Activation of dopamine D4 receptor inhibits an L-type calcium current in cerebellar granule cells

Dopamine receptor D4 (DRD4) polymorphisms with reduced functional potency intensify atrophy in syndrome-specific sites of frontotemporal dementia

OBJECTIVE

We aimed to understand the impact of dopamine receptor D4 (DRD4) polymorphisms on neurodegeneration in patients with dementia. We hypothesized that DRD4dampened-variants with reduced functional potency would be associated with greater atrophy in regions with higher receptor density. Given that DRD4 is concentrated in anterior regions of the limbic and cortical forebrain we anticipated genotype effects in patients with a more rostral pattern of neurodegeneration.

METHODS

337 subjects, including healthy controls, patients with Alzheimer's disease (AD) and frontotemporal dementia (FTD) underwent genotyping, structural MRI, and cognitive/behavioral testing. We conducted whole-brain voxel-based morphometry to examine the relationship between DRD4 genotypes and brain atrophy patterns within and across groups. General linear modeling was used to evaluate relationships between genotype and cognitive/behavioral measures.

RESULTS

DRD4 dampened-variants predicted gray matter atrophy in disease-specific regions of FTD in anterior cingulate, ventromedial prefrontal, orbitofrontal and insular cortices on the right greater than the left. Genotype predicted greater apathy and repetitive motor disturbance in patients with FTD. These results covaried with frontoinsular cortical atrophy. Peak atrophy patterned along regions of neuroanatomic vulnerability in FTD-spectrum disorders. In AD subjects and controls, genotype did not impact gray matter intensity.

CONCLUSIONS

We conclude that DRD4 polymorphisms with reduced functional potency exacerbate neuronal injury in sites of higher receptor density, which intersect with syndrome-specific regions undergoing neurodegeneration in FTD.

Bidirectional modulation of hippocampal synaptic plasticity by Dopaminergic D4-receptors in the CA1 area of hippocampus

Long-term potentiation (LTP) is the persistent increase in the strength of the synapses. However, the neural networks would become saturated if there is only synaptic strenghthening. Synaptic weakening could be facilitated by active processes like long-term depression (LTD). Molecular mechanisms that facilitate the weakening of synapses and thereby stabilize the synapses are also important in learning and memory. Here we show that blockade of dopaminergic D4 receptors (D4R) promoted the formation of late-LTP and transformed early-LTP into late-LTP. This effect was dependent on protein synthesis, activation of NMDA-receptors and CaMKII. We also show that GABA_A-receptor mediated mechanisms are involved in the enhancement of late-LTP. We could show that short-term plasticity and baseline synaptic transmission were unaffected by D4R inhibition. On the other hand, antagonizing D4R prevented both early and late forms of LTD, showing that activation of D4Rs triggered a dual function. Synaptic tagging experiments on LTD showed that D4Rs act as plasticity related proteins rather than the setting of synaptic tags. D4R activation by PD 168077 induced a slow-onset depression that was protein synthesis, NMDAR and CaMKII dependent. The D4 receptors, thus exert a bidirectional modulation of CA1 pyramidal neurons by restricting synaptic strengthening and facilitating synaptic weakening.

Dopamine, cognitive function, and gamma oscillations: role of D4 receptors

Cognitive deficits in individuals with schizophrenia (SCZ) are considered core symptoms of this disorder, and can manifest at the prodromal stage. Antipsychotics ameliorate positive symptoms but only modestly improve cognitive symptoms. The lack of treatments that improve cognitive abilities currently represents a major obstacle in developing more effective therapeutic strategies for this debilitating disorder. While D4 receptor (D4R)-specific antagonists are ineffective in the treatment of positive symptoms, animal studies suggest that D4R drugs can improve cognitive deficits. Moreover, recent work from our group suggests that D4Rs synergize with the neuregulin/ErbB4 signaling pathway, genetically identified as risk factors for SCZ, in parvalbumin (PV)-expressing interneurons to modulate gamma oscillations. These high-frequency network oscillations correlate with attention and increase during cognitive tasks in healthy subjects, and this correlation is attenuated in affected individuals. This finding, along with other observations indicating impaired GABAergic function, has led to the idea that abnormal neural activity in the prefrontal cortex (PFC) in individuals with SCZ reflects a perturbation in the balance of excitation and inhibition. Here we review the current state of knowledge of D4R functions in the PFC and hippocampus, two major brain areas implicated in SCZ. Special emphasis is given to studies focusing on the potential role of D4Rs in modulating GABAergic transmission and to an emerging concept of a close synergistic relationship between dopamine/D4R and neuregulin/ErbB4 signaling pathways that tunes the activity of PV interneurons to regulate gamma frequency network oscillations and potentially cognitive processes.

Thyroid hormone and adrenergic signaling interact to control pineal expression of the dopamine receptor D4 gene (Drd4)

Dopamine plays diverse and important roles in vertebrate biology, impacting behavior and physiology through actions mediated by specific G-protein-coupled receptors, one of which is the dopamine receptor D4 (Drd4). Here we present studies on the >100-fold daily rhythm in rat pineal Drd4 expression. Our studies indicate that Drd4 is the dominant dopamine receptor gene expressed in the pineal gland. The gene is expressed in pinealocytes at levels which are approximately 100-fold greater than in other tissues, except the retina, in which transcript levels are similar. Pineal Drd4 expression is circadian in nature and under photoneural control. Whereas most rhythmically expressed genes in the pineal are controlled by adrenergic/cAMP signaling, Drd4 expression also requires thyroid hormone. This advance raises the questions of whether Drd4 expression is regulated by this mechanism in other systems and whether thyroid hormone controls expression of other genes in the pineal gland.

Genetics of dopamine receptors and drug addiction: a comprehensive review

Drug dependence is a chronic, relapsing disorder in which compulsive drug-seeking and drug-taking behaviours persist despite serious negative consequences. Addictive substances, such as opioids, ethanol, psychostimulants and nicotine, induce pleasant states or relieve distress, effects that contribute to their recreational use. Dopamine is critically involved in drug addiction processes. However, the role of the various dopaminergic receptor subtypes has been difficult to delineate. Here, we will review the information collected implicating the receptors of the D1 family (DRD1 and DRD5) and of the D2 family (DRD2, DRD3 and DRD4) in drug addiction. We will summarize the distribution of these receptors in the brain, the preclinical experiments carried out with pharmacological and transgenic approaches and the genetic studies carried out linking genetic variants of these receptors to drug addiction phenotypes. A meta-analysis of the studies carried out evaluating DRD2 and alcohol dependence is also provided, which indicates a significant association. Overall, this review indicates that different aspects of the addiction phenotype are critically influenced by dopaminergic receptors and that variants of those genes seem to influence some addiction phenotypes in humans.

Essential roles of dopamine D4 receptors and the type 1 adenylyl cyclase in photic control of cyclic AMP in photoreceptor cells

Light and dopamine regulate many physiological functions in the vertebrate retina. Light exposure decreases cyclic AMP formation in photoreceptor cells. Dopamine D(4) receptor (D(4)R) activation promotes light adaptation and suppresses the light-sensitive pool of cyclic AMP in photoreceptor cells. The key signaling pathways involved in regulating cyclic AMP in photoreceptor cells have not been identified. In the present study, we show that the light- and D(4)R-signaling pathways converge on the type 1 Ca(2+)/calmodulin-stimulated adenylyl cyclase (AC1) to regulate cyclic AMP synthesis in photoreceptor cells. In addition, we present evidence that D(4)R activation tonically regulates the expression of AC1 in photoreceptors. In retinas of mice with targeted deletion of the gene (Adcy1) encoding AC1, cyclic AMP levels and Ca(2+)/calmodulin-stimulated adenylyl cyclase activity are markedly reduced, and cyclic AMP accumulation is unaffected by either light or D(4)R activation. Similarly, in mice with disruption of the gene (Drd4) encoding D(4)R, cyclic AMP levels in the dark-adapted retina are significantly lower compared to wild-type retina and are unresponsive to light. These changes in Drd4-/- mice were accompanied by significantly lower Adcy1 mRNA levels in photoreceptor cells and lower Ca(2+)/calmodulin-stimulated adenylyl cyclase activity in retinal membranes compared with wild-type controls. Reduced levels of Adcy1 mRNA were also observed in retinas of wild-type mice treated chronically with a D(4)R antagonist, L-745870. Thus, activation of D(4)R is required for normal expression of AC1 and for the regulation of its catalytic activity by light. These observations illustrate a novel mechanism for cross-talk between dopamine and photic signaling pathways regulating cyclic AMP in photoreceptor cells.

Dopamine D4 receptors regulate intracellular calcium concentration in cultured chicken cone photoreceptor cells: relationship to dopamine receptor-mediated inhibition of cAMP formation

Dopamine is a retinal neuromodulator secreted from amacrine and interplexiform cells. Activation of dopamine D4 receptors on photoreceptor cells reduces a light-sensitive pool of cAMP. The aim of the present study was to evaluate the role of dopamine receptors and cAMP in the regulation of intracellular Ca(2+) concentrations ([Ca(2+)](i)) in photoreceptor cells of chick retina. Retinal cells from 6 day-old chicken embryos were isolated and cultured for 5-7 days prior to experiments. Cone photoreceptors were the predominant cell type in these cultures. Dopamine and agonists of dopamine D4 receptors suppressed K(+)-stimulated uptake of (45)Ca(2+) and [Ca(2+)](i), measured with the Ca(2+)-sensitive fluorescent dye fura-2AM. The effects of the agonists were blocked by dopamine D2/D4 receptor antagonists or by pertussis toxin. 8Br-cAMP, a cell-permeable analog of cAMP, had no effect on inhibition of K(+)-stimulated (45)Ca(2+) influx or [Ca(2+)](i) by dopamine D2/D4 receptor agonists. Quinpirole inhibited the increase in cAMP level elicited by K(+), which requires Ca(2+) influx through voltage-gated Ca(2+) channels, but not that induced by the calcium ionophore A23187. Moreover, dopamine had no effect on either forskolin-stimulated or Ca(2+)/calmodulin-stimulated adenylyl cyclase activity in cell membranes prepared from the cultured cells. These data indicate that the decrease of cAMP elicited by dopamine D4 receptor stimulation may be secondary to decreased [Ca(2+)](i).

Direct and remote modulation of L-channels in chromaffin cells: distinct actions on alpha1C and alpha1D subunits?

Understanding precisely the functioning of voltage-gated Ca2+ channels and their modulation by signaling molecules will help clarifying the Ca(2+)-dependent mechanisms controlling exocytosis in chromaffin cells. In recent years, we have learned more about the various pathways through which Ca2+ channels can be up- or down-modulated by hormones and neurotransmitters and how these changes may condition chromaffin cell activity and catecolamine release. Recently, the attention has been focused on the modulation of L-channels (CaV 1), which represent the major Ca2+ current component in rat and human chromaffin cells. L-channels are effectively inhibited by the released content of secretory granules or by applying mixtures of exogenous ATP, opioids, and adrenaline through the activation of receptor-coupled G proteins. This unusual inhibition persists in a wide range of potentials and results from a direct (membrane-delimited) interaction of G protein subunits with the L-channels co-localized in membrane microareas. Inhibition of L-channels can be reversed when the cAMP/PKA pathway is activated by membrane permeable cAMP analog or when cells are exposed to isoprenaline (remote action), suggesting the existence of parallel and opposite effects on L-channel gating by distinctly activated membrane autoreceptors. Here, the authors review the molecular components underlying these two opposing signaling pathways and present new evidence supporting the presence of two L-channel types in rat chromaffin cells (alpha1C and alpha1D), which open new interesting issues concerning Ca(2+)-channel modulation. In light of recent findings on the regulation of exocytosis by Ca(2+)-channel modulation, the authors explore the possible role of L-channels in the autocontrol of catecholamine release.

Dopamine D4 receptor-mediated presynaptic inhibition of GABAergic transmission in the rat supraoptic nucleus

The mechanism by which dopamine induces or facilitates neurohypophysial hormone release is not completely understood. Because oxytocin- and vasopressin-secreting supraoptic neurons are under the control of a prominent GABAergic inhibition, we investigated the possibility that dopamine exerts its action by modulating GABA-mediated transmission. Whole cell voltage-clamp recordings of supraoptic neurons were carried out in acute hypothalamic slices to determine the action of dopamine on inhibitory postsynaptic currents. Application of dopamine caused a consistent and reversible reduction in the frequency, but not the amplitude, of miniature synaptic events, indicating that dopamine was acting presynaptically to reduce GABAergic transmission. The subtype of dopamine receptor involved in this response was characterized pharmacologically. Dopamine inhibitory action was greatly reduced by two highly selective D4 receptor antagonists L745,870 and L750,667 and to a lower extent by the antipsychotic drug clozapine but was unaffected by SCH 23390 and sulpiride, D1/D5 and D2/D3 receptor antagonists, respectively. In agreement with these results, the action of dopamine was mimicked by the potent D4 receptor agonist PD168077 but not by SKF81297 and bromocriptine, D1/D5 and D2/D3 receptor agonists, respectively. Dopamine and PD168077 also reduced the amplitude of evoked inhibitory postsynaptic currents, an effect that was accompanied by an increase in paired-pulse facilitation. These data clearly indicate that D4 receptors are located on GABA terminals in the supraoptic nucleus and that their activation reduces GABA release in the supraoptic nucleus. Therefore dopaminergic facilitation of neurohypophysial hormone release appears to result, at least in part, from disinhibition of magnocellular neurons caused by the depression of GABAergic transmission.

D2-mediated modulation of N-type calcium currents in rat globus pallidus neurons following dopamine denervation

We have studied the effects of dopamine and the D2-like agonist quinpirole on calcium currents of neurons isolated from the striatum and the globus pallidus (GP). Experiments were performed in young adult rats, either in control conditions or following lesion of the nigrostriatal pathway by the unilateral injection of 6-hydroxydopamine (6-OHDA) in the substantia nigra. Apomorphine-driven contralateral turning, 15 days after lesioning, assessed the severity of the dopamine denervation. In addition, the loss of tyrosine hydroxylase immunohistochemistry confirmed the extent of the toxin-induced damage. In both striatal medium spiny (MS) and GP neurons of control animals dopamine and quinpirole promoted a very modest inhibition of calcium conductance. Following 6-OHDA, the inhibition was unaltered in MS (from 10 to 12%), but significantly augmented in GP neurons (21% vs. 9%). Interestingly, analogous inhibition was observed in GP neurons dissociated 20 h after reserpine treatment. Further features of the D2 response were thus studied only in neurons isolated from 6-OHDA-lesioned GP. The D2 modulation was G-protein-mediated but not strictly voltage-dependent. omega-Conotoxin-GVIA occluded the response implying the involvement of N-type calcium channels. The effect of quinpirole developed fast and was insensitive to alterations of cytosolic cAMP. The incubation in phorbol esters or OAG blocked the D2 effect, supporting the involvement of PKC. These findings suggest that postsynaptic D2-like receptors are functionally expressed on GP cell bodies and may supersensitize following dopamine-denervation. A direct D2 modulation of calcium conductance in GP may alter GP firing properties and GABA release onto pallidofugal targets.

Activation of melatonin receptor increases a delayed rectifier K+ current in rat cerebellar granule cells

The present study was initiated to investigate the effect of melatonin on K+ current in rat cerebellar granule cells for 2 to 6 days in culture (DIC). The whole-cell configuration of the conventional patch-clamp technique was used to record the outward K+ current. Two types of outward K+ current, a transient outward K+ current and a delayed rectifier K+ current, were separated by different voltage protocols and a specific blocker of K+ channel. Application of melatonin (10 microM) by a brief pressure ejection induced a significant and reversible increase of the delayed rectifier K+ current amplitude in 78% of the cells tested. The activated effect of melatonin on the K+ current was independent of the time in culture, and the percentage of activation remained at a relatively stable level from 2 DIC to 6 DIC; but that was dependent on the concentration of melatonin applied. The activation of the K+ current induced by melatonin presented no desensitization after repeated application of melatonin. The effect of melatonin on the K+ current can be mimicked by 2-iodomelatonin, a melatonin receptor agonist. With the addition of guanosine-5'-O-(3-thiophosphate) in the pipette solution, melatonin caused a stronger activation effect on the K+ channels, and an irreversible increase of the current amplitude in some granule cells tested. Pretreatment of cells with PTX suppressed the action of melatonin on the K+ current in most granule cells studied. In addition, the activation curves and inactivation curves tested with the steady-state activation and inactivation protocols were unchanged by melatonin, suggesting that melatonin did not modulate the channel's activation and inactivation properties. Our results demonstrated the presence of a functional melatonin receptor in cultured cerebellar granule cells from neonatal cerebellum. Activating the receptor can modulate the outward K+ currents by coupling to a PTX-sensitive G protein.

Dopamine D4 receptor activation inhibits presynaptically glutamatergic neurotransmission in the rat supraoptic nucleus

Oxytocin and vasopressin release from magnocellular neurons of the supraoptic nucleus is under the control of glutamate-dependent excitation. The supraoptic nucleus also receives a generalized dopaminergic input from hypothalamic sources. To determine if dopamine can influence this excitatory drive onto the magnocellular neurons, we used whole-cell patch clamp to record the effect of dopamine on evoked and miniature excitatory postsynaptic currents in rat hypothalamic slices. Dopamine exposure (30 microM to 1 mM) induced a large and reversible reduction in the amplitude of evoked excitatory postsynaptic current in nearly all magnocellular cells tested. D4 receptors appeared to mediate dopamine's activity, based on inhibition of the response with 50 microM clozapine, but not by SCH 23390 or sulpiride, and mimicry of dopamine's action with the D4 specific agonist, PD 168077. Analysis of paired-pulse experiments and miniature postsynaptic currents indicated that dopamine's action involved a presynaptic mechanism, since the frequency of miniature postsynaptic currents was reduced with dopamine exposure without any change in current kinetics or amplitude, while the paired-pulse ratio increased. We therefore have demonstrated for the first time a role for dopamine D4 receptors in the supraoptic nucleus in the presynaptic inhibition of glutamatergic neurotransmission onto magnocellular neurons.

The chemokine interleukin-8 acutely reduces Ca(2+) currents in identified cholinergic septal neurons expressing CXCR1 and CXCR2 receptor mRNAs

The chemokine IL-8 is known to be synthesized by glial cells in the brain. It has traditionally been shown to have an important role in neuroinflammation but recent evidence indicates that it may also be involved in rapid signaling in neurons. We investigated how IL-8 participates in rapid neuronal signaling by using a combination of whole-cell recording and single-cell RT-PCR on dissociated rat septal neurons. We show that IL-8 can acutely reduce Ca(2+) currents in septal neurons, an effect that was concentration-dependent, involved the closure of L- and N-type Ca(2+) channels, and the activation of G(ialpha1) and/or G(ialpha2) subtype(s) of G-proteins. Analysis of the mRNAs from the recorded neurons revealed that the latter were all cholinergic in nature. Moreover, we found that all cholinergic neurons that responded to IL-8, expressed mRNAs for either one or both IL-8 receptors CXCR1 and CXCR2. This is the first report of a chemokine that modulates ion channels in neurons via G-proteins, and the first demonstration that mRNAs for CXCR1 are expressed in the brain. Our results suggest that IL-8 release by glial cells in vivo may activate CXCR1 and CXCR2 receptors on cholinergic septal neurons and acutely modulate their excitability by closing calcium channels.

Dopamine D4 receptors and development of newer antipsychotic drugs

The last ten years have witnessed the generation of a large amount of information on the neurobiology of dopamine receptors. Molecular biology and pharmacology studies have revealed existence of at least five dopamine receptor subtypes, namely D1, D2, D3, D4 and D5. The discovery of D4 receptors and the putative affinity of clozapine for D4 receptors have kindled development of selective D4 receptor antagonists for the treatment of schizophrenia. Studies on expression of D4 receptor proteins have shown selective localisation of D4 receptors in mesolimbic/mesocortical areas which could probably explain the lack of motor side effects with atypical antipsychotics like clozapine and olanzapine. However, neuropathological and genetic studies on the role of D4 receptors in the pathophysiology of schizophrenia and preliminary clinical studies with selective D4 receptor antagonists have been disappointing. There have been, however, complimentary findings between selective D4 receptor antagonism and genetic approaches such as antisense treatment or gene targeting. The therapeutic potential of D4 receptors as a target for developing antipsychotics will be known only when selective D4 receptor-antagonists with varying D2/D4 and D4/5-HT2A ratios are developed and tested in psychiatric patients.

Differential modulation of rod and cone calcium currents in tiger salamander retina by D2 dopamine receptors and cAMP

Synaptic transmission from vertebrate photoreceptors involves activation of L-type calcium currents (ICa). Dopamine is an important circadian neuromodulator in the retina and photoreceptors possess D2 dopamine receptors. We examined modulation of ICa by dopamine and cAMP in retinal slices and isolated cells of larval tiger salamander. Results show that dopamine and a D2 agonist, quinpirole, enhanced ICa in rods and red-, blue- and UV-sensitive small single cones but inhibited ICa in red-sensitive large single cones. A D1 agonist, SKF-38393, was without effect. Quinpirole effects were blocked by pertussis toxin (PTx) pretreatment indicating involvement of PTx-sensitive G-proteins. Like dopamine, inhibition of cAMP-dependent protein kinase (PKA) by Rp-cAMPS enhanced ICa in rods and small single cones, but inhibited ICa in large single cones. In contrast, forskolin and Sp-cAMPS, which stimulate PKA, inhibited ICa in rods and small single cones but enhanced ICa in large single cones. Sp-cAMPS also occluded effects of quinpirole. These results suggest that D2 receptors modulate ICa via inhibition of cAMP. Differences among the responses of photoreceptors to cAMP are consistent with the possibility that small single cones and rods may possess different Ca2+ channel subtypes than large single cones. The results with dopamine and quinpirole showing inhibition of ICa in large single cones and enhancement of rod ICa were unexpected because previous studies have shown that dopamine suppresses rod inputs and enhances cone inputs into second-order neurons. The present results therefore indicate that the dopaminergic enhancement of cone inputs does not arise from modulation of photoreceptor ICa.

4-aminopyridine, a specific blocker of K(+) channels, inhibited inward Na(+) current in rat cerebellar granule cells

The effects of 4-aminopyridine (4-AP), a specific blocker of outward K(+) current, on voltage-activated transient outward K(+) current (I(K(A))) and inward Na(+) current (I(Na)) were investigated on cultured rat cerebellar granule cells using the whole cell voltage-clamp technique. At the concentration of 1-5 mM, 4-AP inhibited both I(K(A)) and I(Na). It reduced the amplitude of peak Na(+) current without significant alteration of the steady-state activation and inactivation properties. The inhibitory effect was not enhanced by repeated depolarizing pulses (0.5 or 0.1 Hz), suggesting that the binding affinity of 4-AP on Na(+) channels is state-independent. In contrast, the effect of 4-AP on Na(+) channels appeared to be voltage-dependent, the weaker inhibition occurred at more depolarization. Moreover, 4-AP slowed both the activation and inactivation kinetics of Na(+) current. These effects were similar to those induced by alpha-scorpion toxin and sea anemone toxins on Na(+) channels in other cell model. Our data demonstrate for the first time that 4-AP is able to block not only A-type K(+) channels, but also Na(+) channels in rat cerebellar granule cells. It is concluded that the inhibition exerted by 4-AP on Na(+) current likely differs from that provoked by local anesthetics. The possibility that the binding site of neurotoxin receptor 3 may be involved is discussed.

Presynaptic dopamine D(4) receptor localization in the rat nucleus accumbens shell

Dopamine D(4) receptors in the nucleus accumbens shell (AcbSh) are thought to play a key role in mediating the locomotor and sensitizing affects of psychostimulants, as well as the therapeutic efficacy of atypical antipsychotic drugs. We used electron microscopic immunocytochemistry to determine the functional sites for endogenous and exogenous D(4) receptor activation in this region. Of 1,090 D(4) receptor-labeled profiles observed in the AcbSh of rat brain, 65% were axons and axon terminals, while 22% were dendrites and dendritic spines. Within axons and terminals, D(4) receptor immunoreactivity was localized to segments of the plasma membrane and membranes of nearby vesicles. The axon terminals were morphologically heterogenous, varying in size and content of either all small synaptic vesicles (ssv), or ssv and large dense-core vesicles. The labeled terminals occasionally formed asymmetric excitatory-type axospinous synapses, but the majority were without recognizable synaptic specializations. In a separate series of tissue sections that were processed for dual-labeling of the D(4) receptor and the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH), 56% of all observed associations were appositions between differentially labeled axonal profiles, and 17% were terminals that contained immunoreactivity for both antigens. Dendritic spines containing D(4) receptor-labeling also received convergent input from TH-immunoreactive terminals and unlabeled terminals forming asymmetric synapses. These results provide the first ultrastructural evidence for a major presynaptic, and a more minor postsynaptic, involvement of D(4) receptors in dopaminergic modulation of excitatory transmission in the AcbSh.

Dopamine D4 receptors

Initial investigations on dopamine D4 receptors generated much interest in the role of this receptor in schizophrenia and other aspects of human behavior, as well as new opportunities for novel therapeutics. However, attempts to treat patients suffering from schizophrenia with dopamine D4 agents have failed to yield satisfactory results so far. An examination of the dopamine D4 literature shows that contrasting and conflicting data seemed to be the norm in this field of research. This paper reviews the literature on the dopamine D4 receptor and discusses many of the associated methodological problems that might have contributed to the paradoxical findings.

Forskolin-induced expression of tyrosine hydroxylase in human foetal brain cortex

Brain-derived neurotrophic factor (BDNF) has previously been shown by this and other laboratories to work in concert with dopamine (DA) to induce the dopaminergic phenotype in foetal rat and human cerebral cortex during specified sensitive developmental stages. In the present study this induction by BDNF/DA was found to be greatly amplified by adding forskolin (fsk: 10 microM) to the rat and human cerebral cortex cultures together with DA (10 microM) and BDNF (50 ng/ml). This amplification was 14-fold for human tissue and 2-fold for rat tissue treated over an 80% shorter period. Compared to treatment with BDNF alone, the additional fsk increased tyrosine hydroxylase-positive (TH+) cell numbers by 220-fold in the human and 26-fold in the rat tissue. Parallel reverse transcription-polymerase chain reaction (RT-PCR) measurement of TH mRNA showed substantial increases above control levels when BDNF/DA or BDNF/DA/fsk treatments were applied. Since fsk boosts intracellular levels of cyclic AMP (cAMP), its amplifying action when added together with BDNF/DA is likely to be due to interactions via the cAMP response element/cAMP response element binding protein (CRE/CREB) systems. This is discussed.

Renal dopamine receptors in health and hypertension

During the past decade, it has become evident that dopamine plays an important role in the regulation of renal function and blood pressure. Dopamine exerts its actions via a class of cell-surface receptors coupled to G-proteins that belong to the rhodopsin family. Dopamine receptors have been classified into two families based on pharmacologic and molecular cloning studies. In mammals, two D1-like receptors that have been cloned, the D1 and D5 receptors (known as D1A and D1B, respectively, in rodents), are linked to stimulation of adenylyl cyclase. Three D2-like receptors that have been cloned (D2, D3, and D4) are linked to inhibition of adenylyl cyclase and Ca2+ channels and stimulation of K+ channels. All the mammalian dopamine receptors, initially cloned from the brain, have been found to be expressed outside the central nervous system, in such sites as the adrenal gland, blood vessels, carotid body, intestines, heart, parathyroid gland, and the kidney and urinary tract. Dopamine receptor subtypes are differentially expressed along the nephron, where they regulate renal hemodynamics and electrolyte and water transport, as well as renin secretion. The ability of renal proximal tubules to produce dopamine and the presence of receptors in these tubules suggest that dopamine can act in an autocrine or paracrine fashion; this action becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension; disruption of the D1 or D3 receptor produces hypertension in mice. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to the hypertension. The molecular basis for the dopaminergic dysfunction in hypertension is not known, but may involve an abnormal post-translational modification of the dopamine receptor.

Review the role of dopamine D4 receptors in schizophrenia and antipsychotic action

For the past 20 years the most enduring explanation for schizophrenia has been the dopamine hypothesis, which proposes that the dopaminergic system is overactive in this widespread disease. Classically, the D2 receptor formed the core of the dopamine hypothesis since there was considerable evidence for elevations of D2 receptor levels in the brains of schizophrenic patients, and because these receptors served as the primary target in mediating antipsychotic effects of most neuroleptics. However, the dopamine D4 receptor has recently received particular attention in this context. This is because the atypical antipsychotic, clozapine, which is effective in treating refractory schizophrenics without the side-effect profile of typical neuroleptics, displays a 10-fold higher affinity for D4 compared to D2 or D3 receptors. Furthermore, the concentration in plasma water of clinical doses ofclozapine correlates well with its in vitro binding affinity for D4, but not D2 or D3 receptors, suggesting that D4 is a potential target in mediating clozapine's antipsychotic effects. As well, marked elevations in the level of a D4-like site (not identical to the D4 receptor) has been seen in the striatum of postmortem schizophrenic brains, but not in control brains. Finally, the most interesting feature of the D4 receptor is perhaps the array of polymorphisms associated with it, creating structural diversity in this receptor that supercedes all other known catecholamine receptors. The existence of these D4 polymorphisms raises the possibility that structural variations of this receptor may be associated with an increased susceptibility to schizophrenia, or observed variations in individual response to clozapine treatment. However, several studies aimed at investigating these hypotheses could not establish a direct role of D4 in schizophrenia. Furthermore, no association was evident between the polymorphic forms of D4 and susceptibility to schizophrenia, or variable clozapine response. Nevertheless, investigations surrounding this receptor has been far from futile. The observations which support the idea that D4 might serve as a target for clozapine have significantly modified and extended our understanding of mechanisms underlying atypical antipsychotic treatment of schizophrenia, as well as the dopamine hypothesis for schizophrenia. Further characterization of this receptor may prove to be crucial in designing highly effective antipsychotic drugs with minimal contraindications.

Signal transduction pathways modulated by the D2 subfamily of dopamine receptors

The D2 subfamily of dopamine receptors includes D2A, D2B, D3, and D4 dopamine receptors. These receptors activate cellular effector systems, principally through pertussis toxin-sensitive G-proteins. Historically, D2-like receptors in brain tissues were recognized as the dopamine receptor subtypes that inhibit adenylyl cyclase. Recent studies, reviewed here, have shown that multiple effector systems can be activated by these receptors, and the potential involvement of these in dopaminergic neutrotransmission is discussed.

Molecular attributes of dopamine receptors: new potential for antipsychotic drug development

The neurotransmitter dopamine (DA) plays a central role in the control of motor function, emotional states, and endocrine physiology. The discovery that schizophrenic symptoms can be alleviated by neuroleptic drugs and the finding that these drugs interact at dopamine receptors has indicated involvement of the dopamine system in schizophrenia. The dopamine system has recently been shown to rely on the activation of five distinct subtypes of DA receptors (D1-D5) identified by molecular cloning, and pharmacological studies have specifically implicated the D2-like receptors (D2R, D3R and D4R) in antipsychotic action. In addition, the localization of D3R and D4R expression in the mesolimbic/mesocortical DA pathways is consistent with their proposed involvement in affective behaviour, and suggests that drugs developed specifically for these receptor subtypes might have potent antipsychotic activity with a lower propensity for extrapyramidal, endocrine, and cognitive side-effects.

Adenosine inhibits L- and N-type calcium channels in pituitary melanotrophs. Evidence for the involvement of a G protein in calcium channel gating

It has been previously demonstrated that activation of A1 adenosine receptors in frog melanotrophs causes inhibition of spontaneous action potential discharges and alpha-melanocyte-stimulating hormone secretion. In the present study, we have investigated the effect of adenosine on high-voltage-activated (HVA) calcium currents in cultured melanotrophs, using the whole-cell variant of the patch-clamp technique with barium as a charge carrier. Adenosine and the specific A1 adenosine receptor agonist R-PIA (50 microM each) produced a decrease of the amplitude of the barium current, while the selective A2 adenosine receptor agonist CGS 21680 did not affect the current. The inhibitory effect of R-PIA was observed throughout the activation range of the current, with stronger responses at more positive potentials. R-PIA inhibited both the L- and N-type components of the current, the effect on the N-component being two-fold higher than on the L-component. The inhibitory effect of R-PIA was rendered irreversible by addition of GTP gamma S (100 microM) to the intracellular solution. Pre-treatment of the cells with pertussis toxin (1 microgram/ml; 12 h) totally abolished the effect of R-PIA on the HVA calcium channels. Conversely, addition of a high concentration of cAMP (100 microM) together with the phosphodiesterase inhibitor IBMX (100 microM) to the intracellular solution did not modify the effect of R-PIA on the current. It is concluded that, in frog melanotrophs, adenosine induces inhibition of L- and N-calcium currents and that this effect is mediated by a pertussis toxin-sensitive G protein. Our data also indicate that the inhibitory effect of adenosine on the calcium currents is not mediated by inhibition of adenylyl cyclase.

Pharmacological modifications in dopaminergic neurotransmission affect the quinpirole-evoked suppression of serotonin N-acetyltransferase activity in chick retina: an impact on dopamine D4-like receptors

Dopamine (DA) plays an important role in the regulation of melatonin biosynthesis in retinas of several vertebrate species. In the retina of chick, the DA receptor controlling melatonin production represents a D4-like subtype. Stimulation of this receptor by quinpirole (QNP) results in a dose-dependent decline of the nighttime activity of serotonin N-acetyltransferase (NAT; a key regulatory enzyme in melatonin biosynthesis) and melatonin level of chick retina. The present study was undertaken to determine whether long-term treatment with antipsychotic drugs (clozapine-30 mg/kg, i.m.; sulpiride-100 mg/kg, i.m.; and raclopride-10 mg/kg, i.p., once daily for 21 days) and L-DOPA (80 mg/kg, i.p., once daily for 7 days) affects the response of the melatonin generating system of chick retina to the suppressive effect of QNP. Chronic administration to chicks of clozapine and sulpiride, but not raclopride, resulted in a markedly increased response of retinal NAT activity to the action of QNP. ED50 values for QNP were 3-times (clozapine) and 4-times (sulpiride) lower than those in the respective vehicle-treated control groups. On the other hand, QNP was significantly less potent in retinas of birds treated with L-DOPA than in control animals; the ED50 value for QNP was 3-times higher in birds injected with L-DOPA than in the vehicle-treated group. These results indicate that long-term treatment with clozapine, sulpiride and L-DOPA may modify the reactivity of D4-like DA receptors regulating NAT activity of chick retina. A possibility of modifications of circadian and electrophysiological processes within the eye following prolonged administration of DA-ergic drugs is discussed.