The effects of microiontophoretically-applied opioids and opiate antagonists on nociceptive responses of neurones of the caudal reticular formation in the rat.

Abstract Opioids, when applied by microiontophoresis to caudal reticular neurones (CRF) in the rat, were found to be predominantly inhibitory in action. When the actions of a series of analogues of enkephalin, that have very different potencies on peripheral tissues, were applied microiontophoretically to caudal reticular neurones, only small differences in their ability to depress the firing of these neurones were observed. Analogues of enkephalin with differing affinities for μ- and δ-receptors also produced comparable inhibitions of neuronal activity. These inhibitory actions were reduced by opiate antagonists. Many caudal reticular neurones have peripheral receptive fields which are activated by noxious stimuli. Excitatory responses produced by noxious stimuli were depressed by microiontophoretically-applied opioids and this effect was antagonised by naloxone or naltrexone. The reduction in firing rate evoked in some neurones by peripheral noxious stimulation was very resistant to change when opiate antagonists were applied microiontophoretically but could sometimes be totally blocked by systemic injection of opiate antagonists.