The effects of manipulations of attentional demand on cortical acetylcholine release

Neuronal activity in the nucleus accumbens is necessary for performance-related increases in cortical acetylcholine release

In vivo microdialysis was used to determine the necessity of neuronal activity in the nucleus accumbens (NAC) for task-induced increases in cortical acetylcholine (ACh) efflux. Rats were trained in a behavioral task in which they were required to perform a defined number of licks of a citric acid solution in order to gain access to a palatable, cheese-flavored food. Upon reaching a consistent level of performance, rats were implanted with microdialysis cannula in the medial prefrontal cortex (mPFC) and either the ipsilateral shell of the NAC or in the dorsal striatum (STR; control site). Dialysis samples from the mPFC were analyzed for ACh concentrations and samples from the NAC were analyzed for dopamine (DA) concentrations. Performance in the task was associated with increases in both ACh efflux in the cortex (150-200%) and DA efflux in the NAC (50-75%). These increases were blocked by administration of tetrodotoxin (TTX; 1.0 microM) via reverse dialysis into the NAC. Administration of TTX into the dorsal STR control site was ineffective in blocking performance-associated increases in cortical ACh. The D2 antagonist sulpiride (10 or 100 microM) administered into the NAC via reverse dialysis was ineffective in blocking increases in cortical ACh efflux. The present data reveal that neuronal activity in the NAC is necessary for behaviorally induced increases in cortical ACh efflux and that this activation does not require increases in D2 receptor activity.

Attentional functions of cortical cholinergic inputs: what does it mean for learning and memory?

The hypothesis that cortical cholinergic inputs mediate attentional functions and capacities has been extensively substantiated by experiments assessing the attentional effects of specific cholinotoxic lesions of cortical cholinergic inputs, attentional performance-associated cortical acetylcholine release, and the effects of pharmacological manipulations of the excitability of basal forebrain corticopetal cholinergic projections on attentional performance. At the same time, numerous animal experiments have suggested that the integrity of cortical cholinergic inputs is not necessary for learning and memory, and a dissociation between the role of the cortical cholinergic input system in attentional functions and in learning and memory has been proposed. We speculate that this dissociation is due, at least in part, to the use of standard animal behavioral tests for the assessment of learning and memory which do not sufficiently tax defined attentional functions. Attentional processes and the allocation of attentional capacities would be expected to influence the efficacy of the acquisition and recall of declarative information and therefore, persistent abnormalities in the regulation of the cortical cholinergic input system may yield escalating impairments in learning and memory. Furthermore, the cognitive effects of loss of cortical cholinergic inputs are augmented by the disruption of the top-down regulation of attentional functions that normally acts to optimize information processing in posterior cortical areas. Because cortical cholinergic inputs play an integral role in the mediation of attentional processing, the activity of cortical cholinergic inputs is hypothesized to also determine the efficacy of learning and memory.

Sensory-interference in rat primary somatosensory cortical neurons

To study sensory interaction in the primary somatosensory cortex (SI), we registered 221 neurons in the SI of pentobarbital-anaesthetized Wistar rats. Tactile stimulation was applied in the receptive field of the SI neuron with an electronically controlled probe (20 ms duration). Tactile stimulation elicited 2.33 +/- 0.13 spikes per stimulus in SI neurons. Simultaneous application of paintbrush tickles of the contralateral limb usually decreased tactile responses (1.59 +/- 0.11 spikes per stimulus). This effect was considered a 'sensory-interference'. Light flashes applied at random did not modify tactile response. Applying atropine (1 mm), a muscarinic receptor antagonist, and bicuculline (1 mm), a GABAA receptor antagonist, to the SI cortex blocked the sensory-interference effect, while application of mecamylamine (10 mm), a nicotinic cholinergic receptor antagonist, did not affect sensory-interference. Results reveal sensory interactions in SI cortex that control tactile responses, and suggest the participation of the basal forebrain in the sensory-interference effect.

Lateralized Attentional Functions of Cortical Cholinergic Inputs

The integrity of the cortical cholinergic input system is necessary for attention performance. This experiment tested hypotheses concerning the lateralized contributions of cortical cholinergic inputs to attention performance by assessing the effects of unilateral lesions of basal forebrain cholinergic neurons on sustained attention performance. Loss of right-hemispheric cortical cholinergic inputs impaired the rats' ability to detect signals but did not affect nonsignal trial performance. Conversely, loss of left-hemispheric cortical cholinergic inputs increased the number of false alarms in nonsignal trials. These data correspond with hypotheses about the mediation of detection processes primarily by right-hemispheric circuits and executive aspects of attention performance by left-hemispheric systems. Cortical cholinergic inputs represent a major component of the brain's lateralized attention systems.

The Cholinergic Lesion of Alzheimer's Disease: Pivotal Factor or Side Show?

A profound loss of cortical cholinergic innervation is a nearly invariant feature of advanced Alzheimer's disease (AD). The temporal course of this lesion and its relationship to other aspects of the disease have not yet been fully clarified. Despite assertions to the contrary, a review of the evidence suggests that a perturbation of cholinergic innervation is likely to be present even in the very early stages of AD. This cholinergic lesion is unlikely to be a major determinant of the clinical symptoms or of the neuropathological lesions. Nonetheless, it almost certainly contributes to the severity of the cognitive and behavioral deficits, especially in the areas of memory and attention. The cholinergic lesion may also influence the progression of the neuropathological process through complex interactions with amyloidogenesis, tau phosphorylation and neuroplasticity.

Cholinergic modulation of visual attention and working memory: dissociable effects of basal forebrain 192-IgG-saporin lesions and intraprefrontal infusions of scopolamine

Two experiments examined the effects of reductions in cortical cholinergic function on performance of a novel task that allowed for the simultaneous assessment of attention to a visual stimulus and memory for that stimulus over a variable delay within the same test session. In the first experiment, infusions of the muscarinic receptor antagonist scopolamine into the medial prefrontal cortex (mPFC) produced many omissions but did not impair rats' ability to correctly detect a brief visual stimulus. However, these animals were highly impaired in remembering the location of that stimulus following a delay period, although in a delay-independent manner. In the second experiment, another group of animals with selective 192 IgG-saporin lesions of the nucleus basalis magnocellularis (nBM) were not impaired under conditions of low-attentional demand. However, when the stimulus duration was reduced, a significant memory impairment was observed, but similar to the results of the first experiment, the nBM-lesioned animals were not impaired in attentional accuracy, although aspects of attention were compromised (e.g., omissions). These findings demonstrate that (1) cortical cholinergic depletion produces dissociable deficits in attention and memory, depending on the task demands, (2) delay-independent mnemonic deficits produced by scopolamine are probably due to impairments other than simple inattention, and (3) working memory deficits are not simply dependent on attentional difficulties per se. Together, these findings implicate the nBM cortical cholinergic system in both attentional and mnemonic processing.

Cholinergic enhancement modulates neural correlates of selective attention and emotional processing

Neocortical cholinergic afferents are proposed to influence both selective attention and emotional processing. In a study of healthy adults we used event-related fMRI while orthogonally manipulating attention and emotionality to examine regions showing effects of cholinergic modulation by the anticholinesterase physostigmine. Either face or house pictures appeared at task-relevant locations, with the alternative picture type at irrelevant locations. Faces had either neutral or fearful expressions. Physostigmine increased relative activity within the anterior fusiform gyrus for faces at attended, versus unattended, locations, but decreased relative activity within the posterolateral occipital cortex for houses in attended, versus unattended, locations. A similar pattern of regional differences in the effect of physostigmine on cue-evoked responses was also present in the absence of stimuli. Cholinergic enhancement augmented the relative neuronal response within the middle fusiform gyrus to fearful faces, whether at attended or unattended locations. By contrast, physostigmine influenced responses in the orbitofrontal, intraparietal and cingulate cortices to fearful faces when faces occupied task-irrelevant locations. These findings suggest that acetylcholine may modulate both selective attention and emotional processes through independent, region-specific effects within the extrastriate cortex. Furthermore, cholinergic inputs to the frontoparietal cortex may influence the allocation of attention to emotional information.

A double dissociation between serial reaction time and radial maze performance in rats subjected to 192 IgG-saporin lesions of the nucleus basalis and/or the septal region

The cholinergic basal forebrain has been implicated in aspects of cognitive function including memory and attention, but the precise contribution of its major components, the basalocortical and the septohippocampal systems, remains unclear. Rats were subjected to lesions of either the nucleus basalis magnocellularis (Basalis), the medial septum/vertical limb of the diagonal band of Broca (Septum), or both nuclei (Basalis + Septum), using the selective cholinotoxin 192 IgG-saporin. Cognitive performance was evaluated in tasks taxing attention (the five-choice serial reaction time task, 5-CSRTT) and spatial working memory (radial arm maze, RAM). Nucleus basalis lesions disrupted performance of the 5-CSRTT, as demonstrated by decreased choice accuracy, increased incidence of missed trials, increased latencies to respond correctly, and a disrupted pattern of response control. Combined lesions of the Basalis and Septum resulted in qualitatively similar deficits to Basalis lesions alone, although interestingly, these rats were unimpaired on measures of response speed, and showed weaker deficits on accuracy and omissions. Decreasing the attentional load by lengthening stimulus duration reversed some of the deficits in Basalis and Basalis + Septum rats, suggesting an attentional deficit rather than motivation or motor perturbations. Performance in rats with septal lesions was only affected when task difficulty was increased. In the RAM an opposing pattern of effects was observed, with Septum and Basalis + Septum rats showing dramatic impairments, and Basalis rats performing normally. Taken together, these data provide clear evidence for a functional dissociation between septohippocampal and basalocortical cholinergic systems in aspects of cognitive function.

Reversal of visual attention dysfunction after AMPA lesions of the nucleus basalis magnocellularis (NBM) by the cholinesterase inhibitor donepezil and by a 5-HT1A receptor antagonist WAY 100635

RATIONALE

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 (AD). In addition to memory dysfunction, attentional functions are also impaired in AD.

OBJECTIVE

We investigated the extent to which the cholinesterase inhibitor donepezil reversed the attentional performance deficit in nucleus basalis magnocellularis (NBM) lesioned rats. We also examined the effects of a selective and potent 5-HT(1A) receptor antagonist, WAY 100635, on the attentional deficit of NBM lesioned rats.

METHODS

We injected alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) into the NBM to selectively destroy cholinergic neurons projecting to the neocortex. Attentional functions were examined using the 5-CSRT task, in which hungry rats were required to locate brief visual targets presented randomly in one of five locations in a specially designed chamber. RESULTS. AMPA lesions of the NBM caused marked reductions in choline acetyltransferase activity (ChAT) ranging from 30 to 46% in medial areas of the cortex (medial-frontal and cingulate) and from 58 to 72% in more lateral areas (anterior-dorso-lateral and parietal). AMPA lesioned rats made fewer correct responses (choice accuracy), longer latency to correct response and an increase in the number of premature and perseverative responses. These impairments showed some recovery over the next 12 weeks. Reducing the duration of the visual stimulus reinstated the impairments in choice accuracy. The anticholinesterase inhibitor donepezil at 1.0 mg/kg but not 0.5 mg/kg reversed the impairments in choice accuracy and correct response latency. The premature and perseverative over-responding of AMPA lesioned rats remained unchanged. A dose of 0.1 mg/kg WAY 100635 to AMPA-lesioned rats improved their choice accuracy but did not shorten correct response latencies. The number of premature responses was reduced by WAY 100635 but perseverative over-responding was not affected.

CONCLUSIONS

The attentional impairments induced due to cortical cholinergic dysfunction may be ameliorated by cholinergic treatments such as cholinesterase inhibitors. In addition, 5-HT(1A) receptors and the cortical cholinergic system exert balanced opposition in regulating attentional performance in the rat. Blockade of 5-HT(1A) receptors may be useful to treat some aspects of attentional dysfunction in AD.

Differential cortical acetylcholine release in rats performing a sustained attention task versus behavioral control tasks that do not explicitly tax attention

The present study used microdialysis techniques to compare acetylcholine release in the frontoparietal cortex of rats performing in a task requiring sustained attention with that of rats performing in two control procedures. The two control procedures were a fixed-interval 9-s schedule of reinforcement assessing primarily the effects of operant responding and comparable reward rates, and an operant procedure designed to test the effects of lever extension to prompt responding. These two control procedures involved comparable sensory-motor and motivational variables to those of the sustained attention task, but did not explicitly tax attentional processes. Performance of the sustained attention task was associated with a significant increase in cortical acetylcholine efflux, reaching a maximum of nearly 140%. Performance of the two control procedures was associated with significantly smaller (approximately 50%) increases in cortical acetylcholine release. This robust dissociation between attentional and control performance-associated increases in cortical acetylcholine release resulted, in part, from the elimination of the pre-task transfer of the animals into the operant chambers and the associated increases in acetylcholine release observed in previous studies. The present results support the hypothesis that demands on attentional performance, as opposed to the frequency of lever pressing, reward delivery and other task-related variables, selectively activate the basal forebrain corticopetal cholinergic system.

High-performance liquid chromatography/tandem mass spectrometry assay for the rapid high sensitivity measurement of basal acetylcholine from microdialysates

A high-throughput liquid chromatography tandem mass spectrometry (LC/MS/MS) method has been developed for the analysis of acetylcholine (ACh) in brain dialysates. This separation of ACh is based on cation exchange chromatography with elution buffer consisting of a mixture of ammonium acetate, ammonium formate and acetonitrile. Using isocratic separation conditions, ACh was resolved within a minute and detected using tandem mass spectrometry in the positive ion electrospray mode. The limit of detection for ACh was found to be 1 fmol on column with a S/N ratio of 3:1. The assay has been used routinely for the measurement of ACh in brain dialysates from awake freely moving rats. Furthermore, separation conditions were modified to allow simultaneous measurement of ACh and the acetylcholine esterase inhibitor, neostigmine.

Stimulation of cortical acetylcholine release following blockade of ionotropic glutamate receptors in nucleus accumbens

In vivo microdialysis techniques were used to determine the ability of glutamate receptors within the nucleus accumbens to trans-synaptically modulate the basal forebrain cortical cholinergic system. Rats were implanted with a dialysis probe in the medial prefrontal cortex to measure changes in cortical acetylcholine efflux and in the ipsilateral nucleus accumbens to locally manipulate glutamate receptor activity. Intra-accumbens perfusion of the broad spectrum ionotropic glutamate receptor antagonist kynurentate (1.0, 5.0 mm) led to a dose-dependent increase (maximum of 200%) in cortical acetylcholine efflux. This stimulated efflux was reproduced with the intra-accumbens perfusion of the AMPA/kainate antagonist DNQX (0.1, 0.25, 2.5 mm; maximum increase of 200%) or the NMDA antagonist D-CPP (10.0, 100.0, 200 micro M; maximum increase of 400%). These results reveal a significant glutamatergic tone within the accumbens of awake rats and support the hypothesis that accumbens efferents to basal forebrain modulate the excitability of the basal forebrain cortical cholinergic system.

Selective lesions of basal forebrain cholinergic neurons produce anterograde and retrograde deficits in a social transmission of food preference task in rats

We examined the performance of Long-Evans rats with 192 IgG-saporin lesions of the medial septum/vertical limb of the diagonal band (MS/VDB) or nucleus basalis magnocellularis/substantia innominata (NBM/SI), which removed cholinergic projections mainly to hippocampus or neocortex, respectively. We studied the effects of these lesions on anterograde and retrograde memory for a natural form of hippocampal-dependent associative memory, the social transmission of food preference. In a study of anterograde memory, MS/VDB lesions did not affect the immediate, 24-h or 3-week retention of the task. In contrast, NBM/SI lesions severely impaired immediate and 24-h retention. In a study of retrograde memory in which rats acquired the food preference 5 days or 1 day before surgery and they were tested 10-11 days after surgery, MS/VDB-lesioned rats showed striking memory deficits for the preference acquired at a long delay (5 days) before surgery, although all lesioned rats exhibited poorer retention on both retest sessions than on their pretest performance. Subsequent testing of new anterograde learning in these rats revealed no disrupting effects of lesions on a standard two-choice test. When rats were administered a three-choice test, in which the target food was presented along with two more options, NBM/SI-lesioned rats were somewhat impaired on a 24-h retention test. These results provide evidence that NBM/SI and MS/VDB cholinergic neurons are differentially involved in a social memory task that uses olfactory cues, suggesting a role for these neurons in acquisition and consolidation/retrieval of nonspatial declarative memory.

Effects of (+/-)-4-[[2-(1-methyl-2-pyrrolidinyl)ethyl]thio]phenol hydrochloride (SIB-1553A), a selective ligand for nicotinic acetylcholine receptors, in tests of visual attention and distractibility in rats and monkeys

Nicotine, a nonselective ligand for nicotinic acetylcholine receptors (nAChRs), has been shown to improve attention and reduce distractibility in humans. Although the numerous side effects induced by nicotine prevent its use as a therapeutic agent, it is hypothesized that subtype-selective nAChR ligands may offer a potential therapeutic benefit to humans with attention deficits. In this study, we evaluated the attention-enhancing properties of (+/-)-4-[[2-(1-methyl-2-pyrrolidinyl)ethyl]thio]phenol hydrochloride (SIB-1553A), a ligand selective for neuronal nAChRs with predominant activity at the human beta 4 subtype. SIB-1553A was evaluated in a test of attention (i.e., five-choice serial reaction time task or SRTT) and distractibility (i.e., delayed matching to sample task with distractor or DMTS-D) in adult rats and monkeys, respectively. SIB-1553A did not improve SRTT performance in normal rats, but reversed deficits induced by the N-methyl-D-aspartate (NMDA) antagonist dizocilpine. In the DMTS-D, SIB-1553A improved accuracy across several doses at the short delay intervals, which were affected most by distracting stimuli in adult monkeys. Subsequent testing with optimal doses for each monkey was also associated with significant improvements in DMTS-D accuracy at short delays, indicating the reproducibility of the drug effect. In both species, SIB-1553A had no significant effects on latencies for sample or choice selection and was not associated with adverse effects at efficacious doses. Although it remains to be further demonstrated, SIB-1553A may act through combined nicotinic and non-nicotinic mechanisms. Collectively, the present data suggest that in specific conditions SIB-1553A may improve certain aspects of attentional function in young adult rats and nonhuman primates without adverse side effects.