Male and female Fmr1 knockout mice on C57 albino background exhibit spatial learning and memory impairments.

Impaired spatial learning is a prominent deficit in fragile X syndrome (FXS). Previous studies using the Fmr1 knockout (KO) mouse model of FXS have not consistently reported a deficit in spatial learning. Fmr1 KO mice bred onto an albino C57BL/6J-Tyr(c-Brd) background showed significant deficits in several primary measures of performance during place navigation and probe trials in the Morris water maze. Fmr1 KO mice were also impaired during a serial reversal version of the water maze task. We examined fear conditioning as an additional cognitive screen. Knockout mice exhibited contextual memory deficits when trained with unsignaled shocks; however, deficits were not found in a separate group of KO mice trained with signaled shocks. No potentially confounding genotypic differences in locomotor activity were observed. A decreased anxiety-like profile was apparent in the open field, as others have noted, and also in the platform test. Also as previously reported, startle reactivity to loud auditory stimuli was decreased, prepulse inhibition and social interaction increased in KO mice. Female Fmr1 KO mice were tested along with male KO mice in all assays, except for social interaction. The female and male KO exhibited very similar impairments indicating that sex does not generally drive the behavioral symptoms of the disorder. Our results suggest that procedural factors, such as the use of albino mice, may help to reliably detect spatial learning and memory impairments in both sexes of Fmr1 KO mice, making it more useful for understanding FXS and a platform for evaluating potential therapeutics.

Netrin-G2 and netrin-G2 ligand are both required for normal auditory responsiveness.

Mice in which netrin-G2 has been genetically inhibited do not startle to an acoustic stimulus, but otherwise perform normally through a behavioral test battery. Light microscopic examination of the inner ear showed no obvious structural abnormalities. Brainstem responses to acoustic stimuli (auditory brainstem responses, ABR) were also present, confirming the lack of any overarching defects in the inner ear or auditory nerve. Genetic inhibition of netrin-G2 ligand produced a nearly identical phenotype, that is, no startle with ABR present, and otherwise normal. This similarity confirms that these two proteins act in the same biological pathway. We have also determined that the affinity between the two proteins is strong, around 2.5 nM, similar to that observed between netrin-G1 and netrin-G1 ligand - 2.3 nM in our hands. The combination of equivalent phenotypes when genetically inhibited coupled with evidence of a strong biochemical interaction supports the notion of a receptor-ligand interaction between these two proteins in vivo. This interaction is critical for auditory synaptic responsiveness in the brain.

Nicotinic agonists competitively antagonize serotonin at mouse 5-HT3 receptors expressed in Xenopus oocytes.

The 5-HT3 receptor (5-HT3R) is part of a superfamily of ligand-gated ion channels which includes nicotinic acetylcholine receptors (nAChR). cRNA derived from the long isoform cloned mouse 5-HT3R was used to drive expression of 5-HT3Rs in Xenopus oocytes. 5-HT-induced currents were monitored using two-electrode voltage-clamp. Eight nicotinic agonists, including ACh and nicotine, but not alpha-anatoxin, were found to antagonize 5-HT-induced currents. With the exception of 3-(2,4)-dimethoxybenzylidene-anabaseine (DMXB-anabaseine; GTS-21) this antagonism appeared to be competitive since it could be overcome by increasing concentrations of 5-HT. Potency of 5-HT3 antagonism was comparable to reported values for nAChR alpha7 activation. These results confirm the notion of families of receptors and further indicate that strong similarities can exist in some critical binding domains.