Direct dopamine D2-receptor-mediated modulation of arachidonic acid release in transfected CHO cells without the concomitant administration of a Ca2+-mobilizing agent

Modulation of CaV1.3b L-type calcium channels by M1 muscarinic receptors varies with CaVβ subunit expression

OBJECTIVES

We examined whether two G protein-coupled receptors (GPCRs), muscarinic M1 receptors (M1Rs) and dopaminergic D2 receptors (D2Rs), utilize endogenously released fatty acid to inhibit L-type Ca2+ channels, CaV1.3. HEK-293 cells, stably transfected with M1Rs, were used to transiently transfect D2Rs and CaV1.3b with different CaVβ-subunits, allowing for whole-cell current measurement from a pure channel population.

RESULTS

M1R activation with Oxotremorine-M inhibited currents from CaV1.3b coexpressed with α2δ-1 and a β1b, β2a, β3, or β4-subunit. Surprisingly, the magnitude of inhibition was less with β2a than with other CaVβ-subunits. Normalizing currents revealed kinetic changes after modulation with β1b, β3, or β4, but not β2a-containing channels. We then examined if D2Rs modulate CaV1.3b when expressed with different CaVβ-subunits. Stimulation with quinpirole produced little inhibition or kinetic changes for CaV1.3b coexpressed with β2a or β3. However, quinpirole inhibited N-type Ca2+ currents in a concentration-dependent manner, indicating functional expression of D2Rs. N-current inhibition by quinpirole was voltage-dependent and independent of phospholipase A2 (PLA2), whereas a PLA2 antagonist abolished M1R-mediated N-current inhibition. These findings highlight the specific regulation of Ca2+ channels by different GPCRs. Moreover, tissue-specific and/or cellular localization of CaV1.3b with different CaVβ-subunits could fine tune the response of Ca2+ influx following GPCR activation.

Inflamed moods: a review of the interactions between inflammation and mood disorders

Mood disorders have been recognized by the World Health Organization (WHO) as the leading cause of disability worldwide. Notwithstanding the established efficacy of conventional mood agents, many treated individuals continue to remain treatment refractory and/or exhibit clinically significant residual symptoms, cognitive dysfunction, and psychosocial impairment. Therefore, a priority research and clinical agenda is to identify pathophysiological mechanisms subserving mood disorders to improve therapeutic efficacy. During the past decade, inflammation has been revisited as an important etiologic factor of mood disorders. Therefore, the purpose of this synthetic review is threefold: 1) to review the evidence for an association between inflammation and mood disorders, 2) to discuss potential pathophysiologic mechanisms that may explain this association and 3) to present novel therapeutic options currently being investigated that target the inflammatory-mood pathway. Accumulating evidence implicates inflammation as a critical mediator in the pathophysiology of mood disorders. Indeed, elevated levels of pro-inflammatory cytokines have been repeatedly demonstrated in both major depressive disorder (MDD) and bipolar disorder (BD) patients. Further, the induction of a pro-inflammatory state in healthy or medically ill subjects induces 'sickness behavior' resembling depressive symptomatology. Potential mechanisms involved include, but are not limited to, direct effects of pro-inflammatory cytokines on monoamine levels, dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis, pathologic microglial cell activation, impaired neuroplasticity and structural and functional brain changes. Anti-inflammatory agents, such as acetyl-salicylic acid (ASA), celecoxib, anti-TNF-α agents, minocycline, curcumin and omega-3 fatty acids, are being investigated for use in mood disorders. Current evidence shows improved outcomes in mood disorder patients when anti-inflammatory agents are used as an adjunct to conventional therapy; however, further research is needed to establish the therapeutic benefit and appropriate dosage.

Elevated immune-inflammatory signaling in mood disorders: a new therapeutic target?

Converging translational evidence has implicated elevated immune-inflammatory signaling activity in the pathoetiology of mood disorders, including major depressive disorder and bipolar disorder. This is supported in part by cross-sectional evidence for increased levels of proinflammatory eicosanoids, cytokines and acute-phase proteins during mood episodes, and prospective longitudinal evidence for the emergence of mood symptoms in response to chronic immune-inflammatory activation. In addition, mood-stabilizer and atypical antipsychotic medications downregulate initial components of the immune-inflammatory signaling pathway, and adjunctive treatment with anti-inflammatory agents augment the therapeutic efficacy of antidepressant, mood stabilizer and atypical antipsychotic medications. Potential pathogenic mechanisms linked with elevated immune-inflammatory signaling include perturbations in central serotonin neurotransmission and progressive white matter pathology. Both heritable genetic factors and environmental factors including dietary fatty-acid composition may act in concert to sustain elevated immune-inflammatory signaling. Collectively, these data suggest that elevated immune-inflammatory signaling is a mechanism that is relevant to the pathoetiology of mood disorders, and may therefore represent a new therapeutic target for the development of more effective treatments.

The utility of (11)C-arachidonate PET to study in vivo dopaminergic neurotransmission in humans

We developed a novel method to study dopaminergic neurotransmission using positron emission tomography (PET) with [1-(11)C]arachidonic acid ([1-(11)C]AA). Previous preclinical studies have shown the utility of [1-(11)C]AA as a marker of signal transduction coupled to cytosolic phospholipase A(2) (cPLA(2)). Using [1-(11)C]AA and [(15)O]water PET, we measured regional incorporation coefficients K(*) for AA and regional cerebral blood flow (rCBF), respectively, in healthy male volunteers given the D(1)/D(2) agonist (10 or 20 μg/kg subcutaneous) apomorphine. We confirmed a robust central dopaminergic response to apomorphine by observing significant increases in the serum concentration of growth hormone. We observed significant increases, as well as decreases in K(*) and increases in rCBF in response to apomorphine. These changes remained significant after covarying for handedness and apomorphine dosage. The magnitude of increases in K(*) was lower than those in our previous animal experiments, likely reflecting the smaller dose of apomorphine used in the current human study. Changes in K(*) may reflect neuronal signaling downstream of activated D(2)-like receptors coupled to cPLA(2). Changes in rCBF are consistent with previous studies showing net functional effects of D(1)/D(2) activation. [1-(11)C]AA PET may be useful for studying disturbances of dopaminergic neurotransmission in conditions such as Parkinson's disease and schizophrenia.

Imaging brain signal transduction and metabolism via arachidonic and docosahexaenoic acid in animals and humans

The polyunsaturated fatty acids (PUFAs), arachidonic acid (AA, 20:4n-6) and docosahexaenoic acid (DHA, 22:6n-3), important second messengers in brain, are released from membrane phospholipid following receptor-mediated activation of specific phospholipase A(2) (PLA(2)) enzymes. We developed an in vivo method in rodents using quantitative autoradiography to image PUFA incorporation into brain from plasma, and showed that their incorporation rates equal their rates of metabolic consumption by brain. Thus, quantitative imaging of unesterified plasma AA or DHA incorporation into brain can be used as a biomarker of brain PUFA metabolism and neurotransmission. We have employed our method to image and quantify effects of mood stabilizers on brain AA/DHA incorporation during neurotransmission by muscarinic M(1,3,5), serotonergic 5-HT(2A/2C), dopaminergic D(2)-like (D(2), D(3), D(4)) or glutamatergic N-methyl-d-aspartic acid (NMDA) receptors, and effects of inhibition of acetylcholinesterase, of selective serotonin and dopamine reuptake transporter inhibitors, of neuroinflammation (HIV-1 and lipopolysaccharide) and excitotoxicity, and in genetically modified rodents. The method has been extended for the use with positron emission tomography (PET), and can be employed to determine how human brain AA/DHA signaling and consumption are influenced by diet, aging, disease and genetics.

Chronic valproate treatment blocks D2-like receptor-mediated brain signaling via arachidonic acid in rats

BACKGROUND AND OBJECTIVE

Hyperdopaminergic signaling and an upregulated brain arachidonic acid (AA) cascade may contribute to bipolar disorder (BD). Lithium and carbamazepine, FDA-approved for the treatment of BD, attenuate brain dopaminergic D(2)-like (D(2), D(3), and D(4)) receptor signaling involving AA when given chronically to awake rats. We hypothesized that valproate (VPA), with mood-stabilizing properties, would also reduce D(2)-like-mediated signaling via AA.

METHODS

An acute dose of quinpirole (1 mg/kg) or saline was administered to unanesthetized rats that had been treated for 30 days with a therapeutically relevant dose of VPA (200 mg/kg/day) or vehicle. Regional brain AA incorporation coefficients, k*, and incorporation rates, J(in), markers of AA signaling and metabolism, were measured by quantitative autoradiography after intravenous [1-(14)C]AA infusion. Whole brain concentrations of prostaglandin (PG)E(2) and thromboxane (TX)B(2) also were measured.

RESULTS

Quinpirole compared to saline significantly increased k* in 40 of 83 brain regions, and increased brain concentrations of PGE(2) in chronic vehicle-treated rats. VPA treatment by itself reduced concentrations of plasma unesterified AA and whole brain PGE(2) and TXB(2), and blocked the quinpirole-induced increments in k* and PGE(2).

CONCLUSION

These results further provide evidence that mood stabilizers downregulate brain dopaminergic D(2)-like receptor signaling involving AA.

Brain arachidonic acid cascade enzymes are upregulated in a rat model of unilateral Parkinson disease

Arachidonic acid (AA) signaling is upregulated in the caudate-putamen and frontal cortex of unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, a model for asymmetrical Parkinson disease. AA signaling can be coupled to D(2)-like receptor initiated AA hydrolysis from phospholipids by cytosolic phospholipase A(2) (cPLA(2)) and subsequent metabolism by cyclooxygenase (COX)-2. In unilaterally 6-OHDA- and sham-lesioned rats, we measured brain expression of cPLA(2), other PLA(2) enzymes, and COX-2. Activity and protein levels of cPLA(2) were significantly higher as was COX-2-protein in caudate-putamen, frontal cortex and remaining brain on the lesioned compared to intact side of the 6-OHDA lesioned rats, and compared to sham brain. Secretory sPLA(2) and Ca(2+)-independent iPLA(2) expression did not differ between sides or groups. Thus, the tonically increased ipsilateral AA signal in the lesioned rat corresponds to upregulated cPLA(2) and COX-2 expression within the AA metabolic cascade, which may contribute to symptoms and pathology in Parkinson disease.

Chronic d-amphetamine depresses an imaging marker of arachidonic acid metabolism in rat brain

Acute d-amphetamine (d-Amph) administration to rats leads to the release of arachidonic acid (AA, 20:4n-6) as a second messenger following indirect agonism at dopamine D2-like receptors in the brain. We hypothesized that chronically administered d-Amph in rats also would alter brain AA metabolism and signalling. To test this, adult male rats were injected i.p. daily for 2 wk with saline or 2.5 mg/kg d-Amph. After a 1-d washout, the unanaesthetized rats were injected acutely with i.v. saline, 1 mg/kg quinpirole (a D2-like receptor agonist) or 5.0 mg/kg SKF-38393 (a D1-like receptor agonist), followed by i.v. [1-14C]AA. The AA incorporation coefficient k* (brain radioactivity/integrated plasma radioactivity), a marker of AA signalling and metabolism, was quantified using autoradiography in each of 62 brain regions. Compared with chronic saline, chronic d-Amph widely decreased baseline values of k* in brain regions having D2-like receptors. On the other hand, chronic amphetamine did not alter the k* responses to quinpirole seen in chronic saline-treated rats. SKF-38393 had minimal effects on k* in both chronic saline-treated and amphetamine-treated rats, consistent with D1-like receptors not being coupled to AA signalling. The ability of chronic d-Amph after 1-d washout to down-regulate baseline values of k* probably reflects neuroplastic changes in brain AA signalling, and may correspond to depressive behaviours noted following withdrawal from chronic amphetamine in humans and in rats.

Imaging apomorphine stimulation of brain arachidonic acid signaling via D2-like receptors in unanesthetized rats

RATIONALE AND OBJECTIVE

Because of the important role of dopamine in neurotransmission, it would be useful to be able to image brain dopamine receptor-mediated signal transduction in animals and humans. Administering the D1-D2 receptor agonist apomorphine may allow us to do this, as the D2-like receptor is reported to be coupled to cytosolic phospholipase A2 activation and arachidonic acid (AA) release from membrane phospholipid.

METHODS

Unanesthetized adult rats were given intraperitoneally apomorphine (0.5 mg/kg) or saline, with or without pretreatment with 6 mg/kg intravenous raclopride, a D2/D3 receptor antagonist. [1-14C]AA was injected intravenously, then AA incorporation coefficients k*--brain radioactivity divided by integrated plasma radioactivity--markers of AA signaling, were measured using quantitative autoradiography in 62 brain regions.

RESULTS

Apomorphine significantly elevated k* in 26 brain regions, including the frontal cortex, motor and somatosensory cortex, caudate-putamen, thalamic nuclei, and nucleus accumbens. Raclopride alone did not change baseline values of k*, but raclopride pretreatment prevented the apomorphine-induced increments in k*.

CONCLUSIONS

A mixed D1-D2 receptor agonist, apomorphine, increased the AA signal by activating only D2-like receptors in brain circuits containing regions with high D2-like receptor densities. Thus, apomorphine might be used with positron emission tomography to image brain D2-like receptor-mediated AA signaling in humans in health and disease.

In vivo imaging of disturbed pre- and post-synaptic dopaminergic signaling via arachidonic acid in a rat model of Parkinson's disease

BACKGROUND

Parkinson's disease involves loss of dopamine (DA)-producing neurons in the substantia nigra, associated with fewer pre-synaptic DA transporters (DATs) but more post-synaptic dopaminergic D2 receptors in terminal areas of these neurons.

HYPOTHESIS

Arachidonic acid (AA) signaling via post-synaptic D2 receptors coupled to cytosolic phospholipase A2 (cPLA2) will be reduced in terminal areas ipsilateral to a chronic unilateral substantia nigra lesion in rats given D-amphetamine, which reverses the direction of the DAT, but will be increased in rats given quinpirole, a D2-receptor agonist.

METHODS

D-amphetamine (5.0 mg/kg i.p.), quinpirole (1.0 mg/kg i.v.), or saline was administered to unanesthetized rats having a chronic unilateral lesion of the substantia nigra. AA incorporation coefficients, k* (radioactivity/integrated plasma radioactivity), markers of AA signaling, were measured using quantitative autoradiography in 62 bilateral brain regions following intravenous [1-(14)C]AA.

RESULTS

In rats given saline (baseline), k* was elevated in 13 regions in the lesioned compared with intact hemisphere. Quinpirole increased k* in frontal cortical and basal ganglia regions bilaterally, more so in the lesioned than intact hemisphere. D-amphetamine increased k* bilaterally but less so in the lesioned hemisphere.

CONCLUSIONS

Increased baseline elevations of k* and increased responsiveness to quinpirole in the lesioned hemisphere are consistent with their higher D2-receptor and cPLA2 activity levels, whereas reduced responsiveness to D-amphetamine is consistent with dropout of pre-synaptic elements containing the DAT. In vivo imaging of AA signaling using dopaminergic drugs can identify pre- and post-synaptic DA changes in animal models of Parkinson's disease.

Dopamine D4 receptor knockout mice exhibit neurochemical changes consistent with decreased dopamine release

Dopamine D4 receptor (D4R) knockout mice (D4R-/-) provided for unique neurochemical studies designed to understand D4R contributions to dopamine (DA) regulation. In this study, post-mortem brain tissue content of DA did not differ between D4R+/+ and D4R-/- mice in the striatum (Str) or nucleus accumbens core (NAc). However, there was a significant decrease (82%) in the content of 3,4-dihydoxyphenylacetic acid (DOPAC), a major metabolite of DA, in the NAc of D4R-/- mice. Microdialysis studies performed in a region of brain spanning of the dorsal Str and NAc showed lower baseline levels of DA and a significant reduction in KCl-evoked overflow of DA in the D4R-/- mice. Baseline extracellular levels of DOPAC and homovanillic acid were also significantly lower in the D4R-/- mice. In vivo chronoamperometric recordings of KCl-evoked release of DA also showed decreased release of DA in the Str and NAc of the D4R-/- mice. These studies demonstrate a role of D4Rs in presynaptic DA regulation and support the hypothesis that alterations in D4Rs may lead to diminished DA function.

Abnormalities in the fatty acid composition of the postmortem orbitofrontal cortex of schizophrenic patients: gender differences and partial normalization with antipsychotic medications

Previous studies have observed significant abnormalities in the fatty acid composition of peripheral tissues from drug-naïve first-episode schizophrenic (SZ) patients relative to normal controls, including deficits in omega-3 and omega-6 polyunsaturated fatty acids, which are partially normalized following chronic antipsychotic treatment. We hypothesized that postmortem cortical tissue from patients with SZ would also exhibit deficits in cortical docosahexaenoic acid (DHA, 22:6n-3) and arachidonic acid (AA; 20:4n-6) relative to normal controls, and that these deficits would be greater in drug-free SZ patients. We determined the total fatty acid composition of postmortem orbitofrontal cortex (OFC) (Brodmann area 10) from drug-free and antipsychotic-treated SZ patients (n=21) and age-matched normal controls (n=26) by gas chromatography. After correction for multiple comparisons, significantly lower DHA (-20%) concentrations, and significantly greater vaccenic acid (VA) (+12.5) concentrations, were found in the OFC of SZ patients relative to normal controls. Relative to age-matched same-gender controls, OFC DHA deficits, and elevated AA:DHA, oleic acid:DHA and docosapentaenoic acid (22:5n-6):DHA ratios, were found in male but not female SZ patients. SZ patients that died of cardiovascular-related disease exhibited lower DHA (-31%) and AA (-19%) concentrations, and greater OA (+20%) and VA (+17%) concentrations, relative to normal controls that also died of cardiovascular-related disease. OFC DHA and AA deficits, and elevations in oleic acid and vaccenic acid, were numerically greater in drug-free SZ patients and were partially normalized in SZ patients treated with antipsychotic medications (atypical>typical). Fatty acid abnormalities could not be wholly attributed to lifestyle or postmortem tissue variables. These findings add to a growing body of evidence implicating omega-3 fatty acid deficiency as well as the OFC in the pathoaetiology of SZ, and suggest that abnormalities in OFC fatty acid composition may be gender-specific and partially normalized by antipsychotic medications.

D-Amphetamine stimulates D2 dopamine receptor-mediated brain signaling involving arachidonic acid in unanesthetized rats

In rat brain, dopaminergic D(2)-like but not D(1)-like receptors can be coupled to phospholipase A(2) (PLA(2)) activation, to release the second messenger, arachidonic acid (AA, 20:4n-6), from membrane phospholipids. In this study, we hypothesized that D-amphetamine, a dopamine-releasing agent, could initiate such AA signaling. The incorporation coefficient, k* (brain radioactivity/integrated plasma radioactivity) for AA, a marker of the signal, was determined in 62 brain regions of unanesthetized rats that were administered i.p. saline, D-amphetamine (2.5 or 0.5 mg/kg i.p.), or the D(2)-like receptor antagonist raclopride (6 mg/kg, i.v.) before saline or 2.5 mg/kg D-amphetamine. After injecting [1-(14)C]AA intravenously, k* was measured by quantitative autoradiography. Compared to saline-treated controls, D-amphetamine 2.5 mg/kg i.p. increased k* significantly in 27 brain areas rich in D(2)-like receptors. Significant increases were evident in neocortical, extrapyramidal, and limbic regions. Pretreatment with raclopride blocked the increments, but raclopride alone did not alter baseline values of k*. In independent experiments, D-amphetamine 0.5 mg/kg i.p. increased k* significantly in only seven regions, including the nucleus accumbens and layer IV neocortical regions. These results indicate that D-amphetamine can indirectly activate brain PLA(2) in the unanesthetized rat, and that activation is initiated entirely at D(2)-like receptors. D-Amphetamine's low-dose effects are consistent with other evidence that the nucleus accumbens, considered a reward center, is particularly sensitive to the drug.

Serum calcium-independent phospholipase A2 activity in bipolar affective disorder

OBJECTIVE

Phospholipases A2 (PLA2) are a family of enzymes involved in membrane phospholipid metabolism and cell signalling. The gene encoding one form, type VI calcium-independent phospholipase A2, is located in a region of DNA that may contain a gene important in the aetiology of psychosis. Moreover, the activity of calcium-independent PLA2 is reported to be elevated in the blood and brain of patients with schizophrenia. In this study we determined whether a similar change takes place in patients with bipolar disorder with and without a history of psychosis.

METHODS

Serum calcium-independent and -dependent PLA2 activities were determined in 24 patients with bipolar I disorder.

RESULTS

Serum calcium-independent and -dependent PLA2 activities in bipolar cases did not differ significantly from that in healthy volunteers (HVs). However, calcium-independent PLA2 activity was significantly (p < 0.05) higher in patients with a history of psychosis compared with those with no history of psychosis (by 55%) or to HVs (by 31%).

CONCLUSIONS

Our data suggest that a subset of bipolar I disorder patients with a history of psychosis have elevated calcium-independent PLA2 activity. Given that this enzyme activity is also increased in schizophrenia, elevated rates of phospholipid turnover mediated by the enzyme could represent a common biochemical feature of psychotic illness.

D2 but not D1 dopamine receptor stimulation augments brain signaling involving arachidonic acid in unanesthetized rats

RATIONALE AND OBJECTIVES

Signal transduction involving the activation of phospholipase A2 (PLA2) to release arachidonic acid (AA) from membrane phospholipids, when coupled to dopamine D1- and D2-type receptors, can be imaged in rats having a chronic unilateral lesion of the substantia nigra. It is not known, however, if the signaling responses occur in the absence of a lesion. To determine this, we used our in vivo fatty acid method to measure signaling in response to D1 and D2 receptor agonists given acutely to unanesthetized rats.

METHODS

[1-(14)C]AA was injected intravenously in unanesthetized rats, and incorporation coefficients k* for AA (brain radioactivity/integrated plasma radioactivity) were measured using quantitative autoradiography in 61 brain regions. The animals were administered i.v. the D2 receptor agonist, quinpirole (1 mg kg(-1), i.v.), the D1 receptor agonist SKF-38393 (5 mg kg(-1), i.v.), or vehicle/saline.

RESULTS

Quinpirole increased k* significantly in multiple brain regions rich in D2-type receptors, whereas SKF-38393 did not change k* significantly in any of the 61 regions examined.

CONCLUSIONS

In the intact rat brain, D2 but not D1 receptors are coupled to the activation of PLA2 and the release of AA.

Chronic lithium chloride administration to unanesthetized rats attenuates brain dopamine D2-like receptor-initiated signaling via arachidonic acid

We studied the effect of lithium chloride on dopaminergic neurotransmission via D2-like receptors coupled to phospholipase A2 (PLA2). In unanesthetized rats injected i.v. with radiolabeled arachidonic acid (AA, 20:4 n-6), regional PLA2 activation was imaged by measuring regional incorporation coefficients k* of AA (brain radioactivity divided by integrated plasma radioactivity) using quantitative autoradiography, following administration of the D2-like receptor agonist, quinpirole. In rats fed a control diet, quinpirole at 1 mg/kg i.v. increased k* for AA significantly in 17 regions with high densities of D2-like receptors, of 61 regions examined. Increases in k* were found in the prefrontal cortex, frontal cortex, accumbens nucleus, caudate-putamen, substantia nigra, and ventral tegmental area. Quinpirole, 0.25 mg/kg i.v. enhanced k* significantly only in the caudate-putamen. In rats fed LiCl for 6 weeks to produce a therapeutically relevant brain lithium concentration, neither 0.25 mg/kg nor 1 mg/kg quinpirole increased k* significantly in any region. Orofacial movements following quinpirole were modified but not abolished by LiCl feeding. The results suggest that downregulation by lithium of D2-like receptor signaling involving PLA2 and AA may contribute to lithium's therapeutic efficacy in bipolar disorder.

Dopamine Receptor Signaling

The D1-like (D1, D5) and D2-like (D2, D3, D4) classes of dopamine receptors each has shared signaling properties that contribute to the definition of the receptor class, although some differences among subtypes within a class have been identified. D1-like receptor signaling is mediated chiefly by the heterotrimeric G proteins Galphas and Galphaolf, which cause sequential activation of adenylate cyclase, cylic AMP-dependent protein kinase, and the protein phosphatase-1 inhibitor DARPP-32. The increased phosphorylation that results from the combined effects of activating cyclic AMP-dependent protein kinase and inhibiting protein phosphatase 1 regulates the activity of many receptors, enzymes, ion channels, and transcription factors. D1 or a novel D1-like receptor also signals via phospholipase C-dependent and cyclic AMP-independent mobilization of intracellular calcium. D2-like receptor signaling is mediated by the heterotrimeric G proteins Galphai and Galphao. These pertussis toxin-sensitive G proteins regulate some effectors, such as adenylate cyclase, via their Galpha subunits, but regulate many more effectors such as ion channels, phospholipases, protein kinases, and receptor tyrosine kinases as a result of the receptor-induced liberation of Gbetagamma subunits. In addition to interactions between dopamine receptors and G proteins, other protein:protein interactions such as receptor oligomerization or receptor interactions with scaffolding and signal-switching proteins are critical for regulation of dopamine receptor signaling.

Psychoses and creativity: is the missing link a biological mechanism related to phospholipids turnover?

Recent evidence suggests that genetic and biochemical factors associated with psychoses may also provide an increased propensity to think creatively. The evolutionary theories linking brain growth and diet to the appearance of creative endeavors have been made recently, but they lack a direct link to research on the biological correlates of divergent and creative thought. Expanding upon Horrobin's theory that changes in brain size and in neural microconnectivity came about as a result of changes in dietary fat and phospholipid incorporation of highly unsaturated fatty acids, we propose a theory relating phospholipase A2 (PLA2) activity to the neuromodulatory effects of the noradrenergic system. This theory offers probable links between attention, divergent thinking, and arousal through a mechanism that emphasizes optimal individual functioning of the PLA2 and NE systems as they interact with structural and biochemical states of the brain. We hope that this theory will stimulate new research in the neural basis of creativity and its connection to psychoses.

Endogenous RGS proteins facilitate dopamine D(2S) receptor coupling to G(alphao) proteins and Ca2+ responses in CHO-K1 cells

The role of RGS proteins on dopaminergic D2S receptor (D2SR) signalling was investigated in Chinese hamster ovary (CHO)-K1 cells, using recombinant RGS protein- and PTX-insensitive G alphao proteins. Dopamine-mediated [35S]GTPgammaS binding was attenuated by more than 60% in CHO-K1 D2SR cells coexpressing a RGS protein- and PTX-insensitive G(alphao)Gly184Ser:Cys351Ile protein versus cells coexpressing a similar amount of PTX-insensitive G alphaoCys351Ile protein. Dopamine-agonist-mediated Ca2+ responses were dependent on the coexpression with a G alphao Cys351Ile protein and were fully abolished upon coexpression with a G alphaoGly184Ser:Cys351Ile protein. These results suggest that interactions between the G alphao protein and RGS proteins are involved in efficient D2SR signalling.

Constitutive coupling of a chimeric dopamine D2/alpha 1B receptor to the phospholipase C pathway: inverse agonism to silent antagonism by neuroleptic drugs

Neuroleptic drugs have been suggested to act as inverse agonists at the dopamine D2 receptor, but no link between therapeutic efficacy and ligand's intrinsic activity could be determined. Since the resolving capacity to monitor inverse agonism at dopamine D2 receptors is limited, we speculated that receptor constitutive activation could be enhanced by constructing chimeric D2/alpha 1B receptors. Marked inverse agonist responses with a series of dopamine antagonists were obtained by: 1) exchange of the D 2short receptor's 3ICL by that of the alpha 1B-adrenoceptor, 2) incorporation of an activating mutation (Ala 279 Glu) in the distal portion of its 3ICL, and 3) coexpression with a G alpha11 protein. This chimeric D2/alpha 1B receptor construct displayed a ligand binding profile comparable to that of the wild-type (wt) D 2short receptor and an effector activation profile close to that of the wt alpha 1B-adrenoceptor. Most of the dopamine antagonists attenuated by -54 to -59% basal inositol phosphates (IP) formation, thus clearly acting as inverse agonists. Ziprasidone behaved as a silent antagonist (+5% versus basal IP level) and antagonized both dopamine-mediated (pK B, 7.61) and tropapride-mediated (pK B, 8.52) IP responses. Clozapine, olanzapine, and raclopride displayed partial inverse agonist properties (-31, -67, and -71% versus tropapride, respectively), whereas bromerguride (+63%) and cis-(+)-5-methoxy-1-methyl-2-(di-n-propylamino tetralin) [(+)-UH 232] (+88%) demonstrated positive agonism. In conclusion, analyses with the chimeric D2/alpha 1B Ala 279 Glu 3ICL receptor construct suggest that neuroleptic drugs can be differentiated on the basis of their intrinsic activity, as they can either activate, inhibit, or be silent at this receptor construct.

Dopamine D(2) receptor-induced COX-2-mediated production of prostaglandin E(2) in D(2)-transfected Chinese hamster ovary cells without simultaneous administration of a Ca(2+)-mobilizing agent

We have earlier demonstrated that dopamine stimulates the liberation of the prostaglandin E(2) (PGE(2)) precursor, arachidonic acid, in Chinese hamster ovary cells transfected with the rat dopamine D(2) receptor (long isoform), also without concomitant administration of a Ca(2+)-releasing agent [Nilsson et al., Br J Pharmacol 1998;124:1651-8]. In the present report, we show that dopamine, under the same conditions, also induces a concentration-dependent increase in the production of PGE(2), with a maximal effect of 235% at approximately 100 microM, and with an EC(50) of 794 nM. The effect was counteracted by the D(2) antagonist eticlopride, pertussis toxin, the inhibitor of intracellular Ca(2+) release TMB-8, incubation in Ca(2+)-free experimental medium, and PKC desensitization obtained by chronic pretreatment with the phorbol ester TPA. It was also antagonized by the non-specific cyclooxygenase (COX) inhibitor, indomethacin, and by the selective COX-2 inhibitor, NS-398, but not by the specific COX-1 inhibitor, valeryl salicylate. Both the non-specific phospholipase A(2) inhibitor, quinacrine, and an inhibitor of cPLA(2) and iPLA(2), AACOF3, counteracted the effect; in contrast, a selective iPLA(2) inhibitor, BEL, and a selective sPLA(2) inhibitor, TAPC, were ineffective. No effects of dopamine were obtained in control cells mock-transfected with the p3C vector only. The results reinforce previous assumptions that dopamine may interact with eicosanoid metabolism by means of D(2) receptor activation, and implicate an involvement of cPLA(2) and COX-2 in this effect. It is suggested that measurement of dopamine-induced PGE(2) production may serve as a convenient way to study D(2) receptor function in vitro.

Decreased activity of brain phospholipid metabolic enzymes in human users of cocaine and methamphetamine

Phospholipids are essential components of cell membranes which may also function to mediate some of the behavioural effects of dopamine receptor stimulation caused by psychostimulant drugs. Neuroimaging and pharmacological data suggest that abnormal brain metabolism of phospholipids might explain some of the consequences of chronic exposure to drugs of abuse including drug craving. We previously reported decreased activity of calcium-stimulated phospholipase A(2) (Ca-PLA(2)) in autopsied putamen of human cocaine users. To establish the specificity of this change in phospholipid metabolism and whether decreased Ca-PLA(2) might be a general feature of all abused drugs which enhance dopaminergic neurotransmission, we measured activity of 11 major phospholipid metabolic enzymes in dopamine-rich (putamen) and poor brain areas of chronic users of cocaine and of methamphetamine. Enzyme changes were restricted to the putamen which showed decreased (-21%, as compared with the control subjects) Ca-PLA(2) activity in users of methamphetamine and reduced (-31%) activity of phosphocholine cytidylyltransferase (PCCT), the rate-limiting enzyme of phosphatidylcholine synthesis, in the cocaine users. We suggest that chronic exposure to psychostimulant drugs might cause a compensatory downregulation of Ca-PLA(2) in dopamine-rich brain areas due to excessive dopamine-related stimulation of the enzyme. Decreased striatal Ca-PLA(2) and/or PCCT activity in cocaine users might also help to explain why CDP choline, which enhances phospholipid synthesis, reduces craving in some users of the drug cocaine.

Somatostatin receptor-mediated arachidonic acid mobilization: evidence for partial agonism of synthetic peptides

1. Somatostatin and the stable octapeptide analogues, octreotide and angiopeptin, were examined for their ability to stimulate the release of tritium from [(3)H]-arachidonic acid pre-loaded CHO-K1 cells expressing human recombinant sst(2), sst(3) or sst(5) receptors. 2. Somatostatin stimulated tritium release (pEC(50)) through the sst(2) (7.8+/-0.1) and sst(5) (7.3+/-0.2), but not the sst(3) receptor. Octreotide behaved as a full (sst(2) receptor) or partial agonist (sst(5) receptor), whereas angiopeptin behaved as a weak partial agonist at both receptor types. 3. Maximum responses to somatostatin through both receptor types were significantly reduced by pertussis toxin, whereas pEC(50) estimates were unaffected. 4. Inhibition of MEK1 or Src, but not PKA, PI 3-kinases or tyrosine kinases, by reportedly selective inhibitors reduced sst(2)-mediated responses by somatostatin, but not angiopeptin. A selective inhibitor of PKC (Ro-31-8220) reduced both somatostatin and angiopeptin responses. 5. These data provide further evidence for partial agonist activity of synthetic peptides of somatostatin. Furthermore, the somatostatin receptor signalling mechanisms which mediate arachidonic acid mobilization appear to be multiple and complex.

Renal dopaminergic mechanisms and hypertension: a chronology of advances

Dopamine (DA) has been shown to influence kidney function through endogenous synthesis and subsequent interaction with locally expressed dopamine receptor subtypes (D1, D5 as D1-like and D2, D3, and D4 as D2-like). DA, and DA-receptor specific agonists and antagonists can alter renal water and electrolyte excretion along with renin release when infused systemically or intrarenally. Such effects are brought about by a combination of renal hemodynamic and direct tubular effects evoked along the full length of the nephron. The cellular mechanisms that direct these dopamine-mediated renal electrolyte fluxes have recently been clarified and include alterations in adenylyl cyclase, phospholipase C, and phospholipase A1 activity. The dopaminergic system also interacts directly with the renal kallikrein-kinin, prostaglandin and other neurohumoral systems. Aberrant renal dopamine production and/or dopamine receptor function have been reported in salt-dependent and low-renin forms of human primary hypertension as well as in genetic models of animal hypertension, including the SHR and Dahl SS rat. DA D1 or D3 receptor knockout mice have been shown to develop hypertension.

Dopamine receptors and groups I and II mGluRs cooperate for long-term depression induction in rat prefrontal cortex through converging postsynaptic activation of MAP kinases

Tetanic stimuli to layer I-II afferents in rat prefrontal cortex induced long-term depression (LTD) of layer I-II to layer V pyramidal neuron glutamatergic synapses when tetani were coupled to bath application of dopamine. This LTD was blocked by the following metabotropic glutamate receptor (mGluR) antagonists coapplied with dopamine: (S)-alpha-methyl-4-carboxyphenylglycine (MCPG; group I and II antagonist), (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; group I antagonist), or (RS)-alpha-methylserine-O-phosphate monophenyl ester (MSOPPE; group II antagonist). This suggests that the dopamine-facilitated LTD requires synaptic activation of groups I and II mGluRs during tetanus. LTD could also be induced by coupling tetani to bath application of groups I and II mGluR agonist (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). In the next series of experiments, coapplication of dopamine and 1S,3R-ACPD, but not application of either drug alone, consistently induced LTD without tetani or even single test stimuli during drug application, suggesting that coactivation of dopamine receptors and the mGluRs is sufficient for LTD induction. Immunoblot analyses with anti-active mitogen-activated protein kinases (MAP-Ks) revealed that D1 receptors, D2 receptors, group I mGluRs, and group II mGluRs all contribute to MAP-K activation in prefrontal cortex, and that combined activation of dopamine receptors and mGluRs synergistically or additively activate MAP-Ks. Consistently, LTD by dopamine + 1S, 3R-ACPD coapplication, as well as the two other forms of LTD (LTD by dopamine + tetani and LTD by 1S,3R-ACPD + tetani), was blocked by bath application of MAP-K kinase inhibitor PD98059. LTD by dopamine + 1S,3R-ACPD coapplication was also blocked by postsynaptic injection of synthetic MAP-K substrate peptide. Our results suggest that dopamine receptors and groups I and II mGluRs cooperate to induce LTD through converging postsynaptic activation of MAP-Ks.

Dopamine receptors and groups I and II mGluRs cooperate for long-term depression induction in rat prefrontal cortex through converging postsynaptic activation of MAP kinases

Tetanic stimuli to layer I-II afferents in rat prefrontal cortex induced long-term depression (LTD) of layer I-II to layer V pyramidal neuron glutamatergic synapses when tetani were coupled to bath application of dopamine. This LTD was blocked by the following metabotropic glutamate receptor (mGluR) antagonists coapplied with dopamine: (S)-alpha-methyl-4-carboxyphenylglycine (MCPG; group I and II antagonist), (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; group I antagonist), or (RS)-alpha-methylserine-O-phosphate monophenyl ester (MSOPPE; group II antagonist). This suggests that the dopamine-facilitated LTD requires synaptic activation of groups I and II mGluRs during tetanus. LTD could also be induced by coupling tetani to bath application of groups I and II mGluR agonist (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). In the next series of experiments, coapplication of dopamine and 1S,3R-ACPD, but not application of either drug alone, consistently induced LTD without tetani or even single test stimuli during drug application, suggesting that coactivation of dopamine receptors and the mGluRs is sufficient for LTD induction. Immunoblot analyses with anti-active mitogen-activated protein kinases (MAP-Ks) revealed that D1 receptors, D2 receptors, group I mGluRs, and group II mGluRs all contribute to MAP-K activation in prefrontal cortex, and that combined activation of dopamine receptors and mGluRs synergistically or additively activate MAP-Ks. Consistently, LTD by dopamine + 1S, 3R-ACPD coapplication, as well as the two other forms of LTD (LTD by dopamine + tetani and LTD by 1S,3R-ACPD + tetani), was blocked by bath application of MAP-K kinase inhibitor PD98059. LTD by dopamine + 1S,3R-ACPD coapplication was also blocked by postsynaptic injection of synthetic MAP-K substrate peptide. Our results suggest that dopamine receptors and groups I and II mGluRs cooperate to induce LTD through converging postsynaptic activation of MAP-Ks.