title: Daily melatonin injections induce cytological changes in pars tuberalis-specific cells similar to short photoperiod.
abstract: Hypophyseal pars tuberalis (PT)-specific cells are known to exhibit remarkable seasonal changes in morphology especially in photoperiodic animals like the Djungarian hamster Phodopus sungorus. Their high density of melatonin-receptors leads to the supposition that fluctuations in circulating melatonin levels are a crucial factor for the morphological alterations induced by photoperiodic signals. To prove this hypothesis the nocturnal elevation of melatonin in long photoperiods was prolonged by late afternoon administration of melatonin. We investigated whether this treatment induces cytological changes usually observable under short photoperiod. Electron microscopy revealed that in contrast to hamsters maintained in long photoperiods PT-specific cells of hamsters injected with melatonin or those kept in short photoperiods appear inactive, containing a relatively high number of secretory granules, sparse endoplasmatic reticulum, irregularly outlined and invaginated cell nuclei and a high amount of glycogen. Furthermore immunoreactivity for the common alpha-chain of glycoprotein hormones and beta-TSH was significantly weaker in hamsters kept in short photoperiods or daily injected with melatonin than untreated or vehicle injected controls in long photoperiod. These results demonstrate that an exogenous prolongation of the elevated nocturnal melatonin levels causes a similar morphological appearance of PT-specific cells as observed in short photoperiods. It is tempting to speculate that the melatonin signal is a direct 'Zeitgeber' for the transduction of photoperiodic information to the secretory activity in this cell type.
authors.affiliation: Institute of Anatomy, Münster, Germany.
authors.forename: T M
authors.initials: TM
authors.lastname: Böckers
authors.forename: P
authors.initials: P
authors.lastname: Niklowitz
authors.forename: J
authors.initials: J
authors.lastname: Bockmann
authors.forename: J D
authors.initials: JD
authors.lastname: Fauteck
authors.forename: W
authors.initials: W
authors.lastname: Wittkowski
authors.forename: M R
authors.initials: MR
authors.lastname: Kreutz
mesh_terms: D000818:Animals; D001835:Body Weight / Q000187:drug effects; D006224:Cricetinae; D005260:Female; D006023:Glycoproteins / Q000378:metabolism; D007150:Immunohistochemistry; D008550:Melatonin / Q000008:administration & dosage / Q000378:metabolism / Q000494:pharmacology*; D008854:Microscopy, Electron; D009929:Organ Size / Q000187:drug effects; D017111:Phodopus; D017440:Photoperiod; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects* / Q000648:ultrastructure; D011956:Receptors, Cell Surface / Q000378:metabolism; D044094:Receptors, Melatonin; D013972:Thyrotropin / Q000378:metabolism
chemical_list: D006023:Glycoproteins; D011956:Receptors, Cell Surface; D044094:Receptors, Melatonin; D013972:Thyrotropin; D008550:Melatonin