Differentiation of Dopamine Receptors in the Periphery - American

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Differentiation of Dopamine Receptors in Periphery L E O N I. G O L D B E R G and

JAI

D.

the

KOHLI

The University of Chicago, Committee on Clinical Pharmacology, Departments of Pharmacological and Physiological Sciences and Medicine, Chicago, I L 60637

Two distinct dopamine (DA) receptors have been studied i n the anesthetized dog. DA1 receptors subserve smooth muscle r e l a x a t i o n and can be charac t e r i z e d by comparing p u t a t i v e DA agonists i n the phenoxybenzamine p r e t r e a t e d renal arterial bed. DA2 receptors located on sympathetic nerve endings subserve inhibition of norepinephrine r e l e a s e and can be c h a r a c t e r i z e d i n the untreated femoral vascular bed by comparing v a s o d i l a t i o n produced by d i p r o p y l DA and p u t a t i v e a g o n i s t s . The s t r u c t u r a l requirements f o r a c t i v a t i o n of DA1 receptors are very strict. P o s i t i v e compounds r e q u i r e 3,4OH groups on the benzene r i n g . S u b s t i t u t i o n s on the alpha and beta carbon and single s u b s t i t u t i o n s on the Ν atom, except f o r N-methyl, are i n a c t i v e . N,N-di-substituted compounds with one group being N-propyl or N-butyl are 30-100 times l e s s a c t i v e than DA. For s e m i - r i g i d analogs r e l a t e d to d i h y d r o x y - a m i n o t e t r a l i n s (ADTN), beta rotamers with 6,7-dihydroxy s u b s t i t u t i o n showed DA1 activity; the alpha rotamers with 5,6-dihydroxy - s u b s t i t u t i o n were i n a c t i v e except f o r N,N-di-n -propyl A-5,6-DTN, which was approximately equipotent to DA. 7,8-Dihydroxybenzazepines have been shown to be full or partial a g o n i s t s . Apomor phine i s a weak partial agonist; isoapomorphine and ergot d e r i v a t i v e s are i n a c t i v e . In c o n t r a s t , many compounds are a c t i v e on DA2 r e c e p t o r s . These include compounds with 1 hydroxy group on the benzene r i n g and ergot compounds such as bromocrip tine. Compounds a c t i v e on both receptors e x h i b i t d i f f e r e n c e s i n potency s e r i e s . Apomorphine and d i p r o p y l DA are much more potent as DA agonists than as DA a g o n i s t s . D i f f e r e n c e s i n antagonists 2

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0097-6156/83/0224-0101$06.00/0 © 1983 American Chemical Society In Dopamine Receptors; Kaiser, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

102

DOPAMINE

RECEPTORS

have a l s o been demonstrated. Most compounds studied antagonize both DA1 and DA r e c e p t o r s . However, d i f f e r e n c e s i n relative potencies have been demonstrated and a s e l e c t i v e DA antagonist, domperidone, has been found. 2


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An important p r a c t i c a l reason f o r c h a r a c t e r i z i n g receptors i s to provide a basis f o r the development of new drugs. I f the molecular requirements f o r the a c t i v a t i o n of a receptor and i t s s p e c i f i c antagonism can be e l u c i d a t e d , m o d i f i c a t i o n s of agonist molecules can be made to produce compounds with q u a l i t a t i v e l y d i f f e r e n t pharmacological a c t i v i t y . In t h i s paper we describe two d i s t i n c t dopamine (DA) receptors i n p e r i p h e r a l t i s s u e s , DA, receptors which subserve smooth muscle r e l a x a t i o n , and DA« receptors which subserve i n h i b i t i o n of norepinephrine r e l e a s e from the p o s t g a n g l i o n i c sympathetic nerve. This s u b d i v i s i o n i s based on demonstration of pronounced d i f f e r e n c e s i n the potency s e r i e s of agonists a c t i v e on both r e c e p t o r s , agonists which act on only one of the two r e c e p t o r s , and s e l e c t i v e antagonism (_1). The DAi and DAp c l a s s i f i c a t i o n d i f f e r s from other c l a s s i f i c a t i o n s based on anatomical l o c a t i o n s ( p r e s y n a p t i c / p o s t s y n a p t i c ) or response ( i n c r e a s e i n adenylate cyclase/no change i n adenylate c y c l a s e ; e x c i t a t i o n / c o n t r a c t i o n ) . The s u b d i v i s i o n of receptors described i n t h i s paper a l s o d i f f e r s from s u b d i v i s i o n s based on the use of binding assays ( 2 ) . DA^

receptors

Almost a l l i n v e s t i g a t i o n s of the s t r u c t u r e a c t i v i t y r e l a t i o n ship (SAR) of DA, receptors were conducted i n vascular preparat i o n s i n which DÂ causes v a s o d i l a t i o n i n v i v o and r e l a x a t i o n i n v i t r o . We have used e s s e n t i a l l y the same technique f o r invest i g a t i n g DA, agonists s i n c e 1965 (1,4). Renal blood flow i s measured by an electromagnetic flow meter i n anesthetized dogs. A f t e r a d m i n i s t r a t i o n of phenoxybenzamine, agonists are i n j e c t e d d i r e c t l y i n t o the renal a r t e r y and t h e i r e f f e c t s on renal blood f l o w are measured. P r i o r a d m i n i s t r a t i o n of an alpha-adrenergic b l o c k i n g agent i s necessary s i n c e DA and most DA agonists cause v a s o c o n s t r i c t i o n by a c t i o n on alpha-adrenergic r e c e p t o r s . Phenoxybenzamine does not s i g n i f i c a n t l y decrease the renal v a s o d i l a t i o n produced by DA (j>) as has been suggested by the i n h i b i t o r y a c t i o n of phenoxybenzamine i n a DA binding assay. Table I l i s t s agonists which have been found to be a c t i v e i n the renal vascular bed of the anesthetized dog. A few i n a c t i v e compounds are included f o r comparison. Each agonist was antagonized s e l e c t i v e l y by a DA antagonist i n a dose which does not a f f e c t v a s o d i l a t i o n produced by bradykinin or i s o p r o t e r e n o l . Compounds shown i n Table I were s t u d i e d i n our l a b o r a t o r y under comparable c o n d i t i o n s (2,4,5,7-10).

In Dopamine Receptors; Kaiser, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Table I


S t r u c t u r a l requirements f o r DA

1

activity

S u b s t i t u t i o n on n i t r o g e n Series

R

3,4-Dihydroxyphenyl ethyl amines

R

l

H

H

H

CH

C

C

C

C

C

C

2

Approximate relative potency

1 1

3

3 7 (n) 3 7 (n) H

C H (n)

0.03

H

C H (n)

0.04

3 7 (n) 3 7 (n) H

C H

0.04

H

phenethyl

0.007

3 7 (n) 4 9 (n) H

C H 2

5

0.007

H

C H

5

0.04

3 7 (n) 3 7 (n) H

C H (n)

inactive

H

C H (n)

inactive

(n)

C H (n)

i nacti ve

H

inactive

3

7

4

9

5

2

n

(n)

2-Aminotetralins no hydroxy

C

5-hydroxy

C

7-hydroxy

C

5,6-dihydroxy

H

3 7 H

7

3

7

3

7

C H (n)

1

H

H

1

H

CH

C H 3

6,7-dihydroxy

3

7

(n)

3 7 (n) C H (n) C

H

3

7

3

7

3

C H (n)

0.25

C H (n)

0.25

3

4

7

g

Continued next page


DOPAMINE


104

RECEPTORS

Table I continued S t r u c t u r a l requirements f o r DA

1

activity

S u b s t i t u t i o n on nitrogen Series

R

l

Approximate relative potency 1

l-Aminomethyl-5,6dihydroxy isochromans

H

H

H

CH

Aporphines

apomorphine

0.01 p a r t i a l agonist

isoapomorphine

inactive

6-n-propyl norapomorphine

0.02

3

3',4'-dihydroxynomifensine

Benzazepines

a

1

0.01 p a r t i a l agonist SK&F 38393

a

0.01 p a r t i a l agonist

SK&F 82526

b

9

2,3,4,5-tetrahydro-7,8-dihydroxy-l-phenyl-lH-3-benzazepine ,

6-chloro-4'-hydroxy analog o f SK&F 38393



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The presence of two OH groups separated from the nitrogen atom by a distance of 7-8 angstrom u n i t s i s a common f e a t u r e of a l l a c t i v e compounds. We had hypothesized p r e v i o u s l y t h a t the beta rotameric form of DA, as t y p i f i e d by A-6,7-DTN, was the p r e f e r r e d conformation. However, we r e c e n t l y reported t h a t d i p r o p y l A-5,6-DTN was a potent DAi agonist [10] and, thus, the rotameric conformation may be of secondary importance. The R enantiomer of A-6,7-DTN i s the a c t i v e form (11). A l l of the agonists l i s t e d i n Table I act on other receptors i n a d d i t i o n to the DA receptor. Important exceptions are the benzazepine d e r i v a t i v e s , SK&F 38393 (12,13), and SK&F 82526 which act s e l e c t i v e l y on DA, receptors (14,15.). f° i s the more a c t i v e enantiomer (16). 1

T h e

ΡΑ}

R

r m

antagonists

DA, antagonists are studied i n the same phenoxybenzaminep r e t r e a t e d dog preparation as DA, a g o n i s t s . We compare r e l a t i v e potencies of DA, antagonists by making simultaneous i n t r a - a r t e r i a l i n j e c t i o n s of DÂ with the antagonist under study. When administered i n t r a - a r t e r i a l l y i n t h i s way, most antagonists have a very short duration of a c t i o n and many compounds can be studied i n one experiment. The i n t r a - a r t e r i a l route of a d m i n i s t r a t i o n a l s o permits estimation of responses p r i o r to r e c i r c u l a t i o n (2,4,17). In a d d i t i o n , we c a l c u l a t e the r e l a t i v e s p e c i f i c i t y of the antagonists by determining the minimum dose r e q u i r e d to attenuate b r a d y k i n i n or i s o p r o t e r e n o l d i v i d e d by the minimum dose required to attenuate DA. As noted i n Table I I , s u l p i r i d e i s the most potent antagonist and also has a large range of s p e c i f i c i t y . In c o n t r a s t , h a l o p e r i d o l and the phenothiazines are much l e s s potent and e x h i b i t a very narrow range of s p e c i f i c i t y . Because of the wide range of s p e c i f i c i t y of s u l p i r i d e and other s u b s t i t u t e d benzamides, i t i s p o s s i b l e to administer these drugs i n t r a v e n o u s l y and d i f f e r e n t i a t e renal or mesenteric vasodil a t i o n produced by DA from n o n - s p e c i f i c v a s o d i l a t i n g agents such as bradykinin (18,19). However, we disagree with Shepperson et a l ^20) who estimated the r e l a t i v e potency of l e s s s p e c i f i c antagonists by the intravenous route. DAQ agonists Several techniques have been employed to assess the e f f e c t s of DA« agonists on the p o s t g a n g l i o n i c sympathetic nerve. For r o u t i n e screening of DA agonists we i n j e c t the compound i n t o the femoral a r t e r y of tne anesthetized dog and compare the r e s u l t a n t v a s o d i l a t i o n with that produced by the standard agonist, d i p r o p y l dopamine (DPDA) (21). Buylaert and h i s a s s o c i a t e s use apomorphine as the standard agonist. The v a s o d i l a t i o n r e s u l t i n g from DPDA and apomorphine i s known to be due to i n h i b i t i o n 2


106

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RECEPTORS

Table I I


DAi receptor antagonists with r e l a t i v e potencies ranges o f s p e c i f i c i t y

Class Butyrophenones

Phenothiazines

Antagonist

Relative potency (moles)

Haloperidol

1.4

X

ΙΟ"

7

10

6

1(T

8

>50

ΙΟ"

7

>50

7

>50

10" 7

Sultopride

2.9

X

io-

Bulbocapnine

4.7

X

10"

8

>50

^8

*Minimal dose a t t e n u a t i n g v a s o d i l a t i n g responses to bradykinin or i s o p r o t e r e n o l d i v i d e d by minimal dose a t t e n u a t i n g v a s o d i l a t i n g responses to dopamine.


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of sympathetic nervous system a c t i v i t y since i t i s e l i m i n a t e d by c u t t i n g the sympathetic nerves or by a d m i n i s t e r i n g a gangl i o n i c blocking agent (22J. The v a s o d i l a t i o n i s s p e c i f i c a l l y antagonized by DA antagonists i n doses which do not attenuate v a s o d i l a t i o n produced by i s o p r o t e r e n o l or b r a d y k i n i n . The advantage of u t i l i z i n g femoral a r t e r y v a s o d i l a t i o n in the i n t a c t dog as an endpoint i s that the r e l a t i v e a c t i v i t y of compounds a c t i v e on DAi and DA receptors can be assessed under s i m i l a r c o n d i t i o n s i n the anesthetized dog. The disadvantage of t h i s technique i s that compounds with potent alpha-adrene r g i c v a s o c o n s t r i c t o r a c t i v i t y or n o n - s p e c i f i c vascular a c t i v i t y cannot be s t u d i e d . However, compounds with alpha-adrenergic a c t i v i t y can be compared with standard agonists i n the canine preparation of Long and a s s o c i a t e s (23_). In t h i s method the p o s t g a n g l i o n i c sympathetic nerves are e l e c t r i c a l l y stimulated and the reduction i n t a c h y c a r d i a induced by e l e c t r i c a l s t i m u l a t i o n of the nerve i s taken as a measure of DA a c t i v i t y . Use of s p e c i f i c DA antagonists i s e s s e n t i a l i n t h i s technique to separate the e f f e c t s of DA from alpha^-adrenergic agonists. In c o n t r a s t to tne r e l a t i v e l y short l i s t of compounds a c t i v e as DA, agonists (Table I ) , a l a r g e number of compounds have been reported to be a c t i v e on DA receptors (2,11,24,25). Table I I I presents a s e l e c t i v e l i s t of these compounds. In a d d i t i o n to the demonstration that more compounds are a c t i v e on the DA r e c e p t o r s , pronounced d i f f e r e n c e s i n potency s e r i e s have been demonstrated i n agonists which are a c t i v e on both r e c e p t o r s . For example, apomorphine i s a weak p a r t i a l agonist of DAi recept o r s , but i s a potent f u l l agonist of DA receptors (2 2z). Furthermore, DPDA, propyl b u t y l DA, propylpentyl DA, p r o p y l i s o b u t y l DA, propylphenethyl DA, and p r o p y l e t h y l DA e x h i b i t d i f f e r e n t potencies as DAi a g o n i s t s , and a l l of them are much l e s s potent than DA as DAi agonists. However, a l l these compounds are equipotent and strong agonists of the DA receptor (21). As with most DA, a g o n i s t s , the m a j o r i t y of DA agonists are a c t i v e on other r e c e p t o r s . In p a r t i c u l a r , many ergot d e r i v a t i v e s e x h i b i t alpha-adrenergic and serotonin a c t i v i t y (26.). In c o n t r a s t , Bach et a l (27.) reported that the pyrazole d e r i v a t i v e (LY 141865) i s devoid of alpha and serotonin agonist and antagonist e f f e c t s . P r e l i m i n a r y s t u d i e s i n our l a b o r a t o r i e s demonstrated t h a t LY 141865 i s a potent DA agonist without alpha-adrenergic or DAj a c t i v i t y . 2


2

2

2

2

2

2

2

9

2

2

2

DAQ

antagonists

A l l compounds studied thus f a r as DAi antagonists also antagonize DA r e c e p t o r s . We r e c e n t l y reported that domperidone i s an e x t r a o r d i n a r i l y s e l e c t i v e DA receptor antagonist (28). In a range of 0.5-5 jjg/kg i t producesdose r e l a t e d antagonism of DPDA induced femoral v a s o d i l a t i o n without a f f e c t i n g the vasodil a t i o n produced by i s o p r o t e r e n o l or b r a d y k i n i n . In a dose of 2

2


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DOPAMINE

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Table I I I Selected l i s t o f DA

2

agonists with r e l a t i v e DAj a c t i v i t y


Relative

activity

Series

Compound

DAo

DA,

Dopamine (DA)

dimethyl DA

active

inactive

dipropyl DA propyl butyl DA propylphenethyl DA propyl ethyl DA

all equipotent and active

weak and graded*

Aporphine

apomorphine

potent f u l l agonist

weak p a r t i a l agonist

6,7-Dihydroxy-2aminotetralin (ADTN)

dimethyl ADTN

active

inactive

primary amine

active

inactive

N-N-dipropyl derivative

active

active

bromocriptine

active

inactive

pergolide

active

inactive

lisuride

active

inactive

lergotrile

active

inactive

active

inactive

5,6-Dihydroxy-2aminotetralin

Ergot derivatives

Piribedil

*For r e l a t i v e a c t i v i t i e s , see Table I .


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5 mg/kg i n t r a v e n o u s l y , domperidone does not attenuate DAi-induced renal v a s o d i l a t i o n . Domperidone i s i n e f f e c t i v e as an antagonist of alpha^-adrenergic receptors as determined by lack of antagonism of bradycardic e f f e c t s of c l o n i d i n e i n the canine c a r d i a c postg a n g l i o n i c nerve p r e p a r a t i o n . Q u a l i t a t i v e d i f f e r e n c e s have a l s o been observed i n r e l a t i v e DA, and DA a c t i v i t y of several antagonists. The d i f f e r e n c e s are s t r i k i n g l y demonstrated with the enantiomers of s u l p i r i d e . ( S ) - s u l p i r i d e i s much more a c t i v e than ( R ) - s u l p i r i d e as a DA antagonist; whereas, ( R ) - s u l p i r i d e i s s l i g h t l y more a c t i v e tnan ( S ) - s u l p i r i d e as a DAj antagonist (11). 2


2

Spectrum of receptor a c t i v i t y of DA agonists The determination o f the r e l a t i v e a c t i v i t y of DA agonists on DA and other receptors i s of c r i t i c a l importance i n the e l u c i dation of the s t r u c t u r a l requirements f o r synthesis of s e l e c t i v e DA, and DA a g o n i s t s . For example, i t should be p o s s i b l e t o compare compounds with mixed receptor a c t i v i t y with the s e l e c t i v e DA agonist SK&F 82526 and the r e l a t i v e l y s e l e c t i v e DAo a g o n i s t , LY 141865 t o e l u c i d a t e the chemical requirements f o r tne d i f f e r e n t i a l receptor a c t i v i t y (Figure 1). Determination of r e l a t i v e receptor a c t i v i t y i s a l s o important f o r developing new DA agonists f o r c a r d i o v a s c u l a r and renal therapy. F o r t u n a t e l y , the r e s u l t s obtained i n the anesthetized dog are r e l e v a n t f o r human s t u d i e s . Indeed, the c l i n i c a l s t u d i e s of DA (4) and p r o p y l b u t y l DA (29) were d i r e c t extensions of canine i n v e s t i g a t i o n s . We compare the r e l a t i v e a c t i v i t y of DAi agonists on the f o l l o w i n g r e c e p t o r s : beta, (both d i r e c t l y and i n d i r e c t l y by r e l e a s e of norepinephrine from myocardial storage s i t e s ) , beta^, alpha, and DA^. Table IV describes the spectrum of a c t i v i t y of several compounds studied i n the p e n t o b a r b i t a l anesthetized dog. The methods used have been described i n d e t a i l (21,30). 2

1

Comparison of data obtained i n v i v o and i n v i t r o A comprehensive reivew of studies of the a c t i o n of DA, agonists and antagonists i n i s o l a t e d blood vessels and organs was r e c e n t l y published (_31). ^ g e n e r a l , there i s good c o r r e l a t i o n between i n v i v o and i n v i t r o data with i s o l a t e d canine blood v e s s e l s . However, exceptions have been reported with i s o l a t e d preparations of other species. The major d i f f e r e n c e s are that s u l p i r i d e and i t s enantiomers appear t o be much more potent i n v i v o than i n v i t r o . Another discrepancy i s that bromoc r i p t i n e , which i s i n a c t i v e on DAi receptors i n v i v o , has been shown t o cause DA-like v a s o d i l a t i o n of the i s o l a t e d perfused r a t kidney and r e l a x a t i o n of r a b b i t mesenteric a r t e r y s t r i p s . These data suggest the p o s s i b i l i t y t h a t both DAi and DA receptors may occur on vascular t i s s u e i n some species. However, other n

2


110

DOPAMINE RECEPTORS


CI

OH SK 8 F 8 2 5 2 6 Figure 1.

L Y 141865

The DA agonist—SK&F

82526—and the DA agonist—LY

t

2

141865.

Table IV Spectrum o f c a r d i o v a s c u l a r receptor a c t i v i t y o f s e l e c t e d DA agonists

β ^Indirect*

α

DAj

DA

Dopamine (DA)

++

50%

0

++

+++

++

Epinine

+++

25%

+

+++

+++

++

A-6,7-DTN

++

100%

0

++++

+++

++

0

0

++

2

A-5,6-DTN

0

0

+++

Di propyl A-5,6-DTN

0

0

0

++++

+++

+++

Dipropyl DA

0

0

0

+

++

++

Propylphenethyl DA

0

0

0

+

+

++

SK&F 38393

0

0

0

0

++++

0

SK&F 82526

0

0

0

0

++++

0

* 0 j a c t i v i t y caused by release o f norepinephrine storage s i t e s .

from

myocardial


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p o s s i b l e mechanisms w i l l have t o be r u l e d out. The major problem i n comparing DAp receptors studied by the d i f f e r e n t techniques i s that alpha -adrenergic a c t i v i t y , which produces s i m i l a r i n h i b i t i o n of norepinephrine r e l e a s e as a c t i v a t i o n of DA r e c e p t o r s , has not been c l e a r l y r u l e d out i n many of the s t u d i e s . 2

2


Determinations o f the DA receptor subtypes r e s p o n s i b l e f o r p h y s i o l o g i c a l and pharmacological a c t i o n s In 1978 we compared the s t r u c t u r e a c t i v i t y requirements f o r the DA, (vascular DA) receptor with other p u t a t i v e DA recept o r s (1,4). The l i m i t e d SAR data a v a i l a b l e at that time supported the existence of at l e a s t two d i f f e r e n t DA r e c e p t o r s . DA-induced responses were d i v i d e d i n t o two categories on the basis of d i f f e r ences i n agonist a c t i v i t y and potency s e r i e s . The f o l l o w i n g phenomena appeared t o be due t o a c t i v a t i o n of DA r e c e p t o r s : i n h i b i t i o n of norepinephrine r e l e a s e from the p o s t g a n g l i o n i c sympathetic nerve; i n h i b i t i o n of g a n g l i o n i c t r a n s m i s s i o n ; i n h i b i t i o n o f p r o l a c t i n r e l e a s e ; and emesis. Review o f the current l i t e r a t u r e has not revealed exceptions t o these c o r r e l a t i o n s . In c o n t r a s t , only v a s o d i l a t i o n has been c l e a r l y r e l a t e d t o DA. r e c e p t o r s . I n t e r e s t i n g l y , the potency s e r i e s of agonists f o r s t i m u l a t i o n of adenylate c y c l a s e and renal v a s o d i l a t i o n i s s i m i l a r (2!). Furthermore, SK&F 82526 i s a c t i v e i n both models (15). However, no c o r r e l a t i o n was found i n the r e l a t i v e actions of antagonists ( 8 ) . F i r s t , s u l p i r i d e , which i s a potent antagon i s t of DA-induced renal v a s o d i l a t i o n , i s i n a c t i v e as an antagon i s t of DA-induced s t i m u l a t i o n of adenylate c y c l a s e (32). Second, ergot d e r i v a t i v e s , which are i n a c t i v e as DA^ a g o n i s t s , are antagon i s t s of DA-induced s t i m u l a t i o n of adenylate c y c l a s e (33). These exceptions prevent us from using the Di and Do s u b d i v i s i o n of Kebabian and Calne (34). We have been unable t o r e l a t e the DAi and DA subtypes to s u b d i v i s i o n s proposed by r a d i o l i g a n d binding assays. This i s an extremely c o n t r o v e r s i a l area: one t o four d i f f e r e n t recept o r subtypes have been postulated by binding assays. The problem with binding assays appears t o be r e l a t e d i n part t o lack of s e l e c t i v i t y of the ligands used. For example, spiperone has been shown t o bind t o serotonin and n o n - s p e c i f i c binding s i t e s (36). In c o n t r a s t , when the s e l e c t i v e DA antagonist, domperidone, i s used as the l i g a n d , a better r e l a t i o n s h i p with DA receptors can be demonstrated (36). A question t o be answered by f u t u r e research i s whether use o f s e l e c t i v e DAi ligands w i l l reveal a potency s e r i e s of agonists s i m i l a r t o the DAi subtype. Both DA, and DA~ agonists cause behavioral changes ana thus demonstration of DAi receptors i n c e n t r a l nervous system binding assays should be p o s s i b l e i f t h i s method i s v a l i d (15,16,37). 2

2

2

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F i n a l l y , behavioral models cannot be used to e s t a b l i s h q u a n t i t a t i v e potency s e r i e s of DA agonists and antagonists because of d i f f e r e n c e s i n blood-brain b a r r i e r p e n e t r a t i o n , r e g i o n a l d i s t r i b u t i o n , and spectra of receptor and non-receptor actions (38). H o p e f u l l y , better c o r r e l a t i o n of p e r i p h e r a l and c e n t r a l receptors w i l l occur as behavioral models improve and more s e l e c t i v e agonists and antagonists become a v a i l a b l e .


Acknowledgments We wish to g r a t e f u l l y acknowledge the cooperation of many chemists who provided us with compounds f o r study. We would a l s o l i k e to thank Ms Dana Glock f o r t e c h n i c a l a s s i s t a n c e and Ms P a t r i c i a Gomben f o r s e c r e t a r i a l a s s i s t a n c e . This work was suppoted by NIH grant GM-22220. Literature

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

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Goldberg, L . I . ; Kohli, J.D. Commun. Psychopharmacol. 1979, 3, 447-56. Goldberg, L . I . ; Volkman, P.H.; K o h l i , J.D. Ann. Rev. Pharmacol. T o x i c o l . 1978, 18, 57-79. McNay, J.L.; Goldberg, L . I . J . Pharmacol. Exp. Ther. 1966, 151, 23-31. Goldberg, L . I . Pharmacol. Rev. 1972, 24, 1-29. Goldberg, L . I . ; S o n n e v i l l e , P.F.; McNay, J.L. J . Pharmacol. Exp. Ther. 1968, 163, 188-97. Creese, I . ; S i b l e y , D.R. Biochem. Pharmacol. 1982, 31, 2568-9. Dolak, T.M.; Goldberg, L . I . Ann. Reports Medicinal Chemistry 1981, Chpt. 11, 103-11. Goldberg, L . I . ; Kohli, J.D. "Advances i n the B i o s c i e n c e s " ; Pergamon Press, New York, 1982; p. 41-9. K o h l i , J.D.; Goldberg, L . I . J. Pharmacy Pharmacol. 1980, 32, 225-6. K o h l i , J.D.; Goldberg, L . I . ; McDermed, J.D. Eur. J . Pharmacol. 1982, 81, 293-9. Goldberg, L . I . ; Kohli, J.D.; L i s t i n s k y , J . J . ; McDermed, J.D. "Catecholamines: Basic and Clinical F r o n t i e r s " ; Pergamon Press, New York, 1979; p. 447-9. Pendleton, R.G.; Samler, L.; K a i s e r , C.; R i d l e y , P.R. Eur. J . Pharmacol. 1978, 51, 19-28. Hahn, R.A.; W a r d e l l , J.R., J r . J . Cardiovasc. Pharmacol. 1980, 2, 583-93. Weinstock, J . ; Wilson, J.W.; Ladd, D.L.; Brush, C.K.; Pfeiffer F.R.; Kuo, G.Y.; Holden, K.G.; Yim, N.C.F.; Hahn, R.A.; W a r d e l l , J.R., Jr.; Tobia, A.J.; S e t l e r , P.E.; Sarau, H.M.; R i d l e y , P.T. J . Med. Chem. 1980, 23, 973-5.


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Hahn, R.A.; W a r d e l l , J.R., Jr.; Sarau, H.M.; R i d l e y , P.T. J . Pharmacol. Exper. Therap. 1982 (in p r e s s ) . K a i s e r , C.; Dandridge, P.Α.; Garvey, E.; Hahn, R.A.; Sarau, H.M.; S e t l e r , P.E.; Bass, L.S.; C l a r d y , J . J . Med. Chem. 1982, 25, 697-703. K o h l i , J.D.; Glock, D.; Goldberg, L . I . "The Benzamides: Pharmacology, Neurobiology, and Clinical Aspects"; Raven P r e s s , New York, 1982; p. 97. Glock, D.; Goldberg, L . I . ; Kohli, J.D. Fed. Proc. 1981, 40, 290 #318. Hahn, R.A.; W a r d e l l , J.R., J r . Arch. Pharmacol. 1980, 314, 177-182. Shepperson, N.B.; Duval, N.; Massingham, R; Langer, S.Ζ. J . Pharmacol. Exp. Ther. 1982, 221, 753-61. K o h l i , J.D.; Weder, A.B.; Goldberg, L . I . ; Ginos, J.Z. J . Pharmacol. Exp. Ther. 1980, 213, 370-4. B u y l a e r t , W.A.; Willems, J.L.; Bogaert, M.G. J . Pharm. Pharmacol. 1978, 30, 113-5. Long, J.P.; H e i n t z , S.; Cannon, J.G.; Kim, J . J . Pharmacol. Exp. Ther. 1975, 192, 336-42. Lokhandwala, M.F.; T a d e p a l l i , A.S.; Jandhyala, B.S. J. Pharmacol. Exp. Ther. 1979, 211, 620-5. B a r r e t t , R.J.; Lokhandwala, M.F. Eur. J . Pharmacol. 1982, 77, 79-83. Berde, B.; Schild, H.O. "Ergot A l k a l o i d s and Related Com pounds"; S p r i n g e r - V e r l a g , New York, 1978. Bach, N.J.; K o r n f e l d , E.C.; Jones, N.D.; Chaney, M.O.; Dorman, D.E.; Paschal, J.W.; Clemens, J.A.; S m a l s t i g , E.B. J . Med. Chem. 1980, 23, 481-91. Glock, D.; Kohli, J.D.; Goldberg, L . I . Fed. Proc. 1982, 41, 1651 #8077. F e n n e l l , W.; T a y l o r , Α.; Brandon, T.; Goldberg, L.; Ginos, J.; Mitchell, J . ; Miller, R. C l i n . Res. 1980, 28, 469A. Meyer, M.B.; Goldberg, L . I . C a r d i o l o g i a 1966, 49, 1-10. Brodde, O.E. L i f e S c i . 1982, 31, 289-306. Spano, P.F.; S t e f a n i n i , E.; Trabucchi, M.; F r e s i a , P. " S u l p i r i d e and other Benzamides"; I t a l i a n Brain Research Foundation Pres, M i l a n , Italy, 1979; pp. 11-31. Kebabian, J.W.; Calne, D.B.; Kebabian, P.R. Commun. Psychopharmacol. 1977, 1, 311-8. Kebabian, J.W.; Calne, D.B. Nature 1979, 277, 93-6. Seeman, P. Biochem. Pharmacol. 1982, 31, 2563-8. Lazareno, S.; Nahorski, S.R. Eur.J.Pharmacol. 1982, 81, 273-85. C o s t a l l , B.; Naylor, R.J.

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R E C E I V E D February 8, 1983



Commentary: Utility and Problems in the Classification of Dopamine Receptors B A R R Y A. BERKOWITZ Smith Kline & French Laboratories, Department of Pharmacology, Research & Development Division, Philadelphia, P A 19101

Dopamine and dopamine receptors have emerged as i n t r i g u i n g and useful t a r g e t s i t e s f o r chemical, b i o l o g i c a l , and therapeutic endeavors. T h i s , when coupled with advances i n p h y s i o l o g i c a l and receptor technologies, has l e d to a number o f viewpoints on the classification o f dopamine receptors. The b r a i n and c a r d i o v a s c u l a r / r e n a l systems have been the areas where the most work has been accomplished. Dopamine receptor s t u d i e s i n the c e n t r a l nervous system have been p r i m a r i l y b i o -chemical, whereas those i n the periphery have been primarily physiological. In t h e i r review Goldberg and Kohli have well summarized the present state of the a r t f o r p h y s i o l o g i c and pharmacologic a n a l y s i s of peripheral dopamine receptors and provide a reasonable classification designated DA1 and D A 2 to i d e n t i f y the two major receptor subtypes. This commentary addresses the utility and problems of dopamine receptor classification.

The studies of Goldberg and colleagues have been and remain pioneering i n not one but at l e a s t two f r o n t s . F i r s t , the concepts and demonstration of vascular dopamine receptors has allowed and stimulated d e t a i l e d studies o f the l o c a t i o n , funct i o n , mechanism, and r o l e of peripheral dopamine and dopamine receptors. Second, t h e i r work l e d to and a c c e l e r a t e d the u t i l i z a t i o n o f dopamine agonists i n c l i n i c a l medicine with the use of intravenously administered dopamine f o r shock and heart f a i l u r e being the best example. In t h e i r communication the evidence i s summarized suggesting two d i s t i n c t types of dopamine receptors i n the p e r i p h e r y , designated DA] and D A 2 . Goldberg and K o h l i ' s c l a s s i f i c a t i o n i s based p r i m a r i l y on i n vivo r e s u l t s using the vasculature and flow of blood to the r e n a l , femoral and other s e l e c t e d 0097-6156/83/0224-0114$06.00/0 © 1983 American Chemical Society


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p e r i p h e r a l v a s c u l a r beds. Most o f the other c l a s s i f i c a t i o n s f o r dopamine receptors have been based on r e s u l t s obtained with the b r a i n and cannot be assumed t o apply t o the vasculature and periphery. I n t e r e s t i n g l y , i t i s u s u a l l y not the o r i g i n a t o r s o f any s p e c i f i c c l a s s i f i c a t i o n system who misuse o r misapply i t but f r e q u e n t l y others who t r y t o apply i t t o d i f f e r e n t areas and f i n d exceptions.


Why c h a r a c t e r i z e dopamine receptors? There are a number o f reasons t o t r y t o c h a r a c t e r i z e dopamine receptor i n a d d i t i o n t o the f a c t t h a t i t i s f a s h i o n a b l e . Two o f the major reasons w e l l described by Goldberg and K o h l i are (1) the importance o f dopamine receptors subtypes r e s p o n s i b l e f o r p h y s i o l o g i c a l and pharmacological a c t i o n s and (2) the u t i l i z a t i o n o f the concept o f s e l e c t i v e dopamine r e ceptors i n the design and c h a r a c t e r i z a t i o n o f new drugs f o r c a r d i o v a s c u l a r and renal t h e r a p i e s . Four Major Reasons f o r Problems i n the C l a s s i f i c a t i o n and I d e n t i f i c a t i o n o f Dopamine Receptors i n the Periphery Heterogeniety and s t r u c t u r e o f the v a s c u l a t u r e . The multitude o f f u n c t i o n s o f the v a s c u l a t u r e , i n c l u d i n g conduit r e s i s t a n c e and f i l t r a t i o n v e s s e l s , may u n f o r t u n a t e l y be sub served by an e q u a l l y complex heterogeniety o f receptors and receptor l o c a t i o n s . This obviously i n c l u d e s dopamine r e c e p t o r s . Unfortunately (or f o r t u n a t e l y , depending on one's p o i n t o f view) the v a s c u l a t u r e i s not "mushy" l i k e the b r a i n . The c o l lagen and connective t i s s u e content o f blood v e s s e l s does not a l l o w the gentle homogenization and t i s s u e d i s r u p t i o n which has allowed extensive biochemical a n a l y s i s and binding s t u d i e s o f the b r a i n dopamine r e c e p t o r ( s ) . Thus, there i s a huge v o i d i n the biochemical analyses o f the p e r i p h e r a l dopamine r e c e p t o r ( s ) and mechanisms. There i s c l e a r l y much t o be done i n t h i s area f o r the f u t u r e . Dopamine has m u l t i p l e a c t i o n s on the c a r d i o v a s c u l a r system. Unfortunately dopamine i s not a s e l e c t i v e drug f o r dopamine r e c e p t o r s . Whereas we may s t r i v e t o define a c t i o n s o f drugs o r neurotransmitters as dopaminergic, these end p o i n t s have been f r e q u e n t l y defined under l e s s than i d e a l c o n d i t i o n s . Dopamine a l s o has prominent e f f e c t s on a - andfc-adrenor e c e p t o r s . Thus, i n order t o define any type o f dopamine receptor o r subtype, most i n v i v o o r i n v i t r o s t u d i e s o f the periphery and c a r d i o v a s c u l a r system must u t i l i z e a v e r i t a b l e pharmacopeia o f drugs t o block other receptors o r mediator. In v i v o , s t u d i e s with dopamine agonists must g e n e r a l l y be performed using phenoxybenzamine o r other α-adrenoreceptor b l o c k i n g agents. In v i t r o , not only must α - b l o c k i n g drugs be


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used, but f r e q u e n t l y , 3 - r e c e p t o r s b l o c k e r as well as c y c l o oxygenase i n h i b i t o r s . While t h i s approach has been necessary,and i s by no means unique f o r dopaminergic pharmacology, i t has y i e l d e d data which w i l l r e q u i r e confirmation once we obtain more s e l e c t i v e drugs. The f a c t that most i n v i v o p r o t o c o l s d i f f e r s u b s t a n t i a l l y i n t h e i r use of drugs from i n v i t r o p r o t o c o l s may well e x p l a i n many of the c o n f l i c t s i n comparing dopaminergic a c t i o n s i n vivo and i n v i t r o . Lack of s e l e c t i v e drugs a c t i v e a t dopamine r e c e p t o r s . O b v i o u s l y , there i s a need f o r s e l e c t i v e agonist and antagonists of dopamine r e c e p t o r s . Almost by d e f i n i t i o n one cannot accur a t e l y describe a receptor without a s e l e c t i v e agonist or a n t a gonist. T h i s has been the d i f f i c u l t case we face with dopamine receptor s c i e n c e . Lack of In V i t r o Model System. Whereas there are a v a r i e t y of i n v i t r o receptor models f o r a - and 3-adrenoreceptors there has been l e s s progress on well accepted models f o r dopamine receptors. T i s s u e c u l t u r e may well be used i n c r e a s i n g l y i n the future as a source of receptor m a t e r i a l . In a d d i t i o n , there needs to be continued work on the use of i s o l a t e d v a s c u l a r t i s sue to probe dopamine r e c e p t o r s . Conclusion The tendency to examine one's data with a c a l c u l a t o r i n one hand and a Greek d i c t i o n a r y i n the other with the i n v e s t i g a t o r poised to name a new receptor need not be o v e r l y encouraged. Where p o s s i b l e , receptor c l a s s i f i c a t i o n and concepts based upon b i o c h e m i c a l , p h y s i o l o g i c a l and pharmacological evidence would seem to serve us b e s t . Goldberg and Kohli have well summarized the present s t a t e of the a r t f o r the p h y s i o l o g i c and pharmac o l o g i c a n a l y s i s of p e r i p h e r a l dopamine r e c e p t o r s . They are to be commended f o r t h e i r care and c o n t r i b u t i o n i n d e f i n i n g dopamine r e c e p t o r s . T h e i r work stands as a reasonable and secure base f o r future s t u d i e s . R E C E I V E D January 7, 1983


Differentiation of Dopamine Receptors in the Periphery - American

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