We have shown that activation of a metabotropic glutamate receptor (mGluR), specifically mGluR4a, enhances glutamate release onto neurones in layer V of the rat entorhinal cortex (EC) (Evans et al. 2000). Glutamate release occurs by both action potential (AP)-dependent and -independent means. By simultaneously recording predominantly action potential-independent spontaneous excitatory postsynaptic currents (sEPSCs) and AP-evoked (e)EPSCs we were able to assess the ability of mGluR4a to differentially modulate these forms of glutamate release.

Slices (450 µm) of EC were prepared from Wistar rats (45-60g) anaesthetised with ketamine (120 mg kg^-1 I.M.) and xylazine (8 mg kg^-1 I.M.). Whole-cell voltage clamp recordings (V_h -60 mV) of sEPSCs were made from visually identified neurones in layer V. An extracellular electrode was placed in the white matter and used to generate eEPSCs at 0.033 Hz. Drugs applied by bath perfusion included (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid (ACPT-1, 20 µM), forskolin (10 µM) and SQ22536 (50 µM). Mean eEPSC amplitudes (± S.E.M.) were compared using Student’s paired t test (95 % confidence limit) on samples of 10 eEPSCs. Interevent intervals (IEI) of sEPSCs were compared using with the Kolmogorov-Smirnov test (again at 95 %) applied to cumulative probability distributions (at least 200 events sampled from each neurone in each situation).

The specific mGluR4a agonist ACPT-1 depressed eEPSC amplitude (29.4 ± 2.1 pA to 18.0 ± 1.4 pA, n = 10, P < 0.05) and simultaneously reduced mean IEI (i.e. increased frequency) of sEPSCs (256 ± 50 ms to 151 ± 25 ms, n = 10, P < 0.01). As we have previously reported that the latter effect was mediated through protein kinase A (PKA, Evans et al. 2001), we examined whether the reduction of eEPSC amplitude depended on the same effector. However, perfusion with the PKA activator, forskolin, decreased IEI (483 ± 71 ms to 221 ± 111 ms, n = 3 P < 0.01), but increased the amplitude of eEPSCs (49.2 ± 4.4 pA to 98.6 ± 8.3 pA, n = 3 P < 0.05). When PKA activity was blocked with the specific adenylyl cyclase inhibitor, SQ 22536, ACPT-1 still depressed eEPSC amplitude (35.4 ± 6.1 pA to 21.9 ± 1.2 pA, n = 6 P < 0.01), but the decrease in sEPSC interval was abolished (212 ± 47 ms v 219 ± 49 ms in ACPT-1). Finally we indirectly assessed whether ACPT-1 could alter presynaptic Ca²⁺ influx. ACPT-1 reduced the paired-pulse ratio of eEPSC amplitude (50 ms interval) from 1.65 ± 0.2 to 1.2 ± 0.2 (n = 6, P < 0.05 t test), suggesting that mGluR4a activation reduces presynaptic Ca²⁺ influx.

Thus, presynaptic mGluRs may simultaneously enhance spontaneous glutamate release and depress AP-driven release through distinct effector systems.