FIGURE 2. C-Fos expression in LHRH neurons. C-Fos (arrows) is expressed 
in LHRH neurons during an LH surge. (A) Bright-field image 
shows that the nuclei of two LHRH neurons in the field contain 
c-Fos. (B) The same section viewed under fluorescence 
microscopy reveals immunofluorescent LHRH neurons (arrows) 
close to the OVLT that express c-Fos in their nuclei. 
LHRH neurons activated when the actions of progesterone were prevented or 
blocked, progesterone-deprived rats showed a reduced intensity of c-Fos 
protein immunoreactivity within their nuclei (figure 6). Thus, progesterone 
appeared not only to recruit more LHRH neurons into the active state but also 
increased the extent of stimulation of individual cells. More generally, these 
data indicate that c-Fos expression is graded to the intensity of the stimulus 
and, as was revealed by progesterone deprivation, is not all or none. Further 
confirmation of this feature has been obtained for hypovolemic stimulation of 
magnocellular neurons of the hypothalamus (Roberts et al., submitted for 
publication), in which, following delivery of increasing hemorrhagic stimuli, c-Fos 
immunoreactivity within vasopressin and oxytocin neurons increased both in 
terms of the number of neurons expressing c-Fos and the intensity of the 
staining within each stimulated neuron. An example showing the general 
changes in c-Fos expression in the paraventricular nucleus is shown in figure 7. 
The changes in c-Fos expression paralleled the changes in hormone secretion. 
FRAs AS MARKERS FOR BASELINE ACTIVITY: THE 
TUBEROINFUNDIBULAR DOPAMINE NEURONS AS THE MODEL SYSTEM 
The analysis of LHRH, vasopressin, and oxytocin neurons, although 
demonstrating that c-Fos can effectively serve as a marker for stimulated 
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