Evidence for L-dopa systems responsible for cardiovascular control in the nucleus tractus solitarii of the rat

The Many Faces of G Protein-Coupled Receptor 143, an Atypical Intracellular Receptor

GPCRs transform extracellular stimuli into a physiological response by activating an intracellular signaling cascade initiated via binding to G proteins. Orphan G protein-coupled receptors (GPCRs) hold the potential to pave the way for development of new, innovative therapeutic strategies. In this review we will introduce G protein-coupled receptor 143 (GPR143), an enigmatic receptor in terms of classification within the GPCR superfamily and localization. GPR143 has not been assigned to any of the GPCR families due to the lack of common structural motifs. Hence we will describe the most important motifs of classes A and B and compare them to the protein sequence of GPR143. While a precise function for the receptor has yet to be determined, the protein is expressed abundantly in pigment producing cells. Many GPR143 mutations cause X-linked Ocular Albinism Type 1 (OA1, Nettleship-Falls OA), which results in hypopigmentation of the eyes and loss of visual acuity due to disrupted visual system development and function. In pigment cells of the skin, loss of functional GPR143 results in abnormally large melanosomes (organelles in which pigment is produced). Studies have shown that the receptor is localized internally, including at the melanosomal membrane, where it may function to regulate melanosome size and/or facilitate protein trafficking to the melanosome through the endolysosomal system. Numerous additional roles have been proposed for GPR143 in determining cancer predisposition, regulation of blood pressure, development of macular degeneration and signaling in the brain, which we will briefly describe as well as potential ligands that have been identified. Furthermore, GPR143 is a promiscuous receptor that has been shown to interact with multiple other melanosomal proteins and GPCRs, which strongly suggests that this orphan receptor is likely involved in many different physiological actions.

Effects of Blockage of Peripheral Choline, Serotonin, and Dopamine Receptors on Heart Rhythm Variability in Rats under Conditions of Stimulation of Neurotransmitter Systems

Stimulation of the serotoninergic system (5-hydroxytryptophan, 50 mg/kg; fluoxetine, 3 mg/kg) induced a significant increase in HR and a reduction in the amplitude of all waves of the heart rhythm variability. Stimulation of the dopaminergic system (L-DOPA and amantadine, 20 mg/kg each) resulted in a moderate increase in HR and amplitudes of low-frequency (LF) and very-low-frequency (VLF) waves of the heart rhythm variability. Successive blockade of nicotinic (hexamethonium, 7 mg/kg) and muscarinic cholinergic receptors (atropine, 1 mg/kg) leads to a significant decrease in the variability of cardiointervals (almost to complete levelling) both under control conditions and after stimulation of the neurotransmitter systems. Serotonin receptor blockade (promethazine, 2 mg/kg) did not affect HR, but reduced the amplitude of LF- and VLF-waves. Under conditions of serotoninergic system stimulation, the blockade of serotonin receptors was followed by a significant HR acceleration without changes in heart rhythm variability; blockade of dopamine receptors (sulpiride, 1 mg/kg) induced HR acceleration and increase in the amplitude of LF- and VLF-waves; blockade of dopamine receptors under conditions of dopamine system stimulation was followed by a significant increase in HR and a decrease in the amplitude of all waves of the heart rhythm variability. It can be hypothesized that serotonin- and dopaminergic systems affect the heart rhythm via cardiomyocyte receptors and via modulation of activity of the adrenergic and cholinergic systems. The effects of serotonin- and dopaminergic systems can be considered as synergic in the CNS, and antagonistic at the periphery.

Overexpression of the gene product of ocular albinism 1 (GPR143/OA1) but not its mutant forms inhibits neurite outgrowth in PC12 cells

Neurite outgrowth is a complex differentiation process regulated by external and/or internal mechanisms. Among external mechanisms, G-protein coupled receptors (GPCRs) have been implicated in this process, but the pathways involved are not fully understood. L-3,4-dihydroxyphenylalanine (l-DOPA) is considered to be inert by itself, and to relieve Parkinson's disease through its conversion to dopamine. We have proposed that l-DOPA acts as a neurotransmitter. GPR143, the gene product of ocular albinism 1 (OA1), was identified as a receptor for l-DOPA. OA1 is an X-linked disorder characterized by all typical visual anomalies associated with hypopigmentation and optic misrouting, resulting in severe reduction of visual acuity. However, the molecular basis for this phenotype remains unknown. To study the function of GPR143, we investigated the phenotypic effect of overexpression of GPR143 in pheochromocytoma (PC12) cells treated with nerve growth factor. Overexpression of mouse GPR143 inhibited neurite outgrowth, and the effect was mitigated by l-DOPA cyclohexylester, an antagonist for l-DOPA. Furthermore, knockdown of G-protein Gα13 attenuated mouse GPR143 induced inhibition of neurite outgrowth. Human wild-type (wt) GPR143 also inhibited neurite outgrowth, but its mutants did not mimic the effect of wt GPR143. Our results provide a mechanism for axon guidance phenotype in ocular albinism 1.

l-DOPA and Its Receptor GPR143: Implications for Pathogenesis and Therapy in Parkinson's Disease

l-3,4-Dihydroxyphenylalanine (l-DOPA) is the most effective therapeutic agent for Parkinson's disease (PD). l-DOPA is traditionally believed to be an inert amino acid that exerts actions and effectiveness in PD through its conversion to dopamine. In contrast to this generally accepted idea, l-DOPA is proposed to be a neurotransmitter. Recently, GPR143 (OA1), the gene product of ocular albinism 1 was identified as a receptor candidate for l-DOPA. GPR143 is widely expressed in the central and peripheral nervous system. GPR143 immunoreactivity was colocalized with phosphorylated α-synuclein in Lewy bodies in PD brains. GPR143 may contribute to the therapeutic effectiveness of l-DOPA and might be related to pathogenesis of PD.

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.

l-3,4-Dihydroxyphenylalanine induces ptosis through a GPR143-independent mechanism in mice

Through its conversion to dopamine by aromatic l-amino acid decarboxylase (AADC), l-3,4-dihydroxyphenylalanine (l-DOPA) replenishes depleted brain dopamine in Parkinson's disease patients. We recently identified GPR143 as a candidate receptor for l-DOPA. In this study, we investigated the behavioral actions of l-DOPA in wild type (wt) and Gpr143-deficient mice. l-DOPA dose-dependently (10-100 mg/kg, i.p.) induced ptosis under treatment with 3-hydroxybenzylhydrazine, a centrally acting AADC inhibitor. This effect was not mimicked by 3-O-methyldopa. l-DOPA-induced ptosis in Gpr143-deficient mice to a similar extent as in wt mice. These results suggest that l-DOPA induces ptosis in a GPR143-independent fashion in mice.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Expression of ocular albinism 1 (OA1), 3, 4- dihydroxy- L-phenylalanine (DOPA) receptor, in both neuronal and non-neuronal organs

Oa1 is the casual gene for ocular albinism-1 in humans. The gene product OA1, alternatively designated as GPR143, belongs to G-protein coupled receptors. It has been reported that OA1 is a specific receptor for 3, 4-dihydroxy- L-phenylalanine (DOPA) in retinal pigmental epithelium where DOPA facilitates the pigmentation via OA1 stimulation. We have recently shown that OA1 mediates DOPA-induced depressor response in rat nucleus tractus solitarii. However, the distribution and function of OA1 in other regions are largely unknown. We have generated oa1 knockout mice and examined OA1 expression in both neuronal and non-neuronal tissues by immunohistochemical analyses using anti-mouse OA1 monoclonal antibodies. In the telencephalon, OA1 was expressed in cerebral cortex and hippocampus. Predominant expression of OA1 was observed in the pyramidal neurons in these regions. OA1 was also expressed in habenular nucleus, hypothalamus, substantia nigra, and medulla oblongata. The expression of OA1 in the nucleus tractus solitarii of medulla oblongata may support the reduction of blood pressure by the microinjection of DOPA into this region. Outside of the nervous system, OA1 was expressed in heart, lung, liver, kidney and spleen. Abundant expression was observed in the renal tubules and the splenic capsules. These peripheral regions are innervated by numerous sympathetic nerve endings. In addition, substantia nigra contains a large population of dopaminergic neurons. Thus, the immunohistochemical analyses suggest that OA1 may modulate the monoaminergic functions in both peripheral and central nervous systems.

The protein Ocular albinism 1 is the orphan GPCR GPR143 and mediates depressor and bradycardic responses to DOPA in the nucleus tractus solitarii

BACKGROUND AND PURPOSE

L-DOPA is generally considered to alleviate the symptoms of Parkinson's disease by its conversion to dopamine. We have proposed that DOPA is itself a neurotransmitter in the CNS. However, specific receptors for DOPA have not been identified. Recently, the gene product of ocular albinism 1 (OA1) was found to exhibit DOPA-binding activity. Here, we have investigated whether OA1 is a functional receptor of DOPA in the nucleus tractus solitarii (NTS).

EXPERIMENTAL APPROACH

We examined immunohistochemical expression of OA1 in the NTS, and the effects of DOPA microinjected into the depressor sites of NTS on blood pressure and heart rate in anaesthetized rats, with or without prior knock-down of OA1 in the NTS, using shRNA against OA1.

KEY RESULTS

Using a specific OA1 antibody, OA1-positive cells and nerve fibres were found in the depressor sites of the NTS. OA1 expression in the NTS was markedly suppressed by microinjection into the NTS of adenovirus vectors carrying the relevant shRNA sequences against OA1. In animals treated with OA1 shRNA, depressor and bradycardic responses to DOPA, but not those to glutamate, microinjected into the NTS were blocked. Bilateral injections into the NTS of DOPA cyclohexyl ester, a competitive antagonist against OA1, suppressed phenylephrine-induced bradycardic responses without affecting blood pressure responses.

CONCLUSION AND IMPLICATIONS

OA1 acted as a functional receptor for DOPA in the NTS, mediating depressor and bradycardic responses. Our results add to the evidence for a central neurotransmitter role for DOPA, without conversion to dopamine.

DOPA cyclohexyl ester, a DOPA antagonist, blocks the depressor responses elicited by microinjections of nicotine into the nucleus tractus solitarii of rats

Nicotinic cholinergic receptors play a role in cardiovascular regulation in the lower brain stem. Herein, we present evidence that l-3,4-dihydroxyphenylalanine (DOPA), a putative neurotransmitter in the central nervous system, is involved in the depressor response to microinjection of nicotine into the nucleus tractus solitarii (NTS). Microinjection of nicotine into the medial area of the NTS led to decreases in arterial blood pressure and heart rate in anesthetized rats. Mecamylamine, a nicotinic receptor antagonist, microinjected into NTS, blocked the depressor and bradycardic responses to nicotine. Nicotine-induced depressor and bradycardic responses were blocked by DOPA cyclohexyl ester (DOPA CHE), an antagonist for DOPA. DOPA CHE did not modify the action of carbachol on excitatory postsynaptic potential in rat cortical slices. These results suggest that endogenous DOPA is involved in nicotine-induced depressor responses in the NTS of anesthetized rats.

Differential effects of methamphetamine and cocaine on behavior and extracellular levels of dopamine and 3,4-dihydroxyphenylalanine in the nucleus accumbens of conscious rats

The central dopamine system plays a prominent role in the effect of psychostimulants such as methamphetamine, cocaine and nicotine. l-3,4-Dihydroxyphenylalanine (DOPA), a precursor of dopamine, has been proposed as a neurotransmitter in the central nervous system. We have studied the effects of these psychostimulants on the release of DOPA and dopamine from the nucleus accumbens shell in conscious rats using in vivo microdialysis. Methamphetamine and cocaine increase the extracellular levels of dopamine. The effect of methamphetamine (1 mg/kg s.c.) on the release of dopamine was almost comparable to that of cocaine (10 mg/kg i.p.). However, methamphetamine increases, but cocaine decreases the extracellular levels of DOPA. In a behavioral study, methamphetamine (1 mg/kg s.c.) induced chewing, walking and sniffing behavior. Cocaine (10 mg/kg i.p.) produces weak effects on these behavioral parameters, when compared to the effects of methamphetamine (1 mg/kg s.c.). The behavioral changes produced by methamphetamine are suppressed by DOPA cyclohexyl ester (30 mg/kg i.p.), a competitive DOPA antagonist. Endogenous DOPA in the nucleus accumbens thus appears to be in involved in the behavioral responses to these psychomotor stimulants.

L-3,4-dihydroxyphenylalanine-induced c-Fos expression in the CNS under inhibition of central aromatic L-amino acid decarboxylase

L-3,4-dihydroxyphenylalanine (DOPA) is a neurotransmitter candidate. To map the DOPAergic system functionally, DOPA-induced c-Fos expression was detected under inhibition of central aromatic L-amino acid decarboxylase (AADC). In rats treated with a central AADC inhibitor, DOPA significantly increased the number of c-Fos-positive nuclei in the paraventricular nuclei (PVN) and the nucleus tractus solitarii (NTS), and showed a tendency to increase in the supraoptic nuclei (SON), but not in the striatum. On the other hand, DOPA with a peripheral AADC inhibitor elevated the level of c-Fos-positive nuclei in the four regions, suggesting that DOPA itself induces c-Fos expression in the SON, PVN and NTS. In rats treated with 6-hydroxydopamine (6-OHDA) to lesion the nigrostriatal dopamine (DA) pathway, DOPA significantly induced c-Fos expression in the four regions under the inhibition of peripheral AADC. However, under the inhibition of central AADC, DOPA did not significantly increase the number of c-Fos-positive nuclei in the four regions, suggesting that DOPA at least in part induces c-Fos expression through its conversion to DA. It was likely that the 6-OHDA lesion enhanced the response to DA, but attenuated that to DOPA itself. In conclusion, we proposed that the SON, PVN and NTS include target sites for DOPA itself.

Nitroxidergic neurons in rat nucleus tractus solitarii express vesicular glutamate transporter 3

Earlier we reported that glutamate transporter (VGLUT) 2 and neuronal nitric oxide synthase (nNOS) are colocalized in some fibers and are present in apposing fibers in the nucleus tractus solitarii (NTS). Those findings provided anatomical support for a hypothesized physiological link between glutamate and nitric oxide (NO.) in the NTS. Recently a third class of VGLUT, VGLUT3, was identified, but its distribution in NTS and its anatomical relationship with nNOS have not been shown. In this study we tested the hypothesis that neurons and fibers containing VGLUT3 lie in close proximity to those containing nNOS and that both proteins colocalize in some neurons and fibers in the NTS. We perfused rats and obtained brain stem sections and nodose ganglion sections for immunofluorescent staining analyzed by confocal microscopy. The NTS contained moderate VGLUT3-immunoreactivity (IR), with the intermediate, medial and interstitial subnuclei containing higher VGLUT3-IR than other subnuclei. Although all three forms of VGLUT were present in the NTS, VGLUT3-IR was not colocalized with either VGLUT1-IR or VGLUT2-IR in either processes or cells in the brain stem. Cells and processes containing both VGLUT3-IR and nNOS-IR were noted in all NTS subnuclei and in the nodose ganglion. Triple immunofluorescent staining revealed that cells double-labeled for nNOS-IR and VGLUT3-IR were all additionally labeled for neuronal nuclear antigen (NeuN), a neuronal marker. These findings support our hypothesis that neurons and fibers containing VGLUT3 lie in close proximity to those containing nNOS and that both proteins colocalize in some neurons and fibers in the NTS.

Soluble guanylate cyclase and neuronal nitric oxide synthase colocalize in rat nucleus tractus solitarii

Nitric oxide has been implicated in transmission of cardiovascular signals in the nucleus tractus solitarii (NTS). Pharmacological studies suggest that activation of neurons by nitric oxide in the NTS may involve soluble guanylate cyclase (sGC). However, anatomical data supporting this suggestion have not been available. In this study, we tested the hypothesis that neurons and fibers containing neuronal nitric oxide synthase (nNOS) lie in close proximity to those containing sGC and the two enzymes colocalize in some neurons and fibers in the NTS. We perfused six rats and obtained brain stem sections for double immunofluorescent staining utilizing antibodies selective for sGC and for nNOS combined with confocal microscopy. The distribution and staining intensity of nNOS-immunoreactivity (IR) was similar to our earlier reports. IR of sGC was present in cell bodies, proximal dendrites and fibers of many brain stem regions. Strong sGC-IR was noted in the hypoglossal, dorsal motor nucleus of vagus and gracilis nuclei. The NTS exhibited moderate sGC-IR. Superimposed images showed that many NTS neurons contained both nNOS-IR and sGC-IR. The percentage of sGC-IR positive cells that were also nNOS-IR positive differed among NTS subnuclei. Similarly, the percentage of nNOS-IR positive cells that were also sGC positive differed among NTS subnuclei. Fibers stained for both nNOS-IR and sGC-IR were also present in NTS subnuclei. In addition, we identified fibers that were stained for nNOS-IR or sGC-IR alone and often found such singly labeled fibers apposed to each other. These data support our hypothesis and provide anatomical support for the suggestion that nitroxidergic activation of the NTS involves sGC.

L-3,4-Dihydroxyphenylalanine as a neurotransmitter candidate in the central nervous system

Historically, 3,4-dihydroxyphenylalanine (DOPA) has been believed to be an inert amino acid that alleviates the symptoms of Parkinson's disease by its conversion to dopamine via the enzyme aromatic L-amino acid decarboxylase. In contrast to this generally accepted idea, we propose that DOPA itself is a neurotransmitter and/or neuromodulator, in addition to being a precursor of dopamine. Several criteria, such as synthesis, metabolism, active transport, existence, physiological release, competitive antagonism, and physiological or pharmacological responses, must be satisfied before a compound is accepted as a neurotransmitter. Recent evidence suggests that DOPA fulfills these criteria in its involvement mainly in baroreflex neurotransmission in the lower brainstem and in delayed neuronal death by transient ischemia in the striatum and the hippocampal CA1 region of rats.

DOPA causes glutamate release and delayed neuron death by brain ischemia in rats

DOPA seems to be a neuromodulator in striata and hippocampal CA1 and a neurotransmitter of the primary baroreceptor afferents terminating in the nucleus tractus solitarii (NTS) and baroreflex pathways in the caudal ventrolateral medulla and rostral ventrolateral medulla in the brainstem of rats. DOPA recognition sites differ from dopamine (DA) D(1) and D(2) and ionotropic glutamate receptors. Via DOPA sites, DOPA stereoselectively releases by itself neuronal glutamate from in vitro and in vivo striata. In the cultured neurons, DOPA and DA cause neuron death via autoxidation. In addition, DOPA causes autoxidation-irrelevant neuron death via glutamate release. Furthermore, DOPA released by four-vessel occlusion seems to be an upstream causal factor for glutamate release and resultant delayed neuron death by brain ischemia in striata and hippocampal CA1. Glutamate has been regarded as a neurotransmitter of baroreflex pathways. Herein, we propose a new pathway that DOPA is a neurotransmitter of the primary aortic depressor nerve and glutamate is that of secondary neurons in neuronal microcircuits of depressor sites in the NTS. DOPA seems to release unmeasurable, but functioning, endogenous glutamate from the secondary neurons via DOPA sites. A common following pathway may be ionotropic glutamate receptors-nNOS activation-NO production-baroreflex neurotransmission and delayed neuron death. However, we are concerned that DOPA therapy may accelerate neuronal degeneration process especially at progressive stages of Parkinson's disease.

Involvement of nitric oxide production via kynurenic acid-sensitive glutamate receptors in DOPA-induced depressor responses in the nucleus tractus solitarii of anesthetized rats

We have proposed the hypothesis that L-3,4-dihydroxyphenylalanine (DOPA) plays a role of neurotransmitter of the primary baroreceptor afferents terminating in the nucleus tractus solitarii (NTS). In the present study, we tried to clarify whether glutamate receptors and/or nitric oxide (NO), important modulators for central cardiovascular regulation, are involved in the DOPA-induced cardiovascular responses in the nucleus. Male Wistar rats were anesthetized with urethane and artificially ventilated. Compounds or antisense oligos (17-mer) for neuronal NO synthase were microinjected into depressor sites of the unilateral nucleus. DOPA 30-300 pmol microinjected into the nucleus dose-dependently induced depressor and bradycardic responses. Prior injection of kynurenic acid (600 pmol) suppressed DOPA (300 pmol)-induced responses by approximately 80%. Prior injection of N(G)-monomethyl-L-arginine 100 nmol, a potent NO synthase inhibitor, reversibly attenuated by approximately 90% DOPA-induced responses, while the D-isomer 100 nmol produced no effect. Furthermore, prior injection of neuronal NO synthase antisense oligos (20 pmol) reversibly reduced by approximately 70% responses to DOPA. Sense or scrambled oligos produced no effect. A NO precursor L-arginine (30 nmol) induced depressor and bradycardic responses, but these responses were not affected by kynurenic acid. These results suggest important roles for glutamate receptors and NO in DOPA induced-depressor and bradycardic responses in the NTS.

Is DOPA a neurotransmitter?

Historically, 3,4-dihydroxyphenylalanine (DOPA) has been considered to be an inert amino acid that alleviates the symptoms of Parkinson's disease by its conversion to dopamine via the enzyme aromatic L-amino acid decarboxylase. In contrast to this generally accepted idea, we propose that DOPA itself is a neurotransmitter and/or neuromodulator in addition to being a precursor of dopamine. Several criteria such as synthesis, metabolism, active transport, existence, physiological release, competitive antagonism and physiological or pharmacological responses must be satisfied before a compound is accepted as a neurotransmitter. Recent evidence suggests that DOPA fulfills these criteria in its involvement in baroreflex neurotransmission.

Inhibition by L-3,4-dihydroxyphenylalanine of hippocampal CA1 neurons with facilitation of noradrenaline and gamma-aminobutyric acid release

Electrophysiological studies were performed to elucidate whether L-3,4-dihydroxyphenylalanine (L-DOPA) acted on hippocampal CA1 neurons, since this drug has been reported to act as a neurotransmitter in the hypothalamus and striatum. Hippocampal slices (450 microM thick) obtained from male Wistar rats (4-7 weeks of age) were placed in a bath (maintained at 30+/-1 degrees C) continuously perfused with artificial cerebrospinal fluid. The population spikes elicited by electrical stimuli applied to the Schaffer collateral/commissural fibers were recorded in the hippocampal CA1 region, using a glass micropipette filled with 3 M NaCl. Drugs were applied in the bath through a perfusion system. The population spikes were inhibited by L-DOPA (1 nM-10 microM) with a bell-shaped concentration-response curve (n=7-15). Maximum inhibitory effects were obtained at 100 nM. L-DOPA cyclohexyl ester, a putative L-DOPA recognition site antagonist, antagonized the L-DOPA-induced inhibition of population spike. However, the inhibition remained unaffected in the presence of 3-hydroxybenzylhydrazine, an aromatic amino acid decarboxylase inhibitor. Furthermore, bath application of either phentolamine, an alpha-adrenoceptor antagonist, or bicuculline, a GABA(A) receptor antagonist, antagonized the inhibitory effects of L-DOPA on population spikes. In addition, bicuculline (1 microM) antagonized the inhibition of population spike induced by 6-fluoronorepinephrine (10 microM), an alpha-adrenoceptor agonist, while phentolamine (10 microM) did not affect the muscimol (1 microM)-induced inhibition. These results suggested that L-DOPA itself acted on L-DOPA recognition sites to release noradrenaline, and that the latter facilitates gamma-aminobutyric acid (GABA) release via alpha-adrenoceptors located on the GABA-containing cells and/or their nerve terminals, thereby inhibiting the population spikes in the hippocampal CA1 field.

DOPA cyclohexyl ester, a competitive DOPA antagonist, protects glutamate release and resultant delayed neuron death by transient ischemia in hippocampus CA1 of conscious rats

In rat striata, DOPA released is a causal factor for glutamate release and resultant delayed neuron death by four-vessel occlusion. Nanomolar DOPA cyclohexyl ester (CHE), a potent and relatively stable competitive DOPA antagonist, protects these events. We tried to clarify whether DOPA CHE protects these events in hippocampal CA1 pyramidal cell layers most vulnerable against ischemia. Five to 10 min ischemia caused slight to mild glutamate release in 10 min samples during microdialysis and mild to severe neuron death 96 h after reperfusion. DOPA and dopamine were under assay limit in this design, but were basally detected by 20 min sampling and released by 20 min ischemia. In 10 min samples, intrahippocampal perfusion of 100 nM DOPA CHE 10 min before ischemia for 70 min did not inhibit glutamate release by 10 min ischemia, while it abolished glutamate release and protected delayed neuron death by 5 min ischemia. DOPA CHE is neuroprotective under a mild ischemic condition in rat hippocampus CA1.

Optical approaches to embryonic development of neural functions in the brainstem

The ontogenetic approach to physiological events is a useful strategy for understanding the functional organization/architecture of the vertebrate brainstem. However, conventional electrophysiological techniques are difficult or impossible to employ in the early embryonic central nervous system. Optical techniques using voltage-sensitive dyes have made it possible to monitor neural activities from multiple regions of living systems, and have proven to be a useful tool for analyzing the embryogenetic expression of brainstem neural function. This review describes recent progress in optical studies made on embryonic chick and rat brainstems. Several technical issues concerning optical recording from the embryonic brainstem preparations are discussed, and characteristics of the optical signals evoked by cranial nerve stimulation or occurring spontaneously are described. Special attention is paid to the chronological analyses of embryogenetic expression of brainstem function and to the spatial patterning of the functional organization/architecture of the brainstem nuclei. In addition, optical analyses of glutamate, GABA, and glycine receptor functions during embryogenesis are described in detail for the chick nucleus tractus solitarius. This review also discusses intrinsic optical signals associated with neuronal depolarization. Some emphases are also placed on the physiological properties of embryonic brainstem neurons, which may be of interest from the viewpoint of developmental neurobiology.

The central aromatic amino acid DOPA decarboxylase inhibitor, NSD-1015, does not inhibit L-DOPA-induced circling in unilateral 6-OHDA-lesioned-rats

The centrally acting aromatic amino acid dopa decarboxylase (AADC) inhibitor, 3-hydroxybenzyl hydrazine (NSD-1015), is widely used to study the neurotransmitter-like actions of L-DOPA. However, the effects of NSD-1015 on L-DOPA-induced motor activity are unclear as both increases and decreases have been reported. We now investigate the effects of NSD-1015 on L-DOPA-induced contralateral circling behaviour in 6-OHDA-lesioned rats and on striatal levels of L-DOPA, 3-O-methyl-DOPA (3-OMD), dopamine, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) using microdialysis techniques. NSD-1015 (50-200 mg/kg i.p.) inhibited AADC activity both in the liver and striatum of normal rats. Administration of NSD-1015 (50-200 mg/kg i.p.), delayed the onset of circling produced by administration of L-DOPA (25 mg/kg i.p.) and carbidopa (12.5 mg/kg i. p.), suggesting blockade of central AADC activity. However, the duration of the L-DOPA-induced circling was prolonged and overall no inhibition of circling behaviour occurred. L-DOPA (25 mg/kg i.p.) plus carbidopa (12.5 mg/kg i.p.) increased extracellular levels of L-DOPA, 3-OMD, dopamine, DOPAC and HVA in the 6-OHDA-lesioned striatum. Pretreatment of rats with the central AADC inhibitor, NSD-1015 (100 mg/kg i.p.), potentiated the increase in dialysate levels of L-DOPA and 3-OMD. However, it did not reduce striatal dopamine levels in the 6-OHDA-lesioned hemisphere, which were elevated following L-DOPA administration. The increases in DOPAC and HVA levels were abolished by NSD-1015 pretreatment. These results suggest that, while NSD-1015 blocks central AADC activity, it also acts as a monoamine oxidase inhibitor so maintaining striatal dopamine concentration by reducing dopamine metabolism. NSD-1015, therefore, may not be an appropriate tool for the study of brain AADC activity and for assessing the neuromodulatory role of L-DOPA.

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.

L-DOPA cyclohexyl ester is a novel potent and relatively stable competitive antagonist against L-DOPA among several L-DOPA ester compounds

We explored L-DOPA esters with chemically bulky structures to find a potent stable competitive antagonist against L-DOPA, compared to DOPA methyl ester (DOPA ME). In anesthetized rats, DOPA cyclohexyl ester (DOPA CHE), DOPA cyclopentyl ester (DOPA CPE) and DOPA cyclopentyldimethyl ester (DOPA CPDME) at 1 microgram microinjected into depressor sites of the nucleus tractus solitarii elicited or tended to elicit more marked antagonism against depressor responses to 60 ng L-DOPA, compared to DOPA ME. At 100 ng, DOPA CHE elicited the most potent antagonism. At 1 microgram, duration of the antagonistic activity of DOPA CHE was approximately three times longer than that of DOPA ME. During microdialysis of the nucleus accumbens, conversion from DOPA CHE at 1 microM perfused via probes to extracellular L-DOPA was the lowest among these compounds and less than one half of that from DOPA ME. Binding studies showed that the recognition site for L-DOPA differs from ionotropic glutamatergic, dopaminergic D1 and D2 receptors. We recently found that L-DOPA evoked by transient ischemia may act as a DOPA CHE-sensitive causal factor for glutamate release and resultant neuronal cell death. DOPA CHE is the most potent, relatively stable competitive antagonist against L-DOPA and is a useful mother compound to develop neuroprotective drugs.

An L-dopaergic relay from the posterior hypothalamic nucleus to the rostral ventrolateral medulla and its cardiovascular function in anesthetized rats

We have proposed that L-3,4-dihydroxyphenylalanine (L-DOPA) is a neurotransmitter in the central nervous system [Misu Y. et al. (1996) Prog. Neurobiol. 49, 415-454]. Herein, we attempt to clarify whether lesions in the posterior hypothalamic nucleus decrease the tissue content of L-DOPA in the rostral ventrolateral medulla. We also attempt to clarify whether or not endogenous L-DOPA is evoked by electrical stimulation of the posterior hypothalamic nucleus. It is possible that evoked L-DOPA functions as a transmitter candidate to activate pressor sites of the rostral ventrolateral medulla in anesthetized rats. Electrolytic lesions were made in the bilateral posterior hypothalamic nucleus by a monopolar direct current of 2 mA for 10 s, 10 days before measurements. The effect of the lesions was to selectively decrease the tissue content of L-DOPA by one-half in the right rostral ventrolateral medulla. Decreases in the amounts of dopamine, noradrenaline or adrenaline were not observed. Decreases were also not evident in the right caudal ventrolateral medulla. During microdialysis of the right rostral ventrolateral medulla, extracellular basal levels of L-DOPA and three types of catecholamine were consistently detectable by high-performance liquid chromatography with electrochemical detection. Tetrodotoxin (1 microM) perfused into the right rostral ventrolateral medulla gradually decreased basal levels of L-DOPA by 25%; it decreased basal levels of noradrenaline and adrenaline by 25-30% and dopamine levels by 40%. Intensive electrical stimulation of the ipsilateral posterior hypothalamic nucleus (50 Hz, 0.3 mA, 0.1 ms duration, twice for 5 min at an interval of 5 min) selectively caused the release of L-DOPA in a repetitive and constant manner. The stimulation was accompanied by hypertension and tachycardia. However, catecholamines were not released. Tetrodotoxin suppressed the release of L-DOPA, but partially inhibited hypertension with only a slight inhibition of tachycardia evoked by stimulation of the posterior hypothalamic nucleus. L-DOPA methyl ester, a competitive L-DOPA antagonist, was bilaterally microinjected into pressor sites of the rostral ventrolateral medulla at 1.5 microg x 2 and 3 microg x 2. The antagonist dose-dependently and consistently antagonized pressor and tachycardiac responses to mild transient stimulation of the unilateral posterior hypothalamic nucleus (33 Hz, 0.2 mA, 0.1 ms duration, for 10 s). In addition, the antagonist alone (3 microg x 2) elicited hypotension and bradycardia. These results show that an L-DOPAergic relay may project from the posterior hypothalamic nucleus directly to pressor sites of the rostral ventrolateral medulla and/or indirectly to certain neurons near pressor sites in microcircuits of the same region. When released, L-DOPA appears to function tonically to activate pressor sites; it also appears to be involved in the maintenance and regulation of blood pressure and heart rate.

L-dopaergic components in the caudal ventrolateral medulla in baroreflex neurotransmission

L-3,4-Dihydroxyphenylalanine (L-DOPA) is probably a transmitter of the primary baroreceptor afferents terminating in the nucleus tractus solitarii; L-DOPA functions tonically to activate depressor sites of the caudal ventrolateral medulla, which receives input from the nucleus tractus solitarii [Misu Y. et al. (1996) Prog. Neurobiol. 49, 415-454]. We have attempted to clarify whether or not L-DOPAergic components within the caudal ventrolateral medulla are involved in baroreflex neurotransmission in anesthetized rats. Electrolytic lesions of the right nucleus tractus solitarii (1 mA d.c. for 10 s, 10 days before measurement) selectively decreased by 45% the tissue content of L-DOPA in the dissected ipsilateral caudal ventrolateral medulla. Electrolytic lesions did not decrease dopamine, norepinephrine and epinephrine levels. During microdialysis of the right caudal ventrolateral medulla, extracellular levels of L-DOPA, norepinephrine, epinephrine and 3,4-dihydroxyphenylacetic acid were consistently detectable using high-performance liquid chromatography with electrochemical detection. However, extracellular dopamine levels were lower than the assay limit. Baroreceptor activation by i.v. phenylephrine selectively evoked L-DOPA without increasing the levels of norepinephrine, epinephrine and 3,4-dihydroxyphenylacetic acid. This L-DOPA release was suppressed by acute lesion in the ipsilateral nucleus tractus solitarii. Intermittent stimulation of the right aortic depressor nerve (20 Hz, 3 V, 0.3 ms duration, for 30 min) repetitively and constantly caused L-DOPA release, hypotension and bradycardia, without increases in levels of norepinephrine, epinephrine and 3,4-dihydroxyphenylacetic acid. Local inhibition of L-DOPA synthesis with alpha-methyl-p-tyrosine (30 microM) infused into the ipsilateral caudal ventrolateral medulla gradually decreased basal levels of L-DOPA and 3,4-dihydroxyphenylacetic acid without decreasing norepinephrine and epinephrine. The inhibition of L-DOPA synthesis interrupted L-DOPA release and decreased by 65% depressor responses elicited by aortic nerve stimulation; however, it produced no effect on bradycardic responses. CoCl2 (119 ng), a mainly presynaptic inhibitory transmission marker, and L-DOPA methyl ester (1 microg), a competitive L-DOPA antagonist, when microinjected into depressor sites of the right caudal ventrolateral medulla, reduced by 60% depressor responses to transient ipsilateral stimulation of the aortic nerve (20 Hz, 3 V, 0.1 ms duration, for 10 s). No changes in bradycardic responses were observed. There may exist an L-DOPAergic relay from the nucleus tractus solitarii to the caudal ventrolateral medulla. L-DOPAergic components in the caudal ventrolateral medulla are involved in baroreflex neurotransmission via a baroreceptor-aortic depressor nerve-nucleus tractus solitarii-caudal ventrolateral medulla relay in the rat.

Some interactions of L-DOPA and its related compounds with glutamate receptors

L-DOPA is probably a transmitter and/or modulator in the central nervous system (1). L-DOPA methyl ester (DOPA ME) is a competitive L-DOPA antagonist. However, it remains to be clarified whether there exist L-DOPAergic receptors. In Xenopus laevis oocytes injected with rat brain poly(A)+ RNA, L-DOPA induced small inward currents with ED50 of 2.2 mM at a holding potential of -70 mV. The currents were abolished by kynurenic acid or CNQX. Similar L-DOPA-currents were seen in oocytes co-injected with AMPA receptors, GluRs1,2,3 and 4. In brain membrane preparations, L-DOPA inhibited specific binding of [3H]-AMPA with IC50 of 260 microM. This inhibition was not modified by 200 microM ascorbic acid, an antioxidant. L-DOPA did not inhibit binding of [3H]-ligands of MK-801, kainate, DCKA and CGP39653. DOPA ME and L-DOPA cyclohexyl ester, a novel, potent and competitive antagonist (2), inhibited specific binding of [3H]-MK-801 with respective IC50 of 1 and 0.68 mM, but elicited no effect on that of the other [3H]-ligands. With low affinities, L-DOPA acts on AMPA receptors, while competitive antagonists act on NMDA ion channel domain. L-DOPAergic agonist and antagonist may not interact on ionotropic glutamate receptors. DOPA ME-sensitive L-DOPA recognition sites (1) seem to differ from glutamate receptors.

The evidence for tonic GABAergic regulation of basal L-DOPA release via activation of inhibitory GABA(A) receptors in the nucleus tractus solitarii of anesthetized rats

We have proposed that DOPA is a neurotransmitter of the primary baroreceptor afferents terminating in the rat nucleus tractus solitarii (NTS). GABA is a putative inhibitory neuromodulator for baroreflex inputs in the NTS. Thus, GABA may inhibit DOPAergic transmission in the NTS. We tried to clarify whether basal DOPA release is inhibited by muscimol, a GABA(A) agonist, and facilitated by bicuculline, a GABA(A) antagonist, during microdialysis of the NTS in anesthetized rats. DOPA release was consistently detectable. Muscimol 10-100 microM perfused via probes gradually inhibited concentration-dependently DOPA release. Peak 30% inhibition occurred 2 h after perfusion. Muscimol (30 microM)-induced inhibition was antagonized by non-effective 10 microM bicuculline. Bicuculline (30 microM) elicited peak 30% facilitation of DOPA release 2 h after perfusion. Endogenous GABA seems to regulate tonically basal DOPA release via activation of inhibitory GABA(A) receptors in the rat NTS. These findings further support the above proposal.

GABA may function tonically via GABA(A) receptors to inhibit hypotension and bradycardia by L-DOPA microinjected into depressor sites of the nucleus tractus solitarii in anesthetized rats

We have proposed that DOPA is a transmitter of the primary baroreceptor afferents terminating in the rat nucleus tractus solitarii (NTS). GABA is a putative inhibitory neuromodulator for baroreflex inputs in the NTS. GABA may inhibit DOPAergic transmission. Drugs were microinjected into depressor sites of the NTS in anesthetized rats. DOPA (10-60 ng) elicited dose-dependent depressor responses. GABA (3-300 ng) elicited dose-dependent pressor responses. Nipecotic acid (100 ng) elicited pressor responses. Bicuculline (10 ng) elicited depressor responses. Responses to DOPA (30 ng) were inhibited by pretreatment with GABA and nipecotic acid, but potentiated by bicuculline, when vascular responses to pretreated drugs returned to basal levels. DOPA ME, a competitive DOPA antagonist, did not displace specific [3H]GABA binding. Prior DOPA ME (1 microg) inhibited by one-half pressor responses to 300 ng GABA. GABA seems to inhibit tonically via GABA(A) receptors depressor responses to DOPA and to elicit pressor responses partially by inhibition of tonic function of endogenous DOPA to activate depressor sites in the NTS. These findings further support the above proposal.

An L-DOPA-like depressor action of L-threo-dihydroxyphenyl-serine in the rat caudal ventrolateral medulla

We have proposed that L-3,4-dihydroxyphenylalanine (L-DOPA) is a neurotransmitter and/or neuromodulator in the central nervous system (1). In this study, we investigated whether or not L-threo-dihydroxyphenylserine (L-threo-DOPS), a synthetic amino acid structurally related to L-DOPA, microinjected into the caudal ventrolateral medulla (CVLM) and the rostral ventrolateral medulla (RVLM) shows cardiovascular actions similar to those of L-DOPA in anesthetized rats. When L-threo-DOPS was microinjected into CVLM, it produced dose-dependent (0.01-3 ng) depressor and bradycardic responses. D-threo-DOPS (3 ng) produced no effect. The responses to L-threo-DOPS (1 ng) were almost completely blocked by L-DOPA methyl ester (1 microg), a competitive antagonist for L-DOPA, supporting the existence of an L-threo-DOPS-sensitive recognition site for L-DOPA in CVLM. Microinjection of L-threo-DOPS into RVLM, however, showed no effect (0.001-100 ng), which contrasted with the cardiopressor action of L-DOPA applied in RVLM. In RVLM, there may exist an L-threo-DOPS-insensitive recognition site for L-DOPA.

The neuroregulatory properties of L-DOPA. A review of the evidence and potential role in the treatment of Parkinson's disease

Accumulating evidence suggests that L-dihydroxyphenylalanine (L-DOPA) has neurotransmitter-like and/or neuromodulatory properties in the CNS. Such evidence is based on a wide range of findings including the existence of specific L-DOPAergic neurons in several regions of the CNS, neurotransmitter-like characteristics and specific pharmacological effects. This review attempts to outline the main evidence for this conception and to relate such findings to L-DOPA treatment effects in Parkinson's disease. In this context L-DOPA in itself has been shown to potentiate D2 receptor-mediated effects, inhibit acetylcholine release and increase the release of L-glutamate, neuropharmacological effects which can be linked to treatment side-effects in advanced Parkinson's disease. It is suggested that supersensitive L-DOPA-mediated effects contribute to the pathogenesis underlying L-DOPA-induced motor complications in advanced Parkinson's disease. However, since specific L-DOPA receptors have yet to be identified, the assessment of the relative importance of L-DOPA-mediated effects in this clinical context must be regarded as incomplete.

Neurobiology of L-DOPAergic systems

L-DOPA is proposed to be a neurotransmitter and/or neuromodulator in CNS. It is released probably from neurons, which may contain L-DOPA as an end-product, and/or from some compartment other than catecholamine-containing vesicles. The L-DOPA itself produces presynaptic and postsynaptic responses. All are stereoselective and most are antagonized by competitive antagonist. In striatum, L-DOPA is neuromodulator, mother of catecholamines, not only a precursor for dopamine but also a potentiator of children for presynaptic beta-adrenoceptors to facilitate dopamine release and postsynaptic D2 receptors, and ACh release inhibitor. All may cooperate for Parkinson's disease. Meanwhile, supersensitization of increase in L-glutamate release to nanomolar levodopa was seen in Parkinson's model rats, which may relate to dyskinesia or "on-off" during chronic therapy. In lower brainstem, L-DOPA tonically activates postsynaptic depressor sites of NTS and CVLM and pressor sites of RVLM. L-DOPA is probably a neurotransmitter of primary baroreceptor afferents terminating in NTS. GABA, the inhibitory neuromodulator for baroreflex in NTS, tonically functions to inhibit, via GABAA receptors, L-DOPA release and depressor responses to levodopa. Levodopa inversely releases GABA. L-DOPAergic monosynaptic relay from NTS to CVLM and from PHN to RVLM is suggested. Tonic L-DOPAergic baroreceptor-aortic nerve-NTS-CVLM relay seems to carry baroreflex information. Disturbance of neuronal activity to release L-DOPA in NTS, loss of the activity in CVLM, enhancement of the activity with decreased decarboxylation and increase in sensitivity to levodopa in RVLM may be involved in maintenance of hypertension in SHR. This is a story of "L-DOPAergic receptors" with extremely high affinity and low density.

Depressor action of L-threo-dihydroxyphenylserine in the rat nucleus tractus solitarii

Microinjections of L-threo-dihydroxyphenylserine (L-threo-DOPS, 0.1-3 ng), a synthetic precursor amino acid of noradrenaline, into the medial area of the nucleus tractus solitarii produced dose-dependent depressor and bradycardic responses in anesthetized rats treated with or without i.p. 3-hydroxybenzylhydrazine, a central inhibitor of L-aromatic amino acid decarboxylase. D-threo-DOPS (3 ng) produced no effect. L-Dihydroxyphenylalanine (L-DOPA) methyl ester (1 microgram), a competitive antagonist of L-DOPA, microinjected into the nucleus tractus solitarii, blocked the depressor and bradycardic responses to L-threo-DOPS itself produces vasodepressor actions without its conversion to noradrenaline, probably via a recognition site for L-DOPA in the rat nucleus tractus solitarii.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

Altered basal release and pressor effect of L-DOPA in the rostral ventrolateral medulla of spontaneously hypertensive rats

1. Transmitter-like L-DOPA functions as a tonic to produce postsynaptic cardiopressor responses in the rostral ventrolateral medulla (RVLM) of rats. We attempted to clarify whether a transmitter-like L-DOPA system is altered in the RVLM of spontaneously hypertensive rats (SHR) to maintain the hypertension. 2. By microdialysis of left RVLM area, the basal L-DOPA release was higher in SHR than in Wistar-Kyoto (WKY) rats. This release was partially inhibited by tetrodotoxin (TTX, 1 mu mol/L) to a similar degree in both strains. TTX-sensitive L-DOPA release was higher in SHR than in WKY. 3. L-DOPA (10-600 ng) and L-glutamate (10-300 ng) microinjected into left RVLM produced dose-dependent hypertension and tachycardia. Pressor but not tachycardiac responses to L-DOPA at lower doses were slightly greater in SHR than in WKY, whereas no difference to L-glutamate was observed in either strain. 4. In RVLM regions, there was no difference of tyrosine hydroxylase activity in SHR or WKY; however, L-aromatic amino acid decarboxylase activity was lower in SHR than in WKY. 5. Enhanced presynaptic neuronal L-DOPA release, including a decrease in decarboxylation and sensitization of postsynaptic pressor sites to L-DOPA in RVLM, may be involved in the maintenance of hypertension in SHR.

Effect of barnidipine on blood flow to major organs and renal function in anaesthetized dogs and spontaneously hypertensive rats

1. The effects of barnidipine on blood flow to major organs and on renal function were investigated in anaesthetized dogs and conscious spontaneously hypertensive rats (SHR), and the results were compared with those for nicardipine, nitrendipine, nisoldipine, manidipine and amlodipine. 2. In anaesthetized dogs, barnidipine (0.3-3 mu g/kg i.v.) dose-dependently decreased blood pressure and increased or preserved blood flow in the vertebral, coronary, femoral and renal arteries. The effect of barnidipine on blood flow was the most potent of the compounds tested. In conscious SHR, barnidipine (0.3-3 mg/kg p.o.) produced a dose-dependent antihypertensive effect and decreased renal vascular resistance. Barnidipine also dose-dependently increased urinary volume. The antihypertensive and diuretic effects of barnidipine were the most potent of the drugs tested. 3. In summary, barnidipine was shown to preserve or increase blood flow to major organs and to produce diuretic activity with a decrease in blood pressure. These findings suggest that barnidipine maintains or promotes renal function at antihypertensive doses.

Altered basal release and depressor effect of L-DOPA in the nucleus tractus solitarii of spontaneously hypertensive rats

1. L-DOPA as a probable neurotransmitter of baroreceptor afferents functions as a tonic to mediate cardiodepressor control in the nucleus tractus solitarii (NTS). We attempted to clarify further whether a transmitter-like L-DOPA system is altered in NTS of adult spontaneously hypertensive rats (SHR). 2. By microdialysis of left NTS area, the basal L-DOPA release was lower in SHR than in Wistar-Kyoto (WKY) rats. This release was partially inhibited by tetrodotoxin (TTX, 1 mu mol/L) to a similar degree in both strains. TTX-sensitive L-DOPA release was lower in SHR than in WKY. 3. L-DOPA (10-300 ng) and L-glutamate (3-100 ng) microinjected into left NTS produced dose-dependent hypotension and bradycardia. No difference of responses to L-glutamate was seen in either strain. However, depressor but not bradycardic responses to L-DOPA at higher doses were slightly greater in SHR than in WKY. 4. In caudal dorsomedial medulla including NTS, tyrosine hydroxylase activity was increased in SHR compared to WKY, while there was no difference in either strain of L-aromatic amino acid decarboxylase activity. 5. Impaired tonic neuronal activity to release L-DOPA in NTS may be involved in the maintenance of hypertension in SHR. An increase in sensitivity of a recognition site for L-DOPA seems to occur as a compensatory mechanism for impairment of the neuronal activity.

Loss of tonic neuronal activity to release L-DOPA in the caudal ventrolateral medulla of spontaneously hypertensive rats

Experiments were designed to clarify whether a tonic L-DOPA system is altered in the caudal ventrolateral medulla (CVLM) of adult spontaneously hypertensive rats (SHR), compared to age-matched Wistar-Kyoto rats (WKY). By microdialysis in CVLM, basal L-DOPA release was constantly detectable and was lower in SHR than that in WKY. This release was reduced by tetrodotoxin perfusion (1 microM) in WKY to a basal level in SHR, whereas no modification occurred with tetrodotoxin in SHR. No difference of tyrosine hydroxylase and DOPA decarboxylase activities in the CVLM region was seen between the two strains. By microinjections into depressor sites of CVLM, L-DOPA (10-300 ng) or L-glutamate (3-300 ng) elicited dose-dependent depressor and bradycardic responses and greater depressor responses to both amino acids were seen at high doses in SHR, compared to WKY. Tonic neuronal activity to release L-DOPA is lost in the CVLM of adult SHR and this loss may contribute to maintenance of hypertension in SHR.

L-DOPA systems for blood pressure regulation in the lower brainstem

We have explored probable neurotransmitter roles of L-3,4-dihydroxyphenylalanine (L-DOPA) in baroreceptor reflex and blood pressure regulation in depressor sites of the nucleus tractus solitarii (NTS) and the caudal ventrolateral medulla (CVLM), and in pressor sites of the rostral ventrolateral medulla (RVLM) in anesthetized rats. During microdialysis of these three areas, the basal L-DOPA release is in part tetrodotoxin (TTX)-sensitive and Ca2(+)-dependent, high K+ Ca2(+)-dependently releases dL-DOPA. L-DOPA microinjected (10-300 ng) dose-dependently produces postsynaptic depressor responses in the NTS and CVLM and pressor responses in the RVLM, and a recognition site for L-DOPA functions tonically to activate depressor neurons in the NTS and CVLM and pressor neurons in the RVLM. It is highly probable that L-DOPA is a neurotransmitter of the baroreceptor afferents terminating in the NTS, which is based on further findings such as (1) antagonism by a competitive L-DOPA antagonist against depressor responses to aortic nerve stimulation, (2) TTX-sensitive L-DOPA release by aortic nerve stimulation, (3) abolition of baroreceptor-stimulated L-DOPA release by bilateral sino-aortic denervation and (4) decreases in tyrosine hydroxylase (TH)- and L-DOPA-immunoreactivities without modifications of dopamine- and DBH-immunoreactivities in the left NTS and ganglion nodosum 7 days after ipsilateral aortic nerve denervation peripheral to the ganglion. In the NTS, GABA tonically functions to inhibit via GABAA receptors L-DOPA release and depressor responses to L-DOPA, whereas L-DOPA induces GABA release. Impaired TTX-sensitive neuronal activity to release L-DOPA in the NTS and enhanced TTX-sensitive neuronal activity including a decrease in decarboxylation of L-DOPA to dopamine and an increase in sensitivity of the recognition site to L-DOPA in the RVLM are relevant to the maintenance of hypertension in spontaneously hypertensive rats. Decreases in the contents of L-DOPA in the right CVLM 10 days after electrical lesion of the ipsilateral NTS suggest a 'L-DOPAergic' and monosynaptic relay from the NTS to the CVLM. L-DOPA seems to play major roles as a neurotransmitter for baroreceptor reflex and blood pressure regulation in the lower brainstem of rats.

Altered tonic L-3,4-dihydroxyphenylalanine systems in the nucleus tractus solitarii and the rostral ventrolateral medulla of spontaneously hypertensive rats

We have proposed that L-3,4-dihydroxyphenylalanine (L-DOPA) is a neurotransmitter in the central nervous system [Y. Misu et al. (1995) Adv. Pharmac. 32, 427-459]. L-DOPA as a probable neurotransmitter for the primary baroreceptor afferents tonically functions to mediate cardiodepressor control in the nucleus tractus solitarii and also tonically functions to mediate cardiopressor control in the rostral ventrolateral medulla of rats. We further attempted to clarify whether a transmitter-like L-DOPA system is altered in these areas of adult spontaneously hypertensive rats. By microdialysis in the left nucleus tractus solitarii area, the basal L-DOPA release was lower in spontaneously hypertensive rats than that in Wistar-Kyoto rats. This release was partially reduced by tetrodotoxin (1 microM) to the same absolute levels in the two strains. Tonic neuronal L-DOPA release is impaired in this nucleus of spontaneously hypertensive rats. This impairment is not secondarily due to decrease in formation or increase in decarboxylation of L-DOPA, since tyrosine hydroxylase activity was increased in spontaneously hypertensive rats, compared to Wistar-Kyoto rats, while no difference of L-aromatic amino acid decarboxylase activity was seen in the caudal dorsomedial medulla including the nucleus. L-DOPA (10-300 ng) microinjected into the nucleus produced dose-dependent hypotension and bradycardia. A maximum depressor response of spontaneously hypertensive rats to L-DOPA at higher doses was slightly greater than that of Wistar-Kyoto rats. On the other hand, in the left rostral ventrolateral medulla, the basal L-DOPA release was higher in spontaneously hypertensive rats than that in Wistar-Kyoto rats. This release was also partially reduced by tetrodotoxin to the same absolute levels in the two strains. Tonic neuronal L-DOPA release is enhanced in spontaneously hypertensive rats. This enhancement seems to include partially a decrease in decarboxylation of L-DOPA, since L-aromatic amino acid decarboxylase activity was decreased in spontaneously hypertensive rats compared to Wistar-Kyoto rats, while no difference in tyrosine hydroxylase activity was seen. L-DOPA (10-600 ng) produced dose-dependent hypertension and tachycardia. Importantly, a pressor response of spontaneously hypertensive rats to L-DOPA at lower doses was slightly greater than that of Wistar-Kyoto rats. L-DOPA seems to play a transmitter-like role in blood pressure regulation at levels of the nucleus tractus solitarii and rostral ventrolateral medulla in rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Is diurnal variation of mood associated with parasympathetic activity?

18 patients with distinct improvement of mood in the evening and 18 patients without, both suffering from major depression and equally treated with tricyclic antidepressants (TCA), and an age- and sex-matched group of 18 normal control subjects underwent a standardized heart-rate (HR) analysis (HRA) in the morning (08:00) and 12 h later in the evening (20:00). The battery of cardiovascular reflex tests included the determination of HR variability (HRV) while resting and during deep breathing, and a spectral HRA. The depressed patients with diurnal variation of mood showed significantly decreased HR and significantly increased HRV parameters while resting and during deep respiration in the evening. On the contrary, patients without diurnal changes of mood just showed a significant HRV increase during deep respiration in the evening. No statistically significant changes of these parameters were detected in the healthy subjects. It is not known if the observed changes of HR parameters representing increment of parasympathetic tone are intrinsically related to the mood swings or if this rhythm just becomes visible under the circumstances of depression. Also, the impact of TCA treatment upon the results remains to be elucidated.

L-dihydroxyphenylalanine and its decarboxylase: new ideas on their neuroregulatory roles

Recent experimental reports concerning L-dihydroxyphenylalanine (L-DOPA) and aromatic L-amino acid decarboxylase (AADC, L-DOPA decarboxylase) are reviewed in this article. Both in vitro and in vivo data now suggest that L-DOPA is an endogenous neuroactive compound that is released from neurons and acts as a neurotransmitter or neuromodulator in the brain. Administration of exogenous L-DOPA affects dopamine receptor status, AADC activity, and mitochondrial oxidation in experimental animals. The type and severity of these effects depend on the duration of the treatment. These findings may partly explain the limited efficacy of L-DOPA therapy in Parkinson's disease (PD). AADC also plays a controlling role in the central nervous system, being a regulatory enzyme in the synthesis of a putative neuromodulator 2-phenylethylamine and other trace amines. Recent experimental findings on AADC activity and localisation are of importance because they suggest that striatal [18F]DOPA uptake used as an indicator of PD progression in positron emission tomography (PET) studies is likely to overestimate nigrostriatal integrity in advanced PD. Possible new PET tracers of presynaptic dopaminergic function are discussed in this context.

Baroreceptor-aortic nerve-mediated release of endogenous L-3,4-dihydroxyphenylalanine and its tonic depressor function in the nucleus tractus solitarii of rats

We have proposed that L-3,4-dihydroxyphenylalanine (L-DOPA) is a neurotransmitter and/or neuromodulator in the central nervous system [Misu Y. and Goshima Y. (1993) Trends pharmac. Sci. 14, 119-123]. This study aimed to explore whether or not endogenous L-DOPA, as a neurotransmitter candidate of the primary baroreceptor afferents, tonically functions to activate depressor neurons in the nucleus tractus solitarii of anesthetized rats. By parallel microdialysis in bilateral nucleus tractus solitarii areas, the basal L-DOPA release was in part inhibited by tetrodotoxin perfusion (1 microM) or Ca2+ deprivation, and was markedly reduced by alpha-methyl-p-tyrosine (200 mg/kg, i.p.), a tyrosine hydroxylase inhibitor. Forty to 100 mM K+ concentration-dependently released L-DOPA. Fifty millimoles K+ repetitively and constantly released L-DOPA. This release was Ca(2+)-dependent. Stimulation of the left aortic nerve (100 Hz, 8 V) repetitively and constantly released L-DOPA and this release was tetrodotoxin-sensitive. Phenylephrine i.v. infused produced L-DOPA release and reflex bradycardia, temporally associated with a rise and subsequent recovery of blood pressure. This release and bradycardia were abolished by denervation of the bilateral carotid sinus and aortic nerves. In addition, L-DOPA methyl ester, a competitive L-DOPA antagonist, when microinjected into depressor sites of the left nucleus tractus solitarii, antagonized depressor responses to mild stimulation (20 Hz, 3 V) of the ipsilateral aortic nerve. This antagonist alone, microinjected bilaterally, elicited a dose-dependent hypertension, which was abolished by alpha-methyl-p-tyrosine. Furthermore, by immunocytochemical analysis seven days after denervation of the left aortic nerve, tyrosine hydroxylase- and L-DOPA-, but not dopamine- and dopamine-beta-hydroxylase-immunoreactivities decreased in the ipsilateral nucleus tractus solitarii and dorsal motor vagus nucleus complex area. In the left ganglion nodosum, denervation decreased staining and number of L-DOPA-immunoreactive cells and staining of tyrosine hydroxylase-immunoreactive cells, but no modification of dopamine-immunoreactive cells was seen. Taken together with previous findings that L-DOPA itself is stereoselectively responsible for cardiovascular control in this nucleus, it is probable that L-DOPA is a neurotransmitter of the primary baroreceptor afferents terminating directly in depressor neurons and/or indirectly in some neurons within a microcircuit, including depressor neurons of the nucleus tractus solitarii. Endogenously released L-DOPA itself tonically functions to activate depressor neurons for regulation of blood pressure in the rat nucleus tractus solitarii.