Methods: 

In the mammalian olfactory bulb, axonless granule cells (GCs) mediate self- and lateral inhibitory interactions between mitral/tufted cells (MTCs) via reciprocal dendrodendritic synapses. Little is known on plasticity at this synapse. MTC activity during odor sensation occurs in repetitive bursts that are synchronized to the respiration rhythm. We have applied a theta-burst type stimulation protocol (TBS) that mimicks this type of activity to excite GCs via the MTC input. We use whole-cell electrophysiological recordings from GCs in conjunction with glomerular stimulation of MTCs in acute brain slices from juvenile rats (P12-P18).

Results: 

The TBS protocol reliably induces long-term depression of the granule cell EPSP (71 ± 21% of control, n = 12, P < 0.005). Preliminary data indicate that TBS has similar effects in adult mouse GCs.

In a "single sniff" paradigm with just one burst, either LTD or LTP were observed (n = 16; LTD: 78 ± 11% of control, n = 8; LTP 148 ± 49%, n = 7). This bidirectional plasticity was independent of the occurrence of GC sodium spikes or the maximal depolarization during the induction, while larger EPSPs showed less plasticity than small EPSPs. In the presence of APV, TBS no longer results in LTD (104 ± 20% of control, n = 4), further underscoring the important role of the NMDA receptor at this synapse.

Conclusion: 

The MTC-GC synapse undergoes substantial plasticity following MTC activity modelled on olfactory input. Thus aspects of olfactory memory may be encoded at this synapse.