SOMATOSTATIN IN THE PREPACEMAKER NUCLEUS OF WEAKLY ELECTRIC FISH, APTERONOTUS-LEPTORHYNCHUS - EVIDENCE FOR A NONSYNAPTIC FUNCTION

Neuropeptides are widely distributed throughout the nervous system and exert a large number of heterogeneous functions. While they are synth esized in the soma, release is thought to take place in axonal termina ls of neurons. A good model system to investigate the role of peptides in the nervous system is provided by the central posterior/prepacemak er nucleus (CP/PPn) of weakly electric gymnotiform fish. A subnucleus of this thalamic complex, the prepacemaker nucleus (PPn), innervates t he pacemaker nucleus (Pn), a medullary cell group controlling the elec tric organ discharge (EOD). Previous immunocytochemical and in situ-hy bridization studies employing topographical criteria indicated that PP n neurons may express the neuropeptide somatostatin (SS). In the prese nt study, we unambiguously identified PPn neurons by in vitro tract tr acing. By combining this technique with SS immunocytochemistry, we fou nd that a large portion of retrogradely labelled PPn neurons exhibited SS-like immunoreactivity (72-89%, n=708 cells in 10 fish examined). S urprisingly, however, neither the proximal PPn axons nor anterogradely labelled terminals innervating the Pn displayed significant amounts o f SS-like immunolabelling (n=10 fish examined in each experiment). The se results and the lack of SS binding sites in the Pn [82] suggest tha t SS expressed by PPn cells is not synaptically released at the target site of their axons, the Pn, but acts via a nonsynaptic mechanism in the CP/PPn proper.