Hong, E., Santhakumar, K., Akitake, C.A., Ahn, S.J., Thisse, C., Thisse, B., Wyart, C., Mangin, J.M., and Halpern, M.E. (2013) Cholinergic left-right asymmetry in the habenulo-interpeduncular pathway. Proc. Natl. Acad. Sci. USA. 110(52):21171-21176.
The habenulo-interpeduncular pathway, a highly conserved cholinergic system, has emerged as a valuable model to study left-right
asymmetry in the brain. In larval zebrafish, the bilaterally paired dorsal habenular nuclei (dHb) exhibit prominent left-right
differences in their organization, gene expression, and connectivity, but their cholinergic nature was unclear. Through the
discovery of a duplicated cholinergic gene locus, we now show that choline acetyltransferase and vesicular acetylcholine transporter homologs are preferentially expressed in the right dHb of larval zebrafish. Genes encoding the nicotinic acetylcholine receptor
subunits α2 and β4 are transcribed in the target interpeduncular nucleus (IPN), suggesting that the asymmetrical cholinergic
pathway is functional. To confirm this, we activated channelrhodopsin-2 specifically in the larval dHb and performed whole-cell
patch-clamp recording of IPN neurons. The response to optogenetic or electrical stimulation of the right dHb consisted of
an initial fast glutamatergic excitatory postsynaptic current followed by a slow-rising cholinergic current. In adult zebrafish,
the dHb are divided into discrete cholinergic and peptidergic subnuclei that differ in size between the left and right sides
of the brain. After exposing adults to nicotine, fos expression was activated in subregions of the IPN enriched for specific nicotinic acetylcholine receptor subunits. Our studies
of the newly identified cholinergic gene locus resolve the neurotransmitter identity of the zebrafish habenular nuclei and
reveal functional asymmetry in a major cholinergic neuromodulatory pathway of the vertebrate brain.