Different distribution of serotonin in an elasmobranch (Scyliorhinus stellaris) and in a teleost (Conger conger) fish.

1. A comparative quantitative study on the occurrence of 5-HT in several tissues and organs of the elasmobranch Scyliorhinus stellaris and the teleost Conger conger has been carried out. 2. In Scyliorhinus, the richest source of the amine is the brain, in which 5-HT is twice as concentrated as in the teleost brain. Significant levels have been also detected in the intestine, followed by gills and heart. 5-HT is also concentrated in other epithelial organs, such as the rectal gland and the olfactory organ. 3. In Conger, the gills show the highest content of 5-HT, in which, like in the kidney, serotonin is 2-3 times more concentrated than in the corresponding organs of the elasmobranch. 4. These differences are discussed in relation to the distinct phylogenetic and ecophysiological features of the two animals. In addition, the possible functional significance of 5-HT in the fish heart is taken into consideration.


INTRODUCTION
The literature concerning the occurrence and function of serotonin (5-HT) in lower vertebrates, and particularly in fish, is sparse. In fact, while some quantitative data from the gastrointestinal tract (Erspamer. 1953) as well as from the nervous system (Correale. 1956: Bogdanski et trl.. 1963: Brodie rt o/.. 1964) are reported and have been related to the well-known neurohumoral function of this amine. there is still a lack of knowledge on the content and function of 5-HT in other piscine organs and tissues. Such a lack is particularly acute considering the multiple role of 5-HT. as indicated by the long phylogenetic history of this substance. which has been firmly recognized (see e.g. Haber et trl.,198 I,for references).
The present comparative work was carried out on an elasmobranch (Sr~liorhms sfrllrrris) and on a teleost (Coryrr coyqer) marine fish for providing some preliminary quantitative information concerning the occurrence of 5-HT in those still undetected tissues and organs in which a multiple function of this agent can be inferred.
weighing between 700 and I300 g, were obtained during the autumn and the winter from the aquarium of the Zoological Station of Naples. where they were kept in holding tanks with circulating sea Mater at 16 18 C for at least IO days. The animals were killed by decerebration. The blood was collected by a syringe inserted into the ventricle. after exposure of the heart by opening the ventral body wall. The * Requests for reprints should be sent to: Prof. Bruno myotomal muscles were dissected from the caudal end of the body. The gill filaments were clipped with surgical scissors from the arches. All organs were immediately weighed after excision from the fish.

The extraction
was performed according to the method of Welsh & Moorhead (1960). HCI (1 N) was added to the tissue (w;v = I :2): the tissue. minced with scissors. was then homogenized with further addition of dtstillcd water (HCI: water = l:2) The homogenates were boiled for I-2 min and centrifuged at low speed (5000~ for IO min). The supernatants were collected and neutralized with I N NaOH. Preliminary experiments based on the extraction of known amounts of 5-HT have shown a recovery of the amine varying betbeen X0 and X5",,. a value rather close to that reported by Welsh (I 964).
The bioassay method employed in this work was that reported by Vane (1957) on a strip of fundus from a rat stomach. This is a very sensitive method, allo\\ing the detection of such loa amounts of S-HT as SOpg or 1 ng.
the amine differs significantly.
In fact, the elasmobranch brain has approximately more than a double amount of 5-HT than the teleost brain. In contrast. compared to the elusmobranch.
CO,I~CV c~omqr~r exhibits a higher content of 5-HT in its gills (\\here the amine is almost three times more concentrated than in the elasmobranch gills). spleen and kidney. The different encephalic content of 5-HT in Sq!liurn and Congrr is of particular interest. since these are two phylogenetically distant fishes and arc also characterized by distinct ecoph!siological habits. Indeed. a comparative survey of the morphological organization of 5-HT neuronal systems in the brain of lower vertebrates (Parent. 19X1) indicates that at this level differences can probably occur in several groups of fishes. On the other hand, in addition to this. the multiple localization of 5-HT in the fish brain includes also other morphological and functional compartments.
such as the pineal, the ncuroendocrine output of melatonin, and the ependymal cells. Thus. the physiological significance of this difference. revealed on the basis of ;I \+ hole organ analysis. requires further zonal stud! in order to he better elucidated.
The finding of the highest content of scrotonin in the gill filaments of Co~/c,r U~U/CV is consistent with an intrinsic role of this amine either in the bronchial microcirculation or in the epithelial transport. or in both.
Important differences in the functional anatomy of the branchial microvasculature.
as well as in the role of the corresponding nerve supply and humoral modulation.
have been recently surveyed in several teleosts (including A~c/~ri//tr) and elasmobranches (including Sc~yliorhimrs) by Dune] & Laurent (1980). Pharmacological experiments in rim in the trout Srrlr~o qlrrirtijlcri have led Thomas cl/ ctl. ( 1979) to suppose that serotonin plays a ma_ior role in the adjustment of the branchial circulation and particularI> of the lamellar perfusion. On the other hand. the dramatic differences in gill permeability and osmoregulation between teleosts and elusmobranchs vcr) likely rc-Hcct a diffcrcnt humoral cndo\\mcnt of the gill lila-ments epithelium. Such a role of 5-HT is strongly supported by our results. The fact that 5-HT levels in Corqrr kidney are three times those in clasmobranch k'idney could be related to the higher levels of excretion and osmoregulation characteristic of the "teleost machine" in comparison with the elasmobranch. This is in agreement both with the direct regulatory role of 5-HT postulated by Erspamcr & Ottolcnghi (1952a.b) on mammalian kidney function. and with the results of Welsh & Moorhcad (1960) who found in a number of invertebrates relatively high levels of 5-HT in poorly innervated excretory organs, including kidney. In the light of this, the relatively high content of the amine in other epithclial tissues such as the rectal gland and the olfactory organ of .'G~~/iod~iwr.s. involved in salt transport and chemosensory functions. respectively. indicates additional organ models for providing an insight into the role of serotonin in epithelial physiolog).
In both fishes. the heart has remarkable amounts of 5-HT. While the role of serotonin on invertebrate heart has been to some extent investigated (Erspamer & Ghiretti. 1951;Kiss & Rozsa. 1978). its role in the vertebrate heart remains still obscure.
The cardiac chambers of the elasmobranches (Saetcrsdal cr 01. . 1975. : Berge. 1979) and teleosts (Lamanski c'f trl.. 1975: Midttun. 19X1) are characterired by remarkable amounts of granule-containing cells. often located just beneath the endocardium. Such ;I topological condition is strategically suited for releasing ph~lrmncologic~llly active substances direct]) into the cardiac lumen. On the other hand, compared to that of higher vertebrates. the fish heart is endowed with a very high catecholamine content that exerts important effects upon the myocardial function and marked vasodilator action on the branchial circulation (Fiinge. 1963: Satchel]. 1971 1. The well-known vasoconstriction efl'ect of 5-HT. together with the significant amount of the amine in the hearts of the two fishes. might well be related to the maintenance of \,ascular control in these animals. Moreover. the fact that S-IIT may release noradrcnalinc from several  1954) prompt further studies on a putative control of serotonin on the mechanism of cardiac secretion of catecholamines in fishes. On the whole. these dntn shed more light on the concept of the fish heart 21s an "endocrine" organ.