The C-terminal neurotensin-(8-13) fragment potently modulates rat neostriatal dopamine D2 receptors

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.

Neurotensin enhances glutamatergic EPSCs in VTA neurons by acting on different neurotensin receptors

Neurotensin (NT) is an endogenous neuropeptide that modulates dopamine and glutamate neurotransmission in several limbic regions innervated by neurons located in the ventral tegmental area (VTA). While several studies showed that NT exerted a direct modulation on VTA dopamine neurons less is known about its role in the modulation of glutamatergic neurotransmission in this region. The present study was aimed at characterising the effects of NT on glutamate-mediated responses in different populations of VTA neurons. Using whole cell patch clamp recording technique in horizontal rat brain slices, we measured the amplitude of glutamatergic excitatory post-synaptic currents (EPSCs) evoked by electrical stimulation of VTA afferents before and after application of different concentrations of NT1-13 or its C-terminal fragment, NT8-13. Neurons were classified as either Ih(+) or Ih(-) based on the presence or absence of a hyperpolarisation activated cationic current (Ih). We found that NT1-13 and NT8-13 produced comparable concentration dependent increase in the amplitude of EPSCs in both Ih(+) and Ih(-) neurons. In Ih(+) neurons, the enhancement effect of NT8-13 was blocked by both antagonists, while in Ih(-) neurons it was blocked by the NTS1/NTS2 antagonist, SR142948A, but not the preferred NTS1 antagonist, SR48692. In as much as Ih(-) neurons are non-dopaminergic neurons and Ih(+) neurons represent both dopamine and non-dopamine neurons, we can conclude that NT enhances glutamatergic mediated responses in dopamine, and in a subset of non-dopamine, neurons by acting respectively on NTS1 and an NT receptor other than NTS1.

Diversity and Bias through Receptor-Receptor Interactions in GPCR Heteroreceptor Complexes. Focus on Examples from Dopamine D2 Receptor Heteromerization

Allosteric receptor-receptor interactions in GPCR heteromers appeared to introduce an intermolecular allosteric mechanism contributing to the diversity and bias in the protomers. Examples of dopamine D2R heteromerization are given to show how such allosteric mechanisms significantly change the receptor protomer repertoire leading to diversity and biased recognition and signaling. In 1980s and 1990s, it was shown that neurotensin (NT) through selective antagonistic NTR-D2 like receptor interactions increased the diversity of DA signaling by reducing D2R-mediated dopamine signaling over D1R-mediated dopamine signaling. Furthermore, D2R protomer appeared to bias the specificity of the NTR orthosteric binding site toward neuromedin N vs. NT in the heteroreceptor complex. Complex CCK2R-D1R-D2R interactions in possible heteroreceptor complexes were also demonstrated further increasing receptor diversity. In D2R-5-HT2AR heteroreceptor complexes, the hallucinogenic 5-HT2AR agonists LSD and DOI were recently found to exert a biased agonist action on the orthosteric site of the 5-HT2AR protomer leading to the development of an active conformational state different from the one produced by 5-HT. Furthermore, as recently demonstrated allosteric A2A-D2R receptor-receptor interaction brought about not only a reduced affinity of the D2R agonist binding site but also a biased modulation of the D2R protomer signaling in A2A-D2R heteroreceptor complexes. A conformational state of the D2R was induced, which moved away from Gi/o signaling and instead favored β-arrestin2-mediated signaling. These examples on allosteric receptor-receptor interactions obtained over several decades serve to illustrate the significant increase in diversity and biased recognition and signaling that develop through such mechanisms.

Cross-receptor interactions between dopamine D2L and neurotensin NTS1 receptors modulate binding affinities of dopaminergics

Dopaminergic systems have been described to functionally interact with the neuromodulatory peptide neurotensin. Employing fluorescence detected coimmunoprecipitation and radioligand binding experiments, we herein demonstrate that coexpression of dopamine D(2L) receptor and the neurotensin receptor subtype NTS(1) leads to physical interaction and the formation of heteromers in transfected human embryonic kidney 293 cells. In this in vitro system, a trans-inhibitory effect on the agonist binding affinity of D(2) was observed in presence of neurotensin. To correlate between the functional properties of dopaminergic agents and the magnitude of neurotensin-induced modulation of D(2L) binding affinities in cells coexpressing D(2L) and NTS(1), a structurally diverse set of dopamine receptor agonists, partial agonists, and antagonists was tested. Ligand specific profiles indicating substantial bias between ligand efficacy and transmodulation were discovered, suggesting a heteromerization-based functional selectivity. In the presence of neurotensin, the novel D(2) agonist FAUC 326 displayed a 34-fold decrease of binding affinity in cells coexpressing D(2L) and NTS(1).

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.

Monitoring neurotensin[8-13] degradation in human and rat serum utilizing matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

A method was developed to quantify neurotensin (NT) fragment [8-13] and a novel NT[8-13] derivative, KK1, in human and rat serum utilizing matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The method allows for simultaneous quantification of the major NT[8-13] metabolite, NT[9-13] (according to molecular mass), and detection of the major KK1 metabolite, KK1M (according to molecular mass). The degradation rates of NT[8-13] and KK1 were calculated to be 24.1+/-1.0 and 193+/-8min in human serum and 5.90+/-0.22 and 153+/-4min in rat serum, respectively. The method utilizes a novel sample drying technique and spectrum acquisition protocol. In addition, an internal standard dissimilar in structure to the analytes was used. This method may be broadly applicable to the quantification of NT[8-13] and other peptide analogues of varying structure.

Neurotensin-induced modulation of dopamine D2 receptors and their function in rat striatum: counteraction by a NTR1-like receptor antagonist

The present study investigated the neurotensin (NT) receptor subtype (NTR) involved in the antagonistic neurotensin modulation of striatal dopamine D2 receptors observed in vitro and in vivo. The NT induced increase of the IC50 values of dopamine (DA) competition for [125I]iodosulpiride binding sites was counteracted by the NTR1-like antagonist SR48692 in rat striatal slices. Intrastriatal perfusion of pergolide induced in the awake rat an inhibition of striatal DA release that was antagonized by NT. This action of NT was counteracted by co-perfusion with the NTR1 like antagonist SR48692. These data indicate that there exists in the striatum at the prejunctional level an intramembrane antagonistic NT receptor/DA D2 receptor-receptor interaction where NTR1 like receptor activation reduces the DA D2 autoreceptor function.

Chronic, but not acute, dosing of antipsychotic drugs alters neurotensin binding in rat brain regions

The present study compared high affinity neurotensin (NT) binding in rat brain following acute or chronic treatment with the classical antipsychotic, haloperidol, and the newer antipsychotic drugs, clozapine and zotepine. Drugs were given orally, as an acute treatment (1 dose) or chronically (21 day dosing) and binding to the NT high affinity receptor was examined in three brain regions; striatum, nucleus accumbens/olfactory tubercle and frontal cortex. Acute dosing with either vehicle, haloperidol, clozapine or zotepine produced no significant changes in NT binding from controls (naïve rats). Chronic (21 day) dosing resulted in an increase in the K:(D:) and B(max) of high affinity receptors in the striatum following haloperidol, but not clozapine, zotepine or vehicles. In contrast, the newer antipsychotics, clozapine and zotepine but not haloperidol or vehicles, significantly altered NT binding in the nucleus accumbens/olfactory tubercle by decreasing the K:(D:) and B(max). Further differentiation between the two newer antipsychotic drugs occurred in the frontal cortex. Clozapine had no significant effect on NT binding, whereas zotepine significantly reduced the K:(D:) of the high affinity receptor with no alteration in B(max). The antipsychotic drugs tested did not interact directly with the NT high affinity receptor. Therefore, they must be acting indirectly via an alternative receptor mechanism to alter NT high affinity binding. In accordance with previously reported NT/dopamine receptor interactions, this would suggest cross-talk between these systems. Overall, these data demonstrate that chronic, but not acute, administration of antipsychotic drugs alters NT binding in the rat brain. In addition, anatomical differences in NT binding arise according to the antipsychotic drug under test. This may be predictive of drug side-effect profile, antipsychotic efficacy or atypicality.

Quantitative receptor radioautography in the study of receptor-receptor interactions in the nucleus tractus solitarii

The nucleus tractus solitarii (NTS) in the dorsomedial medulla comprises a wide range of neuropeptides and biogenic amines. Several of them are related to mechanisms of central blood pressure control. Angiotensin II (Ang II), neuropeptide Y (NPY) and noradrenaline (NA) are found in the NTS cells, as well as their receptors. Based on this observation we have evaluated the modulatory effect of these peptide receptors on alpha 2-adrenoceptors in the NTS. Using quantitative receptor radioautography, we observed that NPY and Ang II receptors decreased the affinity of alpha 2-adrenoceptors for their agonists in the NTS of the rat. Cardiovascular experiments agreed with the in vitro data. Coinjection of a threshold dose of Ang II or of the NPY agonists together with an ED50 dose of adrenergic agonists such as NA, adrenaline and clonidine counteracted the depressor effect produced by the alpha 2-agonist in the NTS. The results provide evidence for the existence of an antagonistic interaction between Ang II AT1 receptors and NPY receptor subtypes with the alpha 2-adrenoceptors in the NTS. This receptor interaction may reduce the transduction over the alpha 2-adrenoceptors which can be important in central cardiovascular regulation and in the development of hypertension.

Neurotensin peptides antagonistically regulate postsynaptic dopamine D2 receptors in rat nucleus accumbens: a receptor binding and microdialysis study

An in vitro receptor binding and in vivo microdialysis study was performed to further investigate the modulation of dopamine (DA) D2 receptors by neurotensin (NT) peptides. Saturation experiments with the D2 agonist [3H]NPA (N-propylnorapomorphine) showed that 10 nM of NT, 10 nM of neuromedin N (NN) and 1 nM of the C-terminal NT-(8-13) fragment significantly increased the KD values by 125%, 181%, and 194%, respectively without significantly affecting the Bmax value of the [3H]NPA binding sites in coronal sections of rat ventral forebrain mainly containing the nucleus accumbens (Acb) and the olfactory tubercle. In line with the previous findings that NT can increase GABA release in the Acb and that NT receptors are not found on DA terminals in this brain region, the present in vivo microdialysis study demonstrated that local perfusion of NT (1 nM) counteracted the D2 agonist pergolide (2 mu M) induced inhibition of GABA, but not of DA release in the rat Acb. This result indicates that NT counteracts the D2 agonist induced inhibition of GABA release in the rat Acb, via an antagonistic postsynaptic NT/D2 receptor interaction as also suggested by the inhibitory regulation of D2 receptor affinity in the Acb by the NT peptides demonstrated in the present receptor binding experiments. Thus, the neuroleptic and potential antipsychotic profile of the NT peptides may involve an antagonistic NT/D2 receptor regulation in the ventral striatum.

Receptor-receptor interactions and their relevance for receptor diversity. Focus on neuropeptide/dopamine interactions

Receptor diversity in combination with receptor-receptor subtype specific interactions, which can be antagonistic or synergistic in character, markedly increase plasticity in WT and VT in the nervous system. In this way switching among transmission lines for the various DA receptor subtypes becomes possible. Some of these aspects are supported by our work on selective modulation of D2 receptors by CCK and NT. Selective regulation of D2 receptors via CCK-8 receptor subtypes and NT receptors may underlie CCK/DA interactions and NT/DA interactions in the basal ganglia. These studies underline the importance of receptor-receptor interactions exerted at the membrane level between neuropeptide receptors and D2 receptors, which are determined at least in part by the ongoing activity at D1 receptors. In the case of both CCK/D2 and NT/D2 receptor interactions, it has been possible, by means of intrastriatal and intraaccumbens microdialysis, to obtain a functional correlate to the receptor interactions found in the membrane preparations from the striatum. Schizophrenia may be in part related to reduced release of CCK and/or NT peptides or to alterations in their receptor interactions with the D2 receptor. This view may lead to new therapeutic approaches.

A single (-)-nicotine injection causes change with a time delay in the affinity of striatal D2 receptors for antagonist, but not for agonist, nor in the D2 receptor mRNA levels in the rat substantia nigra

The in vitro and in vivo effects of (-)-nicotine on dopamine D2 receptors in the rat neostriatum have been studied using biochemical binding, in situ hybridization and immunocytochemistry. A single i.p. injection (1 mg/kg) of (-)-nicotine resulted in a reduction of the KD value of the D2 antagonist [3H]raclopride binding sites in rat neostriatal membrane preparations at 12 h without any significant change in the Bmax value. This action of (-)-nicotine was counteracted by pretreatment 15 min earlier with the nicotine antagonist mecamylamine (1 mg/kg, i.p.). However, the KD and the Bmax values of the D2 agonist [3H]NPA binding sites in the rat neostriatal membrane preparations were not significantly affected 0.5-48 h after a single i.p. injection with 1 mg/kg of (-)-nicotine. No significant change in neostriatal D2 receptor mRNA levels was observed at any time interval after the (-)-nicotine injection. No significant change was observed in tyrosine hydroxylase (TH) immunoreactivity in either the substantia nigra or the neostriatum, nor in nigral TH mRNA levels during the time interval studied (4-24 h posttreatment). Furthermore, addition of low (10 nM) or high (1 microM) concentrations of (-)-nicotine in vitro to rat neostriatal membranes did not alter the characteristics of [3H]raclopride or [3H]NPA binding. These results indicate that a single (-)-nicotine injection can produce a selective and delayed increase in the affinity of D2 receptors for the antagonist, but not for the agonist without modifying the levels of D2 receptor mRNA, probably via the activation of central nicotinic receptors.

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.

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.

Neuromedin N is a potent modulator of dopamine D2 receptor agonist binding in rat neostriatal membranes

In the concentration range of 1-10 nM, neuromedin N produced a significant concentration-related increase in the Kd values of [3H]L-(-)-N-propylnorapomorphine binding sites in rat neostriatal membranes with a peak action at 10 nM (36% increase versus the control group mean value). The Bmax values were not affected by neuromedin N. Neurotensin at 10 nM induced an increase in the Kd values, which was not affected by a threshold concentration of neuromedin N (0.1 nM). In view of the higher potency of neuromedin N versus neurotensin to modulate neostriatal D2 receptors in contrast to the higher potency of neurotensin versus neuromedin N to bind to the cloned neurotensin receptors, it seems possible that the neuromedin N activated neostriatal neurotensin receptors controlling the D2 receptors represent a distinct subtype of neurotensin receptors.