Evidence for a peripheral action of thyrotropin releasing hormone on gastrointestinal transit in mice

The effect of the two analogs of thyrotropin-releasing hormone (TRH), namely MK-771 and DN-1417, administered peripherally, on the actions of morphine and loperamide on gastrointestinal transit, was investigated in mice. The actions of naltrexone and methyl naltrexone on these effects were also determined in order to assess the central or peripheral mechanism of action. The administration of morphine sulfate (5 mg/kg) inhibited gastrointestinal transit, as measured by the charcoal meal test. The effect of morphine was potentiated by MK-771 and DN-1417 at the 10 mg/kg dose. The peripheral administration of DN-1417 inhibited gastrointestinal transit but MK-771 had no effect. Naltrexone antagonized the potentiating effect of the peptides on the effect of morphine on gastrointestinal transit. Methyl naltrexone, which crosses the blood-brain barrier with difficulty, also antagonized the potentiating effect of the peptides on the action of morphine on gastrointestinal transit. A subthreshold dose (0.5 mg/kg) of loperamide, a peripherally acting opiate, which had no effect on gastrointestinal transit by itself, when combined with MK-771 or DN-1417 significantly inhibited gastrointestinal transit. The administration of methyl naltrexone had little effect on loperamide-induced inhibition of gastrointestinal transit but antagonized the potentiating effect of MK-771 and DN-1417. It is concluded that the effects of analogs of thyrotropin releasing hormone on gastrointestinal transit involve peripheral sites of action, the effect of DN-1417 was not antagonized by naloxone and that the inhibitory effect of DN-1417 on gastrointestinal transit is not mediated by opiate receptors.