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1. GABAB autoreceptors are a subclass of GABAB receptors that inhibit the release of [3H]GABA from GABAergic nerve terminals. Baclofen is an agonist that reduces [3H]GABA, whilst the antagonist (+)-(S)-5,5-dimethylmorpholinyl-2-acetic acid (Sch 50911) enhances [3H]GABA release in electrically-stimulated rat neocortical brain slices preloaded with [3H]GABA. Here, the pharmacological actions of a series of compounds derived from the positive allosteric modulator, 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol (CGP7930), were examined on GABAB autoreceptors.2. The compound, 3-(3,5-ditbutyl-4-hydroxyphenyl)-2,2-dimethyl-1-oximinopropane (compound 2), at 10 μmol/L had little effect on the stimulation-induced overflow of [3H]GABA when superfused alone, but when superfused in the presence of baclofen (2 μmol/L) inhibited the overflow of [3H]GABA. These effects were reversed by Sch 50911 (10 μmol/L). Although compounds 1-(4-chlorophenyl)-3-(4-hydroxy-3,5-diisopropylphenyl)-2-methyl-1-oximinopropane (compound 1), 1-[(3,5-ditbutyl-4-hydroxyphenyl)methyl]-1-oximinomethylcyclohexane (compound 3), 3-(3,5-ditbutyl-4-hydroxyphenyl)-1,2-diphenyl-1-oximinopropane (compound 4) and 4-(3,5-ditbutyl-4-hydroxyphenyl)-3-methyl-2-oximinobutane (compound 5) (each at 10 μmol/L) tended to reduce the stimulation-induced overflow in the presence of baclofen, an effect reversed by Sch 50911, their status as modulators is not confirmed in the present study.3. Another derivative, 3-(3,5-ditbutyl-4-hydroxyphenyl)-1-(4-chlorophenyl)-2-methyl-1-oximinopropane (compound 6) (10 μmol/L), acted as an agonist as it inhibited the release of [3H]GABA by 32% (EC50 of 3.3 μmol/L), an effect reversed by Sch 50911 (10 μmol/L). The other compounds, 1-[(3,5-ditbutyl-4-hydroxyphenyl)methyl]-1-methyl-2-oximinocyclohexane (compound 7), 4-(3,5-ditbutyl-4-hydroxyphenyl)-3,3-dimethyl-2-oximinobutane (compound 8) and 4-(4-hydroxy-3,5-diisopropylphenyl)-3,3-dimethyl-2-oximinobutane (compound 9) (each at 10 μmol/L), were inactive.4. These findings indicate that this series of compounds show different modes of activity at GABAB autoreceptors.