European Journal of Pharmacology, 235 (1993) 153-155 153 © 1993 Elsevier Science Publishers B.V. All rights reserved 0014-2999/93/$06.00 EJP 21223 Short communication The action of new potent GABA B receptor antagonists in the hemisected spinal cord preparation of the rat Felix Brugger, Urs Wicki, Hans-Rudolf Olpe, Wolfgang Froestl and Stuart Mickel Research and Det,elopment Department, Pharmaceuticals Dit~ision, Ciba-Geigy Ltd., CH-4002 Basel, Switzerland Received 21 December 1992, revised MS received 15 February 1993, accepted 23 February 1993 CGP 52432 (3-N-(3,4-dichlorobenzyl)aminopropyl-P-diethoxymethylphosphinic acid), CGP 54062 (3-N[1-(R,S)-(3,4-dichloro- phenyl)ethyl]amino-2-(S)-hydroxypropyl-P-benzyl-phosphinic acid), CGP 54626 (3-N[[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2- (S)-hydroxypropyl-P-cyclohexylmethylphosphinic acid) and CGP 55845 (3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxy- propyl-P-benzyl-phosphinic acid) are novel selective GABA B receptor antagonists. The apparent K~ values for the complex formed between the GABAB receptor and these compounds were determined using the monosynaptic reflex in the hemisected rat spinal cord preparation in vitro. CGP 55845 was found to be the most potent GABAn receptor antagonist tested (apparent K d = 30 nM). On the same preparation 0.3 ~zM CGP 55845 was equipotent with 100 p~M of CGP 35348 (P-(3-aminopropyl)-P-di- ethoxymethyl-phosphinic acid) for reversal of the depressant action of (R)-(-)-baclofen. GABA~ receptor antagonists; Spinal cord; Monosynaptic reflex; Baclofen; (In vitro) 1. Introduction (Froestl et al., 1992). The IC,s0 values as determined in radio receptor binding assays ([3H]CGP 27492 (amino- Baclofen has been used for almost 20 years for the propyl-phosphinic acid) as ligand) were 65, 13, 7 and 5 treatment of spasticity and is the main selective nM for CGP 52432, CGP 54062, CGP 54626 and CGP GABA B receptor agonist employed for in vivo and in 55845 respectively (H. Bittiger personal communica- vitro studies. It reduces muscle tone in patients with tion). The apparent K d values of these compounds for spinal transections and muscle rigidity or tonic stretch reversal of baclofen-induced depression of the monosy- reflexes in decerebrated animals. It has therefore been naptic reflex in the hemisected rat spinal cord prepara- suggested that the therapeutically relevant effect of tion in vitro are reported below. It is also reported that baclofen results from a direct action at the spinal level these compounds when applied in the absence of ba- (Birkmayer, 1972). Pierau and Zimmerman (1973) clofen had no significant effect on the monosynaptic showed that the depressant effect of baclofen on spinal reflex. reflex activity is primariliy presynaptic through a de- crease of transmitter released from the afferent fibers. Davies (1981) showed that the spinal monosynaptic 2. Materials and methods reflex is very sensitive to the action of baclofen. No major postsynaptic effects of baclofen on the mem- Experiments were carried out using hemisected brahe of the motoneurones have been reported. The spinal cords from 6-10 day old rats (Tif: RAI f(SPF)) GABA B receptor antagonists, phaclofen, 2-hydroxy- essentially as described by Evans (1978). Briefly, under saclofen and CGP 35348, have been shown to reverse urethane anaesthesia, the spinal cord with attached the presynaptic depressant action of baclofen with Kis dorsal and ventral roots (L3-L6) was excised, sagitally in the low micromolar range (Seabrook et al., 1990). hemisected and transferred to a perfusion block. The Recently a series of novel highly potent and selec- preparation was superfused (1 ml/min) with gassed tive GABA~3 receptor antagonists has been described (95% Oe, 5% COe) artificial cerebrospinal fluid (ACSF) which comprised (in mM): NaC1 120; KC1 2.5; KH2PO 4 1.25; CaC12 2.5; MgSO 4 2; NaHCO 3 30 and glucose 10, at 25°C. A dorsal root (L4) was stimulated maximally (3 Correspondence to: F. Brugger, Research and Development Depart- ment, Pharmaceuticals Division, Ciba-Geigy Ltd., K 125/7.19, CH- times threshold) using a pair of silver wire electrodes (1 4002 Basel, Switzerland. stimulus/min). The resultant reflexes were recorded