A Novel Patterning Of Interneurons In Hindbrain Reveals New Principles Of Organization

The work described here focuses on a novel patterning of interneurons where stripes of excitatory and inhibitory neurotransmitter phenotypes are present in an alternating fashion from medial to lateral in the hindbrain of larval zebrafish. I investigated the hypothesis that these stripes represent an underlying template for the organization of neural circuits in hindbrain. Using photoconvertible proteins, I tracked cells of different ages within stripes identified by transgenic lines expressing fluorescent proteins exclusively in neurons with shared neurotransmitter or transcription factor expression. I found that stripes maintain an age-related organization from ventral to dorsal for old to young cells in most hindbrain regions at 4 dpf, a time when zebrafish are freely swimming. Using dye backfills and stochastic labeling of membrane-tagged fluorescent proteins, I examined the projection patterns of neurons within inhibitory stripes and found that neurons within a stripe share a common morphology and mediolateral projection pattern. This broad patterning exists in a specific circuit, the startle response circuit of the Mauthner cell, where lateral stripe neurons project laterally onto the lateral dendrite and middle stripe neurons project more medially onto ventral dendrite regions. Since an age-related patterning exists within one stripe, I looked for differences within one stripe, the medial excitatory stripe identified by expression of the alx transcription factor. Using photoconvertible proteins, stochastic expression of membrane-tagged fluorescent proteins, whole-cell patch clamp, and calcium imaging, I found that both structural and functional patterning within a single stripe where an orderly patterning based on the age of a neuron exists for the cell body position within the stripe, the projections of these cells within the neuropil, the input resistance of these cells, and possibly, recruitment during different speeds of swimming. I also showed that there is a topological transformation from spinal cord to hindbrain both by age and by transcription factor expression. These findings indicate that a broad organization of hindbrain exists, where stripes represent a simple toolkit that is comprised of interneuron types arrayed into stripes, and, within a stripe, cells comprising a single class of interneurons are organized dorsoventrally within a stripe by age, excitability and wiring