Inhibition of serotonin outflow by nociceptin/orphaninFQ in dorsal raphe nucleus slices from normal and stressed rats: Role of corticotropin releasing factor

In the dorsal raphe nucleus (DRN) many inputs converge and interact to modulate serotonergic neuronal activity and the behavioral responses to stress. The effects exerted by two stress-related neuropeptides, corticotropin releasing factor (CRF) and nociceptin/orphaninFQ (N/OFQ), on the outflow of ³H]5-hydroxytryptamine were investigated in superfused rat dorsal raphe nucleus slices.Electrical stimulation (100 mA, 1 ms for 2 min) evoked a frequency-dependent peak of ³H]5-hydroxytryptamine outflow, which was sodium and calcium-dependent. Corticotropin releasing factor (1–100 nM), concentration-dependently inhibited the stimulation (3 Hz)-evoked ³H]5-hydroxytryptamine outflow; the inhibition by 30 nM corticotropin releasing factor (to 68 ± 5.7%) was prevented both by the non selective CRF receptor antagonist alpha-helicalCRF(9-41) (α-HEL) (300 nM) and by the CRF₁ receptor antagonist antalarmin (ANT) (100 nM). The CRF₂ agonist urocortin II (10 nM) did not modify ³H]5-hydroxytryptamine outflow, ruling out the involvement of CRF₂ receptors. Bicuculline (BIC), a GABA_A antagonist (10 μM), prevented the inhibitory effect of corticotropin releasing factor (30 nM), supporting the hypothesis that the inhibition was mediated by increased γ-aminobutyric acid (GABA) release. Nociceptin/orphaninFQ (1 nM–1 μM) exerted an antalarmin- and bicuculline-insensitive inhibition on ³H]5-hydroxytryptamine outflow, with the maximum at 100 nM (to 63 ± 4.2%), antagonized by the NOP receptor antagonist UFP-101 (1 μM). Dorsal raphe nucleus slices prepared from rats exposed to 15 min of forced swim stress displayed a reduced ³H]5-hydroxytryptamine outflow, in part reversed by antalarmin and further inhibited by nociceptin/orphaninFQ. These findings indicate that (i) both corticotropin releasing factor and nociceptin/orphaninFQ exert an inhibitory control on dorsal raphe nucleus serotonergic neurons; (ii) the inhibition by corticotropin releasing factor involves γ-aminobutyric acid neurons; (iii) nociceptin/orphaninFQ inhibits dorsal raphe nucleus serotonin system in a corticotropin releasing factor- and γ-aminobutyric acid-independent manner; (iv) nociceptin/orphaninFQ modulation is still operant in slices prepared from stressed rats. The nociceptin/orphaninFQ-NOP receptor system could represent a new target for drugs effective in stress-related disorders.