Brain Research, 117 (1976) 147-152
147
(~] Elsevier/North-Holland Biomedical Press, Amsterdam - Printed in The Netherlands
Non-GABA mediated primary afferent depolarization
A. H. REPKIN, P. WOLF* and E. G. ANDERSON Department of Pharrnacology, College of Medicine, University of Illinois at the Medical Center, Chicago, Xl/. (u.s.A.)
(Accepted August 6th, 1976)
Primary afferent depolarization (PAD), the process postulated to underlie presynaptic inhibitionS, 6, is most readily monitored by observing changes in potentials recorded from dorsal roots and evoked by stimulation of adjacent afferents. The potentials usually recorded are the dorsal root potential (DRP), which reflects electrotonically conducted currents generated by afferent terminal depolarization 1,12, and the dorsal root reflex (DRR), which is manifested as a synchronous antidromic discharge in dorsal roots generated during the rapid depolarizing phase of the DRpT, 12. While studying the pharmacology of the DRP and D R R we have consistently observed a second form of antidromic activity in the dorsal root, that of spontaneously occurring antidromic action potentials, which is physiologically and pharmacologically distinct from the evoked DRR. These potentials appear to be a unique indicator of the degree of tonic PAD and, hence, an extremely useful monitor of afferent terminal polarization. The following report introduces and describes this phenomenon. Cats pretreated with atropine methyl nitrate (0.1 mg/kg) and anesthetized with ether were spinalized by transection of the cord at the C~ level following bilateral vagotomy and anemic decerebration. The general anesthetic was discontinued and the animals were artificially ventilated (12-15 respirations/min, end-tidal CO2, 3 ~_ 0 . 5 ~ ) with room air. Gallamine triethiodide, administered periodically, limited movement. The lumbar spinal cord was exposed from the L5 to S~ segments and covered with a pool filled with light mineral oil. Both oil pool and body temperature were maintained at 37 zk 0.5 °C. The dura mater was cut longitudinally, all dorsal roots thus exposed were severed. Recordings were made with bipolar platinum electrodes from a dorsal rootlet dissected from an L7 dorsal root. The proximal pole of the recording electrode was placed as close as possible to the cord without touching it; the distal pole supported the remainder of the rootlet. Potentials recorded from this rootlet were either evoked by stimulation of the remainder of the L7 dorsal root or occurred spontaneously. These latter potentials were led to an amplitude discriminator after amplification. The discriminator output (Schmidt-trigger pulses) was either converted * Visiting fellow to the Department of Pharmacology, University of Illinois at the Medical Center, on a grant of the Janggen-Poehn-Stiftung,St. Gallen, Switzerland. Current address : Neurochirurgische Universiffitsklinik, Zurich, Switzerland.
148
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C
events 500
0
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100msec
0.5my 5
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0
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Fig. I. A: spontaneously occurring action potentials (dorsal root discharge; DRD) recorded from a dorsal rootlet with bipolar electrodes. B : ratemeter recording of DRD generation over a 3 h period. C: effect of evoked depolarization of primary afferents on DRD generation. Upper trace: poststimulus time histogram (50 sweeps) showing the number (ordinate) and temporal distribution (abscissa) of DRD occurring prior to and following stimulation (arrow) of a neighboring dorsal root. Lower trace: computer average of 10 evoked depolarizations. to rates of occurrence and plotted with a rectilinear recorder or served as input signals for computer analysis. Under the above conditions, action potentials were observed to occur spontaneously in recordings from both whole dorsal roots or dorsal rootlets (Fig. 1A) but not in ventral roots. This activity, which we have termed the dorsal root discharge (DRD), occurs in only a minority of afferent fibers, since only a small number of active units were observed in dorsal rootlets containing hundreds of fibers. The number of axons firing in a given population was strongly influenced by the previous history of the preparation. In our experience, the D R D was abolished by local or general anoxic insult occurring during any stage of the experiment, by spinal cord edema, or by barbiturate anesthetics in doses as low as 10 mg/kg. On the other hand, traumatic insult during cord dissection frequently induced injury discharges which were manifest as high frequency bursts of antidromic action potentials which obscured the recording of the D R D . These injury discharges, unlike the D R D , were unaffected by anoxia. Thus, both their pattern of occurrence and response to decreased ventilation distinguished them from the D R D . The D R D originated from within the cord and not at the severed end of the dorsal rootlet. Thus, the application of 1 ~ lidocaine to the distal portion of the rootlet did not affect discharge frequency. Spontaneous activity was abolished, however, bylido-
149 caine applied between the recording electrodes and the spinal cord, indicating that these potentials travel away from the cord. Lidocaine applied to the site of cord transection (C)1 was also without effect on the DRD. This eliminates the possibility that spontaneously occurring potentials are generated by injury to ascending sensory collaterals severed by spinalization. These findings indicate that the DRD is generated wholly within the spinal cord. In addition, several experiments were performed in animals with intact nervous systems in which recordings were made from sural nerves. These preparations were anesthetized and artificially ventilated with ether during nerve dissection. Wound edges were then infiltrated with local anesthetic and ether withdrawn. Recordings from the sural nerve were characterized by a large amount of multiple unit activity which was increased by stroking or pinching of the leg. Lidocaine applied between the peripheral receptors and the recording electrodes abolished this activity and revealed spontaneous action potentials traveling antidromically in the nerve trunk. Thus, it appears that the D R D is normally overridden by orthodromic activity and is manifested peripherally only when the sensory neurons are deprived of their afferent activity. Furthermore, the data suggest that the DRD occurs in sensory fibers since, with the possible exception of small diameter autonomic efferents, the sural nerve contains only sensory axonsS, 11. These observations also suggest that the D R D does not arise from injuries suffered by the spinal cord during dissection. In summary, it appears that a spontaneous antidromic discharge occurs in sensory fibers which arises from a depolarizing influence exerted onto primary afferents within the spinal cord. The conduction velocity of fibers carrying the DRD was estimated using two electrodes placed on the dorsal rootlet and separated by 2.5-3 cm, each recorded against ground. Activity from a single unit was detected by the proximal electrode and led to an amplitude discriminator with a narrow window, the output of which triggered an oscilloscope. The potentials recorded from the distal electrode were displayed on a second oscilloscope beam, thus the potential of interest was time-locked to the trigger. This procedure allowed identification of a single unit recorded from both electrodes in spite of differences in the amplitude and shape of the potential 'seen' by the two electrodes. Conduction velocities calculated from the time delay between electrodes ranged between 40-80 m/sec (n ~ 40). Although measurements were subject to significant error due to the short distance between electrodes, they clearly fall into the A fiber range. Fig. l B illustrates the variation with time of DRD generation in a population of afferents. The relatively constant activity, maintained over a period in excess of 3 h, suggests that spontaneous potentials arise from a tonic depolarization of both a sustained and a constant magnitude. This behavior contrasts with some kinds of injury discharges in dorsal roots which decrease in frequency with time. Other characteristics of DRD generation were its relative stability in the face of marked changes in blood pressure induced by the administration of epinephrine or decamethonium. Only when blood pressure fell precipitously was there a reduction in the rate of DRD generation. The DRD is, however, temperature sensitive, increasing in rate with cooling of the oil pool surrounding the spinal cord and decreasing in frequency with warming above
150 body temperature. These and other characteristics of DRD behavior are the subject of a more extensive treatment which is currently in preparation. The interaction between spontaneous and elicited depolarizations of afferent fibers was characterized by observing the generation of antidromic action potentials during the time course of evoked dorsal root potentials. A high-pass filter eliminated the longer lasting evoked depolarizations (DRP) so that only transient potentials were 'seen' by the amplitude discriminator. The DRR, being a transient depolarization, was also represented in the discriminator output during the rising phase of the DRP The data are presented in the form of a poststimulus time histogram (upper trace, Fig. 1C) showing the number and temporal distribution ofantidromic activity occurring prior to and following electrical stimulation of neighboring dorsal root fibers. Also illustrated is the DRP evoked by this stimulation (lower trace, Fig. 1C). Comparison of the histogram and the DRP shows an increased occurrence of antidromic potentials during the DRP which parallels the intensity of the evoked depolarization. This
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0.2 mv
B events/sec
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,°° I 0 L_ 5rain 2 0 0 mg/kg semicarbazide HGI Fig, 2. A: effect of i.v. administration of 0.5 mg/kg (bar, upper record) of bicuculline on spontaneously occurring (upper record) and evoked (lower record) dorsal root activity. Upper record is a ratemeter recording of the DRD. Lower record is a computer average of the D R P and D R R (10 sweeps) recorded prior to (1) and following (2) bicuculline injection. B: ratemeter recording of D R D generation prior to and 4 h after i.v. administration of 200 mg/kg semicarbazide (30 min injection time).
151 correlation suggests that changes in the polarization of the afferent fiber are signaled by changes in the rate of D R D generation. In view of the possibility that the DRD represents a tonic component of the same mechanism responsible for generation of the D R R and DRP, the effects of bicuculline and semicarbazide, which block evoked potentials by dissimilar mechanisms, were examined on the DRD. As previously demonstrated 10, the i.v. injection of 0.5 mg/kg of bicuculline, a postsynaptic GABA antagonist 3,4, abolished the D R R and markedly reduced the DRP (Fig. 2A1 and 2). This dose, however, had no effect on the DRD (Fig. 2A). The efficacy of bicuculline in reducing presumably GABA-mediated phasic potentials such as the DRP and D R R compared to the incapacity of the drug to affect tonic potentials (DRD) suggests that GABA may not be involved in the mediation of the latter. This was confirmed in experiments in which GABA was depleted from the spinal cord by semicarbazide-induced blockade of GABA synthesis 2,9. Semicarbazide (200 mg/kg) abolished the DRP and D R R within 4 h of administration and also produced a drastic reduction in spinal cord GABA (0.007/zM GABA/g of spinal cord in the experiment illustrated in Fig. 2B as compared to 1.l ± 0.07 (S.E.M.) in untreated control animals). The DRD was, however, unaffected (Fig. 2B). This persistence of spontaneous activity after semicarbazide indicates that the DRD is not GABAmediated. Although both the DRD and the DRR are antidromic action potentials recorded from dorsal roots, they respond in a distinctly different manner to experimentally produced depolarizations of the afferent terminal. DRR generation is dependent upon the rate of afferent terminal depolarization and occurs only during the most rapidly rising phase of the DRP 7. This suggests that the DRR-generating mechanism accommodates rapidly to phasic, presumably GABA-mediated, depolarizations. In contrast, the DRD was observed to occur at a relatively constant rate for extended periods of time (Fig. 1B), but also reflected the time course and intensity of evoked depolarizations (Fig. I C). These observations suggest that the afferent fiber is normally subjected to a sustained depolarizing influence to which the DRD-generating mechanism does not accommodate. Thus, the DRD appears to reflect a process, intrinsic to the spinal cord, which maintains the primary afferent fiber in a depolarized condition - - a process which is distinct both physiologically and pharmacologically from classically described depolarizing influences onto the afferent terminal. We thank Ms. A. Williams for technical assistance and Dr. R. A. Levy tbr suggestions. This publication was supported by NIH Grant NS 05611.
l Barron, D. H. and Matthews, B. H. C., The interpretation of potential changes in the spinal cord, J. Physiol. (Lond.), 92 (1938) 276-321. 2 Bell, J. A. and Anderson, E. G., Semicarbazide induced depletion of ~-aminobutyric acid and blockade of presynaptic inhibition, Brain Research, 43 (1972) 161-169. 3 Curtis, D. R., Duggan, A. W., Felix, D. and Johnston, G. A. R., Bicuculline and central inhibition, Nature (Lond.), 226 (1970) 1222-1224.
152 4 Curtis, D. R., Duggan, A. W., Felix, D. and Johnston, G. A. R., Bicuculline, an antagonist of GABA and synaptic inibition in the spinal cord of the cat, Brain Research, 32 (1971) 69-96. 5 Eccles, J. C., Eccles, R. M. and Magni, F., Central inhibitory action attributable to presynaptic depolarization produced by muscle afferent volleys, J. PhysioL (Lond.), 159 (1961) 147-166. 6 Eccles, J. C., Magni, F. and Willis, W. D., Depolarization of central terminals of Group I afferent fibers from muscle, J. Physiol. (Lond.), 160 (1962) 62-93. 7 Eccles, J. C. and Malcolm, J. L., Dorsal root potentials of the spinal cord, J. Neurophysio/., 9 (1946) 139-160. 8 Inberg, K. R., Regeneration of the motor and sensory fibers in the sciatic nerve and the suralis nerve of the cat, Aetaphysiol. stand., 18 (1949) 308-323. 9 Killam, K. F., Convulsant hydrazides. II. Comparison of electrical changes and enzyme inhibition induced by the administration of thiosemicarbazide, J. Pharmacol. exp. Ther., 119 (1957) 263-271. 10 Levy, R. A., Repkin, A. H. and Anderson, E. G., The effect of bicuculline on primary afferent depolarization, Brain Research, 32 (1971) 261-265. 11 Nakanishi, T. and Norris, F. H., Jr., Motor fibers in rat sural nerve, Exp. Neurol., 26 (1970) 433435. 12 Schmidt, R. F., Presynaptic inhibition in the vertebrate central nervous system, Ergebn. Physiol., 63 (1971) 20-101.
147
(~] Elsevier/North-Holland Biomedical Press, Amsterdam - Printed in The Netherlands
Non-GABA mediated primary afferent depolarization
A. H. REPKIN, P. WOLF* and E. G. ANDERSON Department of Pharrnacology, College of Medicine, University of Illinois at the Medical Center, Chicago, Xl/. (u.s.A.)
(Accepted August 6th, 1976)
Primary afferent depolarization (PAD), the process postulated to underlie presynaptic inhibitionS, 6, is most readily monitored by observing changes in potentials recorded from dorsal roots and evoked by stimulation of adjacent afferents. The potentials usually recorded are the dorsal root potential (DRP), which reflects electrotonically conducted currents generated by afferent terminal depolarization 1,12, and the dorsal root reflex (DRR), which is manifested as a synchronous antidromic discharge in dorsal roots generated during the rapid depolarizing phase of the DRpT, 12. While studying the pharmacology of the DRP and D R R we have consistently observed a second form of antidromic activity in the dorsal root, that of spontaneously occurring antidromic action potentials, which is physiologically and pharmacologically distinct from the evoked DRR. These potentials appear to be a unique indicator of the degree of tonic PAD and, hence, an extremely useful monitor of afferent terminal polarization. The following report introduces and describes this phenomenon. Cats pretreated with atropine methyl nitrate (0.1 mg/kg) and anesthetized with ether were spinalized by transection of the cord at the C~ level following bilateral vagotomy and anemic decerebration. The general anesthetic was discontinued and the animals were artificially ventilated (12-15 respirations/min, end-tidal CO2, 3 ~_ 0 . 5 ~ ) with room air. Gallamine triethiodide, administered periodically, limited movement. The lumbar spinal cord was exposed from the L5 to S~ segments and covered with a pool filled with light mineral oil. Both oil pool and body temperature were maintained at 37 zk 0.5 °C. The dura mater was cut longitudinally, all dorsal roots thus exposed were severed. Recordings were made with bipolar platinum electrodes from a dorsal rootlet dissected from an L7 dorsal root. The proximal pole of the recording electrode was placed as close as possible to the cord without touching it; the distal pole supported the remainder of the rootlet. Potentials recorded from this rootlet were either evoked by stimulation of the remainder of the L7 dorsal root or occurred spontaneously. These latter potentials were led to an amplitude discriminator after amplification. The discriminator output (Schmidt-trigger pulses) was either converted * Visiting fellow to the Department of Pharmacology, University of Illinois at the Medical Center, on a grant of the Janggen-Poehn-Stiftung,St. Gallen, Switzerland. Current address : Neurochirurgische Universiffitsklinik, Zurich, Switzerland.
148
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I
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100msec
0.5my 5
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0
I hr
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Fig. I. A: spontaneously occurring action potentials (dorsal root discharge; DRD) recorded from a dorsal rootlet with bipolar electrodes. B : ratemeter recording of DRD generation over a 3 h period. C: effect of evoked depolarization of primary afferents on DRD generation. Upper trace: poststimulus time histogram (50 sweeps) showing the number (ordinate) and temporal distribution (abscissa) of DRD occurring prior to and following stimulation (arrow) of a neighboring dorsal root. Lower trace: computer average of 10 evoked depolarizations. to rates of occurrence and plotted with a rectilinear recorder or served as input signals for computer analysis. Under the above conditions, action potentials were observed to occur spontaneously in recordings from both whole dorsal roots or dorsal rootlets (Fig. 1A) but not in ventral roots. This activity, which we have termed the dorsal root discharge (DRD), occurs in only a minority of afferent fibers, since only a small number of active units were observed in dorsal rootlets containing hundreds of fibers. The number of axons firing in a given population was strongly influenced by the previous history of the preparation. In our experience, the D R D was abolished by local or general anoxic insult occurring during any stage of the experiment, by spinal cord edema, or by barbiturate anesthetics in doses as low as 10 mg/kg. On the other hand, traumatic insult during cord dissection frequently induced injury discharges which were manifest as high frequency bursts of antidromic action potentials which obscured the recording of the D R D . These injury discharges, unlike the D R D , were unaffected by anoxia. Thus, both their pattern of occurrence and response to decreased ventilation distinguished them from the D R D . The D R D originated from within the cord and not at the severed end of the dorsal rootlet. Thus, the application of 1 ~ lidocaine to the distal portion of the rootlet did not affect discharge frequency. Spontaneous activity was abolished, however, bylido-
149 caine applied between the recording electrodes and the spinal cord, indicating that these potentials travel away from the cord. Lidocaine applied to the site of cord transection (C)1 was also without effect on the DRD. This eliminates the possibility that spontaneously occurring potentials are generated by injury to ascending sensory collaterals severed by spinalization. These findings indicate that the DRD is generated wholly within the spinal cord. In addition, several experiments were performed in animals with intact nervous systems in which recordings were made from sural nerves. These preparations were anesthetized and artificially ventilated with ether during nerve dissection. Wound edges were then infiltrated with local anesthetic and ether withdrawn. Recordings from the sural nerve were characterized by a large amount of multiple unit activity which was increased by stroking or pinching of the leg. Lidocaine applied between the peripheral receptors and the recording electrodes abolished this activity and revealed spontaneous action potentials traveling antidromically in the nerve trunk. Thus, it appears that the D R D is normally overridden by orthodromic activity and is manifested peripherally only when the sensory neurons are deprived of their afferent activity. Furthermore, the data suggest that the DRD occurs in sensory fibers since, with the possible exception of small diameter autonomic efferents, the sural nerve contains only sensory axonsS, 11. These observations also suggest that the D R D does not arise from injuries suffered by the spinal cord during dissection. In summary, it appears that a spontaneous antidromic discharge occurs in sensory fibers which arises from a depolarizing influence exerted onto primary afferents within the spinal cord. The conduction velocity of fibers carrying the DRD was estimated using two electrodes placed on the dorsal rootlet and separated by 2.5-3 cm, each recorded against ground. Activity from a single unit was detected by the proximal electrode and led to an amplitude discriminator with a narrow window, the output of which triggered an oscilloscope. The potentials recorded from the distal electrode were displayed on a second oscilloscope beam, thus the potential of interest was time-locked to the trigger. This procedure allowed identification of a single unit recorded from both electrodes in spite of differences in the amplitude and shape of the potential 'seen' by the two electrodes. Conduction velocities calculated from the time delay between electrodes ranged between 40-80 m/sec (n ~ 40). Although measurements were subject to significant error due to the short distance between electrodes, they clearly fall into the A fiber range. Fig. l B illustrates the variation with time of DRD generation in a population of afferents. The relatively constant activity, maintained over a period in excess of 3 h, suggests that spontaneous potentials arise from a tonic depolarization of both a sustained and a constant magnitude. This behavior contrasts with some kinds of injury discharges in dorsal roots which decrease in frequency with time. Other characteristics of DRD generation were its relative stability in the face of marked changes in blood pressure induced by the administration of epinephrine or decamethonium. Only when blood pressure fell precipitously was there a reduction in the rate of DRD generation. The DRD is, however, temperature sensitive, increasing in rate with cooling of the oil pool surrounding the spinal cord and decreasing in frequency with warming above
150 body temperature. These and other characteristics of DRD behavior are the subject of a more extensive treatment which is currently in preparation. The interaction between spontaneous and elicited depolarizations of afferent fibers was characterized by observing the generation of antidromic action potentials during the time course of evoked dorsal root potentials. A high-pass filter eliminated the longer lasting evoked depolarizations (DRP) so that only transient potentials were 'seen' by the amplitude discriminator. The DRR, being a transient depolarization, was also represented in the discriminator output during the rising phase of the DRP The data are presented in the form of a poststimulus time histogram (upper trace, Fig. 1C) showing the number and temporal distribution ofantidromic activity occurring prior to and following electrical stimulation of neighboring dorsal root fibers. Also illustrated is the DRP evoked by this stimulation (lower trace, Fig. 1C). Comparison of the histogram and the DRP shows an increased occurrence of antidromic potentials during the DRP which parallels the intensity of the evoked depolarization. This
A
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(2)
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B events/sec
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,°° I 0 L_ 5rain 2 0 0 mg/kg semicarbazide HGI Fig, 2. A: effect of i.v. administration of 0.5 mg/kg (bar, upper record) of bicuculline on spontaneously occurring (upper record) and evoked (lower record) dorsal root activity. Upper record is a ratemeter recording of the DRD. Lower record is a computer average of the D R P and D R R (10 sweeps) recorded prior to (1) and following (2) bicuculline injection. B: ratemeter recording of D R D generation prior to and 4 h after i.v. administration of 200 mg/kg semicarbazide (30 min injection time).
151 correlation suggests that changes in the polarization of the afferent fiber are signaled by changes in the rate of D R D generation. In view of the possibility that the DRD represents a tonic component of the same mechanism responsible for generation of the D R R and DRP, the effects of bicuculline and semicarbazide, which block evoked potentials by dissimilar mechanisms, were examined on the DRD. As previously demonstrated 10, the i.v. injection of 0.5 mg/kg of bicuculline, a postsynaptic GABA antagonist 3,4, abolished the D R R and markedly reduced the DRP (Fig. 2A1 and 2). This dose, however, had no effect on the DRD (Fig. 2A). The efficacy of bicuculline in reducing presumably GABA-mediated phasic potentials such as the DRP and D R R compared to the incapacity of the drug to affect tonic potentials (DRD) suggests that GABA may not be involved in the mediation of the latter. This was confirmed in experiments in which GABA was depleted from the spinal cord by semicarbazide-induced blockade of GABA synthesis 2,9. Semicarbazide (200 mg/kg) abolished the DRP and D R R within 4 h of administration and also produced a drastic reduction in spinal cord GABA (0.007/zM GABA/g of spinal cord in the experiment illustrated in Fig. 2B as compared to 1.l ± 0.07 (S.E.M.) in untreated control animals). The DRD was, however, unaffected (Fig. 2B). This persistence of spontaneous activity after semicarbazide indicates that the DRD is not GABAmediated. Although both the DRD and the DRR are antidromic action potentials recorded from dorsal roots, they respond in a distinctly different manner to experimentally produced depolarizations of the afferent terminal. DRR generation is dependent upon the rate of afferent terminal depolarization and occurs only during the most rapidly rising phase of the DRP 7. This suggests that the DRR-generating mechanism accommodates rapidly to phasic, presumably GABA-mediated, depolarizations. In contrast, the DRD was observed to occur at a relatively constant rate for extended periods of time (Fig. 1B), but also reflected the time course and intensity of evoked depolarizations (Fig. I C). These observations suggest that the afferent fiber is normally subjected to a sustained depolarizing influence to which the DRD-generating mechanism does not accommodate. Thus, the DRD appears to reflect a process, intrinsic to the spinal cord, which maintains the primary afferent fiber in a depolarized condition - - a process which is distinct both physiologically and pharmacologically from classically described depolarizing influences onto the afferent terminal. We thank Ms. A. Williams for technical assistance and Dr. R. A. Levy tbr suggestions. This publication was supported by NIH Grant NS 05611.
l Barron, D. H. and Matthews, B. H. C., The interpretation of potential changes in the spinal cord, J. Physiol. (Lond.), 92 (1938) 276-321. 2 Bell, J. A. and Anderson, E. G., Semicarbazide induced depletion of ~-aminobutyric acid and blockade of presynaptic inhibition, Brain Research, 43 (1972) 161-169. 3 Curtis, D. R., Duggan, A. W., Felix, D. and Johnston, G. A. R., Bicuculline and central inhibition, Nature (Lond.), 226 (1970) 1222-1224.
152 4 Curtis, D. R., Duggan, A. W., Felix, D. and Johnston, G. A. R., Bicuculline, an antagonist of GABA and synaptic inibition in the spinal cord of the cat, Brain Research, 32 (1971) 69-96. 5 Eccles, J. C., Eccles, R. M. and Magni, F., Central inhibitory action attributable to presynaptic depolarization produced by muscle afferent volleys, J. PhysioL (Lond.), 159 (1961) 147-166. 6 Eccles, J. C., Magni, F. and Willis, W. D., Depolarization of central terminals of Group I afferent fibers from muscle, J. Physiol. (Lond.), 160 (1962) 62-93. 7 Eccles, J. C. and Malcolm, J. L., Dorsal root potentials of the spinal cord, J. Neurophysio/., 9 (1946) 139-160. 8 Inberg, K. R., Regeneration of the motor and sensory fibers in the sciatic nerve and the suralis nerve of the cat, Aetaphysiol. stand., 18 (1949) 308-323. 9 Killam, K. F., Convulsant hydrazides. II. Comparison of electrical changes and enzyme inhibition induced by the administration of thiosemicarbazide, J. Pharmacol. exp. Ther., 119 (1957) 263-271. 10 Levy, R. A., Repkin, A. H. and Anderson, E. G., The effect of bicuculline on primary afferent depolarization, Brain Research, 32 (1971) 261-265. 11 Nakanishi, T. and Norris, F. H., Jr., Motor fibers in rat sural nerve, Exp. Neurol., 26 (1970) 433435. 12 Schmidt, R. F., Presynaptic inhibition in the vertebrate central nervous system, Ergebn. Physiol., 63 (1971) 20-101.
