UNILATERAL OPTIC-NERVE TRANSECTION DECREASES 2-[I-125]-IODOMELATONIN BINDING IN RETINORECIPIENT AREAS AND VISUAL PATHWAYS OF CHICK BRAIN

In chick brain, specific 2-[I-125]-iodomelatonin-binding was localized primarily in the visual system, i.e., retinorecipient and relay nucle i and fiber tracts of the tectofugal, thalamofugal, circadian and acce ssory visual pathways. Unilateral transection of the optic nerve (ONX) significantly reduced the binding of 2-[I-125]-iodomelatonin (75 pM) in many, but not all, primary retinal targets and visual pathways at 7 and 14 days, but not 1 day, postlesion. As measured using quantitativ e autoradiography, 2-[I-125]-iodomelatonin binding was decreased by 90 % in both the central portion of the lesioned optic tract and one of i ts targets, the nucleus of the basal optic root (nBOR). Other retinore cipient areas exhibiting substantial decreases (60%) in 2-[I-125]-iodo melatonin-binding included the optic tectum, lateroventral and dorsola teral geniculate nuclei and tectal gray area contralateral to the lesi on. These findings are consistent with the hypothesis that melatonin r eceptors are located presynaptically on incoming optic nerve terminals in many retinorecipient areas. This localization may account for most of the binding sites in nBOR. In other primary visual areas, however, melatonin receptors also appear to be located on postsynaptic cells a nd/or non-retinal afferents. ONX had no significant effect on 2-[I-125 ]-iodomelatonin binding in two retinorecipient areas, the visual supra chiasmatic nucleus and the dorsolateral anterior thalamus, which are p art of the circadian/oculomotor and thalamofugal pathways, respectivel y. An unexpected consequence of ONX was that 2-[I-125]-iodomelatonin b inding was decreased in certain secondary (nucleus rotundus, isthmi nu clei) and tertiary level (ectostriatum) nuclei along the prominent tec tofugal visual pathway. Binding in the tectorecipient nucleus triangul aris was not significantly altered, however. Analysis of secondary lev el relay nuclei in the oculomotor pathway revealed that binding after ONX was decreased in the ipsilateral Edinger-Westphal nucleus but not in the oculomotor nuclei. Selective transsynaptic changes in 2-[I-125] -iodomelatonin binding after lesion of the visual input most likely re flect activity-dependent regulation and functional plasticity of centr al melatonin receptors.