Opioid receptors mediate a postsynaptic facilitation and a presynaptic inhibition at the afferent synapse of axolotl vestibular hair cells

Abstract

This study was designed to determine the effects of opiate drugs on the electrical activity of afferent neurons and on the ionic currents of hair cells from semicircular canals. Experiments were done on larval axolotls ( Ambystoma tigrinum). The multiunit spike activity of afferent neurons was recorded in the isolated inner ear under both resting conditions and mechanical stimulation. Ionic currents were recorded using voltage clamp of hair cells isolated from the semicircular canal. In the isolated inner-ear preparation, microperfusion of either non-specific opioid receptor antagonist naloxone (10 nM to 1 mM), μ receptor agonist [D-Ala 2, N-Me-Phe 4,Gly 5-ol]-enkephalin (1 pM to 10 μM), or κ receptor antagonist nor-binaltorphimine (10 nM to 100 μM) elicited a dose-dependent long-lasting (>5 min) increase of the electrical discharge of afferent neurons. The μ receptor agonist funaltrexamine (1 nM to 100 μM) and the κ receptor agonist U-50488 (1 nM to 10 μM) diminished the basal spike discharge of vestibular afferents. The δ receptor agonist D-Pen 2-D-Pen 5-enkephalin (1 nM to 10 mM) and the antagonist naltrindole (1 nM to 10 mM) were without a significant effect. The only drug that displayed a significant action on hair-cell ionic currents was trans-(±)-3,4-dichloro- N -methyl- N -(2-[1-pyrrolidinyl]-cyclohexyl) benzeneacetamide methanesulfonate (U-50488) that reduced the Ca 2+ current in a dose-dependent fashion. On its own, μ receptor agonist [D-Ala 2, N-Me-Phe 4,Gly 5-ol]-enkephalin (0.01 and 10 μM) significantly potentiated the response of afferent neurons to the excitatory amino acid agonist (±)-α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (0.1 μM), while synaptic transmission was blocked by the use of high-Mg 2+, low-Ca 2+ solutions. Our data indicate that the activity of vestibular afferent neurons may be regulated in a complex fashion by opioid receptors: μ opioid receptors mediating an excitatory, postsynaptic modulatory input to afferent neurons, and κ receptors mediating an inhibitory, presynaptic input to hair cells.