Differential effects of halothane and isoflurane on lumbar dorsal horn neuronal windup and excitability

BACKGROUND

Windup of spinal nociceptive neurones may underlie temporal summation of pain, influencing the minimum alveolar concentration (MAC) of anaesthetics required to prevent movement to supramaximal stimuli. We hypothesized that halothane and isoflurane would differentially affect windup of dorsal horn neurones.

METHODS

We recorded 18 nociceptive dorsal horn neurones exhibiting windup to 1 Hz electrical hindpaw stimuli in rats. Effects of 0.8 and 1.2 MAC isoflurane and halothane were recorded in the same neurones (counterbalanced, crossover design). Windup was calculated as the total number of C-fibre (100-400 ms latency) plus afterdischarge (400-1000 ms latency) spikes/20 stimuli (area under curve, AUC) or absolute windup (C-fibre plus afterdischarge-20 x initial response).

RESULTS

Increasing isoflurane from 0.8 to 1.2 MAC did not affect AUC, but increased absolute windup from 429 (62) to 618 (84) impulses/20 stimuli (P<0.05) and depressed the initial C-fibre response from 14 (3) to 8 (2) impulses (P<0.05). Increasing halothane from 0.8 to 1.2 MAC depressed AUC from 690 (79) to 537 (65) impulses/20 stimuli (P<0.05) and the initial response from 18 (2) to 13 (2) impulses (P<0.05), but absolute windup was not affected. Absolute windup was 117% greater during 1.2 MAC isoflurane compared with 1.2 MAC halothane.

CONCLUSIONS

Windup was significantly greater under isoflurane than halothane anaesthesia at 1.2 MAC, whereas the initial C-fibre response was suppressed more by isoflurane. These findings suggest that these two anaesthetics have mechanistically distinct effects on neuronal windup and excitability.

Carbon dioxide depresses the F wave by a central, not peripheral, mechanism during isoflurane anesthesia

Carbon dioxide (CO(2)) has anesthetic properties and has been reported to depress the F wave of the evoked electromyogram; the F wave is thought to reflect motoneuron excitability. Anesthetics such as isoflurane also depress the F wave. Because CO(2) can depress muscle contractile function, as well as spinal cord neurons, it is unclear whether CO(2) depresses the F wave via a central or peripheral mechanism. We anesthetized rabbits with isoflurane (1.4%) and prepared for hindlimb bypass (with a membrane oxygenator) whereby the partial pressures of CO(2) in the hindlimb muscle and torso could be independently adjusted. The F wave was recorded from the hindlimb plantar muscles when the CO(2) was normal to the hindlimb and torso, and when it was increased (to approximately 90 mm Hg) in the hindlimb, the torso, or both. Increasing the CO(2) to just the hindlimb had no significant effect on the F-wave amplitude, but increasing the CO(2) to the torso depressed the F wave to 52% +/- 32% of control; adding CO(2) to the hindlimb during torso hypercarbia did not result in any additional depression of the F wave. CO(2) depressed the F wave via a central, not peripheral, mechanism, although the precise mechanism is unknown.

Isoflurane depresses windup of C fiber-evoked limb withdrawal with variable effects on nociceptive lumbar spinal neurons in rats

Windup is a progressive increase in responses of nociceptive spinal cord neurons to repeated electrical C fiber stimulation. We hypothesized that isoflurane would depress windup at approximately the minimum alveolar anesthetic concentration (MAC) required to suppress purposeful movement in response to noxious stimulation. We recorded windup responses in single lumbar spinal neurons (n = 17) to a series of 15 repetitive electrical stimuli delivered at 1 Hz to the hindpaw at C fiber strength; hindpaw withdrawal force was simultaneously recorded. The total number of action potentials per 15 stimuli (mean +/- sem as a percentage of each neuron's maximal response) was 83% +/- 5%, 84% +/- 5%, 67% +/- 7%, and 57% +/- 8% at 0.7, 0.9, 1.1, and 1.4 MAC, respectively. The 0.9 and 1.1 MAC values differed significantly from each other, whereas the 0.7 and 0.9 MAC values differed from the 1.4 MAC value (P < 0.05). The reduced firing was attributed to a depression of the initial C fiber-evoked responses in most units, and a reduction in windup slope over the initial 5 stimuli in 6 units. Muscle force was 67%, 11%, and 4% of the 0.7 MAC value at 0.9, 1.1, and 1.4 MAC, respectively. Isoflurane depressed excitability and variably affected windup of lumbar spinal cord neurons, while uniformly depressing windup of limb withdrawals in a concentration-dependent manner.

Role of TNF-alpha in sensitization of nociceptive dorsal horn neurons induced by application of nucleus pulposus to L5 dorsal root ganglion in rats

Herniation of the nucleus pulposus (NP) from lumbar intervertebral discs commonly results in radiculopathic pain and paresthesia (sciatica). While traditionally considered the result of mechanical compression of the dorsal root ganglion (DRG) and/or spinal nerve root, recent studies implicate pro-inflammatory mediators released from or evoked by NP, a possibility that was presently investigated. Single-unit recordings were made from L5 wide dynamic range dorsal horn neurons in pentobarbital-anesthetized rats. Autologous NP was harvested from a coccygeal disc and placed onto the exposed L5 DRG. A control group had subcutaneous adipose tissue or saline placed similarly. To test involvement of tumor necrosis factor-alpha (TNF-alpha), a third group received autologous NP plus local soluble TNF-alpha receptor type 1 (0.013 microg) which binds TNF-alpha to prevent its action. In each group, neuronal responses to graded heat (38-50 degrees C) and mechanical (von Frey filaments 4-76 g) stimuli were recorded prior to and at three successive hourly intervals following each treatment. Responses to noxious heat and mechanical stimuli were significantly enhanced 1 h post-NP and remained elevated thereafter. Thermally and mechanically evoked responses were not significantly affected in control rats or those treated with NP + soluble TNF-alpha receptor type 1. These results indicate that sensitization of nociceptive spinal neuronal responses develops quickly following exposure of the DRG to NP, and that TNF-alpha is involved. This electrophysiological model of herniated NP may prove useful in further characterizing the role of inflammatory mediators in hyperalgesia and allodynia resulting from lumbar disc herniation.

Mustard oil has differential effects on the response of trigeminal caudalis neurons to heat and acidity

Topical application of mustard oil (allyl isothiocyanate) to the skin or injection into joints induces hyperalgesia, allodynia, and neuroinflammation. However, when applied to the oral or nasal mucosa, mustard oil evokes a desensitizing pattern of irritation. Presently we investigated the responses of neurons in superficial laminae of trigeminal subnucleus caudalis (Vc) to noxious thermal (53 degrees C) and chemical (pentanoic acid; 200 mM) stimuli prior to and following lingual mustard oil application. A low concentration of mustard oil (0.125%) applied by constant flow (0.5 ml/min; 15 min), initially excited Vc neurons followed by partial desensitization. Responses to noxious heat were unchanged following mustard oil. A high concentration of mustard oil (1.25%) initially excited Vc neurons followed quickly (within 20 s) by nearly complete desensitization. The desensitization was transient since reapplication of mustard oil approximately 20 min later elicited a comparable response that also rapidly desensitized. Mustard oil also transiently cross-desensitized Vc responses to pentanoic acid (to 52%), in striking contrast to noxious heat-evoked responses which were significantly sensitized to approximately 160% of pre-mustard oil levels. The data suggest that the effect of mustard oil on subsequent lingual nociceptive responses is concentration dependent, transient, and modality specific.

Spinal c-fos expression associated with spontaneous biting in a mouse model of dry skin pruritus

As a model of dry skin pruritus in mice, one hind paw was treated twice daily with a mixture of acetone/diethylether/water (AEW); controls received water only. A protective collar prevented the animals from accessing the treatment area. At 16 days, the collar was removed and AEW-treated mice exhibited marked biting of the treated paw; the number and cumulative duration of bites was significantly greater than in controls. After 3 additional treatment days (collars intact), animals were perfused for c-fos immunohistochemistry. There was significantly more fos-like immunoreactivity in the ipsilateral lumbar spinal cord of AEW-treated animals, with the majority in superficial laminae. It is proposed that biting of the dry skin reflects pruritus, and that neurons predominantly in superficial laminae of the dorsal horn may signal itch sensation.

An in vivo method for recording single unit activity in lumbar spinal cord in mice anesthetized with a volatile anesthetic

We describe a method to record single unit neuronal activity from mouse spinal cord using volatile anesthesia. The small size of the mouse can complicate usual methods that are used for single-unit recording in rats, but simple modifications can significantly increase the number of successful recordings. Stabilization of the vertebral column is particularly important, as are adequate ventilation of the animal, control of body temperature and accurate determination of anesthetic concentrations in respiratory gas samples.

Antinociception induced by chronic exposure of rats to cigarette smoke

To investigate if chronic exposure to cigarette smoke induces analgesia, rats were exposed to concentrated cigarette smoke in an environmental chamber over four successive 5-day blocks (6 h/day), with 2 smoke-free days between blocks. A control group was exposed to room air. Tail flick latencies increased significantly (analgesia) during each smoke exposure block, with a relative decline in analgesia across blocks (tolerance) and a return to control levels during the first three smoke-free interludes while remaining higher after the conclusion of the 4-week exposure period. Mechanical (von Frey) withdrawal thresholds declined over time in smoke-exposed and control groups, with the smoke-exposed group showing significantly lower thresholds. Plasma nicotine reached 95.4 +/- 32 (S.D.) ng/ml at the end of weekly smoke exposure and declined to 44.9 +/- 10.6 ng/ml 24 h after withdrawal. Rats lost weight during smoke exposure and quickly regained weight during smoke-free interludes and at the cessation of smoke exposure. Analgesia may contribute to the initiation of smoking, and rapid reversal of the analgesic effect following acute exposure may contribute to the difficulty in quitting smoking.

Halothane depresses C-fiber-evoked windup of deep dorsal horn neurons in mice

A progressive increase in the response of a nociceptive spinal neuron to repeated electrical C-fiber stimulation reflects a phenomenon called windup. Second order neurons in the dorsal horn, as well as motoneurons, can develop windup. Inhaled anesthetics act primarily in spinal cord to suppress movement induced by noxious stimulation. We hypothesized that halothane would depress neuronal windup in mice at concentrations that also prevented movement. We measured windup in deep dorsal horn neurons in lumbar spinal cord at 0.75 MAC (the minimum alveolar concentration of anesthetic that prevents movement in 50% of subjects in response to noxious stimulation), 0.9 MAC, and 1.1 MAC. The change from 0.75 to 0.9 MAC did not significantly decrease windup (-11+/-22%), but the change from 0.9 to 1.1 MAC decreased windup (-35+/-7%, P<0.01). We conclude that halothane depresses neuronal windup in the range that prevents movement, and that the effect on windup might play a role in halothane's immobilizing action.

Isoflurane differentially modulates medullary on and off neurons while suppressing hind-limb motor withdrawals

BACKGROUND

Isoflurane acts primarily in the spinal cord to block movement; however, it is unclear how supraspinal sites might contribute to anesthetic effects on quantified parameters of movement such as force.

METHODS

The authors investigated the effects of isoflurane on spontaneous and noxious heat-evoked activity of nociceptive reflex-modulating ON and OFF cells in the rostral ventromedial medulla of rats. Single ON or OFF neurons were recorded simultaneously with hind-limb withdrawal force elicited by graded noxious thermal hind paw stimulation. Isoflurane concentrations were administered in reference to each animal's minimum alveolar concentration (MAC) of isoflurane.

RESULTS

From 0.65 to 1.15 MAC, isoflurane dose-dependently reduced spontaneous activity of ON cells by 70% (P < 0.001). OFF-cell spontaneous activity was dose-dependently increased 138% (P < 0.001). ON-cell heat-evoked activity was depressed 95% by isoflurane from 0.65 to 1.15 MAC (P < 0.001). Isoflurane-induced changes in ON- and OFF-cell activity paralleled similar reductions in withdrawal force, with the largest change in both neuronal activity and withdrawal force occurring between 0.85 and 1.15 MAC. For the lowest stimulus temperature, excitatory responses of ON cells and inhibitory responses of OFF cells were significantly greater for trials in which withdrawals occurred than for trials in which no withdrawal occurred, suggesting that responses in both classes of neurons were related to movement rather than the stimulus alone.

CONCLUSIONS

The results show that isoflurane modulation of ON- and OFF-cell activity corresponds to anesthetic-induced reductions in hind-limb withdrawal force, and therefore, the effects of isoflurane on these classes of neurons in rostral ventromedial medulla might contribute to motor depression.

Spectral Analysis of Movement Patterns During Anesthesia

It remains unclear how anesthetics produce immobility, an end-point used in determining anesthetic potency. Understanding how movement (in response to noxious stimulation) is ablated by anesthetics could be aided by using spectral analysis of the high and low frequency components of complex movement patterns. We therefore applied a spectral analysis to previously published movement data from rats anesthetized with isoflurane and halothane at 0.6, 0.9, and 1.1 minimum alveolar concentration (MAC). We recorded isometric forces of hindlimb movement elicited by noxious mechanical stimulation of the hindpaw. The movement patterns were subjected to spectral analysis to determine force amplitude for each frequency component. When halothane was increased from 0.6 to 0.9 MAC, force amplitude decreased only for the lowest-frequency (<1 Hz) components, in part related to the generally lower high-frequency forces at 0.6 MAC. Between 0.6 and 0.9 MAC isoflurane amplitude was reduced for most frequencies in the 0-10 Hz range. For both halothane and isoflurane at 1.1 MAC, as expected, force amplitude substantially decreased at all frequencies. We conclude that spectral analysis is useful to describe and quantify the effects of anesthetics on complex movement patterns resulting from noxious stimuli applied during anesthesia.

IMPLICATIONS

Complex movement can occur when a noxious stimulus is applied to an anesthetized animal. The frequency components of these movement patterns can be described and quantified by spectral analysis, thus providing a useful tool to investigate the immobilizing properties of anesthetics.

Mecamylamine reduces nicotine cross-desensitization of trigeminal caudalis neuronal responses to oral chemical irritation

We investigated the role of neuronal nicotinic acetylcholine receptors (nAChRs) in nicotine cross-desensitization of chemonociceptive responses of trigeminal subnucleus caudalis (Vc) neurons in rats. Vc responses to lingually applied pentanoic acid were significantly reduced following nicotine, and this was prevented when the nAChR antagonist mecamylamine was applied before or after nicotine. A peripheral site of nicotine cross-desensitization is suggested via a nAChR-mediated reduction in acidic excitation of lingual nociceptors that project to Vc.

Opioid modulation of scratching and spinal c-fos expression evoked by intradermal serotonin

We investigated a spinal site for opioid modulation of itch-related scratching behavior in rats. Intradermal 5-HT (2%, 10 microl) elicited intermittent bouts of hindlimb scratching directed toward the injection site (nape of neck) beginning within minutes and lasting >1 hr. 5-HT-evoked scratching was significantly reduced by systemic administration of the opiate antagonist naltrexone but was not affected by systemic morphine at a dosage (3 mg/kg) that induces analgesia. Intradermal 5-HT elicited a significant increase in c-fos-like immunoreactivity (FLI) in superficial laminas I-III at the lateral aspect of the cervical C3-C6 dorsal horn compared with controls receiving intradermal saline. Neither systemic morphine nor naltrexone significantly affected counts of 5-HT-evoked FLI. The lack of effect of morphine suggests that intradermal 5-HT activates dorsal horn neurons, signaling itch but not pain. Attenuation of 5-HT-evoked scratching but not spinal FLI by naltrexone suggests a supraspinal site for its antipruritic action. In contrast, morphine significantly attenuated FLI elicited by intradermal capsaicin, a chemical that induces pain but not scratching.

c-fos expression in superficial dorsal horn of cervical spinal cord associated with spontaneous scratching in rats with dry skin

Using a dry skin model in rats, we assessed spontaneous itch-related scratching behavior and associated c-fos expression in the superficial dorsal horn of the spinal cord. The number of spontaneous bouts of hind limb scratching directed toward the nape of the neck was significantly higher after 5 days of topical application of acetone-diethylether-water (AEW) compared to pre-treatment levels or to control animals treated with water only. In AEW-treated animals, neurons expressing Fos-like immunoreactivity (FLI) were observed in superficial laminae of the dorsal horn at C3-C5 levels. There was a significant, positive correlation between the number of neurons in lamina I expressing FLI and the number and cumulative duration of spontaneous scratching bouts. These results suggest that this model may be useful to assess mechanisms of dry skin pruritus, and that lamina I neurons are activated as a consequence of itching and/or scratching in this model.

Spinal anaesthesia indirectly depresses cortical activity associated with electrical stimulation of the reticular formation

BACKGROUND

Neuraxial blockade reduces the requirements for sedation and general anaesthesia. We investigated whether lidocaine spinal anaesthesia affected cortical activity as determined by EEG desynchronization that occurs following electrical stimulation of the midbrain reticular formation (MRF).

METHODS

Six goats were anaesthetized with isoflurane, and cervical laminectomy performed to permit spinal application of lidocaine. The EEG was recorded before, during and after focal electrical stimulation (0.1, 0.2, 0.3 and 0.4 mA) in the MRF while keeping the isoflurane concentration constant.

RESULTS

During lidocaine spinal anaesthesia, the spectral edge frequency (SEF) after MRF electrical stimulation (13.6 (SD 1.0) Hz, averaged across all stimulus currents) was less than the SEF during control and recovery periods (18.6 (3.6) Hz and 17.2 (2.2) Hz, respectively; P<0.05). Bispectral index values were similarly affected: 69 (10) at control compared with 55 (6) during the spinal block (P<0.05).

CONCLUSIONS

These results suggest that lidocaine spinal anaesthesia blocks ascending somatosensory transmission to mildly depress the excitability of reticulo-thalamo-cortical arousal mechanisms.

5-Hydroxytryptamine (5-HT)2 receptor involvement in acute 5-HT-evoked scratching but not in allergic pruritus induced by dinitrofluorobenzene in rats

We investigated the role of serotonin (5-hydroxytryptamine; 5-HT)2 and 5-HT3 receptor subtypes in acute itch-associated scratching behavior as well as in an allergic pruritus model in rats. Intradermal 5-HT evoked hind limb scratching directed toward the injection site in naïve rats. Scratching behavior was significantly reduced by pretreatment with the 5-HT2 receptor antagonist ketanserin. Intradermal injection of alpha-methylserotonin, a 5-HT2 receptor agonist, also elicited scratching behavior in a dose-dependent manner, indicating that acute 5-HT-induced scratching is mediated via peripheral 5-HT2 receptors. To produce a model of allergic pruritus, skin was sensitized by topical application of 5% dinitrofluorobenzene (DNFB). One month later, repeated challenge of the skin with 0.2% DNFB at weekly intervals elicited scratching as part of the immediate allergic response. Scratching was not affected by ketanserin or by the 5-HT3 receptor antagonist ondansetron, indicating that neither 5-HT2 nor 5-HT3 receptors is involved in itch-associated scratching behavior caused by allergic skin dermatitis in rats.

Oral irritation by mustard oil: self-desensitization and cross-desensitization with capsaicin

We investigated the temporal pattern of oral irritation elicited by sequential application of mustard oil (allyl-isothiocyanate), and whether it exhibits self-desensitization and cross-desensitization with capsaicin. Mustard oil (0.125%, 40 micro l) was sequentially applied to one side of the tongue at 1 min intervals, and subjects rated the intensity of the irritant sensation elicited by each stimulus. Ratings successively declined across trials, indicating desensitization. In contrast, sequential application of capsaicin (10 ppm) elicited irritation that increased in intensity across trials (sensitization). To test for self-desensitization by mustard oil, a 10 min hiatus was imposed following the series of unilateral mustard oil stimuli, after which mustard oil was applied to both sides of the tongue. In a two-alternative forced-choice paradigm, subjects chose which side had stronger irritation and also independently rated the irritant intensity on each side. A significant majority of subjects chose the side not previously receiving mustard oil as more intense, and assigned significantly higher intensity ratings to that side, indicating self-desensitization. In two additional sessions, the same paradigm was used to show mustard oil cross-desensitization of irritation elicited by capsaicin, and capsaicin cross-desensitization of irritation from mustard oil. In a final session, sequential application of mustard oil at faster (20 s) intervals initially evoked a sensitizing pattern followed by desensitization. The temporal patterns of oral irritation exhibited by mustard oil, and its reciprocal cross-desensitization with capsaicin, are similar to those of menthol and nicotine.

Isoflurane depresses diffuse noxious inhibitory controls in rats between 0.8 and 1.2 minimum alveolar anesthetic concentration

Diffuse noxious inhibitory control (DNIC) occurs when the response to a noxious stimulus is inhibited by a second, spatially remote noxious stimulus. The minimum alveolar anesthetic concentration (MAC) to suppress movement is not altered by a second remote noxious stimulus. We hypothesized that DNIC would be depressed in the peri-MAC range. Rats were anesthetized with isoflurane, and MAC was measured. We recorded dorsal horn neuronal responses to noxious thermal stimulation of the hindpaw, with or without concomitant supramaximal noxious mechanical stimulation of the tail or contralateral hindpaw. At 0.8 MAC, the tail clamp decreased neuronal responses 70% compared with control heat-evoked responses (from 1032 +/- 178 impulses per minute to 301 +/- 135 impulses per minute; P < 0.05). The tail clamp had no significant effect on neuronal responses at 1.2 MAC (from 879 +/- 139 impulses per minute to 825 +/- 191 impulses per minute; P > 0.05). Similarly, 1.2 MAC isoflurane significantly depressed DNIC elicited by hindpaw clamping. In another group, the cervical spinal cord was reversibly blocked by cooling to determine whether the inhibition was mediated supraspinally. With spinal cord cooling, the counterstimulus-evoked inhibition was not observed at 0.8 MAC. These results suggest that DNIC involves supraspinal structures and is present at sub-MAC isoflurane concentrations but is depressed at more than 1 MAC.

IMPLICATIONS

Diffuse noxious inhibitory control (DNIC) occurs when a noxious stimulus is perceived as being less painful when a second noxious stimulus is applied elsewhere on the body. DNIC is present in anesthetized animals, although how anesthesia affects it is unknown. We found that isoflurane depressed DNIC in the transition from 0.8 to 1.2 minimum alveolar anesthetic concentration, suggesting that DNIC is depressed in the anesthetic range needed to suppress movement.

Quantitative assessment of directed hind limb scratching behavior as a rodent itch model

Hind limb scratching is used increasingly as an itch model in rodents. Scratching is usually quantified as the number of scratching bouts over a 60 min period. Since the antipruritic effect of scratching should depend on the total time of skin contact, then the duration of scratching bouts and within-bout scratching frequency may also be important factors. Therefore, we measured these parameters during episodes of scratching directed toward the site (nape of neck) of intradermal injection of serotonin in Sprague-Dawley rats. Serotonin elicited significantly more scratching bouts than saline. There was a biphasic pattern of scratching over time, with peaks at 10-20 and 40-50 min. Although cumulative bout duration (2-min intervals) had a similar biphasic distribution, the mean individual bout duration (2.1 s) did not change significantly over time. Within-bout scratching frequency remained constant over time at 8 Hz. The number of scratching bouts was suppressed in a dose dependent manner by naltrexone (3 and 5 mg/kg), while the individual bout duration and the within-bout frequency were not significantly different compared with serotonin-evoked scratching without naltrexone. These results validate the total number of scratching bouts as an indicator of the magnitude of itch-related scratching.

Trigeminal modulation of gustatory neurons in the nucleus of the solitary tract

Electrophysiological methods were used to investigate the effects of trigeminal nerve stimulation or transection on responses of single gustatory neurons in the nucleus of the solitary tract (NTS) to tastants (NaCl, sucrose, citric acid, monosodium glutamate) in pentobarbital-anesthetized rats. Unilateral transection of the lingual nerve, or the mandibular branch of the trigeminal nerve, resulted in significant reductions (by 21 and 29%, respectively; P<0.01) in tastant-evoked responses, with no further effect following bilateral transection. Electrical stimulation of the central cut end of the mandibular nerve directly excited nine of 14 gustatory NTS units. For these units, central mandibular stimulation facilitated the tastant-evoked responses in six, depressed responses in three, and had no effect in five. Facilitation of tastant-evoked responses peaked 4 min after mandibular stimulation and recovered within 8 min. Electrical stimulation of the peripheral cut end of the mandibular nerve significantly reduced tastant-evoked responses in nine other NTS units, with a maximal reduction at 4 min post-stimulation followed by recovery. Stimulation of the superior cervical sympathetic ganglion did not affect NTS tastant-evoked responses. These results suggest the presence of complex central modulation of NTS neurons by trigeminal afferents, as well as a peripheral depressant effect on gustatory processing possibly mediated via neuropeptide release from trigeminal nerve endings in the tongue.

Peri-MAC depression of a nociceptive withdrawal reflex is accompanied by reduced dorsal horn activity with halothane but not isoflurane

BACKGROUND

Anesthetics act in the spinal cord to suppress movement evoked by a noxious stimulus, although the exact site is unknown.

METHODS

This study investigated sensorimotor processing in hind limb withdrawal reflexes, and effects of two general anesthetics, halothane and isoflurane, on simultaneously recorded responses of single dorsal horn neurons and hind limb withdrawal force, elicited by graded noxious thermal hind paw stimulation in rats. Minimum alveolar anesthetic concentration (MAC) needed to block gross movement to a supra-maximal mechanical stimulus was determined for each animal.

RESULTS

Between 0.9 and 1.1 MAC, halothane and isoflurane greatly reduced or abolished withdrawal force (79 and 89% reduction, respectively). Halothane (0.75-1.4 MAC) depressed heat-evoked neuronal responses in a concentration-related manner (41% reduction between 0.9 and 1.1 MAC averaged across all stimulus temperatures, P < 0.05) and decreased stimulus-response function slopes, with corresponding reductions in withdrawal force. In contrast, isoflurane did not reduce neuronal responses in the 0.75-1.4 MAC range and slightly facilitated responses (by 16%) when concentration increased from 0.9 to 1.1 MAC, despite a concurrent withdrawal force reduction. Anesthetic depression of heat-evoked withdrawal force correlated well with MAC determination using a supra-maximal mechanical stimulus. At sub-MAC anesthetic concentrations, some units exhibited firing rate changes that preceded and paralleled moment-to-moment changes in force during a given withdrawal.

CONCLUSIONS

Halothane reduces noxious-evoked movement at least partly via depression of dorsal horn neurons, whereas isoflurane suppresses movement by an action at more ventral sites in the spinal cord.