Lamotrigine blocks NMDA receptor-initiated arachidonic acid signalling in rat brain: implications for its efficacy in bipolar disorder

Abstract

An up-regulated brain arachidonic acid (AA) cascade and a hyperglutamatergic state characterize bipolar disorder (BD). Lamotrigine (LTG), a mood stabilizer approved for treating BD, is reported to interfere with glutamatergic neurotransmission involving N-methyl-d-aspartate receptors (NMDARs). NMDARs allow extracellular calcium into the cell, thereby stimulating calcium-dependent cytosolic phospholipase A2 (cPLA2) to release AA from membrane phospholipid. We hypothesized that LTG, like other approved mood stabilizers, would reduce NMDAR-mediated AA signalling in rat brain. An acute subconvulsant dose of NMDA (25 mg/kg) or saline was administered intraperitoneally to unanaesthetized rats that had been treated p.o. daily for 42 d with vehicle or a therapeutically relevant dose of LTG (10 mg/kg.d). Regional brain AA incorporation coefficients k* and rates J in, and AA signals, were measured using quantitative autoradiography after intravenous [1-14C]AA infusion, as were other AA cascade markers. In chronic vehicle-treated rats, acute NMDA compared to saline increased k* and J in in widespread regions of the brain, as well as prostaglandin (PG)E2 and thromboxane B2 concentrations. Chronic LTG treatment compared to vehicle reduced brain cyclooxygenase (COX) activity, PGE2 concentration, and DNA-binding activity of the COX-2 transcription factor, NF-κB. Pretreatment with chronic LTG blocked the acute NMDA effects on AA cascade markers. In summary, chronic LTG like other mood stabilizers blocks NMDA-mediated signalling involving the AA metabolic cascade. Since markers of the AA cascade and of NMDAR signalling are up-regulated in the post-mortem BD brain, mood stabilizers generally may be effective in BD by dampening NMDAR signalling and the AA cascade.

Figure 1

Coronal brain autoradiographs showing effects of NMDA and LTG on regional AA incorporation coefficients k* in rats. Values of k* (ml/s/g brain) × 10⁻⁴ are given on a color scale from 7 (purple) to 23 (yellow-orange). CPu, caudate putamen; Hipp, hippocampus; LTG, lamotrigine; NMDA, N-methyl-D-aspartic acid, 25 mg/kg, i.p.; Mot, motor cortex; PFr, prefrontal cortex; Som, somatosensory cortex. NMDA, N-methyl-D-aspartate, 25 mg/kg, i.p..

Figure 2

Effects of chronic LTG on cPLA₂, COX, and NF-kB DNA binding activities. (A) Effects of chronic LTG on whole brains: cPLA₂-IV activity. Data are means ± SD (n=8) and were analyzed with unpaired t-test. (B) Effects of chronic LTG on whole brain cyclooxygenase (COX) activity. Data are means ± SD (n=8) and were analyzed with unpaired t-test. *** p <0.001. PGE₂, prostaglandin E₂. (C) Effects of chronic LTG on NF-kB DNA binding activity. Values are represented as % control (n = 8). *** p < 0.001.