Thermal characterization and transmitter analysis of single units in the preoptic and anterior hypothalamus of conscious ducks

With a multibarrel assembly combining one carbon fiber micropipette as recording electrode and 6 pipettes for microiontophoretic application of drugs, the activity of neurons in the preoptic and anterior hypothalamic (POAH) region was extracellularly recorded in situ in conscious ducks implanted chronically with a device permitting hypothalamic thermal stimulation. Among 355 neurons 17% were identified as warm-responsive (warm units) and 20% as cold-responsive (cold units). In 58 warm and 56 cold units control discharge rates at 40°C local temperature (F₄₀) and temperature coefficients (F/T) were determined and presented as means ± SEM. The F₄₀ values of warm units (35.2±2.3 Imp · s^–1) were significantly higher than of cold units (16.3±1.8 Imp · s^–1). The F/T values (+1.77±0.15 and –1.77±0.19 Imp · s^–1 · °C^–1) of warm and cold units were not different in absolute terms. In pilot experiments either activation or inhibition by lowering whole-body temperature was observed in both warm and cold units. Microiontophoretic application of one or more of the amines acetylcholine (ACh), 5-hydroxytryptamine (5-HT), and noradrenaline (NA) to warm and cold units revealed differences in their responsiveness to ACh, which more consistently stimulated cold units. NA inhibited the majority of warm units; 5-HT stimulated the majority of cold units. In both warm and cold units NA and ACh differed in their actions, with the latter amine more consistently producing activation.