Inhibition of nicotinic responses of bovine adrenal chromaffin cells by the protein kinase C inhibitor, Ro 31-8220.

1. The effects of the protein kinase C inhibitor, Ro 31-8220, on the responses of cultured bovine adrenal chromaffin cells to nicotine, phorbol 12, 13-dibutyrate (PDBu) and K+ have been investigated. 2. Tyrosine hydroxylase activity was measured in situ in intact cells by measuring 14CO2 evolved following the hydroxylation and rapid decarboxylation of [14C]-tyrosine offered to the cells. Secretion of endogenous adrenaline and noradrenaline was measured by use of h.p.l.c. with electrochemical detection. Cyclic AMP levels were measured in cell extracts by RIA. 3. Ro 31-8220 produced a concentration-dependent inhibition of 300 nM PDBu-stimulated tyrosine hydroxylase activity with an IC50 of < 2 microM and complete inhibition at 10 microM. It had no effect on the responses to forskolin. 4. Ro 31-8220 produced a concentration-dependent inhibition of 5 microM nicotine-stimulated tyrosine hydroxylase activity, adrenaline and noradrenaline secretion and cellular cyclic AMP levels, with an IC50 of about 3 microM and complete inhibition by 10 microM. At concentrations up to 10 microM, Ro 31-8220 had little or no effect on the corresponding responses to 50 mm K+. 5. A structural analogue of Ro 31-8220, bisindolylmaleimide V, that lacks activity as a protein kinase C inhibitor, had no effect up to 10 microM on PDBu-stimulated tyrosine hydroxylase activity or on nicotine-stimulated cyclic AMP levels or noradrenaline secretion and only marginal inhibitory effects on nicotine-stimulated tyrosine hydroxylase activity and adrenaline secretion. 6. A structurally related protein kinase C inhibitor, bisindolylmaleimide I, inhibited PDBu-stimulated tyrosine hydroxylase activity with an IC50 of < 1 microM and complete inhibition by 3 microM, but had essentially no effect on nicotine stimulated tyrosine hydroxylase activity or catecholamine secretion. 7. The results suggest that Ro 31-8220 is not only a protein kinase C inhibitor but is also a potent inhibitor of nicotinic receptor responses in adrenal chromaffin cells by a mechanism unrelated to protein kinase C inhibition. The results are consistent with Ro 31-8220 being a nicotinic receptor antagonist.     

Effects of acute diazepam and clobazam on spontaneous locomotor activity and central amine metabolism in rats.

Following 2 h after a single injection of diazepam ( 10 mg/kg) although no effect was found on tyrosine hydroxylase in striatum (a region abundant in dopaminergic nerver endings), its activity in pons-medulla, a region known to be rich in noradrenergic cell bodies, was enhanced by 17%. Diazepman elevated the levels of norepinephrine and dopamine in several brain regions. However, it decreased the concentration of homovanillic acid and augmented that of 3,4-dihydroxyphenlacetic acid, an intraneuronal metabolite of dopamine, suggesting that this anti-anxiety agent reduced the neuronal release of dopamine and possible norepinephrine. The enhanced activity of tyrosine hydroxylase in pons-medulla appears to reflect a specific compensatory response to the postulated low levels of catecholamines in the synaptic clefts. Acute diazepam treatment decreased tryptophan hydroxylase activity and the endogenous levels of its substrate, tryptophan in mid-brain region; however, the levels of 5-hydroxytryptamine and its metabolite 5-hydroxyindoleacetic acid were markedly increased in hypothalamus, mid-brain and pons-medulla, suggesting that diazepam decreased the synthesis as well as the neuronal release of 5-hydroxytryptamine. Clobazam ( 10 mg/kg) which is a 1,5-benzodiazepine, failed to exert any significant effect on behavioural activity and catecholamine metabolism although like diazepam, it also enhanced the levels of 5-hydroxytryptamine and 5-hydroxyindoleacetic acid. It is suggested that whereas diazepam may exert its behavioural suppressant effect by reducing the turnover of actecholamine, its anxiolytic action may be related to decreased turnover of brain 5-HT.     

Effects of the noradrenaline metabolites on tyrosine hydroxylase activity in guinea-pig atria

Summary The effects of noradrenaline, its five metabolites and metanephrine, were studied on tyrosine hydroxylase activity in guinea-pig atria. The deaminated metabolite, (±)-3,4-dihydroxyphenylglycol (DOPEG), was equipotent with (±)-noradrenaline in its inhibitory action on tyrosine hydroxylase activity in the homogenates of guinea-pig atria. The inhibition by DOPEG was competitive with the cofactor, reduced pteridine. The deaminated acid, 3,4-dihydroxymandelic acid (DOMA) and the O-methylated deaminated acid, 3-methoxy, 4-hydroxymandelic acid (VMA) had 1/50th and 1/30th, respectively, the potency of noradrenaline in inhibiting tyrosine hydroxylase. The rest of the metabolites did not inhibit tyrosine hydroxylase in homogenates in concentrations up to 1.0 mM. In intact guinea-pig atria noradrenaline was considerably more potent than DOPEG in inhibiting tyrosine hydroxylase. Normetanephrine 1.4×10^–4 M inhibited tyrosine hydroxylase in the intact tissue but failed to inhibited the enzyme in the homogenate even in higher concentrations. The effect of normetanephrine in the intact tissue is related to the ability of this compound to release endogenous noradrenaline.A reserpine-like agent, Ro 4-1284, did not inhibit tyrosine hydroxylase activity in the homogenate but in the intact tissue the inhibition was more than 50%. This effect of Ro 4-1284 in the intact tissue appears to be related to the releasing effects of this agent and to an increase in the axoplasmic levels of DOPEG.Since the formation of the deaminated glycol, DOPEG, represents the main metabolic pathway for the neurotransmitter in adrenergic nerve endings, the present results are compatible with the view that, in addition to the pool of extravesicular noradrenaline, the cytoplasmic concentration of DOPEG could also participate in the regulation of the activity of tyrosine hydroxylase.     

Effects of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) on alpha2-adrenoceptors which regulate the synthesis and release of noradrenaline in the rat brain

N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) induces a degeneration of noradrenergic axons originating in the locus coeruleus. The sensitivity of alpha2-adrenoceptors which regulate the synthesis and release of noradrenaline was investigated in three brain regions which receive an unequal innervation from locus coeruleus, 21 days after DSP4 (50 mg/kg) administration. After giving treated rats a dopa decarboxylase inhibitor, the in vivo tyrosine hydroxylase activity and the tissue concentrations of noradrenaline were also evaluated. Relevant reductions of noradrenaline levels were found in hippocampus and parietal cortex (91% and 77.5%, respectively; P<0.001) together with a less pronounced reduction in hypothalamus (32%, P<0.01). The administration of the neurotoxin led to decreases of the basal tyrosine hydroxylase activity, determined as the accumulation of 3,4-dihydroxyphenylalanine, in hippocampus and parietal cortex (75% and 50.5%, respectively; P<0.001), but not in hypothalamus. The inhibitory effect of clonidine on tyrosine hydroxylase activity was markedly reduced in hippocampus of rats treated with DSP4 (10+/-5% vs 57+/-3% in the control group, P<0.001) but was not changed in parietal cortex and hypothalamus. Moreover, in hippocampus, a lack of functionality of the alpha2-adrenoceptors which regulate K(+)-evoked [3H]noradrenaline release was determined. However, in cortical synaptosomes the concentration-effect curve for the oxymetazoline shifted to the right. The administration of the neurotoxin did not modify the inhibitory effects of the agonist in hypothalamus. These results support the previously described selectivity of DSP4 for noradrenergic terminals arising from locus coeruleus and suggest a more severe lesioning of the hippocampus than the parietal cortex.     

Effects of db cAMP on tyrosine hydroxylase activity of ganglia and nerve endings

Summary Preincubation of intact superior cervical ganglia or nictitating membrane for 2 h with dibutyryl cyclic AMP (db cAMP) increased the hydroxylation of tyrosine. This effect was not blocked by the protein synthesis inhibitor, cycloheximide. The K _m of tyrosine hydroxylase for the substrate, tyrosine, and for the cofactor, reduced pteridine, were decreased by db cAMP. There were no changes in the V _max of the enzyme. The inhibitory potency of noradrenaline on the hydroxylation of tyrosine was also decreased. Thus an inductive effect may be ruled out. The activation of the enzyme was only observed when the tissues were preincubated with the db cAMP and not when the cyclic nucleotide was added to the isolated enzyme.Preincubation of cervical ganglia for 4 h with db cAMP increased activity of decarboxylase and monoamine oxidase in tissue homogenates without changing the tyrosine hydroxylase activity.     

Long-term effects of p-chloroamphetamine on tryptophan hydroxylase activity and on the levels of 5-hydroxytryptamine and 5-hydroxyindole acetic acid in brain

The i.p. administration of a single dose of 10 mg/kg of p-chloroamphetamine to rats causes a reduction in the activity of cerebral tryptophan hydroxylase and a decrease in the levels of 5-hydroxytryptamine and 5-hydroxyindole acetic acid in brain for as long as 4 months after injection. 2 weeks after injection, the activity of tryptophan hydroxylase and the level of 5-hydroxytryptamine in brain are reduced by 59 and 53% respectively. The irreversible tryptophan hydroxylase inhibitor, p-chlorophenylalanine, also decreases the brain level of 5-hydroxytryptamine; however, this effect has disappeared 2 weeks after injection.     

Time course of the effects of 6-hydroxydopamine on catecholamine-containing neurones in rat hypothalamus and striatum

1. The effects of intraventricular injection of 6-hydroxydopamine (6-OHDA) on tyrosine hydroxylase activity, uptake of ³H-noradrenaline and endogenous catecholamine concentration in rat hypothalamus and striatum were investigated at various times after the injection of 6-OHDA.

2. In the hypothalamus after the injection of 250 μg of 6-OHDA there was a rapid decrease in tyrosine hydroxylase activity, ³H-noradrenaline uptake and noradrenaline content, which was essentially complete within 2 hours.

3. In the striatum after this dose of 6-OHDA there was a much slower reduction in tyrosine hydroxylase activity and ³H-noradrenaline uptake during the first 48 h after drug injection. For the first 24 h the dopamine concentration in this brain area was increased significantly above control values, but had fallen below control values by 48 hours.

4. After the injection of a smaller dose of 6-OHDA (25 μg) the only detectable change in the striatum was a rapid increase in the dopamine concentration. In the hypothalamus this dose induced a rapid depletion of noradrenaline, not accompanied initially by any significant reduction in tyrosine hydroxylase activity.

5. These results are consistent with the hypothesis that 6-OHDA causes a rapid degeneration of catecholamine-containing nerve terminals in the central nervous system (CNS). These degenerative changes, indicated by the loss of tyrosine hydroxylase and noradrenaline uptake sites, did not appear to be preceded by an initial displacement of endogenous catecholamines by 6-OHDA, except possibly at early times after the administration of small doses of the drug.

     

Effect of intraventricular injections of 6-hydroxydopamine in neonatal rats on the catecholamine levels and tyrosine hydroxylase activity in brain regions at maturity.

Intraventricular injections of 6-hydroxydopamine (6-OHDA, 25 μg) on days 1 and 2 after birth produced a marked change in tyrosine hydroxylase activity in only one rat brain region, the corpus striatum, whereas the ability of brain tissue to synthesise catecholamines, as measured by the rate of tyrosine hydroxylation in brain homogenates, was significantly reduced in amygdala, cortex, hippocampus, hypothalamus, septum and thalamus, and greatly increased in both dorsal and ventral pons. Both increases and decreases in the rate of tyrosine hydroxylation in all brain regions with the exception of the corpus striatum and septum were blocked by pretreatment of the newborn rats with desmethylimipramine (DMI) 45 min before the 6-OHDA. Similarly, norepinephrine (NE) levels were reduced in amygdala, cortex, hippocampus, hypothalamus, septum and thalamus, and increased in cerebellum, medulla, midbrain and pons. These 6-OHDA-induced changes in NE were also blocked by DMI pretreatment. The NE and dopamine (DA) levels and the rate of tyrosine hydroxylation were markedly reduced in striatum, and DMI again blocked the depletion of NE but potentiated both the decrease in DA and the decreased rate of tyrosine hydroxylation. It is suggested that the pattern of decreased NE levels in forebrain regions and increased NE levels in hindbrain after neonatal 6-OHDA treatment does not depend on degeneration of forebrain terminals, since at suitably low 6-OHDA levels the NE changes can be shown to occur in the absence of changes in the tyrosine hydroxylase activity.     

Evidence for the involvement of central serotonin in mechanism of domestication of silver foxes

Silver foxes selected for more than 30 years for tame behavior and displaying no defensive reaction to human contact were shown to have a higher serotonin level in midbrain and hypothalamus, and a higher 5-hydroxyindole acetic acid (5-HIAA) content in midbrain, hypothalamus and hippocampus in comparison to nonselected wild silver foxes bred in captivity over the same time span. Tryptophan hydroxylase (TPH) activity in midbrain and hypothalamus in domesticated foxes was increased as compared with their aggressive/defensive counterparts. Monoamine oxidase type A (MAO A) activity was decreased with an increased K_m and unchanged V_max in domesticated foxes. No changes in specific [³H]ketanserin or [³H]8-OH-DPAT binding in frontal cortex was revealed. A reduced density (B_max) of 5HT_1A receptors in hypothalamic membranes in domesticated foxes was shown. It is suggested that the brain serotonergic system is involved in the mechanism of domestication converting wild aggressive/defensive animals into tame ones.     

Depletion of brain noradrenaline and dopamine by 6-hydroxydopamine

1. After intracisternal administration, 6-hydroxydopamine had a greater effect on brain noradrenaline than on dopamine.2. Administration of two doses of 6-hydroxydopamine increased the depletion of noradrenaline but not of dopamine.3. Small doses of 6-hydroxydopamine decreased the concentration of noradrenaline with little or no effect on dopamine. Tyrosine hydroxylase activity was not reduced with these treatments.4. While pargyline pretreatment offered no advantage in the depletion of brain noradrenaline after 6-hydroxydopamine, depletion of brain dopamine was greatly potentiated by this treatment. The reduction of striatal tyrosine hydroxylase activity observed after 6-hydroxydopamine was also potentiated by pargyline pretreatment.5. The amounts of labelled noradrenaline and dopamine formed from (3)H-tyrosine were greatly reduced by 6-hydroxydopamine treatment. After (3)H-DOPA, formation of noradrenaline was greatly reduced while formation of labelled dopamine was only moderately reduced suggesting that decarboxylation of DOPA can occur in other than catecholamine containing neurones.6. Desmethylimipramine and imipramine inhibited depletion of noradrenaline produced by 6-hydroxydopamine but did not alter depletion of dopamine. Reserpine did not inhibit depletion of catecholamines produced by 6-hydroxydopamine.7. Administration of 6-hydroxydopamine to developing rats lowered both noradrenaline and dopamine concentrations as well as the tyrosine hydroxylase activity in the brains of these animals.     

Effect of low-dose lithium administration and subsequent withdrawal on biogenic amines in rat brain.

1 The effects of low-dose lithium administration (2 mEq/kg, daily) and its subsequent withdrawal have been examined with reference to changes in biogenic amine systems in several discrete regions of rat brain. 2 Increased levels of striatal tyrosine and midbrain tryptophan were found following lithium administration together with slight decreases in striatal tyrosine hydroxylase and midbrain tryptophan hydroxylase activities. Withdrawal resulted in a decrease in tyrosine content with increased tyrosine hydroxylase activity, whilst tryptophan levels and tryptophan hydroxylase activity were increased. 3 Lithium treatment and withdrawal resulted in altered levels of noradrenaline and dopamine, these changes being regionally variable. 3-Methoxy-4-hydroxyphenylglycol content was depressed in both treated and withdrawal rats as were 3,4-dihydroxyphenylacetic acid levels. Homovanillic acid decreased as a result of lithium treatment, but was greatly elevated in the withdrawal group. 4 5-Hydroxytryptamine content decreased in some brain regions following lithium treatment with return towards control values in withdrawal rats. 5-Hydroxyindoleacetic acid levels also displayed a regional variation as a result of lithium treatment and withdrawal. 5 The implications of these observations in elucidating the pharmacological effect of lithium treatment and its subsequent withdrawal are discussed.     

Ipsapirone, a new anxiolytic drug, stimulates catecholamine turnover in various regions of the rat brain

Ipsapirone, a new anxiolytic drug with a high affinity to 5-HT1A receptors, given in a dose of 10 mg/kg ip markedly accelerated noradrenaline disappearance after inhibition of tyrosine hydroxylase with alpha-methyl-p-tyrosine (250 mg/kg ip) in the cortex, hippocampus and hypothalamus of male Wistar rats. It also increased disappearance of dopamine and the level of 3,4-dihydroxyphenylacetic acid and homovanillic acid in the striatum. At the same time, the level of 5-hydroxyindoleacetic acid was decreased in the cortex, striatum, hypothalamus, but not changed in the hippocampus. 8-OH-DPAT, a selective agonist of 5-HT1A receptors, used in a dose of 5 mg/kg sc was less effective, having accelerated noradrenaline disappearance in the cortex and hypothalamus, and having increased only the level of homovanillic acid in the striatum. The effect of ipsapirone on catecholamine turnover might be secondary in relation to inhibition of the serotonin neurons. A direct interaction between ipsapirone and its metabolite 1-PP with alpha 1- and alpha 2-adrenoceptors is very likely, too.     

Increase of tyrosine hydroxylase levels and activity during morphine withdrawal in the heart

Our previous studies have shown an enhanced activity of the noradrenergic pathways innervating the heart in rats withdrawn from morphine. However, the possible adaptive changes that can occur in these pathways during morphine dependence are not known. We studied the alterations in tyrosine hydroxylase (the rate-limiting enzyme in catecholamines biosynthesis) and tyrosine hydroxylase activity in the heart (right and left ventricle) during morphine withdrawal. In the same paradigm, we measured Fos expression as a marker of neuronal activation and the normetanephrine/noradrenaline ratio (an index of noradrenaline turnover). We evaluated the levels of tyrosine hydroxylase and Fos by quantitative Western blot analysis, and noradrenaline turnover using high-performance liquid chromatography (HPLC). Dependence on morphine was induced by a 7-day s.c. implantation of morphine pellets. Morphine withdrawal was precipitated on day 8 by an injection of naloxone (5 mg/kg s.c.). The results show a significant increase in tyrosine hydroxylase levels and activity in the right and left ventricle 30 or 90 min after naloxone precipitated withdrawal in parallel with an increase in noradrenaline turnover. Morphine withdrawal also induced an increase in the Fos expression, which indicates an activation of cardiac cellular activity. Our results suggest that an increase in tyrosine hydroxylase protein levels and tyrosine hydroxylase enzyme activity might contribute to the enhanced noradrenergic activity in the heart in response to morphine withdrawal.     

Evidence for the role of adrenocortical hormones in the regulation of noradrenaline and dopamine metabolism in certain brain areas.

1 Bilateral adrenalectomy suppressed body growth and increased the activity of tyrosine hydroxylase in rat striatum in a time-dependent manner. Fifteen days after adrenalectomy, the concentrations of noradrenaline were decreased significantly in hypothalamus and striatum, as were those of dopamine in brain stem and striatum. 2 Catechol-O-methyltransferase failed to change in response to adrenalectomy, but the activity of monoamine oxidase in cortex was significantly increased 7 days after surgery. These changes in various neurochemical parameters were even more pronounced 15 days after adrenal ablation. 3 Administration of corticosterone (10 mg/kg i.p.) to adrenalectomized rats effectively reversed the observed effects on brain amine metabolism. Corticosterone treatment for 7 days beginning from the 8th day of adrenalectomy virtually restored the concentrations of noradrenaline and dopamine as well as the activities of striatal tyrosine hydroxylase and cerebrocortical monoamine oxidase to the values seen for sham-operated controls. 4 Our data suggest that changes seen in brain noradrenaline and dopamine of adrenalectomized rats are specific to adrenocortical steroids and that these hormones play a role in the regulation of catecholamine formation.     

Effects of N‐(2‐chloroethyl)‐N‐ethyl‐2‐bromobenzylamine (DSP4) on α2‐Adrenoceptors which Regulate the Synthesis and Release of Noradrenaline in the Rat Brain

Abstract: N‐(2‐chloroethyl)‐N‐ethyl‐2‐bromobenzylamine (DSP4) induces a degeneration of noradrenergic axons originating in the locus coeruleus. The sensitivity of α₂‐adrenoceptors which regulate the synthesis and release of noradrenaline was investigated in three brain regions which receive an unequal innervation from locus coeruleus, 21 days after DSP4 (50 mg/kg) administration. After giving treated rats a dopa decarboxylase inhibitor, the in vivo tyrosine hydroxylase activity and the tissue concentrations of noradrenaline were also evaluated. Relevant reductions of noradrenaline levels were found in hippocampus and parietal cortex (91% and 77.5%, respectively; P<0.001) together with a less pronounced reduction in hypothalamus (32%, P<0.01). The administration of the neurotoxin led to decreases of the basal tyrosine hydroxylase activity, determined as the accumulation of 3,4‐dihydroxyphenylalanine, in hippocampus and parietal cortex (75% and 50.5%, respectively; P<0.001), but not in hypothalamus. The inhibitory effect of clonidine on tyrosine hydroxylase activity was markedly reduced in hippocampus of rats treated with DSP4 (10±5% vs 57±3% in the control group, P<0.001) but was not changed in parietal cortex and hypothalamus. Moreover, in hippocampus, a lack of functionality of the α₂‐adrenoceptors which regulate K⁺‐evoked [³H]noradrenaline release was determined. However, in cortical synaptosomes the concentration‐effect curve for the oxymetazoline shifted to the right. The administration of the neurotoxin did not modify the inhibitory effects of the agonist in hypothalamus. These results support the previously described selectivity of DSP4 for noradrenergic terminals arising from locus coeruleus and suggest a more severe lesioning of the hippocampus than the parietal cortex.     

The effect of (+/-)mianserin and its enantiomers on the behavioural hyperactivity of the olfactory-bulbectomized rat

The effects of (+/-)mianserin and its enantiomers on the behavioural hyperactivity of the olfactory bulbectomized (OB) rat and on the levels of some central neurotransmitters were investigated. The reversal of hyperactivity by racemic mianserin was found to be independent of dose within a non-sedative range of 2-10 mg/kg and occurred following seven days of treatment. Chronic administration of racemic mianserin, over the dose range 2-15 mg/kg elevated the steady-state concentration of noradrenaline in the amygdaloid cortex and midbrain. Of the two enantiomers, only the behaviourally active (+)-(S)-form increased the steady state concentration of noradrenaline in these two areas of the brain. The reversal of the hyperactivity by (+/-)mianserin and the (+)-(S)-enantiomer was not associated with a change in the concentration of serotonin or 5-hydroxyindole acetic acid. The behaviourally inactive enantiomer (-)-(R) decreased the concentration of 5-hydroxyindole acetic acid, an effect which may be associated with the sedative action of the drug. The results suggest that the reversal of the behavioural hyperactivity of the olfactory bulbectomized (OB) rat by (+/-)mianserin and its (+)-(S)-enantiomer is associated with an increase in the steady-state concentration of noradrenaline, in the areas of the brain studied.     

Effect of antidepressant drugs on monoamine synthesis in brain in vivo.

The activity of tryptophan and tyrosine hydroxylase were estimated in vivo by measuring the accumulation during 30 min of 5-hydroxytryptophan (5-HTP) and 3,4-dihydroxyphenylalanine (DOPA), respectively, after inhibition of aromatic amino acid decarboxylase by administration of m-hydroxybenzylhydrazine (NSD 1015) (100 mg/kg, i.p.). Whereas the activity of tyrosine hydroxylase in the dopamine-rich striatum was sensitive to haloperidol, which caused a significant increase in accumulation of DOPA, there was no effect of haloperidol in the predominantly noradrenergic frontoparietal cortex, confirming that the activity of tyrosine hydroxylase, measured in the frontoparietal cortex, is essentially localized in noradrenergic neurones. In the frontoparietal cortex of the rat the in vivo activity of tryptophan and tyrosine hydroxylase were equipotently attenuated by imipramine, while the selective blocker of the uptake of noradrenaline, desipramine and the selective blocker of the uptake of serotonin, citalopram, reduced only tyrosine or tyrosine hydroxylase respectively. Milnacipran, an antidepressant which inhibits the uptake of both monoamines to a similar extent, decreased the synthesis of both monoamines equipotently. The monoamine oxidase inhibitor, clorgyline, also reduced the synthesis of both monoamines. Thus, the in vivo inhibition of the synthesis of monoamines would appear to be mediated by an increase in synaptic concentration of monoamines, resulting from the inhibition of the uptake or catabolism of monoamines. Chronic administration of citalopram led to a significant increase of the basal synthesis of 5-hydroxytryptamine (5-HT). Milnacipran, given chronically, significantly enhanced the basal synthesis of both 5-HT and noradrenaline (NA).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of Clonidine on Tyrosine Hydroxylase Activity in the Adrenal Medulla and Brain of Spontaneously Hypertensive Rats

Abstract: In the present study, we evaluated the effect of the α₂‐adrenoceptor agonist clonidine on tyrosine hydroxylase activity in adrenal medulla and brain of spontaneously hypertensive rats and Wistar Kyoto rats. Six‐week‐old animals were treated with clonidine (100 µg/kg body weight, daily, i.p.) for 4 weeks. Treatment with clonidine significantly reduced mean arterial blood pressure in spontaneously hypertensive rats to values similar to normotensive Wistar Kyoto rats. In the adrenal medulla of spontaneously hypertensive rats, clonidine treatment produced a significant increase in tyrosine hydroxylase activity with higher V_max values and no changes in K_M values. This effect was accompanied by a significant increase in tyrosine hydroxylase protein expression and of noradrenaline and adrenaline levels. In the brain of spontaneously hypertensive rats, treatment with clonidine produced a significant decrease in tyrosine hydroxylase activity with lower V_max values and no changes in K_M values accompanied by a significant decrease in tyrosine hydroxylase protein expression and of dopamine and noradrenaline levels. In Wistar Kyoto rats, clonidine treatment had no effect on tyrosine hydroxylase activity and protein expression or catecholamine levels in adrenal medulla or brain. Clonidine treatment significantly reduced noradrenaline and adrenaline plasma levels in spontaneously hypertensive rats and Wistar Kyoto rats. In conclusion, treatment with the α₂‐adrenoceptor agonist clonidine prevented the increase in mean arterial blood pressure in young spontaneously hypertensive rats. This effect was accompanied by opposite effects on tyrosine hydroxylase activity in spontaneously hypertensive rat adrenal medulla and brain: an increase in adrenal medulla and a decrease in brain, bringing sympathetic function to a similar profile found in normotensive Wistar Kyoto rats.     

Increased brain serotonergic and noradrenergic activity after repeated systemic administration of the beta-2 adrenoceptor agonist salbutamol,a putative antidepressant drug

Subchronic (5 mg/kg SC, twice daily for 14 days) but not acute administration of the beta-2-adrenoceptor agonist salbutamol to rats caused a significant increase in the accumulation of 5-hydroxytryptophan in the limbic forebrain, the corpus striatum and the cerebral cortex when measured during 30 min after inhibition of l-amino acid decarboxylase by NSD 1015 (100 mg/kg IP). Simultaneously assayed tryptophan concentrations in the same brain regions were not affected. These results indicate an increase in the in vivo rate of tryptophan hydroxylation in the brain, produced by subchronic salbutamol administration.The effect of salbutamol treatment on brain catecholamine (CA) utilization was estimated by studying the disappearance of CA in the brain after inhibition of tyrosine hydroxylase by alpha-methyltyrosine methyl ester (H 44/68) 250 mg/kg IP, 3.5 h before sacrifice. Subchronically but not acutely administered salbutamol caused both a significant increase in endogenous noradrenaline (NA) levels and an increased NA utilization. Dopamine levels and turnover were, however, not altered by either acute or subchronic treatment.The activation, probably centrally elicited, of brain NA and 5-hydroxytryptamine systems by the subchronic salbutamol regimen supports the concept of beta-adrenoceptor mediated regulation of brain monoamine systems, and could contribute to the clinically reported antidepressant activity of beta-2-adrenoceptor agonists.     

Direct central effects of acute methylenedioxymethamphetamine on serotonergic neurons

Acute peripheral administration of either the (+) or (-) stereoisomer of methylenedioxymethamphetamine (MDMA) to rats results in a rapid loss of tryptophan hydroxylase (TPH) activity in several brain regions. This decline in enzyme activity precedes a decrease in serotonin (5-HT) concentrations in the same areas. An initial rise in the concentration of 5-hydroxyindole acetic acid after drug administration suggests that an increase in the turnover of 5-HT is an early event in the development of these changes. Unsuccessful attempts to reproduce the in vivo effects of MDMA on TPH activity using in vitro preparations such as cortical slices or the mouse mastocytoma cell line, P-815, suggested a requirement for an intact neuronal system or metabolism of the drug. Injection of MDMA directly into several brain regions also had no effect on TPH activity or 5-HT concentrations. However, when brain concentrations of MDMA were maintained using a constant i.c.v. infusion, TPH activity declined as observed following peripheral administration. The results, therefore, indicate that the acute effect of MDMA on 5-HT synthesis is a direct central effect of the drug which may be triggered by a sustained increase in transmitter turnover.