Fragile X syndrome is a leading heritable cause of mental retardation that results from the loss of FMR1 gene function. A Drosophila model for Fragile X syndrome, based on the loss of dfmr1 activity, exhibits phenotypes that bear similarity to Fragile X-related symptoms. Herein, we demonstrate that treatment with metabotropic glutamate receptor (mGluR) antagonists or lithium can rescue courtship and mushroom body defects observed in these flies. Furthermore, we demonstrate that dfmr1 mutants display cognitive deficits in experience-dependent modification of courtship behavior, and treatment with mGluR antagonists or lithium restores these memory defects. These findings implicate enhanced mGluR signaling as the underlying cause of the cognitive, as well as some of the behavioral and neuronal, phenotypes observed in the Drosophila Fragile X model. They also raise the possibility that compounds having similar effects on metabotropic glutamate receptors may ameliorate cognitive and behavioral defects observed in Fragile X patients.

Two functional types of nicotinic acetylcholine receptors (nAChRs) are expressed when human embryonic kidney cells are permanently transfected with equal amounts of human ␣4 and ␤2 subunit cDNAs. Most (82%) of these nAChRs exhibit an EC 50 of 74 Ϯ 6 M for ACh, a much lower sensitivity than the remaining fraction (EC 50 of 0.7 Ϯ 0.4 M) or than expected from expression of equal amounts of ␣4 and ␤2 mRNAs in Xenopus laevis oocytes. We have found three conditions that can increase the number of nAChRs with high sensitivity to activation. These are: 1) transient transfection with additional ␤2 subunits, 2) overnight incubation in nicotine, or 3) overnight culture at 29°C. Using metabolic labeling with [ 35 S]methionine to measure subunit stoichiometry, we found that the majority of nAChRs had a stoichiometry of (␣4) 3 (␤2) 2 . Overnight treatment with nicotine increased the number of nAChRs and increased the proportion of the (␣4) 2 (␤2) 3 stoichiometry. Alternate ␣4␤2 nAChR stoichiometries with distinct functional properties raise the possibility for an interesting mode of synaptic regulation for nicotinic signaling in the mammalian brain.The ␣4␤2 nAChR is the predominant nAChR subtype in the mammalian brain that has high affinity for nicotine. nAChRs composed of ␣4 and ␤2 subunits modulate neurotransmitter release (Dani, 2001) and play a direct role in addiction to nicotine (Picciotto et al., 1998;Marubio et al., 1999). Mutations in ␣4␤2 nAChRs have been linked to autosomal-dominant nocturnal frontal lobe epilepsy (Weiland et al., 2000). They also are thought to be involved in Alzheimer's and Parkinson's diseases (Rusted et al., 2000).Two different approaches showed independently that chick ␣4␤2 nAChRs have a stoichiometry of (␣4) 2 (␤2) 3 when expressed in Xenopus laevis oocytes from cRNAs or cDNAs injected at a 1:1 (␣/␤) ratio (Anand et al., 1991;Cooper et al., 1991). A more recent study showed that when the rat ␣4/␤2 subunit ratio is varied, nAChRs of two functional classes are formed in oocytes (Zwart and Vijverberg, 1998). When the ␣4/␤2 ratio was 1:9, nAChRs were formed that were more sensitive to activation and desensitized more slowly. However, when the ratios were 1:1 or 9:1, nAChRs appeared that were less sensitive to activation and desensitized more rapidly. These findings raised the possibility that ␣4␤2 nAChRs can also exist in a stoichiometry that differs from (␣4) 2 (␤2) 3 .Here, we report that the majority (82%) of ␣4␤2 nAChRs expressed in a stable HEK cell line exhibit sensitivity to activation by ACh that is much lower (EC 50 ϭ 74 Ϯ 6 M) than when ␣4␤2 nAChRs are expressed in Xenopus laevis , 1997)]. This confirms the observations by Buisson and Bertrand (2001) in another independently derived line. They also reported that nicotine and other nicotinic agents increased the proportion of high-sensitivity nAChRs and speculated that the increase was caused by slow conversion of existing low-sensitivity nAChRs to high-sensitivity nAChRs. However, it is well known that nicotine, other nicotinic ...

Fragile X mental retardation is a prominent genetic disorder caused by the lack of the FMR1 gene product, a known RNA binding protein. Specific physiologic pathways regulated by FMR1 function have yet to be identified. Adult dfmr1 (also called dfxr) mutant flies display arrhythmic circadian activity and have erratic patterns of locomotor activity, whereas overexpression of dFMR1 leads to a lengthened period. dfmr1 mutant males also display reduced courtship activity which appears to result from their inability to maintain courtship interest. Molecular analysis fails to reveal any defects in the expression of clock components; however, the CREB output is affected. Morphological analysis of neurons required for normal circadian behavior reveals subtle abnormalities, suggesting that defects in axonal pathfinding or synapse formation may cause the observed behavioral defects.

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We have evaluated the role of the Drosophila mushroom bodies (MBs) in courtship conditioning, in which experience with mated females causes males to reduce their courtship toward virgins (Siegel and Hall, 1979). Whereas previous studies indicated that MB ablation abolished learning in an olfactory conditioning paradigm (deBelle and Heisenberg, 1994), MB-ablated males were able to learn in the courtship paradigm. They resumed courting at naive levels within 30 min after training, however, while the courtship of control males remained depressed 1 hr after training. We also describe a novel courtship conditioning paradigm that established long-term memory, lasting 9 days. In MB-ablated males, memory dissipated completely within 1 day. Our results indicate that the MBs are not required for learning and immediate recall of courtship conditioning but are required for consolidation of short-term and long-term associative memories.