Sensitization of nociceptive cutaneous nerve fibers from the rat's tail by noxious mechanical stimulation

Summary This single fiber study on rat tail nerve afferents attempts to establish a peripheral neural correlate for the hyperalgesia to mechanical stimulation which follows injury to the skin. Mechano-heat sensitive C fibers (MH-C or polymodal nociceptors) and high-threshold mechanoreceptive A delta fibers (HTM-A delta) were examined with a series of constant noxious pressure stimulations (4-6-8-4 N on 25 mm², 120 s each, 5 min intervals). These injurious stimuli were either directed to the most sensitive spot of the receptive fields (central stimulation) or closely outside their borders (1–5 mm). With this protocol no clear sensitization was seen in MH-C fibers apart from a stronger dynamic response to central stimulation in some of them. In contrast, most HTM-A delta units, irrespective of the site of noxious stimulation, developed spontaneous activity, lowering of their von Frey thresholds and expansion of their receptive fields. All HTM-A delta units responded to outside stimulation: upon the first stimulus (4 N) there was a delayed discharge of continuously increasing frequency (recruited response), but the onset of the last stimulation (4 N repeated) evoked vigorous dynamic responses in many fibers. The recruitment of HTM-A delta nociceptor activity may contribute to post-injury hyperalgesia to mechanical stimulation and it may counteract adaptation of the single afferent fiber during prolonged noxious influence.On leave from the Laboratoire de Physiologie, Faculté de Médecine Lyon-Sud, Chemin du Petit Revoyet, F-69600 Ouillins, France     

Perfusion of the mechanically compressed lumbar ganglion with lidocaine reduces mechanical hyperalgesia and allodynia in the rat

The rat L(5) dorsal root ganglion (DRG) was chronically compressed by inserting a hollow perforated rod into the intervertebral foramen. The DRG was constantly perfused through the hollow rod with either lidocaine or normal saline delivered by a subcutaneous osmotic pump. Behavioral evidence for neuropathic pain after DRG compression involved measuring the incidence of hindlimb withdrawals to both punctate indentations of the hind paw with mechanical probes exerting different bending forces (hyperalgesia) and to light stroking of the hind paw with a cotton wisp (tactile allodynia). Behavioral results showed that for saline-treated control rats: the withdrawal thresholds for the ipsilateral and contralateral paws to mechanical stimuli decreased significantly after surgery and the incidence of foot withdrawal to light stroking significantly increased on both ipsilateral and contralateral hind paws. Local perfusion of the compressed DRG with 2% lidocaine for 7 days at a low flow-rate (1 microl/h), or for 1 day at a high flow-rate (8 microl/h) partially reduced the decrease in the withdrawal thresholds on the ipsilateral foot but did not affect the contralateral foot. The incidence of foot withdrawal in response to light stroking with a cotton wisp decreased significantly on the ipsilateral foot and was completely abolished on the contralateral foot in the lidocaine treatment groups. This study demonstrated that compression of the L(5) DRG induced a central pain syndrome that included bilateral mechanical hyperalgesia and tactile allodynia. Results also suggest that a lidocaine block, or a reduction in abnormal activity from the compressed ganglia to the spinal cord, could partially reduce mechanical hyperalgesia and tactile allodynia.     

Cardiovascular and neuronal responses to head stimulation reflect central sensitization and cutaneous allodynia in a rat model of migraine

Reduction of the threshold of cardiovascular and neuronal responses to facial and intracranial stimulation reflects central sensitization and cutaneous allodynia in a rat model of migraine. Current theories propose that migraine pain is caused by chemical activation of meningeal perivascular fibers. We previously found that chemical irritation of the dura causes trigeminovascular fibers innervating the dura and central trigeminal neurons receiving convergent input from the dura and skin to respond to low-intensity mechanical and thermal stimuli that previously induced minimal or no responses. One conclusion of these studies was that when low- and high-intensity stimuli induce responses of similar magnitude in nociceptive neurons, low-intensity stimuli must be as painful as the high-intensity stimuli. The present study investigates in anesthetized rats the significance of the changes in the responses of central trigeminal neurons (i.e., in nucleus caudalis) by correlating them with the occurrence and type of the simultaneously recorded cardiovascular responses. Before chemical stimulation of the dura, simultaneous increases in neuronal firing rates and blood pressure were induced by dural indentation with forces >/= 2.35 g and by noxious cutaneous stimuli such as pinching the skin and warming > 46 degrees C. After chemical stimulation, similar neuronal responses and blood pressure increases were evoked by much smaller forces for dural indentation and by innocuous cutaneous stimuli such as brushing the skin and warming it to >/= 43 degrees C. The onsets of neuronal responses preceded the onsets of depressor responses by 1.7 s and pressor responses by 4.0 s. The duration of neuronal responses was 15 s, whereas the duration of depressor responses was shorter (5.8 s) and pressor responses longer (22.7 s) than the neuronal responses. We conclude that the facilitated cardiovascular and central trigeminal neuronal responses to innocuous stimulation of the skin indicate that when dural stimulation induces central sensitization, innocuous stimuli are as nociceptive as noxious stimuli had been before dural stimulation and that a similar process might occur during the development of cutaneous allodynia during migraine.     

Responses to spinal microstimulation in the chronically spinalized rat and their relationship to spinal systems activated by low threshold cutaneous stimulation

We describe the responses evoked by microstimulation of interneuronal regions of the spinal cord in unanesthetized rats chronically spinalized at T10–T12. One to three weeks after spinalization, sites in the lumbar spinal cord were stimulated using trains of low current microstimulation. The isometric force produced by stimulation of a spinal site was measured at the ankle. Responses were reliably observed from stimulation of a region within the first 1250 μm from the dorsal surface of the spinal cord. These responses were clearly not due to direct motoneuronal activation and were maintained after chronic deafferentation. The force evoked by microstimulation and measured at the ankle varied smoothly across the workspace. Simultaneous stimulation of two sites in the spinal cord produced a response that was a simple linear summation of the responses evoked from each of the sites alone. Microstimulation generally produced a highly non-uniform distribution of response directions, biased toward responses which pulled the limb toward the body. Within these distributions there appeared to be two main types of responses. These different types of responses were preferentially evoked by microstimulation of different rostrocaudal regions of the spinal cord. This anatomical organization paralleled the spinal cutaneous somatotopy, as assessed by recording cutaneous receptive fields of neurons at sites to which the microstimulation was applied. This relationship was maintained after chronic deafferentation. The findings described here in the rat spinal cord in large part replicate those previously described in the frog. Received: 27 July 1998 / Accepted: 15 June 1999     

Afferent flow patterns in a cat cutaneous nerve during painful and painless mechanical stimulation of the skin

Quantitative characteristics of afferent flows coding information from a number of receptors were obtained by the gliding impulses method. The frequency spectrum of activity in a cutaneous nerve, the relative numbers of active A, A, and C fibers and their distribution by impulse transition frequency during stimulation of the cat's skin with pins and needles were determined. The afferent flow recorded in the nerve during pricking of the skin is characterized by high density, due to the number of excited fibers and the frequency of activity in them. The higher density of the afferent flow during the application of a painful than of a painless stimulus is mainly due to activity in C fibers. Unmyelinated fibers subjected to the action of the same stimulus and of chemically active substances liberated from the cells during tissue injury are excited directly and generate high-frequency spikes which increase the flow density in the nerve. The number of active myelinated fibers and the spike frequency during the action of a painful stimulus are only a little greater than the corresponding characteristics of the afferent discharge during painless stimulation.Translated from Neirofiziologiya, Vol. 8, No. 4, pp. 391–399, July–August, 1976.     

Continuous perfusion with morphine of the orbitofrontal cortex reduces allodynia and hyperalgesia in a rat model for mononeuropathy

Recent imaging reports demonstrate the activation of the orbitofrontal cortical (OFC) area during acute and chronic pain. The aim of this study was to compare the effects of chronic perfusion of this area with morphine on nociception in control rats and in rats subjected to mononeuropathy. Chronic perfusion of morphine, using miniosmotic pumps, produced significant and naloxone-reversible depression of tactile and cold allodynias and thermal hyperalgesia, observed in neuropathic rats, while it produced significant elevation and naloxone insensitive increase of acute nociceptive thresholds in control rats. The observed results support the idea that this area is a component of a flexible cerebral network involved in pain processing and perception.     

Thermal and mechanical responses of the amphibian skin to nerve stimulation.

The isolated calf skin of the toad and bullfrog was found to generate a burst of heat in response to stimulation of the cutaneous nerve. The electric responses of the skin evoked by nerve stimulation invariably lagged behind the heat burst. The generation of a heat burst was followed by a slow mechanical response of the skin. Suppression of these responses by adrenergic blocking agents suggests that heat is produced at the junction between the gland cells and the sympathetic nerve fibers. Large mechanical changes in the skin were observed when noradrenaline was applied to the inner surface of the skin.     

Peripheral glutamate receptors contribute to mechanical hyperalgesia in a neuropathic pain model of the rat

We hypothesized that glutamate (Glu) released from the peripheral terminals of primary afferents contributes to the generation of mechanical hyperalgesia following peripheral nerve injury. Nerve injury was performed on rats with a lumbar 5 spinal nerve lesion (L5 SNL), which was preceded by L5 dorsal rhizotomy (L5 DR) to avoid the potential central effects induced by L5 SNL through the L5 dorsal root. Mechanical hyperalgesia, as evidenced by a reduction in paw withdrawal threshold (PWT), was short-lasting (<6 days) after L5 DR, but persistent (>42 days) after L5 SNL preceded by L5 DR. When an intraplantar injection into the affected hind paw was given immediately before L5 SNL, non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (20 nmol), group-I metabotropic Glu (mGlu) receptor antagonist DL-amino-3-phosphonopropionic acid (DL-AP3; 70 nmol), and selective group-II mGlu receptor agonist 4-aminopyrrolidine-2,4-dicarboxylate (APDC; 20 nmol) delayed the onset of PWT reduction for 1-4 days. However, this onset was not affected by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4,-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX; 100 nmol). When the same injection was given after L5 SNL-induced mechanical hyperalgesia had been established, MK-801 reversed the PWT reduction for 30-75 min, whereas NBQX, DL-AP3, or APDC had no effect. These results suggest that the manipulation of the peripheral Glu receptors reduces neuropathic pain, by blocking NMDA and group-I mGlu receptors and by stimulating group-II mGlu receptor during the induction phase of neuropathic pain, but only by blocking the NMDA receptor during its maintenance phase.     

Vasomotor response to thermal stimulation of the scrotal skin in rats

Summary In 27 anesthetized rats arterial blood pressure and electrical activity of dorsal horn neurones were recorded during thermal stimulation of the scrotal skin. In 19 of these animals blood pressure increased during scrotal skin warming, showing a distinct threshold at about 37.5°C in 16 rats. This effect was reproducible during the whole experiment although quantitative changes of the response occurred. In most experiments a transient response was seen at the beginning of the vasomotor reaction. The mean increase of blood pressure to a superthreshold warming step, derived from 11 experiments in which more than 3 temperatures were tested and in which the blood pressure was above 100 mm Hg, was 25±14 mm Hg. The observed threshold of the vasomotor response corresponds well with the threshold of activity increase in warm sensitive dorsal horn neurones to scrotal skin warming. It is suggested that the blood pressure reaction is mediated by scrotal skin warm receptors, which supports the notion that some organ specificity exists in thermoreceptors of the scrotal skin.     

The kainate receptor antagonist 2S,4R-4-methylglutamate attenuates mechanical allodynia and thermal hyperalgesia in a rat model of nerve injury.

Opioids and receptor antagonists of excitatory amino acids attenuate mechanical allodynia and thermal hyperalgesia in animal models of neuropathic pain. Recently, a kainate receptor antagonist, 2S,4R-4-methylglutamate, has been developed but has not been tested for antinociceptive effects in animal models of neuropathic pain. We evaluated whether 2S,4R-4-methylglutamate attenuated responses to mechanical and thermal stimuli in uninjured (control) rats and increased responsiveness in rats with chronic constriction injury. Rats were tested for a number of withdrawal responses using a calibrated von Frey filament (mechanical stimulus) and withdrawal latencies from a radiant heat source (thermal stimulus). In control rats, 2S,4R-4-methylglutamate produced a small but significant decrease in responses from the mechanical stimulus (25 mg/kg) and significantly increased withdrawal latencies from the thermal stimulus at the highest dose administered (100 mg/kg). In addition, 2S,4R-4-methylglutamate greatly attenuated increased responsiveness in rats with chronic constriction injury. At four to eight days following chronic constriction injury, animals that displayed increased responsiveness to mechanical and thermal stimuli were injected intraperitoneally with either dizocilpine maleate (0.1 mg/kg), morphine (4 mg/kg), vehicle as controls, or 2S,4R-4-methylglutamate (25, 50, 75 or 100 mg/kg). 2S,4R-4-Methylglutamate (25, 50, 75 and 100 mg/kg) significantly attenuated the frequency of responses to mechanical stimuli (Wilcoxon, P < 0.05) and the latency of responses to thermal stimuli (analysis of variance and Duncan's, P < 0.05). Dizocilpine maleate and morphine, as expected, also reduced these responses. These results suggest that, in addition to opioid and N-methyl-D-aspartate receptors, kainate receptors may play a role in the maintenance of mechanical allodynia and thermal hyperalgesia associated with peripheral nerve injury.     

Unilateral lesions of mesostriatal dopaminergic pathway alters the withdrawal response of the rat hindpaw to mechanical stimulation

To investigate the role mesostriatal dopamine system plays in pain processing, we examined the withdrawal response of rat hindpaws to mechanical stimulus at 1, 4, and 12 weeks after unilateral 6-hydroxydopamine (6-OHDA) lesions of the mesostriatal pathway. In all of the 6-OHDA rats examined, almost no tyrosine hydroxylase (TH) immunoreactivity was detected in the substantia nigra, ventral tegmental area, and striatum ipsilateral to 6-OHDA lesions. Alteration in the withdrawal response in this model animal was evaluated by comparing the latency of withdrawal reflex following the mechanical stimulus to the hindpaw. The latency of withdrawal response in the 6-OHDA rats was significantly reduced in the side ipsilateral to 6-OHDA lesions at all times observed, whereas that was not changed through the period observed in the contralateral side, indicating that dopamine depletion in the mesostriatal system has the influence on withdrawal response to the mechanical stimulus. These results show that the unilateral dopamine depletion causes hypersensitivity to the mechanical stimulus in the ipsilateral side, suggesting that, at least in part, dopamine in the mesostriatal system may be involved in sensory processing including pain sensation induced by mechanical stimulation.     

Apparent hyperalgesia in the mouse tail-flick test due to increased tail skin temperature after lesioning of serotonergic pathways

The relationship between tail skin temperature and responsiveness to noxious radiant heat in the tail-flick test was investigated in mice. A significant negative correlation between tail skin temperature and tail-flick latency was found when the tail skin temperature was increased by elevating the ambient temperature. After intracerebroventricular injection of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT, 80 micrograms) tail skin temperatures were increased and tail-flick latencies reduced. In contrast, administration of the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA, 400 mg kg-1 for 5 consecutive days) lead to a slight lowering of tail temperatures and a tendency towards elevation of tail-flick latencies. The results show that factors which affect tail skin temperature also influence the tail-flick test in mice. The divergent effects of 5,7-DHT and PCPA on tail-flick responsiveness may be due to the different effects of these compounds on the tail skin temperature. The results suggest that the reduced tail-flick latency after partial destruction of serotonergic pathways by 5,7-DHT is due primarily to the increased tail skin temperature. The dependence of tail-flick latency on tail skin temperature limits the usefulness of the tail-flick test unless changes in tail skin temperature are controlled for.     

Change in the reflex organization of the visual receptor in response to thermal cutaneous stimulation

Summary It was shown by perimetry that when the skin is heated or cooled the setting of the human visual system under constant conditions of dark adaptation changes in such a way that the reciprocal relationship between the rod and cone system is retained. Heating the skin caused the rod field to be reduced, but the cone field was increased. Cooling caused the reverse change, i.e., the rod field increased and the cone field was reduced. These functional changes in the visual system induced by cutaneous thermal stimulation are evidently brought about reflexly.(Presented by Active Member SSSR V. V. Parin) Translated from Byulletin' Éksperimental'noi Biologii i Meditsiny Vol. 56, No. 10, pp. 10–13, October, 1963     

Differential effects of morphine and clonidine on visceral and cutaneous spinal nociceptive transmission in the rat

1. The effect of morphine or clonidine administered systemically on visceral and cutaneous spinal nociceptive transmission was examined in 45 dorsal horn neurons in spinalized, decerebrate rats: 17 "cutaneous" dorsal horn neurons located in the L3-L5 spinal segments were excited by heating the glabrous skin of the hindpaw (48 degrees C, 15 s) and 28 "visceral" dorsal horn neurons located in the T13-L2 spinal segments were excited by colorectal distension (80 mmHg, 20 s). The 28 visceral dorsal horn neurons were subclassified as 18 short-latency abrupt neurons (SL-A), which were excited by colorectal distension at short latency (less than 1 s) and whose activity abruptly returned to base line following termination of the distending stimulus, and as 10 short-latency-sustained (SL-S) neurons, which also were excited at short latency (less than 1 s) by colorectal distension, but whose activity was sustained above base line for 4-31 s following termination of the distending stimulus. 2. Morphine produced a dose-dependent, naloxone-reversible inhibition of both spontaneous activity and/or neuronal responses during heating or colorectal distension of 8 SL-A, 7 SL-S, and 11 cutaneous dorsal horn neurons. Comparison of the effective doses of morphine to produce a 50% reduction in the response of the neurons (ED50s) during colorectal distension or heating demonstrated that, at the intensities of distension and heating employed, SL-S neurons were affected at the least dosage (ED50 = 0.46 mumol/kg), followed by SL-A neurons (ED50 = 1.95 mumol/kg) and cutaneous neurons (ED50 = 6.12 mumol/kg). Effects on spontaneous activity were variable: at low doses morphine produced an increase in the spontaneous activity of 2 SL-A and 5 cutaneous neurons; greater doses (up to 42 mumol/kg) inhibited in all of the SL-A and SL-S neurons, but not three cutaneous neurons studied. With the exclusion of these three neurons, the ED50s for inhibition of spontaneous activity were comparable to the ED50s for inhibition of neuronal responses during colorectal distension or heating of the hindpaw in all three neuronal groups. 3. Clonidine produced a dose-dependent, yohimbine- or phentolamine-reversible inhibition of both spontaneous activity and neuronal responses during heating or colorectal distension of 10 SL-A, 3 SL-S, and 6 cutaneous dorsal horn neurons.(ABSTRACT TRUNCATED AT 400 WORDS)