Phenylglycine derivatives as antagonists of group III metabotropic glutamate receptors expressed on neonatal rat primary afferent terminals

Abstract

1. Three novel phenylglycine analogues; (RS)-alpha-methyl-3-chloro-4-phosphonophenylglycine (UBP1110), (RS)-alpha-methyl-3-methoxy-4-phosphonophenylglycine (UBP1111) and (RS)-alpha-methyl-3-methyl-4-phosphonophenylglycine (UBP1112) antagonised the depression of the fast component of the dorsal root-evoked ventral root potential induced by (S)-AP4 with apparent K(D) values of: 7.4+/-2.3, 5.4+/-0.6 and 5.1+/-0.3 micro M (all n=3), respectively. 2. A Schild analysis of the antagonism of (S)-AP4 induced depression of synaptic transmission by UBP1112 revealed a pA(2) value of 5.3 and a slope of 0.81+/-0.26 (n=9). 3. None of the phenylglycines tested were potent antagonists of responses mediated by group II mGlu receptors (apparent K(D) values >480 micro M). UBP1112 when tested at a concentration of 1 mM had little or no activity on (S)-3,5-DHPG-, NMDA-, AMPA- or kainate-induced responses on motoneurones. 4. UBP1110, UBP1111 and UBP1112 are at least 100-fold selective for group III over group I and II mGlu receptors expressed in the spinal cord making them the most potent, selective, antagonists yet tested at (S)-AP4 sensitive receptors in the spinal cord.

Figure 1

Structures of MCPG, MPPG and CPPG and five new analogues: UBP1110, UBP1111, UBP1112, UBP1130 and UBP1113.

Figure 2

(a) Typical trace showing that (2R,4R)-APDC (3 μM) depressed the fDR-VRP by 58 and 22% in the absence and presence, respectively, of 1 mM UBP1111. (b) Typical trace showing that (S)-AP4 (1 μM) depressed the fDR-VRP by 52 and 32% in the absence and presence, respectively, of 10 μM UBP1111. The experiments were performed in Mg²⁺/(R)-AP5-containing medium. Control depressions of the fDR-VRP were achieved prior to application of antagonist, which was then superfused over the preparation for 15 min before the application of the same concentration of agonist. The reversal of the depressant effects of the agonists can clearly be seen for the antagonist. Recovery of the original response was achieved after a 30 min washout of the antagonist. C, control; R, recovery. Note that the fDR-VRP did not change in amplitude in the presence of UBP1111. Similar results were obtained for all of the novel antagonists.

Figure 3

Time course analysis showing the recovery of the fDR-VRP with time. (a) 3 μM (2R,4R)-APDC produced approximately a 50% reduction in the fDR-VRP. This depression was attenuated in the presence of 1 mM UBP1111. Recovery of the original response occurred within 30 min of washout of UBP1111. (b) 3 μM (S)-AP4 produced approximately a 50% reduction in the fDR-VRP. This was attenuated in the presence of 10 μM UBP1111. Recovery of the original response occurred within 30 min of washout of UBP1111. Similar results were obtained for all of the novel antagonists.

Figure 4

Concentration–respone curves for the depression of the fDR-VRP in neonatal rat motoneurones by (S)-AP4 and the antagonism of the (S)-AP4 depression by (a) 20 μM UBP1110 (n=3), (b) 10 μM UBP1111 (n=6), (c) 15 μM UBP1112 (n=3) and (d) 200 μM UBP1130 (n=3). Each point represents the mean depression±s.e.m. from at least three independent preparations. (e) Schild's analysis of the antagonism of (S)-AP4-induced responses by UBP1112.

Figure 5

Concentratison–response curves for the depression of the fDR-VRP in neonatal rat motoneurones by group II mGlu receptor agonists. (a) (1S,3S)-ACPD and the antagonism of the (1S,3S)-ACPD-induced depression by 1 mM UBP1110. Each point represents the mean depression±s.e.m. from three independent preparations. (b) (2R,4R)-APDC and the antagonism of the (2R,4R)-APDC-induced depression by 1 mM UBP1112. Each point represents the mean depression±s.e.m. from four independent preparations.

Figure 6

(a) A trace from a typical experiment showing that 1 mM UBP1112 showed little or no significant antagonist effects against NMDA, AMPA, kainate or (S)-3,5-DHPG-induced depolarisations of neonatal rat motoneurones: a(i) in the absence of antagonist; a(ii) in the presence of 1 mM UBP1112 (15 min preincubation); a(iii) after a 30 min washout of UBP1112. (b) A trace from a typical experiment showing that 1 mM UBP1110 showed little or no significant antagonist effects against NMDA, AMPA, kainate or (S)-3,5-DHPG-induced depolarisations of neonatal rat motoneurones: b(i) in the absence of antagonist; b(ii) in the presence of 1 mM UBP1110 (15 min preincubation); b(iii) after a 30 min washout of UBP1110. It is worth noting that neither UBP1110 nor UBP1112 (1 mM) had depolarising or depressant effects when applied alone.