Melatonin from slow-release implants upregulates claudin-2 in the ovine choroid plexus

In ewes, the turnover rate of cerebrospinal fluid (CSF), mainly produced by choroid plexus (ChP), is photoperiodically modulated and is higher during short days (SDs) than long days (LDs). We demonstrated that, melatonin from continuous slow-release implants increases the expression of aquaporins, water channel-forming proteins engaged in transcellular water transport (across the plasma membrane of the cells), in the ovine ChP. This study evaluated the effect of slow-release melatonin implants on the expression of claudin-2 (CLDN2), a pore-forming protein that allows the paracellular passage (between the cells) of select inorganic cations and water, in the ovine ChP. The studies were conducted on ovariectomized, estradiol-implanted ewes during seasonal anestrus (May/June). The ewes were implanted with slow-release-melatonin implants (n = 6, Melovine 18 mg) or sham-implanted (n = 6). Blood samples were collected for melatonin and prolactin measurements. The ewes were sacrificed 40 days after the melatonin/sham implantation, and the ChPs from the brain ventricles were collected for real-time PCR and Western blot analyses. Plasma melatonin concentration reached the median value of 120.4 pg/ml (range: min/max = 29.6/447.0) or was below the detection limit 40 days after the melatonin/sham implantation, respectively. The area under the curve of the plasma prolactin concentration was significantly (P < 0.05) higher in sham-implanted ewes than in melatonin-implanted ewes. CLDN2 expression in the ChP was significantly (P < 0.05) higher in melatonin-implanted ewes than in sham-implanted ewes at both the mRNA and protein levels. This is the first evidence for the photoperiodic regulation of CLDN2 expression in the ovine ChP, since it has been shown that slow-release melatonin implants during LDs, mimicking SDs, increased the expression of CLDN2. This may partially explain the higher turnover rate of CSF observed in ewes during SDs.