The synthetic polyribonucleotide
pyrogen Poly I:
Poly C (800 micrograms.kg-1) was injected intramuscularly on alternate days into pregnant and non-pregnant female guinea pigs. Pregnant animals, close to term, had smaller
fevers in response to the
pyrogen than did non-pregnant animals. Repeated
injections of the
pyrogen caused sequentially smaller
fevers for the first 3-4
injections, particularly in non-pregnant animals, and this appeared to be like the tolerance usually developed to repeated
injections of
endotoxin. Continued
pyrogen injections then caused, in non-pregnant animals,
fevers of increasing magnitude until the original
fever levels were reached, whereas in pregnant guinea pigs the
fever responses remained reduced until parturition. The development of tolerance was associated with an increase in immunoreactivity for
arginine vasopressin (AVP) in some neurons in the medial part of the paraventricular nucleus, and in terminals in the lateral septum and amygdala similar to changes found in these areas at term of pregnancy. These observations raise the possibility that AVP in these regions may have a role in the development of tolerance to
pyrogens, and further quantitative studies of the AVP content of, and release from, nerve terminals projecting to the limbic system seem warranted.