Serotonin Stimulates Thyrotropin-Releasing Hormone Release from Superfused Rat Hypothalami

Abstract

Spontaneous and induced TRH release from fragments of the male rat mediobasal hypothalamus (MBH) superfused in vitro for several h were studied by means of a specific and sensitive RIA. Recovery of exogenous TRH added to superfusion chambers with or without tissues was 98%. The spontaneous release of TRH over the initial 60 min of superfusion was 0.31 pg/MBH · min. The MBH TRH content was 2128 pg/MBH or 238 pg/mg after 4 h of superfusion. Results from serial dilutions of MBH superfusates and MBH extracts showed inhibition curves parallel to that of synthetic TRH. In a chromatographic column system, similar samples eluted undistinguishable from synthetic TRH. A high dose of K⁺ given as a pulse triggered an immediate TRH release (280% of basal) that peaked during the first 10 min of collection. MBHs stimulated with a 1-h 60-mM K⁺ infusion sustained a 7-fold increase in TRH release. Pulses of serotonin (initial concentration in chambers, 2 × 10^-8 M) stimulated TRH release to 466% of basal control values. Moreover, 2 × 10^-l0 and 2 × 10^-12 M serotonin increased TRH release to 413% and 225%, respectively. This stimulatory effect of serotonin was blocked by pretreatment with methysergide (2 × 10^-6 M). A constant infusion of serotonin (2× 10^-8 M) for 1 h resulted in episodic release of TRH. The stimulatory effects of potassium and serotonin on TRH release were also confirmed in incubation experiments. These results show that hypothalamic fragments superfused in vitro release immunoactive TRH which has a similar mobility to synthetic TRH in a chromatographic column system. It has been demonstrated that very low concentrations of serotonin directly activate the release of TRH from MBH fragments in vitro. The effect of serotonin can be blocked by an antagonist of this drug. Depolarization of the nerve terminals by continuous infusion of K⁺ can sustain an increased release of TRH for at least 1 h. In addition, these results add new support to the previous demonstration from this laboratory of the usefulness of the superfusion system as a technique for studying the dynamic pattern of the release of neuropeptides from brain fragments.