Effect of monoamines on firing rate and thermal sensitivity of neurons in the preoptic area of awake rabbits

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Single units in the preoptic/anterior hypothalamus (POAH) were studied by changing local temperature of the POAH with a water-perfused thermode. Norepinephrine (NE) or 5-hydroxytryptamine (5-HT) was injected either into the lateral ventricle or directly into the POAH. In some experiments the POAH temperature was changed both before and after the injection of a monoamine. The firing activity of cold-sensitive cells was increased by intraventricular and direct injections of NE and decreased by 5-HT. Ventricular 5-HT tended to facilitate warm-sensitive neurons. Ventricular NE and direct injections of NE and 5-HT had uniform effects on the activity of warm-sensitive neurons. Ventricular NE inhibited the majority of thermal-insensitive neurons. 5-HT consistently converted warm- and cold-sensitive units into thermal-insensitive units. NE converted cold-sensitive units into warm-sensitive units or increased the cold sensitivity of cold-sensitive neurons. Some insensitive units were converted into warm- or cold-sensitive units by the monoamines. The effects of 5-HT and NE on cold-sensitive neurons conforms to past investigations dealing with the role of these neurotransmitters in thermoregulation. Recording single-neuron activity in the unanesthetized preparation demonstrates that monoamine-induced changes in firing rate and thermal sensitivity of POAH neurons precedes changes in the thermoregulatory motor output.