Oxytocin acts at V1 receptors to excite sympathetic preganglionic neurones in neonate rat spinal cord in vitro

Intracellular recordings were made from sympathetic preganglionic neurones (SPNs) in transverse slices of thoraco-lumbar spinal cord of young rats (12–20 days old). A small group of SPNs generally having higher membrane potentials (− 70mV) compared to a remaining group (− 66mV) showed spontaneous oscillations of their membrane potential. Oxytocin superfused in concentrations of 0.1–30 μM had four effects on SPNs, inducing slow depolarisation, EPSPs, IPSPs and brief rhythmic oscillations. The slow depolarisation was unaffected by TTX whereas this abolished the other changes. The oxytocin-induced depolarisation was associated with a slow inward current and was not reversed at membrane potentials negative toE_K, it increased at more positive potentials and was still present in low Ca²⁺ and high Mg²⁺ solutions. These features of the oxytocin induced current are similar to those of the TTX resistant voltage dependent Na⁺ current described in brainstem autonomic neurones. Vasopressin superfused at concentrations of 0.1 μM to 30 μM had similar effects on SPNs to those of oxytocin. A comparison of the effects of oxytocin and vasopressin on the same neurones revealed that oxytocin was almost 10 times less potent than vasopressin. The effects of oxytocin were not mimicked by a selective oxytocin agonist but were mimicked by a selective vasopressin V_1a agonist and blocked by a selective V_1a antagonist. Therefore it is concluded that the effects of oxytocin on SPNs are mediated by the vasopressin V_1a receptor. It is suggested that oxytocin and vasopressin terminals in the lateral horn are part of a descending system controlling oscillating networks of SPNs in the spinal cord.