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Department of Physiology, Faculty of Medicine, Toyama Medical and Pharmaceutical University, Japan.

1. The activity of seventy-eight single neurones in the ventromedial hypothalamus (v.m.h.) was recorded in sixty-three freely behaving rats, and the effects of feeding, intraventricular (I.C.V.) administrations of noradrenaline, glucose, NaCl and ambient temperature on neuronal activity were analysed. If I.C.V. NaCl had an effect, intraperitoneal (I.P.) NaCl, mannitol and polyethylene glycol were also tested. 2. Neurones in the v.m.h. were classified into three groups according to diurnal variations and their relations to electroencephalogram (e.e.g.) and responses to feeding: diurnal-e.e.g. related (57/78, 73.1%); diurnal-e.e.g. independent (17/78, 21.8%); non-diurnal-e.e.g. independent (4/78, 5.1%). Of fifty-seven e.e.g.-related neurones, twenty-six decreased activity during feeding episodes. Of seventeen e.e.g.-independent neurones, eight increased activity gradually during feeding and sustained the increase after the feeding episode. The response magnitude of two e.e.g.-independent neurones depended on the kind of food available. 3. Of twenty-five e.e.g.-related neurones tested, twelve responded to I.C.V. noradrenaline, but not to I.C.V. glucose or NaCl. Neurones independent of e.e.g. responded variously to I.C.V. noradrenaline, glucose and NaCl. When I.C.V. NaCl had an effect, I.P. NaCl, mannitol and polyethylene glycol had the same effect. The activity of three neurones was increased by I.C.V. glucose and decreased by I.C.V. noradrenaline, but was not changed by I.C.V. NaCl. The activity of three was increased by I.C.V. glucose and decreased by I.C.V. NaCl and by I.C.V. noradrenaline. The activity of five was increased, and that of three was decreased by I.C.V. glucose, NaCl and noradrenaline. Collectively, fourteen of twenty-four tested neurones responded to I.C.V. glucose, twenty-six of forty-one tested neurones responded to I.C.V. noradrenaline and eleven of twenty-six tested neurones responded to I.C.V. NaCl. 4. Increase of ambient temperature changed the activity of five e.e.g.-independent neurones. Directions of these activity changes were the same as directions of responses to NaCl; two up, three down. 5. The results suggest two main neuronal groups in the v.m.h.: the e.e.g.-related group is involved in the processing of information about sleep-arousal. The e.e.g.-independent group contributes to the long-term processing of information concerned with the regulation of the internal environment such as glucose level, osmotic pressure, NaCl level, the trigger mechanism for feeding, ambient temperature, food preference, etc.

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