The Journal of Neurophysiology Vol. 80 No. 6 December 1998, pp. 3356-3360
Copyright ©1998 by the American Physiological Society

RAPID COMMUNICATION

ATP P_2× Receptors and Sensory Synaptic Transmission Between Primary Afferent Fibers and Spinal Dorsal Horn Neurons in Rats

Ping Li, Amelita A. Calejesan, and Min Zhuo

Department of Anesthesiology, Washington University, School of Medicine, St. Louis, Missouri 63110-1093

	ABSTRACT

Abstract Introduction Methods Results Discussion References

Li, Ping, Amelita A. Calean, and Min Zhuo. ATP P_2× receptors and sensory synaptic transmission between primary afferent fibers and spinal dorsal horn neurons in rats. J. Neurophysiol. 80: 3356-3360, 1998. Glutamate is a major fast transmitter between primary afferent fibers and dorsal horn neurons in the spinal cord. Recent evidence indicates that ATP acts as another fast transmitter at the rat cervical spinal cord and is proposed to serve as a transmitter for nociception and pain. Sensory synaptic transmission between dorsal root afferent fibers and neurons in the superficial dorsal horn of the lumbar spinal cord were examined by whole cell patch-clamp recording techniques. Experiments were designed to test if ATP could serve as a transmitter at the lumbar spinal cord. Monosynaptic excitatory postsynaptic currents (EPSCs) were completely abolished after the blockade of both glutamatergic -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate and N-methyl-D-aspartate receptors. No residual current was detected, indicating that glutamate but not ATP is a fast transmitter at the dorsal horn of the lumbar spinal cord. Pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), a selective P_2× receptor antagonist, produced an inhibitory modulatory effect on fast EPSCs and altered responses to paired-pulse stimulation, suggesting the involvement of a presynaptic mechanism. Intrathecal administration of PPADS did not produce any antinociceptive effect in two different types of behavioral nociceptive tests. The present results suggest that ATP P_2×2 receptors modulate excitatory synaptic transmission in the superficial dorsal horn of the lumbar spinal cord by a presynaptic mechanism, and such a mechanism does not play an important role in behavioral responses to noxious heating. The involvement of other P_2× subtype receptors, which is are less sensitive to PPADS, in acute nociceptive modulation and persistent pain remains to be investigated.

	INTRODUCTION

Abstract Introduction Methods Results Discussion References

The superficial dorsal horn of the spinal cord is important for sensory transmission, including information coding nociceptive information (Christensen and Perl 1970; Light et al. 1979; Light and Perl 1979; Kumazawa and Perl 1978; Pearson 1952; Rethelyi et al. 1989; Sugiura et al. 1986). Glutamate is a major neurotransmitter, and fast synaptic transmission in the dorsal horn is mainly mediated by -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate and N-methyl-D-aspartate (NMDA) receptors (Glaum et al. 1994; Hori et al. 1996; Nastrom et al. 1994; Randic et al. 1993; Schneider and Perl 1988, 1994; Yoshimura and Jessell 1990; see Yaksh and Malmberg 1994 for a review). ATP is also proposed as another fast transmitter for nociception (see Burnstock and Wood 1996; Kennedy and Leff 1995 for reviews). In the spinal dorsal horn, several ATP receptor subtypes including P2×_2,4,6 receptor subtypes are expressed in presynaptic terminals of primary afferent fibers and postsynaptic dorsal horn neurons (Brake et al. 1994; Buell et al. 1996; Collo et al. 1996; Cook et al. 1997; Vulchanova et al. 1996; see Burnstock and Wood 1996 for a review). Although a presynaptic action of ATP P_2× receptors on glutamate release was recently reported (Gu and MacDermott 1997), the involvement of postsynaptic P_2× receptors in synaptic transmission between primary afferent fibers and dorsal horn neurons is still unclear. ATP may serve as a neuromodulator to enhance glutamate induced responses by a postsynaptic mechanism (Li and Perl 1995) or induce postsynaptic currents in a subpopulation of dorsal horn neurons (Bardoni et al. 1997). In this study, we used in vitro whole cell patch-clamp recording techniques and in vivo behavioral tests to examine the role of P_2× receptors in sensory synaptic transmission and behavioral nociception.

	METHODS

Abstract Introduction Methods Results Discussion References

Slice preparation

Transverse spinal cord slices from postnatal (P4-P17) rats (Sprague-Dawley; Harlan) were prepared with the method of Takahashi (1990) with some modifications. Rats were deeply anesthetized with halothane and then cooled on ice for 10 min. The lumbar spinal cord was exposed, and ice-cold physiological saline solution (all in mM: 113 NaCl, 3 KCl, 25 NaHCO₃, 1 NaH₂PO₃, 2 CaCl₂, 1 MgCl₂, and 25 D-glucose, equilibrated with 95% O₂-5% CO₂, pH 7.3) was poured over it to cool the spinal cord. A segment of lumbar spinal cord was quickly removed and immersed in oxygenated, ice-cold saline for 3 min. The dura was removed and cross-section slices (150-200 µm in thickness) were cut with a Vibratome. The slices were kept in a filtering funnel filled with oxygenated saline at room temperature for 2 h.

Whole cell patch-clamp recording

Whole cell voltage-clamp experiments were performed with an Axopatch 200B patch-clamp amplifier. The slice was continuously perfused with oxygenated saline. Visually guided whole cell patch-clamp recordings were performed with the aid of an Axioskop microscope. The exact sites of recorded neurons were determined by biocytin labeling. Biocytin (0.5%) was added into the recording solution to label the cell. Whole cell patch-clamp recordings were made with 5-10 M electrodes without fire polishing. The recording pipette solution contained (in mM) 110 CsMeSO_3, 5 MgCl₂, 1 ethylene glycol-bis(-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), 40 N-2-hydroxdyethylpiperazine-N'-2-ethanesulfonic acid sodium, 2 MgATP, 0.1 Na₃GTP (pH 7.2, HCl), and the osmolarity was adjusted to 295-300 mosmol. After touching the soma with the pipette tip, gentle suction was applied to improve the seal until a high resistance (>10 G) was reached. Membrane potential was clamped at 70 mV (liquid junction potential not corrected). Typical series resistance was 14-39 M and monitored throughout the experiments.

View larger version (19K): [in this window] [in a new window]   FIG. 1. Excitatory monosynaptic responses in the superficial dorsal horn of the lumbar spinal cord are mediated by glutamatergic by -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors and N-methyl-D-aspartate (NMDA) receptors. A: evoked excitatory postsynaptic currents (EPSCs) induced electrical stimulation of the dorsal root nerves. Five responses induced by stimulation at a low intensity were displayed (with a 20-s interval). The time of stimulation is indicated by an arrow. B: evoked EPSCs induced electrical stimulation of the dorsal root nerves. In the same experiment as A, electrical stimulation at a high intensity elicited polysynaptic responses. Two responses induced at a 20-s interval were displayed. C: EPSCs induced by electrical stimulation of dorsal root nerves were completely blocked by application of both a AMPA/kainate receptor antagonist CNQX (10 µM) and a NMDA receptor antagonist AP-5 (50 µM). D: evoked monosynaptic EPSCs induced by placing a stimulating electrode at the dorsal root entry zone (DREZ) in the dorsal horn. The time of stimulation is indicated by an arrow. Five responses induced at a 20-s interval were displayed. One response of reduced magnitude was induced by stimulation at a low intensity. No polysynaptic response was observed. E: EPSCs induced by stimulation at the DREZ were completely blocked by application of both an AMPA/kainate receptor antagonist CNQX (10 µM) and a NMDA receptor antagonist AP-5 (50 µM). F: distribution of cells, which were labeled and from which no residual current was recorded after the blockade of glutamatergic receptors, illustrated on a representative coronal spinal cord slice (Paxinos and Watson 1997). Twenty-eight of 56 cells were successfully labeled.

View larger version (19K): [in this window] [in a new window]   FIG. 2. Presynaptic ATP P2× receptors regulate the release of glutamate. A: example of EPSCs to a paired-pulse stimulation delivered at a 50-ms interval before (left) or 10 min after (right) application of 10 µM pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). The time of stimulation is indicated by arrows. B: summary of data from 6 experiments. The paired-pulse stimulation ratio (P ratio = the 1st peak current/the 2nd peak current × 100) was calculated before and 10 min after 10 µM PPADS. Data are presented as means ± SE, * P < 0.05. C: model explaining the effect of PPADS on fast synaptic transmission in the lumbar spinal cord. At normal condition, fast synaptic transmission is mediated by postsynaptic glutamatergic AMPA and/or kainate receptors. ATP released from primary afferent fibers and/or spinal cord interneurons binds to presynaptic P2× receptors and enhances the release of glutamate (Glu). ATP may also act on postsynaptic P2× receptors to modulate glutamatergic synaptic transmission (see Li and Perl 1995).

View larger version (39K): [in this window] [in a new window]   FIG. 3. Blockade of spinal P2× receptors by PPADS is not antinociceptive. A: intrathecal administration of PPADS at 2 different doses (0.59 and 5.9 µg in 10 µl) did not significantly affect the tail-flick response latency (n = 10). Morphine (10 µg in 10 µl; n = 5) but not saline (10 µl; n = 5) produced significant increases in tail-flick response latency (12 s); * P < 0.05. B: hot-plate latency at 50°C was not significantly affected by intrathecal administration of PPADS (n = 6). Morphine (10 µg in 10 µl; n = 5) but not saline (10 µl; n = 5) produced significant increases in the hot-plate response latency; * P < 0.05.

Postsynaptic excitatory postsynaptic potentials (EPSCs) were evoked at 0.02-0.05 Hz with a bipolar tungsten electrode placed at the dorsal root entry zone (DREZ) or dorsal root afferent fibers. EPSCs were filtered at 1 kHz and digitized at 5 kHz. Monosynaptic EPSCs were identified with two criteria (Hori et al. 1996); 1) the response latency did not change with increasing intensities of electrical stimulation, and 2) EPSCs followed high-frequency stimulation (50 Hz) with reduced amplitude and without changing response latency. Inhibitory influences were removed by including bicuculline methoiodide (10 µM) and strychnine hydrochloride (1 µM) in the perfusion solution.

Biocytin staining

Slices were fixed in cold 10 mM phosphate-buffered saline (PBS; pH 7.2) containing 4% paraformaldeyhde (1-7 days at 4°C) and were then rinsed several times for 10 min each in PBS. To block endogenous peroxidases, slices were transferred into 1% H₂O₂ and 10% methanol in PBS for another 10 min. After one rinse in PBS, slices were pretreated in 2% Triton-100 in PBS for 1 h and incubated with avidin-biotin complex (1:100 in PBS; Vector Labs) solution for 2 h. Slices were reacted with diaminobenzidine (0.05%) containing 0.02% cobalt chloride, 0.02% nickel ammonium sulfate, and 0.01% hydrogen peroxide dissolved in PBS for 10-30 min. The slices were thoroughly rinsed and mounted onto gelatin-coated slides.

Behavioral nociceptive tests

Adult, male rats (Sprague-Dawley; Harlan) were used. The spinal nociceptive tail-flick reflex was evoked by noxious heat focused on the underside of the tail. The latency to reflexive removal of the tail from the heat was measured. A cutoff time of 12 s was employed to minimize damage to the skin of the tail. The hot-plate reflex was measured with a controlled metal plate at 50°C. A cutoff time of 65 s was used. The nociceptive responses were licking, lifting hindpaw, or jumping off the hot-plate, and the latency for response was recorded.

Intrathecal drug administration

The intrathecal injection was carried out as described previously (Hylden and Wilcox 1980) with some modifications. To avoid possible stress induced during intrathecal injection, rats were anesthetized with halothane (2%) during injection. After the injection, it took 2-3 min for rats to recover. The volume of the injection was 10 µl. Although baseline nociceptive thresholds were not affected by intrathecal administration of saline, we cannot completely exclude the potential damage introduced by intrathecal administration.

Data and analysis

Data are presented as means ± SE. Statistical comparisons were made with the use of analyses of variance (Newman-Keuls tests for post hoc comparison) or Student's t-test. P < 0.05 was considered significant.

	RESULTS

Abstract Introduction Methods Results Discussion References

To induce synaptic responses in the superficial dorsal horn, stimulating electrodes were placed at the DREZ (n = 120) or attached dorsal root nerves (n = 10). EPSCs were recorded from dorsal horn neurons in both cases. Direct stimulation of the dorsal root nerves induced monosynaptic EPSCs at low intensities (Fig. 1A); however, polysynaptic responses were often recorded when the intensity was increased (n = 10; see Fig. 1B). To stimulate a smaller population of afferent fibers, we placed stimulating electrodes near the DREZ. Short and constant latency fast EPSCs were induced. These EPSCs were monosynaptic because the response latency did not change with increasing intensities of electrical stimulation. Furthermore, they followed high-frequency stimulation (50 Hz) with reduced amplitude and without changing in latency. The amplitudes of EPSCs were stable when single stimulation pulses were delivered at 0.02 Hz.

To test if ATP may also be involved in fast synaptic transmission, we carried out experiments with two types of glutamate receptor antagonists, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 µM) for AMPA/kainate receptors and AP-5 (50 or 100 µM) for NMDA receptors, to block glutamate receptor-mediated synaptic responses. If there are other fast transmitters involved, we should have been able to detect some residual currents. In a total of 56 neurons tested, fast EPSCs were completely blocked by co-application of CNQX and AP-5 (Fig. 1E); 28 neurons were successfully labeled. They were located in lamina I (n = 5), II (n = 14), and III (n = 5). Similar results were obtained with direct stimulation of dorsal root nerves (n = 10; Fig. 1C). The intensity of electrical stimulation used was strong enough to activate high-threshold nociceptive unmyelinated C fibers because at the same intensity slow, substance P/neurokinin A-mediated synaptic currents were induced when six pulses of stimuli were delivered at 25 Hz (Li and Zhuo, unpublished data).

P_2× receptors are highly expressed in dorsal root ganglia (Brake et al. 1994; Chen et al. 1995; Cook et al. 1997), and activation of presynaptic P_2× receptors elicited glutamate release (Gu and MacDermott 1997). Several drugs were used to block P_2× receptors. Among them, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) is a selective antagonist for P_2× receptors (see Dale and Gilday 1996). We thus investigated possible presynaptic effects of PPADS. EPSCs to a paired-pulse stimulation at a 50-ms interval were recorded before and after application of 10 µM PPADS (PPADS at this dose did not affect other receptors) (see Dale and Gilday 1996). Interestingly, EPSCs were significantly decreased (n = 6, from 80.7 ± 19.8 pA to 38.9 ± 9.7 pA; P < 0.05). This inhibition was accompanied by significant changes in the ratio of paired-pulse responses (Fig. 2). In an example shown in Fig. 2, paired-pulse depression was observed before the application. At 10 min after PPADS, paired-pulse facilitation was induced (Fig. 2). Similar results were obtained from another five experiments (Fig. 2), indicating that PPADS produced its effect by blocking presynaptic P_2× receptors.

A previous study showed that PPADS did not produce any antinociceptive effect in the tail-flick (TF) test (Driessen et al. 1994). We reexamined the effect of PPADS in the TF reflex and also carried out experiments with the hot-plate (HP) test. Intrathecal administration of PPADS (0.6 or 5.9 µg) into the lumbar spinal cord produced no significant change in response latencies (Fig. 3). The skin temperature of the tail was not significantly affected (mean 22.5 ± 0.5°C, n = 5). At a higher dose (59 µg), PPADS induced abnormal aggressive behavior and made measurements impossible (n = 3). Rats became hypersensitive to gentle touch and thus no more testing was carried out. Intrathecal injection of vehicle saline (10 µl) affected neither the TF nor HP latency (n = 5; Fig. 3). Morphine, however, produced significant antinociceptive effects in the TF (n = 5) and HP (n = 5) tests when administered intrathecally at a similar quantity (10 µg; Fig. 3).

	DISCUSSION

Abstract Introduction Methods Results Discussion References

At the superficial dorsal horn of the lumbar spinal cord, we confirmed results of previous studies that glutamate is a major fast excitatory transmitter. No evidence was found for the involvement of P_2× receptors in synaptic transmission between primary afferent fibers and dorsal horn neurons. Consistent with previous reports (Li and Perl 1995; Bardoni et al. 1997), our results suggest that P_2× receptors do not play a major role in sensory transmission between primary afferent fibers and dorsal horn neurons at the lumbar spinal cord. In support of studies using cultured neurons (Gu and MacDermott 1997), presynaptic P_2× receptors regulate the presynaptic release of glutamate. Behavioral tests showed no antinociceptive effect produced after the blockade of spinal P_2× receptors by PPADS; if anything, there is a hyperalgesic effect at a high dose.

ATP has been proposed as an excitatory transmitter for primary afferent fibers (Jahr and Jessell 1983; Salter and Henry 1985; Salter and Hicks 1994). P_2× receptors are ligand-gated ion channels and were identified in dorsal root ganglion cells as well as the superficial dorsal horn of the spinal cord (Brake et al. 1994; Buell et al. 1996; Collo et al. 1996; Cook et al. 1997; Vulchanova et al. 1996; see Burnstock and Wood 1996 for a review). At the cervical spinal cord, EPSCs induced by focal electrical stimulation were mediated by P_2× receptors in a subpopulation (<5%) of dorsal horn neurons (Bardoni et al. 1996), providing evidence that ATP may contribute to sensory synaptic transmission. However, other studies failed to show any postsynaptic response mediated by P_2× receptors in cultured neurons or the lumbar spinal cord (Gu and MacDermott 1997; Li and Perl 1995). We found that at the lumbar spinal cord fast EPSCs were mediated by glutamatergic AMPA/kainate receptors, consistent with other reports (Hori et al. 1996; Jahr and Jessell 1985; Randic et al. 1993; Schneider and Perl 1988, 1994; Yoshimura and Jessell 1990). However, ATP may act as a neuromodulator to enhance synaptic transmission in the superficial dorsal horn of the spinal cord (Gu and MacDermott 1997; Li and Perl 1995). We showed that PPADS significantly decreased the amplitude of fast EPSCs and altered paired-pulse responses, indicating the involvement of a presynaptic mechanism (Schulz et al. 1994; Zalutsky and Nicoll 1990). Our finding of a presynaptic action of P_2× receptors, however, does not exclude a potential postsynaptic modulatory effect of P_2× as reported by Li and Perl (1995).

The involvement of P_2× receptors in nociception is less clear. Behavioral studies with different antagonists find that inhibition of P_2× receptors in the spinal cord is antinociceptive (Driessen et al. 1994). However, some of these drugs are not selective for P_2× receptors and could affect other transmitter receptors as well (e.g., NMDA receptors) (see Dale and Gilday 1996). PPADS, a more selective antagonist than other drugs, did not produce any antinociceptive effect in this study (see also Driessen et al. 1994). This suggests that P_2×2 receptors did not contribute to spinal nociceptive transmission. However, these studies do not completely exclude the involvement of other P_2× subtype receptors (such as P_2×4,6), which are less sensitive to PPADS (Buell et al. 1996; Collo et al. 1996) in acute nociceptive modulation as well as persistent pain.

In summary, our results suggest that ATP P_2×2 receptors modulate excitatory synaptic transmission in the superficial dorsal horn of the lumbar spinal cord by a presynaptic mechanism, and such a mechanism does not play an important role in behavioral nociceptive responses to noxious heating to the hindpaw or tail.

	ACKNOWLEDGEMENTS

  We thank G. Maccaferri for providing the protocol for biocytin staining and J. Li and X. D. Yang for technical advice. We also thank A. Warner for helpful comments on language revision.

  This work was supported in part by grants from the National Institute of Drug Abuse (DA-10833) and The McDonnell Center for Higher Brain Function in Washington University.

	FOOTNOTES

  Address for reprint requests: M. Zhuo, Dept. of Anesthesiology, Campus Box 8054, Washington University, School of Medicine, St. Louis, MO 63110.

  Received 18 May 1998; accepted in final form 10 August 1998.

	REFERENCES

Abstract Introduction Methods Results Discussion References

• BARDONI, R., GOLDSTEIN, P. A., LEE, C. J., GU, J. G., MACDERMOTT, A. B. ATP P_2× receptors mediate fast synaptic transmission in the dorsal horn of the rat spinal cord. J. Neurosci. 17: 5297-5304, 1997.[Abstract/Free Full Text]
• BRAKE, A. J., WAGENBACH, M. J., JULIUS, D. New structural motif for ligand-gated ion channels defined by an ionotropic ATP receptor. Nature 371: 519-523, 1994.[Medline]
• BUELL, G., LEWIS, C., COLLO, G., NORTH, R. A., SURPRENANT, A. An antagonist-insensitive P_2× receptor expressed in epithelia and brain. EMBO. J. 15: 55-62, 1996.[Medline]
• BURNSTOCK, G., WOOD, J. N. Purinergic receptors: their role in nociception and primary afferent neurotransmission. Curr. Opin. Neurobiol. 6: 526-532, 1996.[Medline]
• CHEN, C. C., AKOPIAN, A. N., SIVILOTTI, L., COLQUHOUN, D., BURNSTOCK, G., WOOD, J. N. A P_2× purinoceptor expressed by a subset of sensory neurons. Nature 377: 428-431, 1995.[Medline]
• CHRISTENSEN, B. N., PERL, E. R. Spinal neurons specifically excited by noxious or thermal stimuli: marginal zone of the dorsal horn. J. Neurophysiol. 33: 293-307, 1970.[Free Full Text]
• COLLO, G., NORTH, R. A., KAWASHIMA, E., MERLO-PICH, E., NEIDHART, S., SURPRENANT, A., BUELL, G. Cloning of P2×₅ and P2×₆ receptors and the distribution and properties of an extended family of ATP-gated ion channels. J. Neurosci. 16: 2495-2507, 1996.[Abstract/Free Full Text]
• COOK, S. P., VULCHANOVA, L., HARGREAVES, K. M., ELDE, R., MCCLESKEY, E. W. Distinct ATP receptors on pain-sensing and stretch-sensing neurons. Nature 387: 505-508, 1997.[Medline]
• DALE, N., GILDAY, D. Regulation of rhythmic movements by purinergic neurotransmitters in frog embryos. Nature 383: 259-263, 1996.[Medline]
• DRIESSEN, B., REIMANN, W., SELVE, N., FRIDERICHS, E., BÜLTMANN, R. Antinociceptive effect of intrathecally administered P₂-purinoceptor antagonists in rats. Brain Res. 666: 182-188, 1994.[Medline]
• GLAUM, S. R., MILLER, R. J., HAMMOND, D. L. Inhibitory actions of µ, ₁ and ₂ opioid agonists on excitatory afferent transmission in the adult rat spinal cord. J. Neurosci. 14: 4965-4971, 1994.[Abstract]
• GU, J. G., MACDERMOTT, A. B. Activation of ATP P2× receptors elicits glutamate release from sensory neuron synapses. Nature 389: 749-753, 1997.[Medline]
• HORI, Y., ENDO, K., TAKAHASHI, T. Long-lasting synaptic facilitation induced by serotonin in superficial dorsal horn neurones of the rat spinal cord. J. Physiol. (Lond.) 492: 867-876, 1996.[Medline]
• HYLDEN, J.L.K., WILCOX, G. L. Intrathecal morphine in mice: a new technique. Eur. J. Pharmacol. 67: 313-316, 1980.[Medline]
• JAHR, C. E., JESSELL, T. M. ATP excites a subpopulation of rat dorsal horn neurones. Nature 304: 730-733, 1983.[Medline]
• JAHR, C. E., JESSELL, T. M. Synaptic transmission between dorsal root ganglion and dorsal horn neurons in culture: antagonism of EPSPs and glutamate excitation by kynurenate. J. Neurosci. 5: 2281-2289, 1985.[Abstract]
• KENNEDY, C., LEFF, P. Painful connection for ATP. Nature 377: 385-386, 1995.[Medline]
• KUMAZAWA, T., PERL, E. R. Excitation of marginal and substantia gelatinosa neurons in the primate spinal cord: indications of their place in dorsal horn functional organization. J. Comp. Neurol. 177: 417-434, 1978.[Medline]
• LI, J., PERL, E. R. ATP modulation of synaptic transmission in the spinal substantia gelatinosa. J. Neurosci. 15: 3357-3365, 1995.[Abstract]
• LIGHT, A. R., PERL, E. R. Reexamination of the dorsal root projection to the spinal dorsal horn including observations on the differential termination of coarse and fine fibers. J. Comp. Neurol. 186: 117-131, 1979.[Medline]
• LIGHT, A. R., TREVINO, D. L., PERL, E. R. Morphological features of functionally defined neurons in the marginal zone and substantia gelatinosa of the spinal dorsal horn. J. Comp. Neurol. 186: 151-171, 1979.[Medline]
• NASTROM, J., SCHNEIDER, S. P., PERL, E. R. Differential L-glutamate responsiveness among superficial dorsal horn neurons. J. Neurophysiol. 72: 2956-2965, 1994.[Abstract/Free Full Text]
• PEARSON, A. A. Role of gelatinous substance of spinal cord in conduction of pain. Arch. Neurol. Psychiatry 68: 515-529, 1952.
• RANDIC, M., JIANG, M. C., CERNE, R. Long-term potentiation and long-term depression of primary afferent neurotransmission in the rat spinal cord. J. Neurosci. 13: 5228-5241, 1993.[Abstract]
• RETHELYI, M., LIGHT, A. R., PERL, E. R. Synaptic ultrastructure of functionally and morphologically characterized neurons of the superficial spinal dorsal horn of cat. J. Neurosci. 9: 1846-1863, 1989.[Abstract]
• SALTER, M. W., HENRY, J. L. Effects of adenosine 5'-monophosphate and adenosine 5'-triphosphate on functionally identified units in the cat spinal dorsal horn. Evidence for a differential effect of adenosine 5'-riphosphate on nociceptive vs non-nociceptive units. Neuroscience 15: 815-825, 1985.[Medline]
• SALTER, M. W., HICKS, J. L. ATP-evoked increases in intracellular calcium in neurons and glia from the dorsal spinal cord. J. Neurosci. 14: 1563-1575, 1994.[Abstract]
• SCHNEIDER, S. P., PERL, E. R. Comparison of primary afferent and glutamate excitation of neurons in the mammalian spinal dorsal horn. J. Neurosci. 8: 2062-2073, 1988.[Abstract]
• SCHNEIDER, S. P., PERL, E. R. Synaptic mediation from cutaneous mechanical nociceptors. J. Neurophysiol. 72: 612-621, 1994.[Abstract/Free Full Text]
• SCHULZ, P. E., COOK, E. P., JOHNSTON, D. Changes in paired-pulse facilitation suggest presynaptic involvement in long-term potentiation. J. Neurosci. 14: 5325-5337, 1994.[Abstract]
• SUGIURA, Y., LEE, C. L., PERL, E. R. Central projections of identified, unmyelinated (C) afferent fibers innervating mammalian skin. Science 234: 358-361, 1986.[Abstract/Free Full Text]
• TAKAHASHI, T. Membrane currents in visually identified motoneurones of neuronal rat spinal cord. J. Physiol. (Lond.) 423: 27-46, 1990.[Abstract/Free Full Text]
• VULCHANOVA, L., ARVIDSSON, U., RIEDL, M., WANG, J., BUELL, G., SURPRENANT, A., NORTH, R. A. Differential distribution of two ATP-gated ion channels (P_2× receptors) determined by immunocytochemistry. Proc. Natl. Acad. Sci. USA 93: 8063-8067, 1996.[Abstract/Free Full Text]
• YAKSH, T. L. AND MALMBERG, A. B. Central pharmacology of nociceptive transmission. In: Textbook of Pain, edited by P. D. Wall and R. Melzack. New York: Churchill Livingstone, 1994, 3rd ed., p. 165-200.
• YOSHIMURA, M., JESSELL, T. M. Amino acid-mediated EPSCs at primary afferent synapses with substantia gelatinosa neurones in the rat spinal cord. J. Physiol. (Lond.) 430: 315-335, 1990.[Abstract/Free Full Text]
• ZALUTSKY, R. A., NICOLL, R. A. Comparison of two forms of long-term potentiation in single hippocampal neurons. Science 248: 1619-1624, 1990.[Abstract/Free Full Text]

0022-3077/98 $5.00 Copyright ©1998 The American Physiological Society