In Vivo Pharmacological Analysis Of Exogenous Glycine In Mitral Cell Of The Zebrafish Larvae Olfactory Bulb

Brain function depends on the homeostatic regulation between excitation and inhibition. Glycine is an important transmitter which can activate not only the inhibitory glycine receptors (GlyRs), but also the glycine site of NMDA receptors (NMDARs) in the central nervous system (CNS). Functional GlyRs are widely distributed in CNS. They can mediate tonic inhibition, which influences neuronal excitability and network activity. In CNS, the study of GlyRs has been largely ignored due to the apparent absence of glycinergic synaptic transmission.Activation of NMDARs results in the opening of an ion channel that is nonselective to cations, which plays a pivotal role in neural development, learning, memory, and synaptic plasticity including long-term Potentiation (LTP) and long-term Depression (LTD).The postsynaptic depolarization or perfusion with Mg2+-free ACSF is necessary due to the properties of the NMDARs, which require the relief of the Mg2+ Pblockade. Activation of NMDARs also require the binding of glutamate and the occupancy of the glycine site of NMDARs. Once NMDARs are activated, Ca2+ influx triggers synaptic plasticity.In addition to the hippocampus and other brain regions, the excitatory NMDARs and inhibitory GlyRs are also widely distributed in olfactory bulb, which involved in transmission of odor information from the nose to the brain. The storage and coding of odor information are carried out mainly in the olfactory bulb. GABA- and glycine-mediated currents show cross-inhibition in olfactory bulb, suggest that they may provide a mechanism to modulate inhibitory synaptic transmission.Using perforated patch-clamp technique, we recorded the mitral cell spontaneous excitatory postsynaptic currents (sEPSCs) in vivo in 7-10 days after fertilization (dpf) zebrafish larvaes. After approximate 3 minutes of perfusion by different concentrations of glycine, we found significant LTP in lower concentrations of glycine (100-200μM) and LTD in higher concentration (500-1000μM). Pharmacological analysis by AP5, strychnine and L-689,560 demonstrated that it resulted from different glycine binding to GlyRs and glycine site of NMDARs. The similar case was found in TBS-induced LTP in mitral cells. Our results suggest that alterations of the concentration of glycine may lead to the modification of synapse transmission and plasticity. The mechanism remains unclear. A better understanding of the mechanism will help us find the right target for drug therapy.