Activation of spinal cord serotonergic neurons accompanies cold-induced sympathoexcitation

These studies examined the hypothesis that serotonergic neurons located in central sites known to be involved with autonomic regulation are activated by cold exposure, a potent stimulator of the sympathetic nervous system. In all experiments, rats were exposed to either 3 degrees C or 22 degrees C for 24 h. Significant increases (p < 0.05) in urinary norepinephrine excretion, depletions of myocardial norepinephrine, and enhanced myocardial L-DOPA accumulation following decarboxylase inhibition provided evidence of sympathoexcitation at 3 degrees C. Accumulations of the serotonin metabolite 5-hydroxyindoleacetic acid, in saline-injected rats, and 5-hydroxytryptophan in decarboxylase-inhibited rats were increased in spinal cord and brainstem regions of cold-exposed rats. Two hours after injection of the serotonin synthesis inhibitor p-chlorophenylalanine, significantly greater depletions of serotonin in spinal cord and 5-hydroxyindoleacetic acid in spinal cord and brainstem of cold-exposed rats were noted; synthesis inhibition also caused a larger drop in body temperature in cold-exposed rats. Microdissections dissections of raphe nuclei and thoracic spinal cord sites indicated that the principal sites of serotonergic activation were the dorsal and intermediate spinal regions, and the raphe magnus. Thus, cold-induced sympathoexcitation was accompanied by activation of serotonergic neurons in spinal cord and brainstem regions known to be involved in autonomic regulation.