Benzodiazepine binding sites in rat interscapular brown adipose tissue: Effect of cold environment, denervation and endocrine ablations

Abstract

3H-Flunitrazepam (FNZP) binding was examined in a crude membrane fraction obtained from rat interscapular brown adipose tissue (IBAT). A single population of binding sites was apparent with dissociation constant (K D) = 0.47 ± 0.04 um and maximal number of binding sites (B max) = 31 ± 5 pmol.mg prot −1. From the activity of several benzodiazepine (BZP) analogs to compete for the binding, the peripheral nature of FNZP binding was tentatively established. Similar BZP binding sites were detectable in isolated IBAT mitochondria. Exposure of rats to 4°C for 15 days decreased B max significantly without affecting K D. Cold-induced decrease in B max of BZP binding was prevented by surgical IBAT denervation. Denervation prevented or impaired the increased activity of the mitochondrial markers succinate dehydrogenase and malate dehydrogenase in IBAT of cold-exposed rats, but did not affect monoamine oxidase activity. Hypophysectomy of rats decreased significantly both K D and B max of IBAT BZP binding. Thyroidectomy, adrenalectomy or ovari ectomy did not affect IBAT BZP binding parameters. The BZP analogs diazepam, clonazepan and Ro 5–4864 decreased significantly quanosine 5' -diphosphate binding (GDP) in IBAT mitochondria while co-incubation of Ro 5–4964 or clonazepam with the peripheral type BZP antagonist PK 11195 did not modify BZP activity on GDP binding. Our results indicate that BZP binding in rat IBAT may belong to the peripheral type, is decreased by a cold environment through activation of peripheral sympathetic nerves and is affected by hypophysectomy. BZP and GDP binding in IBAT mitochondria seem not to be functionally related.