The influence of the ventrolateral medulla on thermoregulatory circulations in the rat.

The rostral ventrolateral medulla (RVLM) was stimulated electrically and chemically (0.1-0.2 microliter, 0.4 M DL-homocysteic acid) in urethane-anaesthetised rats. Changes in the vasomotor tone of the cutaneous circulation of the tail, front and hind feet, the nose and the fur covered areas of the back, proximal hind leg and neck were assessed indirectly by measurement of skin temperatures. Electrical stimulation of the RVLM at 25 Hz (2-5 min) produced sustained decreases in skin temperatures and differed from the effect induced by stimulation at 100 Hz or chemical stimulation, in that the concomitant increases in blood pressure and respiration, as well as the accompanying bradycardia or tachycardia, were minimal and within 10-15% of basal control levels. In the hyperthermic animal changes in the skin temperatures of the tail and feet were observed on stimulation of sites lateral to the rostral third of the inferior olive (IO) and which extended through the nuc. paragigantocellularis lateralis to an area medial to the caudal half of the facial nucleus (FN). There was some degree of topographical organisation with the front and hind feet areas overlying the more extensive tail area in a rostro-caudal orientation. The representation of the other areas of the body was more limited. Neck and nose temperatures changed on stimulation of a small area of the RVLM adjacent to the caudal pole of FN, while the skin of the back and upper hind limb responded to stimulation of a limited area lateral to the rostral pole of IO. The unequal representation of the cutaneous surface probably reflects the degree of vascularisation and the importance of the tail and plantar foot surfaces in thermoregulatory responses.