Noradrenergic mechanisms in mediation of stress-induced hyperalgesia in rats.

Exposure to a mild stressor (15 min of vibration) produced analgesia in some rats and hyperalgesia in other rats from the same batch treated in the same way. Rats which responded with decreased tail-flick latencies (TFL) showed signs of hyperemotionality during the stress procedure. Stress-induced hyperalgesia was abolished by the administration of diazepam (2.5 mg/kg i.p.) and clonidine (25 micrograms/kg i.p.). It is suggested that the reversal of hyperalgesia was due to the anxiolytic properties of the drugs. Yohimbine, an alpha 2-adrenoceptor antagonist (5 mg/kg i.p.), antagonized the effect of clonidine. The influence of clonidine on stress-induced hyperalgesia may be mediated by alpha 2-adrenoceptors.