IN-VITRO TRH RELEASE FROM HYPOTHALAMUS SLICES VARIES DURING THE DIURNAL CYCLE

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.