Basal forebrain magnocellular neurons stain for nerve growth factor receptor: correlation with cholinergic cell bodies and effects of axotomy

Ngfr+ cholinergic projection from SI/nBM to mPFC selectively regulates temporal order recognition memory

Acetylcholine regulates various cognitive functions through broad cholinergic innervation. However, specific cholinergic subpopulations, circuits and molecular mechanisms underlying recognition memory remain largely unknown. Here we show that Ngfr+ cholinergic neurons in the substantia innominate (SI)/nucleus basalis of Meynert (nBM)-medial prefrontal cortex (mPFC) circuit selectively underlies recency judgements. Loss of nerve growth factor receptor (Ngfr-/- mice) reduced the excitability of cholinergic neurons in the SI/nBM-mPFC circuit but not in the medial septum (MS)-hippocampus pathway, and impaired temporal order memory but not novel object and object location recognition. Expression of Ngfr in Ngfr-/- SI/nBM restored defected temporal order memory. Fiber photometry revealed that acetylcholine release in mPFC not only predicted object encounters but also mediated recency judgments of objects, and such acetylcholine release was absent in Ngfr-/- mPFC. Chemogenetic and optogenetic inhibition of SI/nBM projection to mPFC in ChAT-Cre mice diminished mPFC acetylcholine release and deteriorated temporal order recognition. Impaired cholinergic activity led to a depolarizing shift of GABAergic inputs to mPFC pyramidal neurons, due to disturbed KCC2-mediated chloride gradients. Finally, potentiation of acetylcholine signaling upregulated KCC2 levels, restored GABAergic driving force and rescued temporal order recognition deficits in Ngfr-/- mice. Thus, NGFR-dependent SI/nBM-mPFC cholinergic circuit underlies temporal order recognition memory.

Adenosine inhibits glutamatergic input to basal forebrain cholinergic neurons

Adenosine has been proposed as an endogenous homeostatic sleep factor that accumulates during waking and inhibits wake-active neurons to promote sleep. It has been specifically hypothesized that adenosine decreases wakefulness and promotes sleep recovery by directly inhibiting wake-active neurons of the basal forebrain (BF), particularly BF cholinergic neurons. We previously showed that adenosine directly inhibits BF cholinergic neurons. Here, we investigated 1) how adenosine modulates glutamatergic input to BF cholinergic neurons and 2) how adenosine uptake and adenosine metabolism are involved in regulating extracellular levels of adenosine. Our experiments were conducted using whole cell patch-clamp recordings in mouse brain slices. We found that in BF cholinergic neurons, adenosine reduced the amplitude of AMPA-mediated evoked glutamatergic excitatory postsynaptic currents (EPSCs) and decreased the frequency of spontaneous and miniature EPSCs through presynaptic A(1) receptors. Thus we have demonstrated that in addition to directly inhibiting BF cholinergic neurons, adenosine depresses excitatory inputs to these neurons. It is therefore possible that both direct and indirect inhibition may synergistically contribute to the sleep-promoting effects of adenosine in the BF. We also found that blocking the influx of adenosine through the equilibrative nucleoside transporters or inhibiting adenosine kinase and adenosine deaminase increased endogenous adenosine inhibitory tone, suggesting a possible mechanism through which adenosine extracellular levels in the basal forebrain are regulated.

Casein kinase II contributes to the synergistic effects of BMP7 and BDNF on Smad 1/5/8 phosphorylation in septal neurons under hypoglycemic stress

The combination of bone morphogenetic protein 7 (BMP7) and neurotrophins (e.g. brain-derived neurotrophic factor, BDNF) protects septal neurons during hypoglycemic stress. We investigated the signaling mechanisms underlying this synergistic protection. BMP7 (5 nM) increased phosphorylation and nuclear translocation of BMP-responsive Smads 1/5/8 within 30 min in cultures of rat embryonic septal neurons. BDNF (100 ng/mL) enhanced the BMP7-induced increase in phospho-Smad levels in both nucleus and cytoplasm; this effect was more pronounced after a hypoglycemic stress. BDNF increased both Akt and Erk phosphorylation, but pharmacological blockade of these kinase pathways (with wortmannin and U0126, respectively) did not reduce the Smad phosphorylation produced by the BMP7 + BDNF combination. Inhibitors of casein kinase II (CK2) activity reduced the (BMP7 + BDNF)-induced Smad phosphorylation, and this trophic factor combination increased CK2 activity in hypoglycemic cultures. These findings suggest that BDNF can increase BMP-dependent Smad phosphorylation via a mechanism requiring CK2.

Identification of a lectin causing the degeneration of neuronal processes using engineered embryonic stem cells

Unlike the mechanisms involved in the death of neuronal cell bodies, those causing the elimination of processes are not well understood owing to the lack of suitable experimental systems. As the neurotrophin receptor p75(NTR) is known to restrict the growth of neuronal processes, we engineered mouse embryonic stem (ES) cells to express an Ngfr (p75(NTR)) cDNA under the control of the Mapt locus (the gene encoding tau), which begins to be active when ES cell-derived progenitors start elongating processes. This caused a progressive, synchronous degeneration of all processes, and a prospective proteomic analysis showed increased levels of the sugar-binding protein galectin-1 in the p75(NTR)-engineered cells. Function-blocking galectin-1 antibodies prevented the degeneration of processes, and recombinant galectin-1 caused the processes of wild-type neurons to degenerate first, followed by the cell bodies. In vivo, the application of a glutamate receptor agonist, a maneuver known to upregulate p75(NTR), led to an increase in the amount of galectin-1 and to the degeneration of neurons and their processes in a galectin-1-dependent fashion. Section of the sciatic nerve also rapidly upregulated levels of p75(NTR) and galectin-1 in terminal Schwann cells, and the elimination of nerve endings was delayed at the neuromuscular junction of mice lacking Lgals1 (the gene encoding galectin-1). These results indicate that galectin-1 actively participates in the elimination of neuronal processes after lesion, and that engineered ES cells are a useful tool for studying relevant aspects of neuronal degeneration that have been hitherto difficult to analyze.

Hibernation model of tau phosphorylation in hamsters: selective vulnerability of cholinergic basal forebrain neurons - implications for Alzheimer's disease

Neurofibrillar tangles made up of 'paired helical filaments' (PHFs) consisting of hyperphosphorylated microtubule-associated protein tau are major hallmarks of Alzheimer's disease (AD). Tangle formation selectively affects certain neuronal types and systematically progresses throughout numerous brain areas, which reflects a hierarchy of neuronal vulnerability and provides the basis for the neuropathological staging of disease severity. Mechanisms underlying this selective neuronal vulnerability are unknown. We showed previously that reversible PHF-like phosphorylation of tau occurs during obligate hibernation. Here we extend these findings to facultative hibernators such as Syrian hamsters (Mesocricetus auratus) forced into hibernation. In this model, we showed in the basal forebrain projection system that cholinergic neurons are selectively affected by PHF-like phosphorylated tau, while gamma-aminobutyric acid (GABA)ergic neurons are largely spared, which shows strong parallels to the situation in AD. Formation of PHF-tau in these neurons apparently does not affect their function as pacemaker for terminating hibernation. We conclude that although formation of PHF-like phosphorylated tau in the mammalian brain follows a certain hierarchy, affecting some neurons more frequently than others, it is not necessarily associated with impaired neuronal function and viability. This indicates a more general link between PHF-like phosphorylation of tau and the adaptation of neurons under conditions of a 'vita minima'.

Rabbit forebrain cholinergic system: morphological characterization of nuclei and distribution of cholinergic terminals in the cerebral cortex and hippocampus

Although the rabbit brain, in particular the basal forebrain cholinergic system, has become a common model for neuropathological changes associated with Alzheimer's disease, detailed neuroanatomical studies on the morphological organization of basal forebrain cholinergic nuclei and on their output pathways are still awaited. Therefore, we performed quantitative choline acetyltransferase (ChAT) immunocytochemistry to localize major cholinergic nuclei and to determine the number of respective cholinergic neurons in the rabbit forebrain. The density of ChAT-immunoreactive terminals in layer V of distinct neocortical territories and in hippocampal subfields was also measured. Another cholinergic marker, the low-affinity neurotrophin receptor (p75(NTR)), was also employed to identify subsets of cholinergic neurons. Double-immunofluorescence labeling of ChAT and p75(NTR), calbindin D-28k (CB), parvalbumin, calretinin, neuronal nitric oxide synthase (nNOS), tyrosine hydroxylase, or substance P was used to elucidate the neuroanatomical borders of cholinergic nuclei and to analyze the neurochemical complexity of cholinergic cell populations. Cholinergic projection neurons with heterogeneous densities were found in the medial septum, vertical and horizontal diagonal bands of Broca, ventral pallidum, and magnocellular nucleus basalis (MBN)/substantia innominata (SI) complex; cholinergic interneurons were observed in the caudate nucleus, putamen, accumbens nucleus, and olfactory tubercule, whereas the globus pallidus was devoid of cholinergic nerve cells. Cholinergic interneurons were frequently present in the hippocampus and to a lesser extent in cerebral cortex. Cholinergic projection neurons, except those localized in SI, abundantly expressed p75(NTR), and a subset of cholinergic neurons in posterior MBN was immunoreactive for CB and nNOS. A strict laminar distribution pattern of cholinergic terminals was recorded both in the cerebral cortex and in CA1-CA3 and dentate gyrus of the hippocampus. In summary, the structural organization and chemoarchitecture of rabbit basal forebrain may be considered as a transition between that of rodents and that of primates.

In vivo labeling of rabbit cholinergic basal forebrain neurons with fluorochromated antibodies

Cholinergic basal forebrain neurons (CBFN) expressing the low-affinity neurotrophin receptor p75 (p75(NTR)) were previously selectively labeled in vivo with carbocyanine 3 (Cy3)-tagged anti-p75(NTR), but the applied 192IgG-conjugates recognized p75(NTR) only in rat. The antibody ME 20.4 raised against human p75(NTR) had been shown to cross-react with the receptor in monkey, raccoon, sheep, cat, dog, pig and rabbit. Hence, for in vivo labeling of rabbit CBFN in the present study, ME 20.4 was fluorochromated with Cy3-N-hydroxysuccinimide ester and purified Cy3-ME 20.4 was injected intracerebroventricularly. Two days post-injection, clusters of Cy3-ME 20.4 were found in CBFN displaying choline acetyltrans-ferase-immunoreactivity. Following photoconversion, electron microscopy revealed fluorochromated antibodies in secondary lysosomes. In conclusion, Cy3-ME 20.4 might become an appropriate marker for CBFN in live and fixed tissues of various mammalian species.

Differential regulation of p75 and trkB mRNA expression after depolarizing stimuli or BDNF treatment in basal forebrain neuron cultures

Extensive evidence suggests that BDNF regulates neural function and architecture after depolarization. Expression of BDNF is increased after depolarization, and the ability of BDNF to modulate synaptic function is well documented. To further investigate BDNF signaling after activity, we analyzed the effects of depolarization or BDNF treatment on receptor mRNA expression in cultured basal forebrain neurons. Levels of mRNA coding for the cognate BDNF receptor, trkB, as well as the common neurotrophin receptor, p75, were quantitated simultaneously using a sensitive solution hybridization technique. Depolarization or BDNF treatment increased p75 mRNA expression 94% and 195%, respectively. In contrast, trkB message decreased 23% after depolarization but was unchanged by BDNF treatment. Together, these changes resulted in significant increases in the p75/trkB ratio after depolarization or BDNF treatment that could alter BDNF binding or signal transduction.

Hippocampal tyrosine kinase A receptors are restricted primarily to presynaptic vesicle clusters

Adult septohippocampal cholinergic neurons are dependent on trophic support for normal functioning and survival; these effects are largely mediated by the tyrosine kinase A receptor (TrkA), which binds its ligand, nerve growth factor (NGF), with high affinity. To determine the subcellular localization of TrkA within septohippocampal terminal fields, two rabbit polyclonal antisera to the extracellular domain of TrkA were localized immunocytochemically in rat dentate gyrus by light and electron microscopy. By light microscopy, TrkA immunoreactivity was found mostly in fine, varicose fibers primarily in the hilus and, to a lesser extent, in the granule cell and molecular layers. By electron microscopy, the central and infragranular regions of the hilus contained the highest densities of TrkA-immunoreactive profiles. Most TrkA-labeled profiles were axons (31% of 3,473), axon terminals (20%), and glia (38%); fewer were dendrites (6%), dendritic spines (5%), and granule cell and interneuron somata (<1%). TrkA immunolabeling in axons and axon terminals was discrete, often concentrated in patches of small synaptic vesicles that were adjacent to somatic and dendritic profiles. TrkA-labeled terminals formed both asymmetric and symmetric synapses, primarily with dendritic shafts and spines. TrkA-immunoreactive glial profiles frequently apposed terminals contacting dendritic spines. The findings that presynaptic profiles contain TrkA immunolabeling in sites of vesicle accumulation suggest that NGF binding to TrkA may influence transmitter release. The presence of TrkA immunoreactivity in somata, dendrites, and glia further suggests that cells within the dentate gyrus may take up NGF.

Infusion of nerve growth factor (NGF) into kitten visual cortex increases immunoreactivity for NGF, NGF receptors, and choline acetyltransferase in basal forebrain without affecting ocular dominance plasticity or column development

Intracerebroventricular or intracortical administration of nerve growth factor (NGF) has been shown to block or attenuate visual cortical plasticity in the rat. In cats and ferrets, the effects of exogenous NGF on development and plasticity of visual cortex have been reported to be small or nonexistent. To determine whether locally delivered NGF affects ocular dominance column formation or the plasticity produced by monocular deprivation in cats at the height of the critical period, we infused recombinant human NGF into the primary visual cortex of kittens using an implanted cannula minipump. NGF had no effect on the normal developmental segregation of geniculocortical afferents into ocular dominance columns as determined both physiologically and anatomically. The plasticity of binocular visual cortical responses induced by monocular deprivation was also normal in regions of immunohistochemically detectable NGF infusion, as measured using intrinsic signal optical imaging and single-unit electrophysiology. Immunohistochemical analysis of the basal forebrain regions of the same animals demonstrated that the NGF infused into cortex was biologically active, producing an increase in the number of NGF-, TrkA-, p75(NTR)-, and choline acetyltransferase-positive neurons in basal forebrain nuclei in the hemisphere ipsilateral to the NGF minipump compared to the contralateral basal forebrain neurons. We conclude that NGF delivered locally to axon terminals of cholinergic basal forebrain neurons resulted in increases in protein expression at the cell body through retrograde signaling.

Expression of Trk isoforms in brain regions and in the striatum of patients with Alzheimer's disease

The TrkAII tyrosine kinase receptor differs from the TrkAI isoform by an insertion of six amino acids in the extracellular domain. We used RT-PCR to determine their respective distribution in rat and human brain. Only trkAII transcripts were detected in 12 rat brain regions, while both trkAI and trkAII transcripts were detected in the cerebellum and pituitary gland. In human, both trkAI and trkAII transcripts were detected in the frontal, temporal, and occipital cortex and thalamus, while only trkAI transcripts were detected in the hippocampus and cerebellum. In the caudate and putamen, trkAII transcripts were exclusively detected. Thereafter, we studied the expression of TrkA isoforms in the striatum of five patients with Alzheimer's disease (AD), four patients with non-AD dementia, seven patients with Parkinson's disease, and six paired nondemented elderly control individuals. In controls and non-AD patients, a constant expression of trkAII transcripts was detected within all striatum parts. In AD patients, a heterogeneous decrease in trkAII expression was observed in the caudate, putamen, and ventral striatum, resulting either in a drop of trkAII transcript levels or in a weak coamplification of trkAII and trkAI transcripts. The alteration of TrkAII gene expression paralleled those of choline acetyltransferase. Together with previous data, this suggests that the alteration of trk gene expression could contribute to a decrease in NGF binding sites and its protective effects on cholinergic neurons of AD patients.

Laser scanning and electron microscopic evidence for rapid and specific in vivo labelling of cholinergic neurons in the rat basal forebrain with fluorochromated antibodies

Recently developed methods for the selective labelling of cholinergic basal forebrain neurons containing the low-affinity neurotrophin receptor p75 (p75(NTR)) in vivo and in vitro are based on carbocyanine 3 (Cy3)-tagged antibodies directed against p75(NTR). The present study focuses on the maintenance of this neuronal label after injection of such fluorescent antibodies into the cerebral ventricle. One, 3, and 10 days after injection this marker exclusively stains neurons immunoreactive for the cholinergic markers choline acetyltransferase and vesicular acetylcholine transporter in the rat medial septum, diagonal band and nucleus basalis. Thirty days after injection the in vivo labelling was nearly abolished. Predominant labelling of lysosomes was shown by electron microscopic analysis following photoconversion of the Cy3-label to an electron-dense reaction product. The pre-labelling of cholinergic neurons might facilitate pharmacological and electrophysiological approaches in living slices and cell culture systems as well as detailed investigations focused on the transport of neurotrophins in vivo and in animals with experimentally altered p75(NTR) expression.

Cholinergic medial septum neurons do not degenerate in aged 129/Sv control or p75(NGFR)-/-mice

Cholinergic medial septum neurons express TrkA and p75 nerve growth factor receptor (p75(NGFR)) and interactions between TrkA and p75(NGFR) are necessary for high-affinity binding and signaling of nerve growth factor (NGF) through TrkA. In adult p75(NGFR)-deficient (-/-) mice, retrograde transport of NGF and other neurotrophins by these neurons is greatly reduced, however, these neurons maintain their cholinergic phenotype and size. Reduced transport of NGF has been proposed to play a role in Alzheimer's disease. Here, we investigated whether chronic and long-term absence of p75(NGFR) (and possibly reduced NGF transport and TrkA binding) would affect the cholinergic septohippocampal system during aging in mice. In young (6-8 months), middle aged (12-18 months), and aged (19-23 months) 129/Sv control mice the total number of choline acetyltransferase-positive medial septum neurons and the mean diameter and cross sectional area of the cholinergic cell bodies were similar. The cholinergic hippocampal innervation, as measured by the density of acetylcholinesterase-positive fibers in the outer molecular layer of the dentate gyrus was also similar across all ages. These parameters also did not change during aging in p75(NGFR) -/- mice and the number and size of the choline acetyltransferase-positive neurons and the cholinergic innervation density were largely similar as in control mice at all ages. These results suggest that p75(NGFR) does not play a major role in the maintenance of the number or morphology of the cholinergic basal forebrain neurons during aging of these mice. Alternatively, p75(NGFR) -/- mice may have developed compensatory mechanisms in response to the absence of p75(NGFR).

Two distinct populations of cholinergic neurons in the septum of raccoon (Procyon lotor): evidence for a separate subset in the lateral septum

The present study focused on cholinergic neurons in the lateral septal region of the raccoon detected by choline acetyltransferase (ChAT)-immunostaining. For comparison of the cholinergic neurons of the medial and lateral septal nuclei, soma sizes were measured, and several antibodies were applied that differentially characterize these cells in several species: low-affinity neurotrophin receptor p75 (p75(NTR)), calbindin-D(28k) (CALB), and constitutive nitric oxide synthase (cNOS). To compare the basic organization of the raccoon septum with that in other mammals, parvalbumin (PARV) immunocytochemistry and Wisteria floribunda-agglutinin (WFA) lectin histochemistry also were used in double-staining experiments. The ChAT-immunoreactive neurons of the rostral lateral septum are arranged in laminae. Accumulations of cholinergic varicosities, often clearly ensheathing noncholinergic neurons, occupy small territories of the rostral septum. Such regions become larger in the caudal septum. They are assumed to correspond to the septohippocampal and septofimbrial nuclei of the rat. In contrast to the large medial septal cholinergic neurons of the raccoon that contain p75(NTR), CALB, and cNOS, the cholinergic neurons of the lateral septum are smaller and do not express these markers. A further peculiarity is that the region of the lateral septum that contains cholinergic neurons corresponds to WFA-labelled extracellular matrix zones that contain chondroitin sulfate proteoglycans. In addition to clustered thread- or ring-like accumulations of the WFA, sparsely labelled perineuronal nets surround the lateral septal cholinergic neurons. Similar to other species that have been investigated, perineuronal nets are completely absent around cholinergic cells of the medial septum. The PARV-containing neurons of this region, however, are enwrapped by perineuronal nets as they are in the rat. Within the medial septum, the PARV-containing neurons are restricted to ventral bilateral territories that are devoid of cholinergic cells. In this respect, they differ from the more vertically arranged PARV-containing medial septal cells in rodents and primates. Apart from striking differences in numbers and distribution patterns, the raccoon lateral septal cholinergic neurons resemble those detected by Kimura et al. (Brain Res [1990] 533:165-170) in the ventrolateral septal region of rat and monkey. Their participation in the functions of the lateral septum remains to be elucidated.

Leukaemia inhibitory factor prevents loss of p75-nerve growth factor receptor immunoreactivity in medial septal neurons following fimbria-fornix lesions

Transection of the fimbria-fornix leads to retrograde degeneration of axotomized septal cholinergic neurons as manifested by loss of choline acetyltransferase and low-affinity nerve growth factor receptor (p75NGFR) immunoreactivity. Nerve growth factor administered into cerebral ventricles at the time of axotomy can prevent these changes, while ciliary neurotrophic factor can prevent the loss of p75NGFR immunostaining. Leukaemia inhibitory factor shares structural homologies with ciliary neurotrophic factor and has similar actions in the nervous system. Both proteins share the same signalling pathways, which involve the interleukin-6 transducing receptor components leukaemia inhibitory factor receptor beta and gp130. In this study, we compared the effects of leukaemia inhibitory factor, ciliary neurotrophic factor and nerve growth factor, administered into cerebral ventricles, on p75NGFR and choline acetyltransferase immunoreactivity in septal neurons after fimbria-fornix transection. We found that leukaemia inhibitory factor, like ciliary neurotrophic factor, prevents the loss of p75NGFR-stained medial septal neurons after fimbria-fornix axotomy, without maintaining choline acetyltransferase expression in these neurons. In addition, p75NGFR-immunostained neurons had significantly smaller mean diameter after axotomy in leukaemia inhibitory factor- and ciliary neurotrophic factor-treated animals as compared with either nerve growth factor-treated or unlesioned animals. These findings suggest that both leukaemia inhibitory factor and ciliary neurotrophic factor can prevent the axotomy-induced cell death of septal cholinergic neurons, but that, in contrast to nerve growth factor, these growth factors do not maintain the expression of choline acetyltransferase or the normal neuronal size of these injured neurons.

Evidence that nerve growth factor influences recent memory through structural changes in septohippocampal cholinergic neurons

We compared, in 4- and 23-month-old Fischer-344 rats, the effects of nerve growth factor (NGF) on basal forebrain cholinergic neurons with behavioral performance in acetylcholine-dependent memory tasks (recent and reference memory). Noncholinergic monoamine markers in target fields of cholinergic neurons were also investigated. We found that NGF has contrasting effects on recent memory in the two age groups in causing improvement in aged rats and deterioration in young rats. In addition, NGF caused significant increase in the size of cholinergic perikarya in all sectors of the basal nucleus complex (BNC). Higher doses of NGF were required to produce hypertrophy in aged animals, a pattern consistent with a lower sensitivity to NGF of aged cholinergic neurons. Analysis of covariance showed that the behavioral effects of NGF were eliminated after covarying out the hypertrophy of cholinergic perikarya. Therefore, NGF causes hypertrophy of cholinergic perikarya regardless of age, and this neurobiological measure correlates with the effects of NGF on recent memory. Reference memory improved moderately only in old rats. This mild effect covaried with an increase in choline acetyltransferase activity in neocortex. Cortical terminal fields of noradrenergic and serotoninergic pathways were not affected by NGF. Taken together, our results indicate that NGF influences recent memory in an age- and transmitter-specific fashion. We postulate that the direct cause of the effects of NGF on memory is not perikaryal hypertrophy per se but rather an increased density of terminals, which always accompanies perikaryal hypertrophy. Although these results continue to support the use of NGF for the treatment of Alzheimer's disease, they raise questions regarding the therapeutic role of NGF for degeneration of BNC neurons occurring in young age.

Ganglioside GM1 potentiates NGF action on axotomised medial septal cholinergic neurons

Transection of the fimbria fornix leads to retrograde degeneration of axotomised septal cholinergic neurons as manifested by loss of choline acetyltransferase and p75NGFR immunoreactivity. Intracerebroventricularly administered nerve growth factor initiated at the time of axotomy can prevent these changes. We have shown that concurrent intraperitoneal administration of GM1 with a low and otherwise unprotective intracerebroventricular dose of nerve growth factor, can also prevent the loss of these fimbria fornix axotomised cholinergic neurons, where GM1 alone does not have this effect. This study further confirms the neuroprotective actions of GM1 and suggests that it may interact to potentiate the effect of nerve growth factor on these axotomised septal cholinergic neurons.

Tyrosine kinase A, galanin and nitric oxide synthase within basal forebrain neurons in the rat

Cholinergic basal forebrain neurons appear to play a key role in cognition and attention. In rat, basal forebrain neurons express multiple proteins including the high-affinity signal transducing tyrosine kinase A receptor for nerve growth factor, the neuropeptide galanin and nitric oxide synthase, a marker for the novel neurotransmitter nitric oxide. The present study was undertaken to define the relationship between neurons expressing each of these markers within the medial septum-vertical limb of the diagonal band, horizontal limb of the diagonal band and nucleus basalis in colchicine pre-treated rats. Tyrosine kinase A-immunopositive neurons were seen throughout all subfields of the basal forebrain. In contrast, nitric oxide synthase- and galanin-immunoreactive neurons were mainly distributed within the septal-diagonal band complex. Co-localization experiments revealed that virtually all nitric oxide synthase-positive neurons (visualized by nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry) also contained tyrosine kinase A, whereas many fewer tyrosine kinase A neurons were nicotinamide adenine dinucleotide phosphate-diaphorase positive within the medial septum-vertical limb of the diagonal band. Within the horizontal limb of the diagonal band, numerous nicotinamide adenine dinucleotide phosphate-diaphorase neurons expressed tyrosine kinase A, whereas only a small number of tyrosine kinase A neurons contained nicotinamide adenine dinucleotide phosphate-diaphorase. Within the nucleus basalis very few neurons were nicotinamide adenine dinucleotide phosphate-diaphorase reactive, and a minor number contained tyrosine kinase A. Additional co-localization experiments revealed minor percentages of neurons containing nicotinamide adenine dinucleotide phosphate-diaphorase and galanin immunoreactivity within the various subfields of the basal forebrain. Within the horizontal limb of the diagonal band minor numbers of nicotinamide adenine dinucleotide phosphate-diaphorase-reactive perikarya displayed galanin. Similarly, only a few galanin-containing neurons expressed nicotinamide adenine dinucleotide phosphate-diaphorase. The existence of tyrosine kinase A, nitric oxide synthase and galanin within select neuronal subgroups of the cholinergic basal forebrain suggests that these perikarya are responsive to a complex set of chemical signals. A greater understanding of the chemical signature of the cholinergic basal forebrain neurons will provide the insight required to develop novel pharmacological approaches aimed at preventing or slowing the degenerative processes that effect these neurons in aging and pathologic disorders.

Selective in vivo fluorescence labelling of cholinergic neurons containing p75(NTR) in the rat basal forebrain

The cholinergic system of the rat basal forebrain is used as a model for the homologous region in humans which is highly susceptible to neuropathological alterations as in Alzheimer's disease. Cholinergic cells in the basal forebrain express the low-affinity neurotrophin receptor p75NTR. This has been utilized for selective immunolesioning of cholinergic neurons after internalization of an immunotoxin composed of anti-p75NTR and the ribosome-inactivating toxin saporin. However, the goal of many studies may be not the lesion, but the identification of cholinergic cells after other experimentally induced alterations in the basal forebrain. Therefore, a novel cholinergic marker was prepared by conjugating the monoclonal antibody 192IgG directed against p75NTR with the bright red fluorochrome carbocyanine 3 (Cy3). Three days after intraventricular injection of Cy3-192IgG the fluorescence microscopic analysis revealed a pattern of Cy3-labelled cells matching the distribution of cholinergic neurons. Apparently the marker was internalized within complexes of p75NTR and Cy3-192IgG which were then retrogradely transported to the cholinergic perikarya of the basal forebrain. In addition to the even labelling of somata, a strong punctate-like Cy3-immunofluorescence was seen in structures resembling lysosomes. The specificity of the in vivo staining was proven by subsequent immunolabelling of choline acetyltransferase (ChAT) with green fluorescent Cy2-tagged secondary antibodies. In the medial septum, the diagonal band and the nucleus basalis only cholinergic neurons were marked by Cy3-192IgG. In parallel experiments, digoxigenylated 192IgG was not detectable within cholinergic basal forebrain neurons after intraventricular injection. Presumably, this modified antibody could not be internalized. On the other hand, digoxigenylated 192IgG was found to be an excellent immunocytochemical marker for p75NTR as shown by double labelling including highly sensitive mouse antibodies directed against ChAT. Based on the present findings, future applications of the apparently non-toxic Cy3-192IgG and other antibodies for fluorescent in vivo and in vitro labelling are discussed.

Co-expression of p75NTR- and calbindin-immunoreactivity in cholinergic neurons of the raccoon basal forebrain

The cholinergic system of the basal forebrain is involved in the modulation of sensory information. This has previously been investigated in the raccoon, an animal especially interesting because of its highly developed somatosensory cortex. The present study focused on the co-expression of the low-affinity neurotrophin receptor p75NTR and calbindin in cholinergic neurons of the raccoon basal forebrain and neostriatum. Carbocyanine immunofluorescence double labelling revealed the co-localization of choline acetyltransferase and p75NTR as well as calbindin in a large portion of basal forebrain neurons, but not in the neostriatum. In contrast, immunolabelling of two other calcium-binding proteins, parvalbumin and calretinin, was found exclusively in non-cholinergic neurons.

Hypertrophy of basal forebrain neurons and enhanced visuospatial memory in perinatally choline-supplemented rats

The effects of choline supplementation during two time-frames of early development on radial-arm maze performance and the morphology of basal forebrain neurons immunoreactive for the P75 neurotrophin receptor (NTR) in male and female Sprague-Dawley rats were examined. In the first experiment, rats were supplemented with choline chloride from conception until weaning. At 80 days of age, subjects were trained once a day on a 12-arm radial maze for 30 days. Compared to control littermates, supplemented rats made fewer working and reference memory errors; however, the memory enhancing effects of choline supplementation were greater in males than females. A morphometric analysis of NTR-immunoreactive cell bodies at three levels through the medial septum/diagonal band (MS/DBv) of these rats revealed that perinatal choline supplementation caused the somata of cells in the MS/DBv to be larger by 8-15%. In a second experiment, choline supplementation was restricted to embryonic days 12-17. A developmental profile of NTR immunoreactive cell bodies in the MS/DBv of 0-, 8-, 16-, 30- and 90-day old male and female rats again revealed that cell bodies were larger in choline-supplemented rats than controls. As in the behavioral studies, the effect of choline supplementation was greater in male than female rats. These data are consistent with the hypothesis that supplementation with choline chloride during early development leads to an increase in the size of cell bodies of NTR-immunoreactive cells in the basal forebrain and that this change may contribute to long-term improvement in spatial memory.

Retrograde degeneration and colchicine protection of basal forebrain cholinergic neurons following hippocampal injections of an immunotoxin against the P75 nerve growth factor receptor

Intracerebroventricular injection of 192 IgG antibody against the p75LNGFR rat low affinity nerve growth factor receptor conjugated with saporin, a ribosome inactivating protein, has been shown to destroy the p75LNGFR-expressing cholinergic neurons of the basal forebrain. We injected this immunotoxin into the hippocampus and studied its retrograde effect upon the cholinergic neurons of the medial septum and the vertical limb of the diagonal band of Broca. Seven days after injection, there was a nearly total depletion of cholinergic axons within the hippocampus. This depletion was associated with a marked and significant decrease in the number of cholinergic neurons of the ipsilateral medial septum and the vertical limb of the diagonal band of Broca. At longer survival times, these changes were more pronounced. Parvalbumin-positive, GABAergic neurons within the same areas of the basal forebrain were not affected by immunotoxin injections. Injections of saporin alone had no effect upon cholinergic neurons. Simultaneous injection of colchicine with the immunotoxin resulted in a significant reduction of retrograde degeneration of cholinergic neurons and relative preservation of hippocampal cholinergic axons. These observations suggest that 192 IgG-saporin is transported retrogradely from the hippocampus to the cholinergic neurons in the medial septum and the vertical limb of the diagonal band of Broca and provide a model for retrograde degeneration of basal forebrain cholinergic neurons following cortically based toxic-pathologic processes.

Development of cholinergic and GABAergic neurons in the rat medial septum: different onset of choline acetyltransferase and glutamate decarboxylase mRNA expression

In the present study, we have investigated the developmental expression of the transmitter-synthesizing enzymes choline acetyltransferase (ChAT) and glutamate decarboxylase (GAD) in rat medial septal neurons by using in situ hybridization histochemistry. In addition, we have employed immunostaining for ChAT and the calcium-binding protein parvalbumin, known to be contained in septohippocampal GABAergic neurons. A large number of GAD67 mRNA-expressing neurons were already observed in the septal complex on embryonic day (E) 17, the earliest time point studied. During later developmental stages, there was mainly an increase in the intensity of labeling. Neurons expressing ChAT mRNA were first recognized at E 20, and their number slowly increased during postnatal development of the septal region. The adult pattern of ChAT mRNA-expressing neurons was observed around postnatal day (P) 16. By using a monoclonal ChAT antibody, the first immunoreactive cells were not seen before P 8. Similarly, the first weakly parvalbumin-immunoreactive neurons were seen in the septal complex by the end of the 1st postnatal week. These results indicate that in situ hybridization histochemistry may be an adequate method to monitor the different development of transmitter biosynthesis in cholinergic and GABAergic septal neurons. Moreover, the late onset of ChAT mRNA expression would be compatible with a role of target-derived factors for the differentiation of the cholinergic phenotype.

Effects of intraseptal injection of 192-IgG-saporin in mature and aged Long-Evans rats

In this study, the effects intraseptal injections of the selective cholinergic immunotoxin, 192-IgG-saporin, were investigated in mature (6-month-old) and aged (24-26-month-old) male Long-Evans rats. Ten days following intraseptal injection of either 192-IgG-saporin or saline, testing began in a battery of behavioral tests modulated by the septohippocampal system including two versions of the Morris water maze (i.e. submerged platform task, and 2-platform spatial discrimination), inhibitory avoidance, and pre-pulse inhibition of acoustic startle. In both mature and aged rats, intraseptal injection of 192-IgG-saporin selectively reduced ChAT activity in the hippocampus and posterior cingulate cortex, without affecting ChAT activity of amygdala or parietal cortex. In general, in all of the behavioral tests analyzed, intraseptal 192-IgG-saporin treatment had no effect in mature animals. Age-related deficits were observed in the spatial memory tasks, however this impairment was largely a function of the poor performance of aged rats treated with the toxin. In addition, an increase in the response to an acoustic startle was found in aged rats treated with 192-IgG-saporin. Thus, although intraseptal injection of 192-IgG-saporin produced similar reductions of ChAT activity, performance of mature and aged rats in tasks believed to be modulated by the septohippocampal pathway tended to be differentially affected in mature and aged rats.

Involvement of nerve growth factor in visual cortex plasticity

The physiological role of nerve growth factor (NGF), the prototype member of the neurotrophin family, has been widely studied. NGF has been shown to promote survival, sprouting and differentiation of sympathetic ganglion cells and sensory neurons in the peripheral nervous system; it has also been shown to support survival and regeneration of cholinergic neurons in the central nervous system. Recent evidence indicates that NGF is also involved in the neuronal plasticity of the visual cortex. Exogenous supplies of NGF have been shown to interfere with normal processes underlying activity- and age-dependent synaptic modifications in both developing and adult visual cortex. In parallel to these physiological effects, numerous neuronal markers in the visual cortex have been found to be influenced by NGF. Several proposals have been introduced to explain the physiological role of NGF in visual cortex plasticity. Although the mechanisms underlying NGF effects in the visual cortex are still under active investigation, current evidence implies that NGF, and perhaps other neurotrophins as well, may be useful for preventing or correcting inappropriate or anomalous connections in the visual cortex, and thus for treating visual dysfunctions such as amblyopia and strabismus.

Differential distribution of exogenous BDNF, NGF, and NT-3 in the brain corresponds to the relative abundance and distribution of high-affinity and low-affinity neurotrophin receptors

To evaluate effective means for delivering exogenous neurotrophins to neuron populations in the brain, we compared the distribution and transport of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin-3 (NT-3) following intracerebral delivery. Rats received an injection of radioiodinated or unlabeled neurotrophin into the lateral ventricle and were killed humanely after 1.5-24 hours. Other rats received continuous infusion of unlabeled neurotrophin into the lateral ventricle, the striatum, or the hippocampus for 3-14 days. The neurotrophins were detected by autoradiography or immunohistochemistry. There were striking differences between BDNF, NGF, and NT-3 in their penetration through brain tissue. These differences occurred regardless of the site or method of delivery, but were most pronounced following a bolus intracerebroventricular (ICV) injection. After ICV injection, NGF was widely distributed in tissues around the ventricles and at the surface of the brain, whereas the penetration of BDNF into brain tissue was distinctly less than that of NGF, and the penetration of NT-3 was intermediate. An ICV injection of NGF produced prominent but transient labeling of cells that contain the low-affinity NGF receptor, whereas an injection of BDNF prominently labeled the ventricular ependyma. During ICV infusion (12 micrograms/day), the distribution of BDNF was no greater than that observed after a 12-micrograms bolus injection. A sixfold increase in the amount of BDNF infused (72 micrograms/day) produced a more widespread distribution in the brain and doubled the depth of penetration into periventricular tissues near the cannula. Corresponding to their differences in penetration, NGF was retrogradely transported by basal forebrain cholinergic neurons after ICV or intrastriatal delivery, whereas NT-3 was transported by a few basal forebrain neurons after ICV delivery, and BDNF was rarely detected in neurons after ICV delivery. Delivery of BDNF directly to the striatum or the hippocampus labeled numerous neurons in nuclei afferent to these structures. In situ hybridization studies confirmed that the high-affinity BDNF receptor (TrkB) was much more widely expressed in neurons than was the high-affinity NGF receptor (TrkA). Moreover, mRNA for truncated forms of TrkB was expressed at high levels in the ependyma, the choroid epithelium, and the gray matter. It is likely that binding of BDNF to TrkB, which appears to be more abundant and ubiquitous than TrkA, restricts the diffusion of BDNF relative to that of NGF.

Distribution of nerve growth factor following direct delivery to brain interstitium

Several studies suggest the potential of nerve growth factor (NGF) in the treatment of patients with Alzheimer's disease. To characterize NGF transport within the brain interstitium, we implanted controlled release polymers containing NGF and [125I]NGF into the brains of adult male rats and measured spatial distributions of NGF for up to one week. NGF concentration in the brain was quantified using ELISA, radiation counting, and autoradiography. At 2 days post-implantation, quantities of NGF in excess of 50 pg per section were detected within thick (1 mm) coronal slices of the hemisphere ipsilateral to the site of implantation up to 3 mm rostral and caudal to the edge of the polymer. Lower levels of radioactivity (> 5 pg but < 50 pg NGF per section) could be detected throughout the rest of the brain. Levels were highest in the tissue sections containing the polymer, reaching 9.5 ng per section. Autoradiography of thin (20 microns) coronal sections indicated that local NGF concentrations immediately adjacent to the polymer approached 40 micrograms/ml. Analysis of sequential sections on the autoradiograph confirmed that NGF was transported only 2-3 mm from the polymer in any direction. At one week post-implantation, the pattern of NGF distribution was similar to that seen at 2 days, and concentrations remained high near the site of the implant. Comparison of local NGF concentration profiles to simple models of diffusion with first-order elimination suggests that the NGF moved through the tissue by diffusion through the interstitial space with a half-life on the order of 0.5 h. The limited range of NGF transport in brain tissue indicates that: (i) protein drug agents such as NGF will probably need to be delivered almost directly to the site of action for efficacy; and (ii) toxicities associated with delivery of NGF and other protein agents to non-target cells, as often occurs with systemic delivery of drugs, may be reduced by local, interstitial delivery since therapy can be restricted to a small volume of the brain.

The effects of AMPA-induced lesions of the medial septum and vertical limb nucleus of the diagonal band of Broca on spatial delayed non-matching to sample and spatial learning in the water maze

These experiments investigated in the rat the impact on spatial delayed non-matching to sample and on acquisition of the Morris water maze of (i) AMPA-induced lesions of the medial septal nucleus, which produced a marked reduction of hippocampal choline acetyltransferase activity and acetylcholine levels (measured using in vivo dialysis) together with lesser reductions in cholinergic markers in the cingulate cortex and (ii) similar AMPA-induced lesions of the vertical limb nucleus of the diagonal band of Broca (vDB), which produced more marked reductions in cholinergic markers in the cingulate cortex than in the hippocampus. Medial septal lesions produced a delay-dependent deficit in spatial working memory, while lesions of the vDB resulted in a delay-independent performance deficit. In addition, rats with vDB lesions adopted biased response strategies during the imposition of long delays. Neither lesion significantly affected the acquisition of a spatial reference memory task, the Morris water maze. The results are discussed in terms of cholinergic- and GABAergic-dependent functions of the hippocampal formation and cingulate cortex in spatial short-term and reference memory.

Synergistic trophic actions on rat basal forebrain neurons revealed by a synthetic NGF/BDNF chimaeric molecule

Basal forebrain cholinergic neurons, which degenerate in Alzheimer's disease, respond to multiple trophic factors, including the neurotrophins, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). This dual responsiveness prompted us to investigate the effects of a synthetic chimaeric molecule, containing the active domains of both NGF and BDNF. The NGF/BDNF chimaeric factor exhibited synergistic actions, and was 100-fold more potent than wild-type BDNF in enhancing survival of cultured dissociated basal forebrain cholinergic neurons. This effect was apparently due to true BDNF/NGF synergy, since addition of the two wild-type trophins simultaneously reproduced the effect of the chimaera. Synergy was selective for neurons which respond to both factors; substantia nigra dopaminergic neurons, which respond to BDNF but not NGF, exhibited no potentiation. The chimaeric factor thus revealed a synergy that may normally occur in the brain, and constitutes a potentially novel therapeutic agent with greater potency than naturally occurring individual trophins.

Dose-response comparison of recombinant human nerve growth factor and recombinant human basic fibroblast growth factor in the fimbria fornix model of acute cholinergic degeneration

Both nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) have been proposed for the treatment of Alzheimer's disease. This study describes a comparative, dose-response analysis of recombinant human (rh)NGF and rhbFGF in a rat unilateral fimbria-fornix model of acute cholinergic neuronal degeneration. Doses for rhNGF were 0.6, 6, 60, 600 and 1,800 ng/rat/day and for rhbFGF were 600, 1,800, 3,000 and 6,000 ng/rat/day, delivered for 4 weeks. The number of surviving septal cholinergic neurons was evaluated using ChAT immunohistochemistry. In control animals, the number of ChAT-positive neurons remaining on the lesioned side was between 22 and 18% compared to the non-lesioned side. Infusion with either neurotrophic factor increased the number of ChAT-positive neurons on the lesioned side in a dose-dependent manner. The maximal response to rhbFGF peaked at 3,000 ng/rat/day with a cell savings of 47%. However, there was evidence of neuropathological changes associated with rhbFGF. In contrast, rhNGF produced a maximal response with an infusion of 600 ng rhNGF/rat/day and a cell savings of 70% and no evidence of neuropathology, indicating that rhNGF was better tolerated and more efficacious than rhbFGF.

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.

Destruction of the cholinergic basal forebrain using immunotoxin to rat NGF receptor: modeling the cholinergic degeneration of Alzheimer's disease

Degeneration of cholinergic neurons in the basal forebrain (CBF) is a prominent neuropathological feature of Alzheimer's disease and is thought responsible for some cognitive deficits seen in patients. An animal model of pure CBF degeneration would be valuable for analysis of the function of these neurons and testing therapeutic strategies. CBF neurons express receptors for nerve growth factor. In order to selectively destroy these neurons, we developed an immunotoxin using monoclonal antibody (192 IgG) to rat NGF receptor (p75NGFr) armed with the ribosome inactivating protein, saporin. In vitro 192-saporin was highly toxic to neurons expressing p75NGFr. Intraventricular injections of 192-saporin destroyed the CBF and impaired passive avoidance learning. These results indicate that 192-saporin treated rats can be used to model a key feature of Alzheimer's disease and that anti-neuronal immunotoxins are a powerful approach to selective neural lesioning.

Trophic-factor modulation of cortical acetylcholinesterase reappearance following transection of the medial cholinergic pathway in the adult rat

Laminar patterns of cortical acetylcholinesterase (AChE) activity are reestablished in the adult, pharmacologically unmanipulated rat following axotomy of the medial cholinergic pathway. The extent to which trophic and/or growth promoting or inhibiting agents modulate AChE fiber reappearance is not fully understood. Such studies, however, would further clarify possible roles for these agents in neuronal plasticity in response to injury, as well as in plastic processes associated with normative functions. In the present experiments, we explored trophic modulation by intracortically infusing nerve growth factor (NGF) or somatostatin into cingulate cortex at a site distal to transection of the medial cholinergic pathway. Comparisons were made with sham-operated or noninfused transected controls, as well as with transected animals infused with renin or antibodies against NGF. Administration began 2 days after axotomy and continued at successive 3-day intervals for 4 weeks. It was found that, proximal to the lesion site, NGF increased and somatostatin decreased optical density of AChE; the number of AChE-containing fibers was unaltered compared to controls. Distal to the knife cut, both NGF and somatostatin increased number of AChE fibers but did not alter overall AChE optical density. Nonetheless, NGF produced an increase in the number of intensely staining puncta both proximal and distal to the cut. Neither renin nor anti-NGF antibodies produced statistically significant effects on optical density or number of fibers at any cortical locus studied. We conclude that NGF and somatostatin have opposite effects on the expression of AChE: whereas NGF increases AChE levels, somatostatin inhibits AChE accumulation in proximal fibers, perhaps by actions on synthesis or transport. Fiber proliferation, which only occurred distally, was affected positively by both NGF and somatostatin, indicating that neurite-promoting effects produced by both agents are confined to tissue regions where neurite extension is stimulated by axotomy. Increases in AChE-positive puncta produced by NGF, however, were not confined to regions of fiber proliferation.

The effects of AMPA-induced lesions of the septo-hippocampal cholinergic projection on aversive conditioning to explicit and contextual cues and spatial learning in the water maze

The environmental context of an animal both subsumes and is associated with the explicit cues that guide its behavioural responses. Recent work in this laboratory suggests that learning about the relationship between the cues which comprise a context depends on the hippocampus. In the present study the role of the cholinergic input to the hippocampus in contextual learning was assessed in rats using a conditioned stimulus/context conditioning paradigm and spatial learning in the Morris water maze. In the former, a place preference apparatus provided the context. The subject was confined in the black chamber and a 'clicker' conditioned stimulus was presented five times in a 20 min period. A trace interval of 5 or 30 s, depending on the group, was interposed between the end of the clicker and a footshock. Theory predicts that animals in the 5 s condition will learn more about the clicker as a predictor of shock and become strongly conditioned, while those in the 30 s condition learn relatively more about the context. Conditioning to the clicker (conditioned stimulus) was measured in a separate lick suppression chamber--presentation of the clicker suppresses drinking, and contextual learning was determined by recording the time spent on the black side of the place preference apparatus when both the black and a familiar white chamber were accessible. Lesions of the medial septum/diagonal band induced by RS-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) enhanced contextual learning in this paradigm but disrupted conditioned stimulus conditioning in the 30 s condition. Acquisition of the Morris water maze was largely unimpaired. The results are suggested to reflect a shift towards the use of hippocampal-dependent contextual learning strategies in lesioned animals.

Estrogen and nerve growth factor-related systems in brain. Effects on basal forebrain cholinergic neurons and implications for learning and memory processes and aging

Estrogen replacement can significantly affect the expression of ChAT and NGF receptors in specific basal forebrain cholinergic neurons. The time-course of the effects is consistent with a direct up-regulation of ChAT followed by either direct or indirect down-regulation of p75NGFR and trkA NGF receptors, possibly due to increased cholinergic activity in the hippocampal formation and cortex and a decrease in hippocampal levels of NGF. Current evidence suggests ChAT, p75NGFR, trkA, and NGF all play a role in regulating cholinergic function in the hippocampal formation and cortex. In addition, all have been implicated in the maintenance of normal learning and memory processes as well as in changes in cognitive function associated with aging and with neurodegenerative disease. It is possible that estrogen may affect cognitive function via effects on NGF-related systems and basal forebrain cholinergic neurons. Effects of estrogen on cognitive function have been reported, as has some preliminary evidence for beneficial effects of estrogen in decreasing the prevalence of and reducing some cognitive deficits associated with Alzheimer's disease. Whether these effects are related to effects on NGF-related systems or basal forebrain cholinergic neurons is currently unknown. Indirect evidence suggests that estrogen interacts with NGF-related systems and that changes in circulating levels of estrogen can contribute to age-related changes in hippocampal levels of NGF. These findings have important implications for consideration of estrogen replacement therapy in pre- and post-menopausal women. Further studies examining effects of different regimens of estrogen replacement as well as estrogen combined with progesterone on NGF and basal forebrain cholinergic neurons in young and aged animals are required. Prospective studies correlating aging and estrogen replacement with numbers of basal forebrain cholinergic neurons and hippocampal and cortical levels of NGF also need to be performed to better assess the potential benefits of estrogen replacement in reducing age- and disease-related cognitive decline.

Reversal of spatial memory impairments in aged rats by nerve growth factor and neurotrophins 3 and 4/5 but not by brain-derived neurotrophic factor

Aged rats, displaying impairments in spatial learning and memory associated with marked cellular atrophy of forebrain cholinergic neurons, received intracerebroventricular infusions of one of the four neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), or neurotrophin 4/5 (NT-4/5), or a combination of NGF and BDNF, or vehicle. During the 4-week infusion period rats receiving NGF, NT-3, or NT-4/5 showed improved acquisition and retention of spatial memory. With NGF and NT-3, but not NT-4/5, this was accompanied by a significant reduction in cholinergic neuron atrophy in septum, nucleus basalis, and striatum. BDNF, in contrast, was without effect either alone or in combination with NGF. These results show that memory deficits associated with aging can be reversed by several members of the neurotrophin family and that this effect may be mediated through activation of multiple neurotrophin receptors associated with cholinergic and possibly noncholinergic systems in the brain.

Nerve growth factor-induced ocular dominance plasticity in adult cat visual cortex

Activity-dependent modifiability of cortical ocular dominance occurs only during early postnatal life, within the so-called "critical period," but not thereafter in adult visual cortex. To examine the role of neurotrophins in the activity- and age-dependent stimulation-induced modifiability of visual cortex, we tested whether intracortical infusion of nerve growth factor could induce ocular dominance plasticity in adult visual cortex. Nerve growth factor was continuously infused, by means of osmotic minipumps, into striate cortex of adult cats for 2 weeks. At the time of minipump implantation, one eyelid of the experimental animals was sutured closed. After 3 weeks of monocular deprivation, the ocular dominance distribution of neurons in the striate cortex was assessed using single unit recording. We found that monocular deprivation imposed on adult animals in conjunction with nerve growth factor infusion causes an ocular dominance shift toward the deprived eye. Although the underlying mechanisms remain uncertain, the results indicate that nerve growth factor can enhance activity-dependent synaptic modification and remodeling in adult visual cortex.

Changes in nerve growth factor immunoreactivity following entorhinal cortex lesions: possible molecular mechanism regulating cholinergic sprouting

To assess the possible role of trophic factors in lesion-induced plasticity, we have used a sensitive immunohistochemical technique to evaluate changes in nerve growth factor (NGF) staining in the hippocampal formation 3, 8, 16, and 30 days following entorhinal cortex lesions. Our results indicate that a band of NGF immunoreactivity appears in the outer molecular layer of the ipsilateral dentate gyrus following entorhinal ablation. The distribution of the NGF-immunoreactive band exactly coincides with the distribution of sprouting cholinergic terminals revealed by acetylcholinesterase histochemistry or NGF-receptor immunostaining. Increased NGF-immunoreactivity is detectable at 3 days postlesion, is most intense at 8 days, and decreases to near control levels by 30 days. Lesion-induced increases in NGF immunostaining also occur in animals in which septohippocampal fibers had been removed by prior destruction of the fimbria-fornix. Increases in NGF-immunoreactivity, however, are substantially reduced in animals receiving intraventricular injections of colchicine, which presumably blocks NGF release. These results indicate that 1) increases in NGF immunostaining, which occur following entorhinal lesions, precede any changes in cholinergic sprouting parameters and are greatest during the period of maximal cholinergic sprouting; 2) increased NGF-immunoreactivity is not due to NGF binding by septohippocampal fibers; and 3) increased NGF-immunoreactivity appears to depend on the release of NGF by neurons that produce it. We hypothesize that, following entorhinal lesions, NGF immunostaining within the hippocampal formation may represent NGF "anchored" within the tissue and that NGF accumulation by such a mechanism may direct the sprouting response of NGF-sensitive cholinergic neurons.

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.

Nerve growth factor reverses spatial memory impairments in aged rats

Aged rats, displaying impairments in spatial learning and memory associated with marked cellular atrophy of forebrain cholinergic neurons, received intracerebroventricular infusions of the neurotrophin nerve growth factor (NGF), or vehicle. During the 4-week infusion period rats receiving NGF showed improved acquisition and retention of spatial memory. With NGF this was accompanied by a significant reduction in cholinergic neuron atrophy in both septum, nucleus basalis and striatum. The cause of learning and memory deficits associated with ageing is not known. In the present paper we show that learning and memory deficits in aged rats can be reversed by NGF.

Behavioral and neuroanatomical consequences of a unilateral intraventricular infusion of AF64A and limitations on the neuroprotective effects of nimodipine

The monoethylcholine aziridinium ion, AF64A, (3 nmol in 1 microliter) or artificial CSF (1 microliter) was infused unilaterally into the right dorsal lateral ventricle of male adult rats. Treatment with the L-type calcium channel antagonist, nimodipine (70 micrograms/kg b.wt.) or its vehicle was administered beginning before and for seven days following surgery. The infusion of AF64A reduced spontaneous alternation rates in the T-maze when compared to CSF and sham infused animals. AF64A-treated animals also took longer to reach the goal area in a complex maze task on specific trials relative to CSF and sham-infused animals. Locomotion and habituation to the open field did not differ between surgery groups. Unilateral AF64A significantly depleted acetylcholinesterase (AChE) positive terminals in the ipsilateral hippocampus and cell bodies in the ipsilateral medial septal area (MSA). Receptors for nerve growth factor (NGF-R), often colocalized with cholinergic cell bodies and terminals, also were depleted in the ipsilateral MSA of AF64A infused animals. Treatment with nimodipine did not have a neuroprotective effect on AF64A animals in either behavioral or histological results. However, some degree of protection was found in the vehicle-treated rats. This effect was likely a consequence of the stress of the injection procedure rather than the content of the vehicle, largely polyethylene glycol 400. Nimodipine-treated animals, regardless of surgery group, exhibited fewer emotional responses and had lower spontaneous alternation rates than untreated animals. The behavioral alterations found in the nimodipine groups are most easily explained in terms of altered emotionality. Overall our findings indicate that AF64A is a potent cholinotoxin that can selectively eliminate the ipsilateral septohippocampal cholinergic system when unilaterally infused into the lateral ventricle. It is possible that the mechanism of action of AF64A, like other nitrogen mustard analogues, involves disruption of basic processes involved in protein synthesis and DNA activities. Because of this, the toxic effects of the aziridinium mustard are independent of extracellular calcium and thus may not be susceptible to protection by calcium channel antagonists.

Neurotrophic factor mRNA expression in dentate gyrus is increased following angular bundle transection

In the central nervous system, the highest levels of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) mRNA are found in the hippocampal formation. In the present study, we report that a unilateral transection of the angular bundle, which relays cortical information via the entorhinal cortex to the hippocampal formation, increases NGF and BDNF mRNA in the ipsilateral dentate gyrus. Within 4 hours following transection, the hybridization signal for NGF and BDNF mRNA increases in stratum granulosum 3- and 5-fold, respectively, compared to control levels. This lesion-induced increase of both mRNA returns to control levels within 24 hours and is maintained for at least 5 days. The induction is not prevented by pretreatment with AP-5, CNQX, or cholinergic denervation due to transection of the fimbria-fornix. Finally, the induction of neurotrophin mRNA is preceded by an increase in c-fos mRNA. These results provide evidence that transection of the cortical input to the hippocampal formation upregulates NGF and BDNF mRNA selectively in stratum granulosum. We suggest that the increased expression of NGF and BDNF mRNA may be an early step in the synaptic rearrangement of neurotrophin responsive cholinergic afferents observed following damage to the entorhinal cortex.

Neurotrophic factor mRNA expression in dentate gyrus is increased following in vivo stimulation of the angular bundle

Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are two structurally-related neurotrophins synthesized in dentate gyrus granule cells and pyramidal neurons of the hippocampal formation. These neurons receive excitatory glutamatergic afferents from the entorhinal cortex via the angular bundle/perforant path. In the present study, we tested whether electrophysiological stimulation of this glutamatergic pathway modifies NGF or BDNF messenger RNA (mRNA) expression in vivo. Within hours following brief trains of high frequency angular bundle stimulation, the levels of mRNA encoding both neurotrophins were increased exclusively in granule cells of the ipsilateral dentate gyrus. The increase in neurotrophic factor mRNA expression was found to be mediated through the N-methyl-D-aspartate (NMDA) glutamate receptor subtype, and occurred in the absence of seizure. These findings provide evidence that neurotrophic factor mRNA levels in the hippocampal formation are increased by direct activation of excitatory afferents originating in the entorhinal cortex. We suggest that the function of some neurotrophin-responsive neuronal populations may depend upon the integrity and activity of neurons in the entorhinal cortex, a population of neurons reported to be compromised in patients with Alzheimer's disease.

Fibers immunoreactive for nerve growth factor receptor in adult rat cortex and hippocampus mimic the innervation pattern of AChE-positive fibers

Numerous reports have indicated that nerve growth factor (NGF) exerts neurotrophic effects on the cholinergic neurons of the basal forebrain. Receptors for NGF (NGFR) have been demonstrated on cholinergic perikarya in the medial septum, diagonal band of Broca, and basal nucleus of Meynert. These neurons provide the major cholinergic innervation to the cerebral cortex and hippocampus, and previous studies have shown that their terminal plexuses also possess NGFR. However, these studies have shown only isolated examples of immunoreactive fibers. In the present paper we confirm and extend the observation of the presence of NGFR immunoreactivity in the hippocampus and cortex of adult rat by showing the entire plexus and demonstrating that the plexus is strikingly similar to the pattern of cholinergic innervation. Fibers stained for acetylcholinesterase (AChE) and NGFR immunoreactivity were found in all layers of the parietal cortex. Within the hippocampus, fibers were observed in all regions, but were most dense in the strata oriens, pyramidale, and radiatum of hippocampal subfields CA1 and CA3. Particularly intense staining was found throughout the dentate gyrus. Partial transections of the fimbria-fornix, which disrupt fibers projecting from the medial septum to the hippocampus, concomitantly abolish the innervation pattern of both NGFR and AChE. These results provide additional evidence that NGFR are associated with septohippocampal and basocortical cholinergic fibers.

Distribution and colocalization of choline acetyltransferase immunoreactivity and NADPH diaphorase reactivity in neurons within the medial septum and diagonal band of Broca in the rat basal forebrain

NADPH diaphorase histochemistry and choline acetyltransferase immunocytochemistry were used to assess quantitatively the presence of nitric oxide synthase in the cholinergic neurons of the magnocellular basal forebrain complex. Virtually all (97%) NADPH diaphorase reactive magnocellular neurons in the medial septum and the vertical and horizontal limbs of the diagonal band of Broca were choline acetyltransferase immunoreactive, whereas only a proportion of the choline acetyltransferase immunoreactive neurons were NADPH diaphorase reactive. Thus NADPH diaphorase histochemistry identified a subpopulation of the magnocellular cholinergic neurons. Occasionally, NADPH diaphorase reactive neurons were observed within the medial septum and diagonal band of Broca that were not choline acetyltransferase immunoreactive, and in general were morphologically distinct from the magnocellular neurons; such neurons are probably representatives within the medial septum and diagonal band of more widely distributed phenotypically distinct populations of NADPH diaphorase reactive neurons. The proportions of the neurons in which choline acetyltransferase and NADPH diaphorase colocalized in the medial septum and in the diagonal bands of Broca were similar in any one coronal section, but there was a considerable difference in the proportions throughout the rostrocaudal extent of these nuclei. In the most rostral sections of the medial septum and diagonal band, approximately 70% of the choline acetyltransferase immunoreactive neurons were NADPH diaphorase reactive, whereas the proportion decreased progressively to about 30% at the level of the decussation of the anterior commissure. To examine further the extent of colocalization throughout the magnocellular basal forebrain complex, sections of the magnocellular preoptic nucleus, substantia innominata, and nucleus basalis magnocellularis were examined. While there was little total colocalization of choline acetyltransferase immunoreactivity and NADPH diaphorase reactivity in any particular section (approximately 18%), almost all of the double labelled neurons were in the substantia innominata, with very few in the other nuclei. Thus although there is a caudal to rostral gradient of the proportion of magnocellular cholinergic neurons that are NADPH diaphorase reactive throughout the entire basal forebrain magnocellular complex, subregions, such as the substantia innominata and magnocellular preoptic nucleus, may not follow this trend. The recent demonstration that the NADPH diaphorase histochemical reaction localizes a nitric oxide synthase suggests that attention should be given to the NADPH diaphorase subpopulation in pathological and experimentally induced alterations of the basal forebrain.

High-affinity nerve growth factor receptor (Trk) immunoreactivity is localized in cholinergic neurons of the basal forebrain and striatum in the adult rat brain

Trk-immunoreactivity was observed in basal forebrain and striatal cholinergic neurons, whereas low-affinity NGF receptor immunoreactivity was observed in basal forebrain but not striatal cholinergic neurons. Since NGF exerts trophic actions on both basal forebrain and striatal cholinergic populations, the presence of Trk in these neurons lends strong support for an essential role of Trk in NGF-responsive neurons, but suggests that the low affinity receptor is not necessary for NGF actions in the striatum.