A comparison of the ability of Leu 8 - and Pro 8 -oxytocin to regulate intracellular Ca 2+ and Ca 2+ -activated K + channels at human and marmoset oxytocin receptors.

The neurohypophyseal hormone oxytocin (OT) regulates biological functions in both peripheral tissues and the central nervous system (CNS). In the CNS, OT influences social processes including peer relationships, maternal-infant bonding and affiliative social relationships. In mammals, the nonapeptide OT structure is highly conserved with leucine in the 8th position (Leu8 -OT). In marmosets ( Callithrix ) a nonsynonymous nucleotide substitution in the OXT gene codes for proline in the 8th residue position (Pro8 -OT). OT binds to its cognate G protein-coupled receptor (OTR) and exerts diverse effects, including stimulation (Gs ) or inhibition (Gi/o ) of adenylyl cyclase, stimulation of potassium channel currents (Gi ) and activation of phospholipase C (Gq ). CHO cells expressing marmoset (mOTR) or human (hOTR) oxytocin receptors were used to characterize OT signaling. At mOTR, Pro8 -OT was more efficacious than Leu8 -OT in measures of Gq activation with both peptides displaying subnanomolar potencies. At hOTR, neither potency nor efficacy of Pro8 -OT and Leu8 -OT differed with respect to Gq signaling. In both mOTR- and hOTR-expressing cells Leu8 -OT was more potent and modestly more efficacious than Pro8 -OT in inducing hyperpolarization. In mOTR cells, Leu8 -OT-induced hyperpolarization was modestly inhibited by pretreatment with pertussis toxin (PTX) consistent with a minor role for Gi/o -activation; however, the Pro8 -OT response in mOTR and hOTR cells was PTX-insensitive. These findings are consistent with membrane hyperpolarization being largely mediated by a Gq signaling mechanism leading to Ca2+ -dependent activation of K+ channels. Evaluation of the influence of apamin, charybdotoxin, paxilline and TRAM-34 demonstrated involvement of both intermediate and large conductance Ca2+ -activated K+ channels.