Effects of the beta 2-adrenoceptor agonist clenbuterol on tyrosine and tryptophan in plasma and brain of the rat

The beta 2-adrenoceptor agonist, clenbuterol (initially 5 mg/kg), was found to significantly reduce plasma tyrosine and raise brain tryptophan levels (P less than 0.01). By comparison, decreases in plasma tryptophan and increases in brain tyrosine were small and often nonsignificant. Amino acid levels measured in different brain regions revealed that the elevations were similar among the cerebellum, striatum, and cortex. These effects were partially blocked by propanolol but not by atenolol. The ED50 was estimated from dose-response curves to be about 0.05 mg/kg for both the decrease in plasma tyrosine and the increase in brain tryptophan. The effects of low doses of clenbuterol were prevented completely by propranolol. Peripheral organs displayed strikingly different patterns of change in amino acid concentrations. Only the spleen had any accumulation of tryptophan, but that was much less than in brain. In contrast, tyrosine and tryptophan were decreased in heart and unaltered in liver; tyrosine was decreased in lung. The elevation in brain tryptophan levels was attenuated by the beta 2-antagonist, ICI 118,551, but not by the beta 1-antagonist, betaxolol; but the reduction in plasma tyrosine was unaffected by either drug. The serotonin antagonist, methysergide, failed to block the effects of clenbuterol. We conclude that changes in amino acid concentrations produced by clenbuterol are mediated by beta 2-adrenoceptor stimulation. Although the increases in brain tyrosine and tryptophan were similar to increases in the plasma ratios of these amino acids to the sum of the other large neutral amino acids competing for transport into the brain, the disparity between the effects of ICI 118,551 in brain and plasma suggests that clenbuterol may also have a direct action in brain to regulate levels of aromatic amino acids. Since clenbuterol has been purported to have an antidepressant effect and since other antidepressants also increase brain tryptophan, this may be a common feature of antidepressant drug action.     

Inhibition by viozan of extravasation induced in rat trachea by capsaicin is mediated exclusively by β2-adrenoceptors

The mechanism by which 2(3H)-benzothiazolone, 4-hydroxy-7-[2-[[2-[[3-(2-phenylethoxy)propyl]-sulphonyl]ethyl]amino]ethyl]-monohydrochloride (AR-C68397AA; viozan), a dual dopamine D2/beta2-adrenoceptor agonist which has shown promise in the treatment of chronic obstructive pulmonary disease (COPD), inhibits the extravasation of plasma protein induced by capsaicin in the tracheas of Brown Norway rats has been re-evaluated. Viozan (10-30 microg/kg given intratracheally; i.t.) inhibited dose-dependently the extravasation of plasma protein tagged with Evans Blue into rat trachea induced by capsaicin (10 microg/kg i.t.). Similar effects were seen with the selective beta2-adrenoceptor agonist, salbutamol (3-10 microg/kg i.t.), but the selective dopamine D2 receptor agonist, quinagolide (10-30 microg/kg i.t.), was inactive. The effects of viozan and salbutamol were abolished by propranolol (3 mg/kg) given intraperitoneally (i.p.) but unaffected by sulpiride (3 mg/kg i.p.). Thus, in c,ontrast to claims in the literature, a functional response to dopamine D2 receptor activation in a preclinical model of oedema arising from sensory nerve fibre activation in the rat lung could not be demonstrated. Moreover, no qualitative difference could be demonstrated between the response to a dual D2/beta2-adrenoceptor agonist and a selective beta2-adrenoceptor agonist. The observations call into question whether a dual D2/beta2-adrenoceptor agonist such as viozan would bring added benefit over established selective beta2-adrenoceptor agonists in the therapy     

Increased insulin is not required for beta2-adrenoceptor-induced increases in mouse brain tryptophan

The current study tested the hypothesis that beta(2)-adrenoceptor-mediated increases in brain tryptophan are caused by increased insulin secretion. Male mice were treated with streptozotocin (40 mg/kg) for 5 days to induce experimental diabetes. Control and diabetic mice were treated with the beta(2)-adrenoceptor agonist, clenbuterol (0.1 mg/kg), 1 h before selected brain regions were dissected for analysis by high performance liquid chromatography (HPLC) with electrochemical detection for tryptophan content, and plasma was collected for analysis of total and free tryptophan and glucose concentrations. Clenbuterol increased brain tryptophan and plasma glucose and decreased plasma total tryptophan but did not alter plasma free tryptophan. There were no significant differences in brain or plasma tryptophan between control and streptozotocin-treated mice. In a separate experiment, pretreatment of the mice with an insulin antibody did not prevent the clenbuterol-induced increases in brain tryptophan. These results suggest that beta(2)-adrenoceptor agonists increase brain tryptophan by a mechanism that does not involve changes in insulin.     

A novel in vivo test for drugs affecting central serotonergic and adrenergic systems

In urethane-anaesthetized rats, myoclonic twitches of the anterior digastricus muscle were evoked by L-5-hydroxy-tryptophan (L-5-HTP, 50-100 mg/kg iv.), the serotonin (5-HT) receptor agonist, quipazine (1-8 mg/kg i.v.) and the 5-HT releaser, fenfluramine (4-8 mg/kg i.v.). The effect of L-5-HTP or quipazine on the frequency of twitches was inhibited by the 5-HT receptor antagonist cyproheptadine. Also L-DOPA (100 mg/kg i.p.) or the alpha 1-adrenoceptor agonist, cirazoline (0.3-3 mg/kg i.v.) evoked twitches of the muscle which were inhibited by the alpha 1-adrenoceptor antagonist, prazosin. In decerebrate, artificially respired rats, neither L-5-HTP nor L-DOPA evoked the twitches. The frequency of twitches evoked by fenfluramine but not by L-DOPA was increased by the alpha 2-adrenoceptor agonist, clonidine (0.2 and 0.4 mg/kg i.v.); clonidine's effect was abolished by the alpha 2-adrenoceptor antagonist, yohimbine. The beta 2-adrenoceptor agonist, salbutamol (0.01-1 mg/kg i.v.) had no effect on fenfluramine-induced twitches. It is concluded that (1) activation of 5-HT receptors or alpha 1-adrenoceptors in the brain of urethane-anaesthetized rats evokes twitches of the anterior digastricus muscle, and (2) this preparation can be utilized as a test to study the action of compounds on central 5-HT and adrenergic systems.     

Blood-brain barrier and neuronal membrane transport of 6-[18F]fluoro-L-DOPA.

The transport of 6-[18F]fluoro-L-3,4-dihydroxyphenylalanine ([18F]FDOPA) across the blood-brain barrier (BBB) and neuronal membranes was compared with that of L-3,4-dihydroxyphenylalanine (L-DOPA) in rats. The carotid injection method was used as a direct measurement of [18F]FDOPA, 1-[14C]-L-DOPA, and 3-[14C]-L-DOPA transport across the BBB, while isolated nerve terminals were used to examine neuronal membrane transport of [3H]-L-DOPA. [18F]FDOPA appeared to use the same large neutral amino acid carrier for BBB transport as L-DOPA and L-phenylalanine. In addition, carbidopa [L-alpha-hydrazino-alpha-methyl-beta-(3,4-dihydroxyphenyl)propionic acid] was found not to have direct interference with the transport carrier on the BBB, but indirectly inhibited aromatic L-amino acid decarboxylase (AAAD) activity in brain endothelium by depletion of pyridoxal phosphate, a necessary cofactor of the enzyme. In striatal and cortical synaptosomes, [3H]-L-DOPA uptake was inhibited by non-radioactive L-DOPA, FDOPA, and 6-fluoro-L-meta-tyrosine (6-FMT). The inhibition was significantly greater in terminals isolated from the striatum than in those from the cerebral cortex. FDOPA, 6-FMT, and L-DOPA equally inhibited the neuronal transport of [3H]-L-DOPA. This suggests that FDOPA and 6-FMT compete with L-DOPA at similar transport sites at the neuronal membrane.     

BIA 3-202, a novel catechol-O-methyltransferase inhibitor, enhances the availability of L-DOPA to the brain and reduces its O-methylation.

1-[3,4-Dihydroxy-5-nitrophenyl]-2-phenyl-ethanone (BIA 3-202) is a new long-acting catechol-O-methyltransferase (COMT) inhibitor with limited access to the brain. The present study evaluated the interference of BIA 3-202 upon levels of L-3,4-dihydroxyphenylalanine (L-DOPA) and metabolites in plasma (3-O-methyl-L-DOPA) and brain [3-O-methyl-L-DOPA, dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA)] in rats orally treated with L-DOPA (20 mg/kg) plus benserazide (30 mg/kg). At different time points (1, 3 and 6 h) after the administration of BIA 3-202 (0, 3, 10 and 30 mg/kg) or L-DOPA plus benserazide, rats were sacrificed and the right striatum was quickly dissected out and stored for the assay of L-DOPA, 3-O-methyl-L-DOPA, dopamine and amine metabolites. Levels of L-DOPA, 3-O-methyl-L-DOPA, dopamine, DOPAC and HVA in the striatum in L-DOPA plus benserazide-treated rats were higher than in vehicle-treated rats. However, this increase in striatal L-DOPA, dopamine, DOPAC and HVA was, in a dose- and time-dependent manner, even higher (P<0.05) in rats given BIA 3-202 (3, 10 and 30 mg/kg). This effect was accompanied by a marked decrease in 3-O-methyl-L-DOPA levels in the striatum of L-DOPA plus benserazide-treated rats. Increases in levels of L-DOPA and decreases in 3-O-methyl-L-DOPA levels in plasma also accompanied the administration of BIA 3-202. BIA 3-202 did not significantly affect levels of DOPAC and HVA in the striatum in vehicle-treated rats. It is concluded that administration of BIA 3-202 enhances the availability of L-DOPA to the brain by reducing its O-methylation in the periphery, which may prove beneficial in parkinsonian patients treated with L-DOPA plus an aromatic amino acid decarboxylase inhibitor.     

Support for limited brain availability of tyrosine in patients with schizophrenia

Several mechanisms have been suggested to account for altered dopaminergic neurotransmission in schizophrenia. The brain is the only organ for which amino-acid transport is limited and competition for transport over the blood-brain barrier (BBB) occurs at physiological plasma concentrations. One line of research suggests that patients with schizophrenia have altered brain levels of the essential amino acid tyrosine, the precursor for the synthesis of dopamine. The most common hypothesis is that less tyrosine is available because of competition with elevated levels of other amino acids. By consequence, the synthesis of dopamine in the brain will decrease. In contrast, another line of evidence suggests a change in the affinity for one of the transport proteins. A limitation of this research has been that the systems for amino-acid transport across the BBB have not been fully characterized at a molecular or functional level. The L system is the major system for transport of tyrosine across cell membranes including the BBB. The A system is also involved in this transport. Earlier in-vitro studies using fibroblasts have demonstrated a normal L system in schizophrenia but nevertheless reduced tyrosine transport. The combination of molecular research, fibroblast techniques, and brain imaging provides a new basis for clinical research on the role of amino-acid membrane transport in schizophrenia.     

Evidence for beta 2-adrenoceptor-mediated facilitation of 3H-noradrenaline release from isolated guinea pig papillary muscles

The effects of beta-adrenoceptor agonists and antagonists on field-stimulated release of radioactivity from superfused guinea pig papillary muscles preincubated with 3H-noradrenaline were studied. Stimulation-evoked overflow of tritium was abolished in the absence of Ca2+ or the presence of tetrodotoxin. Isoprenaline (1 mumol/L) caused a slight facilitation of evoked overflow, whereas phentolamine (1 mumol/L) exerted a strong facilitatory action. However, when phentolamine (1 mumol/L) was present throughout superfusion, isoprenaline and the selective beta 2-adrenoceptor agonist, zinterol, caused concentration-dependent increases (half-maximal effects at 1 nmol/L). The effects of the agonists were inversely related to stimulation frequency. Furthermore, the concentration-response curve of isoprenaline was shifted to the right by the selective beta 2-adrenoceptor antagonist, ICI 118,551, but not by the selective beta 1-adrenoceptor antagonist, ICI 89,406. Schild-plot analysis revealed competitive antagonism and a pA2 value of 9.04 for ICI 118,551. Both ICI 118,551 and ICI 89,406, as well as beta-adrenoceptor antagonists with intrinsic sympathomimetic activity (pindolol and celiprolol; 1 mumol/L), had no effect on stimulation-evoked overflow of tritium (phentolamine present). It is concluded that guinea pig papillary muscles are endowed with prejunctional beta 2 adrenoceptors facilitating impulse-evoked noradrenaline release. The facilitation is markedly promoted by blockade of prejunctional alpha adrenoceptors.     

The effects of central aromatic amino acid DOPA decarboxylase inhibition on the motor actions of L-DOPA and dopamine agonists in MPTP-treated primates

1. Endogenous L-DOPA may act as a neuromodulator contributing to the production of motor activity. We now investigate the effects of the centrally acting aromatic amino acid dopa decarboxylase (AADC) inhibitor NSD-1015 (3-hydroxybenzyl hydrazine) on the motor actions of L-DOPA and dopamine agonist drugs in MPTP treated common marmosets. 2. Pretreatment with NSD-1015 (10 - 50 mg kg(-1); i.p.) worsened baseline motor deficits in MPTP-treated common marmosets. Similarly, it abolished L-DOPA (5 - 18 mg kg(-1) s.c.) induced locomotor activity and reversal of disability. NSD-1015 pretreatment inhibited dopamine formation and elevated L-DOPA levels in plasma. 3. The increase in locomotor activity and improvement in disability produced by the administration of the D-1 agonist A-86929 (0.03 - 0. 04 mg kg(-1) s.c.) or the D-2 agonist quinpirole (0.05 - 0.3 mg kg(-1) i.p.) was abolished by NSD-1015 (25 mg kg(-1) i.p.) pretreatment. While the effects of a low dose combination of A-86929 (0.04 mg kg(-1) s.c.) and quinpirole (0.05 mg kg(-1) i.p.) were inhibited by NSD-1015 (25 mg kg(-1) i.p.), there was little effect on the action of a high dose combination of these drugs (0.08 mg kg(-1) A-86929 and 0.1 mg kg(-1) quinpirole). 4. Following central AADC inhibition with NSD-1015 (25 mg kg(-1) i.p.), locomotor behaviour induced by administration of high dose combinations of A-86929 (0.08 mg kg(-1) s.c.) and quinpirole (0.1 mg kg(-1) i.p.) was unaffected by L-DOPA (5 mg kg(-1) s.c.) pretreatment. 5. These results do not support a role for endogenous L-DOPA in spontaneous or drug induced locomotor activity. Rather, they strengthen the argument for the importance of endogenous dopaminergic tone in the motor actions of dopamine agonists.     

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.     

Characterization of atypical beta-adrenoceptors in the guinea pig duodenum.

The atypical beta-adrenoceptors mediating relaxation in the guinea pig duodenum were studied using catecholamines (isoprenaline, noradrenaline and adrenaline), a selective beta3-adrenoceptor agonist BRL37344 ((R*,R*)-(+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phe noxyacetic acid sodium salt) and a non-conventional partial beta3-adrenoceptor agonist CGP12177A ((-)-4-(3-t-butylamino-2-hydroxypropoxy)benzimidazol-2-one)). Catecholamines and beta3-adrenoceptor agonists induced concentration-dependent relaxation in this preparation. Propranolol (1 microM) produced only small rightward shifts in the concentration-response curves of these agonists. In the presence of propranolol (1 microM), however, a non-selective beta1-, beta2- and beta3-adrenoceptor antagonist bupranolol caused a concentration-dependent rightward shift of the concentration-response curves for catecholamines and beta3-adrenoceptor agonists. Schild plot analyses of the effects of bupranolol against these agonists gave pA2 values of 6.02 (isoprenaline), 5.98 (noradrenaline), 5.93 (adrenaline), 6.51 (BRL37344) and 5.70 (CGP12177A), respectively, and all Schild slopes were not significantly different from unity. These results suggest that atypical beta-adrenoceptors are present in the guinea pig duodenum and involved in mediating the functional relaxant response.     

The interaction between salmeterol and beta 2-adrenoceptor agonists with higher efficacy on guinea-pig trachea and human bronchus in vitro.

1. In guinea-pig tracheal preparations precontracted with 1 mumol l-1 carbachol, formoterol, procaterol, fenoterol, salmefamol, salbutamol and terbutaline (in that order of potency) caused a concentration-dependent and almost complete, relaxation. However, under these conditions, the maximum relaxation by salmeterol was approximately 30% of the maximum attainable relaxation. 2. We have therefore explored the ability of salmeterol to inhibit the relaxant response to beta 2-adrenoceptor agonists of different chemical structure and relatively higher efficacy in smooth muscle preparations from guinea-pig trachea and human bronchus. 3. With 1 mumol l-1 salmeterol in the organ bath, the concentration-effect curves for the other agonists were shifted to the right in a variable way by 1.8-2.8 log units, fenoterol and salbutamol being the extremes. 4. When 20 mumol l-1 sulfonterol, another low efficacy beta 2-adrenoceptor agonist, was substituted for salmeterol, the difference in the magnitude of the rightward shift between fenoterol and salbutamol was eliminated. 5. In the human bronchus, formoterol and terbutaline had a higher apparent efficacy than salmeterol. With 1 mumol l-1 salmeterol in the organ bath, the concentration-effect curve for formoterol was shifted 2.7 log units to the right. 6. Salmeterol inhibits, competitively, relaxant responses to beta 2-adrenoceptor agonists with higher efficacy. The degree of inhibition seems to be dependent on the agonist used. This contrasts with results obtained with sulfonterol and suggests that salmeterol interacts with the beta 2-adrenoceptor in a complex way.     

Dynamics of experimental vasogenic brain oedema in the rat: changes induced by adrenergic drugs

The effects of adrenergic drugs on the formation and resolution of cerebral oedema in a rat model of cold-induced vasogenic brain oedema were studied. Evans blue dye extravasation, water content and ultrastructural alterations (pinocytotic vesicle formation in capillary endothelial cells and apparent water accumulation in the brain parenchyma) were evaluated in parietal cortex. Previous administration of the alpha-adrenoceptor antagonist phenoxybenzamine produced a reduction of Evans blue extravasation and water content, diminished vesicle formation and reduced water accumulation. Previous administration of the beta2-adrenoceptor agonist clenbuterol reduced Evans blue extravasation and water content, but did not change vesicle frequency. The effects of clenbuterol on Evans blue passage to the brain were blocked by timolol (beta-adrenoceptor antagonist) but not by metoprolol (selective beta1-adrenoceptor antagonist). When given after the application of cold, clenbuterol was also able to reduce Evans blue and water content in the brain. Isoprenaline (beta-adrenoceptor agonist that does not cross the blood-brain barrier) showed a reduction in Evans blue extravasation only when given intracerebroventricularly. Vinblastine (a drug that prevents vesicle formation) produced a reduction of the amount of pinocytotic vesicles. We conclude that there is an influence of the central adrenergic nervous system on the formation and/or resolution of vasogenic brain oedema and that the alterations on water movement and Evans blue transport mediated by adrenergic drugs seem to be due, at least in part, to alterations of pinocytotic activity in capillary endothelial cells.     

Peripheral mediation of effects of clenbuterol on locomotor and investigatory behavior in rats

Clenbuterol is one of the few beta adrenergic agonists that readily passes the blood-brain barrier. Hence, the behavioral effects in rats of systemic administrations of clenbuterol have been used as a reflection of the activation of central beta receptors. The present experiments were designed to test the hypothesis that the reduction in locomotor activity induced by clenbuterol is mediated by central rather than peripheral beta receptors. First, dose-dependent reductions in ambulation, holepoking, and rearing were established following intraperitoneal injections of 0.004 to 1.0 mg/kg clenbuterol. These effects were then found to be similar to those of 0.4 mg/kg isoproterenol, a mixed beta adrenergic agonist that does not enter the brain after systemic administration. The behaviorally suppressive effects of either 0.4 mg/kg isoproterenol or 0.05 mg/kg clenbuterol were found to be completely antagonized by pretreatment with a 10.0 mg/kg dose of nadolol, a beta antagonist that does not penetrate the brain when administered systemically. Nadolol itself had no significant effects on behavior. These results indicate that these behavioral effects of systemic administrations of clenbuterol are mediated by the activation of peripheral rather than central beta adrenergic receptors.     

Studies on the mechanisms by which clenbuterol, a beta-adrenoceptor agonist, enhances 5-HT-mediated behaviour and increases metabolism of 5-HT in the brain of the rat

The head twitch response in mice produced by injection of 5-hydroxytryptophan (100 mg/kg i.p.) and carbidopa (25 mg/kg i.p.) was enhanced by administration of clenbuterol (0.5 mg/kg i.p.), a beta-adrenoceptor agonist. Clenbuterol also enhanced the hyperactivity syndrome in rats produced by quipazine (25 mg/kg i.p.), a 5-hydroxytryptamine (5-HT) agonist. This enhancement was not prevented by depletion of 5-HT in brain with p-chlorophenylalanine or after pretreatment with prazosin. The behavioural responses of the rats to administration of the alpha 2-adrenoceptor agonist, clonidine, was unaltered by acute or longer-term administration of clenbuterol. Following chronic administration of clenbuterol (5 mg/kg daily for 14 days), a procedure resulting in down-regulation of central beta-adrenoceptors, a larger dose of clenbuterol was necessary to enhance the quipazine-induced hyperactivity, suggesting that the mechanism of enhancement involved central post-synaptic beta-adrenoceptors. Further evidence for this conclusion was that a lesion of central noradrenaline pathways produced by 6-hydroxydopamine did not abolish the clenbuterol-induced enhancement of the quipazine-mediated behaviour. The binding characteristics of 5-HT2-receptors were unchanged by acute or chronic administration of clenbuterol. Clenbuterol (5 mg/kg) increased the percentage of plasma free (non-albumin bound) tryptophan, plasma free fatty acid concentration and the concentration of tryptophan and 5-hydroxyindoleacetic acid (5-HIAA) in the brain. The increase in 5-HT turnover in brain was prevented by pretreatment with the beta 1-adrenoceptor antagonist atenolol, which enters the brain poorly. It is therefore suggested that the clenbuterol-induced increase in 5-HT metabolism results from the increase in the concentration of plasma free fatty acid which increases plasma free tryptophan and thus increases the concentration of tryptophan in brain and 5-HT synthesis in brain. The clenbuterol-induced enhancement of 5-HT-mediated behaviour is therefore not associated with its effect on 5-HT metabolism. The data are discussed in relation to that obtained after administration of antidepressant drugs.     

Studies on the activity of L-threo-3,4-dihydroxyphenylserine (L-DOPS) as a catecholamine precursor in the brain. Comparison with that of L-dopa

L-Threo-3,4-dihydroxyphenylserine (L-DOPS) was compared with L-3,4-dihydroxyphenylalanine (L-DOPA) with respect to their activities as central amine precursors. The apparent Km value (the substrate affinity) of L-DOPS for aromatic L-amino acid decarboxylase was nearly equal to that of L-DOPA, whereas the vmax value (the rate of decarboxylation) of L-DOPS was much smaller than that of L-DOPA, the penetration of L-DOPS into the brain through the blood-brain barrier was found to be smaller (about one-fourth) than that of L-DOPA but, for an amine precursor, it was still substantial. Unlike L-DOPA, L-DOPS did not cause a marked accumulation of norepinephrine (NE), the corresponding catecholamine in the brain, but nialamide, a monoamine oxidase inhibitor significantly enhanced the L-DOPS-induced rise of NE. Moreover, the brain concentration of 3-methoxy-4-hydroxy-phenylethyleneglycol (MHPG), the principal end metabolite of NE, was increased markedly by L-DOPS. These results suggest that L-DOPS may act as an NE precursor in the brain and activate NE neurons by increasing the turnover rate of NE.     

Beta-adrenoceptor responses of the airways: for better or worse?

Beta2-adrenoceptor agonists are the first-line treatment of asthma and chronic obstructive pulmonary disease (COPD), in which a short-acting beta2-adrenoceptor agonist is used as required for relief of bronchoconstriction. A long-acting beta2-adrenoceptor agonist may be added to an inhaled corticosteroid as step 3 in the management of chronic asthma. Long-acting beta2-adrenoceptor agonists may also be added in treatment of COPD. This review examines the beneficial and detrimental effects of beta2-adrenoceptor agonists. The beneficial effects of beta2-adrenoceptor agonists are mainly derived from their bronchodilator activity which relieves the bronchiolar narrowing and improves air flow. The potential anti-inflammatory actions of stabilizing mast cell degranulation and release of inflammatory and bronchoconstrictor mediators, is considered. Other potential beneficial responses include improvements in mucociliary clearance and inhibition of extravasation of plasma proteins that is involved in oedema formation in asthma. The side effects of beta2-adrenoceptor agonists are primarily related to beta2-adrenoceptor-mediated responses at sites outside the airways. Of major concern has been the development of tolerance and this is discussed in relation to incidence of increased morbidity and mortality to asthma over the past three decades. A clinical aspect of beta2-adrenoceptor pharmacology in recent years has been the recognition of genetic polymorphism of the receptor and how this affects responses to and tolerance to beta2-adrenoceptor agonists. A controversial feature of beta2-adrenoceptor agonists is their stereoisomerism and whether the inactive (S)-isomer of salbutamol had detrimental actions in the commercially used racemate. The consensus is that despite these adverse properties, beta2-adrenoceptor agonist remains the most useful pharmacological agents in the management of asthma and COPD.     

beta-Adrenoceptor agonists enhance 5-hydroxytryptamine-mediated behavioural responses.

The beta-adrenoceptor agonists, salbutamol, terbutaline and clenbuterol, were investigated for their effect on 5-hydroxytryptamine-mediated (5-HT) hyperactivity. 2 The lipophilic beta-adrenoceptor agonist, clenbuterol (5 mg/kg) enhanced the behaviours induced by quipazine (25 mg/kg), including headweaving, forepaw treading and hind-limb abduction and thus increased automated activity recording. Clenbuterol (5 mg/kg) also enhanced the hyperactivity syndrome produced by the 5-HT agonist, 5-methoxy N,N-dimethyltryptamine (2 mg/kg) and the combination of tranylcypromine (10 mg/kg) and L-tryptophan (50 mg/kg). Salbutamol and terbutaline potentiated quipazine-induced hyperactivity only when given at the higher dose of 20 mg/kg. 3 The effect of clenbuterol in enhancing quipazine hyperactivity was blocked by the centrally acting beta 1-adrenoceptor antagonist, metoprolol (5 mg/kg), but not by the beta 2-adrenoceptor antagonist, butoxamine (5 mg/kg) or the peripherally acting beta 1-adrenoceptor antagonist, atenolol (5 mg/kg). 4 Clenbuterol (5 mg/kg) did not enhance the circling responses produced by methamphetamine (0.5 mg/kg) in unilateral nigrostriatal-lesioned rats. 5 The results suggest that beta-adrenoceptor agonists in common with some established antidepressant treatments produce enhancement of 5-HT-mediated behavioural responses.     

Effect of the beta-adrenoceptor agonist flerobuterol on serotonin synthesis in the rat brain

The influence of 2- and 14-day treatments with flerobuterol, a preferential beta(2)-adrenoceptor agonist, on regional serotonin (5-HT) synthesis in the rat brain was studied by autoradiography using alpha-[(14)C]methyl-L-tryptophan. Flerobuterol was delivered at a rate of 0.5 mg/kg/day using osmotic pumps implanted s.c. The 2-day flerobuterol treatment significantly increased plasma Trp, both free and total, and decreased plasma Leu and Ile. This resulted in a significant increase in the facilitated transport of Trp. There was a significant increase in the synthesis of 5-HT in the 2-day treatment group in the dorsal and median raphe as well as in all postsynaptic structures, with the exception of the hypothalamus. In contrast, after a 14-day treatment, the enhanced facilitated transport of Trp was no longer present, and the increase in the rate of 5-HT synthesis persisted only in the parietal and occipital cortex and the superior colliculus. These data suggest that flerobuterol, similar to other beta-adrenergic agonists, acutely increases 5-HT synthesis, in part, through an elevation of brain Trp availability.     

Brain serotonergic activity and plasma amino acid levels in genetically obese Zucker rats

In order to test the hypothesis that serotonergic activity is abnormal in the brains of genetically obese Zucker rats, levels of serotonin (5-HT); its amino acid precursor, tryptophan (Trp), and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA) were measured in eight brain regions in groups of obese and non-obese male rats. Plasma albumin levels as well as levels of amino acids and related compounds in plasma and in a cortical sample were also determined in the same animals. While Trp was lower in several brain regions of the obese animals, the only region showing a depressed level of 5-HT in the obese group was the mesencephalon. Obese animals also had a lower amount of 5-HIAA in the diencephalon, but no other differences were significant. Both elevations and depressions were observed in cortical amino acid levels in obese animals. The level of plasma albumin was increased in the obese group. Free Trp was decreased in the plasma of obese rats while levels of other amino acids (methionine, leucine, isoleucine, valine and phenylalanine) which compete with Trp for transport across the blood-brain barrier were elevated. Thus the combination of lower plasma free Trp and increased levels of competitive amino acids appears to contribute to decreased levels of Trp in the brain of genetically obese rats.