Memory-related acetylcholine efflux from rat prefrontal cortex and hippocampus: a microdialysis study

Role of hippocampal and prefrontal cortical cholinergic transmission in combination therapy valproate and cannabidiol in memory consolidation in rats: involvement of CREB- BDNF signaling pathways

PURPOSE

Cognitive disorders are associated with valproate and drugs used to treat neuropsychological diseases. Cannabidiol (CBD) has beneficial effects on cognitive function. This study examined the effects of co-administration of CBD and valproate on memory consolidation, cholinergic transmission, and cyclic AMP response element-binding protein (CREB)-brain-derived neurotrophic factor (BDNF) signaling pathway in the prefrontal cortex (PFC) and hippocampus (HPC).

METHODS

One-trial, step-through inhibitory test was used to evaluate memory consolidation in rats. The intra-CA1 injection of physostigmine and atropine was performed to assess the role of cholinergic transmission in this co-administration. Phosphorylated CREB (p-CREB)/CREB ratio and BDNF levels in the PFC and HPC were evaluated.

RESULTS

Post-training intraperitoneal (i.p.) valproate injection reduced memory consolidation; however, post-training co-administration of CBD with valproate ameliorated memory impairment induced by valproate. Post-training intra-CA1 injection of physostigmine at the ineffective doses in memory consolidation (0.5 and 1 µg/rat), plus injection of 10 mg/kg of CBD as an ineffective dose, improved memory loss induced by valproate, which was associated with BDNF and p-CREB level enhancement in the PFC and HPC. Conversely, post-training intra-CA1 injection of ineffective doses of atropine (1 and 2 µg/rat) reduced the positive effects of injection of CBD at a dose of 20 mg/kg on valproate-induced memory loss associated with BDNF and p-CREB level reduction in the PFC and HPC.

CONCLUSION

The results indicated a beneficial interplay between valproate and CBD in the process of memory consolidation, which probably creates this interaction through the BDNF-CREB signaling pathways in the cholinergic transmission of the PFC and HPC regions.

Developmental Age and Biological Sex Influence Muscarinic Receptor Function and Neuron Morphology within Layer VI of the Medial Prefrontal Cortex

Acetylcholine (ACh) neurotransmission within the medial prefrontal cortex (mPFC) plays an important modulatory role to support mPFC-dependent cognitive functions. This role is mediated by ACh activation of its nicotinic (nAChR) and muscarinic (mAChR) classes of receptors, which are both present on mPFC layer VI pyramidal neurons. While the expression and function of nAChRs have been characterized thoroughly for rodent mPFC layer VI neurons during postnatal development, mAChRs have not been characterized in detail. We employed whole-cell electrophysiology with biocytin filling to demonstrate that mAChR function is greater during the juvenile period of development than in adulthood for both sexes. Pharmacological experiments suggest that each of the M1, M2, and M3 mAChR subtypes contributes to ACh responses in these neurons in a sex-dependent manner. Analysis of dendrite morphology identified effects of age more often in males, as the amount of dendrite matter was greatest during the juvenile period. Interestingly, a number of positive correlations were identified between the magnitude of ACh/mAChR responses and dendrite morphology in juvenile mice that were not present in adulthood. To our knowledge, this work describes the first detailed characterization of mAChR function and its correlation with neuron morphology within layer VI of the mPFC.

GAS5 which is regulated by Lhx8 promotes the recovery of learning and memory in rats with cholinergic nerve injury

Damage to the cholinergic system in central nervous system injuries such as traumatic brain injury (TBI) and neurodegenerative diseases leads to impaired learning and cognition. Neural stem cells (NSCs) have self-renewal capacity and multi-directional differentiation potential and considered the best source of cells for cell replacement therapy. However, how to promote the differentiation of NSCs into neurons is a major challenge in current research. Lhx8 has a specific effect on the development of the cholinergic nervous system, but its exact function is unclear. In this study, we found that Lhx8 could regulate the expression of Growth arrest-specific (GAS)5 which has been implicated in cancer but was less studied in the nervous system. Additionally, results from PCR, fluorescence in situ hybridization, and immunocytochemical analyses showed that GAS5 is mainly expressed in the cytoplasm of hippocampal neural stems cells and promotes their differentiation into neurons; the Morris water maze test demonstrated that GAS5 overexpression restored learning and memory in rats with cholinergic injury. These findings indicate that GAS5, which is regulated by Lhx8, improve brain function following cholinergic nerve injury.

Opioid withdrawal and memory consolidation

It is well established that learning and memory are central to substance dependence. This paper specifically reviews the effect of opioid withdrawal on memory consolidation. Although there is evidence that opioid withdrawal can interfere with initial acquisition and retrieval of older memories, there are several reasons to postulate a facilitatory action on the consolidation of newly acquired memories. In fact, there is substantial evidence that memory consolidation is facilitated by the release of stress hormones, that it requires the activation of the amygdala, of central noradrenergic and cholinergic pathways, and that it involves long-term potentiation. This review highlights evidence that very similar neurobiological processes are involved in opioid withdrawal, and summarizes recent results indicating that naltrexone-precipitated withdrawal enhanced consolidation in rats. From this neurocognitive perspective, therefore, opioid use may escalate during the addiction cycle in part because memories of stimuli and actions experienced during withdrawal are strengthened.

Regulation of Cognitive Processing by Hippocampal Cholinergic Tone

Cholinergic dysfunction has been associated with cognitive abnormalities in a variety of neurodegenerative and neuropsychiatric diseases. Here we tested how information processing is regulated by cholinergic tone in genetically modified mice targeting the vesicular acetylcholine transporter (VAChT), a protein required for acetylcholine release. We measured long-term potentiation of Schaffer collateral-CA1 synapses in vivo and assessed information processing by using a mouse touchscreen version of paired associates learning task (PAL). Acquisition of information in the mouse PAL task correlated to levels of hippocampal VAChT, suggesting a critical role for cholinergic tone. Accordingly, synaptic plasticity in the hippocampus in vivo was disturbed, but not completely abolished, by decreased hippocampal cholinergic signaling. Disrupted forebrain cholinergic signaling also affected working memory, a result reproduced by selectively decreasing VAChT in the hippocampus. In contrast, spatial memory was relatively preserved, whereas reversal spatial memory was sensitive to decreased hippocampal cholinergic signaling. This work provides a refined roadmap of how synaptically secreted acetylcholine influences distinct behaviors and suggests that distinct forms of cognitive processing may be regulated in different ways by cholinergic activity.

A robust in vivo-like persistent firing supported by a hybrid of intracellular and synaptic mechanisms

Persistent firing is believed to support short-term information retention in the brain. Established hypotheses make use of the recurrent synaptic connectivity to support persistent firing. However, this mechanism is known to suffer from a lack of robustness. On the other hand, persistent firing can be supported by an intrinsic cellular mechanism in multiple brain areas. However, the consequences of having both the intrinsic and the synaptic mechanisms (a hybrid model) on persistent firing remain largely unknown. The goal of this study is to investigate whether a hybrid neural network model with these two mechanisms has advantages over a conventional recurrent network based model. Our computer simulations were based on in vitro recordings obtained from hippocampal CA3 pyramidal cells under cholinergic receptor activation. Calcium activated non-specific cationic (CAN) current supported persistent firing in the Hodgkin-Huxley style cellular models. Our results suggest that the hybrid model supports persistent firing within a physiological frequency range over a wide range of different parameters, eliminating parameter sensitivity issues generally recognized in network based persistent firing. In addition, persistent firing in the hybrid model is substantially more robust against distracting inputs, can coexist with theta frequency oscillations, and supports pattern completion.

Long-term changes in the CA3 associative network of fear-conditioned mice

The CA3 associative network plays a critical role in the generation of network activity patterns related to emotional state and fear memory. We investigated long-term changes in the corticosterone (CORT)-sensitive function of this network following fear conditioning and fear memory reactivation. In acute slice preparations from mice trained in either condition, the ratio of orthodromic population spike (PS) to antidromic PS was reduced compared to unconditioned animals, indicating a decrease in efficacy of neuronal coupling within the associative CA3 network. However, spontaneous sharp wave-ripples (SW-R), which are thought to arise from this network, remained unaltered. Following CORT application, we observed an increase in orthodromic PS and a normalization to control levels of their ratio to antidromic PS, while SW-R increased in slices of fear conditioned and fear reactivated mice, but not in slices of unconditioned controls. Together with our previous observations of altered hippocampal gamma activity under these learning paradigms, these data suggest that fear conditioning and fear reactivation lastingly alters the CORT-sensitive configuration of different network activity patterns generated by the CA3 associational network. Observed changes in the mRNA expression of receptors for glutamate, GABA and cannabinoids in the stratum pyramidale of area CA3 may provide a molecular mechanism for these adaptive changes.

Corticosterone and corticotropin-releasing factor acutely facilitate gamma oscillations in the hippocampus in vitro

Stressful experiences do not only cause peripheral changes in stress hormone levels, but also affect central structures such as the hippocampus, implicated in spatial orientation, stress evaluation, and learning and memory. It has been suggested that formation of memory traces is dependent on hippocampal gamma oscillations observed during alert behaviour and rapid eye movement sleep. Furthermore, during quiescent behaviour, sharp wave-ripple (SW-R) activity emerges. These events provide a temporal window during which reactivation of memory ensembles occur. We hypothesized that stress-responsive modulators, such as corticosterone (CORT), corticotropin-releasing factor (CRF) and the neurosteroid 3α, 21-dihydroxy-5α-pregnan-20-one (THDOC) are able to modulate gamma oscillations and SW-Rs. Using in vitro hippocampal slices, we studied acute and subacute (2 h) impact of these agents on gamma oscillations in area cornu ammonis 3 of the ventral hippocampus induced by acetylcholine (10 μm) combined with physostigmine (2 μm). CORT increased the gamma oscillations in a dose-dependent fashion. This effect was mediated by glucocorticoid receptors. Likewise, CRF augmented gamma oscillations via CRF type 1 receptor. Lastly, THDOC was found to diminish cholinergic gamma oscillations in a dose-dependent manner. Neither CORT, CRF nor THDOC modulated gamma power when pre-applied for 1 h, 2 h before the induction of gamma oscillations. Interestingly, stress-related neuromodulators had rather mild effects on spontaneous SW-R compared with their effects on gamma oscillations. These data suggest that the alteration of hippocampal gamma oscillation strength in vitro by stress-related agents is an acute process, permitting fast adaptation to new attention-requiring situations in vivo.

Direct excitation of parvalbumin-positive interneurons by M1 muscarinic acetylcholine receptors: roles in cellular excitability, inhibitory transmission and cognition

Parvalbumin-containing (PV) neurons, a major class of GABAergic interneurons, are essential circuit elements of learning networks. As levels of acetylcholine rise during active learning tasks, PV neurons become increasingly engaged in network dynamics. Conversely, impairment of either cholinergic or PV interneuron function induces learning deficits. Here, we examined PV interneurons in hippocampus (HC) and prefrontal cortex (PFC) and their modulation by muscarinic acetylcholine receptors (mAChRs). HC PV cells, visualized by crossing PV-CRE mice with Rosa26YFP mice, were anatomically identified as basket cells and PV bistratified cells in the stratum pyramidale; in stratum oriens, HC PV cells were electrophysiologically distinct from somatostatin-containing cells. With glutamatergic transmission pharmacologically blocked, mAChR activation enhanced PV cell excitability in both CA1 HC and PFC; however, CA1 HC PV cells exhibited a stronger postsynaptic depolarization than PFC PV cells. To delete M1 mAChRs genetically from PV interneurons, we created PV-M1 knockout mice by crossing PV-CRE and floxed M1 mice. The elimination of M1 mAChRs from PV cells diminished M1 mAChR immunoreactivity and muscarinic excitation of HC PV cells. Selective cholinergic activation of HC PV interneurons using Designer Receptors Exclusively Activated by Designer Drugs technology enhanced the frequency and amplitude of inhibitory synaptic currents in CA1 pyramidal cells. Finally, relative to wild-type controls, PV-M1 knockout mice exhibited impaired novel object recognition and, to a lesser extent, impaired spatial working memory, but reference memory remained intact. Therefore, the direct activation of M1 mAChRs on PV cells contributes to some forms of learning and memory.

Cholinergic depletion in nucleus accumbens impairs mesocortical dopamine activation and cognitive function in rats

In rats, selective depletion of the cholinergic interneurons in the ventral striatum (nucleus accumbens or N.Acc.) results in heightened behavioural sensitivity to amphetamine and impaired sensorimotor gating processes, suggesting a hyper-responsiveness to dopamine (DA) activity in the N.Acc. We hypothesized that local cholinergic depletion may also trigger distal functional alterations, particularly in prefrontal cortex (PFC). Adult male Sprague-Dawley rats were injected bilaterally in the N.Acc. with an immunotoxin targeting choline acetyltransferase. Two weeks later, cognitive function was assessed using the delayed alternation paradigm in the T-maze. The rats were then implanted with voltammetric recording electrodes in the ventromedial PFC to measure in vivo extracellular DA release in response to mild tail pinch stress. The PFC was also examined for density of tyrosine hydroxylase (TH)-labelled varicosities. In another cohort of control and lesioned rats, we measured post mortem tissue content of DA. Depletion of cholinergic neurons (restricted to N.Acc.) significantly impaired delayed alternation performance across delay intervals. While (basal) post mortem indices of PFC DA function were unaffected by N.Acc. lesions, in vivo mesocortical DA activation was markedly reduced; this deficit correlated significantly with cognitive impairments. TH-labelled varicosities however, were unaffected in cortical layer V relative to controls. These data suggest that selective depletion of cholinergic interneurons in N.Acc. triggers widespread functional impairments in mesocorticolimbic DA function and cognition. The possible relevance of these findings is also discussed in relation to schizophrenia, where reduced density of cholinergic neurons in ventral striatum has been reported.

The effects of galantamine on nicotine withdrawal-induced deficits in contextual fear conditioning in C57BL/6 mice

Current smoking cessation aids are relatively ineffective at maintaining abstinence during withdrawal. Nicotine withdrawal is associated with a variety of symptoms including cognitive deficits and targeting these deficits may be a useful strategy for maintaining abstinence. Galantamine is an acetylcholinesterase inhibitor and allosteric modulator of nicotinic acetylcholine receptors (nAChRs) with cognitive enhancing effects that may alleviate cognitive deficits associated with nicotine withdrawal. The effects of galantamine on nicotine withdrawal-induced deficits in contextual fear conditioning in C57BL/6 mice were examined. An initial acute dose-response experiment revealed that 0.5 and 1mg/kg galantamine had no effect on fear conditioning. To determine if galantamine would reverse nicotine withdrawal-related deficits in contextual fear conditioning, mice were implanted with osmotic mini-pumps that delivered chronic saline or 6.3mg/kg/d nicotine for 12 days and then pumps were removed. Training and testing of fear conditioning occurred 24 and 48 h later, respectively. Nicotine withdrawal disrupted contextual fear conditioning, which was reversed with 1 but not 0.5mg/kg galantamine. Across all conditions in both studies 2mg/kg galantamine led to high levels of freezing that were likely due to nonspecific effects. The ability of galantamine to reverse nicotine withdrawal-deficits in contextual conditioning is likely mediated through enhanced levels of acetylcholine via inhibition of acetylcholinesterase, potentiation of hippocampal α4β2* nAChRs, or both. The present study suggests that acetylcholinesterase inhibitors and/or drugs that act as allosteric modulators of nAChRs might be targets for smoking cessation aids because they may alleviate withdrawal symptoms such as cognitive deficits that can lead to relapse.

Sex differences in the septo-hippocampal cholinergic system in rats: behavioral consequences

The hippocampus is processing temporal and spatial information in particular contexts or episodes. Using freely moving rats, we monitored extracellular levels of acetylcholine (ACh), a critical neurotransmitter activating hippocampal circuits. We found that the ACh release in the dorsal hippocampus increases during the period of learning or exploration, exhibiting a sex-specific 24-h release profile. Moreover, neonatal increase in circulating androgen not only androgenizes behavioral and hormonal features, but also produces male-type ACh release profile after the development. The results suggest neonatal sexual differentiation of septo-hippocampal cholinergic system. Environmental conditions (such as stress, housing or food) of animals further affected the ACh release.Although recent advances of neuroscience successfully revealed molecular/cellular mechanism of learning and memory, most research were performed using male animals at specific time period. Sex-specific or time-dependent hippocampal functions are still largely unknown.

Gonadal hormones modulate the potency of the disruptive effects of donepezil in male rats responding under a nonspatial operant learning and performance task

In contrast to estrogen in female rats, testosterone in male rats may decrease cholinergic activity in the brain, thereby attenuating behaviors mediated by the cholinergic system. To investigate this possibility, the interactive effects of the gonadal hormones and donepezil, an acetylcholinesterase (AChE) inhibitor, on the responding of male rats were examined under a multiple schedule of repeated acquisition and performance of response sequences and on AChE activity in specific brain regions. Donepezil dose-effect curves (0.56-10 mg/kg) were determined in males that were gonadally intact, gonadectomized (GX), GX with testosterone replacement (GX+T) or GX with estradiol replacement (GX+E). In all four groups, donepezil produced dose-dependent rate-decreasing and error-increasing effects in the acquisition and performance components. However, disruptions of response rate and accuracy in both components occurred at lower doses in GX and GX+E males than in intact males. The GX+E males also had the highest percentage of errors under control (saline) conditions in the acquisition components. In terms of AChE activity, GX males had higher levels in the prefrontal cortex, striatum and hippocampus, but lower levels in the midbrain, compared with intact males; hypothalamic and cortical levels were comparable for the GX and intact groups. Together, these results in male rats indicate that the potency of donepezil's disruptive effects on the responding under a complex operant procedure requiring learning and performance of response sequences depends upon the gonadal hormone status, and that the effects of testosterone on cholinergic activity vary among brain regions.

WIN55,212-2 induced deficits in spatial learning are mediated by cholinergic hypofunction

Cannabinoids acting on CB(1) receptors induce learning and memory impairments. However, the identification of novel non-CB(1) receptors which are insensitive to the psychoactive ingredient of marijuana, Delta(9)-tetrahydrocannabinol (Delta(9)-THC) but sensitive to synthetic cannabinoids such as WIN55,212-2 (WIN-2) or endocannabinoids like anandamide lead us to question whether WIN-2 induced learning and memory deficits are indeed mediated by CB(1) receptor activation. Given the relative paucity of receptor subtype specific antagonists, a way forward would be to determine the transmitter systems, which are modulated by the respective cannabinoids. This study set out to evaluate this proposition by determination of the effects of WIN-2 on acquisition of spatial reference memory using the water maze in rats. Particular weight was given to performance in trial 1 of each daily session as an index of between-session long-term memory, and in trial 4 as an index of within-session short-term memory. Intraperitoneal (i.p.) administration of WIN-2 (1 mg/kg and 3 mg/kg) prior to training impaired long-term, but not short-term memory. This deficit was not reversed by the CB(1) antagonists/inverse agonists Rimonabant (3mg/kg i.p.) and AM281 (0.5 mg/kg i.p.), but recovered in the presence of the cholinesterase inhibitor rivastigmine (1 mg/kg). Reversal by rivastigmine was specific to WIN-2, as it failed to reverse MK801 (0.08 mg/kg) induced learning impairments. Collectively, these data suggest that in this spatial reference memory task WIN-2 causes a reduction in cholinergic activation, possibly through a non-CB(1)-like mechanism, which affects long-term but not short-term spatial memory.

Chronic prenatal ethanol exposure increases disinhibition and perseverative responding in the adult guinea pig

Cognitive and behavioural deficits, including increased impulsivity and perseveration, are associated with chronic prenatal ethanol exposure (CPEE) in humans. We tested whether these same deficits occur in the guinea pig after CPEE treatment. Pregnant guinea pigs received oral administration of ethanol (4 g/kg maternal body weight/day), or isocaloric-sucrose/pair-feeding throughout gestation. Young adult offspring were trained in lever-pressing paradigms to work for a sucrose-pellet food reward. CPEE increased No-Go, but not Go, responses in the Go/No-Go paradigm, indicative of a disinhibition deficit in these animals. Perseverative responses in the Cued Alternation task were also increased in CPEE offspring. These data show that CPEE induces behavioural deficits in the guinea pig that are remarkably similar to the executive function deficits that follow prenatal ethanol exposure in humans.

Activational and organisational effects of gonadal steroids on sex-specific acetylcholine release in the dorsal hippocampus

Acetylcholine (ACh) release in the dorsal hippocampus increases during stress, exploration or learning, exhibiting sex-specific 24-h release profile. We review the role of gonadal steroids on the ACh release in the dorsal hippocampus. In our studies, we found that male rats showed higher extracellular ACh levels than females, but gonadectomy decreased ACh levels in both sexes of rats and subsequently eliminated the sex difference. To examine the sex difference under comparable gonadal steroid levels, we implanted steroid capsules after gonadectomy. Oestradiol supplementation maintained circulating oestradiol to the levels in proestrous female rats, whereas testosterone capsules maintained circulating testosterone to the levels similar to intact male rats. Under comparable gonadal steroids levels, ACh levels were sex-specific. Testosterone replacement in orchidectomised rats clearly restored ACh levels, which were greater than ovariectomised testosterone-primed rats. Similarly, oestradiol replacement in ovariectomised rats successfully restored ACh levels, which were higher than orchidectomised oestradiol-primed rats. These results suggest sex-specific activational effects of gonadal steroids on ACh release. To further examine the organisational effect, female pups were neonatally treated with oil, testosterone, oestradiol, or dihydrotestosterone. These rats were bilaterally ovariectomised and a testosterone capsule was implanted at postnatal week 8. Neonatal treatment of either testosterone or oestradiol clearly increased ACh levels, whereas neonatal dihydrotestosterone treatment failed to change levels. These results suggest that: (i) gonadal steroids maintain the sex-specific ACh release in the dorsal hippocampus and (ii) neonatal activation of oestrogen receptors is sufficient to mediate masculinisation of the septo-hippocampal cholinergic system.

Gonadal steroid hormones maintain the stress-induced acetylcholine release in the hippocampus: simultaneous measurements of the extracellular acetylcholine and serum corticosterone levels in the same subjects

To examine the role of gonadal steroid hormones in the stress responses of acetylcholine (ACh) levels in the hippocampus and serum corticosterone levels, we observed these parameters simultaneously in intact, gonadectomized, or gonadectomized steroid-primed rats. In both sexes of rats, neither gonadectomy nor the replacement of gonadal steroid hormone affected the baseline levels of ACh. However, gonadectomy severely attenuated the stress response of ACh, whereas the replacement of corresponding gonadal hormone successfully restored the response to intact levels. The gonadal hormones affected the serum corticosterone levels in a different manner; the testosterone replacement in orchidectomized rats suppressed the baseline and the stress response of corticosterone levels, whereas the 17beta-estradiol replacement in ovariectomized rats increased the levels. We further found that letrozole or flutamide administration in intact male rats attenuated the stress response of ACh. In addition, flutamide treatment increased the baseline levels of corticosterone, whereas letrozole treatment attenuated the stress response of corticosterone. Moreover, we found a low positive correlation between the ACh levels and corticosterone levels, depending on the presence of gonadal steroid hormone. We conclude that: 1) gonadal steroid hormones maintain the stress response of ACh levels in the hippocampus, 2) the gonadal steroid hormone independently regulates the stress response of ACh in the hippocampus and serum corticosterone, and 3) the sex-specific action of gonadal hormone on the cholinergic stress response may suggest a neonatal sexual differentiation of the septohippocampal cholinergic system in rats.

Subsequent exposure to the choline uptake enhancer MKC-231 antagonizes phencyclidine-induced behavioral deficits and reduction in septal cholinergic neurons in rats

This study examined the effects of subsequent, subchronic, treatment with choline uptake enhancer MKC-231 on the behavioral and cellular deficits induced by repeated PCP exposure in rats. Prior subchronic PCP exposure resulted in increased locomotion following an acute PCP or cocaine challenge, but resulted in decreased locomotor activity in response to a carbachol-challenge. MKC-231 significantly antagonized the alterations in the locomotor responses to cocaine and carbachol, but not to PCP. In the novel object recognition test, repeated PCP exposure caused cognitive deficits in rats, and the PCP-induced cognitive deficits were antagonized by MKC-231. In contrast, no effects of PCP exposure were shown in the repeated passive avoidance test. Furthermore, repeated PCP exposure decreased a number of choline acetyltransferase (ChAT)-positive cells in the medial septum and increased dynorphin A expression in the ventral striatum. Moreover, MKC-231 significantly antagonized the changes in septal ChAT-positive cells, but not the changes in ventrostriatal dynorphin A expression. These results suggest that MKC-231 could be a therapeutic drug for the treatment of schizophrenia.

Dissociation of cholinergic function in spatial and procedural learning in rats

The cholinergic system has long been known for its role in acquisition and retention of new information. Scopolamine, a muscarinic acetylcholine receptor antagonist impairs multiple memory systems, and this has promoted the notion that drug-induced side effects are responsible for diminished task execution rather than selective impairments on learning and memory per se. Here, we revisit this issue with the aim to dissociate the effects of scopolamine (0.2-1.0 mg/kg) on spatial learning in the water maze. Experiments 1 and 2 showed that acquisition of a reference memory paradigm with constant platform location is compromised by scopolamine independent of whether the animals are pre-trained or not. Deficits were paralleled by drug induced side-effects on sensorimotor parameters. Experiment 3 explored the role of muscarinic receptors in acquisition of an episodic-like spatial delayed matching to position (DMTP) protocol, and scopolamine still caused a learning deficit and side-effects on sensorimotor performance. Rats extensively pre-trained in the DMTP protocol with 30 s and 1 h delays over several months in experiment 4 and tested in a within-subject design under saline and scopolamine had no sensorimotor deficits, but spatial working memory remained compromised. Experiment 5 used the rising Atlantis platform in the DMTP paradigm. Intricate analysis of the amount of dwelling and its location revealed a clear deficit in spatial working memory induced by scopolamine, but there was no effect on sensorimotor or procedural task demands. Apart from the well-known contribution to sensorimotor and procedural learning, our findings provide compelling evidence for an important role of muscarinic acetylcholine receptor signaling in spatial episodic-like memory.

Insights into the Role of Dopamine Receptor Systems in Learning and Memory

It is well established that learning and memory are complex processes involving and recruiting different brain modulatory neurotransmitter systems. Considerable evidence points to the involvement of dopamine in various aspects of cognition, and interest has been focused on investigating the clinical relevance of dopamine systems to age-related cognitive decline and manifestations of cognitive impairment in schizophrenia, Alzheimer's disease, Parkinson's disease and other neurodegenerative diseases. In the past decade or so, in spite of the molecular cloning of the five dopamine receptor subtypes, their specific roles in brain function remained inconclusive due to the lack of completely selective ligands that could distinguish between the members of the D1-like and D2-like dopamine receptor families. One of the most important advances in the field of dopamine research has been the generation of mutant mouse models permitting evaluation of the dopaminergic system using gene targeting technologies. These mouse models represent an important approach to explore the functional roles of closely related receptor subtypes. In this review, we present and discuss evidence on the role of dopamine receptors in different aspects of learning and memory at the cellular, molecular and behavioral levels. We compare evidence using conventional pharmacological, lesion or electrophysiological studies with results from mice with targeted deletions of different subtypes of dopamine receptor genes. We particularly focus on dopamine D1 and D2 receptors in an effort to delineate their specific roles in various aspects of cognitive function. We provide strong evidence, from our own recent work as well as others, that dopamine is part of the network that plays a very important role in cognitive function, and that although multiple dopamine receptor subtypes contribute to different aspects of learning and memory, the D1 receptor seems to play a more prominent role in mediating plasticity and specific aspects of cognitive function, including spatial learning and memory processes, reversal learning, extinction learning, and incentive learning.

Developmental modulation of synaptic transmission by acetylcholine in the primary visual cortex

Despite the evidence that cortical synaptic organization and cognitive functions are influenced by the activity of the cholinergic system during postnatal development, so far no information is available on the effects produced by acetylcholine (ACh) on synaptic transmission. In the present article, we show that the ability of visual cortex slices to respond to ACh depends on postnatal age. In adulthood, ACh exerts mainly a facilitatory action on synaptic transmission, depressing field potential (FP) amplitude only if applied at high concentrations (millimolar range). During early postnatal development, at postnatal day 13 (P13), facilitation by ACh was lacking, with depression of FP observed with concentration of ACh in the micromolar range. The magnitude of ACh facilitatory effects increases with age. The time course of ACh-dependent facilitation overlaps the developmental maturation of acetylcholinesterase (AChE), suggesting a close relationship between ACh action and AChE activity. Thus, age-dependent modification of the cholinergic modulatory action may affect cortical maturation by regulating the magnitude of synaptic transmission.

Timing and Hippocampal Theta in Animals

All animals have at least two different internal clocks, one governing cognition of time of day, and the other concerning awareness of seconds and minutes. In the latter case, organisms show scalar properties. The timing mechanisms in the brain may function similarly throughout the animal kingdom, but this is not yet clear. Previous studies have shown that the hippocampus is intricately involved with the process of interval timing. Data concerning electrophysiological field potentials in the hippocampus show obviously rhythmic activity, known as hippocampal theta activity. An information-processing model of interval timing postulates three distinct stages: a clock, a memory, and a decision stage /11/. The timing process includes memory processing, which means that the hippocampus works together with working memory to estimate current time passing.

Postoperative delirium in elderly patients after major abdominal surgery

Background

The aim of this study was to investigate the occurrence of postoperative delirium (POD) in elderly patients undergoing major abdominal surgery and to identify factors associated with delirium in this population.

Methods

Data were collected prospectively from 51 patients aged 65 years or more. Delirium was diagnosed by the Confusion Assessment Method and from the medical records. The Mini Mental State Examination (MMSE) was used to identify cognitive impairment.

Results

POD occurred in 26 of 51 patients. Delirium lasted for 1–2 days in 14 patients (short POD group) and 3 days or more in 12 patients (long POD group). The latter patients had significantly greater intraoperative blood loss and intravenous fluid infusion, a higher rate of postoperative complications, a lower MMSE score on postoperative day 4 and a longer hospital stay than patients without POD. Patients in the short POD group were significantly older than those in the long POD group and those who did not develop delirium.

Conclusion

Approximately half of the elderly patients in this study developed POD. Bleeding was found to be an important risk factor for delirium.

Complementary remedy of aged-related learning and memory deficits via exogenous choline acetyltransferase

The present study aimed to examine whether the aged mice with naturally occurring cognitive deficits in learning and memory would benefit from supplementation of choline acetyltransferase (ChAT), the biosynthetic enzyme for neurotransmitter acetylcholine. Delivered by protein transduction domain (PTD), ChAT could pass through the blood-brain barrier, enter the neurons, interact with heat shock protein 70kDa, and retain enzyme activity. In behavior tests, PTD-ChAT given to the aged and memory-deficient mice almost completely reversed the behavioral changes, such as impairment of memory retention in the step-through test (an index of long-term memory) and prolonged swimming time in water maze test (an index of spatial recognition memory). The results suggest a novel and potential therapeutic use of PTD-ChAT in the age-related cognitive deficits.

Intrahippocampal muscimol shifts learning strategy in gonadally intact young adult female rats

Learning strategy preferences depend upon circulating estrogen levels, with enhanced hippocampus-sensitive place learning coinciding with elevated estrogen levels. The effects of estrogen on strategy may be mediated by fluctuations in GABAergic function, given that inhibitory tone in the hippocampus is low when estrogen is high. We investigated the effects on learning strategy of intrahippocampal injections of a GABA(A) agonist in gonadally intact female rats. On the day of training, rats received 0.3 microL intrahippocampal infusions of muscimol (0.26 nmol or 2.6 nmol) or saline 20 min prior to training on a T-maze in which place (hippocampus-sensitive) or response (striatum-sensitive) strategies offer effective solutions. Muscimol treatment increased the use of the response strategy in a dose-dependent manner without influencing learning speed, indicating that muscimol modulated strategy and not learning ability. Furthermore, the muscimol-related shift to response strategies varied across the estrous cycle. The results indicate that increasing inhibition in the hippocampus biases rats away from hippocampus-sensitive place learning strategies and toward hippocampus-insensitive response learning strategies without a learning deficit. Furthermore, rats at proestrus demonstrated the most dramatic shift in learning strategy following muscimol treatment compared with control conditions, while rats at estrus demonstrated the most complete bias toward response strategies. The enhanced use of hippocampus-sensitive strategies at proestrus likely results from reduced hippocampal inhibition.

Dopamine D3 receptor antagonists as therapeutic agents

The behavioral and pathophysiological role of the dopamine D(3) receptor, which was deduced from anatomical, lesion and drug treatment studies in the ten years following cloning of the receptor, indicated that its functions differed from those of the D(2) receptor. There is increasingly strong evidence that D(3) receptor antagonists will be effective antipsychotic agents. In this regard, an amelioration of the negative and cognitive symptoms of schizophrenia holds the most promise for D(3) receptor antagonists, a concept currently under clinical evaluation. In addition, D(3) receptors could be involved in behavioral sensitization and the potential application of D(3) receptor antagonists in the treatment of drug abuse is undergoing intensive experimental investigation.

Reversal of clozapine effects on working memory in rats with fimbria-fornix lesions

Clozapine is an effective antipsychotic drug, but its effects on cognitive function are unclear. Previously, we found that clozapine caused a working memory deficit, which was reversed by nicotine. Hippocampal systems are important in determining clozapine effect on memory. In the current study, the memory effects of clozapine and nicotine administration were determined in rats with lesions of the fimbria-fornix, a fiber bundle which carries cholinergic and other projections between the septum and the hippocampus. Female Sprague-Dawley rats were trained on a win-shift procedure in the radial-arm maze, in which each arm entry was rewarded once per session. Then, 13 rats received bilateral knife-cut lesions of the fimbria-fornix, while 14 rats underwent sham surgery. The rats were tested after subcutaneous injections with combinations of clozapine (0 and 1.25 mg/kg) and nicotine (0, 0.2, and 0.4 mg/kg). In sham-operated rats, clozapine caused a significant (P<0.005) working memory impairment. Fimbria-fornix lesions also caused a significant (P<0.05) memory impairment. Interestingly, clozapine had the opposite effect on working memory in the lesioned vs sham-operated rats. In contrast to its effects in controls, clozapine (1.25 mg/kg) significantly (P<0.05) attenuated the working memory deficit caused by fimbria-fornix lesions. Nicotine (0.2 mg/kg) did not quite significantly improve memory in lesioned rats. The effects of clozapine and nicotine were not additive in the lesioned rats. This study demonstrates the efficacy of clozapine in improving working memory in fimbria-fornix-lesioned rats, whereas it causes impairments in intact rats. Therapeutic treatment with clozapine in people with malfunctions of the hippocampus such as seen in schizophrenia may improve cognitive performance, whereas the same doses of clozapine may impair memory in individuals without hippocampal malfunction.

Acetylcholine modulates cortical synaptic transmission via different muscarinic receptors, as studied with receptor knockout mice

The central cholinergic system plays a crucial role in synaptic plasticity and spatial attention; however, the roles of the individual cholinergic receptors involved in these activities are not well understood at present. In the present study, we show that acetylcholine (ACh) can facilitate or depress synaptic transmission in occipital slices of mouse visual cortex. The precise nature of the ACh effects depends on the ACh concentration, and is input specific, as shown by stimulating different synaptic pathways. Pharmacological blockade of muscarinic receptor (mAChR) subtypes and the use of M1-M5 mAChR-deficient mice showed that specific mAChR subtypes, together with the activity of the cholinesterases (ChEs), mediate facilitation or depression of synaptic transmission. The present data suggest that local ACh, acting through mAChRs, regulates the cortical dynamics making cortical circuits respond to specific stimuli.

Cholinergic parameters and the retrieval of learned and re-learned spatial information: a study using a model of Wernicke-Korsakoff Syndrome

This is a factorial (2 x 2 x 2) spatial memory and cholinergic parameters study in which the factors are chronic ethanol, thiamine deficiency and naivety in Morris water maze task. Both learning and retention of the spatial version of the water maze were assessed. To assess retrograde retention of spatial information, half of the rats were pre-trained on the maze before the treatment manipulations of pyrithiamine (PT)-induced thiamine deficiency and post-tested after treatment (pre-trained group). The other half of the animals was only trained after treatment to assess anterograde amnesia (post-trained group). Thiamine deficiency, associated to chronic ethanol treatment, had a significant deleterious effect on spatial memory performance of post-trained animals. The biochemical data revealed that chronic ethanol treatment reduced acetylcholinesterase (AChE) activity in the hippocampus while leaving the neocortex unchanged, whereas thiamine deficiency reduced both cortical and hippocampal AChE activity. Regarding basal and stimulated cortical acetylcholine (ACh) release, both chronic ethanol and thiamine deficiency treatments had significant main effects. Significant correlations were found between both cortical and hippocampal AChE activity and behaviour parameters for pre-trained but not for post-trained animals. Also for ACh release, the correlation found was significant only for pre-trained animals. These biochemical parameters were decreased by thiamine deficiency and chronic ethanol treatment, both in pre-trained and post-trained animals. But the correlation with the behavioural parameters was observed only for pre-trained animals, that is, those that were retrained and assessed for retrograde retention.

Effects of isoflurane on prefrontal acetylcholine release and hypothalamic Fos response in young adult and aged rats

This experiment investigated the influence of age on prefrontal acetylcholine (ACh) release and Fos response in the hypothalamic paraventricular nucleus and the nucleus tractus solitarius (NTS) of rats following isoflurane anesthesia. It is known that isoflurane decreases acetylcholine release in most brain regions. In the present study, we found that the level of prefrontal acetylcholine was significantly lower in 28-month-old rats (14% of baseline) than in 3-month-old rats (38% of baseline) during 2 h of isoflurane anesthesia (P < 0.05). The old rat group showed significantly greater Fos induction in the paraventricular nucleus compared to the young adult rat group (P < 0.05), indicating that the old rats were subjected to stress. No difference in Fos response was noted in the nucleus tractus solitarius. The old rats displayed a significant increase in feeding behavior during the 3-h recovery period (P < 0.05), but there was no difference in overall acetylcholine levels. Taken together, these findings suggest that isoflurane anesthesia influences old rats more profoundly than young adult rats with regard to reductions in acetylcholine release and stress responses. This may have implications for understanding the development of postoperative delirium in aged patients.

Neurokinin-1 receptor antagonism by SR140333: enhanced in vivo ACh in the hippocampus and promnestic post-trial effects

Substance P (SP) has memory-promoting, reinforcing and anxiolytic-like effects when applied systemically or centrally. Such effects may be mediated by the neurokinin-1 (NK-1) receptor, since SP preferentially binds to this receptor. We measured the effects of a selective non-peptide NK-1 receptor antagonist, SR140333 (1, 3 and 9 mg/kg i.p.) on ACh levels in frontal cortex, amygdala and hippocampus by microdialysis and HPLC. Levels of ACh in the hippocampus increased dose-dependently immediately after treatment. The same doses of SR140333 given post-trial had minor facilitative effects on inhibitory avoidance learning and open-field habituation, but did not have reinforcing effects in a conditioned place preference (CPP) task. The selective action of NK-1 receptor antagonism on hippocampal ACh may be related to its positive influence on learning.

The acetylcholine fiber density of the neocortex is altered by isolated rearing and early methamphetamine intoxication in rodents

Alterations in the cholinergic physiology of the brain were the first to be observed when research on environmental influences on postnatal brain development began 35 years ago. Since then, the effects of isolated rearing (IR) or early pharmacological insults have been shown not only on the physiology, but also the anatomy of a variety of transmitter systems. The cholinergic fiber density, however, still remained to be assessed. We therefore used a histochemical procedure to stain cholinergic fibers in the brains of young adult gerbils reared either in groups in enriched environments or isolated in standard makrolon cages. Half of the animals from each rearing condition had received a single high dose of methamphetamine on postnatal day 14. Fiber densities were measured by computerized image analysis in the medial and orbital prefrontal cortex (PFC), dysgranular and granular insular cortex, sensorimotor cortices, and the entorhinal cortex of both hemispheres. Isolation rearing increased the cholinergic fiber densities in the prefrontal cortices of the left hemisphere and in the entorhinal cortex of the right hemisphere by about 10%, with no effect in the respective contralateral side. The early methamphetamine intoxication showed no influence in prefrontal and entorhinal cortices, but diminished the acetylcholine (ACh) innervation of the forelimb area of cortex in both hemispheres in IR gerbils and of the left hemisphere in ER gerbils, and reduced the acetylcholine innervation in the hindlimb area in both sides in both rearing groups. These results demonstrate that (a) cholinergic fiber density is differentially regulated in different cortical areas and (b) the plasticity of the cholinergic system can only be understood in the interplay with other neuromodulatory innervations.

Septohippocampal acetylcholine: involved in but not necessary for learning and memory?

The neurotransmitter acetylcholine (ACh) has been accorded an important role in supporting learning and memory processes in the hippocampus. Cholinergic activity in the hippocampus is correlated with memory, and restoration of ACh in the hippocampus after disruption of the septohippocampal pathway is sufficient to rescue memory. However, selective ablation of cholinergic septohippocampal projections is largely without effect on hippocampal-dependent learning and memory processes. We consider the evidence underlying each of these statements, and the contradictions they pose for understanding the functional role of hippocampal ACh in memory. We suggest that although hippocampal ACh is involved in memory in the intact brain, it is not necessary for many aspects of hippocampal memory function.

Changes in acetylcholine extracellular levels during cognitive processes

Measuring the changes in neurotransmitter extracellular levels in discrete brain areas is considered a tool for identifying the neuronal systems involved in specific behavioral responses or cognitive processes. Acetylcholine (ACh) is the first neurotransmitter whose diffusion from the central nervous system was investigated and whose extracellular levels variations were correlated to changes in neuronal activity. This was done initially by means of the cup technique and then by the microdialysis technique. The latter, notwithstanding some technical limitations, makes it possible to detect variations in extracellular levels of ACh in unrestrained, behaving animals. This review summarizes and discusses the results obtained investigating the changes in ACh release during performance of operant tasks, exposition to novel stimuli, locomotor activity, and the performance of spatial memory tasks, working memory, and place preference memory tasks. Activation of the forebrain cholinergic system has been demonstrated in many tasks and conditions in which the environment requires the animal to analyze novel stimuli that may represent a threat or offer a reward. The sustained cholinergic activation, demonstrated by high levels of extracellular ACh observed during the behavioral paradigms, indicates that many behaviors occur within or require the facilitation provided by the cholinergic system to the operation of pertinent neuronal pathways.

Attentional functions of cortical cholinergic inputs: what does it mean for learning and memory?

The hypothesis that cortical cholinergic inputs mediate attentional functions and capacities has been extensively substantiated by experiments assessing the attentional effects of specific cholinotoxic lesions of cortical cholinergic inputs, attentional performance-associated cortical acetylcholine release, and the effects of pharmacological manipulations of the excitability of basal forebrain corticopetal cholinergic projections on attentional performance. At the same time, numerous animal experiments have suggested that the integrity of cortical cholinergic inputs is not necessary for learning and memory, and a dissociation between the role of the cortical cholinergic input system in attentional functions and in learning and memory has been proposed. We speculate that this dissociation is due, at least in part, to the use of standard animal behavioral tests for the assessment of learning and memory which do not sufficiently tax defined attentional functions. Attentional processes and the allocation of attentional capacities would be expected to influence the efficacy of the acquisition and recall of declarative information and therefore, persistent abnormalities in the regulation of the cortical cholinergic input system may yield escalating impairments in learning and memory. Furthermore, the cognitive effects of loss of cortical cholinergic inputs are augmented by the disruption of the top-down regulation of attentional functions that normally acts to optimize information processing in posterior cortical areas. Because cortical cholinergic inputs play an integral role in the mediation of attentional processing, the activity of cortical cholinergic inputs is hypothesized to also determine the efficacy of learning and memory.

Impaired and spared cholinergic functions in the hippocampus after lesions of the medial septum/vertical limb of the diagonal band with 192 IgG-saporin

To lesion the cholinergic input to the hippocampus, rats received injections of 192 IgG-saporin into the medial septum/vertical limb of the diagonal band (MS/VDB). The lesions produced near-total loss of choline acetyltransferase (ChAT)-positive neurons in the MS/VDB. The loss was accompanied, however, by only partial decreases (to 40% of control levels) in acetylcholine (ACh) release in the hippocampus. Moreover, ACh release in the hippocampus increased when lesioned and control rats were tested on a spontaneous alternation task, indicating that there was significant residual cholinergic function in the hippocampus. The lesions were sufficient to impair spontaneous alternation scores. However, this impairment could be reversed by either systemic or intra-hippocampal injections of the indirect cholinergic agonist, physostigmine, providing additional evidence of residual and effective cholinergic functions in the hippocampus of lesioned rats. Moreover, systemic injections of physostigmine at doses that produced mild tremors in control rats led to more severe tremors in the lesioned rats, suggesting upregulation of cholinergic mechanisms after saporin lesions, likely in brain areas other than the hippocampus. Thus, these findings provide evidence for decreases in cholinergic input to the hippocampus accompanied by deficits on a spontaneous alternation tasks. The findings also provide evidence for considerable residual cholinergic input to the hippocampus after saporin lesions of the MS/VDB. Together, the results suggest that 192 IgG-saporin lesions of the MS/VDB, using methods often employed, do not fully remove septohippocampal cholinergic input to the hippocampus but are nonetheless sufficient to produce impairments on a task impaired by hippocampal lesions.

Delay-dependent working memory impairment in young-adult and aged 5-HT1BKO mice as assessed in a radial-arm water maze

Serotonin (5-HT) plays a modulatory role in mnemonic functions, especially by interacting with the cholinergic system. The 5-HT1B receptor is a key target of this interaction. The 5-HT1B receptor knockout mice were found previously to exhibit a facilitation in hippocampal-dependent spatial reference memory learning. In the present study, we submitted mice to a delayed spatial working memory task, allowing the introduction of various delays between an exposure trial and a test trial. The 5-HT1BKO and wild-type mice learned the task in a radial-arm water maze (returning to the most recent presented arm containing the escape platform), and exhibited a high level of performance at delays of 0 and 5 min. However, at the delay of 60 min, only 5-HT1BKO mice exhibited an impairment. At a delay of 90 min, all mice were impaired. Treatment by scopolamine (0.8 mg/kg) induced the same pattern of performance in wild type as did the mutation for short (5 min, no impairment) and long (60 min, impairment) delays. The 22-month-old wild-type and knockout mice exhibited an impairment at short delays (5 and 15 min). The effect of the mutation affected both young-adult and aged mice at delays of 15, 30, and 60 min. Neurobiological data show that stimulation of the 5-HT1B receptor inhibits the release of acetylcholine in the hippocampus, but stimulates this in the frontal cortex. This dual function might, at least in part, explain the opposite effect of the mutation on reference memory (facilitation) and delay-dependent working memory (impairment). These results support the idea that cholinergic-serotonergic interactions play an important role in memory processes.

Effects of acetylcholine antagonist injection into the prefrontal cortex on the progress of lever-press extinction in rats

To determine the relationship between cholinergic modulation within the rat medial prefrontal cortex (mPFC) and the progress of lever-press extinction, we conducted an experiment in which muscarinic and nicotinic acetylcholine (ACh) receptor antagonists were microinjected into the mPFC. The muscarinic antagonist injected immediately before the initial extinction training did not affect the progress of extinction during the training (no within-session effect), but disrupted a second re-training session the next day (across-session effect). By contrast, the nicotinic antagonist disrupted the progress of extinction both within and across training sessions. These results confirm that ACh in the mPFC modulates lever-press extinction and suggest that nicotinic and muscarinic receptors are involved in short- and long-term memory processes.

Dietary cytidine (5')-diphosphocholine supplementation protects against development of memory deficits in aging rats

The present study was designed to assess the effect of supplementation with dietary cytidine (5')-diphosphocholine (CDP-choline), a source of cytidine and choline, on memory in young and older rats. Although the hippocampal-dependent memory deficits in aged rats are well documented, cognitive functioning in early aging has not been as thoroughly evaluated. Female Sprague-Dawley rats (3 or 15 months of age) consumed either a control diet or a diet supplemented with CDP-choline (approximately 500 mg/kg/day) for 8 weeks, after which they were trained to perform spatial and cued versions of the Morris water maze. Compared with young rats, aged rats exhibited a selective deficit in spatial memory tasks that required rats to retain information for 24 h or longer. CDP-choline supplementation protected against the development of this deficit, but had no memory-enhancing effect in normal young rats. These findings suggest that early-aged rats display a selective impairment in hippocampal-dependent long-term memory, and that dietary CDP-choline supplementation can protect against this deficit.