RECENT ADVANCES IN OPIOID PHARMACOLOGY

89

RECENT ADVANCES Ir1 OPIOID

PHARMACOLOGY

A. L i v i n g s t o n D e p a r t m e n t of P h a r m a c o l o g y T h e h l e d i c a l S c h o o l , RRISTOL, RS8 1TD

I n some w a y s i t c a n be c o n s i d e r e d t h a t a l l r e s e a r c h o n o p i o i d r e c e p t o r s a n d e n d o g e n o u s o p i o i d s is r e c e n t , s i n c e i t

is j u s t o v e r a d e c a d e s i n c e t h e s t u d i e s o f 7 e r t a n d S n y d e r (1973)

and

Terenius

opioid

receptor

(1973)

existed

in

demonstrated nervous

a

that

tissue

and

specific

consquently

s t r e n g t h e n e d t h e e a r l i e r s u g g e s t i o n s oP n o t o n l y a s p e c i f i c

s i t e for m o r p h i n e - l i k e

receptor h o o d of

a n endogenous ligand

drues b u t also t h e l i k e l i -

for

thesc

receptors

in

brain

tissues. Opioid Receptors E a r l y s t u d i e s on t h e r e l a t i v e p o t e n c i e s o f o p i o i d s h a d s u g g e s t e d t h a t more C l a n o n e r e c e p t o r the e x p e r i m e n t s of

type night exist, but

l h r t i n e t a1 ( 1 9 7 6 ) f o r m e d t h e b a s i s o f

t h c c u r r e n t l y used c l a s s i f i c a t i o n .

T h e y f o u n d t h a t by u s i n g

a series of s u b s t i t i i t i o n tests on opioid-dependant

dogs t h e y

could c l a s s i f y t h e o p i o i d r e c e p t o r s i n t o t h r e e g r o u p s :

mu

recrptor

r?ceptor sigma

relating

relating

receptor

t o rnorhpine and a n a l g e s i a ;

to k e t o c y c l a z o c i n e a n d s e d a t i o n relating

to

SKF

10,047

the

t h e kappa ;and the

(n-allyl-nor-

n e t a z o c i n e ) and psychotomirnrtic e f f e c t s . This

clnssification

morphine-like

compounds

charaterises quite

the

satisfactorily.

majority

of

However,

90

a f t e r t h e d i s c o v e r y of t h e e n d o g e n o u s p e p t i d e s ,

it

lins,

was

receptor

t o be

found

the s m a l l p e p t i d e s

v i t y of

to propose

necessary

t h e d e l t a receptor, t o account

type,

t h e enkepha-

a

fourth

for t h e a c t i -

i n b i o a s s a y p r e p a r a t i o n s of t h e

mouse v a s d e f e r e n s ( d e l t a coming from " d e f e r e n s " )

(Lord et

a l , 1977). The c h a r a c t e r i s a t i o n of any r e c e p t o r type hinges on t h e

a speciPic ligand for that receptor.

a v a i l a b i l i t y of

such as morphine and fentanyl

Orugs

were P a i r l y s p e c i f i c f o r t h e

mu r e c e p t o r , w h i l s t compounds s u c h a s t h e e n d o g e n o u s p e p t i d e

or b e t t e r D - A l a

leu-enkephalin, fairly

specific

facilitated

et

(Chang

studies

of

al,

Nowever, on

lack of

by

ligands

sites

receptor Smith

and

and

provided

consequently in

the

Simon,

brain 1980).

a s p e c i f i c kappa ligand i n h i b i t e d t h e

Cillan

until

site

receptor

k a p p a a g o n i s t U50,488H. t h e u s e of

the

1979;

this

demonstrated

receptor

delta

mapping

P-Leu-enkephalin,

et

a1

its

(1983), u s i n g

existence the

was

specific

Receptor s t u d l e s also b e n e f i t

from

s p e c i f i c a n t a g o n i s t s a n d , w h i l s t n a l o x o n e c a n be

f a i r l y s p e c i f i c For

t h e mu r e c e p t o r and I C I

154139 f a i r l y

s p e c i f i c f o r t h e d e l t a receptor, t h e r e are no specilic a n t a gonists

for

Receptors siderable suggested

the for

the

attention that

they

in did

kappa

or

sigma

opioids

the not

last

sigma have

decade.

in

exist

sites.

Tiowever,

Gillan

and

received

Some

the brain,

1 0 , 0 4 7 b i n d s t o mu a n d k a p p a s i t e s a s a c l l a s sig'na

receptors.

workers

s i n c e SKF

ttic

Yosterlitz

con-

so c a l l e d

(1982)

and

S n y d e r and G o o d n a n ( 1 9 8 0 ) d e m o n s t r a t e d t.!lat s t o r p l i i n ? , w h i c h has

high

affinity

for mu,

delta

and

kappa

r:ccptors,

wn:;

t o d i s p l a c e a l l t h e b i n d i n g of S K F 1 0 , 0 4 7 a n d t h a t

unable

t h e r e m a i n d e r was d i s p l a c e a b l e b y c o m p o u n d s s u c h a s c y c l a z o -

c i n e w h i c h a r e p r o p o s e d t o h a v e sigma a c t i v i t y .

The p i c t u r e

h a s b e e n c o m p l i c a t e d by s t u d i e s I n v o l v i n g t h e p s y c h o t o m i n e -

tic

agent

phencyclidine

to

(similar

ketamine

and

tiletamine).

was n o t e d

It

that

the psychotic

behaviour

i n d u c e d by

p h e n c y c l i d i n e (DCP) a n d S K F 1 0 , 0 4 7 was v e r y s i m i l a r a n d t h a t the

tissue. PC?

sites

binding

Indeed,

and

sigma

had

similar

R

receptor

in

brain

(1979) s u g g e s t e d t h a t

Zukin and Zukin

opioid

distribution

be

might

same

the

the

thing.

‘ { o w e v e r , more r e c e n t s t u d i e s ( T a m , 1983) h a v e s h o w n t h a t t h e b i n d i n g a f f i n i t i e s of PCP a n d ZKF 1 0 , 0 4 7 t o b r a i n t i s s u e a r e not

exactly

the

sites

binding

in

brain

suggest

that

similar properties

!!ore

differrnt. ag?nts

may

be

recentor

acid

SpiPFel t!it’

heink:

mu

AI

1.t

rvceptor opioid

tisauc

recent

actirlc

as

for

rt,spir?tclry

These

have

suggested

an

the

that

been

have

existencr

receptor.

put of

forward.

an

epsilon

t’le e n d o g e n o u s

t h r h i g h potency of

(19384) h a v e p r o p o s e d

witb

and

tile d r u g

both

e x c i t a t o r y amino

aspartate

i n t o T U I ;tnd 1x12 s i t e s ,

clcpri.:;sion,

some

binclllig s i t e s a r e p r o b a b l y

types

;?nal::rasia

dif-

findings

on t h e r a t v a s d c f e r e n s b i o a s s a y ,

c o n c t ? r r i t ~ ~ l si t h

these

10,047 share

SXF

N-nettiyl

proposed

Pasternaic

rr?cczptor

and

studies

receptor

(1979)

Prndorpiiin

compounds.

PCP

the

of

distribution

i n d e p e n d e n t l y on

known

t o itccoiint

:and

whilst

thr

s h o w s some s i g n i f i c a n t

their actual

opioid

Ot!ier

:Vustc=r

that

f o r t h e two g r o u p s of

ferences woulrl

same, a n d

sub-division

with

th?

whilst of

t h e mu1 s i t e

mu2

ricptazinol

site

with

s h o w i n g it

92

relative

affinity

for

the

site.

mu1

Both

of

these

s u g g e s t i o n s have t h e i r s u p p o r t e r s and d e t r a c t o r s and at the moment t h e r e is i n s u f f i c i e n t e v i d e n c e t o r e g a r d t h e t h e o r i e s

a s proven. Endogenous opioids F o l l o w i n g t h e d e m o n s t r a t i o n of

a n i n t e n s i v e s e a r c h was u n d e r t a k e n t o l o o k

sites i n b r a i n , for

the

endogenous

ligands

Hughes and co-workers tides

in

brain

specific opioid binding

showed

for

these

receptors.

In

two p e n t a p e p -

t h e presence o f

T h e s e p e p t i d e s were named Flet a n d

tissue.

d e p e n d i n g o n w h e t h e r t h e r e was a m e t h i o n i n e

Leu-enkephalin,

or l e u c i n e r e s i d u e i n t h e f i f t h aminoacid p o s i t i o n . a f t e r w a r d s a much l e n g t h was

also

larger

isolated

peptide consisted of tide,

known

3s

tains

a

(Cox

Shortly

one m i n o acid This

l97G).

larger

an a l r e a d y d e s c r i b e d polypepfound

in

the

anterior

pendorphin.

to n o t e

sequence of

thirty

et al.

lipotrophin

is i n t e r e s t i n g

the

peptide of

p a r t of

p i t u i t a r y , a n d was named It

1975

that

amino a c i d s

a l s o con-

pendorphin found

in

!net-enkephalin

a n d t h i s l e d some sorlcers to s u S g e s t t h a t m P t - e n ! i r p h a l i n

w s

a

the

breakdown

case.

product

pendorphin,

of

Investigation

but

this

t h e b i n d i n g a c t i v i t y or

of

is

not

t’iesr p c p -

p rndorpliin

hail

s t r o n g mu b i n d i n C a f f i n i t y , m e t - P n k r p h a l i n

hail a f f i n i t y

lor

b o t h mu a n d d e l t a s i t e s ;ind l e u - e n ! c e p h a l i n

hnd , m a i n l y d e l t a

t i d e s to b r a i n o p i o i d

binding

activity.

sites describcd, the

three

A t

this

t h e mu,

endogenous

these sites,

r e c e p t o r s showed t 9 a t

stace

delta.

peptides

t:ic,r:?

w r e

four

rtacrtptor

4 a p p a ant1 FjiZfiia s i t e s , b u t

founti

nnly

t h e mu a n d t h e delta s i t e s .

n c t 4

on

two of

Cons(,:luc,ntly,

thr

93

i n v e s t i g a t i o n of

endogenous o p i o i d s continued and

t o t h e d e s c r i p t i o n o f X-neo-endorphin and

delta

activity

discovery

it

although

contained

enkephalin, ligand.

dynorphin

of

et a l ,

(Kaneawa

the

to

seemed

be

amino the

w h i c h a g a i n s h o w e d inu 1979) a n d

et

(Goldstein

al,

acid

a

finally

the

which,

1979)

sequence

elusive

Since these discoveries,

t h i s led

for

Leu-

endogenous

kappa

l a r g e n u m b e r of

other

p e p t i d e s h a v e b e e n d e s c r i b e d i n b r a i n t i s s u e , many o f w h i c h may i n t e r a c t w i t h o p i o i d r e c e p t o r s , b u t t o d a t e o n l y l i g a n d s

for

mu,

delta

and

kappa

sites

have

been

characterised

l e a v i n g t h e sigma b i n d i n g s i t e w i t h o u t a n e n d o g e n o u s l i g a n d . T h e s e q u e n c e h o m o l o g y of

these peptides

in

itself

led

T h e e s t i m a t i o n a n d l o c a l i s a t i o n of

t o a n u m b e r O P problems.

many p e p t i d e s h a d d e p e n d e d o n t h e u s e of a n t i b o d y t e c h n i q u e s s u c h a s r a d i o immun o a s s n y a n d immun o c y t o 1o g i c a 1 1oca 1i s a t i o n . Since both

t h e antibodies used,

i f

are raised

the antibodies

s h a r e s e q u e n c e s of

g e n s which

l i t y of

aminoacids

overlap i n antibody recognition

good e x a m p l e of

t h i s problem was w i t h

or

t h e radioimrnunoassay molecule

t h e s p e c i f i c i t y of

t h e s e t e c h n i q u e s depend upon

p lipotropin

of

c o n s i d c r a b l e cross

P

then

to anti-

the possibi-

is much g r e a t e r .

A

t h e carly s t u d i e s on

e n d o r p h i n ; s i n c e o n e t h i r d of

was r e p r e s e n t e d

reactivity existed.

hy

the

pendorphin

Since

a

Plipotropin

u a s o f t e n r e l e a s e d i n t o t h e c i r c u l a t i o n a t t h e same t i m e a s

p

endorphin,

endorphin

were

lipotropin.

in

of

the

fact

studies

measuring

measuring,

a t

circulating

least

in

M o s t a s s a y s a r e now d o n e i n p a r a l l r l

nntihody to t,ndorphin)

many

part,

p p

using one

P l i p o t r o p i n (which d o e s not cross react w i t h ( j

a n d one w h i c h r e a c t s t o b o t h a n d t h e n e s t i m a t i n g

94

the

i n t h e two r e s u l t s .

the d i f f e r e n c e

P e n d o r p h i n by

A

s i m i l a r p r o b l e m e x i s t e d f o r t h e e n k e p h a l i n s , s i n c e t h e y were very

s i m i l a r , a n d t h i s was c o m p o u n d e d b y t h e d i f f i c u l t y i n

e x t r a c t i n g t h e enkephalins due to rapid enzymatic hydroly-

sis.

e t a1 ( 1 9 8 0 )

T h i s p r o b l e m was s o l v e d b y C l e m e n t - J o n e s

A common b r e a k d o w n p r o d u c t o f met-

i n a v e r y neat m a n n e r .

e n k e p h a l i n is t h e s u l p h o x i d e w h i c h is much more s t a b l e . technique

invloved preparing

t h e problem of b r e a k d o w n

dised chemically during extraction,

T h i s a n t i b o d y t h u s showed a h i g h s p e c i f i c i t y for

is solved.

met-enkephalin With seen

t h u s excluding leu-

i n t h e t i s s u e is o x i -

Tf a l l t h e m e t - e n k e p h a l i n

enkephalin.

to t h e sulphoxide

a n antobody

is o n t h e m e t h i o n i n e c o m p o n e n t ) ,

(which

The

and

eliminated

hindsight,

to

exist

other

involved

problem

the

areas

where

apparent

of

breakdown.

problems

could

measurements

be

leu-

of

e n k e p h a l i n i n some t i s s u e s w h i c h were s u b s e q u e n t l y s h o w n t o c o n t a i n d y n o r p h i n w h i c h was i n f a c t c r o s s - r e a c t i n g

a n t i b o d y d u e t o i t s homology w i t h t h e smaller

leu-enkephalin peptide.

Yowever,

sequenced

these

as

more

are

problems

the

difficulty

Fortunately, becoming

the

less

likely

with

t h e precursor m o l t ~ c u l e sof identieied luatr3.d.

and

sequenced

more

-

identified

easily

unknown unknown

of

the

r e c o ~ n i s e d 2nd

peptides

opioid

isolation

and

thrsc pcptidrs. and

and

l e u e n k e p i i a l i n problea h i g h -

when

cxistence

:ire

pepttdes

d e a l t w i t h , b u t thc. d y n o r p h i n lights

with the

their

par-nt

exist.

peptifirs

is

sequencing of

Thr-v ?NA

i i a v ? bccri code

~ v n -

T h e s e t’lree p r e c u r s o r s a r e l o r t l l e i ~ n d o r p h i t l s , t h t ,

cnlcephalins

:ind

amino a c i d s a n d

the

dynorpliins.

Facii

contains

:ih>ut ?50

thc possible c l t > n v a l : c s i t e s Iiav*. b ~ c nirir.n-

95

tified, allowing the prediction of other potentially active peptide

It is interesting to note that the

sequences.

p lipotropin,

endorphin precursor is also the precursor f o r adrenocorticotrophic

hormone,

and

melanocyte

stimulating

hormone, whilst the enkephalin presursor contains both met and leu enkephalin sequences in the ratio of six to one as well as several other peptides.

The dynorphin precursor

also contains the sequence for leu-enkephalin and endorphin

amongst

others

The technique of

(Weber

et

neo-

al,

1983).

immunocytochemistry has allowed the

investigation of the localisation of the endogenous opioid peptides, their

which

has provided

function.

strongly

For instance,

localised

pituitary,

some

in

as well

the

as certain

strong

indications of

p endorphin

hypothalmus peripheral

seems and

to be

anterior

tissues.

The

enklephalins have been demonstrated i n brain areas such as the periaquaductal grey matter, the spinal cord, the locus coeruleus and the pertventricular areas of the midbrain, all of which are associated by electrophysioloeical techniques

Thus, it would appear that the enkepha-

with pain pathways.

lins are much more likely to be involved with the modulafion of pain perception than the endorphins.

The dynorphins have

a different distribution to the enkephalins and endorphins, being found i n certain hypothalamic and amygdaloid nuclei and in different regions to met enkephalin in the spinal cord.

Enkephlins

and

dynorphins

are

also

found

in

peripheral tissues such as the adrenal medulla and gut wall. The question

of

interest is, of course, can we u s e

PndOgenOllS opioid peptides a s analgesics?

And a s usual in

t h e a n s w e r is y e s a n d n o .

science, render

the

peptides

converting

more s t a b l e t o e n z y m a t i c b r e a k d o w n b y

t o amides or b y p e r f o r m i n g o t h e r chemical

them

manipulations.

I t has been p o s s i b l e t o

i t has not p r o v e d p o s s i b l e t o admi-

However,

nister

them

t o p r o d u c e a n a l g e s i a by

direct

injection

any

totally

than

o r t h e spaces asso-

i n t o t h e CNS i t s e l f

c i a t e d w i t h i t , s u c h as t h e v e n t r i c l e s of obviously

other route

t h e brain.

This

l i m i t s t h e i r u s e as t h e r a p e u t i c a g e n t s .

An a l t e r n a t i v e a p p r o a c h h a s been t o f i n d w a y s oP i n h i b i t i n g the

enzymes

responsible

endrogenous peptides.

the

normal

Unfortunately,

best i n h i b i t o r s Pound, injected

of

the

t h e enzymes i n v o l v e d

directly

t i d e s and

their

into

the

dynorphin, interest

non-peptide

the

endogenous

to

brain

enzyme i n h i b i t o r s ,

to

turned

also have to

b e s t a t i n and thiorplian,

SO, a f t e r a n i n i t i a l s u r g e of

again

breakdown

b r e a k d o w n a r e n o t p a r t i c u l a r l y s p e c i f i c a n d t h e two

in this

be

for

interest the

effective.

i n o p i o i d pep-

i n v e s t i g a t i o n s have

analesics. kappa

be

The

discovery

agonist,

has

oP

directed

t h e k a p p a o p i o i d s s i n c e i t h a s b e e n shown

towards

t h a t opioids acting at

t h e s e r e c e p t o r s show g o o d a n a l g e s i a

under certain conditions.

If

t h e opioids are administered

i n t r a t h e c a l l y t o e x e r t t h e i r e f f e c t s on t h e s p i n a l c o r d , t h e mu

agonists

w i l l

produce

in

analgesia

both

thermal

and

v i s c e r a l chemical t e s t s , d e l t a a g o n i s t s w i l l produce annlge-

sia

i n thermal

analgesia 1984). the

chemical

whilst

visceral

kappa

tests

Consequently, s i n c e non-peptide

kappa

which,

in

tests o n l y ,

receptor,

unlike

t h e mu

such

as

agonists,

agoriists

( ~ c h m a i r s s and Y a k s h , o p i o i d s which act on

pentnzocine, do not

produce

are

produce

avai lablr respiratory

97

depression,

the

possibility

of

their

clinical

use

in

v e s c e r a l p a i n is c o m i n g c l o s e r . One f i e l d

of

o p i o i d pharmacology

which

still not

has

b e e n f u l l y e x p l a i n e d i s t h e r o l e o f t h e sigma r e c e p t o r . date,

no endogenous

ligand h a s been

a common e f f e c t o n t h e n - m e t h y l

a s p a r t a t e receptor.

a n a n t a g o n i s t f o r t h e receptor

of

a r e s t i l l areas w h e r e g r e a t a d v a n c e s n a y be m a d e . b e more

t h e s e d e v e l o p m e n t s would with

Another

Thus,

a n e n d o g e n o u s p s y c h o t o m i m e t i c compound a n d

t h e p o s s i b l e development

dealing

the

receptor now a p p e a r s t o be v i a

l i n k with t h e "phencyclidine"

t h e e x i s t e n c e of

demonstrated and

To

psycotic field

likely

conditions

which

has

not

However

concern

to

those

rather

than

analgesia.

fully

been

explained,

a l t h o u g h e a r l y r e s u l t s were e n c o u r a g i n g , is t h a t of a c u p u n c ture.

The

initial

suggestions

that

oversimplification.

However,

process

p endorphin

release e n d o g e n o u s o p i o i d s s u c h as an

the

might

are p r o b a b l y

t h e understanding

of

the

e x i s t e n c e of

endogenous s u b s t a n c e s which c a n modulate p a i n

transmission

certainly provides a s c i e n t i f i c basis f o r t h e

explanation

how

of

the

technique

might

operate.

Tn summary t h e n , t h e e n d o g e n o u s o p i o i d s a n d t h e i r m a n i pulation

provide

analgesics,

a

basis

for

the

rational

b u t w e m u s t a l w a y s reinember

t h e only neurotransmitter agents involved

development

that

they

of

are n o t

i n p a i n pathways

and o t h e r d r u g s which modify b o t h c h o l i n e r g i c and a d r e n e r g i c transmission

can

S p e n c e r , 1976).

act

as

adequate

analgesics

(Sewell

and

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