Visual discrimination and P300 are affected in parallel by cholinergic agents in the behaving monkey

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

Cerebral metabolic effects of acetyl-l-carnitine in rats during aging

The aim of the present study was to investigate the neuronal structures that mediate the antiaging properties of acetyl-l-carnitine (ALCAR). The regional cerebral metabolic rates for glucose (rCMRglc) have been determined with the quantitative autoradiographic [(14)C]2-deoxyglucose procedure at different times after i.v. administration of saline or ALCAR 500 mg/kg to naïve, non pretreated 3-, 12- and 24-month-old rats and to 24-month-old rats pretreated with ALCAR (100 mg/kg/day, for 3 months). rCMRglc increased maximally at 30 min after ALCAR in 3-, 12- and 24-month old rats (14, 15 and 15 areas affected, 19, 24 and 22% mean increments). Peak metabolic activations occurred with similar magnitude in motor, visual, limbic and thalamic areas in all age rats and with larger magnitude in hippocampal and thalamic areas in aged rats. Cerebral metabolic activations subsided by 60 min after ALCAR in 3-month rats (3 brain regions affected, 4% decrease) and persisted by that time in 12- and 24-month-old rats (14 and 12 regions affected, 15 and 20% increases). Cerebral activations were enhanced in aged rats after chronic treatment with ALCAR (24 brain regions affected, 20% mean increase). Hence, during aging, metabolic responsivity to ALCAR is maintained in most brain areas and increased in limbic and thalamic regions. Increased responsivity to ALCAR may result from undetermined pharmacokinetic factors and/or from a higher sensitivity and contribute to the aging reversal properties of ALCAR.

Linkage and linkage disequilibrium of evoked EEG oscillations with CHRM2 receptor gene polymorphisms: implications for human brain dynamics and cognition

Event-related oscillations (ERO) offer an alternative theoretical and methodological approach to the analysis of event-related EEG responses. The P300 event-related potential (ERP) is elicited through the superposition of the delta (1-3 Hz) and theta (3-7 Hz) band oscillatory responses. The cholinergic neurotransmitter system has a key function in modulating excitatory post-synaptic potentials caused by glutamate, and therefore influences P300 generation and the underlying oscillatory responses. Here we report significant linkage and linkage disequilibrium between target case frontal theta band, visual evoked brain oscillations and a single nucleotide polymorphism (SNP) from the cholinergic muscarinic receptor gene (CHRM2) on chromosome 7. We also demonstrate significant linkage disequilibrium between CHRM2 SNPs and target case parietal delta band visual evoked oscillations (LD P<0.001). These findings were not observed for the equivalent non-target case data, suggesting a role for the CHRM2 gene in higher cognitive processing in humans.

Effects of muscarinic and adrenergic agonism on auditory P300 in the macaque

Homologs of human endogenous evoked potentials are known in several species of nonhuman primates, but the neurotransmitter substrates of these potentials remain uncertain. In particular, the role of central cholinergic and adrenergic systems is not yet clearly defined. We recorded cognitive evoked potentials from the scalp in four adult bonnet macaque monkeys during a passive version of the auditory oddball paradigm with unique novel stimuli under saline control conditions. In two subjects each, cognitive evoked potentials were also recorded following intramuscular administration of the m1 muscarinic agonist AF102B or of the alpha-2A noradrenergic agonist guanfacine. On saline, large positivities resembling the human P300 were recorded over midline sites in response to rare or novel auditory stimuli in all four monkeys. The amplitude of these positivities was sensitive to the delivery of fruit-juice reward in association with rare stimuli in three monkeys tested. At cognition-enhancing doses, AF102B enlarged the amplitude of P300-like positivities in both monkeys tested; guanfacine enlarged the amplitude of P300-like positivities in one of two monkeys tested. These results add to existing evidence of human-like endogenous late positivities in monkeys that are influenced by the cholinergic and adrenergic systems, and suggest a possible role of m1 muscarinic and alpha-2A noradrenergic receptor subtypes.

The correlation between P300 alterations and regional cerebral blood flow in non-demented Parkinson's disease

P300 was evoked by a visual oddball and an S1-S2 task in 22 non-demented Parkinson's disease (PD) patients (13 in the early stage, nine in the late stage) and 18 normal controls. Reaction time was also measured. All patients undertook the (99)Tc-ECD SPECT examination. Quantitative regional cerebral blood flow (rCBF) was obtained by overlying SPECT image on the 3D-magnetic resonance image. In the PD patients in the late stage, P300 latency to S2-same and reaction time were significantly prolonged, while rCBF at bilateral frontal, temporal, and the right parietal lobes was decreased. P300 latency to S2-same was significantly correlated with the rCBF at bilateral temporal lobes. Reaction time was significantly correlated with the rCBF at the right frontal and parietal lobes, as well as the temporal and occipital lobes. The results suggest that P300 changes in non-demented PD in the late stage could be related to the temporal lobe dysfunction.

Scopolamine impairs memory performance and reduces frontal but not parietal visual P3 amplitude

It has been suggested that the P3 event-related potential (ERP) may mark the operation of certain working or long-term memory processes. It has also been reported that cholinergic blockade by scopolamine induces significant memory impairment and is associated with an increased latency, as well as amplitude reduction or abolition of the auditory P3, thus supporting hypothesised links between P3 and long-term memory function. An intriguing anomaly is that, while visual P3 latency is also increased by scopolamine, amplitude is not changed. The aim of this study was to make a more detailed assessment of the effects of scopolamine on the visual P3 at a drug dose known to induce memory impairment. After drug administration, memory performance was significantly impaired and visual P3 latency was significantly increased. There was little evidence of parietal P3 amplitude reduction, but frontal P3 amplitude was significantly reduced in both target and non-target conditions. These findings, when considered in the light of a more recent study of the effects of scopolamine on auditory P3, suggest that cholinergic blockade produces a common effect in both visual and auditory modalities of significant frontal P3 amplitude reduction, but no significant parietal P3 amplitude reduction. These results are consistent with the view that there are modality-independent generators of the parietal and frontal P3. The finding of drug-induced memory impairment and modulations of frontal ERP deflections is also consistent with recent evidence of a significant role for regions of the frontal lobe in encoding and retrieval of long-term memories.

Cholinergic modulation, visual function and Alzheimer's dementia

Electrophysiological evidence at a cellular level and in vivo macroelectrode recordings converge in indicating a degree of specificity of acetylcholine action in vision. Acetylcholine (ACh) function is also thought to play a significant role in memory, learning and other cognitive processes. In this respect, ACh action is suggested to serve in both sensory and cognitive processes. The pharmacological blocking of brain muscarinic transmission has been proposed as a model of geriatric memory impairment and Alzheimer's dementia. Visual electrophysiological testing is deemed of diagnostic specificity for this disease. ACh brain neurotransmission, however, mostly contributes to the modulation of nonspecific aspects of cognition, such as arousal or attention. Alzheimer's dementia results from complex neuron alterations [which also affect muscarinic receptors among other (sub)cellular structures] rather than simply reflecting ACh impoverishment. A substantial loss of retinal ganglion cells is documented in patients with Alzheimer's disease and is consistent with electrophysiological observations. However, it is unclear to what extent the dysfunction of the visual system observable in Alzheimer's dementia is qualitatively different from that occurring spontaneously during aging. The dissimilarities between the effect of acute muscarinic blocking (e.g. by scopolamine) and dementia outnumber the similarities. Accordingly, the conventional ACh agonist-antagonist model of dementia now appears questionable, and replacement treatment with compounds enhancing ACh function proved disappointing. It is suggested that (nonspecific) ACh action becomes function-specific, as determined by the architecture of local brain circuits in which it is involved.

Beneficial effects of thiamine on recognition memory and P300 in abstinent cocaine-dependent patients

The present study evaluated the effects of thiamine vs. placebo on memory task performance and event-related electroencephalographic potentials in eight abstinent cocaine-dependent patients. Patients orally ingested 5 g of thiamine and 5 g of a lactose placebo on two separate days scheduled approximately 1 week apart. The order of administration was randomized. Double-blind procedures were followed. Approximately 3 h after ingesting the capsules, patients completed Sternberg's (1975) memory scanning task during which performance and event-related potentials (P300) were recorded simultaneously. Thiamine was found to significantly improve recognition accuracy and P300 amplitude, at the midline parietal (Pz) electrode. The improvement was most reliable under conditions of increased memory load. These preliminary findings justify a further examination of the relation between thiamine's hypothesized effects on central nervous system cholinergic function, and the direct and indirect effects of cocaine abuse.