Effects of highly selective kappa-opioid agonists on EEG power spectra and behavioural correlates in conscious rats

The present study compares the electroencephalographic (EEG) and behavioural effects of highly selective kappa-opioid agonists, spiradoline (U62066), enadoline (CI-977), BRL 52656, and BRL 53001, after SC administration to conscious rats. All compounds caused a distinctive dose- and time-related EEG effect and behavioural profile. The EEG power spectra (PS) of treated animals were characterised by power reduction indicative of CNS activation and an unusual spectral peak at 4-7 Hz, compared to controls. Behaviourally, however, the animals appeared to be sedated, with a reduction of locomotor activity, loss of postural tone, ataxia, and unusual hyperreactivity. The compounds differed in their liability to produce these CNS effects, with kappa-opioid analgesics such as enadoline and BRL 53001 showing a reduced propensity at equivalent antinociceptive doses. Where tested, these effects were inhibited by the nonselective opioid antagonist naloxone. The non-brain penetrating kappa-opioid agonist, BRL 52974, failed to produce alterations in EEG PS or behaviour, up to a dose of 10 mg/kg, indicating that the effects depend on activation of centrally located kappa-receptors.