FREQUENCY-DEPENDENT DEPRESSION OF EXCITATORY SYNAPTIC TRANSMISSION ISINDEPENDENT OF ACTIVATION OF MCPG-SENSITIVE PRESYNAPTIC METABOTROPIC GLUTAMATE RECEPTORS IN CULTURED HIPPOCAMPAL-NEURONS

1. A paired-pulse paradigm, and a high-frequency train followed by a t est pulse, were used to investigate the possible role of presynaptic m etabotropic glutamate receptors (mGluRs) in frequency-dependent modula tion of the amplitude of excitatory postsynaptic currents (EPSCs). Pai red whole cell patch-clamp recordings from monosynaptically connected hippocampal neurons maintained in very low-density cultures were perfo rmed, using the mGluR antagonist (RS)-alpha-methyl-4-carboxyphenylglyc ine (MCPG, 500 mu M) and the mGluR agonist (1S,3R)-1-aminocyclopentane -1,3-dicarboxylic acid [(1S,3R)-ACPD, 100 mu M]. 2. Paired-pulse depre ssion (PPD) was observed in all the excitatory pairs recorded. The ave rage PPD ratio (amplitude of the 2nd EPSC divided by the amplitude of the 1st EPSC) was 0.80 +/- 0.1 (SD) (n = 8). Application of the mGluR antagonist MCPG had no effect on the amplitude of the EPSCs and did no t affect the ratio of the two EPSCs (PPD ratio 0.79 +/- 0.2). 3. The a mplitudes of 10 successive EPSCs stimulated at a high frequency (20 Hz ) decremented on average in both 4 mM extracellular Ca2+ (n = 5) and i n 1 mM extracellular Ca2+ (n = 6). In all pairs tested, posttetanic de pression (PTD) was observed (PTD ratio 0.7 +/- 0.2). Bath application of MCPG (500 mu M) did not affect the amplitudes of the EPSCs during t he train; MCPG also did not affect PTD. 4. The mGluR agonist (1S,3R)-A CPD depressed the amplitudes of the EPSCs in both the paired-pulse (1s t EPSC, 35 +/- 9%; 2nd EPSC, 36 +/- 10%) and posttetanic pulse (1 and 4 mM extracellular Ca2+) paradigms. The amount of depression observed, both PPD and PTD, remained unaffected by application of (1S,3R)-ACPD. Coapplication of the antagonist MCPG (500 mu M) blocked the effects o f (1S,3R)-ACPD (100 mu M). 5. We conclude that frequency-dependent dep ression of EPSC amplitudes occurs independent of endogenous activation of MCPG-sensitive mGluRs in cultured hippocampal neurons. Moreover, w e demonstrate that exogenous activation of mGluRs by the agonist (1S,3 R)-ACPD can produce additional EPSC depression above that already pres ent due to frequency-dependent mechanisms.