Blood level and brain distribution of thyrotropin-releasing hormone(TRH) determined by radioimmunoassay after intravenous administration in rats.

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Abstract
The blood level and brain distribution of immunoreactive TRH [thyrotropin-releasing hormone] after i.v. administration of TRH tartrate monohydrate (TRH-T) were investigated by radioimmunoassay in rats. Optimal condition for complete inactivation of TRH-degradative enzymes in brain by a microwave irradiation was 5kW power for 1.5 s. The endogenous TRH was present at the highest concentration in the hypothalamus, followed by the brain stem and thalamus, and was the lowest in the cerebral cortex including hippocampus and cerebellum. The time-course of TRH blood levels after TRH-T administration at 0.5, 5 and 25 mg/kg fitted to a 2 compartment open model; and the half-lives [t1/2] in .beta.-phase increased dose-dependently. The administered TRH was incorporated rapidly from blood into whole brain and the peak brain level corresponding to 0.108-0.166% of the total dose administered was attained 1 min after the administration. The brain TRH was eliminated with a t1/2 of .apprx. 3 min. The administered TRH was distributed throughout the brain, at the highest concentration in the hypothalamus, followed by the brain stem, cerebellum, thalamus and cerebral cortex. TRH tended to be eliminated most rapidly from the cerebellum.