Glutamate- and GABA-mediated neuron–satellite cell interaction in nodose ganglia as revealed by intracellular calcium imaging

In the sensory ganglia, neurons are devoid of synaptic contacts, and ganglion neurons surrounded by one of glial cells, satellite cells. Recent studies suggest that neurons and satellite cells interact through neurotransmitters. In the present study, intracellular Ca²⁺ (Ca²⁺]_i) dynamics of neurons and satellite cells from one of viscerosensory ganglia, nodose ganglion (NG), were investigated by stimulation with glutamate and its agonist and/or the antagonist of the GABA_A receptor bicuculline. In the specimens containing neurons with satellite cells, glutamate and a metabotropic glutamate receptor (mGluR) agonist t-ACPD evoked Ca²⁺]_i increases in both neurons and surrounding satellite cells. Moreover, bicuculline also induced Ca²⁺]_i increases in neurons and satellite cells. However, in the isolated neurons, bicuculline did not cause an increase in Ca²⁺]_i, suggesting that satellite cells are equipped with the ability to release GABA. In the neurons associated with satellite cells, the delay time until the onset of a response was shorter in the case of glutamate stimulation with bicuculline than that without bicuculline (107.3 ± 93.4 vs. 231.8 ± 97.0 s, p < 0.01). Furthermore, immunoreactivities for glutamate transporter, GLAST, and GABA transporter, GAT-3, were observed in both neurons and satellite cells of NG. In conclusion, the levels of Ca²⁺]_i of NG neurons and surrounding satellite cells are increased by glutamate through at least mGluRs, and endogenous GABA modulates these responses; GABA inhibition is dependent on a close association between neurons and satellite cells. Such neuron–glia interaction in the nodose ganglion may regulate sensory information from visceral organs.