GONADAL-GABAERGIC INTERACTION IS AN IMPORTANT FACTOR INVOLVED IN PHOTOPERIOD-INDUCED 2-[I-125] IODOMELATONIN BINDING CHANGES IN THE JAPANESE-QUAIL BRAIN

The type of mechanism(s) by which melatonin alone and/or through the i ntervention of other putative neurotransmitters is able to control cir cadian rhythms remains unresolved. Comparison of 2-[I-125] iodomelaton in binding pattern in the brain of castrated and gonadally intact Japa nese quail (Coturnix japonica), using quantitative receptor autoradiog raphy, displayed that the combination of the intact gonadal condition and a long-day (16L:8D) photostimulatory schedule is responsible for m ajor binding changes. In fact, high and low binding levels were obtain ed in the suprachiasmatic area and nucleus ectomamillaris (p < 0.01) a nd in the nucleus preopticus anterior and paleostriatum primitivum (p < 0.001), respectively. A gonadal modulatory role was not always evide nt in all brain areas as revealed by long-day photic cycles producing diminished (p < 0.01) binding levels in the anterior neostriatum and t he nucleus rotundus of both castrated and gonadally intact animals, al though elevated values were also found in the substantia grisea centra lis (p < 0.05) of the same animals. Saturation binding studies reveale d that gonadal and/or photic effects induce alterations in the number of binding sites, whereas the affinity constant varied only in some hy pothalamic sites. Testing of GABAergic activity on 2-[I-125] iodomelat onin binding levels showed that this inhibitory neurotransmitter was r esponsible for increasing low receptor values. Moreover, GABA-dependen t influences were shown to be mediated via a GABA, receptor subtype si nce bicuculline (specific antagonist of this site) inhibited the eleva ted GABA-induced melatonin binding levels in the above brain sites of the gonadally intact quail exposed to both photoperiod cycles. Even in this case, melatonin binding changes were due to the variations in th e number of binding sites. The apparent GABAergic-gonadal influence re sulting in changes of the 2-[I-125] iodomelatonin binding values, unde r the different photic conditions, provides evidences of other probabl e neural mechanisms that entrain circadian rhythmicity in neuroendocri ne activities and in sociosexual behaviors.