Differential modulation of alpha2-adrenoceptor subtypes in rat kidney by chronic desipramine treatment

Postsynaptic alpha-2 adrenergic receptors are critical for the antidepressant-like effects of desipramine on behavior

The antidepressant desipramine inhibits the reuptake of norepinephrine (NE), leading to activation of both pre- and postsynaptic adrenergic receptors, including alpha-1, alpha-2, beta-1, and beta-2 subtypes. However, it is not clear which adrenergic receptors are involved in mediating its antidepressant effects. Treatment of mice with desipramine (20 mg/kg, i.p.) produced an antidepressant-like effect, as evidenced by decreased immobility in the forced-swim test; this was antagonized by pretreatment with the alpha-2 adrenergic antagonist idazoxan (0.1-2.5 mg/kg, i.p.). Similarly, idazoxan, administered peripherally (0.5-2.5 mg/kg, i.p.) or centrally (1-10 microg, i.c.v.), antagonized the antidepressant-like effect of desipramine in rats responding under a differential-reinforcement-of-low-rate (DRL) 72-s schedule, ie, decreased response rate and increased reinforcement rate. By contrast, pretreatment with the beta-adrenergic antagonists propranolol and CGP-12177 or the alpha-1 adrenergic antagonist prazosin did not alter the antidepressant-like effect of desipramine on DRL behavior. The lack of involvement of beta-adrenergic receptors in mediating the behavioral effects of desipramine was confirmed using knockout lines. In the forced-swim test, the desipramine-induced decrease in immobility was not altered in mice deficient in beta-1, beta-2, or both beta-1 and beta-2 adrenergic receptors. In addition, desipramine (3-30 mg/kg) produced an antidepressant-like effect on behavior under a DRL 36-s schedule in mice deficient in both beta-1 and beta-2 adrenergic receptors. As antagonism of presynaptic alpha-2 adrenergic receptors facilitates NE release, which potentiates the effects of desipramine, the present results suggest that postsynaptic alpha-2 adrenergic receptors play an important role in its antidepressant effects.

Norepinephrine transporter knockout-induced up-regulation of brain alpha2A/C-adrenergic receptors

The norepinephrine transporter (NET) is responsible for the rapid removal of norepinephrine released from sympathetic neurons; this release is controlled by inhibitory alpha(2)-adrenergic receptors (alpha(2)ARs). Long-term inhibition of the NET by antidepressants has been reported to change the density and function of pre- and postsynaptic ARs, which may contribute to the antidepressant effects of NET inhibitors such as desipramine. NET-deficient (NET-KO) mice have been described to behave like antidepressant-treated mice. By means of quantitative real-time PCR we show that mRNAs encoding the alpha(2A)-adrenergic receptor (alpha(2A)AR) and the alpha(2C)-adrenergic receptor (alpha(2C)AR) are up-regulated in the brainstem, and that alpha(2C)AR mRNA is also elevated in the hippocampus and striatum of NET-KO mice. These results were confirmed at the protein level by quantitative autoradiography. The NET-KO mice showed enhanced binding of the selective alpha(2)AR antagonist [(3)H]RX821002 in several brain regions. Most robust increases (20-25%) in alpha(2)AR expression were observed in the hippocampus and in the striatum. Significant increases (16%) were also seen in the extended amygdala and thalamic structures. In an 'in vivo' test, the alpha(2)AR agonist clonidine (0.1 mg/kg) caused a significantly greater reduction of locomotor activity in NET-KO mice than in wild-type mice, showing the relevance of our findings at the functional level.

Effect of desipramine on Ca2+ levels and growth in renal tubular cells

The in vitro effect of desipramine on renal tubular cell is unknown. In Madin-Darby canine kidney (MDCK) cells, the effect of desipramine on intracellular Ca2+ concentration ([Ca2+]i) was measured by using fura-2. Desipramine (>25 microM) caused a rapid and sustained rise of [Ca2+]i in a concentration-dependent manner (EC50=50 microM). Desipramine-induced [Ca2+]i rise was prevented by 40% by removal of extracellular Ca2+ but was not altered by L-type Ca2+ channel blockers. In Ca2+-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, caused a monophasic [Ca2+]i rise, after which desipramine failed to release more Ca2+; in addition, pretreatment with desipramine partly decreased thapsigargin-induced [Ca2+]i increase. U73122, an inhibitor of phospholipase C, did not change desipramine-induced [Ca2+]i rise. Incubation with 10-100 microM desipramine enhances or inhibits cell proliferation in a concentration- and time-dependent manner. The inhibitory effect of desipramine on proliferation was not extracellular Ca2+-dependent. Apoptosis appears to contribute to desipramine-induced cell death. Together, these findings suggest that desipramine increases baseline [Ca2+]i in renal tubular cells by evoking both extracellular Ca2+ influx and intracellular Ca2+ release, and can cause apoptosis.