We have previously described a daily rhythm in thyrotropin releasing h
ormone (TRH) and TRH mRNA in the rat hypothalamus. To determine whethe
r TRH release fluctuates in a diurnal manner, we have measured basal a
nd potassium stimulated release from hypothalamic slices, and compared
it to release from olfactory bulb slices, during the diurnal cycle. B
asal TRH release was higher at 7:00 h than at any other time (1:00, 13
:00 or 19:00 h) in either hypothalamus or olfactory bulb. The ratio of
stimulated over basal release was higher in the hypothalamus at 19:00
h, when TRH content was highest. Potassium stimulated TRH release fro
m olfactory bulb was not different from basal release at any time. TRH
release fluctuations were not due to a rhythm of extracellular inacti
vation: the activity of pyroglutamyl aminopeptidase II, an ectoenzyme
responsible for TRH inactivation, was constant throughout the cycle. O
ur data demonstrate that diurnal variations of TRH release occur in vi
tro and that the enhanced responsiveness to potassium stimulation in h
ypothalamus is correlated with increased levels of peptide.