Functional dopamine D2 receptors on rat vagal afferent neurones

Dopamine D2-Subtype Receptors Outside the Blood-Brain Barrier Mediate Enhancement of Mesolimbic Dopamine Release and Conditioned Place Preference by Intravenous Dopamine

Dopamine (DA) is a cell-signaling molecule that does not readily cross the blood-brain barrier. Despite this, peripherally administered DA enhances DA levels in the nucleus accumbens and alters DA-related behaviors. This study was designed to investigate whether DA subtype-2 receptors are involved in the enhancement of nucleus accumbens (NAc) DA levels elicited by intravenous DA administration. This was accomplished by using microdialysis in the NAc and extracellular single unit recordings of putative DA neurons in the ventral tegmental area (VTA). Additionally, the reinforcing properties of intravenous DA were investigated using a place conditioning paradigm and the effects of intravenous DA on ultrasonic vocalizations were assessed. Following administration of intravenous dopamine, the firing rate of putative DA neurons in the VTA displayed a biphasic response and DA levels in the nucleus accumbens were enhanced. Pretreatment with domperidone, a peripheral-only DA D2 receptor (D2R) antagonist, reduced intravenous DA mediated increases in VTA DA neuron activity and NAc DA levels. Pretreatment with phentolamine, a peripheral α-adrenergic receptor antagonist, did not alter the effects of IV DA on mesolimbic DA neurotransmission. These results provide evidence for peripheral D2R mediation of the effects of intravenous DA on mesolimbic DA signaling.

Dopamine receptors in emesis: Molecular mechanisms and potential therapeutic function

Dopamine is a member of the catecholamine family and is associated with multiple physiological functions. Together with its five receptor subtypes, dopamine is closely linked to neurological disorders such as schizophrenia, Parkinson's disease, depression, attention deficit-hyperactivity, and restless leg syndrome. Unfortunately, several dopamine receptor-based agonists used to treat some of these diseases cause nausea and vomiting as impending side-effects. The high degree of cross interactions of dopamine receptor ligands with many other targets including G-protein coupled receptors, transporters, enzymes, and ion-channels, add to the complexity of discovering new targets for the treatment of nausea and vomiting. Using activation status of signaling cascades as mechanism-based biomarkers to foresee drug sensitivity combined with the development of dopamine receptor-based biased agonists may hold great promise and seems as the next step in drug development for the treatment of such multifactorial diseases. In this review, we update the present knowledge on dopamine and dopamine receptors and their potential roles in nausea and vomiting. The pre- and clinical evidence provided in this review supports the implication of both dopamine and dopamine receptor agonists in the incidence of emesis. Besides the conventional dopaminergic antiemetic drugs, potential novel antiemetic targeting emetic protein signaling cascades may offer superior selectivity profile and potency.

Anti-incretin effect: The other face of Janus in human glucose homeostasis

The provocative idea that type 2 diabetes (T2D) may be a surgically treated disorder is based on accumulating evidence suggesting impressive remission rates of obesity and diabetes following bariatric surgery interventions. According to the "anti-incretin" theory, ingestion of food in the gastrointestinal (GI) tract, apart from activating the well-described incretin effect, also results in the parallel stimulation of a series of negative feedback mechanisms (anti-incretin effect). The primary goal of these regulations is to counteract the effects of incretins and other postprandial glucose-lowering adaptive mechanisms. Disruption of the equilibrium between incretins and anti-incretins could be an additional pathway leading to the development of insulin resistance and hyperglycemia. This theory provides an alternative theoretical framework to explain the mechanisms behind the optimal effects of metabolic surgery on T2D and underlines the importance of the GI tract in the homeostatic regulation of energy balance in humans. The anti-incretin concept is currently based on a limited amount of evidence and certainly requires further validation by additional studies. The aim of the present review is to discuss and critically evaluate recent evidence on the anti-incretin theory, providing an insight into current state and future perspectives.

Time-dependent assessment of stimulus-evoked regional dopamine release

To date, the spatiotemporal release of specific neurotransmitters at physiological levels in the human brain cannot be detected. Here, we present a method that relates minute-by-minute fluctuations of the positron emission tomography (PET) radioligand [11C]raclopride directly to subsecond dopamine release events. We show theoretically that synaptic dopamine release induces low frequency temporal variations of extrasynaptic extracellular dopamine levels, at time scales of one minute, that can evoke detectable temporal variations in the [11C]raclopride signal. Hence, dopaminergic activity can be monitored via temporal fluctuations in the [11C]raclopride PET signal. We validate this theory using fast-scan cyclic voltammetry and [11C]raclopride PET in mice during chemogenetic activation of dopaminergic neurons. We then apply the method to data from human subjects given a palatable milkshake and discover immediate and-for the first time-delayed food-induced dopamine release. This method enables time-dependent regional monitoring of stimulus-evoked dopamine release at physiological levels.

L-DOPA inhibits excitatory synaptic transmission in the rat nucleus tractus solitarius through release of dopamine

The mode of action of L-DOPA on excitatory synaptic transmission in second-order neurons of the nucleus tractus solitarius (NTS) was studied using the rat brainstem slices. Superfusion of L-DOPA (10μM) reduced the frequency of miniature excitatory postsynaptic currents (mEPSCs) without any effect on the amplitude. A low concentration (1μM) was ineffective on the mEPSCs, and the highest concentration (100μM) exerted a stronger inhibitory effect. L-DOPA (10μM) decreased the amplitude of EPSCs (eEPSCs) evoked by electrical stimulation of the tractus solitarius and increased the paired-pulse ratio. The inhibitory effects of L-DOPA on mEPSCs and eEPSCs were similar to those of dopamine (100μM). The effects of L-DOPA were blocked by a competitive antagonist, L-DOPA methyl ester (100μM) and also by a D₂ receptor antagonist, sulpiride (10μM), while those of dopamine were blocked by the latter but not by the former. In reserpine (5mg/kg, s.c.)-treated rats, the effects of L-DOPA on both mEPSCs and eEPSCs were completely abolished, but those of dopamine remained unchanged. The present results suggest a possibility that L-DOPA may induce the release of dopamine from the axon terminals in the NTS and the released dopamine suppresses the glutamatergic transmission through activation of the presynaptic D₂ receptors.

Oleic Derivatives of Dopamine and Respiration

Ventilatory inhibition is considered an undesirable pharmacological side effect of pharmacotherapy in neurodegenerative conditions underlain by brain dopamine deficiency. In this context, oleic derivatives of dopamine or N-acyl-dopamines are novel substances that may be of high therapeutic interest as having the ability to cross the blood-brain barrier and acting in dopamine-like manner. In the present study we seek to define the influence of N-acyl-dopamines on lung ventilation and its hypoxic responses in the rat. We found that N-oleoyl-dopamine decreased both normoxic and peak hypoxic ventilation in response to 8% acute hypoxia, on average, by 31% and 41%, respectively. Its metabolite, 3'-O-methyl-N-oleoyl-dopamine, caused a 15% ventilatory decrease each, whereas an oleic ester derivative, 3'-O-oleoyl-N-oleoyl-dopamine, caused 11% and 19% ventilatory decreases, respectively. All three N-acyl-dopamines investigated displayed an inhibitory effect on ventilation. The findings indicate that 3'-O-methyl-N-oleoyl-dopamine and 3'-O-oleoyl-N-oleoyl-dopamine performed better than N-oleoyl-dopamine in term of less ventilatory suppression, albeit the differences among the three compounds were modest. We conclude that N-acyl-dopamines are worthy of intensified explorations as potential carriers of dopamine molecule in view of the lack of clinically effective methods of dopamine delivery into the brain in neurodegenerative conditions.

Breathing in Parkinsonism in the Rat

Parkinsonism is underlain by dopamine (DA) deficiency in the mid-brain, a neurotransmitter innately involved with respiratory regulation. However, the state of respiration in parkinsonism is an unsettled issue. In this study we seek to determine ventilation and its responses to hypoxia in a reserpine--alpha-methyl-tyrosine model of parkinsonism in the rat. We also attempted to differentiate between the role of discrete brain and carotid body DA stores in the modulation of the hypoxic ventilatory response (HVR). To this end we used domperidone, a peripheral D2 receptor antagonist, and levodopa, a central D2 receptor agonist. The HVRs to acute 12% and 8% hypoxia were studied in a whole body plethysmograph in the same rats before and after the induction of parkinsonic symptoms in conscious rats. We found that resting ventilation and the HVR were distinctly reduced in parkinsonism. The reduction was particularly evident in the peak hypoxic hyperpneic augmentation. Domperidone, which enhanced ventilation in the control healthy condition, failed to reverse the reduced parkinsonic HVR. In contrast, levodopa, which did not appreciably affected ventilation in the healthy condition, caused the parkinsonic HVR to return to and above the baseline healthy level. The findings demonstrate the predominance of a lack of the central DA stimulatory element and minimize the role of carotid body DA in the ventilatory impediment of parkinsonism. In conclusion, the study provides the pathophysiological savvy concerning the respiratory insufficiency of parkinsonism, a sequela which carries a risk of chronically impaired blood oxygenation, which may drive the disease worsening.

Behavioural and anatomical characterization of mutant mice with targeted deletion of D1 dopamine receptor-expressing cells: response to acute morphine

Considerable topographic overlap exists between brain opioidergic and dopaminergic neurons. Pharmacological blockade of the dopamine D(1) receptor (Drd1a) reverses several behavioural phenomena elicited by opioids. The present study examines the effects of morphine in adult mutant (MUT) mice expressing the attenuated diphtheria toxin-176 gene in Drd1a-expressing cells, a mutant line shown previously to undergo post-natal striatal atrophy and loss of Drd1a-expression. MUT and wild-type mice were assessed behaviourally following acute administration of 10 mg/kg morphine. Treatment with morphine reduced locomotion and rearing similarly in both genotypes but reduced total grooming only in MUT mice. Morphine-induced Straub tail and stillness were heightened in MUT mice. Chewing and sifting were decreased in MUT mice and these effects were not modified by morphine. Loss of striatal Drd1-positive cells and up-regulated D(2)-expression, as reflected in down-regulated D(1)-like and up-regulated D(2)-like binding, respectively, is not uniform along the cranio-caudal extent in this model but appears to be greater in the caudal striatum. Preferential caudal loss of µ-opioid-expression, a marker for the striosomal compartment, was seen. These data indicate that Drd1a-positive cell loss modifies the exploratory behavioural response elicited by morphine, unmasking novel morphine-induced MUT-specific behaviours and generating a hypersensitivity to morphine for others.

Mechanisms of Broad-Spectrum Antiemetic Efficacy of Cannabinoids against Chemotherapy-Induced Acute and Delayed Vomiting

Chemotherapy-induced nausea and vomiting (CINV) is a complex pathophysiological condition and consists of two phases. The conventional CINV neurotransmitter hypothesis suggests that the immediate phase is mainly due to release of serotonin (5-HT) from the enterochromaffin cells in the gastrointestinal tract (GIT), while the delayed phase is a consequence of release of substance P (SP) in the brainstem. However, more recent findings argue against this simplistic neurotransmitter and anatomical view of CINV. Revision of the hypothesis advocates a more complex, differential and overlapping involvement of several emetic neurotransmitters/modulators (e.g. dopamine, serotonin, substance P, prostaglandins and related arachidonic acid derived metabolites) in both phases of emesis occurring concomitantly in the brainstem and in the GIT enteric nervous system (ENS) [1]. No single antiemetic is currently available to completely prevent both phases of CINV. The standard antiemetic regimens include a 5-HT₃ antagonist plus dexamethasone for the prevention of acute emetic phase, combined with an NK₁ receptor antagonist (e.g. aprepitant) for the delayed phase. Although NK₁ antagonists behave in animals as broad-spectrum antiemetics against different emetogens including cisplatin-induced acute and delayed vomiting, by themselves they are not very effective against CINV in cancer patients. Cannabinoids such as D⁸-THC also behave as broad-spectrum antiemetics against diverse emetic stimuli as well as being effective against both phases of CINV in animals and patients. Potential side effects may limit the clinical utility of direct-acting cannabinoid agonists which could be avoided by the use of corresponding indirect-acting agonists. Cannabinoids (both phyto-derived and synthetic) behave as agonist antiemetics via the activation of cannabinoid CB₁ receptors in both the brainstem and the ENS emetic loci. An endocannabinoid antiemetic tone may exist since inverse CB₁ agonists (but not the corresponding silent antagonists) cause nausea and vomiting.

Age-related behavioural phenotype and cellular characterisation of mice with progressive ablation of D1 dopamine receptor-expressing cells

In this study we characterize the behavioural and cellular phenotype of mutant (MUT) mice with progressive loss of D1 dopamine receptor (Drd1a)-expressing cells. Adult [14-19 weeks] MUT mice showed intact working memory in the spontaneous alternation test but evidenced anxiety-like behaviour in the elevated plus maze and the light-dark test. The ethogram of mature adult MUT [average age 22 weeks] was compared with that of young adult MUT mice [average age 12 weeks]. While MUT mice evidenced hyperactivity over initial exploration at both time points, the topography of hyperactivity shifted. Moreover, initial hyperactivity was sustained over habituation at 12 weeks, but not at 22 weeks. Thus, by 22 weeks MUT mice evidenced shifts in, and mitigation of, these early phenotypic effects. However, orofacial behaviours of chewing and sifting were reduced similarly at 12 and 22 weeks. These data support the hypothesis that aspects of the mutant phenotype change with time. Quantitative autoradiography at 20 weeks revealed loss of D1-like dopamine receptor binding in the entire basal ganglia, with upregulated D2-like binding. There appear to be topographically specific interactions between normal maturational processes and compensatory mechanisms evoked subsequent to targeted ablation of D1 dopamine receptor-expressing cells. Understanding the mechanistic bases of mitigation vs persistence of individual phenotypes in relation to neural adaptation consequent to cell loss may lead to novel therapeutic strategies for basal ganglia disorders.

Dopamine D2 receptor mRNA expression is increased in the jugular-nodose ganglia of rats with nitrogen dioxide-induced chronic bronchitis

The bronchodilatatory effect of inhaled dopamine or dopamine D(2) receptor agonists in cases of bronchial constriction may involve the suppression of pathologically increased airway sensory nerve activity. The aim of this study is to investigate the regulation of the dopamine D(2) receptor mRNA expression in the ganglia of rats with nitrogen dioxide-induced chronic bronchitis compared with that in ganglia of healthy control animals. Rats were exposed to nitrogen dioxide (10 ppm, 20 d) and dopamine D(2) receptor mRNA levels in sensory ganglia (jugular-nodose, trigeminal, cervical dorsal root and thoracic dorsal root ganglia) were examined by quantitative real-time polymerase chain reaction and compared to control tissues. Whereas for trigeminal and dorsal root ganglia the dopamine D(2) receptor expression levels showed no difference between both animal groups, there was a significant (p<0.05) increase in the jugular-nodose ganglia with a 2.1-fold factor. The increase of dopamine D(2) receptor mRNA in jugular-nodose sensory neurons which innervate the airways may represent a neurochemical basis for the effects seen in man and animal models following topical administration of dopamine or dopamine agonists onto the respiratory epithelium.

Central Overexpression of Angiotensin AT1AReceptors Prevents Dopamine D2Receptor Regulation of Alcohol Consumption in Mice

BACKGROUND

While angiotensin receptors are found on the soma and terminals of dopaminergic neurons, controversy surrounds the potential role of angiotensin in alcohol consumption.

METHODS

Using a transgenic mouse with a brain-specific overexpression of angiotensin AT(1A) receptors (NSE-AT(1A) mice), we have examined the role of angiotensin in alcohol consumption and alcohol-induced regulation of the dopaminergic system.

RESULTS

The functional relevance of the overexpressed AT(1A) receptors was confirmed by an exaggerated rehydration response following 24-hour dehydration. NSE-AT(1A) mice showed a high preference for alcohol (similar to wild-type mice); yet, raclopride treatment had no effect on alcohol consumption in NSE-AT(1A) mice, while significantly reducing consumption in wild-type mice. In contrast, NSE-AT(1A) mice showed enhanced sensitivity to raclopride compared with wild types in terms of D(2) receptor up-regulation within the ventral mesencephalon. In addition, striatal D(2) receptors in NSE-AT(1A) mice were sensitive to up-regulation by chronic alcohol consumption.

CONCLUSIONS

Collectively, these data imply that while expression of angiotensin AT(1A) receptors on striatal neurons has no impact upon basal alcohol consumption or preference, AT(1A) receptors do modulate the sensitivity of dopamine D(2) receptors to regulation by alcohol and the ability of a D(2) receptor antagonist to reduce consumption.

The CRF1 receptor antagonist, antalarmin, reverses isolation-induced up-regulation of dopamine D2 receptors in the amygdala and nucleus accumbens of fawn-hooded rats

We have previously shown that Fawn-Hooded (FH) rats reared in isolation display an anxiety-like phenotype and an enhanced acquisition of ethanol seeking behaviour. Furthermore, antalarmin, a selective corticotrophin-releasing factor type 1 (CRF1) receptor antagonist, reduces isolation-induced acquisition and maintenance of volitional ethanol consumption in this strain. The aim of this study was to investigate the ability of CRF1 receptor antagonism by antalarmin to impact upon brain chemistry in both isolated and group-housed FH rats. To achieve this, FH rats were reared, from weaning, in either group-housed or isolation-housed conditions and at 12 weeks of age were treated with antalarmin (20 mg/kg, i.p; n = 10 per group) or vehicle (1 mL/kg, i.p; n = 10 per group) bi-daily for ten consecutive days before being killed and their brains removed for neurochemical analyses. Autoradiography and in situ hybridization was employed to analyse changes in the dopaminergic and neurotrophin systems. Isolation rearing increased dopamine D2 receptor density in the central amygdala and nucleus accumbens, an effect reversed by antalarmin treatment. Conversely, treatment with antalarmin had no impact upon the isolation-induced alterations of the mRNA encoding brain-derived neurotrophic factor or the TrkB receptor. Collectively, these findings demonstrate that multiple signalling systems are susceptible to modulation by social isolation and that antalarmin can reverse some, but not all, isolation-induced alterations in brain chemistry.

A murine model of hyperdopaminergic state displays altered respiratory control

The dopamine transporter (DAT) protein plays an important role in the termination of dopamine signaling. We addressed the hypothesis that loss of DAT function would result in a distinctive cardiorespiratory phenotype due to the significant role of dopamine in the control of breathing, especially with respect to chemical control, metabolism, and thermoregulation. The DAT knockout mouse (DAT-/-) displays a state of functional hyperdopaminergia characterized by marked novelty driven hyperactivity. Certain behavioral and drug responses in these mice are reminiscent of endophenotypes of individuals with attention deficit hyperactivity disorders (ADHD). We performed experiments on conscious, unrestrained DAT-/- mice (KO) and littermate DAT+/+ wild-type (WT) controls. Ventilation was measured by the barometric technique during normoxia, hypoxia, or hypercapnia. We measured core body temperature and CO2 production as an index of metabolism. DAT-/- mice displayed a significantly lower respiratory frequency than WT mice, reflecting a prolonged inspiratory time. DAT-/- mice exhibited a reduced ventilatory response to hypoxia characterized by an attenuation of both the respiratory frequency and tidal volume responses. Both groups showed similar metabolic responses to hypoxia. Circadian measurements of body temperature were significantly lower in DAT-/- mice than WT mice during inactive periods. We conclude that loss of the DAT protein in this murine model of altered dopaminergic neurotransmission results in a significant respiratory and thermal phenotype that has possible implications for understanding of conditions associated with altered dopamine regulation.

5-HT activates vagal afferent cell bodies in vivo: Role of 5-HT2 and 5-HT3 receptors

Occipital artery (OA) injections of 5-HT elicit pronounced reductions in heart rate and mean arterial blood pressure (MAP) in urethane-anesthetized rats by activation of vagal afferent cell bodies in the ipsilateral nodose ganglion. In contrast, internal carotid artery (ICA) and i.v. injections elicit similar cardiovascular responses by activation of peripheral vagal afferent terminals. The aim of this study was to examine the roles of 5-HT3 and 5-HT2 receptors in the 5-HT-induced activation of vagal afferent cell bodies and peripheral afferent terminals in urethane-anesthetized rats. OA, ICA and i.v. injections of 5-HT elicited dose-dependent reductions in heart rate and MAP that were virtually abolished after i.v. administration of the 5-HT3 receptor antagonists, MDL 7222 or ICS 205-930. The responses elicited by the OA injections of 5-HT were markedly diminished after i.v. injection of the 5-HT2 receptor antagonists, xylamidine or ketanserin, whereas the responses elicited by i.v. or ICA injections of 5-HT were not affected. The present findings suggest that (1) 5-HT3 and 5-HT2 receptor antagonists gain ready access to nodose ganglion cells upon i.v. administration, and (2) functional 5-HT3 and 5-HT2 receptors exist on the cell bodies of vagal afferent neurons mediating the cardiovascular responses elicited by OA injections of 5-HT. These findings also support a wealth of evidence that 5-HT3 receptors exist on the peripheral terminals of vagal afferents, and although they do not discount the possibility that 5-HT2 receptors exist on peripheral vagal afferent terminals, it appears that activation of these receptors does not have pronounced effects on 5-HT3 receptor activity on terminals that mediate the hemodynamic responses to 5-HT.

Occipital artery injections of 5-HT may directly activate the cell bodies of vagal and glossopharyngeal afferent cell bodies in the rat

The primary objective of this study was to determine whether circulating factors gain direct access to and affect the activity of vagal afferent cell bodies in the nodose ganglia and glossopharyngeal afferents cell bodies in the petrosal ganglia, of the rat. We found that the occipital and internal carotid arteries provided the sole blood supply to the nodose ganglia, and that i.v. injections of the tracer, Basic Blue 9, elicited strong cytoplasmic staining in vagal and glossopharyngeal afferent cell bodies that was prevented by prior ligation of the occipital but not the internal carotid arteries. We also found that occipital artery injections of 5-HT elicited pronounced dose-dependent reductions in heart rate and diastolic arterial blood pressure that were (1) virtually abolished after application of the local anesthetic, procaine, to the ipsilateral nodose and petrosal ganglia, (2) markedly attenuated after transection of the ipsilateral vagus between the nodose ganglion and brain and virtually abolished after subsequent transection of the ipsilateral glossopharyngeal nerve between the petrosal ganglion and the brain, (3) augmented after ipsilateral transection of the aortic depressor and carotid sinus nerves, and (4) augmented after transection of all ipsilateral glossopharyngeal and vagal afferent nerves except for vagal cardiopulmonary afferents. These findings suggest that blood-borne 5-HT in the occipital artery gains direct access to and activates the cell bodies of vagal cardiopulmonary afferents of the rat and glossopharyngeal afferents of undetermined modalities.

Comparison of ethanol preference and neurochemical measures of mesolimbic dopamine and adenosine systems across different strains of mice

BACKGROUND

To extend the known phenotype of strains commonly used in the development of mutant mice, ethanol, saccharin, and caffeine preferences were examined in C57Bl/6J, CD-1, and hybrid C57Bl/6J x CD-1 mice. As dopaminergic mechanisms are inherently involved in the neuronal processing of many drugs of abuse (including ethanol), and an important role for adenosine-dopamine interactions has also been reported, the dopaminergic and purinergic neurochemical profiles of mice were compared against the consummatory phenotype observed.

METHODS

Ethanol (5% v/v), saccharin (0.1% w/v), and caffeine (0.1% w/v) consumption and preference were examined using a 2-bottle free-choice paradigm. Dopamine and adenosine receptor and transporter mRNA and protein density were quantified using in situ hybridization histochemistry and in vitro autoradiography, respectively.

RESULTS

C57Bl/6J and hybrid C57Bl/6J x CD-1 mice demonstrated a clear ethanol preference, voluntarily consuming large quantities of ethanol when given the choice between drinking vessels containing either ethanol or water. Conversely, CD-1 mice were characterized as ethanol-avoiding under the present paradigm. Differences in D(1) receptor mRNA between the strains were consistent with the observed behavioral differences in ethanol preference. The high ethanol-preferring phenotype of C57Bl/6J mice could not be directly linked to alterations in dopamine transporter neurochemistry and/or enkephalin levels as proposed by earlier researchers. Ethanol-seeking behavior appeared to correlate with D2 receptor expression, however, with evidence that ethanol-preferring mice also exhibit an increased density of D2 receptors within limbic dopaminergic projection nuclei. Interestingly, strain differences in the expression of the ethanol-sensitive nucleoside transporter paralleled differences in ethanol consumption, a novel finding consonant with purinergic involvement in dopamine-related behaviors.

CONCLUSIONS

This study has highlighted the relevance of alterations in dopamine receptor expression and purinergic modulation within the mesolimbic pathway and predisposition toward the development of ethanol-seeking behavior.

Tyrosine hydroxylase is expressed in a subpopulation of small dorsal root ganglion neurons in the adult mouse

The expression of tyrosine hydroxylase (TH) was studied in adult mouse dorsal root ganglia (DRGs) and spinal cord by means of immunohistochemistry and in situ hybridization. TH immunoreactivity and TH mRNA were present in 10-15% of lumbar DRG neurons, in most cases being small/medium-sized. Only very few of these neurons coexpressed calcitonin gene-related peptide (CGRP), and only around 6% bound isolectin B4 (IB4). Dopamine beta-hydroxylase-positive(+) or aromatic amino acid decarboxylase (AADC)+ DRG neurons were rare and did not colocalize TH. No evidence for dopamine transporter expression was obtained. Axotomy of the sciatic nerve only showed a tendency towards reduction in the number of TH+ neurons. In the dorsal horn of the spinal cord, moderately dense and widespread TH+ nerve terminals were observed, mainly in the gray matter and they did not show a typical primary afferent pattern. Also, dorsal rhizotomy or peripheral axotomy had no apparent effect on TH-LI in the dorsal horn. In the skin, along with an abundant TH+ innervation of blood vessels and sweat gland acini, a number of fibers was observed in close relation to the skin surface, some even penetrating into the epithelium. These results demonstrate presence, in normal adult mouse DRGs, of a subpopulation of TH+, essentially CGRP- and IB4-negative small/medium-sized neurons. No evidence for transport of TH into central afferents was obtained, but the enzyme may be present in some sensory fibers in the skin. The fact that neither AADC nor the dopamine transporter could be visualized suggests of non-dopaminergic transmitter phenotype, but the levels of these two dopaminergic markers may be too low to be detected with the present methodology. A further alternative is that L-DOPA after release is extracellularly converted to dopamine.

Receptor crosstalk: characterization of mice deficient in dopamine D1 and adenosine A2A receptors

Here we report the development of D1A2A receptor knockout mice to investigate whether interactions between dopamine D1 and adenosine A2A receptors participate in reward-related behavior. The combined deletion of D1 and A2A receptors resulted in mice with decreased weight and appetitive processes, reduced rearing and exploratory behaviors, increased anxiety, and a significantly poorer performance on the rotarod, compared to wild-type littermates. D1A2A receptor knockout mice shared phenotypic similarities with mice deficient in D1 receptors, while also paralleling behavioral deficits seen in A2A receptor knockout mice, indicating individual components of the behavioral phenotype of the D1A2A receptor knockout attributable to the loss of both receptors. In contrast, ethanol and saccharin preference in D1A2A receptor knockout mice were distinctly different from that observed in derivative D1 or A2A receptor-deficient mice. Compared to wild types, preference and consumption of ethanol were decreased in D1A2A receptor knockout mice, the reduction in ethanol consumption greater even than that seen in D1 receptor-deficient mice. Preference and consumption of saccharin were also reduced in D1A2A receptor knockout mice, whereas saccharin preference was similar in wild-type, D1, and A2A receptor knockout mice. These data suggest an interaction of D1 and A2A receptors in the reinforcement processes underlying the intake of rewarding substances, whereby the A2A receptor seems involved in goal-directed behavior and the motor functions underlying the expression of such behaviors, and the D1 receptor is confirmed as essential in mediating motivational processes related to the repeated intake of novel substances and drugs.

Genetic interdependence of adenosine and dopamine receptors: Evidence from receptor knockout mice

Dopamine and adenosine receptors are known to share a considerable overlap in their regional distribution, being especially rich in the basal ganglia. Dopamine and adenosine receptors have been demonstrated to exhibit a parallel distribution on certain neuronal populations, and even when not directly co-localized, relationships (both antagonistic and synergistic) have been described. This study was designed to investigate dopaminergic and purinergic systems in mice with ablations of individual dopamine or adenosine receptors. In situ hybridization histochemistry and autoradiography was used to examine the level of mRNA and protein expression of specific receptors and transporters in dopaminergic pathways. Expression of the mRNA encoding the dopamine D2 receptor was elevated in the caudate putamen of D1, D3 and A2A receptor knockout mice; this was mirrored by an increase in D2 receptor protein in D1 and D3 receptor knockout mice, but not in A2A knockout mice. Dopamine D1 receptor binding was decreased in the caudate putamen, nucleus accumbens, olfactory tubercle and ventral pallidum of D2 receptor knockout mice. In substantia nigra pars compacta, dopamine transporter mRNA expression was dramatically decreased in D3 receptor knockout mice, but elevated in A2A receptor knockout mice. All dopamine receptor knockout mice examined exhibited increased A2A receptor binding in the caudate putamen, nucleus accumbens and olfactory tubercle. These data are consistent with the existence of functional interactions between dopaminergic and purinergic systems in these reward and motor-related brain regions.

The acute anti-craving effect of acamprosate in alcohol-preferring rats is associated with modulation of the mesolimbic dopamine system

Acamprosate (Campral) is a drug used clinically for the treatment of alcoholism. In order to examine further the time-course and mechanism of action of acamprosate, the effect of acute and repeated acamprosate administration was examined on (i) operant ethanol self-administration and (ii) voluntary home cage ethanol consumption by alcohol-preferring Fawn-Hooded, iP and Alko Alcohol (AA) rats. Acutely, acamprosate was shown to cause a significant decrease in operant ethanol self-administration by Fawn-Hooded and alcohol-preferring iP rats in part by decreasing the motivational relevance of a specific ethanol cue; however, repeated injection of acamprosate led to tolerance to this effect. Voluntary alcohol consumption in the home cage in Fawn-Hooded and AA rats was also reduced by an acute acamprosate injection; however, again tolerance developed to repeated injections. In a separate experiment, the effect of acamprosate on markers of the dopaminergic system was examined. Interestingly, acute acamprosate was also shown to cause increased dopamine transporter density and decreased dopamine D2-like receptor density within the nucleus accumbens but not in the caudate-putamen, suggesting a link between the decreased motivational salience of the ethanol cue and altered dopaminergic signalling within the nucleus accumbens. With repeated injections of acamprosate, markers of the dopaminergic system returned to steady state levels with a similar temporal profile to the development of tolerance in the behavioural studies. Along with previous studies, our findings indicate that acamprosate modulates the mesolimbic dopaminergic system and may thereby decrease ethanol reinforcement processes; however, these effects undergo tolerance in alcohol-preferring rats and may in part explain the fact why some subjects are non-responders to chronic acamprosate treatment.

Chronic corticotropin-releasing factor type 1 receptor antagonism with antalarmin regulates the dopaminergic system of Fawn-Hooded rats

Corticotropin-releasing factor is a neuropeptide associated with the integration of physiological and behavioural responses to stress and also in the modulation of affective state and drug reward. The selective, centrally acting corticotropin-releasing factor type 1 receptor antagonist, antalarmin, is a potent anxiolytic and reduces volitional ethanol consumption in Fawn-Hooded rats. The efficacy of antalarmin to reduce ethanol consumption increased with time, suggestive of adaptation to reinforcement processes and goal-directed behaviour. The aim of the present study was to examine the effects of chronic antalarmin treatment on reward-related regions of Fawn-Hooded rat brain. Bi-daily antalarmin treatment (20 mg/kg, i.p.) for 10 days increased tyrosine hydroxylase messenger RNA expression throughout the ventral mesencephalon. Following chronic antalarmin the density of dopaminergic terminals within the basal ganglia and amygdaloid complex were reduced, as was dopamine transporter binding within the striatum. Receptor autoradiography indicated an up-regulation of dopamine D2, but no change in D1, binding in striatum, and Golgi-Cox analysis of striatal medium spiny neurones indicated that chronic antalarmin treatment increased spine density. Thus, chronic antalarmin treatment modulates dopaminergic pathways and implies that chronic treatment with drugs of this class may ultimately alter postsynaptic signaling mechanisms within the basal ganglia.

The Metabotropic Glutamate 5 Receptor Antagonist 3-[(2-Methyl-1,3-thiazol-4-yl)ethynyl]-pyridine Reduces Ethanol Self-Administration in Multiple Strains of Alcohol-Preferring Rats and Regulates Olfactory Glutamatergic Systems

The metabotropic glutamate 5 receptor (mGlu5) receptor has been implicated as having a role in pain modulation, anxiety, and depression, as well as drug-seeking behavior. In the present study, we examined the effect of the selective mGlu5 receptor antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP) on operant ethanol self-administration by two strains of rats, the Fawn-Hooded (FH) rat and the inbred alcohol-preferring (iP) rat. MTEP (2 mg/kg i.p.) caused a significant reduction in responding for ethanol by both strains of rats; however, in the iP rats, MTEP also induced apparent sedation at this dose, although still reduced alcohol responding at lower doses. Chronic MTEP (2 mg/kg/day) caused a significant reduction in ethanol consumption by FH rats in a two-bottle preference test; however, chronic treatment with this dose had no effect on anxiety-like behavior or depressive-like behavior in FH rats, suggesting the dose used was subthreshold for anxiolytic or antidepressive-like effects. Finally, repeated dosing with MTEP (2 mg/kg i.p.) caused significant reductions in expression of the mRNA encoding the NR1 subunit of the N-methyl-D-aspartate receptor and the GluR2 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor in the cingulate cortex. A significant decrease in NR1 expression also occurred in the piriform cortex. Chronic MTEP also caused a significant decrease in mGlu5 gene expression and a significant increase in dopamine transporter and dopamine D(2)-like receptor binding within the olfactory tubercle. Collectively, these data suggest that MTEP can reduce alcohol-seeking behavior in different rodent models of alcoholism, and this effect is associated with regulation of cortical glutamate systems, particularly those in olfactory-related regions.

Lack of adenosine A1 and dopamine D2 receptor-mediated modulation of the cardiovascular effects of the adenosine A2A receptor agonist CGS 21680

Some behavioral and biochemical effects of the systemically administered adenosine A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680) in rats are potentiated by adenosine A(1) receptor agonists and counteracted by dopamine D2 receptor agonists. In the present study we compared potentiating and antagonistic interactions between CGS 21680 and adenosine A(1) and dopamine D2 receptor agonists on motor activity and on cardiovascular responses (arterial blood pressure and heart rate). The motor-depressant effects produced by CGS 21680 (0.5 mg/kg, i.p.) were potentiated by the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA, 0.3 mg/kg, i.p.) and counteracted by the dopamine D2 receptor agonist quinpirole (0.5 mg/kg, i.p.). In contrast, neither CPA nor quinpirole significantly modified the decrease in arterial pressure or the increase in heart rate induced by CGS 21680. However, the adenosine A(2A) receptor antagonist 3-(3-hydroxypropyl)-8-(m-methoxystyryl)-7-methyl-1-propargylxanthine phosphate disodium salt (MSX-3, 3 mg/kg, i.p.) counteracted both the motor-depressant and cardiovascular effects of CGS 21680. Therefore, the effects of the systemically administered adenosine A(2A) receptor agonist CGS 21680 on cardiovascular function, in contrast to its effects on motor behavior, appear to be independent of the effects of adenosine A(1) and dopamine D2 receptor activity.

Pressor Responsiveness to Intravenous Quinpirole is Blunted in Malnourished, Conscious Rats: Central vs. Peripheral and Spinal Mechanisms

In conscious rats, intravenous treatment with the dopamine D2-like receptor agonist quinpirole, elicited a pressor effect, which is attributed to central dopamine D2 receptor-mediated activation of sympathetic outflow associated with arginine vasopressin release. This prominent central effect is opposed to peripheral sympathoinhibitory and spinal depressor effects. The present study investigated the effects of pre- and postnatal undernutrition on the central pressor responsiveness to quinpirole. Malnourished (MalN) rats were obtained by feeding dams a multideficient diet (providing 8% protein) during pregnancy and nursing. At 90 days of age, MalN rats weighed significantly less than control (CNT) rats born to dams fed standard commercially diet (23% protein) during pregnancy and nursing. Baseline mean arterial pressure and heart rate in MalN rats were comparable to those of CNT. Intravenous treatment with quinpirole (0.3 mg/kg) in MalN conscious rats induced a pressor effect, which was significantly reduced in both magnitude and duration, when compared with CNT rats. In both groups studied, pressor response to quinpirole was fully abolished by the peripheral and central dopamine D2 receptor antagonist, metoclopramide (5 mg/kg, i.v.) whereas was significantly enhanced after pretreatment with either intravenous (0.5 mg/kg) or intrathecal (40 microg per rat at T9-T10 level) domperidone, a dopamine D2 receptor antagonist that does not cross the blood-brain barrier. However, even under peripheral and spinal dopamine D2 receptor blockade, maximum pressor effect of quinpirole remained significantly reduced in MalN when compared with CNT rats. Neither the maximum pressor nor the bradycardiac responses to intravenous phenylephrine or arginine vasopressin differed between CNT and MalN rats. This study shows that undernutrition imposed during fetal life and suckling blunted the pressor response to quinpirole in conscious rats. This blunted response appears mainly related to desensitization of brain dopamine D2 receptors rather than enhanced peripheral and/or spinal dopamine D2 receptor-mediated depressor effect or vascular hyporesponsiveness to alpha1-adrenoceptor and vasopressin receptor stimulation.

Non-angiotensin II [125I] CGP42112 binding is a sensitive marker of neuronal injury in brainstem following unilateral nodose ganglionectomy: Comparison with markers for activated microglia

Previously we reported that a non-angiotensin II [(125)I] CGP42112 binding site is up-regulated in rat brainstem nuclei as a result of unilateral nodose ganglionectomy. In the present study, we compared non-angiotensin II [(125)I] CGP42112 binding with microglia/macrophage activation following nodose ganglionectomy, using both in vitro autoradiography and immunohistochemistry. Specific [(125)I] CGP42112 binding was observed in the nucleus of the solitary tract (NTS) and revealed an AT(2) receptor component as well as a non-angiotensin II receptor component. Subsequent to unilateral nodose ganglionectomy, [(125)I] CGP42112 binding in the ipsilateral NTS was increased approximately two-fold and was also induced in the ipsilateral dorsal motor nucleus (DMX) and the nucleus ambiguus (n.amb). This non-angiotensin II [(125)I] CGP42112 binding site was displaced by CGP42112 but not other ligands. Increased [(3)H] PK11195 binding (a known marker of reactive gliosis) was also observed in the same brainstem nuclei as non-angiotensin II [(125)I] CGP42112 binding after nodose ganglionectomy. The similarity in binding patterns between [(125)I] CGP42112 and [(3)H] PK11195 was shown to be primarily due to retrograde degeneration in the ipsilateral NTS, DMX and n.amb, as both radioligands were localized to similar cellular targets within the interstial space and over cellular debris. Immunohistochemical data confirmed reactive gliosis within the ipsilateral NTS, DMX and n.amb, following nodose ganglionectomy, which was predominantly characterized by an increase in OX-42 immunoreactivity (a marker for activated microglia/macrophages), with only a small increase in glial fibrillary acidic protein immunoreactivity (a marker of astrogliosis) detected. These data demonstrate for the first time that non-angiotensin II [(125)I] CGP42112 binding is associated with activated microglia, as well as macrophages, following unilateral nodose ganglionectomy. Furthermore, these studies also demonstrate the potential use of non-angiotensin II [(125)I] CGP42112 binding as a marker for quantitating inflammatory events which occur as a result of damage to the CNS.

Systemic apomorphine alters HPA axis responses to interleukin-1 beta administration but not sound stress

Apomorphine is a dopamine receptor agonist that was recently licensed for the treatment of erectile dysfunction. However, although sexual activity can be stressful, there has been little investigation into whether treatments for erectile dysfunction affect stress responses. We have examined whether a single dose of apomorphine, sufficient to produce penile erections (50 microg/kg, i.a.), can alter basal or stress-induced plasma ACTH levels, or activity of central pathways thought to control the hypothalamic-pituitary-adrenal axis in rats. An immune challenge (interleukin-1 beta, 1 microg/kg, i.a.) was used as a physical stressor while sound stress (100 dB white noise, 30 min) was used as a psychological stressor. Intravascular administration of apomorphine had no effect on basal ACTH levels but did substantially increase the number of Fos-positive amygdala and nucleus tractus solitarius catecholamine cells. Administration of apomorphine prior to immune challenge augmented the normal ACTH response to this stressor at 90 min and there was a corresponding increase in the number of Fos-positive paraventricular nucleus corticotropin-releasing factor cells, paraventricular nucleus oxytocin cells and nucleus tractus solitarius catecholamine cells. However, apomorphine treatment did not alter ACTH or Fos responses to sound stress. These data suggest that erection-inducing levels of apomorphine interfere with hypothalamic-pituitary-adrenal axis inhibitory feedback mechanisms in response to a physical stressor, but have no effect on the response to a psychological stressor. Consequently, it is likely that apomorphine acts on a hypothalamic-pituitary-adrenal axis control pathway that is unique to physical stressors. A candidate for this site of action is the nucleus tractus solitarius catecholamine cell population and, in particular, A2 noradrenergic neurons.

The effect of isolation rearing on volitional ethanol consumption and central CCK/dopamine systems in Fawn-Hooded rats

Numerous studies have demonstrated that socially isolating rats (from weaning) produces a sustained anxious phenotype and an enhanced response to psychostimulant drugs such as amphetamine and cocaine. In addition, isolation rearing has been shown to induce significant changes in the mesolimbic dopamine system. These data indicate that isolation rearing not only induces an anxiogenic phenotype but also induces neurochemical changes in reward nuclei of the brain, which is correlated with an enhanced response to psychostimulants. For these reasons, the effect of isolation rearing on volitional ethanol consumption was examined in Fawn-Hooded (FH) rats and correlated with neurochemical changes in central dopamine and cholecystokinin systems. Social isolation from weaning produced an anxiogenic phenotype as measured by a decreased time spent on the open arms of an elevated plus-maze. Interestingly, isolation-rearing induced a greater proportion of FH rats to acquire preference for ethanol while having no effect on the amount of ethanol consumed by alcohol-preferring rats. In addition, isolation rearing induced a number of changes in central CCK/dopamine systems.

The neurochemical effects of anxiolytic drugs are dependent on rearing conditions in Fawn-Hooded rats

There is a vast literature examining the neurochemical effects of anxiolytics throughout the rat brain; however, although the behavioural actions of anxiolytic drugs are routinely assessed in animal models of anxiety, the majority of neurochemical studies have been performed in rats with relatively 'normal' behavioural phenotypes. Since there is significant evidence that an anxious phenotype is associated with numerous neurochemical alterations, it is feasible that the central effects of anxiolytics may vary depending on the underlying behavioural state (and corresponding neuropathology) of the experimental animal. For this reason, the aim of the present study was to examine the effect of chronic anxiolytic drug administration on the central CCK and dopamine systems in anxious (isolated from weaning) and nonanxious (group-housed) Fawn-Hooded (FH) rats. It is important to note that these studies were performed in rats with continued access to ethanol, which may affect the responses to anxiolytic treatment. Chronic anxiolytic treatment with the selective CCK-B (CCK(2)) receptor antagonist, Ci-988 (0.3 mg/kg/day ip) or diazepam (2 mg/kg/day ip), induced numerous effects throughout the central nervous system (CNS), with Ci-988 inducing significant changes in the density of dopamine D(2) receptors, and diazepam producing marked changes in both dopamine D(2) and CCK-B receptor binding density as well as preproCCK mRNA expression. Interestingly, the neurochemical effects of these anxiolytic drugs varied significantly depending on the rearing conditions of the rats, demonstrating the importance of using adequate animal models when correlating the behavioural and central effects of drugs acting throughout the CNS.

Activation of dopamine D2-like receptors attenuates pulmonary C-fiber hypersensitivity in rats

This study was performed to determine whether activation of dopamine D2-like receptors inhibits the hyperresponsiveness of pulmonary C fibers induced by inflammatory mediators such as prostaglandin E2 (PGE2). In anesthetized, open-chest rats, constant infusion of PGE2 (1.5-4.5 microg/kg per minute, 2 minutes) significantly enhanced the C-fiber response to capsaicin injection. At 20 minutes after pretreatment with quinpirole (3 mg/kg, intravenous), a D2-like receptor agonist, the hyperresponsiveness to capsaicin of the same C fibers induced by PGE2 infusion was markedly attenuated, and this inhibitory effect lasted for more than 90 minutes. The effect of quinpirole was dose dependent and was antagonized by pretreatment with domperidone (5 mg/kg, intravenous), a D2-like receptor antagonist, administrated 10 minutes before the quinpirole injection. In a separate series of experiments, C-fiber responses to injections of phenyl biguanide and lactic acid and to constant-pressure lung inflation were augmented by PGE2; these potentiating responses were also significantly reduced by quinpirole. Furthermore, the effect of quinpirole was equally effective in inhibiting the increase in excitability of pulmonary C fibers induced by alveolar hypercapnia or constant infusion of adenosine. In conclusion, these results clearly show that activation of the dopamine D2-like receptors attenuates the hyperresponsiveness of pulmonary C fibers to both chemical stimuli and lung inflation.

Dopamine modulates synaptic transmission in the nucleus of the solitary tract

10.1152/jn.00224.2002. Dopamine (DA) modulates the cardiorespiratory reflex by peripheral and central mechanisms. The aim of this study was to examine the role of DA in synaptic transmission of the nucleus tractus solitarius (NTS), the major integration site for cardiopulmonary reflexes. To examine DA's role, we used whole cell, voltage-clamp recordings in a rat horizontal brain stem slice. Solitary tract stimulation evoked excitatory postsynaptic currents (EPSCs) that were reduced to 70 +/- 5% of control by DA (100 microM). The reduction in EPSCs by DA was accompanied by a decrease in the paired pulse depression ratio with little or no change in input resistance or EPSC decay, suggesting a presynaptic mechanism. The D1-like agonist SKF 38393 Br (30 microM) did not alter EPSC amplitude, whereas the D2-like agonist, quinpirole HCl (30 microM), depressed EPSCs to 73 +/- 4% of control. The D2-like receptor antagonist, sulpiride (20 microM), abolished DA modulation of EPSCs. Most importantly, sulpiride alone increased EPSCs to 131 +/- 10% of control, suggesting a tonic D2-like modulation of synaptic transmission in the NTS. Examination of spontaneous EPSCs revealed DA reversibly decreased the frequency of events from 9.4 +/- 2.2 to 6.2 +/- 1.4 Hz. Sulpiride, however, did not alter spontaneous events. Immunohistochemistry of NTS slices demonstrated that D2 receptors colocalized with synaptophysin and substance P, confirming a presynaptic distribution. D2 receptors also localized to cultured petrosal neurons, the soma of presynaptic afferent fibers. In the petrosal neurons, D2 was found in cells that were TH-immunopositive, suggesting they were chemoreceptor afferent fibers. These results demonstrate that DA tonically modulates synaptic activity between afferent sensory fibers and secondary relay neurons in the NTS via a presynaptic D2-like mechanism.

The effect of chronic ethanol consumption and withdrawal on mu-opioid and dopamine D(1) and D(2) receptor density in Fawn-Hooded rat brain

Previous studies have implicated the dopamine and opioid systems in the induction and maintenance of ethanol consumption. This study investigated, in alcohol-preferring Fawn-Hooded (FH) rats, whether chronic free-choice ethanol consumption and subsequent withdrawal cause alterations in central mu-opioid, dopamine D(1), and D(2) receptor density using autoradiography. FH rats were given a free choice between a 5% ethanol solution and tap water (n = 25) and displayed a mean ethanol consumption of 5.6 g/kg/day. A parallel group of FH rats (n = 5) only had access to tap water. Rats were then withdrawn from ethanol for 0, 1, 2, 5, or 10 days and killed by cervical dislocation and decapitation. Increases in mu-opioid receptor density were observed in the nucleus accumbens and ventral tegmental area upon withdrawal compared with the ethanol naive group. In the lateral amygdala, binding in all withdrawal groups was significantly different from the ethanol naive FH rats, and also from the chronic ethanol rats. An increase in dopamine D(1) receptor density was observed in the substantia nigra, pars reticulata in the 5- and 10-day withdrawal groups compared with ethanol naive. Accumbal dopamine D(2) receptor density (+25-30%) increased in the 10-day withdrawal group compared with both naive and chronic ethanol groups. These findings demonstrate that the opioid and dopamine systems are susceptible to modulation by chronic ethanol consumption and withdrawal in the FH rat. Furthermore, although acute ethanol withdrawal results in modulation of mu-opioid receptors, effects on dopamine receptors are delayed and only become evident 5 to 10 days after withdrawal.

The highly selective CRF(2) receptor antagonist K41498 binds to presynaptic CRF(2) receptors in rat brain

1. Novel analogues of antisauvagine-30 (aSvg-30), a selective antagonist for CRF(2) receptors, have been synthesized and characterized in vitro and in vivo. 2. The analogues were tested for their ability to compete for [(125)I-Tyr(0)]Svg binding and to inhibit Svg-stimulated adenylate cyclase activity in human embryonic kidney (HEK) 293 cells, permanently transfected with cDNA coding for the human CRF(1) (hCRF(1)), hCRF(2alpha) and hCRF(2beta) receptor. One analogue [D-Phe(11), His(12), Nle(17)]Svg(11-40), named K41498, showed high affinity binding to hCRF(2alpha) (K(i)=0.66+/-0.03 nM) and hCRF(2beta) (K(i)=0.62+/-0.01 nM) but not the hCRF(1) receptor (k(i)=425+50 nM) and decreased Svg-stimulated cAMP accumulation in hCRF(2) expressing cells. In conscious Wistar-Kyoto rats, K41498 (1.84 microg, i.v.) antagonized the hypotensive response to systemic urocortin (1.4 microg, i.v.), but did not block the pressor response to centrally administered urocortin (2.35 microg, i.c.v.). 3. K41498 was subsequently radio-iodinated, and in autoradiographic studies, specific (sensitive to rat urocortin, astressin and aSvg30, but insensitive to antalarmin) binding of (125)I-K41498 (100 pM) was detected in the heart and in selected brain regions including the nucleus tractus solitarius (NTS), spinal trigeminal nucleus, lateral septum and around the anterior and middle cerebral arteries. 4. Following unilateral nodose ganglionectomy, binding of (125)I-K41498 was reduced by 65% in the ipsilateral NTS, indicative of presynaptic CRF(2) receptors on vagal afferent terminals. 5. These data demonstrate that K41498 is a useful tool to study native CRF(2) receptors in the brain and periphery.

Blockade of spinal dopamine D2 receptors enhances the pressor effect of intravenous quinpirole in normotensive, conscious rats

The present investigation was undertaken to examine whether in conscious intact rats blockade of spinal dopamine D2 receptors enhances the pressor effect of intravenous quinpirole. In saline-pretreated rats, intravenous quinpirole (1 mg/kg) induced a significant pressor effect, which reached a maximum (17.71+/-0.60 mmHg) within the first min. after injection. Pretreatment with intravenous (0.5 mg/kg) or intrathecal (40 microg/rat at T9-T10) domperidone, a dopamine D2 receptor antagonist that does not cross the blood-brain barrier, significantly enhanced the maximal pressor response to quinpirole (25.60+/-1.52 and 24.00+/-1.72 mmHg, respectively). The pressor effect of quinpirole was also significantly enhanced after combined pretreatment with intravenous and intrathecal domperidone, and its maximum (31.60+/-2.31 mmHg) was significantly higher than that recorded in animals pretreated with intrathecal or intravenous domperidone alone. Intravenous pretreatment with metoclopramide (5 mg/kg) fully abolished the quinpirole-induced pressor effect. These results show that in conscious intact rats, blockade of spinal dopamine D2 receptors enhances the pressor response to systemic quinpirole, suggesting that this agonist can decrease blood pressure through a spinal dopaminergic mechanism. Thus, our previous hypothesis that the entire effect of intravenous quinpirole on blood pressure in conscious rats can be composed of a central pressor action, a peripheral sympathoinhibitory depressor effect and also a spinal depressor effect is strongly supported by the present findings.

Characterisation of vasopressin V(1A), angiotensin AT(1) and AT(2) receptor distribution and density in normotensive and hypertensive rat brain stem and kidney: effects of restraint stress

In the present study, we have examined neurochemical correlates that may be involved in the differential cardiovascular responses observed in normotensive and hypertensive rats during stress. Using a restraint stress paradigm, both normotensive Wistar Kyoto (WKY) and Spontaneously Hypertensive rats (SHR) underwent acute (1 h restraint in a perspex tube), chronic (1 h restraint for ten consecutive days) or no restraint (control) stress. Following cessation of restraint, rats were processed by incubating sections of brain stem and kidney with [125I]-HO-LVA (0.03 nM) or [125I]Sar(1)Ile(8)-AngiotensinII (0.5 nM), in the presence of PD123319 (10 microM) or losartan (10 microM), to determine the distribution and density of vasopressin V(1A), angiotensin AT(1) and AT(2) receptors, respectively. Analysis of autoradiograms indicated changes in the density of radioligand binding in acutely and chronically-stressed rats, as compared to controls. For example, V(1A) binding in the medial nucleus tractus solitarius (SolM) decreased in the WKY but increased in the SHR. AT(1) binding in SolM did not significantly change in the WKY but decreased in the SHR with repeated restraint. In kidney slices, AT(1) binding decreased with stress in the WKY (-17%) but increased in SHR (+10-15%). AT(2) binding in the kidney showed a pattern similar to that of AT(1) binding in SHR, but not WKY. Graded increases in V(1A) binding were measured in kidney medulla and cortex of both strains (+50-60% with chronic restraint). These results suggest that physiological adaptation to restraint is associated with specific changes in V(1A), AT(1) and AT(2) receptor density within brain nuclei and kidney.

Calcium mobilization elicited by two types of nicotinic acetylcholine receptors in mouse substantia nigra pars compacta

Nicotinic acetylcholine receptors (nAChRs) are expressed in the midbrain ascending dopaminergic system, a target of many addictive drugs. Here we assessed the intracellular Ca2+ level by imaging fura-2-loaded cells in substantia nigra pars compacta in mouse brain slices, and we examined the influence on this level of prolonged exposures to nicotine using mice lacking the nAChR beta2-subunit. In control cells, superfusion with nicotine (10-100 microM) caused a long-lasting rise of intracellular Ca2+ level which depended on extracellular Ca2+. This nicotinic response was almost completely absent in beta2-/- mutant mice, leaving a small residual response to a high concentration (100 microM) of nicotine which was inhibited by the alpha7-subunit-selective antagonist, methyllycaconitine. Conversely, the alpha7-subunit-selective agonist choline (10 mM) caused a methyllycaconitine-sensitive increase in intracellular Ca2+ level both in wild-type and beta2-/- mutant mice. Nicotine-elicited Ca2+ mobilization was reduced by the Na+ channel blocker tetrodotoxin (TTX) and by T-type Ca2+ channel blocking agents, whereas the choline-elicited Ca2+ increase was insensitive to TTX. Neither nicotine nor choline produced Ca2+ increase following inhibition of the release of Ca2+ from intracellular stores by dantrolene. These results demonstrate that in nigral dopaminergic neurons, nicotine can elicit Ca2+ mobilization via activation of two distinct nAChR subtypes: that of beta2-subunit-containing nAChR followed by activation of Na+ channel and T-type Ca2+ channels, and/or activation of alpha7-subunit-containing nAChR. The Ca2+ influx due to nAChR activation is subsequently amplified by the recruitment of intracellular Ca2+ stores. This Ca2+ mobilization may possibly contribute to the long-term effects of nicotine on the dopaminergic system.

Quinpirole treatment increases renal sympathetic nerve activity and baroreflex gain in conscious rabbits: a spectral study

Intravenous administration of 0.3 mg/kg of quinpirole to conscious rabbits that had been pretreated with domperidone caused a marked increase in blood pressure and renal sympathetic nerve activity with a peak at 5-10 min after injection (25% and 3-fold increase, respectively). Spectral analysis of the blood pressure-renal sympathetic nerve activity relationship in the 0.2-0.4 Hz domain showed that baroreflex gain was markedly increased at 5-10 min (4-fold) and at 20-25 min after injection (3.7-fold). These results show that administration of the dopamine D(2)/D(3) receptor agonist quinpirole causes profound and long-lasting changes in the central integration of the sympathetic baroreceptor-vasomotor reflex.

Brain dopamine D2 receptor mRNA levels are elevated in young spontaneously hypertensive rats

Levels of brain dopamine D2 receptor expression were compared between spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) controls by quantitative in situ hybridisation, using a complementary RNA probe for D2 receptor mRNA. In SHR which were 6 weeks of age, significantly higher levels of D2 receptor mRNA were found in the caudate-putamen (42%), nucleus accumbens (23%), olfactory tubercle (17%) and substantia nigra (38%) compared to age-matched WKY controls. D2 receptor mRNA levels were also higher in the substantia nigra (27%) of 12-14-week old SHR compared to WKY. The increased levels of dopamine D2 receptor gene expression displayed in young prehypertensive SHR could implicate altered central dopaminergic activity in the pathogenesis of hypertension.

Visualisation of AMPA binding sites in the brain stem of normotensive and hypertensive rats

The present study has employed in vitro receptor autoradiography with (S)-[(3)H]-5-fluorowillardiine (10 nM) to visualise the presence of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) binding sites in the brain stems of adult (16-18 weeks) normotensive (Wistar-Kyoto (WKY) and Don Ryu (DRY)) and Spontaneously Hypertensive (SHR) rats. Similar topographic distribution and density of (S)-[(3)H]-5-fluorowillardiine binding was observed in the nucleus tractus solitarius (NTS) of all three strains. Specific (S)-[(3)H]-5-fluorowillardiine binding sites were also visualised in sections of nodose ganglion from adult WKY rats, demonstrating that vagal afferent perikarya possess AMPA binding sites. However, while unilateral vagal deafferentation did not result in a significant decrease in binding site density in the caudal half of the rat NTS, the visualisation of AMPA binding sites on the nodose ganglion is consistent with the existence of a population of binding sites on vagal afferent terminals. In the caudal half of the rat NTS, AMPA binding sites appear to be predominantly postsynaptic in nature.

Targeted expression of a toxin gene to D1 dopamine receptor neurons by cre-mediated site-specific recombination

Idiopathic Parkinson's disease involves the loss of midbrain dopaminergic neurons, resulting in the presynaptic breakdown of dopaminergic transmission in the striatum. Huntington's disease and some neurodegenerative diseases with Parkinsonian features have postsynaptic defects caused by striatal cell death. Mice were generated in which an attenuated form of the diphtheria toxin gene (tox-176) was expressed exclusively in D1 dopamine receptor (D1R)-positive cells with the aim of determining the effect of this mutation on development of the basal ganglia and on the locomotor phenotype. Transgenic mice expressing Cre, a site-specific DNA recombinase, were crossed with a second line in which a transcriptionally silenced tox-176 gene was inserted into the D1R gene locus by homologous recombination. Young doubly transgenic mutant mice expressing the tox-176 gene displayed bradykinesia, dystonia, and had falls caused by myoclonic jerks. The mutant brain had evidence of apoptosis and reactive gliosis and, consistent with the D1R expression pattern, the striatum was reduced in volume, and the Islands of Calleja were absent. In contrast, the cortex was of normal thickness. D1Rs were not detectable in mutants by in situ hybridization or ligand autoradiography, whereas D2 dopamine receptor (D2R) mRNA and protein was present in the striatum. In addition, substance P and dynorphin, neuropeptides known to be expressed in D1R-positive striatonigral projection neurons were not detectable. Enkephalin, a marker found in D2-positive striatopallidal projection neurons was expressed in the mutant brain. The mutant represents a novel neurodegenerative disease model with a dramatic extrapyramidal phenotype.

Targeted expression of a toxin gene to D1 dopamine receptor neurons by cre-mediated site-specific recombination

Idiopathic Parkinson's disease involves the loss of midbrain dopaminergic neurons, resulting in the presynaptic breakdown of dopaminergic transmission in the striatum. Huntington's disease and some neurodegenerative diseases with Parkinsonian features have postsynaptic defects caused by striatal cell death. Mice were generated in which an attenuated form of the diphtheria toxin gene (tox-176) was expressed exclusively in D1 dopamine receptor (D1R)-positive cells with the aim of determining the effect of this mutation on development of the basal ganglia and on the locomotor phenotype. Transgenic mice expressing Cre, a site-specific DNA recombinase, were crossed with a second line in which a transcriptionally silenced tox-176 gene was inserted into the D1R gene locus by homologous recombination. Young doubly transgenic mutant mice expressing the tox-176 gene displayed bradykinesia, dystonia, and had falls caused by myoclonic jerks. The mutant brain had evidence of apoptosis and reactive gliosis and, consistent with the D1R expression pattern, the striatum was reduced in volume, and the Islands of Calleja were absent. In contrast, the cortex was of normal thickness. D1Rs were not detectable in mutants by in situ hybridization or ligand autoradiography, whereas D2 dopamine receptor (D2R) mRNA and protein was present in the striatum. In addition, substance P and dynorphin, neuropeptides known to be expressed in D1R-positive striatonigral projection neurons were not detectable. Enkephalin, a marker found in D2-positive striatopallidal projection neurons was expressed in the mutant brain. The mutant represents a novel neurodegenerative disease model with a dramatic extrapyramidal phenotype.

Dopaminergic modulation of respiratory motor output in peripherally chemodenervated goats

We examined the ventilatory effects of exogenous dopamine (DA) and norepinephrine (NE) administration in chloralose-anesthetized, paralyzed, artificially ventilated adult goats before and after carotid body denervation (CBD). Intravenous (iv) DA bolus injections and slow iv infusions caused dose-dependent inhibition of phrenic nerve activity (PNA) in carotid body (CB)-intact animals during normoxia and hyperoxia but not during hypercapnia. NE administration in CB-intact goats caused dose-dependent inhibition of PNA of similar magnitude to DA trials. The DA D2-receptor agonists quinelorane and quinpirole inhibited PNA, whereas the DA D1-receptor agonist SKF-81297 had no effect. After CBD, the ventilatory depressant effects of DA persisted, but responses were significantly attenuated compared with CB-intact trials. CBD abolished the inhibitory effect of low-dose NE administration but did not alter ventilatory responses to high-dose NE injection. The peripheral DA D2-receptor antagonist domperidone substantially attenuated the inhibitory effects of DA bolus injections and infusions and reversed the inhibitory ventilatory effect of high-dose DA administration to excitation in some animals. The alpha-adrenoceptor antagonist phentolamine had no effect on DA-induced ventilatory depression. Beta-Adrenoceptor stimulation with isoproterenol produced similar hemodynamic effects to DA administration but had no effect on PNA. We conclude that DA and NE exert both CB-mediated and non-CB-mediated inhibitory effects on respiratory motor output in anesthetized goats. The ventilatory depressant effects that persist in peripherally chemodenervated animals are DA D2-receptor mediated, but their exact location remains speculative.

The distribution of nitric oxide synthase-, adenosine deaminase- and neuropeptide Y-immunoreactivity through the entire rat nucleus tractus solitarius: Effect of unilateral nodose ganglionectomy

The present study has employed immunocytochemistry on free-floating sections of adult rat medulla oblongata to characterise the distribution of nitric oxide synthase- (NOS), adenosine deaminase- (ADA) and neuropeptide Y- (NPY) immunoreactivity (IR) throughout the entire rostro-caudal axis of the nucleus tractus solitarius (NTS). In addition, unilateral nodose ganglionectomy was performed in a group of rats to determine whether any observed immunoreactivity was associated with central vagal afferent terminals. NOS-IR was found throughout the entire NTS, in cells, and both varicose and non-varicose fibres. Furthermore, unilateral nodose ganglionectomy resulted in a clear reduction in NOS-IR (visualised with diaminobenzidine) in a highly restricted portion of the ipsilateral medial NTS. Similarly, ADA- and NPY-containing cells, fibres and terminals were also found throughout the adult rat NTS. However, following unilateral nodose ganglionectomy, there was no apparent reduction in either ADA-IR or NPY-IR on the denervated side of the NTS. These data indicate a role for nitric oxide, purines and neuropeptide Y as neuromodulators within the rat NTS, although only nitric oxide appears to be primarily associated with vagal afferent input. Adenosine deaminase and neuropeptide Y-containing neurons appear to be predominantly postsynaptic to vagal input, although their possible association with vagal afferents cannot be completely excluded.

Bradykinin B2 receptors in nodose ganglia of rat and human

The present study has employed in vitro electrophysiology to characterise the ability of bradykinin to depolarise the rat isolated nodose ganglion preparation, containing the perikarya of vagal afferent neurons. Both bradykinin and kallidin elicited a concentration-dependent (1-100 nM) depolarisation when applied to the superfusate bathing the nodose ganglia, whereas the bradykinin B1 receptor agonist, des-Arg9-bradykinin, was only effective in the micromolar range. Furthermore, the electrophysiological response to bradykinin was antagonised by the bradykinin B2 receptor antagonist, D-arginyl-L-arginyl-L-prolyl-trans-4-hydroxy-L-prolylglycyl-3-(2-t hienyl)-L-alanyl-L-seryl-D-1,2,3,4-tetrahydro-3-isoquinolinecarbonyl+ ++-L-(2alpha,3beta,7abeta)-octahydro-1H-indole-2-carbonyl-L- arginine (Hoe 140), in a concentration-related manner. To determine the anatomical location of functional bradykinin B2 receptors, in vitro autoradiography with [125I]para-iodophenyl Hoe 140 was performed on sections of rat and human inferior vagal (nodose) ganglia and confirmed the presence of binding over vagal perikarya. Collectively, these data provide evidence for functionally relevant bradykinin B2 receptors on vagal afferent neurons, which are apparently also present on human vagal perikarya.

Pretreatment with the dopamine agonist quinpirole inhibits central antihypertensive mechanisms in rats

1. Intravenous or central treatment of spontaneously hypertensive rats (SHR) with the dopamine D2 receptor agonist quinpirole caused a short-lasting pressor response with little effect on heart rate. 2. At 30 min after intravenous administration of quinpirole, the antihypertensive effect of rilmenidine was significantly inhibited. This interaction of quinpirole and rilmenidine was similarly observed when quinpirole was administered either intravenously (0.3 or 0.1 mg/kg), in the lateral cerebral ventricles (0.1 mg/kg) or intracisternally (0.1 mg/kg) or when rilmenidine was administered intravenously (1 mg/kg) or intracisternally (0.1 mg/kg). 3. The apparent desensitization to the antihypertensive effect of rilmenidine 30 min after pretreatment with quinpirole was not observed after a 4 or 24 h interval. 4. These data suggest that quinpirole has prolonged effects on central sympathetic vasomotor mechanisms that are the target of centrally acting antihypertensive drugs. These and previous results show a functional interaction between central dopamine D2 receptor activation and sympathetic responses mediated by a wide range of different receptors, including imidazoline and 5-hydroxytryptamine 5-HT1A-receptors and alpha 2-adrenoceptors.

Complex interactions between nitric oxide and adenosine receptors in the rat isolated nodose ganglion

The present study has employed in vitro electrophysiology, utilising the isolated rat nodose ganglion preparation, to determine whether nitric oxide (NO) and adenosine interact with each other in vagal afferent neurons. The nucleophile NO donor, diethylamine-NO, caused reproducible, concentration-related depolarisations of the isolated rat nodose ganglia. Pre-incubation of the isolated rat nodose ganglia with the adenosine A2A receptor agonists CGS 21680 (2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride) and DPMA (N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine) (both 1 microM) resulted in a functional antagonism of the ability of diethylamine-NO to depolarise the preparation. A similar effect was observed with adenosine (10 microM) only in the presence of the adenosine A1 receptor antagonist PACPX (1,3-dipropyl-8-(2-amino-4-chlorophenyl)-xanthine, 100 nM). Conversely, the adenosine A1 receptor agonists ENBA (N6-[2-endo-norbomyl]adenosine, 1 microM) and cyclohexyladenosine (100 nM) potentiated the effect of diethylamine-NO on isolated rat nodose ganglia. Inclusion of either adenosine A3 agonists or ATP had no effect on the diethylamine-NO concentration-response curve. These data suggest an ability of NO to interact, in opposing manner, with adenosine A2A and A1 receptors in rat vagal afferent neurons. On the other hand, neither A3 receptors nor ATP appear capable of interacting with NO.

Cholecystokinin, dopamine D2 and N-methyl-D-aspartate binding sites in the nucleus of the solitary tract of the rat: possible relationship to ingestive behavior

Receptor autoradiography was used to investigate the distribution of brainstem binding sites for cholecystokinin, dopamine and N-methyl-D-aspartate with particular reference to the nucleus of the solitary tract of the rat, an area involved in the control of ingestive behavior. Binding sites for the A and B subtypes of the cholecystokinin receptor, labeled with [(125)I]cholecystokinin octapeptide sulfate in the presence or absence of antagonists for the devazepide (A) or L-365,260 (B) receptor, were present throughout the caudal rostral extent of the nucleus of the solitary tract, the A type predominating in the commissural, medial and gelatinous part and the B type in the lateral part. In the most rostral part of the medial nucleus of the solitary tract, both A and B receptors were present. Dopamine D2 receptors, labeled with [(125)I]NCQ-298, were found in all parts of the nucleus of the solitary tract. No binding to the dopamine D1 receptor, labeled with [(125)I]SCH-23982, was found in the brainstem. N-Methyl-D-aspartate receptors, labeled with [(3)H]dizocilpine maleate, were also present in the entire caudorostral extent of the nucleus of the solitary tract. Binding to cholecystokinin A receptors was co-distributed with [(125)I]NCQ-298 and [(3)H]dizocilpine maleate binding in the caudal and rostral parts of the nucleus of the solitary tract, and binding to cholecystokinin B receptors overlapped with [(125)I]NCQ-298 and [(3)H]dizocilpine maleate binding in the rostral nucleus of the solitary tract. These results are consistent with the hypothesis that cholecystokinin, dopamine and glutamate interact in the nucleus of the solitary tract in the control of ingestive behavior.