Relationship of presympathetic-premotor neurons to the serotonergic transmitter system in the rat brainstem

Numerous physiological conditions and emotionally motivated behaviors require concomitant activation of somatomotor and sympathetic efferents. Using a virally mediated retrograde transsynaptic tract-tracing approach, we have previously determined locations of presympathetic-premotor neurons (PSPMNs) in the rat brainstem. These putative dual-function neurons send projections to somatomotor and sympathetic targets and likely participate in sympatho-somatomotor integration. A significant portion of these neurons is found within brainstem areas known to contain serotonergic neurons. Thus, we hypothesized that some of the PSPMNs utilize serotonin as their neurotransmitter. To test this hypothesis we first produced an antibody against TPH2, a brain-specific isoform of tryptophan hydroxylase (serotonin synthetic enzyme). We identified PSPMNs by using recombinant strains of the pseudorabies virus (PRV) for transsynaptic tract-tracing. PRV-152, a strain that expresses enhanced green fluorescent protein, was injected into sympathectomized gastrocnemius muscle, while PRV-BaBlu, which expresses beta-galactosidase, was injected into the adrenal gland in the same animals. Using immunofluorescent methods we determined whether coinfected neurons expressed TPH2. Our findings demonstrate that TPH2-positive PSPMNs are present at different rostrocaudal levels of the brainstem. Just over half of them are found at the pontomedullary junction within raphe obscurus, raphe magnus, and gigantocellular nucleus pars alpha. These cells may play a role in mediating responses to acute pain stimuli and/or participate in the central control of exercise. Overactivity of these serotonergic sympatho-somatomotor circuits may also play a role in the pathophysiology of serotonin syndrome.