Metabotropic glutamate receptors modulate synaptic transmission in the perforant path: pharmacology and localization of two distinct receptors

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Metabotropic glutamate receptors (mGluRs) have emerged as an interesting family of eight different receptor subtypes that can be divided into three groups according to their pharmacology and sequence similarity. In the present study, the specific mGluR agonists (1 S,3 R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1 S,3 R)-ACPD) and l(+)-2-amino-4-phosphonobutyric acid (L-AP4) depressed field excitatory postsynaptic potentials (fEPSPs) in the rat dentate gyrus evoked by perforant path stimulation in a concentration-dependent, rapid and reversible manner (EC 50: L-AP4 5.9±1.6 μM, (1 S,3 R)-ACPD 80±34 μM). In a `paired-pulse' stimulation protocol, the first fEPSP showed a stronger reduction, resulting in `paired-pulse' facilitation. The effects of L-AP4 but not of (1 S,3 R)-ACPD could be antagonized by the group III mGluR antagonists ( S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP4) and ( R S)- α-methyl-4-phosphonophenylglycine (MPPG). Moreover, (1 S,3 R)-ACPD was still potently depressing fEPSPs after preperfusion of near saturating concentrations of L-AP4. Together, the results suggest that both substances act on different mGluRs. The effects of (1 S,3 R)-ACPD could not be further differentiated by selective group I or group II mGluR agonists. Although (2 S,1′ S,2′ S)-2-carboxycyclopropylglycine (L-CCG-I) blocked fEPSPs at concentrations ≫1 μM, these effects, as well as L-AP4 effects, were potently antagonized by MAP4. This suggests that mGluR8 might be responsible for the actions of L-AP4 and L-CCG-I. The two different mGluRs showed a distinct distribution when fEPSPs were recorded simultaneously in the outer and middle molecular layer (OML/MML): The L-AP4 sensitive receptor, possibly mGluR8, seems to be located in the OML while (1 S,3 R)-ACPD showed its main effect in the MML.