β-Funaltrexamine Discriminates between Two Subtypes of μ 2-Opioid Receptors in Electrically Stimulated Longitudinal Muscle of Guinea Pig Ileum

Abstract

1. Morphine inhibited twitch responses of the longitudinal muscle of the guinea pig to electrical stimulation. A concentration–response curve of morphine was partly inhibited by a 60-min pretreatment of ileal strip with 10 −10 M of β-funaltrexamine (β-FNA). However, the 60-min pretreatment with a higher concentration (10 −9 M) of β-FNA had no further significant inhibitory effect on the curve of morphine, suggesting that β-FNA discriminated between two subtypes of μ 2 receptors, β-FNA-sensitive and -resistant receptors. 2. The μ 2 receptors in synaptosomal fraction were selectively labeled by [ 3H]naloxone, and the labeled receptors were competitively inhibited by morphine. The competitive-inhibition curve of morphine showed the presence of high- and low-affinity sites. β-FNA eliminated the high-affinity site only when endogenous GTP or GTPγ-S was present. 3. β-FNA discriminated between two subtypes of μ 2 receptors.