Lecture Notes in Computer Science
Corticopetal Acetylcholine: Possible Scenarios on the
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- 1 Introduction 1.1 Corticopetal Acetylcholine
- 1.2 Attentions, Cortical State Transitions and Cholinergic Control System from NBM
- 2 Neural Correlate of Conscious Percepts and the Role of the Corticopetal ACh 2.1 Neural Correlates of Conscious Percepts and Transient Synchrony
- 2.2 Role of the Corticopetal ACh: A Working Hypothesis
- 3 Do the Existing Experimental Data Support the Working Hypothesis 3.1 Introductory Remarks: Transient Synchronization by Virtue of Pre- and
- 3.2 Controversy on Experimental Data
- 3.3 Possible Scenarios
- 4 Concluding Discussions
Corticopetal Acetylcholine: Possible Scenarios on the Role for Dynamic Organization of Quasi-Attractors Hiroshi Fujii 1,2 , Kazuyuki Aihara 2,3 , and Ichiro Tsuda 4,5
Department of Information and Communication Sciences, Kyoto Sangyo University, Kyoto 603-8555, Japan fujii@bacco.kyoto-su.ac.jp 2 Institute of Industrial Science, the University of Tokyo, Tokyo 153-8505 aihara@sat.t.u-tokyo.ac.jp 3 ERATO, Japan Science and Technology Agency, Tokyo 151-0065, Japan 4 Research Institute for Electronic Science, Hokkaido University,
Sapporo 060-0812, Japan tsuda@math.sci.hokudai 5 Center of Excellence (COE)in Mathematics, Department of Mathematics, Hokkaido University, Sapporo 060-0810, Japan Abstract. A new hypothesis on a possible role for the corticopetal acetylcholine (ACh) is provided from a dynamical systems standpoint. The corticopetal ACh helps to transiently organize a global (inter- and intra-cortical) quasi-attractors via gamma range synchrony when it is behaviorally needed as top-down attentions and expectation.
Achetylcholine (ACh) is the first substance identified as a neurotransmitter by Otto Loewi [19]. Although it is increasingly recognized that ACh plays a critical role, not only in arousal and sleep, but in higher cognitive functions as attention, conscious flow, and so on, the question on the way in which ACh works in those cognitive processes remains a mystery [11]. The corticopetal ACh, originated in the nucleus basalis of Meinert (NBM), a part of the basal forebrain (BF), is the primary source of cortical ACh, and the major target of BF projections is the cortex [21]. Behavioral studies and those using immunotoxin as well provide consistent evidence of the role of ACh in top-down attentions. A blockage of NBM ACh, either by deasease-related or drug-induced, causes a severe loss of attentions: selective attention, sustained attention, and divided attention together with a shift of attention. ACh concerns conscious flow (Perry & Perry [24]). Continual death of cholinergic neurons in NBM causes Lewy Body Dementia (LBD), one of the most salient symptoms of which is the complex visual hallucination (CVH) [1]. 1
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Perry and Perry [24] noted those hallucinatory LBD patients who see: “integrated images of people or animals which appear real at the time”, “insects on walls”, or “bicycles outside the fourth storey window”. Images are generally vivid and colored, continue for a few minutes (neither seconds nor hours). It is to be noted that “many of those experiences are enhanced during eye closed and relieved by visual input”, and “nicotinic anatagonists, such as mecamylamine, are not reported to induce hallucinations”.
Corticopetal Acetylcholine: Possible Scenarios on the Role for Dynamic Organization 171
1.2 Attentions, Cortical State Transitions and Cholinergic Control System from NBM Top-down flow of signals which accompanies attentions, expectation and so on may cause state transitions in the “down stream” cortices. Fries et al. [7] reported an increase of synchrony in high-gamma range in accordance of selective attention. (See, also Jones [15], Buschman et al. [2].) Metherate et al. [22] stimulated NBM in in vivo preparations of auditory cortex. Of particular interest in their observations is that NBM ACh produced a change in
oscillations to low-amplitude, fast (20-40 Hz) (i.e., gamma) oscillations. A shift of spike discharge pattern from phasic to tonic was also observed. 2 They pointed out that in view of the wide spread projections of NBM neurons, larger intercortical networks could also be modified. Together with Fries et al. data, it is suggested that NBM cholinergic projections may induce a state transition as a shift of frequency, and change of discharge pattern in neocortical neurons. This may be consistent with the observation made by Kay et al. [16] . During perceptual processing in the olfactory-limbic axis, a cascade of brain events at the successive stages of the task as “expectation” and/or “attention” was observed. ‘Local’ transitions of the olfactory structures indicated by modulations of EEG signals as gamma amplitude, periodicity, and coherence were reported to exist. Kay et al. also observed that the local dynamics transiently falls into attractor-like states. Such ‘local’ transitions of states are generally postulated to be triggered by ‘top- down’ glutamatergic spike volleys from “upper stream” organizations. However, such brain events of state transitions with a change in synchrony could be a result of collaboration of descending glutamatergic spike volleys and ascending ACh afferents from NBM. (See, also [26]). 3
Corticopetal ACh 2.1 Neural Correlates of Conscious Percepts and Transient Synchrony The corticopetal ACh pathway might be the critical control system which may trigger various kinds of attentions receiving convergent inputs from other sensory and association areas as Sarter et al. [25], [26] argued. In order to discuss the role of the
2 NBM contains cholinergic neurons and non-cholinergic neurons. GABAergic neurons are at least twice more numerous than cholinergic neurons [15] The Metherate et al. observations (above) may be the result of collective functioning of both the cholinergic and GABAergic projections. Wenk [31] argued another possibility that the NBM ACh projections on the reticular thalamic nucleus might cause the cortical state change. 3 Triangular Attentional Pathway: The above arguments may better be complemented by the triangular amplification circuitry, a pathway consisting of parietal cortex → prefrontal cortex → NBM → sensory cortex [10]. This may constitute the cholinergic control system from NBM, i.e., the top-down attentional pathway for cholinergic modulations of responses in cortical sensory areas. 172 H. Fujii, K. Aihara, and I. Tsuda corticopetal ACh related to attentions, we first begin with the question: What is the
The recent experiments by Kenet et al. [17] show the possibility that in a background (or spontaneous) state where no external stimuli exist, the visual cortex
a number of pre-existing and intrinsic internal states which represent features, and the cortex fluctuates between such multiple intrinsic states even when no external inputs exist. (See, also Treisman et al. [27].) Attention and Dynamical Binding Through Synchrony In order that perception of an object makes sense, its features, or fragmentary sub- assemblies, must be bound as a unity. How does the “binding” of those local fragmentary dynamics into a global dynamics is done? A widely accepted view is that top-down signals as expectation, attentions, and so on may play the role of such an
concerned assemblies representing stimuli or events. We postulate that such a process is a basis of global intra- and inter-cortical conjunctions of brain activities. See, Varela et al. [30],Womelsdorf et al. [32]. See, also Dehaene and Changeux [5].) The neural correlate of conscious percepts is a globally integrated state of related brain networks mediated by synchrony over gamma and other frequency bands. In mathematical terms, such transient processes of synchrony of global, but between selected groups of neurons may be described as a transitory state of approaching a global attractor. We note that such a “transitory” state may be conceptualized as an
stay there for a while, but may simultaneously possess repelling orbits from itself. The proper Milnor attractor and its perturbed structures can be a specific representation of attractor ruins [8]. However, since the concept of attractor ruins may include a wider class of non-classical attractors than the Milnor attractor, we may use the term “attractor ruins” in this paper to include possible but unknown classes of ruins. 2.2 Role of the Corticopetal ACh: A Working Hypothesis How can top-down attentions, expectation, etc. contribute to conscious perception with the aid of ACh? Assuming the arguments in the preceding section, this question could be translated into: “How do the corticopetal ACh projections work for the emergence of globally organized attractor ruins via transient synchrony?” We summarize our tentative proposition as a Working Hypothesis in the following. Working Hypothesis: The role of the corticopetal ACh accompanied with top-down contextual signals as attentions and so on is the mediator for dynamically organizing quasi-attractors, which are required in conscious perception, or in executing actions. ACh “integrates” multiple “floating subnetworks” into “a transient synchrony group” in the gamma frequency range. Such a transiently emerging synchrony group can be regarded as an attractor ruin in the dynamical systems-theoretic sense. Corticopetal Acetylcholine: Possible Scenarios on the Role for Dynamic Organization 173
3 Do the Existing Experimental Data Support the Working Hypothesis? 3.1 Introductory Remarks: Transient Synchronization by Virtue of Pre- and Post-synaptic ACh Modulations ACh may have both pre-synaptic and post-synaptic effects on individual neurons in the cortex. First, top-down glutamatergic spike volleys flow into cortical layers, which might convey contextual information on stimuli. The corticopetal ACh arrives concomitantly with the glutamatergic volleys. If ACh release modulates synaptic connectivity between cortical neurons even in an effective sense by virtue of “pre-synaptic modulations”, metamorphosis of the attractor landscape 4 should be inevitable. Post-synaptic influences of the corticopetal ACh on individual neurons – either inhibitory or excitatory, might cause deep effects on their firing behavior, and might induce a state transition with a collective gamma oscillation. A consequence of the three effects together might trigger a specific group of networks to oscillate in gamma frequency with phase synchrony. These are all speculative stories based on experimental evidence. We need at least to examine the present status on the experimental data concerning the cortical ACh influence on individual neurons. This may be the place to add a comment on the specificity of the corticopetal ACh projections on the cortex, which may be a point of arguments. It is reported that the cholinergic afferents specialized synaptic connections with post synaptic targets, rather than releasing ACh non-specifically (Turrini et al..[29].)
Let us review quickly the existing experimental data. As noted before, “there exist little consensus among researchers for more than a half century” [11]. The following is not intended to give a complete review, but to give a preliminary knowledge which may be of help to understand the succeeding discussions. ACh have two groups of receptors, one is the muscarinic receptors, mAChRs with 5 subtypes, and the other is nicotinic receptors, nAChRs with 17 subtypes.
The nAChR is a relatively simple cationic (Na + and Ca 2+ ) channel, the opening of which leads to a rapid depolarization followed by desensitization. Most of mAChRs activation exhibits the slower onset and longer lasting G-protein coupled second messenger generation.
Here the primary interest is in mAChRs. 5 The functions of mAChRs are reported to be two-fold: one is pre-synaptic, and the other is post- synaptic modulations.
4 “Attractor landscape” is usually used for potential systems. Here, we use it to mean the landscape of “basins” (absorbing regions), of classical and non-classical attractors as attractor ruins.
5 The nicotinic receptors, nAChRs may work as a disinhibition system to layer 2/3 pyramidal neurons
174 H. Fujii, K. Aihara, and I. Tsuda Post-synapticModulations The results of traditional studies may be divided into two opposing data. The majority view is that mAChRs function as excitatory transmitter for post-synaptic neurons (see, e.g., McCormick [20]) , while there are minority data that claim inhibitory functioning. The latter, however, has been considered to be a consequence of ACh excitation of interneurons, which in turn may inhibit post-synaptic pyramidal neurons (PYR). Recently, Gulledge et al. [11], [12] stated that transient mAChR activation generates strong and direct transient inhibition of neocortical PYR. The underlying ionic process is the induction of calcium release from internal stores, and subsequent activation of small-conductance (SK–type) calcium-activated potassium channels. The authors claim that the traditional data do not describe the actions of transient mAChR activation, as is likely to happen during synaptic release of ACh by the following reasons. 1.
of muscarinic agonists (1–100 mM). Extracellular concentrations of ACh in the cortex are at least one order of magnitude lower than those required to depolarize PYR in vitro. 2. Phasic (transient) application vs. bath application: Most data depended on experiments with bath applications, which may correspond to prolonged, tonic mAChR stimulation. The ACh release accompanied with attentions, etc., would better correspond to a transient puff application as the authors’ experiment.
The specificity of ACh afferents on postsynaptic targets was already noted. [29]. Pre-synaptic Modulations Experimental works on pre-synaptic modulations are mostly based on ACh bath applications, and the modulation data were measured in terms of local field potentials (LFP). Most results claimed the pathway specificity of modulations. Typically, it was concluded that muscarinic modulation can strongly suppress intracortical(IC) synaptic
μ M muscarine decreases both IC and TC pathway transmission, and that those data were presynaptic effects, since membrane potential and input resistance were unchanged. Recently, Kuczewski et al. [18] studied the same problem, and obtained different results from the previous ones. Low ACh (less than 100 μ
and TC pathways, i.e., for the layer 2/3 and layer 4. 3.3 Possible Scenarios The lack of consistent experimental data makes our job complicated. The situation might be compared to playing a jigsaw puzzle with, many pieces missing and some pieces mingled from other jigsaw pictures. What we can do at this moment may be to propose possible alternatives of scenarios for the role of the corticopetal ACh.
Corticopetal Acetylcholine: Possible Scenarios on the Role for Dynamic Organization 175
The following are the list of prerequisites and evidences on which our arguments should be based.
1.
Two modulations may occur simultaneously inside the 6 layers of the cortex. The firing characteristics of individual neurons, and the strength of synaptic connections may change dynamically either as post-synaptic or pre-synaptic modulations. Virtually no models, to our knowledge, have been proposed, which took the net effects of the two modulations into account. 2.
The interaction of ACh with top-down glutamatergic spike volleys should be considered. The majority of neurons alter in response to combined exposure to both acetylcholine and glutamate concomitantly. (Perry & Perry [24].) 3.
As a post-synaptic influence, ACh release may change the firing regime of neurons, and induce gamma oscillation [6], [23], [31]. As to pre-synaptic modulations, the details of synaptic processes appear to be largely unknown. The significance of experimental studies can not be over- emphasized. Now let us try to draw a dessin for possible scenarios on the role of corticopetal ACh. Here we may put three corner stones for the models: 1.
Who triggers the gamma oscillation? 2.
Who (and how to) modulates the effective connectivity? 3.
What is the mechanism of phase synchrony and what is the role of it? Scenario I The basic idea is that in the default, low level state of ACh, globally organized attractors do not virtually exist, and may take the form of floating fragmentary dynamics. Then, ACh release may help to strengthen the synaptic connections pre- synaptically. One of the roles of post-synaptic modulation is to start up the gamma oscillation. (Here the influence of GABAergic projections from NBM might play a role.) Another, but important role will be stated later.
The effective modulation of synaptic connectivity might be carried by, rather than the pre-synaptic modulation, the phase synchrony of the gamma oscillation itself which is triggered by the post-synaptic modulation. Such a mechanism for the change of synaptic connectivity, and resulting binding of fragmentary groups of neurons was proposed by Womelsdorf et al. [32]. They claimed that the mutual influence among neuronal groups depends on the phase relation between rhythmic activities within the groups. Phase relations supporting interactions among the groups preceded those interactions by a few milliseconds, consistent with a mechanistic role. See, also Buzsaki [3]. For the case of Scenario II, the role of the post-synaptic modulation is to start up the gamma oscillation, and the reset of its phase, as Gulledge and Stuart [11] suggested. The transient hyper-polarization may play the role of referee to start up the oscillation among the related groups in unison.
The Scenario I makes the post- synaptic modulation carry the two roles of starting up the gamma oscillation, and 176 H. Fujii, K. Aihara, and I. Tsuda resetting its phase. For the pre-synaptic modulation a bigger role of realization of attractors by virtue of synaptic strength modulation is assigned.
The critical role of the corticopetal ACh in cognitive functions, together with its relation to some disease-related symptoms as complex visual hallucinations in DLB, and its apparent involvements in the neocortical state change have motivated us to the study of the functional role(s) of the corticopetal ACh from dynamical systems standpoints. Cognitive functions are phenomena carried by the brain dynamics. We hope that understanding the cognitive dynamics with the dynamical systems language would open new theoretical horizons. It is of some help to consider the conceptual difference of the two “forces” which flow into the 6 layers of the neocortex. Glutamate spike volleys could be, if viewed as an event in a dynamical system, an external force, which may kick the orbit to another orbit, and may sometimes to out of the “basin” of the present attractor beyond the border – the separatrix. In contrary to this situation, ACh projections – though transient, could be regarded as a slow parameter working as a bifurcation parameter that modifies the landscape itself. What we are looking at in the preceding arguments is that the two phenomena happen concomitantly at the 6 layers of the cortex. Hasselmo and McGaughy [13] emphasized the ACh role in memory as: “high acetylcholine sets circuit dynamics for attention and encoding; low acetylcholine sets dynamics for consolidation”, which is based on some experimental data on selective pre-synaptic depression and facilitation. However, in view of the potential role of attentions in local bindings or global integrations, we may pose alternative (but not necessarily exclusive,) scenarios on the ACh function as temporarily modifying the quasi-attractor landscape, in collaboration with glutamatergic spike volleys. Rather, we speculate that the process of memorization itself would realized through such a dynamic formation of attractor ruins, for which mAChR may play a role. Download 12.42 Mb. Do'stlaringiz bilan baham: |
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