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

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

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

A specific form of cognitive rigidity following excitotoxic lesions of the basal forebrain in marmosets

The effects of N-methyl-D-aspartate-induced lesions of the basal forebrain were studied on performance of a series of visual discrimination tests that examined a range of cognitive functions in the marmoset. These included the ability to attend to the various dimensional properties of stimuli and to use just one of these properties in order to solve a discrimination (intra-dimensional shift); to switch attention from one dimension to another (extra-dimensional shift); to learn the reinforcement value of specific exemplars within a dimension (new learning); and to relearn their reinforcement value following reversal of the reward contingencies (serial reversals). Lesions of the basal forebrain did not impair the ability either to attend selectively to the dimensional properties of the stimuli or to switch attention from one dimension to the other. However, the lesion did affect various aspects of associative learning including a transient impairment of new learning and a marked disruption of serial reversal learning. The reversal deficit could be characterised as a tendency to perseverate on the previously correct stimulus and as a failure to to show the formation of a reversal learning set. In addition, the lesion prevented disruption of performance of a well-learned discrimination when novel exemplars from the irrelevant dimension were introduced (probe test). It is suggested that the functional effects of the basal forebrain lesion reflect impaired learning of stimulus-reward associations and behavioural rigidity. The finding, however, that there was no effect of the lesion on attentional set-shifting suggests that any loss of inhibitory control was specific to the level of stimulus-response or stimulus-reward associations, inhibitory control at the level of attentional selection remaining intact. The similarity of the effects of damage to the basal forebrain to those seen following damage to the orbitofrontal cortex and the amygdala are discussed in the context of the close anatomical and functional relationships that exist among these three structures.

Effects of tetrahydroaminoacridine on spatial navigation of nucleus-basalis- and frontal-cortex-lesioned rats

The present study investigates the effects of tetrahydroaminoacridine (THA: 1 and 3 mg/kg) on water maze (WM) spatial learning performance of intact, nucleus-basalis- (NB) lesioned, frontal-cortex- (FR) lesioned, or NB + FR-lesioned rats. NB lesions did not impair WM learning and had no effect on the WM performance deficit in FR-lesioned rats. THA at 1 or 3 mg/kg did not improve WM spatial memory of intact, NB-, FR-, or NB + FR-lesioned rats. These results suggest that 1) the cholinergic NB system is not a prerequisite for frontally mediated acquisition of WM performance, 2) THA treatment does not enhance spatial memory, and 3) THA is not effective in alleviating cognitive deficits induced by degeneration of the frontal cortex.

The neurotoxic actions of ibotenic acid on cholinergic and opioid peptidergic systems in the central nervous system of the rat

The neurotoxic effects produced by ibotenic acid (IA) induced chemical lesions of the central nervous system (CNS) cholinergic system were examined on the opioid peptidergic system in adult rats. Forebrain cholinergic systems were bilaterally lesioned by the infusion of IA (1 or 5 micrograms/site) into the nucleus basalis magnocellularis (NBM). One week after the injections, the animals were sacrificed, and activities of acetylcholinesterase (AChE), choline acetyltransferase (ChAT) and concentrations of beta-endorphin (beta-End) and Met-enkephalin (Met-Enk) were measured in different brain regions. Animals treated with IA showed a decrease in the activity of ChAT (-24%), AChE (-36%) and beta-End level (-33%) in the frontoparietal cortex (FC). For the first time we report that these changes were associated with a compensatory increase in the activity of ChAT (+27%), AChE (+25%), beta-End level (+66%) in the remaining part of the cortex, i.e. cortex devoid of frontal cortex (C-FC). Met-enkephalin level increased by 59% in the frontoparietal cortex and did not change in the cortex devoid of frontal cortex upon IA treatment. These results suggest that IA treatment results in changes in the activity of cortical ChAT and AChE, and beta-End level in the same direction. Injection of IA in the NBM did not cause a change in the activity of ChAT or AChE in other brain regions such as hippocampus, striatum or midbrain. In addition to cortex devoid of frontal cortex, midbrain also showed a significant increase in the beta-End level in the IA treated animals. However, pituitary beta-End decreased in the neurotoxin treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Widespread neuronal degeneration after ibotenic acid lesioning of cholinergic neurons in the nucleus basalis revealed by in situ hybridization

In efforts to test the cholinergic hypothesis for Alzheimer's disease and to create an animal model for this disease, ibotenic acid has been used to lesion cholinergic neurons in the basal forebrain. In this study we have used in situ hybridization with oligonucleotide probes specific for mRNAs encoding choline acetyltransferase and glutamic acid decarboxylase, respectively, to study the effects of such a lesion. Our results show that lesion paradigms normally used to induce neuronal degeneration in nucleus basalis by ibotenic acid not only lesion the cholinergic neurons within this nucleus, but in addition, a major fraction of gamma-aminobutyric acid (GABA) neurons in nucleus basalis, substantia innominata, globus pallidus and ventral pallidum.

Progressive decline in spatial learning and integrity of forebrain cholinergic neurons in rats during aging

Rats distributed over five different age groups, 3, 12, 18, 24 and 30 months of age, were screened for their spatial learning and memory ability in the Morris water maze, and the degree of place navigational impairments was correlated with morphological changes in the four major forebrain cholinergic cell groups (medial septum, MS; vertical limb of the diagonal band of Broca, VDB; nucleus basalis magnocellularis, NBM; and striatum) using choline acetyltransferase (ChAT) and nerve growth factor receptor (NGFr) histochemistry. Impaired place navigation developed progressively with age, such that 8% of the 12-month-old rats, 45% of the 18-month-old, 53% of the 24-month-old, and over 90% of the 30-month-old rats were behaviorally impaired. Significant reductions in the number of ChAT/NGFr-positive cell bodies, amounting to between 19 and 45%, were observed in all four cell groups, and the remaining cells were reduced in size (6-24% reduction in cross-sectional area in the oldest age groups). Although the morphological changes were less severe and tended to develop later than the behavioral impairments, there was overall a significant correlation between water maze performance and ChAT/NGFr-positive cell counts, and to a lesser degree also cell size in all four cell groups. These changes were also highly correlated with age. The highest correlations were seen in MS, VDB and NBM, which are known to play a role in spatial memory performance in young rats. The results indicate that degenerative and/or atrophic changes in the forebrain cholinergic system and decline in spatial learning ability are parallel processes during aging. Although the magnitude of the morphological changes does not appear to be substantial enough, by itself, to explain the severe spatial learning impairments that develop in the oldest animals, the present data are consistent with the view that impaired function in the forebrain cholinergic system can contribute to age-dependent cognitive decline in rodents.

Blockade of conditioned taste aversion by scopolamine and N-methyl scopolamine: associative conditioning, not amnesia

The anticholinergic, scopolamine, consistently disrupts one-trial passive avoidance conditioning but the effects of such drugs on one-trial conditioned taste aversion (CTA) are variable and contradictory. In the present study, treatment of rats with scopolamine impaired the suppression of sucrose intake by post-ingestion administration of lithium chloride (LiCl) in a two-bottle choice test. A similar effect was obtained by using N-methyl scopolamine which penetrates the brain only to a limited degree on acute administration. The blockade of CTA could be prevented in three ways: (i) by exposing the rats to sucrose only on the training day, (ii) by pre-exposing the rats to both sucrose and scopolamine, and (iii) by using a less palatable sucrose/ascorbate mixture. The results demonstrate that the effect of scopolamine on taste aversion is not mediated by the central nervous system, and can be modified by altering the novelty and relative salience of the taste conditioned stimulus. These experiments suggest that conditioned associations between taste and LiCl, and scopolamine and LiCl may underlie the blockade of CTA by scopolamine.

Disruptive effects of muscimol infused into the basal forebrain on conditional discrimination and visual attention: differential interactions with cholinergic mechanisms

The behavioral effects of GABAergic manipulation of the basal forebrain were investigated using two behavioral tasks, which previous studies have shown to yield dissociable effects following quisqualate-induced lesions of the basal forebrain: a five-choice serial reaction time task, involving approaching the location of a brief visual stimulus that is associated with reward; and a conditional visual discrimination task, requiring retrieval of information about a discriminative stimulus that stays constant over time. Following acquisition of the tasks, chronic guide cannulae were stereotaxically implanted into the basal forebrain. Those animals trained on the conditional visual discrimination task showed a dose-dependent reduction in choice accuracy and a lengthening of latency to respond correctly to the visual stimulus following administration of the GABA-A agonist, muscimol (1, 2, 3 ng/microliters/hem). While certain of these deficits, for example response latency, could be restored to control levels by co-administration of the GABA-A antagonist, bicuculline, none of the behavioural impairments could be significantly attenuated by systemic by systemic co-administration of the cholinesterase inhibitor, physostigmine (0.05, 0.1, 0.2 mg/kg, IP). Similarly, a dose dependent effect of muscimol (1, 1.5, 2 ng/microliters/hem) on choice accuracy and correct response latency was observed on performance of the five-choice attentional task. However, in contrast to the conditional task, significant attenuation of the impairment in choice accuracy was obtained following administration of physostigmine (0.05 and 0.1 mg/kg). Attenuation of muscimol-induced deficits by administration of bicuculline was also observed. It is therefore evident that although manipulation of GABAergic activity in the region of the basal forebrain produces profound deficits in different tasks of cognitive function, only some of these may be due to modulation of the magnocellular cholinergic projection to the neocortex.

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.

Effects of THA on passive avoidance retention performance of intact, nucleus basalis, frontal cortex and nucleus basalis + frontal cortex-lesioned rats

Unilateral quisqualic acid lesions of the nucleus basalis magnocellularis (NBM) produced marked choline acetyltransferase depletion (-67% ipsilateral to lesion) and impaired passive avoidance (PA) retention at 24 hours. Pretraining injections of tacrine (THA: 1, 3 and 5 mg/kg), an anticholinesterase, failed to facilitate PA retention in intact rats. However, the retention performance of NBM-lesioned rats was improved by pretraining administration of THA at 3 mg/kg but not at either 1 or 5 mg/kg. Frontal cortex lesioning did not impair PA retention, and THA at 3 mg/kg had no effect on the PA retention of frontal cortex-lesioned rats. THA at 3 mg/kg failed to improve retention performance of NBM + frontal cortex-lesioned rats. After 10 days of chronic treatment with THA, NBM lesion-induced PA retention deficits were partially restored at both 3- and 5-mg/kg doses. The results suggest that 1) the insult to cholinergic neurons in the NBM may be involved in the PA memory consolidation deficit induced by nonselective quisqualic acid lesioning; 2) the beneficial effects of THA on NBM lesion-induced PA retention deficit occur in a narrow dose range; 3) the alleviating effects of THA on NBM lesion-induced PA memory deficits are blocked by frontal cortex lesions; and 4) the dose-response window for THA-induced PA retention performance improvement is broadened by repeated treatment.

Increased GABAergic transmission aggravates nucleus basalis magnocellularis lesion-induced behavioral deficits

Quisqualic acid NBM lesions had no effect on water maze performance, but slightly impaired passive avoidance acquisition. GammavinylGABA treatment alone had no effect on the passive avoidance and water maze performance, but aggravated acquisition deficit in rats subjected to NBM lesioning. However, gammavinylGABA-treated NBM-lesioned rats reached control level of performance.

Comparison of the effects of acute and chronic ibotenic and quisqualic acid nucleus basalis lesioning

The present study examines the effects of acute (1 month recovery) and chronic (8 month recovery) bilateral quisqualic (quis) and ibotenic (ibo) acid nucleus basalis (NB) lesioning on the activity of cholinergic neurons and on passive avoidance (PA) and water-maze (WM) performance. Our data demonstrate that A: The activity of choline acetyltransferase (ChAT) in cortical tissue and the number of ChAT positive neurons in the NB were decreased 1 and 8 months after quis or ibo NB lesioning. B: Ibo NB lesioning produced a greater nonspecific subcortical cell loss than quis NB lesioning. C: PA retention was impaired by acute and chronic quis and ibo NB lesioning. D: Acute ibo NB lesioning impaired acquisition and reversal learning in WM performance whereas chronic ibo NB lesioning impaired only reversal WM learning. Acute and chronic quis NB lesioning impaired reversal WM learning. The present results suggest that NB cholinergic neurons do not recover spontaneously from excitotoxin-induced damage and that they may be importantly involved in inhibitory avoidance and spatial reversal learning performance.

Behavioral impairments after lesions of the nucleus basalis by ibotenic acid and quisqualic acid

Ibotenic acid (IBO) or quisqualic acid (QUIS) was infused into the region of the nucleus basalis magnocellularis (NBm) in F344 rats in order to behaviorally and biochemically characterize the effects of these two neurotoxins. QUIS infusion resulted in a slightly higher depletion of choline acetyltransferase (ChAT) activity in both anterior and posterior regions of cortex than did lesions caused by infusion of IBO. Both QUIS- and IBO-treated rats demonstrated significantly longer latencies than controls to find a hidden platform in a Morris water maze task. In addition, QUIS-treated rats performed significantly better than IBO-treated rats in the water maze. Analysis of swim speed and open field behavior did not show significant differences in general motor activity. Passive avoidance retention was unaffected by either neurotoxin. Cortical amino acid levels, [3H]neurotensin binding, dopamine, norepinephrine, and serotonin levels were unaffected by either neurotoxin. The levels of HVA and 5-HIAA in the IBO and QUIS groups were significantly reduced compared to controls, but were not significantly different from each other. Histological examination showed greater damage to non-NBm structures with IBO than with QUIS, including the basolateral nucleus of the amygdala and the reticular formation of the thalamus. The greater behavioral deficit seen after IBO lesions may be due to damage to other areas rather than differences in the extent of depletion of corticai ChAT, amino acids, catecholamines or indolamines.

Effects of THA on passive avoidance and spatial performance in quisqualic acid nucleus basalis-lesioned rats

Bilateral quisqualic acid nucleus basalis (NB) lesions impaired passive avoidance (PA) retention. NB lesions did not impair acquisition performance (stable platform location) in the water maze (WM). However, NB-lesioned rats were impaired in learning the new location of the escape platform in WM. Pretraining injections of tacridine (an anticholinesterase, THA) at 3 mg/kg, but not at 1 mg/kg, slightly improved PA retention performance in NB-lesioned rats. THA (1 or 3 mg/kg) did not alleviate NB lesion-induced WM defect. The results further suggest that loss of NB neurons impair PA acquisition and relearning of the new platform location in WM, and that cholinergic neuron loss may be at least partially involved in the NB lesion-induced performance defect.

Comparison of quisqualic and ibotenic acid nucleus basalis magnocellularis lesions on water-maze and passive avoidance performance

The present study compares water-maze (WM) (reference and working memory) and passive avoidance (PA) (acquisition and retention) deficits induced by ibotenic (ibo) and quisqualic (quis) acid nucleus basalis magnocellularis (NBM) lesions. Ibo lesions produced a large subcortical cell loss and a decrease in frontal cortex (FR) choline acetyltransferase (ChAT) activity. Ibo lesions impaired WM acquisition and PA acquisition and retention performance. Quis NBM lesions were restricted to the ventromedial pallidum, but ChAT activity was decreased in FR. Quis NBM lesions impaired PA acquisition and retention, but had no effect on the reference or working memory WM performance.

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.

A comparison of excitotoxic lesions of the basal forebrain by kainate, quinolinate, ibotenate, N-methyl-D-aspartate or quisqualate, and the effects on toxicity of 2-amino-5-phosphonovaleric acid and kynurenic acid in the rat

1. It has been suggested that an NMDA1 receptor subtype might be activated by N-methyl-D-aspartate (NMDA) and ibotenate and an NMDA2 subtype by NMDA or quinolinate, and that the NMDA2 site might be more susceptible to blockade by kynurenic acid. 2. Experiments were carried out to examine the ability of 2-amino-5-phosphonovaleric acid (AP5) and kynurenic acid to antagonize the neurotoxic properties of kainate, ibotenate, NMDA, quinolinate and quisqualate injected into the rat basal forebrain. 3. Following histological analysis of the injection sites, lesion volume was assessed parametrically. Each of the toxins except quisqualate was found to make lesions of parvocellular neurones within the basal forebrain with a relative order of potency: kainate much greater than quinolinate greater than ibotenate = NMDA. 4. Equimolar doses of AP5 abolished the toxicity produced by quinolinate and NMDA; toxicity to kainate and ibotenate was attenuated to approximately 40% of the toxin-alone condition. 5. The antagonistic properties of kynurenate were dose-dependent: equimolar kynurenate had no effect on quinolinate but attenuated the actions of ibotenate, kainate and NMDA; 2 x equimolar kynurenate had no effect on quinolinate or ibotenate but attenuated the toxicity of kainate and NMDA; and 3 x equimolar kynurenate had no effect on the toxicity of kainate or ibotenate, attenuated the actions of NMDA and abolished the toxic action of quinolinate. 6. The results are discussed in terms of the actions of the various toxins at different receptors, differentially sensitive to AP5 and kynurenate.

Quisqualate lesions of rat NBM: selective effects on working memory in a double Y-maze

Some authors have reported that quisqualic acid lesions of the nucleus basalis magnocellularis (NBM), although producing large cortical cholinergic losses, have little effect on memory. The purpose of the present study was to investigate the effects of quisqualic acid lesions of the NBM on working and reference memory in a double Y-maze. Each trial started with placement into one of the two end arms of the first Y-maze, and the correct response was to go down the stem (reference memory). Access was then given to the second Y-maze, the correct response being conditional upon the side of the first Y-maze from which that trial had begun (working memory). Rats were trained to an 88% correct criterion and were then given either bilateral quisqualic acid (60 nM, 0.5 microliters) or sham lesions (0.9% saline, 0.5 microliters) of the NBM. One week postsurgery, rats were tested on the double Y-maze task with delays of 0, 5 or 30 seconds being introduced prior to both the working and reference memory choice. NBM lesions produced a 63.2 +/- 6.2% decrease of cortical choline acetyltransferase (ChAT) compared to unoperated controls. Delays affected only the working memory of the sham group. Rats with lesions showed a significant impairment of working memory at all delays, but no change in reference memory. Results indicate that quisqualic acid lesions of the NBM that produce significant reductions in cortical ChAT selectively impair working memory.

The role of interactions between the cholinergic system and other neuromodulatory systems in learning and memory

Extensive evidence indicates that disruption of cholinergic function is characteristic of aging and Alzheimer's disease (AD), and experimental manipulation of the cholinergic system in laboratory animals suggests age-related cholinergic dysfunction may play an important role in cognitive deterioration associated with aging and AD. Recent research, however, suggests that cholinergic dysfunction does not provide a complete account of age-related cognitive deficits and that age-related changes in cholinergic function typically occur within the context of changes in several other neuromodulatory systems. Evidence reviewed in this paper suggests that interactions between the cholinergic system and several of these neurotransmitters and neuromodulators--including norepinephrine, dopamine, serotonin, GABA, opioid peptides, galanin, substance P, and angiotensin II--may be important in learning and memory. Thus, it is important to consider not only the independent contributions of age-related changes in neuromodulatory systems to cognitive decline, but also the contribution of interactions between these systems to the learning and memory deficits associated with aging and AD.

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.

Impairment of radial-arm maze performance in rats following lesions involving the cholinergic medial pathway: reversal by arecoline and differential effects of muscarinic and nicotinic antagonists

Pharmacologic studies have indicated that accurate performance on the radial-arm maze depends upon the integrity of both nicotinic and muscarinic cholinergic neurotransmitter systems and that these systems interact in a complex fashion. Although numerous studies have suggested that pathways deriving from the basal nuclear complex of the forebrain are critical for the cholinergic modulation of learning and memory, most have focussed on the septohippocampal projection, and none have specifically targeted the medial or lateral systems. In Experiment 1, cortical knife cuts interrupting the medial cholinergic pathway were made at the level of the caudate-putamen nucleus. Such transections produced a robust but temporary disruption of choice accuracy performance in the radial-arm maze. Recovery of this behavior occurred within 10 days and before cholinergic fiber regeneration, suggesting that compensatory changes could have taken place in non-ablated neuronal circuits. In Experiment 2, daily postsurgical administration of arecoline, an agonist with predominantly muscarinic actions, was found to virtually eliminate the adverse behavioral effects of medial pathway transections, indicating that the deficit could be attributable, in part, to disruption of cholinergic projections. In Experiment 3, the effects of scopolamine, a muscarinic antagonist, and mecamylamine, a nicotinic antagonist, were examined in rats with medial cholinergic pathway transections after behavior had returned postsurgically to control levels. Although both drugs attenuated radial-arm maze performance before knife cuts, only scopolamine reduced choice accuracy following surgery. We conclude that the medial cholinergic pathway, particularly its nicotinic actions, plays an important role in cognitive function, at least as exemplified by radial-arm maze performance. Muscarinic mechanisms associated with other telencephalically projecting cholinergic networks, as well as possibly with the medial pathway itself, appear to operate interactively with nicotinic influences.

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.

Basal forebrain cholinergic system: a functional analysis

This chapter has been organized empirically, focusing on the types of approaches that have been taken to understand BFCS function. This approach reflects the state of our knowledge about the behavioral and psychological functions of the BFCS. Considerable information has been gathered in the very short time that the BFCS has been the object of intense investigation. The results from the neurotoxic lesions and from the HACU studies provide some points of consistency and some puzzling differences. Both approaches to the study of basal forebrain function suggest that the MSA is involved in tasks that require spatial working memory; MSA lesions impaired choice accuracy, and HACU in the HIP was increased after performance. The pattern of results in simpler tasks is more difficult to interpret. In a left-right reference memory discrimination in a T-maze, MSA lesions did not impair acquisition or performance, whereas HACU in the HIP was activated during performance. This pattern of results suggests that although the MSA is engaged during this type of task, its activity is not necessary for normal performance. These, and other comparisons indicate the need for a systematic analysis of task demand (Olton, 1989b). Parametric manipulations of different task demands in a systematic fashion can indicate the extent to which the BFCS is involved in the function associated with each parametric manipulation. Ultimately, of course, the organization of this material should focus on particular psychological functions, rather than the techniques and procedures used to gather the information. Achieving this goal is going to require careful attention to the design of behavioral experiments so that definitive conclusions can be made about the extent to which the BFCS is involved in a given psychological function. A systematic application of task analysis can achieve this goal (Olton, 1986, 1989a, 1989b). For example, BFCS lesions in rats impair choice accuracy in spatial working memory tasks, and performance in these tasks engages the HACU system, at least in the HIP. If the spatial functions of this task involve the BFCS, then a nonspatial version of the task should produce a different pattern of results. If the spatial nature of the task is unimportant for BFCS function, then a nonspatial version of the task should produce the same results. By systematically changing one characteristic of the task at a time, the contribution of each component can be assessed.(ABSTRACT TRUNCATED AT 400 WORDS)

Dissociative effects of ibotenic and quisqualic acid-induced basal forebrain lesions on cortical acetylcholinesterase-positive fiber density and cytochrome oxidase activity

The behavioral effects of excitatory amino acid-induced basal forebrain lesions have been conventionally attributed to the loss of cholinergic neurons innervating cortical areas. However, comparative examinations of quisqualic acid- and ibotenic acid-induced lesions to this region have suggested that the behavioral consequences of ibotenate-induced lesions may not be exclusively related to the loss of cholinergic neurons [Etherington R. et al. (1987) Neurosci. Res. Commun. 1, 135-143; Robbins T. W. et al. (1989) Neuroscience 28, 337-352]. These findings prompted the present investigation of the effects of quisqualic acid- and ibotenic acid-induced basal forebrain lesions on cortical cholinergic fiber density and cytochrome oxidase activity. Parallel brain sections from rats with unilateral lesions produced by each toxin were examined for cytochrome oxidase activity and acetylcholinesterase-positive fiber density, at a period of four, eight and 20 days postlesion. Quisqualic acid-induced lesions resulted in a greater loss of cortical acetylcholinesterase-positive fibers than did ibotenic acid-induced lesions, but the latter lesions produced a greater reduction in cytochrome oxidase activity. These results suggest that the loss of cortical cholinergic afferents does not contribute to the cortical metabolic decrease induced by infusions of ibotenic acid into the basal forebrain. Thus, the behavioral and metabolic consequences of ibotenic acid-induced lesions may be due to the destruction of an additional, noncholinergic pathway.

Sprouting of cholinergic axons does not occur in the cerebral cortex after nucleus basalis lesions

Different doses of the excitotoxin quisqualate were used to make lesions in the caudal part of the ferret nucleus basalis, i.e. the part that projects to the visual cortex. The higher doses of the excitotoxin destroyed all nerve growth factor receptor-immunoreactive cells in the caudal nucleus basalis and gave rise to up to 75% loss of acetylcholinesterase-containing axons in the visual cortex. In sections stained for Nissl substance there was generalized tissue damage around the injection sites and extensive loss of all neuron types in areas surrounding the caudal nucleus basalis. Lower doses of the excitotoxin damaged only a proportion of the nerve growth factor receptor-immunoreactive neurons in the caudal nucleus basalis and produced a much lower depletion of acetylcholinesterase-positive fibres in the visual cortex. The only damage seen in sections stained for Nissl substance was a loss of magnocellular neurons in the vicinity of the injection sites. A quantitative morphological approach was used to show that either one week or three months after the lesions there was a linear correlation between the proportion of acetylcholinesterase-positive axons lost in the visual cortex and the proportion of nerve growth factor receptor-immunoreactive cells that had disappeared from the caudal nucleus basalis. Since the correlation lines for the short-term (one week) survival and the long-term (three months) survival experiments coincided, this indicated that no collateral sprouting of cholinergic axons had occurred in the visual cortex of the long-term survival animals regardless of size of the lesion in the nucleus basalis.

The effect of gamma-vinyl-GABA on the performance of nucleus basalis-lesioned rats in spatial navigation task

The present study investigates whether the stimulation of gamma-aminobutyric acid (GABA)ergic system affects spatial navigation deficits induced by the lesioning of the nucleus basalis (NB). Thus, the effect of gamma-vinyl-GABA treatment which elevates the GABA levels in brain was studied on water maze task both in unoperated and NB-lesioned (ibotenic acid) rats. The subchronic administration of gamma-vinyl-GABA aggravated dose-dependently NB lesion-induced deficits, although it did not impair the performance of unoperated rats in this task. The imbalance between GABAergic system and cholinergic or non-cholinergic systems of the NB may contribute to spatial navigation deficits in rats.

Elevation of Met-enkephalin-like immunoreactivity in the rat striatum and globus pallidus following the focal injection of excitotoxins

The present study examined the effects of excitotoxins which activate distinct excitatory amino acid (EAA) receptor subtypes on the levels of Methionine-enkephalin-like immunoreactivity (ME-i.r.) in the striatum and globus pallidus, with a view to developing a model of the striatopallidal enkephalin deficit that prevails in Huntington's disease (HD). Each of the 4 excitotoxins, N-methyl-D-aspartate (NMDA, 50-150 nmol), quisqualate (QUIS, 26.5-102 nmol), kainate (KA, 0.5-7 nmol) and quinolinate (QUIN, 18-288 nmol), were unilaterally infused into the right striatum under halothane anaesthesia. Seven days after the injection, levels of ME-i.r. in the ipsilateral and contralateral striatum or globus pallidus were measured by radioimmunoassay (RIA). Injection of each of the 4 excitotoxins produced dose-related and bilateral elevations in ME-i.r. in both brain regions. Generally, the excitotoxin-induced contralateral response mirrored that on the ipsilateral side and the globus pallidus showed a greater change in ME-i.r. levels than did the striatum. The rank order of apparent efficacy for these 4 agents, based on the magnitude of the maximal effect produced by the excitotoxin, was QUIN = KA greater than NMDA = QUIS. In contrast, the rank order of apparent potency, based on the doses producing a maximal effect, was KA greater than QUIS greater than QUIN greater than NMDA. Histological examination of brain sections revealed that in all cases of excitotoxin injection, the dose producing a maximal increase in ME-i.r. was associated with tissue damage in the injection area. However, no tissue damage was apparent in the globus pallidus or the contralateral striatum. To determine the involvement of EAA receptors in the observed elevations of ME-i.r., the action of 3 EAA antagonists was evaluated in co-injection experiments. Kynurenate (KYN), but not CNQX, antagonized the actions of QUIS on pallidal ME-i.r. levels. Both KYN and CPP, a potent NMDA receptor antagonist, blocked the effect of QUIN. The possibility that contralateral changes in the striatum or globus pallidus were due to mobilization of an endogenous EAA was investigated by injection of CPP into the striatum contralateral to the QUIN infusion. This injection of CPP (1.8-3.6 nmol) did not block the QUIN-induced contralateral response, but reduced the elevation in ME-i.r. in the ipsilateral pallidum. Although the excitotoxin-induced changes in ME-i.r. levels do not appear to correspond to the enkephalin deficit seen in HD, such a deficit may be discernible in different parameters of enkephalinergic cell function.(ABSTRACT TRUNCATED AT 400 WORDS)

Memory following cholinergic (NBM) and noradrenergic (DNAB) lesions made singly or in combination: potentiation of disruption by scopolamine

Groups of rats were trained on either delayed matching or nonmatching to position tasks, then divided into four subgroups and given the following bilateral lesions: (a) SHAM [vehicle injection into the nucleus basalis magnocellularis (NBM) and dorsal noradrenergic bundle (DNAB)], (b) DNAB (6-hydroxydopamine lesion of the DNAB, vehicle into the NBM), (c) NBM (quisqualic acid lesion of the NBM, vehicle into the DNAB) and (d) DUAL (neurotoxin lesions of both DNAB and NBM). Following postoperative recovery, the DUAL lesion subjects were slightly impaired, but by the seventh day of testing all groups were performing at similar levels. This strongly suggests that quisqualate lesions of the NBM are not sufficient to produce severe and lasting mnemonic disorders resembling those seen in Alzheimer's disease (AD). These data also indicate that the noradrenergic system may not be of critical importance with respect to cognition. It was reasoned that an additional anticholinergic treatment might exacerbate an underlying deficiency. All groups were injected, peripherally, with the cholinergic antagonist scopolamine (0-0.5 mg/kg). This drug dose-dependently disrupted performance in all groups. Moreover, the highest dose had a marked effect in the DUAL group, impairing performance even when no mnemonic burden was present (at zero delay). The results suggest that cholinergic NBM and noradrenergic DNAB lesions produce only transient mnemonic deficiencies. A combination of the two can be disruptive, but longer term task (or reference) memory is the primary process affected, and only under certain conditions. The implication of these findings to research concerning animal models relating to Alzheimer's disease is discussed.

Cholinergic-dopaminergic interactions in cognitive performance

Both acetylcholinergic (ACh) and dopaminergic (DA) systems have been found to be crucial for the maintenance of accurate cognitive performance. In a series of studies examining those aspects of cognitive function revealed by the radial-arm maze, we have found that these two neurotransmitter systems interact in a complex fashion. Choice accuracy deficits in the radial-arm maze can be induced by blockade of either muscarinic- or nicotinic-ACh receptors. The choice accuracy deficit induced by blockade of muscarinic receptors with scopolamine can be reversed by the DA receptor blocker, haloperidol. The specific DA D1 blocker SCH 23390 also has this effect, whereas the specific D2 blocker raclopride does not, implying that it is D1 blockade that is critical for reversing the scopolamine effect. On the other hand, the choice accuracy deficit induced by nicotinic blockade with mecamylamine is potentiated by haloperidol. This effect is also seen with the D2 antagonist raclopride, but not with the D1 antagonist SCH 23390, implying that it is the D2 receptor which is important for the potentiation of the mecamylamine effect. The relevance of the D2 receptor for nicotinic actions on cognitive function is emphasized by the finding that the selective D2 agonist LY 171555 reverses the choice accuracy deficit caused by mecamylamine. Nicotinic and muscarinic blockade are synergistic in the deficit they produce. Antagonist doses subthreshold when given alone produce a pronounced impairment when given together. This latter deficit can be reversed by the D2 agonist LY 171555. These studies have outlined the complex nature of ACh-DA interactions with regard to cognitive function. Possible neural circuits for these interactions are discussed. The effectiveness of these selective DA treatments in reversing cognitive deficits due to ACh underactivation suggests a novel approach to treating cognitive dysfunction in syndromes such as Alzheimer's disease.

Effects of concurrent manipulations of nicotinic and muscarinic receptors on spatial and passive avoidance learning

The present study investigates the effects of concurrent manipulations of nicotinic and muscarinic cholinergic receptors on spatial and passive avoidance learning/retention in rats. Daily pretraining test injections of combinations of the subthreshold doses of muscarinic (scopolamine 0.3 mg/kg) and nicotinic (mecamylamine 2.5 mg/kg or 10 mg/kg) antagonists impaired acquisition of the water-maze task (WM). Drug-induced deficits were also observed during the retention trial: the groups injected with scopolamine 0.3 mg/kg, mecamylamine 10 mg/kg and scopolamine 0.3 mg/kg in combination with mecamylamine 2.5 mg/kg showed reduced spatial bias compared with controls. Single preretention test injections of the combination of subthreshold doses of mecamylamine (10 mg/kg) and scopolamine (0.8 mg/kg) impaired memory retrieval in WM. Combined pretraining injections of subthreshold doses of scopolamine (1.0 mg/kg) and mecamylamine (10 mg/kg) induced a severe passive avoidance impairment comparable to 2.0 mg/kg of scopolamine. However, preretention test injections did not impair passive avoidance retention. Either single or combined injections of hexamethonium (5.0 mg/kg, SC) and methylscopolamine (1.0 mg/kg) did not impair either passive avoidance or water-maze performance. The present results suggest that 1) nicotinic and muscarinic systems jointly modulate performance in spatial and avoidance learning tasks and 2) cholinergic antagonists affect acquisition functions more effectively than retention ability. These findings may be relevant to the clinical disorders, like Alzheimer's disease, which are associated with a loss of both cholinergic neurons and nicotinic receptors.

Neurotensin regulation of endogenous acetylcholine release from rat cerebral cortex: effect of quinolinic acid lesions of the basal forebrain

The effects of neurotensin (NT) on endogenous acetylcholine (ACh) release from basal forebrain, frontal cortex, and parietal cortex slices were tested. The results show that NT differentially regulates evoked ACh release from frontal and parietal cortex slices without altering either spontaneous or evoked ACh release from basal forebrain slices. In the frontal cortex, NT significantly inhibited evoked ACh release by a tetrodotoxin (TTX)-insensitive mechanism, suggesting an action directly on cholinergic terminals. In the parietal cortex, NT enhanced evoked ACh release by a TTX-sensitive mechanism, suggesting an action of NT on the cholinergic neuron or in close proximity to the cholinergic neuron. The effects of NT on ACh release were confined to evoked ACh release; that is, spontaneous ACh release was not affected. NT did not affect spontaneous or potassium-evoked ACh release from occipital cortex slices. The second set of experiments tested the effects of quinolinic acid (QUIN) lesions of the basal forebrain cell bodies on the NT-induced regulation of evoked ACh release in the cerebral cortex. QUIN lesions of basal forebrain cell bodies caused decreases in choline acetyltransferase activity (27 and 28%), spontaneous ACh release (14 and 21%), and evoked ACh release (38 and 44%) in frontal and parietal cortex, respectively. In addition, 11 days following QUIN lesions of basal forebrain cell bodies, the action of NT to regulate evoked ACh release in frontal cortex or parietal cortex was no longer observed. The results suggest that in the rat frontal and parietal cortex, NT differentially regulates the activity of cholinergic neurons by decreasing and increasing evoked ACh release, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Chromaffin cell grafts to rat cerebral cortex reverse lesion-induced memory deficits

Adrenal chromaffin cells were isolated from donor adult rats and transplanted to the cerebral cortex of bilaterally nucleus basalis magnocellularis-lesioned rats. Chromaffin cell grafts to lesioned animals completely reversed the spatial memory deficit seen in lesioned alone animals on a T-maze alternation task. Although chromaffin cell grafts have been used previously to reverse motor abnormalities arising from defective nigro-striatal aminergic transmission, the present report is the first evidence that chromaffin cell transplants can reverse deficits in memory function. Grafts also enhanced cortical acetylcholinesterase staining.

The correlation of passive avoidance deficit in aged rat with the loss of nucleus basalis choline acetyltransferase-positive neurons

The present study investigates the effects of ageing on the number of choline acetyltransferase (ChAT)-positive neurons in the nucleus basalis (NB) and the correlation between the number of ChAT-positive neurons and passive avoidance (PA) retention in young (3-month-old) and aged (26-month-old) rats. The results indicate that the number of ChAT-positive neurons is decreased in aged rats and that the degree of loss of NB neurons is related to the degree of PA retention deficit in aged rats.

Dementia: animal models of the cognitive impairments following damage to the basal forebrain cholinergic system

The finding that patients with Alzheimer's disease (AD) have significant degeneration of neurons in the basal forebrain cholinergic system (BFCS) stimulated a great deal of research to determine the cognitive impairments resulting from selective damage to this area. The experiments reviewed here indicate that lesions of the nucleus basalis magnocellularis (NBM) and of the medial septal area (MSA) reproduce the behavioral symptoms following lesions of their respective target sites, the frontal cortex (FC) and the hippocampus (HIP). Impairments of recent memory are one of the most striking symptoms in AD patients at the beginning of their disease, and lesions of the BFCS induce similar impairments. Comparisons of the effects of the lesions produced by different neurotoxins, ibotenic (IBO) acid and quisqualic (QUIS) acid, have raised questions about the role of cholinergic and noncholinergic neurotransmitter systems in the basal forebrain. The implications of these data for the cholinergic hypothesis of mnemonic functions are discussed.

Release of acetylcholine, gamma-aminobutyrate, dopamine and glutamate, and activity of some related enzymes, in rat gustatory neocortex

The gustatory neocortex (GN), final relay along the gustatory pathway, is a region of the brain involved in the neural integration of feeding behavior. Since information on the neurotransmitters in this nucleus is scarce, the aim of the present work was to establish whether acetylcholine (ACh), gamma-aminobutyric acid (GABA), dopamine and glutamate may act as transmitters within this structure. It was found that GN slices are able to release labeled GABA, ACh and glutamate but not dopamine. Additionally, it was possible to detect significant glutamic acid decarboxylase, choline acetyltransferase and acetylcholinesterase activities in GN homogenates. The activity of the two enzymes involved in acetylcholine metabolism was higher than that observed in other cortical regions. These findings suggest that GABA, ACh and glutamate probably are neurotransmitters in the GN, whereas dopamine is not.

Neurochemical recovery in the neocortex after colchicine lesions of the nucleus basalis magnocellularis in rats

Neurochemical recovery was investigated in male, Fischer-344 rats up to 3 months after lesions of the nucleus basalis. Bilateral injections of colchicine (1.0 micrograms/site) into the nucleus basalis magnocellularis (NBM) resulted in a 30% decrease in choline acetyltransferase (ChAT) activity in frontal cortex 4 weeks after surgery, compared to unlesioned controls. ChAT activity in the frontal cortex gradually recovered to control levels by 12 weeks. The loss of ChAT-immunoreactive neurons in the NBM observed 4 weeks after surgery was still evident 12 weeks after surgery. These results suggest that surviving cholinergic neurons in the NBM contribute to recovery of ChAT activity in the neocortex.

Nucleus basalis magnocellularis and memory: differential effects of two neurotoxins

Although the cholinergic system is involved in memory, noncholinergic systems may also contribute to memory. Lesions of the nucleus basalis magnocellularis (NBM) produce behavioral impairments and reduction of cholinergic markers in the frontal cortex (FC). The present study compared the behavioral effects of lesions made with two different neurotoxins, ibotenic (IBO) acid and quisqualic (QUIS) acid. IBO or QUIS was injected into the NBM, and rats were tested in three different tasks: cued delayed nonmatch-to-sample (CDNMS), spatial delayed nonmatch-to-sample (SDNMS), and spatial two-choice simultaneous discrimination (STCSD). IBO producted a greater behavioral impairment than QUIS in the CDNMS and the SDNMS, although QUIS produced a greater drop in choline acetyltransferase (ChAT) activity in the cortex than IBO. At the end of behavioral testing, IBO rats, but not QUIS rats, were impaired in the retention of both tasks. The fact that QUIS lesions produced a greater loss of NBM cholinergic neurons, as determined by decreased ChAT activity, but less of a behavioral impairment in both a spatial and nonspatial task, suggests that the loss of noncholinergic NBM neurons must contribute to the memory impairments following NBM cell loss.

Effects of quisqualic acid nucleus basalis lesioning on cortical EEG, passive avoidance and water maze performance

The study examines the effects of unilateral quisqualic acid nucleus basalis (NB) lesioning on cortical EEG and learning behavior. Lesions produced both gliosis in the ventral pallidum and a marked reduction in the cortical ChAT activity. Normal cortical EEG activity was abolished on the side of NB lesion, i.e., slow wave activity and the incidence of high voltage spindles was higher on the side of lesion compared with the control side. NB lesioning impaired passive avoidance retention, but not spatial learning ability. These results suggest that EEG and passive avoidance deficits induced by NB quisqualic acid lesion may result from the damage specifically to cholinergic neurons. Thus, the restoration of EEG and passive avoidance performance defects in quisqualic-lesioned rats may be used as an index of the efficacy of the cholinergic replacement therapies.

Neurotoxic lesions of the nucleus basalis induced by colchicine: effects on spatial navigation in the water maze

Neuronal loss in the nucleus basalis magnocellularis (NBM) has been consistently associated with learning and memory impairments. Previous studies have used excitotoxicants such as kainic acid or ibotenic acid to examine the behavioral consequences of NBM lesions. In the present study, rats were given bilateral injections of the neurotoxicant colchicine (1.0 micrograms/site) into the NBM and examined for changes in learning and memory. Unlike excitotoxicants, which can produce extensive subcortical damage, colchicine produced a lesion limited to the site of injection. Histological studies demonstrated that colchicine decreased the number of choline acetyltransferase (ChAT)-positive cells in the NBM, and resulted in a marked loss of cortical acetylcholinesterase staining. Separate neurochemical analysis showed that colchicine lesions decreased ChAT activity in the neocortex but not the hippocampus or caudate nucleus. Similar to previous studies, rats with NBM lesions showed a large deficit in a passive avoidance task. Lesions of the NBM impaired acquisition of a reference memory task in the Morris water maze. However, the deficit was transient and with continued training lesioned rats performed as well as controls. In a reversal test in the water maze the learning deficit reappeared. These data suggest that colchicine may be useful in producing lesions of the NBM, which primarily affects the rate of acquisition of a spatial reference memory task.

Similar memory impairments found in medial septal-vertical diagonal band of Broca and nucleus basalis lesioned rats: are memory defects induced by nucleus basalis lesions related to the degree of non-specific subcortical cell loss?

The function of nucleus basalis (NB) and medial septal-vertical diagonal band of Broca (MS-VDBB) in a place navigation task requiring reference memory was investigated. Two subclasses of nucleus basalis ibotenic acid-lesioned rats could be identified: a group having both extensive non-specific subcortical damage and severely impaired learning behavior, and a less impaired group with correspondingly less subcortical damage. The depletion of cortical cholinergic enzymes was slightly higher in the group of NB-lesioned rats with extensive subcortical lesions than in the group with smaller lesioned areas. In the hippocampus of both of these NB-lesioned groups, cholinergic innervation remained unchanged. Ibotenic acid lesioning restricted to the MS-VDBB depleted hippocampal cholinergic innervation, but not the innervation of the frontal cortex, and also led to impaired learning behavior. Of all the lesioned rats, the most impaired were the NB-lesioned rats with large non-specific subcortical lesion.

The effects of ibotenic acid lesions of the nucleus basalis and cholinergic-rich neural transplants on win-stay/lose-shift and win-shift/lose-stay performance in the rat

Rats were trained to criterion performance in 2-lever operant conditional memory tasks that required them to follow either a Win-stay/Lose-shift, or else a Win-shift/Lose-stay response rule. Substantial impairments in performance of both pretrained conditional tasks were seen following ibotenic acid lesions of the nucleus basalis, but not of the globus pallidus. The deficit in both tasks was apparent at all inter-response retention intervals, indicating that nucleus basalis lesions produced a general impairment in the performance of the complex conditional operant tasks, and not a specific deficit in short-term memory. The nucleus basalis lesion rats were then divided into groups matched for equivalent performance. One group was given cell suspension grafts into neocortex of E15 cholinergic-rich forebrain tissue; a second group was given similar grafts of E17 hippocampal tissue; and a third group was given sham transplants. Testing beginning 3 months post-transplant showed that there was no evidence of recovery of performance on these tasks in the cholinergic-rich transplanted groups compared to the controls. However, the rats with cholinergic-rich transplants subsequently showed a significant improvement in retention of a step-through passive avoidance task. The results indicate that either cholinergic deafferentation of the neocortex is not critical for the observed deficits in the operant conditional tasks, or recovery of function following cholinergic-rich transplants is task-specific, in that more complex cognitive tasks may require different levels of graft-host neural integration.

Cholinergic neurons of the pontomesencephalic tegmentum release acetylcholine in the basal nuclear complex of freely moving rats

Two major systems of cholinergic projection neurons are found within the centrum of the mammalian brain: the basal nuclear complex, projecting predominantly to the cerebral cortex, amygdala, and hippocampus, and the pontomesencephalotegmental network, innervating primarily the thalamus. Neurons comprising the latter network also project to the basal forebrain, but the functional properties of that fiber connection, if any, are unknown. In an attempt to address this issue, the extracellular concentration of acetylcholine was measured in the basal nuclear complex of freely moving rats, both singularly and in combination with lesions and pharmacologic manipulations. Acetylcholine release monitored in the presence of physostigmine sulfate in the basal forebrain was (a) calcium-dependent, (b) increased by systemic scopolamine injection, the rise persisting in the presence of quisqualate lesions of the basal nuclear complex, (c) blocked by tetrodotoxin, and (d) abolished by ablation of cell bodies in the pontomesencephalic tegmentum, which also produced a decrease of choline acetyltransferase activity in the nucleus basalis/substantia innominata region, but not by quisqualate lesions of the basal forebrain. It is concluded from these data that the calcium-dependent release of acetylcholine in the basal nuclear complex (a) is largely axonal in nature, (b) derives substantially from axons of the cholinergic pontomesencephalic tegmentum, and (c) appears to be controlled by presynaptic muscarinic receptors on axon terminals of the latter system. The pontomesencephalotegmental cholinergic complex might thus influence cortical acetylcholine release, in part at least, by means of serial-order cholinergic-cholinergic interactions in the basal nuclear complex.

Choline acetyltransferase activity and [3H]vesamicol binding in the temporal cortex of patients with Alzheimer's disease, Parkinson's disease, and rats with basal forebrain lesions

[3H]Vesamicol binding was characterized in human brain post mortem. The number of binding sites was then determined in parallel with choline acetyltransferase activity in the temporal cortex of patients with Alzheimer's disease, demented and non-demented patients with Parkinson's disease, and in the cerebral cortex of rats with quisqualic acid lesions of the nucleus basalis magnocellularis. Whereas choline acetyltransferase activity decreased in patients with Alzheimer's or Parkinson's disease indicating loss of cholinergic innervation, the number of binding sites for [3H]vesamicol was the same as or higher than in controls. Similar results were obtained with the lesioned rats. It is suggested that the increase in binding sites may reflect compensatory regulation of the spared neurons at the level of the synaptic vesicle.

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.

Memory deficits following nucleus basalis magnocellularis lesions may be mediated through limbic, but not neocortical, targets

In order to test the contribution of the target areas of the nucleus basalis magnocellularis to the mediation of item and order recognition memory for spatial locations, one set of rats received lesions of the dorsolateral frontal cortex, parietal cortex, or basolateral amygdala after training in an order recognition memory task, whereas another set of animals received lesions of the nucleus basalis magnocellularis or basolateral amygdala in an item recognition memory task. Animals with basolateral amygdala lesions displayed a deficit for order recognition memory, but no deficit for item recognition memory, a pattern equivalent to that found for animals with nucleus basalis magnocellularis lesions. In contrast, animals with dorsolateral frontal cortex displayed no deficit, and animals with parietal cortex lesions displayed only a partial deficit for order recognition memory, results that differ from those found for animals with nucleus basalis magnocellularis lesions. It appears that the nucleus basalis magnocellularis influences item and order recognition memory for lists of spatial locations primarily through projections to limbic but not neocortical targets.

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)