Evidence for cholecystokinin-dopamine receptor interactions in the central nervous system of the adult and old rat. Studies on their functional meaning

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.

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.

Growth Factors in the Carotid Body-An Update

The carotid body may undergo plasticity changes during development/ageing and in response to environmental (hypoxia and hyperoxia), metabolic, and inflammatory stimuli. The different cell types of the carotid body express a wide series of growth factors and corresponding receptors, which play a role in the modulation of carotid body function and plasticity. In particular, type I cells express nerve growth factor, brain-derived neurotrophic factor, neurotrophin 3, glial cell line-derived neurotrophic factor, ciliary neurotrophic factor, insulin-like-growth factor-I and -II, basic fibroblast growth factor, epidermal growth factor, transforming growth factor-α and -β, interleukin-1β and -6, tumor necrosis factor-α, vascular endothelial growth factor, and endothelin-1. Many specific growth factor receptors have been identified in type I cells, indicating autocrine/paracrine effects. Type II cells may also produce growth factors and express corresponding receptors. Future research will have to consider growth factors in further experimental models of cardiovascular, metabolic, and inflammatory diseases and in human (normal and pathologic) samples. From a methodological point of view, microarray and/or proteomic approaches would permit contemporary analyses of large groups of growth factors. The eventual identification of physical interactions between receptors of different growth factors and/or neuromodulators could also add insights regarding functional interactions between different trophic mechanisms.

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 A_2A and the dopamine D₂ receptors likely represents a prototypical example. The pharmacological evidence obtained, together with the tissue distribution of the A_2A-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.

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.

The role of cholecystokinin in the induction of aggressive behavior: a focus on the available experimental data (review)

Cholecystokinin (CCK) is a neuropeptide that is (among others) reportedly involved in the pathophysiology of psychiatric disorders. The excitatory role of CCK in negative affective emotions as well as in aversive reactions, antisocial behaviors and memories, has been indicated by numerous electrophysiological, neurochemical and behavioral methodologies on both animal models for anxiety and human studies. The current review article summarizes the existing experimental evidence with regards to the role of CCK in the induction of aggressive behavior, and: (a) synopsizes the anatomical circuits through which it could potentially mediate all types of aggressive behavior, as well as (b) highlights the potential use of these experimental evidence in the current research quest for the clinical treatment of mood and anxiety disorders.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Heterodimers and receptor mosaics of different types of G-protein-coupled receptors

Through an assembly of interacting GPCRs, heterodimers and high-order heteromers (termed receptor mosaics) are formed and lead to changes in the agonist recognition, signaling, and trafficking of participating receptors via allosteric mechanisms, sometimes involving the appearance of cooperativity. This field has now become a major research area, and this review deals with their physiology being integrators of receptor signaling in the CNS and their use as targets for novel drug development based on their unique pharmacology.

Molecular mechanisms and therapeutical implications of intramembrane receptor/receptor interactions among heptahelical receptors with examples from the striatopallidal GABA neurons

The molecular basis for the known intramembrane receptor/receptor interactions among G protein-coupled receptors was postulated to be heteromerization based on receptor subtype-specific interactions between different types of receptor homomers. The discovery of GABAB heterodimers started this field rapidly followed by the discovery of heteromerization among isoreceptors of several G protein-coupled receptors such as delta/kappa opioid receptors. Heteromerization was also discovered among distinct types of G protein-coupled receptors with the initial demonstration of somatostatin SSTR5/dopamine D2 and adenosine A1/dopamine D1 heteromeric receptor complexes. The functional meaning of these heteromeric complexes is to achieve direct or indirect (via adapter proteins) intramembrane receptor/receptor interactions in the complex. G protein-coupled receptors also form heteromeric complexes involving direct interactions with ion channel receptors, the best example being the GABAA/dopamine D5 receptor heteromerization, as well as with receptor tyrosine kinases and with receptor activity modulating proteins. As an example, adenosine, dopamine, and glutamate metabotropic receptor/receptor interactions in the striatopallidal GABA neurons are discussed as well as their relevance for Parkinson's disease, schizophrenia, and drug dependence. The heterodimer is only one type of heteromeric complex, and the evidence is equally compatible with the existence of higher order heteromeric complexes, where also adapter proteins such as homer proteins and scaffolding proteins can exist. These complexes may assist in the process of linking G protein-coupled receptors and ion channel receptors together in a receptor mosaic that may have special integrative value and may constitute the molecular basis for some forms of learning and memory.

Identification of overlapping AP-2/NF-kappa B-responsive elements on the rat cholecystokinin gene promoter

In this study we evaluate both proximal and more distal transcriptional regulation of the 5' flanking region of the rat cholecystokinin gene in transfected GH3 (rat pituitary tumor) cells. Transcriptional activity was measured on the intact (-400 to +73) 5' flanking region of cholecystokinin (CCK), as well as with DNA constructs, which were deleted in both the conventional 5' to 3', as well as an unconventional 3' to 5' direction. Our in vivo studies indicate complex phorbol ester and forskolin interactions in the 10-base pair region between -130 and -140. We conclude, there are at least two transcriptional factors involved in regulation of the rat CCK transcription in this region. In vitro studies utilizing heterologous nuclear (HeLa) extract, as well as purified transcription factors AP-2 and NF-kappa B, identify overlapped AP-2- and NF-kappa B-responsive elements within the 17-base pair sequence between -149 and -134 of the distal 5' flanking region. In this region complex transcriptional regulation occurs, which indicates inhibition of AP-2 CCK promoter complexing by NF-kappa B. Six-point mutations introduced into this sequence prevent AP-2 and NF-kappa B binding to CCK promoter, as well as its transcriptional activation by phorbol ester and forskolin in GH3 cells.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Extracellular cholecystokinin levels in the rat spinal cord following chronic morphine exposure: an in vivo microdialysis study

Conflicting results concerning the issue of whether or not chronic morphine exposure induces an increase in CCK biosynthesis have been found in many CNS sites, including the spinal cord, where CCK activity may contribute to the facilitation of the development of opiate tolerance. The present study was undertaken in order to monitor the extracellular level of CCK under spontaneous and stimulus-evoked release in the spinal cord dorsal horn of drug naive and morphine tolerant rats. Tolerance was induced by implantation of two morphine pellets (2x75 mg) which induced a stable morphine plasma concentration after 48 h post-implantation. The tail-flick test and naloxone precipitated withdrawal were used as indexes of tolerance and dependence to morphine. The effect of morphine-pellet implantation on basal and K+-induced release of CCK-like immunoreactivity (CCK-LI) in the rat dorsal horn were monitored with in vivo microdialysis 96 h after implantation of morphine or placebo pellets, when rats showed tolerance and dependence. Basal CCK levels were below the detection limit of the assay (0.6 pM) in both tolerant and normal animals. K+ (100 mM) in the perfusion medium induced a more than 3-fold increase of the extracellular level of CCK-LI in control animals, and a more than 4-fold increase on CCK-LI in morphine-pellet implanted animals. However, this difference was not significant. In addition, naloxone (2 mg/kg; i.v.), did not induce any change in the extracellular level of CCK in either group. The present study suggests that the modulatory interaction between CCK and opioids in the development of tolerance in the spinal cord may occur without necessarily increasing the extracellular level of CCK. Another possible explanation of the finding is that the microdialysis technique is not sensitive enough to detect differences in unstimulated CCK levels in normal and tolerant animals.

Evidence for a differential cholecystokinin-B and -A receptor regulation of GABA release in the rat nucleus accumbens mediated via dopaminergic and cholinergic mechanisms

In the present study we characterized the cholecystokinin receptor regulation of (i) the dopamine D2 agonist binding sites in striatal sections including the nucleus accumbens and (ii) GABA and dopamine release in the central part of the rat nucleus accumbens, by combining the in vitro filter wipe-off and the in vivo microdialysis techniques. In the binding study we demonstrate that sulphated cholecystokinin octapeptide (1 nM) increased (219 +/- 30%) the KD value of the D2 agonist [3H]N-propylnorapomorphine binding sites in sections from the striatum including the accumbens. This effect was counteracted by the cholecystokinin-B antagonist PD134308 (50 nM). In a parallel study using microdialysis in the central nucleus accumbens, we found that local perfusion with sulphated cholecystokinin octapeptide (1 microM) induced an increase in GABA (135 +/- 7%) and dopamine (146 +/- 8%) release which was unaffected by the cholecystokinin-A antagonist L-364,718 (10 nM). In contrast, when the cholecystokinin-B antagonist PD134308 (10 nM) was co-perfused with the peptide it prevented the increase in dopamine and decreased GABA release (-24 +/- 2%). This reduction was counteracted by the addition to the perfusate medium of the cholecystokinin-A antagonist or the cholinergic muscarinic M2 receptor antagonist AF-DX 116 (0.1 microM). Taken together, these data demonstrate that the facilitation by sulphated cholecystokinin octapeptide of GABA and dopamine release in the central accumbens probably reflects an inhibitory effect of the peptide on both pre- and postsynaptic D2 receptors, mediated via cholecystokinin-B receptor activation. In addition, for the first time we provide evidence for a differential cholecystokinin-A and -B receptor-mediated regulation of GABA transmission in the central accumbens, where the cholecystokinin-B receptor exerts a dominant excitatory influence while the cholecystokinin-A receptor mediates an inhibition of GABA release via a local muscarinic M2 receptor.

Effect of angiotensin converting enzyme inhibitors on quality of life in hypertensive patients. Pharmacodynamic basis

The aim of this review is to comment the results described in the literature concerning the possible pharmacodynamic mechanisms involved in the improvement of quality of life of angiotensin converting enzyme inhibitors that is just a working hypothesis. These drugs, widely used in the treatment of hypertension, prevent the formation of angiotensin II and the generation of free radicals, as well as the hydrolysis of bradykinin, enkephalins and endorphins. Different mechanisms have been implicated on quality of life: 1) increase of bradykinin levels in the central nervous system that would trigger the release of nitric oxide (NO), noradrenaline, acetylcholine, excitatory amino acids and vasopressin which are involved in memory and cognition; 2) increase of brain blood supply by enhanced NO synthesis; 3) interference with cholinergic mechanisms in the central nervous system by angiotensin II inhibition of acetylcholine release; 4) decrease of endorphin metabolism; and 5) interaction with hypothalamic-pituitary-adrenal axis that releases ACTH and vasopressin.

Cholecystokinin octapeptide and the D2 antagonist raclopride induce Fos-like immunoreactivity in the shell part of the rat nucleus accumbens via different mechanisms

Induction of neuronal Fos-like immunoreactivity (IR) in the rat brain by cholecystokinin octapeptide (CCK-8) and the dopamine (DA) D2 receptor antagonist raclopride was demonstrated. In vivo treatment with the CCK-8 (0.01, 0.1 and 1 nmol/rat, i.c.v.) or the D2 antagonist raclopride (0.1, 0.5 and 1 mg/kg, i.p.) alone increased in a dose-dependent way the Fos-like ir profiles in the shell part of the rat nucleus accumbens (AcbSh). Combined treatment with CCK-8 (0.1 nmol/rat) and raclopride (0.5 mg/kg) caused significant additive increases in the Fos-like ir profiles in the AcbSh. In the central caudate-putamen, the medial olfactory tubercle, and the frontal cerebral cortex where either compound alone was weakly active or inactive, the combined treatment with both compounds led to a significant induction of neuronal Fos-like ir profiles. These results suggest that the blockade of D2 and activation of CCK transduction lines can induce Fos-like IR via different mechanisms. They may produce additive effects in AcbSh and synergistic effects in the caudate-putamen and the olfactory tubercle on the induction of neuronal Fos-like IR and thus on long-term regulation of gene expression in the striatum.

Improved event-related potential signs of selective attention after the administration of the cholecystokinin analog ceruletide in healthy persons

Cholecystokinin (CCK) is co-localized with dopamine (DA) in neurons of the mesolimbic-frontocortical dopamine (DA) system, considered essential for the pathology of psychotic behavior and associated attention deficits. The present experiments in 13 healthy men aimed at examining the effects of the CCK analog ceruletide on attention as reflected by event-related brain potentials (ERPs). Subjects were tested according to a double-blind cross-over design on three occasions, following intravenous infusion of placebo, 0.5 microgram ceruletide, and 2.5 micrograms ceruletide. ERPs were recorded during the subject's performance on an auditory selective attention task including the concurrent presentation of frequent standard tones and infrequent deviant tones which the subject had to listen to, or to ignore. The processing negativity (PN) over frontocentral cortical areas, reflecting selective attention, was higher after ceruletide than placebo, this increase being most pronounced after the 2.5 micrograms dose (placebo -1.29 +/- 0.38 microV versus ceruletide -3.02 +/- 0.65 microV, p < .05). ERP signs of a general increase in cortical arousal after ceruletide did not reach significance. Likewise, mismatch negativity, an indicator of preattentive processing of stimulus deviance, was not significantly affected by the peptide. The results indicate that ceruletide affects human brain function primarily by improving selective attention.

Cholecystokinin octapeptide in vitro and ex vivo strongly modulates striatal dopamine D2 receptors in rat forebrain sections

Receptor autoradiographic experiments together with the filter wipe-off technique were performed to investigate the effects of cholecystokinin octapeptide (CCK-8) on dopamine D2 receptors. In vitro studies showed that 1 nM CCK-8 significantly increased the KD value of binding sites for the D2 agonist [3H]N-propylnorapomorphine (NPA) in the rostral and caudal parts of the nucleus accumbens by 48 and 148% respectively. In contrast, 1 nM CCK-8 significantly decreased the IC50 value of dopamine for binding sites for the D2 antagonist [125I]iodosulpride in the rostral and caudal parts of the caudate-putamen by 46 and 56% respectively, and in the rostral and caudal parts of the nucleus accumbens (areas of CCK-dopamine coexistence) by 57 and 75% respectively. Ex vivo studies demonstrated that 30 min after an intraventricular injection of 1 nmol/rat CCK-8 the KD value of [3H]NPA binding sites in the caudal part of the forebrain and the IC50 value of dopamine for [125I]iodosulpride binding sites in the caudal part of the nucleus accumbens were significantly increased by 160% and decreased by 77% respectively. These results indicate for the first time that in sections CCK-8 in vitro and ex vivo can strongly regulate D2 receptor affinity in the striatum. The present studies also provide evidence for stronger modulation of D2 receptors by CCK-8 in the area of CCK/dopamine coexistence in the nucleus accumbens than in other basal ganglion areas, supporting the existence of CCK/D2 receptor interactions in cotransmission. The stronger interactions found in sections than in membrane preparations may indicate the requirement of intracellular mechanisms and/or a more intact membrane structure for optimal receptor-receptor interactions.

Possible mechanisms for the powerful actions of neuropeptides

In order to understand the mechanisms underlying the powerful actions of neuropeptides, the present article has emphasized the unique ability of neuropeptides to act as VT signals, which via high-affinity G-protein coupled receptors can exert long-lasting actions and control synaptic transmission via receptor-receptor interactions. Also of substantial importance is the ability of neuropeptides to act as a set of signals via the formation of different types of active fragments, which can act as negative-feedback or positive-feedback signals to modulate the response elicited by the parent peptide and to give origin to syndromic responses. Also in the actions of the fragments on the neuronal network, receptor-receptor interactions may play an important role both by modulating the parent peptide receptors and by modulating other types of VT and/or WT receptors. Future work will have to evaluate the role of neuropeptides as transcellular signals and as regulators of neuronal excitabilities after the formation of carbamates, but certainly new important developments are within the horizon of today's research.

Dopamine D1 receptors are involved in the modulation of D2 receptors induced by cholecystokinin receptor subtypes in rat neostriatal membranes

The action of cholecystokinin octapeptide (CCK-8) on rat neostriatal dopamine (DA) D2 receptors was evaluated in membrane binding experiments. 0.1 nM of CCK-8 increased the Kd value of the D2 agonist [3H]N-propylnorapomorphine (NPA) binding sites by 42%. The CCKB antagonist PD134308 blocked this action. Kinetic analysis demonstrated that this effect of CCK-8 was related to a reduction by 45% of the association rate constant of [3H]NPA. In contrast, 1 nM of CCK-8 decreased the KH and the KL values of DA for the D2 antagonist [3H]raclopride binding sites by 56% and 50%, respectively. Both the CCKA antagonist L364718 and the CCKB antagonist PD134308 blocked this effect. The D1 antagonist SCH23390 counteracted the CCK-8 induced decrease in the KH and the KL values of DA, and allowed 1 nM of CCK-8 to produce a significant increase in the IC50 value of NPA for the [3H]raclopride binding sites. These results indicate that CCK-8 can reduce the affinity of the neostriatal D2 agonist binding sites, but increase the affinity of D2 receptors for DA. D1 receptors may exert a switching role in the modulation of the neostriatal D2 receptors by the CCK receptors.

Ultrastructural study of CCK and tyrosine hydroxylase immunoreactivity in the rat nucleus accumbens

The cholecystokinin (CCK)- and tyrosine hydroxylase (TH)-like immunoreactive (LI) axons and boutons were studied in the caudal and medial parts of the rat nucleus accumbens (NAC), using the indirect immunoperoxidase technique, at the electron microscopic level. Both CCK- and TH-LI boutons contained clear synaptic vesicles and large granular vesicles of similar size, but the CCK-LI boutons contained more large granular vesicles than TH-LI boutons. The CCK-LI and TH-LI boutons were heterogeneous. This finding might be related to the various immunoreactive neuronal types innervating the caudomedial NAC. However, the CCK-LI boutons (containing mostly small, round, clear synaptic vesicles) formed mainly asymmetrical synaptic contacts with dendritic spines whereas the TH-LI boutons (containing medium-sized as well as small, round, clear synaptic vesicles) formed mostly symmetrical synaptic contacts with dendritic shafts.

Receptor-receptor interactions as an integrative mechanism in nerve cells

Several lines of evidence indicate that interactions among transmission lines can take place at the level of the cell membrane via interactions among macromolecules, integral or associated to the cell membrane, involved in signal recognition and transduction. The present view will focus on this last subject, i.e., on the interactions between receptors for chemical signals at the level of the neuronal membrane (receptor-receptor interaction). By receptor-receptor interaction we mean that a neurotransmitter or modulator, by binding to its receptor, modifies the characteristics of the receptor for another transmitter or modulator. Four types of interactions among transmission lines may be considered, but mainly intramembrane receptor-receptor interactions have been dealt with in this article, exemplified by the heteroregulation of D2 receptors via neuropeptide receptors and A2 receptors. The role of receptor-receptor interactions in the integration of signals is discussed, especially in terms of filtration of incoming signals, of integration of coincident signals, and of neuronal plasticity.

Neurotensin and cholecystokinin octapeptide control synergistically dopamine release and dopamine D2 receptor affinity in rat neostriatum

Combined perfusion of the neostriatum with 1 nM of cholecystokinin octapeptide (CCK-8) and 0.01, 0.1 or 1 nM of neurotensin was done in the halothane-anesthetized rat after systemic apomorphine treatment (0.05 mg/kg, s.c.). Neurotensin (1 nM) plus CCK-8 (1 nM) effectively counteracted the apomorphine-induced inhibition of neostriatal perfusate levels of dopamine (DA). With a constant concentration of CCK-8 (1 nM), the apomorphine-induced inhibition of DA release was counteracted dose relatedly by neurotensin in concentrations of 0.01, 0.1 and 1 nM. The results of binding experiments demonstrated that threshold concentrations of CCK-8 and neurotensin significantly increased the KD values of the high-affinity D2 receptors without significant alterations in the low-affinity D2 receptors or in the proportion of D2 receptors in the high-affinity state. Thus, neurotensin and CCK receptors may regulate synergistically, via intramembrane interactions with the D2 receptors, the binding characteristics and the signal transduction of D2 autoreceptors in the neostriatum. The combined presence of very low concentrations of CCK-8 and neurotensin in the extracellular fluid may be sufficient to regulate D2 receptor transduction, underlining the important role of these peptide receptor interactions with the D2 receptors.

Electrophysiological evidence of modulatory interaction between dopamine and cholecystokinin in the nucleus accumbens

Dopamine has been shown to modulate responses of accumbens neurones to excitatory inputs from the amygdala. The demonstration that cholecystokinin (CCK) co-exists and appears to be co-released with dopamine in the accumbens suggests that the modulatory action of dopamine in the accumbens may in turn be modified by CCK. This possibility was investigated in the present study. Single unit recordings were obtained in the medial and caudal accumbens of urethane-anaesthetized rats. These neurones were strongly excited by amygdala stimulation, and concurrent stimulation of the ventral tegmental area (VTA) at 10 Hz attenuated the responses, presumably due to dopamine release. Iontophoretic application of proglumide (PRG) at 30 nA enhanced the attenuating effect of VTA stimulation on the excitatory response to amygdala stimulation. Exogenous dopamine produced a similar attenuation in response and the attenuation was in turn suppressed by concurrent iontophoresis of sulphated CCK fragments applied at a current titrated not to produce significant effect on the spontaneous activity of the neurone nor its response to amygdala stimulation. These results demonstrate that exogenous and endogenous CCK can modify the postsynaptic action of dopamine in the nucleus accumbens in addition to modulating its release shown in other studies, and further suggests that CCK is likely an endogenous functional antagonist of dopamine, serving a comodulatory role in regulating synaptic transmission in the ventral striatum.

Changes in pituitary-adrenal activity affect the apomorphine- and cholecystokinin-8-induced changes in striatal dopamine release using microdialysis

The effects of apomorphine (0.05 mg/kg, i.p.) and cholecystokinin-8 (sulphated; CCK-8) were analyzed on the levels of dopamine and its metabolites using intrastriatal microdialysis in the adrenalectomized rat with or without corticosterone replacement treatment (5 mg/kg, twice daily for 7 days, last dose given 2 h before killing). Adrenalectomy did not affect the basal release of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) or homovanillic acid (HVA). However, the apomorphine-induced decrease in dopamine release was attenuated following adrenalectomy. Furthermore, there was an enhancement of the apomorphine-induced decrease in DOPAC levels without any modulation of the apomorphine-induced effects on HVA levels. In contrast, the CCK-8-induced increase in dopamine levels was potentiated following adrenalectomy. This potentiation was fully counteracted by replacement treatment with corticosterone. These results indicate that corticosterone may be involved in the regulation of dopamine release, perhaps through glucocorticoid receptors in nigral dopamine cells controlling inter alia the synthesis of G-proteins involved in the regulation of dopamine autoreceptors and CCK-8 receptors located on dopamine nerve terminals or of the receptor proteins themselves.

Regional distribution of cholecystokinin binding sites in macaque basal ganglia determined by in vitro receptor autoradiography

Cholecystokinin binding sites were labeled with [3H]cholecystokinin-8, [125I]cholecystokinin-33, and [125I]cholecystokinin-8 in major structures of macaque basal ganglia by in vitro receptor autoradiography. Analysis of autoradiograms revealed areas of heavy cholecystokinin binding in the neostriatum and substantia nigra that were set off, often quite sharply, from the adjacent globus pallidus and subthalamic nucleus where labeling was, by contrast, very light. Heavy label characterized the ventromedial and posterior parts of the caudate nucleus and adjacent putamen, binding was of moderate intensity in central areas of these regions, while, the dorsolateral margin of the head of the caudate and precommissural putamen, the dorsolateral one-third of the body of the caudate, and all but the most medial and ventral portions of the posterior putamen lateral to the pallidum were sparsely labeled. The pattern of cholecystokinin binding within the neostriatum was mottled; patches of reduced label stood out from the background of more prominent binding. However, those patches were only imperfectly correlated with the striosomal organization of both the caudate nucleus and putamen as revealed by acetylcholinesterase staining. Cholecystokinin binding in the substantia nigra was also intricately patterned. Moderately dense, vertically orientated bands of label were found in the dorsal one-third to half of the pars reticulata, providing a marked contrast to the near background levels in the ventral pars reticulata and overlying pars compacta. The present study shows that heavy cholecystokinin binding is confined to particular areas within the primate basal ganglia; the pattern of label within the substantia nigra and neostriatum can be linked to intrinsic and afferent connections of these structures. The confinement of binding sites to the dorsal pars reticulata suggests an association with dendrites of pars compacta neurons which invade this region; this interpretation is consistent with recent evidence of depletion of nigral cholecystokinin binding sites in macaques following chemical lesion of dopaminergic cells of the par compacta. In the neostriatum the distribution of binding shows overlap with its topographically organized corticostriatal innervation; portions of heavily labeled striatum coincide with regions innervated by association cortex of the frontal and temporal lobes, whereas regions of diminished binding correspond to areas innervated mainly by sensory and motor cortex. These latter findings suggest that cholecystokinin may have a particularly strong influence on cognitive aspects of striatal function.

Effects of short- and long-term haloperidol administration and withdrawal on regional brain cholecystokinin and neurotensin concentrations in the rat

The effects of oral administration of the neuroleptic, haloperidol, on regional brain concentrations of cholecystokinin (CCK) and neurotensin were examined in the rat. Both short-term (3 weeks) and long-term (8 months) haloperidol administration increased the concentration of CCK in the substantia nigra. While short-term administration significantly increased the concentration of CCK in the ventral tegmental area and decreased the concentration of CCK in the cortex, including the medial prefrontal cortex, these effects were not observed following long-term drug administration. In contrast, long-term, but not short-term, haloperidol administration decreased the concentration of CCK in the olfactory tubercle. Withdrawal from long-term haloperidol did not alter CCK concentrations in any of the brain regions examined. Short-term haloperidol administration significantly increased the concentration of neurotensin in the caudate-putamen. Both short- and long-term administration increased the concentration of neurotensin in the nucleus accumbens, but only the increased following long-term administration reached statistical significance. Withdrawal from long-term haloperidol administration slightly decreased the concentrations of neurotensin in the caudate-putamen and nucleus accumbens. These results indicate that dopamine receptor blockade can affect both CCK- and neurotensin-containing neural systems. Furthermore, these two neuropeptides are affected differently depending upon the duration of haloperidol administration and withdrawal from this drug. The results raise the possibility that chronic administration of haloperidol may be toxic to some neurotensin-containing neurons in the basal ganglia.

CCK-8 injected into the nucleus accumbens attenuates the supersensitive locomotor response to apomorphine in 6-OHDA and chronic-neuroleptic treated rats

Postsynaptic dopamine-cholecystokinin (CCK) interactions in the nucleus accumbens were studied in two behavioral preparations of DA receptor supersensitivity: chronic-neuroleptic treated and 6-hydroxydopamine (6-OHDA) denervated rats. Subcutaneous (SC) injections of apomorphine (APO; 0.15 mg/kg) in experiment 1 produced marked hyperlocomotion in rats following 12 days of pretreatment with cis-[Z]-flupenthixol (2 mg/kg; twice per day). Bilateral intra-accumbens (N.Acc.) microinjections of CCK-8 (2 ng and 2 micrograms) reliably reduced APO-stimulated hyperlocomotion. An intermediate CCK dose (20 ng) was without effect. No change in APO responsivity following chronic vehicle treatment was observed and the baseline APO response was not altered by CCK at any dose. Denervation of mesolimbic dopamine (DA) terminals by intra-N.Acc. injections of 6-hydroxydopamine (6-OHDA; 8 micrograms/side) in experiment 2 similarly resulted in intense locomotor hyperactivity after APO stimulation (0.1 mg/kg; SC). Bilateral intra-N.Acc. injections of CCK-8 (1, 10, 100 ng, and 1 micrograms) significantly attenuated the supersensitive locomotor response to APO. As in experiment 1, CCK produced "biphasic" dose-response effects with strong attenuation that persisted throughout the entire 60-min test at both high (1 microgram) and low (1 ng) doses. Intermediate CCK doses (10 and 100 ng) produced only short-term reductions in activity. Hypomotility induced by APO in SHAM-lesioned rats was not effectively reversed by CCK treatments. CCK had no effect on unstimulated baseline locomotor activity in either 6-OHDA or SHAM-lesioned rats. These results provide further evidence that CCK-8 modulates mesolimbic DA activity by functionally opposing the postsynaptic effects of DA in the region of the nucleus accumbens.

neuropeptides in long ascending spinal tract cells in the rat: evidence for parallel processing of ascending information

A study has been made of the involvement of spinal peptidergic neurons in ascending tracts at lumbar-sacral levels in rats, by combining the retrograde transport of a protein-gold complex with immunocytochemistry. Ten neuropeptides have been considered for their presence in the cells of origin of the following six ascending tracts, including some involved in pain transmission: the spinosolitary tract, the medial and lateral spinoreticular tracts, the spinomesencephalic tract, the spinothalamic tract and the postsynaptic dorsal column tract. Although there was overlap in the distribution of several of the types of peptidergic cells and some ascending tract cells only a very small percentage of long ascending tract cells were found to contain neuropeptides. Most (90%) of those peptidergic ascending tract cells, however, were clearly congregated in two distinct spinal regions: the lateral spinal nucleus and the region surrounding the central canal (including lamina X). Ascending tract cells in both of these regions contained a wide variety of neuropeptides. Immunoreactivities for a total of seven different peptides were seen. The lateral spinal nucleus had the highest percentage of neuropeptide containing ascending tract cells; cells of all the four populations of peptidergic neurons lying in this region were involved in supraspinal projections; they stained for vasoactive intestinal polypeptide, bombesin, substance P or dynorphin and their axons projected in the spinomesencephalic, spinoreticular and spinosolitary tracts. The region surrounding the central canal contained bombesin-, enkephalin-, cholecystokinin- and somatostatin-immunoreactive ascending tract cells; these cells were found at the origin of the spinothalamic, spinomesencephalic, spinoreticular and spinosolitary tracts. In this region only the cells staining for substance P were not involved in supraspinal projections. The peptidergic ascending tract cells in other spinal regions were few; they were found in either lamina I or lateral part of lamina V. Ascending tract lamina I cells reacted for dynorphin or vasoactive intestinal polypeptide and their axons projected in the spinosolitary and spinomesencephalic tracts. Ascending tract lamina V cells reacted for somatostatin and were found at the origin of the medial component of the spinoreticular tract. It is proposed that peptidergic ascending tract cells form minor but distinct subgroups within each ascending tract. Each of the ascending tracts are divisible into peptide- and nonpeptide-containing groups of cells which convey information in a parallel fashion.(ABSTRACT TRUNCATED AT 400 WORDS)