Activity-dependent Fos-like expression was investigated immunohistochemically in rainbow trout (Oncorhynchus mykiss) that had performed vibratory-evoked startle responses. We found significantly higher numbers of Fos-like-immunoreactive neurons in the reticular formation, in the octavolateral area, and in several cranial nerve motor nuclei in the brain and in the motor column of the spinal cord of startled fish than in control fish. In one fish, in which stimulation did not evoke startle responses, substantial numbers of positive cells occurred in the brain, primarily in the magnocellular octavolateral nucleus. We observed Fos-like-immunoreactive neurons in cell groups that are known to participate in the startle response (e.g., the Mauthner cell) as well as in cell groups that have been proposed but until now not shown to be involved.
Cells of origin of descending spinal pathways in eels of different body lengths (15–59 cm) were identified by labelling them retrogradely with horseradish peroxidase from the rostral spinal cord. The aim was to determine whether changes in size, number and distribution of these neurons take place in line with changes in the size of the spinal targets that occur with increasing body length. The somal cross-sectional areas of the labelled cells were measured from horizontal frozen sections and counts were made of the axons within the ventral funiculus in 1 µm transverse sections of the pectoral spinal cord. The distributions and numbers of labelled cells in the brain, and the number of axons in the ventral funiculus in the pectoral spinal cord, were similar for all lengths of fish studied. The somata of most of the labelled cells in the brain increase in size in fish up to a length of 40 cm. Some reticulospinal neurons are individually identifiable and most of these show a distinct increase in size; however, the cells that have relatively large somata in small fish show no change in size as the fish lengthen.
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.
To determine whether the topographically organized retinal input to the optic tectum is subsequently mapped onto the reticular formation, the responses of antidromically identified reticulospinal cells to tectal surface stimulation were investigated in 45 decerebrated, paralysed trout. The tectum was stimulated through a silver ball surface electrode at 24 different locations, and extracellular recordings were made from the rhombencephalic brainstem with glass microelectrodes filled with a 10% solution of horseradish peroxidase (HRP) in Tris buffer (pH 7.4). After recording, the HRP was, in some cases, iontophoretically expelled from the pipette to identify its location and visualized in histological sections by a modified Hanker-Yates method. Individual reticulospinal neurons discharged 1-4 spikes at short latency in response to stimulation of each of the 24 tectal locations. From one tectal location per cell, this initial response was followed by a late, sustained burst. With a short stimulus train (6 pulses, 55 Hz) the burst could last for over a second with discharge rates of up to 500 Hz. Sometimes this burst could be evoked from neighbouring tectal locations, but only by greatly increasing the stimulus strength. We conclude that the reticular formation receives a highly divergent monosynaptic connection from all locations of the tectum and that the longer latency, sustained burst response is due to a mapped connection between the tectum and the reticular formation. Since the late burst could be preceded by a silent period lasting for approximately 32 ms, we cannot rule out a dependence on interneurons situated between the tectum and the reticulospinal cells.
We have investigated the detail with which supraspinal neurons innervate different regions in the spinal cord. Horseradish peroxidase was applied at different levels of the cord: (1) to the cut surface; (2) injected iontophoretically into one side; or (3) into one of the motoneuron pools innervating different muscle compartments. In all cases, labeled neurons were found throughout the brain distributed over nuclear groups identified in previous fish studies. Some cells from all but one of the nuclei have axons that extend over the whole length of the cord and about 50% of all neurons project to post-anal levels. No topographical distribution relating somal position and target location was found. Sixty seven percent of the neurons send axons to the ipsilateral side of the cord, although most nuclear groups provide a bilateral projection. Nucleus ruber projects entirely contralaterally. The descending and magnocellular octavolateral nuclei, and the descending nucleus of the trigeminal nerve project entirely ipsilaterally. Neurons that project to the spinal motoneuron pools innervating the myotomal red- or white-muscle are distributed throughout the same brain stem nuclei, but the cell bodies of those neurons innervating white-muscle motoneurons are larger. Nineteen pairs of cells were found to be consistently identifiable and all project to all levels of the cord; they were only labeled from injections made into the white-muscle motoneuron pool.
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