Rolf H. Prager

Phaclofen: a peripheral and central baclofen antagonist

Phaclofen, the phosphonic acid derivative of baclofen, reversibly antagonized the depression of the cholinergic twitch response of the guinea pig ileum and distal colon by either baclofen or GABA. When administered microelectrophoretically, phaclofen reversibly blocked the presumed presynaptic reduction by baclofen of the monosynaptic excitation of spinal interneurones by impulses in primary afferent fibres of the cat but did not block the postsynaptic depressant action of baclofen on these neurones. Phaclofen may thus be useful in determining the physiological significance of central and peripheral bicuculline-insensitive receptors with which GABA and (-)-baclofen interact.

Terpinen-4-ol, the main component of the essential oil of Melaleuca alternifolia (tea tree oil), suppresses inflammatory mediator production by activated human monocytes

Abstract:Objective and Design: To evaluate potential anti-inflammatory properties of tea tree oil, the essential oil steam distilled from the Australian native plant, Melaleuca alternifolia.¶Material and Methods: The ability of tea tree oil to reduce the production in vitro of tumour necrosis factor-α (TNFα), interleukin (IL)-1β, IL-8, IL-10 and prostaglandin E2 (PGE2) by lipopolysaccharide (LPS)-activated human peripheral blood monocytes was examined.¶Results: Tea tree oil emulsified by sonication in a glass tube into culture medium containing 10% fetal calf serum (FCS) was toxic for monocytes at a concentration of 0.016% v/v. However, the water soluble components of tea tree oil at concentrations equivalent to 0.125% significantly suppressed LPS-induced production of TNFα, IL-1β and IL-10 (by approximately 50%) and PGE2 (by approximately 30%) after 40 h. Gas chromatography/ mass spectrometry identified terpinen-4-ol (42%), α-terpineol (3%) and 1,8-cineole (2%, respectively, of tea tree oil) as the water soluble components of tea tree oil. When these components were examined individually, only terpinen-4-ol suppressed the production after 40 h of TNFα, IL-1β, IL-8, IL-10 and PGE2 by LPS-activated monocytes. Conclusion: The water-soluble components of tea tree oil can suppress pro-inflammatory mediator production by activated human monocytes.

2-Hydroxy-saclofen: an improved antagonist at central and peripheral GABAB receptors

2-hydroxy-saclofen (2-OH-S), a sulphonic analogue of baclofen, slightly increased the twitch height and reversibly antagonised the GABA- and baclofen-induced depression of twitch contractions in the guinea pig vas deferens and isolated ileum, causing a parallel dextral shift in the baclofen dose-response curve in a competitive manner (pA2 = 5.0) in the latter tissue. 2-OH-S (10-50 microM) reversibly elevated the spike height and antagonised the baclofen (8-20 microM)-induced suppression of ictal discharges in rat cortical slices superfused in Mg2+-free Krebs solution, the spike height declining to control level within 15 min of washout. The antagonism by 2-OH-S on GABAB receptor-mediated actions is selective, as 2-OH-S did not affect depressive responses to adenosine or morphine, or contractile responses to GABA (GABAA receptor-mediated), acetylcholine and carbachol in the ileum. Compared to phaclofen, 2-OH-S is a more potent competitive antagonist of GABAB receptor-mediated actions in the central and peripheral nervous system.

The water-soluble components of the essential oil of Melaleuca alternifolia (tea tree oil) suppress the production of superoxide by human monocytes, but not neutrophils, activated in vitro

Abstract:Objective: To evaluate the regulatory properties of the essential oil of Melaleuca alternifolia (tea tree oil) on the production of oxygen derived reactive species by human peripheral blood leukocytes activated in vitro.¶Materials and methods: The ability of tea tree oil to reduce superoxide production by neutrophils and monocytes stimulated with N-formyl-methionyl-leucyl-phenylalanine (fMLP), lipopolysaccharide (LPS) or phorbol 12-myristate 13-acetate (PMA) was examined.¶Results: The water-soluble fraction of tea tree oil had no significant effect on agonist-stimulated superoxide production by neutrophils, but significantly and dose-dependently suppressed agonist-stimulated superoxide production by monocytes. This suppression was not due to cell death. Chemical analysis identified the water-soluble components to be terpinen-4-ol, α-terpineol and 1,8-cineole. When examined individually, terpinen-4-ol significantly suppressed fMLP- and LPS- but not PMA-stimulated superoxide production; α-terpineol significantly suppressed fMLP-, LPS- and PMA-stimulated superoxide production; 1,8-cineole was without effect.¶Conclusion: Tea tree oil components suppress the production of superoxide by monocytes, but not neutrophils, suggesting the potential for selective regulation of cell types by these components during inflammation.¶

Baclofen antagonism by 2-hydroxy-saclofen in the cat spinal cord.

When administered microelectrophoretically, a sulphonic acid derivative of baclofen, 3-amino-2-(4-chlorophenyl)-2-hydroxy-propylsulphonic acid, reversibly reduced the presynaptic reduction by (-)-baclofen of the monosynaptic excitation of spinal interneurones by impulses in low threshold primary afferent fibres of the cat as well as the postsynaptic depression by (-)-baclofen of the firing of these neurones. This compound, 2-hydroxy-saclofen, may be useful in assessing the physiological significance of central baclofen receptors.

The actions of 2-hydroxy-saclofen at presynaptic GABAB receptors in the rat hippocampus

The actions of 2-hydroxy-saclofen (2-OH-S), a recently developed analog of baclofen, were studied at presynaptic GABAB receptors in the rat hippocampal slice. Baclofen (0.5-20 microM) reduces the amplitude of excitatory postsynaptic potentials (EPSPs) recorded from hippocampal CA1 pyramidal neurons. In the presence of 200-500 microM 2-OH-S, the synaptic depressant action of baclofen is significantly reduced. These data show that 2-OH-S is an effective antagonist at presynaptic GABAB receptors on excitatory terminals in the hippocampus.

Antagonism at GABAB receptors by saclofen and related sulphonic analogues of baclofen and GABA

Saclofen (the direct sulphonic analogue of baclofen) is a competitive antagonist of baclofen at GABAB receptors in guinea pig ileum and rat cortical slices (estimated pA2 = 5.3), at least twice as potent as 2-hydroxy-saclofen (pA2 = 5). A series of related sulphonic analogues also antagonised baclofen in the guinea pig ileum, including 2-hydroxy-saclofen amide (pA2 = 3.3), 3-amino-2-hydroxy-2-phenyl-propylsulphonic acid (pA2 = 3.5), 3-amino-2-(benzo-(b)-furan-2-yl)-propylsulphonic acid (pA2 = 4.3), and 3-amino-2-(4-chlorophenyl)-prop-1-enesulphonic acid (pA2 = 2.5-3), but none were more active than saclofen which is the most potent specific GABAB antagonist yet found.

Blockade of the late IPSP in rat CA1 hippocampal neurons by 2-hydroxy-saclofen

The effects of the GABAB receptor antagonist 2-hydroxy-saclofen were studied using intracellular recording of synaptic potential from CA1 hippocampal neurons. 2-Hydroxy-saclofen (50-200 microM) reversibly blocked the late, GABAB receptor-mediated inhibitory postsynaptic potential (IPSP) but not the early, GABAA receptor-mediated IPSP. In addition, the hyperpolarizing response to baclofen was reduced by similar concentrations of 2-hydroxy-saclofen. This suggests that 2-hydroxy-saclofen is a potent antagonist at postsynaptic GABAB receptors on hippocampal neurons.

Arylalkylamines are a novel class of positive allosteric modulators at GABAB receptors in rat neocortex

Using grease-gap recording from rat neocortical slices, the gamma-aminobutyric acid(B) (GABA(B)) receptor agonists baclofen (3-100 microM) and SKF 97541 (3-aminopropyl-methylphosphinic acid) (1-30 microM) elicited reversible and concentration-dependent hyperpolarizing responses, with EC(50) values of 10 and 3 microM, respectively. The hyperpolarizations were antagonised by the GABA(B) receptor antagonist Sch 50911 ((+)-(S)-5,5-dimethylmorpholinyl-2-acetic acid) (1, 5 and 10 microM). Fendiline (N-[3,3-diphenylpropyl)-alpha-methylbenzylamine) (5-50 microM) and its congeners, prenylamine (N-[3,3-diphenylpropyl)-alpha-methylphenylethylamine) (10-100 microM) and F551 (N-[3,3-diphenylpropyl)-alpha-methyl-3-methoxybenzylamine) (1-30 microM) reversibly enhanced hyperpolarizing responses to the agonists; such effects were reduced by Sch 50911. These arylalkylamines produced leftward shifts of the concentration-response curves, with a marked increase in the maximal hyperpolarization obtained, compared with the agonists alone, F551 being the most potent. These findings suggest that these arylalkylamines represent a new class of positive modulators of GABA(B) receptor-mediated function.

GABAB-receptor-mediated actions of baclofen in rat isolated neocortical slice preparations: antagonism by phosphono-analogues of GABA

Baclofen, 3-amino-propylphosphonic acid (3-APPA), and beta-phenyl-GABA (BPG), each reduced the frequency of spontaneous paroxysmal discharges in rat neocortical slices maintained in Mg2+-free medium, reversibly antagonised by phaclofen and 4-amino-butylphosphonic acid (4-ABPA). At lower concentrations, not influencing the discharges, both 3-APPA and BPG also reversibly antagonised this action of baclofen. However, des-chloro-phaclofen was inactive. Thus, phaclofen and 4-ABPA are GABAB-receptor antagonists in neocortex, whereas both 3-APPA and BPG have partial agonist/antagonist activity at cortical GABAB-receptors.