Fast-spiking Cell to Pyramidal Cell Connections Are the Most Sensitive to Propofol-induced Facilitation of GABAergic Currents in Rat Insular Cortex

Extrasynaptic GABAA receptors in central medial thalamus mediate anesthesia in rats

Neuronal depression in the thalamus underlies anesthetic-induced loss of consciousness, while the precise sub-thalamus nuclei and molecular targets involved remain to be elucidated. The present study investigated the role of extrasynaptic GABAA receptors in the central medial thalamic nucleus (CM) in anesthesia induced by gaboxadol (THIP) and diazepam (DZP) in rats. Local lesion of the CM led to a decrease in the duration of loss of righting reflex induced by THIP and DZP. CM microinjection of THIP but not DZP induced anesthesia. The absence of righting reflex in THIP-treated rats was consistent with the increase of low frequency oscillations in the delta band in the medial prefrontal cortex. CM microinjection of GABAA receptor antagonist SR95531 significantly attenuated the anesthesia induced by systemically-administered THIP, but not DZP. Moreover, the rats with declined expression of GABAA receptor δ-subunit in the CM were less responsive to THIP or DZP. These findings explained a novel mechanism of THIP-induced loss of consciousness and highlighted the role of CM extrasynaptic GABAA receptors in mediating anesthesia.

Physiological features of parvalbumin-expressing GABAergic interneurons contributing to high-frequency oscillations in the cerebral cortex

Parvalbumin-expressing (PV+) inhibitory interneurons drive gamma oscillations (30-80 Hz), which underlie higher cognitive functions. In this review, we discuss two groups/aspects of fundamental properties of PV+ interneurons. In the first group (dubbed Before Axon), we list properties representing optimal synaptic integration in PV+ interneurons designed to support fast oscillations. For example: [i] Information can neither enter nor leave the neocortex without the engagement of fast PV+ -mediated inhibition; [ii] Voltage responses in PV+ interneuron dendrites integrate linearly to reduce impact of the fluctuations in the afferent drive; and [iii] Reversed somatodendritic Rm gradient accelerates the time courses of synaptic potentials arriving at the soma. In the second group (dubbed After Axon), we list morphological and biophysical properties responsible for (a) short synaptic delays, and (b) efficient postsynaptic outcomes. For example: [i] Fast-spiking ability that allows PV+ interneurons to outpace other cortical neurons (pyramidal neurons). [ii] Myelinated axon (which is only found in the PV+ subclass of interneurons) to secure fast-spiking at the initial axon segment; and [iii] Inhibitory autapses - autoinhibition, which assures brief biphasic voltage transients and supports postinhibitory rebounds. Recent advent of scientific tools, such as viral strategies to target PV cells and the ability to monitor PV cells via in vivo imaging during behavior, will aid in defining the role of PV cells in the CNS. Given the link between PV+ interneurons and cognition, in the future, it would be useful to carry out physiological recordings in the PV+ cell type selectively and characterize if and how psychiatric and neurological diseases affect initiation and propagation of electrical signals in this cortical sub-circuit. Voltage imaging may allow fast recordings of electrical signals from many PV+ interneurons simultaneously.

HC. Relevance of Cortical and Hippocampal Interneuron Functional Diversity to General Anesthetic Mechanisms: A Narrative Review

General anesthetics disrupt brain processes involved in consciousness by altering synaptic patterns of excitation and inhibition. In the cerebral cortex and hippocampus, GABAergic inhibition is largely mediated by inhibitory interneurons, a heterogeneous group of specialized neuronal subtypes that form characteristic microcircuits with excitatory neurons. Distinct interneuron subtypes regulate specific excitatory neuron networks during normal behavior, but how these interneuron subtypes are affected by general anesthetics is unclear. This narrative review summarizes current principles of the synaptic architecture of cortical and interneuron subtypes, their contributions to different forms of inhibition, and their roles in distinct neuronal microcircuits. The molecular and cellular targets in these circuits that are sensitive to anesthetics are reviewed in the context of how anesthetics impact interneuron function in a subtype-specific manner. The implications of this functional interneuron diversity for mechanisms of anesthesia are discussed, as are their implications for anesthetic-induced changes in neural plasticity and overall brain function.

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.

EDITOR’S PERSPECTIVE

Propofol decreases spike firing frequency with an increase in spike synchronization in the cerebral cortex

Little is known about how propofol modulates the spike firing correlation between excitatory and inhibitory cortical neurons in vivo. We performed extracellular unit recordings from rat insular cortical neurons, and classified neurons with high spontaneous firing frequency, bursting, and short spike width as high frequency with bursting neurons (HFB; pseudo fast-spiking GABAergic neurons) and other neurons with low spontaneous firing frequency and no bursting were classified as non-HFB. Intravenous administration of propofol (12 mg/kg) from the caudal vein reduced the firing frequency of HFB, whereas propofol initially increased (within 30 s) and then decreased the firing frequency of non-HFB. Both HFB and non-HFB spontaneous action potential discharge was depressed by propofol with a greater depression seen for HFB. Cross-correlograms and auto-correlograms demonstrated propofol-induced increases in the ratio of the peak, which were mostly observed around 0-10 ms divided to baseline amplitude. The analysis of interspike intervals showed a decrease in spike firing at 20-100 Hz and a relative increase at 8-15 Hz. These results suggest that propofol induces a larger suppression of firing frequency in HFB and an enhancement of synchronized neural activities in the α frequency band in the cerebral cortex (192 words).

Extracellular glucose-dependent IPSC enhancement by leptin in fast-spiking to pyramidal neuron connections via JAK2-PI3K pathway in the rat insular cortex

Leptin is produced in the adipocytes and plays a pivotal role in regulation of energy balance by controlling appetite and metabolism. Leptin receptors are widely distributed in the brain, especially in the hypothalamus, hippocampus, and neocortex. The insular cortex (IC) processes gustatory and visceral information, which functionally correlate to feeding behavior. However, it is still an open issue whether and how leptin modulates IC neural activities. Our paired whole-cell patch-clamp recordings using IC slice preparations demonstrated that unitary inhibitory postsynaptic currents (uIPSCs) but not uEPSCs were potentiated by leptin in the connections between pyramidal (PNs) and fast-spiking neurons (FSNs). The leptin-induced increase in uIPSC amplitude was accompanied by a decrease in paired-pulse ratio. Under application of inhibitors of JAK2-PI3K but not MAPK pathway, leptin did not change uIPSC amplitude. Variance-mean analysis revealed that leptin increased the release probability but not the quantal size and the number of release site. These electrophysiological findings suggest that the leptin-induced uIPSC increase is mediated by activation of JAK2-PI3K pathway in presynaptic FSNs. An in vivo optical imaging revealed that leptin application decreased excitatory propagation in IC induced by electrical stimulation of IC. These leptin-induced effects were not observed under the low energy states: low glucose concentration (2.5 mM) in vitro and one-day-fasting condition in vivo. However, leptin enhanced uIPSCs under application of low glucose with an AMPK inhibitor. These results suggest that leptin suppresses IC excitation by facilitating GABA release in FSN→PN connections, which may not occur under a hunger state.

Orexin facilitates GABAergic IPSCs via postsynaptic OX1 receptors coupling to the intracellular PKC signalling cascade in the rat cerebral cortex

Orexin has multiple physiological functions including wakefulness, appetite, nicotine intake, and nociception. The cerebral cortex receives abundant orexinergic projections and expresses both orexinergic receptor 1 (OX₁R) and 2 (OX₂R). However, little is known about orexinergic regulation of GABA-mediated inhibitory synaptic transmission. In the cerebral cortex, there are multiple GABAergic neural subtypes, each of which has its own morphological and physiological characteristics. Therefore, identification of presynaptic GABAergic neural subtypes is critical to understand orexinergic effects on GABAergic connections. We focused on inhibitory synapses at pyramidal neurons (PNs) from fast-spiking GABAergic neurons (FSNs) in the insular cortex by a paired whole-cell patch-clamp technique, and elucidated the mechanisms of orexin-induced IPSC regulation. We found that both orexin A and orexin B enhanced unitary IPSC (uIPSC) amplitude in FSN→PN connections without changing the paired-pulse ratio or failure rate. These effects were blocked by SB-334867, an OX₁ receptor (OX₁R) antagonist, but not by TCS-OX2-29, an OX₂R antagonist. [Ala¹¹, D-Leu¹⁵]-orexin B, a selective OX₂R agonist, had little effect on uIPSCs. Variance-mean analysis demonstrated an increase in quantal content without a change in release probability or the number of readily releasable pools. Laser photolysis of caged GABA revealed that orexin A enhanced GABA-mediated currents in PNs. Downstream blockade of G_q/11 protein-coupled OX₁Rs by IP₃ receptor or protein kinase C (PKC) blockers and BAPTA injection into postsynaptic PNs diminished the orexin A-induced uIPSC enhancement. These results suggest that the orexinergic uIPSC enhancement is mediated via postsynaptic OX₁Rs, which potentiate GABA_A receptors through PKC activation.

Trigeminal Nerve Transection-Induced Neuroplastic Changes in the Somatosensory and Insular Cortices in a Rat Ectopic Pain Model

The primary sensory cortex processes competitive sensory inputs. Ablation of these competitive inputs induces neuroplastic changes in local cortical circuits. However, information concerning cortical plasticity induced by a disturbance of competitive nociceptive inputs is limited. Nociceptive information from the maxillary and mandibular molar pulps converges at the border between the ventral secondary somatosensory cortex (S2) and insular oral region (IOR); therefore, S2/IOR is a suitable target for examining the cortical changes induced by a disturbance of noxious inputs, which often causes neuropathic pain and allodynia. We focused on the plastic changes in S2/IOR excitation in a model of rats subjected to inferior alveolar nerve transection (IANX). Our optical imaging using a voltage-sensitive dye (VSD) revealed that the maxillary molar pulp stimulation-induced excitatory propagation was expanded one to two weeks after IANX at the macroscopic level. At the cellular level, based on Ca²⁺ imaging using two-photon microscopy, the amplitude of the Ca²⁺ responses and the number of responding neurons in S2/IOR increased in both excitatory and inhibitory neurons. The in vitro laser scanning photostimulation (LSPS) revealed that Layer II/III pyramidal and GABAergic fast-spiking neurons in S2/IOR received larger excitatory inputs from Layer IV in the IANX models, which supports the findings obtained by the macroscopic and microscopic optical imaging. Furthermore, the inhibitory postsynaptic inputs to the pyramidal neurons were decreased in the IANX models, suggesting suppression of inhibitory synaptic transmission onto excitatory neurons. These results suggest that IANX induces plastic changes in S2/IOR by changing the local excitatory and inhibitory circuits.

Opposite Roles in Short-Term Plasticity for N-Type and P/Q-Type Voltage-Dependent Calcium Channels in GABAergic Neuronal Connections in the Rat Cerebral Cortex

Neurotransmitter release is triggered by Ca²⁺ influx through voltage-dependent Ca²⁺ channels (VDCCs). Distinct expression patterns of VDCC subtypes localized on the synaptic terminal affect intracellular Ca²⁺ dynamics induced by action potential-triggered Ca²⁺ influx. However, it has been unknown whether the expression pattern of VDCC subtypes depends on each axon terminal or neuronal subtype. Furthermore, little information is available on how these VDCC subtypes regulate the release probability of neurotransmitters. To address these questions, we performed multiple whole-cell patch-clamp recordings from GABAergic neurons in the insular cortex of either the male or the female rat. The paired-pulse ratio (PPR; 50 ms interstimulus interval) varied widely among inhibitory connections between GABAergic neurons. The PPR of unitary IPSCs was enhanced by ω-conotoxin GVIA (CgTx; 3 μm), an N-type VDCC blocker, whereas blockade of P/Q-type VDCCs by ω-agatoxin IVA (AgTx, 200 nm) decreased the PPR. In the presence of CgTx, application of 4 mm [Ca²⁺]_o or of roscovitine, a P/Q-type activator, increased the PPR. These results suggest that the recruitment of P/Q-type VDCCs increases the PPR, whereas N-type VDCCs suppress the PPR. Furthermore, we found that charybdotoxin or apamin, blockers of Ca²⁺-dependent K⁺ channels, with AgTx increased the PPR, suggesting that Ca²⁺-dependent K⁺ channels are coupled to N-type VDCCs and suppress the PPR in GABAergic neuronal terminals. Variance-mean analysis with changing [Ca²⁺]_o showed a negative correlation between the PPR and release probability in GABAergic synapses. These results suggest that GABAergic neurons differentially express N-type and/or P/Q-type VDCCs and that these VDCCs regulate the GABA release probability in distinct manners.SIGNIFICANCE STATEMENT GABAergic neuronal axons target multiple neurons and release GABA triggered by Ca²⁺ influx via voltage-dependent Ca²⁺ channels (VDCCs), including N-type and P/Q-type channels. Little is known about VDCC expression patterns in GABAergic synaptic terminals and their role in short-term plasticity. We focused on inhibitory synaptic connections between GABAergic neurons in the cerebral cortex using multiple whole-cell patch-clamp recordings and found different expression patterns of VDCCs in the synaptic terminals branched from a single presynaptic neuron. Furthermore, we observed facilitative and depressive short-term plasticity of IPSCs mediated by P/Q-type and N-type VDCCs, respectively. These results suggest that VDCC expression patterns regulate distinctive types of synaptic transmission in each GABAergic axon terminal even though they are branched from a common presynaptic neuron.

Substance-Specific Differences in Human Electroencephalographic Burst Suppression Patterns

Different anesthetic agents induce burst suppression in the electroencephalogram (EEG) at very deep levels of general anesthesia. EEG burst suppression has been identified to be a risk factor for postoperative delirium (POD). EEG based automated detection algorithms are used to detect burst suppression patterns during general anesthesia and a burst suppression ratio (BSR) is calculated. Unfortunately, applied algorithms do not give information as precisely as suggested, often resulting in an underestimation of the patients’ burst suppression level. Additional knowledge of substance-specific burst suppression patterns could be of great importance to improve the ability of EEG based monitors to detect burst suppression. In a re-analysis of EEG recordings obtained from a previous study, we analyzed EEG data of 45 patients undergoing elective surgery under general anesthesia. The patients were anesthetized with sevoflurane, isoflurane or propofol (n = 15, for each group). After skin incision, the used agent was titrated to a level when burst suppression occurred. In a visual analysis of the EEG, blinded to the used anesthetic agent, we included the first distinct burst in our analysis. To avoid bias through changing EEG dynamics throughout the burst, we only focused on the first 2 s of the burst. These episodes were analyzed using the power spectral density (PSD) and normalized PSD, the absolute burst amplitude and absolute burst slope, as well as permutation entropy (PeEn). Our results show significant substance-specific differences in the architecture of the burst. Volatile-induced bursts showed higher burst amplitudes and higher burst power. Propofol-induced bursts had significantly higher relative power in the EEG alpha-range. Further, isoflurane-induced bursts had the steepest burst slopes. We can present the first systematic comparison of substance-specific burst characteristics during anesthesia. Previous observations, mostly derived from animal studies, pointing out the substance-specific differences in bursting behavior, concur with our findings. Our findings of substance-specific EEG characteristics can provide information to help improve automated burst suppression detection in monitoring devices. More specific detection of burst suppression may be helpful to reduce excessive EEG effects of anesthesia and therefore the incidence of adverse outcomes such as POD.

Actions of Propofol on Neurons in the Cerebral Cortex

Propofol is primarily a hypnotic, and is widely used for induction and maintenance of anesthesia, as well as for sedation in various medical procedures. The exact mechanisms of its action are not well understood, although its neural mechanisms have been explored in in vivo and in vitro experiments. Accumulating evidence indicates that one of the major targets of propofol is the cerebral cortex. The principal effect of propofol is considered to be the potentiation of GABA_A receptor-mediated inhibitory synaptic currents, but propofol has additional roles in modulating ion channels, including voltage-gated Na⁺ channels and several K⁺ channels. We focus on the pharmacological actions of propofol on cerebrocortical neurons, particularly at the cellular and synaptic levels.

Comparison of αβδ and αβγ GABAA receptors: Allosteric modulation and identification of subunit arrangement by site-selective general anesthetics

GABA_A receptors play a dominant role in mediating inhibition in the mature mammalian brain, and defects of GABAergic neurotransmission contribute to the pathogenesis of a variety of neurological and psychiatric disorders. Two types of GABAergic inhibition have been described: αβγ receptors mediate phasic inhibition in response to transient high-concentrations of synaptic GABA release, and αβδ receptors produce tonic inhibitory currents activated by low-concentration extrasynaptic GABA. Both αβδ and αβγ receptors are important targets for general anesthetics, which induce apparently different changes both in GABA-dependent receptor activation and in desensitization in currents mediated by αβγ vs. αβδ receptors. Many of these differences are explained by correcting for the high agonist efficacy of GABA at most αβγ receptors vs. much lower efficacy at αβδ receptors. The stoichiometry and subunit arrangement of recombinant αβγ receptors are well established as β-α-γ-β-α, while those of αβδ receptors remain controversial. Importantly, some potent general anesthetics selectively bind in transmembrane inter-subunit pockets of αβγ receptors: etomidate acts at β⁺/α^- interfaces, and the barbiturate R-5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid (R-mTFD-MPAB) acts at α⁺/β^- and γ⁺/β^- interfaces. Thus, these drugs are useful as structural probes in αβδ receptors formed from free subunits or concatenated subunit assemblies designed to constrain subunit arrangement. Although a definite conclusion cannot be drawn, studies using etomidate and R-mTFD-MPAB support the idea that recombinant α1β3δ receptors may share stoichiometry and subunit arrangement with α1β3γ2 receptors.

Histamine H3 Heteroreceptors Suppress Glutamatergic and GABAergic Synaptic Transmission in the Rat Insular Cortex

Histamine H₃ receptors are autoreceptors that regulate histamine release from histaminergic neuronal terminals. The cerebral cortex, including the insular cortex (IC), expresses abundant H₃ receptors; however, the functions and mechanisms of H₃ receptors remain unknown. The aim of this study was to elucidate the functional roles of H₃ in synaptic transmission in layer V of the rat IC. Unitary excitatory and inhibitory postsynaptic currents (uEPSCs and uIPSCs) were obtained through paired whole-cell patch-clamp recording in cerebrocortical slice preparations. The H₃ receptor agonist, R-α-methylhistamine (RAMH), reduced the uEPSC amplitude obtained from pyramidal cell to pyramidal cell or GABAergic interneuron connections. Similarly, RAMH reduced the uIPSC amplitude in GABAergic interneuron to pyramidal cell connections. RAMH-induced decreases in both the uEPSC and uIPSC amplitudes were accompanied by increases in the failure rate and paired-pulse ratio. JNJ 5207852 dihydrochloride or thioperamide, H₃ receptor antagonists, inhibited RAMH-induced suppression of uEPSCs and uIPSCs. Unexpectedly, thioperamide alone increased the uIPSC amplitude, suggesting that thioperamide was likely to act as an inverse agonist. Miniature EPSC or IPSC recordings support the hypothesis that the activation of H₃ receptors suppresses the release of glutamate and GABA from presynaptic terminals. The colocalization of H₃ receptors and glutamate decarboxylase or vesicular glutamate transport protein 1 in presynaptic axon terminals was confirmed through double pre-embedding microscopy, using a combination of pre-embedding immunogold and immunoperoxidase techniques. The suppressive regulation of H₃ heteroreceptors on synaptic transmission might mediate the regulation of sensory information processes, such as gustation and visceral sensation, in the IC.

Spike Timing Rigidity Is Maintained in Bursting Neurons under Pentobarbital-Induced Anesthetic Conditions

Pentobarbital potentiates γ-aminobutyric acid (GABA)-mediated inhibitory synaptic transmission by prolonging the open time of GABA_A receptors. However, it is unknown how pentobarbital regulates cortical neuronal activities via local circuits in vivo. To examine this question, we performed extracellular unit recording in rat insular cortex under awake and anesthetic conditions. Not a few studies apply time-rescaling theorem to detect the features of repetitive spike firing. Similar to these methods, we define an average spike interval locally in time using random matrix theory (RMT), which enables us to compare different activity states on a universal scale. Neurons with high spontaneous firing frequency (>5 Hz) and bursting were classified as HFB neurons (n = 10), and those with low spontaneous firing frequency (<10 Hz) and without bursting were classified as non-HFB neurons (n = 48). Pentobarbital injection (30 mg/kg) reduced firing frequency in all HFB neurons and in 78% of non-HFB neurons. RMT analysis demonstrated that pentobarbital increased in the number of neurons with repulsion in both HFB and non-HFB neurons, suggesting that there is a correlation between spikes within a short interspike interval (ISI). Under awake conditions, in 50% of HFB and 40% of non-HFB neurons, the decay phase of normalized histograms of spontaneous firing were fitted to an exponential function, which indicated that the first spike had no correlation with subsequent spikes. In contrast, under pentobarbital-induced anesthesia conditions, the number of non-HFB neurons that were fitted to an exponential function increased to 80%, but almost no change in HFB neurons was observed. These results suggest that under both awake and pentobarbital-induced anesthetized conditions, spike firing in HFB neurons is more robustly regulated by preceding spikes than by non-HFB neurons, which may reflect the GABA_A receptor-mediated regulation of cortical activities. Whole-cell patch-clamp recording in the IC slice preparation was performed to compare the regularity of spike timing between pyramidal and fast-spiking (FS) neurons, which presumably correspond to non-HFB and HFB neurons, respectively. Repetitive spike firing of FS neurons exhibited a lower variance of ISI than pyramidal neurons both in control and under application of pentobarbital, supporting the above hypothesis.