Studies in suppression of nocifensive reflexes measured with tail flick electromyograms and using intrathecal drugs in barbiturate anesthetized rats

Cauda equina repair in the rat: part 1. Stimulus-evoked EMG for identifying spinal nerves innervating intrinsic tail muscles

Cauda equina injuries may produce severe leg and pelvic floor dysfunction, for which no effective treatments exist. We are developing a rat cauda equina injury model to allow nerve root identification and surgical repair. One possible difficulty in implementing any repair strategy after trauma in humans involves the correct identification of proximal and distal ends of nerve roots separated by the injury. Two series of studies were carried out. In Series 1, we electrically stimulated segmental contributors to the dorsal and ventral caudales nerves in order to characterize the recruitment patterns of muscles controlling rat tail movements. In Series 2, we attempted to identify individual nerve roots forming the cauda equina by both level of origin and function (i.e., dorsal or ventral), based solely upon the recruitment patterns in response to electrical stimulation. For Series 1 studies, electrical stimulation of the segmental contributors showed that all nerve roots-from the sixth lumbar to the first coccygeal-contributed to recruitment of muscles found at the base of the tail. Intrinsic tail muscles lying more distally in the tail showed a more root-specific pattern of innervation. For Series 2, the rate of successful identification of an unknown nerve root as being ventral was very high (>95%), and only somewhat lower (approximately 80%) for dorsal roots. Correctly identifying the level of origin of that root was more difficult, but for ventral roots this rate still exceeded 90%. Using the rat cauda equina model, we have shown that stimulus-evoked EMG can be used to identify ventral nerve roots innervating tail muscles with a high degree of accuracy. These findings support the feasibility of using this conceptual approach for identifying and repairing damaged human cauda equina nerve roots based on stimulus-evoked recruitment of muscles in the leg and pelvic floor.

Commonly used preclinical models of pain

Pain cuts across gender, age, and disease and is the most common reason people seek health-related treatment. Certain pain states do not respond to standard therapies, leaving nurses with few options to successfully care for patients in pain. Preclinical studies use many models to investigate the mechanisms and treatments for pain states similar to those encountered in humans. Within Cervero and Laird's conceptual framework of experimental pain, the authors present several commonly used preclinical models. Phase 1 pain models measure responses to a brief, controlled stimulus that minimizes tissue damage. Phase 2 pain models use topical or injected irritants, which cause inflammation and persistent pain that change peripheral and central neuronal responses. Phase 3 pain models produce neuropathic pain through partial or complete ligation of peripheral nerves. The use of preclinical models of pain can lead to the development of better pharmacological and nonpharmacological therapies that enhance nursing practice.

Acupuncture modulation of capsaicin-induced inflammation: effect of intraperitoneal and local administration of naloxone in rats. A blinded controlled study

OBJECTIVE

It is believed that acupunctural stimulation induces an analgesic response mainly through a central mechanism: that is, through an increase in the production of opioid peptides and their release at different levels in the nervous system. We sought to establish whether the modulating effect of acupuncture on experimental neurogenic edema can be attributed to a central mechanism only or whether a peripheral mechanism could also exist. Intraperitoneal administration was compared to local administration in the same paw in rats that were injected with capsaicin and in the same dermatome of the acupunctural stimulation.

MATERIALS AND METHODS

Experimentation was conducted on 105 male Sprague-Dawley rats weighing 180-220 g, divided into 7 groups as follows: group 1, control; groups 2-4 (15 animals), stimulated with manual acupuncture; group 3 also treated with intraperitoneal naloxone 1 mg/kg; group 4 also treated locally with naloxone (20 microg); groups 5-7 (15 animals), stimulated with 5 Hz and 5 mA electroacupuncture (EAP); group 6 also treated with intraperitoneal naloxone, 1 mg/kg, group 7 also treated locally with naloxone (20 microg).

RESULTS

The results indicate that the administration of 1 mg/kg of naloxone intraperitoneally can inhibit the modulating effect of acupunctural stimulation. Equally effective in inhibiting the modulating effect of acupunctural stimulation, although not having a systemic effect, is a 20-microg dose of naloxone administered peripherally on the site of edema induction.

CONCLUSION

It is possible to conclude that both systemic and peripheral mechanisms seem to be implicated in the modulating effect of acupuncture on the neurogenic inflammation mechanism.

Motor cortex and pyramidal tract axons responsible for electrically evoked forelimb flexion: refractory periods and conduction velocities

Double-pulse methods are used here to measure the refractory periods and conduction velocities of the pyramidal tract axons which cause forelimb flexion in pentobarbital anesthetized rats. In the refractory period experiments, conditioning and test pulses were delivered to the motor cortex, the ipsilateral internal capsule, or the ipsilateral pyramid, and the maximum force exerted by the contralateral forelimb was measured at various conditioning-test intervals. The movements increased as conditioning-test interval increased from 0.5 to 1.0 ms in pyramid sites, from 0.6 to 1.5 in internal capsule sites, and from 0.6 to 2.0 ms in surface cortical sites, suggesting longer refractory periods for the substrates at more rostral sites. In cortical sites, as the conditioning-test interval increased from 4.0 to 20.0 ms, the movements decreased gradually to the single-pulse level, suggesting decreasing temporal summation at longer conditioning-test intervals. In the collision experiments, when conditioning pulses were delivered to one site and test pulses to a second site, the movements increased at conditioning-test intervals that were longer by 0.5-1.3 ms than the refractory periods in either site. This suggests that collisions occurred between orthodromic and antidromic action potentials in the pyramidal tract axons responsible for the limb movement. The collision-like increase was greater between internal capsule and pyramid than between cortex and pyramid, or between cortex and internal capsule. The estimated conduction times were 0.9-1.5 ms between cortex and pyramid, 0.4-0.8 ms between cortex and internal capsule, and 0.5-0.8 ms between internal capsule and pyramid. The range of conduction velocities, therefore, was quite narrow between all pairs (8.8-16.8 m/s). The largest pyramidal tract axons appear to be responsible for most of the force of forelimb flexion in pentobarbital anesthetized rats.

Evaluation of opioid-induced antinociceptive effects in anaesthetized and conscious animals

1. The activity profiles of opioid agonists and non-steroidal analgesic agents have been compared against different nociceptive stimuli in the mouse and rat. 2. Opioid agonists, but not non-steroidal analgesic agents, inhibited reflex depressor responses evoked by visceral distension in anaesthetized rats. The ranked order of potency of opioids in the visceral distension reflex was identical to that observed in the mouse writhing assay. 3. Opioid-induced inhibition of reflex depressor responses and writhing was observed with ligands acting on mu- and kappa-, but not delta-receptors. Antinociceptive activity of opioids in the rat cold water tail-flick assay was restricted to mu-receptor agonists. 4. Morphine- and ethylketocyclazocine (EKC)-induced inhibition of the visceral distension reflex was blocked by naloxone, but not by the quaternary opioid antagonist N-methylnalorphine. 5. Direct cardiovascular effects were observed with ligands for the mu- and kappa-receptor. Blood pressure changes induced by morphine and Tyr.D-Ala.Gly.MePhe.Gly-ol (DAGOL), but not EKC, were blocked by N-methylnalorphine. Pretreatment with 16-methylcyprenorphine (M8008) antagonized morphine-, DAGOL- and EKC-induced cardiovascular effects, but not those of dynorphin-(1-13) or U50488. 6. It is concluded that reflex circulatory responses evoked by visceral distension in anaesthetized rats are a valid index for the evaluation of opioid-induced antinociception. A simultaneous assessment of cardiovascular effects of opioids was achieved.

Electroacupuncture suppression of a nociceptive reflex is potentiated by two repeated electroacupuncture treatments: the first opioid effect potentiates a second non-opioid effect

Electroacupuncture (EA) produces suppression of the nociceptive tail flick reflex in rats. Two 10 min treatments given 90 min apart, cause a potentiation of the EA effect produced by the second treatment. This potentiation can be prevented by giving naltrexone before the first and second treatments (either intravenously or intrathecally administered). However, this potentiation cannot be reversed by naltrexone give only once, before the second EA treatment. These results suggest that the first EA effect which is opioid in nature, sets up a cascade effect, potentiating a second non-opioid EA effect.

Electroacupuncture suppresses a nociceptive reflex: naltrexone prevents but does not reverse this effect

Rats were anesthetized with a continuous infusion of pentobarbital. First we showed that electroacupuncture (EA) suppressed the tail flick nociceptive reflex. Naltrexone (a long acting opiate antagonist) was given either intravenously or intrathecally to attempt to block this EA effect. The naltrexone prevented this EA effect when injected before the onset of EA treatment, but failed to reverse the EA effects when injected immediately after termination of EA treatment. We discuss the possibility that the endorphins set up a cascade effect whereby the subsequent EA suppression of tail flick is no longer dependent on endorphin and hence cannot be affected by opiate antagonists. This finding may clarify some of the confusion in the acupuncture endorphin literature.