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Bojidar Stankov, Morten Møller, Valeria Lucini, Simona Capsoni and Franco Fraschini

Stankov B, Møller, M, Lucini V, Capsoni S, Fraschini F. A carnivore species (Canis familiaris) expresses circadian melatonin rhythm in the peripheral blood and melatonin receptors in the brain. Eur J Endocrinol 1994;131:191–200. ISSN 0804–4643

Dogs kept under controlled photoperiodic conditions of 12 h light and 12 h dark expressed a clear diurnal melatonin rhythm in the peripheral blood, with a swift peak restricted to the late part of the scotophase. The highest density of high-affinity, G-protein-linked 2-[125I]iodomelatonin binding sites was found in the pars tuberalis of the pituitary gland. Binding sites were found also in the pars distalis, and light microscopy/high-resolution autoradiography showed that binding was located exclusively over the chromophobe and basophilic cells forming the adenopituitary zona tuberalis, well developed in this species, and extending into the gland as a continuation of pars tuberalis. Cords of basophilic cells located in the pars distalis proper also expressed high receptor density. The eosinophils in the adenohypophysis and the neural lobe were devoid of binding. Heavily labeled were the external laminar and the mitral cell layers of the olfactory bulbs, but no binding was detected in the filae nervi olfactorii or tractus olfactorius. The hypothalamic suprachiasmatic nuclei were discernible clearly. Quantitative autoradiography inhibition experiments revealed that the apparent melatonin inhibitory constant (ic 50) in all those areas was around 0.1 nmol/l, which is a physiologically appropriate value considering the peripheral blood melatonin levels. Co-incubation with guanosine 5′-O-(3-thiotriphosphate) (GTPΓS) led to a consequential decrease in the binding density. The specific binding observed in other areas (hippocampus, frontal, parietal, occipital cortex and cerebellum) was rather weak, diffuse and could not be attributed to a particular layer; the apparent ic 50 for melatonin was about 1 μmol/l, and co-incubation with GTPΓS did not modify the binding density. Collectively, these data show that the dog posesses all the prerequisites for an efficient network adapted to photoperiodic time measurements. A circadian melatonin signal in the peripheral blood and an apparently functional readout receptor system located in key positions within the brain are both present in this species.

Bojidar Stankov, Chair of Chemotherapy, Department of Pharmacology, University of Milan, 32 Via Vanvitelli, 20129 Milano, Italy