Functional topography of the dorsomedial hypothalamus

The dorsomedial hypothalamus (DMH) has been proposed to mediate, perhaps integrate, autonomic components of both the defense reaction to acute stress and thermoregulatory responses to cold. We made a finer‐grained functional map of the DMH than achieved hitherto, to see whether ‘cold’ and ‘defense’ patterns might be generated by distinct DMH cell groups. We simultaneously recorded phrenic nerve activity and single cutaneous vasoconstrictor (CVC) fibers supplying plantar skin in 14 vagotomized, ventilated, urethane‐anesthetized rats (1–1.4 g/kg, i.v.). To characterize the response to ambient cooling, we cooled the trunk skin by a water jacket: this reversibly activated plantar CVC fibers and depressed phrenic burst rate. D,L‐homocysteic acid (DLH; 15 nl, 50 mM) microinjected into 76 DMH sites to activate small groups of cell bodies never reduced phrenic activity. Increased phrenic burst rate was evoked most strongly from caudal regions, plantar CVC activity from rostral regions: the ratio of the two responses varied systematically with location. In paired comparisons they were uncorrelated. Partly overlapping but distinct contour maps of response magnitude could be plotted over the DMH region for phrenic rate, CVC fibers and heart rate. These findings indicate that DMH neurons show functional topography but limited capacity to integrate autonomic/respiratory motor patterns. Supported by ARC.