Dopamine D2 receptor isoforms expressed in AtT20 cells differentially couple to G proteins to acutely inhibit high voltage-activated calcium channels

Bromocriptine monotherapy overcomes prostate cancer chemoresistance in preclinical models

Chemoresistance is a major obstacle in the clinical management of metastatic, castration-resistant prostate cancer (PCa). It is imperative to develop novel strategies to overcome chemoresistance and improve clinical outcomes in patients who have failed chemotherapy. Using a two-tier phenotypic screening platform, we identified bromocriptine mesylate as a potent and selective inhibitor of chemoresistant PCa cells. Bromocriptine effectively induced cell cycle arrest and activated apoptosis in chemoresistant PCa cells but not in chemoresponsive PCa cells. RNA-seq analyses revealed that bromocriptine affected a subset of genes implicated in the regulation of the cell cycle, DNA repair, and cell death. Interestingly, approximately one-third (50/157) of the differentially expressed genes affected by bromocriptine overlapped with known p53-p21- retinoblastoma protein (RB) target genes. At the protein level, bromocriptine increased the expression of dopamine D2 receptor (DRD2) and affected several classical and non-classical dopamine receptor signal pathways in chemoresistant PCa cells, including adenosine monophosphate-activated protein kinase (AMPK), p38 mitogen-activated protein kinase (p38 MAPK), nuclear factor kappa B  (NF-κB), enhancer of zeste homolog 2 (EZH2), and survivin. As a monotherapy, bromocriptine treatment at 15 mg/kg, three times per week, via the intraperitoneal route significantly inhibited the skeletal growth of chemoresistant C4-2B-TaxR xenografts in athymic nude mice. In summary, these results provided the first preclinical evidence that bromocriptine is a selective and effective inhibitor of chemoresistant PCa. Due to its favorable clinical safety profiles, bromocriptine could be rapidly tested in PCa patients and repurposed as a novel subtype-specific treatment to overcome chemoresistance.

Anterior pituitary gland synthesises dopamine from l-3,4-dihydroxyphenylalanine (l-dopa)

Prolactin (PRL) is a hormone principally secreted by lactotrophs of the anterior pituitary gland. Although the synthesis and exocytosis of this hormone are mainly under the regulation of hypothalamic dopamine (DA), the possibility that the anterior pituitary synthesises this catecholamine remains unclear. The present study aimed to determine if the anterior pituitary produces DA from the precursor l-3,4-dihydroxyphenylalanine (l-dopa). Accordingly, we investigated the expression of aromatic l-amino acid decarboxylase (AADC) enzyme and the transporter vesicular monoamine transporter 2 (VMAT2) in the anterior pituitary, AtT20 and GH3 cells by immunofluorescence and western blotting. Moreover, we investigated the production of DA from l-dopa and its release in vitro. Then, we explored the effects of l-dopa with respect to the secretion of PRL from anterior pituitary fragments. We observed that the anterior pituitary, AtT20 and GH3 cells express both AADC and VMAT2. Next, we detected an increase in DA content after anterior pituitary fragments were incubated with l-dopa. Also, the presence of l-dopa increased DA levels in incubation media and reduced PRL secretion. Likewise, the content of cellular DA increased after AtT20 cells were incubated with l-dopa. In addition, l-dopa reduced corticotrophin-releasing hormone-stimulated adrenocorticotrophic hormone release from these cells after AADC activity was inhibited by NSD-1015. Moreover, DA formation from l-dopa increased apoptosis and decreased proliferation. However, in the presence of NSD-1015, l-dopa decreased apoptosis and increased proliferation rates. These results suggest that the anterior pituitary synthesises DA from l-dopa by AADC and this catecholamine can be released from this gland contributing to the control of PRL secretion. In addition, our results suggest that l-dopa exerts direct actions independently from its metabolisation to DA.

Cancer and the Dopamine D2 Receptor: A Pharmacological Perspective

The dopamine D₂ receptor (D₂R) family is upregulated in many cancers and tied to stemness. Reduced cancer risk has been correlated with disorders such as schizophrenia and Parkinson's disease, in which dopaminergic drugs are used. D₂R antagonists are reported to have anticancer efficacy in cell culture and animal models where they have reduced tumor growth, induced autophagy, affected lipid metabolism, and caused apoptosis, among other effects. This has led to several hypotheses, the most prevalent being that D₂R ligands may be a novel approach to cancer chemotherapy. This hypothesis is appealing because of the large number of approved and experimental drugs of this class that could be repurposed. We review the current state of the literature and the evidence for and against this hypothesis. When the existing literature is evaluated from a pharmacological context, one of the striking findings is that the concentrations needed for cytotoxic effects of D₂R antagonists are orders of magnitude higher than their affinity for this receptor. Although additional definitive studies will provide further clarity, our hypothesis is that targeting D₂-like dopamine receptors may only yield useful ligands for cancer chemotherapy in rare cases.

Dopamine receptor D2S gene transfer improves the sensitivity of GH3 rat pituitary adenoma cells to bromocriptine

Bromocriptine, a dopamine agonist (DA), has been used in the treatment of prolactinomas. Recent studies have indicated that dopamine 2 receptor short isoform (D2S) may play an important role in suppressing PRL synthesis and prolactinoma cell growth under DA treatment. In the current study, we investigated the role of D2S in the therapeutic action of bromocriptine in GH3 using both in vitro and in vivo approaches. Infection of adenovirus-D2S increased D2S expression in GH3 cells (P<0.05). D2S expression significantly decreased the GH3 cell viability subjected to bromocriptine treatment in vitro (P<0.05). In nude mice, adenovirus-D2S transfection sensitized GH3 xenograft to bromocriptine treatment evidenced by the significant inhibition of D2S expressed tumor growth as compared with vector control. Furthermore, decrease of Bcl-2 expression, increase of Bax, and active Caspase-3 were found in D2S expressed GH3 xenograft subjected to bromocriptine treatment. In summary, our study indicates that D2S expression plays a critical role in the therapeutic action of bromocriptine in pituitary adenomas and that adenovirus-mediated D2S gene transfer combined with bromocriptine may provide a novel treatment for DA-resistant prolactinomas.

Differential effects of nerve growth factor on expression of dopamine 2 receptor subtypes in GH3 rat pituitary tumor cells

Nerve growth factor (NGF) can increase expression of dopamine 2 receptors (D2R) in GH3 rat pituitary tumor cells (GH3 cells). As D2R exists in long (D2L) and short (D2S) isoforms, effects of NGF on D2R subtypes have not been accurately evaluated. In this study, we compared mRNA levels of D2R subtypes in GH3 cells treated with or without NGF with real-time RT-PCR. In addition, we also evaluated the relationship between GH3 cell growth after bromocriptine treatment and mRNA levels of D2R subtypes. We found that D2R total, D2L, and D2S mRNA in GH3 cells were significantly increased after NGF treatment, compared with the vehicle group. Moreover, NGF increased the ratio of D2S to D2L. GH3 cell survival rate after bromocriptine treatment was negatively correlated with D2R total mRNA, and D2S may be more potent than D2L in inhibiting cell growth. Cell apoptosis rate was highly elevated in GH3 cells treated with NGF and bromocriptine, compared with the control group or the group treated with NGF or bromocriptine alone. Our data provide preliminary evidence that the effect of NGF was more prominent on expression of D2S than D2L, in addition, D2S might have a greater impact suppressing GH3 cells growth than D2L.

The histamine autoreceptor is a short isoform of the H₃ receptor

BACKGROUND AND PURPOSE The histamine H(3) receptor was identified as the autoreceptor of brain histaminergic neurons. After its cloning, functional H(3) receptor isoforms generated by a deletion in the third intracellular loop were found in the brain. Here, we determined if this autoreceptor was the long or the short isoform. EXPERIMENTAL APPROACH We hypothesized that the deletion would affect H(3) receptor stereoselectivity. The effects of the enantiomers of two chiral ligands, N(α)-methyl-α-chloromethylhistamine (N(α) Me-αClMeHA) and sopromidine, were investigated on cAMP formation at the H(3(445)) and H(3(413)) receptor isoforms, common to all species. They were further compared with their effects at autoreceptors. They were also compared on [(35)S]GTPγ[S] binding to membranes of rat cerebral cortex, striatum and hypothalamus, the richest area in autoreceptors. KEY RESULTS The stereoselectivity of N(α) Me-αClMeHA enantiomers as agonists was similar at the H(3(413)) receptor isoform and autoreceptors, but lower at the long isoform. While (S) sopromidine did not discriminate between the isoforms, (R) sopromidine was an antagonist at the H(3(413)) receptor isoform and autoreceptors, but a full agonist at the long isoform. In rat brain, stereoselectivity of N(α) Me-αClMeHA was higher in the hypothalamus than in cerebral cortex or striatum, whereas the opposite pattern was found for sopromidine. CONCLUSIONS AND IMPLICATIONS The pharmacological profiles of H(3) receptor isoforms differed markedly, showing that the function of autoreceptors was fulfilled by a short isoform, such as the H(3(413)) receptor. Development of drugs selectively targeting autoreceptors might enhance their therapeutic efficacy and/or decrease incidence of side effects.

Role of dopamine receptors in ADHD: a systematic meta-analysis

The dopaminergic system plays a pivotal role in the central nervous system via its five diverse receptors (D1-D5). Dysfunction of dopaminergic system is implicated in many neuropsychological diseases, including attention deficit hyperactivity disorder (ADHD), a common mental disorder that prevalent in childhood. Understanding the relationship of five different dopamine (DA) receptors with ADHD will help us to elucidate different roles of these receptors and to develop therapeutic approaches of ADHD. This review summarized the ongoing research of DA receptor genes in ADHD pathogenesis and gathered the past published data with meta-analysis and revealed the high risk of DRD5, DRD2, and DRD4 polymorphisms in ADHD.

Correlation of alternative splicing of the D2 dopamine receptor mRNA and estrogen receptor mRNA in the prolactinomas and gonadotrope tumors

OBJECTIVE

Estradiol (E2) acts to modulate the ratio of two dopamine D2 receptor isoforms (D2L/D2S) by the nuclear estrogen receptor (ER) and to reduce dopamine's inhibitory action on PRL secretion. Here we demonstrate the correlation between the expression of ER mRNA and D2R mRNA isoforms in pituitary neoplasms cells.

METHODS

Twenty-four human pituitary adenomas (14 prolactinomas and 10 gonadotrope tumors) were examined for the expression of both ER mRNA and D2R mRNA by means of semi-quantitative RT-PCR analysis.

RESULTS

No significant difference was found in ERbeta mRNA expression levels between prolactinomas and gonadotrope tumors (P = 0.871), but there was a significant difference in the expression of ERalpha mRNA (P = 0.003). The significant difference was found between the two pituitary adenomas types in both levels of D2S and D2L mRNA expression (P = 0.036 and 0.007 respectively). Furthermore, both levels of expression in prolactinomas were significantly higher than that in gonadotrope tumors. Additionally, a negative correlation between D2S and ERalpha mRNA expression and a positive correlation between D2L and ERalpha mRNA expression were found in these tumors.

CONCLUSION

This study for the first time shows a good correlation between expression of ER and D2R isoforms in prolactinomas and gonadotrope tumors. Reducing the amount of the ERalpha in neoplasm cells can alter the ratio of D2L/D2S, which may increase the drug sensitivity of pituitary adenomas.

Dopamine, vesicular transporters and dopamine receptor expression and localization in rat thymus and spleen

The localization of dopamine stores and the expression and localization of vesicular monoamine transporter (VMAT) type-1 and 2 and of dopamine D1-like and D2-like receptor subtypes were investigated in rat thymus and spleen by immunohistochemical, immunochemical techniques and by RT-PCR. In the thymus dopamine immunoreactivity was developed in the cortico-medullary junction and in the medulla, but not in the thymic cortex. In the spleen, dopamine stores were found in reticular structures in the white pulp border and in the white pulp, but not in the red one. Both thymus and spleen expressed VMAT-1 and VMAT-2 immunoreactivity as well as dopamine D1, D2, D3, D4 and D5 receptor immunoreactivity. Immunohistochemistry revealed VMAT-1, VMAT-2 and dopamine D1, D2, D3, D4 and D5 receptor immunoreactivity primarily in the thymic cortical-medulla transitional zone and to a lesser extent in the medulla but not in the cortex. In the spleen, VMAT-1, VMAT-2 and dopamine D1, D2, D3, D4 and D5 receptor immunoreactivity was located primarily in the white pulp border and to a lesser extent in the white pulp. These findings indicate that both thymus and spleen express a dopaminergic system characterized by the presence of dopamine, vesicular monoamine transporters and the five subtypes of dopamine receptors. The presence of these dopaminergic markers suggests that dopamine likely originating from immune cells and/or from sympathetic neuroeffector plexus is released in the lymphoid microenvironment. Based on the microanatomical localization of dopaminergic markers investigated, a role of dopamine in maturation and selection of lymphocytes and activation of immune responses is suggested.

Differential desensitization of dopamine D2 receptor isoforms by protein kinase C: The importance of receptor phosphorylation and pseudosubstrate sites

Altered regulation of dopamine D(2) receptors is implicated in addiction, schizophrenia and movement disorders, as well as lactotroph growth and regulation. Dopamine D(2S) and dopamine D(2L) receptors are alternately-spliced variants that differ by 29 amino acids in the third intracellular (i3) domain and display different sensitivity to desensitization by protein kinase C (PKC). In the present studies we determined the specific phosphorylation sites on the dopamine D(2S) receptor that confer PKC-mediated desensitization. In dopamine D(2L) receptors, we identified a PKC pseudosubstrate site responsible for the relative insensitivity of the receptor to PKC-induced uncoupling. In transiently transfected Ltk(-) fibroblast cells, 2-min preactivation of PKC with 12-O-tetradecanoyl 4beta-phorbol 13alpha-acetate (TPA) completely inhibited calcium mobilization induced by the dopamine D(2S) receptor, but not the dopamine D(2L) variant. Point mutation of i3 PKC sites Ser228/229Gly rendered the dopamine D(2S) receptor resistant to PKC action, with lesser effects of other Ser and Thr mutations. Inactivation of the PKC pseudosubstrate motif in the dopamine D(2L) receptor sensitized the receptor to PKC, and this was reversed by mutation of i3 PKC sites Ser228/229. A phospho-specific antibody generated against phospho-Ser228/229 demonstrated PKC-induced phosphorylation at these sites of dopamine D(2S), but not D(2L) receptors, in Ltk(-) cells. Conversely, the pseudosubstrate dopamine D(2L) receptor mutant displayed PKC-induced phosphorylation at Ser228/229, which was abolished when these sites were mutated. Similar phosphorylation results were observed using GH4 cells stably transfected with dopamine D(2) receptors and mutants. Thus the relative location of phosphorylation and pseudosubstrate sites provides an important determinant substrate sensitivity to PKC.

Role of a Galphai2 protein splice variant in the formation of an intracellular dopamine D2 receptor pool

Treatment of D2-receptor-expressing cells with specific drugs upregulates the receptor number at the cell surface independently of protein synthesis, leading to the concept of an intracellular receptor pool. However, how this pool is operating is still an enigma. Here, we report that a splice variant of the Galphai2 protein, protein sGalphai2, plays a crucial role in the maintenance of this D2-receptor pool. Co-expression of sGi2 with D2 receptor reduced receptor localization to cell surface by one-third. This effect is associated with specific intracellular protein-protein interaction and the formation of a sGi2-D2-receptor complex. It has been suggested that the formation of this complex serves to prevent D2 receptors from reaching the cell membrane. Treatment of D2-receptor-expressing cells with agonists increased the number of cell surface D2 receptors and coincided with a reduction in these receptors from intracellular complexes, suggesting that agonist treatment released D2 receptors from the complex allowing them to localize to the cell membrane. Thus, in addition to elucidating how the intracellular pool of D2 receptor functions, our findings uncover a novel mechanism regulating the density of cell surface D2 receptors.

Novel insights in dopamine receptor physiology

The dopaminergic system has a pivotal role in the central nervous system but also plays important roles in the periphery, mainly in the endocrine system. Dopamine exerts its functions via five different receptors, named D(1)-D(5), belonging to the category of G protein coupled membrane receptors. Dopamine receptors are heterogeneously expressed in different cells, tissues and organs, where they stimulate or inhibit different functions, including neurotransmission and hormone synthesis and secretion. In particular, the dopamineric system has a pivotal role in the physiological regulation of the hypothalamus-pituitary-adrenal axis. Recent data have demonstrated the expression and function of dopamine receptors not only in endocrine organs but also in endocrine tumors, mainly those belonging to the hypothalamus-pituitary-adrenal axis, and also in the so-called 'neuroendocrine' tumors. These data confirm the important role of the dopaminergic system in this endocrine axis, as well as in the neuroendocrine system. This review summarizes the main structural and functional characteristics of dopamine receptors, emphasizing the most recent novelties, and focused on the physiological and pathological regulation of the hypothalamus-pituitary-adrenal axis by the dopaminergic system. In addition, the recent findings on the relationship between dopamine receptors and neuroendocrine tumors are summarized.

Expression of D2 receptor isoforms in cultured neurons reveals equipotent autoreceptor function

Alternative splicing of the dopamine D2 receptor gene produces two distinct isoforms referred to as D2long (D2L) and D2short (D2S). In mesencephalic dopamine neurons, inhibition of the firing rate through activation of somatodendritic D2 receptors and blockade of neurotransmitter release through stimulation of terminal D2 receptors represent major roles of D2 autoreceptors. Recently, data obtained from D2L-deficient mice suggested that D2S acts as the preferential D2 autoreceptor. In the present study, we investigate whether this D2 isoform-specific autoreceptor function is linked to differences in the subcellular localization and/or signaling properties of the D2S and D2L using mesencephalic neurons transfected with enhanced green fluorescent protein (EGFP)-tagged receptors. Our results show that EGFP-tagged D2S and D2L are localized to the axonal and somatodendritic compartments of mesencephalic neurons. In addition, we demonstrate that EGFP-tagged D2S and D2L regulate cellular excitability, neurotransmitter release and basal levels of intracellular calcium with similar effectiveness. Overall, our morphological and electrophysiological studies suggest that the major D2 autoreceptor function attributed to D2S is likely explained by the predominant expression of this isoform in dopamine neurons rather than by distinct subcellular localization and signaling properties of D2S and D2L.

Direct analysis of the genes encoding G proteins G alpha T2, G alpha o, G alpha Z in ADHD

We have followed up the extensive replicated evidence that the dopamine DRD4 receptor is involved in the aetiology of ADHD by undertaking direct analysis of genes encoding other proteins in this effector system. We prioritised the genes encoding G protein alpha subunits G alpha(T2), G alpha(o), G alpha(Z) as these have been shown to transduce the effects of ligand binding at DRD4. We screened the exons of all three genes for sequence variation in 28 unrelated subjects with ADHD and identified 13 novel polymorphisms. All were tested for possible association with ADHD using a combination of pooled and individual genotyping. The results of our study do not suggest that polymorphisms in these genes contribute to susceptibility to ADHD.

S32504, a novel naphtoxazine agonist at dopamine D3/D2 receptors: I. Cellular, electrophysiological, and neurochemical profile in comparison with ropinirole

S32504 [(+)-trans-3,4,4a,5,6,10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth[1,2-b]-1,4-oxazine] displayed marked affinity for cloned, human (h)D(3) receptors (pK(i), 8.1) at which, in total G-protein ([(35)S]GTPgammaS binding, guanosine-5'-O-(3-[(35)S]thio)-triphosphate), Galpha(i3) (antibody capture/scintillation proximity), and mitogen-activated protein kinase (immunoblot) activation procedures, it behaved as an agonist: pEC(50) values, 8.7, 8.6, and 8.5, respectively. These actions were blocked by haloperidol and the selective D(3) receptor antagonist S33084 [(3aR,9bS)-N-[4-(8-cyano-1,3a,4,9b-tetrahydro-3H-benzopyrano[3,4-c]pyrrole-2-yl)-butyl]-(4-phenyl) benzamide)]. S32504 showed lower potency at hD(2S) and hD(2L) receptors in [(35)S]GTPgammaS binding (pEC(50) values, 6.4 and 6.7) and antibody capture/scintillation proximity (hD(2L), pEC(50), 6.6) procedures. However, reflecting signal amplification, it potently stimulated hD(2L) receptor-coupled mitogen-activated protein kinase (pEC(50), 8.6). These actions were blocked by haloperidol and the selective D(2) receptor antagonist L741,626 [4-(4-chlorophenyl)-1-(1H-indol-3-ylmethyl)piperidin-4-ol]. The affinity of S32504 for hD(4) receptors was low (5.3) and negligible for hD(1) and hD(5) receptors (pK(i), <5.0). S32504 showed weak agonist properties at serotonin(1A) ([(35)S]GTPgammaS binding, pEC(50), 5.0) and serotonin(2A) (G(q), pEC(50), 5.2) receptors and low affinity for other (>50) sites. In anesthetized rats, S32504 (0.0025-0.01 mg/kg, i.v.) suppressed electrical activity of ventrotegmental dopaminergic neurons. Correspondingly, S32504 (0.0025-0.63 mg/kg, s.c.) potently reduced dialysis levels (and synthesis) of dopamine in striatum, nucleus accumbens, and frontal cortex of freely moving rats, actions blocked by haloperidol and L741,626 but not by S33084. In contrast, S32504 only weakly inhibited serotonergic transmission and failed to affect noradrenergic transmission. Actions of S32504 were expressed stereospecifically versus its less active enantiomer S32601 [(-)-trans-3,4,4a,5,6,10b-hexahydro-9-carbomoyl-4-propyl-2H-naphth[1,2-b]-1,4-oxazine]. Although the D(3)/D(2) agonist and antiparkinsonian agent ropinirole mimicked the profile of S32504, it was less potent. In conclusion, S32504 is a potent and selective agonist at dopamine D(3) and D(2) receptors.

Ethanol and estradiol modulate alternative splicing of dopamine D2 receptor messenger RNA and abolish the inhibitory action of bromocriptine on prolactin release from the pituitary gland

BACKGROUND

Several reports show evidence for the existence of high levels of prolactin (PRL) in alcoholic men and women. Previously we have shown that ethanol increases PRL release both in vivo and in vitro. How ethanol increases PRL release is not well understood.

METHODS

In this study, we determined the effects of ethanol in the presence and absence of estradiol-17 beta on PRL messenger RNA (mRNA) levels, dopamine D2 receptor mRNA splicing, and the PRL-inhibitory response of a dopaminergic agent, bromocriptine, in the pituitary of Fischer-344 rats and in primary cultures of anterior pituitary cells. Real-time reverse transcriptase-polymerase chain reaction was used for mRNA detection, and radioimmunoassay was used for hormone detection.

RESULTS

Estradiol and ethanol alone increased PRL mRNA expression in the pituitary gland. Ethanol also potentiated estradiol action on PRL mRNA expression in the pituitary. Determination of the D2 receptor splicing, by determining the changes in the percentage of D2 receptor mRNA expressed as its long form (D2L) and as its short form (D2S), revealed that both ethanol and estradiol altered D2 receptor splicing. Ethanol and estradiol, alone and together, increased the percentage of the D2L receptor but decreased the D2S receptor percentage. Similarly, ethanol and estradiol alone and in combination increased D2L, but decreased the D2S receptor percentage in primary cultures of pituitary cells. Evaluation of bromocriptine's inhibition of PRL release in primary cultures of pituitary cells indicated that ethanol reduced the ability of this D2 receptor agonist to inhibit PRL release.

CONCLUSIONS

These results confirm estradiol's inhibition of D2 function and provide novel evidence that ethanol, like estradiol, reduces dopamine's ability to inhibit PRL release by modifying alternative splicing of the dopamine D2 receptor in the pituitary.

The familial hemiplegic migraine mutation R192Q reduces G-protein-mediated inhibition of P/Q-type (Ca(V)2.1) calcium channels expressed in human embryonic kidney cells

Familial hemiplegic migraine is associated with at least 13 different missense mutations in the alpha1A Ca(2+) channel subunit. Some of these mutations have been shown to affect the biophysical properties of alpha1A currents. To date, no study has examined the influence of such mutations on the G-protein regulation of channel function. Because G-proteins inhibit movement of the voltage sensor, we examined the effects of the R192Q mutation, which neutralizes a positive charge in the first S4 segment. Human wild-type (WT) or R192Q mutant channels were expressed in human embryonic kidney tsA-201 cells along with dopamine D2 receptors. Application of quinpirole induced fast (approximately 1 s), pertussis toxin-sensitive inhibition of alpha1A(WT) and alpha1A(R192Q) Ca(2+) currents, consistent with the activation of a membrane-delimited pathway. alpha1A(WT) Ca(2+) currents were inhibited by 62.9 +/- 0.9 % (n = 27), whereas alpha1A(R192Q) Ca(2+) currents were inhibited by only 47.9 +/- 1.8 % (n = 35; P < 0.001). Concentration-response analysis showed that only the extent of inhibition was affected, with no change in agonist potency (EC(50) = 1 nM). Prepulse facilitation, which is a characteristic of voltage-dependent inhibition, was also reduced by the R192Q mutation. However, the kinetics of facilitation and slow activation were not affected, suggesting that G-protein-Ca(2+) channel affinity was unchanged. These results show that the R192Q mutation reduces the G-protein inhibition of P/Q-type Ca(2+) channels, probably by altering mechanisms by which Gbetagamma subunit binding induces a change in channel gating. Altered G-protein modulation and the consequent reduced presynaptic inhibition may contribute to migraine attacks by favouring a persistent state of hyperexcitability.

Differential actions of antiparkinson agents at multiple classes of monoaminergic receptor. II. Agonist and antagonist properties at subtypes of dopamine D(2)-like receptor and alpha(1)/alpha(2)-adrenoceptor

The accompanying multivariate analysis of the binding profiles of antiparkinson agents revealed contrasting patterns of affinities at diverse classes of monoaminergic receptor. Herein, we characterized efficacies at human (h)D(2SHORT(S)), hD(2LONG(L)), hD(3), and hD(4.4) receptors and at halpha(2A)-, halpha(2B)-, halpha(2C)-, and halpha(1A)-adrenoceptors (ARs). As determined by guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding, no ligand displayed "full" efficacy relative to dopamine (100%) at all "D(2)-like" sites. However, at hD(2S) receptors quinpirole, pramipexole, ropinirole, quinerolane, pergolide, and cabergoline were as efficacious as dopamine (E(max)100%); TL99, talipexole, and apomorphine were highly efficacious (79-92%); piribedil, lisuride, bromocriptine, and terguride showed intermediate efficacy (40-55%); and roxindole displayed low efficacy (11%). For all drugs, efficacies were lower at hD(2L) receptors, with terguride and roxindole acting as antagonists. At hD(3) receptors, efficacies ranged from 33% (roxindole) to 94% (TL99), whereas, for hD(4) receptors, highest efficacies (approximately 70%) were seen for quinerolane, quinpirole, and TL99, whereas piribedil and terguride behaved as antagonists and bromocriptine was inactive. Although efficacies at hD(2S) versus hD(2L) sites were highly correlated (r = 0.79), they correlated only modestly to hD(3)/hD(4) sites (r = 0.44-0.59). In [(35)S]GTPgammaS studies of halpha(2A)-ARs, TL99 (108%), pramipexole (52%), talipexole (51%), pergolide (31%), apomorphine (16%), and quinerolane (11%) were agonists and ropinirole and roxindole were inactive, whereas piribedil and other agents were antagonists. Similar findings were obtained at halpha(2B)- and halpha(2C)-ARs. Using [(3)H]phosphatidylinositol depletion, roxindole, bromocriptine, lisuride, and terguride displayed potent antagonist properties at halpha(1A)-ARs. In conclusion, antiparkinson agents display diverse agonist and antagonist properties at multiple subtypes of D(2)-like receptor and alpha(1)/alpha(2)-AR, actions, which likely contribute to their contrasting functional profiles.

Functional selectivity of dopamine receptor agonists. II. Actions of dihydrexidine in D2L receptor-transfected MN9D cells and pituitary lactotrophs

D(2)-like dopamine receptors mediate functional changes via activation of inhibitory G proteins, including those that affect adenylate cyclase activity, and potassium and calcium channels. Although it is assumed that the binding of a drug to a single isoform of a D(2)-like receptor will cause similar changes in all receptor-mediated functions, it has been demonstrated in brain that the dopamine agonists dihydrexidine (DHX) and N-n-propyl-DHX are "functionally selective". The current study explores the underlying mechanism using transfected MN9D cells and D(2)-producing anterior pituitary lactotrophs. Both dopamine and DHX inhibited adenylate cyclase activity in a concentration-dependent manner in both systems, effects blocked by D(2), but not D(1), antagonists. In the MN9D cells, quinpirole and R-(-)-N-propylnorapomorphine (NPA) also inhibited the K(+)-stimulated release of [(3)H]dopamine in a concentration-responsive, antagonist-reversible manner. Conversely, neither DHX, nor its analogs, inhibited K(+)-stimulated [(3)H]dopamine release, although they antagonized the effects of quinpirole. S-(+)-NPA actually had the reverse functional selectivity profile from DHX (i.e., it was a full agonist at D(2L) receptors coupled to inhibition of dopamine release, but a weak partial agonist at D(2L) receptor-mediated inhibition of adenylate cyclase). In lactotrophs, DHX had little intrinsic activity at D(2) receptors coupled to G protein-coupled inwardly rectifying potassium channels, and actually antagonized the effects of dopamine at these D(2) receptors. Together, these findings provide compelling evidence for agonist-induced functional selectivity with the D(2L) receptor. Although the underlying molecular mechanism is controversial (e.g., "conformational induction" versus "drug-active state selection"), such data are irreconcilable with the widely held view that drugs have "intrinsic efficacy".

Constitutive expression of functional GABA(B) receptors in mIL-tsA58 cells requires both GABA(B(1)) and GABA(B(2)) genes

Studies of gamma-aminobutyric acid (GABA)(B) receptor function in heterologous cell systems have suggested that expression of two distinct seven transmembrane G-protein coupled receptor subunits is necessary for receptor activation and signal transduction. Some results suggest that both receptor proteins must be inserted into the plasma membrane to create heterodimers; however, it is possible that subunit monomers or homodimers are functional in cells which constitutively express GABA(B) receptors. A new pituitary intermediate lobe melanotrope cell clone (mIL tsA58) has been isolated which constitutively expresses GABA(B), D(2) and corticotrophin releasing factor receptors. Here, we report on characterization of the GABA(B) receptors. Solution hybridization-nuclease protection assays reveal the presence of GABA(B(1)) and GABA(B(2)) transcripts. Western blots show GABA(B(1a)) and one of two GABA(B(2)) proteins. Addition of the GABA(B) agonist baclofen to cultured mIL-tsA58 (mIL) cells inhibits high voltage activated Ca(2+) channels, as measured by agonist-induced inhibition of the K(+)-depolarization-stimulated increase in Ca(2+) influx. CGP55845, a GABA(B) antagonist, blocks the response to baclofen. Knockdown of either GABA(B(1)) or GABA(B(2)) subunits with selective antisense oligodeoxynucleotides reduced GABA(B) protein levels and completely abolished the GABA(B) receptor response in the mIL cells. Taken together, these results indicate that functionally active GABA(B) receptors in mIL cells require the constitutive expression of both GABA(B) genes. This is a physiologic validation of results from recombinant overexpression in naive cells and shows that the mIL cell line is a useful model for studying GABA(B) receptor expression, regulation and function.

Coupling of dopamine receptor subtypes to multiple and diverse G proteins

The family of five dopamine receptors subtypes activate cellular effector systems through G proteins. Historically, dopamine receptors were thought to only stimulate or inhibit adenylyl cyclase, by coupling to either G(s)alpha or G(i)alpha, respectively. Recent studies in transfected cells, reviewed here, have shown that multiple and highly diverse signaling pathways are activated by specific dopamine receptor subtypes. This multiplicity of signaling responses occurs through selective coupling to distinct G proteins and each of the receptors can interact with more than one G protein. Although some of the multiple coupling of dopamine receptors to different G proteins occurs from within the same family of G proteins, these receptors can also couple to G proteins belonging to different families. Such multiple interactions between receptors and G proteins elicits functionally distinct physiological effects which acts to enhance and subsequently suppress the original receptor response, and to activate apparently distinct signaling pathways. In the brain, where coexpression of functionally distinct receptors in heterogeneous cells further adds to the complexity of dopamine signaling, minor alterations in receptor/G protein coupling states during either development or in adults, may underlie the imbalanced signaling seen in dopaminergic-linked diseases such as schizophrenia, Parkinson's disease and attention deficit hyperactivity disorder.