Prostaglandin E2 (PGE2) inhibits glutamatergic synaptic transmission in dorsolateral periaqueductal gray (dl-PAG)

Abstract

The purpose of this study was to determine the role of prostaglandin E(2) (PGE(2)) in modulating neuronal activity of the dorsolateral periaqueductal gray (dl-PAG) through excitatory and inhibitory synaptic inputs. First, whole cell voltage-clamp recording was performed to obtain excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) of the dl-PAG neurons. Our results show that PGE(2) significantly decreased the frequency of miniature EPSCs and amplitude of evoked EPSCs. The effects were mimicked by sulprostone, an agonist to PGE(2) EP(3) receptors. In contrast, PGE(2) had no distinct effect on IPSCs. In addition, spontaneous action potential of the dl-PAG neurons was recorded using whole cell current-clamp methods. PGE(2) significantly attenuated the discharge rate of the dl-PAG neurons. The decreased firing activity was abolished in the presence of glutamate NMDA and non-NMDA receptor antagonists. The results from the current study provide the first evidence indicating that PGE(2) inhibits the neuronal activity of the dl-PAG via selective attenuation of glutamatergic synaptic inputs, likely due to the activation of presynaptic EP(3) receptors.

Figure 1

The electrophysiological activity was recorded from the dorsolateral PAG (dl-PAG). At the end of each experiment, recording sites were examined under a microscope using differential interference contrast. The representative locations (solid circles) of recorded neurons are shown with anterior-posterior (AP) coordinates of the sections using Swanson’s rat brain maps. AQ, cerebral aqueduct; DR, dorsal nucleus raphe; IC, inferior colliculus external nucleus.

Figure 2

PGE₂ decreased the frequency of glutamatergic mEPSCs of the dl-PAG neurons. The effect was observed in eight neurons tested. A: Representative tracings from a dl-PAG neuron show that 5μM of PGE₂ attenuated the frequency of mEPSCs, and that the mEPSCs recovered during washout and completely abolished in the presence of 20 μM of CNQX. B&C: The cumulative probability analysis shows that PGE₂ increased the inter-event interval of mEPSCs but did not alter the distribution pattern of the amplitude of the mEPSCs. D&E: Average data show the effects of PGE₂ on the frequency and amplitude of mEPSCs of the dl-PAG neurons. *P<0.05, vs. control and washout.

Figure 3

EP₃ activation decreased the frequency of mEPSCs of the dl-PAG neurons. Sulprostone (5μM) was used to activate EP₃ in eight neurons. A: Representative tracings from a dl-PAG neuron show that sulprostone inhibited the frequency of mEPSCs, and the mEPSCs recovery during washout. B&C: Sulprostone increased the inter-event interval of mEPSCs without altering the distribution pattern of the amplitude of the mEPSCs. D&E: Average data show the effect of sulprostone. *P<0.05, vs. control and washout.

Figure 4

PGE₂ and sulprostone attenuated the peak amplitude of eEPSCs of the dl-PAG neurons and increased the PPR of eEPSCs. A&B: Typical traces from a dl-PAG neuron and summarized data (n=8) showing the peak amplitude of eEPSCs during control, PGE₂ and washout; and the PPR of eEPSCs. C&D: Typical traces from a dl-PAG neuron and summarized data (n=8) showing the peak amplitude of eEPSCs during control, sulprostone and washout; and the PPR of eEPSCs. *P<0.05, vs. control and washout for the amplitude; and vs. control for the PPR. The traces are average of 10 consecutive responses. Stimulation artifacts are removed and indicated by arrows.

Figure 5

PGE₂ had no distinct effects on GABAergic IPSCs of the dl-PAG neurons. Representative tracings from a dl-PAG neuron (A) and average data (B) show that the frequency and amplitude of spontaneous mIPSCs were not altered by bath application of 5μM of PGE₂. The results were seen in ten neurons tested. Effects of PGE₂ on eIPSCs were further examined in the dl-PAG neurons. Averaged traces of 10 consecutive responses from a dl-PAG neuron (C) and average data of eight neurons (D) show that 5μM of PGE₂ did not significantly alter the peak amplitude and PPR of eIPSCs. The eIPSCs were completely abolished with bicuculline.

Figure 6

PGE₂ had an inhibitory effect on the firing activity of the dl-PAG neurons. A: Original tracings from a dl-PAG neuron show the spontaneous discharge activity during control, PGE₂ (5 μM) perfusion and washout. B: Average data (n=8). *P<0.05, vs. control and washout. C&D: Original tracings from a dl-PAG neuron and average data (n=9) show the spontaneous discharge activity during control, CNQX (20 μM) plus AP-5 (50 μM), and PGE₂ perfusion in the presence of the glutamate receptors antagonists. The effect of PGE₂ was abolished after CNQX and AP-5 application.