Brain Research, 210 (1981) 413-418 413 © Elsevier/North-Holland Biomedical Press ¢z-Adrenoreceptors in rat brain are decreased after long-term tricyclic antide- pressant drug treatment CHARLES B. SMITH, JESISISA. GARCIA-SEVILLA and PEGGIE J. HOLLINGSWORTH Department of Pharmacology, University of Michigan Medical School, Ann Arbor, Mich. 48109 (U.S.A.) (Accepted November 13th, 1980) Key words: a~-adrenoreceptor - - chronic amitriptyline - - tricyclic antidepressant After twp weeks of twice-daily administration of amitriptyline to rats, the binding of [3H]clonidine to presynaptic a2-adrenoreceptors was decreased in membranes isolated from 5 areas of the rat brain. After one day of treatment, binding did not differ from saline treated controls. In vitro, a high concentration of amitriptyline caused a competitive inhibition of [aH]clonidine binding but did not alter the number of binding sites. The decrease in the number of a~-adrenoreceptor binding sites after two weeks of amitriptyline treatment would explain the subsensitivity of these receptors which occurs after prolonged administration of antidepressant drugs. An important mechanism by which the neuronal release of norepinephrine is regulated is by stimulation of a presynaptic autoreceptor, the a2-inhibitory adreno- receptor 5,12. When this receptor is stimulated, the further release of noradrenaline is inhibited which thus provides a means for autoregulation of the amount of norepine- phrine within the synaptic cleft. Recently, we reported that the administration of various antidepressant drugs for 2-3 weeks leads to an increased norepinephrine release from adrenergic neurons in the isolated rat left atrium which is secondary to the development of subsensitivity of the presynaptic a2-adrenoreceptor2,a. This subsensitivity of the presynaptic a2-adrenoreceptor might be an important mechanism by which antidepressant drugs could enhance the neuronal release of norepinephrine. The purpose of the present study was to determine whether the long-term admini- stration of amitriptyline, a widely used tricyclic antidepressant, causes changes in presynaptic a2-adrenoreceptors in various areas of the rat brain which could result in increased norepinephrine release. Receptor binding techniques provide a powerful means of evaluating changes in the number and/or affinities of receptors. [aH]Cloni- dine is an agent which has high affinity for the presynaptic a2-adrenoreceptor4,16. In order to evaluate changes in this receptor, we studied the specific binding of [aH]clonidine to receptors upon neural membranes prepared from homogenates of various areas of the rat brain. Homogenates were made from the following brain areas isolated from male, Sprague-Dawley rats (220-240 g): amygdala, hippocampus, anterior caudate nucleus,