Antagonism of GABA(B) receptors by morpholino-2-acetic acid derivatives Sch 54679 and Sch 51324 in rat brain.

In rat neocortical slices maintained in Mg2+-free Krebs medium, baclofen depressed the rate of spontaneous discharges in a concentration-dependent manner (EC50 = 4.5 microM). This depression was reversibly antagonised by 5-(S,R)-hydroxymethyl-5-methylmorpholinyl-2-(R,S)-acetic acid (Sch 54679) and 2-(R,S)-5-[spirocyclopentyl]-morpholinyl-acetic acid (Sch 51324) (respective pA2 values of 5.8+/-0.15 and 5.4+/-0.2). In electrically-stimulated slices preloaded with [3H]gamma-aminobutyric acid (GABA), Sch 54679 (EC50 = 3 microM) was 2.3 times more potent than Sch 51324 (EC50 = 7 microM) in increasing [3H]GABA release through antagonism of GABA(B) autoreceptors. These structurally novel analogues may be pharmacologically useful for elucidating GABA(B) receptor functions.