Halothane induces depressor responses to noxious stimuli in the rat

Local anesthesia in piglets undergoing castration-A comparative study to investigate the analgesic effects of four local anesthetics on the basis of acute physiological responses and limb movements

Surgical castration of male piglets without analgesia is a painful procedure. This prospective, randomized and double-blinded study aimed to evaluate the analgesic effects of four different local anesthetics for piglet castration during the first week of life. In total, 54 piglets aged 3 to 7 days were distributed into 6 treatment groups: handling (H); castration without pain relief (sodium chloride, NaCl); and castration with a local anesthetic: 4% procaine (P), 2% lidocaine (L), 0.5% bupivacaine (B) or 20 mg/ml mepivacaine (M). By excluding stress and fear as disruptive factors via a minimum anesthesia model, all piglets received individual minimum alveolar concentration (MAC) isoflurane anesthesia. Twenty minutes before castration, all treatment groups except group H received one injection per testis. Then, 0.5 ml of a local anesthetic or NaCl was injected intratesticularly (i.t.), and 0.5 ml was administered subscrotally. Acute physiological responses to noxious stimuli at injection and castration were evaluated by measuring blood pressure (BP), heart rate (HR), cortisol, epinephrine, norepinephrine and chromogranin A (CgA); limb movements were quantified. The results confirm that castration without analgesia is highly painful. Surgical castration without pain relief revealed significant changes in mean arterial blood pressure (MAP) and HR. Local anesthetic administration significantly reduced changes in BP and HR associated with castration. Piglets receiving a preoperative local anesthetic exhibited the fewest limb movements during castration, while the NaCl group exhibited the most. Injection itself was not associated with significant changes in MAP or HR. However, many piglets exhibited limb movements during injection, indicating that the injection itself causes nociceptive pain. No significant differences were found between groups regarding parameters of plasma cortisol, catecholamines and CgA. In conclusion, all four local anesthetics administered are highly effective at reducing signs of nociception during castration under light isoflurane anesthesia. However, injection of a local anesthetic seems to be painful.

Heart Rate Changes in Response to Mechanical Pressure Stimulation of Skeletal Muscles Are Mediated by Cardiac Sympathetic Nerve Activity

Stimulation of mechanoreceptors in skeletal muscles such as contraction and stretch elicits reflexive autonomic nervous system changes which impact cardiovascular control. There are pressure-sensitive mechanoreceptors in skeletal muscles. Mechanical pressure stimulation of skeletal muscles can induce reflex changes in heart rate (HR) and blood pressure, although the neural mechanisms underlying this effect are unclear. We examined the contribution of cardiac autonomic nerves to HR responses induced by mechanical pressure stimulation (30 s, ~10 N/cm²) of calf muscles in isoflurane-anesthetized rats. Animals were artificially ventilated and kept warm using a heating pad and lamp, and respiration and core body temperature were maintained within physiological ranges. Mechanical stimulation was applied using a stimulation probe 6 mm in diameter with a flat surface. Cardiac sympathetic and vagus nerves were blocked to test the contribution of the autonomic nerves. For sympathetic nerve block, bilateral stellate ganglia, and cervical sympathetic nerves were surgically sectioned, and for vagus nerve block, the nerve was bilaterally severed. In addition, mass discharges of cardiac sympathetic efferent nerve were electrophysiologically recorded. Mechanical stimulation increased or decreased HR in autonomic nerve-intact rats (range: −56 to +10 bpm), and the responses were negatively correlated with pre-stimulus HR (r = −0.65, p = 0.001). Stimulation-induced HR responses were markedly attenuated by blocking the cardiac sympathetic nerve (range: −9 to +3 bpm, p < 0.0001) but not the vagus nerve (range: −75 to +30 bpm, p = 0.17). In the experiments with cardiac sympathetic efferent nerve activity recordings, mechanical stimulation increased, or decreased the frequency of sympathetic nerve activity in parallel with HR (r = 0.77, p = 0.0004). Furthermore, the changes in sympathetic nerve activity were negatively correlated with its tonic level (r = −0.62, p = 0.0066). These results suggest that cardiac sympathetic nerve activity regulates HR responses to muscle mechanical pressure stimulation and the direction of HR responses depends on the tonic level of the nerve activity, i.e., bradycardia occurs when the tonic activity is high and tachycardia occurs when the activity is low.

Types of skin afferent fibers and spinal opioid receptors that contribute to touch-induced inhibition of heart rate changes evoked by noxious cutaneous heat stimulation

Background

In anesthetized rats and conscious humans, a gentle touch using a soft disc covered with microcones (with a texture similar to that of a finger), but not with a flat disc, inhibits nociceptive somatocardiac reflexes. Such an inhibitory effect is most reliably evoked when touch is applied to the skin ipsilateral and closest to nociceptive inputs. However, the mechanism of this inhibition is not completely elucidated. We aimed to clarify the types of cutaneous afferent fibers and spinal opioid receptors that contribute to antinociceptive effects of microcone touch.

Results

The present study comprised two experiments with urethane-anesthetized rats. In the first experiment, unitary activity of skin afferent fibers was recorded from the saphenous nerve, and responses to a 10-min touch using a microcone disc and a flat disc (control) were compared. Greater discharge rate during microcone touch was observed in low-threshold mechanoreceptive Aδ and C afferent units, whereas many Aβ afferents responded similarly to the two types of touch. In the second experiment, the effect of an intrathecal injection of opioid receptor antagonists on the inhibitory effects of microcone touch on heart rate responses to noxious heat stimulation was examined. The magnitude of the heart rate response was significantly reduced by microcone touch in rats that received saline or naltrindole (δ-opioid receptor antagonist) injections. However, such an inhibition was not observed in rats that received naloxone (non-selective opioid receptor antagonist) or Phe-Cys-Tyr-Trp-Orn-Thr-Pen-Thr-NH₂ (CTOP; μ-opioid receptor antagonist) injections.

Conclusions

Microcone touch induced greater responses of low-threshold mechanoreceptive Aδ and C afferent units than control touch. The antinociceptive effect of microcone touch was abolished by intrathecal injection of μ-opioid receptor antagonist. These results suggest that excitation of low-threshold mechanoreceptive Aδ and C afferents produces the release of endogenous μ-opioid ligands in the spinal cord, resulting in the inhibition of nociceptive transmission that contributes to somatocardiac reflexes.

Pain tests provoke modality-specific cardiovascular responses in awake, unrestrained rats

Nociception modulates heart rate (HR) and mean arterial pressure (MAP), suggesting their use of HR and MAP as indicators of pain in animals. We explored this with telemetric recording in unrestrained control and neuropathic (spinal nerve ligation) rats. Plantar stimulation was performed emulating techniques commonly used to measure pain, specifically brush stroke, von Frey fiber application, noxious pin stimulation, acetone for cooling, and radiant heating, while recording MAP, HR, and specific evoked somatomotor behaviors (none; simple withdrawal; or sustained lifting, shaking, and grooming representing hyperalgesia). Pin produced elevations in both HR and MAP, and greater responses accompanied hyperalgesia behavior compared to simple withdrawal. Von Frey stimulation depressed MAP, and increased HR only when stimulation produced hyperalgesia behavior, suggesting that minimal nociception occurs without this behavior. Brush increased MAP even when no movement was evoked. Cold elevated both HR and MAP whether or not there was withdrawal, but MAP increased more when withdrawal was triggered. Heating, consistently depressed HR and MAP, independent of behavior. Other than a greater HR response to pin in animals made hyperalgesic by injury, cardiovascular events evoked by stimulation did not differ between control and neuropathic animals. We conclude that (a) thermoregulation rather than pain may dominate responses to heat and cooling stimuli; (b) brush and cooling stimuli may be perceived and produce cardiovascular activation without nocifensive withdrawal; (c) sensations that produce hyperalgesia behavior are accompanied by greater cardiovascular activation than those producing simple withdrawal; and (d) von Frey stimulation lacks cardiovascular evidence of nociception except when hyperalgesia behavior is evoked.

Changes in blood pressure induced by electrical stimulation of the femur in anesthetized rats

The effects of electrical stimulation of the femur, on blood pressure, were examined in anesthetized rats. Two small holes, 3-4 mm apart, were manually drilled into the femur down to the bone marrow. Following this, two stainless-steel electrodes were inserted into the holes, and an electrical square wave current was passed between the electrodes. In central nervous system-intact rats, electrical stimulation of the femur at 5 and 10 mA at 20 Hz for 20 s produced an intensity-dependent decrease in mean arterial blood pressure. This response was abolished by severance of the femoral and sciatic nerves ipsilateral to the stimulation. Furthermore, the renal sympathetic efferent nerve activities (as a representative index of vasoconstrictor activities) decreased following the electrical stimulation of the femur. However, in acutely-spinalized rats (spinalized at the cervical level) the same stimulation increased renal sympathetic efferent nerve activities and mean arterial blood pressure. It was concluded that high-intensity electrical stimulation of the femur reflexively affected blood pressure. It can be inferred that the osteal high-threshold receptors and/or fibers are involved in the afferent nerve pathway, and the efferent nerve pathway is the sympathetic vasoconstrictor nerve. The excitatory response properties at the propriospinal level are modified into an inhibitory response by supraspinal structures.

Intrathecal picrotoxin minimally alters electro-encephalographic responses to noxious stimulation during halothane and isoflurane anesthesia

BACKGROUND

Isoflurane and halothane act in the spinal cord to blunt ascending transmission of impulses to the brain resulting from noxious stimulation. Because intrathecal picrotoxin (an antagonist at the gamma-aminobutyric acid-A receptor) partially reverses the immobilizing effect of isoflurane and halothane, we hypothesized that the electroencephalographic response to noxious stimulation would likewise be partially reversed by intrathecal picrotoxin.

METHODS

Rats were anesthetized with isoflurane (n = 8) or halothane (n = 8) and a laminectomy performed. Following determination of minimum alveolar concentration (MAC), the electroencephalogram (EEG) was recorded during separate applications of a hindpaw clamp, tail clamp and electrical current to the tail at 0.8 and 1.2 MAC. Picrotoxin was then applied to the exposed spinal cord and the EEG response to noxious stimulation again determined.

RESULTS

The EEG was more active during halothane anesthesia than isoflurane (spectral edge frequency for 95% power: 25.6 +/- 2.1 Hz vs. 23.1 +/- 1.6 Hz, P < 0.05). Noxious stimulation usually caused the EEG to shift to higher frequencies (e.g. for 0.8 MAC halothane, median edge frequency for 50% power: from 7.6 +/- 3.1 Hz to 10.7 +/- 2.6 Hz, P < 0.05). Picrotoxin minimally affected this response.

CONCLUSIONS

Noxious stimulation evokes an EEG response that is minimally altered by intrathecal picrotoxin. This suggests that isoflurane and halothane do not have GABAergic actions in the spinal cord that indirectly suppress the EEG response.

Castration of piglets: the analgesic effects of intratesticular and intrafunicular lidocaine injection

OBJECTIVE

The aim of this study was to evaluate the analgesic effect of intratesticular and intrafunicular lidocaine for the surgical castration of piglets and to investigate the degree of nociception induced by lidocaine injection.

STUDY DESIGN

Prospective controlled experimental study.

ANIMALS

Forty-seven male Norwegian landrace piglets with normal testicular anatomy, aged 22 (+/-2.6 SD) days and weighing 7.4 +/- 1.4 kg.

MATERIALS AND METHODS

Anaesthesia was induced and maintained using halothane delivered in oxygen. End-tidal halothane was stabilized at 1.3% for 20 minutes before mean arterial blood pressure (MAP) pulse rate and electroencephalography (EEG) monitoring began. After 5 minutes of data collection, scrotal skin was desensitized with lidocaine before either an intrafunicular (IF) (n = 15) or an intratesticular (IT) (n = 16) lidocaine injection was made. Pigs in the control group (n = 16) did not receive lidocaine. Ten minutes later, a scalpel and an emasculator were used to cut the funiculus spermaticus. The MAP, pulse rate and EEG were monitored continuously for 5 minutes after castration.

RESULTS

During castration, MAP increased significantly, while pulse rate and EEG theta power fell significantly more in control, compared with the IT or IF groups. EEG alpha power fell more in the control group than in the IF group. No significant differences were found between the IF and IT groups. EEG, MAP and pulse rate responses to castration in the control group were significantly larger than the response to lidocaine injection.

CONCLUSION/CLINICAL RELEVANCE

Injecting lidocaine into the funiculus spermaticus or into the testes is effective in reducing signs of nociception caused by castration. Lidocaine injection is less noxious than castration without local anaesthetic.

Exercise pressor reflex in decerebrate and anesthetized rats

I investigated whether muscular contraction evokes cardiorespiratory increases (exercise pressor reflex) in alpha-chloralose- and chloral hydrate-anesthetized and precollicular, midcollicular, and postcollicular decerebrated rats. Mean arterial pressure (MAP), heart rate (HR), and minute ventilation (Ve) were recorded before and during 1-min sciatic nerve stimulation, which induced static contraction of the triceps surae muscles, and during 1-min stretch of the calcaneal tendon, which selectively stimulated mechanosensitive receptors in the muscles. Anesthetized rats showed various patterns of MAP response to both stimuli, i.e., biphasic, depressor, pressor, and no response. Sciatic nerve stimulation to muscle in precollicular decerebrated rats always evoked spontaneous running, so the exercise pressor reflex was not determined from these preparations. None of the postcollicular decerebrated rats showed a MAP response or spontaneous running. Midcollicular decerebrated rats consistently showed biphasic blood pressure response to both stimulations. The increases in MAP, HR, and Ve were related to the tension developed. The static contractions in midcollicular decerebrated rats (381 +/- 65 g developed tension) significantly increased MAP, HR, and Ve from 103 +/- 12 to 119 +/- 24 mmHg, from 386 +/- 30 to 406 +/- 83 beats/min, and from 122 +/- 7 to 133 +/- 25 ml/min, respectively. After paralysis, sciatic nerve stimulation had no effect on MAP, HR, or Ve. These results indicate that the midcollicular decerebrated rat can be a model for the study of the exercise pressor reflex.

[Critical analysis of animal models of acute pain. II]

OBJECTIVE

To analyse models of acute pain in experimental animals.

DATA SOURCES

References were obtained from computerised bibliographic data banks (Medline and others) and the authors' personal documents.

DATA SYNTHESIS

The majority of tests permit only a measurement of threshold, whereas clinical pain is almost always prolonged. The relationships between tests of acute pain and motor activity are reviewed from a number of standpoints; in particular we consider the influence, which postural adjustments of the animal may exert on motor responses in the limbs and the significance of flexor and extensor reflexes. In analysing the problem of the sensitivity of tests, we raise the following questions: 1) what type(s) of fibres underlie the observed responses and might these be different depending on whether one is stimulating a healthy or an inflamed tissue; 2) what significance do measurements of "latency" have when a stimulus is increasing; 3) how valid are the methods of analysing the results? The influence of species and the genetic line used in tests and the specificity and predictivity of tests are considered. Finally, we review those factors, which may distort behavioural measurements in animals, notably--pharmacokinetics, interactions between heterotopic stimuli, environmental factors and related psychophysiological/psychological considerations (subjective "undesirable" phenomena, learning phenomena). We pay particular attention to related physiological functions (thermoregulation, vasomotricity, blood pressure). These considerations lead us to re-position nociception within a much larger homeostatic framework which in addition to pain, includes phenomena such as anxiety and vegetative functions. They also suggest that we should define an "effective stimulus" as one, which activates nociceptive nerve terminals after a physical stimulus, has passed through a "peripheral lens" which regulates its intensity for reasons, which are physical, albeit of biological origin. Finally they remind us that the "system of pain" forms part of a whole set of subsystems--sensory, motor, vegetative, emotional, motivational--which scientific method, being reductionist by nature, cannot study in its entirety. However one must consider results of nociceptive tests within this general context.

CONCLUSION

It is only by taking the approach described in this review, that fundamental and clinical research can interact usefully.

Cerebral cortex regional blood flow and tissue oxygen tension during the trigeminal depressor response in rabbits

The trigeminal depressor response (TDR) is characterized with profound hypotension caused by sympathetic outflow decreases. The purpose of this study was: (1) to compare the hemodynamic changes during acute hypotension induced by the TDR with those induced by electrical stimulation of the vagal nerve (VS) as models of sympathetic inhibition and vagal activation and (2) to investigate the effects of nitrous oxide (N2O), a well-known sympathomimetic, on the hemodynamic changes during the TDR. Male Japan White rabbits were anesthetized with halothane in oxygen and mechanically ventilated. The electrode pair for the TDR was inserted into the submucosal tissue of the animal's tongue. The pair for the VS was applied to the isolated left vagal trunk. Both the TDR and the VS produced similar decreases in mean arterial pressure (MAP) and cerebral cortex regional blood flow (rCBF), although the decreases in heart rate (HR) and aortic blood flow were greater during the VS. Cerebral cortex tissue oxygen tension in both groups decreased slightly. 50% N2O, producing MAP elevation and HR decrease, ameliorated the hemodynamic changes including rCBF reduction during the TDR. A sudden diminution of sympathetic tone following noxious stimulation of the orofacial area, as in the case of the TDR, may be a possible trigger mechanism for vasodepressor reactions in dental patients. The sympathomimetic and possible antinociceptive effects of N2O may explain, at least in part, the preventive effects on vasodepressor reactions during dental procedures.

Focal heat stimulation for the determination of the minimum alveolar concentration of halothane in the rabbit

A focal heat stimulus of 54.37 +/- 0.07 (SD) degrees C was applied for 30 s to the inner aspect of the pinna of the ear for the determination of the minimum alveolar concentration (MAC) of halothane in New Zealand White rabbits. The latency before head movement was measured electromanometrically. The MAC value was 1.05 +/- 0.09 (SD)%. Other physiological responses occurred inconsistently and could not be used as reliable end points for the determination of the MAC in the rabbit.

Cardiovascular responses to noxious stimuli during isoflurane anesthesia are minimally affected by anesthetic action in the brain

We investigated the ability of isoflurane to ablate cardiovascular responses to a noxious stimulus (dew-claw clamp) in 10 goats during isoflurane delivery throughout the body and with selective delivery to either the head or torso. Whole-body minimum alveolar anesthetic concentration (MAC) and the concentration blocking autonomic responses (MAC-BAR) were determined. Next, cranial blood (jugular vein) was drained into an oxygenator and reinfused into the head (carotid artery). In six animals (Group I) isoflurane to the torso was maintained at 0.2%-0.3%, and the cranial isoflurane concentration adjusted to determined MAC-BAR; in three animals (Group II) cranial isoflurane was maintained at 0.2%-0.3%, and the end-tidal isoflurane adjusted to determine MAC-BAR. Whole-body MAC was 1.4% +/- 0.3%; whole-body MAC-BAR was 3.7% +/- 0.4%, a concentration range associated with hypotension (46 +/- 8 mm Hg). During bypass, MAC-BAR was 5.6% +/- 2.4% for Group I (P < 0.05) and 2.2% +/- 0.7% for Group II (P < 0.05). Changes in blood pressure, but not heart rate, correlated with the isoflurane concentration, both during the whole-body period and bypass period for Groups I and II. We conclude that isoflurane suppresses the arterial blood pressure response to noxious stimuli, but only with concomitant hypotension, and that the brain has little influence on this response, as its MAC-BAR substantially exceeded whole-body MAC-BAR.

Hemodynamic stress activates locus coeruleus neurons of unanesthetized rats

The effects of hypotensive stress elicited by nitroprusside infusion on discharge activity of noradrenergic locus coeruleus (LC) neurons of unanesthetized rats were characterized. Nitroprusside (75 micrograms/30 microliters/min, 15 min IV infusion) decreased mean arterial pressure of unanesthetized rats by 50 +/- 2 mmHg (n = 5). Simultaneous recordings of LC spontaneous discharge revealed an increase in discharge rate (197 +/- 87%) that was associated with hypotension. A lower concentration of nitroprusside (10 micrograms/30 microliters/min) that decreased blood pressure of halothane-anesthetized rats by 55 +/- 2 mmHg was much less effective in producing hypotension and did not increase LC discharge when administered to unanesthetized rats. Prior administration of the CRF antagonist, alpha helical CRF9-41 (50 micrograms, ICV) greatly attenuated LC activation by nitroprusside. These findings demonstrate that LC activation elicited by nitroprusside is dependent on the magnitude of hypotension. The present results also demonstrate that nitroprusside is a less potent hemodynamic challenge in unanesthetized rats. Finally, LC activation associated with nitroprusside administration to unanesthetized rats is mediated to a large extent by CRF, confirming findings in anesthetized rats.