Angiotensin II inhibits GABAergic synaptic transmission in dorsolateral periaqueductal gray neurons

Abstract

The purpose of this study was to determine the role of angiotensin II (Ang II) in modulating inhibitory and excitatory synaptic inputs to the dorsolateral periaqueductal gray (dl-PAG). The whole cell voltage-clamp recording was performed to examine inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs) of the dl-PAG neurons. Ang II, at the concentration of 2microM, decreased the frequency of miniature IPSCs from 0.83+/-0.02 to 0.45+/-0.03Hz (P<0.05) in 10 tested neurons. This did not significantly affect the amplitude and decay time constant. The effect of Ang II on miniature IPSCs was blocked by the prior application of Ang II AT1 receptor antagonist losartan, but not by AT2 receptor antagonist PD123319. Additionally, Ang II decreased the amplitude of evoked IPSCs from 148+/-15 to 89+/-7pA (P<0.05), and increased the paired-pulse ratio from 96+/-5% to 125+/-7% (P<0.05) in eight tested neurons. In contrast, Ang II had no distinct effects on the EPSCs. Our data suggest that Ang II inhibits GABAergic synaptic inputs to the dl-PAG through activation of presynaptic AT1 receptors.

Fig. 1

Ang II decreased the frequency of GABAergic mIPSCs of the dl-PAG neurons. The effect was observed in 10 neurons tested. (A) Representative tracings from a dl-PAG neuron show that 2 µM of Ang II attenuated the frequency of mIPSCs, and that the mIPSCs recovered during washout and completely abolished in the presence of 20 µM of bicuculline. (B and C) The cumulative probability analysis shows that Ang II increased the inter-event interval of mIPSCs but did not alter the distribution pattern of the amplitude of the mIPSCs. (D and E) Average data show the effects of Ang II on the frequency and amplitude of mIPSCs of the dl-PAG neurons. *P < 0.05, vs. control and washout.

Fig. 2

Effects of Ang II on the mIPSCs with the prior application of losartan and PD123319. (A) Summary data showing the antagonizing effect of losartan on Ang II inhibition of the mIPSCs frequency in eight dl-PAG neurons. (B) The mIPSCs amplitude of dl-PAG neurons after Ang II with the prior losartan. (C) Summary data showing that PD123319 had no effect on Ang II attenuation of the mIPSCs frequency in eight dl-PAG neurons. (D) The mIPSCs amplitude of dl-PAG neurons after Ang II with the prior PD123319. *P < 0.05, vs. control and washout.

Fig. 3

Ang II attenuated the peak amplitude of eIPSCs of the dl-PAG neurons and increased the PPR of eIPSCs. (A and B) Typical traces from a dl-PAG neuron and average data showing the peak amplitude of eIPSCs during control, Ang II and washout (n = 8); and the PPR of eIPSCs (n = 8). *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 were removed and indicated by arrows.

Fig. 4

Ang II had no distinct effects on glutamatergic EPSCs 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 mEPSCs were not altered by bath application of 2 µM of Ang II. The results were seen in seven neurons tested. Effects of Ang II on eEPSCs were further examined in 19 dl-PAG neurons. Averaged traces of 10 consecutive responses from a dl-PAG neuron (C) and average data (D) show that 2 µM of Ang II did not significantly alter the peak amplitude (n = 10) and PPR (n = 9) of eEPSCs. The mEPSCs and eEPSCs were completely abolished with CNQX.