Reciprocal connections between the preglomerular nucleus and the central posterior/prepacemaker nucleus in the diencephalon of weakly electric fish, Apteronotus leptorhynchus

Abstract

The central posterior/prepacemaker nucleus of gymnotiform fish is a bilateral cell group located in the dorsal thalamus. This complex consists of approximately 10,000 neurons which can be divided into several subpopulations. One subpopulation comprised of a few hundreds of neurons projects to the pacemaker nucleus in the medulla oblongata, thus constituting the prepacemaker nucleus portion of this complex. By employing in vitro tract-tracing techniques, we have, in the present investigation, examined the pattern of connectivity formed by the central posterior/prepacemaker nucleus with a diencephalic cell group, the preglomerular nucleus. As demonstrated by anterograde and retrograde tracing, a subpopulation of several hundreds of neurons located in the central posterior/prepacemaker nucleus project to the ipsi- and contralateral preglomerular nucleus. Double-labelling experiments revealed that at least a fraction of these neurons also innervate the pacemaker nucleus. In the preglomerular nucleus, a large number of neurons give rise to projections that terminate in the ipsilateral central posterior/prepacemaker nucleus. The reciprocal connection between the central posterior/prepacemaker nucleus and the preglomerular nucleus may be used to relay sensory information directly conveyed to one of the two nuclei indirectly to the other nucleus. The existence of at least some central posterior/prepacemaker nucleus neurons projecting to both the preglomerular nucleus and the pacemaker nucleus may provide the morphological basis for the transmission of an efference copy of electromotor information produced by neurons in the central posterior/prepacemaker nucleus to the preglomerular nucleus.