The regulation of cardiovascular function by sensory activation is readily evident even outside the experimental setting, for instance during routine surgical manipulations, but little work has been done to characterize this relationship for stimuli that are commonly used for cutaneous pain testing. Unlike the predictable elevation in HR and BP evoked by persistent noxious stimulation from which an animal cannot withdraw, our present findings show diverse cardiovascular responses to brief cutaneous stimuli that are modality specific (). There are several reasons for the divergence of our new observations from previous studies. First, in the present study, animals could terminate the stimulus by withdrawing their limb, such that the duration of stimulation above the noxious threshold was very brief. Secondly, many prior measurements have been obtained during anesthesia, which substantially alters the extent and even direction of cardiovascular events induced by sensory stimulation [11
]. Finally, activation of cutaneous primary afferent fibers initiates not only sensory experience, but also triggers processes such as thermoregulation and arousal that together produce cardiovascular response patterns that differ by stimulus modality.
Interpretation of our cardiovascular observations during noxious mechanical stimulation appears straightforward. Application of a pin increases MAP and HR, similar to prior reports that used sustained noxious mechanical stimulation [1
]. The amplified response of HR in hyperalgesic animals after SNL supports the validity of this well-accepted model of neuropathic pain, and suggests that HR monitoring could provide additional insight into pain perception. The generation of a greater HR elevation accompanying hyperalgesia-type behavior suggests that this sustained and complex behavior denotes a distinct, more intense nociceptive event, which is compatible with our recent observation that this behavior is selectively associated with aversiveness in an operant model using conditioned place avoidance [47
]. Although a gold standard for a pain experience per se
is inevitably lacking in animal research, a specific association of hyperalgesia-type behavior with accentuated cardiovascular activation supports the validity of this behavior type as an indicator of pain.
Using a commonly employed paradigm for punctate mechanical stimulation with von Frey fibers, the amplitudes of MAP and HR responses are generally small, consistent with the low intensity of these stimuli that straddle the threshold for triggering a behavioral response. Unexpectedly, MAP is depressed by applications of von Frey fibers. This effect is similar after touches that produce no movement and those that cause withdrawal, which suggests that these stimuli are perceived without producing nociception. Since fully noxious stimuli such as pin application increase MAP, it is possible that most von Frey fiber applications fail to generate nociception and pain, but rather trigger somatomotor and vasomotor events through an arousal mechanism. The few applications (9% in our present data) that result in hyperalgesia-type behavior provoke elevated HR, in contrast to depressed HR in the absence of withdrawal, which likely indicates activation of nociceptive pathways when hyperalgesia-type behavior occurs. These observations raise the possibility that the conventional format for sensory testing with von Frey fibers, in which a simple withdrawal event is equated with nociception, may be an unreliable test for pain. This speculation is supported by the failure of von Frey stimulation to produce conditioned place avoidance, even after nerve injury [47
Brushing the plantar skin to produce dynamic mechanical stimulation reliably elevates MAP even without nerve injury, and may do so without any form of foot withdrawal. Stroking with a soft brush is not conventionally considered a painful stimulus, but this is open to question since the animal’s experience cannot be known. In most cases, we observed cardiovascular activation by brushing with no accompanying nocifensive somatomotor behavior. While this suggests that these perceptions produced arousal without nociception, this cardiovascular activation contrasts with the depression induced by von Frey touches that likewise produce no movement, for which we have no explanation.
Together, our data show that cutaneous mechanical stimulation affects cardiovascular parameters in divergent patterns that depend on the manner of application. Early studies that used direct electrical stimulation of peripheral nerves in anesthetized animals demonstrated the ability of afferent sensory activity to either depress or elevate BP as well as peripheral sympathetic vasomotor neuronal activity [8
]. Even when both C-type as well as A-type cutaneous afferent fibers are stimulated, low frequency activity (1–10Hz) decreases MAP within 5 seconds, whereas higher frequency activity (≥50Hz) elevates sympathetic activity and MAP [22
]. The frequency of afferent traffic elicited by noxious pin stimulation is likely higher than that evoked by threshold stimulation with von Frey fibers [4
], which may explain the divergent effects of pin and von Frey stimulation upon MAP. Also, since sympathetic activation is proportionate to the total number of neuronal depolarizations in a cutaneous afferent train [38
], brush stimulation may thereby produce a vasopressor effect through the sustained nature of the afferent traffic it produces, the spatial summation of stimulating a large cutaneous area, and the particular potency of moving mechanical stimuli in producing afferent pulse trains [17
Cooling of the plantar skin in our study provoked the greatest increase in MAP and HR of the several modalities tested. Since cardiovascular responses to cooling are comparable in control and injured animals, it is possible that the form of surface cooling we used is noxious in control animals. However, our data reveal cardiovascular responses even when no somatomotor behavior is triggered, making it unlikely that the stimulation is consistently painful. Compared to applications that trigger no somatomotor behavior, cooling that produces foot withdrawal and especially hyperalgesia-type behavior is accompanied by amplified MAP responses, which might suggest a component of nociception in these cases. It is well established that local nonpainful cutaneous cooling may induce global vasoconstriction and elevation of MAP and HR [24
], which could be the predominating influence upon vasomotor control under the conditions of our study. It is also possible that somatomotor behavior is dictated by central thermoregulatory processing rather than nociceptive systems. Since evaporative cold stimulation cannot be terminated immediately by limb withdrawal, the intensity of the circulatory response may be due in part to persistence of the stimulation.
Plantar heat produces overall depression of MAP and HR, even in control animals. This contrasts with published demonstrations of the pressor effect of sustained cutaneous heating [1
], but the method used in the present study allowed the animals to minimize exposure to stimulation above the nociceptive threshold. Notably, we observed comparable cardiovascular depression upon plantar warming even in the absence of paw withdrawal, which further suggests a minimal role of nociceptive activation. The application of non-noxious heat to small areas of skin has long been recognized as a potent trigger of global suppression of sympathetic activity and systemic vasodilatation [10
], in a fashion opposite to that triggered by local cooling, and this thermoregulatory pathway could account for cardiovascular influences during thermal threshold testing. An alternative explanation for the depressor response stems from the selective activation of C-type nociceptive fibers during the relatively slow pace at which we heated the skin [48
]. In contrast to Aδ fiber stimulation, such as that caused by punctate mechanical stimuli or rapid skin warming, preferential stimulation of C-type fibers by slow warming selectively activates a distinct, coordinated passive coping response that includes cardiovascular depression [26
Pain research has been hobbled by the difficulty of drawing inferences relevant to human pain from models employing animal subjects [31
]. Although quantifying somatomotor behavior induced by cutaneous stimulation is the accepted standard for sensory testing in animals, this has been adopted largely out of convenience, and doubts have been raised regarding the validity of this approach [25
]. Our study reveals unexpected complexity in the phasic responses of MAP and HR induced by cutaneous stimulation, probably due to the participation of central circuits serving processes other than sensory discrimination and pain, such as arousal and thermoregulation. Although this restricts the usefulness of cardiovascular measures as independent gauges of nociception and pain, our findings also show that particular escape behaviors are selectively linked to specific patterns of phasic cardiovascular reflexes. Specifically HR activation that is characteristic of nociception is accentuated after noxious pin stimuli when sustained and complex hyperalgesia-type behavior is provoked. Furthermore, punctate von Frey stimulation only triggers cardiovascular activation when hyperalgesia-type behavior occurs. These observations indicate that HR measurement after intense mechanical stimulation may provide additional quantification of the painful experience, and also highlight the value of higher-level integrated behaviors as an outcome measure, rather than relying solely on the threshold or latency for simple withdrawal of the stimulated extremity.