Anatomical distribution and physiological effects of enkephalin in rat inferior olive

The opioid peptide enkephalin was evaluated as a possible synaptic transmitter in the inferior olivary complex using anatomical and physiological techniques. With standard immunohistochemical procedures, fibres and terminals containing enkephalin-like immunoreactivity were found to be heterogenously distributed in the inferior olive of the rat. Enkephalin-like immunoreactivity was found in the medial and dorsal accessory nuclei, but was sparse in the principal nucleus. Iontophoresis or pressure microapplication of Leu5-enkephalin, or a peptidase-resistant enkephalin analogue, depressed both spontaneous and amino acid-evoked firing of inferior olivary complex cells. The effect appeared to be postsynaptic, since the excitatory effects of iontophoretically-applied DL-homocysteic acid were suppressed. No selective effect on a calcium-mediated component of the action potential was seen. We conclude that enkephalin may act as a neurotransmitter or neuromodulator at afferent synapses in the inferior olive.