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) ionotophoretica lly onto neurons located in functionally distinct thalamic structures of ur ethane-anesthetized rats. Extracellular single unit recordings were made in the media and lateral ventroposterior nucleus, posterior thalamic nucleus, zona incerta, lateral posterior nucleus, laterodorsal nucleus, ventrolater al 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 hi gh threshold bursts. 3 In about 20% of the neurons NC increased the firing rate. In most cells N C-induced increases in discharge rate could be blocked by the GABAA recepto r antagonists bicuculline and SR 95531. 4 The NC receptor ligands [Phe(1) psi (CH2-NH)Gly(2)] nociceptin(1-13)NH2, Ac-RYYRIK-NH2 and [Nphe(1)]NC(1-13)NH2 were also evaluated. All these pepti des inhibited NC-induced changes in firing rate. In addition, in some neuro ns where NC inhibited firing, [Nphe(1)]NC(1-13)NH2 and Ac-RYYRIK-NH2 elicit ed per se an increase in firing rate, suggesting the existence of tonic inn ervation of thalamic neurons by NC-containing fibres. 5 In NC-inhibited neurons nocistatin induced a significant increase in firi ng rate. 6 The present study demonstrated that NC regulates various thalamic nuclei related not only to somatosensory, but also to the visual and motor functio ns.