Figure 4

Morphology of IPN neurons. (a-g, f') Three-dimensional projections of confocal z-stacks showing electroporated IPN neurons expressing membrane-Cherry (gold) and the surrounding IPN neuropil (grey), visualized using the Tg(ET16:GFP) transgenic line. All panels are dorsal views, anterior top, except (f'), which is a lateral view, anterior right. (a'-e') Maximum intensity projections of deconvolved confocal z-stacks reveal the detailed morphology of IPN neurons. (a, a') An interneuron in the vIPN that extends neurites exclusively within the vIPN neuropil. The soma (arrow) is located on the R of the midline but the neurites enter the neuropil on both L and R sides. A second, more weakly labeled neuron is also visible. (b, b') The soma of this neuron, as with most IPN neurons, occupies the central cellular 'core' of the nucleus whilst its arbor extends radially to synapse with afferent habenular axons that encircle the core. (c, c') A vIPN neuron with a flattened dendritic arbor. In this specimen two neurons were labeled. One of these (soma marked by arrow in (c')) radiates an extensive dendritic arbor exclusively on the R of the midline within the vIPN neuropil. The lateral projection (inset in (c')) reveals the arbor is extremely flattened along the DV axis (arrowheads in (c')). (d, d') Neuron with a dorsally located soma (just outside the dIPN neuropil), which elaborates two arbors in spatially distinct regions of the IPN. The larger of these arbors (arrowhead in (d')) is connected to the cell body (arrow in (d')) by a long, unbranched process. As is also the case for some of the other interneurons, it is unclear whether these arbors are axonal, dendritic or both in nature. (e, e') A dIPN projection neuron that extends an axon just caudal to the IPN, most probably synapsing with neurons of the serotonergic raphé nucleus. This neuron has dendritic processes located both in the dorsal (D) and ventral (V) neuropils. (f, f') In this specimen, three projection neurons are labeled. Axons extend around the IPN (arrowhead in (f)) as well as caudally and dorsally (arrows in (f, f')). (g) Projection neurons extending anteriorly directed axons that pass around the IPN, cross the midline and continue rostrally (arrow). Scale bar in (a'): 20 μm