Rapid development of tolerance to sub-anaesthetic dose of ketamine: an oculomotor study in macaque monkeys.

BACKGROUND: Ketamine, a non-competitive N-methyl-D: -aspartic acid antagonist, has been widely used for anaesthetic purposes. At sub-anaesthetic dosage, it induces a dissociative state similar to schizophrenia. The discovery of this effect on dissociative state has led to its use as a pharmacological model of schizophrenia and has also been responsible for its illegal use as a recreational drug. Whereas the former has provided invaluable information, the latter has demonstrated that repeated administration of ketamine induces tolerance. Surprisingly, a review of the relevant literature shows that tolerance to sub-anaesthetic doses of ketamine is largely unreported in neuropharmacological studies.
METHODS: In order to investigate this caveat, we have performed a post hoc analysis of the behavioural effects induced by repeated injections of sub-anaesthetic doses of ketamine observed in five consecutive monkeys performing two oculomotor tasks. Ketamine effects were quantified by the animals' performances and latencies in a prosaccade and an antisaccade task, two oculomotor paradigms that are impaired after ketamine administration.
RESULTS: Although the result of the initial injections confirmed a clear behavioural effect of ketamine injections in all monkeys, subsequent administrations showed that a tolerance eventually appeared in all monkeys. The profile of this tolerance exhibited however a large inter-subject variability.
CONCLUSIONS: Psychopharmacological experiments using ketamine as a pharmacological model of psychosis should therefore consider the kinetic and time course of these effects in each individuals and take them into account in the design of experimental protocols.