Oxytocin Modulates Glutamatergic Synaptic Transmission between Cultured Neonatal Spinal Cord Dorsal Horn Neurons

Abstract

The functional characteristics of binding sites for the neuropeptide oxytocin (OT) detected by radioautography in laminae I and II of the dorsal horn (DH) and on cultured neonatal DH neurons were studied on the latter using perforated patch-clamp recordings. The neurons were identified by their spike discharge properties and on the basis of the presence of met-enkephalin-like and glutamate decarboxylase-like immunoreactivities. OT (100 nm) never induced any membrane current at a holding potential of −60 mV but increased the frequency of spontaneously occurring AMPA receptor-mediated EPSCs or the mean amplitude of electrically evoked EPSCs in a subset (35%) of neurons. The frequency of miniature EPSCs (m-EPSCs) recorded in the presence of 0.5 μmtetrodotoxin was also increased by OT (100 nm) without any change in their mean amplitude, indicating an action at a site close to the presynaptic terminal. The decay kinetics of any type of EPSC were never modified by OT. The effect of OT was reproduced by [Thr⁴,Gly⁷]-OT (100 nm), a selective OT receptor agonist, and blocked by d(CH₂)₅-[Tyr(Me)²,Thr⁴,Tyr-NH₂⁹]-ornithine vasotocin (100 nm), a specific OT receptor antagonist. Reducing the extracellular Ca²⁺concentration from 2.5 to 0.3 mmin the presence of Cd²⁺(100 μm) reversibly blocked the effect of OT on m-EPSCs. The OT receptors described here may represent the substrate for modulatory actions of descending hypothalamo-spinal OT-containing pathways on the nociceptive system.