sentence: Here , LTP can be triggered not only by electrical stimulation ( e.g ., theta-burst , delta-burst ) , activation of metabotropic glutamate receptors ( mGluRs ) and brain-derived neurotrophic factor / tropomyosin receptor kinase B ( BDNF / TrkB ) signaling ( Bortolotto et al ., 1994 ; Balschun et al ., 1999 ; Huang et al ., 2013 ; Schildt et al ., 2013 ) but by learning itself ( Whitlock et al ., 2006 ) .