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 m etabolic 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 di stribution of nuclear particles in the cell nuclei of supraoptic neuro ns of 3-month-old male Sprague-Dawley rats were analyzed after osmotic ally induced activation of transcription by periods of acute (I day) a nd chronic (6 days) dehydration, and after halting the stimulation by rehydration of animals. The nuclear volume and the ultrastructure of c hromatin were assessed on ultrathin sections. The number and distribut ion of nuclear particles were assessed on freeze-fracture replicas. Th e initial phase of osmotically induced enhancement of transcription wa s accompanied by an increase in nuclear volume and by a partial replac ement of nuclear particles of large diameter (> 11 nm) by smaller nucl ear particles. This latter change affected predominantly the nuclear p eriphery (0-1 000 nm from the nuclear membrane) and occurred simultane ously 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 part icles by smaller ones was enhanced in the nuclear periphery and was pa rtially propagated to the interior of the nucleus. After suppression o f cellular activation by rehydration of animals, the number of nuclear particles returned to control levels in the nuclear periphery while i n the center of the nucleus the number of small particles decreased an d the number of large particles increased as compared to control value s. These results, together with the observation that in unstimulated c ells the nuclear periphery and the nuclear interior differ in their co mposition of nuclear particles, evidence a structural and functional c ompartmentalization in the cell nucleus of supraoptic neurons.