Emerging evidence suggests that blockade of hyperpolarization-activated current (Ih) produces ana... more Emerging evidence suggests that blockade of hyperpolarization-activated current (Ih) produces analgesia acting at peripheral sites. However, little is known about the role of this current in central pain-processing structures. The aim of the present work was to characterize the Ih in deep dorsal horn neurons and to assess the role of the current in the transmission of somatosensory signals across spinal circuits. To these purpose in vitro preparations of the spinal cord from mice pups were used in combination with whole cell recordings to characterize the current in native neurons. Extracellular recordings from sensory and motor pathways were performed to assess the role of the current in spinal somatosensory processing. Cesium chloride and ZD7288 were used as current blockers. Most deep dorsal horn neurons showed a functional Ih that was blocked by ZD7288 and cesium. Ih blockade caused hyperpolarization, increased input resistance and potentiation of synaptic responses. Excitatory effects of Ih blockade on synaptic transmission were confirmed in projecting anterolateral axons and ventral roots. Ih modulation by cAMP produced a rightward shift in the voltage dependency curve and blocked excitatory effects of ZD7288 on sensory pathways. Results indicate that Ih currents play a stabilizing role in the spinal cord controlling transmission across sensory and motor spinal pathways via cellular effects on input resistance and excitability. In addition, results suggest that current modulation may alter significantly the role of the current in somatosensory processing.
In the context of neuropathic pain, the contribution of regeneration to the development of positi... more In the context of neuropathic pain, the contribution of regeneration to the development of positive symptoms is not completely understood. Several efforts have been done to described changes in axotomized neurons, however, there is scarce data on changes occurring in intact neurons, despite experimental evidence of functional changes. To address this issue, we analysed by immunohistochemistry the presence of phosphorylated signal transducer and activator of transcription 3 (pSTAT3), an accepted marker of regeneration, within DRGs where axotomized neurons were retrogradely labelled following peripheral nerve injury. Likewise, we have characterized abnormal electrophysiological properties in intact fibres after partial nerve injury.
Epidemiological studies link long term exposure to xenoestrogen Bisphenol-A to adverse cardiovasc... more Epidemiological studies link long term exposure to xenoestrogen Bisphenol-A to adverse cardiovascular effects. Our previous results show that BPA induces hypertension by a mechanism involving CamKII activation and increased redox stress caused by eNOS uncoupling. Recently, CamKII sustained activation has been recognized as a central mediator of programmed cell death in cardiovascular diseases, including necroptosis. However, the role of necroptosis in cardiac response to BPA had not yet been explored. Mice exposed to BPA for 16 weeks showed altered heart function, electrical conduction, and increased blood pressure. Besides, a stress test showed ST-segment depression, indicative of cardiac ischemia. The hearts exhibited cardiac hypertrophy and reduced vascularization, interstitial edema, and large hemorrhagic foci accompanied by fibrinogen deposits. BPA initiated a cardiac inflammatory response, up-regulation of M1 macrophage polarization, and increased oxidative stress, coinciding ...
American Journal of Physiology-Regulatory, Integrative and Comparative Physiology
The contribution of changes in ureter motility produced by a stone to the pain of ureteric calcul... more The contribution of changes in ureter motility produced by a stone to the pain of ureteric calculosis is unclear. In this study we measured ureter motility as changes in intraureter pressure in anesthetized rats 1, 4, and 8 d ys after implantation of an artificial calculus (n = 33) and compared it with motility in normal (n = 8) and ligated (n = 4) ureters. Partial obstruction of the ureter by the stone produced a 478% increase in the amplitude of contractions, a 70% decrease in the rate of contractions, and a 66% decrease in the baseline pressure. The pressures reached during contractions were equivalent to those evoking nociceptive reactions in animals and humans. These changes persisted in rats that had spontaneously eliminated the stone. Complete obstruction of the ureter by the stone or by ligation abolished contractions. We conclude that the increased motility caused by a stone likely contributes to the development and maintenance of visceral pain and referred hyperalgesia in ...
Roza, Carolina, Jennifer M. A. Laird, and Fernando Cervero. Spinal mechanisms underlying persiste... more Roza, Carolina, Jennifer M. A. Laird, and Fernando Cervero. Spinal mechanisms underlying persistent pain and referred hyperalgesia in rats with an experimental ureteric stone. J. Neurophysiol. 79: 1603–1612, 1998. Spinal neurons processing information from the ureter have been characterized in rats 1–4 days after the implantation of an experimental ureteric stone and compared with those of normal rats. The effects of a conditioning noxious stimulation of the ureter in the presence of the hyperalgesia evoked by the calculosis also were examined. Extracellular recordings were performed at the T12–L1 segments of the spinal cord. In rats with calculosis, more neurons expressed a ureter input (53 vs. 42% in normal rats); such cells being more likely to show background activity, at a higher rate than normals (6.6 ± 1.2 vs. 3.2 ± 0.9 spikes/s; mean ± SE) and increasing with the continuing presence of the stone. The threshold pressure for a ureteric response was higher than in normal rats (...
Visceral pain is qualitatively distinct from other pain types; it is poorly localized, difficult ... more Visceral pain is qualitatively distinct from other pain types; it is poorly localized, difficult to quantify, and accompanied by marked autonomic changes. Acute visceral pain may be an indication of a medical emergency requiring urgent surgical or clinical intervention. However, chronic visceral pain, which contributes significantly to lifelong morbidity, occurs most frequently in the absence of any distinct pathology making it difficult to treat. This chapter reviews our current understanding of how visceral pain is detected in the periphery, and processed within the spinal cord and central nervous system. We focus on recent work that has identified pro-nociceptive changes in the bowel of patients with chronic visceral pain and discuss how these findings could lead to the development of novel viscero-specific analgesics. Finally, we consider how the microbiota can act locally to shape the detection of pain in the periphery and centrally to modulate our perception of visceral pain.
Superficial laminae of the spinal cord possess a considerable number of neurons with spontaneous ... more Superficial laminae of the spinal cord possess a considerable number of neurons with spontaneous activity as reported in vivo and in vitro preparations of several species. Such neurons may play a role in the development of the nociceptive system and/or in the spinal coding of somatosensory signals. We have used electrophysiological techniques in a horizontal spinal cord slice preparation from adult mice to investigate how this activity is generated and what are the main patterns of activity that can be found. The results show the existence of neurons that fire regularly and irregularly. Within each of these main types, it was possible to distinguish patterns of spontaneous activity formed by single action potentials and different types of bursts according to intra-burst firing frequency. Activity in neurons with irregular patterns was blocked by a mixture of antagonists of the main neurotransmitter receptors present in the cord. Approximately 82% of neurons with a regular firing pat...
Pflügers Archiv - European Journal of Physiology, 2016
The superficial dorsal horn contains large numbers of interneurons which process afferent and des... more The superficial dorsal horn contains large numbers of interneurons which process afferent and descending information to generate the spinal nociceptive message. Here, we set out to evaluate whether adjustments in patterns and/or temporal correlation of spontaneous discharges of these neurons are involved in the generation of central sensitization caused by peripheral nerve damage. Multielectrode arrays were used to record from discrete groups of such neurons in slices from control or nerve damaged mice. Whole-cell recordings of individual neurons were also obtained. A large proportion of neurons recorded extracellularly showed well-defined patterns of spontaneous firing. Clock-like neurons (CL) showed regular discharges at ∼6 Hz and represented 9 % of the sample in control animals. They showed a tonic-firing pattern to direct current injection and depolarized membrane potentials. Irregular fast-burst neurons (IFB) produced short-lasting high-frequency bursts (2-5 spikes at ∼100 Hz) at irregular intervals and represented 25 % of the sample. They showed bursting behavior upon direct current injection. Of the pairs of neurons recorded, 10 % showed correlated firing. Correlated pairs always included an IFB neuron. After nerve damage, the mean spontaneous firing frequency was unchanged, but the proportion of CL increased significantly (18 %) and many of these neurons appeared to acquire a novel low-threshold A-fiber input. Similarly, the percentage of IFB neurons was unaltered, but synchronous firing was increased to 22 % of the pairs studied. These changes may contribute to transform spinal processing of nociceptive inputs following peripheral nerve damage. The specific roles that these neurons may play are discussed.
Kv7.2 channel expression has been reported to decrease in dorsal root ganglia (DRG) following the... more Kv7.2 channel expression has been reported to decrease in dorsal root ganglia (DRG) following the induction of a peripheral neuropathy while other experiments show that Kv7.2 accumulates in peripheral neuromas. The mechanisms underlying these novel expression patterns are poorly understood. Here we use immunofluorescence methods to analyze Kv7.2 protein expression changes in sensory neurons following peripheral axotomy and the potential role of axonal transport. Results indicate that DRG neurons express Kv7.2 in ~16% of neurons and that this number decreases by about 65% after axotomy. Damaged neurons were identified in DRG by application of the tracer Fluoro-ruby at the site of injury during surgery. Reduction of Kv7.2 expression was particularly strong in damaged neurons although some loss was also found in putative uninjured neurons. In parallel to the decrease in the soma of axotomized sensory neurons, Kv7.2 accumulated at neuromatose fiber endings. Blockade of axonal transport ...
The spinal cord is the first relay center for nociceptive information. Following peripheral injur... more The spinal cord is the first relay center for nociceptive information. Following peripheral injury, the spinal cord sensitizes. A sign of spinal sensitization is the hyper-reflexia which develops shortly after injury and can be detected in the isolated spinal cord as a "memory of pain." In this context, it is easy to understand that many analgesic compounds target spinally located sites of action to attain analgesia. In vitro isolated spinal cord preparations have been used for a number of years, and experience on the effects of compounds of diverse pharmacological families on spinal function has accumulated. Recently, we have proposed that the detailed study of spinal segmental reflexes in vitro may produce data relevant to the evaluation of the analgesic potential of novel compounds. In this review, we describe the main features of segmental reflexes obtained in vitro and discuss the effects of compounds of diverse chemical nature and pharmacological properties on such reflexes. Our aim was to compare the different profiles of action of the compounds on segmental reflexes in order to extract clues that may be helpful for pharmacological characterization of novel analgesics.
Mechanosensitive cation channels are thought to be crucial for different aspects of mechanopercep... more Mechanosensitive cation channels are thought to be crucial for different aspects of mechanoperception, such as hearing and touch sensation. In the nematode C. elegans, the degenerins MEC-4 and MEC-10 are involved in mechanosensation and were proposed to form mechanosensitive cation channels. Mammalian degenerin homologues, the H(+)-gated ASIC channels, are expressed in sensory neurones and are therefore interesting candidates for mammalian mechanosensors. We investigated the effect of an ASIC2 gene knockout in mice on hearing and on cutaneous mechanosensation and visceral mechanonociception. However, our data do not support a role of ASIC2 in those facets of mechanoperception.
The M-current has been proposed as a potential target for analgesia under neuropathic pain condit... more The M-current has been proposed as a potential target for analgesia under neuropathic pain conditions. M-currents and/or their molecular correlates, KCNQ proteins, have been demonstrated in key elements of the nociceptive system including spinal and dorsal root ganglion neurons. Here we demonstrate that retigabine, a selective KCNQ channel opener, applied at neuromatose endings modulates the excitability of axotomized fibres inhibiting ectopic discharges. Responses to mechanical and chemical stimulation were obtained from intact and previously axotomized Adelta- and C-fibres using in vitro preparations and extracellular electrophysiological recording techniques. Application of retigabine (10 microM) produced an estimated approximately 80% reduction in the number of discharges produced by mechanical and chemical stimulation of most axotomized fibres tested (24/27). The electrical threshold of stimuli applied to the neuroma was found to increase in the presence of retigabine (+17.5+/-2.3%) and to decrease in the presence of a high potassium medium (-16.5+/-3.7%). This indicates that retigabine produces a hyperpolarization and a subsequent reduction of the excitability in aberrant sensory endings. Application of XE-991 (10 microM), a KCNQ channel blocker, had no effect on responses to stimulation of the neuroma but blocked the effects of retigabine indicating a specific involvement of KCNQ channels. In contrast to the strong effects on ectopic discharges, retigabine did not change responses to stimulation recorded from intact receptors. Results indicate that KCNQ channel opening at axotomized endings may constitute a novel and selective mechanism for modulation of some neuropathic pain symptoms.
Cold allodynia is a common complaint in patients with peripheral neuropathies. However, cold sens... more Cold allodynia is a common complaint in patients with peripheral neuropathies. However, cold sensitivity of the different types of sensory afferents present in injured nerves is poorly known. We recorded activity evoked by cold in intact sensory fibers of the skin-saphenous nerve preparation and in axotomized sensory fibers of approximately 21 days-old neuromas of the saphenous nerve of mice, in vitro. Sixteen percent of the axotomized units responded to cooling with an accelerating discharge, which stopped immediately during rewarming. This response was similar to that observed in the intact cold-sensitive fibers. Temperature threshold distribution was broad in intact and axotomized cold fibers (30.7-22 degrees C and 34.5-14.5 degrees C, respectively). One-third of the axotomized cold-sensitive fibers were mechanosensitive and none of them displayed spontaneous activity at baseline temperature. In contrast, 33% of intact cold-sensitive fibers exhibited low rates of ongoing discharges. In 60% of the cold-sensitive, axotomized units, cold threshold was shifted towards warmer values by the TRPM8 agonist L-menthol. Seventy percent of axotomized, cold-insensitive units developed sensitivity to cold when exposed to 4-aminopyridine and their mean cold threshold (approximately 28 degrees C) was unaffected by menthol. Their response properties differed greatly from those of cold-sensitive units. In conclusion, the transducing capacity to cold stimuli is substantially recovered in neuromas. Furthermore, axotomized fibers maintain the 4-AP-sensitive, voltage-activated, transient potassium conductance that counteracts the depolarizing effects of cold in the majority of intact, cold-insensitive primary afferents. Our results indicate that injured nociceptors do not develop abnormal cold sensitivity, suggesting that other mechanisms underlie the cold-induced allodynia following peripheral nerve injury.
Emerging evidence suggests that blockade of hyperpolarization-activated current (Ih) produces ana... more Emerging evidence suggests that blockade of hyperpolarization-activated current (Ih) produces analgesia acting at peripheral sites. However, little is known about the role of this current in central pain-processing structures. The aim of the present work was to characterize the Ih in deep dorsal horn neurons and to assess the role of the current in the transmission of somatosensory signals across spinal circuits. To these purpose in vitro preparations of the spinal cord from mice pups were used in combination with whole cell recordings to characterize the current in native neurons. Extracellular recordings from sensory and motor pathways were performed to assess the role of the current in spinal somatosensory processing. Cesium chloride and ZD7288 were used as current blockers. Most deep dorsal horn neurons showed a functional Ih that was blocked by ZD7288 and cesium. Ih blockade caused hyperpolarization, increased input resistance and potentiation of synaptic responses. Excitatory effects of Ih blockade on synaptic transmission were confirmed in projecting anterolateral axons and ventral roots. Ih modulation by cAMP produced a rightward shift in the voltage dependency curve and blocked excitatory effects of ZD7288 on sensory pathways. Results indicate that Ih currents play a stabilizing role in the spinal cord controlling transmission across sensory and motor spinal pathways via cellular effects on input resistance and excitability. In addition, results suggest that current modulation may alter significantly the role of the current in somatosensory processing.
In the context of neuropathic pain, the contribution of regeneration to the development of positi... more In the context of neuropathic pain, the contribution of regeneration to the development of positive symptoms is not completely understood. Several efforts have been done to described changes in axotomized neurons, however, there is scarce data on changes occurring in intact neurons, despite experimental evidence of functional changes. To address this issue, we analysed by immunohistochemistry the presence of phosphorylated signal transducer and activator of transcription 3 (pSTAT3), an accepted marker of regeneration, within DRGs where axotomized neurons were retrogradely labelled following peripheral nerve injury. Likewise, we have characterized abnormal electrophysiological properties in intact fibres after partial nerve injury.
Epidemiological studies link long term exposure to xenoestrogen Bisphenol-A to adverse cardiovasc... more Epidemiological studies link long term exposure to xenoestrogen Bisphenol-A to adverse cardiovascular effects. Our previous results show that BPA induces hypertension by a mechanism involving CamKII activation and increased redox stress caused by eNOS uncoupling. Recently, CamKII sustained activation has been recognized as a central mediator of programmed cell death in cardiovascular diseases, including necroptosis. However, the role of necroptosis in cardiac response to BPA had not yet been explored. Mice exposed to BPA for 16 weeks showed altered heart function, electrical conduction, and increased blood pressure. Besides, a stress test showed ST-segment depression, indicative of cardiac ischemia. The hearts exhibited cardiac hypertrophy and reduced vascularization, interstitial edema, and large hemorrhagic foci accompanied by fibrinogen deposits. BPA initiated a cardiac inflammatory response, up-regulation of M1 macrophage polarization, and increased oxidative stress, coinciding ...
American Journal of Physiology-Regulatory, Integrative and Comparative Physiology
The contribution of changes in ureter motility produced by a stone to the pain of ureteric calcul... more The contribution of changes in ureter motility produced by a stone to the pain of ureteric calculosis is unclear. In this study we measured ureter motility as changes in intraureter pressure in anesthetized rats 1, 4, and 8 d ys after implantation of an artificial calculus (n = 33) and compared it with motility in normal (n = 8) and ligated (n = 4) ureters. Partial obstruction of the ureter by the stone produced a 478% increase in the amplitude of contractions, a 70% decrease in the rate of contractions, and a 66% decrease in the baseline pressure. The pressures reached during contractions were equivalent to those evoking nociceptive reactions in animals and humans. These changes persisted in rats that had spontaneously eliminated the stone. Complete obstruction of the ureter by the stone or by ligation abolished contractions. We conclude that the increased motility caused by a stone likely contributes to the development and maintenance of visceral pain and referred hyperalgesia in ...
Roza, Carolina, Jennifer M. A. Laird, and Fernando Cervero. Spinal mechanisms underlying persiste... more Roza, Carolina, Jennifer M. A. Laird, and Fernando Cervero. Spinal mechanisms underlying persistent pain and referred hyperalgesia in rats with an experimental ureteric stone. J. Neurophysiol. 79: 1603–1612, 1998. Spinal neurons processing information from the ureter have been characterized in rats 1–4 days after the implantation of an experimental ureteric stone and compared with those of normal rats. The effects of a conditioning noxious stimulation of the ureter in the presence of the hyperalgesia evoked by the calculosis also were examined. Extracellular recordings were performed at the T12–L1 segments of the spinal cord. In rats with calculosis, more neurons expressed a ureter input (53 vs. 42% in normal rats); such cells being more likely to show background activity, at a higher rate than normals (6.6 ± 1.2 vs. 3.2 ± 0.9 spikes/s; mean ± SE) and increasing with the continuing presence of the stone. The threshold pressure for a ureteric response was higher than in normal rats (...
Visceral pain is qualitatively distinct from other pain types; it is poorly localized, difficult ... more Visceral pain is qualitatively distinct from other pain types; it is poorly localized, difficult to quantify, and accompanied by marked autonomic changes. Acute visceral pain may be an indication of a medical emergency requiring urgent surgical or clinical intervention. However, chronic visceral pain, which contributes significantly to lifelong morbidity, occurs most frequently in the absence of any distinct pathology making it difficult to treat. This chapter reviews our current understanding of how visceral pain is detected in the periphery, and processed within the spinal cord and central nervous system. We focus on recent work that has identified pro-nociceptive changes in the bowel of patients with chronic visceral pain and discuss how these findings could lead to the development of novel viscero-specific analgesics. Finally, we consider how the microbiota can act locally to shape the detection of pain in the periphery and centrally to modulate our perception of visceral pain.
Superficial laminae of the spinal cord possess a considerable number of neurons with spontaneous ... more Superficial laminae of the spinal cord possess a considerable number of neurons with spontaneous activity as reported in vivo and in vitro preparations of several species. Such neurons may play a role in the development of the nociceptive system and/or in the spinal coding of somatosensory signals. We have used electrophysiological techniques in a horizontal spinal cord slice preparation from adult mice to investigate how this activity is generated and what are the main patterns of activity that can be found. The results show the existence of neurons that fire regularly and irregularly. Within each of these main types, it was possible to distinguish patterns of spontaneous activity formed by single action potentials and different types of bursts according to intra-burst firing frequency. Activity in neurons with irregular patterns was blocked by a mixture of antagonists of the main neurotransmitter receptors present in the cord. Approximately 82% of neurons with a regular firing pat...
Pflügers Archiv - European Journal of Physiology, 2016
The superficial dorsal horn contains large numbers of interneurons which process afferent and des... more The superficial dorsal horn contains large numbers of interneurons which process afferent and descending information to generate the spinal nociceptive message. Here, we set out to evaluate whether adjustments in patterns and/or temporal correlation of spontaneous discharges of these neurons are involved in the generation of central sensitization caused by peripheral nerve damage. Multielectrode arrays were used to record from discrete groups of such neurons in slices from control or nerve damaged mice. Whole-cell recordings of individual neurons were also obtained. A large proportion of neurons recorded extracellularly showed well-defined patterns of spontaneous firing. Clock-like neurons (CL) showed regular discharges at ∼6 Hz and represented 9 % of the sample in control animals. They showed a tonic-firing pattern to direct current injection and depolarized membrane potentials. Irregular fast-burst neurons (IFB) produced short-lasting high-frequency bursts (2-5 spikes at ∼100 Hz) at irregular intervals and represented 25 % of the sample. They showed bursting behavior upon direct current injection. Of the pairs of neurons recorded, 10 % showed correlated firing. Correlated pairs always included an IFB neuron. After nerve damage, the mean spontaneous firing frequency was unchanged, but the proportion of CL increased significantly (18 %) and many of these neurons appeared to acquire a novel low-threshold A-fiber input. Similarly, the percentage of IFB neurons was unaltered, but synchronous firing was increased to 22 % of the pairs studied. These changes may contribute to transform spinal processing of nociceptive inputs following peripheral nerve damage. The specific roles that these neurons may play are discussed.
Kv7.2 channel expression has been reported to decrease in dorsal root ganglia (DRG) following the... more Kv7.2 channel expression has been reported to decrease in dorsal root ganglia (DRG) following the induction of a peripheral neuropathy while other experiments show that Kv7.2 accumulates in peripheral neuromas. The mechanisms underlying these novel expression patterns are poorly understood. Here we use immunofluorescence methods to analyze Kv7.2 protein expression changes in sensory neurons following peripheral axotomy and the potential role of axonal transport. Results indicate that DRG neurons express Kv7.2 in ~16% of neurons and that this number decreases by about 65% after axotomy. Damaged neurons were identified in DRG by application of the tracer Fluoro-ruby at the site of injury during surgery. Reduction of Kv7.2 expression was particularly strong in damaged neurons although some loss was also found in putative uninjured neurons. In parallel to the decrease in the soma of axotomized sensory neurons, Kv7.2 accumulated at neuromatose fiber endings. Blockade of axonal transport ...
The spinal cord is the first relay center for nociceptive information. Following peripheral injur... more The spinal cord is the first relay center for nociceptive information. Following peripheral injury, the spinal cord sensitizes. A sign of spinal sensitization is the hyper-reflexia which develops shortly after injury and can be detected in the isolated spinal cord as a "memory of pain." In this context, it is easy to understand that many analgesic compounds target spinally located sites of action to attain analgesia. In vitro isolated spinal cord preparations have been used for a number of years, and experience on the effects of compounds of diverse pharmacological families on spinal function has accumulated. Recently, we have proposed that the detailed study of spinal segmental reflexes in vitro may produce data relevant to the evaluation of the analgesic potential of novel compounds. In this review, we describe the main features of segmental reflexes obtained in vitro and discuss the effects of compounds of diverse chemical nature and pharmacological properties on such reflexes. Our aim was to compare the different profiles of action of the compounds on segmental reflexes in order to extract clues that may be helpful for pharmacological characterization of novel analgesics.
Mechanosensitive cation channels are thought to be crucial for different aspects of mechanopercep... more Mechanosensitive cation channels are thought to be crucial for different aspects of mechanoperception, such as hearing and touch sensation. In the nematode C. elegans, the degenerins MEC-4 and MEC-10 are involved in mechanosensation and were proposed to form mechanosensitive cation channels. Mammalian degenerin homologues, the H(+)-gated ASIC channels, are expressed in sensory neurones and are therefore interesting candidates for mammalian mechanosensors. We investigated the effect of an ASIC2 gene knockout in mice on hearing and on cutaneous mechanosensation and visceral mechanonociception. However, our data do not support a role of ASIC2 in those facets of mechanoperception.
The M-current has been proposed as a potential target for analgesia under neuropathic pain condit... more The M-current has been proposed as a potential target for analgesia under neuropathic pain conditions. M-currents and/or their molecular correlates, KCNQ proteins, have been demonstrated in key elements of the nociceptive system including spinal and dorsal root ganglion neurons. Here we demonstrate that retigabine, a selective KCNQ channel opener, applied at neuromatose endings modulates the excitability of axotomized fibres inhibiting ectopic discharges. Responses to mechanical and chemical stimulation were obtained from intact and previously axotomized Adelta- and C-fibres using in vitro preparations and extracellular electrophysiological recording techniques. Application of retigabine (10 microM) produced an estimated approximately 80% reduction in the number of discharges produced by mechanical and chemical stimulation of most axotomized fibres tested (24/27). The electrical threshold of stimuli applied to the neuroma was found to increase in the presence of retigabine (+17.5+/-2.3%) and to decrease in the presence of a high potassium medium (-16.5+/-3.7%). This indicates that retigabine produces a hyperpolarization and a subsequent reduction of the excitability in aberrant sensory endings. Application of XE-991 (10 microM), a KCNQ channel blocker, had no effect on responses to stimulation of the neuroma but blocked the effects of retigabine indicating a specific involvement of KCNQ channels. In contrast to the strong effects on ectopic discharges, retigabine did not change responses to stimulation recorded from intact receptors. Results indicate that KCNQ channel opening at axotomized endings may constitute a novel and selective mechanism for modulation of some neuropathic pain symptoms.
Cold allodynia is a common complaint in patients with peripheral neuropathies. However, cold sens... more Cold allodynia is a common complaint in patients with peripheral neuropathies. However, cold sensitivity of the different types of sensory afferents present in injured nerves is poorly known. We recorded activity evoked by cold in intact sensory fibers of the skin-saphenous nerve preparation and in axotomized sensory fibers of approximately 21 days-old neuromas of the saphenous nerve of mice, in vitro. Sixteen percent of the axotomized units responded to cooling with an accelerating discharge, which stopped immediately during rewarming. This response was similar to that observed in the intact cold-sensitive fibers. Temperature threshold distribution was broad in intact and axotomized cold fibers (30.7-22 degrees C and 34.5-14.5 degrees C, respectively). One-third of the axotomized cold-sensitive fibers were mechanosensitive and none of them displayed spontaneous activity at baseline temperature. In contrast, 33% of intact cold-sensitive fibers exhibited low rates of ongoing discharges. In 60% of the cold-sensitive, axotomized units, cold threshold was shifted towards warmer values by the TRPM8 agonist L-menthol. Seventy percent of axotomized, cold-insensitive units developed sensitivity to cold when exposed to 4-aminopyridine and their mean cold threshold (approximately 28 degrees C) was unaffected by menthol. Their response properties differed greatly from those of cold-sensitive units. In conclusion, the transducing capacity to cold stimuli is substantially recovered in neuromas. Furthermore, axotomized fibers maintain the 4-AP-sensitive, voltage-activated, transient potassium conductance that counteracts the depolarizing effects of cold in the majority of intact, cold-insensitive primary afferents. Our results indicate that injured nociceptors do not develop abnormal cold sensitivity, suggesting that other mechanisms underlie the cold-induced allodynia following peripheral nerve injury.
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Papers by Carolina Roza