On the distribution patterns of D1, D2, tyrosine hydroxylase and dopamine transporter immunoreactivities in the ventral striatum of the rat

Dysfunctional Heteroreceptor Complexes as Novel Targets for the Treatment of Major Depressive and Anxiety Disorders

Among mental diseases, major depressive disorder (MDD) and anxiety deserve a special place due to their high prevalence and their negative impact both on society and patients suffering from these disorders. Consequently, the development of novel strategies designed to treat them quickly and efficiently, without or at least having limited side effects, is considered a highly important goal. Growing evidence indicates that emerging properties are developed on recognition, trafficking, and signaling of G-protein coupled receptors (GPCRs) upon their heteromerization with other types of GPCRs, receptor tyrosine kinases, and ionotropic receptors such as N-methyl-D-aspartate (NMDA) receptors. Therefore, to develop new treatments for MDD and anxiety, it will be important to identify the most vulnerable heteroreceptor complexes involved in MDD and anxiety. This review focuses on how GPCRs, especially serotonin, dopamine, galanin, and opioid heteroreceptor complexes, modulate synaptic and volume transmission in the limbic networks of the brain. We attempt to provide information showing how these emerging concepts can contribute to finding new ways to treat both MDD and anxiety disorders.

Immunolabeling Provides Evidence for Subregions in the Songbird Nucleus Accumbens and Suggests a Context-Dependent Role in Song in Male European Starlings (Sturnus vulgaris)

Birdsong is well known for its role in mate attraction during the breeding season. However, many birds, including European starlings (Sturnus vulgaris), also sing outside the breeding season as part of large flocks. Song in a breeding context can be extrinsically rewarded by mate attraction; however, song in nonbreeding flocks, referred to here as gregarious song, results in no obvious extrinsic reward and is proposed to be intrinsically rewarded. The nucleus accumbens (NAC) is a brain region well known to mediate reward and motivation, which suggests it is an ideal candidate to regulate reward associated with gregarious song. The goal of this study was to provide new histochemical information on the songbird NAC and its subregions (rostral pole, core, and shell) and to begin to determine subregion-specific contributions to gregarious song in male starlings. We examined immunolabeling for tyrosine hydroxylase (TH), neurotensin, and enkephalin (ENK) in the NAC. We then examined the extent to which gregarious and sexually motivated song differentially correlated with immunolabeling for the immediate early genes FOS and ZENK in each subdivision of the NAC. We found that TH and ENK labeling within subregions of the starling NAC was generally similar to patterns seen in the core and shell of NACs in mammals and birds. Additionally, we found that gregarious song, but not sexually motivated song, positively correlated with FOS in all NAC subregions. Our observations provide further evidence for distinct subregions within the songbird NAC and suggest the NAC may play an important role in regulating gregarious song in songbirds.

On the G Protein-Coupled Receptor Neuromodulation of the Claustrum

G protein-coupled receptors modulate the synaptic glutamate and GABA transmission of the claustrum. The work focused on the transmitter–receptor relationships in the claustral catecholamine system and receptor–receptor interactions between kappa opioid receptors (KOR) and SomatostatinR2 (SSTR2) in claustrum. Methods used involved immunohistochemistry and in situ proximity ligation assay (PLA) using confocal microscopy. Double immunolabeling studies on dopamine (DA) D1 receptor (D1R) and tyrosine hydroxylase (TH) immunoreactivities (IR) demonstrated that D1R IR existed in almost all claustral and dorsal endopiriform nucleus (DEn) nerve cell bodies, known as glutamate projection neurons, and D4R IR in large numbers of nerve cell bodies of the claustrum and DEn. However, only a low to moderate density of TH IR nerve terminals was observed in the DEn versus de few scattered TH IR terminals found in the claustrum. These results indicated that DA D1R and D4R transmission in the rat operated via long distance DA volume transmission in the rat claustrum and DEn to modulate claustral-sensory cortical glutamate transmission. Large numbers of these glutamate projection neurons also expressed KOR and SSTR2 which formed KOR-SSTR2 heteroreceptor complexes using PLA. Such receptor–receptor interactions can finetune the activity of the glutamate claustral-sensory cortex projections from inhibition to enhancement of their sensory cortex signaling. This can give the sensory cortical regions significant help in deciding on the salience to be given to various incoming sensory stimuli.

Where Is Dopamine and how do Immune Cells See it?: Dopamine-Mediated Immune Cell Function in Health and Disease

Dopamine is well recognized as a neurotransmitter in the brain, and regulates critical functions in a variety of peripheral systems. Growing research has also shown that dopamine acts as an important regulator of immune function. Many immune cells express dopamine receptors and other dopamine related proteins, enabling them to actively respond to dopamine and suggesting that dopaminergic immunoregulation is an important part of proper immune function. A detailed understanding of the physiological concentrations of dopamine in specific regions of the human body, particularly in peripheral systems, is critical to the development of hypotheses and experiments examining the effects of physiologically relevant dopamine concentrations on immune cells. Unfortunately, the dopamine concentrations to which these immune cells would be exposed in different anatomical regions are not clear. To address this issue, this comprehensive review details the current information regarding concentrations of dopamine found in both the central nervous system and in many regions of the periphery. In addition, we discuss the immune cells present in each region, and how these could interact with dopamine in each compartment described. Finally, the review briefly addresses how changes in these dopamine concentrations could influence immune cell dysfunction in several disease states including Parkinson's disease, multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease, as well as the collection of pathologies, cognitive and motor symptoms associated with HIV infection in the central nervous system, known as NeuroHIV. These data will improve our understanding of the interactions between the dopaminergic and immune systems during both homeostatic function and in disease, clarify the effects of existing dopaminergic drugs and promote the creation of new therapeutic strategies based on manipulating immune function through dopaminergic signaling. Graphical Abstract.

Brain Dopamine Transmission in Health and Parkinson's Disease: Modulation of Synaptic Transmission and Plasticity Through Volume Transmission and Dopamine Heteroreceptors

This perspective article provides observations supporting the view that nigro-striatal dopamine neurons and meso-limbic dopamine neurons mainly communicate through short distance volume transmission in the um range with dopamine diffusing into extrasynaptic and synaptic regions of glutamate and GABA synapses. Based on this communication it is discussed how volume transmission modulates synaptic glutamate transmission onto the D1R modulated direct and D2R modulated indirect GABA pathways of the dorsal striatum. Each nigro-striatal dopamine neuron was first calculated to form large numbers of neostriatal DA nerve terminals and then found to give rise to dense axonal arborizations spread over the neostriatum, from which dopamine is released. These neurons can through DA volume transmission directly influence not only the striatal GABA projection neurons but all the striatal cell types in parallel. It includes the GABA nerve cells forming the island-/striosome GABA pathway to the nigral dopamine cells, the striatal cholinergic interneurons and the striatal GABA interneurons. The dopamine modulation of the different striatal nerve cell types involves the five dopamine receptor subtypes, D1R to D5R receptors, and their formation of multiple extrasynaptic and synaptic dopamine homo and heteroreceptor complexes. These features of the nigro-striatal dopamine neuron to modulate in parallel the activity of practically all the striatal nerve cell types in the dorsal striatum, through the dopamine receptor complexes allows us to understand its unique and crucial fine-tuning of movements, which is lost in Parkinson's disease. Integration of striatal dopamine signals with other transmitter systems in the striatum mainly takes place via the receptor-receptor interactions in dopamine heteroreceptor complexes. Such molecular events also participate in the integration of volume transmission and synaptic transmission. Dopamine modulation of the glutamate synapses on the dorsal striato-pallidal GABA pathway involves D2R heteroreceptor complexes such as D2R-NMDAR, A2AR-D2R, and NTSR1-D2R heteroreceptor complexes. The dopamine modulation of glutamate synapses on the striato-entopeduncular/nigral pathway takes place mainly via D1R heteroreceptor complexes such as D1R-NMDAR, A2R-D1R, and D1R-D3R heteroreceptor complexes. Dopamine modulation of the island/striosome compartment of the dorsal striatum projecting to the nigral dopamine cells involve D4R-MOR heteroreceptor complexes. All these receptor-receptor interactions have relevance for Parkinson's disease and its treatment.

The role of catecholamines in HIV neuropathogenesis

The success of anti-retroviral therapy has improved the quality of life and lifespan of HIV + individuals, transforming HIV infection into a chronic condition. These improvements have come with a cost, as chronic HIV infection and long-term therapy have resulted in the emergence of a number of new pathologies. This includes a variety of the neuropathological and neurocognitive effects collectively known as HIVassociated neurocognitive disorders (HAND) or NeuroHIV. These effects persist even in the absence of viral replication, suggesting that they are mediated the long-term changes in the CNS induced by HIV infection rather than by active replication. Among these effects are significant changes in catecholaminergic neurotransmission, especially in dopaminergic brain regions. In HIV-infected individuals not treated with ARV show prominent neuropathology is common in dopamine-rich brain regions and altered autonomic nervous system activity. Even infected individuals on therapy, there is significant dopaminergic neuropathology, and elevated stress and norepinephrine levels correlate with a decreased effectiveness of antiretroviral drugs. As catecholamines function as immunomodulatory factors, the resultant dysregulation of catecholaminergic tone could substantially alter the development of HIVassociated neuroinflammation and neuropathology. In this review, we discuss the role of catecholamines in the etiology of HIV neuropathogenesis. Providing a comprehensive examination of what is known about these molecules in the context of HIV-associated disease demonstrates the importance of further studies in this area, and may open the door to new therapeutic strategies that specifically ameliorate the effects of catecholaminergic dysregulation on NeuroHIV.

Heterogeneous extracellular dopamine regulation in the subregions of the olfactory tubercle

Recent studies show that dense dopamine (DA) innervation from the ventral tegmental area to the olfactory tubercle (OT) may play an important role in processing multisensory information pertaining to arousal and reward, yet little is known about DA regulation in the OT. This is mainly due to the anatomical limitations of conventional methods of determining DA dynamics in small heterogeneous OT subregions located in the ventral most part of the brain. Additionally, there is increasing awareness that anteromedial and anterolateral subregions of the OT have distinct functional roles in natural and psychostimulant drug reinforcement as well as in regulating other types of behavioral responses, such as aversion. Here, we compared extracellular DA regulation (release and clearance) in three subregions (anteromedial, anterolateral, and posterior) of the OT of urethane-anesthetized rats, using in vivo fast-scan cyclic voltammetry following electrical stimulation of ventral tegmental area dopaminergic cell bodies. The neurochemical, anatomical, and pharmacological evidence confirmed that the major electrically evoked catecholamine in the OT was DA across both its anteroposterior and mediolateral extent. While both D2 autoreceptors and DA transporters play important roles in regulating DA evoked in OT subregions, DA in the anterolateral OT was regulated less by the D2 receptors when compared to other OT subregions. Comparing previous data from other DA rich ventral striatum regions, the slow DA clearance across the OT subregions may lead to a high extracellular DA concentration and contribute towards volume transmission. These differences in DA regulation in the terminals of OT subregions and other limbic structures will help us understand the neural regulatory mechanisms of DA in the OT, which may elucidate its distinct functional contribution in the ventral striatum towards mediating aversion, reward and addiction processes.

The role of transmitter diffusion and flow versus extracellular vesicles in volume transmission in the brain neural-glial networks

Two major types of intercellular communication are found in the central nervous system (CNS), namely wiring transmission (point-to-point communication, the prototype being synaptic transmission with axons and terminals) and volume transmission (VT; communication in the extracellular fluid and in the cerebrospinal fluid (CSF)) involving large numbers of cells in the CNS. Volume and synaptic transmission become integrated inter alia through the ability of their chemical signals to activate different types of receptor protomers in heteroreceptor complexes located synaptically or extrasynaptically in the plasma membrane. The demonstration of extracellular dopamine (DA) and serotonin (5-HT) fluorescence around the DA and 5-HT nerve cell bodies with the Falck-Hillarp formaldehyde fluorescence method after treatment with amphetamine and chlorimipramine, respectively, gave the first indications of the existence of VT in the brain, at least at the soma level. There exist different forms of VT. Early studies on VT only involved spread including diffusion and flow of soluble biological signals, especially transmitters and modulators, a communication called extrasynaptic (short distance) and long distance (paraaxonal and paravascular and CSF pathways) VT. Also, the extracellular vesicle type of VT was demonstrated. The exosomes (endosome-derived vesicles) appear to be the major vesicular carriers for VT but the larger microvesicles also participate. Both mainly originate at the soma-dendritic level. They can transfer lipids and proteins, including receptors, Rab GTPases, tetraspanins, cholesterol, sphingolipids and ceramide. Within them there are also subsets of mRNAs and non-coding regulatory microRNAs. At the soma-dendritic membrane, sets of dynamic postsynaptic heteroreceptor complexes (built up of different types of physically interacting receptors and proteins) involving inter alia G protein-coupled receptors including autoreceptors, ion channel receptors and receptor tyrosine kinases are hypothesized to be the molecular basis for learning and memory. At nerve terminals, the presynaptic heteroreceptor complexes are postulated to undergo plastic changes to maintain the pattern of multiple transmitter release reflecting the firing pattern to be learned by the heteroreceptor complexes in the postsynaptic membrane.

Volume Transmission in Central Dopamine and Noradrenaline Neurons and Its Astroglial Targets

Already in the 1960s the architecture and pharmacology of the brainstem dopamine (DA) and noradrenaline (NA) neurons with formation of vast numbers of DA and NA terminal plexa of the central nervous system (CNS) indicated that they may not only communicate via synaptic transmission. In the 1980s the theory of volume transmission (VT) was introduced as a major communication together with synaptic transmission in the CNS. VT is an extracellular and cerebrospinal fluid transmission of chemical signals like transmitters, modulators etc. moving along energy gradients making diffusion and flow of VT signals possible. VT interacts with synaptic transmission mainly through direct receptor-receptor interactions in synaptic and extrasynaptic heteroreceptor complexes and their signaling cascades. The DA and NA neurons are specialized for extrasynaptic VT at the soma-dendrtitic and terminal level. The catecholamines released target multiple DA and adrenergic subtypes on nerve cells, astroglia and microglia which are the major cell components of the trophic units building up the neural-glial networks of the CNS. DA and NA VT can modulate not only the strength of synaptic transmission but also the VT signaling of the astroglia and microglia of high relevance for neuron-glia interactions. The catecholamine VT targeting astroglia can modulate the fundamental functions of astroglia observed in neuroenergetics, in the Glymphatic system, in the central renin-angiotensin system and in the production of long-distance calcium waves. Also the astrocytic and microglial DA and adrenergic receptor subtypes mediating DA and NA VT can be significant drug targets in neurological and psychiatric disease.

Volume transmission and its different forms in the central nervous system

Volume transmission (VT) is a widespread mode of intercellular communication that occurs in the extracellular fluid (ECF) and in the cerebrospinal fluid (CSF) of the brain with VT signals moving from source to target cells via energy gradients leading to diffusion and convection (flow). The VT channels are diffuse forming a plexus in the extracellular space, while in wiring transmission (WT) the channels (axons, terminals) are private. The speed is slow (seconds-minutes) in VT while rapid in the millisecond range in WT. The extracellular space is the substrate for VT, which is modulated by the extracellular matrix. Extrasynaptic VT is linked to synaptic transmission and likely often takes place due to incomplete diffusion barriers with the synaptic transmitter reaching extrasynaptic domains of the pre-and post-synaptic membrane of the synapse, the astroglia, and even adjacent synapses. Indications exist for the existence of striatal D2-like receptor-mediated extrasynaptic form of dopamine (DA) VT at the local circuit level in vivo in the human striatum. Synaptic glutamate via extrasynaptic VT can act on extrasynaptic metabotropic glutamate receptors located on the astroglia leading to Ca(2+) mediated astrocytic glutamate release into the extracellular space (ECS). Long distance peptide VT and CSF VT is the major long distance VT with distances more than 1 mm and flow in the CSF. Indications for long distance VT of beta-endorphin and oxytocin are obtained. We propose that monogamy in the female prairie vole may take place through an increase in oxytocin VT, especially in nucleus accumbens. Release of extracellular vesicles containing receptors, proteins, RNAs and mtDNA from cellular networks in the central nervous system (CNS) into the ECF and CSF may be a fundamental communication in the CNS. It represents a special form of volume transmission, the Roamer subtype of VT. It may greatly contribute to dynamic events of synaptic plasticity but also to spread of pathological proteins in protein conformational disorders. VT also occurs in the peripheral nervous system and associated cells. Short and long distance VT may take place in meridian channels via diffusion and flow in the interstitial fluid. Acupuncture can produce VT signals by releasing transmitters and modulators from nerve terminals and mast cells.

Distribution and compartmental organization of GABAergic medium-sized spiny neurons in the mouse nucleus accumbens

The nucleus accumbens (NAc) is a critical brain region involved in many reward-related behaviors. The NAc comprises major compartments the core and the shell, which encompass several subterritories. GABAergic medium-sized spiny neurons (MSNs) constitute the output neurons of the NAc core and shell. While the functional organization of the NAc core outputs resembles the one described for the dorsal striatum, a simple classification of the NAc shell neurons has been difficult to define due to the complexity of the compartmental segregation of cells. We used a variety of BAC transgenic mice expressing enhanced green fluorescence (EGFP) or the Cre-recombinase (Cre) under the control of the promoter of dopamine D1, D2, and D3 receptors and of adenosine A2a receptor to dissect the microanatomy of the NAc. Moreover, using various immunological markers we characterized in detail the distribution of MSNs in the mouse NAc. In addition, cell-type specific extracellular signal-regulated kinase (ERK) phosphorylation in the NAc subterritories was analyzed following acute administration of SKF81297 (a D1R-like agonist), quinpirole (a D2 receptors (D2R)-like agonist), apomorphine (a non-selective DA receptor agonist), raclopride (a D2R-like antagonist), and psychostimulant drugs, including cocaine and d-amphetamine. Each drug generated a unique topography and cell-type specific activation of ERK in the NAc. Our results show the existence of marked differences in the receptor expression pattern and functional activation of MSNs within the shell subterritories. This study emphasizes the anatomical and functional heterogeneity of the NAc, which will have to be considered in its further study.

Extrasynaptic neurotransmission in the modulation of brain function. Focus on the striatal neuronal-glial networks

Extrasynaptic neurotransmission is an important short distance form of volume transmission (VT) and describes the extracellular diffusion of transmitters and modulators after synaptic spillover or extrasynaptic release in the local circuit regions binding to and activating mainly extrasynaptic neuronal and glial receptors in the neuroglial networks of the brain. Receptor-receptor interactions in G protein-coupled receptor (GPCR) heteromers play a major role, on dendritic spines and nerve terminals including glutamate synapses, in the integrative processes of the extrasynaptic signaling. Heteromeric complexes between GPCR and ion-channel receptors play a special role in the integration of the synaptic and extrasynaptic signals. Changes in extracellular concentrations of the classical synaptic neurotransmitters glutamate and GABA found with microdialysis is likely an expression of the activity of the neuron-astrocyte unit of the brain and can be used as an index of VT-mediated actions of these two neurotransmitters in the brain. Thus, the activity of neurons may be functionally linked to the activity of astrocytes, which may release glutamate and GABA to the extracellular space where extrasynaptic glutamate and GABA receptors do exist. Wiring transmission (WT) and VT are fundamental properties of all neurons of the CNS but the balance between WT and VT varies from one nerve cell population to the other. The focus is on the striatal cellular networks, and the WT and VT and their integration via receptor heteromers are described in the GABA projection neurons, the glutamate, dopamine, 5-hydroxytryptamine (5-HT) and histamine striatal afferents, the cholinergic interneurons, and different types of GABA interneurons. In addition, the role in these networks of VT signaling of the energy-dependent modulator adenosine and of endocannabinoids mainly formed in the striatal projection neurons will be underlined to understand the communication in the striatal cellular networks.

Aspects on the integrative actions of the brain from neural networks to "brain-body medicine"

"Integration" is a key term in describing how nervous system can perform high level functions. A first condition to have "integration" is obviously the presence of efficient "communication processes" among the parts that have to be combined into the harmonious whole. In this respect, two types of communication processes, called wiring transmission (WT) and volume transmission (VT), respectively, were found to play a major role in the nervous system, allowing the exchange of signals not only between neurons, but rather among all cell types present in the central nervous system (CNS). A second fundamental aspect of a communication process is obviously the recognition/decoding process at target level. As far as this point is concerned, increasing evidence emphasizes the importance of supramolecular complexes of receptors (the so called receptor mosaics) generated by direct receptor-receptor interactions. Their assemblage would allow a first integration of the incoming information already at the plasma membrane level. Recently, evidence of two new subtypes of WT and VT has been obtained, namely the tunnelling nanotubes mediated WT and the microvesicle (in particular exosomes) mediated VT allowing the horizontal transfer of bioactive molecules, including receptors, RNAs and micro-RNAs. The physiological and pathological implications of these types of communication have opened up a new field that is largely still unexplored. In fact, likely unsuspected integrative actions of the nervous system could occur. In this context, a holistic approach to the brain-body complex as an indissoluble system has been proposed. Thus, the hypothesis has been introduced on the existence of a brain-body integrative structure formed by the "area postrema/nucleus tractus solitarius" (AP/NTS) and the "anteroventral third ventricle region/basal hypothalamus with the median eminence" (AV3V-BH). These highly interconnected regions operate as specialized interfaces between the brain and the body integrating brain-borne and body-borne neural and humoral signals.

Increased affinity of dopamine for D(2) -like versus D(1) -like receptors. Relevance for volume transmission in interpreting PET findings

Evidence indicates that dopamine (DA) mainly acts as a volume transmission (VT) transmitter through its release into the extracellular fluid where the D(1) -like and D(2) -like receptors are predominantly extrasynaptic. It was therefore of interest to compare the affinities of the two major families of DA receptors. [(3)H] raclopride /DA and [(3)H] SCH23390/DA competition assays compared the affinity of DA at D(2) -like and D(1) -like receptors in rat dorsal striatal membrane preparations as well as in membrane preparations from CHO cell lines stably transfected with human D(2L) and D(1) receptors. The IC(50) values of DA at D(2) -like receptors in dorsal striatal membranes and CHO cell membranes were markedly and significantly reduced compared with the IC(50) values of DA at D(1) -like receptors. These IC(50) values reflect differences in both the high and low affinity states. The K(iH) value for DA at [(3)H] raclopride-labeled D(2) -like receptors in dorsal striatum was 12 nM, and this can help explain PET findings that amphetamine-induced increases in DA release can produce an up to 50% decrease of [(11)C] raclopride binding in the dorsal striatum in vivo. These combined results give indications for the existence of striatal D(2) -like receptor-mediated DA VT at the local circuit level in vivo. The demonstration of a K(iH) value of 183 nM for DA at D(1) antagonist-labeled D(1) -like receptors instead gives a likely explanation for the failure of a reduction of D(1) -like receptor binding after amphetamine-induced DA release in PET studies using the D(1) -like antagonist radioligands [(11)C] SCH23390 and [(11)C] NNC. It seems difficult to evaluate the role of the extrasynaptic D(1) receptors in VT in vivo with the PET radioligands available for this receptor.

Neuregulin links dopaminergic and glutamatergic neurotransmission to control hippocampal synaptic plasticity

Neuregulin-1 (NRG-1) and its receptor ErbB4 are genetically associated with schizophrenia, a complex developmental disorder of high heritability but unknown etiology that has been proposed to result from deficits in functional connectivity and synaptic plasticity. Based on pharmacological evidence, imbalances in dopaminergic and glutamatergic transmission systems are believed to contribute to its pathophysiology, but genetic data supporting a causative role for either are sparse. Stimulation of NRG-1/ErbB4 signaling inhibits or reverts hippocampal long-term potentiation (LTP) at glutamatergic synapses between Schaeffer collateral afferents and CA1 pyramidal neurons (SC-->CA1). We have recently demonstrated that NRG-1 regulates glutamatergic plasticity by rapidly increasing extracellular hippocampal dopamine levels and activation of D4 dopamine receptors.7 These new findings position the NRG-1/ErbB4 signaling pathway at the crossroads between dopaminergic and glutamatergic neurotransmission and offer novel ways to consolidate genetic, functional and pharmacological data toward a better understanding of the etiological processes underlying schizophrenia, and the role of NRG-1 for normal synaptic function and plasticity. The currently available data suggest that hippocampal interneurons might play a crucial role in mediating NRG-1 induced depotentiation. This interpretation is in line with other evidence pointing towards an involvement of GABAergic cells in the etiology of schizophrenia.

The neurobiology of pair bonding: insights from a socially monogamous rodent

The formation of enduring relationships between adult mates (i.e., pair bonds) is an integral aspect of human social behavior and has been implicated in both physical and psychological health. However, due to the inherent complexity of these bonds and the relative rarity with which they are formed in other mammalian species, we know surprisingly little about their underlying neurobiology. Over the past few decades, the prairie vole (Microtus ochrogaster) has emerged as an animal model of pair bonding. Research in this socially monogamous rodent has provided valuable insight into the neurobiological mechanisms that regulate pair bonding behaviors. Here, we review these studies and discuss the neural regulation of three behaviors inherent to pair bonding: the formation of partner preferences, the subsequent development of selective aggression toward unfamiliar conspecifics, and the bi-parental care of young. We focus on the role of vasopressin, oxytocin, and dopamine in the regulation of these behaviors, but also discuss the involvement of other neuropeptides, neurotransmitters, and hormones. These studies may not only contribute to the understanding of pair bonding in our own species, but may also offer insight into the underlying causes of social deficits noted in several mental health disorders.

Characterization of the dopamine system in the brain of the túngara frog, Physalaemus pustulosus

Dopamine is an evolutionarily ancient neurotransmitter that plays an essential role in mediating behavior. In vertebrates, dopamine is central to the mesolimbic reward system, a neural network concerned with the valuation of stimulus salience, and to the nigrostriatal motor system and hypothalamic nuclei involved in the regulation of locomotion and social behavior. In amphibians, dopaminergic neurons have been mapped out in several species, yet the distribution of dopaminoreceptive cells is unknown. The túngara frog, Physalaemus pustulosus, is an excellent model system for the study of neural mechanisms by which valuations of stimuli salience and social decisions are made, especially in the context of mate choice. In order to better understand where dopamine acts to regulate social decisions in this species, we have determined the distribution of putative dopaminergic cells (using tyrosine hydroxylase immunohistochemistry) and cells receptive to dopaminergic signaling (using DARPP-32 immunohistochemistry) throughout the brain of P. pustulosus. The distribution of dopaminergic cells was comparable to other anurans. DARPP-32 immunoreactivity was identified in key brain regions known to modulate social behavior in other vertebrates including the proposed anuran homologues of the mammalian amygdalar complex, nucleus accumbens, hippocampus, striatum, preoptic area, anterior hypothalamus, ventromedial hypothalamus, and ventral tegmental area/substantia nigra pars compacta. Due to its widespread distribution, DARPP-32 likely also plays many roles in non-limbic brain regions that mediate non-social information processing. These results significantly extend our understanding of the distribution of the dopaminergic system in the anuran brain and beyond.

Dopamine regulation of social choice in a monogamous rodent species

There is growing appreciation that social decision making in humans is strongly influenced by hedonic and emotional processing. The field of social neuroeconomics has shown that neural systems important for reward are associated with social choice and social preferences in humans. Here, we show that the neurobiology of social preferences in a monogamous rodent species, the prairie vole, is also regulated by neural systems involved in reward and emotional processing. Specifically, we describe how mesolimbic dopamine transmission differentially mediates the formation and maintenance of monogamous pair bonds in this species. Thus, reward processing exerts tremendous regulation over social choice behaviors that serve as the foundation of a rather complex social organization. We conclude that prairie voles are an excellent model system for the neuroscience of social choice and that complex social decision-making can be robustly explained by reward and hedonic processing.

Metabotropic glutamate type 5, dopamine D2 and adenosine A2a receptors form higher-order oligomers in living cells

G protein-coupled receptors are known to form homo- and heteromers at the plasma membrane, but the stoichiometry of these receptor oligomers are relatively unknown. Here, by using bimolecular fluorescence complementation, we visualized for the first time the occurrence of heterodimers of metabotropic glutamate mGlu(5) receptors (mGlu(5)R) and dopamine D(2) receptors (D(2)R) in living cells. Furthermore, the combination of bimolecular fluorescence complementation and bioluminescence resonance energy transfer techniques, as well as the sequential resonance energy transfer technique, allowed us to detect the occurrence receptor oligomers containing more than two protomers, mGlu(5)R, D(2)R and adenosine A(2A) receptor (A(2A)R). Interestingly, by using high-resolution immunoelectron microscopy we could confirm that the three receptors co-distribute within the extrasynaptic plasma membrane of the same dendritic spines of asymmetrical, putative glutamatergic, striatal synapses. Also, co-immunoprecipitation experiments in native tissue demonstrated the existence of an association of mGlu(5)R, D(2)R and A(2A)R in rat striatum homogenates. Overall, these results provide new insights into the molecular composition of G protein-coupled receptor oligomers in general and the mGlu(5)R/D(2)R/A(2A)R oligomer in particular, a receptor oligomer that might constitute an important target for the treatment of some neuropsychiatric disorders.

Role of the amygdaloid cholecystokinin (CCK)/gastrin-2 receptors and terminal networks in the modulation of anxiety in the rat. Effects of CCK-4 and CCK-8S on anxiety-like behaviour and [3H]GABA release

The amygdala plays a key role in fear and anxiety. The intercalated islands are clusters of glutamate-responsive GABAergic neurons rich in cholecystokinin (CCK)-2 receptors which control the trafficking of nerve impulses from the cerebral cortex to the central nucleus of amygdala. In this study, the nature of the CCK-glutamate-GABA interactions within the rat rostral amygdala, and their relevance for anxiety, were studied. CCK/gastrin-like immunoreactive nerve terminals were found to be mainly restricted to the paracapsular intercalated islands and the rostrolateral part of the main intercalated island. Behaviourally, the bilateral microinjection of CCK-4 (0.043-4.3 pmol/side) or CCK-8S (4.3 pmol/side) into the rostrolateral amygdala reduced the open-arm exploration in the elevated plus-maze without affecting locomotion. In contrast, neither CCK-4 nor CCK-8S (0.043-4.3 pmol/side) had any effects in the shock-probe burying test as compared with their saline-treated controls. Biochemically, CCK-4 (0.3 and 1.5 microm), unlike CCK-8S, enhanced significantly the K(+)-stimulated release of [(3)H]GABA from amygdala slices. These effects were fully prevented by prior superfusion of the slices with either the selective CCK-2 receptor antagonist CR2945 (3 microm), or 6,7-dinitroquinoxaline-2,3(1H,4H)-dione (DNQX), 10 microm, a glutamatergic (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptor antagonist. It is suggested that CCK modulates glutamate-GABA mechanisms by acting on CCK-2 receptors via volume transmission occurring at the level of the basolateral amygdaloid nucleus and/or by synaptic or perisynaptic volume transmission in the region of the rostrolateral main and paracapsular intercalated islands, resulting in subsequent disinhibition of the central amygdaloid nucleus and anxiety or panic-like behaviour.

Dopamine and cyclic-AMP regulated phosphoprotein-32-dependent modulation of prefrontal cortical input and intercellular coupling in mouse accumbens spiny and aspiny neurons

The roles of dopamine and cyclic-AMP regulated phosphoprotein-32 (DARPP-32) in mediating dopamine (DA)-dependent modulation of corticoaccumbens transmission and intercellular coupling were examined in mouse accumbens (NAC) neurons by both intracellular sharp electrode and whole cell recordings. In wild-type (WT) mice bath application of the D2-like agonist quinpirole resulted in 73% coupling incidence in NAC spiny neurons, compared with baseline (9%), whereas quinpirole failed to affect the basal coupling (24%) in slices from DARPP-32 knockout (KO) mice. Thus, D2 stimulation attenuated DARPP-32-mediated suppression of coupling in WT spiny neurons, but this modulation was absent in KO mice. Further, whole cell recordings revealed that quinpirole reversibly decreased the amplitude of cortical-evoked excitatory postsynaptic potentials (EPSPs) in spiny neurons of WT mice, but this reduction was markedly attenuated in KO mice. Bath application of the D1/D5 agonist SKF 38393 did not alter evoked EPSP amplitude in WT or KO spiny neurons. Therefore, DA D2 receptor regulation of both cortical synaptic (chemical) and local non-synaptic (dye coupling) communications in NAC spiny neurons is critically dependent on intracellular DARPP-32 cascades. Conversely, in fast-spiking interneurons, blockade of D1/D5 receptors produced a substantial decrease in EPSP amplitude in WT, but not in KO mice. Lastly, in putative cholinergic interneurons, cortical-evoked disynaptic inhibitory potentials (IPSPs) were attenuated by D2-like receptor stimulation in WT but not KO slices. These data indicate that DARPP-32 plays a central role in 1) modulating intercellular coupling, 2) cortical excitatory drive of spiny and aspiny GABAergic neurons, and 3) local feedforward inhibitory drive of cholinergic-like interneurons within accumbens circuits.

Differential effects of blockade of dopamine D1-family receptors in nucleus accumbens core or shell on reinstatement of heroin seeking induced by contextual and discrete cues

In humans, exposure to environmental contexts previously associated with heroin intake can provoke drug relapse, but the neuronal mechanisms mediating this relapse are unknown. Using a drug relapse model, we found previously that reexposing rats to heroin-associated contexts, after extinction of drug-reinforced responding in different contexts, reinstates heroin seeking. This effect is attenuated by inhibition of glutamate transmission in the ventral tegmental area and medial accumbens shell, components of the mesolimbic dopamine system. Here, we explored the role of dopamine of the accumbens in context-induced reinstatement by using the D1-family receptor antagonist SCH 23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride]. Rats were trained to self-administer heroin for 12 d; drug infusions were paired with a discrete tone-light cue. Subsequently, the heroin-reinforced lever pressing was extinguished in the presence of the discrete cue in a context that differed from the drug self-administration context in terms of visual, auditory, tactile, and circadian cues. When tested in the original drug self-administration context, systemic and medial or lateral accumbens shell SCH 23390 injections attenuated context-induced reinstatement of heroin seeking, whereas accumbens core SCH 23390 injections were ineffective. In contrast, core but not lateral or medial shell SCH 23390 injections attenuated discrete-cue-induced reinstatement in a nondrug context after extinction of lever presses without this cue. Results indicate that activation of medial and lateral accumbens shell D1-family dopamine receptors mediate context-induced reinstatement of heroin seeking and provide the first demonstration for a role of lateral shell dopamine in conditioned drug effects. Results also demonstrate novel dissociable roles of accumbens core and shell in context- versus discrete-cue-induced reinstatement of heroin seeking.

On the role of receptor–receptor interactions and volume transmission in learning and memory

Learning and memory seem to be inherent to a biological neural network. To emerge, they need an extensive functional connectivity, enabling a large repertoire of possible responses to stimuli, and sensitivity of the connectivity to activity, allowing for the selection of adaptive responses. According to the classical view about the organization of the CNS, the connectivity issue is realized by the huge amount of synaptic contacts each neuron establishes, while the adaptation of the network to specific tasks is obtained by mechanisms of activity-dependent synaptic plasticity. The discovery of direct receptor-receptor interactions at the level of the plasma membrane and the existence in the brain of two main modes of communication, the wiring transmission (such as the synaptic transmission) and the volume transmission (based on the diffusion of signals in the extracellular space), provided a broader view of the functional organization of the CNS with potential important consequences on the understanding of learning and memory processes. Owing to receptor-receptor interactions clusters of receptors, the receptor mosaics (RM), can be formed at the plasma membrane where they can work as collective functional units. As a consequence, the connections between the cells become themselves networks (molecular networks) able to adapt their function according to the stimuli they receive. Learning, therefore, can occur also at the level of RMs. Thus, memory formation seems not only to be a distributed process, but also to follow a hierarchical morpho-functional organization. Furthermore, the combination of the two different forms of transmission could allow processes of correlation and coordination to be established between networks and network elements without the need of additional physical connections, leading to a significant increase of the degrees of freedom available to the CNS for learning.

From the Golgi–Cajal mapping to the transmitter-based characterization of the neuronal networks leading to two modes of brain communication: Wiring and volume transmission

After Golgi-Cajal mapped neural circuits, the discovery and mapping of the central monoamine neurons opened up for a new understanding of interneuronal communication by indicating that another form of communication exists. For instance, it was found that dopamine may be released as a prolactin inhibitory factor from the median eminence, indicating an alternative mode of dopamine communication in the brain. Subsequently, the analysis of the locus coeruleus noradrenaline neurons demonstrated a novel type of lower brainstem neuron that monosynaptically and globally innervated the entire CNS. Furthermore, the ascending raphe serotonin neuron systems were found to globally innervate the forebrain with few synapses, and where deficits in serotonergic function appeared to play a major role in depression. We propose that serotonin reuptake inhibitors may produce antidepressant effects through increasing serotonergic neurotrophism in serotonin nerve cells and their targets by transactivation of receptor tyrosine kinases (RTK), involving direct or indirect receptor/RTK interactions. Early chemical neuroanatomical work on the monoamine neurons, involving primitive nervous systems and analysis of peptide neurons, indicated the existence of alternative modes of communication apart from synaptic transmission. In 1986, Agnati and Fuxe introduced the theory of two main types of intercellular communication in the brain: wiring and volume transmission (WT and VT). Synchronization of phasic activity in the monoamine cell clusters through electrotonic coupling and synaptic transmission (WT) enables optimal VT of monoamines in the target regions. Experimental work suggests an integration of WT and VT signals via receptor-receptor interactions, and a new theory of receptor-connexin interactions in electrical and mixed synapses is introduced. Consequently, a new model of brain function must be built, in which communication includes both WT and VT and receptor-receptor interactions in the integration of signals. This will lead to the unified execution of information handling and trophism for optimal brain function and survival.

One century of progress in neuroscience founded on Golgi and Cajal's outstanding experimental and theoretical contributions

Since the discovery and mapping of the neuronal circuits of the brain by Golgi and Cajal neuroscientists have clearly spelled the fundamental questions which should be answered to delineate the arena for a scientific understanding of brain function: How neurons communicate with each other in a network? Is there some basic principle according to which brain networks are organised? Is it possible to map out brain regions specialised in carrying out some specific task? As far as the first point is concerned it is well known that Golgi and Cajal had opposite views on the interneuronal communication. Golgi suggested protoplasmic continuity and/or electrotonic spreading of currents between neurons. Cajal proposed the so-called "neuron doctrine", which maintained that neurons could communicate only via a specialised region of contiguity, namely the synapse. The present paper has the first and second points as main topics and last century progresses in these fields are viewed as developments of Golgi and Cajal's findings and above all, hypotheses. Thus, we will briefly discuss these topics moving from the transmitter based mapping, which brought neurochemistry into the Golgi-Cajal mapping of the brain with silver impregnation techniques. The mapping of transmitter-identified neurons in the brain represents one of the major foundations for neuropsychopharmacology and a reference frame for the biochemical and behavioural investigations of brain function. Biochemical techniques allowed giving evidence for multiple transmission lines in synapses interacting via receptor-receptor interactions postulated to be based on supramolecular aggregates, called receptor mosaics. Immunocytochemical and autoradiographic mapping techniques allowed the discovery of extra-synaptic receptors and of transmitter-receptor mismatches leading to the introduction of the volume transmission concept by Agnati-Fuxe teams. The Volume Transmission theory proposed the existence of a three-dimensional diffusion of e.g. transmitter and ion signals, released by any type of cell, in the extra-cellular space and the cerebrospinal fluid of the brain. Thus, a synthesis between Golgi and Cajal's views became possible, by considering two main modes of intercellular communication: volume transmission (VT) and wiring transmission (WT) (a prototype of the latter one is synaptic transmission) and two types of networks (cellular and molecular networks) in the central nervous system. This was the basis for the suggestion of two fundamental principles in brain morphological and functional organisation, the miniaturisation and hierarchic organisation. Finally, moving from Apathy's work, a new model of brain networks has recently been proposed. In fact, it has been proposed that a network of fibrils enmeshes the entire CNS forming a global molecular network (GMN) superimposed on the cellular networks.

Cellular and subcellular rat brain spermidine synthase expression patterns suggest region-specific roles for polyamines, including cerebellar pre-synaptic function

In the brain, the polyamines spermidine (Spd) and spermine (Spm) serve highly specific functions by interacting with various ion channel receptors intimately involved with synaptic signaling. Both, glial cells and neurons contain Spd/Spm, but release and uptake mechanisms could re-distribute polyamines between cell types. The cellular and subcellular localization of polyamine biosynthetic enzymes may therefore offer a more appropriate tool to identify local sources of enhanced Spd/Spm synthesis, which may be related with specific roles in neuronal circuits and synaptic function. A recently characterized antibody against Spd synthase was therefore used to screen the rat brain for compartment-specific peaks in enzyme expression. The resulting labeling pattern indicated a clearly heterogeneous expression predominantly localized to neurons and neuropil. The highest levels of Spd synthase expression were detected in the accumbens nucleus, taenia tecta, cerebellar cortex, cerebral cortical layer I, hippocampus, hypothalamus, mesencephalic raphe nuclei, central and lateral amygdala, and the circumventricular organs. Besides a diffuse labeling of the neuropil in several brain areas, the distinct labeling of mossy fiber terminals in the cerebellar cortex directly indicated a synaptic role for Spd synthesis. Electron microscopy revealed a preferential distribution of the immunosignal in synaptic vesicle containing areas. A pre-synaptic localization was also observed in parallel and climbing fiber terminals. Electrophysiological recordings in acute cerebellar slices revealed a Spd-induced block of evoked extracellular field potentials resulting from mossy fiber stimulation in a dose-dependent manner.

Increase in A2A receptors in the nucleus accumbens after extended cocaine self-administration and its disappearance after cocaine withdrawal

Effects of extended cocaine self-administration and its withdrawal have been studied on A(2A) and D(2) receptor binding characteristics and expression in the nucleus accumbens and the anterior and posterior dorsal striatum of the rat (Rattus norvegicus). Biochemical binding techniques have been used with the D(2)-like receptor antagonist radioligand [(3)H]-Raclopride and the A(2A) receptor antagonist radioligand [(3)H]-ZM 241385 and immunoblots to study their expression. A substantial and significant increase in functional A(2A), but not in functional D(2) receptors, was observed in the nucleus accumbens immediately following 10 days of cocaine self-administration which returned to normal levels after 7 days of drug withdrawal. In contrast, in the posterior dorsal striatum significant reductions in A(2A) expression were observed immediately after cocaine self-administration which was associated with a trend for a reduction of the A(2A) receptor antagonist binding sites. In cocaine withdrawal groups, significant increases in the density and K(d) value of D(2)-like antagonist binding sites were observed in the nucleus accumbens in the absence of changes in D(2) expression, suggesting an up-regulation of D(3) receptors in this region after cocaine withdrawal. A(2A) receptor increases in the nucleus accumbens induced by cocaine may represent a compensatory up-regulation to counteract cocaine-induced increases in D(2) signaling and D(3) signaling which is in line with its disappearance in the 7-day withdrawal group displaying increased reinforcing efficacy of cocaine. A(2A) agonists may therefore represent cocaine antagonist drugs to be used in treatment of cocaine addiction acting inter alia by antagonizing signaling in accumbens A(2A)/D(2) and A(2A)/D(3) heteromers.

The nigrostriatal DA pathway and Parkinson's disease

The discovery of the nigrostriatal DA system in the rat was made possible by the highly specific and sensitive histochemical fluorescence method of Falck and Hillarp in combinations with electrolytic lesions in the substantia nigra and removal of major parts of the neostriatum. Recent work on DA neuron evolution shows that in the Bottlenose Dolphin the normal DA cell groups of the substantia nigra are very cell sparse, while there is a substantial expansion of the A9 medial and A10 lateral subdivisions forming an impressive "ventral wing" in the posterior substantia nigra. The nigrostriatal DA pathway mainly operates via Volume Transmission. Thus, DA diffuses along concentration gradients in the ECF to reach target cells with high affinity DA receptors. A novel feature of the DA receptor subtypes is their physical interaction in the plasma membrane of striatal neurons forming receptor mosaics (RM) with the existence of two types of RM. The "functional decoding unit" for DA is not the single receptor, but rather the RM that may affect not only the integration of signals in the DA neurons but also their trophic conditions. In 1991 A2A receptor antagonists were indicated to represent novel antiparkinsonian drugs based on the existence of A2A/D2 receptor-receptor interactions and here P2X receptor antagonists are postulated to be neuroprotective drugs in treatment of Parkinson's Disease.

The brain as a system of nested but partially overlapping networks. Heuristic relevance of the model for brain physiology and pathology

A new model of the brain organization is proposed. The model is based on the assumption that a global molecular network enmeshes the entire central nervous system. Thus, brain extra-cellular and intra-cellular molecular networks are proposed to communicate at the level of special plasma membrane regions (e.g., the lipid rafts) where horizontal molecular networks can represent input/output regions allowing the cell to have informational exchanges with the extracellular environment. Furthermore, some "pervasive signals" such as field potentials, pressure waves and thermal gradients that affect large parts of the brain cellular and molecular networks are discussed. Finally, at least two learning paradigms are analyzed taking into account the possible role of Volume Transmission: the so-called model of "temporal difference learning" and the "Turing B-unorganised machine". The relevance of this new view of brain organization for a deeper understanding of some neurophysiological and neuropathological aspects of its function is briefly discussed.

Anxiolytic-like effects of the selective metabotropic glutamate receptor 5 antagonist MPEP after its intra-amygdaloid microinjection in three different non-conditioned rat models of anxiety

The intercalated islands, clusters of dopamine D1-rich GABAergic neurons, are interposed between the basolateral and central nuclei of the amygdala, and control the traffic of nerve impulses between these two structures. Metabotropic glutamate receptor 5- (mGluR5)-like immunoreactivity was studied by immunohistochemistry in this part of the amygdala and was found to be mainly restricted to the central and basolateral nuclei and to a lesser extent to the medial paracapsular intercalated islands. The role of the metabotropic glutamate receptor 5 in the modulation of anxiety has been studied in this region by microinjection of small volumes of the mGluR5 antagonist 2-methyl-6(phenylethenyl) pyridine (MPEP), with restricted diffusion from its injection site, into the rostral amygdala near the basolateral and central amygdaloid nuclei and the intercalated islands, and the behavior of the animals was evaluated using three non-conditioned models of anxiety. Anxiolytic-like effects were observed after MPEP administration in all tests used. In the White and Black Box test, MPEP (2 nmol per side) significantly increased the time spent in the white compartment of the box. In line with these results, MPEP (8 nmol per side) increased the exploration of the open arms of the Elevated Plus-Maze. Burying behavior latency was increased and burying behavior itself was decreased in the Shock-Probe Burying test. It is suggested that anxiolytic effects of MPEP may be mediated by blockade of mGluR5 in the basolateral and/or central amygdaloid nuclei, reducing glutamate transmission in the basolateral amygdaloid nuclei and glutamate output from the central amygdala.

Volume transmission and wiring transmission from cellular to molecular networks: history and perspectives

The present paper deals with a fundamental issue in neuroscience: the inter-neuronal communication. The paper gives a brief account of our previous and more recent theoretical contributions to the subject and also reports new recent data that support some aspects of our proposal on two major modes of communication in the central nervous system: the wiring and the volume transmission. There exist two competing theories on inter-neuronal communication: the neuron doctrine and the theory of the diffuse nerve network, supported by Cajal and Golgi, respectively (see their respective Nobel Lectures). The present paper gives a brief account of a view on inter-neuronal communication in the brain, the volume and wiring transmission concept that to a great extent reconcile these two theories. Thus, the theory of volume and wiring transmission are summarized and its recent developments that allow to extend these two modes of communication from the cellular network to the molecular network level is also briefly illustrated. The explanatory value of this broadened view is further enhanced by our recent proposal on the existence of a Global Molecular Network enmeshing the entire central nervous system. It may be interesting to note that also the Global Molecular Network theory is reminiscent of the old reticular theory of Apathy. Finally, the so-called 'tide hypothesis' for diffusion of signals in the brain is briefly discussed and its possible extension to the molecular level is for the first time introduced. Early indirect evidence supporting volume transmission in the brain was the discovery of transmitter-receptor mismatches. Thus, as an experimental part of the present paper a new approach to evaluate transmitter-receptor mismatches is given and evidence for inter-relationships between temperature micro-gradients and mismatches is provided.

Uncoupling protein 2/3 immunoreactivity and the ascending dopaminergic and noradrenergic neuronal systems: Relevance for volume transmission

Uncoupling proteins in the inner mitochondrial membrane uncouples oxidative phosphorylation from ATP synthesis. It has been suggested that these proteins are involved in thermogenesis as well as in the regulation of reactive oxygen species production in the mitochondria. The present work was conducted to investigate the localization of the uncoupling protein 2-like immunoreactivity (uncoupling protein 2/3 immunoreactivity) in the main catecholaminergic projection fields in the rat brain as well as in the areas of the dopaminergic and noradrenergic nerve cell groups. In particular, the relationships of tyrosine hydroxylase, dopamine beta-hydroxylase and uncoupling protein 2/3 immunoreactivity were assessed by double immunolabeling and confocal laser microscopy analysis associated with computer-assisted image analysis. Uncoupling protein 2/3 immunoreactivity was observed in discrete dopaminergic terminals in the nucleus accumbens and in the cerebral cortex whereas it was found in scattered noradrenergic terminals in the caudate putamen and Islands of Calleja Magna. One interesting finding was that uncoupling protein 2/3 immunoreactivity together with tyrosine hydroxylase immunoreactivity in the shell of nucleus accumbens was observed surrounding the previously characterized D1 receptor rich nerve cell column system characterized by a relative lack of tyrosine hydroxylase immunoreactivity. Moreover, in animal models of dopaminergic pathway degeneration, plastic changes in uncoupling protein 2/3 terminals have been shown in the cerebral cortex and striatum as seen from the increased size and intensity of uncoupling protein 2/3 immunoreactivity of their varicosities. Taken together, these findings open up the possibility that uncoupling protein 2/3 could play an important role modulating the dopaminergic and noradrenergic neurotransmission within discrete brain regions.

The distribution of dopamine D1 receptor and μ-opioid receptor 1 receptor immunoreactivities in the amygdala and interstitial nucleus of the posterior limb of the anterior commissure: Relationships to tyrosine hydroxylase and opioid peptide terminal systems

Mismatches between dopamine innervation and dopamine D1 receptor (D1) distribution have previously been demonstrated in the intercalated cell masses of the rat amygdala. Here the distribution of enkephalin and beta-endorphin immunoreactive (IR) nerve terminals with respect to their mu-opioid receptors is examined in the intercalated cell masses, along with a further immunohistochemical analysis of the dopamine/D1 mismatches. A similar analysis is also made within the extended amygdala. A spatial mismatch in distribution patterns was found between the mu-opioid receptor-1 immunoreactivity and enkephalin IR in the main intercalated island of the amygdala. Discrete cell patches of dopamine D1 receptor and mu-opioid receptor-1 IR were also identified in a distinct region of the extended amygdala, the interstitial nucleus of the posterior limb of the anterior commissure, medial division (IPACM), which displayed sparse tyrosine hydroxylase or enkephalin/beta-endorphin IR nerve terminals. Furthermore, distinct regions of the main intercalated island that showed dopamine/D1 receptor matches (the rostral and rostrolateral parts) were associated with strong dopamine and cyclic AMP regulated phosphoprotein, 32 kDa-IR in several D1 IR neuronal cell bodies and dendrites, whereas this was not the case for the dopamine/D1 mismatch areas (the rostromedial and caudal parts) of the main intercalated island. The lack of correlation between the terminal/receptor distribution patterns suggests a role for volume transmission for mu-opioid receptor- and dopamine D1 receptor-mediated transmission in distinct regions of the amygdala and extended amygdala. This may have implications for amygdaloid function, where slow long lasting responses may develop as a result of volume transmission operating in opioid peptide and dopaminergic communication.

Overlapping intracellular and differential synaptic distributions of dopamine D1 and glutamate N-methyl-D-aspartate receptors in rat nucleus accumbens

The dopamine D1 receptor (D1R) in the nucleus accumbens (Acb) shell is highly implicated in psychostimulant-evoked locomotor activity and reward, whereas the D1R in the Acb core is more crucial for appetitive instrumental learning. These behavioral effects depend in part on interactions involving glutamatergic N-methyl-D-aspartate (NMDA) receptors, whose essential NR1 subunit has physical associations with the D1R. To determine the relevant sites for D1R activation and interactions involving NMDA receptors, we examined the electron microscopic immunolabeling of D1R and NR1 C-terminal peptides in rat Acb shell and core. In each Acb subdivision, the D1Rs were located principally on extrasynaptic plasma membranes of dendritic shafts and spines and more rarely were associated with cytoplasmic endomembranes. Many D1R-labeled somata and dendrites also contained NR1 immunoreactivity. In comparison with D1R, NR1 immunoreactivity was more often seen in the cytoplasm and near asymmetric synapses on somatodendritic profiles. In these profiles, notable overlapping distributions of D1R and NR1 occurred near endomembranes. The exclusively D1R- or D1R- and NR1-containing dendrites were most prevalent in the Acb shell, but were also present in the Acb core. In each region, NR1 was also detected in axon terminals without D1R, which formed excitatory-type synapses with D1R-labeled dendrites. These results provide ultrastructural evidence that D1Rs in the Acb have subcellular distributions supporting, 1) intracellular cotrafficking with NR1 and 2) modulation of the postsynaptic excitability in spiny neurons affected by presynaptic NMDA receptor activation. The region-specific differences in receptor distributions suggest a major, but not exclusive, involvement of Acb D1R in reward-related processing.

Localisation of p2x2 receptor subunit immunoreactivity on nitric oxide synthase expressing neurones in the brain stem and hypothalamus of the rat: a fluorescence immunohistochemical study

A large body of evidence suggests that nitric oxide (NO) and ATP act as neurotransmitters in the regulatory mechanisms concerning several autonomic functions at the level of both the hypothalamus and the brain stem. In the present study, we investigated whether neuronal NO synthase containing neurones also express P2X(2) receptor subunit of the ATP-gated ion channel via double-labelling fluorescence immunohistochemistry. Our data demonstrate that a high percentage of neuronal NO synthase-immunoreactive neurones are also P2X(2)-immunoreactive in the rostral ventrolateral medulla (98%) and supraoptic nucleus of the hypothalamus (92%). Significant numbers of neuronal NO synthase-immunoreactive neurones are also P2X(2)-immunoreactive in the subpostremal (48%) and commissural (65%) subdivisions of the nucleus tractus solitarius. In the caudal ventrolateral medulla and raphe obscurus, 96% and 89%, respectively, of neuronal NO synthase containing neurones also express P2X(2) receptor subunit. In contrast to the supraoptic nucleus, there was a lower percentage of co-localisation between NO synthase and P2X(2) receptor subunit in the paraventricular nucleus of the hypothalamus. In summary, this study demonstrates for the first time that there is a widespread co-localisation of neuronal NO synthase and P2X(2) receptor subunit in the hypothalamus and brain stem of the rat. Further studies are required to elucidate whether NO and ATP functionally interact within the hypothalamus and the brain stem.

The dopamine D1 receptor-rich main and paracapsular intercalated nerve cell groups of the rat amygdala: relationship to the dopamine innervation

The intercalated cell masses are GABAergic neurons interposed between the major input and output structures of the amygdala. Dopaminergic projections to the main and paracapsular intercalated islands were examined by determining the relationship of the dopamine nerve-terminal networks to the D1-receptor immunoreactive staining of cells within the intercalated islands, using double-fluorescence immunolabelling procedures in combination with confocal laser microscopy. The relationship of terminals positive for both tyrosine hydroxylase and dopamine beta-hydroxylase (noradrenaline and/or adrenaline) to terminals positive for tyrosine hydroxylase but negative for dopamine beta-hydroxylase (dopamine terminals) was studied in relation to the D1-receptor immunoreactivity in adjacent sections at various rostrocaudal levels. The microscopy and image analysis revealed that there was only a minor dopaminergic innervation of the D1 receptor-immunoreactive cells in the rostromedial and caudal component of the main intercalated island, suggesting volume transmission as the main communication mode for dopamine in these regions. In contrast, the D1 receptor-immunoreactive areas in the rostrolateral part of the main island and also the paracapsular intercalated islands showed a high degree of dopaminergic innervation, indicating that synaptic and perisynaptic dopamine transmission plays a dominant role in these regions. It is known that amygdala neurons are involved in the elicitation and learning of fear-related behaviors. We suggest that slow dopaminergic volume transmission in the rostromedial and caudal parts of the main intercalated island may have a role in tonic excitatory modulation in these parts of the main island, allowing GABAergic activity to develop in the central amygdaloid nucleus and thereby contributing to inhibition of fear-related behavioral and autonomic responses. In contrast, a faster synaptic and perisynaptic dopaminergic transmission in the rostrolateral part of the main intercalated island and in the paracapsular intercalated islands may have a role in allowing a more rapid elicitation of fear-related behaviors.

A critical role for nucleus accumbens dopamine in partner-preference formation in male prairie voles

Although the role of nucleus accumbens (NAcc) dopamine (DA) in reward learning has been extensively studied, few investigations have addressed its involvement in learning socially relevant information. Here, we have examined the involvement of NAcc DA in social attachment of the "monogamous" prairie vole (Microtus orchrogaster). We first demonstrated that DA is necessary for the formation of social attachment in male prairie voles, because administration of haloperidol blocked, whereas apomorphine induced, partner-preference formation. We then provided the first descriptions of DA neuroanatomy and tissue content in vole NAcc, and mating appeared to induce a 33% increase in DA turnover. We also showed that administration of haloperidol directly into the NAcc blocked partner preferences induced by mating and apomorphine. In addition, administration of apomorphine into the NAcc but not the caudate putamen induced partner preferences in the absence of mating. Together, our data support the hypothesis that NAcc DA is critical for pair-bond formation in male prairie voles.

Dopamine: a potential substrate for synaptic plasticity and memory mechanisms

It is only recently that a number of studies on synaptic plasticity in the hippocampus and other brain areas have considered that a heterosynaptic modulatory input could be recruited as well as the coincident firing of pre- and post-synaptic neurons. So far, the strongest evidence for such a regulation has been attributed to dopaminergic (DA) systems but other modulatory pathways have also been considered to influence synaptic plasticity. This review will focus on dopamine contribution to synaptic plasticity in different brain areas (hippocampus, striatum and prefrontal cortex) with, for each region, a few lines on the distribution of DA projections and receptors. New insights into the possible mechanisms underlying these plastic changes will be considered. The contribution of various DA systems in certain forms of learning and memory will be reviewed with recent advances supporting the hypothesis of similar cellular mechanisms underlying DA regulation of synaptic plasticity and memory processes in which the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) pathway has a potential role. To summarize, endogenous DA, which depends on the activity patterns of DA midbrain neurons in freely moving animals, appears as a key regulator in specific synaptic changes observed at certain stages of learning and memory and of synaptic plasticity.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

Intracerebral infusion of H-dopamine and H-mannitol in the striatum of halothane-anaesthetized male rats. A dual-probe microdialysis study of long-distance diffusion

This report characterizes an in vivo intracerebral long-distance diffusion model using dual-probe microdialysis. Two probes 1 mm apart were implanted into the striatum of control and 6-hydroxydopamine (6-OHDA)-lesioned halothane-anaesthetized male rats. Either tritiated dopamine (500 nM 3H-DA) or mannitol (1.5 microM 3H-mannitol) was infused continuously for 5 h, while samples were collected from the other probe. Samples (10 microl) were counted by liquid scintillation. For the DA-infused rats, another 10 microL was separated with high-pressure liquid chromatography (HPLC)-electrochemical detection into individual fractions containing 3,4-dihydroxy phenylacetic acid (DOPAC) and homovanillinic acid (HVA), and counted for beta-decay. The total transfer of 3H-labelled compounds described the overall effect of cellular uptake, metabolism and clearance into the microcirculation, and was compared with that of an extracellular marker, 3H-mannitol. The migration reached steady-state levels, generating an equilibrium between delivery and removal from the extracellular space. The half-time of the steady-state values, t50%, was in all cases lower in 6-OHDA-treated rats compared with control. In addition, the t50% values of 3H-mannitol were lower than those following the 3H-dopamine infusion in both control or 6-OHDA-lesioned rats. However, it was not possible to detect any unmetabolized 3H-dopamine at the 1 mm distance. In conclusion, the dual-probe microdialysis approach proved to be a valid method to study in vivo diffusion and migration in the brain, and the intracerebral spread of compounds highly depends on the nature of the compound infused.