Regional distribution of in vitro release of thyrotropin releasing hormone in rat brain.

To increase our knowledge of the TRH functions in brain and the processes of TRH compartmentalization and release, we studied the in vitro release of endogenous TRH in different brain areas. We also determined the correlation between TRH levels and release under both basal and stimulated conditions. TRH concentration was measured in tissues and media by specific radioimmunoassay. TRH-like material detected in olfactory bulb and hypothalamic incubates (basal or K+ stimulated) were shown to be chromatographically identical to synthetic TRH. Different brain regions showed high variability in the basal release of TRH (1-20% of tissue content). This suggests the existence of different pools. The response to depolarizing stimulus (56 mM K+) was significant only in the following regions: median eminence, total hypothalamus, preoptic area, nucleus accumbens-lateral septum, amygdala, mesencephalon, medulla oblongata and the cervical region of the spinal cord. These regions have been shown to contain a high number of receptors, a high concentration of TRH nerve endings and are susceptible to TRH effects. These results support the hypothesis that TRH functions as neuromodulator in these areas.