Role of dopamine receptors in the mechanism of formation of stress-induced stomach lesions

Experiments on rats with the use of different exposures to stress (generalized electrization and "social stress") have demonstrated that stimulation of dopamine receptors localized in the central nervous system is one of the reasons for stress-induced gastric lesions, particularly for massive hemorrhages. Stimulation of peripheral dopamine receptors seems to have a gastroprotective action. As judged from the intensity of the effects of the dopamine agonists, apomorphine and L-DOPA, on stress-induced lesions of the gastric mucosa, stimulation of D2- rather than of D1-dopamine receptors is of greater importance in stress.     

Antistress Effect of Nitric Oxide

Rat strains feature different resistances to stress. The increased production of nitric oxide (NO) in the August strain prevents the appearance of ulcerous lesions of gastric mucosa and behavioral changes induced by restraint stress. Wistar rats feature a lower level of NO production and are more sensitive to restraint stress compared to the August rats according to both the ulcerous gastric lesions and behavioral parameters. The stress-induced release of catecholamines was mimicked by experimental hyperfunction of the dopaminergic (DA) system induced by L-DOPA. The NO synthase inhibitor N-nitro-L-arginine (L-NNA) enhanced the L-DOPA-induced behavioral changes. This effect was more pronounced in the August strain. The administration of the exogenous NO donor, dinitrosyl iron complexes (DNIC), limited the behavioral disturbances induced by L-DOPA in both rat strains. The protective effect of DNIC in conditions of the DA system hyperfunction is similar to the effect of a D₂blocker sulpiride. Thus, NO has a central antistress effect apparently mediated by limiting the release of catecholamines.     

Decarboxylation of Exogenous l-3,4-Dihydroxyphenylalanine in Rat Striatum as Studied by In Vivo Voltammetry

An in vivo voltammetric technique was used to determine whether striatal nondopaminergic neurons take up and decarboxylate exogenous L-3,4-dihydroxyphenylalanine (L-DOPA) and release it as dopamine. After the striatal serotonergic neurons of the rat had been destroyed by intraventricular injection of 5,7-dihydroxytryptamine, L-DOPA was administered intraperitoneally. It was found that changes in the dopamine concentration in the striatal extracellular fluid of the rat were the same as those in the nonlesioned rat. L-DOPA was also administered to the rat after the striatal perikarya had been destroyed by the intrastriatal injection of kainate. The striatal dopamine concentrations of the lesioned rat changed in parallel with 5,7-dihydroxytryptamine-lesioned rats, as well as the nonlesioned rats. Moreover, when normal rats were administered L-DOPA, the dopamine concentration was not increased in the cerebellum, where dopamine neurons do not exist. From these observations, it is concluded that exogenous L-DOPA is taken up, decarboxylated to dopamine, and released only in the striatal dopamine neurons.     

Role of striatal L-DOPA in the production of dyskinesia in 6-hydroxydopamine lesioned rats

We explored possible differences in the peripheral and central pharmacokinetics of L-DOPA as a basis for individual variation in the liability to dyskinesia. Unilaterally, 6-hydroxydopamine (6-OHDA) lesioned rats were treated chronically with L-DOPA for an induction and monitoring of abnormal involuntary movements (AIMs). Comparisons between dyskinetic and non-dyskinetic cases were then carried out with regard to plasma and striatal L-DOPA concentrations, tissue levels of dopamine (DA), DA metabolites, and serotonin. After a single intraperitoneal injection of L-DOPA, plasma L-DOPA concentrations did not differ between dyskinetic and non-dyskinetic animals, whereas peak levels of L-DOPA in the striatal extracellular fluid were about fivefold larger in the former compared with the latter group. Interestingly, the time course of the AIMs paralleled the surge in striatal L-DOPA levels. Intrastriatal infusion of L-DOPA by reverse dialysis concentration dependently induced AIMs in all 6-OHDA lesioned rats, regardless of a previous priming for dyskinesia. Steady-state levels of DA and its metabolites in striatal and cortical tissue did not differ between dyskinetic and non-dyskinetic animals, indicating that the observed difference in motor response to L-DOPA did not depend on the extent of lesion-induced DA depletion. These results show that an elevation of L-DOPA levels in the striatal extracellular fluid is necessary and sufficient for the occurrence of dyskinesia. Individual differences in the central bioavailability of L-DOPA may provide a clue to the varying susceptibility to dyskinesia in Parkinson's disease.     

The effect of intrastriatal injection of liposome-entrapped tyrosinase on the dopamine levels in the rat brain.

Parkinson's disease is a neurodegenerative disorder which is mainly characterized by degeneration of the dopaminergic cells in the nigro-striatal system. Due to a lowered L-tyrosine 3-monooxygenase activity, L-tyrosine is not sufficiently transformed to L-DOPA. To date the most common therapy is the administration of the dopamine precursor L-DOPA, with severe collateral effects. Therefore, the substitution of the lacking tyrosine hydroxylase with tyrosinase might be a novel therapeutical approach that would generate specifically L-DOPA from L-tyrosine. We present here evidence that stereotaxic injection of liposome-entrapped tyrosinase is able to significatively increase the levels of dopamine in the rat brain. The catecholamines L-DOPA, dopamine, L-epinephrine, L-norepinephrine were extracted by acid treatment from the brains and detected by HPLC.     

Some parasympathetic neurons in the guinea-pig heart express aspects of the catecholaminergic phenotype in vivo

Summary In a histochemical study of intrinsic cardiac ganglia of the guinea-pig in whole-mount preparations, it was found that some 70–80% of the neurons express aspects of the catecholaminergic phenotype. These neurons have an uptake mechanism for L-DOPA, and contain the enzymes for converting L-DOPA, (but not D-DOPA) to dopamine and noradrenaline, i.e. aromatic L-aminoacid decarboxylase and dopamine -hydroxylase. Monoamine oxidase is also present within some of the neurons. In these respects, the neurons resemble noradrenergic neurons of sympathetic ganglia, so we refer to them as intrinsic cardiac amine-handling neurons. However, these neurons do not contain tyrosine hydroxylase and show little or no histochemically detectable uptake of -methyldopa, dopamine or noradrenaline, even after depletion of endogenous stores of amines by pre-treatment with reserpine. Noradrenergic fibres from the sympathetic chain form pericellular baskets around nerve cell bodies. The uptake of L-DOPA into nerve cell bodies is not prevented by treatment with 6-hydroxydopamine sufficient to cause transmitter-depletion or degeneration of the extrinsic noradrenergic fibres. Such degeneration experiments suggest that axons of the amine-handling neurons project to cardiac muscle, blood vessels and other intrinsic neurons. The cardiac neurons do not show any immunohistochemically detectable serotonergic characteristics; there is no evidence for uptake of the precursors L-tryptophan and 5-hydroxytryptophan or 5-HT itself, whereas the extrinsic noradrer ergic nerve fibres within the ganglia can take up 5-HT when it is applied in high concentrations.Abbreviations AChE acetylcholinesterase - DBH-IR dopamine -hydroxylase-like immunoreactivity - L-DOPA L-dihydroxyphenylalamine - 5-HT-IR 5-hydroxytryptamine-like immunoreactivity - 6-OHDA 6-hydroxydopamine - methyldopa L--methyl-dihydroxyphenylalanine - MAO monoamine oxidase - NPY neuropeptide Y - SIF small intensely fluorescent cells - TH-IR tyrosine hydroxylase-like immunoreactivity - VIP vasoactive intestinal polypeptide     

Effects of fasting and food restriction on sympathetic activity in brown adipose tissue in mice

Summary The activity of the sympathetic nervous system in mice that were either fed ad libitum, food restricted or fasted was estimated by measuring the accumulation of dopamine following the inhibition of dopamine -hydroxylase activity. Mice in each group were injected with the dopamine -hydroxylase inhibitor 1-cyclohexyl-2-mercaptoimidazole and were exposed to either 30°C (warm) or 4°C (cold). Mice were killed 1 h after the injection. Both heart and brown adipose tissue were then quickly removed and homogenized in ice-cold perchloric acid. Dopamine and noradrenaline were determined using high performance liquid chromatography. Regardless of whether mice were warm or cold exposed, both content and concentration of brown adipose tissue and dopamine were predictably higher in 1-cyclohexyl-2-mercaptoimidazole-injected mice than in non-injected animals. In mice fed ad libitum, post-injection content and concentration of dopamine in both brown adipose tissue and heart were higher in cold-exposed mice than in warm-exposed animals. In food-restricted and fasted mice, post-injection concentrations of dopamine in brown adipose tissue were higher in cold-exposed mice than in warm-exposed animals. In food-restricted and fasted mice there was no difference between warm- and cold-exposed animals with respect to post-injection contents and concentrations of dopamine in heart tissue. In fasted mice there was no difference between warm- and cold-exposed animals in post-injection content of dopamine in brown adipose tissue. This study provides further evidence that fasting, in contrast to food restriction, may blunt the tissue sympathetic nervous system response in brown adipose tissue of cold-exposed mice.Abbreviations BAT brown adipose tissue - CHMI 1-cyclohexyl-2-mercaptoimidazole - DA dopamine - DHBA dihydroxybenzylamine - EDTA ethylenediaminetetra-acetic acid - HPLC high performance liquid chromatography - NA noradrenaline - PCA perchloric acid - SNS sympathetic nervous system     

Effects of serotonin and stress on the state of the gastric mucosa in rats with the intact and transsected vagus nerve

In experiments on vagotomized and intact rats with the use of two models of experimental gastric ulceration (injection of serotonin and stress) it was demonstrated that the inhibitory action of vagotomy on haemorrhagic gastric effectiveness was more pronounced in stress than after serotonin application. Vagotomy decreased stress-induced erosive lesions but increased serotonin-induced erosions that may be a result of the increase of gastric tissue sensitivity to this amine which developed simultaneously with significant decrease of its level in gastric wall after vagotomy. Serotonin-antagonist peritol decreased stress-induced gastric disturbances in vagotomized rats more significantly than in intact rats; this suggested the great role of serotonin in anti-ulcerogenic effect of vagotomy.     

3-METHOXY-4-HYDROXYPHENYLETHANOL IN THE RAT BRAIN

Abstract— The neutral dopamine metabolite, 3-methoxy4hydroxypenylethanol (MOPET) can be measured in the rat brain by a GLC method using a pentaflouropropionic derivative and electron capture detector. The identity of MOPET is verified by mass spectrographic analyses.
The endogenous level of MOPET of 16.6 ng/g whole rat brain can be raised more than four-fold by intraperitoneal injection of L-DOPA, dopamine or MOPET. In contrast intraventricular injection of dopamine or intraperitoneal injection of L-DOPA plus a peripheral decarboxylase inhibitor (Ro 4-4602)., results in small and insignificant increase bf MOPET in the CNS.
It is concluded that MOPET is probably of low significance to central dopamine metabolism and that MOPET found in the rat brain is predominantly of peripheral origin.     

Conditioned posture changes in dogs after systemic administration of dopaminergic agents

Influence of systemic injection of some dopaminergic drugs on conditioned postural rearrangement prior to instrumental movement realization and on other motor components of instrumental reaction as well as on the performance of the instrumental task itself--was studied in chronic experiments in 5 dogs on a model of instrumental defensive reflexes connected with maintenance of a certain posture. Drugs were used influencing the nigrostriate dopaminergic system, i.e. dopamine agonist L-DOPA and haloperidol blocking dopamine striate receptors. All the motor components of the instrumental reaction and first of all conditioned postural rearrangement were modified by systemic haloperidol injection. Initial components of the postural rearrangement were modified to the greatest extent, in particular the period of preparation of the animal to the posture change increased. On the contrary, the latency of initiation of postural rearrangement was sharply shortened by systemic injection of L-DOPA. On the other hand, the main component of the postural change, i.e. redistribution of body mass among the bearing limbs (the values of which significantly increased after preliminary stimulation of the head of the caudate nucleus) changed insignificantly during modulation of the striatum dopamine level.     

Striatal L-DOPA Decarboxylase Activity in Parkinson's Disease In Vivo: Implications for the Regulation of Dopamine Synthesis

Abstract: L-DOPA is a large neutral amino acid subject to transport out of, as well as into, brain tissue. Competition between dopamine synthesis and L-DOPA egress from striatum must favor L-DOPA egress if decarboxylation declines relatively more than transport in Parkinson's disease. To test this hypothesis, we injected patients with Parkinson's disease with a radidabeled analogue of L-DOPA and recorded regional brain radioactivity as a function of time by means of positron emission tomography. We simultaneously estimated the activity of the decarboxylating enzyme and the amino acid transport. In the striatum of patients, we found the L-DOPA decarboxylase activity to be reduced in the head of the caudate nucleus and the putamen. However, the rate of egress of the DOPA analogue was unaffected by the disease and thus inhibited dopamine synthesis more than predicted in the absence of L-DOPA egress.     

Naloxone reverses L-dopa induced overstimulation effects in a Parkinson's disease animal model analogue

Chronic L-DOPA treatment of Parkinson's disease frequently leads to the development of motoric overstimulation and hyperkinetic movements. To investigate this problem in the laboratory, rats surgically altered by unilateral 6-hydroxydopamine lesions (6-OHDA) were chronically treated with one L-DOPA (10 mg/kg i.p.) injection per day for 20 days. In this 6-OHDA rotation model, the unilateral dopamine denervation results in a profound contralateral sensory-motor neglect and the animals spontaneously rotate in a direction ipsilateral to the dopamine depleted hemisphere. Initially, the L-DOPA treatment did not alter the response bias but after several weeks, the response bias was reversed and the animals rotated in the formerly akinetic direction, contralaterally, at a significantly higher level. Using this overstimulation effect as an analogue of the clinically observed L-DOPA overstimulation, animals were given naloxone in conjunction with the L-DOPA treatment. Naloxone (0.10, 0.25 and 0.50 mg/kg i.p.) produced a dose related decrease in the L-DOPA induced contralateral rotation. Consistent with an expected selective effect on the L-DOPA induced rotation, a dose related increase in ipsilateral rotation was observed. These results suggest that naloxone can attenuate the overstimulation effect of L-DOPA and that this effect is not attributable to non-specific response suppression effects.     

Prolactin inhibits the development of stress-induced ulcers in the rat

Hyperprolactinaemia, as induced by pituitary homografts under the kidney capsule, was accompanied by an inhibition of development of gastric ulcers following the application of cold-plus-restraint stress in male rats. This effect was mimicked by intracisternal administration of a low dose of the hormone. Peripheral injection of the dopamine receptor antagonist, domperidone, also inhibited the development of stress-induced ulcers. However, no effect was found after peripheral injection of another dopamine receptor antagonist, haloperidol. This latter drug appeared to antagonize the cytoprotective effect of prolactin (PRL) on stress-induced ulcers. Furthermore, peripheral injection of the prostaglandin synthesis inhibitor, indomethacin, increased the incidence of gastric ulcers in hyperprolactinaemic rats subjected to cold -plus-restraint stress. These data suggest that the cytoprotective effect of PRL on development of gastric ulcers in stressed animals may involve both central (i.e. dopamine transmission) and peripheral (i.e. prostaglandin synthesis) mechanisms.     

Influence of dopamine energic pharmacology drugs on secretion of the adrenocorticotropic hormone from the hypophysis

To elucidate the role of dopamine as a neuromediator in the adrenocorticotropic hormone (ACTH) secretion, investigations were carried out with dopaminergic pharmacology drugs on male white Wistar rats. In the first series of experiments, the effects of 200 mg/kg body wt L-DOPA, of the combination of 200 mg/kg L-DOPA and 50 mg/kg body wt carbidopa, and of 2.5 mg/kg body wt bromocriptine, after a single intraperitoneal injection of ACTH in the serum of rats after 30, 90 and 120 min, following the injection, were studied. In the second series of experiments, the effect of 200 mg/kg body wt L-DOPA, of the combination of 200 mg/kg body wt L-DOPA and 50 mg/kg body wt carbidopa, of 1 mg/kg body wt bromocriptine, after intraperitoneal injection, on the concentration of ACTH in the serum within 7 days, were assessed. The inhibition of agonists of dopamine after ACTH secretion with repeated application has been shown. Using a radioimmunology assay with test kits, the amount of ACTH in the serum was determined.     

Dopamine antibodies in the pathogenesis of parkinsonism]

L-DOPA and dopamine (DA) binding antibodies were found in the blood serum of Parkinsonian patients and middle-aged and elderly normal persons. DA-binding serum gamma-globulins of parkinsonian patients injected into rat caudate nuclei induced the pathogenetic mechanism of Parkinson's syndrome (generator of pathologically enhanced excitation) in these brain part and evoked main parkinsonian symptoms (oligokinesia, rigidity, tremor). The serum gamma-globulins of Parkinsonian patients without Da-antibodies caused less pronounced EEG disturbances. Parkinsonian symptoms developed rarely and were shorter and less pronounced compared with the DA-antibody effect. The DA binding antibodies role in Parkinson's syndrome pathogenesis and is L-DOPA therapeutic tolerance formation was discussed.     

A site-specific mutation of tyrosine hydroxylase reduces feedback inhibition by dopamine in genetically modified cells grafted in parkinsonian rats

Aromatic L-amino acid decarboxylase (AADC) is necessary for conversion of L-DOPA to dopamine. Therefore, AADC gene therapy has been proposed to enhance pharmacological or gene therapies delivering L-DOPA. However, addition of AADC to the grafts of genetically modified cells expressing tyrosine hydroxylase (TH) and GTP cyclohydrolase 1 (GCH1), which produce L-DOPA in parkinsonian rats, resulted in decreased production of L-DOPA and dopamine owing to feedback inhibition of TH by dopamine. End-product feedback inhibition has been shown to be mediated by the regulatory domain of TH, and site-specific mutation of serine 40 makes TH less susceptible to dopamine inhibition. Therefore, we investigated the efficacy of using TH with serine 40 mutated to leucine (mTH) in an ex vivo gene-therapy paradigm. Primary fibroblasts (PF) from Fischer 344 rats were transduced with retrovirus to express mTH or wild-type rat TH cDNA (wtTH). Both cell types were also transduced with GCH1 to provide the obligate TH cofactor, tetrahydrobiopterin. PF transfected with AADC were used as coculture and cografting partners. TH activities and L-DOPA production in culture were comparable between PFwtTHGC and PFmTHGC cells. In cocultures with PFAADC cells, PFmTHGC cells showed significant reduction in the inhibitory effect of dopamine compared with PFwtTHGC cells. In vivo microdialysis measurement showed that cografting PFAADC cells with PFmTHGC cells resulted in smaller decreases in L-DOPA and no reduction in dopamine levels compared with cografts of PFAADC cells with PFwtTHGC cells, which decreased both L-DOPA and dopamine levels. Maintenance of dopamine levels with lower levels of L-DOPA would result in more focused local delivery of dopamine and less potential side-effects arising from L-DOPA diffusion into other structures. These data support the hypothesis that mutation of serine 40 attenuates TH end-product inhibition in vivo and illustrates the importance of careful consideration of biochemical pathways and interactions between multiple genes in gene therapy.     

Occurrence of L-DOPA and dopamine in plants and cell cultures ofMucuna pruriens and effects of 2,4-d and NaCl on these compounds

The development of the L-DOPA-content of roots, stems and leaves ofMucuna pruriens during growth of the plants is described. Besides L-DOPA, the leaves, but not the stems and the roots, also contain the related catechol dopamine. The time course of dopamine accumulation is compared to that of L-DOPA. In cell suspension cultures ofM. pruriens dopamine can be detected as well. Its level is strongly increased by addition of the growth regulator 2,4-d to the medium, a condition that suppresses cell growth and L-DOPA-accumulation. Dopamine induction appears to be a specific metabolic effect of 2,4-d. Salt stress, as caused by the addition of NaCl, gives no induction of dopamine formation, whereas L-DOPA is released into the medium.     

Long-Term L-DOPA Treatment Causes Indiscriminate Increase in Dopamine Levels at the Cost of Serotonin Synthesis in Discrete Brain Regions of Rats

(1) The treatment of choice for Parkinson’s disease (PD) is 3,4-dihydroxyphenylalanine (L-DOPA) with peripheral decarboxylase inhibitor, but long-term therapy leads to motor and psychiatric complications. In the present study we investigated 5-hydroxytryptamine (5-HT) and dopamine concentrations in serotonergic and dopaminergic nuclei following chronic administration of L-DOPA to find whether the neurotransmitter synthesis in these brain areas are compensated. (2) Rats were administered L-DOPA (250 mg/kg) and carbidopa (25 mg/kg) daily for 59 and 60 days, and killed on the 60th day, respectively at 24 h and 30 min after the last dose. L-DOPA, norepinephrine, 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), dopamine, homovanillic acid (HVA), and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured in striatum, nucleus raphe dorsalis (NRD), nucleus accumbens (NAc), substantia nigra, cerebellum, and cortex employing HPLC-electrochemical procedure. (3) Prolonged treatment of L-DOPA caused depression in the animals as revealed in a forced swim test. Serotonin content was significantly decreased in all brain regions studied 30 min after long-term L-DOPA, except in NAc. The cortex and striatum showed lowered levels of this indoleamine 24 h after 59 doses of L-DOPA. Dopamine, HVA, and DOPAC concentrations were significantly higher in all the regions studied after 30 min, and in the cerebellum after 24 h of L-DOPA. The levels of DOPAC were elevated in all the brain areas studied 24 h after prolonged L-DOPA treatment. (4) The present results suggest that long-term L-DOPA treatment results in significant loss of 5-HT in serotonergic and dopaminergic regions of the brain. Furthermore, while L-DOPA metabolism per se was uninfluenced, dopamine synthesis was severely impaired in all the regions. The imbalance of serotonin and dopamine formation may be the cause of overt cognitive, motor, and psychological functional aberrations seen in parkinsonian patients following prolonged L-DOPA treatment.     

Germination growth response of different plant species to the allelochemical L-3,4-dihydroxyphenylalanine (L-DOPA)

The aim of this study was to investigate the seed germination response of different plant families to L-3,4-dihydroxyphenylalanine (L-DOPA), one of the strongest allelochemicals in nature. Three types of responses in terms of colouration changes on filter paper were obtained; black and gray (Gramineae and Compositae), no change (Leguminosae, Brassicaceae, and Cucurbitaceae) and an obstructed-circle around the seeds with black colouration on the outer side of the circle (Hydrophyllaceae) when L-DOPA solution was applied during seed germination. Radicle growth in the Gramineae and Leguminosae families was inhibited less by a single treatment of L-DOPA solution (250 g/ml) than in the other families. However, continuous treatment with L-DOPA demonstrated that the Gramineae family was less affected in terms of the inhibition of radicle growth than the Leguminosae family. When more seeds were added to the L-DOPA solution less inhibition of radicle growth was observed in all plants tested. The EC₅₀ of L-DOPA for bluebell (Hydrophyllaceae), white clover (Leguminosae), and lettuce (Compositae) was approximately 200, 100, and 50 g/ml, respectively. However, in perennial ryegrass (Gramineae) no EC₅₀ was observed even at 250 g/ml L-DOPA. In the Gramineae family, addition of more seeds into the L-DOPA solution increased the colouration on the filter paper. These results demonstrated that each seed functions to oxidize or dissolve L-DOPA. In the Gramineae, Leguminosae, Compositae, and Hydrophyllaceae, increasing the number of seeds imbibed in the L-DOPA solution increased the rate of L-DOPA disappearance from the petri-dish. Of the Grammaceous plants tested, only perennial ryegrass, which showed fairly weak allelopathic activity, metabolised L-DOPA to dopamine. Although the relationships between the changes in colouration of the filter paper and the inhibition of radicle growth in these experiments are still unknown, there appears to be a strong response in each species to protect the cell from L-DOPA damage.     

Neurochemical changes following the administration of precursors of biogenic amines.

Male Wistar rats were injected intraperitoneally with either saline, L-trytophan, D,L-5hydroxytryptophan, L-tyrosine, L-DOPA or choline and killed by the near-freezing method 15 and 45 min after injection. The brains were removed, pulverized and acetylcholine, dopamine, norepinephrine, 5-hydroxytryptamine, aspartate, glutamate, glycine and γ-aminobuttyric acid wwere extracted and concurrently measured using previously reported methods. Compared to saline injected controls, some precursors not only resulted in changes in the specific neurotransmiter systems being pertubated, but also changes in the content of other neurotransmitters.