Chronic antidepressant treatment enhances alpha-adrenergic and serotonergic responses in the facial nucleus

In addition to their well recognized activity in blocking uptake of biogenic amines, tricyclic antidepressants have recently been shown, with chronic treatment, to alter neurotransmitter receptor sensitivity. In this study, the responsiveness of facial motoneurons to norepinephrine (NE) and serotonin (5-HT) was assessed with single unit recording and microiontophoretic techniques. Treatment of rats with daily intraperitoneal injections of several clinically effective tricyclics for 14–20 days was found to enhance responses to NE, 5-HT, and to an intravenously administered 5-HT agonist, 5-MeODMT. These changes in sensitivity were not seen in animals chronically treated with saline, chlorpromazine, or fluoxetine, and thus appear specific to antidepressants. Acute effects of tricyclics on NE and 5-HT responses were variable, dependent on the specific drug tested, and appear to have no necessary relation to the pronounced sensitization produced by chronic treatment.