Fast-spiking Interneurons Contribute to Propofol-induced Facilitation of Firing Synchrony in Pyramidal Neurons of the Rat Insular Cortex.

BACKGROUND: The general anesthetic propofol induces frontal alpha rhythm in the cerebral cortex at a dose sufficient to induce loss of consciousness. The authors hypothesized that propofol-induced facilitation of unitary inhibitory postsynaptic currents would result in firing synchrony among postsynaptic pyramidal neurons that receive inhibition from the same presynaptic inhibitory fast-spiking neurons. METHODS: Multiple whole cell patch clamp recordings were performed from one fast-spiking neuron and two or three pyramidal neurons with at least two inhibitory connections in rat insular cortical slices. The authors examined how inhibitory inputs from a presynaptic fast-spiking neuron modulate the timing of spontaneous repetitive spike firing among pyramidal neurons before and during 10 µM propofol application. RESULTS: Responding to activation of a fast-spiking neuron with 150-ms intervals, pyramidal cell pairs that received common inhibitory inputs from the presynaptic fast-spiking neuron showed propofol-dependent decreases in average distance from the line of identity, which evaluates the coefficient of variation in spike timing among pyramidal neurons: average distance from the line of identity just after the first activation of fast-spiking neuron was 29.2 ± 24.1 (mean ± SD, absolute value) in control and 19.7 ± 19.2 during propofol application (P < 0.001). Propofol did not change average distance from the line of identity without activating fast-spiking neurons and in pyramidal neuron pairs without common inhibitory inputs from presynaptic fast-spiking neurons. The synchronization index, which reflects the degree of spike synchronization among pyramidal neurons, was increased by propofol from 1.4 ± 0.5 to 2.3 ± 1.5 (absolute value, P = 0.004) and from 1.5 ± 0.5 to 2.2 ± 1.0 (P = 0.030) when a presynaptic fast-spiking neuron was activated at 6.7 and 10 Hz, respectively, but not at 1, 4, and 13.3 Hz. CONCLUSIONS: These results suggest that propofol facilitates pyramidal neuron firing synchrony by enhancing inhibitory inputs from fast-spiking neurons. This synchrony of pyramidal neurons may contribute to the alpha rhythm associated with propofol-induced loss of consciousness.

A Case of Successful Tracheal Tube Exchange With McGrath MAC for Tube Damage During Oral Surgery.

A patient undergoing a bilateral sagittal split and LeFort 1 maxillary osteotomy performed under general anesthesia required emergent intraoperative exchange of a potentially damaged nasotracheal tube. This exchange was smoothly performed under constant indirect visualization using the McGrath MAC video laryngoscopy system. After the exchange, ventilation of the patient dramatically improved. The removed endotracheal tube was torn 19 cm from the distal tip. The McGrath MAC was useful for visualizing the glottis and confirming the entire course of the tube exchange despite the patient's having a difficult airway (Cormack-Lehane grade 3).

Role of specific presynaptic calcium channel subtypes in isoflurane inhibition of synaptic vesicle exocytosis in rat hippocampal neurones.

BACKGROUND: P/Q- and N-type voltage-gated calcium channels (VGCC) are the principal subtypes mediating synaptic vesicle (SV) exocytosis. Both the degree of isoflurane inhibition of SV exocytosis and VGCC subtype expression vary between brain regions and neurotransmitter phenotype. We hypothesised that differences in VGCC subtype expression contribute to synapse-selective presynaptic effects of isoflurane. METHODS: We used quantitative live-cell imaging to measure exocytosis in cultured rat hippocampal neurones after transfection of the fluorescent biosensor vGlut1-pHluorin. Selective inhibitors of P/Q- and N-type VGCCs were used to isolate subtype-specific effects of isoflurane. RESULTS: Inhibition of N-type channels by 1 µM ω-conotoxin GVIA reduced SV exocytosis to 81±5% of control (n=10). Residual exocytosis mediated by P/Q-type channels was further inhibited by isoflurane to 42±4% of control (n=10). The P/Q-type channel inhibitor ω-agatoxin IVA at 0.4 µM inhibited SV exocytosis to 29±3% of control (n=10). Residual exocytosis mediated by N-type channels was further inhibited by isoflurane to 17±3% of control (n=10). Analysis of isoflurane effects at the level of individual boutons revealed no difference in sensitivity to isoflurane between P/Q- or N-type channel-mediated SV exocytosis (P=0.35). There was no correlation between the effect of agatoxin (P=0.91) or conotoxin (P=0.15) and the effect of isoflurane on exocytosis. CONCLUSIONS: Sensitivity of SV exocytosis to isoflurane in rat hippocampal neurones is independent of the specific VGCC subtype coupled to exocytosis. The differential sensitivity of VGCC subtypes to isoflurane does not explain the observed neurotransmitter-selective effects of isoflurane in hippocampus.

Propofol-induced spike firing suppression is more pronounced in pyramidal neurons than in fast-spiking neurons in the rat insular cortex.

Propofol is a major intravenous anesthetic that facilitates GABAA receptor-mediated inhibitory synaptic currents and modulates inward current (Ih), K+, and voltage-gated Na+ currents. This propofol-induced modulation of ionic currents changes intrinsic membrane properties and repetitive spike firing in cortical pyramidal neurons. However, it has been unknown whether propofol modulates these electrophysiological properties in GABAergic neurons, which express these ion channels at different levels. This study examined whether pyramidal and GABAergic neuronal properties are differentially modulated by propofol in the rat insular cortical slice preparation. We conducted multiple whole-cell patch-clamp recordings from pyramidal neurons and from GABAergic neurons, which were classified into fast-spiking (FS), low threshold spike (LTS), late-spiking (LS), and regular-spiking nonpyramidal (RSNP) neurons. We found that 100µM propofol hyperpolarized the resting membrane potential and decreased input resistance in all types of neurons tested. Propofol also potently suppressed, and in most cases eliminated, repetitive spike firing in all these neurons. However, the potency of the propofol-induced changes in membrane and firing properties is particularly prominent in pyramidal neurons. Using a low concentration of propofol clarified this tendency: 30µM propofol decreased the firing of pyramidal neurons but had little effect on GABAergic neurons. Pre-application of a GABAA receptor antagonist, picrotoxin (100µM), diminished the propofol-induced suppression of neural activities in both pyramidal and FS neurons. These results suggest that GABAergic neurons, especially FS neurons, are less affected by propofol than are pyramidal neurons and that propofol-induced modulation of the intrinsic membrane properties and repetitive spike firing are principally mediated by GABAA receptor-mediated tonic currents.

Opioid subtype- and cell-type-dependent regulation of inhibitory synaptic transmission in the rat insular cortex.

The insular cortex (IC) plays a principal role in the regulation of pain processing. Although opioidergic agonists depress cortical excitatory synaptic transmission, little is known about opioidergic roles in inhibitory synaptic transmission. In the IC, the opioid receptors differentially regulate the excitatory propagation: agonists of the mu (MOR), delta (DOR), and kappa (KOR) exhibit suppressive, facilitative, and little effects, respectively. Thus, we aimed to examine the effects of opioid receptor agonists on unitary inhibitory postsynaptic currents (uIPSCs) in the IC. Pyramidal and GABAergic neurons in the rat IC were recorded by a multiple whole-cell patch-clamp technique. [D-Ala2,N-Me-Phe4,Gly5-ol]-Enkephalin acetate salt (DAMGO), an MOR agonist, reduced uIPSC amplitude by 74% in fast-spiking GABAergic interneuron (FS)→FS connections without a significant effect on FS→pyramidal cell (Pyr) connections. These effects of DAMGO were also observed in non-FS→FS and non-FS→Pyr connections: DAMGO reduced the uIPSC amplitude in non-FS→FS but not in non-FS→Pyr connections. DAMGO-induced depression of uIPSCs was blocked by the MOR antagonist, D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2. The DOR agonist, [D-Pen2,5]-Enkephalin hydrate (DPDPE), reduced uIPSC amplitude by 39% in FS→FS and by 49% in FS→Pyr connections, which was antagonized by the DOR antagonist, naltrindole. However, DPDPE had little effect on non-FS→FS/Pyr connections. (±)-trans-U-50488 methanesulfonate salt (U50488), a KOR agonist, had little effect on uIPSC in FS→FS/Pyr connections. These results suggest that MOR-induced uIPSC depression in FS→FS and non-FS→FS, but not FS→Pyr and non-FS→Pyr connections, results in the depression of excitatory propagation in the IC, which may be an underlying mechanism of the powerful analgesic effects of MOR agonists.

Opposite effects of mu and delta opioid receptor agonists on excitatory propagation induced in rat somatosensory and insular cortices by dental pulp stimulation.

The insular cortex (IC) contributes to nociceptive information processing. IC neurons express opioid receptors, including the mu (MOR), kappa (KOR), and delta (DOR) subtypes. Opioidergic agonists suppress excitatory synaptic transmission in the cerebral cortex. In addition, morphine injection into the IC reduces responses to noxious thermal stimuli. However, the mechanisms of the opioid-dependent modulation of cortical excitation at the macroscopic level, which bridge the cellular and behavioral findings, have remained unknown. The present in vivo optical imaging study aimed to examine the effects of the agonists of each subtype on cortical excitatory propagation in the IC and the neighboring cortices, the primary (S1) and secondary somatosensory (S2) areas. To assess the opioidergic effects on the cortical circuits, we applied electrical stimulation to the maxillary 1st molar pulp, which induced excitation in the ventral part of S1 and the S2/insular oral region (IOR). The initial excitatory response was observed 10-14ms after stimulation, and then excitation propagated concentrically. DAMGO (10-100µM), an MOR agonist, suppressed the amplitude of cortical excitation and shrank the maximum excitation areas in S1 and S2/IOR. In contrast, 10-100µM DPDPE, a DOR agonist, increased the amplitude of excitation and expanded the area of maximum excitation. U50488 (10-100µM), a KOR agonist, had little effect on cortical excitation. These results suggest that MOR-induced suppression of excitatory propagation in the IC is an underlying mechanism of the powerful analgesic effects of MOR agonists. In contrast, DOR may play a minor role in suppressing acute pain.

Fast-spiking cell to pyramidal cell connections are the most sensitive to propofol-induced facilitation of GABAergic currents in rat insular cortex.

BACKGROUND: Propofol facilitates γ-aminobutyric acid-mediated inhibitory synaptic transmission. In the cerebral cortex, γ-aminobutyric acidergic interneurons target both excitatory pyramidal cells (Pyr) and fast-spiking (FS) and non-FS interneurons. Therefore, the propofol-induced facilitation of inhibitory transmission results in a change in the balance of excitatory and inhibitory inputs to Pyr. However, it is still unknown how propofol modulates γ-aminobutyric acidergic synaptic transmission in each combination of Pyr and interneurons. METHODS: The authors examined whether propofol differentially regulates inhibitory postsynaptic currents (IPSCs) depending on the presynaptic and postsynaptic cell subtypes using multiple whole cell patch clamp recording from γ-aminobutyric acidergic interneurons and Pyr in rat insular cortex. RESULTS: Propofol (10 µM) consistently prolonged decay kinetics of unitary IPSCs (uIPSCs) in all types of inhibitory connections without changing paired-pulse ratio of the second to first uIPSC amplitude or failure rate. The FS→Pyr connections exhibited greater enhancement of uIPSC charge transfer (2.2 ± 0.5 pC, n = 36) compared with that of FS→FS/non-FS connections (0.9 ± 0.2 pC, n = 37), whereas the enhancement of charge transfer in non-FS→Pyr (0.3 ± 0.1 pC, n = 15) and non-FS→FS/non-FS connections (0.2 ± 0.1 pC, n = 36) was smaller to those in FS→Pyr/FS/non-FS. Electrical synapses between FS pairs were not affected by propofol. CONCLUSIONS: The principal inhibitory connections (FS→Pyr) are the most sensitive to propofol-induced facilitation of uIPSCs, which is likely mediated by postsynaptic mechanisms. This preferential uIPSC enhancement in FS→Pyr connections may result in suppressed neural activities of projection neurons, which in turn reduces excitatory outputs from cortical local circuits.

Postsynaptic cell type-dependent cholinergic regulation of GABAergic synaptic transmission in rat insular cortex.

The cerebral cortex consists of multiple neuron subtypes whose electrophysiological properties exhibit diverse modulation patterns in response to neurotransmitters, including noradrenaline and acetylcholine (ACh). We performed multiple whole cell patch-clamp recording from layer V GABAergic interneurons and pyramidal cells of rat insular cortex (IC) to examine whether cholinergic effects on unitary inhibitory postsynaptic currents (uIPSCs) are differentially regulated by ACh receptors, depending on their presynaptic and postsynaptic cell subtypes. In fast-spiking (FS) to pyramidal cell synapses, carbachol (10 µM) invariably decreased uIPSC amplitude by 51.0%, accompanied by increases in paired-pulse ratio (PPR) of the second to first uIPSC amplitude, coefficient of variation (CV) of the first uIPSC amplitude, and failure rate. Carbachol-induced uIPSC suppression was dose dependent and blocked by atropine, a muscarinic ACh receptor antagonist. Similar cholinergic suppression was observed in non-FS to pyramidal cell synapses. In contrast, FS to FS/non-FS cell synapses showed heterogeneous effects on uIPSC amplitude by carbachol. In roughly 40% of pairs, carbachol suppressed uIPSCs by 35.8%, whereas in a similar percentage of pairs uIPSCs were increased by 34.8%. Non-FS to FS/non-FS cell synapses also showed carbachol-induced uIPSC facilitation by 29.2% in about half of the pairs, whereas nearly 40% of pairs showed carbachol-induced suppression of uIPSCs by 40.3%. Carbachol tended to increase uIPSC amplitude in interneuron-to-interneuron synapses with higher PPR, suggesting that carbachol facilitates GABA release in interneuron synapses with lower release probability. These results suggest that carbachol-induced effects on uIPSCs are not homogeneous but preiotropic: i.e., cholinergic modulation of GABAergic synaptic transmission is differentially regulated depending on postsynaptic neuron subtypes.

Presynaptic interneuron subtype- and age-dependent modulation of GABAergic synaptic transmission by beta-adrenoceptors in rat insular cortex.

beta-Adrenoceptors play a crucial role in the regulation of taste aversion learning in the insular cortex (IC). However, beta-adrenergic effects on inhibitory synaptic transmission mediated by gamma-aminobutyric acid (GABA) remain unknown. To elucidate the mechanisms of beta-adrenergic modulation of inhibitory synaptic transmission, we performed paired whole cell patch-clamp recordings from layer V GABAergic interneurons and pyramidal cells of rat IC aged from postnatal day 17 (PD17) to PD46 and examined the effects of isoproterenol, a beta-adrenoceptor agonist, on unitary inhibitory postsynaptic currents (uIPSCs). Isoproterenol (100 microM) induced facilitating effects on uIPSCs in 33.3% of cell pairs accompanied by decreases in coefficient of variation (CV) of the first uIPSC amplitude and paired-pulse ratio (PPR) of the second to first uIPSC amplitude, whereas 35.9% of pairs showed suppressive effects of isoproterenol on uIPSC amplitude obtained from fast spiking (FS) to pyramidal cell pairs. Facilitatory effects of isoproterenol were frequently observed in FS-pyramidal cell pairs at > or =PD24. On the other hand, isoproterenol suppressed uIPSC amplitude by 52.3 and 39.8% in low-threshold spike (LTS)-pyramidal and late spiking (LS)-pyramidal cell pairs, respectively, with increases in CV and PPR. The isoproterenol-induced suppressive effects were blocked by preapplication of 100 microM propranolol, a beta-adrenoceptor antagonist. There was no significant correlation between age and changes of uIPSCs in LTS-/LS-pyramidal cell pairs. These results suggest the presence of differential mechanisms in presynaptic GABA release and/or postsynaptic GABA(A) receptor-related assemblies among interneuron subtypes. Age- and interneuron subtype-specific beta-adrenergic modulation of IPSCs may contribute to experience-dependent plasticity in the IC.

Presynaptic and postsynaptic modulation of glutamatergic synaptic transmission by activation of alpha(1)- and beta-adrenoceptors in layer V pyramidal neurons of rat cerebral cortex.

Adrenergic agonists have different modulatory effects on excitatory synaptic transmission depending on the receptor subtypes involved. The present study examined the loci of alpha(1)- and beta-adrenoceptor agonists, which have opposite effects on excitatory neural transmission, involved in modulation of glutamatergic transmission in layer V pyramidal cells of rat cerebral cortex. Phenylephrine, an alpha(1)-adrenoceptor agonist, suppressed the amplitude of AMPA receptor-mediated excitatory postsynaptic currents evoked by repetitive electrical stimulation (eEPSCs, 10 pulses at 33 Hz). The coefficient of variation (CV) of the 1st eEPSC amplitude and paired-pulse ratio (PPR), which were sensitive to extracellular Ca(2+) concentration, were not affected by phenylephrine. Phenylephrine suppressed miniature EPSC (mEPSC) amplitude without changing its frequency. In contrast, isoproterenol, a beta-adrenoceptor agonist, strongly increased the amplitude of the 1st eEPSC compared with that of the 2nd to 10th eEPSCs, which resulted in a decrease in PPR. Isoproterenol-induced enhancement of eEPSC amplitude was accompanied by a decrease in CV. Isoproterenol increased the frequency of mEPSCs without significant effect on amplitude. Phenylephrine suppressed inward currents evoked by puff application of glutamate, AMPA, or NMDA, whereas isoproterenol application was not accompanied by significant changes in these inward currents. These findings suggest that phenylephrine decreases eEPSCs through postsynaptic AMPA or NMDA receptors, while the effects of isoproterenol are mediated by facilitation of glutamate release from presynaptic terminals without effect on postsynaptic glutamate receptors. These two different mechanisms of modulation of excitatory synaptic transmission may improve the "signal-to-noise ratio" in cerebral cortex.