It is widely believed that the brain processes information and stores memories by modifying and stabilizing synaptic connections between neurons. In experimental models of synaptic plasticity, such as long-term potentiation (LTP), the stabilization of changes in synaptic strength requires rapid de novo RNA and protein synthesis. Candidate genes, which could underlie activity-dependent plasticity, have been identified on the basis of their rapid induction in brain neurons. Immediate-early genes (IEGs) are induced in hippocampal neurons by high-frequency electrical stimulation that induces LTP and by behavioral training that results in long-term memory (LTM) formation. Here, we investigated the role of the IEG Arc (also termed Arg3.1) in hippocampal plasticity. Arc protein is known to be enriched in dendrites of hippocampal neurons where it associates with cytoskeletal proteins (Lyford et al., 1995). Arc is also notable in that its mRNA and protein accumulate in dendrites at sites of recent synaptic activity (Steward et al., 1998). We used intrahippocampal infusions of antisense oligodeoxynucleotides to inhibit Arc protein expression and examined the effect of this treatment on both LTP and spatial learning. Our studies show that disruption of Arc protein expression impairs the maintenance phase of LTP without affecting its induction and impairs consolidation of LTM for spatial water task training without affecting task acquisition or short-term memory. Thus, Arc appears to play a fundamental role in the stabilization of activity-dependent hippocampal plasticity.

Imaging recognition of multidrug-resistance by 99mTc-labeled sestamibi, tetrofosmin and furifosmin in mice bearing human breast tumors was evaluated using a high-resolution SPECT, FASTSPECT. Imaging results showed that the washout rates in drug-resistant MCF7/D40 tumors were significantly greater than that in drug-sensitive MCF7/S tumors. Furifosmin exhibited greater washout from both MCF7/S and MCF7/D40 than sestamibi, while tetrofosmin washout was greater than sestamibi in MCF7/D40 only. Feasibility of the monocationic agents for characterizing MDR expression was well clarified with FASTSPECT imaging.

Background-99m Tc-sestamibi (MIBI) and 99m Tc-tetrofosmin (TF) are avid transport substrates recognized by the multidrug resistance (MDR) P-glycoprotein (Pgp). This study was designed to compare the properties of MIBI and TF in assessing the inhibition of Pgp by PSC833 in severe combined immunodeficient mice bearing MCF7 human breast tumors using SPECT imaging.Methods-Animals with drug-sensitive (MCF/WT) and drug-resistant (MCF7/AdrR) tumors were treated by PSC833 and by carrier vehicle 1 h before imaging, respectively. Dynamic images were acquired for 30 min after intravenous injection of MIBI/TF using a SPECT system, FastSPECT. The biodistribution of MIBI and TF was determined at the end of the imaging session.Results-MCF7/WT in the absence and presence of PSC833 could be visualized by MIBI and TF imaging within 5 min and remained detectable for 30 min postinjection. MCF7/AdrR could be visualized only 2-5 min without PSC833 treatment but could be detected for 30 min with PSC833, very similar to MCF7/WT. MCF7/AdrR without PSC833 showed significantly greater radioactive washout than MCF7/WT and MCF7/AdrR with PSC833 treatment. PSC833 increased the accumulation (%ID/g) in MCF7/AdrR 3.0-fold (1.62 ± 0.15 vs. 0.55 ± 0.05, P <.05) for TF and 1.9-fold (1.21 ± 0.04 vs. 0.64 ± 0.05, P <.05) for MIBI but did not affect MCF7/WT.Conclusions-The feasibility of MIBI and TF for assessment of MDR expression and inhibition was demonstrated in mice through FastSPECT imaging. The results indicate that TF may be at least comparable with MIBI in recognizing Pgp expression and modulation.

Objectives Prompt identification of necrosis and apoptosis in the infarct core and penumbra region is critical in acute stroke for delineating the underlying ischemic/reperfusion molecular pathologic events and defining therapeutic alternatives. The objective of this study was to investigate the capability of 99mTc-labeled duramycin in detecting ischemia-reperfusion injury in rat brain after middle cerebral artery (MCA) occlusion. Methods Ischemic cerebral injury was induced in ten rats by vascular insertion of a nylon suture in the left MCA for 3 hr followed by 21–24 hr reperfusion. After i.v. injection of 99mTc-duramycin (1.0–3.5 mCi), dynamic cerebral images were acquired for 1 hr in six rats using a small-animal SPECT imager. Four other rats were imaged at 2 hr post-injection. Ex vivo images were obtained by autoradiography after sacrifice. Histologic analyses were performed to assess cerebral infarction and apoptosis. Results SPECT images showed that 99mTc-duramycin uptake in the left cerebral hemisphere was significantly higher than that in the right at 1 and 2 hr post-injection. The level of radioactive uptake in the ischemic brain varied based on ischemic severity. The average ratio of left cerebral hot-spot uptake to right hemisphere radioactivity, as determined by computerized ROI analysis, was 4.92±0.79. Fractional washout at 1 hr was 38.2±4.5% of peak activity for left cerebral hot-spot areas and 80.9±2.0% for remote control areas (P<0.001). Based on triphenyltetrazolium chloride staining and autoradiograph image data, the hotspot uptake may be associated primarily with the ischemic penumbra, in which high apoptotic activity was observed by cleaved caspase-3 immunocytochemical staining. Conclusions 99mTc-duramycin SPECT imaging may be useful for detecting and quantifying ongoing apoptotic neuronal cell loss induced by ischemia-reperfusion injury.

Freezing (immobility) in the presence of aversive stimuli is a species-specific behavior that is used as an operational measure of fear. Conditioning of this response to discrete sensory stimuli and environmental context cues has been used as a tool to study the neuropsychology of memory dynamics and their development over the lifespan. Three age groups of F344 rats (3, 9, and 27 month) received tone–foot shock pairing (or tone only) in a distinctive chamber on two consecutive days. Separate subgroups of rats from each age group were then tested, at retention intervals of 1, 20, 40, or 60 days, for context-mediated fear in the environment in which they were trained, for generalization of the fear response to a novel chamber, and for fear of the tone. Beginning at day 20, the 27-month-old rats exhibited less freezing behavior than did younger rats when tested in the conditioning context. This age difference was a result of freezing behavior becoming progressively stronger with time in the two younger age groups, a phenomenon that has been referred to as memory incubation. Incubation of the contextual fear response was not detected in the old rats. In a novel context, all age groups exhibited significantly more freezing than did control animals. There was also pronounced incubation of this generalized freezing response, and the extent of incubation declined significantly with age. In the novel context, the freezing response to the tone was robust in all age groups and increased over time, in constant proportion to the degree of freezing elicited by the novel context itself, prior to tone onset. The fact that old animals are known to be relatively selectively impaired in forms of memory that depend on a functional hippocampus suggests a possible explanation for the reduced incubation effects seen in old rats; however, whether the increased expression of fear over time is mediated by a hippocampal-dependent memory consolidation process or whether it reflects a generalized increase in the gain of the circuitry mediating the fear response itself, remains to be determined.