Sustained elevation in hippocampal NGF-like biological activity following medial septal lesions in the rat

Comparison of target innervation by sympathetic axons in adult wild type and heterozygous mice for nerve growth factor or its receptor trkA

Nerve growth factor (NGF), a neurotrophin required for the survival and maintenance of postganglionic sympathetic neurons, mediates its trophic effects by activation of its high-affinity receptor trkA. Null mutant mice lacking either NGF or trkA have profound sympathetic deficits, thus revealing the vital importance of NGF synthesis in target tissues and trkA expression by sympathetic neurons. In this study, we sought to assess whether sympathetic neurons of the superior cervical ganglion (SCG) display alterations in their neurochemical phenotype in adult mice carrying one mutated allele for either NGF or trkA, and whether such differences result in altered patterns of innervation to the submandibular salivary gland and pineal gland. In comparison with adult siblings, levels of trkA protein in the SCG were reduced in age-matched NGF(+/-) and trkA(+/-) mice. While NGF(+/-) mice also had significantly fewer sympathetic axons innervating both the submandibular salivary gland and pineal gland, densities of sympathetic axons in both tissues reached normal levels in trkA(+/-) mice. These findings reveal that while levels of trkA are reduced in SCG neurons of adult NGF(+/-) and trkA(+/-) mice (compared with their wild type counterparts), sympathetic axons are capable of achieving normal patterns of target innervation in trkA(+/-) mice but not in NGF(+/-) mice. As NGF protein levels are not depleted in the submandibular salivary gland and pineal gland of NGF(+/-) mice, a loss of sympathetic neurons [Nat Neurosci 1999; 2:699-705], in combination with reduced levels of trkA protein, may account for perturbed patterns of sympathetic innervation to peripheral tissues.

Prolonged exposure to elevated levels of endogenous nerve growth factor affects the morphological and neurochemical features of sympathetic neurons of postnatal and adult mice

It is well documented that acute increases of target-derived nerve growth factor affect the morphological and neurochemical features of post-ganglionic sympathetic neurons. It has yet to be determined, however, whether similar changes are still evident after prolonged exposure to increased levels of endogenous nerve growth factor. Using a transgenic line of mice which overexpresses nerve growth factor in the brain commencing after the first week of postnatal life and continuing into adulthood, we have shown previously that sympathetic axons sprout into the nerve growth factor-rich cerebellum of these animals; no such axons are seen in the cerebellum of age-matched wild type animals. The aim of this study was to examine and characterize the effects of chronically elevated levels of endogenous nerve growth factor on sympathetic neurons of the superior cervical ganglion. In comparison to adult wild type mice, adult transgenic animals possessed hypertrophied ganglia which displayed both an increase in sympathetic somal size and a decrease in their density. At the electron microscope level, sympathetic somata of the adult transgenic animals had numerous electron-dense lysosome-like structures in the cytoplasm, as compared to that seen in the sympathetic somata of adult wild type animals. Immunodetection of nerve growth factor in the sympathetic somata revealed that the staining intensity in postnatal (day 28) transgenic mice was greater than that in age-matched wild type mice. By adulthood, however, such differences in the intensities of nerve growth factor immunostaining were no longer evident. In situ hybridization analyses of trkA receptor messenger RNA revealed that levels of expression among somata of similar sizes were comparable between the transgenic and wild type neuronal populations of both postnatal day 28 and adult animals. A small subpopulation of sympathetic somata in postnatal transgenic mice displayed a marked increase in p75NTR messenger RNA expression in comparison to somata of a similar size in age-matched wild type animals. By adulthood, the proportion of sympathetic somata in the transgenic animals possessing elevated levels of p75NTR messenger RNA expression had increased. These results reveal that chronically elevated levels of endogenous nerve growth factor in the postnatal and adult mouse brain can induce both structural and neurochemical remodelling of sympathetic neurons. The preferential increase in p75NTR messenger RNA expression among sympathetic somata of transgenic mice may be required for their growth of collateral axons into the nerve growth factor-rich cerebellum during postnatal development and may facilitate the increased immunodetection of nerve growth factor on these aberrant sympathetic axons in adult transgenic animals.

Lesion-induced plasticity of central neurons: sprouting of single fibres in the rat hippocampus after unilateral entorhinal cortex lesion

In response to a central nervous system trauma surviving neurons reorganize their connections and form new synapses that replace those lost by the lesion. A well established in vivo system for the analysis of this lesion-induced plasticity is the reorganization of the fascia dentata following unilateral entorhinal cortex lesions in rats. After general considerations of neuronal reorganization following a central nervous system trauma, this review focuses on the sprouting of single fibres in the rat hippocampus after entorhinal lesion and the molecular factors which may regulate this process. First, the connectivity of the fascia dentata in control animals is reviewed and previously unknown commissural fibers to the outer molecular layer and entorhinal fibres to the inner molecular layer are characterized. Second, sprouting of commissural and crossed entorhinal fibres after entorhinal cortex lesion is described. Single fibres sprout by forming additional collaterals, axonal extensions, boutons, and tangle-like axon formations. It is pointed out that the sprouting after entorhinal lesion mainly involves unlesioned fibre systems terminating within the layer of fibre degeneration and is therefore layer-specific. Third, molecular changes associated with axonal growth and synapse formation are considered. In this context, the role of adhesion molecules, glial cells, and neurotrophic factors for the sprouting process are discussed. Finally, an involvement of sprouting processes in the formation of neuritic plaques in Alzheimer's disease is reviewed and discussed with regard to the axonal tangle-like formations observed after entorhinal cortex lesion.

Cholinergic immunolesions by 192IgG-saporin--useful tool to simulate pathogenic aspects of Alzheimer's disease

Alzheimer's disease, the most common cause of senile dementia, is characterized by intracellular formation of neurofibrillary tangles, extracellular deposits of beta amyloid as well as cerebrovascular amyloid accumulation and a profound loss of cholinergic neurons within the nucleus basalis Meynert with alterations in cortical neurotransmitter receptor densities. The use of the cholinergic immunotoxin 192IgG-saporin allows for the first time study of the impact of cortical cholinergic deafferentation on cortical neurotransmission, learning, and memory without direct effects on other neuronal systems. This model also allows the elucidation of contributions of cholinergic mechanisms to the establishment of other pathological features of Alzheimer's disease. The findings discussed here demonstrate that cholinergic immunolesions by 192IgG-saporin induce highly specific, permanent cortical cholinergic hypoactivity and alterations in cortical neurotransmitter densities comparable to those described for Alzheimer's disease. The induced cortical cholinergic deficit also leads to cortical/hippocampal neurotrophin accumulation and reduced amyloid precursor protein (APP) secretion, possibly reflecting the lack of stimulation of postsynaptic M1/M3 muscarinic receptors coupled to protein kinase C. This immunolesion model should prove useful to test therapeutic strategies based on stimulation of cortical cholinergic neurotransmission or amelioration of pathogenic aspects of cholinergic degeneration in the basal forebrain. Application of the model to animal species that can develop beta-amyloid plaques could provide information about the contribution of cholinergic function to amyloidogenic APP processing.

Damage to the NBM leads to a sustained lesion-induced increase in functional NGF in the cortex

Bilateral infusions of colchicine or vehicle were made in the nucleus basalis magnocellularis of rats. Cortical homogenates were assessed in a choline acetyltransferase (ChAT) activity assay and a PC12 cell neurite outgrowth assay 3, 7, 14, 28, 35 or 84 days post-lesion. Cortical ChAT activity was initially decreased by 30%, followed by recovery to control levels at 84 days post-lesion. Cortical nerve growth factor (NGF) activity was increased 35% above control levels at all time points following colchicine infusion, while addition of a NGF antibody blocking all activity. The data show an increase in functional NGF in the target area of the lesioned neurons preceding and coinciding with regional recovery of a cholinergic marker, suggesting that NGF has a role in the cholinergic recovery.

Axonal regeneration and limited functional recovery following hippocampal deafferentation

Although central neurons do not naturally recover following injury, damaged adult septal neurons can regenerate when nerve growth factor (NGF) is provided along with a suitable cellular substrate. This study investigates the outgrowth of axotomized septal neurons grafted with primary fibroblasts genetically modified to produce NGF. Confocal microscope images of double staining for neuritic markers (neurofilament or low-affinity NGF receptor) and the astrocytic marker glial fibrillary acidic protein (GFAP) demonstrated that regenerating neurites crossed dense buildups of astrocytic processes at the edges of NGF-producing grafts and were in apposition with astrocytic processes within NGF-producing grafts. Immunoreactivity for acetylcholinesterase and low-(p75) and high-affinity (TrkA) NGF receptors was dense in NGF-producing grafts but absent in control grafts. NGF-grafted rats exhibited significantly increased hippocampal density of p75-immunoreactive fibers and significantly decreased ectopic hippocampal sympathetic ingrowth as compared to control-grafted rats. Rats with unilateral fimbria-fornix lesions and NGF-producing grafts exhibited ameliorated performance on a simple memory task. These findings demonstrate that implantation of NGF-producing grafts to the lesion cavity allows axotomized septal cholinergic neurons to reinnervate the hippocampus, and that rats receiving these grafts show a partial recovery of function.

An early lesion of the lateral septal nuclei produces changes in the forced swim test depending on gender

1. Several pharmacological maneuvers in very young rats produce later changes resembling human depression. 2. Rats were submitted to a wide lesion in lateral septal region at 8th day after birth and forced to swim at maturity. 3. Male lesioned group showed the highest amount of immobility; whereas, female sham lesion group showed a greater response to treatments. 4. A gender-dependent sensitivity to early lateral septal nucleus lesions and to antidepressants are concluded.

Selective rostral transection of the fornix spares the hippocampal commissural pathway in the rat: a Phaseolus vulgaris leucoagglutinin tracing study

This study describes an approach for disconnecting the septal region from the hippocampus by fimbria-fornix lesions while sparing the commissural projections. After a frontal cut through the rostral fornix, commissural fibres were labelled with the anterograde tracer Phaseolus vulgaris leucoagglutinin. The commissural fibre bundle located in the posterior-basal fornix (ventral hippocampal commissure) remained unaffected by the rostral fornix transection, whereas the absence of septal fibres in the hippocampus could be verified using AChE histochemistry. Thus, using this approach, selective studies of the septo-hippocampal projection can be performed while leaving the overwhelming portion of the commissural fibre system intact.

Nerve growth factor immunoreactivity and sympathetic sprouting in the rat hippocampal formation

Several lines of evidence support a role for nerve growth factor (NGF) in the sympathetic sprouting response that occurs following septal cholinergic denervation of the rat hippocampal formation. The present study was undertaken to compare the distribution of NGF-like immunoreactivity and the topography of sympathetic sprouting in rats receiving medial septal lesions. Comparisons were made using adjacent sections of the hippocampal formation stained either for NGF-like immunoreactivity or for NGF receptor-immunoreactivity (p75, to visualize sympathetic fibers). p75-immunoreactive sympathetic axons were localized within the same regions exhibiting NGF-like staining, i.e., the hilus of the dentate gyrus and stratum lucidum in the CA3 area. Furthermore, the sympathetic fibers that invaded the hippocampal formation exhibited NGF-like immunostaining. These results provide additional evidence in support of NGF's role in this collateral sprouting response in the mature rat CNS.

Nerve growth factor and Alzheimer's disease

Nerve growth factor (NGF) is a well-characterized protein that exerts pharmacological effects on a group of cholinergic neurons known to atrophy in Alzheimer's disease (AD). Considerable evidence from animal studies suggests that NGF may be useful in reversing, halting, or at least slowing the progression of AD-related cholinergic basal forebrain atrophy, perhaps even attenuating the cognitive deficit associated with the disorder. However, many questions remain concerning the role of NGF in AD. Levels of the low-affinity receptor for NGF appear to be at least stable in AD basal forebrain, and the recent finding of AD-related increases in cortical NGF brings into question whether endogenous NGF levels are related to the observed cholinergic atrophy and whether additional NGF will be useful in treating this disorder. Evidence regarding the localization of NGF within the central nervous system and its presumed role in maintaining basal forebrain cholinergic neurons is summarized, followed by a synopsis of the relevant aspects of AD neuropathology. The available data regarding levels of NGF and its receptor in the AD brain, as well as potential roles for NGF in the pathogenesis and treatment of AD, are also reviewed. NGF and its low affinity receptor are abundantly present within the AD brain, although this does not rule out an NGF-related mechanism in the degeneration of basal forebrain neurons, nor does it eliminate the possibility that exogenous NGF may be successfully used to treat AD. Further studies of the degree and distribution of NGF within the human brain in normal aging and in AD, and of the possible relationship between target NGF levels and the status of basal forebrain neurons in vivo, are necessary before engaging in clinical trials.

Increased NGF-like activity in young but not aged rat hippocampus after septal lesions

Sympathetic sprouting in the hippocampus following septal denervation is thought to involve nerve growth factor (NGF). This sprouting response is dramatically reduced in aged rats, but immunological assays reveal no age-related decline in hippocampal NGF levels. In the present study, both a bioassay and an immunoassay were used to examine the effect of a medial septal lesion on hippocampal NGF levels in young adult (2-5 months) and aged (24 months) Fischer 344 rats. No significant differences were detected between normal young and aged rats, in agreement with earlier results. Following medial septal lesions, however, only young rats demonstrated significant increases in hippocampal NGF-like activity. These results support the hypothesis that the age-related deficit in sympathetic sprouting results from an attenuated neurotrophic response to hippocampal denervation.

Acetylcholinesterase activity and post-lesional plasticity in the hippocampus of young and aged rats

Applying quantitative microscopic histochemistry, the activity of acetylcholinesterase was determined in the various layers of the rat hippocampus at three different levels along the rostrocaudal extent. Two age groups of animals were examined: young adults (two to three months old) and aged subjects (26 months old). Young adults were divided into controls, and animals killed eight and 35 days following bilateral ibotenate lesioning of the medial septum-diagonal band complex. Aged rats were divided into controls and animals 35 days post-lesion. Analysis of variance revealed that the mean acetylcholinesterase activities of the entire hippocampus of individuals were not significantly different between young and aged rats when averaged across controls and 35 days post-lesion. There was a significant decrease of acetylcholinesterase activity (-52%) in young adults eight days post-lesion as compared to controls, but a significant increase (+63%) took place until 35 days post-lesion as compared to eight days post-lesion. Significantly lower activities existed, however, in young (-22%) and aged rats (-18%) 35 days post-lesion as compared to controls. This decrease in mean activity was not age dependent. As acetylcholinesterase is considered to be a good cholinergic indicator in the hippocampus, the results suggest a homotypic collateral sprouting from spared cholinergic afferents following ibotenate lesion of the medial septum-diagonal band complex in young and aged rats. Based on the data obtained, it is reasonable to assume that there was no difference in the post-lesional plasticity of neuronal acetylcholinesterase between young adult and aged rats.

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.

Quisqualic acid-induced lesion of the nucleus basalis of Meynert in young and aging rats: plasticity of surviving NGF receptor-positive cholinergic neurons

Previous studies have demonstrated that cholinergic neurons in the adult rat forebrain, i.e., septal region, are able to respond and regenerate after damage followed by exogenous treatment with beta-nerve growth factor. Furthermore, it has been shown that an age-related loss of NGF-receptor (NGFr) immunoreactivity occurs in cholinergic septal neurons. Since the regenerative capacity of cholinergic neurons is of importance for potential therapeutic strategies during the course of age-related neurodegenerative diseases, we have compared NGFr positive neurons in young adult (3 months old) and in aging (18-24 months old) rats in their ability to produce a physiological plasticity response after surviving an excitotoxic lesion of the nucleus basalis of Meynert (NBM). In aging control rats, NGFr immunoreactivity within NBM neurons was significantly reduced, in analogy to data obtained earlier from studies about septal neurons in aged rats. After lesion with quisqualic acid, a severe cell loss as well as atrophy of remaining cholinergic neurons was observed in both groups. Investigation of the NBM at various times after the lesion demonstrated signs of axonal or dendritic sprouting and local regeneration, with a maximum seen 3 months after the lesion. No age-related differences in the response could be found. However, despite local fiber growth, no reinnervation of the frontal and parietal cortex could be noted, as demonstrated by acetylcholinesterase histochemistry. Our findings suggest that, despite a relatively early onset of NGFr decline during lifetime, cholinergic cells keep the capacity for a plastic response, although they ultimately fail to reinnervate the neocortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Hippocampal NGF levels are not reduced in the aged Fischer 344 rat

The sympathetic sprouting response that occurs in the rat hippocampal formation following septal denervation is reduced in aged rats. Since considerable evidence implicates NGF-like activity in eliciting the sprouting, the simplest explanation for the age-related decline in sympathetic sprouting is a reduction in hippocampal NGF levels. In the present study, hippocampal NGF levels were measured using a 2-site ELISA in four different age groups of Fischer 344 rats. There was no decline in NGF levels with age, nor did we find any differences between male and female rats. This contradicts an earlier report in which a 40% reduction in hippocampal NGF protein levels was found in aged rats. Possible reasons for this discrepancy are discussed. The present results do not support the hypothesis that the age-related decline in sympathetic sprouting is due to a reduction in total hippocampal NGF levels.