Psychopharmacology (2003) 169:35–41 DOI 10.1007/s00213-003-1467-1 ORIGINAL INVESTIGATION M. Diez-Ariza · C. Redondo · M. García-Alloza · B. Lasheras · J. Del Río · M. J. Ramírez Flumazenil and tacrine increase the effectiveness of ondansetron on scopolamine-induced impairment of spatial learning in rats Received: 7 July 2002 / Accepted: 11 March 2003 / Published online: 4 July 2003  Springer-Verlag 2003 Abstract Rationale: Cholinergic receptor blockade pro- duces memory deficits in animal models. These deficits can be prevented by 5-HT 3 receptor antagonists, such as ondansetron, which increases acetylcholine release. We investigated the effects on cognitive performance of combined treatments of ondansetron with either flumaze- nil, a GABA A receptor benzodiazepine site antagonist, or tacrine, a cholinesterase inhibitor, which are also able to prevent scopolamine-induced cognitive impairment. Methods: Spatial learning and memory was assessed by studying the effects of single and combined treatments on acquisition and retention of the Morris water maze task in rats. Results: Scopolamine (0.6 mg/kg) induced signifi- cant learning and retention deficits. Both ondansetron (0.1 mg/kg) and tacrine (3 mg/kg) partially prevented the scopolamine-induced learning deficit. A full reversal was only found after the combined treatment of ondansetron with flumazenil (10 mg/kg) and also after tacrine in combination with ondansetron. Likewise, scopolamine- induced retention deficit was fully counteracted by the combined treatment of ondansetron with either flumazenil or tacrine, and only partially by any of the single treatments tested. Conclusions: The scopolamine-induced impairment of learning and retention in the water maze is fully prevented by ondansetron when given in combina- tion with either flumazenil or tacrine, suggesting that both combined treatments result in a potentiated cholinergic function and may constitute the basis of a new therapy for cognitive disorders. Keywords Serotonin · GABA · Ondansetron · Tacrine · Flumazenil · Morris water maze Introduction Serotonin (5-HT) seems to play an important role in cognitive processes, and the involvement of different 5-HT receptor subtypes in learning and memory has been repeatedly suggested (see reviews by Buhot 1997; Costall and Naylor 1997; Meneses 1998; Buhot et al. 2000). Blockade of serotonin 5-HT 3 receptors by selective antagonists, such as ondansetron, improves the perfor- mance of rodents and primates in behavioural tests to evaluate cognitive function (Greenshaw 1993; Buhot 1997; Bloom and Morales 1998), and reverses the memory loss induced by anticholinergic treatment or associated with aging (Hodges et al. 1996; Arnsten et al. 1997; Carli et al. 1997). In clinical studies, the attenuation by 5-HT 3 receptor antagonists of age-related cognitive decline and scopolamine-induced deficit in spatial and verbal memory has been shown (Little et al. 1995; Costall and Naylor 1997) but not always (Broocks et al. 1998). Activation and restoration of cholinergic function remains a major objective in the development of pharmacological approaches towards the treatment of cognitive dysfunctions associated with aging and demen- tia (Gallagher and Colombo 1995; Knopman 2001). Accordingly, the increased release of cortical acetylcho- line (ACh) induced by 5-HT 3 receptor antagonists, initially reported by Barnes et al. (1989), is considered a rational mechanism involved in the cognition enhancing property of this class of drugs. In this regard, previous studies from our group showed that the 5-HT 3 receptor antagonist, ondansetron, produced an enhancement in ACh release in slices from rat entorhinal cortex and this effect was potentiated by coadministration of GABA A receptor antagonists such as bicuculline and flumazenil (Ramirez et al. 1996; Diez-Ariza et al. 1998). In a more recent study (Diez-Ariza et al. 2002), it was found that these GABA A antagonists also potentiated cortical ACh release induced by ondansetron in freely moving rats. GABA A receptor antagonists at the benzodiazepine site enhance cognitive processes when administered alone (Lal and Forster 1990; Prather et al. 1992) and flumazenil M. Diez-Ariza · C. Redondo · M. García-Alloza · B. Lasheras · J. Del Río · M. J. Ramírez ( ) ) Department of Pharmacology, School of Medicine, University of Navarra, Irunlarrea 1, 31008 Pamplona, Spain e-mail: mariaja@unav.es Fax: +34-948-425649