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Neurobiology used here- is a chronic treatment, allow­ ing the neural circuitry to develop compen­ Memory floxed satory mechanisms. Other hippocampal synapses support LTP, but (as these mice Richard G. M. Morris and Roger J. Morris show) they cannot compensate for defective CAl pyramidal neurons in supporting s the generations come and go, new ed currents in the CAl region, yet signalling learning. Amemories are made while old ones was normal in other parts of the hippocam­ Striking new findings were obtained fade away. The arrival of a new gener­ pus. Moreover, whereas long-term potentia­ using single-cell recording techniques. The ation of mouse mutants, together with tion (LTP) and long-term depression were skull of a 30-gram mouse might seem to be advances in their electrophysiological assess­ absent in the CAl neurons, short-term too small to support a headcap containing a ment, have now been heralded with the pub­ plasticity was intact. The mice also showed moveable electrode headstage and imped­ lication offour papers by Susumu Tonegawa, reduced spatial learning in a water-maze ance-reducing circuits. But in a technical Eric Kandel and their colleagues, in Cell' -4• (where the animal is required to use distal tour de force, McHugh et aC miniaturized The importance of these new approaches cues to swim to a platform that is hidden in a the device that was first developed by Bruce goes well beyond the data obtained - they large pool). McNaughton and John O'Keefe, and they are a step towards bridging the intellectual These findings confirm, with greater were rewarded for their efforts with two divide between in vivo systems and cellular anatomical precision, earlier studies using new results. First, although the CAl pyra­ accounts ofneuronal memory mechanisms. antagonists to the NMDA receptor, and they midal cells that fired in localized areas of Tonegawa and colleagues1- 3 have broken add one crucial piece of information. Gene space ('place fields') were clearly observed, new ground by creating a mouse in which a deletion - even with the Cre/loxP system the areas that they covered were larger in targeted gene (a subunit of an excitatory glu­ tamate receptor) has been inactivated, but Restricting gene inactivation to a subset of neurons only in specific neurons within a subregion of the hippocampus known as CAl. Many of Cre is a bacteriophage these neurons are 'place' cells: they fire recombinase that excises impulses only when the mouse is at a certain DNA between a pair of loxP NMDA R1 Neo location. In an analysis oflearning and place palindromic sequences X o , , o cells in the mutant mice, they find that they called loxP sites 11 • If the learn a spatial task poorly; that brain slices loxP sites are introduced from them show a precise regional deficit in into a mouse so as to an important form ofsynaptic plasticity; and flank a targeted gene that single-neuron recordings in the whole (referred to as CA 1 neuronsi\ All other neurons animals reveal peculiarities in the firing the floxed gene, for Cre expressed Cre not expressed pattern in the CAl region. Kandel and col­ 'flanked by loxP'), that leagues4 describe a mouse that has an engi­ gene will be excised in NMDARt neered alteration in synaptic plasticity of a cells that express active 0 , D o , , o Gene excised Gene functional different kind, but which also shows changes Cre'2. in the patterns ofcell firing. Transgenic (light blue) was added to normal unless the Cre The first generation of mouse mutants technology- pro-nuclear the 5' end. recombinase was that were developed to explore memory and injection of DNA into a The expression also expressed. The behaviour were created using homologous fertilized oocyte- is used pattern of the aCaMKI/ transgenic mice recombination to inactivate genes of to express Cre. The Cre promoter is influenced by were then produced interese. Powerful though this technique gene is placed under the where the construct by injecting the may be, the regionally and temporally unre­ control of regulatory inserts into the host floxed embryonic stem stricted nature of the resulting gene deletion elements that cause the genome - a factor that cells into a can lead to developmental defects or prema­ Cre protein to be varies in every transgenic blastocyst. ture death, and other pleiotropic effects can expressed only in line made. Fortunately, in The two elements play havoc with attempts to interpret the specific subsets of cells. one cell line, Cre was were brought together by effect ofthe deletion in the adult brain. Some Tonegawa's group1-3 expressed in almost all mating the mice, then of these problems are avoided by using the used the promoter for the CA 1 pyramidal neurons, backcrossing to obtain Cre!loxP system to restrict inactivation of a-subunit of and only in these the F2 recombinant the targeted gene to a specific subset of cells Ca2•-calmodulin protein cells. generation, in which (see panel). Building on Mark Mayford's kinase II (aCaMKI~ (a), a Tonegawa's group some mice were characterization of the a -calcium- calmod­ gene that is normally created a second line of homozygous for floxed ulin-dependent kinase II promoter expressed postnatally in transgenic mice by NMDAR1, and contained (aCaMKI[)6, Tsien etal.1'2 produced a mouse forebrain neurons'3. The homologous one copy of the Cre in which the gene for the Rl subunit oftheN­ promoter (red) was recombination in gene whose expression methyl-o-aspartate (NMDA) receptor was added upstream of the embryonic stem cells. was restricted to CA 1 excised only inCA 1 pyramidal neurons. Cre gene (dark green), The flanking loxP sites neurons (c). In these Having trapped their mouse, it was time and a nuclear localization were specifically mice, the NMDAR1 gene to get it to tell its tale. The Rl subunit medi­ signal (black) was added introduced into introns was excised (in the ates a slow-rising and long-lasting excitatory to the 5' end of Cre to so as not to inactivate third postnatal week of postsynaptic current that is critical for cer­ ensure the transport of the targeted (NMDAR1) life) only from CA 1 tain forms of synaptic plasticity7 and spatial Cre to the nucleus. A gene (b): so the floxed pyramidal neurons2 . learning8 in the hippocampus. The mutants polyadenylation signal gene was functionally R.J.M.&R.G.M.M. had no detectable NMDA-receptor-mediat-

680 NATURE IVO L 385120 FEBRUARY 1997