In situ hybridization was used to measure the expression of members of the 
Fos/Jun family of immediate-early genes in hypothalamic neurons in vivo 
following defined stimuli that utilize different afferent pathways. Only c-jun 
messenger RNA was expressed in the hypothalamic supraoptic and paraventricular 
nuclei of control animals. Intravenous infusions of sodium chloride solutions of 
different tonicity produced a range of plasma osmolalities within physiological 
limits. While the induction of c-fos and jun B messenger RNAs followed the 
stimulus intensity, the expression of c-jun was repressed at low levels of 
stimulation. A higher level of osmotic stimulation was able to co-induce c-jun 
with the c-fos, jun B and fos B genes, suggesting that other signalling pathways 
may then be activated. Parturition or systemic administration of 
cholecystokinin, that activate supraoptic and paraventricular neurons via 
ascending afferent pathways from the brainstem, both induced c-fos, but not the 
other genes, in the magnocellular nuclei. Use of double in situ hybridization 
confirmed that, unlike with osmotic stimulation, induction of c-fos only 
occurred in oxytocin neurons. These two stimuli did not cause a concomitant 
repression of c-jun messenger RNA expression in magnocellular oxytocin neurons. 
These patterns of induction provide evidence for the differential regulation of 
members of this family of genes in a physiological context.