Mu and delta receptors belong to a family of receptors that are coupled to potassium channels.

TLDR

Many vertebrate neurotransmitter receptors, including acetylcholine M2, norepinephrine α2, dopamine D2, 5‑HT1, adenosine A1, GABAB, and somatostatin, are coupled to a conserved inwardly rectifying potassium conductance that mediates their effects. The authors compared mu and delta opioid agonist effects by voltage‑clamp recording of membrane currents in rat locus coeruleus neurons and guinea pig submucous plexus cells, and used intracellular GTPγS to probe G‑protein coupling. Both mu and delta opioid agonists increased the same inwardly rectifying potassium conductance, strongly hyperpolarized the membrane, and the conductance properties were indistinguishable, indicating that both receptors use a common G‑protein‑coupled effector.

Abstract

The effects of agonists at mu and delta opioid receptors were compared by measuring membrane currents under voltage clamp from neurons of the rat nucleus locus coeruleus and guinea pig submucous plexus. In each tissue, the appropriate selective agonist (Tyr-D-Ala-Gly-MePhe-Gly-ol for mu receptors in locus coeruleus or Tyr-D-Pen-Gly-Phe-D-Pen for delta receptors in submucous plexus) increased the conductance of an inwardly rectifying potassium conductance and strongly hyperpolarized the membrane. The properties of the potassium conductance affected by the two opioids could not be distinguished. Experiments with intracellular application of guanosine 5'-[gamma-thio]triphosphate indicated that a guanine nucleotide-binding regulatory protein was involved in the coupling between opioid receptor and potassium channel, but there was no evidence for activation of either cAMP-dependent protein kinase or protein kinase C. It is noted that a number of vertebrate neurotransmitter receptors are coupled to potassium channels. It is noted that a number of vertebrate neurotransmitter receptors are coupled to potassium channels. The potassium conductance associated with these channels has properties similar to the conductance activated by mu and delta opioids; this family includes the following receptors: acetylcholine M2, norepinephrine alpha 2, dopamine D2, 5-hydroxytryptamine 5-HT1, adenosine A1, gamma-aminobutyric acid GABAB, and somatostatin. It is suggested that this conductance is a conserved neuronal effector coupled to one of the receptor types that mediates the effects of each of several major transmitters. The mu and delta opioid receptors appear to be unusual in that both utilize this same effector mechanism.