Lack of Usefulness of DN‐1417 for Characterization of a CNS Receptor for Thyrotropin‐Releasing Hormone

Abstract

CNS receptors for thyrotropin-releasing hormone (TRH) and its analogs are likely to mediate the experimentally and clinically observed net excitatory effect of these peptides on lower motor neurons. Previous findings suggest that several types of TRH receptors with distinct TRH analog specificities may be present in rat CNS. In particular, based on competition isotherm assays with unlabeled analog gamma-butyrolactone-gamma-carbonyl-L-histidyl-L-prolineamide (DN-1417). Funatsu et al. claim the existence of a limbic forebrain site that binds this peptide and TRH with high affinity but that does not bind [3-methyl-histidyl2]-TRH (MeTRH). Using saturation and competition isotherm experiments, we have examined the binding of [3H]TRH and [3H]DN-1417 in three regions of rat CNS: pyriform cortex/amygdala, limbic forebrain, and lumbosacral spinal cord. In all three regions, saturation assays with [3H]TRH (0.4-100 nM) resolved only a single, saturable receptor with high affinity (KD = 12-14 nM) for TRH; in no case could more than one saturable site be identified. When [3H]DN-1417 was substituted as the assay ligand, no high-affinity binding component for this analog could be detected in the three regions. Competition curves for the binding of unlabeled DN-1417 to limbic forebrain and lumbosacral spinal cord ([3H]TRH as assay ligand) were monophasic (not biphasic like those of Funatsu et al.) and indicative of low-affinity binding of DN-1417 in these regions (Ki values = 2-3 microM; in agreement with values obtained in similar assays with [3H]MeTRH).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological and biochemical comparison of thyrotropin releasing hormone (TRH) and di‐methyl proline‐TRH on pituitary GH3 cells

“…Whilst it may be tempting to speculate about the existence of multiple TRH-receptor subtypes (as has been suggested in brain tissue, Funatsu et al, 1985), given that the available ligands for this receptor are all agonists, the biphasic nature of the competition curves could equally well result from agonist-induced affinity changes and receptor kinetics. Indeed, other workers (Hawkins et al, 1987) have questioned the existence of more than one TRH-receptor subtype. It is interesting to note, from a structure-activity point of view, that the order of potency of the displacing ligands seems not to be related to simple structural modifications of the parent compound.…”