Dept. of Biology, Brandeis University, Waltham, MA 02454.
In addition to contributing postsynaptic currents, NMDA receptors (NMDARs) can occur presynaptically. Here we investigate their function at excitatory synapses onto layer 5 (L5) and L2/3 pyramidal neurons of rat visual cortex.
In L5 thick tufted pyramidal cells, APV (200 ÁM) reversibly reduced the frequency (66▒5%, p<0.0001, n=12) but not the amplitude (p=0.85) of spontaneous AMPA mEPSCs, suggesting that NMDARs normally elevate mEPSC frequency, perhaps by contributing to the resting calcium levels in the presynaptic terminal. APV also reversibly reduced AMPA mEPSC frequency (69.9▒0.4%, p<10-6, n=6) in L2/3 pyramids, without affecting amplitude (p=0.97). Consistent with no activation of postsynaptic NMDARs (holding potential = -90mV, 1mM Mg2+), mEPSC decay time course was not altered (p=0.99).
To examine the functional role of presynaptic NMDARs, we studied the effects of NMDAR blockers (APV, MK801, or ifenprodil) on evoked neurotransmission in L5-to-L5 pairs. Wash-in of NMDAR blockers reduced AMPA receptor mediated responses evoked by 30 Hz (after/before=48▒5%, n=17), but not 0.1 Hz (105▒5%, n=15; p<10-8) spiking. Switching from 0.1 to 30 Hz firing for 15 min suppressed neurotransmission in the presence of MK801 (77.9▒2.8%, n=3), but not in control experiments (100▒6.3%, n=4; p<0.05), suggesting that the presynaptic NMDARs are necessary for maintenance of neurotransmission during high-frequency firing.
In L5 pairs, the NR2B-specific NMDAR blocker ifenprodil reduced neurotransmission evoked at 30 Hz (51▒10%, n=4), but not at 0.1 Hz (103▒6.6%, n=6; p<0.01) in P14-P16 rats. At this age, postsynaptic NMDARs are no longer ifenprodil sensitive. Ifenprodil wash-in did not affect the NMDA:AMPA ratio (103▒6%, p=0.62, n=4), although MK801/APV did (83▒6%, n=15; p<0.05). Hence if presynaptic NMDARs undergo a developmental subunit switch, it occurs at a later age compared to postsynaptic NMDARs.
(This research was funded by NIH)