Involvement of multiple opiate receptors in opioid kindling

D-Tyr-Ser-Gly-Phe-Leu-Thr (DSLET), beta-endorphin, morphiceptin and morphine were microinjected at 48-h intervals into the amygdala or hippocampus of awake rats in an attempt to identify the opiate receptor types involved in opioid kindling. DSLET, beta-endorphin, morphiceptin and morphine were injected into the lateral ventricle to assess the possibility of kindling seizures by this route. The delta-receptor agonist DSLET effectively kindled convulsions when microinjected into amygdala or ventral hippocampus. The convulsions were suppressed or strongly attenuated by ICI 174,864, a specific antagonist of the delta-receptor, microinjected into the same brain site, but were not affected by ICI 174,864 administered peripherally. When microinjected into amygdala or hippocampus, beta-endorphin and morphiceptin also kindled convulsions, which were antagonized by naloxone but not by ICI 174,864. Morphine evoked EEG epileptiform activity but did not kindle convulsions from limbic brain sites. DSLET occasionally evoked epileptiform spiking and submaximal convulsions when injected into ventricle, and morphiceptin evoked epileptiform spiking only, but tolerance to these effects occurred after repetition of the injections. Thus, convulsions can be kindled by activation of either mu-, delta- or epsilon-receptors when opioids are injected directly into limbic tissue. However, the ability of these compounds to kindle seizures is markedly reduced when they are administered into ventricle. The striking differences between the present results and previous results obtained by peripheral or intraventricular administration of opioid peptides suggest that the route of administration, among other variables, is a crucial factor in assessing the epileptogenic properties of opioid peptides.