Inhibitors of oxidative and hydrolytic endocannabinoid degradation do not enhance depolarization-induced suppression of excitation on dorsal cochlear nucleus glycinergic neurons.

Abstract

Neurons from the dorsal cochlear nucleus (DCN) present endocannabinoid (EC) dependent short-term synaptic plasticity in the form of depolarization-induced suppression of excitation (DSE). Postsynaptic calcium influx promotes EC synthesis and depression of neurotransmission. ECs can be degraded by a hydrolytic and an oxidative pathway, the latter via the enzyme cyclooxygenase 2 (COX-2). Hyperactivity in the DCN is related to the development of tinnitus, which can be induced by high doses of salicylate, a COX-2 inhibitor. Since EC-dependent plasticity in the DCN can affect its excitation-inhibition balance, we investigated the impact of inhibitors of both oxidative and hydrolytic EC metabolism on the DSE from the synapses between the parallel fibers and cartwheel neurons (PF-CW) in the DCN. We found that inhibitors of COX-2 (ibuprofen and indomethacin) did not alter DSE at the PF-CW synapse. Salicylate also did not alter DSE. However, we found that inhibitors of the hydrolytic pathway did not affect DSE magnitude, but surprisingly speeded DSE decay. We conclude that oxidative EC degradation in the PF-CW synapse is not relevant for termination of DSE and are probably not important for controlling this form of synaptic plasticity in the DCN PF-CW synapse. The lack of effect on DSE of high doses of salicylate also suggests that it is not acting by increasing DSE in the PF-CWC synapse. However, the counter intuitive effect of the hydrolytic inhibitors shows that increasing EC on this synapse have more complex effects on DSE.