Organisation of the zebrafish spinal cord: distribution of motoneuron dendrites and 5-HT containing cells

In order to find arguments for a selective innervation and modulation of the fast and slow spinal motoneurons in the zebrafish, we determined: the territories occupied by the dendritic trees of the large spinal motoneurons innervating the fast white muscle (WMNs) and those of the smaller motoneurons (RIMNs) innervating the slower red and intermediate fibers; the distribution of 5-hydroxytryptamine (5-HT) immunoreactive cells, which constitute one of the systems for the modulation of motoneuron activity. Motoneurons were either retrogradely labelled with horseradish peroxidase or iontophoretically filled with Lucifer yellow. The 5-HT cells were identified immunohistochemically. We found that the dendritic territories of the WMNs and those of the RIMNs partly overlap but they also occupy unique areas. The unique area of the WMN dendrites is dorsal to the lateral neuropil, where axon collaterals of the bulbospinal tract invade the motor column; the unique area of the RIMN dendrites lies in the ventral part of the cord, near the ventral commissural tract. 5-HT immunoreactive cells were observed in the ventromedial part of the spinal cord and among the motoneuron somata. The large overlap in dendrite territories between the WMNs and RIMNs argues for a motoneuron recruitment pattern as may be expected from the size principle; while the distinct areas occupied by dendrites of either WMNs or RIMNs hints at a selective innervation. The presence of the 5-HT cells close to the motoneuron somata and in the ventral area, where dendrites of the RIMNs ramify, suggests that the activities of the motoneuron types can be selectively modulated.