338
Brain Research, 549 (1991) 338-341 © 1991 Elsevier Science Publishers B.V. 0006-8993/91/$03.5(I ADONIS 0006899391246658
BRES 24665
Lumbar intrathecal morphine alters activity of putative nociceptive modulatory neurons in rostral ventromedial medulla M.M. Heinricher and K. Drasner Departments of Neurology and Anesthesia, University of California-San Francisco, San Francisco, CA 94143 (U.S.A.)
(Accepted 19 February 1991) Key words: Pain modulation; Antinociception; Opioid; Nucleus raphe magnus; Pain; Rat
Two physiologically and pharmacologically distinct classes of putative nociceptive modulatory neurons have been identified in the rostral ventral medulla (RVM) of the lightly anesthetized rat: on-cells and off-cells. We have previously shown that administration of morphine either systemically or by microinjection into the periaqueductal gray (PAG) produces an increase in the activity of all off-cells and a depression of the activity of all on-cells concomitant with inhibition of the tail flick reflex. We now demonstrate that morphine applied intrathecally has effects on RVM neurons that are indistinguishable from those of systemic or PAG administration. This may contribute to the known multiplicative effects of concurrent administration of opioids at spinal and supraspinal sites.
Behavioral studies have demonstrated that morphine has an antinociceptive effect when microinjected at s e v e r a l brainstem sites 15. A m o n g these is the rostral ventromedial medulla (RVM), which projects through the dorsolateral funiculus to the dorsal horn where it acts to modulate nociceptive transmission 4. Application of opioid compounds to the spinal cord by means of an intrathecal catheter is also antinociceptive 12'~3 and the antinociception resulting from systemic administration of morphine has been demonstrated to involve a synergistic interaction between spinal and supraspinal sites ~6. Recent electrophysiological studies have identified two classes of presumed nociceptive modulating neurons in R V M 7. 'Off-cells' are characterized by a pause in firing just prior to the occurrence of the tail flick response (TF), a nocifensive reflex evoked by the application of noxious heat to the tail. 'On-cells' show a burst of activity just prior to the T E Off-cells uniformly become continuously active following systemic or P A G administration of morphine 5,8. On-cell firing is depressed by morphine applied iontophoretically 1°, or when administered systemically 2 or in the P A G 5. The present study examines the effect of intrathecally applied morphine on the activity of identified on- and off-ceUs in RVM. Forty-two male Sprague-Dawley rats (275-350 g) were used in these experiments. A lumbar intrathecal catheter (PE-10 tubing) was implanted under pentobarbital anesthesia TM and the animals allowed to recover for a minim u m of 5 days prior to electrophysiological experiments.
For single unit recording, rats were initially anesthetized with pentobarbital (60 mg/kg, i.p.), and a catheter was placed in each external jugular vein. The animals were then placed in a stereotaxic frame and a small craniectomy was made to allow placement of a recording microelectrode in the RVM. Following these surgical procedures, the animals were maintained in a lightly anesthetized state using a continuous infusion of methohexital (15-30 mg/kg/h, i.v.) as previously described 3. Body temperature was maintained at approximately 37 °C using a circulating water blanket. The TF was evoked using a feedback-controlled projector lamp focused on the blackened ventral surface of the tail. The occurrence of the TF was signalled by a force transducer attached to the tail. Baseline latencies were 4-5 s. A gold- and platinum-plated stainless-steel microelectrode stereotactically placed in R V M was used for both stimulation and extracellular recording. The electrode w a s advanced until the TF could be inhibited using currents of 10 ~ A or less (400 ~s pulses, 50 H z continuous train). Recording was begun at this point. Units were characterized according to the classification system of Fields et al. 7 Peri-event histograms of cell activity relative to the occurrence of the TF and the tail heat stimulus were generated (5 trials), and spontaneous activity of the cell was then monitored for a period of 20-30 min. Morphine sulfate (MS, 10 or 15/~g in 5/~1 of a balanced salt solution), lidocaine HCI (5-10 ¢tl of a 10% solution) or vehicle (5/~1) was then injected intrathecally using a 10
Correspondence: M.M. Heinricher, Department of Neurology, Box 0114, University of California, San Francisco, CA 94143, U.S.A.
339
CON
MOR
NAL
1 min
Fig. 1. Ratemeter record shows effect of intrathecal MS injection on spontaneous firing of an off-cell. CON, firing pattern prior to MS, showing periodic fluctuations in firing, with active periods of up to several minutes in length alternating with silent periods. MOR, cell became continuously active following injection of 10 gg of MS. NAL, MS effect was reversed by naloxone (0.25 mg/kg, i.v.). Calibration: firing rate in spikes/s.
/tl syringe. This was followed by an 8/~1 vehicle flush to clear the catheter, after which the TF was tested at 5 min intervals until latency reached the 10 s cut-off value. Lidocaine was given intravenously in animals without intrathecal catheters in four control experiments. Naloxone (0.25 or 0.5 mg/kg, i.v.) was given 30-40 min after the morphine injection. Only one cell was studied per animal. At the end of the experiment, an electrolytic lesion was made at the recording site. The rat was sacrificed with an overdose of methohexital and perfused intracardially with saline followed by 10% formalin. Catheters were flushed with Methylene blue, and the spinal cord examined to confirm correct catheter placement at the lumbar enlargement. Recording sites were histologically verified to be in the RVM as described in
tail were attenuated or abolished by intrathecal morphine. Noxious pinch delivered to the hindlimb also had no effect on cell activity, however, pinching the forelimb was sufficient to provoke a brief withdrawal and a pause in off-cell firing and burst of on-cell activity. Morphineinduced changes in spontaneous and evoked activity were uniformly antagonized by systemically administered naloxone. Cell activity was unchanged in the 3 cases in which the MS injection failed to produce antinociception. Similarly, in 5 animals, intrathecal administration of the vehicle alone had no effect on TF latency or neuronal discharge. A total of 7 cells (3 off-cells and 4 on-cells) was examined before and after intrathecal lidocaine administration (0.5-1.0 mg). This injection blocked Changes in cell activity previously evoked by application of noxious pinch to the hindlimbs, but not those resulting from pinching the forelimbs or muzzle. In 3 animals, lidocaine injection resulted in the inhibition of the TF, and changes in cell activity associated with tail heating were also eliminated. Changes in spontaneous activity were similar to those seen after MS injection, in that on-cell activity was depressed (2 of 4 cells tested) or eliminated (2 of 4 cells), and off-cells fired continuously (3 of 3 cells). This dose of lidocaine given intravenously had no discernible effect on cell activity (n = 4), although 3 mg produced a decrease in the spontaneous discharge of one on-cell, and an increase in the discharge of one off-cell. We have previously shown that the two physiologically characterized classes of presumed nociceptive modulating
CON
p r e v i o u s w o r k 2'3,7.
Eleven off-cells and 15 on-cells were studied in 26 experiments in which MS was injected into the intrathecal space. In all but 3 cases, this injection resulted in an increase in TF latency to the 10 s cut-off value within 5-15 min of completing the injection, and produced differential effects on the firing of cells of the two classes. Prior to the injection, cells of both classes exhibited an irregularly cyclic firing pattern, as previously described 3. Following intrathecal morphine administration, off-cell discharge was shifted to a continuous pattern (Fig. 1). On-cells invariably showed a profound depression of activity, usually to complete silence (Fig. 2). The slowing of the off-cell and acceleration of the on-cell characteristically associated with heating of the
MOB
NAL
1 1 mln
"
Fig. 2. Effect of intrathecal MS injection on the spontaneous firing of an on-cell CON, firing prior to MS. MOR, cell firing was completely suppressed by MS (15 ~g). NAL, spontaneous firing was
restored by naloxone (0.25 mg/kg, i.v.). Calibration: firing rate in spikes/s.
340 neurons in the R V M reliably show differential responses to morphine given systemically or in the P A G , with off-cells becoming continuously active and on-cells silent 2'5'8. The present results demonstrate an effect of morphine when its site of action is limited to the spinal cord. As would have been predicted, delivery of morphine to the lumbar intrathecal space eliminated changes in R V M neuronal activity related to noxious stimulation of the tail and caudal body surface. However, changes in the ongoing activity of these neurons that were indistinguishable from those produced by systemic or P A G morphine administration were also seen. H o w spinal morphine alters the spontaneous firing of on- and off-cells in R V M is not clear, but presumably requires a brainstem relay such as nucleus reticularis gigantocellularis, since direct spinal inputs to the R V M are limited to its more lateral aspects 1. From a functional point of view, several possibilities should be considered. The first is opioid activation of an ascending projection that exerts a net inhibitory effect on the firing of on-cells and a net excitatory effect on the firing of off-cells. This seems unlikely for several reasons. Superfusion of the spinal cord with morphine has been shown to suppress the activity of dorsal horn neurons, including identified projection neurons 6'11. It should be noted however that some neurons were excited by morphine at low doses 6. More importantly, intrathecal lidocaine injection also results in an increase in off-cell firing and depression of on-cell discharge. This observation favors the hypothesis that a suppression of activity in some ascending projection is the basis for the effect of spinal morphine on these brainstem neurons. Under the conditions of the present experiments, and in the absence of any drug treatment or acute noxious stimulation, on- and off-cells typically show cyclic alternations between periods in which on-cells are silent and off-cells active, and periods in which off-cells are silent and on-cells active 3. That is, periods of spontaneously occurring discharge of cells within each class are in phase, and the firing of the two classes shows a reciprocal
pattern. The timing of this pattern can be explicitly controlled by noxious stimulation; thus, noxious pinch delivered anywhere on the body or a silrel TF trial will initiate a variable length period in which the activity of on-cells is maintained while off-cells remain silent. Significant blockade by morphine of activity in relevant ascending pathways could therefore shift the balance in excitability of the two classes, permitting a sustained period of off-cell firing and on-cell silence. Under these conditions, noxious pinch delivered to the rostral body (served by afferents arising from the unblocked trigeminal system and cervical spinal cord) would still elicit a burst of on-cell firing and a pause in off-cell firing, but ongoing activity in the off-cell, rather than on-cell, population, would be favored. These findings have implications for the mechanisms of opioid analgesia obtained using different routes of administration. Clearly, activation of brainstem modulatory systems is not a major factor in the antinociception produced by intrathecal morphine administration, since nociceptive transmission is not eliminated at sites remote from the drug application 12. However, we have shown that there is a small, but significant, association between the pattern of spontaneous firing of on- and off-cells and nociceptive responsiveness, as measured by the TF latency. On average, TF latency is almost 1 s longer when measured during periods of off-cell firing and on-cell silence 9. Thus, the shift to a period of sustained off-cell firing produced by morphine acting at the cord would tend to reinforce direct spinal effects of the drug, and could produce subtle increases in nociceptive threshold over the entire body. Such an action mediated through the R V M could contribute to the known multiplicative antinociptive effects of concurrent administration of opioids at spinal and supraspinal sites 16.
1 Abols, I.A. and Basbaum, A.I., Afferent connections of the rostral medulla of the cat: a neural substrate for midbrainmedullary interactions in the modulation of pain, J. Comp. Neurol., 201 (1981) 285-297. 2 Barbaro, N.M., Heinricher, M.M. and Fields, H.L., Putative pain modulating neurons in the rostral ventral medulla: reflex related activity predicts effects of morphine, Brain Research, 366 (1986) 203-210. 3 Barbaro, N.M., Heinricher, M.M. and Fields, H.L., Putative nociceptive modulatory neurons in the rostral ventromedial medulla of the rat display highly correlated firing patterns, Somatosen. Mot. Res., 6 (1989) 413-425. 4 Basbaum, A.I. and Fields, H.L., Endogenous pain control systems: brainstem spinal pathways and endorphin circuitry, Annu. Rev. Neurosci., 7 (1984) 309-338.
5 Cheng, Z.-E, Fields, H.L. and Heinricher, M.M., Morphine microinjected into the periaqueductal gray has differential effects on three classes of medullary neurons, Brain Research, 375 (1986) 57-65. 6 Dickenson, A.H. and Sullivan, A.F., Electrophysiological studies on the effects of intrathecal morphine on nociceptive neurones in the rat dorsal horn, Pain, 24 (1986) 211-222. 7 Fields, H.L., Bry, J., Hentall, I. and Zorman, G., The activity of neurons in the rostral medulla of the rat during withdrawal from noxious heat, J. Neurosci., 3 (1983) 2545-2552. 8 Fields, H.L., Vanegas, H., Hentall, I.D. and Zorman, G., Evidence that disinhibition of brain stem neurones contributes to morphine analgesia, Nature, 306 (1983) 684-686. 9 Heinricher, M.M., Barbaro, N.M. and Fields, H.L., Putative nociceptive modulatory neurons in the rostral ventromedial
This work was supported by a grant from the National Institute on Drug Abuse (DA 01949). We wish to thank Howard Fields for his helpful advice and generous assistance in completing this work, Michael M. Morgan for critical comments on the manuscript, Beth Budra for artwork, and Pat Littlefield for histological work.
341
10
11 12 13
medulla of the rat: firing of on- and off-cells is related to nociceptive responsiveness, Somatosen. Mot. Res., 6 (1989) 427-439. Heinricher, M.M., Morgan, M.M. and Fields, H.L., Morphine applied iontophoretically depresses activity of on-cells in rostral ventromedial medulla of lightly anesthetized rats, Soc. Neurosci. Abstr., 16 (1990) 98. Hylden, J.L.K. and Wilcox, G.L., Antinoeiceptive effect of morphine on rat spinothalamic tract and other dorsal horn neurons, Neuroscience, 19 (1986) 393--401. Yaksh, T.L., Spinal opiate analgesia: characteristics and principles of action, Pain, 11 (1981) 293-346. Yaksh, T.L. and Noueihed, R., The physiology and pharmacol-
ogy of spinal opiates, Annu. Rev. Pharmacol. Toxicol., 25 (1985) 433-462. 14 Yaksh, T.L. and Rudy, T.A., Chronic catheterization of the spinal subarachnoid space, Physiol. Behav., 17 (1976) 10311036. 15 Yaksh, T.L. and Rudy, T.A., Narcotic analgesics: CNS sites and mechanisms of action as revealed by intracerebral injection techniques, Pain, 4 (1978) 299-359. 16 Yeung, J.C. and Rudy, T.A., Multiplieative interaction between narcotic agonisms expressed at spinal and supraspinal sites of antinociceptive action as revealed by concurrent intrathecal and intracerebroventricular injections of morphine, J. Pharmacol. Exp. Ther., 215 (1980) 633-642
Brain Research, 549 (1991) 338-341 © 1991 Elsevier Science Publishers B.V. 0006-8993/91/$03.5(I ADONIS 0006899391246658
BRES 24665
Lumbar intrathecal morphine alters activity of putative nociceptive modulatory neurons in rostral ventromedial medulla M.M. Heinricher and K. Drasner Departments of Neurology and Anesthesia, University of California-San Francisco, San Francisco, CA 94143 (U.S.A.)
(Accepted 19 February 1991) Key words: Pain modulation; Antinociception; Opioid; Nucleus raphe magnus; Pain; Rat
Two physiologically and pharmacologically distinct classes of putative nociceptive modulatory neurons have been identified in the rostral ventral medulla (RVM) of the lightly anesthetized rat: on-cells and off-cells. We have previously shown that administration of morphine either systemically or by microinjection into the periaqueductal gray (PAG) produces an increase in the activity of all off-cells and a depression of the activity of all on-cells concomitant with inhibition of the tail flick reflex. We now demonstrate that morphine applied intrathecally has effects on RVM neurons that are indistinguishable from those of systemic or PAG administration. This may contribute to the known multiplicative effects of concurrent administration of opioids at spinal and supraspinal sites.
Behavioral studies have demonstrated that morphine has an antinociceptive effect when microinjected at s e v e r a l brainstem sites 15. A m o n g these is the rostral ventromedial medulla (RVM), which projects through the dorsolateral funiculus to the dorsal horn where it acts to modulate nociceptive transmission 4. Application of opioid compounds to the spinal cord by means of an intrathecal catheter is also antinociceptive 12'~3 and the antinociception resulting from systemic administration of morphine has been demonstrated to involve a synergistic interaction between spinal and supraspinal sites ~6. Recent electrophysiological studies have identified two classes of presumed nociceptive modulating neurons in R V M 7. 'Off-cells' are characterized by a pause in firing just prior to the occurrence of the tail flick response (TF), a nocifensive reflex evoked by the application of noxious heat to the tail. 'On-cells' show a burst of activity just prior to the T E Off-cells uniformly become continuously active following systemic or P A G administration of morphine 5,8. On-cell firing is depressed by morphine applied iontophoretically 1°, or when administered systemically 2 or in the P A G 5. The present study examines the effect of intrathecally applied morphine on the activity of identified on- and off-ceUs in RVM. Forty-two male Sprague-Dawley rats (275-350 g) were used in these experiments. A lumbar intrathecal catheter (PE-10 tubing) was implanted under pentobarbital anesthesia TM and the animals allowed to recover for a minim u m of 5 days prior to electrophysiological experiments.
For single unit recording, rats were initially anesthetized with pentobarbital (60 mg/kg, i.p.), and a catheter was placed in each external jugular vein. The animals were then placed in a stereotaxic frame and a small craniectomy was made to allow placement of a recording microelectrode in the RVM. Following these surgical procedures, the animals were maintained in a lightly anesthetized state using a continuous infusion of methohexital (15-30 mg/kg/h, i.v.) as previously described 3. Body temperature was maintained at approximately 37 °C using a circulating water blanket. The TF was evoked using a feedback-controlled projector lamp focused on the blackened ventral surface of the tail. The occurrence of the TF was signalled by a force transducer attached to the tail. Baseline latencies were 4-5 s. A gold- and platinum-plated stainless-steel microelectrode stereotactically placed in R V M was used for both stimulation and extracellular recording. The electrode w a s advanced until the TF could be inhibited using currents of 10 ~ A or less (400 ~s pulses, 50 H z continuous train). Recording was begun at this point. Units were characterized according to the classification system of Fields et al. 7 Peri-event histograms of cell activity relative to the occurrence of the TF and the tail heat stimulus were generated (5 trials), and spontaneous activity of the cell was then monitored for a period of 20-30 min. Morphine sulfate (MS, 10 or 15/~g in 5/~1 of a balanced salt solution), lidocaine HCI (5-10 ¢tl of a 10% solution) or vehicle (5/~1) was then injected intrathecally using a 10
Correspondence: M.M. Heinricher, Department of Neurology, Box 0114, University of California, San Francisco, CA 94143, U.S.A.
339
CON
MOR
NAL
1 min
Fig. 1. Ratemeter record shows effect of intrathecal MS injection on spontaneous firing of an off-cell. CON, firing pattern prior to MS, showing periodic fluctuations in firing, with active periods of up to several minutes in length alternating with silent periods. MOR, cell became continuously active following injection of 10 gg of MS. NAL, MS effect was reversed by naloxone (0.25 mg/kg, i.v.). Calibration: firing rate in spikes/s.
/tl syringe. This was followed by an 8/~1 vehicle flush to clear the catheter, after which the TF was tested at 5 min intervals until latency reached the 10 s cut-off value. Lidocaine was given intravenously in animals without intrathecal catheters in four control experiments. Naloxone (0.25 or 0.5 mg/kg, i.v.) was given 30-40 min after the morphine injection. Only one cell was studied per animal. At the end of the experiment, an electrolytic lesion was made at the recording site. The rat was sacrificed with an overdose of methohexital and perfused intracardially with saline followed by 10% formalin. Catheters were flushed with Methylene blue, and the spinal cord examined to confirm correct catheter placement at the lumbar enlargement. Recording sites were histologically verified to be in the RVM as described in
tail were attenuated or abolished by intrathecal morphine. Noxious pinch delivered to the hindlimb also had no effect on cell activity, however, pinching the forelimb was sufficient to provoke a brief withdrawal and a pause in off-cell firing and burst of on-cell activity. Morphineinduced changes in spontaneous and evoked activity were uniformly antagonized by systemically administered naloxone. Cell activity was unchanged in the 3 cases in which the MS injection failed to produce antinociception. Similarly, in 5 animals, intrathecal administration of the vehicle alone had no effect on TF latency or neuronal discharge. A total of 7 cells (3 off-cells and 4 on-cells) was examined before and after intrathecal lidocaine administration (0.5-1.0 mg). This injection blocked Changes in cell activity previously evoked by application of noxious pinch to the hindlimbs, but not those resulting from pinching the forelimbs or muzzle. In 3 animals, lidocaine injection resulted in the inhibition of the TF, and changes in cell activity associated with tail heating were also eliminated. Changes in spontaneous activity were similar to those seen after MS injection, in that on-cell activity was depressed (2 of 4 cells tested) or eliminated (2 of 4 cells), and off-cells fired continuously (3 of 3 cells). This dose of lidocaine given intravenously had no discernible effect on cell activity (n = 4), although 3 mg produced a decrease in the spontaneous discharge of one on-cell, and an increase in the discharge of one off-cell. We have previously shown that the two physiologically characterized classes of presumed nociceptive modulating
CON
p r e v i o u s w o r k 2'3,7.
Eleven off-cells and 15 on-cells were studied in 26 experiments in which MS was injected into the intrathecal space. In all but 3 cases, this injection resulted in an increase in TF latency to the 10 s cut-off value within 5-15 min of completing the injection, and produced differential effects on the firing of cells of the two classes. Prior to the injection, cells of both classes exhibited an irregularly cyclic firing pattern, as previously described 3. Following intrathecal morphine administration, off-cell discharge was shifted to a continuous pattern (Fig. 1). On-cells invariably showed a profound depression of activity, usually to complete silence (Fig. 2). The slowing of the off-cell and acceleration of the on-cell characteristically associated with heating of the
MOB
NAL
1 1 mln
"
Fig. 2. Effect of intrathecal MS injection on the spontaneous firing of an on-cell CON, firing prior to MS. MOR, cell firing was completely suppressed by MS (15 ~g). NAL, spontaneous firing was
restored by naloxone (0.25 mg/kg, i.v.). Calibration: firing rate in spikes/s.
340 neurons in the R V M reliably show differential responses to morphine given systemically or in the P A G , with off-cells becoming continuously active and on-cells silent 2'5'8. The present results demonstrate an effect of morphine when its site of action is limited to the spinal cord. As would have been predicted, delivery of morphine to the lumbar intrathecal space eliminated changes in R V M neuronal activity related to noxious stimulation of the tail and caudal body surface. However, changes in the ongoing activity of these neurons that were indistinguishable from those produced by systemic or P A G morphine administration were also seen. H o w spinal morphine alters the spontaneous firing of on- and off-cells in R V M is not clear, but presumably requires a brainstem relay such as nucleus reticularis gigantocellularis, since direct spinal inputs to the R V M are limited to its more lateral aspects 1. From a functional point of view, several possibilities should be considered. The first is opioid activation of an ascending projection that exerts a net inhibitory effect on the firing of on-cells and a net excitatory effect on the firing of off-cells. This seems unlikely for several reasons. Superfusion of the spinal cord with morphine has been shown to suppress the activity of dorsal horn neurons, including identified projection neurons 6'11. It should be noted however that some neurons were excited by morphine at low doses 6. More importantly, intrathecal lidocaine injection also results in an increase in off-cell firing and depression of on-cell discharge. This observation favors the hypothesis that a suppression of activity in some ascending projection is the basis for the effect of spinal morphine on these brainstem neurons. Under the conditions of the present experiments, and in the absence of any drug treatment or acute noxious stimulation, on- and off-cells typically show cyclic alternations between periods in which on-cells are silent and off-cells active, and periods in which off-cells are silent and on-cells active 3. That is, periods of spontaneously occurring discharge of cells within each class are in phase, and the firing of the two classes shows a reciprocal
pattern. The timing of this pattern can be explicitly controlled by noxious stimulation; thus, noxious pinch delivered anywhere on the body or a silrel TF trial will initiate a variable length period in which the activity of on-cells is maintained while off-cells remain silent. Significant blockade by morphine of activity in relevant ascending pathways could therefore shift the balance in excitability of the two classes, permitting a sustained period of off-cell firing and on-cell silence. Under these conditions, noxious pinch delivered to the rostral body (served by afferents arising from the unblocked trigeminal system and cervical spinal cord) would still elicit a burst of on-cell firing and a pause in off-cell firing, but ongoing activity in the off-cell, rather than on-cell, population, would be favored. These findings have implications for the mechanisms of opioid analgesia obtained using different routes of administration. Clearly, activation of brainstem modulatory systems is not a major factor in the antinociception produced by intrathecal morphine administration, since nociceptive transmission is not eliminated at sites remote from the drug application 12. However, we have shown that there is a small, but significant, association between the pattern of spontaneous firing of on- and off-cells and nociceptive responsiveness, as measured by the TF latency. On average, TF latency is almost 1 s longer when measured during periods of off-cell firing and on-cell silence 9. Thus, the shift to a period of sustained off-cell firing produced by morphine acting at the cord would tend to reinforce direct spinal effects of the drug, and could produce subtle increases in nociceptive threshold over the entire body. Such an action mediated through the R V M could contribute to the known multiplicative antinociptive effects of concurrent administration of opioids at spinal and supraspinal sites 16.
1 Abols, I.A. and Basbaum, A.I., Afferent connections of the rostral medulla of the cat: a neural substrate for midbrainmedullary interactions in the modulation of pain, J. Comp. Neurol., 201 (1981) 285-297. 2 Barbaro, N.M., Heinricher, M.M. and Fields, H.L., Putative pain modulating neurons in the rostral ventral medulla: reflex related activity predicts effects of morphine, Brain Research, 366 (1986) 203-210. 3 Barbaro, N.M., Heinricher, M.M. and Fields, H.L., Putative nociceptive modulatory neurons in the rostral ventromedial medulla of the rat display highly correlated firing patterns, Somatosen. Mot. Res., 6 (1989) 413-425. 4 Basbaum, A.I. and Fields, H.L., Endogenous pain control systems: brainstem spinal pathways and endorphin circuitry, Annu. Rev. Neurosci., 7 (1984) 309-338.
5 Cheng, Z.-E, Fields, H.L. and Heinricher, M.M., Morphine microinjected into the periaqueductal gray has differential effects on three classes of medullary neurons, Brain Research, 375 (1986) 57-65. 6 Dickenson, A.H. and Sullivan, A.F., Electrophysiological studies on the effects of intrathecal morphine on nociceptive neurones in the rat dorsal horn, Pain, 24 (1986) 211-222. 7 Fields, H.L., Bry, J., Hentall, I. and Zorman, G., The activity of neurons in the rostral medulla of the rat during withdrawal from noxious heat, J. Neurosci., 3 (1983) 2545-2552. 8 Fields, H.L., Vanegas, H., Hentall, I.D. and Zorman, G., Evidence that disinhibition of brain stem neurones contributes to morphine analgesia, Nature, 306 (1983) 684-686. 9 Heinricher, M.M., Barbaro, N.M. and Fields, H.L., Putative nociceptive modulatory neurons in the rostral ventromedial
This work was supported by a grant from the National Institute on Drug Abuse (DA 01949). We wish to thank Howard Fields for his helpful advice and generous assistance in completing this work, Michael M. Morgan for critical comments on the manuscript, Beth Budra for artwork, and Pat Littlefield for histological work.
341
10
11 12 13
medulla of the rat: firing of on- and off-cells is related to nociceptive responsiveness, Somatosen. Mot. Res., 6 (1989) 427-439. Heinricher, M.M., Morgan, M.M. and Fields, H.L., Morphine applied iontophoretically depresses activity of on-cells in rostral ventromedial medulla of lightly anesthetized rats, Soc. Neurosci. Abstr., 16 (1990) 98. Hylden, J.L.K. and Wilcox, G.L., Antinoeiceptive effect of morphine on rat spinothalamic tract and other dorsal horn neurons, Neuroscience, 19 (1986) 393--401. Yaksh, T.L., Spinal opiate analgesia: characteristics and principles of action, Pain, 11 (1981) 293-346. Yaksh, T.L. and Noueihed, R., The physiology and pharmacol-
ogy of spinal opiates, Annu. Rev. Pharmacol. Toxicol., 25 (1985) 433-462. 14 Yaksh, T.L. and Rudy, T.A., Chronic catheterization of the spinal subarachnoid space, Physiol. Behav., 17 (1976) 10311036. 15 Yaksh, T.L. and Rudy, T.A., Narcotic analgesics: CNS sites and mechanisms of action as revealed by intracerebral injection techniques, Pain, 4 (1978) 299-359. 16 Yeung, J.C. and Rudy, T.A., Multiplieative interaction between narcotic agonisms expressed at spinal and supraspinal sites of antinociceptive action as revealed by concurrent intrathecal and intracerebroventricular injections of morphine, J. Pharmacol. Exp. Ther., 215 (1980) 633-642
