Effect of the NMDA receptor antagonist, MK-801, on locomotor activity and on the metabolism of dopamine in various brain areas of mice.

Various doses (0.1-0.5 mg/kg i.p.) of the N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, produced a dose-dependent increase in well-coordinated locomotor activity of NMRI mice. Higher doses (greater than 0.5 mg/kg) produced a typical motor syndrome characterized by head weaving, body rolling, ataxia and salivation. MK-801, 0.2 mg/kg i.p., a dose which produced marked locomotor stimulation, increased the rate of disappearance of dopamine in the striatum and in the limbic forebrain of the animals, whereas the rate of disappearance of noradrenaline remained unchanged in the limbic forebrain and in the hippocampus. MK-801 increased the rate of tyrosine hydroxylation (measured as the accumulation of 3,4-dihydroxyphenylalanine (DOPA) after inhibition of DOPA decarboxylase) in the striatum with no change in DOPA formation in the limbic forebrain. The levels of 3,4-dihydroxyphenylacetic acid (DOPAC) remained unchanged both in the striatum and in the limbic forebrain following the administration of MK-801. It is concluded that MK-801 may facilitate the activation of dopaminergic mechanisms through an indirect (perhaps by reducing glutamatergic activity) rather than a direct effect on dopamine neurons.

Postsynaptic dopamine/adenosine interaction: I. Adenosine analogues inhibit dopamine D2-mediated behaviour in short-term reserpinized mice.

Mice pretreated with reserpine 5 mg/kg (4 h prior to the start of motor activity recording) showed locomotor activation after the administration of the D-2 agonist bromocriptine (5 mg/kg). This bromocriptine-induced locomotor activity was dose dependently inhibited by the co-administration of a D-2 antagonist (sulpiride) and dose dependently potentiated by a D-1 agonist (CY 208-243). The potentiating effect of the D-1 agonist could be inhibited by either a D-1 or a D-2 antagonist (SCH 23390 1 mg/kg or sulpiride 100 mg/kg, respectively). The bromocriptine-induced locomotor activity was not altered by either blockade of D-1 dopaminergic receptors (SCH 23390 1 mg/kg) or by co-administration of a greater dose of reserpine (10 mg/kg) plus the dopamine synthesis inhibitor, alpha-methyl-p-tyrosine (200 mg/kg). The adenosine agonists, L-PIA (a preferentially A-1 adenosine agonist) and NECA (an A-1 and A-2 adenosine agonist with above 10-fold greater affinity for A-2 than L-PIA) inhibited in a dose-dependent manner the effect of bromocriptine, NECA being above ten times more potent than L-PIA. The findings show that bromocriptine stimulates postsynaptic D-2 receptors in dopamine-depleted mice and that this effect can be inhibited by adenosine stimulation. The existence of a postsynaptic D-2/A-2 interaction is suggested, the stimulation of A-2 receptors causing an inhibition of responses elicited by postsynaptic D-2 stimulation.

Postsynaptic dopamine/adenosine interaction: II. Postsynaptic dopamine agonism and adenosine antagonism of methylxanthines in short-term reserpinized mice.

Caffeine and its first-stage metabolites (paraxanthine, theophylline and theobromine) caused a significant potentiation of the locomotor activity induced by bromocriptine, 5 mg/kg, in mice pretreated with reserpine, 5 mg/kg (4h prior to the start of motor activity recordings). None of these substances significantly enhanced locomotor activity in reserpinized mice when administered alone. The rank order of potency was caffeine greater than paraxanthine greater than theophylline greater than theobromine. A high dose of a D-2 antagonist (sulpiride 100 mg/kg) caused a marked inhibition of the locomotor activity induced by bromocriptine, 5 mg/kg, plus 25 mg/kg of caffeine, paraxanthine or theophylline. However, a high dose of a D-1 antagonist (SCH-23390 1 mg/kg) caused a significant decrease of the locomotor activity induced by bromocriptine 5 mg/kg, plus 25 mg/kg of caffeine or paraxanthine, but did not change the locomotor activity caused by bromocriptine, 5 mg/kg, plus theophylline 25 mg/kg. The inhibitory effect of 5'-(N-ethyl)carboxamido-adenosine (NECA), 0.025 mg/kg, on bromocriptine-induced locomotor activation in reserpinized mice was reversed by the simultaneous administration of 10, 25 and 50 mg/kg of caffeine, paraxanthine or theophylline. The rank order of potency for reversal was theophylline greater than paraxanthine = caffeine. We suggest that methylxanthines act postsynaptically by potentiating the effects of D-2 stimulation and that this potentiation can be produced by D-1 agonism (paraxanthine or caffeine) and by adenosine antagonism (theophylline, paraxanthine or caffeine), most probably involving A-2 receptors.

B-HT 920 is a full agonist at both pre- and postsynaptic D-2 dopamine receptors.

Stimulation of presynaptic D-2 dopamine receptors by B-HT 920 or by apomorphine inhibited the synthesis of dopamine in the corpus striatum of gammabutyrolactone-treated mice to about the same extent. Stimulation of postsynaptic D-2 dopamine receptors by B-HT 920 given in combination with the D-1 receptor agonist SKF38393 enhanced the motor activity of reserpine-treated mice at least as much as observed following the combined D-1/D-2 receptor agonist apomorphine. Since B-HT 920 is as effective as apomorphine in these models, B-HT 920 appears to be a full agonist at both pre- and post-synaptic D-2 dopamine receptors.

Stimulation of postsynaptic D2- dopamine receptors by B-HT 958 is revealed by co-treatment with the D1- receptor agonist SKF 38393.

The motor activity of reserpine-treated mice was used to study effects of B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo-[5,4-d]- azepine) on postsynaptic dopamine and noradrenaline receptors. The motor activity was only slightly stimulated by B-HT 958 or by the D1- receptor agonist SKF 38393 but it was markedly increased by the two drugs given in combination. The effect of B-HT 958 peaked earlier following low rather than high doses. The enhanced motor activity was inhibited by the D2- receptor antagonist sulpiride or the D1- receptor antagonist SCH 23390, indicating that it was caused by stimulation of both receptor types. The results suggest that B-HT 958 stimulates postsynaptic D2- receptors in addition to D2- autoreceptors and that its blockade of postsynaptic alpha 2-adrenoceptors is of no importance for the motor activity.

Transfer of DOPA from the sympatho-adrenal system to the pancreas, liver and kidney via the blood circulation.

The concentration of 3,4-dihydroxyphenylalanine (DOPA) was low in the pancreas, liver, kidney, spleen, salivary glands, heart and adrenal glands of untreated mice, but increased following inhibition of DOPA decarboxylase by 3-hydroxybenzylhydrazine. The ratio of the accumulation of DOPA to the concentration of noradrenaline (i.e. the density of sympathetic nerves) was greater in the kidney, liver and, particularly, pancreas than in the other organs studied, suggesting that DOPA occurred outside sympathetic nerves in these organs. The tyrosine hydroxylase inhibitor alpha-methyltyrosine almost completely inhibited the accumulation of DOPA in all organs. The DOPA accumulation was enhanced in all organs by the alpha-adrenoceptor antagonists phentolamine and yohimbine. The results indicate that the DOPA was formed in the sympatho-adrenal system. In the pancreas, liver and kidney, most of the DOPA accumulated might have been formed outside these organs and transferred there via the bloodstream.

Neurotransmitters, neuropeptides and binding sites in the rat mediobasal hypothalamus: effects of monosodium glutamate (MSG) lesions.

Indirect immunofluorescence histochemistry and receptor autoradiography were used to study the localization of transmitter-/peptide-containing neurons and peptide binding sites in the mediobasal hypothalamus in normal rats and in rats treated neonatally with repeated doses of the neurotoxin monosodium-glutamate (MSG). In the arcuate nucleus, the results showed a virtually complete loss of cell bodies containing immunoreactivity for growth hormone-releasing factor (GRF), galanin (GAL), dynorphin (DYN), enkephalin (ENK), corticotropin-like intermediate peptide (CLIP), neuropeptide Y (NPY), and neuropeptide K (NPK). Tyrosine hydroxylase(TH)-glutamic acid decarboxylase(GAD)-, neurotensin(NT)- and somatostatin(SOM)-immunoreactive (IR) cells were, however, always detected in the ventrally dislocated, dorsomedial division of the arcuate nucleus. In the median eminence, marked decreases in numbers of GAD-, NT-, GAL-, GRF-, DYN-, and ENK-IR fibers were observed. The numbers of TH-, SOM- and NPY-IR fibers were in contrast not or only affected to a very small extent, as revealed with the immunofluorescence technique. Biochemical analysis showed a tendency for MSG to reduce dopamine levels in the median eminence of female rats, whereas no effect was observed in male rats. Autoradiographic studies showed high to moderate NT binding sites, including strong binding over presumably dorsomedial dopamine cells. In MSG-treated rats, there was a marked reduction in GAL binding in the ventromedial nucleus. The findings implicate that most neurons in the ventrolateral and ventromedial arcuate nucleus are sensitive to the toxic effects of MSG, whereas a subpopulation of cells in the dorsomedial division of the arcuate nucleus, including dopamine neurons, are not susceptible to MSG-neurotoxicity. The results indicate, moreover that the very dense TH-IR fiber network in the median eminence predominantly arises from the dorsomedial TH-IR arcuate cells, whereas the GAD-, NT-, GAL-, GRF- and DYN-IR fibers in the median eminence to a large extent arise from the ventrolateral arcuate nucleus. Some ENK- and NPK-positive cells in the arcuate nucleus seem to project to the lateral palisade zone of the median eminence, but most of the ENK-IR fibers in the median eminence, located in the medial palisade zone, seem to primarily originate from an area(s) located outside the arcuate nucleus, presumably the paraventricular nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of D1 dopamine receptors reveals direct effects of the preferential dopamine autoreceptor agonist B-HT 920 on postsynaptic dopamine receptors.

Possible postsynaptic effects of the preferential dopamine autoreceptor agonist B-HT 920 were studied by means of the mouse motor activity. In reserpine-treated mice, B-HT 920 did not cause any motor activity by itself but it markedly potentiated the slight stimulating effect of the D1 dopamine agonist SKF 38 393. The effect was blocked by either the D2-receptor antagonist sulpiride or the D1-receptor antagonist SCH 23 390, indicating that motor activity is dependent on simultaneous activation of both dopamine receptor types. The hyperactivity produced by 0.1 mg kg-1 B-HT 920 in combination with SKF 38 393 in reserpine-treated mice was at least as great as that following a maximal dose of apomorphine, indicating that B-HT 920 is a full agonist at postsynaptic D2 receptors. The effect of 0.1 mg kg-1 B-HT 920 peaked earlier than those of 1 mg kg-1 and particularly, 10 mg kg-1 suggesting additional effects of the later two doses. B-HT 920 stimulates dopamine autoreceptors almost maximally following 0.1 or 1 mg kg-1 but only the latter dose (with or without SKF 38 393) caused hyperactivity of mice not treated with reserpine. This finding indicates that the postsynaptic D2 receptors are less sensitive to B-HT 920 than the D2 dopamine autoreceptors.

Inhibitory role of D-1 dopamine receptors for the jerks induced by B-HT 920 in rats.

Jerks of the head and upper trunk produced by the dopamine agonist B-HT 920 in reserpine-treated rats were abolished by the D-1 dopamine receptor agonist SKF 38393. The combined treatment with SKF 38393 and B-HT 920 instead resulted in stereotypies and locomotion. B-HT 920 also caused jerks when given after the D-1 receptor antagonist SCH 23390 to rats not pretreated with reserpine. The results indicate that B-HT 920 induces jerks by stimulation of postsynaptic D-2-like dopamine receptors provided that the D-1 receptors are not activated.

Are the alpha 2-adrenoceptors in the noradrenaline cell body region of physiological significance?

Preganglionic stimulation of the cervical sympathetic increased the content of 3,4-dihydroxyphenylacetic acid (DOPAC) in the superior cervical ganglion of rats treated with the dopamine beta-hydroxylase inhibitor FLA-63. It also increased and decreased the concentrations of dopamine and noradrenaline, respectively, in the salivary gland. The effects were partially inhibited by the alpha 2-adrenoceptor agonist clonidine via a yohimbine-sensitive mechanism. The alpha 2-adrenoceptor antagonist yohimbine by itself did not enhance the stimulation-evoked increase in ganglionic DOPAC, but it markedly potentiated the stimulation-evoked changes in dopamine and noradrenaline in the salivary gland. The results indicate that there are inhibitory alpha 2-adrenoceptors both in the somatodendritic and in the axon terminal region of the noradrenaline neurons but that only the alpha 2-receptors of the axon terminals are physiologically stimulated.

Stimulation of the synthesis of catecholamines in a sympathetic ganglion via cholinergic and non-cholinergic mechanisms.

Electrical stimulation of the preganglionic sympathetic neurons rapidly and markedly elevated the contents of the primary dopamine (DA) and noradrenaline (NA) metabolites, i.e., 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylethylene glycol (DOPEG) in the superior cervical ganglion and it enhanced the accumulation of DOPAC and DA following inhibition of the DA-beta-hydroxylase by FLA-63. The stimulation also increased the concentration of DA and decreased the concentration of NA in the salivary gland both without and with DA-beta-hydroxylase inhibition. Chlorisondamine inhibited the increase in ganglionic DOPAC following preganglionic stimulation (5 Hz, 30 min) by 30-50% and it even better reduced the biochemical changes in the salivary gland. Atropine did not produce any clearcut inhibition of the stimulation-induced effects on the superior cervical ganglion or the salivary gland, nor did it enhance the effect of chlorisondamine. The results suggest that nicotine, but not muscarine receptors in the cell body region of the postganglionic NA neurons partially mediate the effects of preganglionic stimulation. The effects remaining after blockade of the nicotine and muscarine receptors might be due to release of a neuropeptide acting on a special receptor. The stimulation-induced increase in the concentration of DOPAC in the superior cervical ganglion might, at least partly, be the result of a depolarization of the NA nerve cell body regions since similar changes were produced by electrical stimulation of the chronically decentralized ganglion.

Very rapid turnover of dopamine in noradrenaline cell body regions.

The contents of dopamine, noradrenaline and their deaminated metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylethylene glycol (DOPEG) were determined in rats in two noradrenaline cell body regions, i.e., the superior cervical ganglion and the locus coeruleus, and in one dopamine cell body region, the substantia nigra. In the two noradrenaline cell body regions, the tyrosine hydroxylase inhibitor alpha-methyltyrosine rapidly lowered the contents of noradrenaline and DOPEG and it lowered the contents of dopamine and DOPAC even more rapidly. The dopamine-beta-hydroxylase inhibitor FLA-63 swiftly elevated the content of dopamine and it lowered the content of noradrenaline in the two noradrenaline regions, but it was ineffective in the substantia nigra. The monoamine oxidase inhibitor pargyline rapidly reduced the deaminated catechols and increased somewhat the contents of the two amines in the superior cervical ganglion and in the locus coeruleus. The alpha-methyltyrosine-induced disappearance of dopamine in the two noradrenaline cell body regions was markedly inhibited by FLA-63 and pargyline in combination, but not by only one of the two drugs. The results indicate that most of the dopamine in the superior cervical ganglion and in the locus coeruleus occurs in the cell body region of noradrenaline neurons, whereas only a minor part of the dopamine in the superior cervical ganglion is present in SIF cells. Axonal transport did not contribute to the disappearance of dopamine in the superior cervical ganglion. A high dose of reserpine reduced the contents of dopamine and noradrenaline in the superior cervical ganglion and in the locus coeruleus, indicating that most of the amines is present in storage granules.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and utilization of catecholamines in the rat superior cervical ganglion following changes in the nerve impulse flow.

The nerve impulse flow to the noradrenaline nerve cell body region of the superior cervical ganglion and to the noradrenaline nerve terminals of the salivary glands was decreased and increased by decentralization and pre-ganglionic electrical stimulation, respectively. The concentrations of dopamine, noradrenaline, and their primary deaminated metabolites (DOPAC, DOPEG) in the ganglion were not changed during the first day after the decentralization but were increased following preganglionic stimulation, particularly that of DOPAC. The disappearances of dopamine and noradrenaline after alpha-methyltyrosine and the disappearance of noradrenaline after inhibition of the dopamine-beta-hydroxylase were not changed by decentralization or stimulation. The accumulation of dopamine following inhibition of the dopamine-beta-hydroxylase was somewhat lowered by decentralization and was markedly enhanced by stimulation. In the noradrenaline nerve terminals of the salivary glands, preganglionic sympathetic stimulation decreased and increased the concentration of noradrenaline and dopamine, respectively. Thus, nerve impulses stimulated the tyrosine hydroxylase activity in the cell body region and the axon terminals of the noradrenaline neurons but they increased the utilization of noradrenaline only in the terminals.

B-HT 958 stimulates dopamine autoreceptors but blocks noradrenaline autoreceptors in the brain.

B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo 5,4-d azepine) blocked the gamma-butyrolactone-induced increase in the synthesis of dopamine and slowed down the alpha-methyltyrosine-induced disappearance of dopamine in the mouse brain by haloperidol-sensitive mechanisms. In reserpine-treated mice, B-HT 958 produced a most a weak locomotion and no change in the apomorphine-induced increase in motor activity. The motor activity of normal mice was reduced by B-HT 958. At high doses, B-HT 958 accelerated the alpha-methyltyrosine-induced disappearance of noradrenaline and it inhibited the effects of clonidine on the turnover and on the synthesis of noradrenaline in the mouse brain. The findings indicate that the dopamine autoreceptors can be selectively stimulated by B-HT 958 but that the alpha 2-adrenoceptors can be blocked following high doses.

Organic acid transport to the blood from the corpus striatum, the thalamus and the cerebellum of the rat.

Conscious rats were given intracerebral injections by preplaced microsyringes. The injectates were 0.3-0.5 microliters of 125I- and 131I-o-iodohippurate. One hour after injection the isotopes present in the unopened cranial cavity were measured by gamma spectrometry. Some animals received 200 mg/kg probenecid intraperitoneally and this reduced the rate of absorption from injectates into the corpus striatum to 65.7 +/- 12.6% of control; from injectates into the cerebellum to 57.1 +/- 9.8% of control. Dye injections showed that injections into the cerebellum did not remain in the parenchyma, in contrast to injections into the corpus striatum or thalamus. Probenecid was also given as 2.9% solution, pH 7, mixed with the iodohippurate in the microsyringe. It had no effect on injections into the cerebellum, reduced the rate of absorption from the corpus striatum to 80.9 +/- 3.2% of controls and that from the thalamus to 88.3 +/- 2.2%. The results indicate parenchymal probenecid-sensitive transport of iodohippurate from the corpus striatum and thalamus but failed to settle the matter for the cerebellum.

Formation of deaminated metabolites of dopamine in noradrenaline neurons.

The deaminated monoamine metabolites 3-methoxy-4-hydroxyphenylethyleneglycol (MOPEG), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were determined electrochemically following organic solvent extraction and reverse-phase, high performance, liquid chromatography in four regions of the mouse brain. In the noradrenaline (NA)-predominant regions (hemispheres, brain stem), the ratio of the concentrations of DOPAC plus HVA to NA plus dopamine (DA) was approximately the same as in the DA-predominant regions (corpus striatum, limbic system). Yohimbine and reserpine elevated the concentrations of DOPAC and HVA both in the NA- and the DA-predominant regions. The effect of yohimbine was somewhat enhanced by the alpha 1-receptor blocking agent prazosin in the NA-predominant regions. The concentration of MOPEG was increased by yohimbine and decreased by reserpine. The concentrations of DOPAC and HVA were lowered by clonidine, but not by apomorphine in the NA-predominant regions of reserpine-treated mice. In the DA-predominant regions, apomorphine, but not clonidine, reduced the concentrations of DOPAC and HVA. The effects of clonidine and apomorphine were reversed by yohimbine and haloperidol, respectively. The results indicate that the concentrations of the acid DA metabolites DOPAC and HVA in the NA-predominant regions reflect the rate of synthesis of DA in the NA neurons.

Synthesis rate of dopamine: difference between corpus striatum and limbic system as a possible explanation of variations in reactions to drugs.

The synthesis rate of dopamine and noradrenaline in different parts of the rat brain was estimated by measuring the accumulation of DOPA following decarboxylase inhibition. Although the synthesis of dopamine was enhanced in all regions by haloperidol and gammabutyrolactone, it was increased to almost 400% of the control in the corpus striatum but to only somewhat more than 200% in the olfactory tubercle and some other limbic regions. The dopamine autoreceptor agonist B-HT 920 was more potent and effective in lowering the synthesis of dopamine in the limbic regions than in the corpus striatum. These differences between the extrastriatal and striatal brain areas in the pharmacological responses might be explained by a 50% higher norma synthesis rate constant of the dopamine in the limbic system than in the corpus striatum. The lower dopamine synthesis rate in the latter region may be due to a negative neuronal feedback from the corpus striatum to the substantia nigra. Functional studies support this proposal. Indeed, B-HT 920 was less effective than haloperidol in changing the direction of head turning of rats with a diencephalic hemisection at doses equieffective in reducing the motor activity. The findings indicate that B-HT 920 is relatively more potent than haloperidol in inhibiting the dopamine neurotransmission in the limbic system than in the corpus striatum.