Potassium channel blockers inhibit D2 dopamine, but not A1 adenosine, receptor-mediated inhibition of striatal dopamine release

Ultra-Processed Food, Reward System and Childhood Obesity

Obesity and overweight are a major public health problem globally. Diet quality is critical for proper child development, and an unhealthy diet is a preventable risk factor for noncommunicable diseases (NCDs), such as obesity. Consumption of sugar-sweetened beverages and ultra-processed foods (UPFs) in childhood may increase the BMI/BMI z-score, body fat percentage, or likelihood of overweight. A strict feeding regulation system allows for sufficient food to be consumed to meet ongoing metabolic demands while avoiding overconsumption. This narrative review explores the issues of obesity and the regulation of food intake related to reward systems and UPF consumption. Nutrient composition alone cannot explain the influence of UPFs on the risk of obesity. Furthermore, the non-nutritional properties of UPFs may explain the mechanisms underlying the relationship with obesity and NCDs. UPFs are designed to be highly palatable, appealing, and energy dense with a unique combination of the main taste enhancer ingredients to generate a strong rewarding stimulus and influence the circuits related to feeding facilitation. How individual UPF ingredients influence eating behavior and reward processes remains not fully elucidated. To increase the knowledge on the relationship between UPFs and pediatric obesity, it may be useful to limit the rapid growth in the prevalence of obesity and subsequent related complications, and to develop new strategies for appropriate food and nutrition policies.

Dopamine D2 autoreceptor interactome: Targeting the receptor complex as a strategy for treatment of substance use disorder

Dopamine D2 autoreceptors (D2ARs), located in somatodendritic and axon terminal compartments of dopamine (DA) neurons, function to provide a negative feedback regulatory control on DA neuron firing, DA synthesis, reuptake and release. Dysregulation of D2AR-mediated DA signaling is implicated in vulnerability to substance use disorder (SUD). Due to the extreme low abundance of D2ARs compared to postsynaptic D2 receptors (D2PRs) and the lack of experimental tools to differentiate the signaling of D2ARs from D2PRs, the regulation of D2ARs by drugs of abuse is poorly understood. The recent availability of conditional D2AR knockout mice and newly developed virus-mediated gene delivery approaches have provided means to specifically study the function of D2ARs at the molecular, cellular and behavioral levels. There is a growing revelation of novel mechanisms and new proteins that mediate D2AR activity, suggesting that D2ARs act cooperatively with an array of membrane and intracellular proteins to tightly control DA transmission. This review highlights D2AR-interacting partners including transporters, G-protein-coupled receptors, ion channels, intracellular signaling modulators, and protein kinases. The complexity of the D2AR interaction network illustrates the functional divergence of D2ARs. Pharmacological targeting of multiple D2AR-interacting partners may be more effective to restore disrupted DA homeostasis by drugs of abuse.

Protein kinase C beta regulates the D₂-like dopamine autoreceptor

The focus of this study was the regulation of the D2-like dopamine autoreceptor (D2 autoreceptor) by protein kinase Cβ, a member of the protein kinase C (PKC) family. Together with the dopamine transporter, the D2 autoreceptor regulates the level of extracellular dopamine and thus dopaminergic signaling. PKC regulates neuronal signaling via several mechanisms, including desensitizing autoreceptors to increase the release of several different neurotransmitters. Here, using both PKCβ(-/-) mice and specific PKCβ inhibitors, we demonstrated that a lack of PKCβ activity enhanced the D2 autoreceptor-stimulated decrease in dopamine release following both chemical and electrical stimulations. Inhibition of PKCβ increased surface localization of D2R in mouse striatal synaptosomes, which could underlie the greater sensitivity to quinpirole following inhibition of PKCβ. PKCβ(-/-) mice displayed greater sensitivity to the quinpirole-induced suppression of locomotor activity, demonstrating that the regulation of the D2 autoreceptor by PKCβ is physiologically significant. Overall, we have found that PKCβ downregulates the D2 autoreceptor, providing an additional layer of regulation for dopaminergic signaling. We propose that in the absence of PKCβ activity, surface D2 autoreceptor localization and thus D2 autoreceptor signaling is increased, leading to less dopamine in the extracellular space and attenuated dopaminergic signaling.

Nicotinic, glutamatergic and dopaminergic synaptic transmission and plasticity in the mesocorticolimbic system: focus on nicotine effects

Cigarette smoking is currently the leading cause of preventable deaths and disability throughout the world, being responsible for about five million premature deaths/year. Unfortunately, fewer than 10% of tobacco users who try to stop smoking actually manage to do so. The main addictive agent delivered by cigarette smoke is nicotine, which induces psychostimulation and reward, and reduces stress and anxiety. The use of new technologies (including optogenetics) and the development of mouse models characterised by cell-specific deletions of receptor subtype genes or the expression of gain-of-function nAChR subunits has greatly increased our understanding of the molecular mechanisms and neural substrates of nicotine addiction first revealed by classic electrophysiological, neurochemical and behavioural approaches. It is now becoming clear that various aspects of nicotine dependence are mediated by close interactions of the glutamatergic, dopaminergic and γ-aminobutyric acidergic systems in the mesocorticolimbic system. This review is divided into two parts. The first provides an updated overview of the circuitry of the ventral tegmental area, ventral striatum and prefrontal cortex, the neurotransmitter receptor subtypes expressed in these areas, and their physiological role in the mesocorticolimbic system. The second will focus on the molecular, functional and behavioural mechanisms involved in the acute and chronic effects of nicotine on the mesocorticolimbic system.

Dopamine signaling in food addiction: role of dopamine D2 receptors

Dopamine (DA) regulates emotional and motivational behavior through the mesolimbic dopaminergic pathway. Changes in DA signaling in mesolimbic neurotransmission are widely believed to modify reward-related behaviors and are therefore closely associated with drug addiction. Recent evidence now suggests that as with drug addiction, obesity with compulsive eating behaviors involves reward circuitry of the brain, particularly the circuitry involving dopaminergic neural substrates. Increasing amounts of data from human imaging studies, together with genetic analysis, have demonstrated that obese people and drug addicts tend to show altered expression of DA D2 receptors in specific brain areas, and that similar brain areas are activated by food-related and drug-related cues. This review focuses on the functions of the DA system, with specific focus on the physiological interpretation and the role of DA D2 receptor signaling in food addiction. [BMB Reports 2013; 46(11): 519-526]

Dopamine signaling in reward-related behaviors

Dopamine (DA) regulates emotional and motivational behavior through the mesolimbic dopaminergic pathway. Changes in DA mesolimbic neurotransmission have been found to modify behavioral responses to various environmental stimuli associated with reward behaviors. Psychostimulants, drugs of abuse, and natural reward such as food can cause substantial synaptic modifications to the mesolimbic DA system. Recent studies using optogenetics and DREADDs, together with neuron-specific or circuit-specific genetic manipulations have improved our understanding of DA signaling in the reward circuit, and provided a means to identify the neural substrates of complex behaviors such as drug addiction and eating disorders. This review focuses on the role of the DA system in drug addiction and food motivation, with an overview of the role of D1 and D2 receptors in the control of reward-associated behaviors.

Regulation of striatal dopamine release by presynaptic auto- and heteroreceptors

Striatal dopamine neurotransmission is critical for normal voluntary movement, affect and cognition. Dysfunctions of its regulation are implicated in a broad range of behaviors and disorders including Parkinson's disease, schizophrenia and drug abuse. Extracellular dopamine levels result from a dynamic equilibrium between release and reuptake by dopaminergic terminals. Both processes are regulated by multiple mechanisms. Here we review data characterizing how dopamine levels are regulated by presynaptic autoreceptors and heteroreceptors, an area intensively investigated due to advances in real time electrochemical detection of extracellular dopamine, i.e., fast-scan cyclic voltammetry and amperometry, and the development of mutant mouse lines with deletions for specific receptors.

Role of Kv1 potassium channels in regulating dopamine release and presynaptic D2 receptor function

Dopamine (DA) release in the CNS is critical for motor control and motivated behaviors. Dysfunction of its regulation is thought to be implicated in drug abuse and in diseases such as schizophrenia and Parkinson's. Although various potassium channels located in the somatodendritic compartment of DA neurons such as G-protein-gated inward rectifying potassium channels (GIRK) have been shown to regulate cell firing and DA release, little is presently known about the role of potassium channels localized in the axon terminals of these neurons. Here we used fast-scan cyclic voltammetry to study electrically-evoked DA release in rat dorsal striatal brain slices. We find that although G-protein-gated inward rectifying (GIRK) and ATP-gated (K_ATP) potassium channels play only a minor role, voltage-gated potassium channels of the Kv1 family play a major role in regulating DA release. The use of Kv subtype-selective blockers confirmed a role for Kv1.2, 1.3 and 1.6, but not Kv1.1, 3.1, 3.2, 3.4 and 4.2. Interestingly, Kv1 blockers also reduced the ability of quinpirole, a D2 receptor agonist, to inhibit evoked DA overflow, thus suggesting that Kv1 channels also regulate presynaptic D2 receptor function. Our work identifies Kv1 potassium channels as key regulators of DA release in the striatum.

Contribution of Kv1.2 voltage-gated potassium channel to D2 autoreceptor regulation of axonal dopamine overflow

Impairments in axonal dopamine release are associated with neurological disorders such as schizophrenia and attention deficit hyperactivity disorder and pathophysiological conditions promoting drug abuse and obesity. The D2 dopamine autoreceptor (D2-AR) exerts tight regulatory control of axonal dopamine (DA) release through a mechanism suggested to involve K(+) channels. To evaluate the contribution of Kv1 voltage-gated potassium channels of the Shaker gene family to the regulation of axonal DA release by the D2-AR, the present study employed expression analyses, real time measurements of striatal DA overflow, K(+) current measurements and immunoprecipitation assays. Kv1.1, -1.2, -1.3, and -1.6 mRNA and protein were detected in midbrain DA neurons purified by fluorescence-activated cell sorting and in primary DA neuron cultures. In addition, Kv1.1, -1.2, and -1.6 were localized to DA axonal processes in the dorsal striatum. By means of fast scan cyclic voltammetry in striatal slice preparations, we found that the inhibition of stimulation-evoked DA overflow by a D2 agonist was attenuated by Kv1.1, -1.2, and -1.6 toxin blockers. A particular role for the Kv1.2 subunit in the process whereby axonal D2-AR inhibits DA overflow was established with the use of a selective Kv1.2 blocker and Kv1.2 knock-out mice. Moreover, we demonstrate the ability of D2-AR activation to increase Kv1.2 currents in co-transfected cells and its reliance on Gβγ subunit signaling along with the physical coupling of D2-AR and Kv1.2-containing channels in striatal tissue. These findings underline the contribution of Kv1.2 in the regulation of nigrostriatal DA release by the D2-AR and thereby offer a novel mechanism by which DA release is regulated.

Third generation antipsychotic drugs: partial agonism or receptor functional selectivity?

Functional selectivity is the term that describes drugs that cause markedly different signaling through a single receptor (e.g., full agonist at one pathway and antagonist at a second). It has been widely recognized recently that this phenomenon impacts the understanding of mechanism of action of some drugs, and has relevance to drug discovery. One of the clinical areas where this mechanism has particular importance is in the treatment of schizophrenia. Antipsychotic drugs have been grouped according to both pattern of clinical action and mechanism of action. The original antipsychotic drugs such as chlorpromazine and haloperidol have been called typical or first generation. They cause both antipsychotic actions and many side effects (extrapyramidal and endocrine) that are ascribed to their high affinity dopamine D(2) receptor antagonism. Drugs such as clozapine, olanzapine, risperidone and others were then developed that avoided the neurological side effects (atypical or second generation antipsychotics). These compounds are divided mechanistically into those that are high affinity D(2) and 5-HT(2A) antagonists, and those that also bind with modest affinity to D(2), 5-HT(2A), and many other neuroreceptors. There is one approved third generation drug, aripiprazole, whose actions have been ascribed alternately to either D(2) partial agonism or D(2) functional selectivity. Although partial agonism has been the more widely accepted mechanism, the available data are inconsistent with this mechanism. Conversely, the D(2) functional selectivity hypothesis can accommodate all current data for aripiprazole, and also impacts on discovery compounds that are not pure D(2) antagonists.

Heteromeric nicotinic acetylcholine-dopamine autoreceptor complexes modulate striatal dopamine release

In the striatum, dopamine and acetylcholine (ACh) modulate dopamine release by acting, respectively, on dopamine D(2) autoreceptors and nicotinic ACh (nACh) heteroreceptors localized on dopaminergic nerve terminals. The possibility that functional interactions exist between striatal D(2) autoreceptors and nACh receptors was studied with in vivo microdialysis in freely moving rats. Local perfusion of nicotine in the ventral striatum (shell of the nucleus accumbens) produced a marked increase in the extracellular levels of dopamine, which was completely counteracted by co-perfusion with either the non-alpha(7) nACh receptor antagonist dihydro-beta-erythroidine or the D(2-3) receptor agonist quinpirole. Local perfusion of the D(2-3) receptor antagonist raclopride produced an increase in the extracellular levels of dopamine, which was partially, but significantly, counteracted by coperfusion with dihydro-beta-erythroidine. These findings demonstrate a potent crosstalk between G protein-coupled receptors and ligand-gated ion channels in dopaminergic nerve terminals, with the D(2) autoreceptor modulating the efficacy of non-alpha(7) nACh receptor-mediated modulation of dopamine release. We further demonstrate physical interactions between beta(2) subunits of non-alpha(7) nicotinic acetylcholine receptors and D(2) autoreceptors in co-immunoprecipitation experiments with membrane preparations from co-transfected mammalian cells and rat striatum. These results reveal that striatal non-alpha(7) nicotinic acetylcholine receptors form part of heteromeric dopamine autoreceptor complexes that modulate dopamine release.

Activation of Dopamine D2 Receptors Linked to Voltage-Sensitive Potassium Channels Reduces Forskolin-Induced Cyclic AMP Formation in Rat Pituitary Cells

3,4-Dihydroxyphenylethylamine (dopamine) D2 receptor agonists, including BHT 920 and bromocriptine, and the potassium channel opener minoxidil share the property of hyperpolarizing the plasma membrane by activating voltage-dependent potassium channels. These drugs were tested for their ability to inhibit the cyclic AMP formation induced by forskolin either in intact or in broken pituitary cells. In contrast to bromocriptine, which was active in both experimental systems, BHT 920 and minoxidil inhibited the forskolin-induced cyclic AMP formation in intact-cell but not in broken-cell preparations. The effects of BHT 920 were (a) concentration dependent, with a calculated IC50 of 0.7 microM, (b) dopaminergic in nature, being specifically antagonized by sulpiride, (c) not additive with those induced by minoxidil, and (d) less effective in the presence of potassium channel blockers, such as 4-aminopyridine and tetraethylammonium. These data indicate that the inhibition of forskolin-induced cyclic AMP formation by BHT 920 in intact pituitary cells is not a primary consequence of receptor occupation, but a late event, possibly related to the opening of voltage-dependent potassium channels elicited by this drug through the activation of a subtype of dopamine D2 receptors uncoupled to adenylyl cyclase.

Effect of Quinine on Autoreceptor-Regulated Dopamine Release in the Rat Striatum

In vivo brain microdialysis was used to examine the role of potassium channel activation in dopamine (DA) autoreceptor function in the striatum of freely moving rats. Local application of the D2 receptor agonists quinpirole or N-0437 through the dialysis probe significantly reduced extracellular concentrations of DA. Local application of the D2 antagonist (-)-sulpiride produced significant increases in DA. Local perfusion with quinine, a K+ channel blocker, completely blocked the (-)-sulpiride-induced increases in DA but did not affect the DA agonist-induced decreases. (-)-Sulpiride completely blocked the effect of quinpirole on DA both in control and in quinine-treated animals. At the highest dose used, quinine caused a large transient increase in extracellular DA. Local application of tetrodotoxin or infusion of Mg2+ in the absence of Ca2+ did not prevent this quinine-induced transient increase in extracellular DA. These results demonstrate that DA autoreceptors in the striatum regulate DA release in awake, behaving animals. Local application of (-)-sulpiride increases DA levels by blocking the tonic activation of autoreceptors by endogenous DA. Quinine blocks the neuroleptic-induced increase in DA, perhaps by preventing the K+ channel opening that would normally accompany endogenous autoreceptor activation. The fact that exogenously applied DA receptor agonists can decrease extracellular DA levels in the presence of quinine suggests that they may be acting at extrasynaptic autoreceptors that are not tonically active in vivo. The effect of DA agonists on this site is via a DA receptor because it is blocked by (-)-sulpiride. However, this receptor does not appear to be coupled to a quinine-sensitive potassium channel.

Dopamine receptor regulation of Ca2+ levels in individual isolated nerve terminals from rat striatum: comparison of presynaptic D1-like and D2-like receptors

We have directly observed the effects of activating presynaptic D1-like and D2-like dopamine receptors on Ca2+ levels in isolated nerve terminals (synaptosomes) from rat striatum. R-(+)-SKF81297, a selective D1-like receptor agonist, and (-)-quinpirole, a selective D2-like receptor agonist, induced increases in Ca2+ levels in different subsets of individual striatal synaptosomes. The SKF81297- and quinpirole-induced effects were blocked by R-(+)-SCH23390, a D1-like receptor antagonist, and (-)-sulpiride, a D2-like receptor antagonist, respectively. SKF81297- or quinpirole-induced Ca2+ increases were inhibited following blockade of voltage-gated calcium channels or sodium channels. In a larger subset of synaptosomes, quinpirole decreased baseline Ca2+. Quinpirole also inhibited veratridine-induced increases in intrasynaptosomal Ca2+ level. Immunostaining confirmed the presynaptic expression of D1, D5, D2 and D3 receptors, but not D4 receptors. The array of neurotransmitter phenotypes of the striatal nerve endings expressing D1, D5, D2 or D3 varied for each receptor subtype. These results suggest that presynaptic D1-like and D2-like receptors induce increases in Ca2+ levels in different subsets of nerve terminals via Na+ channel-mediated membrane depolarization, which, in turn, induces the opening of voltage-gated calcium channels. D2-like receptors also reduce nerve terminal Ca2+ in a different but larger subset of synaptosomes, consistent with the predominant presynaptic action of dopamine in the striatum being inhibitory.

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.

D2 receptors inhibit the secretory process downstream from calcium influx in dopaminergic neurons: implication of K+ channels

Dopaminergic (DAergic) neurons possess D2-like somatodendritic and terminal autoreceptors that modulate cellular excitability and dopamine (DA) release. The cellular and molecular processes underlying the rapid presynaptic inhibition of DA release by D2 receptors remain unclear. Using a culture system in which isolated DAergic neurons establish self-innervating synapses ("autapses") that release both DA and glutamate, we studied the mechanism by which presynaptic D2 receptors inhibit glutamate-mediated excitatory postsynaptic currents (EPSCs). Action-potential evoked EPSCs were reversibly inhibited by quinpirole, a selective D2 receptor agonist. This inhibition was slightly reduced by the inward rectifier K(+) channel blocker barium, largely prevented by the voltage-dependent K(+) channel blocker 4-aminopyridine, and completely blocked by their combined application. The lack of a residual inhibition of EPSCs under these conditions argues against the implication of a direct inhibition of presynaptic Ca(2+) channels. To evaluate the possibility of a direct inhibition of the secretory process, spontaneous miniature EPSCs were evoked by the Ca(2+) ionophore ionomycin. Ionomycin-evoked release was insensitive to cadmium and dramatically reduced by quinpirole, providing evidence for a direct inhibition of quantal release at a step downstream to Ca(2+) influx through voltage-dependent Ca(2+) channels. Surprisingly, this effect of quinpirole on ionomycin-evoked release was blocked by 4-aminopyridine. These results suggest that D2 receptor activation decreases neurotransmitter release from DAergic neurons through a presynaptic mechanism in which K(+) channels directly inhibit the secretory process.

Differential effect of quinpirole and 7-OH-DPAT on the spontaneous [(3)H]-dopamine efflux from rat striatal synaptosomes

The effect of quinpirole and 7-OH-DAPT, two D(2)-like agonists, were examined using superfused rat striatal synaptosomes to study the autoregulation of spontaneous [(3)H]-dopamine ([(3)H]-DA) release. Basal [(3)H]-DA efflux was Ca(2+)-dependent by approximately 45% and was inhibited by cadmium 10 microM by 24%. Quinpirole (1 nM to 3 microM) inhibited spontaneous [(3)H]-DA efflux in a concentration-dependent manner (pEC(50) = 7.56 +/- 0.07 and E(max) = 26 +/- 0.09%) and this effect was competitively antagonized by haloperidol (0.3-1 nM) (apparent pA(2) = 9.61 +/- 0.08). In addition, activation of the D(2) DA autoreceptor by quinpirole only modulates the calcium-dependent component of [(3)H]-DA efflux. Low concentrations of a putative-selective D(3) DA agonist, (+/-)-7-OH-DPAT (0.03-0.1 microM), inhibited spontaneous [(3)H]-DA release by 13% (P < 0.05), but higher drug concentrations (> or =1 microM) increased basal [(3)H]-DA efflux in a concentration-dependent, nonsaturable, but reversible manner. Haloperidol (1-10 nM) reversed the (+/-)-7-OH-DPAT-induced inhibition, but not the increase in [(3)H]-DA outflow. The effect of (+/-)-7-OH-DPAT was mimicked by (+)-7-OH-DPAT. However, another putative D(3) DA agonist, PD 128,907 (1 nM to 3 microM), decreased spontaneous tritium efflux (maximal inhibition of 19 +/- 3.06% at 3 microM, P < 0.01). The effect of 7-OH-DPAT 10 microM was independent of the presence of extracellular Ca(2+), since its effect on basal [(3)H]-DA outflow was not significantly modified in a 200 nM free-Ca(2+) medium. In addition, the 7-OH-DPAT-induced enhancement of basal [(3)H]-DA efflux does not involve depolarization of nerve terminals or the reversal of the DA uptake system, as tetrodotoxin (1 microM) and nomifensine (1microM) did not modify the effect of 7-OH-DPAT 10 microM. The present data indicate that activation of D(2) DA autoreceptor subtype by quinpirole inhibits Ca(2+)-dependent spontaneous [(3)H]-DA efflux. 7-OH-DPAT activates the D(2) DA autoreceptor at low concentrations, whereas its action in releasing [(3)H]-DA effect is not receptor-mediated and could involve other mechanisms other than either conventional vesicular exocytosis or the DA uptake system.

Presynaptic effect of 7-OH-DPAT on evoked [3H]-acetylcholine release in rat striatal synaptosomes

The objective of the present experiments was to study the presynaptic effect of 7-hydroxy-N,N-di-n-propyl-2-aminotetraline (7-OH-DPAT, a D(2)-like dopamine receptor agonist) on [3H]-acetylcholine ([3H]-ACh) release induced by potassium (15 mM, 25 mM and 60 mM), potassium channel-blockers (4-aminopyridine, 4-AP; tetraethylammonium, TEA and quinine) and veratridine to gain insight into the mechanisms involved in the activation of the D(2) dopamine-receptor subtype located at striatal cholinergic nerve terminals. 7-OH-DPAT (1 microM) inhibited the evoked [3H]-ACh release induced by K(+) 15 mM in a similar percentage than that obtained during basal conditions (30% and 27%, respectively). Nevertheless, in the presence of 25 mM and 60 mM of K(+) the inhibitory effect of 7-OH-DPAT was completely abolished. 4-AP (1-100 microM) and TEA (1 and 5 mM) significantly enhanced [3H]-ACh release, showing 69.32%+/-7.60% (P<0.001) and 52.27%+/-5.64% (P<0.001), respectively, at the highest concentrations tested. In these conditions, 7-OH-DPAT (1 microM) inhibited the release induced by potassium channel-blockers approximately 25-27%. Quinine (0.1-1 microM) did not alter [3H]-ACh release either in the presence or absence of 7-OH-DPAT. Veratridine 10 microM evoked [3H]-ACh release in the presence of a low-calcium medium, but in such conditions 7-OH-DPAT (1 microM) did not modify the neurotransmitter release in the absence or presence of veratridine. Present data indicate that activation of the presynaptic D(2) dopamine receptor inhibits the [3H]-ACh release by increasing K(+) conductance, as high K(+) concentrations abolished the inhibitory control of 7-OH-DPAT on [3H]-ACh release. This effect could be mediated by potassium channels different from those sensitive to 4-AP, TEA and quinine. In addition, the presynaptic D(2) dopamine-receptor activation seems to not involve changes in intracellular Ca(2+).

The effects of quinine and 4-aminopyridine on conditioned place preference and changes in motor activity induced by morphine in rats

1. The effects of two unselective potassium (K(+)-) channel blockers, quinine (12.5, 25 and 50 mg/kg) and 4-aminopyridine (1 and 2 mg/kg), on conditioned place preference and biphasic changes in motor activity induced by morphine (10 mg/kg) were tested in Wistar rats. Quinine is known to block voltage-, calcium- and ATP-sensitive K(+)-channels while 4-aminopyridine is known to block voltage-sensitive K(+)-channels. 2. In the counterbalanced method, quinine attenuated morphine-induced place preference, whereas 4-aminopyridine was ineffective. In the motor activity test measured with an Animex-activity meter neither of the K(+)-channel blockers affected morphine-induced hypoactivity, but both K(+)-channel blockers prevented morphine-induced secondary hyperactivity. 3. These results suggest the involvement of quinine-sensitive but not 4-aminopyridine-sensitive K(+)-channels in morphine reward. It is also suggested that the blockade of K(+)-channels sensitive to these blockers is not sufficient to prevent morphine-induced hypoactivity whereas morphine-induced hyperactivity seems to be connected to both quinine- and 4-aminopyridine-sensitive K(+)-channels.

D2-Like dopamine autoreceptor activation reduces quantal size in PC12 cells

D2-like dopamine autoreceptors regulate dopamine release and are implicated in important actions of antipsychotic drugs and rewarding behaviors. To directly observe the effects of D2 autoreceptors on exocytic neurotransmitter release, we measured quantal release of dopamine from pheochromocytoma PC12 cells that express D2 and D4 autoreceptors. High potassium-evoked secretion in PC12 cells produced a unimodal population of quantal sizes. We found that exposures to the D2-like agonist quinpirole that inhibited tyrosine hydroxylase activity by approximately 50% also reduced quantal size by approximately 50%. The reduced quantal size was blocked by the D2 antagonist sulpiride and reversed by L-DOPA. Quinpirole also decreased the frequency of stimulation-evoked quantal release. Together, these findings indicate effects on quantal neurotransmission by D2-like dopamine autoreceptors previously distinguished as synthesis-modulating autoreceptors that regulate tyrosine hydroxylase activity versus impulse-regulating autoreceptors that modulate membrane potential. The results also provide an initial demonstration of a receptor-mediated mechanism that alters quantal size.

Lack of autoreceptor-mediated inhibitory control of dopamine release in striatal synaptosomes of D2 receptor-deficient mice

Mouse purified striatal synaptosomes were used to study the release of newly synthesised [3H]-dopamine ([3H]-DA) or of previously taken up [3H]-DA. Quinpirole (QP, 10 microM), a D2/D3 dopaminergic agonist, was found to reduce the release of newly synthesised [3H]-DA with a larger amplitude when 4-aminopyridine (100 microM) instead than veratridine (1 microM) or potassium (25 mM) was used to evoke DA release. Among the different D2/D3 dopaminergic agonists tested R(-)-propylnorapomorphine (NPA) and quinpirole were the most potent. These compounds reduced, in a concentration-dependent manner, the 4-aminopyridine-evoked release of [3H]-DA previously taken up by synaptosomes (50% maximal inhibition). In contrast, the D3 agonist PD-128,907 had little effect even when used at 100 nM. The QP (100 nM)-induced response was completely antagonised by sulpiride (1 microM). Strikingly, the NPA (100 nM) and PD-128,907 (100 nM)-evoked responses were completely suppressed in D2 receptor-deficient mice. This data strongly suggest that only D2 but not D3 receptors are involved in the autoreceptor-mediated inhibition of the evoked release of [3H]-DA. Interestingly, while amphetamine-induced release of [3H]-DA was not modified, a slight reduction of [3H]-DA efflux induced by the dopamine (DA) uptake inhibitor cocaine was observed in D2 receptor-deficient mice.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Effect of quinine on autoreceptor-regulated serotonin release in the rat hippocampus

The involvement of K+ channels in the autoregulation of terminal serotonin (5-hydroxytryptamine, 5-HT) release was investigated by microdialysis in the hippocampus of conscious rats. Extracellular 5-HT was increased concentration-dependently by the K+ channel blocker quinine (10, 100 and 1000 microM in perfusate), and tetrodotoxin (10 microM) but not fluoxetine (5 microM) exerted a partially attenuating influence. The 5-HT1/2/6 receptor antagonist methiothepin (50 microM) increased dialysate 5-HT, most likely through 5-HT1B autoreceptors tonically activated in the hippocampus of awake rats as opposed to the previously reported lack of effect 5-HT1B autoreceptor blockade in anesthetized rats. The effect of methiothepin was greatly reduced by preperfusion with quinine (100 microM), consonant with a role for quinine-sensitive K+ channels in the autoregulation of 5-HT release in the hippocampus by 5-HT receptor antagonism. In contrast, the reduction in dialysate 5-HT induced by the 5-HT1 receptor agonist RU 24969 (1 microM), in the presence of fluoxetine (5 microM), persisted in the co-presence of quinine, consonant with the involvement of (extrasynaptic?) 5-HT autoreceptors not coupled with quinine-sensitive K+ channels.

Attenuation of the locomotor activating effects of D-amphetamine, cocaine, and scopolamine by potassium channel modulators

1. Locomotor activating effects of D-amphetamine, cocaine, and scopolamine were determined alone and after pretreatment with K-channel modulators in mice. 2. When administered alone, D-amphetamine (1.0- 30 mg/kg) and cocaine (3.0- 56 mg/kg) produced inverted U-shaped dose-effect curves characteristic of psychomotor stimulant drugs. 3. When administered alone, scopolamine (3.0-56 mg/kg) also produced dose-dependent increases in locomotor activity but these effects plateaued with similar increases in locomotor activity induced by 10-56 mg/kg of scopolamine. 4. Pretreatment with the K-channel blockers 4-aminopyridine (0.3-1.7 mg/kg), quinine (30-100 mg/kg) or apamin (0.3-1.0 mg/kg) attenuated the locomotor increases induced by d-amphetamine, cocaine, and scopolamine. 5. Like the K-channel blockers, pretreatment with the K-channel openers cromakalim (1.0-3.0 mg/kg) and pinacidil (3.0-10 mg/kg) also attenuated the locomotor increases induced by D-amphetamine and scopolamine but did not modify the locomotor activating effects of cocaine. 6. These results demonstrate that K-channel modulation modifies the effects of D-amphetamine, cocaine, and scopolamine. 7. The results also demonstrate that K-channel openers can differentially alter the behavioral effects of cocaine and D-amphetamine.

Properties of 3,4-diaminopyridine-evoked dopamine and acetylcholine release in rabbit caudate nucleus slices: involvement of facilitatory adenosine A2 receptors or nitric oxide?

The 3H-overflow from slices of the rabbit caudate nucleus preincubated with tritiated dopamine (DA), or choline, and then superfused and stimulated twice with 3,4-diaminopyridine (3,4-DAP; 25 microM, 1 min), was explored as an in vitro model for evoked release of DA, or acetylcholine (ACh), respectively. In both cases the 3,4-DAP-evoked 3H-overflow was tetrodotoxin-sensitive and Ca(2+)-dependent and hence most probably represents action potential-induced exocytotic release of DA or ACh, respectively. Using pairs of preferential agonists/antagonists it was shown, that evoked DA release was inhibited via presynaptic D2 autoreceptors (quinpirole/domperidone) and kappa-opioid receptors (U-50488H/norbinaltorphimine). No evidence was found for the presence of presynaptic adenosine A1 or A2 receptors on dopaminergic terminals. Moreover, 3,4-DAP-evoked DA release was unaffected by increased intracellular cyclic AMP levels or by drugs affecting the NO/guanylate cyclase pathway. In a similar manner it was shown that 3,4-DAP-evoked ACh release was inhibited via presynaptic muscarine autoreceptors (oxotremorine/atropine) and dopamine D2 heteroreceptors (quinpirole/domperidone). Again, no evidence for the involvement of the NO/guanylate cyclase system in the modulation of ACh release was found, whereas the presence of inhibitory adenosine A1 receptors, but not of facilitatory A2 receptors, could be clearly established. It is concluded, that 3,4-DAP-evoked 3H-overflow from rabbit caudate nucleus slices preincubated with [3H]DA or [3H]choline, represents a simple and useful in vitro model for action potential-induced DA or ACh release, respectively. Moreover, at least in this model or rabbit brain region, facilitatory adenosine A2 receptors and the NO/guanylate cyclase system seem not to be involved in the release of these transmitters.

Systemic and intrastriatal theophylline have opposite effects on dopamine and dopamine metabolites measured by intrastriatal microdialysis in the rat

Using a model of intrastriatal microdialysis, we studied the effect of theophylline, an A1 and A2A adenosine receptor antagonist on striatal dopamine (DA) and DA metabolites. Systemic administration of theophylline (10 and 50 mg/kg) significantly reduced striatal extracellular (EC) levels of DA and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and 4-hydroxy-3-methoxy-phenylacetic acid (HVA). Intrastriatal administration of theophylline (10(-2) M) significantly increased DA and its metabolites (DA1 + 120%; DOPAC, +28%; HVA, +30%). Contradictory effects of systemic and intrastriatal theophylline point to theophylline interactions with different receptors possibly at different locations.

ATP-sensitive K+ channel openers block sulpiride-induced dopamine release in the rat striatum

In vivo brain microdialysis was used to investigate the role of ATP-sensitive K+ (KATP) channel openers in dopamine release regulated by dopamine autoreceptors in the rat striatum. Local infusion of two KATP channel openers, nicorandil (10(-5)-10(-3) M) and cromakalim (10(-5)-10(-3) M), into the striatum thorough the dialysis membrane produced dose-dependent decreases in extracellular concentrations of dopamine. Local application of the dopamine D2 receptor antagonist, (-)-sulpiride (10(-5) M), produced significant increases in extracellular concentrations of dopamine. Both nicrorandil (10(-5) M) and cromakalim (10(-4) M) blocked significantly (-)-sulpiride (10(-5) M)-induced increases in dopamine levels in the striatum. These results suggest that activation of KATP channels in the striatum causes decreases in endogenous dopamine release in vivo. Furthermore, the sulpiride-induced increases in dopamine levels caused by blocking the tonic activation of dopamine autoreceptors were inhibited by activation of KATP channel. These data indicate that KATP channels may be present in nigrostriatal dopaminergic terminals and that striatal dopamine autoreceptors inhibit dopamine release tonically by activation of KATP channels.

K-channel blockers attenuate the presynaptic effects of the D2/D3 agonist quinpirole in monkeys

The present study investigated whether potassium channel blockade could modify the behavioral effects of the dopamine D2/D3 receptor agonist quinpirole in squirrel monkeys. The duration of immobility and/or unusual postures indicative of catalepsy-associated behavior or the duration of scratching, known to be related to the effects of low and high doses, respectively, of quinpirole, were scored during 5-min observation periods in three squirrel monkeys. Saline or incremental doses of quinpirole were administered 10 min before each observation period. Administration of saline did not increase the durations of catalepsy-associated behavior (8% of the observational period) or scratching (< 1% of the observational period). Low doses of quinpirole (0.003-0.03 mg/kg) dose dependently increased the duration of catalepsy-associated behavior to approximately 54% of the observational periods. Higher doses of quinpirole (0.1-0.3 mg/kg) did not increase the duration of catalepsy; rather, these doses increased the duration of scratching to approximately 57% of the observational periods. The differential induction of catalepsy-associated behavior or scratching is believed to be related to, respectively, pre- and postsynaptic activity of quinpirole on dopamine D2/D3 receptors. Pretreatment with the potassium channel blockers apamin, 4-aminopyridine, and amodiaquin attenuated the effects of quinpirole (0.03 mg/kg) on catalepsy-associated behavior, with cataleptic postures maintained for 27, 21, and 24% of the observational periods, respectively. In contrast, pretreatment with potassium channel blockers did not consistently affect the scratching induced by quinpirole. In addition, apamin did not attenuate the catalepsy-associated behavior induced by the postsynaptic D2 receptor antagonist haloperidol (0.01-0.1 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Antagonistic interaction between adenosine A2A receptors and dopamine D2 receptors in the ventral striopallidal system. Implications for the treatment of schizophrenia

Recent studies have shown the existence of a specific antagonistic interaction between adenosine A2a receptors and dopamine D2 receptors in the brain. This A2a-D2 interaction seems to be essential for the behavioural effects of adenosine agonists and antagonists, like caffeine. In the present study quantitative receptor autoradiography and brain microdialysis were combined to demonstrate a powerful antagonistic A2a-D2 interaction in the ventral striopallidal system. In the presence of the A2a agonist (2-p-carboxyethyl)phenylamino-5'-N carboxamidoadenosine, dopamine exhibited a lower efficacy in displacing the radiolabelled D2 receptor antagonist [125I]iodosulpiride from the rat ventral striatum, specially in the nucleus accumbens. A tonic dopaminergic modulation of the striopallidal neurons from the ventral striopallidal system was demonstrated by a dual-probe approach, by infusing selective dopamine agonists and antagonists in the nucleus and by measuring dopamine extracellular levels in the nucleus accumbens and GABA extracellular levels in the nucleus accumbens and in the ipsilateral ventral pallidum. The infusion of (2-p-carboxyethyl)phenylamino-5'-N-carboxamidoadenosine in the nucleus accumbens induced the same postsynaptic changes as the D2 antagonist raclopride, i.e. an increase in pallidal GABA extracellular levels, without changing those levels in the nucleus accumbens. Furthermore, the coinfusion in the nucleus accumbens of low concentrations of (2-p-carboxyethyl) phenylamino-5'-N-carboxamido-adenosine and raclopride, which were ineffective when administered alone, induced a significant increase in pallidal gamma-aminobutyric acids extracellular levels. These results suggest that A2a agonists, alone or in combination with D2 antagonists, could be advantageous antischizophrenic drugs, as blockage of D2 receptors in the ventral striopallidal system appears to be associated with the antipsychotic activity of neuroleptics but not with their extrapyramidal motor-side effects.

Role of NMDA, AMPA and kainate receptors in mediating glutamate- and 4-AP-induced dopamine and acetylcholine release from rat striatal slices

Striatal slices, preincubated with [3H]dopamine and [14C]choline, were superfused continuously and subjected to electrical field stimulation (3 Hz) and perfused with amino acid analogues or 4-amino pyridine (4-AP). The released radioactivity was used to monitor release of the neurotransmitters dopamine (DA) and acetylcholine (ACh). Glutamate, NMDA (in the absence of Mg2+), AMPA, kainic acid, domoate and 4-AP all induced DA and ACh release in a concentration-dependent manner. The DA and ACh release induced by NMDA (15 microM) and glutamate (1 mM) was essentially abolished by Mg2+ (1.15 mM), whereas release induced by AMPA (100 microM), kainic acid (100 microM) or 4-AP (30 microM) was not reduced. Tetrodotoxin (1 microM) essentially abolished the effects of NMDA, markedly reduced the effects of glutamate, AMPA and 4-AP, whereas the effect of kainic acid was only modestly affected. MK-801 (30 nM) reduced NMDA-induced DA release by some 70% and ACh release by 30%. MK-801 reduced 4-AP-induced DA release by 40% but not ACh release. CNQX in a concentration (10 microM) that scarcely affected NMDA-induced ACh release, but blocked that induced by AMPA, kainic acid or domoate, reduced the ACh release induced by 4-AP. In summary, DA and ACh release from rat striatum can be stimulated by activation of NMDA and non-NMDA glutamate receptors, and this mechanism is activated by the potassium channel blocker 4-AP.

The effect of theophylline on parkinsonian symptoms

Adenosine is known to inhibit the release of dopamine from central synaptic terminals. The present open trial was therefore conducted to determine whether the adenosine receptor-antagonist theophylline would be of value in Parkinson's disease. Fifteen parkinsonian patients were treated for up to 12 weeks with a slow release oral theophylline preparation (150 mg day-1), yielding serum theophylline levels of 4.44 mg L-1 after one week. The patients exhibited significant improvements in mean objective disability scores and 11 reported moderate or marked subjective improvement. It is suggested that theophylline might be a useful adjunct to the routine therapy of parkinsonian patients.

Effect of calcium channel blockers on norepinephrine release and modulation by prejunctional D2 dopamine receptors

Influx of calcium through voltage-dependent N-type calcium channels promotes the release of norepinephrine from nerve terminals. Contributions by L- and N-type calcium channels at different levels of nerve stimulation were examined in perfused rat tail arteries loaded with [3H]norepinephrine. Nifedipine had no effect while omega-conotoxin reduced tritium efflux by 69 to 82% depending on the stimulation intensity. Thus, N-type calcium channels predominated in the control of norepinephrine release, and the relative contribution of L- and N-type channels did not change when stimulation intensity was altered. We also explored the effect of calcium channel blockers on modulation of norepinephrine release by D2 dopamine receptors. Inhibition of stimulation-evoked tritium efflux by the D2 agonist N-0923 was similar in the absence and presence of nifedipine and/or omega-conotoxin. We conclude that D2 dopamine receptors are not coupled to L-type calcium channels; however, the role of N-type calcium channels requires further investigation.

Quinine and 4-aminopyridine inhibit the stimulatory output of dopamine in nucleus accumbens and the behavioural activity produced by morphine

We have tested the effects in rats of two potassium channel blocking drugs, 4-aminopyridine and quinine, on morphine-induced stimulation of behavioural activity and on dopamine outflow in nucleus accumbens using microdialysis. Morphine (1 mg/kg s.c.) increased dopamine output by 123% in nucleus accumbens. This dose of morphine also stimulated behavioural activity which in the early part of the time course corresponded closely with the increase of dopamine outflow in nucleus accumbens. Both of these effects were maximal 60-80 min after the morphine administration. 4-Aminopyridine (1 mg/kg i.p.) or quinine (50 mg/kg i.p.) injected 20 min before morphine inhibited the maximal effect on dopamine output by 88 and 80% respectively. Pretreatment with the two potassium channel blocking drugs also resulted in a reduction of morphine-induced stimulation of behavioural activity, 4-aminopyridine by 77% and quinine by 66%. In summary this study demonstrates that two drugs known to block potassium channels inhibit two effects of morphine associated with mesolimbic dopamine function.

Endogenous GABA release from rat striatal slices: effects of the GABAB receptor antagonist 2-hydroxy-saclofen

The reproducibility of endogenous GABA release evoked by multiple periods of electrical field stimulation was examined in rat striatal slices. In these experiments, NO-328 was used to block GABA uptake, and evoked GABA release (overflow) was completely Ca2+ dependent. A seemingly invariant observation in these experiments was that spontaneous GABA release (outflow) progressively decreased as a function of superfusion time and that GABA overflow decreased 25-30% in response to the second of two periods of stimulation (S2/S1 ratios = 0.70 to 0.75). The attenuation of GABA release was not explained by the amount of GABA lost to the superfusion buffer (fractional release), direct depletion of releasable pools of GABA, or slice viability. Furthermore, the decreases in GABA release were not dependent on stimulation frequency (5-15 Hz) or the absolute amount of GABA evoked by electrical stimulation. However, the GABAB receptor antagonist 2-hydroxy-saclofen (2-OH-saclofen; 316 microM) not only enhanced GABA overflow, when superfused throughout both periods of stimulation, but also resulted in S2/S1 ratios of unity. When 2-OH-saclofen was superfused throughout the second stimulation period only, GABA overflow was almost two-fold greater than that evoked by the initial period of stimulation (2-OH-saclofen-free). In addition, these S2 responses were approximately 30% greater than S1 responses that were observed when 2-OH-saclofen was present throughout the entire superfusion period. These results indicate that activation of GABAB receptors was involved in the progressive attenuation of GABA release and further emphasize that GABAB receptors play an important role in modulating endogenous GABA release from striatal slices.

Dendrotoxin blocks a class of potassium channels that are opened by inhibitory presynaptic modulators in rat cortical synaptosomes and slices

1. Rat cortical synaptosomes were prelabeled with radioactive acetylcholine and the release induced by veratridine was determined in the absence and presence of the inhibitory presynaptic modulators, 2-chloroadenosine, carbamylcholine, clonidine, and morphine. All four agents inhibited the evoked release of acetylcholine and this inhibition was reversed by dendrotoxin. 2. Using perfused cortical slices and an extracellular K-sensitive electrode, all modulators again increased K efflux that was blocked by dendrotoxin. In contrast, glybenclamide and tetraethylammonium did not block the modulator-induced efflux.

Influences of different adenosine receptor subtypes on catalepsy in mice

The effects of adenosine A1 and A2 receptors on catalepsy were studied in mice. The adenosine agonists 5-N'-ethylcarboxamide-adenosine (NECA), N6-phenylisopropyladenosine (PIA) and N6-cyclohexyladenosine (CHA) induced dose dependent catalepsy. The A1 adenosine antagonist 8-phenyltheophylline (8-PT) potentiated catalepsy induced by NECA, R-PIA and CHA. However, theophylline did not potentiate but inhibited the responses induced by NECA, R-PIA and CHA. Neither 8-PT nor theophylline alone has any effect on catalepsy in mice. It is concluded that catalepsy induced by the adenosine agonists may be due to A2 receptor stimulation and that the A1 antagonism may potentiate the response.