A comparison between the hypothermia induced by intra-ventricular injections of thyrotropin releasing hormone, noradrenaline or calcium ions in unanaesthetized cats.

1 The hypothermia produced by intraventricular injections of thyrotropin releasing hormone (TRH) in unanaesthetized cats has been investigated. 2 TRH is more potent than either noradrenaline or calcium ions. It is estimated that the equi-potent molar ratio for TRH: noradrenaline:calcium is 1:900:27,000. 3 TRH injections is also produce profuse salivation, tachypnoea, cutaneous vasodilatation and frequently defaecation and vomiting. It is considered that the increased respiration is a major cause of the hypothermia. 4 Prior administration of phentolamine antagonized noradrenaline-induced hypothermia but did not affect hypothermia produced by TRH or calcium ions. Pretreatment with alpha-methyltyrosine did not affect the hypothermia induced by TRH, calcium ions or noradrenaline. 5 The calcium antagonists verapamil and xylocaine did not antagonize hypothermia induced by an injection of calcium ions. 6 The constituent amino acids of TRH did not produce hypothermia either individually or collectively. Thyroxine sodium produced a rise in temperature that was slow in onset, consistent with its known metabolic effects. TSH produced a small hypothermia unrelated to dose.