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

Different cholinergic cell groups in the basal forebrain regulate social interaction and social recognition memory

Social behaviour is a complex construct that is reported to include several components of social approach, interaction and recognition memory. Alzheimer's disease (AD) is mainly characterized by progressive dementia and is accompanied by cognitive impairments, including a decline in social ability. The cholinergic system is a potential constituent for the neural mechanisms underlying social behaviour, and impaired social ability in AD may have a cholinergic basis. However, the involvement of cholinergic function in social behaviour has not yet been fully understood. Here, we performed a selective elimination of cholinergic cell groups in the basal forebrain in mice to examine the role of cholinergic function in social interaction and social recognition memory by using the three-chamber test. Elimination of cholinergic neurons in the medial septum (MS) and vertical diagonal band of Broca (vDB) caused impairment in social interaction, whereas ablating cholinergic neurons in the nucleus basalis magnocellularis (NBM) impaired social recognition memory. These impairments were restored by treatment with cholinesterase inhibitors, leading to cholinergic system activation. Our findings indicate distinct roles of MS/vDB and NBM cholinergic neurons in social interaction and social recognition memory, suggesting that cholinergic dysfunction may explain social ability deficits associated with AD symptoms.

Cholinergic modulation of spatial learning, memory and navigation

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

Mechanisms and use of neural transplants for brain repair

Under appropriate conditions, neural tissues transplanted into the adult mammalian brain can survive, integrate, and function so as to influence the behavior of the host, opening the prospect of repairing neuronal damage, and alleviating symptoms associated with neuronal injury or neurodegenerative disease. Alternative mechanisms of action have been postulated: nonspecific effects of surgery; neurotrophic and neuroprotective influences on disease progression and host plasticity; diffuse or locally regulated pharmacological delivery of deficient neurochemicals, neurotransmitters, or neurohormones; restitution of the neuronal and glial environment necessary for proper host neuronal support and processing; promoting local and long-distance host and graft axon growth; formation of reciprocal connections and reconstruction of local circuits within the host brain; and up to full integration and reconstruction of fully functional host neuronal networks. Analysis of neural transplants in a broad range of anatomical systems and disease models, on simple and complex classes of behavioral function and information processing, have indicated that all of these alternative mechanisms are likely to contribute in different circumstances. Thus, there is not a single or typical mode of graft function; rather grafts can and do function in multiple ways, specific to each particular context. Consequently, to develop an effective cell-based therapy, multiple dimensions must be considered: the target disease pathogenesis; the neurodegenerative basis of each type of physiological dysfunction or behavioral symptom; the nature of the repair required to alleviate or remediate the functional impairments of particular clinical relevance; and identification of a suitable cell source or delivery system, along with the site and method of implantation, that can achieve the sought for repair and recovery.

Distinct roles of basal forebrain cholinergic neurons in spatial and object recognition memory

Recognition memory requires processing of various types of information such as objects and locations. Impairment in recognition memory is a prominent feature of amnesia and a symptom of Alzheimer’s disease (AD). Basal forebrain cholinergic neurons contain two major groups, one localized in the medial septum (MS)/vertical diagonal band of Broca (vDB), and the other in the nucleus basalis magnocellularis (NBM). The roles of these cell groups in recognition memory have been debated, and it remains unclear how they contribute to it. We use a genetic cell targeting technique to selectively eliminate cholinergic cell groups and then test spatial and object recognition memory through different behavioural tasks. Eliminating MS/vDB neurons impairs spatial but not object recognition memory in the reference and working memory tasks, whereas NBM elimination undermines only object recognition memory in the working memory task. These impairments are restored by treatment with acetylcholinesterase inhibitors, anti-dementia drugs for AD. Our results highlight that MS/vDB and NBM cholinergic neurons are not only implicated in recognition memory but also have essential roles in different types of recognition memory.

Diffusion-weighted magnetic resonance imaging detection of basal forebrain cholinergic degeneration in a mouse model

Loss of basal forebrain cholinergic neurons is an early and key feature of Alzheimer's disease, and magnetic resonance imaging (MRI) volumetric measurement of the basal forebrain has recently gained attention as a potential diagnostic tool for this condition. The aim of this study was to determine whether loss of basal forebrain cholinergic neurons underpins changes which can be detected through diffusion MRI using diffusion tensor imaging (DTI) and probabilistic tractography in a mouse model. To cause selective basal forebrain cholinergic degeneration, the toxin saporin conjugated to a p75 neurotrophin receptor antibody (mu-p75-SAP) was used. This resulted in ~25% loss of the basal forebrain cholinergic neurons and significant loss of terminal cholinergic projections in the hippocampus, as determined by histology. To test whether lesion of cholinergic neurons caused basal forebrain, hippocampal, or whole brain atrophy, we performed manual segmentation analysis, which revealed no significant atrophy in lesioned animals compared to controls (Rb-IgG-SAP). However, analysis by DTI of the basal forebrain area revealed a significant increase in fractional anisotropy (FA; +7.7%), mean diffusivity (MD; +6.1%), axial diffusivity (AD; +8.5%) and radial diffusivity (RD; +4.0%) in lesioned mice compared to control animals. These parameters strongly inversely correlated with the number of choline acetyl transferase-positive neurons, with FA showing the greatest association (r(2)=0.72), followed by MD (r(2)=0.64), AD (r(2)=0.64) and RD (r(2)=0.61). Moreover, probabilistic tractography analysis of the septo-hippocampal tracts originating from the basal forebrain revealed an increase in streamline MD (+5.1%) and RD (+4.3%) in lesioned mice. This study illustrates that moderate loss of basal forebrain cholinergic neurons (representing only a minor proportion of all septo-hippocampal axons) can be detected by measuring either DTI parameters of the basal forebrain nuclei or tractography parameters of the basal forebrain tracts. These findings provide increased support for using DTI and probabilistic tractography as non-invasive tools for diagnosing and/or monitoring the progression of conditions affecting the integrity of the basal forebrain cholinergic system in humans, including Alzheimer's disease.

Gene regulation in the rat prefrontal cortex after learning with or without cholinergic insult

The prefrontal cortex is essential for a wide variety of higher functions, including attention and memory. Cholinergic neurons are thought to be of prime importance in the modulation of these processes. Degeneration of forebrain cholinergic neurons has been linked to several neurological disorders. The present study was designed to identify genes and networks in rat prefrontal cortex that are associated with learning and cholinergic-loss-memory deficit. Affymetrix microarray technology was used to screen gene expression changes in rats submitted or not to 192 IgG-saporin immunolesion of cholinergic basal forebrain and trained in spatial/object novelty tasks. Results showed learning processes were associated with significant expression of genes, which were organized in several clusters of highly correlated genes and would be involved in biological processes such as intracellular signaling process, transcription regulation, and filament organization and axon guidance. Memory loss following cortical cholinergic deafferentation was associated with significant expression of genes belonging to only one clearly delineated cluster and would be involved in biological processes related to cytoskeleton organization and proliferation, and glial and vascular remodeling, i.e., in processes associated with brain repair after injury.

Acetylcholine and attention

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

Gene expression profile in rat hippocampus with and without memory deficit

The cholinergic neuronal system, through its projections to the hippocampus, plays an important role in learning and memory. The aim of the study was to identify genes and networks in rat hippocampus with and without memory deficit. Genome-scale screening was used to analyze gene expression changes in rats submitted or not to intraparenchymal injection of 192 IgG-saporin and trained in spatial/object novelty tasks. Results showed learning processes were associated with significant expression of genes that could be grouped into several clusters of similar expression profiles and that are involved in biological functions, namely lipid metabolism, signal transduction, protein metabolism and modification, and transcription regulation. Memory loss following hippocampal cholinergic deafferentation was associated with significant expression of genes that did not show similar cluster organization. Only one cluster of genes could be identified; it included genes that would be involved in tissue remodeling. More important, most of the genes significantly altered in lesioned rats were down-regulated.

Behavioral and immunohistological effects of cholinergic damage in immunolesioned rats: Alteration of c-Fos and polysialylated neural cell adhesion molecule expression

The aim of this study was to determine the brain structures as well as the plasticity events associated with the behavioral effects of cholinergic damage. Rats were submitted to injection of 192 IgG-saporin in the medial septum/diagonal band of Broca complex and the nucleus basalis magnocellularis. The immunohistochemical expression of c-Fos protein and PSA-NCAM (polysialylated neural cell adhesion molecule) and the behavioral performances in the nonmatching-to-position task were assessed at various post-lesion times. Thus, 3 days after injection of the immunotoxin, increased c-Fos labeling was observed in the areas of infusion, indicating these cells were undergoing some plastic changes and/or apoptotic processes. A drastic increase was observed in the number of PSA-NCAM positive cells and in their dendritic arborization in the dentate gyrus. At 7 days post-lesion, no behavioral deficit was observed in immunolesioned rats despite the drastic loss of cholinergic neurons. These neurons showed decreased c-Fos protein expression in the piriform and entorhinal cortex and in the dentate gyrus. In the latter, PSA-NCAM induction was high, suggesting that remodeling occurred, which in turn might contribute to sustaining some mnemonic function in immunolesioned rats. At 1 month, cholinergic neurons totally disappeared and behavioral deficits were drastic. c-Fos expression showed no change. In contrast, the increased PSA-NCAM-labeling observed at short post-lesion times was maintained but the plastic changes due to this molecule could not compensate the behavioral deficit caused by the immunotoxin. Thus, as the post-lesion time increases, a gradual degeneration process should occur that may contribute to mnemonic impairments. This neuronal loss leads to molecular and cellular alterations, which in turn may aggravate cognitive deficits.

Lesions to the nucleus basalis magnocellularis lower performance but do not block the retention of a previously acquired learning set

Rats were first trained to acquire an olfactory discrimination learning set (ODLS) on 40 olfactory-unique discrimination problems. Following acquisition of ODLS, animals were lesioned bilaterally in the nucleus basalis magnocellularis (nBM) using either quisqualic acid (QUIS) or 192 IgG-saporin (SAP). QUIS animals performed significantly worse than control animals following surgery and SAP animals performed transiently worse than control animals. Despite lowered performances, both QUIS and SAP animals performed significantly better than expected by chance on trial 2 indicating retention of the ODLS previously acquired. Implications for the role of the nBM in aspects of cognitive flexibility and its role in acquisition versus retention are discussed.

Up-regulation of cation-independent mannose 6-phosphate receptor and endosomal-lysosomal markers in surviving neurons after 192-IgG-saporin administrations into the adult rat brain

The cation-independent mannose 6-phosphate receptor (CI-MPR) is a single transmembrane domain glycoprotein that plays a major role in the trafficking of lysosomal enzymes from the trans-Golgi network to the endosomal-lysosomal (EL) system. Because dysfunction of EL system is associated with a variety of neurodegenerative disorders, it is possible that the CI-MPR may have a role in regulating neuronal viability after toxicity/injury. In the present study, we report that 192-IgG-saporin-induced loss of basal forebrain cholinergic neurons causes a transient up-regulation of CI-MPR protein levels in surviving neurons of the basal forebrain and frontal cortex but not in the brainstem region, which was relatively spared by the immunotoxin. This was accompanied by a parallel time-dependent increase in other EL markers, ie, cathepsin D, Rab5, and LAMP2 in the basal forebrain region, whereas in the frontal cortex the levels of cathepsin D, and to some extent Rab5, were increased. Given the critical role of the EL system in the clearance of abnormal proteins in response to changing conditions, it is likely that the observed increase in the CI-MPR and components of the EL system in surviving neurons after 192-IgG-saporin treatment represents an adaptive mechanism to restore the metabolic/structural abnormalities induced by the loss of cholin-ergic neurons.

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

Infusion of 192 IgG-saporin (SAP) into the medial septum (MS) of rats selectively destroys cholinergic neurons projecting to the hippocampus and impairs acquisition of a delayed matching to position (DMP) T-maze task. The present study evaluated whether introduction of a mild aversive stimulus 30 min prior to training would attenuate the deficit in DMP acquisition caused by the SAP lesions. Male Sprague-Dawley rats received medial septal infusions of either artificial cerebrospinal fluid or SAP (0.22 microg in 1.0 microl). Fourteen days later, all animals were trained to perform the DMP task. Half of the SAP-treated animals and controls received an intraperitoneal injection of saline each day, 30 min prior to training. Results show that intraperitoneal saline attenuated the impairment in DMP acquisition in SAP lesioned rats. These results suggest that a mild aversive stimulus can attenuate cognitive deficits caused by medial septal cholinergic lesions.

The septohippocampal cholinergic system and spatial working memory in the Morris water maze

The objective of the present study was to determine whether a systematic optimization of Morris water maze (mwm) testing parameters could reveal a significant role of the septohippocampal cholinergic system in spatial working memory. Young adult rats were lesioned using 192 IgG-saporin infused bilaterally into the medial septum. Lesions were near complete as measured by choline acetyltransferase (ChAT) activity and immunohistochemistry. Behavioral testing was performed in three phases. In the first, lesioned and unlesioned rats were trained in the mwm focusing on working memory, which was tested using novel platform locations daily. In the second phase, the optimal locations were retested with increasing intertrial intervals (ITI). In the third phase, intracerebroventricular infusions of nerve growth factor (NGF) were employed to enhance cholinergic activity of the unlesioned rats and potentially further separate group performance. Neither the standard or increased ITI resulted in a consistent significant difference in spatial working memory between groups. In addition, NGF treatment also failed to induce a significant difference in behavioral performance. In conclusion, impairments in working memory as assessed by the mwm could not be revealed despite a greater than 90% loss of hippocampal ChAT and the use of optimal testing parameters and NGF treatment.

Selective loss of basal forebrain cholinergic neurons by 192 IgG-saporin is associated with decreased phosphorylation of Ser glycogen synthase kinase-3beta

Glycogen synthase kinase-3beta (GSK-3beta) is a multifunctional enzyme involved in a variety of biological events including development, glucose metabolism and cell death. Its activity is inhibited by phosphorylation of the Ser9 residue and up-regulated by Tyr216 phosphorylation. Activated GSK-3beta increases phosphorylation of tau protein and induces cell death in a variety of cultured neurons, whereas phosphorylation of phosphatidylinositol-3 (PI-3) kinase-dependent protein kinase B (Akt), which inhibits GSK-3beta activity, is one of the best characterized cell survival signaling pathways. In the present study, the cholinergic immunotoxin 192 IgG-saporin was used to address the potential role of GSK-3beta in the degeneration of basal forebrain cholinergic neurons, which are preferentially vulnerable in Alzheimer's disease (AD) brain. GSK-3beta co-localized with a subset of forebrain cholinergic neurons and loss of these neurons was accompanied by a transient decrease in PI-3 kinase, phospho-Ser473Akt and phospho-Ser9GSK-3beta levels, as well as an increase in phospho-tau levels, in the basal forebrain and hippocampus. Total Akt, GSK-3beta, tau and phospho-Tyr216GSK-3beta levels were not significantly altered in these brain regions in animals treated with 192 IgG-saporin. Systemic administration of the GSK-3beta inhibitor LiCl did not significantly affect cholinergic marker or phospho-Ser9GSK-3beta levels in control rats but did preclude 192-IgG saporin-induced alterations in PI-3 kinase/phospho-Akt, phospho-Ser9GSK-3beta and phospho-tau levels, and also partly protected cholinergic neurons against the immunotoxin. These results provide the first evidence that increased GSK-3beta activity, via decreased Ser9 phosphorylation, can mediate, at least in part, 192-IgG saporin-induced in vivo degeneration of forebrain cholinergic neurons by enhancing tau phosphorylation. The partial protection of these neurons following inhibition of GSK-3beta kinase activity suggests a possible therapeutic role for GSK-3beta inhibitors in attenuating the loss of basal forebrain cholinergic neurons observed in AD.

Loss of forebrain cholinergic neurons and impairment in spatial learning and memory in LHX7-deficient mice

The identification of the genetic determinants specifying neuronal networks in the mammalian brain is crucial for the understanding of the molecular and cellular mechanisms that ultimately control cognitive functions. Here we have generated a targeted allele of the LIM-homeodomain-encoding gene Lhx7 by replacing exons 3-5 with a LacZ reporter. In heterozygous animals, which are healthy, fertile and have no apparent cellular deficit in the forebrain, b-galactosidase activity reproduces the pattern of expression of the wild-type Lhx7 locus. However, homozygous mutant mice show severe deficits in forebrain cholinergic neurons (FCNs), while other classes of forebrain neurons appear unaffected. Using the LacZ reporter as a marker, we show that in LHX7-deficient mice FCN progenitors survive but fail to generate cholinergic interneurons in the striatum and cholinergic projection neurons in the basal forebrain. Analysis of behaviour in a series of spatial and non-spatial learning and memory tasks revealed that FCN ablation in Lhx7 mutants is associated with severe deficits in spatial but only mild impairment of non-spatial learning and memory. In addition, we found no deficit in long-term potentiation in mutant animals, suggesting that FCNs modulate hippocampal function independently of its capacity to store information. Overall our experiments demonstrate that Lhx7 expression is required for the specification or differentiation of cholinergic forebrain neurons involved in the processing of spatial information.

Evaluation of cholinergic markers in Alzheimer's disease and in a model of cholinergic deficit

Cognitive deficits in neuropsychiatric disorders, such as Alzheimer's disease (AD), have been closely related to cholinergic deficits. We have compared different markers of cholinergic function to assess the best biomarker of cognitive deficits associated to cholinergic hypoactivity. In post-mortem frontal cortex from AD patients, acetylcholine (ACh) levels, cholinacetyltransferase (ChAT) and acetylcholinesterase (AChE) activity were all reduced compared to controls. Both ChAT and AChE activity showed a significant correlation with cognitive deficits. In the frontal cortex of rats with a selective cholinergic lesion, all cholinergic parameters measured (ACh levels, ChAT and AChE activities, "in vitro" and "in vivo" basal ACh release) were significantly reduced. AChE activity was associated to ChAT activity, and even more, to "in vivo" and "in vitro" basal ACh release. Quantification of AChE activity is performed by an easy and cheap method and therefore, these results suggest that determination of AChE activity may be used as an effective first step method to evaluate cholinergic deficits.

Time course of behavioral changes following basal forebrain cholinergic damage in rats: Environmental enrichment as a therapeutic intervention

The present experiment was designed to study changes in behavior following immunolesioning of the basal forebrain cholinergic system. Rats were lesioned at 3 months of age by injection of the 192 IgG-saporin immunotoxin into the medial septum area and the nucleus basalis magnocellularis, and then tested at different times after surgery (from days 7-500) on a range of behavioral tests, administered in the following order: a nonmatching-to-position task in a T-maze, an object-recognition task, an object-location task, and an open-field activity test. The results revealed a two-way interaction between post-lesion behavioral testing time and memory demands. In the nonmatching-to-position task, memory deficits appeared quite rapidly after surgery, i.e. at a post-lesion time as short as 1 month. In the object-recognition test, memory impairments appeared only when rats were tested at late post-lesion times (starting at 15 months), whereas in the object-location task deficits were apparent at early post-lesion times (starting from 2 months). Taking the post-operative time into account, one can hypothesize that at the shortest post-lesion times, behavioral deficits are due to pure cholinergic depletion, while as the post-lesion time increases, one can speculate the occurrence of a non-cholinergic system decompensation process and/or a gradual degeneration process affecting other neuronal systems that may contribute to mnemonic impairments. Interestingly, when middle-aged rats were housed in an enriched environment, 192 IgG-saporin-lesioned rats performed better than standard-lesioned rats on both the nonmatching-to-position and the object-recognition tests. Environment enrichment had significant beneficial effects in 192 IgG-saporin-lesioned rats, suggesting that lesioned rats at late post-lesion times (over 1 year) still have appreciable cognitive plasticity.

No facilitation of amphetamine- or cocaine-induced hyperactivity in adult rats after various 192 IgG-saporin lesions in the basal forebrain

Lesions of basal forebrain cholinergic neurons by intracerebroventricular (i.c.v.) injections of 192 IgG-saporin increased the locomotor response to 0.5 and 1.5 mg/kg of D-amphetamine in adult rats [A. Mattsson, S.O. Ogren, L. Olson, Facilitation of dopamine_mediated locomotor activity in adult rats following cholinergic denervation, Exp Neurol. 174 (2002) 96-108.]. In the present study, adult male rats were subjected to bilateral injections of 192 IgG-saporin either into the septum (Sp), the nucleus basalis magnocellularis (Nbm), both structures (SpNbm) or i.c.v. Locomotor activity was assessed in the home cage 23 days after surgery, and, subsequently, thrice after an intraperitoneal injection of D-amphetamine (1 mg/kg) and twice after an injection of cocaine (15 mg/kg). Analysis of AChE-stained material showed that Sp lesions induced preferentially hippocampal denervation, Nbm lesions induced preferentially cortical denervation, while both SpNbm and i.c.v. lesions deprived the hippocampus and the cortex of almost all AChE-positive reaction products. The spontaneous and drug-induced locomotor activity of all lesioned rats did not differ significantly from that of control rats, except in rats subjected to i.c.v. injections, in which the locomotor response was significantly increased after the second administration of cocaine. In addition, in Nbm and SpNbm rats, the locomotor reaction to cocaine was weaker right after the second injection. The present results do not confirm the report by Mattsson et al. on the potentiation of amphetamine-induced locomotion by i.c.v. injections of 192 IgG-saporin, but suggest that cocaine-induced locomotion can be increased by such lesions and, to some respect, attenuated by cholinergic damage in the Nbm.

Reduction in hippocampal cholinergic innervation is unrelated to recognition memory impairment in aged rhesus monkeys

Alterations in the basal forebrain cholinergic system have been widely studied in brain aging and Alzheimer's disease, but the magnitude of decline and relationship to cognitive impairment are still a matter of debate. The rhesus monkey (Macaca mulatta) provides a compelling model to study age-related memory decline, as the pattern of impairment closely parallels that observed in humans. Here, we used antibodies against the vesicular acetylcholine transporter and a new stereological technique to estimate total cholinergic fiber length in hippocampal subregions of behaviorally characterized young and aged rhesus monkeys. The analysis revealed an age-related decline in the length of cholinergic fibers of 22%, which was similar across the hippocampal subregions studied (dentate gyrus granule cell and molecular layers, CA2/3-hilus, and CA1), and across the rostral-caudal extent of the hippocampus. This effect, however, was unrelated to performance on the delayed nonmatching-to-sample task, a test of recognition memory sensitive to hippocampal system dysfunction and cognitive aging in monkeys. These findings indicate that a decline in cholinergic input fails to account for the influence of normal aging on memory supported by the primate hippocampal region.

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.

Involvement of cholinergic neuronal systems in intravenous cocaine self-administration

Recent studies suggest the participation of cholinergic neurons in the brain processes underlying reinforcement. The involvement of cholinergic neurons in cocaine self-administration has been recently demonstrated in studies using muscarinic and nicotinic agonists and antagonists, microdialysis, assessment of choline acetyltransferase activity and acetylcholine (ACh) turnover rates. The present experiment was initiated to identify subsets of cholinergic neurons involved in the brain processes that underlie cocaine self-administration by lesioning discrete populations with a selective neurotoxin. Rats were trained to self-administer cocaine and the cholinergic neurotoxin 192-IgG-saporin or vehicle was then bilaterally administered into the posterior nucleus accumbens (NAcc)-ventral pallidum (VP). The 192-IgG-saporin induced lesions resulted in a pattern of drug-intake consistent with either a shift in the dose intake relationship to the left or downward compared to sham-treated controls. A second experiment used a self-administration threshold procedure that demonstrated this lesion shifted the dose intake relationship to the left compared to the sham-vehicle treated rats. The magnitude and extent of the lesion was assessed by measuring the expression of p75 (the target for 192-IgG-saporin) and choline acetyltransferase (ChAT) in the NAcc, VP, caudate nucleus-putamen (CP) and vertical limb of the medial septal nucleus-diagonal band (MS-DB) of these rats using real time reverse transcriptase-polymerase chain reaction. Significant reductions in gene expression for p75 (a selective marker for basal forebrain cholinergic neurons) and ChAT were seen in the MS-DB and VP while only small decreases were seen in the NAcc and CP of the 192-IgG-saporin treated rats. These data indicate that the overall influence of cholinergic neurons in the MS-DB and VP are inhibitory to the processes underlying cocaine self-administration and suggest that agonists directed toward subclasses of cholinergic receptors may have efficacy as pharmacotherapeutic adjuncts for the treatment of cocaine abuse.

Pretraining or previous non-spatial experience improves spatial learning in the Morris water maze of nucleus basalis lesioned rats

Previous experiments have shown that infusions of ibotenic acid in the nucleus basalis magnocellularis (NBM) induce a strong impairment in spatial navigation for a hidden platform in the Morris water maze. This effect was initially attributed to a cholinergic deficit, but later studies showed that performance level did not correlate with the degree of cholinergic denervation. Therefore, this impairment is due to a combined cholinergic and non-cholinergic deficit. However, it is not clear in which particular processes the NBM is involved. In this study we have evaluated the origin of behavioural impairment in spatial navigation in the water maze after an ibotenic acid-induced lesion of NBM. In the first experiment, Wistar rats were trained preoperatively in an allocentric navigation task. Postoperatively, they were tested in the same task. All lesioned animals showed a performance level similar to controls. Lesions did not impede the acquisition of new positions in the water maze, nor did affect the ability of animals to remember new platform positions after an intertrial interval of 20s, even if animals had received only allocentric experience with the platform position, or allocentric and path integration information concurrently. Lesions also failed to affect the ability to locate a hidden platform in a new environment. However, hippocampal infusions of scopolamine (5 microg) produced a severe impairment in NBM-damaged animals, without impairing performance of controls. In the second experiment Wistar rats with the same lesion were first trained in a visual-guided task in the water maze, and subsequently evaluated in the spatial task. In both tasks lesioned animals were not different from controls. These results suggest that the NBM played an important role during acquisition phases but not in the execution of spatial navigation. Moreover, the excessive emotional response displayed by lesioned animals is postulated as a relevant cause for the impairment observed in spatial navigation after NBM damage.

Age-related vulnerability to diencephalic amnesia produced by thiamine deficiency: the role of time of insult

Age is a risk factor for the development of many neurological disorders, including alcohol-related neurological disorders. A rodent model of Wernicke-Korsakoff Syndrome (WKS), acute pyrithiamine-induced thiamine deficiency (PTD), produces diencephalic damage and impairments of memory similar to what is seen in WKS patients. Advanced age increases the vulnerability to the cascade of acute and some chronic neurological events caused by PTD treatment. Interactions between PTD treatment and age at the time of treatment (3, 10, or 21 months), in addition to the effects of an increased recovery period, were examined relative to spatial memory impairment and neuropathology in Fischer 344 rats. Although acute neurological disturbances and medial thalamic brain lesions were more prevalent in middle-aged and senescent rats exposed to PTD treatment, relative to young rats, behavioral data did not support the view that PTD and aging have synergistic effects. In addition, both advanced age and PTD treatment result in a loss of basal forebrain cholinergic neurons, though there was no interaction. Despite the fact that no convincing evidence was found for an effect of extended recovery time on neuropathology measures, young rats given an extensive recovery period displayed less working memory impairment. In summary, these data provide evidence for an increased susceptibility of the aged rat to the acute neurological consequences and diencephalic pathology associated with PTD treatment and indicated a similar vulnerability of the middle-aged rat. However, the synergistic interaction between aging and PTD treatment in thalamic tissue loss did not express behaviorally.

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

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

Behavioral patterns under cholinergic control during development: lessons learned from the selective immunotoxin 192 IgG saporin

The immunotoxin 192 IgG saporin (192 IgG-sap) offers a valuable tool to investigate the role of the developing basal forebrain cholinergic system in modulating behavioral functions in developing, as well as adult rats. After neonatal 192 IgG-sap lesions, rats display reduced ultrasonic vocalizations as neonates, deficits in passive avoidance learning as juveniles, and altered reactions to spatial novelty as adults. These data suggest that neonatal cholinergic depletion affects cognitive performance in juvenile and adult rats. Additionally, neonatal cholinergic depletion alters ultrasonic vocalizations, which could then alter establishing normal mother-infant relationships, and thus compound the pup's cognitive deficits. These findings underscore the importance of assessing behavior during ontogeny, as well as in adulthood.

A double dissociation between serial reaction time and radial maze performance in rats subjected to 192 IgG-saporin lesions of the nucleus basalis and/or the septal region

The cholinergic basal forebrain has been implicated in aspects of cognitive function including memory and attention, but the precise contribution of its major components, the basalocortical and the septohippocampal systems, remains unclear. Rats were subjected to lesions of either the nucleus basalis magnocellularis (Basalis), the medial septum/vertical limb of the diagonal band of Broca (Septum), or both nuclei (Basalis + Septum), using the selective cholinotoxin 192 IgG-saporin. Cognitive performance was evaluated in tasks taxing attention (the five-choice serial reaction time task, 5-CSRTT) and spatial working memory (radial arm maze, RAM). Nucleus basalis lesions disrupted performance of the 5-CSRTT, as demonstrated by decreased choice accuracy, increased incidence of missed trials, increased latencies to respond correctly, and a disrupted pattern of response control. Combined lesions of the Basalis and Septum resulted in qualitatively similar deficits to Basalis lesions alone, although interestingly, these rats were unimpaired on measures of response speed, and showed weaker deficits on accuracy and omissions. Decreasing the attentional load by lengthening stimulus duration reversed some of the deficits in Basalis and Basalis + Septum rats, suggesting an attentional deficit rather than motivation or motor perturbations. Performance in rats with septal lesions was only affected when task difficulty was increased. In the RAM an opposing pattern of effects was observed, with Septum and Basalis + Septum rats showing dramatic impairments, and Basalis rats performing normally. Taken together, these data provide clear evidence for a functional dissociation between septohippocampal and basalocortical cholinergic systems in aspects of cognitive function.

The role of the septo-hippocampal cholinergic projection in T-maze rewarded alternation

Administration of 192IgG-saporin, a cholinergic neurotoxin, to the medial septum destroys the cell bodies from which the septo-hippocampal cholinergic projection originates, leading to reductions in both hippocampal acetylcholinesterase (AChE) and choline acetyltransferase (ChAT). Despite reports that 192IgG-saporin-induced cholinergic loss leads to post-operative impairments in acquisition and performance of spatial memory tasks, a number of other reports have described intact spatial memory performance following these lesions. Factors that might account for these different outcomes include variations in toxin injection sites or volumes, and post-operative testing at times that might permit regeneration of damaged neuronal processes. We, therefore, assessed the effects of intraseptal microinjection of 192IgG-saporin, in rats, on the post-operative retention of pre-operatively acquired discrete-trial rewarded alternation in the T-maze. This design allowed us to assess the effects of the lesion 7 days post-surgery, at which point, at best, incomplete neuronal regeneration would have been expected to have occurred. The lesion led to a profound loss of hippocampal AChE staining, and a clear inflammatory response, as assessed by proliferation of OX42-stained macrophages in the medial septum and diagonal band nuclei, but there was no impairment in spatial working memory.

Switching memory systems during learning: changes in patterns of brain acetylcholine release in the hippocampus and striatum in rats

This experiment measured acetylcholine (ACh) release simultaneously in the hippocampus and striatum while rats were trained in a cross maze. Consistent with past findings, rats initially showed learning on the basis of place (i.e., turning to the correct position relative to the room), but after extensive training, rats shifted to learning on the basis of response (i.e., turning to the right/left to find the food). Profiles of ACh release in the hippocampus and striatum were markedly different during training. In the hippocampus, ACh release increased by approximately 60% at the onset of training and remained at that level of release throughout training, even after the rats began to show learning on the basis of turning rather than place. In the striatum, increases in ACh release occurred later, reaching asymptotic increases of 30-40%, coincident with a transition from expressing place learning to expressing response learning. These findings suggest that the hippocampal and striatal systems both participate in learning in this task, but in a manner characterized by differential activation of the neural systems. The hippocampal system is apparently engaged first before the striatum is activated and, to the extent the hippocampus is important for place learning, promotes the use of a place solution to the maze. Later in training, although the hippocampus remains activated, the striatum is also activated in a manner that may enable the use of a response strategy to solve the maze. These findings may offer a neurobiological marker of a transition during skill learning from declarative to procedural learning.

192 IgG-saporin lesions to the nucleus basalis magnocellularis (nBM) disrupt acquisition of learning set formation

Rats with bilateral 192 IgG-saporin lesions to the nucleus basalis magnocellularis (nBM) were tested on olfactory discrimination learning set (ODLS), olfactory discrimination reversal learning set (DRLS), and open field activity. Control animals demonstrated learning set in both the ODLS and DRLS tasks. The nBM-lesioned animals showed initial acquisition impairment in learning set in the ODLS task but eventually demonstrated learning set in both ODLS and DRLS tasks. There were no group differences in open-field activity. Results suggest that removal of the nBM cholinergic system through 192 IgG-saporin lesions impairs early acquisition of learning set compared to control animals, but does not prevent later use of learning set formation. Implications for the non-cholinergic basal forebrain cells in learning set are discussed.

Excitotoxic lesioning of the rat basal forebrain with S-AMPA: consequent mineralization and associated glial response

Regional depositions of calcium within the basal ganglia, cortex, cerebellum, and white matter and at perivascular sites have been observed in several pathological conditions. These generally indicate signs of ongoing apoptosis or necrotic processes, whereby the activation of glutamate receptors causes a rise in intracellular calcium levels leading to mineralization of neurons, and ultimately to cell death. The selective degeneration of cholinergic neurons in the basal forebrain is a major neuropathological component of Alzheimer's disease, and may result in abnormal deposition of calcium. In experimental models, selective lesions of the basal forebrain can be induced by intraparenchymal infusions of excito- or immunotoxins targeting cholinergic neurons. Excitotoxic lesions are often accompanied by calcium deposition within affected areas. In a previous study we also noted the presence of unusual deposition in areas close to the site of injections following unilateral S-AMPA-induced lesions of the basal forebrain (T. Perry, H. Hodges, and J. A. Gray, 2001, Brain Res. Bull. 54, 29-48). In this paper, we have characterized these deposits histologically and evaluated the microglial (CD11b) and astrocytic (GFAP) responses at 8 and 16 weeks following lesioning of the nucleus basalis magnocellularis with S-AMPA. The resulting deposits were heterogeneous in morphology and composed primarily of calcium. Small granular deposits were detected around blood vessels, whereas larger calcospherites were situated within the parenchyma. These deposits were more widely dispersed at 16 weeks postlesioning, affected neighboring nuclei, and displayed a progressive increase in size and frequency of occurrence. However, calcification within these regions was differentially associated with microglial and astrocytic reactivity at the two time points. Both microglial and astrocytic responses were pronounced at 8 weeks, whereas at 16 weeks, astrocytic reactivity prevailed and the microglial response was markedly attenuated. Importantly, the pattern of reactivity for microglia detected at 8 weeks was specifically localized to vulnerable nucleated areas prior to their substantial accumulation of calcium deposits, which was clearly evident by 16 weeks. We suggest that the initial microglial response could be used as a selective predictor of tissue necrosis and subsequent calcification, and that astrocytes, which form a glial scar in the affected tissues, may contribute toward the buildup of calcium deposits. The functional relevance of these findings is discussed.

NGF induces appearance of adult-like response to spatial novelty in 18-day male mice

We investigated the effects of Nerve Growth Factor (NGF) administration on the maturation of reactivity to spatial and non-spatial novelty in developing mice. CD-1 mice of both sexes received intracerebral administration of NGF on postnatal day (pnd) 15, and their response to object displacement (spatial novelty) and object substitution (object novelty) were assessed in a spatial open-field with four objects on pnd 18 or 28. On pnd 18, NGF induced only in males precocious appearance of spatial novelty discrimination, while increasing choline acetyltransferase activity in neocortex and hippocampus of both sexes. The behavioral and neurochemical effects disappeared by pnd 28. NGF triggers adult-like responding to spatial novelty in developing mice and such effect is gender-specific.

Protection and reversal of excitotoxic neuronal damage by glucagon-like peptide-1 and exendin-4

Glucagon-like peptide-1 (7-36)-amide (GLP-1) is an endogenous insulinotropic peptide that is secreted from the L cells of the gastrointestinal tract in response to food. It has potent effects on glucose-dependent insulin secretion, insulin gene expression, and pancreatic islet cell formation. In type 2 diabetes, GLP-1, by continuous infusion, can normalize blood glucose and is presently being tested in clinical trials as a therapy for this disease. More recently, GLP-1 has been found to have central nervous system (CNS) effects and to stimulate neurite outgrowth in cultured cells. We now report that GLP-1, and its longer-acting analog exendin-4, can completely protect cultured rat hippocampal neurons against glutamate-induced apoptosis. Extrapolating these effects to a well defined rodent model of neurodegeneration, GLP-1 and exendin-4 greatly reduced ibotenic acid-induced depletion of choline acetyltransferase immunoreactivity in basal forebrain cholinergic neurons. These findings identify a novel neuroprotective/neurotrophic function of GLP-1 and suggest that such peptides may have potential for halting or reversing neurodegenerative processes in CNS disorders, such as Alzheimer's disease, and in neuropathies associated with type 2 diabetes mellitus.

Effect of 192 IgG-saporin on circadian activity rhythms, expression of P75 neurotrophin receptors, calbindin-D28K, and light-induced Fos in the suprachiasmatic nucleus in rats

Photic entrainment of circadian rhythms in mammals is mediated through a direct retinal projection to the core region of the suprachiasmatic nucleus (SCN), the circadian clock. A proportion of this projection contains the low-affinity p75 neurotrophic receptor (p75NTR). Neonatal monosodium glutamate (MSG) treatment, which dramatically reduces p75NTR immunoreactivity in the SCN has no impact on photic entrainment. In order to clarify the contribution of p75NTR fibers in photic entrainment, targeted lesions of the p75NTR-immunoreactive SCN plexus were performed using intracerebroventricular (ICV) or intrahypothalamic injections of the immunotoxin 192 IgG-saporin (SAP) in rats. SAP treatment effectively abolished p75NTR immunoreactivity within the SCN core. ICV SAP treatment produced three different behavioral activity patterns: Animals became arrhythmic, displayed a shorter free-running period, or remained rhythmic following the lesion. Arrhythmic animals had large hypothalamic lesion which encompassed the entire SCN. In rhythmic rats, ICV-SAP significantly reduced immunostaining for calbindin-D28k (CaBP) in the SCN, and rats with shortened free-running periods had the lowest number of CaBP immunoreactive cells. ICV SAP also attenuated light-induced Fos expression in the SCN core. Despite lack of p75NTR and reduced CaBP and Fos expression in the SCN, SAP-treated rhythmic rats displayed normal photic entrainment. Intrahypothalamic SAP treatment reduced CaBP expression in the SCN but had no effect on light-induced Fos expression, free-running rhythms, or photic entrainment. The data show that p75NTR-immunoreactive elements in the SCN are not required for photic entrainment.

Changes in the septohippocampal cholinergic system following removal of molar teeth in the aged SAMP8 mouse

We investigated the effect of dysfunctional teeth on age-related changes in the septohippocampal cholinergic system by assessing acetylcholine (ACh) release and choline acetyltransferase (ChAT) activity in the hippocampus and ChAT immunohistochemistry in the medial septal nucleus and the vertical limb of the diagonal band in young-adult and aged SAMP8 mice after removal of their upper molar teeth (molarless condition). Aged molarless mice showed decreased ACh release and ChAT activity in the hippocampus and a reduced number of ChAT-immunopositive neurons in the medial septal nucleus compared to age-matched control mice, whereas these effects were not seen in young-adult mice. The results suggest that the molarless condition in aged SAMP8 mice may enhance an age-related decline in the septohippocampal cholinergic system.

Effects of 192 IgG-saporin on acetylcholinesterase histochemistry in male and female rats

Sex hormones may exert neuroprotective effects in various models of brain lesions. Male and female Long-Evans rats were subjected to intracerebroventricular injections of 2 microg 192 IgG-saporin or vehicle. Starting 2 days before surgery, half the male rats were treated with estradiol for 7 days. Three weeks after surgery, they were sacrificed for histochemical staining of acetylcholinesterase (AChE) and densitometric evaluations. The lesion induced a substantial to dramatic decrease of the AChE-positive fiber density in the cingulate, somatosensory, piriform, retrosplenial and perirhinal cortices, and in the hippocampus. Weak effects were found in the striatum. There was no significant decrease in the dorsal thalamus. Sex had no significant effect on AChE-positive staining in any brain area. In males, estradiol treatment did not alter the effects of 192 IgG-saporin. These results show that sex or estradiol treatment in male rats does not interfere with the immunotoxic effects of intracerebroventricular injections of 192 IgG-saporin on cholinergic projections from the basal forebrain.

Selective immunolesions of CH4 cholinergic neurons do not disrupt spatial memory in rats

Adult male Long-Evans rats were subjected to bilateral lesions of the cholinergic neurons in the nucleus basalis magnocellularis (NBM) by injection of 0.2 or 0.4 microg 192-IgG-saporin in 0.4 microl phosphate-buffered saline. Control rats received an equivalent amount of phosphate-buffered saline. Starting 2 weeks after surgery, all rats were tested for locomotor activity in their home cage, beam-walking performance, T-maze alternation rates (working memory), reference and working memory performance in a water-maze task, and memory capabilities in the eight-arm radial maze task using uninterrupted and interrupted (delay of 2 min, 2 h and 6 h after four arms had been visited) testing procedures. Histochemical analysis showed a significant decrease of acetylcholinesterase (AChE)-positive reaction products (30-66%) in various cortical regions at the 0.2-microg dose. At the dose of 0.4 microg, there was an additional, although weak, damage to the hippocampus (17-30%) and the cingulate cortex (34%). The behavioral results showed only minor impairments in spatial memory tasks, and only during initial phases of the tests (reference memory in the water maze, working memory in the radial maze). The behavioral effects of the dramatic cholinergic lesions do not support the idea of a substantial implication of cholinergic projections from the NBM to the cortex in the memory processes assessed in this study, but they remain congruent with an involvement of these projections in attentional functions.

Selective behavioral and neurochemical effects of cholinergic lesions produced by intrabasalis infusions of 192 IgG-saporin on attentional performance in a five-choice serial reaction time task

The effects of the cholinergic immunotoxin 192 IgG-saporin (SAP) (0.0, 0.15, or 0.45 microg/microl; 0.5 microl/hemisphere) infused into the area of the nucleus basalis magnocellularis (NBM) of rats were tested in a five-choice serial reaction time task (5CSRTT) designed to assess visual attention. The effects of this manipulation on acetylcholine efflux in the medial frontal cortex were determined using in vivo microdialysis during the 5CSRTT. Rats with extensive lesions of the NBM (SAP HIGH) showed an array of behavioral deficits in the 5CSRTT hypothesized to represent deficits in central executive function that were associated with severe deficits in accuracy. Lengthening the stimulus duration ameliorated these deficits. Rats with restricted lesions of the NBM (SAP LOW) showed impairments over time on task when tested under standard conditions that were exacerbated by increases in the event rate. The number of choline acetyltransferase-immunoreactive cells in the area of the NBM but not the vertical limb of the diagonal band correlated significantly with accuracy in the task. SAP HIGH rats had significantly lower levels of cortical acetylcholine (ACh) efflux relative to SHAM both before and during the 5CSRTT. SAP LOW rats showed significantly higher levels of cortical ACh efflux before but not during the 5CSRTT. Cortical ACh efflux increased in all rats with the onset of the attentional task. These data provide the first direct evidence for a relationship between selective damage in the basal forebrain with decreased cortical ACh efflux and impaired attentional function.

Competition between memory systems: acetylcholine release in the hippocampus correlates negatively with good performance on an amygdala-dependent task

Lesions of the amygdala impair acquisition of a food conditioned place preference (CPP) task. In contrast, lesions of the fornix facilitate acquisition on this task, showing that an intact hippocampal system can interfere with learning an amygdala-dependent task. Our recent findings indicate that acetylcholine (ACh) release in the hippocampus increases while rats perform a hippocampus-dependent spontaneous alternation task. To the extent that ACh output in the hippocampus reflects activation of that brain area in learning and memory, the results obtained with fornix lesions suggest that ACh release in the hippocampus might be negatively correlated with learning on a CPP task. Using in vivo microdialysis, release of ACh was measured in the hippocampus while rats learned and were tested on an amygdala-dependent CPP task and a hippocampus-dependent spontaneous alternation task. Release of ACh in the hippocampus increased when rats were tested on either task. The magnitude of the increase in release of hippocampal ACh was negatively correlated with good performance on the amygdala-dependent CPP task. These findings suggest that ACh release may reflect activation and participation of the hippocampus in learning and memory, but in a manner that can be detrimental to performance on a task dependent on another brain area.