Nuclear compartmentalization in transcriptionally activated hypothalamic neurons.

Transcription of cell-specific vasopressin and oxytocin genes as well as transcription of those housekeeping genes responsible for general metabolic activation and cellular hypertrophy is induced in supraoptic hypothalamic neurons by rises in plasma osmolarity. In this study, the nuclear volume, the ultrastructure of chromatin and the number and distribution of nuclear particles in the cell nuclei of supraoptic neurons of 3-month-old male Sprague-Dawley rats were analyzed after osmotically induced activation of transcription by periods of acute (1 day) and chronic (6 days) dehydration, and after halting the stimulation by rehydration of animals. The nuclear volume and the ultrastructure of chromatin were assessed on ultrathin sections. The number and distribution of nuclear particles were assessed on freeze-fracture replicas. The initial phase of osmotically induced enhancement of transcription was accompanied by an increase in nuclear volume and by a partial replacement of nuclear particles of large diameter (> 11 nm) by smaller nuclear particles. This latter change affected predominantly the nuclear periphery (0-1,000 nm from the nuclear membrane) and occurred simultaneously with a partial decondensation of chromatin clusters that may be related to chromatin unfolding. In chronically stimulated animals, the decondensation of chromatin and the replacement of large nuclear particles by smaller ones was enhanced in the nuclear periphery and was partially propagated to the interior of the nucleus. After suppression of cellular activation by rehydration of animals, the number of nuclear particles returned to control levels in the nuclear periphery while in the center of the nucleus the number of small particles decreased and the number of large particles increased as compared to control values. These results, together with the observation that in unstimulated cells the nuclear periphery and the nuclear interior differ in their composition of nuclear particles, evidence a structural and functional compartmentalization in the cell nucleus of supraoptic neurons.