Comparative actions of the trimetoquinol, tetrahydropapaveroline and salsolinol isomers in beta-adrenoceptor systems

Revisiting the controversial role of salsolinol in the neurobiological effects of ethanol: old and new vistas

The possible involvement of salsolinol (Sal), an endogenous condensation product of ACD (the first metabolite of ethanol) and dopamine, in the neurochemical basis underlying ethanol action has been repeatedly suggested although it has not been unequivocally established, still being a controversial matter of debate. The main goal of this review is to evaluate the presumed contribution of Sal to ethanol effects summarizing the reported data since the discovery in the 1970s of Sal formation in vitro during ethanol metabolism until the more recent studies characterizing its behavioral and neurochemical effects. Towards this end, we first analyze the production and detection of Sal, in different brain areas, in basal conditions and after alcohol consumption, highlighting its presence in regions especially relevant in regulating ethanol-drinking behaviour and the importance of the newly developed methods to differentiate both enantiomers of Sal which could help to explain some previous negative findings. Afterwards, we review the behavioral and neurochemical studies. Finally, we present and discuss the previous and current enunciated mechanisms of action of Sal in the CNS.

Intravenous and intratracheal administration of trimetoquinol, a fast-acting short-lived bronchodilator in horses with ‘heaves’

REASON FOR PERFORMING STUDY

Trimetoquinol (TMQ) is a potent beta-adrenoceptor agonist bronchodilator used in human medicine but has not been evaluated for potential use as a therapeutic agent for horses with 'heaves'.

OBJECTIVES

To assess the pharmacodynamics of TMQ in horses with 'heaves' to determine potential therapeutic effects.

METHODS

Increasing doses of TMQ were administered to horses with 'heaves' by i.v. and intratracheal (i.t.) routes. Doses ranged 0.001-0.2 microg/kg bwt i.v. and 0.01-2 microg/kg bwt i.t. Cardiac and airways effects were assessed by measurement of heart rate (HR) and maximal change in pleural pressure (deltaPplmax), respectively. Side effects of sweating, agitation and muscle trembling were scored subjectively. Duration of action to i.v. (0.2 microg/kg bwt) and i.t. (2 microg/kg bwt) TMQ was evaluated over 6 h.

RESULTS

Intravenous TMQ was an exceptionally potent cardiac stimulant. Heart rate increased at 0.01 microg/kg bwt, and was still increasing after administration of highest dose, 0.2 microg/kg bwt. Airway bronchodilation, measured as a decrease in deltaPplmax, also commenced at 0.01 microg/kg bwt. By the i.t. route, TMQ was 50-100-fold less potent than by i.v. Side effects included sweating, agitation and muscle trembling. Overall, the onset of HR and bronchodilator effects was rapid, within about 3 min, but effects were over at 2 h.

CONCLUSION

When administered i.v. and i.t., TMQ is a highly potent cardiac stimulant and a modest bronchodilator. It may not be an appropriate pharmacological agent by i.v. and i.t. routes for the alleviation of signs in horses with 'heaves'. Further studies of TMQ by oral and aerosol routes are necessary.

POTENTIAL RELEVANCE

In horses, TMQ is a fast-acting bronchodilator with a short duration of action. It could be used as a rescue agent during an episode of 'heaves'. The i.v. and i.t. administration of TMQ is associated with side effects, similar to those reported for all other beta-agonists. However, other routes, such as aerosol and oral, may prove useful and safe for the alleviation of bronchoconstriction typical of 'heaves'.

Plasma and urinary concentrations of trimetoquinol by LC-MS-MS following intravenous and intra-tracheal administration to horses with heaves

Trimetoquinol (TMQ) is a very potent and fast acting bronchodilator in horses with heaves. This study assessed the plasma and urinary concentrations of TMQ in horses with heaves following administration via the intravenous (IV, 0.2 microg/kg) and intra-tracheal (IT, 2 microg/kg) routes. TMQ was administered to six horses affected with heaves (RAO - Recurrent Airway Obstruction, used interchangeably) by the above routes and plasma and urine samples collected and stored at -20 degrees C until analyzed. Solid Phase Extraction (SPE) of TMQ was followed by highly sensitive ESI(+)-LC-MS-MS (ElectroSpray Ionization, positive mode - Liquid Chromatography - Mass Spectrometry - Mass Spectrometry); with a Limit of Detection (LOD) estimated at 1 pg/mL. Following IV administration, TMQ plasma levels peaked at 1 min at 707 pg/mL, and at 9 min at 306 pg/mL following IT administration. Our results show that TMQ plasma concentrations decline rapidly following IV administration, which is consistent with the fast onset and short duration of TMQ effect that was observed in our previous studies. On the other hand, IT administration showed a very unique plasma concentration pattern. From a regulatory standpoint, the current available TMQ ELISA kit was also used in an attempt to detect TMQ from the plasma and urine samples. We report that the ELISA kit was unable to detect TMQ from any of the samples generated in these studies.

Synthesis and evaluation of trimetoquinol derivatives: novel thromboxane A2/prostaglandin H2 antagonists with diminished beta-adrenergic agonist activity

Trimetoquinol (TMQ, 1) is a unique catecholamine with a strong stereodependence for agonism at beta-adrenergic (S > > R) and antagonism at thromboxane A2/prostaglandin H2 (TP; R > > S) receptors. Our laboratory has reported the effects of N-alkylation and modification of the trisubstituted benzyl group in these receptor systems. For iodinated derivative 5, maintaining potency in TP receptor systems (112%) was coupled with maintaining limited potency in beta-adrenergic receptor systems (34% for beta 1 and 47% for beta 2). In this study, several diverse TMQ derivatives were prepared to probe for binding interactions specific to a particular receptor system. Planar amidine 2, which was designed to explore the importance of TMQ's chiral center, showed a dramatic loss of potency (< 1%) in each receptor system. Likewise, the homologation of a previously described N-benzyl derivative (3) to the N-phenylethyl derivative 4 also showed reduced potency (< 3%) in both receptor systems. However, modification of the trimethoxybenzyl group of TMQ to a 4-hydroxy-3-nitrobenzyl group (7) provided a unique lead for TMQ derivatives with significant potency in TP receptor systems (91%) and reduced potency in beta-adrenergic receptor systems (4% for beta 1 and 19% for beta 2).

Demonstration of highly specific and sensitive antibodies to a naturally occurring tetrahydroisoquinoline alkaloid, salsolidine

1. We report for the first time on the production and characterization of antibodies against a naturally occurring tetrahydroisoquinoline, namely salsolidine (6,7-dimethoxy-1-methyl-1,2,3,4-tetrahydroisoquinoline). 2. Immunogen synthesis was carried out by coupling the hapten salsolidine to bovine serum albumin (BSA) as carrier protein on the basis of reductive amination. 3. By immunization of rabbits with salsolidine-BSA conjugate antisalsolidine antibodies were produced. 4. At a final dilution of 1:1700 the highest-titre antiserum bound 35% of 0.21 pmol [3H]salsolidine. This antiserum was used to develop a radioimmunoassay for salsolidine. 5. Cross-reactivity studies revealed a high specificity of the antiserum to the hapten. 6. The antibodies had a high affinity to salsolidine (Ka = 1.5 x 10(9) M-1). 7. Standard curves covered a measuring range of 0.5-70 pmol/tube and the detection limit was found to be 0.27 pmol/tube.

Pharmacologic antagonism of thromboxane A2 receptors by trimetoquinol analogs in vitro and in vivo

Although (-)-(S)-trimetoquinol [1-(3,4,5-trimethoxy-benzyl)- 6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline; TMQ] is recognized as a potent bronchodilator, (+)-(R)-TMQ is a selective antagonist of human platelet aggregation and serotonin secretion induced by thromboxane A2 (TXA2) agonists. To confirm the pharmacological actions of TMQ analogs, the interaction of the drugs with TXA2 receptors was examined in human platelets and in a mouse sudden death model. The inhibitory potencies of TMQ analogs (pIC50 values) for displacement of [3H]SQ 29,548 binding to platelets showed excellent correlation with the respective pIC50 (-log IC50) values for U46619-induced aggregation (r = 0.99, P less than 0.01) and serotonin secretion (r = 0.99, P less than 0.01) in human platelet-rich plasma and for whole blood aggregation (r = 0.99, P less than 0.01). In each system, the rank order of inhibitory potencies was rac-iodoTMQ greater than or equal to (+)-(R)-TMQ greater than rac-TMQ much greater than (-)-(S)-TMQ. Antithrombotic effects of TMQ analogs were evaluated in a mouse sudden death model. In vivo antithrombotic potencies of these compounds were consistent with the in vitro potencies as TXA2 receptor antagonists in platelet systems. Administration of rac-iodoTMQ, (+)-(R)-TMQ and rac-TMQ 15 min before the injection of U46619 (800 micrograms/kg, iv) protected mice against U46619-induced sudden death. On the other hand, (-)-(S)-TMQ did not protect animals against death. Protection of U46619-induced cardiopulmonary thrombosis by TMQ analogs was seen at doses of 3-100 mg/kg.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological properties of novel bicyclic isoquinoline analogs in isolated guinea pig atria, trachea and in human platelets: relationship to trimetoquinol

1. Antiplatelet and beta-adrenoceptor activities of a set of secondary and tertiary N-methyl substituted amine analogs of trimetoquinol (TMQ, I and II, respectively) and 5,8-ethano-l-(p-methoxybenzyl)-1,2,3,4,5,6,7,8-octahydroisoquin oline (bicyclic isoquinoline compounds III and IV, respectively) were examined. 2. Compounds III and IV induced relaxations of guinea pig trachea which were blocked by propranolol whereas neither compound acted as an agonist nor antagonist of beta-adrenoceptors (chronotropy) in guinea pig atria. TMQ analogs (I and II) were agonists in both beta-adrenoceptor systems. 3. When tested in human platelets, compounds III and IV, like the TMQ analogs, blocked several inducers of the prostaglandin-dependent and -independent pathways, and the alpha 2-adrenoceptor-mediated pathway of platelet activation. 4. The bicyclic isoquinoline analogs (III and IV) possessed more selective beta 2-adrenoceptor stimulatory activity and equal or greater inhibitory activity against inducers of the prostaglandin-independent pathways of platelet function than the corresponding TMQ analogs (I and II). 5. These chemically novel lipophilic bicyclic compounds provide a new lead to the development of agents useful for the treatment of asthma and thrombotic disorders.

The route and significance of endogenous synthesis of alkaloids in animals

There is now substantial evidence that several TIQs and beta-carbolines are present in vivo and increase during certain pathological conditions. It still remains to be determined, however, precisely what roles they play in endogenous functions and whether or not they are critical for the expression of these pathological conditions. Accumulating biochemical information continues to support the notion that these compounds can act as false transmitters. The exciting new findings, which will certainly receive a great deal more attention, concern the interaction of some of the beta-carbolines with the benzodiazepine receptor. Determining if a beta-carboline is an endogenous receptor ligand will attract further research interest on the theoretical and specifically clinically-directed levels. Biochemical, morphological, and behavioral data indicate that some of the condensation products can act as neurotoxins. Very few experiments have included an examination of long-term effects of exposure to one of these alkaloids, so the amount of information on this issue is limited. Chronic rather than acute administration of an alkaloid is more likely to mimic the pathological states in which these compounds are hypothesized to play a role. Biochemically, both the dopaminergic and serotonergic systems have been shown to be affected by chronic treatments with certain alkaloids. Progressive and long-term behavioral alterations also have been reported. Such changes may reflect an adaptation to an increase or decrease in activity of particular systems or a neurotoxic action.

Stereoselective interaction of tetrahydroisoquinolines in beta-adrenoceptor systems

In selected beta1- (heart, lipolysis) and beta2-adrenoceptor (trachea) systems, the interaction of racemic-trimetoquinol (TMQ) and the erythro- and threo-diastereomers of 1-(3',4',5'-trimethoxy-alpha-hydroxybenzyl)-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (alpha-hydroxy TMQ) was investigated. Each tetrahydroisoquinoline possessed agonist activity in these beta-adrenoceptor systems. The rank order of potency observed for these compounds was racemic-TMQ greater than erythro-alpha-hydroxy TMQ greater than threo-alpha-hydroxy TMQ. Using isolated fat adipocytes, a favorable correlation was observed between the elevation in c-AMP and pharmacological response for the TMQ stereoisomers and diastereomers of alpha-hydroxy TMQ. The rise in intracellular c-AMP produced by (-)- and (+)-TMQ in fat cells was blocked by the presence of propranolol, and not in the presence of phentolamine. Since considerably higher concentrations (greater 10(-4) M) of these compounds were required to produce a significant inhibition of c-AMP phosphodiesterase activity in adipose tissue, it is proposed that the lipolytic response is a result of stereoselective interaction of these tetrahydroisoquinolines at the level of membrane-bound adenylate cyclase.

Effects of salsolinol on smooth muscle responses to various biogenic amines

Isolated smooth muscle tissues from rats, guinea pigs and rabbits were used to investigate pharmacological effects of salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline). By itself, salsolinol did not produce any profound or sustained sympathomimetic or 5-hydroxytryptamine-like activity in the concentrations used in these experiments (7.7 and 77 muM). Salsolinol was found to antagonize the contraction of the rabbit aorta induced by noradrenaline but did not affect the adrenaline-induced relaxation of the guinea pig tracheal chain. On both the rat fundus and oestrus rat uterus, salsolinol exhibited significant anti-5-hydroxytryptamine activity but responses of these tissues to acetylcholine and histamine remained unaffected, indicating that the action was not non-specific. These results suggest that in vivo formation of salsolinol after ethanol ingestion could produce significant pharmacological effects.