Potent NMDA‐like actions and potentiation of glutamate responses by conformational variants of a glutamate analogue in the rat spinal cord

Abstract

1 Neuropharmacological actions of all possible-state isomers of a-(carboxycyclopropyl)glycine (CCG), conformationally restricted analogues of glutamate, were examined for electrophysiological effects in the isolated spinal cord of the newborn rat. 2 Eight CCG stereoisomers demonstrated a large variety of depolarizing activities. Among them, the (2R, 3S, 4S) isomers of CCG (D-CCG-II) showed the most potent depolarizing activity, followed by the (2S, 3R, 4S) isomer (L-CCG-IV). 3 The depolarization evoked by L-CCG-IV, D-CCG-II and other D-CCG isomers was effectively depressed by N-methyl-D-aspartate (NMDA) antagonists. D-CCG-II was about 5 times more potent than NMDA in causing a depolarization. 4 The (2S, 3S, 4S) isomer of CCG (L-CCG-I) was more potent than L-glutamate in causing a depolarization of -spinal motoneurones. The depolarization was slightly depressed by NMDA antagonists, but residual amplitudes of responses to L-CCG-I in the presence of NMDA antagonists were almost insensitive to 6,7-dinitro-quinoxaline-2,3-dione (DNQX) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), suggesting that L-CCG-I might be a novel potent agonist. 5 After application of the (2S, 3S, 4R) isomer of CCG (L-CCG-III), responses to L-glutamate, Dand L-aspartate were markedly enhanced. The enhancement lasted for a period of several hours without a further application of L-CCG-III. 6 L-CCG-III also caused a depolarization, but it seemed unlikely that the potentiation of the glutamate response was directly related to the depolarization evoked by L-CCG-III. 7 The potentiation might be due to inhibition of uptake processes, but L-CCG-III was superior to L-(-)-threo-3-hydroxyaspartate, a potent uptake inhibitor of L-glutamate and L-aspartate, in enhancing the response to L-glutamate in terms of amplitude and duration of responses. 8 CCG isomers should provide useful pharmacological tools for analysis of glutamate neurotransmitter systems.

Novel kainate derivatives: potent depolarizing actions on spinal motoneurones and dorsal root fibres in newborn rats

“…Spinal motoneurones When kainate derivatives tested in the present study ( Figure 1) (Figure 3), and the peak amplitudes of depolarizing responses to excitatory amino acids sometimes varied in accordance with the duration of drug application (Shinozaki et al, 1989b (Figure 4), probably HMPPA: 4-(2-hydroxymethylpyridine)-2-carboxy-3-pyrrolidineacetic acid; CPPA: 4-(2-carboxypyridine)-2-carboxy-3-pyrrolidineacetic acid; Kain: kainic acid; MPPA: 4{2-methylpyridine)-2-carboxy-3-pyrrolidineacetic acid; FPA: 4-phenyl-2-carboxy-3-pyr-rolidineacetic acid; CNOPA: 4-(2-carboxypyridine N-oxide)-2-carboxy-3-pyrrolidineacetic acid; MKPA: 4-methylketone-2-carboxy-3-pyrroli-dineacetic acid. When L-glutamate and the kainoids tested here were added to the perfusion fluid for a period of more than 30s, the amplitude of their depolarizing responses was not maintained, but rapidly declined despite the presence of these excitatory amino acids.…”

“…The methods used for the electrophysiological experiments in the isolated newborn rat spinal cord were essentially similar to those described previously (Shinozaki et al, 1989b). The spinal cords of 1-7 day old Wistar rats were used for the experiments.…”

A novel metabotropic glutamate receptor agonist: marked depression of monosynaptic excitation in the newborn rat isolated spinal cord

“…The pharmacological profile of the recently cloned 'mGluR2' glutamate receptors which is negatively coupled to adenyl cyclase would be expected to be different from that of the metabotropic glutamate receptor coupled to phospholipase C. In the present study, DCG-IV did not cause any depolarization of motoneurones except for that mediated by NMDA receptors, while L-CCG-I and (lS,3R)-ACPD caused a depolarization that was to some extent decreased by 4C3H-PG; but both reduced preferentially the monosynaptic excitation of motoneurones at much lower concentrations than the threshold concentration for depolarization. The quisqualate-induced depolarization also consisted of various components including a metabotropic one, which was quite resistant to CPP and CNQX (Shinozaki et al, 1989b). Quisqualate stimulated the cloned 'mGluR2' glutamate receptor expressed in Chinese hamster ovary cells less effectively than trans-ACPD (Nakanishi, 1992).…”

“…CCG is a conformationally restricted glutamate analogue (Shinozaki et al, 1989b;, and L-CCG-I is almost equal to (lS,3R)-ACPD and more potent than L-glutamate on a molar basis in causing a temperaturedependent depolarization of the newborn rat spinal motoneurone (Ishida et al, 1990b). The depolarizing responses to L-CCG-I and (IS,3R)-ACPD were neither depressed by selective NMDA antagonists nor by a non-NMDA blocker, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), demonstrating a clear preference for non-NMDA, non-kainate and non-AMPA receptors (Shinozaki et al, 1989b).…”

A novel antagonist, phenylbenzene ω‐phosphono‐α‐amino acid, for strychnine‐sensitive glycine receptors in the rat spinal cord

“…The detailed procedures of the binding experiments have been described previously (Maruyama & Takeda, 1989 Shinozaki et al, 1989). Addition of L-glutamate (0.1-1 mM), GABA (0.03-1 mM), glycine (100 !LM-1 mM), dopamine (30-3001M), ACh (10-300 JiM), noradrenaline (3-30 pM), 5-HT (1-30 jiM), substance P (0.3-3 JiM) and histamine (0.03-1 mM) to the TTXcontaining (0.5 jiM) perfusing fluid (ACSF) for a period of 10 s caused depolarization of spinal motoneurones of newborn rats in a concentration-dependent manner.…”