Norvaline2-TRH : binding to TRH receptors in rat brain homogenates

Abstract Norvaline 2 -thyrotropin-releasing hormone ([Nva 2 ]TRH) has been described as a thyrotropin-releasing hormone (TRH) analog with no thyrotropin (TSH)-releasing capacity but enhanced analeptic activity compared with TRH, as shown by the reversal of haloperidol-induced catalepsy. We have evaluated the receptor-binding properties of [Nva 2 ]TRH in homogenates of rat anterior pituitary, hypothalamus, brainstem and cortex tissue, using [ 3 H]TRH and [ 3 H][3-Me-His 2 ]TRH as radioligands. Apparent K i values at high affinity TRH-binding sites, labelled predominantly by [ 3 H][3-Me-His 2 ]TRH, ranged from 17.0 to 36.9 μM in all tested regions. Additionally, [Nva 2 ]TRH was shown to compete with [ 3 H]TRH at low affinity TRH-binding sites with similar affinities. It is concluded that the loss of TSH-releasing activity of [Nva 2 ]TRH appears to be due to a drastic reduction in binding affinity to the high affinity TRH receptor subtype. Its analeptic activity, however, may be mediated by low affinity TRH binding sites which are predominantly labelled by [ 3 H]TRH or by yet unidentified mechanisms.