A detailed knowledge of the processes controlling neuronal excitatory amino acid (EAA) release is necessary for a proper understanding of central excitatory neurotransmission. Here, we report the actions of the selective metabotropic glutamate (mGlu) 5 receptor antagonist, MPEP, following both local and systemic administration, on (S)-3,5-DHPG evoked efflux of pre-accumulated D-[³H]asp in the rat corpus striatum in vivo.

Microdialysis probes were implanted bilaterally into the corpus striatum of male Wistar rats (300-350g), as previously described (Patel et al., 2001). All experiments were performed in freely moving animals over a 2 day period, 24 hrs following implantation. After equilibration, striatal tissue was preloaded by 'reverse dialysis' with D-[³H]asp by perfusion for 45 mins at 1 µl/min (1 µCi/µl; 5 µM final concentration). Perfusion was then performed at 7.5 µl/min with samples collected every 15 mins and 3H content determined by liquid scintillation spectrometry. Drugs were diluted in aCSF and applied locally via the probe by 'reverse dialysis'. For systemic administration, MPEP was prepared in 50% PEG 400 and sterile water and injected intraperitoneally (i.p.)D-[³H]Asp efflux rapidly reached a stable baseline which showed significant (60%) Ca²⁺-dependency (P < 0.05, n= 5) and tetrodotoxin insensitivity (P > 0.05, n = 5). Addition of 100 mM KCl to the dialysis stream (15 mins) evoked a 5 fold increase in release of D-[³H]asp. Local application of the selective Group 1 mGlu receptor agonist, (S)-3,5-DHPG (1 - 300 µM), caused a concentration-dependent enhancement of D-[³H]asp release (max. 110.5 ± 26.3 % over basal; P < 0.01, n = 6). A sub-maximal increase of 91.8 ± 19.7 % over basal was observed with 100 µM (S)-3,5-DHPG (P <0.01, n = 6). This concentration was used in subsequent experiments testing the effects of mGlu receptor antagonists on (S)-3,5-DHPG-evoked D-[³H]asp efflux. When co-perfused with (S)-3,5-DHPG (100 µM), the potent, non-competitive mGlu1-selective antagonist CPCCOEt (100-300 µM) failed to significantly alter the agonist-evoked responses (P > 0.05, n = 6). Co-perfusion of the mGlu5-selective, non-competitive antagonist MPEP (300 µM) with (S)-3,5-DHPG, however, significantly inhibited these responses by 66.5 % (P < 0.05; n = 6). MPEP (3-60 mg/kg) was applied i.p. 1 hr prior to local administration of (S)-3,5-DHPG. At 60 mg/kg, MPEP had a marked inhibitory effect on the agonist-evoked responses, reducing them by 75.2 % (P < 0.05; n = 5). Neither MPEP nor CPCCOEt showed significant effects when applied alone, either locally or systemically.

These data confirm a role for the mGlu5 receptor subtype in the control of neuronal EAA release in the rat striatum in vivo and highlight the potential therapeutic usefulness of MPEP in disorders involving abnormal or excessive EAAergic neurotransmission in the mammalian brain.

We thank Dr. F. Gasparini (Novartis Pharma AG) for the generous gift of MPEP.

Patel et al., (2001) Neuroscience 102, 101-111.

AF is a BBSRC CASE award student.