Rapid modifications of somatostatin neuron activity in the periventricular nucleus after acute stress

We have previously reported that stress induces a rapid increase in hypothalamic somatostatin (SS) release. In the present work, we investigated whether SS synthesis is also affected by this treatment. Male rats were subjected to 15-min immobilization (IMO) stress, and measurements of both SS mRNA levels and SS mRNA-containing cells were analyzed in the periventricular nucleus (PeV) by radioactive and nonradioactive in situ hybridization (ISH), respectively. In addition, SS content and total SS mRNA were measured in the whole hypothalamus by radioimmunoassay (RIA) and northern blot analysis, respectively. ISH was conducted by applying either a radioactive-labeled (S-35) or a digoxigenin (DIG)-labeled oligonucleotide probe on histological sections containing the periventricular region of the anterior hypothalamic area (AHA). ISI-I analysis using radioactive label showed a significant increase in SS mRNA levels in stressed rats. In contrast, stress treatment decreased the number of DIG-labeled cells expressing SS mRNA in this region by 35% as compared to the same histological sections from naive control rats. In addition, a significant decrease in the total SS mRNA DIG-labeled area was observed. Finally, SS content and SS mRNA measured in the whole hypothalamus of stressed rats were markedly inhibited as compared to control rats. Our data show that IMO stress induces a significant and rapid increase in SS mRNA level accompanied by a decrease in the number of cells expressing SS mRNA in the PeV-AHA. The present results suggest that a subset of PeV SS neurons, which became silent at the onset of stress, are regulated independently of the remaining whole mass of PeV neurons. This differential control is in line with the cellular heterogeneity described in periventricular SS-producing neurons and with the multiple hypothalamic and pituitary functions assigned to SS.