Effects of morphine and barbiturate on the SI and SII potentials evoked by tooth pulp stimulation of rats

European Journal of Pharmacology, 36 (1976) 347--353

347

© North-Holland Publishing Company, Amsterdam -- Printed in The Netherlands

EFFECTS OF MORPHINE AND BARBITURATE ON THE SI AND SII POTENTIALS EVOKED BY TOOTH PULP STIMULATION OF RATS YOSHIO SHIGENAGA * and REIZO INOKI

Department of Pharmacology, Osaka University Dental School, 32 Joan-chio, Kita-ku, Osaka, Japan Received 1 July 1975, revised MS received 2 December 1975, accepted 8 December 1975

Y. SHIGENAGA and R. INOKI, Effects of morphine and barbiturate on the SI and SH potentials evoked by tooth pulp stimulation of rats, European J. Pharmacol. 36 (1976) 347--353. The effects of morphine and barbiturate on the evoked potentials recorded from the primary and secondary somatic sensory areas of rats were investigated. The electric stimulation of contra- and ipsilateral tooth pulp (CTP and ITP) was used. The afferent impulse from dental pulp projected to the sensory face areas I and II (SI and SII). Morphine in doses of 2.5--10 mg/kg definitely inhibited SI and SII potentials evoked by CTP stimulation. Morphine also inhibited SII potentials evoked by ITP stimulation, while it rather enhanced SI potentials evoked by ITP stimulation. Pentobarbital sodium in doses of 4--16 mg/kg tended to inhibit SI potentials, but showed no effect or rather an enhancement on SII potentials evoked by CTP stimulation. Pentobarbital sodium enhanced SI and SII potentials evoked by ITP stimulation. In a large dose of 32 mg/kg, pentobarbital sodium inhibited SI and SII potentials evoked by ITP and CTP stimulations. The results suggest that SII is more closely related to the analgesia due to morphine than is SI. Tooth pulp stimulation

Morphine

Barbiturate

1. Introduction

It has been reported that morphine selectively inhibits the diffuse thalamocortical projecting system, the reticular activating system and the thalamocortical reverbrating circuit, as well as the spinal afferent pathway (Fujita et al., 1953, 1954; Straw and Mitchell, 1964). In the previous studies, it has been shown that rat's t o o t h pulp afferents which mainly consist of A5 fibers, project into the ventrobasal complex and posterior nuclear region of the thalamus, and also that the contra- and ipsilateral cortical projections of the tooth pulp afferents converge in two regions, somatic sensory face areas I (SI) and II (SII) (Shigenaga et al., 1973, 1974). The present study was * Present address: Department of Physiology, University of Melbourne, Parkville, Victoria, Australia.

SI and SII potentials

performed to investigate the action of morphine, in comparison with that of pentobarbital sodium, on the cortical evoked potentials in SI and SII resulting from tooth pulp stimulation.

2. Materials and methods 33 adult male rats weighing 250--280 g were used. The surgical procedures were performed under ether anesthesia. After the proced .ures were completed, the animals were immobilized with gallamine triethiodide (20--30 mg/kg, i.p.) under artificial respiration. The experiments began after at least 2 hr had elapsed from discontinuance of ether anesthesia. The lower incisor teeth were drilled and bipolar stimulating electrodes, 0.2 m m in tip diameter and with a tip separation of 1--2

348 mm, were inserted into the pulp. The animal was then fixed to a stereotaxic apparatus and the hemi- or bilateral parietal part of the calvaria was removed. Single rectangular pulses of 0.1 msec duration and 500 pA--1 mA intensity were applied to the pulp each second. The evoked potentials were recorded from SI and SII, through a chloride silver electrode applied directly to the cortex. In order to record transcallosal responses, the contralateral cortex to the recording site was exposed and bipolar stimulating electrodes, 0.1 m m in tip diameter and with a tip" separation of about 0.2 mm, were, symmetrically placed on SI and SII. A single rectangular pulse of 0.05 msec duration and 100--300 pA intensity was applied. The indifferent electrode was located in the frontal bone. The exposed cortex and electrodes were covered with a 2--3 m m layer of 2% agar in physiological saline solution or with a mineral jelly. Electrode orientations were referred to the previous report of Shigenaga et al. (1974). Potentials were amplified through a preamplifier (Nihonkoden AVB-2) and displayed on a dual beam oscilloscope (Nihonkoden VC-7). 22 animals were used to examine a relationship between dose and duration of action using morphine in doses of 2.5, 5 and 10 mg/kg. In two of the animals, the antagonistic effect of levallorphan on morphine action was examined. These effects of morphine and levallorphan on the cortical evoked potentials were evaluated from the superimposed recording technique. In general, drug effects on the cortical evoked potentials were determined by comparing the average of the amplitudes and latencies of 20 potentials before and after drug administration. Parameters for measurement of the amplitude and latency of the evoked potentials are shown in fig. 1. All drugs were given i.p. and a cumulative drug administration technique was employed. Interval of administration of morphine and pentobarbital sodium was about 90 and 30 min respectively. Drugs used were morphine hydrochloride

y. SHIGENAGA, R. INOKI

Fig. 1. Diagrammatic representation of the evoked potential. PA: amplitude of surface positive phase; NA: amplitude of surface negative phase; PL: latency of surface positive phase.

(2.5, 5 and 10 mg/kg), levallorphan tartrate (1 mg/kg) and pentobarbital sodium (4, 8, 16 and 32 mg/kg).

3. Results

3.1. Effect o f morphine on the evoked potentials in SII Typical examples of the SII potentials elicited by stimulation of the contra- and ipsilateral t o o t h pulp (CTP and ITP) and the effects of morphine and levallorphan are depicted in fig. 2. Both amplitudes of the surface positive and negative potentials were inhibited by morphine in doses of 2.5, 5 and 10 mg/kg dose dependently. The peak time of inhibition was seen 60--90, 60--120 and 60--180 min after administration for doses of 2.5, 5 and 10 mg/kg respectively. These inhibitory effects were restored about 120 and 180 min after the administration of morphine in doses of 2.5 and 5 mg/kg respectively. In a dose of 10 mg/kg, complete restoration was not observed even 240 rain after the administration of morphine. The inhibition of the amplitude due to morphine in a dose of 10 mg/kg was restored 20 min after the administration of

MORPHINE AND TOOTH PULP STIMULATION

A

B

c

i" !

349

•

2

3

5

Fig. 2. Effects of morphine and levallorphan on SII potentials following CTP and ITP stimulations. A, B and C indicate SII potentials following CTP stimulation and D indicates SII potentials following ITP stimulation. Columns A, B, C and D indicate recordings of the changes due to 2.5, 5, 10 and 10 mg/kg of morphine respectively. C and D show the antagonistic actions of levallorphan (1 mg/kg) on the morphine effect. Uppermost tracings of each column are controls. A-2, A-3, A-4 and A-5 are 45, 60, 90 and 120 min after morphine (2.5 mg/kg) respectively. B-2, B-3, B-4 and B-5 are 60, 90, 120 and 180 rain after morphine (5 mg/kg). C-2, D-2, C-3, D-3 and C-4, D-4 are 45, 90 and 180 rain after morphine (10 mg/kg) and C-5, D-5 are 20 rain after levallorphan. All records are made up of 20--30 superimposed tracings. A positive phase of potentials indicates upward deflexion. Time mark is 5 msec.

l e v a l l o r p h a n in a d o s e o f 1 m g / k g as s h o w n in fig. 2 C-5 and D-5. M o r p h i n e in d o s e s o f 2.5, 5 a n d 10 m g / k g significantly inhibited t h e negative a n d positive p h a s e o f t h e e v o k e d p o t e n t i a l elicited b y C T P s t i m u l a t i o n a n d t h e negative p h a s e o f t h o s e elicited b y I T P s t i m u l a t i o n . This inhibit i o n was d o s e d e p e n d e n t . T h e negative p h a s e t e n d e d t o be m o r e i n t e n s e l y inhibited t h a n t h e positive one. M o r p h i n e did n o t p r o l o n g t h e l a t e n c y o f t h e p o t e n t i a l s significantly. Transcallosal p o t e n t i a l s w e r e u n a f f e c t e d b y m o r p h i n e in a n y d o s e used.

3.2. Effect o f pentobarbital sodium on the evoked potentials in SII No e f f e c t o f p e n t o b a r b i t a l s o d i u m in doses o f less t h a n 16 m g / k g o n t h e positive a n d negative phases o f t h e p o t e n t i a l s e v o k e d b y CTP s t i m u l a t i o n was observed, e x c e p t for an increase in a m p l i t u d e o f t h e negative p h a s e b y t h e 8 m g / k g dose. T h e a m p l i t u d e s o f t h e positive p h a s e o f t h e e v o k e d p o t e n t i a l b y I T P s t i m u l a t i o n w e r e increased b y less t h a n 16 m g / k g o f p e n t o b a r b i t a l s o d i u m . T h e amplit u d e s o f t h e negative p h a s e o f t h e e v o k e d

350

Y. S H I G E N A G A , R. INOKI

potential elicited by ITP stimulation were also increased by 4 mg/kg of pentobarbital sodium, but not altered by 8 and 16 mg/kg. Following CTP and ITP stimulations, pentobarbital sodium in an anesthetic dose of 32 mg/kg inhibited the amplitudes of both negative and positive phases of the evoked potentials. The negative phase was more markedly inhibited than the positive one. The latency was not altered by the administration of pentobarbital sodium. The transcallosal potentials were enhanced by doses of 4, 8, 16 and 32 mg/kg of pentobarbital sodiufn. 3.3. Effect o f morphine on the evoked potentials in S I The negative and positive phases of the evoked potentials elicited by CTP stimulation were inhibited by morphine in doses of 2.5, 5 and 10 mg/kg. The negative phase of the evoked potentials elicited by ITP stimulation, on the contrary, was increased by morphine in doses of 5 and 10 mg/kg. The transcallosal potentials were also slightly enhanced by mor-

phine in doses of 2.5, 5 and 10 mg/kg. Latencies of any evoked potentials elicited by CTP and ITP stimulations were not altered by morphine. These results are shown in table 1. 3.4. Effect o f pentobarbital sodium on the evoked potentials in S I Pentobarbital sodium in doses of 4, 8 and 16 mg/kg inhibited the amplitudes of the negative phase of the potentials elicited by CTP stimulation, but increased those of the positive and negative phases of the evoked potentials elicited by ITP stimulation. Pentobarbital sodium in a dose of 32 mg/kg markedly inhibited the positive and negative phases of the potentials evoked by CTP stimulation and the negative phase of the potentials evoked by ITP stimulation. In the latter case, inhibition of the negative phase of the potential evoked by CTP stimulation was more persistent than that by ITP stimulation. The transcallosal potentials were enchanced by pentobarbital sodium in doses of 4, 8, 16 and 32 mg/kg. Latencies of the positive and negative phase

TABLE 1 Effect o f m o r p h i n e o n t h e a m p l i t u d e o f t h e cortical evoked p o t e n t i a l s r e c o r d e d f r o m SII and SI. Recording sites

Doses (mg/kg)

P e r c e n t change in t h e peak a m p l i t u d e * at stimulating site ** o f CTP

ITP

CXC

PA * * *

NA ***

NA

PA + N A

SII

2.5 5 10

--25+- 9 ¢ ¢ ( s ) ¢ - - 3 6 ± 8 ¢¢ --47 ± 10 ¢¢

- - 3 0 ± 5 ¢¢ (s) - - 4 2 ± 6 ¢¢ --63 ± 10 ¢¢

--17 ± 9 ¢ ¢ ( s ) - - 4 0 ± 8 ¢¢ - - 5 6 ± 8 ¢¢

+8++5± +3 ±

SI

2.5 5 10

- - 3 0 ± 10 ¢ ¢ ( s ) t --40-+ 6 t ¢ --47 ± 8 ¢¢

- - 2 2 ± 5 ¢¢ (s) - - 4 4 ± 14 ? ? - - 5 5 ± 11 ¢¢

--10± 6~(3) +16 ± 4 .¢¢ +20 +_.7 ¢¢

+10± 6 ¢¢(s) +20 ± 1 6 t t +25 ± 8 ¢¢

*

9(s) 5 8

Mean p e r c e n t change (± s t a n d a r d deviation) in average a m p l i t u d e o f 20 r e s p o n s e s b e f o r e and after drug a d m i n i s t r a t i o n . + and - - indicate increase and decrease in a m p l i t u d e respectively. ** CTP, ITP and CXC designate c o n t r a l a t e r a l t o o t h pulp, ipsilateral t o o t h pulp and c o n t r a l a t e r a l c o r t e x to the recording site. * * * PA and N A designate a m p l i t u d e o f surface positive and negative phases o f t h e cortical evoked p o t e n t i a l . t N u m b e r s in parentheses indicate n u m b e r o f animals. tt Significant inhibition or e n h a n c e m e n t (p < 0.05), as calculated by S t u d e n t ' s t-test.

MORPHINE AND TOOTH PULP STIMULATION

351

TABLE 2 Effect of pentobarbital sodium on the amplitude of the cortical evoked potentials recorded from SII and SI. See table 1 for explanation of superscripts. Recording site

Doses (mg/kg)

Percent change in the peak amplitude * at stimulating site ** of CTP

ITP

PA ***

NA ***

PA

NA

PA + NA

+20-+ 9"r'~(s) +23± 16¢'r +20+- 1 5 ¢ ¢ --]4+- 8¢'i"

+15-+ 12"t'~(s) +5+ 6 --3+- 9 --33+- 9 ¢¢

+24++16 + +24+ +26 +

12"t"r (s) 6 ¢¢ 12"i"i" 13 ¢¢

+4± 3 ¢ ¢ (s) +13-+ 7"¢¢ +21-+ 13 ¢¢ +2-+ 16

+14+ 8"l"~(s) +28+ 1 6 ¢ ¢ +10+- 11 --40+- 5 ¢ ' t

+15 + +16++16-+ +18-+

5"r'~(s) 8 ¢¢ 9 ¢¢ 4 ¢'~

SII

4 8 16 32

--9+ 8 ( s ) ¢ +2+- 18 --3-+ 21 --27+ 13¢¢

--1-+ +21+--3+ --58+

SI

4 8 16 32

--4-+ --10-+ --9-+ --48-+

- - 2 0 + 6 "~¢(s) --15-+ 13 --21-+ 9 ¢ ¢ --55-+ 10"i'¢

A

CXC

B

12(s)'l" 15 17 18 ¢¢

C

10(s) 15"r¢ 10 lO¢'t

D

E

2 3

Fig. 3. Effect of pentobarbital sodium on SII and SI potentials following CTP and ITP stimulations and on transcallosal potential in SI. A and B indicate SII potentials following CTP and ITP stimulations respectively. C and D indicate SI potentials following CTP and ITP stimulations respectively. E indicates transcallosal potentials in SI. Uppermost tracings of each column are controls. Tracings 2, 3, 4 and 5 of each column are 30 min after pentobarbital sodium in doses of 4, 8, 16 and 32 mg/kg, respectively. All records are made up of 20--30 superimposed tracings. A positive phase of potentials indicates upward deflexion. Time mark is 5 msec.

352

of the potentials evoked by CTP and ITP stimulations were prolonged by the 32 mg/kg dose of pentobarbital sodium. These results are presented in table 2. An example of the actual potentials is shown in fig. 3.

4. Discussion

It has been demonstrated that t o o t h pulp afferents project into two regions in the cortex, corresponding to somatic sensory face areas I and II (SI and SII) as found by other workers (Kerr et al., 1955; Melzack and Haugen, 1957; Andersson et al., 1973; DarianSmith et al., 1966). It has also been demonstrated that cortical potentials elicited by ITP stimulation completely disappeared after removal of the contralateral cortex (Shigenaga, 1971). This fact implied that impulses of the ITP afferents projected to SI and SII via the contralateral cortex. Morphine showed an inhibitory action on the SI potentials evoked by CTP stimulation, b u t enhanced the SI potentials evoked by ITP stimulation. These results may imply that the cortical neurones receiving the transcallosal input are stimulated by morphine. However, morphine inhibited the SII potentials evoked by CTP and ITP stimulations; this inhibition had a significant dose--response relationship. This suggests that the SII area is a more important site for the analgesic mechanism than is SI. These results agreed with Mizoguchi's (Mizoguchi, 1964) and Melzack's reports (Melzack and Haugen, 1957), which suggested an existence of two kinds of afferents in the classical pathway, and that one sensitive to both nitrogen oxide and morphine, was relevant to pain sensation. From the findings of no depression by 2 mg/ kg of morphine on the cortical potentials evoked by the pulp stimulation in the dog, Chin and Domino (1961) concluded that the neural mechanisms of morphine analgesia for tooth pulp pain were complex and a depression or blockade of afferent impulses arising from the peripheral t o o t h pain receptor could not simply explain the analgesic mechanism

Y. SHIGENAGA, R. INOKI

of morphine. However, Chin and Domino did not try to examine the effects of morphine on SII potentials. Because of the recording site selected, no morphine depression might be conceivable. Soto Moyano et al. ( 1 9 7 5 ) a l s o reported that morphine in dose of 5 mg/kg did not modify the evoked potential registered in the primary somesthetic cortical area following electrical stimulation of the contralateral upper lip in the guinea pig. Mizoguchi (1964) reported that morphine in doses of 2--6 mg/kg depressed the evoked potentials on the trigeminal afferent via the spinal trigeminal complex following single stimulation of the maxillar canine pulp in the dog, while morphine did not depress those via the trigeminal principal sensory nucleus. It is well known that a small dose of barbiturate enhanced the primary response, as a result of a release phenomenon due to an inhibitory action on the ascending or descending inhibitory systems of the reticular formation (Dempsey et al., 1941). In the present study, it was observed that pentobarbital sodium in a sub-anesthetic dose scarcely inhibited SI potentials evoked by CTP stimulation, b u t enhanced those by ITP stimulation. In contrast, SII potentials evoked by CTP and ITP stimulations were rather enhanced by the sub-anesthetic doses of pentobarbital sodium. These results indicate that central depressant drugs of both morphine and barbiturate types have similar effects on SI responses, but different effects on SII responses. It can be stated that impulses projecting to SII are especially affected by morphine and that SII plays a significant role in the analgesic mechanism.

References Andersson, S.A., O. Keller and L. Vyklick~, 1973, Cortical activity evoked from tooth pulp afferent, Brain Res. 50, 473. Chin, J.H. and E.F. Domino, 1961, Effects of morphine on brain potentials evoked by stimulation of the tooth pulp of the dog, J. Pharmacol. Expti. Therap. 132, 74.

MORPHINE AND TOOTH PULP STIMULATION Darian-Smith, I., J. Isbister, H. Mok and T. Yokota, 1966, Somatic sensory cortical projection areas excited by tactile stimulation of the cat: a triple representation, J. Physiol. (London) 182, 671. Dempsey, E.W., R.S. Morison and B.R. Morison, 1941, Some afferent diencephalic pathways related to cortical potentials in the cat, Amer. J. Physiol. 131,718. Fujita, S., M. Yasuhara, S. Yamamoto and K. Ogiu, 1953, Studies on sites of action of analgesics. 1. The effect of analgesics on afferent pathways of several nerves, Jap. J. Pharmacol. 3, 27. Fujita, S., M. Yasuhara, S. Yamamoto and K. Ogiu, 1954, Studies on sites of action of analgesics. 2. The effect of analgesics on afferent pathways o f pain, Jap. J. Pharmacol. 4, 41. Kerr, D.I.B., F.P. Haugen and R. Melzack, 1955, Responses evoked in the brain stem by tooth stimulation, Amer. J. Physiol. 183, 253. Melzack, R. and F.P. Haugen, 1957, Responses evoked at the cortex by tooth stimulation, Amer. J. Physiol. 190, 570. Mizoguchi, K., 1964, The sites of action of morphine and the antagonistic action of levallorphan on the

353 central nervous system of the dog, Folia Pharmacol. Jap. 60, 326. Shigenaga, Y., 1971, Pain pathways from the tooth pulp of the rats and effects of morphine and barbiturate, Folia Pharmacol. Jap. 67, 22. Shigenaga, Y., S. Matano, K. Okada and A. Sakai, 1973, The effects of tooth pulp stimulation in the thalamus and hypothalamus of the cat, Brain Res. 63, 402. Shigenaga, Y., S. Matano, M. Kusuyama and A. Sakai, 1974, Cortical neurons responding to electrical stimulations of the rat's incisor pulp, Brain Res. 67,153. Soto Moyano, R., D. Kayser, Y. Grall, A. Hernandez, S. Ruiz and C. Paeile, 1975, Effect of pentazocine on the evoked potentials recorded in the primary somesthetic cortical areas o f guinea pigs, Brain Res. 88, 475. Straw, R.N. and C.L. Mitchell, 1964, The effects of morphine, pentobarbital, pentazocine and nalorphine on bioelectrical potentials evoked in the brain stem of the cat by electrical stimulation of the tooth pulp, J. Pharmacol. Exptl. Therap. 146, 7.