Receptor binding of fluorinated histidine analogs of thyrotropin-releasing hormone in various regions of the brain

Binding properties of (4(5)-fluoro-imidazole-His super(2))-TRH (4(5)-F-TRH), (2-trifluoromethyl-imidazole-His super(2))-TRH (2-CF sub(3)-TRH) and (4(5)-trifluoromethyl-imidazole-His super(2))-TRH (4(5)-CF sub(3)-TRH), three novel TRH analogs, have been evaluated in rat pituitary, hypothalamus, brainstem and cortex tissue. 4(5)-F-TRH, previously shown to elicit arterial pressor responses and prolactin release similar to those of TRH, binds to TRH receptors with low, micromolar affinity. It is concluded that the biological effects of these analogs are mediated either through low affinity TRH binding sites not recognized by ( super(3)H)(3Me-His super(2))-TRH or through mechanisms not involving TRH receptors as such.