Responses of preoptic thermosensitive neurons to medial forebrain bundle stimulation

Single-unit responses of neurons in the preoptic and anterior hypothalamus (PO/AH) to local thermal stimulation and electrical stimulation of the medial forebrain bundle (MFB) were studied in urethane-anesthetized male rats. In a total of 286 units (112 warm-units, 37 cold-units and 137 thermally insensitive units), 109 units (49 warm-units, 13 cold-units and 47 thermally insensitive units) responded to single pulse stimulation of MFB. The units initially inhibited by MFB stimulation corresponded to 64.2% (70 of 109), the units with facilitatory responses were 27.5% (30 of 109) and the antidromically activated units were 8.3% (9 of 109). High incidence of inhibition by noradrenaline (NA) applied iontophoretically was observed in the neurons inhibited by the MFB stimulation. Iontophoretic application of dichloroisoproterenol to 2 warm-units blocked both the NA-induced inhibition and the MFB-induced inhibition. These ascending and descending connections of the MFB with PO/AH thermosensitive neurons may be part of the neural circuits responsible for thermoregulation.