Dopamine D1 Receptor Activation Research Articles

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.

Dual Control of Dopamine Synthesis and Release by Presynaptic and Postsynaptic Dopamine D2 Receptors

Dysfunctions of dopaminergic homeostasis leading to either low or high dopamine (DA) levels are causally linked to Parkinson's disease, schizophrenia, and addiction. Major sites of DA synthesis are the mesencephalic neurons originating in the substantia nigra and ventral tegmental area; these structures send major projections to the dorsal striatum (DSt) and nucleus accumbens (NAcc), respectively. DA finely tunes its own synthesis and release by activating DA D2 receptors (D2R). To date, this critical D2R-dependent function was thought to be solely due to activation of D2Rs on dopaminergic neurons (D2 autoreceptors); instead, using site-specific D2R knock-out mice, we uncover that D2 heteroreceptors located on non-DAergic medium spiny neurons participate in the control of DA levels. This D2 heteroreceptor-mediated mechanism is more efficient in the DSt than in NAcc, indicating that D2R signaling differentially regulates mesolimbic- versus nigrostriatal-mediated functions. This study reveals previously unappreciated control of DA signaling, shedding new light on region-specific regulation of DA-mediated effects.

Antidepressant-like effect of creatine in mice involves dopaminergic activation

Creatine has been shown to play a significant role in health and disease. However, studies concerning its effect on mood are scarce. This study investigated the effect of creatine (p.o.) in the tail suspension test, a predictive test of antidepressant activity. Creatine reduced the immobility time in the tail suspension test (0.1-1000 mg/kg, male and female mice), without affecting locomotor activity. Furthermore, the involvement of the dopaminergic system in creatine-induced antidepressant-like effect in male mice in the tail suspension test was investigated. The anti-immobility effect of creatine (1 mg/kg) was prevented by the pre-treatment of mice with haloperidol (0.2 mg/kg, intraperitoneal (i.p.) route, non-selective dopamine receptor antagonist), (R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390; 0.05 mg/kg, subcutaneous (s.c.) route, dopamine D₁ receptor antagonist) and sulpiride (50 mg/kg, i.p., dopamine D₂ receptor antagonist). Creatine (0.01 mg/kg, sub-effective dose) in combination with sub-effective doses of (1-phenyl-7,8-dihydroxy-2,3,4,5-tetrahydro-1H-3-benzazepine) hydrochloride (SKF38393; 0.1 mg/kg, s.c., dopamine D₁ receptor agonist), apomorphine (0.5 µg/kg, i.p., preferential dopamine D₂ receptor agonist) or bupropion (1 mg/kg, p.o., dopamine reuptake inhibitor with subtle activity on noradrenergic reuptake) reduced the immobility time in the tail suspension test as compared with either drug alone. These results indicate that the antidepressant-like effect of creatine is likely mediated by an activation of dopamine D₁ and D₂ receptors.

Salsolinol Facilitates Glutamatergic Transmission to Dopamine Neurons in the Posterior Ventral Tegmental Area of Rats

Although in vivo evidence indicates that salsolinol, the condensation product of acetaldehyde and dopamine, has properties that may contribute to alcohol abuse, the underlying mechanisms have not been fully elucidated. We have reported previously that salsolinol stimulates dopamine neurons in the posterior ventral tegmental area (p-VTA) partly by reducing inhibitory GABAergic transmission, and that ethanol increases glutamatergic transmission to VTA-dopamine neurons via the activation of dopamine D1 receptors (D1Rs). In this study, we tested the hypothesis that salsolinol stimulates dopamine neurons involving activation of D1Rs. By using whole-cell recordings on p-VTA-dopamine neurons in acute brain slices of rats, we found that salsolinol-induced increase in spike frequency of dopamine neurons was substantially attenuated by DL-2-amino-5-phosphono-valeric acid and 6, 7-dinitroquinoxaline-2, 3-dione, the antagonists of glutamatergic N-Methyl-D-aspartic acid and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors. Moreover, salsolinol increased the amplitude of evoked excitatory postsynaptic currents (EPSCs) and the frequency but not the amplitude of spontaneous EPSCs. Additionally, SKF83566, a D1R antagonist attenuated the salsolinol-induced facilitation of EPSCs and of spontaneous firing of dopamine neurons. Our data reveal that salsolinol enhances glutamatergic transmission onto dopamine neurons via activation of D1Rs at the glutamatergic afferents in dopamine neurons, which contributes to salsolinol's stimulating effect on p-VTA dopamine neurons. This appears to be a novel mechanism which contributes toward rewarding properties of salsolinol.

Distinct roles for direct and indirect pathway striatal neurons in reinforcement

Dopamine signaling is implicated in reinforcement learning, but the neural substrates targeted by dopamine are poorly understood. Here, we bypassed dopamine signaling itself and tested how optogenetic activation of dopamine D1- or D2-receptor-expressing striatal projection neurons influenced reinforcement learning in mice. Stimulating D1-expressing neurons induced persistent reinforcement, whereas stimulating D2-expressing neurons induced transient punishment, demonstrating that activation of these circuits is sufficient to modify the probability of performing future actions.

Increase of ryanodine receptors by dopamine D1 receptors is negatively regulated by γ‐aminobutyric acid type B receptors in primary cultures of mouse cerebral cortical neurons

Although upregulation of ryanodine receptor (RyR)-1 and -2 is mediated through the activation of dopamine D1 receptors (D1DRs) in the development of psychostimulant-induced place preference, little is known about how such increased expressions of RyRs are negatively regulated. This study investigated negative regulatory mechanisms of increase of RyR-1 and -2 expression by D1DR stimulation with its full agonist, SKF82958 or A 68930, using cultures of mouse cerebral cortical neurons. Sustained exposure to SKF82958 or A 68930 of the neurons increased RyR-1 and -2 proteins in a dose- and time-dependent-manner. The SKF82958-induced increases of RyR-1 and -2 proteins were significantly suppressed by SCH23390 (a selective D1DR antagonist). In addition, the SKF82958- or A 68930-induced increases of RyR-1 and -2 proteins were completely abolished by baclofen (a selective γ-aminobutyric acid type B [GABA(B)] receptor agonist), whereas muscimol (an agonist specific to GABA(A) receptors) had no effect. SKF82958 or A 68930 significantly increased intracellular cAMP level, which was completely suppressed by baclofen. Furthermore, sustained exposure to phorbol 12,13-dibutyrate, a protein kinase C activator, did not change the expression of RyR-1 or -2 proteins. Immunohistochemical study showed colocalizaton of immunoreactivities for three types of proteins, D1DRs and GABA(B) receptor R1 and R2 subunits in the same neuronal bodies, suggesting that the neurochemical changes induced by the activation of D1DRs and GABA(B) receptors occur in the same neurons. These results indicate that RyR-1 and -2 expression facilitated by D1DR stimulation are negatively regulated by GABA(B) receptor via suppression of cAMP production.

Temporally Dependent Changes in Cocaine-Induced Synaptic Plasticity in the Nucleus Accumbens Shell are Reversed by D1-Like Dopamine Receptor Stimulation

Dopaminergic and glutamatergic inputs to the nucleus accumbens shell have a central role in reward processing. Non-contingent cocaine administration generates a number of long-term AMPA receptor-dependent changes in synaptic efficacy. However, the synaptic consequences of cocaine self-administration and the potential role of dopamine in these processes remain unclear. Here, we examined the influence of D1 dopamine receptor (D1DR) activation on excitatory synaptic plasticity in the accumbens shell of adult rats following cocaine self-administration. Our results indicated that during the first 2 days following cocaine exposure both pre- and post-synaptic mechanisms contribute to a net decrease in AMPA receptor-mediated signaling. This is reflected by decreased frequency of miniature EPSCs (mEPSCs) attributable to enhanced cannabinoid receptor activity, decreased mEPSC amplitude, and increased paired-pulse ratio of evoked EPSCs. In contrast, the only changes observed in the shell 3-4 weeks following cocaine self-administration were increased mEPSCs amplitudes and AMPA/NMDA ratios. We further found that although these cocaine-induced neuroadaptations during early and late abstinence have different synaptic expression mechanisms, they were normalized by stimulation of D1DRs. Thus, pre-exposure to the D1DR agonist, SKF38393, during the initial period of abstinence increased excitatory synaptic strength, but reduced excitatory signaling after weeks of abstinence. Taken together, these results indicate that the direction of changes in excitatory transmission induced by cocaine self-administration switches over the first few weeks of abstinence. Moreover, D1DRs gate the stability of these cocaine-induced changes at glutamatergic synapses in the accumbens shell by utilizing multiple temporally distinct mechanisms, which has implications for the treatment of cocaine craving and addiction.

Attention Deficit Hyperactivity Disorder Pharmacogenetics: The Dopamine Transporter and D4 Receptor

Attention Deficit Hyperactivity Disorder Pharmacogenetics: The Dopamine Transporter and D4 Receptor

SKF 38393 reverses cocaine-conditioned place preference in mice

Cocaine is a psychotropic drug with a high potential for abuse due to its euphoric effects. Efforts to develop medications for the treatment of cocaine dependence have not been clinically successful. Some studies using animal models have shown positive effects of dopaminergic agents such as partial agonists of the dopamine D1 receptor. Thus, this study aimed to examine the effect of the dopamine D1 receptor partial agonist SKF 38393 on cocaine craving. Adult male C57BL/6J mice were injected with cocaine for 10 days in a conditioned place preference apparatus using a biased procedure and subsequently treated for three consecutive days with SKF 38393. The results showed that SKF 38393 was able to block the preference of cocaine-conditioned animals for the compartment paired with the drug without showing effects on locomotor activity. The results of this study suggest that partial activation of D1 dopamine receptors may be necessary for the development of pharmacotherapies for cocaine addiction.

Behavioral effects of non-viral mediated RNA interference of synapsin II in the medial prefrontal cortex of the rat

Synapsin II is a synaptic vesicle-associated phosphoprotein that has been implicated in the pathophysiology of schizophrenia. Researchers have demonstrated reductions in synapsin II mRNA and protein in post-mortem prefrontal cortex and hippocampus samples from patients with schizophrenia. Synapsin II protein expression has been shown to be regulated by dopamine D1 and D2 receptor activation. Furthermore, behavioral testing of the synapsin II knockout mouse has revealed a schizophrenic-like behavioral phenotype in this mutant strain, suggesting a relationship between dysregulated and/or reduced synapsin II and schizophrenia. However, it remains unknown the specific regions of the brain of which perturbations in synapsin II play a role in the pathophysiology of this disease. The aim of this project was to evaluate animals with a selective knock-down of synapsin II in the medial prefrontal cortex through the use of siRNA technology. Two weeks after continuous infusion of synapsin II siRNAs, animals were examined for the presence of a schizophrenic-like behavioral phenotype. Our results reveal that rats with selective reductions in medial prefrontal cortical synapsin II demonstrate deficits in sensorimotor gating (prepulse inhibition), hyperlocomotion, and reduced social behavior. These results implicate a role for decreased medial prefrontal cortical synapsin II levels in the pathophysiology of schizophrenia and the mechanisms of aberrant prefrontal cortical circuitry, and suggest that increasing synapsin II levels in the medial prefrontal cortex may potentially serve as a novel therapeutic target for this devastating disorder.

Characteristics of Antiamnestic Effects of Blockade and Activation of Dopamine D1 Receptors after Stress Stimulation in Mice

Changes in the effects of D1-receptor activation and blockade on the amnesia induced by delay in unsafe compartment in mice after forced swimming were studied using conditioned passive avoidance test. The most pronounced antiamnestic effects in mice with behavioral despair reaction were observed after administration of D1 receptor antagonist SCH23390, while D1 receptor antagonist SKF38393 was most effective in mice without preliminary stimulation.

Cut-loading: A Useful Tool for Examining the Extent of Gap Junction Tracer Coupling Between Retinal Neurons

In addition to chemical synaptic transmission, neurons that are connected by gap junctions can also communicate rapidly via electrical synaptic transmission. Increasing evidence indicates that gap junctions not only permit electrical current flow and synchronous activity between interconnected or coupled cells, but that the strength or effectiveness of electrical communication between coupled cells can be modulated to a great extent1,2. In addition, the large internal diameter (~1.2 nm) of many gap junction channels permits not only electric current flow, but also the diffusion of intracellular signaling molecules and small metabolites between interconnected cells, so that gap junctions may also mediate metabolic and chemical communication. The strength of gap junctional communication between neurons and its modulation by neurotransmitters and other factors can be studied by simultaneously electrically recording from coupled cells and by determining the extent of diffusion of tracer molecules, which are gap junction permeable, but not membrane permeable, following iontophoretic injection into single cells. However, these procedures can be extremely difficult to perform on neurons with small somata in intact neural tissue.Numerous studies on electrical synapses and the modulation of electrical communication have been conducted in the vertebrate retina, since each of the five retinal neuron types is electrically connected by gap junctions3,4. Increasing evidence has shown that the circadian (24-hour) clock in the retina and changes in light stimulation regulate gap junction coupling3-8. For example, recent work has demonstrated that the retinal circadian clock decreases gap junction coupling between rod and cone photoreceptor cells during the day by increasing dopamine D2 receptor activation, and dramatically increases rod-cone coupling at night by reducing D2 receptor activation7,8. However, not only are these studies extremely difficult to perform on neurons with small somata in intact neural retinal tissue, but it can be difficult to adequately control the illumination conditions during the electrophysiological study of single retinal neurons to avoid light-induced changes in gap junction conductance.Here, we present a straightforward method of determining the extent of gap junction tracer coupling between retinal neurons under different illumination conditions and at different times of the day and night. This cut-loading technique is a modification of scrape loading9-12, which is based on dye loading and diffusion through open gap junction channels. Scrape loading works well in cultured cells, but not in thick slices such as intact retinas. The cut-loading technique has been used to study photoreceptor coupling in intact fish and mammalian retinas7, 8,13, and can be used to study coupling between other retinal neurons, as described here.

Cut-loading: A Useful Tool for Examining the Extent of Gap Junction Tracer Coupling Between Retinal Neurons

In addition to chemical synaptic transmission, neurons that are connected by gap junctions can also communicate rapidly via electrical synaptic transmission. Increasing evidence indicates that gap junctions not only permit electrical current flow and synchronous activity between interconnected or coupled cells, but that the strength or effectiveness of electrical communication between coupled cells can be modulated to a great extent1,2. In addition, the large internal diameter (~1.2 nm) of many gap junction channels permits not only electric current flow, but also the diffusion of intracellular signaling molecules and small metabolites between interconnected cells, so that gap junctions may also mediate metabolic and chemical communication. The strength of gap junctional communication between neurons and its modulation by neurotransmitters and other factors can be studied by simultaneously electrically recording from coupled cells and by determining the extent of diffusion of tracer molecules, which are gap junction permeable, but not membrane permeable, following iontophoretic injection into single cells. However, these procedures can be extremely difficult to perform on neurons with small somata in intact neural tissue. Numerous studies on electrical synapses and the modulation of electrical communication have been conducted in the vertebrate retina, since each of the five retinal neuron types is electrically connected by gap junctions3,4. Increasing evidence has shown that the circadian (24-hour) clock in the retina and changes in light stimulation regulate gap junction coupling3-8. For example, recent work has demonstrated that the retinal circadian clock decreases gap junction coupling between rod and cone photoreceptor cells during the day by increasing dopamine D2 receptor activation, and dramatically increases rod-cone coupling at night by reducing D2 receptor activation7,8. However, not only are these studies extremely difficult to perform on neurons with small somata in intact neural retinal tissue, but it can be difficult to adequately control the illumination conditions during the electrophysiological study of single retinal neurons to avoid light-induced changes in gap junction conductance. Here, we present a straightforward method of determining the extent of gap junction tracer coupling between retinal neurons under different illumination conditions and at different times of the day and night. This cut-loading technique is a modification of scrape loading9-12, which is based on dye loading and diffusion through open gap junction channels. Scrape loading works well in cultured cells, but not in thick slices such as intact retinas. The cut-loading technique has been used to study photoreceptor coupling in intact fish and mammalian retinas7, 8,13, and can be used to study coupling between other retinal neurons, as described here.

L-DOPA Impairs Proteasome Activity in Parkinsonism through D1Dopamine Receptor

Aberrant membrane localization of dopamine D(1) receptor (D1R) is associated with L-DOPA-induced dyskinesia (LID), a major complication of L-DOPA treatment in Parkinson's disease (PD). Since the proteasome plays a central role in modulating neuronal response through regulation of neurotransmitter receptor intraneuronal fate, we hypothesized that the ubiquitine-proteasome proteolytic pathway could be impaired in LID. Those LIDs are actually associated with a striatum-specific decrease in proteasome catalytic activity and accumulation of polyubiquitinated proteins in experimental rodent and monkey parkinsonism. We then demonstrated that such decreased proteasome catalytic activity (1) results from D1R activation and (2) feed-back the D1R abnormal trafficking, i.e., its exaggerated cell surface abundance. We further showed that the genetic invalidation of the E3 ubiquitin-protein ligase parkin PD gene leads to exaggerated abnormal involuntary movements compared with wild-type mice. We thus established in an unprecedented series of experimental models that impairment of the ubiquitine-proteasome system at specific nodes (E3 ligase parkin, polyubiquitination, proteasome catalytic activity) leads to the same phenomenon, i.e., aberrant behavioral response to dopamine replacement therapy in PD, highlighting the intimate interplay between dopamine receptor and proteasome activity in a nondegenerative context.

Activation of dopamine D1 receptors in the medial septum improves scopolamine-induced amnesia in the dorsal hippocampus

In the present study, we investigated the influence of intra-medial septum (intra-MS) injections of dopamine D1 receptor agents on amnesia induced by intra-CA1 injections of a muscarinic acetylcholine receptor antagonist, scopolamine. This study used a step-through inhibitory (passive) avoidance task to assess memory in adult male Wistar rats. The results showed that in the animals that received post-training intra-MS injections of saline, intra-CA1 administrations of scopolamine (0.75, 1, and 2μg/rat) decreased inhibitory avoidance (IA) memory consolidation as evidenced by a decrease in step-through latency on the test day, which was suggestive of drug-induced amnesia. Post-training intra-MS injections of a dopamine D1 receptor agonist, SKF38393 at doses of 0.1, 0.15, and 0.3μg/rat had no effect, but at dose of 0.5μg/rat impaired IA memory consolidation. Interestingly, intra-MS injections of SKF38393 (0.15, 0.3 and 0.5μg/rat) significantly prevented amnesia induced by intra-CA1 injections of scopolamine (1μg/rat). Intra-MS injections of a dopamine D1 receptor antagonist, SCH23390 (0.5 and 0.75μg/rat) by itself impaired IA memory consolidation, and also at dose of 0.75μg/rat increased amnesia induced by intra-CA1 administrations of an ineffective dose of scopolamine (0.5μg/rat). Post-training intra-MS injections of ineffective doses of SCH23390 (0.1, 0.3 and 0.5μg/rat) prevented an effective dose of SKF38393 response to the impaired effect of scopolamine. These results suggest that dopamine D1 receptors in the MS via projection neurons to the hippocampus affect impairment of memory consolidation induced by intra-CA injections of scopolamine.

Binding of Losartan to Angiotensin AT1 Receptors Increases Dopamine D1 Receptor Activation

Signaling through both angiotensin AT1 receptors (AT1R) and dopamine D1 receptors (D1R) modulates renal sodium excretion and arterial BP. AT1R and D1R form heterodimers, but whether treatment with AT1R antagonists functionally modifies D1R via allosterism is unknown. In this study, the AT1R antagonist losartan strengthened the interaction between AT1R and D1R and increased expression of D1R on the plasma membrane in vitro. In rat proximal tubule cells that express endogenous AT1R and D1R, losartan increased cAMP generation. Losartan increased cAMP in HEK 293a cells transfected with both AT1R and D1R, but it did not increase cAMP in cells transfected with either receptor alone, suggesting that losartan induces D1R activation. Furthermore, losartan did not increase cAMP in HEK 293a cells expressing AT1R and mutant S397/S398A D1R, which disrupts the physical interaction between AT1R and D1R. In vivo, administration of a D1R antagonist significantly attenuated the antihypertensive effect of losartan in rats with renal hypertension. Taken together, these data imply that losartan might exert its antihypertensive effect both by inhibiting AT1R signaling and by enhancing D1R signaling.

Dopamine D4 receptor transmission in the prefrontal cortex controls the salience of emotional memory via modulation of calcium calmodulin-dependent kinase II.

Dopamine (DA) signaling in the medial prefrontal cortex (mPFC) plays a critical role in the processing of emotional information and memory encoding. Activation of DA D4 receptors within the prelimbic (PLC) division of the mPFC bidirectionally modulates emotional memory by strongly potentiating the salience of normally nonsalient emotional memories but blocking the acquisition of suprathreshold emotionally salient fear memories. Previous in vitro studies have shown that activation of cortical DA D4 receptors can bidirectionally modulate levels of α-calcium calmodulin-dependent kinase II (α-CaMKII), a molecule essential for learning and memory. Using an olfactory fear conditioning procedure in rats combined with microinfusions into the mPFC, we examined the potential role of D4 receptor-mediated control of emotional memory salience through signaling via CaMKII, cAMP/protein kinase A (PKA), and protein phosphatase-1 (PP1) signaling. We report that CaMKII blockade prevents the ability of intra-mPFC DA D4 receptor activation to potentiate the salience of subthreshold fear memory. In contrast, blockade of either cAMP/PKA or PP1 signaling pathways rescued the blockade of suprathreshold fear memory via intra-mPFC D4 receptor activation. Our results demonstrate that modulation of emotional memory salience via intra-mPFC DA D4 receptor transmission depends upon downstream signaling via CaMKII, cAMP/PKA, and PP1 substrates.

Dopamine D1 Receptor-Mediated Intracellular Responses in the Hypothalamus after Co-Administration of Caffeine with MDMA

Markers of dopamine D(1) receptor activation were determined to elucidate intracellular mechanisms associated with the combined effects of caffeine and 3,4 methylenedioxymethamphetamine (MDMA), reported previously to produce increased toxicity, when compared with either drug alone. Caffeine (10 mg/kg) and MDMA (15 mg/kg) were administered to male Sprague Dawley rats alone and in combination. One hour after drug administration, core body temperature and phosphorylation of the dopamine D(1) -related intracellular markers, cAMP response element binding protein (CREB), the dopamine and c-AMP-regulated phosphoprotein of 32 kDa (DARPP-32) and expression of the immediate early gene and cellular activation marker c-fos were determined in the hypothalamus. Co-administration of caffeine with MDMA increased core body temperature when compared with MDMA or caffeine treatment alone. Pre-treatment with the dopamine D(1) receptor antagonist SCH 23390 (1 mg/kg, i.p.), 30 min. prior to caffeine and MDMA administration, produced a hypothermic response to MDMA that was unaffected by caffeine. Co-administration of caffeine with MDMA increased p-CREB, p-DARPP-32 and c-fos expression when compared with either treatment alone. Pre-treatment with SCH-23390 attenuated the changes in p-CREB, p-DARPP and c-fos. The results show an enhanced intracellular response when caffeine is combined with MDMA but not with either agent alone suggestive of synergistic intracellular actions convergent on a dopamine D(1) receptor signalling pathway. A dopamine-related synergy associated with the combined administration of caffeine and MDMA may have important use and safety implications for recreational drug users.

Synergistic Activation of Dopamine D1 and TrkB Receptors Mediate Gain Control of Synaptic Plasticity in the Basolateral Amygdala

Fear memory formation is thought to require dopamine, brain-derived neurotrophic factor (BDNF) and zinc release in the basolateral amygdala (BLA), as well as the induction of long term potentiation (LTP) in BLA principal neurons. However, no study to date has shown any relationship between these processes in the BLA. Here, we have used in vitro whole-cell patch clamp recording from BLA principal neurons to investigate how dopamine, BDNF, and zinc release may interact to modulate the LTP induction in the BLA. LTP was induced by either theta burst stimulation (TBS) protocol or spaced 5 times high frequency stimulation (5xHFS). Significantly, both TBS and 5xHFS induced LTP was fully blocked by the dopamine D1 receptor antagonist, SCH23390. LTP induction was also blocked by the BDNF scavenger, TrkB-FC, the zinc chelator, DETC, as well as by an inhibitor of matrix metalloproteinases (MMPs), gallardin. Conversely, prior application of the dopamine reuptake inhibitor, GBR12783, or the D1 receptor agonist, SKF39393, induced robust and stable LTP in response to a sub-threshold HFS protocol (2xHFS), which does not normally induce LTP. Similarly, prior activation of TrkB receptors with either a TrkB receptor agonist, or BDNF, also reduced the threshold for LTP-induction, an effect that was blocked by the MEK inhibitor, but not by zinc chelation. Intriguingly, the TrkB receptor agonist-induced reduction of LTP threshold was fully blocked by prior application of SCH23390, and the reduction of LTP threshold induced by GBR12783 was blocked by prior application of TrkB-FC. Together, our results suggest a cellular mechanism whereby the threshold for LTP induction in BLA principal neurons is critically dependent on the level of dopamine in the extracellular milieu and the synergistic activation of postsynaptic D1 and TrkB receptors. Moreover, activation of TrkB receptors appears to be dependent on concurrent release of zinc and activation of MMPs.

The Extracellular cAMP-Adenosine Pathway Regulates Expression of Renal D1 Dopamine Receptors in Diabetic Rats

Activation of D1 dopamine receptors expressed in the kidneys promotes the excretion of sodium and regulates sodium levels during increases in dietary sodium intake. A decrease in the expression or function of D1 receptors results in increased sodium retention which can potentially lead to the development of hypertension. Studies have shown that in the absence of functional D1 receptors, in null mice, the systolic, diastolic, and mean arterial pressures are higher. Previous studies have shown that the expression and function of D1 receptors in the kidneys are decreased in animal models of diabetes. The mechanisms that down-regulate the expression of renal D1 receptor gene in diabetes are not well understood. Using primary renal cells and acutely isolated kidneys from the streptozotocin-induced rat diabetic model, we demonstrate that the renal D1 receptor expression is down-regulated by the extracellular cAMP-adenosine pathway in vitro and in vivo. In cultures of primary renal cells, a 3 mm, 60-h cAMP treatment down-regulated the expression of D1 receptors. In vivo, we determined that the plasma and urine cAMP levels as well as the expression of 5'-ectonucleotidase, tissue-nonspecific alkaline phosphatase, and adenosine A2a receptors are significantly increased in diabetic rats. Inhibitors of 5'-ectonucleotidase and tissue-nonspecific alkaline phosphatase, α,β-methyleneadenosine 5'-diphosphate, and levamisole, respectively, blocked the down-regulation of D1 receptors in the primary renal cells and in the kidney of diabetic animals. The results suggest that inhibitors of the extracellular cAMP-adenosine pathway reverse the down-regulation of renal D1 receptor in diabetes.