Inhibitory Actions of Nociceptin (Orphanin FQ) on Rat Supraoptic Nucleus Oxytocin and Vasopressin Neurones in Vitro

Abstract

In the cloning of genes for the mu, kappa and delta opioid receptors, a previously unsuspected fourth member of this seven trans-membrane-helix G-protein coupled receptor family was identified¹. This orphan receptor, now named opioid receptor-like 1, ORL-1² weakly binds opioids and the opioid antagonist naloxone¹; it is expressed in brain, notably in the supraoptic nucleus (SON;^3,4). Soon after identification of the ORL-1 receptor, an endogenous heptadecapeptide ligand was isolated^4,5; this is homologous to dynorphin 1–17, but is the product of a distinct gene⁶. Initial behavioural studies after central administration in mice indicated a pronociceptive, anti-opioid, action hence the peptide was named nociceptin, and alternatively, because of its selective high affinity for the orphan ORL-1 receptor, orphanin FQ (OFQ,-FQ describing aminoacids 1 and 17, whereas the classical opioid peptides have Y at position 1; OFQ and the opioid peptides share GGF at positions 2,3,4 in the amino terminal ‘message’^4,5). The nociceptin gene is expressed in the SON⁷, and because previous studies have shown inhibitory actions of opioids within the SON on both oxytocin and vasopressin neurones⁸, we have investigated whether nociceptin could act to oppose opioids on SON neurones. We used coronal 350 µm brain slices, cut on a Vibraslice, from 100–200 g female Sprague-Dawley rats for in vitro whole cell patch recording with current-clamp in an interface chamber with continous superfusion measuring effects on spontaneous firing-rate. Oxytocin and vasopressin neurones were distinguished by their different responses to hyperpolarising pulses⁹. Briefly, oxytocin neurones show slow depolarisation during a sustained (1.2 s) hyperpolarising (100–300 pA) pulse, with a burst of action potentials at the end of the pulse, due to outward rectification; vasopressin neurones do not show these responses.