Early functional development of interneurons in the zebrafish olfactory bulb.

In the adult olfactory bulb (OB) of vertebrates, local GABAergic interneurons (INs) mediate recurrent and lateral inhibition between the principal neurons of the OB, the mitral cells (MCs), and play pivotal roles in the processing of odor-evoked activity patterns. The properties and functions of INs in the developing OB are, however, not well understood. We studied the functional development of INs in the OB of living zebrafish larvae 3-6 days postfertilization using anatomical techniques and in-vivo two-photon Ca2+ imaging. We identified MCs and INs by cell-type-specific expression of transgenic fluorescent markers and found that the IN:MC ratio was lower than in the adult fish. Moreover, the fraction of INs responding with Ca2+ signals to a set of natural odors was substantially lower than in adults. Odors of different chemical classes evoked overlapping patterns of Ca2+ signals that were concentrated in the center of the IN layer. The GABA(A) receptor agonists GABA and muscimol strongly suppressed odor responses, whereas a GABA(A) receptor antagonist enhanced responses and altered the spatial distribution of odor-evoked activity. These results indicate that IN odor responses at early developmental stages are sparse and exhibit no obvious chemotopic organization. Nevertheless, GABAergic signaling is already inhibitory at early stages of OB development and strongly influences odor-evoked activity patterns. Hence, INs already participate in the processing of odor information at very early stages of OB development even though the majority of INs emerge only at later stages.

Adaptive wavefront correction in two-photon microscopy using coherence-gated wavefront sensing.

The image quality of a two-photon microscope is often degraded by wavefront aberrations induced by the specimen. We demonstrate here that resolution and signal size in two-photon microcopy can be substantially improved, even in living biological specimens, by adaptive wavefront correction based on sensing the wavefront of coherence-gated backscattered light (coherence-gated wavefront sensing, CGWS) and wavefront control by a deformable mirror. A nearly diffraction-limited focus can be restored even for strong aberrations. CGWS-based wavefront correction should be applicable to samples with a wide range of scattering properties and it should be possible to perform real-time pixel-by-pixel correction even at fast scan speeds.