Cholinergic lesions produce task-selective effects on delayed matching to position and configural association learning related to response pattern and strategy

Neurobehavioral basis of Maier 3-table and other matching-to-place tasks

The Maier 3-table task comprises three phases conducted each day. During the exploration phase, rats explore the entire apparatus. During the information phase, the rats are placed on one of the three tables where food is found. During the test phase, the animals are placed at the starting point on one of the two remaining tables and must enter the goal table where they previously ate. The acquisition of the Maier 3-table task was slowed down after lesions of the septum, fornix, hippocampus, medial prefrontal cortex, or posterior parietal cortex. Because of its time-consuming nature, the Maier 3-table task has more recently been superseded by appetitive matching-to-place in Y- or T-mazes or the circular water maze, because experimenters skip over the exploration phase. Nevertheless, like the Maier 3-table task, the acquisition of the Y- or T-maze matching-to-place task was retarded after lesions of the medial septum or medial prefrontal cortex, more particularly its prelimbic-infralimbic part. Like the previous task, the water-maze version is sensitive to lesions of the medial septum or retrosplenial cortex. Despite methodological differences between the three procedures, these results indicate common neurobiological bases of matching-to-place learning.

Medial septum tau accumulation induces spatial memory deficit via disrupting medial septum-hippocampus cholinergic pathway

Tau accumulation and cholinergic impairment are characteristic pathologies in Alzheimer's disease (AD). However, the causal role of tau accumulation in cholinergic lesion is elusive. Here, we observed an aberrant tau accumulation in the medial septum (MS) of 3xTg and 5xFAD mice, especially in their cholinergic neurons. Overexpressing hTau in mouse MS (MShTau ) for 6 months but not 3 months induced spatial memory impairment without changing object recognition and anxiety-like behavior, indicating a specific and time-dependent effect of MS-hTau accumulation on spatial cognitive functions. With increasing hTau accumulation, the MShTau mice showed a time-dependent cholinergic neuron loss with reduced cholinergic projections to the hippocampus. Intraperitoneal administration of donepezil, a cholinesterase inhibitor, for 1 month ameliorated the MS-hTau-induced spatial memory deficits with preservation of MS-hippocampal cholinergic pathway and removal of tau load; and the beneficial effects of donepezil was more prominent at low dose. Proteomics revealed that MS-hTau accumulation deregulated multiple signaling pathways with numerous differentially expressed proteins (DEPs). Among them, the vacuolar protein sorting-associated protein 37D (VP37D), an autophagy-related protein, was significantly reduced in MShTau mice; the reduction of VP37D was restored by donepezil, and the effect was more significant at low dose than high dose. These novel evidences reveal a causal role of tau accumulation in linking MS cholinergic lesion to hippocampus-dependent spatial cognitive damages as seen in the AD patients, and the new tau-removal and autophagy-promoting effects of donepezil may extend its application beyond simple symptom amelioration to potential disease modification.

Structural MRI of the basal forebrain as predictor of cognitive response to galantamine in healthy older adults-A randomized controlled double-blinded crossover study

INTRODUCTION

Cholinesterase inhibitors can enhance cognitive functions in healthy elderly and delay cognitive decline in patients with Alzheimer`s disease (AD). However, not everyone benefits from this treatment (non-responders). Current studies show clinical meaningful improvements only in one third of AD patients treated with cholinesterase inhibitors.

METHODS

Here we investigate structural magnetic resonance imaging of the basal forebrain cholinergic system volume (BFvol) as a potential predictor of cognitive response to a single dose of galantamine in healthy adults (n = 18; 59 to 75 years).

RESULTS

We observed that the cognitive response to galantamine, more specifically the attention-dependent filtering performance in a delayed match-to-sample working memory task, correlated with BFvol: Only participants with high BFvol showed a significant positive effect of galantamine on the ability to filter out distracting information during the working memory encoding process.

DISCUSSION

Future studies need to assess whether BFvol may serve as a predictor of the galantamine response in AD patients, too.

Configural Cues Associated with Reward Elicit Theta Oscillations of Rat Retrosplenial Cortical Neurons Phase-Locked to LFP Theta Cycles

Previous behavioral studies implicated the retrosplenial cortex (RSC) in stimulus-stimulus associations, and also in the retrieval of remote associative memory based on EEG theta oscillations. However, neural mechanisms involved in the retrieval of stored information of such associations and memory in the RSC remain unclear. To investigate the neural mechanisms underlying these processes, RSC neurons and local field potentials (LFPs) were simultaneously recorded from well-trained rats performing a cue-reward association task. In the task, simultaneous presentation of two multimodal conditioned stimuli (configural CSs) predicted a reward outcome opposite to that associated with the individual presentation of each elemental CS. Here, we show neurophysiological evidence that the RSC is involved in stimulus-stimulus association where configural CSs are discriminated from each elementary CS that is a constituent of the configural CSs, and that memory retrieval of rewarding CSs is associated with theta oscillation of RSC neurons during CS presentation, which is phase-locked to LFP theta cycles. The results suggest that cue (elementary and configural CSs)-reinforcement associations are stored in the RSC neural circuits, and are retrieved in synchronization with LFP theta rhythm.

Saporin from Saponaria officinalis as a Tool for Experimental Research, Modeling, and Therapy in Neuroscience

Saporin, which is extracted from Saponaria officinalis, is a protein toxin that inactivates ribosomes. Saporin itself is non-selective toxin but acquires high specificity after conjugation with different ligands such as signaling peptides or antibodies to some surface proteins expressed in a chosen cell subpopulation. The saporin-based conjugated toxins were widely adopted in neuroscience as a convenient tool to induce highly selective degeneration of desired cell subpopulation. Induction of selective cell death is one of approaches used to model neurodegenerative diseases, study functions of certain cell subpopulations in the brain, and therapy. Here, we review studies where saporin-based conjugates were used to analyze cell mechanisms of sleep, general anesthesia, epilepsy, pain, and development of Parkinson’s and Alzheimer’s diseases. Limitations and future perspectives of use of saporin-based toxins in neuroscience are discussed.

Effects of Cholinergic Lesions and Cholinesterase Inhibitors on Aromatase and Estrogen Receptor Expression in Different Regions of the Rat Brain

Cholinergic projections have been shown to interact with estrogens in ways that influence synaptic plasticity and cognitive performance. The mechanisms are not well understood. The goal of this study was to investigate whether cholinergic projections influence brain estrogen production by affecting aromatase (ARO), or influence estrogen signaling by affecting estrogen receptor expression. In the first experiment, ovariectomized rats received intraseptal injection of the selective immunotoxin 192IgG-saporin to destroy cholinergic inputs to the hippocampus. In the second experiment ovariectomized rats received daily intraperitoneal injections of the cholinesterase inhibitors donepezil or galantamine for 1 week. ARO activity and relative levels of ARO, ERα, ERß, and GPR30 mRNAs were quantified in the hippocampus, frontal cortex, amygdala and preoptic area. Results show that the cholinergic lesions effectively removed cholinergic inputs to the hippocampus, but had no significant effect on ARO or on relative levels of ER mRNAs. Likewise, injections of the cholinesterase inhibitors had no effect on ARO or ER expression in most regions of the brain. This suggests that effects of cholinergic inputs on synaptic plasticity and neuronal function are not mediated by effects on local estrogen production or ER expression. One exception was the amygdala where treating with galantamine was associated with a significant increase in ARO activity. The amygdala is a key structure involved in registering fear and anxiety. Hence this finding may be clinically relevant to elderly patients who are treated for memory impairment and who also struggle with fear and anxiety disorders.

Multimodal Encoding of Novelty, Reward, and Learning in the Primate Nucleus Basalis of Meynert

Associative learning is crucial for daily function, involving a complex network of brain regions. One region, the nucleus basalis of Meynert (NBM), is a highly interconnected, largely cholinergic structure implicated in multiple aspects of learning. We show that single neurons in the NBM of nonhuman primates (NHPs; n = 2 males; Macaca mulatta) encode learning a new association through spike rate modulation. However, the power of low-frequency local field potential (LFP) oscillations decreases in response to novel, not-yet-learned stimuli but then increase as learning progresses. Both NBM and the dorsolateral prefrontal cortex encode confidence in novel associations by increasing low- and high-frequency LFP power in anticipation of expected rewards. Finally, NBM high-frequency power dynamics are anticorrelated with spike rate modulations. Therefore, novelty, learning, and reward anticipation are separately encoded through differentiable NBM signals. By signaling both the need to learn and confidence in newly acquired associations, NBM may play a key role in coordinating cortical activity throughout the learning process.SIGNIFICANCE STATEMENT Degradation of cells in a key brain region, the nucleus basalis of Meynert (NBM), correlates with Alzheimer's disease and Parkinson's disease progression. To better understand the role of this brain structure in learning and memory, we examined neural activity in the NBM in behaving nonhuman primates while they performed a learning and memory task. We found that single neurons in NBM encoded both salience and an early learning, or cognitive state, whereas populations of neurons in the NBM and prefrontal cortex encode learned state and reward anticipation. The NBM may thus encode multiple stages of learning. These multimodal signals might be leveraged in future studies to develop neural stimulation to facilitate different stages of learning and memory.

Central relaxin-3 receptor (RXFP3) activation increases ERK phosphorylation in septal cholinergic neurons and impairs spatial working memory

The medial septum/diagonal band (MS/DB) is a relay region connecting the hypothalamus and brainstem with the hippocampus, and both the MS/DB and dorsal/ventral hippocampus receive strong topographic GABA/peptidergic projections from the nucleus incertus of the pontine tegmentum. The neuropeptide relaxin-3, released by these neurons, is the cognate ligand for a G_i/o-protein-coupled receptor, RXFP3, which is highly expressed within the MS/DB, and both cholinergic and GABAergic neurons in this region of rat brain receive relaxin-3 positive terminals/boutons. Comprehensive in vitro studies have demonstrated that the cell signaling pathways altered by RXFP3 stimulation, include inhibition of forskolin-activated cAMP levels and activation of ERK phosphorylation. In this study we investigated whether intracerebroventricular (icv) injection of RXFP3-A2, a selective relaxin-3 receptor agonist, altered ERK phosphorylation levels in the MS/DB of adult male rats. We subsequently assessed the neurochemical phenotype of phosphorylated (p) ERK-positive neurons in MS/DB after icv RXFP3-A2 administration by dual-label immunostaining for pERK and neuronal markers for cholinergic and GABAergic neurons. Central RXFP3-A2 injection significantly increased levels of pERK immunoreactivity (IR) in MS/DB at 20 and 90 min post-injection, compared to vehicle and naive levels. In addition, RXFP3-A2 increased the number of cells expressing pERK-IR in the MS/DB at 90 (but not 20) min post-injection in cholinergic (but not GABAergic) neurons, which also expressed putative RXFP3-IR. Moreover, icv injection of RXFP3-A2 impaired alternation in a delayed spontaneous T-maze test of spatial working memory. The presence of RXFP3-like IR and the RXFP3-related activation of the MAPK/ERK pathway in MS/DB cholinergic neurons identifies them as a key target of ascending relaxin-3 projections with implications for the acute and chronic modulation of cholinergic neuron activity and function by relaxin-3/RXFP3 signaling.

The effect of the steroid sulfatase inhibitor (p-O-sulfamoyl)-tetradecanoyl tyramine (DU-14) on learning and memory in rats with selective lesion of septal-hippocampal cholinergic tract

Dehydroepiandrosterone sulfate (DHEAS), is an excitatory neurosteroid synthesized within the CNS that modulates brain function. Effects associated with augmented DHEAS include learning and memory enhancement. Inhibitors of the steroid sulfatase enzyme increase brain DHEAS levels and can also facilitate learning and memory. This study investigated the effect of steroid sulfatase inhibition on learning and memory in rats with selective cholinergic lesion of the septo-hippocampal tract using passive avoidance and delayed matching to position T-maze (DMP) paradigms. The selective cholinergic immunotoxin 192 IgG-saporin (SAP) was infused into the medial septum of animals and then tested using a step-through passive avoidance paradigm or DMP paradigm. Peripheral administration of the steroid sulfatase inhibitor, DU-14, increased step-through latency following footshock in rats with SAP lesion compared to both vehicle treated control and lesioned animals (p<0.05). However, in the DMP task, steroid sulfatase inhibition impaired acquisition in lesioned rats while having no effect on intact animals. These results suggest that steroid sulfatase inhibition facilitates memory associated with contextual fear, but impairs acquisition of spatial memory tasks in rats with selective lesion of the septo-hippocampal tract.

Chronic treatment with a GPR30 antagonist impairs acquisition of a spatial learning task in young female rats

We hypothesize that the beneficial effects of estradiol on cognitive performance may be mediated through GPR30, a putative membrane target of estrogens. Recently we showed that administration of a selective GPR30 agonist (G-1) to ovariectomized rats enhanced acquisition of a delayed matching-to-position (DMP) T-maze task and increased potassium-stimulated acetylcholine release in the hippocampus, similar to estradiol (E2) (Hammond et al., 2009). The present study tested whether treating with a selective GPR30 antagonist (G-15) would impair spatial learning in gonadally intact rats and in ovariectomized (OVX) rats treated with E2. As predicted, G-15 dose-dependently impaired DMP acquisition both in gonadally intact rats and in OVX rats treated with E2. G-15 specifically reduced the rate of acquisition, and this effect was associated with an increased predisposition to adopt a persistent turn. In contrast, G-15 alone at the highest dose had no significant effect on DMP acquisition in OVX controls. The effects were task dependent, as similar effects of G-15 were not observed in gonadally intact rats tested on an operant discrimination/reversal learning task motivated by the same food reward. This suggests that the effects on DMP acquisition were not due to effects on motivation for food. Effects of G-15 on DMP acquisition were similar to previously published work showing significant impairment produced by selective cholinergic denervation of the hippocampus. These data suggest that GPR30 can play an important role in mediating the effects of estradiol on spatial learning, possibly by mediating estradiol effects on basal forebrain cholinergic function.

Selective cholinergic depletion in medial septum leads to impaired long term potentiation and glutamatergic synaptic currents in the hippocampus

Cholinergic depletion in the medial septum (MS) is associated with impaired hippocampal-dependent learning and memory. Here we investigated whether long term potentiation (LTP) and synaptic currents, mediated by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors in the CA1 hippocampal region, are affected following cholinergic lesions of the MS. Stereotaxic intra-medioseptal infusions of a selective immunotoxin, 192-saporin, against cholinergic neurons or sterile saline were made in adult rats. Four days after infusions, hippocampal slices were made and LTP, whole cell, and single channel (AMPA or NMDA receptor) currents were recorded. Results demonstrated impairment in the induction and expression of LTP in lesioned rats. Lesioned rats also showed decreases in synaptic currents from CA1 pyramidal cells and synaptosomal single channels of AMPA and NMDA receptors. Our results suggest that MS cholinergic afferents modulate LTP and glutamatergic currents in the CA1 region of the hippocampus, providing a potential synaptic mechanism for the learning and memory deficits observed in the rodent model of selective MS cholinergic lesioning.

Recognition of novel objects and their location in rats with selective cholinergic lesion of the medial septum

The importance of cholinergic neurons projecting from the medial septum (MS) of the basal forebrain to the hippocampus in memory function has been controversial. The aim of this study was to determine whether loss of cholinergic neurons in the MS disrupts object and/or object location recognition in male Sprague-Dawley rats. Animals received intraseptal injections of either vehicle, or the selective cholinergic immunotoxin 192 IgG-saporin (SAP). 14 days later, rats were tested for novel object recognition (NOR). Twenty-four hours later, these same rats were tested for object location recognition (OLR) (recognition of a familiar object moved to a novel location). Intraseptal injections of SAP produced an 86% decrease in choline acetyltransferase (ChAT) activity in the hippocampus, and a 31% decrease in ChAT activity in the frontal cortex. SAP lesion had no significant effect on NOR, but produced a significant impairment in OLR in these same rats. The results support a role for septo-hippocampal cholinergic projections in memory for the location of objects, but not for novel object recognition.

Galanthamine plus estradiol treatment enhances cognitive performance in aged ovariectomized rats

We hypothesize that beneficial effects of estradiol on cognitive performance diminish with age and time following menopause due to a progressive decline in basal forebrain cholinergic function. This study tested whether galanthamine, a cholinesterase inhibitor used to treat memory impairment associated with Alzheimer's disease, could enhance or restore estradiol effects on cognitive performance in aged rats that had been ovariectomized in middle-age. Rats were ovariectomized at 16-17 months of age. At 21-22 months of age rats began receiving daily injections of galanthamine (5mg/day) or vehicle. After one week, half of each group also received 17ß-estradiol administered subcutaneously. Rats were then trained on a delayed matching to position (DMP) T-maze task, followed by an operant stimulus discrimination/reversal learning task. Treatment with galanthamine+estradiol significantly enhanced the rate of DMP acquisition and improved short-term delay-dependent spatial memory performance. Treatment with galanthamine or estradiol alone was without significant effect. Effects were task-specific in that galanthamine+estradiol treatment did not significantly improve performance on the stimulus discrimination/reversal learning task. In fact, estradiol was associated with a significant increase in incorrect responses on this task after reversal of the stimulus contingency. In addition, treatments did not significantly affect hippocampal choline acetyltransferase activity or acetylcholine release. This may be an effect of age, or possibly is related to compensatory changes associated with long-term cholinesterase inhibitor treatment. The data suggest that treating with a cholinesterase inhibitor can enhance the effects of estradiol on acquisition of a DMP task by old rats following a long period of hormone deprivation. This could be of particular benefit to older women who have not used hormone therapy for many years and are beginning to show signs of mild cognitive impairment. Potential mechanisms for these effects are discussed.

Donepezil plus estradiol treatment enhances learning and delay-dependent memory performance by young ovariectomized rats with partial loss of septal cholinergic neurons

Effects of estrogen therapy on cognitive performance appear to diminish with age and time following the loss of ovarian function. We hypothesize that this is due to a reduction in basal forebrain cholinergic function and that treatment with a cholinergic enhancer can reverse the effect. This study tested whether combining the cholinesterase inhibitor donepezil with estradiol treatment can enhance/restore estradiol effects on cognitive performance in young ovariectomized rats with selective lesions of septal cholinergic neurons. 192IgG-saporin was injected directly into the medial septum to produce selective cholinergic lesions. Rats were then treated with donepezil (Don, daily injections of 3mg/kg/day, i.p.) or vehicle, and then with 17β-estradiol (E2, administered by silastic capsule implanted s.c.) or an empty capsule. Rats were trained on a delayed matching-to-position (DMP) T-maze task which previous studies have shown is sensitive to ovariectomy and estrogen replacement. Results show that neither estradiol nor donepezil alone significantly enhanced acquisition of the DMP task in rats with cholinergic lesions. Combination therapy was effective, however, depending on the severity of the lesion. Don+E2 significantly enhanced acquisition of the task in rats with partial lesions (<50% loss of cholinergic neurons), but not in rats with severe lesions. This effect was due largely to a reduction in perseverative behavior. Don+E2 also improved working memory in rats with partial lesions, as evidenced by significantly better performance than controls during increased intertrial delays. These findings suggest that even partial loss of septal cholinergic neurons can reduce effects of estrogen therapy on cognitive performance, and demonstrate that combining a cholinesterase inhibitor with estrogen therapy can help to restore beneficial effects on performance. We propose that combination therapy may have similar beneficial effects in women, particularly in older women who have not used estrogen therapy for many years and are beginning to show signs of cognitive impairment or early Alzheimer's disease.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

Localization of pre- and postsynaptic cholinergic markers in rodent forebrain: a brief history and comparison of rat and mouse

Rat and mouse models are widely used for studies in cognition and pathophysiology, among others. Here, we sought to determine to what extent these two model species differ for cholinergic and cholinoceptive features. For this purpose, we focused on cholinergic innervation patterns based on choline acetyltransferase (ChAT) immunostaining, and the expression of muscarinic acetylcholine receptors (mAChRs) detected immunocytochemically. In this brief review we first place cholinergic and cholinoceptive markers in a historic perspective, and then provide an overview of recent publications on cholinergic studies and techniques to provide a literature survey of current research. Next, we compare mouse (C57Bl/J6) and rat (Wistar) cholinergic and cholinoceptive systems simultaneously stained, respectively, for ChAT (analyzed qualitatively) and mAChRs (analyzed qualitatively and quantitatively). In general, the topographic cholinergic innervation patterns of both rodent species are highly comparable, with only considerable (but region specific) differences in number of detectable cholinergic interneurons, which are more numerous in rat. In contrast, immunolabeling for mAChRs, detected by the monoclonal antibody M35, differs markedly in the forebrain between the two species. In mouse brain, basal levels of activated and/or internalized mAChRs (as a consequence of cholinergic neurotransmission) are significantly higher. This suggests a higher cholinergic tone in mouse than rat, and hence the animal model of choice may have consequences for cholinergic drug testing experiments.

Sympathetic sprouting in visual cortex stimulated by cholinergic denervation rescues expression of two forms of long-term depression at layer 2/3 synapses

Cholinergic innervation of hippocampus and cortex is required for some forms of learning and memory. Several reports have shown that activation of muscarinic m1 receptors induces a long-term depression (mLTD) at glutamate synapses in hippocampus and in several areas of cortex, including perirhinal and visual cortices. This plasticity likely contributes to cognitive function dependent upon the cholinergic system. In rodent models, degeneration of hippocampal cholinergic innervation following lesion of the medial septum stimulates sprouting of adrenergic sympathetic axons, originating from the superior cervical ganglia (SCG), into denervated hippocampal subfields. We previously reported that this adrenergic sympathetic sprouting occurs simultaneously with a reappearance of cholinergic fibers in hippocampus and rescue of mLTD at CA3-CA1 synapses. Because cholinergic neurons throughout basal forebrain degenerate in aging and Alzheimer's disease, it is critical to determine if this compensatory sprouting occurs in other regions impacted by cholinergic cell loss. To this end, we investigated whether lesion of the nucleus basalis magnocellularis (NbM) to cholinergically denervate cortex stimulates adrenergic sympathetic sprouting and the accompanying increase in cholinergic innervation. Further, we assessed whether the presence of sprouting positively correlates with the ability of glutamate synapses in acute visual cortex slices to express mLTD and low frequency stimulation induced LTD (LFS LTD), another cholinergic dependent form of plasticity in visual cortex. We found that both mLTD and LFS LTD are absent in animals when NbM lesion is combined with bilateral removal of the SCG to prevent possible compensatory sprouting. In contrast, when the SCG remain intact to permit sprouting in animals with NbM lesion, cholinergic fiber density is increased concurrently with adrenergic sympathetic sprouting, and mLTD and LFS LTD are preserved. Our findings suggest that autonomic compensation for central cholinergic degeneration is not specific to hippocampus, but is a general repair mechanism occurring in other brain regions important for normal cognitive function.

Estrogen therapy and cognition: a review of the cholinergic hypothesis

The pros and cons of estrogen therapy for use in postmenopausal women continue to be a major topic of debate in women's health. Much of this debate focuses on the potential benefits vs. harm of estrogen therapy on the brain and the risks for cognitive impairment associated with aging and Alzheimer's disease. Many animal and human studies suggest that estrogens can have significant beneficial effects on brain aging and cognition and reduce the risk of Alzheimer's-related dementia; however, others disagree. Important discoveries have been made, and hypotheses have emerged that may explain some of the inconsistencies. This review focuses on the cholinergic hypothesis, specifically on evidence that beneficial effects of estrogens on brain aging and cognition are related to interactions with cholinergic projections emanating from the basal forebrain. These cholinergic projections play an important role in learning and attentional processes, and their function is known to decline with advanced age and in association with Alzheimer's disease. Evidence suggests that many of the effects of estrogens on neuronal plasticity and function and cognitive performance are related to or rely upon interactions with these cholinergic projections; however, studies also suggest that the effectiveness of estrogen therapy decreases with age and time after loss of ovarian function. We propose a model in which deficits in basal forebrain cholinergic function contribute to age-related changes in the response to estrogen therapy. Based on this model, we propose that cholinergic-enhancing drugs, used in combination with an appropriate estrogen-containing drug regimen, may be a viable therapeutic strategy for use in older postmenopausal women with early evidence of mild cognitive decline.

Chronic treatment with estrogen receptor agonists restores acquisition of a spatial learning task in young ovariectomized rats

Previous work has shown that continuous estradiol replacement in young ovariectomized rats enhances acquisition of a delayed matching-to-position (DMP) T-maze task over that of ovariectomized controls. The mechanism by which estradiol confers this benefit has not been fully elucidated. This study examined the role of selective estrogen receptor agonists of ERalpha, ERbeta, and GPR30 in the enhancement of spatial learning on a DMP task by comparing continuous estradiol replacement with continuous administration of PPT (an agonist of ERalpha), DPN (an agonist of ERbeta), or G-1 (an agonist of GPR30) relative to gonadally intact and ovariectomized vehicle-treated controls. It was found that ovariectomy impaired acquisition on this task, whereas all ER selective agonists restored the rate of acquisition to that of gonadally intact controls. These data suggest that estradiol can work through any of several estrogen receptors to enhance the rate of acquisition on this task.

Donepezil treatment restores the ability of estradiol to enhance cognitive performance in aged rats: evidence for the cholinergic basis of the critical period hypothesis

Recent studies suggest that the ability of estradiol to enhance cognitive performance diminishes with age and/or time following loss of ovarian function. We hypothesize that this is due, in part, to a decrease in basal forebrain cholinergic function. This study tested whether donepezil, a cholinesterase inhibitor, could restore estradiol effects on cognitive performance in aged rats that had been ovariectomized as young adults. Rats were ovariectomized at 3 months of age, and then trained on a delayed matching to position (DMP) T-maze task, followed by a configural association (CA) operant condition task, beginning at 12-17 or 22-27 months of age. Three weeks prior to testing, rats started to receive either donepezil or vehicle. After one week, half of each group also began receiving estradiol. Acclimation and testing began seven days later and treatment continued throughout testing. Estradiol alone significantly enhanced DMP acquisition in middle-aged rats, but not in aged rats. Donepezil alone had no effect on DMP acquisition in either age group; however, donepezil treatment restored the ability of estradiol to enhance DMP acquisition in aged rats. This effect was due largely to a reduction in the predisposition to adopt a persistent turn strategy during acquisition. These same treatments did not affect acquisition of the CA task in middle-aged rats, but did have small but significant effects on response time in aged rats. The data are consistent with the idea that estrogen effects on cognitive performance are task specific, and that deficits in basal forebrain cholinergic function are responsible for the loss of estradiol effect on DMP acquisition in aged ovariectomized rats. In addition, the data suggest that enhancing cholinergic function pharmacologically can restore the ability of estradiol to enhance acquisition of the DMP task in very old rats following long periods of hormone deprivation. Whether donepezil has similar restorative effects on other estrogen-sensitive tasks needs to be explored.

Premarin improves memory, prevents scopolamine-induced amnesia and increases number of basal forebrain choline acetyltransferase positive cells in middle-aged surgically menopausal rats

Conjugated equine estrogen (CEE) is the most commonly prescribed estrogen therapy, and is the estrogen used in the Women's Health Initiative study. While in-vitro studies suggest that CEE is neuroprotective, no study has evaluated CEE's effects on a cognitive battery and brain immunohistochemistry in an animal model. The current experiment tested whether CEE impacted: I) spatial learning, reference memory, working memory and long-term retention, as well as ability to handle mnemonic delay and interference challenges; and, II) the cholinergic system, via pharmacological challenge during memory testing and ChAT-immunoreactive cell counts in the basal forebrain. Middle-aged ovariectomized (Ovx) rats received chronic cyclic injections of either Oil (vehicle), CEE-Low (10 microg), CEE-Medium (20 microg) or CEE-High (30 microg) treatment. Relative to the Oil group, all three CEE groups showed less overnight forgetting on the spatial reference memory task, and the CEE-High group had enhanced platform localization during the probe trial. All CEE groups exhibited enhanced learning on the spatial working memory task, and CEE dose-dependently protected against scopolamine-induced amnesia with every rat receiving the highest CEE dose maintaining zero errors after scopolamine challenge. CEE also increased number of ChAT-immunoreactive neurons in the vertical diagonal band of the basal forebrain. Neither the ability to remember after a delay nor interference, nor long-term retention, was influenced by the CEE regimen used in this study. These findings are similar to those reported previously for 17 beta-estradiol, and suggest that CEE can provide cognitive benefits on spatial learning, reference and working memory, possibly through cholinergic mechanisms.

Selective lesion of septal cholinergic neurons in rats impairs acquisition of a delayed matching to position T-maze task by delaying the shift from a response to a place strategy

This study tested the hypothesis that septal cholinergic lesions impair acquisition of a delayed matching to position (DMP) T-maze task in male rats by affecting learning strategy. Rats received either the selective cholinergic immunotoxin, 192 IgG-saporin (SAP) or artificial cerebrospinal fluid directly into the medial septum. Two weeks later, animals were trained to acquire the DMP task. SAP-treated rats took significantly longer to acquire the task than corresponding controls. Both SAP-treated and control rats adopted a persistent turn and utilized a response strategy during early periods of training. By the time rats reached criterion the persistent turn was no longer evident, and all rats had shifted to an allocentric strategy, i.e., were relying on extramaze cues to a significant degree. During the acquisition period, SAP-treated rats spent significantly more days showing a persistent turn and using a response strategy than corresponding controls. The added time spent using a response strategy accounted entirely for the added days required to reach criterion among the SAP-treated rats. This suggests that the principal mechanism by which septal cholinergic lesions impair DMP acquisition in male rats is by increasing the predisposition to use a response vs. a place strategy, thereby affecting the ability to switch from one strategy to another.

Organization of food protection behavior is differentially influenced by 192 IgG-saporin lesions of either the medial septum or the nucleus basalis magnocellularis

Converging lines of evidence have supported a role for the nucleus basalis magnocellularis (NB) in attentional mechanisms; however, debate continues regarding the role of the medial septum in behavior (MS). Recent studies have supported a role for the septohippocampal system in the online processing of internally generated cues. The current study was designed to investigate a possible double dissociation in rat food protection behavior, a natural behavior that has been shown to depend on external and internal sources of information. The study examined the effects of intraparenchymal injections of 192 IgG-saporin into either the MS or NB on the organization of food protection behavior. NB cholinergic lesions reduced the number of successful food protection behaviors while sparing the temporal organization of food protection behavior. In contrast, MS cholinergic lesions disrupted the temporal organization of food protection behavior while sparing the ability to successfully protect food items. These observations are consistent with a double dissociation of NB and MS cholinergic systems' contributions to processing external and internal sources of information and provide further evidence for the septohippocampal system's involvement in processing internally generated cues.

Behavioral and immunohistological effects of cholinergic damage in immunolesioned rats: Alteration of c-Fos and polysialylated neural cell adhesion molecule expression

The aim of this study was to determine the brain structures as well as the plasticity events associated with the behavioral effects of cholinergic damage. Rats were submitted to injection of 192 IgG-saporin in the medial septum/diagonal band of Broca complex and the nucleus basalis magnocellularis. The immunohistochemical expression of c-Fos protein and PSA-NCAM (polysialylated neural cell adhesion molecule) and the behavioral performances in the nonmatching-to-position task were assessed at various post-lesion times. Thus, 3 days after injection of the immunotoxin, increased c-Fos labeling was observed in the areas of infusion, indicating these cells were undergoing some plastic changes and/or apoptotic processes. A drastic increase was observed in the number of PSA-NCAM positive cells and in their dendritic arborization in the dentate gyrus. At 7 days post-lesion, no behavioral deficit was observed in immunolesioned rats despite the drastic loss of cholinergic neurons. These neurons showed decreased c-Fos protein expression in the piriform and entorhinal cortex and in the dentate gyrus. In the latter, PSA-NCAM induction was high, suggesting that remodeling occurred, which in turn might contribute to sustaining some mnemonic function in immunolesioned rats. At 1 month, cholinergic neurons totally disappeared and behavioral deficits were drastic. c-Fos expression showed no change. In contrast, the increased PSA-NCAM-labeling observed at short post-lesion times was maintained but the plastic changes due to this molecule could not compensate the behavioral deficit caused by the immunotoxin. Thus, as the post-lesion time increases, a gradual degeneration process should occur that may contribute to mnemonic impairments. This neuronal loss leads to molecular and cellular alterations, which in turn may aggravate cognitive deficits.