The inability of chronic mianserin to block central alpha 2-adrenoceptors.

An investigation was undertaken of the effect of long-term mianserin administration on the clonidine-induced decrease in rat brain MHPG-SO4 content. The dose of clonidine (0.1 mg/kg) chosen was one considered to selectively activate central alpha 2-adrenoceptors. Mianserin (10 mg/kg) given once daily for 9 and 15 days failed to alter the response to clonidine. This contrasts with the observations that acute and 5 day mianserin treatment blocks the clonidine-induced reduction in rat brain MHPG-SO4 levels. The possibility of mianersin-induced changes in the sensitivity of alpha 2-adrenoceptors is discussed. Chronic mianserin administration was associated with elevated levels of MHPG-SO4. This may have resulted from blockade of alpha 1-adrenoceptors.