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

Spontaneous exploration of a 6-arm radial tunnel maze by basal forebrain lesioned rats: effects of the benzodiazepine receptor antagonist beta-carboline ZK 93 426

Nine days following ibotenic acid induced basal forebrain lesions or a sham-operation, rats were allowed to explore an automated six-arm radial tunnel maze. From each session, several measures of locomotor and exploratory activity were registered. Lesioned and sham-operated animals were treated with either the benzodiazepine receptor antagonist beta-carboline ZK 93 426 (5 mg/kg; IP) or vehicle (Cremofor EL 10% in saline; IP; n = 10 for each group). Treatment was carried out 30 min before each session during acquisition (seven sessions) and reversals of the maze configuration (seven sessions). Eight days following the 14th session, the animals were retested without any further drug treatment. The main results suggest that the lesion resulted in locomotor hyperactivity, an increase in the number of blind arm entries, and of choice stereotypy. Treatment with ZK 93 426 attenuated the lesion-induced alterations of locomotor and exploratory activities. During the retest, the lesioned, previously vehicle-treated rats revisited arms which they had already explored during this session more frequently than the lesioned, previously ZK-treated rats; the latter group did not differ from the sham-lesioned controls. It is concluded that basal forebrain lesioned animals explored the tunnel maze less efficiently than sham-lesioned controls and that the lesioned animals benefited from the treatment with ZK 93 426. Although the specificity of the lesion in terms of destruction of cholinergic neurons remains unsettled, and although the psychological significances of the behavioral measures obtained from the tunnel maze are not yet fully understood, these results suggest that antagonists or partial inverse agonists at the benzodiazepine receptor may be able to normalize basal forebrain lesion-induced behavioral alterations.

Enhanced detection of nucleus basalis magnocellularis lesion-induced spatial learning deficit in rats by modification of training regimen

Bilateral excitotoxic lesions of the nucleus basalis magnocellularis (NBM) in the rat cause deficits in the water maze, a spatial memory paradigm. Previous investigations aimed at reversing the water maze performance deficit with anticholinesterase treatments have been unable to demonstrate a consistent drug effect due to the relatively good acquisition of the task seen following NBM lesions. The present investigation tested three different water maze training regimens designed to separate the learning curves. F-344 rats received bilateral NBM injections of ibotenic acid; sham-operated rats served as controls. The animals were tested in three groups in the water maze as follows: (1) four trials per day with no intertrial interval (standard paradigm), (2) four trials per day with a 10-minute intertrial interval, and (3) two trials per day with no intertrial interval. Each group was tested in the water maze for five consecutive days, followed by two days of rest, and then tested for an additional five days. The two-trial per day paradigm was more difficult than the standard paradigm for both lesions and controls and yielded the most difference between lesions and controls as compared to the other two testing regimens. The 10-min intertrial interval schedule was more difficult than the standard paradigm for lesioned animals but acquisition was not affected in control rats. These data demonstrate that the nucleus basalis lesions cause a deficit in the water maze task regardless of training parameters. Further, while all rats showed some acquisition of the water maze task, training schedule affected the level of learning of both lesioned and control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Frontal cortex, timing and memory

Two sets of experiments examine the psychological functions and neural organization of the frontal lobes. The first set investigates the effects of lesions of the frontal cortex (FC) on the ability to perform temporal discriminations, using the techniques and theoretical framework of scalar timing theory. FC lesions changed the reference memory for the expected time of reinforcement, so that rats expected reinforcement later than it actually occurred. These results demonstrate that the FC modulates temporal memory. The second set of experiments examined the behavioral effects of lesions in the nucleus basalis magnocellularis (NBM), an area in the basal forebrain that has a significant projection to the frontal cortex. NBM lesions produced impairments in many different tasks assessing both recent and long-term memory. A comparison of the behavioral and neurochemical effects of different types of lesions in the NBM examines the role of cholinergic and noncholinergic neurotransmitters in these behavioral deficits. These data demonstrate that a "frontal syndrome" can follow selective lesions in the NBM, and indicate that the NBM must have a strong role in frontal lobe function.

The role of cortical sympathetic ingrowth in the behavioral effects of nucleus basalis magnocellularis lesions

Following cholinergic denervation of the neocortex by nucleus basalis magnocellularis (NBM) lesions, peripheral sympathetic fibers grow into the neocortex. Two experiments were performed to determine the behavioral effects of this neuronal rearrangement. Group I animals underwent training on a standard radial-8-arm maze task, while Group II animals learned a modified version (i.e. 4 arms baited). Following acquisition, NBM lesions were performed. Animals with lesions but without sympathetic ingrowth performed consistently better in both behavioral paradigms, than animals with NBM lesions and sympathetic ingrowth. These studies suggest that cortical sympathetic ingrowth can alter behavior and is detrimental to the learning of a spatial memory paradigm.

Basal forebrain efferents to the medial dorsal thalamic nucleus in the rhesus monkey

Thalamic efferent connections of the basal forebrain (BF); medial septal nucleus (MS), vertical limb of the diagonal band (VDB), horizontal limb of the diagonal band (HDB), nucleus basalis (NB), and ventral pallidum (VP) were investigated in twelve rhesus monkeys. In five animals, injections of radioactively labeled amino acids were placed in the BF. In four animals, the injections involved different divisions of the NB, HDB, and the most ventral part of the VDB. In those four cases, labeled fibers in the medial forebrain bundle were observed traveling caudally towards the hypothalamus where some turned dorsally to enter the inferior thalamic peduncle. These fibers terminated in the ventral half of the magnocellular part of the medial dorsal thalamic nucleus (MDmc). In a fifth case, the amino acid injection involved most of the MS and the VDB. Labeled fibers traveled caudally from the injection site and entered the stria medullaris. These fibers then traveled caudally before turning ventrally to terminate in the dorsal half of MDmc. To determine which of the diverse neuronal types in the BF gives rise to these thalamic projections, in two monkeys injections of horseradish peroxidase (HRP) were placed into MDmc. Labeled neurons were observed throughout the full extent of the NB, the VDB, the MS, and part of the VP. In order to determine the extent of the cholinergic input to MDmc from the BF, one of the HRP cases was processed for the simultaneous visualization of HRP, and acetylcholinesterase (AChE), the hydrolytic enzyme for acetylcholine, and a second case was processed for simultaneous visualization of HRP, and choline acetyltransferase (ChAT), the synthetic enzyme for acetylcholine. We observed that 30-50% of the HRP-labeled neurons were putatively cholinergic. In order to determine if the NB projection to MD is a collateral of the NB projection to orbital frontal cortex, one fluorescent retrograde tracer was injected into the orbital frontal cortex and one into MD. This case showed that approximately 5% of the BF neurons that project to MDmc also project to the orbital frontal cortex. These results confirm a significant subcortical projection by which the cholinergic system of the basal forebrain may influence higher cortical functions through the thalamus.

Anatomical substrate for neurotensin-acetylcholine interactions in the rat basal forebrain

We have previously shown by combined radioautography and acetylcholinesterase histochemistry that the distribution of 125I-neurotensin (NT) binding sites was in register with that of cholinergic neurons in the rat nucleus basalis magnocellularis (NBM). The present study utilized three experimental approaches to elaborate on the type and cellular localization of NT binding sites in the NBM. Competition studies using levocabastine, a selective blocker of the low affinity NT binding component, revealed that most of the 125I-NT binding sites labeled in the NBM are of the levocabastine-insensitive high affinity type, known to correspond to the physiologically active receptor. Ibotenic acid-induced lesions of the NBM produced a marked reduction in both cholinesterase reactivity and cellular 125I-NT binding suggesting that most of the labeled sites are associated with the cholinergic neurons themselves rather than with an afferent input to those cells. Finally, examination of the high resolution radioautographic distribution of 125I-NT binding sites in semithin sections revealed that a proportion of 125I-NT-labeled receptors is associated with the plasma membrane of magnocellular perikarya and proximal processes, thereby providing an anatomical substrate for a local action of NT in the NBM.

AF64A-induced working memory impairment: behavioral, neurochemical and histological correlates

The present studies examined the behavioral, neurochemical and histological consequences of intraventricular administration of ethylcholine aziridinium ion (AF64A). Male Long-Evans rats were trained to perform a radial arm maze task in which a one hour delay was imposed between the fourth and fifth arm selections. Following acquisition, animals were bilaterally injected with AF64A (3 nmol/side) or CSF into the lateral ventricles and allowed 14 days to recover before behavioral testing resumed. AF64A-treated animals were markedly impaired in their ability to perform this working/episodic memory task at a variety of delay intervals. In contrast to a long-lasting impairment on the radial maze task, these animals showed no impairment in their ability to acquire a simple discrimination task (reference/skill memory). Neurochemical analysis revealed a significant (50%) decrease in choline acetyltransferase (ChAT) activity in the hippocampus (HPC) 90 days following surgery. ChAT activity was not affected in the striatum, frontal and parietal cortices, cingulate or amygdala. Regional concentrations of catecholamines and indoleamines were not affected in any of these brain regions. Histological analysis of animals receiving unilateral injections of AF64A (3 nmol) into the right lateral ventricle revealed decreases in ChAT-immunoreactive (ChAT-IR) cells within the medial septum/vertical limb diagonal band (MS/VLDB), but not in nucleus accumbens, striatum or basal nucleus regions. These data suggest that: (1) intraventricular administration of AF64A can markedly impair working/episodic, as opposed to reference/skill memory, processes; (2) AF64A can be used to selectively alter presynaptic cholinergic indices within the hippocampus; and (3) the behavioral deficits resulting from AF64A administration are most likely a consequence of altered septohippocampal cholinergic function.

Reevaluation of the contribution of the basal forebrain cholinergic system to memory

In this review the relationship between the basal forebrain cholinergic system and memory is explored. It appears that different components of the cholinergic forebrain system [e.g., medial septum (MS) and nucleus basalis magnocellularis (NBM)] have dissociable mnemonic functions in animals, and comparable mnemonic functions in humans are dissociable as well. Furthermore, the cholinergic input from MS to the hippocampus and NBM to basolateral amygdala appear to be of critical importance in mediating mnemonic function. Finally, comparable impairments in memory function are found in basal forebrain-damaged rats and patients with dementia of the Alzheimer's type, suggesting that there might be important homologies between animals and humans in terms of basal forebrain-mediated memory function.

Complex maze learning in rodents as a model of age-related memory impairment

Research is reviewed concerning the age-related learning deficit observed in a 14-unit T-maze (Stone maze). Rats and mice of several strains representing different adult age groups are first trained to criterion in one-way active avoidance in a straight runway. Then training in the Stone maze is conducted which involves negotiation of five maze segments to avoid footshock. Results indicate a robust age-related impairment in acquisition observed in males and females, and in outbred, inbred, and hybrid strains. Pharmacological studies using the muscarinic antagonist, scopolamine, in young and aged rats indicate cholinergic involvement for accurate encoding during acquisition of this task. Retention aspects of storage and retrieval do not appear to be affected by scopolamine treatment. Bilateral electrolytic lesions to the fimbria-fornix of young rats also produce an acquisition deficit to implicate involvement of the septo-hippocampal cholinergic system in Stone maze learning. A salient feature of Stone maze performance is the tendency to demonstrate an alternation strategy in solving the maze. This strategy is exacerbated by impairment of cholinergic neurotransmission with either scopolamine treatment or fimbria-fornix lesions. Various models of hippocampal function are applied toward the psychological characterization of the Stone maze task without complete success. Future research is outlined to provide more thorough psychological characterization of maze performance, to analyze the specificity of cholinergic involvement in the task, and to test possible therapeutic interventions for alleviating the age-related impairments observed.

Scopolamine in rats impairs acquisition but not retention in a 14-unit T-maze

To follow up a previous report noting that scopolamine impaired acquisition performance of young rats in a shock-motivated 14-unit T-maze, the present study assessed the effects of muscarinic antagonism on retention aspects of the same task. The broader objective was to further the investigation of possible defects in cholinergic neurotransmission that might underlie the age-related impairments previously observed in this task. Young (3-month) male F-344 rats were given preliminary training to criterion in one-way active avoidance in a straight runway. Then on the first day of complex maze training, each rat received 5 acquisition (AQ) trials followed by a second 10-trial retention (RET) session conducted the following day. Subjects were assigned to one of eight groups receiving an intraperitoneal injection of either scopolamine hydrochloride (1.0 mg/kg) or saline as follows: (a) 30 min prior to training on the first day (PRE-AQ); (b) 30 min prior to training on both the first and second day (PRE-AQ-RET); (c) immediately after completing the trial on the first day (POST-AQ); (d) 30 min prior to testing on the second day (PRE-RET). Dependent measures included errors, alternation errors, run time, number of shocks, and total shock received. On the first day of maze training, all performance measures except for alternation errors were significantly higher for the two acquisition groups (PRE-AQ and PRE-AQ-RET) compared to all other groups which did not differ significantly.(ABSTRACT TRUNCATED AT 250 WORDS)

Memory disturbances following ibotenic acid injections in the nucleus basalis magnocellularis of the rat

The behavioral effects of lesions of the nucleus basalis magnocellularis (NBM) on two spatial discrimination tasks (place navigation and cross maze) were examined in the rat. These tasks were designed to test reference memory. Lesions by bilateral injection of ibotenic acid into the NBM led to a severe and permanent impairment in the learning of the cross maze task. In the learning of the place navigation task, the rats with lesions showed only a transient deficit. Immediately after the removal of the platform, the rats with lesions explored the quadrant (NE) previously containing the platform as long as controls and above chance levels. The rats with lesions did not extinguish exploration like the controls, seen as a reduction both in time spent in the NE quadrant and in swimming activity. Taken together, the results showed that (1) NBM lesions impair reference memory, but (2) spare other aspects of memory. On the basis of the results in the place navigation task, procedural memory was assumed to remain intact after lesion of the NBM. Biochemical assays of choline acetyltransferase (ChAT) in various brain regions in the lesioned animals demonstrated a reduced ChAT activity in the neocortical projections of the NBM but not in the hippocampus. However, it cannot be decided from this work whether behavioral deficits result from the lesion of cholinergic or of non-cholinergic cells in the NBM.

Galanin impairs acquisition but not retrieval of spatial memory in rats studied in the Morris swim maze

The effect of intraventricular administration of the neuropeptide galanin on acquisition and retrieval in a modified Morris swim maze was studied in rats. Galanin induced a significant deficit in the acquisition of the task while no effects on the retrieval were observed. No deficits were seen 24 h after the last treatment. Galanin did not increase the number of failures to reach the platform. It is suggested that endogenous galanin modulates learning possibly via the galanin-containing cholinergic neurons in the septum-basal forebrain area projecting to the hippocampus and cortex.

Animal models of Alzheimer's disease: experimental cholinergic denervation

Several animal models of AD have been developed, based upon the consistent finding of a presynaptic cholinergic deficit in AD. Significant cell loss in the NBM, the primary cortical cholinergic afferent, has been reported in AD. Lesions of the corresponding nuclei in the rodent and primate produce a persistent cholinergic deficit, but no consistent change in other neurotransmitter systems. Significant mnestic and cerebral metabolic deficits are observed acutely after lesion, which are responsive to pharmacological reversal and recover over time. Administration of AF64A produces similar mnestic and cholinergic deficits as NBM lesion, but these effects may be less responsive to pharmacological reversal. Administration of scopolamine, a muscarinic receptor antagonist, produces transient receptor blockade, mnestic deficits and deficits in cerebral metabolism, which can be reversed with a variety of pharmacological agents. The primary dissociations between these models and the deficits in AD are the lack of pharmacological response and recovery of function in AD patients and the presence of non-cholinergic neurochemical and cytoskeletal abnormalities. Future research should focus upon the systematic production and analysis of non-cholinergic neurotransmitter and cytoskeletal abnormalities to determine the contribution of these factors to the pathology seen in AD and the production of deficit in aged animals, which may more closely approximate the deficits in AD. The analysis of factors involved in recovery of function and pharmacological response in animal models may provide insight into potential treatment approaches to AD.

Persisting behavioral and neurochemical deficits in rats following lesions of the basal forebrain

The effects of excitotoxic lesions of the nucleus basalis magnocellularis on cortical cholinergic activity and passive avoidance performance were examined in rats at 6, 14, 84 and 180 days after lesioning. Lesioned rats showed significant impairment of passive avoidance retention at every time point tested, with no evidence of behavioral recovery compared to unoperated and sham-lesioned (i.e., vehicle-injected) control rats. Cortical choline acetyltransferase (CAT) activity was reduced relative to controls at all time points examined, with the greatest reduction (i.e., 28%) occurring at approximately 14 days after lesioning. The levels of CAT activity at 180 postlesioning remained reduced compared to control animal levels, but less so than at 14 days after lesioning, indicating partial recovery. No changes in cholinergic muscarinic binding were observed at any time following lesioning. The results indicate that the behavioral and neurochemical effects of NbM lesions persist for at least 6 months following lesioning, but that partial, gradual recovery of cholinergic activity occurs.

Brainstem afferents to the magnocellular basal forebrain studied by axonal transport, immunohistochemistry, and electrophysiology in the rat

Brainstem afferents to the magnocellular basal forebrain were studied by using tract tracing, immunohistochemistry and extracellular recordings in the rat. WGA-HRP injections into the horizontal limb of the diagonal band (HDB) and the magnocellular preoptic area (MgPA) retrogradely labelled many neurons in the pedunculopontine and laterodorsal tegmental nuclei, dorsal raphe nucleus, and ventral tegmental area. Areas with moderate numbers of retrogradely labelled neurons included the median raphe nucleus, and area lateral to the medial longitudinal fasciculus in the pons, the locus ceruleus, and the medial parabrachial nucleus. A few labelled neurons were seen in the substantia nigra pars compacta, mesencephalic and pontine reticular formation, a midline area in the pontine central gray, lateral parabrachial nucleus, raphe magnus, prepositus hypoglossal nucleus, nucleus of the solitary tract, and ventrolateral medulla. A similar but not identical distribution of labelled neurons was seen following WGA-HRP injections into the nucleus basalis magnocellularis. The possible neurotransmitter content of some of these afferents to the HDB/MgPA was examined by combining retrograde Fluoro-Gold labelling and immunofluorescence. In the mesopontine tegmentum, many retrogradely labelled neurons were immunoreactive for choline acetyltransferase. In the dorsal raphe nucleus, some retrogradely labelled neurons were positive for serotonin and some for tyrosine hydroxylase (TH); however, the majority of retrogradely labelled neurons in this region were not immunoreactive for either marker. The ventral tegmental area, substantia nigra pars compacta, and locus ceruleus contained retrogradely labelled neurons which were also immunoreactive for TH. Of the retrogradely labelled neurons occasionally observed in the nucleus of the solitary tract, prepositus hypoglossal nucleus, and ventrolateral medulla, some were immunoreactive for either TH or phenylethanolamine-N-methyltransferase. To characterize functionally some of these brainstem afferents, extracellular recordings were made from antidromically identified cortically projecting neurons, mostly located in the HDB and MgPA. In agreement with most previous studies, about half (48%) of these neurons were spontaneously active. Electrical stimulation in the vicinity of the pedunculopontine tegmental and dorsal raphe nuclei elicited either excitatory or inhibitory responses in 21% (13/62) of the cortically projecting neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Radial-arm maze deficits produced by colchicine administered into the area of the nucleus basalis are ameliorated by cholinergic agents

Rats were given bilateral injections of colchicine into the area of the nucleus basalis. Colchicine produced dose-dependent alterations in the acquisition of a food-reinforced working-memory task. Colchicine-induced deficits in maze performance were attenuated by cholinergic agents, including physostigmine, RS-86 (2-ethyl-8-methyl-2,8-diazospiro-(4,5)-decan-1,3-dione-hydro bromide) and nicotine. Naloxone and vasopressin did not affect radial-arm maze performance of colchicine-treated rats. Subsequent neurochemical analysis showed that colchicine decreased choline acetyltransferase (ChAT) activity and levels of norepinephrine, dopamine, 3,4-dihydroxyphenylacetic acid, serotonin and 5-hydroxyindoleacetic acid in the neocortex. However, ChAT activity and other neurochemical measures were not altered in the hippocampus or corpus striatum. Histological assessment indicated damage limited to the injection in the area of the nucleus basalis and enlarged cerebrolateral ventricles. These data suggest the possible utility of the colchicine model in the study of cognitive deficits associated with neurodegenerative diseases.

Place navigation in rats is impaired by lesions of medial septum and diagonal band but not nucleus basalis magnocellularis

The role of forebrain cholinergic projections in place navigation learning was assessed in two experiments. Following surgery, rats were required to learn the spatial location of an underwater platform on the basis of distal room cues. Bilateral injections of ibotenic acid into the nucleus basalis magnocellularis depleted choline acetyltransferase (ChAT) from the anterior and temporoparietal cortex but not the hippocampus. Separate histological studies confirmed the accuracy of the lesions and demonstrated a marked loss of cortical acetylcholinesterase. These rats subsequently showed no deficits in spatial learning or memory. In a second experiment, bilateral lesions of the vertical limb of the diagonal band of Broca and medial septum depleted ChAT from the hippocampus and posterior cortex but not the anterior cortex. Histological studies confirmed the accuracy of the lesion and showed a pronounced loss of acetylcholinesterase from the hippocampus. These rats were deficient in spatial learning and showed reduced spatial bias during transfer tests. The data are discussed in the light of the hypothesis that the cholinergic innervation of the hippocampus plays a key role in spatial reference memory processes involved in place navigation.

Effects of nucleus basalis lesions on the muscarinic and nicotinic modulation of [3H]acetylcholine release in the rat cerebral cortex

Presynaptic muscarinic and nicotinic receptors in the cerebral cortex reportedly inhibit and increase acetylcholine (ACh) release, respectively. In this study, we investigated whether these receptors reside on cholinergic nerve terminals projecting to the cerebral cortex from the nucleus basalis magnocellularis (nbm). Adult male rats received unilateral infusions of ibotenic acid (5 micrograms/1 microliter) in the nbm. Two weeks later, cerebral cortical cholinergic markers (choline acetyltransferase activity, high-affinity choline uptake, and coupled ACh synthesis) were significantly reduced in synaptosomes prepared from the lesioned hemispheres compared to contralateral controls. The depolarization-induced release of [3H]ACh from these synaptosomes was also reduced in the lesioned hemispheres, reflecting the reduced synthesis of transmitter. However, the nbm lesions had no effect on the inhibition of release induced by 100 microM oxotremorine. Synaptosomal [3H]ACh release was not altered by nicotine or the nicotinic agonists anabaseine and 2-(3-pyridyl)-1,4,5,6-tetrahydropyrimidine. Nicotine (10-100 microM) did increase [3H]ACh release in control and lesioned hemispheres in cortical minces, but to a similar extent. These results suggest that neither muscarinic nor nicotinic receptors modulating ACh release reside on nbm-cholinergic terminals.

Long-term neuropathological and neurochemical effects of nucleus basalis lesions in the rat

The long-term effects of excitotoxic lesions in the nucleus basalis magnocellularis of the rat were found to mimic several neuropathological and chemical changes associated with Alzheimer's disease. Neuritic plaque-like structures, neurofibrillary changes, and neuronal atrophy or loss were observed in the frontoparietal cortex, hippocampus, amygdala, and entorhinal cortex 14 months after the lesions were made. Cholinergic markers in neocortex were reduced, while catecholamine and indoleamine metabolism was largely unaffected at this time. Bilateral lesions of the nucleus basalis magnocellularis increased somatostatin and neuropeptide Y in the cortex of the rat by at least 138 and 284 percent, respectively, suggesting a functional interaction between cholinergic and peptidergic neurons that may differ from that in Alzheimer's disease.

Short-term memory, exploration and locomotor activity in aged rats

Behavioral profiles of young (3-6 months) and aged (24-27 months) rats were compared in three respects: (1) short-term memory, (2) exploration and (3) locomotor activity. Compared to young rats, aged rats were impaired in the 8-arm radial maze acquisition, but not in the delayed reinforced alternation acquisition. They had lower scores of spontaneous alternation, of exploration of a novel object and of a novel environment. Their exploratory activity was lower in a simple environment but similar in a complex environment. Their spontaneous locomotor activity was lower during the dark part of the cycle (8 p.m.-8 a.m.) but not different during the light part of the cycle (8 a.m.-8 p.m.). These results suggest that with respect to short-term memory and exploration, differences between aged and young rats may be influenced by a "complexity" factor and may be secondary to differences in motivation and reactions to novelty.

The effects of excitotoxic lesions of the substantia innominata, ventral and dorsal globus pallidus on the acquisition and retention of a conditional visual discrimination: implications for cholinergic hypotheses of learning and memory

The effects of ibotenic acid-induced lesions of the ventral pallidum/substantia innominata region, the dorsal pallidum or both on the acquisition and retention of a conditional visual discrimination have been studied in the rat. Lesions of the ventral pallidum and large lesions of the dorsal and ventral pallidum severely impaired both the acquisition and retention of the conditional discrimination. Dorsal pallidal lesions had similar, but less marked effects. The same lesions also impaired the retention of a passive avoidance task, but had no effect on a conditioned taste aversion. Neurobiological investigations revealed that the lesions destroyed cholinergic neurons in the magnocellular nucleus basalis and caused reductions in cortical choline acetyltransferase activity of about 30-40%. Tract-tracing experiments indicated that the lesions destroyed, in particular, cholinergic neurons projecting to the frontal dorsolateral cortex and also those projecting to more posterior cortex, but not the occipital lobes. Contingency analysis of the behavioural, neurochemical and neuroanatomical data indicated that those animals with the largest decreases in choline acetyltransferase activity, or the largest areas of neuronal loss in the ventral and dorsal globus pallidus, were most impaired in the retention of the conditional discrimination. The results do not, therefore, indicate a simple relationship between cholinergic neuronal loss and the retention of response rules essential for performance of the task ("reference memory"). The relevance of the results to cholinergic hypotheses of learning and memory is discussed.

Age-related shrinkage of cortically projecting cholinergic neurons: a selective effect

The number and size of basal forebrain neurons that provide the cholinergic innervation for the cerebral cortex, amygdala, and hippocampus were studied in young and aged mice. The results showed that these neurons became substantially smaller with increasing age. This effect was relatively selective, since the immediately adjacent cholinergic neurons in the striatum did not show a change of similar magnitude. The shrinkage of these basal forebrain neurons may account for the decline of cholinergic innervation that occurs with age. In the material that we examined, aging did not influence the number of cholinergic neurons in the basal forebrain, only their size. It seems, therefore, that the age-related changes in cholinergic function (and their putative behavioral consequences) are not associated with a substantial component of irreversible cell death.

Cholinergic-dependent cognitive deficits in Parkinson's disease

In a double-blind cross-over study, the effects of a subthreshold dose of scopolamine (0.25 mg) on memory were compared in 32 control subjects and 32 parkinsonian patients who were without any sign of intellectual and mnemic impairment. Although the scores of the controls in the memory test battery showed no deterioration after the administration of scopolamine, the same dose resulted in significantly reduced memory performance in parkinsonian patients in two tests which involved the recognition of meaningless drawings. The selective vulnerability of parkinsonian subjects without cognitive impairment to a subthreshold dose of scopolamine suggests the existence of an underlying alteration of central cholinergic transmission. The neuropsychological findings in our study agree with postmortem biochemical data, which showed decreased cortical choline acetyltransferase activity in all parkinsonian patients, suggesting the existence of neuronal compensation in parkinsonian patients who are without cognitive impairment.

AF64A (ethylcholine aziridinium ion), a cholinergic neurotoxin, selectively impairs working memory in a multiple component T-maze task

The present study examined the nature of the cognitive deficits associated with a selective decrease of cholinergic activity in the hippocampus. Male Fischer rats were trained to perform a multiple component T-maze task which simultaneously assessed their ability to perform on the basis of trial-specific information (working memory) and trial-independent information (reference memory). Following 125 acquisition trials rats were bilaterally injected with AF64A (3 nmol/side) or artificial CSF into the lateral ventricles and allowed 14 days to recover before behavioral testing resumed. The controls rapidly returned to their preoperative level of performance on both components of the maze task. AF64A-treated animals were transiently impaired on the reference memory task. Their performance rapidly improved and they were performing at preoperative levels within 4 days of testing. In contrast, these animals exhibited a marked and long-lasting impairment in their performance of the working memory component. After behavioral testing was completed, neurochemical analysis revealed that AF64A produced a significant decrease in choline acetyltransferase (ChAT) activity in the hippocampus (43%) 42 days following surgery. This dosing regimen produced no alterations of striatal or cortical ChAT activity. These data suggest that alterations of hippocampal cholinergic activity severely impair an animal's ability to perform working memory tasks.

Behavioural, biochemical and histochemical effects of different neurotoxic amino acids injected into nucleus basalis magnocellularis of rats

Lesions of the nucleus basalis magnocellularis in rats have been used to investigate functions of the extrinsic cortical cholinergic system which originates from these neurons. These lesions also produce extensive non-specific subcortical damage and associated regulatory and neurological impairments, causing doubt about the specificity of consequent functional impairments. Here, nucleus basalis magnocellularis lesions made with four different neurotoxic amino acids (kainic acid, ibotenic acid, N-methyl-D-aspartate, and quisqualic acid) have been compared. Quisqualic acid produced less subcortical damage and lesser neurological and regulatory impairments than the other toxins at doses that produced comparable cholinergic deafferentation of the neocortex, as assessed both histologically and biochemically. This suggests that these impairments are non-specific rather than specific consequences of cholinergic cell loss. The effects on learning a spatial navigation task were more ambiguous, suggesting the involvement of both cholinergic and non-cholinergic systems. Impairment of a passive shock avoidance task was as great following quisqualic acid as the other neurotoxins, which may suggest a more direct relationship specifically with the decline in cortical cholinergic activity. It is concluded that in the absence of availability of a specific cholinergic neurotoxin, quisqualic acid produces less non-specific neuroanatomical and neurological side effects than the more widely used toxins N-methyl-D-aspartate, kainic acid or ibotenic acid.

Blockade of spatial learning by the M1 muscarinic antagonist pirenzepine

Two experiments were conducted to determine the effects of the M1 muscarinic receptor antagonist pirenzepine on place navigation in a water maze. In the first experiment rats were required to learn the location of a hidden platform following intracerebroventricular injections of equimolar doses of pirenzepine or scopolamine methylbromide. Both drugs dose-dependently impaired spatial learning according to both escape latency data and transfer test analysis. Pirenzepine was approximately 3 times less potent than scopolamine, a potency ratio which suggests M1 receptor mediation of the impairment. In the second experiment pirenzepine (1 approximately 92.3 micrograms/rat ICV) was injected prior to training on a simultaneous place discrimination task in the water maze. Impairments of choice accuracy were found with a dose of 20 micrograms/rat in the absence of any marked increases in either errors of omission or choice latency. These data suggest that M1 receptor blockade impairs processes which are involved in spatial learning.

Evidence for neocortical involvement in reference memory

Rats were trained on an eight-arm radial maze task using a procedure that provides for an assessment of both working and reference memory. Following training, rats received parietal cortex, medial prefrontal cortex, visual cortex, or nucleus basalis magnocellularis lesions. Rats with visual cortex lesions showed no change in performance on either working or reference memory. Rats with parietal cortex lesions displayed a temporary deficit in reference, but no deficit on working memory. Animals with medial prefrontal cortex lesions showed a temporary deficit on both working and reference memory. Rats with extensive lateral frontal and parietal cortex depletion of acetylcholinesterase following nucleus basalis magnocellularis lesions had a marked disruption only of reference but not of working memory. It is concluded that neocortex and possibly the cholinergic projections to neocortex play an important role in mediating reference memory.

Cholinergic function and memory: extensive inhibition of choline acetyltransferase fails to impair radial maze performance in rats

The present study investigated the effects of a potent inhibitor of choline acetyltransferase (ChAT), BW813U, on the choice accuracy of rats in the radial arm maze. BW813U (100 mg/kg, IP) produced a rapid (within 1 hour) and substantial decrease in ChAT activity throughout the brain, ranging from 66% (hippocampus) to 80% (caudate nucleus) that lasted up to 5 days. A single injection (50 mg/kg, IP) into rats with lesions (using ibotenic acid) in the nucleus basalis magnocellularis and medial septal area, decreased ChAT activity by 75% and 60% in the cortex and hippocampus, respectively. Lesioned and unlesioned rats were trained on the radial arm maze until they reached a criterion level of performance. Each rat then received an injection of BW813U (50 or 100 mg/kg, IP). Choice accuracy was not impaired at any time following the injection. The lack of effect on performance may be due to 2 possible factors: The radial maze retention paradigm chosen may not be sufficiently difficult, or the decrease in acetylcholine production was not sufficient to affect behavior. Compensation by non-cholinergic neural systems might account for the insensitivity of the rats to significant cholinergic depletion.

[3H]ketanserin (serotonin type 2) binding increases in rat cortex following basal forebrain lesions with ibotenic acid

The response of the serotonergic system following injury to the basal forebrain cholinergic system was investigated in rats. The density of 5-hydroxytryptamine (serotonin) type 2 (S2) receptor sites in the frontal cortex and hippocampus was determined 1 week and 4 months after production of lesions by injections of ibotenic acid into the medial septum and nucleus basalis magnocellularis. One week later, the number of S2 receptor sites in the frontal neocortex, as defined by [3H]ketanserin binding, was unchanged. Four months later, the number of [3H]ketanserin binding sites (and Bmax) was increased and high-affinity [3H]serotonin uptake was decreased in the frontal neocortex, but not in the hippocampus, relative to unlesioned controls. Choline acetyltransferase (acetyl-CoA:choline O-acetyltransferase; EC 2.3.1.6) activity was decreased significantly in the frontal neocortex and hippocampus 1 week and 4 months after surgery. The change in frontal neocortical S2 receptor site density was inversely related to the level of choline acetyltransferase activity, was specific for cholinergic denervation associated with the cortex but not the hippocampus, and may represent a localized denervation supersensitivity due to degeneration of median raphe cortical afferents.

Kynurenic acid-induced protection of neurochemical and behavioural deficits produced by quinolinic acid injections into the nucleus basalis of rats

Injection of the endogenous tryptophan metabolite, quinolinic acid (120 nmol in 1.0 microliter) unilaterally into the basal forebrain of rats resulted in a significant ipsilateral decrease in cortical choline acetyltransferase activity, suggesting that cholinergic cells of the nucleus basalis magnocellularis (nbm) were damaged. Injected animals also showed a significant deficit in performance on an 8-arm radial maze, compared to sham operated controls, indicating an impairment of memory. Co-injection of another endogenous tryptophan metabolite, kynurenic acid (360 nmol in 1.0 microliter) with quinolinic acid afforded an almost complete protection against the neurotoxic and memory-impairing effects of quinolinic acid alone. These findings support previous reports that kynurenic acid can protect against the neurotoxic effects of quinolinic acid and indicate for the first time that kynurenic acid can also protect against impairments of memory produced by injection of quinolinic acid into the nbm.

Medial septal and nucleus basalis magnocellularis lesions produce order memory deficits in rats which mimic symptomatology of Alzheimer's disease

Rats with electrolytic lesions of the medial septum or ibotenic acid lesions of the nucleus basalis magnocellularis (NBM) were tested in an order memory task for an 8-item list of varying spatial locations within an 8-arm radial maze. Results indicated that rats with small medial septal lesions resulting in small AchE depletion of dorsal hippocampal formation were impaired only for the first, but not the last choice orders of the list. Animals with large medial septal lesions resulting in large AchE depletion of the dorsal hippocampal formation displayed an order memory deficit for all the choice orders of the list. In contrast, rats with small NBM lesions resulting in small AchE depletion of parietal and part of frontal cortex were impaired only for the last, but not the first choice orders of the list. Animals with large NBM lesions resulting in large AchE depletion of parietal and part of frontal cortex displayed an order memory deficit for all the choice orders of the list. The relationship between these findings and mnemonic symptomatology of Alzheimer's disease was discussed, as was the possible meaning of these results in providing an animal model for studying certain aspects of the disease.

Behavioral recovery following bilateral lesions of the nucleus basalis does not occur spontaneously

Recent studies have shown that rats given bilateral ibotenic acid lesions of the nucleus basalis (NBM) exhibit significant impairments on tasks requiring recent or trial-specific memory. However, despite the persistence of cholinergic deficiencies in the cortical projection area, the memory impairments gradually recover over a period of several months of training. Moreover, in one study, the behavioral recovery on a radial arm maze retention task was shown to generalize to a completely different behavior paradigm (passive avoidance) on which the animals had received no prior experience. The present study was performed to determine the extent to which this generalized recovery of performance on memory tasks is dependent upon extensive post-lesion training. Rats were given ibotenic acid lesions of the NBM and were then passively detained in their home cages for six months. Contrary to animals which had received post-surgical radial arm maze experience, the animals detained in their home cages displayed a significant retention impairment when tested on the passive avoidance task, suggesting that the experience the animals receive is an important factor for whether post-lesion functional recovery occurs. This study also confirms that the loss of cholinergic markers following bilateral, NBM lesions persists for at least several months, or longer.

Memory impairments following basal forebrain lesions

The functional contribution of the nucleus basalis magnocellularis (NBM) and medial septal area (MSA) to memory was evaluated in 4 behavioral tasks. The tasks were postoperative acquisition of a win-stay spatial discrimination in a T-maze, a win-shift spatial discrimination on a radial arm maze, active avoidance in a shuttle box, and passive avoidance in a shuttle box. Bilateral lesions were made by injecting ibotenic acid (IBO) into the NBM or MSA. Control rats received operations in which no neurotoxin was injected. When compared to controls, rats with lesions in either the NBM or MSA had significantly impaired choice accuracy in the T-maze and radial maze tasks, took significantly fewer trials to reach criterion in the acquisition, but not the retention of an active avoidance task, and significantly more trials to reach criterion in the passive avoidance task. The results show that equivalent behavioral changes are obtained from lesions in the NBM and MSA in tasks that vary in their type of motivation, reinforcement, response-reinforcement contingency, and response. These behavioral changes suggest that the NBM and MSA may both be involved in memory.