Expression of Chemokine Ligands cxcl12a and cxcl12b in the Habenular Region in Danio rerio (zebrafish)

Abstract

The habenula (Hb) is a bilateral cluster of neurons in the forebrain. The interpeduncular nucleus (IPN) is an important component of the midbrain. Together, they make up the Hb-IPN conduction system. This system plays an important role in sleep, reward-based decision making, and various other behaviors and processes. These important structures are found in all vertebrates, but the understanding of the development of this pathway is in the preliminary stages.  Previous studies in zebrafish show that chemokine signaling plays an important role in Hb axonal outgrowth. Chemokine (c-x-c) motif receptor 4b (Cxcr4b), which is made in the dorsal habenula (dHb), helps direct axons toward the IPN. Cxcl12a and Cxcl12b are the two ligands in this signaling pathway, which offer guidance cues to dHb axons, though their functions are not yet entirely clear. cxcl12a mutants show dysfunctional dHb axonal projections, with many axons projecting anteriorly, in the opposite direction of the IPN. Mutants with ectopic cxcl12b expression show similar results. Dorsal views show cxcl12a and cxcl12b are both transcribed in regions posterior to the dHb; cxcl12b is also transcribed anterior to the dHb. However, the relative expression patterns of these ligands is unknown, and may give us a clearer understanding of their respective functions in directing dHb axons. To do this, we will employ in situ hybridization, which will allow us to simultaneously visualize where cxcl12a and cxcl12b are being transcribed along the path of the developing dHb axons.