early postnatal period did not influence the plastic changes induced in the corticotectal projection by unilateral enucleation.

Changes of the cholinergic input to the superior colliculus following enucleation in neonatal and adult rats

The effects of neonatal and adult enucleation on the adult pattern of cholinergic inputs to the rat superior colliculus (SC) was analysed. In the superficial layers immunohistochemical labelling revealed that choline acetyltransferase (ChAT) was predominantly confined to single boutons which were almost continuously distributed throughout the rostrocaudal and lateromedial axes. In these layers a higher density of boutons was observed in the stratum zonale (SZ) and lower stratum griseum superficiale (SGSl) than in the upper stratum griseum superficiale (SGS(u)) and stratum opticum (SO). In intermediate collicular layers ChAT-immunostaining was mainly found in axonal profiles which were arranged in a patchy fashion. Neonatal enucleation caused a drastic increase in bouton density in the SZ, SGS(u) and SGSl. The density of boutons was particularly high in the SGS(u), giving the appearance of an almost homogeneous distribution of boutons from the collicular surface down to the upper limit of SO. Visual deafferentiation at the adult stage was followed by an increase in the bouton density exclusively in the SZ. Neonatal enucleation produced a dorsoventral enlargement of the region containing patches of ChAT staining which was slightly greater following adult deafferentiation. The results described here show that after visual deafferentiation an increase in ChAT innervation to superficial and intermediate collicular layers occurs, providing new information regarding plasticity in the visual system. In view of previous data on cholinergic function in the central nervous system, such an increase could compensate for the loss of retinal excitatory input by facilitating neuronal responses in the SC.

Chronic NMDA receptor blockade from birth increases the sprouting capacity of ipsilateral retinocollicular axons without disrupting their early segregation

We have investigated the role of the NMDA glutamate receptor (NMDAR) in the genesis and regulation of structural plasticity during synaptogenesis in the visual layers of the rat superior colliculus (sSC). In this neuropil, three projections compete for synaptic space during development. By fluorescently labeling the projections of both eyes and imaging them with confocal microscopy, we can quantify the sprouting of the ipsilateral retinal projection that follows removal of a portion of the contralateral retinal and/or corticocollicular projection. Using these techniques we have studied the effects of NMDAR blockade under different levels of competition. NMDARs were chronically blocked from birth [postnatal day 0 (P0)] by suspending the competitive antagonist 2-amino-5-phosphonopentanoic acid in the slow release plastic Elvax, a slab of which was implanted over the sSC. Such treatment alone does not impair the normal segregation of the retinal projections. However, if sprouting of the ipsilateral projection is initiated with a small contralateral retinal lesion at P6, this sprouting can be further increased by blocking NMDARs from birth. Sprouting of the ipsilateral retinal projection is also induced by retinal lesions made at P10/P11, but NMDAR blockade does not augment the sprouting induced by this later lesion. However, when combined with simultaneous ablation of the ipsilateral visual cortex, P10/P11 lesions show increased sprouting after NMDAR blockade. These data indicate that P0 NMDAR blockade does not eliminate synaptic competition in the sSC. Instead, early elimination of NMDAR function appears to facilitate sprouting that is gated in a stepwise manner by the other visual afferents.

Organization of the corticotectal projection in hamsters with neonatally elevated levels of serotonin in the superior colliculus

A previous study from this laboratory showed that elevated serotonin (5-HT) levels in the hamsters superior colliculus (SC), induced by a single subcutaneous injection of 5,7-dihydroxytryptamine (5,7-DHT) at birth, resulted in an abnormally widespread distribution of the uncrossed retinotectal projection. The present study investigated whether the corticotectal projection in such animals was altered. Adult normal and 5,7-DHT-treated hamsters were injected with horseradish peroxidase (HRP) into occipital cortex and processed for anterograde tracing of corticotectal terminals in the SC. Quantitative analysis showed that normal and 5,7-DHT-treated hamsters were not significantly different in total labeling or in the gradient of labeling density within the SC. These data indicate that corticotectal axons achieve normal terminal fields after neonatal elevation of 5-HT in the SC.

The role of nitric oxide in development of the patch-cluster system and retinocollicular pathways in the rodent superior colliculus

Nitric oxide (NO) has been implicated as a retrograde signal in the process of refining axonal pathways during brain development. To determine some of the factors involved in this process, we have used two model pathway systems in the rat and mouse superior colliculus (SC). The first, the patch-cluster system, consists of clusters of neurons in the intermediate gray layer (igl) which transiently express NO during development and which receive input from a cholinergic pathway from the parabrachial brainstem as well as from other pathways containing different transmitters. The second system, the retinocollicular pathway, consists of glutamatergic fibers that project to the superficial gray layer. We have used both nitric oxide synthase inhibition (nw-nitro-L-arginine, NoArg) and single (nNOS) and double (nNOS and eNOS) gene knockout mice to examine the effect that reduction in NOS has upon the development of these two systems. The onset of NOS expression in rat, as revealed by nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) labeling, occurred in igl cells as early as postnatal day P5, with clusters being well-established by P14. Cholinergic fibers were first visible at P10 and formed obvious patches and tiers by P14. Intraperitoneal injections of NoArg from P1-P22 had no effect upon the development of these cholinergic patches. The pathway also developed normally in both single and double-knockout mice. In contrast, the ipsilateral retinocollicular pathway was altered in the double, but not in the single knockout mouse. This pathway is exuberant during the first week of life, being distributed across much of the mediolateral axis of the rostral SC. By P8-P15, this pathway has retracted to the most mediorostral SC. This refinement was delayed substantially in the double NOS gene knockout mouse. Ipsilateral fibers were found within 3-5 separate medio-lateral patches within the rostral 600 microns of SC at P15, and patches of abnormal size and extent were also seen at P18. We conclude from these results that NO plays a role in pathway development in the rodent SC, but only in glutamatergic pathways and only when both endothelial and neuronal forms of NOS have been deleted. The mechanism of this effect must involve pathway elimination in situations where there is non-correlated electrical activity. It is likely that NO promotes fiber retraction rather than fiber stabilization in these developing nerve fibers.

Extent and duration of recovered pupillary light reflex following retinal ganglion cell axon regeneration through peripheral nerve grafts directed to the pretectum in adult rats

The functional reinnervation of the olivary pretectal nucleus (OPN) was studied in adult rats with peripheral nerve (PN) grafts bridging the interrupted retinopretectal pathway. Functional recovery was assessed quantitatively using established pupillometry techniques. The effect of intravitreal tuftsin fragment 1-3 (tuftsin 1-3) injections during the grafting procedure was also studied. A total of 53 adult rats received autologous PN grafts connecting the ocular stump of the transected optic nerve to the ipsilateral OPN. The contralateral eye was enucleated to remove the input from that eye to the OPN. A pupillary light reflex was elicited from 35 of the 53 PN-grafted animals and in the best cases, a response was obtained which compared closely to that recorded from control animals. Tuftsin 1-3 was found to increase the rate of recovery of the response. The response amplitude of PN-grafted rats was generally found to diminish with repeated stimulus presentation and also appeared to deteriorate with age. This was in contrast to control animals' responses. However, a PLR could still be elicited in 3 of the 6 animals studied 15 months after PN-grafting. These findings indicate that a near-normal PLR function can be restored using a peripheral nerve graft, but there are a number of factors that are likely to compromise optimal outcome.

Unilateral enucleation of adult rats does not effect the synapse-to-neuron ratio within the stratum griseum superficiale of the superior colliculi

Ninety-day-old hooded male rats were anaesthetised with an intraperitoneal injection of a mixture of xylazine and ketamine and had their right eyes removed. Groups of non-enucleated control and enucleated rats were killed at either 150 or 390 days of age by intracardiac perfusion with fixatives. Stereological methods were used to estimate the synapse-to-neuron ratios within the stratum griseum superficiale (SGS) layers of both the ipsi- and contra-lateral superior colliculi. The enucleation had no significant effects on this ratio irrespective of the side or age of the brains examined. This experiment shows that a constant synapse-to-neuron ratio may be maintained within the SGS layer of the rat superior colliculus despite the inevitable loss of synaptic contacts due to the anterograde transneuronal degeneration initiated by the enucleation.

Unilateral eye enucleation in adult rats causes neuronal loss in the contralateral superior colliculus

Several studies have reported the morphological changes induced by unilateral enucleation during early neonatal life on the developing visual system. This study has examined cellular changes in the superior colliculi by removal of a single eye in adult rats. Anaesthetised male hooded rats aged 90 d had their right eyes removed. Groups of nonenucleated control and enucleated rats were killed when aged either 150 or 390 d. The brains were removed and both the right and left superior colliculi dissected out. The volume of the stratum griseum superficiale (SGS) within these colliculi was estimated stereologically by light microscopy, as well as the numerical density and total number of neurons within this cell layer. The volume of the cell layer was reduced by about 40% on the side contralateral to the enucleated eye but not on the ipsilateral side at both survival periods examined. The numerical density of neurons within the SGS was unaffected by the enucleation so that the colliculi contralateral to the enucleated eye showed a substantial loss of neurons within this cells layer. This study demonstrates the importance of the retinal ganglion cell input, even in adult animals, for maintaining the viability of neurons in the SGS layer of the superior colliculus.

Mitochondrial- and nuclear-encoded subunits of cytochrome oxidase in neurons: differences in compartmental distribution, correlation with enzyme activity, and regulation by neuronal activity

Cytochrome oxidase (CO), a mitochondrial energy-generating enzyme, contains both mitochondrial- and nuclear-encoded subunits. In neurons, local levels of CO activity vary among different neuronal compartments, reflecting local demands for energy. The goals of the present study were to determine if compartmental distribution of CO subunit proteins from the two genomes was correlated with local CO activity, and if their expression was regulated proportionately in neurons. The subcellular distributions of mitochondrial-encoded CO III and nuclear-encoded CO Vb proteins were quantitatively analyzed in mouse cerebellar sections subjected to postembedding immunocytochemistry. Local levels of subunit proteins were also compared to local CO activity, as revealed by CO cytochemistry. In order to study the regulation of subunit protein expression, we assessed changes in immunoreactivity of the two CO subunits as well as changes in CO activity in mouse superior colliculus after 1 to 7 days of monocular enucleation. We found that immunoreaction product for both CO III and CO Vb existed almost exclusively in mitochondria, but their compartmental distributions were different. CO III was nonhomogeneously distributed among different neuronal compartments, where its local level was positively correlated with that of CO activity. In contrast, the subcellular distribution of CO Vb was relatively uniform and did not bear a direct relationship with that of CO activity. Moreover, the two subunit proteins were disproportionately regulated by neuronal activity. CO III and CO activity exhibited parallel decreases after the deprivation of afferent input, and their changes were earlier and to a greater degree than that of CO Vb proteins. Thus, the present findings indicate that the local expression and/or distribution of CO subunit proteins from the two genomes may involve different regulatory mechanisms in neurons. Our data also suggest that the activity-dependent regulation of mitochondrial-encoded CO subunits is likely to play a major role in controlling the local levels of CO content and its activity.

Studies on the identity of the rat optic nerve transmitter

The possible role of glutamate, aspartate, sulfur-containing excitatory amino acids and gamma-glutamyl peptides as major transmitters in the rat optic nerve was evaluated. Four days following optic nerve lesion the K(+)-evoked Ca(2+)-dependent glutamate release was reduced to 31 +/- 16% (+/- S.D., n = 9) comparing release from slices of the denervated (contralateral to the lesion) and non-denervated (ipsilateral) superior colliculus, indicative of a major transmitter function for glutamate. However, significant decreases in glutamate release could not be detected seven days following the lesion (n = 5). Other studies have shown that optic nerve denervation induce formation of synapses of non-retinal origin and cause other cellular changes which may reduce the effect of deafferentation on glutamate release after 7 days. No significant change was observed in aspartate release following the lesion. The concentrations of cysteine sulfinate, cysteate, homocysteine sulfinate, homocysteate and O-sulfo-serine in the optic layers of the superior colliculus were below 1 nmol/g tissue (n = 6). Theoretical considerations indicate that this level is too low for a function of any of these as a major optic nerve transmitter. All postsynaptic components in the rat superior colliculus response, evoked by electrical optic nerve stimulation, were reduced by kynurenate (1-10 mM), a broad spectrum glutamate-receptor antagonist. The study gives further support for the view that glutamate is a major transmitter in the rat optic nerve.

Novel synapses compensate for a neuron ablated in embryos

In leeches, as well as mammals, neuronal death in adults produces lasting deficits, whereas the embryonic nervous system is believed to be more plastic. Killing the single S interneuron in an adult leech ganglion permanently interrupts the chain of S cells linked by electrical synapses along the entire animal. Axons that synapsed with the ablated neuron do not change length in response to cell ablation, but they will grow if another axon of the same neuron is injured. In the present experiments, the S cell and surrounding cells in one ganglion were ablated with a fine pin during embryogenesis (day 8-11). Effects were evaluated 1-4 months later. Cell-specific monoclonal antibody confirmed S cell deletions. Intracellular injection of horseradish peroxidase and 6-carboxyfluorescein dye showed that intact S cells' axons projected twice their usual length into the lesioned ganglion and formed electrical synapses with homologues of their usual synaptic targets. Conduction was often restored by these connections, which replaced those of the deleted S cell. Therefore, in both adults and embryos, growing S interneurons respond to loss of a target by greater growth. However, only on the small scale of the embryo is growth sufficient to reach suitable targets.

Development of the retinofugal projections in the embryonic and larval zebrafish (Brachydanio rerio)

Studies of the projection from the vertebrate retina have contributed significantly to current concepts of neural development. The zebrafish has recently become a favored system for the study of development in general and neural development in particular. Although the development of both the optic nerve and the retinotectal projection of the zebrafish has been described, the retinofugal projection in its entirety has not. This paper describes it and also addresses the issue of projectional exuberance: i.e., transient projections to targets that are not innervated in the adult. The retinofugal projection of embryonic and larval zebrafish (32 hours to 7 days post-fertilization) was labeled by intraocular injection of DiI (1,1'-dioctadecyl-3,3,3',3',tetramethylindocarbocyanine perchlorate) and then studied in wholemounts and sections. The first optic axons crossed the chiasm at 32 hours post-fertilization and projected in a straight line to reach the tectum at about 44 hours. At 48 hours, a few optic axons deviated along either the tract of the posterior commissure or the tract of the postoptic commissure. By 72 hours (about the time of hatching) optic axons arborized in ten distinct regions, termed arborization fields. At 6-7 days post-fertilization, the same ten arborization fields (nine contralateral, one bilateral) were evident. Most of the arborization fields were located in the superficial neuropil and were not associated with morphologically identifiable clusters of somata. On the basis of various landmarks, the ten arborization fields are identified as precursors of retinorecipient nuclei previously described in other adult cypriniform fishes. The development was characterized by the nearly complete absence of any transient projections. Thus, the idea that axonal outgrowth is initially exuberant and trimmed back later is not supported by these results.

Evidence for prenatal competition among the central arbors of trigeminal primary afferent neurons: single axon analysis

Previous studies from this laboratory have demonstrated that prenatal damage to vibrissae follicles results in significant increases in the brainstem representations of the remaining vibrissae as demonstrated by staining for the mitochondrial enzyme cytochrome oxidase (CO). Because CO is primarily a postsynaptic marker, these results do not directly address the question of whether there were changes in the projections of primary afferent fibers. To address this issue, we made intra-axonal recordings from individual vibrissa-related primary afferents in rats that sustained damage to vibrissae follicles on embryonic day 17, and then injected horseradish peroxidase (HRP) into these axons to visualize their terminal arbors in the brainstem at the level of trigeminal subnucleus interpolaris (SpI). All vibrissae-related primary afferents responded to deflection of one and only one vibrissa, and the terminal arbors of axons (N = 47) recovered from animals that sustained fetal peripheral lesions were significantly larger than those (N = 23) from normal rats. Fibers from fetally damaged animals had increased total fiber lengths and numbers of branch points. These results indicate that reduced competition among primary afferent axons results in increases in the terminal arbors that remain. These increases occur without any significant alteration in their peripheral receptive fields.

Olfactory ensheathing glia and platelet-derived growth factor B-chain reactivity in the transplanted rat olfactory bulb

Using a monoclonal antibody against the B-chain of platelet-derived growth factor as a marker, we have examined the behavior of olfactory ensheathing glia in the normal and transplanted rat olfactory bulb. In the normal postnatal olfactory bulb, these glia are found to ensheath the bundles of incoming primary olfactory nerve fibers as well as those in the olfactory nerve layer. Olfactory marker protein antibody was used to identify the olfactory nerve proper. Within the transplant, the same glia: (1) ensheath bundles of both primary olfactory and non-primary olfactory axons, (2) ensheath axonal bundles deep within the donor tissue, and (3) eventually permit radiation of individual axons from bundles to surrounding neuropil. We believe that ensheathing glia (being rich in growth-related factors and extracellular matrix molecules) may be useful in providing trophic support and guidance for the reconstruction of developmentally or traumatically damaged neuronal pathways not directly related to the olfactory system. The evidence presented here indicates that ensheathing glia are capable of existing in deep brain areas and ensheathing other than primary olfactory axons. The special molecular characteristics of these glia along with the morphological findings presented here provide a foundation for further studies of these unique glia and their potential utility in the restoration of damaged neural pathways.

Rapid plastic response following early retinal lesions in rats

Following an early retinal lesion, aberrant uncrossed projections from the opposite, undamaged, retina form in the target visual nuclei. The present study has examined the development of such aberrant projections by making retinal lesions in newborn rat pups, and then examining the nature of the uncrossed retinocollicular projection at different ages following the lesion. Intravitreal injections of horseradish peroxidase were made into the intact eye, and the uncrossed projection was subsequently revealed histochemically. A mature aberrant projection forms as early as postnatal day 9. On postnatal days 5 and 2, aberrant projections are discernable amongst the exuberant uncrossed terminals of normal developing rats, although the former have not matured to form the dense terminal fields characteristic of older projections. Aberrant projections were also detectable as early as 12 h following the lesion, revealed as a relative increase in the density of uncrossed label. These results indicate that lesion-induced plastic responses by intact retinal arbors are initiated shortly after the insult, and they caution the use of retinal lesions in studies of normal retinotopic connectivity during development.

Plasticity of neuronal connections in developing brains of mammals

Although mature nervous systems show substantial malleability following various surgical or environmental manipulations, developing brains show far more prominent plasticity, particularly in terms of morphological features. Neuronal circuits, for example, can be dramatically rewired following neonatal but not adult brain lesions. It remains unknown why neuronal circuits in developing brains show such remarkable plasticity. A number of anatomical and physiological studies suggest that there are transient projections in developing brains and they are eliminated by cell death and/or collateral elimination as development proceeds. This raises a possibility that aberrant projections observed following various surgical or environmental manipulations such as partial denervation, results from retention or stabilization of transient projections. However, evidence suggests that cell death does not play an important role in developmental fine-tuning of neuronal projections. Furthermore, although the elimination of axon collaterals takes place, individual neurons appear to elaborate axonal arbors in appropriate target areas, resulting in a net increase in the size of axonal arbor emerging from individual neurons. In accord with these observations, the number of synapses appear to increase during the period when axonal elimination proceeds. Taken together, reinforcement of appropriate projections rather than elimination of excessive connections plays a major role in developmental specification of neuronal connections. Appearance of aberrant projections after partial denervation may not be a consequence of disordered axonal growth, since they form topographic maps which precisely mirrors those for normal projections. They may be induced due to reinforcement of pre-existing neuronal connections rather than to construction of novel pathways. Observations of axonal morphology in denervated areas indicate that lesion-induced enlargement of projections is due to transformation of axonal morphology, from simple and poorly branched to multiply branched. Perhaps such simple and poorly branched axons in inappropriate target areas may represent ones in the course of elimination but they may serve as a source of sprouting when denervated. In other words, after total elimination of axons any surgical or environmental manipulation cannot induce enlargement of projections. The mechanisms underlying such modifiability of neuronal connections remains unclarified but possible participation of an activity-dependent competitive mechanism is discussed.

Immunohistochemical changes of neuronal calcium-binding proteins parvalbumin and calbindin-D-28k following unilateral deafferentation in the rat visual system

The neuron-specific calcium-binding proteins, parvalbumin and calbindin-D-28k, were studied in the subcortical visual system of normal and unilaterally deafferented albino rats. Immunohistochemistry with monoclonal antibodies was used on vibratome sections through optic tract (OT), dorsal lateral geniculate nucleus (dLGN), olivary pretectal nucleus (OPN), and superior colliculus (SC). In controls, OT stained strongly for parvalbumin and weakly for calbindin-D-28k. The dLGN contained a plexus of parvalbumin-positive fibers. In dLGN, calbindin-D-28k-antibodies showed strong labeling of some neurons with long dendrites and weak staining of the cytoplasm in other neurons. In OPN, parvalbumin stained a ring of neurons and terminals in the shell region, whereas calbindin-D-28k was contained in medial cell populations. In SC, parvalbumin was contained in fibers, terminals, and neurons throughout the visual layer. Calbindin-D-28k showed a laminar distribution of neurons with a predominance in deep portions of superficial grey matter and in ventral portions of stratum opticum. Following unilateral deafferentation induced by optic nerve section, retinal axons showed immunohistochemical changes related to Wallerian degeneration and target neurons reacted by changes of calcium-binding proteins. Parvalbumin and calbindin-D-28k immunostaining decreased during Wallerian degeneration of OT. In the deafferented dLGN, immunohistochemical labeling for calbindin-D-28k declined in strongly stained neurons from 4 to 21 days after lesion. Measurement of dendritic length per number of cells or per area of dLGN showed a significant decline for the contralateral side at 4, 8, and 21 days (ANOVA, P less than 0.05). In deafferented OPN, terminal-like staining for parvalbumin decreased and neuronal labeling was enhanced. In deafferented SC, the neuronal and dendritic staining for parvalbumin increased beginning from Day 1 on and persisting at Day 21, whereas fibers and terminal-like elements decreased in staining. Measurement of parvalbumin-positive neurons per area of SC showed a significant increase of labeling in the contralateral side from Day 1 to Day 21 (ANOVA, P less than 0.05). These studies show that cellular responses to deafferentation of visual neurons involve a regulation of calcium-binding proteins. The decline in staining for calbindin-D-28k in dLGN may relate to reduced retinal afferent activity. The progressive cellular changes in parvalbumin staining may be related to unmasking of intrinsic neurons after removal of parvalbumin-containing, afferent fibers and terminals. Additionally, the changes of parvalbumin labeling in SC neurons may reflect a plastic reorganization of local circuits known to occur in rat SC in response to deafferentation.

Sequential events of degeneration and synaptic remodelling in the viper optic tectum following retinal ablation. A degeneration, radioautographic and immunocytochemical study

The ultrastructural changes taking place in the retino-recipient layers of the viper optic tectum were examined between 5 and 122 days after retinal ablation. The initial degeneration of retinotectal terminals proceeds at widely different rates and is characterized by a marked degree of polymorphism in which a number of different patterns can be discerned. In the final stages of degeneration, either both the degenerating bouton and the distal portion of the postsynaptic element are engulfed by reactive glia, or, more frequently, only the degenerating terminal is eliminated and the postsynaptic differentiation remains. The free postsynaptic differentiations are reoccupied predominantly by boutons containing pleiomorphic vesicles and which are for the most part gamma-aminobutyric acid (GABA)ergic, thus forming heterologous synapses; less frequently these sites are occupied by boutons of the ipsilateral visual contingent to form homologous synapses. These two processes, both of which depend on terminal axonal sprouting, take place within the first 3 postoperative months. They are followed by a decrease in the number of heterologous synapses and a concurrent increase in the number of homologous synapses newly formed by optic boutons generated by collateral preterminal sprouting of ipsilateral retinotectal fibres. The data suggest that partial deafferentation of the optic tectum induces a transitory GABAergic innervation of free postsynaptic sites prior to the restoration of new retinal synaptic contacts.

The impact of intracerebral retinal transplants on types of behavior exhibited by host rats

Retinae transplanted over the midbrain of newborn rats establish functional connections with host brain centers, which provide a substrate for several distinct visual functions. These responses provide insight into the relationship between anatomy and behavior under normal conditions and after brain injury, as well as into the strategies used by an animal to extract significant information from its visual environment.

Reinnervation of cerebellar Purkinje cells by climbing fibres surviving a subtotal lesion of the inferior olive in the adult rat. II. Synaptic organization on reinnervated Purkinje cells

A salient feature of the cerebellar Purkinje cells is the highly ordered distribution of their excitatory afferents on the dendritic tree. Climbing fibres synapse exclusively on the proximal dendrites, whereas parallel fibres articulate with the distal branches, the so-called spiny branchlets. This input organization is lost following the removal of climbing fibres. Such denervation results in the formation of a large number of new spines on the proximal dendrites, and these become contacted by sprouting parallel fibres, which thereby extend their domain of innervation. We have previously shown that the climbing fibres surviving a subtotal lesion of the inferior olive sprout and reinnervate neighbouring Purkinje cells. In the present ultrastructural study, we have investigated the features of Purkinje cells reinnervated by sprouting climbing fibres. The objectives were to examine the fine morphology of the newly formed synapses and to determine whether the modifications of Purkinje cell morphology and afferent organization are reversed by this reinnervation. Surviving climbing fibres were labelled by the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) and immunohistochemically visualized by means of the gold-substituted silver peroxidase technique, 2 and 6 months after 3-acetylpyridine lesions of the inferior olive in adult rats. Sprouting climbing fibres and newly formed arborizations were identified in the light microscope, isolated, and cut in serial ultrathin sections for electron microscopic analysis. The labelled boutons belonging to newly formed terminal plexuses exhibited the typical morphological features of climbing fibre terminals, i.e., a high number of round synaptic vesicles and a few small mitochondria. Most frequently they formed asymmetric synapses on stubby thorns protruding from the proximal Purkinje cell dendrites. In some instances, however, the postsynaptic element consisted of long slender spines or spines showing an atypical morphology. A number of labelled boutons was also in contact with the perikarya of reinnervated Purkinje cells, either articulating with spines or synapsing directly on the smooth somatic surface. The proximal dendrites of denervated Purkinje cells were characterized by large numbers of spines, which were frequently postsynaptic to parallel fibres. By contrast, Purkinje cells reinnervated by the sprouting climbing fibres generally showed a lower number of spines on their proximal dendrites, indicating a reversal of this morphological change. The aberrant parallel fibre input was also decreased on reinnervated dendrites or had completely disappeared. Nevertheless, some reinnervated Purkinje cells showed the persistence of some parallel fibre synapses on their proximal dendrites. On occasion, climbing fibre and parallel fibre boutons synapsed on the same spine.