Absence of hippocampal mossy fiber sprouting in transgenic mice overexpressing brain-derived neurotrophic factor

Excess neuronal activity upregulates the expression of two neurotrophins, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) in adult hippocampus. Nerve growth factor has been shown to contribute the induction of aberrant hippocampal mossy fiber sprouting in the inner molecular layer of the dentate gyrus, however the role of prolonged brain-derived neurotrophic factor exposure is uncertain. We examined the distribution and plasticity of mossy fibers in transgenic mice with developmental overexpression of brain-derived neurotrophic factor. Despite 2--3-fold elevated BDNF levels in the hippocampus sufficient to increase the intensity of neuropeptide Y immunoreactivity in interneurons, no visible changes in mossy fiber Timm staining patterns were observed in the inner molecular layer of adult mutant hippocampus compared to wild-type mice. In addition, no changes of the mRNA expression of two growth-associated proteins, GAP-43 and SCG-10 were found. These data suggest that early and persistent elevations of brain-derived neurotrophic factor in granule cells are not sufficient to elicit this pattern of axonal plasticity in the hippocampus.

Neurotrophin-4/5 promotes dendritic outgrowth and calcium currents in cultured mesencephalic dopamine neurons

Ca(2+) currents and their modulation by neurotrophin-4/5 were studied in cultured mesencephalic neurons. Tyrosine hydroxylase-positive neurons consistently had larger somas than tyrosine hydroxylase-negative neurons. Neurons with larger somas were therefore targeted for recording. In both control and neurotrophin-4/5-treated cultured neurons, isolation of Ca(2+) currents in cultured mesencephalic neurons revealed prominent low- and high-voltage-activated currents. These currents were separable based upon their voltage dependence of activation, the response to replacement of Ca(2+) with Ba(2+) and the response to Ca(2+) channel blockers. Replacement of Ca(2+) with Ba(2+) resulted in a slight reduction of low-voltage-activated currents and a significant enhancement of high-voltage-activated currents. Cd(2+) blocked a larger fraction of the high-voltage-activated current than Ni(2+). The synthetic conotoxins SNX-124 and SNX-230 selectively blocked high-voltage-activated currents. Morphological analysis of mesencephalic cultures pretreated with neurotrophin-4/5 revealed an increase in soma size and dendritic length in tyrosine hydroxylase-positive neurons. In agreement with the neurotrophin-4/5 induction of growth, neurotrophin-4/5 also increased cell capacitance in whole-cell recordings. Neurotrophin-4/5 significantly enhanced both low- and high-voltage-activated currents, but normalization for changes in capacitance revealed only a significant increase in high-voltage-activated current density. This study demonstrates the existence of low-voltage-activated and multiple classes of high-voltage-activated calcium currents in cultured mesencephalic neurons. Morphological and physiological data demonstrate that the increases in calcium currents due to neurotrophin-4/5 pretreatment are associated with somatodendritic growth, but an increase in high-voltage-activated Ca(2+) channel expression also occurred.

Cerebellar brain-derived neurotrophic factor-TrkB defect associated with impairment of eyeblink conditioning in Stargazer mutant mice

In the spontaneous ataxic mutant mouse stargazer, there is a selective reduction of brain-derived neurotrophic factor (BDNF) mRNA expression in the cerebellum. BDNF protein levels in the cerebellum are reduced by 70%. Despite normal levels of full-length and truncated TrkB receptor, constitutive and neurotrophin-4/5-induced tyrosine phosphorylation was significantly reduced in several signal transduction molecules, including phospholipase-Cgamma1, erk1, and erk2. Morphological examination revealed an increased number of external granule cells at postnatal day 15 and the presence of abnormal neurons resembling immature granule cells in the adult. These abnormalities are associated with a severe impairment in the acquisition of classical eyeblink conditioning, indicating cerebellar malfunction. Our data suggest that normal BDNF expression and TrkB signal transduction in the cerebellum are necessary for learning and plasticity in this model.

Impaired eye-blink conditioning in waggler, a mutant mouse with cerebellar BDNF deficiency

In addition to their trophic functions, neurotrophins are also implicated in synaptic modulation and learning and memory. Although gene knockout techniques have been used widely in studying the roles of neurotrophins at molecular and cellular levels, behavioral studies using neurotrophin knockouts are limited by the early-onset lethality and various sensory deficits associated with the gene knockout mice. In the present study, we found that in a spontaneous mutant mouse, waggler, the expression of brain-derived neurotrophic factor (BDNF) was selectively absent in the cerebellar granule cells. The cytoarchitecture of the waggler cerebellum appeared to be normal at the light microscope level. The mutant mice exhibited no sensory deficits to auditory stimuli or heat-induced pain. However, they were massively impaired in classic eye-blink conditioning. These results suggest that BDNF may have a role in normal cerebellar neuronal function, which, in turn, is essential for classic eye-blink conditioning.

Elevated mRNA expression of brain-derived neurotrophic factor in retinal ganglion cell layer after optic nerve injury

PURPOSE

Recent studies show that exogenous brain-derived neurotrophic factor (BDNF) can promote retinal ganglion cell survival in vivo and in vitro. BDNF is expressed by a subpopulation of cells in the ganglion cell layer (GCL). To investigate whether endogenous BDNF may play a role in neuronal protection after ganglion cell trauma, BDNF expression in the retina was examined after optic nerve (ON) injury.

METHODS

The optic nerve in Sprague-Dawley rats was crushed intraorbitally posterior to the optic disc. For controls, the optic nerve on the opposite side in each animal was similarly exposed but was not crushed. After intervals of 6 hours to 6 weeks, eye tissues were processed for in situ hybridization, Northern blot, and RNase protection assay using radiolabeled rat riboprobes.

RESULTS

After ON injury, BDNF expression was significantly elevated in cells restricted to the GCL, and more cells demonstrated expression of BDNF than were observed in the controls. Elevated BDNF expression was first observed at 24 hours, peaked at 48 hours, and declined to the basal level 2 weeks after ON injury. Quantitative analysis showed a fivefold to sixfold increase in the number of BDNF-positive cells and a 54% increase in BDNF signal intensity in individual cells in the GCL 48 hours after ON injury. In control retinas without ON injury, BDNF expression was localized to some cells in the GCL, as was observed in normal eyes without surgery. Northern blot and RNase protection assay demonstrated a 38% elevation in BDNF expression above control levels 48 hours after ON injury.

CONCLUSIONS

These results indicate that cells in the GCL can upregulate gene expression of BDNF in response to ganglion cell axonal injury and suggest that endogenous BDNF may contribute to a natural neuroprotective process after ON injury.

Laminin-alpha2 chain-like antigens in CNS dendritic spines

The laminin-alpha2 chain is a component of brain capillary basement membranes and appears also to be present in neurons of rat, rabbit, pig and non-human primate brain as evidenced by immunohistochemistry. In the present study, we have further characterized this very distinct neuronal laminin-alpha2 chain-like immunoreactivity in the hippocampus of various species. Immunoelectron microscopy with poly- and monoclonal antibodies to the laminin-alpha2 chain G-domain localized laminin-alpha2 chain immunoreactivity in adult rat and rabbit hippocampus to dendritic processes, primarily to dendritic spines. In the developing rat hippocampus, spine-associated laminin-alpha2 chain-like immunoreactivity first appeared at a time corresponding to that of active synaptogenesis. After an entorhinal cortex lesion in adult rats, the time course of denervation-induced loss and reactive reappearance of spines in the molecular layer of the dentate gyrus was correlated closely to the loss and reappearance of laminin-alpha2 chain immunoreactivity. Immunoblot analysis of normal adult rat, rabbit and pig brain revealed a protein similar in size to the reported 80-kDa laminin-alpha2 chain fragment of human placenta as well as 140/160-kDa proteins. These results suggest the presence of proteins with antigenic homology to the laminin-alpha2 chain and/or laminin-alpha2 isoforms in dendrites and dendritic spines in selected areas of the brain, predominately in the hippocampus and other limbic structures. Given the adhesion and neurite promoting functions of laminins, it is possible that neuronal laminin-alpha2 chain-like proteins play a role in synaptic function and plasticity in the CNS.

Axotomized septal cholinergic neurons rescued by nerve growth factor or neurotrophin-4/5 fail to express the inducible transcription factor c-Jun

The inducible transcription factor c-Jun increases in neurons in response to axotomy by unknown mechanisms, and it has been postulated that c-Jun may regulate genes involved in promoting either degeneration or regeneration of axotomized neurons. In this report, we investigated the effect of daily or twice daily intraventricular administration of the neurotrophins nerve growth factor or neurotrophin-4/5 on the decrease in choline acetyltransferase expression and the increase in c-Jun expression in rat medial septum/diagonal band neurons three, seven and 14 days following unilateral, complete, fornix fimbria lesion. We also examined whether medial septum/diagonal band neurons might die by apoptosis within two weeks of fornix fimbria lesion using terminal deoxynucleotidyl transferase-mediated dUTP biotin nick end labelling. Our results show that both nerve growth factor and neurotrophin-4/5 maintain the phenotype of basal forebrain cholinergic neurons following axotomy. Furthermore, using double-labelling immunofluorescence, we found that while c-Jun was expressed in cholinergic neurons in control-treated rats seven days following fornix fimbria lesion, cholinergic neurons rescued by either nerve growth factor or neurotrophin-4/5 in neurotrophin-treated rats failed to express c-Jun. At no time-point (three, seven or 14 days post-axotomy) did any neurons in the medial septum/diagonal band stain positive for terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling, suggesting that medial septum/diagonal band neurons do not undergo apoptosis within the first two weeks following axotomy at the time-points observed by us. Therefore, these results show that both nerve growth factor and neurotrophin-4/5 rescue the phenotype of axotomized cholinergic neurons and that these rescued neurons fail to express c-Jun in response to axotomy. In addition, since neither nerve growth factor nor neurotrophin-4/5 induced c-Jun in medial septum/diagonal band cholinergic neurons, it seems unlikely that the neurotrophic effects of nerve growth factor and neurotrophin-4/5 on cholinergic neurons are mediated via c-Jun expression. Furthermore, since axotomy failed to increase terminal deoxynucleotidyl transferase-mediated dUTP biotin nick end labelling in septal neurons, it appears unlikely that c-Jun expression in these axotomized neurons is related to neuronal degeneration via apoptosis.

Ligand-induced down-regulation of Trk messenger RNA, protein and tyrosine phosphorylation in rat cortical neurons

Chronic exposure of brain neurons to nerve growth factor in vitro and in vivo results in increased levels of the nerve growth factor receptor TrkA. In contrast, in the present study, we have found that chronic exposure of rat embryonic cortical neurons to brain-derived neurotrophic factor (BDNF) leads to a pronounced reduction of the levels of protein and messenger RNA for the full-length but not the truncated BDNF receptor TrkB. Similar effects were observed with the other TrkB ligands neurotrophin-3 and neurotrophin-4/5. After pretreatment with BDNF, neurotrophin-3 or neurotrophin-4/5, subsequent tyrosine phosphorylation responses of the remaining Trks to the same factors were greatly reduced. Three days exposure of rat embryonic cortical neurons to BDNF induced an absolute refractory period of several hours, with no subsequent response to the same factor. Similar but less pronounced refractory effects were observed with neurotrophin-3 and neurotrophin-4/5. Our results suggest a negative regulatory effect of BDNF and other TrkB ligands on TrkB receptors. Down-regulation of the TrkB response by its ligands might play a role in the control of BDNF action during early development, when BDNF levels significantly increase. Our findings are also of potential clinical relevance, since the possibility of ligand-induced down-regulation of the receptor response needs to be addressed when considering BDNF or other neurotrophins for the therapy of neurodegeneration.

Cloning/brain localization of mouse glutamylcysteine synthetase heavy chain mRNA

Glutathione (GSH) is considered the primary molecule responsible for peroxide removal from the brain. Inhibition of its rate-limiting synthetic enzyme, glutamylcysteine synthetase (GCS), results in morphological damage to both cortical and nigral neurons in rodents. Here, we report cloning of the catalytic heavy chain GCS mRNA from mouse and its localization in the murine brain. Heavy chain GCS appears to be localized in glial populations in the hippocampus, cerebellum and olfactory bulb, with lower levels of expression in the cortex and substantia nigra. Variations in GCS levels and subsequent GSH synthesis may explain differences in susceptibility to neuropathology associated with oxidative stress noted in these various brain regions.

Excitotoxic lesion of rat brain with quinolinic acid induces expression of p53 messenger RNA and protein and p53-inducible genes Bax and Gadd-45 in brain areas showing DNA fragmentation

Several recent studies have demonstrated that expression of the tumour-suppressor gene p53 increases within the nervous system after injury. In various cell lines wild-type-p53, induced by DNA damage, has been shown to function to halt cell-cycle progression and under certain circumstances to induce programmed-cell death or apoptosis. Since wild type-p53 can act as a transcription factor to regulate the expression of p53-responsive genes it is possible that either, or both, functions of p53 are mediated by down-stream effector genes. However wild-type-p53 only weakly activates transcription and it remains to be determined whether p53-responsive genes are expressed in lesioned brain. Here we report that excitotoxic lesion of rat brain with the N-methyl-D-aspartate receptor agonist, quinolinic acid, induces expression of p53 messenger RNA and protein in brain regions showing delayed DNA fragmentation and that expression of p53 messenger RNA precedes DNA damage detected by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labelling. In addition, using in situ hybridization and immunocytochemistry we demonstrate increased expression of the p53-responsive gene Gadd-45 (preceding p53 expression) and re-expression of the p53-responsive gene Bax (following p53 expression), in these same areas. Bax has been shown to promote neuronal death by interacting with Bcl-2 family members while Gadd-45 expression has been associated with suppression of the cell-cycle and DNA repair. These results suggest that p53 protein may function as an active transcription factor in lesioned brain perhaps initiating the re-expression of Bax in injured brain regions. However, since Gadd-45 precedes p53 expression it appears unlikely that p53 is involved in regulating the early expression of Gadd-45. Taken together however, these results suggest that p53, Bax and Gadd-45 may play important roles in the response (damage/recovery) of the brain following excitotoxic injury.

Intraparenchymal NGF injections in adult and aged rats induce long-lasting Trk tyrosine phosphorylation

Neurotrophic factors, particularly the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) and related molecules are proposed for the experimental treatment of neurode-generative disease. Earlier observations had suggested down-regulation of the neurotrophin receptor response with chronic stimulation. We therefore tested for effects of acute and chronic NGF treatment in vivo on the tyrosine phosphorylation response of Trk-type neurotrophin receptors in adult and aged rats. Rats were treated for 1 week with daily injections of NGF directly into the striatum. Surprisingly, this chronic neurotrophin treatment induced long-lasting tyrosine phosphorylation of Trk type receptors beyond the last injection. A similar result was obtained with 1 week of daily injections of BDNF into the hippocampus. Persistent TRK tyrosine phosphorylation was also observed after single neurotrophin injections. With 1 microgram of NGF injected, Trk-type receptors were maximally stimulated from immediately after the injection until 3 days after the treatment. Maintaining Trk tyrosine phosphorylation required maintained energy levels in the tissue. Incubation of microslices of brain tissue from NGF-injected animals in glucose-free buffer completely abolished all Trk tyrosine phosphorylation signals. Recovery of tissue in presence of glucose restored the signals in microslices derived from NGF-injected animals, in absence of acute NGF treatment. This result, together with dose-response comparisons after 2-h and 2-day survival times suggest that Trk protein remains tyrosine phosphorylated due to trophic protein which is only slowly being cleared out of the tissue during several days after the injection. Experiments with aged rats indicated similar extent and duration of Trk receptor activation after NGF administration in young adult and in aged brain.

Neurotrophins and Alzheimer's disease: beyond the cholinergic neurons

Improvement of the cholinergic deficit in Alzheimer's disease (AD) by intracerebral application of nerve growth factor (NGF) serves as a paradigmatic example for a novel approach to the treatment of neurodegeneration. The first part of this paper presents and discusses experiments which were performed in our laboratory to study the NGF receptor response after intracerebral NGF treatment in vivo. We found that intraparenchymal NGF elicits prolonged tyrosine phosphorylation of Trk type NGF receptors. Our results indicate that intraparenchymal injections are preferable to intraventricular application for targeting specific neuronal populations with minimal side effects. Besides the cholinergic deficit, severely disturbed brain energy metabolism, particularly in cortical association areas, is another consistent feature of AD. Metabolic hypofunction is observed early in the disease progression and correlates with the cognitive impairment. Cell culture findings are presented which indicate that brain-derived neurotrophic factor (BDNF), and other neurotrophins with activity on the TrkB tyrosine kinase receptor, increase mRNA levels and biochemical activity of enzymes of the glycolytic pathway in brain cells. Treatment with these factors was also found to stimulate glucose utilization in rat embryonic cortex cells in primary cultures. Our observations suggest that selected neurotrophins should become useful not only for the treatment of the cholinergic deficit in AD, but also of the cortical metabolic hypofunction associated with this disease.

Trophic effect of exogenous nerve growth factor on rat striatal cholinergic neurons: comparison between intraparenchymal and intraventricular administration

Penetration into the brain is an important consideration in the pharmacological use of neurotrophic factors for the treatment of brain neurodegeneration, e.g., in Alzheimer's disease. Furthermore, intracerebroventricular treatment with nerve growth factor (NGF) has been found to induce side effects, including aberrant sympathetic sprouting and weight loss. Such findings suggest that direct intraparenchymal application of minimal amounts of trophic factors might be therapeutically desirable. We compared the effectiveness of intrastriatal and intracerebroventricular administrations of NGF on striatal cholinergic neurons in adult rats. Daily intrastriatal administration for 1 week of > or = 50 ng of NGF resulted in an increase in mRNA levels for choline acetyltransferase (ChAT) in striatal cholinergic cells to approximately 2-fold over control. A daily intraventricular dose of 4.5 micrograms of NGF was required for a similar response. Both 5 and 50 ng of NGF/day failed to induce an effect on transmembrane protein tyrosine kinase trkA mRNA levels, but injections of 750 or 1500 ng/day of NGF up-regulated trkA mRNA expression to approximately 2-fold of control. NGF delivered intracerebroventricularly failed to induce an observable change in striatal trkA mRNA, even at a dosage of 4.5 micrograms of NGF/day. These quantitative differences in NGF actions were reflected at the level of NGF receptors. Using Western blotting procedures, we found pronounced tyrosine phosphorylation of Trk-type proteins 2 hr after intrastriatal injection of 50 ng of NGF. Maximal responses were seen with either 150 or 750 ng of NGF. For maximal activation of Trks by intraventricular NGF injection, 4.5 micrograms of NGF was required. Taken together, our results strongly favor intraparenchymal injections or infusions of NGF, and possibly other trophic factors, for therapeutical applications to maximize the effects on the targeted neuronal populations and to minimize undesirable side effects.

NT-4/5 protects against adrenalectomy-induced apoptosis of rat hippocampal granule cells

Adrenalectomy (ADX) in rats has been shown to induce apoptosis of hippocampal granule cells. We tested whether neurotrophins are able to protect hippocampal neurons in this neurodegeneration model. Acid fucshsin stain was used to identify pyknotic cells in ADX rats treated for 4 days with NT-3, NT-4-5 or cytochrome-C, as a control protein. Cytochrome-C injections slightly decreased cell death on the ipsilateral side. NT-3 did not further promote this effect. Significantly less cell death was observed bilaterally in hippocampus treated with NT-4/5. TUNEL end labeling also confirmed the results. Our results demonstrated that NT-4/5, but not NT-3, promotes hippocampal neuron survival in adrenalectomized rats. They further show that injections of a control solution can induce a local protective effect.

Selective failure of brain-derived neurotrophic factor mRNA expression in the cerebellum of stargazer, a mutant mouse with ataxia

In search of the possible involvement of neurotrophic factors in inherited neurological disease, we examined brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin-3 (NT-3) mRNA expression patterns in the ataxic mutant mouse stargazer (stg). Using in situ hybridization, we found a selective and near total reduction in BDNF mRNA in the cerebellar granule cell layer. NT-3 or NGF mRNA expression in the cerebellum was normal. Northern blot analysis demonstrated a 70% reduction in BDNF mRNA in the whole cerebellum. BDNF mRNA levels in other mutant brain regions were unchanged. Absence of BDNF mRNA in granule cells was observed at postnatal age (P15), coincident with the onset of ataxia, and expression levels failed to follow the developmental increase found in the wild type at later ages (P20 and P30). Despite the severe BDNF reduction, in situ hybridization patterns for both the full-length and the truncated BDNF TrkB receptor mRNA were unaltered. No major cytoarchitectural abnormalities were apparent in the stg/stg cerebellum. BDNF expression in a related ataxic mutant, tottering, was unaltered. These data show that BDNF can be regulated selectively in distinct brain regions, possibly by differential activation of its multiple promoters. Absence of cerebellar granule cell BDNF mRNA in stg/stg mice demonstrates that sustained expression of this neurotrophin is not required for cell survival in the developing cerebellar cortex. Our data, in contrast, suggest a role of BDNF in maturation of specific cerebellar neurons and pathways. Early failure of cerebellar BDNF expression may be related to the ataxic phenotype in stg mice.

Heterodimeric neurotrophins induce phosphorylation of Trk receptors and promote neuronal differentiation in PC12 cells

Neurotrophins are a family of highly conserved proteins that affect the development and maintenance of distinct neuronal populations. Neurotrophins exist in vivo as homodimers, but we show that neurotrophins can exist as heterodimers in vitro and are pluripotent, being able to bind and to activate different Trk tyrosine kinase receptors as well as promote neuronal differentiation in PC12 cells as effectively as wild type homodimers. These asymmetric neurotrophin dimers allow unique characterization of neurotrophin structure-function relationships with Trk receptors. The chimeric Trk activities of these heterodimers suggest an alternative model of neurotrophin-Trk receptor activation in which the critical Trk-interacting elements may be attributed to a single protomer.

Response of embryonic rat hippocampal neurons in culture to neurotrophin-3, brain-derived neurotrophic factor and basic fibroblast growth factor

Primary cultures of rat hippocampal cells have been used to evaluate trophic effects of neurotrophin-3, brain-derived neurotrophic factor, nerve growth factor, and basic fibroblast growth factor. There was little survival in cultures prepared from embryonic day 17 embryos and grown in defined medium without growth factors. Addition of basic fibroblast growth factor produced a massive increase in the number of neurons present in the cultures seven days after plating. This action reflected proliferation of neuronal precursor cells rather than increased survival of initially plated neurons. Brain-derived neurotrophic factor was ineffective under these conditions, whereas neurotrophin-3 produced a very small, but statistically significant increase in neuronal survival in the range of 20%. However, hippocampal neurons were responsive to brain-derived neurotrophic factor and neurotrophin-3 as demonstrated under culture conditions, resulting in survival in absence of the neurotrophins. Acute administration of brain-derived neurotrophic factor and neurotrophin-3 to hippocampal cultures grown at high density stimulated the hydrolysis of phosphatidylinositol, a response earlier shown to be mediated by tyrosine receptor kinase neurotrophin receptors. Furthermore, when such cultures were grown in presence of neurotrophin-3 rates of glutamate and GABA uptake were increased. In contrast to the findings obtained in cultures of embryonic day 17, cultures prepared from embryonic day 14 or 15 animals were viable in absence of exogenous growth factors. The specific neurotrophin receptor inhibitor, K-252b reduced survival in these cultures and this effect was partly overcome by exogenous neurotrophin-3. Our findings suggest that hippocampal neuron survival at early embryonic stages may involve paracrine neurotrophin mechanisms, whereas the survival of hippocampal neurons of embryonic day 17 is not markedly enhanced by brain-derived neurotrophic factor or neurotrophin-3. However, at this embryonic stage there is a functional response to both neurotrophins as made evident by the activation of tyrosine kinase receptor-linked signal transduction mechanisms and by the stimulation of transmitter-specific differentiation.

Functional consequences of a single nerve growth factor administration following septal damage in rats

This study examined how possible nerve growth factor (NGF)-induced behaviour changes after septal damage might be modulated by the lesion extent, the dose of NGF administered and the delay between surgery and the onset of testing. In a first experiment, young rats which received electrolytic septal lesions of high or low intensity (inducing respectively large and mild lesions) were treated with 10 or 30 micrograms NGF administered intrahippocampally in a single injection. They were tested 4 months postoperatively for open field ambulation, spontaneous alternation and radial maze performance. It was observed that irrespective of the severity of the lesions rats were impaired in the spontaneous alternation and radial maze tests; however, no obvious changes appeared in the open field test. While an NGF injection did not affect behavioural performances in rats with large lesions, it was capable of ameliorating behavioural deficits in the spontaneous alternation and radial maze tests of rats with mild lesions in both NGF dosage groups. It was also seen that lesions produced a general decrease in hippocampal choline acetyltransferase (ChAT) activity, which was not significantly affected by an NGF administration. There was no significant correlation between ChAT activity and behavioural performance of NGF-treated rats. In a second experiment, young rats received mild septal lesions and were treated with 10 micrograms NGF. These rats were tested 2 weeks postoperatively for radial maze performance. NGF rats exhibited similar behaviour to controls with regard to all of the variables measured. The present results suggest that a single NGF administration spares some abilities to use spatial information efficiently providing lesions are partial.

Synthesis and biological evaluation of radioiodinated N-2-(4-piperidyl)ethyl benzamides

Three iodinated benzamides, 5-7, analogues of the potent acetylcholinesterase inhibitor 1-benzyl-4-[N-[4'-(benzylsulfonyl) benzoyl-N-methylamino]ethyl]piperidine (2), were synthesized and evaluated as potential anticholinesterase agents. All three compounds were found to be three orders of magnitude less potent than the parent compound. However, receptor screening revealed that compounds 5-7 exhibit nanomolar affinity for the sigma binding site. Both [125I]5 and [125I]7 were synthesized and evaluated in rats. Following the intravenous administration of [125I]5 into rats, 1.59% of the injected dose was found in the rat brain within 5 min. The level of radioactivity in the brain remained steady for 2 h, the duration of the study. In contrast, 0.42% of the injected dose was detected in the rat brain following the i.v. injection of [125I]7. Coadministration of either [125I]5 or [125I]7 with 0.5 mumol/kg of haloperidol resulted in a 56-73% reduction in the level of radioactivity in the rat brain, suggesting that these compounds bind to the sigma binding site in vivo. Planar imaging studies with [123I]5 revealed significant accumulation of radioactivity within the monkey brain, with a half-life of 6 h. Compound [123I]5 may be potentially useful for studying sigma receptor distribution in the human brain.

Nerve growth factor regulation of choline acetyltransferase gene expression in rat embryo basal forebrain cultures

Nerve growth factor (NGF) increases the activity of choline acetyltransferase (ChAT), the synthetic enzyme for acetylcholine, in rat basal forebrain neurons both in vivo and in vitro. In poly(A)+ RNA isolated from cultures prepared from the embryonic (E15) rat basal forebrain, radiolabeled probes from the human ChAT gene detected a 3,700 nt and a less abundant 2,300 nt transcript. After growth in the presence of NGF, the abundance of both mRNAs was increased approximately twofold, paralleling the increase in ChAT enzyme activity. In vivo, the human ChAT probes detected a single 3,700 nt form of ChAT mRNA in both embryonic and adult rat basal forebrain. These results suggest that the NGF-mediated increase in ChAT activity in basal forebrain cultures is regulated at the transcriptional level.

Tyrosine hydroxylase mRNA expression by dopaminergic neurons in culture: effect of 1-methyl-4-phenylpyridinium treatment

To enable us to study expression of tyrosine hydroxylase [TH; tyrosine 3-monooxygenase; L-tyrosine tetrahydropteridine:oxygen oxidoreductase (3-hydroxylating); EC 1.14.16.2] as a measure of dopaminergic neuron function in future experiments, methods were developed to quantify TH mRNA levels in cultures of dopaminergic mesencephalic cells. The model of selective dopaminergic toxicity of 1-methyl-4-phenylpyridinium (MPP+) was used to verify the specificity of our methods. Fetal (embryonic day 15) rat ventral mesencephalic cell cultures were treated with 15 microM MPP+ for 48 h, conditions previously shown to reduce the number of TH-immunoreactive neurons, TH activity, and dopamine uptake to 5-10% of control values. This treatment decreased the number of neurons labeled by TH in situ hybridization to 9% of untreated controls and caused a strong reduction of the abundance of TH mRNA in Northern blots. Our findings establish TH mRNA expression as a parameter for future studies of toxic and trophic effects on cultured dopaminergic neurons, and they support the view that MPP+ destroys dopaminergic neurons.

Global in vivo replacement of choline by N-aminodeanol. Testing a hypothesis about progressive degenerative dementia: I. Dynamics of choline replacement

Severe disruption of certain cholinergic pathways is a characteristic feature of Alzheimer's disease. Attempts to establish animal models by interfering with cholinergic function have not been very successful. We now present data which show a substantial and progressive replacement of free and phospholipid-bound choline by the novel choline isostere N-amino-N,N-dimethylaminoethanol during its dietary administration in place of choline. Free choline in blood fell to approximately 20% of controls after 10 to 30 days on diet. Phospholipid-bound choline in plasma was reduced to less than 15%, and in erythrocytes to about 22%. After 120 days of diet free and bound choline were reduced in most tissues to approximately 30% of controls. Only liver retained more than 80% of free choline. Acetylcholine was decreased to 33 to 50% of control. Total true and false transmitter in experimental animals was in all tissues less that acetylcholine in controls, suggesting that muscarinic transmission would be impaired. Moderate reduction of choline acetyltransferase activity was seen in striatum and myenteric plexus, and of QNB-binding in hippocampus, striatum and myenteric plexus.

Toxicity of 1-methyl-4-phenylpyridinium for rat dopaminergic neurons in culture: selectivity and irreversibility

Cultures of dissociated embryonic rat mesencephalic cells were exposed to 10 microM 1-methyl-4-phenylpyridinium (MPP+), a concentration shown earlier to result in loss of greater than 85% of tyrosine hydroxylase (TH)-positive neurons without affecting the total number of cells observed by phase-contrast microscopy. To characterize better the selectivity of the toxic action of MPP+, other parameters were measured reflecting survival and function of dopaminergic or nondopaminergic neurons. Exposure of cultures to 10 microM MPP+ for 48 h reduced TH activity to 11% of control values without reducing protein levels. [3H]Dopamine uptake was reduced to less than 4% of control values, whereas the uptake of gamma-[3H]aminobutyric acid ([3H]GABA) was not affected in these cultures. This same treatment failed to reduce the number of cholinergic cells visualized in septal cultures and did not affect either choline acetyltransferase activity or high-affinity choline uptake. To assess for possible recovery of dopaminergic neurons, cultures were exposed to 10, 1.0, or 0.1 microM MPP+ for 48 h and then kept for up to 6 days in MPP(+)-free medium. After exposure to 10 microM MPP+, the number of TH-positive neurons, their neurite density, TH activity, and [3H]dopamine uptake remained at constant, reduced levels throughout the period of observation after termination of exposure, whereas GABA uptake remained normal. Treatment with lower concentrations of MPP+, i.e., 1.0 and 0.1 microM, induced less pronounced dopaminergic toxic effects. However, no recovery was seen after posttreatment incubation in toxin-free medium. These findings provide evidence that MPP+ treatment results in highly selective and irreversible toxicity for cultured dopaminergic neurons.

Selective and nonselective stimulation of central cholinergic and dopaminergic development in vitro by nerve growth factor, basic fibroblast growth factor, epidermal growth factor, insulin and the insulin-like growth factors I and II

To study the selectivity of neurotrophic actions in the brain, we analyzed the actions of several known growth factors on septal cholinergic, pontine cholinergic, and mesencephalic dopaminergic neurons in culture. Similar to nerve growth factor (NGF), basic fibroblast growth factor (bFGF) stimulated choline acetyltransferase activity in septal cultures. In contrast to NGF, bFGF also enhanced dopamine uptake in mesencephalic cultures and stimulated cell proliferation in all 3 culture types. Insulin and the insulin-like growth factors I and II stimulated transmitter-specific differentiation and cell proliferation in all culture types. Epidermal growth factor (EGF) produced a small increase in dopamine uptake by mesencephalic cells and stimulated cell proliferation in all culture types. In septal cultures, bFGF was most effective when given at early culture times, NGF at later times. The stimulatory actions of bFGF and insulin did not require the presence of glial cells and were not mediated by NGF. In mesencephalic cultures, the stimulation of dopamine uptake by bFGF and EGF was dependent on glial proliferation. The results suggest different degrees of selectivity of the neurotrophic molecules. NGF and, very similarly, bFGF seem to influence septal cholinergic neurons directly and rather selectively, whereas the neurotrophic actions of insulin and the insulin-like growth factors appear to be more general.

Function of neurotrophic factors in the adult and aging brain and their possible use in the treatment of neurodegenerative diseases

This review summarizes the current knowledge of characterized neurotrophic factors, including nerve growth factor (NGF) which serves as paradigmatic example when studying novel molecules. Special consideration is given to the function of neurotrophic factors in the adult and aging brain. Strategies are discussed for the eventual development of pharmacological applications of these molecules in the treatment of neurodegenerative diseases.

Development of cholinergic pedunculopontine neurons in vitro: comparison with cholinergic septal cells and response to nerve growth factor, ciliary neuronotrophic factor, and retinoic acid

The well-documented role of nerve growth factor (NGF) in the function of cholinergic neurons in the mammalian basal forebrain can be regarded as a paradigm for the action of trophic substances on CNS neurons. Although several growth factors have been identified in recent years, the specificities and importance of such factors for the development of the nervous system are still unknown. In the present study it has been tested whether NGF affects the group of pedunculopontine cholinergic neurons. This population, which has been described in detail only recently, is located more caudally than but resembles, in some aspects, the basal forebrain cholinergic neurons. The cell bodies are located in the metencephalic pedunculopontine and dorsolateral tegmental nuclei. Similar to the forebrain cholinergic neurons, they are medium to large in size and ascend centrally with long axons. Projection areas are widespread throughout the mesencephalon and diencephalon. Dissociated pontine and septal cells of fetal rat brain (embryo ages E14 to E17) were grown in culture for 7 to 14 days in the presence or absence of NGF. Furthermore, a possible action of retinoic acid and ciliary neuronotrophic factor (CNTF) on cholinergic neurons of both the basal forebrain and the pontine area were tested. Differentiation of cultured cholinergic neurons was assessed by biochemical determination of choline acetyltransferase (ChAT) activity and by immunocytochemical staining for ChAT. NGF in concentrations of 1 to 1,000 ng/ml medium increased the number of immunostained cells and the staining intensity in ChAT immunocytochemistry and enhanced ChAT activity by at least 100% above control levels in septal cultures, thus confirming earlier results. In marked contrast, the same concentrations of NGF failed to influence ChAT activity or immunocytochemical staining in cultures of the pontine area. Retinoic acid (10(-8) M to 10(-5) M) and CNTF (0.2 and 2.0 ng/ml, corresponding to 1 and 10 trophic units, as defined in the ciliary ganglion cell assay) failed to enhance ChAT activity in either culture system and did not potentiate the NGF-mediated increase of ChAT activity in septal cultures. Our results, which indicate that pedunculopontine cholinergic neurons do not respond to NGF during development, are in line with those of NGF-receptor visualization studies that failed to demonstrate such receptors on cholinergic pontine cells in postnatal and adult rats. The findings further underline the specificity of NGF action in the central nervous system and, in particular, do not support the idea of transmitter-specific neurotrophic factors.(ABSTRACT TRUNCATED AT 400 WORDS)

Chronic administration of nerve growth factor and other neurotrophic factors to the brain

Intracerebral chronic administration of nerve growth factor (NGF) is able to counteract age-related cholinergic dysfunction. This finding suggests that intracerebral administration of neurotrophic factors may become a generally valuable approach when attempting to ameliorate age-related neuronal deficits in experimental animals and humans. This commentary briefly discusses possible strategies for the development of such "neurotrophic factor treatments."