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Effects of dopamine agonists and antagonists on gastric acid secretion and stress responses in rats
Life Sciences, Vol. 41, pp. 1397-1408 Printed in the U.S.A.
Pergamon Journals
E F F E C T S OF D O P A M I N E A G O N I S T S AND A N T A G O N I S T S ON G A S T R I C ACID S E C R E T I O N AND R E S P O N S E S IN RATS Gary
B. Glavin*
and Aisha
STRESS
M. Dugani
D e p a r t m e n t of P h a r m a c o l o g y and T h e r a p e u t i c s , F a c u l t y of Medicine, U n i v e r s i t y of Manitoba, VT0 B a n n a t y n e Avenue, Winnipeg, C A N A D A R3C OW3 (Received in final form July 9, 1987) Summary The d o p a m i n e a g o n i s t s and p r o m o t e r s b r o m o c r i p t i n e , bupropion, and p - h y d r o x y m e t h y l p h e n i d a t e (a p e r i p h e r a l l y a c t i n g m e t h y l p h e n i d a t e analog) reduced basal g a s t r i c acid s e c r e t i o n in rats, while the d o p a m i n e a n t a g o n i s t s h a l o p e r i dol, p i m o z i d e and m e t o c l o p r a m i d e a u g m e n t e d g a s t r i c acid output. Stress ulcer f o r m a t i o n and plasma c o r t i c o s t e r o n e levels w e r e m a r k e d l y r e d u c e d by l-dopa given either i n t r a p e r i t o n e ally or i n t r a c e r e b r o v e n t r i c u l a r l y as well as by i n t r a p e r i t o neally administered p-hydroxymethylphenidate. Domperidone, a p e r i p h e r a l d o p a m i n e r e c e p t o r blocker, p r o d u c e d v a r i a b l e effects on stress responses, i n d i c a t i n g a w i d e r s p e c t r u m of a c t i o n than h i t h e r t o r e a l i z e d for this compound. The results s t r o n g l y support a role for both central and p e r i p h e r a l dopam i n e r g i c a c t i v i t y in r e d u c i n g the p a t h o l o g i c a l c o n s e q u e n c e s of e x p r o s u r e to stress. Stress ulcer is a serious m e d i c a l d i s o r d e r w i t h a high m o r t a l i t y risk e s p e c i a l l y among infants and the e l d e r l y (1,2). The p a t h o g e n e s i s of these lesions remains largely unknown, however, recent r e s e a r c h has e m p h a s i z e d both central and p e r i p h e r a l m o n o a m i n e n e u r o t r a n s m i t t e r s as factors in stress u l c e r o g e n e s i s . R e c e n t reports by H e r n a n d e z et al. (3) suggest an i m p o r t a n t role for d o p a m i n e (DA) in a m e l i o r a t i n g stress ulcer f o r m a t i o n in rats. DA a g o n i s t s such as apomorphine, or DA a c t i v a t o r s like d - a m p h e t a mine, m e t h y l p h e n i d a t e , and p - h y d r o x y m e t h y l p h e n i d a t e (POHMP, a p e r i p h e r a l l y - a c t i n 9 a n a l o g of m e t h y l p h e n i d a t e ) reduced cold-restraint-stress-induced ulcers in rats. This effect was o b t u n d e d by p r e t r e a t i n 9 rats w i t h domperidone, a p e r i p h e r a l l y - a c t i n g DA r e c e p t o r antagonist. B e c a u s e of the p e r i p h e r a l s e l e c t i v i t y of the m e t h y l p h e n i d a t e analog and of domperidone, H e r n a n d e z s u g g e s t e d that p e r i p h e r a l DA r e c e p t o r s t i m u l a t i o n is p r i m a r i l y r e s p o n s i b l e for the g a s t r i c c y t o p r o t e c t i o n o b s e r v e d w i t h a d m i n i s t r a t i o n of DA a g o n i s t s and promoters. This s u g g e s t i o n was s t r o n g l y s u p p o r t e d in a s u b s e q u e n t study of H e r n a n d e z et al. (4). Both g a s t r i c and brain DA r e c e p t o r a c t i v i t y w e r e m e a s u r e d in rats s u b j e c t e d to c o l d - r e s t r a i n t stress. No changes w e r e o b s e r v e d in brain DA receptors, however, an i n c r e a s e in g a s t r i c [ H] D A b i n d i n g was observed after lh of stress, w h e n a p p r o x i m a t e l y 30% of the animals
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for c o r r e s p o n d e n c e .
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had d e v e l o p e d ulcers. However, a role for c e n t r a l d o p a m i n e r g i c f u n c t i o n in m e d i a t i n g stress r e s p o n s e s cannot be ruled out. H e r n a n d e z et al. (4,5) d e m o n s t r a t e d that the e n d o g e n o u s brain and gut t r i d e c a p e p t i d e neurotensin, has p r o f o u n d a n t i - u l c e r o g e n i c effects w h i c h appear to be m e d i a t e d by c e n t r a l DA activation. Some e v i d e n c e exists w h i c h c h a r a c t e r i z e d the p e r i p h e r a l DA r e c e p t o r subtype i n v o l v e d in m e d i a t i n g g a s t r i c function. S a n d r o c k (6) d e t e c t e d a p o p u l a t i o n of specific, h i g h - a f f i n i t y DA r e c e p t o r s in the rodent g a s t r i c m u c o s a and s u b m u c o s a using [ H]DA. These r e c e p t o r s w e r e b e l i e v e d to be of the DA] subtype (positively c ~ u p l e d to a d e n y l a t e cyclase), b e c a u s e 6f the a b s e n c e of s p e c i f i c [ ~ H ] h a l o p e r i d o l b i n d i n g (DA r e c e p t o r ligand). S a n d r o c k also showed t h a t ~ t h e d u o d e n a l u l c e r o g e n c y s t e a m i n e caused a rapid increase in [JH]DA b i n d i n g to d u o d e n a l mucosa, s u g g e s t i n g a role for DA in d u o d e n a l ulcer disease. Szabo (7,8) showed that cys t e a m i n e - i n d u c e d and l - m e t h y l - 4 - p h e n y l - l , 2 , 3 , 6 - t e t r a h y d r o p y r i d i n e ( M P T P ) - i n d u c e d d u o d e n a l ulcer f o r m a t i o n w e r e i n h i b i t e d by the DA a g o n i s t b r o m o c r i p t i n e and by the s e l e c t i v e MAO R i n h i b i t o r l-deprenly. Szabo (9) also found a s i g n i f i c a n t n e g a t i v e r e l a t i o n s h i p b e t w e e n w h o l e brain DA levels and the s e v e r i t y of M P T P - i n d u c e d d u o d e n a l ulceration, s u g g e s t i n g a p o s s i b l e role for central DA in the g e n e s i s of p e r i p h e r a l g a s t r o i n t e s t i n a l disease. However, few studies have d i r e c t l y e x a m i n e d this question. C o s t a l l et al.(10) i n f u s e d a novel DA a g o n i s t ( 2 - D i - n - p r o p y l a m i n o - 5 , 6 - d i h y p r o x t e t r a lin) into the c e r e b r a l v e n t r i c l e s (icv) of rats and p r o d u c e d a d o s e - d e p e n d e n t r e d u c t i o n in both the volume and acid c o n c e n t r a t i o n of basal g a s t r i c secretion. DA a n t a g o n i s t s (haloperidol and metoclopramide) b l o c k e d the t e t r a l i n - i n d u c e d r e d u c t i o n in g a s t r i c acid secretion. Gupta et al. (ii) a d m i n i s t e r e d l-dopa icv in rats and found no effect on stress ulcer d e v e l o p m e n t . V e r y recently, S i k i r i c et al. (12) noted that d o p a m i n e a n t a g o n i s t s alone, in the a b s e n c e of other stressors, induced g a s t r i c lesions in rats. In addition, we r e c e n t l y r e p o r t e d that icv l - d e p r e n l y v i r t u a l l y a b o l i s h e d stress ulcer f o r m a t i o n (13). Still other reports present c o n f l i c t i n g data. For example, G r o i s m a n et al. (14) suggested that m e t o c l o p r a m i d e , a DA r e c e p t o r antagonist, had a n t i u l c e r o genic effect in rats, In addition, Hara et al. (15) o b s e r v e d that DA r e c e p t o r b l o c k a d e w i t h h a l o p e r i d o l did not e x a c e r b a t e a c t i v i t y - s t r e s s g a s t r i c ulcer f o r m a t i o n in rats, and Drago et al. (16) found that d o m p e r i d o n e d e c r e a s e d ulcer f o r m a t i o n in rats. Finally, in a recent c o m p r e h e n s i v e review, W i l l e m s et al. (17) p r e s e n t e d e v i d e n c e s u g g e s t i n g that DA and l-dopa i n c r e a s e basal g a s t r i c acid output, w h e r e a s these c o m p o u n d s d e c r e a s e s t i m u l a t e d (pentagastrin) acid output. In the p r e s e n t study, we e x a m i n e d g a s t r o i n t e s t i n a l responses to DA a g o n i s t s and a n t a g o n i s t s in both r e s t i n g and stressc h a l l e n g e d conditions. We used the c h r o n i c g a s t r i c c a n n u l a prep a r a t i o n for a s s e s s i n g c o n c i o u s g a s t r i c s e c r e t o r y r e s p o n s e s to d o p a m i n e r g i c agents and c o l d - r e s t r a i n t for e x a m i n a t i o n of DA effects on g a s t r i c ulcer and plasma c o r t i c o s t e r o n e r e s p o n s e s to stress. Materials
and M e t h o d s
G a s t r i c Acid: M a l e S p r a g u e - D a w l e y rats (200 +__ i0 g at the start of the experiments) were r a n d o m l y a s s i g n e d to t r e a t m e n t conditions (n=6 per group) and w e r e p r e p a r e d w i t h chronic i n d w e l l i n g g a s t r i c c a n n u l a e as d e s c r i b e d p r e v i o u s l y (18). F o l l o w i n g a 14-day
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r e c o v e r y period, all c a n n u l a e b e c a m e firmly a n c h o r e d and p r o d u c e d no u n t o w a r d effects. W h e n g a s t r i c s e c r e t i o n was to be c o l l e c t e d at the same time each day ( 0 8 3 0 - 1 2 0 0 h on each c o l l e c t i o n day), the c a n n u l a plug was r e m o v e d and the s t o m a c h r i n s e d w i t h 20-30 ml of d i s t i l l e d water. The c a n n u l a was left open and the stomach, a l l o w ed to d r a i n for 30 m i n prior to three c o n s e c u t i v e lh g a s t r i c sec r e t i o n c o l l e c t i o n periods. F o l l o w i n g c o l l e c t i o n , the c a n n u l a plug was r e p l a c e d and 96h e l a p s e d b e t w e e n s u c c e s s i v e c o l l e c t i o n s f r o m the same animal. The v o l u m e of s e c r e t i o n from each lh coll e c t i o n p e r i o d was recorded, the s a m p l e s c e n t r i f u g e d at 2500 x g for i0 m i n to r e m o v e r e s i d u a l debris, pH d e t e r m i n e d and 1.0 ml a l i q u o t s of the s u p e r n a t a n t t i t r a t e d to pH 7.0 w i t h 0.01N NaOH. A c i d o u t p u t was e x p r e s s e d as m E q / 1 0 0 g body w e i g h t / h . The first lh c o l l e c t i o n p e r i o d c o n s i s t e d of a p r e - d r u g injection b a s e l i n e . At the b e g i n n i n g of the s e c o n d hour, all coll e c t i o n v i a l s w e r e c h a n g e d and i n j e c t i o n s (i.0 ml) of vehicle; b r o m o c r i p t i n e m e s y l a t e (Sandoz) at doses of 1.0, 2.0 and 4.0 m g / k g ip; b u p r o p i o n HCI ( b u r r o u g h s - W e l l e o m e ) at doses of 12.5, 25.0 and 50.0 m g / k g ip; h a l o p e r i d o l (McNeil) at doses of 0.i, 0.25 and 0.50 m g / k g im; p i m o z i d e (McNeil) at doses of 0.25, 0.50, and 1.0 m g / k g ip; d o m p e r i d o n e HCI (Janssen) at doses of 0.50, 1.0, and 2.0 m g / k g ip; m e t o c l o p r a m i d e HCI (Nordic) at doses of 5.0, i0.0, and 20.0 m g / k g ip and p - h y d r o x y m e t h y l p h e n i d a t e (a gift of Dr. D.E. H e r n a n d e z , U n i v e r s i t y of S o u t h e r n C a l i f o r n i a , Los Angeles) at doses of 1.0, 2.0 and 4.0 m g / k g ip w e r e a d m i n i s t e r e d . The following v e h i c l e s w e r e used: for b r o m o c r i p t i n e , 4% (v/v) ethanol; for b u p r o p i o n , d i s t i l l e d water; for h a l o p e r i d o l , M c N e i l lactic acid vehicle, pH < 4.0; for p i m o z i d e , t a r t a r i c a c i d / d i s t i l l e d water. At the b e g i n n i n g of the t h i r d hour, vials w e r e a g a i n c h a n g e d (no f u r t h e r i n j e c t i o n s w e r e given) and a s e c o n d p o s t - d r u g i n j e c t i o n c o l l e c t i o n p e r i o d of lh ensued. All rats w e r e t e s t e d over five c o l l e c t i o n p e r i o d s (each of 3h d u r a t i o n ) , s e p a r a t i o n by 96h, in the f o l l o w i n g order: vehicle, first (lowest) dose, s e c o n d dose, t h i r d (highest) dose, and v e h i c l e again. It s h o u l d be n o t e d that a p e r i o d of 96h b e t w e e n s u c c e s s i v e d r u g t r e a t m e n t is s u f f i c i e n t to p r e v e n t any r e s i d u a l e f f e c t s of the p r e v i o u s drug t r e a t m e n t in e i t h e r i n c r e a s i n g or d e c r e a s i n g dose r e g i m e n (unpublished observations). Thus, each a n i m a l s e r v e d as its own control and all d r u g i n j e c t i o n s w e r e p r e c e d e d and f o l l o w e d by a v e h i cle i n j e c t i o n c o l l e c t i o n period. S t r e s s u l c e r and c o r t i c o s t e r o n e d e t e r m i n a t i o n s : Rats w e r e r a n d o m l y a s s i g n e d to t r e a t m e n t g r o u p s (n=6) and d e p r i v e d of food, but not water, for 24h prior to b e i n g i m m o b i l i z e d for 3h as desc r i b e d p r e v i o u s l y (19,20). F o l l o w i n g the r e s t r a i n t period, all rats w e r e d e c a p i t a t e d at the same time each day (ll00h). Trunk b l o o d was c o l l e c t e d into h e p a r i n i z e d tubes, the p l a s m a s e p a r a t e d by c e n t r i f u c a t i o n at 5000 x g for 20 m i n and f r o z e n at -70°C until a s s a y e d for c o r t i c o s t e r o n e a c c o r d i n g to a f l u o r o m e t r i c m e t h o d s e n s i t i v e to c o r t i c o s t e r o n e (+ 5 ug%) but not to c o r t i s o l (21). The s t o m a c h w e r e removed, o p e n e d a l o n g the g r e a t e r c u r v a t u r e , r i n s e d and c l e a n e d w i t h d i s t i l l e d w a t e r f o l l o w e d by 10% f o r m a l d e hyde. T h e y w e r e t h e n e v a l u a t e d for u l c e r s u n d e r a d i s s e c t i n g m i c r o s c o p e w i t h an o c u l a r m i c r o m e t e r by an o b s e r v e r w h o was una w a r e of t r e a t m e n t c o n d i t i o n s . The n u m b e r and c u m u l a t i v e length ( e x p r e s s e d in m i l l i m e t e r s ) of the u l c e r s w e r e recorded. Drugs u s e d were: l - d o p a (Sigma) at doses of 0.5, 1.0, 2.0 m g / k g i p and 2.0 m g / k g / d a y ip for 4 days; d o m p e r i d o n e HCI at doses of 0J25,0.25, 0.5, 1.0, and 2.0 m g / k g ip; metoclopramide HCI at
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doses of 5.0, i0°0 and 20.0 m g / k g ip and p - h y d r o x y m e t h y l p h e n i date at doses of 1.0, 2.0, and 4.0 m g / k g ip. All drugs were d i s s o l v e d in d i s t i l l e d water, w h i c h was used for the v e h i c l e injections in a c o m p a r a b l e v o l u m e (i.0 ml), and w e r e a d m i n i s t e r e d 5 m i n p r i o r to a p p l i c a t i o n of r e s t r a i n t stress. ICV C a n n u l a t i o n : Rats w e r e r a n d o m l y a s s i g n e d to t r e a t m e n t c o n d i t i o n s (n=6 per group). Under sodium pentobarbital anaesthesia (50.0 m g / k g ip), 2 3 - g a u g e c a n n u l a g u i d e s w e r e c h r o n i c a l l y i m p l a n t e d in the r i g h t l a t e r a l v e n t r i c l e u s i n g the f o l l o w i n g coordinates: 0.8 mm p o s t e r i o r to bregma, 1.5 m m lateral to the m i d l i n e and 3.5 m m v e n t r a l to the skull s u r f a c e (i0). After 5 days of recovery, rats w e r e d e p r i v e d of food, but not water, for 24h and g i v e n i n j e c t i o n s p r i o r to r e s t r a i n t stress. Domperidone (0.5, 1.0 and 2.0 ug), l-dopa (0.I, 0.5, 1.0, and 2.0 ug), POHMP (1.0, 5.0 and i0.0 ug), or v e h i c l e ( d i s t i l l e d water) w e r e i n f u s e d over 4 m i n w i t h a Sage I n s t r u m e n t s i n f u s i o n p u m p at a rate of 1.0 ul/min. I n f u s i o n o c c u r r e d 30 m i n p r i o r to r e s t r a i n t . F o l l o w i n g the stress period, all rats w e r e s a c r i f i c e d and e x a m i n e d in the m a n n e r d e s c r i b e d above. S t a t i s t i c a l analysis: All d a t a w e r e a n a l y z e d w i t h a n a l y sis of v a r i a n c e f o l l o w e d by T u k e y m u l t i p l e c o m p a r i s o n tests w h e r e s t a t i s t i c a l s i g n i f i c a n c e ( c o n s i d e r e d at the 0.05 level) was obtained. All data are e x p r e s s e d as m e a n ~ SEM. Results G a s t r i c acid secretion: T a b l e 1 s u m m a r i z e s the e f f e c t s of d o p a m i n e a g o n i s t s and a n t a g o n i s t s on basal g a s t r i c acid secretion. The DA a g o n i s t s b r o m o c r i p t i n e and b u p r o p i o n s i g n i f i c a n t l y d e c r e a s e d basal acid o u t p u t r e l a t i v e to b a s e l i n e secretion. This was true for all doses of b r o m o c r i p t i n e and for the h i g h e r doses of b u p r o p i o n (25.0 and 5.0 mg/kg). The p e r i p h e r a l l y s e l e c t i v e DA agonist, POHMP, d e c r e a s e d g a s t r i c acid s e c r e t i o n at lower doses (1.0 and 2.0 mg/kg) but not at the h i g h e s t dose used in this study (4.0 mg/kg)/ The DA a n t a g o n i s t s h a l o p e r i d o l , p i m o z i d e and m e t o c l o p r a mide, s i g n i f i c a n t l y a u g m e n t e d g a s t r i c acid s e c r e t i o n at the h i g h e r dose used (0.25 and 0.50 m g / k g for h a l o p e r i d o l ; 0.50 and 1.00 m g / k g for p i m o z i d e ; and i0.0 and 20.0 m g / k g for m e t o c l o p r a mide). D o m p e r i d o n e , however, e x h i b i t e d a b i p h a s i c e f f e c t on g a s t r i c secretion. At doses of 0.50 and 1.00 mg/kg, d o m p e r i d o n e p r o d u c e d a slight ( n o n - s i g n i f i c a n t ) e l e v a t i o n of g a s t r i c secretion. However, at the h i g h e s t dose used (2.0 mg/kg), g a s t r i c acid o u t p u t was s i g n i f i c a n t l y r e d u c e d r e l a t i v e to b a s e l i n e levels. S t r e s s u l c e r and c o r t i c o s t e r o n e : Table of i n t r a p e r i t o n e a l l y a d m i n i s t e r e d d o p a m i n e r g i c s t r e s s - i n d u c e d u l c e r formation.
2 shows the e f f e c t s agents on r e s t r a i n t
l - d o p a at doses of 1.0 and 2.0 m g / k g p r o v i d e d s i g n i f i c a n t p r o t e c t i o n a g a i n s t stress u l c e r f o r m a t i o n in terms of both u l c e r f r e q u e n c y (number of u l c e r s w h i c h d e v e l o p e d ) and u l c e r s e v e r i t y (ulcer length) r e l a t i v e to both v e h i c l e and all o t h e r drug treatments. A r e g i m e n of l - d o p a at a dose of 2.0 m g / k g for four days p r i o r to stress r e d u c e d u l c e r f o r m a t i o n r e l a t i v e to v e h i c l e t r e a t e d rats but was less p r o t e c t i v e than a c u t e l y a d m i n i s t e r e d ldopa at doses of 1.0 and 2.0 mg/kg. Domperidone significantly
vehicle-pre 12.5 mg/kg 25.0 mg/kg 50.0 mg/kg vehicle-post
vehicle-pre 1.O mg/kg 2.0 mg/kg 4.0 mg/kg vehicle-post
vehicle-pre 0.10 mg/kg 0.25 mg/kg 0.50 mg/kg vehicle-post
Bupropion
p-hydroxymethylphenidate
Haloperidol
(I.I0) (1.22)÷ (2.311 (4.71, + (I.71)
(1.91) i (1.20)~ (2.41)" (3.70) (2.20)
(0.80)** (0.20)** (0.01)** (0.20) (0.53)
(I.12), (0.03). (0.10). (0.51) (0.81)
vehicle-pre
12.98 13.43 21 63 21 29 13.22
13.89 1 32 7 30 16.59 12.86
13.43 11.26 0.06 0.40 13.40
13.75 0.23 0.86 1.14 12.23
Acid Output (mEq/lO0 g / h )
++Significantly greater than pimozide vehicle-pre, pimozide 0.25 mg/kg, and vehicle-post; p < 0.01.
+ Significantly greater than haloperidol vehicle-pre, haloperidol 0.I0 mg/kg and vehiclepost; p < 0.01.
**Significantly less than bupropion vehicle-pre and vehicle-post; p < 0.01.
* Significantly less than bromocriptine and vehicle-post; p < 0.01.
vehicle-pre 1.0 mg/kg 2.0 mg/kg 4.0 mg/kg vehicle-post
Dose
Bromocriptine
Drug
TABLE I and Antagonists (Mean f SEM)
vehicle-pre 5.0 mg/kg 10.0 mg/kg 20.0 m/gkg vehicle-post
vehicle-pre 0.50 mg/kg 1.00 mg/kg 2.00 mg/kg vehicle-post
vehicle-pre 0.25 mg/kg 0.50 mg/kg 1.00 mg/kg vehicle-post
Dose
phenidate vehicle-pre, 1.0 mg/ vehicle-post; p < 0.01.
P < 0.01. °°Significantly less than p-by phenidate vehicle-pre, metoclo and vehicle-post; p < 0.01. | Significantly less t h a n p-hy
o Significantly less than domp domperidone 0.50 and 1 . 0 mg/kg
Metoclopramide
Domperidone
Pimozide
Drug
Effects of Dopamine Agonists on Gastric Acid Secretion
1402
Dopamine and Stress Responses
TABLE
Vol. 41, No. ii, 1987
2
E f f e c t s of i n t r a p e r i t o n e a l l y - a d m i n i s t e r e d a g o n i s t s and a n t a g o n i s t s on stress ulcer (Mean + SEM)
Drug/Dose
No.
of Ulcers
Vehicle
12.0
3.6
l-dopa
0.5 m g / k g 1.0 m g / k g 2.0 m g / k g 2.0 m g / k g / d a y / 4 days
7.4 2.8 4.8 12.8
2.2 0.6 1.0 2.5
Domperidone
15.0 16.0 21.0 22.0 16.3
0.125 m g / k g 0.25 m g / k g 0.50 m g / k g 1.0 m g / k g 2.0 m g / k g
Metoclopramide
5.0 m g / k g i0.0 m g / k g 20.0 m g / k g
p-hydroxymethylphenidate
Significantly
less
1.0 m g / k g 2.0 m g / k g 4.0 m g / k g
than v e h i c l e
dopamine formation
C u m u l a t i v e Ulcer L e n g t h (mm)
29.0
(7.8)
22.8 7.8 11.8 17.6
(5.2), (2.8), (2.4)*** (2.8)
3.5 3.1 ** 5.1 ** 3.0 3.3)
31.6 32.7 43.3 41.7 31.3
(2.7) (6.7) (i0.2~, (1.9 (1.8
8.7 10.8 9.7
0.9) 1.4) 2.7)
16.0 22.8 38.0
(6.0 (4.8 (6.6
11.3 13.1 12.6
2.1) 2.9) 2.2)
13.2 12.3 19.1
(1.9)** (2.8) (4.1)
and all
, ,
other
drugs
(p<0.05).
S i g n i f i c a n t l y g r e a t e r than vehicle; d o m p e r i d o n e 0.125 mg/kg; all doses of l-dopa; and m e t o c l o p r a m i d e 5.0 and i0.0 m g / k g (p<0.05). Significantly
less
than v e h i c l e
(p<0.05).
e x a c e r b a t e d stress g a s t r i c ulcer f o r m a t i o n at a dose of 1.0 mg/kg, but did not do so at any other dose t e s t e d in this study. Metoc l o p r a m i d e did not s i g n i f i c a n t l y a f f e c t stress ulcer d e v e l o p m e n t at any dose examined, p-hydroxymethylphenidate significantly r e d u c e d r e s t r a i n t stress ulcer f o r m a t i o n at doses of 2.0 and 4.0 mg/kg. Table 3 s u m m a r i z e s the effects of the icy a d m i n i s t r a t i o n of d o p a m i n e r g i c drugs on stress ulcer formation. All doses of POHMP and all doses of l-dopa except 0.1 ug s i g n i f i c a n t l y r e d u c e d ulcer d e v e l o p m e n t . U n e x p e c t e d l y , all doses of d o m p e r i d o n e also s i g n i f i c a n t l y a t t e n u a t e d ulcer development.
stressed
Table 4 summarizes the p l a s m a c o r t i c o s t e r o n e r e s p o n s e s rats given i n t r a p e r i t o n e a l i n j e c t i o n s of d o p a m i n e r g i c
in
Vol. 41, No. ii, 1987
Dopamine and Stress Responses
TABLE
3
E f f e c t s of i n t r a c e r e b r o v e n t r i c u l a r administration d o p a m i n e r g i c agents on stress ulcer f o r m a t i o n (Mean + SEM)
Drug/Dose
Vehicle
Domperidone
l-dopa
No.
& stress
0.i 0.5 1.0 2.0
of Ulcers
23.6
0.5 ug 1.0 ug 2.0 ug ug ug ug ug
p-hydroxymethylphenidate
1.0 ug 5.0 ug i0.0 ug
Significantly
less
than v e h i c l e
+Significantly
less
than
l-dopa
1403
of
C u m u l a t i v e Ulcer L e n g t h (mm)
(3.6)
44.1
(3.8)
12.7 12.0 11.6
5.2), 5.2), 1.8)
26.7 18.7 20.7
(2.7), (1.8), (6.3)
17.7 10.7 9.3 12.3
8.2) 1.2), 3.2), 2.9)
38.3 20.0 i0.0 11.3
(1.2),+ (4.2),+ (4.6),+ (1.8)
14.3 8.3 8.7
5.3), 4.3), 4.3)
20.3 (1.8), 13.7 (7.2) , 23.3. (14.2)
(p<0.05). 0.i ug
(p<0.05).
drugs. l-dopa at all doses s i g n i f i c a n t l y r e d u c e d the stressi n d u c e d rise in p l a s m a c o r t i c o s t e r o n e levels. Metoclopramide did not s i g n i f i c a n t l y affect the c o r t i c o s t e r o n e r e s p o n s e to res t r a i n t stress. D o m p e r i d o n e p r o d u c e d v a r i a b l e effects. Doses of 0.25 and 1.0 m g / k g s i g n i f i c a n t l y r e d u c e d c o r t i c o s t e r o n e levels, w h i l e a dose of 2.0 m g / k g s i g n i f i c a n t l y e l e v a t e d this r e s p o n s e r e l a t i v e to all other groups, p-hydroxymethylphenidate signific a n t l y a t t e n u a t e d c o r t i c o s t e r o n e levels at the two lowest doses used.
rats
T a b l e 5 shows the p l a s m a c o r t i c o s t e r o n e g i v e n icv a d m i n i s t r a t i o n of l-dopa, POHMP
data for s t r e s s e d and domperidone.
l-dopa at all doses except 0.i ug, s i g n i f i c a n t l y attenuated the s t r e s s - i n d u c e d rise in p l a s m a c o r t i c o s t e r o n e . POHMP did not s i g n i f i c a n t l y a f f e c t c o r t i c o s t e r o n e r e s p o n s e s to stress. With d o m p e r i d o n e treatment, only the 1.0 ug dose of the drug signific a n t l y r e d u c e d c o r t i c o s t e r o n e levels r e l a t i v e to v e h i c l e - t r e a t e d s t r e s s e d c o n t r o l rats. DISCUSSION The p r e s e n t
studies
demonstrate
that
the d o p a m i n e
agonists
1404
Dopamine and Stress Responses
Vol. 41, No. Ii, 1987
TABLE 4 Effects of intraperitoneally-administered l-dopa, domperidone and metoclopramide, on plasma corticosterone responses to restraint stress (Mean + SEM)
Drug/Dose
Vechicle
Plasma corticosterone levels (ug%)
(no stress)
39.8
5.3)
Vechicle & stress
69.8
6.3)#
l-dopa 0.5 mg/kg 1.0 mg/kg 2.0 mg/kg 2.0 mg/kg/day/4 days
20.9 27.8 24.6 24.6
4.0):: 4.1),+ 4.5),+ 3.7)
Domperidone 0.125 mg/kg 0.25 mg/kg 0.50 mg/kg 1.0 mg/kg 2.0 mg/kg
58.4 47.6 64.9 44.1 82.9
5.8)~ 7.1)# 6.9), 6.2)**# 4.9)
Metoclopramide
59.8 62.1 60.3
5.1)# 6.3 # 6.7 #
44.2 38.8 58.6
4.3 , 5.5 5.1)#
5.0 mg/kg i0.0 mg/kg 20.0 mg/kg
p-hydroxymethylphenidate
Significantly
1.0 mg/kg 2.0 mg/kg 4.0 mg/kg
less than vehicle and stress
(p<0.05).
+Significantly less than all doses of domperidone and metoclopramide (p<0.05). Significantly greater than all other groups
(p<0.05).
# Significantly greater than non-stressed controls
(p<0.01).
bromocriptine, POHMP, and bupropion significantly decrease gastric acid secretion. In addition, ip administered l-dopa and POHMP reduced stress ulcer formation and plasma corticosterone levels. ICV l-dopa, but not POHMP, also decreased corticosterone levels. The dopamine receptor antagonists haloperidol, pimozide and metoclopramide significantly increased gastric acid secretion. IP domperidone inhibited acid secretion, increased stress ulcer formation and decreased corticosterone levels. In addition, icv domperidone decreased ulcer formation and plasma corticosterone
Vol. 41, No. ii, 1987
Dopamine and Stress Responses
TABLE
1405
5
E f f e c t s of i n t r a c e r e b r o v e n t r i c u l a r administration of l-dopa, d o m p e r i d o n e and p - h y d r o x y m e t h y l p h e n d i a t e on p l a s m a c o r t i c o s t e r o n e r e s p o n s e s to r e s t r a i n t stress (Mean + SEM)
Drug/Dose
Vehicle
& stress
Domperidone
l-dopa
Plasma corticosterone levels (ug%)
0.1 0.5 1.0 2.0
0.5. 1.0 2.0
ug ug ug
ug ug ug ug
p-hydroxymethylphenidate
* Significantly
less
1.0 ug 5.0 ug 10.0 ug
than v e h i c l e
61.2
5.7)
60.4 48.5 61.6
6.1), 5.4) 6.3)
54.8 44.1 39.8 41.5
3.7), 2.9), 3.5), 3.7)
51.2 56.8 66.1
6.6) 5.9) 7.1)
(p<0.05).
levels. Our g a s t r i c s e c r e t i o n data are, in general, c o n s i s t e n t w i t h t h o s e of C o s t a l l et al. (10,22). T h e y s u g g e s t e d that the m e c h a n i s m s of a c t i o n of DA a g o n i s t - i n d u c e d e f f e c t s on g a s t r i c s e c r e t i o n i n v o l v e b o t h b e t a - a d r e n e r g i c receptors, w h i c h m e d i a t e e f f e c t s on g a s t r i c s e c r e t o r y volume, as w e l l as DA r e c e p t o r s w h i c h m e d i a t e e f f e c t s on g a s t r i c acid c o n c e n t r a t i o n . Costall et al. (22) f u r t h e r s u g g e s t e d that beta ~ - a d r e n o c e p t o r s w e r e the s p e c i f i c r e c e p t o r s u b t y p e i n v o l v e d in DA e f f e c t s on g a s t r i c secretion, since the a c i d - r e d u c i n g e f f e c t s of a p o m o r p h i n e c o u l d be b l o c k e d by p r o p a n o l o l but not by a t e n o l o l , y o h i m b i n e , or prazosin. S a n d r o c k (6) s u g g e s t e d that DAm r e c e p t o r s m e d i a t e dopal m i n e a g o n i s t e f f e c t s on d u o d e n a l u l c e r o g e n e s i s , however, t h e i r role in g a s t r i c u l c e r o g e n e s i s and g a s t r i c acid s e c r e t i o n r e m a i n s to be d e t e r m i n e d . D o m p e r i d o n e , at a dose of 2.0 mg/kg, r e d u c e d g a s t r i c acid output. This f i n d i n g is not c o n s i s t e n t w i t h the r e p o r t e d a c t i o n of this c o m p o u n d as a p e r i p h e r a l DA r e c e p t o r a n t a g o n i s t . However, a r e c e n t r e p o r t (23) s u g g e s t s a c l i n i c a l use for d o m p e r i d o n e in " n o n - u l c e r d y s p e p s i a " due to its a p p a r e n t a b i l i t y to r e d u c e gastric e m p t y i n g time. Interestingly, metoclopramide, a procaine derivative with a s i m i l a r m o d e of a c t i o n to that of d o m p e r i d o n e (12), s i g n i f i c a n t l y i n c r e a s e d g a s t r i c acid s e c r e t i o n at the h i g h e r d o s e s u s e d in this study. It is p o s s i b l e that i n t e r a c t i o n
1406
Dopamlne and Stress Responses
Vol. 41, No. ii, 1987
w i t h n o n - d o p a m i n e r e c e p t o r s and/or e f f e c t s on gut m o t i l i t y or m u c o s a l blood flow u n d e r l i e the d i s c r e p a n t f i n d i n g s o b s e r v e d w i t h these c o m p o u n d s and we are i n v e s t i g a t i n g these p o s s i b l i l i t i e s . The p e r i p h e r a l l y - a c t i n g m e t h y l p h e n i d a t e analog, POHMP, m a r k e d l y r e d u c e d g a s t r i c acid s e c r e t i o n at lower doses. This effect may a c c o u n t for its a n t i - u l c e r p r o p e r t i e s w h e n g i v e n either p e r i p h e r a l l y (24) or c e n t r a l l y (this study). We also observed that POHMP r e d u c e d stress ulcer f o r m a t i o n and p l a s m a cortic o s t e r o n e levels, again, at the lowest doses used h e r e i n (i.0 and 2.0 mg/kg). Our g a s t r i c ulcer data are in a g r e e m e n t w i t h the m a j o r i t y of the l i t e r t u r e d e m o n s t r a t i n g a p r o t e c t i v e effect of DA a g o n i s t s and p r o m o t e r s on stress u l c e r o g e n e s i s . In particular, H e r n a n d e z et al. (3,4,5,24,25) have shown that a v a r i e t y of DA a g o n i s t s including d-amphetamine, methylphenidate, apomorphine, and POHMP p r o t e c t a g a i n s t c o l d - r e s t r a i n t s t r e s s - i n d u c e d g a s t r i c ulcers. H e r n e n d e z et al. showed also that p r e - t r e a t m e n t w i t h d o m p e r i d o n e b l o c k e d the p r o t e c t i v e effect of the DA agonists. In the p r e s e n t studies, d o m p e r i d o n e p r e - t r e a t m e n t was not used, however, intrap e r i t o n e a l a d m i n i s t r a t i o n of d o m p e r i d o n e alone, r e s u l t e d in augm e n t e d stress ulcer f o r m a t i o n at a dose of 1.0 m g / k g w h i l e icy a d m i n i s t r a t i o n of d o m p e r i d o n e r e d u c e d ulcer d e v e l o p m e n t at all doses tested. These d i f f e r e n t i a l results o b t a i n e d w i t h d o m p e r i done suggest that this c o m p o u n d exerts effects m o r e w i d e s p r e a d than simple b l o c k a d e of p e r i p h e r a l DA receptors. Indeed, domp e r i d o n e has been r e p o r t e d to i n t e r a c t w i t h a l p h a - a d r e n e r g i c r e c e p t o r s in the aorta as w e l l as in other t i s s u r e s (26). The effects o b s e r v e d w i t h the icv a d m i n i s t r a t i o n of dopam i n e r g i c agents c o n f i r m and e x t e n d our recent report w h i c h showed that the s e l e c t i v e MAO_ inhibitor, l-deprenyl, m a r k e d l y a t t e n u a t e s stress ulcer f o r m a t l o n in rats (13). POHMP, l-dopa, and d o m p e r i done given icy, all s i g n i f i c a n t l y r e d u c e d stress ulcer d e v e l o p ment. That l-dopa and POHMP r e d u c e d stress ulcer is not s u r p r i s ing in light of our e a r l i e r report as w e l l as those H e r n a n d e z et al. (3,5). H o w e v e D ~ e f i n d i n g that icv d o m p e r i d o n e e x e r t e d significant a n t i - u l c e r o g e n i c a c t i v i t y at all doses used is p u z z l i n g in v i e w of its r e p o r t e d DA r e c e p t o r a n t @ g o n i s t activity. Radioligand b i n d i n g e x p e r i m e n t s u n d e r w a y using [ ~ H ] d o m p e r i d o n e may help to m o r e c l e a r l y i d e n t i f y the r e c e p t o r s ( b o t h d o p a m i n e and n o n - d o p a mine) to w h i c h d o m p e r i d o n e binds and may help c l a r i f y the m e c h a n sims u n d e r l y i n g the i n t e r e s t i n g effects of this compound. P l a s m a c o r t i c o s t e r o n e levels are r o u t i n e l y used as an index of stress, p r e s u m a b l y r e f l e c t i n g h y p o t h a l a m i c - p i t u i t a r y a d r e n o c o r t i c a l a r o u s a l in r e s p o n s e to n o x i o u s stimulation. Thus, agents w h i c h a t t e n u a t e the s t r e s s - i n d u c e d rise in p l a s m a cortic o s t e r o n e are t h o u g h t to have at least some central action. l-dopa, given both p e r i p h e r a l l y and c e n t r a l l y in this study, s i g n i f i c a n t l y r e d u c e d c o r t i c o s t e r o n e levels in s t r e s s e d rats. P e r i p h e r a l a d m i n i s t r a t i o n of the DA a n t a g o n i s t m e t o c l o p r a m i d e , was w i t h o u t effect on c o r t i c o s t e r o n e activity, w h i l e d o m p e r i d o n e again p r o d u c e d u n e x p e c t e d results. Given p e r i p h e r a l l y , d o m p e r i done at doses of 0.25 and 1.00 m g / k g s i g n i f i c a n t l y r e d u c e d cortic o s t e r o n e levels, w h i l e a dose of 2.0 m g / k g m a r k e d l y a u g m e n t e d p l a s m a levels of the steroid. G i v e n icy, only a dose of 1.0 ug of d o m p e r i d o n e r e d u c e d c o r t i c o s t e r o n e levels. The same dose also
Vol. 41, No. ii, 1987
Dopamine and Stress Responses
1407
a t t e n u a t e d stress u l c e r f o r m a t i o n s i g n i f i c a n t l y . P e r i p h e r a l adm i n i s t r a t i o n of POHMP a t t e n u a t e d c o r t i c o s t e r o n e levels at the same d o s e s at w h i c h a r e d u c t i o n in u l c e r f o r m a t i o n was o b s e r v e d (i.0 and 2.0 mg/kg), h o w e v e r c e n t r a l a d m i n i s t r a t i o n of POHMP did not i n f l u e n c e c o r t i c o s t e r o n e levels. As w i t h the u l c e r data, the m a j o r i t y of the o b s e r v e d c o r t i c o s t e r o n e r e s p o n s e s s u p p o r t our p r e v i o u s r e p o r t (Glavin et al., 13). We have also n o t e d that icv i n f u s i o n of the r e l a t i v e l y - s e l e c t i v e M A O R i n h i b i t o r l-deprenyl can c a u s e a s t r i k i n g a t t e n u a t i o n of p l a s m a c o r t i c o s t e r o n e levels in r e s p o n s e to s t r e s s ( u n p u b l i s h e d o b s e r v a t i o n s ) . These o b s e r v a t i o n s are all c o n s i s t e n t w i t h a c e n t r a l a n t i - s t r e s s role for d o p a m i n e . In summary, the p r e s e n t data s u p p o r t a s t r e s s - r e d u c i n g role for DA a g o n i s t s and an e x a c e r b a t o r y e f f e c t of DA a n t a g o n i s t s on r e s p o n s e s to stress. In p a r t i c u l a r , l - d o p a and POHMP a p p e a r to have m a r k e d a n t i - u l c e r e f f e c t s w i t h p e r i p h e r a l as w e l l as w i t h central administration. An o b s e r v e d e x c e p t i o n to this g e n e r a l p a t t e r n is d o m p e r i d o n e , the e f f e c t s of w h i c h are m i x e d w i t h e i t h e r p e r i p h e r a l or c e n t r a l a d m i n i s t r a t i o n . We are c u r r e n t l y e x a m i n i n g the e f f e c t s and m e c h a n i s m s of a c t i o n of this i n t e r e s t i n g c o m p o u n d w i t h r a d i o l i g a n d b i n d i n g a s s a y s in both b r a i n and gut in order to c l a r i f y its m o d e of action. N o n e t h e l e s s , our data s t r o n g l y suggest that both c e n t r a l and p e r i p h e r a l d o p a m i n e r g i c a c t i v i t y p l a y an i m p o r t a n t role in stress u l c e r e t i o l o g y and that intact a n d / o r a u g m e n t e d d o p a m i n e f u n c t i o n is a c r i t i c a l d e t e r m i n a n t of r e s i s t ance to stress. References i. 2. 3.
4. 5. 6. 7. 8. 9. i0. II. 12.
13. 14.
15. 16.
D. Fernandez, S t r e s s 2, 16-22, (1981). P. Lamy, G a s t r o e n t e r o l . Clin. Biol. 9, 102-105 (1985). D. H e r n a n d e z , J.W. Adcock, R.C. Orlando, K.S. Patrick, C.B. N e m e r o f f and A.J. Prange, Life Sci. 35, 2 4 5 3 - 2 4 5 8 (1984). D° H e r n a n d e z , G.A. Mason, C.H. W a l k e r and V.L. M o s k a l u k , Clin. Res. 34, 182A (1986). D. H e r n a n d e z , D. Stanley, J. M e l v i n and A. Prange, P h a r m a c o l . Biochem. Behav. 22, 409-413 (1985). A. S a n d r o c k , G a s t r o e n t e r o l . 80, 1362 (1981). S. Szabo, Lancet. 2, 880-882 (1979). S. Szabo, A. Brown, G. Pihan, H. Dali and J.L. N e u m e y e r , Proc. Soc. Exp. Biol. Med. 180, 567-571 (1985). S. Szabo, L a b Invest. 51, 121-147 (1984). B. Costall, R.J. N a y l o r and C.C.W. Tan, Eur. J. P h a r m a c o l . 117, 61-69 (1985). M. Gupta, G.P. G u p t a and K.P. B h a r a g a v a , Ind. J. Med. Res. 7_88, 2 8 1 - 2 8 3 (1983). P. Sikiric, J. Geber, D. Ivanovic, E. Suchanek, V. Gjuris, M. T u c a n - F o r e t i c , S. Mise, G. C v i t a n o v i c and I. Rotkvic, Eur. J. P h a r m a c o l . 131 105-109 (1986). G. Glavin, A. D u g a n i and C. Pinsky, N e u r o s c i . Lett. 70, 379-381 (1986). S. G r o i s m a n , I.S. Chekman, V.P. K h o k h o l y a , T.G. K a r e v i n a and R.D. Samilova, Bull. Exp. Biol. Med. 99, 272-275 (1985). C. H a r a and N. Ogawa, Japan. J. P h a r m a c o l . 35, 474-477 (1984). F. Drago, G° C o n t i n e l l a , G. C o n f o r t o and U. S c a p a g n i n i ,
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L i f e Sci. 36, 1 9 1 - 1 9 7 (i985). J. W i l l e m s , W.A. B u y l a e r t , R.A. L e f e b v r e a n d M.G. B o g a e r t , P h a r m a c o l . Rev. 37, 1 6 5 - 2 0 7 (1985). W. Pare, K. Isom, G. V i n c e n t a n d G. G l a v i n , L a b A n i m . Sci, 2_/7, 2 4 4 - 2 4 7 (1977). G. G l a v i n , B r a i n Res. B u l l . 5, 5 1 - 5 8 (1980). W. P a r e a n d G. G l a v i n , N e u r o s c i . B i o b e h a v . Rev. i_O0, 3 3 9 370 (1986). J. v a n d e r V i e s , A c t a E n d o c r i n o l . 3_88, 3 9 9 - 4 0 6 (1961). B. C o s t a l l , R.J. N a y l o r and C . C . W . Tan, Eur. J. P h a r m a c o l . 116, 2 7 9 - 2 8 5 (1985). S. S a r i n , P. S h a r m a , Y. C h a w l a , P. G o p i n a t h a n d S. N u n o y . Ind. J. Med. Res. 83, 6 2 3 - 6 2 8 (1986). D. H e r n a n d e z , R.C. O r l a n d o , J.W. A d c o c k , K.S. P a t r i c k , C.B. N e m e r o f f and A.J. P r a n g e , C l i n . Res. 31, 6 8 3 A (1983). D. H e r n a n d e z , D. S t a n l e y , J. M e l v i n and A. P r a n g e , B r a i n Res. 381, 1 5 9 - 1 6 3 (1986). C. E n n i s a n d B. Cox, J. P h a r m . P h a r m a c o l . 32, 4 3 4 - 4 3 5 (1980).
Pergamon Journals
E F F E C T S OF D O P A M I N E A G O N I S T S AND A N T A G O N I S T S ON G A S T R I C ACID S E C R E T I O N AND R E S P O N S E S IN RATS Gary
B. Glavin*
and Aisha
STRESS
M. Dugani
D e p a r t m e n t of P h a r m a c o l o g y and T h e r a p e u t i c s , F a c u l t y of Medicine, U n i v e r s i t y of Manitoba, VT0 B a n n a t y n e Avenue, Winnipeg, C A N A D A R3C OW3 (Received in final form July 9, 1987) Summary The d o p a m i n e a g o n i s t s and p r o m o t e r s b r o m o c r i p t i n e , bupropion, and p - h y d r o x y m e t h y l p h e n i d a t e (a p e r i p h e r a l l y a c t i n g m e t h y l p h e n i d a t e analog) reduced basal g a s t r i c acid s e c r e t i o n in rats, while the d o p a m i n e a n t a g o n i s t s h a l o p e r i dol, p i m o z i d e and m e t o c l o p r a m i d e a u g m e n t e d g a s t r i c acid output. Stress ulcer f o r m a t i o n and plasma c o r t i c o s t e r o n e levels w e r e m a r k e d l y r e d u c e d by l-dopa given either i n t r a p e r i t o n e ally or i n t r a c e r e b r o v e n t r i c u l a r l y as well as by i n t r a p e r i t o neally administered p-hydroxymethylphenidate. Domperidone, a p e r i p h e r a l d o p a m i n e r e c e p t o r blocker, p r o d u c e d v a r i a b l e effects on stress responses, i n d i c a t i n g a w i d e r s p e c t r u m of a c t i o n than h i t h e r t o r e a l i z e d for this compound. The results s t r o n g l y support a role for both central and p e r i p h e r a l dopam i n e r g i c a c t i v i t y in r e d u c i n g the p a t h o l o g i c a l c o n s e q u e n c e s of e x p r o s u r e to stress. Stress ulcer is a serious m e d i c a l d i s o r d e r w i t h a high m o r t a l i t y risk e s p e c i a l l y among infants and the e l d e r l y (1,2). The p a t h o g e n e s i s of these lesions remains largely unknown, however, recent r e s e a r c h has e m p h a s i z e d both central and p e r i p h e r a l m o n o a m i n e n e u r o t r a n s m i t t e r s as factors in stress u l c e r o g e n e s i s . R e c e n t reports by H e r n a n d e z et al. (3) suggest an i m p o r t a n t role for d o p a m i n e (DA) in a m e l i o r a t i n g stress ulcer f o r m a t i o n in rats. DA a g o n i s t s such as apomorphine, or DA a c t i v a t o r s like d - a m p h e t a mine, m e t h y l p h e n i d a t e , and p - h y d r o x y m e t h y l p h e n i d a t e (POHMP, a p e r i p h e r a l l y - a c t i n 9 a n a l o g of m e t h y l p h e n i d a t e ) reduced cold-restraint-stress-induced ulcers in rats. This effect was o b t u n d e d by p r e t r e a t i n 9 rats w i t h domperidone, a p e r i p h e r a l l y - a c t i n g DA r e c e p t o r antagonist. B e c a u s e of the p e r i p h e r a l s e l e c t i v i t y of the m e t h y l p h e n i d a t e analog and of domperidone, H e r n a n d e z s u g g e s t e d that p e r i p h e r a l DA r e c e p t o r s t i m u l a t i o n is p r i m a r i l y r e s p o n s i b l e for the g a s t r i c c y t o p r o t e c t i o n o b s e r v e d w i t h a d m i n i s t r a t i o n of DA a g o n i s t s and promoters. This s u g g e s t i o n was s t r o n g l y s u p p o r t e d in a s u b s e q u e n t study of H e r n a n d e z et al. (4). Both g a s t r i c and brain DA r e c e p t o r a c t i v i t y w e r e m e a s u r e d in rats s u b j e c t e d to c o l d - r e s t r a i n t stress. No changes w e r e o b s e r v e d in brain DA receptors, however, an i n c r e a s e in g a s t r i c [ H] D A b i n d i n g was observed after lh of stress, w h e n a p p r o x i m a t e l y 30% of the animals
Author
for c o r r e s p o n d e n c e .
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had d e v e l o p e d ulcers. However, a role for c e n t r a l d o p a m i n e r g i c f u n c t i o n in m e d i a t i n g stress r e s p o n s e s cannot be ruled out. H e r n a n d e z et al. (4,5) d e m o n s t r a t e d that the e n d o g e n o u s brain and gut t r i d e c a p e p t i d e neurotensin, has p r o f o u n d a n t i - u l c e r o g e n i c effects w h i c h appear to be m e d i a t e d by c e n t r a l DA activation. Some e v i d e n c e exists w h i c h c h a r a c t e r i z e d the p e r i p h e r a l DA r e c e p t o r subtype i n v o l v e d in m e d i a t i n g g a s t r i c function. S a n d r o c k (6) d e t e c t e d a p o p u l a t i o n of specific, h i g h - a f f i n i t y DA r e c e p t o r s in the rodent g a s t r i c m u c o s a and s u b m u c o s a using [ H]DA. These r e c e p t o r s w e r e b e l i e v e d to be of the DA] subtype (positively c ~ u p l e d to a d e n y l a t e cyclase), b e c a u s e 6f the a b s e n c e of s p e c i f i c [ ~ H ] h a l o p e r i d o l b i n d i n g (DA r e c e p t o r ligand). S a n d r o c k also showed t h a t ~ t h e d u o d e n a l u l c e r o g e n c y s t e a m i n e caused a rapid increase in [JH]DA b i n d i n g to d u o d e n a l mucosa, s u g g e s t i n g a role for DA in d u o d e n a l ulcer disease. Szabo (7,8) showed that cys t e a m i n e - i n d u c e d and l - m e t h y l - 4 - p h e n y l - l , 2 , 3 , 6 - t e t r a h y d r o p y r i d i n e ( M P T P ) - i n d u c e d d u o d e n a l ulcer f o r m a t i o n w e r e i n h i b i t e d by the DA a g o n i s t b r o m o c r i p t i n e and by the s e l e c t i v e MAO R i n h i b i t o r l-deprenly. Szabo (9) also found a s i g n i f i c a n t n e g a t i v e r e l a t i o n s h i p b e t w e e n w h o l e brain DA levels and the s e v e r i t y of M P T P - i n d u c e d d u o d e n a l ulceration, s u g g e s t i n g a p o s s i b l e role for central DA in the g e n e s i s of p e r i p h e r a l g a s t r o i n t e s t i n a l disease. However, few studies have d i r e c t l y e x a m i n e d this question. C o s t a l l et al.(10) i n f u s e d a novel DA a g o n i s t ( 2 - D i - n - p r o p y l a m i n o - 5 , 6 - d i h y p r o x t e t r a lin) into the c e r e b r a l v e n t r i c l e s (icv) of rats and p r o d u c e d a d o s e - d e p e n d e n t r e d u c t i o n in both the volume and acid c o n c e n t r a t i o n of basal g a s t r i c secretion. DA a n t a g o n i s t s (haloperidol and metoclopramide) b l o c k e d the t e t r a l i n - i n d u c e d r e d u c t i o n in g a s t r i c acid secretion. Gupta et al. (ii) a d m i n i s t e r e d l-dopa icv in rats and found no effect on stress ulcer d e v e l o p m e n t . V e r y recently, S i k i r i c et al. (12) noted that d o p a m i n e a n t a g o n i s t s alone, in the a b s e n c e of other stressors, induced g a s t r i c lesions in rats. In addition, we r e c e n t l y r e p o r t e d that icv l - d e p r e n l y v i r t u a l l y a b o l i s h e d stress ulcer f o r m a t i o n (13). Still other reports present c o n f l i c t i n g data. For example, G r o i s m a n et al. (14) suggested that m e t o c l o p r a m i d e , a DA r e c e p t o r antagonist, had a n t i u l c e r o genic effect in rats, In addition, Hara et al. (15) o b s e r v e d that DA r e c e p t o r b l o c k a d e w i t h h a l o p e r i d o l did not e x a c e r b a t e a c t i v i t y - s t r e s s g a s t r i c ulcer f o r m a t i o n in rats, and Drago et al. (16) found that d o m p e r i d o n e d e c r e a s e d ulcer f o r m a t i o n in rats. Finally, in a recent c o m p r e h e n s i v e review, W i l l e m s et al. (17) p r e s e n t e d e v i d e n c e s u g g e s t i n g that DA and l-dopa i n c r e a s e basal g a s t r i c acid output, w h e r e a s these c o m p o u n d s d e c r e a s e s t i m u l a t e d (pentagastrin) acid output. In the p r e s e n t study, we e x a m i n e d g a s t r o i n t e s t i n a l responses to DA a g o n i s t s and a n t a g o n i s t s in both r e s t i n g and stressc h a l l e n g e d conditions. We used the c h r o n i c g a s t r i c c a n n u l a prep a r a t i o n for a s s e s s i n g c o n c i o u s g a s t r i c s e c r e t o r y r e s p o n s e s to d o p a m i n e r g i c agents and c o l d - r e s t r a i n t for e x a m i n a t i o n of DA effects on g a s t r i c ulcer and plasma c o r t i c o s t e r o n e r e s p o n s e s to stress. Materials
and M e t h o d s
G a s t r i c Acid: M a l e S p r a g u e - D a w l e y rats (200 +__ i0 g at the start of the experiments) were r a n d o m l y a s s i g n e d to t r e a t m e n t conditions (n=6 per group) and w e r e p r e p a r e d w i t h chronic i n d w e l l i n g g a s t r i c c a n n u l a e as d e s c r i b e d p r e v i o u s l y (18). F o l l o w i n g a 14-day
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r e c o v e r y period, all c a n n u l a e b e c a m e firmly a n c h o r e d and p r o d u c e d no u n t o w a r d effects. W h e n g a s t r i c s e c r e t i o n was to be c o l l e c t e d at the same time each day ( 0 8 3 0 - 1 2 0 0 h on each c o l l e c t i o n day), the c a n n u l a plug was r e m o v e d and the s t o m a c h r i n s e d w i t h 20-30 ml of d i s t i l l e d water. The c a n n u l a was left open and the stomach, a l l o w ed to d r a i n for 30 m i n prior to three c o n s e c u t i v e lh g a s t r i c sec r e t i o n c o l l e c t i o n periods. F o l l o w i n g c o l l e c t i o n , the c a n n u l a plug was r e p l a c e d and 96h e l a p s e d b e t w e e n s u c c e s s i v e c o l l e c t i o n s f r o m the same animal. The v o l u m e of s e c r e t i o n from each lh coll e c t i o n p e r i o d was recorded, the s a m p l e s c e n t r i f u g e d at 2500 x g for i0 m i n to r e m o v e r e s i d u a l debris, pH d e t e r m i n e d and 1.0 ml a l i q u o t s of the s u p e r n a t a n t t i t r a t e d to pH 7.0 w i t h 0.01N NaOH. A c i d o u t p u t was e x p r e s s e d as m E q / 1 0 0 g body w e i g h t / h . The first lh c o l l e c t i o n p e r i o d c o n s i s t e d of a p r e - d r u g injection b a s e l i n e . At the b e g i n n i n g of the s e c o n d hour, all coll e c t i o n v i a l s w e r e c h a n g e d and i n j e c t i o n s (i.0 ml) of vehicle; b r o m o c r i p t i n e m e s y l a t e (Sandoz) at doses of 1.0, 2.0 and 4.0 m g / k g ip; b u p r o p i o n HCI ( b u r r o u g h s - W e l l e o m e ) at doses of 12.5, 25.0 and 50.0 m g / k g ip; h a l o p e r i d o l (McNeil) at doses of 0.i, 0.25 and 0.50 m g / k g im; p i m o z i d e (McNeil) at doses of 0.25, 0.50, and 1.0 m g / k g ip; d o m p e r i d o n e HCI (Janssen) at doses of 0.50, 1.0, and 2.0 m g / k g ip; m e t o c l o p r a m i d e HCI (Nordic) at doses of 5.0, i0.0, and 20.0 m g / k g ip and p - h y d r o x y m e t h y l p h e n i d a t e (a gift of Dr. D.E. H e r n a n d e z , U n i v e r s i t y of S o u t h e r n C a l i f o r n i a , Los Angeles) at doses of 1.0, 2.0 and 4.0 m g / k g ip w e r e a d m i n i s t e r e d . The following v e h i c l e s w e r e used: for b r o m o c r i p t i n e , 4% (v/v) ethanol; for b u p r o p i o n , d i s t i l l e d water; for h a l o p e r i d o l , M c N e i l lactic acid vehicle, pH < 4.0; for p i m o z i d e , t a r t a r i c a c i d / d i s t i l l e d water. At the b e g i n n i n g of the t h i r d hour, vials w e r e a g a i n c h a n g e d (no f u r t h e r i n j e c t i o n s w e r e given) and a s e c o n d p o s t - d r u g i n j e c t i o n c o l l e c t i o n p e r i o d of lh ensued. All rats w e r e t e s t e d over five c o l l e c t i o n p e r i o d s (each of 3h d u r a t i o n ) , s e p a r a t i o n by 96h, in the f o l l o w i n g order: vehicle, first (lowest) dose, s e c o n d dose, t h i r d (highest) dose, and v e h i c l e again. It s h o u l d be n o t e d that a p e r i o d of 96h b e t w e e n s u c c e s s i v e d r u g t r e a t m e n t is s u f f i c i e n t to p r e v e n t any r e s i d u a l e f f e c t s of the p r e v i o u s drug t r e a t m e n t in e i t h e r i n c r e a s i n g or d e c r e a s i n g dose r e g i m e n (unpublished observations). Thus, each a n i m a l s e r v e d as its own control and all d r u g i n j e c t i o n s w e r e p r e c e d e d and f o l l o w e d by a v e h i cle i n j e c t i o n c o l l e c t i o n period. S t r e s s u l c e r and c o r t i c o s t e r o n e d e t e r m i n a t i o n s : Rats w e r e r a n d o m l y a s s i g n e d to t r e a t m e n t g r o u p s (n=6) and d e p r i v e d of food, but not water, for 24h prior to b e i n g i m m o b i l i z e d for 3h as desc r i b e d p r e v i o u s l y (19,20). F o l l o w i n g the r e s t r a i n t period, all rats w e r e d e c a p i t a t e d at the same time each day (ll00h). Trunk b l o o d was c o l l e c t e d into h e p a r i n i z e d tubes, the p l a s m a s e p a r a t e d by c e n t r i f u c a t i o n at 5000 x g for 20 m i n and f r o z e n at -70°C until a s s a y e d for c o r t i c o s t e r o n e a c c o r d i n g to a f l u o r o m e t r i c m e t h o d s e n s i t i v e to c o r t i c o s t e r o n e (+ 5 ug%) but not to c o r t i s o l (21). The s t o m a c h w e r e removed, o p e n e d a l o n g the g r e a t e r c u r v a t u r e , r i n s e d and c l e a n e d w i t h d i s t i l l e d w a t e r f o l l o w e d by 10% f o r m a l d e hyde. T h e y w e r e t h e n e v a l u a t e d for u l c e r s u n d e r a d i s s e c t i n g m i c r o s c o p e w i t h an o c u l a r m i c r o m e t e r by an o b s e r v e r w h o was una w a r e of t r e a t m e n t c o n d i t i o n s . The n u m b e r and c u m u l a t i v e length ( e x p r e s s e d in m i l l i m e t e r s ) of the u l c e r s w e r e recorded. Drugs u s e d were: l - d o p a (Sigma) at doses of 0.5, 1.0, 2.0 m g / k g i p and 2.0 m g / k g / d a y ip for 4 days; d o m p e r i d o n e HCI at doses of 0J25,0.25, 0.5, 1.0, and 2.0 m g / k g ip; metoclopramide HCI at
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doses of 5.0, i0°0 and 20.0 m g / k g ip and p - h y d r o x y m e t h y l p h e n i date at doses of 1.0, 2.0, and 4.0 m g / k g ip. All drugs were d i s s o l v e d in d i s t i l l e d water, w h i c h was used for the v e h i c l e injections in a c o m p a r a b l e v o l u m e (i.0 ml), and w e r e a d m i n i s t e r e d 5 m i n p r i o r to a p p l i c a t i o n of r e s t r a i n t stress. ICV C a n n u l a t i o n : Rats w e r e r a n d o m l y a s s i g n e d to t r e a t m e n t c o n d i t i o n s (n=6 per group). Under sodium pentobarbital anaesthesia (50.0 m g / k g ip), 2 3 - g a u g e c a n n u l a g u i d e s w e r e c h r o n i c a l l y i m p l a n t e d in the r i g h t l a t e r a l v e n t r i c l e u s i n g the f o l l o w i n g coordinates: 0.8 mm p o s t e r i o r to bregma, 1.5 m m lateral to the m i d l i n e and 3.5 m m v e n t r a l to the skull s u r f a c e (i0). After 5 days of recovery, rats w e r e d e p r i v e d of food, but not water, for 24h and g i v e n i n j e c t i o n s p r i o r to r e s t r a i n t stress. Domperidone (0.5, 1.0 and 2.0 ug), l-dopa (0.I, 0.5, 1.0, and 2.0 ug), POHMP (1.0, 5.0 and i0.0 ug), or v e h i c l e ( d i s t i l l e d water) w e r e i n f u s e d over 4 m i n w i t h a Sage I n s t r u m e n t s i n f u s i o n p u m p at a rate of 1.0 ul/min. I n f u s i o n o c c u r r e d 30 m i n p r i o r to r e s t r a i n t . F o l l o w i n g the stress period, all rats w e r e s a c r i f i c e d and e x a m i n e d in the m a n n e r d e s c r i b e d above. S t a t i s t i c a l analysis: All d a t a w e r e a n a l y z e d w i t h a n a l y sis of v a r i a n c e f o l l o w e d by T u k e y m u l t i p l e c o m p a r i s o n tests w h e r e s t a t i s t i c a l s i g n i f i c a n c e ( c o n s i d e r e d at the 0.05 level) was obtained. All data are e x p r e s s e d as m e a n ~ SEM. Results G a s t r i c acid secretion: T a b l e 1 s u m m a r i z e s the e f f e c t s of d o p a m i n e a g o n i s t s and a n t a g o n i s t s on basal g a s t r i c acid secretion. The DA a g o n i s t s b r o m o c r i p t i n e and b u p r o p i o n s i g n i f i c a n t l y d e c r e a s e d basal acid o u t p u t r e l a t i v e to b a s e l i n e secretion. This was true for all doses of b r o m o c r i p t i n e and for the h i g h e r doses of b u p r o p i o n (25.0 and 5.0 mg/kg). The p e r i p h e r a l l y s e l e c t i v e DA agonist, POHMP, d e c r e a s e d g a s t r i c acid s e c r e t i o n at lower doses (1.0 and 2.0 mg/kg) but not at the h i g h e s t dose used in this study (4.0 mg/kg)/ The DA a n t a g o n i s t s h a l o p e r i d o l , p i m o z i d e and m e t o c l o p r a mide, s i g n i f i c a n t l y a u g m e n t e d g a s t r i c acid s e c r e t i o n at the h i g h e r dose used (0.25 and 0.50 m g / k g for h a l o p e r i d o l ; 0.50 and 1.00 m g / k g for p i m o z i d e ; and i0.0 and 20.0 m g / k g for m e t o c l o p r a mide). D o m p e r i d o n e , however, e x h i b i t e d a b i p h a s i c e f f e c t on g a s t r i c secretion. At doses of 0.50 and 1.00 mg/kg, d o m p e r i d o n e p r o d u c e d a slight ( n o n - s i g n i f i c a n t ) e l e v a t i o n of g a s t r i c secretion. However, at the h i g h e s t dose used (2.0 mg/kg), g a s t r i c acid o u t p u t was s i g n i f i c a n t l y r e d u c e d r e l a t i v e to b a s e l i n e levels. S t r e s s u l c e r and c o r t i c o s t e r o n e : Table of i n t r a p e r i t o n e a l l y a d m i n i s t e r e d d o p a m i n e r g i c s t r e s s - i n d u c e d u l c e r formation.
2 shows the e f f e c t s agents on r e s t r a i n t
l - d o p a at doses of 1.0 and 2.0 m g / k g p r o v i d e d s i g n i f i c a n t p r o t e c t i o n a g a i n s t stress u l c e r f o r m a t i o n in terms of both u l c e r f r e q u e n c y (number of u l c e r s w h i c h d e v e l o p e d ) and u l c e r s e v e r i t y (ulcer length) r e l a t i v e to both v e h i c l e and all o t h e r drug treatments. A r e g i m e n of l - d o p a at a dose of 2.0 m g / k g for four days p r i o r to stress r e d u c e d u l c e r f o r m a t i o n r e l a t i v e to v e h i c l e t r e a t e d rats but was less p r o t e c t i v e than a c u t e l y a d m i n i s t e r e d ldopa at doses of 1.0 and 2.0 mg/kg. Domperidone significantly
vehicle-pre 12.5 mg/kg 25.0 mg/kg 50.0 mg/kg vehicle-post
vehicle-pre 1.O mg/kg 2.0 mg/kg 4.0 mg/kg vehicle-post
vehicle-pre 0.10 mg/kg 0.25 mg/kg 0.50 mg/kg vehicle-post
Bupropion
p-hydroxymethylphenidate
Haloperidol
(I.I0) (1.22)÷ (2.311 (4.71, + (I.71)
(1.91) i (1.20)~ (2.41)" (3.70) (2.20)
(0.80)** (0.20)** (0.01)** (0.20) (0.53)
(I.12), (0.03). (0.10). (0.51) (0.81)
vehicle-pre
12.98 13.43 21 63 21 29 13.22
13.89 1 32 7 30 16.59 12.86
13.43 11.26 0.06 0.40 13.40
13.75 0.23 0.86 1.14 12.23
Acid Output (mEq/lO0 g / h )
++Significantly greater than pimozide vehicle-pre, pimozide 0.25 mg/kg, and vehicle-post; p < 0.01.
+ Significantly greater than haloperidol vehicle-pre, haloperidol 0.I0 mg/kg and vehiclepost; p < 0.01.
**Significantly less than bupropion vehicle-pre and vehicle-post; p < 0.01.
* Significantly less than bromocriptine and vehicle-post; p < 0.01.
vehicle-pre 1.0 mg/kg 2.0 mg/kg 4.0 mg/kg vehicle-post
Dose
Bromocriptine
Drug
TABLE I and Antagonists (Mean f SEM)
vehicle-pre 5.0 mg/kg 10.0 mg/kg 20.0 m/gkg vehicle-post
vehicle-pre 0.50 mg/kg 1.00 mg/kg 2.00 mg/kg vehicle-post
vehicle-pre 0.25 mg/kg 0.50 mg/kg 1.00 mg/kg vehicle-post
Dose
phenidate vehicle-pre, 1.0 mg/ vehicle-post; p < 0.01.
P < 0.01. °°Significantly less than p-by phenidate vehicle-pre, metoclo and vehicle-post; p < 0.01. | Significantly less t h a n p-hy
o Significantly less than domp domperidone 0.50 and 1 . 0 mg/kg
Metoclopramide
Domperidone
Pimozide
Drug
Effects of Dopamine Agonists on Gastric Acid Secretion
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Dopamine and Stress Responses
TABLE
Vol. 41, No. ii, 1987
2
E f f e c t s of i n t r a p e r i t o n e a l l y - a d m i n i s t e r e d a g o n i s t s and a n t a g o n i s t s on stress ulcer (Mean + SEM)
Drug/Dose
No.
of Ulcers
Vehicle
12.0
3.6
l-dopa
0.5 m g / k g 1.0 m g / k g 2.0 m g / k g 2.0 m g / k g / d a y / 4 days
7.4 2.8 4.8 12.8
2.2 0.6 1.0 2.5
Domperidone
15.0 16.0 21.0 22.0 16.3
0.125 m g / k g 0.25 m g / k g 0.50 m g / k g 1.0 m g / k g 2.0 m g / k g
Metoclopramide
5.0 m g / k g i0.0 m g / k g 20.0 m g / k g
p-hydroxymethylphenidate
Significantly
less
1.0 m g / k g 2.0 m g / k g 4.0 m g / k g
than v e h i c l e
dopamine formation
C u m u l a t i v e Ulcer L e n g t h (mm)
29.0
(7.8)
22.8 7.8 11.8 17.6
(5.2), (2.8), (2.4)*** (2.8)
3.5 3.1 ** 5.1 ** 3.0 3.3)
31.6 32.7 43.3 41.7 31.3
(2.7) (6.7) (i0.2~, (1.9 (1.8
8.7 10.8 9.7
0.9) 1.4) 2.7)
16.0 22.8 38.0
(6.0 (4.8 (6.6
11.3 13.1 12.6
2.1) 2.9) 2.2)
13.2 12.3 19.1
(1.9)** (2.8) (4.1)
and all
, ,
other
drugs
(p<0.05).
S i g n i f i c a n t l y g r e a t e r than vehicle; d o m p e r i d o n e 0.125 mg/kg; all doses of l-dopa; and m e t o c l o p r a m i d e 5.0 and i0.0 m g / k g (p<0.05). Significantly
less
than v e h i c l e
(p<0.05).
e x a c e r b a t e d stress g a s t r i c ulcer f o r m a t i o n at a dose of 1.0 mg/kg, but did not do so at any other dose t e s t e d in this study. Metoc l o p r a m i d e did not s i g n i f i c a n t l y a f f e c t stress ulcer d e v e l o p m e n t at any dose examined, p-hydroxymethylphenidate significantly r e d u c e d r e s t r a i n t stress ulcer f o r m a t i o n at doses of 2.0 and 4.0 mg/kg. Table 3 s u m m a r i z e s the effects of the icy a d m i n i s t r a t i o n of d o p a m i n e r g i c drugs on stress ulcer formation. All doses of POHMP and all doses of l-dopa except 0.1 ug s i g n i f i c a n t l y r e d u c e d ulcer d e v e l o p m e n t . U n e x p e c t e d l y , all doses of d o m p e r i d o n e also s i g n i f i c a n t l y a t t e n u a t e d ulcer development.
stressed
Table 4 summarizes the p l a s m a c o r t i c o s t e r o n e r e s p o n s e s rats given i n t r a p e r i t o n e a l i n j e c t i o n s of d o p a m i n e r g i c
in
Vol. 41, No. ii, 1987
Dopamine and Stress Responses
TABLE
3
E f f e c t s of i n t r a c e r e b r o v e n t r i c u l a r administration d o p a m i n e r g i c agents on stress ulcer f o r m a t i o n (Mean + SEM)
Drug/Dose
Vehicle
Domperidone
l-dopa
No.
& stress
0.i 0.5 1.0 2.0
of Ulcers
23.6
0.5 ug 1.0 ug 2.0 ug ug ug ug ug
p-hydroxymethylphenidate
1.0 ug 5.0 ug i0.0 ug
Significantly
less
than v e h i c l e
+Significantly
less
than
l-dopa
1403
of
C u m u l a t i v e Ulcer L e n g t h (mm)
(3.6)
44.1
(3.8)
12.7 12.0 11.6
5.2), 5.2), 1.8)
26.7 18.7 20.7
(2.7), (1.8), (6.3)
17.7 10.7 9.3 12.3
8.2) 1.2), 3.2), 2.9)
38.3 20.0 i0.0 11.3
(1.2),+ (4.2),+ (4.6),+ (1.8)
14.3 8.3 8.7
5.3), 4.3), 4.3)
20.3 (1.8), 13.7 (7.2) , 23.3. (14.2)
(p<0.05). 0.i ug
(p<0.05).
drugs. l-dopa at all doses s i g n i f i c a n t l y r e d u c e d the stressi n d u c e d rise in p l a s m a c o r t i c o s t e r o n e levels. Metoclopramide did not s i g n i f i c a n t l y affect the c o r t i c o s t e r o n e r e s p o n s e to res t r a i n t stress. D o m p e r i d o n e p r o d u c e d v a r i a b l e effects. Doses of 0.25 and 1.0 m g / k g s i g n i f i c a n t l y r e d u c e d c o r t i c o s t e r o n e levels, w h i l e a dose of 2.0 m g / k g s i g n i f i c a n t l y e l e v a t e d this r e s p o n s e r e l a t i v e to all other groups, p-hydroxymethylphenidate signific a n t l y a t t e n u a t e d c o r t i c o s t e r o n e levels at the two lowest doses used.
rats
T a b l e 5 shows the p l a s m a c o r t i c o s t e r o n e g i v e n icv a d m i n i s t r a t i o n of l-dopa, POHMP
data for s t r e s s e d and domperidone.
l-dopa at all doses except 0.i ug, s i g n i f i c a n t l y attenuated the s t r e s s - i n d u c e d rise in p l a s m a c o r t i c o s t e r o n e . POHMP did not s i g n i f i c a n t l y a f f e c t c o r t i c o s t e r o n e r e s p o n s e s to stress. With d o m p e r i d o n e treatment, only the 1.0 ug dose of the drug signific a n t l y r e d u c e d c o r t i c o s t e r o n e levels r e l a t i v e to v e h i c l e - t r e a t e d s t r e s s e d c o n t r o l rats. DISCUSSION The p r e s e n t
studies
demonstrate
that
the d o p a m i n e
agonists
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Dopamine and Stress Responses
Vol. 41, No. Ii, 1987
TABLE 4 Effects of intraperitoneally-administered l-dopa, domperidone and metoclopramide, on plasma corticosterone responses to restraint stress (Mean + SEM)
Drug/Dose
Vechicle
Plasma corticosterone levels (ug%)
(no stress)
39.8
5.3)
Vechicle & stress
69.8
6.3)#
l-dopa 0.5 mg/kg 1.0 mg/kg 2.0 mg/kg 2.0 mg/kg/day/4 days
20.9 27.8 24.6 24.6
4.0):: 4.1),+ 4.5),+ 3.7)
Domperidone 0.125 mg/kg 0.25 mg/kg 0.50 mg/kg 1.0 mg/kg 2.0 mg/kg
58.4 47.6 64.9 44.1 82.9
5.8)~ 7.1)# 6.9), 6.2)**# 4.9)
Metoclopramide
59.8 62.1 60.3
5.1)# 6.3 # 6.7 #
44.2 38.8 58.6
4.3 , 5.5 5.1)#
5.0 mg/kg i0.0 mg/kg 20.0 mg/kg
p-hydroxymethylphenidate
Significantly
1.0 mg/kg 2.0 mg/kg 4.0 mg/kg
less than vehicle and stress
(p<0.05).
+Significantly less than all doses of domperidone and metoclopramide (p<0.05). Significantly greater than all other groups
(p<0.05).
# Significantly greater than non-stressed controls
(p<0.01).
bromocriptine, POHMP, and bupropion significantly decrease gastric acid secretion. In addition, ip administered l-dopa and POHMP reduced stress ulcer formation and plasma corticosterone levels. ICV l-dopa, but not POHMP, also decreased corticosterone levels. The dopamine receptor antagonists haloperidol, pimozide and metoclopramide significantly increased gastric acid secretion. IP domperidone inhibited acid secretion, increased stress ulcer formation and decreased corticosterone levels. In addition, icv domperidone decreased ulcer formation and plasma corticosterone
Vol. 41, No. ii, 1987
Dopamine and Stress Responses
TABLE
1405
5
E f f e c t s of i n t r a c e r e b r o v e n t r i c u l a r administration of l-dopa, d o m p e r i d o n e and p - h y d r o x y m e t h y l p h e n d i a t e on p l a s m a c o r t i c o s t e r o n e r e s p o n s e s to r e s t r a i n t stress (Mean + SEM)
Drug/Dose
Vehicle
& stress
Domperidone
l-dopa
Plasma corticosterone levels (ug%)
0.1 0.5 1.0 2.0
0.5. 1.0 2.0
ug ug ug
ug ug ug ug
p-hydroxymethylphenidate
* Significantly
less
1.0 ug 5.0 ug 10.0 ug
than v e h i c l e
61.2
5.7)
60.4 48.5 61.6
6.1), 5.4) 6.3)
54.8 44.1 39.8 41.5
3.7), 2.9), 3.5), 3.7)
51.2 56.8 66.1
6.6) 5.9) 7.1)
(p<0.05).
levels. Our g a s t r i c s e c r e t i o n data are, in general, c o n s i s t e n t w i t h t h o s e of C o s t a l l et al. (10,22). T h e y s u g g e s t e d that the m e c h a n i s m s of a c t i o n of DA a g o n i s t - i n d u c e d e f f e c t s on g a s t r i c s e c r e t i o n i n v o l v e b o t h b e t a - a d r e n e r g i c receptors, w h i c h m e d i a t e e f f e c t s on g a s t r i c s e c r e t o r y volume, as w e l l as DA r e c e p t o r s w h i c h m e d i a t e e f f e c t s on g a s t r i c acid c o n c e n t r a t i o n . Costall et al. (22) f u r t h e r s u g g e s t e d that beta ~ - a d r e n o c e p t o r s w e r e the s p e c i f i c r e c e p t o r s u b t y p e i n v o l v e d in DA e f f e c t s on g a s t r i c secretion, since the a c i d - r e d u c i n g e f f e c t s of a p o m o r p h i n e c o u l d be b l o c k e d by p r o p a n o l o l but not by a t e n o l o l , y o h i m b i n e , or prazosin. S a n d r o c k (6) s u g g e s t e d that DAm r e c e p t o r s m e d i a t e dopal m i n e a g o n i s t e f f e c t s on d u o d e n a l u l c e r o g e n e s i s , however, t h e i r role in g a s t r i c u l c e r o g e n e s i s and g a s t r i c acid s e c r e t i o n r e m a i n s to be d e t e r m i n e d . D o m p e r i d o n e , at a dose of 2.0 mg/kg, r e d u c e d g a s t r i c acid output. This f i n d i n g is not c o n s i s t e n t w i t h the r e p o r t e d a c t i o n of this c o m p o u n d as a p e r i p h e r a l DA r e c e p t o r a n t a g o n i s t . However, a r e c e n t r e p o r t (23) s u g g e s t s a c l i n i c a l use for d o m p e r i d o n e in " n o n - u l c e r d y s p e p s i a " due to its a p p a r e n t a b i l i t y to r e d u c e gastric e m p t y i n g time. Interestingly, metoclopramide, a procaine derivative with a s i m i l a r m o d e of a c t i o n to that of d o m p e r i d o n e (12), s i g n i f i c a n t l y i n c r e a s e d g a s t r i c acid s e c r e t i o n at the h i g h e r d o s e s u s e d in this study. It is p o s s i b l e that i n t e r a c t i o n
1406
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w i t h n o n - d o p a m i n e r e c e p t o r s and/or e f f e c t s on gut m o t i l i t y or m u c o s a l blood flow u n d e r l i e the d i s c r e p a n t f i n d i n g s o b s e r v e d w i t h these c o m p o u n d s and we are i n v e s t i g a t i n g these p o s s i b l i l i t i e s . The p e r i p h e r a l l y - a c t i n g m e t h y l p h e n i d a t e analog, POHMP, m a r k e d l y r e d u c e d g a s t r i c acid s e c r e t i o n at lower doses. This effect may a c c o u n t for its a n t i - u l c e r p r o p e r t i e s w h e n g i v e n either p e r i p h e r a l l y (24) or c e n t r a l l y (this study). We also observed that POHMP r e d u c e d stress ulcer f o r m a t i o n and p l a s m a cortic o s t e r o n e levels, again, at the lowest doses used h e r e i n (i.0 and 2.0 mg/kg). Our g a s t r i c ulcer data are in a g r e e m e n t w i t h the m a j o r i t y of the l i t e r t u r e d e m o n s t r a t i n g a p r o t e c t i v e effect of DA a g o n i s t s and p r o m o t e r s on stress u l c e r o g e n e s i s . In particular, H e r n a n d e z et al. (3,4,5,24,25) have shown that a v a r i e t y of DA a g o n i s t s including d-amphetamine, methylphenidate, apomorphine, and POHMP p r o t e c t a g a i n s t c o l d - r e s t r a i n t s t r e s s - i n d u c e d g a s t r i c ulcers. H e r n e n d e z et al. showed also that p r e - t r e a t m e n t w i t h d o m p e r i d o n e b l o c k e d the p r o t e c t i v e effect of the DA agonists. In the p r e s e n t studies, d o m p e r i d o n e p r e - t r e a t m e n t was not used, however, intrap e r i t o n e a l a d m i n i s t r a t i o n of d o m p e r i d o n e alone, r e s u l t e d in augm e n t e d stress ulcer f o r m a t i o n at a dose of 1.0 m g / k g w h i l e icy a d m i n i s t r a t i o n of d o m p e r i d o n e r e d u c e d ulcer d e v e l o p m e n t at all doses tested. These d i f f e r e n t i a l results o b t a i n e d w i t h d o m p e r i done suggest that this c o m p o u n d exerts effects m o r e w i d e s p r e a d than simple b l o c k a d e of p e r i p h e r a l DA receptors. Indeed, domp e r i d o n e has been r e p o r t e d to i n t e r a c t w i t h a l p h a - a d r e n e r g i c r e c e p t o r s in the aorta as w e l l as in other t i s s u r e s (26). The effects o b s e r v e d w i t h the icv a d m i n i s t r a t i o n of dopam i n e r g i c agents c o n f i r m and e x t e n d our recent report w h i c h showed that the s e l e c t i v e MAO_ inhibitor, l-deprenyl, m a r k e d l y a t t e n u a t e s stress ulcer f o r m a t l o n in rats (13). POHMP, l-dopa, and d o m p e r i done given icy, all s i g n i f i c a n t l y r e d u c e d stress ulcer d e v e l o p ment. That l-dopa and POHMP r e d u c e d stress ulcer is not s u r p r i s ing in light of our e a r l i e r report as w e l l as those H e r n a n d e z et al. (3,5). H o w e v e D ~ e f i n d i n g that icv d o m p e r i d o n e e x e r t e d significant a n t i - u l c e r o g e n i c a c t i v i t y at all doses used is p u z z l i n g in v i e w of its r e p o r t e d DA r e c e p t o r a n t @ g o n i s t activity. Radioligand b i n d i n g e x p e r i m e n t s u n d e r w a y using [ ~ H ] d o m p e r i d o n e may help to m o r e c l e a r l y i d e n t i f y the r e c e p t o r s ( b o t h d o p a m i n e and n o n - d o p a mine) to w h i c h d o m p e r i d o n e binds and may help c l a r i f y the m e c h a n sims u n d e r l y i n g the i n t e r e s t i n g effects of this compound. P l a s m a c o r t i c o s t e r o n e levels are r o u t i n e l y used as an index of stress, p r e s u m a b l y r e f l e c t i n g h y p o t h a l a m i c - p i t u i t a r y a d r e n o c o r t i c a l a r o u s a l in r e s p o n s e to n o x i o u s stimulation. Thus, agents w h i c h a t t e n u a t e the s t r e s s - i n d u c e d rise in p l a s m a cortic o s t e r o n e are t h o u g h t to have at least some central action. l-dopa, given both p e r i p h e r a l l y and c e n t r a l l y in this study, s i g n i f i c a n t l y r e d u c e d c o r t i c o s t e r o n e levels in s t r e s s e d rats. P e r i p h e r a l a d m i n i s t r a t i o n of the DA a n t a g o n i s t m e t o c l o p r a m i d e , was w i t h o u t effect on c o r t i c o s t e r o n e activity, w h i l e d o m p e r i d o n e again p r o d u c e d u n e x p e c t e d results. Given p e r i p h e r a l l y , d o m p e r i done at doses of 0.25 and 1.00 m g / k g s i g n i f i c a n t l y r e d u c e d cortic o s t e r o n e levels, w h i l e a dose of 2.0 m g / k g m a r k e d l y a u g m e n t e d p l a s m a levels of the steroid. G i v e n icy, only a dose of 1.0 ug of d o m p e r i d o n e r e d u c e d c o r t i c o s t e r o n e levels. The same dose also
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a t t e n u a t e d stress u l c e r f o r m a t i o n s i g n i f i c a n t l y . P e r i p h e r a l adm i n i s t r a t i o n of POHMP a t t e n u a t e d c o r t i c o s t e r o n e levels at the same d o s e s at w h i c h a r e d u c t i o n in u l c e r f o r m a t i o n was o b s e r v e d (i.0 and 2.0 mg/kg), h o w e v e r c e n t r a l a d m i n i s t r a t i o n of POHMP did not i n f l u e n c e c o r t i c o s t e r o n e levels. As w i t h the u l c e r data, the m a j o r i t y of the o b s e r v e d c o r t i c o s t e r o n e r e s p o n s e s s u p p o r t our p r e v i o u s r e p o r t (Glavin et al., 13). We have also n o t e d that icv i n f u s i o n of the r e l a t i v e l y - s e l e c t i v e M A O R i n h i b i t o r l-deprenyl can c a u s e a s t r i k i n g a t t e n u a t i o n of p l a s m a c o r t i c o s t e r o n e levels in r e s p o n s e to s t r e s s ( u n p u b l i s h e d o b s e r v a t i o n s ) . These o b s e r v a t i o n s are all c o n s i s t e n t w i t h a c e n t r a l a n t i - s t r e s s role for d o p a m i n e . In summary, the p r e s e n t data s u p p o r t a s t r e s s - r e d u c i n g role for DA a g o n i s t s and an e x a c e r b a t o r y e f f e c t of DA a n t a g o n i s t s on r e s p o n s e s to stress. In p a r t i c u l a r , l - d o p a and POHMP a p p e a r to have m a r k e d a n t i - u l c e r e f f e c t s w i t h p e r i p h e r a l as w e l l as w i t h central administration. An o b s e r v e d e x c e p t i o n to this g e n e r a l p a t t e r n is d o m p e r i d o n e , the e f f e c t s of w h i c h are m i x e d w i t h e i t h e r p e r i p h e r a l or c e n t r a l a d m i n i s t r a t i o n . We are c u r r e n t l y e x a m i n i n g the e f f e c t s and m e c h a n i s m s of a c t i o n of this i n t e r e s t i n g c o m p o u n d w i t h r a d i o l i g a n d b i n d i n g a s s a y s in both b r a i n and gut in order to c l a r i f y its m o d e of action. N o n e t h e l e s s , our data s t r o n g l y suggest that both c e n t r a l and p e r i p h e r a l d o p a m i n e r g i c a c t i v i t y p l a y an i m p o r t a n t role in stress u l c e r e t i o l o g y and that intact a n d / o r a u g m e n t e d d o p a m i n e f u n c t i o n is a c r i t i c a l d e t e r m i n a n t of r e s i s t ance to stress. References i. 2. 3.
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