Synergistic anticholinergic and antiserotonergic effects in humans

Synthesis and preliminary biological evaluation of gabactyzine, a benactyzine-GABA mutual prodrug, as an organophosphate antidote

Organophosphates (OPs) are inhibitors of acetylcholinesterase and have deleterious effects on the central nervous system. Clinical manifestations of OP poisoning include convulsions, which represent an underlying toxic neuro-pathological process, leading to permanent neuronal damage. This neurotoxicity is mediated through the cholinergic, GABAergic and glutamatergic (NMDA) systems. Pharmacological interventions in OP poisoning are designed to mitigate these specific neuro-pathological pathways, using anticholinergic drugs and GABAergic agents. Benactyzine is a combined anticholinergic, anti-NMDA compound. Based on previous development of novel GABA derivatives (such as prodrugs based on perphenazine for the treatment of schizophrenia and nortriptyline against neuropathic pain), we describe the synthesis and preliminary testing of a mutual prodrug ester of benactyzine and GABA. It is assumed that once the ester crosses the blood-brain-barrier it will undergo hydrolysis, releasing benactyzine and GABA, which are expected to act synergistically. The combined release of both compounds in the brain offers several advantages over the current OP poisoning treatment protocol: improved efficacy and safety profile (where the inhibitory properties of GABA are expected to counteract the anticholinergic cognitive adverse effects of benactyzine) and enhanced chemical stability compared to benactyzine alone. We present here preliminary results of animal studies, showing promising results with early gabactyzine administration.

The role of serotonin in declarative memory: A systematic review of animal and human research

The serotonergic system is involved in diverse cognitive functions including memory. Of particular importance to daily life are declarative memories that contain information about personal experiences, general facts, and events. Several psychiatric or neurological diseases, such as depression, attention-deficit-hyperactivity disorder (ADHD), and dementia, show alterations in serotonergic signalling and attendant memory disorders. Nevertheless, understanding serotonergic neurotransmission and its influence on memory remained a challenge until today. In this systematic review, we summarize recent psychopharmacological studies in animals and humans from a psychological memory perspective, in consideration of task-specific requirements. This approach has the advantage that comparisons between serotonin (5-HT)-related neurochemical mechanisms and manipulations are each addressing specific mnemonic circuits. We conclude that applications of the same 5-HT-related treatments can differentially affect unrelated tasks of declarative memories. Moreover, the analysis of specific mnemonic phases (e.g., encoding vs. consolidation) reveals opposing impacts of increased or decreased 5-HT tones, with low 5-HT supporting spatial encoding but impairing the consolidation of objects and verbal memories. Promising targets for protein synthesis-dependent consolidation enhancements include 5-HT₄ receptor agonists and 5-HT₆ receptor antagonists, with the latter being of special interest for the treatment of age-related decline. Further implications are pointed out as base for the development of novel therapeutic targets for memory impairment of neuropsychiatric disorders.

A roadmap for development of neuro-oscillations as translational biomarkers for treatment development in neuropsychopharmacology

New treatment development for psychiatric disorders depends critically upon the development of physiological measures that can accurately translate between preclinical animal models and clinical human studies. Such measures can be used both as stratification biomarkers to define pathophysiologically homogeneous patient populations and as target engagement biomarkers to verify similarity of effects across preclinical and clinical intervention. Traditional "time-domain" event-related potentials (ERP) have been used translationally to date but are limited by the significant differences in timing and distribution across rodent, monkey and human studies. By contrast, neuro-oscillatory responses, analyzed within the "time-frequency" domain, are relatively preserved across species permitting more precise translational comparisons. Moreover, neuro-oscillatory responses are increasingly being mapped to local circuit mechanisms and may be useful for investigating effects of both pharmacological and neuromodulatory interventions on excitatory/inhibitory balance. The present paper provides a roadmap for development of neuro-oscillatory responses as translational biomarkers in neuropsychiatric treatment development.

No interaction between rivastigmine and citalopram on memory and novelty processing in healthy human volunteers

BACKGROUND

Animal literature suggests an interaction between acetylcholine and serotonin on cognitive functions.

AIMS

The aim of the current study was to assess whether both neurotransmitters interact during memory and novelty processing in humans.

METHODS

We tested the interaction between acetylcholine and serotonin on cognitive functions in healthy volunteers by means of treatment with rivastigmine and citalopram, respectively.

RESULTS

The main result of the study showed that during the verbal learning task participants significantly recalled fewer words after citalopram treatment than after rivastigmine or placebo during both the immediate and delayed recall tasks. Rivastigmine was not able to reverse the impairing effect of citalopram.

CONCLUSIONS

This finding is in line with previous studies in which we manipulated acetylcholine and serotonin in different manners. Taken together, these studies in humans do not support the notion from animal studies that these two neurotransmitters interact on cognitive functions.

Parallel improvement of cognitive functions and P300 latency following donepezil treatment in patients with Alzheimer's disease: a case-control study

PURPOSE

To evaluate the effect of donepezil, one of the cholinesterase inhibitors, on P300 measurements in patients with Alzheimer's disease (AD) and investigate the relationship between the subfactors of cognitive performance and P300 components.

METHODS

One hundred outpatients with AD were evaluated for cognitive function (cognitive ability screening instrument) and event-related potentials before and after 22 to 23 weeks of treatment with donepezil (5 mg/day). Twenty age-matched normal control subjects were recruited.

RESULTS

The patients with AD showed prolonged P300 and N200 latency, no significant differences in N100 and P200 components, and poor performance in neuropsychological assessments compared with control subjects at baseline. After donepezil treatment, the patients with AD had reduction in P300 latency at Pz lead, which was associated with a parallel improvement in cognitive function in terms of remote memory, recent memory, visual instruction, and orientation. The pre-post treatment difference of P300 latency significantly correlated with the cognitive ability screening instrument score difference and recent memory score difference, respectively.

CONCLUSIONS

The patients with AD still had intact early sensory processing but impaired higher-level cognitive processes that could influence behavior deviation. The donepezil treatment, which enhances higher-level cognitive processing time, revealed that P300 latency decreases as cognitive capability increases, especially improved in recent memory.

Brain Network Activation (BNA) reveals scopolamine-induced impairment of visual working memory

The overarching goal of this event-related potential (ERP) study was to examine the effects of scopolamine on the dynamics of brain network activation using a novel ERP network analysis method known as Brain Network Activation (BNA). BNA was used for extracting group-common stimulus-activated network patterns elicited to matching probe stimuli in the context of a delayed matching-to-sample task following placebo and scopolamine treatments administered to healthy participants. The BNA extracted networks revealed the existence of two pathophysiological mechanisms following scopolamine, disconnection, and compensation. Specifically, weaker frontal theta and parietal alpha coupling was accompanied with enhanced fronto-centro-parietal theta activation relative to placebo. In addition, using the characteristic BNA network of each treatment as well as corresponding literature-guided selective subnetworks as combined biomarkers managed to differentiate between individual responses to each of the treatments. Behavioral effects associated with scopolamine included delayed response time and impaired response accuracy. These results indicate that the BNA method is sensitive to the effects of scopolamine on working memory and that it may potentially enable diagnosis and treatment assessment of dysfunctions associated with cholinergic deficiency.

Cholinergic modulation of auditory processing, sensory gating and novelty detection in human participants

RATIONALE

Suppression of redundant auditory information and facilitation of deviant, novel, or salient sounds can be assessed with paired-click and oddball tasks, respectively. Electrophysiological correlates of perturbed auditory processing found in these paradigms are likely to be a trait marker or candidate endophenotype for schizophrenia.

OBJECTIVE

This is the first study to investigate the effects of the muscarinic M1 antagonist biperiden and the cholinesterase inhibitor rivastigmine on auditory-evoked potentials (AEPs), sensory gating, and mismatch negativity (MMN) in young, healthy volunteers.

RESULTS

Biperiden increased P50 amplitude and prolonged N100 and P200 latency in the paired-click task but did not affect sensory gating. Rivastigmine was able to reverse the effects of biperiden on N100 and P200 latency. Biperiden increased P50 latency in the novelty oddball task, which was reversed by concurrent administration of rivastigmine. Rivastigmine shortened N100 latency and enhanced P3a amplitude in the novelty oddball paradigm, both of which were reversed by biperiden.

CONCLUSION

The muscarinic M1 receptor appears to be involved in preattentive processing of auditory information in the paired-click task. Additional effects of biperiden versus rivastigmine were reversed by a combination treatment, which renders attribution of these findings to muscarinic M1 versus muscarinic M2-M5 or nicotinic receptors much more difficult. It remains to be seen whether the effects of cholinergic drugs on AEPs are specifically related to the abnormalities found in schizophrenia. Alternatively, aberrant auditory processing could also be indicative of a general disturbance in neural functioning shared by several neuropsychiatric disorders and/or neurodegenerative changes seen in aging.

Altered neural cholinergic receptor systems in cocaine-addicted subjects

Changes in the brain's cholinergic receptor systems underlie several neuropsychiatric disorders, including Alzheimer's disease, schizophrenia, and depression. An emerging preclinical literature also reveals that acetylcoholine may have an important function in addictive processes, including reward, learning, and memory. This study was designed to assess alterations in cholinergic receptor systems in limbic regions of abstinent cocaine-addicted subjects compared with healthy controls. On three separate days, 23 1- to 6-week abstinent, cocaine- (and mostly nicotine-) addicted subjects and 22 sex-, age-, and race-matched control subjects were administered the muscarinic and nicotinic cholinergic agonist physostigmine, the muscarinic antagonist scopolamine, and saline. Regional cerebral blood flow (rCBF) after each infusion was determined using single photon emission-computed tomography. Both cholinergic probes induced rCBF changes (p<0.005) in relatively distinct, cholinergic-rich, limbic brain regions. After physostigmine, cocaine-addicted subjects showed altered rCBF, relative to controls, in limbic regions, including the left hippocampus, left amygdala, and right insula. Group differences in the right dorsolateral prefrontal cortex, posterior cingulate, and middle temporal gyrus were also evident. Scopolamine also revealed group differences in the left hippocampus and right insula as well as the posterior cingulate and middle temporal gyrus. Cocaine addicted and controls differ in their subcortical, limbic, and cortical response to cholinergic probes in areas relevant to craving, learning, and memory. Cholinergic systems may offer a pharmacologic target for cocaine addiction treatment.

The P300 component in patients with Alzheimer's disease and their biological children

OBJECTIVE

There are few studies examining P300 in the biological children of patients with Alzheimer's disease (AD). In addition to examining P300 in patients with AD, the current study examined the utility of P300 as a preclinical marker in the offspring of AD patients.

METHODS

P300 was elicited from an AD group, their biological children, and two age- and gender-matched control groups using the auditory oddball paradigm. Each group consisted of 20 subjects each. ERPs recorded from sites Fz, Cz, and Pz were analysed using analysis of variance.

RESULTS

Amplitudes were significantly smaller in the AD group when compared to controls. Both amplitude and latency values in the FH+ group were significantly impaired when compared to its control group.

CONCLUSION

These findings replicate previous P300 amplitude abnormalities found in patients with AD. Further, participants with a family history of AD demonstrate possible preclinical evidence at the electrophysiological level. Comparisons with other findings and theoretical implications are discussed.

Cholinergic modulation of preattentive auditory processing in aging

Auditory event-related potential (ERP) components P50 and N100 are thought to index preattentive auditory processing underlying stimulus detection, whereas a subsequent component termed mismatch negativity (MMN) has been proposed to reflect comparison of incoming stimuli to a short-lived sensory memory trace of preceding sounds. Existing evidence suggests impairment of preattentive auditory processing in aging, which appears to be accompanied by decline of cholinergic activity. Previous studies indicate that scopolamine, which is a centrally acting muscarinic receptor antagonist, modulates preattentive auditory processing in young subjects. It has remained elusive, however, to which extent scopolamine affects preattentive auditory processing in aged subjects. We measured auditory responses simultaneously with electroencephalogram (EEG) and magnetoencephalogram (MEG) from nine non-demented elderly subjects after intravenous injection of scopolamine or glycopyrrolate, the latter being a peripherally acting cholinergic antagonist, using a double blind protocol. Scopolamine significantly delayed electric P50, both electric and magnetic N100 responses, whereas subsequent MMN and P200 responses were not altered by scopolamine. Our results indicate that the cholinergic system modulates auditory processing underlying stimulus detection in aging. In addition, auditory evoked responses appear to have different age-related sensitivity to cholinergic modulation. The combined MEG/EEG measurements using particularly auditory N100 response might offer an objective tool to monitor cholinergic activity in aging and Alzheimer's disease (AD).

Development of Acute Tolerance During Steady-State Arterial Alcohol Concentrations: A Study of Auditory Event-Related Potentials in Rats

BACKGROUND

The blood alcohol clamp is a method whereby alcohol is infused intravenously to maintain a predetermined arterial alcohol concentration (AAC) for an indefinite period of time. The objective of this study was to use the clamp to examine the effects of alcohol on event-related potentials (ERPs) in rats and to assess the development of tolerance during a single alcohol exposure.

METHODS

Adult male Wistar rats that had a chronic implant of EEG electrodes overlying the frontal cortex and were equipped with cannulae in the jugular vein, were clamped at 75 or 150 mg/dl via an intravenous infusion of 20% (v/v) alcohol. Auditory ERPs were recorded before the alcohol infusion (baseline) and at 5, 15, 120, 135, or 195 min after steady-state AAC was achieved. In a separate group of rats, test-retest reliability was examined by acquiring ERPs two to three times in the same rat at 60-min intervals. Dependent variables were calculated as changes from baseline for each time point for P1-N1 amplitude and P1 and N1 latencies.

RESULTS

In the test-retest study, there were no differences in any of the dependent variables over time, indicating that the measures were stable and repeatable. Estimated AACs of 75 and 150 mg/dl significantly (p = 0.0001) decreased P1-N1 amplitude in a dose-related manner. During both clamps, the alcohol effect peaked at 120 min (p < 0.03) and decreased thereafter. Alcohol had no effect on P1 or N1 latencies.

CONCLUSIONS

Pharmacologically relevant AACs significantly decreased the amplitude but not the latencies of the long-latency components of the rat auditory ERP. Acute tolerance developed because the amplitude of the ERP component recovered as AACs were held relatively constant.

Objective markers of drug effects on brain function from recordings of scalp potential in healthy volunteers

In order to stress the importance of P300 responses in drug development, we describe the spatiotemporal characteristics of this objective, evoked event-related potential. These brain activations reflect mnemonic function, in which limbic structures play a role. It is demonstrated that a pharmacological challenge concerning, for example, the cholinergic system in young healthy volunteers induces modifications in P300 reminiscent of the aging brain. We use this type of observation to build a model in which it can be verified whether the deterioration can be counteracted by treatment with “cognition-enhancing” drugs. If we accept the extrapolation of the pharmacological effects to symptomatology, scalp potential analysis offers an appropriate tool for the study of drug interactions in early proof-of-concept models.

Pharmacokinetic-pharmacodynamic modeling of the electroencephalogram effects of scopolamine in healthy volunteers

Scopolamine is a muscarinic receptor antagonist commonly used as a pharmacological model substance based on the "cholinergic hypothesis" of memory loss in senile dementia of the Alzheimer type. The objective of the study was to relate pharmacodynamic electroencephalogram (EEG) changes and scopolamine serum concentration using pharmacokinetic-pharmacodynamic (PK-PD) modeling techniques. This was a randomized, three-way crossover, open-label study involving 10 healthy nonsmoking young male volunteers who received either scopolamine 0.5 mg as an intravenous (i.v.) infusion over 15 minutes or an intramuscular (i.m.) injection or a placebo. The pharmacodynamic EEG measure consists of the total power in delta, theta, alpha, and beta bands over frontal, central, and occipital brain areas. The values of the pharmacokinetic parameters of scopolamine after i.v. infusion were clearance (CL) 205 +/- 36.6 L/h, volume of distribution (Vd) 363 +/- 66.7 L, distribution half-life (t1/2 alpha) 2.9 +/- 0.67 min, and terminal half-life (t1/2 beta) 105.4 +/- 9.94 min (mean +/- SEM). Mean peak serum concentrations (Cmax) were 4.66 and 0.96 ng/ml after i.v. and i.m. administration, respectively (p < 0.05). The area under the serum concentration versus time curve (AUC) after i.m. administration (81.27 +/- 11.21 ng/ml/min) was significantly lower compared to the value after i.v. infusion (157.28 +/- 30.86 ng/ml/min) (mean +/- SEM, p < 0.05). Absolute bioavailability of scopolamine after i.m. injection was 57% +/- 0.08% (mean +/- SEM). After both i.v. and i.m. administration, scopolamine induced a decrease in EEG alpha power (7.50-11.25 Hz) over frontal, central, and occipital brain areas compared to placebo (p < 0.05). The individual concentration-EEG effect relationships determined after i.v. infusion of scopolamine were successfully characterized by a sigmoidal Emax model. The averaged values of the pharmacodynamic parameters were E0 = 0.58 microV2, Emax = 0.29 microV2, EC50 = 0.60 ng/ml, and gamma = 1.17. No time delay between serum concentrations and changes in alpha power was observed, indicating a rapid equilibration between serum and effect site. The results provide the first demonstration of a direct correlation between serum concentrations of scopolamine and changes in total power in alpha frequency band in healthy volunteers using PK-PD modeling techniques. As regards the effect on the EEG, 0.5 mg of scopolamine administered i.v. appears to be a suitable dose.

[The p300 cognitive event-related potential. I. Theoretical and psychobiologic perspectives]

The P300 is a positive wave which arises when an attended stimulus is detected. Its parameters depend on a number of variables, in particular the subject's mental state, the task that has to be accomplished, the significance of the stimulus, and the degree of attention. It can be recorded with accuracy, and the different stages of information processing can therefore be analyzed. The P300 wave shows the modifications in neuronal activity which take place during the cognitive process: P300 latency provides an indirect indication of the duration of the processes involved in stimulus discrimination while its amplitude, which is influenced by a number of variables, provides an index of the intensity of the energetic activation or arousal involved. The P300 wave consists of several components which reflect distinct information-processing events (P3a, P3b, P3e, P-SR, P-CR). According to the theoretical models, it is hypothesized that P300 could either represent the adaptation of the working memory to further environmental input ('context updating'), or indicate a closing process ('context closure') in information processing. As regards the physiological aspect of P300 and its association with cortical networks, various studies have suggested that several cortical generators of P300 may co-exist: the medial temporal lobe, the temporo-parietal junction, and the medial and lateral frontal lobe. Psychopharmacological studies have shown that different neurotransmitter systems are involved in the generation and modulation of P300, namely the cholinergic, noradrenergic, dopaminergic, serotoninergic and gabaergic systems. It appears that the noradrenergic agonists increase the amplitude of P300, dopaminergic agonists may have a biphasic effect (increase/reduction), while cholinergic antagonists and gabaergic agonists reduce P300 amplitude and prolong its latency.

Schizophrenia-like deficits in auditory P1 and N1 refractoriness induced by the psychomimetic agent phencyclidine (PCP)

OBJECTIVES

The amplitude of the cortically generated auditory event-related potential (ERP) components P1 and N1 decreases as the interval between successive stimuli (ISI) decreases. Although the phenomenon of P1 and N1 refractoriness is well established, the underlying mechanisms are poorly understood. The present study investigates P1 and N1 refractoriness in the awake monkey in order to investigate underlying mechanisms.

METHODS

Auditory ERP were obtained in response to repetitive auditory stimuli presented at 5 levels of ISI between 150 ms and 9 s, prior to and following administration of the selective N-methyl-D-aspartate (NMDA) antagonist phencyclidine (PCP).

RESULTS

P1 and N1 amplitude declined in monkeys with decreasing ISI, with similar temporal characteristics to that observed in humans. PCP inhibited P1 and N1 generation at long, but not short, ISI producing a pattern similar to that recently observed in schizophrenic subjects.

CONCLUSIONS

The present findings suggest that the primate P1/N1 model may be useful for investigating mechanisms underlying impaired information processing in schizophrenia, and that NMDA receptor dysfunction may play a key role in information processing dysfunction associated with schizophrenia.

P300 event-related potential and serotonin-1A activity in depression

The identification of the brain structures and neurotransmitters responsible for the generation and/or modulation of P300 could lead to important clinical implications. Since serotonin disturbances seem to play a critical role in depression, the aim of the study was to assess the possible relationships between the P300 event-related brain potential and serotonergic activity in depression. The study was conducted among 45 major depressive inpatients, and serotonergic activity was assessed by prolactin (PRL) response to flesinoxan (a 5-HT1A agonist). Results showed a significant negative correlation between P300 amplitude and PRL response to flesinoxan (r = -0.40, P = 0.007 at Cz; r = -0.47, P = 0.001 at Pz). In contrast, both P300 latency and reaction time were not related to endocrine response. This study supports a role for serotonin-1A in the neurobiological modulation of P300 amplitude.

Scopolamine model of dementia: electroencephalogram findings and cognitive performance

BACKGROUND

Memory and cognitive functions are known to decline with advancing age. Studies have suggested that this may be due to a decrease in cholinergic function in the brains of elderly people. This review aims to assess studies documented in the literature dealing with the 'scopolamine model' of dementia.

METHODS

Sources included MedLine searches from the last 10 years (search for 'scopolamine model', 'dementia', 'electroencephalogram', 'cognition') and references from original and review articles. The aim was to include human and animal studies occupying the cholinergic hypothesis in cognitive dysfunction. Electroencephalographic (EEG) and cognition findings were considered.

RESULTS

Scopolamine influences delta, theta, alpha and beta activity in EEG and partially mimics the EEG changes found in patients with senile dementia or dementia of the Alzheimer type. Effects on different cognitive functions have been extensively documented.

CONCLUSION

Scopolamine produces similar memory deficits seen in the elderly, but the drug cannot induce the full range of deficits seen in patients with Alzheimer's disease. Various aspects of memory were unaffected by scopolamine administration. Memory improvements in elderly subjects can be achieved after cholinergic stimulation.