Control of Size and Excitability of Mechanosensory Receptive Fields in Dorsal Column Nuclei by Homolateral Dorsal Horn Neurons

Robert W. Dykes1 and A. D. Craig2

1 Département de Physiologie, Université de Montréal, Montreal, Quebec H3C 3J7, Canada; and 2 Division of Neurobiology, Barrow Neurological Institute, Phoenix, Arizona 85013

    ABSTRACT
Abstract
Introduction
Methods
Results
Discussion
References

Dykes, Robert W. and A. D. Craig. Control of size and excitability of mechanosensory receptive fields in dorsal column nuclei by homolateral dorsal horn neurons. J. Neurophysiol. 80: 120-129 1998. Both accidental and experimental lesions of the spinal cord suggest that neuronal processes occurring in the spinal cord modify the relay of information through the dorsal column-lemniscal pathway. How such interactions might occur has not been adequately explained. To address this issue, the receptive fields of mechanosensory neurons of the dorsal column nuclei were studied before and after manipulation of the spinal dorsal horn. After either a cervical or lumbar laminectomy and exposure of the dorsal column nuclei in anesthetized cats, the representation of the hindlimb or of the forelimb was defined by multiunit recordings in both the dorsal column nuclei and in the ipsilateral spinal cord. Next, a single cell was isolated in the dorsal column nuclei, and its receptive field carefully defined. Each cell could be activated by light mechanical stimuli from a well-defined cutaneous receptive field. Generally the adequate stimulus was movement of a few hairs or rapid skin indentation. Subsequently a pipette containing either lidocaine or cobalt chloride was lowered into the ipsilateral dorsal horn at the site in the somatosensory representation in the spinal cord corresponding to the receptive field of the neuron isolated in the dorsal column nuclei. Injection of several hundred nanoliters of either lidocaine or cobalt chloride into the dorsal horn produced an enlargement of the receptive field of the neuron being studied in the dorsal column nuclei. The experiment was repeated 16 times, and receptive field enlargements of 147-563% were observed in 15 cases. These data suggest that the dorsal horn exerts a tonic inhibitory control on the mechanosensory signals relayed through the dorsal column-lemniscal pathway. Because published data from other laboratories have shown that receptive field size is controlled by signals arising from the skin, we infer that the control of neuronal excitability, receptive field size and location for lemniscal neurons is determined by tonic afferent activity that is relayed through a synapse in the dorsal horn. This influence of dorsal horn neurons on the relay of mechanosensory information through the lemniscal pathways must modify our traditional views concerning the relative independence of these two systems.

    INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References

Relay cells in the somatosensory pathways receive synaptic influences from one or more classes of cutaneous mechanoreceptors. As well, they are modulated by descending signals and by local excitatory and inhibitory circuitry. Within the limits of those anatomically defined connections, the overt receptive field of a neuron, as well as its size, location, and threshold, are determined by the balance of excitation and inhibition impinging on the cell. Manipulations of the skin surface alter this balance, allowing the relay neuron to express previously existing but suppressed afferent signals. Numerous studies of central relay regions after peripheral nerve lesions and amputations document results of changing the relative contributions of these influences (e.g., Florence and Kaas 1995; Kaas 1991; Koeber and Brown 1995).

Most relevant to the present experiments is the study by Calford and Tweedale (1991), who, by applying capsaicin on peripheral nerves of monkeys and flying foxes, blocked tonic afferent signals travelling in unmyelinated axons serving the skin and thereby producing dramatic enlargements in the receptive fields of mechanosensory cortical neurons. They concluded that tonic activity in the small-diameter axons of peripheral nerves drove central inhibitory processes controlling receptive field size. Because most small-diameter axons of peripheral nerves synapse in the dorsal horn (DH) and because their activity is carried by second-order neurons projecting from the DH, it follows that the signals controlling inhibitory influences on central mechanosensory neurons may be relayed through DH neurons.

Although the hypothesis that the receptive field size of somatosensory cortical neurons is controlled by tonic influences relayed through the spinal DH may be a novel, there is growing evidence that the classic dichotomy between the lemniscal and spinothalamic pathways is not as complete as once imagined. For example, psychophysical studies suggest that small-diameter fibers have an influence on tactile sensations; Apkarian et al. (1994) showed that heat-induced pain modifies tactile perceptions. Behavioral responses occur to cutaneous stimuli applied to previously anesthetic skin after spinal cord lesions in monkeys, suggesting that DH mechanisms normally suppress perception of light tactile stimuli (Denny-Brown et al. 1973; Kirk and Denny-Brown 1970).

At the first synapse in the dorsal column-lemniscal pathway, the cutaneous mechanosensory receptive field is determined by interactions among excitatory and inhibitory influences. For example, Petit and Schwark (1993, 1996) showed that the receptive field of a neuron in the cat dorsal column nuclei (DCN) will shift to a new location when a small quantity of capsaicin or lidocaine is injected subcutaneously into the existing receptive field. Panetsos et al. (1995) showed that this change is perpetuated along the sensory pathway to the somatosensory cortex, and a new, well-organized receptive field serving the same class of sensory receptors appears on the skin adjacent to the original receptive field. We suspect that significant modulation of receptive fields in the DCN occurs through influences mediated by smaller fibers that relay in the DH. Berkley and Hubscher (1995) summarized electrophysiological evidence for interactions between DCN and the nonlemniscal spinal routes in rats. The older literature reveals other data consistent with the idea that an inhibitory mechanism influencing mechanosensory pathways is located in the DH. For example, by manipulating transmission in the spinal cord and DH, Dostrovsky and his colleagues (Dostrovsky 1980; Dostrovsky and Millar 1977; Dostrovsky et al. 1976) showed that the lemniscal pathways are influenced by activities in the spinal DH. Recently, Northgrave and Rasmusson (1996) suggested that this would be one possible mechanism to explain their observations of changes in receptive field locations of mechanosensory neurons in the cuneate nuclei of raccoons.

To explicitly test the hypothesis that neuronal activity in the DH affects receptive field size of DCN neurons, we recorded from single cells in the DCN of cats before and after inactivating regions of the DH with lidocaine or cobalt chloride.

    METHODS
Abstract
Introduction
Methods
Results
Discussion
References

Adult mongrel cats weighing 4.0-5.7 kg were anesthetized with intraperitoneal injections of pentobarbital sodium (Nembutal, 35 mg/kg from a solution of 50 mg/ml) and maintained areflexic by smaller doses administered through an intravenous catheter placed in the cephalic or femoral vein. Most animals also were given 10 mg of dexamethasone (0.2 ml of a 50 mg/ml solution of Solucortef, UpJohn, 5 mg). After shaving the relevant limb, head, and back, the animal was suspended in a stereotaxic device adapted for surgical exposure of the spinal cord. Laminectomies were performed over the DCN and also over either the cervical or lumbosacral spinal cord (Dykes et al. 1982). Body temperature was maintained with a feedback-controlled heating pad and heat-lamp. In one-half of the experiments pancuronium bromide (0.2 ml of 2 mg/ml solution, Astra Pharmaceuticals) was injected, and the animals were respired artificially while CO2 was monitored and maintained at an end-tidal value of 3.5%. In those cases, blood pressure was monitored with a catheter in the carotid artery.

In the area of the spinal laminectomy, a low-impedance microelectrode (0.5-1.0 MOmega ) was inserted into laminae IV and V (1.0-1.5 mm deep) of the left or right DH, and the skin surface was stimulated to elicit multiunit activity. We recorded receptive fields defined in this manner on caricatures of the body and noted the electrode entry points on a drawing of the spinal cord. By repeating this process along the length of the exposed cord in both the medial and lateral parts of the DH, a rough map of the somatotopy of the DH was created (Brown and Fuchs 1975).

At this point, the low-impedance electrode was moved to the exposed DCN and a similar map was created for the representation of the limb in either the cuneate or gracilis nucleus. When we found a multiunit receptive field in the DCN that corresponded to a readily accessible region of the limb mapped in the DH, we changed to a higher impedance tungsten-in-glass electrode (1.5-4.0 MOmega , 1-2 µm tip) useful for isolating single units. The DCN were covered with 4% agar to improve mechanical stability, and a single unit with a mechanosensory receptive field was isolated (Figs. 1 and Fig. 3). At the end of the recordings, lesion was made with 10 µA of current administered through the electrode for 10 s.


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FIG. 1. A: sketch showing the locations of the major blood vessels of the lumbosacral spinal cord used as landmarks while making a map of layers IV and V of the dorsal horn (DH). Receptive fields found at 5 of the recording sites are numbered. Site 3, marked with an asterisk served the same body part as the site in the gracilis nucleus also marked with an asterisk. B: locations of receptive fields found at each numbered recording site. C: sketch of the vascular pattern over the dorsal column nuclei (DCN). indicating some of the recording sites studied to find a region in the gracilis nucleus with a receptive field corresponding to one of those located in the lumbar DH. Circles indicate the receptive field locations of neurons at the indicated recording sites. The asterisk indicates the site where the cell in Fig. 2A was recorded.

The mapping procedures for cutaneous receptive fields are well-established (Craig and Tapper 1978; Dykes et al. 1980; Leclerc et al. 1993). With qualitative stimuli, one can distinguish between several classes of mechanoreceptors, nociceptors, and thermoreceptors. These distinctions and the definition of the receptive field border are reproducible over time and among observers (Stryker et al. 1987). In the region being studied, the hair was cut to a uniform length of ~1 mm. Manually held, blunt probes were used to outline the receptive field by light pressure or movements of a few hairs. The receptive field border was taken as that point where a reproducible response could be elicited with each stimulus, and the boundary of each field was confirmed by the other experimenter.

Receptive field boundaries were drawn on the skin of the animal, and the outline subsequently was transferred to paper. Those receptive fields on a curved skin surface were projected perpendicularly from the skin to the paper so that the actual area on the skin surface was proportional to the area on the paper. Changes in receptive field size were recorded in the same manner. All measurements of receptive field area were made relative to the receptive field area in the control condition so that any distortions or changes of scale between animals and between body parts were eliminated. Because the hypothesis being tested suggested a possible contribution from small-diameter afferent fibers having ongoing activity, such as thermoreceptors, both warm and cold stimuli in the form of blunt metal objects (1-2 cm diam) cooled in ice water or warmed with an infrared lamp to temperatures between 0-10 and 40-42°C, respectively, were applied to the skin surface on and near the receptive field. These stimuli produced distinct, but innocuous sensations in the experimenters. Such stimuli that differ from the ambient skin temperature by >8°C provoke maximal responses in thermoreceptors (Darian-Smith et al. 1973). Noxious stimuli were limited to several pinches with toothed forceps after the mechanosensory receptive field had been mapped. Injections of drugs were performed with a glass micropipette attached to the end of a Hamilton syringe. This arrangement, with the tip of the pipette filled with one of the two solutions, was mounted on an electrode holder of the Kopf stereotaxic device and positioned over the desired location in the spinal cord.

Guided by published experiments, we used 100-400 nl of 2% (20 mg/ml) lidocaine chlorohydrate (Xylocaine, Astra Pharmaceuticals, Mississauga, Ontario, Canada) or 4 mM CoCl2 in distilled water. Lidocaine blocks conduction in both neurons and axons of passage, whereas the CoCl2 acts only at synaptic junctions where it prevents depolarization by blocking potassium channels (Lee and Malpeli 1986). Because both substances were effective, we could attribute our observations to synaptically mediated processes. Both substances diffuse rapidly in regions of grey matter, and effects are seen virtually instantaneously (Malpeli 1983, Malpeli et al. 1981). In contrast, diffusion within white matter such as the optic tract may be slower because optic tract axons may take <= 5 min to be blocked (J. G. Malpeli, personal communication). A 100 nl injection of either substance seems to be effective within a volume having a radius of ~500 µm (Malpeli 1983; Malpeli et al. 1986). Mayers (1966) described a spherical distribution for similar volumes of dyes, and Martin (1991) attempted to make estimates of such injections using autoradiographic methods in rat cortex. The cube-root relationship between the volume and the radius of a sphere suggests that a 400 µL injection would spread ~1.6 times as far as a 100 µL injection. At the end of the experiment, the animal was killed with an overdose of anesthetic and perfused through the heart with 500 ml of cold saline followed by 1 l of 10% phosphate-buffered formalin. The relevant sections of the cord were removed and placed in refrigerated, buffered formalin overnight or until they were prepared for frozen sections by immersion in 25% sucrose for 48 h. The tissue was sectioned on a freezing microtome at 60 µm and lesions were located in cresyl violet-stained sections.

    RESULTS
Abstract
Introduction
Methods
Results
Discussion
References

Receptive field enlargements

The nature of the experiment and the desire to avoid residual effects of previous manipulations of the DH precluded examination of more than one DCN neuron per nucleus. On some occasions, we isolated a second cell in the contralateral nucleus, thereby allowing us to study 16 cells in nine cats (Table 1). In 10 cases, 200-600 nl of lidocaine was injected into the DH and 100-400 nl of CoCl2 was used in the other 6 cases. In all but one of these experiments, our injections into the DH produced an enlargement of the mechanosensory receptive field of the neuron isolated in the DCN. The effects of the injected substance could begin within 5-10 s, but in other cases, it took 1-2 min. The effect generally lasted between 3 to 20 min after the pipette was removed, with one case lasting 1.5 h, the duration of the effect apparently being related to the rate of dissipation of the injected drug. The greatest receptive field enlargements were observed on the limbs and smallest on the digit tips, but in terms of the percentage increase, the changes were similar in both locations. Warm and cold objects placed on or adjacent to the receptive field produced no change in its size or responsiveness.

 
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TABLE 1. Cells isolated and receptive fields mapped before and after spinal injection of lidocaine or cobalt chloride

An experiment in the gracile nucleus and lumbosacral DH is summarized in Fig. 1. The unit isolated in the DCN had a receptive field on the medial side of the ankle. Once its receptive field was mapped, the site in the DH serving the same body part was located from the previously prepared map, and a pipette filled with lidocaine was positioned at that point. While still recording from the cell in the gracilis nucleus, the pipette was inserted ~1,000 µm into the cord DH and 200 nl of lidocaine was injected. During the next few minutes, the receptive field of the gracile neuron gradually enlarged (Fig. 2A). Thirty minutes later, when the pipette was removed, the receptive field still was enlarged. During the next 15 min, the receptive field gradually decreased to nearly its original size, leaving only a slight expansion on the proximal side. Three minutes later the pipette was reinserted, 200 µl of lidocaine was injected again, and the observations were repeated. This time more attention was given to the pattern of the neuronal discharge; the spontaneous activity increased within 1 min, the cell began discharging in bursts of two to three spikes and stimulation of the receptive field produced a more robust response than it had previously. Within 10 min, the receptive field had increased to the size observed after the first injection.


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FIG. 2. A: receptive field of a single unit in the gracilis nucleus before and after injection of 200 µl of lidocaine into the lumbar DH at the site indicated by the asterisk in Fig. 1C. B: receptive field of the 2nd cell recorded in this experiment from the contralateral gracilis nucleus. Receptive field on the heel enlarged to encompass most of the foot when 400 nl of lidocaine was injected into the DH at a site serving the heel of the foot (C). After the lidocaine had dissipated, the receptive field returned to its original size except for a site on the lateral side of the metacarpal pad and the lateral side of the receptive field where it responded to touch with several action potentials.

Later, in the contralateral DCN, another cell was isolated having a receptive field on the heel of the foot. Initially the spontaneous activity was <1/s. The effective stimuli suggested that the cell served mechanoreceptors optimally activated by vibrations or rapid movements. At the site in the DH where the corresponding receptive field was located, 400 nl of lidocaine was injected at a depth of 1,000 µm. Within 10-20 s, the spontaneous activity increased to 5 Hz, and the cell began to discharge in bursts of three spikes. Within 20 min, the receptive field had enlarged to more than twice its original size (Fig. 2B). An hour after the lidocaine pipette was removed, the original receptive field was still slightly larger than the original, and action potentials could still be provoked from a small region of skin adjacent to the metatarsal pad that had not been part of the original receptive field (Fig. 2C). One hour and 30 min later the receptive field had returned to its original size.

After performing six experiments in the gracile nucleus wherein 10 cells were studied, the same experiment was performed in the cuneate nucleus. In one experiment (Fig. 3A), the receptive field of a cuneate neuron was located in the hairy skin on the ventral aspect of the fifth digit. Within 1 min after a lidocaine injection into the corresponding part of the cervical DH, the receptive field began to enlarge. Within 8 min, it attained the size illustrated in Fig. 3B. The spontaneous activity increased, and bursts of 10-15 spikes appeared. An additional 200 nl of lidocaine produced little additional increase in the size of the receptive field or in the rate of spontaneous activity. Tests with 128- and 256-Hz tuning forks suggested that this neuron received an important contribution from pacinian corpuscles. After 40 min, the pipette containing lidocaine was removed, but the receptive field failed to decrease in size. To understand why reversal did not occur, a lesion was made to mark the recording site in the cuneate nucleus (Fig. 3C), and the recording electrode was removed and inserted into the DH. No neural activity was found at the site of the injection pipette in the DH even though somatically driven activity was found at sites 1-2 mm away. Those nearby sites contained neurons serving skin on portions of the forelimb adjacent to the site served by the studied receptive field, suggesting that this region of the DH had been irreversibly damaged in some way during the lidocaine injection procedure.


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FIG. 3. A: vascular pattern of the cervical DH in the region of the cuneate nucleus. Each point is a recording site within the cuneate nucleus having the receptive field indicated below. B: receptive field enlargement of a rapidly adapting cuneate neuron with a receptive field located initially on the volar surface of the first digit. After 400 nl of lidocaine was injected into the DH, the receptive field enlarged to encompass much of the ventral surface of the paw. C: thionin-stained section showing the recording site (right-arrow) in the cuneate nucleus.

The receptive fields of each of the six cells studied in the cuneate nucleus showed an enlargement after the injection. The size of the enlargement varied from one case to another and the temporal development of the enlargement varied from the immediate effect described earlier to one case where the enlargement took 20 min to develop. We believe that the variability in the timing and magnitude of the response depended upon the accuracy of the placement of the injection pipette. For example, one case where no effect was observed after injection at one site, the pipette was reinserted in a second site whereupon the lidocaine injection produced a rapid enlargement. These differences may be explained by the hypothesis that there is a somototopic order to the DH influences on cells of the DCN and that in those cases where our injections took longer to act, the injections were made farther away from the region of the DH having the greatest influence and only after some time did the lidocaine diffuse to those neurons controlling the receptive field of the neuron being studied. The range of receptive field changes observed in these experiments is shown in Fig. 4.


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FIG. 4. A: examples of receptive field enlargements of single neurons in the DCN after lidocaine injections into the DH. Areal increases ranged from 174 to 549% of the original receptive field. B: examples of the enlargements of receptive fields observed in single neurons of the DCN after injections of cobalt chloride into the DH at the corresponding point in the spinal cord representation of the body surface. Enlargements ranged from 147 to 563% of the original receptive field size. Actual sizes vary depending upon the body part from which the diagram was made. All measures are expressed as a percentage of the control receptive field size.

Nature of the adequate stimulus

Convergence of afferent signals from several axons onto a single postsynaptic cell is characteristic of all synaptic relay regions, and the DCN are no exception. The literature provides evidence from both physiological and anatomic studies of convergence upon neurons in the DCN and the enlargements observed in our studies are consistent with such an arrangement. However, convergence also may provide an opportunity for receptors of different class to converge on the postsynaptic neuron. Thus we searched for evidence of inputs from other receptor classes that might be uncovered during the DH injections. Despite considerable attention to this question by careful application of sustained and moving mechanical stimuli as well as thermal and noxious stimuli, we could not demonstrate convergence of inputs from other classes of receptors in the control state. The majority of the cells studied were activated by movements of hairs (Table 1). Except for two cells that were slowly adapting, the DCN neurons adapted rapidly to sustained mechanical deformation of the skin or to the bending of hairs. Under control conditions, we saw no effects of either thermal and noxious stimuli, however, noxious stimuli were limited to a few pinches with toothed forceps in and around the mechanosensory receptive field to avoid sensitization of the skin surface.

These manipulations were repeated when the DH injection produced an enlarged receptive field but no evidence of inputs from other receptor classes could be observed. Thus we never saw a rapidly adapting response change to a slowly adapting response. There were never responses produced by massage of muscles or flexion of joints, and thermal stimuli did not alter either the evoked or spontaneous activity. These subjective tests obviously do not allow us to distinguish between classes of rapidly adapting mechanoreceptors that may converge onto the same cell. As well, it is possible that the greater intensity of the mechanosensory responses observed during the DH injections might have masked some more subtle effects of other receptor classes that, as a result, went unnoticed.

Evidence for a synaptic relay in the DH-mediating influences controlling receptive field size

Injections of lidocaine may have blocked conduction in axons passing through the injected region of the DH. Thus the effects of lidocaine injections into the DH might have been attributable to blocking large, myelinated axons that take a circuitous route to the DCN. We could eliminate this possibility with injections of CoCl2 because this substance does not block axons but transiently blocks synaptic transmission without affecting fibers of passage (Lee and Malpeli 1986). In each of the six experiments employing CoCl2, an increased responsiveness occurred and receptive field enlargement was observed in the DCN (Fig. 4B). The effect of as little as 200 nl of CoCl2 was observed within seconds; the receptive field became more sensitive and within a few minutes began to enlarge. In one case, another 200 nl, injected 18 min after the first, produced further enlargement of the receptive field, suggesting that the pipette in that experiment might have been some distance from the most effective site in the DH. Twenty minutes after the pipette was removed, the receptive field had returned to a size similar to the original. The bursts of action potentials elicited by stimulation of the receptive field during injections of CoCl2 were similar to those seen during lidocaine injections.

Spinal cord transections

To identify possible routes that postsynaptic fibers might take to influence neurons in the DCN, we terminated four experiments by transecting parts of the spinal cord white matter between the injection site and the DCN with the point of a No. 11 scalpel blade. Transection of the contralateral cord (2 cases) produced little or no effect except possibly a slight increase in responsiveness within the original receptive field. This suggests that the contralateral spinal cord does not carry the major portion of the axons responsible for our observations. In contrast, in three of four cases when the ipsilateral dorsolateral cord was sectioned lateral to the dorsal columns within several centimeters of the injection site, the receptive field in the DCN immediately enlarged to a size similar to that observed during the lidocaine or CoCl2 block of the DH, suggesting that the initial part of the pathway influencing receptive field size in the DCN is located in the ipsilateral quadrant.

    DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References

Our results show that neural activity passing through the ipsilateral DH affects receptive field size and responsiveness of cutaneous mechanosensory neurons in the DCN. The effect of the CoCl2 injections suggests that these signals are relayed by second-order neurons located in the deeper layers of the DH; blocking this synaptic relay seems to have relieved a tonic inhibition on both cuneate and gracile neurons. Not only did the receptive field size increase, but the spontaneous activity and response magnitude increased as well. However, there was no obvious change in the nature of the somatic stimuli capable of activating the DCN cells. These influences from the DH may have a somatotopic organization because some injection sites were more effective than others.

The DH mechanism that we manipulated is a tonic influence and must be part of dynamic equilibrium of excitation and inhibition that controls the size and location of the overt receptive field (Dostrovsky and Millar 1977; Dostrovsky et al. 1976). Afferent fibers ending in the DH must provide a constant background signal, preparing the canvas upon which mechanosensory events are sculpted. Perl (1984) posited a comparable role for tonic activity in DH neurons when he hypothesized that spontaneous activity in wide-dynamic-range neurons could serve as the gain control mechanism in the sensory pathways. That this phenomenon is so important even in anesthetized animals suggests that it is likely to be a dominant control mechanism in the lemniscal pathways. Our observations, as well as those of Petit and Schwark (1993, 1996) and Panetsos et al. (1995) support the idea that this tonic effect on DCN neurons arises from activity in peripheral nerves. The data are also consistent with the arguments that small afferent peripheral nerve fibers convey the relevant signal (Calford and Tweedale 1991). Our demonstration that the neural activity is blocked by CoCl2 shows that the first synapse of the pathway is in the DH. The DH neurons responsible for these effects were not located in the histology, but because the injection site was ~1 mm below the cord dorsum and because the effective spread of the injected substances was probably <500 µm (Malpeli 1983; Malpeli et al. 1981, 1986) we can predict that the relay neurons were located in layers IV and/or V of the spinal grey matter, a location consistent with the known locations of other neurons with long axons. The acute transections suggest that the second-order axons begin their ascent in the ipsilateral dorsal quadrant. Thus our data are consistent with the horseradish peroxidase-labeling studies of Rustioni and Kaufman (1977) in the cat. These workers reported that the cells of origin of the postsynaptic dorsal column fibers ending in the DCN arose from medial lamina VI in the upper cervical cord and from lamina IV and medial V in the brachial and lumbar regions ipsilateral to their target.

Although we did not systematically examine the hypothesis with regularly spaced injections along the length of the spinal DH, the variable delays before the onset of the effects of our injection and the need to reposition the injection pipette on several occasions suggests to us that there may be a somatotopically ordered relationship between the processes in the DH and their targets in the DCN. The only other explanation we can offer for such differences in effectiveness of our injections is that the injection pipette missed the grey matter and the injections were placed inadvertently in the adjacent white matter. This remains as an interesting hypothesis to examine with future experiments.

Both Petit and Schwark (1993, 1996) and Calford and Tweedale (1991) argued for spontaneous activity in unmyelinated fibers as the driving force for tonic afferent inhibition of the lemniscal mechanosensory pathway. Calford and Tweedale (1991) took this position because capsaicin injections block unmyelinated but not myelinated fibers, and Petit and Schwark (1993) argued this because lidocaine blocks unmyelinated fibers before blocking myelinated fibers. It is difficult to suggest which class of small afferent fibers might produce such a tonic afferent activity. Northgrave and Rasmusson (1996) argue that it is unlikely to be mechanoreceptors. The smaller myelinated fibers such as cold fibers have a significant ongoing activity, but in our experiments, cooling or warming the skin never produced changes in receptive field size. Although little spontaneous activity has been reported in other classes of unmyelinated fibers, they outnumber myelinated fibers in peripheral nerves by 5-10 times. Thus with sufficient convergence, a low level of ongoing activity in many unmyelinated primary afferent axons could generate an important tonic drive on DH neurons. It has been shown that low levels of activity in unmyelinated fibers do not necessarily provoke sensations (Gybels et al. 1979). Newer neuroanatomic methods have unveiled an unexpected diversity of unmyelinated or lightly myelinated sensory axons serving skin, and one or several of these could provide the needed tonic activity (Fundin et al. 1995, 1997).

The route whereby these tonic influences might arrive in the DCN is not clear. A number of possibilities exist (Berkley and Hubscher 1995). We can rule out direct access via small myelinated primary afferent axons that travel in the dorsal columns (Briner et al. 1988; Patterson et al. 1989, 1990) because our data show that there is at least one synapse in the pathway at the level of the DH. This leaves open, however, relay through the postsynaptic dorsal column system (Angau-Petit 1975; Enevoldson and Gordon 1989a; Petit 1972; Petit et al. 1969; Rustioni 1974, 1975; Rustioni and Kaufman 1977; Uddenberg 1966), a relay by one or more brain stem nuclei, or an indirect descending path dependent upon relay from some higher level (Noble and Riddell 1989).

Beginning with the postsynaptic fibers system in the dorsal columns, we can argue that these fibers, travelling both through deep portions of the dorsal columns as well as through the dorsolateral funiculus, would be the most direct route (Angau-Petit 1975; Enevoldson and Gordon 1989a; Petit 1972; Petit et al. 1969; Rustioni 1973, 1974). It is interesting to note in this context, that the dorsal column-postsynaptic system contains fibers that synapse predominantly in the border zones outside the cell cluster regions, that is, to regions where GABAergic inhibitory interneurons predominate (Cliffer and Willis 1994; Fyffe et al. 1986a,b; Gordon and Grant 1982; Rustioni 1975) and that cold block of the dorsal columns can shift receptive field location (Dostrovsky et al. 1976). If the neurons making up the dorsal column-postsynaptic system were involved, one can imagine very complex interactions because these cells, in turn, receive descending influences and are modulated by segmental inhibitory mechanisms (Noble and Riddell 1989: Pubols et al. 1991). It is also possible that other pathways travelling in the dorsolateral quadrant give collaterals to inhibitory interneurons in the DCN. For example DH neurons that project rostrally give collateral branches into the DCN (Craig and Tapper 1978; Eneveldson and Gordon 1989b). These neurons receive sensory signals from large areas of the body (Craig and Tupper 1978; Willis et al. 1974), and their level of ongoing activity (Surmeister et al. 1986a,b) and the amount of convergence they express depends upon the state of the spinal cord (Cervero et al. 1977; Collins and Ren 1987; Collins et al. 1987, 1990).

DH neurons that project to brain stem nuclei are another candidate (Gordon and Grant 1982). These cells could relay through a polysynaptic route to inhibit neurons in the DCN (Blomqvist and Broman 1993; Conti et al. 1990; Dostrovsky 1980; Weinberg and Rustioni 1989). Finally, the classic literature demonstrated the strong descending influence on the DCN provided by recurrent axons from the pyramidal tract (Andersen et al. 1964, Gorden and Jukes 1964; Winter 1965), showing that these axons provide a strong inhibitory drive on cells in the DCN. Because these axons arise from cortical pyramidal neurons, the tonic influence from the skin would have to be relayed to the cortex before it could modulate the excitability of the neurons in the DCN. However, after Pettit and Schwark (1993) removed the sensory and motor cortex, they were able to repeat their observation of receptive field enlargement after cutaneous anesthesia, suggesting that their observations and perhaps ours were not dependent upon cortical influences.

Based on psychophysical arguments, Apkarian et al. (1994) suggest that the DH provides a kind of gate controlling the central relay of all tactile information analogous to the gate for noxious information originally hypothesized by Melzack and Wall (1965). This hypothesis, if expressed as a mechanism in the DH capable of regulating the information relayed through the dorsal column-lemniscal system, is consistent with our data. Recently, with the use of optical imaging techniques in the cortex of squirrel monkey, Tommerdahl et al. (1996a,b) showed noxious thermal stimuli evoked increased activity in areas 3a and reduced mechanically evoked activity in the area. Because the stimuli activated noci-receptors, this experiment demonstrates an effect of nonlemniscal inputs in somatosensory cortex. When the dorsal columns were transected and tactile stimuli were administered, the activity pattern in the cortex fell below control levels, suggesting that a strong inhibitory influence on primary somatosensory cortex is mediated by nonlemniscal pathways. These observations point in the same direction as our studies; there are very important interactions between the lemniscal and nonlemniscal sensory pathways and the nonlemniscal inputs can suppress neuronal activity in somatosensory cortex evoked by cutaneous mechanoreceptors.

Evidence for a significant modulation of cutaneous perceptual experience by DH mechanisms also comes from the experiments of Denny-Brown et al. (Denny-Brown et al. 1973; Kirk and Denny-Brown 1970). After isolation of a dermatome in macaque monkeys by section of three adjacent nerve roots on each side of the tested nerve, these workers showed that tonic influences from dorsal root ganglia at least five segments away entered Lissaur's tract and influenced perception of tactile stimuli applied to the isolated dermatome. Selective lesions of Lissaur's tract showed that both excitatory and inhibitory influences travel longitudinally to modulate neighboring dermatomes. These DH mechanisms were sufficiently powerful that they could completely suppress tactile information from as much as 50% of the skin surface innervated by the test dermatome for months at a time.

In conclusion, we have demonstrated that a synaptic mechanism in the DH projects a tonic inhibitory influence, at least in part, through the dorsolateral funiculus to control the relay of mechanosensory information through the DCN and thereby throughout the lemniscal pathways. This knowledge must modify our traditional view of the relative independence of the lemniscal pathways from other long ascending pathways.

    ACKNOWLEDGEMENTS

  We acknowledge the advice and support of Dr. Alexander Myasnikov, K. Krout, and E. O'Campo and the secretarial help of L. Imbeault.

  Funding was provided by the Medical Research Council of Canada and The Atkinson Memorial Pain Research Fund administered by the Barrow Neurological Foundation.

    FOOTNOTES

  Address for reprint requests: R. W. Dykes, Département de Physiologie, Faculté de Médecine, Université de Montréal, C.P. 6128, Succursale Centre-ville, Montréal, Québec, H3C 3J7 Canada.

  Received 20 May 1997; accepted in final form 2 April 1998.

    REFERENCES
Abstract
Introduction
Methods
Results
Discussion
References

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