A Self-Similarity Logic May Shape the Organization of the Nervous System.

From the morphological point of view, the nervous system exhibits a fractal, self-similar geometry at various levels of observations, from single cells up to cell networks. From the functional point of view, it is characterized by a hierarchical organization in which self-similar structures (networks) of different miniaturizations are nested within each other. In particular, neuronal networks, interconnected to form neuronal systems, are formed by neurons, which operate thanks to their molecular networks, mainly having proteins as components that via protein-protein interactions can be assembled in multimeric complexes working as micro-devices. On this basis, the term "self-similarity logic" was introduced to describe a nested organization where, at the various levels, almost the same rules (logic) to perform operations are used. Self-similarity and self-similarity logic both appear to be intimately linked to the biophysical evidence for the nervous system being a pattern-forming system that can flexibly switch from one coherent state to another. Thus, they can represent the key concepts to describe its complexity and its concerted, holistic behavior.

Modulation of Neuron and Astrocyte Dopamine Receptors via Receptor-Receptor Interactions.

Dopamine neurotransmission plays critical roles in regulating complex cognitive and behavioral processes including reward, motivation, reinforcement learning, and movement. Dopamine receptors are classified into five subtypes, widely distributed across the brain, including regions responsible for motor functions and specific areas related to cognitive and emotional functions. Dopamine also acts on astrocytes, which express dopamine receptors as well. The discovery of direct receptor-receptor interactions, leading to the formation of multimeric receptor complexes at the cell membrane and providing the cell decoding apparatus with flexible dynamics in terms of recognition and signal transduction, has expanded the knowledge of the G-protein-coupled receptor-mediated signaling processes. The purpose of this review article is to provide an overview of currently identified receptor complexes containing dopamine receptors and of their modulatory action on dopamine-mediated signaling between neurons and between neurons and astrocytes. Pharmacological possibilities offered by targeting receptor complexes in terms of addressing neuropsychiatric disorders associated with altered dopamine signaling will also be briefly discussed.

Striatal astrocytic A2A-D2 receptor-receptor interactions and their role in neuropsychiatric disorders.

It is now generally accepted that astrocytes are active players in synaptic transmission, so that a neurocentric perspective of the integrative signal communication in the central nervous system is shifting towards a neuro-astrocentric perspective. Astrocytes respond to synaptic activity, release chemical signals (gliotransmitters) and express neurotransmitter receptors (G protein-coupled and ionotropic receptors), thus behaving as co-actors with neurons in signal communication in the central nervous system. The ability of G protein-coupled receptors to physically interact through heteromerization, forming heteromers and receptor mosaics with new distinct signal recognition and transduction pathways, has been intensively studied at neuronal plasma membrane, and has changed the view of the integrative signal communication in the central nervous system. One of the best-known examples of receptor-receptor interaction through heteromerization, with relevant consequences for both the physiological and the pharmacological points of view, is given by adenosine A2A and dopamine D2 receptors on the plasma membrane of striatal neurons. Here we review evidence that native A2A and D2 receptors can interact through heteromerization at the plasma membrane of astrocytes as well. Astrocytic A2A-D2 heteromers were found able to control the release of glutamate from the striatal astrocyte processes. A2A-D2 heteromers on striatal astrocytes and astrocyte processes are discussed as far as their potential relevance in the control of glutamatergic transmission in striatum is concerned, including potential roles in glutamatergic transmission dysregulation in pathological conditions including schizophrenia or the Parkinson's disease. This article is part of the Special Issue on "The receptor-receptor interaction as a new target for therapy".

Brain Structure and Function: Insights from Chemical Neuroanatomy.

We present a brief historical and epistemological outline of investigations on the brain's structure and functions. These investigations have mainly been based on the intermingling of chemical anatomy, new techniques in the field of microscopy and computer-assisted morphometric methods. This intermingling has enabled extraordinary investigations to be carried out on brain circuits, leading to the development of a new discipline: "brain connectomics". This new approach has led to the characterization of the brain's structure and function in physiological and pathological conditions, and to the development of new therapeutic strategies. In this context, the conceptual model of the brain as a hyper-network with a hierarchical, nested architecture, arranged in a "Russian doll" pattern, has been proposed. Our investigations focused on the main characteristics of the modes of communication between nodes at the various miniaturization levels, in order to describe the brain's integrative actions. Special attention was paid to the nano-level, i.e., to the allosteric interactions among G protein-coupled receptors organized in receptor mosaics, as a promising field in which to obtain a new view of synaptic plasticity and to develop new, more selective drugs. The brain's multi-level organization and the multi-faceted aspects of communication modes point to an emerging picture of the brain as a very peculiar system, in which continuous self-organization and remodeling take place under the action of external stimuli from the environment, from peripheral organs and from ongoing integrative actions.

Receptor-receptor interactions and microvesicle exchange as mechanisms modulating signaling between neurons and astrocytes.

It is well known that astrocytes play a significant metabolic role in the nervous tissue, maintaining the homeostasis of the extracellular space and of the blood-brain barrier, and providing trophic support to neurons. In addition, however, evidence exists indicating astrocytes as important elements for brain activity through signaling exchange with neurons. Astrocytes, indeed, can sense synaptic activity and their molecular machinery responds to neurotransmitters released by neurons with cytoplasmic Ca2+ elevations that, in turn, stimulate the release of neuroactive substances (gliotransmitters) influencing nearby neurons. In both cell types the recognition and transduction of this complex pattern of signals is mediated by specific receptors that are also involved in mechanisms tuning the intercellular cross-talk between astrocytes and neurons. Two of these mechanisms are the focus of the present discussion. The first concerns direct receptor-receptor interactions leading to the formation at the cell membrane of multimeric receptor complexes. The cooperativity that emerges in the actions of orthosteric and allosteric ligands of the monomers forming the assembly provides the cell decoding apparatus with sophisticated and flexible dynamics in terms of recognition and signal transduction pathways. A further mechanism of plasticity involving receptors is based on the transfer of elements of the cellular signaling apparatus via extracellular microvesicles acting as protective containers, which can lead to transient changes in the transmitting/decoding capabilities of the target cell. This article is part of the Special Issue on "The receptor-receptor interaction as a new target for therapy".

Heteromerization of Dopamine D2 and Oxytocin Receptor in Adult Striatal Astrocytes.

The ability of oxytocin (OT) to interact with the dopaminergic system through facilitatory D2-OT receptor (OTR) receptor-receptor interaction in the limbic system is increasingly considered to play roles in social or emotional behavior, and suggested to serve as a potential therapeutic target. Although roles of astrocytes in the modulatory effects of OT and dopamine in the central nervous system are well recognized, the possibility of D2-OTR receptor-receptor interaction in astrocytes has been neglected. In purified astrocyte processes from adult rat striatum, we assessed OTR and dopamine D2 receptor expression by confocal analysis. The effects of activation of these receptors were evaluated in the processes through a neurochemical study of glutamate release evoked by 4-aminopyridine; D2-OTR heteromerization was assessed by co-immunoprecipitation and proximity ligation assay (PLA). The structure of the possible D2-OTR heterodimer was estimated by a bioinformatic approach. We found that both D2 and OTR were expressed on the same astrocyte processes and controlled the release of glutamate, showing a facilitatory receptor-receptor interaction in the D2-OTR heteromers. Biochemical and biophysical evidence confirmed D2-OTR heterodimers on striatal astrocytes. The residues in the transmembrane domains four and five of both receptors are predicted to be mainly involved in the heteromerization. In conclusion, roles for astrocytic D2-OTR in the control of glutamatergic synapse functioning through modulation of astrocytic glutamate release should be taken into consideration when considering interactions between oxytocinergic and dopaminergic systems in striatum.

Intercellular Communication in the Central Nervous System as Deduced by Chemical Neuroanatomy and Quantitative Analysis of Images: Impact on Neuropharmacology.

In the last decades, new evidence on brain structure and function has been acquired by morphological investigations based on synergic interactions between biochemical anatomy approaches, new techniques in microscopy and brain imaging, and quantitative analysis of the obtained images. This effort produced an expanded view on brain architecture, illustrating the central nervous system as a huge network of cells and regions in which intercellular communication processes, involving not only neurons but also other cell populations, virtually determine all aspects of the integrative function performed by the system. The main features of these processes are described. They include the two basic modes of intercellular communication identified (i.e., wiring and volume transmission) and mechanisms modulating the intercellular signaling, such as cotransmission and allosteric receptor-receptor interactions. These features may also open new possibilities for the development of novel pharmacological approaches to address central nervous system diseases. This aspect, with a potential major impact on molecular medicine, will be also briefly discussed.

Heterodimer of A2A and Oxytocin Receptors Regulating Glutamate Release in Adult Striatal Astrocytes.

BACKGROUND: Roles of astrocytes in the modulatory effects of oxytocin (OT) in central nervous system are increasingly considered. Nevertheless, OT effects on gliotransmitter release have been neglected. METHODS: In purified astrocyte processes from adult rat striatum, we assessed OT receptor (OTR) and adenosine A2A receptor expression by confocal analysis. The effects of receptors activation on glutamate release from the processes were evaluated; A2A-OTR heteromerization was assessed by co-immunoprecipitation and PLA. Structure of the possible heterodimer of A2A and OT receptors was estimated by a bioinformatic approach. RESULTS: Both A2A and OT receptors were expressed on the same astrocyte processes. Evidence for A2A-OTR receptor-receptor interaction was obtained by measuring the release of glutamate: OT inhibited the evoked glutamate release, while activation of A2A receptors, per se ineffective, abolished the OT effect. Biochemical and biophysical evidence for A2A-OTR heterodimers on striatal astrocytes was also obtained. The residues in the transmembrane domains 4 and 5 of both receptors are predicted to be mainly involved in the heteromerization. CONCLUSIONS: When considering effects of OT in striatum, modulation of glutamate release from the astrocyte processes and of glutamatergic synapse functioning, and the interaction with A2A receptors on the astrocyte processes should be taken into consideration.

Modulating brain integrative actions as a new perspective on pharmacological approaches to neuropsychiatric diseases.

A critical aspect of drug development in the therapy of neuropsychiatric diseases is the "Target Problem", that is, the selection of a proper target after not simply the etiopathological classification but rather the detection of the supposed structural and/or functional alterations in the brain networks. There are novel ways of approaching the development of drugs capable of overcoming or at least reducing the deficits without triggering deleterious side effects. For this purpose, a model of brain network organization is needed, and the main aspects of its integrative actions must also be established. Thus, to this aim we here propose an updated model of the brain as a hyper-network in which i) the penta-partite synapses are suggested as key nodes of the brain hyper-network and ii) interacting cell surface receptors appear as both decoders of signals arriving to the network and targets of central nervous system diseases. The integrative actions of the brain networks follow the "Russian Doll organization" including the micro (i.e., synaptic) and nano (i.e., molecular) levels. In this scenario, integrative actions result primarily from protein-protein interactions. Importantly, the macromolecular complexes arising from these interactions often have novel structural binding sites of allosteric nature. Taking G protein-coupled receptors (GPCRs) as potential targets, GPCRs heteromers offer a way to increase the selectivity of pharmacological treatments if proper allosteric drugs are designed. This assumption is founded on the possible selectivity of allosteric interventions on G protein-coupled receptors especially when organized as "Receptor Mosaics" at penta-partite synapse level.

Man is a "Rope" Stretched Between Virosphere and Humanoid Robots: On the Urgent Need of an Ethical Code for Ecosystem Survival.

In this paper we compare the strategies applied by two successful biological components of the ecosystem, the viruses and the human beings, to interact with the environment. Viruses have had and still exert deep and vast actions on the ecosystem especially at the genome level of most of its biotic components. We discuss on the importance of the human being as contraptions maker in particular of robots, hence of machines capable of automatically carrying out complex series of actions. Beside the relevance of designing and assembling these contraptions, it is of basic importance the goal for which they are assembled and future scenarios of their possible impact on the ecosystem. We can't procrastinate the development and implementation of a highly inspired and stringent "ethical code" for human beings and humanoid robots because it will be a crucial aspect for the wellbeing of the mankind and of the entire ecosystem.

Receptor-Receptor Interactions and Glial Cell Functions with a Special Focus on G Protein-Coupled Receptors.

The discovery that receptors from all families can establish allosteric receptor-receptor interactions and variably associate to form receptor complexes operating as integrative input units endowed with a high functional and structural plasticity has expanded our understanding of intercellular communication. Regarding the nervous system, most research in the field has focused on neuronal populations and has led to the identification of many receptor complexes representing an important mechanism to fine-tune synaptic efficiency. Receptor-receptor interactions, however, also modulate glia-neuron and glia-glia intercellular communication, with significant consequences on synaptic activity and brain network plasticity. The research on this topic is probably still at the beginning and, here, available evidence will be reviewed and discussed. It may also be of potential interest from a pharmacological standpoint, opening the possibility to explore, inter alia, glia-based neuroprotective therapeutic strategies.

Adenosine A2A-dopamine D2 receptor-receptor interaction in neurons and astrocytes: Evidence and perspectives.

The discovery of receptor-receptor interactions in the early 1980s, together with a more accurate focusing of allosteric mechanisms in proteins, expanded the knowledge on the G protein-coupled receptor (GPCR)-mediated signaling processes. GPCRs were seen to operate not only as monomers, but also as quaternary structures shaped by allosteric interactions. These integrative mechanisms can change the function of the GPCRs involved, leading to a sophisticated dynamic of the receptor assembly in terms of modulation of recognition and signaling. In this context, the heterodimeric complex formed by the adenosine A2A and the dopamine D2 receptors likely represents a prototypical example. The pharmacological evidence obtained, together with the tissue distribution of the A2A-D2 heteromeric complexes, suggested they could represent a target for new therapeutic strategies addressing significant disorders of the central nervous system. The research findings and the perspectives they offer from the therapeutic standpoint are the focus of the here presented discussion.

Exosomes From Astrocyte Processes: Signaling to Neurons.

It is widely recognized that extracellular vesicles subserve non-classical signal transmission in the central nervous system. Here we assess if the astrocyte processes, that are recognized to play crucial roles in intercellular communication at the synapses and in neuron-astrocyte networks, could convey messages through extracellular vesicles. Our findings indicate, for the first time that freshly isolated astrocyte processes prepared from adult rat cerebral cortex, can indeed participate to signal transmission in central nervous system by releasing exosomes that by volume transmission might target near or long-distance sites. It is noteworthy that the exosomes released from the astrocyte processes proved ability to selectively target neurons. The astrocyte-derived exosomes were proven positive for neuroglobin, a protein functioning as neuroprotectant against cell insult; the possibility that exosomes might transfer neuroglobin to neurons would add a mechanism to the potential astrocytic neuroprotectant activity. Notably, the exosomes released from the processes of astrocytes maintained markers, which prove their parental astrocytic origin. This potentially allows the assessment of the cellular origin of exosomes that might be recovered from body fluids.

A New Integrative Theory of Brain-Body-Ecosystem Medicine: From the Hippocratic Holistic View of Medicine to Our Modern Society.

Humans are increasingly aware that their fate will depend on the wisdom they apply in interacting with the ecosystem. Its health is defined as the condition in which the ecosystem can deliver and continuously renew its fundamental services. A healthy ecosystem allows optimal interactions between humans and the other biotic/abiotic components, and only in a healthy ecosystem can humans survive and efficiently reproduce. Thus, both the human and ecosystem health should be considered together in view of their interdependence. The present article suggests that this relationship could be considered starting from the Hippocrates (460 BC-370 BC) work "On Airs, Waters, and Places" to derive useful medical and philosophical implications for medicine which is indeed a topic that involves scientific as well as philosophical concepts that implicate a background broader than the human body. The brain-body-ecosystem medicine is proposed as a new more complete approach to safeguarding human health. Epidemiological data demonstrate that exploitation of the environment resulting in ecosystem damage affects human health and in several instances these diseases can be detected by modifications in the heart-brain interactions that can be diagnosed through the analysis of changes in heart rate variability.

A2A-D2 Heteromers on Striatal Astrocytes: Biochemical and Biophysical Evidence.

Our previous findings indicate that A2A and D2 receptors are co-expressed on adult rat striatal astrocytes and on the astrocyte processes, and that A2A-D2 receptor⁻receptor interaction can control the release of glutamate from the processes. Functional evidence suggests that the receptor⁻receptor interaction was based on heteromerization of native A2A and D2 receptors at the plasma membrane of striatal astrocyte processes. We here provide biochemical and biophysical evidence confirming that receptor⁻receptor interaction between A2A and D2 receptors at the astrocyte plasma membrane is based on A2A-D2 heteromerization. To our knowledge, this is the first direct demonstration of the ability of native A2A and D2 receptors to heteromerize on glial cells. As striatal astrocytes are recognized to be involved in Parkinson's pathophysiology, the findings that adenosine A2A and dopamine D2 receptors can form A2A-D2 heteromers on the astrocytes in the striatum (and that these heteromers can play roles in the control of the striatal glutamatergic transmission) may shed light on the molecular mechanisms involved in the pathogenesis of the disease.

Receptor-Receptor Interactions as a Widespread Phenomenon: Novel Targets for Drug Development?

The discovery of receptor-receptor interactions (RRI) has expanded our understanding of the role that G protein-coupled receptors (GPCRs) play in intercellular communication. The finding that GPCRs can operate as receptor complexes, and not only as monomers, suggests that several different incoming signals could already be integrated at the plasma membrane level via direct allosteric interactions between the protomers that form the complex. Most research in this field has focused on neuronal populations and has led to the identification of a large number of RRI. However, RRI have been seen to occur not only in neurons but also in astrocytes and, outside the central nervous system, in cells of the cardiovascular and endocrine systems and in cancer cells. Furthermore, RRI involving the formation of macromolecular complexes are not limited to GPCRs, being also observed in other families of receptors. Thus, RRI appear as a widespread phenomenon and oligomerization as a common mechanism for receptor function and regulation. The discovery of these macromolecular assemblies may well have a major impact on pharmacology. Indeed, the formation of receptor complexes significantly broadens the spectrum of mechanisms available to receptors for recognition and signaling, which may be implemented through modulation of the binding sites of the adjacent protomers and of their signal transduction features. In this context, the possible appearance of novel allosteric sites in the receptor complex structure may be of particular relevance. Thus, the existence of RRI offers the possibility of new therapeutic approaches, and novel pharmacological strategies for disease treatment have already been proposed. Several challenges, however, remain. These include the accurate characterization of the role that the receptor complexes identified so far play in pathological conditions and the development of ligands specific to given receptor complexes, in order to efficiently exploit the pharmacological properties of these complexes.

Homocysteine and A2A-D2 Receptor-Receptor Interaction at Striatal Astrocyte Processes.

The interaction between adenosine A2A and dopamine D2 receptors in striatal neurons is a well-established phenomenon and has opened up new perspectives on the molecular mechanisms involved in Parkinson's disease. However, it has barely been investigated in astrocytes. Here, we show by immunofluorescence that both A2A and D2 receptors are expressed in adult rat striatal astrocytes in situ, and investigate on presence, function, and interactions of the receptors in the astrocyte processes-acutely prepared from the adult rat striatum-and on the effects of homocysteine on the A2A-D2 receptor-receptor interaction. We found that A2A and D2 receptors were co-expressed on vesicular glutamate transporter-1-positive astrocyte processes, and confirmed that A2A-D2 receptor-receptor interaction controlled glutamate release-assessed by measuring the [3H]D-aspartate release-from the processes. The complexity of A2A-D2 receptor-receptor interaction is suggested by the effect of intracellular homocysteine, which reduced D2-mediated inhibition of glutamate release (homocysteine allosteric action on D2), without interfering with the A2A-mediated antagonism of the D2 effect (maintained A2A-D2 interaction). Our findings indicate the crucial integrative role of A2A-D2 molecular circuits at the plasma membrane of striatal astrocyte processes. The fact that homocysteine reduced D2-mediated inhibition of glutamate release could provide new insights into striatal astrocyte-neuron intercellular communications. As striatal astrocytes are recognized to be involved in Parkinson's pathophysiology, these findings may shed light on the pathogenic mechanisms of the disease and contribute to the development of new drugs for its treatment.

Near-infrared laser photons induce glutamate release from cerebrocortical nerve terminals.

Although photons have been repeatedly shown to affect the functioning of the nervous system, their effects on neurotransmitter release have never been investigated. We exploited in vitro models that allow effects involving neuron-astrocyte network functioning to be detected (mouse cerebrocortical slices) and dissected these effects at cerebrocortical nerve endings and astrocyte processes. Infrared light proved able to induce glutamate release by stimulating glutamatergic nerve endings.

G protein-coupled receptor-receptor interactions give integrative dynamics to intercellular communication.

The proposal of receptor-receptor interactions (RRIs) in the early 1980s broadened the view on the role of G protein-coupled receptors (GPCR) in the dynamics of the intercellular communication. RRIs, indeed, allow GPCR to operate not only as monomers but also as receptor complexes, in which the integration of the incoming signals depends on the number, spatial arrangement, and order of activation of the protomers forming the complex. The main biochemical mechanisms controlling the functional interplay of GPCR in the receptor complexes are direct allosteric interactions between protomer domains. The formation of these macromolecular assemblies has several physiologic implications in terms of the modulation of the signaling pathways and interaction with other membrane proteins. It also impacts on the emerging field of connectomics, as it contributes to set and tune the synaptic strength. Furthermore, recent evidence suggests that the transfer of GPCR and GPCR complexes between cells via the exosome pathway could enable the target cells to recognize/decode transmitters and/or modulators for which they did not express the pertinent receptors. Thus, this process may also open the possibility of a new type of redeployment of neural circuits. The fundamental aspects of GPCR complex formation and function are the focus of the present review article.

The brain as a "hyper-network": the key role of neural networks as main producers of the integrated brain actions especially via the "broadcasted" neuroconnectomics.

Investigations of brain complex integrative actions should consider beside neural networks, glial, extracellular molecular, and fluid channels networks. The present paper proposes that all these networks are assembled into the brain hyper-network that has as fundamental components, the tetra-partite synapses, formed by neural, glial, and extracellular molecular networks. Furthermore, peri-synaptic astrocytic processes by modulating the perviousness of extracellular fluid channels control the signals impinging on the tetra-partite synapses. It has also been surmised that global signalling via astrocytes networks and highly pervasive signals, such as electromagnetic fields (EMFs), allow the appropriate integration of the various networks especially at crucial nodes level, the tetra-partite synapses. As a matter of fact, it has been shown that astrocytes can form gap-junction-coupled syncytia allowing intercellular communication characterised by a rapid and possibly long-distance transfer of signals. As far as the EMFs are concerned, the concept of broadcasted neuroconnectomics (BNC) has been introduced to describe highly pervasive signals involved in resetting the information handling of brain networks at various miniaturisation levels. In other words, BNC creates, thanks to the EMFs, generated especially by neurons, different assemblages among the various networks forming the brain hyper-network. Thus, it is surmised that neuronal networks are the "core components" of the brain hyper-network that has as special "nodes" the multi-facet tetra-partite synapses. Furthermore, it is suggested that investigations on the functional plasticity of multi-partite synapses in response to BNC can be the background for a new understanding and perhaps a new modelling of brain morpho-functional organisation and integrative actions.