Fibroblast growth factor-2 acutely influences the impulse activity of rat dorsal horn neurones

The neurotrophic and neuroprotective actions of fibroblast growth factor-2 (FGF-2) are well-established. The signal cascade mediating these effects includes steps that are likely to influence also the electrical properties of neurones. However, the possibility that FGF-2 may acutely affect the processing of neuronal impulse activity is largely unexplored. In the present study the impulse activity of single dorsal horn neurones was recorded in the rat during ionophoretical administration of FGF-2 close to the neurones. Before and during FGF-2 ionophoresis the receptive field of each cell was tested with defined mechanical stimuli. At a concentration of 10 nM in the ionophoresis pipette, FGF-2 reduced the responses of the cells to mechanical stimulation. There was no preferential action of FGF-2 on a particular functional type of dorsal horn neurone; both non-nociceptive and nociceptive cells exhibited a reduced mechanical responsiveness. The background (ongoing) activity was also depressed in most neurones. The results of the study show that in addition to neurotrophic and neuroprotective actions FGF-2 has an acute inhibitory influence on the impulse activity of spinal sensory neurones. This depression of neuronal activity could add to the neuroprotective action of FGF-2 by counteracting glutamate excitotoxicity following a central nervous trauma.

Contribution of tactile information to accuracy in pointing movements

We examined the contribution of tactile cues to accuracy during point-to-point movements. We used a task in which the experimenter guided either the left or right hand of the subject to a spatial location during the reference movement. During the subsequent test movement subjects were asked to point with the right hand to the remembered location without vision. Subjects contacted the target with their fingertip either during the reference movement, both the reference and test movements, or neither movement (i.e., the fingertip was held above the target surface). To differentiate between the contribution of tactile and proximal deep pressure information, the left index finger was anesthetized in a subsequent experiment. When subjects contacted the surface with the fingertip of the reference hand alone, error in movement direction decreased. When subjects made fingertip contact during the reference and test movements, gain error also decreased. Anesthesia of the fingertip degraded accuracy, suggesting that tactile information, independent of information from proximal deep pressure receptors, influenced movement accuracy. Thus, tactile information contributed to accuracy in pointing movements. We suggest that forces at fingertip contact may provide information regarding the orientation of the finger and forearm in space, which is used to replicate final arm posture. In addition, tactile cues at the beginning and end of the movement may be used to scale movement amplitude.

Spinal delta-opioid receptors mediate suppression of systemic SNC80 on excitability of the flexor reflex in normal and inflamed rat

Due to low central nervous system (CNS) bioavailability of delta-opioid peptides, little is known about the effect of systemic administration of delta-opioid receptor ligands. The present study examined the effect of non-peptidergic delta-opioid receptor agonists, (+)-4-[(alphaR)-alpha-((2R,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80) and (-)dibenzoyl-L-tartaric acid salt (SNC86), on the activity of alpha-motoneurons in decerebrate-spinal rats. The flexor reflex was facilitated by C-afferent conditioning inputs, shown by a decrease in mechanical threshold and increase in touch- and pinch-evoked responses. Systemic administration of SNC80 (10 micromol/kg) prevented and reversed the neuronal hyperactivity. We further examined the effect of this agonist on the hypersensitivity of the flexor reflex induced by intraplantar injection of Freund's adjuvant. SNC80 dose-dependently (1, 3, 5 and 10 micromol/kg) increased the mechanical threshold and decreased touch-, pinch- and Abeta-afferent inputs-evoked responses. Similar effects were seen with SNC86 (5 micromol/kg). Pretreatment with either naloxone (20 micromol/kg, i.p.) or (Cyclopropylmethyl)-6,7-dehydro-4,5alpha-epoxy-14beta-ethoxy-5beta-methylindolo [2',3':6',7']morphinan-3-ol hydrochloride (SH378; 5 micromol/kg, intraarterially (i.a.)), a novel selective delta-opioid receptor antagonist, completely abolished the anti-hypersensitivity effect of SNC80. The effect of SNC80 remained following intrathecal administration of mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP; 1.5 nmol). These results indicate that systemic injection of SNC80 exerted antihypersensitivity in models of both acute and tonic nociception and these effects are mediated mainly through a spinal delta-opioid mechanism.

Cholecystokinin receptor antagonists increase the rat pup's preference toward maternal-odor and rug texture

The role of the cholecystokinin (CCK) system in mediating the infant's natural preferences toward maternal-related stimuli was examined by peripheral administration of selective CCK(A) and CCK(B) receptor antagonists (Devazepide and L-365,260, respectively) to 11-12-day-old rats and presenting them with a 3-minute preference test. In Experiment 1, the choice was between two floor textures, rug and plywood; the time spent on the relatively preferred side (rug) was measured. In Experiment 2, the odor of maternal faces emanated from one end of the test arena; time spent near that end was measured. These sensory stimuli were chosen as they represent olfactory and tactile aspects of the dam and nest. Compared to controls, both CCK receptor antagonists selectively increased the time spent on the preferred side, in both experiments, without affecting axillary temperature or locomotor activity. The results suggest that CCK may mediate and attenuate the infant's attraction toward naturally preferred stimuli.

Characterization of the glutamatergic system for induction and maintenance of allodynia

Glutamate is the main excitatory neurotransmitter in the central nervous system and has been shown to be involved in spinal nociceptive processing. We previously demonstrated that intrathecal (i.t.) administration of prostaglandin (PG) E(2) and PGF(2 alpha) induced touch-evoked pain (allodynia) through the glutamatergic system by different mechanisms. In the present study, we characterized glutamate receptor subtypes and glutamate transporters involved in induction and maintenance of PGE(2)- and PGF(2 alpha)-evoked allodynia. In addition to PGE(2) and PGF(2 alpha), N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), but not kainate, induced allodynia. PGE(2)- and NMDA-induced allodynia were observed in NMDA receptor epsilon 4 (NR2D) subunit knockout (GluR epsilon 4(-/-)) mice, but not in epsilon 1 (NR2A) subunit knockout (GluR epsilon 1(-/-)) mice. Conversely, PGF(2 alpha)- and AMPA-induced allodynia were observed in GluR epsilon 1(-/-) mice, but not in GluR epsilon 4(-/-) mice. The induction of allodynia by PGE(2) and NMDA was abolished by the NMDA receptor epsilon 2 (NR2B) antagonist CP-101,606 and neonatal capsaicin treatment. PGF(2 alpha)- and AMPA-induced allodynia were not affected by CP-101,606 and by neonatal capsaicin treatment. On the other hand, the glutamate transporter blocker DL-threo-beta-benzyloxyaspartate (DL-TBOA) blocked all the allodynia induced by PGE(2), PGF(2 alpha), NMDA, and AMPA. These results demonstrate that there are two pathways for induction of allodynia mediated by the glutamatergic system and suggest that the glutamate transporter is essential for the induction and maintenance of allodynia.

Long-term cholinergic enhancement of evoked potentials in rat hindlimb somatosensory cortex displays characteristics of long-term potentiation

Pairing a cutaneous electrical stimulus of the hind-paw with stimulation of the basal forebrain produces long-term cholinergic enhancement of the responsiveness to a tactile stimulus. A short period of pairing (20 trials) increased the area of the two main components of the evoked potential by 37.1 +/- 13.5% (+/- SEM) and 37.9 +/- 6.8%, respectively. The effects lasted for the duration of the experiment (> 2 h). The enhancement could be blocked by either MK-801, an NMDA receptor antagonist or by L-NAME, a nitric-oxide-synthase inhibitor when they were given prior to pairing. Control experiments with skin stimulation alone and basal forebrain stimulation alone had only small long-term effects (approximately 10%) on the size of the evoked potential. Thus, long-term cholinergic enhancement, attributable to disinhibition and increased release of acetylcholine in the cortex during neuronal excitation by other sources, and so named because it is blocked by atropine, may be a form of long-term potentiation. The existence of such a mechanism for the control of cortical neuronal plasticity identifies the basal forebrain as a powerful modulator of long-lasting changes in cortical neuronal excitability.

Simultaneous intrastriatal 6-hydroxydopamine and quinolinic acid injection: a model of early-stage striatonigral degeneration

Animal models reproducing early stages of striatonigral degeneration (SND), the core pathology underlying parkinsonism in multiple system atrophy, are lacking. We have developed a new model of early-stage SND by using a simultaneous unilateral administration of quinolinic acid (QA) and 6-hydroxydopamine (6-OHDA) into the putaminal equivalent of the rat striatum. Spontaneous and drug-induced behavior, thigmotactic scanning, paw reaching deficits, and histopathology were studied in rat groups: group 1 (control), group 2 (QA), group 3 (6-OHDA), and group 4 (QA + 6-OHDA). The double toxin administration resulted in reduction of the spontaneous and the amphetamine-induced ipsiversive bias in the 6-OHDA group and in a reduction of the apomorphine-induced ipsiversive rotations in the QA group. Simultaneous QA and 6-OHDA also reduced the thigmotactic bias observed in the 6-OHDA rats. Combined toxin elicited a nonsignificant contralateral deficit in paw reaching but a significant deficit on the ipsilateral side. Histopathology revealed a significant reduction of the lesioned striatal surface (-27%) with neuronal loss and increased astrogliosis in group 4 compared to group 2, consistent with an exacerbation of QA toxicity by additional 6-OHDA. By contrast, the mean loss of the TH-positive neurons in the ipsilateral substantia nigra pars compacta (SNc) of group 4 was less marked (-15%) than in the 6-OHDA group (-36%), indicating a possible protective action of intrastriatal QA upon 6-OHDA retrograde SNc degeneration. This study shows that a combined unilateral intrastriatal administration of QA and 6-OHDA may serve as a model of early stage SND which is more suitable for early therapeutic interventions.

Topical amethocaine gel in the newborn infant: how soon does it work and how long does it last?

AIM

To explore the time of onset and duration of action of topical amethocaine gel in the newborn infant.

DESIGN

A randomised double blind placebo controlled trial.

SUBJECTS

Thirty six infants were studied after 30 minutes application and 36 after 60 minutes application. A total of 56 infants (gestation 27-42 weeks, weight 0. 79-4.1 kg) were studied in the first two weeks after delivery.

METHOD

1.5 g amethocaine or placebo was applied to the dorsum of either foot, occluded, and then left for 30 or 60 minutes. Local anaesthesia was assessed by observing the cutaneous withdrawal response to graded nylon filaments (von Frey hairs). If there was a difference between feet in filament thickness required to elicit a response, the infant was studied in an identical manner at hourly intervals until the difference had disappeared.

RESULTS

Evidence of local anaesthetic action of amethocaine was seen in 23 of 36 (64%) infants after 30 minutes and 26 of 36 (72%) infants after 60 minutes application (no significant difference). Evidence of local anaesthetic action was independent of gestation and order of testing. Amethocaine responders showed a significantly deeper anaesthetic action than placebo responders. The median duration of action was 1.5 hours (range 0.5-3.5) after 30 minutes and three hours (range 1-5) after 60 minutes (p<0.001).

CONCLUSION

Topical amethocaine gel has a local anaesthetic action after 30 minutes application, but application for 60 minutes results in longer duration of action.

Large cortical lesions produce enduring forelimb placing deficits in un-treated rats and treatment with NMDA antagonists or anti-oxidant drugs induces behavioral recovery

Previous studies have utilized a lesion model of cortical injury that produces transient behavioral impairments to investigate the recovery of function process. To better understand the recovery process, it would be beneficial to use a lesion model that produces more severe, enduring, behavioral impairments. The purpose of experiment 1 was to validate whether large lesions of the sensorimotor cortex (SMC), which included the rostral forelimb and caudal forelimb regions, produced enduring behavioral deficits. Rats were given large unilateral electrolytic lesions of the SMC, administered either the N-methyl-D-aspartate (NMDA) antagonist, MK-801 or saline 16 h after injury, and tested on a battery of behavioral tests. Enduring behavioral deficits were observed, for at least 6 months, on two tests of forelimb placing while transient deficits were observed on the foot-fault and somatosensory neutralization tests. Administration of MK-801 facilitated recovery on the somatosensory neutralization test; however, it did not induce recovery on either forelimb placing test. A second experiment was performed to determine if earlier administration of MK-801, the NMDA antagonist magnesium chloride (MgCl(2)), or the anti-oxidant N-tert-butyl-alpha-phenylnitrone (PBN) could induce behavioral recovery in this chronic model. Treatment with these drugs induced behavioral recovery on the forelimb placing tests, whereas, the saline-treated rats did not show any signs of behavioral recovery for at least 3 months. Anatomical analysis of the striatum showed that MK-801 and MgCl(2) but not PBN reduced the extent of lesion-induced striatal atrophy. These results suggest that administration of MK-801, MgCl(2), or PBN shortly after cortical injury can induce recovery of function when recovery is otherwise not expected in un-treated rats.

The effect of oral-gustatory, tactile-bucal, and tactile-manual stimulation on the behavior of the hands in newborns

The effect of stimulation (oral-gustatory with sucrose, tactile-bucal, and tactile-manual) on the frequency of hand contact with the oral (hand-mouth and hand sucking) and perioral (hands near the mouth) regions was compared in 24 full-term newborns. The 16-minute evaluation was divided into four equal periods, without intervals: (A) Baseline, no stimulation applied; (B) tactile-bucal, the newborn was allowed to suck the distal phalange of the researcher's little finger; (C) tactile-manual, the newborn was allowed to grasp the researcher's index finger; (D) 0.3 ml sucrose solution was administered orally to the newborns. After every three newborns were tested, the sequence of stimulation application was changed. The frequencies of the hand behaviors were calculated. It was observed that the sucrose significantly increased the frequency of hand sucking (p<0.05). Sucrose was, therefore, the most effective stimulus for eliciting the behavior of hand sucking.

Residual neurologic deficits 30 years after occupational exposure to elemental mercury

A battery of tests of peripheral and central nervous system function was administered to 205 former workers of a large heavy industrial plant, 104 of whom were previously exposed to inorganic mercury. The mean age of those examined was 71 years. Exposed subjects had participated in a urine-mercury exposure monitoring program during the time of operation of a process that required the use of mercury and its subsequent clean-up. Mercury exposure had been high (mean peak urine mercury concentration was >600 microg/l) and had ended 30 years or more prior to the investigation. Peripheral nerve function outcomes that were statistically significantly associated with cumulative mercury exposure after controlling for covariates included classification as having peripheral neuropathy, peroneal motor nerve conduction velocity, ulnar motor nerve conduction velocity, and peroneal motor nerve F-wave latency. Quantitative assessment of resting tremor was nearly significantly associated with cumulative mercury exposure (p=0.07). Among tests of central nervous system function, results of the Handeye Coordination test were significantly associated with cumulative mercury exposure after controlling for covariates. Cumulative mercury exposure was not observed to be associated with a quantitative measure of dementia or with a number of cognitive neurobehavioral test outcomes. The statistically significant associations with mercury exposure were observed in spite of greater mortality among the exposed group than the unexposed group. These results suggest that substantial occupational mercury exposure can have long-term adverse effects on the peripheral nervous system detectable decades after cessation of exposure. Such long-term adverse effects were not observed for a measure of dementia or other measures of cognitive function.

[Acetylsalicylic acid effects on pain sensitivity to myocardial ischemia and skin sensitivity in patients with angina pectoris]

AIM

To clarify if pain-relieving action of acetylsalicylic acid (ASA) is associated with lowered sensitivity of anginal patients to pain due to myocardial ischemia.

MATERIALS AND METHODS

A double blind randomized placebo-controlled trial enrolled 10 males aged 42-69 years with stable effort angina (EA) of functional class II-III. When exposed to exercise tolerance test (treadmill, stress-system Sicard 460S, computed ECG), the patients developed EA attack with at least 1 mm decline of ST segment on ECG. The exercise test was made before, 2 and 4 hours after administration of ASA and placebo. Sensitivity to ischemia was estimated by the total depth of the ST segment decline in 11 ECG leads (sigma ST) registered at the attack onset. Tactile and pain thresholds (TT and PT) were studied with a highly reproducible technique. TT and PT were measured before, 2 and 4 hours after ASA and placebo administration.

RESULTS

2 and 4 hours after intake of 100 mg of ASA, sigma ST and TT significantly rose compared to the baseline level and placebo. PT significantly rose vs the baseline level.

CONCLUSION

ASA deteriorates sensitivity of anginal patients to myocardial ischemia, skin tactile and pain sensitivity and thus can deprive the EA patient of the pain attack signal. This leads to the risk of overexercising and emergence of painless myocardial ischemia.

Preinjury treatment with morphine or ketamine inhibits the development of experimentally induced secondary hyperalgesia in man

We examine the effect of morphine or ketamine (N-methyl-D-aspartate receptor antagonist; NMDA) treatment on secondary hyperalgesia. Drug treatment started preinjury and continued into the early postinjury period. Hyperalgesia was induced by a local 1 degrees burn injury covering 12.5 cm(2) on the medial side of the calf. In this double-blind, cross-over study, 12 healthy volunteers received, on 3 separate days and in randomized order: (1) placebo; (2) morphine, bolus 150 microg/kg + infusion 1 microg/kg per min and 0.5 microg/kg per min; and (3) ketamine, bolus 60 microg/kg + infusion 6 microg/kg per min and 3 microg/kg per min. Bolus + infusion started 30 min before injury and ended 50 min after it. The area of secondary hyperalgesia was quantitated using punctate (von Frey filaments) and brush stimuli (electric brush). On the day of placebo, all subjects developed an area of hyperalgesia to punctate and brush stimuli outside the thermal injury (secondary hyperalgesia). We show that ketamine or morphine treatment starting preinjury significantly reduces this development (P<0.01, both). In a previous study, we found that postinjury treatment alone with morphine did not affect secondary hyperalgesia, whereas ketamine did so significantly. The differential response to morphine administered pre- or postinjury may be relevant to the recently shown NMDA receptor mediated interaction of central hyperexcitability and morphine antinociception. The effect of ketamine supports the hypothesis of the role of NMDA receptor mediation in central hyperexcitability.

ATP in human skin elicits a dose-related pain response which is potentiated under conditions of hyperalgesia

Despite the considerable interest in the possibility that ATP may function as a peripheral pain mediator, there has been little quantitative study of the pain-producing effects of ATP in humans. Here we have used iontophoresis to deliver ATP to the forearm skin of volunteers who rated the magnitude of the evoked pain on a visual analogue scale. ATP consistently produced a modest burning pain, which began within 20 s of starting iontophoresis and was maintained for several minutes. Persistent iontophoresis of ATP led to desensitization within 12 min but recovery from this was almost complete 1 h later. Different doses of ATP were delivered using different iontophoretic driving currents. Iontophoresis of ATP produced a higher pain rating than saline, indicating that the pain was specifically caused by ATP. The average pain rating for ATP, but not saline, increased with increasing current. Using an 0.8 mA current, subjects reported pain averaging 27.7 +/- 2.8 (maximum possible = 100). Iontophoresis of ATP caused an increase in blood flow, as assessed using a laser Doppler flow meter. The increase in blood flow was significantly greater using ATP than saline in both the iontophoresed skin (P < 0.01) and in the surrounding skin, 3 mm outside the iontophoresed area (P < 0.05). The pain produced by ATP was dependent on capsaicin-sensitive sensory neurons, since in skin treated repeatedly with topical capsaicin pain was reduced to less than 25% of that elicited on normal skin (2.1 +/- 0.4 compared with 9.3 +/- 1.5 on normal skin). Conversely, the pain-producing effects of ATP were greatly potentiated in several models of hyperalgesia. Thus, with acute capsaicin treatment when subjects exhibited touch-evoked hyperalgesia but no ongoing pain, there was a threefold increase in the average pain rating during ATP iontophoresis (22.7 +/- 3.1) compared with pre-capsaicin treatment (7.8 +/- 2.6). Moreover, ATP iontophoresed into skin 24 h after solar simulated radiation (2 x minimal erythymic dose) resulted in double the pain rating of normal skin, increasing from 15.3 +/- 4.1 to 32.7 +/- 4.1. The pain response to saline was not significantly altered after UV irradiation at any time-point studied. We conclude that ATP produces pain by activating capsaicin-sensitive nociceptive afferents when applied to skin. The possibility that ATP activates nociceptors indirectly via its degradation products cannot be ruled out. The effects of ATP are dose-dependent and responses desensitize only slowly. In inflammatory conditions, ATP may be a potent activator of nociceptors and an endogenous mediator of pain.

Thermal hyperalgesia and mechanical allodynia produced by intrathecal administration of the human immunodeficiency virus-1 (HIV-1) envelope glycoprotein, gp120

Astrocytes and microglia in the spinal cord have recently been reported to contribute to the development of peripheral inflammation-induced exaggerated pain states. Both lowering of thermal pain threshold (thermal hyperalgesia) and lowering of response threshold to light tactile stimuli (mechanical allodynia) have been reported. The notion that spinal cord glia are potential mediators of such effects is based on the disruption of these exaggerated pain states by drugs thought to preferentially affect glial function. Activation of astrocytes and microglia can release many of the same substances that are known to mediate thermal hyperalgesia and mechanical allodynia. The aim of the present series of studies was to determine whether exaggerated pain states could also be created in rats by direct, intraspinal immune activation of astrocytes and microglia. The immune stimulus used was peri-spinal (intrathecal, i.t.) application of the Human Immunodeficiency Virus type 1 (HIV-1) envelope glycoprotein, gp120. This portion of HIV-1 is known to bind to and activate microglia and astrocytes. Robust thermal hyperalgesia (tail-flick, TF, and Hargreaves tests) and mechanical allodynia (von Frey and touch-evoked agitation tests) were observed in response to i.t. gp120. Heat denaturing of the complex protein structure of gp120 blocked gp120-induced thermal hyperalgesia. Lastly, both thermal hyperalgesia and mechanical allodynia to i.t. gp120 were blocked by spinal pretreatment with drugs (fluorocitrate and CNI-1493) thought to preferentially disrupt glial function.

Subjective magnitude of tactile roughness

The subjective experience of tactile roughness was judged by subjects using the method of absolute magnitude estimation (AME). The stimuli were 11 grades of sandpaper having particle diameters ranging from 16 to 905 microm. All of the estimates resulted in power functions when assigned numbers were plotted as a function of particle diameter. It was determined that on the finger pad of the index finger and the thumb there was no difference between the active and passive modes of stimulation and that there was no difference in roughness estimates made on the finger and on the thumb. When the finger and thumb were stimulated simultaneously, higher numbers were assigned for a given stimulus indicating the presence of a form of spatial summation at these sites. The pleasantness of the tactile sensation, as assessed using AME, was inversely related to the roughness estimates. Furthermore, hydration of the stratum corneum with water and three concentrations of surfactant solutions reduced the sensation of roughness below that of normally hydrated skin.

Local anaesthetic effect of topical amethocaine gel in neonates: randomised controlled trial

AIM

To assess the efficacy of amethocaine as a topical local anaesthetic in neonates.

METHODS

A randomised, double blind controlled trial compared 4% amethocaine gel (Ametop) with placebo in 60 healthy neonates (29 to 42 weeks of gestation) in the first week after birth. Either 1.5 g 4% w/w amethocaine (gel) or 1.5 g placebo gel were applied to the dorsum of one foot. No gel was applied to the other foot. Each foot was occluded and left for one hour. Local anaesthesia was then assessed by eliciting the cutaneous withdrawal reflex in response to stimulation with a series of graded nylon filaments (von Frey hairs). The reflex was first elicited from the control and then the treated foot. The difference in filament thickness and deforming weight required to elicit the reflex was recorded.

RESULTS

In infants treated with amethocaine, 17 of 31 (54. 8%) showed evidence of local anaesthetic action compared with five of 29 (17.2%) in the placebo group (p=0.003). The mean difference in deforming weight required to elicit the reflex was 18.8 g in the amethocaine group compared with 3.9 g in the placebo group (p=0.02). The apparent local anaesthetic action of the placebo can be explained by habituation to repeated stimulation.

CONCLUSIONS

It is concluded that topical amethocaine gel has a local anaesthetic action on neonatal skin which merits further investigation. An effective and safe surface local anaesthetic would be valuable for the relief of procedure related pain in neonates.

Secondary hyperalgesia to punctate mechanical stimuli. Central sensitization to A-fibre nociceptor input

Tissue injury induces enhanced pain sensation to light touch and punctate stimuli in adjacent, uninjured skin (secondary hyperalgesia). Whereas hyperalgesia to light touch (allodynia) is mediated by A-fibre low-threshold mechanoreceptors, hyperalgesia to punctate stimuli may be mediated by A- or C-fibre nociceptors. To disclose the relative contributions of A- and C-fibres to the hyperalgesia to punctate stimuli, the superficial radial nerve was blocked by pressure at the wrist in nine healthy subjects. Secondary hyperalgesia was induced by intradermal injection of 40 microg capsaicin, and pain sensitivity in adjacent skin was tested with 200 micron diameter probes (35-407 mN). The progress of conduction blockade was monitored by touch, cold, warm and first pain detection and by compound sensory nerve action potential. When A-fibre conduction was blocked completely but C-fibre conduction was fully intact, pricking pain to punctate stimuli was reduced by 75%, but burning pain to capsaicin injection remained unchanged. In normal skin without A-fibre blockade, pain ratings to the punctate probes increased significantly by a factor of two after adjacent capsaicin injection. In contrast, pain ratings to the punctate probes were not increased after capsaicin injection when A-fibre conduction was selectively blocked. However, hyperalgesia to punctate stimuli was detectable immediately after block release, when A-fibre conduction returned to normal. In conclusion, the pricking pain to punctate stimuli is predominantly mediated by A-fibre nociceptors. In secondary hyperalgesia, this pathway is heterosynaptically facilitated by conditioning C-fibre input. Thus, secondary hyperalgesia to punctate stimuli is induced by nociceptive C-fibre discharge but mediated by nociceptive A-fibres.

Mechanical allodynia in rats is blocked by a Ca2+ permeable AMPA receptor antagonist

Mild thermal injury to the hindpaw induces tactile allodynia distal to the injury. The allodynia is blocked by non-NMDA, but not NMDA, antagonists. The calcium permeable subtype of non-NMDA receptors is blocked by Joro spider toxin (JSTX). We injected JSTX or saline intrathecally followed after 5 min, 6 or 24 h by thermal injury. Rats receiving saline had decreased mechanical thresholds. Rats receiving 3 microg JSTX 5 min or 6 h prior to burn showed no allodynia. JSTX had no prominent side effects at doses between 1 and 5 microg. JSTX (5 microg) had no effect on thermal threshold. These results are consistent with the hypothesis that spinal mechanisms leading to tactile allodynia in this injury model act via a calcium permeable AMPA linkage.

Comparison of sedative effects of romifidine following intravenous, intramuscular, and sublingual administration to horses

OBJECTIVE

To compare sedative effects of romifidine following IV, IM, or sublingual (SL) administration in horses.

ANIMALS

30 horses that required sedation for routine tooth rasping.

PROCEDURE

Horses (n = 10/group) were given romifidine (120 microg/kg) IV, IM, or SL. Heart rate, respiratory rate, head height, distance between the ear tips, thickness of the upper lip, response to auditory stimulation, response to tactile stimulation, and degree of ataxia were recorded every 15 minutes for 180 minutes. Tooth rasping was performed 60 minutes after administration of romifidine, and overall adequacy of sedation was assessed.

RESULTS

IV and IM administration of romifidine induced significant sedation, but SL administration did not induce significant sedative effects. Scores for overall adequacy of sedation after IV and IM sedation were not significantly different from each other but were significantly different from scores for horses given romifidine SL. Sedative and other effects varied among groups during the first 60 minutes after drug administration; thereafter, effects of IV and IM administration were similar.

CONCLUSIONS AND CLINICAL RELEVANCE

Onset of action was fastest and degree of sedation was greater after IV, compared with IM, administration of romifidine, but duration of action was longer after IM administration. Sublingual administration did not result in clinically important sedative effects.

Increase in the threshold of pain and touch sensation in the human face with clonidine plus 30% nitrous oxide

OBJECTIVE

This study was undertaken to assess the effects of clonidine combined with 30% nitrous oxide on tactile and pain sensations in the human face.

STUDY DESIGN

Thirty-three subjects were involved in the study. The subjects were divided into 4 groups: 100% oxygen with placebo; 30% N2O with placebo; 100% oxygen with clonidine (0.075 mg), and 30% N2O with clonidine. Three tests for the threshold of pain sensation and tactile sensation were made at 60 minutes before and 0, 15, and 30 minutes during N2O or O2 inhalation.

RESULTS

(1) The N2O with clonidine significantly increased the threshold of pain and tactile sensation in comparison with the other 3 treatments. (2) In terms of pain sensation, both N2O and clonidine showed significant increases in threshold of pain in comparison with the control values.

CONCLUSIONS

These results indicate that the analgesic effects of 30% nitrous oxide are enhanced when use of the gas is combined with prior clonidine administration.

Assessment of the level of sensory block after subarachnoid anesthesia using a pressure palpator

In a cross-over study, we compared two methods of assessing the level of sensory block during subarachnoid anesthesia: the traditional pinprick sensation or a novel pressure palpator exerting a pressure of 650 g. Fifty patients scheduled for transurethral surgery under subarachnoid anesthesia were randomly assigned to be tested for spread of sensory block. In Group 1, the pressure palpator was followed by pinprick; in Group 2, the reverse sequence was used. Evaluation was performed 15 and 25 min after the subarachnoid injection of 2 mL of 5% lidocaine hyperbaric solution. In Group 1, the level of sensory block assessed with the pressure palpator was 1.7 +/- 3.2 cm (0.5 +/- 1.2 dermatomes) higher than that with the pinprick at 15 min, and 2.2 +/- 3.4 cm (0.6 +/- 1.0 dermatomes) higher than that with the pinprick 25 min after the block. In Group 2, the difference was accentuated. The level of sensory block assessed by pinprick 15 min after subarachnoid lidocaine was 5.7 +/- 4.8 cm (1.2 +/- 0.9 dermatomes) lower than the level with the pressure palpator, and 4.2 +/- 3.3 cm (0.9 +/- 0.6 dermatomes) lower than that with the pressure palpator at 25 min. In all instances, the pressure palpator gave a significantly higher assessment than the pinprick. We conclude that the pressure palpator, when preceded by the pinprick test, is associated with an increased threshold. This method may be useful in assessing the sensory block produced by subarachnoid anesthesia.

IMPLICATIONS

A novel pressure palpator that maintains the integrity of the epidermis was used to assess the level of sensory block after subarachnoid anesthesia and was compared with the standard method of the pinprick sensation. This method assessed the block consistently higher than the pinprick method, but it may have advantages as a noninvasive sensory test.

Low-dose lidocaine suppresses experimentally induced hyperalgesia in humans

BACKGROUND

The antinociceptive effects of systemically administered local anesthetics have been shown in various conditions, such as neuralgia, polyneuropathy, fibromyalgia, and postoperative pain. The objective of the study was to identify the peripheral mechanisms of action of low-dose local anesthetics in a model of experimental pain.

METHODS

In a first experimental trial, participants (n=12) received lidocaine systemically (a bolus injection of 2 mg/kg in 10 min followed by an intravenous infusion of 2 mg x kg(-1) x h(-1) for another 50 min). In a second trial, modified intravenous regional anesthesia was administered to exclude possible central analgesic effects. In one arm, patients received an infusion of 40 ml lidocaine, 0.05%; in their other arm, 40 ml NaCl, 0.9%, served as a control. In both trials, calibrated tonic and phasic mechanical and chemical (histamine) stimuli were applied to determine differentially the impairment of tactile and nociceptive perception.

RESULTS

Mechanical sensitivity to touch, phasic mechanical stimuli of noxious intensity, and heat pain thresholds remained unchanged after systemic and regional application of the anesthetic. In contrast, histamine-induced itch (intravenous regional anesthesia), axon reflex flare (systemic treatment), and development of acute mechanical hyperalgesia during tonic pressure (12 N; 2 min) of an interdigital web was significantly suppressed after both treatments.

CONCLUSIONS

Increasing painfulness during sustained pinching has been attributed to excitation and simultaneous sensitization of particular Adelta- and C-nociceptors. This hyperalgesic mechanism seems to be particularly sensitive to low concentrations of lidocaine. These findings confirm clinical experience with lidocaine in pain states dominated by hyperalgesia.

Local anesthetic effect of tramadol, metoclopramide, and lidocaine following intradermal injection

BACKGROUND AND OBJECTIVES

We observed clinically that tramadol and metoclopramide appear to have local anesthetic action. Tramadol is a central-acting analgesic. Metoclopramide is a commonly used antiemetic. The local anesthetic effect of tramadol in reducing propofol injection pain has never been mentioned, although it was speculated with metoclopramide.

METHODS

We conducted a double-blind, placebo-controlled study by injecting tramadol or metoclopramide intradermally in 10 healthy volunteers (5 men, 5 women; age 25-56 years). Each subject received 0.5 mL of four solutions in random order on the volar side of the forearm. These solutions were 25 mg tramadol, 5 mg metoclopramide, 5 mg lidocaine, and 0.5 mL normal saline. Pain on injections and the degree of local anesthesia (tested by pinprick, light touch, and cold) at each site was reported on a 0-3 scale at designed time intervals.

RESULTS

Like 1% lidocaine, tramadol and metoclopramide demonstrated loss of sensation for pinprick, light touch, and cold for 15 minutes after intradermal injection (P < .01 ).

CONCLUSIONS

Intradermal tramadol or metoclopramide can produce local anesthetic effect.

Cutaneous responsiveness of lumbar spinal dorsal horn neurons is reduced by general anesthesia, an effect dependent in part on GABAA mechanisms

Extracellular activity was recorded from single spinal dorsal horn neurons in both chronic cat and acute rat models. This was done to define the effects of anesthesia on the processing of sensory information elicited by nonnoxious tactile stimulation of peripheral receptive fields (RFs). In the chronic cat model, baseline data were obtained in physiologically intact, awake, drug-free animals before anesthetic administration (halothane 1.0-2.0%). This made it possible to compare and contrast activity of each cell in the drug-free and anesthetized state. Halothane effects were confirmed in the acute rat model (anesthetized, spinally transected, and in some cases decerebrate). In addition, the gamma-aminobutyic acid-A (GABAA)-receptor antagonist picrotoxin (2 mg/kg) was administered intravenously to verify that the observed halothane effect on spinal dorsal horn neurons was mediated by an interaction with GABAA-receptor systems. Halothane effects on three separate measures of response to nonnoxious tactile stimuli were observed in the chronic cat model. Halothane produced a significant, dose-dependent reduction in the low-threshold RF area of the neurons studied. Halothane also caused a significant reduction in neuronal response to RF brushing (dynamic stimulus) and to maintained contact with the RF (static stimulus). A dose dependency was not observed with these latter two effects. Neurons with a predominant rapidly adapting response seemed to be less susceptible to halothane suppression than slowly adapting cells. In the acute rat model an increase in halothane caused a reduction in neuronal response similar to that seen in the cat. The intravenous administration of 2 mg/kg of picrotoxin by itself caused no significant change in RF size or response to brushing. However, the same amount of picrotoxin did cause a 50% reversal of the halothane-induced reduction in RF size without causing a significant change in the halothane effect on response to RF brushing. In contrast to work recently reported in a chronic sheep model, halothane causes a significant reduction in spinal dorsal horn neuronal response to tactile stimulation of peripheral RFs. This effect is caused by, in part, but not exclusively, to GABAA-neurotransmitter systems. However, the relative influence of GABAA systems may vary with the nature of the stimulus.