Task-dependent memory loss and recovery following unilateral nucleus basalis lesion: behavioral and neurochemical correlation

Effects of Zizyphus jujube extract on memory and learning impairment induced by bilateral electric lesions of the nucleus Basalis of Meynert in rat

Alzheimer's disease (AD) is a common neurodegenerative condition that affects the elderly population. Its primary symptom is memory loss. The memory dysfunction in AD has been associated with cortical cholinergic deficiency and loss of cholinergic neurons of the nucleus basalis of Meynert (NBM). Zizyphus jujube (ZJ) activates choline acetyltransferase and may have beneficial effects in AD patients. This study investigates the effect of ZJ extract in intact rats and in rat model of AD. 49 male Wistar rats were divided into seven equal groups (1-control, without surgery, received water), 2-AD (bilateral NBM lesion, received water), 3 and 4-AD + ZJ (NBM bilateral lesion, received ZJ extract 500 and 1,000 mg/kg b.w. per day for 15 days), 5-sham (surgery: electrode introduced into NBM without lesion, received water), 6 and 7-without surgery and lesion, received ZJ extract-the same as groups 3 and 4). The learning and memory performance were assessed using passive avoidance paradigm, and the memory cognition for spatial learning and memory was evaluated by Morris water maze. In shuttle box test ZJ extract (500 and 1,000 mg) significantly increased step-through latency in AD + ZJ groups compared with AD group. In Morris water maze test (in probe day), both AD + ZJ groups receiving extract (500 and 1,000 mg) demonstrated significant preference for the quadrant in which the platform was located on the preceding day as compared with AD group. Our results suggested that ZJ has repairing effects on memory and behavioral disorders produced by NBM lesion in rats and may have beneficial effects in treatment of AD patients.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

Effects of electrical stimulation of the nucleus basalis on two-way active avoidance acquisition, retention, and retrieval

This study assessed the role of the nucleus basalis magnocellularis (NBM) in specific memory phases of two-way active avoidance conditioning. We evaluated the effects of NBM electrical stimulation applied during different phases of the avoidance task. Rats were trained in a 30-trial acquisition session, and were tested again 24 and 48 h later. NBM stimulation was applied at different stages of memory formation of the conditioning: (1) immediately before the first training session to determine the effects on acquisition of the two-way avoidance task; (2) immediately after the first training session to evaluate effects on memory consolidation; and (3) immediately before the 24-h retention session to analyze the effects on the retrieval process. NBM stimulation before training significantly improved the acquisition of the task, without affecting subsequent retention at either 24 or 48 h. Stimulation of the NBM immediately after the first training session slightly impaired performance in the 24-h retention session. Stimulation of the NBM immediately before the 24-h retention session did not affect performance in either the 24 or 48-h retention sessions. Therefore, the NBM may play a more important role in acquisition of memory in aversively motivated conditioning tasks than in consolidation or retrieval of such memories. These results are discussed in the context of attention enhancement and cortical and amygdala activation.

Parafascicular electrical stimulation attenuates nucleus basalis magnocellularis lesion-induced active avoidance retention deficit

Previous experiments from our laboratory showed that retention of two-way active avoidance learning is improved by post-training intracranial electrical stimulation (ICS) of the parafascicular nucleus (PF) and impaired by pre-training electrolytic lesions of the nucleus basalis magnocellularis (NBM). The question investigated here was whether post-training PF ICS is able to attenuate the active avoidance retention deficit observed in rats lesioned pre-training in the NBM. To this goal, the following experimental design was used: rats bilaterally lesioned in the NBM and stimulated in the PF, rats lesioned in the NBM, rats stimulated in the PF, control rats implanted in the PF, and sham-operated rats were first trained in a shuttle-box for a single 30-trial session and tested again following two successive retention intervals (24 h and 11 days). The results showed that: (1) NBM lesions impaired the 11-day performance without affecting either the acquisition or the 24-h retention of the avoidance learning; (2) PF ICS treatment in unlesioned rats improved performance in both retention sessions only when the stimulation was applied in the posterior region of the nucleus; and (3) stimulation of the posterior PF compensated the 11-day retention impairment induced by NBM lesions. These results are discussed in relation to the interaction of arousal systems in the modulation of cognitive processes.

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

Progressive brain dysfunction following intracerebroventricular infusion of beta(1-42)-amyloid peptide

The behavioral, neurochemical and histological changes of rats subjected to 3 days treatment with intracerebroventricular infusion of beta-amyloid peptides(Abeta)(1-42) were investigated 20 days and 80 days after the surgery. Abeta(1-42) produced a dose-dependent and a time-dependent impairment in the spontaneous alternation performance in the Y-maze (spatial working memory), place navigation task in a water maze (spatial reference memory) and passive avoidance retention (non-spatial long-term memory) at doses of 10 and 20 microg/rat. The learning impairments were more severe at 80 days than 20 days after infusion of Abeta(1-42). At 25 days after the infusion, a significant decrease in hemicholinium-3 (HC-3) binding was observed only in the hippocampus, although choline acetyltransferase (ChAT) activity was unchanged in the brain regions tested as compared with the vehicle (Abeta(40-1)) treatment. In contrast, the reduction in ChAT activity 85 days after Abeta(1-42) infusion was significant in hippocampus and striatum. HC-3 binding was also significantly decreased in the posterior cortex, hippocampus and striatum. In the histological analysis, brain atrophy was observed inasmuch as ventricular enlargement and neuronal damage in the CA1 area of the hippocampus were seen 85 days after Abeta(1-42) infusion. These results suggest that the rats subjected to intracerebroventricular infusion of Abeta(1-42) suffered from progressive brain dysfunction, and could be useful as an animal model for evaluating the developmental processes at the early and/or middle stage of Alzheimer's-type dementia.

Behavioral and morphological consequences of primary astrocytes transplanted into the rat cortex immediately after nucleus basalis ibotenic lesion

Adult male rats received transplants of dissociated 30-day old cultured cortical astrocytes into the ipsilateral frontal and parietal cortex immediately after unilateral ibotenic acid lesion of the NBM or after sham injury. We hypothesized that transplants of astrocytes into the acetylcholine-deprived cortex might provide trophic support to terminals arising from damaged NBM neurons. Twenty four hours after transplantation and every other day for 11 days post surgery, the animals were tested for locomotion and habituation in an open field. NBM lesion reduced vertical movements only as compared to no lesion and no transplant counterparts. Nine days after surgery rats with NBM lesion and astrocyte-transplants into the cortex were as impaired in the acquisition of a passive avoidance (PA) task as untreated counterparts. Animals with no lesions and transplants into the cortex also had significant PA acquisition deficits. All rats with ibotenic lesion were significantly impaired on PA retention as compared to rats with no lesions. Astrocyte-transplants survived up to 2 months after cortical implantation but these transplants produced severe laminar disruption and gliosis. This effect was greater in rats with NBM lesion than in intact animals with transplants into the cortex. These data show that astrocyte-transplants do not promote functional recovery after NBM lesion and suggest an immune rejection of the astrocyte transplants by the host brain.

The H1- and H2-histamine blockers chlorpheniramine and ranitidine applied to the nucleus basalis magnocellularis region modulate anxiety and reinforcement related processes

This study examined the effects of the H1-antagonist chlorpheniramine and the H2-antagonist ranitidine on reinforcement and anxiety-parameters following unilateral injection into the vicinity of the nucleus basalis magnocellularis (NBM). In Experiment 1, rats with chronically implanted cannulae were injected with chlorpheniramine or ranitidine (each at doses of 0.1, 1, 10 and 20 microg) and were placed into one of four restricted quadrants of a circular open field (closed corral) for a single conditioning trial. During the test for conditioned corral preference, when provided a choice between the four quadrants, only those rats injected with 10 or 20 microg chlorpheniramine spent more time in the treatment corral, indicative of a positively reinforcing action. None of the other doses of chlorpheniramine or of the H2-antagonist influenced rats' preference behavior. In Experiment 2, the elevated plus-maze (EPM) was used to gauge possible anxiolytic or anxiogenic effects of intra-basalis injection of chlorpheniramine or ranitidine (each at doses of 0.1, 1, 10 and 20 microg). A single injection of chlorpheniramine at 0.1 or 20 microg as well as ranitidine at 20 microg was found to exert anxiolytic-like effects in the EPM. Both compounds elevated the time spent on the open arms and increased scanning over the edge of an open arm. None of the other doses of the H1- and H2-antagonist influenced rats' behavior in the EPM. In sum, these findings show that H1- and H2-receptor antagonists differentially modulate reinforcement and fear-related processes in the NBM and thus, provide the first evidence for a behavioral relevance for the histaminergic innervation of this brain site.

Effect of Trasina, an ayurvedic herbal formulation, on experimental models of Alzheimer's disease and central cholinergic markers in rats

Trasina is a herbal formulation of some Indian medicinal plants classified in Ayurveda, the classic Indian system of medicine, as Medhyarasayanas or drugs reputed to improve memory and intellect. Earlier experimental and clinical investigations have indicated that the formulation has a memory-facilitating action. In this investigation, the effect of Trasina, after subchronic administration for 21 days, was assessed on two rodent models simulating some biochemical features known to be associated with Alzheimer's disease (AD). The models, in rats, included intracerebroventricularly (i.c.v.) administered colchicine (15 micrograms/rat) and lesioning of nucleus basalis magnocellularis (nbm) by ibotenic acid (10 micrograms/rat). Retention of an active avoidance response was used as the memory parameter. In addition, the effect of Trasina was evaluated on i.c.v. colchicine-induced depletion of acetylcholine (ACh) concentrations, reduction in choline acetyltransferase (ChAT) activity, and decrease in muscarinic cholinergic receptor (MCR) binding in rat brain frontal cortex and hippocampus. The behavioral and biochemical investigations were done 7, 14, and 21 days after colchicine or ibotenic acid lesioning. Trasina (200 and 500 mg/kg) was administered orally (p.o.) once daily for 21 days, the first drug administration being given just prior to lesioning. Colchicine and ibotenic acid induced marked retention deficit of active avoidance learning that was attenuated in a dose-dependent manner by Trasina after 14 and 21 days of treatment. Frontal cortical and hippocampal ACh concentrations, ChAT activity and MCR binding was significantly reduced after colchicine treatment. Trasina (200 and 500 mg/kg) reversed these deficits after 14 and 21 days of treatment. The findings indicate that the herbal formulation exerts a significant nootropic effect after subchronic treatment that may be due to reversal of perturbed cholinergic function.

Simple and configural association learning in rats with bilateral quisqualic acid lesions of the nucleus basalis magnocellularis

We hypothesized that bilateral quisqualic acid lesions of the nucleus basalis magnocellularis (NBM) in rats would impair configural but not simple association learning. In experiment 1, rats were tested in a negative patterning operant discrimination where they were food-reinforced for responding to a light or a tone (L+, T+) but not for responding to the configural stimulus consisting of the light and tone presented simultaneously (LT-). Consistent with our hypothesis, NBM-lesioned rats showed a transient but significant impairment, responding normally to L+ and T+ but responding more often to LT-, in addition to responding more often during the inter-trial interval (ITI) than controls. In experiment 2, rats were tested in a simple operant discrimination where rats were food-reinforced for responding to a light (L+) but not for responding to a tone (T-). Although NBM-lesioned rats again responded normally to L+ as predicted, NBM-lesioned rats were transiently impaired, making more T- responses and more ITI responses than controls. Together, these results suggest that the NBM is involved in both configural and simple association learning but that this involvement is limited to learning to withhold responding to non-reinforced contextual or discrete stimuli. Finally, rats from experiment 2 underwent extinction trials, where results showed no difference between NBM-lesioned and control groups, suggesting that the NBM is not involved in the extinction of conditioned responding to previously reinforced stimuli.

Astrocytes grafted into rat nucleus basalis magnocellularis immediately after ibotenic acid injection fail to survive and have no effect on functional recovery

In order to determine if the "trophic" properties of astrocytes makes them appropriate for use as a therapeutic agent to excitotoxic brain damage, adult male rats received grafts of cultured cerebral cortical astrocytes into the NBM immediately after infusion of ibotenic acid into the same structure. Twenty four hours after grafting and every other day for 11 days post surgery, the animals were tested for locomotor activity and habituation in an open field. Animals with NBM lesions had significantly reduced rearing activity as compared to counterparts with no lesions. Nine days after surgery, rats with NBM lesions and astrocyte grafts were as impaired in the acquisition of passive avoidance (PA) as their untreated counterparts. All animals with ibotenic lesions were impaired on PA retention compared to rats with no lesions. There was no difference between animals that had received grafts and those that had not. Fourteen days after grafting, all brains were processed for Nissl stain, acetylcholinesterase (AChE) histochemistry, GFAP immunocytochemistry, and bisbenzamide fluorescent microscopy. Decreases in the number of neurons in the NBM as well as decreases in the density of AChE staining in the ipsilateral cortex (the area of innervation of the NBM cholinergic neurons) was evident in all animals with NBM lesions. In addition, a large number of host reactive astrocytes were seen within the NBM, its vicinity, and in the ipsilateral neocortex. Grafted astrocytes survived and integrated into the host tissue when they were grafted into the brain of intact animals but no living grafted astrocytes were found in animals injected with ibotenate. In this latter case, two weeks after grafting, instead of surviving astrocytes only fluorescent tissue 'masses' were seen in the NBM, surrounded by a cavity. Grafted astrocytes did not have any effect on the extension of the lesion caused by ibotenic acid infusion. These results suggest that the concentration of ibotenic acid used to injure the NBM killed not only the host cholinergic neurons but also the grafted astrocytes. The failure of astrocytes to ameliorate the behavioral deficits caused by ibotenic acid lesions of the NBM may be due to the ibotenic acid creating a lethal environment for the grafted and freshly dissociated, cultured astrocytes.

Induction of a physiological memory in the cerebral cortex by stimulation of the nucleus basalis

Auditory cortical receptive field plasticity produced during behavioral learning may be considered to constitute "physiological memory" because it has major characteristics of behavioral memory: associativity, specificity, rapid acquisition, and long-term retention. To investigate basal forebrain mechanisms in receptive field plasticity, we paired a tone with stimulation of the nucleus basalis, the main subcortical source of cortical acetylcholine, in the adult guinea pig. Nucleus basalis stimulation produced electroencephalogram desynchronization that was blocked by systemic and cortical atropine. Paired tone/nucleus basalis stimulation, but not unpaired stimulation, induced receptive field plasticity similar to that produced by behavioral learning. Thus paired activation of the nucleus basalis is sufficient to induce receptive field plasticity, possibly via cholinergic actions in the cortex.

Spatial memory disturbance after focal cerebral ischemia in rats

We investigated the impairment of retention of spatial memory in rats with chronic focal cerebral ischemia, and examined the correlation between this impairment and pathological outcomes. A preoperative acquisition trial of the Morris water-maze task was performed twice a day for 14 successive days, and then the middle cerebral artery (MCA) was occluded. A retention trial was performed 8 weeks after MCA occlusion. Escape latency and swimming path length to the platform were significantly increased in MCA-occluded rats compared with those of sham-operated rats, and these deficits significantly correlated with massive shrinkage of the brain. Retention latency of a passive avoidance task, which was trained preoperatively, was also significantly shortened in MCA-occluded rats compared with that in sham-operated rats. These results suggest that chronic focal cerebral ischemia causes prolonged spatial memory disturbance in rats and is associated with pathological changes, and that this rat model may be useful for assessing not only anterograde but also retrograde amnesia caused by focal cerebral ischemia.

Differential changes in cholinergic markers from selected brain regions after specific immunolesion of the rat cholinergic basal forebrain system

The aim of this study was to characterize the effects of cortical cholinergic denervation on cholinergic parameters in the cerebral cortex and basal forebrain using a novel immunotoxin (conjugate of the monoclonal antibody 192IgG against the low-affinity nerve growth factor receptor armed with cytotoxin saporin) to efficiently and selectively lesion cholinergic neurons in rat basal forebrain. Seven days following an intracerebroventricular injection of the cholinergic immunotoxin 192IgG-saporin the binding levels of nicotinic and M1- and M2-muscarinic acetylcholine receptors (mAChR), high-affinity choline uptake sites, as well as the m1-m4 mAChR mRNA were determined in coronal brain sections by both receptor autoradiography and in situ hybridization, and quantified by image analysis. Hemicholinium-3 binding to high-affinity choline uptake sites was decreased by up to 45% in all cortical regions and in the hippocampus after a single injection of the immunotoxin compared to controls. In contrast, M1-mAChR sites were increased over the corresponding control value in the anterior parts of cingulate, frontal, and piriform cortex by about 20%, in the hindlimb/forelimb areas (18%), in the parietal cortex (35%), in the occipital cortex area 2 (17%), as well as in the temporal cortex (25%) following immunolesion. M2-mAChR levels were found to be significantly increased in the posterior part of the parietal cortex area 1 (by about 22%) and in the occipital cortex area 2 (20%) only. With respect to laminar cortical localization, M2-mAChRs and choline uptake sites were altered in all cortical layers, whereas M1-mAChRs were preferentially affected in the upper cortical layers by the immunolesion. The increase in M1-mAChR binding in the temporal and occipital cortex as a consequence of the immunolesion was complemented by an increase in the amount of m1 and m3 mAChR mRNA of about 20% in these regions. The elevated levels of M2-mAChR sites in the occipital and temporal cortex following immunolesion were accompanied by an increase in the m4 (by 25%) but not m2 mAChR mRNA. There was no effect of the immunolesion on the m1-m4 mAChR mRNA in frontal cortical regions. in the basal forebrain, however, immunolesioning caused about a 40% decrease in the level of m2 mAChR mRNA in the medial and lateral septum as well as in the vertical and horizontal limb of the diagonal band, whereas M1- and M2-mAChR binding and the levels of m1, m3, and m4 mAChR mRNA were not affected by the immunolesion in any of the basal forebrain nuclei studied.(ABSTRACT TRUNCATED AT 400 WORDS)

Ibotenic acid lesion of nucleus basalis magnocellularis differentially affects cholinergic, glutamatergic and GABAergic markers in cortical rat brain regions

The present study was undertaken to study the effect of reduced cortical cholinergic activity on gamma-aminobutyric acid (GABA)ergic and glutamatergic mechanisms in cholinoceptive cortical target regions which are assumed to play an important role for realizing cognitive functions. The densities of cortical muscarinic cholinergic receptor subtypes and corresponding receptor genes m1 through m4, N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) and kainate glutamate receptor subtypes as well as GABAA and benzodiazepine receptors were measured in rats 1 week after unilateral ibotenic acid lesion of the nucleus basalis magnocellularis (Nbm) applying quantitative receptor autoradiography and in situ hybridization. Ibotenic acid lesion resulted in a striking loss of acetylcholinesterase (AChE) staining in the lesioned Nbm which is associated with a 60% decrease in AChE staining and a 30% reduction in [3H]hemicholinium-3 binding in frontal and parietal cortical regions as well fore- and hindlimb areas ipsilateral to the lesion, being more prominent in the more rostral cortical regions. M1-muscarinic cholinergic receptor binding was not changed in any of the cortical regions studied 1 week after lesion. M2-muscarinic receptor binding levels are slightly increased in the parietal cortex only. The lesion-induced increase in parietal cortical M2-muscarinic receptor binding is complemented by an increase in the hybridization signal for the corresponding m4-mRNA transcript. In cortical regions displaying a reduced activity of AChE and decreased levels of high-affinity choline uptake sites due to forebrain cholinergic lesion, NMDA receptor binding was markedly reduced in comparison to the unlesioned brain side whereas AMPA and kainate binding has been significantly increased in these regions. Muscimol binding to GABAA receptors was increased in the rostral portions of frontal and parietal cortices as compared with the unlesioned brain side. Binding levels of benzodiazepine receptors were not affected by the lesion in any of the cortical regions studied. The differential changes in glutamate and GABA receptor subtypes following lesion might be regarded as the consequence of a cortical reorganization compensating for the reduced cholinergic presynaptic input. The data further suggest that presynaptic cortical cholinergic deficits might affect both glutamatergic and GABAergic functions with different intensity and different directions.

Receptor function in cortical rat brain regions after lesion of nucleus basalis

The present study was undertaken to study the interaction of cholinergic and glutamatergic mechanisms in cholinoceptive cortical target regions which is assumed to play an important role for realizing cognitive functions. The densities of cortical muscarinic cholinergic receptor subtypes and corresponding receptor genes m1 through m4, as well as NMDA, AMPA and kainate glutamate receptor subtypes were measured in rats one week after unilateral mechanical lesion of the anterior part of the nucleus basalis magnocellularis (NbM) applying quantitative receptor autoradiography and in situ hybridization. The studies revealed that in cortical regions displaying a low amount of acetylcholinesterase activity due to forebrain cholinergic lesion, NMDA receptor binding was markedly reduced in comparison to the unlesioned side, whereas AMPA and kainate binding has been significantly increased in these regions. M1-muscarinic cholinergic receptor binding was not changed in any of the cortical regions studied, whereas M2-receptor densities are slightly reduced in frontal and parietal cortices following lesion. These alterations in cortical M2-muscarinic receptor binding are complemented by corresponding changes in the m2- and m4-mRNA transcripts. The comparison of binding profiles through selected cortical regions of both lesioned and normal brain side revealed that lesion of the NbM affects NMDA receptors in all cortical layers of the lesioned side, whereas AMPA receptors are affected preferentially in the upper and kainate receptors preferentially in the middle and deeper cortical layers. The differential changes in glutamate receptor subtypes following lesion might be regarded as the consequence of a cortical reorganization compensating for the reduced cholinergic presynaptic input. The data further suggest that presynaptic cortical cholinergic deficits might affect glutamatergic functions with different intensity and different directions.

Recovery of choline acetyltransferase activity without sprouting of the residual acetylcholine innervation in adult rat cerebral cortex after lesion of the nucleus basalis

In view of the divergent literature concerning the long-term effects of ibotenic acid lesions of the nucleus basalis of Meynert (NBM) on the choline acetyltransferase (ChAT) activity in adult rat cerebral cortex, we have critically reassessed the issue of an eventual recovery of this enzymatic activity by sprouting of the residual acetylcholine (ACh) innervation. At short (1 week) and long survival time (3 months) after unilateral ibotenic acid lesion, ChAT activity was biochemically measured in the ipsi and contralateral fronto-parietal cortex of several rats in which the extent of ACh neuronal loss in NBM was also estimated by counts of ChAT-immunostained cell bodies on the lesioned vs. non-lesioned side. In other lesioned rats, particular attention was paid to the distribution of the residual cortical ACh (ChAT-immunostained) innervation, and that of immunostained vasoactive intestinal polypeptide (VIP) axon terminals known to belong in part to intrinsic cortical ACh neurons which co-localize this peptide. One week after NBM lesion, profound decreases of ipsilateral cortical ChAT activity were tightly correlated with the extent of ACh cell body loss in the nucleus. A significant recovery of cortical ChAT activity could be documented after 3 months, despite persistence of NBM cell body losses as severe as after 1 week. At both survival times, the number of ChAT-immunostained axons was markedly reduced throughout the ipsilateral fronto-parietal cortex, demonstrating that most ACh fibers of extrinsic origin had been permanently removed. This result also indicated that the long-term recovery of ChAT activity had occurred without sprouting of the residual ACh innervation. The laminar distribution and number of VIP-immunostained terminals remained the same on the lesioned and intact side and comparable to normal, ruling out an extensive sprouting of intrinsic ACh/VIP or VIP alone fibers. The return to a near normal cortical ChAT activity in severely ACh-denervated cortex suggested that the intrinsic ACh innervation was primarily responsible for this recovery.

Brief exposure to an enriched environment improves performance on the Morris water task and increases hippocampal cytosolic protein kinase C activity in young rats

This study was designed to determine whether brief exposure to an enriched environment around the time of weaning would affect learning and memory processes in young rats. In addition, this study sought to determine if experience in an enriched environment would alter hippocampal protein kinase C (PKC) which is thought to be a possible neural substrate that underlies learning and memory processes. Animals were either reared in an enriched environment or standard laboratory cages starting at 15 days old. After 6 (21 days old) or 12 (27 days old) days subjects were either tested in the Morris water task, or had the hippocampus removed for biochemical analysis of PKC activity. Morris water task results showed that compared to laboratory reared controls, the performance of subjects reared in the enriched environment for 12 days, but not 6 days, was improved. In addition, 12 days of exposure to the enriched environment, but not 6 days, produced more cytosolic hippocampal PKC activity. The particulate fraction appeared not to be affected by rearing in the enriched environment. Brief exposure to an enriched environment around weaning, therefore, both improved Morris water task performance and increased hippocampal PKC activity. These outcomes suggest that performance in the Morris water task and hippocampal PKC may be functionally related.

Age-dependent cerebral metabolic effects of unilateral nucleus basalis magnocellularis ablation in rats

To investigate the age-dependent functional importance of cholinergic neocortical inputs, and to explore whether cortical cholinergic denervation in aged animals might better model the cerebral metabolic changes of Alzheimer's disease, the effects of unilateral ablation of the nucleus basalis magnocellularis (NBM) on cerebral glucose metabolism were studied in young and aged rats. Regional cerebral metabolic rates for glucose (rCMRglc) were determined, using the [14C]deoxyglucose method, in 48 brain regions of 3- and 24-month old Fischer-344 rats at 3, 7, 14 and 28 days after stereotaxic injection of ibotenate into the right NBM, and in sham-operated animals at 3 and 14 days later. For both ages the peak effect of unilateral NBM ablation occurred 3 days later: in young rats, rCMRglc was significantly reduced (compared to the contralateral side) in all 24 anterior cortical areas examined (mean decline 20%), whereas in aged animals, only 9 of 24 areas showed a significant decline in glucose utilization, and the magnitude of rCMRglc reduction (9%) was smaller. Near complete recovery of rCMRglc occurred by 7 days in young and old rats. We conclude that the basalocortical cholinergic projection plays a smaller role in neocortical function of aged rats, possibly because its tonic activity is reduced. Both young and aged rats undergo cortical metabolic normalization after unilateral NBM ablation; hence the NBM-lesioned aged rat is not a better model of the progressive decline in rCMRglc that occurs in Alzheimer's disease.

Behavioural, biochemical and histological effects of AF64A following injection into the third ventricle of the mouse

Behavioural, biochemical and histological effects were assessed following AF64A injected into the third ventricle of female NMRI mice. Doses from 3 to 7 nmol produced significant changes in behaviour, causing hyperactivity, reduced hole-board exploration, rotational behaviour in a symmetrical Y-maze corresponding to a loss of alternation, abnormal behaviour in a plus-maze task of fear/anxiety with markedly increased exploration of the open arms and finally deficits in passive avoidance responding and spatial orientation in a Morris-type water maze. In this latter test, a cue learning deficit was noted for the two highest doses only. No histological changes of consequence were observed up to 5 nmol. Beyond this dose, at 6 and particularly 7 nmol, necrosis of parts of the hippocampus and septum was apparent. ChAT and AChE activity were decreased in the hippocampus but not in the cortex although the decreases were smaller than generally reported for AF64A-treated rats. ChAT and AChE reductions correlated highly with hyperactivity in the open-field and to a lesser extent, with spatial learning deficits. Monoaminergic activity was also affected in the hippocampus, but not in the cortex, at 4 nmol and above. NE and particularly 5-HT and 5-HIAA levels were reduced although the rate of 5-HT turnover was unaltered. A highly significant correlation was obtained between 5-HT effects and the increased open arm exploration in the plus-maze task of fear/anxiety. The behavioural effects and biochemical changes lasted at least 8-9 weeks postop.

Learning deficits after unilateral AF64A lesions in the rat basal forebrain: role of cholinergic and noncholinergic systems

Rats were given unilateral infusions of ethylcholine aziridinium ion (AF64A) into the basal forebrain (BF). BF-lesioned rats had significant acquisition and retention deficits in two different types of learning tasks (water maze and active avoidance). Choline acetyltransferase activity was lower than control in the frontal cortex but not in the hippocampus or striatum. AF64A markedly reduced the levels of norepinephrine, dopamine, and serotonin in all brain regions studied. However, L-glutamic acid decarboxylase activity was not altered by AF64A injection. Cholinergic agents (physostigmine and arecoline) ameliorated the AF64A-induced learning deficits in the water maze task but not in the active avoidance task. Noncholinergic agents (desipramine and L-dopa) ameliorated the AF64A-induced avoidance deficits in the active avoidance task but not in the water maze task. 5-Methoxy-N,N-dimethyltryptamine did not improve either active avoidance or water maze learning. These results suggest that intra-BF injection of AF64A produces extensive brain dysfunction and that different neuronal systems are involved in associative and spatial learning.

GM1 ganglioside effects on astroglial response in the rat nucleus basalis magnocellularis and its cortical projection areas after electrolytic or ibotenic lesions

We examined the effects of chronic GM1 ganglioside injections on the astroglial response to bilateral electrolytic or ibotenic acid lesions in the nucleus basalis magnocellularis (NBM) within the NBM and in three cortical projection areas of NBM neurons. Glial fibrillary acidic protein (GFAP) immunohistochemistry was used to visualize the reactive astrocytes. Twenty-six days after injury, extensive astrogliosis was observed within the NBM after both types of lesions. An increased number of GFAP-positive cells were found in the cortex of saline-treated rats following electrolytic but not ibotenic lesions. We suggest that the loss of fibers of passage within the lesion area may account for the difference in cortical gliosis following the two types of damage. Although 17 days of GM1 injections did not affect astrocyte morphology within the NBM, ganglioside treatment reduced the number of GFAP-positive cells after electrolytic but not after ibotenic lesions. Within the cortex, a decrease in GFAP immunoreactivity, size, and number of astrocytes was only observed after electrolytic lesion. These data indicate that a decrease in the astroglial response to injury is the result of an interaction between the type of injury (electrolytic lesion) and chronic GM1 treatment.

Dissociable effects on spatial maze and passive avoidance acquisition and retention following AMPA- and ibotenic acid-induced excitotoxic lesions of the basal forebrain in rats: differential dependence on cholinergic neuronal loss

Excitotoxic lesions of the basal forebrain were made by infusing either alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) or ibotenic acid. Acquisition and performance of spatial learning in the Morris water maze, over a ten day, two trials per day, training regimen were unaffected by the AMPA-induced lesions which reduced cortical choline acetyltransferase activity by 70%. However, acquisition was significantly impaired in rats with ibotenic acid-induced lesions that reduced cortical choline acetyltransferase by 50%. Additionally, ibotenic acid-lesioned rats swam further than either sham or AMPA-lesioned rats, in the "training" quadrant during a probe trial, in which the escape platform was removed, suggesting a perseverative search strategy. Lesions induced with AMPA, but not ibotenate, significantly impaired the acquisition of "step-through" passive avoidance. Both AMPA- and ibotenate-induced lesions significantly impaired the 96 h retention of passive avoidance, but the effect of AMPA was greater on latency measures. Histological analysis revealed that AMPA infusions destroyed more choline acetyltransferase-immunoreactive neurons than did ibotenate infusions but, unlike ibotenate, spared the overlying dorsal pallidum and also parvocellular, non-choline acetyltransferase-immunoreactive neurons in the ventral pallidal/substantia innominata region of the basal forebrain. The impairment in acquisition of the water maze following ibotenate-induced basal forebrain lesions therefore appears unrelated to damage to cholinergic neurons of the nucleus basalis of Meynert and to depend instead on damage to pallidal and other neurons in this area. The AMPA- and perhaps also the ibotenate-induced impairment in the retention of passive avoidance appears to be more directly related to destruction of cholinergic neurons of the nucleus basalis. These data are discussed in the context of cortical cholinergic involvement in mnemonic processes.