Actions of GABA, glycine, methionine-enkephalin and β-endorphin compared with electrical stimulation of nucleus raphe magnus on responses evoked by tooth pulp stimulation in the medial reticular formation in the cat

Abstract

In decerebrate, cerebellectomised cats, a comparison was made between the effects of electrical stimulation in nucleus raphe magnus (NRM) and iontophoretic application of GABA, glycine, met-enkephalin and β-endorphin on the responses of neurones in the medial brain stem reticular formation to tooth pulp stimulation. NRM stimulation, GABA, glycine and enkephalin produced a short lasting inhibition of tooth pulp evoked responses whilst the time course of the inhibition produced by β-endorphin was much slower, often lasting up to 1 h following a 3–7 min ejection period. The effects of GABA and glycine could be antagonised by iontophoresis of bicuculline and strychnine respectively whilst intravenous injection of naloxone antagonised the inhibition induced by the opioid peptides. In most neurones tested, inhibition of tooth pulp evoked responses by NRM stimulation was blocked by iontophoretic application of bicuculline but not by strychnine or naloxone (i.v.). We conclude that GABA may act as a transmitter which mediates the inhibitory effects of NRM on the responses of reticular neurones to tooth pulp stimulation. Thus GABA may be involved in stimulation produced analgesia.