Neuropeptide FF selectively attenuates the effects of nociceptin on acutely dissociated neurons of the rat dorsal raphe nucleus

Abstract

Intracellular Ca 2+ concentration ([Ca 2+] i) was measured in neurons, acutely dissociated from the rat dorsal raphe nucleus (DRN), with the fluorescent calcium probe Fluo3. Nociceptin (300 nM) had no effect on resting [Ca 2+] i but reduced the magnitude of the [Ca 2+] i transient triggered by depolarization in 90% of neurons having polygonal or fusiform perikarya. In 94% of neurons with the same morphology 5-HT (30 μM) also reduced the magnitude of the [Ca 2+] i transient. The selective 5-HT 1A receptor antagonist 4-iodo- N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]- N-2-pyridinyl-benzamide hydrochloride (p-MPPI) (0.4 μM) strongly attenuated (by 72±7%, n=4) this effect. The responses to nociceptin and 5-HT were not affected by BaCl 2 (100 μM). The neuropeptide FF analog [ d-Tyr 1, ( N-Me)Phe 3]NPFF (1DMe) altered neither the resting [Ca 2+] i nor the [Ca 2+] i transient triggered by depolarization but dose-dependently decreased the effect of nociceptin (EC 50=1.8 nM, maximal reduction: 68±5%). 1DMe had no effect on the response to 5-HT. Another neuropeptide FF analog, exhibiting a different pharmacological activity in mice and rats, [ d-Tyr 1, d-Leu 2, d-Phe 3]NPFF (1 μM) also reduced the effect of nociceptin by 74±11% ( n=4). Few neurons (5 out of 42), either with polygonal/fusiform or smaller ovoid cell bodies, responded to the μ-opioid receptor agonist [ d-Ala 2, ( N-Me)Phe 4, Gly-ol 5]-enkephalin (DAGO) with a decrease in the depolarization-induced [Ca 2+] i transient. 1DMe (100 nM) attenuated this response by 69±14%. These results suggest that, at the cellular level, neuropeptide FF selectively counteracts the effects of opioid receptor activation.