EFFECTS OF CYCLOHEXIMIDE ON THE STRUCTURAL ORGANIZATION OF THE NUCLEOLUS AND THE COILED BODY IN NORMAL AND STIMULATED SUPRAOPTIC NEURONS OFTHE RAT

This study was designed to determine the effects of cycloheximide, a p rotein synthesis inhibitor that interferes with rRNA synthesis and pro cessing, on the nucleoli and coiled bodies of supraoptic nucleus neuro ns from normally-hydrated and osmotically-stimulated rats. The number of nucleoli and the nucleolar size were estimated on smear preparation s of previously silver-impregnated supraoptic nucleus. No significant differences were registered in the mean number of nucleoli per call in cycloheximide-treated rats. The number of nucleoli per neuron remaine d constant, at about 1.3, in all animal groups, suggesting that the nu cleoli number is strictly regulated in differentia red neurons. By con trast, a significant reduction in the average nucleolar volume of supr aoptic nucleus neurons was detected in cycloheximide-treated groups of rats in comparison with their equivalent non-treated groups. By elect ron microscopy, most nucleoli and coiled bodies of supraoptic nucleus neurons exhibited cycloheximide-induced alterations in their fine stru cture and configuration. Nucleolar changes included the occurrence of a few large fibrillar centres, the formation or microspherules and sma ll intranucleolar vacuoles or dilated interstices, and the partial seg regation of nucleolar components coupled with the transformation of re ticulated nucleoli - a nucleolar configuration characteristic of supra optic nucleus neurons of non-cycloheximide-treated ted rats - into com pact ones. The redistribution of nucleolar components might reflect th e interference with rDNA transcription, and also supports the hypothes is that the normal assembly of these components into the nucleolus dep ends upon ongoing nucleolar transcription. Concerning coiled bodies, m ost of them revealed ultrastructural alterations, particularly segrega tion of the amorphous matrix, compactation of coiled threads and forma tion of coiled body-derived dense bodies of fibrillar nature. Moreover , cycloheximide also induced the formation of smaller dense bodies - h ere referred to as dense microbodies - which presumably represent a di stinct nuclear entity different from coiled bodies. Ultrastructural si lver staining of nuclear bodies showed a selective silver reaction on the dense fibrillar component of normal and altered coiled bodies, as well as on the dense microbodies. The possible relationship between th e nucleolus and both coiled bodies and dense microbodies is discussed.