Interaction of L-prolyl-L-leucyl glycinamide with dopamine D2 receptor: evidence for modulation of agonist affinity states in bovine striatal membranes

Stapling Amantadine to Melanostatin Neuropeptide: Discovery of Potent Positive Allosteric Modulators of the D2 Receptors

This work describes the synthesis and pharmacological and toxicological evaluation of melanostatin (MIF-1) bioconjugates with amantadine (Am) via a peptide linkage. The data from the functional assays at human dopamine D2 receptors (hD2R) showed that bioconjugates 1 (EC50 = 26.39 ± 3.37 nM) and 2 (EC50 = 17.82 ± 4.24 nM) promote a 3.3- and 4.9-fold increase of dopamine potency, respectively, at 0.01 nM, with no effect on the efficacy (Emax = 100%). In this assay, MIF-1 was only active at the highest concentration tested (EC50 = 23.64 ± 6.73 nM, at 1 nM). Cytotoxicity assays in differentiated SH-SY5Y cells showed that both MIF-1 (94.09 ± 5.75%, p < 0.05) and carbamate derivative 2 (89.73 ± 4.95%, p < 0.0001) exhibited mild but statistical significant toxicity (assessed through the MTT reduction assay) at 200 μM, while conjugate 1 was found nontoxic at this concentration.

Allosteric Modulators of Dopamine D2 Receptors for Fine-Tuning of Dopaminergic Neurotransmission in CNS Diseases: Overview, Pharmacology, Structural Aspects and Synthesis

Allosteric modulation of G protein-coupled receptors (GPCRs) is nowadays a hot topic in medicinal chemistry. Allosteric modulators, i.e., compounds which bind in a receptor site topologically distinct from orthosteric sites, exhibit a number of advantages. They are more selective, safer and display a ceiling effect which prevents overdosing. Allosteric modulators of dopamine D₂ receptor are potential drugs against a number of psychiatric and neurological diseases, such as schizophrenia and Parkinson's disease. In this review, an insightful summary of current research on D₂ receptor modulators is presented, ranging from their pharmacology and structural aspects of ligand-receptor interactions to their synthesis.

Allosteric modulation of dopamine D2L receptor in complex with Gi1 and Gi2 proteins: the effect of subtle structural and stereochemical ligand modifications

Background

Allosteric modulation of G protein-coupled receptors (GPCRs) is nowadays one of the hot topics in drug discovery. In particular, allosteric modulators of D₂ receptor have been proposed as potential modern therapeutics to treat schizophrenia and Parkinson’s disease.

Methods

To address some subtle structural and stereochemical aspects of allosteric modulation of D₂ receptor, we performed extensive in silico studies of both enantiomers of two compounds (compound 1 and compound 2), and one of them (compound 2) was synthesized as a racemate in-house and studied in vitro.

Results

Our molecular dynamics simulations confirmed literature reports that the R enantiomer of compound 1 is a positive allosteric modulator of the D_2L receptor, while its S enantiomer is a negative allosteric modulator. Moreover, based on the principal component analysis (PCA), we hypothesized that both enantiomers of compound 2 behave as silent allosteric modulators, in line with our in vitro studies. PCA calculations suggest that the most pronounced modulator-induced receptor rearrangements occur at the transmembrane helix 7 (TM7). In particular, TM7 bending at the conserved P7.50 and G7.42 was observed. The latter resides next to the Y7.43, which is a significant part of the orthosteric binding site. Moreover, the W7.40 conformation seems to be affected by the presence of the positive allosteric modulator.

Conclusions

Our work reveals that allosteric modulation of the D_2L receptor can be affected by subtle ligand modifications. A change in configuration of a chiral carbon and/or minor structural modulator modifications are solely responsible for the functional outcome of the allosteric modulator.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s43440-021-00352-x.

Allosteric Modulator Leads Hiding in Plain Site: Developing Peptide and Peptidomimetics as GPCR Allosteric Modulators

Allosteric modulators (AMs) of G-protein coupled receptors (GPCRs) are desirable drug targets because they can produce fewer on-target side effects, improved selectivity, and better biological specificity (e.g., biased signaling or probe dependence) than orthosteric drugs. An underappreciated source for identifying AM leads are peptides and proteins-many of which were evolutionarily selected as AMs-derived from endogenous protein-protein interactions (e.g., transducer/accessory proteins), intramolecular receptor contacts (e.g., pepducins or extracellular domains), endogenous peptides, and exogenous libraries (e.g., nanobodies or conotoxins). Peptides offer distinct advantages over small molecules, including high affinity, good tolerability, and good bioactivity, and specific disadvantages, including relatively poor metabolic stability and bioavailability. Peptidomimetics are molecules that combine the advantages of both peptides and small molecules by mimicking the peptide's chemical features responsible for bioactivity while improving its druggability. This review 1) discusses sources and strategies to identify peptide/peptidomimetic AMs, 2) overviews strategies to convert a peptide lead into more drug-like "peptidomimetic," and 3) critically analyzes the advantages, disadvantages, and future directions of peptidomimetic AMs. While small molecules will and should play a vital role in AM drug discovery, peptidomimetics can complement and even exceed the advantages of small molecules, depending on the target, site, lead, and associated factors.

Discovery of New Potent Positive Allosteric Modulators of Dopamine D2 Receptors: Insights into the Bioisosteric Replacement of Proline to 3-Furoic Acid in the Melanostatin Neuropeptide

The control of Parkinson's disease (PD) is challenged by the motor and non-motor fluctuations as well as dyskinesias associated with levodopa long-term therapy. As such, pharmacological alternatives to reduce the reliance on this drug are needed. Melanostatin (MIF-1), a positive allosteric modulator (PAM) of D₂ receptors (D₂R), is being explored as a novel pharmacological approach focused on D₂R potentiation. In this work, 3-furoic acid (3-Fu) was successfully employed as an l-proline (Pro) surrogate, affording two potent MIF-1 analogues, methyl 3-furoyl-l-leucylglycinate (4a) and 3-furoyl-l-leucylglycinamide (6a). In this series, the C-terminal carboxamide moiety was found crucial to enhancing the potency and toxicological profile, yet it is not considered a requisite for the PAM activity. Conformational analysis excludes 4a from adopting the claimed type II β-turn. The discovery and validation of 6a as a lead compound open a new avenue for the development of a novel class of anti-Parkinson therapeutics targeting the D₂R.

Synthesis, Pharmacological, and Biological Evaluation of 2-Furoyl-Based MIF-1 Peptidomimetics and the Development of a General-Purpose Model for Allosteric Modulators (ALLOPTML)

This work describes the synthesis and pharmacological evaluation of 2-furoyl-based Melanostatin (MIF-1) peptidomimetics as dopamine D₂ modulating agents. Eight novel peptidomimetics were tested for their ability to enhance the maximal effect of tritiated N-propylapomorphine ([³H]-NPA) at D₂ receptors (D₂R). In this series, 2-furoyl-l-leucylglycinamide (6a) produced a statistically significant increase in the maximal [³H]-NPA response at 10 pM (11 ± 1%), comparable to the effect of MIF-1 (18 ± 9%) at the same concentration. This result supports previous evidence that the replacement of proline residue by heteroaromatic scaffolds are tolerated at the allosteric binding site of MIF-1. Biological assays performed for peptidomimetic 6a using cortex neurons from 19-day-old Wistar-Kyoto rat embryos suggest that 6a displays no neurotoxicity up to 100 μM. Overall, the pharmacological and toxicological profile and the structural simplicity of 6a makes this peptidomimetic a potential lead compound for further development and optimization, paving the way for the development of novel modulating agents of D₂R suitable for the treatment of CNS-related diseases. Additionally, the pharmacological and biological data herein reported, along with >20 000 outcomes of preclinical assays, was used to seek a general model to predict the allosteric modulatory potential of molecular candidates for a myriad of target receptors, organisms, cell lines, and biological activity parameters based on perturbation theory (PT) ideas and machine learning (ML) techniques, abbreviated as ALLOPTML. By doing so, ALLOPTML shows high specificity Sp = 89.2/89.4%, sensitivity Sn = 71.3/72.2%, and accuracy Ac = 86.1%/86.4% in training/validation series, respectively. To the best of our knowledge, ALLOPTML is the first general-purpose chemoinformatic tool using a PTML-based model for the multioutput and multicondition prediction of allosteric compounds, which is expected to save both time and resources during the early drug discovery of allosteric modulators.

Amide Bond Bioisosteres: Strategies, Synthesis, and Successes

The amide functional group plays a key role in the composition of biomolecules, including many clinically approved drugs. Bioisosterism is widely employed in the rational modification of lead compounds, being used to increase potency, enhance selectivity, improve pharmacokinetic properties, eliminate toxicity, and acquire novel chemical space to secure intellectual property. The introduction of a bioisostere leads to structural changes in molecular size, shape, electronic distribution, polarity, pK_a, dipole or polarizability, which can be either favorable or detrimental to biological activity. This approach has opened up new avenues in drug design and development resulting in more efficient drug candidates introduced onto the market as well as in the clinical pipeline. Herein, we review the strategic decisions in selecting an amide bioisostere (the why), synthetic routes to each (the how), and success stories of each bioisostere (the implementation) to provide a comprehensive overview of this important toolbox for medicinal chemists.

Synthesis, Pharmacological, and Biological Evaluation of MIF-1 Picolinoyl Peptidomimetics as Positive Allosteric Modulators of D2R

This work describes the synthesis and pharmacological evaluation of picolinoyl-based peptidomimetics of melanocyte stimulating hormone release inhibiting factor 1 (MIF-1) as dopamine modulating agents. Eight novel peptidomimetics were tested for their ability to enhance the maximal effect of tritiated N-propylapomorphine ([³H]-NPA) at dopamine D₂ receptors (D₂R). Methyl picolinoyl-l-valyl-l-alaninate (compound 6b) produced a statistically significant increase in the maximal [³H]-NPA response at 0.01 nM (11.9 ± 3.7%), which is close to the effect of MIF-1 in this assay at same concentration (18.3 ± 9.1%). Functional assays measuring cAMP mobilization in the presence of dopamine corroborate the activity of peptidomimetic 6b as a positive allosteric modulator (PAM) of D₂R. In this assay, 6b produced a typical bell-shaped dose-response curve similar to that of the parent neuropeptide (18.3 ± 7.1% for 6b vs 15.4 ± 5.5% for MIF-1, both at 0.1 nM). Dose-response curves for dopamine in the presence of 6b show EC₅₀ (0.33 ± 0.21 μM for 6b vs 0.17 ± 0.07 μM for MIF-1) and E_max (86.0 ± 5.4% for 6b vs 93.6 ± 4.4% for MIF-1) comparable to those of MIF-1, both at 0.01 nM. Furthermore, peptidomimetic 6b was tested for agonist activity at the human D₂R and the results show that it displays no intrinsic agonism effect, endorsing its activity as a PAM of D₂R. Cytotoxic and neurotoxic assays were performed for peptidomimetic 6b using HEK 293T cells and cortex neurons from 19 day old Wistar-Kyoto rat embryos, respectively, suggesting this analogue displays no toxicity effect in these assays up to 100 μM. Conformational energy minimization for 6b shows that this peptidomimetic cannot adopt the postulated type-II β-turn bioactive conformation, endorsing the possibility of an extended bioactive conformation as claimed by other researchers as a second bioactive conformation of MIF-1. Overall, the pharmacological and toxicological profile of peptidomimetic 6b together with its favorable druglike properties and structural simplicity makes it a potential lead compound for further development and optimization.

Perturbation Theory/Machine Learning Model of ChEMBL Data for Dopamine Targets: Docking, Synthesis, and Assay of New l-Prolyl-l-leucyl-glycinamide Peptidomimetics

Predicting drug-protein interactions (DPIs) for target proteins involved in dopamine pathways is a very important goal in medicinal chemistry. We can tackle this problem using Molecular Docking or Machine Learning (ML) models for one specific protein. Unfortunately, these models fail to account for large and complex big data sets of preclinical assays reported in public databases. This includes multiple conditions of assays, such as different experimental parameters, biological assays, target proteins, cell lines, organism of the target, or organism of assay. On the other hand, perturbation theory (PT) models allow us to predict the properties of a query compound or molecular system in experimental assays with multiple boundary conditions based on a previously known case of reference. In this work, we report the first PTML (PT + ML) study of a large ChEMBL data set of preclinical assays of compounds targeting dopamine pathway proteins. The best PTML model found predicts 50000 cases with accuracy of 70-91% in training and external validation series. We also compared the linear PTML model with alternative PTML models trained with multiple nonlinear methods (artificial neural network (ANN), Random Forest, Deep Learning, etc.). Some of the nonlinear methods outperform the linear model but at the cost of a notable increment of the complexity of the model. We illustrated the practical use of the new model with a proof-of-concept theoretical-experimental study. We reported for the first time the organic synthesis, chemical characterization, and pharmacological assay of a new series of l-prolyl-l-leucyl-glycinamide (PLG) peptidomimetic compounds. In addition, we performed a molecular docking study for some of these compounds with the software Vina AutoDock. The work ends with a PTML model predictive study of the outcomes of the new compounds in a large number of assays. Therefore, this study offers a new computational methodology for predicting the outcome for any compound in new assays. This PTML method focuses on the prediction with a simple linear model of multiple pharmacological parameters (IC₅₀, EC₅₀, K_i, etc.) for compounds in assays involving different cell lines used, organisms of the protein target, or organism of assay for proteins in the dopamine pathway.

Preclinical pharmacokinetic and toxicological evaluation of MIF-1 peptidomimetic, PAOPA: examining the pharmacology of a selective dopamine D2 receptor allosteric modulator for the treatment of schizophrenia

Schizophrenia is a mental illness characterized by a breakdown in cognition and emotion. Over the years, drug treatment for this disorder has mainly been compromised of orthosteric ligands that antagonize the active site of the dopamine D2 receptor. However, these drugs are limited in their use and often lead to the development of adverse movement and metabolic side effects. Allosteric modulators are an emerging class of therapeutics with significant advantages over orthosteric ligands, including an improved therapeutic and safety profile. This study investigates our newly developed allosteric modulator, PAOPA, which is a specific modulator of the dopamine D2 receptor. Previous studies have shown PAOPA to attenuate schizophrenia-like behavioral abnormalities in preclinical models. To advance this newly developed allosteric drug from the preclinical to clinical stage, this study examines the pharmacokinetic behavior and toxicological profile of PAOPA. Results from this study prove the effectiveness of PAOPA in reaching the implicated regions of the brain for therapeutic action, particularly the striatum. Pharmacokinetic parameters of PAOPA were found to be comparable to current market antipsychotic drugs. Necropsy and histopathological analyses showed no abnormalities in all examined organs. Acute and chronic treatment of PAOPA indicated no movement abnormalities commonly found with the use of current typical antipsychotic drugs. Moreover, acute and chronic PAOPA treatment revealed no hematological or metabolic abnormalities classically found with the use of atypical antipsychotic drugs. Findings from this study demonstrate a better safety profile of PAOPA, and necessitates the progression of this newly developed therapeutic for the treatment of schizophrenia.

Development of peptidomimetic ligands of Pro-Leu-Gly-NH(2) as allosteric modulators of the dopamine D(2) receptor

A variety of stable, small-molecule peptidomimetic ligands have been developed to elucidate the mechanism by which the neuropeptide Pro-Leu-Gly-NH₂ (PLG) modulates dopaminergic neurotransmission. Photoaffinity labeling ligands based upon PLG peptidomimetics have been used to establish that PLG binds to the D₂ dopamine receptor at a site that is different from the orthosteric site, thus making PLG and its peptidomimetics allosteric modulators of the dopamine receptor. Through the design, synthesis and pharmacological evaluation of conformationally constrained peptidomimetics containing lactam, bicyclic, and spiro-bicyclic scaffolds, support was provided for the hypothesis that the bioactive conformation of PLG is a type II β-turn. In addition, studies with peptidomimetics designed to mimic either a type VI β-turn or polyproline II helix conformation yielded molecules that were able to modulate dopamine receptors because of their ability to place the carboxamide NH₂ pharmacophore in the same topological space as that seen in the type II β-turn. Extensive studies with the spiro-bicyclic PLG peptidomimetics also established that both positive and negative modes of modulation were possible for the same series of peptidomimetics simply as a result of minor differences in the stereochemistry about the bridgehead carbon within the scaffold. This information was used to transform existing positive modulators into negative modulators, which demonstrated that small structural changes in the spiro-bicyclic dopamine receptor modulators are capable of causing major changes in the modulatory activity of PLG peptidomimetics.

Synthesis of an MIF-1 analogue containing enantiopure (S)-α-trifluoromethyl-proline and biological evaluation on nociception

The synthesis and the effect of a novel MIF-1 analogue on nociception during acute pain in rat model are reported. The synthesis of this enantiopure trifluoromethyl group containing tripeptide was performed through a peptide coupling reaction between the HCl. Leu-Gly-NH2 and the (S)-α-Tfm-proline. The analgesic effect of the CF3-(MIF-1) 2 has been evaluated in vivo on rat model by paw pressure (PP) and hot plate (HP) tests and compared to the native peptide MIF-1. Highest analgesic effect was observed with CF3-(MIF-1) 2 only in PP test. In order to study the mechanisms of nociception induced by the studied peptides, the involvement of the opioid and the nitric oxideergic systems was investigated. The results are in favor of a participation of both system since pretreatment, 20 min before injection of the CF3-(MIF-1) 2, with the non-competitive antagonist of opiate receptors naloxone, the nitric oxide synthase (NOS) inhibitor l-N(G)-nitroarginine ester (l-NAME) or the nitric oxide (NO) donor l-arginine (l-Arg) significantly decreased the pain perception in PP and HP tests.

Transformation of Pro-Leu-Gly-NH2 peptidomimetic positive allosteric modulators of the dopamine D2 receptor into negative modulators

The synthesis of dimethyl derivatives of 5.6.5 spiro bicyclic lactam Pro-Leu-Gly-NH(2) peptidomimetics was carried out to test the hypothesis that by placing methyl groups on the β-methylene carbon of the thiazolidine ring steric bulk would be introduced into the topological space that the β-methylene carbon is believed to occupy in the negative allosteric modulators of the dopamine D(2) receptor. With such a modification, a positive allosteric modulator would be converted into a negative allosteric modulator. This hypothesis was shown to be correct as 3a and 4a where found to be negative allosteric modulators, whereas their unmethylated derivatives were positive allosteric modulators of the dopamine D(2) receptor.

PAOPA, a potent analogue of Pro-Leu-glycinamide and allosteric modulator of the dopamine D2 receptor, prevents NMDA receptor antagonist (MK-801)-induced deficits in social interaction in the rat: implications for the treatment of negative symptoms in schizophrenia

The aim of this study was to investigate whether a potent analogue of the endogenous brain peptide l-prolyl-l-leucyl-glycinamide (PLG), (3(R)-[(2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide (PAOPA), can prevent the induction of social withdrawal caused by sub-chronic treatment with the non-competitive NMDA (N-methyl-l-aspartate) receptor antagonist, MK-801. Results indicate that MK-801 (0.5 mg/kg) significantly decreased social interaction following sub-chronic treatment (7 days). Treatment with PAOPA (1 mg/kg) blocked the effects of MK-801, and increased the amount of time spent in social interaction in comparison to control animals. These results provide evidence for the development of peptidomimetic compounds for the treatment of social withdrawal and related negative symptoms associated with schizophrenia.

Specific binding of photoaffinity-labeling peptidomimetics of Pro-Leu-Gly-NH2 to the dopamine D2L receptor: evidence for the allosteric modulation of the dopamine receptor

The present study was undertaken to investigate the mechanistic role of l-prolyl-l-leucyl-glycinamide (PLG) in modulating agonist binding to the dopamine D(2L) receptor. Competition and displacement assays indicate that the photoaffinity-labeling peptidomimetics of PLG, 3(R)-[(4(S)-(4-azido-2-hydroxy-benzoyl) amino-2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide hydrochloride (1a) and 3(R)-[(4(S)-(4-azido-2-hydroxy-5-iodo-benzoyl)amino-2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide hydrochloride (1b) bind at the same site as PLG. Autoradiography was used to establish the covalent binding of [(125)I]-1b to an approximately 51kDa protein in bovine striatal membranes. Western blot analysis with a dopamine D(2L)-specific antibody, in combination with autoradiography, following a two-dimensional gel separation, suggested this approximately 51kDa protein to be the dopamine D(2L) receptor. Further evidence for binding of 1b to dopamine D(2L) was provided by samples immunoprecipitated with the D(2L) antibody. These samples were analyzed by western blotting in parallel with autoradiography of [(125)I]-1b labeled protein. Both methods revealed bands at approximately 51kDa. Furthermore, PLG is shown to compete with 1b for binding to the dopamine D(2L) receptor as determined by autoradiography, as well as competition experiments with PLG and 1a. Collectively, these findings suggest the successful development of a photoaffinity-labeling agent, compound 1b, that has been used to elucidate the interaction of PLG specifically with the dopamine D(2L) receptor.

Brain Activation by Peptide Pro-Leu-Gly-NH(2) (MIF-1)

MIF-1 (Pro-Leu-Gly-NH(2)) is a tripeptide for which the therapeutic potential in Parkinson's disease and depression has been indicated by many studies. However, the cellular mechanisms of action of MIF-1 are not yet clear. Here, we show the specific brain regions responsive to MIF-1 treatment by c-Fos mapping, and determine the kinetics of cellular signaling by western blotting of pERK, pSTAT3, and c-Fos in cultured neurons. The immunoreactivity of c-Fos was increased 4 hours after MIF-1 treatment in brain regions critically involved in the regulation of mood, anxiety, depression, and memory. The number of cells activated was greater after peripheral treatment (intravenous delivery) than after intracerebroventricular injection. In cultured SH-SY5Y neuronal cells, c-Fos was induced time- and dose-dependently. The activation of cellular c-Fos was preceded by a transient increase of mitogen-activated protein kinase pERK but a reduction of phosphorylated Signal Transducer and Activator of Transcription (pSTAT3) initially. We conclude that MIF-1 can modulate multiple cellular signals including pERK, and pSTAT3 to activate c-Fos. The cellular activation in specific brain regions illustrates the biochemical and neuroanatomical basis underlying the therapeutic effect of MIF-1 in Parkinson's disease and depression.

Allosteric modulation of the dopamine D2 receptor by Pro-Leu-Gly-NH2 peptidomimetics constrained in either a polyproline II helix or a type II beta-turn conformation

Type II beta-turn mimics and polyproline II helix mimics based upon diastereoisomeric 5.6.5 spiro bicyclic scaffolds have provided two pairs of Pro-Leu-Gly-NH(2) (PLG) peptidomimetics with contrasting dopamine receptor modulating activities. Compounds 1a and 3a were found to be positive allosteric modulators of the dopamine receptor, while the corresponding diastereoisomeric compounds 1b and 3b provided the first PLG peptidomimetics with the ability to decrease the binding of agonists to the dopamine receptor. The positive allosteric modulating activity of 3a supported the hypothesis that a polyproline II helix conformation is the bioactive conformation for the PLG analogue Pro-Pro-Pro-NH(2). The results also show that a change in the bridgehead chirality of the 5.6.5 scaffold brings about opposite effects in terms of the modulation of the dopamine receptor.

Synthesis and evaluation of novel pyridine based PLG tripeptidomimetics

Analogues of the pyridine based PLG (Pro-Leu-Gly-NH(2)) peptidomimetic were synthesized and evaluated as dopamine modulating agents. Modifications in the position corresponding to the leucine side chain in PLG afforded derivatives , and , substituted with H, Me and Bn instead of the isobutyl group, respectively. Changes in the proline residue produced derivative , substituted with a symmetrical piperidine ring instead of the pyrrolidine ring and , in which the pyrrolidine ring is connected to the pyridine ring via a hydroxymethyl group instead of a keto function. The peptidomimetics were tested for their ability to enhance the maximal effect of N-propylapomorphine (NPA) at dopamine D2 receptors in the functional cell-based R-SAT assay. Compounds , , and , produced a statistically significant increase in the maximal NPA response at 10 nM (117 +/- 6%, 118 +/- 6%, and 116 +/- 3%, respectively), which is similar to the effect of PLG in this assay, whereas was able to potentiate the response to a similar extent at 1 nM concentration (115 +/- 5%). All derivatives produced a bell-shaped dose-response curve and none of the compounds were active at the D2 receptor alone, which indicates that the mechanism behind the activity of both the pyridine based mimetics and PLG is the same. Interestingly, l-Pro-d-Leu-Gly-NH(2) was found to be more potent than PLG and produced a 119 +/- 1% increase in the NPA response at 1 nM.

Allosteric modulation of the dopamine receptor by conformationally constrained type VI beta-turn peptidomimetics of Pro-Leu-Gly-NH2

A peptidomimetic of Pro-Leu-Pro-NH2, 7, possessing an indolizidinone type VI beta-turn mimic was synthesized via improved high-yielding protocols for the preparation and Cbz protection of alpha-allylproline. Bicyclic peptidomimetic 7 and spirobicylic peptidomimetic 8 enhanced the binding of [3H] N-propylnorapomorphine to dopamine receptors indicating that a type VI beta-turn is a possible bioactive conformation of the homochiral Pro-Leu-Pro-NH2 and Pro-Pro-Pro-NH 2 analogues of Pro-Leu-Gly-NH2 at the dopamine receptor allosteric regulatory site.

MIF-1 and its peptidomimetic analogs attenuate haloperidol-induced vacuous chewing movements and modulate apomorphine-induced rotational behavior in 6-hydroxydopamine-lesioned rats

Two melanocyte-stimulating hormone release inhibiting factor-1 (MIF-1) also known as L-prolyl-L-leucyl-glycinamide (PLG) peptidomimetic analogs, 3(R)-[[[2(S)-pyrrolidinyl]carbonyl]-amino]-3-(butyl)-2-oxo-1-pyrrolidineacetamide trifluoroacetate (A) and 3(R)-[[[2(S)-pyrrolidinyl]carbonyl]amino]-3-(benzyl)-2-oxo-1-pyrrolidineacetamide trifluoroacetate (B), were evaluated for their ability to modulate dopaminergic activity by measuring apomorphine-induced rotations in 6-hydroxydopamine (6-OHDA)-lesioned rats, and haloperidol (HP)-induced vacuous chewing movements (VCMs) in rats; animal models of Parkinson's disease (PD) and human tardive dyskinesia (TD), respectively. In the 6-OHDA model, both analogs were found to potentiate the contralateral rotational behavior induced by apomorphine dose-dependently and with approximately the same potency. Furthermore, each analog was able to significantly attenuate HP-induced VCMs with almost equal efficacy. The potency and efficacy of these analogs were significantly greater than their parent compound, PLG. These results suggest that both analogs can modulate dopaminergic activity in vivo, likely by the same mechanisms recruited by PLG previously reported.

Design and synthesis of photoaffinity-labeling ligands of the L-prolyl-L-leucylglycinamide binding site involved in the allosteric modulation of the dopamine receptor

Pro-Leu-Gly-NH(2) (PLG), in addition to its endocrine effects, possesses the ability to modulate dopamine D(2) receptors within the central nervous system. However, the precise binding site of PLG is unknown. Potential photoaffinity-labeling ligands of the PLG binding site were designed as tools to be used in the identification of the macromolecule that possesses this binding site. Six different photoaffinity-labeling ligands were designed and synthesized on the basis of the gamma-lactam PLG peptidomimetic 1. The 4-azidobenzoyl and 4-azido-2-hydroxybenzoyl photoaffinity-labeling moieties were placed at opposite ends of PLG peptidomimetic 1 to generate a series of ligands that potentially could be used to map the PLG binding site. All of the compounds that were synthesized possessed activity comparable to or better than PLG in enhancing [(3)H]-N-propylnorapomorphine agonist binding to dopamine receptors. Photoaffinity ligands that were cross-linked to the receptor preparation produced a modulatory effect that was either comparable to or greater than the increase in agonist binding produced by the respective ligands that were not cross-linked to the dopamine receptor. The results indicate that these photoaffinity-labeling agents are binding at the same allosteric site as PLG and PLG peptidomimetic 1.

Modulation of agonist binding to human dopamine receptor subtypes by L-prolyl-L-leucyl-glycinamide and a peptidomimetic analog

The present study was undertaken to investigate the role of the hypothalamic tripeptide L-prolyl-L-leucyl-glycinamide (PLG) and its conformationally constrained analog 3(R)-[(2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide (PAOPA) in modulating agonist binding to human dopamine (DA) receptor subtypes using human neuroblastoma SH-SY5Y cells stably transfected with respective cDNAs. Both PLG and PAOPA enhanced agonist [3H]N-propylnorapomorphine (NPA) and [3H]quinpirole binding in a dose-dependent manner to the DA D2L,D2S, and D4 receptors. However, agonist binding to the D1 and D3 receptors and antagonist binding to the D2L receptors by PLG were not significantly affected. Scatchard analysis of [3H]NPA binding to membranes in the presence of PLG revealed a significant increase in affinity of the agonist binding sites for the D2L, D2S, and D4 receptors. Analysis of agonist/antagonist competition curves revealed that PLG and PAOPA increased the population and affinity of the high-affinity form of the D2L receptor and attenuated guanosine 5'-(beta,gamma-imido)-triphosphate-induced inhibition of high-affinity agonist binding sites for the DA D2L receptor. Furthermore, direct NPA binding with D2L cell membranes pretreated with suramin, a compound that can uncouple receptor/G protein complexes, and incubated with and without DA showed that both PLG and PAOPA had only increased agonist binding in membranes pretreated with both suramin and DA, suggesting that PLG requires the D2L receptor/G protein complex to increase agonist binding. These results suggest that PLG possibly modulates DA D2S, D2L, and D4 receptors in an allosteric manner and that the coupling of D2 receptors to the G protein is essential for this modulation to occur.

Design, Synthesis and Evaluation of a PLG Tripeptidomimetic Based on a Pyridine Scaffold

A 2,3,4-substituted pyridine derivative has been identified as a potential tripeptidomimetic scaffold. The design of the scaffold was based on conformational and electrostatic comparisons with a natural tripeptide. The scaffold has been used in the synthesis of a Pro-Leu-Gly-NH2 (PLG) mimetic. The different substituents in the 2-, 3-, and 4-positions of the pyridine ring were introduced via an aromatic nucleophilic substitution reaction, a "halogen-dancing" reaction, and a Grignard coupling of a Boc-protected amino aldehyde, respectively. The synthetic route involves eight steps and provides the mimetic in 20% overall yield. The pyridine based PLG-mimetic was evaluated for its ability to enhance the maximum response of the dopamine agonist N-propylapomorphine (NPA) at human D2 receptors using a cell based assay (the R-SAT assay). The dose-response curve of the mimetic was found to exhibit a down-turn phase, similar to that of PLG. In addition, the mimetic was more potent than PLG to enhance the NPA response; the maximum response was found to be 146% at 10 nM concentration, as compared to 115% for PLG at the same concentration. Interestingly, conformational analysis by molecular modeling showed that the pyridine mimetic cannot adopt a type II beta-turn conformation that previously has been suggested to be the bioactive conformation of PLG.

Iso-lactam and reduced amide analogues of the peptidomimetic dopamine receptor modulator 3(R)-[(2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide

An analogue of the highly potent gamma-lactam Pro-Leu-Gly-NH(2) peptidomimetic, 3(R)-[(2(S)-pyrrolidinylcarbonyl) amino]-2-oxo-1-pyrrolidineacetamide (2), 4(R)-[[2(S)-pyrrolidinylcarbonyl]amino]-2-oxo-1-pyrrolidineacetamide (3), in which the lactam carbonyl moiety has been placed in a different position with respect to the 3-amino group was synthesized. Also, a series of analogues of 2, compounds 4-6, were synthesized in which each of the amide bonds of 2 were systematically replaced with a reduced amide bond surrogate. The analogues were tested for their ability to enhance the binding of [3H]N-propylnorapomorphine to dopamine receptors in a functional in vitro assay utilizing bovine striatal membranes. Peptidomimetic 3 was shown to be more potent than 2, while 4 and 5 were significantly less effective than 2. Peptidomimetic 6 had a pharmacological profile similar to that of 2.

Synthesis and dopamine receptor modulating activity of 3-substituted gamma-lactam peptidomimetics of L-prolyl-L-leucyl-glycinamide

gamma-Lactam peptidomimetic 2 of Pro-Leu-Gly-NH(2) (PLG) was substituted at the 3-position with isobutyl, butyl, and benzyl moieties to give the PLG peptidomimetics 3-5, respectively. These compounds were synthesized to test the hypothesis that attaching a hydrophobic moiety to the lactam ring to mimic the isobutyl side chain of the leucyl residue of PLG would increase the dopamine receptor modulating activity of such peptidomimetics. These peptidomimetics were tested for their ability to enhance the binding of [(3)H]-N-propylnorapomorphine to dopamine receptors isolated from bovine striatal membranes. The rank order of effectiveness of the 3-substituent was benzyl > n-butyl > isobutyl > H.

PLG regulates hnRNP-L expression in the rat striatum and pre-frontal cortex: identification by ddPCR

Central dopaminergic systems are implicated in schizophrenia and Parkinson's disease, and are known to be modulated by the endogenous tripeptide Pro-Leu-Gly-NH(2) (PLG or MIF-1, melanocyte-stimulating hormone release inhibiting factor-1). Differential display polymerase chain reaction (ddPCR) was utilized to identify genes that are regulated by protracted PLG treatment (20 mg/kg, i.p. for 28 days) in male Sprague-Dawley rats. A total of 2400 genes were screened and 3 down-regulated bands were identified in the PLG-treated samples. Sequencing analysis revealed a total of six unique cDNA species. One fragment possessed a high degree of homology with Mus musculus hnRNP-L (protein L) mRNA (GenBank #AB009392) (termed PRG1: PLG regulated gene 1). Elongation of the PRG1 cDNA, by RACE-PCR, provided an 835 bp sequence with 95% homology to AB009392 over a 743 bp span. Open reading frame analysis provided a putative amino acid sequence consistent with the identity of PRG1 as rat hnRNP-L. Northern hybridization experiments with PRG1 revealed a 2.3 kb mRNA species that was decreased by 65% in the PLG-treated tissue. Western blot analysis revealed significantly decreased hnRNP-L levels in the striatum and pre-frontal cortex (but not the nucleus accumbens) by 71 and 61%, respectively of PLG-treated animals. The identification of altered expression of hnRNP-L following PLG treatment provides insight into the long-term effects of PLG and may provide insight into its molecular mechanism of action.

L-prolyl-l-leucyl-glycinamide and its peptidomimetic analog 3(R)-[(2(S)-pyrrolidylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide (PAOPA) attenuate haloperidol-induced c-fos expression in the striatum

Acute treatment of rats with haloperidol results in a rapid and transient increase in striatal c-fos mRNA and Fos immunoreactivity. The induction of immediate early genes by haloperidol may be involved in the development of extrapyramidal side effects. L-Prolyl-L-leucyl-glycinamide (PLG, or MIF-1) has been observed to antagonize the development of haloperidol-induced D(2) receptor supersensitivity in rats. We investigated the modulatory effects of PLG on haloperidol-induced c-fos and Fos protein expression in the rat striatum. We report that coadministration of either PLG or the potent analog of PLG, 3(R)-[(2(S)-pyrrolidylcarbonyl)amino]-2-oxo-1-pyrrolidineacetam ide (PAOPA), attenuated haloperidol-induced c-fos and Fos expression. Haloperidol induced [2 mg/kg, intraperitoneally (i.p.)] c-fos and Fos expression by 500% and 100%, respectively. These responses were attenuated by 170% and 75%, respectively, when coadministered with PLG (20 mg/kg, i.p.) or by 79% by PAOPA (10 microg/kg, i.p.).

Synthesis and dopamine receptor modulating activity of unsubstituted and substituted triproline analogues of L-prolyl-L-leucyl-glycinamide (PLG)

Triprolines Pro-Pro-Pro-NH2 (4), Pro-Pro-D-Pro-NH2 (5), Pro-Pro(trans-3-Me)-D-Pro-NH2 (6), and Pro-Pro(cis-3-Me)-D-Pro-NH2 (7) were made as conformationally constrained analogues of Pro-Leu-Gly-NH2. Triprolines 4-6 produced significant increases in the high- and low-affinity state ratio (RH/RL) of the dopamine receptor, but only 4 was found to increase apomorphine induced rotations in 6-hydroxydopamine-lesioned rats.

Synthesis and dopamine receptor modulating activity of substituted bicyclic thiazolidine lactam peptidomimetics of L-prolyl-L-leucyl-glycinamide

6-Substituted bicyclic thiazolidine lactam peptidomimetics of Pro-Leu-Gly-NH(2) (1) were synthesized to test the hypothesis that incorporation of a hydrophobic side chain into the bicyclic thiazolidine lactam scaffold would further enhance the dopamine receptor modulating activity of such peptidomimetics. The substituents employed were the isobutyl, butyl, and benzyl groups to give peptidomimetics 3-5, respectively. These peptidomimetics were evaluated in vivo as modulators of apomorphine-induced rotational behavior in the 6-hydroxydopamine-lesioned rat model of hemiparkinsonism and were compared with the unsubstituted bicyclic thiazolidine lactam Pro-Leu-Gly-NH(2) peptidomimetic 2. Peptidomimetics 3-5 each affected rotational behavior in a bell-shaped dose-response relationship producing maximal increases of 44% (1 microgram/kg,ip), 56% (0.1 microgram/kg,ip), and 30% (1 microgram/kg, ip), respectively. In comparison, unsubstituted peptidomimetic 2 increased rotational behavior by only 23% at a dose of 0.1 microgram/kg, ip.

Synthesis and dopamine receptor modulating activity of novel peptidomimetics of L-prolyl-L-leucyl-glycinamide featuring alpha,alpha-disubstituted amino acids

In the present study, L-prolyl-L-leucyl-glycinamide (1) peptidomimetics 3a-3d and 4a-4d were synthesized utilizing alpha, alpha-disubstituted amino acids. These analogues were designed to explore the conformational effects of constraints at the phi3 and psi3 torsion angles. Constrained conformations were verified by the use of X-ray crystallography and circular dichroism. The effects of Pro-Leu-Gly-NH2 analogues 3a-3d and 4a-4d on enhancing rotational behavior induced by apomorphine in the 6-hydroxydopamine-lesioned animal models of Parkinson's disease were studied. The ability of these peptidomimetics to increase the binding of agonist N-propylnorapomorphine (NPA) to the dopamine D2 receptor was also examined. Extended analogue Pro-Leu-Deg-NH2 was the most active compound of this series. It was 10 times more potent and almost 2 times more effective than 1 in increasing apomorphine-induced rotations (56 +/- 15% at 1.0 mg/kg ip) and in enhancing [3H]NPA specific binding (40%).

Design, synthesis, and dopamine receptor modulating activity of spiro bicyclic peptidomimetics of L-prolyl-L-leucyl-glycinamide

In the present study, the synthesis of the 5.5.6. and 5.6.5. spiro bicyclic lactam PLG peptidomimetics, compounds 3 and 4, respectively, was undertaken. These peptidomimetics were designed to examine the following: (1) the effect that changing the size of the thiazolidine and lactam ring systems would have on the ability of these systems to mimic the type-II beta-turn and (2) the effect that these structural perturbations would have on the ability of the peptidomimetics to modulate dopamine receptors. Through the use of the [3H]spiroperidol/N-propylnorapomorphine (NPA) dopamine D2 receptor competitive binding assay, 3 and 4, at a concentration of 100 nM, decreased the dissociation constant of the high-affinity state of the dopamine receptor for the agonist. These effects were observed when either Gpp(NH)p was absent or present and they were comparable to those produced by PLG at a concentration of 1 microM. Peptidomimetics 3 and 4 also increased the percentage of D2 receptors that existed in the high-affinity state. Even with Gpp(NH)p present, 3 and 4 were able to return the RH/RL ratios to values observed in the respective controls where Gpp(NH)p was absent. Furthermore, both peptidomimetics were able to attenuate the Gpp(NH)p-induced shift to the low-affinity state to a greater extent than PLG. Peptidomimetics 3 and 4 were evaluated in vivo as modulators of apomorphine-induced rotational behavior in the 6-hydroxydopamine-lesioned rat model of hemiparkinsonism, and each affected the rotational behavior in a bell-shaped dose-response relationship producing increases of 95 +/- 31% (0.01 mg/kg, ip) and 88 +/- 14% (0.001 mg/kg, ip), respectively. In comparison, the previously reported 5.5.5. spiro bicyclic lactam 2 increased rotational behavior by 25 +/- 11% (0.01 mg/kg, ip).

Modulatory effects of PLG and its peptidomimetics on haloperidol-induced catalepsy in rats

A behavioral model of dopaminergic function in the rat was used to examine the anticataleptic effects of L-prolyl-L-leucyl-glycinamide (PLG) and peptidomimetic analogs of PLG. Administration of 1 mg/kg PLG intraperitoneally significantly attenuated haloperidol (1 mg/kg)-induced catalepsy (as measured by the standard horizontal bar test), whereas doses of 0.1 and 10 mg/kg PLG did not. Eight synthetic PLG peptidomimetics (Calpha, alpha-dialkylated glycyl residues with lactam bridge constraint [1-4] and without [5-8]) were tested in the same manner (at a dose of 1 microg/kg) and categorized according to their activity, i.e. very active (5), moderately active (2, 3, 4, and 6), and inactive (1, 7, and 8). The catalepsy-reversal action of the diethylglycine-substituted peptidomimetic 5 was examined further and found to exhibit a U-shaped dose-response effect with an optimal dose of 1 microg/kg. The similarity between the effects of PLG and the synthetic peptidomimetics suggests a common mechanism of action. Finally, the synthetic peptidomimetics examined here, particularly peptidomimetic 5, were more effective than PLG in attenuating haloperidol-induced catalepsy.

Beta-analogs of PLG (L-prolyl-L-leucyl-glycinamide): ex-chiral pool syntheses and dopamine D2 receptor modulating effects

Starting from (S)- and (R)-aspartic acid enantiomerically pure beta-proline derivatives were synthesized. These chiral building blocks were transformed into beta-analogs of the dopamine receptor modulating peptide PLG. According to dopamine receptor binding studies, significant enhancement of [3H]pramipexole binding was observed for the isomeres 1a,b and 2a-c. The derivative 1b revealed an activity comparable to PLG.

Design, synthesis, and dopamine receptor modulating activity of diketopiperazine peptidomimetics of L-prolyl-L-leucylglycinamide

The diketopiperazine "C5" conformational mimic has been incorporated into the L-prolyl-L-leucylglycinamide (PLG, 1) structure and into the bicyclic lactam PLG peptidomimetic structure 3 to give compounds 5 and 6, respectively. These analogues were designed to explore the idea that the N-terminal "C5" conformation, which was found in the crystal structure of 2 and which was mimicked in 4 by the diketopiperazine function, was a factor in the high potency of these two agents. Through the use of the [3H]spiroperidol/N-propylnorapomorphine (NPA) D2 receptor competitive binding assay, both 5 and 6 were found to increase the affinity of the dopamine receptor for agonists and both were found to increase the percentage of D2 receptors which existed in the high-affinity state. These effects were observed when Gpp(NH)p was either absent or present, and they were analogous to the effects observed previously for PLG and the PLG peptidomimetics 2 and 4. However, the potency seen with 5 and 6 was less than that seen for 2 and 4, suggesting that while the N-terminal "C5" conformation may play a role in the potency of the gamma-lactam peptidomimetics of PLG, it does not appear to be the primary factor. In the 6-hydroxydopamine-lesioned animal model of Parkinson's disease, 5 altered apomorphine-induced rotational behavior in a dose-dependent manner. The maximum effect occurred at a dose of 0.01 mg/kg i.p. and resulted in a 52.27 +/- 13.96% (p < 0.001, n = 7) increase in rotations compared to apomorphine administered alone.

Modulation of dopamine receptor agonist-induced rotational behavior in 6-OHDA-lesioned rats by a peptidomimetic analogue of Pro-Leu-Gly-NH2 (PLG)

The present study was undertaken to determine if the previously reported in vitro interactions of the Pro-Leu-Gly-NH2 (PLG) peptidomimetic analogue 3(R)-[(2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacet amide (PAOPA) with the dopaminergic system could be exhibited in an in vivo animal model using 6-hydroxydopamine (6-OHDA)-lesioned rats. In this model, PAOPA was found to potentiate the contralateral rotational behavior induced by either apomorphine or L-DOPA. PAOPA was 100-fold more potent than PLG, and produced a fourfold greater response than PLG when administered i.p. PAOPA also potentiated contralateral rotations induced by SKF-38393 and quinpirole. In summary, the results of this study indicate that PAOPA, a conformationally constrained peptidomimetic analogue of PLG, can modulate dopaminergic activity in vivo with higher potency and efficacy than PLG.

Modulation of dopaminergic neurotransmission in the 6-hydroxydopamine lesioned rotational model by peptidomimetic analogues of L-prolyl-L-leucyl-glycinamide

Melanocyte stimulating hormone release inhibiting factor (MIF-1), also known as L-prolyl-L-leucyl-glycinamide (PLG), has previously been found to have the ability to modulate dopamine D2-receptor agonist binding both in the striatum and limbic regions. In the present study the 6-hydroxydopamine unilateral lesion model of apomorphine-induced rotational behaviour, in Wistar rats, was used to assess the dopaminergic modulatory activity of PLG and two novel analogues, L-prolyl-L-prolyl-L-prolinamide (analogue A) and (2S, 5R, 7R)-1-Aza-7[3'(S)-1-(2',5'-dioxo-pyrrolidino[2,1-c]piperazino++ +)] -8-oxo-4-thiabicyclo[3.30]octane-2-carboxamide (analogue B). PLG and the two novel analogues showed a bell-shaped dose-response relationship, suggesting that analogue A, B and PLG all manifest their effect through a similar mechanism and exhibit a window of therapeutic efficacy. Analogue A was a 100 times, while analogue B was 10 times, more potent than PLG in increasing the contralateral rotational response when given in combination with apomorphine. Analogue A was also more efficacious than PLG or analogue B at increasing apomorphine-induced contralateral rotations. Intrastriatal administration of either analogue A or B resulted in a greater increase in apomorphine-induced rotations than the most efficacious intraperitoneally delivered dose. The results of the present study suggest that PLG and its two novel analogues are able to modulate dopamine receptor activity and may be possible therapeutic agents for the treatment of Parkinsonian symptoms as well as tardive dyskinesia.

Effect of BCH 325 (Pro-D-Phe-Pro-Gly) on central dopaminergic functions

Three behavioral models were used to characterize the pharmacological action of BCH 325 on central dopaminergic transmission. The effect of acute SC treatment with BCH 325 upon dopaminergic mechanisms affecting motor activity was studied on the climbing behavior of mice. It was shown that the beta-casomorphin analogue evoked a dose-dependent increase in apomorphine (APO)-induced hypoactivity that was reversed by sulpiride (SULP). In in vitro studies on slices of nucleus accumbens of mice it could be demonstrated that 10(-6) M APO caused a reduction of K(+)-stimulated [14C]dopamine (DA) release that was potentiated following simultaneous incubation with 10(-6) M BCH 325. To prove a postsynaptic influence in D1 receptor-mediated behavior pattern, the action of BCH 325 was studied on bromocriptine (BROMO)-evoked yawning behavior of rats after pretreatment with reserpine (RES) or saline. The peptide could not influence the BROMO yawning after saline administration, but it was able to normalize the number of yawns, which were reduced after RES. To investigate the effect of BCH 325 on postsynaptic D2 receptors, jerking behavior on RES-pretreated rats after a high dose of BROMO was used. Following RES pretreatment only, the number of BROMO-induced jerks was decreased by treatment of rats with 0.5 mumol/kg BCH 325. In contrast, the jerking behavior was enhanced by 0.5 mumol/kg BCH 325 in rats that were additionally treated with alpha-methyl-p-tyrosine (MPT). In biochemical studies on slices of the nucleus accumbens of mice, the in vivo pretreatment with RES caused a reduction of K(+)-stimulated [14C]DA release that was blocked by the SC administration of 0.5 mumol/kg BCH 325.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of brain dopamine transmission by sex steroids

Sex steroid hormones influence the dopaminergic systems of the hypothalamus as well as the extrahypothalamic regions of the brain in controlling movement and behavior in both humans and animals. This review focuses on the effects of sex steroids on dopaminergic activity in extrahypothalamic brain areas. Among sex steroids, estrogens have been most extensively investigated, and many studies report that estrogens affect behaviors mediated by the basal ganglia, such as in humans suffering from extrapyramidal disorders. Epidemiological and clinical evidence also suggests an influence of estrogens on the vulnerability threshold for schizophrenia and sex differences in the clinical expression of this disease. Clinical observations point to a role of androgenic hormones in Gilles de la Tourette's syndrome. In normal humans, sex steroids were also shown to influence motor and cognitive performance. Biochemical and behavioral studies in animals have also shown the effect of sex steroids on dopaminergic activity in the basal ganglia; however, both activating and inhibiting effects have been reported. This may partly be explained by effects of the dose, duration of treatment, interval between steroid administration and testing the behavior measured, and the part of the basal ganglia from which the behavior is elicited. In view of the numerous variables that influence net dopaminergic response to steroids, focus will be on the literature using similar experimental conditions to assess the effect of in vivo chronic steroid treatment, acute short-term steroid treatment and the estrous cycle as well as in vitro effects of steroids on dopamine receptors. These experimental paradigms point to two general mechanisms of action of steroids: a rapid short-term non-genomic membrane effect and a slower long-term possibly genomic effect of steroids on dopamine systems. Combining dopaminergic drugs with sex steroids could improve efficacy or reduce side effects associated with these drugs. Examples of such combined treatments in rats and monkeys are presented for delta 9-tetrahydrocannabinol, cocaine, neuroleptics, apomorphine and L-DOPA. A better understanding of steroid-dopamine interactions and the possible isolation of conditions to have only pro or anti dopaminergic activity could then be used to develop combined therapies or to optimize drug treatments that would take into account the patient's sex and endocrine status.

Modulation by estradiol and progesterone of the GTP effect on striatal D-2 dopamine receptors

Agonist binding properties of rat striatal D-2 dopamine (DA) receptors were investigated after in vivo or in vitro estradiol or progesterone exposures in order to elucidate the mechanism of action of steroid hormones on DA receptors. Chronic estradiol treatment of ovariectomized rats (10 micrograms, twice each day, for 2 weeks) increased lateral striatum total receptor density and left unchanged the proportion and affinity of the agonist high- and low-affinity states of this receptor in the striatum. In addition, when GTP was added in DA competition for [3H]spiperone binding experiments, D-2 receptors in the medial part of the striatum from estrogen-treated animals were more sensitive to GTP than those in the lateral part, whereas GTP had equal activity in both parts of the striatum in vehicle-treated rats. With apomorphine, but not with DA competition for [3H]spiperone binding, addition of estradiol (1 nM) to striatal homogenates of intact male rats prevented the expected shift of the high- to the low-affinity state of D-2 receptors, normally induced by GTP (100 microM) under these conditions. This effect of estradiol was not observed in the presence of 4 mM MgCl2, while in vitro progesterone (100 nM) had no effect in either the absence or presence of MgCl2. In addition, in vivo chronic progesterone treatment of ovariectomized rats left striatal [3H]spiperone density and affinity unchanged. Moreover, 1 nM estradiol increased the IC50 of GTP for inhibition of [3H]N-propylnorapomorphine binding to the high-affinity state of striatal D-2 receptors. This effect was also observed but decreased by 2-fold in the presence of MgCl2. Our data suggest that estradiol in vivo and in vitro interferes with the effect of GTP on striatal D-2 DA receptors.

Tyr-MIF-1 attenuates development of tolerance to spiperone-induced catalepsy in rats

Because the tripeptide MIF-1 (Pro-Leu-Gly-NH2) is known to attenuate the effects of neuroleptic-induced catalepsy as well as neuroleptic-induced proliferation of dopamine (DA) receptors, we studied the related naturally occurring peptide, Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) for similar properties. Male rats were treated SC for 11 consecutive days with either the DA D1 receptor antagonist SCH 23390 HCl (0.50 mg/kg per day), the DA D2 receptor antagonist spiperone HCl (0.30 mg/kg per day), or vehicle. Half the rats were cotreated daily with Tyr-MIF-1 (1.0 mg/kg per day). The cataleptic effects of SCH 23390 were not altered by Tyr-MIF-1. Tolerance to SCH 23390-induced catalepsy did not develop during the 11-day treatment, and Tyr-MIF-1 had no effect on SCH 23390-induced catalepsy. However, tolerance developed to spiperone-induced catalepsy, and Tyr-MIF-1 attenuated this development of tolerance (p < 0.001). Locomotor and stereotyped activities of the DA D1 and D2 agonists, SKF 39393 (3.0 mg/kg) and quinpirole (3.0 mg/kg) were not affected by Tyr-MIF-1 after treatment with the DA antagonists was discontinued. Tyr-MIF-1 did not alter the Bmax or Kd for in vitro binding of [3H]SCH 23390 and [3H]spiperone to homogenates of the striatum. These findings indicate that Tyr-MIF-1 is able to selectively affect the development of receptor tolerance to a DA D2 receptor antagonist, and that this effect is unrelated to changes in affinity or numbers of D2 receptors.

Striatal dopamine D1-like receptors have higher affinity for dopamine in ethanol-treated rats

Experiments were carried out with brain tissues of ethanol-experienced (long-term ethanol intake but withdrawn) vs. ethanol-naive animals. The in vitro 3H antagonist binding of [3H]SCH 23390 and of [3H]spiperone to striatal dopamine D1- and D2-like receptors revealed no significant changes in KD and Bmax values. Displacement of the 3H antagonist binding by dopamine indicated high- and low-affinity states, which also showed no significant alteration at the D2-like receptor but a 5-fold increase of dopamine affinity at the high-affinity state of the D1-like receptor of the ethanol-experienced rats.