Inhibition of striatal and retinal dopamine release via nociceptin/orphanin FQ receptors

We determined the effects of nociceptin/orphanin FQ and the NOP receptor ligands acetyl-Arg-Tyr-Tyr-Arg-Ile-Lys-NH2 (Ac-RYYRIK-NH2) and naloxone benzoylhydrazone on transmitter release in vitro. The electrically evoked tritium overflow from guinea-pig and mouse striatal slices and guinea-pig retinal discs preincubated with [3H]-dopamine was inhibited by nociceptin/orphanin FQ (pEC50 7.9, 7.6 and 8.6; Emax 30, 50 and 55%). Ac-RYYRIK-NH2 0.032 μM and naloxone benzoylhydrazone 5 μM antagonized the effect of nociceptin/orphanin FQ in striatal slices of the guinea-pig (apparent pA2 9.1 and 6.8) and the mouse (apparent pA2 9.2 and 7.5) and strongly attenuated the effect of nociceptin/orphanin FQ 0.1 μM in guinea-pig retinal discs. Ac-RYYRIK-NH2 0.032 μM did not affect the evoked overflow by itself whereas naloxone benzoylhydrazone 5 μM inhibited it in each tissue. The electrically evoked tritium overflow from mouse brain cortex slices preincubated with [3H]-noradrenaline was inhibited by nociceptin/orphanin FQ (pEC50 7.9, Emax 85%), Ac-RYYRIK-NH2 (pEC50 8.3, Emax 47%) but not affected by naloxone benzoylhydrazone 5 μM. Ac-RYYRIK-NH2 and naloxone benzoylhydrazone showed apparent pA2 values of 8.6 and 6.9. In conclusion, the inhibitory effect of nociceptin/orphanin FQ on dopamine release in the striatum and retina and on noradrenaline release in the cerebral cortex is mediated via NOP receptors. Ac-RYYRIK-NH2 behaves as an extremely potent NOP receptor antagonist in the striatum and retina and as a partial agonist in the cortex. Keywords: NOP receptor, nociceptin, orphanin FQ, dopamine release, noradrenaline release, striatum, guinea-pig retina, mouse cerebral cortex, Ac-RYYRIK-NH2, naloxone benzoylhydrazone Introduction A fourth type of opioid receptor, which has a homology in the amino acid sequence to the three classical opioid receptors of about 50%, was first identified in 1994 (for review, see Mogil & Pasternak, 2001). This receptor, termed NOP according to the nomenclature of IUPHAR (International Union of Pharmacology; Cox et al., 2000), is activated by a heptadecapeptide, termed nociceptin/orphanin FQ(N/OFQ) (for review, see Mogil & Pasternak, 2001). The NOP and the classical opioid receptors resemble each other with respect to the transduction mechanisms, i.e. inhibition of adenylyl cyclase (Hawes et al., 2000) and of voltage-dependent Ca2+ channels and activation of voltage-dependent K+ channels (Moran et al., 2000). However, the NOP receptor markedly differs from the classical opioid receptors in its pharmacological properties (e.g. lack of antagonistic effect of the opioid receptor blocking agent naloxone) (Calo et al., 2000) and its distribution in the brain (Mollereau & Mouledous, 2000). The most striking difference, however, occurs with respect to the influence on nociception. Thus, N/OFQ elicits hyperalgesia rather than analgesia when administered at a supraspinal site (for review, see Mogil & Pasternak, 2001). The action of N/OFQ is, however, not restricted to nociception since the heptadecapeptide also affects feeding (Polidori et al., 2000), learning and memory (Noda et al., 2000) and anxiety (Jenck et al., 1997) as well as numerous vegetative functions (Mogil & Pasternak, 2001). One aspect in which the classical opioid and the NOP receptors resemble each other is that they occur presynaptically on a variety of peripheral and central neurones where they cause inhibition of the release of the respective neurotransmitter (Moran et al., 2000; Schlicker & Morari, 2000). N/OFQ e.g. affects the release of dopamine and this has been shown by direct measurement of transmitter release or electrophysiological techniques for the (i) nigrostriatal (Konya et al., 1998; Schlicker et al., 1998a; Shieh & Pan, 2001), (ii) mesolimbic (Murphy et al., 1996; Murphy & Maidment, 1999; Lutfy et al., 2001; Shieh & Pan, 2001; Zheng et al., 2002), (iii) tuberoinfundibular (Wagner et al., 1998; Shieh & Pan, 2001) and (iv) incertohypothalamic system (Shieh & Pan, 2001). The involvement of NOP receptors in the effect of N/OFQ has so far been proven only in the studies by Shieh & Pan (2001; use of an antisense oligodeoxynucleotide) and by Zheng et al. (2002; use of the weakly potent NOP receptor antagonist [Phe1Ψ(CH2-NH)Gly2]-N/OFQ(1-13)NH2). The need for such experiments is highlighted by a study on conscious rats in which intrastriatal administration of N/OFQ increased dopamine release in a manner sensitive to naloxone thereby arguing against the involvement of NOP receptors (Konya et al., 1998). We therefore examined in two in vitro models of the striatum, i.e. superfused guinea-pig and mouse striatal slices, in which N/OFQ inhibits dopamine release in the presence of naloxone (Schlicker et al., 1998a), whether this effect is counteracted by NOP receptor antagonists. We used the selective and highly potent NOP receptor antagonist acetyl-Arg-Tyr-Tyr-Arg-Ile-Lys-NH2 (Ac-RYYRIK-NH2) (Dooley et al., 1997; Berger et al., 1999) and a non-selective and less potent NOP receptor antagonist, naloxone benzoylhydrazone, used by our own group (Schlicker et al., 1998b; Malinowska et al., 2000a) and by other investigators (Bucher, 1998; Siniscalchi et al., 1999; Ho et al., 2000; Bigoni et al., 2002). Furthermore, we studied whether N/OFQ affects dopamine release also in guinea-pig retinal discs since the influence of N/OFQ on the retinal dopamine system has so far not been examined. Finally, we compared the potencies of Ac-RYYRIK-NH2 and naloxone benzoylhydrazone at the NOP receptor causing inhibition of noradrenaline release in the mouse brain cortex, i.e. at an NOP receptor previously identified and characterized by our group (Schlicker et al., 1998b; Bauer et al., 1999; Werthwein et al., 1999).