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Evidence against a serotonin involvement in the tonic descending inhibition of nociceptor-driven neurons in the cat spinal cord
Brain Research, 199 (1980) 225-230 © Elsevier/North-Holland Biomedical Press
225
Evidence against a serotonin involvement in the tonic descending inhibition of nociceptor-driven neurons in the cat spinal cord
PETER J. SOJA and JOHN G. SINCLAIR*
Division of Pharmacology and Toxicology, Faculty of Pharmaceutical Sciences, University of British Columbia, Vancouver, B.C. V6T 1 W5 ( can,~da)
(Accepted June 12th, 1980) Key words: spinal cord - - serotonin - - analgesia
To determine whether serotonin (5-HT) is involved in the powerful tonic descending inhibition which exists on dorsal horn nociceptor-driven neurons, their response to a noxious stimulus was tested with drugs that enhance (fluoxetine) or decrease (p-chlorophenylalanine) 5-HT synaptic activity. Neither the response with the spinal cord intact nor the enhanced response with the spinal cord cold blocked was altered by these drugs. Thus we conclude that 5-HT is not involved in this tonic descending inhibition. Substantial evidence exists that brain stem stimulation produced analgesia (SPA) is due, at least in part, to an activation of a descending inhibitory system which depresses transmission of nociceptive information at the spinal cord level. Several brain stem sites, particularly those in the ventrolateral periaqueductal gray (PAG), have been shown to elicit antinociceptive effects and inhibit nociceptor-driven dorsal horn neuronsl,15A 7. Sectioning the dorsolateral funiculi ( D L F ) of the spinal cord has been shown to reverse the SPA elicited by P A G simulation 2-4. Serotonin (5-HT) has been implicated in SPA since many of the SPA sites are rich in 5-HT neurons and 5-HT neurons descend in the DLF. Further support for a 5H T involvement in SPA is provided by the reports that p-chlorophenylalanine (pCPA), a tryptophan hydroxylase inhibitor 14, antagonizes the SPA in rats 1 as well as the inhibition of nociceptor-driven dorsal horn neurons elicited by P A G stimulation in cats 7. Similar effects are produced by LSD, presumably by altering 5-HT synaptic activity, although the mechanism is not clearg, 11. Evidence also exists that the opiates, at the brain stem level, activate a descending system involving 5-HT which elicits antinociception effects via the spinal cord. Microinjection of opiates into the P A G produces antinociceptionla, 2a and inhibition of spinal cord nociceptor-driven neurons 5. The antinociception is antagonized by D L F lesions a-4 or intrathecal administration of the putative 5-HT blocker, methysergide 2a. Recently Yaksh and Tyce 25 have shown that microinjection of morphine into the P A G of rats produces a release of 5-HT from the spinal cord. * To whom all correspondence should be addressed.
226
Several investigators have shown that dorsal horn neurons of the cat spinal cord, which respond to a noxious stimulus applied to their receptive field, are under a powerful tonically active descending inhibition from the brain stem 6,1°,'~1. It is not known, however, whether the system(s) activated by SPA or opiates as described above is the same as that producing the tonic inhibition. Therefore, the present study was initiated to determine whether 5-HT is involved in the tonic descending inhibition of nociceptor-driven neurons. These experiments were performed on 18 cats of either sex (2.0-5.0 kg) which were initially anesthetized with a halothane-oxygen mixture and subsequently maintained with chloralose (60 mg/kg i.v.). The surgical preparation of the animals was essentially the same as that described by Handwerker et al. 10. Following a laminectomy between 1-1 and $1, a thermode shaped to fit the cord was positioned at L2-L4 for applying a cold block to the spinal cord. The hind limbs were shaved and the feet fastened with plaster to a wooden platform with the toe pads facing up. The blood pressure and end-tidal CO,) levels were continuously monitored, the latter being maintained between 4.0 and 4.5~. A bilateral pneumothorax was performed to minimize spinal cord movement and the animal was paralyzed with Flaxedil and artificially respired. The body temperature and spinal cord oil pool temperature were automatically maintained at 37 °C. To produce the spinal cold block, a refrigerant with a reservoir temperature of - - 5 °C was circulated through the thermode placed on the spinal cord. The effectiveness of the block was initially determined by noting the disappearance of the surface N-wave following stimulation of the dorsal columns rostral to the thermode 1°. A fall in blood pressure of about 25 mm Hg was found to be a good indication of a block. Single unit extracellular activity in the L7 region was recorded in the conventional manner using glass microelectrodes filled with 2.0 ~ Pontamine sky blue in 0.5 M sodium acetate which permitted marking of recording sites 12. A search stimulus (l. 5 V, 0.1 msec, 1.0 Hz) was applied to the tibial nerve or L7 dorsal root. Neurons encountered were initially classified according to their response to natural stimuli. Some cells responded only to non-noxious forms of stimuli to their receptive field, such as touch or hair movement. Others responded to this type of stimuli but also elicited a sustained response while maintaining a noxious pinch to the receptive field. Most of the latter group, referred to as wide dynamic range neurons, were also activated by noxious radiant heat (47-53 °C) and were located in lamina V with some overlap into laminae IV and VI. The noxious radiant heat was applied for 10-15 sec at 2-3 rain intervals by means of a focused halogen projector lamp with feedback controP °. Fluoxetine hydrochloride, a specific 5-HT neuronal uptake blocker 8,2~, was tested on neurons which responded consistently to a series of noxious radiant heat pulses. This drug was tested in doses of 4-6 mg/kg (1 cell at 4 mg/kg; 7 cells at 6 mg/kg), slowly administered, using only one cell per animal. We found fluoxetine to be a very potent agent in blocking 5-HT synaptosomal uptake xg. Furthermore, we found the dose range used in these experiments to have pronounced effects on other systems involving 5-HT in the cat 18 and rabbit :tg. If 5-HT is involved in the tonic descending
227
O
o
o I
I
20Sec
Fig. 1. The effect of fluoxetine (4 mg/kg i.v.) on the response of a dorsal horn neuron activated by noxious radiant heat (H) of 50 °C. A: control responses. B: 6 and 9 min after initiating the infusion of fluoxetine. C: 20 and 24 min after completing the injection.
inhibition on these neurons one would expect fluoxetine to enhance this inhibition and thus decrease the response to noxious radiant heat. However, fluoxetine failed to influence the response to noxious radiant heat in the majority of cells tested (Figs. 1 and 2) and decreased the response on only one cell.
o
"E o U "62OO-
c 1500 a. a: 1 0 0 "i-
z
0
6mg/kg
Fluoxetine
5o0 × 0
I
I
I
-5
0
5
I
i
10 15 Time (rain)
I
20
25
%
Fig. 2. The effect of fluoxetine (6 mg/kg i.v.) on the responses of dorsal horn neurons to noxious radiant heat. Fluoxetine was infused over the time period indicated by the bar. The numbers of discharges were counted over a 25 sec period beginning at the start of a 10-15 see radiant heat pulse. This value at time zero was equated to 100700. Values before and after time zero were calculated based on this figure. Each point represents the mean 4- S.E.M., n = 7.
228 The other approach taken was to determine the extent of tonic inhibition on these nociceptor-driven neurons in non-pretreated andp-CPA pretreated animals. The 5 animals receiving p-CPA were pretreated 48 and 24 h prior to recording with a dose of" 300 mg/kg i.p. This dose was reported to decrease cat spinal cord levels by 90 ~(i as well as producing hind limb ataxia, poverty of movement and prolonged meowing ')0. These symptoms were present in all thep-CPA treated animals used in the present study. We have also previously reported that this dose of p-CPA was very effective in antagonizing other 5-HT systems in cats TM. In both groups of animals, neurons which responded consistently to noxious radiant heat pulses were also tested following the application of a spinal cold block. Responses to the noxious heat were again recorded following rewarming of the cord. If 5-HT is involved in the tonic descending inhibition, one would expect to see a decrease in the tonic inhibition in the p-CPA pretreated animals as reflected by little difference in the response to noxious radiant heat in the intact vs the cold blocked state of the spinal cord. However, Fig. 3 clearly shows that cold blocking the spinal cord results in an equivalent increase in the response to noxious radiant heat in both groups of animals. Our findings confirm the existence of strong tonic descending inhibition on spinal cord nociceptor-driven neurones. Of the 39 cells tested after cold blocking the spinal cord, 37 showed a markedly increased response to noxious radiant heat. Two neurons, however, exhibited a decreased response following the cold block, an observation also made by Wall el on one cell. However, the results strongly suggest that 5oHT is not involved in the tonic descending inhibition which exists on nociceptor-driven spinal cord neurons. Therefore, if 5-HT is involved in the descending systems activated by SPA or opiates they would appear not to be the same as the tonically active descending system which impinges on these neurons. Consistent with this notion are reports that sectioning the spinal cord DLF a or pretreatment with p-CPA 1 antagonize SPA but have little or no effect in altering the baseline latency of the tailflick response. Similarly, Yaksh et al. 24 T
1500. .
I
CONTROL n=21
I
p-CPA n =16
O 1000.
500,
0" INTA~-I-
COLD BLOGK
Fig. 3. A comparison of the responses of dorsal horn neurones to noxious radiant heat with the spinal cord intact or reversibly cold blocked in non-pretreated and p-CPA pretreated cats.
229 f o u n d t h a t lesioning the r a p h e nuclei failed to alter the nociceptive t h r e s h o l d in the tail flick a n d h o t plate tests a l t h o u g h they a t t e n u a t e d the effects o f m o r p h i n e . F u r t h e r more, M a l e c a n d L a n g w i n s k i 16 r e p o r t e d t h a t fluoxetine d i d n o t alter the h o t plate r e a c t i o n time in rats a l t h o u g h it did p o t e n t i a t e the effects o f m o r p h i n e . These studies i m p l y t h a t the t o n i c descending inhibition is n o t influenced by these treatments. F u r t h e r w o r k is n e e d e d to clarify the involvement o f 5 - H T in descending systems m o d u l a t i n g nociceptive p a t h w a y s at the spinal c o r d level. This w o r k was s u p p o r t e d b y the M e d i c a l Research C o u n c i l o f C a n a d a a n d the B. C. H e a l t h Care Research F o u n d a t i o n . 1 Akil, H. and Mayer, D.J. Antagonism of stimulation-produced analgesia by p-CPA, a serotonin synthesis inhibitor, Brain Research, 44 (1972) 492-497. 2 Basbaum, A.I., Clanton, C.H. and Fields, H.L., Opiate and stimulation-produced analgesia: functional anatomy of a medullospinal pathway, Proc. nat. Acad. Sci. (Wash.), 73 (1976) 4685-4688. 3 Basbaum, A.I., Marley, N. and O'Keefe, J., Spinal cord pathways involved in the production of analgesia by brain stimulation. In J.J. Bonica and D. Albe-Fessard (Eds.) Advances in Pain Research and Therapy, Vol. 1. Raven Press, New York, 1976, pp. 511-515. 4 Basbaum, A.I., Marley, N.J.E., O'Keefe, J., and Clanton, C.H., Reversal of morphine and stimulus-produced analgesia by subtotal spinal cord lesions, Pain, 3 (1977) 43-56. 5 Bennett, G.J. and Mayer, D.J., Inhibition of spinal cord interneurons by narcotic microinjection and focal electrical stimulation in the periaqueductal gray matter, Brain Research, 172 (1979) 243-257. 6 Brown, A.G., Effects of descending impulses on transmission through the spinocervical tract, J. Physiol. (Lond.), 219 (1971) 103-125. 7 Carstens, E. and Zimmermann, M., Pharmacologically distinct systems in medial and lateral midbrain mediating descending inhibition of spinal nociceptive transmission in the cat, Neurosci. Abstr., 5 (1979) 607. 8 Fuller, R.W., Perry, K.W. and Molloy, B.B., Effect of 3-(p-trifluoromethyl-phenoxy)-N-methyl3-phenylpropylamine on the depletion of brain serotonin by 4-chloroamphetamine, J. Pharmacol. exp. Ther., 193 (1975) 796-803. 9 Guilbaud, G., Besson, J.-M., Oliveras, J.-L. and Liebeskind, J.C. Suppression by LSD of the inhibitory effect exerted by dorsal raphe stimulation on certain spinal cord interneurons in the cat, Brain Research, 61 (1973), 417-422. 10 Handwerker, H.O., Iggo, A. and Zimmermann, M., Segmental and supraspinal actions on dorsal horn neurons responding to noxious and non-noxious skin stimuli, Pain, 1 (1975) 147-165. 11 Hayes, R.L., Newlon, P.G., Rosecrans, J.A. and Mayer, D.J., Reduction of stimulation-produced analgesia by lysergic acid diethylamide, a depressor of serotonergic neural activity, Brain Research, 122 (1977) 367-372. 12 Hellon, R.F., The marking of electrode tip positions in nervous tissue, J. Physiol. (Lond.), 214 (1971) 12P. 13 Jacquet, Y. and Lajtha, A., Paradoxical effects after microinjection of morphine in the periaqueductal gray matter in the rat, Science, 185 (1974) 1055-1057. 14 Koe, B.K. and Weissman, A., p-Chlorophenylalanine: a specific depletor of brain serotonin, J. Pharmacol. exp. Ther., 154 (1966) 499-516. 15 Liebeskind, J.C., Guilbaud, G., Besson, J.-M. and Oliveras, J.-L., Analgesia from electrical stimulation of the periaqueductal gray matter in the cat: behavioral observations and inhibitory effects on spinal cord interneurons, Brain Research, 50 (1973) 441-446. 16 Malec, D. and Langwinski, R., Effect of quipazine and fluoxetine on analgesic-induced catalepsy and antinociception in the rat, J. Pharm. Pharmacol., 32 (1980) 71-73. 17 Mayer, D.J., Wolfe, T.L., Akil, H., Carder, B. and Liebeskind, J.C., Analgesia from electrical stimulation in the brainstem of the rat, Science, 174 (1971) 1351-1354. 18 Sastry, B.S.R. and Sinclair, J.G., Tonic inhibitory influence of a supraspinal monoaminergic system on presynaptic inhibition of an extensor monosynaptic reflex, Brain Research, 124 (1977) 109-120.
230 19 Sinclair, J.G. and Lo, G.F., The blockade of serotonin uptake into synaptosomes: relationship to an interaction with monoamine oxidase inhibitors, Canad. J. Physiol. Pharmacol., 55 (1977) 180-187. 20 Taber, C.A. and Anderson, E.G. Paradoxical blockade by p-chlorophenylalanine of 5-hydroxytryptophan facilitatory actions on spinal reflexes, J. Pharmacol. exp. Ther., 187 (1973) 229-238. 21 Wall, P.D., The laminar organization of dorsal horn and effects of descending impulses, J. Physh, l. (Lond.), 188 (1967) 403-423. 22 Wong, D.T., Bymaster, F.P. Horng, J.S. and Molloy, B.B., A new selective inhibitor for uptake of serotonin into synaptosomes of rat brain: 3-(p-trifluoromethylphenoxy)-N-methyl-3-phenylpropylamine, J. Pharmacol. exp. Ther., 193 (1975) 804-811. 23 Yaksh, T.L., Direct evidence that spinal serotonin and noradrenaline terminals mediate the spinal antinociceptive effects of morphine in the periaqueductal gray, Brain Research, 160 (1979) 180-185. 24 Yaksh, T.L., Plant, R.L. and Rudy, T.A., Studies on the antagonism by raphe lesions of the antinociceptive actions of systemic morphine, Europ. J. PharmacoL, 41 (1977) 399-408. 25 Yaksh, T.L. and Tyce, G.M. Microinjection of morphine into the periaqueductal gray evokes the release of serotonin from the spinal cord, Brain Research, 171 (1979) 176-181. 26 Yaksh, T.L., Yeung, J.C. and Rudy, T.A., Systemic examination in the rat of brain sites sensitive to the direct application of morphine: observation of differential effects within the periaqueductal gray, Brain Research, 114 (1976) 83-103.
225
Evidence against a serotonin involvement in the tonic descending inhibition of nociceptor-driven neurons in the cat spinal cord
PETER J. SOJA and JOHN G. SINCLAIR*
Division of Pharmacology and Toxicology, Faculty of Pharmaceutical Sciences, University of British Columbia, Vancouver, B.C. V6T 1 W5 ( can,~da)
(Accepted June 12th, 1980) Key words: spinal cord - - serotonin - - analgesia
To determine whether serotonin (5-HT) is involved in the powerful tonic descending inhibition which exists on dorsal horn nociceptor-driven neurons, their response to a noxious stimulus was tested with drugs that enhance (fluoxetine) or decrease (p-chlorophenylalanine) 5-HT synaptic activity. Neither the response with the spinal cord intact nor the enhanced response with the spinal cord cold blocked was altered by these drugs. Thus we conclude that 5-HT is not involved in this tonic descending inhibition. Substantial evidence exists that brain stem stimulation produced analgesia (SPA) is due, at least in part, to an activation of a descending inhibitory system which depresses transmission of nociceptive information at the spinal cord level. Several brain stem sites, particularly those in the ventrolateral periaqueductal gray (PAG), have been shown to elicit antinociceptive effects and inhibit nociceptor-driven dorsal horn neuronsl,15A 7. Sectioning the dorsolateral funiculi ( D L F ) of the spinal cord has been shown to reverse the SPA elicited by P A G simulation 2-4. Serotonin (5-HT) has been implicated in SPA since many of the SPA sites are rich in 5-HT neurons and 5-HT neurons descend in the DLF. Further support for a 5H T involvement in SPA is provided by the reports that p-chlorophenylalanine (pCPA), a tryptophan hydroxylase inhibitor 14, antagonizes the SPA in rats 1 as well as the inhibition of nociceptor-driven dorsal horn neurons elicited by P A G stimulation in cats 7. Similar effects are produced by LSD, presumably by altering 5-HT synaptic activity, although the mechanism is not clearg, 11. Evidence also exists that the opiates, at the brain stem level, activate a descending system involving 5-HT which elicits antinociception effects via the spinal cord. Microinjection of opiates into the P A G produces antinociceptionla, 2a and inhibition of spinal cord nociceptor-driven neurons 5. The antinociception is antagonized by D L F lesions a-4 or intrathecal administration of the putative 5-HT blocker, methysergide 2a. Recently Yaksh and Tyce 25 have shown that microinjection of morphine into the P A G of rats produces a release of 5-HT from the spinal cord. * To whom all correspondence should be addressed.
226
Several investigators have shown that dorsal horn neurons of the cat spinal cord, which respond to a noxious stimulus applied to their receptive field, are under a powerful tonically active descending inhibition from the brain stem 6,1°,'~1. It is not known, however, whether the system(s) activated by SPA or opiates as described above is the same as that producing the tonic inhibition. Therefore, the present study was initiated to determine whether 5-HT is involved in the tonic descending inhibition of nociceptor-driven neurons. These experiments were performed on 18 cats of either sex (2.0-5.0 kg) which were initially anesthetized with a halothane-oxygen mixture and subsequently maintained with chloralose (60 mg/kg i.v.). The surgical preparation of the animals was essentially the same as that described by Handwerker et al. 10. Following a laminectomy between 1-1 and $1, a thermode shaped to fit the cord was positioned at L2-L4 for applying a cold block to the spinal cord. The hind limbs were shaved and the feet fastened with plaster to a wooden platform with the toe pads facing up. The blood pressure and end-tidal CO,) levels were continuously monitored, the latter being maintained between 4.0 and 4.5~. A bilateral pneumothorax was performed to minimize spinal cord movement and the animal was paralyzed with Flaxedil and artificially respired. The body temperature and spinal cord oil pool temperature were automatically maintained at 37 °C. To produce the spinal cold block, a refrigerant with a reservoir temperature of - - 5 °C was circulated through the thermode placed on the spinal cord. The effectiveness of the block was initially determined by noting the disappearance of the surface N-wave following stimulation of the dorsal columns rostral to the thermode 1°. A fall in blood pressure of about 25 mm Hg was found to be a good indication of a block. Single unit extracellular activity in the L7 region was recorded in the conventional manner using glass microelectrodes filled with 2.0 ~ Pontamine sky blue in 0.5 M sodium acetate which permitted marking of recording sites 12. A search stimulus (l. 5 V, 0.1 msec, 1.0 Hz) was applied to the tibial nerve or L7 dorsal root. Neurons encountered were initially classified according to their response to natural stimuli. Some cells responded only to non-noxious forms of stimuli to their receptive field, such as touch or hair movement. Others responded to this type of stimuli but also elicited a sustained response while maintaining a noxious pinch to the receptive field. Most of the latter group, referred to as wide dynamic range neurons, were also activated by noxious radiant heat (47-53 °C) and were located in lamina V with some overlap into laminae IV and VI. The noxious radiant heat was applied for 10-15 sec at 2-3 rain intervals by means of a focused halogen projector lamp with feedback controP °. Fluoxetine hydrochloride, a specific 5-HT neuronal uptake blocker 8,2~, was tested on neurons which responded consistently to a series of noxious radiant heat pulses. This drug was tested in doses of 4-6 mg/kg (1 cell at 4 mg/kg; 7 cells at 6 mg/kg), slowly administered, using only one cell per animal. We found fluoxetine to be a very potent agent in blocking 5-HT synaptosomal uptake xg. Furthermore, we found the dose range used in these experiments to have pronounced effects on other systems involving 5-HT in the cat 18 and rabbit :tg. If 5-HT is involved in the tonic descending
227
O
o
o I
I
20Sec
Fig. 1. The effect of fluoxetine (4 mg/kg i.v.) on the response of a dorsal horn neuron activated by noxious radiant heat (H) of 50 °C. A: control responses. B: 6 and 9 min after initiating the infusion of fluoxetine. C: 20 and 24 min after completing the injection.
inhibition on these neurons one would expect fluoxetine to enhance this inhibition and thus decrease the response to noxious radiant heat. However, fluoxetine failed to influence the response to noxious radiant heat in the majority of cells tested (Figs. 1 and 2) and decreased the response on only one cell.
o
"E o U "62OO-
c 1500 a. a: 1 0 0 "i-
z
0
6mg/kg
Fluoxetine
5o0 × 0
I
I
I
-5
0
5
I
i
10 15 Time (rain)
I
20
25
%
Fig. 2. The effect of fluoxetine (6 mg/kg i.v.) on the responses of dorsal horn neurons to noxious radiant heat. Fluoxetine was infused over the time period indicated by the bar. The numbers of discharges were counted over a 25 sec period beginning at the start of a 10-15 see radiant heat pulse. This value at time zero was equated to 100700. Values before and after time zero were calculated based on this figure. Each point represents the mean 4- S.E.M., n = 7.
228 The other approach taken was to determine the extent of tonic inhibition on these nociceptor-driven neurons in non-pretreated andp-CPA pretreated animals. The 5 animals receiving p-CPA were pretreated 48 and 24 h prior to recording with a dose of" 300 mg/kg i.p. This dose was reported to decrease cat spinal cord levels by 90 ~(i as well as producing hind limb ataxia, poverty of movement and prolonged meowing ')0. These symptoms were present in all thep-CPA treated animals used in the present study. We have also previously reported that this dose of p-CPA was very effective in antagonizing other 5-HT systems in cats TM. In both groups of animals, neurons which responded consistently to noxious radiant heat pulses were also tested following the application of a spinal cold block. Responses to the noxious heat were again recorded following rewarming of the cord. If 5-HT is involved in the tonic descending inhibition, one would expect to see a decrease in the tonic inhibition in the p-CPA pretreated animals as reflected by little difference in the response to noxious radiant heat in the intact vs the cold blocked state of the spinal cord. However, Fig. 3 clearly shows that cold blocking the spinal cord results in an equivalent increase in the response to noxious radiant heat in both groups of animals. Our findings confirm the existence of strong tonic descending inhibition on spinal cord nociceptor-driven neurones. Of the 39 cells tested after cold blocking the spinal cord, 37 showed a markedly increased response to noxious radiant heat. Two neurons, however, exhibited a decreased response following the cold block, an observation also made by Wall el on one cell. However, the results strongly suggest that 5oHT is not involved in the tonic descending inhibition which exists on nociceptor-driven spinal cord neurons. Therefore, if 5-HT is involved in the descending systems activated by SPA or opiates they would appear not to be the same as the tonically active descending system which impinges on these neurons. Consistent with this notion are reports that sectioning the spinal cord DLF a or pretreatment with p-CPA 1 antagonize SPA but have little or no effect in altering the baseline latency of the tailflick response. Similarly, Yaksh et al. 24 T
1500. .
I
CONTROL n=21
I
p-CPA n =16
O 1000.
500,
0" INTA~-I-
COLD BLOGK
Fig. 3. A comparison of the responses of dorsal horn neurones to noxious radiant heat with the spinal cord intact or reversibly cold blocked in non-pretreated and p-CPA pretreated cats.
229 f o u n d t h a t lesioning the r a p h e nuclei failed to alter the nociceptive t h r e s h o l d in the tail flick a n d h o t plate tests a l t h o u g h they a t t e n u a t e d the effects o f m o r p h i n e . F u r t h e r more, M a l e c a n d L a n g w i n s k i 16 r e p o r t e d t h a t fluoxetine d i d n o t alter the h o t plate r e a c t i o n time in rats a l t h o u g h it did p o t e n t i a t e the effects o f m o r p h i n e . These studies i m p l y t h a t the t o n i c descending inhibition is n o t influenced by these treatments. F u r t h e r w o r k is n e e d e d to clarify the involvement o f 5 - H T in descending systems m o d u l a t i n g nociceptive p a t h w a y s at the spinal c o r d level. This w o r k was s u p p o r t e d b y the M e d i c a l Research C o u n c i l o f C a n a d a a n d the B. C. H e a l t h Care Research F o u n d a t i o n . 1 Akil, H. and Mayer, D.J. Antagonism of stimulation-produced analgesia by p-CPA, a serotonin synthesis inhibitor, Brain Research, 44 (1972) 492-497. 2 Basbaum, A.I., Clanton, C.H. and Fields, H.L., Opiate and stimulation-produced analgesia: functional anatomy of a medullospinal pathway, Proc. nat. Acad. Sci. (Wash.), 73 (1976) 4685-4688. 3 Basbaum, A.I., Marley, N. and O'Keefe, J., Spinal cord pathways involved in the production of analgesia by brain stimulation. In J.J. Bonica and D. Albe-Fessard (Eds.) Advances in Pain Research and Therapy, Vol. 1. Raven Press, New York, 1976, pp. 511-515. 4 Basbaum, A.I., Marley, N.J.E., O'Keefe, J., and Clanton, C.H., Reversal of morphine and stimulus-produced analgesia by subtotal spinal cord lesions, Pain, 3 (1977) 43-56. 5 Bennett, G.J. and Mayer, D.J., Inhibition of spinal cord interneurons by narcotic microinjection and focal electrical stimulation in the periaqueductal gray matter, Brain Research, 172 (1979) 243-257. 6 Brown, A.G., Effects of descending impulses on transmission through the spinocervical tract, J. Physiol. (Lond.), 219 (1971) 103-125. 7 Carstens, E. and Zimmermann, M., Pharmacologically distinct systems in medial and lateral midbrain mediating descending inhibition of spinal nociceptive transmission in the cat, Neurosci. Abstr., 5 (1979) 607. 8 Fuller, R.W., Perry, K.W. and Molloy, B.B., Effect of 3-(p-trifluoromethyl-phenoxy)-N-methyl3-phenylpropylamine on the depletion of brain serotonin by 4-chloroamphetamine, J. Pharmacol. exp. Ther., 193 (1975) 796-803. 9 Guilbaud, G., Besson, J.-M., Oliveras, J.-L. and Liebeskind, J.C. Suppression by LSD of the inhibitory effect exerted by dorsal raphe stimulation on certain spinal cord interneurons in the cat, Brain Research, 61 (1973), 417-422. 10 Handwerker, H.O., Iggo, A. and Zimmermann, M., Segmental and supraspinal actions on dorsal horn neurons responding to noxious and non-noxious skin stimuli, Pain, 1 (1975) 147-165. 11 Hayes, R.L., Newlon, P.G., Rosecrans, J.A. and Mayer, D.J., Reduction of stimulation-produced analgesia by lysergic acid diethylamide, a depressor of serotonergic neural activity, Brain Research, 122 (1977) 367-372. 12 Hellon, R.F., The marking of electrode tip positions in nervous tissue, J. Physiol. (Lond.), 214 (1971) 12P. 13 Jacquet, Y. and Lajtha, A., Paradoxical effects after microinjection of morphine in the periaqueductal gray matter in the rat, Science, 185 (1974) 1055-1057. 14 Koe, B.K. and Weissman, A., p-Chlorophenylalanine: a specific depletor of brain serotonin, J. Pharmacol. exp. Ther., 154 (1966) 499-516. 15 Liebeskind, J.C., Guilbaud, G., Besson, J.-M. and Oliveras, J.-L., Analgesia from electrical stimulation of the periaqueductal gray matter in the cat: behavioral observations and inhibitory effects on spinal cord interneurons, Brain Research, 50 (1973) 441-446. 16 Malec, D. and Langwinski, R., Effect of quipazine and fluoxetine on analgesic-induced catalepsy and antinociception in the rat, J. Pharm. Pharmacol., 32 (1980) 71-73. 17 Mayer, D.J., Wolfe, T.L., Akil, H., Carder, B. and Liebeskind, J.C., Analgesia from electrical stimulation in the brainstem of the rat, Science, 174 (1971) 1351-1354. 18 Sastry, B.S.R. and Sinclair, J.G., Tonic inhibitory influence of a supraspinal monoaminergic system on presynaptic inhibition of an extensor monosynaptic reflex, Brain Research, 124 (1977) 109-120.
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