Opioid-independent hyperalgesia induced in mice by pentobarbitone at low dosage

Refinement of the use of non-human primates in scientific research. Part III: refinement of procedures

There is an ethical and scientific need to minimise the harm experienced by animals used in scientific procedures and to maximise their well-being. Welfare can be improved by the refinement of practice, particularly if these refinements are applied to every aspect of the life of an animal used in the laboratory, from birth to death. Primates are considered likely to have a greater capacity for suffering than other sentient species and therefore refinement of their use is particularly important. The refinement of the human impact on laboratory-housed primates and of housing and husbandry practices are dealt with in parts I and II of this three-part review. In part III, methods of refinement that can be applied specifically to the use of primates in procedures, are summarised and discussed, together with a description of some current practices, and the scientific evidence that suggests that they should no longer be used. Methods of refinement of identification, capture and restraint, sampling, administration of substances, humane endpoints, and euthanasia are included. If these methods are used, taking into account species-specific differences and needs, it is concluded that harm can be minimised and primate welfare improved.

Paradoxical Modulation of Nociception in Mice by Barbiturate Agonism and Antagonism: Is a GABA Site Involved in Nociception?

In a battery of four acute and chronic nociceptive tests, the GABA antagonist picrotoxin produces a uniform and sustained analgesia in mice. By contrast, barbiturates which have been presumed to act at the same receptor produce mixed and paradoxical actions. At a standard time of 10 min after drug administration a convulsant barbiturate [5-ethyl-5-(3'-methyl-but-2'-enyl)-barbituric acid] produced analgesia in three tests but had no effect in the fourth; a pure hypnotic barbiturate (amylobarbitone) produced hyperalgesia in three tests but analgesia in the fourth; while the mixed hypnotic-convulsant pentobarbitone produced hyperalgesia in two of the tests and was without any effect in the other two. There was no pattern in these results with respect to acute or chronic nociceptive tests. Surprisingly, with extended observation using the tail-flick test both pentobarbitone and the pure hypnotic (amylobarbitone) gave early hyperalgesia followed by analgesia; the convulsant barbiturate gave only analgesia. The results suggest a role for GABAA receptors in the transmission of nociceptive information; they also suggest that barbiturates act at quite a different receptor.

Anxiolytic-like effects of meprobamate. Interactions with an opiate antagonist in Swiss and BALB/c mice

Naloxone has previously been shown to block the effects of benzodiazepines in the Swiss but not in the BALB/c strain. We have also reported that naloxone potentiates subeffective doses of benzodiazepines in Swiss mice. In the present studies we first determined whether naloxone could block anxiolytic-like effects of meprobamate in Swiss and BALB/c mice. Then we evaluated if subeffective doses of meprobamate could be potentiated in Swiss as well as in BALB/c mice. The elevated plus-maze test and the light/dark choice procedure were used. The lowest dose of meprobamate with anxiolytic-like effects was 60 mg/kg in the BALB/c mice. This dose was effective in both the plus-maze and in the light/dark choice procedure. In Swiss mice the same dose was effective in the plus-maze, whereas 120 mg/kg was required in the light/dark choice procedure. When an effective dose of meprobamate was combined with naloxone, 10 mg/kg, no blockade of anxiolytic-like effects was obtained in any strain in any procedure. To the contrary, when a subeffective dose of meprobamate was combined with naloxone, 10 mg/kg, an anxiolytic-like effect was obtained in both strains in both procedures. The present series of experiment shows that the ability of naloxone to block anxiolytic-like drug effects do not apply to meprobamate. However, the naloxone-induced potentiation of subeffective doses previously observed after treatment with benzodiazepines or buspirone was present also after treatment with meprobamate. Moreover, although blockade of anxiolytic-like drug effects with naloxone has not been observed in BALB/c mice, potentiation was as evident in that strain as in the Swiss. This suggests that the mechanisms behind naloxone's blockade of anxiolytic-like effects are independent from those behind its potentiation of such effects.

The role of subtypes of the opioid receptor in the anxiolytic action of chlordiazepoxide

Previous studies have shown that the opiate antagonist naloxone blocks the anxiolytic-like effects of benzodiazepines in several models of anxiety, including the elevated plus-maze. Although naloxone preferentially binds to the mu opioid receptor, its selectivity is rather low. The opioid receptor subtype important for anxiolytic-like actions of benzodiazepines in the plus-maze remains, therefore, unknown. In the present experiments, the ability of antagonists selective for subtypes of the opioid receptor to block the anxiolytic-like effects of chlordiazepoxide in the elevated plus-maze was evaluated in Swiss mice. Chlordiazepoxide, 5 mg/kg, increased the proportion as well as the number of open arms entries without modifying closed arms entries. Lower doses of the benzodiazepine were ineffective. The mu receptor antagonist beta-funaltrexamine, 10 and 20 mg/kg, the delta antagonist naltrindole, 10 mg/kg, and the kappa antagonist nor-binaltorphimine, 2.5 and 5 mg/kg, were then combined with chlordiazepoxide, 5 mg/kg. beta-funaltrexamine, 10 mg/kg, reduced the effects of the benzodiazepine while the dose of 20 mg/kg completely blocked the effects. Nor-binaltorphimine was ineffective at a dose of 2.5 mg/kg, but completely inhibited the actions of chlordiazepoxide when the dose was 5 mg/kg. Naltrindole was ineffective. None of the antagonists affected plus-maze behavior when administered alone. It was concluded that the mu and kappa receptors are important for the anxiolytic-like actions of chlordiazepoxide in the elevated plus maze.

Analgesia and abuse potential: an accidental association or a common substrate?

The fact that centrally acting analgesics have abuse potential commensurate with their analgesic activity raises the question of whether these effects are related. The abuse potential of drugs depends on their ability to produce reinforcing effects, which are mediated by a neural system that includes the ventral tegmental dopamine cells and their connections with the ventral striatum. Morphine and amphetamine are both powerful analgesics and have high abuse potential. Their analgesic and reinforcing effects are mediated by similar receptors, similar sites of action, and overlapping neural substrates. These coincidences suggest that reinforcers may produce analgesia by transforming the aversive affective state evoked by pain into a more positive affective state. The implications of this hypothesis and its relation to other known mechanisms of analgesia are discussed. The hypothesis predicts that drugs with reinforcing effects should produce analgesia. A survey of drugs acting through 21 classes of receptors reveals that in 13 classes there is evidence for both analgesic and reinforcing effects that are approximately equipotent. The GABA(A) agonists were found to be the only drugs with confirmed abuse potential that lack analgesic activity. The interpretation of this and several other anomalous cases is discussed.

Pentobarbital, diazepam, and ethanol abolish the interphase diminution of pain in the formalin test: evidence for pain modulation by GABAA receptors

There are two phases to the behavioral response to injection of formalin. After an initial vigorous response, a period of reduced pain occurs 10 to 15 min after formalin, followed by reemergence of pain-related behaviors. These phases are believed to represent acute chemical stimulation of afferent neurons followed by injury-related inflammatory pain. Pentobarbital (10, 15, or 25 mg/kg), diazepam (0.5, 1.5, or 5.0 mg/kg), or ethanol (0.5, 1.0, or 1.5 g/kg) attenuated the diminution of pain between the two phases, so that pain was continuous throughout 60 min of testing, but had no effect on pain scores during the peaks of either phase. The effects of pentobarbital and diazepam were blocked by picrotoxin (2.5 mg/kg), which itself had no effect. Ro 15-1788 also blocked the effect of diazepam. Picrotoxin did not effectively antagonize the effect of ethanol. A high dose of picrotoxin (5.0 mg/kg) caused seizures in some rats and also eliminated the interphase depression of pain. The results suggest that the biphasic time course of formalin pain is produced by a central antinociceptive mechanism that is inhibited by GABAA receptors.

Stress analgesia: the opioid analgesia of long swims suppresses the non-opioid analgesia induced by short swims in mice

In mice, room temperature swimming for as short a period as 15 sec has been found to induce a non-opioid analgesia with a time course of 10-12 min. As the duration of the swim is increased, an opioid analgesia develops with a longer persistence (25-30 min); the development of the opioid analgesia appears to suppress the expression of the non-opioid analgesia so that none of the latter is evident after 3 min swims. The characteristics of the tail-flick nociceptive test are also described.

The antinociceptive actions of kava components in mice

1. The antinociceptive properties of the aqueous extract of the intoxicating beverage kava, and of the lipid soluble extract (kava resin) were tested in mice, by the tail immersion and abdominal constriction methods. Both extracts showed analgesic effects in both tests. 2. Eight purified pyrones from the lipid soluble extract were also tested for activity in the tail immersion test, and kawain, dihydrokawain, methysticin and dihydromethysticin were found to be very effective in producing analgesia. Using the tail immersion test the time course of action of the extracts of the four effective pyrones of kava were studied. 3. Naloxone, in doses which inhibited morphine-induced analgesia in both tests, was completely ineffective in reversing the antinociceptive activities of the kava extracts, showing that analgesia produced by kava occurs via non-opiate pathways.