Monoamine metabolism in rat brain regions following long term alcohol treatment

Summary Female Wistar rats (150–200 g) were treated with ethanol (15% w/v) for 21 days and compared with control rats given water. Ethanol administration produced a reduction of fluid and food consumption and changes in the metabolism of cerebral monoamines. There was an increase in serotonin (5-HT) turnover statistically significant in the striatum, and a decrease in noradrenaline (NA) turnover in ethanol rats as compared to controls.Endogenous NA levels were significantly increased in the diencephalon and dopamine (DA) levels were increased in the striatum. After inhibition of catecholamine synthesis with-methyltyrosine (-MT), NA depletion was significantly retarded but no changes in DA depletion were noted. DOPA accumulation after decarboxylation inhibition showed no significant change in any brain region studied.     

Hyperthyroidism: Specifically increased response to central NA-(α-)receptor stimulation and generally increased monoamine turnover in brain

Summary Repeated treatment with thyroxine (T₄) caused in mice enhanced response,i.e. locomotor stimulation, to central noradrenaline (NA)-receptor activation by clonidine but not to central dopamine (DA)-receptor activation by apomorphine or ET 495 after previous depletion of endogenous catecholamines (CA) by reserpine and inhibition of the CA-synthesis by-methyl-p-tyrosine. Increasing the dosese of the receptor agonists in control animals did not increase the locomotor stimulation but merely prolonged the effect. In chronically T₄-treated rats with a significantly elevated plasma level of T₄ and T₃ the apomorphine-induced stereotypies were not increased and the response to small amounts of DA locally applied in N. accumbens was not significantly affected by the T₄-pretreatment. The synthesis rates of NA and DA, reflected in the amount of L-Dopa accumulated in predominantly NA- and DA-rich brain parts, respectively, during 30 min after administration of the centrally active, aromatic L-amino acid decarboxylase inhibitor NSD 1015, were both increased by T₄-pretreatment. Also the brain serotonin (5-hydroxytryptamine, 5-HT)-synthesis was increased, measured in an analogous way. In contrast, utilization of brain CA or 5-HT seemed not affected by the T₄-pretreatment, as disclosed by unchanged disappearance rates of NA, DA or 5-HT after inhibition of tyrosine hydroxylase and tryptophan hydroxylase, respectively. Release of brain CA, as reflected in the amount of O-methylated metabolites after monoamine oxidase inhibition, was not affected by the hormone pretreatment. We conclude that moderate hyperthyroidism is associated with a specific, increased sensitivity at or beyond central NA-(-)receptors. This effect may be implicated in: 1. The potentiation by thyroid hormone of tricyclic antidepressive drugs; 2. the affective symptoms in hyperthyroidism,e.g. anxiety and increased susceptibility to emotional stress; 3. the cardiovascular symptoms in hyperthyroidism.     

Effect of gammahydroxybutyric acid on catecholamine synthesis and utilization in the developing rat brain

Summary Gammahydroxybutyric acid (GHBA) was administered subcutaneously, 750mg/kg, to 1, 4, 14 and 28 days old rats 30 or 90 min before sacrifice. Whole brain and regional brain levels of tyrosine, dopamine (DA) and noradrenaline (NA) were measured. In some experiments the tyrosine hydroxylase activity was studied by measuring the accumulation of dihydroxyphenylalanine (DOPA) after inhibition of aromatic L-aminoacid decarboxylase. GHBA induced an increase in tyrosine and DA levels at the various ages except at 1 day of postnatal age. The effect of GHBA on the accumulation of DOPA after inhibition of aromatic L-aminoacid decarboxylase varied with age. Thus, tyrosine hydroxylase activity seemed to be enhanced in the 4 days old rats after 90 min and after 30 min in the 28 days old rats. Ninety minutes after GHBA administration to the 28 days old animals, DOPA accumulation reached or was slightly below control levels. Brain NA levels were not affected by GHBA administration. Regional analysis of DA and NA after inhibition of tyrosine hydroxylase with-methyl-tyrosine demonstrated a reduced disappearance of DA after GHBA in the striatum region already from 4 days of postnatal age. GHBA administration did not affect the nerve impulse release of NA in any of the brain regions studied.It may be concluded that GHBA acts inhibitory on brain DA neurons during early postnatal development.     

Early effects of monoamine oxidase inhibitors on dopamine metabolism and monoamine oxidase activity in the neostriatum of the rat

Summary Dopamine levels in the neostriatum of the rat were estimated at various times after pargyline (75 mg/kg), pheniprazine (4 mg/kg) and tranylcypromine (20 mg/kg) intraperitoneal injections. Depending on the inhibitor used, the amine levels reached 135 to 150% of control values within 10 min. During the 20 min which followed this rapid linear rise, DA levels increased in tissues at a much slower rate. As indicated by the estimation of MAO activity using¹⁴C-tyramine or kynuramine as substrates, the three drugs induced a complete inhibition of the enzyme activity within 5 min. A similar effect was seen after pheniprazine and tranylcypromine with¹⁴C-dopamine as substrate, but the maximal inhibition never exceeded 80% after pargyline treatment. As revealed by the in vivo 3 min initial accumulation of³H-H²O seen at the end of a local infusion of L. 3. 5-³H-tyrosine, the rate of the first step of DA synthesis was unchanged 5 min after pargyline treatment but significantly reduced 10 min after the drug injection. This explains the sudden interruption in the rise of DA levels observed shortly after the MAO inhibitors injection. DA synthesis rate was calculated from the initial (5 min) rate of DA accumulation and was found to be 148 to 190 nmole/g/hr depending on the inhibitor used.     

Temporal changes in catecholamine synthesis of rat forebrain structures after axotomy

Summary Axotomy was performed by means of a complete transverse cerebral hemisection in rats at the level of the caudal hypothalamus. An inhibitor of the aromatic amino acid decarboxylase, NSD 1015 (3-hydroxybenzyl-hydrazine) was injected at the time of hemisection or 1 to 72 h post operation and the animals killed 30 min later. DOPA, accumulating as a consequence of decarboxylase inhibition, tyrosine, noradrenaline (NA), and dopamine (DA) were measured. DOPA accumulation on the lesioned side was increased four fold in the corpus striatum and 50% in the dopamine-rich part of the limbic forebrain when measured directly after hemisection. When measured 1 1/2 h after hemisection DOPA synthesis had almost returned to control values and stayed constant up to 12 h, while DA levels had increased in c. striatum and limbic forebrain to 90%, and 70% above the respective control value. After 1–3 days the tyrosine hydroxylation rate on the lesioned side declined below control values, and was accompanied by a drastic fall in DA concentration. There were only slight changes on the intact side. In the predominantly NA-innervated hemisphere portion changes in DOPA synthesis were less marked.The data give support to the hypothesis that axotomy of dopaminergic neurons gives rise to an increase in tyrosine hydroxylation due to a receptor-mediated feedback mechanism. The stimulation of DOPA synthesis is probably antagonized by end product inhibition at times when DA levels are high. The rapid rise in DA after axotomy is caused by increased synthesis, while at the same time DA release and metabolisn are reduced. The marked decrease in tyrosine hydroxylation rate and DA, setting in about 1 day after axotomy is probably due to anterograde degeneration.With 2 FiguresFor a preliminary report, seeKehr et al. (1973).     

Regional concentrations of noradrenaline and dopamine in the frontal cortex of the rat: dopaminergic innervation of the prefrontal subareas and lateralization of prefrontal dopamine

Catecholamine levels in the two subareas of the prefrontal cortex and in one non-prefrontal region of the rat frontal lobe were measured radioenzymatically. In contrast with noradrenaline (NA), the distribution of dopamine (DA) in the frontal lobe is markedly heterogeneous. DA levels of the orbitofrontal and medial prefrontal subarea are, respectively, 3 and 4 times higher than those of a non-prefrontal region of the frontal lobe, confirming the expectation of neuroanatomical findings. Furthermore, it appears that at the population level, DA levels of the medial prefrontal subarea are lateralized, the left hemisphere being significantly higher than the right hemisphere.     

Effect of the activation of alpha-adrenoreceptors on the synthesis and release of noradrenaline by peripheral adrenergic nerves in vivo

Summary The synthesis and release of noradrenaline (NA) in the heart and submaxillary glands were studied in the rat following s.c. injections of oxymetazoline (50g/kg) or noradrenaline (500g/kg). NA release was evaluated from the decline in tissular specific radioactivity after administration of³H-NA and NA synthesis by the estimation of the amounts of³H-NA synthesized from³H-tyrosine (TY) or³H-Dopa, 30 min after the injection.Oxymetazoline treatment delayed the release of NA, the NA biological half-lives rising from 12 up to 36 hours in the heart and from 5.9 up to 21 hours in sub-maxillary glands. This inhibitory effect on NA release was interpreted as the consequence of the stimulation of -adrenoreceptors. Thirty minutes after its injection, oxymetazoline increased both NA endogenous levels and³H-NA amounts formed from³H-TY:³H-NA specific activities were not significantly altered.NA treatment led to an acceleration of NA release in the heart (NA biological half-life decreasing from 12 to 2.2 hours) but not in sub-maxillary glands. After injection of³H-TY, the amounts of³H-NA found in the heart and sub-maxillary glands were strongly reduced. Similar results were observed in the heart using³H-Dopa as a precursor. These data are interpreted as the consequence of the removal of the newly synthesized³H-NA by exogenous NA.The results obtained with oxymetazoline point out a dissociation between the NA release which is reduced and the NA synthesis which is unaltered. This indicates that NA synthesis rate by sympathetic nerve terminals is not immediately regulated by its release intensity. These data do not support the end-product feedback inhibition hypothesis according to which tyrosine hydroxylase is regulated by the intraneuronal NA concentration.     

Dose-dependent pharmacokinetics of α-methyl-p-tyrosine (α-MT) and comparison of catecholamine turnover rates after two doses of α-MT

Summary Groups of rats were injected i.p. with 0.407 or 1.02 mmoles/kg of D, L--methyl-p-tyrosine methylester HCl (-MT). The time-courses for-MT in plasma and brain were followed together with the endogenous brain dopamine (DA) and noradrenaline (NA) contents.The elimination of-MT from plasma and brain was markedly delayed after the high-MT dose compared with the low dose. At 40 hours after the injection of 1.02 mmoles/kg of-MT both plasma and brain levels were high, whereas no-MT could be detected in plasma or brain at 16 hours after the lower dose.The brain catecholamines were decreased to very low values after the higher-MT dose (DA 14% and NA 10% of controls at 8 and 24 hours respectively). There was no complete recuperation at 40 hours of any of the amines. After the lower-MT dose, the DA concentration was back to control levels at 16 hours and NA at 12 hours. Between 16–40 hours after the high-MT dose a majority of the rats showed prominent signs of sedation, weight loss and dehydration. No such signs were observed in rats receiving 0.407 mmoles/kg. During the first hour after the-MT injection the declines of DA and NA respectively were almost identical for both-MT doses. When the whole time-course (0–8 hours) after the high dose was considered, biphasic declines were obtained for both DA and NA, suggesting at least two different catecholamine pools. However, due to toxic effects after the high-MT dose, turnover data have to be interpreted with caution.     

Comparison of the effects of dopamine D 1 and D 2 receptor antagonists on rat striatal,limbic and nigral dopamine synthesis and utilisation

Summary The effects of dopamine (DA) antagonists upon DA synthesis and utilisation in the rat striatum, olfactory tubercle and substantia nigra have been studied. The concentrations of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), the rate of depletion of DA afterin vivo inhibition of tyrosine hydroxylase by H 44/68, and the accumulation of L-DOPA afterin vivo inhibition of 1-aromatic amino acid decarboxylase by NSD 1015 were measured in the study. Haloperidol (0.23mol/kg i. p.), sulpiride (293mol/kg i. p.) and remoxipride (5.6mol/kg i. p.) increased both DA synthesis and utilisation in the striatum and olfactory tubercle. A lower dose of sulpiride (45mol/kg i. p.) increased DA synthesis and utilisation in the olfactory tubercle alone. None of the compounds, at the doses used, affected either DOPAC and HVA concentrations or the rate of utilisation of DA in the substantia nigra. Sulpiride (293mol/kg i. p.) and remoxipride, however, produced a modest rise in nigral DA synthesis. The dopamine D 1-selective antagonist SCH 23390 had only modest effects on striatal, limbic and nigral DA synthesis and utilisation at the doses tested (0.078 and 0.36mol/kg i. p.).     

Dihydalazine inhibits tyrosine hydroxylasein vivo in the rat

Summary The effect of dihydralazine on monoamine metabolism in the rat was investigated. Dihydralazine, 5 mg/kg i.v., reduced noradrenaline (NA) in the heart. After pretreatment with phenoxybenzamine an NA depletion was evident also in the brain. Dihydralazine did not affect the utilization of NA or dopamine in the brain as judged by the disappearance rates of these amines following synthesis inhibition by-methyl-p-tyrosine. However, dihydralazine reduced the synthesis of monoamines as evidenced by a decreased accumulation of the monoamine precursor dihydroxyphenylalanine (and 5-hydroxytryptamine) subsequent to treatment with NSD 1015.It is concluded that dihydralazine inhibits central tyrosine hydroxylase (and tryptophan hydroxylase) in the rat.     

Comparison of dopamine synthesis regulation in the terminals of nigrostriatal,mesolimbic, tuberoinfundibular and tuberohypophyseal neurons

Summary The rate of accumulation of DOPA after the administration of a decarboxylase inhibitor (NSD 1015) was determined in the striatum, olfactory tubercle, median eminence and posterior pituitary of the rat brain in order to obtain an index of the rate of synthesis of dopamine (DA) in the terminals of nigrostriatal, mesolimbic, tuberoinfundibular and tuberohypophyseal neurons, respectively. In all brain regions an increase in the concentration of DA by the administration of a monoamine oxidase inhibitor decreased DOPA accumulation while a decrease in the concentration of DA by the administration of reserpine increased DOPA accumulation. These results indicate that end product inhibition plays a role in regulating DA synthesis in all four neuronal systems. Injections of DA agonists decreased and DA antagonists increased the accumulation of DOPA in striatum, olfactory tubercle and posterior pituitary, but not in median eminence. The administration of-butyrolactone (GBL) and baclofen increased the concentration of DA and the accumulation of DOPA in the striatum, olfactory tubercle and posterior pituitary, and these effects were reversed by the administration of apomorphine. On the other hand, GBL and baclofen had no effect on the concentration of DA or the accumulation of DOPA in the median eminence. These two drugs did, however, reduce the-methyltyrosine-induced decline of DA in the median eminence suggesting that they inhibit the activity of tuberoinfundibular nerves just as they do DA nerves in other systems. These results suggest that the regulation of DA synthesis in terminals of nigrostriatal, mesolimbic and tuberohypophyseal nerves is different from that in tuberoinfundibular nerves in that the latter nerves appear to lack an autoreceptor regulatory mechanism.     

Emerging dysfunctions consequent to combined monoaminergic depletions in Parkinsonism

The loss of dopamine (DA) neurons has been the pathophysiological focus of the devastating conditions of Parkinson's disease, but depletion of DA alone in animal models has failed to simultaneously elicit both the motor and non-motor deficits of PD. The present study aimed to investigate, in rats, the respective role of dopamine (DA), noradrenaline (NA) and serotonin (5-HT) depletions on motor and non-motor behaviors and on subthalamic (STN) neuronal activity. We show that NA or DA depletion significantly decreased locomotor activity and enhanced the proportion of bursty and irregular STN neurons. Anxiety-like states required DA depletion plus the depletion of 5-HT or NA. Anhedonia and "depressive-like" behavior emerged only from the combined depletion of all three monoamines, an effect paralleled by an increase in the firing rate and the proportion of bursty and irregular STN neurons. Here, we provide evidence for the exacerbation of behavioral deficits when NA and/or 5-HT depletions are combined with DA depletion, bringing new insight into the combined roles of the three monoamines in PD.     

Antagonism of morphine-induced central stimulation in mice by small doses of catecholamine-receptor agonists

Summary The morphine (75 mg/kg i.p.) induced stimulation of motor activity in mice was significantly suppressed by small doses of central catecholamine (CA) receptor agonists, apomorphine (0.2 mg/kg) and clonidine (0.05 mg/kg). In the same dose, and at the same time interval as the behavioural stimulation was obtained, morphine did not significantly affect thein vivo rate of tyrosine hydroxylation in two dopamine (DA)-rich mouse brain regions, the corpus striatum and the limbic system, or in the noradrenaline (NA)-rich, but DA-poor hemispheres, measured as the Dopa-accumulation during 30 min after inhibition of aromatic amino-acid decarboxylase by 3-hydroxybenzylhydrazine (NSD 1015) 150 mg/kg. The apomorphine induced reduction in Dopa accumulation in the DA-rich brain regions was not significantly affected by morphine. The disappearance rate of brain NA after inhibition of tyrosine hydroxylase by-methyltyrosine methylester (250 mg/kg), the utilization of NA, was accelerated by morphine, whereas that of DA was not affected. Clonidine (0.05 mg/kg) retarded selectively brain NA utilization, and also suppressed the morphine-induced increase in NA utilization.In conclusion, morphine's stimulation of motor activity in mice, an effect which previously has been found to be correlated with its dependence producing action, could be inhibited by apomorphine or clonidine in small doses which inhibit brain DA- and NA-neurons, respectively. Thus, we have now shown the psychomotor stimulation by two euphoriant and dependenceproducing drugs, ethanol and morphine, to be suppressed by CA autoreceptor activation.     

Evidence for an increased catecholamine synthesis in rat adrenal glands following stimulation of peripheral dopamine receptors

Summary In studies on peripheral dopamine (DA) turnover in our department evidence has accumulated that changes in adrenal DA levels induced by varying degrees of neurogenic stimulation roughly reflect changes in the catecholamine (CA) synthesis rate. The question arises if changes in DA levels in rat adrenals induced by different DA D-2 receptor agonists and previously reported from our laboratory, also indicate changes in CA synthesis. After various periods of drug administration rats were killed by decapitation and tissue CA levels in adrenals and forebrain were determined by HPLC-EC. The potent inhibitor of DA--hydroxylase FLA 63 (40 mg/kg i.p.) increased adrenal DA by 186% after 1 h and by 423% after 3 h. The DA D-2 agonist quinpirole (0.2 mg/kg s.c, 30 min) itself increased adrenal DA by 55–60% compared to control. In FLA 63 pretreated rats quinpirole increased adrenal DA levels by further 127% (FLA 63 — 1 h), resp. 122% (FLA 63 — 3h) than did FLA 63 itself. The DA D-2 receptor antagonist domperidone (3 mg/kg s.c, 150 min) blocked the quinpirole effect both in saline and FLA 63 (3 h) pretreated rats. Adrenal DOP AC was changed in similar manner as adrenal DA in FLA 63 pretreated rats. No significant changes either in adrenal NA or A were observed after FLA 63 pretreatment.Under the present experimental conditions adreanal DA may thus mainly be looked upon as an intermediate in the synthesis of NA and A, and the elevation of DA induced by DA D-2 receptor stimulation as a consequence of increased catecholamine synthesis.     

The effect of β-alanine on motor behaviour,body temperature and cerebral monoamine metabolism in rat

Summary Intracerebroventricular (ICV) injection of-alanine produced a decrease in rectal temperature, inhibition of exploratory behaviour and motility, and changes in the metabolism of cerebral monoamines. Dopa and 5-HTP accumulation after inhibition of L-aromatic amino acid decarboxylase, NSD 1015 (3-hydroxybencylhydrazine HCl, 100mg/kg i.p.) was found to be significantly increased in all the dissected cerebral regions of animals treated with-alanine, as compared to the controls. Levels of tyrosine and tryptophan did not show any significant change. Endogenous levels of dopamine (DA), noradrenaline (NA), serotonin (5-HT) and 5-hydroxyindolacetic acid (5-HIAA), did not change. After inhibition of the catecholamines synthesis with-methyltyrosine (-MT), dopamine depletion was retarded and noradrenaline accelerated, but without reaching statistical significance. After intraperitoneal (i.p.) injection of-alanine, significant changes in motor behaviour were found. Body temperature and metabolism of brain catecholamine were unchanged. This lack of effect could be explained by poor penetration through the blood-brain barrier.     

Studies on rat brain catecholamine synthesis and Β-adrenoceptor number following administration of electroconvulsive shock,desipramine and clenbuterol

Summary The effects of administration to rats of repeated electroconvulsive shock (ECS), clenbuterol and desipramine (DMI) on-adrenoceptor number in cortex, and noradrenaline (NA) and dopamine (DA) turnover in whole brain has been investigated by examining the rate of decline of NA concentration (k_NA) following injection of-methyl-p-tyrosine. A single injection of clenbuterol (5 mg/kg) raised brain NA content and decreased the rate constant (k_NA), leaving the turnover rate unaltered. Acute DMI injection decreased k_NA and turnover rate, while a single ECS did not change NA metabolic rate. Repeated treatment with either ECS (5 seizures over 10 days), clenbuterol (5 mg/kg for 14 days) or DMI (5 mg/kg twice daily for 14 days) decreased-adrenoceptor density in cortex. No change in NA content, rate constant or turnover rate was observed after repeated ECS or clenbuterol administration. Ninety min after the last dose of DMI brain NA content was significantly decreased but k_NA was unchanged compared with control animals, possibly because of the presence of subsensitive presynaptic ₂-adrenoceptors. At 18 hours after the last dose brain NA content was still lower than control animals but k_NA was enhanced. This is presumably a withdrawal effect, the uptake inhibitory effect of the drug now being decreased. The treatments had little effect on DA turnover apart from DMI decreasing synthesis rate. Clearly there is no obvious relationship between the ability of antidepressant treatments to alter NA turnover and decrease-adrenoceptor number.     

Interaction of haloperidol and γ-butyrolactone with (+)-amphetamine-induced changes in monoamine synthesis and metabolism in rat brain

Summary The interaction of (+)-amphetamine with haloperidol and-butyrolactone on synthesis of monoamines in rat brain regions was investigated using anin vivo method, in which the accumulation of dopa and 5-hydroxytryptophan (5-HTP) was measured after inhibition of the aromatic amino acid decarboxylase by means of 3-hydroxybenzylhydrazine. The accumulation of 3-methoxytyramine (3-MT) after inhibition of the monoamine oxidase with pargyline was taken as an indicator of thein vivo release of dopamine (DA) into the extraneuronal space.(+)-Amphetamine at doses of 1-3 mg/kg caused an increase of dopa formation in the DA-rich areas c. striatum and mesolimbic forebrain but had no effect on dopa formation in neocortex and 5-HTP formation in all brain regions investigated. At a dose of 10 mg/kg (+)-amphetamine decreased dopa as well as 5-HTP formation in all brain regions studied.3-MT accumulation in whole rat brain was stimulated by (+)-amphetamine as well as haloperidol and inhibited by-butyrolactone.Combined treatment with haloperidol and (+)-amphetamine not only potentiated the stimulation of dopa formation but also the increase of 3-MT accumulation. Pretreatment with-butyrolactone antagonized the stimulation of 3-MT formation induced by (+)-amphetamine at a dose of 10 mg/kg. This dose of (+)-amphetamine markedly counteracted the-butyroiactoneinduced increase in dopa formation especially in the DA-rich areas.The data support the view that impulse flow in DA neurons facilitates the effect of (+)-amphetamine on DA release and DA synthesis. Inhibition of catecholamine synthesis after high doses of (+)-amphetamine may be due to an increase in cytoplasmatic DA concentration causing end-product inhibition of tyrosine hydroxylase.A preliminary report has been given recently (Kehr, 1976 b).Data of this communication are part of W.S.'s thesis for an M.D.     

Lipopolysaccharide administration produces time-dependent and region-specific alterations in tryptophan and tyrosine hydroxylase activities in rat brain

Summary. The present study examined the effect of systemic administration of lipopolysaccharide (LPS; 100 and 250 μg/kg, i.p.) on tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH) activities in frontal cortex, striatum and midbrain of the rat. Enzyme activities were determined by measuring accumulation of the transient intermediates 5-hydroxytrptophan (5-HTP) and L-dihydroxyphenylalanine (L-DOPA) following in vivo administration of the decarboxylase inhibitor, NSD 1015. TPH activity was increased 2 hours after administration of LPS (100 and 250 μg/kg) in both frontal cortex and midbrain, and a secondary increase was seen in the midbrain 12 hours after challenge. LPS provoked an increase in TH activity in the midbrain only, and this was evident for up to 24 hours after LPS administration. Thus in addition to previous studies demonstrating that LPS increases in vivo NA, DA and 5-HT release, this study shows that LPS increases the activity of the rate-limiting enzymes responsible for their synthesis. Received May 4, 2000; accepted June 16, 2000     

Evidence for co-release of noradrenaline and dopamine from noradrenergic neurons in the cerebral cortex.

The aim of this study was to determine whether extracellular dopamine (DA) in the prefrontal cortex (PFC) might originate other than from DA neurons, also from noradrenergic (NA) ones. To this aim, we compared the levels of DA and NA in the dialysates from the PFC, a cortical area innervated by NA and DA neurons, and cortices that receive NA but minor or no DA projections such as the primary motor, the occipital-retrosplenial, and the cerebellar cortex. Moreover, the effect of alpha(2)-ligands and D(2)-ligands that distinctly modify NA and DA neuronal activity on extracellular NA and DA in these areas was studied. Extracellular NA concentrations were found to be similar in the different cortices, as expected from the homogeneous NA innervation, however, unexpectedly, also DA concentrations in the PFC were not significantly different from those in the other cortices. The alpha(2)-adrenoceptor agonist clonidine, intraperitoneally (i.p.) injected or locally perfused into the PFC, reduced not only extracellular NA levels, as expected from its ability to inhibit NA neuron activity, but also markedly reduced extracellular DA levels. Conversely, the alpha(2)-adrenoceptor antagonist idazoxan, i.p. injected or locally perfused into the PFC, not only increased extracellular NA levels, in line with its ability to activate NA neuron activity, but also increased those of DA. Conversely, in contrast to its ability to inhibit DA neuronal activity, the D(2) receptor agonist quinpirole only modestly and transiently reduced extracellular DA levels, while gamma-butyrolactone failed to modify DA levels in the PFC; conversely, haloperidol, at variance from its ability to activate DA neurons, failed to significantly modify extracellular DA levels in the PFC. Both haloperidol and quinpirole were totally ineffective after local perfusion into the PFC. Systemically injected or locally perfused, clonidine and idazoxan also modified both DA and NA concentrations in dialysates from primary motor, occipital-retrosplenial and cerebellar cortices as observed in the PFC. Finally, i.p. injected or locally perfused, clonidine reduced and idazoxan increased extracellular NA levels in the caudate nucleus, but neither alpha(2)-ligand significantly modified extracellular DA levels. Our results suggest that extracellular DA in the PFC, as well as in the other cortices, may depend on NA rather than DA innervation and activity. They suggest that dialysate DA reflects the amine released from NA neurons as well, where DA acts not only as NA precursor but also as co-transmitter. The co-release of NA and DA seems to be controlled by alpha(2)-receptors located on NA nerve terminals.     

Noradrenaline and dopamine-beta-hydroxylase levels in rat salivary glands after preganglionic nerve stimulation: Evidence for re-use of amine storage granules in transmitter release

Summary The levels of endogenous noradrenaline (NA) in rat salivary glands were reduced to about 40 per cent after 1 hour of preganglionic stimulation at 4 Hz. Following 2 to 3 hours of rest the NA values had recovered to normal and remained normal for 6 hours after the stimulation. The recovered NA could be depleted by renewed nerve stimulation. After treatment with reserpine the NA disappeared almost completely. The recovered NA is discussed to be mainly present in amine storage granules that are re-used for storage and transmitter release. The axonal down-transport of new granules to the nerve terminals appears to be too small, as based on earlier results, for being of any quantitative importance in the experimental situation. The dopamine--hydroxylase (DBH) level in the glands did not significantly change after the period of stimulation. The DBH content remained mainly unchanged following either post-stimulation recovery, repeated stimulation, or reserpine treatment. First when the glands were stimulated for 4 hours, there was a small decrease of the DBH levels. The mainly unchanged levels of DBH after stimulation does not appear to be explained by trapping in the tissue of released DBH, or recovery of the DBH by a marked axonal down-transport of new granules, as based on earlier results. The results of the DBH estimations support the view that the granules are not consumed to any marked extent at nerve activity, but can be re-used for nerve transmission. Furthermore, the results are in agreement with lack of correlation between released NA and released DBH.