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Lack of usefulness of dn 1417 for characterization of a cns receptor for trh


, : Lack of usefulness of dn 1417 for characterization of a cns receptor for trh. Journal of Neurochemistry 49(1): 239-245

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 in 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 .mu.M); in agreement with values obtained in similar assays with [3H]MeTRH. We found that bacitracin in the assay buffer was useful in preventing the deamidation of [3H]TRH otherwise seen during incubation with limbic forebrain membranes. Our data do not support the existence of TRH receptor subtypes in the CNS. Rather, they favor the view that the neurotransmitter-like effects of high- and low-affinity TRH agonists are mediated by a single type of TRH receptor.

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