Cellular substrates for interactions between neurons containing phenylethanolamine N‐methyltransferase and GABA in the nuclei of the solitary tracts

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Abstract
Adrenaline and γ‐aminobutyric acid (GABA) have been implicated in autonomic functions involving the intermediate and caudal portions of the medial nuclei of the solitary tracts (m‐NTS). We sought to determine whether there was a cellular basis for direct intracellular or synaptic interactions between these transmitters in neurons in the m‐NTS of rat brain by using dual‐labeling immunocytochemical methods. Light microscopy revealed immunoautoradiographic labeling for the adrenaline‐synthesizing enzyme phenylethanolamine N‐methyltransferase (PNMT) in perikarya and processes in close proximity to cells demonstrating peroxidase reaction product for GABA. Electron microscopy of the intermediate m‐NTS at the level of the area postrema further established the localization of immunoautoradiographic and peroxidase labels for PNMT and GABA in common as well as separate perikarya and dendrites. All axon terminals were labeled separately for PNMT and GABA. The PNMT‐labeled terminals formed both symmetric and asymmetric synapses, whereas the GABA‐labeled terminals formed exclusively symmetric synapses. Twenty‐four percent (n = 42) of the PNMT‐ and 39% (n = 128) of the GABA‐labeled terminals formed synaptic junctions on unlabeled soma and dendrites. Occasionally both types of terminals converged on a common unlabeled dendrite and on GABA‐labeled dendrites. Only 3% of the PNMT‐ and 12% of the GABA‐containing terminals formed synapses on PNMT‐labeled soma and dendrites, whereas 7% of each type formed synapses with GABA‐labeled profiles. The remaining labeled terminals lacked synaptic relations within the sections examined. The autoradiographic results were confirmed and extended by means of immunogold labeling for PNMT in combination with peroxidase‐antiperoxidase localization of the GABA‐synthesizing enzyme glutamic acid decarboxylase (GAD). GAD‐labeled terminals formed symmetric synapses with dendrites that were either unlabeled or contained low levels of PNMT (gold particles) or PNMT and GAD. We conclude that in caudal, more cardiovascular portions of the NTS, adrenaline and GABA may coexist, but they are more commonly detected in separate populations of neurons having receptive sites for both transmitters and innervating certain common target neurons.

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