Effects of Propofol on Electrical Synaptic Strength in Coupling Reticular Thalamic GABAergic Parvalbumin-Expressing Neurons

Abstract

Electrical synapses between neurons exhibit a high degree of plasticity, which makes critical contributions to neuronal communication. The GABAergic parvalbumin-expressing (PV+) neurons in the thalamic reticular nucleus (TRN) interact with each other through electrical and chemical synapses. Plasticity of electrical synaptic transmission in TRN plays a key role in regulating thalamocortical and corticothalamic circuits and even the formation of consciousness. We here examined the effects of propofol, a commonly used general anesthetic agent, on the strength of electrical synapses between TRN PV+ neurons by fluorescence-guided patch-clamp recording and pharmacological methods. Results show that 100 μM propofol reduced the electrical synaptic strength between TRN PV+ neurons. Notably, the propofol-induced depression of electrical synaptic strength between TRN PV+ neurons was diminished by saclofen (10 μM, antagonist of GABA_B receptors), but not blocked by gabazine (10 μM, antagonist of GABA_A receptors). Application of baclofen (10 μM, agonist of GABA_B receptors), similar to propofol, also reduced the electrical synaptic strength between TRN PV+ neurons. Moreover, the propofol-induced depression of electrical synaptic strength between TRN PV+ neurons was abolished by 9-CPA (100 μM, specific adenylyl cyclase inhibitor), and by KT5720 (1 μM, selective inhibitor of PKA). Our findings indicate that propofol acts on metabotropic GABA_B receptors, resulting in a depression of electrical synaptic transmission of coupled TRN PV+ neurons, which is mediated by the adenylyl cyclase-cAMP-PKA signaling pathway. Our findings also imply that propofol may change the thalamocortical communication via inducing depression of electrical synaptic strength in the TRN.

Keywords: electrical synapses; general anesthesia; propofol; synaptic strength; thalamic reticular nucleus.

FIGURE 1

Recording of the electrical strength of coupled TRN neurons. (A) Low-magnification photomicrograph showing two recording electrodes in the TRN region of a thalamic slice preparation. IC, internal capsule; VPL, ventral posterolateral nucleus. (B) High-magnification view of a pair of TRN neurons (labeled TRN1 and TRN2) with adjoining somata with tips of recording electrodes. (C) The TRN neurons were confirmed as PV+ neurons by somatic tdTomato fluorescence. (D) Recording traces of voltage responses to hyperpolarizing current injected to each TRN neurons. Left, current injection into TRN 1 produced a direct voltage deflection of TRN 1 (black trace) and a non-direct (via electrical synaptic transmission) voltage deflection of TRN 2 (gray trace). Right is the reverse. (E) electrical transmission asymmetry of cc plotted against Rin ratio. (F) Electrical transmission asymmetry of Gc plotted against Rin ratio.

FIGURE 2

Propofol reduces the electrical synaptic strength of electrical coupled TRN neurons. (A) Example traces of voltage responses of a coupled pair of TRN neurons before (I), during (II), and after (III) propofol application (100 μM). (B) Time course calculations of Gc for traces recorded from the pair of TRN neurons shown in A, correspondence time points of the traces in A were marked as I to III. (C) Average Gc values taken 10 min before, 10 min after propofol application, and 10 min after propofol washout (means ± SD, n = 21 pairs). Average Gc values at 10 min before propofol application and 10 min after drug washout were taken to calculate ΔGc. **p < 0.01, repeated-measures ANOVA.

FIGURE 3

Effects of GABA_A receptor antagonist gabazine on propofol-induced depression of electrical synaptic strength of TRN neurons. (A) With 10 μM gabazine in the ACSF perfusion, example traces of voltage responses of a coupled pair of TRN neurons before (I), during (II), and after (III) propofol application. (B) Time course calculations of Gc for all traces of the pair of TRN neurons shown in A, correspondence time points of the traces in A were marked as I to III. (C) Average Gc values taken 10 min before, 10 min after propofol application, and 10 min after propofol washout (means ± SD, n = 15 pairs). (D) Compare of the mean and deviation of Gc (in 10 min) in control conditions, and in 10 μM gabazine. Average Gc values at 10 min before propofol application and 10 min after drug washout were taken to calculate ΔGc. **p < 0.01, repeated-measures ANOVA.

FIGURE 4

Effects of GABA_B receptor antagonist saclofen on propofol-induced depression of electrical synaptic strength of TRN neurons. (A) With 10 μM saclofen in the ACSF perfusion, example traces of voltage responses of a coupled pair of TRN neurons before (I), during (II), and after (III) propofol application. (B) Time course calculations of Gc for all traces of the pair of TRN neurons shown in A, correspondence time points of the traces in A were marked as I to III. (C) Average Gc values taken 10 min before, 10 min after propofol application, and 10 min after propofol washout (means ± SD, n = 15 pairs). (D) example traces of voltage responses of a coupled pair of TRN neurons before (I), during (II), and after (III) baclofen (10 μM) application. (E) Time course calculations of Gc for all traces of the pair of TRN neurons shown in D, correspondence time points of the traces in D were marked as I to III. (F) Average Gc values taken 10 min before, 10 min after propofol application, and 10 min after propofol washout (means ± SD, n = 10 pairs). (G), compare of the mean and deviation of Gc (in 10 min) in control conditions, and in 10 μM saclofen and baclofen. Average Gc values at 10 min before drug application and 10 min after drug washout were taken to calculate ΔGc. **p < 0.01, repeated-measures ANOVA.

FIGURE 5

Effects of specific inhibitor of adenylyl cyclase and PKA on propofol-induced depression of electrical synaptic strength of TRN neurons. (A) With 100 μM 9-CPA in the ACSF perfusion, example traces of voltage responses of a coupled pair of TRN neurons before (I), during (II), and after (III) propofol application. (B) Time course calculations of Gc for all traces of the pair of TRN neurons shown in A, correspondence time points of the traces in A were marked as I to III. (C) Average Gc values taken 10 min before, 10 min after propofol application, and 10 min after propofol washout (means ± SD, n = 7 pairs). (D) With 1 μM KT5720 in the ACSF perfusion, example traces of voltage responses of a coupled pair of TRN neurons before (I), during (II), and after (III) propofol application. (E) Time course calculations of Gc for all traces of the pair of TRN neurons shown in D, correspondence time points of the traces in D were marked as I to III. (F) Average Gc values taken taken 10 min before, 10 min after propofol application, and 10 min after propofol washout (means ± SD, n = 9 pairs). (G) Compare of the mean and deviation of Gc (in 10 min) in control conditions, and in 100 μM 9-CPA and 1 μM KT572. Average Gc values at 10 min before propofol application and 10 min after drug washout were taken to calculate ΔGc.