Abstract
Abstract: :
Purpose: Pineal melatonin acts as an endocrine signal of the environmental light/dark (L/D) cycle. We address whether ocular melatonin contributes to a general endocrine signalling role, or whether it acts as a local L/D signal in the mouse retina. Methods: We compare diurnal profiles of in vivo methoxy–indoleamine (MID) content in the retina and pineal, and examine the release of MIDs from eye–cups in vitro of +/+ C3H mice. Mice were entrained to a 12:12 LD cycle and tissues were collected at 4 hour intervals. Pineals and retinae were dissected, homogenised, chloroform extracted and assayed for MID content using HPLC. Additionally, eye–cups were cultured for ∼ 4 days and MID release into the culture media determined using HPLC. Retinal tissue was subsequently processed for histology and immunocytochemistry. Results: The pineal showed a diurnal rhythm in melatonin content, with low values during day and peak values of approximately 200 pg/pineal gland at night. Melatonin content in the retina fell below levels of detection over the entire L/D cycle. By contrast, rhythmic levels of 5–methoxytryptophol (MTOL) were detected, and showed a diurnal change in content with low levels during the day and peak values at night of approximately 125 pg/eye. After four days in culture the tissue preservation of cultured eye–cups was good, and choline acetyltransferase and tyrosine hydroxylase immunostaining was preserved. Despite this high level of preservation, our preliminary studies failed to detect the release of either melatonin or MTOL into the culture medium from eye–cups. Conclusions: As shown previously, the pineal gland shows an in vivo rhythm of melatonin content with peak values at night and undetectable levels during the day. In the retina, melatonin content was undetectable over the entire L/D cycle whereas MTOL showed a strong diurnal variation. These results suggest that melatonin is rapidly catabolised within the retina to MTOL and that melatonin does not have any extraocular signalling role. Our preliminary experiments failed to find evidence for the release of either melatonin or MTOL from cultured eye–cups. We are currently refining our culture conditions, and studying MID release from both the retina and pineal.
Keywords: retina • melatonin • retinal culture