Responses of anterior hypothalamic-preoptic thermosensitive neurons to substance P and capsaicin

The effects of capsaicin and substance P, applied locally, on the activity of thermosensitive and thermally-insensitive neurons in the anterior hypothalamic-preoptic area were studied with the use of multibarrelled microelectrodes in the urethane-anesthetized rat. In a total of 34 thermosensitive neurons (21 warm-units and 13 cold-units) in the anterior hypothalamic-preoptic area, only 7 units responded to substance P and 27 units (79.4%) were not affected. In contrast, 23 of 57 (40.4%) thermally-insensitive neurons responded to substance P. Of 74 thermosensitive and thermally-insensitive neurons tested with both substance P and capsaicin, only 7 units (9.5%) showed the same direction of change in activity in response to the application of both drugs. The majority of the neurons responded to only one of the drugs (32 units, 43.2%) or responded to substance P and capsaicin in opposite directions (9 units, 12.2%). A substance P antagonist, (D-Pro2, D-Trp7,9)-substance P, blocked the substance P-induced excitatory responses in 5 of 6 neurons tested in the anterior hypothalamic-preoptic area, but not the capsaicin-induced excitatory or inhibitory responses (n = 5). It is unlikely that substance P in the anterior hypothalamic-preoptic area participates in the hypothermic effects of capsaicin, observed after local injection.