Identification of different cell types in the command (pacemaker) nucleus of several gymnotiform species by retrograde transport of horseradish peroxidase

Abstract

The medullary nucleus controlling electric organ discharge in the weakly electric Gymnotiformes, Eigenmannia sp., Sternopygus sp., Apteronotus albifrons and sp. and Gymnotus carapo was exam ined using the method of retrograde axonal transport of horseradish peroxidase applied to the transsectioned spinal cord. The minimal rate of retrograde transport in the spinal axons of the medullary command center measured for a distance of eight cm is 40 mm/day. Beyond a minimal period, survival time (2–70 days) after injection was not a determining factor for the percentage of cells labelled in the medullary nucleus. The form and frequency of the electric organ discharge were identical before and after injection of horseradish peroxidase, as well as after several weeks of survival, even when all cells of the medullary command nucleus were filled with the enzyme. The medullary nucleus is a well-defined midline agglomeration of cells in the reticular formation, comprised of two types of round, multipolar cells, the ‘large’ cells having approximately twice the diameter of the ‘small’ cells. The two types of cells are intermingled in the medullary nucleus in Eigenmannia and Apteronotus, whereas in Sternopygus, Gymnotus, Hypopomus and Gymnorhamphichthys, they form two separate groups, the small cells lying dorsally to the large cells. The number of large cells as determined in serial photography in single individuals was 28 for Gymnorhamphichthys, 32 for Eigenmannia, 34 for Hypopygus, 52 for Hypopomus, 63 for Gymnotus and 197 for Apteronotus. In the wave species there were at least twice as many small cells as large ones, whereas in the pulse species examined the proportions found were 1:1 in Gymnotus and Hypopomus, 3:1 in Hypopygus and 1:2 in Gymnorhamphichthys. In Gymnotus and Apteronotus the electromotoneurons which innervate the electric organ and which are served by the axons leading from the medullary command nucleus, are distributed along the spinal cord in a manner approximately proportionate to the volume occupied by the electric organ at various levels. In contrast, the electromotoneuron distribution in Eigenmannia remains the same throughout the fish, although the electric organ surface increases toward the tail. There are up to over four times as many electromotoneurons at each level in Apteronotus as in the other species. In all species examined only the large cells in the medullary nucleus were labelled by horseradish peroxidase injected in the spinal cord. Labelling revealed dendro-dendritic connections between large cells in Eigenmannia and Gymnotus; no such connections were observed in Apteronotus. With the exception of Apteronotus, the percentage of cells labelled depended upon the caudal-rostral level of the horseradish peroxidase injection; the more anteriorly the enzyme was injected, the more large cells were labelled. In Apteronotus however, all the large cells were marked, regardless of the injection level. The results indicate that in Apteronotus the axons of all large cells contact the electromotoneurons at all spinal levels. On the contrary, in Gymnotus and Eigenmannia, certain axons of the large cells contribute to certain spinal levels. It seems that an organotopic organization exists within the medullary nucleus for these two species.