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

Local injections of excitatory amino acid agonists alter the glutamatergic and dopaminergic transmissions in the rat striatum

This study examined the effects of kainic, ibotenic, and quisqualic acid-induced lesions of the rat striatum on biochemical markers of the glutamatergic corticostriatal and dopaminergic nigrostriatal afferent transmissions. Fifteen to 21 days after striatal injections of these various compounds, significant reductions in the high-affinity glutamate uptake rate, due to decreases in the Vmax of the transport process, were measured. Interestingly, the relationship between these decreases in the Vmax and the decreases in the levels of biochemical markers for the intrinsic striatal cholinergic and GABAergic neurons differed depending on the excitotoxin used. These findings suggest that excitatory amino acid agonists-induced alterations of the glutamatergic terminal activity may not depend only on the loss of cholinergic and GABAergic striatal neurons. In contrast, the observed changes in the dopamine and metabolite contents seemed to be related to the extent of the striatal neuronal degeneration induced by each excitotoxin. All in all, these results indicate that excitatory amino acid agonists can impair the activity and/or the integrity of the two main striatal afferent pathways, through presumably different mechanisms.

Mnemonic deficits in the double Y-maze are related to the effects of nucleus basalis injections of ibotenic and quisqualic acid on choline acetyltransferase in the rat amygdala

Many researchers have reported that the magnitude of decrease in cortical choline acetyltransferase (ChAT) following excitotoxic lesions of the nucleus basalis magnocellularis (nbm) is unrelated to the degree of cognitive impairment. Recently, an explanation has been offered for this lack of correlation: different excitotoxins, when injected into the nbm, differentially affected cholinergic projections to the cortex and amygdala, and those excitotoxins previously reported to produce the greatest mnemonic deficits produced the largest decreases in amygdaloid ChAT. The present study evaluated the role of amygdalofugal cholinergic projections in memory by comparing the effects of intra-nbm ibotenic and quisqualic acid on cortical and amygdaloid ChAT and on mnemonic performance in the double Y-maze. Rats were trained in the double Y-maze until working and reference memory choice accuracy stabilized to a criterion of > or = 78% correct. Rats then were given either bilateral quisqualic acid (60 nmol in 0.5 microliter), bilateral ibotenic acid (50 nmol in 0.5 microliter), or sham (0.9% saline in 0.5 microliter) lesions of the nbm, and again were tested on the maze. Quisqualate produced a selective impairment of working memory, a large (51%) decrease in cortical ChAT and a small (17%) decrease in amygdaloid ChAT; ibotenate, on the other hand, produced a greater impairment of working memory, an impairment of reference memory, a similar (51%) decrease in cortical ChAT, but a greater (30%) decrease in amygdaloid ChAT. These results suggest that the cholinergic projections from the nbm to the cortex and amygdala play an important role in memory. They suggest that excitotoxins producing greater depletions of amygdaloid ChAT produce greater mnemonic deficits.

Amelioration of sensory attention and sensorimotor deficits by chromaffin cell grafts to the cerebral cortex of nucleus basalis magnocellularis lesioned rats

Rats that have received lesions to the nucleus basalis magnocellularis display with a variety of behavioral deficits; among these are decreases in performance of maze tests as well as deficiencies on measures of general health, sensory attention and sensorimotor abilities. We have previously shown that grafts of chromaffin cells placed in the cerebral cortex of nucleus basalis magnocellularis lesioned rats can ameliorate the lesion-induced deficits in performance of a task involving spatial memory. In the present study, we find that lesion-induced deficits in the sensory attention measure of exploration of the environment (head scanning) as well as the sensorimotor behavior involving a rat righting itself when placed nose down on an inclined grid are evident at 8 weeks post-lesion in lesioned-alone rats; these deficits are significantly ameliorated by chromaffin cell grafts in the cerebral cortex placed two weeks following the lesion procedure. These findings may have relevance to the use of chromaffin cells for grafting in neurodegenerative disorders in which sensorimotor or attention deficit components are involved.

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

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

A primate model of Alzheimer's disease

Animal models of Alzheimer's Disease (AD) are designed to duplicate a subset of selected neuropathological, biochemical and behavioral changes that are associated with AD. One well-studied model is based upon the assumption that the destruction of basal forebrain cholinergic neurons by injection of a neurotoxin, such as ibotenic acid, is sufficient to reproduce the cognitive impairments associated with AD. Monkeys have been trained and tested in a variety of behavioral tasks that are selective for learning and memory deficits. Typically, performance was only slightly impaired, and usually recovered to control levels with continued testing. Monkeys with basal forebrain lesions were sensitive to cholinergic antagonists but did not show a consistent benefit from treatment with cholinergic agonists. Furthermore, the memory deficits did not correlate with the degree of cholinergic cell loss. Electrophysiological studies suggest that cholinergic basal forebrain cells may help to evaluate afferent sensory stimuli for degree of novelty or familiarity, or the association of the stimuli with a subsequent reward. Consistent with these findings, monkeys with basal forebrain lesions were significantly impaired in an attention task that tested spatial orienting ability. These recent studies suggest that monkeys with basal forebrain lesions may be useful as models of the attentional deficits associated with AD.

Excitotoxic lesions of basal forebrain cholinergic neurons: effects on learning, memory and attention

A substantial body of literature has suggested that the memory and learning deficits associated with Alzheimer's disease are attributable to degeneration of the cholinergic magnocellular neurons of the nucleus basalis of Meynert (nbM). Subsequently, lesion-induced damage to the cholinergic projections from the nbM to the neocortex has been utilized extensively as an animal model of dementia. Ibotenic acid lesions of the basal forebrain have been found, for example, to produce deficits in a wide variety of tasks involving learning and memory. However, recently, with the availability of more potent cholinergic excitotoxins such as AMPA, it has become apparent that nbM lesions do not provide a simple animal model of the cognitive deficits in ageing and Alzheimer's disease. Further analysis suggests that many of the learning and memory impairments traditionally attributed to the cholinergic corticopetal system are due not to destruction of cholinergic neurons in the nbM, but instead result from the disruption of cortico-striatal outputs passing through the dorsal and ventral globus pallidus. Furthermore, experiments utilizing quisqualic acid and AMPA have revealed that the most convincing deficit observed as a result of such lesions is in visual attention. This role for the basal forebrain-cortical cholinergic system in attentional function is further supported by results obtained from complementary pharmacological studies. This does not exclude a role for acetylcholine in learning and memory processes. Rather, such cognitive processes appear to depend not upon the integrity of the nbM itself, but upon more rostral elements of the cholinergic basal forebrain system.

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

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

Where does damage lead to enhanced food aversion: the ventral pallidum/substantia innominata or lateral hypothalamus?

It is well known that lesions of the lateral hypothalamus (LH) produce aphagia. Several previous studies have reported that lateral hypothalamus damage produces food aversion in addition to aphagia. However, damage to other regions near the LH also produce aphagia and enhanced aversion. The purpose of this study was to resolve where the site or sites for aversion-inducing lesions is/are located. Small, bilateral excitotoxin lesions (QUIN, 10 micrograms in 1 microliter or IBO, 15 micrograms in 1 microliter) or bilateral sham injections of vehicle were made into the globus pallidus (GP), the ventral pallidum/substantia innominata (VP/SI) or the lateral hypothalamus (LH). Affective reactions to taste were elicited by infusing sucrose solutions (1 M) into the mouth via chronic oral cannulae. The number of aversive responses (gapes, chin-rubbing, head-shaking and forelimb flails) emitted was tallied. Individual lesions were mapped and a single 'necessary and sufficient' site for damage-induced aversion was identified (the area of overlapping damage common to all rats that showed enhanced aversive reactions). To identify the lesions, two lesion-mapping techniques were used: (1) a conventional neuron-counting procedure in which an attempt is made to count all neurons within a brain region, and (2) a new modified 'fractionator' procedure consisting of exhaustive 400 x magnification counts at point locations within a brain region. Results indicated that aversive reactions to food are enhanced only following bilateral neuron loss (> 70%) from the caudal ventromedial VP/SI alone. This shared site has a lateral diameter of 1.0 mm, a dorsoventral diameter of 0.5 mm and a rostrocaudal diameter of 1.0 mm. Damage restricted to the LH never produced enhanced aversion even when it produced aphagia. The crucial region for aversion is located ventral and medial to the globus pallidus and dorsal and lateral to the lateral hypothalamus.

The behavioral effects of serotonin synthesis inhibition and quisqualic acid induced lesions of the nucleus basalis magnocellularis in rats

1. To investigate the role of the cholinergic and serotonergic systems in the regulation of cognitive functions, the effects of concurrent lesioning of nucleus basalis magnocellularis (NB) with quisqualic acid (quis) and inhibition of brain serotonin synthesis by systemic p-chlorophenylalanine (PCPA) treatment on passive avoidance (PA) retention and water maze (WM) spatial navigation performance were studied in rats. 2. Quis NB lesioning induced a marked reduction (-62%) in frontal cortical choline-acetyltransferase activity, impaired retention of PA, and slightly and transiently impaired acquisition of WM spatial navigation. 3. PCPA (400 mg/kg/day x 3, i.p.) treatment depleted frontal cortical concentrations of both serotonin (82% depletion) and its major metabolite 5-HIAA (90% depletion) and slightly affected the noradrenergic and dopaminergic systems. PCPA treatment alone had no effect on WM or PA behavior, but potentiated the PA retention deficit and slightly aggravated the WM deficit in rats subjected to quis NB lesioning. 4. The present results further support the view that serotonergic and NB neurons interact in the regulation of cognitive functions.

Anticholinergic sensitivity in the aging rat septohippocampal system as assessed in a spatial memory task

The effects of central cholinergic blockade on spatial memory were tested in aged and basal forebrain-lesioned rats using the Morris Water Maze. In Experiment 1, aged rats (18-21 months old) were characterized as behaviorally impaired or nonimpaired based on water maze performance prior to an atropine sulfate challenge. In the atropine test (50 mg/kg, IP), both the impaired and the nonimpaired rats showed a severe disruption of their search behavior compared to young subjects. This effect was due to blockade of central receptors since peripheral cholinergic blockade using atropine methylbromide did not produce any impairments. Experiment 2 investigated effects of atropine on rats with septal lesions (SL), nucleus basalis lesions (NBL), and rats with both lesions combined (SL + NBL). Before drug treatment, the groups with septal lesions (SL and SL + NBL groups) displayed a moderate impairment in locating the platform site. However, similar to the aged rats, the septal-lesioned rats exhibited severe impairments in the water maze during atropine treatment. This effect was not seen in the normal controls or in the NBL rats. Aged rats, either impaired or nonimpaired in a spatial memory task, showed a pronounced sensitivity to pharmacological blockade of central cholinergic neurotransmission which resulted in severe deficits in spatial navigation in the water maze. Since the same behavioral deficit was produced by cholinergic blockade in young rats with septal lesions, we concluded that the impaired water maze performance seen in the aged rats during cholinergic blockade resulted from impaired function in the septohippocampal system.

Combined serotonergic-cholinergic lesions do not disrupt memory in rats

Rats were trained on a delayed nonmatching to position task, divided into four groups and given the following lesions: (a) SHAM (vehicle injection into nucleus basalis magnocellularis (NBM) and raphé nuclei (RN), (b) RN (5,7-dihydroxytryptamine lesions of raphé, vehicle into NBM), (c) NBM (quisqualic acid lesion of NBM, vehicle into RN), and (d) COMB (lesions of both RN and NBM). RN lesions had no effect on performance measures including accuracy (percent correct), errors of omission, bias, latencies, and magazine response rate. NBM lesions produced delay-independent (nonmnemonic) disruptions, but performance improved over the 20 days' test. The effects of COMB lesions were no worse than NBM lesions alone. The results suggest that (a) the serotonergic system is not essential for performance in this task, (b) NBM lesions transiently impair nonmnemonic aspects of performance, and (c) serotonergic-cholinergic interactions may not be essential for some cognitive processes.

Basal forebrain grafts in the rat neocortex restore in vivo acetylcholine release and respond to behavioural activation

Acetylcholine release in the frontal cortex of awake rats after acute or chronic lesions of the nucleus basalis magnocellularis and grafting of cholinergic-rich basal forebrain tissue was studied by in vivo microdialysis. Three to four weeks and five months after a unilateral quisqualic acid lesion of the nucleus basalis, and five months after lesion and cortical implantation of a basal forebrain cell suspension, acetylcholine release was characterized during a range of pharmacological and behavioural manipulations. Neostigmine (5 microM) was added to the perfusion fluid in order to inhibit the degradation of acetylcholine. The extracellular levels of acetylcholine in normal animals increased three- to four-fold when KCl (100 mM) was added to the perfusion medium and was reduced by 80% after addition of tetrodotoxin (1 microM). The nucleus basalis lesion resulted in a 60% reduction in baseline acetylcholine levels compared to normal and the response to KCl-evoked depolarization was significantly reduced. There were no differences between the acute and chronic lesion groups during any of the manipulations performed. Rats with grafts showed baseline levels of acetylcholine about 70% higher than normal, and responded to both KCl (two-fold increased acetylcholine release) and tetrodotoxin (85% reduced levels). All groups showed lower acetylcholine levels during halothane anaesthesia (on average 70-85% reduction). Sensory stimulation by handling resulted in a two-fold increase in acetylcholine release in normal animals, whereas the absolute responses in the lesioned controls were significantly weaker. Rats with grafts increased their acetylcholine release after handling to an extent not different to normal or lesioned controls. Immobilization stress induced an almost two-fold increase in cortical acetylcholine levels in normal rats, whereas the effect in the lesion-only groups was very weak. The grafts responded to the immobilization with an enhanced acetylcholine overflow that was significantly higher than in lesioned controls. The results showed that the reduction in frontocortical acetylcholine release induced by excitotoxic lesions of the nucleus basalis did not recover spontaneously over several months. Intracortical cholinergic-rich grafts obtained from the fetal basal forebrain provided a source of acetylcholine release with firing-dependent properties which could be modulated by behaviourally stressful stimuli. The ability of the grafts to respond to behavioural manipulation strongly suggests that the host brain can functionally influence graft neuronal activity during ongoing behaviour. Host control of graft activity may play a role in the recovery of the lesion-induced deficits seen with these types of grafts.

Differential alterations of cortical cholinergic and neurotensin markers following ibotenic acid lesions of the nucleus basalis magnocellularis

The present study determined whether cortical cholinergic neurons recover functionally following the loss of afferent projections from the nucleus basalis magnocellularis (nbm). At various time points following ibotenic acid lesions of the nbm, choline acetyltransferase (ChAT) activity or the capacity of cortical cholinergic neurons to synthesize [3H]acetylcholine (ACh) from the precursor molecule [3H]choline were measured in the frontoparietal cortex. First, cortical ChAT activity was decreased by 21% and 35% on the side ipsilateral to the lesion at 1 and 2 weeks following the nbm lesion, respectively. By 6 weeks following nbm lesions, cortical ChAT activity returned to control levels and remained at control levels at 10 weeks following nbm lesions. However, by 13 weeks following nbm lesions, we observed a 21% increase in ChAT activity on the side ipsilateral to the lesion. ChAT activity in the nbm remained unchanged over the time course studied. Secondly, there was a parallel reduction (by 43%) in the capacity of frontoparietal cortex slices from the side ipsilateral to the lesion to synthesize [3H]ACh by 2 weeks following nbm lesions. By 13 weeks following the lesion there was a significant increase (29%) in the synthetic capacity of cortical cholinergic neurons compared to the 2 week time point. Third, the content of neurotensin in the frontoparietal cortex was significantly decreased by 25% and 36%, at 2 weeks and 13 weeks following nbm lesions, respectively. Neurotensin levels in the nbm were not affected by ibotenic acid lesions. In contrast, [125I]neurotensin binding sites in the frontal or parietal cortex were not altered at 2 weeks following nbm lesions.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of repeated administration of propentofylline on memory impairment produced by basal forebrain lesion in rats

The effects of repeated propentofylline administration on impairments of learning and memory in rats with basal forebrain lesions were investigated in several behavioral tasks (water maze, habituation and passive avoidance tasks). Rats were subjected to all the tasks in sequence. Basal forebrain lesions produced by bilateral injections of ibotenic acid (approximately 6 micrograms on each side) severely impaired performance in water maze, habituation and passive avoidance tasks. Repeated administration of propentofylline (10 and 25 mg/kg per day for 14 days, p.o.) improved the deficits of performance in a water maze task, even when administration began one week after the basal forebrain lesions were produced. The impaired performance in habituation and passive avoidance tasks was also markedly ameliorated after repeated administration (24 and 26 days) of propentofylline. The rats with basal forebrain lesions exhibited a significant decrease in choline acetyltransferase activity in the cortex. Propentofylline significantly increased hippocampal choline acetyltransferase activity in basal forebrain-lesioned rats compared with that in vehicle-treated basal forebrain-lesioned rats. However, cortical choline acetyltransferase activity in basal forebrain-lesioned animals was not affected by repeated propentofylline administration. These results indicate that repeated administration of this agent ameliorated the impaired performance of basal forebrain-lesioned rats in part by increasing hippocampal choline acetyltransferase activity. Propentofylline might be useful for the treatment of amnesia and dementia.

Effects of excitotoxic lesions of the nucleus basalis magnocellularis on conditioned taste aversion and inhibitory avoidance in the rat

The role of the nucleus basalis magnocellularis (NBM) in a variety of learning tasks is well known. Lesions of this nucleus result in a reduction of cholinergic transmission throughout a vast portion of the cortex. Because cholinergic transmission in the insular cortex seems to be important for the acquisition of conditioned taste aversion, the aim of the present work was to study the effects of bilateral chemically induced lesions of the NBM on this conditioning, as correlated with some cholinergic markers in the insular cortex. The effect on inhibitory avoidance was also studied. Lesions prevented the acquisition of the aversion and disrupted retention of the task in previously trained animals. Learning in the inhibitory avoidance paradigm was also notably affected. Postlesion reductions of choline acetyltransferase and acetylcholinesterase activities and of K(+)-stimulated [3H]acetylcholine release were found in the insular cortex. Further, in intact rats labeling of NBM neurons was observed by retrograde tracing after injection of Fluoro-Gold into the insular cortex. These findings indicate that the NBM is involved in the neural integration of feeding behavior and that its cholinergic projection to the insular cortex is one of the implicated neurotransmitter systems.

Lesions of the nucleus basalis magnocellularis in immature rats: short- and long-term biochemical and behavioral changes

Short- and long-term effects of unilateral lesions of the nucleus basalis magnocellularis (NBM) on cortical choline acetyltransferase (ChAT) activity and passive avoidance conditioned responses were examined in immature rats. The lesions were made by stereotaxic injection of quisqualic acid on postnatal days 14 (P14), 17 (P17), and 21 (P21). A marked loss of ChAT activity was found 7 days after surgery in all age groups of lesioned rats. Unoperated P14 rats were unable to perform the passive avoidance conditioned responses. Acquisition began on P17. Lesions made on P17 and P21 strongly impaired the acquisition and retention of the task, evaluated 7 days postoperation. No biochemical but a partial behavioral recovery was observed 3 months after surgery in rats lesioned on P14. On the contrary, despite a persistent decrease in cortical ChAT activity, rats lesioned on P21 were able to acquire and retain the passive avoidance conditioned response. These results indicate that destruction of NBM cholinergic neurons shortly after birth is not compensated for by the developmental plasticity of the residual neurons but results in permanent cholinergic hypofunction. They also demonstrate that cholinergic NBM neurons play an important role in the acquisition and retention of a passive avoidance task; nevertheless, a behavioral recovery may take place 3 months after the lesion, even in the presence of a persistent cholinergic hypofunction.

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

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

Galanin-immunoreactivity in the nucleus basalis of Meynert in the rat: age-related changes and differential response to lesion-induced cholinergic cell loss

The neuropeptide Galanin (Gal) is known to play a functional role in the basal forebrain cholinergic system. In our study, the morphology and density of the Gal-immunoreactive (Gal-IR) fiber network within the cholinergic nucleus basalis of Meynert (NBM) was investigated 1, 3 and 6 months after stereotaxic lesion with quisqualic acid in young adult (3 months old) and late middle-aged (20 months old) rats. Quantitative densitometry showed a significantly reduced Gal-IR fiber network in 20-month-old control rats. After lesion-induced cholinergic cell loss, no further changes in Gal-IR were noted in this group of aging rats during the period of investigation. In contrast, young adult animals displayed a significant increase of Gal-IR fiber density 6 months after NBM lesion. However, no hyperinnervation of individual surviving cholinergic neurons was seen. The results obtained in an animal model of cholinergic deficit support the hypothesis of age-related neuroplasticity of specific transmitter and peptide systems. Adaptive changes in Gal may play a role for the modulation of cholinergic function and could be of importance in human age-related neurodegenerative disorders, i.e. Alzheimer's disease.

Effects of NBM lesions with two neurotoxins on spatial memory and autoshaping

Four groups of Wistar rats received either vehicle, quisqualate, or one of two different ibotenic acid infusions into the basal forebrain. Following recovery from surgery, all rats were tested in three distinct behavioral paradigms: the Bättig radial arm maze, the Barnes circular platform, and autoshaping in an operant chamber. The results showed that the size and site of the ibotenic acid lesion had a profound effect on acquisition performance in some, but not all, procedures. Performance in the Bättig maze and acquisition of a food-rewarded lever press were in particular disrupted by ibotenic acid lesions. The severity of the reduction in cortical choline acetyltransferase (ChAT) did not correlate with performance in the tests. Quisqualate produced the largest reduction in ChAT levels but had no significant effect on performance in any of the three procedures used. Anatomic analysis revealed severe nonspecific damage to the striatum following ibotenic acid that was more pronounced in the group receiving a highly concentrated solution of ibotenic acid as compared to rats infused with a greater volume but less concentrated solution of the neurotoxin. Striatal damage was much less severe following quisqualic acid infusions. However, both types of neurotoxins produced equivalent nonspecific degeneration of the reticular thalamic nucleus. These data confirm reports that nonspecific damage appears to define the severity of ibotenic acid lesions on subsequent behavioral performance.

Comparison of the effects of single and combined neurotoxic lesions of the nucleus basalis magnocellularis and dorsal noradrenergic bundle on learning and memory in the rat

Groups of rats received bilateral destruction of either the nucleus basalis magnocellularis (NBM) by infusion of ibotenic acid, the dorsal noradrenergic bundle (DNB) by infusion of 6-hydroxydopamine, or both NBM + DNB (COMB). These lesion groups along with sham operated controls were trained on a food reinforced spatial delayed conditional discrimination task in a T-maze. All four groups were able to attain a criterion of 85% correct responses despite significant group differences in the number of trials to criterion and number of correct responses during training. Performance of the DNB and NBM groups on acquisition trials was significantly lower than controls, significantly higher than the COMB group, but not significantly different from each other. When tested at delays of 30-300 s, all groups demonstrated parallel rates of decline in performance. On reversal learning trials, the NBM and COMB groups were significantly impaired, whereas the performance of the DNB group was better than controls. No significant interaction between the DNB and NBM lesions was observed on any of the behavioral measures. Biochemical analyses demonstrated significant reductions of choline acetyltransferase (ChAT) activity in cortex but not hippocampus of the NBM and COMB groups, and a significant reduction of norepinephrine (NE) in cortex and hippocampus of the DNB and COMB but not the NBM group. The concentration of other monoamine and amino acid neurotransmitters in the lesion groups were unchanged from controls. These results suggest that DNB and NBM lesions produce separate and independent cognitive impairments that do not severely disrupt retention of trial independent (reference memory) and trial dependent (working memory) information on this T-maze task.

Degeneration of rat thalamic reticular neurons following intrathalamic domoic acid injection

Domoic acid (DA), an analog of kainic acid, produces attentional deficits in humans who have ingested shell fish contaminated with this excitotoxin. The thalamic reticular nucleus (RT), by virtue of its location, connections and intrinsic properties, has been implicated in attentional processes. This study demonstrated the vulnerability of RT neurons following intrathalamic DA injections in rats. Lesions were characterized by almost total neuronal loss throughout the RT and sparing of adjacent populations of relay neurons in the VL and VPL. Los of RT neurons may underlie some types of attentional deficits observed in humans following DA poisoning.

Effects of medial dorsal thalamic and ventral pallidal lesions on the acquisition of a conditioned place preference: further evidence for the involvement of the ventral striatopallidal system in reward-related processes

In our previous work, it has been established that the basolateral amygdala and ventral striatum are part of a neural system that is involved in reward-related processes. However, it is unclear how information processed in this limbic-motor interface may come to affect incentive motivational responses. The present experiments have investigated the involvement of post-striatal elements of the ventral striatopallidal system in the rat. Lesions of the anterior or posterior domains of the ventral pallidum, which receives the major outflow from the ventral striatum, or the nucleus medialis dorsalis of the thalamus, which receives projections from both the ventral pallidum and also the basolateral amygdala, were made by infusing the excitotoxin, ibotenic acid. The effects of the lesions on the acquisition of a place preference conditioned by exposure of hungry rats to sucrose were then measured. Lesions of either the anterior or posterior ventral pallidum significantly attenuated, whereas lesions of the medial dorsal thalamus completely abolished, the acquisition of a conditioned place preference, provided that the latter lesions included the medial-lateral extent of the nucleus. Medial dorsal thalamic lesions did not damage the stria medullaris or medial habenula. Ingestion of sucrose following 23 h deprivation was unaffected by either ventral pallidal or medial dorsal thalamus lesions and thus disruption of place preference acquisition was not secondary to changes in primary motivation. The results indicate that reward-related processes, as measured in the place preference conditioning paradigm, may depend upon ventral striatopallidal outflow that engages medial dorsal thalamus-frontal cortex mechanisms, in addition to the previously highlighted direct outflow to brainstem elements of the motor system.

The neuropharmacological and neurochemical basis of place learning in the Morris water maze

The Morris water maze (MWM) offers several advantages over other methods of studying the neurochemical basis of learning and memory, particularly with respect to its ability to dissociate deficits in memory formation from deficits in sensory, motor, motivational and retrieval processes. The contributions of nearly all of the major neurotransmitter systems have been investigated and consistent patterns have emerged. Normal function in glutamatergic and cholinergic systems is necessary for spatial learning, as blockade of NMDA receptors and cholinergic hypofunction prevents spatial learning but does not impair recall. Peptides such as adrenal and sex hormones and somatostatin may also be necessary for spatial learning. In contrast, activity in either GABAergic or opioidergic systems impairs spatial learning, though by quite different means. GABAergic activity prevents memory function, whereas opioidergic activity reduces motivation. Normal monoaminergic activity is necessary for normal performance in the MWM, but not for spatial learning per se. However, noradrenergic and serotonergic systems may enhance cholinergic-mediated mnemonic processes. Further research into the relative contributions of different receptor subtypes as well as interactions between neurochemical systems should provide significant advances in our understanding of the neural basis of learning and memory in mammals.

Scopolamine injected into the rat amygdala impairs working memory in the double Y-maze

Recent neurochemical results suggest the hypothesis that the nucleus basalis magnocellularis (nbm) cholinergic projection to the amygdala may play a role in memory. The present study investigated the effects of intra-amygdaloid injections of the cholinergic antagonist scopolamine on working and reference memory in the double Y-maze. Rats were pretrained until working and reference memory choice accuracy stabilized to a criterion of > or = 86% correct. Bilateral cannulae were then surgically implanted in the basolateral amygdaloid complex. Rats (n = 9) received scopolamine in doses of 8.0, 24.0, and 72.0 micrograms/0.5 microliter and saline (0.5 microliter) in a counterbalanced order with retraining to criterion between injections. Intra-amygdaloid scopolamine produced a dose-dependent and differential impairment of working and reference memory. A dose of 24.0 micrograms impaired working memory without significantly affecting reference memory; doses of 8.0 micrograms and 72.0 micrograms affected neither and both types of memory, respectively. Results implicate amygdaloid acetylcholine in memory.

Muscimol injections into the nucleus basalis magnocellularis of rats: selective impairment of working memory in the double Y-maze

Anatomical and neurochemical results suggest that the cortico- and amygdalopetal cholinergic neurons of the nucleus basalis magnocellularis (NBM) may receive GABAergic inputs. The present experiments were undertaken to evaluate the possible influence of intra-NBM injections of the GABAA agonist, muscimol, on memory. In two experiments, rats were chronically implanted with guide cannulae placed bilaterally into the NBM. Rats were trained to a criterion of at least 83% correct on each component in a double Y-maze task that allowed a dissociation of working and reference memory. The task began with placement into one of the two end arms of the first Y-maze and the reference memory task was to go to the stem for food. Access to the second Y was then given and the working memory task was to go to the goal arm opposite the arm in the first maze from which that trial began. In experiment 1, pre-trained rats (n = 7) received muscimol (0.5 microliter) in doses of 0, 0.01, 0.1 and 1.0 microgram in a counterbalanced order with re-training to criterion between injections. In experiment 2, pre-trained rats (n = 8) received saline, muscimol (0.1 microgram), the GABAA antagonist, bicuculline (0.01 microgram), and muscimol + bicuculline. Results of experiment 1 revealed that intra-NBM muscimol produced a dose-dependent and differential impairment of working and reference memory. A dose of 0.1 microgram impaired working memory without significantly affecting reference memory; doses of 0.01 microgram and 1.0 microgram affected neither and both types of memory, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversal of learning and memory impairments following lesion of the nucleus basalis magnocellularis (NBM) by concurrent noradrenergic depletion using DSP4 in the rat

In the following study the behavioural effects of simultaneous lesion of the nucleus basalis magnocellularis (NBM) using ibotenic acid and noradrenergic depletion following a single i.p. administration of DSP4 (50 mg/kg) were examined in the rat. NBM lesion induced a deficit in acquisition of a reinforced T-maze alternation task, a working memory adaptation of a spatial navigation task in a water maze and 24 h retention in a passive avoidance task compared to sham controls. No effect of the lesion on a reference memory version of spatial navigation in a water maze task was found. Animals that received a combination of NBM lesion and DSP4 treatment showed no impairment on any of the tasks that were impaired by NBM lesion alone. This indicates a reversal of the learning and memory deficits consequent to NBM lesion by simultaneous noradrenergic depletion. NBM lesion induced a significant reduction in choline-acetyltransferase activity in the frontal cortex, and DSP4 induced a significant decrease in noradrenaline concentration in occipital cortex and hippocampus, confirming the effects of these treatments. These results suggest an interaction between central noradrenergic and cholinergic systems in learning and memory processes.

Behavioral response of rats with cortical lesions to cholinomimetics

This study examined the performance of cortically lesioned rats and their response to cholinomimetic agents in passive avoidance and water maze tasks. Lesions encompassing mainly the frontal and parietal cortices produce a deficit in a 5-day passive avoidance retention test. This deficit was attenuated by the intraperitoneal (IP) administration of muscarinic agonists or an anticholinesterase. In the Morris water maze task, lesioned vehicle-treated animals showed greater escape latency times when compared to their sham counterparts. Cholinomimetics, injected daily during acquisition, improved mean escape latency time on days 3 and 4 of testing. The performance of the various groups in a cued version of the water task did not differ. This work demonstrates that performance deficits arising from neocortical loss can be attenuated by cholinergic drugs.

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

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

Iron accumulation in the rat basal ganglia after excitatory amino acid injections--dissociation from neuronal loss

The current study examines in an animal model the relation of excessive iron accumulation in the basal ganglia to the pathology of Parkinsonism and Hallervoden-Spatz disease. Following a unilateral microinjection of excitatory amino acids, kainate, or quinolinate to the anterior olfactory nucleus/ventral striatal region, an increase in histochemical iron concentration was observed in the ipsilateral ventral pallidum, the islands of calleja, the globus pallidus, the entopeduncular nucleus, the ventral thalamus, and the substantia nigra pars reticulata. The iron was observed both in glia and as intensification of patches in the neuropil. In a second group of rats, after microinjection of ibotenate or quisqualate to the nucleus basalis of Meynert, iron accumulated in the ipsilateral entopeduncular nucleus and pars reticulata of substantia nigra. Increased iron accumulation, compared to that in the contralateral side, was stable for months after a single microinjection. In the basal ganglia distal from the site of EAA injection, no gross morphological changes were associated with the increased iron accumulation. The implications of these findings to the pathology of Parkinson's and Hallervorden-Spatz diseases are discussed.

Drug discrimination learning in rats with excitotoxic lesions of nucleus basalis and ventral globus pallidus

Rats can readily acquire conditional discriminations in which mixtures of drugs serve as compound internal discriminative stimuli. Excitotoxic lesions in the region of the nucleus basalis have been shown to impair the acquisition of conditional discriminations based upon external visual stimuli, but nothing was known about their effects on discrimination of internal stimuli. A baseline of undiscriminated bar-pressing for food reinforcers was established prior to surgery. Lesions were made by infusing either ibotenic or quisqualic acid bilaterally into the basal forebrain (the ibotenate-induced lesions had been shown previously to impair cortical cholinergic function and to produce non-specific damage). After surgery, rats were trained to discriminate effects of drug mixtures using a standard, two-bar operant conditioning procedure. The ibotenate, but not the quisqualate, lesion impaired the acquisition of a discrimination of a mixture of (+)-amphetamine plus pentobarbitone, while neither lesion impaired acquisition with a mixture of (-)-nicotine plus midazolam. The ibotenate lesions also reduced overall rates of responding in both experiments. Thus, the deficit in the acquisition of drug discrimination in rats with ibotenate lesions had some pharmacological specificity, but could not be related easily to disturbances in neocortical cholinergic function. In comparisons with other published data, the results suggest a possible dichotomy in the processing of interoceptive and external information in the basal forebrain, a major target of ventral striatal overflow.

Effects of lesion of the cholinergic basal forebrain nuclei on the activity of glutamatergic and GABAergic systems in the rat frontal cortex and hippocampus

The effects of cholinergic basal forebrain lesions on the activity of the glutamatergic and GABAergic systems were investigated in the rat frontal cortex and hippocampus. Bilateral quisqualic acid injections in the nucleus basalis magnocellularis (NBM) at the origin of the main cholinergic innervation to the neocortex induced a cholinergic deficit in the cerebral cortex 15 days later, as shown by the marked selective decrease in cortical choline acetyltransferase (CAT) activity observed. Concurrent alterations in the kinetic parameters of high affinity glutamate uptake consisting mainly of a decrease in the Vmax were observed in the cerebral cortex. These changes presumably reflect a decreased glutamatergic transmission and provide support for the hypothesis that cortical glutamatergic neurons may undergo the influence of cholinergic projections from the NBM. Surprisingly, similar alterations in the glutamate uptake process were found to occur at hippocampal level in the absence of any significant change in the hippocampal cholinergic activity. These data indicate that the NBM may contribute to regulating hippocampal glutamatergic function without interfering with the hippocampal cholinergic innervation that mainly originates in the medial septal area-diagonal band (MSA-DB) complex. No change in parameters of GABAergic activity, namely the glutamic acid decarboxylase (GAD) activity and high affinity GABA uptake, were observed in any of the structures examined. In a second series of experiments involving bilateral intraventricular injections of AF64A, marked survival time-dependent decreases in CAT and high affinity choline uptake activities but no significant change in the high affinity glutamate uptake rate were observed in the hippocampus. No significant change in either parameters of cholinergic activity or in the glutamate uptake was concurrently observed in the cerebral cortex. The GABAergic activity was again unaffected whatever the survival time and the structure considered. Taken as a whole, these data suggest that basal forebrain projections originating in the NBM may play a major role in regulating glutamatergic but not GABAergic function in both the cerebral cortex and the hippocampus; whereas the glutamatergic and GABAergic activities in these two structures may not be primarily under the influence of the cholinergic projections from the MSA-DB complex.

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

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

Excitotoxic lesions of the pedunculopontine tegmental nucleus of the rat. I. Comparison of the effects of various excitotoxins, with particular reference to the loss of immunohistochemically identified cholinergic neurons

The pedunculopontine tegmental nucleus (PPTg) has been shown to have cholinergic connections with the thalamus and basal ganglia. The ability of various doses of the excitotoxins (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) (AMPA), folate, ibotenate, kainate, N-methyl-D-aspartate (NMDA), quinolinate and quisqualate to make lesions in the PPTg was examined, with particular reference to their ability to destroy cholinergic neurons identified using choline acetyltransferase (ChAT) immunohistochemistry. All of the toxins induced convulsive activity on recovery from surgical anesthesia and all except folate made lesions in the PPTg and surrounding structures. The size of the lesions was computed following examination of Cresyl violet stained sections. The largest lesions were made by kainate = AMPA greater than NMDA = ibotenate greater than quisqualate = quinolinate. All of the toxins destroyed cholinergic neurons, higher doses producing greater loss than lower. The ratio of cholinergic cell loss to general neuronal loss (assessed by Cresyl violet staining) was also computed, revealing marked differences between the toxins. Statistical analysis showed that there were significant differences between excitotoxins in terms of this ratio, but these were accounted for by the low dose of quinolinate (24 nmol) producing a significantly greater ratio of damage (12.18:1) than every other toxin. (Next highest ratio: quisqualate 60 nmol, 6.22:1.) Between the other toxins (kainate, AMPA, ibotenate, quisqualate, NMDA and the high dose of quinolinate) there were no statistically significant differences. Intense calcium deposits (stained by Alizarin red) were found frequently and often defined the borders of the lesion. Tyrosine hydroxylase immunohistochemistry revealed axons running below and into the area of lesioned tissue suggesting strongly that fibers were undamaged by the lesions. We conclude that in the PPTg, different excitotoxins make discriminably different lesions, both quantitatively and qualitatively. Unlike excitotoxic lesions in the basal forebrain quinolinate, not quisqualate, made the most selective lesions of cholinergic neurons and, unlike excitotoxic lesions in the septal nuclei, non-myelinated fibers were spared by ibotenate. The implications of these data for research into brainstem mechanisms of Parkinson's disease are discussed.

Excitotoxic lesions of the pedunculopontine tegmental nucleus of the rat. II. Examination of eating and drinking, rotation, and reaching and grasping following unilateral ibotenate or quinolinate lesions

The pedunculopontine tegmental nucleus (PPTg) contains a population of cholinergic neurons thought to be part of the ascending reticular activating system, and non-cholinergic neurons. In the previous study it was shown that various excitotoxins made effective lesions of cholinergic neurons in the PPTg but that quinolinate made smaller lesions in the non-cholinergic population, making it more selective than any other excitotoxin. The purpose of the present experiment was, first, to make lesions of cholinergic neurons throughout the length of the PPTg by infusing toxin at two different sites within it; and second, to examine simple motor activities in rats bearing either quinolinate or ibotenate lesions of the PPTg, and contrast these with the deficits seen after 6-hydroxydopamine (6-OHDA) induced lesions of mesostriatal dopamine (DA)-containing neurons. Post-mortem examination was carried out using choline acetyltransferase (ChAT) and tyrosine hydroxylase (TOH) immunohistochemistry, and routine Nissl staining. Both quinolinate and ibotenate destroyed approximately 75% of ChAT-positive neurons in the PPTg, but damage to non-cholinergic neurons (assessed by Nissl staining) was twice as great following ibotenate as quinolinate. 6-OHDA induced almost complete lesions of mesostriatal DA neurons, assessed by TOH immunohistochemistry. DA depleted rats showed deficits in drinking and spilled more food in the first 2 weeks after surgery, and were unable to reach or grasp food pellets in the staircase test. They also showed strong ipsilateral turning in response to amphetamine and contralateral turning to apomorphine. Quinolinate lesioned rats had no eating or drinking impairment in the home cage but showed a reaching (though not grasping) disability in the staircase test. They had a mild ipsilateral bias following amphetamine. Ibotenate lesioned rats, despite having larger lesions than the quinolinate, showed no deficits in eating or drinking in the home cage, or reaching or grasping disabilities in the staircase test. They did have a mild contralateral bias in response to amphetamine. This dissociation of the effects of quinolinate and ibotenate lesions of the PPTg is consistent with the suggestion that the PPTg has two functionally distinct components, and is attributed to the differential lesion of non-cholinergic neurons by the two excitotoxins.

Place and taste aversion learning: role of basal forebrain, parietal cortex, and amygdala

Animals with nucleus basalis magnocellularis (NBM), parietal cortex, dorsolateral frontal cortex, amygdala or control lesions were tested in a neophobia and taste aversion learning task. Only animals with basolateral amygdala lesions were impaired in taste aversion learning and in displaying neophobia to a novel flavor. This finding suggested a dissociation between the function of the NBM component of the basal forebrain cholinergic system and the amygdala. The same animals with NBM or control lesions were then tested for acquisition of a spatial navigation task using a dry-land version (cheese board) of the Morris water maze. Animals with NBM lesions were impaired in this task relative to control animals. Animals with parietal cortex lesions displayed a comparable deficit in the place navigation task. These findings suggest parallel functions for the NBM component of the basal forebrain system and the parietal cortex. The role of the NBM in mediating memory appears to be limited in that it does not play a role in all learning situations.

Attenuation by the benzodiazepine receptor antagonist, ZK 93 426, of the deficit in spatial navigation induced by nucleus basalis lesions

The effects of the benzodiazepine receptor antagonist, beta-carboline ZK 93,426 treatment were studied both in NB-lesioned (ibotenic acid) and in unoperated Kuo-Wistar rats in a water maze task. The ZK 93,426 administered in the doses of 1 and 5 mg/kg, 30 min prior to the testing in a water maze apparatus, attenuated the NB lesion-induced spatial navigation deficit, although it had no effect on the performance of unoperated rats. The results suggest functional interactions between GABAergic system and ibotenic acid-induced lesion of the basal forebrain in rats.

Quisqualic acid-induced lesion of the nucleus basalis of Meynert in young and aging rats: plasticity of surviving NGF receptor-positive cholinergic neurons

Previous studies have demonstrated that cholinergic neurons in the adult rat forebrain, i.e., septal region, are able to respond and regenerate after damage followed by exogenous treatment with beta-nerve growth factor. Furthermore, it has been shown that an age-related loss of NGF-receptor (NGFr) immunoreactivity occurs in cholinergic septal neurons. Since the regenerative capacity of cholinergic neurons is of importance for potential therapeutic strategies during the course of age-related neurodegenerative diseases, we have compared NGFr positive neurons in young adult (3 months old) and in aging (18-24 months old) rats in their ability to produce a physiological plasticity response after surviving an excitotoxic lesion of the nucleus basalis of Meynert (NBM). In aging control rats, NGFr immunoreactivity within NBM neurons was significantly reduced, in analogy to data obtained earlier from studies about septal neurons in aged rats. After lesion with quisqualic acid, a severe cell loss as well as atrophy of remaining cholinergic neurons was observed in both groups. Investigation of the NBM at various times after the lesion demonstrated signs of axonal or dendritic sprouting and local regeneration, with a maximum seen 3 months after the lesion. No age-related differences in the response could be found. However, despite local fiber growth, no reinnervation of the frontal and parietal cortex could be noted, as demonstrated by acetylcholinesterase histochemistry. Our findings suggest that, despite a relatively early onset of NGFr decline during lifetime, cholinergic cells keep the capacity for a plastic response, although they ultimately fail to reinnervate the neocortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of regional cerebral blood flow by cholinergic fibers originating in the basal forebrain

We review mainly recent studies on vasodilative regulation of cortex and hippocampus by central cholinergic nerves originating in the basal forebrain. We also briefly review the influence of other central noradrenergic fibers originating in the locus ceruleus, serotonergic fibers originating in the dorsal raphe nucleus, dopaminergic fibers originating in the substantia nigra, and peripheral sympathetic and parasympathetic nerve fibers upon regulation of regional cerebral blood flow. Local metabolites have long been considered to play an important physiological role in regulating regional cerebral blood flow. However, the evidence reviewed here emphasizes that the regulation of regional cerebral blood flow by these central cholinergic nerves is independent of regional metabolism. We propose through this review that although studies investigating neural regulation of cortical and hippocampal blood flow by cholinergic fibers originating in the basal forebrain have added much to the understanding of regulation of regional cerebral blood flow further studies are needed to determine the physiological relevance of regional cerebral blood flow in relation to higher nervous functions such as memory, learning, and personality, and changes in these cognitive functions with aging and pathology such as Alzheimer's disease.

N-methyl-D-aspartate unilaterally injected into the dorsal striatum of rats produces contralateral circling: antagonism by 2-amino-7-phosphonoheptanoic acid and cis-flupenthixol

To evaluate the possible contribution of dorsal striatal glutamate receptors to motor behavior, circling responses were observed in rats following unilateral intrastriatal microinjections of the agonist, N-methyl-D-aspartate (NMDA) or the antagonist, 2-amino-7-phosphonoheptanoic acid (APH). The role of dopamine (DA) in NMDA-produced circling also was evaluated. In experiment 1, an NMDA dose of 5.0 micrograms (in 0.5 microliter), but not 0.5 or 0.05 microgram produced significant contraversive circling. In experiment 2, an APH dose of 10.0 micrograms but not 1.0 or 0.1 microgram produced significant ipsiversive circling. In experiment 3, microinjection of the ineffective 0.1 microgram dose of APH or a dose (20 micrograms) of the DA antagonist, cis-flupenthixol, that did not produce circling when administered alone, significantly reduced the circling response produced by the 5.0 micrograms dose of NMDA. As NMDA produced circling in the same direction as that seen following similar unilateral injections of locomotion-stimulating DA agonists, the present results suggest that glutamate, acting via NMDA receptors in the dorsal striatum, may exert an excitatory influence on motor systems. The observation that a DA receptor blocker antagonized the NMDA response further suggests that the observed motor excitatory effect of glutamate at NMDA receptors requires concurrent stimulation of DA receptors in the same region of the striatum.

Excitotoxic lesions of the septum produce anxiolytic effects in the elevated plus-maze and the shock-probe burying tests

Our previous research has shown that electrolytic lesions of the posterior septum result in dramatic, antianxiety effects in two different animal models of anxiolytic drug action, i.e., a selective increase in open-arm activity in the elevated plus-maze test, and a selective abolition of defensive burying in the shock-probe burying test. Although these results suggest that posterior regions of the septum play an important role in the expression of anxiety in these tests, it is unclear whether destruction of septal nuclei themselves mediated these effects, since electrolytic lesions also destroy fibers of passage. Accordingly, in the present experiments, the anxiolytic effects of electrolytic lesions of the septum were compared to those of excitotoxic lesions, which preferentially destroy cell bodies, leaving fibers of passage intact. In the first experiment, both electrolytic and kainic acid lesions of the posterior septum produced complete anxiolytic effects in the elevated plus-maze (an increase in the percentage of open-arm entries and percentage of time in open arms), and partial anxiolytic effects in the shock-probe test (an increase in contact-induced probe shocks), compared to sham-lesioned controls. These antianxiety effects could not be attributed to an increase in general activity, or a decrease in reactivity to shock. In the second experiment, excitotoxic lesions of the posterior septum were produced by a more selective agent, quisqualic acid. Quisqualic acid, like electrolytic lesions, produced clear, anxiolytic effects in both the plus-maze and the shock-probe tests, compared to sham-lesioned control. Taken together, these results strongly suggest that cells originating in posterior regions of the septum mediate anxiety-related responses.

Attentional functions of the forebrain cholinergic systems: effects of intraventricular hemicholinium, physostigmine, basal forebrain lesions and intracortical grafts on a multiple-choice serial reaction time task

Degeneration of the cholinergic magnocellular neurons in the basal forebrain and their cortical projections is a major feature of the neuropathology of Alzheimer's disease. In the present study, two experiments examined the disruptive effects on visual attentional performance of two different manipulations that reduce central cholinergic function. In Expt. I, pharmacological manipulation of the cholinergic system was investigated using icv administration of hemicholinium (HC-3), a high affinity choline uptake blocker, administered either alone or in conjunction with the anticholinesterase, physostigmine. The results revealed impairments in the ability of the rats to localize brief visual targets in a serial reaction time task, as shown in particular by a reduction in choice accuracy and lengthening of the latency to respond correctly to the visual stimulus. Cholinergic specificity was supported by the reversal of these behavioural impairments by pre-treatment with the anticholinesterase, physostigmine. In Expt. II, quisqualate-induced lesions of the basal forebrain produced behavioural deficits at 3 weeks post-lesion surgery similar to those observed following icv infusion of HC-3. In an attempt to restore the extrinsic cortical cholinergic innervation by reinnervation of the deafferented cortex, embryonic basal forebrain cholinergic cells were transplanted into the cortex of lesioned animals. After three months recovery, impairments in performance on the baseline schedule of the task were no longer apparent in lesioned animals. However, behavioural deficits, observed predominantly as a lengthening of correct response latency, could be reinstated in the lesioned animals by interpolation of distracting bursts of white noise during each trial, and this deficit was ameliorated by the cholinergic grafts. Furthermore, a non-specific effect of both cholinergic and non-cholinergic grafts in controlling the increase in perseverative time-out responses which occurred as a result of the basal forebrain lesion was consistently observed. These results suggest that cholinergic dysfunction can produce deficits in visual attention which can be ameliorated by cholinergic treatments such as physostigmine or cholinergic-rich cortical grafts. These data provide support for a role for the basal forebrain-neocortical cholinergic projection in attentional function.

Effects of vinconate, a novel vinca alkaloid, on spatial learning deficits induced by the basal forebrain lesion in rats

We investigated the effects of vinconate, a novel vinca alkaloid, on spatial learning deficits induced by the basal forebrain (BF) lesion in rats. Bilateral BF lesions were produced by injecting ibotenic acid (6 micrograms/0.5 microliter/side). In BF-lesioned rats, impairment of spatial learning in escaping onto the platform during training and decrease in spatial bias during the spatial probe trial in Morris's water maze task were both observed. Vinconate (5 and 10 mg/kg) treatment shortened the increase of escape latency to the platform in BF-lesioned rats and significantly reversed the decrease in spatial bias induced by the BF lesion. Vinconate (10 mg/kg) attenuated the decrease in choline acetyltransferase activity in the frontoparietal cortex caused by the BF lesion. The present study suggests that vinconate has an antiamnesic effect on the BF-lesion-induced amnesia by ameliorating the dysfunction in cholinergic neurons.

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

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

Attempts to make models for Alzheimer's disease

Profound reductions in cortical acetylcholine levels together with degeneration of cholinergic neurons in the basal forebrain have been reported in patients with Alzheimer's disease. A similar loss of the cholinergic neurons of the basal forebrain and impairment of learning and memory occur in animals injected with a nerve growth factor-diphtheria toxin conjugate, suggesting that this animal model is suitable to analyze cholinergic roles on learning and memory processes, and also the pathogenesis of Alzheimer's disease. In addition, animal models constructed by electrolytic or neurotoxic lesioning of the basal magnocellular nucleus, and models made by transgenetic technology were described.

Loss of cholinergic neurons in the nucleus basalis induces neocortical electroencephalographic and passive avoidance deficits

The present experiments were designed to examine the hypothesis that the degeneration of cholinergic nucleus basalis is related to the cognitive and neurophysiological deficits found in old age. Aged (26 months) rats were impaired both in the acquisition of spatial (water-maze) task and retention of passive avoidance task. During aging, neocortical electroencephalographic fast activity was decreased and high-voltage spindles increased. Loss of choline acetyltransferase-positive neurons correlated with the high-voltage spindle incidence and passive avoidance retention deficit. Unilateral ibotenate nucleus basalis lesioning decreased choline acetyltransferase activity in the cortex and produced a large nonspecific subcortical cell loss in young rats. Ibotenate-lesioned rats were impaired in spatial learning and passive avoidance retention in young rats. Quisqualic acid produced a greater decrease in cortical choline acetyltransferase activity and smaller nonspecific subcortical cell loss than ibotenate lesioning. Spatial learning was not impaired, but passive avoidance performance was disrupted. Slow waves and high-voltage spindles were increased and beta activity decreased on the side of either quisqualate or ibotenate nucleus basalis lesioning. These results demonstrate that age-related neurophysiological and cognitive deficits result partially from the loss of cholinergic neurons in the nucleus basalis and that quisqualic acid nucleus basalis-lesioning in young rats may be used as a pharmacological model of the age-related cholinergic neuron loss.