Antianalgesic action of thiamylal sodium in cats

Thiopentone induced enhancement of somatic motor responses to noxious stimulation: influence of GABAA receptor modulation

PURPOSE

This study was conducted to determine whether hyperalgesic effects of subanaesthetic concentrations of thiopentone could be attributed to GABAA receptor effects.

METHODS

All studies were performed on 50 rats in a prospective, randomized, blinded fashion using saline-injected animals as controls. Using a modified Randall-Selitto technique, the motor behavior stimulated by noxious stimulation was quantified by determining the lowest tail pressure required to provoke a withdrawal response (somatic motor response threshold, SMRT). In the first protocol (21 rats), we studied the effects of 0.5, 1.5 and 5 mg.kg-1 i.v. of the GABAA agonist, muscimol, on SMRT. In the second protocol (20 rats), the effects of administration of saline, muscimol 0.5 mg.kg-1, or the competitive GABAA antagonist, bicuculline 0.25 mg.kg-1, upon the SMRT-reducing effects of a standardized thiopentone infusion were observed.

RESULTS

No dose of muscimol produced hyperalgesia. The highest dose of muscimol used (5 mg.kg-1) produced pronounced analgesic effects, raising the SMRT above 750 g. No change in SMRT was detected with the smaller doses of muscimol. Given in combination with muscimol (0.5 mg.kg-1), thiopentone produced analgesia, as shown by an increase in SMRT (P = 0.009). In the bicuculline treated animals, SMRT decreased linearly with increasing plasma thiopentone concentrations (P < 0.001). The slope of the relationship in the bicuculine group was not significantly different from that observed in the saline-treated group, indicating that bicuculline did not block the hyperalgesic effects of thiopentone.

CONCLUSION

The results of these studies suggest that hyperalgesia associated with thiopentone is not mediated primarily by GABAA receptors.

The effect of pentobarbital sodium on the dorsal horn of the spinal cord

The effect of intravenously administered pentobarbital sodium on the activity of single unit in Rexed lamina V of the transected feline lumbar spinal cord was studied using an extracellular microelectrode recording technique. Pentobarbital sodium 1.0 mg·kg(-1), 2.5 mg·kg(-1), and 5.0 mg·kg(-1) administered intravenously suppressed both the spontaneous and the evoked activity in Rexed lamina V cells, known to respond principally to noxious stimuli, in a dose-dependent manner. The maximum depression of cell activity occurred within 5 min after intravenous administration. The recovery of cell activity occurred within 70 min after intravenous administration of pentobarbital sodium. We conclude that pentobarbital sodium intravenously administered has a suppressive effect on single unit activity of cells in Rexed lamina V and probably has an analgesic effect. Its suppressive effect is dose-dependent.

Pentobarbital prevents the development of C-fiber-induced hyperalgesia in the rat

Noxious stimuli applied to the skin can produce long-lasting, C-fiber-dependent, secondary hyperalgesia that is mediated by central mechanisms. NMDA receptor antagonists and low doses of morphine can preferentially block the development of hyperalgesia without significantly altering unpotentiated responses to nociceptive stimuli. The aim of our study was to determine if low doses of pentobarbital can also preferentially alter either hyperalgesic or unpotentiated responses to nociceptive heat stimuli in spinalized and intact rats. Our results demonstrate the following. (1) Mustard oil applied above the ankle joint or electrical stimulation of the sciatic nerve at C-fiber intensity in spinalized, unanesthetized rats decreased the latency to withdrawal of the foot from water maintained at 47-49 degrees C. This secondary hyperalgesia to thermal stimulation persisted for at least 1 h and was most likely mediated by central mechanisms. (2) Pentobarbital in both spinalized and spinal cord-intact rats prevented the development of the late component (42-120 min) but only partially decreased the early (2-6 min) component of hyperalgesia. In contrast, pentobarbital had relatively minimal effects on unpotentiated withdrawal responses. Thus, pentobarbital is similar to morphine in its ability to prevent hyperalgesia, but may differ from the anesthetic isoflurane, which does not interfere with the development of hyperalgesia.

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

We report on the effects of somatic conditioning stimuli on the reflexive nocifensive tail flick electromyograms (TFEMGs) in the rat anesthetized with continuous pentobarbital infusions. Rather than using a reflexive tail movement as a measure of noxious responses, TFEMGs were recorded from the base of the rat's tail as this gave more reliable results in the anesthetized state. In order to demonstrate the compatibility of this model with previous tail flick studies, we demonstrated an inhibition of TFEMGs by intrathecal morphine which was reversed by intravenous naloxone. The TFEMG latencies were then shown to be increased by electroacupuncture. This effect was antagonized by intrathecal naltrexone pretreatment. All of these results under barbiturate anesthesia resembled those observed previously in awake rats. The constant pentobarbital infusion maintained a stable baseline state, as reflected by TFEMG latencies and blood pressure recordings during the prolonged time-course of each experiment (approximately 1 h). We feel that this method of studying electroacupuncture suppression of nocifensive reflexes is valuable for its relative simplicity and reliability. It also has the virtue of avoiding the pitfalls of such studies in awake animals.

Cortical nociceptive responses and behavioral correlates in the monkey

Experiments were performed to characterize cerebral cortical activity and pain behavior elicited by electrical stimulation of the tooth pulp in unanesthetized monkeys. Four monkeys were trained on two different operant paradigms: two on a simple escape task and two on an appetitive tolerance-escape task. All monkeys were implanted with bipolar stimulating electrodes in the right maxillary canine tooth and subdural recording electrodes over the left primary (SI) and/or secondary (SII) somatosensory cortices. Subdural tooth pulp-evoked potentials (TPEPs) recorded over the SII consisted of components P1 (27.5 ms), N1 (40.3 ms), P2 (84.0 ms), N2 (163.5 ms), P3 (295.3 ms), and N3 (468.0 ms). The long latency component (P3-N3) was found exclusively over the SII and was elicited by high intensity stimulation. The appearance of component P3-N3 required the recruitment of A delta nerve fibers into the maxillary nerve compound action potential and was correlated with high frequencies of escape. Administration of morphine sulfate (4 mg/kg, i.m.) caused a contemporaneous reduction in escape frequency and in the amplitude of P3-N3 recorded over the SII. The relationships between TPEP amplitude, escape behavior and A delta nerve fiber activity strongly suggest that the SII is involved with nociception and pain behavior.

The effects of small doses of barbiturate on the activity of primate nociceptive tract cells

The activity of neurons identified as or similar to spinothalamic tract cells was compared in anesthetized, intact monkeys and in unanesthetized, decerebrate or spinalized animals. Background activity and responses to A- and C-fiber volleys were smaller in decerebrate than in the other preparations, suggesting the presence of a tonic descending inhibition of nociceptive tract cells in the decerebrate state. Injections of small doses of sodium pentobarbital caused a slight increase in sensitivity of peripheral receptive fields to mechanical stimuli in some cells, and an increase in responses to squeezing the skin with forceps and to C-fiber volleys. Larger doses of pentobarbital reduced the responses to A- and C-fiber volleys. These observations suggest that the activity of nociceptive tract cells is well preserved in the presence of the doses of pentobarbital we routinely use and are consistent with the hyperalgesic state reported to be produced by small doses of barbiturate.