YM992 [(S)-2-[[(7-fluoro-4-indanyl)oxy]methyl]morpholine monohydrochloride] is a selective serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitor (SSRI) and a potent 5-HT(2A) antagonist. The aim of the present study was to assess, using in vivo extracellular unitary recordings, the effect of acute and sustained administration of YM992 (40 mg kg(-1) day(-1) s.c., using osmotic minipumps) on the spontaneous firing activity of locus coeruleus (LC) norepinephrine (NE) neurons. Acute intravenous injection of YM992 (4 mg kg(-1)) significantly decreased NE neuron firing activity by 29% and blocked the inhibitory effect of a subsequent injection of the 5-HT(2) agonist DOI [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride]. A 2-day treatment with YM992 decreased the firing rate of NE neurons by 66%, whereas a partial recovery was observed after a 7-day treatment and a complete one after a 21-day treatment. Following the injection of the alpha(2)-adrenoceptor antagonist idazoxan (1 mg kg(-1) i.v.), NE neuron firing was equalized in controls and 2-day YM992-treated rats. This put into evidence an increased degree of activation of alpha(2)-adrenergic autoreceptors in the treated rats. The suppressant effect of the alpha(2)-adrenoceptor agonist clonidine was significantly decreased in long-term YM992-treated rats. The recovery of LC firing activity after long-term YM992 administration could thus be explained by a decreased sensitivity of alpha(2)-adrenergic autoreceptors. Sustained SSRI administration leads to a gradual reduction of the firing activity of NE neurons during long-term administration, whereas YM992 produced opposite effects. The exact basis for the increased synaptic availability of NE by YM992 remains to be elucidated. This NE activity, resulting from 5-HT reuptake inhibition plus 5-HT(2A) receptor antagonism, might confer additional benefits in affective and anxiety disorders.