Chronic imipramine but not bupropion increases arachidonic acid signaling in rat brain: is this related to 'switching' in bipolar disorder?

Abstract

Agents effective against mania in bipolar disorder are reported to decrease turnover of arachidonic acid (AA) in phospholipids and expression of calcium-dependent AA-selective cytosolic phospholipase A(2) (cPLA(2)) in rat brain. In contrast, fluoxetine, an antidepressant that is reported to switch bipolar depressed patients to mania, increases cPLA(2) expression and AA turnover in rat brain. We therefore hypothesized that antidepressants that increase switching to mania generally increase cPLA(2) and AA turnover in brain. To test this hypothesis, adult male CDF-344 rats were administered imipramine and bupropion, with reported high and low switching rates, respectively, at daily doses of 10 and 30 mg kg(-1) i.p., respectively, or i.p. saline (control) for 21 days. Frontal cortex expression of different PLA(2) enzymes and AA turnover rates in brain when the rats were unanesthetized were measured. Compared with chronic saline, chronic imipramine but not bupropion significantly increased cortex cPLA(2) mRNA activity, protein and phosphorylation, expression of the cPLA(2) transcription factor, activator protein-2alpha (AP-2alpha) and AA turnover in phospholipids. Protein levels of secretory phospholipase A(2), calcium-independent phospholipase A(2), cyclooxygenase (COX)-1 and COX-2 were unchanged, and prostaglandin E(2) was unaffected. These results, taken with prior data on chronic fluoxetine in rats, suggest that antidepressants that increase the switching tendency of bipolar depressed patients to mania do so by increasing AA recycling and metabolism in brain. Mania in bipolar disorder thus may involve upregulated brain AA metabolism.

Figure 1

Cytosolic phospholipase A₂ (cPLA₂) mRNA (a), protein (b), phosphorylated protein (c) and enzyme activity (d) in brains of rats chronically administered saline (control), imipramine (10mgkg⁻¹) or bupropion (30mgkg⁻¹) for 21 days. Data are expressed as the relative level of cPLA₂ normalized to the endogenous control (β-globulin) using the ΔΔC_T method. Protein levels are ratios of optical densities of cPLA₂ or phosphorylated cPLA₂ to β-actin, expressed as percent of control. For activity assay details, see the ‘Materials and methods’. Data were compared using one-way ANOVAwith Tukey’s post hoc test for multiple comparisons. Data are means±s.e.m., n = 10 rats per group. Means within a row not sharing a common superscript are statistically different, P < 0.05.

Figure 2

Secretory phospholipase A₂ (sPLA₂) (a) and calcium-independent phospholipase A₂ (iPLA₂) (b) protein levels in brains of rats chronically administered saline (control), imipramine (10mgkg⁻¹) or bupropion (30mgkg⁻¹) for 21 days. Data are ratios of optical densities of sPLA₂ or iPLA₂ to β-actin, expressed as percent of controls and were compared using a one-way ANOVA with Tukey’s post hoc test for multiple comparisons. Data are means±s.e.m., n = 10 rats per group. No significant difference was detected.

Figure 3

Cyclooxygenase-1 (COX-1) (a) and cyclooxygenase-2 (COX-2) (b) protein levels and prostaglandin E₂ (PGE₂) concentration (c) in brains of rats chronically administered saline (control), imipramine (10mgkg⁻¹) or bupropion (30mgkg⁻¹) for 21 days. Data are ratios of optical densities of COX-1 or COX-2 to β-actin, expressed as percent of controls and were compared using a one-way ANOVAwith Tukey’s post hoc test for multiple comparisons. Data are means±s.e.m., n=10 rats per group for Western blots, n = 5 or 7 per group for PGE₂ concentration. No significant difference was detected.

Figure 4

Activator protein-2α (AP-2α) (a), AP-2β (b), nuclear factor-kappa B p65 (NF-κB p65) (c) and NF-κB p50 (d) protein levels in brains of rats chronically administered saline (control), imipramine (10mgkg⁻¹) or bupropion (30mgkg⁻¹) for 21 days. Data are ratios of optical densities to β-actin, expressed as percent of controls and were compared using a one-way ANOVA with Tukey’s post hoc test for multiple comparisons. Data are means±s.e.m., n = 10 rats per group. Means within a row not sharing a common superscript are statistically different, P < 0.05.

Figure 5

Cytosolic phospholipase A₂ (cPLA₂) (a), secretory phospholipase A₂ (sPLA₂) (b), calcium-independent phospholipase A₂ (iPLA₂) (c), cyclooxygenase-1 (COX-1) (d) and cyclooxygenase-1 (COX-2) (e) protein levels in brains of rats acutely administered saline (control), imipramine (10mgkg⁻¹) or bupropion (30mgkg⁻¹). Data are ratios of optical densities of protein to β-actin, expressed as percent of controls and were compared using a one-way ANOVA with Tukey’s post hoc test for multiple comparisons. Data are means±s.e.m., n = 10 rats per group. No significant difference was detected.