Prostaglandin E2 depresses solitary tract-mediated synaptic transmission in the nucleus tractus solitarius

Prostaglandin E2 (PGE2) is a prototypical inflammatory mediator that excites and sensitizes cell bodies [Kwong K, Lee LY (2002) PGE2 sensitizes cultured pulmonary vagal sensory neurons to chemical and electrical stimuli. J Appl Physiol 93:1419-1428; Kwong K, Lee LY (2005) Prostaglandin E2 potentiates a tetrodotoxin (TTX)-resistant sodium current in rat capsaicin-sensitive vagal pulmonary sensory neurons. J Physiol 56:437-450] and peripheral nerve terminals [Ho CY, Gu Q, Hong JL, Lee LY (2000) Prostaglandin E (2) enhances chemical and mechanical sensitivities of pulmonary C fibers in the rat. Am J Respir Crit Care Med 162:528-533] of primary vagal sensory neurons. Nearly all central nerve terminals of vagal afferents are in the nucleus tractus solitarius (NTS), where they operate with a high probability of release [Doyle MW, Andresen MC (2001) Reliability of monosynaptic sensory transmission in brain stem neurons in vitro. J Neurophysiol 85:2213-2223]. We studied the effect of PGE2 on synaptic transmission between tractus solitarius afferent nerve terminals and the second-order NTS neurons in brain stem slices of Sprague-Dawley rats. Whole-cell patch recording in voltage clamp mode was used to study evoked excitatory postsynaptic glutamatergic currents (evEPSCs) from NTS neurons elicited by electrical stimulation of the solitary tract (ST). In 34 neurons, bath-applied PGE2 (200 nM) decreased the evEPSC amplitude by 49±5%. In 22 neurons, however, PGE2 had no effect. We also tested 15 NTS neurons for capsaicin sensitivity. Seven neurons generated evEPSCs that were equally unaffected by PGE2 and capsaicin. Conversely, evEPSCs of the other eight neurons, which were PGE2-responsive, were abolished by 200 nM capsaicin. Furthermore, the PGE2-induced depression of evEPSCs was associated with an increase in the paired pulse ratio and a decrease in both the frequency and amplitude of the spontaneous excitatory postsynaptic currents (sEPSCs) and TTX-independent spontaneous miniature excitatory postsynaptic currents (mEPSCs). These results suggest that PGE2 acts both presynaptically on nerve terminals and postsynaptically on NTS neurons to reduce glutamatergic responses.