Inhibitory action of nociceptin/orphanin FQ on functionally different thalamic neurons in urethane-anaesthetized rats

1. In this study we administered nociceptin/orphanin FQ (NC) ionotophoretically onto neurons located in functionally distinct thalamic structures of urethane-anesthetized rats. Extracellular single unit recordings were made in the medial and lateral ventroposterior nucleus, posterior thalamic nucleus, zona incerta, lateral posterior nucleus, laterodorsal nucleus, ventrolateral nucleus and reticular nucleus. 2. NC decreased the firing rate in 60% of thalamic neurons. This decrease in firing rate was accompanied by a significant reduction in the number of high threshold bursts. 3. In about 20% of the neurons NC increased the firing rate. In most cells NC-induced increases in discharge rate could be blocked by the GABA(A) receptor antagonists bicuculline and SR 95531. 4. The NC receptor ligands [Phe(1)Psi(CH(2)-NH)Gly(2)] nociceptin(1-13)NH(2), Ac-RYYRIK-NH(2) and [Nphe(1)]NC(1-13)NH(2) were also evaluated. All these peptides inhibited NC-induced changes in firing rate. In addition, in some neurons where NC inhibited firing, [Nphe(1)]NC(1-13)NH(2) and Ac-RYYRIK-NH(2) elicited per se an increase in firing rate, suggesting the existence of tonic innervation of thalamic neurons by NC-containing fibres. 5. In NC-inhibited neurons nocistatin induced a significant increase in firing rate. 6. The present study demonstrated that NC regulates various thalamic nuclei related not only to somatosensory, but also to the visual and motor functions.