Responses of young and aged rat CNS to partial cholinergic immunolesions and NGF treatment

Do two models of acute and chronic stress stimulation influence the amount of nerve growth factor (NGF) and its receptor TrkA in the hippocampal neurons of middle aged rats?

Our study aimed to explore the influence of two different stressors: acute (once for 15 min) and chronic (15 min daily for 21 days) exposure to high light open field (HL-OF) or forced swim (FS) on the density of nerve growth factor (NGF) and tyrosine kinase A (TrkA) immunoreactive neurons in the hippocampal CA1 and CA3 pyramidal cell layers and dentate gyrus (DG) granule cell layer in middle aged (360 days old; P360; P, postnatal day) rats. In contrast to non-stressed animals, acute HL-OF stimulation resulted in an increase (p<0.001) in the density of NGF-ir cells in CA1, CA3, DG, whereas chronic HL-OF produced no changes in all hippocampal regions. The rats which underwent acute and chronic FS tests showed no statistically significant differences in the density of NGF-ir containing cells in the CA1, CA3, and DG subfields compared with control rats. Except for DG, where after 21 days of FS the density of TrkA-ir neurons was found to increase (p<0.05) in comparison to unstressed rats, no changes were noted in the density of TrkA-ir in the studied hippocampal structures as a result of acute and chronic HL-OF or FS exposure. These results indicate that acute HL-OF stress stimulation was the only factor inducing changes in the density of NGF-ir containing neurons in the hippocampal CA1, CA3, and DG of middle aged rats. In respect of the density of NGF-ir and TrkA-ir cells in the hippocampal structures, prolonged exposure to HL-OF or FS stressors did not constitute an aggravating factor for rats in the studied ontogenetic period.

Selective loss of basal forebrain cholinergic neurons by 192 IgG-saporin is associated with decreased phosphorylation of Ser glycogen synthase kinase-3beta

Glycogen synthase kinase-3beta (GSK-3beta) is a multifunctional enzyme involved in a variety of biological events including development, glucose metabolism and cell death. Its activity is inhibited by phosphorylation of the Ser9 residue and up-regulated by Tyr216 phosphorylation. Activated GSK-3beta increases phosphorylation of tau protein and induces cell death in a variety of cultured neurons, whereas phosphorylation of phosphatidylinositol-3 (PI-3) kinase-dependent protein kinase B (Akt), which inhibits GSK-3beta activity, is one of the best characterized cell survival signaling pathways. In the present study, the cholinergic immunotoxin 192 IgG-saporin was used to address the potential role of GSK-3beta in the degeneration of basal forebrain cholinergic neurons, which are preferentially vulnerable in Alzheimer's disease (AD) brain. GSK-3beta co-localized with a subset of forebrain cholinergic neurons and loss of these neurons was accompanied by a transient decrease in PI-3 kinase, phospho-Ser473Akt and phospho-Ser9GSK-3beta levels, as well as an increase in phospho-tau levels, in the basal forebrain and hippocampus. Total Akt, GSK-3beta, tau and phospho-Tyr216GSK-3beta levels were not significantly altered in these brain regions in animals treated with 192 IgG-saporin. Systemic administration of the GSK-3beta inhibitor LiCl did not significantly affect cholinergic marker or phospho-Ser9GSK-3beta levels in control rats but did preclude 192-IgG saporin-induced alterations in PI-3 kinase/phospho-Akt, phospho-Ser9GSK-3beta and phospho-tau levels, and also partly protected cholinergic neurons against the immunotoxin. These results provide the first evidence that increased GSK-3beta activity, via decreased Ser9 phosphorylation, can mediate, at least in part, 192-IgG saporin-induced in vivo degeneration of forebrain cholinergic neurons by enhancing tau phosphorylation. The partial protection of these neurons following inhibition of GSK-3beta kinase activity suggests a possible therapeutic role for GSK-3beta inhibitors in attenuating the loss of basal forebrain cholinergic neurons observed in AD.

Methanesulfonyl fluoride, an acetylcholinesterase inhibitor, attenuates simple learning and memory deficits in ischemic rats

Methanesulfonyl fluoride (MSF), a highly selective CNS inhibitor of acetylcholinesterase, has been recently demonstrated to promote improvement in cognitive performance in patients with senile dementia of Alzheimer type. Because a similar cognitive impairment may accompany stroke, we investigated in the present study whether treatment with MSF could produce beneficial effects in adult rats subjected to an experimental stroke model. Sprague-Dawley rats received transient 60 min intraluminal occlusion of the right middle cerebral artery (MCAo) and were given i.p. injections of either MSF (1 mg/kg at 24 and 48 h post-MCAo and 0.3 mg/kg thereafter every other day) or the vehicle, peanut oil, for 4 weeks. Behavioral tests and biochemical assays were performed at 28 days post-surgery. MSF treatment produced about 90% inhibition of acetylcholinesterase in the brain. Ischemic animals that received the vehicle displayed significant elevated body swing biased activity (84.8 +/- 10%) and significantly prolonged acquisition (398 +/- 62 s) and shortened retention (79 +/- 26 s) of the passive avoidance task. Interestingly, while the ischemic animals that received the MSF exhibited elevated body swing biased activity (87.7 +/- 8%), they performed significantly better in the passive avoidance task (255 +/- 36 s and 145 +/- 18 s in acquisition and retention) than the vehicle-treated animals. Moreover, whereas brains from both groups of animals revealed similar extent and degree of cerebral infarction, the MSF-treated ischemic animals showed more intense immunoreactivity, as well as a significantly higher number (10-15% increase) of septal choline acetyltransferase-positive cells than the vehicle-treated ischemic animals. These results show that MSF, possibly by preserving a functional cholinergic system, attenuated stroke-induced deficits in a simple learning and memory task.

Hippocampal neurotrophin levels after injury: Relationship to the age of the hippocampus at the time of injury

Aging impairs the competence of the hippocampus for synaptic reorganization after injury. This potentially is due to the inability of the aging hippocampus to up-regulate the critical neurotrophic factors for prolonged periods after injury to levels at which they can stimulate neurite outgrowth and facilitate synaptic reorganization. We hypothesize that the concentrations of neurotrophins in the hippocampus after injury depend on the age at the time of injury. We quantified the concentrations of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin-3 (NT-3) in the hippocampus of young, middle-aged, and aged Fischer 344 rats at 4 days after kainic acid (KA)-induced injury. In comparison with the age-matched intact hippocampus, the KA-lesioned hippocampus exhibited increased levels of BDNF and NGF in all three age groups. In contrast, the NT-3 concentration was unaltered after KA lesion. Notwithstanding similar percentage increases in BDNF after injury, the lesioned middle-aged and aged hippocampus contained 45-52% less BDNF than the lesioned young hippocampus. NGF and NT-3 levels after injury were comparable across the three age groups, however. Furthermore, lower BDNF concentration in the injured aging hippocampus was associated with normal astrocytic response but significantly diminished microglial reaction. Thus, in comparison with the injured young hippocampus, the injured aging hippocampus contains considerably less BDNF but similar levels of NGF and NT-3. Lower BDNF levels in the injured aging hippocampus might underlie the diminished spontaneous healing response observed in the aging hippocampus after injury, particularly in terms of synaptic reorganization and dentate neurogenesis.

Behavioral Effects of Basal Forebrain Cholinergic Lesions in Young Adult and Aging Rats

The interactive effects of age and cholinergic damage were assessed behaviorally in young and middle-aged rats. Rats were lesioned at either 3 or 17 months of age by injection of 192 IgG-saporin immunotoxin into the medial septum and the nucleus basalis magnocellularis, and they were then tested on a range of behavioral tasks: a nonmatching-to-position task in a T-maze, an object-recognition task, an object-location task, and an open-field activity test. Depending on the task used, only an age or a lesion effect was observed, but there was no Age X Lesion interaction. Middle-aged and young rats responded to the cholinergic lesions in the same manner. These results show that in the middle-aged rats in which cholinergic transmission was affected, additional injury to the system was not always accompanied by major cognitive dysfunctions.

NGF acts via p75 low-affinity neurotrophin receptor and calpain inhibition to reduce UV neurotoxicity

The relative roles of the high-affinity nerve growth factor (NGF) receptor, TrkA, and low-affinity p75 neurotrophin receptor (p75NTR) in neuronal survival are an active research area. We reported previously that UV treatment induces a calpain-dependent, delayed neuronal death. We show here that NGF inhibits this UV-induced cortical neuron death. Interestingly, NGF neuroprotection requires p75NTR. Because it has been reported that NGF binding to p75NTR leads to ceramide generation, we evaluated whether ceramide was also neuroprotective. We found that ceramide also inhibits UV toxicity, and that the actions of ceramide and NGF were not additive. Moreover, cycloheximide inhibited ceramide and NGF neuroprotection, suggesting that their actions require new protein synthesis. Consistent with this possibility, we found that NGF activates the expression of genes such as calbindin. Lastly, we explored the role of calpain in NGF actions. NGF and ceramide both reduced the level of calpain activation after UV treatment. This NGF effect was p75NTR dependent. Overall, we interpret these results as consistent with an NGF neuroprotective pathway wherein p75NTR activation leads sequentially to ceramide generation, new protein synthesis, and inhibition of calpain activation. Overall, these results provide insight into a p75NTR dependent pathway of NGF neuroprotection.

Pretraining or previous non-spatial experience improves spatial learning in the Morris water maze of nucleus basalis lesioned rats

Previous experiments have shown that infusions of ibotenic acid in the nucleus basalis magnocellularis (NBM) induce a strong impairment in spatial navigation for a hidden platform in the Morris water maze. This effect was initially attributed to a cholinergic deficit, but later studies showed that performance level did not correlate with the degree of cholinergic denervation. Therefore, this impairment is due to a combined cholinergic and non-cholinergic deficit. However, it is not clear in which particular processes the NBM is involved. In this study we have evaluated the origin of behavioural impairment in spatial navigation in the water maze after an ibotenic acid-induced lesion of NBM. In the first experiment, Wistar rats were trained preoperatively in an allocentric navigation task. Postoperatively, they were tested in the same task. All lesioned animals showed a performance level similar to controls. Lesions did not impede the acquisition of new positions in the water maze, nor did affect the ability of animals to remember new platform positions after an intertrial interval of 20s, even if animals had received only allocentric experience with the platform position, or allocentric and path integration information concurrently. Lesions also failed to affect the ability to locate a hidden platform in a new environment. However, hippocampal infusions of scopolamine (5 microg) produced a severe impairment in NBM-damaged animals, without impairing performance of controls. In the second experiment Wistar rats with the same lesion were first trained in a visual-guided task in the water maze, and subsequently evaluated in the spatial task. In both tasks lesioned animals were not different from controls. These results suggest that the NBM played an important role during acquisition phases but not in the execution of spatial navigation. Moreover, the excessive emotional response displayed by lesioned animals is postulated as a relevant cause for the impairment observed in spatial navigation after NBM damage.

The influence of ageing on the insulin signalling system in rat lacrimal and salivary glands

PURPOSE

Ageing adversely affects the structure and function of lacrimal and salivary glands (LG and SG) and leads to marked insulin resistance that correlates with reduced insulin signal transduction. The aim of this study was to investigate whether ageing affects insulin signal transduction in LG and SG in vivo.

METHODS

Male Wistar rats aged 20 months and 2 months (control group) were compared (n=8/group). Samples were removed under anaesthesia after i.v. injection of insulin, homogenized, immunoprecipitated with anti-insulin receptor (IR), Shc and STAT-1 antibodies and immunoblotted with antiphosphotyrosine antibody.

RESULTS

The 20-month-old rats were significantly hyperinsulinaemic and presented a reduced rate of blood glucose disappearance in response to insulin, compared to the 2-month-old rats. The level of phosphorylation determined by densitometry in the older group of rats showed that ageing significantly reduced insulin-induced IR phosphorylation in LG and SG and STAT-1 phosphorylation in SG, compared to in the control group, but did not alter Shc phosphorylation.

CONCLUSIONS

Ageing influences insulin signal transduction in the LG and SG of rats. Considering the major anabolic actions of insulin, these observations may help to explain the mechanisms of LG and SG dysfunctions observed in ageing.

Attenuated transcriptional responses to oxidative stress in the aged rat brain

The aged nervous system displays impaired cognitive functions, and these impairments are exacerbated in several neurodegenerative diseases. A role for oxidative stress has been suggested for several of these age-associated dysfunctions. In addition, recovery from more acute traumatic insults that also generate oxidative stress is impaired in the aged. Here we examine the response of aged rat hippocampi to normobaric hyperoxia treatments and demonstrate an attenuation in the DNA binding activity of the AP-1 and nuclear factor-kappa B transcription factors, which are important components of stress response signal transduction pathways and can determine shifts in cellular commitments to necrosis, apoptosis, or functional recovery in the central nervous system.

Impaired spatial learning in aged rats is associated with loss of p75-positive neurons in the basal forebrain

We investigated age-related changes in the number and size of neurons positive for the p75 neurotrophin receptor in the cholinergic basal forebrain of female Dark Agouti rats. Since the integrity of these neurons is known to be closely associated with performance in tests of spatial learning ability, we also investigated the incidence of age-related spatial learning impairments, using the Barnes maze. Spatial learning impairments occurred with increasing frequency with age. No rats showed impairment at six months, but 50% were impaired at 14 months and 71% at 26 months. There was no correlation between age and decreased number of p75-positive neurons in the rostral basal forebrain, which consists of the medial septum and vertical limb of the diagonal band of Broca. In the caudal basal forebrain, which consists of the horizontal limb and the nucleus of Meynert, there was a 13% reduction in the number of p75-positive neurons at 17 months compared to six months, and a 30% reduction at 26 months. There was a strong correlation between the presence of spatial learning impairment and a reduction in the number of p75-positive neurons. This correlation was most evident in the rostral basal forebrain, but was also present in the caudal basal forebrain. In the rostral basal forebrain, all learning impaired rats had fewer p75-positive neurons than the average number in unimpaired rats. A close correspondence between the presence of p75 and choline acetyltransferase was evident in basal forebrain neurons of learning-impaired and unimpaired rats. Gross pathological changes to the morphology of p75-positive neurons were relatively frequent in learning-impaired rats. These changes consisted of hypertrophy, appearance of vacuoles, and marginalisation of the cytoplasm. The results indicate the susceptibility of p75-positive neurons to degenerative changes with aging, and show that the loss of these neurons in the basal forebrain was strongly correlated with impairment in spatial learning.

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.

Behavioral training-induced c-Fos expression in the rat nucleus basalis of Meynert during aging

The present study investigated the behavioral training-induced c-Fos expression in the nucleus basalis of Meynert (nbM) in differently aged rats. This study demonstrated that the c-Fos expression in nbM was significantly increased and the peak occurred at 2 h after dark-avoidance training. Although the increase of c-Fos expression was also observed after pseudotraining, the number of Fos-like immunoreactive neurons in pseudotrained rats was significantly less than that in dark-avoidance trained rats at each time-point. This result suggested that c-Fos expression might be involved in learning and memory processes. In addition, all the pseudotraining-, training- and memory arousing-induced c-Fos expression was decreased with increasing age, and the decrease was more notable in trained and memory aroused rats. This suggested that the total number of nbM neurons and/or the sensitivity of nbM neurons to experimental manipulations, especially learning and memory performance, might reduce during aging.

Par-4 induces cholinergic hypoactivity by suppressing ChAT protein synthesis and inhibiting NGF-inducibility of ChAT activity

Profound reductions in choline acetyl-transferase (ChAT) activity are reliable markers for cholinergic hypoactivity associated with cognitive function deficit in Alzheimer's disease (AD). Par-4 (prostate apoptosis response-4) is a novel mediator of neuronal apoptosis associated with the pathogenesis of AD. Par-4 contains a leucine zipper domain (Leu.zip) that presumably mediates protein-protein interactions critical for its functions in apoptosis. Par-4 activity can be effectively blocked by overexpression of Leu. zip because it exerts a dominant negative action possibly by competitively blocking the interaction of Par-4 with other proteins. Whether Par-4 participates in regulation of cholinergic signaling has not been determined. We report that overexpression of Par-4 results in apoptotic and non-apoptotic reductions in ChAT activity in transfected PC12 cells following exposure to a toxic concentration (50 microM) of aggregated amyloid beta peptide 1-42 (Abeta 1-42) and a non-toxic concentration (1 microM) of soluble Abeta 1-42, respectively. Non-apoptotic reduction in ChAT activity induced by Par-4 can be completely blocked by co-overexpression of Leu.zip, indicating that enhanced Par-4 activity is a necessary event for cholinergic hypoactivity in PC12 cells. Further studies found that Par-4 induces non-apoptotic reduction in ChAT activity by: (1) reducing ChAT protein levels following exposure to non-toxic concentration of Abeta, and (2) blocking the cellular capability to increase ChAT activity following exposure to nerve growth factor (NGF). The role of Par-4 in inducing cholinergic hypoactivity may have significant implications in the understanding and the treatment of memory impairment in AD.

Nerve growth factor brain concentration and stress: changes depend on type of stressor and age

In the relationship between the hippocampus and the hypothalamo-pituitary-adrenocortical axis, trophic and tropic actions of nerve growth factor are involved in parallel with those on the cholinergic nuclei of the basal forebrain. Here, we report the changes produced by stress activation of the hypothalamo-pituitary-adrenocortical axis on hippocampal and basal forebrain nerve growth factor concentrations in 3-month-old male Wistar rats. The stressors used were: restraint; cold exposure; foot-shock; and rotatory platform. Restraint stress tended to reduce nerve growth factor in the hippocampus and reduced it significantly in the basal forebrain. Nerve growth factor levels in the hippocampus were not modified by cold exposure. However, a single unrepeated exposure significantly increased nerve growth factor in the basal forebrain. Both acute and chronic foot-shock reduced nerve growth factor in the hippocampus, leaving the levels in the basal forebrain unmodified. Acute but not chronic rotatory platform reduced nerve growth factor in the hippocampus, while showing a tendency, more pronounced after chronic application, toward an increase in the basal forebrain. Since with aging both activity of the hypothalamus-pituitary-adrenal axis and nerve growth factor trophic and tropic functions change, we studied the effect of restraint and cold stress in the 24-month-old male rat. The variations in nerve growth factor concentrations in the basal forebrain following stress activation are no longer present in the aged rat. The picture that emerges is indicative of a complex relationship between stress and nerve growth factor which is influenced by the kind of stressor and by age. Lack of uniformity in the effects produced by different stressors might reside in different qualitative and/or quantitative degree of involvement of neurotransmitters and/or neurohormones for each of them.

NGF-mediated alteration of NF-kappaB binding activity after partial immunolesions to rat cholinergic basal forebrain neurons

There are age-associated cognitive and cholinergic deficits in the neurotrophin-dependent cholinergic basal forebrain neurons (CBFNs). There are also increases in the activity of the transcription factor NF-kappaB in the aged rodent brain that may reflect chronic enhancement of stress response signaling. We used partial immunolesions (PIL) to CBFN to examine the role of endogenous NGF on choline acetyltransferase (ChAT) activity and NGF-mediated NF-kappaB alteration after cholinergic deafferentation. We injected 192 IgG-saporin, an immunotoxin selectively taken up by neurotrophin receptor p75(NTR)-bearing neurons, into lateral ventricles, followed by infusions of anti-NGF to assess NF-kappaB, ChAT and NGF responses to PIL after anti-NGF infusion. Treatment with anti-NGF decreased ChAT activity by 17-34% in the cortex, hippocampus, and olfactory bulb and PIL decreased ChAT activity by 47-73%. Changes in AChE activity levels paralleled those observed for ChAT after PIL. NGF protein levels in the olfactory bulb, but not the cortex or hippocampus, increased significantly after PIL treatment. Infusion of anti-NGF abolished the PIL-induced eight-fold NGF increase in CNS. NF-kappaB binding activity to the IgG-kappaB and ChAT specific NF-kappaB consensus sequences, increased in the cortex but not hippocampus after PIL followed by anti-NGF infusion. It is likely that immunolesion-induced changes in ambient NGF levels may perturb NF-kappaB activity.

Differential alterations of NF-kappaB to oxidative stress in primary basal forebrain cultures

Oxidative stress has been linked to neuronal cell death resulting from either acute insults due to ischemia, trauma, excitotoxicity, or chronic neurodegenerative diseases. Cholinergic basal forebrain neurons (CBFNs) compete for nerve growth factor (NGF) synthesized in the hippocampus and cortex via retrograde transport. NGF affects CBFN survival and cholinergic function via activation of the NF-kappaB transcription factor and this signaling pathway appears to be impaired in aged rats. Here, we demonstrate that activation of NF-kappaB in basal forebrain primary culture via treatment with hydrogen peroxide or TNF-alpha is predominantly restricted to CBFNs, and that NF-kappaB activation appears to mostly affect p65 translocation to the nucleus, but not the p50 subunit. These results are consistent with NF-kappaB activation being a part of recovery processes after acute oxidative stress. Since p50 or p49 (also called p52) binding to promoter sites does not stimulate transcription - both p50 and p49 lack an activating domain - and p65 does contain an activating domain and thus can act as a transcription enhancer, differential translocation of different NF-kappaB dimers can act as repressors of constitutive activity or enhancers. These results are in agreement with the hypothesis that p50/p65 is the active trans-activating species of NF-kappaB, as compared to p50/p50 homodimers which bind to NF-kappaB binding sites but do not trans-activate promoters. Our results also suggest that selective activation of different NF-kappaB dimer species may have regulatory significance in neuronal responses to acute or chronic insults to CNS. Thus, increased p65 translocation could have enhancing effects while increased p50 translocation could have a repressor role. Manipulation of the types of NF-kappaB species being translocated could provide a basis for therapeutic strategies.

Repeated immunolesions display diminished stress response signal

Cholinergic basal forebrain neurons (CBFNs) retrogradely transport neurotrophins released in the hippocampus and cortex as part of a general response to injury in a process that is impaired in the aged rodent and can be spared by the exogenous addition of pharmacological doses of nerve growth factor (NGF). This observation suggests that components of stress response signal transduction pathways in the aged CNS can be exogenously activated. The extent and mechanism of the endogenous stimulation of NGF in response to injury can be mimicked via treatment with 192 IgG-saporin of rat CNS, an immunolesion model. Here we report on the use of a conditioning lesion paradigm to determine if repeated partial immunolesions have a conditioning effect on the immunolesion-induced increases in NGF protein or decreases in choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity. We report that chronic repeated immunolesions, as used here, were not as effective as a one time equivalent immunolesion in terms of induced NGF protein increases or decreasing ChAT and AChE activity in the hippocampus and cortex. Thus, chronic lesions resulting in cholinergic impairment typical of the aged CNS may differ from acute toxic models as a result of desensitization due to a conditioning effect of chronic subthreshold lesioning events in the CNS.

Effect of NGF treatment on outcome measures in a rat model of middle cerebral artery occlusion

Ischemic insults to the brain result in a time-dependent increase in neuronal death that is responsible for some of the functional deficits associated with stroke. Our working hypothesis is that ischemia results in a prompt depletion of high energy phosphate species resulting in decreased pH and glutathione levels in brain in a temporal and spatial pattern that disrupts nerve growth factor homeostasis and increases neuronal apoptosis. Here we show hemispheric depletion of active phosphate species after ischemia. Also, we observed that the striatum is an early target for oxidative stress that is followed by energy metabolic impairment and altered neurotrophin levels that were detected by noninvasive magnetic resonance imaging (MRI) measurements of cytotoxicity and conventional biochemical determinations of apoptosis, glutathione, and nerve growth factor (NGF) protein levels in a pattern distinct from that observed in the hippocampus. Furthermore, early assessment of intracellular pH by 31P-magnetic resonance spectroscopy (31P-MRS) was a predictor of later infarct development as determined by MRI. We also show that pretreatment with pharmacological doses of NGF did not have overall significant beneficial consequences on irreversible ischemia in an intraluminal unilateral irreversible model of stroke in rat brain.

Age and species-dependent differences in the neurokinin B system in rat and human brain

Neurokinin B and its cognate neurokinin-3 receptor are expressed more in the forebrain than in brain stem structures but little is known about the primary function of this peptide system in the central processing of information. In general, few studies have specifically addressed age-related changes of tachykinins, notably the changes in number and/or distribution of the neurokinin B-expressing and neurokinin-3 receptor-bearing neurons. Data on functions and changes of neurokinins in physiological aging are limited and apply mainly to the substance P/neurokinin-1 receptor system. In the present study, we analyzed neurokinin B/neurokinin-3 receptor system in young (5 months) versus middle aged (15 months) and old rats (23-25 months) and also in aging human brains. For the majority of the immunohistochemically examined regions of the rat brain, there was no statistically significant change in neuronal number and size of the neurokinin B and neurokinin-3 receptor staining. In the adult human brain, there was no age-associated change of the number or size of neurokinin-B-positive neurons. However, we found a major decline in number of neurokinin-3 receptor-expressing neurons between young/middle aged (30 years to 69 years) versus old (70 years and older) adults. Interestingly, numbers of neurokinin-3 receptor-positive microglia increased whereas the neurokinin-3 receptor-positive astrocytes remained unchanged in both aging rat and human brains. Finally, in addition to assessing the morphological and quantitative changes of the neurokinin B/neurokinin-3 receptor system in the rat and human brain, we discuss functional implications of the observed interspecies differences.

APE/Ref-1 responses to oxidative stress in aged rats

Chronic oxidative stress has been hypothesized to be a major contributor to the aging process. The continued exposure to reactive oxygen species (ROS) generated by oxidative metabolism or environmental sources can damage critical cellular structures and be responsible for some age-related pathology. The exposure of rodents to 100% oxygen, isobaric hyperoxia, increases ambient ROS levels and significantly increases apoptosis in brain. The deleterious effects of ROS also include increased lipid peroxidation, protein oxidation, and DNA damage. Although differences in the relative amounts of oxidative stress in young and old brains have been observed, the mechanisms responsible for impaired aging-associated DNA repair processes have not been characterized. We measured DNA levels of the DNA repair enzyme apurinic/apyrimidinic endonuclease (APE/Ref-1) protein by Western blot analysis in the brains of young (3-month) and old (30-month) male rats exposed to isobaric hyperoxia. Given that APE/Ref-1 is the rate-limiting enzyme in the repair pathway of apurinic/apyrimidinic sites generated in DNA by oxidative damage, we assumed that APE/Ref-1 protein levels were a good reflection of ongoing DNA base excision repair. Isobaric hyperoxia stimulated APE/Ref-1 expression in the hippocampus and basal forebrain of young rats experiencing 100% oxygen for 6 hr, while aged rats showed no significant changes in APE/Ref-1 protein levels in all brain areas at any time tested (0-48 hr) after hyperoxia. Differences in the stress-induced levels of expression of DNA repair enzymes may contribute to apoptotic increases and pathology associated with the aging process.

Responses in the aged rat brain after total immunolesion

In the present study, we compare the effects of cholinergic deafferentation of the hippocampus, cortex, and olfactory bulb of young and aged rats on nerve growth factor (NGF) protein levels in these areas. We also describe glial responses to intraventricular injections of the immunotoxin, 192 IgG-saporin in the aged. Choline acetyltransferase (ChAT) activity was dramatically decreased in the basal forebrain and target areas of the cholinergic basal forebrain neurons (CBFNs) in the young immunolesioned rats and to a lesser extent in their aged counterparts. After total immunolesion, NGF protein levels significantly increased in the hippocampus, cortex, and olfactory bulb of the young rats but not of the aged rats, except for small increases in the olfactory bulb after two weeks. After immunolesion NGF protein levels in the basal forebrain increased in young rats and less so in the aged rats. The total immunolesions had no effects on NGF and BDNF mRNA levels in the hippocampus and cortex. Two weeks after injection of the immunotoxin, the profiles of AChE- and p75NTR-positive cells significantly decreased in medial septum, vertical and horizontal limbs of diagonal band and nucleus basalis of Meynert. There was also an increase in microglia while but not astrocytes in the subnuclei of basal forebrain. In conclusion, 192 IgG-saporin was effective in producing cholinergic lesions in both young and aged rat brains, the lesion-induced NGF response was partially extinguished in the aged rat brains and immunolesions induced a microglial response in aged brain.

Long term changes in brain cholinergic markers and nerve growth factor levels after partial immunolesion

There are deficits in cholinergic basal forebrain neurons (CBFNs) in the aged brain and patients suffering Alzheimer's disease associated with a partial loss of the CBFNs. To mimic this partial loss and assess its long term effects on residual cholinergic activity and resultant target-derived nerve growth factor (NGF) levels, we produced a partial immunolesion to CBFNs with 192 IgG-saporin, an immunotoxin selectively taken up by p75NTR-bearing neurons. We measured two cholinergic markers, choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity, and NGF protein levels at 10 days, 1, 6 and 12 months postlesion. There were no significant changes in the cholinergic markers and the NGF protein levels in the sham-treated animal controls during the one year experiment. Ten days after 192 IgG-saporin treatment, ChAT activity decreased to 35-50% of controls in the olfactory bulb, hippocampus, and cortex. There was a minor but significant recovery of ChAT activity one year after the immunolesion in the hippocampus. Changes in AChE activity mirrored the ChAT changes but were less robust. There were transient increases in NGF protein levels in the hippocampus and cortex that returned to basal levels at 6 months and 12 months postlesion, respectively. In summary, partial immunolesions resulted in partial region-specific and time-dependent recoveries of cholinergic activity in the target areas of the basal forebrain after a partial elimination of CBFNs and a return to basal levels of NGF protein consistent with the hypothesis that the remaining CBFNs compensated for losses of ChAT and NGF due to changes in cholinergic innervation of basal forebrain target areas.