Propofol Inhibits Cerebellar Parallel Fiber-Purkinje Cell Synaptic Transmission via Activation of Presynaptic GABAB Receptors in vitro in Mice

Etomidate enhances cerebellar CF-PC synaptic plasticity through CB1 receptor/PKA cascade in vitro in mice

Etomidate (ET) is a widely used intravenous imidazole general anesthetic, which depresses the cerebellar neuronal activity by modulating various receptors activity and synaptic transmission. In this study, we investigated the effects of ET on the cerebellar climbing fiber-Purkinje cells (CF-PC) plasticity in vitro in mice using whole-cell recording technique and pharmacological methods. Our results demonstrated that CF tetanic stimulation produced a mGluR1-dependent long-term depression (LTD) of CF-PC excitatory postsynaptic currents (EPSCs), which was enhanced by bath application of ET (10 µM). Blockade of mGluR1 receptor with JNJ16259685, ET triggered the tetanic stimulation to induce a CF-PC LTD accompanied with an increase in paired-pulse ratio (PPR). The ET-triggered CF-PC LTD was abolished by extracellular administration of an N-methyl-(D)-aspartate (NMDA) receptor antagonist, D-APV, as well as by intracellular blockade of NMDA receptors activity with MK801. Furthermore, blocking cannabinoids 1 (CB1) receptor with AM251 or chelating intracellular Ca2+ with BAPTA, ET failed to trigger the CF-PC LTD. Moreover, the ET-triggered CF-PC LTD was abolished by inhibition of protein kinase A (PKA), but not by inhibition of protein kinase C inhibiter. The present results suggest that ET acts on postsynaptic NMDA receptor resulting in an enhancement of the cerebellar CF-PC LTD through CB1 receptor/PKA cascade in vitro in mice. These results provide new evidence and possible mechanism for ET anesthesia to affect motor learning and motor coordination by regulating cerebellar CF-PC LTD.

CGP7930 - An allosteric modulator of GABABRs, GABAARs and inwardly-rectifying potassium channels

Type-A and -B GABA receptors (GABAARs/GABABRs) control brain function and behaviour by fine tuning neurotransmission. Over-time these receptors have become important therapeutic targets for treating neurodevelopmental and neuropsychiatric disorders. Several positive allosteric modulators (PAMs) of GABARs have reached the clinic and selective targeting of receptor subtypes is crucial. For GABABRs, CGP7930 is a widely used PAM for in vivo studies, but its full pharmacological profile has not yet been established. Here, we reveal that CGP7930 has multiple effects not only on GABABRs but also GABAARs, which for the latter involves potentiation of GABA currents, direct receptor activation, and also inhibition. Furthermore, at higher concentrations, CGP7930 also blocks G protein-coupled inwardly-rectifying K+ (GIRK) channels diminishing GABABR signalling in HEK 293 cells. In male and female rat hippocampal neuron cultures, CGP7930 allosteric effects on GABAARs caused prolonged rise and decay times and reduced the frequency of inhibitory postsynaptic currents and potentiated GABAAR-mediated tonic inhibition. Additional comparison between predominant synaptic- and extrasynaptic-isoforms of GABAAR indicated no evident subtype selectivity for CGP7930. In conclusion, our study of CGP7930 modulation of GABAARs, GABABRs and GIRK channels, indicates this compound is unsuitable for use as a specific GABABR PAM.

Glucagon-like peptide-1 facilitates cerebellar parallel fiber glutamate release through PKA signaling in mice in vitro

Glucagon-like peptide-1 (GLP-1) is mainly secreted by preproglucagon neurons; it plays important roles in modulating neuronal activity and synaptic transmission through its receptors. In the present study, we investigated the effects of GLP-1 on parallel fiber-Purkinje cell (PF-PC) synaptic transmission in mouse cerebellar slices using whole-cell patch-clamp recording and pharmacology methods. In the presence of a γ-aminobutyric acid type A receptor antagonist, bath application of GLP-1 (100 nM) enhanced PF-PC synaptic transmission, with an increased amplitude of evoked excitatory postsynaptic synaptic currents (EPSCs) and a decreased paired-pulse ratio. The GLP-1-induced enhancement of evoked EPSCs was abolished by a selective GLP-1 receptor antagonist, exendin 9-39, as well as by the extracellular application of a specific protein kinase A (PKA) inhibitor, KT5720. In contrast, inhibiting postsynaptic PKA with a protein kinase inhibitor peptide-containing internal solution failed to block the GLP-1-induced enhancement of evoked EPSCs. In the presence of a mixture of gabazine (20 μM) and tetrodotoxin (1 μM), application GLP-1 significantly increased frequency, but not amplitude of miniature EPSCs via PKA signaling pathway. The GLP-1-induced increase in miniature EPSC frequency was blocked by both exendin 9-39 and KT5720. Together, our results indicate that GLP-1 receptor activation enhances glutamate release at PF-PC synapses via the PKA signaling pathway, resulting in enhanced PF-PC synaptic transmission in mice in vitro. These findings suggest that, in living animals, GLP-1 has a critical role in the modulation of cerebellar function by regulating excitatory synaptic transmission at PF-PC synapses.

GLP-1 enhances hyperpolarization-activated currents of mouse cerebellar Purkinje cell in vitro

Glucagon-like peptide-1 (GLP-1) is mainly secreted by preglucagonergic neurons in the nucleus tractus solitarius, which plays critical roles in regulation of neuronal activity in the central nervous system through its receptor. In the cerebellar cortex, GLP-1 receptor is abundantly expressed in the molecular layer, Purkinje cell (PC) layer and granular layer, indicating that GLP-1 may modulate the cerebellar neuronal activity. In this study, we investigated the mechanism by which GLP1 modulates mouse cerebellar PC activity in vitro. After blockade of glutamatergic and GABAergic synaptic transmission in PCs, GLP1 increased the spike firing rate accompanied by depolarization of membrane potential and significantly depressed the after-hyperpolarizing potential and outward rectifying current of spike firing discharges via GLP1 receptors. In the presence of TTX and Ba2+, GLP1 significantly enhanced the hyperpolarized membrane potential-evoked instant current, steady current, tail current (I-tail) and hyperpolarization-activated (IH) current. Application of a selective IH channel antagonist, ZD7288, blocked IH and abolished the effect of GLP1 on PC membrane currents. The GLP1 induced enhancement of membrane currents was also abolished by a selective GLP1 receptor antagonist, exendin-9-39, as well as by protein kinase A (PKA) inhibitors, KT5720 and H89. In addition, immunofluorescence detected GLP1 receptor in the mouse cerebellar cortex, mostly in PCs. These results indicated that GLP1 receptor activation enhanced IH channel activity via PKA signaling, resulting in increased excitability of mouse cerebellar PCs in vitro. The present findings indicate that GLP1 plays a critical role in modulating cerebellar function by regulating the spike firing activity of mouse cerebellar PCs.

Nongenetic and genetic predictors of haemodynamic instability induced by propofol and opioids: A retrospective clinical study

AIM

Propofol and opioids are commonly used in anaesthesia, but are highly susceptible to haemodynamic instability, thereby threatening the patient's surgical safety and prognosis. The purpose of this study was to investigate the predictors of haemodynamic instability and establish its predictive model.

METHODS

A total of 150 Chinese patients undergoing thyroid or breast surgery participated in the study, with target-controlled infusion concentrations of propofol, opioids dosage, heart rate (HR), mean arterial pressure (MAP) and Narcotrend Index recorded at key points throughout the procedure. The Agena MassARRAY system was used to genotype candidate single nucleotide polymorphisms related to pharmacodynamics and pharmacokinetics of propofol and opioids.

RESULTS

Among nongenetic factors, baseline HR (R = -.579, P < .001) and baseline MAP (R = -.725, P < .001) had a significant effect on the haemodynamic instability. Among genetic factors, the CT/CC genotype of GABRB1 rs4694846 (95% confidence interval [CI]: -11.309 to -3.155), AA/AG of OPRM1 rs1799971 (95%CI: 0.773 to 10.290), AA of CES2 rs8192925 (95%CI: 1.842 to 9.090) were associated with higher HR instability; the AA/GG genotype of NR1I2 rs6438550 (95%CI: 0.351 to 7.761), AA of BDNF rs2049046 (95%CI: -9.039 to -0.640) and GG of GABBR2 rs1167768 (95%CI: -10.146 to -1.740) were associated with higher MAP instability. The predictive models of HR and MAP fluctuations were developed, accounting for 45.0 and 59.2% of variations, respectively.

CONCLUSION

We found that cardiovascular fundamentals and genetic variants of GABRB1, GABBR2, OPRM1, BDNF, CES2 and NR1I2 are associated with cardiovascular susceptibility, which can provide a reference for haemodynamic management in clinical anaesthesia.

Mechanisms Underlying Mu Opioid Receptor Effects on Parallel Fiber-Purkinje Cell Synaptic Transmission in Mouse Cerebellar Cortex

μ-opioid receptors (MOR) are widely expressed in the brain, varying in density in different areas. Activation of MORs underlies analgesia, euphoria, but may lead to tolerance, dependence, and ultimately opioid addiction. The Purkinje cell (PC) is the only efferent neuron in the cerebellar cortex and receives glutamatergic synaptic inputs from the parallel fibers formed by the axons of granule cells. Studies have shown that MORs are expressed during the development of cerebellar cells. However, the distribution of MOR and their effects on PF-PC synaptic transmission remain unclear. To examine these questions, we used whole-cell patch clamp recordings and pharmacological methods to determine the effects and mechanisms of MOR activation on synaptic transmission at PF-PC synapses. The MOR-selective agonist DAMGO significantly reduced the amplitude and area under the curve (AUC) of PF-PC evoked (e) EPSCs, and increased the paired-pulse ratio (PPR).DAMGO-induced inhibitory effects on PF-PC eEPSCs and PPR were abolished by MOR specific blocker CTOP. Further, DAMGO significantly reduced the frequency of PF-PC mEPSCs, but had no obvious effect on their amplitude, suggesting a presynaptic site of action. The DAMGO-induced reduction in the frequency of PF-PC mEPSCs also was blocked by CTOP. A protein kinase A (PKA) inhibitor PKI added in the pipette solution did not affect the inhibitory effects on PF-PC mEPSCs induced by DAMGO. Both the PKA inhibitor K5720 and MEK inhibitor U0126 in artificial cerebrospinal fluid (ACSF) prevented the inhibitory effects of DAMGO on PF-PC mEPSCs. These findings reveal that MORs are expressed in presynaptic PF axon terminals, where DAMGO can activate presynaptic MORs to inhibit PF-PC synaptic transmission by regulating the release of glutamate. G-protein-dependent cAMP-PKA signaling pathway may be involved in this process.

A single nucleotide polymorphism-based formula to predict the risk of propofol TCI concentration being over 4 µg mL-1 at the time of loss of consciousness

We aim to develop a formula based on single nucleotide polymorphisms (SNPs) to predict whether the propofol target-controlled infusion (TCI) concentration would be over 4 μg mL^-1 at the time of loss of consciousness (LOC). We recruited 184 patients undergoing thyroid or breast surgeries with propofol anaesthesia. A total of 48 SNPs of CYP2B6, CYP2C9, UGT1A9, HNF4A, ABCB1, ABCC4, ABCG2, GABRA2, GABRA4, GABRB1, GABRB3, GABRG2, GABBR2, GAD1, SLC1A3, BDNF, and NRXN1, previously associated with propofol metabolic and pharmacology pathway, were genotyped. The formula was developed in the training cohort using the least absolute shrinkage and selection operator logistic regression model, and then validated in the testing cohort. The SNPs, GABBR2 rs1167768, GABBR2 rs1571927, NRXN1 rs601010, BDNF rs2049046, GABRA4 rs1512135, UGT1A9 rs11692021, GABBR2 rs2808536, HNF4A rs1884613, GABRB3 rs2017247, and CYP2B6 rs3181842 were selected to construct the SNP-based formula, which was used to calculate the risk score for over 4 μg mL^-1 TCI concentration of propofol at the time of LOC. Patients in the high-risk group were more likely to require a propofol concentration higher than 4 μg mL^-1 and presented a longer LOC latency. The SNP-based formula may significantly improve the safety and effectiveness of propofol-induced anaesthesia.

Nicotine depresses facial stimulation-evoked molecular layer interneuron-Purkinje cell synaptic transmission via α7 nicotinic acetylcholine receptors in mouse cerebellar cortex

Nicotine modulates cerebellar physiology function by interacting with nicotinic acetylcholine receptors (nAChRs) and is involved in modulation of cerebellar cortical circuitry functions. Here, we investigated the effect of nicotine on sensory stimulation-evoked molecular layer interneuron-Purkinje cell (MLI-PC) synaptic transmission mouse cerebellar cortex using in vivo cell-attached recording technique and pharmacological methods. The results show that micro-application of nicotine to the cerebellar molecular layer significantly decreased sensory stimulation-evoked MLI-PC synaptic transmission in mouse cerebellar cortex. Nicotine-induced depression in sensory stimulation-evoked MLI-PC synaptic transmission was abolished by either a non-selective nAChR blocker, hexamethonium, or the α7-nAChR antagonist methyllycaconitine (MLA), but not the selective α4β2-nAChR antagonist dihydro-β-erythroidine. Notably, molecular layer micro-application of nicotine did not significantly affect the number of spontaneous or facial stimulation-evoked action potentials of MLIs. Moreover, nicotine produced significant increases in the amplitude and frequency of miniature inhibitory postsynaptic currents of PCs, which were abolished by MLA in cerebellar slices. These results indicate that micro-application of nicotine to the cerebellar molecular layer depresses facial stimulation-induced MLI-PC synaptic transmission by activating α7 nAChRs, suggesting that cholinergic inputs modulate MLI-PC synapses to process sensory information in the cerebellar cortex of mice in vivo.

Propofol facilitates climbing fiber-Purkinje cell synaptic transmission via NMDA receptor in vitro in mice

Propofol is generally used for the induction and maintenance of anesthesia in clinical procedures via activation of γ -aminobutyric acid A (GABA_A) receptors. When administered at the clinical dose, propofol use is associated with movement disorders, including dystonia and ataxia, suggesting that propofol administration impacts the function of cerebellar neuronal circuitry. In this study, we investigated the effect of propofol on climbing fiber (CF)-Purkinje cell (PC) synaptic transmission in mouse cerebellar slices in the absence of GABAergic inhibition using a whole-cell recording technique and pharmacological methods. Our results showed that bath application of propofol enhanced CF-PC synaptic transmission, which was demonstrated by an increased amplitude and area under the curve (AUC) of the excitatory postsynaptic currents (EPSCs) accompanied by a decrease in the paired-pulse ratio (PPR). The propofol-induced increase in the amplitude of P1 was concentration-dependent with a half effective concentration (EC₅₀) of 20.9 μM. The propofol-induced increases in the amplitude and AUC of CF-PC EPSCs were abolished by an N-Methyl-D-aspartate (NMDA) receptor blocker. Furthermore, the application of NMDA enhanced CF-PC EPSCs and overwhelmed the effect of propofol on CF-PC EPSCs. Moreover, intracellular blockade of NMDA receptors attenuated the propofol-induced enhancement of CF-PC synaptic transmission but strengthened the propofol-induced change in the PPR. These results indicate that propofol enhances CF-PC synaptic transmission by activation of NMDA receptors in the mouse cerebellar cortex, suggesting that propofol administration might be involved in propofol-induced dysfunction of the cerebellum via NMDA receptors.

Noradrenaline inhibits complex spikes activity via the presynaptic PKA signaling pathway in mouse cerebellar slices

Norepinephrine (NA) is an important neurotransmitter of the cerebellum that regulates synaptic transmission, motor regulation and motor learning under certain conditions via adrenergic receptors (ARs). We previously found that NA depressed cerebellar climbing fiber-Purkinje cell (CF-PC) synaptic transmission via α2-ARs in vivo in mice. We here investigated the mechanisms of NA inhibited CF-PC synaptic transmission in acute cerebellar slices using the whole-cell recording technique and pharmacological methods. Bath application of NA (10 μM) depressed CF-PC synaptic transmission, which exhibited a time-dependent decrease in amplitude of excitatory postsynaptic currents (N1), accompanied by an increase in the paired-pulse ratio (PPR). The NA-induced depression of CF-PC synaptic transmission was significantly prevented by inhibition of protein kinase A (PKA) with either H-89 or KT5720. Furthermore, the NA-induced inhibition of CF-PC synaptic transmission was rescued by activation adenylate cyclase (AC), and the AC-induced enhancement of CF-PC synaptic transmission was depressed by NA. Moreover, inhibition of AC with SQ22536, produced a significant depression of CF-PC synaptic transmission and abrogated the NA-induced depression of CF-PC synaptic transmission. However, the NA-induced depression of CF-PC synaptic transmission was not blocked by intracellular inhibition of PKA with a cell impermeable PKA inhibitor, PKI, or by extracellular inhibition of protein kinase C. These results indicate that NA activates presynaptic α2-AR, resulting in a depression of mouse cerebellar CF-PC synaptic transmission through the AC-PKA signaling pathway.

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

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.