Behavioural response to SKF 38393 and quinpirole following chronic antidepressant treatment

Stressors can affect immobility time and response to imipramine in the rat forced swim test

We subjected Wistar rats to the forced swim test (FST) to compare the effects of two doses of imipramine in physically stressed rats (P: unavoidable electric footshocks), emotionally stressed rats (E: odors), or non-stressed rats (C). Stress or control sessions lasted 35 days. Drug treatments began on day 21 and continued for the next 14 days. E rats were placed for 10 min, once per day for 35 days, in a small non-movement-restricting cage impregnated with urine collected from a P rat. E and P rats exhibited opposite changes in locomotion. After 21 days of stress sessions, P rats displayed the longest immobility times in the FST, followed by E rats. In the P group, on day 7 of treatment (day 28 of the study), imipramine (2.5 mg/kg) reduced immobility time to baseline values. In the E group, immobility time decreased only after 14 days of treatment with the low imipramine dose. The high dose of imipramine (5.0 mg/kg) reduced immobility time at day 7 of treatment in all groups. In conclusion, physical and emotional stress similarly increased immobility time in the FST, but emotional stress appears to be more resistant to imipramine treatment.

The role of dopamine in the mechanism of action of antidepressant drugs

The present paper reviews evidence on the effect of antidepressant treatments on dopamine transmission. Chronic treatment with antidepressant drugs potentiates the behavioural stimulant responses elicited by the stimulation of dopamine receptors, including reward-related behaviours. Moreover, antidepressants affect dopamine release in several brain areas. The reviewed literature is discussed in terms of the possible mechanisms underlying antidepressant-induced supersensitivity to dopamine-mediated behavioural responses, and of the possible implications for the therapeutic effect of these drugs. It is concluded that the potentiation of dopaminergic neurotransmission induced by chronic antidepressant treatments might contribute to their therapeutic effect.

Repeated administration of fluoxetine, desipramine and tranylcypromine increases dopamine D2-like but not D1-like receptor function in the rat

We tested the effect of repeated treatment (twice daily for 14 days) of rats with the antidepressant drugs fluoxetine, desipramine and tranylcypromine, on the behavioural response to the non-selective dopamine (DA) receptor agonist, apomorphine, the D1-like receptor agonists, SKF 38393 and SKF 81297 and the D2-like receptor agonists, RU 24213 and quinpirole. Agonist-induced behaviour was monitored by automated activity meters and direct observation using a checklist scoring method. Fluoxetine, desipramine and tranylcypromine enhanced (albeit to a varying degree) the behavioural responses to apomorphine (0.75 mg/kg, s.c.), quinpirole (0.25 mg/kg, s.c.) and RU 24213 (0.75 mg/kg, s.c.). In contrast, fluoxetine, desipramine and tranylcypromine did not increase the behavioural responses to SKF 38393 (7.5 mg/kg, s.c.) and SKF 81297 (0.5 mg/kg, s.c.). Finally, fluoxetine, despiramine and tranylcypromine did not modify the behavioural responses to the concomitant administration of SKF 38393 (7.5 mg/kg, s.c.) and quinpirole (0.25 mg/kg, s.c.). Our data suggest that repeated administration of fluoxetine, desipramine and tranylcypromine increases central DA D2-like but not D1-like receptor function.

Monoamine oxidase inhibitor sensitive site implicated in sensitization to quinpirole

Clorgyline (1.0 mg/kg/day) administered via osmotic minipumps blocked the development of locomotor sensitization to the dopamine receptor agonist quinpirole (0.5 mg/kg every 3 days for 8 injections). In male rats already well sensitized to quinpirole, the continuous infusion of clorgyline (1.0 mg/kg/day for 28 days) produced a progressive decline in locomotor activity, despite a continued regimen of quinpirole injections (0.5 mg/kg every 3 days). It is suggested that the development, as well as the maintenance, of locomotor sensitization to quinpirole is modulated by the activation of an monoamine oxidase inhibitor-sensitive site. This site may be a dopamine D2 receptor-linked monoamine oxidase inhibitor-displaceable quinpirole binding site, the enzyme monoamine oxidase-A, or other clorgyline binding sites.

Indirect dopamine agonists effects on despair test: dissociation from hyperactivity

Both dexamphetamine and the pure dopamine reuptake inhibitor GBR 12783 elicit a stimulation of locomotion and increase swimming activity in the behavioral despair test in mice. The dopamine D1 dopamine receptor antagonist SCH 23390 dose dependently (7.5-30 micrograms/kg s.c.) antagonized the stimulant locomotor effect on both drugs but did not prevent their antiimmobility effect on the behavioral despair test. The D2 dopamine receptor antagonist haloperidol dose dependently (12.5-50 micrograms/kg i.p.) antagonized the effects of dexamphetamine on both locomotor activity and behavioral despair test. By contrast, haloperidol inhibited the effects of GBR 12783 in the forced swimming test but not on locomotion. It is concluded that indirect dopamine agonists are effective on the behavioral despair test independently of a stimulation of locomotor activity. Their effects on the despair test depend on the stimulation of D2 but not D1 dopamine receptors.