1 04

Increased secre t ion o f cort isol , the most i m p o r t a n t co r t icos te ro id o f tcleost llsh, is a p r i m a r y ind ica to r o f stress. A l though cort isol secre t ion is u n d e r mul t ip le con t ro l , the p i tu i t a ry g land has .i d o m i n a t i n g role. I;or all ve r t eb ra te s A( ' I I 1 is general is cons ide red the most i m p o r t a n t fac tor m e d ia t i n g p i tu i ta ry con t ro l o f cor t icos te ro id secre t ion d u r in g stress. a M S l 1 m a \ he o f i m p o r t a n c e in add i t ion to or ins tead o f A C T H , d e p e n d i n g on the type o f stressor. K e y w o r d s : CX-MSH, A C T H , c o r t i s o l , s t r e s s r e s p o n s e , t c l e o s t l l s h . T rad i t iona l ly , cort icosteroids have been a t t r ibu ted a m a jo r role in stress physiology. For m an y ver tebra tes the involvement o f hypo ­ tha lam us a n d p i tu i tary gland in the control o f cort icosteroid secretion has been established, with C R H , A Y P and A C T H as the most im p o r ­ tant secretagogues, and cortisol or cor t icos terone as the ad renocor t ica l end p roduc ts o f the h y p o tha lam o-p i tu i ta ry -ad rena l axis ( C h r o u s o s & G o l d , 1992). In general , this ve r teb ra te p a t te rn does also apply to teleost fishes ( S u m p t e r et a/., 1994). In this p a p e r the p i tu i ta ry involve­ ment in the control o f cortisol, the main end p ro d u c t o f the hypotha lam o-p i tu i ta ry in te r rcna l HPIaxis in fish, will be reviewed. In these animals the exper im en ta l evidence for a p ro m in e n t role of A C T H as a cor t ico tropic h o rm o n e du r ing stress is in fact r a th e r limited, and the cor t ico tropic function o f M S H m ay have been underes t im a ted . Cortisol. In fish, cortisol has been implica ted in the formation and mobil iza t ion o f energy substrates. It fu r ther st imulates the dilferentiaCTJ J tion o f chlor ide cells, the main ion t ranspor t ing cells in the gills. This effect likely is o f adap t ive significance, mainly because o f potent ia l d am a g e to the gills by stressors W e n d e l a a r B o n g a & L o c k , 1992). C hron ica l ly e levated cortisol levels have been implicated in the re d u c ­ tion o f growth an d rep roduc t ion , as well as in im m unesuppress ion associated with p ro longed exposure to stressors. The endocr ine control o f cortisol secretion in teleost fish is complex. Atrial na tr iure t ic factor, * Address for co r respondence 1 04 S.K. WENDELAAR BONGA, P.HM. BALM & A.E. LAMKRS angio tensin II and uro tensins all have d irect s t imula tory effects on cortisol release in t rou t ( A r n o l d R e e d & B a l m e n t , 1994). H ow ever , these secretaffoffues most likely are no m o re than modulating: the O O J o cor t ico tropic act ions o f hormone(s) o r ig ina t ing from the p i tu i ta ry gland: studies on hypophysec tom ized fish have ind ica ted tha t the p i tu i ta ry g land dom ina tes the endocr ine control o f cortisol secret ion ( Y o u n g , 1993:. A C T H and a M S H are the main cand ida tes for p i tu ­ itary control . J A C T H . T h e r e is extensive histological evidence for the invo lvem ent o of A C T H in the stress response to stressors in fish (see D o n a l d s o n , 1981). H ow ever , few reports on p la sm a A C T H levels are available, because o f lack o f specific an d sensitive assays for most species an d the presence o f different A C T H l ik e pept ides released by the p i tu i tary , as ind ica ted by studies on goldfish, salnionids an d tilapia (see B a l m et a l., 1994 ). A rise o f p lasm a A C T H im m unoreac t iv i ty has been observed in sa lmonids subjected to han d l in g and con f inem en t ( S u m p t e r et al., 1986; P i c k e r i n g et al. , 1987). H ow ever , the rm a l shock had no effect in b row n t rou t P i c k e r i n g et a l., 1986 even though p la sm a cortisol levels were highly elavated. B a l m et al. 1994 recently showed that in the ti lapia Oreochromis mossambicus two types o f pept ides acco u n ted for most of the A C T H im m u n o re a c t iv i ty released by the p i tu i ta ry pars distalis, with similar cor t ico tropic potency. T h e im m ed ia te rise in cortisol following han d l ing stress in ti lapia was not p re c ed ed or a cc o m p a n ied by a rise in A C T H . W h e n tilapia were conf ined in pairs , a de tec tab le rise in p lasm a A C T H was only found in one o f the animals , which also showed a h igher cortisol level. In the o ther , p e rh ap s socially d o m in an t , fish o f each pair , the A C T H levels were not significantly elevated, and the rise in p lasm a cortisol was only m arg ina l B a l m et a l., 1994). T hese results show tha t the stress-related rise in cortisol in fish is not exclu­ sively d e p e n d e n t on elevated p lasm a A C T H . M S H . a M S H an d ß -e n d o rp h in are the most im p o r ta n t h o rm on es released by the M S H cells. Before secretion. M S H can be acetyla ted , a J ' J process tha t results in th ree ho rm ona l ly active forms o f M S H : des-, m onoand d i-ace ty la ted M S H . T h ese forms have also been dem on# s t ra ted in fish F o l l e n i u s et a l., 1986). A l though a M S H derives its n am e from its capac i ty for p igm ent dispersion in skin m e lanophores , this act ion is only found in a limited n u m b e r o f ver tebra tes . T he re fo re , J * o th e r functions o f the MSH cells are ind ica ted and several reports have suggested a role in stress adap ta t ion . S u m p t e r el al. 1985) showed for b row n t rou t that p lasm a levels o f a-MSH, ß -en d o rp h in a n d cortisol were raised in fish subjected to hand l ing a n d con f inem en t com bined with a therm al shock. In ra inbow trout res t ra ined out of the water , p lasma ACTH, a-MSH and cortisol were e levated S u m p t e r et a l., STRESS RESPONSE IN TELEOST FISH 1 05 1986h Acidified w a te r activates the MSH cells an d elevates p lasm a MSH levels ( L a m e r s et al., 1991 . All three forms of a-MSH were released in vivo as well as in vitro, a-MSH st imulates cortisol release from ti lapia in tcrrenal tissue incuba ted in vitro, with di-acetyl a-MSH showing the highest cor t ico trop ic activity, followed by the des-acetyl and the m ono-ace ty l form L a m e r s et a i , 1992 . Recently, B a l m et al. repo r ted that N -ace ty l -ß -en d o rp h in po ten t ia tes the c o r ­ t icotropic activity o f a-MSH in tilapia. W h e n the concen t ra t ion differ­ ence be tween both h o rm o n e s — plasm a a-MSH concen t ra t ions are an o rd e r o f m ag n i tu d e h igher than those o f A C T H in t rout ( S u m p t e r et al., 1986 a n d t i lapia ( B a l m et a l., 1995)— we may conclude tha t the c o r ­ t icotropic act ion o f t i lapia a-MSH is physiologically relevant. A fur­ the r ind ica t ion o f a role for a-MSH in the HPI-axis o f t i lapia was the d em o n s t ra t io n o f a feedback re la t ionship with cortisol. High cortisol levels induced in vivo by cor t iso l-conta in ing food dec reased the sensitivitv o f the a-MSH cells to T R H 10-fold and that to C R H 100-fold J ( L a m e r s et al., 1994).