Distribution of GAP-43 mRNA in the adult rat brain

Analysis of medaka GAP43 gene promoter activity in transgenic lines

We produced transgenic medaka fish lines that mimicked the expression of the GAP43 gene. Fish lines with the proximal 2-kilobase (kb) 5'-untranslated region (UTR) as the expression promoter specifically expressed enhanced green fluorescent protein (EGFP) in neural tissues, such as the brain, spinal cord, and peripheral nerves, and its expression decreased with growth, but persisted until adulthood. A functional analysis of the promoter using partially deleted UTRs revealed that functions related to neural tissue-specific promoter activity were widely distributed in the region upstream of the proximal 400-b. Furthermore, the distal half of the 2-kb UTR contributed to expression throughout the brain, while the region 400-b upstream of the proximal 600-b was strongly associated with expression in specific areas, such as the telencephalon. In addition, a region from 957 to 557 b upstream of the translation initiation site was important for the long-term maintenance of promoter activity into adulthood. Among the transcription factors with recognition sequences in this region, Sp1 and CREB1 have been suggested to play important roles in the GAP43 promoter expression characteristics, such as strong expression in the telencephalon and long-term maintenance of expression.

Amygdala stimulation promotes recovery of behavioral performance in a spatial memory task and increases GAP-43 and MAP-2 in the hippocampus and prefrontal cortex of male rats

The relationships between affective and cognitive processes are an important issue of present neuroscience. The amygdala, the hippocampus and the prefrontal cortex appear as main players in these mechanisms. We have shown that post-training electrical stimulation of the basolateral amygdala (BLA) speeds the acquisition of a motor skill, and produces a recovery in behavioral performance related to spatial memory in fimbria-fornix (FF) lesioned animals. BLA electrical stimulation rises bdnf RNA expression, BDNF protein levels, and arc RNA expression in the hippocampus. In the present paper we have measured the levels of one presynaptic protein (GAP-43) and one postsynaptic protein (MAP-2) both involved in synaptogenesis to assess whether structural neuroplastic mechanisms are involved in the memory enhancing effects of BLA stimulation. A single train of BLA stimulation produced in healthy animals an increase in the levels of GAP-43 and MAP-2 that lasted days in the hippocampus and the prefrontal cortex. In FF-lesioned rats, daily post-training stimulation of the BLA ameliorates the memory deficit of the animals and induces an increase in the level of both proteins. These results support the hypothesis that the effects of amygdala stimulation on memory recovery are sustained by an enhanced formation of new synapses.

Modulation of hippocampal neuronal activity by So-ochim-tang-gamibang in mice subjected to chronic restraint stress

Background

So-ochim-tang-gamibang (SOCG) is a decoction formula which has been used to improve mental activity in traditional Korean medicine. The present study was performed to evaluate whether the treatment of SOCG was involved in activating hippocampal neurons in mice which were subjected to chronic restraint stress (CRS).

Methods

Mice were subjected to CRS for 2 weeks to induce depressive-like behaviors. SOCG was orally administered for the same period. mRNA expression in the hippocampus was analyzed by RT-PCR. Levels of serotonin receptor 5-HT1AR in the hippocampus were determined by western blotting and by immunofluorescence staining in coronal brain sections. Cultured neurons were prepared from the dorsal root ganglia (DRG) in mice to examine the effects of CRS and SOCG treatment on neurite outgrowth. Depressive-like behaviors of experimental animals were measured by open field test (OFT) and forced swimming test (FST).

Results

mRNA levels of serotonin 1A and 1B receptors (5-HT1AR and 5-HT1BR) were decreased in the hippocampus of CRS animals and increased by SOCG treatment. Signals of 5-HT1AR protein in CA3 pyramidal cells were decreased by CRS but elevated back to levels in control animals after SOCG treatment. Phospho-Erk1/2 protein in CA3 cells showed similar pattern of changes as in 5-HT1AR, suggesting coordinated regulation after SOCG treatment in CRS animals. Axonal growth-associated protein GAP-43 levels were also decreased by CRS and then increased by SOCG treatment. In vivo administration of SOCG improved neurite outgrowth of primary DRG neurons from CRS animals and also increased 5-HT1AR protein signals. Behavioral tests of open field and forced swimming showed that immobility time periods were significantly decreased by SOCG treatment.

Conclusions

Our data suggest that SOCG treatment may increase synaptic responsiveness to serotonergic neuronal inputs by upregulating 5-HT1AR in the hippocampal neurons.

A Shift from a Pivotal to Supporting Role for the Growth-Associated Protein (GAP-43) in the Coordination of Axonal Structural and Functional Plasticity

In a number of animal species, the growth-associated protein (GAP), GAP-43 (aka: F1, neuromodulin, B-50, G50, pp46), has been implicated in the regulation of presynaptic vesicular function and axonal growth and plasticity via its own biochemical properties and interactions with a number of other presynaptic proteins. Changes in the expression of GAP-43 mRNA or distribution of the protein coincide with axonal outgrowth as a consequence of neuronal damage and presynaptic rearrangement that would occur following instances of elevated patterned neural activity including memory formation and development. While functional enhancement in GAP-43 mRNA and/or protein activity has historically been hypothesized as a central mediator of axonal neuroplastic and regenerative responses in the central nervous system, it does not appear to be the crucial substrate sufficient for driving these responses. This review explores the historical discovery of GAP-43 (and associated monikers), its transcriptional, post-transcriptional and post-translational regulation and current understanding of protein interactions and regulation with respect to its role in axonal function. While GAP-43 itself appears to have moved from a pivotal to a supporting factor, there is no doubt that investigations into its functions have provided a clearer understanding of the biochemical underpinnings of axonal plasticity.

Transplantation of olfactory ensheathing cells attenuates acute carbon monoxide poisoning-induced brain damages in rats

In this study, the therapeutic effect of olfactory ensheathing cells (OEC) transplantation on brain damage was evaluated on acute carbon monoxide (CO) poisoning rat model. Two weeks after primary culture, OECs were microinjected into hippocampus of CO poisoning rats. Survival of OECs in the host was observed and quantified. OECs survived at 2 weeks, but surviving cell number was found sharply decreased at 6 weeks and reduced to less than 10(3) at 8 weeks after transplantation. At 2 weeks after transplantation, motor function test and cerebral edema assay were performed and followed by pathological examination including hematoxylin and eosin and immunohistochemistry staining to observe the neuron injury and synapsin I and growth associated protein-43 (GAP-43) expression. Furthermore, biomarkers of oxidative stress and apoptosis related proteins in the hippocampus were detected. The results showed that CO exposure led to neurological dysfunction and cerebral edema in rats. After OEC transplantation, neurological function was significantly improved and the cerebral edema was alleviated. In addition, the numbers of neurons and Nissl bodies were increased and synapsin I and GAP-43 protein expressions were upregulated in the hippocampus. Compared with CO poisoned rats, superoxide dismutase activity and glutathione content were both increased and methane dicarboxylic aldehyde level was decreased in the hippocampus of OEC transplanted rats. Moreover, OEC transplantation reduced apoptosis induced by CO exposure. The Bcl-2 expression was significantly upregulated and Bax expression was significantly downregulated. The activity of caspase-3 and the cleaved-poly ADP-ribose polymerase expression were decreased. Taken together, our data suggest that OEC attenuates brain damages induced by acute CO poisoning within 2 weeks after transplantation.

Gap43 transcription modulation in the adult brain depends on sensory activity and synaptic cooperation

Brain development and learning is accompanied by morphological and molecular changes in neurons. The growth associated protein 43 (Gap43), indicator of neurite elongation and synapse formation, is highly expressed during early stages of development. Upon maturation of the brain, Gap43 is down-regulated by most neurons with the exception of subdivisions such as the CA3 region of hippocampus, the lateral superior olive (LSO) and the central inferior colliculus (CIC). Little is known about the regulation of this mRNA in adult brains. We found that the expression of Gap43 mRNA in specific neurons can be modulated by changing sensory activity of the adult brain. Using the central auditory system of rats as a model, Gap43 protein and mRNA levels were determined in LSO and CIC of hearing-experienced rats unilaterally or bilaterally deafened or unilaterally stimulated by a cochlear implant (CI). Our data indicate that Gap43 is a marker useful beyond monitoring neuronal growth and synaptogenesis, reflecting also specific patterns of synaptic activities on specific neurons. Thus, unilateral loss of input to an adult auditory system directly causes asymmetrical expression of Gap43 mRNA between LSOs or CICs on both sides of the brainstem. This consequence can be prevented by simple-patterned stimulation of a dysfunctional ear by way of a CI. We suggest that as a function of input balance and activity pattern, Gap43 mRNA expression changes as cells associate converging afferent signals.

In vivo single branch axotomy induces GAP-43-dependent sprouting and synaptic remodeling in cerebellar cortex

Plasticity in the central nervous system in response to injury is a complex process involving axonal remodeling regulated by specific molecular pathways. Here, we dissected the role of growth-associated protein 43 (GAP-43; also known as neuromodulin and B-50) in axonal structural plasticity by using, as a model, climbing fibers. Single axonal branches were dissected by laser axotomy, avoiding collateral damage to the adjacent dendrite and the formation of a persistent glial scar. Despite the very small denervated area, the injured axons consistently reshape the connectivity with surrounding neurons. At the same time, adult climbing fibers react by sprouting new branches through the intact surroundings. Newly formed branches presented varicosities, suggesting that new axons were more than just exploratory sprouts. Correlative light and electron microscopy reveals that the sprouted branch contains large numbers of vesicles, with varicosities in the close vicinity of Purkinje dendrites. By using an RNA interference approach, we found that downregulating GAP-43 causes a significant increase in the turnover of presynaptic boutons. In addition, silencing hampers the generation of reactive sprouts. Our findings show the requirement of GAP-43 in sustaining synaptic stability and promoting the initiation of axonal regrowth.

Structural plasticity of climbing fibers and the growth-associated protein GAP-43

Structural plasticity occurs physiologically or after brain damage to adapt or re-establish proper synaptic connections. This capacity depends on several intrinsic and extrinsic determinants that differ between neuron types. We reviewed the significant endogenous regenerative potential of the neurons of the inferior olive (IO) in the adult rodent brain and the structural remodeling of the terminal arbor of their axons, the climbing fiber (CF), under various experimental conditions, focusing on the growth-associated protein GAP-43. CFs undergo remarkable collateral sprouting in the presence of denervated Purkinje cells (PCs) that are available for new innervation. In addition, severed olivo-cerebellar axons regenerate across the white matter through a graft of embryonic Schwann cells. In contrast, CFs undergo a regressive modification when their target is deleted. In vivo knockdown of GAP-43 in olivary neurons, leads to the atrophy of their CFs and a reduction in the ability to sprout toward surrounding denervated PCs. These findings demonstrate that GAP-43 is essential for promoting denervation-induced sprouting and maintaining normal CF architecture.

Impaired sprouting and axonal atrophy in cerebellar climbing fibres following in vivo silencing of the growth-associated protein GAP-43

The adult mammalian central nervous system has a limited ability to establish new connections and to recover from traumatic or degenerative events. The olivo-cerebellar network represents an excellent model to investigate neuroprotection and repair in the brain during adulthood, due to its high plasticity and ordered synaptic organization. To shed light on the molecular mechanisms involved in these events, we focused on the growth-associated protein GAP-43 (also known as B-50 or neuromodulin). During development, this protein plays a crucial role in growth and in branch formation of neurites, while in the adult it is only expressed in a few brain regions, including the inferior olive (IO) where climbing fibres (CFs) originate. Following axotomy GAP-43 is usually up-regulated in association with regeneration. Here we describe an in vivo lentiviral-mediated gene silencing approach, used for the first time in the olivo-cerebellar system, to efficiently and specifically downregulate GAP-43 in rodents CFs. We show that lack of GAP-43 causes an atrophy of the CF in non-traumatic conditions, consisting in a decrease of its length, branching and number of synaptic boutons. We also investigated CF regenerative ability by inducing a subtotal lesion of the IO. Noteworthy, surviving CFs lacking GAP-43 were largely unable to sprout on surrounding Purkinje cells. Collectively, our results demonstrate that GAP-43 is essential both to maintain CFs structure in non-traumatic condition and to promote sprouting after partial lesion of the IO.

Aquaporins and glia

Glial cells coordinate the differentiation, metabolism, and excitability of neurons; they modulate synaptic transmission and integrate signals emanating from neurons and other glial cells. Several evidences underlying the relation between these pathways and the regulatory mechanisms of ion concentration, supporting the role of Aquaporins (AQPs) in these processes. The goal of this review is to summarize the localization of different isoforms of AQPs in relation to glial cells both in central and peripheral nervous system, underlying AQP involvement in physiological and in pathophysiological conditions such as brain edema, glioma and epilepsy.

Immunohistochemical comparison of whisker pad cutaneous innervation in Swiss Webster and hairless mice

To establish the mouse mutant, hairless (Hr), as a useful model for future analyses of target-ending interactions, we assessed the cutaneous innervation in the whisker pad after loss of primary hair targets. Postnatal (P) development of fur in Hr begins similarly to that of "normal" Swiss Webster (SW) mice. Around P10, hairs are shed and the follicles rendered permanently incompetent. Hair loss progresses rostrocaudally until the entire skin is denuded. Substantial alterations in the distribution and density of sensory and autonomic endings in the mystacial pad vibrissal and intervibrissal fur innervation were discovered. Pilo-neural complexes innervating fur hairs were dismantled in Hr. Epidermal innervation in SW was rich; only a few endings expressed growth-associated protein-43 kdal (GAP), suggesting limited changes in axonal elongation. Innervation in Hr formed a dense layer passing upward through the thickened epidermis, with substantial increases among all types of endings. Vibrissal follicle-sinus complexes were also hyperinnervated. Endings in Hr vibrissae and fur were strongly GAP-positive, suggesting reorganization of innervation. Dermal and vascular autonomic innervation in both strains co-localized tyrosine hydroxylase and neuropeptide Y, but only in Hr did neuropeptide Y co-localize calcitonin gene-related peptide (CGRP) and express GAP immunolabeling. Stereological quantitation of trigeminal ganglia revealed no differences in neuron number between Hr and SW, although there were small increases in cell volume in Hr trigeminal ganglion cells. These results suggested that a form of collateral sprouting was active in Hr mystacial pads, not in response to local injury, but as a result of loss of primary target tissues.

Co-expression of GAP-43 and nNOS in avulsed motoneurons and their potential role for motoneuron regeneration

Neuronal nitric oxide synthase (nNOS) is induced after axonal injury. The role of induced nNOS in injured neurons is not well established. In the present study, we investigated the co-expression of nNOS with GAP-43 in spinal motoneurons following axonal injury. The role of induced nNOS was discussed and evaluated. In normal rats, spinal motoneurons do not express nNOS or GAP-43. Following spinal root avulsion, expression of nNOS and GAP-43 were induced and colocalized in avulsed motoneurons. Reimplantation of avulsed roots resulted in a remarkable decrease of GAP-43- and nNOS-IR in the soma of the injured motoneurons. A number of GAP-43-IR regenerating motor axons were found in the reimplanted nerve. In contrast, the nNOS-IR was absent in reimplanted nerve. These results suggest that expression of GAP-43 in avulsed motoneurons is related to axonal regeneration whereas nNOS is not.

Molecular mechanisms, biological actions, and neuropharmacology of the growth-associated protein GAP-43

GAP-43 is an intracellular growth-associated protein that appears to assist neuronal pathfinding and branching during development and regeneration, and may contribute to presynaptic membrane changes in the adult, leading to the phenomena of neurotransmitter release, endocytosis and synaptic vesicle recycling, long-term potentiation, spatial memory formation, and learning. GAP-43 becomes bound via palmitoylation and the presence of three basic residues to membranes of the early secretory pathway. It is then sorted onto vesicles at the late secretory pathway for fast axonal transport to the growth cone or presynaptic plasma membrane. The palmitate chains do not serve as permanent membrane anchors for GAP-43, because at steady-state most of the GAP-43 in a cell is membrane-bound but is not palmitoylated. Filopodial extension and branching take place when GAP-43 is phosphorylated at Ser-41 by protein kinase C, and this occurs following neurotrophin binding and the activation of numerous small GTPases. GAP-43 has been proposed to cluster the acidic phospholipid phosphatidylinositol 4,5-bisphosphate in plasma membrane rafts. Following GAP-43 phosphorylation, this phospholipid is released to promote local actin filament-membrane attachment. The phosphorylation also releases GAP-43 from calmodulin. The released GAP-43 may then act as a lateral stabilizer of actin filaments. N-terminal fragments of GAP-43, containing 10-20 amino acids, will activate heterotrimeric G proteins, direct GAP-43 to the membrane and lipid rafts, and cause the formation of filopodia, possibly by causing a change in membrane tension. This review will focus on new information regarding GAP-43, including its binding to membranes and its incorporation into lipid rafts, its mechanism of action, and how it affects and is affected by extracellular agents.

Axonal competition in the synaptic wiring of the cerebellar cortex during development and in the mature cerebellum

Purkinje cell (PC) dendrites are made by a proximal dendritic domain, which is provided with scattered clusters of spines innervated by a single climbing fiber (CF) and by a distal domain with a high density of spines innervated by parallel fibers (PFs). Following block of electrical activity a spine increase occurs in the proximal domain and the new spines are innervated by the PFs while the number of synaptic contacts formed by the CF is reduced. Also the GABAergic input expands its territory of innervation on the proximal domain, which undergoes a profound restructuring of the glutamate and GABA receptors. Excitatory-like postsynaptic assemblies appear not only on the new spines, but also on the smooth region of the dendrite and both of them may be innervated by GABAergic terminals. In this case GABA receptors coexist with the glutamate receptors leading to the formation of hybrid synapses. In contrast, PF synapses contain solely glutamate receptors. Thus, the expression of glutamate receptors appears to be an intrinsic property of the PC, while the expression of the GABA receptors is induced by the presence of GABAergic terminals. The data highlight an important feature of the CF input; its electrical activity, in addition to inducing a powerful phasic excitation and a tonic inhibition, controls the finer architecture of the cerebellar cortex.

Dorsal cochlear nucleus responses to somatosensory stimulation are enhanced after noise-induced hearing loss

Multisensory neurons in the dorsal cochlear nucleus (DCN) achieve their bimodal response properties [Shore (2005) Eur. J. Neurosci., 21, 3334-3348] by integrating auditory input via VIIIth nerve fibers with somatosensory input via the axons of cochlear nucleus granule cells [Shore et al. (2000) J. Comp. Neurol., 419, 271-285; Zhou & Shore (2004)J. Neurosci. Res., 78, 901-907]. A unique feature of multisensory neurons is their propensity for receiving cross-modal compensation following sensory deprivation. Thus, we investigated the possibility that reduction of VIIIth nerve input to the cochlear nucleus results in trigeminal system compensation for the loss of auditory inputs. Responses of DCN neurons to trigeminal and bimodal (trigeminal plus acoustic) stimulation were compared in normal and noise-damaged guinea pigs. The guinea pigs with noise-induced hearing loss had significantly lower thresholds, shorter latencies and durations, and increased amplitudes of response to trigeminal stimulation than normal animals. Noise-damaged animals also showed a greater proportion of inhibitory and a smaller proportion of excitatory responses compared with normal. The number of cells exhibiting bimodal integration, as well as the degree of integration, was enhanced after noise damage. In accordance with the greater proportion of inhibitory responses, bimodal integration was entirely suppressive in the noise-damaged animals with no indication of the bimodal enhancement observed in a sub-set of normal DCN neurons. These results suggest that projections from the trigeminal system to the cochlear nucleus are increased and/or redistributed after hearing loss. Furthermore, the finding that only neurons activated by trigeminal stimulation showed increased spontaneous rates after cochlear damage suggests that somatosensory neurons may play a role in the pathogenesis of tinnitus.

Aquaporin 1 - a novel player in spinal cord injury

The role of water channel aquaporin 1 (AQP-1) in uninjured or injured spinal cords is unknown. AQP-1 is weakly expressed in neurons and gray matter astrocytes, and more so in white matter astrocytes in uninjured spinal cords, a novel finding. As reported before, AQP-1 is also present in ependymal cells, but most abundantly in small diameter sensory fibers of the dorsal horn. Rat contusion spinal cord injury (SCI) induced persistent and significant four- to eightfold increases in AQP-1 levels at the site of injury (T10) persisting up to 11 months post-contusion, a novel finding. Delayed AQP-1 increases were also found in cervical and lumbar segments, suggesting the spreading of AQP-1 changes over time after SCI. Given that the antioxidant melatonin significantly decreased SCI-induced AQP-1 increases and that hypoxia inducible factor-1alpha was increased in acutely and chronically injured spinal cords, we propose that chronic hypoxia contributes to persistent AQP-1 increases after SCI. Interestingly; AQP-1 levels were not affected by long-lasting hypertonicity that significantly increased astrocytic AQP-4, suggesting that the primary role of AQP-1 is not regulating isotonicity in spinal cords. Based on our results we propose possible novel roles for AQP-1 in the injured spinal cords: (i) in neuronal and astrocytic swelling, as AQP-1 was increased in all surviving neurons and reactive astrocytes after SCI and (ii) in the development of the neuropathic pain after SCI. We have shown that decreased AQP-1 in melatonin-treated SCI rats correlated with decreased AQP-1 immunolabeling in the dorsal horns sensory afferents, and with significantly decreased mechanical allodynia, suggesting a possible link between AQP-1 and chronic neuropathic pain after SCI.

Expression of protein kinase-C substrate mRNA in the motor cortex of adult and infant macaque monkeys

To understand the molecular and cellular bases of plasticity in the primate motor cortex, we investigated the expression of three protein kinase-C (PKC) substrates: GAP-43, myristoylated alanine-rich C-kinase substrate (MARCKS), and neurogranin, which are key molecules regulating synaptic plasticity. Prominent signals for the three mRNAs were primarily observed in pyramidal cells. Large pyramidal cells in layer V, from which the descending motor tract originates, contained weaker hybridization signals for GAP-43 and neurogranin mRNAs than did the smaller pyramidal cells. We also performed double-label in situ hybridization showing that GAP-43 and neurogranin mRNAs were expressed in a subset of MARCKS-positive neurons. Quantitative analysis showed that the expression was different between the layers: layer VI contained the strongest and layer II the weakest signals for all three mRNAs. The expression levels of GAP-43 and MARCKS mRNA in layer V were higher than in layer III, while the expression level of neurogranin mRNA in layer V was almost the same as in layer III. Developmental analysis from the newborn to adult indicated that the expression levels of the three mRNAs were higher in the infant motor cortex than in the adult. The expression of both GAP-43 and neurogranin mRNAs transiently increased over several months postnatally. The present study showed that the expression of the three PKC substrates was specific to cell types, cortical layers, and postnatal developmental stage. The specific expression may reflect functional specialization for plasticity in the motor cortex of both infants and adults.

Activity-dependent axonal and synaptic plasticity in the cerebellum

The cerebellum is a brain region endowed with a high degree of plasticity also in adulthood. After damage or alteration in the patterns of activity, it is able to undergo remarkable changes in its architecture and to form new connections based upon a process of synaptic reorganization. This review addresses cellular and molecular mechanisms that regulate the competition between two inputs belonging to different neuronal populations in innervating two contiguous but separate domains of the same target cell. The two inputs are the parallel fibers, the axon of the cerebellar granule cells, and the olivocerebellar neurons, that terminate as climbing fibers in the cerebellar cortex. The target is the Purkinje cell characterized by two dendritic domains that are different in size and number of spines, upon which the two afferent inputs impinge. Both inputs express several genes related to plasticity throughout the life span conferring the ability to remodel their synapses. In addition, we provided evidence that climbing fibers and Purkinje cells show remarkable reciprocal trophic interactions that are required for the maintenance of the correct synaptic connectivity. Through their activity, climbing fibers sustain the competition with parallel fibers by displacing this input to the distal territory of the Purkinje cell dendrite. In addition, they operate on the Purkinje cells through AMPA receptor suppressing spines in the territory surrounding their synapses. In this way, climbing fibers are able to optimize spine distribution and functional connectivity.

Glial fibrillary acidic protein (GFAP)-positive radial-like cells are present in the vicinity of proliferative progenitors in the nucleus tractus solitarius of adult rat

The dorsal vagal complex (DVC), an integrative center of autonomic functions located dorsally in the caudal brainstem, comprises the nucleus tractus solitarius (NTS), the area postrema (AP), and the dorsal motor nucleus of the vagus nerve (DMNX). Recently, this area of the brainstem was shown to retain, during adulthood, the expression of developmental markers, which is consistent with several forms of morphological and functional plasticity. These data led us to attempt to determine the structural organization and phenotypical characteristics of the astroglial compartment in the adult DVC. We report a strikingly high density of glial fibrillary acidic protein (GFAP) immunoreactive cells in the NTS and the DMNX compared to other brainstem structures. Furthermore, we observed a subpopulation of atypical GFAP+ cells in the NTS. These cells expressed vimentin and nestin and displayed unbranched processes that radiate rostrocaudally from cuboid cell bodies located in the 4th ventricle wall. Interestingly, these radiating cells were found in close association with neural progenitors whose proliferation was stimulated by intracerebroventricular injection of epidermal growth factor/basic fibroblast growth factor or lesion of the vagus nerve. Newly born neurons in the NTS identified by doublecortin (DCX) immunolabeling were also preferentially found in the vicinity of the radiating cells. Altogether, these results indicate that the adult NTS retains, during adulthood, astroglial cells that display morphological and phenotypical features seen during development. The overlap in the distribution of proliferative neural progenitors, newborn neurons, and radiating GFAP-positive cells suggest a possible role of the glial compartment of the NTS in functional plasticity in this structure.

Regulation of intrinsic neuronal properties for axon growth and regeneration

Regulation of neuritic growth is crucial for neural development, adaptation and repair. The intrinsic growth potential of nerve cells is determined by the activity of specific molecular sets, which sense environmental signals and sustain structural extension of neurites. The expression and function of these molecules are dynamically regulated by multiple mechanisms, which adjust the actual growth properties of each neuron population at different ontogenetic stages or in specific conditions. The neuronal potential for axon elongation and regeneration are restricted at the end of development by the concurrent action of several factors associated with the final maturation of neurons and of the surrounding tissue. In the adult, neuronal growth properties can be significantly modulated by injury, but they are also continuously tuned in everyday life to sustain physiological plasticity. Strict regulation of structural remodelling and neuritic elongation is thought to be required to maintain specific patterns of connectivity in the highly complex mammalian CNS. Accordingly, procedures that neutralize such mechanisms effectively boost axon growth in both intact and injured nervous system. Even in these conditions, however, aberrant connections are only formed in the presence of unusual external stimuli or experience. Therefore, growth regulatory mechanisms play an essentially permissive role by setting the responsiveness of neural circuits to environmental stimuli. The latter exert an instructive action and determine the actual shape of newly formed connections. In the light of this notion, efficient therapeutic interventions in the injured CNS should combine targeted manipulations of growth control mechanisms with task-specific training and rehabilitation paradigms.

Spontaneous Electrical Activity and Structural Plasticity in the Mature Cerebellar Cortex

The Purkinje cell of the cerebellar cortex presents two distinct dendritic domains: a distal one, with spiny branchlets and a high density of spines innervated by many parallel fibers, and a proximal one, with a few clusters of spines innervated by a single climbing fiber terminal arbor. In adult rats, after 7 days of blocked electrical activity by the administration of TTX into the cerebellar parenchyma, the proximal dendritic domain of the Purkinje cell shows a remarkable growth of new spines that are innervated by parallel fibers. At the same time, the climbing fiber terminal arbor tends to become atrophic. In contrast, in the branchlets, spine density remains unmodified. These changes are reversible when TTX is removed. TTX treatment also leads to a decrease in spine size both in the branchlets and in the new spines of the proximal dendritic compartment. Spontaneous electrical activity should therefore be regarded not simply as noise, but as a significant signal for maintaining the typical profile of afferent innervation of the Purkinje cell and for preventing spines from shrinking.

Expression of protein kinase C-substrate mRNAs in the basal ganglia of adult and infant macaque monkeys

We performed in situ hybridization histochemistry on the monkey basal ganglia to investigate the mRNA localization of three protein kinase C substrates (GAP-43, MARCKS, and neurogranin), of which expression plays a role in structural changes in neurites and synapses. Weak hybridization signals for GAP-43 mRNA and intense signals for both MARCKS and neurogranin mRNAs were observed in the adult neostriatum. All three of the mRNAs were expressed in both substance P-positive direct pathway neurons and enkephalin-positive indirect pathway neurons. In the nucleus accumbens, the hybridization signals for the three mRNAs were weaker than those in the neostriatum. Double-label in situ hybridization histochemistry in the neostriatum revealed that GAP-43 and neurogranin mRNAs were expressed in a subset of MARCKS-positive neurons. While intense hybridization signals for MARCKS mRNA were observed in all of the other basal ganglia regions such as the globus pallidus, substantia innominata, subthalamic nucleus, and substantia nigra, intense signals for GAP-43 mRNA were restricted to the substantia innominata and substantia nigra pars compacta. No signal for neurogranin mRNA was observed in the basal ganglia regions outside the neostriatum and the nucleus accumbens. These results indicate that the protein kinase C substrates are abundant in some specific connections in cortico-basal ganglia circuits. Developmental analysis showed that the expression level in the putamen and nucleus accumbens, but not in the caudate nucleus, was higher in the infant than in the adult, suggesting that synaptic maturation in the caudate nucleus occurs earlier than that in the putamen and nucleus accumbens.

Altered expression of MAP-2, GAP-43, and synaptophysin in the hippocampus of rats with chronic cerebral hypoperfusion correlates with cognitive impairment

Chronic cerebral hypoperfusion causes cognitive impairment, but the underlying molecular mechanism is not well understood. We used permanent occlusion of bilateral common carotid arteries (2-VO) to induce chronic cerebral hypoperfusion in male Wistar rats. Cognitive impairment and the expression patterns of MAP-2, GAP-43, and synaptophysin were examined. We found that both learning capacity and memory were gradually impaired in the rats with chronic cerebral hypoperfusion concomitant with increased duration of 2-VO treatment. Four weeks of 2-VO treatment resulted in down-regulation of synaptophysin expression at the protein levels, and a further decrease was observed at 10-20 weeks, although mRNA levels remained the same. Ten weeks of 2-VO treatment lead to down-regulation of MAP-2 expression at both the mRNA and protein levels with a further decrease at 20 weeks. Interestingly, GAP-43 mRNA was significantly up-regulated by 2-VO treatment, although the protein levels were not altered. Therefore, the cognitive impairment caused by chronic cerebral hypoperfusion may be partially explained by reduced expression of synaptophysin and MAP-2 at the protein level. The reduction in MAP-2 expression may be attributed to the inhibition of transcription, while the reduction in synaptophysin expression might be due to the inhibition of translation. Up-regulation of GAP-43 mRNA in the rat hippocampus with 2-VO treatment suggests that a compensatory mechanism may antagonize ischemic challenges.

Is postnatal neocortical maturation hierarchical?

To understand the postnatal development of the cerebral cortex we must know how changes in one cortical area depend on inputs from other cortical areas. Do cortical areas serving early stages of processing (primary sensory receiving areas) mature first, passing relatively stable outputs about sensorimotor relationships to cortical areas involved in higher stages of processing that are still developing? And, if some areas mature later than others, do they have functions that can account for aspects of adolescent behavior? Some observations support concurrent maturation in all cortical areas, others support a hierarchical sequence. Here, evidence on this important issue is evaluated, and means of obtaining reliable information are presented.

A field guide to the anterior olfactory nucleus (cortex)

While portions of the mammalian olfactory system have been studied extensively, the anterior olfactory nucleus (AON) has been relatively ignored. Furthermore, the existing research is dispersed and obscured by many different nomenclatures and approaches. The present review collects and assembles the relatively sparse literature regarding the portion of the brain situated between the olfactory bulb and primary olfactory (piriform) cortex. Included is an overview of the area's organization, the functional, morphological and neurochemical characteristics of its cells and a comprehensive appraisal of its efferent and afferent fiber systems. Available evidence suggests the existence of subdivisions within the AON and demonstrates that the structure influences ongoing activity in many other olfactory areas. We conclude with a discussion of the AON's mysterious but complex role in olfactory information processing.

Developmental changes in the expression of growth-associated protein-43 mRNA in the monkey thalamus: northern blot and in situ hybridization studies

The expression of growth-associated protein-43 has been related to axonal elongation and synaptic sprouting. Using the Northern blot analysis, we investigated the developmental changes of growth-associated protein-43 mRNA in the thalamus of macaque monkeys. The amount of growth-associated protein-43 mRNA was high at embryonic day 125, and decreased at postnatal day 1. It increased again at postnatal day 8, reached its peak value at postnatal days 50-70, and then decreased gradually until postnatal year 1. We previously reported that the amount of growth-associated protein-43 mRNA in the cerebral cortex decreased roughly exponentially during perinatal and postnatal periods and that it approached the asymptote by postnatal day 70 [Oishi T, Higo N, Umino Y, Matsuda K, Hayashi M (1998) Development of GAP-43 mRNA in the macaque cerebral cortex. Dev Brain Res 109:87-97]. The present findings may indicate that extensive synaptic growth of thalamic neurons continues even after that of cortical neurons has finished. We then performed in situ hybridization to investigate whether the expression level of growth-associated protein-43 mRNA was different among various thalamic nuclei. In the infant thalamus (postnatal days 70-90), moderate to intense expression of growth-associated protein-43 mRNA was detected in all thalamic nuclei. Quantitative analysis in the infant thalamus indicated that the expression levels were different between the nuclear groups that are defined by the origin of their afferents. The expression in the first order nuclei, which receive their primary afferent fibers from ascending pathways [Guillery RW (1995) Anatomical evidence concerning the role of the thalamus in corticocortical communication: a brief review. J Anat 187 (Pt 3):583-592], was significantly higher than that in the higher order nuclei. While moderate expression was also detected in the adult dorsal thalamus, the expression in the first order nuclei was almost the same as that in the higher order nuclei. Thus, the in situ hybridization experiments indicated that the transient postnatal increase in the amount of growth-associated protein-43 mRNA, which was shown by the Northern blot analysis, was mainly attributed to enhanced expression in the first order nuclei during the postnatal period. This may be a molecular basis for environmentally induced modification of thalamocortical synapses.

A soluble Nogo receptor differentially affects plasticity of spinally projecting axons

In the central nervous system, regeneration of injured axons and sprouting of intact axons are suppressed by myelin-derived molecules that bind to the Nogo receptor (NgR). We used a soluble form of the NgR (sNgR), constructed as an IgG of the human NgR extracellular domain, to manipulate plasticity of uninjured primary afferent and descending monoaminergic projections to the rat spinal cord following dorsal rhizotomy. Rats with quadruple dorsal rhizotomies were treated with intrathecal sNgR or saline, or were left untreated for 2 weeks. Rhizotomy alone resulted in sprouting of serotonergic axons and to a lesser extent, tyrosine-hydroxylase (TH)-expressing axons, while axons expressing dopamine-beta-hydroxylase (DbetaH) were unaffected. Human IgG immunohistochemistry revealed that sNgR infused into the intrathecal space penetrated approximately 300 microm into spinal white and grey matter. Separate axonal populations differed in their responses to intrathecal sNgR: TH-expressing and DbetaH-expressing axons responded most and least vigorously, respectively. Serotonergic axons were identified by serotonin (5-HT) or serotonin transporter (SERT) immunohistochemistry. Interestingly, a large increase in 5-HT compared to SERT-positive axons density in both saline and sNgR-treated rats indicated that serotonergic axons both sprouted and increased their transmitter content in response to rhizotomy and sNgR treatment. Calcitonin gene-related peptide-positive axons were largely depleted ipsilaterally by rhizotomy, and sNgR increased axon density only in deeper contralateral laminae (III-V). GAP-43 immunohistochemistry revealed a small increase in axon density following dorsal rhizotomy that was further augmented by sNgR treatment. These results reveal a differential effect of myelin antagonism on distinct populations of spinally projecting axons.

Cellular remodeling in mammalian retina: results from studies of experimental retinal detachment

Retinal detachment, the separation of the neural retina from the retinal pigmented epithelium, starts a cascade of events that results in cellular changes throughout the retina. While the degeneration of the light sensitive photoreceptor outer segments is clearly an important event, there are many other cellular changes that have the potential to significantly effect the return of vision after successful reattachment. Using animal models of detachment and reattachment we have identified many cellular changes that result in significant remodeling of the retinal tissue. These changes range from the retraction of axons by rod photoreceptors to the growth of neurites into the subretinal space and vitreous by horizontal and ganglion cells. Some neurite outgrowths, as in the case of rod bipolar cells, appear to be directed towards their normal presynaptic target. Horizontal cells may produce some directed neurites as well as extensive outgrowths that have no apparent target. A subset of reactive ganglion cells all fall into the latter category. Muller cells, the radial glia of the retina, undergo numerous changes ranging from proliferation to a wholesale structural reorganization as they grow into the subretinal space (after detachment) or vitreous after reattachment. In a few cases have we been able to identify molecular changes that correlate with the structural remodeling. Similar changes to those observed in the animal models have now been observed in human tissue samples, leading us to conclude that this research may help us understand the imperfect return of vision occurring after successful reattachment surgery. The mammalian retina clearly has a vast repertoire of cellular responses to injury, understanding these may help us improve upon current therapies or devise new therapies for blinding conditions.

Axonal and synaptic remodeling in the mature cerebellar cortex

By blocking electrical activity in the cerebellar cortex the Purkinje cell dendrites become a uniform territory with a high density of spines all bearing the glutamate receptor delta2 subunit (GluRdelta2) and being mainly innervated by parallel fibers. Such a subunit, which is constitutively targeted specifically to the parallel fiber synapses, appears in the spines contacted by the climbing fibers before they disconnect from the target. A similar pattern of hyperspiny transformation and innervation occurs a few days after a subtotal lesion of the inferior olive, the source of climbing fibers. During the climbing fiber reinnervation process which follows the removal of the electrical block or by collateral sprouting of surviving inferior olive neurons, the new active climbing fibers establish synaptic contacts with proximal dendritic spines that bear GluRdelta2s. After, they repress these subunits and displace the parallel fibers to the distal dendritic territory. These findings suggest the following operational principle in the axonal competition for a common target. The Purkinje cells have an intrinsic phenotypic profile which is compatible with the parallel fiber innervation, this mode being operational in targets innervated by a single neuronal population, like the neuromuscular system. An additional input, the climbing fibers, in order to achieve its own territory on the proximal dendrite needs the ability to displace the competitor. Such an inhibition is activity-dependent and the activity needs to be present in order to allow the climbing fiber to maintain its territory, even when the developmental period is over.

Reparative mechanisms in the cerebellar cortex

In the adult brain, different neuronal populations display different degrees of plasticity. Here, we describe the highly different plastic properties of inferior olivary neurones and Purkinje cells. Olivary neurones show a basal expression of growth-associated proteins, such as GAP-43 and Krox24/EGR-1, and remarkable remodelling capabilities of their terminal arbour. They also regenerate their transected neurites into growth-permissive territories and may reinnervate the lost target. Sprouting and regrowing olivary axons are able to follow specific positional information cues to establish new connections according to the original projection map. In addition, they set a strong cell body reaction to injury, which in specific olivary subsets is regulated by inhibitory target-derived cues. In contrast, Purkinje cells do not have a constitutive level of growth-associated genes, and show little cell body reaction, no axonal regeneration after axotomy, and weak sprouting capabilities. Block of myelin-derived signals allows terminal arbour remodelling, but not regeneration, while selective over-expression of GAP-43 induces axonal sprouting along the axonal surface and at the level of the lesion. We suggest that the high constitutive intrinsic plasticity of the inferior olive neurones allows their terminal arbour to sustain the activity-dependent ongoing competition with the parallel fibres in order to maintain the post-synaptic territory, and possibly underlies mechanisms of learning and memory. Such a plasticity is used also as a reparative mechanism following axotomy. In contrast, in Purkinje cells, poor intrinsic regenerative capabilities and myelin-derived signals stabilise the mature connectivity and prevent axonal regeneration after lesion.

GAP-43 mRNA detection by in situ hybridization, direct and indirect in situ RT-PCR in hippocampal and cerebellar tissue sections of adult rat brain

The growth-associated protein GAP-43 is a presynaptic membrane phosphoprotein that is expressed at high levels during development and axonal growth. To evaluate the cellular distribution of GAP-43 mRNA in the hippocampus and cerebellum of adult rats we applied in situ hybridization (ISH) as well as direct and indirect in situ RT-PCR using biotin as a reporter molecule. ISH resulted in a positive signal in most cerebellar granular cells and in 30% of hippocampal CA3 neurons. Direct in situ RT-PCR yielded cells with strong signals in every region investigated, with elevated background levels most likely related to incorporation of labeled nucleotides into non-specific amplicons through internal priming and DNA repair activity. Indirect in situ RT-PCR turned out to be the best approach for detecting GAP-43 mRNA positive cells. Cerebellar cells exhibiting a positive signal for GAP-43 mRNA were of the granular cell type (98%). Hippocampal neurons with a positive reaction for GAP-43 mRNA included all the neuron groups analyzed, namely CA1 (99%) and CA3 pyramidal cells (94%) and dentate gyrus granule cells (92%). Dentate gyrus granule cells have not tested positive for GAP-43 mRNA detection by molecular morphology analysis. These data show that in normal rats GAP-43 mRNA is present in different cell populations of hippocampal formation, supporting the role of this protein in the ongoing processes of synaptic plasticity.

Plasticity following Injury to the Adult Central Nervous System: Is Recapitulation of a Developmental State Worth Promoting?

The adult central nervous system (CNS) appears to initiate a transient increase in plasticity following injury, including increases in growth-related proteins and generation of new cells. Recent evidence is reviewed that the injured adult CNS exhibits events and patterns of gene expression that are also observed during development and during regeneration following damage to the mature peripheral nervous system (PNS). The growth of neurons during development or regeneration is correlated, in part, with a coordinated expression of growth-related proteins, such as growth-associated-protein-43 (GAP-43), microtubule-associated-protein-1B (MAP1B), and polysialylated-neural-cell-adhesion-molecule (PSA-NCAM). For each of these proteins, evidence is discussed regarding its specific role in neuronal development, signals that modify its expression, and reappearance following injury. The rate of adult hippocampal neurogenesis is also affected by numerous endogenous and exogenous factors including injury. The continuing study of developmental neurobiology will likely provide further gene and protein targets for increasing plasticity and regeneration in the mature adult CNS.

Cell type- and region-specific expression of protein kinase C-substrate mRNAs in the cerebellum of the macaque monkey

We performed nonradioactive in situ hybridization histochemistry in the monkey cerebellum to investigate the localization of protein kinase C-substrate (growth-associated protein-43 [GAP-43], myristoylated alanine-rich C-kinase substrate [MARCKS], and neurogranin) mRNAs. Hybridization signals for GAP-43 mRNA were observed in the molecular and granule cell layers of both infant and adult cerebellar cortices. Signals for MARCKS mRNA were observed in the molecular, Purkinje cell, and granule cell layers of both infant and adult cortices. Moreover, both GAP-43 and MARCKS mRNAs were expressed in the external granule cell layer of the infant cortex. In the adult cerebellar vermis, signals for both GAP-43 and MARCKS mRNAs were more intense in lobules I, IX, and X than in the remaining lobules. In the adult hemisphere, both mRNAs were more intense in the flocculus and the dorsal paraflocculus than in other lobules. Such lobule-specific expressions were not prominent in the infant cerebellar cortex. Signals for neurogranin, a postsynaptic substrate for protein kinase C, were weak or not detectable in any regions of either the infant or adult cerebellar cortex. The prominent signals for MARCKS mRNA were observed in the deep cerebellar nuclei, but signals for both GAP-43 and neurogranin mRNAs were weak or not detectable. The prominent signals for both GAP-43 and MARCKS mRNAs were observed in the inferior olive, but signals for neurogranin were weak or not detectable. The cell type- and region-specific expression of GAP-43 and MARCKS mRNAs in the cerebellum may be related to functional specialization regarding plasticity in each type of cell and each region of the cerebellum.

Extrinsic regulation of injury/growth-related gene expression in the inferior olive of the adult rat

Successful axon regeneration relies on the capability of the lesioned neurons to up-regulate a specific set of injury/growth-associated genes. In the adult central nervous system, the strength of the cell body response is generally related to the distance of the injury site from the perikaryon, being stronger for proximal lesions. Nevertheless, inferior olive (IO) cells react to injury and regenerate their axons even after distal transections. To investigate the mechanisms that regulate the IO growth properties, we examined the expression of injury/growth markers (nitric oxide synthase, growth-associated protein 43 and c-Jun) after target deletion or axotomy performed at different sites along the olivocerebellar pathway. Both axon injury and target loss disclose two subsets of IO neurons distributed within precise subnuclei: one subset up-regulates all markers in all conditions, whereas the other shows a mild c-Jun expression but remains unresponsive even after a very proximal axotomy. These observations indicate that distinct subpopulations of IO cells respond to different regulatory strategies. Unresponsive neurons appear insensitive to environmental positive or negative cues, suggesting that they are intrinsically unable to set up a cellular reaction to injury. In contrast, cell body changes in reactive neurons are elicited after the removal of retrogradely transported target-derived inhibitory signals. Target loss also induces degeneration of IO cells, whose survival remains partially dependent on Purkinje targets in adulthood. Thus, the intrinsic regenerative potential of a functionally homogeneous population is regulated by multiple mechanisms, specific for distinct neuronal subsets.

Evidence that ganglion cells react to retinal detachment

Growth associated protein 43 (GAP 43) is involved in synapse formation and it is expressed in the retina in a very specific pattern. Although GAP 43 is downregulated at the time of synapse formation, it can be re-expressed following injury such as axotomy or ischemia. Because of this we sought to characterize the expression of GAP 43 after retinal detachment (RD). Immunoblot, immunocytochemical and quantitative polymerase chain reaction (QPCR) techniques were used to assess the level of GAP 43 expression after experimental RD. GAP 43 was localized to three sublaminae of the inner plexiform layer of the normal retina. GAP 43 became upregulated in a subset of retinal ganglion cells following at least 7 days of RD. By immunoblot GAP 43 could be detected by 3 days. QPCR shows the upregulation of GAP 43 message by 6hr of detachment. To further characterize changes in ganglion cells, we used an antibody to neurofilament 70 and 200kDa (NF) proteins. Anti-NF labels horizontal cells, ganglion cell dendrites in the inner plexiform layer, and ganglion cell axons (fasicles) in the normal retina. Following detachment it is upregulated in horizontal cells and ganglion cells. When detached retina was double labelled with anti-GAP 43 and anti-NF, some cells were labelled with both markers, while others labelled with only one. We have previously shown that second order neurons respond to detachment; here we show that third order neurons are responding as well. Cellular remodelling of this type in response to detachment may explain the slow recovery of vision that often occurs after reattachment, or those changes that are often assumed to be permanent.

Corticothalamic cells in layers 5 and 6 of primary and secondary sensory cortex express GAP-43 mRNA in the adult rat

The expression of a presynaptic phosphoprotein, growth-associated protein (GAP)-43, is associated with synaptogenesis during development and synaptic remodeling in the adult. This study examined GAP-43 mRNA expression and distribution in primary and secondary areas of visual, auditory, and somatosensory cortex of the adult rat, by in situ hybridization with a digoxigenin-coupled mRNA probe, focusing particularly on the corticothalamic cells in layers 5 and 6. In the six cortical areas studied, GAP-43 mRNA was expressed predominantly in layers 5 and 6 and was greater in secondary than primary areas. There were densely labeled cells in layers 5 and 6 of all areas, which showed a restricted sublaminar distribution in primary areas and more even distribution in secondary areas. Combining retrograde transport of rhodamine beads with in situ hybridization in visual and auditory cortex showed that corticothalamic cells in layers 5 and 6 express GAP-43 mRNA. There are more of these GAP-43 mRNA positive corticothalamic cells in layer 5 of secondary areas than in primary areas. The evidence suggests that in the adult rat, plasticity related to GAP-43 is present in primary and secondary sensory cortex and more so in secondary areas.

Acoustic trauma induces reemergence of the growth- and plasticity-associated protein GAP-43 in the rat auditory brainstem

We explored the consequences of unilateral acoustic trauma to intracochlear and central nervous system structures in rats. An acoustic trauma, induced by applying click stimuli of 130 dB (sound pressure level; SPL) for 30 minutes, resulted in an instant and permanent threshold shift of 95.92 +/- 1.08 dB (SEM) in the affected ear. We observed, as a consequence, a structural deterioration of the organ of Corti. Deprivation-dependent changes of neurons of the auditory brainstem were determined using antibodies against neurofilament and the growth-associated protein GAP-43 and compared with those following cochleotomy, studied earlier. By 231 days posttrauma, spiral ganglion cell bodies and their processes were almost entirely lost from all cochlear regions with destroyed organ of Corti. In the lateral superior olive (LSO) ipsilateral to the trauma, cell bodies of lateral olivocochlear neurons turned transiently GAP-43 positive within the first 1.5 years posttrauma. The time course of emergence and disappearance of this population of neurons was similar to that found after cochleotomy. Additionally, after noise trauma, principal cells in contralateral LSO and in medial superior olive (MSO) on both sides of the brainstem developed an expression of GAP-43 that began 3 and 16 days posttrauma, respectively, and lasted for at least 1 year. Such cells were rarely observed after cochleotomy. An unequivocal rise in GAP-43 immunoreactivity was also found in the neuropil of the inferior colliculus and the ventral cochlear nucleus, both preferentially on the acoustically damaged side. We conclude that the degree and specific cause of sudden unilateral deafness entail specific patterns of plasticity responses in the auditory brainstem, possibly to prevent the neural network dedicated to locate sounds in the environment from delivering erroneous signals centralward.

Long-term injured purkinje cells are competent for terminal arbor growth, but remain unable to sustain stem axon regeneration

Long-distance axon regeneration requires the activation of a specific set of neuronal growth-associated genes. Adult Purkinje cells fail to upregulate these molecules in response to axotomy and show extremely weak regenerative properties. Nevertheless, starting from several months after injury, transected Purkinje axons undergo spontaneous sprouting. Here, we asked whether long-term injured Purkinje cells acquire novel intrinsic growth properties that enable them to upregulate growth-associated genes and sustain axon regeneration. To test this hypothesis, we examined axon growth and cell body changes in adult rat Purkinje neurons following axotomy and implantation of embryonic neocortical tissue or Schwann cells into the injury track. Purkinje cells that survived over 6 months after injury/transplantation displayed profuse sprouting in the injured cerebellum and developed extensive networks of terminal branches into embryonic neocortical grafts. In addition, severed Purkinje axons exposed to these transplants 6 months after injury grew faster than their counterparts confronted with the same environment immediately after axotomy. Nevertheless, long-term injured Purkinje cells failed to regenerate stem neurites into Schwann cell grafts, and, under all experimental conditions, they did not upregulate growth-associated molecules, including c-Jun, GAP-43, SNAP-25, and NADPH-diaphorase. These results indicate that the long-term injured Purkinje cells remain unable to activate the gene program required to sustain axon regeneration and their plasticity is restricted to terminal arbor remodeling. We propose that the delayed growth of injured Purkinje cells reflects an adaptive phenomenon by which the severed axon stump develops a new terminal arbor searching for alternative connections with local partners.

Growth-associated protein 43 is up-regulated in the ganglion cells of the ischemic rat retina

We investigated the expression and cellular localization of growth-associated protein (GAP)-43 in the rat retina following ischemia induced by transiently increased intraocular pressure. In the normal retina, GAP-43 immunoreactivity was restricted to profiles in the inner plexiform layer. Following ischemia and reperfusion, immunoreactivity appeared in ganglion cells. The cell density of labeled ganglion cells peaked three days post-lesion and then decreased at seven days. Quantitative evaluation by immunoblotting confirmed that GAP-43 expression increased at three days (to 190% of control levels) and then slightly decreased at seven days. Our findings suggest that some ganglion cells have the potential to regenerate through the up-regulation of GAP-43 in the ischemic rat retina.

Cellular distribution of GAP-43 mRNA in hippocampus and cerebellum of adult rat brain by in situ RT-PCR

The growth-associated protein GAP-43 is a presynaptic membrane phosphoprotein that plays a key role in guiding the growth of axons and in modulating the formation of new synapses. To identify the cells that synthesize GAP-43 mRNA, we applied direct in situ reverse transcription-polymerase chain reaction (in situ RT-PCR) in cerebellum and hippocampus of adult rat brain. In situ RT-PCR revealed GAP-43 mRNA in cerebellar granule cells, in Purkinje cells and in some interneurons of the molecular layer. Previous in situ hybridization studies had demonstrated a dense label throughout the granular layer of the cerebellar cortex but no labeling of other cerebellar neurons. Hippocampal cells showing distinct GAP-43 mRNA signal after in situ RT-PCR were CA1 and CA3 pyramidal neurons, CA4 hilar cells, and dentate gyrus granule cells, whereas in situ hybridization studies had detected GAP-43 mRNA only in CA3 and CA1 pyramidal neurons. Our data indicate that GAP-43 mRNA is widely distributed, suggesting that many cell types are potentially involved in synaptic plasticity events. (J Histochem Cytochem 49:1195-1196, 2001)

Role of glutamate delta -2 receptors in activity-dependent competition between heterologous afferent fibers

A principle that regulates detailed architecture in the brain is that active terminals have a competitive advantage over less active terminals in establishing synaptic connections. This principle is known to apply to fibers within a single neuronal population competing for a common target domain. Here we uncover an additional rule that applies when two neuronal populations compete for two contiguous territories. The cerebellar Purkinje cell dendrites have two different synaptic domains with spines innervated by two separate excitatory inputs, parallel fibers (PFs) and climbing fibers (CFs). Glutamate delta-2 receptors are normally present only on the PF spines where they are important for their innervation. After block of activity by tetrodotoxin, numerous new spines form in the CF domain and become innervated mainly by PFs; all spines, including those still innervated by the CFs, bear delta-2 receptors. Thus, in the absence of activity, PFs gain a competitive advantage over CFs. The entire dendritic arbor becomes a uniform territory with the molecular cues associated with the PFs. To access their proper territory and maintain synaptic contacts, CFs must be active and locally repress the cues of the competitor afferents.

NMDA receptor and nitric oxide synthase activation regulate polysialylated neural cell adhesion molecule expression in adult brainstem synapses

Here we report that synapses in the adult dorsal vagal complex, a gateway for many primary afferent fibers, express a high level of the polysialylated neural cell adhesion molecule (PSA-NCAM). We show that electrical stimulation of the vagal afferents causes a rapid decrease of PSA-NCAM expression both in vivo and in acute slices. Inhibition of NMDA receptor activity completely prevented the decrease. Blockade of calmodulin activation, neuronal nitric oxide (NO) synthase, or soluble guanylyl cyclase and chelation of extracellular NO mimicked this inhibition. Our data provide a mechanistic framework for understanding how activity-linked stimulation of the NMDA-NO-cGMP pathway induces rapid changes in PSA-NCAM expression, which may be associated with long-term depression.

Neuronal BC1 RNA: co-expression with growth-associated protein-43 messenger RNA

Brain-specific cytoplasmic RNA 1 (BC1-RNA), a non-coding RNA polymerase III transcript, is a neuronal RNA that is specifically targeted to dendritic domains. It is co-localized with components of the dendritic protein synthetic machinery, and it has been suggested to operate in the regulation of local translation-related processes in postsynaptic microdomains, thus subserving long-term synaptic plasticity in neurons. To probe the relevance of BC1 expression in neuronal plasticity, we have analyzed the expression pattern of BC1 RNA in the rat nervous system. We found that BC1 RNA is expressed by a specific subset of neurons (but not by non-neuronal cells) in the central and peripheral nervous system of the adult rat. The BC1 labeling pattern indicates that the subcellular location of the RNA is typically postsynaptic which, depending on cell type, manifests itself in a predominantly somatic, somatodendritic, or dendritic location. Our results further show that BC1-expressing neurons typically co-express the messenger RNA for growth-associated protein-43 (GAP-43). Such co-expression was observed in diverse brain areas, including the olfactory bulb, neocortex, and hippocampus, among others. While BC1 RNA was in many neuronal cell types detectable in distal dendritic domains, GAP-43 messenger RNA was typically more restricted to neuronal perikarya. In the mature nervous system, expression of GAP-43 has been described as an intrinsic determinant of predominantly presynaptic plasticity, while BC1 RNA has been implicated in postsynaptic plasticity. Co-expression of both RNAs, as reported here, thus identifies a distinct subset of neurons in the rat nervous system that exhibits both types of plasticity.

Differential effects of amphetamine and phencyclidine on the expression of growth-associated protein GAP-43

The purpose of the present study was to test changes in the expression of growth-associated protein (GAP-43) after chronic treatment with two different psychotomimetic drugs: amphetamine and phencyclidine. Rats were treated chronically for 7 days (twice daily) with 5 mg/kg of amphetamine and phencyclidine and sacrificed after 2, 5 or 7 days of treatment, and following 7, 14 or 21 days of recovery after full treatment (7 days). Separate groups of rats were treated on the same regiment with haloperidol, and control group was treated with vehicle. To determine the effects of different psychotomimetic drugs on the expression of GAP-43 we have used Northern blotting and quantitative in situ hybridization. Treatment with amphetamine induced decrease of GAP-43 mRNA expression, that was detected also during recovery period, up to 14 days after the last day of 7 days treatments. On the contrary, PCP induced increase of GAP-43 mRNA expression, that was detectable from the first days of treatment until 21 days after the last day of treatment. Treatment with haloperidol did not produce significant changes in GAP-43 mRNA expression. It can be suggested that GAP-43 upregulation upon phencyclidine treatment occurs as a result of functional activation of pathways able to participate in remodeling, while amphetamine showed neurotoxic effect, decreasing expression of GAP-43 mRNA.

Role of GAP-43 in mediating the responsiveness of cerebellar and precerebellar neurons to axotomy

To determine whether the competence for axonal sprouting and/or regeneration in the cerebellar system correlates with GAP-43 expression, we have studied GAP-43 mRNA and protein expression in the postlesioned cerebellum and inferior olive. Purkinje cells transiently express GAP-43 during their developmental phase (from E15 to P5 in the rat) which consists of fast axonal growth and the formation of the corticonuclear projection. Adult Purkinje cells, which in control adult rats do not express GAP-43, are extremely resistant to the effects of axotomy but cannot regenerate axons. However, a late and protracted sprouting of axotomized Purkinje cells occurs spontaneously and correlates with a mild expression of GAP-43 mRNA. In contrast, inferior olivary neurons, despite their high constitutive expression of GAP-43, do not sprout but retract their axons and die after axotomy. Furthermore, mature Purkinje cells in cerebellar explants of transgenic mice that overexpress GAP-43 do not regenerate after axotomy, even in the presence of a permissive substrate (cerebellar embryonic tissue) and, contrary to the case in wild-type mice, they do not survive in the in vitro conditions and undergo massive cell death. These results show that the expression of GAP-43 is not only associated with axonal growth, but also with neuronal death.

Co-induction of nitric oxide synthase, bcl-2 and growth-associated protein-43 in spinal motoneurons during axon regeneration in the lizard tail

In lizards, tail loss transects spinal nerves and the cut axons elongate in the regrowing tail, providing a natural paradigm of robust regenerative response of injured spinal motoneurons. We previously ascertained that these events involve nitric oxide synthase induction in the axotomized motoneurons, suggesting a correlation of this enzyme with regeneration-associated gene expression. Here we investigated, in lizards, whether the cell death repressor Bcl-2 protein and growth-associated protein-43 (GAP-43) were also induced in motoneurons that innervate the regenerated tail in the first month post-caudotomy. Single and multiple immunocytochemical techniques, and quantitative image analysis, were performed. Nitric oxide synthase, GAP-43 or Bcl-2 immunoreactivity was very low or absent in spinal motoneurons of control lizards with intact tail. Nitric oxide synthase and GAP-43 were induced during the first month post-caudotomy in more than 75% of motoneurons which innnervate the regenerate. Bcl-2 was induced in approximately 95% of these motoneurons at five and 15days, and in about 35% at one month. The intensity of Bcl-2 and GAP-43 immunostaining peaked at five days, and nitric oxide synthase at 15days; immunoreactivity to these proteins was still significantly high at one month. Immunofluorescence revealed co-localization of nitric oxide synthase, GAP-43 and Bcl-2 in the vast majority of motoneurons at five and 15days post-caudotomy. These findings demonstrate that co-induction of nitric oxide synthase, Bcl-2 and GAP-43 may be part of the molecular repertoire of injured motoneurons committed to survival and axon regeneration, and strongly favor a role of nitric oxide synthase in motoneuron plasticity.

Evolution of the Purkinje cell response to injury and regenerative potential during postnatal development of the rat cerebellum

To understand the mechanisms leading to the progressive loss of intrinsic neuronal growth properties during central nervous system development, we have investigated the evolution of the response to injury and regenerative potential of immature Purkinje cells, axotomized at different postnatal ages from postnatal day (P)3 to P12. In adult rodents, these neurons are characterised by a weak cell body response to axotomy, which is associated with a remarkable resistance to injury and a poor regenerative capability. During the first postnatal week, Purkinje cells are strongly sensitive to injury and massively degenerate within a few days. Immature Purkinje cells react to neurite transection by a strong upregulation of c-Jun, accompanied by a moderate, but consistent, expression of the growth-associated protein (GAP)-43. In contrast, nicotinamide adenine dinucleotide monophosphate (NADPH)-diaphorase reactivity, which can be activated by adult Purkinje neurons, is not modified in their juvenile counterparts. The severed Purkinje axons show a vigorous regenerative sprouting both into the lesioned cerebellar environment and into embryonic neocortical tissue transplanted into the injury site. The typical adult features of the response to injury progressively develop during the second postnatal week, when the injured neurons acquire resistance, cell body changes become milder, the regenerative potential declines, and the severed axons undergo characteristic morphological modifications, including torpedoes and the hypertrophy of recurrent collateral branches. This complete reversal of the features and the outcome of the Purkinje cell reaction to axotomy likely results from the profound changes that occur in the maturing Purkinje cells and/or in their microenvironment during this phase of cerebellar development.

Regulation of GAP-43 protein and mRNA in nigrostriatal dopaminergic neurons after the partial destruction of dopaminergic terminals with intrastriatal 6-hydroxydopamine

Changes in the level of GAP-43 and its mRNA in nigrostriatal dopaminergic neurons in an animal model of the presymptomatic period of Parkinson's disease were measured to find the characteristic features of GAP-43 in nigrostriatal dopaminergic neurons. Since the dopaminergic neurons possess a relatively large amount of GAP-43 protein and mRNA, the dopaminergic neurons must be endowed with specific functions related to those of GAP-43. In this study, dopaminergic axon terminals were partially destroyed by intrastriatal 6-hydroxydopamine (6-OHDA). Rats were decapitated 3, 14, and 56 days following treatment. Levels of GAP-43 and tyrosine hydroxylase (TH) in the striatum were detected by immunoblotting and quantified. The number of GAP-43 mRNA-positive neurons and that of TH mRNA-positive neurons in the substantia nigra pars compacta (SNc) were detected by in situ hybridization using alkaline phosphatase (ALP)-labeled probes. Levels of GAP-43 in the striatum showed no significant alteration during the period of the experiment, although levels of TH were gradually restored. The number of GAP-43 mRNA-positive neurons as well as that of TH mRNA-positive neurons in the SNc decreased. These results suggests that dopaminergic neurons restore their axon terminals with little change in GAP-43, and that transcription and/or stability of GAP-43 mRNA in the dopaminergic neurons are susceptible to the toxin, although the dopaminergic neurons can maintain the translational product in the terminals. This feature may be related with a degeneration of dopaminergic neurons in Parkinson's disease.