ROLE OF L-TYPE CALCIUM CHANNELS ON YOHIMBINE-PRECIPITATED CLONIDINE WITHDRAWAL IN-VIVO AND IN-VITRO

This study was designed to elucidate the possible participation of L-t ype calcium channels in the expression of clonidine-withdrawal precipi tated by yohimbine in clonidine-dependent animals. Mice implanted for 5 days with osmotic minipumps containing the alpha2-adrenoceptor agoni st clonidine showed symptoms of a withdrawal syndrome (jerks, headshak es, defecations and weight loss) when yohimbine, an alpha2-adrenocepto r antagonist, was injected. Similarly, isolated rat ilea incubated wit h clonidine in vitro showed a withdrawal contracture when yohimbine wa s added to the organ bath. The effects of L-type calcium channel block ers (verapamil and diltiazem) and the stimulant Bay K 8644 on these tw o different types of withdrawal responses were evaluated. A dose-depen dent decrease in yohimbine-precipitated clonidine withdrawal in vivo w as observed when verapamil (10-40 mg/kg, sc. and 120 gg/mouse, i.cv.) or diltiazem (5 - 20 mg/kg, sc. and 160 mug/mouse, i.c.v.) were admini stered to mice dependent on clonidine. No effect was found after Bay K 8644 (0.5 - 5 mg/kg, s.c. and 1-5 mug/mouse) was injected under these conditions. In vitro, both verapamil (0.1-5 muM) and D-CiS-diltiazem (1 - 50 muM) concentration-dependently reduced the height of the yohim bine-precipitated withdrawal contracture in rat ileum incubated with c lonidine. Furthermore, the effect of diltiazem was stereospecific, as D-cis-diltiazem 10 muM markedly inhibited clonidine withdrawal, wherea s the same concentration of L-CiS-diltiazem had no effect. In contrast , the calcium channel stimulant Bay K 8644 (0.1 - 1 muM) increased the height of the ileum withdrawal contrature. These results confirm that yohimbine-precipitated clonidine withdrawal can be obtained both in v ivo and in vitro, and suggest that the expression of these abstinence responses involves activation of L-type calcium channels. The present results, together with those of previous studies of the effects of cal cium channel-acting drugs on ethanol-, opiate- and benzodiazepine-with drawal, suggest that L-type calcium channels play an important role in the expression of the withdrawal responses to CNS depressant drugs.