Daily Melatonin Injections Induce Cytological Changes in Pars Tuberalis‐Specific Cells Similar to Short Photoperiod

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 α‐chain of glycoprotein hormones and β‐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.