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

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. 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. 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 GABAA receptor antagonists bicuculline and SR 95531. The NC receptor ligands [Phe1Ψ(CH2-NH)Gly2] nociceptin(1-13)NH2, Ac-RYYRIK-NH2 and [Nphe1]NC(1-13)NH2 were also evaluated. All these peptides inhibited NC-induced changes in firing rate. In addition, in some neurons where NC inhibited firing, [Nphe1]NC(1-13)NH2 and Ac-RYYRIK-NH2 elicited per se an increase in firing rate, suggesting the existence of tonic innervation of thalamic neurons by NC-containing fibres. In NC-inhibited neurons nocistatin induced a significant increase in firing rate. The present study demonstrated that NC regulates various thalamic nuclei related not only to somatosensory, but also to the visual and motor functions. Keywords: Thalamus, nociceptin/orphanin F/Q, [Phe1Ψ(CH2-NH)Gly2]Nociceptin(1-13)NH2, Ac-RYYRIK-NH2, [Nphe1]NC(1-13)NH2, bicuculline, iontophoresis, urethane Introduction Over the last few years, a new G-protein-coupled receptor (ORL1) that shows high homology with opioid receptors, but is not activated by the known opioid ligands, has been identified and cloned. In 1995 its endogenous ligand was simultaneously identified by two groups (Meunier et al., 1995; Reinscheid et al., 1995). Despite consistent inhibitory actions of NC at the cellular level in various brain sites (Moran et al., 2000), the effect of NC on pain behaviour remains controversial. The thalamus, especially the somatosensory thalamus, is involved in pain perception (Willis, 1997). Somatic sensory information is relayed to the cortex through major thalamic nuclei, the medial ventroposterior nucleus (VPM), the lateral ventroposterior nucleus (VPL), and the posterior thalamic nucleus (PO). In the rat the former two nuclei are also referred to as the ventrobasal complex. Whereas the VPL receives inputs from dorsal column nuclei and the spinal cord, the VPM receives information from trigeminal nuclei (Price, 1995). The zona incerta (ZI) is also involved in somatosensory processing (Nicolelis et al., 1992) in addition to other functions (Kim et al., 1992; Spencer et al., 1988). The lateral posterior nucleus (LP) as well as the laterodorsal nucleus (LD) receives fibres from several vision-related structures. The ventrolateral nucleus (VL) is thought to be a motor nucleus, relaying activity from the cerebellum (Price, 1995). Each of these nuclei are connected to a restricted sector of the reticular nucleus (Rt), which is composed of GABAergic cells that receive synapses from both thalamocortical and corticothalamic fibres (Jones, 1985). In situ hybridization analysis in rats has revealed that NC receptor mRNA is expressed in all these different thalamic nuclei (Neal et al., 1999). In the present study we have evaluated the effects of NC on spontaneous firing rates and burst patterns in these thalamic nuclei. Furthermore, we have attempted a pharmacological characterization of the receptor mediating the effects of NC using the ORL-1 receptor ligands [Phe1Ψ(CH2-NH)Gly2]Nociceptin(1-13)NH2 (F/G) (Guerrini et al., 1998) and Acetyl-Arg-Tyr-Tyr-Arg-Ile-Lys-NH2 (AcRNH2) (Dooley et al., 1997; Berger et al., 1999; 2000a), which have been shown to exert both agonist and antagonist activity in different in vitro and in vivo systems (Calo' et al., 2000a), and the recently identified selective and pure ORL-1 receptor antagonist, [Nphe1]NC(1-13)NH2 (Nphe) (Guerrini et al., 2000). In addition, the actions of nocistatin, a biologically active peptide derived from the same precursor as NC (Okuda Ashitaka et al., 1998) were evaluated on thalamic firing rate and compared with NC-induced effects.