Pharmacological analysis of ionotropic glutamate and GABA receptor function in neuronal circuits of the zebrafish olfactory bulb

In the olfactory bulb and other brain areas, basic cellular and synaptic properties of individual neurons have been studied extensively in reduced preparations. Nevertheless, it is still poorly understood how intactions between multiple neurons shape spatio-temporal activity patterns and give rise to the computational properties of the the intact circuit. In this thesis, I used pharmacological manipulations of excitatory and inhibitory neurotransmitter receptors to examine the synaptic interactions underlying spontaneous and odor-evoked activity patterns in the intact olfactory bulb of zebrafish. Electrophysiological and one- and two-photon calcium imaging methods were used to record activity from the principal neurons of the OB (mitral cells, MCs), their sensory input, and local interneurons. The combined blockade of AMPA/kainate and NMDA receptors abolished odor-evoked excitation of MCs, indicating that sensory input to the OB is mediated by ionotropic glutamate receptors. Surprisingly, however, the blockade of AMPA/Kainiate receptors alone increased the mean response of MCs and decreased the mean response of interneurons (INs), and the blockade of NMDA receptors caused little or no change in the mean responses of MCs and INs. In addition, antagonists of both glutamate receptor types had diverse effects on the magnitude and time course of individual MC and IN responses and, thus, changed spatio-temporal activity patterns across neuronal populations. The blockade of GABA(A) receptors increased spontaneous and odor evoked firing rates of mitral cells and often induced rhythmic bursting. Moreover, the blockade of, GABA(A) or AMPA/kainate receptors abolished fast oscillatory activity in the local field potential. Blockade of GABA(B) receptors reduced calcium influx in afferent sensory axons and modulated response time courses of mitral cells. These results indicate that (1) IN activity during an odor response depends mainly on AMPA/Kainiate receptor input, (2) interactions between MCs and INs regulate the total OB output activity, (3) AMPA/Kainiate receptors and GABA(A) receptors underly the synchronization of odor-dependent neuronal ensembles and (4) odor-specific patterns of OB output activity are shaped by circuits containing iGlu receptors and GABA receptors. These results provide insights into the mechanisms underlying the processing of odor-encoding activity patterns in the OB