Basal forebrain cell loss following fimbria/fornix transection

Grafting of genetically modified cells: effects of acetylcholine release in vivo

In this study, microdialysis was used to investigate functional recovery of central cholinergic neurons in the forebrain of rats with cortical devascularizing lesions. Mature male rats were unilaterally lesioned by disruption of the pia arachnoid vessels and genetically modified fibroblasts secreting nerve growth factor (NGF) were placed at the site of the lesion. One month following surgery, microdialysis probes were installed in the remaining cortex and were perfused with artificial cerebrospinal fluid (csf) containing neostigmine (5 nM) and/or KCl (100 mM). The basal (non-stimulated) release of acetylcholine (ACh) in the cortex was similar in all experimental groups, whereas KCl stimulated release of ACh was significantly augmented (P < 0.05) in the ipsilateral remaining cortex in lesioned animals that have been implanted with fibroblasts secreting NGF. These results suggest that NGF secreted by genetically engineered fibroblasts modulates neuroplasticity in the adult mammalian CNS and may favour recovery of cortical function following injury.

Removal of the hippocampus and transection of the fornix produce comparable deficits on delayed non-matching to position by rats

Rats with radiofrequency lesions of the fimbria/fornix or with extensive aspiration lesions of the hippocampal region (the hippocampus proper, dentate gyrus, and subicular complex) were tested on their performance of a delayed non-matching to position task which had been learnt before surgery. On a given trial, one of two sample levers was presented in a random manner. Following a response on this lever and a subsequent delay, both levers were presented and reward was now contingent on a response on the lever that was not used as the sample. Both lesions produced equivalent performance deficits on this test of spatial working memory, the pattern of these deficits being consistent with a mnemonic impairment. The lack of difference between these two groups on a variety of performance measures indicates that hippocampal connections passing through the fornix are not only necessary for this test, but that non-fornical hippocampal connections appear unable on their own to maintain accurate responding.

Fine structure of rat septohippocampal neurons: II. A time course analysis following axotomy

Previous light microscopic immunocytochemical studies with antibodies against transmitter-synthesizing enzymes have suggested that septohippocampal neurons undergo retrograde degeneration following transection of their axons by cutting the fimbria-fornix. However, a fine-structural analysis of the degeneration process in these cells is lacking so far. Here we have identified septohippocampal neurons by retrograde tracing with Fluoro-Gold. Thereafter, the fimbria-fornix was transected bilaterally. Fine-structural changes in prelabeled septohippocampal neurons were then studied after varying survival times up to 10 weeks. Examination under the fluorescence microscope of Vibratome sections through the septal region revealed numerous retrogradely labeled cells after all survival times following axotomy. These neurons were then intracellularly injected with the fluorescent dye Lucifer Yellow in order to stain their dendritic arbor. Many cells were found after each survival time that displayed characteristics of septohippocampal neurons in control rats (see Naumann et al., J Comp Neurol 325:207-218, 1992). In addition, increasing with survival time, there were many shrunken neurons with a reduced dendritic arbor. Representative examples of both normal appearing and shrunken neurons were photoconverted for subsequent electron microscopic analysis. Relatively few signs of neuronal degeneration were found at each survival time analyzed. The majority of cells, including the heavily shrunken ones, displayed fine-structural characteristics of normal neurons. However, a few degenerating neurons and reactive glial cells were present in all survival stages. We conclude that axotomized septohippocampal projection neurons cease the expression of transmitter-synthesizing enzymes and shrink, but many more cells survive for extended periods of time without target-derived neurotrophic factor than was assumed in previous light microscopic studies.

Studies related to the use of colchicine as a neurotoxin in the septohippocampal cholinergic system

Colchicine has been shown to be neurotoxic to cholinergic neurons in the medial septum 1 week following intracerebroventricular injections. The experiments described here were designed to examine the selectivity of this effect over a longer time course, and to examine the role of axoplasmic transport in the neurotoxic effect. As previously reported, 1 week after intracerebroventricular injections of colchicine, the numbers of choline acetyltransferase (ChAT)-immunoreactive neurons in the medial septum-diagonal band complex (MSDB) were reduced to 38% of control; this reduction was stable 2 and 3 weeks post injection. Injections of colchicine placed into the body of the fornix produced similar results. GAD-immunoreactive somata, the other major population of neurons in the MSDB, were unaffected 3 weeks following colchicine, as previously reported 1 week following similar injections. The normal AChE staining pattern in the hippocampus, particularly the dentate gyrus, was depleted following either ICV or intrafornical injections of colchicine. This depletion was more severe with longer survival times. Injections of lumicolchicine, an isomer of colchicine which does not bind tubulin, had no effect on ChAT-immunoreactive neurons in the MSDB or on AChE staining in the hippocampus. Injections of colchicine, but not of lumicolchicine, partially blocked the retrograde transport of the fluorescent dye Fluoro-Gold from the hippocampus to the MSDB. In addition, the content of NGF in the hippocampus rose 84% above control values 2 weeks following colchicine and remained elevated at three weeks. Together these results indicate that colchicine is selectively toxic for cholinergic neurons in the septohippocampal system, and suggest that the alkaloid's neurotoxic effects work via the blockade of axoplasmic transport.

Restoration of ascending noradrenergic projections by residual locus coeruleus neurons: compensatory response to neurotoxin-induced cell death in the adult rat brain

There is clinical and experimental evidence that monoamine neurons respond to lesions with a wide range of compensatory adaptations aimed at preserving their functional integrity. Neurotoxin-induced lesions are followed by increased synthesis and release of transmitter from residual monoamine fibers and by axonal sprouting. However, the fate of lesioned neurons after long survival periods remains largely unknown. Whether regenerative sprouting may contribute significantly to recovery of function following lesions which induce cell loss has been questioned. We have previously analyzed the response of locus coeruleus (LC) neurons to systemic administration of the noradrenergic (NE) neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) to adult rats. This drug causes ablation of nearly all LC axon terminals within 2 weeks after administration, followed by a profound loss of LC cell bodies 6 months later. The present study was conducted to determine the fate of surviving LC neurons and to characterize their potential for regenerative sprouting during a 16 month period after DSP-4 treatment. The time-course and extent of LC neuron degeneration were analyzed quantitatively in Nissl-stained sections, and the regenerative response of residual neurons was characterized by dopamine-beta-hydroxylase immunohistochemistry. The results document that LC neurons degenerate gradually after DSP-4 treatment, cell loss reaching on average 57% after 1 year. LC neurons which survive the lesion exhibit a vigorous regenerative response, even in those animals in which cell loss exceeds 60-70%. This regenerative process leads progressively to restoration of the NE innervation pattern in the forebrain, with some regions becoming markedly hyperinnervated. In stark contrast to the forebrain, very little reinnervation takes place in the brainstem, cerebellum and spinal cord. These findings suggest that regenerative sprouting of residual neurons is an important compensatory mechanism by which the LC may regain much of its functional integrity in the presence of extensive cell loss. Furthermore, regeneration of LC axons after DSP-4 treatment is region-specific, suggesting that the pattern of reinnervation is controlled by target areas. Elucidation of the factors underlying recovery of LC neurons after DSP-4 treatment may provide insights into the compensatory mechanisms of central neurons after injury and in disease states.

Stability of septohippocampal neurons following excitotoxic lesions of the rat hippocampus

The present study examined the effects of removing hippocampal nerve growth factor (NGF)-producing neurons upon cholinergic and noncholinergic septohippocampal projecting neurons. To deplete septal/diagonal band neurons of their intrinsic source of NGF, rats received unilateral intrahippocampal injections of ibotenic acid and were sacrificed 2-24 weeks later. Choline acetyltransferase and parvalbumin immunohistochemistry failed to reveal changes in the number of cholinergic or gamma-aminobutyric acid-containing neurons, respectively, within the septal/diagonal band region ipsilateral to the hippocampal lesion at any time point examined. Additionally, immunocytochemical localization of nonphosphorylated and phosphorylated neurofilament proteins did not reveal abnormal staining characteristics within the septal/diagonal band complex, suggesting that this lesion does not alter cytoskeletal features of neurons which project to the hippocampus. Selected rats received unilateral hippocampal lesions and 3 months later were injected with fluorogold into the remaining hippocampal remnant and with wheat germ agglutinin conjugated to horse radish peroxidase into the intact contralateral hippocampus. Both retrograde tracers were predominantly transported to their respective ipsilateral septum and vertical limb of the diagonal band. This indicates that following the lesion, septal/diagonal band neurons still project ipsilaterally and sprouting to the NGF-rich contralateral side does not occur. RNA blot analysis revealed a decrease in NGF mRNA expression within the lesioned hippocampus with a maximum reduction of approximately 70%. In contrast, no change in NGF mRNA expression was observed within the ipsilateral septum relative to the contralateral side. The present study demonstrates that removal of hippocampal target neurons does not alter the number, morphology, or projections of both cholinergic and noncholinergic septal/diagonal band neurons.

Human nerve growth factor prevents degeneration of basal forebrain cholinergic neurons in primates

Basal forebrain cholinergic neurons respond to nerve growth factor (NGF), and it has been suggested that the administration of NGF might prevent their degeneration in patients with Alzheimer's disease. One major prerequisite to be fulfilled before the consideration of clinical trials of NGF in patients with Alzheimer's disease is the demonstration that human NGF affects basal forebrain cholinergic neurons in primates. In the present study, we used a recombinant human nerve growth factor (rhNGF), which we previously showed to be active on rat basal forebrain cholinergic neurons, in nonhuman primates with a unilateral transection of the fornix (a well-established model for the induction of retrograde degenerative changes in septal cholinergic neurons). After the lesion, one group of animals received rhNGF and a second group received vehicle solution for 2 weeks. In animals receiving vehicle, the medial septal nucleus ipsilateral to the lesion showed reductions in number (55%) and size of cell bodies immunoreactive for NGF receptor and choline acetyltransferase. In Nissl stains, many cells showed reduced size and basophilia. The rhNGF completely prevented alterations in the number and size of NGF receptor- and choline acetyltransferase-immunoreactive neurons in the medial septal nucleus and reversed atrophy in a subpopulation of large, basophilic medial septal nucleus neurons, as identified by Nissl stains. The effects of rhNGF were identical to those of mouse NGF, which we have previously used in the same primate lesion paradigm. The restoration of the phenotype of injured cholinergic septal neurons by rhNGF in the monkey raises the possibility that this factor may be used to ameliorate acetylcholine-dependent memory impairments that occur in aged nonhuman primates. In concert, results of the present investigation provide critical information for the future use of NGF in patients with neurological disorders that affect NGF-responsive cells in the peripheral and central nervous systems.

Effects of sciatic nerve transplants after fimbria-fornix lesion: examination of the role of nerve growth factor

At two weeks post-transplantation, sciatic nerves inserted into the lesioned septo-hippocampal pathway contain NGF levels more than twice that of normal nerves. These transplanted nerves also contain regenerating cholinergic axons. Moreover, transplanted animals exhibit septal NGF levels that are significantly greater than in animals with lesions only. These results suggest a role for NGF in the ingrowth of axons into the transplants and in the increase in ChAT(+) septal neurons previously observed at this post-transplant time.

Loss of AChE- and NGFr-labeling precedes neuronal death of axotomized septal-diagonal band neurons: reversal by intraventricular NGF infusion

The time course of cellular changes in the medial septum (MS) and vertical limb of the diagonal band area (VDB) after a complete unilateral fimbria-fornix (FF) transection has been studied using prelabeling of the septohippocampal neurons by bilateral hippocampal injections of the fluorescent retrograde tracer Fluoro-Gold (FG), in combination with acetylcholine esterase (AChE) histochemistry and nerve growth factor receptor (NGFr) immunocytochemistry. The results show that the long-term disappearance of AChE-positive and NGFr-positive cells represents a combination of down-regulation of the marker proteins, cell shrinkage, and an actual cell loss. By 4 weeks after lesion the loss of FG-prelabeled cells amounted to 50% in MS and 30% in VDB. A further 25-30% of the MS neurons survived (as indicated by the presence of FG label), but were undetectable by the AChE and NGFr markers. Down-regulation of the marker proteins and cell shrinkage preceded the cell loss by more than a week: while shrinkage and reduced numbers of AChE/NGFr positive cells was evident already by 4-7 days, an actual cell loss (i.e., loss of FG-prelabeled cells) became evident only at 4 weeks after lesion. Continuous intraventricular NGF infusion (0.15 micrograms/day) was capable of counteracting all three types of changes. Infusion over 2 weeks reversed both atrophy and loss of AChE/NGFr staining, whereas infusion over 4 weeks completely prevented the later occurring cell loss. In addition, the NGF infusions induced significant hypertrophy in the undamaged cholinergic neurons in both nucleus basalis and striatum. It is concluded that down-regulation of marker proteins, such as AChE and NGFr, and cellular atrophy precede cell death in the axotomized septohippocampal system and that about 1/3 of the axotomized septal cholinergic neurons may survive for a long time in a down-regulated atrophic state. Exogenous NGF can prevent both the atrophic and the degenerative processes.

Septal choline acetyltransferase immunoreactive neurons: dose-dependent effects of AF64A

Two experiments were performed. In the first, the cholinotoxin, AF64A (0.5, 1.0 or 1.5 nmol/ventricle), or vehicle (3.0 microliters) was injected (ICV) bilaterally into male rats (n = 23). Choline acetyltransferase (ChAT) immunoreactive (IR) perikarya in the four subgroups of the septal complex were visualized by immunocytochemistry (PAP method) 28 days postinjection, and counted using a microprojector (x40). The 0.5 nmol/ventricle dose of AF64A significantly reduced (31%) the number of ChAT-IR cell bodies in the intermediate subgroup (rostral extension of the nucleus basalis/substantia innominata). Higher doses did not produce additional reductions. The highest dose (1.5 nmol/ventricle) of AF64A resulted in significant decreases in ChAT-IR cell bodies in the dorsal (51%) and midline (35%) subgroups (medial septum), but did not affect the number of ventral subgroup (diagonal band of Broca) ChAT-IR neurons. In the second experiment, electrolytic lesions were placed in the corpus callosum, cingulum and overlying cingulate gyrus, in order to simulate the nonselective damage seen following the 1.5 nmol/ventricle dose of AF64A. In comparison to the surgical controls (n = 3), the electrolytic lesions (n = 6) failed to significantly affect the number of ChAT-IR perikarya in any of the septal subdivisions. Thus the distinct subgroups of septal ChAT-IR neurons are differentially sensitive to the toxic effects of ICV administered AF64A: intermediate much greater than dorsal greater than midline much greater than ventral subgroup.

Recombinant human nerve growth factor prevents retrograde degeneration of axotomized basal forebrain cholinergic neurons in the rat

Cholinergic neurons in the basal forebrain magnocellular complex (BFMC) respond to nerve growth factor (NGF) during development and in adult life, and it has been suggested that the administration of NGF might ameliorate some of the abnormalities that occur in neurological disorders associated with degeneration of this population of neurons. A prerequisite for the introduction of NGF in clinical trials is the availability of active recombinant human NGF (rhNGF). The present investigation was designed to test, in vivo, the efficacy of a preparation of rhNGF. Axons of cholinergic neurons of the BFMC in the rat were transected in the fimbria-fornix; this manipulation alters the phenotype and, eventually, causes retrograde degeneration of these neurons. Our investigation utilized two lesion paradigms (resection and partial transection of fibers in the fimbria-fornix), two different strains of rats, and two delivery systems. Following lesions, animals were allowed to survive for 2 weeks, during which time one group received intraventricular mouse NGF (mNGF), a second group received rhNGF, and a third group received vehicle alone. In animals receiving vehicle, there was a significant reduction in the number (resection: 70%; transection: 50%) and some reduction in size of choline acetyltransferase- or NGF receptor-immunoreactive cell bodies within the medial septal nucleus ipsilateral to the lesion. Treatment with either mNGF or rhNGF completely prevented these alterations in the number and size of cholinergic neurons. The rhNGF was shown to be equivalent in efficacy with mNGF. Thus, rhNGF is effective in preventing axotomy-induced degenerative changes in cholinergic neurons of the BFMC. Our results, taken together with the in vitro effects of rhNGF (42), indicate that an active rhNGF is now available for further in vivo studies in rodents and primates with experimentally induced or age-associated lesions of basal forebrain cholinergic neurons. These investigations provide essential information for the consideration of future utilization of rhNGF for treatment of human neurological disorders, including Alzheimer's disease.

Long-term administration of mouse nerve growth factor to adult rats with partial lesions of the cholinergic septohippocampal pathway

Nerve growth factor (NGF), a neurotrophic factor acting on cholinergic neurons of the basal forebrain, has been proposed as a treatment for Alzheimer's disease. Experimental support for its pharmacological use is derived from short-term studies showing that intraventricular administration of NGF during 2-4 weeks protects cholinergic cell bodies from lesion-induced degeneration, stimulates synthesis of choline acetyltransferase, and improves various behavioral impairments. To investigate the consequences of long-term NGF administration, we tested whether cholinergic cell bodies are protected from lesion-induced degeneration and whether cholinergic axons are stimulated to regrow into the denervated hippocampus following fimbrial transections. We found that intraventricular injections of NGF twice a week for 5 months to adult rats resulted in extended protection of cholinergic cell bodies from lesion-induced degeneration and did not produce obvious detrimental effects on the animals. NGF treatment mildly stimulated growth of cholinergic neurites within the 2-mm area directly adjacent to the fimbrial lesion but it failed to induce significant homotypic growth of cholinergic neurites into the deafferented hippocampus.

Response of the monkey cholinergic septohippocampal system to fornix transection: a histochemical and cytochemical analysis

Transection of the fimbria-fornix pathway is a paradigm that has been richly exploited in rats to assess the structural and functional correlates of cognitive behavior, neural grafting, and growth factor administration. Principally, the degeneration of cholinergic neurons within the septal/diagonal band region has received detailed attention following this manipulation. In contrast, no studies have examined the response of the cholinergic septal/diagonal neurons following axotomy in nonhuman primates. This study examined the neuronal and glial responses within the septal region to selective fornix transection (without cingulate gyrus ablation) in four Cebus apella monkeys. One month following unilateral transection of the fornix by means of an open microsurgical approach, a comprehensive loss of acetylcholinesterase [AChE]-containing fibers was observed throughout the hippocampal formation and dentate gyrus ipsilateral to the lesion. Decreases in AChE fiber densities were also observed within the entorhinal cortex ipsilateral to the lesion. No such changes in AChE-fiber density were consistently observed within the subicular region. The decrease in hippocampal AChE-positive fibers was paralleled by a 49.5% reduction in cholinergic medial septal neurons as revealed by Nissl stains and immunohistochemical staining for the receptor for nerve growth factor, a marker of cholinergic basal forebrain neurons in primates. In contrast, no significant changes in the number of neurons within the vertical limb of the diagonal band were noted. Following the transection, a relatively intense reactive gliosis was observed within the dorsal half of the septal region ipsilateral to the transection and within the overlying transected corpus callosum. These data provide the foundation in nonhuman primates on which novel therapeutic factors can be evaluated in paradigms relevant to the study of Alzheimer's disease.

Fate of septohippocampal neurons following fimbria-fornix transection: a time course analysis

Neurons in the medial septum (MS) and vertical limb of the diagonal band (vDB) undergo degenerative changes following transection of their axons. These changes have been well studied by histological techniques such as Nissl stains and immunocytochemistry. A dramatic loss of stained neurons occurs following axotomy and this has been interpreted as indicative of neuronal death. However, since the staining intensity and the size of affected neurons may be reduced by axotomy, it is possible that the apparent neuronal death may actually be due to a decrease in somal size or the ability to detect neurons by routine histological methods. The present study describes the effects of axotomy on MS and vDB neurons which have been labeled by hippocampal injections of the retrograde tracer, Fluoro-Gold (FG), prior to transection of the fimbria-fornix and supracallosal stria. The number of FG-labeled neurons in the MS decreased by 21% at three weeks, 36% at six weeks, and 31% at ten weeks after fimbria-fornix transection. The reduction was statistically significant at 6 and 10 weeks. The number of FG-labeled neurons in the vDB showed no reduction at three weeks but was decreased by 31% and 37% at six and ten weeks, respectively. This was statistically significant only at 10 weeks. By comparison, the number of neurons immunoreactive for choline acetyltransferase (ChAT) was reduced by 75-80% at these time points. The size (area and diameter) of FG-labeled somata decreased in both the MS and vDB within three weeks following fimbria-fornix transection and remained relatively constant at the six- and ten-week time points.(ABSTRACT TRUNCATED AT 250 WORDS)

Loss of true blue labelling from the medial septum following transection of the fimbria-fornix: evidence for the death of cholinergic and non-cholinergic neurons

Many neurons in the medial septal nucleus lose their transmitter-associated enzyme staining following axotomy in the proximal fimbria-fornix (FF), but it is not clear if these neurons have died or persist in a shrunken and subfunctional state. To investigate this further, septal neurons projecting through the FF were labelled with the fluorescent dye, True blue, by retrograde transport from multiple bilateral injection sites in the hippocampus. True blue-labelled neurons and cholinergic neurons immunohistochemically stained for choline acetyltransferase (ChAT) were then quantitatively compared in neighbouring sections through the medial septum 28 days after complete unilateral transections of the proximal FF. The number of True blue and ChAT positive cells ipsilateral to the FF lesion showed significant (P less than 0.001) declines of 51.4% and 71.1%, respectively, relative to the unlesioned side. Cell loss was considerably more severe among large neurons, such that 78.0% and 92.7% of True blue and ChAT labelled cells larger than the normal mean, but only 40.1% and 68.0% of True blue and ChAT labelled cells smaller than the normal mean size were lost. This indicates either that larger neurons were more prone to cell loss, or that some (but not all) large neurons persisted in a shrunken form. Histograms showed no increase in cell number in any of the smaller size categories and a substantial decrease in most cases, indicating that shrinkage alone could not account for the loss of all large neurons. Since True blue can remain present in brainstem cholinergic neurons surviving for over 365 days after axotomy, loss of True blue suggests breakdown of membrane integrity and cell death.(ABSTRACT TRUNCATED AT 250 WORDS)