Descending control of electroreception. I. Properties of nucleus praeeminentialis neurons projecting indirectly to the electrosensory lateral line lobe.

The first-order CNS processing region within the electrosensory system, the electrosensory lateral line lobe, receives massive descending inputs from the nucleus praeeminentialis as well as the primary afferent projection. The n. praeeminentialis receives its input from the electrosensory lateral line lobe as well as from higher centers; hence this nucleus occupies an important position in a feedback loop within the electrosensory system. This report describes the physiological properties of a category of n. praeeminentialis neurons characterized by very high spontaneous firing frequency, but relatively poor sensitivity to electrolocation targets as compared to neurons in the electrosensory lateral line lobe. These neurons are specialized to encode long-term changes in electric organ discharge amplitude with high resolution. Intracellular recording and Lucifer yellow staining of these neurons show that they are the previously described multipolar neurons of the n. praeeminentialis, and they project bilaterally to the posterior eminentia granularis. Posterior eminentia granularis efferents project to the electrosensory lateral line lobe forming its dorsal molecular layer. Hence, these multipolar cells influence the electrosensory lateral line lobe circuitry indirectly. The information that the multipolar cells encode regarding the electric organ discharge amplitude may be needed for a gain control mechanism operative within the electrosensory lateral line lobe. Previous studies have shown that the indirect projection from the n. praeeminentialis to the electrosensory lateral line lobe must be intact for this gain control mechanism to operate.