Organisation of the Zebrafish Spinal Cord: Distribution of Motoneuron Denfrites and 5-HT containing Cells

Abstract

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.

Serotonergic Modulation of Locomotion in Zebrafish—Endogenous Release and Synaptic Mechanisms

“…In larval zebrafish, the major serotonergic innervation of the spinal cord originates from the raphe region, which stretches from the caudal midbrain to the anterior medulla oblongata, and there are also intraspinal neurons positive for 5-HT (McLean and Fetcho, 2004;Brustein and Drapeau, 2005). This innervation pattern persists into adulthood (Van Raamsdonk et al, 1996;Kaslin and Panula, 2001), suggesting that 5-HT responsible for the modulation of the locomotor activity may arise from intraspinal neurons as well as supraspinal projections from the raphe.…”

“…In the spinal cord of both larval and adult zebrafish, 5-HT innervation arises from descending raphe projections and intraspinal neurons (Van Raamsdonk et al, 1996;Kaslin and Panula, 2001;McLean and Fetcho, 2004). At the larval stage, 5-HT does not affect the frequency of the spontaneous swimming and only decreases the quiescent period between consecutive swimming episodes (Brustein and Drapeau, 2005).…”

Serotonergic systems in the spinal cord of the amphibian urodele Pleurodeles waltl

“…Our data show that the P. waltl spinal cord itself contains serotonergic perikarya, which send their axons in a plexus located on the ventrolateral surface. Intraspinal serotonergic neurons have also been found in fish (Parent and Northcutt, 1982;Ritchie and Leonard, 1982;Van Raamsdonk et al, 1996), Urodela (Gruberg and Harris, 1981;Jovanovic et al, 1996), turtle (Kiehn et al, 1992), and young opossum (DiTirro et al, 1983) but not in chameleon (Bennis et al, 1990). In mammals, 5-HT-containing intraspinal neurons were detected following pharmacological pretreatment to increase the level of 5-HT expression (Lamotte et al, 1982), whereas other studies have reported that serotonergic innervation is derived only from supraspinal structures (Kojima and Sano, 1983).…”

Regenerating descending axons preferentially reroute to the gray matter in the presence of a general macrophage/microglial reaction caudal to a spinal transection in adult zebrafish

“…It is unclear to what degree the cellular specificity of synaptic contacts of descending axons with the spinal motor system (Fetcho, 1992a;Van Raamsdonk et al, 1996) is reestablished. It is likely that a relatively crude reconnection with the spinal intrinsic circuitry is sufficient to produce meaningful motor output after a spinal lesion (McClellan, 1998;Bernhardt, 1999a).…”