Abstract
Opioid receptors are expressed in the vestibular endorgans (afferent neurons and hair cells) and are activated by the efferent system, which modulates the discharge of action potentials in vestibular afferent neurons (VANs). In mammals, VANs mainly express the μ opioid-receptor, but the function of this receptors activation and the cellular mechanisms by which they exert their actions in these neurons are poorly studied. To determine the actions of μ opioid receptor (MOR) and cell signaling mechanisms in VANs, we made perforated patch-clamp recordings of VANs that were obtained from postnatal days 7 to 10 (P7–10) rats and then maintained in primary culture. The MOR agonist [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) inhibited the total voltage-gated outward current; this effect was prevented by the perfusion of a Ca2+-free extracellular solution. We then studied the voltage-gated calcium current (Ica) and found that DAMGO Met-enkephalin or endomorphin-1 inhibited the ICa in a dose-response fashion. The effects of DAMGO were prevented by the MOR antagonist (CTAP) or by pertussis toxin (PTX). The use of specific calcium channel blockers showed that MOR activation inhibited T-, L- and N-type ICa. The use of various enzyme activators and inhibitors and of cAMP analogs allowed us to demonstrate that the MOR acts through a cAMP dependent signaling mechanism. In current clamp experiments, MOR activation increased the duration and decreased the amplitude of the action potentials and modulated the discharge produced by current injection. Pre-incubation with PTX occluded MOR activation effect. We conclude that MOR activation inhibits the T-, L- and N-type ICa through activation of a Gαi/o protein that involves a decrease in AC-cAMP-PKA activity. The modulation of ICa may have an impact on the synaptic integration, excitability, and neurotransmitter release from VANs.
Highlights
The μ opioid receptor (MOR) participates in reward-seeking behavior, analgesia, physical dependence, and the modulation of respiratory frequency (Fields, 2011; Raehal et al, 2011)
The maximum inhibition was attained with 1 μM DAMGO that inhibited 58 ± 7% of the current at −47 mV (LVA) and 35 ± 4% at −14 mV (HVA, n = 15, P = 0.01); these results indicate that MOR activation inhibits both the low voltage activated calcium channels (LVA) and high voltage activated calcium channels (HVA) Ca2+ current components
The inhibition produced by DAMGO at both ages (P7–10 vs. P28–30) was not significantly different for the HVA current (P = 0.30) but was significantly lower for the LVA in P28–30 rats (P = 0.04); these findings suggest that the LVA current decreased with age or that the LVA current uncouples from the second messenger system in older ages; changes in the MOR expression or sensitivity should in principle be reflected in both the LVA or the HVA current components
Summary
The μ opioid receptor (MOR) participates in reward-seeking behavior, analgesia, physical dependence, and the modulation of respiratory frequency (Fields, 2011; Raehal et al, 2011). The MOR is a member of the G protein coupled receptors; it is predominantly coupled to a Gαi/o protein, which decreases the adenylyl cyclase activity, thereby lowering the cellular level of cAMP and the PKA activity (Law, 2011). Among other cellular processes affected by opioid receptors, the MOR modulates voltage-dependent calcium, sodium and potassium channels (Witkowski and Sulczyk, 2006; Law, 2011). Studies in the vestibular system showed that opioid peptides and their receptors are expressed both at the central and at the peripheral levels. Recordings in slices obtained from the medial vestibular nucleus (MVN) of the rat have shown that delta-opioid receptor agonists decreased the neuronal tonic discharge (Sulaiman and Dutia, 1998). The use of preproenkephalin antisense and naloxone significantly delayed vestibular compensation (Kitahara et al, 2006)
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