The μ-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr- NH2 (CTOP) [but not D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP)] produces a nonopioid receptor-mediated increase in K+ conductance of rat locus ceruleus neurons

The somatostatin analogues D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH₂ (CTOP) and D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH₂ (CTAP) have been used widely as selective antagonists of μ-opioid receptors. Actions of CTOP and CTAP on the membrane properties of rat locus ceruleus neurons were studied using intracellular recordings of membrane currents in superfused brain slices. CTOP increased a K⁺ conductance with an EC₅₀ of 560 nM. The maximal conductance increase produced by CTOP (10 μM) was similar to that produced by high concentrations of the μ-opioid agonists D-Ala-Met- enkephalinglyol (1 μM) and Met-enkephalin (10 μM, as well as an α₂- adrenoceptor agonist (UK14304, 3 μM) and somatostatin (1 μM). The K⁺ current produced by CTOP was not antagonized by naloxone (1 μM), suggesting it was not mediated by μ-opioid receptors. The K⁺ currents induced by high concentrations of CTOP desensitized to 42% of the initial maximum after prolonged superfusion (t(1/2) = 247 sec). In the presence of fully desensitized CTOP responses, somatostatin (1 μM) still produced near- maximal K⁺ currents; i.e., there was no cross-desensitization, which suggests that CTOP might act on a receptor distinct from somatostatin receptors. However, the converse did not apply; high concentrations of CTOP (30 μM) did not produce any additional current in the presence of desensitized somatostatin responses. No cross-desensitization was observed between CTOP (10-30 μM) and Met-enkephalin (30 μM) or nociceptin (3 μM) regardless of the order of drug application. Cyclo-(7-aminoheptanoyl-Phe-D- Trp-Lys-Thr[Bzf], antagonized both somatostatin- (K(D) = 10 μM) and CTOP- (K(D) = 8 μM) induced K⁺ currents with similar potency. Concentrations of CTOP (100 nM) that produced a small K⁺ current partially antagonized the actions of Met-enkephalin (10 μM) on μ-opioid receptors. In contrast to CTOP, CTAP produced no K⁺ current at concentrations of 300 nM and 1 μM and little current at 10 μM. CTAP potently antagonized K⁺ currents produced by the μ-opioid receptor agonist D-Ala-Met-enkephalinglyol, with an equilibrium dissociation constant of 4 nM (Schild analysis). CTAP did not antagonize K⁺ currents produced by CTOP or somatostatin. These results demonstrate that CTOP is a potent and efficacious agonist at nonopioid receptors, whereas CTAP is a potent μ-opioid receptor antagonist with little nonopioid agonist activity in rat locus ceruleus neurons. The receptor activated by CTOP has yet to be fully resolved but seems to be similar to the somatostatin type 2 receptor or perhaps to a receptor closely related to somatostatin or opioid receptors.