A membrane phosphoprotein associated with neural development, axonal regeneration, phospholipid metabolism, and synaptic plasticity

Expression of the growth-associated protein GAP-43 in adult rat retinal ganglion cells following axon injury

We have studied the expression of the growth-associated protein GAP-43 after injury to the axons of adult rat retinal ganglion cells (CNS neurons that do not normally regenerate injured axons). Both the biosynthetic labeling of GAP-43 and the GAP-43 immunoreactivity of the retina increased after axotomy, but only when the injury was within 3 mm of the eye. These results suggest the following conclusions: First, axon injury is sufficient to alter GAP-43 expression in CNS neurons, even in the absence of regeneration. Second, mechanisms that regulate GAP-43 expression are sensitive to the length of uninterrupted axon remaining after injury. Finally, the conditions that favor increased GAP-43 are similar to those that favor regrowth of injured CNS axons into grafts of peripheral nerve, suggesting that GAP-43 induction is accompanied by an increased potential of injured CNS neurons to regenerate.

Dibutyryl-cAMP induces SNAP-25 translocation into the neurites in PC12

SNAP-25 immunoreactivity was translocated into the endings of the processes induced in PC12 cells by dibutyryl-cAMP-treatment. Conversely, the protein was not present in the endings of the processes seen after NGF-treatment unless dibutyryl-cAMP was used simultaneously. This redistribution of SNAP-25 immunoreactivity appeared to be dependent upon new protein synthesis. Finally, dibutyryl-cAMP was capable of inducing SNAP-25 expression.

Redistribution of GAP-43 during growth cone development in vitro; immunocytochemical studies

The growth-associated protein GAP-43 (B-50, F1, pp46), has been found in elongating axons during development and regeneration, and has also been associated with synaptic plasticity in mature neurons. We have examined the loss of GAP-43 labelling from cerebellar granule cells with immunocytochemical localization of a polyclonal antibody to GAP-43. One day after plating, the plasma membrane of cell bodies, neurites and growth cones were all labelled with anti-GAP-43. By 10 days, most of the cell body labelling was lost, and by 20 days the neuritic and growth cone labelling was greatly reduced. Beginning at six days, anti-GAP-43 labelling of growth cones, which was initially uniform, became clustered. When growth cones were double-labelled with antibodies to GAP-43 and the synaptic vesicle protein, p65, inverse changes in the distribution of label was observed. While growth cone labelling with anti-p65 increased from three to 20 days in culture, GAP-43 label began to be lost from some growth cones by six days and showed continuing decline through 20 days. For individual growth cones, the loss of GAP-43 appeared to parallel the accumulation of p65, and first growth cones to lose GAP-43 appeared to be the first to accumulate p65 label. When cultures were grown on a substrate of basement membrane material, the time frames of neuritic outgrowth, loss of GAP-43 labelling, and increase in p65 labelling were all accelerated. At five days, labelling for GAP-43 was weak and labelling for p65 was strong, in a pattern comparable to that seen in older cultures on a polylysine substrate. These results suggest several conclusions concerning the expression and loss of GAP-43 in cultured cerebellar granule neurons. First, GAP-43 label is initially distributed in all parts of these cells. With increasing time in culture the label is first lost from cell bodies and later from neurites and growth cones. Second, the loss of GAP-43 label from growth cones is correlated with the appearance of the synaptic vesicle protein p65. Finally, in vitro developmental changes in the loss of GAP-43 can be altered by changing the growth substrate.

Microencephaly reduces the phosphorylation of the PKC substrate B-50/GAP43 in rat cortex and hippocampus

The administration of the antimitotic agent methylazoxymethanol (MAM) to rats at day 15 of gestation results in a consistent loss of intrinsic neurons primarily in cortex and hippocampus. These animals when adult, show a cognitive impairment, if tested in specific behavioural tasks. B-50/GAP43 is a neuronal phosphoprotein, specific substrate for protein kinase C (PKC) and involved in the development and plasticity of synaptic connections. Since B-50/GAP43 has been implicated in functional modulation of synapses and in the molecular mechanism underlying cognitive processes, we studied the phosphorylation of B-50 in cortex and hippocampus of control and MAM-treated rats. Here we report that B-50 in MAM-treated rats shows a marked reduction in the phosphate incorporation in the areas affected by the prenatal treatment. In situ hybridization studies demonstrate that the mRNA levels for B-50 are not altered in MAM-treated rats and that the relative amount of the protein, as revealed by Western blot analysis, is also not affected in microencephalic rats. These results suggest that microencephalic animals might represent a useful experimental model to study biochemical correlates of cognitive impairment and synaptic plasticity.

Ultrastructural localization of adrenocorticotrophic hormone and the phosphoprotein B-50/growth-associated protein 43 in freeze-substituted, Lowicryl HM20-embedded mesencephalic central gray substance of the rat

Previous studies have shown that the endogenous phosphorylation of the neuron-specific protein B-50 in isolated synaptic plasma membranes is inhibited by adrenocorticotrophic hormone(1-24). The aim of this study is to examine if there is a specific neuroanatomical interaction of adrenocorticotrophic hormone and B-50 in the mesencephalic central gray substance of the rat. With light microscopy, high B-50 immunoreactivity was detected throughout the mesencephalic central gray substance, overlapping with those areas where adrenocorticotrophic hormone-immunoreactive fibres were present. To study the ultrastructural localization of B-50 and adrenocorticotrophic hormone, we employed a method of immunogold labelling on ultrathin sections of freeze-substituted and Lowicryl HM20-embedded fixed brain tissue. This offered optimal morphological preservation together with high retention of antigenicity. At the electron microscopic level, adrenocorticotrophic hormone immunoreactivity was detected in dense-core secretory granules present in non-junctional regions of axoinal varicosities. This suggests a non-synaptic release of adrenocorticotrophic hormone from the axons. Using double immunolabelling techniques we showed that in adrenocorticotrophic hormone-innervated areas of the mesencephalic central gray substance B-50 immunoreactivity was present at plasma membranes of all unmyelinated axons and axonal varicosities and virtually absent in dendrites. The result on B-50 localization agrees well with previous studies in the hippocampus [Van Lookeren Campagne et al. 1990 J. Neurocytol. 19, 948-961] and in the pyramidal tract [Gorgels et al. 1989 J. Neurosci. 9, 3861-3869] of the rat and suggests that in the mature rat central nervous system, B-50 expression in axons is a general phenomenon. For the adrenocorticotrophic hormone-innervated areas, we discuss the proposal that non-synaptically released adrenocorticotrophic hormone modulates B-50 phosphorylation in axons and axon terminals.

Mapping the development of the rat brain by GAP-43 immunocytochemistry

Growth-associated protein-43 (GAP-43) is a phosphoprotein of the nerve terminal membrane which has been linked to the development and restructuring of axonal connections. Using a monospecific antibody prepared in sheep against purified GAP-43, we examined the temporal and spatial changes in the distribution of this protein from embryonic stage day 13 (E13) to adulthood. At stages in which neurons are still dividing and migrating, levels of GAP-43 are extremely low, as is seen in the cortical plate throughout the embryonic period. With the onset of process outgrowth, intense GAP-43 immunoreactivity appears along the length of axons: by E13, such staining is already strong in the brainstem, where it continues up through the first postnatal week and then disappears. In the neocortex, intense fiber staining first appears several days later but ends at the same time as in the brainstem. At the end of the period of intense axonal staining there is a brief interval in which high levels of GAP-43 immunostaining are seen in the neuropil. In regions of the brain in which specific developmental events have been characterized anatomically and physiologically, the period of dense neuropil staining coincides with the formation of axonal end-arbors, the beginning of synaptogenesis, and the time at which synaptic organization can be modified by the impingent pattern of activity (i.e. the critical period). Over the next few days, staining in neuropil declines sharply in most regions except for certain structures in the rostral neuraxis which may be sites of ongoing synaptic remodeling.

GAP-43 expression in developing cutaneous and muscle nerves in the rat hindlimb

The expression of the growth associated protein, GAP-43, in developing rat hindlimb peripheral nerves has been studied using immunocytochemistry. GAP-43, is first detected in lumbar spinal nerves at embryonic day (E)12 as the axons grow to the base of the hindlimb. It is expressed along the whole length of the nerves as well as in the growth cones. GAP-43 staining becomes very intense over the next 36 h while the axons remain in the plexus region at the base of the limb bud before forming peripheral nerves at E14. It remains intense along the length of the growing peripheral nerves, the first of which are cutaneous, branching away from the plexus and growing specifically to the skin, their axon tips penetrating the epidermis of the proximal skin at E15 and the toes at E19. GAP-43-containing terminals form a dense plexus throughout the epidermis which subsequently withdraws subepidermally in the postnatal period. GAP-43 staining is also evident along the growing muscle nerves during muscle innervation, which follows behind that of skin. Axons branch over the surface of proximal muscles at E15 but do not form terminals until E17. As target innervation proceeds, GAP-43 staining declines in the proximal part of the nerve but remains intense in the distal portions. Overall GAP-43 expression in the hindlimb decreases in the second postnatal week as axon growth and peripheral terminal formation decline.

Developmental changes in B-50 (GAP-43) in primary cultures of cerebral cortex: B-50 immunolocalization, axonal elongation rate and growth cone morphology

Changes in neurite outgrowth parameters and in the immunolocalization of the neuronal growth-associated protein B-50 (GAP-43) were studied in cultured neocortex as a function of development. In addition, we studied the effects of chronic blockade of bioelectric activity (BEA) with tetrodotoxin (TTX) on these parameters. Axonal outgrowth rate in control cultures reached a maximum at 8 days in vitro (DIV) and declined to a low level at 21 DIV. B-50 staining shifted from the perikaryon to the axons and growth cones during the first 3 DIV. In axons the intensity of B-50 staining increased towards the growth cone. Within growth cones, the central/basal region and filopodia were intensely stained, whereas lamellipodia showed only marginal staining. Growth cone size gradually decreased after 3 DIV, due to the successive loss of lamellipodia and filopodia, and became club-shaped during the second week, until by 21 DIV growth cones were completely lost, and axons started retracting and degenerated. In the central area of the cultures, growth cones also decreased in size with time, but became stabilized as presynaptic elements onto other neurons. Acute addition of TTX did not affect the outgrowth rate at 6 DIV. Chronic TTX treatment led to an earlier retraction and degeneration of axons than in control cultures and to a loss of B-50-stained cells and varicosities during the third week, but did not affect growth cone morphology or B-50 staining. The regressive phenomena are probably due to an increased neuronal cell death shown to occur after chronic TTX treatment. The developmental changes in axonal elongation rate and growth cone morphology may be related to developmental changes in the content and/or phosphorylation of B-50 (GAP-43, which are studied in the same cultures in the following paper (Ramakers et al. (1991) Int. J. Devl Neurosci. 9, 231-241].

Developmental changes in B-50 (GAP-43) in primary cultures of cerebral cortex: content and phosphorylation of B-50

The content and phosphorylation of the neuronal growth-associated protein B-50 (GAP-43) were studied in cultured neocortex as a function of normal development and development in the presence of tetrodotoxin (TTX), a blocker of bioelectric activity (BEA). The observations were correlated with previous morphological findings on neurite outgrowth and B-50 immunolocalization in the same cultures. In control cultures, the concentration of B-50 reached a maximum at 7 days in vitro (DIV) and decreased thereafter, whereas the concentration of neuron specific enolase (NSE), which was used as a neuronal reference marker, rose till 28 DIV and leveled off towards 42 DIV. The degree of basal phosphorylation of B-50 (relative to that of total protein) decreased after the first week in vitro. Stimulation of B-50 phosphorylation by phorbol ester also decreased with age in vitro, indicating that changes in B-50 phosphorylation were mainly due to changes in protein kinase C (PKC) activity. The chronic presence of TTX led to a reduced content of B-50 and NSE after 14 DIV. The basal phosphorylation of B-50 was neither affected by acute nor chronic TTX treatment. However, upon stimulation of PKC with phorbol esters, some alterations of B-50 phosphorylation were revealed in cultures grown in TTX. These biochemical observations are in line with the absence of effects of TTX on neurite outgrowth during the first 2 weeks in culture, and later effects of TTX on neuronal survival. The developmental changes in B-50 concentration and phosphorylation largely correlate with previous morphological observations on axonal outgrowth and growth cone shape in the same cultures. We suggest that B-50 phosphorylation plays an important role in transducing extracellular signals into directed neurite outgrowth.

GAP-43 expression in the developing rat lumbar spinal cord

The expression of the growth-associated protein GAP-43, detected by immunocytochemistry, has been studied in the developing rat lumbar spinal cord over the period E11 (embryonic day 11), when GAP-43 first appears in the spinal cord, to P29 (postnatal day 29) by which time very little remains. Early GAP-43 expression in the fetal cord (E11-14) is restricted to dorsal root ganglia, motoneurons, dorsal and ventral roots and laterally positioned and contralateral projection neurons and axons. Most of the gray matter is free of stain. The intensity of GAP-43 staining increases markedly as axonal growth increases, allowing clear visualization of the developmental pathways taken by different groups of axons. Later in fetal life (E14-19), as these axons find their targets and new pathways begin to grow, the pattern of GAP-43 expression changes. During the period, GAP-43 staining in dorsal root ganglia, motoneurons, and dorsal and ventral roots decreases, whereas axons within the gray matter begin to express the protein and staining in white matter tracts increases. At E17-P2 there is intense GAP-43 labelling of dorsal horn neurons with axons projecting into the dorsolateral funiculus and GAP-43 is also expressed in axon collaterals growing into the gray matter from lateral and ventral white matter tracts. At E19-P2, GAP-43 is concentrated in axons of substantia gelatinosa. Overall levels decline in the postnatal period, except for late GAP-43 expression in the corticospinal tract, and by P29 only this tract remains stained.

Time-dependent differences in the increase in GAP-43 expression in dorsal root ganglion cells after peripheral axotomy

Peripheral axotomy of primary afferent neurons results in the up-regulation of the growth-associated phosphoprotein GAP-43, by dorsal root ganglion cells. We have studied the temporal sequence of GAP-43 expression in those dorsal root ganglion neurons with unmyelinated axons (the small dark cells) and in those with myelinated axons (the large light cells) after sciatic nerve section in the adult rat. Immunoreactivity for the RT 97 neurofilament epitope, which is detectable only in large light dorsal root ganglion cells, was used to differentiate the two types of dorsal root ganglion cell. Within two days of a sciatic nerve section the number of GAP-43-immunoreactive profiles in the ipsilateral ganglion had increased five-fold and this increase persisted for 80 days post-section. While 50% of the small numbers of GAP-43-positive cells in control ganglia were RT 97 positive, only 8% of the large number of GAP-43-immunoreactive cells four days post-section, were RT 97 positive. By 14 days the number of RT 97-positive/GAP-43-positive cells had increased to 29%. This was paralleled by an increase in GAP-43 immunoreactivity in large diameter profiles at 14 days. The signals that alter GAP-43 expression in unmyelinated (small, RT 97 -ve) and myelinated (large, RT 97 +ve) afferents after peripheral nerve injury appear to operate with different time-courses.

Regulation of gene expression in the olfactory neuroepithelium: a neurogenetic matrix

The olfactory neuroepithelium exhibits neurogenesis throughout adult life, and in response to lesions, a phenomenon that distinguishes this neural tissue from the rest of the mammalian brain. The newly formed primary olfactory neurons elaborate axons into the olfactory bulb. Thus, denervation and subsequent reinnervation of olfactory bulb neurons may occur throughout life. This unique ability of the olfactory neuroepithelium to generate new neurons from a population of precursor cells present in the basal cell layer of this tissue makes it a valuable model in the study of neural development and regeneration. The molecular processes underlying the neurogenic properties of the olfactory neuroepithelium are poorly understood. Here we have reviewed our studies on the expression of B50/GAP43 during ontogeny of the olfactory system and following lesioning. This analysis includes the characterization of the expression of OMP, a protein expressed in mature olfactory neurons, as well as PKC and calmodulin. The latter two molecules are of particular interest to the function of B50/GAP43 since the degree of phosphorylation of B50/GAP43 appears to determine B50/GAP43's ability to bind calmodulin (see also Storm, chapter 4, this volume). In the mature olfactory epithelium B50/GAP43 expression is restricted to a subset of cells located in the basal region. Since the expression of B50/GAP43 is high in developing and regenerating nerve cells we are confident that the B50/GAP43 positive cells are new neurons derived from the stem cells in the basal region of the epithelium. B50/GAP43 is absent from the stem cells themselves and also from the mature OMP-expressing neurons. On the basis of the patterns of B50/GAP43 and OMP expression two stages could be discriminated in the regeneration of the olfactory epithelium. First, as an immediate response to lesioning a large population of B50/GAP43 positive, OMP negative neurons are formed. Subsequently, during the second stage, these newly formed differentiating neurons mature as evidenced by a decrease in B50/GAP43 and an increase in OMP expression. The second stage in the regeneration process is only manifested if the regenerating neurons can reach their target cells in the olfactory bulb. Hence, bulbectomy results in the arrest of the reconstituted olfactory epithelium in an immature state. The differential patterns of B50/GAP43 expression following peripheral lesioning and bulbectomy suggest the existence of a target derived signal molecule involved in the down-regulation of B50/GAP43 expression in olfactory neurons that have established synaptic contacts in the olfactory bulb (see also Willard, chapter 2, this volume, "the suppressor hypothesis").(ABSTRACT TRUNCATED AT 400 WORDS)

Selective expression of protein F1/(GAP-43) mRNA in pyramidal but not granule cells of the hippocampus

Protein F1/GAP-43 is a protein kinase C substrate associated with axonal growth and synaptic plasticity. We used in situ hybridization in rat brain to determine the cellular distribution of its gene expression. Throughout the septotemporal axis of the adult hippocampus, pyramidal cells express F1/GAP-43 mRNA, but granule cells do not. To determine if F1/GAP-43 expression in granule cells ever occurs, we studied its expression in development during mossy fiber outgrowth, when expression should be maximal. Quantitation of relative hybridization levels in the hippocampus revealed a modest increase in granule cell F1/GAP-43 mRNA coincident with mossy fiber outgrowth. But even the peak hybridization in granule cells on day 16 was 75% less than in pyramidal cells. The distribution of grains was over the entire granule cell layer at day 9, but was restricted by day 20 to the inner aspect of the layer, the site of the youngest cells which are still sending out axonal processes. Cell-selective expression of F1/GAP-43 within a particular brain structure was not restricted to the hippocampus. In cerebellum, F1/GAP-43 hybridization was detected in granule cells but not Purkinje cells; in olfactory bulb, mitral cells but not internal granule cells; in habenula, cells in the lateral but not medial nucleus; in substantia nigra, pars compacta cells but not cells in pars reticulata. Neurons containing biogenic amines exhibited intense F1/GAP-43 hybridization: substantia nigra pars compacta (dopamine), the locus coeruleus (norepinephrine), and dorsal raphe (serotonin). In contrast, cholinergic neurons exhibited little (basal forebrain) or no (medial habenula) hybridization. F1/GAP-43 expression is not restricted to a specific cell type and is not correlated with axon length. High F1/GAP-43 expression is apparent in many neurons having either neuromodulatory or memory storage functions. We propose that F1/GAP-43 is important for accelerating process outgrowth and synaptic remodeling, rather than directing growth itself.

Immunoreactive GAP-43 in the neuropil of adult rat neostriatum: localization in unmyelinated fibers, axon terminals, and dendritic spines

GAP-43 is a neuron-specific phosphoprotein that has been implicated in neuronal development, axonal regeneration, and synaptic plasticity. Although in mammals the caudate-putamen is among those brain areas that retain a high content of GAP-43 throughout life, the role of the phosphoprotein in the neostriatum is unknown. In order to understand better the possible function(s) of GAP-43 in the adult striatum, its cellular localization was examined with immunohistochemistry at the light and electron microscopic levels by using a sheep polyclonal antibody. At the light microscopic level immunoreactive GAP-43 was abundant throughout the neostriatal neuropil but was absent from neuronal somata. At the ultrastructural level, labeling was most prevalent in small unmyelinated axons (0.12-0.15 microns diameter). Reaction product was distributed along fibers in discrete patches about 1 micron apart and in preterminal sites from which vesicle-filled boutons arose. Staining was also present in small (0.35 microns) axon terminals that contained round vesicles and formed asymmetric synapses, mostly with thin spines. Following unilateral cortical lesions, some degenerating cortical axons in the neostriatum exhibited GAP-43 labeling. Unexpectedly, in normal striatum, GAP-43 was also occasionally found in the heads of dendritic protrusions and in thin spines that received asymmetric contacts. We speculate that in the adult neostriatum, the protein may be important in the remodeling of synapses onto medium spiny neurons that involve, in part, the corticostriatal pathway.

Memory formation in the chick depends on membrane-bound protein kinase C

The role of protein kinase C (PKC) in the formation of memory for a one-trial passive avoidance task in 1-day-old chicks has been studied, following earlier observations that training on this task results in transient and lateralised changes in the phosphorylation state of presynaptic B-50 protein, a PKC substrate. In accord with hypotheses that the activity of PKC is regulated by translocation from cytosol to membrane, a significant increase was found in the fraction of the alpha/beta forms of the enzyme, assayed immunologically, present in a synaptic-membrane-bound, Triton-extractable form in the left intermediate medial hyperstriatum ventrale (IMHV) of chicks 30 min after training on the passive avoidance task. Two inhibitors of PKC, melittin (10 microliters, 120 microM) and H7 (10 microliters, 10 mM), if injected intracerebrally 10 min prior to or 10 min after training, were without effect on the general behaviour of the chicks or their training. However, these injections of the inhibitors produced amnesia in birds tested 3 h later. This effect was lateralised; only left hemisphere injections of the inhibitors produced amnesia. A possible state-dependency interpretation of these results was ruled out. The results are discussed in the context of hypotheses as to the regulatory role of PKC in neural plasticity and memory formation.

Ultrastructural double localization of B-50/GAP43 and synaptophysin (p38) in the neonatal and adult rat hippocampus

B-50/GAP43, a neuron-specific phosphoprotein, is highly expressed in developing nervous tissue. Monospecific polyclonal affinity-purified B-50 antibodies were used to document the ultrastructural distribution of B-50 in the hippocampus of 90-day-old (P90) and 1-day-old (P1) rats. Double-labelling immunoprocedures were performed to compare the localization of B-50 and synaptophysin (p38), a protein specific for synaptic vesicles. By immunofluorescence light microscopy B-50 and p38 were similarly distributed in the CA1 neuropil of P90 rats. In contrast, in P1 rats B-50 was more widely distributed than p38. By electron microscopy of P90 hippocampus, B-50 was located at the plasma membranes of axon shafts and of p38-immunoreactive axon terminals. Some B-50 was found in the cytosol of axon terminals. B-50 was absent at the plasma membranes of apical dendrites and of pyramidal cells. In the P1 rat hippocampus, B-50 was detected at the plasma membrane of growth cones, axon terminals and axon shafts, but not in their cytosol. The plasma membranes of pyramidal cell bodies and their processes extending into the stratum radiatum were without B-50. B-50-immunoreactive organelles of the lysosomal family were found in the cytosol of pyramidal cells of the hippocampus of P1 and P90 rats. This ultrastructural study shows that during development of the stratum radium in the hippocampal field CA1, the localization of B-50 persist at the plasma membrane of axons and axon terminals in P1 and P90 rats. This localization of B-50 is consistent with the suggestion that B-50 acts as a regulator of neurotransmitter release and intracellular messengers.

Evidence that protein kinase M does not maintain long-term potentiation

We have shown that the induction but not maintenance of long-term potentiation (LTP) in the Schaffer collateral-CA1 synaptic zone of the rat hippocampus is blocked by the extracellular application of the protein kinase inhibitor staurosporine. This compound was also found to block the induction of LTP in the perforant path-granule cell synaptic zone of the intact hippocampus. We have determined that staurosporine is membrane-permeable and can be detected inside cells by fluorescence microscopy. When cultured fetal hippocampal neurons were treated with staurosporine, fluorescence was observed throughout the cytoplasm and in neurites. Other cell types gave similar results. It has been proposed that constitutively active cytosolic protein kinase M or other protein kinases maintain long-term potentiation. Since staurosporine has access to the cytosol and inhibits protein kinase M in vitro, our results suggest that this enzyme is not responsible for the maintenance of LTP. This conclusion may extend to other protein kinases as well, since staurosporine has been shown to inhibit a variety of these enzymes.

The initial period of peripheral nerve regeneration and the importance of the local environment for the conditioning lesion effect

The aim of this study was to investigate the early period of neurite outgrowth in the regenerating rat sciatic nerve and to determine if the non-neuronal cells were important for the conditioning lesion effect. Regeneration distance was evaluated with the pinch-reflex test 6 h to 5 days after a test crush lesion. The regeneration velocity accelerated during approximately 3 days, whereupon outgrowth continued with a constant velocity. In unconditioned nerves the initial delay was 2.8 h and the constant rate of regeneration was 3.2 mm/day. In nerves with a distal conditioning lesion the initial delay was 2.4 h and the rate of regeneration increased by 52%. When the test crush was applied at the same place as the conditioning crush the initial delay was 1.9 h and the rate of regeneration increased by 61%. The conditioning lesion effect was not influenced by the distance between the cell body and the conditioning crush lesion. Furthermore, the conditioning lesion effect could not be expressed if conditioned axons grew into a freeze injured nerve section. Incorporation of [3H]thymidine increased in the regenerating nerve segment. The increase occurred earlier if this segment had been subjected to a conditioning crush lesion. The results of these experiments showed that peripheral neurites start to regenerate within a few hours after an injury, suggesting that growth cone formation is independent of the cell body reaction. A conditioning crush lesion increases the regeneration velocity and its acceleration, and the conditioning lesion effect cannot be expressed in the absence of living Schwann and other non-neuronal cells.

Abnormal retinal projections alter GAP-43 patterns in the diencephalon

In Syrian hamsters, mature retinal terminals contain only low levels of the growth-associated protein, GAP-43, whereas the lateral posterior nucleus (LP) of the thalamus contains high levels of this protein. Damage to the superior colliculus in neonatal hamsters induces retinal terminals to form dense patches of innervation in the LP, an area which otherwise receives little if any direct retinal input. The present study used GAP-43 antibodies to examine the interaction between abnormally routed optic fibers and the cells in the anomalous thalamic target zone. Immunohistochemistry revealed very little GAP-43 in the abnormal retinal projection to the LP, indicating that the normal developmental decline in GAP-43 levels occurs even in an inappropriate extracellular environment. Moreover, retinal fibers were found to exclude the protein from its normal territory, forming negatively-stained islands in those regions of the LP containing the retinal terminals. In order to identify the normal source of GAP-43-positive terminals in the LP, we surgically removed two major extrinsic afferents to this region, or we chemically eliminated local interneurons. Whereas removing projections from the SC or posterior cortex did not alter GAP-43 immunoreactivity in the LP, destruction of local interneurons with ibotenic acid resulted in markedly diminished levels of this protein. These results show that retinal terminals induced to form in an abnormal target area undergo their normal diminution of GAP-43, and that these retinal projections displace other GAP-43-rich terminals in the LP that appear to arise from local interneurons.

Neural bases of recognition memory investigated through an analysis of imprinting

Through a learning process known as imprinting, the young of some animals, including the domestic chick, come to recognize an object by being exposed to it. Visually naive chicks vigorously approach a wide range of objects. After an adequate period of exposure to one object chicks selectively approach it in a recognition test. The nervous system of dark-reared chicks is not a tabula rasa, as chicks have predispositions to approach some stimuli rather than others. Nevertheless, visual imprinting leads to changes in a nervous system that may not have been 'marked' by previous visual experience, and so encourages the hope of discovering the neural bases of the learning process. The intermediate and medial part of the hyperstriatum ventrale, a sheet of cells within the cerebral hemispheres, plays a crucial role in visual imprinting, particularly in the memory process of recognition. The cellular and sub-cellular changes that take place in this part of the hyperstriatum ventrale after imprinting are described. The right and left hyperstriatum ventrale regions play different roles in the imprinting process, and evidence is given for the existence of multiple memory systems in the chick brain.

Protein phosphorylation in intact superior cervical ganglion during regeneration

The incorporation of radioactive phosphate into proteins of both normal and regenerating superior cervical ganglion nerve of the rat is reported. Incorporation studies carried out by in vitro and in vivo methods are compared. In the in vitro method, excised intact ganglia or their homogenates were incubated in the presence of inorganic phosphate or ATP, respectively, under various conditions. Proteins were analyzed by gel electrophoresis followed by autoradiography, in which quantitative but not qualitative differences between regenerating and control cases were apparent. In the in vivo procedure, inorganic phosphate was injected into the living animal 4 h before removal of ganglia. At least fivefold more proteins became labeled in vivo than in vitro, whereas no similarity in the pattern of labeling between the two methods was observed. For example, the most heavily labeled protein in the in vivo method, tentatively identified as microtubule-associated protein-2, was not detected on autoradiograms of proteins labeled by the in vitro method. In this latter method, an 85-kDa species and growth-associated protein-43 were always labeled, and the extent of their phosphorylation was enhanced by the additional presence of phosphatidylserine and Ca2+, a result indicating that these labeled species are substrates of protein kinase C. The in vitro conditions also led to the labeling of proteins identified as alpha- and beta-tubulin. Comparison of the methods suggests that removal of the ganglion interferes with the function of protein phosphorylation systems and that this effect involves elements of the cytoskeleton.

Neurotransmitter release

Axon terminals release more than one physiologically active substance. Synaptic messengers may be stored in two different types of vesicles. Small electron-lucent vesicles mainly store classical low molecular weight transmitter substances and the larger electron-dense granules store and release proteins and peptides. Release of the two types of substances underlies different physiological control. Release of messenger molecules from axon terminals is triggered by influx of Ca2+ through voltage sensitive Ca2+ channels and a rise in cytosolic Ca2+ concentrations. Neither the immediate Ca2+ target(s) nor the molecular species involved in synaptic vesicle docking, fusion and retrieval are known. It is, however, likely that steps involved in the molecular cascade of transmitter release include liberation of vesicles from their association with the cytonet and phosphorylation by protein kinase C of proteins which have the ability to alter between membrane bound and cytoplasmic forms and thus facilitate or initiate the molecular interaction between synaptic vesicles and the plasma membrane.

GAP-43 in adult visual cortex

GAP-43 was purified from cat brain by a rapid isolation procedure and was used to raise highly specific polyclonal antibodies in rabbits. Immunoblots of proteins from adult cat, monkey and human visual cortex as well as bovine cortex also showed specific staining of a single protein that was present in both soluble and membrane fractions. Immunocytochemistry of both cat and human adult visual cortex showed that GAP-43 has a laminar distribution.

The pattern of GAP-43 immunostaining changes in the rat hippocampal formation during reactive synaptogenesis

The reactive synaptogenesis that takes place in the rat hippocampal formation after certain experimental manipulations affords an opportunity to investigate the molecular events that underlie structural remodeling in the adult CNS. Between 2 and 4 days after lesioning the perforant pathway, levels of the synaptic phosphoprotein, GAP-43 (B50, F1, pp46, neuromodulin), were found to increase markedly in the inner molecular layer (iml) of the dentate gyrus, coincident with the time at which commissural-associational (CA) fibers begin to sprout axon collaterals into dendritic portions denervated by the lesion. GAP-43 immunostaining in the iml began to decline by 8 days but continued to define an expanded CA projection for at least one month. In the outer molecular layer (oml), GAP-43 levels decreased after the loss of perforant pathway terminals and did not return for 2-3 weeks, the time at which sprouting of septal inputs into this layer can be visualized by cholinesterase histochemistry. These results demonstrate that GAP-43 levels change during reactive synaptogenesis, and point to differences among neural systems in their expression of this protein.

Effect of conditioning lesions on regeneration of goldfish optic axons: time course of the cell body reaction to axotomy

The time course of the cell body reaction to axotomy was determined in goldfish retinal ganglion cells by measuring cell body size and the amount of labelled protein conveyed by fast axonal transport to the optic tectum, both of which increase during regeneration of the optic axons. Following a single testing lesion of the optic nerve, the regenerating axons began to innervate the tectum at about 14 days after the lesion and the cell body reaction began to decline 2-3 weeks thereafter. If the testing lesion had been preceded by a conditioning lesion 2 weeks earlier, the time for the regenerating axons to arrive in the tectum was reduced by a week, because of the faster rate of axonal outgrowth, but the interval between their arrival and the beginning of the decline of the cell body reaction was unchanged. Electrophysiological measurements showed that synaptic transmission was initiated earlier when the axons reached the tectum faster. These results indicate that the mechanisms initiating the recovery of cell body metabolism are independent of those governing the rate of axonal outgrowth. The recovery of the cell body may begin shortly after synapses are established, regardless of whether they are correctly or incorrectly targetted. The correctness of the target may be a separate factor in determining how rapidly and completely the cell body recovers.

Depolarization-induced phosphorylation of the protein kinase C substrate B-50 (GAP-43) in rat cortical synaptosomes

We studied the molecular events underlying K(+)-induced phosphorylation of the neuron-specific protein kinase C substrate B-50. Rat cortical synaptosomes were prelabelled with 32P-labelled orthophosphate. B-50 phosphorylation was measured by an immunoprecipitation assay. In this system, various phorbol esters, as well as a synthetic diacylglycerol derivative, enhance B-50 phosphorylation. K+ depolarization induces a transient enhancement of B-50 phosphorylation, which is totally dependent on extracellular Ca2+. Also, the application of the Ca2+ ionophore A23187 induces B-50 phosphorylation, but the magnitude and kinetics of A23187-induced B-50 phosphorylation differ from those induced by depolarization. The protein kinase inhibitors 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), and staurosporine antagonize K(+)- as well as PDB-induced B-50 phosphorylation, whereas trifluoperazine and calmidazolium are ineffective under both conditions. We suggest that elevation of the intracellular Ca2+ level after depolarization is a trigger for activation of protein kinase C, which subsequently phosphorylates its substrate B-50. This sequence of events could be of importance for the mechanism of depolarization-induced transmitter release.

Neuroplasticity in the olfactory system: differential effects of central and peripheral lesions of the primary olfactory pathway on the expression of B-50/GAP43 and the olfactory marker protein

The regeneration of the olfactory neuroepithelium following olfactory bulbectomy or peripheral deafferentation was studied with mRNA probes and antibodies for B-50/GAP43 and for olfactory marker protein (OMP). Two stages in the regeneration of the olfactory epithelium could be discerned with these reagents. The first stage occurs following either peripheral deafferentation of the olfactory epithelium with Triton X-100 (TX-100) or after bulbectomy and is characterized by the formation of a large population of immature olfactory receptor neurons. These newly formed neurons express B-50/GAP43, a phosphoprotein related to neuronal growth and plasticity. During the second stage of the regeneration process the newly formed olfactory neurons mature, as evidenced by a decrease in their expression of B-50/GAP43 and an increase in the expression of OMP. This stage is only manifested if the developing neurons have access to the target olfactory bulb. Formation of a full complement of OMP-expressing neurons occurs only after peripheral lesion with TX-100. In contrast, following bulbectomy the reconstituted olfactory epithelium lacks its normal target and is compromised in its ability to recover from nerve damage, as evidenced by the presence of a large number of B-50/GAP43-expressing neurons up to 3 months after the lesion and its failure to establish a full complement of OMP-expressing neurons. These results demonstrate that the olfactory epithelium is capable of replacing its sensory neurons independently of the presence of its target, the olfactory bulb. However, the differential patterns of expression of B-50/GAP43 and OMP at long times after peripheral lesion with TX-100 or bulbectomy illustrate the profound effect the olfactory bulb has on neuronal maturation in reconstituted olfactory neuroepithelium.

G0 is a major growth cone protein subject to regulation by GAP-43

G0, a GTP-binding protein that transduces information from transmembrane receptors, has been found to be a major component of the neuronal growth cone membrane. GAP-43, an intracellular growth cone protein closely associated with neuronal growth, stimulates GTP-gamma-S binding to G0. It does so through an amino-terminal domain homologous to G-linked transmembrane receptors. Thus, G0 in the growth cone may be regulated by intracellular as well as extracellular signals.

Anatomical studies of dorsal column axons and dorsal root ganglion cells after spinal cord injury in the newborn rat

The response of dorsal column axons was studied after neonatal spinal overhemisection injury (right hemicord and left doral funiculus). Rat pups (N = 11) received this spinal lesion at the C2 level within 30 hours after birth. The cauda equina was exposed 3 months later in one group of chronic operates (N = 5) and in a group of normal adults (N = 2), and all spinal roots from L5 caudally were cut bilaterally; 4 days later the spinal cord and medulla were processed for Fink-Heimer impregnation of degenerating axons and terminals. In a second group of chronic operates (N = 6) and normal adult controls (N = 4) the left sciatic nerve was injected with a cholera toxin-HRP conjugate (C-HRP), followed by a 2-3 day transganglionic transport period, and then the spinal cord and medulla were processed with tetramethylbenzidine histochemistry. Both control groups have a consistent dense projection in topographically adjacent regions of the dorsal funiculus and gracile nucleus. However, there is no sign of axonal growth around the lesion in either group of chronic experimental operates. Instead, there is a decreased density of projection within the dorsal funiculus near the lesion site. Many remaining C-HRP labeled axons in the experimental operates have abnormal, thick varicosities and swollen axonal endings (5-10 microns x 10-30 microns) within the dorsal funiculus through several spinal segments caudal to the lesion. Ultrastructural analysis of the dorsal funiculus in three other chronic experimental operates reveals the presence of numerous vesicle filled axonal profiles and reactive endings which appear similar to the C-HRP labeled structures. Transganglionic labeling after C-HRP sciatic nerve injections (N = 4) and retrograde labeling of L4, L5 dorsal root ganglion neurons after fast blue injections of the gracile nucleus (N = 6) both suggest that all dorsal column axons project to the gracile nucleus in the newborn rat. Dorsal root ganglion (DRG) cell survival following the neonatal overhemisection injury was also examined in the L4 and L5 DRG. DRG neurons that project to the gracile nucleus were prelabeled by injecting fast blue into this nucleus at birth two days prior to the cervical overhemisection spinal injury. Both normal littermates (N = 9) and spinally injured animals (N = 12) were examined after postinjection survival periods of 10 or 22 days.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification of the growth-associated protein GAP-43 by reversed phase chromatography: amino acid sequence analysis and cDNA identification

GAP-43 is a neuronal phosphoprotein. Increased synthesis and axonal transport of GAP-43 has been associated with axon growth, and altered phosphorylation of GAP-43 has been associated with changes in synaptic efficacy. Here we report a rapid and effective procedure employing reverse-phase HPLC for the purification of GAP-43 from rat brain. To characterize the protein purified by this procedure, we generated proteolytic fragments and determined their amino acid sequences. These directly determined sequences, corresponding to 56% of the GAP-43 amino acids, confirm recently reported sequences deduced from the nucleotide sequences of cDNAs. Using oligonucleotide probes constructed according to these amino acid sequences, we identified GAP-43 cDNAs in a library prepared from neonatal rat superior cervical ganglion cells. One of these cDNAs was 1.1 kB in size; it hybridized specifically with a 1.5 kB RNA from brain, but not from liver, and contained the entire coding sequence for GAP-43. This cDNA differed from recently reported cDNAs in its 3' untranslated region.

4-Aminopyridine stimulates B-50 (GAP43) phosphorylation and [3H]noradrenaline release in rat hippocampal slices

In situ phosphorylation of the presynaptic protein kinase C substrate B-50 was investigated in rat hippocampal slices incubated with the convulsant drug 4-aminopyridine (4-AP). Phosphorylation of B-50 was significantly enhanced 1 min after the addition of 4-AP (100 microM). This increase by 4-AP was concentration dependent (estimated EC50 30-50 microM). Concomitant with the changes in B-50 phosphorylation, 4-AP also dose-dependently stimulated [3H]noradrenaline [( 3H]NA) release from the slices. 4-AP stimulated [3H]NA release within 5 min to seven times the control level. The B-50 phosphorylation induced by 4-AP remained elevated after removal of the convulsant, this is contrast to B-50 phosphorylation induced by depolarization with K+. A similar persistent increase was observed for [3H]NA release after a 5-min incubation period with 4-AP. These results give more insight into the molecular mechanisms underlying 4-AP-induced epileptogenesis and provide further evidence for the correlation between B-50 phosphorylation and neurotransmitter release in the hippocampal slice.

Chicken growth-associated protein GAP-43 is tightly bound to the actin-rich neuronal membrane skeleton

We have identified the chicken equivalent of growth-associated protein GAP-43 in a detergent-resistant membrane skeleton from cultures of chick neurones and embryonic chick brain. Antisera to the membrane skeleton protein, the 3D5 antigen, precipitate the translation product of chick GAP-43 cDNA, and the 3D5 antigen is also detected by antisera against synthetic peptides from the known amino acid sequence of rat GAP-43. The chick protein and the rat GAP-43 are biochemically similar proteins that both serve as major targets of phosphorylation by endogenous protein kinase C. The detergent-resistant complex in which GAP-43 is found also contains actin (approximately 5% of the total protein) and a neurone-specific cell surface glycoprotein. We suggest that the membrane skeleton of neurones may be a primary site of action of GAP-43.

4-Aminopyridine stimulates B-50 (GAP-43) phosphorylation in rat synaptosomes

Recently, we have shown that stimulation of [3H]-noradrenaline release from hippocampal slices by 4-aminopyridine (4-AP) is accompanied by an enhancement of the phosphorylation of B-50, a major presynaptic substrate of protein kinase C (PKC). PKC has been implicated in the regulation of transmitter release. In this study, we investigated the effects of 4-AP on B-50 phosphorylation in synaptosomes from rat brain and compared the effects of 4-AP with those of depolarization with K+, in order to gain more insight into the mechanism of action of 4-AP. B-50 phosphorylation was stimulated by incubation with 4-AP for 2 minutes at concentrations ranging from 10 microM to 5 mM. 4-AP (100 microM) stimulated B-50 phosphorylation already within 15 seconds; longer incubations revealed a sustained increase in the presence of 4-AP. B-50 phosphorylation was also stimulated by depolarization with 30 mM K+ for 15 seconds. The effects of both 4-AP or K+ depolarization on B-50 phosphorylation were abolished at low extracellular Ca2+ concentrations. The increase in B-50 phosphorylation induced by 4-AP seemed to be dependent on the state of depolarization, since the effect of 4-AP was largest under nondepolarizing conditions. Comparing the effects of 4-AP and K+ depolarization on B-50 phosphorylation suggests that a different mechanism of action is involved. These results indicate that the stimulation of B-50 phosphorylation by 4-AP in hippocampal slices can be attributed to a direct action of 4-AP on presynaptic terminals. In addition, our results support the hypothesis that B-50 phosphorylation by PKC is involved in Ca2(+)-dependent transmitter release evoked by 4-AP.

The expression of B-50/GAP-43 during development of rat spinal neurons in culture is regulated by interneuronal contact

The neuron-specific phosphoprotein B-50 (GAP-43) is associated with neuritogenesis during development and regeneration. We monitored B-50 in cultured spinal neurons (fetal rat) with an enzyme-linked immunoadsorbent assay. B-50 levels increased from 24 to 72 h, then decreased. Other cultures, fixed at 24 h intervals and incubated with anti-B-50 immunoglobulins and fluorescent conjugates, showed that B-50 was present in somata after 24 h, but mainly in neurites after 48 h; after 72-96 h neurons migrated into clusters and B-50 was detected only in free neurites at the perimeter of the culture. We conclude that B-50 expression is down-regulated by neurite-cell or cell-cell contact.

Neuronal gene expression in amyotrophic lateral sclerosis

To characterize neuronal gene expression in amyotrophic lateral sclerosis (ALS), we quantitated one glial and three neuronal mRNAs in spinal cords of 7 subjects with ALS and 11 controls. The ALS cases showed no loss of mRNA for the neurofilament light subunit when assessed with in situ hybridization. Northern analysis, and RNase protection assay; and no loss of mRNA for amyloid precursor protein or a growth-associated protein (GAP-43/B-50) on Northern analysis. ALS cords also showed no significant change in glial mRNA. Our findings indicate that expression of these neuronal mRNAs is well maintained in ALS-afflicted spinal cord. They do not support the hypothesis of a generalized impairment of neuronal gene transcription in the pathogenesis of this disorder.

Binding of the neuronal protein B-50, but not the metabolite B-60, to calmodulin

Protein kinase C phosphorylates the neurone-specific protein B-50 at a single Ser41 residue, which is also the point for a major proteolytic cleavage in vitro, and probably in vivo, that produces a B-50 phosphorylation-inhibiting N-terminal fragment and a large C-terminal metabolite B-60 (B-50(41-226]. The intact purified protein will bind to calmodulin in the absence of calcium, but the interaction has an absolute requirement for dephospho-B-50. In an attempt to unify two aspects of B-50 biochemistry, we have examined the interaction of B-50 binding to calmodulin and B-50 proteolysis. HPLC- and affinity-purified B-50 bound to calmodulin, but purified B-60 did not. To ensure that this effect was not due to the phosphorylation state of pure, isolated B-60, the metabolite was generated in vitro using a Triton extract of synaptosomal plasma membranes, which contains the as yet uncharacterized B-50 protease. B-60 derived from dephospho-B-50 also failed to bind calmodulin. The results demonstrate a direct connection between B-50 binding to calmodulin and B-50 proteolysis. The position of the proposed calmodulin-binding domain within intact B-50 is discussed in light of the failure of calmodulin to bind B-60.

GAP-43 as a 'calmodulin sponge' and some implications for calcium signalling in axon terminals

In most neurons, the maturation of axonal growth cones to become stable synaptic terminals is accompanied by a dramatic decline in the abundance of a major growth cone component, GAP-43. Accumulation of GAP-43 persists, however, in a minority of mature synaptic terminals. What properties of axons and their terminals are affected by these changes in GAP-43 expression? Storm and colleagues first noted that the membrane- and calmodulin-binding properties of GAP-43 (a.k.a. P-57 or neuromodulin) could allow it to sequester a large fraction of calmodulin to the submembranous regions, and to release free calmodulin in response to protein kinase C activation. Analysis of evolutionarily conserved sequences in GAP-43 indicates that these properties are central to the biological effects of the protein. If GAP-43 is presumed to inactivate bound calmodulin, the network of GAP-43 in an axon terminal could be considered a regulatable calmodulin buffer, or 'calmodulin sponge', absorbing free calmodulin and releasing it in response to activation of protein kinase C. Such a calmodulin sponge has properties that could be useful in modulating the responses of membrane and cytoskeletal assembly events to calcium signals in growth cones, and in mediating long-term potentiation of neurotransmitter release from some pre-synaptic terminals.

Distribution of growth-associated protein, B-50 (GAP-43) in the mammalian enteric nervous system

The presence of the growth-associated protein, B-50 (also known as GAP-43) was investigated in the adult mammalian enteric nervous system. The small intestine of rat, ferret and human was examined by immunohistochemistry. Dense B-50-like immunoreactivity was localized in nerves throughout the wall of the rat, ferret and human small intestine, notably in the myenteric and submucous plexuses, where in the ferret ileum it co-localized with vasoactive intestinal polypeptide-immunoreactive fibre groups. Material with the biochemical and immunological characteristics of rat B-50 was extracted from the rat ileum. In-situ hybridization demonstrated that enteric neurons express B-50. These findings are consistent with a role for B-50 in the documented plasticity of the adult enteric nervous system.

Chicken growth-associated protein (GAP)-43: primary structure and regulated expression of mRNA during embryogenesis

Growth-associated protein (GAP)-43 is a neuron-specific phosphoprotein whose expression is associated with axonal outgrowth during neuronal development and regeneration. In order to investigate the expression of this gene product in the early developing nervous system we have isolated and sequenced a cDNA for chicken GAP-43. The predicted amino acid sequence for chicken GAP-43 displays extensive similarity to that of the mammalian protein, particularly in the amino-terminal region, to which functional domains of the protein have been assigned. The cDNA hybridizes with two RNAs of differing molecular weights on Northern blots; both appear to be regulated similarly. These RNAs first appear in the brain on embryonic day 3 (E3), suggesting that GAP-43 begins to be expressed when neuroblasts become post-mitotic. In situ hybridization analysis reveals that GAP-43 RNA is expressed by several neural structures in the chick embryo, including derivatives of the neural tube, neural crest, and neuroectodermal placodes.

Transfection of PC12 cells with the human GAP-43 gene: effects on neurite outgrowth and regeneration

The neuronal growth associated protein GAP-43 is expressed at high levels during axonal growth and regeneration. In this report, we describe the transfection of the nerve growth factor (NGF)-responsive pheochromocytoma cell line PC12 with the human GAP-43 cDNA under the control of the Moloney murine leukemia virus long terminal repeat (MoMuLV LTR). Two PC12 subclones were isolated that constitutively expressed GAP-43 from the transfected cDNA and showed increased responsiveness to NGF. Of the two transfected PC12 subclones, the subclone expressing the most human GAP-43 RNA showed an accelerated initial neurite outgrowth response and a 10-fold increased sensitivity to NGF. Neurite regeneration was significantly enhanced in both transfected subclones and, in contrast to untreated PC12 cells, could occur transiently in the absence of added NGF. These results suggest that GAP-43 may potentiate the action of NGF on neurite initiation and regeneration.

Effects of aging on the dynamics of information processing and synaptic weight changes in the mammalian hippocampus

It is clear that the properties of LTE make it a plausible mechanism for associative information storage at some synapses in the central nervous system. While many of the factors that regulate LTE's induction and expression have been discovered and a strong case is being developed for its role in learning and memory processes, until we understand more clearly the mechanisms underlying both the expression and maintenance of LTE, an understanding of its change with age will be difficult. Judging by the progress that has been made over the past several years in uncovering some of the molecular events that are critical for LTE's expression, one may be optimistic that answers will be forthcoming reasonably soon. Of particular importance to aging mammals, such answers may provide insights into why older organisms show faster forgetting. This may have a profound impact on therapeutic strategies for memory disorders in both normal and pathological conditions of aging.

The growth-associated protein GAP-43 appears in dorsal root ganglion cells and in the dorsal horn of the rat spinal cord following peripheral nerve injury

When adult dorsal root ganglion cells are dissociated and maintained in vitro, both the small dark and the large light neurons show increases in the growth-associated protein GAP-43, a membrane phosphoprotein associated with neuronal development and plasticity. Immunoreactivity for GAP-43 appears in the cytoplasm of the cell bodies as early as 3.5 h post axotomy and is present in neurites and growth cones as soon as they develop. At early stages of culture (4 h to eight days) satellite/Schwann cells are also immunoreactive for GAP-43. Neurons in isolated whole dorsal root ganglion maintained in vitro become GAP-43-immunoreactive between 2 and 3 h after axotomy. It takes three days however, after cutting or crushing the sciatic nerve in adult rats in vivo, for GAP-43 immunoreactivity to appear in the axotomized dorsal root ganglion cells. GAP-43 immunoreactivity can be detected in the central terminals of primary afferent neurons in the superficial laminae of the dorsal horn of the lumbar enlargement four days after sciatic cut or crush. The intensity of the GAP-43 staining reaches a peak at 21 days and becomes undetectable nine weeks following crush injury and 36 weeks following sciatic nerve cut. The pattern of GAP-43 staining is identical to the distribution of sciatic small-calibre afferent terminals. Little or no staining is present in the deep dorsal horn, but GAP-43 does appear in the ipsilateral gracile nucleus 22 days after sciatic injury. In investigating the mechanism of GAP-43 regulation, blockade of axon transport in the sciatic nerve with vinblastine (10(-5) M-10(-4) M) or capsaicin (1.5%) was found to produce a pattern of GAP-43 immunoreactivity in the dorsal horn identical to that found with crush, while electrical stimulation of the sciatic nerve had no effect. Axotomy of primary sensory neurons or the interruption of axon transport in the periphery therefore acts to trigger GAP-43 production in the cell body. The GAP-43 is transported to both the peripheral and the central terminals of the afferents. In the CNS the elevated GAP-43 levels may contribute to an inappropriate synaptic reorganization of afferent terminals that could play a role in the sensory disorders that follow nerve injury.