Computational models of schizophrenia and dopamine modulation in the prefrontal cortex

Catechol-O-methyltransferase valine(158)methionine polymorphism modulates brain networks underlying working memory across adulthood

BACKGROUND

Cognitive abilities decline with age with large individual variability. Genetic variations have been suggested to be an important source for some of this heterogeneity. Among these variations, those related to the dopaminergic system, particularly the valine(158)methionine polymorphism in catechol-O-methyltransferase (COMTval(158)met), have been implicated in modulating age-related changes in executive function.

METHODS

We studied 75 subjects (age 21-90 years) using functional neuroimaging while they performed a low-level working memory (WM) task to explore the effects of aging, of the COMTval(158)met polymorphism, and their interactions on the physiological patterns of interconnected cortical activity engaged by WM.

RESULTS

Our results show that val homozygotes and older subjects showed increased activity in dorsolateral prefrontal cortex (DLPFC) and decreased activity in ventrolateral prefrontal cortex (VLPFC) relative to met homozygotes and younger subjects, respectively. Interestingly, there were also independent effects of the COMTval(158)met polymorphism and age on the strength of connectivity between brain regions within the left prefrontal-parietal network; val homozygotes and older subjects showed greater connectivity between the DLPFC and other brain regions within the network and met homozygotes showed greater connectivity between the VLPFC and other brain regions within the network. Furthermore, the greater functional connectivity strength of DLPFC in val homozygotes relative to met homozygotes was much more pronounced in older adults

CONCLUSIONS

Our findings suggest that the COMTval(158)met polymorphism modulates both the activity and functional connectivity of brain regions within WM networks and most importantly that this effect is exaggerated with increasing age, contributing to the variability in age-related decline in executive cognition.

Neurobiological and pharmacological aspects of atypical antipsychotic drugs

Az atípusos (második generációs) antipszichotikumok forradalmi változást hoztak a szkizofrénia és az egyéb pszichotikus zavarok farmakoterápiájában. A jelenleg alkalmazott atípusos szerek fő jellemzője a viszonylag gyenge D 2 -receptor-gátlás, jóval erősebb szerotonin-5-HT 2A -blokkolás mellett. Ezen jellemzők tehetők részben felelőssé a második generációs szerek hatékonyságáért a negatív, a kognitív és az affektív tünetek területén, amely a konvencionális antipszichotikumokra nem vagy csak alig jellemző. A második generációs szerek közül több molekulánál jellemző a D 2 -receptorról történő gyors disszociáció, amely szintén az atípusos hatásmechanizmus egyik fontos részjelensége lehet. A többféle jeltovábbító rendszer befolyásolása révén az atípusos antipszichotikumok konvencionális társaiknál jóval nagyobb szereppel bírnak a neuronalis plaszticitás megőrzésében. A multiplex neurobiológiai hatásmechanizmus révén a második generációs szerek neuroprotektív mechanizmusaik révén a szkizofrénia kezelésében új minőséget jelentenek.

Probability distributions of Markovian sodium channel states during propagating axonal impulses with or without recovery supernormality

This study addressed a macroscopic neurophysiological phenomenon - supernormality during the recovery phase of propagating axonal impulses - in explicit chemical terms. Excitation was reconstructed numerically using the kinetic scheme of multiple-state probabilistic transitions within a population of voltage-dependent sodium channels (NaCh) derived by Vandenberg and Bezanilla ("PC" scheme). Each NaCh transition was characterized as a reversible Markov process with voltage-dependent rate constants associated with each respective directional transition. While recovery reconstructed with the Hodgkin-Huxley formalism included a supernormal period, the PC scheme did not. The present analysis showed that the occurrence and degree of supernormality with the PC scheme was determined by the relative speed of the transitions within the closed loop of the kinetic scheme; supernormality was promoted by speeding these kinetics. The analysis also showed that concurrent with supernormality, the faster loop kinetics caused (1) an elevation in the C(1) --> C(2) transitions, and (2) a reduction in the I(4) --> I(5) transitions. Thus, macroscopic functionality in information processing could be expressed in terms of probabilistic interstate transitions among a population of NaCh molecules.

Dopamine and psychosis: theory, pathomechanisms and intermediate phenotypes

Schizophrenia is a chronic, severe, and disabling brain disorder arising from the adverse interaction of predisposing risk genes and environmental factors. The psychopathology is characterized by a wide array of disturbing cognitive, emotional, and behavioral symptoms that interfere with the individual's capacity to function in society. Contemporary pathophysiological models assume that psychotic symptoms are triggered by a dysregulation of dopaminergic activity in the brain, a theory that is tightly linked to the serendipitous discovery of the first effective antipsychotic agents in the early 1950s. In recent years, the availability of modern neuroimaging techniques has significantly expanded our understanding of the key mediator circuits that bridge the gap between genetic susceptibility and clinical phenotype. This paper discusses the pathophysiological concepts, molecular mechanisms and neuroimaging evidence that link psychosis to disturbances in dopamine neurotransmission.

Single-trial P3 amplitude and latency informed event-related fMRI models yield different BOLD response patterns to a target detection task

Using single-trial parameters as a regressor in the General Linear Model (GLM) is becoming an increasingly popular method for informing fMRI analysis. However, the parameter used to characterise or to differentiate brain regions involved in the response to a particular task varies across studies (e.g. ERP amplitude, ERP latency, reaction time). Furthermore, the way in which the single-trial information is used in the fMRI analysis is also important. For example, the single-trial parameters can be used as regressors in the GLM or to modify the duration of the events modelled in the GLM. The aim of this study was to investigate the BOLD response to a target detection task when including P3 amplitude, P3 latency and reaction time parameters in the GLM. Simultaneous EEG-fMRI was recorded from fifteen subjects in response to a visual choice reaction time task. Including P3 amplitude as a regressor in the GLM yielded activation in left central opercular cortex, left postcentral gyrus, left insula, left middle frontal gyrus, left insula and left parietal operculum. Using P3 latency and reaction time as an additional regressor yielded no additional activation in comparison with the conventional fMRI analysis. However, when P3 latency or reaction time was used to determine the duration of events at a single-trial level, additional activation was observed in the left postcentral gyrus, left precentral gyrus, anterior cingulate cortex and supramarginal gyrus. Our findings suggest that ERP amplitudes and latencies can yield different activation patterns when used to modify relevant aspects of the GLM.

Involvement of the prefrontal cortex in problem solving

To achieve a behavioral goal in a complex environment, such as problem-solving situations, we must plan multiple steps of action. On planning a series of actions, we anticipate future events that will occur as a result of each action, and mentally organize the temporal sequence of events. To investigate the involvement of the lateral prefrontal cortex (PFC) in such multistep planning, we examined neuronal activity in the PFC while monkeys performed a maze path-finding task. In this task, we set monkeys the job of capturing a goal in the maze by moving a cursor on the screen. Cursor movement was linked to movements of each wrist. To dissociate the outcomes of the intended action from the motor commands, we trained the monkeys to use three different hand-cursor assignments. We found that monkeys were able to perform this task in a flexible manner. This report first introduces a problem-solving framework for studying the function of the PFC, from the view point of cognitive science. Then, this chapter will cover the neuronal representation of a series of actions, goal subgoal transformation, and synchrony of PFC neurons. We reported PFC neurons reflected final goals and immediate goals during the preparatory period. We also found some PFC neurons reflected each of all forthcoming steps of actions during the preparatory period and increased their activity step by step during the execution period. Recently, we found that the transient increase in synchronous activity of PFC neurons was involved in goal subgoal transformations. Our data suggest that the PFC is involved primarily in the dynamic representation of multiple future events that occur as a consequence of behavioral actions in problem-solving situations.

Decision making in recurrent neuronal circuits

Decision making has recently emerged as a central theme in neurophysiological studies of cognition, and experimental and computational work has led to the proposal of a cortical circuit mechanism of elemental decision computations. This mechanism depends on slow recurrent synaptic excitation balanced by fast feedback inhibition, which not only instantiates attractor states for forming categorical choices but also long transients for gradually accumulating evidence in favor of or against alternative options. Such a circuit endowed with reward-dependent synaptic plasticity is able to produce adaptive choice behavior. While decision threshold is a core concept for reaction time tasks, it can be dissociated from a general decision rule. Moreover, perceptual decisions and value-based economic choices are described within a unified framework in which probabilistic choices result from irregular neuronal activity as well as iterative interactions of a decision maker with an uncertain environment or other unpredictable decision makers in a social group.

The orbitofrontal cortex and beyond: from affect to decision-making

The orbitofrontal cortex represents the reward or affective value of primary reinforcers including taste, touch, texture, and face expression. It learns to associate other stimuli with these to produce representations of the expected reward value for visual, auditory, and abstract stimuli including monetary reward value. The orbitofrontal cortex thus plays a key role in emotion, by representing the goals for action. The learning process is stimulus-reinforcer association learning. Negative reward prediction error neurons are related to this affective learning. Activations in the orbitofrontal cortex correlate with the subjective emotional experience of affective stimuli, and damage to the orbitofrontal cortex impairs emotion-related learning, emotional behaviour, and subjective affective state. With an origin from beyond the orbitofrontal cortex, top-down attention to affect modulates orbitofrontal cortex representations, and attention to intensity modulates representations in earlier cortical areas of the physical properties of stimuli. Top-down word-level cognitive inputs can bias affective representations in the orbitofrontal cortex, providing a mechanism for cognition to influence emotion. Whereas the orbitofrontal cortex provides a representation of reward or affective value on a continuous scale, areas beyond the orbitofrontal cortex such as the medial prefrontal cortex area 10 are involved in binary decision-making when a choice must be made. For this decision-making, the orbitofrontal cortex provides a representation of each specific reward in a common currency.