Structural and functional characterisation of M/T cells using Ca2+ Imaging and Activity Correlation Imaging in dendritic networks of the developing Xenopus brain
von Camille Inès Alexandra Okom
Datum der mündl. Prüfung:2016-12-09
Betreuer:Prof. Dr. Dr. Detlev Schild
Gutachter:Prof. Dr. Dr. Detlev Schild
Gutachter:Prof. Dr. André Fiala
EnglischOlfactory information collected in the nose is conveyed from the axon terminals of olfactory receptor neurons to the dendritic tufts of mitral/tufted (M/T) cells across glutamatergic synapses. These synaptic interactions occur within glomeruli, the spherical neuropil structures where presynaptic and postsynaptic fibres intermingle. The second-order projection neurons, the M/T cells carry out sensory processing and integration of the olfactory signals. In Xenopus laevis larvae, sensitivity to temperature drops is encoded by a group of receptor neurons terminating a particular glomerulus, called the γ-glomerulus. Interestingly, several studies brought evidence that some M/T cells receive their input from more than one glomerulus. Yet, the implications of such a connectivity in terms of multi-processing and encoding of olfactory information have not yet been investigated. We hypothesised that the multiple glomerular innervation by M/T cells would enable them to collect and integrate both chemical and thermic information. Consequently, Ca2+ imaging experiments performed in olfactory bulb slices revealed that most temperature-sensitive M/T cells also responded to chemical odourants, indicating that the dual-sensitivity of these cells is a function of their one-to-many glomerular connectivity. Furthermore, Xenopus M/T cells display upon odourant stimulation three types of temporal activity patterns: activated, inhibited, unresponsive, which corroborate previous research showing that reciprocal and lateral M/T cell inhibition affects odour representations in the rodent bulb. Inhibitory periglomerular and granule cells establish contact with M/T cells through dendrodendritic synapses and constitute a scalable inhibitory lever on M/T cell outputs. In order to test the effect of GABA-mediated inhibition on Xenopus M/T cells, the Ca2+ dynamics of M/T ensembles were compared in the presence or absence of gabazine, a GABA-A receptor antagonist. The blockade of inhibition drastically altered the patterns of odour responses in M/T cells. Moreover, gabazine did not desynchronise correlated patterns of spontaneous activity across these neurons, but enhanced them. These results demonstrate that GABAergic inhibition of M/T cells drives odour discrimination and contrast enhancement of overlapping odour signals in these neurons. In contrast, it plays a minor role, if any, in the coupling of M/T cells’ baseline activities. Finally, simultaneous patch-clamp recordings and Ca2+ imaging proved that action potentials (APs) and AP-related Ca2+ signals backpropagate in the dendrites of Xenopus M/T cells. Spontaneous and somatically evoked APs activated Ca2+ channels generating Ca2+ spikes were observed in dendritic branches and glomerular endings. Besides, the cross-correlation analysis of Ca2+ fluctuations along M/T cell dendrites exposed differential activities of dendritic sub-compartments. Early evidence suggests that gabazine enhances the uniformity of backpropagating spikes along dendrites.
Keywords: olfaction, Xenopus, mitral cells, thermosensitivity, inhibition, dendrite, calcium imaging