Age-related sex differences in spatial learning and basal forebrain cholinergic neurons in F344 rats

Basal forebrain cholinergic neurons are important for spatial learning in rodents. Spatial learning ability is reportedly better in males than females, and declines with age. To examine the role of cholinergic function in sex- or age-related differences in spatial learning, we compared the size of basal forebrain cholinergic neurons (BFCN) of young and aged male and female Fischer 344 (F344) rats that had been trained in the Morris water maze. Young male and female rats were equally proficient in finding the platform during training trials, but probe tests revealed that young male rats had better knowledge of the platform's precise location. Impairments in spatial learning were observed in aged rats, and the advantage of males over females was lost. BFCN were significantly larger in young male than young female rats, and were correlated with spatial memory performance for both groups. BFCN were smaller in aged than young males; no change was seen between young and aged females. In the groups of aged rats the correlation between neuron size and spatial memory was lost. The present findings provide further evidence of a role for the basal forebrain cholinergic system in spatial learning, but reveal a complex interaction between sex, age and behavioral performance.     

The role of cholinergic and GABAergic medial septal/diagonal band cell populations in the emergence of diencephalic amnesia

The septohippocampal pathway, which is mostly composed of cholinergic and GABAergic projections between the medial septum/diagonal band (MS/DB) and the hippocampus, has an established role in learning, memory and disorders of cognition. In Wernicke-Korsakoff's syndrome (WKS) and the animal model of the disorder, pyrithiamine-induced thiamine deficiency (PTD), there is both diencephalic damage and basal forebrain cell loss that could contribute to the amnesic state. In the current experiment, we used the PTD animal model to access both cholinergic (choline acetyltransferase [ChAT] immunopositive) and GABAergic (parvalbumin [PV]; calbindin [CaBP]) neuronal loss in the MS/DB in relationship to midline-thalamic pathology. In addition, to gain an understanding about the role of such neuropathology in behavioral dysfunction, animals were tested on a non-rewarded spontaneous alternation task and behavioral performance was correlated to neuropathology. Unbiased stereological assessment of neuronal populations revealed that ChAT-positive neurons were significantly reduced in PTD rats, relative to control pair-fed rats, and thalamic mass and behavioral performance correlated with ChAT neuronal estimates. In contrast, both the PV- and CaBP-positive neurons in the MS/DB were not affected by PTD treatment. These results support an interactive role of both thalamic pathology and cholinergic cell loss in diencephalic amnesia.     

Impaired spatial learning in aged rats is associated with loss of p75-positive neurons in the basal forebrain

We investigated age-related changes in the number and size of neurons positive for the p75 neurotrophin receptor in the cholinergic basal forebrain of female Dark Agouti rats. Since the integrity of these neurons is known to be closely associated with performance in tests of spatial learning ability, we also investigated the incidence of age-related spatial learning impairments, using the Barnes maze. Spatial learning impairments occurred with increasing frequency with age. No rats showed impairment at six months, but 50% were impaired at 14 months and 71% at 26 months. There was no correlation between age and decreased number of p75-positive neurons in the rostral basal forebrain, which consists of the medial septum and vertical limb of the diagonal band of Broca. In the caudal basal forebrain, which consists of the horizontal limb and the nucleus of Meynert, there was a 13% reduction in the number of p75-positive neurons at 17 months compared to six months, and a 30% reduction at 26 months. There was a strong correlation between the presence of spatial learning impairment and a reduction in the number of p75-positive neurons. This correlation was most evident in the rostral basal forebrain, but was also present in the caudal basal forebrain. In the rostral basal forebrain, all learning impaired rats had fewer p75-positive neurons than the average number in unimpaired rats. A close correspondence between the presence of p75 and choline acetyltransferase was evident in basal forebrain neurons of learning-impaired and unimpaired rats. Gross pathological changes to the morphology of p75-positive neurons were relatively frequent in learning-impaired rats. These changes consisted of hypertrophy, appearance of vacuoles, and marginalisation of the cytoplasm.The results indicate the susceptibility of p75-positive neurons to degenerative changes with aging, and show that the loss of these neurons in the basal forebrain was strongly correlated with impairment in spatial learning.     

Markers for biogenic amines in the aged rat brain: relationship to decline in spatial learning ability

The major goal of the study was to evaluate the relationship of brain aging to individual differences in functional decline in rats. Forebrain choline-acetyltransferase (ChAT) and monoamines, including their metabolites, were examined in young and aged male Long-Evans rats in relation to their spatial learning ability. Aged rats that were unimpaired on a spatial learning task exhibited few changes in neurochemistry relative to the young group: each change in this subgroup was also evident in the remaining aged animals that were behaviorally impaired. Additional changes in neurochemical measures only found in the behaviorally impaired aged animals included decreased ChAT in the basal forebrain, striatum, and frontal cortex. A cluster analysis using the 15 neurochemical measures that were sensitive to aging yielded groupings of aged animals that differed with respect to their spatial learning ability, but not in their cue learning latencies. In this analysis the activity of ChAT in the basal forebrain and striatum appeared to be the best predictors of spatial learning impairment.     

Neuroinflammation not associated with cholinergic degeneration in aged-impaired brain

Degeneration of the cholinergic neurons in the basal forebrain and elevation of inflammatory markers are well-established hallmarks of Alzheimer's disease; however, the interplay of these processes in normal aging is not extensively studied. Consequently, we conducted a neuroanatomical investigation to quantify cholinergic neurons and activated microglia in the medial septum/vertical diagonal band (MS/VDB) of young (6 months) and aged (28 months) Fisher 344 × Brown Norway F₁ rats. Aged rats in this study were impaired relative to the young animals in spatial learning ability as assessed in the Morris water maze. Stereological analysis revealed no difference between aged and young rats in the total numbers of cholinergic neurons, demonstrating that loss of cholinergic neurons is not a necessary condition to observe impaired spatial learning in aged rats. In this same region, the total number of activated microglia was substantially greater in aged rats relative to young rats. Jointly, these data demonstrate that aging is characterized by an increase in the basal inflammatory state within the MS/VDB, but this inflammation is not associated with cholinergic neuron death.     

Loss of NMDA, but not GABA-A, binding in the brains of aged rats and monkeys.

In this quantitative neurochemical study we investigated age-related changes in the GABAergic, glutamatergic, and cholinergic neurotransmitter systems in rats and rhesus monkeys. Sixteen young (5 months) and 20 aged (24 months) rats and seven "young" (4-9 years), six "adult" (20-25 years), and five "aged" (29-34 years) monkeys were studied. NMDA-displaceable 1-[3H]glutamate binding was significantly decreased in many neocortical and subcortical regions examined in aged rats and monkeys. The level of choline acetyltransferase (ChAT) activity and [3H]muscimol binding were unchanged in aged animals.     

Age-related recurrence of basal forebrain lesion-induced cholinergic deficits

Lesions of basal forebrain cholinergic nuclei projecting in neocortex have recently been employed as an animal model for the cholinergic deficits in Alzheimer's disease. However, unlike Alzheimer's patients, whose deterioration appears to be progressive and irreversible, basalis lesioned rats usually recover both behaviorally and neurochemically within several months after the lesion. We now demonstrate that this recovery may be a function of the age of the rat and that cholinergic deficits re-occur in the aged rat. Choline acetyltransferase (ChAT) activity and [3H]hemicholinium-3 ([3H]HCh-3) binding are reduced in cortex ipsilateral to ibotenic acid lesions in the 12-month postlesion rat following an initial recovery to normal levels by about 3 months postlesion. The recurrence of decrease of cholinergic markers is not a consequence of a non-specific age-related decline since the activity of glutamic acid decarboxylase remains constant between 3 and 12 months postlesion.     

Histo- and immunohistochemical changes in acetylcholinesterase and choline acetyltransferase activities in the amygdaloid complex in aged rats.

Histo- and immunohistochemical distribution of acetylcholinesterase (AChE) and choline acetyltransferase (ChAT) in the amygdaloid nuclei of young adult (3 month old) and aged (26 month old) Wistar rats was compared. AChE staining and ChAT immunoreactivity showed the same regional variations in the amygdaloid nuclei of young adult rats. The density and staining of AChE- and ChAT-positive fibres, terminals, and nerve cells were reduced in aged rat amygdala. Moreover, heavily stained aberrant fibres and coarse terminals were located around the nerve cells, blood vessels, and occasionally in patches. In aged rats, atrophic AChE positive and ChAT immunoreactive nerve cells exhibited serpentine-like, thicker, and less extensively branched dendrites than those in young adult rats. These changes are similar to the age-related changes in the cholinergic enzymes in other brain regions which are targets to the basal forebrain.     

胰岛素与尼莫地平对大鼠基底前脑胆碱能神经元及空间学习记忆的影响

目的探讨胰岛素与尼莫地平对基底前脑胆碱能神经元与学习记忆在1型糖尿病脑病发生中的影响。方法将成年雄性Wistar大鼠随机分成糖尿病组、干预组、载体组及正常对照组。用链脲佐菌素成功建立成1型糖尿病模型12周后,对干预组大鼠每日皮下注射长效胰岛素(3 IU)、腹腔注射尼莫地平(20 mg/kg),连续用药6周,在相同条件下对载体组大鼠注射等体积的无药物液体,但对糖尿病组或正常对照组大鼠未进行任何处理。应用Morris水迷宫及胆碱乙酰转移酶(ChAT)免疫组化方法,分别测定各组大鼠的空间学习记忆能力及基底前脑胆碱能神经元的变化。结果糖尿病组大鼠内侧隔核、斜角带核垂直支、斜角带核水平支的ChAT阳性神经元数均明显减少(P<0.05),空间学习记忆能力也明显下降(P<0.05);干预组大鼠以上三个核团的ChAT阳性神经元数与学习记忆能力均明显大于糖尿病组大鼠(P<0.05),但干预组的各项指标仍然明显低于载体组大鼠或正常对照组大鼠(P<0.05)。结论在糖尿病的长期自然发展过程中,若不进行治疗则可累及到基底前脑胆碱能神经元,导致学习记忆障碍,这可能是引发1型糖尿病脑病的一个负性因素;此时联合应用胰岛素与尼莫地平仍可有效遏制该脑病向纵深发展的恶性势头。     

Amelioration of cholinergic neurons dysfunction in aged rats depends on the continuous supply of NGF

The present study was designed to examine whether NGF-induced improvement in morphology of senile basal forebrain cholinergic neurons persist after discontinuation of NGF treatment. Trophic effect of continuous intraventricular infusion of NGF was tested in the 4- and 28 months old male Wistar rats immediately after cessation of NGF and 3 or 6 weeks after termination of treatment. Immunohistochemical procedure for ChAT, TrkA, and p75(NTR) receptor has been applied to identify cholinergic cells in the basal forebrain structures. Using the quantitative image analyzer, morphometric and densitometric parameters of cholinergic cells were measured. In untreated 28-month-old rats a reduction in the number, size and intensity of staining of cholinergic neurons was observed in all basal forebrain structures. NGF significantly improved morphological parameters of ChAT- and TrkA-positive cells in aged rats. In 28-month-old rats tested within 3 and 6 weeks after discontinuation of infusion a renewed progressive deterioration of cholinergic phenotype of basal forebrain neurons was observed when compared with the NGF-treated immediately tested group. The parallel staining for p75(NTR) revealed normal morphology of the basal forebrain neurons, despite of the age of rats or the NGF treatment. Analysis of Nissl stained sections also showed that 28-month-old rats did not display significant losses of neurons in the basal forebrain when compared with the young animals. These findings demonstrate that senile impairment of cholinergic neurons is induced by a loss of cholinergic phenotype rather than an acute degeneration of cell bodies. NGF may be beneficial in enhancing cholinergic neurochemical parameters, but the protective effects seem to be dependent on the continuous supply of NGF.     

Concurrent visualization of choline acetyltransferase-like immunoreactivity and retrograde transport of neocortically injected markers in basal forebrain neurons of cat and rat

Magnocellular neurons in the basal forebrain of rats and cats were retrogradely labeled with Fast Blue or horseradish peroxidase injected into the neocortex. Using antisera against choline acetyltransferase (ChAT) a direct double-labeling technique was carried out and it was demonstrated that retrogradely transported markers and ChAT-like immunoreactivity occur within the same neurons. These findings strongly support the cholinergic nature of basal forebrain projection to the neocortex.     

Experimental approaches to age-related cognitive impairments

Rats exhibit morphological, biochemical, and metabolic changes in their brains, as well as cognitive deficits, with aging. Aged rats were found to be significantly impaired compared to young rats in a water maze task and test of motor coordination, and show reduced locomotor activity and exploration. Although aged rats did exhibit deficits as a group, not all aged rats were impaired. Additionally, the subgroup that was impaired on one task was not necessarily the subgroup that was impaired on another task. The cholinergic projection neurons in the basal forebrain region were significantly atrophied in the aged rodent. The degree of atrophy was highly correlated with the cognitive impairment exhibited on the Morris water maze task. Swollen choline acetyltransferase (ChAT)-positive "plaque-like" structures were observed in the neocortex of the aged but not the young rats. Declines in cholinergic activity in the brain has also been observed during aging. Biochemical measurements of ChAT in the basal forebrain region of aged rats revealed small but consistent decreases in ChAT activity compared to young rats. General metabolic activity, measured by the 2-deoxyglucose method, was also decreased in the hippocampal CA1 and CA3 fields, the dentate gyrus, the medial septal-diagonal band area, and the prefrontal cortex of aged rats. There was a significant correlation between the decrease in glucose utilization and deficits on the Morris water maze. Most aged rats exhibit pathological EEG patterns as reflected by frequent long-duration high voltage neocortical spindles (HVS) during immobility. Bilateral lesions of the nucleus basalis and scopolamine treatment increased the incidence of HVS, thereby mimicking changes in the aged brain. We attempted to ameliorate the cognitive deficits observed in subgroups or impaired rats by either: (1) implanting fetal cells of basal forebrain origin into the hippocampus, or (2) infusing nerve growth factor (NGF) chronically into the lateral ventricle. The grafts appeared to facilitate an improvement in the ability of the impaired aged rats to perform in the Morris water maze. This improved performance was reversed by injections of atropine at doses that did not affect the behavior of young animals that performed well in the same task. These results suggest that enhancement of the cholinergic system could have an effect on the performance of the impaired aged animals. The study of the effects of infusions of NGF clearly demonstrate that the ability of impaired aged rats to remember what they had previously learned was increased after NGF treatment.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of gonadectomy on immunoreactivity for choline acetyltransferase in the cortex, hippocampus, and basal forebrain of adult male rats

Androgens are known to affect cognitive and mnemonic aspects of spatial processing. The cholinergic system is thought to play an important role in cognition and memory, but little is known about the interaction between androgen and cholinergic neurons. The present study focused on the effects of testosterone on the cholinergic neurons in the anterior cingulate cortex, the posterior parietal cortex, the hippocampus, and the basal forebrain including the medial septum, i.e., regions related to spatial processing. We examined choline acetyltransferase (ChAT) immunoreactivity in three groups of adult male rats: sham-operated (Sham), 28-day gonadectomized (GDX), and 28-day gonadectomized with immediate implantation of testosterone propionate (GDX+TP). Comparison of the Sham and GDX+TP groups demonstrated that the GDX group had significantly decreased cell counts of ChAT-immunoreactive neurons in anterior cingulate cortex layer II/III, posterior parietal cortex layer II/III, and the medial septum, but not in the other basal forebrain subregions examined (the horizontal part of the diagonal band of Broca and the substantia innominata). The GDX group also had significantly reduced hippocampal ChAT-immunoreactive fiber pixel density. The GDX+TP group maintained ChAT-immunoreactive cell counts in the anterior cingulate cortex, posterior parietal cortex, and medial septum equivalent to those in the Sham group. Less than 1% of identified cells showed colocalization of immunoreactivity for ChAT and androgen receptor in the cell bodies of the cortex and basal forebrain.Our observations demonstrate that the presence or absence of testosterone for 4 weeks influenced the cholinergic population region-specifically in the adult rat brain.     

Presynaptic markers of cholinergic function in the rat brain: relationship with age and cognitive status

The nature of age-related changes in cholinergic function and their relationship to age-related behavioral decline were examined in the present study. Male Fischer-344 rats of four ages (four, 11, 17 and 23 months) were tested in a battery of cognitive tasks. Discrete microdissections of brain areas involved in cognitive function were performed, and activity of choline acetyltransferase and levels of hemicholinium-3 binding were determined to assess the integrity of cholinergic innervation. Age-related changes in cholinergic markers occurred predominantly in the medial septal area and its target areas (hippocampus and cingulate cortex), and were also present in the posterior caudate. However, most of the age-related changes in cholinergic markers were already present at ages at which behavioral impairment was not yet maximal. There were some consistent correlations between behavioral and neurochemical measures, independent of age, but these accounted for relatively small proportions of variance in behavioral performance. For most of these correlations, lower levels of presynaptic cholinergic markers were related to better behavioral performance. In brain areas in which correlations changed with age, lower levels of presynaptic cholinergic markers were associated with better performance in young rats, whereas higher levels were associated with better performance in aged rats. Recent lesion studies using a toxin selective for basal forebrain cholinergic neurons have suggested that these neurons do not play as central a role in learning and memory in young and aged animals as was previously thought. When considered in this context, the present results suggest that preserved cholinergic function in old age might act indirectly to sustain cognitive ability. Changes in cholinergic function may represent one of a number of age-related neurobiological events that underlie behavioral impairments, or may be a permissive factor for other age-related processes that are more directly responsible for cognitive impairments.     

Spatial reference and working memory across the lifespan of male Fischer 344 rats

Loss of mnemonic function is among the earliest and most disconcerting consequences of the aging process. This study was designed to provide a comprehensive profile of spatial mnemonic abilities in male Fischer 344 (F344) rats across the lifespan. Young, middle-aged, and aged F344 rats were trained in spatial reference and working memory versions of the water maze task. There was a progressive age-related decline in spatial reference memory across the lifespan. Reliable individual differences were observed among aged rats, with some aged rats performing as well as young cohorts and others performing outside this range. An age-related delay-dependent decline was observed on a working memory version of the water maze task although no relationship between performance on reference and working memory tasks was present. Notably, middle-aged rats were impaired relative to young on both tasks. Together these data demonstrate that individual differences in spatial reference memory exist among aged F344 rats and provide novel data demonstrating an unrelated decline in working memory across the lifespan, suggesting that age-related mnemonic dysfunction may occur across multiple brain systems.     

Baro-activated neurons with pulse-modulated activity in the rat caudal ventrolateral medulla express GAD67 mRNA

GABAergic neurons in the caudal ventrolateral medulla (CVLM) are believed to mediate the sympathetic baroreceptor reflex by inhibiting presympathetic neurons in the rostral ventrolateral medulla (RVLM). Accordingly, some CVLM neurons are activated by increased arterial pressure (AP; baro-activated), have activity strongly modulated by the AP pulse (pulse-modulated), and can be antidromically activated from the RVLM. This study examined whether baro-activated, pulse-modulated CVLM neurons are indeed GABAergic and examined their structures. We recorded extracellularly from 19 baro-activated, pulse-modulated CVLM neurons in chloralose-anesthetized rats. Most of these cells (13/19) were silenced by decreasing AP with nitroprusside, but some (6/19) remained active at low AP levels. They were also excited by phenyl biguanide (17/17) but inhibited by noxious tail pinch (8/11). Twelve baro-activated cells were filled with biotinamide and examined for expression of GAD67 mRNA. Because adjacent vagal motor neurons are also activated by increased AP, we examined choline acetyltransferase (ChAT) immunoreactivity. Most baro-activated cells (9/12) expressed high levels of GAD67 mRNA, the rest (3/12) displayed lower levels of GAD67 mRNA, but none showed ChAT immunoreactivity. In contrast, adjacent baro-inhibited CVLM cells had no GAD67 mRNA (n = 5) but were instead tyrosine hydroxylase immunoreactive (n = 7). Reconstruction of baro-activated CVLM neurons revealed axons that projected dorsomedially and rostrally with several axon collaterals. These data demonstrate the existence of GABAergic CVLM neurons with the physiological characteristics expected of interneurons that mediate the sympathetic baroreceptor reflex. In addition, baro-activated GABAergic CVLM neurons appear to integrate several types of inputs and provide inhibition to multiple targets.     

Neurotrophic effect of brain-derived neurotrophic factor on basal forebrain cholinergic neurons in culture from postnatal rats.

We examined the effect of brain-derived neurotrophic factor (BDNF) on cholinergic neurons in culture from postnatal rat basal forebrain by assay of choline acetyltransferase (ChAT) activity and cytochemical staining for acetylcholinesterase (AChE). BDNF was found to increase the ChAT activities but failed to promote the survival of AChE-positive neurons in cultures from neonatal (P3) rats, suggesting that its main role is cholinergic differentiation. In contrast, an enhancement of the survival of AChE-positive neurons and of ChAT activity was observed in cultures from P15-16 rats, suggesting that BDNF's main action is the maintenance of cholinergic neurons. Our results indicate a similarity between BDNF and nerve growth factor effects on the responses of cholinergic neurons of postnatal rat basal forebrain in culture.     

Age and sex-dependent decreases in ChAT in basal forebrain nuclei

Microdissection techniques were utilized to measure the activity of choline acetyltransferase (ChAT) (enzyme responsible for synthesis of acetylcholine) in individual basal forebrain nuclei of aged (24 month) and young (4 month) male and female rats. Small but consistent decreases in the activity of ChAT in aged rats were found, and the location of the changes was dependent on the sex of the rat. Aged female rats showed approximately 30% lower ChAT and 40% lower acetylcholinesterase (AChE) activity in the ventral globus pallidus (vGP). Aged males did not show decreased ChAT in the vGP but activity in the medial aspect of the horizontal diagonal band nucleus was 50% lower than in the young males. ChAT activity in four other closely aligned basal forebrain nuclei was not different between the young and aged rats. Analysis of cell number, density and area in the vGP by AChE histochemistry showed no significant differences between aged and young females. In addition, age and sex-dependent changes were measured in pituitary glucose-6-phosphate dehydrogenase activity. The relationship of the changes to age-dependent decrements in memory, the possible influence of gonadal hormones on aging, and the mechanisms responsible for age-related declines in ChAT activity are discussed.     

Basal Forebrain Cholinergic Deficits Reduce Glucose Metabolism and Function of Cholinergic and GABAergic Systems in the Cingulate Cortex

Purpose

Reduced brain glucose metabolism and basal forebrain cholinergic neuron degeneration are common features of Alzheimer''s disease and have been correlated with memory function. Although regions representing glucose hypometabolism in patients with Alzheimer''s disease are targets of cholinergic basal forebrain neurons, the interaction between cholinergic denervation and glucose hypometabolism is still unclear. The aim of the present study was to evaluate glucose metabolism changes caused by cholinergic deficits.

Materials and Methods

We lesioned basal forebrain cholinergic neurons in rats using 192 immunoglobulin G-saporin. After 3 weeks, lesioned animals underwent water maze testing or were analyzed by ¹⁸F-2-fluoro-2-deoxyglucose positron emission tomography.

Results

During water maze probe testing, performance of the lesioned group decreased with respect to time spent in the target quadrant and platform zone. Cingulate cortex glucose metabolism in the lesioned group decreased, compared with the normal group. Additionally, acetylcholinesterase activity and glutamate decarboxylase 65/67 expression declined in the cingulate cortex.

Conclusion

Our results reveal that spatial memory impairment in animals with selective basal forebrain cholinergic neuron damage is associated with a functional decline in the GABAergic and cholinergic system associated with cingulate cortex glucose hypometabolism.     

Dissociation between the attentional functions mediated via basal forebrain cholinergic and GABAergic neurons

The role of basal forebrain corticopetal cholinergic projections in attentional functions has been extensively investigated. For example, 192 IgG-saporin-induced loss of cortical cholinergic inputs was repeatedly demonstrated to result in a selective impairment in the ability of rats to detect signals in a task designed to assess sustained attention performance. The loss of cortical cholinergic inputs correlated highly with the decrease in the hit rate. Little is known about the functions of basal forebrain non-cholinergic neurons, particularly corticopetal GABAergic neurons, largely because of the absence of specific research tools to manipulate selectively this projection. As basal forebrain lesions produced with ibotenic acid were previously observed to potently destroy non-cholinergic, particularly GABAergic neurons while producing only moderate decreases in the density of cortical cholinergic inputs, the present experiment examined the effects of such lesions on sustained attention performance and then compared these effects with the immunohistochemical and attentional consequences of selective cholinotoxic lesions produced by intra-basal forebrain infusions of 192 IgG-saporin. In contrast to the selective decrease in hits previously observed in 192 IgG-saporin-lesioned animals, the attentional performance of ibotenic acid-lesioned animals was characterized by a selective increase in the relative number of false alarms, that is 'claims' for signals in non-signal trials. Analyses of the response latencies suggested that this effect of ibotenic acid was due to impairments in the animals' ability to switch from the processing of the response rules for signal trials to those for non-signal trials. As expected, 192 IgG-saporin did not affect the number of basal forebrain parvalbumin-positive neurons, that are presumably GABAergic, but decreased cortical acetylcholinesterase-positive fiber density by over 80%. Conversely, in ibotenic acid-lesioned animals, basal forebrain parvalbumin-positive cells were decreased by 60% but cortical acetylcholinesterase-positive fiber density was only moderately reduced (less than 25%).These data form the basis for the development of the hypothesis that basal forebrain GABAergic neurons mediate executive aspects of attentional task performance. Such a function may be mediated in parallel via basal forebrain GABAergic projections to the cortex and the subthalamic nucleus.