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

Abstract

1. We determined the effects of nociceptin/orphanin FQ and the NOP receptor ligands acetyl-Arg-Tyr-Tyr-Arg-Ile-Lys-NH(2) (Ac-RYYRIK-NH(2)) and naloxone benzoylhydrazone on transmitter release in vitro. 2. The electrically evoked tritium overflow from guinea-pig and mouse striatal slices and guinea-pig retinal discs preincubated with [(3)H]-dopamine was inhibited by nociceptin/orphanin FQ (pEC(50) 7.9, 7.6 and 8.6; E(max) 30, 50 and 55%). Ac-RYYRIK-NH(2) 0.032 microM and naloxone benzoylhydrazone 5 microM antagonized the effect of nociceptin/orphanin FQ in striatal slices of the guinea-pig (apparent pA(2) 9.1 and 6.8) and the mouse (apparent pA(2) 9.2 and 7.5) and strongly attenuated the effect of nociceptin/orphanin FQ 0.1 microM in guinea-pig retinal discs. Ac-RYYRIK-NH(2) 0.032 microM did not affect the evoked overflow by itself whereas naloxone benzoylhydrazone 5 microM inhibited it in each tissue. 3. The electrically evoked tritium overflow from mouse brain cortex slices preincubated with [(3)H]-noradrenaline was inhibited by nociceptin/orphanin FQ (pEC(50) 7.9, E(max) 85%), Ac-RYYRIK-NH(2) (pEC(50) 8.3, E(max) 47%) but not affected by naloxone benzoylhydrazone 5 microM. Ac-RYYRIK-NH(2) and naloxone benzoylhydrazone showed apparent pA(2) values of 8.6 and 6.9. 4. 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-NH(2) behaves as an extremely potent NOP receptor antagonist in the striatum and retina and as a partial agonist in the cortex.

Figure 1

Effect of nociceptin/orphanin FQ (N/OFQ) on the electrically evoked tritium overflow from superfused brain slices and retinal discs. Guinea-pig (gp) and mouse striatal slices and guinea-pig retinal discs preincubated with [³H]-dopamine ([³H]-DA) were superfused with medium containing nomifensine 10 μM, (−)-sulpiride 3.2 μM and naloxone 10 μM; tritium overflow was evoked at 1 Hz. Mouse cortex slices preincubated with [³H]-noradrenaline ([³H]-NA) were superfused with medium containing desipramine 1 μM, rauwolscine 1 μM and naloxone 10 μM; tritium overflow was evoked at 0.3 Hz. N/OFQ was added to the medium from 62 min of superfusion onward. Tritium overflow was evoked twice, after 40 (S₁) and 90 min (S₂) of superfusion, and the ratio of the overflow evoked by S₂ over that evoked by S₁ was determined. Tritium overflow is given as per cent of the S₂/S₁ value in the corresponding controls (not shown). Means±s.e.mean of 5–19 experiments. *P<0.05, **P<0.001.

Figure 2

Interaction of Ac-RYYRIK-NH₂ (A, C) and naloxone benzoylhydrazone (NalBzOH) (B, D) with the effect of nociceptin/orphanin FQ (N/OFQ) on the electrically (1 Hz) evoked tritium overflow from superfused guinea-pig (gp) (A, B) and mouse (C, D) striatal slices preincubated with [³H]-dopamine ([³H]-DA). The medium contained nomifensine 10 μM, (−)-sulpiride 3.2 μM, naloxone 10 μM and when necessary Ac-RYYRIK-NH₂ or NalBzOH throughout superfusion and N/OFQ from 62 min of superfusion onward. Tritium overflow was evoked twice, after 40 (S₁) and 90 min (S₂) of superfusion, and the ratio of the overflow evoked by S₂ over that evoked by S₁ was determined. Tritium overflow is given as per cent of the S₂/S₁ value in the corresponding controls (not shown). Means±s.e.mean of 4–19 experiments (guinea-pig striatal slices) and 4–15 experiments (mouse striatal slices). *P<0.05, **P<0.001.

Figure 3

Interaction of Ac-RYYRIK-NH₂ and naloxone benzoylhydrazone (NalBzOH) with the effect of nociceptin/orphanin FQ (N/OFQ) on the electrically (1 Hz) evoked tritium overflow from superfused guinea-pig (gp) retinal discs preincubated with [³H]-dopamine ([³H]-DA). The medium contained nomifensine 10 μM, (−)-sulpiride 3.2 μM, naloxone 10 μM and when necessary Ac-RYYRIK-NH₂ or NalBzOH throughout superfusion and N/OFQ from 62 min of superfusion onward. Tritium overflow was evoked twice, after 40 (S₁) and 90 min (S₂) of superfusion, and the ratio of the overflow evoked by S₂ over that evoked by S₁ was determined. Tritium overflow is given as per cent of the S₂/S₁ value in the corresponding controls. Means±s.e.mean of 7–10 experiments. *P<0.05, **P<0.001.

Figure 4

Interaction of Ac-RYYRIK-NH₂ with nociceptin/orphanin FQ (N/OFQ) (A) and of naloxone benzoylhydrazone (NalBzOH) with Ac-RYYRIK-NH₂ (B) in superfused mouse brain cortex slices preincubated with [³H]-noradrenaline ([³H]-NA). The medium contained desipramine 1 μM, rauwolscine 1 μM and naloxone 10 μM. In panel (A), Ac-RYYRIK-NH₂ was present in the medium throughout superfusion whereas N/OFQ was added from 62 min onward. In panel (B), NalBzOH was present in the medium throughout superfusion whereas Ac-RYYRIK-NH₂ was added from 62 min onward. Two periods of electrical stimulation (0.3 Hz) were administered, after 40 (S₁) and 90 min (S₂) of superfusion, and the ratio of the overflow evoked by S₂ over that evoked by S₁ was determined. Tritium overflow is given as per cent of the S₂/S₁ value in the corresponding controls (not shown). Means±s.e.mean of 4–6 experiments. *P<0.05, **P<0.001.