Aversive stimulus attenuates impairment of acquisition in a delayed match to position T-maze task caused by a selective lesion of septo-hippocampal cholinergic projections

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.

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.

In vivo cross-sectional characterization of cerebral alterations induced by intracerebroventricular administration of streptozotocin

Cerebral aging is often associated with the occurrence of neurodegenerative diseases leading to dementia. Animal models are critical to elucidate mechanisms associated to dementia and to evaluate neuroprotective drugs. Rats that received intracerebroventricular injection of streptozotocin (icv-STZ) have been reported as a model of dementia. In these animals, this drug induces oxidative stress and brain glucose metabolism impairments associated to insulin signal transduction failure. These mechanisms are reported to be involved in the pathogenesis of Alzheimer's disease and other dementia. Icv-STZ rats also display memory impairments. However, little is known about the precise location of the lesions induced by STZ administration. In this context, the present study characterized the cerebral lesions induced by two-doses of icv-STZ by using high-field magnetic resonance imaging to easily and longitudinally detect cerebral abnormalities and by using immunohistochemistry to evaluate neuronal loss and neuroinflammation (astrocytosis and microgliosis). We showed that, at high doses, icv-STZ induces severe and acute neurodegenerative lesions in the septum and corpus callosum. The lesions are associated with an inflammation process. They are less severe and more progressive at low doses. The relevance of high and low doses of icv-STZ to mimic dementia and evaluate new drugs is discussed in the final part of this article.

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.

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.

Acetylcholine and attention

Historically, ACh has been implicated in learning and short-term memory functions. However, more recent studies have provided support for a role of cortical ACh in attentional effort, orienting and the detection of behavioral significant stimuli. The current review article summarizes studies in animals and humans which have investigated the role of ACh in attention and cognition. An attempt has been made to differentiate between brain regions involved in attentional processes versus those important for other cognitive functions. To this purpose, various experimental methods and interventions were used. Animal behavioral studies have injected the selective immunotoxin IgG-saporin to induce specific cholinergic lesions, employed electrochemical techniques such as microdialysis, or have administered cholinergic compounds into discrete parts of the brain. Human studies that give some indication on the link between central cholinergic signaling and cognition are obviously confined to less invasive, imaging methods such as fMRI. The brain areas that are deemed most important for intact attentional processing in both animals and humans appear to be the (pre)frontal, parietal and somatosensory (especially visual) regions, where ACh plays a vital role in the top-down control of attentional orienting and stimulus discrimination. In contrast, cholinergic signaling in the septohippocampal system is suggested to be involved in memory processes. Thus, it appears that the role of ACh in cognition is different per brain region and between nicotinic versus muscarinic receptor subtypes.

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.

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.