Monoaminergic nerves in the central and peripheral nervous systems of fishes.

Since thc introduction of the Falck--Hillarp fluoresccnce technique (Falck, 1962: Falck et al.. 1962) for the localisation of biogenic monoamines, and the unequivocal demonstration of monoaminergic nerve terminals at a fine structural level with the aid of the selective markers 5and 6-hydroxydopamine, thcre have been major advances in the understanding of the anatomy of vertebrate monoaminergic nervous systems. In view of the clinical significance of the adrenergic nervous system in man, research into monoamines has been primarily concentrated on mammals. The distribution of monoamines in both the peripheral and central nervous systems of the rat is now known in detail, and the recent application of immuno-histochemical techniques for localising monoamine synthesising enzymes at both the light and electron microscopical levels hits resulted in even greater sensitixit.', and specificity of monoaminc localization (H6kfelt et uL, 1973. 1974). Submammalian vertebrates, on the other hand, have received far less attention in this respect and of these, fish have received least attention of all. The purpose of this review is to survey the literature relating to identified monoaminergic nerves in the peripheral and central nervous systems of teleosts. elasmobranchs and cyclostomes. Whilst there have been several studies of the distribution and concentration of monoamines present in central nervous tissue of fish {see Holzbauer & Sharman, 1972), the distribution of monoamine-containing nerves in fish has not been reviewed exclusively and there are doubtless large gaps in our knowledge to be revealed. The pre,~nt author intends to confine this review to the distribution of identitied monoaminergic nerves in fish without recot.rse to physiological and pharmacological reports except where they are relevant in corroborating functional interpretations. Identification of monoaminergic nerves by the Falck-Hillarp ltuorescence histochemical technique is highly reliable, as long as appropriate controls are employed. Electron microscopic identification of monoaminergic nerves, however, is less easy since the uhrastructural appearance of neurotransmitter storage vesicles is highly dependent on the mode of fixation. Potassium permanganate tixation is the most reliable method for visualising the dense core of monoaminergic storage vesicles in normal mare-