Age and sex-dependent decreases in ChAT in basal forebrain nuclei

hnRNP A1B, a Splice Variant of HNRNPA1, Is Spatially and Temporally Regulated

RNA binding proteins (RBPs) play a key role in cellular growth, homoeostasis and survival and are tightly regulated. A deep understanding of their spatiotemporal regulation is needed to understand their contribution to physiology and pathology. Here, we have characterized the spatiotemporal expression pattern of hnRNP A1 and its splice variant hnRNP A1B in mice. We have found that hnRNP A1B expression is more restricted to the CNS compared to hnRNP A1, and that it can form an SDS-resistant dimer in the CNS. Also, hnRNP A1B expression becomes progressively restricted to motor neurons in the ventral horn of the spinal cord, compared to hnRNP A1 which is more broadly expressed. We also demonstrate that hnRNP A1B is present in neuronal processes, while hnRNP A1 is absent. This finding supports a hypothesis that hnRNP A1B may have a cytosolic function in neurons that is not shared with hnRNP A1. Our results demonstrate that both isoforms are differentially expressed across tissues and have distinct localization profiles, suggesting that the two isoforms may have specific subcellular functions that can uniquely contribute to disease progression.

M(4) muscarinic receptors and locomotor activity regulation

M(4) muscarinic receptors (M(4) MR) represent a subfamily of G-protein coupled receptors serving a substantial role in spontaneous locomotor activity regulation, cognition and modulation of cholinergic system. With increasing body of literature discussing the role of M(4) MR some controversies arose. Thus, we try here to summarize the current evidence regarding the M(4) MR, with the special focus on their role in Locomotor activity control. We review the molecular function of M(4) MR in specific brain areas implicated in locomotor regulation, and shortly in other CNS processes that could be connected to locomotor activity. We also focus on brain areas implicated in locomotor activity biorhythm changes like suprachiasmatic nucleus, subparaventricular zone posterior hypothalamic area, striatum and thalamus. Gender-related aspects and differences in locomotor activity in males and females are discussed further.

Reprint of: From the 90׳s to now: A brief historical perspective on more than two decades of estrogen neuroprotection

Historical perspective abstract:From the 90׳s to now: a historical perspective on more than two decades of estrogen neuroprotection: In the early 90׳s, estrogens were known to exert organizational and activational effects on reproductive tissues and sexual behavior. As well, the role of sex and gonadal hormones in altering the risk for developing Alzheimer׳s Disease (AD) was only beginning to be elucidated. Preliminary investigations suggested that estrogen-containing therapies typically given for the management of disruptive menopausal symptoms could reduce AD risk, attenuate disease-associated cognitive deficits, and modulate brain substrates known to be dysregulated by the condition, such as the cholingeric system. The findings from our seminal paper demonstrating cognitive benefits and cholinergic impacts with exogenous estrogen treatment in a rodent model of surgical hormone depletion provided initial support for use of estrogen-containing therapies as a treatment for age-related brain disorders. We then went on to demonstrate neuroprotective actions of estrogen in several other in vivo and in vitro models of neurological challenge, including stroke and AD. Further, our findings of the chemical structure requirements for estrogen׳s neuroprotective effects identified a novel approach for optimizing future estrogen-containing hormone therapy options. These early efforts laid the groundwork for later, large-scale clinical investigations into the potential of estrogen-based menopausal hormone therapies for the prevention of a variety of age-related disorders. Although findings of these studies were equivocal, the neuroprotective actions of estrogen, and specifically 17β-estradiol, identified by early investigations, remain well-documented. Future development of interventions that optimize cognitive aging are crucial and, with proper understanding of the factors that influence the realization of beneficial impacts, estrogen-containing treatments may still be among these.

ORIGINAL ARTICLE ABSTRACT

Ovarian steroid deprivation results in a reversible learning impairment and compromised cholinergic function in female Sprague-Dawley rats: We hypothesized that estradiol (E2) serves as a neurotrophomodulatory substance for basal forebrain cholinergic neurons thought to be involved in learning and memory. Learning/memory was assessed using the two-way active avoidance paradigm and the Morris water task. Female Sprague-Dawley rats were either ovariectomized (OVX) or OVX for 3 weeks, followed by s.c. implantation of a Silastic pellet containing 17-βE2 (E2 pellet), resulting in a replacement of E2 to physiological levels. Ovary-intact (INTACT) animals served as our positive control. Active avoidance behavior and choline acetyltransferase (ChAT) activity in the frontal cortex and hippocampus were assessed at 5 and 28 weeks postovariectomy while performance on the Morris water task and high-affinity choline uptake (HACU) were measured only at the 5-week time point. At the 5-week time point, E2 replacement caused a significant elevation in the level of active avoidance performance relative to OVX animals. At the 28-week time point, OVX animals demonstrated a significantly lower number of avoidances relative to controls (61%) whereas E2-pellet animals not only demonstrated superior performance relative to OVX animals but also showed an accelerated rate of learning. Morris water task performance, on the other hand, was not significantly affected by estrogenic milieu despite a trend towards better performance in the E2-pellet group. Neurochemical analyses revealed that 5 weeks of ovariectomy was sufficient to reduce HACU in both the frontal cortex and hippocampus by 24 and 34%, respectively, while E2 replacement was successful in elevating HACU relative to OVX animals in both regions. ChAT activity was decreased in the hippocampus but not the frontal cortex of 5-week OVX animals. E2 replacement resulted in a reversal of this effect. At the 28-week time period, an unexpected decrease in ChAT activity was observed across all treatment groups. Interestingly, E2-pellet animals demonstrated the least severe decline in ChAT. This phenomenon was most evident in the frontal cortex where ChAT decreased by 61 and 56% in INTACT and OVX animals, respectively, whereas the decline in E2-pellet animals was only 16% over the same time period, suggesting a previously unreported cytoprotective effect of E2. Taken together, these findings demonstrate important effects of estrogens on cholinergic neurons and support the potential use of estrogen therapy in treatment of dementias in postmenopausal women. © 1994. This article is part of a Special Issue entitled SI:50th Anniversary Issue.

From the 90's to now: A brief historical perspective on more than two decades of estrogen neuroprotection

Historical perspective abstract:From the 90's to now: a historical perspective on more than two decades of estrogen neuroprotection: In the early 90's, estrogens were known to exert organizational and activational effects on reproductive tissues and sexual behavior. As well, the role of sex and gonadal hormones in altering the risk for developing Alzheimer's Disease (AD) was only beginning to be elucidated. Preliminary investigations suggested that estrogen-containing therapies typically given for the management of disruptive menopausal symptoms could reduce AD risk, attenuate disease-associated cognitive deficits, and modulate brain substrates known to be dysregulated by the condition, such as the cholingeric system. The findings from our seminal paper demonstrating cognitive benefits and cholinergic impacts with exogenous estrogen treatment in a rodent model of surgical hormone depletion provided initial support for use of estrogen-containing therapies as a treatment for age-related brain disorders. We then went on to demonstrate neuroprotective actions of estrogen in several other in vivo and in vitro models of neurological challenge, including stroke and AD. Further, our findings of the chemical structure requirements for estrogen's neuroprotective effects identified a novel approach for optimizing future estrogen-containing hormone therapy options. These early efforts laid the groundwork for later, large-scale clinical investigations into the potential of estrogen-based menopausal hormone therapies for the prevention of a variety of age-related disorders. Although findings of these studies were equivocal, the neuroprotective actions of estrogen, and specifically 17β-estradiol, identified by early investigations, remain well-documented. Future development of interventions that optimize cognitive aging are crucial and, with proper understanding of the factors that influence the realization of beneficial impacts, estrogen-containing treatments may still be among these.

ORIGINAL ARTICLE ABSTRACT

Ovarian steroid deprivation results in a reversible learning impairment and compromised cholinergic function in female Sprague-Dawley rats: We hypothesized that estradiol (E2) serves as a neurotrophomodulatory substance for basal forebrain cholinergic neurons thought to be involved in learning and memory. Learning/memory was assessed using the two-way active avoidance paradigm and the Morris water task. Female Sprague-Dawley rats were either ovariectomized (OVX) or OVX for 3 weeks, followed by s.c. implantation of a Silastic pellet containing 17-βE2 (E2 pellet), resulting in a replacement of E2 to physiological levels. Ovary-intact (INTACT) animals served as our positive control. Active avoidance behavior and choline acetyltransferase (ChAT) activity in the frontal cortex and hippocampus were assessed at 5 and 28 weeks postovariectomy while performance on the Morris water task and high-affinity choline uptake (HACU) were measured only at the 5-week time point. At the 5-week time point, E2 replacement caused a significant elevation in the level of active avoidance performance relative to OVX animals. At the 28-week time point, OVX animals demonstrated a significantly lower number of avoidances relative to controls (61%) whereas E2-pellet animals not only demonstrated superior performance relative to OVX animals but also showed an accelerated rate of learning. Morris water task performance, on the other hand, was not significantly affected by estrogenic milieu despite a trend towards better performance in the E2-pellet group. Neurochemical analyses revealed that 5 weeks of ovariectomy was sufficient to reduce HACU in both the frontal cortex and hippocampus by 24 and 34%, respectively, while E2 replacement was successful in elevating HACU relative to OVX animals in both regions. ChAT activity was decreased in the hippocampus but not the frontal cortex of 5-week OVX animals. E2 replacement resulted in a reversal of this effect. At the 28-week time period, an unexpected decrease in ChAT activity was observed across all treatment groups. Interestingly, E2-pellet animals demonstrated the least severe decline in ChAT. This phenomenon was most evident in the frontal cortex where ChAT decreased by 61 and 56% in INTACT and OVX animals, respectively, whereas the decline in E2-pellet animals was only 16% over the same time period, suggesting a previously unreported cytoprotective effect of E2. Taken together, these findings demonstrate important effects of estrogens on cholinergic neurons and support the potential use of estrogen therapy in treatment of dementias in postmenopausal women. © 1994. This article is part of a Special Issue entitled SI:50th Anniversary Issue.

Understanding the cognitive impact of the contraceptive estrogen Ethinyl Estradiol: tonic and cyclic administration impairs memory, and performance correlates with basal forebrain cholinergic system integrity

Ethinyl Estradiol (EE), a synthetic, orally bio-available estrogen, is the most commonly prescribed form of estrogen in oral contraceptives, and is found in at least 30 different contraceptive formulations currently prescribed to women as well as hormone therapies prescribed to menopausal women. Thus, EE is prescribed clinically to women at ages ranging from puberty to reproductive senescence. Here, in two separate studies, the cognitive effects of cyclic or tonic EE administration following ovariectomy (Ovx) were evaluated in young female rats. Study I assessed the cognitive effects of low and high doses of EE, delivered tonically via a subcutaneous osmotic pump. Study II evaluated the cognitive effects of low, medium, and high doses of EE administered via a daily subcutaneous injection, modeling the daily rise and fall of serum EE levels with oral regimens. Study II also investigated the impact of low, medium and high doses of EE on the basal forebrain cholinergic system. The low and medium doses utilized here correspond to the range of doses currently used in clinical formulations, and the high dose corresponds to doses prescribed to a generation of women between 1960 and 1970, when oral contraceptives first became available. We evaluate cognition using a battery of maze tasks tapping several domains of spatial learning and memory as well as basal forebrain cholinergic integrity using immunohistochemistry and unbiased stereology to estimate the number of choline acetyltransferase (ChAT)-producing cells in the medial septum and vertical/diagonal bands. At the highest dose, EE treatment impaired multiple domains of spatial memory relative to vehicle treatment, regardless of administration method. When given cyclically at the low and medium doses, EE did not impact working memory, but transiently impaired reference memory during the learning phase of testing. Of the doses and regimens tested here, only EE at the highest dose impaired several domains of memory; tonic delivery of low EE, a dose that corresponds to the most popular doses used in the clinic today, did not impact cognition on any measure. Both medium and high injection doses of EE reduced the number of ChAt-immunoreactive cells in the basal forebrain, and cell population estimates in the vertical/diagonal bands negatively correlated with working memory errors.

CHRNA7 polymorphisms and response to cholinesterase inhibitors in Alzheimer's disease

Background

CHRNA7 encodes the α7 nicotinic acetylcholine receptor subunit, which is important to Alzheimer's disease (AD) pathogenesis and cholinergic neurotransmission. Previously, CHRNA7 polymorphisms have not been related to cholinesterase inhibitors (ChEI) response.

Methods

Mild to moderate AD patients received ChEIs were recruited from the neurology clinics of three teaching hospitals from 2007 to 2010 (n = 204). Nine haplotype-tagging single nucleotide polymorphisms of CHRNA7 were genotyped. Cognitive responders were those showing improvement in the Mini-Mental State Examination score ≧2 between baseline and 6 months after ChEI treatment.

Results

AD women carrying rs8024987 variants [GG+GC vs. CC: adjusted odds ratio (AOR) = 3.62, 95% confidence interval (CI) = 1.47–8.89] and GG haplotype in block1 (AOR = 3.34, 95% CI = 1.38–8.06) had significantly better response to ChEIs (false discovery rate <0.05). These variant carriers using galantamine were 11 times more likely to be responders than female non-carriers using donepezil or rivastigmine.

Conclusion

For the first time, this study found a significant association between CHRNA7 polymorphisms and better ChEI response. If confirmed by further studies, CHRNA7 polymorphisms may aid in predicting ChEI response and refining treatment choice.

Translational cognitive endocrinology: designing rodent experiments with the goal to ultimately enhance cognitive health in women

Understanding the cognitive impact of endogenously derived, and exogenously administered, hormone alterations is necessary for developing hormone treatments to support healthy brain function in women, especially during aging. The increasing number of studies in the burgeoning area of translational cognitive neuroendocrinology has revealed numerous factors that influence the extent and direction of female steroid effects on cognition. Here, we discuss the decision processes underlying the design of rodent hormone manipulation experiments evaluating learning and memory. It is noted that even when beginning with a clear hypothesis-driven question, there are numerous factors to consider in order to solidify a sound experimental design that will yield clean, interpretable results. Decisions and considerations include: age of animals at hormone administration and test, ovariectomy implementation, when to administer hormones relative to ovarian hormone loss, how and whether to monitor the estrous cycle if animals are ovary-intact, dose of hormone, administration route of hormone, hormone treatment confirmation protocols, handling procedures required for hormone administration and treatment confirmation, cognitive domains to be tested and which mazes should be utilized to test these cognitive domains, and control measures to be used. A balanced view of optimal design and realistic experimental practice and protocol is presented. The emerging results from translational cognitive neuroendocrinology studies have been diverse, but also enlightening and exciting as we realize the broad scope and powerful nature of ovarian hormone effects on the brain and its function. We must design, implement, and interpret hormone and cognition experiments with sensitivity to these tenets, acknowledging and respecting the breadth and depth of the impact gonadal hormones have on brain functioning and its rich plasticity. This article is part of a Special Issue entitled Hormone Therapy.

Males, but not females, lose tyrosine hydroxylase fibers in the medial prefrontal cortex and are impaired on a delayed alternation task during aging

The structure of the prefrontal cortex (PFC) is particularly vulnerable to the effects of aging, and behaviors mediated by the PFC are impaired during aging in both humans and animals. In male rats, behavioral deficits have been correlated with a decrease in dopaminergic functioning. However, studies have found that anatomical changes associated with aging are sexually dimorphic, with males experiencing greater age-related loss than females. The present study investigated the effects of sex and aging on performance of a delayed alternation t-maze, a task mediated by the medial prefrontal cortex (mPFC), and on tyrosine hydroxylase (TH) immunoreactivity in this brain region using adult (7 months) and aged (21 months) male and female F344 rats. There was a sex by age interaction in performance of the delayed alternation task such that adult males performed better than aged males, but aged females were not different than adult females. Adult males performed better than adult females across all delays; however, this sex difference was reversed during aging and aged males performed worse than aged females. In addition, TH immunoreactivity decreased during aging in layers 2/3 in the male, but not female mPFC. Thus females were less sensitive to the effects of aging on the prefrontal dopaminergic system and on performance of a delayed alternation task. These effects may be due to decreases in testosterone in aging males, as well as the protective effects of ovarian hormones, which continue to be secreted after cessation of the estrous cycle in aging females.

Effect of bite-raised condition on the hippocampal cholinergic system of aged SAMP8 mice

Occlusal disharmony induces chronic stress, which results in learning deficits in association with the morphologic changes in the hippocampus, e.g., neuronal degeneration and increased hypertrophied glial fibrillary acidic protein-positive cells. To investigate the mechanisms underlying impaired hippocampal function resulting from occlusal disharmony, we examined the effects of the bite-raised condition on the septohippocampal cholinergic system by assessing acetylcholine release in the hippocampus and choline acetyltransferase immunoreactivity in the medial septal nucleus in aged SAMP8 mice that underwent the bite raising procedure. Aged bite-raised mice showed decreased acetylcholine release in the hippocampus and a reduced number of choline acetyltransferase-immunopositive neurons in the medial septal nucleus compared to age-matched control mice. These findings suggest that the bite-raised condition in aged SAMP8 mice enhances the age-related decline in the septohippocampal cholinergic system, leading to impaired learning.

Tissue-Specific Effects of Loss of Estrogen during Menopause and Aging

The roles of estrogens have been best studied in the breast, breast cancers, and in the female reproductive tract. However, estrogens have important functions in almost every tissue in the body. Recent clinical trials such as the Women's Health Initiative have highlighted both the importance of estrogens and how little we know about the molecular mechanism of estrogens in these other tissues. In this review, we illustrate the diverse functions of estrogens in the bone, adipose tissue, skin, hair, brain, skeletal muscle and cardiovascular system, and how the loss of estrogens during aging affects these tissues. Early transcriptional targets of estrogen are reviewed in each tissue. We also describe the tissue-specific effects of selective estrogen receptor modulators (SERMs) used for the treatment of breast cancers and postmenopausal symptoms.

Acetyl-L-carnitine improves aged brain function

The effects of acetyl-L-carnitine (ALCAR), an acetyl derivative of L-carnitine, on memory and learning capacity and on brain synaptic functions of aged rats were examined. Male Fischer 344 rats were given ALCAR (100 mg/kg bodyweight) per os for 3 months and were subjected to the Hebb-Williams tasks and AKON-1 task to assess their learning capacity. Cholinergic activities were determined with synaptosomes isolated from brain cortices of the rats. Choline parameters, the high-affinity choline uptake, acetylcholine (ACh) synthesis and depolarization-evoked ACh release were all enhanced in the ALCAR group. An increment of depolarization-induced calcium ion influx into synaptosomes was also evident in rats given ALCAR. Electrophysiological studies using hippocampus slices indicated that the excitatory postsynaptic potential slope and population spike size were both increased in ALCAR-treated rats. These results indicate that ALCAR increases synaptic neurotransmission in the brain and consequently improves learning capacity in aging rats.

The COMT Val158 Met polymorphism as an associated risk factor for Alzheimer disease and mild cognitive impairment in APOE 4 carriers

BACKGROUND

The aim of this study is to examine the influence of the catechol-O-methyltranferase (COMT) gene (polymorphism Val158 Met) as a risk factor for Alzheimer's disease (AD) and mild cognitive impairment of amnesic type (MCI), and its synergistic effect with the apolipoprotein E gene (APOE).A total of 223 MCI patients, 345 AD and 253 healthy controls were analyzed. Clinical criteria and neuropsychological tests were used to establish diagnostic groups.The DNA Bank of the University of the Basque Country (UPV-EHU) (Spain) determined COMT Val158 Met and APOE genotypes using real time polymerase chain reaction (rtPCR) and polymerase chain reaction (PCR), and restriction fragment length polymorphism (RFLPs), respectively. Multinomial logistic regression models were used to determine the risk of AD and MCI.

RESULTS

Neither COMT alleles nor genotypes were independent risk factors for AD or MCI. The high activity genotypes (GG and AG) showed a synergistic effect with APOE epsilon4 allele, increasing the risk of AD (OR = 5.96, 95%CI 2.74-12.94, p < 0.001 and OR = 6.71, 95%CI 3.36-13.41, p < 0.001 respectively). In AD patients this effect was greater in women.In MCI patients such as synergistic effect was only found between AG and APOE epsilon4 allele (OR = 3.21 95%CI 1.56-6.63, p = 0.02) and was greater in men (OR = 5.88 95%CI 1.69-20.42, p < 0.01).

CONCLUSION

COMT (Val158 Met) polymorphism is not an independent risk factor for AD or MCI, but shows a synergistic effect with APOE epsilon4 allele that proves greater in women with AD.

Behavioral and degeneration changes in the basal forebrain systems of aged rats: a quantitative study in the region of the basal forebrain after levo-acetyl-carnitine treatments assessed by Abercrombie estimation

One group of six male control rats [21 months old] and one group of six male rats of the same age, singularly stored in a cage, and treated with acetyl-l-carnitine-HCl (ALCAR: 60 mg/kg/day/p.o.) for six months were tested in the spatial learning/memory Morris maze-water task and for atrophy and cell loss in seven myelo- and cytostructurally defined basal forebrain (BF) cholinergic regions [Gritti et al., 1993 J Comp Neurol 329: 438-457]. Coronal sections 25 mum thick were cut through the BF regions and processed every 200 mum for choline acetyltransferase (ChAT) immunohistochemistry. The ALCAR-treated rats had significantly shorter exit times on the Morris maze-water task test than the control rats (ANOVA-enzyme: F(1,39)=112.5, P=0.0001; sessions: F(3,39)=10.41, P=0.0001; interaction: F(3,39)=5.09, P=0.0044). Degenerative morphological changes in the BF ChAT-positive cells were observed in the control rats, but not in the treated animals, in: the diagonal band of Broca, the magnocellular preoptic nucleus, the olfactory tubercle, the substantia innominata, and the globus pallidus (ANOVA-enzyme: F(1,2)=14, P=0,0003; structures: F(6,7)=4, P=0,0018; interaction: F(6,7)=3, P=0,0043). In the diagonal band of Broca (P<0.0494) and in the magnocellular preoptic nucleus (P<0.0117) there were significantly fewer ChAT-positive neurons in the aged control rats than in the ALCAR-treated rats. These results demonstrate that in rats aged from 15 to 21 months ALCAR treatment significantly attenuated spatial learning/memory impairment on the Morris maze-water task and also importantly reduced the degeneration in size and number of cholinergic cells in the BF.

Sex differences in water maze performance and cortical neurotrophin levels of LHX7 null mutant mice

Mice lacking both alleles of the LIM-homeobox gene Lhx7 display dramatically reduced number of forebrain cholinergic neurons. Given the fact that sex differences are consistently observed in forebrain cholinergic function, in the present study we investigated whether the absence of LHX7 differentially affects water maze performance in the two sexes. Herein we demonstrate that LHX7 null mutants display a sex-dependent impairment in water maze, with females appearing more affected than males. Moreover, neurotrophin assessment revealed a compensatory increase of brain-derived neurotrophic factor and neurotrophin 3 in the neocortex of both male and female mutants and an increase of nerve growth factor levels only in the females. Nevertheless, the compensatory increase of cortical neurotrophin levels did not restore cognitive abilities of Lhx7 homozygous mutants. Finally, our analysis revealed that cortical neurotrophin levels correlate negatively with water maze proficiency, indicating that there is an optimal neurotrophin level for successful cognitive performance.

Evidence for increased NADPH-diaphorase-positive neurons in the central auditory system of the aged rat

CONCLUSIONS

The age-related increase in the production of nitric oxide (NO) suggests that this increase was related to neuron aging. Additional studies may provide information regarding aging-related changes in the central auditory system.

OBJECTIVES

Although NO has been associated with aging, it is unclear whether specific areas of the central auditory system are involved. We therefore assayed aging-related changes in NADPH-diaphorase (NADPH-d), a selective histochemical marker for NO, in the neurons of the central auditory system and other brain regions.

MATERIALS AND METHODS

The numbers of NADPH-d-stained neurons and the area and staining density of cell bodies were examined in aged (24 months old) and younger (4 months old) Wistar rats.

RESULTS

The number of NADPH-d-positive neurons in the inferior colliculus was significantly increased in aged rats (p<0.05), whereas the area of NADPH-d-positive neurons in all areas did not differ significantly between aged and younger rats (p>0.05). The staining densities of NADPH-d-positive neurons in the inferior colliculus, the auditory cortex, and the visual cortex were significantly greater in aged compared with younger rats (p<0.05).

Specific mechanism for blood inflow stimulation in brain area prone to Alzheimer's disease lesions

The present study describes the specific two-stage mechanism that intensifies blood supply to the brain area comprising amygdala, hippocampus, olfactory bulb, entorhinal cortex, and neocortex (AHBC). Cholinergic neurons from the nuclei of basal forebrain induce vasodilatory effect through release of acetylcholine. In physiological aging the efficacy of this neuronal system declines, while intensive formation of amyloidogenic peptides starts. These peptides at low, picomolar concentrations activate alpha7 nicotinic acetylcholine receptors, thus enhancing angiogenesis and in so doing restoring blood supply to the AHBC area.

The length of hippocampal cholinergic fibers is reduced in the aging brain

Cholinergic deficits occur in the aged hippocampus and they are significant in Alzheimer's disease. Using stereological and biochemical approaches, we characterized the cholinergic septohippocampal pathway in old (24 months) and young adult (3 months) rats. The total length of choline acetyltransferase (ChAT)-positive fibers in the dorsal hippocampus was significantly decreased by 32% with aging (F((1,9))=20.94, p=0.0014), along with the levels of synaptophysin, a presynaptic marker. No significant changes were detected in ChAT activity or in the amounts of ChAT protein, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), tropomyosin related kinase receptor (Trk) A, TrkB, or p75 neurotrophin receptor (p75(NTR)) in the aged dorsal hippocampus. The number and size of ChAT-positive neurons and the levels of ChAT activity, NGF and BDNF were not statistically different in the septum of aged and young adult rats. This study suggests that substantial synaptic loss and cholinergic axonal degeneration occurs during aging and reinforces the importance of therapies that can protect axons and promote their growth in order to restore cholinergic neurotransmission.

Evidence for interplay between thyrotropin-releasing hormone (TRH) and its structural analogue pGlu-Glu-Pro-NH2 ([Glu2]TRH) in the brain: an in vivo microdialysis study

Local perfusion of pGlu-Glu-Pro-NH2, an endogenous peptide structurally related to thyrotropine-releasing hormone (TRH), via in vivo microdialysis into the rat hippocampus did not change the basal level of extracellular acetylcholine. However, co-perfusion of pGlu-Glu-Pro-NH2 with TRH in equimolar concentrations yielded a significant attenuation of TRH-induced acetylcholine release. The results have supported the study's hypothesis that pGlu-Glu-Pro-NH2 opposes the cholinergic effect of TRH in the mammalian central nervous system. The enantiomer pGlu-d-Glu-Pro-NH2 affected neither basal extracellular nor TRH-induced increase of acetylcholine levels.

Sex influences on cholinesterase inhibitor treatment in elderly individuals with alzheimer's disease

BACKGROUND

The second generation of cholinesterase inhibitors (ChEIs) is approved in the United Kingdom for the treatment of mild to moderate Alzheimer's disease (AD). The UK National Institute of Clinical Excellence has raised questions, however, about whether ChEIs are cost-effective for the treatment of dementia. To address these concerns, it is important to identify factors that predict which patients may have the best response to ChEl treatment.

OBJECTIVE

We reviewed animal studies and human clinical studies to address whether sex can predict and influence the response to ChEI treatment based on differences in neuroanatomy, pharmacokinetics, and prevalence of dementia.

METHODS

Relevant articles examining the use of ChEIs in humans with dementia (especially in AD) and in animals were identified through searches of several databases, including MEDLINE, PubMed, and EMBASE for general medical topics, the Cochrane Controlled Clinical Trials Register and CINAHL DIRECT for nursing and allied health issues, and PsycLIT for reviews of psychology and psychiatry topics (1980 June 2006). Articles reviewed were limited to those that discussed the use of ChEIs in relation to sex.

RESULTS

Animal studies have produced a substantial amount of evidence to support the hypothesis that sex may influence the response to ChEIs and, in particular, that testosterone may play a significant role in producing this difference by its influence on the entry of ChEIs into the brain. The results of clinical studies in humans, on the other hand, have been mixed. Two double-label and open-label clinical studies suggested that there may be a 3-way interaction between apolipoprotein E genotype, sex, and tacrine (range, P = 0.03 to P = 0.05). Seven double-blind, open-label clinical trials and 13 case studies of donepezil, rivastigmine, and galantamine produced little evidence of an association between treatment outcomes (as measured with clinical rating scales) and sex, although in an open-label 2-year study in women with AD treated with donepezil, women had lower mortality rates than men (10% and 20%, respectively; P = 0.003). One study produced weak evidence that women treated with ChEIs may experience more adverse effects than men, but this may have been attributable to low body weight rather than to sex differences.

CONCLUSIONS

A substantial relation has not been established between sex and the second-generation ChEIs currently used in clinical settings for the treatment of AD. If an interaction between sex and ChEI treatment does exist, as suggested in 10 of the studies we analyzed, it is likely to be small and subtle, with much individual variation, as is the case with most neurologic sex differences. Nevertheless, sexual dimorphism in response to ChEI therapy warrants further investigation, especially in regard to its role in the development of novel AD therapies.

Effects of neonatal handling on the basal forebrain cholinergic system of adult male and female rats

Neonatal handling is an early experience which results in improved function of the hypothalamic-pituitary-adrenal axis, increased adaptability and coping as a response to stress, as well as better cognitive abilities. In the present study, we investigated the effect of neonatal handling on the basal forebrain cholinergic system, since this system is known to play an important role in cognitive processes. We report that neonatal handling results in increased number of choline-acetyl transferase immunopositive cells in the septum/diagonal band, in both sexes, while no such effect was observed in the other cholinergic nuclei, such as the magnocellular preoptic nucleus and the nucleus basalis of Meynert. In addition, neonatal handling resulted in increased M1 and M2 muscarinic receptor binding sites in the cingulate and piriform cortex of both male and female rats. A handling-induced increase in M1 muscarinic receptor binding sites was also observed in the CA3 and CA4 (fields 3 and 4 of Ammon's horn) areas of the hippocampus. Furthermore, a handling-induced increase in acetylcholinesterase staining was found only in the hippocampus of females. Our results thus show that neonatal handling acts in a sexually dimorphic manner on one of the cholinergic parameters, and has a beneficial effect on BFCS function, which could be related to the more efficient and adaptive stress response and the superior cognitive abilities of handled animals.

Sexually dimorphic effects of the Lhx7 null mutation on forebrain cholinergic function

It has been reported recently that mice lacking both alleles of the LIM-homeobox gene Lhx7, display dramatically reduced number of forebrain cholinergic neurons. In the present study, we investigated whether the Lhx7 mutation affects male and female mice differently, given the fact that gender differences are consistently observed in forebrain cholinergic function. Our results show that in adult male as well as female Lhx7 homozygous mutants there is a dramatic loss of choline acetyltransferase immunoreactive forebrain neurons, both projection and interneurons. The reduction of forebrain choline acetyltransferase immunoreactive neurons in Lhx7 homozygous mutants is accompanied by a decrease of acetylcholinesterase histochemical staining in all forebrain cholinergic neuron target areas of both male and female homozygous mutants. Furthermore, there was an increase of M1-, but not M2-, muscarinic acetylcholine receptor binding site density in the somatosensory cortex and basal ganglia of only the female homozygous mutant mice. Such an increase can be regarded as a mechanism acting to compensate for the dramatically reduced cholinergic input, raising the possibility that the forebrain cholinergic system in female mice may be more plastic and responsive to situations of limited neurotransmitter availability. Finally, our study provides additional data for the sexual dimorphism of the forebrain cholinergic system, as female mice appear to have a lower density of M1-muscarinic acetylcholine receptors in the striatal areas of the basal ganglia and a higher density of M2-muscarinic acetylcholine receptors, in a number of cortical areas, as well as the striatal areas of the basal ganglia.

Estrogen-metabolizing gene COMT polymorphism synergistic APOE epsilon4 allele increases the risk of Alzheimer disease

Alzheimer disease (AD) is a polygenic multifactorial disorder. Several studies suggested that the neuroprotective effect of estrogen was based on an APOE-dependent mechanism. The goals of the current study were to determine if the genes involved in estrogen metabolism were linked to the risk of AD and find out if there was an interaction between estrogen-metabolizing gene polymorphisms and the APOE epsilon4 allele in the risk of prevalent AD. We investigated 66 patients with AD and 86 age- and gender-matched normal subjects. The polymorphisms of APOE and estrogen-metabolizing genes CYP17, CYP1A1 and COMT were examined. No association was found between each estrogen-metabolizing gene polymorphism and AD. However, the COMT HH genotype and APOE epsilon4 allele had a synergistic effect on the risk of AD. Taking subjects with epsilon4-epsilon4-/HH- as reference, the risk of developing AD in subjects with one epsilon4 allele (epsilon4+epsilon4-/HH-) was 2.6 (95% confidence interval, CI, 0.7- 9.1); however, the risk in subjects with both HH and one epsilon4 (epsilon4+epsilon4-/HH+) increased to 3.6 (95% CI 1.2-10.6). The subjects with homozygous epsilon4 still had the highest risk in developing AD (odds ratio 6.6, 95% CI 0.6-69.6). The p value of the linear trend test for this regression model was 0.004. It is possible that a high metabolism of estrogen by COMT may have reduced the protective effect of estrogen in AD. Further studies to clarify this interaction may improve our understanding of the generic risks for AD.

Further evidence for the cholinergic hypothesis of aging and dementia from the canine model of aging

Memory decline in human aging and dementia is linked to dysfunction of the cholinergic system. Aging dogs demonstrate cognitive impairments and neuropathology that models human aging and dementia. This paper reviews recent evidence suggesting cholinergic involvement in canine cognitive aging based on studies with the anti-cholinergic drug, scopolamine, and a novel acetylcholinesterase inhibitor, phenserine. In particular, we examine: (1) the cognitive specificity of scopolamine's impairment in dogs, (2) the effect of age on scopolamine impairment and (3) the effect of phenserine on cognitive performance in dogs. Our findings indicate that working memory performance is disrupted by scopolamine at doses that do not disrupt non-cognitive behavior or long-term, semantic-like, memory, as indicated by performance of previously learned discriminations. This pattern of deficits is also seen in human and canine aging. We demonstrate that aged dogs are more sensitive to the impairing effects of scopolamine than young dogs, suggesting a decrease in cholinergic tone with increasing age. Dogs receiving phenserine demonstrate improved learning and memory compared to placebo controls. Our findings suggest that cholinergic decline could result in memory impairment, but that the memory impairment may be secondary to deficits in attention and/or encoding of new information. Together, these results suggest that the canine cholinergic system declines with age and that the aged dog is a unique model for screening therapeutics and for examining the relationship between amyloid pathology and cholinergic dysfunction in age-dependent cognitive decline.

Sexually dimorphic effects of hippocampal cholinergic deafferentation in rats

To determine whether the basal forebrain-hippocampal cholinergic system supports sexually dimorphic functionality, male and female Long-Evans rats were given either selective medial septum/vertical limb of the diagonal band (MS/VDB) cholinergic lesions using the neurotoxin 192 IgG-saporin or a control surgery and then postoperatively tested in a set of standard spatial learning tasks in the Morris water maze. Lesions were highly specific and effective as confirmed by both choline acetyltransferase/parvalbumin immunostaining and acetylcholinesterase histochemistry. Female controls performed worse than male controls in place learning and MS/VDB lesions failed to impair spatial learning in male rats, both consistent with previous findings. In female rats, MS/VDB cholinergic lesions facilitated spatial reference learning. A subsequent test of learning strategy in the water maze revealed a female bias for a response, relative to a spatial, strategy; MS/VDB cholinergic lesions enhanced the use of a spatial strategy in both sexes, but only significantly so in males. Together, these results indicate a sexually dimorphic function associated with MS/VDB-hippocampal cholinergic inputs. In female rats, these neurons appear to support sex-specific spatial learning processes.

Sexually dimorphic aging of dendritic morphology in CA1 of hippocampus

During aging, rats of both sexes experience a decline in performance on hippocampal-dependent tasks. Investigations into the neuroanatomical correlates of this functional decline have been conducted almost exclusively in male subjects. In the present study, dendritic spine density in stratum radiatum and complexity of the entire apical dendritic tree were quantified using Golgi-Cox-stained tissue in young (3-5 months) and aged (19-22 months) rats of both sexes. Because both cognitive decline and hippocampal morphology may be influenced by ovarian hormonal state, young adult females were examined during either proestrus or estrus, and aged females were examined in one of two reproductively senescent states: persistent estrus or persistent diestrus. A sex difference in dendritic branching of CA1 pyramidal cells was found among young adults. However, this difference disappeared during aging, due to a reduction in branching with age for males but not for females. Spine density was not influenced by age or sex, nor did ovarian hormone status influence either measure. These results are consistent with our previous findings in the rat medial prefrontal cortex and primary motor cortex and with the human literature, which indicate that age-related atrophy of cognitive brain regions is more severe for males than females.

Age-related sex differences in spatial learning and basal forebrain cholinergic neurons in F344 rats

Basal forebrain cholinergic neurons are important for spatial learning in rodents. Spatial learning ability is reportedly better in males than females, and declines with age. To examine the role of cholinergic function in sex- or age-related differences in spatial learning, we compared the size of basal forebrain cholinergic neurons (BFCN) of young and aged male and female Fischer 344 (F344) rats that had been trained in the Morris water maze. Young male and female rats were equally proficient in finding the platform during training trials, but probe tests revealed that young male rats had better knowledge of the platform's precise location. Impairments in spatial learning were observed in aged rats, and the advantage of males over females was lost. BFCN were significantly larger in young male than young female rats, and were correlated with spatial memory performance for both groups. BFCN were smaller in aged than young males; no change was seen between young and aged females. In the groups of aged rats the correlation between neuron size and spatial memory was lost. The present findings provide further evidence of a role for the basal forebrain cholinergic system in spatial learning, but reveal a complex interaction between sex, age and behavioral performance.

Neuroprotection associated with running: is it a result of increased endogenous neurotrophic factors?

The possible neuroprotective effect of physical exercise was investigated in rats after middle cerebral artery occlusion (MCAO), a focal stroke model. It was found that physical exercise in the form of a 12-week treadmill running programme reduced the volume of infarction caused by MCAO. At the molecular level, reverse transcription polymerase chain reaction revealed that the runner had increased gene expression for nerve growth factor (NGF) over the nonrunner with or without MCAO. Expression of the NGF receptors, p75, was increased only in the absence of MCAO. In addition, runners showed a significantly higher number of cholinergic neurons, which constitutively expressed p75, in the horizontal diagonal band of Broca. The present findings suggest that neuroprotection after physical exercise may be a result of an increase in an endogenous neurotrophic factor nerve growth factor and the proliferation of its receptive cholinergic neurons.

NGF induces appearance of adult-like response to spatial novelty in 18-day male mice

We investigated the effects of Nerve Growth Factor (NGF) administration on the maturation of reactivity to spatial and non-spatial novelty in developing mice. CD-1 mice of both sexes received intracerebral administration of NGF on postnatal day (pnd) 15, and their response to object displacement (spatial novelty) and object substitution (object novelty) were assessed in a spatial open-field with four objects on pnd 18 or 28. On pnd 18, NGF induced only in males precocious appearance of spatial novelty discrimination, while increasing choline acetyltransferase activity in neocortex and hippocampus of both sexes. The behavioral and neurochemical effects disappeared by pnd 28. NGF triggers adult-like responding to spatial novelty in developing mice and such effect is gender-specific.

Sex differences in androgen receptor immunoreactivity in basal forebrain nuclei of elderly and Alzheimer patients

The vertical limb of the diagonal band of Broca (VDB or Ch2) and the nucleus basalis of Meynert (NBM or Ch4) are major cholinergic nuclei of the human basal forebrain, a complex that is affected in Alzheimer's disease (AD). Sex hormones influence the function of these cholinergic neurons in animals and humans and we showed earlier that estrogen and androgen receptors (AR) are present in both the VDB and the NBM of young patients of 20-39 years of age. The aim of the present study was to investigate whether AR expression changes in relation to aging and AD. In both brain areas of male and female patients over the age of 56 nuclear staining had almost disappeared and cytoplasmic AR expression was decreased. This decrease was most pronounced in the VDB of men. In addition, the proportion of neurons showing cytoplasmic AR expression was higher in control aged women than in control aged men in both the VDB and the NBM. Surprisingly, cytoplasmic ARs were significantly decreased in the VDB and the NBM only in AD women and not in AD men. These observations suggest the possible involvement of androgens in the functional changes of the basal forebrain nuclei in aging and AD.

Combined 192 IgG-saporin and 5,7-dihydroxytryptamine lesions in the male rat brain: a neurochemical and behavioral study

In a previous experiment [Eur J Neurosci 12 (2000) 79], combined intracerebroventricular injections of 5,7-dihydroxytryptamine (5,7-DHT; 150 microg) and 192 IgG-saporin (2 microg) in female rats produced working memory impairments, which neither single lesion induced. In the present experiment, we report on an identical approach in male rats. Behavioral variables were locomotor activity, T-maze alternation, beam-walking, Morris water-maze (working and reference memory) and radial-maze performances. 192 IgG-saporin reduced cholinergic markers in the frontoparietal cortex and the hippocampus. 5,7-DHT lesions reduced serotonergic markers in the cortex, hippocampus and striatum. Cholinergic lesions induced motor deficits, hyperactivity and reduced T-maze alternation, but had no other effect. Serotonergic lesions only produced hyperactivity and reduced T-maze alternation. Beside the deficits due to cholinergic lesions, rats with combined lesions also showed impaired radial-maze performances. We confirm that 192 IgG-saporin and 5,7-DHT injections can be combined to produce concomitant damage to cholinergic and serotonergic neurons in the brain. In female rats, this technique enabled to show that interactions between serotonergic and basal forebrain cholinergic mechanisms play an important role in cognitive functions. The results of the present experiment in male rats are not as clear-cut, although they are not in obvious contradiction with our previous results in females.

Cholinergic innervation of pial arteries in senescent rats: an immunohistochemical study

Perivascular acetylcholine (ACh)-immunoreactive nerve fibres were demonstrated in basilar and middle cerebral arteries, in pial arteries and arterioles and in intracerebral arteries of male Fisher 344 rats of 6 months (young), 15 months (adult) and 22 months (senescent). Analysis included whole mounts of basilar and middle cerebral arteries, of pial arteries and sections of brain including pia-arachnoid membrane to demonstrate the localization of nerve fibres throughout the wall of pial and of intracerebral arteries. ACh-immunoreactive nerve fibres were demonstrated by indirect immunohistochemistry using a polyclonal anti-ACh antibody and their relative density was quantified. Perivascular ACh-immunoreactive nerve fibres were located in basilar and middle cerebral arteries, in pial arteries and arterioles and in intracerebral arteries. These fibres were found in the adventitia and adventitia-media border with a higher density in pial rather than in intracerebral arteries. A decrease of ACh-immunoreactive nerve fibres was observed both in pial and intracerebral arteries of adult or senescent rats compared to younger cohorts. The direct demonstration of ACh-immunoreactive nerve fibres in the cerebrovascular tree may contribute to evaluate the influence of experimental and pathological conditions on cerebrovascular cholinergic neuroeffector mechanisms, including a role of cholinergic innervation in the pathophysiology of cerebrovascular disease of the elderly.

Estrogen effects on object memory and cholinergic receptors in young and old female mice

We investigated whether object recognition memory is modulated by estrogen in young (5 month) and aged (24 month) female C57Bl/6J mice, and if cholinergic muscarinic receptors might contribute to this response. Mice that were ovariectomized, or ovariectomized plus estradiol-treated three weeks before behavioral testing or quantitative autoradiography were compared to intact mice. Memory for a previously encountered object deteriorated significantly between 3 and 6h after initial exposure, regardless of animal age. In both young and aged mice, estradiol-treated mice showed significantly greater recall than did ovariectomized mice. In both age groups, the apparent number of [(3)H]pirenzepine/M(1)-like and [(3)H]AFDX384/M(2)-like muscarinic receptor binding sites was reduced in the basal forebrain as well as its projection areas following ovariectomy, but this decrease was not alleviated by estrogen. Aging poorly affected object memory, but reduced muscarinic binding in some cortical subregions and in the caudate nucleus. These findings suggest that estrogen effects on memory in C57Bl/6J mice are not due to changes in the number of muscarinic receptors.

Spatial reference memory and neocortical neurochemistry vary with the estrous cycle in C57BL/6 mice

Estrous cycle-related variations of spatial reference memory and neurochemistry in intact female mice were examined. Spatial reference memory was tested in cycling females, ovariectomized (OVX) females, and males by using a 1-day water maze protocol. Choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) activities were measured in the hippocampus and neocortex. Estrus females exhibited worse spatial acquisition and 30-min retention than did proestrus and metestrus females, higher neocortical ChAT activity than proestrus females, and higher neocortical GAD activity than OVX females and males. Neocortical, rather than hippocampal, neurochemistry was more sensitive to hormonal modulation, suggesting that hormonal mediation of neocortical function may play a critical role in regulating spatial reference memory in female mice.

Sexual diergism in rat hypothalamic-pituitary-adrenal axis responses to cholinergic stimulation and antagonism

The hypothalamic-pituitary-adrenal (HPA) axis has differential physiological activity in male and female animals (sexual diergism). Central cholinergic systems stimulate this endocrine axis. In the present study we investigated muscarinic and nicotinic cholinergic influences on HPA axis activity in male and female rats by pretreatment with selective cholinergic receptor antagonists followed by stimulation with physostigmine (PHYSO), an acetylcholinesterase inhibitor. Hormonal measures were plasma arginine vasopressin (AVP), adrenocorticotropic hormone (ACTH), and corticosterone (CORT). Male rats had significantly greater AVP and ACTH responses to PHYSO alone than did females. Scopolamine (SCOP) enhanced the AVP response to PHYSO to a greater extent in males than in females. In contrast, mecamylamine (MEC) enhanced the AVP response in females but decreased it in males. SCOP potentiated, and MEC inhibited, the stimulatory effect of PHYSO on ACTH in both sexes, but SCOP potentiation was greater in males, and MEC inhibition was greater in females. Absolute CORT increases following PHYSO were greater in females, but percent increases over baseline were greater in males. Similar to their effects on ACTH responses, MEC attenuated, and SCOP enhanced, CORT responses to PHYSO. These results suggest that cholinergic receptor subtypes may influence HPA axis activity differentially in male and female rats.

Galanin regulates the postnatal survival of a subset of basal forebrain cholinergic neurons

The neuropeptide galanin colocalizes with choline acetyltransferase, the synthetic enzyme for acetylcholine, in a subset of cholinergic neurons in the basal forebrain of rodents. Chronic intracerebroventricular infusion of nerve growth factor induces a 3- to 4-fold increase in galanin gene expression in these neurons. Here we report the loss of a third of cholinergic neurons in the medial septum and vertical limb diagonal band of the basal forebrain of adult mice carrying a targeted loss-of-function mutation in the galanin gene. These deficits are associated with a 2-fold increase in the number of apoptotic cells in the forebrain at postnatal day seven. This loss is associated with marked age-dependent deficits in stimulated acetylcholine release, performance in the Morris water maze, and induction of long-term potentiation in the CA1 region of the hippocampus. These data provide unexpected evidence that galanin plays a trophic role to regulate the development and function of a subset of septohippocampal cholinergic neurons.

Effects of age on gene expression during estrogen-induced synaptic sprouting in the female rat

Age and estrogen treatment influenced fiber outgrowth and compensatory neuronal sprouting after unilateral entorhinal cortex lesions (ECL) which model Alzheimer disease-like deafferentation in the dentate gyrus of the hippocampus. In young F344 rats (3 months old), ovariectomy (OVX) decreased reactive fiber outgrowth by 60%. Sprouting in middle-aged rats (18 months old) was reduced in intact females; no further reduction was caused by OVX. Several astrocyte mRNAs were measured in the dentate gyrus of young and middle-aged female rats in three different estrogen states (sham OVX, OVX, or OVX + estradiol) 1 week after ECL. Glial fibrillary acidic protein (GFAP) mRNA was twofold greater in middle-aged rats than young, although both ages showed threefold increases in response to ECL. In prior studies GFAP was found to be decreased by estradiol treatment 3-4 days after ECL; in this study GFAP mRNA had returned to sham OVX levels in young rats by 7 days post-ECL. Surprisingly, estradiol treatment increased GFAP mRNA levels by 25% above OVX in middle-aged rats. Apolipoprotein E (apoE) mRNA was decreased 20% by age in the dentate, although both age groups showed a 25% increase in apoE mRNA in response to ECL. Apolipoprotein J (apoJ) mRNA was increased 20% in the dentate gyrus of middle-aged rats, and both age groups responded to ECL with a 65% increase in apoJ mRNA. The estrogen state did not alter levels of either apolipoprotein mRNA in the deafferented dentate. The data suggest that the estrogen-induced decrease of GFAP in response to lesions does not persist at 7 days post-ECL during sprouting. Overall effects of age on the dentate gyrus include elevated GFAP mRNA and decreased apoE mRNA. The cortical wound site showed consistent enhancement of GFAP mRNA in both age groups by estradiol above sham OVX and greater responses in middle-aged rats.

Functional sex differences ('sexual diergism') of central nervous system cholinergic systems, vasopressin, and hypothalamic-pituitary-adrenal axis activity in mammals: a selective review

Sexual dimorphism of the mammalian central nervous system (CNS) has been widely documented. Morphological sex differences in brain areas underlie sex differences in function. To distinguish sex differences in physiological function from underlying sexual dimorphisms, we use the term, sexual diergism, to encompass differences in function between males and females. Whereas the influence of sex hormones on CNS morphological characteristics and function of the hypothalamic-pituitary-gonadal axis has been well-documented, little is known about sexual diergism of CNS control of the hypothalamic-pituitary-adrenal (HPA) axis. Many studies have been conducted on both men and women but have not reported comparisons between them, and many animal studies have used males or females, but not both. From a diergic standpoint, the CNS cholinergic system appears to be more responsive to stress and other stimuli in female than in male mammals; but from a dimorphic standpoint, it is anatomically larger, higher in cell density, and more stable with age in males than in females. Dimorphism often produces diergism, but age, hormones, environment and genetics contribute differentially. This review focuses on the sexual diergism of CNS cholinergic and vasopressinergic systems and their relationship to the HPA axis, with resulting implications for the study of behavior, disease, and therapeutics.

Effects of an acidic fibroblast growth factor fragment analog on learning and memory and on medial septum cholinergic neurons in senescence-accelerated mice

We examined the effects of repeated subcutaneous injections of an acidic fibroblast growth factor fragment analog, [Ala16] acidic fibroblast growth factor (1-29), on learning and memory and on the choline acetyltransferase immunoreactivity of forebrain neurons in senescence-accelerated mice. One group of accelerated senescence-prone mice (accelerated senescence-prone-8) received [Ala16] acidic fibroblast growth factor (1-29), whereas the other group of accelerated senescence-prone-8 mice and a group of accelerated senescence-resistant mice (control) received vehicle solution. Injections began at three weeks after birth and were given weekly for 10 months. In a passive avoidance test, the mean retention latency at three, six and nine months of age was significantly longer in controls (vehicle-treated accelerated senescence-resistant-1) and acidic fibroblast growth factor fragment-treated accelerated senescence-prone-8 than in vehicle-treated accelerated senescence-prone-8 mice, and the latency in acidic fibroblast growth factor fragment-treated accelerated senescence-prone-8 mice was significantly shorter than that in controls only at nine months of age. In the Morris water maze task, the mean latency to climb onto the platform was significantly longer in acidic fibroblast growth factor fragment- and vehicle-treated accelerated senescence-prone-8 mice than in controls. However, the mean latency in the third and fourth trial blocks was significantly shorter for acidic fibroblast growth factor fragment-treated accelerated senescence-prone-8 than for vehicle-treated accelerated senescence-prone-8 mice. In the probe trials, controls and acidic fibroblast growth factor fragment-treated accelerated senescence-prone-8 mice spent significantly more time in the quadrant in which the platform had previously been located than in the other three quadrants. In acidic fibroblast growth factor fragment-treated accelerated senescence-prone-8 mice, the density of medial septum neurons intensely stained for choline acetyltransferase was significantly greater than that in vehicle-treated accelerated senescence-prone-8 mice, but significantly less than that in controls. The results indicate that the beneficial effect of [Ala16] acidic fibroblast growth factor (1-29) on learning and memory function in accelerated senescence-prone-8 mice may be related to a preservation of function in medial septum cholinergic neurons.

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

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

The effects of neonatal basal forebrain lesions on cognition: towards understanding the developmental role of the cholinergic basal forebrain

Abnormal development of the cholinergic basal forebrain has been implicated in numerous developmental disabilities such as Rett Syndrome and Down Syndrome. This review summarizes recent data using two rodent animal models that involve interrupting cholinergic basal forebrain projections on postnatal day 1 and postnatal day 7 when basal forebrain fibers are beginning to innervate their neocortical and hippocampal targets, respectively. In one model, electrolytic lesions in mice aimed at the basal forebrain on postnatal day 1 transiently reduce cholinergic markers in neocortex which induce permanent alterations in neocortical anatomy that correlate with impairments on cognitive tasks. Furthermore, the lesion effects are sex dependent. In another model, 192 IgG saporin lesions in rats on postnatal day 7 permanently reduce cholinergic markers in neocortex and hippocampus, and result in mild impairments in spatial processing, acquisition and exploratory activities. These data suggest that during the first postnatal week of development the cholinergic basal forebrain system is critical for normal neocortical differentiation and, possibly synaptogenesis in neural circuits that will be important for spatial memory and acquisition of spatial data. During the second postnatal week of development, the cholinergic basal forebrain system appears to take on a role largely similar to its adult role in selective attention and processing of new information. These studies also suggest strongly that interrupting cholinergic basal forebrain innervation of neocortex and hippocampus leads to anatomical and neurochemical abnormalities that may serve as neural substrates for some of the cognitive deficits seen in disorders such as Rett Syndrome and Down Syndrome.

Increased synaptic sprouting in response to estrogen via an apolipoprotein E-dependent mechanism: implications for Alzheimer's disease

Estrogen replacement therapy appears to delay the onset of Alzheimer's disease (AD), but the mechanisms for this action are incompletely known. We show how the enhancement of synaptic sprouting by estradiol (E2) in response to an entorhinal cortex (EC) lesion model of AD may operate via an apolipoprotein E (apoE)-dependent mechanism. In wild-type (WT) mice, ovariectomy decreased commissural/associational sprouting to the inner molecular layer of the dentate gyrus, with synaptophysin (SYN) as a marker. E2 replacement returned SYN in the inner layer to levels of EC-lesioned, ovary-bearing controls and increased the area of compensatory synaptogenesis in the outer molecular layer. In EC-lesioned apoE-knock-out (KO) mice, however, E2 did not enhance sprouting. We also examined apoJ (clusterin) mRNA, which is implicated in AD by its presence in senile plaques, its transport of Abeta across the blood-brain barrier, and its induction by neurodegenerative lesioning. ApoJ mRNA levels were increased by E2 replacement in EC-lesioned WT mice but not in apoE-KO mice. These data suggest a mechanism for the protective effects of estrogens on AD and provide a link between two important risk factors in the etiology of AD, the apoE epsilon4 genotype and an estrogen-deficient state. This is also the first evidence that SYN, a presynaptic protein involved in neurotransmitter release, is regulated by E2 in the adult brain, and that apoE is necessary for the induction of apoJ mRNA by E2 in brain injury.

Effects of estrogen replacement on choline acetyltransferase and trkA mRNA expression in the basal forebrain of aged rats

The effects of one week of estrogen replacement on choline acetyltransferase (ChAT) and trkA mRNA expression are examined in young and aged rodents to determine whether estrogen continues to affect cholinergic neurons in aging brain. Significant increases in ChAT and trkA are observed in the nucleus basalis of Meynert (nBM) of both age groups. ChAT expression is also increased in the HDB without changes in trkA expression. Results indicate modulation of ChAT expression by estrogen is retained in the aged rodent brain and suggests the possibility that changes in ChAT expression may be dissociated from concurrent alterations in trkA.

Thyroid hormone-induced plasticity in the adult rat brain

It is well known that thyroid hormone plays a crucial role in the development and maturation of the nervous system. However, little is known about the role of thyroid hormone in the adult brain. In this short review we have dwelt on this point, with regard to the role of thyroid hormone on neuropeptide gene expression regulation in the paraventricular nucleus of the hypothalamus and in extrahypothalamic brain areas, on neurotrophin and neurotrophin receptor expression in the hippocampus and basal forebrain in basal conditions, and after neurotoxic challenges. Effects of hypothyroidism are discussed in view of a possible role of thyroid status in brain aging quality.

Sexually dimorphic effects of anti-NGF treatment in neonatal rats

This study investigated how chronic perinatal reduction of nerve growth factor (NGF) affected brain cholinergic markers in the two sexes. Rats received anti-NGF on postnatal days (PNDs) 2-12, and choline acetyltransferase (ChAT) activity was measured on PND 16. Anti-NGF significantly reduced cortical ChAT activity in males, but not in females; no sex-dependent effects were found in hippocampus or striatum. These data suggest sexual dimorphism in cholinergic responsiveness to NGF.

Quantitative morphological changes in neurons from the dorsal lateral geniculate nucleus of young and old rats

BACKGROUND

We studied the morphological changes occurring in neurons from the dorsal lateral geniculate nucleus (dLGN) during aging by analysing the size and shape of cell bodies and nuclei.

METHODS

Male albino Wistar rats, aged 3, 18, 24, and 30 months, were used. After appropriate tissue preparation and following the usual histological procedure, the profiles of 1,920 neuronal bodies and nuclei were drawn using a camera lucida. Data was later recorded and processed with a semiautomatic image analyser.

RESULTS AND CONCLUSIONS

We observed that dLGN neurons do not change in size from the age of 3-24 months. Between 24 and 30 months, the soma and nucleus of the cell undergo hypertrophy, 32.8% and 35.6%, respectively, when compared to those from 3-month-old animals (P < 0.01). Furthermore, we found a high correlation between cell body size/nucleus size, which does not disappear with age. The r values (correlation coefficient) were 0.7998, 0.8662, 0.8433 and 0.7304, and R2 (determination coefficient) was equal to 0.6397, 0.7504, 0.7112, and 0.5335. These latter values show that in 63.97%, 75.04%, 71.12%, and 53.35% of cases, respectively, modifications in somata size were accompanied by similar changes in nucleus size, and vice-versa. The study of the shape of the soma and nucleus of the cell revealed that both structures have a rounded-oval configuration that does not change in a significant way from adulthood to old age.

Glial cell line-derived neurotrophic factor induces the dopaminergic and cholinergic phenotype and increases locomotor activity in aged Fischer 344 rats

Glial cell line-derived neurotrophic factor has been shown to affect dopaminergic and cholinergic neuron markers and functions in young rats. However, it is not known if the response to exogenous glial cell line-derived neurotrophic factor is augmented during normal aging. Thus, the effects of chronic intraventricular infusions of glial cell line-derived neurotrophic factor were determined in young adult (three-months-old) and aged (24-months-old) Fischer 344 (F344) male rats. The effects of glial cell line-derived neurotrophic factor were compared to the effects of the neurotrophin nerve growth factor. Growth factors were administered at a dose of 10 mg/day for 14 days. Locomotor activity and weight changes were also examined in all rats. Aged F344 rats showed significantly reduced (by 75-80%) locomotor activity compared to young rats. In glial cell line-derived neurotrophic factor-treated aged and young rats there was significantly increased (242% and 149%, respectively) locomotor activity measured at seven days. There was also a significant increase in locomotor activity measured 14 days after the start of infusion. Both glial cell line-derived neurotrophic factor and nerve growth factor reduced weight gain by 10% in young and old F344 rats. Two weeks following the start of nerve growth factor or glial cell line-derived neurotrophic factor administration the brains were used for neurochemical analyses. Glial cell line-derived neurotrophic factor significantly increased tyrosine hydroxylase activity in the substantia nigra and striatum of aged rats and in the substantia nigra of young rats. Nerve growth factor treatment did not significantly affect tyrosine hydroxylase activity. However, glial cell line-derived neurotrophic factor and nerve growth factor increased choline acetyltransferase activity in the septum, hippocampus, striatum and cortex of aged rats and in the hippocampus and striatum of young rats to a comparable degree. These findings indicate that specific dopaminergic and cholinergic neuron populations remain responsive to glial cell line-derived neurotrophic factor during the life span of the rat and may be involved in maintaining phenotypic expression within multiple neuronal populations. Additionally, the glial cell line-derived neurotrophic factor-induced up-regulation of brain neurotransmitter systems may be responsible for increased locomotor activity in F344 rats.

Expression of insulin receptor-related receptor in the rat brain examined by in situ hybridization and immunohistochemistry

Insulin receptor-related receptor (IRR) is a member of the insulin receptor family. However, its endogenous ligand and physiological roles are unknown. To elucidate the physiological roles of IRR, an orphan receptor, in the brain, we examined its expression at mRNA and protein levels in the brain by in situ hybridization and immunohistochemistry, respectively. The expression of IRR mRNA in the brain was highly restricted to the forebrain including the nucleus of the diagonal band, medial septal nucleus, ventral pallidum, accumbens nucleus and caudate putamen, and the brainstem including the prepositus hypoglossal nucleus, medial vestibular nucleus, gigantocell reticular nucleus, paragigantocellular nucleus and ventral cochlear nucleus. Most IRR mRNA-positive cells in the forebrain but not in the brainstem were cholinergic neurons. However, most IRR mRNA in the forebrain and brainstem was coexpressed with that of trkA, a high-affinity receptor for nerve growth factor. IRR-immunoreactive cell bodies were also detected in the forebrain and brainstem. The pattern of IRR immunoreactivity was similar to that of IRR mRNA. Its restricted pattern indicates that IRR plays unique roles in the brain, in contrast to insulin and insulin-like growth factor-I receptors, other members of the insulin receptor family, which are widely expressed in the brain.

Potassium, but not atropine-stimulated cortical acetylcholine efflux, is reduced in aged rats

Using in vivo microdialysis, cortical acetylcholine (ACh) efflux was measured in freely moving Brown Norway/Fischer 344 F1 rats, aged 4 or 22 months. The effects of local, intracortical perfusion of atropine (1.0 or 100.0 microM) via the dialysis probe were compared to local K+ (100.0 mM) stimulation in the presence of elevated extracellular Ca2+ (2.5 mM). Basal cortical ACh efflux in aged rats was similar to that of young animals. Administration of atropine (1.0 or 100.0 microM) via the cortical dialysis probe substantially increased cortical ACh efflux, but did not differentially stimulate ACh efflux in young and aged rats. In contrast, ACh efflux stimulated locally with K+ and Ca2+ was significantly reduced in aged rats relative to young adults. The implications of the dissociable effects of K(+)-depolarization and muscarinic blockade for local regulation of cortical ACh efflux in aged animals are discussed.