Effects of biogenic amines on central thermoresponsive neurones in the rabbit

1. Single unit activities were recorded with five-barrelled micropipettes from the thermo-responsive neurones in the preoptic area and the mid-brain reticular formation in urethanized rabbits. 5-hydroxytryptamine (5-HT), noradrenaline (NA) and acetylcholine (ACh) were applied micro-iontophoretically to the immediate vicinity of the recording cells.2. Out of seventeen warm-responsive neurones recorded in the preoptic area, fifteen neurones responded to 5-HT with the increase in firing rate and two showed no response. Thirteen out of seventeen warm-units decreased their firing rate in response to application of NA and four were not affected. ACh had no effect on any of the warm-units examined.3. Six out of seven cold-units in the preoptic area were depressed by 5-HT, while NA excited five of six units studied. None of the cold-units were influenced by ACh.4. These results are in good agreement with the changes in rectal temperature produced by 5-HT and NA micro-injected into the hypothalamus in rabbits.5. In the mid-brain reticular formation, 5-HT excited all of fourteen cold-responsive neurones. Of these, eight cold-units were depressed and six were unaffected by NA, while ACh excited six units and had no effect on eight units. All of the five warm-responsive units were inhibited by 5-HT and none were influenced by NA. Thus, the responses of reticular thermoresponsive neurones to 5-HT and NA were opposite to those of the preoptic thermo-responsive neurones.