INCREASE IN RAT REGIONAL BRAIN CYCLIC NUCLEOTIDES BY THYROTROPIN-RELEASING HORMONE (TRH) AND ITS ANALOG DN-1417

Abstract

The effects of thyrotropin-releasing hormone (TRH) and its analog γ-butyrolactone-γ-carbonyl-L-histidyl-L-prolinamide citrate(DN-1417) on adenosine 3', 5'-monophosphate (cyclic AMP) and guanosine 3', 5'-monophosphate (cyclic GMP) levels in rat brain were investigated using a radioimmunoassay method. The time course of elevation of these nucleotides in various brain regions after administration of DN-1417 showed a peak at 5 to 15 min followed by a gradual decrease. DN-1417 (1 to 10 mg/ kg i.p.) caused a dose-related increase in cyclic AMP levels in the cerebellum, cerebral cortex, striatum, nucleus accumbens, thalamus, hypothalamus and brain stem; whereas significant increases in cyclic GMP were observed in the cerebellum, nucleus accumbens and brain stem. TRH (3 to 10 mg/kg i.p.) caused significant increases of cyclic AMP in the cerebellum, cerebral cortex, nucleus accumbens, thalamus and brain stem and also caused an increase in cerebellar cyclic GMP. With one exception, DN-1417, apomorphine (Apo), methamphetamine (MAP) (all, 3 mg/kg i.p.) and TRH (10 mg/kg i.p.)-induced increases in cyclic nucleotides were blocked by pimozide (1 mg/kg i.p., 4 hr before), a dopamine receptor blocker; the exception was a TRH-induced increase in cerebellar cyclic GMP. These increases were not blocked by propranolol (10 mg/kg i.p., 30 min before), an adrenergic β-receptor blocker. α-Methyl-p-tyrosine (α-MT, 250 mg/kg i.p., 4 hr before), a tyrosine hydroxylase inhibitor, almost completely blocked DN-1417- and MAP-induced increases in cyclic nucleotides, slightly blocked TRH-effects, and had no effect on Apo-effects. These in vivo results were confirmed in an in vitro system using brain slices. The addition of DN-1417 (10^-4 M) or TRH (10^-3 M) significantly enhanced the spontaneous [³H]-dopamine and [³H]-norepinephrine release from the superfused slices of the rat nucleus accumbens and cerebral cortex in vitro. The addition of DN-1417 (10^-4 M) or TRH (10^-4M) had no effect on the activities of adenylate cyclase and guanylate cyclase, although only a high concentration (10^-3 M) of DN-1417 inhibited the cyclic AMP- and cyclic GMP-hydrolytic activities in various brain region homogenates. These results suggest that DN-1417 does not produce an increase in the levels of cyclic nucleotides by direct receptor-enzyme activation, but that DN-1417 like MAP causes the increase through endogenous catecholaminergic, particularly dopaminergic activation.