Role of prefrontal cortex and striatal output systems in short-term memory deficits associated with ageing, basal forebrain lesions, and cholinergic-rich grafts

The cholinergic hypothesis of geriatric memory dysfunction suggests (a) that basal forebrain lesions in animals should mimic cognitive and mnemonic impairments of human dementia and (b) that cholinergic grafts in the cortex and hippocampus may alleviate such impairments, whether induced by basal forebrain lesions or due to the intrinsic processes of ageing. Our own studies addressing these issues are reviewed. Although aged rats manifest impairments in short-term memory that are reversed by cholinergic grafts in the cortex and hippocampus, basal forebrain lesions have produced ambiguous results, which in part are attributable to nonspecific effects of the lesions. Acetylcholinesterase histochemistry and the topography of NBM-cortical connections indicate that basal forebrain lesions that include the NBM in general spare the cholinergic innervation of the prefrontal cortex, but can damage prefrontal cortical outflows via the globus pallidus. Two experiments are presented to indicate that the medial prefrontal cortex and its ventral striatal outputs provide a critical substrate for normal short-term memory performance in delayed matching and nonmatching tasks. These observations can resolve many of the discrepancies in previous lesion and graft studies.     

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

192IgG-saporin (SAP) was used to selectively destroy cholinergic neurons in the rostral basal forebrain (e.g., medial septum (MS) and vertical limb of the diagonal band of Broca (VDB)) and/or the caudal basal forebrain (e.g., nucleus basalis magnocellularis (NBM)) of ovariectomized Sprague-Dawley rats. The effects of these lesions on two different cognitive tasks, a delayed matching to position (DMP) T-maze task, and a configural association (CA) operant conditioning task, were evaluated and compared. Injecting SAP into either the MS or NBM significantly impaired acquisition of the DMP task. Analysis showed that the effects were due largely to an affect on response patterns adopted by the rats during training, as opposed to an effect on working memory performance. Notably, the impairment in DMP acquisition did not correlate with the degree of cholinergic denervation of the hippocampus. Despite the deficit, most animals eventually learned the task and reached criterion; however by the end of training, controls and animals that received SAP into either the MS or NBM appeared more likely to use an allocentric place strategy to solve the task, whereas animals that received SAP into both the MS and NBM were more likely to use an egocentric response strategy. Cholinergic lesions also produced a small but significant affect on acquisition of the CA task, but only with respect to response time, and only in the SAP-NBM-treated animals. SAP-NBM lesions also produced small but significant impairments in both the number of responses and response time during the acquisition of simple associations, possibly reflecting an effect on alertness or attention. Notably, the effects on CA acquisition were small, and like the effects on DMP acquisition did not correlate with the degree of cholinergic denervation of the hippocampus. We conclude that selective basal forebrain cholinergic lesions produce learning deficits that are task specific, and that cholinergic denervation of either the frontal cortex or hippocampus can affect response patterns and strategy in ways that affect learning, without necessarily reflecting deficits in working memory performance.     

脑内乙酰胆碱与认知活动的关系

脑内细胞外乙酰胆碱（ACh）的变化主要反映胆碱能神经元的活动,皮层和海马等脑区的ACh主要来源于基底前脑胆碱能神经元的纤维投射。应用微透析等技术在体检测清醒、自由活动动物认知过程中脑内乙酰胆碱的含量,可以研究ACh与特定行为反应和认知活动之间的关系。研究发现当机体需要对新刺激进行分析时,在学习与记忆、空间工作记忆、注意、自发运动和探究行为等认知活动中,基底前脑胆碱能神经元被激活,脑内ACh的释放也随之改变。结果提示脑内胆碱能递质系统活动与认知过程密切相关     

Opposite effects of a single versus repeated doses of gabapentin on retention performance of an inhibitory avoidance response in mice

CF-1 male mice were trained in an inhibitory avoidance (IA) task. A single gabapentin (GBP) administration (50mg/kg, ip) immediately after training enhanced retention performance when mice were tested 8 days after training. On the contrary, when the same dose of the anticonvulsant drug was given twice a day for 7 days (repeated treatment), a significant impairment on retention performance 12h after the last injection of GBP was observed. When the retention test was delayed 7 days after the end of the repeated treatment, the retention performance was not significant different from the control group, whereas if the retention test was delayed 14 days, retention performance was higher than control group but similar to that observed when GBP was administered once immediately after training. The impairment on retention performance was correlated with a significant decrease in the high affinity choline uptake in the hippocampus at the end of the retention test. The pretest administration of the direct muscarinic cholinergic agonist oxotremorine (50 microg/kg, ip) reversed the impairment on retention performance. This reversion was prevented by the muscarinic cholinergic antagonist scopolamine (0.5 mg/kg, ip). Taken together, these results suggest that the impairment on retention performance of an IA task in mice induced by repeated administration of GBP affected memory retrieval but not memory consolidation and that this impairment may be attributable to a reduction on central cholinergic activity.     

Extensive lesions of cholinergic basal forebrain neurons do not impair spatial working memory

A recent studysuggests that lesions to all major areas of the cholinergic basal forebrain in the rat (medial septum, horizontal limb of the diagonal band of Broca, and nucleus basalis magnocellularis) impair a spatial working memory task. However, this experiment used a surgical technique that mayhave damaged cerebellar Purkinje cells. The present studytested rats with highlyselective lesions of cholinergic neurons in all major areas of the basal forebrain on a spatial working memorytask in the radial arm maze. In postoperative testing, there were no significant differences between lesion and control groups in working memory, even with a delayperiod of 8 h, with the exception of a transient impairment during the first 2 d of postoperative testing at shorter delays (0 or 2 h). This finding corroborates other results that indicate that the cholinergic basal forebrain does not playa significant role in spatial working memory. Furthermore, it underscores the presence of intact memoryfunctions after cholinergic basal forebrain damage, despite attentional impairments that follow these lesions, demonstrated in other task paradigms.     

Deficit in selective and divided attention associated with cholinergic basal forebrain immunotoxic lesion produced by 192-saporin; motoric/sensory deficit associated with Purkinje cell immunotoxic lesion produced by OX7-saporin

The immunotoxin 192-saporin, infused intracerebroventricularly into rats, destroys cholinergic neurons in the basal forebrain nuclei. Doses required for complete cholinergic loss also kill some Purkinje cells. The immunotoxin OX7-saporin, when infused intraventricularly into rats, destroys Purkinje cells in a pattern similar to that produced by 192-saporin, without affecting cholinergic neurons. Thus, we used OX7-saporin to distinguish behavioral effects of 192-saporin due to cerebellar damage versus those due to cholinergic cell loss. Three doses of 192-saporin (1.6, 2.6, and 3.3 micrograms/rat) were chosen along with a dose of OX7-saporin (2.0 micrograms/rat) that produced Purkinje loss equivalent to the two highest doses of 192-saporin. Groups of Fischer-344 rats were trained in the multiple choice reaction time task and retested with more complex tasks after lesioning. They were also tested in the water maze, passive avoidance, acoustic startle, and open field. The OX7-saporin group exhibited changes in many tests suggesting hypermotility and sensory deficits. The 192-saporin groups differed from the OX7-saporin group when they displayed deficits in multiple choice reaction time tasks in which novel challenges were introduced, including sessions with a noise distractor, shortened and lengthened intertrial intervals, and use of nine instead of five sources of light stimulus. The 192-saporin groups showed no impairment in the other tasks. The cholinergic basal forebrain lesion may mask some of the effects of cerebellar damage up to a threshold after which effects of Purkinje cell loss predominate when 192-saporin is administered intraventricularly.     

The NK3 receptor agonist senktide ameliorates scopolamine-induced deficits in memory for object, place and temporal order

Senktide, a potent neurokinin-3 receptor (NK₃-R) agonist, increases acetylcholine (ACh) release in the striatum, the prefrontal cortex (Schäble et al., 2011), the amygdala and hippocampus, presumably via postsynaptic mechanisms. A promnestic action of NK₃-R agonists has been described in a variety of learning/memory tasks. The memory-enhancing effects of NK₃-R agonists and their activating influence on ACh suggest a possible role of the NK₃-R in learning and memory via cholinergic modulation. Deterioration of the cholinergic system in the basal forebrain has been associated with learning and memory deficits and cholinergic agents have promnestic effects in a variety of learning paradigms. The anticholinergic drug, scopolamine, a muscarinic ACh receptor antagonist, incurs deficits in a variety of learning tasks and provides a useful tool to investigate the role of the cholinergic systems in mechanisms underlying learning and memory. The aim of this study was to ascertain the effect of the NK₃-R agonist, senktide, in the scopolamine-induced deficit model. We hypothesized that senktide treatment would attenuate scopolamine-induced (subcutaneous – s.c. 0.75 mg/kg) memory impairment in three novelty preference paradigms based on spontaneous object exploration: namely object recognition, object–place recognition and object recognition for temporal order. Administration of senktide reversed the scopolamine-induced memory deficits by re-establishing object recognition (s.c. 0.2 mg/kg), object–place recognition (0.2 and 0.4 mg/kg), as well as object recognition for temporal order (0.4 mg/kg) in adult Wistar rats. These results indicate memory enhancing effects of senktide in animals subjected to scopolamine-induced memory impairments and indicate that the promnestic action of NK₃-R agonists is mediated by muscarinic cholinergic mechanisms.     

Cortical neuroanatomical correlates of behavioral deficits produced by lesion of the basal forebrain in rats

Morphological changes in the frontoparietal cortex were assessed in rats that exhibited deficits in a go/no go alternation task due to electrolytic lesion of the basal forebrain. Cortical area, laminar thickness, neuronal density, and soma area were examined in frontal, hindlimb, forelimb, and parietal areas of the cortex. Quantitative morphological analysis of the frontoparietal cortex in lesioned rats revealed a decrease in laminar thickness due to reduced soma size in particular cortical laminae. Neuronal density was not affected. These effects were present in all cortical areas examined and most pronounced in laminae II-III. Similar morphological changes were observed in the same cortical areas following lesions of the basal forebrain made with ibotenic acid, allowing a discrimination of lesion effects from those induced by damage to fibers of passage or differential behavioral testing. Lesions of the basal forebrain have previously been shown to produce both behavioral deficits and changes in cortical cholinergic activity. The cortical morphological changes observed in the present study following basal forebrain lesion provide further evidence for the importance of ascending cholinergic inputs to the cortex and their role in learning and memory.     

Environmental enrichment results in cortical and subcortical changes in levels of synaptophysin and PSD-95 proteins

Experience-dependent plasticity is thought to involve selective change in pre-existing brain circuits, involving synaptic plasticity. One model for looking at experience-dependent plasticity is environmental enrichment (EE), where animals are exposed to a complex novel environment. Previous studies using electron microscopy showed that EE resulted in synaptic plasticity in the visual cortex and hippocampus. However, the areas in the brain that have been examined following EE have been limited. The present study quantified potential synaptic plasticity throughout the brains of C57BL/6 mice using an enzyme-linked immunosorbent assay (ELISA) for two synaptic proteins, synaptophysin and PSD-95. EE resulted in increased synaptophysin and PSD-95 levels through major brain regions, including anterior and posterior areas of the forebrain, hippocampus, thalamus, and hypothalamus. However, no changes in synaptophysin were detected in the cerebellum. These results demonstrate that EE results in an increase in levels of both pre- and post-synaptic proteins in multiple regions of the brain, and it is possible that such changes represent the underlying synaptic plasticity occurring in EE.     

Differing time dependencies of object recognition memory impairments produced by nicotinic and muscarinic cholinergic antagonism in perirhinal cortex

The roles of muscarinic and nicotinic cholinergic receptors in perirhinal cortex in object recognition memory were compared. Rats' discrimination of a novel object preference test (NOP) test was measured after either systemic or local infusion into the perirhinal cortex of the nicotinic receptor antagonist methyllycaconitine (MLA), which targets alpha-7 (α7) amongst other nicotinic receptors or the muscarinic receptor antagonists scopolamine, AFDX-384, and pirenzepine. Methyllycaconitine administered systemically or intraperirhinally before acquisition impaired recognition memory tested after a 24-h, but not a 20-min delay. In contrast, all three muscarinic antagonists produced a similar, unusual pattern of impairment with amnesia after a 20-min delay, but remembrance after a 24-h delay. Thus, the amnesic effects of nicotinic and muscarinic antagonism were doubly dissociated across the 20-min and 24-h delays. The same pattern of shorter-term but not longer-term memory impairment was found for scopolamine whether the object preference test was carried out in a square arena or a Y-maze and whether rats of the Dark Agouti or Lister-hooded strains were used. Coinfusion of MLA and either scopolamine or AFDX-384 produced an impairment profile matching that for MLA. Hence, the antagonists did not act additively when coadministered. These findings establish an important role in recognition memory for both nicotinic and muscarinic cholinergic receptors in perirhinal cortex, and provide a challenge to simple ideas about the role of cholinergic processes in recognition memory: The effects of muscarinic and nicotinic antagonism are neither independent nor additive.     

The Effects of Selective Cholinergic Basal Forebrain Lesions and Aging upon Expectancy in the Rat

The effects of selective cholinergic cell loss within the basal forebrain (BF) were determined using a task that requires shifting of attention between two visual stimuli. Discriminability between two stimuli and response bias were determined in young and old F-344 rats given BF injections of IgG-192 saporin (100 ng). The lesion reduced ChAT activity in the frontal and parietal cortices, hippocampus, and olfactory bulbs. The lesion did not significantly alter Na⁺/K⁺-ATPase activity in cortex, hippocampus, or olfactory bulbs, or endogenous levels of neuropeptide Y and neurokinin B within the BF. The BF lesions impaired both stimulus discriminability and response bias in young and old rats. The BF lesions had a significantly greater effect upon stimulus discriminability and response bias in aged rats, compared to young rats, only when the stimulus duration was very brief, i.e., when the task was most difficult to solve. At longer stimulus durations, aging and lesions showed no interaction. The results suggest that the selective loss of cholinergic cells in the BF, but not normal aging, impairs the ability to discriminate between independent sensory stimuli. The loss of these cells confers a response bias in simple operant tasks involving motor responses to reward-related visual stimuli.     

Dissociating Basal Forebrain and Medial Temporal Amnesic Syndromes: Insights from Classical Conditioning

In humans, anterograde amnesia can result from damage to the medial temporal (MT) lobes (including hippocampus), as well as to other brain areas such as basal forebrain. Results from animal classical conditioning studies suggest that there may be qualitative differences in the memory impairment following MT vs. basal forebrain damage. Specifically, delay eyeblink conditioning is spared after MT damage in animals and humans, but impaired in animals with basal forebrain damage. Recently, we have likewise shown delay eyeblink conditioning impairment in humans with amnesia following anterior communicating artery (ACoA) aneurysm rupture, which damages the basal forebrain. Another associative learning task, a computer-based concurrent visual discrimination, also appears to be spared in MT amnesia while ACoA amnesics are slower to learn the discriminations. Conversely, animal and computational models suggest that, even though MT amnesics may learn quickly, they may learn qualitatively differently from controls, and these differences may result in impaired transfer when familiar information is presented in novel combinations. Our initial data suggests such a two-phase learning and transfer task may provide a double dissociation between MT amnesics (spared initial learning but impaired transfer) and ACoA amnesics (slow initial learning but spared transfer). Together, these emerging data suggest that there are subtle but dissociable differences in the amnesic syndrome following damage to the MT lobes vs. basal forebrain, and that these differences may be most visible in non-declarative tasks such as eyeblink classical conditioning and simple associative learning.     

Dissociating basal forebrain and medial temporal amnesic syndromes: Insights from classical conditioning

In humans, anterograde amnesia can result from damage to the medical temporal (MT) lobes (including hippocampus), as well as to other brain areas such as basal forebrain. Results from animal classical conditioning studies suggest that there may be qualitative differences in the memory impairment following MT vs. basal forebrain damage. Specifically, delay eyeblink conditioning is spared after MT damage in animals and humans, but impaired in animals with basal forebrain damage. Recently, we have likewise shown delay eyeblink conditioning impairment in humans with amnesia following anterior communicating artery (ACoA) aneurysm rupture, which damages the basal forebrain. Another associative learning task, a computer-based concurrent visual discrimination, also appears to be spared in MT amnesia while ACoA amnesics are slower to learn the discriminations. Conversely, animal and computational models suggest that, even though MT amnesics may learn quickly, they may learn qualitatively differently from controls, and these differences may result in impaired transfer when familiar information is presented in novel combinations. Our initial data suggests such a two-phase learning and transfer task may provide a double dissociation between MT amnesics (spared initial learning but impaired transfer) and ACoA amnesics (slow initial learning but spared transfer). Together, these merging data suggest that there are subtle but dissociable differences in the amnesic syndrome following damage to the MT lobes vs. basal forebrain, and that these differences may be most visible in non-declarative tasks such as eyeblink classical conditioning and simple associative learning.     

Behavioural state differentially engages septohippocampal cholinergic and GABAergic neurons in R6/1 Huntington's disease mice

Huntington's disease (HD) is a neurodegenerative condition characterised by progressive motor, psychological and cognitive decline. R6/1 HD transgenic mice model the clinical hippocampal-dependent cognitive deficits observed in patients. Cholinergic and GABAergic septohippocampal projections play important roles in hippocampal-dependent cognition. The current study examined neuronal activity of cholinergic and GABAergic septohippocampal projections in response to arousal elicited during differing behavioural states. The different behavioural states examined were; home cage (controls), acute exploration of a novel enriched environment and either spontaneous wakefulness (dark phase) or spontaneous sleep (light phase). We employed triple-label immunohistochemistry using c-Fos as an indirect marker of neuron activation and parvalbumin and choline acetyltransferase (ChAT) to label GABAergic and cholinergic neurons in the basal forebrain, respectively. The Y-maze was used to assess short-term hippocampal-dependent memory independently during either the dark or light phase and revealed a memory deficit in R6/1 HD mice compared to wild types that was particularly prominent during the dark phase. Three-way ANOVA of basal forebrain cholinergic and GABAergic activity through co-expression of c-Fos revealed overt responses to differing behavioural states. Both genotypes increased cholinergic neuron activity in response to exploring a novel enriched environment and also an increase during the dark phase compared to the light phase. Novel enriched environment exploration caused a larger response of GABAergic neuron activity in R6/1 HD mice, which also failed to increase the activity of GABAergic neurons during the dark phase compared to the light phase as observed for wildtype mice. Basal levels of c-Fos-positive cells were greatly increased in the hippocampal granule cell layer of R6/1 HD mice during both circadian phases. The differential activation of septohippocampal cholinergic and GABAergic neurons in R6/1 HD mice in response to differing behavioural states may be associated with impaired hippocampal-dependent short-term memory.     

Cholinergic modulation of Pavlovian fear conditioning in rats: differential effects of intrahippocampal infusion of mecamylamine and methyllycaconitine

The cholinergic system has consistently been implicated in Pavlovian fear conditioning. Considerable work has been done to localize specific nicotinic receptor subtypes in the hippocampus and determine their functional importance; however, the specific function of many of these subtypes has yet to be determined. An alpha7 nicotinic antagonist methyllycaconitine (MLA) (35 microg), and a broad spectrum non-alpha7 nicotinic antagonist mecamylamine (35 microg) was injected directly into the dorsal hippocampus or overlying cortex either 15 min pre-, 1 min post-, or 6h post-fear conditioning. One week after conditioning, retention of contextual and cue (tone) conditioning were assessed. A significant impairment in retention of contextual fear was observed when mecamylamine was injected 15 min pre- and 1 min post-conditioning. No significant impairment was observed when mecamylamine was injected 6h post-conditioning. Likewise, a significant impairment in retention of contextual fear was observed when MLA was injected 1 min post-conditioning; however, in contrast, MLA did not show any significant impairments when injected 15 min pre-conditioning, but did show a significant impairment when injected 6h post-conditioning. There were no significant impairments observed when either drug was injected into overlying cortex. No significant impairments were observed in cue conditioning for either drug. In general, specific temporal dynamics involved in nicotinic receptor function were found relative to time of receptor dysfunction. The results indicate that the greatest deficits in long-term retention (1 week) of contextual fear are produced by central infusion of MLA minutes to hours post-conditioning or mecamylamine within minutes of conditioning.     

Tuning the brain by learning and by stimulation of the nucleus basalis

Although it is well-established that the cerebral cortex is a substrate for learning, memory and higher cognitive functions, rather less is known about the mechanisms by which experiences are acquired and stored in the cortex. The role of the basal forebrain cholinergic system (BFCS) in learning-induced plasticity is underlined by a recent report by Kilgard and Merzenich[1]. In this article I will discuss the findings of Kilgard and Merzenich in the context of other developments in our understanding of the BFCS and its role in learning-induced plasticity. However, before the discussion I would like to provide some essential background information.     

Effects of intrahippocampal injections of the cholinergic neurotoxin AF64A on open-field activity and avoidance learning in the rat

The effects of direct intrahippocampal administration of the cholinergic neurotoxin, AF64A, were investigated in male rats. Bilateral injections of AF64A (5 nmole/2 microliters) produced a significant decrease in choline acetyltransferase (CAT) activity in the dorsal hippocampus (25%) and overlying frontoparietal cortex (30%) but no changes in the striatum. Rats lesioned with AF64A exhibited increased levels of open-field activity, which was most marked at 1 week after the lesion; however, the rates of intrasession habituation were similar in lesioned and control rats. Lesioned rats also displayed deficits in acquisition and retention of a passive avoidance task and less dramatic deficits in acquisition of two-way shuttle box avoidance. These findings indicate that lesioning of cholinergic terminals in the hippocampus and/or cerebral cortex with AF64A leads to long-term deficits in learning and memory as well as increases in open-field activity.     

Spatial learning in the 5-HT1B receptor knockout mouse: selective facilitation/impairment depending on the cognitive demand

Age-related memory decline is associated with a combined dysfunction of the cholinergic and serotonergic systems in the hippocampus and frontal cortex, in particular. The 5-HT1B receptor occupies strategic cellular and subcellular locations in these structures, where it plays a role in the modulation of ACh release. In an attempt to characterize the contribution of this receptor to memory functions, 5-HT1B receptor knockout (KO) mice were submitted to various behavioral paradigms carried out in the same experimental context (water maze), which were aimed at exposing mice to various levels of memory demand. 5-HT1BKO mice exhibited a facilitation in the acquisition of a hippocampal-dependent spatial reference memory task in the Morris water maze. This facilitation was selective of task difficulty, showing thus that the genetic inactivation of the 5-HT1B receptor is associated with facilitation when the complexity of the task is increased, and reveals a protective effect on age-related hippocampal-dependent memory decline. Young-adult and aged KO and wild-type (WT) mice were equally able to learn a delayed spatial matching-to-sample working memory task in a radial-arm water maze with short (0 or 5 min) delays. However, 5-HT1BKO mice, only, exhibited a selective memory impairment at intermediate and long (15, 30, and 60 min) delays. Treatment by scopolamine 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. Taken together, these studies revealed a beneficial effect of the mutation on the acquisition of a spatial reference memory task, but a deleterious effect on a working memory task for long delays. This 5-HT1BKO mouse story highlights the problem of the potential existence of "global memory enhancers."     

Translational Rodent Models of Korsakoff Syndrome Reveal the Critical Neuroanatomical Substrates of Memory Dysfunction and Recovery

Investigation of the amnesic disorder Korsakoff Syndrome (KS) has been vital in elucidating the critical brain regions involved in learning and memory. Although the thalamus and mammillary bodies are the primary sites of neuropathology in KS, functional deactivation of the hippocampus and certain cortical regions also contributes to the chronic cognitive dysfunction reported in KS. The rodent pyrithiamine-induced thiamine deficiency (PTD) model has been used to study the extent of hippocampal and cortical neuroadaptations in KS. In the PTD model, the hippocampus, frontal and retrosplenial cortical regions display loss of cholinergic innervation, decreases in behaviorally stimulated acetylcholine release and reductions in neurotrophins. While PTD treatment results in significant impairment in measures of spatial learning and memory, other cognitive processes are left intact and may be recruited to improve cognitive outcome. In addition, behavioral recovery can be stimulated in the PTD model by increasing acetylcholine levels in the medial septum, hippocampus and frontal cortex, but not in the retrosplenial cortex. These data indicate that although the hippocampus and frontal cortex are involved in the pathogenesis of KS, these regions retain neuroplasticity and may be critical targets for improving cognitive outcome in KS.