Dopaminergic Receptors as Neuroimmune Mediators in Experimental Autoimmune Encephalomyelitis.

The dopaminergic system plays an essential role in maintaining homeostasis between the central nervous system (CNS) and the immune system. Previous studies have associated imbalances in the dopaminergic system to the pathogenesis of multiple sclerosis (MS). Here, we examined the protein levels of dopaminergic receptors (D1R and D2R) in different phases of the experimental autoimmune encephalomyelitis (EAE) model. We also investigated if the treatment with pramipexole (PPX)-a dopamine D2/D3 receptor-preferring agonist-would be able to prevent EAE-induced motor and mood dysfunction, as well as its underlying mechanisms of action. We report that D2R immunocontent is upregulated in the spinal cord of EAE mice 14 days post-induction. Moreover, D1R and D2R immunocontents in lymph nodes and the oxidative damage in the spinal cord and striatum of EAE animals were significantly increased during the chronic phase. Also, during the pre-symptomatic phase, axonal damage in the spinal cord of EAE mice could already be found. Surprisingly, therapeutic treatment with PPX failed to inhibit the progression of EAE. Of note, PPX treatment inhibited EAE-induced depressive-like while failed to inhibit anhedonic-like behaviors. We observed that PPX treatment downregulated IL-1ß levels and increased BNDF content in the spinal cord after EAE induction. Herein, we show that a D2/D3 receptor-preferred agonist mitigated EAE-induced depressive-like behavior, which could serve as a new possibility for further clinical trials on treating depressive symptoms in MS patients. Thus, we infer that D2R participates in the crosstalk between CNS and immune system during autoimmune and neuroinflammatory response induced by EAE, mainly in the acute and chronic phase of the disease.

Dopamine D1 and D2 receptors mediate analgesic and hypnotic effects of l-tetrahydropalmatine in a mouse neuropathic pain model.

RATIONALE: Levo-tetrahydropalmatine (l-THP), an active ingredient of Corydalis yanhusuo, has been reported to be a partial agonist for dopamine D1 receptors (D1R) and an antagonist for D2R. Although it has been safely used clinically in China for decades as an analgesic with sedative/hypnotic properties, there are few studies that address the mechanisms by which l-THP exerts its beneficial effects in chronic pain-induced sleep disturbance. OBJECTIVES: To investigate the effects and mechanisms of l-THP on sleep disturbance in a neuropathic pain-like condition. METHODS: A mouse model of chronic neuropathic pain induced by partial sciatic nerve ligation (PSNL) was employed. The antinociceptive and hypnotic effects of l-THP were evaluated by measurement of mechanical allodynia, thermal hyperalgesia, and electroencephalogram (EEG) recordings in PSNL mice. Pharmacological approaches and c-Fos expression were used to clarify the mechanisms of l-THP. RESULTS: Intraperitoneal injection of l-THP at 5 and 10 mg/kg not only significantly increased the mechanical threshold by 134.4% and 174.8%, and prolonged the thermal latency by 49.4% and 69.2%, but also increased non-rapid eye movement sleep by 17.5% and 29.6%, and decreased sleep fragmentation in PSNL mice, compared with the vehicle control. Moreover, the antinociceptive effect of l-THP was prevented by D1R antagonist SCH23390 or D2R agonist quinpirole; meanwhile, the hypnotic effect of l-THP was blocked by quinpirole rather than by SCH23390. Immunohistochemistry demonstrated that l-THP inhibited c-Fos overexpression induced by PSNL in the cingulate cortex and the periaqueductal gray. CONCLUSIONS: These findings indicated that l-THP exerted analgesic effects by agonism D1R and antagonism D2R, and the antagonism of D2R mediated the hypnotic effect of l-THP in PSNL mice.

Guibourtia tessmannii-induced fictive ejaculation in spinal male rat: involvement of D1, D2-like receptors.

CONTEXT: Guibourtia tessmannii (Caesalpiniaceae) is a plant traditionally used as aphrodisiac. We previously reported the pro-ejaculatory effects of the aqueous and methanol extracts of G. tesmannii in spinal male rat. However, the mechanism underlying such effects has not been elucidated. OBJECTIVE: This study characterizes the dopaminergic sub-type receptors involved in G. tesmannii-induced ejaculation in male Wistar rat. MATERIALS AND METHODS: Urethane-anesthetized spinal male rats were intravenously treated with saline solution (1 mL/kg, control); dopamine (0.1 µmol/kg, reference); aqueous or methanol extracts of G. tesmannii (20 mg/kg) in the absence or presence of haloperidol (0.26 µmol/kg), a nonspecific dopaminergic receptor antagonist, Sch23390 (0.26 µmol/kg), a specific D1-like receptor antagonist or, sulpiride (0.26 µmol/kg), a specific D2-like receptor antagonist. Electromyography of the bulbospongiosus muscles and intraseminal pressure were recorded after urethral, penile and drug stimulations. RESULTS: Urethral and penile stimulations, intravenous injection of dopamine or, aqueous and methanol extracts of G. tesmannii always triggered the expression of rhythmic contraction of the bulbospongiosus muscles with an average mean of 3.33 ± 0.43; 7.83 ± 0.85; 9.80 ± 0.86; 0.83 ± 0.54 and 2.67 ± 0.95 contractions, respectively. The intraseminal pressure was more expressed after urethral and penile stimulations (15.66 ± 1.58 and 13.60 ± 2.40 mmHg, respectively). In rats pretreated with haloperidol, Sch23390 or sulpiride, no ejaculation was recorded after intravenous injection of G. tesmannii extracts or dopamine. DISCUSSION AND CONCLUSION: Guibourtia tesmannii-induced ejaculation requires the integrity of D1 and D2-like receptors. These findings further justify the ethno-medicinal claims of G. tesmannii as an aphrodisiac.

Dopamine D1 Receptor-Mediated Transmission Maintains Information Flow Through the Cortico-Striato-Entopeduncular Direct Pathway to Release Movements.

In the basal ganglia (BG), dopamine plays a pivotal role in motor control, and dopamine deficiency results in severe motor dysfunctions as seen in Parkinson's disease. According to the well-accepted model of the BG, dopamine activates striatal direct pathway neurons that directly project to the output nuclei of the BG through D1 receptors (D1Rs), whereas dopamine inhibits striatal indirect pathway neurons that project to the external pallidum (GPe) through D2 receptors. To clarify the exact role of dopaminergic transmission via D1Rs in vivo, we developed novel D1R knockdown mice in which D1Rs can be conditionally and reversibly regulated. Suppression of D1R expression by doxycycline treatment decreased spontaneous motor activity and impaired motor ability in the mice. Neuronal activity in the entopeduncular nucleus (EPN), one of the output nuclei of the rodent BG, was recorded in awake conditions to examine the mechanism of motor deficits. Cortically evoked inhibition in the EPN mediated by the cortico-striato-EPN direct pathway was mostly lost during suppression of D1R expression, whereas spontaneous firing rates and patterns remained unchanged. On the other hand, GPe activity changed little. These results suggest that D1R-mediated dopaminergic transmission maintains the information flow through the direct pathway to appropriately release motor actions.

Reward-modulated motor information in identified striatum neurons.

It is widely accepted that dorsal striatum neurons participate in either the direct pathway (expressing dopamine D1 receptors) or the indirect pathway (expressing D2 receptors), controlling voluntary movements in an antagonistically balancing manner. The D1- and D2-expressing neurons are activated and inactivated, respectively, by dopamine released from substantia nigra neurons encoding reward expectation. However, little is known about the functional representation of motor information and its reward modulation in individual striatal neurons constituting the two pathways. In this study, we juxtacellularly recorded the spike activity of single neurons in the dorsolateral striatum of rats performing voluntary forelimb movement in a reward-predictable condition. Some of these neurons were identified morphologically by a combination of juxtacellular visualization and in situ hybridization for D1 mRNA. We found that the striatal neurons exhibited distinct functional activations before and during the forelimb movement, regardless of the expression of D1 mRNA. They were often positively, but rarely negatively, modulated by expecting a reward for the correct motor response. The positive reward modulation was independent of behavioral differences in motor performance. In contrast, regular-spiking and fast-spiking neurons in any layers of the motor cortex displayed only minor and unbiased reward modulation of their functional activation in relation to the execution of forelimb movement. Our results suggest that the direct and indirect pathway neurons cooperatively rather than antagonistically contribute to spatiotemporal control of voluntary movements, and that motor information is subcortically integrated with reward information through dopaminergic and other signals in the skeletomotor loop of the basal ganglia.

Dopamine D1 receptor signaling system regulates ryanodine receptor expression in ethanol physical dependence.

BACKGROUND: Ryanodine receptors (RyRs) amplifying activity-dependent calcium influx via calcium-induced calcium release play an important role in central nervous system functions including learning, memory, and drug abuse. In this study, we investigated the role and the regulatory mechanisms of RyR expression under continuous exposure of mice to ethanol (EtOH) vapor for 9 days. METHODS: The model of EtOH physical dependence was prepared as follows: 8-week-old male ddY mice were exposed to EtOH vapor for 9 days. Protein and mRNA of RyR-1, RyR-2, and RyR-3 in the frontal cortex and limbic forebrain were determined by Western blot and real-time RT-PCR analysis, respectively. RESULTS: Exposure of mice to EtOH vapor for 9 days induced significant withdrawal signs when estimated with withdrawal score, which was dose-dependently suppressed by intracerebroventricular administration of dantrolene, an RyR antagonist. Protein levels of RyR-1 and RyR-2 in the frontal cortex and limbic forebrain significantly increased during EtOH vapor exposure for 9 days with increased expression of their mRNA, whereas that of RyR-3 in these 2 brain regions showed no changes. Increased proteins and mRNA of RyR-1 and RyR-2 were completely abolished by SCH23390, a selective antagonist of dopamine D1 receptors (D1DRs), but not by sulpiride, a selective antagonist of D2DRs. CONCLUSIONS: RyRs play a critical role in the development of EtOH physical dependence and that the up-regulation of RyRs in the brain of mouse, showing EtOH physical dependence is regulated by D1DRs.

Visual input modulates audiomotor function via hypothalamic dopaminergic neurons through a cooperative mechanism.

Visual cues often modulate auditory signal processing, leading to improved sound detection. However, the synaptic and circuit mechanism underlying this cross-modal modulation remains poorly understood. Using larval zebrafish, we first established a cross-modal behavioral paradigm in which a preceding flash enhances sound-evoked escape behavior, which is known to be executed through auditory afferents (VIII(th) nerves) and command-like neurons (Mauthner cells). In vivo recording revealed that the visual enhancement of auditory escape is achieved by increasing sound-evoked Mauthner cell responses. This increase in Mauthner cell responses is accounted for by the increase in the signal-to-noise ratio of sound-evoked VIII(th) nerve spiking and efficacy of VIII(th) nerve-Mauthner cell synapses. Furthermore, the visual enhancement of Mauthner cell response and escape behavior requires light-responsive dopaminergic neurons in the caudal hypothalamus and D1 dopamine receptor activation. Our findings illustrate a cooperative neural mechanism for visual modulation of audiomotor processing that involves dopaminergic neuromodulation.

Dopamine depresses melanin concentrating hormone neuronal activity through multiple effects on α2-noradrenergic, D1 and D2-like dopaminergic receptors.

Two neuronal populations of the lateral hypothalamus that, respectively, produce melanin-concentrating hormone (MCH) and orexin peptides are crucially involved in control of metabolism, feeding and related goal-oriented behaviors. In contrast to orexin neurons, mainly involved in short-term regulation of feeding, MCH neurons participate in long-term control of energy storage and body weight. Beyond its effect on feeding, MCH has also been shown to be involved in regulation of seeking behavior and addiction through modulation of dopamine (DA) metabolism. This regulation is essential for reinforcement-associated behaviors. Moreover, drugs of abuse, which increase extracellular DA levels, are known to decrease food intake. Consistent with this observation, DA has been shown to modulate orexin neurons of the lateral hypothalamus. However, no study is available concerning the effects of DA on MCH neurons. Whole-cell patch-clamp recordings were done in hypothalamic mouse brain slices. MCH neurons were identified by Tau-Cyan-GFP labeling using a transgenic mouse model (MCH-GFP). First, we show that DA (10-200 µM) induces an outward current in MCH neurons. However, this current is not due to activation of DA receptors, but mediated through activation of α2-noradrenergic receptors and subsequent opening of G-protein activated inward rectifier K+ (GIRK) channels. Current-clamp experiments revealed that this GIRK-activation leads to hyperpolarization, thus decreasing excitability of MCH neurons. Furthermore, we confirm that MCH neurons receive mainly GABAergic inputs rather than glutamatergic ones. We show that DA modulates these inputs in a complex manner: at low concentrations, DA activates D1-like receptors, promoting presynaptic activity, whereas, at higher concentrations (100 µM), D2-like receptor activation inhibits presynaptic activity. Overall, DA should lead to a decrease in MCH neuron excitability, likely resulting in down-regulation of MCH release and feeding behavior.

Psychotropic drug-induced locomotor hyperactivity and prepulse inhibition regulation in male and female aromatase knockout (ArKO) mice: role of dopamine D1 and D2 receptors and dopamine transporters.

RATIONALE AND OBJECTIVES: The aim of the present study was to investigate the possible role of oestrogen in schizophrenia by comparing aromatase knockout (ArKO) mice, which are unable to produce oestrogen, with wild-type controls using two behavioural animal models with relevance to the illness, psychotropic drug-induced locomotor hyperactivity and prepulse inhibition (PPI). RESULTS: Baseline PPI was not different between ArKO and controls. Treatment with apomorphine, MK-801 and amphetamine caused disruption of PPI in all groups. However, in female but not male ArKO mice, the effect of both apomorphine and amphetamine was reduced. In female ArKO mice, amphetamine-induced hyperlocomotion was markedly reduced, but in male mice, the genotype difference was far smaller. Female but not male ArKO mice also showed a reduction of phencyclidine-induced locomotor hyperactivity. The density of dopamine transporters, but not D1 and D2 receptors, was significantly increased in the caudate putamen of male but not female ArKO mice compared to wild-type mice. This could represent a compensatory dopaminergic upregulation in male ArKO mice. CONCLUSION: Because of their lack of oestrogen production, it was anticipated that ArKO mice would display enhanced effects of amphetamine on locomotor activity and PPI. Instead, in these animals, aromatase knockout appeared to be 'protective'. This may represent limitations in the ability to model a complex illness such as schizophrenia in a constitutive knockout model, such as ArKO mice. Moreover, the current results may point at the involvement of other sex steroids, which are also altered in ArKO mice, in dopaminergic control of behaviour.

D2-like but not D1-like dopamine receptors are involved in the ventrolateral orbital cortex-induced antinociception: a GABAergic modulation mechanism.

The ventrolateral orbital cortex (VLO) is part of an endogenous analgesic system consisting of an ascending pathway from the spinal cord to VLO via the thalamic nucleus submedius (Sm) and a descending pathway to the spinal cord relaying in the periaqueductal gray (PAG). This study examines whether activation of D(1)-like and D(2)-like dopamine receptors in VLO produces antinociception and whether GABAergic modulation is involved in the VLO, D(2)-like dopamine receptor activation-evoked antinociception. The radiant heat-evoked tail flick (TF) reflex was used as an index of nociceptive response in lightly anesthetized rats. Microinjection of the D(2)-like (D(2)/D(3)) dopamine receptor agonist quinpirole (0.1-2.0 microg), but not D(1)-like (D(1)/D(5)) receptor agonist SKF-38393 (1.0, 5.0 microg), into VLO produced dose-dependent antinociception which was antagonized by the D(2)-like (D(2)/D(3)) receptor antagonist raclopride (1.5 microg). We also found that VLO application of the GABA(A) receptor antagonist bicuculline or picrotoxin (100 ng) enhanced the quinpirole-induced inhibition of the TF reflex, whereas the GABA(A) receptor agonist muscimol (250 ng) or THIP (1.0 microg) significantly attenuated the quinpirole-induced inhibition. These results suggest that D(2)-like, but not D(1)-like, dopamine receptors are involved in VLO-induced antinociception and that GABAergic disinhibitory mechanisms participate in the D(2)-like receptor mediated effect. These findings provide support for the hypothesis that D(2)-like receptor activation may inhibit the inhibitory action of the GABAergic interneurons on the output neurons projecting to PAG leading to activation of the brainstem descending inhibitory system and depression of nociceptive inputs at the spinal dorsal horn.

Acute dopamine D(1) and D(2) receptor stimulation does not modulate mismatch negativity (MMN) in healthy human subjects.

RATIONALE: Schizophrenia is commonly associated with an impairment in pre-attentive change detection, as represented by reduced mismatch negativity (MMN), an auditory event related potential. While the neurochemical basis of MMN has been linked to the integrity of the glutamatergic system involving N-methyl-D-aspartate (NMDA) receptors, the role of the dopaminergic system and in particular, the role of D(1) and D(2) receptors on MMN is yet to be determined. OBJECTIVES: The aim of the present project was to investigate the acute effects of dopamine D(2) (bromocriptine) and D(1)/D(2) (pergolide) receptor stimulation on the human MMN in healthy subjects. METHODS: Fifteen healthy male subjects participated in a double-blind, placebo-controlled, cross-over design in which each subject was tested under three acute treatment conditions separated by a 1-week wash out period; placebo, bromocriptine (2.5 mg) and pergolide (0.1 mg). The subjects were exposed to a duration-MMN paradigm with 50 ms standard tones (91%) and 100 ms deviant tones (9%). RESULTS: The results showed that neither D(2) receptor stimulation with bromocriptine, nor simultaneous D(1) and D(2) receptor stimulation with pergolide, modulated MMN. CONCLUSIONS: These findings suggest that acute D(1) and D(2) receptor stimulation does not modulate MMN. While the role of dopamine cannot be completely ruled out, the findings support the view that the aberrant MMN reported in schizophrenia may be linked primarily to glutamate dysfunction involving NMDA receptors.

Oxidative stress causes renal dopamine D1 receptor dysfunction and hypertension via mechanisms that involve nuclear factor-kappaB and protein kinase C.

Renal dopamine, via activation of D1 receptors, plays a role in maintaining sodium homeostasis and BP. There exists a defect in renal D1 receptor function in hypertension, diabetes, and aging, conditions that are associated with oxidative stress. However, the exact underlying mechanism of the oxidative stress-mediated impaired D1 receptor signaling and hypertension is not known. The effect of oxidative stress on renal D1 receptor function was investigated in healthy animals. Male Sprague-Dawley rats received tap water (vehicle) and 30 mM L-buthionine sulfoximine (BSO), an oxidant, with and without 1 mM tempol for 2 wk. Compared with vehicle, BSO treatment caused oxidative stress and increase in BP, which was accompanied by defective D1 receptor G-protein coupling and loss of natriuretic response to SKF38393. BSO treatment also increased NF-kappaB nuclear translocation, protein kinase C (PKC) activity and expression, G-protein-coupled receptor kinase-2 (GRK-2) membranous translocation, and D1 receptor serine phosphorylation. In BSO-treated rats' supplementation of tempol decreased oxidative stress, normalized BP, and restored D1 receptor G-protein coupling and natriuretic response to SKF38393. Tempol also normalized NF-kappaB translocation, PKC activity and expression, GRK-2 sequestration, and D1 receptor serine phosphorylation. In conclusion, these results show that oxidative stress activates NF-kappaB, causing an increase in PKC activity, which leads to GRK-2 translocation and subsequent D1 receptor hyper-serine phosphorylation and uncoupling. The functional consequence of this phenomenon was the inability of SKF38393 to inhibit Na/K-ATPase activity and promote sodium excretion, which may have contributed to increase in BP. Tempol reduced oxidative stress and thereby restored D1 receptor function and normalized BP.

Oxidative stress reduces renal dopamine D1 receptor-Gq/11alpha G protein-phospholipase C signaling involving G protein-coupled receptor kinase 2.

The dopamine D1 receptors (D1R), expressed in renal proximal tubules, participate in the regulation of sodium transport. A defect in the coupling of the D1R to its G protein/effector complex in renal tubules has been reported in various conditions associated with oxidative stress. Because G protein-coupled receptor kinases (GRKs) are known to play an important role in D1R desensitization, we tested the hypothesis that increased oxidative stress in obese Zucker rats may cause GRK2 upregulation and, subsequently, D1R dysfunction. Lean and obese rats were given normal diet or diet supplemented with antioxidant lipoic acid for 2 wk. Compared with lean rats, obese rats exhibited oxidative stress, D1R were uncoupled from G(q/11)alpha at basal level, and SKF-38393 failed to elicit D1R-G protein coupling, stimulate phospholipase C (PLC), and inhibit Na-K-ATPase activity. These animals showed increased basal protein kinase C (PKC) activity and membranous translocation of GRK2 and increased GKR2-G(q/11)alpha interaction and D1R serine phosphorylation. Enzymatic dephosphorylation of D1R restored SKF-38393-induced adenylyl cyclase stimulation but not PLC activation. Treatment of obese rats with lipoic acid restored D1R-G protein coupling and SKF-38393-induced PLC stimulation and Na-K-ATPase inhibition. Lipoic acid treatment also normalized PKC activity, GRK2 sequestration, and GKR2-G(q/11)alpha interaction. In conclusion, these data show that oxidative stress increases PKC activity causing GRK2 membranous translocation. GRK2 interacts with G(q/11)alpha and acts, at least in part, as a regulator of G protein signaling leading to the D1R-G(q/11)alpha uncoupling, causing inability of SKF-38393 to stimulate PLC and inhibit Na/K-ATPase. Lipoic acid, while reducing oxidative stress, normalized PKC activity and restored D1R-G(q/11)alpha-PLC signaling and the ability of SKF-38393 to inhibit Na-K-ATPase activity.

Peripheral modulation of dopaminergic receptors affects erectile responses in rats.

Clinical observations have suggested that dopaminergic mechanisms are involved in the regulation of male sexual responses, including erection. Apomorphine was initially reported to exert its erectogenic effects by modifying central dopaminergic activity. This study aimed primarily at evaluating and investigating the effect of apomorphine on erectile functions in rats and its potential effects on the cardiovascular system, as well as the possible role of dopaminergic stimulation in the peripheral control of erection. Measurement of intracavernosal pressure changes elicited by electrical stimulation of the cavernosal nerve in anaesthetized rats and mating tests were used. SCH23390, the D1 receptor antagonist, attenuated penile response to electrical stimulation. Intravenous administration of apomorphine in low dose (100 microg/kg), but not in high dose, significantly potentiated erectile responses to electrical stimulation. Intracavernosally injected apomorphine (50 microg/kg) significantly potentiated the filling rate of the corpora cavernosa 5 min. after injection, and did not induce erection in absence of electrical stimulation. In addition, apomorphine amplified the male sexual and copulatory behaviour by reducing ejaculation, mount and intromission latencies, and significantly increasing the number of ejaculations per session. However, apomorphine produced rapid and long-lasting hypotension and potentiated the hypotension and tachycardia associated with nerve-stimulated penile erection. Our results suggest that D1-dopaminergic receptors may be functionally involved in the peripheral mediation of penile erection. Apomorphine may amplify sexual and copulatory behaviour and may also, by a complementary role, amplify neurogenically mediated erections by acting in the periphery.

Omega-3 fatty acids and monoamine neurotransmission.

We proposed several years ago that the behavioral effects of n-3 PUFA deficiency observed in animal models might be mediated through the dopaminergic and serotonergic systems that are very involved in the modulation of attention, motivation and emotion. We evaluated this hypothesis in an extended series of experiments on rats chronically diet-deficient in alpha-linolenic acid, the precursor of long-chain n-3 PUFA, in which we studied several parameters of these neurotransmission systems. The present paper synthesizes the main data we obtained on interactions between n-3 PUFA status and neurotransmission in animal models. We demonstrated that several parameters of neurotransmission were affected, such as the vesicular pool of dopamine and serotonin, thus inducing several regulatory processes such as modification of cerebral receptors in specific brain areas. We also demonstrated that (i) a reversal diet with adequate n-6 and n-3 PUFA given during the lactating period to rats originating from alpha-linolenic acid-deficient dams was able to restore both the fatty acid composition of brain membranes and several parameters of the dopaminergic and serotonergic neurotransmission, and (ii) when given from weaning, this reversal diet allowed partial recovery of biochemical parameters, but no recovery of neurochemical factors. The occurrence of profound n-3 PUFA deficiency during the lactating period could therefore be an environmental insult leading to irreversible damage to specific brain functions. Strong evidence is now showing that a profound n-3 PUFA experimental deficiency is able to alter several neurotransmission systems, at least the dopaminergic and serotonergic. Whether these experimental findings can be transposed to human pathophysiology must be taken cautiously, but reinforces the hypothesis that strong links exist between the PUFA status, aspects of brain function such as neurotransmission processes and behavior.

Forebrain D1 function and sensorimotor gating in rats: effects of D1 blockade, frontal lesions and dopamine denervation.

Prefrontal D1 hypoactivity is implicated in the pathophysiology of schizophrenia, and might contribute to sensorimotor gating deficits in schizophrenia patients, based on evidence that D1 blockade in the medial prefrontal cortex (MPFC) reduces prepulse inhibition of startle (PPI) in animal models. PPI is disrupted by systemic and intra-MPFC infusion of the D1 antagonist, SCH23390. We investigated the role of the MPFC in the PPI-disruptive effects of systemic SCH23390 administration, and more generally, in the dopaminergic regulation of PPI. PPI was measured in rats after forebrain manipulations, including systemic administration of SCH23390, ibotenic acid lesions of the MPFC, and 6OHDA-induced dopamine (DA) depletion from MPFC or nucleus accumbens. Systemic SCH23390 disrupted PPI; these effects were not opposed by ibotenic acid lesions of the MPFC. PPI remained intact after MPFC DA depletion, but--as predicted by Bubser and Koch [M. Bubser, M. Koch, Prepulse inhibition of the acoustic startle response of rats is reduced by 6 hydroxydopamine lesions of the medial prefrontal cortex, Psychopharmacology 113 (1994) 487-492]--a reduction in PPI from pre- to post-surgery correlated significantly with MPFC DA loss. The effects of systemic SCH23390 were not opposed by NAC DA depletion. D1 receptors regulate PPI in rats, but this effect does not appear to be mediated either by the MPFC or by increased mesolimbic DA activity.

Tempol reduces oxidative stress and restores renal dopamine D1-like receptor- G protein coupling and function in hyperglycemic rats.

Dopamine via activation of renal D1-like receptors inhibits the activities of Na-K-ATPase and Na/H exchanger and subsequently increases sodium excretion. Decreased renal dopamine production and sodium excretion are associated with hyperglycemic conditions. We have earlier reported D1-like receptor-G protein uncoupling and reduced response to D1-like receptor activation in streptozotocin (STZ)-treated hyperglycemic rats (Marwaha A, Banday AA, and Lokhandwala MF. Am J Physiol Renal Physiol 286: F451-F457, 2004). The present study was designed to test the hypothesis that oxidative stress associated with hyperglycemia increases basal D1-like receptor serine phosphorylation via activation of the PKC-G protein receptor kinase (GRK) pathway, resulting in loss of D1-like receptor-G protein coupling and function. We observed that STZ-treated rats exhibited oxidative stress as evidenced by increased lipid peroxidation. Furthermore, PKC activity and expression of PKC-betaI- and -delta-isoforms were increased in STZ-treated rats. In addition, in STZ-treated rats there was increased GRK2 translocation to proximal tubular membrane and increased basal serine D1-like receptor phosphorylation. Supplementation with the antioxidant tempol lowered oxidative stress in STZ-treated rats, led to normalization of PKC activity, and prevented GRK2 translocation. Furthermore, tempol supplementation in STZ-treated rats restored D1-like receptor-G protein coupling and inhibition of Na-K-ATPase activity on D1-like receptor agonist stimulation. The functional consequence was the restoration of the natriuretic response to D1-like receptor activation. We conclude that oxidative stress associated with hyperglycemia causes an increase in activity and expression of PKC. This leads to translocation of GRK2, subsequent phosphorylation of the D1-like receptor, its uncoupling from G proteins and loss of responsiveness to agonist stimulation.

Overexpression of dopamine receptor genes and their products in the postnatal rat brain following maternal n-3 fatty acid dietary deficiency.

A combination of PCR-Select cDNA subtraction and gene array hybridization was used to identify differentially expressed genomic markers in brains of rats fed for 3 weeks in utero and 2 weeks after birth on an n-3 polyunsaturated fatty acid (PUFA)-deficient diet supplied to dams. Total RNA was isolated, switch mechanism at 5'-end of the RNA transcripts (SMART) applied and used for PCR-Select subtraction of PUFA-deficient and adequately-fed control preparations. Subtracted and amplified ds-cDNA end-products were fragmented, terminally labeled with biotin-ddUTP and hybridized with a RN-U34A gene array. A 10-fold increase in potential genes with log2(Tester/Driver) = 1.4 was found compared with traditional gene array technology when the same chip was tested using non-subtracted targets. Reverse transcription-real-time relative PCR confirmed 30% of the transcripts. Among the validated transcripts, D1 and D2 receptors for dopamine (DA), were most prominent among a number of over-expressed neurotransmitter receptors and retinoic acid receptor (RXR alpha-2 and alpha-1). Immunohistochemical staining of brain sections from 2-week-old pups revealed a substantial enrichment of the D2 receptor in discrete regions of the mesolimbic and mesocortical pathways as well as in a large number of brain areas from the n-3 PUFA-deficient pups. Punches of the same areas run on western blots showed similar results. The overwhelming expression of D1 and D2 receptors may be attributed to a behavioral hypersensitivity caused by the possible impairment of DA production during brain development, which may have implications in certain disorders of the nervous system.

Dopamine D(2)-like receptor function is converted from excitatory to inhibitory by thyroxine in the developmental hippocampus.

The mechanism by which a lack of thyroid hormone in the early development of the brain causes permanent mental retardation in cretins is currently unknown. On the other hand, an abnormality in dopamine-related brain function is believed to underlie some forms of mental illness. In this study, we demonstrate that although the activation of a dopaminergic D(2)-like receptor inhibited glutamatergic transmission in the hippocampal slices of normal adult rats, indicating the inhibitory action of the D(2)-like receptor on glutamatergic transmission, it markedly enhanced glutamatergic transmission both in a mutant hypothyroid rat with a missense mutation in thyroglobulin and in hypothyroid rats treated with methylmercaptoimidazole (MMI), indicating the excitatory action of the D(2)-like receptor on glutamatergic transmission. Paired pulse facilitation of field excitatory postsynaptic potentials was reduced by the activation of the D(2)-like receptors from MMI-induced hypothyroid rats, suggesting a presynaptic locus of the excitatory action of the D(2)-like receptors. In normal rats, the excitatory D(2)-like dopamine receptors were observed in the developing stages and were completely replaced by normal inhibitory responses up to adulthood. Furthermore, the continuous supplement of thyroxine from birth exerted a normalising effect on the abnormal excitatory property of D(2)-like dopamine receptors in the hippocampal slices of MMI-treated hypothyroid rats. From these results, it is suggested that thyroxine may play a crucial role in reversing the excitatory property of D(2)-like dopaminergic receptors in the immature brain to an inhibitory one in the mature brain. Moreover, we suggest that the abnormal excitatory property of D(2)-like dopaminergic receptors may develop in response to a lack of thyroxine and may contribute to some central nervous system deficits, including cognitive dysfunctions accompanied by hypothyroidism.

Prefrontal D1 and ventral hippocampal N-methyl-D-aspartate regulation of startle gating in rats.

BACKGROUND: Sensorimotor gating, as measured by prepulse inhibition of the startle reflex, is deficient in schizophrenia patients, and in rats after specific manipulations of limbic cortico-striato-pallido-thalamic circuitry. For example, prepulse inhibition in rats is disrupted after D1 blockade in the medial prefrontal cortex, and after N-methyl-D-aspartate infusion into the ventral hippocampus. In the present study, we examined whether these two substrates form part of an integrated circuit regulating sensorimotor gating, which might contribute to the loss of prepulse inhibition in patient populations. METHODS: Prepulse inhibition was assessed in male Sprague-Dawley rats after systemic or intra-medial prefrontal cortex administration of the D1 antagonist, SCH 23390. Separate rats received intra-medial prefrontal cortex infusion of the retrograde transported label Fluoro-Gold. In rats with sham or electrolytic lesions of the medial prefrontal cortex, prepulse inhibition was tested after infusion of N-methyl-D-aspartate or vehicle into ventral hippocampus regions that were determined to send projections to the medial prefrontal cortex. RESULTS: Prepulse inhibition was disrupted after systemic SCH 23390 treatment and after infusion of SCH 23390 into medial prefrontal cortex sites within the prelimbic and cingulate cortices. Fluoro-Gold infusion into these medial prefrontal cortex sites labeled cells in the ventral hippocampus complex, including regions CA1 and entorhinal cortex. N-methyl-D-aspartate infusions into these ventral hippocampus regions disrupted prepulse inhibition in rats after sham but not electrolytic lesions of the medial prefrontal cortex. CONCLUSIONS: Prepulse inhibition appears to be regulated by interacting substrates within the ventral hippocampus and MPFC. Specifically, NMDA activation of the ventral hippocampus appears to disrupt prepulse inhibition in a manner that is dependent on the integrity of infralimbic or cingulate cortical regions that also support a D1-mediated regulation of prepulse inhibition. Conceivably, dysfunction within these hippocampal-frontal circuits may contribute to sensorimotor gating deficits in schizophrenia.