Intraseptal infusion of the cholinergic agonist carbachol impairs delayed-non-match-to-sample radial arm maze performance in the rat

Western diet consumption impairs memory function via dysregulated hippocampus acetylcholine signaling

Western diet (WD) consumption during early life developmental periods is associated with impaired memory function, particularly for hippocampus (HPC)-dependent processes. We developed an early life WD rodent model associated with long-lasting HPC dysfunction to investigate the neurobiological mechanisms mediating these effects. Rats received either a cafeteria-style WD (ad libitum access to various high-fat/high-sugar foods; CAF) or standard healthy chow (CTL) during the juvenile and adolescent stages (postnatal days 26-56). Behavioral and metabolic assessments were performed both before and after a healthy diet intervention period beginning at early adulthood. Results revealed HPC-dependent contextual episodic memory impairments in CAF rats that persisted despite the healthy diet intervention. Given that dysregulated HPC acetylcholine (ACh) signaling is associated with memory impairments in humans and animal models, we examined protein markers of ACh tone in the dorsal HPC (HPCd) in CAF and CTL rats. Results revealed significantly lower protein levels of vesicular ACh transporter in the HPCd of CAF vs. CTL rats, indicating chronically reduced ACh tone. Using intensity-based ACh sensing fluorescent reporter (iAChSnFr) in vivo fiber photometry targeting the HPCd, we next revealed that ACh release during object-contextual novelty recognition was highly predictive of memory performance and was disrupted in CAF vs. CTL rats. Neuropharmacological results showed that alpha 7 nicotinic ACh receptor agonist infusion in the HPCd during training rescued memory deficits in CAF rats. Overall, these findings reveal a functional connection linking early life WD intake with long-lasting dysregulation of HPC ACh signaling, thereby identifying an underlying mechanism for WD-associated memory impairments.

Western diet consumption impairs memory function via dysregulated hippocampus acetylcholine signaling

Western diet (WD) consumption during development yields long-lasting memory impairments, yet the underlying neurobiological mechanisms remain elusive. Here we developed an early life WD rodent model to evaluate whether dysregulated hippocampus (HPC) acetylcholine (ACh) signaling, a pathology associated with memory impairment in human dementia, is causally-related to WD-induced cognitive impairment. Rats received a cafeteria-style WD (access to various high-fat/high-sugar foods; CAF) or healthy chow (CTL) during the juvenile and adolescent periods (postnatal days 26-56). Behavioral, metabolic, and microbiome assessments were performed both before and after a 30-day healthy diet intervention beginning at early adulthood. Results revealed CAF-induced HPC-dependent contextual episodic memory impairments that persisted despite healthy diet intervention, whereas CAF was not associated with long-term changes in body weight, body composition, glucose tolerance, anxiety-like behavior, or gut microbiome. HPC immunoblot analyses after the healthy diet intervention identified reduced levels of vesicular ACh transporter in CAF vs. CTL rats, indicative of chronically reduced HPC ACh tone. To determine whether these changes were functionally related to memory impairments, we evaluated temporal HPC ACh binding via ACh-sensing fluorescent reporter in vivo fiber photometry during memory testing, as well as whether the memory impairments could be rescued pharmacologically. Results revealed dynamic HPC ACh binding during object-contextual novelty recognition was highly predictive of memory performance and was disrupted in CAF vs. CTL rats. Further, HPC alpha-7 nicotinic receptor agonist infusion during consolidation rescued memory deficits in CAF rats. Overall, these findings identify dysregulated HPC ACh signaling as a mechanism underlying early life WD-associated memory impairments.

Cholinergic suppression of hippocampal sharp-wave ripples impairs working memory

Learning and memory are assumed to be supported by mechanisms that involve cholinergic transmission and hippocampal theta. Using G protein-coupled receptor-activation-based acetylcholine sensor (GRAB_ACh3.0) with a fiber-photometric fluorescence readout in mice, we found that cholinergic signaling in the hippocampus increased in parallel with theta/gamma power during walking and REM sleep, while ACh3.0 signal reached a minimum during hippocampal sharp-wave ripples (SPW-R). Unexpectedly, memory performance was impaired in a hippocampus-dependent spontaneous alternation task by selective optogenetic stimulation of medial septal cholinergic neurons when the stimulation was applied in the delay area but not in the central (choice) arm of the maze. Parallel with the decreased performance, optogenetic stimulation decreased the incidence of SPW-Rs. These findings suggest that septo-hippocampal interactions play a task-phase-dependent dual role in the maintenance of memory performance, including not only theta mechanisms but also SPW-Rs.

Cholinergic modulation of spatial learning, memory and navigation

Spatial learning, including encoding and retrieval of spatial memories as well as holding spatial information in working memory generally serving navigation under a broad range of circumstances, relies on a network of structures. While central to this network are medial temporal lobe structures with a widely appreciated crucial function of the hippocampus, neocortical areas such as the posterior parietal cortex and the retrosplenial cortex also play essential roles. Since the hippocampus receives its main subcortical input from the medial septum of the basal forebrain (BF) cholinergic system, it is not surprising that the potential role of the septo-hippocampal pathway in spatial navigation has been investigated in many studies. Much less is known of the involvement in spatial cognition of the parallel projection system linking the posterior BF with neocortical areas. Here we review the current state of the art of the division of labour within this complex 'navigation system', with special focus on how subcortical cholinergic inputs may regulate various aspects of spatial learning, memory and navigation.

An inhibitor of soluble epoxide hydrolase ameliorates diabetes-induced learning and memory impairment in rats

BACKGROUND

Pharmacological inhibition of soluble epoxide hydrolase (sEH) enhances the synaptic function in the CNS and has a protective role in cognitive decline. We hypothesized that the sEH inhibitor TPPU might prevent the diabetes-induced decline in learning and memory which is associated with an alteration in the level of neurotransmitters and oxidative stress.

METHODS

Type 1 diabetes was induced in rats and the animals were treated with TPPU for 8 weeks. The learning and memory functions were assessed by the Barnes maze and a step-down test. Indicators of oxidative stress, levels of neurotransmitters, and activity of acetylcholinesterase were measured in the discrete regions of the brain.

RESULTS

Our results revealed that treatment with TPPU significantly improves learning and memory performance in diabetic rats along with decreasing the level of blood sugar. Moreover, treatment with TPPU significantly prevented the diabetes-induced alteration in levels of neurotransmitters, the activity of acetylcholinesterase and preserved anti-oxidant defence system.

CONCLUSION

Inhibition of the sEH alleviates diabetes-induced decline in learning and memory.

Impaired object-location learning and recognition memory but enhanced sustained attention in M2 muscarinic receptor-deficient mice

RATIONALE

Muscarinic acetylcholine receptors are known to play key roles in mediating cognitive processes, and impaired muscarinic cholinergic neurotransmission is associated with normal ageing processes and Alzheimer's disease. However, the specific contributions of the individual muscarinic receptor subtypes (M1-M5) to cognition are presently not well understood.

OBJECTIVES

The aim of this study was to investigate the contribution of M2-type muscarinic receptor signalling to sustained attention, executive control and learning and memory.

METHODS

M2 receptor-deficient (M2^-/-) mice were tested on a touchscreen-operated task battery testing visual discrimination, behavioural flexibility, object-location associative learning, attention and response control. Spontaneous recognition memory for real-world objects was also assessed.

RESULTS

We found that M2^-/- mice showed an enhancement of attentional performance, but significant deficits on some tests of learning and memory. Executive control and visual discrimination were unaffected by M2-depletion.

CONCLUSIONS

These findings suggest that M2 activation has heterogeneous effects across cognitive domains, and provide insights into how acetylcholine may support multiple specific cognitive processes through functionally distinct cholinergic receptor subtypes. They also suggest that therapeutics involving M2 receptor-active compounds should be assessed across a broad range of cognitive domains, as they may enhance some cognitive functions, but impair others.

Critical role of CA1 muscarinic receptors on memory acquisition deficit induced by total (TSD) and REM sleep deprivation (RSD)

AIM

Despite different theories regarding sleep physiological function, an overall census indicates that sleep is useful for neural plasticity which eventually strengthens cognition and brain performance. Different studies show that sleep deprivation (SD) leads to impaired learning and hippocampus dependent memory. According to some studies, cholinergic system plays an important role in sleep (particularly REM sleep), learning, memory, and its retrieval. So this study has been designed to investigate the effect of CA1 Cholinergic Muscarinic Receptors on memory acquisition deficit induced by total sleep deprivation (TSD) and REM sleep deprivation (RSD).

METHOD

A modified water box (locomotor activity may be provide a limiting factor in this method of SD) or multiple platforms were used for induction of TSD or RSD, respectively. Inhibitory passive avoidance apparatus has been used to determine the effects of SD and its changes by physostigmine (as cholinesterase inhibitor) or scopolamine (muscarinic receptor antagonist) on memory formation. Because locomotor activity and pain perception induce critical roles in passive avoidance memory formation, we also measured these factors by open field and hot-plate instruments, respectively.

RESULTS

The results showed that TSD and RSD for 24 hours impaired memory formation but they did not alter locomotor activity. TSD also induced analgesia effect, but RSD did not alter it. Intra-CA1 injection of physostigmine (0.0001μg/rat) and scopolamine (0.01μg/rat) did not alter memory acquisition in the sham-TSD or sham-RSD, by themselves. Moreover, intra-CA1 injection of sub-threshold dose of physostigmine (0.0001μg/rat) and scopolamine (0.01μg/rat) could restore the memory acquisition deficit induced by RSD, while scopolamine could restore TSD-induced amnesia. Both drugs reversed analgesia induced by TSD. None of previous interventions altered locomotor activity.

CONCLUSION

According to this study, CA1 cholinergic muscarinic receptors play an important role in amnesia induced by both TSD and RSD. However further studies are needed for showing cellular and molecular mechanisms of surprising result of similar pharmacological effects using compounds with opposite profiles.

Hippocampal acetylcholine depletion has no effect on anxiety, spatial novelty preference, or differential reward for low rates of responding (DRL) performance in rats

We investigated the role of the septo-hippocampal cholinergic projection in anxiety, spatial novelty preference, and differential reward for low rates of responding (DRL) performance. Cholinergic neurons of the rat medial septum (MS) and the vertical limb of the diagonal band of Broca (VDB) were lesioned using the selective immunotoxin, 192 IgG-saporin. Rats were then tested on several behavioral tests previously shown to be sensitive to either (a) hippocampal lesions or (b) nonselective MS/VDB lesions which target both cholinergic and γ-aminobutyric acid (GABA)-ergic projections, or both. Saporin lesions substantially reduced hippocampal cholinergic innervation, resulting in an absence of acetyl cholinesterase staining and markedly reduced choline acetyltransferase activity (mean reduction: 80 ± 5%; range: 50-97%). However, the saporin-lesioned rats did not differ from control rats in any of the behavioral tests. Thus we found no evidence from these lesion studies that the septo-hippocampal cholinergic projection plays an essential role in anxiety, spatial novelty preference, or DRL.

Sustaining high acetylcholine levels in the frontal cortex, but not retrosplenial cortex, recovers spatial memory performance in a rodent model of diencephalic amnesia

Although the thalamus and/or mammillary bodies are the primary sites of neuropathology in cases of diencephalic amnesia such as Wernicke Korsakoff Syndrome (WKS), there is also functional deactivation of certain cortical regions that contribute to the cognitive dysfunction. Acetylcholine (ACh) is a key neurotransmitter that modulates neural processing within the cortex and between the thalamus and cortex. In the pyrithiamine-induced thiamine deficiency (PTD) rat model of WKS, there are significant reductions in cholinergic innervation and behaviorally stimulated ACh efflux in the frontal (FC) and retrosplenial (RSC) cortices. In the present study, ACh released levels were site-specifically amplified with physostigmine (0.5 μg, 1.0 μg) in the FC and the RSC, which was confirmed with in vivo microdialysis. Although physostigmine sustained high ACh levels in both cortical regions, the effects on spatial memory, assessed by spontaneous alternation, were different as a function of region (FC, RSC) and treatment (PTD, pair-fed [PF]). Higher ACh levels in the FC recovered the rate of alternation in PTD rats as well as reduced arm-reentry perseverative errors. However, higher ACh levels within the FC of PF rats exacerbated arm-reentry perseverative errors without significantly affecting alternation rates. Maintaining high ACh levels in the RSC had no procognitive effects in PTD rats, but rather impaired alternation behavior in PF rats. These results demonstrate that diverse cortical regions respond differently to intensified ACh levels-and the effects are dependent on thalamic pathology. Thus, pharmacotherapeutics aimed at enhancing cognitive functions must account for the unique features of cortical ACh stimulation and the connective circuitry with the thalamus.

Possible role of acetylcholine in regulating spatial novelty effects on theta rhythm and grid cells

Existing pharmacological and lesion data indicate that acetylcholine plays an important role in memory formation. For example, increased levels of acetylcholine in the hippocampal formation are known to be associated with successful encoding while disruption of the cholinergic system leads to impairments on a range of mnemonic tasks. However, cholinergic signaling from the medial septum also plays a central role in generating and pacing theta-band oscillations throughout the hippocampal formation. Recent experimental results suggest a potential link between these distinct phenomena. Environmental novelty, a condition associated with strong cholinergic drive, has been shown to induce an expansion in the firing pattern of entorhinal grid cells and a reduction in the frequency of theta measured from the LFP. Computational modeling suggests the spatial activity of grid cells is produced by interference between neuronal oscillators; scale being determined by theta-band oscillations impinging on entorhinal stellate cells, the frequency of which is modulated by acetylcholine. Here we propose that increased cholinergic signaling in response to environmental novelty triggers grid expansion by reducing the frequency of the oscillations. Furthermore, we argue that cholinergic induced grid expansion may enhance, or even induce, encoding by producing a mismatch between expanded grid cells and other spatial inputs to the hippocampus, such as boundary vector cells. Indeed, a further source of mismatch is likely to occur between grid cells of different native scales which may expand by different relative amounts.

Acetylcholine release in the hippocampus and prelimbic cortex during acquisition of a socially transmitted food preference

Interference with cholinergic functions in hippocampus and prefrontal cortex impairs learning and memory for social transmission of food preference, suggesting that acetylcholine (ACh) release in the two brain regions may be important for acquiring the food preference. This experiment examined release of ACh in the hippocampus and prefrontal cortex of rats during training for social transmission of food preference. After demonstrator rats ate a food with novel flavor and odor, a social transmission of food preference group of rats was allowed to interact with the demonstrators for 30 min, while in vivo microdialysis collected samples for later measurement of ACh release with HPLC methods. A social control group observed a demonstrator that had eaten food without novel flavor and odor. An odor control group was allowed to smell but not ingest food with novel odor. Rats in the social transmission but not control groups preferred the novel food on a trial 48 h later. ACh release in prefrontal cortex, with probes that primarily sampled prelimbic cortex, did not increase during acquisition of the social transmission of food preference, suggesting that training-initiated release of ACh in prelimbic cortex is not necessary for acquisition of the food preference. In contrast, ACh release in the hippocampus increased substantially (200%) upon exposure to a rat that had eaten the novel food. Release in the hippocampus increased significantly less (25%) upon exposure to a rat that had eaten normal food and did not increase significantly in the rats exposed to the novel odor; ACh release in the social transmission group was significantly greater than that of the either of the control groups. Thus, ACh release in the hippocampus but not prelimbic cortex distinguished well the social transmission vs. control conditions, suggesting that cholinergic mechanisms in the hippocampus but not prelimbic cortex are important for acquiring a socially transmitted food preference.

Acetylcholinesterase activity in an experimental rat model of Type C hepatic encephalopathy

Patients with liver malfunction often suffer from hepatic encephalopathy, a neurological complication which can affect attention and cognition. Diverse experimental models have been used to study brain alterations that may be responsible for hepatic encephalopathy symptoms. The aim of the study was to determine whether cognitive impairment found in cirrhosis could be due to disturbance of acetylcholinesterase activity. Acetylcholinesterase activity was assessed in the brains of Wistar rats with thioacetamide-induced cirrhosis. The cirrhotic group displayed up-regulation of acetylcholinesterase levels in the entorrhinal cortex, anterodorsal and anteroventral thalamus and accumbens, whereas down-regulation was found in the CA1, CA3 and dentate gyrus of the hippocampus. Our results indicate that the experimental model of hepatic encephalopathy by chronic administration of thioacetamide presents alterations of acetylcholinesterase activity in brain limbic system regions, which play a role in attention and memory.

Effects of asiatic acid on passive and active avoidance task in male Spraque-Dawley rats

AIM OF THE STUDY

Centella asiatica has a reputation to restore declining cognitive function in traditional medicine. To date, only a few compounds that show enhancing learning and memory properties are available. Therefore, the present study investigates the effects of for acute administration of asiatic acid (A-A) isolated from Centella asiatica administration on memory and learning in male Spraque-Dawley rats.

MATERIALS AND METHODS

4-5 weeks Spraque-Dawley rats were administered with concentration 1, 3, 5, 10, 30 mg/kg of A-A, baclofen, scopolamine and saline intra peritoneally and were evaluated for passive avoidance (PA), active avoidance (AA) and changes in blood pressure (BP).

RESULTS

Treatment 30 mg/kg of A-A resulted in significantly dose-dependently improved memory, with increased retention latency to enter difference compartment in PA test compared to baclofen, saline and scopolamine. Furthermore, 30 mg/kg of A-A was significantly higher on learning abilities on 1st day but there was no significantly difference on avoidance memory ability after 7 days of retention. Low reading in blood pressure dose-dependent significantly difference was observed in the 30 mg/kg of A-A group compared to saline group.

CONCLUSIONS

Administration A-A facilitated PA and AA on memory and learning and but had no effect on active avoidance on memory. Hence, may serve useful memory and learning with less effect in blood pressure in promoting memory and learning increases.

Differential effects of systemic and intraseptal administration of the acetylcholinesterase inhibitor tacrine on the recovery of spatial behavior in an animal model of diencephalic amnesia

Several lines of evidence suggest that acetylcholinesterase inhibitors (AChE) have their cognitive enhancing effects by stimulating cholinergic receptors within the medial septum. However, intraseptal administration of cholinergic enhancing drugs produce mixed results that appear to depend on both the integrity of the medial septum as well as task demands. Three experiments were conducted to determine the relationship between increased cholinergic activity within the medial septum and hippocampus and behavioral recovery in a model of diencephalic amnesia produced by pyrithiamine-induced thiamine deficiency (PTD). In Experiment 1, systemic tacrine (0.0, 0.75, 1.5mg/kg) was administered to PTD and pair-fed (PF) rats prior to a spontaneous alternation task. Without tacrine, PF rats alternated at a higher rate than PTD rats. Both doses of tacrine increased alternation in PTD rats to within the range of PF rats. In Experiment 2, three doses of intraseptal tacrine (2.5, 5.0, 12.5microg) were administered to PTD and PF rats and changes in hippocampal acetylcholine efflux were assessed. Both the 5.0 and 12.5microg doses significantly increased hippocampal acetylcholine levels, but the change was greater in the PTD rats. In Experiment 3, despite the fact that both intraseptal doses of tacrine (5.0, 12.5microg) increased hippocampal acetylcholine levels, only 5.0microg significantly improved alternation scores in PTD rats. Thus, when there is basal forebrain cholinergic cell loss in conjunction with diencephalic pathology, the therapeutic range of AChE-I in the medial septum and the effective doses do not directly map onto changes in acetylcholine efflux in the hippocampus.

A model of episodic memory: mental time travel along encoded trajectories using grid cells

The definition of episodic memory includes the concept of mental time travel: the ability to re-experience a previously experienced trajectory through continuous dimensions of space and time, and to recall specific events or stimuli along this trajectory. Lesions of the hippocampus and entorhinal cortex impair human episodic memory function and impair rat performance in tasks that could be solved by retrieval of trajectories. Recent physiological data suggests a novel model for encoding and retrieval of trajectories, and for associating specific stimuli with specific positions along the trajectory. During encoding in the model, external input drives the activity of head direction cells. Entorhinal grid cells integrate the head direction input to update an internal representation of location, and drive hippocampal place cells. Trajectories are encoded by Hebbian modification of excitatory synaptic connections between hippocampal place cells and head direction cells driven by external action. Associations are also formed between hippocampal cells and sensory stimuli. During retrieval, a sensory input cue activates hippocampal cells that drive head direction activity via previously modified synapses. Persistent spiking of head direction cells maintains the direction and speed of the action, updating the activity of entorhinal grid cells that thereby further update place cell activity. Additional cells, termed arc length cells, provide coding of trajectory segments based on the one-dimensional arc length from the context of prior actions or states, overcoming ambiguity where the overlap of trajectory segments causes multiple head directions to be associated with one place. These mechanisms allow retrieval of complex, self-crossing trajectories as continuous curves through space and time.

Zero net flux estimates of septal extracellular glucose levels and the effects of glucose on septal extracellular GABA levels

Although hippocampal infusions of glucose enhance memory, we have found repeatedly that septal glucose infusions impair memory when gamma-aminobutyric acid (GABA) receptors are activated. For instance, hippocampal glucose infusions reverse the memory-impairing effects of co-infusions of the GABA agonist muscimol, whereas septal glucose infusions exacerbate memory deficits produced by muscimol. One potential explanation for these deleterious effects of glucose in the septum is that there are higher levels of endogenous extracellular fluid glucose concentrations in the septum than in the hippocampus. Another hypothesis is that septal glucose infusions impair memory by increasing septal GABA synthesis or release, which is possible because elevating glucose increases GABA levels in other brain regions. To test these hypotheses, Experiment 1 quantified extracellular fluid glucose levels in the septum and hippocampus using zero net flux in vivo microdialysis procedures in conscious, freely moving rats. Experiment 2 determined whether septal infusions of glucose would increase GABA concentrations in dialysates obtained from the septum. The results of Experiment 1 indicated that extracellular fluid glucose levels in the hippocampus and septum are comparable. The results of Experiment 2 showed that co-infusions of glucose with muscimol, at doses that did not affect memory on their own, decreased percent alternation memory scores. However, none of the infusions significantly affected GABA levels. Collectively, these findings suggest that the memory-impairing effects of septal infusions of glucose are not likely due to regional differences in basal extracellular fluid glucose concentrations and are not mediated via an increase in septal GABA release.

Blocking GABA-A receptors in the medial septum enhances hippocampal acetylcholine release and behavior in a rat model of diencephalic amnesia

Wernicke-Korsakoff syndrome (WKS), a form of diencephalic amnesia caused by thiamine deficiency, results in severe anterograde memory loss. Pyrithiamine-induced thiamine deficiency (PTD), an animal model of WKS, produces cholinergic abnormalities including decreased functional hippocampal acetylcholine (ACh) release and poor spatial memory. Increasing hippocampal ACh levels has increased performance in PTD animals. Intraseptal bicuculline (GABA(A) antagonist) augments hippocampal ACh release in normal animals and we found it (0.50 microg/microl and 0.75 microg/microl) also increased in-vivo hippocampal ACh release in PTD animals. However, the 0.75 microg/microl dose produced a greater change in hippocampal ACh release in control animals. The 0.50 microg/microl dose of bicuculline was then selected to determine if it could enhance spontaneous alternation performance in PTD animals. This dose of bicuculline significantly increased hippocampal ACh levels above baseline in both PTD and control rats and resulted in complete behavioral recovery in PTD animals, without altering performance in control rats. This suggests that balancing ACh-GABA interactions in the septohippocampal circuit may be an effective therapeutic approach in certain amnestic syndromes.

Increasing hippocampal acetylcholine levels enhance behavioral performance in an animal model of diencephalic amnesia

Pyrithiamine-induced thiamine deficiency (PTD) was used to produce a rodent model of Wernicke-Korsakoff syndrome that results in acute neurological disturbances, thalamic lesions, and learning and memory impairments. There is also cholinergic septohippocampal dysfunction in the PTD model. Systemic (Experiment 1) and intrahippocampal (Experiment 2) injections of the acetylcholinesterase inhibitor physostigmine were administered to determine if increasing acetylcholine levels would eliminate the behavioral impairment produced by PTD. Prior to spontaneous alternation testing, rats received injections of either physostigmine (systemic=0.075 mg/kg; intrahippocampal=20, 40 ng/muL) or saline. In Experiment 2, intrahippocampal injections of physostigmine significantly enhanced alternation rates in the PTD-treated rats. In addition, although intrahippocampal infusions of 40 ng of physostigmine increased the available amount of ACh in both pair-fed (PF) and PTD rats, it did so to a greater extent in PF rats. The increase in ACh levels induced by the direct hippocampal application of physostigmine in the PTD model likely increased activation of the extended limbic system, which was dysfunctional, and therefore led to recovery of function on the spontaneous alternation task. In contrast, the lack of behavioral improvement by intrahippocampal physostigmine infusion in the PF rats, despite a greater rise in hippocampal ACh levels, supports the theory that there is an optimal range of cholinergic tone for optimal behavioral and hippocampal function.

Activation of the medial septum reverses age-related hippocampal encoding deficits: a place field analysis

When a rat runs through a familiar environment, the hippocampus retrieves a previously stored spatial representation of the environment. When the environment is modified a new representation is seen, presumably corresponding to the hippocampus encoding the new information. The medial septum is hypothesized to modulate whether the hippocampus engages in retrieval or encoding. The cholinergic agonist carbachol was infused into the medial septum, and hippocampal CA1 place cells were recorded in freely moving rats. In a familiar environment, septal activation impaired the retrieval of a previously stored hippocampal place cell representation regardless of age. When the environment was changed, medial septal activation impaired the encoding process in young, but facilitated the encoding of the new information in aged rats. Moreover, the improved encoding was evident during a subsequent exposure to the modified environment 24 h later. The findings support the role the septum plays in modulating hippocampal retrieval/encoding states. Furthermore, our data indicate a mechanism of age-related cognitive impairment.

Differential cholinergic modulation of synaptic encoding and gain control mechanisms in rat hippocampus

Recent studies have highlighted a variety of cognitive effects caused by cholinolytic drug injections into different cortical structures. These findings were largely interpreted as evidence for location-specific cholinergic modulation of synaptic encoding mechanisms. Here, using evoked field responses in anaesthetized rat dorsal hippocampus we show that in addition to reinforcement of synaptic connections (long-term potentiation, LTP), endogenous acetylcholine also regulates firing gain of CA1 pyramidal neurons (EPSP-spike potentiation). Gain augmentation upon increase in cholinergic drive involves evoked synchronous firing at both apical and basal afferent projections, unlike enhancement of activity-induced LTP constrained to the basal afferent system. These data indicate that acetylcholine can act as an effective input and gain controller in the hippocampus. Modulation of synaptic plasticity would determine the relative dominance of afferent inputs while the facilitation of synchronous firing is likely to promote a more generalized spread of excitation and long range communication within the limbic cortex.

Multiple Gq-coupled receptors converge on a common protein synthesis-dependent long-term depression that is affected in fragile X syndrome mental retardation

Gq-coupled, M1 muscarinic acetylcholine receptors (mAChRs) facilitate hippocampal learning, memory, and synaptic plasticity. M1 mAChRs induce long-term synaptic depression (LTD), but little is known about the underlying mechanisms of mAChR-dependent LTD and its link to cognitive function. Here, we demonstrate that chemical activation of M1 mAChRs induces LTD in hippocampal area CA1, which relies on rapid protein synthesis, as well as the extracellular signal-regulated kinase and mammalian target of rapamycin translational activation pathways. Synaptic stimulation of M1 mAChRs, alone, or together with the Gq-coupled glutamate receptors (mGluRs), also results in protein synthesis-dependent LTD. New proteins maintain mAChR-dependent LTD through a persistent decrease in surface AMPA receptors. mAChRs stimulate translation of the RNA-binding protein, Fragile X mental retardation protein (FMRP) and FMRP target mRNAs. In mice without FMRP (Fmr1 knock-out), a model for human Fragile X syndrome mental retardation (FXS), both mGluR- and mAChR-dependent protein synthesis and LTD are affected. Our results reveal that multiple Gq-coupled receptors converge on a common protein synthesis-dependent LTD mechanism, which is aberrant in FXS. These findings suggest novel therapeutic strategies for FXS in the form of mAChR antagonists.

Hippocampal infusions of glucose reverse memory deficits produced by co-infusions of a GABA receptor agonist

Although septal infusions of glucose typically have positive effects on memory, we have shown repeatedly that this treatment exacerbates memory deficits produced by co-infusions of gamma-aminobutyric acid (GABA) receptor agonists. The present experiments tested whether this negative interaction between glucose and GABA in the medial septum would be observed in the hippocampus, a brain region where glucose typically has positive effects on memory. Specifically, we determined whether hippocampal infusions of glucose would reverse or exacerbate memory deficits produced by hippocampal co-infusions of the GABA receptor agonist muscimol. Fifteen minutes prior to either assessing spontaneous alternation (SA) or continuous multiple trial inhibitory avoidance (CMIA) training, male Sprague-Dawley-derived rats were given bilateral hippocampal infusions of vehicle (phosphate-buffered saline [PBS], 1 microl/2 min), glucose (33 or 50 nmol), muscimol (0.3 or 0.4 microg, SA or 3 microg, CMIA) or muscimol and glucose combined in one solution. The results indicated that hippocampal infusions of muscimol alone decreased SA scores and CMIA retention latencies. More importantly, hippocampal infusions of glucose, at doses that had no effect when infused alone, attenuated (33 nmol) or reversed (50 nmol) the muscimol-induced memory deficits. Thus, although co-infusions of glucose with muscimol into the medial septum impair memory, the present findings show that an opposite effect is observed in the hippocampus. Collectively, these findings suggest that the memory-impairing interaction between glucose and GABA in the medial septum is not a general property of the brain, but rather is brain region-dependent.

The role of acetylcholine in learning and memory

Pharmacological data clearly indicate that both muscarinic and nicotinic acetylcholine receptors have a role in the encoding of new memories. Localized lesions and antagonist infusions demonstrate the anatomical locus of these cholinergic effects, and computational modeling links the function of cholinergic modulation to specific cellular effects within these regions. Acetylcholine has been shown to increase the strength of afferent input relative to feedback, to contribute to theta rhythm oscillations, activate intrinsic mechanisms for persistent spiking, and increase the modification of synapses. These effects might enhance different types of encoding in different cortical structures. In particular, the effects in entorhinal and perirhinal cortex and hippocampus might be important for encoding new episodic memories.

Functional role of entorhinal cortex in working memory processing

Our learning and memory system has the challenge to work in a world where items to learn are dispersed in space and time. From the information extracted by the perceptual systems, the learning system must select and combine information. Both these operations may require a temporary storage where significance and correlations could be assessed. This work builds on the common hypothesis that hippocampus and subicular, entorhinal and parahippocampal/postrhinal areas are essential for the above-mentioned functions. We bring up two examples of models; the first one is modeling of in vivo and in vitro data from entorhinal cortex layer II of delayed match-to-sample working memory experiments, the second one studying mechanisms in theta rhythmicity in EC. In both cases, we discuss how cationic currents might be involved and relate their kinetics and pharmacology to behavioral and cellular experimental results.

Intraseptal tacrine-induced disruptions of spatial memory performance

The medial septal nucleus regulates the physiology and emergent functions (e.g., memory formation) of the hippocampal formation. This nucleus is particularly rich in cholinergic receptors and is a putative target for the development of cholinomimetic cognitive enhancing drugs. Several studies have examined the direct effects of intraseptal cholinomimetic treatments and the results have been somewhat conflicting with both promnestic and amnestic effects. Several variables (e.g., age, task difficulty, timing of drug administration) may influence treatment outcome. The present study examined the effects of intraseptal infusion of the acetylcholinesterase inhibitor tacrine (0-25 microg/0.5 microl) on spatial memory performance. Tacrine was infused into the medial septum just prior to testing. Tacrine infusions did not significantly affect the number of correct choices in the first eight entries, or the number of correct choices until an error. This treatment did not alter the angle of arm entries, or impair the animals' ability to complete the task (enter all baited arms). However, tacrine produced a linear dose-dependent increase in errors, doubling (12.5 microg) and tripling (25.0 microg) the number of errors made before rats completed the task. The deficit demonstrates that activation of intraseptal cholinergic receptors can disrupt spatial memory performance. These findings are discussed with regards to septohippocampal-dependent memory processes and the development of therapeutic strategies in the treatment of age-related memory disorders.

Timing of administration mediates the memory effects of intraseptal carbachol infusion

Medial septal neurons innervate the entire hippocampal formation. This input provides a potent regulation of hippocampal formation physiology (e.g. theta) and memory function. Medial septal neurons are rich in cholinergic receptors and thus are potential targets for the development of cognitive enhancers. Direct intraseptal infusion of cholinomimetics alters hippocampal physiology and can produce either promnestic or amnestic effects. Several variables (e.g. age of animal, integrity of septohippocampal circuits, task difficulty) may influence treatment outcome. We have previously demonstrated that intraseptal carbachol (12.5-125 ng) infusion immediately after the sample session of a delayed-non-match-to-sample radial maze paradigm produces a dose-dependent amnesia. The present study examined whether manipulating the timing of intraseptal carbachol infusion with respect to the sample session would alter the amnestic effect. A within-subjects design was used to examine the effect of intraseptal carbachol (125 ng/0.5 microl) in a delayed-non-match to sample radial maze task. During a sample session, rats retrieved rewards from six of 12 maze arms. At the test session (3 h later), only the alternate set contained reward and entries into the sample set arms constituted errors. Intraseptal carbachol was administered: 1) 30 min prior; 2) immediately prior; 3) immediately after and 4) 90 min after the sample session. Intraseptal carbachol prior to the sample had no effect on any index of accuracy. Infusion immediately after the sample, or delayed 90 min into the retention interval, produced an acute amnesia. These findings demonstrate that the timing of treatment is a critical variable in determining the memory effects of septohippocampal manipulations and that dynamic changes in cholinergic tone are important for memory.