Basal forebrain lesions produce a dissociation of trial-dependent and trial-independent memory performance

Tests for learning and memory in rodent regulatory studies

For decades, regulatory guidelines for safety assessment in rodents for drugs, chemicals, pesticides, and food additives with developmental neurotoxic potential have recommended a single test of learning and memory (L&M). In recent years some agencies have requested two such tests. Given the importance of higher cognitive function to health, and the fact that different types of L&M are mediated by different brain regions assessing higher functions represents a step forward in providing better evidence-based protection against adverse brain effects. Given the myriad of tests available for assessing L&M in rodents this leads to the question of which tests best fit regulatory guidelines. To address this question, we begin by describing the central role of two types of L&M essential to all mammalian species and the regions/networks that mediate them. We suggest that the tests recommended possess characteristics that make them well suited to the needs in regulatory safety studies. By brain region, these are (1) the hippocampus and entorhinal cortex for spatial navigation, which assesses explicit L&M for reference and episodic memory and (2) the striatum and related structures for egocentric navigation, which assesses implicit or procedural memory and path integration. Of the tests available, we suggest that in this context, the evidence supports the use of water mazes, specifically, the Morris water maze (MWM) for spatial L&M and the Cincinnati water maze (CWM) for egocentric/procedural L&M. We review the evidentiary basis for these tests, describe their use, and explain procedures that optimize their sensitivity.

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.

Sexually dimorphic effects of hippocampal cholinergic deafferentation in rats

To determine whether the basal forebrain-hippocampal cholinergic system supports sexually dimorphic functionality, male and female Long-Evans rats were given either selective medial septum/vertical limb of the diagonal band (MS/VDB) cholinergic lesions using the neurotoxin 192 IgG-saporin or a control surgery and then postoperatively tested in a set of standard spatial learning tasks in the Morris water maze. Lesions were highly specific and effective as confirmed by both choline acetyltransferase/parvalbumin immunostaining and acetylcholinesterase histochemistry. Female controls performed worse than male controls in place learning and MS/VDB lesions failed to impair spatial learning in male rats, both consistent with previous findings. In female rats, MS/VDB cholinergic lesions facilitated spatial reference learning. A subsequent test of learning strategy in the water maze revealed a female bias for a response, relative to a spatial, strategy; MS/VDB cholinergic lesions enhanced the use of a spatial strategy in both sexes, but only significantly so in males. Together, these results indicate a sexually dimorphic function associated with MS/VDB-hippocampal cholinergic inputs. In female rats, these neurons appear to support sex-specific spatial learning processes.

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

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

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.

Neural Transplantation in Animal Models of Dementia

Neural transplantation provides a powerful novel technique for investigating the neurobiological basis and potential strategies for repair of a variety of neurodegenerative conditions. The present review considers applications of this technique to dementia. After a general introduction (section 1), attempts to replace damaged neural systems by transplantation are considered in the context of distinct animal models of dementia. These include grafting into aged animals (section 2), into animals with neurotransmitter-selective lesions of subcortical nuclei, in particular involving basal forebrain cholinergic systems (section 3), and into animals with non-specific lesions of neocortical and hippocampal systems (section 4). The next section considers the alternative use of grafts as a source of growth/trophic factors to inhibit degeneration and promote regeneration in the aged brain (section 5). Finally, a number of recent studies have employed transplanted tissues to model and study the neurodegenerative processes associated with ageing and Alzheimer's disease taking place within the transplant itself (section 6). It is concluded (section 7) that although neural transplantation does not offer any immediate prospect of therapeutic repair in clinical dementia, the technique does offer a powerful neurobiological tool for studying the neuropathological processes involved in both spontaneous degeneration and specific diseases of ageing. New understandings derived from neural transplantation may be expected to lead to rational development of novel strategies to inhibit the neurodegenerative process and to promote regeneration in the aged brain.

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.

Nucleus basalis lesions: implication of basoamygdaloid cholinergic pathways in memory

Previous studies have shown a lack of association between cortical choline acetyltransferase (ChAT) activity and severity of memory impairment following excitotoxic lesions of the nucleus basalis magnocellularis (NBM). It recently has been proposed that the differential effects of NBM injections of various excitotoxins on amygdaloid and cortical ChAT may explain this result. The present study evaluated the mnemonic effect of unilateral intra-NBM infusions of the excitotoxins phthalic acid and quisqualic acid, which decrease ChAT activity primarily in the amygdala and cortex, respectively. Rats were trained in a double Y-maze, lesioned, and allowed to recover for 1 week prior to memory assessment. Behavioral results showed impaired working but not reference memory following phthalic acid lesions, and no significant effect following quisqualic acid lesions. Biochemical analysis in a second group of subjects confirmed that phthalic acid lesions produced a large decrease in basolateral amygdaloid ChAT, but had little effect on cortical ChAT activity. Conversely, quisqualic acid lesions produced a large decrease in cortical, but not basolateral amygdaloid, ChAT activity. These results suggest that the NBM amygdalopetal cholinergic pathways play a role in mnemonic functioning.

Spatial working memory over long retention intervals: dependence on sustained cholinergic activation in the septohippocampal or nucleus basalis magnocellularis-cortical pathways?

Previous direct neurochemical studies of the temporal dynamics of cholinergic activation in the septohippocampal and nucleus basalis magnocellularis-cortical pathways at various stages during repeated testing of mice with selective spatial reference or working memory protocols [Durkin and Toumane (1992), Behav. Brain Res. 50, 43-52] showed that the post-test durations of cholinergic activation in each pathway varied as a function of the type of memory tested and the level of task mastery. Since (i) the hippocampal formation is considered to constitute a critical component of a temporary memory buffer, and (ii) working memory items are not thought to be submitted to consolidation and permanent storage, we postulated that the duration of testing-induced cholinergic activation in the septohippocampal pathway may govern the maintenance of the working memory trace over the retention interval. In order to test directly this hypothesis C57 B1/6 mice were extensively trained (one trial/day, 25-30 days) on an identical selective working memory task to attain high levels of retention (> 80% correct), but using either 5 min (Group 1), or 60 min (Group 2) retention intervals. At various times (30 s-75 min) following the initial acquisition phase of the test, cholinergic activity in the hippocampus and frontal cortex was quantified using measures of high-affinity choline uptake. Whereas cholinergic activation was observed in both pathways at 30 s post-acquisition and throughout the 5 min retention interval in Group 1, the situation in Group 2 is different, activation of the septohippocampal pathway being maintained for only 15 min, while activation in the nucleus basalis magnocellularis-cortical pathway is maintained for the totality of the 1 h retention interval. The nucleus basalis magnocellularis-cortical cholinergic pathway, in addition to its role in long-term reference memory storage processes may, thus, via an intervention in the temporal encoding of information, also subsume a complementary intermediate-term buffer storage role in working memory situations requiring retention intervals in excess of 15 min in mice. This secondary, "backup", function of the nucleus basalis magnocellularis-cortical pathway would thus liberate the septohippocampal complex from its primary active role in the temporary maintenance and/or accessibility of the working memory trace in these particular cases requiring long retention intervals.

Developmental regulation of adult cortical morphology and behavior: an animal model for mental retardation

The purpose of this study was to examine the behavioral performance in adult mice which, as neonates, had received lesions to cortically projecting, cholinergic basal forebrain neurons. The nucleus basalis magnocellularis (nBM) provides the primary cholinergic innervation to cerebral cortex. Lesions in the nBM in neonatal mice result in transient cholinergic denervation and persistent abnormalities in cortical morphology and cytoarchitecture. These cortical abnormalities resemble pathologies observed in a number of developmental disabilities in humans, including Down Syndrome. Balb/CByJ mice received lesions to the nBM 12-24 hr after birth; littermates served as controls. Behavioral testing began 8 weeks after the lesion and included assessments of spontaneous motor activity, retention (a passive avoidance task) and cognition (the Morris swim task). Following behavioral testing, a subset of mice was killed for Nissl and acetylcholinesterase (AChE) histology. The cortical morphology in these brains was evaluated and ranked by the experimenter, who was blind to the lesion and behavioral studies. The lesioned mice exhibited increased spontaneous activity as compared to littermate controls. The lesioned mice were also severely impaired in performance of the retention and cognitive task; they showed decreased passive avoidance retention latencies and increased swim maze latencies as compared to controls. The brains of all of the lesioned mice exhibited cortical morphological abnormalities that ranged from slight to severe. Cortical AChE intensity and distribution in the brains of the lesioned mice, however, were comparable to those of controls. In correlation studies of behavioral and morphological data, motor activity did not correlate with either passive avoidance retention or swim maze latencies. Additionally, cortical cytoarchitectural abnormalities did not correlate with motor activity. Cortical cytoarchitectural abnormalities did, however, correlate with both passive avoidance and swim maze latencies, i.e. severely abnormal cortical morphology predicted low passive avoidance retention latencies and high swim maze latencies. These data indicate that cortical cytoarchitectural abnormalities resulting from nBM lesions in neonates correlate with impairments on the cognitive task, but not with the activity measures, in adult mice. Thus, in this lesion model, and by extrapolation in developmental disabilities in humans, structural changes in the cortex which result from transient disruption of cortical cholinergic innervation may lead to persistent cognitive impairments in adulthood.

The effects of ibotenic acid lesions of the basal forebrain on visual discrimination performance in rats

Rats were trained to stable performance in a conditional brightness discrimination task and then received infusions of ibotenic acid or vehicle into the basal forebrain. Following 2 weeks of recovery, animals were retested in the original discrimination. Lesioned rats tended to performed badly on the first day of testing as measured by all parameters (percent correct responding, latency to respond, and missed trials) but thereafter, most rats recovered quickly to prelesion levels. In keeping with previous reports, an approximately 30% reduction in choline acetyltransferase activity was observed in the lesioned animals. Four rats showed no recovery over a period of several months; however, an analysis of the choline acetyltransferase in several brain regions revealed no obvious differences to those animals in which performance recovered. Postlesion testing with the putative nootropic beta-carboline ZK 93426 showed no major differences to the effects observed in control animals. Scopolamine had similar negative effects in both groups tested. These data indicate that deficits induced by lesions of the basal forebrain do not correlate with reductions in cholinergic activity.

Differential effect of functional ablation of thalamic reticular nucleus on the acquisition of passive and active avoidance

The possible contribution of inadvertent damage of the thalamic reticular nucleus to memory impairment caused by lesion of nucleus basalis magnocellularis (NBM) was examined. Rats carrying chronically implanted cannulae received unilateral injection of 3 ng tetrodotoxin (TTX) into the reticular nucleus either 60 min before (PRE) or 2 min after (POST) acquisition of a combined passive avoidance (PAR)--active avoidance (AAR) task. Three days later retrieval was tested during unilateral TTX blockade of the reticular nucleus in the same (IPSI) or in the opposite (CONTRA) hemisphere. Unilateral inactivation of the reticular nucleus affected neither acquisition nor retrieval of PAR, but interfered with AAR acquisition under the PRE conditions. AAR reacquisition was impaired in the PRE-CONTRA but not in the other groups. The effects of reticular nucleus blockade (AAR disruption without PAR impairment) contrast with AAR facilitation and PAR disruption after NBM lesions. It is concluded that the consequences of NBM damage are not enhanced by unintentional thalamic encroachment.

Mechanical deafferentation of basal forebrain-cortical pathways and neurotoxic lesions of the nucleus basalis magnocellularis: comparative effect on spatial learning and cortical acetylcholine release in vivo

Rats were assigned to one of the following treatments: bilateral cut of basal forebrain-cortical fibers (DEAFF), ibotenic (IBO) or quisqualic (QUIS) acid lesions of the NBM and sham operations (SHAM). They were trained to perform a radial eight-arm maze task with all the paths or only four paths baited. Cortical cholinergic release measured by microdialysis in vivo and choline acetyltransferase activity were also assessed in the four lesion conditions. The results show that, in the full baited maze task, only the DEAFF group showed a severe spatial learning impairment. In the four-baited path task, the DEAFF group was still more impaired than the other groups but a performance deficit also emerged in rats with IBO lesions. Neurochemical data indicated that cortical choline acetyltransferase activity was reduced by 25% after IBO lesions, by 52% after DEAFF and by 46% after Quis lesions. However, cortical cholinergic release, which dropped in the same fashion after DEAFF or QUIS lesions, was unaffected by IBO lesions. Thus, in spite of the distinctive patterns of behaviour exhibited by the three lesioned groups, no correlation between cortical cholinergic deficiencies and spatial learning impairment was found. The similar behavioural effects produced by DEAFF and fornix sections suggests that, among the basal forebrain-cortical pathways, descending fibers projecting onto the septo-hippocampal system could exert a strong control on spatial learning performance.

Impaired acquisition in a 14-unit T-maze following medial septal lesions in rats is correlated with lesion size and hippocampal acetylcholinesterase staining

Septohippocampal cholinergic system involvement in acquisition of an aversively motivated 14-unit T-maze was evaluated in 4-month-old male Fischer-344 rats. Each rat was assigned to one of two groups that received either a bilateral electrolytic lesion to the medial septal area (MSA) or a sham operation. One week after surgery, each rat began pretraining in one-way active avoidance (footshock = 0.8 mA) consisting of 10 trials per day on each of 3 consecutive days. Criterion for successful completion of pretraining was 8/10 avoidances on the third day. On the day following completion of pretraining, each rat received 10 trials in a shock-motivated 14-unit T-maze. The performance requirement was to move through each of five maze segments within 10 s to avoid footshock (0.8 mA). A second 10-trial session was provided 24 h later. Performance measures included errors, alternation errors, runtime, shock frequency, and duration. Following maze training, each rat was sacrificed, and formalin-fixed brains were frozen for histology, which included procedures for thionin Nissl and acetylcholinesterase (AChE) staining. MSA-lesioned rats were observed to be significantly impaired on all measures of maze performance compared to sham-operated controls. Densitometric analysis of hippocampal AChE staining revealed a 30% reduction in relative AChE staining of MSA-lesioned rats compared to sham-operated controls. Lesion size was observed to be highly positively correlated with maze errors. A negative correlation of mean error score with density of AChE staining was observed for MSA-lesioned rats, but not for sham-operated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitotoxic lesions of rat basal forebrain: differential effects on choline acetyltransferase in the cortex and amygdala

Previous studies have shown that basal forebrain lesions using different excitotoxins produce similar decreases in cortical choline acetyltransferase, but differential effects on memory. However, basal forebrain cholinergic neurons send efferents to the amygdala and cortex. The present studies compared the effects of several excitotoxins on choline acetyltransferase levels in both of these structures. Lesions of the basal forebrain were made in rats by infusing different doses of either alpha-amine-3-hydroxy-5-methyl-4-isoxazole propionic acid, ibotenic acid, quisqualic acid, quinolinic acid or N-methyl-D-aspartic acid and measuring choline acetyltransferase seven days later. All of the excitotoxins exerted a differential response on cholinergic neurons of the basal forebrain projecting to the cortex or amygdala. Quinolinic acid was a more potent neurotoxin to cholinergic neurons innervating the amygdala than those projecting to the cortex. In contrast, quisqualic acid and alpha-amine-3-hydroxy-5-methyl-4-isoxazole were more potent neurotoxins to the cortical projection. alpha-Amine-3-hydroxy-5-methyl-4-isoxazole propionic acid was the most potent excitotoxin for destroying cholinergic neurons innervating either the cortex or amygdala. A parallel neurotoxic response was obtained in the cortex and amygdala following infusion of ibotenic acid or N-methyl-D-aspartic acid with little selectivity for choline acetyltransferase depletion in the cortex or amygdala. Histological analysis of the injection site revealed that acetylcholinesterase-positive neurons were destroyed by the excitotoxins in a dose-dependent manner. Excitotoxins (ibotenic acid, quinolinic acid, N-methyl-D-aspartic acid) that produce the greatest impairments in memory were found to produce the greatest depletion of choline acetyltransferase in the amygdala.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a role of the neuropeptide galanin in spatial learning

The neuropeptide galanin coexists with acetylcholine (ACh) in the basal forebrain cholinergic neurons and modulates cholinergic activity in the forebrain. The cholinergic forebrain neurons appear to play a significant role in learning and memory, as suggested by a severe loss of these neurons in Alzheimer's disease. The involvement of endogenous galanin in learning is demonstrated here by the use of the recently synthesized high-affinity galanin antagonist M35 [galanin(1-13)-bradykinin(2-9) amide] (Kd = 0.1 nM). Intracerebroventricular (i.c.v.) administration of M35 (6 but not 3 nmol) produced a significant (P < 0.025) facilitation of acquisition in a spatial learning test (Morris swim maze) without any increase in swim speed. Thus, M35 (6 nmol) shortened the escape latency, reduced the number of failures to reach the platform, and shortened the path length to reach the hidden platform. M35 (3 and 6 nmol) tended to enhance retention performance seven days after the last training session. Receptor autoradiographic studies on the distribution of [125I]M35 following i.c.v. administration show that it binds preferentially in the periventricular regions including the hippocampus. These results suggest that galanin may modulate spatial learning and memory and that galanin antagonists may provide a new principle in the treatment of Alzheimer's disease.

Lesions of the medial septum which produce deficits in working/spatial memory do not impair long-term potentiation in the CA3 region of the rat hippocampus in vivo

The effects of removing the septohippocampal pathway on the ability to induce long-term potentiation (LTP) in the CA3 region of the hippocampus was examined in vivo in rats. The septal input to the hippocampus was destroyed by electrolytic lesioning of the medial septum (MS). Prior to electrophysiological investigation, working/spatial memory of lesioned and control rats was tested using an 8-arm radial maze task. Maze performance was significantly impaired in animals with MS lesions. LTP inducibility was examined in the commissural fimbrial fibre- and mossy fibre (mf)-CA3 pathways in MS-lesioned and control rats. The pre-tetanus values in MS-lesioned rats tended to be smaller than those in controls, in both pathways. High-frequency stimulation of the commissural fibres resulted in a sustained increase in the orthodromic population spike and EPSP amplitude in both control and MS-lesioned rats. The magnitude of potentiation was similar in both groups. In control rats, high-frequency stimulation of the mf potentiated the amplitude of both the population spike and EPSP; in MS-lesioned rats, the EPSP amplitude alone was significantly increased by mf high-frequency stimulation. Hippocampal acetylcholinesterase (AChE) content was severely reduced bilaterally in MS-lesioned rats with working/spatial memory impairments, indicating that the lesions were effective in destroying the cholinergic septohippocampal input. These findings suggest that, in contrast to working/spatial memory processes, LTP at CA3 synapses is not dependent upon the integrity of the septohippocampal pathway.

Delayed matching-to-sample performance by rats in a new avoidance-motivated maze: response to scopolamine and fimbria-fornix lesions

A new avoidance-motivated detour-maze in which memory for an immediately preceding sample event could be assessed was evaluated by testing seven 6-month-old male F-344 rats with a delayed matching-to-sample (DMTS) paradigm. Rats first received extensive pretraining with this paradigm over several months and after a minimum of 1,390 choice-trials demonstrated great proficiency in this maze. Studies were then conducted to establish cholinergic and hippocampal involvement in the DMTS task by using drug manipulations (scopolamine and physostigmine), and after lesions to the fimbria-fornix (FF) pathway. A high dose (1.0 mg/kg) of scopolamine but not a low dose (0.3 mg/kg) significantly interfered with choice accuracy as measured by errors and trials to criterion; physostigmine (0.01 and 0.03 mg/kg) had no significant effect; and fimbria lesions significantly disrupted both choice accuracy and runtime performance. Disruption was most pronounced on difficult problems (different paths to the goal). After lesions only, considerable within-trial perseverative errors occurred during the early postlesion weeks on four difficult problems from among the 18 tested. Results were discussed in terms of (a) specificity of this disruption, (b) indications of proactive interference effects, and (c) the movement-related excitation component of maze learning. The present results accord with earlier findings of disruption by scopolamine and FF lesions in a 14-unit T-maze, both mazes having similar performance requirements of shock avoidance and multiple 90-degree turns along the paths to the goal. The present results affirm that this new detour maze provides a viable approach for assessing cognitive performance in a within-subject design and thereby offers new possibilities for testing various aspects of cognitive processing, particularly for aged rodent models, in a complex aversive situation.

Spatial learning in two inbred strains of mice: genotype-dependent effect of amygdaloid and hippocampal lesions

Spatial learning performance and maze-running strategies were estimated in two inbred strains of mice, C57BL/6 and DBA/2, submitted to an 8-arm radial maze task. Subsequently the genotype-dependent effect of hippocampus and amygdala on the mastering of this task was examined as a function of the different acquisition model provided by each strain. The results firstly show that unoperated C57BL/6 mice reach a higher level of performance and develop a stronger preference for adjacent arms - 45 degrees angle - turns than unoperated DBA/2 mice. In the high learner C57BL/6 strain, both hippocampal and amygdaloid lesions impair performance and modify maze-running strategies. With practice, however, the difference between amygdala-lesioned mice and controls disappears while that between hippocampus-lesioned mice and controls persists. Conversely, in the low learner DBA/2 strain, hippocampal lesions have a negative effect on a single parameter of performance, while amygdaloid lesions only affect maze-running strategies. Taken together, these results confirm the specific control exerted by the hippocampus on spatial learning. Moreover, they suggest that the amygdala can parallel the role of the hippocampus as far as the baseline level of performance of the strain considered is high.

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

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

Cholinergic mechanisms in learning, memory and dementia: a review of recent evidence

The discovery in the late 1970s that cholinergic neurons in the basal forebrain degenerate in Alzheimer's disease (AD) greatly accelerated research on the role of cholinergic mechanisms in learning and memory. As is often the case in science, the early enthusiasm for the cholinergic hypothesis has been tempered by the results of subsequent research. Although there is substantial pharmacological evidence that unspecified cholinergic systems in the brain play important roles in some forms of learning and memory, recent findings in humans indicate that antimuscarinic drugs do not model the deficits seen in AD. In addition, the goal of elucidating the functions of these basal forebrain neurons in animals has proved to be difficult and is yet to be achieved. Despite substantial effort, therefore, the cognitive and behavioral consequences of cholinergic pathology in AD remain unknown. Under these circumstances, attempts to develop cholinergic pharmacotherapies for these deficits in AD are based on questionable assumptions.

The role of cholinergic projections from the nucleus basalis in memory

The behavioral effects of lesions of the nucleus basalis magnocellularis (NBM) are reviewed, focusing on the anatomical extent of the lesion, the involvement of neurotransmitter systems and the alterations in memory processes. Most behavioral deficits after NBM lesions can be attributed to damage to the NBM itself, although during spontaneous or pharmacologically induced recovery, other brain structures might play a role. The neurochemical deficit underlying the behavioral impairments is most likely the decrease in cholinergic functioning, since, for example, enhancement of cholinergic functioning is sufficient for behavioral improvement. However, since the lesions are not specific for cholinergic neurons, the extent to which noncholinergic damage causes behavioral deficits is still unclear. Finally, lesions of the NBM impair memory, but affect also other behavioral processes, such as discrimination and habituation. A common process underlying these various impairments could be that of insufficiently focused processing of stimuli.

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.

A critical evaluation of monkey models of amnesia and dementia

In this review we consider various models of amnesia and dementia in monkeys and examine the validity of such models. In Section 2 we describe the various types of memory tests (tasks) available for use with monkeys and discuss the extent to which these tasks assess different facets of memory according to present theories of human memory. We argue that the rules which govern correct task performance are best regarded as a form of semantic rather than procedural memory, and that when information about stimulus attributes or reward associations is stored long-term then that knowledge is semantic. The demonstration of episodic memory in monkeys is problematic and the term recognition memory has been used too loosely. In particular, it is difficult to dissociate episodic memory for stimulus events from the use of semantic memory for the rule of the task, since dysfunction of either can produce impairment on performance of the same task. Tasks can also be divided into those which assess memory for stimulus-reward associations (evaluative memory) and those which tax stimulus-response associations including spatial and conditional responding (non-evaluative memory). This dissociation cuts across the distinction between semantic and episodic memory. In Section 3 we examine the usefulness of the classification of tasks described in Section 2 in clarifying our understanding of the contribution of the temporal lobes and the cholinergic system to memory. We conclude that evaluative and non-evaluative memory are mediated by separate parallel systems involving the amygdala and hippocampus, respectively.

Behavioral pharmacology and biochemistry of central cholinergic neurotransmission

Systemically administered cholinergic (muscarinic) receptor antagonists can impair the acquisition and post-acquisition performance of a variety of learned behaviors. acquisition performance of a variety of learned behaviors. At present, there is no consensus about the psychological mechanisms underlying these deficits. Behavioral inhibition, working (short-term) memory, reference (long-term) memory, attention, movement and strategy selection, and stimulus processing are among the constructs that have been proposed as underlying the effects of muscarinic receptor blockade. On the basis of neuroanatomical and neuropharmacological considerations it is contended that debates about the nature of the mediating events are pointless because they are on an anatomy that does not exist. Specifically, given that cholinergic neurons innervate almost the entire neuraxis and that muscarinic cholinergic receptors are distributed throughout the central nervous system, it is virtually certain that systemically applied antimuscarinic drugs will influence a broad spectrum of brain functions. In addition, the nature of the deficits produced by scopolamine and atropine, which are competitive antagonists, will depend on the regional endogenous rate of acetylcholine release, which may in turn be influenced by the particular environment and/or level of training imposed on the animal. As the literature seems to indicate, therefore, the effects of competitive antagonists will vary as a function of both the behavioral test and the level of training. Accordingly, attempts at unitary formulations of central cholinergic function are ill-conceived and illusory. Another approach to understanding central cholinergic function has been based on the use of local injections of excitotoxins into brain regions such as the basal forebrain that contain cholinergic neurons. Recent published reports indicate, that many of the behavioral deficits observed after ibotenic acid lesions of the basal forebrain are due primarily to the loss of non-cholinergic neurons. The inherent limitations of the excitotoxin lesion approach for unravelling the functions of central cholinergic systems are such that they cannot produce definitive information and might best, therefore, be abandoned. At present, a reliable selective toxin for cholinergic neurons is not available and urgently required. Until such a compound is identified, local intracerebral applications of antimuscarinic agents may be the preferred procedure for studying the behavioral correlates of regional blockade of cholinergic activity. Brain microdialysis in freely moving animals also holds considerable promise with respect to defining the circumstances under which acetylcholine is released in discrete regions of the central nervous system. At present, the function of central cholinergic systems and the possible role of each in learning and memory remain poorly understood.

Effects of cholinergic-rich neural grafts on radial maze performance of rats after excitotoxic lesions of the forebrain cholinergic projection system--I. Amelioration of cognitive deficits by transplants into cortex and hippocampus but not into basal forebrain

After ibotenate (10.0 mg/ml) lesions to the nucleus basalis and medial septal regions, at the source of the cortical and hippocampal branches of the forebrain cholinergic projection system, rats displayed long-lasting stable impairment in reference and working memory in both spatial (place) and associative (cue) radial maze tasks. Cell suspension transplants of cholinergic-rich fetal basal forebrain tissue dissected at embryonic day 15 substantially improved all aspects of radial maze performance to a comparable degree whether sited in cortex, hippocampus, or both regions of the host brain. No additive effects were obtained with grafts in both terminal regions, but total graft volume, assessed stereologically, showed a significant negative correlation with error scores. Rats with behaviourally effective grafts, like controls, were disrupted in the place task when tested in dim light which obscured extra-maze spatial cues. Lesioned rats were not affected by change in lighting. Grafts of cholinergic-poor fetal hippocampal tissue did not improve radial maze performance; neither did grafts of cholinergic-rich tissue placed within the host basal forebrain lesion sites. In rats with cholinergic-rich terminal grafts, cortical and hippocampal choline acetyltransferase activity was restored to control level, commensurate with site of transplant, whereas it was significantly reduced in lesioned animals and those with functionally ineffective grafts. The indiscriminate error pattern and insensitivity to changes in lighting shown by lesioned rats suggested that lesioning primarily disrupted attention rather than short- or long-term spatial or associative memory processes. Since rats with cholinergic-rich grafts showed both reduced errors and recovery of stimulus control, the data indicated that grafts affected information processing, rather than changes in motor or motivational processes. Changes in choline acetyltransferase activity and the behavioural efficacy of cholinergic-rich grafts are consistent with the involvement of acetylcholine in the behavioural deficits and recovery displayed by lesioned and grafted groups, but do not rule out contributions from other factors. The equipotency of grafts within each terminal region suggests also that there may be a considerable degree of functional cooperation between the two branches of the forebrain cholinergic projection system. Functional recovery may involve local, nonspecific synaptic or paracrine mechanisms within the target regions, since grafts were efficacious only when placed in the terminal areas, but not when sited homotopically in the basal forebrain, indicating that they did not achieve any functionally significant structural repair to the host brain at that site.

Effects of combined serotonin depletion and lesions of the nucleus basalis magnocellularis on acquisition of a complex spatial discrimination task in the rat

The purpose of the present experiment was to determine the effects of lesions of cholinergic neurons originating from the nucleus basalis magnocellularis (NBM), alone or in combination with central serotonin depletion, on learning and memory in rats trained in the Stone 14-unit T-maze--a complex, positively-reinforced spatial discrimination task. Lesion of cholinergic neurons within the NBM was accomplished by bilateral infusion of ibotenic acid. Serotonin depletion was accomplished by the systemic administration of p-chloroamphetamine (PCA). The results show that PCA-induced serotonin depletion enhanced learning. This effect was completely prevented by NBM lesions, despite the fact that NBM lesions alone did not affect the performance of rats in this task. The results of this study support the view that the cholinergic and serotonergic systems may functionally interact in learning and memory processes. The significance of this interaction in the etiology and treatment of dementia should be further investigated.

Psychobiological evidence for the distinction between episodic and semantic memory

The evidence for discrete neurobiological mechanisms that underlie episodic and semantic memory is reviewed. Published data from three separate lines of research are considered: studies of human amnesic patients, psychopharmacological studies of normal human subjects, and studies of working and reference memory in rodents, a distinction that is arguably analogous to the episodic/semantic dichotomy. It is concluded that the available evidence does not indicate that episodic and semantic memory are mediated by discrete neural subsystems. An alternative model of human memory is discussed, based on the concept of parallel distributed processing.

Alterations in neocortical expression of nicotinic acetylcholine receptor mRNAs following unilateral lesions of the rat nucleus basalis magnocellularis

We investigated the effect of a unilateral lesion of the nucleus basalis magnocellularis (nbm) on the expression of nicotinic acetylcholine receptors (nAChRs) in the rat cerebral cortex. Cortical nAChR concentration as determined by [3H]nicotine binding was unaffected by the nbm lesion. Expression levels of nAChR subunit mRNAs were measured using cDNA clones coding for the receptor subunits, alpha-3, alpha-4, and beta-2. At 1 week postlesion, expression levels of alpha-4, and beta-2 were increased by an average of 82% and 19%, respectively. On the other hand, expression levels of these mRNAs on the lesioned side 4 weeks after lesioning did not differ from those on the control side. Expression of alpha-3 was not altered by the nbm lesion. These results imply regulation of nAChR transcripts by cell to cell interactions. Co-increase of alpha-4 and beta-2 transcripts may provide supporting evidence for the occurrence of supersensitivity in deafferentated cholinergic neurons.

Scopolamine differentially disrupts the behavior of male and female Wistar rats in a delayed nonmatching to position procedure

Evidence is available that pharmacological interference with the cholinergic system may disrupt behavior in experimental procedures designed to investigate learning and memory processes. Recently it has been suggested that the cholinergic system may be sexually dimorphic. The present experiment was designed to investigate whether or not manipulation of the cholinergic system differentially affected memory processes in both sexes. Male and female Wistar rats were exposed to a delayed nonmatching to position procedure and were challenged with increasing doses of scopolamine hydrobromide (a central and peripheral muscarinic receptor blocker) and scopolamine methyl bromide (which does not pass the blood-brain barrier). Response accuracy decreased in both sexes as the delay interval duration increased. Behavioral differences between saline-treated males and females were not observed. Response accuracy decreased dose-dependently after subjects were injected with scopolamine hydrobromide. Response accuracy also decreased after treatment with scopolamine methyl bromide, but to a smaller extent. Males showed less accurate responding after treatment with either drug than females. These results provide behavioral evidence for the hypothesis that cholinergic functioning may differ between the sexes.

Ultrastructural localization of [125I]neurotensin binding sites to cholinergic neurons of the rat nucleus basalis magnocellularis

The distribution of specifically-labeled neurotensin binding sites was examined in relation to that of cholinergic neurons in the rat nucleus basalis magnocellularis at both light and electron microscopic levels. Lightly prefixed forebrain slices were either labeled with [125I](Tyr3) neurotensin alone or processed for combined [125I]neurotensin radioautography and acetylcholinesterase histochemistry. In light microscopic radioautographs from 1-microns-thick sections taken from the surface of single-labeled slices, silver grains were found to be preferentially localized over perikarya and proximal processes of nucleus basalis cells. The label was distributed both throughout the cytoplasm and along the plasma membrane of magnocellular neurons all of which were found to be cholinesterase-positive in a double-labeled material. Probability circle analysis of silver grain distribution in electron microscopic radioautographs confirmed that the major fraction (80-89%) of specifically-labeled binding sites associated with cholinesterase-reactive cell bodies and dendrites was intraneuronal. These intraneuronal sites were mainly dispersed throughout the cytoplasm and are thus likely to represent receptors undergoing synthesis, transport and/or recycling. A proportion of the specific label was also localized over the nucleus, suggesting that neurotensin could modulate the expression of acetylcholine-related enzymes in the nucleus basalis. The remainder of the grains (11-20%) were classified as shared, i.e. overlied the plasma membrane of acetylcholinesterase-positive neuronal perikarya and dendrites. Extrapolation from light microscopic data, combined with the observation that shared grains were detected at several contact points along the plasma membrane of cells which also exhibited exclusive grains, made it possible to ascribe these membrane-associated receptors to the cholinergic neurons themselves rather than to abutting cellular profiles. Comparison of grain distribution with the frequency of occurrence of elements directly abutting the plasma membrane of neurotensin-labeled/cholinesterase-positive perikarya indicated that labeled cell surface receptors were more or less evenly distributed along the membrane as opposed to being concentrated opposite abutting axon terminals endowed or not with a visible junctional specialization. The low incidence of labeled binding sites found in close association with abutting axons makes it unlikely that only this sub-population of sites corresponds to functional receptors. On the contrary, the dispersion of labeled receptors seen here along the plasma membrane of cholinergic neurons suggests that neurotensin acts primarily in a paracrine mode to influence the magnocellular cholinergic system in the nucleus basalis.

Therapeutic effect of THA on hemicholinium-3-induced learning impairment is independent of serotonergic and noradrenergic systems

Tetrahydroaminoacridine (THA: Tacrine) has previously been shown to reverse deficits in spatial discrimination learning induced by hemicholinium-3 (HC-3). In the present experiments the effects of prior depletion of serotonin (5-HT) or noradrenaline (NA) on this reversal were examined. In the first experiment 5-HT lesions were made by injecting 5,7-DHT (2 x 50 micrograms/5 microliters) into the lateral ventricles of rats pretreated with desmethylimipramine (DMI 25 mg/kg IP). A permanently indwelling guide tube was then implanted over the right lateral ventricle. Subsequent testing under drug-free conditions, revealed no effect of the lesion on the number of trials needed to attain criterion (nine consecutive correct choices) in two-platform spatial discrimination learning in a watermaze. Using a latin square design rats were then tested for the effects of HC-3 and THA. HC-3 (5 micrograms/5 microliters ICV) or placebo (CSF) were injected 60 min before the start of a 30-trial training session. THA (4.6, 10 mg/kg SC) or placebo were then injected 15 min before training. Choice accuracy but not choice latency was significantly impaired by HC-3 and the effect was reversed by THA in both sham operated and 5-HT lesioned rats. In the second experiment two injections of DSP-4 (50 mg/kg IP) were given, following cannulation, to deplete forebrain NA. The lesion had no effect on spatial learning under drug-free conditions and failed to block the THA-induced reversal of spatial discrimination learning deficits following HC-3. These results confirm that forebrain Ach depletion by HC-3 impairs spatial discrimination learning and that the deficit is reversed by THA.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative effects of ibotenic acid- and quisqualic acid-induced lesions of the substantia innominata on attentional function in the rat: further implications for the role of the cholinergic neurons of the nucleus basalis in cognitive processes

Two experiments examined the effects of excitotoxic lesions of the substantia innominata on cholinergic activity in the neocortex and on performance in a paradigm measuring selective attention in the rat. In Expt. 1, ibotenate-induced lesions produced approximately 30% reductions in cortical choline acetyltransferase (ChAT) activity, and damage to wide regions of the substantia innominata and ventral pallidum. The rats were impaired in their ability to localize brief visual targets in a serial reaction time task, as measured by reduced choice accuracy. This impairment was particularly evident at short stimulus durations, but the lesioned rats did not exhibit evidence of primary visual sensory dysfunction and exhibited only minor deficits when the stimuli were presented unpredictably. The deficit was exacerbated when distracting white noise was interpolated into the task. The rats with lesions were also slower to respond correctly, probably resulting partly from the adoption of a speed/error trade-off strategy, and were slower to collect earned food pellets, although they made no more errors of omission than controls. In Expt. 2, quisqualate-induced lesions produced fewer signs of non-specific damage and 50% reductions in cortical ChAT activity. This lesion produced generally qualitatively similar, but weaker effects to those of ibotenate-induced lesions. It was notable that many of the deficits following either ibotenate- or quisqualate-induced lesions lasted for several months after surgery. The results are discussed in terms of the cholinergic hypothesis of cognitive dysfunction. It is argued that lesions of the substantia innominata, including the magnocellular cholinergic neurons of the nucleus basalis of Meynert, produce deficits in attentional processing, which may not result from damage specifically to cholinergic cells. However, the longevity of the effects makes these preparations suitable for further exploration of the restorative effects of cholinergic treatments.

Comparison of retention performance between young rats with fimbria-fornix lesions and aged rats in a 14-unit T-maze

Young (3-months) and aged (24-months) male F-344 rats were pretrained in one-way active avoidance in a straight runway for 3 days. Then two 10-trial daily sessions were given in a 14-unit T-maze in which the response requirement was to negotiate each of 5 maze segments within 10 s to avoid footshock. One day or one week after acquisition, bilateral electrolytic lesions were made in the fimbria-fornix of young rats (1-day lesion or 1-week lesion). Corresponding sham operations were made for remaining young rats (1-day sham or 1-week sham). Aged animals did not receive any surgical treatment. One week after surgery, a 10-trial retention test was conducted to assess the lesion effects on retention and to manipulate the interval between acquisition and lesions. Aged animals were tested in the maze 1 week after acquisition. Results revealed that rats with fimbria-fornix lesions exhibited significant impairment compared to sham-operated groups on all retention performance measures including errors, runtime, number of shocks, duration of shock, and alternation errors. The number of errors and alternation errors of lesioned animals were still higher than those of sham-operated animals at the second half of the retention test, whereas other non-cognitive measures for lesioned animals recovered to control levels. The interval between acquisition training and lesions had no influence on retention performance. Although performance of aged rats during acquisition and retention trials was significantly worse than that of young controls and lesioned animals, a similar recovery pattern during retention testing was found for young rats with fimbria-fornix lesions and aged rats, i.e. both groups showed significant declines in non-cognitive measures with less decline in cognitive measures. These results suggest that the fimbria-fornix is partially involved in retention of 14-unit T-maze performance and that the age-related retention deficit observed in this task may be related to impaired transmission through this pathway.

Different long-term effects of bilateral and unilateral nucleus basalis lesions on rat cerebral cortical neurotransmitter content

Young adult rats received either unilateral or bilateral ibotenic acid infusions in their nucleus basalis, destroying most of the cholinesterase-staining neurons in that region. Cerebral cortex levels of choline acetyltransferase, somatostatin, neuropeptide Y, and monoamines were then assayed 2.5 and 10 months after bilateral lesions, or, 2.5, 10, and 14 months after unilateral lesions. Entorhinal and cerebral cortex levels of several amino acid transmitters were also measured. As expected, choline acetyltransferase activity was decreased in the frontal cortex ipsilateral to the ibotenic acid infusion in unilaterally or bilaterally lesioned animals. Parietal cortex concentrations of somatostatin and neuropeptide Y were altered by lesioning in a complicated, time-dependent manner. Thus, while unilateral lesions transiently decreased or had no effect on these neuropeptide levels, bilateral lesions elevated the level of each neuropeptide by over 100% at 10 months. Other cortical transmitter systems investigated appeared to be less affected by nucleus basalis-lesions. Unilateral lesions had no effect on prefrontal cortex norepinephrine, serotonin, or dopamine content at 14 months post-lesioning. These different neurochemical effects of unilateral and bilateral nucleus basalis lesions may be important for developing a model for the trans-synaptic effects of cortical cholinergic deafferentation.

Colchicine administered into the area of the nucleus basalis decreases cortical nicotinic cholinergic receptors labelled by [3H]-acetylcholine

Lesions in the nucleus basalis in the rat are known to decrease presynaptic markers for acetylcholine, including levels of cholineacetyltransferase (CHAT), high affinity uptake of choline and levels of acetylcholinesterase. Effects of lesions of the nucleus basalis on populations of nicotinic and muscarinic receptors are less well understood. After bilateral injection of the neurotoxic agent, colchicine into the nucleus basalis in the rat, levels of CHAT in the cerebral cortex were reduced 44%. Muscarinic cholinergic [( 3H]QNB) and dopaminergic [( 3H]spiroperidol) binding was not changed in the cortex, hippocampus or striatum. However, significant decreases in nicotinic binding sites, labelled by [( 3H]acetylcholine), were observed in the frontal cortex of nucleus basalis treated animals; scatchard plot analysis indicated a significant decrease in the number, but not affinity, of nicotinic binding sites. Colchicine injected into the nucleus basalis had no effect on the binding of [3H]acetylcholine in the hippocampus, but decreased binding of [3H]acetylcholine in the striatum. Subsequent experiments, in which colchicine was administered into the striatum at a site above the nucleus basalis had no significant effect on nicotinic binding in the striatum or frontal cortex. These results support the hypothesis that degeneration of the nucleus-basalis-cortical cholinergic pathway results in a loss of presynaptic nicotinic binding sites in the cortex as well as in the striatum (through transsynaptic degeneration of the cortico-striatal pathway).

Effects of Nucleus Basalis Magnocellularis Lesions in Rats on Delayed Matching and Non-Matching to Position Tasks

The effects of quisqualic acid lesions of the nucleus basalis magnocellularis on short-term memory capacities of the rat have been investigated using the delayed matching and non-matching to position tasks. The lesions do not disrupt performance of either task by pretrained animals, but do disrupt the ability to acquire the non-matching contingency, and to reverse to the non-matching task when trained on the matching task. The unidirectional nature of the reversal deficit has been replicated. The generalized disruption of performance of either task by the muscarinic antagonist scopolamine was comparable in lesioned and control rats. The lesions were associated with extensive loss of acetylcholinesterase staining in the basal forebrain and in the neocortex, and 55% depletions of choline acetyltransferase activity in the neocortex but not in the hippocampus. These observations demonstrate that the cholinergic projection from nucleus basalis to the neocortex is not critical for normal short-term memory, but that lesions involving this system do disrupt specific types of conditional discrimination learning.

Scopolamine and methylscopolamine differentially affect fixed-consecutive-number performance of male and female Wistar rats

Male and female Wistar rats were trained on a fixed-consecutive-number schedule in which a response on a food lever was followed by the presentation of reinforcement when at least three, but not more than seven responses had been completed on a work lever. Subjects were treated with different doses of the centrally acting cholinergic antagonist scopolamine hydrobromide or the more peripherally active cholinergic antagonist scopolamine methylbromide (0.08, 0.16 or 0.32 mg/ml/kg) once behavior had stabilized. Scopolamine hydrobromide and scopolamine methylbromide dose-dependently decreased response rates in males and females. Scopolamine methylbromide decreased response rates more than equivalent doses of scopolamine hydrobromide and the rate-suppressant effects of both drugs were more marked in males than in females. Scopolamine hydrobromide dose-dependently decreased response accuracy, but differences between males and females were not observed. Response accuracy also decreased after scopolamine methylbromide, but did not vary as a function of the dose of the drug. The decrease in response accuracy induced by both drugs was attributable to an increase in the percentage of trials with a premature switch from the work lever to the food lever. Both scopolamine hydrobromide and scopolamine methylbromide dose-dependently increased the number of premature switches. Differences between males and females were not observed. Administration of scopolamine hydrobromide and scopolamine methylbromide also decreased the number of obtained reinforcers in a dose-dependent manner. Females obtained significantly fewer reinforcers than males, while scopolamine methylbromide affected the number of obtained reinforcers to a larger extent than scopolamine hydrobromide.

Nucleus basalis lesions impair memory in rats trained on nonspatial and spatial discrimination tasks

Ibotenic acid-induced lesions of the nucleus basalis of Meynert (nbM) in rats produced severe memory impairments when subjects were required to acquire a brightness discrimination and learn its reversal (Experiment 1). Lesion-induced impairments were also observed when a complex (30-choice) spatial discrimination task served as the assessment instrument (Experiment 2). The memory deficits observed in Experiment 2 were evident despite an approximate 4-month postoperative recovery period and prior brightness discrimination training. Identical nbM lesions failed to produce memory decrements when rats were trained and tested 24 hr later on a simple (2-choice) right vs. left spatial discrimination task. It is concluded that nbM lesions impair memory for both nonspatial and spatial tasks and that, at least with tests of spatial memory, task difficulty may be an important determinant of this impairment.

Basal forebrain lesions impair tactile discrimination and working memory

Rats received bilateral injections of the excitotoxin, N-methyl-D,L aspartate, which resulted in degeneration of basal forebrain cholinergic (BFC) neurons in the nucleus basalis magnocellularis. Most tests of general neurological function revealed no differences between control rats and those with BFC lesions and where differences were found they appeared to be due to hyperemotionality. Rats with BFC lesions demonstrated significant deficits in working memory, as evaluated in an 8-arm radial maze. In addition, these rats showed a severe impairment in tactile discrimination learning, an effect of BFC lesions not previously demonstrated. We propose that cholinergic deafferentation of the somatosensory cortex with consequent disruption in somatosensory information processing might account at least in part for this effect.

The effects of scopolamine and methylscopolamine on visual and auditory discriminations in male and female Wistar rats

The present experiment was designed to investigate whether or not the administration of scopolamine hydrobromide would differentially disrupt auditory or visual discrimination performance in male and female Wistar rats. Two groups of male and female Wistar rats were trained to discriminate between a continuous and intermittent visual stimulus, while two other groups were trained to discriminate between a continuous or intermittent auditory stimulus in a discrete-trial discrimination procedure. Once discrimination performance had stabilized, subjects were treated with different doses (0.125, 0.25, 0.50 or 1.0) of scopolamine hydrobromide or scopolamine methylbromide. Treatment effects were assessed with respect to discrimination performance, as well as with respect to the number of trials which were not completed. Scopolamine hydrobromide, but not scopolamine methylbromide, disrupted visual and auditory discrimination performance. The auditory discrimination was more seriously disrupted. However, both the administration of scopolamine hydrobromide and of scopolamine methylbromide increased the number of trials which were not completed suggesting that the accuracy of visual and auditory discriminations after drug treatment may have been influenced by other variables than drug effects on memory processes. Sex differences were not observed, neither with respect to discrimination performance, nor with respect to the number of trials which were not completed.

Hemicholinium-3 impairs spatial learning and the deficit is reversed by cholinomimetics

The effects of hemicholinium-3 (HC-3) on spatial discrimination learning were studied. Rats were equipped with indwelling cannulae in the right lateral ventricle and, following recovery, were trained on a two platform spatial discrimination task in a water maze. In this task a visible escape platform remains in a fixed position in the pool during a single training session, whilst the location of an identical "float" (which affords no escape) is randomly varied. For each session the location of the fixed escape platform was changed and the rats were retrained to criterion following pretreatment either with artificial cerebrospinal fluid (CSF) or HC-3 (2.5, 5.0 micrograms/rat/ICV) 1 h before training. Each rat received every treatment according to a latin square design. The results showed that spatial learning was dose dependently impaired by HC-3, choice accuracy being reduced to chance levels by the higher dose. There was no evidence of motoric difficulty, as choice latencies were not significantly increased. Experiments were then conducted to test for reversal of the deficit using a range of psychotropic drugs. Rats were treated with CSF or HC-3 (5 micrograms/rat ICV) 60 min prior to testing and test drugs were injected 15 min before testing. Some doses of physostigmine (46-460 micrograms/kg/SC) and tetrahydroaminoacridine (THA) (2.2-10 mg/kg/SC) reversed the spatial learning deficit. The muscarinic agonists arecoline (0.046-1 mg/kg/SC), aceclidine (1-10 mg/kg/SC), oxotremorine (30-100 micrograms/kg/SC) and RS-86 (0.46, 1.0 microgram/kg/SC) were also effective. Pilocarpine (0.22-2.2 mg/kg/SC) showed marginal activity and isoarecoline (4.6-10 mg/kg/SC) was inactive. Nicotine (0.32, 1, 3.2 mg/kg/SC) and piracetam (10, 30, 100 mg/kg IP) were also inactive. The alpha 2 agonist, clonidine (46, 100 micrograms/kg SC) and the antagonist idazoxan (32, 100 micrograms/kg SC) were also inactive. Learning deficits were not reversed by haloperidol (20, 60 micrograms/kg), amphetamine (0.1, 0.46 mg/kg), the selective 5-HT1A agonist 8-OH-DPAT (30, 100 micrograms/kg) or by the benzodiazapine antagonist ZK-93426 (1, 3.2, 10 mg/kg). The results show that forebrain Ach depletion by HC-3 impairs spatial discrimination learning and these deficits are reversed by cholinesterase inhibitors and some muscarinic receptor agonists. Some degree of pharmacological selectivity is indicated by the failure of a range of other drugs to reverse the impairments.

Comparative effects of quisqualic and ibotenic acid-induced lesions of the substantia innominata and globus pallidus on the acquisition of a conditional visual discrimination: differential effects on cholinergic mechanisms

Two experiments tested the hypothesis that the deficits in conditional discrimination learning produced by ibotenic acid-induced lesions of the ventral pallidum and substantia innominata are produced by loss of the magnocellular cholinergic cells in the nucleus basalis and adjacent regions. Experiment 1 replicated the previously reported deficit in conditional learning produced by ibotenate-induced lesions of the ventral pallidum/substantia innominata, but failed to demonstrate any restoration of learning by a subchronic regimen of the acetylcholinesterase inhibitor physostigmine sufficient to produce significant (30%), but equivalent, degrees of inhibition in the frontal cortex of ventral pallidum/substantia innominata-lesioned or sham-operated rats. Experiment 2 examined the effects of quisqualic acid-induced lesions of the ventral pallidum/substantia innominata. According to most of the measures of learning employed, the quisqualic acid-induced lesion of the ventral pallidum/substantia innominata failed to impair conditional learning, even though the quisqualate-induced lesion produced greater degrees of cholinergic neuron destruction than the ibotenate-induced lesion, as measured in terms of reductions in cortical choline acetyltransferase activity (44% vs 27%). Although consideration of individual data suggested that very high (60%) levels of choline acetyltransferase reduction in Experiment 2 might have detrimental effects of conditional learning, the overall failure of the quisqualate-induced lesions of the ventral pallidum/substantia innominata to impair learning is to be contrasted with the significant behavioural effects of ibotenate-induced lesions. Histological and immunocytochemical analysis showed that the quisqualate-induced lesion, unlike that produced by ibotenate, tended to produce less damage to the overlying dorsal globus pallidus and to parvocellular neurons of the ventral pallidum/substantia innominata, thus implicating these nonspecific effects of ibotenate-induced lesions in their behavioural effects. The present results question previous interpretations of the behavioural effects of ibotenate-induced lesions of the ventral pallidum/substantia innominata in terms of damage inflicted on the cortically-projecting cholinergic cells of the nucleus basalis, and suggest that quisqualic acid, although also nonspecific in its excitotoxic effects, is nevertheless more selective for producing damage to cholinergic neurons in the ventral pallidum/substantia innominata than ibotenic acid.

Comparison of behavioral effects of nucleus basalis magnocellularis lesions and somatosensory cortex ablation in the rat

Cholinergic neurons in the nucleus basalis region of the forebrain project to various portions of the cerebral cortex, including somatosensory cortex. Degeneration of these neurons and their cortical projections is a major feature of the neuropathology of Alzheimer's disease. Injecting an excitotoxin into the basal forebrain to destroy nucleus basalis neurons provides a potentially useful animal model for studying the role of these neurons in Alzheimer's disease. Previously, we demonstrated that rats with nucleus basalis excitotoxin lesions performed poorly on a tactile discrimination task and on a test of working memory. In an effort to clarify further the role of impaired memory versus other types of impairment (e.g. disrupted somatosensory processing due to cholinergic deafferentation of somatosensory cortex), we compared a group of rats with bilateral nucleus basalis excitotoxin lesions and a group with bilateral somatosensory cortical ablations on a variety of behavioral tasks. Rats with nucleus basalis lesions performed as well as controls on a battery of neurological tests but exhibited increased emotionality unlike rats with somatosensory cortical ablations which performed poorly on the battery but were not hyperemotional. The two lesion groups were impaired significantly and to a comparable degree in performing two-choice tactile discriminations in a T-maze. In contrast, only rats with nucleus basalis lesions showed deficits in working memory as tested in an eight-arm radial maze. Both lesion groups performed comparably to sham controls on a test of reference memory involving a black/white discrimination in a T-maze. The findings suggest that rats with nucleus basalis lesions manifest disturbances in several of the same spheres (emotionality, somatosensory information processing, memory) that are disrupted in Alzheimer's disease and further confirm the utility of the excitotoxin lesion approach for studying the pathophysiology of Alzheimer's disease.

TRH analog MK-771 reverses neurochemical and learning deficits in medial septal-lesioned rats

Microinjection of ibotenic acid into medial septum of rats decreased choline acetyltransferase (CAT) and high-affinity choline uptake (HACU) activities in hippocampus and retarded the learning of a spatial memory task in the radial-arm maze. Administration of MK-771, a stable TRH analog, to such animals restored HACU activity in hippocampus to normal levels. Daily treatment of rats with MK-771 prior to maze running also restored the animals' learning ability. MK-771 did not enhance hippocampal HACU activity or maze performance in sham-lesioned rats. These results suggest that MK-771 reversed the ibotenic acid-induced memory deficit by restoring septohippocampal cholinergic function. MK-771 and other TRH analogs may represent novel agents for improving memory deficits produced by cholinergic insufficiency in Alzheimer's disease.