Abstract
The pineal gland, a circumventricular organ, plays an integrative role in defense responses. The injury-induced suppression of the pineal gland hormone, melatonin, which is triggered by darkness, allows the mounting of innate immune responses. We have previously shown that cultured pineal glands, which express toll-like receptor 4 (TLR4) and tumor necrosis factor receptor 1 (TNFR1), produce TNF when challenged with lipopolysaccharide (LPS). Here our aim was to evaluate which cells present in the pineal gland, astrocytes, microglia or pinealocytes produced TNF, in order to understand the interaction between pineal activity, melatonin production and immune function. Cultured pineal glands or pinealocytes were stimulated with LPS. TNF content was measured using an enzyme-linked immunosorbent assay. TLR4 and TNFR1 expression were analyzed by confocal microscopy. Microglial morphology was analyzed by immunohistochemistry. In the present study, we show that although the main cell types of the pineal gland (pinealocytes, astrocytes and microglia) express TLR4, the production of TNF induced by LPS is mediated by microglia. This effect is due to activation of the nuclear factor kappa B (NF-kB) pathway. In addition, we observed that LPS activates microglia and modulates the expression of TNFR1 in pinealocytes. As TNF has been shown to amplify and prolong inflammatory responses, its production by pineal microglia suggests a glia-pinealocyte network that regulates melatonin output. The current study demonstrates the molecular and cellular basis for understanding how melatonin synthesis is regulated during an innate immune response, thus our results reinforce the role of the pineal gland as sensor of immune status.
Highlights
The pineal gland, a circumventricular organ, plays an integrative role in the neuro-endocrine-immune response [1,2,3]
Cellular Distribution of toll-like receptor 4 (TLR4) in the Rat Pineal Gland TLR4 expression was detected by immunohistochemistry in frozen pineal gland sections from rats perfused at ZT 6
TLR4-positive immunostaining co-localized with both astrocyte (GFAP, Fig. 1A-D) and microglia (ED-1, Fig. 1F-I) markers, and was present on isolated pinealocytes (Fig. 1E), indicating that these three cell types present in the pineal gland express TLR4 and are able to respond to LPS
Summary
The pineal gland, a circumventricular organ, plays an integrative role in the neuro-endocrine-immune response [1,2,3]. The major cellular component of the pineal gland is the pinealocyte. In rats, this cell type comprises around 90% of the gland and the remainder consists mainly of glial cells and nerve fibers [4]. The pinealocytes are responsible for synthesizing melatonin, the darkness hormone, while the functions of astrocytes and microglia in the pineal gland are currently unclear. Gastrointestinal cells, immunocompetent cells, and astrocytes synthesize melatonin, which plays a local role related to tissue protection [8,9,10,11,12,13,14,15,16]
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have