RECIPROCAL CONNECTIONS BETWEEN THE PREGLOMERULAR NUCLEUS AND THE CENTRAL POSTERIOR PREPACEMAKER NUCLEUS IN THE DIENCEPHALON OF WEAKLY ELECTRIC FISH, APTERONOTUS-LEPTORHYNCHUS/

The central posterior/prepacemaker nucleus of gymnotiform fish is a bi lateral cell group located in the dorsal thalamus. This complex consis ts of approximately 10,000 neurons which can be divided into several s ubpopulations. One subpopulation comprised of a few hundreds of neuron s projects to the pacemaker nucleus in the medulla oblongata, thus con stituting the prepacemaker nucleus portion of this complex. By employi ng in vitro tract-tracing techniques, we have, in the present investig ation, examined the pattern of connectivity formed by the central post erior/prepacemaker nucleus with a diencephalic cell group, the preglom erular 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/prepa cemaker nucleus. The reciprocal connection between the central posteri or/prepacemaker nucleus and the preglomerular nucleus may be used to r elay sensory information directly conveyed to one of the two nuclei in directly to the other nucleus. The existence of at least some central; posterior/prepacemaker nucleus neurons projecting to both the preglome rular nucleus and tile pacemaker nucleus may provide the morphological basis for the transmission of an efference copy of electromotor infor mation produced by neurons in the central posterior/prepacemaker nucle us to the preglomerular nucleus. (C) 1997 IBRO. Published by Elsevier Science Ltd.