Two dopamine receptors: biochemistry, physiology and pharmacology

Effects of lithium carbonate on apomorphine-induced sniffing behaviour in rats

Effects of lithium carbonate (Li2CO3) on sniffing induced by apomorphine have been tested in rats. Intraperitoneal administration of different doses of apomorphine (0.25, 0.5 and 1 mg/kg) induced a dose-dependent sniffing response. Chronic Li2CO3 exposure (0.1% in drinking water for 30-35 days) but not acute administration of the drug (320 mg/kg, intraperitoneally) decreased the response of apomorphine. The response to chronic Li2CO3 exposure was observed when apomorphine was injected 60 min., 24 hr or 72 hr after Li2CO3withdrawal, with maximum effect observed when the drug was administered 72 hr after withdrawal of Li2CO3. Blockade of sniffing induced by apomorphine by the D1 dopamine receptor antagonist, SCH23390 (0.005 mg/kg, intraperitoneally) or the D2 dopamine receptor antagonist, sulpiride (25 mg/kg, intraperitoneally) was not increased in acute Li2CO3-treated animals. In animals which were treated chronically with Li2CO3, the blockade of apomorphine response by sulpiride but not by SCH23390 was potentiated. It is concluded that chronic treatment of animals with Li2CO3 is able to alter D2 dopamine receptors response.

The effects of dopamine receptor agents on naloxone-induced jumping behaviour in morphine-dependent mice

In the present study, the effects of dopamine receptor agonists and antagonists on naloxone-induced jumping in morphine-dependent mice were examined. Mice were rendered dependent as described in the methods section. Naloxone was injected to elicit jumping (as withdrawal sign). The first group received dopamine receptor drugs before naloxone injection to test the effects of the drugs on the expression of jumping. Administration of the dopamine D1/D2 receptor agonist, apomorphine (0.25, 0.5 and 1 mg/kg), decreased jumping, but not diarrhoea, induced by naloxone. The effect of apomorphine on jumping was reduced by the dopamine D2 receptor antagonist, sulpiride. The dopamine D2 receptor agonist, quinpirole (0.1, 0.3 and 0.5 mg/kg), increased jumping, while it decreased diarrhoea in mice. Different doses of sulpiride did not alter jumping, but one dose of the drug (12.5 mg/kg) decreased jumping. Neither the dopamine D1 receptor agonist, SKF38393 (1-phenyl-7,8-dihydroxy-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride; 8 and 16 mg/kg), nor the dopamine D1 receptor antagonist, SCH23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-benzazepine-7-ol maleate; 5, 10 and 25 mg/kg), altered jumping, but they decreased diarrhoea. The second group of animals received the drugs during the development of dependence. Administration of quinpirole (0.1, 0.3 and 0.5 mg/kg), but not bromocriptine (4, 8 and 16 mg/kg), apomorphine (0.25, 0.5, 1 and 2 mg/kg) or sulpiride (12.5, 25 and 50 mg/kg) decreased naloxone-induced jumping and diarrhoea. A dose of SKF38393 (8 mg/kg) decreased jumping, while both SKF38393 (4 and 16 mg/kg) and SCH23390 (5 and 10 microg/kg) increased diarrhoea. It is concluded that activation of both dopamine D1 and D2 receptors may suppress naloxone-induced jumping in morphine-dependent mice, and that stimulation of dopamine D1 receptors during development of morphine dependence may increase diarrhoea through peripheral mechanism.

Dopamine receptor mechanism(s) and morphine tolerance in mice

Based on our previous demonstration of the involvement of dopamine-2 (D2) dopamine receptors in morphine antinociception, we examined the role of D2 dopamine receptors in the expression and development of tolerance to morphine antinociception in mice. Tolerance to morphine antinociception was assessed by the tail-flick response after the administration of morphine (50 mg/kg) once daily for 3 days. The D2 dopamine receptor agonist, quinpirole (0.01, 0.02 and 0.03 mg/kg), but not the D2 dopamine receptor antagonist, sulpiride (12.5, 25 and 50 mg/kg), increased morphine antinociception in morphine non pre-exposed mice. The response of quinpirole was decreased by the lower doses of sulpiride. Both quinpirole and sulpiride decreased the expression and development of tolerance to antinociception induced by morphine (1.5, 3 and 6 mg/kg). The effect of quinpirole on the expression and development of tolerance, was reduced by a lower and per se non-effective dose of sulpiride. It was concluded that D2 dopaminergic receptors may play a part in the expression and development of tolerance to the antinociceptive effect of morphine.

Further characterization of preproenkephalin mRNA-containing cells in the rodent globus pallidus

The globus pallidus (external pallidum of primates) is an essential nucleus within basal ganglia circuitry, in part because it receives at least one-half of striatal efferent projections. Neurons of the globus pallidus can be divided into subpopulations based on anatomical, physiological, and chemical features. Globus pallidus neurons project to several structures (the striatum, subthalamic nucleus, entopeduncular nucleus, and substantia nigra pars reticulata), have one of two alternative waveforms (positive/negative versus negative/positive), contain either the calcium binding protein parvalbumin or the neuropeptide precursor preproenkephalin mRNA and show differential immediate early gene responses to dopamine receptor agonists and antagonists. The objective of the present study was to characterize in greater detail the preproenkephalin mRNA-containing pallidal neurons using Sprague-Dawley rats. In situ hybridization for preproenkephalin mRNA was combined with immunocytochemical detection of: (i) the neuron-specific nuclear protein, NeuN, (ii) FluoroGold-labeled pallidostriatal and pallidosubthalamic cells, or (iii) Fos induced by either systemic combined D1-class/D2-class dopamine receptor agonists or a D2-class receptor antagonist. These experiments demonstrated that a substantial population (42%) of globus pallidus neurons contains preproenkephalin mRNA, and that globus pallidus neurons retrogradely labeled after FluoroGold injections into the striatum are more frequently preproenkephalinergic, compared to the population of pallidosubthalamic neurons. Furthermore, systemic administration of a D2 receptor antagonist, eticlopride, induced Fos immunoreactivity predominantly in globus pallidus neurons expressing preproenkephalin mRNA, while combined administration of D1 and D2 receptor agonists induced Fos predominantly in pallidal neurons lacking preproenkephalin mRNA.These results support the conclusion that preproenkephalin mRNA identifies one of the two major subpopulations of pallidal neurons. This preproenkephalin mRNA-expressing pallidal subpopulation preferentially targets the striatum and is more readily activated in its immediate early gene expression by D2 receptor antagonists than by dopamine receptor agonists. This projection provides a pallidal substrate for the dopaminergic regulation of striatal information processing.

Differential Actions of Endogenous and Iontophoretic Dopamine in Rat Striatum

We have voltammetrically monitored dopamine release in the rat striatum evoked either by stimulation of the median forebrain bundle or by iontophoresis. Neuronal spike activity was recorded with the same microelectrode alternately with voltammetric scans. Low levels of stimulation-evoked dopamine (< 1 microM) excited all striatal units tested. However, this effect could be converted into an inhibition at higher levels of release. In contrast to the results with neuronally released dopamine, excitation was never seen when dopamine was applied by iontophoresis. Inhibition was, however, observed at high concentrations (> 1 - 5 microM) of exogenous dopamine. These data show that endogenous dopamine at low levels may be predominantly an excitatory transmitter in the striatum, but exogenous (iontophoretic) dopamine does not have the same effect on striatal unit activity as the endogenous material.

Localization of peripheral dopamine D1 and D2 receptors in rat corpus cavernosum

OBJECTIVE

To detect and locate anatomically peripheral dopamine D1 and D2 receptors in rat cavernosa, as dopamine is important in sexual drive and penile erection through receptors located in the central nervous system.

MATERIALS AND METHODS

Corpora cavernosa were obtained from Sprague-Dawley rats; total RNA and membrane proteins were extracted and cryostat sections prepared. The rat brain hypothalamus was used as a control for dopamine D1 and D2 receptors. The presence and expression of peripheral dopamine D1 and D2 receptor mRNAs in rat corpus cavernosa was assessed using reverse transcription and polymerase chain reaction (RT-PCR), and Northern blot hybridization using (32)P-UTP-labelled RNA probes. Concurrently, corresponding proteins from D1 and D2 receptors were assayed and detected by a Western blotting technique. The anatomical location of dopamine D1 and D2 receptor mRNAs in rat penile tissues was identified by in situ hybridization using (35)S-UTP-labelled RNA probes in cryostat sections. Immunohistochemical staining was used to locate peripheral dopamine D1 and D2 receptor proteins in rat corpora cavernosa.

RESULTS

Dopamine D1 and D2 receptor gene expression was detected in rat corpora cavernosa. In situ hybridization signals for dopamine D1 and D2 receptor mRNAs were localized to corpus cavernosal tissues and dorsal vessels in the rat penis. Western blot analyses showed peripheral dopamine D1 and D2 receptor proteins in rat corpora cavernosa. Immunohistochemically, peripheral dopamine D1 and D2 receptor proteins were detected in dorsal nerves, dorsal vessels and corpus cavernosal smooth muscle of the rat penile tissues.

CONCLUSIONS

Peripheral dopamine D1 and D2 receptors are present in the corpora cavernosa of rats. The functional significance of these receptors and signal transduction pathways in modulating the vascular tone of the penis warrants further investigation.

Neurotensin-induced modulation of dopamine D2 receptors and their function in rat striatum: counteraction by a NTR1-like receptor antagonist

The present study investigated the neurotensin (NT) receptor subtype (NTR) involved in the antagonistic neurotensin modulation of striatal dopamine D2 receptors observed in vitro and in vivo. The NT induced increase of the IC50 values of dopamine (DA) competition for [125I]iodosulpiride binding sites was counteracted by the NTR1-like antagonist SR48692 in rat striatal slices. Intrastriatal perfusion of pergolide induced in the awake rat an inhibition of striatal DA release that was antagonized by NT. This action of NT was counteracted by co-perfusion with the NTR1 like antagonist SR48692. These data indicate that there exists in the striatum at the prejunctional level an intramembrane antagonistic NT receptor/DA D2 receptor-receptor interaction where NTR1 like receptor activation reduces the DA D2 autoreceptor function.

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.

The role of the globus pallidus D2 subfamily of dopamine receptors in pallidal immediate early gene expression

The globus pallidus plays an important role in basal ganglia circuitry, representing the first relay nucleus of the 'indirect pathway' of striatal efferents. In contrast to the well-characterized actions of dopamine on striatal neurons, the functional role of the dopamine innervation of globus pallidus is less well understood. Previous research showed that systemic administration of either a dopamine D2 receptor antagonist or combined dopamine D1 and D2 receptor agonists induces Fos, the protein product of the immediate early gene c-fos, in neurons of globus pallidus [Ruskin and Marshall (1997) Neuroscience 81, 79-92]. To determine whether the ability of the D2 receptor antagonist, sulpiride, to induce Fos in rat pallidal neurons is mediated by D2-like receptors in striatum or globus pallidus, intrastriatal or intrapallidal sulpiride infusions were conducted. The diffusion of intrastriatal sulpiride was estimated by measuring this antagonist's competition for N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)-induced D2 receptor inactivation. The phenotype of the striatal neurons expressing Fos after intrastriatal infusion was assessed by combining Fos immunocytochemistry with D2 receptor mRNA in situ hybridization. Intrastriatal infusions of (-)-sulpiride (10-200 ng) dose-dependently increased the number of striatal cells expressing Fos; and the Fos-immunoreactive striatal cells were D2 receptor mRNA-expressing, the same population in which systemic D2 receptor antagonists induce Fos. Intrastriatal infusions of high (5 microg), but not low (10-200 ng), (-)-sulpiride doses also induced Fos in globus pallidus cells but the sulpiride appeared to spread to the globus pallidus. Direct intrapallidal infusions of (-)-sulpiride (50-100 ng) dose-dependently induced Fos in globus pallidus with minimal influence on striatum or other basal ganglia structures. Using sensitive in situ hybridization conditions, prominent labeling of D2 receptor mRNA was evident in globus pallidus. D2 receptor mRNA was densest in a lateral 200 microm wide band that follows the curvature of the pallidal/striatal boundary. Cellular analysis revealed silver clusters associated with D2 receptor mRNA labeling over globus pallidus neurons that were immunoreactive for neuron-specific nuclear protein. These results strongly suggest that the dopaminergic innervation of globus pallidus, acting through D2-like receptors internal to this structure, can control gene expression in pallidal neurons.

Localization of peripheral dopamine D1 and D2 receptors in rat and human seminal vesicles

Dopamine, an established neurotransmitter in the central nervous system, is recognized for its role in penile erection and ejaculation in rats. However, its complete mechanism of action in the genitourinary tract is unknown. The objective of this study was to investigate the existence and expression of peripheral dopamine D1 and D2 receptor messenger RNAs (mRNAs) and corresponding proteins in rat and human seminal vesicles. The seminal vesicle tissues of male Sprague-Dawley rats and human radical prostatectomy specimens were used to extract total RNA and proteins, and to prepare slide sections. Rat hypothalamus tissue served as a control for dopamine D1 and D2 receptors. Testing for the presence and expression of peripheral dopamine D1 and D2 receptor mRNAs in rat and human seminal vesicle tissues was performed by reverse transcription-polymerase chain reaction. Western blotting was used to detect corresponding proteins of D1 and D2 receptors. Immunohistochemical staining using rabbit antipeptide polyclonal antibodies was employed to identify and anatomically localize dopamine D1 and D2 receptor proteins in rat and human seminal vesicles. Dopamine D1 and D2 receptor transcripts were detected in both human and rat seminal vesicle tissues. Western blot analysis demonstrated that peripheral dopamine D1 and D2 receptor proteins exist in both human and rat seminal vesicle tissues. Immunohistochemical analysis demonstrated the localization of peripheral dopamine D1 and D2 receptors to the smooth muscle layer of human and rat seminal vesicles. The results of this study demonstrate that peripheral dopamine D1 and D2 receptors are present in the seminal vesicle tissue in both rats and humans. Although these results suggest that seminal emission may be mediated in part by the stimulation of peripheral dopamine receptors located in the seminal vesicles, the functional significance of dopamine in male reproductive tract has yet to be fully defined.

Effects of morphine withdrawal on catecholaminergic neurons on heart right ventricle; implication of dopamine receptors

The purpose of our study was to examine the effects of D1-and D2-dopamine receptors blockade on the changes in the ventricular content of catecholamines in rats withdrawn from morphine. Rats were given morphine by subcutaneous (sc) implantation of morphine pellets for 5 days. On the eighth day, morphine withdrawal was induced by sc administration of naloxone (1 mg/kg), and rats were killed 30 min later. Pretreatment with SCH 23390 (dopamine D1, D5 receptor antagonist) 15 min prior to naloxone administration suppressed some the behavioural signs of morphine withdrawal, whereas eticlopride (dopamine D2, D3, D4 receptor antagonist) did not. In addition, biochemical analysis indicate that SCH 23390 completely abolished the withdrawal-induced increase in noradrenaline and dopamine turnover in the right ventricle. By contrast, eticlopride did not block the hyperactivity of catecholaminergic neurons in the heart during morphine withdrawal. These data suggest that the hyperactivity of catecholaminergic neurons in the heart during morphine withdrawal is dependent upon D1 dopamine receptor activation. In addition, our results exclude the involvement of D2 dopamine receptors.Key words: morphine withdrawal, right ventricle, catecholaminergic activity.

Neuromodulation of Na+ channels: an unexpected form of cellular plasticity

Voltage-gated Na+ channels set the threshold for action potential generation and are therefore good candidates to mediate forms of plasticity that affect the entire neuronal output. Although early studies led to the idea that Na+ channels were not subject to modulation, we now know that Na+ channel function is affected by phosphorylation. Furthermore, Na+ channel modulation is implicated in the control of input-output relationships in several types of neuron and seems to be involved in phenomena as varied as cocaine withdrawal, hyperalgesia and light adaptation. Here we review the available evidence for the regulation of Na+ channels by phosphorylation, its molecular mechanism, and the possible ways in which it affects neuronal function.

Intrasubthalamic injection of 6-hydroxydopamine induces changes in the firing rate and pattern of subthalamic nucleus neurons in the rat

The subthalamic nucleus (STN) receives dopaminergic projections from the substantia nigra pars compacta (SNc). To investigate the role of direct and indirect dopaminergic influences on STN neurons, the spontaneous activity was studied in four groups of animals: normal rats, rats with intrasubthalamic or intranigral injection of 6-hydroxydopamine (6-OHDA), and sham STN injection rats by using extracellular recordings 4 weeks postsurgery. After intrasubthalamic injection of 6-OHDA, the mean firing rate significantly decreased (7.29 +/- 0.39 spikes/sec, P < 0.01 vs. 11.13 +/- 0.59 spikes/sec in normal or 11.26 +/- 0.57 spikes/sec in sham group), and the percentage of STN neurons discharging regularly decreased significantly (81%, P < 0.05 vs. 90% in normal group or P < 0.01 vs. 92% in sham group) and that of bursty cells increased (19%, P < 0.05 vs. 10%; in normal group or P < 0.01 vs. 8% in sham group). In the group of rats with SNc lesion, the firing rate of subthalamic neurons did not show a significant difference (11.61 +/- 0.81 spikes/sec) compared with normal group. However, the firing pattern was dramatically changed: 74% of cells exhibited bursty pattern and only 26% of cells discharged regularly or slightly irregularly. Immunohistochemical results showed that intrasubthalamic injection of 6-OHDA induced a marked degeneration of dopaminergic cells in the lateral part of the ipsilateral SNc, whereas 6-OHDA injection into the SNc induced a total in situ lesion of dopamine cells. These results suggest that the SNc exerts an excitatory influence on STN neurons and that the loss of this dopaminergic projection could, at least partially, account for the changes in the firing pattern of STN neurons in the 6-OHDA rat model of parkinsonism.

Lithium attenuates the activation-euphoria but not the psychosis induced by d-amphetamine in schizophrenia

d-Amphetamine (20 mg) was administered intravenously in a double-blind design to 17 schizophrenic patients with and without concurrent 3-week lithium carbonate pretreatment to evaluate the lithium attenuation of d-amphetamine effects which we had observed in affective disorder patients. Lithium significantly attenuated the acute d-amphetamine-induced changes in an activation-euphoria cluster and in the mannerisms and posturing item of the Brief Psychiatric Rating Scale. Because the psychosis-increasing effects of d-amphetamine were not significantly decreased, it is conceivable that the d-amphetamine-induced changes in activation and euphoria and in psychosis are regulated by different dopamine mechanisms. The small clinical effects seen at 3 weeks of lithium treatment in schizophrenic patients could be mediated by dopamine.

A dopamine-secreting pheochromocytoma

We describe a patient with pheochromocytoma, which secretes dopamine. He was admitted to hospital because of chronic diarrhea. After surgical resection of the tumor, dramatic cessation of the diarrhea and blood pressure elevation were observed. Decreased expression of dopamine beta-hydroxylase in the tumor was considered a possible mechanism of producing a pathophysiological concentration of dopamine. This case shows that excessive excretion of dopamine, a vasodilative hormone, may affect blood pressure.

Modulation of a 40-kDa catecholamine regulated protein by dopamine receptor antagonists

Previous reports have shown that catecholamine regulated proteins (CRP) are central nervous system specific and covalently bind to catecholamines. In the present study, we report the subcellular localization and differential modulation of a 40-kDa catecholamine regulated protein (CRP40) by dopamine D1 and D2 receptor antagonists. CRP40 was found to be localized with nuclear and synaptosomal/mitochondrial and fractions. Chronic treatment with dopamine D2 receptor antagonist haloperidol in rats significantly increased the levels of CRP40 in the striatum, whereas, chronic R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH 23390) dopamine D1 receptor antagonist administration significantly decreased striatal CRP40 levels. Moreover, acute haloperidol treatment did not alter the levels of CRP40 in any of the brain regions. Despite a sequence homology with the heat shock protein 70 (HSP70), levels of HSP70 remained unchanged after either drug treatment, suggesting a distinct function of CRP40 than HSP70. These results further suggest that CRP40 play an important role in dopaminergic neuronal function and the dopamine D1 receptor-mediated signaling pathway may be involved in the regulation of CRP40.

Blunted pressor responsiveness to intravenous quinpirole in conscious, chronic spinal cord-transected rats: peripheral vs. spinal mechanisms

Intravenous quinpirole (1 mg/kg) in conscious rats with chronic spinal cord transection (at T5-T7) induced an initial pressor effect, which was significantly reduced in both magnitude and duration compared with that in sham-operated rats, which was then followed by a long-lasting depressor effect. To distinguish the spinal and/or peripheral origin of this phenomenon, conscious, spinal cord-transected rats were also pretreated with either intravenous (0. 5 mg/kg), intrathecal (40 microg/kg) or combined intravenous and intrathecal domperidone, a dopamine D(2) receptor antagonist that does not cross the blood-brain barrier. Intravenous pretreatment with domperidone enhanced, but did not completely restore, the pressor effect of quinpirole, and had no effect upon the depressor component. However, both the depressor component and the reduction of the pressor effect induced by spinal section were fully abolished by intrathecal or combined intrathecal and intravenous domperidone. Quinpirole-induced changes in mean aortic pressure were also fully abolished by intravenous pretreatment with metoclopramide (5 mg/kg). Neither the pressor nor the bradycardiac response to intravenous phenylephrine differed between sham-operated and spinal rats. These results suggest that the blunted pressor response to quinpirole after spinal cord transection is related to an enhanced spinal dopamine D(2) receptor-mediated depressor effect rather than to hypersensitivity of peripheral dopamine D(2) receptors or vascular hyporesponsiveness to alpha(1)-adrenoceptor stimulation. Thus, in conscious intact rats, the prominent central pressor effect of quinpirole seems to oppose, not only a peripheral sympathoinhibitory depressor effect, as previously thought, but also a spinal depressor effect.

Dopaminergic modulation of neuronal excitability in the striatum and nucleus accumbens

The striatum and its ventral extension, the nucleus accumbens, are involved in behaviors as diverse as motor planning, drug seeking, and learning. Invariably, these striatally mediated behaviors depend on intact dopaminergic innervation. However, the mechanisms by which dopamine modulates neuronal function in the striatum and nucleus accumbens have been difficult to elucidate. Recent electrophysiological studies have revealed that dopamine alters both voltage-dependent conductances and synaptic transmission, resulting in state-dependent modulation of target cells. These studies make clear predictions about how dopamine, particularly via D1 receptor activation, should alter the responsiveness of striatal neurons to extrinsic excitatory synaptic activity.

Effect of nicotine on sniffing induced by dopaminergic receptor stimulation

Effects of nicotine on sniffing induced by amphetamine and apomorphine have been tested in rats. Intraperitoneal (i.p.) administration of nicotine (0.5 and 1 mg/kg), amphetamine (1-6 mg/kg) or apomorphine (0.1-1 mg/kg) induced sniffing. Nicotine (0. 25-1 mg/kg) potentiates sniffing induced by amphetamine (1 mg/kg). Nicotine (1 mg/kg) also potentiates the response induced by different doses (0.1-1 mg/kg) of apomorphine. Atropine induced sniffing and increased the response of both amphetamine and nicotine. Higher doses of hexamethonium decreased the sniffing response induced by amphetamine and the response induced by combination of amphetamine and nicotine. Sulpiride reduced the response induced by nicotine or amphetamine plus nicotine, while SCH23390 reduced normal sniffing behaviour of the animals and sniffing induced by either amphetamine or amphetamine plus nicotine. The data may indicate that nicotinic receptor mechanism(s) may be involved in the sniffing induced by dopaminergic receptor stimulation.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

Localization of dopamine D1A and D1B receptor mRNAs in the forebrain and midbrain of the domestic chick

The distribution and cellular localization of dopamine D1A and D1B receptor mRNAs in the forebrain and midbrain of the domestic chick were examined using in situ hybridization histochemistry with 35[S]-dATP labeled oligonucleotide probes, visualized with film and emulsion autoradiography. Labeling for D1A receptor mRNA was intense in the medial and lateral striatum, and moderately abundant in the pallial regions termed the archistriatum and the neostriatum, in the hypothalamic paraventricular nucleus region, and in the superficial gray layer of optic tectum of the midbrain. D1B receptor mRNA was abundant in the medial and lateral striatum, and in the pallial region termed the hyperstriatum ventrale, and moderately abundant in the intralaminar dorsal and posterior thalamus and in the superficial gray of the optic tectum. At the cellular level, about 75% of neurons in the medial striatum and 59% of neurons in the lateral striatum were labeled for D1A receptor mRNA, whereas about 39% of the neurons in the medial striatum and 21% in the lateral striatum were labeled for D1B receptor mRNA. Large striatal neurons were not labeled for D1A or D1B receptor mRNA. The data suggest that while both D1A and D1B receptors mediate dopaminergic responses in many neurons of the avian striatum, primarily D1A receptors mediate dopaminergic responses in the archistriatum and the neostriatum, while primarily D1B receptors mediate dopaminergic responses in the hyperstriatum ventrale and the thalamus.

Dopamine and 7-OH-DPAT may act on D(3) receptors to inhibit tuberoinfundibular dopaminergic neurons

Whether the tuberoinfundibular dopaminergic (TIDA) neurons resided in the dorsomedial arcuate nucleus (dmARN) can respond to dopamine and a dopamine D(3) receptor agonist, 7-hydroxydipropylaminotetralin (7-OH-DPAT), was the focus of this study. In studies using extracellular single-unit recording of dmARN neurons in brain slices obtained from ovariectomized rats, dopamine and 7-OH-DPAT inhibited 60.1% (n = 141) and 80.9% (n = 47) of recorded dmARN neurons, respectively. Other dopamine D(1) or D(2) receptor agonists were not as effective. Intracerebroventricular injection of 7-OH-DPAT (10(-9) mol/3 microl) in ovariectomized, estrogen-primed rats significantly lowered the TIDA neuronal activity as determined by 3, 4-dihydroxyphenylacetic acid (DOPAC) levels in the median eminence. Co-administration of a putative D(3) receptor antagonist, U-99194A, could prevent the effect of 7-OH-DPAT. Unilateral microinjection of 7-OH-DPAT or dopamine itself (10(-11)-10(-9) mol/0.2 microl) into the right dmARN exhibited the same inhibitory effect on TIDA neurons. In all, dopamine may act on D(3) receptors to exhibit an inhibitory effect on its own release from the TIDA neurons.

D(1) dopamine receptor activation reduces GABA(A) receptor currents in neostriatal neurons through a PKA/DARPP-32/PP1 signaling cascade

Dopamine is a critical determinant of neostriatal function, but its impact on intrastriatal GABAergic signaling is poorly understood. The role of D(1) dopamine receptors in the regulation of postsynaptic GABA(A) receptors was characterized using whole cell voltage-clamp recordings in acutely isolated, rat neostriatal medium spiny neurons. Exogenous application of GABA evoked a rapidly desensitizing current that was blocked by bicuculline. Application of the D(1) dopamine receptor agonist SKF 81297 reduced GABA-evoked currents in most medium spiny neurons. The D(1) dopamine receptor antagonist SCH 23390 blocked the effect of SKF 81297. Membrane-permeant cAMP analogues mimicked the effect of D(1) dopamine receptor stimulation, whereas an inhibitor of protein kinase A (PKA; Rp-8-chloroadenosine 3',5' cyclic monophosphothioate) attenuated the response to D(1) dopamine receptor stimulation or cAMP analogues. Inhibitors of protein phosphatase 1/2A potentiated the modulation by cAMP analogues. Single-cell RT-PCR profiling revealed consistent expression of mRNA for the beta1 subunit of the GABA(A) receptor-a known substrate of PKA-in medium spiny neurons. Immunoprecipitation assays of radiolabeled proteins revealed that D(1) dopamine receptor stimulation increased phosphorylation of GABA(A) receptor beta1/beta3 subunits. The D(1) dopamine receptor-induced phosphorylation of beta1/beta3 subunits was attenuated significantly in neostriata from DARPP-32 mutants. Voltage-clamp recordings corroborated these results, revealing that the efficacy of the D(1) dopamine receptor modulation of GABA(A) currents was reduced in DARPP-32-deficient medium spiny neurons. These results argue that D(1) dopamine receptor stimulation in neostriatal medium spiny neurons reduces postsynaptic GABA(A) receptor currents by activating a PKA/DARPP-32/protein phosphatase 1 signaling cascade targeting GABA(A) receptor beta1 subunits.

Prenatal cocaine exposure increases serotonergic inhibition of electrically evoked acetylcholine release from rat striatal slices at adulthood

This study tests the hypothesis that prenatal cocaine (pCOC) exposure (20 mg/kg, bidaily from embryonic days 15-21) modifies 5-HT(3) receptor regulation of electrically-evoked [(3)H]acetylcholine (ACh) overflow from adult male and female (proestrus, diestrus) rat striatal slices. Also, the influence of endogenous dopamine (DA) on serotonin (5-HT) regulation of ACh overflow was determined by assessing the effects alpha-methyl-para-tyrosine (AMPT) pretreatment or sulpiride. Phenylbiguanide (PBG, 5-HT(3) agonist) superfusion dose-dependently inhibited ACh overflow in all groups except the diestrus pCOC group in which there was an enhanced sensitivity to PBG. PBG (10, 30, and 60 microM) produced greater effects in the pCOC male than in the prenatal saline (pSAL) group. The pCOC male group also exhibited greater sensitivity to PBG (30 and 60 microM) than the pCOC proestrus group. PBG inhibition of ACh overflow was comparable in the pSAL male and female (proestrus) groups. PBG inhibition of ACh overflow was greater in the pCOC diestrus group than in the pCOC proestrus (10, 30, and 60 microM), the pSAL diestrus (10 and 30 microM), and the pCOC male (10 microM) conditions. In slices from untreated rats superfused with 30 microM PBG, AMPT pretreatment (68% DA loss) reduced inhibition of ACh overflow, and 1 microM sulpiride increased ACh overflow. ICS205-930 (5-HT(3) antagonist) reduced effectiveness of PBG indicating 5-HT(3) receptor specificity for PBG. In summary, pCOC exposure enhances modulatory effects of 5-HT (via 5-HT(3) receptors) on striatal ACh release in male and females rats and the inhibitory actions of 5-HT(3) receptors are mediated by DA.

Effects of adrenoceptor agents on apomorphine-induced licking behavior in rats

In the present study, intraperitoneal (IP) administration of the dopaminergic receptor agonist apomorphine (0.1, 0.25, and 0.5 mg/kg) induced a dose-dependent licking in rats. The intraperitoneal injection of the alpha1'''adrenoceptor agonist phenylephrine (1-8 mg/kg) but not the alpha2-adrenoceptor agonist clonidine (0.025-0.05 mg/kg) decreased licking induced by apomorphine. The alpha-adrenoceptor antagonists prazosin, phenoxybenzamine, and yohimbine also reduced the apomorphine response significantly. The response induced by phenylephrine was decreased by a dose of prazosin. The beta1-adrenenocepor agonist dobutamine and beta2-adrenenocepor agonist salbutamol did not alter the apomorphine response. However, beta2-adrenenocepor antagonists atenolol and propranolol reduced the apomorphine effect. It may be concluded that alpha1- and possibly beta1-adrenoceptor mechanisms may be involved in modulation of licking behavior.

Role of the superior colliculus in the motor effects of cannabinoids and dopamine

We studied the cellular distribution of CB1 cannabinoid receptors in the superior colliculus of the rat using an antibody raised against the N-terminal of the receptor. The effect of unilateral cannabinoid receptor stimulation in the intermediate layers of the superior colliculus on rotational behavior in rats was also explored. The antibody against CB1 receptors outlined the crossed descending system of the superior colliculus (predorsal bundle output system) as well as the collicular commisure. The potent cannabinoid agonist CP55,940 (5 microgram/0.25 microliter) induced strong contralateral turning when microinjected unilaterally into the lateral intermediate layers of the superior colliculus. The levels of turning obtained with the intracollicular administration of the cannabinoid were comparable to the highest levels obtained with dopamine agonists in the basal ganglia. The D(2) dopamine agonist quinpirole or the D(1) dopamine agonist SKF82958 reversed this contralateral rotation but failed to affect motor behavior on their own. A new motor pathway for cannabinoids is discussed.

Effects of intrasubthalamic injection of dopamine receptor agonists on subthalamic neurons in normal and 6-hydroxydopamine-lesioned rats: an electrophysiological and c-Fos study

Subthalamic neuronal activity is controlled by a dopaminergic innervation, which may act via D1 and D2 dopamine receptors. This study investigates the effect of apomorphine and the selective D1 and D2 agonists, SKF 82958 and quinpirole respectively, in normal and 6-hydroxydopamine-lesioned rats. The effect of microinjection of these drugs into the subthalamic nucleus was assessed by recording unit activity and the expression of the c-Fos-immunoreactive protein in the subthalamic nucleus. Dopaminergic agonists reduced the discharge rate and did not induce c-Fos expression in the normal rat. Apomorphine and quinpirole increased the discharge rate and induced a strong expression of c-Fos-like immunoreactive proteins, whereas SKF 82958 induced a decrease of the discharge rate and a slight expression of c-Fos in 6-hydroxydopamine-lesioned rats. The striking contrast in the changes obtained with apomorphine and quinpirole in normal and 6-hydroxydopamine-lesioned rats is discussed in relation to a hyperexpression of D2 dopaminergic receptors on the GABAergic terminals into the subthalamic nucleus. These results show that, in normal rats, dopamine agonists exert an inhibitory control on subthalamic neurons via D1 and D2 receptors. However, in 6-hydroxydopamine-lesioned rats, the hyperactivity of subthalamic neurons is also reduced by D1 receptor agonist but not by D2 dopamine agonists. This last result points out one aspect of the complex mechanisms underlying the physiopathology of Parkinson's disease.

Modification of naloxone-induced withdrawal signs by dextromethorphan in morphine-dependent mice

In the present study the effect of dextromethorphan on naloxone-induced withdrawal signs in morphine-dependent mice was examined. In addition, the modulatory role of dopaminergic mechanisms upon the effect of dextromethorphan was investigated. Mice were rendered dependent on morphine by subcutaneous (s.c.) injections of morphine sulfate three times a day for 3 days, and withdrawal signs were induced by intraperitoneal (i.p.) administration of naloxone 2 h after the 10th injection of morphine sulfate on day 4. Dextromethorphan (20-50 mg/kg, i.p.) caused a significant decrease in withdrawal jumping, paw-shakes, grooming, burrows, writhing and diarrhea in morphine-dependent mice. The mixed dopamine D1/D2 receptor agonist apomorphine (0.5 and 1 mg/kg, s.c.) reduced the response induced by dextromethorphan. The effect of apomorphine was blocked by the dopamine D1 receptor antagonist SCH 23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7- ol maleate) (0.5 and 1 mg/kg, i.p.) but not by the dopamine D2 receptor antagonist sulpiride (25 and 50 mg/kg, s.c.) nor the peripheral dopamine receptor antagonist domperidone (5 and 10 mg/kg, s.c.). These results suggest that the dopaminergic system(s) may in part mediate the suppressive action of the NMDA receptor antagonist dextromethorphan on naloxone-induced withdrawal signs in morphine-dependent mice.

Dopaminergic neurotransmission at the paraventricular nucleus of hypothalamus in central regulation of penile erection in the rat

PURPOSE

To investigate whether the paraventricular nucleus of hypothalamus (PVN) is involved in the central regulation of apomorphine-induced penile erection in the rat, and to decipher dopamine receptor subtypes in the PVN that are involved in apomorphine-induced penile erection.

MATERIALS AND METHODS

Male adult Sprague-Dawley rats (200 to 300 gm.) anesthetized with pentobarbital sodium were used. The intracavernous pressure (ICP), recorded along with systemic and mean arterial pressure (SAP, MAP) as well as heart rate (HR), was measured via a 26-gauge needle inserted into one corpus cavernosum. The PVN was activated by stereotaxically delivered apomorphine hydrochloride (0.1 nmol./100 nl.). Injection of saline into PVN served as a vehicle control. To investigate the participation of dopamine receptor subtypes in the PVN on apomorphine-induced penile erection, D1 or D2 receptor antagonist, SCH-23390 (100 pmol./100 nl.) or sulpiride (100 pmol./100 nl.) respectively, was administered into the PVN prior to subcutaneous application of apomorphine (80 microg./kg.). The effects on ICP of microinjection of D1, D2 or D3 receptor agonist, SKF-38393 (200 pmol./100 nl.), lisuride (200 pmol./100 nl.) or 7-hydroxy-DPAT (200 pmol./100 nl.) respectively, into the PVN were also evaluated.

RESULTS

The mean resting ICP was 5.2+/-0.4 mm. Hg. Upon administration of apomorphine into the PVN, there was a significant increase in ICP that peaked at 50.7+/-5.3 mm. Hg and persisted for 45.2+/-18.0 minutes after an onset latency of 677.7+/-311.6 seconds. Yawning and teeth gnashing were also observed in most of animals during the period of ICP increase. There was no significant change in SAP, MAP or HR. In addition, there was no elevation in ICP after administration of saline to the PVN or direct injection of apomorphine into the cavernous tissue. Microinjection of D1 or D2 receptor antagonist into the PVN blocked the increase in ICP after subcutaneous administration ofapomorphine. Direct application of D2, but not D1 or D3 receptor agonist into the PVN, on the other hand, increased the ICP.

CONCLUSIONS

Our results demonstrate that application of apomorphine to the paraventricular nucleus of hypothalamus elicited penile erection in the rat. Such an increase in ICP to apomorphine was due mainly to activation of the D2 receptor subtype in the PVN. These observations indicate that PVN may be involved in the central regulation of apomorphine-induced penile erection in the rat.

Opposing role of dopamine D1 and D2 receptors in modulation of rat nucleus accumbens noradrenaline release

The role of dopamine receptors in the modulation of nucleus accumbens noradrenaline release was investigated in superfused rat brain slices. At concentrations of </=1 microM, dopamine enhanced, whereas at higher concentrations dopamine inhibited electrically evoked [3H]noradrenaline release. The D1 receptor agonist SKF-38393 increased, whereas the D2 agonist quinpirole inhibited evoked [3H]noradrenaline release. These effects were attenuated by the D1 antagonist SCH-23390 and the D2 antagonist (-)-sulpiride, respectively, indicating that accumbens noradrenaline release is regulated by stimulatory D1 and inhibitory D2 receptors. Whereas (-)-sulpiride enhanced, SCH-23390 did not reduce evoked accumbens [3H]noradrenaline release, indicating a tonic activation of D2 receptors only. Given the similar apparent affinity of dopamine for D1 and D2 receptors in striatal slices, the lack of tonic D1 receptor activation suggests that D1, unlike D2, receptors are extrasynaptically localized. No dopaminergic modulation of noradrenaline release was observed in rat medial prefrontal cortex or amygdala slices. To examine the regulation of accumbens noradrenaline release under conditions of increased dopaminergic activity, measurements were made using slices of amphetamine-pretreated rats. In these slices, the electrically evoked release of [3H]dopamine and [3H]noradrenaline was enhanced. The increasing effect of (-)-sulpiride on noradrenaline release was augmented, and SCH-23390 almost completely reversed this enhancement of [3H]noradrenaline release. These data suggest that whereas although under a moderate dopaminergic tone, accumbens noradrenaline release is mainly regulated by inhibitory D2 receptors, under circumstances of increased dopaminergic activity, recruitment of extrasynaptic stimulatory D1 receptors contributes to enhancement of noradrenaline release.

Paradoxical locomotor behavior of dopamine D1 receptor transgenic mice

The behavioral effects of augmenting dopamine D1 receptor expression in the brain were investigated in mice incorporating additional copies of the mouse D1 receptor gene. Two transgenic lines showed increases in brain D1 receptor binding sites, which were greatest in extrastriatal regions. The full D1 agonist SKF 81297, when administered systemically to control animals, stimulated a dose-dependent increase in locomotor activity. In contrast, in D1 receptor overexpressing transgenic mice, this drug caused a marked suppression of locomotion due to a decrease in the frequency of movement initiation. Amphetamine and cocaine induced comparable locomotor activation in both transgenic animals and their control littermates. In the transgenic animals, D1 agonist-induced rearing and climbing behaviors were suppressed. However, on rotarod testing, the agonist-treated transgenic and control mice performed comparably, indicating that sensorimotor coordination was unaffected. These studies demonstrate that altering the levels of D1 receptor expression reverses the effects of D1 agonism on locomotor initiation and rearing.

Effects of dopaminergic agents on antinociception in formalin test

Morphine caused a dose-related antinociception in early phase and late phase of formalin test in mice. The D2 dopamine agonist quinpirole, but not the D1 dopamine agonist SKF 38393, increased the antinociceptive effect of morphine in both phases of the test. The antinociceptive effect of quinpirole also was decreased by sulpiride or domperidone pretreatment in the early phase of test. The D1 antagonist SCH23390, the D2 antagonist sulpiride, or the peripheral D2 dopamine antagonist domperidone, increased the morphine effect. Single administration of SKF38393, quinpirole, SCH23390, sulpiride, and domperidone also induce antinociception. The response of SCH23390, but not that of other dopamine agents, was antagonized with naloxone. The effects of the drugs alone and in combination with morphine have been discussed.

Nicotine-induced grooming: a possible dopaminergic and/or cholinergic mechanism

The ability of nicotine, to induce grooming in rats was studied. Grooming was induced by i.p. injection of different doses (0.0675-0.5 mg/kg) of nicotine to rats. The effect was dose-dependent. However, the response was decreased with increasing doses of the drug from 0.25-0.5 mg/kg. Administration of the dopamine (DA) D1/D2 receptor agonist apomorphine (0.025-5 mg/kg, i.p.) also caused grooming in a dose-dependent manner. High doses of apomorphine (0.1-0.5 mg/kg, i.p.) also induced a lower degree of response. Combination of a low dose of nicotine (0.0675 mg/kg) with different doses of apomorphine did not show any interaction. However, there was an interaction between a high dose of nicotine and apomorphine. Thus, combination of a higher dose of nicotine (0.125 mg/kg) with apomorphine, reduced apomorphine-induced grooming. The muscarinic receptor antagonist atropine (5 and 10 mg/kg), peripheral nicotinic receptor antagonist hexamethonium (5 and 10 mg/kg), central nicotinic receptor antagonist mecamylamine (1 and 3 mg/kg) and D1 DA receptor antagonist SCH23390 (0.05 and 0.1 mg/kg) all decreased the response to nicotine. Atropine, mecamylamine and SCH23390 by themselves reduced spontaneous grooming. It is concluded that nicotine elicits grooming indirectly through a possible D1 dopaminergic mechanism. However, muscarinic and nicotinic cholinergic mechanism(s) may be involved.

The type IV phosphodiesterase inhibitors, Ro 20-1724 and rolipram, block the initiation of cocaine self-administration

The hypothesis that the selective activation of cyclic AMP (cAMP) signal transduction pathways will suppress the initiation of cocaine self-administration was examined in this investigation. To test this hypothesis, the effects of the administration of the cAMP-specific (type IV) phosphodiesterase inhibitors, rolipram and Ro 20-1724, on cocaine self-administration were determined. The effects of Ro 20-1724 treatment on operant responding for food also were examined. Both cocaine and food were delivered following a fixed-ratio 5 schedule. A significant increase in the latency for the delivery of the first cocaine infusion and a reduction in the number of infusions obtained per session were produced by treatment with either rolipram or Ro 20-1724. Similar effects on responding for food were seen with Ro 20-1724 administration. Responding after drug-induced delays tended to be at control levels. These results suggest that cAMP-specific phosphodiesterase inhibitors may inhibit the initiation of operant responding for either cocaine or food. However, the extent to which these actions involve specific effects on central motivational systems as opposed to other mechanisms remains to be determined.

The alleged dopamine D1 receptor agonist SKF 83959 is a dopamine D1 receptor antagonist in primate cells and interacts with other receptors

So far, no clear correlation has been found between the effects of dopamine D1 receptor agonists on motor behavior in primate models of Parkinson's disease and their ability to stimulate adenylate cyclase in rats, the benzazepine SKF 83959 (3-methyl-6-chloro-7,8-hydroxy-1-[3-methylphenyl]-2,3,4,5-tetrahydro-]H- 3-benzazepine) being the most striking example. Since this discrepancy might be attributed to: (A) the different species used to study these effects or (B) the interaction of SKF 83959 with other catecholamine receptors, the aims of this study were: (1) to study the ability of SKF 83959 to stimulate adenylate cyclase in cultured human and monkey glial cells equipped with dopamine D1 receptors and (2) to evaluate the affinity for and the functional interaction of SKF 83959 with other catecholamine receptors. Binding studies revealed that SKF 83959 displayed the highest affinity for the dopamine D1 receptor (pKi=6.72) and the alpha2-adrenoceptor (pKi=6.41) and moderate affinity for the dopamine D2 receptor and the noradrenaline transporter. In monkey and human cells, SKF 83959 did not stimulate cyclic adenosine monophosphate (cAMP) formation to a significant extent, but antagonized very potently the dopamine-induced stimulation of cAMP formation in both cell types. The compound stimulated basal dopamine outflow and inhibited depolarization-induced acetylcholine release only at concentrations > 10 microM. Finally, SKF 83959 concentration dependently increased electrically evoked noradrenaline release, indicating that it had alpha2-adrenoceptor blocking activity and interfered with the noradrenaline transporter. In conclusion, SKF 83959 is a potent dopamine D1 receptor and alpha2-adrenoceptor antagonist. Thus, the anti-parkinsonian effects of SKF 83959 in primates are not mediated by striatal dopamine D1 receptors coupled to adenylate cyclase in a stimulatory way.

Behavioral effects of dopamine agonists and antagonists: influence of estrous cycle, ovariectomy, and estrogen replacement in rats

The influence of the hormonal condition on the reactivity of central dopamine (DA) receptors was studied in male and in intact and ovariectomized (OVX) female rats. They were injected with selective DA agonists, acting either on D1 (SKF 38393, 2.5 or 10 mg/kg) or D2 receptors (PPHT, 31.3 or 125 microg/kg), or with selective DA antagonists, acting either on D1 (SCH 23390, 6.25 or 25 microg/kg), or D2 receptors (sulpiride, 10 or 40 mg/kg). The acquisition of an avoidance conditioning response (CAR) and the performance of some spontaneous motor behaviors were tested. Both D1 and D2 agonists and antagonists impaired the acquisition of CARs in diestrous, OVX, and male rats. Nevertheless, the effects of these drugs during estrus and in estradiol-primed OVX rats were different according to the drug and the dose injected. Whereas SKF 38393 failed to induce significative changes, PPHT and low doses of SCH 23390 and sulpiride improved the acquisition of CARS in those groups. The effects on conditioning were not accompanied with equivalent changes in spontaneous motor activity. Estradiol level fluctuations that occur in female rats within the estrous cycle or in OVX rats primed with estradiol would be responsive of changes in the response to DA agents. Although the reactivity of central DA systems is differentially affected by the hormonal condition of the rat, the precise mechanism of this modulatory action remains unknown.

Effects of D1 and D2 dopamine receptor antagonists on cocaine-induced self-stimulation and locomotor activity in rats

To clarify the involvement of D1 and D2 dopamine systems in intracranial self-stimulation (ICSS) and locomotor activity in rats, we studied the acute effects of cocaine and the interaction between cocaine and dopamine antagonists with respect to these behaviors. Although cocaine (5.0, 10.0, or 20.0 mg/kg) dose-dependently increased locomotor activity, it augmented the rate of ICSS only at 5.0 mg/kg. The failure of high doses of cocaine to augment purpose-oriented behavior such as ICSS may result from its induction of a manic-like state. The D1 dopamine receptor antagonist SCH23390 (0.02, 0.1, or 0.5 mg/kg) or the D2 antagonist nemonapride (0.04, 0.2, or 1.0 mg/kg) significantly decreased cocaine augmentation of ICSS. The higher two doses of either antagonist also produced a significant decrease in cocaine-induced locomotor activity. We therefore suspect that cocaine's augmentative effect on those behaviors, especially ICSS, requires activation of both D1 and D2 dopamine receptors.

A phosphoinositide-linked dopamine D1 receptor mediates repetitive jaw movements in rats

BACKGROUND

We have demonstrated that rats injected with D1 agonists SKF 38393 or A68930 demonstrate repetitive jaw movements (RJM). These agonist-induced movements in rats are similar in their appearance to those induced in rats by long-term treatment with antipsychotic drugs. Over recent years D-1 receptors were discovered which showed linkage not only to c-AMP but also to PI hydrolysis. We examined the effect of EEDQ inactivation of D1 receptors on D-1 mediated PI hydrolysis and RJM.

METHODS

Twenty four hours following EEDQ or vehicle administration D-1 agonists or vehicle were administered. The number of RJM episodes was assessed in EEDQ and vehicle treated rats. D-1 receptor density and inositol phosphate formation were determined in the striata.

RESULTS

EEDQ administration resulted, 24 hours later, in 70-80% selective depletion of D-1 receptors in the striata but did not modify the rate of RJM induced by D-1 agonists. There was no significant difference in D-1 mediated PI hydrolysis in EEDQ treated rats when compared to vehicle treated group.

CONCLUSIONS

The present data support the earlier demonstration of D-1 agonist induced RJM, an effect mediated by a subpopulation of a D-1 receptor subtype and constitute the first behavioral evidence for the existence of a behavioral response mediated by D-1 like dopamine receptors linked to an alternate second messenger system-PI hydrolysis.

Effects of the Aconitum alkaloid songorine on synaptic transmission and paired-pulse facilitation of CA1 pyramidal cells in rat hippocampal slices

1. The present study investigated the electrophysiological effects of songorine (1 100 microM), an alkaloid occurring in plants of the Aconitum genus, in rat hippocampal slices. 2. Songorine (10-100 microM) evoked a concentration-dependent increase in the amplitude of the orthodromic population spike and in the slope of the field e.p.s.p. The enhancement was long-lasting and was not reversed by up to 90 min of washout. Songorine failed to affect size and shape of the presynaptic fiber spike which represents the compound action potential of the Schaffer collaterals. This indicates that enhancement of the synaptic response is no consequence of an increased afferent excitability. 3. The antidromically evoked population spike was not affected by songorine at concentrations up to 100 microM suggesting that the enhancement of the orthodromic population spike and of the field e.p.s.p. was not due to an increase in pyramidal cell excitability. 4 The input-output curve for the postsynaptic population spike was shifted to the left implying that a presynaptic fiber spike of the same size elicited a larger postsynaptic response, indicating a decrease in threshold for generation of the population spike. 5. The songorine-evoked increase in excitability was not affected by the NMDA receptor antagonist, D-AP5. However, the effect of songorine was completely abolished by the selective dopamine D2 receptor antagonist sulpiride (0.1 microM) as well as by haloperidol (10 microM) and was mimicked by application of the dopamine releaser, amantadine (100 mM). In contrast, the selective D1 receptor antagonist, SCH23390, did not block the action of songorine. 6. The results indicate that the plant alkaloid songorine enhances excitatory synaptic transmission which may be due to an agonistic action at D2 receptors.

Involvement of spinal dopamine receptors in mediation of the hypotensive and bradycardic effects of systemic quinpirole in anaesthetised rats

This study examined the involvement of spinal dopamine D2 receptors in the cardiovascular effects induced by intravenous administration of the selective dopamine D2 receptor agonist quinpirole, as has been previously reported for the hypotensive action of systemic bromocriptine. In normotensive pentobartitone-anaesthetised rats, intravenous injection of quinpirole (25 to 1000 microg/kg) decreased mean aortic pressure and heart rate in a dose-related manner. The intravenous (0.5 mg/kg) or intrathecal (40 microg/rat at T9-T10) pretreatment with domperidone, a dopamine D2 receptor antagonist that does not cross the blood-brain barrier, significantly reduced the maximal hypotensive and bradycardic responses to intravenous quinpirole (1000 microg/kg). In contrast, the latter effects were fully abolished either by intravenous metoclopramide (5 mg/kg) or combined pretreatment with intravenous and intrathecal domperidone. In addition, when injected intrathecally at the T9-T10 level of the spinal cord, quinpirole (7.7 to 61.4 microg/rat) also produced dose-dependent depressor and bradycardic effects which could be blocked by intrathecal, but not intravenous, domperidone pretreatment. This suggests that, in anaesthetised normotensive rats, the hypotensive and bradycardic responses to intravenous quinpirole are fully mediated by dopamine D2 receptors, some of which are located in the peripheral circulation and some of which are located within the spinal cord. The latter finding is novel, suggesting that partial spinal mediation may not be peculiar to bromocriptine, as was previously thought. Rather, partial spinal mediation may be common to most dopamine D2 receptor agonists.

Selective blockade of a slowly inactivating potassium current in striatal neurons by (+/-) 6-chloro-APB hydrobromide (SKF82958)

The ion channels of rat striatal neurons are known to be modulated by stimulation of D1 dopamine receptors. The susceptibility of depolarization-activated K+ currents to be modulated by the D1 agonist, 6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetra-hydro-1H-3-benzaze pine (APB) was investigated using whole-cell voltage-clamp recording techniques from acutely isolated neurons. APB (0.01-100 microM) produced a concentration-dependent reduction in the total K+ current. At intermediate concentrations (ca. 10 microM), APB selectively depressed the slowly inactivating A-current (I(As)). A similar effect was produced by application of the D1 agonist, 7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1-H-2-benzazepine (SKF38393, 10 microM). APB reduced I(As) rapidly, having onset and recovery time constants of 1.2 sec and 1.6 sec, respectively. Unexpectedly, the effect of APB could not be mimicked by application of Sp-adenosine 3',5'-cyclic monophosphothioate triethylamine (Sp-cAMPS, 100-200 microM), a membrane-permeable analog of cyclic AMP (cAMP), or by pretreatment with forskolin (25 microM), an activator of adenylyl cyclase. The reduction in I(As) also was not blocked by pretreatment with the D1 receptor antagonist, R(+)-SCH23390 hydrochloride (SCH23390, 10-20 microM). In addition, intracellular dialysis with guanosine-5'-O-(2-thiodiphosphate (GDP-beta-S, 200 microM) did not preclude the APB-induced inhibition of I(As), nor did dialysis with guanosine-5'-O-(3-thiotriphosphate (GTP-gamma-S, 400 microM) prevent reversal of the effect. The effect of APB was produced by a reduction in the maximal conductance of I(As) without changing the voltage-dependence of the current. Collectively, these results argue that APB does not inhibit I(As) through D1 receptors coupled to stimulation of adenylyl cyclase, but rather by allosterically regulating or blocking the channels giving rise to this current.

Action sequencing is impaired in D1A-deficient mutant mice

The role of dopamine in the production of behaviour is multifarious in that it can influence different aspects of movement (e.g. movement initiation, sensorimotor integration, and movement sequencing). A characteristic of the dopamine system which seems to be critical for the expression of this diverse influence is its varied receptor population. Previous studies have shown that specific receptor subtype activation leads to specific behavioural responses or alterations of selective aspects of movement. It is known that one of the important influences of dopamine includes sequential co-ordination of 'syntactic' patterns of grooming movements because moderate loss of the dopaminergic nigrostriatal projections specifically disrupts these patterns without affecting grooming actions in a general fashion (Berridge, K.C. Psychobiology, 15, 336, 1989). The specific receptors of the dopamine family which play a key part in this co-ordination of movement sequences is not known. In the present study, we examined the serial order of particular syntactic sequences or chains of grooming actions in mice lacking D1A receptors to explore the relationship between this receptor subtype and movement sequencing. Mutant mice had shorter grooming bouts and a disruption of the organization of sequential patterns compared with wild-type littermate controls. Sequential disruption was reflected in the failure of D1A mutants to follow the syntactic pattern of grooming to completion. This sequential disruption deficit appeared to be specific, as mutant mice initiated more syntactic chains than wild-type controls even though they were less likely to complete them. These results support the hypothesis that D1A receptor activation plays a part in the sequencing of natural action. This conclusion has important implications for the understanding of the functional heterogeneity of dopamine receptor subtypes and of the aetiology of symptoms observed in patients with basal ganglia disease.

Vascular dopamine-I receptors and atherosclerosis

Vascular smooth muscle cell (VSMC) migration and proliferation are believed to play key roles in atherosclerosis. To elucidate the role of vascular dopamine D1-like receptors in atherosclerosis, the effects of dopamine, specific D1-like agonists SKF 38,393, and YM 435 on platelet-derived growth factor (PDGF) BB-mediated VSMC migration, proliferation, and hypertrophy were studied. We observed that cells stimulated by 5 ng/ml PDGF BB showed increased migration, proliferation and hypertrophy. These effects were prevented by coincubation with dopamine, SKF 38,393, or YM 435 at 1-10 mumol/l, and this prevention was reversed by Sch 23,390 (1-10 mumol/l), a specific D1-like antagonist. These actions are mimicked by 1-10 mumol/l forskolin, a direct activator of adenylate cyclase and 8-bromocyclic AMP at 0.1-1 mmol/l. The actions are blocked by a specific protein kinase A (PKA) inhibitor N-[2-(p-bromocinnamylamino) ethyl]-5-isoquinoline-sulfonamide (H 89), but are not blocked by its negative control, N-[2-(N-formyl-p-chlorocinnamylamino) ethyl]-5-isoquinoline sulfonamide (H 85). PDGF-BB (5 ng/ml)-mediated activation of phospholipase D (PLD), protein kinase C (PKC) and mitogen-activated protein kinase (MAPK) activity were significantly suppressed by coincubation with dopamine. These results suggest that vascular D1-like receptor agonists inhibit migration, proliferation and hypertrophy of VSMC, possibly through PKA activation and suppression of activated PLD, PKC and MAPK activity.

Infusion of a D-1 receptor agonist into the nucleus accumbens enhances cocaine-induced behavioural sensitization

The present study was designed to evaluate the effect of dopamine (DA) D-1 receptor activation in the nucleus accumbens (NAC) on the locomotor sensitizing effects of repeated intra-accumbens (intra-NAC) infusions of cocaine. Repeated infusion of cocaine (10 microg/0.5 microl daily for 2 days) resulted in an enhanced locomotor response to a subsequent intra-NAC and systemic (i.p.) challenging dose of the psychostimulant. Pretreatment with the selective D-1 agonist SK&F82958 (1.0 microg) markedly enhanced the sensitizing effects of both intra-NAC and systemic cocaine. The effect of SK&F82958 was completely blocked by systemic administration of the D-1 antagonist SCH23390 (0.1 mg/kg, i.p.). These data give further support to the idea that activation of D-1 receptors plays an important role in the induction of locomotor sensitization and show that the NAC may, in fact, be an anatomical locus of initiation of behavioural sensitization.

The effect of acute lithium and rubidium pretreatment on apomorphine-induced pecking in pigeons

The effects of different doses of lithium (5-320 mg/kg intramuscularly) and rubidium (0.25 32 mg/kg intramuscularly) on apomorphine-induced pecking were investigated in pigeons. These two cations did not induce pecking by itself. Intramuscular administration of apomorphine (a mixed D1/D2 dopamine receptors agonist, 0.1-1.6 mg/kg) induced pecking in a dose-dependent manner. SCH 23390 (D1 dopamine receptor antagonist, 0.02-0.08 mg/kg) and sulpiride (D2 dopamine receptor antagonist, 25-100 mg/kg) decreased apomorphine-induced pecking dose-dependently. Combination of SCH 23390 (0.04 mg/kg) with sulpiride (50 mg/kg) caused a stronger inhibitory effect on apomorphine response. This indicates that both D1 and D2 dopamine receptors are involved in apomorphine-induced pecking. The response induced by apomorphine (0.2-0.8 mg/kg) was decreased in animals pretreated with lithium and rubidium. In these conditions, SCH 23390 and sulpiride produced a larger inhibitory effect on the apomorphine response, suggesting that acute lithium and rubidium pretreatment inhibit pecking by interfering with dopaminergic mechanisms.

The neuropharmacology of yawning

Yawning is a phylogenetically old, stereotyped event that occurs alone or associated with stretching and/or penile erection in humans and in animals from reptiles to birds and mammals under different conditions. Although its physiological function is still unknown, yawning is under the control of several neurotransmitters and neuropeptides at the central level as this short overview of the literature on the neurochemistry of yawning shows. Among these substances, the best known are dopamine, excitatory amino acids, acetylcholine, serotonin, nitric oxide, adrenocorticotropic hormone-related peptides and oxytocin, that facilitate yawning and opioid peptides that inhibit this behavioral response. Some of the above compounds interact in the paraventricular nucleus of the hypothalamus to control yawning. This hypothalamic nucleus contains the cell bodies of oxytocinergic neurons projecting to extra-hypothalamic brain areas that play a key role in the expression of this behavioral event. When activated by dopamine, excitatory amino acids and oxytocin itself, these neurons facilitate yawning by releasing oxytocin at sites distant form the paraventricular nucleus, i.e. the hippocampus, the pons and/or the medulla oblongata. Conversely, activation of these neurons by dopamine, oxytocin or excitatory amino acids, is antagonized by opioid peptides, that, in turn, prevent the yawning response. The activation and inhibition, respectively of these oxytocinergic neurons is related to a concomitant increase and decrease, respectively, of paraventricular nitric oxide synthase activity. However, other neuronal systems in addition to the central paraventricular oxytocinergic neurons are involved in the control of yawning, since they do not seem to be involved in the expression of yawning induced by the stimulation of acetylcholine or serotoninergic receptors, nor by adrenocorticotropic hormone (ACTH) and related peptides. Nitric oxide is also involved in the induction of yawning by the latter compounds and neuronal links, for instance between dopamine and acetylcholine and dopamine and serotonin, seem to be involved in the yawning response. Finally, other neurotransmitters, i.e. gamma-aminobutyric acid (GABA) and noradrenaline, and neuropeptides, i.e. neurotensin and luteinizing hormone-releasing hormone (LH-RH), influence this behavioral response. In conclusion, in spite of some recent progress, little is known of, and more has to be done to identify, the neurochemical mechanisms underlying yawning at the central level.

Amphetamine reinstates polydipsia induced by chronic exposure to quinpirole, a dopaminergic D2 agonist, in rats

The hypothesis that the combined activation of D1 and D2 dopaminergic receptors is instrumental in inducing amphetamine (AMPH)-mediated hyperdipsia was tested in rats. The D1 agonist SKF-38393 (SKF) and the D2 agonist quinpirole (QNP) were i.p. injected, alone or in combination, to male rats for 10 days. After 2 days of wash-out, a single dose of AMPH (3 mg/kg) was administered. Intake of water and food and diuresis were daily measured at 2, 5 and 24 h. In two further experiments the higher dose of QNP (0.56 mg/kg) was given with two different doses of the D1 antagonist SCH-23390 (SCH), or, respectively, of the peripheral D2 antagonist domperidone (DMP). In a fourth experiment, the possibility that QNP, given alone or in combination with SKF, produces an AMPH-like internal state was evaluated by using a drug-discrimination paradigm. Results show that chronic administration of QNP produced a significant increase of 24 h water intake that was reinstated by AMPH. This QNP effect was only partially prevented by DMP, suggesting a main central mechanism of action. By itself D1 receptor manipulation did not affect water intake, but influenced QNP polydipsia that, accordingly, was enhanced by the lower dose of SKF (0.3 mg/kg) and inhibited by the lower dose of SCH (0.01 mg/kg). In rats trained to discriminate AMPH from solvent, QNP partially generalized for the AMPH stimulus, an effect that was potentiated by SKF. In conclusion, a D1-modulated sensitization of D2 dopaminergic mechanisms is probably involved in AMPH-induced hyperdipsia.

Effect of cerebral ischemia on dopamine receptors and uptake sites in the gerbil hippocampus

Dopamine D1 and D2 receptors and uptake sites were studied in the gerbil hippocampus, parietal cortex and thalamus 1 h to 7 days after 10 min of cerebral ischemia using the occlusion of bilateral common carotid arteries. [3H]SCH23390 ([N-methyl-3H]R[+]-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-7-ol-be nzazepine) and [3H]mazindol were used as markers of dopamine D1 receptors and uptake sites, respectively. [3H]Nemonapride was used to label dopamine D2 receptors. No obvious alteration in [3H]SCH23390 and [3H]mazindol binding was found in the hippocampus up to 48 h after ischemia. These bindings showed a significant reduction in the hippocampus after 7 days of recirculation. In contrast, [3H]nemonapride binding was unaffected in the hippocampus during the recirculation periods. The parietal cortex and thalamus also exhibited no significant changes in [3H]SCH23390, [3H]nemonapride and [3H]mazindol binding after ischemia. MAP2 (microtubule-associated protein 2) immunoreactivity was unchanged in all regions up to 48 h after ischemia. Thereafter, a marked loss of MAP2-immunoreactive neurons was observed in the hippocampal CA1 and CA3 neurons 7 days after recirculation. These findings were consistent with histological observations with cresyl violet staining. Our results demonstrate that dopamine D1 receptors and dopamine uptake sites in the hippocampus are susceptible to cerebral ischemia, whereas dopamine D2 receptors in this region are particularly resistant. Furthermore, these findings suggest that dopamine transmission may not be major factor in producing ischemic hippocampal damage.

Antagonistic effects of isofloxythepin on dopamine D1 and D2 receptors and behaviors in rats

1. In vitro, isofloxythepin competed for the binding of [3H]SCH 23390 to striatal D1 receptors and for the binding of [3H]spiperone to striatal D2 receptors with IC50 values of 6.1 (+/- 1.2) x 10(-10)M and 8.4 (+/- 2.6) x 10(-10)M, respectively. Isofloxythepin further inhibited the D1 receptor-mediated, dopamine-stimulated adenylate cyclase in the striatal tissue. 2. Fifteen hours after rats were injected with a single dose of isofloxythepin (1 mg/kg, SC), the ex vivo binding curve of [3H]spiperone to striatal D2 receptors was markedly inhibited, whereas the binding curve of [3H]SCH 23390 to striatal D1 receptors was unaffected. 3. Fifteen hours after isofloxythepin pretreatment, D1 agonist SKF 38393 (15 mg/kg, IP)-stimulated grooming behavior was not affected, whereas the D2 agonist quinpirole (3 mg/kg, IP)-stimulated stereotyped behavior was completely abolished. 4. On the basis of the findings from in vivo studies, we conclude that, although isofloxythepin is found to have high affinity for both D1 and D2 receptors in vitro, it lacks D1 antagonistic potency in vivo.