The effects of 5-hydroxytryptamine 1A (5-HT 1A ) receptor ligands on aversive learning were examined in the passive avoidance (PA) task in mice. Anxiety and autonomic functions were investigated using the elevated plus-maze and heart rate measurements. The main findings from this study are as follows. 1) Pretraining administration of the 5-HT 1A receptor agonist 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide] facilitated PA retention at low doses (0.01 and 0.03 mg/kg) but impaired PA retention at higher doses (0.1-1.0 mg/kg), consistent with previous findings in the rat. Serotonin (5-hydroxytryptamine; 5-HT) is a biogenic amine involved in a wide range of physiological functions, including learning and memory (Ö gren, 1985;Buhot et al., 2000). The ascending 5-HT neurons originating from the brainstem raphe nuclei innervate virtually all regions of the rat forebrain, allowing for widespread neuromodulatory actions (Dahlström and Fuxe, 1964) mediated by 14 identified receptor subtypes (Hoyer et al., 2002). Among the 5-HT receptor subtypes implicated in cognitive functions, the 5-HT 1A receptors are of special interest because they are abundant in brain

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