Glial and potassium responses to an afferent volley: suppression by enkephalin in the rat spinal cord in vitro.

A transient rise in the extracellular potassium concentration following activation of internuncial neurons by an afferent volley was recorded intracellularly from glial cells and by K+-sensitive electrodes in the superficial dorsal horn of the rat spinal cord in vitro. Methionine (Met)-enkephalin did not affect the extracellular potassium level at rest but suppressed the potassium response elicited by afferent volleys. This effect was abolished by naloxone. In the chronically deafferent spinal cord combined with the block of synaptic transmission by Mg2+, Met-enkephalin suppressed the transient rise of the extracellular potassium concentration induced by L-glutamate. It is concluded that synaptic excitation of interneurons in the superficial dorsal horn is suppressed by the postsynaptic action of Met-enkephalin.