Various Types of Thyrotropin-Releasing Hormone Receptors in Discrete Brain Regions and the Pituitary of the Rat

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Abstract
Receptors for thyrotropin-releasing hormone (TRH) in the rat brain and the pituitary are heterogenous. The receptors were classified into four types according to the dissociation constant (KD). High-affinity receptors (KD < 3 nM) are present in the pituitary, hypothalamus, amygdala, and limbic forebrain which contains the nucleus accumbens and the septum. Intermediate-affinity receptors (KD, 5–16 nM) are evidently present in the frontal cortex, hippocampus, striatum, thalamus, and the brainstem, but may also be present in other regions. Lowaffinity TRH receptors (KD, 50–80 nM) are seen in the limbic forebrain, amygdala, and the hypothalamus. Verylow-affinity receptors (KD, 215 nM) exist in the pituitary. Experiments using DN-1417 (γ-butyrolactone-γ-carbonyl-histidyl-prolinamide citrate), a synthetic TRH analogue with a more potent central activity, indicated the presence of TRH receptors having a high affinity to DN-1417 at least in the limbic forebrain but not in the pituitary. This type of receptor is not labeled by [3H](3methyl-histidine2)-TRH. Density of the TRH receptor is the highest in the pituitary and next highest in the amygdala.