Single stress induces long-lasting elevations in vasopressin mRNA levels in CRF hypophysiotrophic neurones, but repeated stress is required to modifyAVP immunoreactivity

Repeated stress is known to induce an increased vasopressin (AVP) expressio n in paraventricular corticotrophin-releasing factor (CRF) neurones which i s supposed to enhance the ACTH-releasing capacity of these cells. To test t he hypothesis that a single stress is sufficient to produce these changes, we used quantitative in-situ hybridization analysis to measure steady state CRF and AVP mRNA, Moreover the colocalized AVP and CRF immunoreactive site s were assessed in the dense core vesicle compartment of CRF axon terminals in the external zone of the median eminence with quantitative immunoelectr on microscopy. Acute immobilization produced a significant increase in the average AVP and CRF mRNA levels (145% and 65%, respectively, above control values) in the medial parvocellular subdivisions of the paraventricular nuc leus (PVN), and these changes persisted for over 4 days after stress. In co ntrast to these changes in AVP mRNA levels, there were no concomitant chang es in AVP immunostaining in CRF terminals and axons during the 4-day period . However, when immobilization stress was repeated daily, the number of CRF terminals containing AVP increased progressively. Moreover, the ratio of A VP and CRF immunoreactivity in the dense core vesicle compartment was incre ased. Taken together, these results provide evidence that single stress exp erience can cause long-lasting changes in AVP and CRF mRNA steady state exp ression that is not apparently accompanied by changes in peptide levels, Th ey also suggest that repeated stress is required for developing progressive shifts in the neurohormone storage pattern of these neurones.