INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

PREVIOUS WORK HAS SHOWN that static skeletal muscle contraction elicits a reflex increase in cardiovascular function seen as a rise in mean arterial pressure (MAP) as well as heart rate (HR; 3, 15, 25, 28, 40). This increase in cardiovascular function is known as the muscle pressor reflex (MPR) and is mediated by the activation of thinly myelinated group III and unmyelinated group IV muscle afferent fibers (25). These fibers respond to mechanical as well as chemical changes that occur in the contracting muscle and are thought to synapse on spinoreticular tract neurons in the dorsal horn of the spinal cord (4, 5, 15, 17, 19, 28, 40). Ascending projections arising from these dorsal horn cells travel, at least in part, in dorsolateral regions of the spinal cord, ultimately exciting neurons in the cardiovascular control centers of the medulla (2, 11, 12, 15, 20, 28). Descending projections from the medulla then excite sympathetic preganglionic neurons located in the intermediolateral area of the spinal cord, which ultimately gives rise to the increase in cardiovascular function.

Activation of other peripheral afferent nerve fibers can elicit a pressor reflex (18, 29-31, 35). Nociceptors are primarily free nerve endings located throughout the periphery that respond to stimuli that threaten or that actually damage tissue (13, 36, 37). Activation of nociceptors causes an increase in cardiovascular function, a nociceptive pressor reflex (NPR; 30, 31). Nociceptive afferent neurons are typically thinly myelinated (group III or A-delta fibers) or unmyelinated (group IV or C fibers; 13, 36, 37). Thus the afferent fibers mediating the NPR and MPR fall within the same classification. It has been postulated that the MPR is mediated by a group of receptors known as ergoreceptors that are separate from nociceptors (14, 19). However, strong experimental evidence that ergoreceptors and nociceptors represent separate groups of receptors is lacking.

Nociceptors also project to other populations of cells in the dorsal horn of the spinal cord. One of these populations is composed of spinothalamic tract neurons (13, 36, 37). The spinothalamic tract is an important pathway in humans for the sensation of pain and temperature (13, 36, 37). Previous work has shown that spinothalamic tract neurons located in the deep dorsal horn are subject to central sensitization as evidenced by an increased responsiveness of these cells to noxious and nonnoxious stimuli (6, 22, 32). This previous work used an intradermal injection of capsaicin (Cap) as a model of peripheral inflammation and the subsequent afferent activation associated with inflammation because Cap is a potent stimulus for unmyelinated afferent fibers (16). This intradermal injection of Cap increased the discharge rate of spinothalamic tract neurons to subthreshold as well as noxious stimuli, indicating sensitization of these cells (6, 7, 22, 32). Because spinothalamic tract neurons in the dorsal horn of the spinal cord can be sensitized by peripheral administration of Cap, we looked at the possibility that the spinoreticular tract neurons responsible for pressor reflexes could also be sensitized.

The purpose of this study was to test the hypothesis that the dorsal horn cells responsible for pressor reflexes could be sensitized. Specifically, we measured the cardiovascular responses evoked by static contraction of skeletal muscle, i.e., the MPR, before and after application of Cap onto the common peroneal nerve. Using separate groups of cats, we measured the cardiovascular responses produced by activation of cutaneous or muscle nociceptors, i.e., an NPR, before and after application of Cap onto the common peroneal nerve. In other words, an NPR was evoked from two types of tissue, skin and skeletal muscle. We hypothesized that because activation of either the MPR or NPR evokes an increase in cardiovascular function, the afferent neurons mediating these reflexes project onto the same population of spinoreticular tract neurons in the dorsal horn of the spinal cord, i.e., they converge in the spinal cord. We also hypothesized that activation of C fibers by applying Cap to the common peroneal nerve would sensitize these spinoreticular tract cells leading to an increase in the MPR as well as the NPR.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Experimental preparation. Adult cats of either sex with a mean body weight of 3.5 ± 0.2 kg were anesthetized by inhaling an isoflurane (5%)-oxygen (2 l/min) mixture. A catheter was inserted into the cephalic vein, and the inhalation anesthetic was removed. Further anesthesia, consisting of -chloralose (80 mg/kg) and urethane (100 mg/kg), was given through the catheter. Polyethylene catheters were placed into an external jugular vein and a common carotid artery. The trachea was exposed, and an endotracheal tube was inserted into the airway. The cats were mechanically ventilated to maintain arterial blood gases within normal limits (pH 7.35-7.4; PCO₂ 35-40 mmHg; PO₂ >80 mmHg). Sodium bicarbonate and/or supplemental oxygen were given as needed to help maintain these limits. Body temperature was constantly measured using a rectal probe (Yellow Springs Instrument, series 400) and was kept between 36.0 and 38.0°C with a heating pad and heat lamp. We periodically checked for the presence of a corneal reflex. If this reflex was present and/or resting arterial blood pressure and/or HR increased, then additional anesthetic, -chloralose (10 mg/kg) or pentobarbital sodium (2-4 mg/kg), was given.

Protocol 1: contraction of triceps surae muscle to evoke the MPR. Contraction of the triceps surae muscle was induced by electrically stimulating the tibial nerve in the popliteal fossa at twice motor threshold, 40 Hz, and 0.025-ms duration for 1 min. Previous work has shown that electrical stimulation of the tibial nerve using these electrical parameters does not directly activate afferent neurons that evoke cardiovascular responses (15). The resting tension was set at 1 kg (L₀ in this preparation) before each contraction, and 15 min was given between muscle contractions to allow the muscle to recover. After at least two contractions of the triceps surae muscle where the pressor reflex was stable, Cap (3% in a 1:1:8 solution of ethanol-Tween 80-saline) was rubbed directly onto the common peroneal nerve via a cotton-tipped applicator, either ipsilateral (n = 7) or contralateral (n = 7) to the contracting muscle. Application of Cap caused a substantial increase in MAP and HR that lasted ~20 min (see RESULTS). Because of this large rise in MAP we waited until MAP returned to about the same baseline (~20 min) before another muscle contraction was evoked. Thereafter the contractions were repeated approximately every 30 min for 1.5 h. After the MPR tests were complete, the cats were paralyzed with pancuronium bromide (0.1 mg/kg). The same electrical stimulus was repeated on the tibial nerve. This was done to verify that the MPR was due to static contraction of skeletal muscle and not the electrical stimulus used to evoke the contraction. Vehicle controls were performed on separate cats (n = 7). The vehicle (1:1:8; ethanol-Tween 80-saline) was rubbed directly onto the common peroneal nerve via a cotton-tipped applicator in the same manner as Cap, and the subsequent contractions were evoked at postapplication times that were similar to the Cap experiments.

Protocol 2: electrical stimulation of saphenous nerve to evoke a cutaneous NPR. A separate group of cats were paralyzed with pancuronium bromide after the aforementioned surgical setup. The saphenous nerve was electrically stimulated with trains of electrical pulses (1 train/s; 750-ms train duration) delivered at 20 Hz and 1-ms duration, and two different intensities were used, high intensity (~250 µV) and low intensity (~25 µV). The nerve was stimulated for 30 s or until the rise in MAP stabilized (usually 20-25 s). The high-intensity stimulation was performed, and then the low-intensity stimulation was evoked after a minimum of 5 min. After at least 10 min, this high-intensity/low-intensity pattern was repeated to ensure that the pressor reflex was stable. Cap was then applied in the same manner as the previous protocol, directly onto the common peroneal nerve ipsilateral (n = 5) or contralateral (n = 5) to the stimulated nerve. The electrical stimulations were repeated at different time points (~ 25, 45, and 75 min postapplication of Cap), represented as Cap 1, Cap 2, and Cap 3, respectively. The order of high intensity/low intensity was counterbalanced for the post-Cap stimulations. In one cat, the low-intensity stimulation failed to produce a significant rise in MAP and HR, and thus the low-intensity data were excluded from this cat.

Protocol 3: electrical stimulation of tibial nerve to evoke a muscle NPR. In another group of cats (n = 6), a branch of the tibial nerve was isolated and placed on the stimulating electrode. Before paralyzing the cat, we visually confirmed that electrical stimulation of the nerve caused contraction of the gastrocnemius muscle. The muscles were then paralyzed, and the nerve was electrically stimulated (higher intensity: ~250 µV) using the same stimulus parameters and time frame as protocol 2. Again, at least two stimulations were elicited to ensure reproducibility of the cardiovascular responses and 10 min was given between electrical stimulations to allow for recovery. Cap was then applied to the ipsilateral common peroneal nerve as described in the two previous protocols. The electrical stimulations were repeated at different time points (~25, 45, and 75 min postapplication of Cap).

Measured and calculated variables. Arterial blood pressure was measured by connecting the common carotid artery catheter to a pressure transducer (Statham model P23ID), and muscle tension was measured using the force transducer. The arterial blood pressure and tension variables were continuously monitored on a four-channel chart recorder (Astromed model 7400) and a personal computer (Biopac acquisition system). MAP and HR were obtained from the arterial blood pressure signal. Baseline values were determined by averaging at least 30 s of data before the muscle contraction or electrical stimulation. The peak values for MAP, HR, and tension represent the peak level the variable reached during the 1 min of muscle contraction or 30 s of electrical stimulation. The peak change in a given variable represents the difference between the peak and baseline values. Time course MAP data for the MPR were measured as the level of MAP at 10-s intervals from the onset to the end of the contraction. To help protect against inconsistencies associated with blood pressure oscillations, MAP values at the 10-s intervals represent the mean of the MAP signal beginning 1 s before through 1 s after the 10-s time point.

Data analysis. Data are expressed as means ± SE. The time course for the MPR was determined as the change in MAP at 10-s time points compared with baseline during the 1-min muscle contraction and was done for the contraction before application of Cap (pre-Cap), the first contraction after application of Cap, and recovery. The time course data were analyzed with a two-way repeated-measures ANOVA with the two factors being time and Cap application (pre-Cap, Cap, and recovery). Baseline and peak hemodynamic data were also analyzed using the 2-way ANOVA, with time (baseline and peak) and Cap application (pre-Cap, Cap, and recovery) being the two factors. A one-way repeated-measures ANOVA was used to compare the peak changes in hemodynamic and tension data for pre-Cap, Cap, and recovery. The one-way ANOVA was also used for analysis of the NPR experiments, except the Cap application levels were pre-Cap, Cap 1, Cap 2, and Cap 3. A Tukey's test was used for post hoc comparisons when applicable. For all analyses, P < 0.05 was used as the level of statistical significance.

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Cardiovascular effects of Cap. Application of Cap to the peroneal nerve caused a robust increase in MAP and HR. Within 30 s after Cap application, MAP and HR began to rise. These increases were sustained for ~15 min, and thereafter MAP and HR began a slow decline, reaching approximate pre-Cap levels after ~20 min. The mean increases in MAP and HR for all the protocols were 63 ± 5 mmHg and 19 ± 2 beats/min, respectively.

Protocol 1: MPR. An example of the cardiovascular effects of static muscle contraction is illustrated in Fig. 1, left. Note that MAP and HR rise in response to the muscle contraction. After Cap was applied to the ipsilateral peroneal nerve and blood pressure was allowed to return toward the pre-Cap baseline, the muscle contraction was repeated. Figure 1, middle, illustrates that in this example the MPR was markedly blunted when the contraction was repeated 21 min after Cap application (even though baseline MAP is below the pre-Cap baseline). Figure 1, right, shows that when the contraction was repeated 41 min after Cap application, the MPR shows some degree of recovery. The time course data for the change in MAP produced by static contraction before and after ipsilateral application of Cap is shown in Fig. 2A. MAP rose sharply at the beginning of the muscle contraction and then reached a plateau that was maintained until the end of the contraction. After the contraction was over, MAP trended back toward baseline. However, the time course of the MAP response was reduced for the first contraction after Cap application. Statistical analysis of these data showed a significant main effect for time (P = 0.003), demonstrating that MAP rose during the muscle contraction. There was also a significant main effect for application of Cap (P = 0.04) and a significant interaction between time and Cap application (P < 0.001). This indicates that the time course for the increase in MAP to muscle contraction was reduced after Cap application. Later contractions showed a time course profile that was not statistically different compared with the control responses, i.e, the pressor response recovered. Figure 2B shows the time course of the pressor response before and after contralateral application of Cap. Similar to the ipsilateral data, Cap transiently reduced the time course of the MPR. Statistical analysis showed that there was a significant main effect for time (P < 0.001) and the interaction between time and Cap (P < 0.001) indicating that the response in MAP to muscle contraction was reduced by Cap application. There was no significant main effect for Cap (P = 0.075). Nevertheless, independent of whether Cap was applied to the ipsilateral or contralateral common peroneal nerve, the MPR was attenuated for a period of time and then recovered back toward control.

View larger version (8K): [in this window] [in a new window]   Fig. 1.   Original records from 1 cat showing the changes in mean arterial pressure (MAP) and heart rate (HR) evoked by static contraction of the triceps surae muscle before application of capsaicin (Cap) to the ipsilateral peroneal nerve (Pre-Cap; left). Note that static contraction of the triceps surae evokes an increase in MAP and HR, i.e., produces a muscle pressor reflex (MPR). Middle: the magnitude of the MPR is greatly attenuated when the contraction is repeated 21 min after Cap (Post-Cap). Although the mean data show a tendency for baseline MAP to be higher for this post-Cap time point (see RESULTS), this figure illustrates that Cap effectively attenuates the MPR even when baseline MAP is lower. Right: a modest degree of recovery of the MPR when the contraction is repeated 41 min after Cap is shown. bpm, Beats/min.

View larger version (24K): [in this window] [in a new window]   Fig. 2.   A: time course of the increases in MAP evoked by a 1-min static contraction (denoted by the solid bar) of the triceps surae muscle before () and 24 ± 3 min after () application of Cap to the ipsilateral peroneal nerve. , Response evoked by static contraction 63 ± 11 min after Cap (Recovery). B: time course of the increases in MAP elicited by static contraction before and after (31 ± 5 min, Cap; 57 ± 4 min, Recovery) application of Cap to the contralateral peroneal nerve. *P < 0.05 compared with pre-Cap; n = 7 for both panels. Note, for clarity, error bars are not depicted for the recovery data.

View larger version (29K): [in this window] [in a new window]   Fig. 3.   Peak increases in MAP, HR, and tension evoked by static contraction before (solid bars; Pre-Cap) application of Cap to the peroneal nerve. The open bars represent the 1st contraction after Cap, and the crosshatched bars represent the 2nd contraction after Cap (Recovery) of the hemodynamic responses. Left, effect of ipsilateral Cap; right, data from contralateral Cap application. Time points for the contractions are indicated in the legend for Fig. 2. *P < 0.05 compared with pre-Cap; n = 7 for both left and right.

Protocol 2: cutaneous NPR. If application of Cap attenuates the pressor response to static muscle contraction, then the same application of Cap should attenuate the pressor response from activation of cutaneous nociceptors if they follow the same pathway. Before application of Cap, high-intensity electrical stimulation of the saphenous nerve caused an increase in MAP and HR as shown in Fig. 4. Approximately 24 min after application of Cap onto the ipsilateral common peroneal nerve, the electrical stimulation evoked a significantly higher peak increase in MAP (P = 0.031), whereas the peak increase in HR also tended to be greater. At later stimulations (~45 and 72 min postapplication of Cap), the electrical stimulation continued to evoke a greater change in MAP and HR compared with preapplication data (Fig. 4; Table 2). Unlike the MPR, the NPR was enhanced after application of Cap and this accentuation was sustained even 72 min after the Cap administration, i.e., the NPR did not recover. For two cats in which it was tested, the NPR was still elevated after 2.5 h. Application of Cap to the contralateral peroneal also accentuated the NPR evoked by saphenous stimulation (Fig. 4; Table 2). Statistical analysis showed that the changes in MAP and HR were accentuated (P = 0.003 and P = 0.008, respectively) after Cap. There were no differences in baseline hemodynamic parameters at the various time points (Table 2).

View larger version (36K): [in this window] [in a new window]   Fig. 4.   A: peak increases in MAP and HR evoked by direct stimulation of cutaneous nociceptors in the saphenous nerve before (solid bars), ~25 (open bars), 45 (crosshatched bars), and 75 (horizontal hatched bars) min after Cap application to the ipsilateral peroneal nerve. The mean time values for post-Cap stimulations are indicated below the HR graph. B: hemodynamic responses to saphenous stimulation before and after Cap application to the contralateral nerve. The mean time values for the post-Cap stimulations for these data are indicated below the HR graph. *P < 0.05 compared with pre-Cap; n = 5.

Protocol 3: muscle NPR. The cats were paralyzed, and the tibial nerve was electrically stimulated at the same parameters as in protocol 2 to activate nociceptors arising from skeletal muscle. Direct activation of tibial nerve afferents increased MAP and HR (Fig. 5; Table 2). This was similar, although quantitatively somewhat smaller, to the effect of saphenous stimulation. Similar to the cutaneous NPR, Cap application to the ipsilateral peroneal nerve accentuated the pressor reflex evoked from skeletal muscle (Fig. 5; Table 2). There was a statistically significant (P = 0.008) increase in the change in MAP (Fig. 5) and the peak level of MAP (P = 0.006; Table 2). Although there was a greater increase in HR after Cap, it did not reach statistical significance (P = 0.237). There were no differences in pre-Cap compared with the post-Cap baseline hemodynamic data for the muscle NPR protocol (Table 2). Thus application of Cap accentuated the NPR evoked from both skin and skeletal muscle.

View larger version (27K): [in this window] [in a new window]   Fig. 5.   Peak increases in MAP and HR evoked by direct stimulation of muscle nociceptors in the tibial nerve before (solid bars), ~25 (open bars), 45 (crosshatched bars), and 75 (horizontal hatched bars) min after Cap application to the ipsilateral peroneal nerve. The mean time values for post-Cap stimulations are indicated in the graph. *P < 0.05 compared with pre-Cap; n = 6.

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

The purpose of this study was to see if the MPR as well as the NPR could be augmented by application of Cap. The results of this study show that the application of Cap attenuates the MPR, yet augments the NPR induced by high-intensity stimulation of the saphenous or tibial nerves. The attenuation of the MPR by Cap was short-lived as the MPR recovered toward control over time. By contrast, accentuation of the NPR was sustained over time. One possibility for the opposite effects of Cap on these two reflexes is that the MPR and the NPR are mediated by different sensory pathways. If so, then results of this study suggest that the cells receiving input from these two anatomic pathways are differentially affected by a sustained activation of C fibers in the common peroneal nerve. Kao (14) introduced the term "ergoreceptors" to refer to sensory afferent fibers arising from skeletal muscle that enhance cardiorespiratory function during muscular work. Kniffki et al. (19) found some afferent neurons that were responsive to muscle contraction and did not appear to behave as nociceptors, but whether or not these contraction-sensitive fibers influence the cardiovascular system was not determined. Although the results of the current study do not prove the existence of ergoreceptors, they are consistent with this concept.

Inflammation of peripheral tissue can result in heightened pain sensation, such that nonnoxious stimuli are now perceived as pain and noxious stimuli elicit a greater sensation of pain. There are two facets involved in this process. The first is a sensitization of primary afferent neurons by products of inflammation, thereby making them more excitable to noxious and nonnoxious stimuli (13, 32, 36, 37). The second involves sensitization of cells in the central nervous system (CNS), termed central sensitization, that occurs in response to the barrage of nociceptor input that occurs when tissue becomes injured and/or inflamed (6, 7, 22, 32). The purpose of the current study was to determine if cells in the CNS involved in cardiovascular regulation could be sensitized, while avoiding the potential complication of sensitizing the primary afferent neurons involved in producing these reflexes.

To prevent sensitization of the primary afferent neurons, Cap was applied to the peroneal nerve, which innervates the ventral muscles and skin of the lower portion of the hindlimb. The dorsal muscle group of the lower hindlimb, i.e., the triceps surae muscle, was contracted to evoke the MPR to ensure that the afferent neurons producing this reflex arose from a region not innervated by nerves that received the Cap insult. The same holds true for the saphenous nerve, which is a cutaneous nerve innervating the dorsal aspect of the lower hindlimb and paw. Furthermore, application of Cap to the contralateral leg still modulated both reflexes. Thus it is very unlikely that the alterations in the MPR and NPR caused by Cap application were due to alterations in the primary afferent neurons mediating the pressor reflexes. On the other hand, the fact that the NPR was accentuated in our study suggests that central cells involved in cardiovascular regulation were sensitized by Cap application. The application of Cap to the common peroneal nerve evoked an increase in MAP and HR that lasted ~20 min. This sustained increase followed a similar time course to the increased discharge rate of spinothalamic tract neurons after the intradermal injection of Cap (6, 7, 22, 32). It is also consistent with the time course of pain sensation reported in human subjects after an intradermal injection of Cap (32). In addition, this application of Cap to a peripheral nerve caused the NPR to be elevated for >1 h, indicating a relatively long-lasting enhancement. These consistencies with previous work suggest that application of Cap to the common peroneal nerve sensitized cells involved in cardiovascular regulation. Furthermore, the signal causing this sensitization, as well as the inhibition of the MPR, crosses the spinal cord as the NPR and MPR were altered by application of Cap to the contralateral peroneal nerve.

Previous work has shown that sensitization of dorsal horn cells by peripheral inflammation involves activation of spinal N-methyl-D-aspartate (NMDA) and neurokinin (NK)-1 receptors (6, 7). Activation of these receptor systems may stimulate a variety of intracellular pathways, including the production of nitric oxide (NO), which has been implicated in producing central sensitization (22, 26, 27, 41). Related to this, we recently showed that administration of L-arginine into the dorsal horn accentuates the MPR (38). It is presumed that this enhancement was due to an L-arginine-induced increase in NO. Thus the L-arginine results suggest that cells in the dorsal horn involved in producing the MPR can be sensitized. However, the results of the current study suggest that applying Cap to a peripheral nerve is not a mechanism for sensitizing these cells. The implication from this is that the C fiber activation associated with acute inflammation or injury does not enhance, in fact it appears to inhibit, the MPR. It is unknown at present if enhancement of the NPR by Cap involves NO.

The basic anatomy for the MPR and NPR is thought to be very similar. Both reflexes are initiated by activating small-diameter afferent fibers that synapse in the dorsal horn of the spinal cord (15, 28, 31, 40). These dorsal horn cells in turn activate, at least in part, spinal sympathetic preganglionic neurons in the intermediolateral portion of the thoracic and upper lumbar spinal cord by stimulating medullary neurons involved in cardiovascular regulation (2, 15, 18, 28, 29, 31, 35). Furthermore, nociceptors are thinly myelinated (group III or A-delta) and unmyelinated (group IV or C fibers) afferent neurons (13, 36, 37). As stated above, the MPR is mediated by the same class of afferent fibers, i.e., groups III and IV (15, 25, 28). Activation of nociceptors causes the release of the excitatory amino acids (EAA) glutamate and aspartate, as well as substance P (SP) release into the dorsal horn of the spinal cord (8, 33, 34). Static contraction causes spinal release of the same neurochemicals (9, 39). In addition, blockade of dorsal horn receptors for EAA and SP, e.g., NMDA, non-NMDA, and NK-1, blunts the MPR (15, 40). Because of the numerous similarities between the MPR and nociceptors, we originally proposed that the afferent pathways mediating the cardiovascular responses to static contraction and nociceptor activation were the same or that they converge onto the same dorsal horn spinoreticular cells. Thus, if C fiber activation sensitizes these spinoreticular cells, both reflexes would be enhanced by Cap application. However, the results of the current study fail to support this original hypothesis.

On the other hand, it is well established that painful stimuli activate a variety of sensory modulating mechanisms including descending "diffuse noxious inhibitory controls" (21). This endogenous sensory modulating system involves descending input to the dorsal horn and various neurochemicals including the opiate peptides, serotonin (5-HT), and ₂-adrenergic agonists (1, 13, 36, 37). Because application of Cap activates C fibers, many of which are nociceptors, it is likely that this endogenous sensory modulating system is engaged. Sorkin and McAdoo (34) showed that intradermal Cap increases 5-HT release in the dorsal horn of the spinal cord, supporting the concept that Cap application activates sensory modulating systems. We and others have shown that activation of dorsal horn opiate, 5-HT, or ₂-adrenergic receptors attenuates the MPR (15, 40). Thus we hypothesize that activation of an endogenous sensory modulating system by Cap inhibits dorsal horn cells receiving ergoreceptive input from the contracting muscle, thereby attenuating the MPR. On the other hand, the sensory modulating system fails to inhibit the dorsal horn cells receiving nociceptive input, because they become sensitized by peripheral Cap application. Recent work indicates that sensitized spinothalamic cells are resistant to the inhibitory actions of GABA and locus ceruleus stimulation (23, 24). More work is needed to test the validity of this working hypothesis.

Although the results of the current study are consistent with hypotheses that ergoreceptors and nociceptors are distinct, an alternative possibility related to the stimulus-response properties of dorsal horn neurons is conceivable. It is likely that sensitization of dorsal horn cells steepens and/or shifts the stimulus-response curve up while activation of the endogenous sensory modulating system causes a rightward shift. Because of these shifts, a stimulus at or near the top of the curve will be augmented while a stimulus at or near the bottom of the curve will exhibit inhibition. It is likely that the high-intensity nerve stimulation used to evoke the NPR represents such a strong stimulus and thus this reflex is augmented. Muscle contraction on the other hand may be a much weaker stimulus and is thus represented on a much lower part of the curve. Even though sensitization steepened the stimulus-response curve, the right shift induced by the endogenous sensory modulating system causes the resultant cardiovascular responses to be smaller. If so, then it is possible that both reflexes converge onto the same population of spinoreticular neurons or the receptors mediating these reflexes are not distinct. Thus ergoreceptors are not distinct receptors, but "ergoreception" represents one portion of a continuum that includes nociception at a "higher" end of this continuum. In other words, the sensory input from a contracting skeletal muscle resides on a much lower point on the stimulus-response curve of dorsal horn cells (and possibly other CNS cells as well) than noxious input. Consistent with this concept is the fact that the NPR evoked by high-intensity stimulation was enhanced, whereas the lower intensity electrical stimulation was unaffected. More work is needed to determine whether the CNS processing of somatic pressor reflexes is indeed a continuum or ergoreceptors are a distinct group of receptors. Regardless, the fact that Cap either enhanced (high intensity) or failed to alter (lower intensity) the NPR, yet it attenuated the MPR, strongly suggests that the contraction-induced cardiovascular responses produced in the anesthetized cat model do not simply represent a noxious reflex response.

We proposed that the attenuation of the MPR is mediated by activation of supraspinal structures, not a self-contained spinal event. Activation of peripheral afferent neurons can diminish sensory transmission at the spinal level, but this is thought to result from activation of large, myelinated neurons, A-beta skin afferent neurons for example (13, 37). Cap primarily activates unmyelinated afferent fibers, and thus attenuation of the pressor reflex by large myelinated neurons is unlikely (16). It is also unlikely that differences in the segmental entry of the primary afferent neurons accounts for the attenuation. Stimuli entering in one spinal segment can induce inhibitory effects in other spinal segments without relaying in the brain. For example, somatic and visceral inputs to the cervical spinal cord can inhibit spinothalamic cells in the lumbar spinal cord via a spinal circuit (10, 42). However, afferent fibers from the saphenous, tibial, and peroneal nerves enter the spinal cord in the same spinal segments, i.e., L₆, L₇, and S₁. Furthermore, the tibial and peroneal nerves are branches of the sciatic. In addition, Cap application attenuated the MPR, but the NPR from the tibial nerve was enhanced. Thus it is likely that attenuation of the MPR involves activation of supraspinal structures associated with an endogenous sensory modulating system.

We proposed that the divergent actions of Cap occur at the level of the first synapse, i.e., the dorsal horn of the spinal cord. Either separate cells receive input from distinct pathways or the dorsal horn integration of these two reflexes is altered. The rationale for this proposal is that numerous studies (see above) have documented that dorsal horn cells are sensitized by Cap or other types of tissue inflammation. In addition, the MPR and other types of sensory input are inhibited at the level of the dorsal horn. However, because this study represents the initial test on the effects of Cap application on somatic pressor reflexes, other possible CNS sites may produce and/or be involved in causing the results observed in the current study. More work is needed to determine what CNS sites are involved.

A limitation to the current study was the manner in which the NPR was evoked. To induce the NPR, the saphenous and tibial nerves were stimulated at an intensity that directly activates all afferent neurons. Although there is no doubt this represents a noxious stimulus and the resultant cardiovascular responses are primarily the result of nociceptor activation (15, 31), it does not represent a "pure" nociceptor stimulus. This is particularly germane to the muscle NPR, because electrical stimulation of this nerve activates afferent neurons from nociceptors and ergoreceptors. This potential complication is unavoidable. Direct electrical stimulations were used because they provide robust and reproducible cardiovascular responses. Although ergoreceptor afferent neurons are activated by direct electrical stimulation, their contribution to the cardiovascular responses appears to be minimal compared with nociceptor activation, because the NPR evoked from the tibial nerve was augmented, while ergoreceptor stimulation associated with muscle contraction (i.e., the MPR) was attenuated.

In summary, the results of this study show that Cap application to a peripheral nerve attenuates the MPR but enhances the NPR. The attenuation of the MPR was not sustained, but the accentuation of the NPR was. These results are consistent with the hypothesis that the MPR is mediated by ergoreceptors. On the other hand, these results may indicate that even though ergoreceptors are not a distinct class of receptors, ergoreception represents an important sensory input for adjusting cardiovascular function that is distinctly different from nociception. In other words, ergoreception and nociception may be separable components within a continuum of sensory input that evokes autonomic adjustments. Furthermore, these data suggest that the CNS processing of ergoreception and nociception is differentially affected by a Cap-induced activation of peripheral C fibers.

Perspectives