Chronic lithium administration down regulates transthyretin mRNA expression in rat choroid plexus

Abstract

Transthyretin (TTR) accounts for a quarter of the protein content of ventricular cerebrospinal fluid (CSF) yet its exact role in the brain remains unknown. Patients with a diagnosis of depression have reduced CSF levels of TTR and the locus encoding the TTR gene has been implicated in a Danish pedigree of bipolar patients. Lithium, the major treatment for bipolar disorder in the UK, was subcutaneously infused into rats for 28 days in the form of lithium chloride using osmotic minipumps. In situ hybridizations using oligonucleotide probes targeted against the TTR transcript were performed on coronal brain sections. Lithium significantly reduced the level of transthyretin mRNA in the rat choroid plexus within the lateral and third ventricle. The down-regulation was confirmed using semi-quantitative reverse transcription PCR on dissected brain tissue. Recent studies in mice suggest that the TTR gene is implicated in depression-like behavior therefore this effect of lithium may be relevant to its use as a mood stabilizer or an adjuvant to antidepressant drugs.

Keywords: (SQ)-RT-PCR; bipolar disorder; in-situ hybridization; minipump; mood stabilizer.

Figure 1

Autoradiographs of coronal sections (AP −2.8 mm) from a typical vehicle-treated (panel A–C) or lithium-treated (panel D–F) animal hybridized with 3’ end ³³P-labelled oligoprobes to TTR (A, D), glyceraldehyde-3-phosphate dehydrogenase (B,E), and β-actin (C,F) mRNA. Note that the TTR signal (indicated by arrows) is restricted to the choroid plexus of the 3^rd ventricle (3^rdV) and lateral ventricle (LV) and is weaker in the lithium-treated brain. The signal distribution for the house-keeping genes G3PDH or β-actin from adjacent sections was similar to that previously described and did not differ between the groups.

Figure 2

Effect of lithium treatment on TTR mRNA expression throughout the brain. Semi-quantitative densitometric analysis revealed a significant decrease in optical density in both the lateral ventricle (A) and the third ventricle (B) in rats treated with lithium chloride (n = 6) compared with vehicle (n = 6) at each level. Data represent mean of 6 optical density readings for lateral ventricle and 3 optical density readings for third ventricle ± SEM.

Figure 3

Semi-quantitative-RT-PCR from lithium (n = 3) and vehicle-treated rats (n = 3). Expression levels were studied using oligonucleotides directed towards TTR: primers 5’- AAT ACG CAG AGG TGG TTT TC-3’ and 5’-GGC ATC TTC CCG AGT TG-3’ and to β-actin: primers 5’-TTG TAA CCA ACT GGG ACG ATA TGG-3’ and 5’-GAT CTT GAT CTT CAT GGT GCT AGG-3’. Shown is an example of the accumulation of product with 30 cycles of PCR for a serial dilution of template cDNA ranging from; lane 1 = 10 ng, 2 = 5 ng, 3 = 2.5 ng, 4 = 1.25 ng and 5 = 0.625 ng. Product sizes are indicated relative to 1 kb DNA markers (M).