NEURONAL CONTROL OF BEHAVIORAL PLASTICITY - THE PREPACEMAKER NUCLEUS OF WEAKLY ELECTRIC GYMNOTIFORM FISH

Gymnotiform fish of the genera Apteronotus and Eigenmannia provide an excellent vertebrate model system to study neural mechanisms controlli ng behavioral plasticity. These teleosts generate, by means of an elec tric organ, quasi-sinusoidal discharges of extremely stable frequency and waveform. Modulations consisting of transient rises in discharge f requency are produced during social encounters, and play an important role in communication. These so-called ''chirps'' exhibit a remarkable sexual dimorphism, as well as an enormous seasonal and individual var iability. Chirping behavior is controlled by a subset of neurons in th e complex of the central posterior/prepacemaker nucleus in the diencep halon. It is hypothesized that the plasticity in the performance of ch irping behavior is, at least in part, governed by two mechanisms: firs t, by seasonally induced structural changes in dendritic morphology of neurons of the prepacemaker nucleus, thus leading to pronounced alter ations in excitatory input. Second, by androgen-controlled changes in the innervation pattern of the prepacemaker nucleus by fibers expressi ng the neuropeptide substance P. In addition to these two dynamic proc esses, cells are generated continuously and at high number in the cent ral posterior/prepacemaker nucleus during adulthood. This phenomenon m ay provide the basis for a ''refreshment'', thus facilitating possible changes in the underlying neural network.