Presynaptic Interneuron Subtype- and Age-Dependent Modulation of GABAergic Synaptic Transmission by β-Adrenoceptors in Rat Insular Cortex

Presynaptic Adrenoceptors

Presynaptic α2-adrenoceptors are localized on axon terminals of many noradrenergic and non-noradrenergic neurons in the peripheral and central nervous systems. Their activation by exogenous agonists leads to inhibition of the exocytotic release of noradrenaline and other transmitters from the neurons. Most often, the α2A-receptor subtype is involved in this inhibition. The chain of molecular events between receptor occupation and inhibition of the exocytotic release of transmitters has been determined. Physiologically released endogenous noradrenaline elicits retrograde autoinhibition of its own release. Some clonidine-like α2-receptor agonists have been used to treat hypertension. Dexmedetomidine is used for prolonged sedation in the intensive care; It also has a strong analgesic effect. The α2-receptor antagonist mirtazapine increases the noradrenaline concentration in the synaptic cleft by interrupting physiological autoinhibion of release. It belongs to the most effective antidepressive drugs. β2-Adrenoceptors are also localized on axon terminals in the peripheral and central nervous systems. Their activation leads to enhanced transmitter release, however, they are not activated by endogenous adrenaline.

Insulin potentiates inhibitory synaptic currents between fast-spiking and pyramidal neurons in the rat insular cortex

Insulin plays roles in brain functions such as neural development and plasticity and is reported to be involved in dementia and depression. However, little information is available on the insulin-mediated modulation of electrophysiological activities, especially in the cerebral cortex. This study examined how insulin modulates the neural activities of inhibitory neurons and inhibitory postsynaptic currents (IPSCs) in rat insular cortex (IC; either sex) by multiple whole-cell patch-clamp recordings. We demonstrated that insulin increased the repetitive spike firing rate with a decrease in the threshold potential without changing the resting membrane potentials and input resistance of fast-spiking GABAergic neurons (FSNs). Next, we found a dose-dependent enhancement of unitary IPSCs (uIPSCs) by insulin in the connections from FSNs to pyramidal neurons (PNs). The insulin-induced enhancement of uIPSCs accompanied decreases in the paired-pulse ratio, suggesting that insulin increases GABA release from presynaptic terminals. The finding of miniature IPSC recordings of the increased frequency without changing the amplitude supports this hypothesis. Insulin had little effect on uIPSCs under the coapplication of S961, an insulin receptor antagonist, or lavendustin A, an inhibitor of tyrosine kinase. The PI3-K inhibitor wortmannin or the PKB/Akt inhibitors, deguelin and Akt inhibitor VIII, blocked the insulin-induced enhancement of uIPSCs. Intracellular application of Akt inhibitor VIII to presynaptic FSNs also blocked insulin-induced enhancement of uIPSCs. In contrast, uIPSCs were enhanced by insulin in combination with the MAPK inhibitor PD98059. These results suggest that insulin facilitates the inhibition of PNs by increases in FSN firing frequency and IPSCs from FSNs to PNs. (250 words).

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

Parvalbumin Interneuron Dysfunction in a Thalamo-Prefrontal Cortical Circuit in Disc1 Locus Impairment Mice

Altered cortical excitation-inhibition (E-I) balance resulting from abnormal parvalbumin interneuron (PV IN) function is a proposed pathophysiological mechanism of schizophrenia and other major psychiatric disorders. Preclinical studies have indicated that disrupted-in-schizophrenia-1 (Disc1) is a useful molecular lead to address the biology of prefrontal cortex (PFC)-dependent cognition and PV IN function. To date, PFC inhibitory circuit function has not been investigated in depth in Disc1 locus impairment (LI) mouse models. Therefore, we used a Disc1 LI mouse model to investigate E-I balance in medial PFC (mPFC) circuits. We found that inhibition onto layer 2/3 excitatory pyramidal neurons in the mPFC was significantly reduced in Disc1 LI mice. This reduced inhibition was accompanied by decreased GABA release from local PV, but not somatostatin (SOM) INs, and by impaired feedforward inhibition (FFI) in the mediodorsal thalamus (MD) to mPFC circuit. Our mechanistic findings of abnormal PV IN function in a Disc1 LI model provide insight into biology that may be relevant to neuropsychiatric disorders including schizophrenia.

Chronic ethanol exposure alters prelimbic prefrontal cortical Fast-Spiking and Martinotti interneuron function with differential sex specificity in rat brain

Chronic ethanol exposure results in numerous neurobiological adaptations that promote deficits in medial prefrontal cortical (mPFC) function associated with blunted inhibitory control and elevated anxiety during withdrawal. Studies exploring alcohol dependence-related changes in this region have largely investigated adaptations in glutamatergic signaling, with inhibitory neurotransmission remaining relatively understudied. To address this, we used biochemical and electrophysiological methods to evaluate the effects of ethanol on the activity of deep-layer prelimbic mPFC Fast-Spiking (FS) and Martinotti interneurons after chronic ethanol exposure in male and female rats. We report that chronic alcohol exposure significantly impairs FS neuron excitability in both males and females. Interestingly, we observed a marked sex difference in the baseline activity of Martinotti cells that furthermore displayed differential sex-specific responses to alcohol exposure. In addition, alcohol effects on Martinotti neuron excitability negatively correlated with hyperpolarization-activated currents mediated by hyperpolarization-activated cyclic nucleotide gated (HCN) channels, indicative of a causal relationship. Analysis of HCN1 protein expression also revealed a substantial sex difference, although no effect of ethanol on HCN1 protein expression was observed. Taken together, these findings further elucidate the complex adaptations that occur in the mPFC after chronic ethanol exposure and reveal fundamental differences in interneuron activity between sexes. Furthermore, this disparity may reflect innate differences in intracortical microcircuit function between male and female rats, and offers a tenable circuit-level explanation for sex-dependent behavioral responses to alcohol.

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).

Diversity and Function of Somatostatin-Expressing Interneurons in the Cerebral Cortex

Inhibitory interneurons make up around 10-20% of the total neuron population in the cerebral cortex. A hallmark of inhibitory interneurons is their remarkable diversity in terms of morphology, synaptic connectivity, electrophysiological and neurochemical properties. It is generally understood that there are three distinct and non-overlapping interneuron classes in the mouse neocortex, namely, parvalbumin-expressing, 5-HT3A receptor-expressing and somatostatin-expressing interneuron classes. Each class is, in turn, composed of a multitude of subclasses, resulting in a growing number of interneuron classes and subclasses. In this review, I will focus on the diversity of somatostatin-expressing interneurons (SOM+ INs) in the cerebral cortex and elucidate their function in cortical circuits. I will then discuss pathological consequences of a malfunctioning of SOM+ INs in neurological disorders such as major depressive disorder, and present future avenues in SOM research and brain pathologies.

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.

Central taste anatomy and physiology

The gustatory system contributes to the flavor of foods and beverages and communicates information about nutrients and poisons. This system has evolved to detect and ultimately respond to hydrophilic molecules dissolved in saliva. Taste receptor cells, located in taste buds and distributed throughout the oral cavity, activate nerve afferents that project to the brainstem. From here, information propagates to thalamic, subcortical, and cortical areas, where it is integrated with information from other sensory systems and with homeostatic, visceral, and affective processes. There is considerable divergence, as well as convergence, of information between multiple regions of the central nervous system that interact with the taste pathways, with reciprocal connections occurring between the involved regions. These widespread interactions among multiple systems are crucial for the perception of food. For example, memory, hunger, satiety, and visceral changes can directly affect and can be affected by the experience of tasting. In this chapter, we review the literature on the central processing of taste with a specific focus on the anatomic and physiologic responses of single neurons. Emphasis is placed on how information is distributed along multiple systems with the goal of better understanding how the rich and complex sensations associated with flavor emerge from large-scale, systems-wide, interactions.

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.

Relationship between the fluorescence intensity of rhodamine-labeled orexin A and the calcium responses in cortical neurons: An in vivo two-photon calcium imaging study

Neural responses to a ligand vary widely between neurons; however, the mechanisms underlying this variation remain unclear. One possible mechanism is a variation in the number of receptors expressed in each neural membrane. Here, we synthesized a rhodamine-labeled orexin A compound, enabling us to quantify the amount of orexin binding to its receptors, OX₁ and OX₂, which principally couple to the G_q/11 protein. The rhodamine intensity and calcium response were measured under tetrodotoxin application from insular cortical glutamatergic neurons in Thy1-GCaMP6s transgenic mice using an in vivo two-photon microscope. Applying rhodamine-labeled orexin A (10 μM) to the cortical surface gradually and heterogeneously increased both the intensity of the rhodamine fluorescence and [Ca²⁺]_i. Calcium responses started simultaneously with the increase in rhodamine-labeled orexin fluorescence and reached a plateau within several minutes. We classified neurons as high- and low-responding neurons based on the peak amplitude of the [Ca²⁺]_i increase. The rhodamine fluorescence intensity was larger in the high-responding neurons than the low-responding neurons. Preapplication of SB334867 and TCS-OX₂-29, OX₁ and OX₂ antagonists, respectively, decreased the proportion of high-responding neurons. These results suggest that the diverse receptor expression level in neural membranes is involved in mechanisms underlying varied neural responses, including [Ca²⁺]_i increases.

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.

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.

Role of Somatostatin-Positive Cortical Interneurons in the Generation of Sleep Slow Waves

During non-rapid eye-movement (NREM) sleep, cortical and thalamic neurons oscillate every second or so between ON periods, characterized by membrane depolarization and wake-like tonic firing, and OFF periods, characterized by membrane hyperpolarization and neuronal silence. Cortical slow waves, the hallmark of NREM sleep, reflect near-synchronous OFF periods in cortical neurons. However, the mechanisms triggering such OFF periods are unclear, as there is little evidence for somatic inhibition. We studied cortical inhibitory interneurons that express somatostatin (SOM), because ∼70% of them are Martinotti cells that target diffusely layer I and can block excitatory transmission presynaptically, at glutamatergic terminals, and postsynaptically, at apical dendrites, without inhibiting the soma. In freely moving male mice, we show that SOM+ cells can fire immediately before slow waves and their optogenetic stimulation during ON periods of NREM sleep triggers long OFF periods. Next, we show that chemogenetic activation of SOM+ cells increases slow-wave activity (SWA), slope of individual slow waves, and NREM sleep duration; whereas their chemogenetic inhibition decreases SWA and slow-wave incidence without changing time spent in NREM sleep. By contrast, activation of parvalbumin+ (PV+) cells, the most numerous population of cortical inhibitory neurons, greatly decreases SWA and cortical firing, triggers short OFF periods in NREM sleep, and increases NREM sleep duration. Thus SOM+ cells, but not PV+ cells, are involved in the generation of sleep slow waves. Whether Martinotti cells are solely responsible for this effect, or are complemented by other classes of inhibitory neurons, remains to be investigated.SIGNIFICANCE STATEMENT Cortical slow waves are a defining feature of non-rapid eye-movement (NREM) sleep and are thought to be important for many of its restorative benefits. Yet, the mechanism by which cortical neurons abruptly and synchronously cease firing, the neuronal basis of the slow wave, remains unknown. Using chemogenetic and optogenetic approaches, we provide the first evidence that links a specific class of inhibitory interneurons-somatostatin-positive cells-to the generation of slow waves during NREM sleep in freely moving mice.

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.

Suppressive regulation of lateral inhibition between medium spiny neurons via dopamine D1 receptors in the rat nucleus accumbens shell

The nucleus accumbens (NAc) shell is closely associated with reward, psychiatric disorders (depression or schizophrenia), and drug abuse. Dopamine, released from the ventral tegmental area, is involved in these physiological functions and pathophysiological changes of NAc shell. Medium spiny neurons (MSNs), which are only GABAergic projection neurons in NAc, also innervate adjacent MSNs, forming the lateral inhibition network. Previous studies demonstrate that dopamine suppresses the lateral inhibition via D₂-like (D₂ and D₃) receptors. However, the regulation to MSN-MSN synaptic transmission via D₁ receptors remained unclear. In present study, aiming to reveal this issue, we examined the effects of the potent dopamine D₁ receptor selective agonist SKF82958 on unitary IPSCs (uIPSCs) between two MSNs. SKF82958 (10μM) decreased the amplitude of uIPSCs in about half of MSNs. The actions of SKF82958 was eliminated by pre-application of SCH23390 (1μM), a dopamine D₁ receptor selective antagonist. These results suggest that lateral inhibition between MSNs was suppressed via the activation of D₁ receptors. Taken our previous findings, dopamine exclusively abolish the lateral inhibition in a stepwise pattern: (1) at low concentration of dopamine, only D₃ receptors take part in the regulation of MSN-MSN synaptic transmissions, (2) dopamine concentration becomes higher, D₂ receptors become involved in the suppression of lateral inhibition, and (3) at the maximal activity of the mesolimbic dopaminergic pathway, all dopamine receptor subtypes (i.e., D₁, D₂, and D₃) are recruited for disinhibition of MSN activities.

Postnatal development and sensory experience synergistically underlie the excitatory/inhibitory features of hippocampal neural circuits: Glutamatergic and GABAergic neurotransmission

During a postnatal critical period balance of excitation/inhibition in the developing brain is highly regulated by environmental signals. Compared to the visual cortex, rare document includes effects of sensory experience on the hippocampus, which is also bombarded by sensory signals. In this study, basic and tetanized field excitatory postsynaptic potentials (fEPSPs) were recorded in CA1 area of hippocampus of light-(LR) and dark-reared (DR) rats (at 2, 4 and 6weeks of age). Also, we assessed age- and activity-dependent changes in the N-Methyl-d-aspartic acid (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors subunit compositions and, GABA producing enzymes. While the sensory deprivation increased amplitude of baseline fEPSPs, it decreased degree of potentiation of post-tetanus responses. Expression of GluA1 and GluA2 subunits of AMPA receptors was increased across age in DR rats. In contrast to LR rats, mRNA and protein expression of GluN1, GluN2A and GluN2B subunits of NMDA receptors was decreased in DR ones. Also, dark rearing diminished expression of GABA synthesis enzymes GAD65 and GAD67. These results indicate that, sensory experience adjusts synaptic plasticity and might also affect the balance of excitation/inhibition in the hippocampus.

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.

Isoproterenol facilitates GABAergic autapses in fast-spiking cells of rat insular cortex

In the cerebral cortex, fast-spiking (FS) cells are the principal GABAergic interneurons and potently suppress neural activity in targeting neurons. Some FS neurons make synaptic contacts with themselves. Such synapses are called autapses and contribute to self-inhibition of FS neural activity. β-Adrenoceptors have a crucial role in regulating GABAergic synaptic inputs from FS cells to pyramidal (Pyr) cells; however, the β-adrenergic functions on FS autapses are unknown. To determine how the β-adrenoceptor agonist isoproterenol modulates inhibitory synaptic transmission in the autapses of FS cells, paired whole-cell patch-clamp recordings were obtained from FS and Pyr cells in layer V of rat insular cortex. Previous studies found that isoproterenol (100 μM) had pleiotropic effects on unitary inhibitory postsynaptic currents (uIPSCs) in FS→Pyr connections, whereas autapses in FS cells were always facilitated by isoproterenol. Facilitation of autapses by isoproterenol was accompanied by decreases in the paired-pulse ratio of second to first uIPSC amplitudes and the coefficient of variation of the uIPSC amplitude, which suggests that β-adrenergic facilitation is likely mediated by presynaptic mechanisms. The discrepancy between isoproterenol-induced modulation of uIPSCs in FS autapses and in FS→Pyr connections may reflect the presence of different presynaptic mechanisms of GABA release in each synapse.

Tachycardic responses to stimulation of β-adrenoceptors in the brain parenchyma in conscious rats

This study aimed to investigate how stimulation of β-adrenoceptors in the anteroventral third ventricular region (AV3V; a pivotal forebrain area for autonomic functions) and other brain regions affects heart rate (HR) in conscious rats. Topical injections of the β-adrenergic agonist isoproterenol (Isop) into the AV3V caused dose-related and reversible increases in HR. Only its highest dose utilized significantly affected blood pressure (BP), inducing a decrease. The tachycardia due to AV3V Isop lasted significantly longer than that elicited by hypotension, and was inhibited by AV3V administration of the β-adrenergic antagonist propranolol or systemic infusion of a ganglion blocker hexamethonium. Plasma noradrenaline indicative of sympathetic nerve activity increased in parallel with rises in HR after the AV3V application of Isop. When Isop was locally injected into various brain regions other than the AV3V, region-related effectiveness in provoking tachycardia was observed that tended to be large in limbic structures and the hypothalamic paraventricular nucleus. No region responded to Isop applications with decreases in HR. These results suggest that β-adrenoceptors in the AV3V and other brain regions may be able to produce tachycardia by enhancing, at least in part, the efferent sympathetic nerve activity controlling cardiac function.

Selective suppression of excitatory synapses on GABAergic interneurons by norepinephrine in juvenile rat prefrontal cortical microcircuitry

The noradrenergic system of the brain is thought to facilitate neuronal processes that promote behavioral activation, alertness, and attention. It is known that norepinephrine (NE) can be significantly elevated in the prefrontal cortex under normal conditions such as arousal and attention, and following the administration of psychostimulants and various other drugs prescribed for psychiatric disorders. However, how NE modulates neuronal activity and synapses in the local prefrontal circuitry remains elusive. In this study, we characterized the actions of NE on individual monosynaptic connections among layer V pyramidal neurons (P) and fast-spiking (FS) GABAergic interneurons in the juvenile (postnatal days 20-23) rat prefrontal local circuitry. We found that NE selectively depresses excitatory synaptic transmission in P-FS connections but has no detectable effect on the excitatory synapses in P-P connections and the inhibitory synapses in FS-P connections. NE apparently exerts distinctly different modulatory actions on identified synapses that target GABAergic interneurons but has no effect on those in the pyramidal neurons in this specific developmental period. These results indicate that, depending on the postsynaptic targets, the effects of NE in prefrontal cortex are synapse-specific, at least in the juvenile animals.

Activation of β2-adrenoceptor enhances synaptic potentiation and behavioral memory via cAMP-PKA signaling in the medial prefrontal cortex of rats

The prefrontal cortex (PFC) plays a critical role in cognitive functions, including working memory, attention regulation, behavioral inhibition, as well as memory storage. The functions of PFC are very sensitive to norepinephrine (NE), and even low levels of endogenously released NE exert a dramatic influence on the functioning of the PFC. Activation of β-adrenoceptors (β-ARs) facilitates synaptic potentiation and enhances memory in the hippocampus. However, little is known regarding these processes in the PFC. In the present study, we investigate the role of β2-AR in synaptic plasticity and behavioral memory. Our results show that β2-AR selective agonist clenbuterol facilitates spike-timing-dependent long-term potentiation (tLTP) under the physiological conditions with intact GABAergic inhibition, and such facilitation is prevented by co-application with the cAMP inhibitor Rp-cAMPS. Loading postsynaptic pyramidal cells with Rp-cAMPS, the PKA inhibitor PKI(5-24), or the G protein inhibitor GDP-β-S significantly decreases, but does not eliminate, the effect of clenbuterol. Clenbuterol suppresses the GABAergic transmission, while blocking GABAergic transmission by the GABA(A) receptor blocker partially mimics the effect of clenbuterol. In behavioral tests, a post-training infusion of clenbuterol into mPFC enhances 24-h trace fear memory. In summary, we observed that prefrontal cortical β2-AR activation by clenbuterol facilitates tLTP and enhances trace fear memory. The mechanism underlying tLTP facilitation involves stimulating postsynaptic cAMP-PKA signaling cascades and suppressing GABAergic circuit activities.

Ca(2+)-activated ion currents triggered by ryanodine receptor-mediated Ca(2+) release control firing of inhibitory neurons in the prepositus hypoglossi nucleus

Spontaneous miniature outward currents (SMOCs) are known to exist in smooth muscles and peripheral neurons, and evidence for the presence of SMOCs in central neurons has been accumulating. SMOCs in central neurons are induced through Ca(2+)-activated K(+) (K(Ca)) channels, which are activated through Ca(2+)-induced Ca(2+) release from the endoplasmic reticulum via ryanodine receptors (RyRs). Previously, we found that some neurons in the prepositus hypoglossi nucleus (PHN) showed spontaneous outward currents (SOCs). In the present study, we used whole cell recordings in slice preparations of the rat brain stem to investigate the following: 1) the ionic mechanisms of SOCs, 2) the types of neurons exhibiting frequent SOCs, and 3) the effect of Ca(2+)-activated conductance on neuronal firing. Pharmacological analyses revealed that SOCs were induced via the activation of small-conductance-type K(Ca) (SK) channels and RyRs, indicating that SOCs correspond to SMOCs. An analysis of the voltage responses to current pulses of the fluorescence-expressing inhibitory neurons of transgenic rats revealed that inhibitory neurons frequently exhibited SOCs. Abolition of SOCs via blockade of SK channels enhanced the frequency of spontaneous firing of inhibitory PHN neurons. However, abolition of SOCs via blockade of RyRs reduced the firing frequency and hyperpolarized the membrane potential. Similar reductions in firing frequency and hyperpolarization were also observed when Ca(2+)-activated nonselective cation (CAN) channels were blocked. These results suggest that, in inhibitory neurons in the PHN, Ca(2+) release via RyRs activates SK and CAN channels, and these channels regulate spontaneous firing in a complementary manner.

Neural processing of gustatory information in insular circuits

The insular cortex is the primary cortical site devoted to taste processing. A large body of evidence is available for how insular neurons respond to gustatory stimulation in both anesthetized and behaving animals. Most of the reports describe broadly tuned neurons that are involved in processing the chemosensory, physiological and psychological aspects of gustatory experience. However little is known about how these neural responses map onto insular circuits. Particularly mysterious is the functional role of the three subdivisions of the insular cortex: the granular, the dysgranular and the agranular insular cortices. In this article we review data on the organization of the local and long-distance circuits in the three subdivisions. The functional significance of these results is discussed in light of the latest electrophysiological data. A view of the insular cortex as a functionally integrated system devoted to processing gustatory, multimodal, cognitive and affective information is proposed.

Short-term plasticity of unitary inhibitory-to-inhibitory synapses depends on the presynaptic interneuron subtype

Excitatory-to-inhibitory cortical synapses exhibit either short-term facilitation or depression, depending on the subtype identity of the postsynaptic interneuron, while the short-term plasticity (STP) of inhibitory-to-excitatory synapses depends on the presynaptic interneuron. However, the rules governing STP of inhibitory-to-inhibitory synapses have not yet been determined. We recorded 109 unitary connections made by the two major inhibitory interneuron subtypes in layer 4 of mouse somatosensory cortex, fast-spiking (FS) and somatostatin-containing (SOM) interneurons, on each other and on excitatory, regular-spiking (RS) neurons. In all pairs, we measured dynamic changes in the postsynaptic response to a 20 Hz train of presynaptic action potentials. In half of our dataset, we also measured kinetic properties of the unitary IPSC: latency, rise time, and decay time constant. We found a pronounced dependency of STP on the presynaptic, but not the postsynaptic, identity: FS interneurons made strongly depressing connections on FS, SOM, and RS targets, while in synapses made by SOM interneurons on FS and RS targets, weak early depression was followed by weak late facilitation. IPSC latency and rise time were also strongly dependent on the presynaptic interneuron subtype, being 1.5-2× slower in output synapses of SOM compared with FS interneurons. In contrast, the IPSC decay time constant depended only on the postsynaptic class, with 1.5× slower decay on excitatory compared with inhibitory targets. The properties of the inhibitory outputs of FS and SOM interneurons reciprocate the properties of their excitatory inputs and imply a dynamic spatiotemporal division of labor between these two major inhibitory subsystems.

Kinetics of GABAB autoreceptor-mediated suppression of GABA release in rat insular cortex

Release of GABA is controlled by presynaptic GABA receptor type B (GABA(B)) autoreceptors at GABAergic terminals. However, there is no direct evidence that GABA(B) autoreceptors are activated by GABA release from their own terminals, and precise profiles of GABA(B) autoreceptor-mediated suppression of GABA release remain unknown. To explore these issues, we performed multiple whole-cell, patch-clamp recordings from layer V rat insular cortex. Both unitary inhibitory and excitatory postsynaptic currents (uIPSCs and uEPSCs, respectively) were recorded by applying a five-train depolarizing pulse injection at 20 Hz. In connections from both fast-spiking (FS) and non-FS interneurons to pyramidal cells, the GABA(B) receptor antagonist CGP 52432 had little effect on the initial uIPSC amplitude. However, uIPSCs, responding to later pulses, were effectively facilitated. This CGP 52432-induced facilitation was prominent in the fourth uIPSCs, which were evoked 150 ms after the first uIPSC. The facilitation of uIPSCs was accompanied by an increase in the paired-pulse ratio. In addition, analysis of the coefficient of variation suggests the involvement of presynaptic mechanisms in CGP 52432-induced uIPSC facilitation. Paired-pulse stimulation (interstimulus interval = 150 ms) of presynaptic FS cells revealed that the second uIPSC was also facilitated by CGP 52432, which had little effect on the amplitude and interevent interval of miniature IPSCs. In contrast, uEPSCs, responding to all five stimulations of a presynaptic pyramidal cell, were less affected by CGP 52432. These results suggest that a single presynaptic action potential is sufficient to activate GABA(B) autoreceptors and to suppress GABA release in the cerebral cortex.

D(2)-like dopamine receptors differentially regulate unitary IPSCs depending on presynaptic GABAergic neuron subtypes in rat nucleus accumbens shell

In the nucleus accumbens (NAc), a medium spiny (MS) neuron receives GABAergic inputs from two major sources: fast-spiking (FS) neurons and other, adjacent MS neurons. These two types of inhibitory synapses are considered to play different roles in output activities, i.e., FS→MS connections suppress output from the NAc whereas MS→MS connections contribute to lateral inhibition. In the present study, we focused on the electrophysiological properties of unitary inhibitory postsynaptic currents (uIPSCs) obtained from MS→MS connections and FS→MS connections and examined the effects of quinpirole, a dopamine D(2)-like receptor agonist, on uIPSCs with multiple whole cell patch-clamp recording. Application of quinpirole (1 μM) reliably suppressed the amplitude of uIPSCs by 29.6% in MS→MS connections, with increases in paired-pulse ratio and failure rate. The suppressive effects of quinpirole on uIPSCs were mimicked by 1 μM PD128907, a D(2/3) receptor agonist, whereas quinpirole-induced suppression of uISPCs was blocked by preapplication of 1 μM sulpiride or 10 μM nafadotride, both D(2/3) receptor antagonists. On the other hand, quinpirole (1 μM) had divergent effects on FS→MS connections, i.e., quinpirole increased uIPSC amplitude in 38.1% of FS→MS connections and 23.8% of FS→MS connections were suppressed by quinpirole. Analysis of coefficient of variation in uIPSC amplitude implied the involvement of presynaptic mechanisms in quinpirole-induced effects on uIPSCs. These results suggest that activation of D(2)-like receptors facilitates outputs from MS neurons in the NAc by reducing lateral inhibition during a dormant period of FS neuron activities.

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.