[en] Dopamine (DA) neurons of the substantia nigra pars compacta (SNc) exhibit two main firing modes, spontaneous single action potential (AP) firing and bursting. The amount of DA released by these neurons in target areas depends on the presynaptic AP firing pattern and is essential to modulate several aspects of behavior such as the control of movement. Bursting activity is mediated by excitatory afferents and specifically by the activation of NMDA receptors (NMDARs). However, the level of activation of NMDARs at these synapses during spontaneous synaptic activity is unknown. We assessed the occupancy of the glycine binding sites of the NMDAR by testing the effects of coagonists and by blocking the uptake of glycine on spontaneous excitatory postsynaptic currents (sEPSCs) in DA neurons of the SNc.
Parasagittal midbrain slices (300 – 350 µm thickness) were cut from the brains 16- to 26- days old Wistar rats using a vibratome. Whole-cell recordings were made using warm (~35C°) standard artificial cerebrospinal fluid. sEPSCs were pharmacologically isolated and recorded at +40 and -70 mV.
A large NMDAR-sEPSC component was present at +40 mV as revealed by the application of 50 µM D-AP5 (n=8, P < 0.001). At -70 mV, D-AP5 had no effect on the EPSC, as expected (n=7, P = 0.52). The mean amplitude of the average NMDAR sEPSC measured at +40 mV increased significantly from 8.6 ± 0.8 pA in control conditions to 10.9 ± 1.0 pA during bath application of 300 μM glycine (n=10, P < 0.001). We also investigated the effect of D-serine, the other coagonist at the NMDAR glycine site. The mean NMDAR EPSC amplitude measured at +40 mV showed a tendency toward larger values, from 6.8 ± 0.8 pA in control conditions to 8.8 ± 1.1 pA in the presence of 100 µM D-serine (n=4). Blockade of the glycine transporter-1 (GlyT1) significantly increased the mean amplitude of NMDAR sEPSCs measured at +40 mV from 7.1 ± 0.5 pA in control conditions to 10.5 ± 0.8 pA in the presence of 5 μM NFPS (n=8, P < 0.001). At -70 mV, NFPS had no effect on the AMPAR-sEPSC component, as expected (n=8, P = 0.23).
In conclusion, our results indicate that the NMDAR glycine sites are not saturated by glycine and D-serine during sEPSCs in control conditions.
