PAIN: PHARMACOLOGICAL ASPECTS

17. PAIN: PHARMACOLOGICAL ASPECTS

S. D. Iversen Department of Experimental Psychology, University of Cambridge, Downing Street, Cambridge, England

Opium has been in use in medicine since the twelfth century. The natural product may contain as much as 10% by weight of morphine and 0.5% of codeine.

As well as its analgesic effect, morphine is

rewarding and man and animals rapidly develop tOlerance and dependence.

The problems of addiction compounded by the depressant

action of morphine on respiratory centres, has led to persistent attempts to synthesise potent analgesic compounds without risk of dependence and less potent as CNS depressants.

These rational

attempts at drug design have not been totally successful and at least under experimental conditions all drugs which are highly effective analgesics tend to induce some dependence.

Pentazocine

(Figure 1) is said to be less addictive than most opiates at equivalent analgesic doses.

Substitution of an allyl group on

the nitrogen atom produces antagonists naloxone.

of morphine, such as

Some of these compounds such as nalorphine are partial

agonists as well as antagonists and have analgesic activity. Unfortunately, nalorphine has unpleasant psychological side effects, which make it unlikely to cause dependence but equally unacceptable as an analgesic drug of choice. In the last decade our understanding of the anatomy, neurophysiology and neuropharmacology of the pain pathways of spinal cord and brain have advanced dramatically and alternative pharmacOlogical strategies for the management of pain suggest themselves.

18. The Anatomy of the Pain Pathways The receptors for pain consist of nonencapsulated endings of peripheral neurofibres; notably the smaller myelinated A fibres and the unmyelinated group C fibres.

These dorsal root

neurones synapse in the substantia gelatinosa of the dorsal horn and thus form the ascending lateral spino-thalamic tract. Together with the trigeminothalamic tract, this tract synapses in the ventral posterior thalamic nucleus and hence projects to somaesthetic cortex.

It seems likely that the neuropeptide

substance P is the transmitter of a class of unmyelinated C fibre and destruction of these SP-containing neurones with the neurotoxin CAPSAICIN results in loss of the responsiveness to one class of painful stimuli, chemical irritants (Iversen et al., 1981). However, it now seems unwarranted to assume that SP is the transmitter for all classes of pain stimuli.

Nevertheless, the develop-

ment of PS antagonists would offer a novel pharmacological approach to the control of certain kinds of pain.

It has been shown in

the trigeminal nucleus slice (Jessell and Iversen, 1977) and in the super fused spinal cord of rat (Yaksh et al., 1980) that morphine is able to inhibit the release of endogenous SP, a presynaptic inhibitory mechanism which may account for some aspects of morphine analgesia.

However, since SP appears to playa relatively rest-

ricted role in the transmission of pain stimuli, this presynaptic site of interaction of morphine with sensory terminals clearly does not entirely account for its profound and generalised analgesic effect.

Intravenous morphine suppresses the firing of nociceptive

but not of non-nociceptive neurones in the spinal cord (Duggan et al., 1977), and of

nucleus lateralis neurones of the thalamus

activated by natural noxious stimuli (Hill and Pepper, 1978).

19. How does morphine act to modulate pain? Pain is a sensation but it also results in affective or emotional responses. to pain?

Does morphine relieve both of these responses

Beecher's (1966) views about pain and narcotic analgesia

in man have been influential for the past 20 years.

He distinguished

between relatively invariant "pain sensations" that relate to the intensity of noxious stimuli and varied "pain reactions" that reflect complex emotional and cognitive responses elicited by such stimuli.

The latter are influenced by personality, past

experiences and experimental context.

He proposed that narcotics

produce clinical analgesia primarily by altering the unpleasantness of pain reactions rather than by altering the intensity of pain sensations.

This view has de-emphasised the role of sensory

processes in narcotic analgesia.

Anecdotally patients under mor-

phine are said to feel the pain but not to care about it. This is an important issue to resolve since at least in principle drugs could be developed which control either the sensation or the reaction

to pain or indeed both dimensions.

However, in a recent study Gracely et al.

(1979) report that in

dental patients fentanyl, a short-acting narcotic reduced the sensory intensity of tooth pulp pain stimuli without reducing their unpleasantness.

Patients were asked to rate subjectively

painful electrical stimulation of tooth pulp on one of two rating scales (sensory intensity and unpleasantness), when they were given a random list of words describing these dimensions.

Using this

elegant psychophysical rating scale they found that fentanyl significantly reduced the perceived sensory intensity and conclude that narcotic analgesia involves effects both on the sensation and

20 affective reaction to pain.

By contrast, the minor tranquiliser,

diazepam reduces the unpleasantness of the pain but not its intensity, confirming a number of observations that anxiolytics and narcotics control different aspects of the response to pain. Thus in evaluating analgesic drugs it is important to employ measurements of more than one aspect of the response to pain. This comment is particularly pertinent to animal studies where little attention has been paid to the behavioural characteristics of the standard tests for analgesia.

Tail flick, flinch-jump,

hot-plate, pinch or pressure and formalin paw test are variously employed but may well assess different aspects of the response to pain.

Tail-flick is likely to be largely mediated at the spinal

level, whereas hot-plate responses are said to involve a larger emotional element.

An evaluation of these testing procedures may

facilitate the development of Where does

morphine act

more selective drugs.

to relieve pain?

It now seems that there are a number of brain circuits which modulate the transmission and behavioural impact of painful stimuli. These exist at all levels of nervous system from cortex to spinal cord, some involve local circuit neurones, others extensive pathways between brain and spinal cord (SC) and in a number of them chemical neurotransmitters have been kentified.

In 1965 Melzack

and Wall published a "gating theory" suggesting that supraspinal levels of the CNS could modulate the transmission of pain stimuli.

The anatomical and chemical basis of this theory has

been realised with time. In the 1960's noradrenaline, dopamine and serotonin containing pathways were described in brain, which did not appear to be correlated in any obvious way with the pain pathways.

The role

of these monoaminergic systems in pain perception has gradually emerged and as we shall see they provide links with the major neurotransmitter involved in pain transmission, the brain opiate

21. system. An important paper was published by Mayer et al.

(1971) in

which they investigated claims that electrical stimulation of certain brain sites induced analgesia.

In the rat they found a

number of sites in the diencephalon and mesencephalon (dorsal

.

tegmentum, especially ventral posterior central gray, ventral tegmentum and thalamus, generically known as periadqueductal gray, PAG) where stimulation produced analgesia (SPA) could be elicited. Referring to the "gating theory" of pain control proposed by Melzack and Wall, they proposed that these sites in the periaqueductal gray (PAG) were activating a descending fibre system to spinal cord, capable of modulating pain transmission.

This

paper predates the discovery of the endogenous opiates of brain but it is interesting that Mayer et al.'s predictions were entirely correct.

It is now thought that the mesencephalic periaqueductal

gray area projects to a final common pathway in the raphe system of the brain stem.

The nucleus raphe rnagnus is the origin of a sero-

tonin (5HT) containing inhibitory pathway projecting to the dorsal and ventral horns of the spinal cord.

ManipUlations which increase

endogenous brain levels of 5HT enhance SPA induced from PAG (Akil and Liebeskind, 1975).

Close to the nucleus

raph~

magnus in

the bulbar retiCUlar formation lies the nucleus reticularis paragigantocellularis (NRPG).

Stimulation of this nucleus also

results in analgesia and it is believed that from this nucleus arises a parallel descending inhibitory noradrenaline pathway to the spinal cord.

SPA is thought to activate these pathways and

thus release of 5HT and NA in the SC is a corollary of the analgesia.

Toxin induced lesions of 5HT and NA in SC block PSA

(Johannesen et al., 1982).

It has also been reported that in

patients suffering from intractable pain, electrical stimulation of periventricular and periaqueductal gray may afford some relief

22. from pain (Hosobuchi et al., 1977), yet while the monoamines clearly playa major role in the extensive circuits concerned in the transmission of sensory

information, including pain, and in the

circuits influencing such transmissions, the most important substrate involved in the modulation of pain was not discovered until 1975. With the introduction of receptor binding techniques the ability of brain to selectively bind specific agonists of all drug classes was soon under investigation.

When it became clear

that brain could bind morphine and other narcotics stereospecifically, the search began for the endogenous morphine-like compound which normally activates such receptors (Hughes et al., 1975). Almost simultaneously three groups reported the occurrence in brain of Leu- and Met-enkephalin, opioid peptides whose structure was immediately recognised to exist within the precursor molecule, so called "Big AeTH", a high molecular weight peptide of pituitary and probably of brain (Figure 2). With a combination of radio-ligand binding, radio-immuno assay to determine levels of enkephalins and autoradiographic teChniques for visualising opiate receptors in situ, a picture was quiCkly built up of the distribution of brain enkephalins and their receptors in rat and other species including monkey (Haber and Elde, 1982). Leu- and Met-enkephalin are widely distributed in brain with particularly high concentrat.ions in the areas known to be involved in pain transmission: the substantia gelatinosa of the dorsal horn of spinal cord, the peri-aqueductal gray, thalamus and the basal ganglia. As well as enkephalin, the brain contains another endorphin, so called

beta-endorphin (Figure 2).

with specific antisera

23. Watson et al.

(1978) were able to distinguish brain enkephalins

from beta-endorphin. of beta-endorphin.

The pituitary contains high concentrations However, while the brain contains some beta-

endorphin fibre systems arising from hypothalamus and projecting to mesencephalic and brain stern sites, the innervation is sparse when compared to the enkephalins.

Despite early reports that

beta-endorphin afforded pain relief in man and the fact that in animals intra-ventricular beta-endorphin induced a profound catatonic state (Bloom et al., 1976), it is now generally thought that this beta-endorphin plays a minor role in the response to pain. One of the densest areas of enkephalin and of opiate receptors is found in lamina I and II of the dorsal horn, the part of the substantia gelatinosa concerned with the integration of incoming sensation, including pain.

As mentioned earlier morphine may

modulate presynaptically the release of SP at this site.

However,

it seems that there are also enkephalin-containing interneurons in this region of cord, which appear not to synapse on SP terminals and may well be part of the descending pain inhibitory systems as well as providing local inhibitory circuits.

stimulation of

the SHT or NA systems of the brain stern inhibit the response of neurons transmitting noxious stimuli in the dorsal horn (Fields et al., 1977) and iontophoretic

applica~ion

cells also result in their inhibition. reversible.

of opiates onto these

These effects are naloxone

Thus at the spinal cord level pain transmission can

be controlled either by manipulating SHT and NA activity or opiate levels.

Furthermore, it seems possible

that opiates

mediate both a link in the spinofugal system and act locally and directly on nociceptive neurones.

24. Experimental work in which drugs are applied locally to the SC (intrathecal) also supports this view.

Intrathecal application

of the antagonists of NA and 5HT, phentolamine and methysergide, block the analgesia (assessed on tailflick) induced by activation of the PAG (Yaksh, 1979).

Intrathecal application of noradrenaline

(Takagi, 1980) onto the lumbar spinal cord induces analgesia and application of the NA antagonist phenoxybenzamine inhibited the analgesia inductd by stimulation of the brain stem inhibitory sites.

Intrathecal manipulation of SC GABA levels with BA clofen

or muscimol is also reported to induce analgesia. intrathecal morphine (Yaksh and RUdy, 1976) in rat,

Equally, (and in rabbit,

cat and monkey) elevates analgesic threshold as measured by tail flick, hot plate, forceps pinch

and shock titration methods.

At the same site naloxone will antagonise the effect of systemic morphine.

After repeated application of morphine to the SC, rats

show some features of morphine withdrawal when challenged with naloxone (Yaksh et al., 1977).

The possible clinical value of

epidurally administered narcotics represents an early and unexpected application of this body of research on the SC. with the discovery of the existence of a wide range of peptide neurotransmitters in CNS, it has been shown that a number of these, somatostatin, VIP, neurotensin and CCK, are localised

in the dorsal horn of SC either in sensory terminals or interneurones.

Neurotensin is particularly notable since it is reported

to alter pain thresholds and it is possible that it acts at least

in part at SC level (Iversen et al., 1981).

However, the role

of non-opioid peptides in pain requires further careful evaluation before undertaking the development of novel peptide interacting drugs for treating pain.

25. Supra-spinal sites of action of narcotics Enkephalins are also found in high concentrations in the PAG, the major brain site implicated in the modulation of pain.

It is

now established that there is a relationship between the opiate substrate of the PAG and the phenomenon of SPA.

At the sites where

electrical stimulation induces analgesia, local injection of morphine has the same effect (Yeung et al., 1977).

However, since

SPA is not completely blocked by naloxone (Akil et al., 1966), it is considered that there are substrates in PAG where analgesia may be induced but which are not dependent on endogenous opioids. However, most of the results suggest that stimulation of PAG releases locally an endogenous opioid which activates the spinofugal pain inhibitory pathway.

Indeed, microinjections of morphine

in PAG release 5HT from SC (Yaksh and Tyle, 1979). Since pain involves an affective as well as a sensory component it is reasonable to ask, but difficult to determine, which aspect of pain transmission is being modified by PAG.

Tests of nociception

are used indiscriminantly and more attention needs to be given to the evaluation of methods for quantifying different aspects of the response to pain.

It is not unreasonable to suppose, however,

that limbic and cortical areas may modulate the emotional aspects of the response.

But we have little information on this issue and

certainly no drugs with which to control different levels of CNS. However, opiate rich sites in the amygdala have been reported where local injections of morphine modify hot plate responding, a test claimed to have a substantial emotional component.

Further-

more, in anatomical studies it has been reported that frontal cortex in rat and monkey projects to PAG, a pathway which would allow higher centres to influence spinofugal systems (Hardy and Leichnetz, 1981a, b).

26. Thus we should not focus all our attention on the PAG and the spinal cord when evaluating the pain mechanisms of brain and their control with drugs or other

therap~utic

techniques.

Wall and Woolf (1980) recently commented "the activity

As

of all

nociceptive cells is controlled by a balance of excitatory and inhibitory mechanisms mediated by many neural circuits, some local within the spinal cord and some with long areas extending from cord to brain". One of these appears to involve forebrain dopamine (DA) neurones arising from substantia nigra (SN) (Jurna and Heinz, 1979).

Opiate receptors are found in abundance on striatal and

limbic DA neurones and systemic morphine increases the firing rate of these neurones.

Drugs, like haloperidol which block DA

transmission impair the response to pain.

This pathway involves

the efferent projections from caudate nucleus to SN but it has been shown that SN stimulation in the absence of the feedback loop modifies the transmission of pain stimuli at the leYel of the spinal cord, suggesting a direct influence of SN neurones to SC perhaps mediated by brain stem (Blinn et al., 1980). Recent Opioid Pharmacology Most of the research described has used morphine itself and naloxone as an antagonist.

Efforts are being made to compare

the effect of a wide range of synthetic agonists, antagonists and mixed agonist/antagonists on the various mOdel systems of SC and brain.

However, one thing is clear.

Analogues of the endogenous

enkephalins are analgesic but these also, like morphine, lead to tOlerance and dependence. The biochemical work of the opiate receptor involving binding assays has revealed the existence of a number of pharmacologically distinguishable receptors.

Certainly, three, Mu, Delta and Kappa,

27. maybe more.

These exist in brain as well as in gut and vas

deferens (Wood, 1982).

The p and

~

receptors of brain do not

relate in any obvious way to leu- and met-enkephalin, appear to co-exist on the same neurones, but like the enkephalins, may be differentially distributed in different brain areas. The task ahead is to try to define the receptor types related to various actions of morphine.

If this proves possible and it

turns out that analgesia is related to one rather than another receptor type, it may be possible to produce more specific drugs. with this approach the exciting possibility exists that an analgesic without

addictive or depressant properties might become

available. Using naloxone as an antagonist, a dose-ratio analysis of the depression by morphine of nociceptive neurones of SC reveals that this opiate depression of single unit activity has the same pharmacological properties as observed with morphine analgesia (Yaksh, 1977).

This suggests that the opiate receptor mediating

cellular depression and those mediating analgesia are the same. However, using naloxazone and correlating opiate binding with behavioural responses, Pasternak et al.

(1980) conclude that the

population of opiate receptors mediating death by respiratory depression and those mediating analgesia are different. In conclusion, the enormous amount of information on the transmitters involved in pain transmission obtained in the last decade has not yet enabled us to produce drugs for the relief of pain which avoid the risks associated with the use of morphine.

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Brain Res. 94, 279-296.

Akil, H., Mayer, D.J. and Liebeskind, J.C. (1976) 961-962. Beecher, H.K. (1966)

Science, 151.

Science, 191,

840-841.

Blinn, G., Heinz, G. and Jurna, I. (1980)

Neuropharm.

Bloom, F., Segal, D., Ling, N. and Guillemin, R. 194, 630-632. Duggan, A.W., Hall, J.G. and Headley, P.M. §i, 65-76.

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(1977)

75-85.

Science

Br. J. Pharmac.

Fields, H.L., Basbaum, A.I., Clanton, C.H. and Anderson, S.D. (1977) Brain Res. 126, 441-453. Gracely, R.H., Dubner, R. and McGrath, P.A. (1979) 203, 1261-1263. Jessell, T.M. and Iversen. L.L. (1977) Haber, S. and Elde, R.

Nature, Lond. 268, 549-551.

(1982) Neuroscience

Hardy, S.G.P. and Leichnetz, G.R. 23, 13-17.

Science

1, 1049-1095.

(1981a) Neuroscience Letters,

ibid (1981b) 97-101. Hill, R.G. and Pepper, C.M. (1978)

Br. J. Pharmac. 64, 137-143.

Hosobuchi, Y., Adams, J.E. and Linchitz, R. 183-186.

(1977) Science 197,

Hughes, J., smith, T.W., Kosterlitz, H.W., Fothergill, L.A., Morgan, D.A. and Morris, H.R. (1975) Nature, Lond. 258, 577-579. Iversen, L.L., Nagy, J., Emson, P.C., Lee, C.M., Hanley, M., Sandberg, B., Ninkovic, M. and Hunt, S. (1981) In Chemical Transmission, 75 years. Editors L. Stjarne, P. Hedquist, H. Lagercrantz and A. Wenmalm. Academic Press, pp. 501-512. Johannessen, J.N., Watkins, L.R., Carlton, S.M. and Mayer, D.J. (1982) Brain Res. 237, 373-386. Jurna, I. and Heinz, G. (1979) Pharmacol. 309, 145-151.

Naunyn-Schmiedeberg's Arch.

Mayer, D.J., Wolfle, T.L., Akil, H., Carder, B. and Liebeskind, J.C. (1971) Science 174, 1351-1354. Melzack, R. and Wall, P.D. (1965)

Science 150, 971-979.

29. Pasternak, G.W., Childers, S.R. and Snyder, S.H. (1980) 208, 514-516. Takagi, H. (1980) 182-184.

Trends in Pharmacological Science.

Science March.

Wall, P.O. and Woolf, C.J. (1980) Nature 287, 185-186. Watson, S.J., Akil, H., Richard, C.W. and Barchas, J.D. Nature, Lond. 275, 226-228. Wilson, P.R. and Yaksh, T.L. (1978) Eur. J. Pharmacol. Wood, S.L. (1982) Yaksh, T.L. (1978) Yaksh, T.

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Yaksh, T.L. and Rudy, T.A. (1977) 411-428. Yaksh, T.L. and Tyee,

G.M. (1979)

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(1977)

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Yaksh, T.L., Jessell, T.M., Gamse, R., MUdge, A.W. and Leeman, S.E. (1980) Nature, Lond. 286, 155-157. Yeung, T.C., Yaksh, T.L. and Rudy, T.A. (1977)

Pain

i,

23-40.

30. AlIO/ttfirt

COdei""

Morphine

"'~. II

°

Q

/CH,

CH.CH.~~CH.~HN'CH' 0v

CH,

II

Herem

°

goorn~.

M.,hado""

.....CH, ~CH.cH-C'CH'

<.J--f::.

-CH,

CH,

N

1

CH, Pentnocine

MereridlM

AII,~orri.".f

Nalorphine

Figure 1. structures of some morphine-like drugs and antagonists.

31.

PRECURSOR RELATIONSHIPS OF CORTICOTROPINS AND PITUITARY ENDORPHINS

"7H

Il-

.....JlcOOM

BIG ACTH f MW 31,000'

i-----'".II;-'- ---------;. I ACT"

,

....J

L ._ - - ' -

Ie

4'

,.,61

!If

1,·.....11,··_.. 1

B

o0

47"6165

ACTH 4-10

M£T-

ENl«P'HALrN

Figure 2. In the pituitary gland, and probably in brain, a single large precursor mOlecule (BIG ACTH) contains within its amino acid sequence the entire ACTH and beta-lipotropin hormone

(~-LPH)

molecules.

In turn, the ACTH sequence

contains within it that of alpha-melanotropin and the beta-LPH endorphin.

(~-MSH);

sequence contains beta-MSH and beta-

Furthermore, the sequence of amino acids

4 - 10 in ACTH is repeated within the beta-LPH sequence. Although the amino acid sequence of met-enkephalin is contained in the first five residues of beta-endorphin, free met-enkephalin has not been detected in significant quantities in the pituitary gland.

Moreover, there is no

evidence that beta-endorphin is the precursor of metenkephalin synthesis in the brain. Childers, 1979).

(From

Synder and