Circadian release of pigment-dispersing factor in the visual system of the housefly, Musca domestica

In the present study we examined profiles of nerve fiber varicosities containing dense core vesicles (DCVs) in the distal medulla of the housefly's optic lobe using electron microscopic methods. These profiles are infrequent among other neuronal profiles and do not contain presynaptic specializations for the release of DCVs. Presynaptic elements surrounded by electrontranslucent vesicles were only occasionally detected, whereas synaptic input sites to the profiles containing DCVs were never observed. Among the varicosities in the distal medulla, those immunoreactive to pigment-dispersing hormone (PDH) are most numerous. The DCVs of PDH-immunoreactive (PDH-ir) varicosities differ by size from DCVs of other profiles. Moreover, in the day/night cycle PDH-ir varicosities show differences in structure revealing the rhythmic accumulation and release of PDF. There were fewer PDH-ir DCV per varicosity profile in flies fixed 1 hour after lights-on than in flies fixed 1 hour after lights-off. Moreover, at the beginning of the day all DCVs harbored an electron-dense matrix, while at the beginning of the night numerous electron-lucent DCVs were observed. By applying a bath stimulation with a high potassium concentration we also showed that depolarizing events are involved in peptide release in the medulla. After potassium treatment immunolabeling with anti-PDH serum was weaker and PDH-ir varicosities were smaller and more distant from each other than in control animals.