Recent studies have established the functional and molecular presence of a high-affinity peptide transporter, PEPT2, in whole tissue rat choroid plexus. However, the precise membrane location and directionality of PEPT2-mediated transport is uncertain at present. In this study, we examined the transport kinetics of a model dipeptide, glycylsarcosine (GlySar), along with the protein expression of PEPT2 using primary cell cultures of choroidal epithelium from neonatal rats. GlySar accumulation and transepithelial transport were 3 to 4 times higher when introduced from the apical as opposed to the basal side of the monolayers. GlySar apical uptake was also stimulated by an inwardly directed proton gradient. The uptake of GlySar was inhibited by di/tripeptides, carnosine, and ␣-amino cephalosporins but was unaffected by amino acids, cephalosporins lacking an ␣-amino group, and organic anions and cations. The Michaelis constant (K m ) of GlySar was 59.6 M for apical uptake and 1.4 mM for basal uptake; this is consistent with the high-affinity properties of PEPT2 at the apical membrane. Immunoblot analyses and immunofluorescent confocal microscopy demonstrated the presence of PEPT2, but not PEPT1, in rat choroid plexus epithelial cells. Moreover, PEPT2 was present in the apical and subapical regions of the cell but was absent in the basolateral membrane. These findings demonstrate, for the first time, that PEPT2 protein is present at the apical membrane of choroidal epithelial cells and that it is functionally active at this membrane surface. The results suggest that PEPT2 may have a role in the efflux of peptides and/or mimetics from cerebrospinal fluid to the blood.

5-Aminolevulinic acid (5-ALA) is a precursor of porphyrins and heme that has been implicated in the neuropsychiatric symptoms associated with porphyrias. It is also being used clinically to delineate malignant gliomas. The blood-CSF barrier may be an important interface for 5-ALA transport between blood and brain as in vivo studies have indicated 5-ALA is taken up by the choroid plexuses whereas the normal blood-brain barrier appears to be relatively impermeable. This study examines the mechanisms of 5-[ 3 H]ALA uptake into isolated rat lateral ventricle choroid plexuses. Results suggest that there are two uptake mechanisms. The first was a Na ϩ -independent uptake system that was pH dependent (being stimulated at low pH). Uptake was inhibited by the dipeptide Gly-Gly and by cefadroxil, an ␣-amino-containing cephalosporin. These properties are the same as the proton-dependent peptide transporters PEPT1 and PEPT2, which have recently been shown to transport 5-ALA in frog oocyte expression experiments. Choroid plexus uptake was not inhibited by captopril, a PEPT1 inhibitor, suggesting PEPT2-mediated uptake. The presence of PEPT2 and absence of PEPT1 in the choroid plexus were confirmed by western blotting. The second potential mechanism was both Na ϩ and HCO 3 Ϫ dependent and appears to be an organic anion transporter, although it is possible that removal of Na ϩ and HCO 3 Ϫ may indirectly affect PEPT2 by affecting intracellular pH. The presence of PEPT2 and a putative Na ϩ /HCO 3 Ϫ -dependent organic anion transporter is important not only for an understanding of 5-ALA movement between blood and brain but also because these transporters may affect the distribution of a number of drugs between blood and CSF.

MPH-MLR 80 mg provides a long-acting biphasic pattern of plasma MPH concentrations with one less peak and trough than Ritalin IR.

The publishers wish to apologise for an error which occurred in the above paper published in J. Neurochem. 89, pp375-382. A paragraph was printed incorrectly on page 377. The correct text is reproduced below.'For concentration-dependent studies in cell cultures, the data were best fit to the equation: carnosine uptake ¼ V max · C/(K m + C) + K d · C, where V max is the maximal rate of saturable carnosine uptake, K m is the Michaelis constant, C is the substrate (carnosine) concentration, and K d is the non-saturable rate constant. To evaluate if more than one class of transporters was operational for carnosine, a WoolfAugustinsson-Hofstee transformation of the saturable portion of uptake (V ¼ carnosine uptake -K d · C) was performed in which: V ¼ V max -K m · V/C.'

PEPT2 is functionally active and localized to the apical membrane of rat choroid plexus epithelial cells. However, little is known about the transport mechanisms of endogenous neuropeptides in choroid plexus, and the role of PEPT2 in this process. In the present study, we examined the uptake kinetics of carnosine in rat choroid plexus primary cell cultures and choroid plexus whole tissue from wild-type (PEPT2 +/+ ) and null (PEPT2 -/-) mice. Our results indicate that carnosine is preferentially taken up from the apical as opposed to basolateral membrane of cell monolayers, and that basolateral efflux in limited. Transepithelial flux of carnosine was not distinguishable from that of paracellular diffusion. The apical uptake of carnosine was characterized by a high affinity (K m ¼ 34 lM), low capacity (V max ¼ 73 pmol/mg protein/min) process, consistent with that of PEPT2. The non-saturable component was small (K d ¼ 0.063 lL/mg protein/min) and, under linear conditions, was only 3% of the total uptake. Studies in transgenic mice clearly demonstrated that PEPT2 was responsible for over 90% of carnosine's uptake in choroid plexus whole tissue. These findings elucidate the unique role of PEPT2 in regulating neuropeptide homeostasis at the blood-cerebrospinal fluid interface.