Cerebral capillary sequestration and blood‐brain barrier (BBB) permeability to apolipoproteins E2 (apoE2), E3 (apoE3), and E4 (apoE4) and to their complexes with sAβ1–40, a peptide homologous to the major form of soluble Alzheimer's amyloid β, were studied in perfused guinea pig brain. Cerebrovascular uptake of three apoE isoforms was low, their blood‐to‐brain transport undetectable, but uptake by the choroid plexus significant. Binding of all three isoforms to sAβ1–40 in vitro was similar with a KD between 11.8 and 12.9 nM. Transport into brain parenchyma and sequestration by BBB and choroid plexus were negligible for sAβ1–40‐apoE2 and sAβ1–40‐apoE3, but significant for sAβ1–40‐apoE4. After 10 min, 85% of sAβ1–40‐apoE4 taken up at the BBB remained as intact complex, whereas free sAβ1–40 was 51% degraded. Circulating apoE isoforms have contrasting effects on cerebral capillary uptake of and BBB permeability of sAβ. ApoE2 and apoE3 completely prevent cerebral capillary sequestration and blood‐to‐brain transport of sAβ1–40. Conversely, apoE4, by entering brain microvessels and parenchyma as a stable complex with sAβ, reduces peptide degradation and may predispose to cerebrovascular and possibly enhance parenchymal amyloid formation under pathological conditions.

We previously demonstrated that metaphit (a phencyclidine analogue with an acylating isothiocyanate group) induces occurrence of audiogenic seizures in mice exposed to audio stimulation 24 h after metaphit administration. We have studied various receptor systems associated with excitatory and inhibitory networks: sites for competitive and noncompetitive antagonists of the N-methyl D-aspartic acid (NMDA) receptor complex, for [3H]muscimol on the gamma-aminobutyric acid (GABA) receptor complex, and for [3H]batrachotoxinin A20-alpha-benzoate on the voltage-dependent sodium channel. Mice were examined for neurochemical changes at 24 h after pretreatment with metaphit, when susceptibility to audiogenic seizures is greatest. Ex vivo receptor binding studies detected no changes; in vivo labeling of the phencyclidine site in the NMDA receptor complex was reduced by 20% in cortical and midbrain regions. A separate group of experiments was aimed at measuring brain levels of metaphit. One minute after retroorbital administration of [3H]metaphit at a dose sufficient to produce susceptibility to audiogenic seizures 24 h later, the brain level of [3H]metaphit (determined by high-performance liquid chromatography, HPLC) was 49 pmol/mg tissue; at 1, 4, and 24 h, the level was 12, 6, and 1.4 pmol/mg tissue or microM if metaphit was evenly distributed throughout the brain. Although the observed metaphit concentrations during the first 4 h are high enough to acylate receptors, no firm evidence for acylation was found for most of the examined receptors. Finally, the time course of the brain level of metaphit showing a continuous decrease is entirely different from that of development of the seizure susceptibility, which peaks at 18-24 h.

Although cellular reprogramming continues to generate new cell types, reprogramming remains a rare cellular event. The molecular mechanisms that limit reprogramming, particularly to somatic lineages, remain unclear. By examining fibroblast-to-motor neuron conversion, we identify a previously unappreciated dynamic between transcription and replication that determines reprogramming competency. Transcription factor overexpression forces most cells into states that are refractory to reprogramming and are characterized by either hypertranscription with little cell division, or hyperproliferation with low transcription. We identify genetic and chemical factors that dramatically increase the number of cells capable of both hypertranscription and hyperproliferation. Hypertranscribing, hyperproliferating cells reprogram at 100-fold higher, near-deterministic rates. We demonstrate that elevated topoisomerase expression endows cells with privileged reprogramming capacity, suggesting that biophysical constraints limit cellular reprogramming to rare events.

(a) A significant cerebrovascular permeability to intact DA is mediated by a MAO-B independent specific transport system at the BBB, (b) this system could be inhibited by D1 and D2 DA receptor antagonists, and (c) DA blood-to-brain transport was inhibited by nicotine.