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 [(3)H]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 [(3)H]5- hydroxytryptamine outflow, which was sodium and calcium-dependent. Corticotropin releasing factor (1-100 nM), concentration-dependently inhibited the stimulation (3 Hz)-evoked [(3)H]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) (alpha-HEL) (300 nM) and by the CRF(1) receptor antagonist antalarmin (ANT) (100 nM). The CRF(2) agonist urocortin II (10 nM) did not modify [(3)H]5- hydroxytryptamine outflow, ruling out the involvement of CRF(2) receptors. Bicuculline (BIC), a GABAA antagonist (10 microM), prevented the inhibitory effect of corticotropin releasing factor (30 nM), supporting the hypothesis that the inhibition was mediated by increased gamma-aminobutyric acid (GABA) release. Nociceptin/ orphaninFQ (1 nM-1 microM) exerted an antalarmin- and bicuculline-insensitive inhibition on [(3)H]5- hydroxytryptamine outflow, with the maximum at 100 nM (to 63+/- 4.2%), antagonized by the NOP receptor antagonist UFP-101 (1 microM). Dorsal raphe nucleus slices prepared from rats exposed to 15 min of forced swim stress displayed a reduced [(3)H]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 gamma-aminobutyric acid neurons; (iii) nociceptin/ orphaninFQ inhibits dorsal raphe nucleus serotonin system in a corticotropin releasing factor- and gamma-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.