Differential Content of Vesicular Glutamate Transporters in Subsets of Vagal Afferents Projecting to the Nucleus Tractus Solitarii in the Rat

Differential Content of Vesicular Glutamate Transporters in Subsets of Vagal Afferents Projecting to the Nucleus Tractus Solitarii in the Rat. Sam M. Hermes, James F. Colbert, and Sue A. Aicher* The Journal of Comparative Neurology 522:642–653 (2014). Vagal afferents are sensory nerves that originate from a variety of areas in the body and send information about the periphery to the brain. The first place they stop or what is called their primary termination site is the nucleus tractus solitariius or NTS. Hopefully the NTS is familiar to everyone as it is also the primary termination site of the arterial baroreceptor afferents. Both vagal (i.e. cardiopulmonary) and arterial baroreceptor afferents are thought to release glutamate as their primary neurotransmitter. Anatomically these afferents can be identified as glutamatergic if they contain one of three types of glutamate transporters (VGLUT1, VGLUT2, or VGLUT3) which help to package and release glutamate and produce primarily excitatory neurotransmission. It has been generally thought that the VGLUTs can identify different populations of neurons in the CNS since they're distribution tends to differ. In the present study, the authors wished to determine if myelinated and non-myelinated vagal afferents could be identified by their VGLUT expression. They did so by utilizing two different tracers: one being cholera toxin B subunit, better know as the CTB we also use in the lab. They also used a isolectin B4 or IB4 which seemed to have some selectivity for unmyelinated nerves in other systems. The authors injected both tracers into the vagus nerve and used confocal microsocopy of the NTS to examine the possibility of labeling these two different populations of fibers. In addition, they used electron microscopy to look at ultrastructural differences. The results demonstrated that CTb was found in both types of afferents (i.e. myelinated and unmyelinated) as well as on nerve terminals in the NTS. In contrast IB4 was observed only in unmyelinated afferents. At the ultrastructural level these tracers were found on primarily excitatory (i.e. glutamatergic) synapses as observed by the presence of asymmetrical post-synaptic densities on the electron micrographs. Interestingly, when they examined the VGLUT distributions, they were different on CTB vs. IB4 labeled neurons. CTb-labeled afferents had mostly VGLUT2 (83%), but IB4-labeled afferents had low levels of vesicular transporters, VGLUT1 (5%) or VGLUT2 (21%). These findings suggest the possibility that glutamate release from unmyelinated vagal afferents are regulated by a distinct, non-VGLUT, mechanism. This would be relevant to our studies since glutamate is the primary excitatory transmitter that not only drives RVLM neurons, but is also used by RVLM neurons to tonically excite sympathetic preganglionic neurons in the spinal cord. No one knows however if the RVLM neurons that control different sympathetic outputs contain different populations of VGLUTs (only because it hasn't been done to my knowledge). ~PJM