Cold stress alters characteristics of sympathetic nerve discharge bursts

Frequency-domain analyses were used to determine the effect of cold stress on the relationships between the discharge bursts of sympathetic nerve pair s, sympathetic and aortic depressor nerve pairs, and sympathetic and phreni c nerve pairs in chloralose-anesthetized, baroreceptor-innervated rats. Sym pathetic nerve discharge (SND) was recorded om the renal, lumbar, splanchni c, and adrenal nerves during decreases in core body temperature from 38 to 30 degrees C. The following observations were made. 1) Hypothermia produced nonuniform changes in the level of activity in regionally selective sympat hetic nerves. Specifically, cold stress increased lumbar and decreased rena l SND but did not; significantly change the level of activity in splanchnic and adrenal nerves. 2) The cardiac-related pattern of renal, lumbar, and s planchnic SND bursts was transformed to a low-frequency (0-2 Hz) pattern du ring cooling, despite the presence of pulse-synchronous activity in arteria l baroreceptor afferents. 3) Peak coherence values relating the discharges between sympathetic nerve pairs decreased at the cardiac frequency but were unchanged at low frequencies (0-2 Hz), indicating that the sources of low- frequency SND bursts remain prominently coupled during progressive reductio ns in core body temperature. 4) Coherence of discharge bursts in phrenic an d renal sympathetic nerve pairs in the 0- to 2-Hz frequency band increased during mild hypothermia (36 degrees C) but decreased during deep hypothermi a (30 degrees C). We conclude that hypothermia profoundly alters the organi zation of neural circuits involved in regulation of sympathetic nerve outfl ow to selected regional circulations.