Regulation of growth-associated protein 43 (GAP-43) messenger RNA associated with plastic change in the adult rat barrel receptor complex

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.

Postnatal environment overrides genetic and prenatal factors influencing offspring obesity and insulin resistance

There is growing evidence that the postnatal environment can have a major impact on the development of obesity and insulin resistance in offspring. We postulated that cross-fostering obesity-prone offspring to lean, obesity-resistant dams would ameliorate their development of obesity and insulin resistance, while fostering lean offspring to genetically obese dams would lead them to develop obesity and insulin resistance as adults. We found that obesity-prone pups cross-fostered to obesity-resistant dams remained obese but did improve their insulin sensitivity as adults. In contrast, obesity-resistant pups cross-fostered to genetically obese dams showed a diet-induced increase in adiposity, reduced insulin sensitivity, and associated changes in hypothalamic neuropeptide, insulin, and leptin receptors, which might have contributed to their metabolic defects. There was a selective increase in insulin levels and differences in fatty acid composition of obese dam milk which might have contributed to the increased adiposity, insulin resistance, and hypothalamic changes in obesity-resistant cross-fostered offspring. These results demonstrate that postnatal factors can overcome both genetic predisposition and prenatal factors in determining the development of adiposity, insulin sensitivity, and the brain pathways that mediate these functions.

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.

The differential distribution of the growth-associated protein-43 in first and higher order thalamic nuclei of the adult rat

Corticothalamic axons from layer 5 of primary and secondary auditory and visual areas have large terminals that make multiple synaptic contacts on proximal dendrites of relay cells in higher order thalamic nuclei and have been termed "driver" inputs. The corticothalamic cells express mRNA for the presynaptic growth-associated protein-43, in the adult rat [Feig SL (2004) Corticothalamic cells in layers 5 and 6 of primary and secondary sensory cortex express GAP-43 mRNA in the adult rat. J Comp Neurol 468:96-111]. In contrast, ascending driver afferents to first order nuclei (e.g. retinal, inferior collicular, and lemniscal) lose growth-associated protein-43 as mature synaptic terminals are established. Levels of immunoreactivity for growth-associated protein-43 are compared for first and higher order visual (lateral geniculate and lateral posterior), auditory (ventral and dorsal divisions of the medial geniculate), and somatosensory (ventral posterior and posterior) thalamic nuclei. At one week postnatal, staining for growth-associated protein-43 is uniform throughout first and higher order thalamic nuclei. By three weeks and thereafter, staining is denser in the higher order than first order thalamic nuclei. Electron microscopy shows growth-associated protein-43 in profiles with characteristics of afferents from layer 5 in LP and medial geniculate nucleus and no such label in retinal afferents in lateral geniculate nucleus. In these nuclei, approximately 25% of the profiles with characteristics of cortical afferents from layer 6 have label for growth-associated protein-43. The superficial layers of the superior colliculus also show growth-associated protein-43 positive profiles with characteristics of terminals from cortical layer 5. Some growth-associated protein-43 positive terminals were also positive for GABA in the thalamic nuclei studied and in the superior colliculus. The data suggest that sensory afferents to first order thalamocortical relays become stabilized once mature synaptic patterns are established, but the higher stages of information processing involving higher order thalamic relays, via cells in cortical layer 5, retain plasticity related to growth-associated protein-43 in the adult.

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.

Activity-dependent maintenance and growth of dendrites in adult cortex

Whereas it is widely accepted that the adult cortex is capable of a remarkable degree of functional plasticity, demonstrations of accompanying structural changes have been limited. We examined the basal dendritic field morphology of dye-filled neurons in layers III and IV of the mature barrel cortex after vibrissal-deafferentation in adult rats. Eight weeks later, the tendency for these neurons to orient their dendritic arbors toward the center of their home barrel was found to be disrupted by the resultant reduced activity of thalamocortical innervation. Measures of spine density and total dendritic length were normal, indicating that the loss of dendritic bias was accompanied by growth of dendrites directed away from the barrel center. This finding suggests that in the mature cortex, the apparently static structural attributes of the normal adult cortex depend on maintenance of patterns of afferent activity; with the corollary that changes in these patterns can induce structural plasticity.

Obesity-prone rats have preexisting defects in their counterregulatory response to insulin-induced hypoglycemia

Rats that develop diet-induced obesity (DIO) on a 31% fat [high-energy (HE)] diet have defective sensing and responding to altered glucose levels compared with diet-resistant (DR) rats. Thus we postulated that they would also have defective counterregulatory responses (CRR) to insulin-induced hypoglycemia (IIH). Chow-fed selectively bred DIO and DR rats underwent three sequential 60-min bouts of IIH separated by 48 h. Glucose levels fell comparably, but DIO rats had 22–29% lower plasma epinephrine (Epi) levels during the first two bouts than DR rats. By the third trial, despite comparable Epi levels, DIO rats had lower 30-min glucose levels and rebounded less than DR rats 85 min after intravenous glucose. Although DIO rats gained more carcass and fat weight after 4 wk on an HE diet than DR rats, they were unaffected by prior IIH. Compared with controls, DR rats with prior IIH and HE diet had higher arcuate nucleus neuropeptide Y (50%) and proopiomelanocortin (POMC; 37%) mRNA and an inverse correlation ( r = 0.85; P = 0.004) between POMC expression and body weight gain on the HE diet. These data suggest that DIO rats have a preexisting defect in their CRR to IIH but that IIH does not affect the expression of their hypothalamic neuropeptides or weight gain as it does in DR rats.

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.

Defense of body weight depends on dietary composition and palatability in rats with diet-induced obesity

Sprague-Dawley rats selectively bred for diet-induced obesity (DIO) or diet resistance (DR) were characterized on diets of differing energy content and palatability. Over 10 wk, DR rats on a high-energy (HE) diet (31% fat) gained weight similarly to DR rats fed chow (4.5% fat), but they became obese on a palatable liquid diet (Ensure). DIO rats gained 22% more weight on an HE diet and 50% more on Ensure than chow-fed DIO rats. DIO body weight gains plateaued when switched from HE diet to chow. But, Ensure-fed DIO rats switched to chow spontaneously reduced their intake and weight to that of rats switched from HE diet to chow. They also reduced their hypothalamic proopiomelanocortin and dynorphin but not neuropeptide Y mRNA expression by 17-40%. When reexposed to Ensure after 7 wk, they again overate and matched their body weights to rats maintained on Ensure throughout. All Ensure-fed rats had a selective reduction in dynorphin mRNA in the ventromedial hypothalamic nucleus. Thus genetic background, diet composition, and palatability interact to produce disparate levels of defended body weight and central neuropeptide expression.

Sibutramine alters the central mechanisms regulating the defended body weight in diet-induced obese rats

Chronic administration of sibutramine lowers body weight, presumably by altering brain monoamine metabolism. Here the effect of sibutramine on sympathoadrenal function (24-h urine norepinephrine and epinephrine levels) and arcuate nucleus (ARC) neuropeptide Y (NPY) and proopiomelanocortin (POMC) expression was assessed in diet-induced obese rats fed a low-fat diet. Chronic (10 wk) sibutramine [5 mg. kg(-1). day(-1) ip; rats fed ad libitum and injected with sibutramine (AS)] lowered body weight by 15% but only transiently (3-4 wk) reduced intake compared with vehicle-treated controls [rats fed chow ad libitum and injected with vehicle daily (AV)]. Other rats food restricted (RS) to 90% of the weight of AS rats and then given sibutramine restored their body weights to the level of AS rats when allowed libitum food intake. After reequilibration, RS rats were again energy restricted to reduce their weight to 90% of AS rats, and additional vehicle-treated rats (RV) were restricted to keep their body weights at the level of AS rats for 3 wk more. Terminally, total adipose depot weights and leptin levels paralleled body weights (AV > AS = RV > RS), although AS rats had heavier abdominal and lighter peripheral depots than RV rats of comparable body weights. Sibutramine treatment increased sympathetic activity, attenuated the increased ARC NPY, and decreased POMC mRNA levels induced by energy restriction in RV rats. Thus sibutramine lowered the defended body weight in association with compensatory changes in those central pathways involved in energy homeostasis.

Expression of GAP-43 and SCG10 mRNAs in lateral geniculate nucleus of normal and monocularly deprived macaque monkeys

We performed nonradioactive in situ hybridization histochemistry (ISH) in the lateral geniculate nucleus (LGN) of the macaque monkey to investigate the distribution of mRNA for two growth-associated proteins, GAP-43 and SCG10. GAP-43 and SCG10 mRNAs were coexpressed in most neurons of both magnocellular layers (layers I and II) and parvocellular layers (layers III-VI). Double-labeling using nonradioactive ISH and immunofluorescence revealed that both GAP-43 and SCG10 mRNAs were coexpressed with the alpha-subunit of type II calcium/calmodulin-dependent protein kinase, indicating that both mRNAs are expressed also in koniocellular neurons in the LGN. We also showed that GABA-immunoreactive neurons in the LGN did not contain GAP-43 and SCG10 mRNAs, indicating that neither GAP-43 nor SCG10 mRNAs were expressed in inhibitory interneurons in the LGN. GABA-immunoreactive neurons in the perigeniculate nucleus, however, contained both GAP-43 and SCG10 mRNAs, indicating that both mRNAs were expressed in inhibitory neurons in the perigeniculate nucleus, which project to relay neurons in the LGN. Furthermore, to determine whether the expression of GAP-43 and SCG10 mRNAs is regulated by visual input, we performed nonradioactive ISH in the LGN and the primary visual area of monkeys deprived of monocular visual input by intraocular injections of tetrodotoxin. Both mRNAs were downregulated in the LGN after monocular deprivation for 5 d or longer. From these results, we conclude that both GAP-43 and SCG10 mRNAs are expressed in the excitatory relay neurons of the monkey LGN in an activity-dependent manner.

Reorganization of adult rat barrel cortex intrinsic signals following kainic acid induced central lesion

A plasticity model studying the adult rat barrel cortex intrinsic signal after a central lesion was developed. Repeated optical imaging studies of the barrel cortex of five rats were performed over variable periods of time (2 days to 6 weeks) after intracortical injection of kainic acid. The signal of the elicited principal whisker corresponding to the injected barrel in the repeat studies relocated to the perimeter of the lesion. The area of the signals of this principal whisker and of surrounding whiskers were larger in the first two weeks studies than those obtained before injection (P<0.01) resulting in increase overlapping of adjacent signals (P=0.01). Even though the signal of the PW remains relocated in the later studies (>2 weeks), all the signals returned to normal size. These findings demonstrate recovery and reorganization of sensory representation in the somatosensory cortex following a central lesion.

Effects of prenatal exposure to ethanol on systems matching: the number of neurons in the ventrobasal thalamic nucleus of the mature rat

Following prenatal exposure to ethanol, rats have a 1/3 fewer neurons in the second order (principal sensory nucleus of the trigeminal nerve) and fourth order neurons (somatosensory cortex) of the trigeminal-somatosensory pathway than do controls. Based on the numerical matching hypothesis, we predict that the number of third-order neurons (in the ventrobasal nucleus of the thalamus; VB) also will show a similar effect of prenatal ethanol exposure. Stereological methods were used to determine the total number of neurons in the VB on postnatal day 30. Surprisingly, prenatal exposure to ethanol had no effect on the VB volume or on the number of VB neurons. Thus, prenatal exposure to ethanol induces numerical imbalances within the trigeminal-somatosensory system.

Nerve growth factor preserves a critical motor period in rat striatum

We previously found the occurrence of a critical motor period during rat postnatal development where circling training starting the 7-day schedule at 30 days-but not before or after-induces a lifetime drop in the binding to cholinergic muscarinic receptors (mAChRs) in striatum. Here, we studied whether nerve growth factor (NGF) participates in this restricted period of muscarinic sensitivity. For this purpose, we administered mouse salival gland 2.5S NGF (1.4 or 0.4 microg/day, infused by means of ALZA minipumps) by intrastriatal unilateral route between days 25 and 39, and then trained rats starting at 40 days. Under these conditions, NGF induced a long-term reduction in the striatal [3H] quinuclidilbenzylate (QNB) binding sites despite the fact that motor training was carried out beyond the natural critical period. Thus, at day 70, measurement of specific QNB binding in infused striata of trained rats showed decreases of 42% (p < .0004) and 33% (p < .02) after administration of the higher and lower NGF doses, respectively, with respect to trained rats treated with cytochrome C, for control. Noncannulated striata of the NGF-treated rats also showed a decrease in QNB binding sites (44%; p < .0001) only at the higher infusion rate. This effect was not found in the respective control groups. Our observations show that NGF modulates the critical period in which activity-dependent mAChR setting takes place during rat striatal maturation.

Gene expression of growth-associated proteins, GAP-43 and SCG10, in the hippocampal formation of the macaque monkey: nonradioactive in situ hybridization study

We performed nonradioactive in situ hybridization histochemistry in the monkey hippocampal formation that includes the hippocampus, the subicular complex, and the entorhinal cortex to detect the expression of mRNA for two growth-associated proteins: GAP-43 and SCG10. Overall, the distribution patterns overlapped but were partially distinct. In the hippocampus, the intense hybridization signals for both GAP-43 and SCG10 mRNAs were observed in the pyramidal cell layer of Ammon's horn, especially in CA3 subfields. The intense hybridization signals were also observed in the stratum oriens of Ammon's horn and the polymorphic layer of the dentate gyrus. In the granule cell layer of the dentate gyrus, many GAP-43 mRNA-positive cells were observed, whereas a few positive cells with weak signals were observed for SCG10 mRNA. Throughout the subicular complex, the hybridization signals for both mRNAs were weak. In the entorhinal cortex, both mRNAs were abundant in the caudal field. These subregion-specific expression of the growth-associated proteins may reflect the functional specialization regarding plasticity in each region of the monkey hippocampal formation.

Enhanced neocortical neural sprouting, synaptogenesis, and behavioral recovery with D-amphetamine therapy after neocortical infarction in rats

BACKGROUND AND PURPOSE

D-Amphetamine administration increases behavioral recovery after various cortical lesions including cortical ablations, contusions, and focal ischemia in animals and after stroke in humans. The purpose of the present study was to test the enhanced behavioral recovery and increased expression of proteins involved in neurite growth and synaptogenesis in D-amphetamine-treated rats compared with vehicle-treated controls after a focal neocortical infarct.

METHODS

Unilateral neocortical ischemia was induced in male spontaneously hypertensive Wistar rats (n=8 per time point per group) by permanently occluding the distal middle cerebral artery and ipsilateral common carotid artery in 2 groups of rats: D-amphetamine treated (2 mg/kg IP injections) and vehicle treated (saline IP injections). To determine the spatial and temporal distribution of neurite growth and/or synaptogenesis, growth-associated protein (GAP-43), a protein expressed on axonal growth cones, and synaptophysin, a calcium-binding protein found on synaptic vesicles, were examined by immunohistochemical techniques, and both density and distribution of reaction product were measured. Since the resulting infarction included a portion of the forelimb neocortex, behavioral assessments of forelimb function using the foot-fault test of Hernandez and Schallert were performed on the same rats used for immunohistochemical studies during the period of drug action and 24 hours later. A Morris water maze and other indices of behavioral assays were also measured similarly. Recovery times were 3, 7, 14, 30, and 60 days postoperatively.

RESULTS

Both GAP-43 and synaptophysin proteins demonstrated statistically significant increases in density and distribution of immunoreaction product as determined by optical density measurements in the neocortex of the infarcted group treated with D-amphetamines compared with vehicle-treated infarcted controls. The GAP-43 was elevated to statistically significant levels in forelimb, hindlimb, and parietal neocortical regions ipsilateral to the infarction only at days 3, 7, and 14. By contrast, the synaptophysin demonstrated no statistically significant changes in expression at 3 or 7 days but demonstrated statistically significant increases at 14, 30, and 60 days in the forelimb, hindlimb, and parietal neocortical regions ipsilateral to the infarction as well as increased distribution in the contralateral parietal neocortex. Behavioral assessment of forelimb function indicated that improved recovery of forelimb placement on the side contralateral to the infarction was statistically significant in the D-amphetamine-treated group compared with the vehicle-treated group (P<0.025). Spatial memory, as measured with the Morris water maze, worsened in the vehicle-treated group compared with the D-amphetamine-treated group at 60 days (P<0.025).

CONCLUSIONS

These data support the occurrence of neurite growth followed by synaptogenesis in the neocortex in a pattern that corresponds both spatially and temporally with behavioral recovery that is accelerated by D-amphetamine treatment. While the specific mechanisms responsible for D-amphetamine-promoted expression of proteins involved in neurite growth and synaptogenesis and of enhanced behavioral recovery are not known, it is suggested that protein upregulation occurs as a result of functional activation of pathways able to remodel in response to active behavioral performance.

Hippocampal and cortical growth-associated protein-43 messenger RNA in schizophrenia

Growth-associated protein-43 is involved in maturational and plasticity-associated processes, and changes in growth-associated protein-43 expression are a marker of altered plasticity following experimental and neuropathological lesions. Using in situ hybridization, we have investigated growth-associated protein-43 mRNA in the medial temporal lobe and cerebral cortex in 11 normal subjects and 11 matched subjects with schizophrenia, a disorder in which perturbed neurodevelopment and aberrant plasticity are implicated. In the schizophrenia group, growth-associated protein-43 messenger RNA was decreased in the medial temporal lobe, primary visual cortex and anterior cingulate gyrus, but was unaltered in the superior temporal and dorsolateral prefrontal cortices. Correlations of growth-associated protein-43 messenger RNA signal between areas were stronger and more numerous in the schizophrenics than in the controls, suggesting a more global regulation of growth-associated protein-43 expression. Finally, the ratio of growth-associated protein-43 messenger RNA to synaptophysin messenger RNA--a putative index of the production of new synapses--was decreased in the medial temporal lobe in the schizophrenics. Our findings imply that neuronal plasticity as indexed by growth-associated protein-43 expression is impaired, and perhaps aberrantly regulated, in schizophrenia. The data support the emerging view that the disease pathophysiology is one which affects the hippocampal and cortical circuitry and that the abnormalities are reflected in the altered expression of specific neuronal genes.

Effects of neuronal proteoglycans on activity-dependent growth responses of fetal hippocampal neurons

Excitatory amino-acid (EAA) neurotransmitters act as molecular signals influencing the structure of neurons during development. However, the signal transduction and effector mechanisms responsible for these effects have yet to be fully elucidated. We have previously provided evidence that EAA agonists induce the synthesis and release of proteoglycans (PGs) with neurite-promoting activity from fetal hippocampal neurons. In the present studies exposure of fetal hippocampal neurons to glutamate (100 microM) for 5 min resulted in increases in the neuron-specific growth-associated genes T alpha 1 alpha-tubulin (T alpha 1), microtubule-associated protein-2 (MAP-2) and growth-associated protein-43 (GAP-43). mRNA levels peaked at between 8 and 12 h following exposure as determined by competitive reverse transcription polymerase chain reaction (RT-PCR). Increases in neurite growth as measured by axonal length, the total length of dendrites, the number of branches per axon, the total length of branches per axon and the total neurite length were also observed 48 h after glutamate exposure. The increase in T alpha 1, MAP-2 and GAP-43 mRNA levels following glutamate exposure was mediated via both N-methyl-D-aspartate and metabotropic receptor activation. Heparin, which inhibits the neurite growth-promoting effects of PGs in vitro, and heparitinase, which catalyzes the cleavage of heparan sulphate, also inhibited the glutamate-dependent induction of T alpha 1, MAP-2 and GAP-43 mRNA expression and neurite growth when added to culture medium following glutamate exposure. Chondroitin sulphate and chondroitinase AC had no effects on the mRNA levels tested or on neurite growth. Therefore, these studies suggest that neuronal PGs regulated by activation of EAA receptors mediate neuronal growth responses.

Changes in extracellular levels of dopamine metabolites in somatosensory cortex after peripheral denervation

This study examined the effects of a nerve transection on monoamine release from primary somatosensory cortex. The technique of microdialysis was employed to sample extracellular levels of norepinephrine (NE), 3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindole-3-acetic acid (5-HIAA) and homovanillic acid (HVA) in the barrel field of freely moving rats following the surgical transection of the contralateral infraorbital nerve. Microdialysates obtained 3, 4, and 5 days after deafferentation were analyzed using high-performance liquid chromatography with electrochemical detection. We found a significant increase in the release of the dopamine metabolites, DOPAC and HVA from the deafferented cortex. Three days after deafferentation the release of DOPAC was three-fold higher in the deafferented than in the control animals, and remained about 100% higher in the next two days in this group of animals. The release of HVA showed a gradual increase following the deafferentation procedure, since a 92% larger value on day 3 increased to a 338% difference on day 5. On the other hand, the release rate of NE and the levels of the serotonin metabolite 5-HIAA were not significantly affected by the deafferentation procedure. These results are discussed in the context of the possible participation of dopamine in the reorganization of the deafferented somatosensory cortex.

Neocortical neural sprouting, synaptogenesis, and behavioral recovery after neocortical infarction in rats

BACKGROUND AND PURPOSE

Neuroanatomical plasticity is well described in lesions of the hippocampus but remains a subject of some controversy in the neocortex. The purpose of the present study was to measure the neocortical distribution and density of expression of proteins known to be involved in neurite growth or synaptogenesis and to correlate the neocortical expression with behavioral recovery after a focal neocortical infarction. Focal neocortical infarction creates a circumscribed lesion in the neocortex that provides a denervation stimulus for neurite growth and synaptogenesis.

METHODS

Unilateral neocortical ischemia was induced in male spontaneously hypertensive Wistar rats (n = 4 per time point) by permanent occlusion of the distal middle cerebral artery and ipsilateral common carotid artery. To determine the spatial and temporal distribution of neurite growth and/or synaptogenesis, GAP-43, a growth-associated protein expressed on axonal growth cones, and synaptophysin, a calcium-binding protein found on synaptic vesicles, were examined by immunohistochemical techniques. The reaction product was measured, and the distribution was recorded. Since the resulting infarction included a portion of the forelimb neocortex, behavioral assessments of forelimb function that used the foot-fault test of Hernandez and Schallert were performed on the same rats used for immunohistochemical studies. Recovery times were 3, 7, 14, 30, and 60 days after surgery.

RESULTS

Both GAP-43 and synaptophysin proteins demonstrated statistically significant increases in the density of immunoreaction product as determined by optical density measurements in the neocortex of infarcted rats compared with sham controls. The GAP-43 was elevated to statistically significant levels in forelimb, hindlimb, and parietal neocortical regions medial and lateral to the infarction only at days 3, 7, and 14. In contrast, synaptophysin demonstrated no statistically significant changes in expression at 3 or 7 days but demonstrated statistically significant increases at 14, 30, and 60 days in the forelimb, hindlimb, and parietal neocortical regions medial and lateral to the infarction as well as in the contralateral parietal neocortex. Behavioral assessment of forelimb function indicated impairment of forelimb placement on the side contralateral to the infarction that trended toward control values at 14 days and was not significantly different from controls by 30 days.

CONCLUSIONS

These data support the occurrence of neurite growth followed by synaptogenesis in the neocortex, ipsilateral and contralateral to neocortical ischemia, in a pattern that corresponds both spatially and temporally with behavioral recovery. Thus, neuroanatomical remodeling in the neocortex provides a mechanism for recovery of function.

Differential expression of p140trk, p75NGFR and growth-associated phosphoprotein-43 genes in nucleus basalis magnocellularis, thalamus and adjacent cortex following neocortical infarction and nerve growth factor treatment

A loss of target-derived neurotrophic factors is hypothesized to be one of the major determinants of central nervous system neuronal degeneration. In order to obtain further insight into early neuronal responses to injury, lesion-induced alterations in the expression of high- and low-affinity nerve growth factor receptors, as well as growth-associated phosphoprotein-43 genes in nucleus basalis magnocellularis, thalamic and neocortical neurons were studied. For this purpose, unilateral cortical devascularization operations were conducted on adult rats. Animals received i.c.v. infusions of vehicle or nerve growth factor (12 micrograms/day) and were killed at one, three, seven and 15 days post-lesion. In situ hybridization studies using 35S-labelled oligonucleotide probes for p75NGFR, p140trk and growth-associated phosphoprotein-43 messenger RNAs reveals that these genes were differentially regulated following the lesion. In the nucleus basalis magnocellularis ipsilateral to the lesion, p140trk gene expression significantly decreased on days 3 and 7, while p75NGFR messenger RNA initially increased on day 3 and decreased on days 7 and 15 after lesion. GAP-43 messenger RNA levels were significantly increased in the nucleus basalis magnocellularis on post-lesion days 3 and 7. Moreover, in contrast to p75NGFR or 140trk, growth-associated phosphoprotein-43 messenger RNA levels were significantly increased in pyramidal neurons located in the remaining cortex adjacent to the cortical lesion at all time points. In the lateral and ventroposterior nuclei of the thalamus, growth-associated phosphoprotein-43 messenger RNA level was slightly increased on days 1 and 3 and was dramatically decreased, significantly below the levels in sham-operated controls, on post-lesion days 7 and 15. During nerve growth factor application, the level of p140trk messenger RNA in the lesioned nucleus basalis magnocellularis returned to values observed in the contralateral nucleus basalis magnocellularis while p75NGFR messenger RNA was increased above values noted in all animals not treated with nerve growth factor. Nerve growth factor treatment did not affect the expression of growth-associated phosphoprotein-43 messenger RNA in any of the areas studied. p140trk messenger RNA was not up-regulated during the time that nerve growth factor was applied, as observed for p75NGFR, but only eight days after interrupting nerve growth factor treatment. Three cell types, nucleus basalis magnocellularis, cortical pyramidal and thalamic neurons, were probably affected in different ways by the devascularization with respect to lesion extent. Consequently, the remaining number of synaptic contacts in each of these brain areas is most likely different which may lead to a differential regulation of growth-associated phosphoprotein-43 messenger RNA.(ABSTRACT TRUNCATED AT 400 WORDS)

Lateralized effect of unilateral somatosensory cortex contusion on behavior and cortical reorganization

Previous studies have shown that rats recover function after unilateral somatosensory cortex lesions, possibly by transfer of information processing to other brain areas not normally involved in those functions. In the present study, adult rats underwent unilateral contusions of the somatosensory cortex with ablation of the barrel receptor field. Behavioral testing with modified beam-walking and sensory neglect tasks demonstrated persistent somatosensory deficits in rats with left contusions but no apparent deficits in right injured animals. After 2 months, the [14C]2-deoxyglucose (2-DG) method was used to show the metabolic activity produced by unilateral stimulation of the facial vibrissae. In left injured animals, cortical metabolic activity rostral and caudal to the injury site was depressed both under basal conditions and during right vibrissal stimulation. On the other hand, comparison of the pattern of [14C]2-DG uptake in the intact, right cortex revealed changes in the pattern of glucose utilization associated with left injury combined with right vibrissal stimulation. Pattern changes were quantified by measuring the area in which glucose utilization was within the highest 25% of this range (high activity area; HAA). Right vibrissal stimulation in left injured rats caused an expansion of this HAA in the intact occipital/temporal cortex. Also, in the intact somatosensory cortex of left injured rats, there was an enlarged HAA whether or not vibrissal stimulation was performed. Thus, a combination of depressed peri-injury metabolic activity and aberrant activity in remote brain areas occurs following unilateral somatosensory cortex injury. It remains to be shown whether these factors ameliorate or contribute to persistent behavioral deficits.

Alpha 1-adrenoceptors in the adult rat barrel field: effects of deafferentation and norepinephrine removal

Norepinephrine (NE), acting on brain adrenoceptors, plays an important role in barrel field neuronal activity and plasticity. For this reason, the distribution of alpha 1- and alpha 2-adrenoceptors in the somatosensory cortex barrel field was studied by autoradiographic techniques in rats undergoing plastic change or NE depletion. In layers IV and V of the cortex, the pattern of alpha 1-adrenoceptors (assessed by [3H]prazosin binding) varied across the barrel field. There was relatively low binding within the barrels themselves, with 21% higher binding in the surrounding septa. alpha 2-Adrenoceptor binding (assessed with [3H]paraminoclonidine) was almost homogeneous across the entire barrel field. Two weeks after noradrenergic deafferentation by unilateral lesioning of the locus coeruleus, there was a 16% upregulation of [3H]prazosin binding. This then returned to control levels of by 8 weeks. Peripheral deafferentation of sensory input to the barrel field produced the opposite effect on alpha 1-adrenoceptors. Unilateral removal of all but the central (C3) vibrissa (which induces plastic changes in the cortical representation of the spared virbrissa) caused a 12% decrease in [3H]prazosin binding in the whole barrel field at 2 weeks after surgery which returned to normal by 8 weeks. Therefore, alpha 1-adrenoceptors in the barrel field of the rat are affected in opposite ways by changes in NE content and afferent sensory input. We hypothesize that alpha 1-adrenoceptor levels are modulated after vibrissectomy through either an indirect reaction to reduced cortical gamma-aminobutyric acid levels, or by a reordering of metabolic priorities during plastic change of the cortical neuronal network.