Reversal by naloxone of spinal antinociceptive effects of fentanyl, ketocyclazocine and midazolam.

Experiments using measurement of electrical-current threshold as a nociceptive test in the skin of the tail and neck in rats demonstrated that fentanyl, ketocyclazocine and midazolam caused spinally mediated antinociception when the drugs were administered intrathecally via chronically implanted lumbar subarachnoid catheters. The benzodiazepine antagonist flumazenil selectively suppressed the midazolam response, indicating that this benzodiazepine exerted its segmental antinociceptive effect via spinal-cord benzodiazepine receptors. Naloxone blocked the responses to both opioids and also midazolam. The dose of naloxone which suppressed the midazolam response was similar to that required to suppress the response to the kappa-opioid agonist. We suggest that the segmental antinociceptive effects of fentanyl and midazolam are mediated via different pathways; the benzodiazepine exerts its antinociceptive action via a spinal-cord opioid pathway which does not involve mu-receptors.

Spinally mediated antinociception following intrathecal chlordiazepoxide--further evidence for a benzodiazepine spinal analgesic effect.

This is a report of the results of 25 experiments in five rats investigating the dose-response relationship for the antinociceptive effects of chlordiazepoxide given intrathecally in the dose range 0.03-0.9 mumol and a further 40 experiments in eight rats investigating the actions of flumazenil and naloxone on this effect. Electrical-current thresholds for pain were measured in the skin of the tail and neck of rats with previously implanted lumbar subarachnoid catheters. Intrathecal chlordiazepoxide produced spinally mediated antinociception, i.e. rises in the current threshold for pain in the tail without a significant change in the neck. This antinociceptive effect was dose dependent. Flumazenil 16.5 mumol kg-1 i.p. reduced the response caused by chlordiazepoxide 0.6 mumol by 78 +/- 6% (mean +/- SEM). By contrast, the same dose of flumazenil did not significantly affect the antinociceptive effect of an equipotent dose of intrathecal fentanyl 0.74 nmol. Naloxone 0.38 mumol kg-1 i.p. abolished the spinally mediated antinociception caused by fentanyl (96 +/- 7% suppression) but did not significantly reduce the effect of chlordiazepoxide (27 +/- 13% suppression). However, a higher dose of naloxone (6.1 mumol kg-1 i.p.) caused significant partial suppression (79 +/- 10.7%) of chlordiazepoxide spinal antinociception. We conclude that chlordiazepoxide produces an antinociceptive effect by combination with benzodiazepine receptors in the spinal cord.

Analgesia mediated by spinal kappa-opioid receptors.

The results from 44 experiments performed on 13 rats with chronically implanted lumbar subarachnoid catheters are reported. ICI 197 067 produced dose-dependent segmental analgesic effects when measurement of electrical current threshold for pain was used as the nociceptive test. Ten microliters of intrathecal ICI 197 067 (0.06 mg ml-1; 1.5 nmol) caused a significant rise in the current threshold for pain in the tail of 1.56 +/- 0.04 x control (mean +/- SEM) but no significant change in pain threshold in the neck (1.03 +/- 0.03 x control). By contrast, simultaneous measurements of tail-flick latency in these animals revealed no significant rise in pain thresholds using this nociceptive test. Intraperitoneally administered naloxone produced a dose-dependent suppression of the spinally mediated analgesic effect produced by ICI 197 067; the ED50 for this effect was 0.79 mumol kg-1, a value very close to the ED50 for naloxone antagonism of ketocyclazocine spinally mediated analgesia. We conclude that ICI 197 067 produces spinally mediated analgesia by binding to spinal-cord kappa-opioid receptors.

On the mechanism by which midazolam causes spinally mediated analgesia.

The electrical current thresholds for pain (ECTP) in the skin of the neck and tail were measured in rats with chronically implanted lumbar subarachnoid catheters. The effects of a benzodiazepine antagonist and a gamma-aminobutyric acid (GABA) antagonist on the analgesic effects of equivalent doses of midazolam, fentanyl, and ketocyclazocine were studied. These were the minimum doses producing maximal segmental analgesia when given intrathecally (i.e., they all caused a significant and maximum increase in ECTP in the tail, which was similar for all three drugs, but no significant change in the ECTP in the neck). Flumazenil (Ro 15-1788) administration caused a parallel shift to the right of the dose-response curve for midazolam spinal analgesia. Segmental analgesia following midazolam was also significantly attenuated (P less than 0.05) when the selective GABA antagonist bicuculline was given intrathecally at the same time as midazolam. The highest dose of bicuculline used (50 pmol) caused no significant attenuation of the segmental analgesic effects of either ketocyclazocine or fentanyl. The authors concluded that the segmental analgesia produced by intrathecal midazolam is mediated by the benzodiazepine-GABA receptor complex that is involved in other benzodiazepine actions.

Cardiovascular effects of propofol in the anaesthetized dog.

This study was designed to investigate if propofol produced cardiovascular effects by direct actions or by indirect actions secondary to depression of the central nervous system. Experiments were performed on chloralose anaesthetized dogs in which all neurogenic cardiovascular reflexes were abolished by bilateral vagotomy and common carotid ligatures, in combination with i.v. bretylium and propranolol. Bolus doses of propofol followed by infusions at rates up to 160 mg kg-1 h-1 produced blood concentrations of propofol from 1.99 to 112 micrograms ml-1. Infusions of hydroxyethyl starch given to maintain central venous pressures and pulmonary artery occlusion pressures at control values were used as an index of changes in capacitance. Blood concentrations of propofol less than 10 micrograms ml-1 caused an increase in mean capacitance of 8.0 (SEM 1) ml kg-1 with no significant changes in systemic vascular resistance, pulmonary vascular resistance or inotropic state of the heart. We conclude that anaesthesia with propofol may be accompanied by decreased cardiac output secondary to reduction in preload by a direct venodilator effect. Our experiments indicate that cardiac output and arterial pressure are preserved well at normal anaesthetic blood concentrations of propofol if the preload is maintained.

Intrathecal midazolam and fentanyl in the rat: evidence for different spinal antinociceptive effects.

The effects of intrathecal midazolam and fentanyl on electrical current threshold for pain were measured using stimulating electrodes in the neck and tail of rats with chronically implanted lumbar subarachnoid catheters. This involved the measurement of the minimum current (50 Hz 2 ms pulses 0-5 mA), which made the rat squeak when applied alternately to electrodes at each skin site. The responses measured in milliamperes were expressed as a number of times control readings. Equieffective doses of both midazolam and fentanyl produced a significant increase in electrical threshold for pain in the tail (mean +/- SEM 3.14 +/- 0.51 and 2.89 +/- 0.35: P less than 0.05; Wilcoxon sum rank test) in the absence of any change in the neck (mean +/- SEM 1.28 +/- 0.13 and 0.96 +/- 0.12, NS), thus demonstrating a spinal effect. Fentanyl caused a significant simultaneous increase in tail flick latency (mean +/- SEM 67.8 +/- 20.1%, P less than 0.05), but midazolam did not (mean +/- SEM 4.22 +/- 2.76%, NS). Intraperitoneal injections of naloxone (0.25 mg/kg) blocked the response to fentanyl in both tests and did not affect the response to midazolam. Intraperitoneal flumazenil (5 mg/kg) blocked the midazolam antinociceptive effect but did not affect the response to fentanyl in either test. Tail withdrawal in response to non-noxious stimulation was preserved in all animals with spinal analgesia, indicating that myelinated afferent and efferent pathways were still functioning. Righting reflex, coordination, motor power, and alertness were also preserved in the presence of both drugs. Both drugs caused spinally mediated antinociceptive effects that were qualitatively different.

Intrathecal midazolam in the rat: evidence for spinally-mediated analgesia.

This study investigated the possible analgesic effect of midazolam as a result of interruption of those spinal cord pathways taken by pain afferents. Experiments were performed on 15 male Wistar rats with chronically implanted lumbar subarachnoid catheters. The threshold for pain induced by brief passage of electric current between pairs of electrodes placed on the tail and the skin of the neck was measured before and after subarachnoid injections of midazolam. Intrathecal midazolam caused a significant (P less than 0.02) increase in the threshold for pain in the tail, but not in the neck; this response was not produced by intrathecal injections of vehicle and was blocked by prior intraperitoneal injections of the benzodiazepine antagonist RO 15-1788. We also performed experiments on frog sciatic nerves which showed that midazolam did not have a local anaesthetic action. We conclude that intrathecal midazolam causes spinally-mediated analgesia by binding to benzodiazepine receptors in the spinal cord.