D1R Activation Research Articles

AB007. Visual signals modulate refractive error development through dopamine receptor signaling

: Myopia prevalence is dramatically increasing in recent years and in cases in which the refractive error is greater than −6.00 D this disease can lead to severe visual impairment as well as even blindness. Changes in visual input affect the balance between ocular growth and refractive power development. If a mismatch occurs during eye development, the severity of this error affects the degree of myopia. In different animal models of this disease, we found that spatial visual stimuli are essential for maintaining a stable refractive status and normal vision. This is evident because the effects of changes in temporal visual stimuli (e.g., flickering light) on this process depend on whether spatial information is present or absent in the visual environment. Furthermore, the frequency, wavelength and intensity of light are involved in controlling refraction development. However, the molecular mechanisms underlying light-induced refraction changes are still unclear. There is definitive evidence that dopamine (DA) is one of the regulators of this process. This retinal neurotransmitter released by dopaminergic amacrine cells appears to play an important role in vision-guided eye growth because its synthesis and release are positively associated with the light intensity and spatial stimuli impinging on the retina. We found that bright light enhances retinal DA synthesis, and attenuates form deprivation myopia (FDM) development via activation of the dopamine receptor 1 (D1R). A nonselective DA receptor agonist apomorphine (APO) inhibited FDM in dopamine receptor 2 (D2R) knockout mice. These individual similar effects of DA and APO in wildtype and D2R knockout mice suggest that D1R activation has a protective effect against myopia development. On the other hand, D2R activation instead appears to promote myopia development because either genetic D2R ablation or pharmacological inactivation of D2R also attenuates myopia development. Based on these results, we hypothesize that the visual environment regulates the retinal DA levels, which in turn affects the relative balance between D1R and D2R activation. When D1R is relatively hyperactivated, the ocular refractive status shifts towards hyperopia. In contrast, such an effect on D2Rpromotes the refractive status to shift in the opposite direction towards myopia.

The role of central and peripheral D2R receptors in the mechanism of colonic vascular permeability during experimental colitis in rats

We investigated the involvement of central and peripheral D2 dopaminergic receptors in the mechanism of vascular permeability in rat's colon during experimental ulcerative colitis. Ulcerative colitis was induced in male white inbred rats by 6 % iodoacetamide enema. For the investigation of central and peripheral D2R, separate and joint injections of D2R antagonist domperidone (2 mg/100 g, per os) and D2R agonist quinpirole (1 mg/100 g, per os) were applied. Central D2R were destroyed by neurotoxin injection – 6OHDA. Colonic vascular permeability was measured by colonic extravasation of 1,5 % Evans blue. It was observed that blockade of peripheral D2R decreased colonic vascular permeability, while simultaneous activation of central D2R and inhibition of peripheral D2R have additive positive effect in prevention of increased colonic vascular permeability during experimental colitis.

D1-like dopamine receptor dysfunction in the lateral habenula nucleus increased anxiety-like behavior in rat

Lateral habenula (LHb) is important for emotional processing. It is a link node between forebrain and midbrain. LHb is reciprocally connected with ventral tegmental area, acting as a regulatory center for the dopaminergic system. However, the role of dopamine receptors in the LHb in emotional processing is less clear. In the present study, the expression of dopamine D1 and D2 receptors in LHb was testified by western blot. In addition, D1- or D2-like receptor agonist or antagonist was bilaterally administered into the LHb, anxiety-like and depressive-like behaviors were tested 15min later in rats. In addition, the effects of LHb dopamine receptor activation and inactivation on aversive learning and memory were assessed. Our results showed that: (1) activation and inhibition of D1R but not D2R in LHb increased anxiety-like behavior but decreased depressive-like behavior in rats. (2) D1R activation and inactivation in LHb impaired aversive memory acquisition but not consolidation in rats, D1R agonist also impaired aversive memory retrieval in rats. These results might provide new clues about how LHb was involved in emotional processing.

Nigrostriatal dopaminergic depletion increases static orofacial allodynia.

BackgroundThis study investigated mesencephalic dopamine depletion effects on static mechanical allodynia (SMA) elicited by chronic constriction of the infraorbitary nerve (CCI-IoN).MethodsDopamine depletion (6-OHDA administration into the medial forebrain bundle) effects on CCI-IoN-induced SMA were explored using behavioral (nocifensive behavior score upon non-noxious stimuli using von Frey filament), pharmacological (bromocriptine injections) and immunohistochemical (PKCγ and pERK1/2) techniques.ResultsThe central dopamine depletion increased significantly the SMA score. Intraperitoneal and intracisternal injections of bromocriptine alleviated the allodynic behavior observed in both CCI-IoN and CCI-IoN + 6-OHDA animal groups. At the cellular level, dopamine depletion induced a significant increase in PKCγ expression in the medullary dorsal horn (MDH) in rat with CCI-IoN + 6-OHDA when compared to sham animals (CCI-IoN only). Similarly, after static non-noxious stimuli, the expression of pain marker proteins pERK1/2 within the MDH revealed significantly a higher number of positive cells in CCI-IoN + 6-OHDA rats when compared to the CCI-IoN group.ConclusionThis study demonstrates that nigrostriatal dopamine depletion exacerbates the neuropathic pain resulting from CCI-IoN. This effect is probably due to an action through descending pain inhibitory systems which increased pain sensitization at the MDH level. It demonstrates also an analgesic effect elicited by D2R activation at the segmental level.Electronic supplementary materialThe online version of this article (doi:10.1186/s10194-016-0607-z) contains supplementary material, which is available to authorized users.

Trans-activation of TRPV1 by D1R in mouse dorsal root ganglion neurons

TRPV1, a ligand-gated ion channel expressed in nociceptive sensory neurons is modulated by a variety of intracellular signaling pathways. Dopamine is a neurotransmitter that plays important roles in motor control, cognition, and pain modulation in the CNS, and acts via a variety of dopamine receptors (D1R–D5R), a class of GPCRs. Although nociceptive sensory neurons express D1-like receptors, very little is known about the effect of dopamine on TRPV1 in the peripheral nervous system. Therefore, in this study, we examined the effects of D1R activation on TRPV1 in mouse DRG neurons using Ca2+ imaging and immunohistochemical analysis. The D1R agonist SKF-38393 induced reproducible Ca2+ responses via Ca2+ influx through TRPV1 rather than Ca2+ mobilization from intracellular Ca2+ stores. Immunohistochemical analysis revealed co-expression of D1R and TRPV1 in mouse DRG neurons. The PLC-specific inhibitor blocked the SKF-38393-induced Ca2+ response, whereas the PKC, DAG lipase, AC, and PKA inhibitors had no effect on the SKF-38393-induced Ca2+ response. Taken together, our results suggest that the SKF-38393-induced Ca2+ response results from the direct activation of TRPV1 by a PLC/DAG-mediated membrane-delimited pathway. These results provide evidence that the trans-activation of TRPV1 following D1R activation may contribute to the modulation of pain signaling in nociceptive sensory neurons.

Social behavior in a genetic model of dopamine dysfunction at different neurodevelopmental time points.

Impairments in social behavior characterize many neurodevelopmental psychiatric disorders. In fact, the temporal emergence and trajectory of these deficits can define the disorder, specify their treatment and signal their prognosis. The sophistication of mouse models with neurobiological endophenotypes of many aspects of psychiatric diseases has increased in recent years, with the necessity to evaluate social behavior in these models. We adapted an assay for the multimodal characterization of social behavior at different development time points (juvenile, adolescent and adult) in control mice in different social contexts (specifically, different sex pairings). Although social context did not affect social behavior in juvenile mice, it did have an effect on the quantity and type of social interaction as well as ultrasonic vocalizations in both adolescence and adulthood. We compared social development in control mice to a transgenic mouse model of the increase in postsynaptic striatal D2R activity observed in patients with schizophrenia (D2R-OE mice). Genotypic differences in social interactions emerged in adolescence and appeared to become more pronounced in adulthood. That vocalizations emitted from dyads with a D2R-OE subject were negatively correlated with active social behavior while vocalizations from control dyads were positively correlated with both active and passive social behavior also suggest social deficits. These data show that striatal dopamine dysfunction plays an important role in the development of social behavior and mouse models such as the one studied here provide an opportunity for screening potential therapeutics at different developmental time points.

Differentiation of forebrain and hippocampal dopamine 1-class receptors, D1R and D5R, in spatial learning and memory.

Activation of prefrontal cortical (PFC), striatal, and hippocampal dopamine 1-class receptors (D1R and D5R) is necessary for normal spatial information processing. Yet the precise role of the D1R versus the D5R in the aforementioned structures, and their specific contribution to the water-maze spatial learning task remains unknown. D1R- and D5R-specific in situ hybridization probes showed that forebrain restricted D1R and D5R KO mice (F-D1R/D5R KO) displayed D1R mRNA deletion in the medial (m)PFC, dorsal and ventral striatum, and the dentate gyrus (DG) of the hippocampus. D5R mRNA deletion was limited to the mPFC, the CA1 and DG hippocampal subregions. F-D1R/D5R KO mice were given water-maze training and displayed subtle spatial latency differences between genotypes and spatial memory deficits during both regular and reversal training. To differentiate forebrain D1R from D5R activation, forebrain restricted D1R KO (F-D1R KO) and D5R KO (F-D5R KO) mice were trained on the water-maze task. F-D1R KO animals exhibited escape latency deficits throughout regular and reversal training as well as spatial memory deficits during reversal training. F-D1R KO mice also showed perseverative behavior during the reversal spatial memory probe test. In contrast, F-D5R KO animals did not present observable deficits on the water-maze task. Because F-D1R KO mice showed water-maze deficits we tested the necessity of hippocampal D1R activation for spatial learning and memory. We trained DG restricted D1R KO (DG-D1R KO) mice on the water-maze task. DG-D1R KO mice did not present detectable spatial memory deficit, but did show subtle deficits during specific days of training. Our data provides evidence that forebrain D5R activation plays a unique role in spatial learning and memory in conjunction with D1R activation. Moreover, these data suggest that mPFC and striatal, but not DG D1R activation are essential for spatial learning and memory.

Nigrostriatal dopaminergic depletion produces orofacial static mechanical allodynia.

Dopamine is implicated in different orofacial pain-related diseases. The mechanisms underlying this invalidating pain are not yet understood. Therefore, the present study investigated if unilateral or bilateral lesions of the medial forebrain bundle (MFB) could induce a trigeminal static mechanical allodynia (SMA) comparable to that obtained after chronic construction injury of the infraorbital nerve (CCI-IoN) in rats. Unilateral and bilateral nigrostriatal lesions were obtained by injecting 6-hydroxydopamine into the MFB, and peripheral lesion was obtained by CCI-IoN. Static allodynia behaviour was tested by a mild non-noxious static von Frey filament stimulus. The analgesic effects of bromocriptine (D2R agonist) were assessed by both intraperitoneal and intracisternal injections. Finally, immunohistochemical study was done to investigate the implication of the protein kinase c isoform gamma (PKCγ) and the phosphorylated form of extracellular signal-related kinase 1/2 (pERK1/2) in pain sensitization at segmental level. 6-OHDA-lesioned animals developed SMA in the orofacial region as assessed by non-noxious stimuli. Intraperitoneal and intracisternal injections of bromocriptine alleviated this allodynic behaviour. Investigations within the medullary dorsal horn revealed an increase in PKCγ expression, a protein implicated in the chronicity of pain, within superficial laminae in 6-OHDA-lesioned rats. Also static mechanical stimulations of the orofacial region evoked increased expression of the molecular pain marker pERK1/2 in 6-OHDA-lesioned rats. Our data show that unilateral and bilateral dopamine depletion promoted trigeminal SMA comparable to that obtained after CCI-IoN. This allodynia can be alleviated by D2R activation, making D2R agonist a potential analgesic for orofacial neuropathic pain.

Cannabinoid CB1 receptors activation and coactivation with D2 receptors modulate GABAergic neurotransmission in the globus pallidus and increase motor asymmetry.

The cannabinoid CB1 (CB1R) and dopaminergic D2 (D2R) receptors modify GABAergic transmission in the globus pallidus. Although dopaminergic denervation produces changes in the expression and supersensitization of these receptors, the consequences of these changes on GABAergic neurotransmission are unknown. The aim of this study was to show the effects of CB1R and D2R activation and coactivation on the uptake and release of [(3) H]GABA in the globus pallidus of hemiparkinsonian rats as well as their effects on motor behavior. The activation of CB1R blocked GABA uptake and decreased GABA release in the globus pallidus in the dopamine denervated side, whereas the co-activation of CB1R-D2R increased GABA release and had no effect on GABA uptake. A microinjection of the CB1R agonist ACEA into the globus pallidus ipsilaterally to a 6-OHDA lesion potentiated turning behavior that was induced by methamphetamine. However, a microinjection of the D2R agonist quinpirole did not modify this behavior, and a microinjection of a mixture of CB1R and D2R agonists significantly potentiated turning behavior. The behavioral effects produced after the activation of the CB1R and the co-activation of CB1R and D2R can be explained by increased GABAergic neurotransmission produced by a block of GABA uptake and an increase in the release of GABA in the globus pallidus, respectively.

Dopamine receptor 1 neurons in the dorsal striatum regulate food anticipatory circadian activity rhythms in mice.

Daily rhythms of food anticipatory activity (FAA) are regulated independently of the suprachiasmatic nucleus, which mediates entrainment of rhythms to light, but the neural circuits that establish FAA remain elusive. In this study, we show that mice lacking the dopamine D1 receptor (D1R KO mice) manifest greatly reduced FAA, whereas mice lacking the dopamine D2 receptor have normal FAA. To determine where dopamine exerts its effect, we limited expression of dopamine signaling to the dorsal striatum of dopamine-deficient mice; these mice developed FAA. Within the dorsal striatum, the daily rhythm of clock gene period2 expression was markedly suppressed in D1R KO mice. Pharmacological activation of D1R at the same time daily was sufficient to establish anticipatory activity in wild-type mice. These results demonstrate that dopamine signaling to D1R-expressing neurons in the dorsal striatum plays an important role in manifestation of FAA, possibly by synchronizing circadian oscillators that modulate motivational processes and behavioral output.

Fast oscillatory activity in the anterior cingulate cortex: dopaminergic modulation and effect of perineuronal net loss.

Dopamine release in the prefrontal cortex plays a critical role in cognitive function such as working memory, attention and planning. Dopamine exerts complex modulation on excitability of pyramidal neurons and interneurons, and regulates excitatory and inhibitory synaptic transmission. Because of the complexity of this modulation, it is difficult to fully comprehend the effect of dopamine on neuronal network activity. In this study, we investigated the effect of dopamine on local high-frequency oscillatory neuronal activity (in β band) in slices of the mouse anterior cingulate cortex (ACC). We found that dopamine enhanced the power of these oscillations induced by kainate and carbachol, but did not affect their peak frequency. Activation of D2R and in a lesser degree D1R increased the oscillation power, while activation of D4R had no effect. These high-frequency oscillations in the ACC relied on both phasic inhibitory and excitatory transmission and functional gap junctions. Thus, dopamine released in the ACC promotes high-frequency synchronized local cortical activity which is known to favor information transfer, fast selection and binding of distributed neuronal responses. Finally, the power of these oscillations was significantly enhanced after degradation of the perineuronal nets (PNNs) enwrapping most parvalbumin interneurons. This study provides new insights for a better understanding of the abnormal prefrontal gamma activity in schizophrenia (SZ) patients who display prefrontal anomalies of both the dopaminergic system and the PNNs.

Voluntary exercise improves performance of a discrimination task through effects on the striatal dopamine system.

We have previously demonstrated that voluntary exercise facilitates discrimination learning in a modified T-maze. There is evidence implicating the dorsolateral striatum (DLS) as the substrate for this task. The present experiments examined whether changes in DLS dopamine receptors might underlie the exercise-associated facilitation. Infusing a D1R antagonist into the DLS prior to discrimination learning facilitated the performance of nonexercising rats but not exercising rats. Infusing a D2R antagonist impaired the performance of exercising rats but not nonexercising rats. Exercise-associated facilitation of this task may rely on an exercise-induced decrease in D1R and increase in D2R activation in the DLS.

Dopamine-enabled anti-Hebbian timing-dependent plasticity in prefrontal circuitry.

Spike timing-dependent plasticity (STDP) of glutamatergic synapses is a Hebbian associative plasticity that may underlie certain forms of learning. A cardinal feature of STDP is its dependence on the temporal order of presynaptic and postsynaptic spikes during induction: pre–post (positive) pairings induce t-LTP (timing-dependent long-term potentiation) whereas post–pre (negative) pairings induce t-LTD (timing-dependent long-term depression). Dopamine (DA), a reward signal for behavioral learning, is believed to exert powerful modulations on synapse strength and plasticity, but its influence on STDP has remained incompletely understood. We previously showed that DA extends the temporal window of t-LTP in the prefrontal cortex (PFC) from +10 to +30 ms, gating Hebbian t-LTP. Here, we examined DA modulation of synaptic plasticity induced at negative timings in layer V pyramidal neurons on mouse medial PFC slices. Using a negative timing STDP protocol (60 post–pre pairings at 0.1 Hz, δt = -30 ms), we found that DA applied during post–pre pairings did not produce LTD, but instead enabled robust LTP. This anti-Hebbian t-LTP depended on GluN2B-containing NMDA receptors. Blocking D1- (D1Rs), but not D2- (D2Rs) class DA receptors or disrupting cAMP/PKA signaling in pyramidal neurons also abolished this atypical t-LTP, indicating that it was mediated by postsynaptic D1R-cAMP/PKA signaling in excitatory synapses. Unlike DA-enabled Hebbian t-LTP that requires suppression of GABAergic inhibition and cooperative actions of both D1Rs and D2Rs in separate PFC excitatory and inhibitory circuits, DA-enabled anti-Hebbian t-LTP occurred under intact inhibitory transmission and only required D1R activation in excitatory circuit. Our results establish DA as a potent modulator of coincidence detection during associative synaptic plasticity and suggest a mechanism by which DA facilitates input-target association during reward learning and top-down information processing in PFC circuits.

Dopamine facilitates dendritic spine formation by cultured striatal medium spiny neurons through both D1 and D2 dopamine receptors

Variations of dopamine (DA) levels induced by drugs of abuse or in the context of Parkinson's disease modulate the number of dendritic spines in medium spiny neurons (MSNs) of the striatum, showing that DA plays a major role in the structural plasticity of MSNs. However, little is presently known regarding early spine development in MSNs occurring before the arrival of cortical inputs and in particular about the role of DA and D1 (D1R) and D2 (D2R) DA receptors. A cell culture model reconstituting early cellular interactions between MSNs, intrinsic cholinergic interneurons and DA neurons was used to study the role of DA in spine formation. After 5 or 10 days in vitro, the presence of DA neurons increased the number of immature spine-like protrusions. In MSN monocultures, chronic activation of D1R or D2R also increased the number of spines and spinophilin expression in MSNs, suggesting a direct role for these receptors. In DA-MSN cocultures, chronic blockade of D1R or D2R reduced the number of dendritic spines. Interestingly, the combined activation or blockade of both D1R and D2R failed to elicit more extensive spine formation, suggesting that both receptors act through a mechanism that is not additive. Finally, we found increased ionotropic glutamate receptor responsiveness and miniature excitatory postsynaptic current (EPSC) frequency in DA-MSN co-cultures, in parallel with a higher number of spines containing PSD-95, suggesting that the newly formed spines present functional post-synaptic machinery preparing the MSNs to receive additional glutamatergic contacts. These results represent a first step in the understanding of how dopamine neurons promote the structural plasticity of MSNs during the development of basal ganglia circuits.

Ischemic-LTP in Striatal Spiny Neurons of both Direct and Indirect Pathway Requires the Activation of D1-Like Receptors and NO/Soluble Guanylate Cyclase/cGMP Transmission

Striatal medium-sized spiny neurons (MSNs) are highly vulnerable to ischemia. A brief ischemic insult, produced by oxygen and glucose deprivation (OGD), can induce ischemic long-term potentiation (i-LTP) of corticostriatal excitatory postsynaptic response. Since nitric oxide (NO) is involved in the pathophysiology of brain ischemia and the dopamine D1/D5-receptors (D1-like-R) are expressed in striatal NOS-positive interneurons, we hypothesized a relation between NOS-positive interneurons and striatal i-LTP, involving D1R activation and NO production. We investigated the mechanisms involved in i-LTP induced by OGD in corticostriatal slices and found that the D1-like-R antagonist SCH-23390 prevented i-LTP in all recorded MSNs. Immunofluorescence analysis confirmed the induction of i-LTP in both substance P-positive, (putative D1R-expressing) and adenosine A2A-receptor-positive (putative D2R-expressing) MSNs. Furthermore, i-LTP was dependent on a NOS/cGMP pathway since pharmacological blockade of NOS, guanylate-cyclase, or PKG prevented i-LTP. However, these compounds failed to prevent i-LTP in the presence of a NO donor or cGMP analog, respectively. Interestingly, the D1-like-R antagonism failed to prevent i-LTP when intracellular cGMP was pharmacologically increased. We propose that NO, produced by striatal NOS-positive interneurons via the stimulation of D1-like-R located on these cells, is critical for i-LTP induction in the entire population of MSNs involving a cGMP-dependent pathway.

395 INTRARENAL ANGIOTENSIN AND DOPAMINE RECEPTOR INTERACTIONS IN THE CONTROL OF SODIUM EXCRETION AND BLOOD PRESSURE

The cardiovascular and renal actions of type-2 angiotensin (Ang) receptors (AT2Rs) generally oppose those mediated by Ang type-1 receptors (AT1Rs). At the level of cell signaling, AT2Rs inhibit MAP kinases ERK-1 and -2 reducing or eliminating AT1R-mediated increases in ERK activity. The bradykinin (BK) - nitric oxide (NO) - cyclic GMP (cGMP) pathway is now recognized as the extracellular signaling pathway relevant to the majority of AT2R actions. From a functional standpoint, AT2Rs induce systemic vasodilation and natriuresis, the latter of which is mediated by des-aspartyl1-Ang II (Ang III) rather than Ang II. AT2R inhibition of renal tubule Na+ transport occurs largely in renal proximal tubule cells (RPTCs) and also to some extent in the thick ascending limb of Henle. The renal dopaminergic system is crucial in the control of renal Na+ excretion and blood pressure. Dopamine (DA) is synthesized from filtered dihydroxyphenylalanine (DOPA) taken up into RPTCs. DA is exported from RPTCs largely into the tubule lumen, where it binds to the D1-like receptor family [D1R (D1 and D5 subtypes)]. D1R activation inhibits Na+ reabsorption through an adenylate cyclase-cyclic AMP (cAMP)-dependent mechanism. The importance of the renal dopaminergic system is underscored by the fact that renal D1Rs account for about 50% of basal Na+ excretion under normal to high Na+ intake conditions. Surprisingly, natriuresis induced by D1R activation in proximal tubules is abolished by concurrent AT2R blockade. We have found that DA-induced natriuresis requires cAMP-stimulated translocation and activation of AT2Rs at the apical plasma membranes of RPTCs. Renal interstitial accumulation of cAMP via direct activation of adenylyl cyclase with forskolin and selective inhibition of cAMP degradation with 3-isobutyl-1-methylxanthine (IBMX)] caused a significant and sustained increase in AT2R-mediated natriuresis. Direct agonist stimulation of D1Rs was not necessary for AT2R-mediated natriuresis since forskolin+IBMX-induced natriuresis persisted in the presence of D1R blockade with SCH-23390. These findings indicate that cAMP is a key mediator of the interaction between renal D1Rs and AT2Rs to induce natriuresis during high-salt states. Targeting the common trafficking pathways of these receptors would affect the natriuretic capacity of two of the most powerful systems governing Na+ homeostasis. Such endeavors hold significant clinical implications in cardiovascular medicine and hypertension where the focus has predominantly been on AT1R blockade and inhibition of the RAS.

Differential effects of dopamine antagonists infused to the medial preoptic area on the sexual behavior of female rats primed with estrogen and progesterone

Dopamine (DA) in the medial preoptic area (mPOA) is important for the control of appetitive aspects of sexual behavior in the female rat. Recently, following infusions of DA agonists to the mPOA of females primed with estradiol benzoate (EB) alone, we found that the ratio of D1R/D2R activity within the mPOA determines the expression of appetitive behaviors (Graham and Pfaus, 2010). To further the knowledge of this mechanism, the present experiments examined the effects of intra-mPOA infusions of selective DA receptor antagonists. Ovariectomized, sexually-experienced rats primed with EB and progesterone (P) were implanted bilaterally with cannulae aimed at the mPOA and infused with 4 doses (0, 0.25, 1.0 and 4.0μg) of the nonselective D1R/D2R antagonist flupenthixol (FLU), and selective D1R or D2R antagonists, SCH 23390 (SCH) or raclopride (RAC), respectively, in a randomized order prior to tests of sexual behavior in bilevel chambers. The high dose of FLU significantly decreased solicitations, hops and darts, and pacing behavior. The high dose of SCH also significantly decreased solicitations. In contrast, the high dose of RAC produced an increase in pacing, and a trend toward an increase in solicitations but no other effect on sexual behavior. These results reinforce the idea that the ratio of D1R/D2R activity within the mPOA of female rats is critical for the expression of appetitive behaviors, and further that this ratio is altered by P which shifts the DA effect to a predominantly facilitative D1R activation.

Altered Functioning of Both Renal Dopamine D1 and Angiotensin II Type 1 Receptors Causes Hypertension in Old Rats

Activation of renal dopamine D1 (D1R) and angiotensin II type 1 receptors (AT(1)Rs) influences the activity of proximal tubular sodium transporter Na,K-ATPase and maintains sodium homeostasis and blood pressure. We reported recently that diminished D1R and exaggerated AT(1)R functions are associated with hypertension in old Fischer 344 × Brown Norway F1 (FBN) rats, and oxidative stress plays a central role in this phenomenon. Here we studied the mechanisms of age-associated increase in oxidative stress on diminished D1R and exaggerated AT(1)R functions in the renal proximal tubules of control and antioxidant Tempol-treated adult and old FBN rats. Although D1R numbers and D1R agonist SKF38393-mediated stimulation of [(35)S]-GTPγS binding (index of D1R activation) were lower, G protein-coupled receptor kinase 4 (kinase that uncouples D1R) levels were higher in old FBN rats. Tempol treatment restored D1R numbers and G protein coupling and reduced G protein-coupled receptor kinase 4 levels in old FBN rats. Angiotensin II-mediated stimulation of [(35)S]-GTPγS binding and Na,K-ATPase activity were higher in old FBN rats, which were also restored with Tempol treatment. We also measured renal AT(1)R function in adult and old Fischer 344 (F344) rats, which, despite exhibiting an age-related increase in oxidative stress and diminished renal D1R function, are normotensive. We found that diuretic and natriuretic responses to candesartan (indices of AT(1)R function) were similar in F344 rats, a likely explanation for the absence of age-associated hypertension in these rats. Perhaps, alterations in both D1R (diminished) and AT(1)R (exaggerated) functions are necessary for the development of age-associated hypertension, as seen in old FBN rats.

Mechanisms of Dopamine D 1 and Angiotensin Type 2 Receptor Interaction in Natriuresis

Renal dopamine D(1)-like receptors (D(1)Rs) and angiotensin type 2 receptors (AT(2)Rs) are important natriuretic receptors counterbalancing angiotensin type 1 receptor-mediated tubular sodium reabsorption. Here we explore the mechanisms of D(1)R and AT(2)R interactions in natriuresis. In uninephrectomized, sodium-loaded Sprague-Dawley rats, direct renal interstitial infusion of the highly selective D(1)R agonist fenoldopam induced a natriuretic response that was abolished by the AT(2)R-specific antagonist PD-123319 or by microtubule polymerization inhibitor nocodazole but not by actin polymerization inhibitor cytochalasin D. By confocal microscopy and immunoelectron microscopy, fenoldopam translocated AT(2)Rs from intracellular sites to the apical plasma membranes of renal proximal tubule cells, and this translocation was abolished by nocodazole. Because D(1)R activation induces natriuresis via an adenylyl cyclase/cAMP signaling pathway, we explored whether this pathway is responsible for AT(2)R recruitment and AT(2)R-mediated natriuresis. Renal interstitial coinfusion of the adenylyl cyclase activator forskolin and 3-isobutly-1-methylxanthine induced natriuresis that was abolished either by PD-123319 or nocodazole but was unaffected by specific the D(1)R antagonist SCH-23390. Coadministration of forskolin and 3-isobutly-1-methylxanthine also translocated AT(2)Rs to the apical plasma membranes of renal proximal tubule cells; this translocation was abolished by nocodazole but was unaffected by SCH-23390. The results demonstrate that D(1)R-induced natriuresis requires AT(2)R recruitment to the apical plasma membranes of renal proximal tubule cells in a microtubule-dependent manner involving an adenylyl cyclase/cAMP signaling pathway. These studies provide novel insights regarding the mechanisms whereby renal D(1)Rs and AT(2)Rs act in concert to promote sodium excretion in vivo.

Inflammatory Nociception Diminishes Dopamine Release and Increases Dopamine D2 Receptor mRNA in the Rat's Insular Cortex

BackgroundThe insular cortex (IC) receives somatosensory afferent input and has been related to nociceptive input. It has dopaminergic terminals and D1 (D1R) -excitatory- and D2 (D2R) -inhibitory- receptors. D2R activation with a selective agonist, as well as D1R blockade with antagonists in the IC, diminish neuropathic nociception in a nerve transection model. An intraplantar injection of carrageenan and acute thermonociception (plantar test) were performed to measure the response to inflammation (paw withdrawal latency, PWL). Simultaneously, a freely moving microdyalisis technique and HPLC were used to measure the release of dopamine and its metabolites in the IC. Plantar test was applied prior, one and three hours after inflammation. Also, mRNA levels of D1 and D2R's were measured in the IC after three hours of inflammation.ResultsThe results showed a gradual decrease in the release of dopamine, Dopac and HVA after inflammation. The decrease correlates with a decrease in PWL. D2R's increased their mRNA expression compared to the controls. In regard of D1R's, there was a decrease in their mRNA levels compared to the controls.ConclusionsOur results showed that the decreased extracellular levels of dopamine induced by inflammation correlated with the level of pain-related behaviour. These results also showed the increase in dopaminergic mediated inhibition by an increase in D2R's and a decrease in D1R's mRNA. There is a possible differential mechanism regarding the regulation of excitatory and inhibitory dopaminergic receptors triggered by inflammation.