The dorsal cochlear nucleus (DCN) in the auditory brainstem integrates auditory and non auditory inputs within its molecular layer. Exposure to loud sound (acoustic overexposure: AOE) has been shown to modulate synaptic excitability within the DCN. Furthermore, hyperexcitability in the DCN has been reported to underlie tinnitus (Zhang & Kaltenbach 1998). The mechanisms responsible for those excitability changes are still poorly understood, particularly in the molecular layer where synaptic plasticity has been reported. Here we investigated whether AOE altered synaptic plasticity within DCN slices of P17-P22 Wistar rats, 4 to 7 days after AOE. Rats were anaesthetized (with fentanyl 0.15mg/kg; fluanisone 5mg/kg, hypnovel 2.5mg/kg, i.p.) and exposed to a loud (110dB SPL) single tone (15kHz) for 6 hours. Control rats were anesthetized (but unexposed). Hearing threshold shifts of 30-40 dB SPL were observed 4 to 7 days after AOE at frequencies exceeding 15kHz (p<0.05, n=5). In unexposed condition, 0.3Hz stimulations of the DCN molecular layer triggered post synaptic field potentials in the DCN fusiform layer that were blocked by the glutamatergic AMPA receptor antagonist, NBQX. Furthermore, high frequency stimulations (HFS; 30 seconds 50 Hz) of the DCN molecular layer triggered an increase of the postsynaptic field potential peak amplitude from 0.35±0.04mV to 0.50±0.06mV (n=21, p<0.05). This long term potentiation (LTP) persisted for over 30mins. However after AOE, HFS were unable to elicit LTP in the DCN (-4±6%; n=20, NS). Paired pulses performed at 50ms showed a paired pulse facilitation (PPF: 1.28±0.11, n=10, p<0.05) in the unexposed condition by contrast to AOE where PPF was absent (0.97±0.07; n=8, P>0.05). This latter indicated that the absence of LTP after AOE was due to higher release probability at DCN multisensory synapses. This was further confirmed by changing extracellular [Ca2+]e to increase or decrease the release probability. In the unexposed condition, there was no PPF (0.92±0.07; n=5, P>0.05) when tests were carried out in 3mM [Ca2+]e (instead of 2mM [Ca2+]e). Increasing [Ca2+]e from 2 to 3mM in the unexposed condition stopped the induction of LTP after HFS (-4±7%; n=7, P>0.05). By contrast after AOE, decreasing [Ca2+]e from 2 to 1mM reproduced the induction of LTP previously observed in the unexposed condition (field potentials increased from 0.37±0.05mV to 0.32±0.06mV; n=11, p<0.05). Paired pulse tests carried out in 1mM [Ca2+]e after AOE also showed PPF (2.7±0.5; n=4, p<0.05). In conclusion, after AOE, the absence of LTP at glutamatergic DCN multisensory synapses is due to an increased release probability and can be reversed by lowering extracellular calcium. This finding is an important step to finding selective therapeutic targets against tinnitus after AOE. (All values are mean±sem and Paired T tests are used).