BACKGROUND AND PURPOSE5-HT6 receptors are abundant in the hippocampus, nucleus accumbens and striatum, supporting their role in learning and memory. Selective 5-HT6 receptor antagonists produce pro-cognitive effects in several learning and memory paradigms while 5-HT6 receptor agonists have been found to enhance and impair memory. EXPERIMENTAL APPROACHThe conditioned emotion response (CER) paradigm was validated in rats. Then we examined the effect of the 5-HT6 receptor antagonist, EMD 386088 (10 mg·kg KEY RESULTSPairing unavoidable foot shocks with a light and tone cue during CER training induced a robust freezing response, providing a quantitative index of contextual memory when the rat was returned to the shock chamber 24 h later. Pretreatment (-20 min pre-training) with scopolamine or MK-801 reduced contextual freezing 24 h after CER training, showing production of memory impairment. Immediate post-training administration of 5-HT6 receptor antagonist, SB-270146, and agonists, EMD 386088 and E-6801, had little effect on CER freezing when given alone, but all significantly reversed scopolamine-and MK-801-induced reduction in freezing. CONCLUSION AND IMPLICATIONSBoth the 5-HT6 receptor agonists and antagonist reversed cholinergic-and glutamatergic-induced deficits in associative learning. These findings support the therapeutic potential of 5-HT6 receptor compounds in the treatment of cognitive dysfunction, such as seen in Alzheimer's disease and schizophrenia.

Roots of tomato seedlings can be induced to coil by treatment with ethylene . The extent of coiling is dependent on the level of ethylene to which the seedlings are exposed and can be prevented by the incorporation of Ag ions into the growing medium . In contrast to all other tomato mutants examined, roots of the mutant diageotropica do not reorientate their growth in response to ethylene . The results of an agar penetration test indicate that roots of this mutant are agravitropic . The relationship between gravitropism and root coiling, and the origin of the ethylene modified growth pattern is discussed .