No evidence for binding of clozapine, olanzapine and/or haloperidol to selected receptors involved in body weight regulation.

The underlying mechanisms of antipsychotic (AP)-induced weight gain are unknown, but both central and peripheral AP target receptors could potentially be involved. This study used radioligand binding assays to compare the binding affinities of clozapine, olanzapine and haloperidol for candidate receptors potentially involved in AP-induced weight gain. Selected candidates derived from known pathways involved in body weight regulation included receptors classified as anorexigenic (bombesin receptor subtype 3, calcitonin gene-related peptide receptor, cholecystokinin receptor, melanocortin-4 receptor, neurotensin receptor 1) or orexigenic (cannabinoid receptor 1, galanin 1 receptor, melanin-concentrating hormone receptor (MCHR), neuropeptide Y1 receptor) as well as receptors involved in physiological actions related to digestion and fluid homeostasis (angiotensin II type 1 receptor, bradykinin B2 receptor, endothelin receptor, neurokinin 1 receptor, vasoactive intestinal polypeptide receptor 1). Clozapine, olanzapine and haloperidol exhibited negligible affinities to all of these receptors except for the MCHR (Ki=501 nM; haloperidol). With respect to other candidates from (neuro)transmitter systems already suggested to be involved in AP-induced weight gain, the binding profile of olanzapine resembled that of clozapine, with high affinity (Ki<10 nM) for serotonin (5-HT) 5-HT2A, 5-HT2C and 5-HT6, muscarinic M1 and histamine H1 receptors. In contrast, the binding profile of haloperidol was substantially different (high affinity only for the dopamine D1 receptor). In conclusion, we have not identified a novel binding site of the two investigated atypical AP that could contribute to the induced weight gain.

Functional asymmetry of the sodium-D-glucose cotransporter expressed in yeast secretory vesicles.

The sodium-D-glucose cotransporter (SGLT1) was expressed in a yeast mutant strain NY 17 (sec6-4) that accumulates secretory vesicles at a nonpermissive temperature because of a block in the delivery of these vesicles to the plasma membrane. By differential centrifugation a microsomal fraction enriched in secretory vesicles was prepared with a high specific activity of the vanadate-sensitive H+-ATPase and invertase. In this membrane fraction one protein band of an apparent molecular weight of 55 kDa representing the nonglycosylated SGLT1 protein could be detected by immunochemical analysis. In addition, higher molecular weight protein bands probably representing dimers and aggregates were found. In transport studies with the microsomes D-glucose fluxes showed asymmetric properties: efflux experiments revealed the typical properties of the SGLT1 such as sodium dependence, inhibition by phlorizin and potential dependence. Influx of D-glucose showed no dependence on sodium and was not inhibited by phlorizin. Furthermore, the transporter exhibited a striking asymmetry with regard to the D-glucose affinity and the sugar specificity. These results suggest that the orientation of the SGLT1 expressed in yeast secretory vesicles is, indeed, inverted with regard to its configuration in the plasma membrane of epithelial cells. Moreover, there are striking functional differences between the periplasmic and cytoplasmic face of the transporter.