Theta LTP can be induced in the olfactory bulb first synapses. (A) Stimulation and recording configuration. A bipolar stimulation pipette was placed on a bundle of ON fibers that innervated the recorded glomerulus. ON, olfactory nerve; GL, glomerular layer; MC, mitral cell. (B–D) LTP of glomerular field EPSPs induced by ON theta burst stimulation (TBS). (B) Time course of glomerular field EPSPs (upper: single example; lower: average traces from N = 34 recordings). Note field EPSP was potentiated following TBS. (C) Paired-pulse ratio (PPR, N = 7) of ON field EPSPs (interval 50 msec) was depressed following TBS. (D) LTP of glomerular field EPSPs was D-APV independent (N = 5). (E–G) TBS of ON induced LTP in postsynaptic external tufted (ET) cells. (E) Time course of EPSCs recorded from ET cells (N = 6). (F) PPR of EPSCs (N = 4). (G) LTP of ET cell EPSCs are D-APV independent (N = 3). (H,I) TBS of ON induced potentiation of periglomerular (PG) cell EPSCs. (H) Time course of EPSCs (N = 8). (I) PPR of EPSCs (N = 4).