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

Intermittent vs Continuous Administration of Nerve Growth Factor to Injured Medial Septal Cholinergic Neurons in Rat Basal Forebrain

Many medial septal neurons of the basal forebrain are dependent on nerve growth factor (NGF) from the hippocampus for survival and maintenance of a cholinergic phenotype. When deprived of their source of NGF by axotomy, medial septal neuronal cell bodies atrophy and lose their cholinergic markers. This is similar to what is observed in the basal forebrain during Alzheimer’s disease (AD). In the present study, medial septal neurons were axotomized in female rats by way of a fimbria/fornix lesion. For fourteen days following axotomy, varying NGF doses (1 – 250 μg/ml) were administered to the lateral cerebral ventricle with either mini-osmotic infusion or daily injection. The responsiveness of medial septal neurons was evaluated with choline acetyltransferase immunohistochemistry. Within the mini-osmotic pumps, NGF activity diminished greatly during the first five days of implantation, but increased dramatically in the CSF after five days of infusion. The responsiveness of medial septal neurons to NGF was dose dependent and the ED₅₀ for NGF injection was determined to be 14.08 μg/ml compared to 27.60 μg/ml for NGF infusion. Intermittent injections at varying intervals were evaluated with 30 μg/ml NGF over a fourteen-day time period (2, 3, 6, or 12 injections). No differences occurred in the number of choline acetyltransferase neurons from rats that received weekly injections to those that received daily injections of NGF. NGF administration has been suggested as a therapy for AD. The results of these studies continue to highlight the need for NGF stability within the delivery system and AD patient CSF, the choice of delivery system, frequency of administration, and the NGF dose for maintaining basal forebrain cholinergic neurons during AD.

The septo-hippocampal system, learning and recovery of function

We understand this review as an attempt to summarize recent advances in the understanding of cholinergic function in cognition. Such a role has been highlighted in the 1970s by the discovery that dementia patients have greatly reduced cholinergic activity in cortex and hippocampus. A brief anatomical description of the major cholinergic pathways focuses on the basal forebrain and its projections to cortex and hippocampus. From this distinction, compelling evidence suggests that the basal forebrain --> cortex projection regulates the excitability of principal cortical neurons and is thereby critically involved in attention, stimulus detection and memory function, although the biological conditions for these functions are still debated. Similar uncertainties remain for the septo-hippocampal cholinergic system. Although initial lesions of the septum caused memory deficits reminiscent of hippocampal ablations, recent and more refined neurotoxic lesion studies which spared non-cholinergic cells of the basal forebrain failed to confirm these memory impairments in experimental animals despite a near total loss of cholinergic labeling. Yet, a decline in cholinergic markers in aging and dementia still stands as the most central piece of evidence for a link between the cholinergic system and cognition and appear to provide valuable targets for therapeutic approaches.

Neuroprotection and neurogenesis: Modulation of cornus ammonis 1 neuronal survival after transient forebrain ischemia by prior fimbria-fornix deafferentation

Severe transient forebrain ischemia causes selective neuronal death in the hippocampal cornus ammonis 1 region. We tested the hypothesis that fimbria-fornix deafferentation can provide long-term protection to cornus ammonis 1 neurons and modulate neurogenesis following ischemia. Fimbria-fornix lesion or sham-fimbria-fornix lesion was performed on Wistar rats 13 days prior to 10 min forebrain ischemia or sham ischemia. Temperature was regulated and rats survived for 7, 14 or 28 days. Immunofluorescent bromodeoxyuridine and neuron specific nuclear protein staining and immunochemistry terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling staining were performed. At 7 days after ischemia, 73%+/-14% of cornus ammonis 1 neurons were damaged, while deafferentation reduced the injury to 36%+/-17% of cornus ammonis 1 neurons. This protection persisted for at least 28 days. Ischemia significantly increased the number of bromodeoxyuridine-positive cells (85-90 cells/section in stroke group vs. 6 to 11 cells/section in normal or sham stroke group), with very few terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling-stained cells adjacent to the hippocampal cornus ammonis 1. Fimbria-fornix lesioning followed by ischemia increased the percentage of new neurons 13-fold over ischemia alone and 6.5-fold over sham lesion plus ischemia. The results indicate that fimbria-fornix deafferentation provides long-term neuroprotection in cornus ammonis 1 following forebrain ischemia and promotes neurogenesis after ischemic insults.

Role of class 3 semaphorins in the development and maturation of the septohippocampal pathway

In examining the role of Class 3 secreted semaphorins in the prenatal and postnatal development of the septohippocampal pathway, we found that embryonic (E14-E16) septal axons were repelled by the cingulate cortex and the striatum. We also found that the hippocampus exerts chemorepulsion on dorsolateral septal fibers, but not on fibers arising in the medial septum/diagonal band complex, which is the source of septohippocampal axons. These data indicate that endogenous chemorepellents prevent the growth of septal axons in nonappropriate brain areas and direct septohippocampal fibers to the target hippocampus. The embryonic septum expressed np-1 and np-2 mRNAs, and the striatum and cerebral cortex expressed sema 3A and sema 3F. Experiments with recombinant semaphorins showed that Sema 3A and 3F, but not Sema 3C or 3E, induce chemorepulsion of septal axons. Sema 3A and 3F also induce growth cone collapse of septal axons. This indicates that these factors are endogenous cues for the early guidance of septohippocampal fibers, including cholinergic and gamma-aminobutyric acid (GABA)ergic axons, during the embryonic stages. During postnatal stages, when target cell selection and synaptogenesis take place, np-1 and np-2 were expressed by septohippocampal neurons at all ages tested. In the target hippocampus, pyramidal and granule cells expressed sema 3E and sema 3A, whereas most interneurons expressed sema 3C, but few expressed sema 3E or 3A. Combined tracing and expression studies showed that GABAergic septohippocampal fibers terminated preferentially onto sema 3C-positive interneurons. In contrast, cholinergic septohippocampal fibers terminated onto sema 3E and sema 3A-expressing pyramidal and granule cells. The data suggest that Class 3 secreted semaphorins are involved in postnatal development. Moreover, because GABAergic and cholinergic axons terminate onto neurons expressing distinct, but overlapping, patterns of semaphorin expression, semaphorin functions may be regulated by different signaling mechanisms at postnatal stages.

Effects of fimbria-fornix transection on calpain and choline acetyl transferase activities in the septohippocampal pathway

The ability of fimbria-fornix bilateral axotomy to elicit calpain and caspase-3 activation in the rat septohippocampal pathway was determined using antibodies that selectively recognize either calpain- or caspase-cleaved products of the cytoskeletal protein alphaII-spectrin. Radioenzymatically determined choline acetyl transferase (ChAT) activity was elevated in the septum at day 5, but reduced in the dorsal hippocampus at days 3, 5 and 7, after axotomy. Prominent accumulation of calpain-, but not caspase-3-, cleaved spectrin proteolytic fragments was observed in both the septum and dorsal hippocampus 1-7 days after axotomy. ChAT-positive neuronal cell bodies in the septum also displayed calpain-cleaved spectrin indicating that calpain activation occurred in cholinergic septal neurons as a consequence of transection of the septohippocampal pathway. Calpain-cleaved alphaII-spectrin immunoreactivity was observed in cholinergic fibers coursing through the fimbria-fornix, but not in pyramidal neurons of the dorsal hippocampus, suggesting that degenerating cholinergic nerve terminals were the source of calpain activity in the dorsal hippocampus following axotomy. Accumulation of calpain-cleaved spectrin proteolytic fragments in the dorsal hippocampus and septum at day 5 after axotomy was reduced by i.c.v. administration of two calpain inhibitors. Calpain inhibition partially reduced the elevation of ChAT activity in the septum produced by transection but failed to decrease the loss of ChAT activity in the dorsal hippocampus following axotomy. These findings suggest that calpain activation contributes to the cholinergic cell body response and hippocampal axonal cytoskeletal degradation produced by transection of the septohippocampal pathway.

Nerve growth factor signaling, neuroprotection, and neural repair

Nerve growth factor (NGF) was discovered 50 years ago as a molecule that promoted the survival and differentiation of sensory and sympathetic neurons. Its roles in neural development have been characterized extensively, but recent findings point to an unexpected diversity of NGF actions and indicate that developmental effects are only one aspect of the biology of NGF. This article considers expanded roles for NGF that are associated with the dynamically regulated production of NGF and its receptors that begins in development, extends throughout adult life and aging, and involves a surprising variety of neurons, glia, and nonneural cells. Particular attention is given to a growing body of evidence that suggests that among other roles, endogenous NGF signaling subserves neuroprotective and repair functions. The analysis points to many interesting unanswered questions and to the potential for continuing research on NGF to substantially enhance our understanding of the mechanisms and treatment of neurological disorders.

Response of abducens internuclear neurons to axotomy in the adult cat

The highly specific projection of abducens internuclear neurons on the medial rectus motoneurons of the oculomotor nucleus constitutes an optimal model for investigating the effects of axotomy in the central nervous system. We have analyzed the morphological changes induced by this lesion on both the cell bodies and the transected axons of abducens internuclear neurons in the adult cat. Axotomy was performed by the transection of the medial longitudinal fascicle. Cell counts of Nissl-stained material and calretinin-immunostained abducens internuclear neurons revealed no cell death by 3 months postaxotomy. Ultrastructural examination of these cells at 6, 14, 24, and 90 days postaxotomy showed normal cytological features. However, the surface membrane of axotomized neurons appeared contacted by very few synaptic boutons compared to controls. This change was quantified by measuring the percentage of synaptic coverage of the cell bodies and the linear density of boutons. Both parameters decreased significantly after axotomy, with the lowest values at 90 days postlesion ( approximately 70% reduction). We also explored axonal regrowth and the possibility of reinnervation of a new target by means of anterograde labeling with biocytin. At all time intervals analyzed, labeled axons were observed to be interrupted at the caudal limit of the lesion; in no case did they cross the scar tissue to reach the distal part of the tract. Nonetheless, a conspicuous axonal sprouting was present at the caudal aspect of the lesion site. Structures suggestive of axonal growth were found, such as large terminal clubs, from which short filopodium-like branches frequently emerged. Similar findings were obtained after parvalbumin and calretinin immunostaining. At the electron microscopy level, biocytin-labeled boutons originating from the sprouts appeared surrounded by either extracellular space, which was extremely dilated at the lesion site, or by glial processes. The great majority of labeled boutons examined were, thus, devoid of neuronal contact, indicating absence of reinnervation of a new target. Altogether, these data indicate that abducens internuclear neurons survive axotomy in the adult cat and show some form of axonal regrowth, even in the absence of target connection.

Pituitary adenylate cyclase-activating polypeptide promotes the survival of basal forebrain cholinergic neurons in vitro and in vivo: comparison with effects of nerve growth factor

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a member of the vasointestinal polypeptide gene family for which neurotrophic activity has been postulated. PACAP mRNA is expressed in the developing and adult hippocampus, which is the principal target region of septal cholinergic neurons. We therefore studied the effects of PACAP on septal cholinergic neurons. In primary cultures from septum of embryonic and postnatal rats, PACAP increased the number of neurons immunohistochemically stained for the low-affinity nerve growth factor (NGF) receptor p75 and for the enzyme choline acetyltransferase (ChAT). PACAP also caused a corresponding increase in ChAT activity. In comparison, NGF had a greater effect than PACAP on the number of p75- and ChAT-positive neurons in these cultures. In vivo, following fimbria fornix transection, the number of immunohistochemically stained septal cholinergic neurons fell significantly to 18% in rats given continuous intracerebroventricular infusion of vehicle, whereas in rats given NGF the number of these neurons did not differ significantly from unoperated controls. In PACAP-treated rats the number was 48% of unoperated values, which represented a significant increase compared with vehicle-treated rats and a significant decrease compared with NGF-treated rats or unoperated controls. Double-staining experiments revealed that most ChAT-positive neurons in rat medial septum also express PACAP receptor 1. Together the results show that PACAP promotes the survival of septal cholinergic neurons in vitro, and after injury in vivo, suggesting that PACAP acts as a neurotrophic factor influencing the development and maintenance of these neurons.

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.

Pig fetal septal neurons implanted into the hippocampus of aged or cholinergic deafferented rats grow axons and form cross-species synapses in appropriate target regions

The anatomical specificity of axon growth from fetal pig septal xenografts was studied by transplanting septal cells from E30-35 pig fetuses into cholinergic deafferented (192-IgG-saporin-infused) rats or into aged rats (> 18 months). Cell suspensions (100,000 cells/microl) were injected bilaterally into the dorsal and ventral hippocampus of immunosuppressed rats (10 mg/kg/day cyclosporine A). To assess axonal growth and synapse formation, acetylcholinesterase histochemistry, an antibody to choline acetyltransferase (ChAT), and three pig-positive/rat-negative antibodies: bovine 70kD neurofilament (NF70), human low-affinity NGF receptor (hNGFr), and human synaptobrevin (hSB) were used. In rats with surviving grafts at 6 months, NF70 axonal labeling was more extensive than either ChAT or hNGFr labeling. All three markers demonstrated graft axons extending selectively through the hippocampal CA fields and the molecular layer of the dentate gyrus. Graft axons did not extend into adjacent entorhinal cortex or neocortex. The distribution of pig hSB-positive synapses correlated with AChE-positive fiber outgrowth in to the host. Electron microscopic analysis of hSB-immunostained hippocampal sections revealed pig presynaptic terminals in contact with normal rat postsynaptic structures in the CA fields and the dentate gyrus. These data demonstrate target-appropriate growth of pig cholinergic axons and the formation of cross-species synapses in the deafferented or aged rat hippocampus.

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.

Recovery of spatial performance in the Morris water maze following bilateral transection of the fimbria/fornix in rats

The present study investigated whether spatial performance in the Morris water maze (MWM) recovers after bilateral transection of the fimbria/fornix (FF) in rats, whether such recovery results from restored or residual spatial cognitive capacity, and what contribution, if any, pre-operative training makes to such recovery. Following surgery, rats were administered extensive training to a constant submerged platform location with frequent probe tests to assess performance strategies. Following the attainment of asymptotic performance levels, rats were tested for acquisition of a second platform location. FF lesions were found to produce a severe impairment both in pre-operatively trained rats (a retention or retrieval deficit) and in naive rats (an acquisition deficit) as shown by the use of indirect routes to the platform on submerged platform trials and an absence of localized searching in the platform's area on probe trials. However, with further training, performance recovered in both groups, such that they eventually used direct escape routes to the submerged platform and showed highly localized searching in its area on probe trials. When tested for acquisition of a second platform location, a substantial deficit reappeared, but was again overcome with additional training. Pre-operative training was found to attenuate the initial post-operative deficit and speed recovery of performance but did not affect asymptotic performance levels nor acquisition of the second platform location. These data show that, though spatial cognition as assessed in the MWM is impaired by FF lesions, spatial performance eventually recovers. Moreover, pre-operative training, though of some initial post-operative benefit, is not essential for this recovery. The deficit shown in acquisition of the second platform location argues against recovery of spatial cognition and suggests that the basis of recovered performance is residual spatial cognitive capacity. Several limitations of this residual capacity are apparent: (i) rate of acquisition of spatial information is reduced; (ii) utilization of spatial information stored pre-operatively is restricted; and (iii) translation of spatial information into navigational behaviour is less efficient. The neural bases of this residual system are speculated to include spared intra-hippocampal storage mechanisms and/or mechanisms involved in extra-hippocampal long-term memory consolidation while the neural bases of the FF's contribution to spatial information storage in the intact brain are speculated to involve theta synchronization of hippocampal activity and the induction and expression of hippocampal long-term potentiation.

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.

Medial septal cholinergic neurons express c-Jun but do not undergo DNA fragmentation after fornix-fimbria transections

We investigated the expression of inducible transcription factors (ITFs) and the fate of medial septal (MS) cholinergic neurons following fornix fimbria (FF) transection c-Jun, but not c-Fos or Krox 24 was induced in nerve growth factor receptor-immunoreactive (NGFr-ir), parvalbumin-negative MS neurons by 48 h and still highly expressed 2 months after transection. JunD was expressed only at 48 h after transection. Levels of choline acetyl transferase immunoreactivity (ChAT-ir) and NGFr-ir decreased substantially 7 and 14 days respectively following FF transection and remained depressed for up to 2 months. We also investigated other measures of nerve cell death and found that there was a time-dependent loss of cresyl violet staining, but no evidence of DNA fragmentation, acidophilia or clusterin expression in the MS region. There was however, good evidence of microglial activation and astrocyte hypertrophy in the MS. These results suggest that axotomized c-Jun-positive septohippocampal neurons lose their cholinergic phenotype but do not die for up to 2 months after FF transection. The function of c-Jun in axotomized MS neurons remains a mystery, but c-Jun expression alone is clearly not sufficient to elicit death of these neurons.

Neurotrophin-4/5 maintains the cholinergic phenotype of axotomized septal neurons

We examined the effect of intraseptal or intracerebroventricular (i.c. v.) infusions of NT-4/5 or intraseptal infusions of NGF on the level of immunohistochemical staining of choline acetyltransferase (ChAT)and the low-affinity nerve growth factor receptor (LNGFR)in the rat medial septum following unilateral transection of the fimbria. The extent of cell loss in the septum ipsilateral to the lesion, determined by cell counts of ChAT-immunopositive neurons and expressed as a ratio comparing the lesioned to the intact sides, was 0.28 in animals that received an infusion of phosphate-buffered saline (PBS). The ratios were 0.97 and 1.07 in animals that received an infusion of NT-4/5 into the ipsilateral ventricle and septum respectively. Septal infusions of NGF produced a ratio of ChAT-immunopositive cells of 1.03. The ratios of LNGFR-immunopositive neurons increased from 0.50 in PBS-infused animals to 0.79 and 0.83 in animals infused with NT-4/5 via the i.c. v. infusion of NT-4/5 or septal infusion of NT-4/5 or NGF. As determined by immunohistochemical staining, NT-4/5 infused into the lateral ventricle was detected in the periventricular portions of the forebrain ipsilateral to the infusion, while NT-4/5 or NGF infused intraseptally was detected in much of the septum, bilaterally. Furthermore, exogenous NT-4/5 or NGF was detected in numerous neuronal perikarya in the medial septal and diagonal band nuclei. These data demonstrate that, as with NGF, i.c.v. as well as septal infusions of NT-4/5 can maintain the phenotype of basal forebrain cholinergic neurons following axotomy.

Axonal regeneration and limited functional recovery following hippocampal deafferentation

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

NGF mRNA is expressed by GABAergic but not cholinergic neurons in rat basal forebrain

Nerve growth factor (NGF) supports the survival and biosynthetic activities of basal forebrain cholinergic neurons and is expressed by neurons within lateral aspects of this system including the horizontal limb of the diagonal bands and magnocellular preoptic areas. In the present study, colormetric and isotopic in situ hybridization techniques were combined to identify the neurotransmitter phenotype of the NGF-producing cells in these two areas. Adult rat forebrain tissue was processed for the colocalization of mRNA for NGF with mRNA for either choline acetyltransferase, a cholinergic cell marker, or glutamic acid decarboxylase, a GABAergic cell marker. In both regions, many neurons were single-labeled for choline acetyltransferase mRNA, but cells containing both choline acetyltransferase and NGF mRNA were not detected. In these fields, virtually all NGF mRNA-positive neurons contained glutamic acid decarboxylase mRNA. The double-labeled cells comprised a subpopulation of GABAergic neurons; numerous cells labeled with glutamic acid decarboxylase cRNA alone were codistributed with the double-labeled neurons. These data demonstrate that in basal forebrain GABAergic neurons are the principal source of locally produced NGF.

Nerve growth factor prevents apoptotic cell death in injured central cholinergic neurons

Experimental lesions have been widely used to induce neuronal degeneration and to test the ability to trophic molecules to prevent lesion-induced alterations, but these studies have not demonstrated unequivocally that afflicted neurons die as a result of these manipulations. The documentation of neuronal death in the above-described models and the time when it occurs after injury are crucial for the interpretation of trophic effects. In the present study, we combined multiple approaches to investigate the nature of retrograde neuronal changes in cholinergic neurons of the medial septal nucleus (MSN) after complete, unilateral transection of the fimbria-fornix (F-F). Projections neurons of the MSN were prelabeled with the fluorescent tracer Fluoro-gold (FG) 1 week prior to lesion. By counting both FG-labeled and choline acetyltransferase (ChAT)-immunoreactive neurons in the MSN at multiple time points postaxotomy, we differentiated the phenotypic response to injury from the degenerative process and established a critical time between the third and fourth weeks postaxotomy, during which approximately 50% of fluorescent perikarya disappear. Working in the previous time window, we identified dying cells by electron microscopy (EM) and terminal transferase-mediated (TdT) deoxyuridine triphosphate (d-UTP)-biotin nick end labeling (TUNEL) and showed that MSN neurons die via apoptosis, beginning at 16 days postaxotomy. An additional group of animals was allowed to survive for 1 month (i.e., 10 days after cell death has been completed); during this period, animals were treated with intraventricular nerve growth factor (NGF). Quantitative analysis of surviving cholinergic perikarya showed that NGF prevented degeneration of the majority of neurons. In concert, the results of the present study establish that NGF does not merely protect the phenotype but also prevents cell death in lesioned central cholinergic neurons.

Cholinergic and GABAergic neurons in the nucleus basalis region of young and aged rats

Antibodies directed against choline acetyltransferase and glutamic acid decarboxylase were used in combination with recently developed stereological techniques to quantify changes in cholinergic, GABAergic, and total neuron number (Nissl-stain) within adjacent tissue sections through the horizontal limb/nucleus basalis in young (3 months, n = 6) and aged (27 months, n = 6) Fischer-344 male rats. Unbiased estimates of total neuron number within these regions were produced using a three-dimensional optical probe, the optical disector, in combination with a systematic random sampling scheme. Estimates of cell counts in immunostained tissue sections were conducted throughout the entire horizontal limb/nucleus basalis region. A significant 30% decrease in both cholinergic and total neuron number was detected in the aged animals; GABAergic neuron number remained unchanged. Total neuron number was significantly correlated with both cholinergic (r = 0.94) and glial cell number (r = 0.63), but not with GABAergic cell number. Based on neuron counts within an individual thick tissue section, the cholinergic neurons comprised only 11-15% of all neurons in the nucleus basalis of young and aged animals. Cholinergic neuron loss accounted for only 20% of the total age-related neuron loss within the horizontal limb/nucleus basalis in Fischer-344 male rats. These results indicate that age-related cholinergic neuron loss within the basal forebrain is reflected in reductions in total neuron number; however, GABAergic neurons, many of which project to the cortex, are unaffected by age. The magnitude of the age-related total neuron loss cannot be entirely accounted for by cholinergic cell loss. Therefore, an unidentified non-cholinergic, non-GABAergic component within the basal forebrain is also lost during aging and may contribute to the cognitive deficits previously ascribed to cholinergic dysfunction.

The role of inducible transcription factors in apoptotic nerve cell death

Recent studies have shown that certain types of nerve cell death in the brain occur by an apoptotic mechanism. Researchers have demonstrated that moderate hypoxic-ischemic (HI) episodes and status epilepticus (SE) can cause DNA fragmentation as well as other morphological features of apoptosis in neurons destined to die, whereas more severe HI episodes lead to neuronal necrosis and infarction. Although somewhat controversial, some studies have demonstrated that protein synthesis inhibition prevents HI-and SE-induced nerve cell death in the brain, suggesting that apoptotic nerve cell death in the adult brain is de novo protein synthesis-dependent (i.e., programmed). The identity of the proteins involved in HI-and SE-induced apoptosis in the adult brain is unclear, although based upon studies in cell culture, a number of potential cell death and anti-apoptosis genes have been identified. In addition, a number of studies have demonstrated that inducible transcription factors (ITFs) are expressed for prolonged periods in neurons undergoing apoptotic death following HI and SE. These results suggest that prolonged expression of ITFs (in particular c-jun) may form part of the biological cascade that induces apoptosis in adult neurons. These various studies are critically discussed and in particular the role of inducible transcription factors in neuronal apoptosis is evaluated.

Ciliary neurotrophic factor supports p75NGFR-immunoreactive non-cholinergic, but not cholinergic, developing septal neurons in vitro

Ciliary neurotrophic factor is known to exert both survival and differentiative actions on a number of neuronal populations of the peripheral and central nervous systems. In this study we have compared the trophic effects of ciliary neurotrophic factor and nerve growth factor on developing septal neurons of the rat in vitro. Fetal septal neurons were grown in vitro under glass coverslips in sandwich culture. Septal cultures grown for 14 days in the continual presence of nerve growth factor contain a population of cholinergic neurons that stain intensely for the low-affinity nerve growth factor receptor (p75NGFR), choline acetyltransferase and acetylcholinesterase. Without added nerve growth factor, few neurons stain for these markers. Ciliary neurotrophic factor addition for 14 days from plating in the absence of exogenous nerve growth factor results in the appearance of a population of neurons that stains for p75NGFR. This population is similar in number to that seen in nerve growth factor-treated cultures but is not immunoreactive for choline acetyltransferase and is significantly smaller in mean cross-sectional area. Delayed addition of nerve growth factor to ciliary neurotrophic factor-supported cultures at 14 days for a further seven days fails to induce choline acetyltransferase immunoreactivity in these p75NGFR-positive septal neurons. In cultures grown in the continual presence of nerve growth factor from plating, removal of nerve growth factor and addition of nerve growth factor antibodies at 14 days results in the death of over 80% of the cholinergic neurons after a further four days. Addition of ciliary neurotrophic factor during the period of nerve growth factor withdrawal appears to preserve a p75NGFR-positive, choline acetyltransferase-negative neuronal population. However, seven day re-addition of nerve growth factor to ciliary neurotrophic factor-treated, nerve growth factor-withdrawn cultures fails to induce choline acetyltransferase immunoreactivity in the ciliary neurotrophic factor-supported p75NGFR-positive septal neurons. Simultaneous treatment of cultures with both ciliary neurotrophic factor and nerve growth factor for 14 days from plating approximately doubles the number of p75NGFR-positive neurons relative to cultures treated with either ciliary neurotrophic factor or nerve growth factor alone, but the number of choline acetyltransferase-positive neurons in these cultures is not significantly greater than that found in cultures treated solely with nerve growth factor. These results suggest that ciliary neurotrophic factor does not support the survival and differentiation of developing septal cholinergic neurons in vitro, but can support the development of a p75NGFR-immunoreactive population of non-cholinergic septal neurons.

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.

Maintaining the neuronal phenotype after injury in the adult CNS. Neurotrophic factors, axonal growth substrates, and gene therapy

Multiple genetic and epigenetic events determine neuronal phenotype during nervous system development. After the mature mammalian neuronal phenotype has been determined it is usually static for the remainder of life, unless an injury or degenerative event occurs. Injured neurons may suffer one of three potential fates: death, persistent atrophy, or recovery. The ability of an injured adult neuron to recover from injury in adulthood may be determined by events that also influence neuronal phenotype during development, including expression of growth-related genes and responsiveness to survival and growth signals in the environment. The latter signals include neurotrophic factors and substrate molecules that promote neurite growth. Several adult CNS regions exhibit neurotrophic-factor responsiveness, including the basal forebrain, entorhinal cortex, hippocampus, thalamus, brainstem, and spinal cord. The specificity of neurotrophic-factor responsiveness in these regions parallels patterns observed during development. In addition, neurons of several CNS regions extend neurites after injury when presented with growth-promoting substrates. When both neurotrophic factors and growth-promoting substrates are provided to adult rats that have undergone bilateral fimbria-fornix lesions, then partial morphological and behavioral recovery can be induced. Gene therapy is one useful tool for providing these substances. Thus, the mature CNS remains robustly responsive to signals that shape nervous system development, and is highly plastic when stimulated by appropriate cues.

Fine structure of rat septohippocampal neurons. III. Recovery of choline acetyltransferase immunoreactivity after fimbria-fornix transection

Most cholinergic projection neurons in the medial septal nucleus (MS) lose their capability to synthesize choline acetyltransferase (ChAT) after axotomy by bilateral fimbria-fornix transection. We have recently shown that identified septohippocampal neurons survive axotomy up to 10 weeks and display fine-structural characteristics of cells in control rats. However, the fate and functional role of these neurons remained unclear. Here we describe observations made in rats which survived axotomy for 6 months. Adult Sprague-Dawley rats were subjected to bilateral transection of the fimbria-fornix system. In some animals septohippocampal projection neurons were labeled by the retrograde fluorescent tracer Fluoro-Gold (FG) prior to axotomy. After varying survival times following fimbria-fornix transection, the animals were fixed and sections of the septal region immunostained for ChAT. Three weeks postlesion, the number of ChAT-positive cells in the MS was reduced to 19% of control, suggesting a severe neuronal loss. However, 10 weeks and 6 months after axotomy this value increased to 28% and 54%, respectively. Fine-structural analysis of ChAT-positive neurons after 6 months survival revealed all characteristics of vital cells including normal input synapses. The majority of these cells could be identified as former septohippocampal projection neurons by the presence of FG. We conclude that many neurons in the MS have the capacity to restore their transmitter synthesis in a long-lasting process following axotomy.

Fluid percussion injury causes loss of forebrain choline acetyltransferase and nerve growth factor receptor immunoreactive cells in the rat

Memory dysfunction is a common sequela of human traumatic brain injury (TBI). Cholinergic forebrain neurons are recognized for their role in memory. We tested the hypothesis that forebrain cholinergic neurons are vulnerable to fluid percussion injury (FPI), a model of human TBI. Rodents were subjected to a moderate parasagittal FPI, sham injury, or fimbria/fornix axotomy and then killed 10 days after the procedure. Additional animals underwent FPI or sham injury and were killed 7, 14, and 28 days after the procedure. Neurons in the medial septal nucleus and vertical limb of the nucleus of the diagonal band of Broca were identified and quantitated using choline acetyltransferase (ChAT) and low affinity nerve growth factor receptor (NGF-R) immunohistochemistry. Our results showed a significant decrease in ChAT (17% +/- 5%) and NGF-R (24% +/- 8%) immunoreactive cells in FPI animals killed after 10 days when compared to sham-injured animals. Animals undergoing fimbria/fornix axotomy showed a greater reduction in ChAT (53% +/- 13%) and NGF-R (55% +/- 5%) immunoreactive cells 10 days postaxotomy. The number of ChAT and NGF-R immunoreactive neurons was reduced at all time points. However, statistical significance was present 10 and 14 days postinjury for ChAT immunoreactive neurons and 10 days only for NGF-R immunoreactive neurons. These studies have shown that FPI produces transient loss of ChAT and NGF-R immunoreactive neurons.

Ventral root avulsion: an experimental model of death of adult motor neurons

The present study proposes a reproducible model of experimental degeneration of adult motor neurons in the rat. Avulsion of ventral roots in the adult lumbar cord transects motor axons at the root exit and leads to retrograde cell death of 80% of motor neurons 2 weeks later; this result follows a series of retrograde changes, including chromatolysis, loss of transmitter phenotype, and accumulation of phosphorylated neurofilaments in perikarya. Glial cells recruited at the site of retrograde injury express both microglia-specific epitopes (as exemplified by OX-42 immunoreactivity) and macrophage-specific markers (e.g., ED-1 immunoreactivity). Macrophage-specific markers become particularly intense 7 days postaxotomy and provide additional evidence of active phagocytosis of injured neurons. Ventral root avulsion is a very useful model for assessing mechanisms of motor neuron death and testing the ability of trophic factors and other agents to preserve the phenotype and promote the survival of adult motor neurons in vivo.

Expression of neurotrophin and trk receptor genes in adult rats with fimbria transections: effect of intraventricular nerve growth factor and brain-derived neurotrophic factor administration

The expression of the specific trk receptors for nerve growth factor and brain-derived neurotrophic factor (trkA and trkB) has been assayed by messenger RNA in situ hybridization in adult rats with partial fimbrial transections along with intraventricular treatment of nerve growth factor or brain-derived neurotrophic factor. In the forebrain, specific hybridization labeling for trkA messenger RNA showed an identical pattern to that of choline acetyltransferase messenger RNA, supporting the view that trkA expression is confined to the cholinergic population in the basal forebrain and the cholinergic interneurons in the striatum. After partial unilateral transections of the fimbria there was a progressive loss of choline acetyltransferase and trkA messenger RNA expression in the septal region ipsilateral to the lesion. Daily intraventricular administration of brain-derived neurotrophic factor or nerve growth factor partially prevented the lesion-induced decrease in the levels of both messengers, the latter being more effective than the former. Grain count analysis of individual cells was used to test whether the two factors upregulated choline acetyltransferase or trkA expression in individual cells surviving the lesion. Brain-derived neurotrophic factor treatment failed to induce any change in the levels of both messengers per neuron in the septal area. In contrast, daily intraventricular administration of nerve growth factor upregulated both choline acetyltransferase and trkA messenger RNA expression in individual neurons. This upregulation was evident on ipsilateral and contralateral sides, suggesting that nerve growth factor is able to upregulate these markers in intact and injured cholinergic cells in the basal forebrain. Similar to the situation in the septum, brain-derived neurotrophic factor did not upregulate choline acetyltransferase or trkA expression in the striatum. However, nerve growth factor administration strongly upregulated choline acetyltransferase messenger RNA expression by individual cholinergic neurons of the striatum. A medial to lateral gradient decrease in this upregulation was detected in the striatum ipsilateral to the side of administration, suggesting a limited diffusion of the nerve growth factor protein from the ventricle into brain parenchyma. In contrast to the strong effect on choline acetyltransferase expression, nerve growth factor treatment was ineffective in altering trkA messenger RNA in the striatum. The contrasting findings between septum and striatum suggest different regulatory mechanisms for trkA messenger RNA expression in the two cholinergic populations. Since nerve growth factor was found to upregulate the expression of its trkA receptor, we tested whether brain-derived neurotrophic factor administration had similar effects on the regulation of its trkB receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Progressive degeneration of nigrostriatal dopamine neurons following intrastriatal terminal lesions with 6-hydroxydopamine: a combined retrograde tracing and immunocytochemical study in the rat

In order to develop a rodent model displaying a progressive degeneration of the dopamine neurons of the substantia nigra, we bilaterally injected the tracer substance FluoroGold into the terminal field of the nigrostriatal projection, i.e. the striatum. One week later, rats received unilateral injections of 20 micrograms 6-hydroxydopamine into one of the two striatal tracer deposits. Groups of animals were killed one, two, four, eight and 16 weeks later. Ipsilateral to the lesion there was a progressive loss of FluoroGold-labelled nigral cells, with cell counts dropping from 96% of the contralateral side at one week to 59% at two weeks, 35% at four weeks, 23% at eight weeks and down to 15% at 16 weeks. Labelled nigral neurons ipsilateral to the lesion showed a moderate to marked atrophy at all investigated time points. The number of tyrosine hydroxylase-immunoreactive cells was decreased to 83% of contralateral at one week, 39% at two weeks, 44% at four weeks, 34% at eight weeks and 52% at 16 weeks postlesion. Rhodamine fluorescence immunocytochemistry showed that the proportion of surviving ipsilateral fluorogold-labelled cells displaying immunoreactivity for tyrosine hydroxylase was 69% at one week postlesion, 51% at two weeks, 63% at four weeks, 69% at eight weeks and 76% at 16 weeks. We conclude that injection of 6-hydroxydopamine into the terminal field of nigral dopaminergic neurons causes a progressive degeneration of these cells, starting between one and two weeks after lesion and continuing over eight to 16 weeks. This degeneration is preceded, and accompanied by, cellular atrophy and a partial loss of marker enzyme expression, thus yielding an animal model which mimics the degenerative processes in Parkinson's disease more closely than the animal models available so far. The present model may be helpful in investigating the in vivo effects of putative neuroprotective agents and neurotrophic factors.

Recovery of choline acetyltransferase activity without sprouting of the residual acetylcholine innervation in adult rat cerebral cortex after lesion of the nucleus basalis

In view of the divergent literature concerning the long-term effects of ibotenic acid lesions of the nucleus basalis of Meynert (NBM) on the choline acetyltransferase (ChAT) activity in adult rat cerebral cortex, we have critically reassessed the issue of an eventual recovery of this enzymatic activity by sprouting of the residual acetylcholine (ACh) innervation. At short (1 week) and long survival time (3 months) after unilateral ibotenic acid lesion, ChAT activity was biochemically measured in the ipsi and contralateral fronto-parietal cortex of several rats in which the extent of ACh neuronal loss in NBM was also estimated by counts of ChAT-immunostained cell bodies on the lesioned vs. non-lesioned side. In other lesioned rats, particular attention was paid to the distribution of the residual cortical ACh (ChAT-immunostained) innervation, and that of immunostained vasoactive intestinal polypeptide (VIP) axon terminals known to belong in part to intrinsic cortical ACh neurons which co-localize this peptide. One week after NBM lesion, profound decreases of ipsilateral cortical ChAT activity were tightly correlated with the extent of ACh cell body loss in the nucleus. A significant recovery of cortical ChAT activity could be documented after 3 months, despite persistence of NBM cell body losses as severe as after 1 week. At both survival times, the number of ChAT-immunostained axons was markedly reduced throughout the ipsilateral fronto-parietal cortex, demonstrating that most ACh fibers of extrinsic origin had been permanently removed. This result also indicated that the long-term recovery of ChAT activity had occurred without sprouting of the residual ACh innervation. The laminar distribution and number of VIP-immunostained terminals remained the same on the lesioned and intact side and comparable to normal, ruling out an extensive sprouting of intrinsic ACh/VIP or VIP alone fibers. The return to a near normal cortical ChAT activity in severely ACh-denervated cortex suggested that the intrinsic ACh innervation was primarily responsible for this recovery.

Specificity of 192 IgG-saporin for NGF receptor-positive cholinergic basal forebrain neurons in the rat

A monoclonal antibody to the rat nerve growth factor (NGF) receptor, 192 IgG, accumulates bilaterally and specifically in cholinergic basal forebrain (CBF) cells following intraventricular injection. An immunotoxin composed of 192 IgG linked to saporin (192 IgG-saporin) has been shown to destroy cholinergic neurons in the basal forebrain. We sought to determine if intraventricular 192 IgG-saporin affected choline acetyltransferase (ChAT) enzyme activity in the CBF terminal projection fields. ChAT assays from 192 IgG-saporin-treated animals showed significant time-dependent decreases in ChAT activity in the neocortex, olfactory bulb and hippocampus, compared to PBS- or OKT1-saporin-injected controls. ChAT and tyrosine hydroxylase activity in the striatum was always unchanged by 192 IgG-saporin. ChAT immunohistochemistry was confirmative of major cell loss in the CBF, while other cholinergic nuclei appeared unremarkable. The data provide further evidence of the selectivity of 192 IgG-saporin in abolishing cholinergic, NGF receptor-positive CNS neurons.

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.

Fine structure of rat septohippocampal neurons: I. Identification of septohippocampal projection neurons by retrograde tracing combined with electron microscopic immunocytochemistry and intracellular staining

In this report the normal dendritic organization and fine structure of identified septohippocampal projection neurons is described as a prerequisite for a time course analysis of retrograde changes in these neurons following axotomy (see Naumann et al., J. Comp. Neurol. 325:219-242, 1992). Septohippocampal projection neurons were retrogradely labeled by injection of the fluorescent tracer Fluoro-Gold into the hippocampus. Next, retrogradely labeled cells in Vibratome sections of the medial septum/diagonal band complex were intracellularly stained with the fluorescent dye Lucifer Yellow (LY). Photooxidation of LY resulted in a stable electron-dense reaction product, which allowed us to study these double-labeled neurons by electron microscopy. Another series of sections containing retrogradely labeled neurons were immunostained for choline acetyltransferase (ChAT) or parvalbumin (PARV). In this way the fine structure of two different chemically characterized subpopulations of septohippocampal neurons could be compared with that of the LY-injected neurons. Intracellular filling of retrogradely labeled neurons with LY stained the cell body and the entire dendritic arbor. Essentially, three classes of neurons could be distinguished, i.e., bipolar cells, multipolar neurons, and an intermediate group. All these neurons displayed smooth, often varicose dendrites lacking spines. Mainly located close to the midline, there was a group of cells with only very few if any LY-stained dendrites. In the electron microscope, the double-labeled neurons were easily identified by numerous electron-dense lysosomes associated with transported Fluoro-Gold and the diffuse reaction product resulting from photooxidation. They displayed fine-structural characteristics as previously described for cholinergic neurons. In fact, our fine-structural analysis of ChAT-positive Fluoro-Gold-labeled neurons, but also of back-filled PARV-positive cells, gave very similar results. All these neurons had infolded nuclei, abundant cytoplasmic organelles, and a few axosomatic synapses. Thus, a plain electron microscopic study does not allow one to distinguish between subpopulations of septohippocampal projection neurons.

LHRH neurons in the medial septal-diagonal band-preoptic area do not project directly to the hippocampus: a double-labeling immunohistochemical study

While neurons containing immunoreactive luteinizing hormone-releasing hormone (LHRH) are scattered primarily in the medial septal-diagonal band of Broca-medial preoptic area (mS-dbB-PO) complex, autoradiographic studies have demonstrated dense concentrations of LHRH receptors in the hippocampus. The route by which LHRH is transported to its hippocampal receptors is unknown. The present study was designed to test the hypothesis that LHRH-containing neurons in the mS-dbB-PO complex project to hippocampal sites containing LHRH receptors, thereby serving as a source of innervation to these receptors. Large (0.10 microliters) or small (0.02 microliters) volumes of the retrograde tracer wheat germ agglutinin (WGA) were injected unilaterally into four separate hippocampal locations in six ovariectomized female rats. In an additional five females, a 0.15 microliter volume of the retrograde tracer fluorogold (FG) was similarly injected. After a five day survival period, the animals were sacrificed. Vibratome sections of the brain were stained for both WGA and LHRH with a dual immunohistochemical technique. Since FG is a fluorescent chromagen, brains of animals injected with FG only required processing for LHRH immunofluorescence. As a positive control, some sections containing retrogradely labeled cells filled with either WGA or FG were processed for choline acetyltransferase (CHAT) immunoreactivity. The WGA and FG injections covered targeted hippocampal sites and neurons containing retrogradely transported WGA or FG were found in abundance in the mS-dbB-PO complex. In accord with previous reports, many CHAT-positive and fewer LHRH-positive neurons were found in this complex. Approximately 5-10% of the CHAT-positive neurons also contained WGA or FG; however, no neurons were found to co-localize LHRH and either of the retrograde tracers. The results indicate that LHRH neurons in the mS-dbB-PO complex do not project directly to hippocampal sites containing LHRH receptors.

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.

The extrinsic modulation of hippocampal theta depends on the coactivation of cholinergic and GABA-ergic medial septal inputs

The long trains of theta field activity recorded from the hippocampal formation of urethane-anesthetized rats are thought to be primarily dependent on cholinergic afferents originating in the medial septum/vertical limb of the diagonal band of Broca (MS/vDBB). Recent anatomical studies have revealed the existence of a septal GABA-ergic input to the hippocampal formation which synapses mainly on intrinsic GABA-ergic interneurons. The present work investigated the possibility that some form of interaction between cholinergic and GABA-ergic MS/vDBB inputs might be required for the generation of hippocampal theta field and cellular activities in urethane-anesthetized rats. Reversible inactivation of the MS/vDBB completely abolished theta field and theta-on cell activities, but "released" theta-off cells. The theta field and theta-on cell activities induced by direct intrahippocampal microinfusions of carbachol were also abolished by MS/vDBB inactivation. We speculated that septal suppression was producing two effects: 1) removing excitatory, cholinergic input; and 2) removing inhibitory control of hippocampal GABA-ergic interneurons, thereby increasing the overall level of hippocampal inhibition. Sequential administration of both carbachol and the GABA-A antagonist, bicuculline, resulted in theta-like oscillations similar to those seen in hippocampal slices bath perfused with carbachol alone. Thus, following MS/vDBB inactivation hippocampal GABA-ergic systems are overactive; this enhances intrinsic inhibition and blocks carbachol theta. By reducing the overall level of inhibition in the hippocampus with bicuculline, it is possible to reinstate its oscillatory properties. Conversely, increasing the level of inhibition in the hippocampus (with muscimol) results in the abolishment of theta field activity and the discharges of both theta-on and theta-off cells. Based on these findings we are proposing that cholinergic and GABA-ergic systems originating in the MS/vDBB act synergistically to modulate hippocampal theta. Cholinergic projections provide the afferent excitatory drive for hippocampal theta-on cells and septal GABA-ergic projections act to reduce the overall level of inhibition by inhibiting hippocampal GABA-ergic interneurons (hippocampal theta-off cells). Both activities must be present for the generation of hippocampal theta field and cellular activities. The balance between the cholinergic and GABA-ergic systems may determine whether hippocampal synchrony (theta) or asynchrony (LIA) occurs.

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.

Progressive decline in spatial learning and integrity of forebrain cholinergic neurons in rats during aging

Rats distributed over five different age groups, 3, 12, 18, 24 and 30 months of age, were screened for their spatial learning and memory ability in the Morris water maze, and the degree of place navigational impairments was correlated with morphological changes in the four major forebrain cholinergic cell groups (medial septum, MS; vertical limb of the diagonal band of Broca, VDB; nucleus basalis magnocellularis, NBM; and striatum) using choline acetyltransferase (ChAT) and nerve growth factor receptor (NGFr) histochemistry. Impaired place navigation developed progressively with age, such that 8% of the 12-month-old rats, 45% of the 18-month-old, 53% of the 24-month-old, and over 90% of the 30-month-old rats were behaviorally impaired. Significant reductions in the number of ChAT/NGFr-positive cell bodies, amounting to between 19 and 45%, were observed in all four cell groups, and the remaining cells were reduced in size (6-24% reduction in cross-sectional area in the oldest age groups). Although the morphological changes were less severe and tended to develop later than the behavioral impairments, there was overall a significant correlation between water maze performance and ChAT/NGFr-positive cell counts, and to a lesser degree also cell size in all four cell groups. These changes were also highly correlated with age. The highest correlations were seen in MS, VDB and NBM, which are known to play a role in spatial memory performance in young rats. The results indicate that degenerative and/or atrophic changes in the forebrain cholinergic system and decline in spatial learning ability are parallel processes during aging. Although the magnitude of the morphological changes does not appear to be substantial enough, by itself, to explain the severe spatial learning impairments that develop in the oldest animals, the present data are consistent with the view that impaired function in the forebrain cholinergic system can contribute to age-dependent cognitive decline in rodents.

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.