Studies on pain. Effects of morphine on a spinal nociceptive flexion reflex and related pain sensation in man

Brain Research, 331 (1985) 105-114 Elsevier

105

BRE 10641

Studies on Pain. Effects of Morphine on a Spinal Nociceptive Flexion Reflex and Related Pain Sensation in Man JEAN-CLAUDE WILLER Lab. Neurophysiologie Clinique, Facultd de Medecine Saint-Antoine, 75571 Paris Cedex 12 (France)

(Accepted July 10th, 1984) Key words: morphine - - nociceptive reflexes - - pain - - thresholds - - man

The nociceptive flexion reflex and the corresponding subjective pain score elicited by sural nerve stimulation were studied in 6 healthy volunteers. A significant correlation was found between the respective recruitment curves of the reflex and of the pain score as a function of stimulus intensity. Consequently, the reflex (Tr) and the pain (Tp) thresholds were found to be almost identical (mean: 10.6 and 10.3 mA, respectively). Similarly, the threshold of the maximal reflex response (Tmr)was very close to that of intolerable pain (Tip): 37.1 and 38.8 mA, respectively. These four parameters were studied before and after intravenous administration of morphine chlorhydrate (0.05, 0.1, 0.2 and 0.3 mg/kg) and subsequent administration of naloxone hydrochloride (0.02 mg/kg; i.v.). While 0.05 mg/kg morphine remained without any effect, higher doses produced an increase in the four thresholds (Tr, Tp, Tmf, Tip). Furthermore, a very significant linear relationship was found between the importance of the increase and the dose of morphine. Morphine also depressed in a dose-dependent fashion, the nociceptive reflexes elicited by a constant stimulation intensity (1.2-1.3 Tr). All these effects were immediately reversed by subsequent naloxone. During all the pharmacological situations, variations in T r and Tp as well as in Tmr and Tip were found to be very significantly linearly related, indicating a close relationship between the effects of morphine on the nociceptive reflex and on the related pain sensation. These results suggest that, in our model involving a brief 'epicritic' nociceptive stimulus, the mechanisms of morphine-induced analgesia in man can be explained by a depressive effect on the nociceptive transmission directly at a spinal level. INTRODUCTION

acute and chronic spinal cats and dogs5,10,23 and in chronic 'spinal' man22, 27.

Psychophysiological and clinical studies have shown that analgesia p r o d u c e d by m o r p h i n e or by other related narcotic agents resulted in an increase in the threshold of e x p e r i m e n t a l l y - i n d u c e d pain produced either by radiant heat, mechanical pressure or electrical stimulationsT,17, 30. F u r t h e r m o r e , in their pioneer study, Wolff et al. 30 r e p o r t e d that intramuscular morphine p r o d u c e d a d o s e - d e p e n d e n t increase in the threshold of a cutaneous pain e x p e r i e n c e d with the radiant heat m e t h o d . A s an a t t e m p t to explain the mechanisms of the analgesic effect of m o r p h i n e , numerous animal studies have shown that this drug clearly depressed the nociceptive transmission at various levels of the central nervous system (see refs. in ref. 4 and 13). M o r p h i n e and derivatives was also shown to block nociceptive reflexes in spinal rats9,

According to these data, a depressive effect directly at a spinal level has been p r o p o s e d by several groups as one of the main mechanisms of m o r p h i n e analgesia (see refs. in refs. 4, 13, 14, 31 and 32), while others favoured a supraspinal level as the major site of action in the m o r p h i n e - i n d u c e d analgesia1,2,6,18-2°. Since this latter hypothesis is mainly based upon indirect arguments, there is still a controversy concerning the location of the main site(s) responsible for the analgesic properties of m o r p h i n e and derivatives. This is particularly clear in h u m a n research since few m e t h o d s which could allow the m e a s u r e m e n t of a specific physiological correlate of pain have been yet described. In such an a t t e m p t , we have previously shown that a nociceptive flexion reflex from a knee-flexor muscle was in a close relation-

Correspondence: J.-C. Wilier, Lab. Neurophysiologie Clinique, Facult6 de Medecine Saint-Antoine, 27 Rue Chaligny, 75571 Paris Cedex 12, France.

0006-8993/85/$03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)

106 ship with the sensation of pain, both elicited by stimulating the ipsilateral sural nerve at the ankle 25. Further studies confirmed these data and revealed that the use of nociceptive flexion reflexes was a useful tool for pain research in man26. Using this method, the present study performed in normal volunteers, shows that intravenous morphine produced a dose-dependent depression in the spinal nociceptive reflexes which paralleled a dose-dependent level of analgesia as revealed by an increase in the response thresholds. MATERIALS AND METHODS The experiments were carried out with 6 unpaid healthy volunteers from the medical staff and investigators (4 men, 2 women, 25-43 years old). These subjects were carefully informed of the goals and procedures of this study, in particular regarding intravenous administration of morphine and of naloxone and thus gave their signed consent according to the ethical principles of the Helsinki convention. During the sessions, the subjects were installed in a reclining position in a comfortable armchair in order to obtain a state of good muscular relaxation. Electrophysiological methods for recording reflex activity from a knee-flexor muscle, elicited by electrical stimulation of sural nerve was derived from those described previously25. Briefly, the sural nerve was stimulated at a rate of 0.25 Hz at its retro-malleolar pathway, using a couple of surface electrodes placed on the degreased skin overlying the nerve 2 cm apart. The electrical stimulus consisted in a train of 5 rectangular pulses (1 ms duration) delivered over 20 ms by a constant-current stimulator. Electromyographic reflex responses were recorded from the ipsilateral biceps femoris muscle, using a couple of surface electrodes placed on the degreased skin over the muscle (Fig. 1 left). These reflex responses were full wave rectified, integrated and expressed as percentages of the maximal control values. However, according to the specific characteristics of the nociceptive flexion reflexSA1,25,26, the time window for integration was between 90 and 180 ms in order to avoid the tactile (RII) component which may occur between 60 and 70 ms as well as the artifactual jumping movement which can sometimes be observed as early as 250 ms after the stimulus onset.

The subjective report of the quality and intensity of the sensation elicited by the sural stimulation was estimated by the subjects on a 10-levels visual scale consisting of 10 switches. Each of these switches was connected to a potentiometer delivering a DC current of 0.2 V (level 1) to 2 V (level 10) increasing in 0.2-V steps. For all subjects, the pain level was defined as level 3. Thus, the first two levels represented tactile sensations, while levels 4-10 corresponded to increasingly painful sensations from just above pain threshold (level 4) to intense and intolerable pain (level 10). Before each session, a 5-min training period was performed in order to familiarize the subjects with this method of self-estimation of sensation. The electrical signals (reflex, sensation and stimulus intensity) were thus connected in parallel to a storage oscilloscope to allow for photographing and the monitoring of the experiments, to a tape-recorder and to a computer for an on-line numerization of data. In these conditions, the intensity of stimulation was delivered randomly between 0 and 50 mA while both the numerized nociceptive reflex and corresponding sensation were plotted against stimulus intensity via a computer program. Usually, both the reflex activity and the subjective rating score increased linearly as a function of stimulus intensity within a limited range as shown in Fig. 1 right. This pattern of response allowed the measurement of both pain and nociceptive reflex thresholds. For this purpose, at least 20-30 x - y points were used for calculating the regression curves. For each curve and for each subject, the significance of the regression coefficient (r) was always between P < 0.01 and P < 0.001. The reflex threshold (Tr) was defined as the abscissa corresponding to the intersection of the reflex linear regression curve with the 10% ordinate line (Fig. 1, upper right). Also, the threshold for obtaining a maximal reflex response (Tmr) was defined as the abscissa corresponding to the intersection of the reflex linear regression curve with the 100% ordinate line (Fig. 1, upper right). In a same fashion, the pain threshold (Tp) was defined as the abscissa corresponding to the intersection of the sensation linear regression curve with the level 3 ordinate line (Fig. 1, lower right). The threshold for intolerable pain (Tip) was defined as the abscissa corresponding to the intersection of the sensation linear regression curve with the level 10 ordinate line (Fig. 1, lower right).

107

NOCICEPTIVE

•

!

I ...............................................

REFLEX (as~o of max) I

I

80

~ ~ f O

.

mr

•

| "~

.

•

r

,o

i.. " ' " ° " ° 0 ~. . . . . .

0

I 0" .......

......

I ee

~

oo.oo.oo..o..o.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I

I

..o . . . . . . . . . . . .

I

,...o.o..,..oo...

I

I

SENSATION ( a . u )

~

I ............

4

lilt

0

I

0

I

_~............

.w.__;,,~__~

!P.

I

20

I

.....

~

....

L__

...............................

I

40

2.

1

mA

Fig. 1. Summary of the experimental design. Left - - upper part: experimental set up for stimulating the sural nerve (Stim.), measuring the stimulus intensity (Probe) and recording reflex activity from the biceps femoris muscle (Bi). Left - - lower part: examples of recruitment of the nociceptive reflex activity as a function of stimulus intensity. 1 = 4 m A ; 2 = 9 m A ; 3 = 12 m A ; 4 = 15 mA. Calibrations: horizontal = 50 ms; vertical = 0.3 inV. Right: method used for calculating - - upper part: the reflex threshold (Tr) and the threshold for obtaining a maximal reflex response (Tmr); - - lower part: the pain threshold (Tp) and the threshold for intolerable pain (Tip) (see text).

Pharmacologicalprocedure At the beginning of each session, a perfusion of isotonic glucose was placed into a vein of the forearm so as to facilitate drug injection and to avoid some possible stressful reaction due to direct intravenous injection during the time course of the experiment, since it has been shown that anxiety and/or stress can modify both nociceptive reflex activity and pain sensationZ4,2s. The effects of 0.05, 0.1, 0.2 and 0.3 mg/kg morphine chlorhydrate (i.v.) were then studied on the parameters described in the previous section, i.e. Tr, Tmr, Tp and Tip. At the end of each experiment, the stereospecificity of the morphine effects was explored with a naloxone hydrochloride (0.02 mg/kg) intravenous injection. Each subject experienced 3 different doses of morphine in a random order and during 3 different experiments.

For the same subject, the time interval between two successive sessions was deliberately that of 6-8 months in order to be sure to avoid the phenomenon of tolerance which could result from repetitiveadministration of narcotic in too short a time period. In these conditions, the general experimental procedure for a given session consisted of the study of the parameters described above (T r, Tmr, Tp, Tip ) i n a control period and after morphine and subsequent naloxone administration. However, just before and during drug injection, the sural nerve stimulus was kept constant at 1.2-1.3 times the reflex threshold in order to study the time course of the drug effect on a constant nociceptive reflex response. Fig. 2 summarizes these experimental conditions. In addition, blood pressure and respiratory activity were clinically monitored at regular 5-8-min intervals throughout

108

CONTROL MORPHzINE I NALOXONE I~:::::::::::::::::::::::: =====================

I

4' random

stim.

~

e

lOmin

~

constant stim.

Fig. 2. Diagram showing the general experimental procedure for each session. Arrows indicate morphine and naloxone injection, respectively. the session which lasted between 90 and 100 min. For each session, numerical data concerning the effects of morphine and of naloxone upon the parameters defined above were expressed as percentages of the control values. Global mean results were expressed in terms of percentage increases in thresholds. Paired t-test and linear regression analysis were used for studying the significance of variations. RESULTS As described above, the experimental procedure consisted of sequences during which both the spinal nociceptive reflex and the related subjective sensation elicited either by constant or random stimulation of sural nerve were studied before and after morphine and subsequent naloxone administration i.v. via the cannula of the isotonic glucose perfusion. In the following sections, we will consider in turn the characteristics of the control (pre-drug) responses and the modification induced upon these responses by the several doses of morphine and by the subsequent naloxone administration.

Characteristics of control responses For all subjects, the general characteristics of control responses are quite similar and are illustrated with a typical individual example in Fig. 1 right. In this experiment, the threshold of the reflex response (Tr) was 10.1 mA, while the threshold of the maximal reflex response (Tmr) was 39.2 mA. Note the linear nature of the intensity-response curve between these values. The corresponding curves obtained for subjective sensations exhibited a similar trend with 9.8 mA and 34.5 mA as thresholds for pain (Tp) and

intolerable pain (Tip) respectively. Such curves were generally very reproducible for a same subject and for the whole group with minimal variations between sequences (see Fig. 3, dotted lines). The mean intensity-response curves obtained with all the subjects for the reflex responses and the pain sensation were remarkably similar. The mean values (_+ S.E.M.) for T, and Tp were 10.6 + 0.3 and 10.3 + 0.8 mA, respectively, while Tmr and Tip were found to be 37.1 + 2.7 and 38.8 _ 3.1 mA, respectively. Similarly, when the stimulus intensity was kept constant at a level which elicited a suprathreshold nociceptive reflex (1.2-1.3 times the reflex threshold), the successive responses did not show any sign of habituation nor of sensitization as can be seen in the control sequences of Fig. 5. As before, the study of the standard errors of the means (S.E.M.) of all the control responses (representing 100%) showed that for all subjects the variations of the successive control responses were found to be in a range of 6.4-10.3%. Moreover, for each subject, the study of the regression curves of these control responses as a function of time showed, in all cases, a lack of significance between these two parameters as revealed by the very low values of the regression (r) coefficient which was found to be in a range of r = 0.032 and r -- 0.081 (0.8 < P < 0.5; n = 30-50).

Effects of intraveous morphine As described in this section, morphine injection resuited in a depression of both nociceptive reflex activity and related pain sensation in a dose-dependent fashion. In all cases, naloxone hydrochloride (0.02 mg/kg) i.v. reversed these effects. As shown in Fig. 3, which illustrates an individual example, the responses were modified in a direct relationship to the dose of morphine given in each case. At 0.05 mg/kg morphine, neither the reflex nor the sensation recruitment curves were different from the controls. At 0.1 mg/kg morphine, there was a slight shift to the right for both the control and the sensation curves. This shift of the courses is clearly due to a change in the slope while the thresholds (T r and Tp) are (in that example) not significantly increased. In contrast, for 0.2 mg/kg and 0.3 mg/kg of morphine administration, the shift to the right of the recruitment curves of both reflex and sensation increased

109

%

SENSATION

REFLEX

a.u

: ...............•...............•...............:.....~,..".

.............:.....

8

~,.#" 40

t

: .............. : ............... .~............... '.~..,.,:'S...'. ............... L..

.............

MORPHINE

.s" ......

.d:'¢"

mg. kg -1

4 f 0

80t '

"

~

- - J l

,...6 ..... /.,..:..::""j"'~

4 t i ..........~ .....

,. ...............

40 •

3 m g k g -1

f

..............

. ..............

:..:.~!::.,.::.::

............

8

.......

/:,,j,

o : ' " ' 2 ~ '...... ~ ~ 0

20

.

40

I

I

.

.................... .

,

,

,

: .............. ;. ............. J....... ,,.:....: ............... : ............... : .....

: ....

.,"... ~ ,6.... p''"

I

I'"..............:...............:............-~!..C:;'-'." ................. ,.." .... ~ I.,::..<:"....

mg. kg -1

801

............................................

I

81-

............... :.............. :............ i ")'~":::"::L'""~': ............... "..

4oI •2

NALOXONE

I'. ..............~...............L.....;..~57.............'................'....

:............... r.............. :............... ;.;;'.::T ........:............:... ....~."

•1

CONTROL

.~* ..."

mA

I 0

S" .." $" y" j,.."

20

• •

40

mA

Fig. 3. Individual example showing the effects of several doses of morphine (indicated on the left part) and of subsequent naloxone (0.02 mg/kg) on the nociceptive reflex activity (left) and the corresponding subjective sensation (right) induced by sural nerve stimulation. For the clarity of the figure, only the resulting regression curves have been indicated for each situation (control, morphine, naloxone).

clearly as a function of the dose, with a maximal effect for the highest dose. Note the progressive increase in T r, Tmr, Tp and Tip and the decrease of the curve's slope as well as being a function of the dose of morphine. Data obtained for all subjects are summarized in Fig. 4, which shows the mean increase in each threshold as a function of the dose of morphine which has been administered. Note that 0.05 mg/kg of morphine did not modify significantly the four parameters considered. In contrast, higher doses clearly induced an increase in the four thresholds. In addition, these increases were very significantly related to the dose of morphine; this relationship was significantly linear in the 0.05-0.3 mg/kg range of morphine tested here. Moreover, for each dose of morphine studied, there was no significant difference between the percentage of increase in T r compared to that of Tp, as well as between the percentage of in-

crease in Tmr compared to that of Tip as revealed by the paired t-test. Morphine injection also depressed the nociceptive reflex activity elicited by a constant stimulation intensity (1.2-1.3 times the reflex threshold). This data is illustrated in Fig. 5 which shows a dose-dependent depressive effect of morphine on an individual example. From this figure, it appears that morphine remains without a clear and significant depressive effect with a 0.05 mg/kg dose (5%) while 0.1, 0.2 and 0.3 mg/kg produced a 20%, 45% and 70% depression in the nociceptive reflex, respectively. Global data are summarized in Fig. 6 which shows the mean value of the depressive effect of morphine as a function of the dose which has been administered. As for the study of the thresholds, there was a significant linear relationship in the importance of the reflex depression as a function of the dose of morphine used in

110 I

I

I

I

ye: 743X- 57 r e : 0.948 * * *

©

¥o

854 X- 60 r°: 0.976 ..1,

r" 200"

. . ~

I

I

I

•

0 "I--

y e 920X- 59 re: 0.997 * * *

.[ ~

~ •¢~ 200'

y o 1086 X- 56

I

..'~

'"'(l| y?

f.-

(n 1 0 0

co I 0 0

k-0

L.. (J

.:_

.c_

o

o

0

~ I

0.05

o

I

I

i

I

I

I

I

0.1

0.2

0.3

0.05

0.1

0.2

0.3

morphine ( mg/kg ) Fig. 4. Global data showing the dose-dependent effects of morphine on the 4 thresholds as defined in Fig. 1 right. Each point indicates

the mean (+ S.E.M.) % increase in the related threshold. Statistical analysis of regression line are indicated on the left hand upper corner of each graph. A

B

100

1'

50

o

eont

.__

m .I

halo

D ' lo0

~ loo1T

/ so 0

5°1 coot

rn .2

nalo

0

l

._

lOmin Fig. 5. Individual example showing the effects of morphine and subsequent naloxone (0.02 mg/kg) on the nociceptive flexion reflex elicited by a constant stimulus intensity (1.2-1.3 times the reflex threshold). In each case, the dose of morphine (in mg/kg) is indicated below the corresponding responses, i.e. = m.05; m.1; m.2 and m.3 correspond to 0.05; 0.1; 0.2 and 0.3 mg/kg morphine. The reflex responses (successive individual responses) are expressed in term of % of the control responses,

this study. In addition, these depressive effects appeared by the first minute following morphine injection, reached a maximal effect by the 3rd to the 5th minute and remained stable at this level during the period which was necessary for collecting the data, i.e. between 30 and 40 min. The classical narcotic side-effects occurred by the first minute following the injection, reached a maximum by the fifth minute and decreased slowly towards a state of drowsiness and of lazyness subjectively reported by the subjects. It is noteworthy to mention here that there was not a clear relationship between the importance of the side-effects and the dose of morphine administered. In contrast, it appeared obvious from our observations that there was an individual sensitivity of the subjective reaction to morphine injection: the same dose could produce minor side-effects in some subjects while being stronger in others. However, we did not observe a clear relationship between the individual importance of the subjective reactions and the importance of effect of morphine on both nociceptive reflex and pain sensation. It must be also mentioned that whatever

111 I

o "E

I

I

I

100 •. . ~ * .

Relationships between the nociceptive reflex and the corresponding pain sensation under morphine administration

..

x IW

50 .m

t.I 0¢-

Y:-246X ÷ 106.8 r: 0.995 * * * I

I

I

I

005

0.1

0.2

0.3

morphine (mg/kg) Fig. 6. Global data showing the relationship between the depression of the nociceptive reflex elicited by a constant stimulus intensity (as in Fig. 5) and the dose of morphine administered. Each point represents the mean + S.E.M. of the reflex responses expressed as a % of control responses (100%). Statistical analysis of the regression line is indicated on the left hand lower corner of the graph. the dose administered, morphine did not produce a major change either in blood pressure or in the respiratory rate as clinically monitored. In 2 subjects, we observed a transient and slight change in the systolic value which increased from 130 to 150-160 mm Hg by the 2 min following morphine injection whatever the dose which was administered. This increase could probably result from a non-specific and anxiogenic reaction of these subjects towards morphine, This increase lasted 3 - 4 min after the injection and then the systolic value returned to its control value for the rest of the experiment. These two subjects did not want to stop the experiment when the investigator asked them if they wished to. All these morphine effects described in the results were completely and immediately (by the first 2 min) reversed by a 0.02 mg/kg naloxone hydrochloride injection. This reversal effect is shown in Figs. 3 and 5. From these figures, it can be seen that naloxone produced often a 'hyper' reversal effect especially on the nociceptive reflex activity (Fig. 5) where the values are higher after naloxone compared to the control ones.

In the first paragraph, it has been shown that the mean intensity-response recruitment curves were almost the same when considering the reflex and the sensation in the several control situations. This similarity indicates the existence of a close relationship between these two parameters. We will now consider the relationships between the nociceptive reflex and pain sensation during the effects of the 4 different doses of morphine described above. In the left part of Fig. 7, the mean increases in T r (abscissa) are plotted against the mean increases in Tp (ordinate) for each dose of morphine. Note the very significant linear relationship between these two parameters. In the right part of the same Fig. 7, such a significant linear relationship can also be observed between the increases in Tmr (abscissa) and Tip (ordinate) for each pharmacological situation. Moreover, when considering the respective equation of these two regression lines, one can see in both cases that the slope of these lines are very near to a 45 ° line (Tg alpha = 1) since Tg alpha = 1.12 (48° slope) for Tp]T r and Tg alpha = 1.20 (50 ° slope) for Tip/Tmr regression curves. DISCUSSION The present study demonstrates that intravenous morphine can result in a dose-response analgesic effect on experimentally-induced pain: pain induced by electrical stimulation of the sural nerve can be gradually lowered by increasing the dose of morphine administered. Simultaneously, morphine was also found to depress a spinal nociceptive flexion reflex. With regard to the sensation and to the reflex activity, both threshold and suprathreshold responses were affected. These results will be discussed from several points of view. However, in the first section, we will consider the methodology used for investigating the experimentally-induced pain. This methodology was derived f r o m earlier studies25, 26 which have shown that there was a very close relationship between the threshold of the nociceptive flexion reflex from the biceps femoris muscle and the threshold of pain elicited by stimulation of the ipsilateral sural nerve. In this work, as in a previous one29, we have further shown that both the development of the re-

112 O.05mg.kg-1

I

I

• .lmg. kg -1

I

I

I

• . 2 m g . k g -1

~,//,,"

~,,,,/"

Y : 1.1x + 2.56

•.3mg.

I

-

-

I

kg -1

I

Y:1.18x+ 2 83

I

//

~

,K/I/////

I..E

.E 2 0 0

200

Or) (D t-"

C

100

=~ 1 0 0

0

0 I 0

100

200

increase

0

in Tr

I 100

I

I 200

~ increase

I

in

Tmr

Fig. 7. Relationships between the % increases in pain and nociceptive reflex obtained with the four doses of morphine used in this study (see symbols in the upper part). Left: T,-Tp relationship. Right: Tmr-Tiprelationship. Statistical analysis of the regression curves are indicated on the left hand upper corner of each graph. Dotted lines show the 45° theoretical regression curves as references with the experimental ones.

flex and of the pain sensation are also closely correlated with suprathreshold stimuli. Of special interest was the observation of a linear relationship for both the reflex response and the pain sensation in a limited range of stimulus intensity. As a consequence, the response thresholds (Tr and Tp) and the maximal response thresholds (Tmr and Tip) were very similar in the control situations. In our experimental conditions, these observations clearly suggest a common spinal mechanism in the development of both nociceptive reflex activity and pain sensation, including maximal responses. These findings are noteworthy for clinical pain studies, particularly, as discussed below, those related to the effects of morphine in the transmission of nociceptive messages and in the mechanisms of analgesia.

(Tp), our data are coherent with the numerous observations previously reported in the literature (see refs. in ref. 3), particularly with those of Wolff et al. 30 which have shown that intramuscular morphine induced a dose-response increase in the pain threshold as measured with thermal radiation applied to the skin of normal subjects. Concerning the threshold of intolerable pain (Tip), we did not find any available previous data which dealt with this parameter in experimental conditions. However, when considering Fig. 4, it appears obviously that the % of increase in Tip as a function of the dose of morphine is more important than the % of increase in Tp in the same pharmacological situation. This suggests that the mechanisms of morphine-induced analgesia could be slightly different when considering the pain threshold or the threshold of intolerable pain.

Effects of morphine on pain sensation The present study shows that intravenous morphine resulted in a dose-dependent increase in both pain threshold and threshold of intolerable pain in normal volunteers. Concerning the pain threshold

Effect of morphine on spinal nociceptive reflexes Our data show that intravenous morphine induced a dose-dependent depressive effect on the recruitment curve of the nociceptive reflex activity. This ef-

113 fect resulted in a dose-dependent increase in both reflex threshold and threshold of maximal reflex. These results are in agreement with those of animal experiments which have shown that intravenous morphine preferentially depressed the spinal polysynaptic reflexes while the monosynaptic ones required higher doses for being affectedS,10,23. This selective depressive effect was also demonstrated upon the reflexly evoked action potentials recorded from the L7 and S1 ventral roots in spinal cats 23. The slower waves representing the polysynaptic reflex discharge were depressed by morphine administration while the earlier monosynaptic spike responses were unaffected. Furthermore, our present data are very similar to those from an earlier report in which we observed that intravenous morphine (0.2-0.3 mg/kg) selectively depressed in a dose-dependent relation the nociceptive polysynaptic reflexes in chronic paraplegic subjects clinically, considered as completely 'spinal' patients 27. These effects were found to be stereospecific since naloxone injection immediately reversed the depressive effects. These data strongly suggest that in our study, the depressive effect of morphine on the spinal nociceptive activity would mainly act via a direct spinal mechanism. This hypothesis is supported by the existence of specific opiate receptors in the superficial layers of the dorsal horn 2~. This idea is also reinforced by the parallelism of the depressive effects of morphine on both pain sensation and nociceptive reflex. As described in the results, intravenous morphine resulted in a very significant dose-dependent linear relationship between the percentage increase in the pain threshold (Tp) and the percentage increase in the nociceptive reflex threshold (Tr) as well as between the percentage increase in the maximal responses (Tip and Tmr). Furthermore, as shown in Fig. 7, in both cases, the slope of the regression curve is very near to 45 ° since we found 48 ° and 50° for the Tp/Tf and Tip/Tmrregression curves, respectively. This latter data clearly suggest that the nociceptive input responsible both for the pain sensation and the reflex activity share some common spinal transmission which is depressed by morphine. Furthermore, a direct spinal effect in the mechanisms of morphine analgesia can also be strongly suggested by a comparison of the present data with those that were described previously with

paraplegic patients 27. As described earlier, intravenous morphine (0.2 and 0.3 mg/kg) resulted in a respective 70% and 90% depression in the patients nociceptive reflexes while identical doses given via the same route induced only a respective 45% and 70% depression of these nociceptive reflexes in normal subjects (see Results). These data would suggest that the spinal cord neurones responsible for the nocicept i v e transmission are more sensitive to the effects of morphine when they are free from some supraspinal influences. This idea is supported by the observations of Le Bars et al. 13-16which have shown that in decerebrate cats, morphine had very few effects on the lamina V dorsal horn neurons contrasting with the powerful depressive effect of this drug in the same animals spinalized with a reversible cold-block. This lack of morphine effect in the decerebrate state was explained by an exacerbation of a tonic inhibitory influence which may mask the depressive effect of morphine. Thus, our results could be interpreted in such a direction, i.e. in paraplegic patients, the spinal cord neurons would be more sensitive to morphine because their spontaneous activity can be increased as a result of the lack of supraspinal inhibitory influence. This increased spontaneous activity was clinically obvious as the whole spinal reflex activity was exacerbated. In contrast, in normal subjects, there would exist descending inhibitory influences which can exert their effect particularly on the nociceptive transmission at the spinal level thus making less effective the depressive morphine effect. It also seems obvious that in that case, the importance of these descending influences could be in relation to the state of wakefulness of a given subject as demonstrated by the inhibitory effect of anxiety on the nociceptive reflexes and on the related pain sensation 28. In conclusion, this study shows that in normal subjects, intravenous morphine results in a dose-dependent inhibition of both spinal nociceptive reflex activity and corresponding pain sensation. A direct depressive effect on the nociceptive transmission at the spinal level is proposed as a possible mechanism for these morphine effects. ACKNOWLEDGEMENT This work was supported by INSERM (C.R.E.) no. 846023.

114 REFERENCES 1 Basbaum, A. and Fields, H. L., Endogenous pain control mechanisms: review and hypothesis, Ann. Neurol., 4 (1978) 451-462. 2 Basbaum, A. and Fields, H. L., Pain control: a new role for the medullary reticular formation. In J. A. Hobson and M. A. B. Brazier (Eds.), The Reticular Formation Revisited, Raven Press, New York, 1980, pp. 329-348. 3 Beecher, H. K., The measurement of pain, Pharmacol. Rev., 9 (1957) 59-209. 4 Besson, J. M., Dickenson, A. H., Le Bars, D. and Oliveras, J. L., Opiate analgesia: the physiology and pharmacology of spinal pain systems. In C. Dumont (Ed.), Advances in Pharmacology and Therapeutics, Vol. 5, Neuropsycho-Pharmacology, Pergamon Press, Oxford, 1978, pp. 61-81. 5 Bonnycastle, D. D., The use of animals in the study of analgetic drugs. In C. A. Keele and R. Smith (Eds.), The Assessment of Pain in Man and Animals, Livingstone, Edinburgh and London, 1962, pp. 231-243. 6 Fields, H. L. and Basbaum, A., Brainstem control of spinal pain transmission neurons, Ann. Rev. Physiol., 40 (1978) 193-221. 7 Gaensler, E. A., Quantitative determination of the visceral pain threshold in man. Characteristics of visceral pain, effect of inflammation and analgesics on the threshold and relationship of analgesia to visceral spasm, J. clin. Invest., 30 (1951) 406-420. 8 Hugon, M., Exteroceptive reflexes to stimulation of the sural nerve in normal man. In J. E. Desmedt (Ed.), New Developments in Electromyography and Clinical Neurophysiology, Karger, Basel, vol. 3, 1973, pp. 713-729. 9 Irwin, S., Houde, R. W., Bennett, D. R., Hendershot, L. C. and Seevers, M. H., The effect of morphine, methadone and meperidine on some reflex responses of spinal animals to nociceptive stimulation, J. Pharmacol. exp. Ther., 101 (1951) 132-143. 10 Koll, W., Physiological observations and pharmacological actions on nociceptive spinal reflexes. In C. A. Keele and R. Smith (Eds.), The Assessment o f Pain in Man and Animals, Livingstone, Edinburgh and London, 1962, pp. 92-103. 11 Kugelberg, E., Eklund, K. and Grimby, L., An electromyographic study of the nociceptive reflexes of the lower limb. Mechanism of the plantar responses, Brain, 83 (1960) 394-410. 12 Le Bars, D. and Besson, J. M., The spinal site of action of morphine in pain relief: from basic research to clinical applications, Trends Pharmacol. Sci., 2 (1981) 323-325. 13 Le Bars, D., Dickenson, A. H. and Besson, J. M., Opiate analgesia and descending control systems. In J. J. Bonica, U. Lindblom and A. Iggo (Eds.), Advances in Pain Research and Therapy, Vol. 5, Raven Press, New York, 1983, pp. 341-372. 14 Le Bars, D., Menetrey, D., Conseiller, C. and Besson, 1. M., Depressive effects of morphine upon lamina V cells activities in the dorsal horn of the spinal cat, Brain Research, 98 (1975) 261-277. 15 Le Bars, D., Guilbaud, G., Jurna, I. and Besson, J. M., Differential effects of morphine on responses of dorsal horn

lamina V type cells elicited by A and C fibre stimulation in the spinal cat, Brain Research, 115 (1976) 518-524. 16 Le Bars, D., Rivot, J. P., Guilbaud, G., Menetrey, D. and Besson, J. M., The depressive effect of morphine on the C fibre response of dorsal horn neurones in the spinal rat pretreated or not by pCPA, Brain Research, 176 (1979) 337-353, 17 Lee, R. E. and Pfeiffer, C. C., Influence of analgesics, dromoran, nisentil and morphine, on pain thresholds in man, J. appl. Physiol., 4 (1951) 193-198. 18 Liebeskind, J. C., Giesler, G. J. and Urca, G., Evidence pertaining to an endogenous mechanism of pain inhibition in the central nervous system. In I. Zotterman (Ed.), Sensory Functions of the Skin in Primates, Pergamon Press, Oxford, 1976, pp. 561-573. 19 Mayer, D. J., Endogenous analgesia systems: neural and behavioral mechanisms. In J. J. Bonica, J. C. Liebeskind and D. Albe-Fessard (Eds.), Advances in Pain Research and Therapy, Vol. 3, Raven Press, New York, 1979, pp. 385-410. 20 Mayer, D. J. and Price, D. D., Central nervous system mechanisms of analgesia, Pain, 2 (1976) 379-404. 21 Pert, C. B., Kuhar, M. J. and Snyder, S. M., Autoradiographic localisation of the opiate receptor in cat brain, Life Sci., 16 (1975) 1849-1854. 22 Roby, A., Wilier, J. C. and Bussel, B., Effect of a synthetic enkephalin analogue on spinal nociceptive messages in humans, Neuropharmacology, 22 (1983) 1121-1125. 23 Wikler, A., Studies on the action of morphine on the central nervous system of the cat, J. Pharmacol. exp. Ther., 80 (1944) 176-187. 24 Wilier, J. C., Influence de l'anticipation de la douleur sur les fr6quences cardiaque et respiratoire et sur le r~flexe nociceptif chez l'homme, Physiol. Behav.. 15 (1975) 411-415. 25 Wilier, J. C., Comparative study of perceived pain and nociceptive flexion reflex in man, Pain, 3 (1977) 69-80. 26 Wilier, J. C., Nociceptive flexion reflexes as a tool for pain research in man. In J. E. Desmedt (Ed.), Motor Control Mechanisms in Health and Disease, Raven Press, New York, 1983, pp. 809-827. 27 Wilier, J. C. and Bussel, B., Evidence for a direct spinal mechanism in morphine induced inhibition of nociceptive reflexes in humans, Brain Research, 187 (1980) 212-215. 28 Wilier, J. C., Dehen, H. and Cambier, J., Stress-induced analgesia in humans: endogenous opioids and naloxone-reversible depression of pain reflexes, Science, 212 (1981) 689-691. 29 Wilier, J. C., Roby, A. and Le Bars, D., Psychophysiological and electrophysiological approaches to the pain relieving effects of heterotopic nociceptive stimuli, Brain, (in press). 30 Wolff, H. G., Hardy, J. D. and Goodell, H., Studies on pain. Measurement of the effect of morphine, codeine and other opiates on the pain threshold and an analysis of their relation to the pain experiences, J. clin. Invest., 19 (1940) 659-680. 31 Yaksh, T. L., Spinal opiate analgesia: characteristics and principles of action, Pain, 11 (1981) 293-346. 32 Yaksh, T. L. and Rudy, T. A., Studies on the direct spinal action of narcotics in the production of analgesia in the rat, J. Pharmacol. exp. Ther., 202 (1977) 411-428.