Early Development of Functional Spatial Maps in the Zebrafish Olfactory Bulb

Abstract

In the adult olfactory bulb (OB), particular chemical classes of odorants preferentially activate glomeruli within loosely defined regions, resulting in a coarse and fractured "chemotopic" map. In zebrafish, amino acids and bile acids predominantly stimulate glomeruli in the lateral and medial OB, respectively. We studied the development of these spatial response maps in zebrafish. At 3 d postfertilization (dpf), the OB contained protoglomerular structures that became refined and more numerous during subsequent days. In a transgenic zebrafish line expressing the Ca 2ϩ indicator protein inverse pericam, mainly in mitral cells, odor responses in the OB were first detected at 2.5-3 dpf. Already at this stage, amino acids and bile acids evoked activity predominantly in the lateral and medial OB, respectively. Twophoton Ca 2ϩ imaging using a synthetic indicator was used to reconstruct activity patterns at higher resolution in three dimensions. Responses to amino acids and bile acids were detected predominantly in the lateral and medial OB, respectively, with little overlap. Between 2.5 and 6 dpf, the number of odor-responsive units increased, but the overall spatial organization of activity persisted. Hence, a coarsespatialorganizationoffunctionalactivitymapsisestablishedveryearlyduringOBdevelopmentwhenglomeruliarenotyetdifferentiated. This spatial organization is maintained during development and growth of neuronal circuits and may have important functions for odor processing in larvae, for the differentiation of glomeruli, and for the refinement of activity maps at later developmental stages.

From the Olfactory Bulb to Higher Brain Centers: Genetic Visualization of Secondary Olfactory Pathways in Zebrafish

“…The three MC subpopulations exhibit distinct spatial distributions that are closely related to the odor map in the OB. Bile salts, putative social pheromones, are supposed to bind OR-type olfactory receptors in ciliated OSNs and activate glomeruli in the dG and avG clusters (Friedrich and Korsching, 1998;Li et al, 2005;Sato et al, 2005). We found that the majority of vglut2a tgϩ MCs and a subset of tbr2a tgϩ MCs are located in the vicinity of the dG and avG clusters, respectively.…”

Dopaminergic Modulation of Mitral Cells and Odor Responses in the Zebrafish Olfactory Bulb

“…In the OB of adult fish, this line expresses the fluorescent marker predominantly or exclusively in MCs (Li et al, 2005;Yaksi and Friedrich, 2006). Optogenetic experiments were performed using a transgenic line that expresses channelrhodopsin-2 fused to yellow fluorescent protein (Chr2YFP) under the control of dlx4/6 promoter/enhancer elements from zebrafish (Zerucha et al, 2000) and the tet-off system [Dlx4/6:itTA/Ptet:Chr2YFP Line 01 (Zhu et al, 2009)].…”

Optical monitoring of brain function in vivo: from neurons to networks

“…7c; [13]). In zebrafish larvae, the approach was used for the analysis of light-evoked responses in the optic tectum [18], odorevoked activity patterns in the olfactory bulb [17], and air puff-evoked Ca 2+ signals in spinal cord neurons [16]. Two-photon imaging studies in rodent primary sensory cortices have established that the coding of spontaneous as well as sensory-driven activity in the upper cortical layers is sparse.…”

“…This technique, termed multi-cell bolus loading (MCBL), enabled the targeted labeling of intact neuronal circuits in the living animal. The technique is applicable in various neuronal tissues and in a variety of species from lower vertebrates to mammals [12][13][14][15][16][17][18]. When combined with appropriate detection techniques, like two-photon laser scanning microscopy and/or brain endoscopy [19,20], it allows the monitoring of both the 'macroscopic' function of brain circuits [20] and the 'microscopic' behavior of individual cells [11,13].…”

“…As already mentioned, the staining protocol is also applicable in lower vertebrates and was used, for example, for staining neurons in the spinal cord ( Fig. 1f; [16]), olfactory bulb [17], and tectum [18] of zebrafish larvae. Interestingly, unlike other common protocols for labeling brain tissue with AM esters of Ca 2+ indicator dyes [22], MCBL-based staining shows little age dependence and can be used not only for staining newborn and juvenile tissues but also for staining adult and even aged tissues [21].…”