Cold Block Research Articles

Inhibition of evoked C-fibre responses in the dorsal horn after contralateral intramuscular injection of capsaicin involves activation of descending pathways

In this study extracellular recordings of nociceptive dorsal horn neurones driven by electrical stimulation of the sciatic nerve were performed in intact urethane-anaesthetized Sprague–Dawley rats. Spikes 0–40, 40–250 and 250–800 ms after stimulus were defined as A- and C-fibre responses and post-discharge, respectively, and the effect of 200 μg capsaicin (8-methyl- N-vanillyl-6-noneamide) injected into the contralateral gastrocnemius-soleus muscle was investigated. In most cells tested, regardless of the size or location of their receptive fields, the injection of capsaicin caused a clear inhibition of the electrically evoked C-fibre responses. In animals with intact descending pathways the mean C-fibre response was inhibited to 51% of baseline 15 min after injection of capsaicin. In contrast, when capsaicin was given during cold block of the spinal cord between the brainstem and the site of recording in the dorsal horn, the same response was inhibited to 91% of baseline. A significant interaction between cold block and capsaicin was detected. We conclude that stimulation of capsaicin-sensitive afferents in the deep tissue in the hind limb can inhibit the electrically evoked C-fibre responses in the dorsal horn by activating inhibitory descending projections from higher centres. The model presented here may be an important tool for further investigations of the endogenous descending antinociceptive system.

Sacral Anterior Root Stimulation and Cryotechnique: A New Option for Selective Urethral Sphincter Block and Reversible Deafferentation in the Future?

A possible application of cryotechnique might be a selective block of nerve fiber activity during sacral anterior root stimulation to achieve selective block of urethral sphincter and reversible deafferentation. In 13 foxhounds, electrical stimulation of sacral anterior roots S2 was performed while the accompanying spinal nerves were simultaneously cooled down from +25°C in a stepwise fashion until a block of urethral sphincter activity was observed. The effects of cold block on the urethral sphincter and bladder were monitored by urodynamic investigation. In 2 additional dogs sacral posterior roots S2 were cooled down to +3°C while accompanying anterior and posterior roots were stimulated distal to the cryothermode. Compound action potentials (APs) were registered proximal to the cryothermode before, during and after cooling and recovery time of cold blocked nerves was evaluated. Complete cold block of the uret hral sphincter during spinal nerve cooling was achieved in all cases. Block temperature averaged +12°C. Detrusor pressure was a mean 5,2 cm water. Recovery time was on average 5 min. The cold block was always reversible. In both dogs of the second series the compound action potentials disappeared nearly completely at +3°C. Three min after the end of the cooling period the appearance of the compound action potentials was back to normal. In this study, cryotechnique proved to be effective for selective and reversible block of nerve fibers during sacral anterior root stimulation. In functional electrical stimulation this technique may lead to an improvement of quality of life in para- or tetraplegic patients resulting in optimization of voiding, standing, walking and grasping and does so without the necessity of surgical dorsal root rhizotomy.

Structural changes in samples cryofixed by contact with a cold metal block.

A common method of cryofixation is to bring a specimen rapidly in contact with a cold metal block. It is usually thought that during this process the surface of the specimen suffers little distortion since it freezes rapidly. Whether this is likely depends on the rate at which samples freeze compared with the speed at which the sample hits the cold block. There is some discrepancy between the published experimentally and theoretically determined freezing rates. As a contribution to this debate the distortion in cryofixed, freeze-substituted, striated muscle fibres has been investigated. In transverse sections, compression can be detected by deviations of the filament lattice from the hexagonal and used to estimate the time of freezing. Some specimens were frozen using a Gatan Cryosnapper, which freezes by catching the specimen between two nitrogen-cooled copper jaws. In addition, the speed with which the jaws close has also been determined. The results suggest that freezing of the well-preserved areas occurs in substantially less than 1 ms. This conclusion is supported by results obtained using metal-mirror apparatus in which the cushioned specimen was dropped onto a nitrogen- or helium-cooled copper block. All the specimens frozen against a cold block have a flat edge whereas muscle fibres are round. At the very edge there is evidence of structural damage as well as the more general lattice distortion.

Endothelial modulation of neural sympathetic vascular tone in canine skeletal muscle.

The effect of nitric oxide synthase (NOS) inhibition and endothelin-A (ETA)-receptor blockade on neural sympathetic control of vascular tone in the gastrocnemius muscle was examined in anesthetized dogs under conditions of constant flow. Muscle perfusion pressure (MPP) was measured before and after NOS inhibition (Nomega-nitro-L-arginine methyl ester; L-NAME) and ETA-receptor blockade [cyclo-(D-Trp-d-Asp-Pro-D-Val-Leu); BQ-123]. Zero and maximum sympathetic nerve activities were achieved by sciatic nerve cold block and stimulation, respectively. In group 1 (n = 6), MPP was measured 1) before nerve cold block, 2) during nerve cold block, and 3) during nerve stimulation. Measurements under these conditions were repeated after L-NAME and then BQ-123. The same protocol was followed in group 2 (n = 6) except that the order of L-NAME and BQ-123 was reversed. MPP and muscle vascular resistance (MVR) increased after L-NAME and then decreased to control values after BQ-123. MVR decreased after BQ-123 alone and, with the addition of L-NAME, increased to a level not different from that observed during the control period. MVR fell during nerve cold block. This response was not affected by administration of L-NAME followed by BQ-123, but it was attenuated by administration of BQ-123 before L-NAME. The constrictor response during sympathetic nerve stimulation was enhanced by L-NAME; no further effect was observed with BQ-123, nor was the response affected when BQ-123 was given first. These findings indicate that endothelin contributes to 1) basal vascular tone in skeletal muscle and 2) the increase in skeletal muscle vascular resistance after NOS inhibition. Finally, nitric oxide "buffers" the degree of constriction in skeletal muscle vasculature during maximal sympathetic stimulation.

Thoracic spinal neuron responses to repeated myocardial ischemia and epicardial bradykinin

Bradykinin has been strongly implicated as a mediator of cardiac nociception. During coronary artery occlusion, the content of bradykinin in coronary sinus blood increases. In non-cardiac tissues nociception to bradykinin exhibits tachyphylaxis, however, this phenomenon has not been rigorously studied in the heart. This raises the question that repeated coronary occlusions may also result in tachyphylaxis, thereby reducing cardiac sensation on subsequent ischemic stimulation. We therefore examined the hypothesis that repetitive episodes of myocardial ischemia and of epicardial application of bradykinin demonstrate tachyphylaxis. Mongrel cats were anesthetized with α-chloralose and heart rate, arterial pressure, and thoracic spinal neuron firing rate were recorded during 60 s of anterior descending coronary occlusion or local epicardial application of bradykinin (10 μM). Neurons were identified by cutaneous receptive fields in the left shoulder area. Sixty-one of 93 neurons tested responded with an increase in firing rate to coronary artery occlusion only ( n=24), bradykinin only ( n=19) or to both ( n=18). On repetitive coronary occlusion, 14 of 25 neurons demonstrated tachyphylaxis compared to 12 of 15 tested with bradykinin ( p<0.05). Similar responses were observed in thoracic spinal neurons that projected to the brain. In neurons demonstrating tachyphylaxis, dorsal cervical cold block partially restored the neuronal activation to coronary occlusion but not to bradykinin. We conclude, based on neuronal responses to repetitive stimuli, that afferent spinal responses to coronary occlusion and bradykinin are different. These data suggest that bradykinin is not the sole mediator of myocardial ischemic pain. The tachyphylaxis to repeated coronary artery occlusions may contribute to the clinical phenomenon of silent myocardial ischemia.

Distribution of bilateral forelimb outputs in the motor cortex of the neonatally hemidecorticated rat

It is known that intracortical microstimulation (ICMS) of the lateral pericentral cortex can evoke masticatory movements and swallowing in awake monkeys. The aim was to determine if the ability of monkeys to carry out mastication is affected by reversible bilateral cold block of the ICMS-defined cortical masticatory area/swallow cortex. A cranial chamber was implanted bilaterally in two monkeys and a warm or cold alcohol–water solution was pumped through thermodes placed bilaterally on the dura overlying the ICMS-defined cortical masticatory area/swallow cortex while monkeys chewed standardised amounts of fruit during pre-cool (thermode temperature, 37 °C), cool (0–4 °C), and post-cool (37 °C) trials. Electromyographic (EMG) activity was recorded from masseter, genioglossus, anterior digastric, geniohyoid and thyrohyoid or perilaryngeal muscles. Vertical and horizontal jaw movements were recorded with a photodiode position transducer, which monitored movements of a light-emitting diode fixed to the mandible. Each masticatory period was divided into a food-preparatory phase, a rhythmic chewing phase and a preswallow phase. Both monkeys could readily accept and ingest the foodstuffs during pre-cool and post-cool trials. In contrast, cold block was associated with masticatory deficits, reflected in both monkeys as impaired food intake or manipulation and difficulty in carrying out a sequence of masticatory cycles, alterations in of the food-preparatory phase, and alterations in masticatory-related EMG patterns of the jaw and tongue muscles. The cold block-induced changes included significant (P<0.05) prolongations of the total masticatory time, the food-preparatory phase duration, and burst durations of the jaw and tongue muscle EMG activities; furthermore, the amplitudes and temporal correlations of the EMG activities of the jaw and tongue muscles were significantly (P<0.05) changed by cold block. These findings provide further evidence that the lateral pericentral cortex has a critical role in the initiation and regulation of masticatory movements in the primate, and that the programming of masticatory muscle activities may be dependent upon corticofugal influences for engaging masticatory motor activities appropriate to the masticatory conditions.

Quantitative analysis of cuneate neurone responsiveness in the cat in association with reversible, partial deafferentation.

1. Partial deafferentation, based on peripheral nerve section or local anaesthetic blockade, has been reported to induce both immediate loss of responsiveness and/or immediate reorganization in receptive fields of neurones in the somatosensory system. In the present study, in anaesthetized cats, we have used a rapid, reversible deafferentation procedure based on cold block of the median nerve in order to evaluate quantitatively the response characteristics of cuneate neurones (n = 39) before, during and after partial deafferentation. 2. The first hypothesis tested was that cuneate neurones with input from ulnar or superficial radial nerve fields in the vicinity of the median nerve field should undergo, in association with median nerve blockade, an increased level of responsiveness to tactile stimuli within the ulnar or radial nerve zone, and an expansion of their cutaneous receptive fields. However, among eighteen cuneate neurones of this type, there was no evidence for any systematic enhancement of responsiveness nor, in at least sixteen of the eighteen neurones, any evidence for receptive field expansion. 3. The second hypothesis tested was that cuneate neurones whose input came from both the median nerve and another peripheral nerve source should undergo, in association with median nerve blockade, an increase in responsiveness to the remaining input and an expansion of the receptive field into the field of that remaining nerve source. However, in none of thirteen neurones of this type tested was there evidence of such a change. 4. The third hypothesis was that cuneate neurones whose control' receptive fields were within the median nerve zone of deafferentation should show an emergence of novel receptive fields and responsiveness from areas around the field of innervation of the median nerve. However, in none of eight neurones of this type was there evidence for such changes in adjacent skin areas. 5. In conclusion, with the use of cold block of the median nerve for partial deafferentation, the present study has confirmed previous findings of denervation-related loss of responsiveness in dorsal column nuclei neurones. The conflicting findings in studies of central nervous system plasticity indicate the need to understand better factors that do and do not lead to acute central changes.

Morphological plasticity induced in the phrenic nucleus following cervical cold block of descending respiratory drive.

Morphological plasticity occurs in the phrenic nucleus within hours following an ipsilateral C2 spinal cord hemisection. The plasticity has been associated with the unmasking of a latent respiratory pathway (the crossed phrenic pathway) which allows recovery of the hemidiaphragm paralyzed by the hemisection during a reflex known as the crossed phrenic phenomenon. This study tests if the plasticity is induced by the generalized effects of spinal cord trauma or the more specific effect of interrupting the main descending respiratory drive to phrenic motoneurons. Electron microscopic quantitative morphometric analysis of the phrenic nucleus neuropil was carried out on four Sprague-Dawley rats (200-250 g) sacrificed 4 h following unilateral reversible cold block of the descending bulbospinal respiratory drive at the second cervical segment of the spinal cord (C2). The data from four sham-operated control animals were compared with those of the experimental group. The following morphological alterations were documented in cold block animals compared to controls: (1) a significant increase in the number of multiple synapses (i.e., terminals with synaptic active zones contacting two or more postsynaptic profiles in the same plane of section), (2) a significant increase in the number of dendrodendritic appositions, and (3) a significant increase in the length of symmetric and asymmetric synaptic active zones. The above changes are similar to the changes induced in the phrenic nucleus following C2 hemisection. We conclude therefore, that injury to the spinal cord is not a requirement for this type of morphological plasticity in the phrenic nucleus, but rather the induced changes are activity-dependent and are likely caused by the interruption of the descending bulbospinal respiratory drive to the phrenic nucleus.

A Comparative Study of HLA-DRB Typing by Transcription-Mediated Amplification with the Hybridization Protection Assay (TMA/HPA) versus PCR/SSOP

To evaluate alternative human leukocyte antigen (HLA)-DNA typing methods, we used a system of transcription-mediated amplification (TMA) with a probe hybridization protection assay (HPA) in a microtiter plate format developed by Chugai Pharmaceutics Ltd. (Tokyo, Japan) to perform intermediate-level DRB typing for 502 individual samples. Two hundred fifty-two samples submitted to our Clinical Immunogenetics Laboratory were prospectively tested concurrently with a locally developed intermediate-level DRB polymerase chain reaction/sequence-specific oligonucleotide probe (PCR/SSOP) assay in a double-blind fashion. In addition, 250 retrospective samples of archived frozen cells or DNA from clinical and research panels, previously typed by allele-level DRB1 PCR/SSOP, were chosen to include 66 distinct DRB1 alleles representing Caucasian, American Black, Asian, and Native American ethnic groups. Among the prospectively typed samples, except for four samples with a TMA/HPA microplate handling problem, a single TMA/HPA allele assignment (1/462 alleles = 0.2%) was discordant with PCR/SSOP. Among the 250 retrospective samples, a single HPA probe for codon 57 aspartic acid consistently cross-reacted with the codon 57 valine sequence of DRB1∗0807. However, TMA/HPA identified six samples with previous PCR/SSOP typing errors, all of which involved identification of sequences at codons 67–71 in samples heterozygous for two DR52-associated DRB1 alleles. Assay turnaround time from sample preparation to results was 11 h for 24 samples or 6–7 h for 1–4 samples. In summary, we found the TMA/HPA DRB typing system to provide rapid, reliable, and accurate HLA-DRB typing results. The current TMA/HPA methodology could be improved by use of a molded plastic cold block to provide more consistent and secure microtiter plate cooling than the current water/ice slurry. Nevertheless, this methodology, based on a microtiter plate format but without the usual plate washing steps of the traditional ELISA, has superior potential for microplate handling and reagent distribution with a robotics system and a work surface incorporating microplate heating and cooling units.

Synchronicity of nociceptive and non-nociceptive adjacent neurons in the spinal dorsal horn of the rat: stimulus-induced plasticity

Current knowledge of spinal processing of sensory information is largely based on single-cell recordings; however, temporal correlation of multiple cell discharges may play an important role in sensory encoding, and single electrode recordings of several neurons may provide insights into the functions of a neuronal network. The technique was applied to the lumbar spinal dorsal horn of pentobarbital-anaesthetized rats during background activity, steady-state noxious heat stimulation (48°C, 100 s), cold block spinalization or radiant heat-induced inflammation of the skin, and the recordings were evaluated by means of auto-correlation, autospectral and cross-correlation analysis. Background patterns obtained by these three methods were extremely stable in time. Autocorrelation with short lag peaks was observed in 72.2% of neurons ( n=223). Background correlated discharges were found in 83.6% of the neuron pairs ( n=134). Cross-correlation with a central peak, suggestive of common input to the recorded cells, was the most common pattern observed in almost all laminae and was associated with high incidence (91.8%) of overlapping receptive fields and with neurons with initial peak autocorrelation pattern. Cross-correlations with central trough were associated with increase autocorrelation patterns. Bilateral peaks in cross-correlation, suggestive of reverberating circuitry, were observed only for pairs of neurons located in laminae IV and V and were associated with rhythmic discharges in one or in both simultaneously-recorded neurons. Lagged peaks or troughs were observed in 4.6% and 2.2% of neuronal pairs, respectively. Long-lasting skin heating induced qualitative changes (pattern changes) in the cross-correlation of 21.6% of the neuron pairs and quantitative changes in 85.7% of them. During skin inflammation qualitative changes in the cross-correlation pattern were observed in 30.8% of the neuron pairs, and quantitative changes (strength and/or synchronization time) in about 57.7% of them. Spinalization induced quantitative changes in cross-correlation in the vast majority of neuron pairs. The results of the present study suggest that discharges of neighbouring spinal dorsal horn neurons are strongly synchronized probably by propriospinal and primary afferent sources. The existence of functional reverberating circuitry was also evidenced. Finally, the functional synchronicity in the spinal dorsal horn presents stimulus-induced plasticity which consists mainly of changes on the strength and/or time of the synchronization and rarely of activation of new connectivities.

Reversible Cervical Hemispinalization of the Rat Spinal Cord by a Cooling Device

Differentiation between traumatic and activity dependent plasticity in the CNS has been a challenge to neuroscientists in the past. We describe a cooling device that allows reversible block of the inspiratory drive to phrenic motoneurons without injury to the spinal cord at the C2 level. Thus, this experimental approach can be used to differentiate between the plasticity induced by blockade of synaptic activity in the phrenic nucleus from the trauma-induced plasticity caused by a C2 spinal cord hemisection which would also interrupt descending inspiratory drive. Complete block of axon transmission of the respiratory pathways running unilaterally in the ventral as well as in the lateral funiculus was achieved by approximation of a cold probe to the ventral surface of the spinal cord. The spinal cord surface temperature was lowered to 7°C. The temperature was maintained by a cold recirculated alcohol system. The efficacy of the reversible block was assessed by bilateral continuous EMG activity recording from the hemidiaphragms ipsilateral and contralateral to the cold application. Quantitative analysis of the EMG hemidiaphragmatic signals was performed in two sham-operated control (no cold application) and an experimental (cold application) group of Sprague–Dawley rats. The control groups were employed to confirm that the surgical exposure of the cord and/or the chronic placement of the probe and the administration of IV dopamine given to maintain stable blood pressure did not affect respiration. No significant change occurred in EMG hemidiaphragmatic activity in control animals. The descending pathway from the rVRG to the phrenic nucleus was completely and continuously blocked for 4 h in all four experimental animals as demonstrated by abolition of the EMG hemidiaphragmatic signal ipsilateral to cold block. In all experimental animals hemidiaphragmatic activity returned when the cold block was removed. The recovered EMG activity was significantly higher than preblock values. Interestingly, EMG activity contralateral to the block did not change significantly from control values after the block was removed, but was significantly enhanced during cold block. The present results suggest that cold block provides a means of studying activity-dependent plasticity in the respiratory pathways of the spinal cord.

Hot mirroring

This paper proposes a storage management scheme for disk arrays, named hot mirroring. In this scheme, storage space is partitioned into two regions. One is the mirrored region, which is characterized by high performance and low storage efficiency. The other is the RAID5 region, which is characterized by low performance and high storage efficiency. Hot data blocks are stored in the former area, while cold blocks are stored in the latter. In addition, mirrored pairs and RAID5 stripes are orthogonally laid out, through which the performance degradation during rebuilding is minimized. Hot block clustering in hot mirroring achieves higher performance than conventional RAID5 arrays. The potential of hot mirroring is examined through extensive simulation.

Restoration of bladder function in spastic neuropathic bladder using sacral deafferentation and different techniques of neurostimulation.

Conventional sacral anterior root stimulation (SARS) results in simultaneous activation of both the detrusor muscle and the external urethral sphincter. We evaluated the possibilities of different neurostimulation techniques to overcome stimulation induced detrusor-sphincter-dyssynergia and to achieve a physiological voiding. The literature was reviewed on different techniques of sacral anterior root stimulation of the bladder and the significance of posterior rhizotomy in patients with supraconal spinal cord injury suffering from the loss of voluntary bladder control, detrusor hyperreflexia and sphincter spasm. The achievement of selective detrusor activation would improve current sacral neurostimulation of the bladder, including the principle of "poststimulus voiding". This is possible with the application of selective neurostimulation in techniques of anodal block, high frequency block, depolarizing prepulses and cold block. Nowadays, sacral deafferentation is a standard therapy in combination with neurostimulation of the bladder because in conclusion advantages of complete rhizotomy predominate. The combination of sacral anterior root stimulation and sacral deafferentation is a successful procedure for restoration of bladder function in patients with supraconal spinal cord injury. Anodal block technique and cryotechnique are excellent methods for selective bladder activation to avoid detrusor-sphincter-dyssynergia and thus improve stimulation induced voiding.

Low dimensional attractors in discharges of sensory neurons of the rat spinal dorsal horn are maintained by supraspinal descending systems.

Background activity was recorded from sensory neurons in laminae I-V of the lumbar spinal dorsal horn of the rat prior to and during cold block spinalization at the cervical cord. To graphically and quantitatively describe the complexity of the discharge patterns, phase space portraits were plotted and the correlation dimension D2 was calculated by using the Grassberger-Procaccia algorithm adopted for point processes, i.e. for series of interspike intervals. The algorithm is validated both for the Baker transformation, which is a simple point process, and for the Lorenz model, whereby a transformation from continuous to point process variables is achieved. A method of surrogate data is provided in order to show the difference between original neuronal patterns and their surrogate stochastic data. Therefore, this method shows that neuronal discharge patterns cannot be fully described in terms of interspike interval histograms. However, in the intact cord most (73%) of the neurons displayed background activity with low (0.28-4.3) D2 values. During spinalization, D2 values significantly increased in 68% of the neurons showing previously low D2 values, irrespective of classification and laminar location of neurons, thus proving that tonic descending systems may maintain a high order in the discharge of sensory dorsal horn neurons.

Effect of reversible dorsal cold block on the persistence of inhibition generated by spinal reflexes.

The effects of bilateral focal cooling of dorsolateral thoracic spinal cord on segmental reflex pathways to the triceps surae muscles were assessed in decerebrate cats from the reflex forces produced by single shocks or trains of electrical stimuli applied to the ipsilateral caudal cutaneous sural and the contralateral tibial nerves. The validity of the dorsal cold block technique as a substitute for acute surgical dorsal hemisection was established by showing that focal cooling reliably reproduced the stretch-induced "clasp knife" inhibition of triceps surae reflexive force seen following dorsal hemisection. Under control (warm) conditions, the inhibitory components of electrically evoked ipsilateral sural and contralateral tibial reflexes faded rapidly during sustained trains, with a resultant production of large-amplitude reflex force as measured from either the entire triceps surae or from the medial gastrocnemius muscle alone. Dorsal cold block greatly reduced the amplitude of reflexive force evoked by sustained electrical stimulation of either nerve. Indeed, the cold block completely reversed the sign of train-evoked reflexes to a net inhibition of reflex force output in one-half of the sural and one-half of the contralateral tibial stimulation experiments. Peak transient forces evoked by single shocks to the sural or contralateral tibial nerves were also sometimes reduced, but this result was more variable than for prolonged nerve stimulation. The persistence of activity in segmental inhibitory pathways during dorsal cold block, as indicated by instances of reflex sign reversal, suggests that descending bulbospinal pathways traversing the dorsolateral funiculi may be responsible for "fading" of segmental inhibitory reflex components in decerebrate cats with intact spinal cords during sustained afferent input. The possibility that the enhanced magnitude and duration of segmental inhibition during cold block will increase the likelihood of disruption of the size principle for motoneuron recruitment is also discussed.

Halothane effects on low-threshold receptive field size of rat spinal dorsal horn neurons appear to be independent of supraspinal modulatory systems

Recent evidence strongly supports the importance of spinal sites of action for the ability of general anesthetic agents to block response to noxious stimuli. This study was designed to examine possible spinal anesthetic effects on non-noxiously evoked activity. Three groups of rats were prepared for acute experiments in which the response of spinal dorsal horn neurons to low threshold receptive field (RF) stimulation was evaluated. In each animal in each group extracellular activity was recorded from a single spinal dorsal horn neuron. A low-threshold RF of each neuron and, at times, the sensitivity to low-threshold stimulation of multiple sites in the RF were determined under baseline conditions (light anesthesia or decerebrate). In Group 1, reversible cooling of the thoracic spinal cord in the presence of either 0.5% or 1% halothane anesthesia caused no change in RF size. However, an increase from 0.5% to 1% halothane caused a 53% decrease in RF size both in the presence and absence of a reversible cold block of the spinal cord. In Group 2, animals with spinal cords transected at the thoracic level had a similar change in low-threshold receptive field size (52%) when halothane concentrations were increased from 0.5% to 1%. Testing sensitivity within the RF areas indicated that the silenced areas at the fringe of the receptive field could still elicit activation of spinal dorsal horn neurons but at a higher threshold. In the final group of animals, decerebration and spinal cord transection allowed us to compare effects of 0.5% and 1% halothane with an anesthetic free baseline. Here, again, a dose-dependent reduction in RF area was observed although the baseline RFs were significantly smaller than those in Groups 1 and 2. These results demonstrate that the reduction in low-threshold receptive field size due to the administration of the inhalation anesthetic halothane occurs in the absence of descending modulation from supraspinal sites. This implicates the spinal dorsal horn as a potentially important site of action for general anesthetics. These results also support the spinal cord as an important tool to study the pharmacology responsible for anesthetic effects on sensory processing.

5-HT1B/1D agonist CGS-12066B attenuates clasp knife reflex in the cat.

1. The effect of intrathecal injection of the selective serotonin (5-HT)1B/1D receptor agonist CGS-12066B maleate (825 nmol) was assessed on stretch-evoked clasp knife inhibition of hindlimb ankle extensor muscle reflex force in precollicular decerebrate cats in which neural transmission in dorsolateral spinal pathways was blocked bilaterally by focal cooling. 2. During cold block, ramp and hold stretches of the medial gastrocnemius muscle (MG) evoked only a brief reflex excitation that was followed by powerful, long-lasting inhibition (the clasp knife reflex). Both the amplitudes of peak force evoked by the ramp and sustained force output during the last 500 ms of the hold phase of the stretch were depressed by > 50%. 3. Reflex force output during the hold portion of stretch was significantly improved on postdrug cold block trials, although peak force remained depressed. CGS-12066B did not significantly alter stretch-evoked force output in decerebrate cats when spinal cord neural transmission was unimpaired. 4. These data suggest that selective 5-HT1B/1D agonists may be of therapeutic usefulness in the treatment of reflex disorders arising from partial spinal cord injury.

Anterograde to retrograde reversal of fast axonal transport within cold blocked and rewarmed intact axons

The possibility that anterograde to retrograde reversal of axonal transport might take place in mid axon at a site distant from any nerve termination was investigated in sciatic nerve preparations from Xenopus laevis. The nerve, containing a pulse of anterogradely transported protein labeled with [ 35S]methionine, was kept in a two-compartment temperature controlled chamber. One compartment containing the proximal nerve was maintained at room temperature throughout the duration of an experiment while the second compartment containing the distal nerve, and separated from the first by a thermal barrier, was initially cooled to 3–4°C and later warmed to room temperature. Transport of labeled proteins in the nerve was detected with a position-sensitive detector of ionizing radiation. With the distal portion of the nerve cold, the pulse of labeled protein transported up to the thermal barrier and stopped. When the distal part of the nerve was warmed to room temperature, retrograde and anterograde pulses of label propagated away from the thermal barrier with no time delay. The retrograde pulse could be collected on the distal side of a proximally placed tie and could be eliminated by treatment of the proximal nerve with vinblastine or dinitrophenol. Functional and structural evidence indicated that the cold block and thermal barrier were not destructive to the axons. Electron microscopy showed that the numerical density of axonal microtubules distal to the cold block was decreased about seven fold during the cold treatment and that this decrease could be prevented by 10 μmol/1 taxol. Taxol also prevented anterograde to retrograde reversal at the thermal barrier when the distal nerve was warmed, but did not prevent continued anterograde transport. We conclude that anterograde to retrograde transport reversal can occur in mid axon and that altered microtubule structure can result in transport reversal.

Vagal cooling and the origin of pulmonary reflexes in cats.

Cooling the vagus nerve is often used to study respiratory reflexes mediated by pulmonary receptors (2,5). Its use is based on the finding that at low temperatures (e.g. 4°C) unmyelinated fibres still transmit impulses whereas transmission in myelinated fibres is fully blocked. This allows distinguishing reflex effects caused by C-fibres from those caused by slowly and rapidly adapting pulmonary receptors (SAR and RAR) (2). The aim of the present study was to investigate whether vagal cooling can also be used to distinguish between reflex effects caused by SAR and RAR. Cooling a nerve fibre increases its refractory period in the cooled area. During this period an afferent impulse in this fibre cannot pass the cold block. As a consequence, the frequency of impulses arriving at the brain stem respiratory centres is reduced. SAR have higher discharge frequencies than RAR. Therefore, the relative reduction in impulse frequency by cooling is larger for SAR than for RAR (5). Our hypothesis is that during vagal cooling the effects of stimulating SAR start to disappear at a higher temperature than the effects of stimulating RAR. In the present study two clearly distinct temperature ranges were identified. It is argued that in the first range (14–10°C) the reflex effects caused by SAR were strongly reduced and in the second (8–4°C) the reflex effects caused by RAR.KeywordsVagus NerveUnmyelinated FibreAfferent ImpulseReflex EffectHigh Discharge FrequencyThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Differential effects of spinalization on discharge patterns and discharge rates of simultaneously recorded nociceptive and non-nociceptive spinal dorsal horn neurons

Recordings were made simultaneously from 2–5 neurons at the same site in the lumbar spinal dorsal horn of pentobarbital-anesthetized rats. Neurons were classified as low-threshold (LT) or multireceptive (MR) according to their responses to non-noxious mechanical or noxious radiant heat stimuli of the skin. At the same recording sites neurons could be encountered which belong to different classes and/or which had mechanoreceptive fields which did not overlap. Cold blocks of the upper or lower thoracic cord or transsections of the upper cervical cord were made to evaluate the effects of spinalization on both the rate and pattern of background activity and/or noxious heat-evoked responses of different dorsal horn neurons under identical experimental conditions. At 24 of 27 recording sites, spinalization had qualitatively or quantitatively different effects on the rate of background activity of simultaneously recorded neurons. Interspike interval (ISI) means of background activity were significantly reduced in 29 of 65 (44.6%) neurons, prolonged in 23 of 65 (35.4%) neurons, or unchanged in 13 of 65 (20%) neurons. MR neurons displayed a significantly higher incidence of decreased background activity 17 of 45 (37.8%) and a lower incidence of increased background activity (18 of 45, 40%) during spinalization that the LT neurons from which 1 of 12 (8.3%) decreased and 8 of 12 (66.6%) increased background activity. Almost all (95.4%) neurons changed their discharge patterns after spinalization. At 9 of 27 recording sites, the discharge patterns of simultaneously recorded neurons were affected differently by spinalization as revealed by the coefficient of dispersion of the interspike intervals (ISI), indicating changes in the tendency to discharge action potential in clusters (bursts). At the same recording sites the level of noxious heat-evoked responses of simultaneously recorded MR neurons was also differentially affected by spinalization. Nociceptive responses were significantly enhanced in 19 of 37 (51.4%) neurons (137.8 ± 142.6%) of control, mean ± SD), reduced in 13 of 37 neurons (35.1%) (by 58.9 ± 20.9%) and/or unchanged in 5 of 37 (13.5%) neurons. It is concluded that no general ‘tone’ of descending antinociception exists and that tonic descending excitatory and inhibitory systems may be active simultaneously modulating both the level and pattern of neuronal discharges.