On the Agonistic Display of the Siamese Fighting Fish

Abstract

The agonistic display of the Siamese fighting fish is characterized by tonic extension of the gill opercula, which is mediated by a single abductor, the opercular dilator muscle. This muscle consists of three bellies, which have different origins and biochemical properties. As part of an effort to define in detail the neural substrate for agonistic display, the distribution, number, and morphology of opercular dilator motoneurons in the trigeminal motor nucleus were examined by retrograde horseradish peroxidase labelling and Golgi-impregnation. The trigeminal motor nucleus consists of three subnuclei: rostral, caudomedial and caudolateral. Neuron numbers among these three subnuclei are relatively constant among animals, and between males and females. Innervation of the opercular dilator muscle originates predominantly from the caudolateral subnucleus; a smaller contingent of motoneurons originates from the caudomedial subnucleus. Labelling of the caudolateral subnucleus is usually intense, whereas caudomedial neurons are weakly labelled. These observations suggest that neurons in the caudomedial subnucleus provide either sparse and widespread innervation, or innervate only a small, circumscribed area of the opercular dilator muscle complex, whereas caudolateral neurons probably provide extensive and dense innervations. The dendritic morphology of trigeminal motoneurons is independent of their targets and of the subnuclei in which they are located. All neurons observed are similar in shape. Primary dendrites of neurons in the rostral and caudomedial subnuclei are long, whereas those in the caudolateral subnucleus are short or absent. Regardless of the position of the somata, dendrites arborize only in the lateral half of the reticular formation in the vicinity of the descending tectobulbal tract, the descending trigeminal tract, and the ascending secondary gustatory tract. The morphological differences suggest that inputs to terminal dendrites have stronger effects on neurons in the caudolateral than those in the other two subnuclei.