airment in behavior, including lethargy, motor coordination problems, and deficits in exploratory activity [51], which preclude detailed behavioral analyses. Similarly, mice expressing (in the entire brain) a substantial part of the GluR-B population in the Q/R site-unedited form become seizure-prone and die prematurely [53]. Some of these problems can be partially overcome by use of spatially and temporally restricted expression systems [54–56], in particular the Cre-lox system, with Cre-recombinase expression in defined brain areas of gene-targeted mice carrying GluR-B alleles marked by loxP sites for Cre-mediated recombination [55,57]. Indeed, restricting the expression of Q/R site-unedited GluR-B to forebrain resulted in almost normal lifespan and an only weakly seizure-prone phenotype [58]. Mice with forebrain-specific GluR-B depletion appeared almost completely normal throughout life with no developmental abnormalities, thus permitting a detailed, quantitative investigation of olfactory behavior.
To allow for the mechanistic separation of olfactory learning, discrimination, and memory, we exploited a well-known phenomenon of transgenes, which concerns heterogeneous expression among different founder lines and even among genetically identical individuals of a given line. Although such “mosaic” expression is usually undesired, here we took advantage of it by ablating GluR-B via gene-targeted, floxed GluR-B alleles with the help