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Effects of intrathecal naloxone and atropine on the nociceptive suppression induced by norepinephrine and serotonin at the spinal level in rats
BRAIN RESEARCH ELSEVIER
Brain Research 666 (1994) 113-116
Short communication
Effects of intrathecal naloxone and atropine on the nociceptive suppression induced by norepinephrine and serotonin at the spinal level in rats Yong-Jie Li, Zhi-Hua Zhang, Jin-Yuan Chen, Jian-Tian Qiao * Department of Neurobiology, Shanxi Medical College, Taiyuan, Shanxi 030001, People's Republic of China Accepted 13 September 1994
Abstract
The interrelations among norepinephrine (NE), serotonin (5-HT), opiate-like substances (OLS), and acetylcholine (ACh) were investigated by using electrophysiological method combining with intrathecal (i.t.) injection. The results show that: (1) pretreatment with i.t. naloxone (Nal) completely reversed the NE-induced suppression of nociceptive discharges in parafascicular (PF) neurons, but partially reversed that of induced by i.t. 5-HT; (2) pretreatment with i.t. atropine (Atr) completely reversed the suppression induced by either NE or 5-HT. The results suggest that OLS may act as a necessary mediator for NE-induced suppression on the spinal transmission of nociceptive inputs, while it is only partially involved in the 5-HT-induced suppression, and moreover, that endogenous ACh is necessary for the performance of nociceptive suppression induced by either spinal NE or 5-HT administration.
Keywords: Parafascicular neuron; Nociceptive suppression; Intrathecal administration; Norepinephrine; Serotonin; Naloxone; Atropine
It has been well established that norepinephrine (NE), serotonin (5-HT), and opiate-like substances (OLS) are involved in the brainstem descending inhibitory mechanisms of nociceptive transmission at the spinal level [2,6]. Recently, ACh is also suggested to be involved in the spinal processing of nociceptive information [4,7,15]. Meanwhile, there is mounting evidence showing interrelations among these neuroactive substances, though the results are equivocal and controversial [5,6,8,16]. The main objective of the present study is to investigate further the role of OLS and ACh in the NE- or 5-HT-induced antinociceptive action at the spinal level. The problem was approached by using a experimental model in which the peripherally evoked nociceptive discharges were recorded in PF neurons in the medial thalami, while related agonists and antagonists were intrathecally injected. In this experimental model, the changes in the nociceptive responses in PF
* Corresponding author. Fax: (86) 0351-40 43 658. 0006-8993/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 0 0 6 - 8 9 9 3 ( 9 4 ) 0 1 0 8 5 -4
will reflect the effects of drugs on the spinal relay circuitry as a whole for the noxious input processing. Male Wistar rats (250-350 g) were anesthetized with sodium pentobarbital (50 mg/kg, i.p.) and i.t. catheters (PE-10) were implanted according to the procedure described by Yaksh and Rudy [14]. Upon completion of experiment, catheter placement was confirmed by pontamine sky blue injection (5 /zl, i.t.). NE bitartrate (Shanghai Tian-Feng Pharmaceutical Plant), 5-HT creatine sulfate (Ciba-Geigy), naloxone hydrochloride (Sigma) and atropine sulfate (Ciba-Geigy) were freshly prepared with normal saline (NS). Each drug was injected in a volume of 10 /zl followed by 10 /~1 NS to flush the catheter of its contents. The rate of injection was about 15/zl/min. The experiment was carried out 8-10 h after initial anesthesia and surgery. Those rats exhibiting any motor deficit after recovering from the surgical procedure were discarded. Rats were reanesthetized with urethane (1.0 g/kg, i.p.) and fixed in a stereotaxic frame. A glass micropipette recording electrode was driven into the PF area according to Paxinos and Watson's
Y.-J. IJ et a l . / Brain Research 666 (1994) 113 116
114
f
\
-100 b |~I,
~
o ~ -2°I
~,_...___~/~ ~ _ _ ~,_.___.._~._____~
~M~¢
_12oL , 0
o NS N S ( n : 6 ) m NS* N E ( n : 8 )
,
,
,
10
20
30 Time
, 40
50
60
(rain)
Fig. 1. T h e t i m e c o u r s e of effects of a d m i n i s t r a t i o n of N E (30 n m o l / 1 0 #xl, i.t., n = 8) or 5 - H T (60 n m o l / 1 0 pA, i.t., n = 9) o n the nociceptive d i s c h a r g e s in P F nociceptive-on neurons. T h e arrows d e n o t e the t i m e of d r u g a d m i n i s t r a t i o n . V a l u e s r e p r e s e n t m e a n + S.E.M. * P < 0.05; * * P < 0.01; * * * P < 0.001.
atlas with coordinates: P 4.0-4.3, L 0.9-1.2, H 5.0-6.5. The signals from the recording electrode were displayed on an oscilloscope and processed on line with ATAC-350 addscope to obtain frequency density histograms on an X - Y recorder. Noxious responses of PF neurons were elicited by high-intensity electrical stimulation (5-10 trains of pulses with 0.4 ms width, 100 Hz, 30-35 V) applied to the contralateral peroneal nerve. In addition, the nociceptive property of neuronal responses was confirmed by natural pain-inducing procedures (tail pinch, puncture of hind limb). At the conclusion of each experiment, pontamine sky blue was injected iontophoretically through the recording electrode to define the location of the last unit recorded in PF histologically. Statistical significance was deter-
mined by means of two-way analysis of variance (ANOVA) and Student's t-test. 92 PF neurons were recorded, and as reported previously [10], three types of neurons could be categorized according to their responses to noxious stimuli. In the present study, only 72 'nociceptive-on' neurons were selected for further observation. This group of neurons was characterized by a remarkable increase of the ongoing spontaneous firing within a few seconds following noxious stimulation. Nociceptive responses were quantified as the total number of spikes in 5 s immediately after the cease of peroneal stimulation and expressed as a percentage of the spontaneous firing control of each neuron. The average percentage increase evoked by noxious inputs was 256_+35% (mean +_ S.E.M). The spontaneous and nociceptive discharges before drug administration were recorded 3 times as control. The same protocol was conducted several times after drug administration at a 5 or 10 min interval. The drug-induced percentage change (DIPC) was calculated quantitatively by the following formula: DIPC = ( B - A ) / A × 100%. Here A and B represent the total numbers of nociceptive discharges in 5 s before and after drug administration, respectively. Administration of either NE (30 n m o l / 1 0 ~l, i.t., n - 8) or 5-HT (60 nmol/10/~1, i.t., n = 9) produced a significant suppression of the nociceptively evoked discharges in PF nociceptive-on neurons. The peak DIPC of 5-HT and NE, as estimated at 5 min after i.t. drug administration, was - 8 1 . 9 -+ 8.7% and - 9 2 . 3 + 5.8%, respectively ( P < 0.001), as shown in Fig. 1. The duration of suppression induced by NE or 5-HT lasted more than 60 rain. As control, i.t. administration of NS (10 #.d) exhibited no significant effect on the nociceptively evoked discharges of PF neurons (Fig. 1).
A
-
2C
2C
-20
-20
- 40
40 U
a_
-60
-60
-130
-80 o NoI÷NS(n : 7 ) • NS-NE(n=B) • NoI*NE ( n = 8 )
-100 !
t 2
-120 l
o
,b
i
20
3b
do
do
-100
• NS÷5-HT(n:9) • Nal +5-HT(n=14)
t -120 0
1~-
20
30
40
50
60
Time (mln~ Time ( m i n ) Fig. 2. Effects of p r e t r e a t m e n t with N a l (240 n m o l / 1 0 ~1, i.t.,) on the control NS injection ( 1 0 / x l , i.t., n = 7) and o n the inhibition i n d u c e d by i.t, N E (30 n m o l / 1 0 / x l , n = 8, (A) or 5 - H T (60 n m o l / 1 0 /~1, n = 14, (B) as c o m p a r e d with that of p r e t r e a t m e n t with NS.
Y.-J. Li et al./ Brain Research 666 (1994) 113-116
Pretreatment with Nal (240 nmol/10 txl, i.t., n = 8) completely antagonized the NE-induced suppression of nociceptive discharges in PF neurons as compared with that of pretreatment with NS ( P < 0.01, Fig. 2A). However, pretreatment with the same dose of Nal (n = 14) only partially antagonized the suppression induced by i.t. 5-HT (Fig. 2B). In another group of experiments, pretreatment with Atr (42 /zg/10 /zl, i.t.) completely antagonized the inhibition produced either by NE (30 nmol/10 ~1, i.t., n = 6) or by 5-HT (60 nmol/10/~1, i.t., n = 7) as shown in Fig. 3A and Fig. 3B, respectively. Atr (42/xg/10 tzl, i.t.) injection alone had no effects on the nociceptive responses in PF. The present study shows that i.t. administration of NE or 5-HT alone markedly suppresses the nociceptive discharges in PF neurons in response to peripheral noxious stimulation. Pretreatment with i.t. Nal completely antagonized the NE-induced suppression of nociceptive discharges, but partially antagonized the 5-HT-induced inhibition with the same Nal dosage. The role of OLS in antinociception produced by NE or 5-HT is not well documented in the literature. For instance, Fields [6] has proposed that the serotonergic descending inhibitory fibers act mainly via the spinal local enkephalinergic neurons, but some investigators do not agree with him [5,8,16]. The controversy could be interpreted by the present results that OLS only partially mediated 5-HT-induced inhibition, which are consistent closely with our previous report [10] that the suppressive effect on PF neurons produced by stimulation of the descending serotonergic fibers behaves differentially to i.t. administered Nal. Therefore, it is postulated that only a small fraction of descending serotonergic fibers exerts their inhibition onto the spinal nociceptive projecting neurons through a way of
115
OLS mediation, while most of them do not. From the data presented here, we postulate that NE-induced inhibition is mediated by OLS and this is supported by Yaksh's recent report that antinociception produced by i.t. a2-receptor agonist (clonidine) could be blocked by Nal [13]. ACh has also been considered playing an important role in the modulation of spinal nociceptive transmission [7,11]. For example, i.t. (or intraventricular) ACh or carbachol administration produces pronounced analgesia and this effect is reversible by Atr [12,15], while i.t. Atr powerfully reduced the analgesia produced by systemic morphine [4]. Our present study shows that ACh is also involved in the i.t. NE- or 5-HT-induced suppression of nociceptive transmission at the spinal level. For it is generally accepted that naturally NE and 5-HT are mostly released from the bulbospinal long-descending fibers at the spinal level [2,9], we are inclined to envisage that ACh might function as a successive link of NE (and OLS) or 5-HT to exhibit their modulatory actions at the spinal level. This postulation is supported by the studies showing that the cholingeric fibers in the dorsal horn originate mainly from local cholinergic neurons in the spinal cord [1,3]. However, another possibility also could not be excluded that some interactions among these neuroactive substances exist at the receptor level when they arrived at the same propriospinal or projecting neurons. This work was supported by the National Natural Science Foundation of China. [1] Barber, R.P., Phelps, P.E., Houser, C.R., Crawford, G.D. and Salvatetra, P.M., The morphology and distribution of neurons containing choline acetyltransferase in the adult rat spinal cord:
A
B 20
20
0 -20 ,
-
- 20i -4C
40 U Q.
U e_ -60 E) -80 -100 120
-6C -8C
t
Ii 0
1 2 o Atr+NS(n 6) • NS+NE(n:8) • AtF+NE(n:6)
f
-12(
2 1;
2'0
310 Time
(rain)
410
50'
o Atr÷NS(n :6) • NS÷5-HT(n:9)
-1OC
;0
t
1j
|" 0
2
• Air +5-HT(n:7) ~ 10
, 30
2'0 Time
, 40
J 50
GO
[rain)
Fig. 3. Effects of pretreatment with Atr ( 4 2 / ~ g / 1 0 / x l , i.t.) on the control and on the inhibition induced by i.t. NE (30 n m o l / 1 0 / x l , n = 6 (A), or 5-HT (60 n m o l / 1 0 / ~ l , n = 7 (B), as compared with that of pretreatment with NS.
116
[2] [3]
[4]
[5]
[6]
[7] [8]
[9] [10]
K-J. Li et al. / Brain Research 060 ~1994) 113 I In
an immunocytochemical study, J. ('omp. Neurol., 229 (1984) 329-346. Besson, J,M. and Chaouch, A., Peripheral and spinal mechanisms of nociception, Physiol. Rel~., 67 (1987) 67 186. Borges, L.F. and Iverson, S,D., Topography of choline acetyltransferase immunoreactive neurons and fibers in the rat spinal cord, Brain Res., 362 (1986) 140-148. Chiang, C.Y. and Zhuo, M., Evidence for the involvement of a descending cholinergic pathway in systemic morphine analgesia, Brain Res., 478 (1989) 393-300. Dubner, R., Ruda, M.A., Miletic, V., Hoffert, M.J. Bennett, G.J., Nishikawa, N. and Coffield, J., Neural circuitry mediating nociception in the medullary and spinal dorsal horns, Adv. Pain Res. Ther., 6 (1984) 151-166, Fields, H.L., Heinricher, M.M. and Mason, P., Neurotransmitters in nociceptive modulation circuits, Annu. Rei~. Neurosci., 14 (1991) 219-245. Green, P.G. and Kitchen, I., Antinociception of opioids and the cholinergic system, Prog. Neurobiol., 26 (1986) 119-146. Hammond, D.L. and Yaksh, T.L., Antagonism of stimulationproduced antinociception by intrathecal administration of methysergide or phentolamine, Brain Res., 298 (1984) 329-337. Jacobs, B.L. and Azmitia, E.C. Structure and function of the brain serotonin system, Physiol. Retd., 72 (1992) 165-229. Li, Y.J., Xie, Y.F. and Qiao, J.T., Effects of intrathecal
[11]
[12]
[13]
[14] [15]
[16]
monoamine antagonists and naloxone on the descending inhibition of the spinal transmission of noxious inpu! in rats: study with a new experimental model, Brain Res., 568 (1991) 131 - 137. Marshall, D.C. and Buccafusco, J.J., Spinal cholinergic neurons and the expression of morphine withdrawal symptolnS in the r~t, J. Neurosci., 7 (1987) 621--628~ Pedigo, N.W., Dewey, W.L. and Harris, L.S., l)eterminati~m and characterization of the antinociceptive activity of intraventricularly administered acetylcholine in the mice, J. Pharmacol. Exp. Ther., t93 (1975) 845-852. Takano, Y., Murata, K., lnoue, H., Takano, M., Sato, [. and Yaksh, T.L., Involvement of spinal opioid receptor in the antinociceptive effect of intrathecally administered alpha-2 adrenoceptor agonist, Abstr, 7th World Con.~res~" on Pain, 1993, P80. Yaksh, T.L. and Rudy, T.A., Chronic catheterization of the spinal subarachnoid space, Physiol. Behac., 17 (1976) 1031-1036. Yaksh, T.L., Dirksen, R. and Harty, G.J., Antinociceptive effects of intrathecally injected cholinomimetic drugs in the rat and cat, Eur. J. Pharmacol., 117 (1985) 81 88. Zhang, Z.H., Yang, S.W., Xie, Y.F. and Qiao, J.T., Effects ot naloxone and aminophylline on the antinociception induced by serotonin and norepinephrine at the spinal level: a study using EMG planimetry of flexor reflex in rats, Chin. J. Physiol. 5ci., 10 (1994) 139-145.
Brain Research 666 (1994) 113-116
Short communication
Effects of intrathecal naloxone and atropine on the nociceptive suppression induced by norepinephrine and serotonin at the spinal level in rats Yong-Jie Li, Zhi-Hua Zhang, Jin-Yuan Chen, Jian-Tian Qiao * Department of Neurobiology, Shanxi Medical College, Taiyuan, Shanxi 030001, People's Republic of China Accepted 13 September 1994
Abstract
The interrelations among norepinephrine (NE), serotonin (5-HT), opiate-like substances (OLS), and acetylcholine (ACh) were investigated by using electrophysiological method combining with intrathecal (i.t.) injection. The results show that: (1) pretreatment with i.t. naloxone (Nal) completely reversed the NE-induced suppression of nociceptive discharges in parafascicular (PF) neurons, but partially reversed that of induced by i.t. 5-HT; (2) pretreatment with i.t. atropine (Atr) completely reversed the suppression induced by either NE or 5-HT. The results suggest that OLS may act as a necessary mediator for NE-induced suppression on the spinal transmission of nociceptive inputs, while it is only partially involved in the 5-HT-induced suppression, and moreover, that endogenous ACh is necessary for the performance of nociceptive suppression induced by either spinal NE or 5-HT administration.
Keywords: Parafascicular neuron; Nociceptive suppression; Intrathecal administration; Norepinephrine; Serotonin; Naloxone; Atropine
It has been well established that norepinephrine (NE), serotonin (5-HT), and opiate-like substances (OLS) are involved in the brainstem descending inhibitory mechanisms of nociceptive transmission at the spinal level [2,6]. Recently, ACh is also suggested to be involved in the spinal processing of nociceptive information [4,7,15]. Meanwhile, there is mounting evidence showing interrelations among these neuroactive substances, though the results are equivocal and controversial [5,6,8,16]. The main objective of the present study is to investigate further the role of OLS and ACh in the NE- or 5-HT-induced antinociceptive action at the spinal level. The problem was approached by using a experimental model in which the peripherally evoked nociceptive discharges were recorded in PF neurons in the medial thalami, while related agonists and antagonists were intrathecally injected. In this experimental model, the changes in the nociceptive responses in PF
* Corresponding author. Fax: (86) 0351-40 43 658. 0006-8993/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 0 0 6 - 8 9 9 3 ( 9 4 ) 0 1 0 8 5 -4
will reflect the effects of drugs on the spinal relay circuitry as a whole for the noxious input processing. Male Wistar rats (250-350 g) were anesthetized with sodium pentobarbital (50 mg/kg, i.p.) and i.t. catheters (PE-10) were implanted according to the procedure described by Yaksh and Rudy [14]. Upon completion of experiment, catheter placement was confirmed by pontamine sky blue injection (5 /zl, i.t.). NE bitartrate (Shanghai Tian-Feng Pharmaceutical Plant), 5-HT creatine sulfate (Ciba-Geigy), naloxone hydrochloride (Sigma) and atropine sulfate (Ciba-Geigy) were freshly prepared with normal saline (NS). Each drug was injected in a volume of 10 /zl followed by 10 /~1 NS to flush the catheter of its contents. The rate of injection was about 15/zl/min. The experiment was carried out 8-10 h after initial anesthesia and surgery. Those rats exhibiting any motor deficit after recovering from the surgical procedure were discarded. Rats were reanesthetized with urethane (1.0 g/kg, i.p.) and fixed in a stereotaxic frame. A glass micropipette recording electrode was driven into the PF area according to Paxinos and Watson's
Y.-J. IJ et a l . / Brain Research 666 (1994) 113 116
114
f
\
-100 b |~I,
~
o ~ -2°I
~,_...___~/~ ~ _ _ ~,_.___.._~._____~
~M~¢
_12oL , 0
o NS N S ( n : 6 ) m NS* N E ( n : 8 )
,
,
,
10
20
30 Time
, 40
50
60
(rain)
Fig. 1. T h e t i m e c o u r s e of effects of a d m i n i s t r a t i o n of N E (30 n m o l / 1 0 #xl, i.t., n = 8) or 5 - H T (60 n m o l / 1 0 pA, i.t., n = 9) o n the nociceptive d i s c h a r g e s in P F nociceptive-on neurons. T h e arrows d e n o t e the t i m e of d r u g a d m i n i s t r a t i o n . V a l u e s r e p r e s e n t m e a n + S.E.M. * P < 0.05; * * P < 0.01; * * * P < 0.001.
atlas with coordinates: P 4.0-4.3, L 0.9-1.2, H 5.0-6.5. The signals from the recording electrode were displayed on an oscilloscope and processed on line with ATAC-350 addscope to obtain frequency density histograms on an X - Y recorder. Noxious responses of PF neurons were elicited by high-intensity electrical stimulation (5-10 trains of pulses with 0.4 ms width, 100 Hz, 30-35 V) applied to the contralateral peroneal nerve. In addition, the nociceptive property of neuronal responses was confirmed by natural pain-inducing procedures (tail pinch, puncture of hind limb). At the conclusion of each experiment, pontamine sky blue was injected iontophoretically through the recording electrode to define the location of the last unit recorded in PF histologically. Statistical significance was deter-
mined by means of two-way analysis of variance (ANOVA) and Student's t-test. 92 PF neurons were recorded, and as reported previously [10], three types of neurons could be categorized according to their responses to noxious stimuli. In the present study, only 72 'nociceptive-on' neurons were selected for further observation. This group of neurons was characterized by a remarkable increase of the ongoing spontaneous firing within a few seconds following noxious stimulation. Nociceptive responses were quantified as the total number of spikes in 5 s immediately after the cease of peroneal stimulation and expressed as a percentage of the spontaneous firing control of each neuron. The average percentage increase evoked by noxious inputs was 256_+35% (mean +_ S.E.M). The spontaneous and nociceptive discharges before drug administration were recorded 3 times as control. The same protocol was conducted several times after drug administration at a 5 or 10 min interval. The drug-induced percentage change (DIPC) was calculated quantitatively by the following formula: DIPC = ( B - A ) / A × 100%. Here A and B represent the total numbers of nociceptive discharges in 5 s before and after drug administration, respectively. Administration of either NE (30 n m o l / 1 0 ~l, i.t., n - 8) or 5-HT (60 nmol/10/~1, i.t., n = 9) produced a significant suppression of the nociceptively evoked discharges in PF nociceptive-on neurons. The peak DIPC of 5-HT and NE, as estimated at 5 min after i.t. drug administration, was - 8 1 . 9 -+ 8.7% and - 9 2 . 3 + 5.8%, respectively ( P < 0.001), as shown in Fig. 1. The duration of suppression induced by NE or 5-HT lasted more than 60 rain. As control, i.t. administration of NS (10 #.d) exhibited no significant effect on the nociceptively evoked discharges of PF neurons (Fig. 1).
A
-
2C
2C
-20
-20
- 40
40 U
a_
-60
-60
-130
-80 o NoI÷NS(n : 7 ) • NS-NE(n=B) • NoI*NE ( n = 8 )
-100 !
t 2
-120 l
o
,b
i
20
3b
do
do
-100
• NS÷5-HT(n:9) • Nal +5-HT(n=14)
t -120 0
1~-
20
30
40
50
60
Time (mln~ Time ( m i n ) Fig. 2. Effects of p r e t r e a t m e n t with N a l (240 n m o l / 1 0 ~1, i.t.,) on the control NS injection ( 1 0 / x l , i.t., n = 7) and o n the inhibition i n d u c e d by i.t, N E (30 n m o l / 1 0 / x l , n = 8, (A) or 5 - H T (60 n m o l / 1 0 /~1, n = 14, (B) as c o m p a r e d with that of p r e t r e a t m e n t with NS.
Y.-J. Li et al./ Brain Research 666 (1994) 113-116
Pretreatment with Nal (240 nmol/10 txl, i.t., n = 8) completely antagonized the NE-induced suppression of nociceptive discharges in PF neurons as compared with that of pretreatment with NS ( P < 0.01, Fig. 2A). However, pretreatment with the same dose of Nal (n = 14) only partially antagonized the suppression induced by i.t. 5-HT (Fig. 2B). In another group of experiments, pretreatment with Atr (42 /zg/10 /zl, i.t.) completely antagonized the inhibition produced either by NE (30 nmol/10 ~1, i.t., n = 6) or by 5-HT (60 nmol/10/~1, i.t., n = 7) as shown in Fig. 3A and Fig. 3B, respectively. Atr (42/xg/10 tzl, i.t.) injection alone had no effects on the nociceptive responses in PF. The present study shows that i.t. administration of NE or 5-HT alone markedly suppresses the nociceptive discharges in PF neurons in response to peripheral noxious stimulation. Pretreatment with i.t. Nal completely antagonized the NE-induced suppression of nociceptive discharges, but partially antagonized the 5-HT-induced inhibition with the same Nal dosage. The role of OLS in antinociception produced by NE or 5-HT is not well documented in the literature. For instance, Fields [6] has proposed that the serotonergic descending inhibitory fibers act mainly via the spinal local enkephalinergic neurons, but some investigators do not agree with him [5,8,16]. The controversy could be interpreted by the present results that OLS only partially mediated 5-HT-induced inhibition, which are consistent closely with our previous report [10] that the suppressive effect on PF neurons produced by stimulation of the descending serotonergic fibers behaves differentially to i.t. administered Nal. Therefore, it is postulated that only a small fraction of descending serotonergic fibers exerts their inhibition onto the spinal nociceptive projecting neurons through a way of
115
OLS mediation, while most of them do not. From the data presented here, we postulate that NE-induced inhibition is mediated by OLS and this is supported by Yaksh's recent report that antinociception produced by i.t. a2-receptor agonist (clonidine) could be blocked by Nal [13]. ACh has also been considered playing an important role in the modulation of spinal nociceptive transmission [7,11]. For example, i.t. (or intraventricular) ACh or carbachol administration produces pronounced analgesia and this effect is reversible by Atr [12,15], while i.t. Atr powerfully reduced the analgesia produced by systemic morphine [4]. Our present study shows that ACh is also involved in the i.t. NE- or 5-HT-induced suppression of nociceptive transmission at the spinal level. For it is generally accepted that naturally NE and 5-HT are mostly released from the bulbospinal long-descending fibers at the spinal level [2,9], we are inclined to envisage that ACh might function as a successive link of NE (and OLS) or 5-HT to exhibit their modulatory actions at the spinal level. This postulation is supported by the studies showing that the cholingeric fibers in the dorsal horn originate mainly from local cholinergic neurons in the spinal cord [1,3]. However, another possibility also could not be excluded that some interactions among these neuroactive substances exist at the receptor level when they arrived at the same propriospinal or projecting neurons. This work was supported by the National Natural Science Foundation of China. [1] Barber, R.P., Phelps, P.E., Houser, C.R., Crawford, G.D. and Salvatetra, P.M., The morphology and distribution of neurons containing choline acetyltransferase in the adult rat spinal cord:
A
B 20
20
0 -20 ,
-
- 20i -4C
40 U Q.
U e_ -60 E) -80 -100 120
-6C -8C
t
Ii 0
1 2 o Atr+NS(n 6) • NS+NE(n:8) • AtF+NE(n:6)
f
-12(
2 1;
2'0
310 Time
(rain)
410
50'
o Atr÷NS(n :6) • NS÷5-HT(n:9)
-1OC
;0
t
1j
|" 0
2
• Air +5-HT(n:7) ~ 10
, 30
2'0 Time
, 40
J 50
GO
[rain)
Fig. 3. Effects of pretreatment with Atr ( 4 2 / ~ g / 1 0 / x l , i.t.) on the control and on the inhibition induced by i.t. NE (30 n m o l / 1 0 / x l , n = 6 (A), or 5-HT (60 n m o l / 1 0 / ~ l , n = 7 (B), as compared with that of pretreatment with NS.
116
[2] [3]
[4]
[5]
[6]
[7] [8]
[9] [10]
K-J. Li et al. / Brain Research 060 ~1994) 113 I In
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