No correlation exists between antidepressant activity and the ability of 5-HT uptake inhibitors to interact with 5-HT receptors of the rat stomach fundus strip

Rapid changes in functional responsiveness of the 5-HT system after single-dose and multiple-dose treatment with antidepressants: effects of maprotiline and oxaprotiline and its enantiomers

Functional responsiveness to 5-HTP (L-5-hydroxytryptophan) and quipazine 24 h after one single dose and 24 h and 72 h after repeated administration (2.5 and 10 mg/kg twice daily for 7 days) of maprotiline, oxaprotiline and its (+) and (-) enantiomers was tested in the rat. All compounds markedly increased the behavioural effects of 5-HTP 24 h after one single injection. This effect was further enhanced after repeated treatment. The response to quipazine was less consistently affected than the response to 5-HTP. Neither drug increased the response to quipazine 24 h after the last injection. However, maprotiline and oxaprotiline enhanced the quipazine-induced head-twitch 72 h after the withdrawal of subchronic treat ment, enantiomers failed to do so. This differential effect of drugs in respect to the interactions with 5-HTP and quipazine is, however, not understood at present. The results of this study indicate that only one single dose of antidepressants may change the functional respon siveness of the 5-HT system and that such changes occur rapidly. The results also provide a possible rationale for the suggested antidepressant activity of levoprotiline, the (-) enantiomer of oxaprotiline and otherwise biochemically inactive compound.

Inhibition of head twitch response to quipazine in rats by chronic amitriptyline but not fluvoxamine or citalopram

Chronic (twice daily/14 days), but not acute, treatment with 10 mg/kg PO amitriptyline reduced the number of quipazine (5 mg/kg)-induced head twitches in rats, measured 2 h (but not 72 h) after the last administration of the drug. Similar treatment with fluvoxamine or citalopram, which are more potent and much more specific serotonin uptake inhibitors than amitriptyline, did not affect the quipazine-induced response. In acute experiments, fluvoxamine (10 mg/kg PO) and citalopram (10 mg/kg PO) potentiated the head twitch reaction induced by L-5-hydroxytryptophan (50 mg/kg IP) given together with Ro 4-4602 (25 mg/kg IP), a peripheral decarboxylase inhibitor. Amitriptyline (10 mg/kg PO) slightly decreased the number of L-5-hydroxytryptophan (5-HTP)-induced head twitches. Higher doses of amitriptyline (20-40 mg/kg PO) also inhibited the quipazine-induced head twitch reaction. The brain level of amitriptyline measured 0.5-24 h after the last oral administration of the chronic dose of 10 mg/kg was always much higher than that observed at the same time intervals after an acute oral dose of 20 or 40 mg/kg. The results obtained indicate that a postsynaptic rather then presynaptic mechanism is responsible for the development of subsensitivity of the central serotonin receptors in the course of chronic treatment with amitriptyline.