Effects of cholinergic-rich neural grafts on radial maze performance of rats after excitotoxic lesions of the forebrain cholinergic projection system--II. Cholinergic drugs as probes to investigate lesion-induced deficits and transplant-induced functional recovery

Central muscarinic cholinergic involvement in serial pattern learning: Atropine impairs acquisition and retention in a serial multiple choice (SMC) task in rats

Atropine sulfate is a muscarinic cholinergic antagonist which impairs acquisition and retention performance on a variety of cognitive tasks. The present study examined the effects of atropine on acquisition and retention of a highly-structured serial pattern in a serial multiple choice (SMC) task. Rats were given daily intraperitoneal injections of either saline or atropine sulfate (50mg/kg) and trained in an octagonal operant chamber equipped with a lever on each wall. They learned to press the levers in a particular order (the serial pattern) for brain-stimulation reward in a discrete-trial procedure with correction. The two groups learned a pattern composed of eight 3-element chunks ending with a violation element: 123-234-345-456-567-678-781-818 where the digits represent the clock-wise positions of levers in the chamber, dashes indicate 3-s pauses, and other intertrial intervals were 1s. Central muscarinic cholinergic blockade by atropine caused profound impairments during acquisition, specifically in the encoding of chunk-boundary elements (the first element of chunks) and the violation element of the pattern, but had a significant but negligible effect on the encoding of within-chunk elements relative to saline-injected rats. These effects persisted when atropine was removed, and similar impairments were also observed in retention performance. The results indicate that intact central muscarinic cholinergic systems are necessary for learning and producing appropriate responses at places in sequences where pattern structure changes. The results also provide further evidence that multiple cognitive systems are recruited to learn and perform within-chunk, chunk-boundary, and violation elements of a serial pattern.

Nicotine modifies in vivo and in vitro rat hippocampal amyloid precursor protein processing in young but not old rats

Previous studies have shown that administration of nicotine modifies the expression and secretion of amyloid precursor protein (APP) in various cell lines. The present study investigated the extent to which chronic subcutaneous nicotine administration influences APP levels and processing in cerebral cortex, striatum and hippocampus of young and old rat brains. The results showed that constant nicotine infusion (0.25 or 4.00mg/kg/day) increased the levels of particulate APP (APPp) but not secreted APP (APPs) in the hippocampus of young rats in vivo. This response to nicotine was not observed in the striatum or cerebral cortex of young rats or in any of the brain regions examined in old animals. Subsequent in vitro analysis demonstrated that nicotine enhanced the release of APPs from hippocampal slice preparations and that this increase was attenuated by mecamylamine, a non-selective nicotinic acetylcholine receptor (nAChR) antagonist. The in vitro effect of nicotine on APPs was age-related, being only detected from hippocampal slices derived from the young but not the older animals. These results suggest that nicotine modulates APP expression and secretion in the hippocampus and that the responses observed to the drug are age-dependent being only detected in younger rats.

Transplantation of primed or unprimed mouse embryonic stem cell-derived neural precursor cells improves cognitive function in Alzheimerian rats

Alzheimer's disease (AD) is a neurodegenerative disorder that is characterized by progressive and irreversible decline of memory. Neuropathological features include the progressive degeneration of cholinergic neurons in the forebrain cholinergic projection system especially nucleus basalis of Meynert (nbM). New cell therapeutic approaches for the replacement of degenerated cells are being researched. The aim of this study was to investigate the production of cholinergic neurons from mouse embryonic stem cells (ESCs) and potential for utilizing ESC-derived neuronal precursor cells (NPCs) and primed NPCs (PNPCs) for cell restorative therapy in a rodent model of AD. NPCs were produced by growth factor-mediated selection under serum-free conditions and differentiated better into cholinergic neurons when NPCs primed with Shh (approximately 22%) in comparison with different cholinergic promoting factors. Behavioral assessment of unilateral nbM ibotenic acid-lesioned rats by Morris water maze and spatial probe test revealed a significant behavioral improvement in memory deficits following transplantation with NPCs and/or PNPCs. Immunohistochemical analysis revealed that the majority (approximately 70%) of the NPCs and/or PNPCs retained neuronal phenotype and approximately 40% of them had a cholinergic cell phenotype following transplantation with no tumor formation, indicating that these may be safe for transplantation. This experimental study has important implications as it suggests that the transplantation of mouse ESC-derived NPCs and/or following commitment to a cholinergic cell phenotype can promote behavioral recovery in a rodent model of AD.

Atropine-induced, state-dependent learning for spatial information, but not for visual cues

This study investigates state-dependent learning employing atropine. The reaction of rats to (1) the presentation of novel stimuli, (2) habituation to intermittent presentations of the same stimulus at the same local, (3) spatial change at the site of stimulus presentation, and (4) a visual stimulus change, was investigated in the straight alleyway test, controlling for the possible development of behavioral and/or pharmacological tolerance. Our findings reveal that rats habituated to stimulus presentation at a specific location, when under an atropine effect, do react to stimulus presentation at another location, or to a different stimulus, when under an atropine effect, indicating that this drug does not interfere with the acquisition of spatial or visual information. Differently, however, rats habituated to stimulus presentation at a specific location in the absence of an atropine effect are unable to react to spatial change when under the atropine effect, but do react to a visual stimulus change. This suggests that atropine interferes either with the retrieval of previously acquired spatial information or with the comparison of previously acquired spatial information with current information, but does not interfere with visual recognition. These findings reveal that atropine interferes with the use of spatial information acquired in the absence of a drug effect.

Autotransplantation of bone marrow-derived stem cells as a therapy for neurodegenerative diseases

Neurodegenerative diseases are characterized by a progressive degeneration of selective neural populations. This selective hallmark pathology and the lack of effective treatment modalities make these diseases appropriate candidates for cell therapy. Bone marrow-derived mesenchymal stem cells (MSCs) are self-renewing precursors that reside in the bone marrow and may further be exploited for autologous transplantation. Autologous transplantation of MSCs entirely circumvents the problem of immune rejection, does not cause the formation of teratomas, and raises very few ethical or political concerns. More than a few studies showed that transplantation of MSCs resulted in clinical improvement. However, the exact mechanisms responsible for the beneficial outcome have yet to be defined. Possible rationalizations include cell replacement, trophic factors delivery, and immunomodulation. Cell replacement theory is based on the idea that replacement of degenerated neural cells with alternative functioning cells induces long-lasting clinical improvement. It is reasoned that the transplanted cells survive, integrate into the endogenous neural network, and lead to functional improvement. Trophic factor delivery presents a more practical short-term approach. According to this approach, MSC effectiveness may be credited to the production of neurotrophic factors that support neuronal cell survival, induce endogenous cell proliferation, and promote nerve fiber regeneration at sites of injury. The third potential mechanism of action is supported by the recent reports claiming that neuroinflammatory mechanisms play an important role in the pathogenesis of neurodegenerative disorders. Thus, inhibiting chronic inflammatory stress might explain the beneficial effects induced by MSC transplantation. Here, we assemble evidence that supports each theory and review the latest studies that have placed MSC transplantation into the spotlight of biomedical research.

Effects of Yukmijihwang-tang derivatives (YMJd) on ibotenic acid-induced amnesia in the rat

The present study investigates the effects of Yukmijihwang-tang Derivatives (YMJd) on learning and memory through the Morris water maze task and the central cholinergic system of rats with excitotoxic medial septum (MS) lesion. In the water maze test, the animals were trained to find a platform in a fixed position for 6 d and then received a 60-s probe trial in which the platform was removed from the pool on the 7th day. Ibotenic lesion of the MS showed the impaired performance in the Morris water maze test and severe cell losses in the MS, as indicated by decreased choline acetyltransferase-immunoreactivity in the medial septum. Daily administrations of YMJd (100 mg/kg, i.p.) for 21 consecutive days produced significant reversals of ibotenic acid-induced deficit in learning and memory. These treatments also reduced the loss of choline acetyltransferase (ChAT) immunoreactivity in the MS induced by ibotenic acid. These results suggest that impairments of spatial learning and memory might be attributable to the degeneration of septohippocampal cholinergic (SHC) neurons and that YMJd treatment ameliorated learning and memory deficits partly due through neuroprotective effects on the central acetylcholine system. Our studies suggest that YMJd might be useful in the treatment of Alzheimer's disease.

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.

Neurobehavioural defects in adult mice neonatally exposed to nicotine: changes in nicotine-induced behaviour and maze learning performance

Neonatal exposure to low doses of nicotine has been shown to disturb the development of low-affinity nicotinic binding sites in the cerebral cortex and to elicit a deviant behavioural response to nicotine in adult mice. In this study, 10-day-old male NMRI mice were exposed to one of three different doses of nicotine (3.3, 33, or 66 microg nicotine-base/kg body wt.) s.c. twice daily on 5 consecutive days to study dose-response effects of nicotine on adult spontaneous and nicotine-induced motor behaviour. The nicotine-induced behaviour test revealed a hypoactive response to nicotine in 4-month-old mice neonatally exposed to 33 or 66 microg nicotine-base, whereas the response to nicotine in control animals and mice exposed to 3.3 microg nicotine-base was an increased activity. Learning and memory functions were also investigated in adult animals neonatally exposed to 66 microg nicotine-base/kg body wt. in the same manner, in the Morris water maze and in the Radial arm maze. In the swim maze and the Radial arm maze tests, no significant differences were observed between nicotine-treated and control animals at the age of 4 months. At 7 months, however, a significant difference in performance was evident, indicating a time-response/time-dependent effect. Furthermore, it was shown that in mice exposed neonatally to a nicotine dose known to inhibit the development of the nicotinic low affinity-binding site (LA), the response to nicotine could not cause any increase in spontaneous motor activity as seen in controls.

Exposure to ethanol and nicotine during the brain growth spurt: spatial DMP performance in male rats

Male Long-Evans rats were reared artificially and, using a 2x2 design, were exposed from postnatal days (PD) 6-9 to ethanol (ET: 6.5 g kg(-1) day(-1) "binge" exposure) and/or nicotine bitartrate (NIC: 6 mg kg(-1) day(-1) continuous exposure) via gastrostomy tubes. Controls were administered maltose-dextrin in amounts isocaloric to ET and/or sodium bitartrate. A fifth suckled-control group was reared normally. NIC accelerated eye opening on PD 14; whereas ET delayed eye opening and hindlimb support on PD 16. Beginning in postnatal week 7, rats were tested on a spatial delayed matching-to-place (DMP) version of the Morris water maze, which entailed a series of problems, each consisting of search and recall trials, that required the rats to use extra-maze cues to locate a hidden escape platform. In Phase 1 of testing, the ET-exposed groups were impaired in the recall trials, but there was no effect of NIC. A longer encoding time (45 vs. 10 s) improved performance across all groups. In contrast, acute administration of NIC (0.1 mg/kg ip) immediately prior to testing in Phase 2 failed to improve performance in any group. In conclusion, these results confirm previous findings of impaired spatial DMP-task performance in ET-exposed rats and further suggest that these memory deficits are amenable to amelioration.

Nicotine and its interaction with beta-amyloid protein: a short review

Two features of Alzheimer's disease (AD) are beta-amyloid protein (betaAP) deposition and a severe cholinergic deficit. beta-Amyloid protein is a 39- to 43-amino acid transmembrane fragment of a larger precursor molecule, amyloid precursor protein. It is a major constituent of senile plaque, a neuropathologic hallmark of AD, and has been shown to be neurotoxic in vivo and in vitro. The cholinergic neurotransmission system is seen as the primary target of AD. However, other systems are also found to show functional deficit. An association between cholinergic deficit and betaAP is suggested by a negative correlation between cigarette smoking and AD. Evidence hitherto suggests that betaAP causes neuronal death possibly via apoptosis by disrupting calcium homeostasis, which may involve direct activation or enhancement of ligand-gated or voltage-dependent calcium channels. Selective second messengers such as protein kinases are triggered that signal neuronal death. Nicotine or acetylcholinesterase inhibitors can partially prevent the neurotoxicity of betaAP in vivo and in vitro. However, the exact mechanism by which nicotine provides its protective effects is not fully understood, but clearly there are protective roles for nicotine. Here, some aspects of betaAP neurotoxicity and nicotinic intervention as a protective agent are discussed.

The developing cholinergic system as target for environmental toxicants, nicotine and polychlorinated biphenyls (PCBs): implications for neurotoxicological processes in mice

During neonatal life, offspring can be affected by toxic agents either by transfer via mother's milk or by direct exposure. In many mammalian species the perinatal period is characterized by a rapid development of the brain - "the brain growth spurt" (BGS). This period in the development of the mammalian brain is associated with numerous biochemical changes that transform the feto-neonatal brain into that of the mature adult. In rodents, the cholinergic transmitter system undergoes a rapid development during the neonatal period, a time when spontaneous motor behaviour also reaches peak activity. We have observed that low-dose exposure to environmental toxicants such as nicotine, polychlorinated biphenyls (PCBs) and polybrominated diphenylethers (PBDE, flame retardants) during the "BGS" can lead to irreversible changes in adult brain function in the mouse. The induction of persistent effects on behaviour and cholinergic nicotinic receptors in the adult animal appears to be limited to a short period during neonatal development. Furthermore, the neurotoxic effects were shown to develop over time, indicating a time-response/time-dependent effect. This indicates that environmental toxicants, such as nicotine, PCBs and probably PBDEs, might be involved in the slow, implacable induction of neurodegenerative disorders and/or interfere with normal aging processes.

Behavioural, histological and immunocytochemical consequences following 192 IgG-saporin immunolesions of the basal forebrain cholinergic system

Use of the selective immunotoxin; 192 IgG-saporin, is helping to elucidate the role of the cholinergic system in cognition by overcoming the problems of interpretation associated with the use of non-specific lesioning agents. In separate studies, we have compared the long- and short-term effects of single site and combined saporin lesions of the nucleus basalis magnocellularis and medial septal area, on spatial learning and memory in radial arm and water maze tasks. At 11 months, only rats with combined lesions showed deficits in both radial and water maze tasks, although terminal cholinergic deafferentation was substantial and extensive tissue loss was seen at the injection sites in both single and combined lesions. However, the extensive tissue loss with long-term lesions suggested that behavioural deficits were not solely attributable to cholinergic deafferentation. In contrast, when rats with combined lesions were tested 5 months after lesioning, no deficits were apparent, although there was almost complete loss of choline acetyltransferase- and nerve growth factor receptor-immunoreactivity in the basal forebrain with no tissue damage at the injection sites. This study supports existing literature that selective loss of cholinergic neurons in the basal forebrain does not produce behavioural impairments in standard tasks of learning and memory, but deficits are apparent when damage is non-selective as occurs late after lesioning, confounding interpretation of behavioural data. It further highlights potential problems with this immunotoxin in long-term studies.

Non-spatial water radial-arm maze learning in mice

Recently, we published a method for examining working and reference memory in mice using a spatial version of the water radial-arm maze. Here we describe a non-spatial version of the same maze. BXSB mice were able to learn the maze as shown by the decrease in the number of working and reference memory errors over sessions. This maze was used to examine learning differences between males and females and between mice with misplaced clusters of neurons in layer I of cortex (ectopias) and those without. In a prior study using the spatial version of the water radial-arm maze, male BXSB mice had poorer working memory than females during the acquisition phase. Similarly, in this study male BXSB mice demonstrated impaired working memory during the asymptotic phase of non-spatial radial-arm maze learning. Two prior studies showed that mice with neocortical ectopias demonstrated working memory impairments compared to non-ectopic littermates in the spatial version of the water radial-arm maze. Contrary to this, in the non-spatial radial-arm maze used here, ectopic mice were not impaired in working memory and showed better memory when the working memory 'load' was the highest. Overall, both versions of the maze can be useful tools to assess spatial and non-spatial working and reference memory in mice.

Immunohistochemical and neurochemical correlates of learning deficits in aged rats

This study examined whether cholinergic and monoaminergic dysfunctions in the brain could be related to spatial learning capabilities in 26-month-old, as compared to three-month-old, Long-Evans female rats. Performances were evaluated in the water maze task and used to constitute subgroups with a cluster analysis statistical procedure. In the first experiment (histological approach), the first cluster contained young rats and aged unimpaired rats, the second one aged rats with moderate impairment and the third one aged rats with severe impairment. Aged rats showed a reduced number of choline acetyltransferase- and p75(NTR)-positive neurons in the nucleus basalis magnocellularis, and choline acetyltransferase-positive neurons in the striatum. In the second experiment (neurochemical approach), the three clusters comprised young rats, aged rats with moderate impairment and aged rats with severe impairment. Alterations related to aging consisted of reduced concentration of acetylcholine, norepinephrine and serotonin in the striatum, serotonin in the occipital cortex, dopamine and norepinephrine in the dorsal hippocampus, and norepinephrine in the ventral hippocampus. In the first experiment, there were significant correlations between water maze performance and the number of; (i) choline acetyltransferase- and p75(NTR)-positive neurons in the nucleus basalis magnocellularis; (ii) choline acetyltransferase-positive neurons in the striatum and; (iii) p75(NTR)-positive neurons in the medial septum. In the second experiment, water maze performance was correlated with the concentration of; (i) acetylcholine and serotonin in the striatum; (ii) serotonin and norepinephrine in the dorsal hippocampus; (iii) norepinephrine in the frontoparietal cortex and; (iv) with other functional markers such as the 5-hydroxyindoleacetic acid/serotonin ratio in the striatum, 3,4-dihydroxyphenylacetic acid/dopamine ratio in the dorsal hippocampus, 5-hydroxyindoleacetic acid/serotonin and homovanillic acid/dopamine ratios in the frontoparietal cortex, and 3,4-dihydroxyphenylacetic acid/dopamine ratio in the occipital cortex. The results indicate that cognitive deficits related to aging might involve concomitant alterations of various neurochemical systems in several brain regions such as the striatum, the hippocampus or the cortex. It also seems that these alterations occur in a complex way which, in addition to the loss of cholinergic neurons in the basal forebrain, affects dopaminergic, noradrenergic and serotonergic processes.

Conditionally immortalized, multipotential and multifunctional neural stem cell lines as an approach to clinical transplantation

Experiments are described using rats with two kinds of brain damage and consequent cognitive deficit (in the Morris water maze, three-door runway, and radial maze): 1) ischemic damage to the CA1 hippocampal cell field after four-vessel occlusion (4VO), and 2) damage to the forebrain cholinergic projection system by local injection of excitotoxins to the nuclei of origin or prolonged ethanol administration. Cell suspension grafts derived from primary fetal brain tissue display a stringent requirement for homotypical cell replacement in the 4VO model: cells from the embryonic day (E)18-19 CA1 hippocampal subfield, but not from CA3 or dentate gyrus or from E16 basal forebrain (cholinergic rich) led to recovery of cognitive function. After damage to the cholinergic system, conversely, recovery of function was seen with cell suspension grafts from E16 basal forebrain or cholinergic-rich E14 ventral mesencephalon, but not with implants of hippocampal tissue. These two models therefore provided a test of multifunctionality for a clonal line of conditionally immortalized neural stem cells, MHP36, derived from the E14 "immortomouse" hippocampal anlage. Implanted above the damaged CA1 cell field in 4VO-treated adult rats, these cells (multipotential in vitro) migrated to the damaged area, reconstituted the gross morphology of the CA1 pyramidal layer, took up both neuronal and glial phenotypes, and gave rise to cognitive recovery. Similar recovery of function and restoration of species-typical morphology was observed when MHP36 cells were implanted into marmosets with excitotoxic CAI damage. MHP36 implants led to recovery of cognitive function also in two experiments with rats with excitotoxic damage to the cholinergic system damage, either unilaterally in the nucleus basalis or bilaterally in both the nucleus basalis and the medial septal area. Thus, MHP36 cells are both multipotent (able to take up multiple cellular phenotypes) and multifunctional (able to repair diverse types of brain damage).

Intrahippocampal cholinergic-rich transplants restore lead-induced deficits: a preliminary study in rats

In the present study restorative potential of fetal cholinergic rich cell suspensions in ameliorating cognitive deficits in rats perinatally exposed to lead was studied. Lactating dams with 1-day old litters were given 0.2% (w/v) lead acetate in drinking water throughout lactation from postnatal day (PND) 1 to PND21 at the end of which the treatment was stopped and the animals were weaned. On PND42 lead exposed rats were given bilateral, intrahippocampal, cholinergic rich fetal neural transplants (approximately 60,000 cells per site) and subsequently assessed 3 and 6 months posttransplantation. Control animals (Sham operated and transplanted) were also run in parallel. Lead exposed rats exhibited a decreased learning ability and locomotor activity. A significant decrease in the levels of acetylcholinesterase and sodium potassium ATPase Na+,K+-ATPase activity was observed in hippocampal region of lead exposed rats. The levels of lead were increased by fivefold in the hippocampal region of lead exposed rats. Transplantation showed marginal improvement in the above impairments at 3 months which were more marked at 6 months. Lead levels at 6 months were not significantly higher in lead exposed rats as compared with the control. Results confirm previous findings that fetal neural transplants help in restoring the lost functional deficits and demonstrate their restorative potential in case of lead induced deficits.

Threshold relationship between lesion extent of the cholinergic basal forebrain in the rat and working memory impairment in the radial maze

The cholinergic basal forebrain (CBF) degenerates in Alzheimer's Disease (AD), and the degree of this degeneration correlates with the degree of dementia. In the present study we have modeled this degeneration in the rat by injecting various doses of the highly selective immunotoxin 192 IgG-saporin (192-sap) into the ventricular system. The ability of 192-sap-treated rats to perform in a previously learned radial maze working memory task was then tested. We report here that 192-sap created lesions of the CBF and, to a lesser extent, cerebellar Purkinje cells in a dose-dependent fashion. Furthermore, we found that rats harboring lesions of the entire CBF greater than 75% had impaired spatial working memory in the radial maze. Correlational analysis of working memory impairment and lesion extent of the component parts of the CBF revealed that high-grade lesions of the hippocampal-projecting neurons of the CBF were not sufficient to impair working memory. Only rats with high-grade lesions of the hippocampal and cortical projecting neurons of the CBF had impaired working memory. These data are consistent with other 192-sap reports that found behavioral deficits only with high-grade CBF lesions and indicate that the relationship between CBF lesion extent and working memory impairment is a threshold relationship in which a high degree of neuronal loss can be tolerated without detectable consequences. Additionally, the data suggest that the CBF modulates spatial working memory via its connections to both the hippocampus and cortex.

Nicotine improves memory in an object recognition task in rats

Nicotine was investigated for its mnemonic effect in a two trials object recognition task. In the first trial, two copies of the same object were presented. In the second trial (24 h after), one of the familiar object and a new object were presented. The time spent exploring the new object by control rats was not significantly different from the exploration time of the familiar object, indicating that they did not remember the familiar object. Rats injected with nicotine before the first trial, after the first trial or before the second trial spent more time in exploring the new object than the familiar one at the second trial. These results suggest that, in normal rats, acute nicotine enhances acquisition, consolidation and restitution of the information in an object recognition task.

Counteractive effects of a partial (sabcomeline) and a full (RS86) muscarinic receptor agonist on deficits in radial maze performance induced by S-AMPA lesions of the basal forebrain and medial septal area

After S-AMPA (8.0 mM) lesions to the nucleus basalis and medial septal regions, at the source of the cortical and hippocampal branches of the forebrain cholinergic projection system, rats displayed long-lasting and relatively stable impairment in long-term reference and short-term working memory in both spatial (place) and associative (cue) radial maze tasks. Treatment with four doses of the partial agonist at the M1 cholinergic muscarinic receptor, sabcomeline (formerly known as SB 202026: 0.01-0.156 mg/kg), substantially reduced working and reference memory errors in both tasks in lesioned rats, in a mainly dose-related manner. These effects were more consistent than those found with the direct muscarinic agonist RS86 (0.05-0.781 mg/kg). The performance of non-lesioned controls was largely unaffected by either treatment. These findings are consistent with previous evidence for cholinergic participation in the radial maze deficits induced by excitotoxic lesions to the forebrain cholinergic projection system. They show that with a relatively selective lesion, which respectively, reduced choline acetyltransferase activity to 36.5 and 22.5% of control level in frontal and dorsolateral cortex, and to 61.8 and 69.2% of control level in dorsal and ventral hippocampus, lesioned rats were responsive to pharmacological treatments aimed to enhance cholinergic function by full or partial agonist activity at M1 receptors. Findings that nicotine (0.1 mg/kg) also reduced radial maze errors in the lesioned animals supports the suggestion that lesion-induced deficits in radial maze performance were amenable to improvement by cholinergic receptor manipulation. However, given the potential adverse side effects of full receptor agonists, which nonselectively target cholinergic receptors throughout the organism, the functional efficacy of sabcomeline, which shows regional selectivity for the central M1 receptor subtypes, suggests that deleterious effects of cholinergic depletion on cognition can be counteracted without incurring the risk of unwanted side effects.

The behavioral functions of the cholinergic basal forebrain: lessons from 192 IgG-saporin

Until recently our understanding of the functional neuroanatomy of the cholinergic basal forebrain (CBF) has been hindered by the lack of a lesioning technique that is truly selective. The development of the immunotoxin 192 IgG-saporin (192-sap) has greatly improved our ability to create specific lesions of the CBF. Rats with such lesions have been studied in a wide variety of behavioral paradigms of learning, memory, and attention. Complete or near-complete destruction of the CBF results in deficits in a variety of behavior paradigms including passive avoidance, spatial tasks (water and radial mazes), delayed matching to position/sample, and attentional tasks. However, interpretation of many experiments is hampered by incomplete lesions and/or concomitant damage to cerebellar Purkinje neurons. Future studies will need to address these issues. Recent development of a similar immunotoxin that is effective in primates should permit more sophisticated behavioral analysis of CBF function. Additionally, immunotoxins selective for other types of neurons, such as the noradrenergic selective anti-DBH-saporin, will permit analysis of the behavioral functions of other diffusely projecting systems and how these other systems may interact with the CBF.

Hippocampal grafts of acetylcholine-producing cells are sufficient to improve behavioural performance following a unilateral fimbria-fornix lesion

Lesions of the septohippocampal pathway produce cognitive deficits that are partially attenuated by grafts of cholinergic-rich tissue into denervated target regions or by systemic administration of cholinomimetic drugs. In the present study, fibroblasts engineered to produce acetylcholine were used to test the hypothesis that restoration of hippocampal acetylcholine in rats with septohippocampal lesions is sufficient to improve cognitive processing post-damage. Rats received unilateral grafts of acetylcholine-producing or control fibroblasts into the hippocampus immediately prior to an aspirative lesion of the ipsilateral fimbria-fornix. Some rats with fimbria-fornix lesions were implanted with acetylcholine-producing or control fibroblasts into the neocortex, another major target of the basal forebrain cholinergic system, to determine if the site of acetylcholine delivery to the damaged brain is critical for functional recovery. Rats were tested in a hidden platform water maze task, a cued water maze task and activity chambers between one and three weeks post-grafting. Compared to unoperated controls, rats with fimbria fornix lesions only were significantly impaired in hidden platform water maze performance. Hippocampal grafts of acetylcholine-producing cells reduced lesion-induced deficits in the water maze, whereas hippocampal control grafts and cortical grafts of either cell type were without effect. Locomotor activity and cued water maze performance were unaffected by the lesion or the implants. Taken together, these data indicate that water maze deficits produced by fimbria fornix lesions, which disrupt a number of hippocampal neurotransmitter systems, can be attenuated by target specific replacement of acetylcholine in the hippocampus and that this recovery occurs in the absence of circuitry repair.

Neurotoxic effects in adult mice neonatally exposed to 3,3'4,4'5-pentachlorobiphenyl or 2,3,3'4,4'-pentachlorobiphenyl. Changes in brain nicotinic receptors and behaviour

The objective of the present study was to investigate whether neonatal exposure to single PCB congeners 3,3',4,4',5-pentachlorobiphenyl (IUPAC 126) (co-planar) and 2,3,3',4,4'-pentachlorobiphenyl (IUPAC 105) (mono-ortho `co-planar like') when given as one single dose (0.14-14 μmol/kg body weight per os) to 10 day old male NMRI mice could induce neurotoxic effects in the adult animal, as earlier seen for some ortho-substituted PCBs. Furthermore, to ascertain whether behavioural aberrations, both in spontaneous behaviour and in learning and memory function, were followed by changes in the cholinergic and/or the dopaminergic system, and whether behavioural changes could worsen with age. It was found that neonatal exposure to 3,3',4,4',5-pentachlorobiphenyl can induce persistent aberrations in spontaneous behaviour and that this derangement can grow worse with age. Furthermore, this exposure affected also learning and memory functions in the adult animal and in the animals showing this deficit, the cholinergic nicotinic receptors in the hippocampus were affected. Exposure to 2,3,3',4,4'-pentachlorobiphenyl, at the same dose or higher, did not cause any significant change in the investigated behavioural variables, spontaneous and swim-maze behaviour.

Effect of haloperidol on nicotine-induced enhancement of vigilance in human subjects

We posed the question whether the cognitive enhancement caused by nicotine in human subjects is mediated by dopamine (DA) release. This issue was addressed by testing performance in the Wesnes and Warburton vigilance task after s.c. nicotine with or without concomitant oral haloperidol. The subjects were moderate (10-14 cigarettes/day) smokers after overnight deprivation of smoking. After an initial practice session, each subject participated in four further sessions spread over 2 weeks, after: placebo/placebo, placebo/haloperidol (5 mg), placebo/nicotine (0.8 mg) or nicotine/haloperidol, double-blind with a balanced ordering of drug combinations. On each occasion, performance was measured on the vigilance task, a finger tapping test, and digit span forward and backward. Nicotine improved detection sensitivity on the vigilance task, and this effect was unchanged by haloperidol; the latter compound reduced forward digit span. Thus, the improved vigilance caused by nicotine does not appear to be mediated by DA systems.

Effects of prenatal ethanol exposure and early experience on radial maze performance and conditioned taste aversion in mice

C57BL/6 mice were intubated on gestational days 14-18 twice daily with 1.58 g/kg ethanol, 4.2 g/kg sucrose, or remained untreated. Offspring of ethanol-treated or lab chow control groups were raised either by group-housed dams and weaned on postnatal day (PND) 28 (enriched condition), or by individually housed dams and weaned on PND 21 (standard condition). Offspring of the sucrose control group were raised by individually housed dams and weaned on PND 21. Groups did not differ in pup weight or litter size. Male and female offspring were assessed for performance in an unbaited radial maze (PND 45-52) and male offspring only were tested for conditioned taste aversion (PND 54-59). As hypothesized, mice prenatally exposed to ethanol and raised under standard conditions failed to develop the conditioned taste aversion response. In contrast, subjects with in utero ethanol exposure that were raised under enriched preweaning conditions developed the taste aversion response. Maze performance improved significantly over days, but no significant effects were detected for either prenatal treatment or preweaning rearing conditions. In conclusion, enriched preweaning rearing conditions abolished the detrimental effects of prenatal ethanol exposure on conditioned taste aversion, but radial maze performance remained unaffected by any treatment in this study.

Acetylcholine, aging, and Alzheimer's disease

A substantial body of literature has suggested that the memory and learning deficits associated with Alzheimer's disease and aging are attributable to degeneration of the cholinergic magnocellular neurons of the nucleus basalis of Meynert (nbM). Subsequently, lesion-induced damage to the cholinergic projections from the nbM to the neocortex has been utilized extensively as an animal model of dementia. In addition, the effect of the normal aging process on deterioration of these neurons and on cognitive function has also been examined. Such studies have revealed, for example, that many of the learning and memory impairments traditionally attributed to the cholinergic corticopetal system are not due to degeneration of the cholinergic neurons of the nbM, but instead may be due to damage of more rostral elements of the cholinergic basal forebrain system. This review will examine the contribution of behavioural animal and human studies to out understanding of the role of the basal forebrain cholinergic neurons in age-related cognitive impairments.

The fimbria-fornix/cingular bundle pathways: a review of neurochemical and behavioural approaches using lesions and transplantation techniques

Extensive lesions of the fimbria-fornix pathways and the cingular bundle deprive the hippocampus of a substantial part of its cholinergic, noradrenergic and serotonergic afferents and, among several other behavioural alterations, induce lasting impairment of spatial learning and memory capabilities. After a brief presentation of the neuroanatomical organization of the hippocampus and the connections relevant to the topic of this article, studies which have contributed to characterize the neurochemical and behavioural aspects of the fimbria-fornix lesion "syndrome" with lesion techniques differing by the extent, the location or the specificity of the damage produced, are reviewed. Furthermore, several compensatory changes that may occur as a reaction to hippocampal denervation (sprouting changes in receptor sensitivity and modifications of neurotransmitter turnover in spared fibres) are described and discussed in relation with their capacity (or incapacity) to foster recovery from the lesion-induced deficits. According to this background, experiments using intrahippocampal or "parahippocampal" grafts to substitute for missing cholinergic, noradrenergic or serotonergic afferents are considered according to whether the reported findings concern neurochemical and/or behavioural effects. Taken together, these experiments suggest that appropriately chosen fetal neurons (or other cells such as for instance, genetically-modified fibroblasts) implanted into or close to the denervated hippocampus may substitute, at least partially, for missing hippocampal afferents with a neurochemical specificity that closely depends on the neurochemical identity of the grafted neurons. Thereby, such grafts are able not only to restore some functions as they can be detected locally, namely within the hippocampus, but also to attenuate some of the behavioural (and other types of) disturbances resulting from the lesions. In some respects, also these graft-induced behavioural effects might be considered as occurring with a neurochemically-defined specificity. Nevertheless, if a graft-induced recovery of neurochemical markers in the hippocampus seems to be a prerequisite for also behavioural recovery to be observed, this neurochemical recovery is neither the one and only condition for behavioural effects to be expressed, nor is it the one and only mechanism to account for the latter effects.

Plastic changes in the cholinergic innervation of the rat cerebral cortex after unilateral lesion of the nucleus basalis with alpha-amino-3-OH-4-isoxozole propionic acid (AMPA): effects of basal forebrain transplants into neocortex

Unilateral AMPA lesions of the nucleus basalis magnocellularis (nbm) produced a nearly complete loss of cholinergic markers in the ipsilateral frontal and parietal cortices with no recovery at 6 months. The loss was associated with compensatory increases in AChE-positive fibre density in the contralateral cortex, in ipsilateral cortical regions not receiving their cholinergic innervation from the nbm and in the size of cholinergic magnocellular neurones in the contralateral nbm. The hypertrophy and increase in AChE-positive fibre density were apparent at 4-6 weeks after lesion and increased with time. Cholinergic transplants to cholinergically deafferented cortex prevented development of the compensatory increases in AChE-positive fibre density and restored AChE-positive fibre density and ChAT activity to control levels in ipsilateral cholinergically deafferented regions, partially after 6-8 weeks and completely after 6 months. In contrast, when cholinergic grafts were placed into unlesioned cortex, axonal outgrowth was localized to the vicinity of the transplant and did not develop with time. These results support the concept that vacant synapses promote and direct axonal outgrowth from transplanted neurones and that grafted cholinergic neurones integrate into the lesioned forebrain cholinergic projections system and prevent the lesion-induced changes in AChE-positive fibre density and ChAT activity.

Central cholinergic systems and cognition

The organization and possible functions of basal forebrain and pontine cholinergic systems are reviewed. Whereas the basal forebrain cholinergic neuronal projections likely subserve a common electrophysiological function, e.g. to boost signal-to-noise ratios in cortical target areas, this function has different effects on psychological processes dependent upon the neural network operations within these various cortical domains. Evidence is presented that (a) the nucleus basalis-neocortical cholinergic system contributes greatly to visual attentional function, but not to mnemonic processes per se; (b) the septohippocampal projection is involved in the modulation of short-term spatial (working) memory processes, perhaps by prolonging the neural representation of external stimuli within the hippocampus; and (c) the diagonal band-cingulate cortex cholinergic projection impacts on the ability to utilize response rules through conditional discrimination. We also suggest that nucleus basalis-amygdala cholinergic projections have a role in the retention of affective conditioning while brainstem cholinergic projections to the thalamus and midbrain dopamine neurons affect basic arousal processes (e.g. sleep-wake cycle) and behavioral activation, respectively. The possibilities and limitations of therapeutic interventions with procholinergic drugs in patients with Alzheimer's disease and other neurodegenerative disorders in which basal forebrain cholinergic neurons degenerate are also discussed.

Maze procedures: the radial-arm and water maze compared

Open mazes are primarily designed to measure place learning and memory, using environmental visuospatial cues. However, maze tasks differ along many dimensions, including (1) types of apparatus, which vary from arenas (water maze: WM) to highly structured routes (radial-arm maze: RAM); (2) availability of visuospatial, associative or sensory cues; (3) task requirements which range from spontaneous exploration to complex sequences of choices; and (4) motivation which may involve aversive escape, the opportunity to shelter or to discover novel objects or food at particular locations. Given this diversity, it is likely that mazes tap a variety of processes that contribute to, or affect spatial learning. Hence 'spatial' abilities measured in one procedure may not resemble those engaged in another, posing problems for the interpretation of drug- or lesion-induced deficits. This review compares two types of maze that exemplify key differences in procedure: the RAM and the WM. (1) Visuospatial, associative and sensory factors contributing to place learning in the two mazes are discussed, together with the types of search strategy that they foster, their differing motivation and vulnerability to effects of non-spatial factors, such as stress and training regime. (2) The equivalence of memory processes (acquisition, working and reference memory) assessed in different mazes is considered, and the extent that these may generalize to non-spatial tasks. (3) Differences in application of the two mazes are evaluated. The WM is well-adapted to the study of selective visuospatial factors in place learning and working memory, but less suitable for repeated measures or for assessment of long-term memory deficits. The RAM detects steady-state reference and working-memory deficits, and is suitable for repeated measures, at the expense of precise analysis of the nature of the processes involved.

Developmental neurotoxicity of four ortho-substituted polychlorinated biphenyls in the neonatal mouse

The objective of the present study was to investigate whether neonatal exposure to single PCB (polychlorinated biphenyl) congeners 2,4,4'-trichlorobiphenyl (IUPAC 28), 2,2',5,5'-tetrachlorobiphenyl (IUPAC 52), 2,3',4,4',5-pentachlorobiphenyl (IUPAC 118) and 2,3,3',4,4',5-hexachlorobiphenyl (IUPAC 156) when given as one single dose (0.7-14 μmol/kg body weight per os) to 10-day-old male NMRI mice could induce persistent neurotoxic effects in the adult animal. Furthermore, to ascertain whether behavioural aberrations, both in spontaneous behaviour and in learning and memory function, were followed by changes in the cholinergic and/or the dopaminergic system. It was found that neonatal exposure to lightly chlorinated ortho-substituted PCBs, 2,4,4'-tri- and 2,2',5,5'-tetrachlorobiphenyls, can induce persistent aberrations in spontaneous behaviour. Neonatal exposure to 2,2',5,5'-tetrachlorobiphenyl also affected learning and memory functions in the adult animal. In the animals showing a deficit in memory and learning function, the cholinergic nicotinic receptors in the cerebral cortex were affected. Exposure to 2,3',4,4',5-penta- and 2,3,3',4,4',5-hexachlorobiphenyl, mono-ortho congeners ('co-planar-like'), in the same dose range did not cause any significant change in the investigated behavioural variables, spontaneous and swim-maze behaviour.

Nicotinic receptors in the brain. Molecular biology, function, and therapeutics

Although the psychological and physiological effects of nicotine have long suggested that nicotine exerts specific actions in the brain, the identification of neuronal nicotinic receptors (nAChRs) only began in the past few years with the development of molecular genetics. It is now clear that neuronal nAChRs form a family of highly heterogenous receptor subtypes, as evidenced by the number of genes encoding nAChR subunits, the diversity of immunopurified receptor proteins, and the multiple functional types of ligand-gated ion channels. Neuronal nAChRs have discrete localizations within the brain, and are involved in modulating neuronal firing and transmitter release. Cumulative evidence from animal and human studies indicates that nicotinic systems play a major role in higher cognitive functions and dysfunctions. In particular, the loss of cortical nAChRs is a neuro-chemical hallmark of Alzheimer (AD) and Parkinson (PD) diseases. In addition, nicotine improves memory and attention in Ad and PD. Our recent studies using electrophysiological biochemical and behavioral approaches suggest that the prefrontal cortex is a major target site for the cognitive actions of nicotine.

Comparison of the effects of the 5-HT3 receptor antagonists WAY-100579 and ondansetron on spatial learning in the water maze in rats with excitotoxic lesions of the forebrain cholinergic projection system

The effects of the 5-HT3 receptor antagonists. WAY-100,579 and ondansetron (both at doses of 0.001, 0.01 and 0.1 mg/kg s.c.) and the muscarinic receptor agonist arecoline (1.0 mg/kg s.c.), on spatial learning and memory in the water maze were examined in rats after combined S-AMPA lesions to the nucleus basalis and medial septal brain regions. Lesioned rats showed substantially increased latency to find the submerged platform, and spent less time searching in the correct quadrant, and more time circling the periphery of the pool, relative to controls. Lesioned rats treated with WAY-100,579, ondansetron and arecoline exhibited marked improvement in these parameters of learning relative to lesioned animals, with arecoline-treated animals showing the most substantial recovery. Linear dose-related trends of improvement were seen with both of the 5-HT3 antagonists. In probe trials, testing retention of the platform position 24 and 72 h after the end of training, control rats exhibited substantial superiority relative to lesioned rats in accuracy of search in the training quadrant and former platform area, matched by rats treated with arecoline on the first, and by rats treated with the two higher doses of WAY-100,579 and ondansetron on the second probe trial. These results are consistent with our previous studies which demonstrated that another selective 5-HT3 receptor antagonist. WAY-100,289, significantly reversed the cognitive deficits in water maze performance induced by ibotenic acid lesions of forebrain cholinergic projection system. Therefore, selective 5-HT3 receptor antagonists may provide a novel effective therapy for treating cognitive deficits associated with degeneration of central cholinergic neurones, such as Alzheimer's disease or age-associated memory impairment.

Relationship between up-regulation of nicotine binding sites in rat brain and delayed cognitive enhancement observed after chronic or acute nicotinic receptor stimulation

(-)-Nicotine tartrate (2 mg/kg), and a nicotinic agonist, RJR 2403 (1.4 mg/kg), and antagonist, mecamylamine (1 mg/kg), were administered to separate groups of rats SC twice daily for 10 days. Two other groups received the same doses of nicotine or RJR 2403 for 1 day followed by saline for 9 days. Twenty-four hours after the final injection, the rats were compared to a 10-day saline-injected group on acquisition of a hidden platform position in the Morris water maze (20 trials, 30-min inter-trial interval). The rats were killed 48 h after the last drug injection and frontal, entorhinal and posterior cingulate cortex and dorsal and ventral hippocampus assayed for [3H]-nicotine binding density. Chronic nicotine significantly increased the number of frontal and entorhinal cortical and dorsal hippocampal, but not posterior cingulate cortical or ventral hippocampal, nicotinic receptors, and improved rate of learning. Chronic mecamylamine and RJR 2403 also significantly increased the number of nicotinic receptors in frontal cortex, though not other regions, but retarded rate of learning. Nicotine given for 1 day 11 days earlier marginally increased nicotinic receptors in entorhinal cortex (but not other regions) and significantly increased rate of learning, though significantly less than 10-day nicotine. Entorhinal cortical and dorsal hippocampal nicotinic receptor numbers were positively associated with rate of learning but not performance at asymptote. Thus cognitive enhancement after chronic nicotine is in part a delayed consequence of nicotine administration 11 days earlier, and may reflect regional changes in nicotinic receptor up-regulation.

Acute and chronic nicotine effects on working memory in aged rats

Acute and chronic nicotine administration has been repeatedly been found in our laboratory to improve working memory performance of normal adult rats in the radial-arm maze. The current study was conducted to determine if acute or chronic nicotine administration would improve working memory performance in aged rats. Sixteen young adult (3-7 months) and 32 aged (24-28 months) male Sprague-Dawley rats were trained on an eight-arm radial maze. A significant age-related choice deficit was seen during the 21 sessions of training. After training, half of the rats in each age group were implanted with nicotine-containing osmotic minipumps and the other half implanted with vehicle-containing pumps. Consistent with previous work, the young adult rats given chronic nicotine (approximately 5 mg/kg per day as measured as nicotine base) showed a significant improvement in working memory performance. In contrast, the aged rats did not show a significant effect of this dose of chronic nicotine. After a 2 week withdrawal period the remaining rats underwent a series of acute drug challenges with nicotinic and muscarinic agonists and antagonists as well as the dopaminergic antagonist haloperidol. Mecamylamine and haloperidol impaired the memory performance of the young adult rats, whereas the aged rats showed no effect. In contrast, scopolamine impaired performance of both young adult and aged rats in a similar manner. Both pilocarpine and nicotine improved the memory performance of the aged rats, but did not improve the young adult rats, possibly due to a ceiling effect on performance. During the cholinergic agonist drug phase, the aged rats which had previously been given chronic nicotine infusions showed better performance than those which had not. The resistance of the aged rats to chronic nicotine-induced working memory improvements and acute mecamylamine-induced working memory deficits may have resulted from the decline in nicotinic receptors seen with aging. Chronic co-administration of the nicotinic antagonist mecamylamine in a previous study was found to abolish the chronic nicotine-induced working memory improvement. The aged rats were resistant to haloperidol-induced deficits which may have resulted from the decrease in dopaminergic receptors seen with aging. Interestingly, acute cholinergic agonists including nicotine did improve working memory performance in the aged rats and previous chronic nicotine infusion was beneficial during the period of acute cholinergic agonist challenge. This suggests that nicotinic treatment may be of use for treating age associated memory impairments but that special dosing regimens may be required.

Behaviour-related effects of nicotine on slow EEG waves in basal nucleus-lesioned rats

The basal magnocellular nucleus is assumed to play a crucial role in cholinergic activation of the cortical EEG. The aim of this study was to establish whether intraperitoneally applied nicotine may counteract the power asymmetry of the slow waves in the cortical EEG of both hemispheres after an unilateral lesion in the basal nucleus. In 17 rats the basal nucleus (substantia innominata/ventral pallidum) was unilaterally lesioned by ibotenic acid. The lesion produced unilateral power increases of all frequencies up to 20 Hz in the frontal EEG that increased with higher arousal level. Additionally, synchronized spike and wave discharges appeared in the frontal EEG. The results indicate that the basal nucleus suppresses especially the delta EEG waves in the frontal motor cortex during motor active behaviour. Nicotine (0.1 and 1 mg/kg) partially counteracts the power asymmetry of frontal slow waves (2-6 Hz) only during exploratory sniffing but not during grooming and waking immobility. Physostigmine (1 mg/kg) was also effective during exploratory sniffing. The results may indicate a role of nicotinic mechanisms in the information input component of exploratory behaviour.

Sensitivity of rat frontal cortical neurones to nicotine is increased by chronic administration of nicotine and by lesions of the nucleus basalis magnocellularis: comparison with numbers of [3H]nicotine binding sites

The effects of chronic nicotine treatment and of unilateral AMPA lesion of the nucleus basalis magnocellularis (nbm) on the sensitivity of frontal cortical neurones to iontophoretically applied nicotine were studied. Chronic nicotine treatment increased the number of [3H]nicotine binding sites from 2.9 to 3.9 pmol g-1 wet weight, and increased the proportion of cortical neurones responding to nicotine from 32.3% to 60.0%. After unilateral nbm lesions, the densities of AChE-positive fibers and [3H]nicotine binding sites were reduced by approximately 97% and 55%, respectively, and the proportion of neurones responding to nicotine increased from 32.3% to 53.8%. The two treatments, chronic nicotine administration and nbm lesion, also increased the size of individual neuronal responses, prolonged their duration, and shortened the response latency. Responses to glutamate were unaffected by either procedures. The results show that the increase in [3H]nicotine binding produced by chronic nicotine administration is associated with an increased response to iontophoretically applied nicotine, suggesting that the receptor upregulation induced by the chronic treatment were functional. Less easily explained is the association between increased sensitivity of frontal cortical neurons to nicotine after nbm lesion with a decreased receptor density. It is suggested that a substantial proportion of nicotinic receptors are located presynaptically, and that their loss after lesion concealed an upregulation at postsynaptic sites.

Acetylcholine: a neurotransmitter for learning and memory?

The cholinergic hypothesis claims that the decline in cognitive functions in dementia is predominantly related to a decrease in cholinergic neurotransmission. This hypothesis has led to great interest in the putative involvement of the cholinergic neurotransmission in learning and memory processes. This review aims to assess the data of studies in which the role of acetylcholine (ACh) in cognitive functions was investigated. For this purpose, studies from three different fields of research, namely: (1) behavioral pharmacology (effects of drugs on behavior); (2) behavioral neuroscience (effects of brain lesions on behavior); and (3) dementia, are discussed separately. The experimental tools that have been used in pharmacological studies may appear to be inadequate to enable conclusions to be drawn about the involvement of ACh in learning and memory processes. Especially, the use of scopolamine as a pharmacological tool is criticized. In the field of behavioral neuroscience a highly specific cholinergic toxin has been developed. It appears that the greater and more specific the cholinergic damage, the fewer effects can be observed at the behavioral level. The correlation between the decrease in cholinergic markers and the cognitive decline in dementia may not be as clearcut as has been assumed. The involvement of other neurotransmitter systems in cognitive functions is briefly discussed. Taking into account the results of the different fields of research, the notion that ACh plays a pivotal role in learning and memory processes seems to be overstated. Even when the role of other neurotransmitter systems in learning and memory is taken into consideration, it is unlikely that ACh has a specific role in these processes. On basis of the available data, ACh seems to be more specifically involved in attentional processes than in learning and memory processes.

Effect of vagal autotransplantation and bifemelane hydrochloride on cholinergic markers and event-related potentials in rats with lesions of the nucleus basalis magnocellularis

In rats lesioned by injecting the ibotenic acid (8 micrograms/site) into the unilateral nucleus basalis magnocellularis (NBM), the effect of treatment with bifemelane hydrochloride (BIF) or autotransplantation of the vagal nodosal ganglion was studied electrophysiologically by serial measurement of the event-related potential (ERP, P300) for 4 weeks. In addition, the effects on cholinergic markers were assessed by determining the specific binding of [3H]QNB (quinuclidinyl benzilate) to the muscarinic acetylcholine receptor (mAChR) as well as the activity of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE). The P300 latency was delayed and its amplitude remained low for 4 weeks in NBM-lesioned rats. In contrast, a return to normal occurred after 2-3 weeks in rats given daily intraperitoneal injections of BIF (15 mg/kg) and in autotransplanted rats. In lesioned rats, the cortical ChAT and AChE activities on the affected side did not recover, but the postsynaptic receptor response was transiently activated soon after lesioning. BIF increased specific mAChR binding (an early increase of affinity and a subsequent increase of receptor density) as well as presynaptic ChAT activity. Transplantation achieved the early activation of mAChR binding (increased receptor density) and continuously increased ChAT activity. Thus, the postsynaptic compensatory receptor mechanism of denervation supersensitivity acted as an early response to the depression of presynaptic cholinergic activity, but it could not improve the P300 response until the subsequent increase of cortical ChAT activity. Improvement of P300 combined with cortical cholinergic recovery after nodosal ganglion grafting or administration of BIF suggests that the neocortical ACh level may play an important role in regulating ERP.

The effects of d-cycloserine, a partial agonist at the glycine binding site, on spatial learning and working memory in scopolamine-treated rats

The present study investigated the effect of d-cycloserine, a partial agonist at the glycine binding site on NMDA receptor complex, on the performance of scopolamine-treated adult rats in a water maze task assessing spatial learning and in a delayed non-matching to position task assessing working memory in a spatial context. In the spatial learning task, scopolamine (0.4 mg/kg, i.p.) impaired acquisition (increased escape latency and distance) and increased swimming speed of rats. D-cycloserine (1.0 mg/kg, i.p.) reversed the deficits in acquisition performance but not the increases in behavioral activity. In the working memory task, scopolamine (0.2 mg/kg, i.p.) produced deficits on nonmnemonic rather than on mnemonic performance factors; scopolamine delay-independently decreased the percent correct responses and reduced behavioral activity of rats. D-cycloserine (1.0, 3.0 and 10 mg/kg, i.p.) did not reverse these performance deficits. When administered alone, the moderate to higher doses of d-cycloserine had no effects on working memory but the lower dose produced slight deficits in mnemonic performance factors; the 1.0 mg/kg dose delay-dependently decreased the percent correct responses without affecting behavioral activity of rats. In the water maze task, d-cycloserine had no effects on acquisition performance or behavioral activity of rats. These results suggest that acute, systemic administration of d-cycloserine does not improve spatial learning or working memory. However, at appropriate doses this agent may be efficacious in disease states of central cholinergic hypofunction since 1.0 mg/kg d-cycloserine was able to reverse the scopolamine-induced deficits in acquisition.

Nicotinic and muscarinic receptors in the rat prefrontal cortex: differential roles in working memory, response selection and effortful processing

The aim of the present study was to evaluate the effects of cholinergic receptor blockade in the rat prefrontal cortex on cognitive processes. The nicotinic antagonists neuronal bungarotoxin and dihydro-beta-erythroidine and the muscarinic antagonist scopolamine were injected into the prelimbic area of the prefrontal cortex. Their behavioural effects were assessed in a T-maze to test reference memory (visual discrimination task) and working memory in delayed matching (MTS) and non-matching to sample (NMTS) tasks. Neuronal bungarotoxin produced a significant decrease in working memory performance in the MTS task but not in the NMTS task. In contrast, scopolamine impaired working memory in both MTS and NMTS tasks. Reference memory was not altered by any of the cholinergic antagonists. These results demonstrate a differential role of nicotinic and muscarinic receptors in the rat prefrontal cortex. Nicotinic transmission appears to be important in delayed response tasks requiring effortful processing for response selection, while the muscarinic system is involved in general working memory processes.

Astrocyte transplants alleviate lesion induced memory deficits independently of cholinergic recovery

Basal forebrain tissue fragments taken from embryonic day 15 were separated into primary astrocytes and primary neurons in culture and grafted to rats with alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid lesions to the nucleus basalis and medial septal regions. The two cell types were compared in two experimental paradigms for their behavioural, biochemical and histochemical effects; standard transplants of whole basal forebrain and sham transplants served as positive and negative controls, respectively. Each transplant cell type was characterised by in vitro immunocytochemistry to assess content and purity. Memory deficits produced by the lesions in a spatial win-stay T-maze task (Experiment 1) and a spatial plus associative radial maze task (Experiment 2) were significantly improved by the astrocyte, but not by the neuronal, primary cell transplants. The astrocyte graft groups performed as well as standard cholinergic rich basal forebrain groups, reaching control levels on both tasks, while the neuronal transplant groups were not significantly different to lesioned (sham transplanted) rats. There was no recovery in choline acetyltransferase activity in brain regions containing astrocyte grafts whereas activity in the neuronal graft regions was increased (often to control levels), similar to recovery produced by basal forebrain grafts. Grafts in all groups survived, transplanted neurons displaying similar morphology and placement in the host brain to unseparated basal forebrain grafts, while astrocytes showed evidence of migration. The cultured astrocytes were estimated to be > 95% pure, showing positive staining for all astrocyte markers and an absence of staining for neuronal markers. The results indicate that the restoration of cognitive function following fetal grafting is not dependent upon a restoration of cholinergic neuronal activity but is more likely mediated via diffuse graft-host communication, with trophic secretion a probable factor. This study emphasizes the usefulness of astrocytes in the repair of central nervous system injury and has implications for therapeutic potential.

Reversal of visual attentional dysfunction following lesions of the cholinergic basal forebrain by physostigmine and nicotine but not by the 5-HT3 receptor antagonist, ondansetron

To investigate further the cholinergic specificity of the effects of basal forebrain lesion-induced disruption of attentional performance, the present study examined the efficacy of various pharmacological agents in improving performance of a five-choice serial reaction time task in rats that had received lesions of the cholinergic basal forebrain. Specifically, the effects of the novel 5-HT3 receptor antagonist, ondansetron (0.3, 1, 10 ng/kg), and of nicotine (0.03, 0.06, 0.1, 0.3 mg/kg) and the anticholinesterase, physostigmine (0.05, 0.1 mg/kg), on attentional function were examined in animals which had received AMPA-induced lesions of the nucleus basalis magnocellularis (nbM). The behavioural impairments observed immediately following the lesion were a reduction were choice accuracy and an increase in correct response latency. Although these impairments showed recovery over the course of the following weeks, the deficit in choice accuracy could be reinstated by reducing the duration of the visual stimulus and thus increasing the attentional load placed on the animals. This reduction in choice accuracy could be dose dependently improved by systemic administration of either physostigmine or nicotine, suggesting that this impairment in attentional function may be attributed to disruption of cholinergic function. The pharmacological specificity of these improvements was supported by the inability of d-amphetamine to improve task performance (0.2, 0.4, 0.8 mg/kg). Ondansetron was also unable to improve accuracy of performance in lesioned animals, but was effective in reducing the anticipatory or premature responding observed in both control and lesioned animals, even when elevated (in the case of controls) by treatment with systemic d-amphetamine. The results of the present study therefore suggest that cholinergic dysfunction can lead to attentional impairments which can be ameliorated by cholinergic treatments such as physostigmine and nicotine, but that ondansetron, despite its proposed ability to release cortical acetylcholine, was unable to restore choice accuracy at the doses employed. The results further suggest a double dissociation of effects on accuracy and the disinhibition of responding.

The selective 5-HT3 receptor antagonist, WAY100289, enhances spatial memory in rats with ibotenate lesions of the forebrain cholinergic projection system

The effects of three doses (0.003, 0.03 and 1.0 mg/kg sc) of the 5-HT3 receptor antagonist, WAY 100289, on spatial learning and memory in the water maze were examined in rats before and after ibotenate lesions to the nucleus basalis and medial septal brain regions at the source of cholinergic projections to cortex and hippocampus. The representative cholinergic nicotinic and muscarinic receptor agonists nicotine (0.1 mg/kg) and arecoline (1.0 mg/kg) were also tested for comparison. Both arecoline and nicotine improved initial acquisition in rats before lesioning, in terms of latency to find a hidden platform and accuracy of search strategy. WAY100289 did not affect the performance of normal rats significantly, apart from some non-significant trends towards improvement with the highest dose. However, in animals showing transient navigational deficits in retention and relearning after lesioning, WAY100289 improved performance at all three doses, though ameliorative effects of nicotine and arecoline were more marked also in lesioned rats. These results show that WAY100289 improved spatial learning in animals impaired after lesions to cholinergic projection nuclei, which may reflect an interaction with cholinergic transmission to enhance cognitive function. However, in the present study, WAY100289 appeared to be less effective than direct cholinergic agonists.

Are the cognitive-enhancing effects of nicotine in the rat with lesions to the forebrain cholinergic projection system mediated by an interaction with the noradrenergic system?

Experiments were conducted to test the hypothesis that the enhancing effect of nicotine on water maze performance in rats with lesions of the forebrain cholinergic projection systems (FCPS) is mediated by an interaction with the noradrenergic system, in particular the ascending dorsal noradrenergic bundle (DNAB) and its projection areas. Three groups of rats received lesions of either: i) the nucleus basalis (NBM) and medial septal area/diagonal band (MSA/DB) by infusion of alpha-amino-3-hydroxy-4-izoxazole propionic acid (AMPA) (FCPS group), ii) DNAB, by infusion of 6-hydroxydopamine (6-OHDA) (NOR group), or iii) both FCPS plus DNAB (COMB group). Control animals received vehicle. Choline acetyltransferase activity was reduced in the cortex and hippocampus of the FCPS and COMB groups and in the hippocampus of the NOR group. NA level was reduced in the cortex and hippocampus of the FCPS and COMB groups, but not the FCPS group. In a reference memory task, the performance of both the NOR and COMB groups, but not the NOR group, was significantly worse than that of controls; there was no effect of nicotine administration (0.1 mg/kg) on escape latency or other measures in this task. In a working memory task, FCPS and COMB rats took longer to find the submerged platform on the second and following trials, and there was a significant enhancement of performance by nicotine in both groups, but not in controls. These results indicate that the enhancing effects of nicotine in rats with FCPS lesions are not mediated by an interaction with the DNAB.

Effects of ABT-418, a novel cholinergic channel ligand, on place learning in septal-lesioned rats

Septal lesions disrupt septohippocampal neurotransmission and impair spatial memory. (-)-Nicotine reduces the memory deficits but has substantial side effect liabilities. Previous studies have demonstrated that ABT-418 is a novel, selective ligand for neuronal nicotinic acetylcholine receptors. In the current study, ABT-418 (0.19 and 1.9 mumol/kg, i.p.) administered before training significantly attenuated lesion-induced deficits in a spatial discrimination version of the Morris water maze. As lesion-induced learning deficits might parallel the cognitive deficits characteristic of Alzheimer's disease, these results suggest that ABT-418 may be useful in the treatment of this condition.